Polímeros: Ciência e Tecnologia (Polimeros) 1st. issue, vol. 30, 2020

Polímeros VOLUME XXX - Issue I - Jan./Mar., 2020

Prof. Hermann Staudinger 23/Mar/1881

100 years of Polymer Science São Paulo 994 St. São Carlos, SP, Brazil, 13560-340 Phone: +55 16 3374-3949 Email: abpol@abpol.org.br 2020

08/Sep/1965

ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.03001

One century of the macromolecule concept Maria Isabel Felisberti1*  Instituto de Química, Universidade Estadual de Campinas – UNICAMP, Campinas, SP, Brasil

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*misabel@unicamp.br

How to cite: Felisberti, M. I. (2020). One century of the macromolecule concept. Polímeros: Ciência e Tecnologia, 30(1), e2020000. https://doi.org/10.1590/0104-1428.03001 Hermann Staudinger (1881-1965) was a German PhD in Organic Chemistry, who proposed that natural rubber was a covalently bound polymer with high molar mass, contradicting the accepted hypothesis of colloidal aggregation of small molecules[1]. He started his carrier as professor of organic chemistry at the Technical University of Karlsruhe, Germany, followed by the Eigenössische Technische Hochschule (ETH) in Zurich, Swiss, and the Albert Ludwigs University in Freiburg, Germany, where he became director of the chemistry department. Parallel to the research in organic chemistry, he was interested in physiology of the active natural compounds[1]. Staudinger was a leading organic chemist[2]. However, in 1920s, he left the classic organic chemistry to dedicate his life to the emerging and challenging polymer science. He faced strong opposition from colleagues, who did not understand why he changed the interesting field of the small and defined molecules to poorly defined molecules, such as rubber and synthetic polymers, at that time denominated as “grease chemistry”. However, he was persistent and able to provide direct and indirect evidences of the macromolecular nature of both natural and synthetic compounds, which at that time was thought to be quite different[1]. His wife Magda Staudinger was a plant physiologist, with relevant contributions to Macromolecular Science, that encouraged Staudinger ideas about the importance of macromolecules for Biochemistry and Biology[1,2]. According to Staudinger, synthetic polymers are models to understand biopolymers and more complex biosystems. He contributed to significant and fundamental advances in macromolecular chemistry; prepared and characterized a variety of macromolecules, including synthetic polymers, biopolymers and modified biopolymers. He demonstrated that synthetic polymers could be formed into fibers, which was thought to be possible just with biopolymers. His studies on crystallization provided evidences that small segments of the polymer chains constitute the unit cell of a crystalline polymer.

From Staudinger law, which correlates the viscosity of a polymer solution with the molar mass, viscosimetry became a powerful technique for polymer characterization, still used

Polímeros, 30(1), e2020000, 2020

in academia and industry. Staudinger first investigated the morphology of macromolecules by using transmission electron microscopy[2]. In 1940, Staudinger founded the Institut für Makromolekulare Chemie (Institute for Macromolecular Chemistry) at the Albert-Ludwigs Universität Freiburg (University of Freiburg), the first European center for inter and multidisciplinary polymer research. In 1941, he became director of the Institute of the Macromolecular Chemistry, which became a Government Research Laboratory. In 1947, he founded the journal Die Makromolekulare Chemie, today Macromolecular Chemistry and Physics. In 1953, Hermann Staudinger was awarded Nobel Prize for Chemistry for the concept of macromolecules and his efforts to stablish the Macromolecular Science[1] . In his Nobel lecture, Hermann Staudinger stated

The macromolecular compounds include the most important substances occurring in nature such as proteins, enzymes, the nucleic acids, besides the polysaccharides such as cellulose, starch and pectins, as well as rubber, and lastly the large number of new, fully synthetic plastics and artificial fibres.

Macromolecular chemistry is very important both for technology and for biology[3:397].

References 1. Cantow, H.-J., & Mülhaupt, R. (2013). Hermann staudinger and polymer research in freiburg. In V. Percec (Ed.), Hierarchical macromolecular structures: 60 years after the staudinger Nobel Prize I. Advances in polymer science (Vol. 261, pp. 21-38).

