Recent Developments in Polymer Recycling by Research Signpost

Recent Developments in Polymer Recycling Editors

A. Fainleib O. Grigoryeva Institute of Macromolecular Chemistry of the National Academy of Sciences of Ukraine Kharkivske shose, 48, 02160 Kyiv, Ukraine

Transworld Research Network, T.C. 37/661 (2), Fort P.O., Trivandrum-695 023 Kerala, India

Published by Transworld Research Network 2011; Rights Reserved Transworld Research Network T.C. 37/661(2), Fort P.O., Trivandrum-695 023, Kerala, India Editors A. Fainleib O. Grigoryeva Managing Editor S.G. Pandalai Publication Manager A. Gayathri Transworld Research Network and the Editors assume no responsibility for the opinions and statements advanced by contributors ISBN: 978-81-7895-524-7

Preface Recycling of polymer wastes is extremely important in a point of view of preservation of the environment and progressive industrial growth. This concerns especially utilization of the crosslinked, thermosetting polymers, composites based on them, for example, scrap automotive tires, tubes. Remember that vulcanized rubbers, thermosetting polymers and composites based on them do not dissolve or melt due to their cross-linked structure, which makes their recycling difficult. Many efforts have been done all over the world in order to find a general concept of recycling and reuse of the scrap elastomers and plastics. Only an insignificant part of them is reprocessed. Accumulation of the polymer wastes may lead to extensive pollution of the atmosphere and the ground through evaporation of toxic aromatic compounds. This book contains the papers on methods of utilization of different families of polymer wastes. Alexander Fainleib, Olga Grigoryeva

Contents

Contributors Chapter 1 Utilization of tire rubber and recycled polyolefins into thermoplastic elastomers Alexander Fainleib, Olga Grigoryeva, Boulos Youssef and Jean-Marc Saiter Chapter 2 Recent advances in the recycling of rubber waste Eldho Abraham, Bibin M Cherian, Elbi P A, Laly A Pothen and Sabu Thomas Chapter 3 Structure, properties and recyclability of natural fibre reinforced polymer composites Deepa B, Laly A. Pothan, Rubie Mavelil-Sam and Sabu Thomas

101

Chapter 4 Recycling of thermosetting polymers: Their blends and composites Raju Thomas, Poornima Vijayan and Sabu Thomas

121

Chapter 5 Recycling of oil and milk pouch polymers and its applications Basudam Adhikari and Arup Choudhury

155

Chapter 6 Recycling of polymer blends Jesmy Jose, Jyotishkumar P, Sajeev M. George and Sabu Thomas

187

Chapter 7 Microbial biodegradation of polyurethane Gary T. Howard

215

Chapter 8 The influence of adhesives on material recycling Hermann Onusseit

239

Chapter 9 Different ways for re-using polymer based wastes. The examples of works done in European countries Jean-Marc Saiter, Parambathmadhom Appu Sreekumar and Boulos Youssef

261

Contributors Alexander Fainleib Institute of Macromolecular Chemistry of the National Academy of Sciences of Ukraine, Kharkivske shose, 48, 02160 Kyiv, Ukraine Olga Grigoryeva Institute of Macromolecular Chemistry of the National Academy of Sciences of Ukraine, Kharkivske shose, 48, 02160 Kyiv, Ukraine Jean-Marc Saiter AMME-LECAP International Laboratory, EA4528, Institut des Matériaux Facultés des Sciences, Université de Rouen, BP 12 76801 Saint Etienne du Rouvray, Cedex, France Boulos Youssef AMME-LECAP International Laboratory, EA4528, Institut des Matériaux Facultés des Sciences, Université de Rouen, BP 12 76801 Saint Etienne du Rouvray, Cedex, France Institut National des Sciences Appliquées, Avenue 76801 Saint Etienne du Rouvray, Cedex, France Eldho Abraham Post Graduate Department of Chemistry, CMS College Kottayam, Kerala, India Bibin Mathew Cherian Post Graduate Department of Chemistry, Bishop Moore College Mavelikara 690110, Kerala, India M. Kottaisamy Centre for Nanotechnology, Kalasalingam University, Anand Nagar Krishnankoil 626 190, Virudhunagar, Tamil Nadu, India Deepa B Post Graduate Department of Chemistry, Bishop Moore College Mavelikara 690110, Kerala, India Laly A. Pothan Post Graduate Department of Chemistry, Bishop Moore College Mavelikara 690110, Kerala, India

Rubie M. Sam Laboratoire de Biophysique et Materiaux Alimentaire AgroParisTech, 1 Avenue des Olympiades, 91744 Massy France Sabu Thomas Department of Polymer Science and Engineering Mahatma Gandhi University, Kottayam 686560 Kerala, India Raju Thomas School of Chemical Sciences, Mahatma Gandhi University Priyadarshini Hills P.O., Kottayam-686 560 Kerala, India Poornima Vijayan School of Chemical Sciences, Mahatma Gandhi University Priyadarshini Hills P.O., Kottayam-686 560, Kerala, India Basudam Adhikari Materials Science Centre, Indian Institute of Technology Kharagpur 721302, India Arup Chowdhuri Department of Chemical & Polymer Engineering, Birla Institute of Technology, Mesra, Ranchi 835215, India Jyotishkumar P School of Chemical Sciences, Mahatma Gandhi University Priyadarshini Hills P.O., Kottayam-686 560, Kerala, India Sajeev Martin School of Chemical Sciences, Mahatma Gandhi University Priyadarshini Hills P.O., Kottayam-686 560, Kerala, India Jesmy Jose School of Chemical Sciences, Mahatma Gandhi University Priyadarshini Hills P.O., Kottayam-686 560, Kerala, India Gary Howard Department of Biological Sciences, Southeastern Louisiana University, Hammond, Louisiana 70402, USA Hermann Onusseit Henkel AG & Co. KGaA, 40589, Duesseldorf, Germany

Parambathmadhom Appu Sreekumar AMME-LECAP International Laboratory, EA4528, Institut des Matériaux Facultés des Sciences, Université de Rouen, BP 12 76801 Saint Etienne du Rouvray, Cedex, France

Transworld Research Network 37/661 (2), Fort P.O. Trivandrum-695 023 Kerala, India

Recent Developments in Polymer Recycling, 2011: 1-46 ISBN: 978-81-7895-524-7 Editors: A. Fainleib and O.Grigoryeva

1. Utilization of tire rubber and recycled polyolefins into thermoplastic elastomers 1

Alexander Fainleib1, Olga Grigoryeva1, Boulos Youssef2,3 and Jean-Marc Saiter2

Institute of Macromolecular Chemistry of the National Academy of Sciences of Ukraine Kharkivske shose, 48, 02160 Kyiv, Ukraine; 2AMME-LECAP International Laboratory, EA4528 Institut des Matériaux, Facultés des Sciences, Université de Rouen, BP 12, 76801 Saint Etienne du Rouvray, Cedex, France; 3Institut National des Sciences Appliquées Avenue 76801 Saint Etienne du Rouvray, Cedex, France

Abstract. In the review the methods of producing thermoplastic elastomers with benefit properties from post-consumer polyethylenes (HDPE, LDPE) and recycled rubbers (ground tire rubber, GTR) using several approaches of their pre-treatment and compatibilization procedures are discussed. Some additions of reactive polyethylenes and rubbers were used for improvement of interface adhesion in the blends studied. The TPE produced were characterized by TGA, DSC, DMTA, rheology measurements, X-ray diffraction and mechanical testing. For all of TPE studied the increasing components compatibility due to the formation of the essential interface layer has been observed. The method of renewal of GTR and its reactivation using its high temperature treatment with bitumen has been developed and successfully applied. High performance thermoplastic elastomers (TPEs) based on postconsumer high-density polyethylene (HDPE-pc), low density polyethylene (LDPE-pc), butadiene rubber (BR), olefinic type Correspondence/Reprint request: Dr. Alexander Fainleib, Institute of Macromolecular Chemistry of the National Academy of Sciences of Ukraine, Kharkivske shose, 48, 02160 Kyiv, Ukraine. E-mail: fainleib@i.ua

Alexander Fainleib et al.

ethylene/propylene/diene monomers (EPDM) containing rubber and ground tire rubber (GTR) treated with bitumen have been prepared by using the dynamic vulcanization technology, and their structure-property relationships were investigated. It was established that at special pre-treatment of GTR by bitumen bestows on the resulting TPEs outstanding mechanical properties. SEM, DSC and DMTA results revealed an improved adhesion between the particles of GTR treated by bitumen and a surrounding thermoplastic matrix compared to the untreated GTR particles. GTR containing TPEs prepared by extrusion technology were reprocessed (by passing through the extruder 6 times) without noting any significant change in their tensile properties, thermal stability and melt viscosity.

1. Introduction Thermoplastic elastomers (TPEs), especially blends of elastomer and thermoplastic obtained by dynamic vulcanization of rubber in thermoplastic and having characteristics of elastomers while maintaining the thermoplasticity are under serious interest of scientists and producers last decade [1-5]. In a point of view of both economical and ecological reason the replacement of virgin components of TPEs (fully or partly) by recycled polymers is very important. The problem is to obtain materials of beneficial properties (preferably, not inferior to traditional TPEs in main properties). Waste plastics, especially polyolefins, and rubbers, including tire rubber, have caused a series of environmental problems. Many approaches have been proposed to use the large amount of waste polymers. The standard use is a replacement of a part of virgin polyolefin, for example polyethylene, by some recycled grades. Similarly, part of virgin rubber is replaced by ground tire rubber (GTR) in less demanding and even in tire formulations [6, 7]. In recent years, a potential way to use GTR in thermoplastic elastomers has been developed. Numerous investigations [8-17] including our own ones have shown that introducing GTR directly into recipes of different polyolefin/rubber TPEs results in drastic decreasing their tensile strength and especially ultimate elongation. This is the result of poor interphase adhesion between the blend components [18, 19]. Various modifiers were used to compatibilize rubber/polyolefin blends with and without reclaimed GTR. A short review about the advantages of functionalization and compatibilization of TPEs including GTR has been recently done by Li et al. [20, 21]. For reactive compatibilization (it seems to be the most effective method), the components of TPEs including GTR should be functionalized or at least their surface has to be activated. It can be done by chemical grafting of reactive monomers onto the polymer surface or, in the case of GTR, by thermal, thermo-mechanical, thermo-chemical, ultrasonic devulcanization techniques, etc. [6, 7, 11, 20].

Utilization of tire rubber and recycled polyolefins into thermoplastic elastomers

2. Comparative characterization of virgin and post-consumer polyolefins In order to predict properties of thermoplastic elastomers prepared from recycled polyolefins and rubbers the following virgin and post-consumer polymers were analyzed and their main characteristics were compared [22, 23]: √

√ √

√

√ √ √

Low density polyethylene (LDPE), trademark "Riblene" FC30 (Polimeri Europe, Rome, Italy), with the following characteristics: Mn=31100, Mw=179200, Mz=487200, Mw/Mn=5.76, Mz/Mw=2.72; Tm=112 oC, Young's modulus, E=220 MPa, tensile strength, TS=18 MPa and elongation at break, EB=600%, MFI190/2.16=0.28 g/10 min and MFI230/2.16=0.8 g/10 min. Linear low density polyethylene (LDPEL), trademark “Lupolen 1840 H” (BASF AG, Ludwigshafen, Germany), MFI190/2.16=1.5 dg/min. Post-consumer low density polyethylene (LDPE-pc) made from greenhouse films of composition LDPE 65-70%, LDPEL 12-17%, EVA copolymer 12-15%, Tm=109 oC, E=180 MPa, TS=16 MPa, EB=500%, MFI190/2.16=0,29 g/10 min and MFI230/2.16=0,95 g/10 min. Post-consumer greenhouse films have been collected in the province of Ragusa (Sicily, Italy) after near one year of exploitation. Post-consumer films have been washed, dried and cut to pieces by an industrial scale. Isotactic polypropylene (PP), trademark “PP-169” (LisichanskNefteOrgsintez, Lisichansk, Ukraine), isotactic index 96%, Mn=150,000, Mw=1,000,000, Mw/Mn=6,7, Young's modulus E=130 MPa, TS=3.7 MPa and EB=530%, MFI190/2.16=3.4 g/10 min. Post-consumer polypropylene (PP-pc), (Roksana Ltd., Kyiv, Ukraine) made from post-consumer packages collected in Kiev (Ukraine), MFI230/2.16=1.9 g/10 min. High-density polyethylene (HDPE), trademark “HDPE 277-73” (KazanOrgSintez, Kazan, Russia), MFI190/5.0=20.6 g/10 min, density 0.961 g/cm3 (at 20 oC). Post-consumer high-density polyethylene (HDPE-pc), MFI190/2.16=2.13 g/10 min, TS= 17.7 MPa and EB=10 %. HDPE-pc made of postconsumer bottle transportation crates collected in Kyiv (Roksana Ltd., Kyiv, Ukraine). Waste of bottle transportation crates were washed, dried and cut to pieces by an industrial apparatus.

The comparison of main characteristics of virgin and post-consumer polyolefins, namely HDPE/HDPE-pc, LDPE/LDPE-pc and PP/PP-pc, was

Alexander Fainleib et al.

fulfilled [22, 23] using Thermogravimetric Analysis (DTA, DTG, TG), Wideangle X-Ray Scattering (WAXS), Dynamic Mechanical Thermal Analysis (DMTA), Differential Scanning Calorimetry (DSC) measurements and mechanical testing. Melt flow index for above mentioned polymers was also determined. The data obtained are summarized in Table 1. The corresponding DTA, DTG and TG curves LDPE and LDPE-pc are given in Figure 1. One can see that the curves (Figure 1a-c) of LDPE and LDPE-pc studied are very similar. TGA curves (Figure 1a) are characterized by presence of one endothermic peak as a result of melting LDPE and LDPE-pc (at 393 K and 388 K, respectively), and a few low-resolved high temperature exothermic peaks due to oxidative destruction of polyethylenes studied. Both LDPE and LDPE-pc have very close temperature of the beginning intensive decomposition near 600 K and char residue values of 3.5 and 5 %, respectively (see Figure 1c). Appearance of additional degradation stage (at 548-693 K) (Figure 1b) and high temperature shift of all TGA, DTG and TG curves, as well as increasing the melting temperature (see Figure 1a) and value of char residue (see Table 2) reflect existence of thermally more stable structures in LDPE-pc in comparison with LDPE. Obviously it is a result of partial degradation of LDPE-pc chains and formation of branched and/or cross-linked polymer chains. WAXS curves of LDPE and LDPE-pc are given in Figure 2a. Both diffractograms show two sharp peaks located at scattering angles of 21.1о and 23.4о (characteristic for orthorhombic crystal cell of polyethylene) identified as the (110) and (200) polyethylene strongest reflections, correspondingly [24]. The diffuse maximum located at 19.5о which corresponds to LDPE-pc amorphous phase scattering maximum [25]. Thus, one can see that there are Table 1. Properties of virgin and post-consumer polyolefins used. Material

Melting temperature, Tm, oC

Degree of crystallinity, Xc, %

Tensile strength, TS, MPa

Elongation at break, EB, %

Flow activation energy, Ea, k/mol

Melt flow index, MFI190/2.16, g/10 min

Gel contenta), wt.%

DSC

DTA

DSC

WAXS

HDPE HDPE-pc

135 135

126 115

62 70

58.6 55.6

19.0 17.7

11 8

29.3 33.0

2.02 2.13

0 0

LDPE LDPE-pc

118 116

120 115

47 43

29.2 27.5

11.0 7.1

668 440

47.4 47.4

0.27 0.28

0 6

PP PP-pc

174 174

165 164

52 48

51.5 40.2

15.3 10.5

484 405

47.9 38.3

3.4 1.9 b)

0 0

determined as o-xylene insoluble fraction; b)measured at 230 oC.

Utilization of tire rubber and recycled polyolefins into thermoplastic elastomers

Table 2. Thermal behavior of LDPE and LDPE-pc. Stages interval of weight loss, Tonset/Tend, K

Tmax rate, K

Weight loss, %

463 / 498

478

-2

503 / 573

553

673 / 773

743

753 / 833

803

Char residue, % :

3.5

LDPE

LDPE-pc 463 / 498

483

-2

498 / 568

533

548 / 693

633

683 / 773

748

773 / 833

813

Char residue, % :

-1

Δ m τ , % m in

-1

Δ m, % 0

0,0

a) Exo

-0,2

-2 0

-4 0

-0,4

-6 0

-0,6 388

-8 0

-0,8 3 93

3 00

450

-10 0 6 00

750

T em pera tu re, K

90 0

-1,0 30 0

4 50

60 0

7 50

T em pera tu re, K

9 00

3 00

450

6 00

7 50

90 0

T em pera tu re, K

Figure 1. Thermogravimetric analysis curves for LDPE (open circle) and LDPE-pc (solid circle): (a) Differential Thermal Analysis (DTA); (b) Differential Thermal Gravimetry (DTG); (c) Thermogravimetry (TG).

Alexander Fainleib et al. 90

21.1

I /1000 pulses

(a)

LD PE-pc

LD PE

19.5

23.4

0 5

2 Θ , degree

I /1000 pulses

20.3

(b)

24.3

36.4

6 31.9

34.6

2 Θ , degree

Figure 2. WAXS curves for: (a) LDPE (open circle) and LDPE-pc (solid circle); (b) BR cured. Table 3. Properties of LDPE and LDPE. Material

LDPE LDPE-pc

Tm, K

Degree of crystallinity, X, %

DSC

DTA

DSC

WAXS

391 389

393 388

47 43

29.2 27.5

Size of crystallites, D, Å

Crystal lattice spacing, d, Å

TS, MPa

EB, %

Gel content a), wt.%

10.7

0.421

11.1

0.421

11.0 7.1

668 440

0 6

Determined as o-xylene insoluble fraction

However, it can be seen that LDPE has a higher value of degree of crystallinity <X> than LDPE-pc. It can be explained by the reduction of molecular weight of LDPE-pc due to additional thermooxidative destruction as well as crosslinking occurred at their outdoor exploiting and reprocessing. It is clear, that LDPE-pc has higher content of amorphous phase than LDPE. WAXS curves of LDPE and LDPE-pc are given in Figure 2a. Both diffractograms show two sharp peaks located at scattering angles of 21.1о and 23.4о (characteristic for orthorhombic crystal cell of polyethylene) identified as the (110) and (200) polyethylene strongest reflections, correspondingly [24]. The diffuse maximum located at 19.5о which corresponds to LDPE-pc amorphous phase scattering maximum [25]. Thus, one can see that there are

Utilization of tire rubber and recycled polyolefins into thermoplastic elastomers

no appreciable differences in a crystal cell or amorphous phase periodicities because both the LDPE have the same mean size of microcrystals (D)=10.7 and 11.1 nm, respectively, and identical crystal lattice spacing (d)=0.421 nm. The data are summarized in Table 3. However, it can be seen that LDPE has a higher value of degree of crystallinity <X> than LDPE-pc. It can be explained by the reduction of molecular weight of LDPE-pc due to additional thermooxidative destruction as well as crosslinking occurred at their outdoor exploiting and reprocessing. It is clear, that LDPE-pc has higher content of amorphous phase than LDPE. The Cp (T) plots of both the LDPEs are shown in Figure 3. One can see that both LDPE and LDPE-pc have typical curves for semicrystalline polyolefins with a phase transition “solid – liquid” in the temperature region 310–391 K and 322–389 K, respectively. Note that the above melting temperature interval and melting peak temperature, Tm=389 K, for LDPE-pc studied are quite similar to the reported values for other post-consumer LDPEs [26]. In contrary to LDPE melting process of crystalline phase of LDPE-pc consists of melting low molecular weight crystallites (probably defected) in the temperature range 322-338 K and melting high molecular weight crystallites at 389 K [27]. We consider that shoulder in the temperature region of 338–350 K without any visible changes in values of Cp relates to a recrystallization of above low molecular weight crystallites of LDPE-pc into the high molecular weight crystallites. The common decrease of values of Cp in temperature region between 322 and 373 K observed for LDPE-pc sample in comparison with LDPE evidences of increasing packing density of polymer due to formation of above mentioned branched or crosslinked polymer chains. It can be seen that LDPE has a higher value of degree of crystallinity <X> than LDPE-pc that is agreed to WAXS data. 391

b) 389

-1

2,5

6,5

Cp, Jg K

-1

Cp, Jg K

-1

3,0

383

2,0

6,0 373

249

1,5

319

5,5

203 350

1,0

338

5,0 0,5

200 250 300 350 400 450

Temperature, K

322 310

275 300 325 350 375 400 425

Temperature, K

Figure 3. Temperature dependence of specific heat capacity (Cp) of: (a) BR cured; (b) LDPE (open circle) and LDPE-pc (solid circle).

Alexander Fainleib et al.

Tensile properties and residual gel content values for LDPE and LDPE-pc samples are presented in Table 1. Increasing residual gel content value and some reduction in tensile properties observed for LDPE-pc confirm the above conclusions about some branching and crosslinking of LDPE-pc chains. Rheological behavior for LDPE and LDPE-pc is shown in Figure 4. One can see that for both LDPE and LDPE-pc at each temperature studied the flow curves are very similar that evidences of close values of viscosity of the polymers investigated. The shear rate dependence of melt viscosity (in Arrenius coordinates) of both LDPE and LDPE-pc is shown in Figure 5. Rheological behavior of all virgin and post-consumer polyolefins studied is presented graphically in Figure 5. Based on the data presented the flow activation energies, E, were calculated. The data are presented in Table 1. 5

log τw, Pa

413 K 433 K 453 K 473 K

3 -2

-1

log γw, s

Figure 4. Dependence of shear stress (τw) vs. shear rate (γw) for LDPE (open circle) and LDPE-pc (solid circle) at different temperatures. log η, Pa s

log η, Pa s

HDPE

LDPE

5 413 K

413 K 433 K

433 K

453 K 473 K

453 K

493 K

473 K

4 473 K

-2

-1

log γw, s

-1

3 2 -1

1 -1

log γw, s

-1

log γw, s

Figure 5. Dependence of shear viscosity (η) vs. shear rate (γw) for virgin (open symbols) and post-consumer (solid symbols) LDPE, HDP and PP at different temperatures.

Utilization of tire rubber and recycled polyolefins into thermoplastic elastomers

3. Reclamation of ground tire rubber The thermo-mechanical method of reclamation (partial devulcanization) and thus activation of GTR (or at least the surface of GTR particles) has been investigated by using a Brabender Plasticorder (model PLE 330) and different treatment conditions [23]. GTR powder (SCANRUB AS, Viborg, Denmark) of high quality large surface/diameter ratio was produced by grinding in an airgab at supersonic speeds. The main characteristics are given in Table 4. As the powder and granulate is made from a large number of different tire types Genan cannot give any extract values for the elastomeric composition of the powder/Granulate. The following can be used as a guideline: Natural rubber ~30 %, SBR (Styrene-butadiene rubber) ~40 %, BR (butadiene rubber) ~20 %, IIR/XIIR (butyl- and halogenated butyl rubber) ~10 %. CR3 has performed the sieve analysis of 2 fractions of GTR powder used and obtained the following results: fraction 0.4<d<0.7 mm (GTR): d<0.2 mm (0.8 %), 0.2<d<0.4 mm (34.6 %), 0.4<d<0.63 mm (53.4 %), and 0.63<d<1.0 mm (11.2 %). The content of sol fraction was 11.4-12.5% (in acetone according to CR3, CR2 and CR4) and 3.1 % determined in toluene. Based on the analysis of the mixing torque curves (here not reported) it was concluded that the devulcanization of GTR is negligible at relatively low temperatures (180-240 oC) and becomes significant only at high temperatures (300 oC). The sol-gel analysis of processed GTR has shown increasing sol-fraction content from ~12.5% for unprocessed GTR to ~16.1% for GTR processed at 240 oC and up to ~21.4% for GTR processed at 300 oC. The final evidence of the thermal activation of the devulcanization process has Table 4. Basic characteristics of GTR.

Characteristic 3

Specific gravity (g/cm ) Ash (%) Acetone extract (%) Carbon black content* (%) Rubber hydrocarbon content (%) Free textile <1.0 mm (wt. %) Free textile >1.0 mm (wt. %) Tensile Strength, TS (MPa) Elongation at break, EB (%) Hardness, Shore A Density (g/cm3) Resiliency (%) Abrasion Loss (mm3)

Standard

Value

ISO 2781

1.1÷1.20

ASTM E1131-86 ISO 1407-81 ASTM E 1131-86 Calculated Genan Genan ISO R 37 ISO R 37 ISO R 868 ISO R 868 DIN 53.512 DIN 53.516

0÷5 11÷17 32÷36 ~42 0÷0.8 0÷1.2 >5.0 >90 72 1.18 40 220

Alexander Fainleib et al.

been confirmed by the measurement of a permanent deformation on materials compressed at a given temperature for 24 hours (ASTM D395/B). The relative results are reported in Table 5. Again, while for the 180 °C processed material no significant differences with the unprocessed GTR can be evidenced, some slight difference is present in the 210 °C and 240 °C processes ones. As for the highest temperature used, in this case no measurement was possible as the material after testing has no consistence and no deformation was measurable. Again, this is a confirmation that relevant changes in the GTR happen only at high temperatures when it is possible to achieve an effective devulcanization of the material. The thermo-chemical method of reclamation (devulcanization) of GTR was also developed. GTR was swollen in a mixture of naphtenic hydrocarbons (industrial processing oil) followed by mixing of swollen GTR with other ingredients (rosin, mineral rubber, indene-coumarone, etc.) by rolls, thermal treatment (T=100-150 oC for 1-10 hours) and second rolling. For the basic GTR and the devulcanized one in regenerate product we have reached an increase of acetone extract value from 12.5 to 18.5 %. These 6 % decrease determines the degree of GTR devulcanization, this fact was confirmed additionally by FTIR data. The molecular mass distribution (MMD) by Size Exclusion Chromatography for acetone soluble fractions of GTR before and after reclaiming has been studied [23]. It was found that the values of molecular masses (Mw, Mn, Mz) of the reclaimed GTR were lower by ~28-43% compared to the initial one. Next considerable efforts were undertaken to find a straightforward method to a better devulcanization of GTR. It was namely suggested that better surface decomposition of GTR allowed a better decomposition of its particles, which can thus participate in the load transfer/dissemination processes during loading and a premature failure of the specimens can be avoided. The concept was to find an ingredient, which may react with sulphur radicals formed due to the break-up of the di-and multi sulphide crosslinks. It was found that bitumen may fulfill this role [28]. Following this presumption a detailed study was devoted to the GTR devulcanization (reclaiming) in bitumen [28-30]. Note that bitumen not only “absorbs” sulphur, it is also a good plasticizer/compatibilizer for various polyolefin blends. The thermochemical method of GTR reclaiming using bitumen as a softening and devulcanizing agent was mainly applied at various processing conditions. The acetone soluble fraction due to bituminous reclamation, depending on the processing conditions, was markedly increased by 18 %. It was presumed that the missing ductility (elongation at break) of the thermoplastic compositions can be enhanced and the ductility requirement for the thermoplastic elastomers can be reached when using a bituminous reclamation for GTR

Utilization of tire rubber and recycled polyolefins into thermoplastic elastomers

Table 5. Deformation measured by compression set of GTR processed at different conditions. Processing conditions GTR as it is 160 °C, 5 min, 100 rpm 180 °C, 5 min, 100 rpm 180 °C, 15 min, 100 rpm 210 °C, 5 min, 100 rpm 210 °C, 5 min, 30 rpm 240 °C, 15 min, 100 rpm 300 °C, 15 min, 100 rpm

at 23 °C 6.90 8.96 9.49 10.42 11.35 13.99 15.75 not measurable

Residual deformation, % at 70 °C 7.41 10.61 10.87 11.27 12.86 14.34 15.07 not measurable

at 100 °C 10.53 11.76 11.76 12.06 14.69 15.97 not measurable not measurable

Table 6. Characteristics of some bitumens used. Characteristics

Bitumen EN 70/100 50 45 18÷20

Bitumen BN-4

Bitumen BN-5

78 30 >240 >360 0.957 31÷32

89 11 >240 >360 1.0091 38÷41

Molar mass distribution a): Mw; Mn; Mz; Mw/Mn

9,324; 5,516; 17,719; 1,69

12,059; 5,647; 26,491; 2.14

13,923; 6,488; 29,436; 2.15

Element content (wt.%) b): С (carbon) Н (hydrogen) S (sulfur) N (nitrogen) Other elements

84.6÷84.82 9.69÷9.56 3.59÷3.39 1.47÷1.42 1.03÷0.43

83.78÷83.90 10.04÷10.27 3.47÷3.57 1.01÷1.20 1.06÷1.7

83.97÷84.07 10.54÷10.72 3.44÷3.62 1.05÷0.92 0.54÷1.13

Softening temperature (oC) Penetration (dmm) Flash Point (oC) Fire point (oC) Density at 20oC (g/ml) Asphaltenes content (wt.%)

determined by CR4 using Size Exclusion Chromatography method; b) determined by CR4 using elementary analysis method [Mazor L. Methods of organic analysis. Akadémiai Kiadó, Budapest 1983].

without sacrificing the set properties. Some characteristics of the bitumens used are shown in Table 6. The step-by-step (discontinuous) and continuous methods of bituminous treatment of GTR were developed [23, 28] and the influence of processing parameters (temperature, time, screw speed, component content, etc.) on properties of GTR containing final TPEs were studied. The step-by-step

Alexander Fainleib et al.

method consisted of such stages: blending of GTR with bitumen, pre-heating the mixture in oven (170 oC for 4-5 hours) followed by rolling of GTR/bitumen blend (for 20-80 min). Finally, different GTR/bitumen containing TPEs have been produced by mastication in the Brabender Plasticorder (model PL 2000) at different processing conditions. The continuous method of bituminous treatment of GTR was performed by using a corotating laboratory twin-screw extruder (Brabender DSE 25). First GTR powder was passed trough the extruder, afterwards, the extruded GTR powder was extruded again together with the bitumen and finally TPE compositions (of different recipe) were produced in the 3rd extrusion run at different processing conditions. The same continuous method of bituminous treatment of GTR was applied by using one-screw extruder (model PLV 150) [28]. First the GRT/bitumen blend was extruded followed by granulation of the extrudate. Afterwards, the granulated GTR/bitumen blend was extruded again together with other components of TPEs. The producing and characterization of TPEs from recycled polyolefins and GTR (or GTR/bitumen blends) will be discussed in Sections 5-9.

4. Reactive compatibilization of components in thermoplastic elastomers based on recycled polyolefins Authors [22, 31] described the method of reactive compatibilization of the recycled LDPE-pc with polybutadiene rubber. The reactive compatibilization method used in this work was realized by introduction of reactive polyethylene copolymer into thermoplastic phase and reactive polybutadiene rubber into the rubber phase to promote the interfacial adhesion by means of chemical interaction between the functional groups of compatibilizing agents in thermoplastic/rubber interface. The chemical reactions occur during melt mixing of components at TPE formation. Schemes of reactions between reactive polyethylenes and reactive polybutadiene rubbers used are shown in Figure 6. Thus, the reactive couples of functional groups can be given as follows: epoxy–carboxyl (1, 2), amine–epoxy (3), amine–anhydride (4), isocyanate–anhydride (5), isocyanate–epoxy (6), isocyanate–carboxyl (7, 8). Tensile properties of TPEs obtained by using different reactive couples are given in Figure 7. One can see that the effect of compatibilization is observed for TPEs obtained by using following reactive couples: PB-NH2/PE-co-GMA, PB-NCO/PE-co-AA and PB-NCO/ PE-co-VA-co-AA. These results indicate that above reactive couples may act as “interfacial agents” promoting adhesion between matrix and dispersed phase in TPEs studied.

Utilization of tire rubber and recycled polyolefins into thermoplastic elastomers

Figure 6. Schemes of reactions between reactive polyethylenes and reactive butadiene rubbers used: PB-E/PE-co-AA (1); PB-COOH/PE-co-GMA (2); PB-NH2/PE-co-GMA (3); PB-NH2/PE-g-MAH (4); PB-NCO/PE-g-MAH (5); PB-NCO/PE-co-GMA (6); PB-NCO/PE-co-AA (7); PB-NCO/PE-co-VA-co-AA (8).

The highest increase in mechanical characteristics is fixed for LDPE-pc (PE-co-AA)/ BR (PB-NCO) TPE, its values of <TS> and <EB> are higher by 31 % and 63 %, respectively, than for the non-modified LDPE-pc/BR TPE. Undoubtedly, this evidences of effective compatibilization of rubber and polyolefin components in the modified TPE in comparison with non-modified one due to reaction between PE-co-AA and PB-NCO in polyolefin/rubber interface and, as a result, improved interfacial adhesion. This conclusion is confirmed by the data presented in Figures 8-10. One can see that introducing PB-NCO/PE-co-AA (NCO/COOH=1/1) compatibilizer increases the tensile properties of TPE obtained (see Figure 7), which achieves maximal values at ~8存10 wt.% of PB-NCO (per BR). However, the further increase of PB-NCO content up to 15 wt.% provides a

Alexander Fainleib et al. EB, %

TS, MPa TS EB

400

0 3

8 300

2 200

100

Figure 7. Tensile properties of unmodified LDPE-pc / BR=60/40 wt.% TPE (0) and the same TPE modified by: PB-E/PE-co-AA (1); PB-COOH/PE-co-GMA (2); PB-NH2/PE-co-GMA (3); PB-NH2/PE-g-MAH (4); PB-NCO/PE-g-MAH (5); PB-NCO/PE-co-GMA (6); PB-NCO/PE-co-AA (7); PB-NCO/PE-co-VA-co-AA (8). All PB-modifiers were used in amount of 7.5 wt.% (per BR) and the ratio of functional groups for PB- and PE-based modifiers was kept 1/1.

Figure 8. Tensile properties of LDPE-pc (PE-co-AA)/BR (PB-NCO) TPEs vs. PBNCO content in BR phase at NCO/COOH ratio equal 1/1.

reduction of EB value. We consider that PB-NCO can also react with unsaturated bonds of the BR inside the rubber phase, which is evident from the gelcontent data presented in Figure 9. One can see a non-linear growth of gel content value at increasing PB-NCO content in BR phase. As can be seen from Figure 10 the addition of PE-co-AA to PB-NCO increases the tensile properties of TPEs obtained, which reach a plateau at NCO/COOH=1/1. The excess of PE-co-AA (â&#x2030;Ľ1.5 e.e.w. per PB-NCO) does

Utilization of tire rubber and recycled polyolefins into thermoplastic elastomers

400

300

200

EB, %

TS, MPa

Figure 9. Gel content of TPEs studied vs. PB-NCO content in BR phase. LDPEpc/BR=60/40 wt.% for all TPEs and the ratio of functional groups for PB- and PE-based modifiers was kept 1/1.

TS EB

1 0,0

0,5

1,0

1,5

100

0 2,0

PE-co-AA content (per PB-NCO, e.e.w.) in LDPE-pc(PE-co-AA)/BR(PB-NCO)

Figure 10. Tensile properties of LDPE-pc (PE-co-AA)/BR (PB-NCO) TPEs vs. PE-co-AA e.e.w. per PB-NCO (at 7.5 wt.% PB-NCO content per BR).

not influence significantly on tensile properties of the TPEs studied. We explain this by lower reactivity of COOH-groups in comparison with NCOgroups. One can see that minimal values of TS and EB are observed for the TPEs modified by PB-NCO (7.5 wt.% per BR) but without any quantity of PE-co-AA. Clearly, that in this case the interfacial adhesion between the TPE basic components is on the level as for unmodified TPE. In addition, decrease of EB value (by ~15%) for this sample in comparison with unmodified LDPE-pc/BR TPE evidences that PB-NCO indeed participates in reaction with unsaturated bonds of BR inside the rubber phase.

Alexander Fainleib et al.

The reason of absence of compatibilizing effect for other reactive couples used is probably connected with kinetic/diffusion peculiarities. One can suppose, that for these couples the reactions of the functional groups in interphase are not so effective and the reagents may be do not have time enough to react completely at the conditions used. LDPE-pc (PE-co-AA)/BR (PB-NCO) TPE has been selected for more detailed investigation of influence of reactive couple content on changing in phase structure, glass transition behavior, degree of crystallinity of polyolefin matrix as well as on thermal and mechanical properties of TPEs studied.

5. Structure-properties relationships for reactively compatibilized thermoplastic elastomers from recycled polyolefins and rubbers Structure-property relationships in LDPE-pc (PE-co-AA) / BR (PB-NCO) TPEs 5.1. Wide-angle X-ray scattering WAXS diffractograms of unmodified and all of modified TPEs studied (Figure 11) show two sharp peaks located at scattering angles of 21.6 and 23.9o (scattering of crystalline phase of polyethylene) as well as the diffuse maximum located at 19.5o (scattering of amorphous phase of polyethylene) [24, 32]. As it was mentioned above three sharp peaks in the region from ~31o to ~36o can be attributed to low molecular weight additives used for TPE curing. The results of WAXS investigation of the phase structure of TPEs show that in comparison to individual LDPE-pc the introduction of BR into LDPE-pc matrix leads to changes in the angular positions of the WAXS diffraction peaks of LDPE-pc component: the peaks with angular positions 2Θ=21.1ο and 2Θ=23.4o move to the angular positions 2Θ=21.6ο and 2Θ=23.9o, respectively. The positions of the diffraction peaks do not change at introduction and further increase of content of reactive couple in modified TPEs. The corresponding calculation have shown that the mean size of microcrystals <D>=11.4-11.5 nm, and the crystal lattice spacing <d>=0.411. One can see some increase of mean size of LDPE-pc component microcrystals and decrease of crystal lattice spacing in TPE samples (see Table 7) in comparison to pure LDPE-pc (see Table 3). As it was mentioned above the angular positions of diffraction peaks are constant for all TPEs studied, but some change of intensities of the mentioned

Utilization of tire rubber and recycled polyolefins into thermoplastic elastomers

21.6

I /1000 pulses

19.5

23.9

2 3 4 0 5

2Î&#x2DC;, degree

Figure 11. Experimental WAXS curves for unmodified LDPE-pc / BR TPE (1) and LDPE-pc (PE-co-AA)/BR (PB-NCO) with = 1.5 wt.% (2), 7.5 wt.% (3) and 10 wt.% (4) of PB-NCO in BR phase. The ratio of PB-NCO/PE-co-AA was kept 1/1 e.e.w. Beginning from the second curve from the bottom, each next curve was shifted upwards by 5 digits.

peaks is observed. This fact has to be reflected in the change of degree of crystallinity <X>, the data are summarized in Table 7. The value of <X> represents the overall crystallinity of blend material and can be compared with the theoretical (additive) value, <X>add, calculated by assuming both retaining by LDPE-pc component its original value of <X>=27.5 %, and the additivity of components contribution. The results show, that the experimental <X> is higher than <X>add, that means, that the rubber component changes the polyethylene crystallization conditions and that its introduction in crystallizable LDPE-pc matrix promotes the phase separation between the crystalline (polyethylene) and amorphous (polyethylene/rubber) phases in TPEs formed. It can be seen that the unmodified TPE has the highest value of <X>. The downtrend of <X> values observed at the introduction of reactive couple in TPEs is an evidence of the destroying some part of crystallites (obviously defected) due to their involving into amorphous phase. This has to be reflected in decrease of onset of melting temperature of crystallites and depression of Tm as it will be shown below by DSC data. In such a case the above downtrend of <X> value can be attributed to the decrease of phase separation of components in modified TPEs. It can be concluded that the experimental results differ from theoretical (additive) data as a result of interactions between the phases, at that the each component affects the microphase structure of another, in TPEs. As a general remark, this fact indicates partial reactively induced compatibilization of BR

Alexander Fainleib et al.

and LDPE-pc in TPEs studied. However, the mentioned distinctions are not so significant, indicating the existence of regions with the structure of individual components in all TPEs. TPEs modified by PB-NCO/PE-co-AA are characterized by higher compatibility of components in comparison to unmodified TPE and the optimal content of the PB-NCO/PE-co-AA modifier corresponds to 7.5 % PB-NCO per BR. These results will be confirmed by DSC and DMTA data below. Table 7. Degree of crystallinity <X> for TPEs produced. Composition, wt.%

DSC

WAXS X, %

Xadda), %

X,%

LDPE-pc / BR = 60 / 40 (unmodified) b) LDPE-pc (PE-co-AA) / BR (PB-NCO) c) : PB-NCO = 1.5

19.3

16.5

18.5

16.4

PB-NCO = 7.5 PB-NCO = 10.0

18.0 18.7

16.3 16.2

17 19

<X>add â&#x20AC;&#x201C; theoretical (additive) degree of crystallinity: <X >add = X(R-LDPE) Âˇ wi , where wi is polyethylene fraction in TPEs; b) LDPE-pc/BR = 60 / 40 ratio is kept for all samples studied; c) NCO / COOH ratio was kept equal 1/1.

5.2. Differential scanning calorimetry Table 7 shows the experimental values of crystallinity degree calculated form DSC data. It is clearly visible the same tendency of <X> value changing and the results obtained agree rather well with WAXS data. Some differences in absolute values have been argued that the two techniques are not directly comparable. Figure 12 shows the Cp (T) plots of TPEs studied. Some depression of melting peak Tm of LDPE-pc component is observed for all TPEs, the summarized data are presented in Table 8. The mentioned melting peak can be related to the melting a crystallizable long polyethylene sequences having a low number of chain defects (branching, graftings, etc.) [32]. Thus, the depression of melting peak in TPEs studied occurs due to increasing defected crystallites content, here mainly because of grafting amide bridges in thermoplastic/rubber interface that has to hamper the crystallization process. One can see that the highest value of depression of melting peak is observed for TPE with content of reactive couple equal 7.5%.

3,2

-1

Cp, Jg K

-1

Utilization of tire rubber and recycled polyolefins into thermoplastic elastomers

2,8

388 K

2,4

2,0

1,6 200

250

300

350

400

Temperature, K

Figure 12. Temperature dependence of specific heat capacity (Cp) of TPEs based on: LDPE-pc/BR (■); LDPE-pc (PE-co-AA)/BR (PB-NCO) TDVs with 1.5 wt.% (□), 7.5 wt.% (○) and 10 wt.% (∇) of PB-NCO in BR phase. The ratio of PB-NCO/PE-co-AA was kept 1/1 e.e.w.

5.3. Dynamic mechanical thermal analysis The DMTA data can give more information about relaxation processes in mixed rubber/polyethylene amorphous phase of TPEs studied. The temperature dependence of storage modulus, E´, loss modulus, E´´, and tangent delta, tan δ are shown in Figure 13 (a, b and c, respectively). In general, one can see that significant differences are observed in relaxation behaviour of LDPE-pc, BR and TPEs based on them, as well as between modified and unmodified TPEs with the same LDPE-pc/BR ratio. One can see that despite of the fact that all TPEs have significant part of crosslinked chains (see Figure 9), they exhibit thermoplastic properties. The character of E´ (T) plots for TPEs at high temperature (>480 K) is typical for thermoplastic polymers and similar to LDPE-pc (see Figure 13, a). We suppose that in all TPEs studied LDPE-pc forms continues phase (matrix) and BR forms a disperse phase. The analyses of temperature dependences of E´´ (Figure 13 b) evidence that even virgin BR and LDPE-pc have two-phase morphology structures. LDPE-pc is characterized by presence of two main transitions: α-transition (Tg1) centred at 333 K is attributed to the relaxation processes in crystalline phase, as mentioned above due to melting and further recrystallization of defected crystallites, and the β-relaxation (Tg2) centred at 243 K is attributed to the relaxation processes of branched chains of amorphous phase of LDPE

Alexander Fainleib et al.

E', MPa

LDPE-pc

0,2

tan δ

100

E", MPa

LDPE-pc

0,1 BR

LDPE-pc

200

250

300

350

400

Temperature, K

450

0 200

250

300

350

400

Temperature, K

0 450

0,0 200

250

300

350

400

450

Temperature, K

Figure 13. Temperature dependence of (a) storage modulus, E´, (b) loss modulus, E´´, and (c) tangent delta (tan δ) for LDPE-pc, BR and TPEs based on: LDPE-pc/BR (■);LDPE-pc (PE-co-AA)/BR (PB-NCO) TPEs with 1.5 wt.% (□), 7.5 wt.% (○) and 10 wt.% (∇) of PB-NCO in BR phase. The ratio of PB-NCO/PE-co-AA was kept 1/1 e.e.w.

[33]. Note, that high temperature shoulder (centred at ~280 K) observed on β-relaxation transition can be attributed to the presence of crosslinked chains in amorphous phase of LDPE-pc. BR cured is characterized by presence of two main transitions too: the α-relaxation (Tg) centred at 253 K is a relaxation of flexible chains of BR and second high-temperature relaxation with Tonset~300 K (see Figure 13 c) is a relaxation of BR segments limited by intermolecular cross-linking. The characters of E″ (T) and tan δ (T) plots evidence that all TPEs studied are characterized by microphase separation of components and have complicated multiphase structure. This conclusion is confirmed by presence of some overlapped transitions observed in the mentioned plots: a lowtemperature transition in the region ~210-300 K that is a result of a superposition of a strong α-relaxation of BR and the weak β-relaxation of LDPE-pc, as well as a high-temperature transition in the region of 320-400 K that is a result of superposition of weak relaxation of BR segments limited by intermolecular cross-linking and the strong α-relaxation of LDPE-pc. At the temperature above ~380 K the melting of crystallizable long polyethylene sequences having a low number of chain defects is started. The temperature positions of α-relaxation peaks taken from corresponding peaks of E″ (T) plot (see Figure 13 b) of the BR-rich and LDPE-pc rich phases in TPEs studied are shown in Table 8. It can be clearly seen that α-relaxation peaks of BR and LDPE-pc are shifted toward one another in modified TPEs in comparison with unmodified TPEs or individual components.

Utilization of tire rubber and recycled polyolefins into thermoplastic elastomers

Table 8. Phase transition temperatures for LDPE-pc, BR and TPEs studied. Composition, wt.%

DSC Tm, K (for LDPE-pc)

DMTA Onset of Tm , K

LDPE-pc 389 322 BR 388 246 LDPE-pc/BR = 60 / 40 (unmodified)b) LDPE-pc (PE-co-AA)/BR (PB-NCO)c) : PB-NCO = 1.5 387 243 PB-NCO = 7.5 385 220 PB-NCO = 10.0 387 250 a) The value has been taken from the E’’ peak; b) LDPE-pc/BR = 60 / 40 ratio is kept for all samples studied; c) NCO / COOH ratio was kept equal 1/1.

α-relaxation peak temperaturea), K BR-rich LDPE-pcrich phase phase (Tg) (Tg1) 353 253 258 333

266 263 265

333 328 337

This fact can be explained by the interaction between BR and LDPE-pc phases due to the formation of the essential interface layer mainly based on PB-NCO/PE-co-AA grafting from rubber and polyethylene phases, respectively. In addition, the relaxation processes in amorphous phases is hampered by presence of LDPE-pc crystallites. However, from the data presented in Figure 13 c one can see that some increase of intensity of relaxation transitions in the temperature region 230-295 K is observed for modified TPEs in comparison to unmodified TPE or pure LDPE-pc. This fact can be attributed to the increasing chain mobility in amorphous phases obviously due to increasing unsoundness of crystalline phase of LDPE-pc. These results are agreed to above WAXS and DSC data. In conclusion, DMTA shows that the TPEs can be considered as multiphase systems having at least one crystalline, two amorphous phases of individual components and regions of mixed compositions. We suppose further that LDPE-pc crystalline phase consists of microphases formed by ‘perfect’ crystallites and by ‘defected’ crystallites, while LDPE-pc amorphous phase consists of microphases formed by crosslinked chains and by branched chains. BR amorphous phase consists of microphase formed by cross-linked segments and of one formed by flexible linear BR chains. The mixed microphase consists of both the components grafted by reactive compatibilizers. Thus, the final properties of TPEs are determined by the heterogeneity of the individual components, as well as by the heterogeneity caused by the thermodynamic

Alexander Fainleib et al.

immiscibility of the components. The degree of compatibilization is determined and changed, to a large extent, by the grafting reaction of PBNCO/PE-co-AA reactive compatibilizer and by the formation of the extensive interfacial layer that leads to improving interfacial adhesion between rubber and polyethylene phases. Thermoplastic elastomers based on recycled polyethylene (LDPE or HDPE) and fresh rubber (BR or styrene-butadiene rubber (SBR)) were prepared by using technologies of dynamic vulcanization and reactive compatibilization or plasticization [16, 34]. Structure-property relationships for TPEs prepared have been investigated and effectiveness of different compatibilizers to promote an interfacial adhesion between the components have been compared [16, 34]. Based on these studies the following conclusions can be drawn: For LDPE-pc/BR based TPEs the highest effectiveness of PE-co-AA/PB-NCO reactive couple was fixed whereas for HDPE-pc/SBR based TPEs the most effective reactive couple was PE-co-GMA (glycidyl methacrylate)/PB-NH2 (amino-functionalized polybutadiene). The values of TS and EB increased respectively by 31% and 63% for LDPE-pc (PE-co-AA) / BR(PB-NCO) TPE, and by 87 % and 182 % for HDPE-pc (PE-co-GMA)/SBR (PB-NH2) TPE. The investigation of phase structure has shown that some destroying and increasing mean size of crystallites, as well as their involving into amorphous phase are observed due to decreasing phase separation of components in all the modified TPEs. Growth of Tg value has been fixed for all the modified TPEs in contrary to unmodified ones or individual components that is a result of formation of mixed polyolefin/rubber amorphous phase with the essential interface layer consisted of both components grafted by reactive compatibilizers used.

6. Thermoplastic elastomers based on recycled HDPE, EPDM and reclaimed ground tire rubber High performance TPEs based on recycled HDPE (or LDPE, or PP), olefinic type ethylene/propylene/diene monomers (EPDM) containing rubber and GTR treated with bitumen were prepared by using the dynamic vulcanization technology, and their structure-property relationships have been investigated. It was established that at special pre-treatment of GTR by bitumen bestows on the resulting TPEs outstanding mechanical properties [17, 28, 29, 30, 34-36]. The method of GTR reclamation used is described in Section 3 of this Chapter. The recipes and processing conditions used for the TPE compositions are given in Table 9 and Table 10, respectively. In one test series (cf. Table 10) a mastication of composition was carried out in the kneading chamber of a

Utilization of tire rubber and recycled polyolefins into thermoplastic elastomers

Brabender plasticorder (model PL 2000) at 160 oC and 80 rpm. Recycled HDPE-pc was melted first for 2 min, then EPDM was added and melted for 2 min, and finally the GTR or GTR/bitumen blend (1/1 by weight) was added and masticated with other components for a further ~10 min. Table 9. Composition recipes used. Composition

Component content, wt.% Recipe “a” Recipe “b” Recipe “c” b) c) 53.3/46.7 61.5/38.5 50/50 40/35/25 40/25/35 50/25/25 40/35/25(1/1) 40/25/35(1/1) 50/25/25(1/1)

HDPE-pc/rubbera) HDPE-pc/rubbera)/GTR HDPE-pc/rubbera) / (GTR/bitumen) a) EPDM was used as a rubber in the blends of the recipes “a” and “b”; SBR was used as a rubber in the blends of the recipe “c”. b) Ratio of HDPE-pc/rubber = 53.3/46.7 (wt.%) is equal to 40/35 (wt.%) in other blends of the recipe “a”. c) Ratio of HDPE-pc/rubber = 61.5/38.5 (wt.%) is equal to 40/25 (wt.%) in other blends of the recipe “b”.

Table 10. Conditions and codes of producing methods used. Conditions of producing method Mastication by using Brabender plasticorder 1. Mastication of composition (in Brabender plasticorder). 1. Heating of GTR/bitumen blend. 2. Mastication of HDPE-pc/rubber/(GTR/bitumen) blend followed by its rolling.

Code A B

1. Heating of GTR/bitumen blend followed by its rolling. 2. Mastication of HDPE-pc/rubber/(GTR/bitumen) blend.

1. Heating of GTR/bitumen blend followed by its rolling. 2. Mastication of HDPE-pc/rubber/(GTR/bitumen) blend followed by its rolling.

Mastication by using single-screw extruder 1. Mastication of GTR/bitumen blend followed by its granulation. 2. Mastication of HDPE-pc/EPDM/(GTR/bitumen) blend.

1. Mastication of GTR/bitumen blend followed by its rolling and granulation. 2. Mastication of HDPE-pc/EPDM/(GTR/bitumen) blend.

1. Heating of GTR/bitumen blend followed by its mastication in extruder and granulation. 2. Mastication of HDPE-pc/EPDM/(GTR/bitumen) blend.

1. Heating of GTR/bitumen blend followed by its rolling, mastication in extruder and granulation. 2. Mastication of HDPE-pc/EPDM/(GTR/bitumen) blend.

Alexander Fainleib et al.

In the other test series (cf. Table 10) a mastication of compositions was carried out by using one-screw extruder (model PLV 150). First, the GRT powder was extruded together with the bitumen at the temperature profile of 155/165/175 °C and the screw speed 40 rpm followed by granulation of the extrudate. Afterwards, the granulated GTR/bitumen blend was extruded again together with other components (T=155/165/175 °C, 40 rpm). Some compositions (cf. Table 10) were prepared by rolling. GTR/bitumen blends and the related TPE compositions were produced on mill rolls at T~60 °C for 40 min.

6.1. Tensile properties Tensile properties of the GTR containing compositions produced by different mastication methods (cf. Table 9) are shown in Table 11. The experimental data clearly demonstrate the beneficial effect of bituminous treatment of GTR and its dependence on the mastication method chosen [28]. One can see that GTR-containing TPEs with suitable tensile properties can be obtained for all recipes used by choosing proper production methods. Based on above-mentioned definition of TPEs (cf. section “Introduction”), one can conclude that the samples B3-B6, B10-11, B14 and B15-B18 satisfy to qualifying standards for TPEs. Note that compression set values are not reported here, however, the related values were below 50 %. It is clearly seen that the requisite condition for producing GTR containing TPEs with suitable properties is the preheating of GTR/bitumen blend before the mastication with the other blend components in Brabender plasticorder or extruder. Sol-gel analysis has shown that heating treatment of GTR leads to decreasing gel-fraction content by ~ 8 % (compared to initial GTR), that is a result of partial devulcanization of GTR. Further decrease of the gel-fraction content up to ~13 % is observed for GTR/bitumen blend after its preheating (calculation has been done per GTR content). It can be concluded that in such a case bitumen acts as a softening and devulcanizing agent for GTR breaking-up sulfuric crosslinks in GTR and therefore leading to activation and functionalization of at least its surface. The reactive sulfur released from GTR and sulfur of bitumen components (cf. Table 6) further take part in covulcanization of pre-heated GTR/bitumen blend with a fresh rubber (EPDM) in the revulcanization step. Indeed it can be seen that the tensile strength and ultimate elongation are higher for all TPEs produced by methods included the procedure of GTR/bitumen pretreatment compared to those produced by the other methods. This suggests an effective interfacial stress transfer between the matrix and the GTR particles [14] due to a better entanglement of the partly devulcanized GTR rubber chains into surrounding matrix.

Utilization of tire rubber and recycled polyolefins into thermoplastic elastomers

One can see that the higher GTR content in TPEs the lower tensile properties of the product. Additional homogenization by rolling pre-heated GTR/bitumen blend and/or final product (cf. Table 11) further improved the tensile strength and, especially, ultimate elongation of the resulting TPEs. This fact evidences of an effective compatibilization of the TPE blend components [37]. Table 11. Tensile properties of individual polymers and blends produced. The data presented in the parentheses represent the properties after ageing at 70 oC for 24 hours. Blend code

B1 B2 B3 B4 B5 B6

B7 B8 B9 B10 B11

B12 B13 B14

B15 B16 B17 B18

Composition

Code of Tensile Ultimate producing strength, elongation, % method MPa Mastication by using Brabender plasticorder

Hardness (Shore A)

Recipe “a” HDPE-pc/EPDM A 13.0 (20.0) 840 (754) HDPE-pc/EPDM/GTR A 4.4 (6.7) 114 (120) HDPE-pc/EPDM/ (GTR/bitumen) A 4.1 (6.5) 168 (176) B 4.9 (8.0) 528 (536) HDPE-pc/EPDM/ (GTR/bitumen) C 6.0 (10.2) 540 (535) HDPE-pc/EPDM/ (GTR/bitumen) HDPE-pc/EPDM/ D 6.1 (10.7) 615 (590) (GTR/bitumen) Recipe “b” HDPE-pc/EPDM A 11.6 (17.9) 750 (593) HDPE-pc/EPDM/ GTR A 3.7 (6.7) 46 (58) HDPE-pc/EPDM/ (GTR/bitumen) A 3.9 (7.0) 85 (97) B 4.0 (5.2) 127 (194) HDPE-pc/EPDM/ (GTR/bitumen) C 5.9 (5.3) 377 (325) HDPE-pc/EPDM/ (GTR/bitumen) Recipe “c” A 6.8 265 HDPE-pc/SBRa) A 7.0 18 HDPE-pc/SBRa)/GTR HDPE-pc/SBRa)/(GTR/bitumen) C 8.4 355 Mastication by using single-screw extruder Recipe “a” HDPE-pc /EPDM/ E 7.5 270 (GTR/bitumen) F 9.0 300 HDPE-pc /EPDM/ (GTR/bitumen) G 9.8 425 HDPE-pc /EPDM/ (GTR/bitumen) H 13.6 515 HDPE-pc /EPDM/ (GTR/bitumen)

Curing system used, phr per 100 phr of SBR in mix formulations: Sulfur 0.9; Tetramethyl thiuram disulfide 0.1 0.1; ZnO 5.0; Stearic acid 2.0; 2-mercaptobenzothiazole 1.0.

93 96 94 93 93 93

90 95 93 93 92

83 79 84

88 89 86 86

Alexander Fainleib et al.

To check the compatibilization efficiency of bitumen in rubber/polyolefin blends, TPEs containing SBR were selected (samples B15-B17). Note that SBR is fare less compatible with HDPE than EPDM. It can be seen that tensile characteristics of HDPE-pc/SBR TPE (sample B12) is much lower compared to HDPE-pc/EPDM TPE (sample B1). Introduction of GTR into HDPE-pc/SBR formulation (sample B13) leads to a dramatic reduction in ultimate elongation compared to HDPE-pc/EPDM/GTR blend (sample B2). However, after bituminous treatment of GTR and further melt production of HDPE-pc/SBR/(GTR/bitumen) TPE (sample B14) the tensile properties were strongly improved compared to the reference HDPE-pc/SBR blend (sample B13) is observed. Hence it can be concluded that bitumen acts as effective compatibilizer for polyolefins and rubbers in TPEs [28]. Note that all EPDM-based compositions exhibit almost similar hardness values, which are near or above 90 Shore A units, whereas the SBR-based compositions exhibit lower hardness values, which are above 80 Shore A. Table 11 indicates some increase of tensile properties for the most of the compositions studied as a result of thermal ageing. It can be concluded here that all the blends studied were found to be remarkably stable to ageing. As it was shown in [14] the enhancement in tensile strength and marginal change in ultimate elongation suggest the formation of some additional crosslinks (post-curing). On the other hand, the increasing both the tensile strength and the ultimate elongation observed for the samples B2-B5 and B8-B10 suggest that a post-curing occurs mainly in rubber phase. TPEs based on LDPE-pc (or HDPE-pc), EPDM and GTR (or GTR/bitumen) were investigated using TMA techniques [38]. It is shown that the presence of GTR/bitumen component leads to decreasing value of thermal expansion coefficient of the resulting TPEs due to diffusion of bitumen into thermoplastic matrix. The compositions of TPEs studied were optimized.

6.2. Thermogravimetric analysis (TGA) The thermal stability of GTR containing compositions (sample B2) and GTR/bitumen containing (sample B6) was studied by TGA and compared to that of the reference HDPE-pc/EPDM TPE (sample B1) [17, 28, 39], the corresponding curves are shown in Figure 14. It can be seen that an introduction of GTR into the basic HDPEpc/EPDM blend is not accompanied by significant changes in thermal stability. Some shift to higher temperatures is observed for the stages of intensive decompositions above ~500 oC only, however, finally a char residue values are near the same for the both samples: 1.77 % for the reference

Utilization of tire rubber and recycled polyolefins into thermoplastic elastomers

HDPE-pc/EPDM (sample B1), and 1.67 % for the HDPE-pc/EPDM/GTR (sample B2). In addition, one can see that the stage of thermal oxidative destruction at 150-250 oC characteristic for the basic HDPE-pc/EPDM blend disappears completely for the GTR containing samples. The thermal behavior of GTR/bitumen containing TPE (sample B6) in the temperature region below ~340 oC is quite similar to the GTR containing blend (sample B2). However, some depression by ~28-33 oC in the temperature of the maximal rate of decomposition in the region from ~340 oC to ~460 oC was observed for the sample B6 compared to the both other samples (B1 and B2), in the temperature region above ~460 oC the thermal behavior of the sample B6 is quite similar to the reference B1. Similar results were obtained [17] for analogous TPEs based on LDPE-pc and PP-pc. -

exo

HDPE -pc:EPDM HDPE-pc:EPDM:GTR HDPE-pc:EPDM:GTR/bitumen

200

400

600

-1,5 0

200

400

600

200

400

Δm τ , % min

-1

0,0

-1

-0,5

-1,0

Δm, %

-50

-100 0

600

Temperature, C

Figure 14. Thermogravimetric analysis of the blendes: (─) HDPE-pc/EPDM (sample B1); (○) HDPE-pc/EPDM/GTR (sample B2), and (▲) HDPE-pc/EPDM/(GTR/bitumen) (sample B6).

Alexander Fainleib et al.

6.3. Rheological properties

Shear viscosity, log η, Pa ⋅ s

The rheological properties of the HDPE-pc/EPDM/GTR (sample B2) and HDPE-pc/EPDM/(GTR/bitumen) (sample B6) were studied and compared with those of the reference HDPE-pc/EPDM (sample B1) [17, 28]. The dependence of shear viscosity (η) versus shear rate (γ) obtained at different temperatures is shown in Figure 15. First of all it should be noted that the measurements were not possible with HDPE-pc/EPDM/GTR blends because of their very high viscosity (>105 Pa·s) caused by the introduction of crosslinked GTR particles. A significant decrease in the melt viscosity was observed for GTR/bitumen containing TPE (sample B6) compared to the reference HDPE-pc/EPDM (sample B1) at a given temperature and a shear rate (except the region of the highest shear rate). Undoubtedly, this is mainly due to the low molecular weight of bitumen which acts as an effective plasticizer in the TPE studied. For both samples B1 and B6 the viscosity decreases with increasing shear rate at a fixed temperature. Shear thinning is typical for most thermoplastic polymers [8]. However, a viscosity increase was observed for sample B1 with rising temperature in the low shear-stress region [28]. On the other hand, in the high shear stress region the opposite tendency is obvious. The above mentioned increase in viscosity can be explained by post-curing of the TPE (sample B1) during the rheological measurement (i.e. thermally induced o

180 C o 190 C o 200 C

-2

-1

10 -1

Shear rate, log γ , s

Figure 15. Shear-viscosity (η) vs. shear-rate (γ) plots for HDPE-pc/EPDM (sample B1, open symbols) and for HDPE-pc/EPDM/(GTR/bitumen) TPE (sample B6, solid symbols) obtained at different temperatures (indicated in the plot). The codes of the samples correspond to the blends in Table 11.

Utilization of tire rubber and recycled polyolefins into thermoplastic elastomers

crosslinking of the EPDM). It is intuitive that the higher temperature, the higher the crosslinking of the product [40]. Note that the gel fraction of sample B1 increased from zero (before mastication) up to ~16 % (after mastication). Note that a remarkable influence of the rubber component on the flow behavior of blends exactly in the range of low shear-stress region occurs, where relaxation processes take place [41] and some agglomerated structures can be formed [42, 43]. Logically, in the high shear-stress region the influence of rubber component on flow behavior of the TPE is insignificant due to the absence of relaxation processes and destruction of agglomerated structures [40]. Very similar results were obtained for TPEs based on LDPE-pc and PP-pc [17].

6.4. Scanning electronic microscopy (SEM) Figure 16 depicts SEM photomicrographs taken from the cryogenic fracture surfaces of the sheets of some blends. They were produced according to recipes “a” (samples B2, B3 and B4) and “b” (samples B8, B9 and B11) via different methods. The codes of the samples correspond to the blends in Table 11. One can clearly seen that GTR particles directly dispersed in HDPE-pc/EPDM blend (samples B2 and B8) are very poorly bonded to the matrix, a lot of large and small size GTR particles are observed outside the matrix indicating for lacking interaction between them [17]. The sample B8 produced with the higher GTR content (35 wt. %) is characterized by deteriorated homogeneity of the surface compared to the samples B2, i.e. increase of an apparent size of de-bonded GTR particles and a presence of cracks (or holes) are observed. Both samples exhibit unacceptable low tensile properties (cf. Table 11), especially, the sample B8 [28]. It can be seen that the surface of the HDPE-pc/EPDM/(GTR/bitumen) blends produced by mastication of compositions by Brabender plasticorder (samples B3 and B9, method “A”) look more homogeneous. Furthermore, the apparent size of the GTR particles is reduced, the small GTR particles are well incorporated into the matrix, whereas the larger ones are partially protruded outside the fracture surface, and there are no visible holes or cracks compared to the corresponding bitumen-free samples. It can be concluded that in such a case bonding between GTR particles and thermoplastic matrix is improved that results into some increasing elongation at break (cf. Table 11). However, the homogeneity level achieved does not provide the high level of tensile properties for samples B3 and B9 (cf. Table 11). We consider that no significant interfacial layer between GTR particles and thermoplastic matrix is formed.

Alexander Fainleib et al.

Figure 16. Typical SEM photomicrographs of cryo fractures cut surfaces of TPEs of recipes “a” (samples B2, B3 and B4) and “b” (samples B8, B9 and B11). The codes of the samples correspond to the blends in Table 11.

A better bonding between GTR particles and matrix have been reached for the samples B4 and B11 produced by the methods “B” and “C”, correspondingly (cf. Table 10), where the partial devulcanization of GTR under the preheating of GTR/bitumen blend is happened. The surface of the samples B4 and B11 is characterized by higher level of homogeneity compared to the other samples. It can be considered that it is a result of formation of a significant interfacial layer of partially devulcanized GTR, bitumen and other components of the blends. Understandably, the samples B4 and B11exhibit high tensile properties (cf. Table 11), especially it is relative to the sample B4.

Utilization of tire rubber and recycled polyolefins into thermoplastic elastomers

6.5. Differential scanning calorimetry (DSC) Typical DSC curves for the individual polymers and for TPEs produced are shown in Figure 17 a and b, respectively, and the corresponding thermal characteristics are summarized in Table 12. Both components (EPDM and HDPE-pc) keep their own amorphous and crystalline phases in TPEs produced, however some reduction in the crystallinity (Xc) can be noticed, especially for the EPDM component [17, 28]. The dramatic reduction in the Xc values of the EPDM component can be explained that its crystallization is hampered due to the presence of HDPE-pc crystallites and intermingling of several EPDM chains with those of the HDPE-pc. The outcome of the latter is a â&#x20AC;&#x153;mixed amorphous phaseâ&#x20AC;? for HDPE-pc/EPDM [17, 28, 39]. Some depression of the melting temperature (Tm) values of both EPDM and HDPE-pc components of the TPEs compared to the individual polymers was observed. This depended on the composition and processing conditions used. It is known that the depression of Tm of polymers in the blends is caused by the formation of less perfect crystallites or crystallites having a smaller size [44]. Irrespective which one is at work Tm decrease is always evidence for improved blend compatibility. a)

ENDO

bitumen

EPDM

HDPE-pc

-100

-50

100

150

B10

200

ENDO

B2 B1

-100

-50

100

150

o Temperature, C

200

Figure 17. Typical DSC traces for (a) individual HDPE-pc, EPDM and bitumen and for (b) TPEs produced by different methods. The codes of the curves correspond to the formulation codes in Table 11.

Alexander Fainleib et al.

Table 12. DSC characteristics for individual components and blends produced. Blend code

Tm, oC

Composition EPDM

B1 B2 B3 B4 B6 B7 B8 B9 B10 B11

HDPE-pc EPDM Recipe “a” HDPE-pc/EPDM HDPE-pc/EPDM/GTR HDPE-pc/EPDM/(GTR/bitumen) HDPE-pc/EPDM/(GTR/bitumen) HDPE-pc/EPDM/(GTR/bitumen) Recipe “b” HDPE-pc/EPDM HDPE-pc/EPDM/GTR HDPE-pc/EPDM/(GTR/bitumen) HDPE-pc/EPDM/(GTR/bitumen) HDPE-pc/EPDM/(GTR/bitumen)

Tm onset / Tm end, oC

HDPE-pc EPDM

Xca), %

HDPE-pc

EPDM

HDPE-pc

136 -

28 / 65

37 / 160 -

65 -

45 46 45 44 44

135 131 130 129 132

26 / 69 26 / 57 27 / 65 29 / 58 29 / 65

73 / 156 73 / 145 73 / 143 73 / 143 70 / 147

6 4 3 3 4

57 63 61 56 62

45 45 42 42 45

135 132 130 131 133

27 / 70 36 / 57 29 / 64 28 / 70 32 / 58

73 / 158 70 / 146 70 / 142 73 / 144 75 / 146

4 3 2 2 0

60 65 64 50 60

a) The Xc (crystallinity) values were calculated taking into account the weight fraction of PE in the EPDM (~71 %) and that of EPDM in the blends; the enthalpy of melting of PE with 100 % degree of crystallinity was taken as 283 J/g.

In addition, for HDPE-pc matrix a significant shift of onset of melting temperature (Tmonset) towards higher temperature and narrowing of a region of crystallites melting (Tm end-Tmonset) is observed in all the blends (cf. Table 12) compared to the individual HDPE-pc. The growth of Tm onset can be caused by a disappearance of smaller the less perfect crystallites due to their involving into the amorphous phase and the narrowing of region of crystallites melting is a result of decreasing crystallite dimension dispersion.

6.6. Dynamic mechanical thermal analysis (DMTA) Temperature dependencies of loss modulus (E"), storage modulus (E′), and loss factor (tan δ) for individual HDPE-pc and EPDM as well as for some blends produced are shown in Figures 18, 19 and 20, respectively [28]. The glass transition temperature (Tg) values defined as a temperature position of E" peak, and corresponding E" values at Tg’s are listed in Table 13. EPDM has one sharp relaxation peak at –36 oC (α-transition), corresponding to the Tg of its amorphous phase (cf. Figure 18). Some rise in E′ and tan δ values around ~50 oC (cf. Figures 19 and 20) can be attributed to the melting of residual polyethylene crystallites that is confirmed by the above DSC data. HDPE-pc has two broad relaxation peaks (cf. Figure 18) at –15 oC (β-transition) assigned to the Tg of its amorphous phase, and the peak around ~60 oC (αc-transition) relating to the vibration and rotational motion of –CH2–

Utilization of tire rubber and recycled polyolefins into thermoplastic elastomers

groups in crystalline phase due to recrystallization of smaller the less perfect crystallites [45]. For both EPDM and HDPE-pc the low temperature transition below -100 oC is observed due to the crankshaft mechanism of –CH2–CH2– polyethylene chain segments [46]. The character of E' (T) plots (cf. Figure 19) of all the blends studied is quite similar to the HDPE-pc that means that HDPE-pc forms a continuous thermoplastic phase (matrix), while the dispersed phase is formed by the EPDM/GTR mixture.

Figure 18. Temperature dependence of loss modulus (E") for HDPE-pc, EPDM and TPEs produced. The codes of the curves correspond to the formulation codes in Table 11.

Figure 19. Temperature dependence of storage modulus (E′) of individual HDPE-pc, EPDM and TPEs produced. The codes of the curves correspond to the formulation codes in Table 11.

34 tanδ

Alexander Fainleib et al. 0,5

EPDM

B7 B8 B9 B11

0,4 0,3 0,2

HDPE-pc

0,1 0,0 -100

-50

100

150 o

Temperature, C

Figure 20. Temperature dependence of loss factor (tan δ) for HDPE-pc, EPDM and TPEs produced. The codes of the curves correspond to the formulation codes in Table 11.

The introduction of GTR into HDPE-pc/EPDM TPE (sample B8) results in essential changing in its viscoelastic properties. New sharp relaxation transition around -50 oC (Tg1) appear (cf. Figures 18 and 20) that is characteristic for rubber component of GTR [21]. A considerable lowering by 9÷14 oC in both other Tg’s (cf. Table 13), as well as some downtrend in the E'=f(T) curve and uptrend in the tan δ=f(T) and E"=f(T) curves were observed compared to the GTR-free sample B7 (cf. Figures 18-20). All these changes suggest a significant growth of chain flexibility of the components of the GTR-containing sample B8 due to disordering the thermoplastic matrix by dispersed crosslinked GTR particles caused by poor interphase adhesion between the components [18, 19]. The tensile characteristics of the sample B8 are much lower compared to the GTR-free sample B7 (cf. Table 11). Introduction of bitumen into HDPER/EPDM/GTR blend (sample B9) yields a convergence between Tg2 and Tg4 values (cf. Table 13) [17, 28] and growth by ~80 % ultimate elongation value compared to the sample B8 (cf. Table 11) that can be interpreted as improved “mixing” of the blend components. However, the disordering thermoplastic matrix by dispersed rubber particles keeps sufficiently high that is reflected by the high values of E" and tan δ (cf. Figures 18 and 20, respectively). As a result, the tensile properties of the sample B9 (cf. Table 11) do not satisfy to qualifying standards for TPEs [37]. It can be concluded that in such a case the processing conditions used (method “A”) do not provide the required devulcanization degree of GTR and interface adhesion between the components. Some growth in the crosslink degree of the amorphous phase of the blend takes place, which is confirmed by both the downtrend of E"=f(T) curve in the temperature region below ~-50 oC and the uptrend of the E'=f(T) curve (cf. Figures 18 and 20, respectively). We consider this to be a result of dynamic vulcanization of dispersed rubber

Utilization of tire rubber and recycled polyolefins into thermoplastic elastomers

phase inside the plastic matrix. So, the bitumen can act as an additional curing agent. This is the reason why the term dynamic vulcanization can be used although bitumen is not at all a traditional curative for rubbers. The HDPE-pc/EPDM/(GTR/bitumen) blend (sample B11) produced by the method “C” is characterized by significant reduction of E" and tan δ values (cf. Figures 18 and 20) as well as a further uptrend of the E'=f(T) curve (cf. Figure 19) compared to sample B9. Undoubtedly, such changes evidence a further growth in the crosslink degree of the dispersed EPDM/GTR rubber phase in sample B11 compared to B9. However, based on the increasing ultimate elongation value of sample B11 (cf. Table 11) an improved interfacial adhesion between the components can be unequivocally quoted. We consider that this is due to the higher degree of bitumen induced devulcanization of GTR at preheating of GTR/bitumen blend before mastication additionally reflected by the significant decrease of Tg1 onset of rubber phase (cf. Figure 18) from –65 oC (for the sample B9) to –75 oC (for the sample B11). This result agrees well with the conclusions made on the base of sol-gel analysis (cf. section “Tensile Properties”). As it was noted above the (re)covulcanization of partly devulcanized GTR with EPDM and bitumen is occurred during mastication of composition in Brabender plasticorder and finally, sample B11 exhibits high values of tensile strength and ultimate elongation (cf. Table 11). Certainly, this is a result of the improved compatibility of the blend components. Based on analysis of the DMTA data it can be concluded that at producing GTR/bitumen-containing TPE by method “C” the bitumen first acts as devulcanizing agent for GTR and then simultaneously as an effective Table 13. DMTA data for individual components and blends produced. Blend code

Tg, oC / E"a), MPa for phases rich in:

Composition

Tg1 / E"

EPDM amorphous Tg2 / E"

HDPE-pc amorphous Tg3 / E"

HDPE-pc crystalline Tg4 / E"

-15 / 65 −b) −b) −b)

60 / 100 68 / 30 54 / 21

GTR

B7 B8

HDPE-pc EPDM HDPE-pc/EPDM HDPE-pc/EPDM/ GTR

-50 / 160

-36 / 102 -31 / 134 -42 / 150

B9 B11

HDPE-pc/EPDM/ GTR/ bitumen HDPE-pc/EPDM/ GTR/ bitumen

-49 / 121 -48 / 63

-35 / 158 -33 / 110

a) b)

E" value taken at Tgi Tg3 is overlapped with Tg2.

−b) −b)

42 / 35 53 / 41

Alexander Fainleib et al.

curing agent for dispersed EPDM/GTR rubber phase and as a compatibilizer for blend components improving the interfacial adhesion between dispersed rubber phase and plastic HDPE-pc [17, 28, 39] or LDPE-pc [39] matrix.

7. Effect of multi-reprocessing on structure and properties of thermoplastic elastomers based on recycled polyolefins and reclaimed ground tire rubber As is known [1], processing of TPEs and preparation of finished products at elevated temperatures and dynamic shear stresses can be accompanied by thermal degradation and crosslinking of polymer chains as well as by the post-vulcanization of rubber through residual unsaturated bonds; as result, all characteristics of TPEs can change. In [47], one can find a detailed description of the mechanisms of the above processes. In the TPEs under study, post-vulcanization can proceed through double bonds of the reclaimed GTR (GTRr), which form during the regeneration of rubber crumbs, unsaturated bonds of EPDM, and free sulfur existing in the system. Crosslinking of the polyolefin matrix can result from the stress-induced degradation of macromolecules and formation of free radicals, which are able to participate in the formation of transversal crosslinks. The above processes primarily affect the flow characteristics of the material. Taking this into account, it seems interesting to study the effect of repeated processing on the rheological characteristics of TPEs under study. Figure 21 presents the • typical dependences of effective viscosity (lg η) on shear rate (lg γ ) for the samples of initial TPE and TPEs after the first, second, and third processing cycles in the extruder (TPE-1, TPE-3, TPE-6) [48]. As follows from Figure 21, under the selected experimental conditions (T=190 and 210 °C), all TPEs preserve their flowability; in this case, the effective viscosity of all TPEs decreases with increasing temperature (from 190 to 210 °C) and with increasing shear rate. This character of rheological curves seems quite expectable and shows that TPEs preserve their thermoplastic characteristics even after six processing cycles in an extruder. Figure 22 presents the • effective viscosity measured at a fixed shear rate ( γ = 0.132 s–1) vs. number of processing cycles of TPE samples. As is seen, viscosity of the test samples increases up to the third processing cycle (sample TPE-3); however, starting with the fourth cycle, viscosity decreases and approaches values typical of the initial TPE or even decreases below this value. The corresponding curves at different (fixed) shear rates show similar profiles. As it is known [49], extrusion processing of polymer materials, including polyolefin–rubber

lg η, Pa⋅s

Utilization of tire rubber and recycled polyolefins into thermoplastic elastomers

1 2 3 4

1' 2' 3' 4'

0,01

0,1

lg γ , s• -1

Figure 21. Viscosity vs. shear rate for the following samples: Initial TPE (1,1'), TPE1 (2, 2′), TPE-3 (3, 3′) and TPE-6 (4, 4′) at 190 оС (1-4) and 210 оС (1′-4′).

η, kPa⋅s

20 1

Figure 22. Viscosity vs. number of processing cycles N of TPE samples at (1) 190 and (2) 210 °C.

TPEs, can be accompanied by two parallel competing processes: crosslinking and thermomechanical degradation of polymer chains of the components. Evidently, crosslinking should increase the melt viscosity, but degradation should decrease it. Therefore, one can conclude that, during the first three processing cycles, processes of post-vulcanization of rubber phase and/or minor crosslinking of the polyolefin matrix play a key role; during further processing of TPE samples (samples TPE-4–TPE-6), thermomechanical degradation processes in polymer chains start to dominate. This conclusion also follows from analysis of the dependences of the activation energy of

Alexander Fainleib et al.

flow Ea for the TPE, TPE-3 and TPE-6 samples over the whole range of (Fig. 23). Indeed, one can see that, for the TPE-3 sample, its Ea value is higher than that of the initial TPE; for the TPE-6 sample, this value is lower. Let us assume that this behavior results from the development of crosslinked regions in TPE-3, and these regions hinder the melt flow of TPE; for sample TPE-6, this behavior can be explained by the degradation of polymer chains. Let us mention that, in this case, we consider the intervals of medium and high shear rates because the character of the flow of the polyolefin matrix in TPE is known to be nearly independent of the presence of the crosslinked dispersed rubber phase [41–43]. The point is that, at high shear rates, relaxation processes are almost completely absent [41] and breakdown of agglomerated structures takes place [42, 43]. Figure 24 presents the curves illustrating the content of gel fractions in the TPE samples, and this evidence supports the above conclusions. As follows from Figure 24, the content of the gel fraction in the TPE samples increases as the number of processing cycles is increased to three; later, this content slightly decreases (cycles 4–6). Hence, as compared with the initial TPE, the effective degree of crosslinking increases from the first to the third cycle and decreases from the fourth to the sixth cycle of TPE processing. In this case, the TPE-3 sample is characterized by the maximum degree of crosslinking. Let us mention that the above difference in the rheological characteristics (η, Ea and wg) of the TPE samples is small; hence, under the selected processing conditions, chemical crosslinking and degradation in TPEs are insignificant and exert almost no effect on the characteristics of the TPE-1–TPE-6 samples. For example, as follows from Figure 24, the density of the samples slightly changes, but its tendency to increase with increasing the number of processing cycles is evident. Figure 25 presents the results of mechanical tests before and after repeated processing of the TPE samples in the extruder. As compared with the initial TPE sample, the strength characteristics of the TPE-1–TPE-6 samples are seen to be somewhat improved. In this case, the maximum increase in tensile strength σbr and relative elongation at break =br amounts to about 15%. It is worth mentioning that, in the TPE1–TPE-3 samples, the strength remains virtually unchanged but =br increases. As it is known [20], increased relative elongation at break in the polyolefin–rubber TPEs suggests better compatibility between components. We can also assume that the above crosslinking processes take place primarily either inside the dispersed rubber (EPDM/GTRr) phase or at the interfacial rubber–polyolefin boundary because, upon crosslinking of thermoplastic polyolefin matrix, =br should be decreased. For the TPE-4–TPE-6 samples, one can observe a decrease in =br

Utilization of tire rubber and recycled polyolefins into thermoplastic elastomers

and a certain increase in σbr. This behavior is likely stem from the development of scarce crosslinks in the polyolefin matrix via the interaction of free radicals, which are formed due to the above degradation processes of PE chains [47]. It seems interesting to study the effect of repeated processing on the thermal stability of the TPE samples. For this purpose, thermogravimetric experiments were conducted [17, 28, 48]. The results obtained suggest that the number of the multi-processing cycles has almost no effect on the thermooxidative degradation of all TPE samples under study, because all curves nearly coincide, and the difference in their thermal characteristics (weight loss, weight loss rate, char residue, etc.) was small. Åà, kJ/mol

2 25

1 3 20

0,01

0,1

lg γ , s

-1

10 •

•

Figure 23. Activation energy of viscous flow vs. shear rate γ for the following samples: (1) initial TPE, (2) TPE-3, and (3) TPE-6.

wg, %

0,960

ρ, g/ñm

0,958

0,956

0,954 0

Figure 24. (1) Content of gel fraction and (2) density vs. number of processing cycles N of TPE samples.

Alexander Fainleib et al. 600

εbr, %

, MPà

550

10 500 2 8 450

400

Figure 25. (1) Tensile strength and (2) relative elongation at break vs. number of processing cycles N of TPE samples. Table 14. Characteristics of the crystalline phase (DSC data) of individual components and HDPE matrix of the TPE samples before and after their multi-reprocessing in extruder. Sample HDPE-pc Initial TPE TPE-1 TPE-2 TPE-3 TPE-4 TPE-5

Тm, оС 135 128 128 128 128 128 128

ΔТm, оС 63 58 54 55 51 51 51

ΔНm, J/g

Χ, %

204 175 172 170 160 165 150

70 60 59 58 55 57 52

Note: The Hm and X values are calculated with allowance for the weight fraction of the PE component in the system.

As was shown earlier in [17, 28, 34, 35, 39], this type of TPE is characterized by a multiphase structure, which involves crystalline and amorphous phases of PE component and mixed amorphous phase, which contains the macromolecules of HDPE, EPDM, and GTRr. It seems interesting to study the effect of the number of processing cycles on the characteristics of crystalline phase of the HDPE matrix in the TPE sample: namely, on changes in its melting temperature Tm and degree of crystallinity X. The relevant data obtained by the DSC method are presented in Table 14. Basic on the DSC results we concluded [48] that these TPEs belong to the

Utilization of tire rubber and recycled polyolefins into thermoplastic elastomers

class of semicrystalline polymer blends. As follows from Table 14, the HDPE-pc component in the TPE samples is characterized by lower values of Tm, ΔTm, ΔHm, and X as compared with those of initial HDPE, and this unequivocally suggests the breakdown of some crystallites. The melting temperature of the TPE, TPE-1–TPE-5 samples is the same, but the melting temperature of the TPE-6 sample is 3°C less. At the same time, one can observe an evident tendency toward narrowing of the melting temperature interval and this effect is most pronounced for the TPE-3–TPE-6 samples. A marked decrease in the heat of fusion ΔHm and, correspondingly, a decrease in the degree of crystallinity are also observed for the TPE-3–TPE-6 samples, especially for the TPE-6 sample. All these facts suggest that, after the third processing cycle, a certain part of the most difficult if PE crystallites breaks down and they are transformed into the amorphous phase of TPE, which, as was shown in [28], presents the mixed HDPE-pc–EPDM–GTRr phase. As follows from the literature data [50], for polyolefin–rubber TPEs, the lower Tm and X of the polyethylene component, the higher the compatibility between components in TPEs. Analysis of thermograms of the TPE samples [48] also shows that, after the third processing cycle, the specific heat capacity Cp of the compositions decreases by 0.5–0.6 J/(g K) over the entire temperature interval under study. Usually, the specific capacity Cp of polymer blends decreases with increasing packing density of macrochains of the system components, and this behavior can be explained either by crystallization or chemical crosslinking [51]. As follows from Table 14, with increasing number of processing cycles, the fraction of the crystalline phase in the TPE samples decreases. Taking into account this observation, the decrease in Cp is likely to be related to crosslinking processes. For the TPE4–TPE-6 samples, a slight increase in Cp can come from the concomitant degradation processes (see the above speculations concerning Rheological characteristics and TGA data). However, the TPE-3–TPE-6 samples are characterized by lower Cp values as compared with those of the initial TPE and TPE-1 and TPE-2 samples, and this observation proves the existence of scarce crosslinks in the HDPE matrix. This evidence agrees fairly well with the physical-mechanical tests (Fig. 25). DMTA studies make it possible to estimate the effect of repeated processing on the characteristics of the amorphous phase of the TPE samples. Figure 26 presents the temperature dependence of mechanical loss tangent for HDPE-pc and EPDM and for the TPE samples. The results of DMTA study of the HDPE-pc, EPDM and their TPEs with GTRr and GTR/bitumen are analyzed in Section 6. Here we can compare the results obtained for these TPEs and for them after multi-reprocessing. The data presented in Figure 26 and Table 14 show that, as compared with the initial TPE, Tg of the TPE-1– TPE-5

Alexander Fainleib et al.

tan δ

lg E', MPà

0,4

b c d e f g

0,3 2

-60

a b c d e f g

0,2

0,1

120

Temperature, °C

-60

60 120 Temperature, °C

Figure 26. Temperature dependences of mechanical loss tangent for (1) HDPE and (2) EPDM and for initial TPE and TPE-1–TPE-6 samples (a–g, respectively).

samples slightly increases with the increasing number of processing cycles. For example, the glass transition temperature of TPE-4 is by 7 °C higher than that of the initial TPE. This is likely to be related to the reduced segmental mobility of polymer chains in the amorphous phase of TPE. In the TPEs under study, this behavior can be a result of either increased density of the network of physical (crystallites) or chemical (transverse) bonds, or of the improved compatibility between crystalline (HDPE-pc) and mixed amorphous phases (HDPE-pc–EPDM–GTRr). Taking into account the fact that the degree of crystallinity of the TPE-1–TPE-5 samples is lower than that of the initial TPE (Table 14) and the content of gel fraction slightly increases (by 1.0–3.5 %, Fig. 24), glass transition temperature Tg of the amorphous phase in the TPE-1–TPE-5 samples is likely to increase due to the reduced phase separation between the crystalline and amorphous phases. This conclusion is also proved by a slight increase in the intensity of the peaks observed for the TPE-1–TPE-5 samples, as compared with the initial TPE. Therefore, the results of DMTA studies show that, with the increasing number of processing cycles, viscoelastic characteristics of TPEs are changed due to a decrease in the degree of crystallinity of the HDPE matrix and due to an increased crosslinking density. In this case, compatibility between crystalline (HDPE) and amorphous (HDPE–EPDM–GTR) phases in TPEs is improved.

Conclusions The effect of compatibilization have been observed for LDPE-pc/BR TPEs prepared in the presence of the following reactive couples: PB-NH2/PE-

Utilization of tire rubber and recycled polyolefins into thermoplastic elastomers

co-GMA, PB-NCO/PE-co-AA and PB-NCO/PE-co-VA-co-AA. The most effective reactive couple was PB-NCO/PE-co-AA and the higher effect of compatibilization was reached at 7.5 % of PB-NCO content per BR when the ratio of functional groups for PB- and PE-based modifiers was kept 1/1. In such a case modified TPE obtained has values of TS and EB higher by 31% and 63%, respectively, than for the non-modified LDPE-pc/BR TPE. The results obtained are explained by realization of reaction between PE-co-AA and PB-NCO in polyolefin/rubber interface and, as a result, by improving interfacial adhesion. In modified TPEs shift and convergence of 留-relaxation transitions of components toward one another is fixed by DMTA that confirms the interaction of BR and LDPE-pc due to the formation of the essential interface layer. WAXS investigation has shown that presence of BR and reactive modifiers do not prevent completely the crystallization process of LDPE-pc at TPEs formation. Depression of Tm has been found for all TPEs studied. DMTA data have shown that some increase of chain mobility in amorphous phases is observed for modified TPEs in comparison to unmodified TPE or pure LDPE-pc obviously due to increasing of unsoundness of crystalline phase of LDPE-pc. It was found that all TPEs studied are characterized by microphase separation of components and have complicated multiphase structure. The comparative analysis of the results obtained by various techniques allows concluding of transformation of the morphology of the basic blend (continuous LDPE-pc phase and dispersed BR phase) to the morphology with the essential interface layer that provides an improvement of mechanical characteristics of modified TPEs produced. The LDPE-pc (PE-co-AA)/BR (PB-NCO) TPEs studied can be classified as so-called semi-interpenetrating polymer networks (semi-IPNs). Based on this study devoted to produce thermoplastic elastomers, TPEs, using ground tire rubber, GTR, the following conclusions can be drawn: 他 Bitumen is a suitable reclaiming agent for GTR under the treatment conditions used. During further melt processing it acts as a curing agent for the rubber components of TPEs and works also as an effective compatibilizer for HDPE-pc/EPDM/GTR compositions. 他 TPEs containing GTR pretreated by bitumen show outstanding mechanical properties, high thermal stability and good reprocessability. In addition, TPE grades containing GTR and recycled HDPE-pc can be produced batch wise and continuously in industrial scale. 他 The performance of TPEs mainly depends on conditions of the GTR reclaiming by bitumen, type of the rubber used, and melt processing parameters.

Alexander Fainleib et al.

¾ Structure-property relationship investigations of the TPEs carried out by SEM, DSC and DMTA methods clearly evidence the improvement in interfacial adhesion between the GTR particles and surrounding thermoplastic matrix when the GTR was partially devulcanized in bitumen. ¾ Analysis of the viscous flow characteristics of TPEs shows that, independently of the number of processing cycles, all samples are characterized by the required flow characteristics at elevated temperatures. Processing of TPEs is accompanied by the competing processes of crosslinking and degradation of macromolecules in a polymer mixture. The results of DSC study and dynamic thermal analysis show that, as the number of processing cycles is increased, phase separation between amorphous and crystalline phases in TPEs decreases. Insignificant intermolecular crosslinking induced by the processing of TPEs appears to have almost no effect on the physicomechanical characteristics of the final material.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

Karger-Kocsis, J. 1999, In: Shonaike GO, and Simon GP, editors. Polymer Blends and Alloys. New York: Marcel Dekker, Chap. 5. Abdou-Sabet, S. 1998, Polym. Mat. Sci. Eng., 79, 86. George, J., Varughese, K.T., and Thomas, S. 2000, Polymer, 41, 1507-17. Michael, H., Scholz, H., and Menning, G. 1999, Kautch. Gummi Kunstst., 52, 510-3. Yang, Y., Chiba, T., Saito, H., and Inoue, T. 1998, Polymer, 39, 3365-72. Cavalieri, F., Padella, F., and Cataldo, F. 2003, J. Appl. Polym. Sci., 90, 1631-8. Grigoryeva, O., Fainleib, A., Starostenko, O., Danilenko, I., Kozak, N., and Dudarenko, G. 2004, Rubber Chem. Tech., 77, 131-46. Nevatia, P., Banerjee, T.S., Dutta, B., Jha, A., Naskar, A.K., and Bhowmick, A.K. 2002, J. Appl. Polym. Sci., 83, 2035-42. McKirahan, J., Liu, P., and Brilhart, M. 1996, ANTEC’96, Society of Plastics Engineering. Bhattacharya, S.N., and Sbarski, I. 2002, Plastics, Rubber and Composites, 27, 317. Fuhrmann, I., and Karger-Kocsis, J. 2003, J. Appl. Polym. Sci., 89, 1622-30. Naskar, A.K., Bhowmick, A.K., and De, S.K. 2001, Polym. Eng. Sci., 41, 1087-98. Adhikari, B., De, D., and Maiti, S. 2000, Prog. Polymer. Sci., 25, 909-48. Rudheshkumar, S., and Karger-Kocsis, J. 2002, Plastics, Rubber and Composites, 31, 1-4. Rudheshkumar, S., Fuhrmann, I., and Karger-Kocsis, J. 2002, Polym. Degr. Stab., 76, 137-44. Grigoryeva, O.P., Fainleib, A.M., Tolstov, A.L., and Starostenko, O.N. 2004, Nonlinear Optics. Quantum Optics, 31, 185-198.

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17. Fainleib, A.M., Grigoryeva, O.P., Tolstov, A.L., and Starostenko O.N. 2004, In Composites Technologies for 2020, Cambridge: Woodhead Publishing Ltd, England, 175-180. 18. Corley, B., and Radusch, H.-J. 1998, J. Macromol. Sci.-Phys. B., 37, 265-73. 19. Orr, C.A., Cernohous, J.J., Guegan, P., Hirao, A., Jeon, H.K., and Macosko, C.W. 2001, Polymer, 42, 8171-8. 20. Li, Y., Zhang, Y., and Zhang, Y.X. 2003, Polym. Test., 22, 859-65. 21. Li, Y., Zhang, Y., and Zhang, Y.X. 2003, Polym. Test., 23, 83-90. 22. Grigoryeva, O., Fainleib, A., Starostenko, O., Tolstov, A., and Brostow, W. 2004, Polym. Int., 53, 1693-1703. 23. Final report on EU INCO-COPERNICUS project RECRUPOT (contract n. ICA2-CT-2001-10003), April, 2004. 24. Krimm, S., and Tobolsky, A. 1961, J. Polym. Sci., 7, 57-76. 25. Hermans, P.H., and Weidinger, A. 1961, Makromol. Chem., 44/46, 24-36. 26. RadonjiÄ?, G., and Gubeljak, N. 2002, Macromol. Mater. Eng., 287, 122-32. 27. Mark, J.E. 1996, Physical Properties of Polymers. Handbook, Woodbury: ATP Press, American Institute of Physics, p.723. 28. Grigoryeva, O., Fainleib, A., Tolstov, A., Starostenko, O., Lievana, E., and Karger-Kocsis, J. 2005, J. Appl. Polym. Sci., 95, 659-671. 29. Fainleib, A.M., Grigoryeva, O.P., Starostenko, O.N., Tolstov, A.L Danylenko, I.Y., Sergeeva, L.M., and Lebedev, E.V. 2004, Pat. Ukraine. N63593. 30. Fainleib, A.M., Grigoryeva, O.P., Starostenko, O.N., Tolstov, A.L., Danilenko, I.Y., Sergeeva, L.M., and Lebedev, E.V. 2004, Pat. Russia, N2241720. 31. Fainleib, A., Grigoryeva, O., Starostenko, O., Danilenko, I., and Bardash, L. 2003, Macromol. Symp., 202, 117-126. 32. Greco, R., Musto, P., Riva, F., and Maglio, G. 1989, J. Appl. Polym. Sci., 37, 789-801. 33. McCrum, N.G., Read, B.E., and Williams, G. 1967, Anelastic and Dielectric Effects in Polymeric Solids, New York: Wiley, p. 355. 34. Grigoryeva, O., Tolstov, A., and Fainleib, A. 2004, Scientific Israel Technological Advantages, 6, 37-44. 35. Grigoryeva, O.P., Lievana, E., Tolstov, A.L., Starostenko, O.N., Fainleib, A.M., and Karger-Kocsis, J. 2004, In Composites Technologies for 2020, Cambridge: Woodhead Publishing Ltd, England, 181-186. 36. Fainleib, A.M., Tolstov, A.L., Grigoryeva, O.P., Starostenko, O.N., Danylenko, I.Y., Bardash, L.V., and Gafurov, U.G. 2006, Pat.Ukraine, N75849. 37. Lievana, E., and Karger-Kocsis, J. 2004, Prog. Rubber, Plastics and Recycling Technol., 20, 1-10. 38. Mamunya, Ye.P., Bey, I.N., Tolstov, A.L., Fainleib, A.M., and Gladkiy, E.P. 2005, Polymernyy J. (Kiev), 27, 117-122. 39. Grigoryeva, O., Fainleib, A., Tolstov, A., Pissis, P., Spanoudaki, A., Vatalis, A., and Delides, C. 2006, J. Therm. Anal. Calorim., 86, 229-233. 40. George, J., Ramamurthy, K., Varughese, K.T., and Thomas, S. 2000, J. Polym. Sci. Part B: Polym. Phys., 38, 1104-22. 41. Munstedt, H. 1981, Polym. Eng. Sci., 21, 259-70. 42. White, J.L., Czarneck, L., and Tanaka, H. 1980, Rubber Chem. Technol., 53, 823-35.

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43. Metzner, A.B. 1985, J. Rheol., 29, 739-45. 44. Mark, J.E. 1996, Physical Properties of Polymers Handbook. Woodbury: American Institute of Physics Press. 45. Akovali, G., and Torun, T.T. 1997, Polym. Int., 42, 307-14. 46. Schatzki, T. 1962, J. Polym. Sci., 57, 496. 47. Grassie, N. 1956, Chemistry of High Polymer Degradation Processes. London: Butterworths. 48. Grigoryeva, O.P., Fainleb, A.M., Shumskii, V.F., Vilenskii, V.A., Kozak, N.V., and Babkina, N.V. 2009, Vysokomol. Soed., Ser. A, 51, 275â&#x20AC;&#x201C;285. 49. Jansson, A., Moller, K., and Gevert, T. 2003, Polym. Degrad. Stab., 82, 37-46. 50. Runt, J.P. 2000, In: Paul DR, and Bucknall CB, editors. Polymer Blends. New York: Wiley, Vol. 1. 51. Vasile, C., and Kulshreshtha, A.K. 2003, In: Handbook of Polymer Blends and Composites. Shawbury: Rapra Technology, Vol. 3B.

Transworld Research Network 37/661 (2), Fort P.O. Trivandrum-695 023 Kerala, India

Recent Developments in Polymer Recycling, 2011: 47-100 ISBN: 978-81-7895-524-7 Editors: A. Fainleib and O.Grigoryeva

2. Recent advances in the recycling of rubber waste 1

Eldho Abraham1,3, Bibin M Cherian2, Elbi P A1, Laly A Pothen3 and Sabu Thomas4

Post Graduate Department of Chemistry, CMS College, Kottayam, Kerala, India Department of Natural Science, College of Agricultural Sciences, São Paulo State University (UNESP), Botucatu 18610-307, São Paulo, Brazil; 3Post Graduate Department of Chemistry Bishop Moore College, Mavelikara 690110, Kerala, India; 4School of Chemical Sciences Mahatma Gandhi University, Kottayam, Kerala, India 2

Abstract. An overview of reclamation of cured rubber with special emphasis on waste latex reclamation is presented in this chapter. Recycling of rubber waste poses a challenging environmental, economical, and social problem. The latex industry has expanded over the years to meet the world demands for gloves, condoms, latex thread, etc. The waste rubber formed in latex-based industries is around 10–15% of the rubber consumed. The formation of a higher percentage of waste latex rubber (WLR) in latex factories is due to the unstable nature of the latex compound and the strict specifications in the quality of latex products. As waste latex rubber (WLR) represents a source of high-quality rubber hydrocarbon, it is a potential candidate for generating reclaimed rubber of superior quality. The role of the different components in the reclamation recipe is explained and the reaction mechanism and chemistry during reclamation are discussed in detail. Different types of reclaiming processes are described with special reference to Correspondence/Reprint request: Dr. Eldho Abraham, Post Graduate Department of Chemistry, CMS College Kottayam, Kerala, India. E-mail: eldhoabraham@gmail.com

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processes, which selectively cleave the cross links in the vulcanized rubber. The stateof-the-art techniques of reclamation with special attention on latex treatment are reviewed. An overview of the latest development concerning the fundamental studies in the field of rubber recycling by means of low-molecular weight compounds is described. A mathematical model description of main-chain and crosslink scission during devulcanization of a rubber vulcanizate is also given.

1. Introduction The white sap of the South American tree Hevea brasiliensis forms the basis of a large and global industry, the rubber industry. The South American Indians were familiar with the material and used it for various purposes already when the first Europeans came in the late 15th century. The Indians called the tree Ca-hu-chu (kautschuk!) or â&#x20AC;&#x2DC;the crying treeâ&#x20AC;&#x2122;. Long before Colombus arrived in the Americas, the native South Americans were using rubber to produce a number of water-resistant products. The Spaniards tried in vain to copy these products (shoes, coats and capes), and it was not until the 18th century that European scientists and manufacturers began to use rubber successfully on a commercial basis. The British inventor and chemist Charles Macintosh, in 1823, established a plant in Glasgow for the manufacture of waterproof cloth and the rainproof garments with which his name has become synonymous. A major breakthrough came in the mid 19th century with the development of the process of vulcanisation. In 1839 Charles Goodyear discovered the process of sulfur vulcanization by combining masticated rubber chemically with sulphur, an irreversible process, that he called vulcanization. This process gives increased strength, elasticity, and resistance to changes in temperature. It also renders rubber impermeable to gases and resistant to heat, electricity, chemical action and abrasion. Vulcanised rubber also exhibits frictional properties highly desired for pneumatic tyre application. Crude latex rubber has few uses. The major uses for vulcanised rubber are for vehicle tyres and conveyor belts, shock absorbers and anti-vibration mountings, pipes and hoses. It also serves some other specialist applications such as in pump housings and pipes for handling of abrasive sludges, power transmission belting, diving gear, water lubricated bearings, etc. Thus the discovery of vulcanization in 1850 meant that a whole range of rubber products became available. Virtually all of the stages of manufacturing, including masticating, compounding, milling, vulcanizing and finishing, are the same today as they were in 1900, although the machinery has been continuously refined and improved. The greatest change in the rubber industry occurred in the early 20th century with the development of the first synthetic rubber called Buna

Recent advances in the recycling of rubber waste

and later synthetic styrene butadine rubber (SBR). However, the economics of rubber production have undergone considerable change. This industry has always been at the mercy of rapid and drastic changes, both in the cost of raw rubber and the prices of finished goods. At present, natural rubber is commercially produced in Indonesia, Thailand, Malaysia, Brazil, India, Sri Lanka and in some African countries like Nigeria and Ivory Coast.

2. Use and reuse of rubber products 2.1. Rubber in everyday life Not only in industrialized countries but also in less developed nations, rubber products are everywhere to be found, though few people recognize rubber in all of its applications. Since 1920, demand for rubber manufacturing has been largely dependent on the automobile industry, the biggest consumer of rubber products. Rubber is used in radio and T.V sets and in telephones. Electric wires are made safe by rubber insulation. Rubber forms a part of many mechanical devices in the kitchen. It helps to exclude draughts and to insulate against noise. Sofas and chairs may be upholstered with foam rubber cushions, and beds may have natural rubber pillows and mattresses. Clothing and footwear may contain rubber: e.g. elasticized threads in undergarments or shoe soles. Most sports equipment, virtually all balls, and many mechanical toys contain rubber in some or all of their parts. Still other applications have been developed due to special properties of certain types of synthetic rubber, and there are now more than 100,000 types of articles in which rubber is used as a raw material.

2.2. Waste rubber as landfills The use of rubber in so many applications results in a growing volume of rubber waste. With the increase in demands, the manufacturing and use of rubber and the rubber products has increased tremendously both in the developed and less developed countries. About 242 million tyres are discarded every year in the United States alone. Less than 7 percent are recycled. 11 percent are incinerated for their fuel value and another 5 percent are exported. The remaining 78 percent are either landfilled, or are illegally dumped. According to a recent report of the US Environmental Protection Agency (U.S EPA), this has resulted in a national stockpile of over 2 billion waste tyres. But with the decreasing scope of available sites and due to the corresponding cost explosion [1] this process of waste rubber disposal is no

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longer feasible. Land filling with waste tire is, also the most unwanted due to environmental problems and has no future possibility. The current rubber waste situation of developed countries is presented in Table I. At the end of 1950s, only about one fifth of the rubber hydrocarbon used by the United States and Europe was reclaimed. By the middle of 1980s less than 1% of the worldwide polymer consumption was in the form of reclaim. At the beginning of 20th century half of the rubber consumed was in the form of reclaim. It is expected that in 21st century most of the scrap rubber will be recycled in the form of reclaim because of day to day increase in environmental awareness. From Table I it is seen that except USA [2], France and Italy [3] the tendency to use scrap rubber as a landfill is decreased. Some countries have already banned the use of discarded tire for land filling. Tires discarded in landfills tend to float on top causing: mosquito breeding and illegal tire disposal are creating problems which can be minimized by recycling. Different ingredients such as stabilizers, flame retardants, colorants, plasticizers etc. were mixed with rubber during compounding. After discarding the tires for landfilling there is a probability of leaching small molecular weight additives from bulk to the surface and from surface to the environment. These small molecular weight additives are not eco-friendly and may kill advantageous bacteria of soil. In this way landfill causes serious environmental problem. Among various methods the least desirable disposal method is discarding the article (or material). This is a situation where not only no value is added to the waste material, but in fact, the value added is negative because of the implicit cost of: (i) transporting the material to the landfill site; and (ii) establishing and maintaining the landfill to satisfy environmental requirements. Table I. Rubber waste situation in 2006 on developed nations. Treatment Land fill

USA UK Germany France Italy Belgium Netherlands Japan (%) (%) (%) (%) (%) (%) (%) (%) 58 23 9 45 40 10 12

Sweden (%) 5

Retreading 19

Energy

Export Recycling

5 7

3 16

16 12

4 16

2 12

25 15

6 19

7 12

2.3. Waste rubber as fuel source Sometimes scrap rubber is used as a fuel. Pyrolysis is one of the thermal approaches to recovering energy and basic materials from waste rubber.

Recent advances in the recycling of rubber waste

Technically, the term 'pyrolysis' covers all forms of heat decomposition, including combustion, although in practice it is normally understood to mean thermal decomposition in a non-reactive (anaerobic, or in the absence of oxygen) atmosphere. Generally 25-50% of the weight of rubber pyrolysed can be recovered in the form of a distillate of approximately 42 MJ/kg calorific value, which is higher than when burning tyres (37.5 MJ/kg) and even higher than coal (29 MJ/kg). The elemental constituents of a rubber compound are almost equal to those found in coal. Indeed coal provides about 30 MJ/kg and rubber provides about 32.5 MJ/kg of heat energy, which compares with liquid fuels at about 56 MJ/kg [4]. Shredded tire chips have been burnt in boilers, but tire grinding and size reduction problem have set back the use of tire chips in boiler. Transportation of tire scrap can cost $0.05/kg, exclusive of grinding costs. The cost of burning one metric ton of tires per hour in an incinerator was ca. $0.20–0.40 per tire in 1974, which increased to $0.35–$0.70 per tire in 1987, and now it is increased further. The Oxford Energy Company incinerates tires and produces electricity. The facility generates 14.4 mW of electricity and costs $38 × 10. Thus in the incineration process discarded rubber is used as a fuel to generate electricity, steam etc. This process is still in use. But it creates new problem of air pollution and is also a low value recovery process of the waste rubber. An environmentally friendly process [5] was developed for recycling rubber waste materials such as waste tires to generate valuable fuels or chemical feedstocks in a closed oxidation process which is free of hazardous emissions. The process involves breakdown of rubber polymer materials by selective oxidation decoupling of C–C, C–S and S–S bonds by water as a solvent at or near its supercritical condition. Adkins [6] invented a method of processing used tires for the recovery of oil, steel, vinyl chloride and carbon. The process includes adding a shredded automobile tire to a batch of isocyanide, polyurethane, latex and soybean oil. The resultant mixture was then heated at 700oF for 10 min to obtain the products. The addition of soybean oil to the bath mixture provides a safer and more economical process.

3. Reclamation of rubber products One of the various problems which mankind faces as it enters into the 21st century is the problem of waste disposal management. Since polymeric materials do not decompose easily, disposal of waste polymers is a serious environmental problem. Large amounts of rubbers are used as tires for aeroplanes, trucks, cars, two wheelers etc. Reclaimed rubber is the product

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resulting when waste vulcanized scrap rubber is treated to produce a plastic material which can be easily processed, compounded and vulcanized with or without the addition of either natural or synthetic rubbers. Regeneration can occur either by breaking the existing cross links in the vulcanized polymer or by promoting scission of the main chain of the polymer or a combination of both processes. Reclaiming of the waste rubber can be a difficult process. There are many reasons, however why waste rubber should be reclaimed or recovered; • • • • • • • •

Recovered rubber can cost half that of natural or synthetic rubber. Recovered rubber has some properties that are better than those of virgin rubber. Producing rubber from reclaim requires less energy in the total production process than does virgin material. It is an excellent way to dispose of unwanted rubber products, which is often difficult. It conserves non-renewable petroleum products, which are used to produce synthetic rubbers. Recycling activities can generate work in developing countries. Many useful products are derived from reused tyres and other rubber products. If tyres are incinerated to reclaim embodied energy then they can yield substantial quantities of useful power. In Australia, some cement factories use waste tyres as a fuel source.

Reclaiming of scrap rubber is, therefore, the most desirable approach to solve the disposal problem. Reclaim is produced from vulcanized rubber granules by breaking down the vulcanized structure using heat, chemicals and mechanical techniques. Reclaim has the plasticity of a new unvulcanized rubber compound, however, the molecular weight is reduced so reclaim compounds have poorer physical properties when compared to new rubber. The main reasons for their use are price and improved processing of rubber compounds. The main processing advantages claimed can be summarised as: - shorter mixing times - lower energy consumption - lower heat development - faster processing on extruder and calenders - lower die swell of the unvulcanized compound - faster curing of the compounds

Recent advances in the recycling of rubber waste

In many rubber products 5-10% reclaim can be added to the new rubber content without serious effects to the physical properties. Far higher percentages (20-40%) are used in products like car mats. Traditionally, however, compounds used in the production of tyre carcasses have been the main outlet for reclaim, due to its processing advantages. In spite of this, the proportion of reclaim in radial tyres is limited to around 2-5%. Reclaiming of scrap rubber products, e.g. used automobile tires and tubes, hoses, conveyor belts etc. is the conversion of a three dimensionally interlinked, insoluble and infusible strong thermoset polymer to a two dimensional, soft, plastic, more tacky, low modulus, processable and vulcanizable essentially thermoplastic product simulating many of the properties of virgin rubber. Recovery and recycle of rubber from used and scrap rubber products can, therefore, save some precious petroleum resources as well as solve scrap/waste rubber disposal problems. Reclaiming processes may be broadly classified into two groups: physical reclaiming processes and chemical reclaiming processes. In a review, Warner [7] has summarized various methods of devulcanization using chemical and physical processes.

3.1. Reclaiming agents, oils and catalysts Many attempts have been made since the beginning of the 20th century for reclaiming of scrap rubber products. As a result, a large number of chemical reclaiming agents for natural and synthetic rubbers, viz. diphenyl disulfide, dibenzyl disulfide, diamyl disulfide [8-9], bis(alkoxy aryl) disulfides [10], butyl mercaptan and thiophenols [11], xylene thiols [12] and other mercaptans [13], phenol sulfides and disulfides [14] have been developed. The following definition was adopted in 1981 by the Rubber Recycling Division of the National Association of Recycling Industries, Inc.: Reclaimed rubber is the product resulting when waste vulcanized scrap rubber is treated to produce a plastic material which can be easily processed, compounded and vulcanized with or without the addition of either natural or synthetic rubbers [15]. Regeneration can occur either by breaking the existing cross links in the vulcanized polymer or by promoting scission of the main chain of the polymer or a combination of both processes. A definition of reclaiming agents, catalysts and reclaiming oils depends on their reaction and function within the process. Yamashita [16] published a review on the different reactions that might occur during reclaiming. The auto-oxidation accelerated degradation reactions are described and these reactions occur particularly in reclaiming processes that involve shearing actions. Without the addition of reclaiming agents, autooxidation reactions will progress through hydroperoxides formed by the attack of oxygen, which is always present in the rubber. Utilization of reclaiming

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agents speeds up and introduces new pathways for the reclamation reactions. Reclaiming catalysts are chemical compounds, which are effective in only small amounts during reclamation. Reclaiming oil has several roles apart from raising the plasticity of reclaimed rubber, such as an accelerating action on the oxidation of the rubber and a gel preventing action by acting as a radical acceptor. It also facilitates the dispersion of the reclaiming agent into the rubber matrix. Hence reclaiming oil with a high compatibility with the rubber should be used. The reclaiming oils often have active double bonds or methylene groups in the molecules, through which they are more easily oxidized than the rubber molecule. It is assumed that these activated molecules formed by the auto-oxidation reaction, accelerate the oxidation of the rubber.

3.2. Proposed reaction mechanism of the reclamation of vulcanized rubber Inspite of a large volume of work on the reclaiming of waste rubbers very little information on the mechanism of reclaiming of rubber supported by straightforward evidences is available. Reclamation can occur by breaking the existing cross links in the vulcanized rubber (scission of the crosslink) or by promoting scission of the main chain of the polymer or by both processes [17]. Amberlang and Smith [18] suggested indirectly the oxidative scission at sulfur crosslinks partly based on speculation and partly based on experimental results. Bennett and Smith [19] reported that an alkyl phenol sulfide reclaiming agent had little activity in the absence of oxygen. The oxygen and reclaiming agent showed exceptional activity in attacking a sulfur cured GR-S gum vulcanizate to produce soluble, low molecular weight fragments under relatively mild experimental conditions. ASTM STP 184 A [20] defined “devulcanization as a combination of depolymerization oxidation and increased plasticity” as they usually occur during the process of reclaiming. But actually devulcanization should be the reverse process of vulcanization. In sulfur vulcanization formation of both the C–S and S–S bond takes place and, therefore, it is expected that during devulcanization only the C–S and S–S bond cleavage should occur. In view of these arguments the conversion of scrap or waste rubbers into usable form by all these above physical and chemical processes may be called reclaiming processes.

3.2.1. Scission of the main chain As an example of this mechanism, the phenyl hydrazine–iron (II) chloride system (PH–FeCl2) is very effective for the oxidative degradation of

Recent advances in the recycling of rubber waste

the rubber molecules at low temperatures. In the degradation reaction of polyisoprene rubber with the PH–FeCl2 system in air, the phenyl hydrazine is the main reagent and FeCl2 acts as catalyst: the rate of degradation of the rubber is determined by the phenyl hydrazine concentration. Phenyl hydrazine is itself easily degraded by oxygen and it is known that nitrogen gas is liberated in this reaction [21]. The rate of degradation is very high in the presence of a metal salt. The initial oxidative degradation of rubber molecules with the PH–FeCl2 system is outlined below (Fig. 1). If sufficient oxygen is present, the various radicals formed by this reaction degrade the rubber molecules as indicated in Fig. 2 [22]. The hydroperoxide is decomposed in the presence of transition metals as shown in Fig. 3.

Figure 1. Oxidation mechanism for the PH–FeCl2 system [adapted from Ref. 21].

Figure 2. Bolland oxidation mechanism (RH = rubber hydrocarbon) [Reproduced with permission from Ref. 22].

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Figure 3. Decomposition of peroxides by ions of metals (redox mechanism) [Reproduced with permission from Ref. 22].

3.2.2. Crosslink scission (triphenyl phosphine) Triphenyl phosphine is known to open the sulfur cross links by a nucleophilic reaction [23] as shown in Fig. 4. The radicals can recombine, but a reaction with a double bond is also possible, with the net effect that the crosslink is broken.

Figure 4. Opening of sulfur crosslinks by triphenyl phosphine [Reproduced with permission from Ref. 23].

3.2.3. Main-chain and crosslink scission Thiols and disulfides interact with radicals formed during the degradation of the rubber network. It is assumed that they initiate an oxidative breakdown of sulfur cross links and a degradation of rubber vulcanized [24] and inhibit gel formation by combination with the radicals. A vulcanizate that is recycled with thiols and disulfides shows a larger degree of network breakdown. In thermo-mechanical processes disulfides and thiols are equally reactive [26]. Aliphatic thiols are found to be less active than aromatic thiols. The efficiency of aromatic compounds seems to increase when alkyl groups or halogens are substituted on the benzene ring. A mechanism that is often proposed for the reaction of disulfides with sulfur vulcanizates is the opening of cross links or the scission of chains by heat and shearing forces and their reaction with disulfides, which prevents recombination. Atmospheric oxygen is also said to prevent recombination by stabilizing the radical sites. Other compounds with a stabilizing effect are antioxidants [26]. The result is a drop in molecular weight of the polymer. During the thermal degradation hydrogen sulfide and thiols are produced.

Recent advances in the recycling of rubber waste

3.2.4. Opening of sulfur crosslinks Various chemical probes are already used in crosslink structure analysis for opening the sulfur cross links [27]. The reactions described in Figs. 5â&#x20AC;&#x201C;9 have been proposed as typical methods for rupturing the sulfur cross links of rubbers [16]. These reactions take place under the given reaction conditions, but the complete reaction is not as simple as shown. Hydrogen addition and reduction reactions are also possible but these are to be avoided from the point of view of reclaiming. During the actual reclaiming process it is likely that the thermally generated polymer radicals are scavenged by the sulfur radicals thereby preventing the recombination of polymer radicals shown in fig 5-9. This may be supported by the peptizing action of organic thiol compounds during mastication of raw rubbers where shear generated polymer radicals are prevented from recombination by the action of thiol radicals [28]. The sulfur analysis of rubber before and after the treatment of diallyl disulfide (DADS) in this study [29] has shown the increase of combined sulfur of the treated rubber. This may be explained due to the attachment of DADS fragments with the rubber molecules. It also appears that above sequences of reactions may occur irrespective of any reclaiming temperature or any reclaiming agent.

Figure 5. Opening of sulfur crosslinks by oxidation; ROOH = organic hydroperoxide [Reproduced with permission from Ref. 16].

Figure 6. Opening of sulfur crosslinks by heat or shear [Reproduced with permission from Ref. 16].

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Figure 7. Opening of sulfur crosslinks by nucleophilic reagents [Reproduced with permission from Ref. 16].

Figure 8. Opening of sulfur crosslinks by rearrangement [Reproduced with permission from Ref. 16].

Figure 9. Opening of sulfur crosslinks by substitution [Reproduced with permission from Ref. 16].

The two principal methods to obtain a re-usable recycled rubber material are: (i) grinding of the rubber and reusing it in the form of a granulate or surface activated powder; (ii) treating the material in a reclaiming process to generate a visco-elastic reclaim. Different processes are developed in order to reclaim vulcanized rubber.

4. Different types of reclaiming processes Rubber may be converted into reclaim by means of a number of processes. In all these the scrap rubber must first be shredded and ground into crumb to permit chemicals and swelling agents to react adequately with the

Recent advances in the recycling of rubber waste

vulcanized structure, to promote good heat transfer, and to remove the fibres by mechanical or chemical action. Each of the processes described below is followed by final processing.

4.1. Mechanical reclaiming process Several processes are used in mechanical reclaiming process, all of which are continuous processes. Fine (fabric-free) scrap rubber is mixed with reclaiming chemicals and fed continuously into an extruder in which the rubber is devulcanized at about 200oC for about 5-10 minutes. The heat is partially generated by the electrical heating of the extruder and the friction of the crumb. The devulcanized rubber is extruded from the machine in a dry form ready for refining. In mechanical reclaiming process crumb rubber is placed in an open two-roll mixing mill and milling is carried out at high temperatures. In this process drastic molecular weight breakdown takes place due to mechanical shearing at high temperatures (above 200oC). A physical process of reclaiming of vulcanized rubber and refining of the reclaimed rubber are described in a US patent by Maxwell [30]. Particulate form of the vulcanized rubber is reclaimed with reclaiming agents by passing the rubber between an essentially smooth stator and an essentially cylindrical rotor arranged to provide an axial shear zone in which the rubber is frictionally propelled by the rotor action. The action may be assisted by mixing a suitable amount of previously reclaimed rubber or of vulcanized rubber with or in advance of the particulate vulcanized rubber, and/or by supplemental heating. In other aspects of the invention previously reclaimed and vulcanized rubber is similarly fed and acted upon as substitute for conventional refining operation. The mechanical reclaiming process of vulcanized natural rubber is also reported by De et al [31]. The reclaimed natural rubber was prepared by milling vulcanized sheets at about 80oC. On a two roll laboratory mill it formed a band on the roll. Next, it was mixed with various rubber additives. In another case, mixing of reclaim rubber (RR) with fresh rubber in various proportions and study of their curing characteristics, mechanical properties etc. were done. But the Mooney viscosity of the reclaimed rubber was very high (>200, i.e. out of scale) indicating that the plasticity of rubber was very low due to the presence of higher percentage of crosslinked rubber. A comparative study of the curing characteristics of the blends of fresh rubber with reclaim rubber indicated that with increase in the reclaim rubber content the cure rate increased but the scorch time, optimum cure time and

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reversion resistance decreased. As the proportion of reclaim rubber in the blends increases modulus, abrasion loss, compression set and hardness increase while tensile strength, elongation at break, tear strength, resilience and flex resistance decrease which shows that increase in the proportion of reclaim rubber, increases the crosslink density. As crosslink density is very high for the natural rubber/reclaim rubber (25/75) blend, so the modulus is high but tensile strength and flex properties are low. Thus reclaim rubber appeared to perform as a non-reinforcing filler in this study.

4.2. Thermal processes Here reclaiming of the rubber that make use of heat and possibly chemicals to plasticize the rubber scrap are summarized [32].

4.2.1. Digester process The digester process is a batch process where the ground scrap material is mixed with fiber dissolving agents, water, plasticizing oils and, if needed, reclaiming agents. They are heated in an autoclave with steam 15 bar (=180oC) for 8-12 hours. The fabric dissolves and the rubber softens. The aqueous phase is separated and the regenerate is dried for final refining. Significant amounts of residual fabric may remain with the reclaim if the digester processes is being used. The reclaiming chemicals used in this process include zinc and calcium chlorides (to dissolve the fibres) together with a very complex mixture of solvents, softening oils, hydrocarbon resins, pine tar and reclaim catalysts.

4.2.2. Pan process The heater or pan process is one of the oldest and most simple processes used in the rubber reclaim industry. Finely ground scrap rubber that has been freed of fibres by mechanical cyclones is mixed with reclaiming chemicals, then placed in open pans in an autoclave and heated with 'live' steam at 15 bar (=180oC) for 4-12 hours. Any residual water is removed and the reclaim is ready for final processing. Reclaiming chemicals are aromatic thiols, disulfides and aromatic oils. Their use allows lower temperatures and shorter reclaiming times and produces a product with superior mechanical properties. The reclaiming chemicals used in the pan process are similar to those used in the digester process except that no zinc or calcium chloride are used.

Recent advances in the recycling of rubber waste

4.2.3. Alkaline process The fiber in the scrap is digested by the use of sodium hydroxide in a high concentration (up to 7%): the cellulose in the fiber-containing scrap is hydrolyzed. After recycling, a washing procedure is required to remove excess de-fiberizing agent. The crumb is then dried and refined. The process is found to be detrimental to SBR containing rubber material, as hardening takes place. Some N,N-dialkyl aryl amine sulfides [14, 33] were shown to be highly active reclaiming agents for vulcanized SBR in alkaline reclaiming processes, and the state of reclaiming was manually assessed through thickness, body and tack evaluation.

4.2.4. Neutral process An improvement to overcome the hardening problem was the development of the neutral process. In this process, zinc chloride and pine oil are used [34]. Calcium chloride or zinc chloride is added for the hydrolysis of the textile. The heater, digester, alkaline and neutral processes use long reaction cycles and are characterized by long recycling time. N,N-dialkyl aryl amine sulfides were shown to be highly active reclaiming agents for vulcanized SBR in neutral reclaiming processes too.

4.2.5. High-pressure steam process More recent developments aimed at shorter reaction times. In the highpressure steam process [34], fiber-free, coarse ground rubber scrap is mixed with reclaiming agents and reclaimed in a high pressure autoclave at a temperature around 280oC for 1â&#x20AC;&#x201C;10 min.

4.2.6. Engelke process In the Engelke process [35] an autoclave is used, in which coarse ground rubber scrap, mixed with plasticizing oil and peptizers. This mixture is heated to very high temperatures (> 250 oC) for 15 min, after which refining and straining takes place.

4.2.7. Continuous steam process A continuous steam process uses temperatures around 260oC and high pressures in a hydraulic column. The rubber is ground and water is used as a carrying medium and to seal the material from extraneous oxygen, because otherwise heat and pressure would cause combustion. Heat and pressure

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combined with the injected chemical agents cause a substantial breakdown of the rubber in suspension.

4.3. Thermo-mechanical reclaiming process During most mechanical processes a strong rise in temperature occurs that aids in degrading the rubber network. Thermo-mechanical recycling of rubber is assumed to be a combination of breaking carbon-to-carbon bonds and sulfur cross links. This results in the formation of soluble branched structures and fragments of gel. The modern material recycling processes all use thermo-mechanical regeneration methods. Recycling chemicals and oils are frequently used in addition to the thermal and mechanical breakdown: disulfides, thiols, amines and unsaturated compounds are the most common recycling chemicals. They are added in quantities of around 1wt% [34]. Softeners lower the thermal degradation resistance of a vulcanizate by weakening the interaction between filler and rubber chains.

4.3.1. Milling process This process [36] involves the thermo-mechanical degradation of the rubber vulcanizate network. The vulcanizate is swollen in a suitable solvent and then transferred to a mill to form a fine powder (~20 Âľm diameter). This powder rubber is revulcanized with curing ingredients. The products thus obtained show slightly inferior properties to those of the original vulcanizates.

4.3.2. High-speed mixing A fast thermo-mechanical recycling process is the high-speed mixing process. The rubber is stirred at a speed of 500 rpm and the temperature rises to 200oC [16]. The process takes 15â&#x20AC;&#x201C;20 min.

4.4. Mechano-chemical methods Plasticization can be improved by using a reclaiming accelerator while applying a mechanical force to the rubber powder in the presence of air at room temperature. In general, at low temperatures a reclaiming catalyst, reclaiming oil and process oil are used jointly with the reclaiming agent. Typical agents are shown in Table II [16]. Peroxide in combination with methyl halide (material 1) is a powerful radical initiator for a redox system. Phenyl hydrazine and ferrous chloride as well as tributylamine and cuprous chloride (materials 2 and 3) form a complex with each other. This complex is easily degraded by oxygen, under formation of the oxidation initiators. These systems degrade diene-based rubber

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in the presence of oxygen at room temperature. Dioxylyldisulfide and 2,20dibenzamidodiphenyldisulfide (materials 4 and 5) are used as peptizing agents. N-cyclohexylbenzothiazole-2-sulfenamide (material 6) is commonly used as vulcanization accelerator and Nisopropyl- N0-phenyl-p-phenylene diamine (material 7) is used as an antioxidant. Thiophenol and nbutylamine (material 8), toluene sulfonic acid and 1, 8-diazabicyclo [5.4.0] undec-7-ene (material 9) as well as the rubber accelerators tetraethylthiuram disulfide and triphenyl phosphine (material 10) were found to increase the plasticity of rubber reclaim. The fact that these reagents behave as reclaiming agents for rubber is explained by their function as a radical acceptor for the rubber radicals that are formed in the mechano-chemical reaction. Table II. Reclaiming agents used in mechano-chemical methods [Reproduced with permission from Ref. 16].

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4.4.1. Trelleborg cold reclaiming (TCR) process In the TCR process, small quantities of recycling agents are mixed into cryogenically ground rubber powder. A short treatment is carried out in a powder mixer at room temperature or at a slightly higher temperature. Phenyl hydrazine-methyl halide or diphenyl guanidine is used to react with the vulcanizates [16].

4.4.2. De-Link process Rudi Kohler [37] reported a new technology for the devulcanization of sulfur cured scrap elastomers using a material termed De-Vulc developed by Sekhar [38]. Such technique of devulcanization was designated as De-Link process. The recycling of rubber crumb with vulcanization accelerators and sulfur on a mill is used in the patented de-link process. The process is not only suitable for NR, but also for EPDM. In this process 100 parts of 40 mesh or finer crumb is mixed with 2â&#x20AC;&#x201C;6 parts of De-Vulc reactant in an open two roll mixing mill for approximately 10 min at temperature below 50oC. De-Vulc reactant is a proprietary material and its nature and composition is not disclosed. The added chemical mixture is prepared from the zinc salt of dimethyldithiocarbamate and mercaptobenzothiazole with stearic acid, sulfur and zinc oxide dispersed in diols [39]. Tetramethyl thiuram disulfide can also be used. It is assumed that the process is based on a proton transfer reaction. The process is more effective for conventional sulfur vulcanizates than semiefficient and efficient sulfur vulcanizates, the number of cross links is decreased by a factor of 2. As the nip opening of the mill was found to have a significant effect on properties, the vulcanizate breakdown is probably caused by mechanical breakdown. Table III. Mechanical properties of reclaimed rubber [Reproduced with permission from Ref. 40].

Recent advances in the recycling of rubber waste

In Table III, NR indicates natural rubber made form virgin materials, NR-D indicates natural rubber with 30% devulcanized rubber added to the blend. Similarly SBR is virgin material, SBR-D indicates SBR with 30% devulcanized SBR added to the blend. It is evident from Table III which was compiled from Ref. [41] that in 30% blend with devulcanized rubber and virgin rubber for NR and SBR, Mooney viscosity, tan δ and 300% modulus are high whereas tensile strength, elongation at break and tear resistance are low [37]. But it was claimed [37] that those tensile properties, tear resistance etc. were very similar to those for the virgin materials.

4.4.3. Swelling in benzene with a sulfoxide Natural rubber vulcanizates are attacked by swelling in benzene with a sulfoxide compound like dimethyl sulfoxide (DMSO), di-n-propyl sulfoxide (DPSO) or a mixture of these with thiophenol, methyl iodide or n-butyl amine in a mechano-chemical process on a mill [42]. A thiol and DMSO react to form a disulfide and a nucleophilic agent CH3-S-CH3. It is reported that these reagents cause selective scission of sulfur bonds. NR is completely degraded by the combination of DMSO and thiophenol. If we apply this technique to SBR, a much less reactivity is reported. Although the degree of swelling in an organic solvent increased, only 2% of sol fraction was formed. The low sol fraction and high swelling ratio is in agreement with the theory of selective crosslink scission [43]. A disadvantage of this process is that solvents like DMSO and methyl iodides are highly toxic.