Cham, Switzerland: Springer International Publishing. http:// dx.doi.org/10.1007/12_2013_257 2. American Chemical Society. (1999). Hermann Staudinger and the Foundation of Polymer Science. International Historic Chemical Landmark. Washington: ACS. Retrieved in 2020, March 26, from http://www.acs.org/content/acs/en/education/ whatischemistry/landmarks/staudingerpolymerscience.html 3. Staudinger, H. (1953). Macromolecular chemistry (pp. 397-419).

Stockholm: The Nobel Prize. Retrieved in 2020, March 26, from https://www.nobelprize.org/uploads/2018/06/staudingerlecture.pdf

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Polímeros E d i t o r i a l C o u nci l

Editorial Committee

Antonio Aprigio S. Curvelo (USP/IQSC) - President

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Members Adhemar C. Ruvolo Filho (UFSCar/DQ) Ailton S. Gomes (UFRJ/IMA) Alain Dufresne (Grenoble INP/Pagora) Bluma G. Soares (UFRJ/IMA) César Liberato Petzhold (UFRGS/IQ) Cristina T. Andrade (UFRJ/IMA) Edson R. Simielli (Simielli - Soluções em Polímeros) Edvani Curti Muniz (UEM/DQI) Elias Hage Jr. (UFSCar/DEMa) José Alexandrino de Sousa (UFSCar/DEMa) José António C. Gomes Covas (UMinho/IPC) José Carlos C. S. Pinto (UFRJ/COPPE) Júlio Harada (Harada Hajime Machado Consutoria Ltda) Luiz Antonio Pessan (UFSCar/DEMa) Luiz Henrique C. Mattoso (EMBRAPA) Marcelo Silveira Rabello (UFCG/UAEMa) Marco-Aurelio De Paoli (UNICAMP/IQ) Osvaldo N. Oliveira Jr. (USP/IFSC) Paula Moldenaers (KU Leuven/CIT) Raquel S. Mauler (UFRGS/IQ) Regina Célia R. Nunes (UFRJ/IMA) Richard G. Weiss (GU/DeptChemistry) Rodrigo Lambert Oréfice (UFMG/DEMET) Sebastião V. Canevarolo Jr. (UFSCar/DEMa) Silvio Manrich (UFSCar/DEMa)

A ss o ci at e E d i t o r s Adhemar C. Ruvolo Filho Alain Dufresne Bluma G. Soares César Liberato Petzhold José António C. Gomes Covas José Carlos C. S. Pinto Paula Moldenaers Richard G. Weiss Rodrigo Lambert Oréfice

D e s k t o p P u b l is h in g

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“Polímeros” is a publication of the Associação Brasileira de Polímeros São Paulo 994 St. São Carlos, SP, Brazil, 13560-340 Phone: +55 16 3374-3949 emails: abpol@abpol.org.br / revista@abpol.org.br http://www.abpol.org.br Date of publication: March 2020

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Polímeros / Associação Brasileira de Polímeros. vol. 1, nº 1 (1991) -.- São Carlos: ABPol, 1991Quarterly v. 30, nº 1 (Jan./Mar. 2020) ISSN 0104-1428 ISSN 1678-5169 (electronic version)

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Editorial Section Editorial................................................................................................................................................................................................E1 News....................................................................................................................................................................................................E2 Agenda.................................................................................................................................................................................................E3 Funding Institutions.............................................................................................................................................................................E4

O r i g in a l A r t ic l e Electrical and spectroelectrochemical investigation of thiophene-based donor-acceptor copolymers with 3,4-ethylenedioxythiophene Marcus Henrique de Araujo, Tulio Matencio, Claudio Luis Donnici and Hállen Daniel Rezende Calado .................................................. 1-10

Synergistic effect of adding lignin and carbon black in poly(lactic acid) Thaís Ferreira da Silva, Fernanda Menezes, Larissa Stieven Montagna, Ana Paula Lemes and Fabio Roberto Passador.......................... 1-10

Thermal treatment of açaí (Euterpe oleracea) fiber for composite reinforcement Felipe Fernando da Costa Tavares, Marcos Danilo Costa de Almeida, João Antonio Pessoa da Silva, Ludmila Leite Araújo, Nilo Sérgio Medeiros Cardozo and Ruth Marlene Campomanes Santana....................................................................................................... 1-9

Preparation and analysis of melamine and melamine-silica as clarifying agents of waste lubricating oil Mirna Sales Loiola Rosa, Timm Knoerzer, Francisco Cardoso Figueiredo and José Ribeiro dos Santos Júnior............................................ 1-7

Heat transfer simulation for decision making in plastic injection mold design Piery Antonio Gruber and Diego Alves de Miranda......................................................................................................................................... 1-9

Nanofibers of gelatin and polivinyl-alcohol-chitosan for wound dressing application: fabrication and characterization Paola Campa-Siqueiros, Tomás Jesús Madera-Santana, Jesús Fernando Ayala-Zavala, Jaime López-Cervantes, María Mónica Castillo-Ortega and Pedro Jesús Herrera-Franco.................................................................................................................. 1-11

Adsorption of terbium ion on Fc/dymethylacrylamide: application of Monte Carlo simulation Norma Aurea Rangel Vázquez........................................................................................................................................................................... 1-8

Electropolymerization of polyaniline nanowires on poly(2-hydroxyethyl methacrylate) coated Platinum electrode Maria Fernanda Xavier Pinto Medeiros, Maria Elena Leyva, Alvaro Antonio Alencar de Queiroz and Liliam Becheran Maron................. 1-7

Disposable coffee capsules as a source of recycled polypropylene Michel Lincoln Bueno Domingues, Jean Rodrigo Bocca, Silvia Luciana Fávaro and Eduardo Radovanovic................................................ 1-9

Physicochemical and drug release properties of microcrystalline cellulose derived from Musa balbisiana Martins Emeje, Marlene Ekpo, Olubunmi Olayemi, Christianah Isimi and Alak Buraghoin.......................................................................... 1-6

Effects of weathering on mechanical and morphological properties cork filled green polyethylene eco-composites Gabriela Celso Melo Soares de Vasconcelos, Laura Hecker de Carvalho, Renata Barbosa, Rita de Cássia de Lima Idalino and Tatianny Soares Alves..................................................................................................................................................................................... 1-11

Influence of Prosopis Juliflora wood flour in Poly Lactic Acid – Developing a novel Bio-Wood Plastic Composite Sachin Sumathy Raj, Thanneerpanthalpalayam Kandasamy Kannan, and Rathanasamy Rajasekar............................................................ 1-11

Cover: Hermann Staudinger - 100 Years of Polymer Science. Arts by Editora Cubo.

Polímeros, 30(1), 2020

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High-tech Thermoplastics for Vehicles of the Future Lanxess will have several emphases during its appearance at the VDI Congress Plastics in Automotive Engineering (PIAE), which is being held virtually due to the COVID-19 pandemic.

“Among other things, we will present lightweight solutions based on our Tepex-branded continuous-fiber-reinforced thermoplastic composites for brake pedals as well as structural components of the vehicle body and high-voltage battery,” says Thomas Malek, Business Development manager for Tepex Automotive in the High Performance Materials (HPM) business unit at Lanxess.

“We will also be concentrating on blow-moldable and injectionmoldable polyamide compounds for tanks and hollow parts for the air management of turbocharged engines.”

A highlight of Lanxess’s presentation will be the tank of a BMW Motorrad road machine, which is made from Durethan, an unreinforced and impact-resistant modified polyamide 6, which is injection-molded into two half-shells, which are welded to the tank by means of hot plate welding. The limit values for fuel emissions through the tank walls are undershot by a considerable margin.

The potential of Tepex-branded composite materials for use in lightweight applications will be demonstrated on several exhibits – for example, on an all-plastic brake pedal developed for a battery-electric sports car will also be showcased. The composite component is around 50% lighter than an equivalent steel construction. It meets all load requirements thanks to the tailor-made fiber layer construction of the Tepex insert and local reinforcement using additional tapes.

A further example of systematic lightweight design using Tepex is an A-pillar with a 3D hybrid design that Porsche has developed for vehicles such as convertibles and roadsters and is employing for the first time in the Porsche 911 convertible.