4.5. Cryomechanical reclaiming process In the mid 1980s, the technique of grinding scrap rubber in cryomechanical process [44] was developed. This reclaiming process involves placing small pieces (1''× 1''× 1/2'') of vulcanized rubber into liquid nitrogen which are transferred to a ball mill and ground in presence of liquid nitrogen to form a fine powder. The particle size of the cryo-ground rubber varies from 30 to 100 mesh for most products. The particle size is controlled by the immersion time in the liquid nitrogen and by the mesh size of screens used in the grinding chamber of the mill. Generally, the cost of the ground rubber increases as the particle size decreases. The cost of 40 mesh ground rubber is usually in the mid to high twenty cents per pound area, while smaller particle sizes like 80–100 mesh cost $0.30–$0.40 per pound [45-46]. It has been reported that using 5–10 phr cryogenically ground rubber in various passenger and truck tire compounds

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shows some economic advantage [47]. The economic benefit for a modest usage (5%) in passenger tires and truck tires has been estimated at approximately $0.10 and $0.54 per tire, respectively. At the 10% usage level the economic benefit correspondingly doubles as shown in Table IV. Table IV. Cost savings per tire using cryogenically ground rubber.

Usage (%) 5 10

Cost savings in $ In passenger tires In truck tires 0.0980 0.5424 0.1861 1.0310

Klingensmith [48] has evaluated the performances of cryogenically ground butyl rubber in the tire inner liner. He showed the effect of mesh size on percent retention of physical properties.

4.5.1. Processing and mixing of cryogenically ground rubber [48] In the processing of cryogenically ground rubber certain particle sizes are more suitable in specific applications. Extrusion: 80–100 mesh cryogenically ground rubber is needed to avoid fracturing and rough edges. In extrusion of thick section 50–60 mesh cryogenically ground rubber can be used depending on the surface smoothness of the final product. The optimum level of cryogenically ground rubber to be added to fresh rubber is 5%. Calendering: for optimum surface smoothness of products which are 0.060" or less thick, the compound requires 80–100 mesh cryogenically ground rubber. Where smoothness is not so important 30–60 mesh can be used. The optimum level of cryogenically ground rubber in calendering is 10%. Molding: the cryogenically ground rubber in all mesh sizes can be used because all mesh sizes help in removing trapped air during molding. The cured rubber particles provide a path for the air to escape by bleeding air from the part. Mold flow: cryogenically ground rubber generally improves mold flow. Shrinkage is usually less for compounds containing cryogenically ground rubber. The shrinkage reduction is proportional to the amount of cryogenically ground rubber in the compound. So less mold flashing was found with increase in the percentage of cryogenically ground rubber.

Recent advances in the recycling of rubber waste

4.5.2. Advantages of using cryogenically ground rubber In the cryogrinding process the equipment cost is less, operating costs are lower, productivity is increased, and the product has better flow characteristics than ambient ground rubber. The unique nature of the surface morphology of the cryoground rubber particle facilitates the ventilation of trapped air in unvulcanized rubber laminate products, particularly tires, thus reducing tendency for cure blistering. Surface oxidation of the cryoground particle is of little concern because of its inherent low surface area, thus differentiating itself from ultrafine, high surface area ambient ground filler (10â&#x20AC;&#x201C;30 mesh). The particle sizes of cryoground rubber are shown in Table V. Table V. Particle sizes of different cryogenically ground rubbers [Reproduced with permission from Ref. 40]. Mesh size

Particle size (Âľm)

40 80 100 200 325 400 500

388 177 149 73.7 44.5 38 20

4.6. Other ground rubber processes Other two types of grinding of rubber are dry ambient grinding and solution or wet ambient grinding. The first step in the manufacture of reclaim is grinding [49-50], of the rubber part to be reclaimed. It is necessary to increase the surface area of the rubber particle which increases the rate of the chemical reaction in reclaiming and also to produce more uniform product. It was found that small particle size vulcanized ground rubber could be added to the rubber compound to reduce the cost. Ground rubber used in compounding varies from 10 to 200 mesh.

4.6.1. Dry ambient grinding This is nothing but a mechanical grinding technique [49]. In this process vulcanized rubber pieces are placed in a serrated grinder for preparation of ground rubber of particle sizes from 10 to 30 mesh. Ambient ground rubber is largely used in tires and mechanical goods. Generally, 5â&#x20AC;&#x201C;20 phr ground rubber is used. With higher particle size of ambient ground rubber,

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smoothness of the product decreases. Although the name of the process is ambient grinding the grinding in fact generates some heat during processing. With high modulus or aged compounds the amount of heat generated can be higher resulting in heating of the rubber and, therefore, degradation of polymer chain takes place. So the name “ambient grinding” is not appropriate. In ambient grinding process some pendant groups are generated which become attached to the virgin rubber matrix producing an intimately bonded mixture [49].

4.6.2. Wet or solution grinding This is a modified ambient grinding process [49] that reduces the particle size of rubber by grinding in a liquid medium. The process involves putting a coarse ground rubber crumbs (10–20 mesh) into a liquid medium, usually water, and grinding between two closely spaced grinding wheels. Here the particle size is controlled by the time spent in the grinding process. In this process particle sizes of 400–500 mesh have been reported. The advantage of the fine particle wet ground rubber is that it allows good processing producing relatively smooth extrudates and calendered sheets. But whether this process helps in heat dissipation through water and reduces polymer chain breakdown is not mentioned.

4.7. Microwave recycling In the microwave recycling method, a controlled dose of microwave electromagnetic energy at specified frequency is used to break the sulfursulfur or carbon-carbon bonds in the crosslinked rubber powder [51]. Thus in this process elastomer waste can be reclaimed without depolymerization to a material capable of being recompounded and revulcanized having physical properties essentially equivalent to the original vulcanizate. By using microwaves, the temperature of the material increases very fast to reach finally 260–350oC [52]. A pre-condition to reach this temperature level for devulcanization is that the vulcanizates should contain carbon black, making them suitable for this method. The waste material for reclamation must have some polarity so that the microwave energy will generate the heat necessary to devulcanize. The devulcanized rubber is not degraded when the material being recycled [53] which normally take place in the usual commercial processes currently being practiced. In this process they claimed that sulfur vulcanized elastomer containing polar groups is suitable for microwave devulcanization. Tyler et al [54] have claimed their microwave

Recent advances in the recycling of rubber waste

devulcanization process as a method of pollution controlled reclaiming of sulfur vulcanized elastomer containing polar groups. Carbon black containing rubber is susceptible to ultra high frequency in a microwave chamber due to interface or ion polarization: accumulation of free electrons at the interface of different phases with a different conductivity and dielectric constant. Microwave energy between 915 and 2450 MHz and between 41 and 177 W h per pound is sufficient to sever all crosslink bonds but insufficient to cleave polymer chain degradation. The microwave energy devulcanization device generates heat at a temperature in excess of 260oC to yield a mass which is fed to an extruder which extrudes the rubber at a temperature of 90â&#x20AC;&#x201C;125oC. The extrudate can be used per se as a compounding stock. Another process was developed for reclaiming waste elastomers by microwave radiation. The process involves the impregnation of the waste rubber with an essential oil and then heat treating the impregnated material under reduced pressure with microwave radiation [55]. The tensile properties of microwave devulcanized EPDM rubber, EPDM hose and IIR are shown in Table VI. From the Table VI it has been found that the tensile properties of devulcanized rubber and virgin rubber-devulcanized rubber blend is almost comparable. The cost of devulcanized hose and inner tube material by microwave method is only a fraction of the cost of the original compound. The transformation from waste to refined stock ready for remixing takes place in only five minutes with usually 90â&#x20AC;&#x201C;95% recovery of the rubber. Therefore, it appears that this microwave technique is a unique method of reclaiming in terms of properties and fastness of the process. This method is very much useful because it provides an economical, ecologically sound method of reusing elastomeric waste to return it to the same process and products in which it was originally generated and it produces Table VI. Physical properties of microwave devulcanized product [Reproduced with permission from Ref. 40].

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a similar product with equivalent physical properties. The properties of the reclaim are reported to be better than those of rubber obtained by other reclaiming methods, and EPDM and IIR are particularly suitable for the process. The process is able to convert vulcanized waste such as EPDM (automotive coolant hoses) into a usable compound in just 5 min [56-57]. The process has the disadvantage that it is difficult to control.

4.8. Ultrasound recycling Next to the microwave radiation, ultrasonic energy is used for the devulcanization of cross linked rubber. The first work with ultrasonic energy was reported by Pelofsky [58] in 1973. In this patented process, solid rubber articles are immersed into a liquid, and then ultrasonic energy is applied whereby the bulk rubber effectively disintegrates and dissolves into the liquid. In this process ultrasonic radiation is in the range of about 20 kHz and at a power intensity of larger than 100W. But in the patent information they did not mention the ultimate properties of the devulcanized rubber. The ultrasonic reclaiming of natural rubber vulcanizate was again reported by Okuda and Hatano [59] in 1987 which was also patented. They subjected the NR vulcanizate to 50 kHz ultrasonic energy for 20 min to achieve devulcanization. Mangaraj and Senapathi [60] indicated in their patent on vulcanization of rubber by ultrasonic radiation a possibility of rubber degradation and devulcanization by ultrasonic energy. Later Levin and coworkers reported [61] the phenomenon of devulcanization by ultrasonic energy. The devulcanization process requires a high energy level to break carbonâ&#x20AC;&#x201C;sulfur and sulfurâ&#x20AC;&#x201C;sulfur bonds. An ultrasonic field creates high frequency extensionâ&#x20AC;&#x201C;contraction stresses in various media [62]. Isayev and his group have made a percolation simulation of the network degradation during ultrasound devulcanization in which they have claimed an excellent agreement of experimental data for SBR and GRT (ground rubber tyre) with the predicted dependence of the gel fraction of devulcanized rubber on crosslink density. At the University of Akron, Ohio, they designed an ultrasonic reactor of a 38.1mm rubber extruder with a length to diameter ratio of 11 and a co-axial cone-shaped ultrasonic die attachment equipped with three temperature-controlled zones. The screw is heated electrically or cooled by water. The scrap rubber is fed into the extruder by a conveyor belt with adjustable output. A 3kW ultrasonic power supply, an acoustic converter and a 76.2mm cone-tipped horn is used. The horn vibrates longitudinally at a 20 kHz frequency and 5-10 mm amplitude. The scrap rubber particles are transported within the extruder to the chamber with the ultrasound horn, and the recycled rubber can exit this chamber through a die. When SBR is

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recycled by ultrasound extrusion [63], cleavages of intermolecular bonds such as C–S and S–S as well as C–C bonds in the main chain take place. An increase in ultrasound amplitude is accompanied by significant decrease in molecular weight of the sol fraction and a decrease of the gel content. Curing behavior, rheological properties, structural characteristics of devulcanized rubber from model SBR and GRT rubbers as well as mechanical properties of vulcanized rubber samples were studied and a possible mechanism of devulcanization was also discussed. They characterized the degree of devulcanization by the measurement of crosslink density and gel fraction of the devulcanized rubber. Later they have published two more papers on the ultrasound devulcanization of sulfur vulcanized SBR and on vulcanization of ultrasonically devulcanized SBR elastomers [64]. An increase in the extruder temperature also results in a higher sol fraction. Conventional sulfur vulcanizates are more susceptible to sol production. This process is characterized by substantial main-chain scission. A continuous ultrasonic devulcanization of unfilled EPDM rubber was also carried out by Yan and Isayev [65], and the mechanical properties of revulcanized EPDM rubber were measured. Gel fraction, crosslink density and dynamic properties were also determined for the virgin vulcanizate, the ultrasonically devulcanized rubber and the revulcanized rubber. The tensile strength of the revulcanized EPDM was much higher than that of the original vulcanizate with elongation at break values being practically unchanged. It is proposed that the improvement in mechanical properties of revulcanized EPDM is mainly due to the extent of non-affine deformation of the bimodal network that possibly appears in the process of revulcanization of ultrasonically devulcanized rubber. For dynamic visco-elastic properties, it is found that devulcanized EPDM is a more elastic material than uncured virgin EPDM and that revulcanized EPDM is less elastic material than the virgin EPDM vulcanizate at the same modulus level. Such devulcanization experiments were carried out by an ultrasound devulcanization reactor developed for the purpose. The ultrasonic reactor is a 1.5 in. rubber extruder with L/D = 11 with a co-axial cone shaped ultrasonic die attachment. There are three temperature-controlled zones. The screw is heated electrically or cooled by water pumped from a thermostat. The die and the ultrasonic horn have sealed inner cavities for running cooling water. A flush mounted thermocouple and a pressure gauge are inserted into the barrel. The temperature and pressure of the rubber at the entrance to the die are measured by a thermocouple and pressure gauge inserted into the barrel. The scrap rubber is fed into the extruder by a conveyor belt with adjustable output. A 3 kW ultrasonic power supply, an acoustic converter, a 1:1 booster and a 3 in. conetipped horn are used. The horn vibrates longitudinally at a 20 kHz frequency and 5–10 mm amplitude. The whole unit is mounted on four rigid bars fixed to

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the extruder flange. Isayev and co-workers [66-67], studied the devulcanization of SBR vulcanizate using the above ultrasonic reactor at various temperatures viz. 121, 149 and 176oC, different clearances at various flow rates and the ultrasonic oscillation amplitudes. The extent of devulcanization was studied by measuring percentage and crosslink density of the gel fraction. It was reported that both crosslink density and the gel fraction decrease in the devulcanization process. For original ground rubber tire the measured gel fraction is 83% and crosslink density of gel is 0.21 kmol/m3, but after ultrasound treatment at 121oC barrel temperature it reduces to 64–65% with crosslink density of 0.02 kmol/m3 The crosslink density also decreases with higher residence time in the treatment zone and with higher specific ultrasonic energy. The mechanical properties of the revulcanized sample were also studied. With decrease in the crosslink density of the devulcanized rubber, the tensile strength of revulcanized samples varies from 1.5 to 10.5 MPa and elongation at break varies from 130 to 250%. Based on the results of mechanical properties Isayev et al [66]. proposed that the devulcanized sample having a crosslink density lower than 0.06 k mol/m3 can be regarded as over treated, and samples with crosslink density higher than 0.10 kmol/m3 can be regarded as undertreated. Thus over treatment causes main chain breakage and undertreatment causes insufficient devulcanization. They also reported that ultrasound treatment of SBR results in low molecular weights of the sol fraction: Mn = 2-4X103. Ultrasonic devulcanization, therefore, causes significant degradation of polymer chains. A simple model based on a purely topological consideration was proposed and simulation of the process was carried out [68–69]. In the model they have assumed a breakup of the main chain bond and crosslink bonds as independent random events. Such random scission of crosslinks and main chain results in the formation of soluble branched rubber chains regarded as fragmented gel structure or microgel. It is found that during ultrasound devulcanization molecular weight of sol fraction decreases from which it may be understood that during ultrasound treatment not only C–S or S–S bonds but also C–C bonds break. Levin et al [70] suggested a revulcanization scheme. They concluded that devulcanized rubber contained a larger amount of sulfidized molecules which were responsible for crosslinking during revulcanization.

4.9. Reclaiming by organic reagents Many attempts have been made since the beginning of 20th century for reclaiming of scrap rubber products. Some chemicals that cause selective scission of sulfur bonds are used as reagents to determine the relative amounts of mono-, di-, and polysulfidic cross links. These reagents are called chemical probes. Thiols in combination with organic bases can selectively cleave sulfur cross links. Hexanethiol was found to cleave di- and polysulfidic cross links,

Recent advances in the recycling of rubber waste

while 2-propane thiol selectively cleaves polysulfidic cross links in a nucleophilic displacement reaction with piperidine as base [71-72]. Thus a large number of chemical reclaiming agents for natural and synthetic rubbers, viz. diphenyl disulfide, dibenzyl disulfide, diamyl disulfide [73-74] bis(alkoxy aryl) disulfides[75], butyl mercaptan and thiophenols [76-77] xylene thiols [78] and other mercaptans [13], phenol sulfides and disulfides [14] have been developed. Cook and co-workers [79] reported the preparation, evaluation and structural correlation of alkyl phenol sulfides as reclaiming agents for styrene butadiene rubber (SBR). The effect of these alkyl phenol sulfides as reclaiming agent was compared with that of many aromatic thiols. Reclaiming of neoprene and nitrile rubbers was also evaluated using alkyl phenol sulfides. The extent of reclaiming of these two rubbers was extensively evaluated and monitored by Mooney plasticity and manual observations, such as sheet thickness, body and tackiness by numerical ratings. The reclaiming was done at 188oC (by 175 psi superheated steam) for 4 h using 5 mesh vulcanized rubber powder. Both the total and combined sulfur in the reclaimed stock were determined, but the method of sulfur estimation was not reported. The thiol–amine combination gives a complex, possibly a piperidinium propane-2-thiolate ion pair, in which the sulfur atom has enhanced nucleophilic properties, and is capable of cleaving organic trisulfides and higher polysulfides within 30 min at 20oC, according to a mechanism as shown in Fig. 10 [80,81]. Disulfides react at a rate, which is slower by a factor of 1000. The polysulfide cleavage is faster due to Pπ–dπ delocalization of the displaced s-electron pair of RSS-- as outlined in Fig. 11:

Figure 10. Cleavage of polysulfide bonds by 2-propane thiol/piperidine probe; iPr = isopropyl and R = rubber polymer [Reproduced with permission from Ref. 80-81].

Figure 11. Cleavage of polysulfide crosslinks and Pπ–dπ delocalization; iPrS- = nucleophilic thiol–amine associate [Reproduced with permission from Ref. 80-81].

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Some N,N-dialkyl aryl amine sulfides [78] were shown to be highly active reclaiming agents for vulcanized SBR in both neutral and alkaline reclaiming processes, and the state of reclaiming was manually assessed through thickness, body and tack evaluation. A review of science and technology of reclaimed rubber was published by Le Beau [82] in 1967. Knorr [26] has shown the action of diaryl disulfide on the natural and synthetic rubber scraps of technical goods. In this process the finely ground fabric free scrap is heated in saturated steam at a very high temperature (150–180oC) with the addition of reclaim oil and Aktiplast 6 (contain disulfides). First finely ground scrap was thoroughly mixed with diaryl disulfide and reclaim oil and allowed to swell for at least 12 h. The material was then placed on talcum powder trays. The layer thickness should not exceed 2–3 cm to allow the oxygen to penetrate the material well. A devulcanizer (or autoclave) is needed of approximately 1.5 m diameter and 3.8 m in length. The trays with scrap are rolled into the devulcanizer. Good circulation of air and steam is necessary for the process. After having sealed the lid, the autoclave is pressurized with 4 bar of air–steam and the fan was turned on for good air circulation. The compressed air valve was then closed and 8–9 bar steam was forced in until the temperature reached 190oC. Depending on the kind of scrap the reclaiming time varied between 3–5 h per charge (200 kg). Rubber network can easily be swollen by methyl iodide which can be removed therefrom by warming under vacuum. Meyer and Hohenemser [83] introduced the use of methyl iodide to estimate monosulfide linkages in vulcanized natural rubber. The level of network bound iodine after reaction for two to three days would reflect the concentration of monosulfide groups since simple saturated monosulfide group reacts as follows. R2S + CH3I → R+2SCH3I-Anderson [84] patented the reclaiming of sulfur vulcanized rubber in the presence of oil, water vapor and aryl disulfide peptizer at elevated temperature in the range of about 175–195oC and at a pressure in the range of about 230–260 psi for 1–4 h. Here aryl disulfide is a mixture of diphenyl disulfide, dicresyl disulfide and dixylyl disulfide. In another attempt rubber like product from rubber scrap was prepared by mixing rubber scrap with sulfur, antioxidant and antiozonants in an apparatus. The reclaiming was carried out at temperature of about 250–450oF and pressure of about 1000–3000 psi for 1–10 min. Sulfur bearing compounds as vulcanizing agents was added in place of free sulfur. The process is particularly suitable for making roofing products [85]. From physical property data (Table VII) it is clear that reclaim rubber possesses very poor mechanical properties. However, the author [26] did not

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mention whether any curative was used with the reclaim rubber. Schnecko [86] has reviewed the present aspects of elastomers recycling and reported the development of some chemical probes for devulcanization of crosslinked rubber. These chemical probes selectively cleave carbon– sulfur and sulfur– sulfur bonds but they do not cleave carbon–carbon bonds. Table VII. Physical properties of reclaimed rubber products [Reproduced with permission from Ref. 26].

In a recent study by Bilgili et al [87] propose a new two stage recycling process to reuse a rubber waste. First, the granulates of the waste were pulverized into small particles using a single screw extruder in the Solid State Shear Extrusion (SSSE) process. Then, the produced powder was compression molded in the absence of virgin rubber. The slabs prepared at various molding conditions were subjected to mechanical, chemical, and microscopic tests. It is found that the slabs have high extensibility with low– medium tensile strength. Compressive creep of the powder, self-adhesion of rubber molecules, and interchange reactions of polysulfidic crosslinks are proposed as the basis of particle bonding. They have demonstrated that rubber slabs can be produced by pulverizing rubber waste using the solid state shear extrusion (SSSE) process and subsequently molding the powder in the absence of virgin rubber. The molding conditions and the powder characteristics significantly affected the failure properties and the crosslink density of the slabs. Higher temperatures and pressures generally improve the bonding of the particles, whereas a high crosslink density and the presence of large amount of coarse particles inhibit the particle bonding. The inferior failure properties of the slabs compared with those of the original vulcanizate can be attributed to a lack of sufficient particle bonding. Compressive creep of the powder, self-adhesion of the rubber molecules, and interchange reactions of polysulfidic crosslinks appear to govern the particle bonding.

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The rubber slabs can be used in the manufacture of noncritical items such as mats, pads, carpet underlay, etc. It is, of course, possible to produce recycled items with any desired shape using different mold geometry. Considering that we could use the pulverized by-product of a rubber company to manufacture these items in the absence of virgin rubber, the process of converting large quantities of factory by-product into useful items using the above two-stage process could be economically feasible. The powder produced by the SSSE process consists of small particles with relatively large surface area. Therefore, by using the above process, the powder could also be used as filler in various applications such as tire manufacturing and production of polymer composites.

4.10. Reclaiming by inorganic compounds Discarded tires and tire factory waste were devulcanized by desulfurization, in presence of sodium, of suspended rubber vulcanizate crumb (10â&#x20AC;&#x201C;30 mesh) in a solvent such as toluene, naphtha, benzene, cyclohexane etc [87]. The alkali metal cleaves mono, di and polysulfidic cross linkages of the swelled and suspended vulcanized rubber crumb at around 300oC in absence of oxygen. As claimed by authors such treatment yielded a rubber polymer having a molecular weight substantially equal to that of rubber prior to vulcanization. Carbon black may also be recovered for reuse and the devulcanized rubber may be subjected to revulcanization without separation of the polymer from the solvent by addition of an appropriate curing composition. Although it appears from the patent that the developed process is a direct reversal of vulcanization without affecting the molecular weight of the polymer. The process may not be economically convenient. Because the process involves swelling of the vulcanized rubber crumb in an organic solvent where in the metallic sodium in molten condition at the process temperature should reach the sulfidic crosslink sites in the bulk of the rubber crumb. Further to this, isolation of the devulcanized product from the solvent may be hazardous and cause pollution. Although the patent has not described the vulcanization characteristics of the devulcanized rubber, the presence of NaS may decrease the scorch safety of the product. Yamashita and co-workers [88-89] have successfully reclaimed powder rubbers using an iron oxide phenyl hydrazine based catalyst. In this process powder rubber from waste tires is treated with phenyl hydrazine and FeCl2, ozonized and treated with H2O2 to give liquid rubber having higher viscosity and better yield than those of similar products without the phenylhydrazineâ&#x20AC;&#x201C; FeCl2 treatment. For example, they reported that 100 g powder rubber from waste tires, 0.5 g phenyl hydrazine in 10 ml benzene and 0.25 g FeCl2 in 5 ml

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MeOH were mixed and kept for a day at room temperature and rolled for 10 min. The above plasticized rubber was ozonized and treated with H2O2 to give liquid rubber having intrinsic viscosity (30oC in benzene) 0.05–0.11 dl/g in 13–25% yield compared with 0.03– 0.05 dl/g and 15–20% yield for liquid rubber obtained without the phenylhydrazine–FeCl2 treatment. Thus from intrinsic viscosity data it is understood that molecular weight of reclaim rubber is very low. During reclaiming by the above process severe breakdown of rubber chains takes place. Kawabata and co-workers [90] have also reclaimed powder rubber using copper (I) chloride–tributyl amine catalyst. They successfully noted that the rate of degradation of isoprene rubber by copper (I) chloride–tributyl amine (Cu2Cl2–Bu3N) mixtures decreased in the order of S-vulcanized rubber> ZnO and tetramethyl thiuram disulfide vulcanized rubber> organic peroxide vulcanized rubber. The sol content and crosslink density of the degradation products indicated that scission in the main chain was as important as breakdown at the crosslinking sites for the sulfur vulcanized and ZnO–TMTD vulcanized samples. But in case of peroxide cured vulcanizates the scission in the main chain was predominant. Therefore, it conclusively proves that during reclaiming process not only the cleavage of carbon–sulfur or sulfur– sulfur bonds takes place but also scission of carbon–carbon bonds of the polymer chain occurs. A novel chemical reclaiming process has been patented wherein reclaiming of pulverized scrap rubber is carried out by a reclaiming composition consisting of reducing agent such as phenyl hydrazine (0.2– 1.0 wt%) and diphenyl guanidine (0.2–0.8 wt%), ferrous chloride and a plasticizer [91]. The reclaiming occurs in a solid phase in oxygenic gas at a temperature of at most 100oC by agitation in a powder mixture for about 30 min.

4.11. Reclaiming by miscellaneous chemicals Vehicle tire scraps containing polyisoprene rubber, SBR, PBR was devulcanized by low temperature phase transfer catalyst. Both the devulcanizing agent composition and the process were patented. The novelty of this process lies in the use of low temperature phase transfer catalyst and a process temperature lower than 150oC. The devulcanized rubber of this invention is distinguishable from conventional reclaimed rubber in that the devulcanized rubber is substantially free from polysulfide crosslink which are selectively broken during the process with negligible main chain scission [92]. Kasai and co-workers [93] reported the use of thiocarboxylic acid as reclaiming agent for crosslinked rubber. In this process waste rubber powder of 30 mesh particle size is mixed with 0.5–10.0 wt% (based on rubber) of

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10% thioacetic acid solution in benzene and left one day at room temperature; stripped of benzene and rolled at <120oC in air to give reclaimed rubber with good mechanical strength. A compound of rubber: 200, ZnO:5; stearic acid:1; sulfur:3; and vulcanizing accelerators: 0.7 parts was heated for 30 min at 145oC to give a vulcanizate having tensile strength 9.0 MPa and elongation at break 410%. Cervinschi and coworkers in 1990 developed a process of reclaiming by hydrocarbon solvent [94]. They have made the waste tires which are swollen by treating at 188–200oC for 24-36h in a mixture of aromatic hydrocarbons 38– 48%, naphthenic hydrocarbons 12–28%, and paraffinic hydrocarbon 35–45%. The swollen rubber in the mixed solvent is passed through a 2 mm orifice of an extruder at 2–6 kN/cm2 pressure to give a paste which is useful in the manufacturing of tire treads. A compound of rubber containing 40–60 parts of above prepared paste and 100–150 parts isoprene rubber, butadiene rubber and SBR was heated for 15 min at 160oC to give a vulcanizate having 300% modulus 3 MPa, elongation at break 500% and Shore A hardness 62. As a solution to waste tire disposal problem used tire was recycled by soaking the tire in an organic solvent e.g. 1,3,5-trimethyl benzene for a sufficient time while reducing its tensile strength by about 50%. Then the soaked rubber was disintegrated by applying a shear force to give a recycled rubber [95]. Scrap rubber containing natural and synthetic can be reclaimed by digester process with the use of a reclaiming oil having molecular weight between 200 and 1000 consisting of benzene, alkyl benzene and alkylated indanes. The composition of this reclaiming oil and the improved digester process using such reclaiming oil has been patented by Bryson [96]. Vulcanized rubber was also reclaimed by the action of transition metal alloys and derivatives [97]. In this process vulcanized rubber was swollen in an organic solvent and then treated with a size reduction agent e.g. transition metal alloy and their derivatives. In a typical composition 150 g of SBR rubber vulcanizate was allowed to swell in 1000 ml benzene for two days, placed in a ball mill in the presence of 2 g powder Mn for 20 h, then beaten in the presence of 0.1 (N) H2SO4 for 4 h, neutralized, washed, dried, homogenized and then compounded to evaluate the properties. The mixing formulation containing reclaim rubber 100; ZnO:0.3; stearic acid:0.2; aromatic oil:0.6; N-phenyl-N-cyclohexyl-p-phenylene diamine (antioxidant):0.1; accelerator:0.1; diphenyl guanidine accelerator:0.5, and sulfur:2.0 parts was vulcanized at 150oC for 45 min to give a vulcanizate having tensile strength 12.6 MPa, elongation at break 220%, Shore A hardness 70 and abrasion loss 5%. 2-Mercaptobenzothiazole was also found to be effective as reclaiming agent [98]. In this process powder rubber from waste tires was kneaded with process oil in the presence of 2-mercapto benzothiazole or its

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cyclohexylamine salt to give reclaim rubber, having Mooney viscosity 31 or 22, respectively. In a typical recipe a mixture of powder rubber:100; reclaiming agent:1; and process oil:10 g was rolled for 30 min. 100 g of the above product was mixed with N-cyclohexyl-2-benzothiozole sulfenamide:1.5, dibenzothiazolyl disulfide:0.5, and sulfur:1.5 g, and vulcanized at 160oC for 10 min to give a vulcanizate having JIS hardness 53, tensile strength 11.9 MPa, and elongation at break 360%.

4.12. Pyrolysis of waste rubber Pyrolysis is one of two thermal approaches to recovering energy and basic materials from waste rubber. Pyrolysis is a controversial, complex, large scale, capital intensive, high-technology approach which is still considered experimental as a method for processing tyres. Technically, the term 'pyrolysis' covers all forms of heat decomposition, including combustion, although in practice it is normally understood to mean thermal decomposition in a non-reactive (anaerobic, or in the absence of oxygen) atmosphere. Pyrolysis is used to describe processes geared towards the recovery of such materials as carbon black, metal, oils, and gasses, as well as those involving the use of waste material for its energy value.

4.12.1. Process Pyrolysis should be understood as the process of heating the rubber under pressure until it decomposes. Three processes are used, involving different thermal ranges: above 600째C; between 400째C and 600째C and below 300째C. The processes differ from each other in production of carbon black, oil and gasses. At high temperatures less carbon black is produced, and more gasses.

4.12.2. Products Whereas the compounds resulting from combustion are relatively predictable, products of pyrolysis differ widely depending on the conditions under which the process is performed. The main products are metals, oils, carbon black, plus a wide range of gaseous and liquid hydrocarbon mixtures, together with varying amounts of residual char. Generally 25-50% of the weight of rubber pyrolysed can be recovered in the form of a distillate of approximately 42 MJ/kg calorific value, which is higher than when burning tyres (37.5 MJ/kg) and even higher than coal (29 MJ/kg). Tests have indicated that this distillate is an excellent low sulphur (less than 0.5%) fuel which can be distilled into a variety of light and heavy fractions, all having similar heating

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values. The quantity of char produced varies from 40 to 50% of the weight of rubber pyrolysed. The char generally has a calorific value of 33 MJ/kg, which is higher than coal, but it contains most of the sulphur. The char is also being investigated for its use (after processing) as an activated carbon which could be used in the tertiary treatment of industrial waste water. The gas produced from tyre pyrolysis ranges from 5-20% by weight of the scrap rubber treated. The composition of the gas varies widely with the pyrolysis conditions, but in all cases a readily combustible high calorific value fuel is produced. The gasses are usually burned to maintain the pyrolysis process and to enable the purification and separation of other fractions. The calorific value depends very much on the percentage of carbon dioxides (CO2) and nitrogen (N2) and may vary between 26,250 and 46,200 kJ/m3. The oil fraction can also be used as fuel. Its caloric value is at the level of heating oil: 40 MJ/kg. This oil is also considered by some to be suitable for use as process oil and as an input material for chemical manufacturing. Even though the process of thermal decomposition is theoretically simple, the process factors need to be very well controlled and the products require extensive processing to produce marketable commodities. The capital investment costs of pyrolysis plants run to several millions of dollars, and cannot take place on a small scale. Although in many laboratories, pilot plant and even commercial attempts have been made to establish economical units over the last 25 years (e.g. Kobe Steel in Japan, Tosco in USA, Tyrolysis in UK, Ebenhausen in Germany and many more) [99] but none has survived. The product spectrum is well known [100]. There are variations by vacuum or in presence of H2, N2 [101] or in molten salts like NaAlO2 [102]. Texaco opened an experimental liquefaction unit about 15 years ago for a mild cracking process below 370oC at atmospheric pressure resulting in light and heavy oil fractions [103]. Finally, it should be emphasized that there are few successful examples of operating pyrolysis plants in the world. The products appear so attractive that attempts are continually being made to get this technical approach going, but they consistently fail either financially or technically.

4.13. Reclaiming of rubbers by the use of a renewable resource material (RRM) In the chemical reclaiming process a large number of chemical reclaiming agents viz. different disulfides, monosulfides, thiols etc. have been used for treatment of scrap ground rubber crumbs or powders at an elevated temperature and under pressure. No report is available on reclaiming of rubber at around ambient temperatures. Almost all disulfides and thiols have very repelling smell and are hazardous. So handling of these disulfides and

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thiols are not very desirable. Being expensive also the use of such chemicals may not be economic for reclaim rubber production. In all the above physical and chemical reclaiming processes except the ultrasound method the extent of reclaiming had not been evaluated or reported. The product of such reclaiming processes was soft and weak mass which was neither characterized nor analyzed for the composition of reclaim. Probably as a result of biased concept such reclaiming was thought to occur by scission of carbon–sulfur and sulfur– sulfur crosslink bonds. The prospect of carbon–carbon bond scission during reclaiming was not investigated. It is either apparent or appropriate to believe that both the physical and chemical reclaiming processes involve polymer chain scission due to mechanical shearing at low or high temperatures, chemical action at high temperatures, thermal scission, or by ultrasound energy at high temperatures. The chain scission of vulcanized rubber during reclaiming is, therefore, supposed to increase plasticity as well as the sol content. The amount of sol as well as the molecular weight of the sol portion of reclaim rubber is supposed to contribute to a great extent to the properties of the reclaim rubber. But except in ultrasound process none has reported the molecular weight of sol. Such information might help in selecting a suitable reclaiming agent as well as a process. In view of the above state of the art in the reclaiming of waste rubbers, De [104] on his PhD work along with Adhikari and coworkers [105-107] have developed a simple process for reclaiming of rubbers with a vegetable product which is ecofriendly and renewable resource material (RRM). The major constituent of RRM is diallyl disulfide. Other constituents of RRM are different disulfides, monosulfides, polysulfides and thiol compounds [108] The reclaiming activity of RRM was studied in natural rubber, styrene butadiene rubber and natural rubber–polybutadiene rubber blend system and compared with the reclaiming activity of synthetic diallyl disulfide [104]. Reclaiming experiments were done using rubbers of known formulations in order to study the chemical and morphological changes occurred during reclaiming operation. The extent of reclaiming was assessed through the measurement of sol content, molecular weight of sol and Mooney viscosity of the reclaim rubber. Tensile properties of this reclaim rubber in a blend with above mentioned virgin rubbers were also evaluated before and after accelerated aging in air.

4.13.1. Reclaiming process using RRM Vulcanized and aged ground rubber of known composition was milled in a two roll mixing mill with simultaneous addition of the RRM and spindle oil or diallyl disulfide (DADS) and spindle oil separately. The reclaiming was

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carried out with different concentrations of reclaiming agents for different milling times at two different temperatures. With progressive milling, band formation took place on the roll surface and the entire mass became sticky. The results in Table VIII showed that 15 min milling of vulcanized rubber with RRM or DADS produced lowest sol content, lowest molecular weight of sol and highest Mooney viscosity of the reclaim rubber whereas 35 min milling provided the highest sol content with the highest molecular weight and the lowest Mooney viscosity. It was found that 10 g of RRM or 2 g DADS was sufficient to obtain reasonable amount of sol with highest molecular weight as well as lowest Mooney viscosity of the reclaim rubber obtained after 35 min milling at 60oC. It is found that the sol fraction gradually increases with the increase in milling time and the highest sol fraction is obtained at 35 min milling showing a major dependence of sol content on milling time because during milling vulcanized rubber samples undergo tremendous mechanical shearing resulting in random polymer chain breakdown. Whatever may be the process of reclaiming, either mechanical or thermal, maximum sol fraction is desirable, the molecular weight of the sol should be as high as possible for improved properties of reclaim. So in any reclaiming process proper care is necessary in setting the reclaiming conditions so that thermal or mechanical Table VIII. Effects of reclaiming agents on sol content, molecular weight of sol and Mooney viscosity of reclaim rubber [Reproduced with permission from Ref. 40].

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shearing has minimum effect on fragmentation of the sol. The reason for increase in sol content with progressive milling lies on the action of DADS either added externally or present as the major constituent of RRM. The DADS breaks into radicals as the temperature rises due to mechanical shearing [33], such radicals combine with the broken polymer chain radical and thereby prevent the recombination of these polymer radicals [104â&#x20AC;&#x201C;106] which explains the increase of sol fraction with increase in milling time.

5. Biotechnological possibilities for reclaiming of rubber One possible way of getting rid of spent rubber could be to degrade it using microorganisms. Biological attack of natural rubber latex is quite facile [109]. Man has tried time and again to consider elastomeric articles as source for microbial attack [110]. Obviously nature is able to take care of its own waste problems but as soon as man become involved and convert the natural rubber polymer into a technical material by sulfur and numerous other ingredients, biological attack is minimized [111]. Spent rubber could be used as substrate for microorganisms provided the structure can be efficiently degraded [112]. Many studies have been made on microbial degradation of rubber materials aiming to either prevent or enhance mineralization [113-114]. Most studies are dealing with micro-organisms belonging to the Actinomycetes [115-116]. An interesting recent approach was reported in a German patent [117] to utilize a chemolithiotrope bacterium in aqueous suspension for attacking powder elastomers on the surface only, so that after mixing with virgin rubber diffusion of soluble polymer chains is facilitated and bonding during vulcanization becomes again possible. A biotechnological process was developed by Straube et al [118] for the devulcanization of scrap rubber by holding the comminuted scrap rubber in a bacterial suspension of chemolithotropic microorganisms with a supply of air until elemental sulfur or sulfuric acid is separated. This seems to be an interesting process which obtains reclaim rubber and sulfur in a simplified manner. Steinbuchel has reported solubilisation of natural rubber of both pure rubber and vulcanized latex by using species of Gordona [119] Biosurfactants were believed to facilitate degradation of the rubber during adhesive growth [120]. Adaptation of microbial enrichment cultures with tire crumb material for several months resulted in enhanced growth of micro organisms especially for natural rubber. Vulcanized rubber in the form of latex is rather easily degraded while rubbers from spent truck tyres are more resistant. Microbial adaptation to growth on polymer material and adhesion of co-substrate increase degradation. The mechanisms behind microbial rubber degradation are often oxidation and chain scission of the polymer

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backbone [121], which does not result in materials suitable for reuse. In a typical process rubber powder, with 1.6% sulfur, was treated with different species of Thiobacillus i.e. T. ferrooxidans, T. thiooxidans, T. thioparus in shake flasks and in a laboratory reactor. The sulfur oxidation depends to a large extent on the particle size. The best results were obtained with T. thioparus with a particle size of 100–200 mm. 4.7% of the total sulfur of the rubber powder was oxidized to sulfate within 40 days [122]. The effect of compounding chemicals on the microbial breakdown of vulcanized natural rubber by a Nocardia sp. has been investigated and found that increased amount of additives such as carbon black (filler) and cyclohexylbenzothiazole sulphenamide (CBS, an accelerator) as well as elemental sulphur increased the resistance towards microbial degradation [123]. The addition of these compounds is most certainly causing a decreased access for the microorganisms to the rubber matrix.

6. Devulcanization in supercritical carbon dioxide A new devulcanization process was developed in which supercritical carbon dioxide (scCO2) was used along with devulcanizing reagents [124125]. Unfilled polyisoprene rubber vulcanizates with different crosslink distributions were prepared by controlling the cure time and the amount of curatives. Each of the vulcanizates was subjected to Soxhlet extraction using azeotropic acetone/chloroform to remove residual curatives. The devulcanization was performed at various temperatures (140–200oC) in the presence of scCO2 for 60 min. The product was fractionated into sol and gel components, and the molecular weight of the sol component and the crosslink density of the gel component were determined. The thiol–amine reagent was found to be the most effective one among several devulcanizing reagents; the molecular weight of the resultant sol component was determined and the crosslink density of the gel component decreased substantially from the initial ones. The yield of the sol component increased with increase in CO2 pressure. In the supercritical fluid state of CO2, the vulcanizate was more efficiently devulcanized than in an ordinary gaseous state of CO2. The sol fraction depended considerably on the crosslink distribution in the vulcanizate. These results suggest that the devulcanizing reagents penetrate and diffuse better into the vulcanizate in the presence of scCO2.

7. Comparative study of recent reclaiming processes Out of these reclaiming processes mainly three of them have been accepted by the industrial community for preparation of reclaim rubber from

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waste rubber products, viz. ultrasound devulcanization, devulcanization by De-Link process and reclaiming by renewable resource material (RRM) and diallyl disulfide. A comparative evaluation of these three processes is presented in Table IX. Thus from the above table it is clear that very high Table IX. Comparative study of recent reclaiming processes [Reproduced with permission from Ref. 40].

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temperature is required for ultrasound devulcanization. But Isayev and co-workers have not mentioned the effect of temperature only without ultrasound energy on devulcanization. But in De-Link process and RRM process nearly ambient temperature is required for reclaiming which is advantageous because at low temperature less energy is consumed for reclaiming and simultaneously polymer degradation can be minimized. For ultrasound devulcanization a special type of ultrasonic reactor is required whereas in De-Link and RRM process only internal mixer or two roll mixing mill is sufficient. Again sol content and molecular weight of sol obtained by ultrasonic devulcanization and RRM process are almost same.

8. Latex products reclaiming The latex industry expanded over the years to meet the world demand for examination gloves, condoms, latex thread, etc. Scrap latex products contain rubber hydrocarbon of very high quality, which is only lightly cross linked. These waste materials were modified into processable materials by novel economic processes and can be effectively utilized by reclaiming processes and may be blended with other polymers. Due to strict specifications for latex products, as much as 15% of the products are sometimes rejected, and these rejects create a major disposal problem for the rubber industry. At the same time, the local authorities prohibit open burning of this waste due to environmental pollution. As latex product waste represents a source of highquality rubber hydrocarbon it is a good candidate for generating reclaimed rubber of superior quality. Thomas et al [126] reported a method for reclaiming latex products. In this process, waste condoms were powdered initially by passing them through a hot two roll mill at 80â&#x20AC;&#x201C;90oC to a size of about 40 mesh. The powdered material was admixed with 10 phr of naphthenic oil and 1 phr of pentachlorothiophenol (PCTP) on a cold mill. The resulting compound was heated in an air oven at 140oC for 30 min. The reclaimed rubber obtained by this process was found to form a smooth band on the mill and contained about 82% of rubber hydrocarbon. However, it was found that only a small amount of this reclaim could be added to raw rubber without adversely affecting the mechanical properties. The addition of 25% reclaimed rubber to filled NR caused a decrease in tensile strength, elongation at break, resilience, tear strength and abrasion resistance. The compression set of the raw rubber was not much affected, but an increase in heat built up and hardness was observed. A compound containing this reclaim showed better processing characteristics.

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The utilisation of cross-linked waste natural rubber as a potential filler in epoxidised natural rubber (ENR) is reported by Mathew et al [127]. The crosslinked waste rubber has been powdered and sieved into different particle sizes. They found that, as the filler content increases, the curing characteristics like optimum cure time, scorch time and induction time decrease. The cure-activating nature of the filler is clear from the increase in cure rate index and rate constant values. The filler helps the compounder by reducing the sticky nature of epoxidised natural rubber compound during mixing. These observations are advantageous as far as processability and productivity are concerned. In the case of the conventional vulcanization system, where sulphur migration is absent, finer filler shows superior tensile performance than size 4 and the mill-sheeted form of the filler. However, in efficient vulcanization systems, where sulphur migration plays a role, the order of performance is inverted. Anderson [128] patented the reclaiming of sulfur vulcanized latex in the presence of oil, water vapor and aryl disulfide peptizer at elevated temperature in the range of about 175â&#x20AC;&#x201C;195oC and at a pressure in the range of about 230â&#x20AC;&#x201C;260 psi for 1â&#x20AC;&#x201C;4 h. Here aryl disulfide is a mixture of diphenyl disulfide, dicresyl disulfide and dixylyl disulfide. In another study diphenyldisulfide is found to be effective for the reclamation of WLR in a thermomechanical process [129]. A stronger reduction of the crosslink density was observed at temperatures of 170 and 180oC when reclaimed with diphenyldisulfide compared to hexadecylamine. In the present study all polyand disulfidic cross links were broken during reclaiming with disulfide at the temperatures mentioned above, indicating that after reclamation the cross links present in WLR are mainly the monosulfides. Main-chain scission to crosslink scission studies showed that reclamation has mainly occurred through the scission of cross links rather than by main-chain scission. The crosslink distribution studies showed that some of the polysulfidic cross links remain in the sample after the treatment with hexadecylamine whereas no polysulfides were found in the samples treated with diphenyldisulfide. This again proves the fact that diphenyldisulfide more effectively breaks the polysulfidic cross links in latex vulcanizate compared to hexadecylamine. Hexadecylamine is found to be ineffective as reclaiming agent, because it results in the formation of additional cross links rather than reclaiming.

9. Applications of recycled/reclaimed rubbers 9.1. Uses of cryogenically ground rubber Cryogenically ground rubber is used in tires, hoses, belts and mechanical goods, wire and cable and in various other applications. It is especially useful

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in producing a product for tire inner liners. The particle size chosen is controlled by cost and fineness needed to produce the desired processing. The finer the particle size, the smoother the calendared sheets and the finer an edge that can be produced on extrusions.

9.1.1. The TAK system The TAK System originated after its inventor. This process was developed by Takallou in 1986 [130]. In this process rubber modified asphalt concrete paving mixture is prepared by adding 3% by weight of fine and coarse rubber particles to a dense graded aggregate mixture. The rubber granulate is prepared from whole tire recycling. Perhaps one of the greatest potential markets for scrap tire generated rubber is as an additive to asphalt pavement. Ground scrap tire rubber can be added to hot mix asphalt in a variety of manners. The use of ground rubber in asphalt pavement is not meant as a means of disposal. Rather, the addition of scrap tire rubber improves certain characteristics of asphalt. In general, scrap tire rubber provides added flexibility, reduces cracking, enhances aging properties, aids in reducing ice formation, facilitates water removal from pavement and reduces road noise. Scrap tire rubber has been used in crack/joint sealers, surface/interlayer membranes, and as an aggregate substitution in hot mix binder. While the positive aspects of rubber-modified asphalt are numerous, it must be recognized that this technology has not been universally accepted by the paving industry. The average net yield of rubber from a used passenger car tire is about 12 lb (after steel and fabric removal). Hence five tires are required to obtain 60 lb of granulated tire rubber which is necessary for production of one ton of rubber modified asphalt concrete mix. Therefore, rubber obtained from 16,000 tires is consumed per mile in a two lane highway with 3 in. of rubber modified asphalt concrete pavement.

9.1.2. Ground rubber in civil engineering applications The civil engineering market encompasses a wide range of uses for scrap tires. In almost all applications, scrap tire material replaces some other material currently used in construction such as lightweight fill materials like expanded shale or polystyrene insulation blocks, drainage aggregate, or even soil or clean fill. A considerable amount of tire shreds for civil engineering applications come from stockpile abatement projects. Tires that are reclaimed from stockpiles are usually dirtier than other sources of scrap tires and are typically rough shredded. Rough tire shreds can be used as embankment fill and in landfill projects.

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Figure 12. Road embankment constructed with shredded tires in El Paso, Texas.

Tire shreds can be used to construct embankments on weak, compressible foundation soils. Tire shreds are viable in this application due to their light weight. For most projects, using tire shreds as a lightweight fill material is significantly cheaper than alternatives. Other uses of tire shreds: subgrade fill and embankments (Fig. 12) include retaining forest roads, protecting coastal roads from erosion, enhancing the stability of steep slopes along highways, and reinforcing shoulder areas. Tire shreds are cost-effective substitutes for traditional materials when they are used to stabilize weak soil, such as constructing road embankments or as a subgrade fill. Additionally, tire shreds provide effective subgrade insulation for roads, walls and bridge abutments. Since its beginnings in the early 1990s, the use of scrap tires in civil engineering applications has had a roller coaster-like existence. Recently, new information has become available --information that should answer many of the doubts, concerns and uncertainties that previously limited the expansion of this market. This market for scrap tires now is poised to provide a myriad of possible uses that can consumer large quantities of scrap tires in a positive manner. Recycled scrap tires play a meaningful role in civil engineering processes, consuming 16 percent of the scrap tire available in 2005. A "civil engineering application" is the use of scrap tires in place of some conventional construction material such as clean fill, aggregate and rock. Scrap tires have been used as lightweight road embankment fill, backfill behind walls, insulation to limit frost penetration beneath roads (Fig. 13), aggregate in leachate and gas collection systems in landfills, and the drainage

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Figure 13. Shredded scrap tires used as road base in Odessa, Texas.

bed for residential septic system. Scrap tires usually are shredded for use in these applications, with the actual size a function of the intended use. The required size can range from a refined two-inch or three-inch square shred to a coarser, three-by twelve-inch shred. In northern climates, excess water is released when subgrade soils thaw in the spring. Placing a 6 to 12-inch thick tire shred layer under the road can prevent the subgrade soils from freezing in the first place. In addition, the high permeability of tire shreds allows water to drain from beneath the roads, preventing damage to road surfaces. Landfill construction and operation is a growing market application for tire shreds. Scrap tire shreds can replace other construction materials that would have to be purchased. Scrap tires may be used as a lightweight backfill in gas venting systems, in leachate collection systems, and in operational liners. They may also be used in landfill capping and closures, and as a material for daily cover. Some states-Alabama, Florida, Georgia, South Carolina, and Virginia-allow tire shreds to be used in construction of drain fields for septic systems. Tire-derived material replaces traditional stone backfill material, but reduces the expense and labor to build the drain fields. Tire chips can also hold more water than stone and can be transported more easily due to their light weight. In a related application, a study was conducted in Indianapolis, Indiana to evaluate the use of scrap tire shreds as replacement for stone in septic systems. The septic system consisted of two in ground trenches, three inches wide and 25 feet long, with shredded scrap tires placed six inches below and two inches above the gravity distribution pipe. One trench contained one inch nominal chips and other trench two inch chips. This system was loaded alternately each month from a three bedroom home. In alternate months the effluent was directed in a standard stone trench 150 feet in total length. The

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system was installed in 1987, and since July 1989 water samples from the stone system and the tire system have been taken each month. Even though the application is one-third smaller in the tire system, results indicate that there does not appear to be significant differences between samples taken from the scrap tire and stone systems. Whole scrap tires can be used in the construction of artificial reefs and breakwaters. Artificial reefs are designed to prevent scouring, project coastal roads and provide habitat to aquatic life, such as filter feeders. Tire reefs are constructed by bundling punctured tires that have been weighed down with concrete and anchoring them to the ocean floor. The largest know scrap tire reef manufactured from approximately 12 million scrap tires is located in Florida. Other states with reef programs include California, Maryland, New Jersey, New York, Virginia and Washington. Breakwaters are used to reduce shoreline erosion. Scrap tire breakwaters are made by tying together tires with rubber strips and nylon bolts. Georgia and New Jersey both have scrap tire breakwaters and report no significant technical difficulties. Floating breakwater designs utilizing scrap tires date back to 1969 with the development the â&#x20AC;&#x153;Wave Mazeâ&#x20AC;?. Major advances took place in 1972 when the Goodyear Tire and Rubber Company refined the design concept. This modular concept has since been the most practical and most utilized design for floating breakwaters. In 1981, the Lorain Port Authority selected the Goodyaer design floating tire breakwater to control wave activity and expand recreational boating opportunities in the east harbor basin of the Port of Lorain (Ohio). Scrap tire material can be used in other various civil enginnering applications including roadway crash barriers and railway crossings. Three states, Alaska, Florida and Texas have reported using scrap tires in such applications without significant technical difficulties. Challenges to using tire shreds in drain fields include tire chip quality (tire chips must be clean cut and be of uniform size) and economics-in some areas, stone is abundant and cheap; tire shreds must be cheaper than stone to be used readily.In defining the various uses for scrap tires, civil engineering applications generally are considered to be those that require a shred no smaller than two inches in any dimension. Applications for smaller-sized, scrap tire derived material, such as the use of particulate rubber as a soil amendment, as a turf top dressing or as an additive to asphalt paving materials, are classified as ground rubber markets. The defining characteristic of a civil engineering application is that the tire-derived material produces a cost effective engineering benefit. In other words, civil engineering applications should not be used just to bury tires.

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The use of scrap tires in civil engineering applications is based upon the unique characteristics of tire shreds, namely lightweight, good insulation properties, very high ability to transmit water, good long-term durability and high compressibility. With these properties, engineers can use tire shreds to solve many of the construction problems that cause them to lie awake at night, and just as important, tire shreds can save their clients money. Not to mention, applications also can consume a very large quantity of scrap tires. The first use of scrap tires in civil engineering applications can be traced back to the mid-1970, when they were used to build breakwaters and artificial reefs. Few scrap tires were used in civil engineering applications between the mid-1970's and 1991, perhaps two to three hundred thousand. (In comparison, more than 240 million scrap tires were generated annually in the U.S.). By 1992, tire shreds were being used in road embankments and being tested as a lightweight backfill for walls. These uses were offered as alternatives to the federal mandate for the use of rubber-modified asphalt, but neither was readily accepted by the highway construction community. The other uses of scrap tire include, • • • • • • •

Playground surface material Gravel substitute Drainage around building foundations and building foundation insulation Erosion control/rainwater runoff barriers (whole tires) Wetlands/marsh establishment (whole tires) Crash barriers around race tracks (whole tires) Boat bumpers at marinas (whole tires)

The number of scrap tires going into civil engineering applications had increased to almost 10 million a year by the end of 1995. Estimates at that time about this market segment were that civil engineering applications would be consuming upwards of 15 million to 20 million scrap tires by 1997. Then came the burning roads. In December 1995 and January 1996, reports about hot spots in two road bed embankments built with scrap tire fill in Washington state and one wall with scrap tire backfill in Colorado were announced to the world. At first, it was steam emanating from vents in the embankments. Soon, there were reports of glowing embers within the embankments. Finally, flames were shooting out of the fills. This news spread; pardon the expression, like wildfire. The impact of these events was dramatic, immediate and profound. In quick order, the majority of civil engineering applications for scrap tires came to an abrupt halt. In 1996, the only major use of scrap tires in civil engineering applications was in landfill construction and operation. The market shrunk to five million tires.

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9.1.3. The wet process This process (Arizona Refinery System) [134] was developed to overcome the problem of fatigue cracking in resurfaced asphalt pavements. The idea is based on using a composite material of hot asphalt cement with 1% by weight of total mixture of ground crumb rubber and diluted with an oil extender for ease of application. At elevated temperatures (300â&#x20AC;&#x201C;4008F) for periods of one-half hour to one hour this reaction forms a thick elastic type material which is then diluted with 5% kerosene to aid in application. At room temperature this asphalt rubber composition is a tough rubbery and elastic binder material. The elastic quality of this mixture is most probably maintained by undissolved rubber particles that serve as units of elastic interference to the propagation of cracking. When a crack begins to propagate through the membrane then it encounters with elastic rubber particle and is stopped or its path of propagation is changed where it will encounter with another elastic rubber particle and so on. Thus propagation of crack is not possible.

10. Advantages of using reclaimed rubber Although reclaim rubber is a product of discarded rubber articles it has gained much importance as additive in various rubber article formulations. It is true that mechanical properties like tensile strength, modulus, resilience, tear resistances etc. are all reduced with the increasing amounts of reclaim rubber in fresh rubber formulation. But at the same time the reclaim rubber provides many advantages if incorporated in fresh rubber.

10.1. Easy breakdown and mixing time During reclaiming process reclaimed rubber has already been plasticized due to a large amount of mechanical working, Therefore, in the consumerâ&#x20AC;&#x2122;s hands it mixes easily than new rubber at lower mixing time with less heat generation. This is particularly advantageous with compounds containing high carbon black loading. In the mixing of tire carcas and side wall stocks also this property is very advantageous because during first banbury pass reclaim rubber is not added rather added during second banbury pass along with the curing agents to a position of the master batch obtained from the first banbury pass. The second pass is much shorter than the first, therefore, an increase in mixing capacity of as much as 40% occurs with a 30% banbury cost saving per pound of rubber. With increase in the ratio of reclaimed RHC to new RHC, the mixing cycle decreases. Furthermore, an all reclaim stock mixes in just one half the time required for an all new rubber stock.

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10.2. Low power consumption during breakdown and mixing Reclaimed rubber consumes less power during breakdown and mixing than new rubber. Rubber Reclaimers Association has done a series of experiments to study the power saving during mixing with reclaim rubber. The first series compared whole tire reclaimed rubber with natural rubber and SBR 1712. Each was mixed with black, filler and oil in proportions to stimulate the composition of the reclaim. Banbury time was kept constant. The savings in power cost per 1000 pounds of reclaim were: 20% vs Natural Rubber 34% vs SBR 1712. The second series show that a mixture of SBR 1712 and BR (without any additives) plus a small proportion of reclaim rubber shows 12% less power consumption than by SBR 1712 alone and 14% less power consumption than for the combination of SBR 1712 1 BR, the mixing time being constant in all the cases. The third series shows that SBR 1712 alone, and SBR 1712 plus increasing proportions of tire reclaimed rubber up to 50% on RHC basis, result in increasing power savings for a constant mixing time.

10.3. Advantages in calendering and extrusion Reclaimed rubber stocks can usually be processed at a lower temperature than those containing virgin rubber alone. It provides generally faster processing during extruding and calendering. Due to the presence of crosslinked gel in reclaimed rubber, it is less thermoplastic than new rubber compounds. Thus when extruded and cured in open steam they tend to hold their shape better. Extruder die swell and calender shrinkage reduce with a proper use of reclaim rubber due to its lower nerve. Fresh rubber calendered sheets show 6â&#x20AC;&#x201C;10% shrinkage. Using of reclaim rubber in tire carcass stocks permits high speed calendering and results in smooth uniform coating. The use of substantial proportion of reclaim rubber in automobile floor mat stocks permits maximum calender speeds which is sometimes twice as large as when very high proportions of SBR are used. Reclaim rubber in tire carcass compound gives better penetration in the fabric and chord than a non-reclaim compound.

10.4. Influence on curing and aging Reclaim rubber containing compounds help to retard and reduce sulfur bloom from both uncured and cured stocks. It cures faster than virgin rubber

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compound, probably due to its combined sulfur and active crosslinking sites. Energy savings thus obtained constitute its usefulness in commercial purpose. During vulcanization reclaim rubber containing stocks show less tendency to revert indicating better aging resistance. Ball and Randall [131], Adhikari et al. [40, 106] and Dierkes [132] observed antiaging characteristics of reclaim rubber. Adhikari et al. observed around 90% retention of tensile properties of NR, SBR and NR-PBR reclaim rubber without using any antioxidant. As per Ball and Randall such aging resistance of reclaim rubber is due to the severe treatment of oxidation, heating, digestion and mechanical shearing which appear to stabilize the hydrocarbon against further changes.

10.5. Influence on tack behaviour The tack of a non-reclaim compound may disappear within 24 h after calendaring whereas, reclaim rubber compound tend to maintain their tack longer than non reclaim compound. Non-reclaim compounds become more tacky in hot weather and dry in cold weather. On the other hand, reclaim rubber compounds are less influenced in tack variation in hot and cold weather. This characteristic of reclaim rubber is exploited for its usefulness in pressure sensitive tape.

10.6. Cost and energy savings Finally, it may be stated that incorporation of reclaim rubber into new rubber compound, not only reduces the cost of the finished product but also saves our united resource of fossil feed stock. Energy consumption in reclaim production from truck treads is 0.09 l of oil equivalent/kg and 0.12 l equivalent/kg from whole tire. These data show that negligible amount of energy in terms of oil equivalent is consumed for reclaim production. Energy consumption in the tire production is 25 l of oil equivalent/tire. But much less energy is consumed in the production and utilization of recycled rubber products than direct production of rubber articles from the virgin raw materials.

Conclusions remarks The increase in the awareness of waste management and environmentrelated issues has led to substantial progress in the utilization of rubber waste. Recycling materials back into its initial use often are more sustainable rather than finding new applications. This paper has presented various aspects of the reclamation and the reuse of different kinds of rubber products. It is expected that the newly developed rubber recycling technologies described in this

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chapter will contribute to both protecting the environment and saving resources with regards to rubber waste generated through out the world.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27.

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122. Loffler, M., Straube, G., and Straube, E. 1993, Biohydrometall Technol Proc Int Biohydrometall Symp 2:673. 123. Tsuchii, A., and Takeda, K. 1990, Appl. Environ. Microbiol., 56, 269–274. 124. Holst, O., Stenberg, B., and Christiansson, M. 1998, Biodegradation., 9, 301–310. 125. Kojima, M., Ogawa, K., Mizoshima, H., Tosaka, M., Kohjiya, S., and Ikeda, Y. 2003, Rubber. Chem. Technol., 76, 957–68. 126. Thomas, K.T., Claramma, N.M., Kuriakose, B., and Thomas, E.V. 1986, Recycling of natural rubber latex waste. In: Paper presented at the International conference on rubber and rubber-like materials, Jamshedpur, India, November. 127. Mathew, G., Singh, R.P., Nair, N.R., and Thomas, S. 2001, Polymer., 42, 2137–65. 128. Anderson, E. Jr. 1985, US Patent 4, 544, 675. 129. Rajan, V.V., Dierkes, W.K., Joseph, R., and Noordermeer, J.W.M. 2005, Rubber Chem Technol., 78, 855–67. 130. Rajalingam, P., and Baker, W.E. 1992, Rubber. Chem. Technol., 65, 908. 131. Ball, J.M., and Randall, R.L. 1949, Rubber Age., 64, 718. 132. Dierkes, W. 1996, Rubber World., 214, 25.

Transworld Research Network 37/661 (2), Fort P.O. Trivandrum-695 023 Kerala, India

Recent Developments in Polymer Recycling, 2011: 101-120 ISBN: 978-81-7895-524-7 Editors: A. Fainleib and O.Grigoryeva

3. Structure, properties and recyclability of natural fibre reinforced polymer composites 1

Deepa B1, Laly A. Pothan1, Rubie Mavelil-Sam1 and Sabu Thomas2 Post Graduate Department of Chemistry, Bishop Moore College, Mavelikara 690110, Kerala, India Department of Polymer Science and Engineering, Mahatma Gandhi University, Kottayam 686560 Kerala, India

Abstract. Natural fibre reinforced polymer composites have emerged as a potential environmentally friendly and cost-effective option to synthetic fibre reinforced composites. In recent years, there has been growing interest in the field of cellulose reinforced thermoplastic and thermoset composites having properties of toughness, resistance to chemical attack and recyclability. Unlike thermoset composites, natural fibres are less frequently used in common thermoplastics such as polyethylene, poly (vinyl chloride) and polystyrene because of difficulties associated with surface interactions between hydrophilic fiber and hydrophobic thermoplastic. Such divergent behavior results in difficulties in compounding these materials and poor mechanical properties. Reports are made on the modification of the polar natural fibre surface by grafting with compatible thermoplastic segments or coating with compatibilizing and coupling agents before the compounding step, addition of compatibilizing and coupling agents in the compounding step, and the modification of the matrix polymer with a polar group. Studies are made on the recyclability and reprocessing of agro based fiber composites and the effect of mechanical and thermal degradation parameters during the recycling processes. Correspondence/Reprint request: Dr. Laly A. Pothan, Post Graduate Department of Chemistry, Bishop Moore College, Mavelikara 690110, Kerala, India. E-mail: lapothan@gmail.com

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1. Introduction Nowadays, the growing environmental awareness throughout the world has triggered a paradigm shift towards designing environmental-friendly materials. Consequently, in recent years, natural fibres have attracted more and more interest as reinforcements for both thermoplastic and thermosetting polymer composites [1-4]. Natural fibres from renewable natural resources offer the potential to act as a biodegradable reinforcing materials alternative for the use of glass or carbon fibres and inorganic fillers. The specific properties of these natural fibres, namely low cost, lightweight, renewable character, carbon dioxide sequesterization, biodegradability, high specific strength and modulus, availability in a variety of forms throughout the world, surface reactivity and the possibility to generate energy, absence of associated health hazards, easy fibre surface modification, and relative non-abrasiveness motivate their association with organic polymers to elaborate composite materials [5, 6]. Biocomposites derived from natural fibres and traditional thermoplastics or thermosets can maintain a balance between economy and environment allowing them to be considered for applications in the fields of automotives, aerospace, defence, marine, sporting goods, building, furniture and packaging industries. These materials provide high durability, design flexibility and lightweight which make them attractive materials in these applications [7, 8]. The versatility, strength and non-corrosive properties of plastics in combination with fibres have helped to establish this class of composites as a potential and viable alternative in several applications. In this chapter we discuss mainly about the structure, properties and recyclability of commonly used natural fibre reinforced polymer composites.

1.1. Natural fibre composites Naturalfibres, used to fill and reinforce both thermoplastics and thermosets, represent one of the fastest-growing types of polymer additives. Fibre reinforced composites (FRC) contain reinforcements having lengths much higher than their cross-sectional dimensions. Fibres are the load-carrying members, while the surrounding matrix keeps them in the desired location and orientation. Natural fibres themselves are cellulose fibre reinforced materials as they consist of microfibrils in an amorphous matrix of lignin and hemicelluloses. These fibres enjoy the right potential for utilisation in composites due to their adequate tensile strength and good specific modulus, thus ensuring a value-added application avenue. Lignocellulosic natural fibres, originated from different plant fibres are suitable raw materials for the production of a wide range of composites for

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different applications. When these fibres are incorporated into a matrix to form a composite, the matrix serves to bind the fibres together, transfer loads to the fibres, and protect them against environmental damage caused by elevated temperature and humidity. The matrix has a strong influence on several mechanical properties of the composite such as transverse modulus and strength, shear properties and properties in compression. The most common matrix materials for composites are thermoset and thermoplastic polymers. The combination of a plastic matrix and reinforcing fibres give rise to composites having the best properties of each component. Since the plastics are soft, flexible, and lightweight as compared to fibres, their combination provides a high strength-to-weight ratio for the resulting composite. The significant weight savings and the ease and low cost of the raw constituent materials make these composites an attractive alternative material to glass and carbon fibres [9]. Material scientists all over the world focus their attention on natural composites reinforced with jute, sisal, banana, coir, pineapple etc., primarily to cut down on the cost of raw materials.