The A-pillars with hybrid inserts are just as stable in the event of a crash as previous designs featuring high-strength steel tubes but they reduce the weight of the vehicle body by a total of around five kilograms.

Tepex also has enormous potential to be applied in structural components and housing parts for high-voltage batteries in electric vehicles. This is due to its inherently outstanding flame-retardant properties that it displays even without flame-retardant additives in various tests based on established norms and standards. Not only do the composite materials here present a lightweight alternative to aluminum, but they also enable cost-effective component solutions thanks to the cost-reducing integration of functions and simple processing without the need for rework in the hybrid molding method.

In addition to the powertrains of electric vehicles, there is also enormous potential for technical thermoplastics from Lanxess in the electric mobility charging infrastructure. The Durethan polyamides and Pocan polybutylene terephthalates are mainly used for components of charging plugs, sockets, and stations as well as wallboxes in garages and carports, for example. The materials are also used in components in inductive wireless charging systems for high-voltage batteries.

Source: Plastics Today - www.plasticstoday.com

Braskem’s bioplastic recognized at UN event as one of Brazil’s most transformational cases in sustainable development

America and the Caribbean (ECLAC) and the Global Compact Network Brazil as one of the most transformational cases in sustainable development in Brazil, in the Industry & Energy category. The recognition was made official during Braskem’s participation in the “Big Push for Sustainability,” a webinar open to the public organized by ECLAC in partnership with the Global Compact Network Brazil, both UN organizations.

Braskem’s production of biobased plastic, which commemorates one decade this year, is the result of years of dedication by the company in the research and development of sustainable products. Produced on an industrial scale since the inauguration of the green ethylene plant located in Triunfo, Rio Grande do Sul, the initiative has made the company the world’s leading producer of biopolymers, with an annual production capacity of 200,000 tons of the material.

The advances in the research and development of sustainable products, such as green polyethylene, green EVA and green ethylene glycol, are part of Braskem’s sustainability practices, which have been an integral part of its corporate DNA since its creation in 2002. In the case of I’m greenT bio-based polyethylene, one of the main advantages is its contribution to reducing greenhouse gas emissions by capturing more than 3 tons of CO2 for each ton produced. The material even retains the same properties, performance and versatility delivered by fossil-based plastics and can be used in existing production and recycling chains.

Thanks to the partnerships Braskem has forged over the last ten years with clients in the plastics chain and with brand owners to encourage the use of plastics that are even more sustainable and minimize environmental impacts, today I’m greenT bio-based polyethylene already can be found in 150 brands worldwide, which includes packaging and products for a wide array of industries, such as food and beverage, personal care and durable goods.

“We believe in the potential of the circular economy to advance sustainability, and biobased plastic is one of our deliveries towards this end. The UN initiative is an important recognition of the journey we are constructing, which reinforces that we are on the right track and amplifies the message that plastics can contribute significantly to the planet’s sustainable development,” said Mateus Schreiner Garcez Lopes, head of Innovation in Renewable Technologies, who presented the case study “Green Polymers: technology for fostering sustainable development” during the ECLAC event.

Braskem’s investments in the area of chemical recycling, one of the biggest challenges of the plastics industry, Braskem also continues to conduct studies with support from universities and research centers to develop technologies that expand existing alternatives for mechanical recycling in order to transform waste plastics, such as supermarket bags and packaging films, into raw materials once again.

The case studies presented by Braskem and companies from other sectors at the ECLAC and Global Compact Network Brazil webinar are part of an online repository with more than 60 initiatives for sustainable development in Brazil. It is a set of initiatives that leverage national and foreign investments to create a Big Push of economic growth, job creation and income generation, reduction in inequality and structural gaps and promotion of environmental sustainability.

According to Carlos Mussi, Director of the ECLAC Office in Brazil, the cases compiled in their repository on the Big Push for Sustainability in Brazil are clear examples of investments with the capacity to deliver social, economic and environmental results in line with a sustainable recovery strategy for the country.

Source: Braskem - www.braskem.com

Braskem’s production of I’m greenT polyethylene, a biobased plastic made from sugarcane, was recognized this Tuesday (May 26) by the United Nations Economic Commission for Latin

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February

July

Performance Polypropylene Europe Date: February 09-10, 2021 Location: Cologne, Germany Website: www.ami.international/events/event?Code=C1119 Polymers and Nanotechnology Date: February 21-24, 2021 Location: Napa Valley, United States Website: www.polyacs.net/21polynano

25th IUPAC International Conference on Physical Organic Chemistry Date: July 10–15, 2021 Location: Hiroshima, Japan Website: icpoc25.jp 84th Prague Meeting on Macromolecules - Frontiers of Polymer Colloids Date: July 18–22, 2021 Location: Prague, Czech Republic Website: mns-20.com 2nd International Conference on Materials and Nanomaterials (MNs-20) Date: July 26–28, 2021 Location: Rome, Italy Website: www.imc.cas.cz/sympo/84pmm

March Oil & Gas Polymer Engineering Texas 2021 Date: March 2-3, 2021 Location: Houston – United States Website: www.ami.international/events/event?Code=C1126 Polymers in Footwear Date: March 23-24, 2021 Location: Portland – United States Website: www.ami.international/events/event?Code= C1120

April PVC Formulation Date: April 19-21, 2021 Location: Cologne, Germany Website: www.ami.international/events/event?Code=C1104 37th International Conference of the Polymer Processing Society Date: April 19-23, 2021 Location: Fukuoka, Japan Website: www.pps-37.org Plastic Pouches Date: April 26-28, 2021 Location: Barcelona, Spain Website: www.ami.international/events/event?Code=C1097

May World Polymer Congress (IUPAC-MACRO2020+) Date: May 16-20, 2021 Location: Jeju Island, South Korea Website: www.macro2020.org Polymers 2021: New Trends in Polymer Science: Health of the Planet, Health of the People Date: May 17-19, 2021 Location: Turin, Italy Website: polymers2021.sciforum.net

June

August International Conference on Electronic Materials (2021 IUMRS-ICEM) XIX Brazilian Materials Research Society Meeting (XIX B-MRS) Date: August 29 – September 2, 2021 Location: Foz do Iguaçu – Brazil Website: www.sbpmat.org.br/19encontro

September CIRM – Workshop — Directed Polymers and Folding Date: September 6-10, 2021 Location: Marseille, France Website: https://conferences.cirm-math.fr/2021-calendar.html 9th International Conference on Fracture of Polymers, Composites and Adhesives Date: September 26-30, 2021 Location: Les Diablerets, Switzerland Website: www.elsevier.com/events/conferences/esistc4conference 13th PVC Formulation Date: September 27-29, 2021 Location: Cologne, Germany Website: www.ami.international/events/event?Code=C1104

October 16th Brazilian Polymer Conference – (16thCBPol) Date: October 24-28, 2021 Location: Ouro Preto, Brazil Website: www.cbpol.com.br

Gordon Research Conference — Polymers Date: June 5-6, 2021 Location: South Hadley, United States Website: www.grc.org/polymers-conference/2021 Biopolymer – Processing & Moulding Date: June 15-16, 2021 Location: Halle (Saale), Germany Website: https://polykum.de/en/biopolymer-mkt-2021 Gordon Research Conference — Polyamines Date: June 27 - July 2, 2021 Location: Waterville Valley, United States Website: www.grc.org/polyamines-conference/2021

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A A A A A A A A A A A A A A A A A A A A A

ABPol Associates Sponsoring Partners

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ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.03519

Electrical and spectroelectrochemical investigation of thiophene-based donor-acceptor copolymers with 3,4-ethylenedioxythiophene Marcus Henrique de Araujo1, Tulio Matencio1, Claudio Luis Donnici1 and Hállen Daniel Rezende Calado1,2*  Departamento de Química, Universidade Federal de Minas Gerais – UFMG, Belo Horizonte, MG, Brasil 2 Centro de Tecnologia de Nanomateriais e Grafeno – CTNano, Belo Horizonte, MG, Brasil

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*hallendaniel@ufmg.br

Abstract This work reports the spectroelectrochemical and electrical behavior of electropolymerized donor-acceptor like (D-A) copolymer films, based on 3,4-ethylenedioxythiophene (EDOT) and beta-substituted electron-acceptor thiophenes.