1.1.1. Thermoplastic and thermoset composites Natural fibre reinforced thermoplastic and thermoset composites constitute an important class of materials with wide variety of applications. Thermoplastic composites are composites that use a thermoplastic polymer as a matrix. A thermoplastic polymer is a long chain polymer that can be either amorphous in structure or semi-crystalline. These polymers are long chain, medium to high molecular weight materials, whose general properties are those of toughness, resistance to chemical attack and recyclability. Simple methods such as extrusion and injection moulding are used for the processing of these composites. An advantage of thermoplastics is that the moulding can be carried out non-isothermally, i.e., they can be rapidly heated and rapidly cooled without any damaging effects to their microstructure. However, polymerized thermoplastics tend to have melt viscosities between 500 and 1000 times that of thermosets, which necessitates higher pressures, causes processing difficulties and adds expenses. In thermoplastics, most of the work reported so far deals with polymers such as polyethylene, polypropylene, polystyrene, and poly (vinyl chloride). The natural fibres used to reinforce thermoplastics mainly include wood, cotton, flax, hemp, jute, sisal, banana, pineapple, rice straw and sugarcane fibres [10]. Thermoset polymers are also used as a matrix material for most structural composite materials. The single biggest advantage of thermoset polymers is that they have a very low viscosity and can thus be introduced into fibres at low pressures. Thermosets are processed by simple processing techniques

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such as hand lay-up and spraying, compression, transfer, resin transfer, injection, compression injection, and pressure bag moulding operations. The use of a few other methods, such as cold press moulding, filament winding, pultrusion, reinforced reaction injection moulding, and vacuum forming, is hardly reported in the case of composites [5]. In these polymers, the fibres are used as unidirectional tapes or mats. These are impregnated with the thermosetting resins and then exposed to high temperature for curing to take place. These composite materials are chemically cured to a highly crosslinked, three-dimensional network structure. These cross-linked structures are highly solvent resistant, tough and creep resistant. The major types of thermosetting materials are epoxy resins and unsaturated polyesters (UP); phenolic resins (including phenol-formaldehyde ones); amino resins (e.g. melamine-formaldehyde and urea-formaldehyde ones), and polyurethane.

1.1.2. Structure and properties of composites A composite may be defined as any substance which is made by physically combining two or more existing materials, selected filler or reinforcing agent and a compatible matrix binder, to produce a multiphase system with different properties from that of the starting materials but in which the constituents retain their identity. The components of a composite do not dissolve or otherwise merge completely with each other, but nevertheless do act in concert. The surface adhesion between the fibre and the polymer plays an important role in the transmission of stress from matrix to the fibre and thus contributes towards the performance of the composite. The properties of the composite cannot be achieved by any of the components acting alone. Overall, the properties of the composite are determined by: i) iii) iv) v)

the properties of the fibre ii) the properties of the resin the ratio of fibre to resin in the composite (Fibre Volume Fraction) the geometry and orientation of the fibres in the composite, and the surface interaction of fibre and resin (the â&#x20AC;&#x2DC;interfaceâ&#x20AC;&#x2122;)

The properties of the fibre

Properties of natural fibres result from their structure and chemical composition. The essential component of all plant fibres is cellulose. Plant fibres are characterized by their cellular structures. Each cell contains crystalline (i.e. ordered) cellulose regions (microfibrils) which are interconnected via lignin and hemicellulose fragments. A cell has one external wall and three thick side walls. The more parallel the microfibrils are arranged to the fibre axis, the higher is the fibre strength.

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Most of the natural fibres have porous structure (seen in cross-section) which can be a factor facilitating their saturation with resins. All of the different fibres used in composites have different properties and so affect the properties of the composite in different ways. The mechanical properties of natural fibres, especially flax, hemp, jute and sisal, are very good and may successfully compete with glass fibre in specific strength and modulus. Natural fibres show higher elongation to break than glass or carbon fibres, which may enhance composite performance. Thermal conductivity of natural fibres is low (0.29– 0.32 W/mK), which thereby makes a good thermal barrier. ii) The properties of the resin Any resin system for use in a composite material will require the following properties: − good mechanical properties − good adhesive properties − good toughness properties − good resistance to environmental degradation iii) The amount of fibre in the composite (‘Fibre Volume Fraction’) The amount of fibre in the composite is largely governed by the manufacturing process used. However, reinforcing fabrics with closely packed fibres will give higher Fibre Volume Fractions (FVF) in a laminate than will those fabrics which are made with coarser fibres, or which have large gaps between the fibre bundles. Fibre diameter is an important factor here with the more expensive smaller diameter fibres providing higher fibre surface areas, spreading the fibre/matrix interfacial loads. As a general rule, the stiffness and strength of a laminate will increase in proportion to the amount of fibre present. However, above about 60-70 % FVF (depending on the way in which the fibres pack together), although the tensile stiffness may continue to increase, the laminate’s strength will reach a peak and then begin to decrease due to the lack of sufficient resin to hold the fibres together properly. iv) The geometry and orientation of the fibres in the composite The geometry of the fibres in a composite is also important since fibres have their highest mechanical properties along their lengths, rather than across their widths. This leads to the highly anisotropic properties of composites, where, unlike metals, the mechanical properties of the composite are likely to be very different when tested in different directions. This means

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that it is very important when considering the use of composites to understand at the design stage, both the magnitude and the direction of the applied loads. When correctly accounted for, these anisotropic properties can be very advantageous since it is only necessary to put material where loads will be applied, and thus redundant material is avoided. v) The surface interaction of fibre and resin (the ‘interface’) In polymer matrix composites, the interface between the reinforcing phase and the bulk phase is paramount to the overall performance of the composite as a structural material. The quality of the fibre/matrix interface is important in the application of natural fibres as reinforcement in plastics. The interface is called the ‘heart of the composite’ and the composite properties depend on the interface. It is not a distinct phase, as the interface does not have a clear boundary. It is more accurately viewed as a transition region that possesses neither the properties of the fibre nor that of the matrix. The interface has been reported to have distinct properties by researchers [11]. The schematic representation of the composite interface is shown in Figure 1. The ultimate mechanical properties of fibre reinforced polymeric composites depend not only on the properties of the fibres and the matrix, but also on the degree of interfacial adhesion between the fibre and the polymer matrix [12].

Figure 1. Schematic representation of the interface.

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The major drawbacks associated with the use of natural fibres as reinforcements in thermoplastic matrix to achieve composite material with improved mechanical properties and dimensional stability are the poor wettability and weak interfacial bonding with the polymer [13, 14]. The polar natural fibre and the non-polar polymer matrix end up in relatively weak bonding. The deficient adhesion leads to a weak load transfer from the matrix to the fibres, which induces a low reinforcing effect. Poor interfacial bonding leads to composites exhibiting rather poor durability and toughness. Another problem is that the processing temperature of composites is restricted to 200 oC as vegetable fibres undergo degradation at higher temperatures; this restricts the choice of matrix materials. Another setback is the high moisture absorption of natural fibres leading to swelling and presence of voids at the interface, which results in poor mechanical properties and reduced dimensional stability of composites. So, in order to reduce the hydrophilic character of the cellulose fibres and to improve their adhesion properties, a pre- treatment of the fibre surface or the incorporation of a surface modifier during processing is required [13, 15]. Several methods used to modify natural fibre surfaces, such as alkali treatment [16, 17], the use of maleic anhydride copolymers [18, 19], pre-impregnation of the fibre [20], and the use of silanes [21-25], have been proposed.

2. Recyclability of natural fibre composites Current environmental concerns have stimulated interest in the recycling of materials, and plastics in particular. There are two concurrent reasons justifying this: the desire not to waste resources, and the problem of disposing scrap material that may otherwise pollute the environment. Composites, and particularly those based on thermosetting polymers, are often perceived as difficult or impossible to recycle, and for that matter, there have been suggestions that their use should be avoided. Yet, their attractive properties make them likely candidates for an ever increasing use in volume manufacturing, particularly in the automotive industry. Scientists working in the field of green materials technology, as well as the policy makers are optimistic about the use and usability of natural fibres to develop greener solutions for the production, consumption and disposal of automotive products. Scientists researching in this field have argued that one way to achieve higher recyclability in an environmentally sound manner would be to substitute commonly used synthetic reinforcing materials in polymer composites, such as glass fibres, by natural fibres. The economic aspects and environmental impact of their use cannot be overestimated. As an example of bio-based materials, natural fibres are

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lighter than conventional fibre materials and therefore can contribute to cost reduction; their production also being more cost effective. It was found that natural fibre reinforced composites can deliver weight savings of about 50 %, and reduce costs by approximately 30 %. The widely perceived consumer demands are also part of the commercial logic to invest in green materials research and production. The use of natural fibres is often seen as the panacea for various environmental problems, such as end-of-life (ELV) vehicles, waste minimization, as well as for projects of economic development [26]. Sustainable bio-based eco-products are products with environmental acceptability which are derived from renewable resources with recycling capabilities and triggered biodegradability. Green polymers, also known as bio-polymers, are derived from natural/agricultural renewable resources. Examples for green polymers are thermoplastic starch, polyhydroxy alkanoates (PHA), polylactic acid (PLA), lignin-based epoxy and soy-based resins as well as epoxidised linseed oil. In search for viable ‘greener’ solutions for more environmental-friendly consumer goods, science and industry turn to nature and in particular to natural fibres [26]. Many investigations have been made on the potential use of natural fibres as reinforcements for so-called ecocomposites, and in several cases the results have shown that they exhibit good stiffness and promising properties. Renewable resources and recyclable thermoplastic polymers provide an attractive eco-friendly quality as well as environmental sustainability to the resulting natural fibre-reinforced composites [27]. Natural fibres such as kenaf, hemp, flax, jute, and sisal offer such benefits as reductions in weight, cost, and CO2, lower reliance on foreign oil sources, recyclability, and the added benefit that these fibre sources are “green” or ecofriendly [28]. Growing environmental awareness and new rules and regulations are forcing industries to seek more ecologically friendly materials for their products. In recent years, the development of biocomposites from biodegradable polymers and natural fibres has attracted great interest, because these materials could allow complete degradation in soil or by composting processes and do not emit any toxic or noxious components [27]. Commonly, natural fibre reinforced petrol-based polymers are called green composites. The use of green/synthetic polymers, reinforced by either synthetic or natural fibres respectively, will limit the environmental friendliness of the resulting composite because of the low biodegradability and problems related to the material recycling. By incorporating natural fibres into green polymer matrices, new ‘‘truly green’’ biodegradable ecocomposites (also called bio-composites) are created. This class of materials is currently under development and is heavily researched.

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The main advantage of â&#x20AC;&#x153;truly greenâ&#x20AC;? composites comes with their disposal â&#x20AC;&#x201C; these materials are compostable and will provide valuable soil amendment products for a sustainable agriculture. Even when incinerated, truly green composites are said to have no impact on global warming, because the carbon dioxide (CO2) set free during thermal incineration equals the CO2 consumed by the crop before harvesting [28]. Glass-reinforced plastics exhibit shortcomings such as their relatively high fibre density (approximately 40 % higher than natural fibres), difficulty to machine, and poor recycling properties; not to mention the potential health hazards posed by glass-fibre particulate [29].

2.1. Recycling of natural fibre reinforced thermoplastic composites Problems associated with recycling of thermoplastics and their composites are much less compared with the recycling of the thermosets. So the widespread use of thermoplastic composites in various industries over the last decades is expected to have a more favourable environmental impact. Recycled thermoplastic composites, however, show a degradation of their mechanical performance, the extent of which depends both on the recycling process as well as on the service conditions history. Furthermore, the possibility for recycling offers sound economical benefits because of the high price of the virgin material [30]. Amongst eco-compatible polymer composites, special attention has been given to polypropylene (PP) composites, due to their added advantage of recyclability. PP cannot be classified as a biodegradable polymer, but it takes an important place in eco-composite materials. For example, Mohanty et al. have demonstrated that NF-reinforced PP composites have the potential to replace glass/PP composites [31]. Significant research efforts have been also spent on eco-composites based on recyclable polymers reinforced with natural fibres. It is found that the recycling processes do not induce very significant changes in flexural strength and thermal stability of the composites. In particular, polypropylene-based composites reinforced with kenaf fibres are less sensitive to reprocessing cycles with respect to PP-based composites reinforced with rice hulls. The response of PP-based composites reinforced with rice hulls or kenaf fibres is promising since their properties remain almost unchanged after recycling processes. Moreover, the recycled composites are suitable for applications as construction materials for indoor applications [27]. The effect of the recycling processes on the property retention of the materials is illustrated in Figure 2.

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Figure 2. Influence of recycling processes on flexural strength and flexural modulus of (a) PP/RH composites and (b) PP/K composites.

From the above figure, it is very clear that the composites reinforced with kenaf show higher modulus and stress at maximum load with respect to the composites reinforced with rice hulls. The flexural strength of PP/RH recycled composites decreases by about 10 % after recycling, although the flexural modulus is practically unchanged. For PP/K composites, the recycling processes induces a slight decrease in the flexural strength after the second recycling (about 5 %) and an increase of the flexural modulus (about 20 %) [27]. Evstatiev et al. have done studies on exploring the potential of a new type of polymer-polymer composites microfibrillar reinforced composites (MFC) for recycling purposes, in order to obtain material with engineering specifications [32-36]. While natural fibres have been used traditionally to fill and reinforce thermosets, natural fibre reinforced thermoplastics, especially polypropylene composites, have attracted greater attention due to their added advantage of recyclability [37]. A few studies have looked at comparative life cycle assessment of specific components made from glass fibre reinforced (GFR) composite materials and natural fibre reinforced (NFR) composite materials

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[38-41]. The study also reports results from sensitivity analyses with respect to recycling at various percentages, pallet life, plastic content, and changes in transport distances, and finds that NFR pallet is environmentally superior under almost all scenarios. However, the environmental impacts of NFR pallet are worse, if its expected life falls below 3 years, compared to that of 5 years for the GFR pallet. Recycling involves mechanical and thermal degradation of both the matrix and the reinforcement. Recycling of biocompostable materials limits their environmental impact, while keeping possible waste management by composting. Besides biodegradation, composite recycling could make these materials more interesting and extend their useful life, reducing the global impact on the environment by minimizing raw material consumption and storing carbon for a longer period. For the matrix PLLA, Pillin et al. studied the thermo-mechanical effects of recycling on the mechanical properties, noting a reduction of stress and strain at break, whereas the Youngâ&#x20AC;&#x2122;s modulus remained constant [42]. Degradation of PLLA can be catalysed at transformation temperature by the presence of air and thus random chain scissions occur [43]. In composites, the mechanical properties of sisal/PP and hemp/PP have been investigated as a function of recycling by Bourmaud et al. [44]. Both tensile modulus and strength were shown to be quite stable after up to seven injection cycles, but the initial value was quite low due to the relatively poor mechanical properties of PP. Many authors have noted a large reduction in fibre length during multiple injection cycles [45-47]. The reduction of dimensions is attributed to shear stresses developed in the injection equipment. Thompson et al. have studied the rheological behaviour of nanofiller/elastomer composites during recycling [48]. Reduction of viscosity during injection cycles was caused by thermooxidative degradation, and the destruction of filler networks was highlighted. Reduction of viscosity during injection cycles can be caused by matrix degradation (chain scissions) and/or reduction of reinforcement size. Duigou et al. studied the recyclability of flax/PLLA biocomposites elaborated with the injection moulding process, and compared their behaviour with that of PP (polypropylene) composites [49]. The evolution of matrix molecular weight and reinforcement geometry during recycling were also been studied. Repeated injection cycles had shown to influence many parameters such as reinforcement geometry, mechanical properties, molecular weight of PLLA, thermal behaviour and rheological behaviour. Observation of tensile fracture surfaces in the SEM gave qualitative information about fibre dispersion and orientation. After the first injection cycle many bundles of fibres could be noted (Figure 3) and thus a lack of homogeneity compared to the fracture surface of a sample subjected to six

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njection cycles (Figure 4). This confirmed bundle division during recycling, and could explain the small change in fibre aspect ratio with injection cycles. Tensile modulus is only slightly influenced by recycling as illustrated in Figure 5. This trend was also noted during previous tests on hemp and sisal/PP composites [44]. A small reduction of PLLA modulus during recycling, shown by Pillin et al. [42] is one of the reasons for this behaviour. Another is the small decrease of fibre aspect ratio during recycling. Figures 6 &7 show that, for both fibre contents, stress and strain at break decrease. This drop may be caused by fibre damage during recycling, the reduction in fibre length resulting in more strain concentrations and a higher risk of debonding.

Figure 3. SEM micrograph of the fracture surface of BC-20 % after one injection.

Figure 4. SEM micrograph of the fracture surface of BC-20 % after six injections.

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Figure 5. Evolution of tensile modulus as a function of injection cycles.

Figure 6. Evolution of tensile strength at yield as a function of injection cycles.

Figure 7. Evolution of strain at break as a function of injection cycles.

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Although biocomposites become more brittle with recycling (Figure 7), they retain a large part of their properties, at least until the third injection cycle. In an industrial situation, 100 % of the recycled biocomposite would not be used (recycled material is always mixed with virgin material). In order to examine the matrix degradation mechanism, molecular weight of PLLA was investigated as a function of injection cycles. Figure 8 shows the evolution of molecular weight for the biocomposites at different fibre contents compared to neat PLLA. Initially, molecular weight of PLLA was 220,000 g/mol. During multiple injections, molecular weight of PLLA decreases dramatically. Fibre content plays an important role in reduction of molecular weight of polymer, since the molecular weight decreases rapidly with higher fibre content (-83 % for BC-20 % and -94 % for BC-30 %). The chemical structure of PLLA is sensitive to hydrolytic degradation and especially to high temperature [42, 50]. Other kinds of degradation processes will also occur during recycling: depolymerisation of macromolecular chains due to residual catalyst, radical and nonradical reactions, cis-elimination, trans-esterification [50, 51], and mechanical degradation due to interactions of the polymer with the equipment and high shear rate in the injection process. Complex degradation processes result in molecular weight reduction during recycling. Figures 9 &10, obtained by DSC, show the influence of injection cycles on glass transition temperature (Tg) and melting enthalpy (Î&#x201D;Hm) of the PLLA and the biocomposites BC-20 % and BC-30 %. All the data are indexed in Table 1.

Figure 8. Evolution of molecular weight as a function of injection cycles.

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Figure 9. Evolution of glass transition temperature as a function of injection cycles.

Figure 10. Evolution of melting enthalpy as a function of injection cycles.

During the first injection, the presence of fibres causes little change in glass transition temperature (Tg) (Figure 9). These results are confirmed by the work on injected, recycled newspaper fibre/ PLLA bio composites of Huda et al. [52]. A drop in PLLA molecular weight was observed as fibre content and number of injection cycles increased. Higher fibre content appeared to accelerate PLLA degradation during recycling. This drop in molecular weight contributed to the lowering of ultimate biocomposite properties. Calorimetric study showed that, depending on the fibre content, glass transition temperature decreased and degree of crystallinity increased with injection cycles. This evolution could be explained by the degradation occurred during processing which induced higher molecular mobility. Newtonian viscosity of these biocomposites also decreased as a function of injection cycles.

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Table 1. Evolution of thermal properties of PLLA, BC-20% and BC-30% as a function of injection cycles.

Figure 11. Evolution of Newtonian viscosity as a function of injection cycles.

Figure 11 presents the evolution of Newtonian viscosity as a function of the number of recycling processes for PLLA, BC-20 % and BC-30 %. During the first injection, it may be noted that for higher fibre content, the viscosity is higher. Several authors have noted that as fibre content increases chain mobility of PLLA is restricted, which may be due to the fibreâ&#x20AC;&#x201C;polymer and

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fibreâ&#x20AC;&#x201C;fibre interactions [53-56]. During recycling, the viscosities of BC-20% and BC-30% decrease (Figure 11). This viscosity drop, either for PLLA, BC-20 % or BC-30 % is very significant, indicating a high degradation rate of the PLLA matrix. Molecular weight reduction (also shown by SEC), reduction of fibre length and aspect ratio are the phenomena that explain this observation. Several authors have described the influence of the length and L/d ratio of fibres on the viscosity of composites [53, 57]. This emphasizes the degradation of the matrix (reduction of molecular weight) through chain scission mechanisms during recycling. With natural fibres as reinforcements, end-of-life composting is possible. Complementary work on degradation mechanisms and their link to environment (temperature, humidity, etc.), as well as detailed damage threshold identifications are in progress.

2.2. Recycling of natural fibre reinforced thermoset composites The polymerâ&#x20AC;&#x201C;polymer interface is a vital factor which plays a crucial role in the ability of polymer composites, especially thermoset polymers and polymer composites, to be repaired, recycled, and bonded [58]. Thermoset matrix recycling is of course unfeasible because of the thoroughly cross-linked nature and the inability to be remoulded. Nevertheless, technologies are now being developed that can reprocess scrap composites to recover some of the value in the material and thus avoid the need of disposing the solid in a landfill, which has been the only option until recently.

2.3. Recycled fibre as reinforcements Kouparttsas et al. [59] have investigated the feasibility of reusing short fibres recovered from recycled thermoset composites for the production of new composites. Glass fibres were recovered from glass-polyester composites, and carbon and aramide fibres from epoxy based composites. In most cases examined, recycling does not adversely affect the mechanical performance of the new composite. This overall behaviour is explained in terms of length preservation, fibre dispersion mechanism and fibre- matrix adhesion. There are positive conclusions drawn from their investigation on the feasibility of reusing short recycled fibres recovered from thermoset composites [59].

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29. Holbery, J., and Houston, D. 2006, Natural-fibre-reinforced polymer composites in automotive applications. JOM, November, 80. 30. Papanicolaou, G.C., Karagiannis, D., Bofilios, D.A., van Lochem, J.H., Henriksen, C., and Lund, H.H. 2008, Polym. Compos., 1026-35. 31. Mohanty, A.K., Drazl, L.T., and Misra, M. 2002, J Adhes. Sci.Technol., 16, 999-1015. 32. Evstatiev, M., and Fakirov, S. 1992, Polymer, 33, 877-80. 33. Fakirov, S., Evstatiev, M., and Schultz, J.M. 1993, Polymer, 34, 4669-79. 34. Fakirov, S., Evstatiev, M., and Petrovich, S. 1993, Macromolecules, 26, 5219-26. 35. Fakirov, S., and Evstatiev, M. 1994, Adv. Materials, 6, 395-8. 36. Evstatiev, M., Fakirov, S., and Kiedrich, K. 2000, In: Cunha, AM and Fakirov, S. editors. Microfibrillar reinforced composite another approach to polymer blends processing, structure development during polymer processing. Dordrecht-BostonLondon: Kluwer Academic Publishers; 311. 37. Geum-Hyun, D., Sun-Young, L., In-Aeh, K., and Young-To, K. 2004, Compos. Struct., 68, 103-8. 38. Corbiere-Nicollier, T., Laban, B.G., Lundquist, L., Leterrier, Y., Manson, J.A.E., and Jolliet, O. 2001, Resource Conservation Recycling, 33, 267-87. 39. Schmidt, W.P., and Beyer, H.M. 1998, Life cycle study on a natural fibre reinforced component. SAE Technical paper 982195, SAE Total Life-cycle Conf. Graz, Austria; December 1-3. 40. Diener, J., Siehler, U. 1999, Angew Makromol. Chem., 272, 1-4. 41. Wetzel, K., Wirth, R., and Flake, R. 1999, Angew Makromol. Chem., 272, 121-7. 42. Pillin, I., Montrelay, N., Bourmaud, A., and Grohens, Y. 2008, Polym. Degr. Stab., 93, 331-28. 43. Liu, X., Zou, Y., Li, W., Cao, G., and Chen, W. 2006, Polym. Degr. Stab., 91, 3259-65. 44. Bourmaud, A., and Baley, C. 2007, Polym. Degr. Stab., 92, 1034-45. 45. Iannace, S., Ali, R., and Nicolais, L. 2001, J. Appl.Polym. Sci., 79, 1084-91. 46. Hernandez, J.P., Raush, T., Rios, A., Strauss, S., and Osswald, T.A. 2002, Eng. Anal. Bound. Elem., 26, 621-8. 47. Moran, J., Alvarez, V., Petrucci, R., Kenny, J., and Vasquez, A. 2007, Appl. J. Polym. Sci., 103, 228-37. 48. Thompson, M.R., and Yeung, K.K. 2006, Polym. Degr. Stab., 91, 2396-407. 49. Duigou, A.L., Pillin, I., Bourmaud, A., Davies, P., and Baley, C. 2008, Composites, A: Appl. Sci. Manufact., 39, 1471-8. 50. Kopinke, F.D., Remmler, M., Mackenzie, K., Mรถder, M., and Wachsen, O. 1996, Polym. Degr. Stab., 53, 329-42. 51. Fan, Y., Nishida, H., Shirai, Y., and Endo, T. 2004, Polym. Degr. Stab., 84, 143-9. 52. Huda, M.S., Drzal, L.T., Mohanty, A.K., and Misra, M. 2006, Compos. Sci. Technol., 66, 1813-24. 53. Kumar, P.R., Nair, K.C.M., Thomas, S., Schit, S.C., and Ramamurthy, K. 2000, Compos. Sci. Technol., 60, 1737-51. 54. Mohanty, A.K., and Nayak, S.K. 2007, J. Mater. Sci. Eng., 443, 202-8.

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55. Alvarez, A., Terenzi, A., Kenny, J.M., and Vasquez, A. 2004, Polym. Eng. Sci., 44, 1907-14. 56. Choi, J.S., Lim, S.T., Choi, H.J., Hong, S.M., Mohanty, A.K., and Drzal, L.T. 2005, Macromol. Symp., 224, 297-307. 57. Guo, R., Azaiez, J., and Bellehumeur, C. 2005, Polym. Eng. Sci., 45, 385-99. 58. Raghavan, J., and Wool, R.P. 1999, J. Appl. Polym. Sci., 71, 775-85. 59. Kouparttsas, C.E., Kawtalis, C.N., Varelidis, P.C., Tsenoglou, C.J., and Papaspyrides, C.D. 2002, Polymer Composites, 23, 682-9.

Transworld Research Network 37/661 (2), Fort P.O. Trivandrum-695 023 Kerala, India

Recent Developments in Polymer Recycling, 2011: 121-153 ISBN: 978-81-7895-524-7 Editors: A. Fainleib and O.Grigoryeva

4. Recycling of thermosetting polymers: Their blends and composites Raju Thomas, Poornima Vijayan and Sabu Thomas School of Chemical Sciences, Mahatma Gandhi University, Priyadarshini Hills P.O. Kottayam-686 560, Kerala, India

Abstract. In many applications thermosets are the materials of choice for long-term use because they are insoluble and infusible high-density networks. Recycling of thermosetting polymers is regarded as one of the urgent problems to be settled because of its technological difficulty. The increased production of thermoset blends and composites in recent years has greatly increased the amount of waste materials. The present chapter reviews the fundamental literature in the recycling of thermosetting polymers, their blends and composites.

1. Introduction The use of polymer materials has simplified the modern life. At the same time, the extensive use of polymer materials in every walks of life have caused serious waste problems. The increased amount of polymer waste has become a serious issue globally and also caused depletion of petroleum resources without which the modern life become impossible for mankind. Among all the possible ways to manage polymer waste, a hierarchy could be established. The most preferred option is the minimization of waste, followed by reuse of materials in the same application, recycling in another application (including recovery of monomers or low-weight molecules), incineration with Correspondence/Reprint request: Dr. Sabu Thomas, School of Chemical Sciences, Mahatma Gandhi University, Priyadarshini Hills P.O., Kottayam-686 560, Kerala, India, E-mail: sabupolymer@yahoo.com

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energy recovery and finally incineration without energy recovery or land filling. A number of studies have been reported in the recycling of polymer wastes. The chapter deals with structure and properties of some important thermosetting polymers and their recycling studies.

2. Structure and properties of thermosetting polymers 2.1. Epoxy resins The terminology â&#x20AC;&#x2DC;epoxy resinâ&#x20AC;&#x2122; is generally applicable to both prepolymers as well as to cured resins. The former contain reactive epoxy groups whereas the cured resin may or may not contain reactive epoxy groups. While the term can be justified in the former case, the cured resins are also called epoxy resins. Epoxy resins typically contain a three membered ring with -O- atom. Different terminologies are also used to specify the group O

such as epoxide, oxirane and ethoxyline group, R CH CH . Commercial epoxy resins usually contain aliphatic, cycloaliphatic, or aromatic backbones. Epoxy resins are highly reactive presumably due to the strained three membered ring structures and react with many nucleophilic and electrophilic reagents. Therefore, a wide variety of organic compounds having active hydrogen atoms can be used as curatives. These include amines (both aliphatic/aromatic and primary/secondary), phenols, carboxylic acids, thiols, anhydrides etc. The general reactions of epoxy resin with these compounds are represented in Scheme 1. 2

NH2

NHR

amine

COOH

COOR

R alcohol

O C

R acid

HO R

thiol O O O

anhydride

C O

Scheme 1. Reactions between epoxy and different curing agents.

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Epoxy resins possess high resistance to chemicals and corrosion. Also, possess moderate toughness, flexibility and excellent mechanical and electrical behavior. It is also used as outstanding adhesives for different substrates. Epoxies are used in tooling, for laminates in flooring and to a small extent in moulding powders and in road surfacing. Epoxy resins are used for encapsulation of miniature components, particularly in space crafts. Epoxy resin laminates are useful in aircraft industry. Carbon fiber/epoxy resin composites are used for structural modification purpose in aeroplanes. Epoxy/aramid fibers find uses in the design of small boats.

2.2. Unsaturated polyester resins Linear unsaturated polyesters, which are often, called prepolymers find industrial applications. Unsaturation is introduced into the resin molecule using an unsaturated dicarboxylic acid such as maleic acid. For example, polyester of the following type is generated between ethylene glycol and maleic acid (cf. Scheme 2).

Scheme 2. Polyester from maleic acid and ethylene glycol.

Commercial unsaturated polyesters are based on phthalic acid, maleic acid, ethylene glycol, and butanediol. The crosslink density, which represents the average number of crosslinks between polyester chains and the average length of the crosslinks, determine the mechanical properties of the product. The crosslink density, in turn, depends on the relative amounts of the unsaturated acids used to prepare the prepolymer. The average length of the crosslinks depends on the relative amounts of the prepolymer and monomer and on the copolymerization behavior of the two double bonds. For example, fumarate-styrene system yields a harder and tougher material than fumaratemethyl methacrylate system. The unsaturated polyester-styrene matrix is employed in fiber-reinforced plastics (FRP) structures. The resins are also useful for decorative coatings. The resin finds use in the manufacturing of large structures such as boats and car bodies since it is curable at room temperature. The powder form of the resin is used in solution or emulsion form as binders for glass-fiber performs and for the manufacture of pre-impregnated cloths.

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2.3. Phenolic resins Phenolic polymers are obtained by the polymerization of phenol with formaldehyde [1]. The polycondensation reaction can be accelerated either by acids or by bases. The reaction yields resole prepolymers (resole phenolics) which are mixtures of mononuclear methylolphenols and various dinuclear and polynuclear compounds. Other products include substitution at o- and ppositions and the type of bridge between the rings (methylene versus ether). The typical ratio of formaldehyde to phenol is 1.2:1. Substituted phenols such as cresols (o-, m-, and p-), p-butylphenol, resorcinol, and bisphenol A are used for specific applications. Other aldehydes such as acetaldehyde, glyoxal, 2-furaldehyde are also used. The composition and molecular weights of the resole depend on the ratio of monomers, pH, temperature and other reaction conditions. For o crosslinking a temperature as high as 180 C is necessary. During the curing process, methylene and ether bridges are formed between benzene rings to yield a network structure of the following type (Scheme 3). OH CH2

CH2

OH CH2

CH2

OH CH2

CH2

O CH2

CH2 OH CH2

CH2

CH2 OH

CH2

Scheme 3. Network structure formation in phenolic resins.

Phenolic mouldings are hard, insoluble and heat resistant materials since they are highly crosslinked and interlocked [2]. The type of resin and filler influence the chemical resistance of the cured material. Cresol and xylenolbased resins are inert towards NaOH attack, whereas simple phenolformaldehyde will be affected. Phenolic mouldings are resistant to acids except 50 % sulphuric acid, formic acid, and oxidizing acids, if the filler used is also resistant. The reins are stable up to 200oC.

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Phenol- formaldehyde mouldings are widely used for domestic plugs and switches. Used in electrical industry where high electrical insulation properties are not needed. It is used for making cases, knobs, handles and telephones. In automobile industry, the resins are used for making fuse-box covers, distributor heads, and other applications where electrical insulation together with adequate heat resistance are needed. Heat resistant grade of the resins are used for saucepan handles, saucepan lid knobs, lamp housings, cooker handles, welding tongs, and electrical iron parts. Since the resin is hard and can be electroplated, it is used in the manufacture of â&#x20AC;&#x2DC;golf ballâ&#x20AC;&#x2122; heads for typewriters. Bottle caps and closures are made from the resin in large quantities. Automatic compression presses and machines suitable for the injection mouldings of thermoplastics are manufactured out of phenolformaldehyde resins.

2.4. Urea-formaldehyde resin It is an aminoplastic, a term generally used to represent resinous polymers formed by the interaction of amines or amides with aldehydes. The cured products form crosslinked insoluble and infusible thermoset. Compared to phenolic resins, the resins are cheaper, light in color, and have better resistance to electrical tracking. However, it exhibits higher water absorption and poor heat resistance. The reaction proceeds in the following route (cf. Scheme 4). The mono and dimethylol derivatives, formed during the reaction, further condense with urea to give the final resin structure. O C

O C

NH2 NH2

+ HCHO

NHCH2OH NHCH2OH

O C

NHCH2OH NH2

NH2 NH2

HCHO

O C

NHCH2OH

O C

NHCH2OH NHCH2NH C

NHCH2OH

+ H2O

NH2

and so on n HCHO

NH2CONH2

H2O

H NHCO NH CH2

OH n

Scheme 4. Urea-formaldehyde resin formation.

There are many desirable properties for U-F moulding powders that enable to keep it in the highest application level. The wide range of colours is a reason for the widespread use of the material. U-F resins do not impart taste

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and odour to foodstuffs and beverages with which they come in contact. Another added advantage is its good electrical insulation properties with particularly good resistance to tracking. The resin can resist continuous heat o up to a temperature of 70 C. Some physical properties of urea-formaldehyde resins are tabulated in the following Table 1. The major application of urea-formaldehyde resin is in the field of electric and electronic applications. It is mainly used for making plugs, sockets and switches. In addition, it is used for domestic applications such as pot and panhandles and tableware. In the sanitary sector, the resins are used as toilet seats and miscellaneous bathroom equipment. The wide colour range and freedom from taste and odour make the material a good choice for the manufacture of bottle caps and closures. However, nowadays, its consumption in this area has been reduced by the development of new thermoplastics. Buttons are made from U-F moulding powders due to its resistance to detergents and dry-cleaning solvents. Miscellaneous uses include meat trays, toys, knobs, lampshades etc. The bulk of U-F resins are used as adhesives for particleboard, plywood and furniture industries. Another application of the resin is in the manufacture of chipboard. U-F resins are also used to make foams. U-F foams are used to place on airport runways to act as an arrester bed to stop aircraft that overshoot during emergency landings or abortive take-offs. Another large scale application of the resin lie in its manufacture of firelighters. Table 1. Properties of urea-formaldehyde resins [2]. Property

Units

α-cellulose filled Wood flour filled Plasticized

Translucent

Specific gravity

—

1.5-1.6

1.5-1.55

Tensile strength

103lbf/in2 MPa

7.5-11.5 52-80

7-9.5 52-80

7-10 48-66

8-12 48-69

Impact strength

ft/bf

0.20-0.35

0.16-0.35

0.16-0.24

0.14-0.2

Cross-breaking strength

103lbf/in2

11-17

11-16.5

13.5-15.5

13-17

Dielectric o strength (90 C)

V/0.001 in

120-200

60-180

100-200

70-130

Volume resistivity

Ωm

1013-1015

1014-1015

—

Water absorption o 24 h at 24 C o 30 min at 100 C

mg mg

50-130 180- 460

40-170 250-600

50-90 300-450

50-100 300-600

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2.5. Melamine-formaldehyde resin Melamine and formaldehyde can also react to give methylol derivatives of melamine such as presented on Scheme 5: NH2

HNCH2OH

H2N

C 3 HCHO

NH2

HOH2CHN

NHCH2OH

Scheme 5. Formation of methylol derivatives of melamine.

The methylol derivative with excess melamine undergoes polycondensation to give linear polymer, which forms three-dimensional network structure with further quantities of melamine monomer (cf. Scheme 6). N NHCH2NH C

C NHCH2NH

N C NH CH2 NH C

C N C

NHCH2NH

C NHCH2NH

Scheme 6. Melamine-formaldehyde resin formation.

The M-F resins are characterized by superior properties. The mineral-filled resins are having low water absorption. The melamine resin is having better resistance to attaining by aqueous solutions such as fruit juice and beverages. Good electrical properties are maintained at elevated temperatures. Better heat resistance and greater hardness are the added advantages. They have a wide color range, track resistance and scratch resistance. Mineral-filled melamine based compositions have superior electrical insulation and heat resistance to the cellulose-filled grades. The resins are

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used for the manufacture of decorative foils in compression moulding. The principal application of the resin is for the manufacture of tableware. A wide color range distribution, surface hardness and stain resistance are the reasons. Cellulose-filled compositions are used at small levels for the manufacture of trays, clock cases and radio cabinets. The mineral-filled compounding are used in electrical applications and knobs and handles for kitchen utensils. M-F resins are widely employed for laminating applications owing to their high hardness, good scratch resistance, freedom from color and heat resistance. They are also used as adhesives. Melamine-formaldehyde condensates are useful in textile industry. They are useful agents for permanent glazing, rot proofing, wool shrinkage control and, with phosphorus compounds, flame proofing. The resin can be used to prepare paper with enhanced wet-strength.

2.6. Polyimides Polyimides have the characteristic functional grouping of the following type [3, 4]. CO N CO

The branched nature of the functional group facilitates the production of polymers. The backbone consists mainly of ring structures and hence high softening points. The polymers exhibit high thermal stability and hence valuable for high temperature applications. Aromatic polyimides are formed by the polycondensation of dianhydrides with diamines. For example, polycondensation of pyromellitic anhydride with p,pâ&#x20AC;˛-diamino diphenyl ether results in the synthesis of polyimides. The reaction is carried out in two steps. In the first step, the reaction is conducted in suitable solvents such as DMF, around 50oC, where polymerization takesplace with the formation of polyamic acid (cf. Scheme 7). n

OC HOOC

COOH

H2N

NH2

Scheme 7. Polyamic acid formation.

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The polyamic acid is then casted as a film, by evaporating the solvent. It is then baked at 300oC in the atmosphere of nitrogen. Polycondensation takes place to form the following product in the second step. This product is converted in to the required shape. Polyimides, which can be either thermoplastic or thermoset, are widely used in aerospace applications. Thermosetting polyimides provide easier processing and higher thermal resistance, while thermoplastic polyimides offer greater toughness. A comparison of the properties of epoxy and polyimide thermoset matrices is furnished in Table 2. The polymer is having excellent resistance to oxidative degradation. Also inactive towards most chemicals other than strong bases and high-energy radiations. The principal application of polyimides is as compressor seals in jet engines. Also, used in data processing equipment such as pressure discs, sleeves, bearings, and as friction elements. They are also used as valve shafts in shut-off values. Due to the heat resistance capacity and deformation resistance of the polymers, they are used in soldering and welding equipment. However, the disadvantage of the polymer is that they may undergo hydrolysis and crack in water or steam at temperatures above 100oC. For such purposes, polyetheretherketones (PEEK) are employed. Table 2. Properties of Composite matrices [2]. Property

Epoxy

Polyimide

Modulus, GPa

2.8-4.2

3.2

Tensile strength, MPa

55-130

Compressive strength, MPa

140

187

Density, g cm-3

1.15-1.2

1.43

Thermal expansion coefficient, 10-6 oC

45-65

3. Recycling of thermosetting polymers The use of plastic and other thermosetting polymers become enormous during the last decades. Hence, it becomes inevitable to clear them so that it may not turn into a major issue in the global conservation resources. The recycling of thermosetting polymers which are usually considered as

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infusible and insoluble, aims to develop reworkable crosslinked polymer, which means crosslinkable-decrosslinkable systems. There are different ways of recycling treatments [5]. Incineration process with considerable noncombustible residues is one among them. The other includes a thermolysis process [6] with a poor value for decomposition products and a mechanical weakening [7]. Another method is mechanical recycling techniques based on granulation and comminution, leading to specific size fractions, which can be incorporated into new sheet moulding compounds (SMC) parts [8], in a thermoplastic matrix [9-11], or in concrete [12]. Solvolysis is another promising method for recovery of composite wastes. Solvolytic process of PET bottles are well-known and industrialized [13]. Chemical recycling process [14-18] was found to the most effective and promising method for thermosetting resins.

3.1. Epoxy resins It was reported that the possibility of recycling amine cured thermosetting resins [19] from epoxy-dissolved nitric acid was successful [20-21]. The decomposition behavior of amine cured bisphenol F type epoxy resin and its chemical recycling has been reported elsewhere [22-23]. Epoxy resin cured with amine hardener and its decomposition in nitric acid was investigated [24]. The purified residue was then recycled to prepare cured resin using an anhydride hardener. The yield of the recycled product as a function of time was studied by immersing in 4M and 6M nitric acid (cf. Figure 1).

Figure 1. Yield of decomposed products in 4M nitric acid solution, at 80 C [24].

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It was examined that the resin decomposed rapidly and disappeared completely at about 100 hrs. The yield of the residue and extract was maximum at 50 hrs and 150 hrs, respectively. Thereafter it decreased. When the concentration of the acid solution was increased to 6M, the yield of the extract became higher as read from the Figure 2. The crystal was formed at 120 hrs due to the breakage of the main chain. In spite of the small decomposition rate and yield, 4M solution was found to be superior to 6M nitric acid. Size exclusion chromatographic analysis was employed to determine the molecular weight of extract collected from 4M and 6M solutions and the same is demonstrated in Figures 3 and 4.

Figure 2. Yield of decomposed products in 6M nitric acid solution, at 80 C [24].

Figure 3. Change of molecular weight distribution of extract with immersion time in o 4M nitric acid solution, at 80 C [24].

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Figure 4. Change of molecular weight distribution of extract with immersion time in o 6M nitric acid solution, at 80 C [24].

Main peaks existed near log M=2.5, 2.7 and 2.73 for the extract collected from 4M nitric acid. For the extract from 6M nitric acid solution, prominent peaks observed near log M=2.5, 2.55, 2.7 and 2.77. The main product â&#x20AC;&#x2DC;extractâ&#x20AC;&#x2122; was assumed to be a mixture of several lower molecular weight compounds and hence similar structures to monomer or dimmer of uncured BPA resin. In addition, the weight distribution from 4M nitric acid suggests that the residue is a mixture of intermediate products with higher molecular weight and can be decomposed again. Flexural strength and modulus of neat as well as recycled resin with different weight percentage of neutralized extract (NE) are furnished in Table 3. Flexural strength of recycled resin becomes maximum at a level of 10-20 % NE extracts. A repolymerization mechanism has been formulated in which the main chain of bisphenol A was easily broken since it possess quaternary carbon atom. Tertiary carbon atom is then generated by the attack of acid. The extract acts as a resin during repolymerization and reacts with curing agent to form crosslinked network. The fine network structure formed during the recycling resulted in rather improved mechanical properties. Table 3. Comparison of mechanical properties [24]. Samples Virgin resin

Flexural strength, MPa 104.2

Flexural modulus, GPa 3.77

Recycled resin (5 %)

118.0

4.08

Recycled resin (10 %)

121.0

3.98

Recycled resin (20 %)

125.0

4.29

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Figure 5. DSC scans of recycled resin in comparison with virgin resin [24].

Differential scanning calorimeter (DSC) was also employed to compare the Tgâ&#x20AC;&#x2122;s of recycled resin as well as that of virgin resin (Figure 5). Both show the existence of one Tg and also the Tg of recycled resin was higher than that of virgin resin which increased with increase in NE content. In a certain study [25], low-stress-type moulding epoxy resin powder containing silicone elastomer was subjected to recycling. The new recycled resin showed better thermal impact resistance than the original moulded resin. Besides, moulding resin powder was found to be suitable as filler for epoxy resin products such as insulating materials, paints and adhesives. The powder was also useful as a decorative agent for an acrylic-resin-type construction material. The study used two model-moulding resins. Model standard resin and model lowâ&#x20AC;&#x201C;stress-type resin. The Table 4 shows the properties of original moulding resin and recycled moulding resin. The flow property of the recycled resin was reduced considerably (68 to 22 cm) where the standard resin powder was recycled into the original standard resin at a recycling ratio of 10 wt %. However, the flow properties improved using an epoxy resin with a melting temperature lower (55oC) than that of the original epoxy resin (70oC). But the moisture resistance and thermal impact resistance of the recycled resin were inferior to those of the original standard resin. The flow properties were also insufficient when the low-stress-type moulding resin powder was recycled in to the original standard resin. The property was improved where spherical silica powder was used, and up to the recycling ratio of 25 wt.%. In addition, heating the moulding resin powder (at o 170 C for 8 hr) improved the flow property. From spectral analysis, it was

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observed that the amount of epoxy groups in the low-stress-type moulding resin powder decreased by heating which lowed the surface reactivity. Nevertheless, the silane elastomer in the low-stress-type resin powder improved the thermal impact resistance of the recycled product. The mouldability, strength and insulating property of epoxy-resin type insulating material with the moulding resin powder were compared with reference material with fused silica filler and the results are tabulated in Table 5. The flexural strength and insulating property of the moulded material with the moulding resin powder were almost same as those the material with silica powder. However, the flow properties were not as good as with silica incorporated material. The surface treatment of moulding resin powder with silane coupling agents improved the strength and insulating property of the moulded material. This is because of the improvement in interfacial adhesion between the matrix and the resin powder. Epoxy silane coupling showed little effect compared to aminosilanes. Table 6 illustrates a comparative account of the strength and thermal expansion properties of epoxy resin moulded with moulding resin powder with that of compounds with conventional fillers such as CaCO3, talc powder and silica powder. Table 4. Properties of original moulding resin and recycled moulding resin [25]. Property

Mouldability Flow (cm) Barcol hardness Burr (mm) Strength properties Flexural strength (kgf/mm2) Flexural modulus (kgf/mm2) Charpy impact strength (kgf/mm1) Thermal mechanical properties, Tg (oC) Thermal expansion coefficient, Îą1/Îą2 (10-5Ă&#x2014;oC-1) Moisture resistance reliability 120oC, 2.3 atm, 200 hr 520 hr Thermal impact resistance reliability, 150-60oC 100 cycles 150 cycles 200 cycles

Original moulding resin Standard Lowresin stresstype resin

Recycled moulding resin Standard resin with low-stressresin powder (25 wt.%) 47 55 1.3

Standard resin with low-stressresin powder heated at 170oC for 8 hr 61 62 2.3

69 74 0.6

73 58 1.0

Standard resin with standard resin powder (10 wt.%) 68 58 1.3

14.7 1418

14.2 1240

17.3 1505

15.8 1391

14.2 1353

2.3

2.8

3.6

2.7

155

171

139

154

2.0/6.5

1.8/7.4

2.2/6.3

2.0/6.6

2.1/6.3

0/10 1/10

0/10 0/10

0/10 4/10

0/10 0/10

3/10 5/10 8/10

0/10 0/10 0/10

7/10 9/10 -

0/10 0/10 1/10

0/10 0/10 0/10

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Table 5. Properties of epoxy resin compounds with moulding resin powders for insulating materials [25]. Moulding resin treated with epoxy silane 1.0 wt.%

Silica powder 60 wt.%

Moulding resin powder 60 wt.%

Moulding resin treated with amino silane 0.5 wt.%

Moulding resin treated with amino silane 1.0 wt.%

199 105

76 72

78 67

75 70

89 68

14.6

14.8

16.1

14.0

1057

696

722

742

678

Insulation resistance at 150oC (1013Ω cm)

4.0

2.0

4.0

141

142

138

143

141

Property

Mouldability Flow (cm) Gell time (s) Flexural strength (kgf mm-2) Flexural modulus (kgf mm-2)

Table 6. Properties of epoxy resin compounds with moulding resin powder for paints and adhesives [25]. Property

None

Moulding resin powder

Silica powder

CaCO3 powder

Talc powder

Average particle size of filler (μm)

—

9.2

Viscosity (Pa s)

2.5

206

Flexural strength (kgf cm-2)

817

800

787

610

577

Charpy impact strength (kgf cm-1)

2.3

2.4

1.6

2.5

Tensile strength (kgf cm-2)

440

474

428

336

280

Thermal expansion (10-5×oC-1)

9.5

Adhesive strength (kgf cm-2)

183

161

151

172

138

The strength of the moulded compound was comparable with that compounded with silica powder, but superior to those containing CaCO3 and talc powder. The thermal expansion was comparable with those having conventional fillers. Viscosity of the recycled resin was higher than those, which contain CaCO3 and silica powder, but lower, than the compound

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containing talc powder. The adhesion property of the material containing moulding resin powder is almost equal to those containing conventional fillers. It was observed that the moulding resin powder is a useful decorating agent for resin-type construction materials. Table 7 furnishes the properties of moulded materials with the powders (maximum particle sizes, 1 and 5 mm). The strength of the materials with powders is sufficient for the construction materials even though they were not as good as that of the original material. The thermal deformation temperature and surface hardness of the materials compounded with powders were superior to that of the original material. Table 7. Properties of acrylic-resin-type construction materials with moulding resin powder [25]. Acrylic resin + aluminium hydroxide (65 wt.%)

Acrylic resin+ moulding resin powder(less than 1mm; 10 wt.%)

Acrylic resin+ moulding resin powder(less than 1mm; 20 wt.%)

Acrylic resin+ moulding resin powder less than 5mm; 10 wt.%)

Flexural strength (kgf cm-2)

550

450

430

360

Charpy impact strength (kgf cm-1)

1.5

1.6

1.7

1.6

Tensile strength (kgf cm-2)

270

290

230

Thermal deformation temperature (oC)

Surface hardness

Property

3.2. Unsaturated polyester resins The solvolysis of sheet-moulding composites (SMC) of unsaturated polyesterâ&#x20AC;&#x201C;styrene thermoset resin incorporated with glass fibers, calcium carbonate filler and thermoplastic poly vinyl acetate as an additive has been reported [26]. SMC are semi-finished products consisting of 25-50 mm chopped glass fibers (GF) and a paste typically of an unsaturated polyester (UP) diluted in styrene. It also consists of fillers (calcium carbonate), a thermoplastic polymer (as a low profile additive), a curing catalytic system, a thickener and a releasing agent. It was observed that aminoalcohols and polyamines allow much higher depolymerization yields, resulting to total

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digestion of polymers. Diethylenetriamine (DETA) was chosen as the solvolytic reagent. Treatment of SMC chips with solvents resulted in a mixture which contains three fractions i.e., glass fibers, fillers and an organic liquid. Glass fibers and fillers did not contain much organic contaminations and the organic liquid can be used as a curing agent for epoxy resin. Solvolysis reactions were done with different solvents on chips of different grades of SMC chips. Results showed that the solvent reactivity depends on solvent in the order polyamines and aminoalcohols > glycols > diacid derivatives. Analysis of the solid fraction is summarized below (Table 8). The study proved that the dismantlement of SMC composites by solvolysis is possible, leading to a valuable liquid fraction and allowing the recovery of two inorganic fractions i.e., the glass fibers and the fillers. Table 8. Analysis of the large sized solid fractions after solvolysis of SMC chips [26]. Reactant MEAa)

DETAa) o

DEGa) 15% NaOH Initial composition o

Time, temperature

20 hr, 170 C 14 hr, 200 C

19 hr, 230 C

Organic matter

0.7 %

0.2 %

24 %

CaCO3

2.4 %

2.9 %

34 %

47 %

GFa)

97 %

67 %

28 %

MEA – monoethanolamine, DETA– diethylenetriamine, DEG – diethyleneglycol, GF– glass fiber

3.3. Phenolic resins Phenolic resins are commonly used as industrial adhesives and in heavyduty automotive parts such as the plastic trim on car bodies and the plastic containers that hold air filters. The resins are formed when phenol and formaldehyde are cured at high temperature and pressure in the presence of catalysts, so that the molecular chains form an interlocking 3D structure that is hard to break. As a result, they cannot be melted and remolded like other plastics. Instead, most of the 2.2 million tons that are manufactured worldwide every year end up in landfills.

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Material recycling is to collect and to reuse the waste plastics as raw materials. Hisashi et al. [27] proposed two methods for the material recycling of waste containing phenolic resin. One is to mix the wastes with virgin compounds to make recycled plastics; another is to reuse them as a filler for thermoplastics. The results obtained are as follows: i) The suppression of generating dust particles could be attained by adding alcohol at mixing crushed materials with virgin compounds. ii) Injection moulding was tried using the recycled plastics, which contained 30 wt.% crushed materials. This gave us satisfactory results in appearance, mechanical property, and moldability. iii) Reuse of the wastes as a filler for thermoplastics was also tried. The heat distortion temperature of the resultant resins was improved by 3 %, compared with that of polystyrene moulding materials, though their tensile strength and flexural stress decreased with increasing content of the wastes. iv) It was confirmed that this recycling technology could be put to practical use and would contribute toward reduction of environmental burdens. There are two difficulties for undertaking material recycling method. The waste plastics should be classified when they are collected to retrieve genuine regenerated plastics. Another is found in the degradation of properties during recycling. Thermal recycling is to convert the waste plastic to fluid fuels. Osaki et al. [28] carried out the granulation of phenolic resin scraps like sprues and runners from injection moulding. The granules were blended with a binder of polypropylene bumper waste granules in the weight ratio 30/70-60/40, then melt extruded and hot-cut. The processability was fairly good up to the ratio 40/60.Satisfactory performance in practical use of the refuse窶電erived fuel compared with coal or other solid fuel made from town refuse. Because this method includes combustion of the product to obtain heat or power, this cannot be an ultimate recycling of plastics from the standpoint of effective uses of resources. Chemical recycling of phenol resin and its compounds has been carried out by some researchers. Supercritical water (SCW) has been growing in importance as a medium for chemical reactions for about ten years. It is wellknown that water under supercritical conditions is much less polar and can homogenize substantial amounts of nonpolar organic compounds [29-31]. Sub- or supercritical fluids have been focused as reaction media for environmental applications from a view point of green chemistry. Supercritical water has been applied to the chemical degradation of phenol

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resin and its model compounds [32-34]. Suzuki et al. [34] decomposed seven prepolymers and molding materials of phenol resin into their monomers by reaction in subcritical and supercritical water under an argon atmosphere. Seven prepolymers of phenol resin were decomposed into their monomers such as phenol, cresols, and p- isopropylphenol by reactions at 523-703 K under an argon atmosphere in subcritical and supercritical water. The total yield of identified products depended on the kind of prepolymers, and the maximum yield reached 78 % in the reaction at 703 K for 0.5 hr. The decomposition reactions were accelerated by the addition of Na2CO3, and the yields of identified monomers reached more than 90 %. Two kinds of molding materials of phenol resin whose content of phenol resin was less than 50 % were also decomposed mainly into phenol and cresols by the reaction in supercritical water. They have confirmed that not only prepolymers of phenol resin but also molding materials of phenol resin were decomposed into their monomers by the reaction in subcritical and supercritical water under an argon atmosphere. The decomposition reaction of thermoplastic resin in subcritical and supercritical water was known; therefore, a chemical recycling process in supercritical water can be applied to the mixture of various plastics including thermoplastic and thermosetting resins. Possible merits of utilizing supercritical methanol over supercritical water are found in the following respects. First, the operation condition will be milder because the critical temperature and pressure of methanol are lower than those of water. This would widen the selection of materials for the reactors. Second, the separation of products from the solvent is much easier than the case using supercritical water, because the boiling point of methanol is lower than that of water. Additionally, alteration or modification of the product distribution would also be expected by changing the solvent. Ozaki et al. [35] studied the chemical recycling of phenol resin by supercritical methanol. The objectives of their study are (1) to know how the conversion of phenol resin is influenced by the reaction conditions and (2) to see what kinds of species are included in the liquid product. In this study, they conducted a preliminary study on the liquefaction of phenol resin by supercritical methanol. The resin is one of the most difficult substances for recycling, because it has a highly cross-linked network structure. The obtained conclusions are as follows: i) Supercritical methanol could liquefy phenol resin, and the reaction was obvious above 400째C to give a conversion higher than 80 %. ii) Both reaction conditions of a longer time at a temperature and a higher reaction temperature for a shorter time resulted in increasing the

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conversion. However, the former condition turned out to be favourable for obtaining a higher yield of liquid products. iii) The liquid product was found to include phenols. iv) The analysis of the solid products revealed a concentration of carbon atoms during the reactions. Phenolic resins, phenol-formaldehyde polymers are previously thought to be nonbiodegradable. The first demonstration of biodegradation of phenolformaldehyde polymers were done by Gusse et al. [36] White-rot fungus is known to decompose organic pollutants such as DDT, TNT, PCBs, and dioxins and can be used to clean up these toxins from the environment. The fungus produces ligninase enzymes, which can break down lignin, the compound that makes up the dry part of wood. Researchers in the department of biology at the University of Wisconsin studied whether the fungus could also degrade phenolic resins, which have a molecular structure similar to that of lignin. They used a generic industrial formula to make the polymers and placed resin chips in cultures with the fungi. The several hundred species of wood-rotting fungi fall into two broad categories, white-rot fungus and brown-rot fungus. The researchers tested 11 different fungi strains, including 5 species of white-rot and 1 species of brown rot fungus. All of the species used in the new research have been previously studied for their ability to biodegrade pollutants. The researchers first realized that the white-rot fungus was degrading the phenolic resins when their colour changed from yellow to a light pink, the colour of the phenol and formaldehyde subunits used to make the resins, says Adam Gusse, a biology graduate student and lead author of the study. They confirmed the presence of those subunits by gas chromatography/ mass spectrometry and by locating pockmarks on the surface of the resin chips with scanning electron microscopy (Figure. 6).

Figure 6. White-rot fungi caused the pockmarks on the surface of the phenolic resin plastic shown in this scanning electron microscope image [36].

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3.4. Urea-formaldehyde Urea-formaldehyde (UF) accounts for about 15 % of the total thermoset resin production. Currently, one of its major applications is in molded products, including electrical equipment, dinnerware, buttons, cosmetic caps, and bottles. However, the same factors that make UF a good choice for many applications, namely its chemical, thermal, and mechanical stability, are also what make recycling such a big challenge. Urea formaldehyde grit (UFG) fillers can be used effectively in blends with HDPE, as one of the possible applications for this recycled thermoset [37]. When compared with more traditional fillers, the UFG is lighter and less expensive, and the modulus gains are similar, although the UFG-based systems are likely to be more brittle. The stressâ&#x20AC;&#x201C;strain behavior at various filler levels is represented in Figure 7.

Figure 7. Stressâ&#x20AC;&#x201C;strain behavior of UFG-filled HDPE at various filler levels (wt.%) [37].

Figure 8. Influence of ionomer modification on the relative tensile modulus of UFG-filled HDPE. The ratio of ionomer to UFG was held constant at 1 to 10 [37].

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More over the addition of zinc-neutralized ionomer can produce significant increases in modulus, at only a modest cost in elongation to break (Figure 8).

3.5. Melamine-formaldehyde Melamine-formaldehyde (MF) resin is used to impregnate paper for the manufacture of both high pressure and particularly low pressure laminates [38]. After the preparation either of the impregnated paper or of the final laminate at the hot press a considerable amount of paper impregnated with non-fully-cured melamine resins is produced by square-trimming the paper. The amount of this dry waste is considerable, running up to 400 tons/year in just a medium sized paper impregnating factory. The fact that the MF resins impregnating the paper might present some residual activity renders possible to consider their utilisation as wood panel binders. This study deals then with the use of melamine-impregnated paper waste either (i) directly, in powder form, as a binder for particleboard or other wood panels, or (ii) indirectly by incorporating the finely ground, powdered resin impregnated paper in the resin itself during its preparation. Powdered melamine waste paper can be used successfully and directly as a binder of particleboard satisfying the relevant standards specification for water resistant and interior grade particleboard according to the relative proportion of waste paper, hence of resin added, and the residual activity of the MF resin impregnating the paper. Equally, powdered melamine waste paper can be used to substitute melamine in the formulation and during the preparation of MUF resin adhesives for wood.

3.6. Polyimides Polyimides are commonly prepared by the reaction of tetracarboxylic acid dianhydrides with diamines. Many of the polyimide-forming chemicals, particularly the dianhydrides, are very expensive. So the recovery of organic values from polyimides, and more particularly to the recycle of useful polyimide forming chemicals, has gained importance. One method for recycling polyimides which comprises heating polyimides with a solution of an alkali metal hydroxide in a dipolar aprotic solvent for a long time and at a temperature and pressure effective to convert polyimides to at least one tetracarboxylic acid or functional derivative thereof and at least one diamine or functional derivative thereof [39]. The product obtained by reaction of the alkali metal hydroxide with the polyimide comprises the polyimide reinforcing agent(s), if any, a functional

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derivative which is usually the alkali metal salt of the tetracarboxylic acid from which the polyimide is derived, and the amine from which the polyimide is derived. Reinforcing agent, especially fibrous filler, may be isolated by washing with organic solvent and water by filtering and drying. When so isolated, it is frequently suitable for recycle to prepare further reinforced polyimide. The amine may be recovered by dissolution in a suitable organic solvent such as ether and/or ethyl acetate to form a solution which may be washed with water, acidified and stripped for recovery of the amine. For recovery of the tetracarboxylic acid, the alkali metal salt may be acidified and extracted with an organic solvent. The organic extracts contain the acid, which may be isolated by conventional means and converted, if desired to a functional derivative such as the dianhydride. Mormann et al. [40] carried out the ammonolysis of bismaleimide thermosets. Polyimides prepared by free radical polymerisation after ammonolysis at 160째C give the corresponding amines and linear polymers with unsubstituted imide and diamide units as demonstrated by Scheme 8.

Scheme 8. Scheme of ammonolysis of bismaleimide thermosets.

In terms of recycling, only the amines will be of immediate use while the linear polymers do not have applications so far. Beyond recycling, the analytical aspect is also important since characterization of the resulting linear polymer will give information on the degree of polymerisation of the maleimide moieties in the thermoset.

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4. Recycling of thermosetting blends 4.1. Epoxy-thermoplastic blends A hybrid of thermosetting and thermoplastic resins improves the properties of the product during use while at the same time making it easy to decompose the resin at the end of its life cycle. The hybridization process involves blending the thermosetting resin with a small amount of thermoplastic resin, which can be broken down by organic solvents or heat, [41-42] schematically represented in Figure 9. Controlling the phase structure of the cured resin by changing the formation temperature, it is possible to obtain a thermosetting/thermoplastic hybrid in which the thermoplastic resin forms a continuous phase structure. A recent study on epoxy resin-PES (Polyether sulfone) blend having lower critical solution temperature (LCST) behaviors at higher formation o temperatures (180 C) result in a continuous phase formation intermediate between thermosetting and thermoplastic resins. Comparative study of the decomposability by organic-solvent treatment with DMF of the cured unblended epoxy resin (Figure 10) with the cured epoxy/ PES hybrid (Figure 11), showed easy decomposition of later [42].

Figure 9. The concept of readily decomposable thermosetting resin [42].

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5. Recycling of thermosetting composites Work on the recycling of thermoset composites can be divided into three categories. In most cases, thermoset composites are milled into very fine powders and are used as filler materials for polymers. For chemical recycling, the polymer is recovered as an organic compound, which may be used as a raw chemical material. Energy recovery means that the caloric content of the polymer matrix is usefully harvested by combustion. Most existing work on the recycling of thermoset composites emphasizes prepregs, but such technology could not be applied to cured systems.

Figure 10. Organic solvent treatment of cured thermosetting resin after treatment o for 10 days at 23 C [42].

Figure 11. Organic solvent treatment of cured thermosetting/thermoplastic hybrid o after treatment for 50 hr at 23 C [42].