Initially, the copolymer films were deposited on indium tin oxide substrates, which spectroelectrochemistry measurements were carried out with an UV-Vis spectrophotometer. Hence, it was possible to observe the electrochromic properties of these materials, visualizing the color changing towards different potentials applied. The experiments have shown that these D-A like copolymers presented good electrochromic properties, such as optical contrast, coloration efficiency, and switching times. Additionally, films prepared on a platinum working electrode were investigated by electrochemical impedance spectroscopy, which has shown the electrical behavior of those copolymers and their potential as candidates to capacitive devices building. Therefore, the combination of electron-donor EDOT with those electron-acceptor monomers is indeed a useful strategy to tailoring and fine-tuning the physicochemical properties of polythiophenes with innovative applications.

Keywords: donor-acceptor copolymers, electrical behavior, polythiophenes, spectroelectrochemistry.

How to cite: Araujo, M. H., Matencio, T., Donnici, C. L., & Calado, H. D. R. (2020). Electrical and Spectroelectrochemical investigation of thiophene-based donor-acceptor copolymers with 3,4-ethylenedioxythiophene. Polímeros: Ciência e Tecnologia, 30(1), e2020001. https://doi.org/10.1590/0104-1428.03519

1. Introduction The polythiophene semiconductor materials have been massively studied for applications as active layer in organic-electronic devices[1,2], and their derivatives have booth environmental and thermic stability, as well good optoelectronic properties, allowing the uses in electrochromic devices[3-6], organic photovoltaic devices[7-9], solar cells[10-12], organic light-emitting diodes[13-15], rechargeable batteries[16-19], among several others. Recently, conducting polymers have also been applied on the design of soft actuators and bioelectronic interfaces[20,21], e.g., the work of Lu et al.[22] who have developed a pure PEDOT:PSS hydrogel with high electrical conductivity, high stretchability, and superior stability as a promising electrical interface with biological tissues for sensing and stimulation.

Although, it should be noted that among all those possible applications, we can highlight the organic light-emitting diodes (OLED) and non-emissive electrochromic devices (ECD), which require a fine-tuning of colors in terms of its tonality, saturation, intensity, and brightness[4]. Different strategies in synthesis have been described to reach new materials with the largest planarity as possible, increasing the chain length and alternating between donating and accepting electron groups, systems named D-A, for increase conjugation[4].

Polímeros, 30(1), e2020001, 2020

The correlation and influence of the monomer structure under the material electrochromic properties can be verified, particularly on the work of Dyer et al.[23].

Such investigation showed that for D-A based polymers, it might be possible to build multicolored electrochromic devices with high efficiency, whose color change follows the composition and photoelectrochemical properties of the material. Nowadays, it can be found a considerable number of publications concerning the D-A-like copolymers with different substituted thiophenes[9,24], which makes it very interesting to put efforts on this research line.

From the 80’s, the use of spectroelectrochemical techniques (coupling spectroscopic and electrochemical techniques) emerged in several research lines, e.g., on the conducting polymers field, where the UV-Vis spectroscopy is widely used to determine electrochromic parameters as well to investigate the electrochemical kinetic[6,25-27]. Worldwide, we can find plenty of works reporting the synthesis of thiophene-based copolymers and their spectroelectrochemical behavior. In this way, Vogel and Holze[28] have published a work presenting the electrosynthesis of aniline-thiophene copolymers with a new spectroelectrochemical behavior with a slight prevailing of aniline in such properties.

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Araujo, M. H., Matencio, T., Donnici, C. L., & Calado, H. D. R. In another example, Zagorska et al.[29] have synthesized copolymers based on 3-alkylthiophenes and thiophene functionalized with an azo chromophore leading to a mix of electrochromic properties. Alakhras has also published a work presenting new copolymers obtained potentiostatically from selenophene and thiophene units, which provided changes on the electro-optical properties[30]. Another work mentions the preparation of a copolymer of 2,5-di(thiophen-2-yl)-1-p-tolyl-1H-pyrrole (DTTP) with 3,4-ethylenedioxythiophene (EDOT)[31].