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5.1. Epoxy-glass fiber composites Disposal of plastic and glass fiber wastes is a serious solid waste problem [43-44]. By the enormous production of computer and communication hardware, epoxy/glass fiber waste become escalated. Thermoset glass fibre composites are milled into very fine powders and are used as filler materials for polymers. In a certain study [45], it was found that glass-fiber-reinforced epoxy resin boards could be used as fillers for epoxy resin products. They could yield products with better strength and thermal expansion properties than those made with conventional fillers. Concrete is one of the most promising substitutes for recycling epoxy/glass waste so as to minimize the environmental impact caused by direct land-fill disposal. Lee et al. [46] have evaluated the feasibility of adding epoxy/glass fiber waste in concrete and discussed its effectiveness and potential in construction application. The ground epoxy/glass particles (EG) were considered as fillers, in which coarser particles (φ>150 μm) partially replace fine aggregate and fine particles (φ<150 μm) acts as ultra fine fillers or supplementary binding material. A series of tests were conducted to assess the properties. Table 9. Mix proportions (kg/m3) [46]. Mix no A0 A1 A2 A3 B0 B1 B2 B3 C0 C1 C2 C3 Mix no. A0 A1 A2 A3 B0 B1 B2 B3 C0 C1 C2 C3

Water 225.0 225.0 222.3 213.2 225.0 225.0 221.0 216.0 225.0 225.0 222.0 216.5 EG particles (φ>150 μm ) 0.0 33.6 66.7 99.2 0.0 35.5 70.5 104.9 0.0 36.9 73.3 109.1

Cement 262.5 262.5 262.5 262.5 225.0 225.0 225.0 225.0 196.9 196.9 196.9 196.9 (φ>75 μm ) 0.0 54.8 108.7 161.9 0.0 57.9 114.9 171.2 0.0 60.3 119.6 178.1

ggbs 90.0 90.0 90.0 90.0 77.1 77.1 77.1 77.1 67.5 67.5 67.5 67.5 Fine aggregate 890.7 795.4 701.6 609.2 941.5 840.8 741.6 644.0 979.6 874.8 771.6 670.0

Fly ash 22.5 22.5 22.5 22.5 19.3 19.3 19.3 19.3 16.9 16.9 16.9 16.9 Coarse aggregate 763.8 763.8 763.8 763.8 763.8 763.8 763.8 763.8 763.8 763.8 763.8 763.8

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In their analysis epoxy/glass fiber waste were collected from copper-clad laminated printed circuit board after separating the metal magnetically. The chopped EG fiber wastes were ground to fine particles to mix with natural fiber aggregate. As observed by SEM the E-glass particles consists of filament shaped fibers with a diameter less than 10 μm and a length greater than 150 μm, which is surrounded by fine epoxy particles and epoxy matrix. In the recycled particles, the weight ratio of epoxy to glass is about 1.5. Epoxy/glass particles, in amounts of 10, 20 and 30 wt.% of fine aggregate, are separated into two parts based on the dividing size of 150 μm. Table 9 illustrates the detailed mix design. According to ASTM C39-99, C469-94 and C469-96 the specimens were tested at ages of 7, 28, 56 and 91 days for compression strength, splitting tensile strength and modulus of elasticity. The details are furnished in Table 10. EG particles, in general, improve concrete strength for a particular water/binder ratio. It is prominent in higher water/binder ratio mixes. Table 10. Mechanical properties of concretes at various ages [46]. Mix no. A0 A1 A2 A3 B0 B1 B2 B3 C0 C1 C2 C3 Mix no. A0 A1 A2 A3 B0 B1 B2 B3 C0 C1 C2 C3

7 days

Compression strength (MPa) at different ages 28 days 56 days 91 days

13.6 28.3 14.6 29.2 14.0 30.3 — 15.2 9.1 20.9 9.6 22.0 11.3 23.6 — 11.7 7.1 15.1 7.3 16.9 8.9 19.1 — 9.3 Elastic modulus (GPa) at different ages 28 days 15.7 17.2 19.8 5.1 14.5 16.2 11.3 4.7 13.4 15.9 17.8 4.4

56 days 18.2 19.5 21.4 — 16.3 22.0 18.8 — 15.8 17.1 20.4 —

91 days 20.4 22.2 24.0 — 16.9 27.8 20.2 — 16.1 19.5 20.7 —

31.9 33.2 33.2 34.2 35.7 36.9 — — 25.9 27.0 27.8 28.5 28.9 30.7 — — 18.8 20.1 21.3 23.2 23.6 25.5 — — Splitting tensile strength (MPa) at different ages 28 days 56 days 91 days 2.7 3.2 3.2 3.1 3.4 3.7 3.8 3.7 4.2 2.1 — — 1.9 2.7 2.8 2.5 3.4 3.4 2.6 3.6 3.7 1.6 — — 1.7 2.1 2.1 15.9 17.1 19.5 2.4 2.9 3.0 1.5 — —

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The compressive strength index

( fc ' fc ' control ) , which represents the

ratio of compressive strength of EG specimen over the compressive strength of control specimen (A0, B0, and C0 mixes), shows that C1 and C2 mix increase about 10 and 20 % respectively, in comparison to the control mix. The filling or complementary cementitious effect of ultra fine EG particles increase in EG1 and EG2, whereas for EG3 a decline is observed due to the inhomogeneous mixture. In addition, the ratio of splitting tensile strength to compressive strength ft increases with the percentage of EG particles. fc ' It was also noted that concretes with 10 and 20 % of EG particles have higher pulse velocity than control specimens for specified concrete ages. The absorption that reflects the condition of the concrete surface porous decreases with the increase in EG particle content. However, it was observed that all mixes using 20 % EG particles have almost the absorption of 4.1 % irrespective of the waster/binder ratio. Surface conditions were significantly improved ultra fine particles, in higher water/binder ratio mixes. The percentage of resistivity increases with EG particles in EG1 and EG2 mixes. Both results suggests that the proper use of fine EG particles can effectively modify the internal micro or meso porous system and reduce concrete permeability. The addition of EG particles shows improvement in sulfate resistance of concrete. Specimens with 20 % EG particles show a reduction of 50 % in weight loss. EG particles reduce concrete porosity and inhibit ions penetration.

( )

5.2. Epoxy–carbon fiber composite Carbon fiber reinforced composites (CFRCs) have increasing number of applications in the aerospace industry and high-grade application products. It also provides the drivers for a recycling solution. Emerging technologies will focus on recovering long, high modulus fibres since this is the most valuable form of CF’s. Around 40 % of long fiber, pre-impregnated (“pre-preg”) material is wasted as off cuts during fabrication, so the recycling process presents, potentially, a high economic value [47]. Milled fiber products are readily available on the market and have little intrinsic value. Landfill taxes and the European Union (EU) end-of-life vehicle directive will penalize those industries failing to take up appropriate recycling technologies [48-49]. Adherent Technologies Inc. has developed a chemical recycling technology that allows the recovery of carbon fibers and also produces a phenolic oil mixture that can be used for the formulation of phenol formaldehyde resins [50].

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Hernanz et al. [51] investigated the chemical recycling of carbon fiber reinforced composites (CFRCs) using sub critical and supercritical alcohols as reactive-extraction media. The epoxy resin that joins the fibers is degraded during the process, producing fibres that retain 85â&#x20AC;&#x201C;99 % of the strength of the virgin fibres. The reactive dissolution of the resin is a non-steady process in which five main mass transfer steps are found (Figure 12). (1) and (2) are diffusion (or dissolution) of the reagent to the surface of the fiber (double-film theory), (3) is the reaction at the surface of the reinforced fiber, (4) is the diffusion (or dissolution) of the products to the bulk fluid and (5) is the external mass transfer by convection in the bulk fluid. The capacity of methanol, ethanol, 1-propanol and acetone as solventreagents for the chemical recycling of carbon fibre reinforced composites has been investigated. The process has worked in batch and in semi-continuous mode at temperatures from 200 to 450oC. The batch system did not have any agitation to avoid fiber damaging. Under these conditions, mass transfer was not favoured. Using a flow system in the semi continuous mode and an alkali as catalyst, the degradation process was improved by increasing the total rate of reaction. Flow rates was in the range from 1.1 to 2.5 kg-alcohol/kgfiber/min and the addition of alkali catalysts such as NaOH, KOH and CsOH from 0.016 to 0.50M were enough to degrade more than 95 % of the resin in less than 15 min. The use of a flow reactor is recommended to enhance the

Figure 12. Mass transfer steps in the reactionâ&#x20AC;&#x201C;extraction process of the epoxy resin from CFRCs. Concentration profile in a single carbon filament [51].

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Figure 13. SEM micrograph (4000Ă&#x2014;) of the recycled carbon fibres treated with 1-propanol at 350 oC. The fibres appear to be very clean and almost resin-free (98.0 wt.% eliminated resin) [51].

mass transfer without damaging the fibres. SEM analyses (Figure 13) and tensile strength tests on the fibres post-treatment showed clean fibres retaining 85â&#x20AC;&#x201C;99 % of the strength of raw fibres. Preliminary GPC analyses assisted by NMR and FTIR spectroscopy were used to estimate the composition of the liquid degradation product stream, however a further investigation in this way is strongly recommended.

Conclusions Recycling of thermoset polymers is a main concern of modern world and a number of methodologies have been developed in this area. The chapter is dedicated to recycling characteristics of thermoset polymers, their blends and composites. Structure and properties of some of the common thermoset polymers are discussed in the first half of this chapter. These include epoxy resin, unsaturated polyester resins, phenolic resins, urea/melamineâ&#x20AC;&#x201C; formaldehyde reins, silicone polymers and polyimides. These polymers have a wide spectrum of applications from domestic to industrial levels. The structure of these polymers, their desirable properties and specific applications are briefed. This is followed by the details of their recycling characteristics. Different types of recycling methods such as incineration, thermolysis and chemical process are available. The properties of original

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polymer as well as recycled products are explained in many cases. Comparable properties are obtained in most cases, supporting recycling process. Recycling of epoxy-thermoplastic blends are explained by decomposition of the resin in organic solvent. Finally recycling of epoxy composites with glass and carbon fibers are reviewed. Thermoset-based products, in particular, thermoset composites and blends constitute a major source material for the production of automotives, aeronautics and electronic components. Increasing use of thermoset composites and blends poses the approaching need to enhance the recycling option. Due to the ability of ionizing radiation to alter the structure and properties of bulk polymeric materials, and the fact that it is applicable to essentially all polymer types, irradiation holds promise for impacting the polymer waste problem. It will be a future research problem to use radiation technology for the recycling of thermoset composites and blends. Also the concept of thermally self-healing polymeric material by Broekhuis and co-workers [52] will move the research on self-healing materials to a new stage, the recycling of thermoset-based plastics and composites.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

Odian, G. 2004, Principles of polymerization, Fourth edition, Wiley interscience, A John Wiley & Sons, Inc. Brydson, J.A., editor. 1999, Plastic materials. Seventh edition, ButterworthHeinemann, Oxford: Elsevier. Jones, J.L., Ochyuski, F.W., Rackley, F.A.1962, Chem. Ind. (London), 1686. Bower, G.M., and Frost, L.W. 1963, J. Polym. Sci., A, 1, 3135. Scheirs, J. Recycling of polymer composites. In Polymer Recycling, Wiley: New York, 1998; p 379-410. Pickering, S.J., Kelly, R.M., Kennerley, J.R., Rudd, C.D., and Fenwick, N.J. 2000, Composites Sci. Technol., 60, 509-23. Graham, W.D. 1995, Polym. Recycl., 1, 87-97. Petterson, J., Nilsson, P.J. 1994, J. Thermoplast. Compos. Mater, 7, 56. Bream, C.E., Hornby, P.R. 2001, J. Mater. Sci., 36, 2965-75. Reygrobellet, J.N. Recyclage de composites fibers de verrepolyester insaturecarbonate de calcium par reincorporation dans des matrixes thermoplastiques.2002, Ph.D Thesis, Montpellier University, France. Kouparitsas, C.E., Kartalis, C.N, Vareilidis, P.C, Tsenoglou, C.J, and Papaspyrides, C.D. 2002, Polym. Compos., 23, 682-9. Yamada, K., Mihashi, H.1995, Proceedings of the International RILEM Workshop; E. & F. N. Spon: London, p. 157-167. Paszun, D., and Spyjach, T. 1997, Ind. Eng. Chem. Res., 36, 1373-83. Karsa, D.R. 1996, The Proc. Of the Symposium on Chemical Aspects of Plastics Recycling, UMIST, Manchester, 199.

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15. Kishiro, O.1998, J. of Chemical society of Japan, 51, 1884-8. 16. Leaversuch, R.D. 1991, Modern Plastics, 68, 40. 17. Nishizaki, H., Sakakibara, M., Yoshida, K., and Endoh, K. 1977, Kagaku-Kaishi, N., 1899-1904. 18. Toselli, M., Impagnaticllo, M., Tramigioli, C., Pilati, F., Mazzoli, G., and Benuzzi, G.1996, Polymer recycling, 2, 1, 27. 19. Tsuda, K., Kubouchi, M., Nishiyama, T., Ono, S., and Hojo, H. 1995, Proc. 10th International Conference on Composite Materials, Vol.V1, Whistler, 215-22. 20. Kubouchi, M., Tsuda, K., Nishiyama, T., and Hojo, H.1995, Adv. Comp. Let., 4, 1, 13-5. 21. Kubouchi, M., Sembokuya, H., Yamamoto, S., Arai, K., and Tsuda, K. 2000 J. Soc. Mater. Sci, Japan, 49, 5, 488-93. 22. Dang, W., Yamaki, K., Sembokuya, H., Kubouchi, M., and Tsuda, K. 2001, Proc. Eco design Tokyo, 980-5. 23. Dang, W., Kubouchi, M., Yamamoto, S., Sembokuya, H., and Tsuda, K. 2002, Polymer, 43, 2953-8. 24. Dilafruz, K., Kubouchi, M., Dang, W., Sembokuya, H., and Tsuda, K. 2003, Proceedings of Ecodesign2003: Third International Symposium on Environmentally Conscious Design and Inverse Manufacturing Tokoyo, Japan, December 8-11. 25. Iji, M. 1998, J. Mat. Sci., 33, 45-53. 26. Vallee, M., Tersac, G., Destais-Orvoen, N., and Durand ,G. 2004, Ind. Eng. Chem. Res., 43, 6317-24 27. Hisashi, T., Yutaka, F., and Kazuaki, S. 2000, J. Network Polymer, Japan, 21, 4, 185-93. 28. Osaki,T. 1996, Kenkyu Hokoku-Fukuoha-kenkogyo Gijutsu Senta (Pub.1997), 7, 61-3. 29. Shaw, R.W., Bill, T.B., Clifford, A.A., Eckert, C.A., Franck, E.U. 1991, Chem. Eng. News Dec 23, 26. 30. Brunner, G.H., Kiran, E., and Sengers, J.M. 1994. In Kluwer, HL, editor. NATO Advanced Study Institute Series E273; Academic Publishers: Dordrecht, The Netherlands. 31. Haschets, C.W., Shine, A.D., and Secor, R.M. 1994, Ind. Eng. Chem. Res. 33, 1040. 32. Goto, J., Otori, T., Adschiri, T., and Arai, K. 1996, Chourinkai Suichuu deno Netsukoukasei Jushi no Bunkai. Nettowakuporima Kouentouronnkai Kouenyoushishuu, 29. 33. Tagaya, H., Suzuki, Y., Kadokawa, J., Karasu, M., and Chiba, K. 1997, Chem. Lett., 49. 34. Suzuki, U., Tagaya, H., Asou, T., Kadokawa, J., and Chiba, K. 1999, Ind. Eng. Chem. Res., 38, 1391. 35. Ozaki, J., Djaja, S.K.I., and Oya. A. 2000, Ind. Eng. Chem. Res., 39, 245-9. 36. Gusse, A.C., Miller, P.D., and Volk, T.J. 2006, Environ. Sci. Technol., 40, 4196-9. 37. Bliznakov, E.D., White, C.C., and Shaw, M.T. 2000, J. Appl. Polym. Sci., 77, 3220â&#x20AC;&#x201C;7.

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38. Fur, X.L., Galhac, M., Zanetti, M., Pizzi, A., and Werkst, H.R. 2004, Eur. J. Wood and Wood Products, 62, 419–23. 39. Wetzel, R.J., Caruso, J.A. 1998, US Patent Issued (5721280) on February 24. 40. Mormann, W., and Frank, P. 2006, Macromol. Symp. 242, 165–73. 41. Mimura, K., and Ito, H. 2003, J. Appl. Polym. Sci. 89, 527. 42. Ito, H., and Mimura, K., Mitsubishi Electric ADVANCE, R&D Progress report, December 2003. 43. Alter, H. 1993, Waste Management and Research, 11, 319-32. 44. Lee, B.K, Ellenbecker, M.J, and Moure-Eraso, R. 2002, Waste Management, 22, 461-70. 45. Arroyo, M., and Bell, M. 2002, J. Appl. Polym. Sci., 83, 2474-84 46. Lee, C.L., Chen. C.H., Huang. R., Wu, J.K. 2008, J. of the Chinese Institute of Engineers, 31, 6, 1-8. 47. Unser, J.F., and Staley, T. 1996, Proceedings of the 41st Int. SAMPE Symposium, Anaheim, CA, USA, 10–20. 48. Conroy, A., Halliwell, S., Reynolds, T. 2006, Compos., A, 37 (8) ,1216–22. 49. Baldwin, R.M., Manley, J.A. 1998, Fuel Process. Technol., 17, 201-7. 50. Kelley, S.S, Wang, X., Khan, M.N., Allred, R.E., Gosau, J.M. 2001, Int SAMPE Symp Exhibition (Proc), 46, 193. 51. Pinero-Hernanz, R., Garcıa-Serna, J., Dodds, C., Hyde, J., Poliakoff, M., Cocero, M.J., Kingman, S., Pickering, S., and Lester, E. 2008, J. of Supercritical Fluids, 46, 83–92. 52. Zhang, Y., Broekhuis, A.A., Picchioni, F. 2009, Macromolecules, 42, 1906-12.

Transworld Research Network 37/661 (2), Fort P.O. Trivandrum-695 023 Kerala, India

Recent Developments in Polymer Recycling, 2011: 155-185 ISBN: 978-81-7895-524-7 Editors: A. Fainleib and O.Grigoryeva

5. Recycling of oil and milk pouch polymers and its applications 1

Basudam Adhikari1 and Arup Choudhury2

Materials Science Centre, Indian Institute of Technology, Kharagpur 721302, India Department of Chemical & Polymer Engineering, Birla Institute of Technology, Mesra Ranchi 835215, India

Abstract. This work deals with the viability of the use of oil pouch (LDPE-LLDPE-nylon 6 and LDPE-LLDPE-PET blends) and recycled milk pouch (50:50 LDPE-LLDPE blend) materials and the scope for improvement of their properties by various techniques. The polymer compositions and thermal stability of the waste oil pouches were studied by various spectroscopic and thermal analyses, respectively. Since post-use oil pouches contain different types of physically inseparable immiscible polymers, the recycling of such waste pouches by melt-extrusion leads to poor quality product. Two efficient reactive compatibilizers, viz., zinc salt of ethylene methacrylic acid copolymer (Surlyn ionomer) and polyethylene grafted maleic anhydride (Fusabond) were used to improve the properties of the recycled oil pouch polymers. Both the thermal stability and mechanical properties of the recycled polymers obtained from these waste oil pouches were significantly improved by the addition of 5 wt.% of compatibilizers. Similarly, the properties of the recycled milk pouch polymer were not as satisfactory as those of the corresponding virgin material but, a significant improvement in viscosity, crystallinity, thermal stability and mechanical properties was achieved by blending with corresponding Correspondence/Reprint request: Dr. Basudam Adhikari, Materials Science Centre, Indian Institute of Technology, Kharagpur 721302, India. E-mail: ba@matsc.iitkgp.ernet.in

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virgin polymer and 0.4 wt.% stabilizer. The viability of the use of recycled milk pouch polymer as matrix in jute fiber reinforced composite for engineering application is also shown. The recycled polymer based jute fiber composites showed inferior mechanical properties compared to those observed for virgin polymer/jute fiber composites. However, the jute-composites made with (50:50) recycled milk pouch-virgin LDPE-LLDPE blend as polymer matrix indicated significantly superior properties in comparison to the recycled milk pouch/jute composites. Overall mechanical performances of the recycled and virgin polymeric composites were correlated by scanning electron microscopy (SEM). Effect of artificial weathering on mechanical properties of jute fiber composites was determined.

1. Introduction In flexible packaging of food articles, polyolefins are widely used polymers (as moisture resistant core layer component) along with nylon 6 or ethylene vinyl acetate (EVA) copolymer or poly (ethylene terephthalate) (PET) as barrier layer component [1-2]. In India, various edible oils and milk are packed in flexible multi-layered plastic pouches. Low-density polyethylene (LDPE) and octene-linear low-density polyethylene (o-LLDPE) are used as packaging polymers often with nylon-6 and PET as barrier layers. In practice, huge consumption of plastic materials in the edible oil and milk packaging has enhanced the volume of waste product in municipal garbage. Therefore, in order to avoid environmental nuisance or pollution, the disposal of such post consumer milk and oil pouches is necessary. Due to short lifetime (around 15-20 days) of such flexible pouches receiving mild handling and less degradation during their service, the properties of the recycled polymers would be very close to those of virgin polymers, provided the recycling is done in a proper way [3]. The thermo-mechanical and thermo-oxidative influences are the major agencies for properties decline of the recycled mass [4-6]. Such thermal degradations propagate through chain scission and cross linking reactions competitively and simultaneously depending upon the processing conditions (temperature, shearing rate and atmosphere), molecular structure of the polymer and type of catalysts used during their manufacturing [6, 7]. In this context, it must be mentioned that after first service life the residual stabilizer is not sufficient to provide necessary stability to the recycled polymer for second life application. Many authors have reported that the thermal stability of the recycled polymer could be successfully improved by the addition of optimum quantity of stabilizer [7-12]. Drake [13] and Drake et al [14] have utilized Irganox 1010, Irgafos 168 as a thermal stabilizer in order to improve the quality of the recycled materials obtained from post-used HDPE bottle crates and got satisfactory results. However, few reports on the successful improvement of the properties of recycled polyolefins by blending with virgin material have been reported [15, 16].

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Apart from thermal degradation, the compatibility problems have also been encountered during mechanical recycling of heterogeneous plastic wastes containing different types of immiscible polymers, e.g., polyolefins, polyester, polyamides etc. [17, 18]. In recent years, a possible approach to recycle heterogeneous plastic wastes into a value added secondary material is compatibilization [19-24] and moreover, most of the compatibilized polymer blends have been commercialized [24-26]. Although, numerous publications are available on the compatibilization of virgin polymer blends [27-31], only a few studies on compatibilization of the recycled polyethylene (PE)/PET and PE/nylon 6 blend systems have been reported. Pawlak et al [32] have pointed out the possibility of recycling post consumer HDPE and PET by using proper compatibilizers and have indicated the viability of using the compatibilized recycled blends for film extrusion, with potential applications in the packaging sector. Akkapeddi et al [33] and Xanthos et al [34] have observed that both ethylene-glycidyl-methacrylate (EGMA) and SEBS-g-MA performed as an effective compatibilizer to improve mechanical properties of the blends of waste polyolefins and PET obtained from American cities. La Mantia and Capizzi [35] have studied the recycling behavior of the compatibilized and non-compatibilized polypropylene (PP)/nylon 6 blends and found a significant improvement in mechanical properties for the compatibilized blends. A report by Humbeeck in this context is quite interesting that the impact strength of the non-compatibilized HDPE/nylon 6 obtained from scrap fuel tanks is found to be comparable to 70 % of that of the virgin HDPE [36]. Presently, recycled plastics are finding growing importance in producing composite products. Natural fibers are now well recognized to produce good reinforcing capability to polymeric composites. Hence, the addition of natural fibers, e.g., wood fibers, sisal fibers, jute fibers etc. to waste polymer could produce newly developed composite materials, which are viable from both the mechanical performance and the economic points of view. Many studies have explored the use of homogeneous polymer waste stream in order to produce fiber-reinforced composite materials [37-40]. Choudhury et al [41, 42] have effectively used Surlyn ionomer and Fusabond as compatibilizing agents in the recycling of LDPE-LLDPE-nylon 6 and LDPE-LLDPE-PET blends based post-use oil pouches. Rheological, thermal and mechanical properties were evaluated for recycled milk pouch films made of LDPE-LLDPE blend, before and after the addition of corresponding virgin resin blend and or small amount of anti-oxidant [43, 44]. In this work thermo-mechanically recycled post consumer milk pouch polymer was also used as matrix material in jute fiber composite for engineering applications [45].

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2. Recycling of PE/nylon 6 and PE/PET based waste oil pouches A huge quantity of post consumer oil pouches (Length: 250 mm; Width: 150 mm; Thickness: 110-120 mm; Color: yellow/white) is disposed to the municipal dustbin. In general, the product of recycled post consumer oil pouches is basically a commingled plastic because of its large diversity of composition in different pouch films of different manufacturers. In fact such flexible pouch films are multi-layered films and are made from blends of mainly polyolefins with either a co-extruded barrier layer or with a laminated barrier layer. Therefore, knowledge of polymer composition in each brand of oil pouch is necessary in order to evaluate the recycling process parameters.

2.1. Composition analysis of post use oil pouches The solvent precipitation method is a most successful way to separate out two physically inseparable polymers by fractionation with a suitable solvent. This process was followed for the separation of the minor barrier layer polymer as insoluble part from the major carrier polymer as soluble part present in oil pouch using xylene as solvent [46]. The unknown additives, if any, in the original pouch film were removed by xylene at this stage. Identification of polymer components as well as a trace of additives trapped within polymer was performed by FT-IR spectroscopy. It was observed that the IR-peaks obtained for the carrier layer polymer present in the waste oil pouches are very close to the corresponding peaks for virgin PE [47, 48] as listed in Table 1. A very weak band was observed at slightly higher region, i.e., at 1384 cm-1 (Table 1) corresponding to symmetric C-H bending of CH3 groups in the branched polyethylene such as LDPE and LLDPE [47]. This observation indicates that the polyethylene used as the carrier layer polymer in the waste oil pouch films is either LDPE or LLDPE or blend of LDPE/LLDPE. The characteristic bands observed at 3289.8 (N-H stretching), 1630 (amide-I C=O stretching), 1539 (amide-II NH/CN coupling) and 684 (amide-V) cm-1 in the IR spectrum of co-extruded barrier layer polymer are well corroborated by the appearance of similar peaks in virgin nylon 6 [49, 50] (Table 2), indicating that the co-extruded barrier layer polymer of the oil pouch film is made of nylon 6. Similarly, the bands (Table 3) obtained for the laminated barrier layer polymer of the post use oil pouches were found to be identical to those of virgin PET [51, 52]. The characteristic C=O stretching frequency (1749.3 cm-1) of ester group of the barrier polymer was observed to be very much close to that of the virgin PET film. Apart from polymeric components, small amounts of pigments, stabilizer, slip agent and labeling dye were also present in the waste

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Table 1. FTIR absorption bands of carrier layer polymer of waste oil pouches and virgin polyethylene films [Refs]. Band for carrier layer polymer (cm-1)

Band for virgin PE,

3349

2918.63

2919

C – H stretching CH2 group (sym)

2847

2851

Stretching of ester carbonyl

1740

N-H bending

1600

C – H bending of CH3 group

1384

1376(LDPE), 1378(o-LLDPE)

1366(HDPE), 1351

721.8

731-720

Band assignment

N-H stretching C – H stretching of CH2 group (asym)

CH2 wagging deformation CH2 rocking

(cm-1)

Table 2. FTIR absorption bands for co-extruded barrier layer polymer and virgin nylon 6 film [Refs]. Band assignment

Band for co-extruded Band for virgin nylon 6, barrier layer polymer

(cm-1)

(cm-1) N-H stretching (free from H-bond)

3445

3289.83

3290

CH2 asymmetric stretch

2920

2930

CH2 symmetric stretch

2852.92

2865

C=O stretching of Ester gr.

1749.45

Amide-I C=O stretch

1630

1637

Amide-II NH/CN coupling

1539

1540

Skeletal deformation involving CONH

1164.8

1170

C-C stretching

1121.09

1120

CH2 rocking, CONH in-plane

968.13

960-977

930

684.07

692-708

N-H stretching (H-bonded)

Crystalline band Amide-V (α)

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oil pouch films. The component polymers present in these waste oil pouch films were also identified by their solubility in specific solvents, wide angle X-Ray diffraction and thermal analyses, and the results supported in favor of polyethylene as the carrier layer polymer and nylon 6 and/or PET as the barrier layer polymer. The compositions of the post used oil pouch films obtained from both the solvent fractionation and the thermo gravimetric analysis (TGA) are included in Table 4. Table 3. FTIR absorption bands for laminated barrier layer polymer and virgin PET film [Refs]. Band assignment

Band for laminated Barrier layer polymer

Band for virgin PET (cm-1)

(cm-1) -OH stretching

3430.74

3420

Aromatic C – H stretching

3057.58

Asymmetric CH2 stretching

2918.91

2955

Symmetric CH2 stretching

2847.4

2897

Stretching of ester carbonyl

1749.3

1745-1750

Aromatic C = C stretching

1569

1575

1350.6

1350

C – O – Ar asymmetric stretching

1269

1265

Crystalline band (strong)

1122

1120

Trans conformer (glycol unit)

965.95

970

Rocking of aromatic C – H out of plane

839.49

845

Out of plane benzene ring C – H deformation

722.92

729

Crystalline band (weak)

Table 4. Composition of the polyethylene/nylon 6 and polyethylene/PET based post use oil pouch films [Refs]. Method

PE/nylon 6 based film PE nylon 6 Additives (wt %) (wt %) (wt %)

PE (wt %)

PE/PET based film PET Additives (wt %) (wt %)

Solvent fractionation

82.79

15.40

1.81

84.42

13.97

1.61

TGA

82.32

14.65

2.64*

80.45

13.52

2.10*

Residue (wt %) from TGA.

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2.2. Compatibilization of recycled blends obtained from waste oil pouches The scope for improvement in properties of the recycled polymer of LDPE/LLDPE/nylon 6 (co-extruded films) and LDPE/LLDPE/PET (laminated films) based post consumer oil pouches was evaluated by the addition of reactive compatibilizer [41, 42]. Two reactive compatibilizers, viz., zinc salt of ethylene methacrylic acid copolymer (Surlyn ionomer; Density: 0.94 g/cc; MFI: 1.5 g/10 min at 190 oC and load 2.16 kg; Melting temperature: 97 oC; Ion type: Zinc) and polyethylene grafted maleic anhydride (Fusabond; Density: 0.93 g/cc; MFI: 1.5 g/10 min at 190 oC and load 2.16 kg; Melting temperature: 120 oC) were used in this recycling process. The effectiveness of these compatibilizers was determined in the upgradation of morphology, thermal and mechanical properties of the recycled blend materials (LDPE/LLDPE/nylon 6 and LDPE/LLDPE/PET blends). The mechanical recycling of these waste oil pouch polymers involved following processing steps: washing/drying in the pre-treatment of post use oil pouches, melt extrusion of waste pouch films and blending of recycled polymer with compatibilizers. During the melt mixing of recycled polymer blends (LDPE/LLDPE/nylon 6 and LDPE/LLDPE/PET) with the compatibilizers (Fusabond or Surlyn ionomer), the possible chemical reactions between terminal groups of barrier layer polymer components [i.e., amine end groups (-NH2) of nylon 6 component and carboxyl (or hydroxyl) groups of PET component] and anhydride groups of Fusabond or carboxylate groups of Surlyn ionomer might take place. At the same time, a specific intermolecular interaction between polyethylene segments of the compatibilizers and those of the LDPE and LLDPE might also occur. The reaction between terminal groups of barrier layer polymer (nylon 6 or PET) and functional groups of corresponding compatibilizer during mixing was assessed by FTIR-analysis. As shown in Figure 1, two characteristic IR-bands in the frequency range of 1700-1500 cm-1, one at ~1613 cm-1 corresponding to C=O stretching vibration of the amide groups (amide I band) and another at ~1567 cm-1 corresponding to NH/C-N coupling of the same (amide II band) were observed for both the compatibilized and noncompatibilized recycled LDPE/LLDPE/nylon 6 blends. It was experimentally observed that peak intensity as well as width of the amide bands (at ~1613 and ~1567 cm-1) increased for both Fusabond and Surlyn ionomer compatibilized recycled blends (LDPE/LLDPE/nylon 6/Fusabond and LDPE/LLDPE/nylon 6/Surlyn ionomer) as compared to non-compatibilized blend. Similarly, a characteristic absorption band at ~1727 cm-1, corresponding to the C=O stretching vibration of carboxyl groups of PET was

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observed in the IR-spectra of compatibilized and non-compatibilized recycled LDPE/LLDPE/PET blends (Figure 2). The peak broadening and minor displacement towards lower frequencies of the PET carbonyl band in case of compatibilized blends compared to non-compatibilized blend indicated formation of strong hydrogen bonding between the carboxyl (or hydroxyl) end groups of the PET and functional groups of the compatibilizers. 0,275 Amide-I C=O stretching

Amide II NH/CN coupling

Absorbance

0,250 (c)

0,225

(b)

(a)

0,200

0,175 1700

1650

1600 1550 -1 Wavenumber, cm

1500

Figure 1. FT-IR spectra of (a) non-compatibilized, (b) Fusabond compatibilized and (c) Surlyn ionomer compatibilized LDPE/LLDPE/nylon 6 recycled blends [41].

Absorbance

0,30

0,35

0,40

0,45

0,50 1900

1850

1800

1750

1700

1650

1600

Wavenumber, cm-1

Figure 2. FT-IR spectra of (a) non-compatibilized, (b) Fusabond compatibilized and (c) Surlyn ionomer compatibilized LDPE/LLDPE/PET recycled blends [42].

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2.3. Morphological study of recycled blends before and after compatibilization The SEM photomicrographs of cryogenically fractured surfaces of the compatibilized and non-compatibilized recycled LDPE/LLDPE/nylon 6 blends are presented in Figure 3. For non-compatibilized recycled blends, the nylon 6 (white spots) particles are irregularly dispersed in the polyethylene matrix as shown in Figure 3a. This phase-separated morphology of noncompatibilized blends is attributed to the large difference between the chemical structure of PE and nylon 6 components. On the other hand, a significant improvement in the phase morphology was observed in the compatibilized recycled blends as shown in Figure 3b-c. In these compatibilized blends the size of the dispersed phases (nylon 6) was remarkably reduced and the dispersion of nylon 6 domains in the PE matrix became more homogeneous (Figure 3b-c). Hence, the compatibilizers (Fusabond or Surlyn ionomer) in the recycled blends enhanced the interfacial adhesion between two immiscible polymer components (i.e., PE and nylon 6). Similarly, the phase morphology of the recycled LDPE/LLDPE/PET blends was also remarkably improved by compatibilized blending as shown in Figure 4. The Surlyn ionomer has shown better compatibilizing effect compared to Fusabond due to strong polar interactions between the â&#x20AC;&#x201C;COOÂŻ groups of Surlyn ionomer and the functional groups of the dispersed phases (nylon 6 or PET).

Figure 3. SEM photomicrograph of the fractured surface of (a) non-compatibilized, (b) Fusabond compatibilized and (c) Surlyn ionomer compatibilized LDPE/LLDPE/nylon 6 recycled blends [41].

2.4. Effect of compatibilizers on melt flow behavior of recycled polymer The melt flow behavior of the non-compatibilized and compatibilized recycled blends was characterized by melt flow index (MFI) measurements. The MFI values of the recycled mass, viz., 3.79 and 5.44 g/10 min for LDPE/LLDPE/nylon 6 and LDPE/LLDPE/PET blends respectively were found to be decreased after blending with compatibilizer. The MFI values

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recorded for Surlyn compatibilized LDPE/LLDPE/nylon 6 and LDPE/LLDPE/PET blends are 2.1 and 3.7 g/10 min, whereas those obtained for Fusabond compatibilized LDPE/LLDPE/ nylon 6 and LDPE/LLDPE/PET blends are 2.4 and 3.84 g/10 min. This clearly indicates that the molecular weight of the recycled polymer is increased by compatibilized blending, since the MFI value is inversely proportional to the molecular weight of the polymer [53]. The lowering of MFI values for compatibilized recycled blends is due to the strong interfacial adhesion between the immiscible polymer components [54] as indicated by SEM photomicrographs (Figures 3 and 4).

Figure 4. SEM photomicrograph of the fractured surface of (a) non-compatibilized, (b) Fusabond compatibilized and (c) Surlyn ionomer compatibilized LDPE/LLDPE/PET recycled blends [42].

2.5. Thermal properties of compatibilized and non-compatibilized recycled oil pouch polymers Thermal properties, viz., glass transition (Tg), melting transition (Tm), degradation temperature (Td) and enthalpy values (Î&#x201D;Hf and Î&#x201D;Hd) of compatibilized and non-compatibilized recycled oil pouch polymers were evaluated by differential scanning calorimetry (DSC) in nitrogen and air [41, 42]. Various thermal response parameters obtained from DSC analyses of these recycled polymers are shown in Tables 5 and 6. The DSC thermograms in nitrogen atmosphere exhibited three distinct melting peaks, two of them in a lower temperature region (108-128 oC), corresponding to crystalline melting (Tm) of the LDPE and LLDPE, and one in a higher temperature region (219-224 or 250-255 oC), corresponding to the crystalline melting (Tm) of nylon 6 or PET. It can also be seen from the data in Table 5 that the melting temperature of the polyethylene components increased significantly by the addition of compatibilizer to the recycled blends, whereas that of nylon 6 or PET component decreased. Interaction between the component polymers in the recycled mass and the compatibilizer might be the reason for such a shift of Tm values [55]. The Tg of nylon 6 and PET components in the compatibilized recycled blends shifted toward lower temperatures relative to the Tg of non-compatibilized blends (Tables 5 and 6).

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Table 5. DSC analysis of non-compatibilized and compatibilized LDPE/LLDPE/nylon 6 and LDPE/LLDPE/PET blend based recycled oil pouches in nitrogen atmosphere [Refs].

Table 6. DSC analysis of non-compatibilized and compatibilized LDPE/LLDPE/nylon 6 and LDPE/LLDPE/PET blend based recycled oil pouches in air [Refs].

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This is due to the fact that the dispersed phase (nylon 6 or PET) get dissolved in the PE matrix in presence of compatibilizer, i.e., the miscibility of the two polymer components is enhanced in the compatibilized blend systems. The percent crystallinity (%, Xcr) of the dispersed components (nylon 6 and PET) is decreased by the addition of compatibilizer to the recycled blends, whereas the % Xcr value of the PE matrix is increased (Table 5). The decrease in crystallinity of the nylon 6 and PET components might be due to strong polar interactions between the dispersed phase and the compatibilizers. On the other hand, compatibilizers accelerate the crystallization of the polyethylene component and they might have been acting as nucleating agents [56]. The DSC measurement in air atmosphere showed a broad endotherm containing two distinct melting peaks for both the non-compatibilized and compatibilized blends [41, 42]. Two melting peaks were observed at ~108 oC and ~123 oC, corresponding to crystalline melting temperatures of LDPE and LLDPE respectively (Table 6) [41, 42]. The melting endotherms of the nylon 6 and PET components were not observed in this heating scan, and instead a broad exothermic peak corresponding to the first stage thermo-oxidative degradation of the polyethylene component was found in the same temperature region. The second stage of degradation (exotherm), which started above 360 oC, might be associated with the formation of a large volume of gaseous products. It can be seen that the onset and peak temperatures of the thermal degradation were higher in the case of compatibilized recycled blends than non-compatibilized blends. This clearly indicates that the thermo-oxidative stability of the recycled polymer was increased by the addition of Fusabond or Surlyn ionomer as compatibilizer, where Surlyn ionomer showed a better effect. On the other hand, DSC measurements in nitrogen atmosphere showed that the thermal degradation (pyrolysis) of the recycled polymers occurred at a higher temperature, i.e., 450-480 oC (Table 5). Since the thermal stability of the recycled polymers in air was lower than in nitrogen, the above observation provides a guideline about the thermal processing of post consumer oil pouches in air.

2.6. Mechanical properties of compatibilized and non-compatibilized recycled polymers Mechanical properties of a polymer blend significantly depend on the degree of compatibility or interfacial adhesion between the polymer components [57, 58], and this phenomenon is well observed in the case of recycled blends obtained from waste oil pouches [41, 42]. The mechanical properties of the recycled LDPE/LLDPE/nylon 6 and LDPE/LLDPE/PET blends, with or without compatibilizer, are shown in Figure 5. As shown in

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Figure 5, the tensile strength, yield strength, tensile modulus and percent elongation at break of compatibilized recycled blends were all higher than those of non-compatibilized recycled blends. The influence of Surlyn ionomer on the tensile properties of the recycled blends was stronger than that of Fusabond, which could be attributed to the higher crystallinity of the Surlyn compatibilized blends [59, 60] as observed in DSC analysis. Tensile modulus value of the recycled blends are increased by 20 % in presence of Surlyn ionomer and 14 % in presence of Fusabond as compared to noncompatibilized polymer (Figure 5d). In contrast to Fusabond, Surlyn ionomer is more efficient and promising compatibilizing agent to improve the mechanical properties of this recycled blend systems to a satisfactory level. Impact strength and hardness were also increased by compatibilization (Figure 5e and 5f) [41, 42]. LDPE/LLDPE/nylon 6 blend LDPE/LLDPE/PET blend

(a)

Yield stress, MPa

Tensile strength, MPa

(b)

LDPE/LLDPE/nylon 6 blend LDPE/LLDPE/PET blend

20 15 10

8 4

5 Fusabond 2 compatibiliser

Number of extrusion cycles

LDPE/LLDPE/nylon 6 blend LDPE/LLDPE/PET blend

(c)

Surlyn 3 compatibiliser

750

450

Without 1 compatibiliser (d)

375

Fusabond 2 compatibiliser

Number of extrusion cycles

Surlyn 3 compatibiliser

LDPE/LLDPE/nylon 6 blend LDPE/LLDPE/PET blend

300

500

225 150

250

Impact strength, J/m

0 35

Without1 compatibiliser

Fusabond 2 compatibiliser

Number of extrusion cycles

LDPE/LLDPE/nylon 6 blend LDPE/LLDPE/PET blend

(e)

30 25 20 15 10

Without1 compatibiliser (f)

Fusabond 2 compatibiliser

Number of extrusion cycles

Surlyn 3 compatibiliser

LDPE/LLDPE/nylon 6 blend LDPE/LLDPE/PET blend

50 40 30 20 10

5 0

75 0

Surlyn 3 compatibiliser

Hardness (Shore D)

Percent elongation

1000

Without 1 compatibiliser

Tensile modulus, MPa

0 Without 1 compatibiliser

Fusabond 2 compatibiliser

Surlyn 3 compatibiliser

Without 1 compatibiliser

Fusabond 2 compatibiliser

Number of extrusion cycles

Surlyn 3 compatibiliser

Figure 5. Mechanical properties of the compatibilized and non-compatibilized recycled oil pouch polymers [41, 42].

3. Recycling of waste milk pouches Like oil pouch waste a huge quantity of post-use milk pouch waste is also being created everyday because of easy packaging, storage, handling and

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distribution of milk. Recycling of such milk pouch polymer wastes can be a solution to waste disposal management. Choudhury et al [43, 44] has carried out some investigation on the recycling of milk pouch polymers. Disposed milk pouches (having MFI=0.758 gm/10 min at 190 oC with 2.16 kg load) were collected from the municipal garbage. These milk pouches were manufactured by Jharkhand State Co-operative Milk Producers Federation Ltd., India for liquid milk packaging using a 50/50 blend of low-density polyethylene (LDPE) and linear low-density polyethylene (LLDPE) obtained from Indian Petrochemical Corporation Ltd.

3.1. Thermal stability and degradation of the post use reclaim milk pouches during multiple extrusion cycles 3.1.1. Multiple extrusions of waste milk pouches The recycled milk pouch material is supposed to be subjected to subsequent processing environments, viz., extrusion, injection molding etc. As the inherent properties of polymer materials, particularly polyethylene, are adversely affected by temperature, shear and oxygen, they could easily undergo numerous thermooxidative and thermomechanical degradations during the reprocessing treatment. Therefore, it is necessary to study the effects of post processing environments on the structural degradation and the subsequent properties. Accordingly the single extruded recycled milk pouch pellets were subjected to multiple extrusions in both stabilized and unstabilized conditions [44]. The extrusion process was carried out in a HAAKE (Rheocord 9000) single-screw extruder in ambient atmosphere. The processing conditions are given in Table 7. Two batches of the materials were prepared; one containing 0.4% (w/w) antioxidant, Irganox B 225 (obtained from Ciba Specialty Chemicals (India) Limited) and another was prepared without antioxidant. For each batch, five consecutive extrusion cycles were performed. The average residence time of each extrusion cycle was 63 s. Table 7. Conditions used in single screw extrusion [Refs]. Temperature (oC)

Process

Extrusion

Screw speed, rpm

Zone 1

Zone 2

Zone 3

Zone 4

Water bath

210

220

230

240

30-35

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3.1.2. Effect of stabilization on rheological properties and melt flow behaviour of multiple-extruded recycled polymer The evaluation of rheological properties of recycled milk pouch material is necessary to assess its reprocessing parameters. The flow curves of the unstabilized and stabilized recycled milk pouches are shown in Figure 6. The curves show that the viscosity of the unstabilized recycled polymer decreases more rapidly than that of the stabilized polymer at higher frequency (shear rate) region [44]. It indicates that the melt strength of the un-stabilized recycled LDPE-LLDPE blend is inferior to that of the stabilized one. For both the unstabilized and stabilized recycled milk pouch polymer, the zero-shear viscosity (complex viscosity at Îł=0.012 rad/s) and MFI values are found to be changed in a reverse manner as a function of number of extrusion cycles as shown in Table 8 [44]. The results shown above, i.e., the decrease in melt flow index and the increase in zero shear viscosity from the beginning of the reprocessing cycles (i.e., from 1st to 3rd extrusion cycle) indicates that thermo-mechanical degradation of the un-stabilized recycled blend is initiated with simultaneous cross-linking reactions leading to the enhancement of molecular weight of the polymer [61]. On the other hand, the increase in MFI values and the decrease in zero shear viscosity at subsequent 4th to 5th extrusion cycles could be attributed to an extensive shearing force at high temperature in presence of atmospheric oxygen, which might lead to intensify the thermo-mechanical chain scission. However, MFI of stabilized recycled material increased from 1st to 5th extrusion cycles, which is an indication of molecular weight decrease of the polymer by chain scission only in presence of antioxidant [44]. Antioxidant prevents crosslinking by coupling with fragmented polymer radicals [44]. Complex viscosity, Pa s

100000

1 Ex (unstabulized) nd 2 Ex (unstabulized) rd 3 Ex (unstabulized) th 4 Ex (unstabulized) th 5 Ex (unstabulized)

10000

1 Ex (stabilized) nd 2 Ex (stabilized) rd 3 Ex (stabilized) th 4 Ex (stabilized) th 5 Ex (stabilized)

1000

100 0,1

100

Frequency, red/s

Figure 6. Flow curves of the unstabilized (solid symbols) and stabilized (open symbols) recycled milk pouches at 220 oC [44].

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Table 8. Zero shear viscosity and melt flow index for un-stabilized and stabilized recycled milk pouches after multiple extrusion cycles [Refs]. No Zero shear viscosity, kPa s Ex Un-stabilized material Stabilized material 1st 2nd 3rd 4th 5th

8.55 9.13 9.33 8.35 7.70

Melt flow index, g/10 min Un-stabilized material

Stabilized material

0.656 0.606 0.582 0.639 0.650

0.770 0.777 0.780 0.795 0.810

8.02 7.81 7.78 7.70 7.60

3.1.3. Determination of gel content of multi-extruded recycled polymer before and after stabilization The degree of cross-linking in both stabilized and unstabilized recycled polymer was determined by measuring the amount of gel formed during reprocessing [44]. The increase of gel content as a function of processing cycles, illustrated in Figure 7, again shows a dominating tendency of the un-stabilized recycled LDPE-LLDPE blend to form cross linked structure in the first three extrusion cycles, whereas the chain scission reaction is found to be dominated in the last two cycles (4th and 5th). However, the addition of 0.4% antioxidant to the recycled material before extrusion has reduced the gel content throughout the whole range of processing cycles (Figure 7).

Gel content, g/100g

Un-stabilized material Stabiized material

14 12 10 8 6 4 2 0 1

Number of extrusion cycles

Figure 7. Variation of gel content as function of number of extrusion cycles for the un-stabilized and stabilized recycled milk pouches [44].

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3.1.4. Thermal analysis of the stabilized and unstabilized multi-extruded recycled polymer The influence of thermomechanical treatment during multiple extrusions of recycled milk pouch polymers (LDPE/LLDPE blend) on their thermal stability was assessed by DSC analysis [44]. The transition temperatures of all samples and their degradation enthalpy values evaluated by DSC analysis in N2 atmosphere are shown in Table 9 and those in air atmosphere are shown in Table 10. In the DSC heating scan, two melting peaks are found for both stabilized and unstabilized recycled polymers. This observation indicates that even after multiple extrusions crystalline phases of both LDPE and LLDPE in the blend remain separated, because LDPE and LLDPE are practically immiscible in crystalline phase [62]. The crystallinity (% Xcr) of both unstabilized and stabilized recycled polymers is found to decrease with increasing the number of extrusion cycles as presented in Tables 9 and 10. The lowering of crystallinity could be attributed Table 9. DSC analysis in nitrogen atmosphere for unstabilized and stabilized recycled milk pouches after multiple extrusion cycles [Refs].

Table 10. DSC analysis in air atmosphere for unstabilized and stabilized recycled milk pouches after multiple extrusion cycles [Refs].

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to the creation of structural irregularity by formation of short branches on polymer backbone chain and groups, viz., hydroxide, hydroperoxide, carbonyl, aliphatic vinyl etc., which reduce the close packing ability of the polymer chains [63]. However, the degree of crystallinity of the recycled LDPE-LLDPE blend was improved by stabilization with antioxidant. An exothermic peak occurred for all samples due to thermooxidative degradation of the polymer when the DSC analysis was run in air. Both the onset and peak temperatures of the oxidation exotherm are found to be remarkably higher in case of the stabilized recycled material in comparison to unstabilized material (Table 10). The above results indicated that the thermal stability of the recycled polymer could be significantly improved by proper stabilization.

3.1.5. Effect of stabilization on mechanical properties of multiextruded recycled polymer In general, thermo-mechanical degradation of recycled polymer during thermal reprocessing leads to poor mechanical properties. However, addition of antioxidant before reprocessing could restrict the thermal degradation of recycled polymer and thus limited the deterioration of mechanical properties [44]. The variation of tensile strength, percent elongation at break, tensile modulus and hardness as a function of number of extrusion cycles is demonstrated in Table 11. The tensile strength of the recycled polymers gradually decreases with increasing number of extrusion cycles, although the decrease is more pronounced for unstabilized polymer compared to stabilized one (Table 11). The percent elongation at break of the unstabilized recycled material exhibited a decreasing trend from 1st to 3rd cycle followed by increasing through 4th and 5th cycle (Table 11). The decrease in percent elongation at break at the initial stage of extrusion cycles (1st to 3rd) is due to a crosslinking reaction [60], whereas the subsequent enhancement (in 4th and 5th cycle) may be due to an extensive chain scission reaction developing free volume between polymer chains allowing the chains to move freely. The Table 11. Mechanical properties of unstabilized and stabilized recycled milk pouch polymer after multiple extrusion cycles [Refs].

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tensile modulus initially rises up to third extrusion step followed by reduction in the last two cycles (4th and 5th cycle) as shown in Table 11. This observation indicates that during the initial stage of multiple reprocessing operation the material nature becomes stiffer and harder, which might be due to cross-linking effect [64] causing the polymer chains pulling closer, leaving less free volume that restricts their mobility [65] and hence produces more stiffer and harder material.

3.2. Blending of recycled milk pouch polymer with virgin LDPELLDPE blend 3.2.1. Blending of recycled polymer and virgin LDPE-LLDPE blend Blending with virgin resin is a well-known potential route to improve the quality of the recycled material. Various formulations (w/w) using recycled milk pouch polymer and virgin LDPE-LLDPE blend, presented in Table 12, are considered [43]. A HAAKE (Rheocord 9000) single-screw extruder was used to prepare the blends under same processing condition as employed in the above processing steps (Table 7). Table 12. Formulations recycled (R)/virgin (V) LDPE-LLDPE blends [Refs].

3.2.2. Rheological properties and melt flow behaviour of recycledvirgin polymer blends The flow curves of the recycled milk pouches and virgin LDPE-LLDPE blends and those of mixtures of the two are shown in Figure 8 [43]. The slopes of the flow curves for each of the blend samples were different, as shown in Figure 8. This indicates that the polymeric structure of the recycled and virgin blend components is not identical to their pure states, and consequently a slight immiscibility might have developed between the polymer components in their blends. Moreover, the nature of the slopes of the flow curves indicates that the recycled blends had a broader molecular weight distribution than that of the virgin material, which is probably due to

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thermo-mechanical degradation. The broader molecular size distribution indicates a more pseudoplastic nature of the recycled blends in the melting phase [65]. The zero-shear viscosity (η0) of the 100 % virgin LDPE-LLDPE blend (RV 100) was significantly higher than that of the 100 % recycled blend (RV 0), whereas the η0 of the formulated blends followed a decreasing trend as the weight percent of the recycled material (RV 0) is increased (Table 13) [43]. The MFI of the recycled polymer (RV 0) was higher than that of the virgin LDPE-LLDPE blend (RV 100), whereas the MFI of the blends of intermediate compositions (i.e., RV 20 to RV 80) sharply decreased with increasing weight percent of virgin material throughout the composition range studied. This observation is in good agreement with the zero shear viscosity data (Table 13). As there is an inverse relation between the MFI of a polymer and its molecular weight [66], the higher MFI value of the 100 % recycled milk pouch is attributed to its having lower molecular weight. Since waste milk pouch films are exposed to the service environment and to more processing cycles during recycling, they cannot avoid degradation at molecular level. 10000

Viscosity (η), Pa s

RV 100 RV 80 RV 60 RV 40 RV 20 RV 0

1000

100 0,1

Shear rate (γ), sec-1

Figure 8. Flow curves of the blends of recycled and virgin LDPE-LLDPE blends [43]. Table 13. Zero shear viscosity and melt flow index of the recycled milk pouch-virgin LDPE-LLDPE blends [Refs].

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3.2.3. Thermal properties of recycled/virgin polymer blends [43] DSC thermograms for each of the blend samples showed a single broad endotherm, consisting of two well-defined crystalline melting peaks at ~110 oC and ~122 oC. The existence of two distinct melting points indicates that even after multiple extrusions crystalline phases of both LDPE and LLDPE in the blend remained immiscible [43]. The lack of co-crystallization between LDPE and LLDPE is perhaps due to a high degree of branching in LDPE [62]. The results of DSC analysis of different formulated blends are presented in Table 14 [43]. As shown in Table 14, the crystallinity was at a maximum for the 100 % virgin LDPE-LLDPE blend (RV 100) and at a minimum for 100 % recycled LDPE-LLDPE blend (RV 0), whereas the crystallinity of the recycled-virgin LDPE-LLDPE blends was gradually enhanced with increasing wt.% of the virgin LDPE-LLDPE blend. The increase in crystallinity of the recycled polymer achieved by blending it with virgin LDPE-LLDPE was also well reflected in the mechanical properties [43]. The thermal stability of the recycled polymer in a N2 atmosphere was inferior to that of the corresponding virgin polymer, which is reflected in the degradation (pyrolysis) temperature (Td) and enthalpy (Î&#x201D;Hd) obtained from DSC analysis (Table 14). However, the thermal stability of the recycled material was significantly improved by blending with virgin polymer. Table 14. Crystalline melting temperature (Tm), percent crystallinity (% Ď&#x2021;cr), degradation temperature (Td) and enthalpy of degradation of the formulated blend samples [Refs].

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3.2.4. Modification of mechanical properties of recycled polymer by blending with virgin polymer The mechanical properties of the virgin and recycled LDPE-LLDPE blends are shown in Table 15 [43]. Table 15 shows that up to ~30 % increase in tensile strength and ~40 % increase in elongation at break can be achieved by addition of 60 wt.% virgin LDPE-LLDPE blend to the recycled material (RV 60). The significant enhancement of tensile strength and elongation at break could be attributed to an increase in the crystallinity as a result of inclusion of more crystalline virgin blend [63]. The tensile modulus of the recycled LDPE-LLDPE blend was lower than that of the virgin polymer (Table 15). This behavior could perhaps be related to polymer chain scission as a result of thermo-oxidative degradation of the polyethylene [63] during recycling. Like tensile modulus, the hardness (Table 15) of the recycled material was also lower than that of the corresponding virgin material, and it again increased with incorporation of virgin LDPE-LLDPE blend. Table 15. Mechanical properties of the formulated recycled-virgin LDPE-LLDPE blends [Refs].

4. Recycled milk pouch and virgin LDPE-LLDPE based jute fiber composites The viability of the thermomechanical recycling of post use milk pouches (50:50 LDPE-LLDPE blend) and their use as polymeric matrices for jutefiber-reinforced composites was investigated [45]. The mechanical, thermal and morphological properties of the jute-fiber-reinforced recycled polymer composites were compared with those of virgin polymer/fiber composites with or without the use of maleic anhydride grafted polyethylene (MA-g-PE) as a coupling agent. The effects of accelerated weathering on the mechanical properties of the composites containing recycled and virgin polymeric matrices were studied.

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4.1. Preparation of composites Composites were prepared by melt mixing of polymers, i.e., recycled LDPE-LLDPE blend, virgin LDPE-LLDPE blend, and 50:50 recycled/virgin LDPE-LLDPE blend with the coupling agent-treated short jute fiber (length: 6 mm, diameter: 30 Îźm) in a HAAKE Rheocord-9000 at 130 oC, using roller blades and mixing chamber of 60 cm3 volumetric capacity [45]. The process was carried out for a period of 10 min at an optimum speed of 50-60 rpm, depending on the quantity of jute fiber. Each batch contained various wt.% of jute (10, 20, and 30 %). Each composite mixture was then homogenized in a two-roll mill (150E-400, Collins, Germany) at 130 oC and compressionmolded with a Delta Malikson 100TY pressman (India), at 100 oC, to produce composite sheets (3Âą0.2 mm thick).

4.2. Mechanical properties of recycled and virgin polymer based jute fiber composites The variation of mechanical properties as a function of fiber loading for each recycled and virgin polymer/jute fiber composites is presented in Table 16 [45]. The mechanical properties of the virgin LDPE-LLDPE/fiber composites were superior to those of the recycled milk pouch/fiber composites. The reason is related to the prehistory of thermo-mechanical degradation of the recycled polymer occurred during recycling operations. The thermomechanical degradation creates structural disorder in the recycled polymer, which results in poor melt strength and inferior processability of the recycled polymer compared to virgin polymer. The lack of suitable induced flow of the melted recycled material during composite fabrication, which requires optimum viscosity of the mass, limits the homogeneous mixing between recycled matrix and jute fiber, and consequently deteriorates the mechanical properties of the composites. However, composites made with 50:50 recycled-virgin LDPE-LLDPE blend as polymer matrix exhibited significantly better mechanical properties compared to recycled LDPELLDPE/fiber composites. It indicates that the 50:50 blending of recycled polymer with virgin LDPE-LLDPE has improved the melt flow properties of the recycled-virgin blend, which is reflected in the enhancement of mechanical properties of the recycled LDPE-LLDPE/fiber composites. As shown in Table 16, the tensile strength of virgin LDPE-LLDPE/jute fiber composites increased with increasing fiber content from 10 to 30 %, whereas for recycled LDPE-LLDPE/jute fiber and recycled-virgin LDPELLDPE/jute fiber composites, tensile strength linearly increased from 10 to 20 % fiber loading followed by a decrease at 30 % fiber loading. The results

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indicate that the fiber loading capacity of recycled polymer is inferior compared to that of virgin polymer at high fiber content (>20 % fiber loading), which is attributed to poor interfacial adhesion between fiber and recycled polymer. The structural irregularity due to thermo-mechanical degradation (cross-linking reaction) of recycled polymer might create disordered fiber distribution and orientation in the matrix or clumping of fibers during mixing process, resulting weak fiber-matrix bonding [67]. The percent elongation at break of the composites is sharply decreased with incorporation of fiber, indicating a ductility fall with fiber reinforcement (Table 16). Tensile modulus of the composites is increased with increasing fiber content (from 10 to 30 %), as shown in Table 16. The recorded modulus is significantly higher for virgin LDPE-LLDPE/jute fiber composite compared to that of recycled milk pouch/jute fiber composites. The reason is ascribed to poor recycled polymerfiber adhesion. The addition of 50 % virgin LLDPE-LLDPE blend to recycled polymer significantly improved the fiber-matrix interfacial adhesion, which is reflected in enhancement in tensile modulus for recycled-virgin LDPE-LLDPE blend/jute fiber composites. As observed in Table 16, the impact strength of virgin polymer/fiber composites is recorded higher than that of the recycled polymer/fiber composites. The results indicate that the virgin polymer composites have better energy absorbing capacity compared to that of recycled polymer composites. This might be due to a lack of interfacial adhesion between recycled polymer and fibers. The impact strength of virgin LDPELLDPE/jute fiber composites increased with increasing the fiber content from 10 to 30 %, while the recycled LDPE-LLDPE/jute fiber composites exhibited that the impact strength initially increased with the incorporation of jute fiber Table 16. Mechanical properties of recycled LDPE-LLDPE, virgin LDPE-LLDPE and virgin-recycled LDPE-LLDPE blends based jute fiber composites [Refs].

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up to 20% loading and subsequently decreased at 30% fiber loading (Table 16). Both toughness and hardness of the recycled LDPE-LLDPE/jute fiber composites are observed to be inferior compared to virgin polymer/jute fiber composites (Table 16).

4.3. Effects of artificial weathering on mechanical properties of the composites The mechanical properties of the recycled and virgin polymer/jute fiber composites after artificial weathering are presented in Table 17 [45]. As observed in the Table 16 and 17, tensile strength of the irradiated composites is gradually decreased with increasing fiber loading, and the values are recorded lower than those observed for non-irradiated composites. Like nonirradiated composites, the percent elongation followed the same decline trend with increasing fiber loading for irradiated composites (Table 17). The reason for this is attributed to photooxidation reactions on the compositeâ&#x20AC;&#x2122;s surface upon UV exposure, resulting in debonding of ester-linkages (sensitive group to photo-cleavage) at the fiber-matrix interface, which promoted microcrack formation and poor stress transmission at the interface. When the photodegraded composite sample is subjected to the mechanical stress, the degraded weak sites act as stress concentrator and crack nuclei. The photooxidation has occurred faster in post-use milk pouch polymer under UV exposure than that in virgin polymer. This resulted a decrease in tensile strength and elongation Table 17. Mechanical properties of recycled LDPE-LLDPE, virgin LDPE-LLDPE and virgin-recycled LDPE-LLDPE blends based jute fiber composites after accelerated weathering [Refs].

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for irradiated recycled polymer/fiber composites. It can be seen that the tensile modulus is remarkably enhanced for irradiated composite materials compared to non-irradiated composites, which may be attributed to crossliking reaction between polymer chain free radicals generated in the matrix phase after irradiation [64]. Accelerated weathering significantly increases both the impact strength and toughness of recycled LDPE-LLDPE/jute fiber composites compared to non-irradiated composites (Table 16 and 17).

4.4. Morphological studies of cryogenically fractured surfaces of composites The mechanical properties of composites could be corroborated with the morphological evidences. The SEM photomicrographs of the cryogenically fractured surfaces of virgin LDPE-LLDPE/jute fiber, recycled LDPELLDPE/jute fiber and virgin-recycled LDPE-LLDPE blend/jute fiber composites are displayed in Figure 9 [45]. The SEM photomicrographs observed in Figure 9a and 9b clearly indicate a significant difference in the interfacial characteristics between virgin LDPE-LLDPE/jute fiber and recycled LDPE-LLDPE/jute fiber composites. In case of recycled LDPELLDPE/jute fiber composites, the large number of fibers appeared to be free

Figure 9. SEM photomicrographs of fracture surfaces of 70 % virgin LDPE-LLDPE blend/30% fiber (a), 70 % recycled LDPE-LLDPE blend/30% fiber (b, d and e), and 70 % recycled-virgin LDPE-LLDPE blend (50:50)/30% fiber (c) composites [45].

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from the polymer matrix and a large sizes of holes resulted due to fiber pull out (Figure 9b). The existence of fiber breakage and fiber agglomerates (clusters) within the recycled matrix is also observed (Figure 9d and e). On the other hand, for virgin LDPE-LLDPE/jute fiber composites, considerably less number of fiber pullout and fiber clustering are observed (Figure 9a). It clearly indicates that the adhesion between jute fiber and virgin polymer is better than that between recycled polymer and jute fiber. A significant improvement in the surface morphology (reduction of fiber pullouts) is observed for composites made with virgin-recycled LDPE-LLDPE blend (Figure 9c) as polymer matrix. Hence, in recycled LDPE-LLDPE/jute fiber composites, the structural irregularity (lack of orientation) of recycled polymer prevents proper fiber-matrix dispersion and weakening of fibermatrix adhesion.

4.5. Thermal properties of recycled and virgin polymer/jute fiber composites In order to assess the effect of jute fiber reinforcement on thermal stability of the recycled and virgin polymer, the thermogravimetric analysis (TGA) was also carried out in nitrogen atmosphere [45]. The results of this analysis are presented in Tables 18. The recycled and virgin polymers exhibit one step thermal degradation (230-550 oC) corresponding to weight loss of polymer, whereas the composite samples show two-step thermal degradation, 1st step degradation corresponding to weight loss of jute fibers (190-390 oC) and the 2nd step degradation (390-510 oC) corresponding to weight loss of polymer matrix. The 1st step degradation of jute fiber might be associated with the dehydration of cellulose unit and thermal cleavage of glycosidic linkage by trans-glycosylation and scission of C-O and C-C bonds. As shown in Table 18, the thermal degradation of the recycled and virgin polymer started at 230 and 265 oC respectively, whereas degradation of polymer matrix in jute fiber composites (2nd step degradation) started between 380395 oC. It is evident that the thermal stability of both recycled and virgin polymer significantly increased by jute fiber reinforcement. For recycled LDPE-LLDPE/jute fiber composites, the 1st step degradation was detected at lower temperature with maximum weight loss compared to those observed for virgin LDPE-LLDPE/jute fiber composites (Table 18). This indicates that the weak fiber-matrix bonding decreases the thermal stability of the recycled LDPE-LLDPE/fiber composites compared to that of virgin LDPELLDPE/fiber composites. However, the thermal stability was significantly improved for recycled/virgin LDPE-LLDPE (50:50) blend/jute fiber composites compared to that of recycled LDPE-LLDPE/jute fiber composites.

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Table 18. Results obtained from thermo-gravimetric analysis (TGA) of recycled and virgin polymer blends and their jute fiber composites [Refs].