Through the spectroelectrochemical characterization, the electrochromic parameters of such copolymer have shown to be better than the homopolymer, such a lower energy band gap, optical contrast of 20% and a switching time less than 1 s[31].

Regarding the electrical behavior of copolymers, Chen et al. [32] have synthesized a copolymer based on 3,4-ethylenedioxythiophene (EDOT) and 3-thienyl ethoxybutanesulfonate (TEBS) showing conductivity almost 6 times higher than compared with pure homopolymer (PEDOT). In the same way, Ates and Ekmen prepared an EDOT-pyrrole (Py) copolymer with a slight gain on the capacitance when compared with both homopolymers that could be a promising electrode material for high-performance electrical energy storage devices[33].

In a different approach, Kulandaivalu et al.[34] prepared a copolymer of EDOT and aniline by electrodeposition and performed electrochemical impedance spectroscopy experiments to understand its electrical behavior and proposed a path to analyze the results, which explained that the incorporation of the EDOT in aniline makes the interfacial resistance of the copolymer to be lower than neat polyaniline.

Yijie et al.[35] have proposed the electrosynthesis of D-A-like copolymers containing a benzothiadiazole unit as the acceptor monomer and different thiophene derivatives as the donating units. In this way, they were able to manage the fine-tuning colors of the materials in booth neutral and oxidized state by varying the electron-rich character of the incorporated thiophene moieties.

We have published a previous work about the synthesis and structural characterization of thiophene monomers: 3-thiophene phenylacetate – PhTAc-2a, 3-thiophene(4-nitrophenyl)acetate – PhTAc-2b, and 3-thiophenephenylcarboxylate – PhTCb, as well as the synthesis and electrochemical characterization of their copolymers with EDOT. In this work, we have given sequence on the characterization of these materials by performing spectroelectrochemical experiments, in order to figure out the electrochromic properties of these donor-acceptor copolymers. Besides, we have run chronoabsorptometry experiments, which allowed us to calculate important electrochromic parameters, such as optical contrast, coloration efficiency, and switching times. Finally, we have studied the electrical profile of those materials by the EIS technique. These experiments and properties for PEDOT film are well known. Therefore, it is essential to mention that we have performed the experiments for PEDOT film just in order to compare its results with the copolymers ones.

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2. Materials and Methods 2.1 Materials The electron-acceptor monomers: 3-thiophene phenylacetate (PhTAc-2a), 3-thiophene(4-nitrophenyl)acetate (PhTAc-2b), and 3-thiophenephenylcarboxylate (PhTCb) were synthesized and characterized on previous work. More detailed information about the electropolymerization and characterization of these copolymers can be found in previous work[36]. 3,4-ethylenedioxythiophene (EDOT, 97%) and sodium perchlorate (NaClO4, 98%) were purchased from Aldrich.

Acetonitrile (ACS) was purchased from Synth. The indium tin oxide (ITO) coated glasses (8-12 Ω; 7.0 × 50.0 × 0.7 mm) were purchased from Delta Technologies.

2.2 Preparation of the D-A copolymers After testing the oxidizing potential for the electron-acceptor monomers[36], solutions of these monomers with EDOT have been prepared in acetonitrile containing 0.1 M of NaClO4 and using a content of 4:1 (moles/moles) of EDOT:monomer.

The films for the spectroelectrochemical characterization were electrodeposited onto ITO coated glass by cyclic voltammetry (20 cycles at a scanning rate of 50 mV s−1) through a PalmSens potentiostat. For the electrochemical impedance spectroscopy measurements, freshly films have been prepared onto a platinum working-electrode (0.01 cm2) also by cyclic voltammetry under the same conditions cited before.

2.3 Spectroelectrochemical characterization After washed with acetonitrile, each thin film substrate was transferred to a quartz cuvette (1.0 cm) filled with a 0.1 M NaClO4/acenotrile solution. On the cuvette, it was attached a platinum wire as counter-electrode and a silver wire as pseudo-reference. This system was put into a spectrophotometer (Varian – Cary 100 Bio) to measure the spectra for different potentials applied. Before the readings, it was applied the conditioning potential during 50 s through a PalmSens potentiostat. The steps of potential were 0.3 V, contemplating the electroactivity window of each copolymer.