Conclusions On the basis of polymeric compositions, there are two types of post use flexible oil pouches available in India. One is composed of LDPE/LLDPE carrier layer (~83 wt.%) with nylon 6 barrier layer (~15 wt.%) and the other is made by LDPE-LLDPE carrier layer (~84 wt.%) with PET barrier layer (~14 wt.%). Recycling of post use multi-layered oil pouches through compatibilization process leads to recycled polymer having better mechanical properties and thermal stability compared to non-compatibilized recycled polymer. This approach could avoid the most difficult and expensive task of separation of polymer components during their recycling and subsequently solve the waste disposal problem. The overall performance of compatibilized recycled polymer, obtained from post use oil pouches, may find suitable applications like shelter films, TV back covers, plastic lumber etc. Recycling of post use milk pouches (50:50 LDPE-LLDPE blend) exhibited appreciable extent of thermo-mechanical degradation, which causes the deterioration of mechanical properties of the recycled polymer. During the multiple extrusions of waste milk pouches, the recycled polymer exhibited greater tendency to undergo cross-liking reaction, i.e., molecular rearrangement over chain scission reaction at the initial stage of extrusion cycles (1st to 3rd cycle), whereas the chain scission reaction is dominated in the 4th and 5th cycles. Restabilization of recycled milk pouch polymer by addition of 0.4 wt.% antioxidant could significantly restrict the thermomechanical degradation of recycled polymer and thus limit the deterioration of mechanical properties. The mechanical, rheological and thermal properties

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of post use milk pouch polymer could be significantly upgraded by blending with virgin polymer. As discussed earlier, mixing of more than 60 % virgin LDPE-LLDPE blend with recycled polymer becomes suitable for film applications. The recycled polymer obtained from waste milk pouches could find suitable application as polymer matrix (as an alternative to virgin polymer) in the development of jute fiber composite, although the properties of recycled polymeric jute composites are found to be somewhat inferior compared to those of virgin polymeric jute composite. However, the utilization of 50:50 virgin-recycled blends as polymer matrix can significantly improve all the properties of the resultant composites. Finally, the discussed results would help to decide suitable processing parameters for recycling of such post consumer flexible oil and milk pouches to produce value added recycled material and thus would help in municipal waste disposal problem.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

Lohfink, G., and Kamal, M. 1993, Polym. Eng. Sci., 33, 1404-16. Nocilla, M.A., and La Mantia, F.P. 1990, Polym. Degrad. Stab., 29, 331-9. Howell, S., and Garry, J. 1992, Hazard Mat., 29, 143. Pfaendner, R., Herbst, H., Hoffmann, K., and Sitek, F. 1995, Angew. Makromol. Chem., 232, 193-8. Loultcheva, M.K., Proietto, M., Jilov, N., and La Mantia, F.P. 1997, Polym. Degrad. Stab., 57, 77-81. Kartalis, C.N., Papaspyrides, C.D., Pfaendner, R., Hoffmann, K., and Herbst, H. 1999, J. Appl. Polym. Sci., 73, 1775-85. Santos, A.S.F., Agnelli, J.A.M., Trevisan, D.W., and Manrich, S. 2002, Polym. Degrad. Stab., 77, 441-7. Zahavich, A.T.P., Latto, B., Takacs, E., and Vlachopoulos, J. 1997, Adv. Polym. Technol., 16, 11-24. Sadrmohaghegh, C., and Scott, G. 1980, Eur. Polym. J., 16, 1037-42. Epacher, E., Tolveth, J., Stoll, K., and Pukanszky, B. 1999, J. Appl. Polym. Sci., 74, 1596-605. Martins, M.H., and De Paoli, M.A. 2002, Polym. Degrad. Stab., 78, 491-5. Holmström, A., and Sörvik, E.M. 1974, J. Appl. Polym. Sci., 18, 761-88. Drake, W.O. 1989, Davos Recycle’89, Ciba-Geigy, Switzerland. Drake, W.O., Franz, P., Hofmann, P., and Sitek, F. 1991, Recycle’91 Davos, Ciba-Geigy, Switzerland. Pattanakul, G., Selke, S., Lai, C., and Miltz, J. 1991, J. Appl. Polym. Sci., 43, 2147. Albano, C., and Sanchez, G. 1999, Polym. Eng. Sci., 39, 1456. Barlow, J.W., and Paul, D.R. 1984, Polym. Eng. Sci., 24, 525.

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18. Wu, S. 1997, Polym. Eng. Sci., 27, 335. 19. Pawlak, A., Morawiec, J., Pazzagli, F., Pracella, M., and Galeski, A. 2002, J. Appl. Polym. Sci., 86, 1473. 20. Kruliš, Z., Kokta, B.V., Horák, Z., Michálková, D., and Fortelný, I. 2001, Macromol. Mat. Eng., 286, 156. 21. Pracella, M., Rolla, L., Chionna, D., and Galeski, A. 2002, Macromol. Chem. Phys., 203, 1473-85. 22. Ram, A., Narkis, M., and Kost, J. 1977, Polym. Eng. Sci., 17, 274. 23. Xanthos, M., Patel, A., Dagli, S.S., Jacob, C., Nosker, T.J., and Renfree, R.W. 1994, Adv. Polym. Sci., 13, 231. 24. Radonjič, G., and Gubeljak, N. 2002, Macromol. Mat. Eng., 287, 122-32. 25. Vargas, L. 1996, Rev. Plast. Mod., 72, 290. 26. Colvin, R., Moore, S., and O’Neill, M. 1998, Mod. Plast., 75, 51. 27. Maity, S.N., Joshi, M., and Misra, A. 1991, J. Appl. Polym. Sci., 43, 311-28. 28. Joshi, M., Maiti, S.N., and Misra, A. 1992, J. Appl. Polym. Sci., 45, 1837-47. 29. Willis, J.M., and Favis, B.D. 1988, Polym. Eng. Sci., 28, 1416-26. 30. Kim, B.K., Park, S.Y., and Park, S.J. 1991, Eur. Polym. J., 27, 349-54. 31. Prusty, M., and Chanda, M. 2002, J. Polym. Mater., 19, 13. 32. Pawlak, A., Morawiec, J., Pazzagli, F., Pracella, M., and Galeski, A. 2002, J. Appl. Polym. Sci., 86, 1473-85. 33. Akkapeddi, M.K., VanBuskirk, B., and Swamikannu, X. 1992, Polym. Mater. Sci. Eng., 67, 317. 34. Xanthos, M., Patel, A., Day, S., Dagli, S.S., Jacob, C., Nosker, T.J., and Renfree, R.W. 1996, Adv. Polym. Sci., 60, 1. 35. La Mantia, F.P., and Capizzi, L. 2001, Polym. Degrad. Stab., 71, 285-91. 36. Humbeeck, W. 2001, NISSAN’S Approach to Design for recycling, Nissan European Technology Centre, Brussels. 37. Miller, N.A., Jones, M.S., and Stirling, C.D. 1998, Polym. Polym. Compos., 6, 97-102. 38. Lu, J.Z., Wu, Q., and Mc Nabb, H.S. 2000, Wood Fiber Sci., 32, 88-104. 39. Peizhang, W., Lan, W., and Licheng, L. 2002, Suliao (China), 31, 41. 40. Selke, S.E., and Wichman, I. 2004, Composites Part A, 35, 321-6. 41. Choudhury, A., Mukherjee, M., and Adhikari, B. 2006, Indian J. Chem. Technol., 13, 233-41. 42. Choudhury, A., Mukherjee, M., and Adhikari, B. 2006, Polym. Polym. Compos., 14, 635-646. 43. Choudhury, A., Mukherjee, M., and Adhikari, B. 2005, Prog. Rubb. Plast. Recycl. Technol., 21, 219-230. 44. Choudhury, A., Mukherjee, M., and Adhikari, B. 2005, Thermochim. Acta, 430, 87-94. 45. Choudhury, A., Mukherjee, M., and Adhikari, B. 2007, Polym. Compos., 28, 78-88. 46. Choudhury, A., Mukherjee, M., and Adhikari, B. 2005, Prog. Rubb. Plast. Recycl. Technol., 21, 117-137.

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47. Gulmine, J.V., Janissek, P.R., Heise, H.M., and Akcelrud, L. 2002, Polym. Testing, 21, 557-63. 48. Mandelkern, L., and Alamo, R.G. 1999, Polymer Data Handbook. New York: Oxford University press. 49. Arimoto, H. 1964, J. Polym. Sci. (Part-A) Polym. Phys., 2, 2283-95. 50. Bradbury, E.M., and Elliott, A. 1963, Polymer, 4, 47-59. 51. Boerio, F.J., and Koenig, J.L. 1971, J. Polym. Sci., 7, 1517-23. 52. Boerio, F.J., Bahl, S.K., and Mcgraw, G.E. 1976, J. Polym. Sci. (Part-B) Polym. Chem., 14, 1029-46. 53. Bremner, T., Cook, D.G., and Rudin, A. 1991, J. Appl. Polym. Sci., 43, 1773. 54. Chuang, H.K., and Han, C.D. 1985, J. Appl. Polym. Sci., 30, 165. 55. Utracki, L.A. 1989, Polymer Alloys and Blends, Hanser-Munich, New York. 56. Guerrero, C., Lozano, T., and Arroyo, E. 2001, J. Appl. Polym. Sci., 82, 1382-90. 57. La Mantia, F.P., and Valenza, A. 1989, Eur. Polym. J., 25, 553-6. 58. Xanthos, M. 1988, Polym. Eng. Sci., 28, 1392-400. 59. Deanin, R.D. 1972, Polymer Structure, Properties and Applications, Cahners Books, Boston. 60. Joshi M., Maiti, S.N., and Misra, A. 1992, J. Appl. Polym. Sci., 45, 1837-47. 61. Sadrmohaghegh, C., and Scott, G. 1980, Eur. Polym. J., 16, 1037-42. 62. Liu, C., Wang, J., and He, J. 2002, Polymer, 43, 3811-8. 63. Popli, R., and Mandelkern, L. 1987, J. Polym. Sci. (Part-A) Polym. Phys., 25, 441-83. 64. Mitterhofer, F. 1980, Polym. Eng. Sci., 20, 692-5. 65. Cho, K., Lee, B.H., Hwang, K.M., and Choe, S. 1998, Polym. Eng. Sci., 38, 1969-75. 66. Bremner, T., Cook, D.G., and Rudin, A. 1990, J. Appl. Polym. Sci., 41, 1617-27. 67. Singleton, A.C.N., Baillie, C.A., Beaumont, P.W.R., and Peijs, T. 2003, Composites: Part A, 34, 519-26. 68. Kumer, A.P., Singh, R.P., and Sarwade, B.D. 2005, Mater. Chem. Phys., 92, 458-69.

Transworld Research Network 37/661 (2), Fort P.O. Trivandrum-695 023 Kerala, India

Recent Developments in Polymer Recycling, 2011: 187-214 ISBN: 978-81-7895-524-7 Editors: A. Fainleib and O.Grigoryeva

6. Recycling of polymer blends Jesmy Jose, Jyotishkumar P, Sajeev M. George and Sabu Thomas School of Chemical Sciences, Mahatma Gandhi University, Priyadarshini Hills P.O. Kottayam-686 560, Kerala, India

Abstract. Polymer blends and alloys offer an interesting solution to obtain multipurpose materials with tailor-made properties. Recycling of these inseparable mixtures is however restricted by processing as well as thermodynamic issues. Regenerative recycling as well as compatibilizer aided blending of post consumer blend recyclates enabled successful restoration of blend properties. The present chapter reviews the state-of the- art scenario in the recycling of post consumer polymer blend wastes.

1. Introduction In the recent decades, an ever growing demand for improved properties such as stiffness, ductility, thermal stability, flame resistance and impact resistance etc had paved the development of blending of polymer mixtures. The total market volume for polymer blends is currently estimated to be more than 1.1 million metric tons a year [1]. It includes a significant number of large volume products such as PPE/HIPS blends (Norly (R)), PC/PBT blends (Xenoxy), and PA/PPE blends (Noryl GTX) [2-5] etc that are being generated to equip the multipurpose needs of plastics industry. Albeit, on par with this ever increasing demand, the dwindling oil reserves: source upon which polymer industry is based, as well as environmental issues have aroused wide Correspondence/Reprint request: Dr. Sabu Thomas, School of Chemical Sciences, Mahatma Gandhi University, Priyadarshini Hills P.O., Kottayam-686 560, Kerala, India. E-mail: sabuchathukulam@yahoo.co.uk

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concern up on the recyclability of these high performance materials. For example, the automotive industry takes up 41 % of the total market volume of polymer alloys and generates around 2.2 million tonnes of plastic scrap per year that are hard to dismantle [1]. The recycling of mixed plastic automotive parts is highly desirable to cut production and material costs, as well as to decrease the impact of end-of-life vehicles (ELV) on the environment (e.g., Directive 2000/53/EC of the EU parliament). In the case of unsorted plastic materials, the scope of conventional production recycling under a â&#x20AC;&#x153;closed loopâ&#x20AC;? recycling process involving the separation, cleaning, and paintremoval procedures is rather restricted as separating them demands high investment in machinery, and poor efficiency: in practice, not more than 95 % purity of the separated fraction is achieved [6-7]. Hence, so far, majority of applications of the inseparable mixtures are downcycling, i.e., these polymers alloys are being used in secondary products like for landfill, energy recovery, etc. An upcycling of the post-consumer blend wastes for value-added goals remains a challenge for the scientific community. Academic work on recycling of polymer blends has been limited, even though recycling industry has seen a spectacular growth in the last decade. Earlier Laverty et al. [8] explored the technical feasibility of recycling binary and ternary combinations of PC, PMMA and several different types of ABS. Later, Liu and Bertilsson found that recycled ABS and PC/ABS (70/30) with a small amount of methyl methacrylate-butadiene-styrene core-shell impact modifiers gives the mixture better impact properties than any of its individual components [9]. Recycling of thermoplastic elastomers based on poly (phenylene ether) (PPE) was studied by Bhowmick and co-workers [10]. The quaternary blend comprising of styrene-ethylene-butylene-styrene (SEBS)/ethylene vinyl acetate (EVA)/PPE-PS (polystyrene) showed improvement in mechanical properties upon recycling, which was correlated with the formation of crosslinked network in the system. The use of reactive compatibilizers in recycling of mixed polymer waste was proposed recently. Recycling of thermoplastic wastes consisting of PE/PP/PS/HIPS blends was investigated by using SEBS/EPR and SBR/EPR as compatibilizers [11]. The SEBS/EPR system allowed blends with better mechanical properties than the SBR/EPR system; this was attributed to the chemical structure similarity between compatibilizers and recycled materials investigated The recycling potential of PC/PBT blend, which are widely used for moulded automobile parts have been studied by Sanchez. The recyclates exhibited mechanical properties comparable to that of the virgin blend [12]. The structure-property correlation was developed through morphological, chemical, and rheological analysis to understand the anomalous enhancement in mechanical properties upon recycling. This chapter addresses the recent concepts in recycling of manufactured polymer blends and inseparable polymer mixtures.

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2. Hard blocks in polymer blend recycling Many of the thermodynamic and processing issues came into play while recycling inseparable polymer mixtures. It is not necessary for a recycled material to posses the properties that are comparable to those of the virgin materials. The properties of the recycled material need only to meet the requirements of the proposed applications. The following section addresses the hard core issues in blend recycling.

2.1. Processing issues Currently available plastic recycling technology relies on compatibility charts to determine the ability of two or more polymers to be processed into single mixed material, whose property profile is considered acceptable for normal applications. For example, Figure 1 provides a relative measure of compatibility for a matrix of polymer pairings that often serve as a good first step towards effective recycling. However, the brevity a short summary of a complex subject often leads to deficiencies in implementation that depends upon the nature of individual components involved. An illustrative example is the recycling of automobile panels made up of PC-ABS blends. Numerous studies revealed effective recycling of compatible PC-ABS mixtures, which was found to be independent of either the blend composition of the chemical constituent of SAN in the ABS [6, 13]. However Figure 2 documents an unacceptable loss of impact strength due to an additive, identified as a fattyacid lubricant, that degraded the PC, causing evolution of volatiles during melt processing [6]. The new â&#x20AC;&#x153;close-loop, regenerative recyclingâ&#x20AC;? is regarded with interest nowadays, for it aims at fully replacing virgin polymer by recycled material thereby making the polymer life virtually endless [14-17]. A very good example is the regenerative recycling of car bumpers made by a poly(propylene) matrix (PP) and an ethylenepropylene rubber copolymer as dispersed phase. A long time of service causes severe ageing of car bumpers combined with an impairment of mechanical properties. In particular, switching from plastic to brittle behaviour and decreasing of the elongation at break are observed. The poor rubber/matrix adhesion caused by the physical ageing of the PP matrix reduces the capability of the rubber particles in stabilizing deformation mechanisms such as crazing and/or shear yielding [8].

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Figure 1. Example of a compatibility chart available in the trade and product literature.

Figure 2. Impact strength, measured by a driven-dart impact tester, of PC-ABS blends containing different compositions of ABS.

However, it has been found that the use of specific additives during the recycling strongly enhances the mechanical properties of these items [15â&#x20AC;&#x201C;18]. Figure 3 compares the impact strength of car bumpers (impact modified PP) for the effect of additives.

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Figure 3. Impact strength of new and used bumpers: A) unprocessed new items; B) unprocessed used items; C) used items recycled without regenerative additives; D) used items recycled with additives. (Recycling in a single screw extruder; stabilizers: 0.7 wt.% of Recycloblend 660 and 0.3 wt.% of Tinuvin 791).

The unprocessed items exhibit impact strength values, which are more than three times lower than those of the new bumpers; an additional decrease in the impact strength is observed after recycling. However, when the reprocessing is carried out in the presence of additives, a considerable enhancement is achieved. The impact strength increases considerably (about 3 times higher than the control material), reaching values characteristic of tough materials and comparable to those of the new bumpers. The on purpose designed additive contained a regenerative agent and an antioxidant system. The mixing action of the recycling process re-establishes the phase compatibility; the antioxidants inhibit oxidation, which speeds up the degradation reactions in the recycling process; eventually the regenerative agent joins short chains possessing suitable reactive groups. This discriminated effect of antioxidants tends to show that the amorphous regions, where additives are dispersed and where oxidation is thought to occur, play a crucial role in the response to the mechanical solicitation. Moreover, the addition of oxirane derivatives as a regenerative agent tends to offset the impairment of properties depending on molecular weight decrease [18â&#x20AC;&#x201C;20]. Table 1 provides some efficient regenerative stabilizers used for recycling of polymer blend systems.

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Table 1. Examples of stabilizers used for the recycling of polymer blends [21]. Restabilization systems for several mixed polymer wastes Stabilizer type - Basic stabilization + low and high molecular weight hindered amine stabilizer (HAS) - Selected oxirane compounds PE/PP bottle fraction 0.2 % Recyclostab 451 HDPE body + PP caps + PET and Recyclostab 451 PVC as impurities Mixed PO + carbon black - Recyclostab 451 (9 antioxidants + costabilizers) - HAS + Chimasorb 944 or Tinuvin 783 (blend of antioxidants) - Tinuvin 622 -Recyclostab 550 costabilizers + HAS PE/PA films coextrudered film HAS + selected antioxidants from food packaging LDPE/PA 6 Mixture Irganox 1098/Irganox 1078-Irgafos 168/Chimassorb 944 HDPE, PO, mixed plastics Recyclostab 411 (antioxidants + costabilizers) PP, PO blends Recyclostab 451 Mixed plastics: 55-60 % PO + 15-20 % - Recyclostab 811 styrenics + 5-8 % PVC + 8 % PET - Aromatic phosphate P-10.1 % + 0.05 % AO-1 + 0.1-0.2 % (HAS-1 + HAS-2/LS-1) + antioxidants (calcium salts organic acids and/or synthetic hydrocalcite + octadecyl (3,5-di-tert-butyl-4-hydroxyphenyl)propionate Waste type PE/EPDM bumper

2.2. Thermodynamic aspects A primary criterion for the economic practicability of the recycling process is that the recyclate should have a high level of final utilizable properties. In this regard, blending technology has been proven to be a promising way to improve the properties of recycled materials [22–25]. The recycling of a ternary mixture of PMMA, ABS and PC is an illustrative example [26]. Addition of PC increases the impact strength, notch impact strength and heat owing to the enhancement of interfacial adhesion by PMMA. Recyclates are considered to be a preferable substitute in molding compositions and are reported to exhibit properties that are practically identical with those of virgin materials. Post consumer polymer blend recyclates differ in structure and polarity with limited miscibility, low tensile and impact properties. Hence compatibilization is also important in polymer recycling. They act as morphology stabilizers, so that they can cause a reduced interfacial tension in the melt, a stabilized phase against growth, an increased adhesion at the phase boundaries and a minimized phase separation in the solid state. Furthermore, extrusion of thermoplastic mixtures in a closed loop recycling system, especially mixtures of PC/PMMA, with the addition of a compatibilizer on the basis of MBS (methylmethacrylate–butadiene–styrene)

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or terpolymer consisting of butadiene â&#x20AC;&#x201D; is described in invention US 5,569,713 [27]. For example, in the work of Liu and Berlitson [9] the toughness of recycled materials was enhanced by blending ABS with PC/ABS and by incorporating methacrylate-butadiene-styrene coreshell impact modifier (MBS). Recycling of other quaternary mixtures such as PET/ABS/PC/SBS or PET/ABS/PC/SEBS, where PET forms the major component are also reported [28]. In order to have useful restabilization process a correct evaluation of the polymer blend and selected compatibilizer has to be made. The properties of recyclates are influenced by primary processing and application as well as by the recovery sources. Recycled materials degrade faster than the equivalent virgin polymers, and hence special stabilizers are required. Addition of compatibilizer to polymer blend waste improves the stability of polymer blend during the second life by maintaining the mechanical properties and thus contributes to the extension of the useful life time. Table 2 gives some examples of the compatibilizers used in recycling industry. A specific case involves the recovery of multilayer material that can be facilitated by addition of compatbilizers [29]. The combination of these layers generally provides to the final material with a mix of the individual performances of the polymers involved, like barrier performance, sealability, moisture or chemical resistance and stiffness that are usually impossible to achieve with one single polymer. The general structure of a multilayer film is shown on Figure 4. The core layer is generally a barrier and the external layers are inert or sealable. Adhesive layers allow a good cohesion between layers. PA6/EVOH/PE multilayer blend is widely used in sausage industry as PA6 provides the mechanical strength and abrasion resistance, EVOH provides oxygen barrier PE provides sealability and protect EVOH against moisture [30]. Figure 5 compares the effect of different compatibilizers on the mechanical properties of recycled PE/EVOH/PE blends.

Figure 4. General structure of a polymer film.

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Table 2. Examples of multilayer structures and recommended compatibilizers. Multilayer type PA/PE

Application Pasta, Meat, cheese, vegetable, fish

Component (Function) PA6 (Oxygen barrier, strength) LDPE, PELLD (Sealing) PA6 (Oxygen, Moisture barrier, abrasion resistance) Ionomer (sealing, clarity, abrasion)

Compatibilizer Fusabond E (PE-MAH) or Surlyn® (EMAA-Zn ionomers) Surlyn®

PA/ionomer

Pasta, Meat, Cheese

PA/EVOH/PE

Sausage casings, Pate

PA6 (Oxygen, Moisture brrier,strength) EVOH (Oxygen barrier) LDPE (Sealing, flexibility, moisture barrier)

Fusabond® E

PE/EVOH/PP

Sausage Casings

PP (Moisture Barrier) EOH (Oxygen barrier) LDPE, LLDPE (Sealing, flexibility, moisture barrier)

Fusabond® P

PE/EVOH/PE

Milk, juices, purees, sauces

LDPE, LLDPE (Sealing, flexibility, moisture barrier) EVOH (Oxygen barrier)

Fusabond® Fusabond® N

PET/PE

Liquid detergents

PET (Oxygen barrier) LDPE, LLDPE (Sealing, flexibility, moisture barrier)

Elvaloy® PTW

Figure 5. Comparison of various compatibilizers for PA6/EVOH/PE blend recycling. PA6, 11 wt.% / EVOH (33 % OH), 7 wt.% / PE, 78 %.

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Figure 6. Effect of different compatibilizers on mechanical properties of PET/LLPDE (75/25) blend coming from multilayer structure recycling.

PET/PE combination of complex film is used for liquid detergent packaging. PET layer provides oxygen barrier and LLDPE layer provides low temperature sealing and flexibility. Figure 6 compares the mechanical properties of PET/LDPE blend from the recycled multilayer structures. For PET/PE recycling it is recommended to use epoxidized modified polyolefin having the ability to react with COOH PET end groups [31].

3. Recycling processes Ideally, a blend recycling module involve the following steps (i) Identification of the recyclate (ii) selection of suitable recycling technique and (iii) characterization of the new products.

3.1. Identification of the blend recyclate The quantification of the chemical and morphological structure of the post consumer blend recyclate is very important from the processing view point. Numerous blend characterization techniques such as thermal, microscopy as well as spectroscopic methods are being currently used [32-36]. Nepote and co-workers [33], for example, analyzed 10 years old post-consumer car bumpers of Fiat car. The IR studies of the blend revealed that in addition to the dispersed rubbery ethylene-propylene phase the precipitated PP matrix contained small amount of EPM. Figure 7 shows the TG and DTG spectrum of the old and new bumpers.

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A pronounced physical ageing was noticed in used bumpers at a temperature lower of 70-100 ยบC. ESCA studies of these materials further reveled that the ageing was due to the preferential surface oxidation of the PP phase. A strong impairment of the mechanical properties has been found in these bumpers. Figure 8 compares the SEM of impact fracture surfaces of the new and aged bumpers. The non-aged bumpers exhibited a complete plastic deformation as observed from the thin tufts of drawn PP covering the entire surface with no evidence of the dispersed phase. However, the aged bumpers show a completely different fracture morphology. The slightly deformed PP phase surrounded by rubbery particles and cavities indicated an almost brittle failure.

Figure 7. TG and DTG curves of the PP component of new and aged bumpers.

(new bumper)

(old bumper)

Figure 8. Scanning electron micrographs of impact fractured surface analysis of new and aged bumpers. Micrographs taken near the notch tip in the region of crack growth.

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3.2. Recycling techniques For dismountable plastic scrap, material-separation technologies such as differential solvation, flotation, and electrostatic and air vortex technologies etc. provide the highest value recycled material and is therefore pursued as the preferred recycling option. Nevertheless, for inseparable plastic wastes such as blends and multiplayer packing wastes, blending seems to be the best option for value-added recycling. In terms of direct blend recycling, reprocessing in melt phase remains the most popular recycling technique. It ensures simplicity and low cost, especially when done 'in-plant' and feeding with scrap of controlled history. Some processes producing inseparable mixture include: (i) co-injection molding (ii) rotary table and multicomponent injection molding [37-38] (iii) coextrusion and lamination [39-40] etc. Nevertheless, these routes often lead to degradable products because of chemical reactions that cause irreversible changes in the polymer structure during processing, thereby further affecting the physical properties and the quality of the recycled product [41-42]. An alternative process, solid-state shear pulverization (S3P), has recently been used to address limitations associated with melt-state processing of polymer blends [43-45]. The process uses a specially designed co-rotating twin-screw extruder with integrated heating and cooling. Figure 9 shows the schematic diagram of the S3P process. As can be seen, this process consists of two zones: the heated zone, for heating and compression, and cooling zone, for cooling and pulverization. The chopped or shredded post-consumer plastic wastes undergoes large compressive shear deformation of granulates, and in turn, storage of a large amount of strain energy in cooling zone. When the stored energy reaches a critical level, the material cannot sustain itself. As a result, the stored energy

Figure 9. Schematic diagram of the single screw Solid State Shear Extrusion pulverization process.

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is converted into surface energy through the formation of new surfaces, and pulverization occurs [46-47]. This technique allows intimate mixing of polymers with very different viscosities. Further, S3P technique enables compatibilization, sold-state dispersion of additives including pigments, as well as a continuous production of recyclate powder with unique shapes and larger surface areas. Moreover, by contrast, S3P maintains blend recyclates in the solid state and avoids the additional heat history that occurs during [other processes], which can be detrimental to the physical properties of pulverized materials. The S3P process directly produces blends with matrix and dispersed phase morphology like those obtained after phase inversion during a long melt-mixing process. This phenomenon is of practical importance because a long processing time is required by conventional melt-mixing to produce stable blend morphology. Figure 10 compares the morphology observed for 90/10 wt.% PS106/HDPE blends processed via twin-screw extrusion or pulverization and annealed at 190 °C for 0, and 480 min [48]. The non-annealed, melt-mixed sample exhibits a fine dispersion of HDPE particles with Dn of 0.9 μm. However, there is a major increase in Dn with increasing annealing time. In contrast, with the PS106/HDPE blend made via pulverization using screw B, Dn remains stable to annealing at a value of 1.0-1.1 μm.

Figure 10. SEM comparison of a 90/10 wt.% PS/HDPE blend prepared by melt mixing via twin-screw extrusion and annealed at 190 °C for (a) 0, (b) 480 min with the same blend prepared by solid-state shear pulverization (screw B) and annealed at 190 °C for (c) 0, (e) 480 min. Size bar ) 3.0 μm in all micrographs. (Note: size bar is substantially reduced with annealing for melt-mixed blends but is invariant with annealing for copulverized blends).

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3.3. Recyclability and material properties The recyclability of thermoplastic materials is contingent upon numerous technical factors including life history prior to recycling and the effects of multiple processing operations upon the physical and chemical integrity of the material. For example, the partial coalescence in multiphase alloy, unless prevented or stabilized, compromises properties of the alloy. Chemical coupling across the interface or the addition of a compatibilizer helps to prevent this from happening; however, prolonged thermal exposure of the alloy may create instability, owing to thermal break down of the compatibilizing species. Extended thermal abuse of PA-PPO alloys (Noryl GTX 910 by General Electric Co.) composed of nylon 66, which forms the continuous matrix, and a dispersed phase of PPO, in which the rubber modifier is found to reside exclusively has also been linked to a breakdown of the morphology, resulting in agglomeration of PPO particles, and is accompanied by transformation to brittle behavior [49]. The separation of latter processes from the effects of degradation of the elastomeric modifier was demonstrated in compositions that contained a fully saturated rubber modifier that did not appear to degrade. The breakdown morphology is seen most easily using TEM and scanning electron microscopy (SEM) techniques, an example of which is shown in Figure 11. In this micrograph, the alloy contains a thermally stable rubber modifier, and the breakdown of the compatibilizer appears to be responsible for the development of large PPO domains that are irregular in shape and up to 15-20 Îźm in diameter. SEM of the solvent etched microtomed surfaces can also be used to provide a semiquantitative assessment of the increase of particle-size distribution caused by agglomeration.

Figure 11. TEM micrographs of a PA-66-PPO alloy after exposure to a processing temperature of 280 Â°C I an injection-molding machine for 20 minutes.

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Dynamic mechanical analysis of the region associated with the polybutadiene β-relaxation revealed a systematic change associated with processing history. This information is summarized in Figure 12, which shows how the peak appears to be smeared slightly to a higher temperature as a result of multiple extrusion. The β-relaxation of the nylon 66 phase (indicated on the Figure 12) interferes with the resolution of the polybutadiene relaxation; however, chemical changes associated with crosslinking and degradation are believed to be responsible for the changes observed. Similar studies on the recycling potential have been performed on numerous blend systems. In CPE/NR blends [50], Sirisinha and co-workers have recently found that with increasing number of recycles, the elastic contribution of the blend decreases which is associated with a noticeable shift in the glass transition temperature of the NR phase of the blends (Figure 13). The above finding further suggested a molecular change in the NR phase via a thermal chain-scission mechanism. Nakason and Saiwari [51] investigated the recyclability of MAH grafted NR/PP blends. Figure 14 shows the tensile strength and hardness of recycled MNR/PP TPVs compared with the virgin material. It is seen that the strength and hardness properties were marginally decreased with increasing number of reprocess cycles. However, in Figure 15, a slight decreasing trend of elongation at break and tendency to recover to the original shape after extension (i.e., increasing trend of tension set) were observed. This is attributed to the degradation of the PP and vulcanized rubber phases. It has

Figure 12. Dynamic mechanical loss (EJ of the low temperature β-relaxation of GTX 91 0 (measured at 1 Hz). 0-7 represents the number of extrusion exposures. The nylon 66 β-relaxation curve is shown at the top.

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Figure 13. Gâ&#x20AC;˛ as a function of the strain amplitude at 1 rad/s for CPE/NR blends with various recycling cycles.

Figure 14. Tensile strength and hardness of recycled 60/40 MNR-8/PP TPVs compared with virgin material.

been well established that PP degrades under a cycle of heat treatments, and hence there is a decrease in its molecular weight, [52-53] viscosity, and mechanical strength [52]. The SEM results (Fig. 16) revealed larger cavitations where the PP phase was previously located. This might be a result of degradation of PP molecules, and hence a lowering of its melt viscosity during injection molding and extrusion processes. Hence, the vulcanized rubber domains were capable of coalescing to form larger particles and also create larger occupied volume by the PP phase, as shown in Figure 16.

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The vulcanized rubber was also capable of degrading under the repeated high heat and shearing force conditions. However, the vulcanized rubber domains are always in an unmelted state. Therefore, the degradation at the surface of the particles is more pronounced thereby causing coalescence in the rubber particles. As a result, decreasing interfacial force between the phases, and hence lowering of the mechanical strength and elastomeric properties in terms of elongation and set properties were observed. Thus,

Figure 15. Elongation at break and tension set of recycled 60/40 MNR-8/PP TPVs compared with virgin material.

Figure 16. SEM micrographs of recycled 60/40 MNR-8/PP TPVs compared with virgin material.

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various properties of the TPV from a final recycling process were more or less in a range of capability to apply in some industrial applications, i.e., the tensile strength, elongation at break, and tension set are 10 MPa, 200, and 21 %, respectively. It was therefore concluded that MNR/PP TPV is recyclable. It still retains useful properties even after experiencing high shear and heat treatment from a number of recycling process.

4. Specific examples of blend recycling The following sections discuss the recycling aspects of specific blends that are important in the industry view point.

4.1. Thermoplastic-thermoplastic blends PC/ABS blends account for over 50 percent of the total blend consumption in automobile sector. According to latest forecasts the demand for PC blends will grow by at least seven percent a year in the period ahead [54]. Due to the exceptional low-temperature toughness (synergistic behavior), low melt viscosity for high shear molding of thin walls (an attribute of SAN), and high-temperature resistance (an attribute of PC), these alloys are used in the construction of vehicles, to manufacture shock absorbers or for other parts that can be highly mechanically loaded. Recycling of PC-ABS blend requires a high processing temperature than ABS alone (260-300 째C) and therefore is more prone to degradation. The processing temperature of a material is specified by the flow of the blends. Blending with PMMA, which has a good flow at the above processing temperatures of PC/ABS produced ternary blends with good processability. Figure 17 gives the viscosity evolution in ternary blends of PMMA/recycled ABS/PC. The viscosity at constant shear rates at 250 째C exhibits a moderate linear decrease upon addition of PMMA, which is especially evident at higher shear rates. Thus even 5 % of PMMA improves the processability of the blend during recycling. As had been anticipated, reprocessing declined the J values of PC/ABS blend by 10 % due to a decrease in the molecular weight of PC [55-60], SAN [61] and thermooxidative degradation of butadiene rubber in the case of PC/ABS [62]. Rybnicek et al [63] recently investigated the effect of recycling on the mechanical properties of PC/ABS blends. Figure 18 compares the impact strength of various virgin and recycled materials. The J values of recycled PC/ABS blend declined by 10 %. However, it is important to note that the toughness was higher than that of the commercial ABS blend Cycoloy HF1100, GE Plastics (compare materials 1, 2, 3 in Figure 18).

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Figure 17. Rheology results of PC/ABS/PMMA blend.

Figure 18. The effect of recycling on toughness.

Moreover, processing a ternary blend PC/ABS 15 % PMMA from virgin and recycled materials does not lead to any recognizable change in toughness (compare materials 3, 4 in Figure 18). Additionally, the content of 2 % of TPE in the PC/ABS/PMMA5 resulted in a drop in toughness of almost 40 %. However, despite this effect, the J value lies at the same level as that for virgin PC, ABS, and PMMA (compare materials 6, 7, 8, 9 in Figure 18). Tensile testing reveals that the addition of 5 % of PMMA increases the yield stress ﾏペ of PC/ABS blend (compare materials 4, 5 in Figure 19). The yield stress ﾏペ and strain ﾎｵy of virgin and recycled ternary blend PC/ABS/ 15% PMMA remain almost unchanged (compare materials 5, 6 in Figure 19). Moreover, the addition of TPE does not affect the deformation behavior and the material is competitive with original materials (compare materials 1, 2, 3, 7 in Figure 19).

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In a similar study Liu and Berlitson [9] obtained recycled ABS/PC blends from dismantled Volvo cars with a 20 % improvement in the sharply impact strength by incorporating methacrylate-butadiene-styrene coreshell impact modifier (MBS). Here, the compatibilizing action of the MBS particles which are trapped in the PC/SAN interphase initiates local shear yeilding of PC phase and thereby retards the coalescence during processing. Figure 20a and b shows the impact fracture surface of ABS/(ABS/PC)/MBS 50/50/5 blend with and without PMMA. The craters of the MBS compatibilizer (0.1 to 0.2 mm) can be found on the surface of the bigger particles of diameter up to 3mm, which are believed to be polycarbonate. However, the presence of 10 % PMMA in the ABS/(Recycled ABS/PC)/PMMA/MBS blend changes the morphology at PC/ABS interface (see Fig. 20b). It was found that the impact strength further improved by 20 % by the introduction of the 10 % of PMMA and hence it was assumed that the interfacial adhesion between ABS and PC would be enhanced by adding PMMA. However, when PA was used instead of PMMA, no such augmentation in toughness was observed, probably due to the incompatibility with PC-ABS blend.

Figure 19. The effect of recycling on tensile properties.

Figure 20. SEM fractography of impact fracture surface of (a) ABS/(ABS/PC) /MBS 50/50/5 and (b) ABS/(ABS/PC)/PMMA/MBS45/45/10/5.

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4.2. Rubber-thermoplastic blends Resistance to cracking and enhanced toughness are the most prized attributes of polymeric materials. The key application of rubber modified thermoplastics is as impact modifiers. Both functionalized rubber and coreshell rubbers serve as effective impact modifiers. Rubber modified PP is an important example. The combination of stiffness coupled with ductility of PP has encouraged the large scale production of rubber modified PP which are being increasingly used in automotive and engineering applications, for example in car dashboards. The recycling of car dash board is very complex because of the time required to dismantle the parts and because of the large number of constitutive elements and of the different polymeric materials, often mutually incompatible, employed in its construction. Ragosta et al. [64] investigated the recycling prospectives of car dash boards supplied by the research centre of Fiat-Elasis (Naples-Italy). The thermo-oxidative and thermo-mechanical degradation phenomena induced a strong worsening in the mechanical and impact properties of the recycled material. Rubber additives improve the fracture toughness of polyolefins [65-66]. Figures 21 and 22 shows the mechanical parameters for used car dash boards modified with varying amount of EPR and an antioxidant. This additive, acting as an oxygen-centered radical scavenger, inhibits autooxidation as soon as it starts, thus enhancing the thermo-oxidative stability. An observed linear decrease in modulus was observed attributed to a softening effect of the rubber which lowers the overall crystallinity of the material. A limited improvement in the mechanical behaviour was observed. An opposite trend is observed for the elongation at break. In fact, this parameter increases linearly with increased EPR content, indicating the compatibilizing action of EPR.

Figure 21. Tensile modulus as a function of the EPR content in recycled PP/EPR blend containing an antioxidant.

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Figure 22. Elongation at break as a function of the EPR content in recycled PP/EPR blend containing an antioxidant.

[a]

[b]

Figure 23. SEM fractography of impact fracture surface of (a) unmodified dashboard; B, dashboard with 20 wt.% of EPR.

The better homogenization of the material achieved by EPR addition improves the flow capability of the recycled product favoring yielding and drawing process. This is evident from the morphology and impact studies. The fracture surfaces (SEM) of the unmodified dash boards show brittle failure (Figure 23a), whereas extensive stress whitening due to plastic deformation have been observed in EPR containing systems (Figure 23b). Moreover, the observed undesired effect of reduced stiffness was overcome by replacing part of the EPR with virgin polypropylene. Figure gives the impact strength and modulus of various formulations.

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Figure 24. Tensile elastic modulus and impact strength of formulations recycled in presence of the antioxidant and containing different amount of EPR and virgin PP: A, unmodified dashboard; B, dashboard with 20 wt.% of EPR; C, dashboard with 15 wt.% of EPR and 5 wt.% of PP; D, dashboard with 10 wt.% of EPR and 10 wt.% of PP.

As seen from the histogram in figure 24, it is sufficient to substitute 5 wt % or 10 wt % of EPR with PP for obtaining materials with balanced properties in terms of modulus and toughness.

4.3. Rubber-rubber blends Recycling of rubber-rubber blends by conventional recycling methods such as incineration is challenging due to toxic emissions. Moreover, reprocessing of mixed elastomer blends is rather tedious. A major share of the rubber wastes consist of waste tires. In the present context of value added recycling, preparation of conductive polymer composites prepared form crushed tire rubber offers an innovative strategy that fits with the durable development concept [67]. Crushed rubber particles from car tire wastes are capable of forming percolation network that shows enhanced conductivity. Figure explains the PTC (positive temperature coefficient) effect obtained with [polycarbonate(PC)-carbon black(CB)] 80/crushed rubber (CR) 20 composite at varying CB content. The values for positive temperature coefficient amplitude observed is the highest ever reported for an amorphous matrix (Figure 25). This increase in conductivity is achieved by the double percolation of CR and CB in PC matrix. A schematic of the percolation network is given by figure. According to Figure 26 one can understand that the increase of conductivity due to CB percolation can only be achieved once conductive pathways are formed at the surface of the CR particles.

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Figure 25. PTC effect obtained with (PC-CB)80/CR20 m/m CPC as a function of CB content for particles of 300<Ď&#x2020;CR<410Îźm.

Figure 26. Structure of CPC based on double percolation, CR and CB.

4.4. Thermoset-rubber/thermoplastic blends Thermosetting based blends are one of the least recycled materials as they can not be remoulded. Despite the obvious difficulties, there are ways to recover some value from thermosetting blend materials. Firstly, it is important to note that most of the weight, as well as much of the value, of reinforced thermosets used are made up of high quality inorganic fillers, i.e. silica and glass fibre fabric. It is possible to recover these materials pyrolysing or solvolysing the polymer matrix [68]. Mechanical recycling of thermosetting blends is possible if the compound material is ground. The

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particles can serve as filler for virgin thermoset resins in PWB manufacture or construction as a blast medium to remove paints or in the construction of landfills. As a filler, they are slightly inferior to pure inorganic filler materials, but are considerably cheaper to produce. Furthermore, the process neither requires nor produces any dangerous substances, with the possible exception of grinding dust. The carbon black in device encapsulation material makes it useful as a decorating agent. The recycling of thermosetting blends is still very much at the experimental stage, and full scale industrial application of the processes described above is not imminent.

Conclusions An upcycling of the inseparable mixtures for value-added goals remains a challenge for the scientific community. According to the more recent international regulations, recyclability of goods is in charge of the producer; the expectations that blends must retain some of their properties in the recycled form introduces a whole new layer of competitive pressure on blend producers around the world. Facing up this problem, new technologies such as restabilization and blending have been developed aiming to upgrade the polymer recyclates during reprocessing. Mechanical behavior of regenerative recycled blends were found to be contingent up on the ageing behavior, mode of action of stabilization package as well as the recyclate blend morphology. However, the regenerative agent may be nearly useless if chain scission is not the dominant process in the recycling operation and the interfacial adhesion between dispersed phase and the matrix does not assure automatically a ductile mechanical behaviour under impact conditions. Therefore, the â&#x20AC;&#x2DC;regenerativeâ&#x20AC;&#x2122; approach demands a detailed knowledge of the chemistry and physics of recycling to suggest the strategy for restoring material performances up to the original level. Blending with a tertiary component aided by a suitable compatibilizer have been found effective in upgrading the performance of recyclates. In this regard, S3P technology provides a break through by achieving an efficient and intimate mixing of the blend recyclates that have significantly dissimilar chemical as well as processing histories. Albeit, the system specificity of the various conventional block and graft copolymer compatibilizers is a major stumbling block in the blending pathway. In a recent break-through, organically modified clay has proven to be an effective compatibilizer for polymer blends. The large surface area of clay enabled the formation of in-situ grafts on the clay surfaces and are preferentially localized at the

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interfaces, which, thereby provide enhanced compatibility (Figure 27). This blending mechanism reflects the composition of the blend and has been found to be fairly system nonspecific. As a result, this turns to be a promising technology for use in processing recycled blends where the composition is often uncertain and price is of general concern.

(a)

(b)

Figure 27. (a) DMA graphs of recycled blends with and without 10 % Cloisite 20A (modulus of the materials with clay has increased nearly 200 %) and (b) TEM image of cross section of Styrofoam boxes/scrap Plexiglas/Cloisite 20A (45/45/10) [69].

References 1. 2. 3. 4. 5. 6. 7. 8.

Engineering Resins, Polymer Alloys and Blends â&#x20AC;&#x201C; Market Survey, 2008, (www.reportlinker.com). Avallea, M., Belingardi, G., and Montaninib, R. 2001, Int, J. Imp. Eng., 25, 5, 455-472. Utracki, L.A., and Favis, B.D. 1989, In: Cheremisinoff NP, editor. Handbook of polymer science and technology, vol. 4. New York: Marcel Dekker Inc. Robeson, L.M. 2007, Polymer Blends: A Comprehensive Review. Munich: Hanser Gardner, p 187. Paul, D.R. 1978, In: Paul DR, and Newman S, editors. Polymer blends. New York: Academic Press. Drelich, J., Payne, T., Kim, J.H., Miller, J.D., Kobler, R., and Christiansen, S. 1998, Polym. Eng. Sci., 38, 9, 1378-1386. Omura, M., Nishida, H., Shirai, Y., and Endo, T. 2005, Polym. Prepr., Japan 54, 1, 1959. Laverty, J.J., Bullach, R.L., Ellis, T.S., and McMinn, T.E. 1996, Polym. Recycling, 2 (3), p 159.

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9. Liu, X., and Bertilsson, H. 1999, J. Appl. Polym. Sci.,74, 3, 510-515. 10. Gupta, S., Pallavi, M.B., Som, A., Krishnamurthy, R., and Bhowmick, A.K. 2008, Polym. Eng. Sci., 48, 3, 496-504. 11. Equiza, N., Yave, W., Quijada, R., and Yazdani-Pedram, M. 2007, Macromol. Mat. Eng., 292, 9, 1001-1011. 12. Sanchez, E. M. S. 2007, Polym. Test., 26, 378-387. 13. Larson, H., and Bertilson, H. 1995, Polymer Recycling, 1, 243-248. 14. Pospısil, J., Sitek, F. A., and Pfaendner, R. 1995, Polym. Degrad. Stab. 48, 351-358. 15. Pospısil, J., Nesˇpurek, S., Pfaendner, R., and Zweifel H. 1997, Trends Polym. Sci., 5, 294-300. 16. Kartalis, C.N., Papaspyrides, C.D., and Pfaendner, R. 2002, J. Appl. Polym Sci., 86, 2472-2485. 17. Tsenoglou, C.J., Kartalis, C.N., Papaspyrides, C.D., and Pfaendner, R. 2002, Adv. Polym. Technol., 21, 260-267. 18. Zeifwel, H. 2000, In: Plastic Additives Handbook. Hanser, p. 986-989. 19. Gensler, R., Plummer, C.J.G., Grain, C., and Kausch, H.H. 2000, Polymer, 41, 3809-3819. 20. Hoffmann, K., Herbst, H., and Pfaendner, R. 1998, European Patent 702704. 21. La Mantia, F. P. 2002, In Hand Book of Plastics Recycling, iSmithers Rapra, p 127-194. 22. Klason, C., Bertilsson, H., and Liu, X. 1998, Makromol Chem Macromol. Symp., 135, 183-186. 23. Liang, R., and Gupta, R.K., 2002, In Proceedings of Annual Technical Conference (ANTEC)—Plastics, Society of Plastics Engineers (SPE). San Francisco 2002, Vol 3, p 2948-2951. 24. Elmaghor, F., Zhang, L., Fan, R., and Li, H. 2004, Polymer, 45, 6719-6724. 25. Balart, R., Lopez, J., Garcıa, D, and Salvador, M.D. 2005, Eur Polym Mater., 41, 2150-2160. 26. Fisher, J.D. 1993, U.S. Patent, 5, 232,986. 27. Lieberman, M. 1996, U.S. Patent, 5,569,713. 28. Woei-Min, T. 2004, U.S. Patent, US 2,004,209,985. 29. Manrich, S., Ravazi, R.F., Danella Jr., O.J., Santana, R.C., and Manrich, S. 2005, REWAS'04 - Global Symposium on Recycling, Waste Treatment and Clean Technology, p. 1671-1680. 30. Laiho, E., Sainio, M., Koski, E., Salste, M. 1999, U.S. Pat., 5, 993,997. 31. Carlos Guerrero, C., Lozano, T., González, V., Arroyo, E., 2001, J. Appl. Polym Sci., 82, 6, 1382-1390. 32. Florestan, J., Lachambre, A., Mermilliod, N., Boulou, J.C., and Marfisi, C. 1994, Resources, Conservation and Recycling, 10, 67-74. 33. Luda, M.P., Ragosta, G., Musto, P., Pollicino, A., Camino, G., Recca, A., and Nepote, V. 2002, Macromol.Mater Eng., 287, 6, 404-411. 34. Hull, J.B., Langton, C.M., and Jones, A.R. 1994, Macromol. Reports, 31, 1191-1199. 35. Mothé, C.G., and Tavares, M.I.B. 1997, Polym. Degrad. Stab., 57, 2, 183-186.

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36. Monteiro, M.R., Moreira, D.G.G., Chinelatto, M.A., Nascente, P.A.P., and AlcaĚ&#x201A;ntara, N.G. 2007, J. Polym. Environment, 15, 13, 195-199. 37. Xanthos, M., and Narh, K.A. 1998, Polymer Composites, 19, 6, 768-780. 38. Tall, S., Albertsson, A.-C., and Karlsson, S. 1998, J. Appl. Polym. Sci., 70, 12, 2381-2390. 39. Tortorella, N., Jung, W.-H., and Beatty, C.L. 2003, Annual Technical Conference - ANTEC, Conference Proceedings, 3, p. 3483-3487. 40. Elmaghor, F., Zhang, L., and Li, H. 2003, J. Appl. Polym. Sci., 88 (12), 2756-2762. 41. Furgiuele, N., Lebovitz, A.H., Khait, K., and Torkelson, J. M. 2000, Macromolecules, 33, 225-228. 42. Furgiuele, N., Lebovitz, A.H., Khait, K., and Torkelson, J.M. 2000, Polym. Eng. Sci., 40, 1447-1457. 43. Ganglani, M., Torkelson, J.M., Carr, S.H., and Khait, K. 2001, J.Appl. Polym. Sci., 80, 671-679. 44. Khait, K., and Torkelson, J.M. 1999, Polym.-Plast. Technol. Eng., 38, 445-457. 45. Lebovitz, A.H., and Torkelson, J.M. 2001, Polym. Mater. Sci. Eng., 85, 530-536. 46. Bilgili, E., Arastoopour, H., and B. Bernstein, 2001, Powder Tech. J., 115, 265-276. 47. Bilgili, E., Arastoopour, H., and B. Bernstein, 2001, Powder Tech. J., 115, 277.-289 48. Lebovitz, A.H., Khait, K., and Torkelson, J.M. 2002, Macromolecules, 33, 225-228. 49. Laverty, J.J., Ellis, T., O'gafu, J., and Kim, S. 1996, Polym. Eng. Sci., 36, 3, 347-357. 50. Prasopnatra, P., Saeoui, P., and Sirisinha, C. 2009, J. Appl. Polym. Sci., 111, 1051-1056. 51. Nakason, C., and Saiwari, S. 2008, J. Appl. Polym. Sci., 110, 4071-4078. 52. Jansson, A., Moller, K., and Gevert, T. 2003, Polym. Degrad. Stab., 82, 37-46. 53. Aurrekoetxea, J., Sarrionadia, M.A., Urrutibeascoa, I., and Maspoch, M.L. 2001, J. Mater. Sci., 36, 2607-2613. 54. Liu, Z.Q., Cunha, A.M., Yi, X.S., and Bernardo, A.C. 2000, J Appl. Polym. Sci., 77, 1393-1400. 55. Abbas, K.B. 1980, Polym. Eng. Sci., 20, 376-382. 56. Bernardo, C.A. In Frontiers in the Science and Technology of Polymer Recycling; Akovali, G., Ed.; Kluwer Academic Publishers: London, 1998; p 371-385. 57. La Mantia, F. P. In Frontiers in the Science and Technology of Polymer Recycling, Akovali, G., Ed., Kluwer Academic Publishers, London, 1998; p 249-269. 58. Shea, J.W., Aloisio, C., and Cammons, R.R. 1975, In Proceedings of Annual Technical Conference (ANTEC), p 614-616. 59. Bledzki, A.K., Barth, C., Hahnsen, H., and Orth, P. 1997, Kunstst. Plast.Eur., 87, 1124-1128. 60. Rybnicek, J. 2001, Diploma Thesis, Czech Technical University, Prague. 61. Bastida, S., Marieta, C., Eguiazabal, J.I., and Nazabal, 1995, J. Eur. Polym. 2. Mater., 331, 643-648. 62. Eguiazabal, J.I., NazaÂ´bal. 1990, J. Polym. Eng. Sci., 30, 527-531. 63. Rybnicek, J., Lach, R., Lapcikova, M., Steidl, J., Krulis, Z., Gellmann, W., and Slouf, M. 2008, J. Appl. Polym. Sci., 109, 3210-3223.

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64. Ragosta, G., Musto, P., Martuscelli, E., and Russo, P. 2000, J. Mater. Sci., 35, 3741-3751. 65. Robertson, R.E. and Paul, D.R. 1973, J. Appl. Polymer Sci., 2579-2595. 66. Paul, D.R., Vinson, C.E., and Locke, C.E. 1972, Polym. Eng. Sci. 12, 157.-166 67. Zribi1, K., Feller1, J.F., Elleuch, K., Bourmaud, A., and Elleuch, B. 2006, Polym. Adv. Techn., 17, 727-731. 68. Pickering, S.J. 2006, Composites Part A: Appl. Sci. Manufacturing, 37, 8, 1206-1215. 69. Si, M., Araki, T., Ade, H., Kilcoyne, A.L.D., Robert Fisher, R., Sokolov, J.C., and Rafailovich, M.H. 2006, Macromolecules, 39, 4793-4801.

Transworld Research Network 37/661 (2), Fort P.O. Trivandrum-695 023 Kerala, India

Recent Developments in Polymer Recycling, 2011: 215-238 ISBN: 978-81-7895-524-7 Editors: A. Fainleib and O.Grigoryeva

7. Microbial biodegradation of polyurethane Gary T. Howard Department of Biological Sciences Southeastern Louisiana University Hammond Louisiana 70402, USA

Abstract. Polyurethane (PU) is a general term used for a class of polymers derived from the condensation of polyisocyanate and polyol. Polyurethanes are an important and versatile class of manmade polymers used in a wide variety of products in the medical, automotive and industrial fields. Depending on the chemical structures of the polyisocyanates and polyols, PU can adopt various forms ranging from flexible to rigid and from low density to solid elastomer. Over three-fourths of the global consumption of PU is in the form of foams amounting to approximately 5 % of the total amount of plastic produced. Despite its xenobiotic origin, PU has been found susceptible to biodegradation by naturally occurring microorganisms. Microbial degradation of PU is dependent on the many properties of the polymer such as molecular orientation, crystallinity, cross-linking and chemical groups present in the molecular chains which determine the accessibility to degrading-enzyme systems. Several reports have appeared in the literature on the susceptibility of PU by fungal attack. Results varied from fungal isolates utilizing colloidal polyester PU as the sole carbon and energy source and displacing esterase activity to their inability to grow solely on PU and the enzymes having to be induced. The majority Correspondence/Reprint request: Dr. Gary T. Howard, Department of Biological Sciences Southeastern Louisiana University Hammond, Louisiana 70402, USA. E-mail: ghoward@selu.edu

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of data in the literature concerning bacterial biodegradation of PU have concentrated on Comamonas acidovorans, Pseudomonas fluorescens and Pseudomonas chlororaphis. These soil isolates are capable of utilizing PU as the sole carbon and energy source. The soluble extra cellular polyurethane-degrading enzymes that these microbes produce have a high similarity to Group I lipases, which contain a G-X-S-X-G serine hydrolase motif. One membrane-bound enzyme expressed by C. acidovorans also contains a serine hydrolase motif; however, the highest degree of similarity for this enzyme is with an acetylcholinesterase. This chapter describes the microorganisms, their enzymes and genes involved in PU degradation. A basic understanding of the biological processes that include the role of polyurethane-degrading enzymes will enhance in the development of new bioremediation techniques of polyurethane waste and the creation of strains for this purpose.

1. Introduction Polyurethanes are present in many aspects of modern life. They represent a class of polymers that have found a widespread use in the medical, automotive and industrial fields. Polyurethanes can be found in products such as furniture, coatings, adhesives, constructional materials, fibers, paddings, paints, elastomers and synthetic skins. Polyurethane should be abbreviated to PUR in compliance with official German and International standards. However, the abbreviation PU is more common in English texts. Advantages of polyurethanes are that they have increased tensile strength and melting points making them more durable [1]. Their resistance to degradation by water, oils, and solvents make them excellent for the replacement of plastics [2]. As coatings, they exhibit excellent adhesion to many substances, abrasion resistance, electrical properties and weather resistance for industrial purposes [2-4]. Depending on the chemical structures of the polyisocyanates and polyols, PU can adopt various forms ranging from flexible to rigid and from low density to solid elastomer. The chemical composition of PU precludes them from being classified as pure plastics but rather as a mixed polymer. The urethane group, which is the basis of this class of plastics, represents a small part of the macromolecule and some PU products do not contain a urethane group. Despite the lack of this base unit, all PU are based on the composition of polyisocyanates. The polyisocyanate polyaddition is distinct from polymerization and polycondensation for the production of synthetic polymers and this feature explains their versatility. The global plastic consumption in 1997 totaled about 145 million tons with polyurethanes comprising a 5 % share resulting in PU being fifth in global plastic consumption [5]. Over three-fourths of the global consumption

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North America Europe Far East Japan Latin America Middle East/Africa

Figure 1. The main global consumers for polyurethane are North America (25 %), Europe (25 %), the Far East (18 %), Japan (7 %), Latin America (7 %), and the remaining split between the Middle East and Africa [5].

of PU is in the form of foams. In the United States alone, the production of PU increased from 45,000 tons in 1960 to 2,722,000 tons in 2004. The main global consumers of polyurethane are summarize in Figure 1.

2. Physical and chemical properties Polyurethanes were first produced and investigated by Dr. Otto Bayer in 1937. Polyurethane is a polymer in which the repeating unit contains a urethane moiety. Urethanes are derivatives of carbamic acids which exist only in the form of their esters [6]. This structure can be represented by the following, generalized amide-ester of carbonic acid:

O R-O-C-N H2 Variations in the R group and substitutions of the amide hydrogen produce multiple urethanes. Although PU may contain urethane groups, other moieties such as urea, ester, ether or an aromatic may be included [2]. The addition of these functional groups may result in fewer urethane moieties in the polymer than functional groups. The urethane linkage results most readily through the reaction of an isocyanate with an alcohol [6, 7]. The hydrogen atom of the hydroxyl group

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is transferred to the nitrogen atom of the isocyanate [1]. The major advantage of PU is that the chain is not composed exclusively of carbon atoms but rather of heteroatoms, oxygen, carbon and nitrogen [1]. The simplest formula for PU is linear and represented by: O

(-R-O-C-NH-R2-NH-C-O-) n

R represents a hydrocarbon containing the alcohol group, R2 is a hydrocarbon chain and n is the number of repetitions. Diisocyanates are employed in PU production reactions because they will react with any compound containing active hydrogen [6]. Table 1. Raw materials for synthesis of polyurethane. Polyisocyanate 2,4-Tolylene diisocyanate 4,4â&#x20AC;&#x2122;-Diphenylmethane diisocyanate 1,3-Xylylene diisocyanate Hexamethylene diisocyanate 1,5-Naphthalene diisocyanate Polyol Polyester-type Poly(butylene adipate) Poly(ethylene butylene adipate) Poly(ethylene adipate) Polycaprolactone Poly(propylene adipate) Poly(ethylene propylene adipate) Polyether-type Poly(oxytetramethylene) glycol Poly(oxypropylene) glycol Poly(oxypropylene)-poly(oxyethylene) glycol Chain extension/crosslinking agent 1,4-Butanediol Ethylene glycol 1,3-Butanediol 2,2-Dimethyl-1,3-propanediol Trimethylopropane Glycerol 1,2,6-Hexanetriol

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For industrial applications, a polyhydroxyl compound can be used. Similarly, polyfunctional nitrogen compounds can be used at the amide linkages. By changing and varying the polyhydroxyl and polyfunctional nitrogen compounds, different PU can be synthesized [6]. Polyester or polyether resins containing hydroxyl groups are used to produce polyester- or polyether-PU respectively [3]. Examples of the raw materials used in the synthesis of PU are summarized in Table 1. Variations in the number of substitutions and the spacing between and within branch chains produce PU ranging from linear to branched and flexible to rigid. Linear PU is used for the manufacture of fibers and molding [3]. Flexible PU is used in the production of binding agents and coatings [2]. Flexible and rigid foamed plastics, which make up the majority of PU produced, can be found in various forms in industry [4]. Using low molecular mass pre-polymers, various block copolymers can be produced. The terminal hydroxyl group allows for alternating blocks, called segments, to be inserted into the PU chain. Variation in these segments results in varying degrees of tensile strength and elasticity. Blocks providing rigid crystalline phase and containing the chain extender are referred to as hard segments [4]. Those yielding an amorphous rubbery phase and containing the polyester/polyether are called soft segments. Commercially, these block polymers are known as segmented PU [8].

3. Polyurethane degradation Research has been initiated to elucidate whether additives to the chemical structure of PU could decrease biodegradation. Kanavel et al [9] observed that sulfur-cured polyester and polyether PU had some fungal inertness. However, they noted that even with fungicides added to the sulfur- and peroxide-cured PU, fungal growth still occurred on the polyester PU and most fungicides had adverse effects on the formulations. Kanavel et al. [9] also recognized the need for physical testing of the PU after extended exposure to the activity of fungi. Santerre et al [10] varied the amount of degradation products released by varying the physical makeup of the polyester PU, as coatings on glass tubes or as films. This implied that while urethane and urea groups are susceptible to hydrolysis, they are not always accessible to the enzyme and degradation may never proceed past the polymer surface. Although the polyether PU showed no significant degradation, they consistently showed higher radiolabel products release from soft-segment-labeled, enzyme-incubated samples than controls. The authors attributed these results to the shielding of ester sites by secondary structures and hydrogen bonding within the hard segment.

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Santerre and Labow [11] tested the effect of hard segment size on the stability of PU against cleavage. Analysis was performed with polyether PU and their susceptibility to cholesterol esterase. Three polyether PU were synthesized with varying molar ratios of [14C]-diisocyanate to chain extender and constant polyether makeup. A 10-fold increase in enzyme concentration of cholesterol esterase previously used [10] was used to approach plateau values for polyether PU hydrolysis. Upon treatment with cholesterol esterase, Santerre and Labow [11] observed that radiolabel release was significantly dependent on the amount of hard segment contained within the polymer. In the polymer with the lowest concentration of hard segment, higher numbers of carbonyl groups are exposed to the surface. With increased hard segment size, a greater number of carbonyl groups are integrated into secondary hard segment structures through hydrogen bonding. The investigators also concluded that an increase in hard segment size does lead to restrictions in polymer chain mobility. In the medical field PU show resistance to macromolecular oxidation, hydrolysis and calcification [12]. Polyurethane elastomers are being used in place of other elastomers due to higher elasticity and toughness, and resistance to tear, oxidation and humidity [2, 6, 13]. In addition, polyether derivatives are inexpensive to produce as prepolymers, which can lower the overall cost of polymer production. Huang and Roby [14] tested the biodegradability of polyamide-urethanes for medical purposes. They synthesized PU with long repeating units and alternating amide and urethane groups from 2-aminoethanol. The resulting partial crystalline fibers were observed to undergo hydrolysis by subtilisin less readily than polyamideesters with degradation proceeding in a selective manner. The amorphous regions on the PU were being degraded prior to the crystalline regions. These fibers showed promise as absorbable sutures and implants where in vivo degradation is needed. The investigators also noted that PU with long repeating units and hydrophilic groups would less likely to pack into high crystalline regions as normal PU, and these polymers were more accessible to biodegradation. Tang et al [15] added surface-modifying macromolecules (SMM) containing fluorinated end groups to the base PU to reduce the material's susceptibility to hydrolysis by lysosomal enzymes. Synthesized polyester urea-urethanes were radiolabled with [14C] and coated onto small hollow tubes. Biodegradation experiments were carried out using methods previously established by Santerre et al [10]. Results indicated that degradation was inhibited by the SMM surface. Different SMM formulations provided varying degrees of enzyme resistance. It was noted that some SMM formulations were incompatible with the PU and led to increased

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biodeterioration. The mechanism of inhibition was not deduced and will be the subject of further study. In an attempt to increase biocompatibility and reduce bacterial adhesion on PU surfaces, Baumgartner et al [16] synthesized phosphonated PU. They used glycerophosphorylcholine (GPC) as the chain extender, which incorporated phosphorylcholine head groups into the PU backbone. This gave the PU surface some characteristics of a red blood cell surface. Physical tests on the PU showed a small decrease in tensile strength and transition temperature with increasing GPC concentration. Water absorption by the PU was increased with increased GPC content. To test bacterial adhesion to the PU, Baumgartner et al [16] used a radial flow chamber. They passed a culture of Staphylococcus aureus across phosphonated and unphosphonated PU at a rate of 8 ml/min. The phosphonated PU showed a decrease in bacterial adhesion with increased GPC content. Lack of degradability and increasing depletion of landfill sites as well as growing water and land problems have led to concern about plastics [17]. As more and more raw materials (e.g. crude oil) become in short supply for the synthesis of plastics, recycling of waste plastics is becoming important [18]. Degradability problems promoted researchers to investigate modification or productions that led to either chemically degradable or biodegradable PU. Huang et al [19] derived polyester PU from polycaprolactonediols in an effort to produce biodegradable PU for use in the medical field. Several different PU were made containing polyester subunits of various lengths. The polymers were subjected to degradation by the enzyme axion and two species of fungi. The enzyme and fungi degraded each PU. In addition, it was also noted that there was an increase in the biodegradability of the polyester PU with increase in the chain length of the polyesters. In a later study, Phua et al [20] observed that two proteolytic enzymes, papain and urease degraded a medical polyester PU. The PU they tested was Biomer速, segmented, cross-linked polyester PU. Although cross linking was previously described as a way of inhibiting degradation [7], papain (molecular weight 20.7 kDa) had little difficulty in diffusing into the film and causing breaks in the structural integrity. Urease activity, because of its size (molecular weight 473 kDa), was limited to the PU surface and therefore was not significant. Phua et al [20] also proposed that papain degraded the polymer by hydrolyzing the urethane and urea linkages producing free amine and hydroxyl groups. The effect of papain on polyether PU was assessed by Marchant et al [21]. Comparison of papain activity to aqueous hydrolysis resulted in both releasing degradation products. Ether linkages were nonenzymatic ally hydrolyzed by water while degradation of the urethane groups was dependent on the presence of the proteolytic enzyme.

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Labrow et al [22] treated polyester PU and polyether PU with human neutrophil elastase and porcine pancreatic elastase. The polyester PU was readily degraded by porcine pancreatic elastase at a rate ten times higher than by human neutrophil elastase. The rate of polyester PU degradation by porcine pancreatic elastase was also ten times higher than its activity against the polyether PU. Human neutrophil elastase had no significant activity against the polyether PU. These results indicate a distinct similarity to the degradation of PU by cholesterol esterase [10, 11, 23]. Inhibition of porcine pancreatic elastase was achieved with the elastase specific inhibitor NMSAAPVCMK.

4. Fungal biodegradation After years of production of PUs, manufacturerâ&#x20AC;&#x2122;s found them susceptible to degradation. Variations in the degradation patterns of different samples of PUs were attributed to the many properties of PUs such as molecular orientation, crystallinity, cross-linking, and chemical groups presented in the molecular chains which determine the accessibility to degrading-enzyme systems [24]. The regularity in synthetic polymers allows the polymer chains to pack easily, resulting in the formation of crystalline regions. This limits accessibility of the polymer chains to degradation whereas; amorphous regions on the PU can degrade more readily. Huang and Roby [14] observed PU degradation proceeded in a selective manner, with the amorphous regions being degraded prior to the crystalline regions. Also, it was observed that PUs with long repeating units and hydrolytic groups would be less likely to pack into high crystalline regions as normal polyurethanes, and these polymers were more accessible to biodegradation. Several investigators have suggested microbial attack on PUs could be through enzymatic action of hydrolases such as ureases, proteases and esterases [25-28]. Several reports have appeared in the literature on the susceptibility of PUs to fungal attack [7, 29, 30]. These studies revealed that polyester-type PUs are more susceptible to fungal attack than other forms. In addition, polyether PUs were noted to be moderately too highly resistant. Boubendir [31] isolated enzymes with esterase and urethane hydrolase activities from the fungi Chaetomium globosum and Aspergillus terreus. These organisms did not grow solely on PU and the enzymes had to be induced. Induction of the enzymes was accomplished by addition of liquid polyester PU to the growth media. Activity of the enzymes was determined by assays based on ethyl carbamate (urethane) as artificial substrate. Four species of fungi, Curvularia senegalensis, Fusarium solani, Aureobasidium pullulans, and Cladosporium sp. were isolated based on their

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ability to utilize a colloidal polyester PU (Impranil DLNTM) as the sole carbon and energy source [32]. Curvularia senegalensis was observed to have a higher PU-degrading activity and therefore subsequent analysis of this fungal isolate was carried out. An extracellular polyurethanase (PUase) displaying esterase activity was purified from this organism. The protein has a molecular mass of 28 kDa, is heat stable at 100 째C for 10 minutes and inhibited by phenylmethylsulphonylfluoride (PMSF). Wales and Sagar [33] proposed a mechanism for the degradation of polyester PUs by extracellular esterases. Polyurethane degradation is the result of synergistic activity between endopolyurethanases and exopolyurethanases. Endoenzymes hydrolyze the PU molecule at random locations throughout the polymer chain leading to loss of tensile strength. Exoenzymes remove successive monomer units from the chain ends however, show little loss of tensile strength.

5. Bacterial biodegradation In a large-scale test of bacterial activity against PUs, Kay et al [34] investigated the ability of 16 bacterial isolates to degrade polyester-PU. Seven of the isolates tested degraded PU when the media was supplemented with yeast extract. Two isolates, Corynebacterium sp. and Pseudomonas aeruginosa, could degrade PU in the presence of basal media. However, none of the isolates grew on PU alone. Physical tests of the degraded polyester PU revealed different but significant decreases in tensile strength and elongation for each isolate. In a further study [35] tested the chemical and physical changes in degraded polyester PU. Polyurethanes taken from Corynebacterium sp. cultures had significant reductions in both tensile strength and elongation after three days of incubation. Infra-red spectrophotometer analysis revealed the ester segment of the polymer to be the main site of attack. The investigators noted that supplementing the media with glucose inhibited esterase production. However, addition of PU did not increase esterase activity. Halim et al [36] tested the growth of several species of bacteria on PU military aircraft paint. The investigators isolated Acinetobacter calcoaceticus, two Pseudomonas sp., Pseudomonas cepacia, and Arthrobacter globiformis. In addition, the U.S. Navy supplied two strains of A. calcoaceticus, Pseudomonas aeruginosa and Pseudomonas putida. All species were capable of utilizing the polyurethane paint as a sole carbon and energy source with the exception of P. cepacia. Using fluorescein diacetate as an esterase substrate, the remaining species showed esterase activity in the absence of PU. This data indicated that the PUases were constitutively expressed.

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5.1. Polyurethane degradation by Bacillus Blake and Howard [37] reported bacterial degradation of a polyester PU (Impranil DLN) by a species of Bacillus. The pattern of degradation involved the binding of cells to the polymer with subsequent floc formation, and the degradation of substrate. The growth of the Bacillus sp. on a solid medium resulted in the visual disappearance of the polyurethane. The complexity of the bacteria-polyurethane interaction was more apparent when grown on a polyurethane liquid medium. Incubation of the Bacillus sp. in media supplemented with polyurethane resulted in the appearance of a chalky precipitate that appeared to be resistant to further degradation. Electrophoretic mobility, electrical impedance, and dynamic light diffraction measurements were performed on the Bacillus-polyurethane system. Bacillus cells had a relatively weak net negative charge corresponding to a zeta potential of -6 mV. Colloidal polyurethane had a strongly negative charge with a zeta potential of -42mV. Complex formation between the PU and cells results in a zeta potential 0f -20 mV. Electrical impedance data showed that on average the Bacillus cell had a volume of around 3.9 mm3 corresponding to a spherical equivalent diameter of just over 2 mm. The majority of the polyurethane particles were sufficiently small to be below the detection limit, 0.6 mm, for electrical impedance. The relative volumes as a function of size for polyurethane and Bacillus were determined by static light diffraction methods. The results from the static light diffraction methods verified that of the electrical impedance results. The above methods were then used to examine the formation of a complex between Bacillus and polyurethane. The electrophoretic mobility data showed that the peaks that were associated with the free polyurethane and the free Bacillus were replaced by a single peak that possessed the size and charge properties anticipated for a complex of the large Bacillus with the strongly negatively charge polyurethane. This evidence was corroborated with electrical impedance measurements that showed there was an increase in the total volume of the Bacillus cells as a function of time as they were mixed with an excess of polyurethane. Evidence that the increase in cell size occurred at the expense of the polyurethane came from light diffraction measurements. Further evidence that the Bacillus cell forms a complex with polyurethane was obtained through microscopic observations. These observations showed that the majority of the cells in the presence of polyurethane were coated with small particles of various dimensions (Figure 2). This evidence indicates that two populations exist in polyurethane cultures: one that is coated with polyurethane and one that is not. At lower

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concentrations of polyurethane, it may be that the two populations of bacteria are dependent on different sources of nutrition. The first population is coated with polyurethane and the polyurethane is metabolized into small, soluble metabolites, which are released into the medium. The second population, which is not covered in polyurethane, uses the small, soluble metabolites produced by the first population to grow. At higher concentrations of polyurethane all the cells present in the media may be coated with polyurethane. The more cells coated with polyurethane the more polyurethane that is degraded and the more metabolites available for growth. This would result in polyurethane-coated cells, which are not free in solution and therefore not detectable. A follow up study [38] revealed that when grown on 1 % Impranil DLN™ YES medium, a lag phase growth was noted for the first 5 h which was followed by logarithmic growth for 8 h, reaching a cell density of 2.60×108 ± 1.17×107. The Monod plot for all concentrations of polyurethane tested did not follow simple Monod kinetics. At higher concentration (9.0 mg/ml to 3.0 mg/ml) of Impranil DLN™ Monod kinetics were not observed. The μ values dramatically decreased at a concentration of 3.0 mg/ml from 1.5 mg/ml to 0.466 doublings/h from 0.721 doublings/h. The μ continued to drop at higher concentrations from 0.466 doublings/h at 3.0 mg/ml to 0.369 doublings/h at 9.0 mg/ml. This dramatic decrease in μ may be explained by observations in a previous study by Blake and Howard [37] that polyurethane was observed to accumulate on the cell surface of a Bacillus sp.

Figure 2. Scanning Electron Micrograph of complex formed between Bacillus cells and polyurethane after a 4 hour exposure.

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5.2. Polyurethane degradation by Pseudomonas Three Pseuomonads have been isolated for their ability to utilize a polyester PU as the sole carbon and energy source. Interestingly, the three species of bacteria produce different PUase activities but are inhibited by serine hydrolase inhibitors. These data suggest that either esterase and/or protease activities are involved in the degradation of Impranil. Growth of Comamonas acidovorans on colloidal polyester-polyurethane resulted in growth parameters for Ks and Îźmax of 0.3 mg/ml and 0.7 doublings/h respectively [39]. A 42kDa PUase enzyme displaying esterase/protease activity has been purified and characterized [39]. NakajimaKambe et al [40, 41] reported a strain of C. acidovorans that could utilize solid polyester PU as the sole carbon and nitrogen source. These authors indicated the role of an extracellular membrane bound esterase activity in PU degradation. Purification of the membrane bound esterase revealed a thermally labile protein having a 62 kDa molecular mass [42]. C. acidovorans strain TB-35 was isolated from soil samples by its ability to degrade polyester PU [40]. Solid cubes of polyester PU were synthesized with various polyester segments. The cubes were completely degraded after 7 days incubation when they were supplied as the sole carbon source and degraded 48 % when they were the sole carbon and nitrogen source. Analysis of the breakdown products of the PU revealed that the main metabolites were derived from the polyester segment of the polymer. Gas chromatographic analysis revealed the metabolites produced were diethylene glycol, trimethylolpropane, and dimethyladipic acid. In agreement with these findings, a later study [43] examined the biodegradation of polyesterpolyurethane foam by P. chlororaphis ATCC 55729. Concentrations of ammonia and diethylene glycol increased over time with an increase of bacterial growth and a decrease in PU mass. A possible biodegradative pathway of PU is shown schematically (Figure 3). Further analysis of strain TB-35 revealed that the degradation products from the polyester PU were produced by an esterase activity [41]. Strain TB-35 possesses two esterase enzymes, a soluble, extracellular and one membrane-bound. The membranebound enzyme was found to catalyze the majority of the polyester PU degradation. The membrane-bound PUase enzyme was purified and characterized [42]. The protein has a molecular mass of 62 kDa, heat stable up to 65 Â°C and inhibited by PMSF. The structural gene, pudA, for the PU esterase was cloned in Escherichia coli. Upon nucleotide sequencing of the open reading frame (ORF), the predicted amino acid sequence contained a Gly-X-Ser-X-Gly motif characteristic of serine hydrolases. The highest degree of homology was detected with the Torpedo californica acetylcholinesterase

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227 Esterase

Esterase

CH3 CH2

N H

C H2

Di-isocyanate

Diethylene Glycol

H2N-R-NH2

Diethylene Glycol

Adipic Acid

Dimethyl Adipic Acid

C H2

Adipic Acid

H2COH

Trimethylol Propane

NH3 + R

Figure 3. Theoretical degradative pathway of polyester-polyurethane by esterase activity of Pseudomonas.

(T ACh E), possessing the Ser-His-Glu catalytic triad, with the glutamate residue replacing the usual aspartate residue. Similarity in the number and positions of cysteine and salt bonds was very apparent between PudA and T AchE, as were also identities of sequences and their positions in the α-helix and β-strand regions between the two. In the neighborhood of the glutamate residue of the Ser199-His433-Glu324 catalytic domain of PudA, there were three hydrophobic domains, one of which constituted the surface-binding domain, which occurred in the C-terminus of most bacterial poly(hydroxyalkanoate) (PHA) depolymerases. Growth of Pseudomonas fluorescens on PU resulted in values of 0.9mg/ml and 1.6 doublings/h for Ks and μmax respectively [44]. Two PUase enzymes have been purified and characterized from this bacterial isolate, a 29kDa protease [44] and a 48kDa esterase [45]. In addition, to the enzymology of the PUases the gene encoding a 48 kDa protein has been cloned and expressed in E. coli [45]. The gene encoding PulA has been sequenced (Genbank, Accession AF144089). The deduced amino acid sequence has 461 amino acid residues and a molecular mass of 49 kDa. The PulA amino acid sequence showed high identity with Group I lipases (58 to 75 %). Growth of Pseudomonas chlororaphis on polyurethane resulted in values of 0.9 mg/ml and 1.3 doublings/h for Ks and μmax respectively [46]. Two PUase enzymes have been purified and characterized, a 65 kDa esterase/protease and a 31 kDa esterase [47]. A third PUase enzyme, 60 kDa esterase, has been partially purified and characterized [46]. Two genes

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encoding polyurethanase activity from P. chlororaphis have been cloned in E. coli [48, 49]. Both genes can be expressed in E. coli. However, the PueA enzyme is secreted in the recombinant E. coli and displays a beta-zone of clearing on polyurethane agar plates while PueB is not secreted in the recombinant E. coli and displays an alpha-zone of clearing on polyurethane agar plates. In addition, PueB has been noted to display esterase activity towards ρ-nitrophenylacetate, ρ-nitrophenylpropionate, ρ-nitrophenylbutyrate, ρ-nitrophenylcaproate, and ρ-nitrophenylcaprylate while PueA has been reported to display esterase activity only towards ρ-nitrophenylacetate and ρnitrophenylpropionate. Upon cloning PueA [48] and PueB [49] from P. chlororaphis in Escherichia coli, the recombinant proteins were noted to have a high homology to Group I lipases. This family of lipases and other serine hydrolases, are characterized by an active serine residue that forms a catalytic triad in which an aspartate or glutamate and a histidine also participate [50-52]. Sequence analysis of the two-polyurethanase genes revealed that both encoded proteins contain serine hydrolase-like active site residues (G-H-S-L-G) and a C-terminal nonapeptide tandem called repeat in toxin (RTX), (G-G-XG-X-D-X-X-X) repeated three times. Group I lipases lack an N-terminal signal peptide but instead contain a C-terminal secretion signal. The secretion of these enzymes occurs in one step through a three-component, ATP-binding cassette (ABC) transporter, Type I secretion system [53]. Proteins secreted by Type I systems typically exhibit two features: (i) an extreme C-terminal hydrophobic secretion signal located within the last 60 amino acids that is not cleaved as part of the secretion process and (ii) -roll structure stabilized by glycine-rich RTX motifs. The RTX repeats form a Ca2+ -roll. These ions coordinated between adjacent coils of the motifs are thought to be important for proper presentation of the secretion signal to the secretion machinery, but their exact role is controversial. Table 2. Identity comparison of PueB and other serine hydrolases. Protein PueB PueA PulA PudA TliA LipA Lipase LipApf33 Lipase

Length (aa/nt) 567/1704 617/1801 451/1353 548/1644 476/1428 613/1789 617/1801 476/1428 449/1338

% Identity (aa/nt)a 100/100 42/59 24/41 11/31 26/40 36/53 39/55 27/41 25/39

Strain

Accession number EF175556 EF175556 AF144089 AB009606 AF083061 BAA02519 BAA84997 BAA36468 JQ1227

Pseudomonas chlororaphis Pseudomonas chlororaphis Pseudomonas fluorescens Comamonas acidovorans Pseudomonas fluorescens B52 Serratia marcescens SM6 Pseudomonas sp. MIS38 Pseudomonas fluorescens 33 Pseudomonas fluorescens SIK W1 a Amino acid and nucleotide identities were determined with Bioedit version 4.8.8 program.

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Figure 4. Single most parsimonious tree inferred from the phylogenetic analysis of polyurethanases and lipases. The numbers above the branches depict total character support/bootstrap support for each branch and node. Branch lengths reflect number of changes estimated along each branch.

Comparison between the amino acid and nucleotide sequences of these two genes revealed that they share 42 % and 59 % identity respectfully (Table 2). Parsimony analysis of the predicted amino acid sequences for the PueA, PueB, PudA, and PulA polyurethanase enzymes and similar lipase enzymes was also performed (Figure 4). Interestingly the polyurethanase enzymes do not form a single cluster, but appear to be distributed among multiple lineages [49]. These analyses suggest that the polyurethanase enzymes thus far studied have evolved from lipases, and are not derived from a single source. Howard et al [54] identified a gene cluster resembling a binding-proteindependent ABC transport system in Pseudomonas chlororaphis in connection with PueA and PueB (Figure 5). The identified ABC transport system consists of three components: an ATPase- binding protein (ABC), an integral membrane protein (MFP), and an outer membrane protein (OMP). The ABC

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pathway has been shown to mediate translocation of an alkaline protease in Pseudomonas aeruginosa [55]. Also, the ABC pathway has been shown to be involved in secretion of a lipase from Serratia marcescens [56], which is located separately from the lipase gene on the chromosome and secretes protease, lipase and S-layer proteins [58]. A gene cluster (accession number AF083061) was identified for an ABC transporter specific for a lipase in Pseudomonas fluorescens SIK W1 [59] and a similar gene cluster (accession number AB015053) was identified in Pseudomonas fluorescens 33 for a lipase gene and two serine proteases [57]. Interestingly, when the two ABC exporter gene clusters of Pseudomonas fluorescens are compared to the ABC exporter gene cluster of the one found in Pseudomonas chlororaphis, a unique gene arrangement is observed (Figure 5). It appears that the novel gene arrangement observed is a combination of the two P. fluorescens gene clusters, and may have resulted through either a rearrangement or an insert ional event between the two ABC gene clusters observed in P. fluorescens. Further investigation of the gene cluster involved growth studies to compare the effects of a PueA deficient strain and a PueB deficient strain with the wild type strain in polyurethane utilization (Table 3). Pseudomonas chlororaphis wild type and its PueA derivatives when grown on 1 % Impranil DLN™ YES medium exhibited a lag phase growth for the first 3 h then was followed by logarithmic growth for 6 h. The wild type reached a cell density of 2.31×108±0.87. The PueA mutant, P. chlororaphis pueA::Kanr, had an 80 %

Figure 5. Comparison of the gene clusters from two strains of Pseudomonas fluorescens and the PUase gene cluster from Pseudomonas chlororaphis. The ABC Reporter Protein, Membrane Fusion Protein and Outer Membrane Protein are involved in Type I translocation of the extracellular protein. The PspA and PspB proteins are serine protease homologues.

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Table 3. Growth kinetic analysis of P. chlororaphis and its derivatives using polyurethane as the sole carbon sourcea. Strain

μmax

doubling time, min 31.5 38.2 29.5

Ks, mg/ml 0.800 0.917 0.710

Cell density, cells/ml 2.31x108±0.87 4.66x107±0.13 2.86x108±0.09

P. chlororaphis (wild type) 1.32 r 1.09 P. chlororaphis pueA::Kan P. chlororaphis pueA::Kanr 1.41 (pPueA-1) P. chlororaphis (pPueA-1) 1.54 27.0 0.649 3.85x108±0.98 r P. chlororaphis pueB::Kan 1.19 34.9 0.893 2.35 x108±0.148 r P. chlororaphis pueB::Kan 1.37 30.4 0.735 3.59 x108±0.187 (pPueB-1) P. chlororaphis (pPueB-1) 1.41 29.5 0.781 3.99 x108±0.813 a The concentrations of Impranil DLN™ used were: 9.0 mg/ml, 6.0 mg/ml, 3.0 mg/ml, 1.5 mg/ml, 0.75 mg/ml, 0.54 mg/ml, 0.375 mg/ml, and 0.18 mg/ml. Each concentration was prepared in triplicate.

decrease in cell number (4.66×107±0.13) whereas, both the complement, P. chlororaphis pueA::Kanr pPueA-1 and P. chlororaphis pPueA-1 had an increase in cell densities, 2.86×108±0.09 (25 % increase) and 3.85×108±0.98 (65 % increase) respectively. The results obtained from the cell densities of each strain were reflected in the growth kinetic studies. Values for Ks and μmax for polyurethane utilization were elucidated by varying the Impranil concentration from 0.18 mg/ml to 9.0 mg/ml. P. chlororaphis wild type exhibited a μmax of 1.32 whereas, the PueA insert ional mutant, P. chlororaphis pueA::Kanr, exhibited a μmax of 1.09. It would be hypothesized that a deletion of the pueA gene would result in a decrease in growth rate. However, the large decrease in growth obtained from the insert ional mutant may indicate that PueA plays a more major role as compared to PueB in polyurethane degradation by P. chlororaphis. When multiple copies of the pueA gene were introduced into either the wild type, P. chlororaphis pPueA-1, a μmax value of 1.54, or the mutant, P. chlororaphis pueB::Kanr, pPueA-1, a μmax value of 1.41, was obtained. An increase in growth rate seems plausible since more PueA produced from the added plasmid would reflect more polyurethane degraded, resulting in an increase in the amount of nutrients available to the cells. The PueB mutant, P. chlororaphis pueB::Kanr, had an 18 % decrease in cell number (2.35×108±0.148) whereas, both the complement, P. chlororaphis pueB::Kanr pPueB-1 and P. chlororaphis pPueB-1 had an increase in cell densities, 3.59×108±0.187 and 3.99×108±0.813 respectively. The results obtained from the cell densities of each strain were reflected in the growth kinetic studies. Values for Ks and μmax for polyurethane utilization were elucidated by varying the Impranil concentration from 0.18 mg/ml to 9.0 mg/ml.

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P. chlororaphis wild type exhibited a μmax of 1.31. When multiple copies of the pueB gene were introduced into the wild type, P. chlororaphis pPueB-1, a μmax value of 1.41 was obtained which was similar to the complement, P. chlororaphis pueB::Kanr pPueB-1, μmax value of 1.37. An increase in growth rate seems plausible since more PueB produced would reflect more polyurethane degraded resulting in an increase in the amount of nutrients available to the cells. However, these values are small and may indicate that PueB plays a minor role as compared to PueA in polyurethane degradation by P. chlororaphis. The insert ional mutant, P. chlororaphis pueB::Kanr, displayed a μmax value of 1.19. Again, it would be hypothesized that the deletion of the pueB gene would result in a decrease in growth rate however; this small variation compared to the wild type suggests that degradation of polyurethane by P. chlororaphis may be more dependent on PueA.

5.3. Binding of polyurethane by polyurethanase enzymes Enzyme molecules can easily come in contact with water-soluble substrates thus allowing the enzymatic reaction to proceed rapidly. However, the enzyme molecules are thought to have an extremely inefficient contract with insoluble substrates (e.g. PU). In order to overcome this obstacle, enzymes that degrade insoluble substrates posses some characteristic that allows them to adhere onto the surface of the insoluble substrate [59-61]. The observations made by Akutsu et al [42] for the polyurethanase PudA indicate that this enzyme degrades PU in a two-step reaction: hydrophobic adsorption onto the PU surface followed by the hydrolysis of the ester bonds of PU. The PU esterase was considered to have a hydrophobic-PU-surface binding domain (SBD) and a catalytic domain. The SBD was show to be essential for PU degradation. This structure observed in PudA has also been reported in poly(hydroxyalkanoate) (PHA) depolymerase, which degrades PHA. PHA is insoluble polyester synthesized as a food reserve in bacteria. In PHA depolymerase enzymes, the hydrophobic SBD has been determined by amino acid sequence analysis and its various physicochemical and biological properties [60, 62]. Another class of enzymes that contain a SBD is cellulases. Several cellulase enzymes have been observed to contain three main structural elements: the hydrolytic domain, a flexible hinge region, and a C-terminus tail region involved in substrate binding [63-65]. Thus far, only two types of PUase enzymes have been isolated and characterized: a cell associated, membrane bound PU-esterase [42] and soluble, extracellular PU-esterases [39, 45, 46]. The two types of PUases seem to have separate roles in PU degradation. The membrane bound PUesterase would allow cell-mediated contact with the insoluble PU substrate

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while, the cell-free extracellular PU-esterases would bind to the surface of the PU substrate and subsequent hydrolysis. Both enzyme actions would be advantageous for the PU-degrading bacteria. The adherence of the bacteria cell to the PU substrate via the PUase would allow for the hydrolysis of the substrate to soluble metabolites which would then be metabolism by the cell. This mechanism of PU degradation would decrease competition between the PU-degrading cell with other cells and also allow for more adequate access to the metabolites. The soluble, extracellular PU-esterase would in turn hydrolyze the polymer into smaller units allowing for metabolism of soluble products and easier access for enzymes to the partially degraded polymer. Studies addressing binding of PUase to soluble PU have been perform. The equilibrium binding of Impranil DLN (polyester-polyurethane) to purified PueA from Pseudomonas chlororaphis was studied by kinetic exclusion assays conducted on a KinExA flow fluorimeter. Briefly, the KinExA comprises an immunoassay instrument that exploits an immobilized form of the polyurethane substrate to separate and quantify the fraction of unoccupied binding sites that remain in solution reaction mixtures of PueA and soluble polyurethane. In this case, the immobilized polyurethane was Bayhydrol 110 adsorption coated onto polystyrene beads, while the soluble polyurethane was Impranil DLN. The results of these binding studies are summarized in Figure 6. Kinetic exclusion assays conducted with 6.6 ﾎｼg/ml PueA in the absence of soluble polyurethane produced fluorescence signals of greater than 2.2 volts with mvolt noise. In the presence of increasing concentrations of soluble Impranil DLN, the fluorescence signal attributed to PueA with unoccupied binding sites decreased to an extrapolated constant value at an infinitely high concentration of the soluble polyurethane that represented nonspecific binding to the beads. The fraction of soluble PueA that contained unoccupied polyurethane binding sites was calculated as the ratio of the difference between the fluorescence signal observed in the absence of Impranil DLN minus that observed in its presence, divided by the difference in fluorescence signals between zero and an infinitely highly high concentration of the soluble polyurethane. The binding data in Figure 6 were fit to a one-site homogeneous binding model with an apparent equilibrium dissociation constant of 220ﾂｱ30 mg/ml Impranil DLN. Since both the soluble Impranil DLN and the immobilized Bayhydrol 110 are hydrolysable substrates for the active PueA enzyme, care was taken to perform individual measurements in such a manner as to minimize the time of exposure of the polyurethane substrates to the active PueA. Thus the PueA窶的mpranil DLN mixtures were assayed within two minutes of mixing, while the PueA captured on the immobilized Bayhydrol was exposed to the fluorescent labeling reagents and wash buffer within

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Figure 6. Equilibrium binding of Impranil DLN to PueA. The concentration of occupied polyurethane binding sites present in different reaction mixtures of PueA and soluble Impranil DLN were determined by kinetic exclusion assays on a flow fluorimeter as described in the text. Each determination was expressed as a fraction of the total PueA in solution and plotted vs. the concentration of free soluble polyurethane. Each datum represents the average of at least two determinations. The parameters for the curve drawn through the data were determined by nonlinear regression analysis using a one-site homogeneous binding model.

Figure 7. Electron micrographs of embedded Bayhydrol 110â&#x201E;˘ polyurethane particles. a) Electron micrograph of polyurethane particles taken at a magnification of 15,000 x. b) Electron micrographs of Immunogold-labeled PueA (1:5,000,000,000 dilution of 0.83 mg/ml PueA) bound to embedded polyurethane particle at 15,000x magnification.

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4 minutes of the initial exposure of the hydrolase to the immobilized substrate. Control experiments demonstrated that much longer exposure times (at least 3-fold longer) were required before a time-dependent deterioration in individual fluorescence signals could be detected. Electron micrographs were used in conjunction with the analysis of binding via the KinExA 3000, Kinetic Exclusion Assay unit. Grids were analyzed at high magnification and electron micrographs were produced from sections incubated in 1:5,000,000 PU and 1:5,000,000,000 PueA (Figure 7). The TEM analysis of PueA, showed PueA to have a high affinity for the polyurethane substrate. Binding was found to be so extensive, that only the most dilute concentrations of PueA could be used, to allow for visualization of areas with individual immunogold labeling.

Conclusions The regularity in synthetic polymers allows polymer chains to pack easily, resulting in the formation of crystalline regions. Crystallinity limits accessibility of polymer chains to degradation whereas; amorphous regions within PU can degrade more readily. In addition, polyester-type PU is considered to be more susceptible to microbial attack than polyether-type PU. The hydrolysis of ester bonds in the polyester segments of PU has been shown to occur through esterase activity. Little information has been reported on the degradation of the isocyanate segment of PU however; the production of ammonia indicates this attack does occur. A diverse group of microorganisms including fungi and bacteria capable of PU degradation can be isolated from soil. The majority of information available to date concerning the mechanisms that bacteria use in biodegradation of PU is from the Pseudomonad group. The esterase enzymes responsible for PU degradation were noted to have a high homology to Group I lipases. Upon nucleotide sequencing of these ORFs, the predicted amino acid sequence all contained a Gly-X-Ser-X-Gly motif characteristic of serine hydrolases. Parsimony analysis of the predicted amino acid sequences for the PueA, PueB, PudA, and PulA polyurethanase enzymes and similar lipase enzymes have been performed. Interestingly the polyurethanase enzymes do not form a single cluster, but appear to be distributed among multiple lineages. These analyses suggest that polyurethanase enzymes thus far studied have evolved from lipases, and are not derived from a single source. Learning more about the pathways for degradation and the genes involved in PU degradation is essential in developing either recombinant derivatives or enriching for indigenous PU-degrading microorganisms for bioremediation.

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References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26.

Bayer, O. 1947, Modern Plastics. 24, 149-52. Saunders, J.H., and Frisch, K.C. 1964, Polyurethanes: Chemistry and Technology, Part II: Technology. New York: Interscience Publ., NY Urbanski, J., Czerwinski, W., Janicka, K., Majewska, F., and Zowall, H. 1977, Handbook of Analysis of Synthetic Polymers and Plastics. Chichester: Ellis Horwood Ltd, UK. Fried, J.R. 1995, Polymer Science and Technology. Englewood Cliffs: Prentice Hall PTR, NJ. Uhlig, K. 1999, Discovering Polyurethanes. Munich: Hanser Publ., Germany. Dombrow, B.A. 1957, Polyurethanes. New York: Reinhold Publ. Co., NY. Kaplan, A.M., Darby, R.T., Greenberger, M., and Rodgers, M.R. 1968, Developments in Industrial Microbiology, 82, 362-71. Young, R.J. and Lovell, P.A. 1994, Introduction to Polymersâ&#x20AC;&#x2122; Chapman & Hall, 2nd Ed. Kanavel, G.A., Koons, P.A., and Lauer, R.E. 1966, Rubber World, 154, 80-8. Santerre, J.P., Labrow, R.S., Duguat, D.G., Erfle, D., and Adams, G.A. 1994, J. Biomed. Mat. Research, 28, 1187-99. Santerre, J.P., and Labrow, R.S. 1997, J. Biomed. Mat. Research, 36, 223-32. Marchant, R.E. 1992, Biodegradability of biomedical polymers, In: Hamid SH, Amin MB, and Maadhah AG, editors. Handbook of Polymer Degradation. New York: Marcel Dekker, Inc., pp 617-631. Ulrich, H. 1983, Polyurethane, In Modern Plastics Encyclopedia, New York: McGraw-Hill, 60, p. 76-84. Huang, S.J., and Roby, M.S. 1986, J. Bioactive Compatible Polymers, 1, 61-71. Tang, Y.W., Santerre, J.P., Labrow, R.R., and Taylor, D.G. 1997, Biomaterials, 18, 37-45. Baumgartner, J.N., Yang, C.Z., and Cooper, S.L. 1997, Biomaterials, 18, 831-7. Kawai, F. 1995. Adv. Biochem. Eng. Biotech., 52, 151-94. Schnabel, W. 1981, Polymer degradation: Principles and Potential Applications, New York: Macmillan Publishing Co., Inc., p. 178-215. Huang, S.J., Macri, C., Roby, M., Benedict, C., and Cameron, J.A. 1981, In: Urethane Chemistry and Applications. Edwards KN, editor. Washington: American Chemical Society, pp 471-487. Phua, S.K., Castillo, E., Anderson, J.M., and Hiltner, A. 1987, J. Biomed. Mat. Research, 21, 231-46. Marchant, R.E., Zhao, Q., Anderson, J.M., and Hiltner, A. 1987, Polymer. 28, 2032-9. Labrow, R.S., Erfle, D.J., and Santerre, J.P. 1996, Biomaterials, 17, 2381-8. Santerre, J.P., Labrow, R.S., and Adams, G.A. 1993, J. Biomed. Mat. Research, 27, 97-109. Pathirana, R.A., and Seal, K.J. 1983, Biodeterioration, 5, 679-89. Evans, D.M., and Levisohn, I. 1968, Int. Biodeterioration. Bull., 4, 89-92. Hole, L.G. 1972, Reports Progr. Appl. Chem., 57, 181-206.

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Flilip, Z. 1978, Eur. J. Appl. Microbiol. Biotech., 5, 225-31. Griffin, G.J.L. 1980, Pure Appl. Chem., 52, 389-407. Darby, R.T., and Kaplan, A.M. 1968, Appl. Microbiol., 16, 900-5. Ossefort, Z.T., and Testroet, F.B. 1966, Rub. Chem. Tech., 39, 1308-27. Boubendir, A. 1993. Dissertation Abstracts International. 53, 4632. Crabbe, J.R., Campbell, J.R., Thompson, L., Walz, S.L., and Schultz, W.W. 1994, Int. Biodeterior. Biodegrad., 33, 103-13. Wales, D.S., and Sagar, B.R. 1988, Mechanistic aspects of polyurethane biodeterioration, In Houghton DR, Smith RN and Eggins HOW, editors. Biodeterioration, 7th edition. London: Elsevier Applied Science, pp 351-8. Kay, M.J., Morton, L.H.G., and Prince, E.L. 1991, Int. Biodeterior. Bull., 27, 205-22. Kay, M.J., McCabe, R.W., and Morton L.H.G. 1993, Int. Biodeterior. Biodegrad., 31, 209-25. Halim El-Sayed, A.H.M.M., Mahmoud, W.M., Davis, E.M., and Coughlin, R.W. 1996, Int. Biodeterior. Biodegrad., 37, 69-79. Blake, R.C., and Howard, G.T. 1998, Int. Biodeterior. Biodegrad., 42, 63-73. Rowe, L., and Howard, G.T. 2002, Int. Biodeterior. Biodegrad., 50, 33-40. Allen, A., Hilliard, N., and Howard, G.T. 1999, Int. Biodeterior. Biodegrad., 43, 37-41. Nakajima-Kambe, T., Onuma, F., Kimpara, N., and Nakahara T. 1995, FEMS Microbiol. Lett., 129, 39-42. Nakajima-Kambe, T., Onuma, F., Akutsu, Y., and Nakahara, T. 1997, J. Ferment. Bioeng., 83, 456-60. Akutsu, Y., Nakajima-Kambe, T., Nomura, N., and Nakahara, T. 1998, Appl. Environm. Microbiol., 64, 62-7. Gautam, R., Bassi, A.S., Yanful, E.K., and Cullen, E. 2007, Int. Biodeter. Biodegrad., 60, 245-9. Howard, G.T., and Blake, R.C. 1999, Int. Biodeter. Biodegrad., 42, 213-20. Vega, R., Main, T., and Howard, G.T. 1999, Int. Biodeter. Biodegrad., 43, 49-55. Ruiz, C., Hilliard, N., and Howard, G.T. 1999, Int. Biodeter. Biodegrad., 43, 7-12. Ruiz, C., Main, T., Hilliard, N., and Howard, G.T. 1999, Int. Biodeter. Biodegrad., 43, 43-7. Stern, R.S., and Howard, G.T. 2000, FEMS Microbiol Lett. 185: 163-168. Howard, G.T., Crother, B., and Vicknair, J. 2001, Int. Biodeter. Biodegrad., 47, 141-9. Jaeger, K.E., Ransac, S., Dijkstra, B.W., Colson, C., Van Heuvel, M., and Misset, O. 1994, FEMS Microbiology Reviews. 15, 29-63 Persson, B., Bentsson-Olivecrona, G., Enerback, S., Olivecrona, T., and Jornvall, H. 1989, Eur. J. Biochem., 179, 39-45. Winkler, F.K., Dâ&#x20AC;&#x2122;Arcy, A., and Hunzinger, W. 1990, Nature, 343, 7-13. Arpigny, J.L., and Jaeger, K.E. 1999, Biochem. J., 343, 177-83. G.T. Howard, R.I. Mackie, I.K.O. Cann, S. Ohene-Adjei, K.S. Aboudehen, B.G.Duos and G.W. Childers. 2007, J. Appl. Microbol., 103, 2074-2083.

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55. Doung, F., Soscia, C., Lazdunski, A., and Marjier, M. 1994, Molecular Microbiology, 11, 1117-26. 56. Akatsuka, H., Kawai, E., Omori, K., and Shibatani, T. 1995, J. Bacteriol., 177, 6381-9. 57. Kawai, E., Akatsuka, H., Idei, A, Shibatani, T., and Omori, K. 1998, Mol. Microbiol., 27, 941-52 58. Ahn, J.H., Pan, J.G., and Rhee, J.S. 1999, J. Bacteriol., 181, 1847-52. 59. Van Tilbeurgh H., Tomme, P., Claeyssens, M., Bhikhahai, R., and Pettersson, G. 1986, FEBS Letters, 204, 223-7. 60. Fukui, T., Narikawa, T., Miwa, K., Shirakura, Y., Saito, T., and Tomita, K. 1988, Biochimica Biophysica ACTA, 952, 164-71. 61. Hansen, C.K. 1992, FEBS Letters, 305, 91-6. 62. Shinomiya, M., Iwata, T., Kasuya, K., and Doi, Y. 1997, FEMS Microbiology Letters. 154, 89-94. 63. Knowles, J., Lehtovaara, P., and Teeri, T. 1987, Trends in Biotechnology, 5, 255-61. 64. Bayer, E.A., Setter, E., and Lamed, R. 1985, J. Bacteriology, 163, 552-9. 65. Langsford, M.L., Gilkes, N.R., Sing, S., Moser, B., Miller, R.C.,Jr., Warren, R.A.J., and Kilburn, D.G. 1987, FEBS Letters, 225, 163-7.

Transworld Research Network 37/661 (2), Fort P.O. Trivandrum-695 023 Kerala, India

Recent Developments in Polymer Recycling, 2011: 239-260 ISBN: 978-81-7895-524-7 Editors: A. Fainleib and O.Grigoryeva

8. The influence of adhesives on material recycling Hermann Onusseit

Henkel AG & Co. KGaA, 40589, Duesseldorf, Germany

1. Introduction As a result of both the dramatic increase in world population and the improved living standard of many people after the industrial revolution, we have seen an extreme increase in consumption of natural resources such as wood, water, coal, metal, and oil. The world-wide consumption of natural resources increased from four to ten billion tons between 1963 and 1995 â&#x20AC;&#x201C; the consumption of resources rose faster than world population. Experts expect that the consumption of natural resources will quintuple by 2050, if it develops equally to population growth. That is to get one's sums wrong because this is more than planet earth can supply in the long term. Already in 1972 the report of the Club of Rome showed the limitation of growth and opened our eyes to the finiteness of material resources. An increasing consumption would lead to a depletion of important mineral and fossil raw materials in a few decades. While at the beginning of the Industrial Revolution nobody much thought about the finiteness of natural resources, during the last decades of the twentieth century the call for processes which carefully treat resources became louder and louder. The finiteness of many natural resources made it Correspondence/Reprint request: Dr. Hermann Onusseit, Henkel AG & Co. KGaA, 40589, Duesseldorf Germany. E-mail: Hermann.Onusseit@henkel.com

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necessary to think about how to save them by developing resource-saving technologies. This leads to the idea of Sustainable Development. Sustainable Development was introduced at the environmental summit in Rio de Janeiro in 1992 as a model for future action. The idea is that people living today should be able to satisfy all their needs without leaving a heritage of pollution, climate problems, and used-up resources for future generations. The solution of this task depends upon our ability to use the existing resources of the earth economically and ecologically efficiently. These existing resources are too precious to destroy them after single use. Methods to stop the impending depletion of our natural resources are economical handling of resources, repeated use of sophisticated goods, and consequent recycling on the highest technical level. To execute this recycling successfully, it is necessary to consider this early during the construction and production of goods. The approach is called closed-cycle economy. This is a kind of economy, which was initially introduced in the nineteen nineties and nowadays seems to be not only an ecological but also economical model of success. Avoiding waste is top priority. Waste has to be recycled as far as possible and material should be recovered. Waste does not have to be discarded but can be a raw material very much in demand with lots of opportunities for use. Waste residues would be reduced to a minimum if material circulation could be closed completely. Recipes which make good ecologic sense cannot be free of charge, but the introduction of new sorting and recycling techniques will lower the costs of recycling considerably and will also improve the quality of secondary raw materials. This is a prerequisite for sophisticated products that must hold their ground in the international market. The objective is to save resources and to find renewable raw materials and energy sources with a sustainable economy for all future generations. Legal regulations reflect this trend in our society. For instance, the introduction of the Packaging Directive in Germany in 1991 was an important step in this direction. The recycling rates and collecting systems predetermined in it have had a strong influence on the recycling of packaging material. On a European level this thought was legally established in the European Packaging Directive (Directive 94/62/EC) dated 20 December 1994. The objective of this directive was to reduce the amount of packaging waste in Europe by 50 % by 2001. At the end of 2001, as a new aim, a recycling rate of 55-70 % by the year 2006 was proposed. The Directive 2004/12/EC of the European parliament and of the council amending Directive 94/62/EC on packaging and packaging waste has set further targets. In order to comply with the objectives of this directive, member states shall take the necessary measures to attain the following targets covering the whole of their territory (Article 6 b), d), e)): No later than 31 December 2008, 60 % as a minimum by weight of packaging waste will be

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recovered or incinerated at waste incineration plants with energy recovery. No later than 31 December 2008, between 55 % as a minimum and 80 % as a maximum by weight of packaging waste will be recycled. No later than 31 December 2008, the following minimum recycling targets for materials contained in packaging waste will be attained: 60 % by weight for glass; 60 % by weight for paper and board; 50 % by weight for metals; 22,5 % by weight for plastics, counting exclusively material that is recycled back into plastics; 15 % by weight for wood. Further regulations that demand a recycling of products according to their use are e. g., the EU "End of Life Vehicles" Directive (2000/53/EEC) from 18 September 2000. By 2005 at least 85 % (80 % material recycling) by weight of scrapped cars was to be reused or recycled, and this must rise to 95 % (85 % material recycling) by the year 2015. To execute the idea of the recycling law it is necessary to think about a material recycling of the used products when designing them. This means that all additives of the process have to be designed in a way that they do not disturb later material recycling. This is especially important for the field of adhesives, because many of todayâ&#x20AC;&#x2122;s products are produced with the help of adhesives.

2. Bonding technologies Most of the products of everyday life consist of combinations of single components. To guarantee the function of these products, these single components need to be assembled. Joining techniques such as welding, brazing, riveting and screwing are used by industry all over the world on a daily basis, but another method of joining has also proven to be highly successful: adhesive bonding. Known for thousands of years, this method has become as important as the other joining techniques as a result of the pace of developments in recent years. In many areas, this bonding technology has even become a key technology: New, hitherto unrealizable combinations of materials, as well as the need for the highest requirements for connections, have made this flexible joining technique the preferred technology, especially in high-technology areas. In hybrid-joining, the unique advantages of using adhesives are combined with the benefits of other joining techniques.

2.1. Adhesive bonding - joining technology with high potential Virtually all solid materials can be connected with one another using adhesive â&#x20AC;&#x201C; for example glass with metal or paper with plastic. High temperatures are not required in the joining process. That is beneficial for the materials and prevents shape-distortion. Additional functions such as corrosion protection, vibration damping, electrical conductivity and sealing to liquids or gases can be

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integrated into the bonded connection. Regarding the designing of adhesive bonded joints, optimum solutions which meet all requirements are determined. Mechanical loads which act on the connections are investigated and also, for example, the effects of liquids, gases, heat, electric current and light. Computer simulations and tests on standard samples give information about the optimum geometric design of the bonded joint, with strength under continuous loads being the most important criteria. The simulation results are then verified by means of load tests on pseudo-components. The successful realization of bonded connections in industrial production requires precise planning of the individual production stages. The joining process must be compatible with previous, subsequent and concurrent production stages. The handling of the parts to be joined is just as important as adapting the technology for applying the adhesive to the use: feeding, mixing, dosing and applying. The combination with other joining techniques is also of key importance. The various options for the hardening step must be taken into account at the planning stage [1]. For high-quality connections, special pre-treatment of the surfaces to be bonded is often necessary. The materials to be bonded are cleaned and activated or modified so that adhesives can adhere better to them. This also gives the surfaces to be joined protection against corrosion. An example is the pretreatment of aluminium for aircraft construction. Industrial adhesives are primarily required to join materials in a fast and safe way, and to provide for trouble-free, inexpensive production in existing manufacturing lines (using automatic machinery as far as possible). Furthermore, they must resist the conditions to which the finished product will be exposed when being used later. Price is another important aspect that must not be underestimated when selecting an adhesive. Ecological aspects, too, play an increasingly important role for the selection of an adhesive system. For example, more and more care has been taken that adhesives can be processed in a way that as little waste and wastewater as possible accrue from their production. In addition to that, the impact of adhesives on the reuse (e.g. returnable products) or recycling of bonded products is a growing matter of interest. As adhesives are important for the production of nearly all goods today, it is increasingly important to understand how adhesive applications influence the idea of the closed-cycle economy. Although adhesives are not recycled per se because of their small amount, they have to be designed in the way that they do not cause troubles when recycling the primary material.

3. Adhesives in recycling processes Today adhesives play a decisive role in the production of almost every good, especially for mass-produced articles. Bags made of paper, graphic products like

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catalogues or magazines, furniture, cars and airplanes − all these products are manufactured with the help of adhesives today. Hence it is getting more important how adhesive applications influence the idea of closed loop economy. In comparison to the materials being bonded by adhesives, the adhesive quantities used are so small that it is normally not worth recycling the adhesive itself. Even though adhesives due to their small amount will not be recycled alone, it must be considered that they should not disturb the recycling of the primary materials. However, today more and more primary materials are retrieved by recycling. This applies particularly to non-durable materials, such as packages are made of. It is expected of the adhesives used for these products that they do not interfere with the recycling of the primary materials, and not affect the quality of the secondary raw materials. Which properties are required from the adhesives, depends on the applied recycling process. Adhesives that consist of organic polymers are normally trouble-free in recycling processes that take place at high temperatures e. g., metal or glass recycling, but the choice of adhesive for plastic and paper products is important because of the low temperatures during the recycling process.

4. Adhesives in high temperature recycling processes High temperature recycling processes are used for the recycling of glass and metals. Temperatures above several hundred degrees are used, and adhesives based on organic material will be burned. A special recycling process is energy recovery. In the “five step hierarchy” in waste management – prevention, reuse, material recycling, energy recovery, and landfill – the waste-to-energy process is another possibility to save resources.

4.1. Glass recycling Glass is one of the oldest materials used for many different applications: it has long been used for packaging beverages like wine and other liquids. Due to its strength, tightness, clarity, and tastelessness it is perfectly suitable for this application. To mark glasses and bottles, labels are bonded onto glass packaging. Glass is also used in adhesively-bonded windows in great quantities. These achieve better thermal insulation of buildings and are an important architectural design tool. Recycling of used glass and bottles has a long tradition. Used glass was already recycled for the production of new glass in the Middle Ages, probably even in ancient Rome. Today recycling of glass is at a very high rate today: At the beginning of the nineteen seventies used glass was systematically brought back to glassworks in large quantities to introduce glass recycling extensively.

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In 1974 in Germany 150,000 tons of domestic sales of 2.3 million tons of container glass were reused, corresponding to a recycling rate of 6.5 %. By 2002 the recycling rate of container glass had increased to approximately 90 % [2]. Because of the willingness of the consumer to sort the different colors and because of very exact quality assurance systems in the recycling facilities, a green bottle consists of more than 90 % of used glass, and brown and white bottles have more than 50 % recycle content. The “Duales” System Germany has set up more than 300,000 glass collection containers throughout the republic to collected used glass packaging. All together more than 1,3 million tons of waste glass packaging were collected by the “Duales” System Germany during the year 2007 [3]. In total, in Germany approximately 2,5 million tons of glass were recycled in 2007. In Europe, in 2006 nearly 10,5 million tons of glass packaging were recycled (compared to 9,8 million tons in 2005) [4]. Thanks to glass recycling, waste disposal in Europe has declined (by 13 % between 1996 and 2006) despite an increase in the demand for glass packaging (by 11 % between 1996 and 2006). In glass recycling the batch is melted at a temperature of about 1550 °C in a tank. Adhesives based on casein or synthetic polymers that are used to label glass bottles, or adhesives based on polysulfide and butyl rubber used for the production of insulating glass, do not disturb the glass recycling process. Because they consist of organic material, they burn at high temperatures without influencing the glass recycling process or the quality of the “new” glass.

4.2. Metal recycling Adhesives are increasingly used for joining metal-to-metal and for joining metal with other materials. Similarly to glass, the recycling of metals has been a common process for a long time. In contrast to many other materials, in the recycling of metal there are no quality losses. Compared to primary metal extraction, a 95 % savings in energy can be achieved by aluminum recycling and a 75 % saving by steel recycling. Great strides have been made especially in the recycling of packaging made of metal, because the filling of food into aluminium cans or tin plate has increased dramatically during recent decades. The economic value of aluminium has always been a reason for bringing the material into the loop of metal extraction, processing, use and recovery. Aluminium has been recycled since the days it was first commercially produced and today recycled aluminium accounts for one-third of global aluminium consumption world-wide. In Europe, aluminium enjoys high recycling rates, ranking from 58 % in beverage cans to 85 % in building and construction and 95 % in transportation. With the introduction of the “Duales” Systems Germany

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in the packaging field there has been set up an exhaustive system for acquisition, sorting and reuse of aluminium packaging. In fact, there have been various recycling initiatives of the aluminium industry before, like melting down beverage cans, but for logistical reasons these programs did not achieve high recycling rates. The "Duales System" in Germany, however, shows a recycling rate of more than 97 % for aluminium packaging in 2002 [3]. The use of eddy current separators in sorting makes it economically attractive to collect and recycle all packaging containing aluminium and aluminium foil, and this contributes to fulfilling the recycling specifications of the packaging directive. During recycling, the aluminium cans are broken up in a shredder into small pieces, and then are conveyed into a de-lacquering oven to remove the paint, the labels, and residual moisture. The hot shredded aluminium is then passed over a small screen to remove any dirt and contaminants and fed directly into a rotary furnace. Heated to 650 째C, the cans melt and blend in with the molten metal already in the furnace. The reclaimed aluminium can be used as new material without loss of quality, and much less energy and water is used than in the production of new aluminium from bauxite. Cans or tins made of tin plate consist of 99,8 % steel with a wafer-thin layer of tin. The recycling of used tin plate cans is possible with easy processes resulting from the unique magnetic properties of steel. Tin plate can also be recovered from mixed waste without problems. According to the latest reuse statistics of the "Duales Systems" German, households collected approx. 188,000 tons of tin plate in 2007 [3]. In Europe the recycling rate of tin plate cans was 66 % in 2006 [5]. As a consequence of the high temperatures of all metal recycling processes, adhesives made of organic polymers, used for labeling cans or for bonding any kind of metals in construction and transportation, incinerate and so make possible trouble-free recycling.

4.3. Energy recovery Burning organic waste is another possibility to save resources. Taking into account that more than 90 % of crude oil is used for energy production, there is a great value in burning organic waste to utilize energy. In principle this can be done with plastic, wood, or paper and board waste. This kind of recycling is especially useful if different kinds of waste are mixed and therefore in material recycling the quality of the secondary material would be worse. Plastics especially continue to decouple growth in demand and material to landfill, both by energy recovery and by material recycling. Adhesives made of organic polymers incinerate and contribute to the energy recovery of waste (cf. Figure 1) [6].

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Change from EU15+2 to EU27+2 in 2005 6000 5000 4000 3000 2000 1000 0 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008

Figure 1. Growth of feedstock (Â ) and mechanical (z) recycling and energy recovery (S).

5. Adhesives in low temperature recycling processes In recycling processes which run at high temperatures, adhesives composed of organic materials show no negative influence. However, in recycling processes that operate at room temperature or just slightly above room temperature, adhesives can cause problems in the process or impact the quality of secondary raw materials. The largest-quantity, most important recycling process that takes place at low temperatures is the recycling of cellulose paper, paperboard and cardboard, and the mechanical recycling of plastics used in packaging.

5.1. Paper recycling In spite of synthetic packaging materials and the rapid growth of electronic media, paper and board consumption is increasing steadily. While in 1950 about 50 million tons of paper were produced world-wide, in 2005 approximately 366 million tons were produced. In the year 2010 four hundred million tons are projected. More than two thirds of this paper is consumed in Europe, Japan and the USA, where only one fifth of the world population lives. To make this increase in paper production possible and for saving resources at the same time, paper recycling has intensified steadily in the last decades and has now reached a high technical level. Most products made of paper have a life span of only a few days (eg, newspapers) or a few weeks (eg, packaging). Therefore it is not striking that the thought of recycling has been a firm component of paper production for a long time. As early as the 13th century waste paper was reused. Not only have the

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Million tons

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Figure 2. European paper recycling (utilization( ), net trade (z), recycling rate(S)).

technical conditions for recycling changed in the following centuries, but also the reasons for recycling. At the beginning, suitable raw material for producing writable material was scarce and the use of waste paper was largely determined by economic interest. Especially in countries with not much wood the consumption of wood could be reduced and the forests could be saved. Today the economic advantages are largely fully exploited. The increase of waste paper use in industrialized countries is determined by problems of disposal. In the sense of resource saving, the recycling of used cardboard packaging materials and other papers is a further example in accordance with the idea of sustainable development. It is a prime example for treating all resources including the renewable ones as carefully as possible. Paper recycling in Europe increased markedly throughout the 1990s. The amount of paper collected and recycled at the end of the decade was roughly two thirds more than at the beginning. This means that the recycling rate (percentage of recovered paper use compared to total paper consumption) was 64,5 % in 2007, compared to about 33 % in 1995 according to the “Monitoring Report” of the European Recovered Paper Council. A total of 58,2 million tonnes of paper and board were recycled in Europe in 2006 and more than half of the paper used in Europe today is now made from recovered fibre. Recycling is a significant part of the paper manufacturing process in Europe but also a large industry in its own right, with links to a number of sectors in the global economy. Building on the success of the initial “European Declaration on Paper Recovery” launched in 2000, which was responsible for pushing Europe’s recycling rate to 64,5 %, the new declaration launched in 2007 covers more European countries, more European organizations and has even greater ambition. The European sectors

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have now joined forces with the common goal of further increasing Europeâ&#x20AC;&#x2122;s recycling rate to 66 % by 2010 (cf. Figure 2) [7]. The new target would mean that some two tonnes of paper is recycled in Europe every second. Twelve different sectors in the paper value chain have pledged their support for the declaration covering all paper and board products, and all aim to make sure that correct systems are in place to push European paper recycling rate even higher. Worldwide, the use of waste paper also increased considerably, from about 85 million tons in 1990 to about 188 million tons in 2005 [8].

5.1.1. Paper recycling processes The prime objective of waste paper recycling is to utilize the fibres contained in pre-or post-consumer waste paper. Non-fibrous components, whether they derive from paper or are added during the processing or the use of paper, should be removed at the highest degree possible to avoid quality defects in the produced papers (specks, holes) and production process faults (e. g., wet web breaking). If you take a closer look at paper and paper products, you notice that even simple papers and boards do not consist solely of cellulose fibres. They contain many additives that guarantee the particular use properties of the papers. In the paper mill many papers are coated to improve surface properties. In processing, most graphic and packaging papers are printed afterwards and then partly varnished or coated. Due to the large number of materials that get into contact with paper during its life cycle, there are very different impurities. Regarding disturbances in production, especially thermoplastic impurities (stickies) must be mentioned. At typical drying temperatures of 80 Â°C to 120 Â°C, many thermoplastic substances get soft and sticky, and so lead to problems in the paper machine drying sections. Thermoplastic deposits can also show sticky properties at or only slightly above room temperature, depending on their glass-transition temperature. In principle all non-paper components that can form sufficient adhesion and cohesion can be a source of sticky impurities (eg, resins from wood, coating binders, inkbinders, coatings, impregnation, adhesives). To fulfill both these requirements, stickies have to be liquid or at least must be soft enough to form sufficient adhesion bonds. At the same time the particles must be big enough and must have enough cohesion to achieve noticeable effects. Thermoplastic particles (stickies) big enough to achieve noticeable effects (sufficient adhesion and cohesion) get into the paper machine principally in two ways: particles come in from the waste paper to the drying section already big enough; or they agglomerate during the recycling process, forming large particles from small ones.

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5.1.2. Adhesives in paper recycling Most products made of paper and paperboard are put together then with the help of adhesives to form complex finished products. As adhesives play an important role in these products, it is not surprising that paper and packaging adhesives have a dominant share of the adhesive market (cf. Figure 3) [9]. If we talk about the influence of adhesives in paper recycling, first it is necessary to make a distinction between external and internal paper recycling.

Woodworking 9%

Othe r 7%

Pape r/Pack aging 27%

Construction/ Handcraft 20%

Transportation 18% Hous ehold/DIY 18%

Shoe s/Le ather 1%

Figure 3. German adhesives marked.

5.1.2.1. External paper recycling Adhesives have always been a decisive factor in the manufacture of packaging materials made from paper, board or cardboard and for the production for graphic art articles. From the beginning of the 20th century, paper, a natural product, was bonded with adhesives based on natural raw materials such as proteins, starch or cellulose. Today, a wide variety of different materials are used in the packaging industry, and many requirements can only be met by synthetic adhesives. External recycling (post-consumer waste) is the recycling of external accumulated paper waste, primarily from used packages or used newspapers. Whereas printing inks are removed by flotation, the most important removal process for most of the other impurities is sorting. If you look at the post-consumer paper recycling process in detail, you will notice that it is primarily a mechanical process (supported by heat and alkali). In this process one tries to weaken the composite of the cellulose

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fibres by applying mechanical power, so that only single cellulose fibres remain. The mechanical power in a pulper or in refining drums is chosen so that it allows fast but careful fibre isolation. These single fibres can be used for new paper (cf. Figure 4). Therefore, for external recycling paper recyclers demand: ‘Non-paper components should be dimensioned and mechanically stable in such a way that they survive as large particles, without being comminuted, in the conditions of pulping and allow mechanical separation by means of punched screens, slot screens and centrifugal purifiers. Relevant examples are cover foils, staples, thick adhesive layers, various product samples. Materials applied in very small dimensions or disintegrating into very small parts are unfavourable, because they can not be removed using today’s conventional sorting methods. Recovered paper components which dissolve in the process under standard conditions of deinking (pH=8–10) and reach the process water pose a risk of unintended spreading to all parts of the paper machine. This results in the requirement that recovered paper should contain as few components as possible which dissolve or disperse in a weakly alkaline medium and form sticky residues or cause discoloration [10].’ To fulfill these requirements the paper mills that produce paper or cardboard out of waste paper have lavish cleaning systems (sorting machines and for graphic papers also deinking systems). After defibrating, the suspension passes through several successive cleaning systems in which impurities are separated by their density, size or shape. Today slotted screens with a slot width up to 0.15 mm are considered most effective as far as sticky removal is concerned.

Figure 4. Paper recycling process.

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Modern recycling processes in paper mills today allow the sorting of big and compact (>0,2 mm) thermoplastic impurities. In spite of the most modern technology it is not possible to remove water-soluble or very small dispersed particles from the water loops by sorting. Although these particles are so small that you would not expect them to cause any harm, they may agglomerate during the process and so grow to bigger particles (the so-called secondary stickies) that then cause the well-known disturbances in the paper machine or the well-known quality flaws of the finished product. Whether an adhesive film is mechanically stable enough to withstand undamaged the paper recycling process, so it is not torn into small pieces which pass all sorting facilities, depends on its inherent strength and on its geometry. Furthermore it has to be considered that the cohesion of all materials decreases as temperature rises. The same adhesive will behave completely different when the adhesive film shows different geometry or the recycling process takes place at different temperatures. Thick, compact adhesive films will normally not be broken at moderate temperatures. However the wish for thick adhesive layers during processing of paper and board often conflicts with the wishes of the paper converter. In general one tries to apply as little adhesive as possible, if only for cost reasons. In addition, modern computer-controlled nozzle systems can optimize the adhesive application. There are today many adhesive application systems that apply single points (up to several hundred separate adhesive points per second) and therefore lead to an extreme cost-saving and clean application of adhesive (cf. Figure 5). Next to the saving of adhesive and efficient production, such systems also have ecological advantages. Closed systems need considerably less cleaning expenditure and in the case of water-borne adhesives very little waste or waste water is produced. Even with mechanically stable, non-water-soluble adhesives, there is the danger that these films created out of small droplets are so thin that they cannot be sorted out in the screening machines of the paper recycling mills. Moreover, very thin films and high temperatures reduce the power that can be absorbed by the adhesive film, and it is easily torn into very small parts. The best classification for adhesives regarding their influence on paper recycling is the general view of adhesive film properties, because in waste paper the adhesive exists as a set or cured film (cf. Figure 6) [11]. In general an adhesive film should have a higher inherent strength than the substrates to be bonded, for example a much higher inherent strength than paper, cardboard or carton or composites (fibre tear), so that it is more easily separable. Non-water-soluble adhesives which are characterized by a high glass transition temperature show enough cohesion in a sufficient layer thickness to survive the pulping process without damage. The problem is

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more complex regarding water-soluble or re-dispersible adhesives, as the environment in the recycling process contributes to the weakening of the mechanical stability of the adhesive film. At the predominant pH-values and temperatures of the aqueous environment in the waste paper mills, considerable stress is put on films that contain hydrophilic groups. This can lead to the complete destruction of these films, as their cohesion gets lost completely. By a good choice of raw materials, however, water-borne adhesives can be formulated in the way they are not torn into such tiny particles in the recycling process, that they remain mechanically sortable (cf. Figure 7). Figure 8 shows the adhesive particle distribution of two waterborne pressure sensitive adhesives (PSA) coatings after the repulping of labels. As you can see clearly, adhesive film A is torn into much smaller particles than adhesive film B. The large particles of adhesive film B can be removed from the cellulose suspension nearly completely by mechanical sorting, as tests have shown. The particle sizes given in Figure 8 are not the real size of the three-dimensional adhesive residues, but sizes that were found in a picture-analytical method. These are two-dimensional measurements that were converted into circles of the same size. From these circles the diameters were calculated [12].

Figure 5. Computer-controlled nozzle application systems.

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Figure 6. Systematic of adhesives films.

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Figure 7. Mechanical separation of adhesives depending of the adhesive film thickness.

Figure 8. Mechanical separation of adhesives.

5.1.2.2. Internal paper recycling Internal paper recycling (pre-consumer waste) describes the recycling of production waste within a paper mill with a processing line on site. An

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example is tissue and towel mills where paper goes directly from the paper machine to rewinders for the production of bathroom tissue or paper toweling. During this processing, adhesives are used for laminating, for the pick-up of the first sheet on the tube, and for the end sheet tiedown, and the waste, or ‘broke’, created here must be returned to the paper mill. Here, in general, only a relatively small amount of rejects are moved back into the paper production process. In contrast to adhesives that get into paper mills by external recycling and have sources varied and unknown, in internal recycling there is only a relatively small amount of adhesive and the types used are known exactly. These mills normally do not have lavish sorting machines. As the additives added to the paper in production cannot be sorted out mechanically, most of the time the additives are required to be completely water-soluble or re-dispersible, even if this pollutes the process water with impurities. Adhesives that are used in this production are normally classified by the European Standard EN 1720 “Adhesives for Paper and Board Packaging and Disposal Sanitary Products Determination of Dispersibility” or by the American TAPPI standard UM 666 “Dispersibility Test for Adhesives”. Today there are many adhesives that fulfill the requirements. Adhesives based on polymers like polyvinyl alcohol, polyvinylpyrrolidone and its copolymers, polyethyloxazoline, copolyesters containing sulphonated material, hydrophilic polyurethane and polyethyleneoxide, but also adhesives based on starch, dextrin and cellulose show good watersolubility or re-dispersibility [13]. In this connection one has to emphasize that soluble or re-dispersible additives in paper mills can lead to two problems: a contamination of the process waters that leads to more waste water pollution; and future agglomeration of dissolved or re-dispersed additives. If they are thermoplastic materials, so-called secondary stickies can be formed.

5.2. Adhesives in plastic recycling processes Bonding is an ideal joining system for plastics to themselves or to other materials, and in the field of packaging many examples can be found. The plastic material with the strongest increase in growth rate in packaging is polyethyleneterephthalate (PET). Light weight, transparency and flexibility characterize this perfect packaging material. The success of PET and especially of PET bottles results from the existence of successful recycling systems, some of which allow a closed circulation from bottle to bottle. Depending on the process the recycling material is sorted, crushed, cleaned and separated from secondary products in several steps. Some processes at the end provide a recycled material that is approved by the American

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watchdog FDA for contact with foodstuff and fulfills the greatest demands regarding purity and quality. With this favourable recycled material new preforms can be manufactured for bottle production and so the circuit is closed. In 2007 about 1,200,000 tons of PET bottles were collected for the different recycling processes in Europe [6]. In addition to the production of new bottles (15,2 %), these used bottles also become fibres for clothes, carpets, PET sheets and binders. Compared to glass and metal can recycling, adhesives that are used for labeling of PET bottles have to be adjusted to the recycling process of PET. Depending on the recycling process, alkali- or water-soluble labeling adhesives are used for disposable bottles as these make possible an easier cleaning of the bottle before recycling. This feature is especially important when selecting a hot melt adhesive, because hot melts normally consist of non-water-soluble thermoplastic polymers. Water- or alkali-soluble or redispersible hot melts developed especially for labeling of PET bottles fulfill all recycling requirements. To reduce the effort during the recycling process, it is important to apply only the minimum amount of adhesive necessary. Special application systems for hot melt adhesives exist which spray only a very light quantity of adhesive onto the bottle for the pick up of the label (cf. Figure 9). Hence, when selecting a labeling adhesive the later recycling of the container should be considered right from the beginning. In recycling, the recycled waste should be as mono-materialistic as possible so that the secondary materials gained are of high quality. Recycling of the plastic base materials is also possible if the quantities and the material of impurities are below a particular level that can be identified very precisely experimentally. Although normally the amount of adhesive applied is very small compared to the parts bonded, a mono-material recycling can be guaranteed even with small amounts by a smart choice of the material of the bonded parts. Examples are pressure sensitive labels used in the automobile industry or electronic industry. These labels are expected to stick to the surfaces of plastic parts safely for decades, but do not disturb the later material recycling of the construction parts. They are made of the same chemical material as the body or some other material that does not cause a quality loss in recycling of the body material. When using such labels, the primary material can be recycled trouble-free without causing quality loss [14]. If plastics undergo a chemical recycling process (decomposition of the polymers into their monomers by depolymerization) small amounts of other organic substances do not disturb, as they can be removed from the monomer stream.

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5.3. Recycling of multilayer packaging material Beverage cartons for short-life goods are made of multilayer layers of cellulose and polyethylene (PE). For long-life goods, there may be an additional aluminium layer between the cellulose and PE layers. These thin materials – about 0,4 mm carton, 0,06 mm PE and 0,0065 mm aluminium – fulfill all requirements for beverage packaging. Recycling of beverage cartons is similar to paper recycling and so can be done without much energy. Not only the high portion of paper (75-80 %), but also the remaining materials polyethylene (21 %) and aluminium (4 % – used only for durable products) are returned into material circulation, which makes good economic sense, too. In 2006, more than 313.000 tons of beverage cartons were collected for recycling in Europe (151.000 tons in Germany). This corresponds to a recycling rate of about 30 % in Europe and 65 % in Germany [15]. In closed pulpers equipped with bars the cartons are crushed, which lets water seep into the carton and makes possible the dispersal of the fibres in a water bath without further additives. In very short time, the cellulose fibres of the carton can be screened out as pulp. Because of their good strength, recycled fibres from beverage cartons are very much in demand. They are used for example for corrugated board and tubular cores, which are needed for rolling up paper, fabrics or carpets. There are several processes to recycle the remaining polyethylene and aluminium foils usefully and at the same time save primary raw materials. These polyethylene and aluminium residues can be used for energy and material utilization in the cement industry. Polyethylene plastic can be used there as a substitute for organic energy sources like oil. Aluminium oxide forms at a temperature of more than 1,450 degrees and can be added to the cement as a necessary loading, so that the cement can solidify faster. In this process, the aluminium residues substitute for some of the primary raw material bauxite. The complete utilization of residue components of beverage cartons - PE and aluminium – can be made with another elaborate process: In a pyrolytic reactor polyethylene is converted at a temperature of 400 °C into process steam and electrical energy. The aluminium is separated as a regained mono-material that can be returned to the aluminium industry in the form of bars. Adhesives like hot melts, used for attaching pour spouts (fitments), do not disturb this process as they can be sorted out easily and can be utilized for their energy content, too.

6. Disbond on command, adhesives with “switches enclosed” Durable goods made of plastic or metal are often a mixture of different materials. In recycling, a separation of the elements is useful, and that separation

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should be as mono-materialistic as possible so that the secondary material gained are of high quality. Basically almost all bondings can be removed relatively easily, often by the use of high temperatures, but cost and possible damage to the primary material must be considered. Just as time was a factor in the original bonding process, so is time also a factor in their later separation. One possibility of “dibond-on-command” is to put energy just into the bonded joint to melt, dissolve, or destroy the organic bond line. This might be especially advantageous on pieces where it is not appropriate to heat the entire piece, or on synthetic materials which would be affected by heat. However, with intelligent solutions systems can be found that combine the advantages of bonding with an easy dismantling or an easy recycling of the primary material. One idea in this direction is the use of nanoparticulate ferrites, which pick up energy from electromagnetic AC-fields, convert it to heat and pass it on to the immediate surroundings of the adhesive layer. This enables a fast, targeted and locally defined energy uptake and thus the "switching" of properties in the surrounding matrix (e.g. polymers). Requirements for a fast debonding with simultaneous intrinsic overheat protection include a tailored combination of ferrite composition, magnetization, solids content, particle size and particle dispersibility in the adhesive [16]. Another possibility to develop adhesives with “switches enclosed”, are adhesives based on binders with di- or polysulfide bonds in the polymers. The joints bonded by these adhesives are debonded by activating the debonding components. through melting, conformation changing or splitting off the protective groups. The activation may be done through the activity of conductive heating, radiation heating, particle radiation, elec. current, elec. or electromagnetic field, ultrasound, pressure wave, or impulse wave. Adhesive containing room temperature stable, thermally activatable components, which cause swelling of the binder matrix, phase transition, decomposition or generation of gases or water vapor are also suitable for separation of adhesive bonds under heating, which permits complete recycling of the separated parts. As thermally activable components malonic, oxalic, adipic, citric, glutaric, ascorbic, or benzoic acid, as well as solid diols, or azo components can be used [17].

Conclusions The recycling of all used products, but especially of mass product articles like packaging or graphic arts, is economically as well as ecologically useful. Legislation in Germany and throughout Europe has contributed to raising to a high level the degree of recycling of all kind of packaging made of glass, metal, plastic, paper and cardboard. To increase this value continuously and to keep recycling more cost-effective, it is necessary for all involved to consider the later recycling of these products.

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Since many products are often a mixture of different materials which are bonded with the help of adhesives, it is important to choose adhesives that do not disturb the recycling of the primary materials. In recycling processes which take place at high temperatures the influence of adhesives that are based on organic polymers can be ignored. In low-temperature recycling technologies, the question whether an adhesive is recycling-friendly or not can only be answered by knowing its application and the recycling process. If the recycling processes are known it is easy to choose suitable adhesives. For plastic and paper recycling there are a lot of adhesives today which fulfill the requirements of recyclers. Concerning the influence of adhesives on the recycling of primary materials there are no special demands for glass, aluminium and tin plate. However, in plastic recycling the requirements of the later recycling processes have to be considered when choosing adhesives. Often water-soluble, lye-soluble or re-dispersible adhesives are demanded. Special developments in the labelling adhesive sector fulfill these requirements. There are systems for beverage carton closing with adhesives that allow trouble-free recycling of the cardboard packaging materials. The right choice of hot melts and a suitable application thickness, or the right choice of adhesive tapes, leads to systems that can be considered recycling-friendly. As demanded by the paper recyclers, an easy and quantitative sorting-out of these systems is possible directly at the beginning of the paper recycling process. Thus adhesives that get into the recycling process do not lead to problems. In paper recycling a lot of adhesives can be sorted out easily. The ability to sort an adhesive film in the recycling process is not only defined by the properties of the adhesive, especially its cohesion, but also by its geometry. Another influential factor is the fixing of the adhesive film to the substrates. Furthermore, the characteristics of an adhesive film are influenced enormously by the pH-value and the temperature of the water in which the recycling process takes place. To be able to draw a really useful conclusion, the only way is to test the finished paper product. In the finished paper product the adhesive film is what it will be like when the product is later recycled. The influences of geometry and substrates can be tested exactly. Only tests on finished products can indicate whether the finished product or components of the product (eg, adhesives, but also coatings, inks or other additives) influence recycling or not. In plastic recycling a lot of adhesives are alkali- or water-soluble and therefore make possible an easier cleaning of plastic articles before actual recycling. Other adhesives may be made of the same chemical material as the plastic article and thus not cause a quality loss in recycling of the plastic material. To simplify future recycling, adhesives have been developed with â&#x20AC;&#x153;switches enclosedâ&#x20AC;?, which allow the system components to be de-bonded into separate parts after use.

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References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

Onusseit, H. 2008, Praxiswissen Klebstechnik, Grundlagen, Hüthig - Jehle - Rehm, p.23-104. Gesellschaft für Glasrecycling und Abfallvermeidung, Ravensburg, Website 2004. Duales System Deutschland AG, Website 2008. Federation Europeenne du Verre d´Emballage, Brüssel, Website 2008. Metallverpackung schaffen Nachhaltigkeit Imagekampagne des Verbands Metallverpackungen e. V., Düsseldorf, 2008. Compelling facts about plastic 2007. An analysis of plastic production, demand and recovery for 2007 in Europe PlasticEurope, EUPC, EuRP and epro, Brüssel, October 2008. Confederation of European Paper Industries Sustainability Report 2007. Matey, M. 1998, In: Blanco A, et al, editor. Improvement of recyclability and the recycling paper industry of the future. p. 23-32. Industrieverband Klebstoffe, Düsseldorf, 2008. Bundesverband Druck e. V. Guide to an optimum utilization of recovered graphic paper, 31 August 1998. Onusseit, H. 2000, Adhesives & Sealants Industry, August, p.24-8. Ackermann, C., and Putz, H.-J. 2001, INGEDE-Symposium, München. Onusseit, H. 2003, Tissue World 2003 Conference, Nice, 24-27 March. Süoss, B. Wirtschaftliches Kunststoff – Recycling Umwelt, March 1999, p.12-3. Naturkostaktiv, Webside 2008. Bond on Command – Nano Antenna Adhesives Sus Tec, Darmstadt, Website, 2008. Ferencz, A., et al. Adhesives containing activatable debonding agents and process for bonding and debonding. Patent Application: DE 99-19904835.

Transworld Research Network 37/661 (2), Fort P.O. Trivandrum-695 023 Kerala, India

Recent Developments in Polymer Recycling, 2011: 261-291 ISBN: 978-81-7895-524-7 Editors: A. Fainleib and O.Grigoryeva

9. Different ways for re-using polymer based wastes. The examples of works done in European countries 1

Jean-Marc Saiter1, Parambathmadhom Appu Sreekumar1 and Boulos Youssef1, 2

AMME-LECAP International Laboratory, EA4528, Institut des Matériaux, Facultés des Sciences Université de Rouen, BP 12, 76801 Saint Etienne du Rouvray, Cedex, France; 2Institut National des Sciences Appliquées, Avenue 76801 Saint Etienne du Rouvray, Cedex, France

Abstract. A world without plastics is not conceivable, but that create a lot of wastes. In this article the problem of plastic wastes, collection, recycling-reusing is analyzed from a point of view associated to the European Union social and technical point of views. Many aspects are analyzed focused on the problem of thermoplastic devices (PE, PP, PVC, PS, PET and some of their blends). We demonstrate that many technical solutions exist to reuse or recycle polymers but sometime the solution can be worst in tem of ecologic balance. Concrete examples are proposed, and a particular aspect concerning polymer blends is analyzed (miscibility). Correspondence/Reprint request: Dr. Jean-Marc Saiter, AMME-LECAP International Laboratory, EA4528 Institut des Matériaux, Facultés des Sciences, Université de Rouen, BP 12, 76801 Saint Etienne du Rouvray Cedex, France. E-mail: jean-marc.saiter@univ-rouen.fr

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1. Introduction It seems difficult today to imagine our world without plastic devices. In fact, since the end of the Second World War (1945), almost all the industrial sectors (such as, agriculture, building construction, communication, packaging, medicine, transport, defence etc.) use at least one plastic material in some way. This is the consequence of what we could define as the second industrial revolution. The first one was the intense use of metals and alloys during the 19th and 20th centuries. As examples, the symbol of the industrial progress during the Universal Exposition of Paris in 1900 was the construction of the Eiffel tower and in USA, the train called Iron Horse. The second industrial revolution can be linked to the requirement of new inexpensive, durable and low-density materials. That was possible with the development of the macromolecular chemistry, which is indispensable from the petroleum industry. The plastic materials, which are used today, are mostly derived from crude oils and natural gas. Since this period, the consumption of plastic materials has been increasing every year; and in 2007 the annual consumption of plastics reached 260×106 Tons. For NAFTA (North American Free Trade Agreement) and EU (European Union) countries, the per capita annual consumption of plastics was estimated to be approximately 100 kg and the extrapolation for 2015 which leads to a value of 140 kg per annum per inhabitant. In the EU, the main producer of plastics is Germany (7.5 % of the world production) followed by Benelux (4.5 %), France (3 %), Italy (2 %), UK and Spain (1.5 %). Looking at only on the fabrication of plastic bags, it is estimated to be between 500 billions to 1000 billions unities every year. The plastic bags represent a good example of the problems that we will have to solve in the near future. Indeed, among this “pharaonic” number of plastic bags, only a few are recycled. In other words, most of those bags are disposed directly to nature, which leads to a drastic environmental pollution (not necessarily toxic but at least visual). The image reported in Figure 1 shows some examples, which we can see all over the world. On the basis of the above facts, we may conclude that it is not possible to imagine the future without taking into account the aspect of end of life, recycling, re-using. If we analyse the situation for France alone, the consumption of plastics is increasing with an amount of 5 million tons per year (among which 85 % belongs to thermoplastics and 15 % to the technical and thermosetting polymers). On the other hand, the repartition of plastics wastages is as follows: low density polyethylene (LDPE) (23 %), polypropylene (PP) (18.5 %), high density polyethylene (HDPE) (17.3 %), polystyrene [PS/expanded PS (12.3 %)], polyvinyl chloride (PVC) (10.7 %), polyethylene terephthalate (PET) (8.5 %) and other types 9.7 % [1].

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Figure 1. Environmental pollution in various parts of the world.

The basic problem in this regard arises mainly from the attitude of the people towards excessive use and careless disposal of the plastics to the surroundings. This situation prevails in many places, not only in under-developed countries in Asia and Africa but also in North America and Europe, where the people are more conscious about the environmental problems. This is particularly true for the polyethylene bags which are progressively banished all over the world. Generally, the plastic materials seem to be terrible for the environment because they takes months to hundred of years to degrade. During the process of decomposition, most of them are able to produce toxic materials consequently polluting the soil and the water sources. Therefore, the following measures should be immediately taken to reduce the synthetic plastics borne hazards: ± To reduce the use of plastics materials (i.e. with a consequence to reduced production). ± To recycle the plastics and enhance the “recyclability”. ± To introduce the end of life of plastics as one of the major parameter. ± To develop new biodegradable plastics possessing comparable properties as the existing petroleum based polymers. Keeping in the ever increasing demand of plastic materials, we will have to solve the fundamental problem of the capability to produce such a big amount of new materials from natural (agriculture) resources.

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Plastics play a bad environmental role during their production (the major chemicals making of plastic are often highly toxic); for some of them during their use (by molecular desorption, migration and so on...) and for quasi all at the end of life (waste). On the other hand many plastic resins are inflammable and have contributed considerably to several accidents worldwide. The toxic substances emitted during the production of plastics are the chemicals like ethylene oxide, benzene, xylene etc. These molecules may cause health hazards in human, from birth defects to cancer, may also adversely affect the nervous, circulatory and immunity system of the body. Since plastics do not undergo bacterial decomposition, land-filling using plastics mean preserving them forever. (In the United States, 80 % of post-consumer plastic waste is sent to landfill, 8 % is incinerated and only 7 % is recycled [2]). The plastic wastes dumped into rivers, streams and seas contaminate the water, soil, marine life and also the air. In context of the severe problems created by plastic wastages to the environment pollution for the collective awareness of citizens, numerous decrees and/or regulatory guidelines concerning the recovery of plastics were imposed by several authorities. The European guideline and regulation for the recycling is as follows [3]: ± Directive 2004/12/EC of 11th February 2004: on packaging and packaging waste and, in particular, introduce measures aimed at facilitating the achievement by end 2011 of a new overall increased packaging waste recovery target of 60 % (with 55 % by way of recycling) in addition to a prescribed range of material specific recycling targets. ± Directive 2000/53/EC of 18th September 2000 implemented by Decree No. 2003-727 of 1st August 2003; This directives is aimed at preventing waste from motor vehicles and vehicle components that have reached the end of their life-cycle and promoting vehicle reuse, recycling and other forms of recovery. It also aims to bring about an improvement in the environmental performance of all the economic operators involved in the life-cycle of vehicles, and especially the operators directly involved in the treatment of end-of-life vehicles. ± Directive 2002/95/EC, which are designed to minimise waste arisings of certain hazardous substances by prohibiting the use of certain heavy metals in electrical and electronic equipment. The Regulations impose obligations on persons who supply electrical and electronic equipment to the Irish market, whether as retailers, importers or manufacturers. ± Directive 2002/96/EC of 27 January 2003 implemented by Decree No. 2005-829 of 20th July 2005 (France) in the field of Waste Electrical and Electronic Equipments (WEEE), are designed to

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promote the recovery of waste electrical and electronic equipment. They will facilitate in particular the achievement of the targets for the collection, treatment, recovery and disposal of waste electrical and electronic equipment in an environmentally sound manner. These regulations are also intended to control, reduce and recover the polymer wastes, and to implement the techniques of eco-design, ecoconsumer for the sustainable development. The target is 85 % reuse/recovery with 80 % reuse/recycling by average weight per vehicle and per year, and from the date of commencement of the Regulations, and 95 % reuse/recovery with 85 % reuse/recycling by average weight per vehicle and per year, by the 1 of January 2015. For the packing materials, the minimum recycling targets for 2008 were set for different materials as follows: glass 60 %, metals 50 %, paper carton 60 %, plastic 22.5 %, 15 % wood [3]. Sustainable development, eco-friendly design, eco-consumption and economic development are very frequently used keywords, which are associated with the usage and recycling of materials especially for the plastics. Since 1992, new technologies have been developed for the recycling of thermoplastics. Recycling of plastics can have several advantages. Some of them are listed below. ± Conservation of non-renewable fossil fuels as plastic production uses 8% of the world’s oil production, 4% as feedstock and 4% during manufacture. ± Reduced consumption of energy. ± Reduced amounts of solid waste going to landfill. ± Reduced emissions of carbon-dioxide (CO2), nitrogen oxides (NOx) and sulphur-dioxide (SO2). ± Reduction in toxic chemicals into water. Generally, among different methods of recycling, the simplest one is to reuse the materials directly or convert to other forms by physical or mechanical methods, consuming minimum amount of energy. As a result, long lasting (durable), multi-trip plastics packaging have become more widespread in recent years, replacing less durable and single-trip alternatives, thereby reducing the amount of wastes. These regenerated or recycled plastics are then sold in competition with virgin thermoplastics pellets. In principle, this type of recycling is very simple when plastics are made of a single type of polymers. The waste is crushed and then added to virgin thermoplastic pellets before lamination. But when plastics are of different varieties, this type of recycling is more complex since these polymers are generally incompatible with each other. The processing temperatures of each

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polymer are different and the mixture of several plastics leads to a reduction of interactions between their chains and consequently to deterioration in mechanical properties. The obstacles for the physical recovery and reuse of mixed plastic materials are numerous: ± Contamination of plastic wastes with other materials such as dirts and metals that can even damage the equipment used in the reprocessing of the waste. ± Plastics mixtures are not homogeneous. They may consist of a large number of grades with different molecular structures and properties; and each plastic component in a mixed waste has different melting behaviour, rheology, and thermal stability. ± Plastic mixtures are usually incompatible to each other and form discrete phases within a continuous phase. ± Plastic wastes have relatively low density compared to virgin grades. ± Difficulty in sorting, screening and identification of materials. ± The decrease in the mechanical and change in the functional properties of new recycled products. ± Difficulties in finding opportunities and consumers for these types of materials. During their fabrication and in order to respect specific applications, additives (as: fillers, plasticisers, pigments, photo or thermal stabilisers) are added to improve the physical and chemical properties of the products. Since these materials have to survive under different physico-chemical conditions such as temperature, sunlight and mechanical loading, a gradual shift in the composition of the material will occur. These ageing phenomena will lead to modifications of the physical, mechanical and chemical properties. Under these conditions, the cleavage of macromolecules can occur, with as a consequence the change of the molar mass and molar mass distribution. Further, the lack of compatibility between the polymers and the presence of impurities (such as stains, paints and pigments minerals) can cause problems during the processing and as a consequence, the material with modest mechanical properties evolve. In conclusion, while considering the recycling of the plastics products, it’s very essential to take into account the disadvantages cited above. Finally, the recycling of thermosetting polymers is very different and also they represent a very small percentage of recycled materials compared to materials from thermoplastic polymers. In conclusion, we will have to keep in mind that recycling is one solution among others. We have also to notice that a recycled material will be different from the initial material and often with worse properties.

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Nevertheless it is not because the quality of the recycled materials is poor compared to virgin materials that we cannot use them for other applications.

2. Sources of plastic wastes Plastic wastes can be classified in three groups: ± Technological wastes, manufacturing wastes or processing wastes: These occur during the production phase of plastic devices and they are usually collected immediately. Directly or after chopping or crushing they can be recycled. ± Industrial wastes, further processing wastes or confectioning wastes: These are usually produced in the course of processing and confectioning of the semi-finished products. They are clean, and either repurchased by the manufacturers of the semi-finished products, or purchased by companies specialized in waste recycling. Thus the problem of their management is solved. ± Wastes of plastic products after they have been used (packaging materials, hollow articles, used products): This category of wastes causes the most significant problem. The main sources of plastic wastages are the industries, agriculture and household materials. Scenario in France ± Wastage of plastic materials during the manufacturing process: Of the 650 million tons of plastics produced in France (2004), the wastage production during the manufacture of polymers is equal to 325 000 tons. This concerns mainly polymers such as polyolefins, polystyrene (PS) and polyvinyl chloride (PVC). ± Production of waste plastics during the processes such as extrusion, blow moulding and injection moulding: They represent almost 290 000 tons. They are often recovered at the location of production and converted into granules and reused with virgin materials. ± Agricultural wastes: These are nearly 190 000 tons in 2004 (131 000 tons of polyolefines and 35 000 tons of PVC). This class includes HDPE or LDPE and PP in the form of films, bags, cans and so on. ± The disposal of waste packaging (mainly polyolefins) in the form of films. They represent a consumption of 250 000 tons. ± Waste Electrical and Electronic goods: They are from electrical appliances of all kinds and represent 350 000 tons of waste thermoplastics.

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Âą Household waste: This class includes about 900 000 tons per year together with 90 000 tons of rubbish bags. In addition, the waste created by public works and building construction sectors consumes nearly 1 300 000 tons of plastics every year. The wastes are mainly films, pieces of pipes, profiles etc. Finally, transportation, vehicles will create 1 800 000 tons of plastic wastes each year, keeping in mind that only 5 % of the total mass of the vehicles is made with plastics (This quantity increases drastically every year). So, waste reduction is one of the key for solving the plastic pollution problems. This is why in France it was created the â&#x20AC;&#x153;Grenelle Environmental Round Tableâ&#x20AC;? in 2008. This meeting has proposed new legislation, with a clear goal: recycling or reused must concern 35 % in 2012 and 45 % by 2015 of the total plastic production. That will lead to annual recycling of wastes which will concern: household wastes (28 M tons), municipal waste (14 M tons) and industrial waste (90 M tons) together amounts to 132 M tons. That will only represent 16 % of the total amount of the wastes produced (849 M tons), including wastes from agricultural and building activities (717 M tons). In France, 38.7 M tons of material are extracted from the stream of waste and transformed into 31.9 M tons of raw materials. For France this sector of activity concerns 2,400 companies employing 31,500 peoples.

3. Various stages of recycling plastic wastes 3.1. Collecting plastics wastes The first stage for plastic recycling is to collect the plastic wastages from different locations. This can be achieved by keeping special containers installed at home, public places, farms etc. These wastages are then collected by the professionals and transported to the recycling sites. In the underdeveloped countries the wastages are collected manually by primary traders. This stage is labour-intensive and requires little capital investment.

3.2. Cleaning and drying The cleaning stage consists of washing and drying the plastic items. Cleaning is important since the clean waste materials fetch better prices and they improve the quality of end products. The plastics can be washed at various stages of recycling process: before, after, or even during sorting. Films and rigid materials are usually cleaned before the size reduction stage. Extra components such as glued paper labels are also removed. The plastic waste material can be washed manually or mechanically. Manual washing may be done

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in oil drums, in tubs or in specially built basins, and the water may be stirred with a paddle. If the waste is greasy, hot water with soap, detergent (e.g. from scrap detergent bottles) or caustic soda should be used [4]. In the mechanical washing, water-filled basin is equipped with a motor that drives a set of paddles at low speed. The plastic materials are left to soak for several hours, while they are stirred continuously by the paddles. Dirt settles out during the process, and the clean plastic material is removed with a drainer. Washing and drying waste plastics are not separate activities but are carried out within the same unit. As with washing, plastics waste can be dried either manually or mechanically.

3.3. Sorting The sorting is meant not only to separate the polymers from recoverable foreign bodies, but also to separate these polymers. Due to great varieties of plastics it’s very difficult to sort them properly. To facilitate identification, in the United States, the Society of Plastics Industry (SPI) has developed a model coding system (using numbers combined with the abbreviations PE, PP, etc.), which is now also being introduced in Europe. This coding system is especially suitable for moulded products where the coding can be engraved onto the moulds. This code is registered within three arrows loop (see below) found in many plastic containers and is given below.

PETE – poly(ethylene terephthalate): Fizzy drink bottles and oven-ready used for meal trays. HDPE – high density polyethylene: Bottles for milk and washing-up used for liquids. PVC – polyvinyl chloride: Food trays, cling film, bottles for squash used for, mineral water and shampoo. LDPE – low density polyethylene: Carrier bags and bin liners PP – polypropylene: Margarine tubs, microwaveable used for meal trays. PS – polystyrene: Yoghurt pots, foam meat or fish trays, Hamburger boxes and egg cartons, vending cups, plastic cutlery, protective packaging for electronic goods and toys. Other – that do not fall in any of the above categories: An example is melamine which is often used in plastic plates and cups.

3.3.1. Manual sorting Manual sorting of plastic wastes require to identify materials by shape, colour, appearance, trademark of the plastics that distinguishes it for visual

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identification by the operators. There are also metal detectors attached to the conveyor which rejects ferrous and non-ferrous metals. Manual sorting techniques can be used where the plastic components are large enough to justify the time and effort involved, since the method is very labour intensive, has bad working environment. The possibility of human errors should not be neglected. Also, it is difficult to differentiate between the resin types used in packages through the visual means employed by-manual sorting.

3.3.2. Sorting based on density Sorting by density technique is carried out in a float-sink tank or hydrocyclone. This method however is not sufficient for the mixtures of polyolefins since these have very similar densities. It is also impossible to separate PVC and PET, since their specific gravities overlap. The density can be altered by different fillers in the materials, which makes it difficult to have a complete separation [5]. In the float-sink separation, the plastics are placed in a fluid that has a density in between the materials making it possible for less dense materials to float and the heavier to sink. Common fluids used are: water for the separation of polyolefins from other plastics, water/methanol mixtures for separation of plastics with lower specific gravities, NaCl solutions and ZnCl2 solutions for plastics with higher specific gravities. Float baths can be arranged in a series, each bath set having the adâ&#x20AC;&#x2122;hoc specific gravity to sort the materials. Pumps provide circulation and direct the flow. The problems with this method are that the separation can be slow, difficult to control and give low-purity products. So, to achieve good separation, long retention times are required and it is essential that the sizes of the material flakes are equal throughout the mix. One of the advantages of this method is that before the plastic mixture is introduced to the separation fluid, the collected materials are exposed to wet grinding, where the paper labels and dirt particles are removed. The hydro-cyclone uses the principle of centrifugal acceleration to separate plastic mixtures. The mixed plastic wastes are separated first from polyolefins, then PS and finally PVC and other materials. Plastics difficult to be separated with float-sink method, such as PE from PP or PS from a mixture of PVC and nylon, can be sorted by using an appropriate medium by means of centrifugation. The technique can selectively separate, wash and dewater plastic flakes from a mixture of plastics waste materials. The apparatus used is a double-cone, solid bowl screw centrifuge. This achieves efficiency of over 99.5 % purity. The separation is fast and has a high selectivity, by using high speed rotation devices.

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3.3.3. Sorting by selective dissolution Sorting by selective dissolution is based on batch dissolution of mixed plastics using solvents. The polymers have different solubility in organic solvents (differences amplified by action of temperature). This process involves steps: to make soluble, then precipitation, then filtration and finally evaporation of solvents. A complete separation of the plastics can be obtained by careful control of temperature and selection of solvent. The same solvent can be used for separation of PS, LDPE, HDPE, PP and PVC, because these plastics dissolve at different temperatures. When the plastic mix is added to the solvent tank, PS dissolves almost immediately. The PS solution is drained and another hotter batch (75 °C) of solvent is added dissolving LDPE. The solution is drained again and a batch of 120 °C warm solvent is added and HDPE dissolves and so on. If PVC and PET are to be separated, a mixture of solvents is used in which PVC dissolves at a lower temperature than PET [6]. The advantages of this method are: individual plastics can be separated from complex mixtures, contaminations such as dirt or soil or food residues do not cause any problems, labour requirement is minimal and the recycled plastics are chemically and functionally equivalent to the virgin plastics. The disadvantage of this technique is the amount of solvent used, even though most of the solvents are recycled within the process. It is also important to control the levels of residual solvent in the recycled plastic and to re-stabilize the separated materials since additives are extracted during dissolution.

3.3.4. Sorting by response to heat Each polymer has its own softening temperature. If a mixture of polymer were heated, polymer which softens faster is then separated from the others. This can be obtained by using, conveyor belt, self agglomeration and extrusionfiltration.

3.3.5. Sorting by sophisticated instruments The automatic detection system can work very quickly even though it is difficult to detect all the characteristics of a polymer in a very short time interval (problems of separation, contaminants). The following methods can be used: Infrared spectroscopy This technique involves irradiating the unsorted, unidentified plastic: with near-infrared waves (λ=600 to 2500 nm). When the infrared light reflects off the surface of the plastic, each resin’s characteristic infrared

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absorption band can be detected. The observed peak can be compared to known polymer peak values to determine the resin type. This separation method has many advantages. Probably the most significant advantage of using spectroscopy is the speed of identification. Because of the great scanning speed of the spectroscopic instrument, many readings of one sample can be taken in short period of time. This allows multiple checks to ensure proper identification. The speed also provides for increased volume of plastics sorted in smaller amounts of time. Labels, or other obstructions like dirt, do not interfere with readings and the colour does not interfere with proper resin identification. For instance, Florestan et al [7] used Fourier Transform Middle Infra Red (FTMIR.) and Fourier Transform Raman (FTRaman) spectroscopies for the automatic identification of plastics. They concluded that the merit of FT-Raman is considerably higher than that of FTMIR. Furthermore FT-Raman can accommodate almost any shape, colour, and surface state of the analysed objects. X-Ray fluorescence spectrophotometer For example, X-Ray fluorescence (XRF) of the chlorine atom is a suitable method for identifying vinyl in a mixed plastics stream. The chlorine X-Ray is weak and does not penetrate paper labels. There is also a rapid decrease in measured chlorine X-Ray intensity as the sample is moved away from the X-Ray source and detector which could be a potential problem for bottles of uneven shape [8]. The efficiency of this technique depends upon the dimension, cleanliness of the materials. Bledzki et al [9] in their work pointed out that spectroscopic analytical method, based on different polymer's chemical structures, enable for the quick and correct identification of polymer wastes regardless of their colour, degree of contamination and moisture content. Radiography (X-Ray absorption spectroscopy and X-Ray fluorescence) and IR (NIR, MIR) methods were found to be industrially useful for sorting PE, PP, PVC, PET and PS bottles, "electrotechnical scrap" and old-car plastic wastes. Martin et al [10] also pointed that XRF instruments help manufacturers to positively identify and enables faster and more accurate sorting of scrap metals and plastics for recycling.

3.3.6. Optical sorting Optical sorting equipment was originally targeted to processors of plastics in the early 1990s. The units enable plastics to be sorted by resin type and by colour. The technology was then adapted to sort glass by colour and to

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remove ceramic contaminants from the incoming stream of material. In the late 1990s, optical separation technology was modified to differentiate between various grades. Within the last few years, the market for optical sorting equipment has shifted toward material recovering facilities (MRFs) in part because of the growing trend toward single-stream collection programs. By automating the sorting process, MRFs are able to sort high volumes of incoming materials using fewer employees and with a higher degree of accuracy, according to the equipment manufacturers. There are several approaches to optical sorting: 1. Infrared (IR) detection of different polymers to sort among the various plastics that are part of the recycled electronics stream; 2. Visible light and image processing to distinguish among different components. 3. Metal detection and separation based on X-Rays, magnetism and eddycurrent devices. The optical sorting process can be divided into three main parts: material preparation, identification then separation. After removing some of the major contaminants, material must be presented to the optical sensors in a single layer for better results, The material is then accelerated through the system using either acceleration conveyors, as in the case of paper and plastic, or vibratory feeders and steep slides in the case of glass. The optical sensors and their placement vary depending on the material to be sorted. For instance, in the case of glass, colour sensors measure the amount of red, green and blue in the glass pieces, accepting or rejecting material based on predetermined calculations. Colour sorts for paper may involve CMYK camera technology, which detects the four printing colours-cyan, magenta, yellow and black. Infrared spectroscopy is used to differentiate among resins like PET from HDPE or PVC and to tell different paper fibres apart. When sorting based on colour, the material is looked at using either transmission or reflection. When the transmission method is used, a light is shown through the material, and the sensors analyze the spectrum in the transmitted light. This method is used when sorting glass or coloured plastics from clear plastics. When sorting paper by grade or plastics by resin, reflection is used. The sensor is placed above the conveyor, peering down, not through, the material. The sensor analyzes the reflected light spectrum. Most optical sorting lines feature easy to-use touch screens and can incorporate pre-programmed sort specifications for particular end users.

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4. Shredder-fragmentation After the cleaning and sorting of plastic wastages, it has to be cut into smaller pieces before recycling. This shredder-fragmentation operation aims to reduce the size of the waste which in turn facilitates not only in the separation of different polymers, but also to recover the micronized powders that used to feed processing machines. The end products of shredding can be irregularly shaped pieces of plastics that can then be sold to reprocessing industries and workshops. In this method depending on the quality and type of raw material and the desired quality of the end product, different types of plastic wastes may be mixed to a certain extent.

5. Plastic waste recycling process Plastic recycling are usually referred to as the processes in which plastic wastes are collected, separated, processed and returned to useful products. Developing an efficient and cost-effective method for recycling plastic wastes that have served their intended purpose, retrieving them from the waste stream and getting them back into the manufacturing process requires collection, sorting and cleaning and finally reclamation which has been discussed in the earlier sections. For homogeneous plastic waste streams, recycling by mechanical (or physical) methods is the economically preferred recovery option. Heterogeneous plastic waste streams, however, are more efficiently treated or handled by chemical and thermal processes, for recovery of basic chemicals and /or energy.

5.1. Mechanical recycling Mechanical recycling is the reprocessing of plastic wastes by physical means into plastic products. The sorted plastics are cleaned and processed directly into end products or into flakes or pellets of consistent quality acceptable for manufactures. The number of steps needs to recycle postconsumer plastics may vary from operation to operation, but typically that involves: inspection for removal of contaminants or further sorting, grinding, washing and drying and conversion into either flakes or pellets. Pellets are made by melting down the dry plastic flakes and then by extruding it into thin strands that are chopped into small, uniform pieces. The molten plastic is forced through a filter to remove any contaminants that may have eluded the washing cycle. The strands are cooled, chopped into pellets and stored for sale and shipment. During the grinding or melting phases, the reprocessed material may be blended with virgin polymer or compounded with additives.

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Mechanical recycling is the preferred recovery route for homogeneous and relatively clean plastic waste streams. It is the second largest recovery technique used after energy recovery in Europe (13.6 % and 14.8 % in 2002 and 2003 respectively of total plastic waste recovered [11]. This technique is also well suited for developing countries since it is less cost-intensive compared to the others. Mechanical recycling presents advantages over chemical recycling, because the equipment used for virgin materials can be also used for recycled one, making investments in complex installations unnecessary. It leads also to a relatively low energy needs and lead to a lower CO2 emission. Finally, a hazardous emission is quasi impossible and low amount of residues is created.

5.2. Feedstock or chemical recycling Chemical recycling or feedstock recycling means that a polymeric product could be reduced into its individual components. Then these components could be fed back as raw material to reproduce the original product or others. Feedstock recycling includes chemical depolymerisation (glycolysis, methanolysis, hydrolysis, ammonolysis etc), gasification and partial oxidation, thermal degradation (thermal cracking, pyrolysis, steam cracking, etc), catalytic cracking and reforming, and hydrogenation. Besides conventional treatments (pyrolysis, gasification), new technological approaches for the degradation of plastics, such as conversion under supercritical conditions and co processing with coal have been tested [12]. This method requires a lot of expertises, capital intensive and is quite cumbersome. Many industrial companies have successfully developed technologies in this recycling field leading to process mixed plastics streams. For instance, we may notice the feedstock recycling performed after depolymerization of PET [13]. This is obtained by using an alkali solution which destroys the polymeric structure into its monomers (terephthalic acid and ethylene glycol) in a relatively short time. The resulting terephthalate salt is treated with sulfuric or hydrochloric acid to yield highly pure terephthalic acid. Results show that the best percentage conversion of PET flakes is 1.5mol/1 NaOH for 2 hours in 200 째C and for PET powder 1.5 mol/1 NaOH for 1.5 hours at 150 째C. An alternative method has been proposed by Patel et al [14] by using an aqueous polyurethane dispersion by the depolymerization of PET waste using 1,4-butanediol.

6. Energy recovery Because recycling plastic wastes lead to decrease the physical and chemical properties, continuous recycling could lead to substandard and low

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quality products. Hence it would no longer be economically profitable to recycle it further. The end of life must be analyzed. As it is not reasonable to live the ultimate waste anywhere, it seems reasonable to use one of their excellent physical properties i.e.: their high calorific value. Energy recovered from plastic waste can make a major contribution to energy production. Plastics can be co-incinerated with other wastes or used as alternative fuel (e.g. coal) in several industrial sectors (such as cement kilns). The energy content of the plastic wastes can be recovered in other thermal and chemical processes such as pyrolysis. Incineration with energy recovery would be the economically preferred option at this stage. In 2003, 4,750,000 tons of postuser plastic wastes collected in Western Europe were reclaimed through energy recovery. This represented 22.5 % of total collectable plastic wastes. Energy recovery remains today the most common recovery route for postuser plastics waste in Western Europe. Nevertheless this option is not without risks. Indeed quasi all the smokes are toxics (heavy metal, dioxin...). So this method requires specific treatment of the smokes in such a way that no particle dispersion in the atmosphere occurs and also a recovering of the toxic ashes. It is interesting to know the real composition of a resin. For instance, we have found for a thermosetting polyester resin (UPR) used for swimming pool side, heavy elements as zirconium, strontium, barium and more than 10 other elements more or less toxic as shown on Table 1 [15]. Capacity expansions and new incineration plants have led to an increase in energy recovery capabilities in countries across Western Europe. Table 1. Composition of an UPR resin with 0.23 % of cobalt octoate from reference 15. Element

Content in PPM

6.6

2.2

1.3

0.1

5.4

0.1

0.4

138

0.4

1.4

0.4

0.2

0.7

7. Mechanical and functional properties of recycled materials The mechanical performances of thermoplastic materials come from low energy interactions or weak bonds between macromolecules (Van der Waals interaction, dipole-dipole interactions or Debye-London type). Special bonds as hydrogen bonds, ionic bonds and polymer chains entanglement occurring on the surface and interfacial regions must also be taken into account [16]. For a mixture of polymers, interactions between macromolecular chains are lower due to the low free surface energy. As a consequence, recycling polymer blends will give systematically materials with poor mechanical

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properties. For instance, bumpers used in automobiles are fabricated using PP, but in practical purpose it’s blended with other polymers and fillers (PP/copolymer of PP-PE/EPDM/talc). So to be well recycled, it is very important to have a maximum of information concerning the true composition. To achieve this goal, a database called "Sigma" was established in 2002.This database was established at the initiative of the “Federation of Plastics” which brings together twelve of the major plastics manufacturers in France (Sommer-Allibert, Plastic Omnium, MGI Coutier, Neyra Holding, etc.) For the same reason and for the programming of Eco Interactive Design Tools in Germany and for the car sector and other program called IMDS (International Material Data Sheet) was proposed. Finally, European Union has introduced rules and regulations for establishing the means of collecting, processing and recovery for such wastes (2000/53/EC). This also gives for the manufacturers an ad’hoc methodology in order to take into account the recycling during the phase of designing. This is the so called “eco design approaches”.

8. Thermodynamics aspect of blending A majority of polymers are immiscible at molecular level even if the macromolecular structures are very close together. For example, both types of polyethylene high and low density cannot be mixed although they have identical chemical structures. If enough time is given to the molecules, the internal disorder of the polymer system will result in phase separation on a macroscopic scale. From a thermodynamic viewpoint, miscibility refers to the molecular level homogeneity that requires the free energy of mixing, ΔGm, must be negative (ΔGm<0). For a binary system, three possible situations depending on the value of ΔGm with the composition can be pointed out: Case I: ΔGm is positive over the whole composition range implicating that the two components are immiscible with each other to any extent. Case II: ΔGm is always negative and thus the components are completely miscible in all proportions. Case III: ΔGm shows a negative curvature in a composition range, and thus the mixture can develop an even lower free energy in this range by splitting into two phases with compositions given by the two minima resulting in a miscibility gap or partial miscibility. In this context, one can conclude that thermodynamic miscibility of polymers requires two criteria to be satisfied at constant temperature and

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pressure [17] in order to ensure stability against phase separation. These include: ΔGm = ( ΔH m − T ΔS m ) < 0

(1)

⎡ ∂ 2 ΔGm ⎤ ρB ⎤ ⎪⎧ ⎡ ρ A ⎪⎫ 2 = RT + − B ⎨ ⎬>0 ⎢ ⎥ ⎢ ⎥ 2 φ φ M M φ ∂ ⎪ ⎪ B B ⎦ i ⎣ ⎦T , P ⎩ ⎣ A A ⎭

(2)

where, ΔH m and ΔS m indicate the change in enthalpy and entropy on mixing respectively, T is the absolute temperature and φi , ρi and M i are the volume fraction, density and molecular weight of component i, respectively. Equation (1) suggests that for spontaneous mixing ΔGm must be negative. ΔSm will always be positive as mixing increases the disorder of the system, thus making the second half of equation (1) negative and favourable to mixing. But in polymer systems, because of the long chain nature of the molecules, ΔSm is often smaller in magnitude than ΔHm. This causes polymer miscibility to rely on specific interactions between the species, leading to a negative value for ΔHm. As already mentioned, many types of interactions may exist between two polymers. These include London dispersion forces between non-polar molecules, Columbic ion-ion and ion-dipole interactions, dipoledipole interactions between permanent or induced dipoles, charge transfer forces, hydrogen bonding, etc. The second thermodynamic requirement given in equation (2) is applicable in the case of partially miscible systems and suggests that the second derivative of ΔGm with respect to the volume fraction of one of the components at constant temperature and pressure should be greater than zero to ensure stability against phase separation since in these systems the composition fluctuation may lead to phase separation. The simplest thermodynamic model for describing the ΔGm of two polymers is based on the extension of the results developed originally for polymer solutions by Flory and Huggins: ⎡ ρ φ ln φ A ρ BφB ln φB ⎤ ΔGm = Bφ AφB + RT ⎢ A A + ⎥ MB ⎣ MA ⎦

(3)

where, B is a binary interaction energy density and R is the universal gas constant. B is related to Flory-Huggin’s (FH) interaction parameter, χ12, by the expression:

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χ12 =

BVref

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(4)

where, Vref is a reference volume that usually is taken as the molar volume of one of the repeat units in the system and B can be estimated from the solubility parameters (δ)of polymers as, B = (δ A − δ B )

(5)

The values of δ can be calculated from molar attraction constants. The value of χ12 associated with the enthalpic interactions of the blend species, may be a priori negative, positive or nil. However, equations (4) and (5) suggest that χ12 cannot have negative values. Finally, to get miscibility, the values of χ12 should be as small as possible. According to Flory-Huggins (FH) model, polymer/polymer miscibility can occur if: ± The polymers are not of very high molecular weight and the combinatorial entropy of mixing, ΔSm, is not negligible. ± The polymers have a very small positive heat of mixing, ΔHm, arising from a very small exchange energy. ± The polymers have a negative heat of mixing arising due to specific interactions. The FH theory is based on simple lattice model of fixed volume and does not take into account the possible volume changes on mixing and is not able to predict the phase segregation on heating (lower critical solution temperature, LCST), phenomena often observed for polymer blends. Other factors such as crystallinity morphology or molecular weight (Mn) can influence the ability for polymers to be miscible. Indeed if two different chemical structures can prevent the mixing of polymers, it has also been established that the semi-crystalline polymer miscibility is linked to the structure of the crystalline mesh of each type of polymer. The best example is one more time obtained with polyethylene, high and low densities, which are not miscible. Concerning the effect of Mn, the interesting work of M. Nahri [18] concerning mixtures of PEG of different molecular weights has shown that it is necessary to add a mixing enthalpy in the thermodynamic equation of phase equilibrium and that miscibility occurs in a specific domain of composition, domain depending also of the values of Mn mixed. For the effect of Mn we may also cited the case of 1,4 and 1,2 polybutadiene which exhibit the following specificity: the miscibility occurs only if the Mn<50,000 [19].

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The polymer miscibility can be calculated from the knowledge of solubility parameter (δ). The value of δ can be determined by various methods described in the literature and some of them are swelling measurement, viscometry, calorimetry and osmometry. Different softwares also have been proposed to allow a direct calculation of ‘δ’ as the QSPR Insight or Synthia, developed by Van Krevelen [20]. In this method, the chemical structure of the polymer is sequenced in different contributed groups for which a database is known. This last method takes into account the specific interactions, the microstructure of the chain compounds, and the effect of additives or impurities. When polymer miscibility is not good, it is often proposed to seek a third molecule able to make compatible the different phases. This compatibilization is very useful for improving the dispersion in polymer blends. It reduces interfacial tension, facilitates dispersion, stabilises the morphology against abusive stresses and strains (arising from processing), enhances adhesion between the phases and improves the overall mechanical properties [21] of the products. The driving forces for the phase segregation of blend components are gravity and interfacial tension. The rate of demixing depends on interfacial tension, viscosity and density differences [22, 23]. Compatibilized blends are not necessarily miscible blends, but still satisfy certain industrial criteria for usefulness, such as mechanical properties. The key to solve problems of coarse morphology in multi phase polymeric materials is to reduce interfacial tension in the melt and to enhance adhesion between the immiscible phases in the solid state. One other solution is to select the most suitable blending technique so that co-continuous or interpenetrating phase morphology can be obtained, which results in direct load sharing. The second solution is the addition of a third homopolymer or block or graft copolymer or low molecular reactive compounds, which is miscible with either of the two phases. This can be considered as non-reactive compatibilisation. The third way is to blend suitably functionalised [24] polymers, which are capable for specific interactions or chemical reactions (reactive compatibilisation). Block or graft copolymers, which act as compatibilisers are of two types: (i) reactive and (ii) non-reactive. Non-reactive compatibilisers have segments capable of specific interaction with each of the blend components. In reactive copolymers, segments are capable of forming strong covalent or ionic bonds with the blend components. Copolymers of both A-B type and A-C type can act as efficient compatibilisers in AB system provided one of the components is miscible with either A or B. Tables 2 and 3 give a few examples of polymer systems compatibilized respectively through non-reactive and reactive copolymers.

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Table 2. Compatibility through non-reactive copolymers. Major component PE or PS PP PE or PP EPDM PS PET PF PVDF PVC SAN

Minor component PS or PE PS or PMMA PP or PE PMMA PA-6 or EPDM HDPE PMMA/PS PS/PPE PS or PE or PP SBR

Compatibilizer S-B, S-EP, S-I-S, S-I-HBD, S-EB-S, PS-g-PE S-E-B-S EPM, EPDM EPDM-g-MMA PS-b-PA-6 or S-EB-S or PPE S-EB-S PF-g-MMA or PF-g-S PS-b-PMMA PCL-b-PS or CPE BR-b-PMMAr

Table 3. Compatibility through reactive copolymers. Major component ABS PP or PS-6 PE PP or PE PA-6

Minor component PA-6/PA6,6 copolymer PA-6 or PP PA-6 or PA6,6 PET Acrylate rubber

Compatibiliser SAN/MA copolymer EPM/MA copolymer Ionomers, carbonyl functionalized PE PP-g-AA, carboxyl functionalized PE EPM-g-MA

There are, however, a large number of polymer pairs that are compatible and have a negative heat of mixing, for example the pair PS/PPO (polystyrene/polyphenylene oxide). It may also include mixtures of PS/PVC and poly methyl methacrylate. This means that the interaction parameter for these systems should be negative i.e. monomers of different chemical nature attract such interactions between polymers with halogen groups and another polymer with oxygen groups (=C=O, H-C-Cl). Co-crosslinking, cross linking, grafting reactions and chain extension may take place in such systems involving low molecular weight compounds as compatibilisers. Block copolymers do both, emulsify the dispersed phase to give smaller particles as well as increase the interfacial adhesion between the phases. The principal role of block copolymer in controlling morphology appears to be in preventing coalescence. Preventing dynamic coalescence leads to size reduction, while preventing static coalescence results in stability of morphology. Reinforcement of the interface is primarily accomplished by the copolymer crossing the interface and entangling with both homopolymers forming â&#x20AC;&#x153;stitchesâ&#x20AC;? [25]. The result is the coupling of the two phases over which stress can be transferred. So it will appear that such blends will be difficult to recycle. It is not reasonable to propose a chemical treatment. All thermal or mechanical actions will lead quasi certainly to a phase dispersion or separation when the

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blend was miscible or will increase the phase separation when the blend was immiscible. All transformations will lead to a heterogeneous mixture with poor mechanical properties.

9. Recycling of thermoplastic materials As an example, the plastic materials used for packaging become wastes shortly after the purchase while the same plastic used in automotive will give a waste after 10 years. Thus it appears that the same plastic base material will not be recycled from the same way. The choice of recycling requires information regarding the characteristics of the waste materials such as scale of the deposit, its location, its nature, degree of contamination by pollutants, and the existence of market opportunities. Recycling will affect the cost of the final product and up to day, it is often observed that a virgin material is cheaper than the recycled one. In spite of this economic point of view, recycling is an obligation (we could say a cultural obligation for developed countries and a vital obligation for the less developed countries or the developing countries). In the following, we will discuss the recycling aspect of some important classes of thermoplastics.

9.1. Recycling of homopolymers Several literature works can be seen regarding the properties of the materials obtained after recycling of homopolymers. Ambrose et al [26] compared the chemical and physical properties of the recycled thermoplastic polyolefin (TPO) materials with similar products manufactured from virgin resins. The properties of a blow-moulded bottle prepared from 100 % postconsumer HDPE showed that the recycled polymer exceeded the materials specifications for virgin plastic designs. Similarly, a sample of PP obtained entirely from shredder residue displayed sufficient material strength for the future separation and reprocessing. Achilias et al [27] indicated that the recycling of plastic wastes from, LDPE, HDPE and PP does not have significant changes in the mechanical properties. This is confirmed by comparing the FTIR spectrum and tensile properties of recycled and virgin plastics. Meran et al [28] reported, however, that the addition of recycled polyolefins to virgin material will decrease the mechanical properties and this decrease is linearly proportional with the added polyolefins blocks. For example, the tensile strength of LDPE decreased linearly with the percentage of recycled materials. For a totally recycled material this reduction was found to be almost 35 %. These studies have demonstrated that the mechanical properties of recycled materials, although degraded to some extent, are still

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acceptable. Avila et al [29] studied the thermo-mechanical recycling of postconsumed plastic bottles, especially made of PET, and its usage as composite materials for engineering applications. The experimental data showed good performance for compression and machinability. Yarahmadi et al [30] proved that PVC floorings as plastic waste can be mechanically recycled in the form in which they were recovered without upgrading, and without the addition of new plasticiser. All the processing methods normally used in plastics (such as injection, extrusion, extrusion/ blow, shrink roto-moulding) can be re-used. The recycled materials produced by polycondensation (such as polyamide, polyester and PET), once used, are no more suitable for all applications. Several pure materials are used in that mode for example: ± PE & PP: Furniture, pallets, noise barriers, road signs, protective sheaths etc. ± PVC: Shuttering pools, noise barriers, plastic flooring, baseboards, foothills of shoes, gloves. ± PET: textile fibres (77 %), bottles, boxes, fittings (9 %), strapping bands (7 %), blisters and cups (6 %). ± HDPE-tubes, cores (39 %); bins, pallets, shuttering (38 %); nets, planks (12 %); bottles (8 %), films (2 %). ± PS: PS foam balls, hangers.

9.1.1. Polyethylene The consumption of polyethylene in France in the year 2007 was 150 000 tons (in Europe almost 850 000 tons per year) which is mainly used as films, moulded objects etc. Agriculture field alone consumes nearly 1 350 000 tons of polyolefins primarily HDPE and LDPE. They are in the form of films or stuffing coverage for agricultural greenhouses. In most cases, the life of plastic films is about 4 to 6 seasons depending on climate and conditions of use. These films after usage are regularly collected by the Committee of Plastics in Agriculture (CPA) which brings farmers, as well as manufacturers and distributors of agricultural plastics together. Usually the films are very thin (25 to 150 μm thick) having density of 0.92. At first they are collected manually and then transported by trucks for various stages of treatments. Before recycling, a pre-treatment is needed to eliminate the large amount of contamination on the sheet by soil, plants and others. The addition of different amount of recycled material to virgin polymeric material can affect the rheological behaviour of the material

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during injection moulding processes. Javierre et al [31] found a 10 % increase in the viscosity of the recycled HDPE when compared to the virgin HDPE. They showed that at the maximum filling pressures and clamping force values can vary around 12 % with the recycled material percentage. It is interesting to note that the tensile properties of the recycled HDPE obtained from the post-consumer milk bottles was not largely different from those of virgin resin and this could be used for various applications [32]. In another study by Adhikary et al [33] showed that the composites obtained from recycled HDPE reinforced by wood flour has an excellent dimensional stability when compared to that made from virgin HDPE. Recycling does not have significant influence on the tensile and flexural properties of the composites. Usage of the compatibiliser such as maleated polypropylene (MAPP) by 3â&#x20AC;&#x201C;5 wt.% in the composite formulation significantly improved both the stability and mechanical properties. Meran et al [28] highlighted that the incorporation of recycled polymers in different amount to the virgin polymer can cause a linear decrease in the tensile properties. The ultimate tensile strength for LDPE, HDPE and PP decreased from 62, 82, 70 MPa to 40, 50, 46 MPa respectively when the amount of recycled polymers increases from 0 to 100%. Another interesting result is that the LDPE, HDPE and PP did not show any problem during compressing process. The recycled HDPE (RHDPE) can be blended with natural fibres by using melt blending and compression moulding techniques. Yong et al [34] investigated the influence of coupling agent type and contents on the compounding rheology, crystallization behaviour, and properties of RHDPE/fibre composites. The use of maleated polyethylene (MAPE), carboxylated polyethylene (CAPE), and titanium-derived mixture improved the compatibility between the bagasse fibre and RHDPE. In this MAPE acted as an effective coupling agent. The mechanical properties of the resultant composites compared well with those of virgin HDPE composites. The modulus and impact strength of the composites increases with MAPE content. The composites had lower crystallization peak temperatures and wider crystalline temperature range than neat RHDPE, and their thermal stability was lower than RHDPE. Oksman and Lindberg [35] reported that tensile strength of recycled PE/wood particle composites was improved with the addition of maleated styrene-ethylene/butylenesstyrene (SEBS-MA) triblock copolymer and reached its maximum level with 4 wt.% SEBS-MA. Paraffin reduced the agglomeration of wood fibre and increased the tensile strength and modulus, but lowered the impact strength of recycled PE/sawdust composites [36].

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9.1.2. Polypropylene PP is one of the main thermoplastic polyolefin and is widely used for the production of stationery articles, ropes, reusable container etc. Like other polyolefins the usage of PP in the world also increases. The consumption of PP in the world is around 32 million tons per year while in Europe itâ&#x20AC;&#x2122;s around 8 million tons. In France the consumption also increased to 1263 000 tons per year in 2006. However, among this amount, 114 328 tons were recycled into other form. The recycled PP can be blended successfully with other polymers as well as in concrete. Bayasi and Zeng [37] studied the effects of polypropylene fibres on the air content, on the slump and inverted slump cone time of concrete mixing and the impact resistance of the concrete. For this purpose, seven mixtures of polypropylene fibres reinforced concrete having various fibre lengths (fibre length 12.7, 19 mm) and fibre content (0.1, 0.3, 0.5 %) were prepared. They reported that air content in the concrete increased with the inclusion of polypropylene fibres, and there was no detectable effect on air content of fresh concrete at volume below 0.3 %. In the case of lump and inverted slump cone, time of concrete mixing i.e. the incorporation of PP fibre to concrete increased inverted slump cone time and the workability appeared insignificant for fibre volume fractions less than or equal to 0.3%. These fibres significantly increased the impact resistance of concrete for volumes that do not affect mix workability (less than 0.5%), while, at higher volume contents, impact resistance may tend to decrease. Arbelaiz et al [38] showed that mechanical recycling is feasible for the MAPP-modified flax fibre bundle/PP composites to re-use this type of material further. They concluded that mechanical property decreases only slightly even though these composites passes several times through injection moulding machine.

9.1.3. Polyethylene terephthalate PET is widely used for making bottles, electrical and electronic instruments, automobiles products, house-wares, lighting products, power tools, material handling equipment, and sporting goods. In France, the usage of PET comes around 400 000 tons per year. Its specific properties such as excellent barrier properties, reflective and opaque, impact resistance increased its usage for the bottles for beverages etc. In the total production, around 30 % of the PET is used for the manufacturing of bottles. So the recycling of PET into another form of materials is very significant to reduce its production. Recovering these wastes using mechanical recycling is one of the more interesting lines of research at

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present. Torres et al [39] and Choi et al [40] investigated the influence of PET bottles lightweight aggregate (WPLA) on the workability (slump) of concrete. They reported that slump value of waste PET bottles lightweight aggregate concrete (WPLAC) increased with the increase in water-cement ratio and the replacement ratio. The improvement ratios of workability represent 52 %, 104 %, and 123 % in comparison with that of normal concrete at the water-cement ratios of 45 %, 49 %, and 53 %, respectively. This may be attributed not only to the spherical and smooth shape but also to the absorption of WPLA. Also the density and modulus of elasticity of concrete mixtures decreased with the increase in WPLA content. Kim et al [41] used the recycled PET from waste bottles in the form of fibre to control plastic shrinkage cracking in cement-based composites. The fibre geometry and volume fraction (0.1â&#x20AC;&#x201C;1.00 %) affected the rate of moisture loss and controlled the plastic shrinkage cracking characteristics. The fibre geometry and fraction by volume did not affect the total moisture loss or moisture loss per hour. At a fraction of 0.25 %, the plastic shrinkage was reduced, but no further improvements were observed when the fraction of fibre was increased to 0.5 %. For the PET mixed with PE, mechanical recycling can be done without a separation of PE from PET, if the amount of PE is low [42]. The presence of PE will significantly decrease material viscosity. This is due to the fact that during the advance of the blend in its molten state, the lower viscosity material (in this case PE) moves to the outer edge of the blend, in contact with the duct walls, and thereby facilitates the flow of the blend. This decrease in viscosity results in a lower PET crystallinity. As a consequence, a lower injection pressures and higher mould temperatures can be used. Nevertheless, PET and PE are incompatible, and a presence of PE above 5 % leads to an important loss of mechanical properties. Similarly the incorporation of PC to PET makes the overall recycling process easier and brings even better properties as compared to the neat PET [39]. Consequently, it seems that blending of PET and PC limits the degradation consequences caused by mechanical and thermal processing during recycling. PET and PC blends show suitable mechanical properties for building applications, near to the properties of PVC. The addition of small amount of recycled PET into PP does not change significantly the PP tensile strength. However, the elongation percentage dramatically decreases, around 82 % for all compositions with recycled PET in relation to pure PP. The MFI value shows a decrease of 17.3 % when 7 % of recycled PET was added to the PP. Due to the formation of agglomerates of recycled PET which can act as a shock-absorber in the bulk of the composite causes an increase in the impact strength value [43].

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9.1.4. Polystyrene The consumption of Expanded Polystyrene (EPS) for packaging in France on 2005 was 46 000 tons in which only 22 000 tons have finished after use in the household trash. The rest of the amount of EPS does not end up in the trash after use. Therefore they can be reused either in the form of expanded or as non-expanded pellets of PS. For this, materials are grinded and can be transformed to another form, which can be used for packaging purpose or blocks and plates for structural applications. Further, it can also be used in the production of lightweight concrete for construction markets or civil engineering as well as in agriculture and horticulture. Extruded polystyrene pellets can be used as raw materials in many applications, i.e. to boxes of compact discs, and of disposable cameras, furniture etc. Different members of European Manufacturers of Expanded Polystyrene (EUMEPS) suggest similar steps. European statistics on the packaging EPS are compiled by EUMEPS, which also share such data with its North American and Asian countries to reach a global mapping of the EPS [44].

9.2. Recycling of multi component polymers The plastics such as PE, PET, PP, PVC collected from diverse sources in nature and of different backgrounds; can be divided into two main categories: Âą Heavy or rigid plastics e.g. flasks, bottles, rigid packaging. Âą Light and flexible plastics e.g. plastic films, over packs, agricultural film, plastic bags. The heavy plastics are simply washed, crushed and then the metallic impurities are separated. The lightweight plastics are shredded, washed, and then densified. The latter enables the formation of granules with a density similar to that of heavy plastics. Compatibilisers can be introduced into the mixture to have a better uniformity. The processing techniques are the same as those used for virgin thermoplastics. In the case of polymer mixtures, the properties depend on several factors: the nature of the mixtures and this constituent, the presence (or absence) of compatibilizers, the presence (or absence) of a fraction of virgin resin and the state of aging materials. For example, recycled polypropylene used in automotive parts with 20 % of nodules of EPDM (ethylene propylene diene monomer) elastomer and 12 % talc slips was investigated by Bahlouli et al [45]. It was found that the morphology of recycled materials is only weakly altered; and the talc particles act as rigidifying material for the matrix. Paula et al [46]

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investigated the polymeric blends prepared from cartooned packings (TetraPak© packings) by mechanical recycling and characterized. LDPE/Al residues from cartooned packaging were blended with recycled HDPE/LDPE and virgin PE resins. It was observed that processability, mechanical properties, chemical resistance and water absorption are dependent on the blend compositions. Also, aluminium was found to remain as isolated particles embedded in the polymeric matrix, and the blend properties and mechanical behaviour of the blend depends on the aluminium dispersion. Several solutions have been proposed to minimize costly sorting procedures for the mixtures of polymers such as blends of immiscible polyolefins or that of PC and acrylonitrile-butadiene-styrene (ABS). A compatibilizer was added to improve interface of PC/ABS [47, 48] and to reduce the particle size of the dispersed phase. Chiang et al [49], Xiaodong et al [50] and Jin et al [51] showed that PMMA can be used as a compatibiliser for the PC/ABS mixture. The influence of compatibility has been studied on a mixture PC/ABS (25/75) with 5% PMMA. The authors [51, 52] showed that the presence of PMMA in the PC/ABS (25/75) mixture does not change the morphology. However, PMMA causes a decrease in the size of the dispersed phase and endows the material with a better adhesion between the phases. Consequently the mechanical properties are improved. These recycled plastics are currently being used in various fields such as [53]: ± ± ± ± ± ±

Outdoor equipments in playgrounds, furniture, for chairs in stadium, In automotives as mudguard of truck, car mats etc. As mats, flooring, tiles, flower pots, garbage cans, tubes, Gates, road cones, paving docks, for drainage, Paving self, anti noise walls, rail guide, embankments road dividers. Other elements of sewers, fences, films for garbage bags, pallet handling etc.

Above fact demonstrate that the recycling of the mixture of plastics can be used to fabricate materials having different properties. Moreover in the case of mixed polymers, the major part of the polymer acts as a continuous phase. The chemical and mechanical properties vary as a function of composition and interaction at the matrix/dispersed-phase interfaces. As mentioned in earlier section, compatibilisers can be added to improve the interaction of the polymers at the interface. The introduction of compatibiliser can avoid coalescence of the properties and improves the dispersion, accession and stabilization phases present. However in many cases, the mechanical properties of mixed plastics are not optimal they are

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offset by the form of manufactured parts. That is why the use of mixed plastic is more acceptable for replacing wood or concrete. They are lighter and more resistant to aggressive media such as insects, weather or the marine environment.

Conclusions During the last two decades, the investigation for the effective methods in recycling of thermoplastics has continued to grow and changed to a huge industrial issue. More effective methods for the recycling of plastics and mixed polymers, more applications of the recycled products in different industrial sectors as road construction, buildings, structural applications etc, should be explored. In addition, more attention has to be focussed on the degradation of chains at the end of life or the presence of additives and impurities and minimize any potential problems that might arise with recycled materials. The objectives of the current researches are to develop high strength polymeric materials by recycling the mixed polymers. In the literature a large number of works can be found on the effects of compatibilization and the incorporation of recycled polymers and fibres into the virgin materials. The main practical difficulty which we will have to face in the future will be to organize the collect and the selection of the different wastes at a very large scale and at a very low cost. Technical solutions exist to recycle plastic materials but an alternative could be the use of biodegradable plastics. Nevertheless this solution presents also disadvantage in term of public health. Up to day the key for using and developing recycling technologies and recycled materials is the price of the petroleum and the cost of transportation which are too small and which make the effective cost of new virgin materials cheaper than recycled one.

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