On the sequence, new copolymer films were prepared, also on ITO coated glass, to run the chronoabsorptometry experiments using the same spectrophotometer and a PGSTAT204 potentiostat to record the chronoamperometry curves. In this case, it was also used a quartz cuvette containing 0.1 M NaClO4/acetonitrile and the same electrodes as described before. Through the potentiostat, the potential was switched starting from the lowest (neutral) to the highest (oxidized).

Each potential was applied during 10 s and, simultaneously, the transmittance variation and chronoamperometry curves have been recorded.

The transmittance variation curves allow determining the response times to the bleaching/coloring processes by measuring the time taken when the transmittance changing reaches 95%. In addition, it is possible to calculate other electrochromic parameters from the transmittance and amperograms curves, such as the optical contrast (ΔT) – the difference between colored and bleached states transmittance, and the coloration efficiency (η) – calculated from the equation[37]: Polímeros, 30(1), e2020001, 2020

Electrical and spectroelectrochemical investigation of thiophene-based donor-acceptor copolymers with 3,4-ethylenedioxythiophene η=

∆OD (1) ∆Q

where ΔOD is the optical density variation in a fixed wavelength; and ΔQ is the sum of injected/ejected charges per surface unit. We can use the following equation to determine the ΔOD value: ∆OD = log(Tred / Tox ) (2)

where Tred is the bleached state transmittance; and Tox is the colored state transmittance.

2.4 Electrochemical Impedance Spectroscopy (EIS) characterization The EIS curves (Nyquist diagrams), were measured with a PGSTAT potentiostat in a three-electrode system (WE = CE = Pt, and RE = Ag/Ag+) using a 0.1 M NaClO4/acetonitrile as electrolytic solution. To scheduling the experiments, two potentials (Edc) were used: -0.5 V (reduced polymer) and 1.5 V (oxidized polymer), excitation amplitude of 10 mV and frequency range from 100 kHz to 10 mHz. The thickness of PEDOT and its copolymers studied by EIS were estimated from the film polymerization charges, assuming 2.25 electrons per monomer and a film density of 1 g cm-3[38].

The representation of the electric behavior of our cell can be assimilated to a Randle circuit, which circuit is usual in electrochemistry[39]. In this equivalent circuit based on the simple process, we have the electrolyte resistance, Rs, the double-layer capacitance, Cdl, and the faradaic impedance Zf. This last one translates the faradic processes that depend on the frequency and consequently cannot be represented by simple linear circuit elements. A typical representation of Zf is a charge transfer resistance in series with a pseudo-capacitance or the same resistor in series with a Warburg impedance which is a type of resistance to mass transfer. This equivalent circuit illustrates the usual behavior of the conductive polymers and corresponds to either the simple diffusion-kinetic model or the distributed transmission line model or the more complex model that describes the mixed electron and ionic conduction taking into account the percolation of charge by electromigration[40].

The determination of the parameters of the circuit was not performed from the simulation of the equivalent circuit.

We calculate these values directly from the impedance diagrams using boundary conditions. Rs is determined through the diagram’s intersections with the Z´ axis at high frequencies, Rct is determined to estimate the semicircle diameter and Cdl is calculated from the following relation: Cdl =

1 2πR ct f

(3)

where f is the frequency that corresponds to the maximum of the half-circle.

The low-frequency capacitance, Clf, is calculated from the following relation: Clf =

1 2πf Z′′

(4)

where Z” is the imaginary part of the EIS spectra.

Polímeros, 30(1), e2020001, 2020

3. Results and Discussions 3.1 Spectroelectrochemistry In order to know the electroactivity window suitable for each copolymer, the films have been characterized by cyclic voltammetry (25 mV s-1) in a monomer-free solution containing 0.1 M of NaClO4/acetonitrile. The voltammograms and the respective chemical structure of these copolymers are presented on Figure 1. Figure 2 shows the spectroelectrochemical results, whose inset graph presents the absorbance variation curves in function of the potential applied, for each corresponding maximum absorbance wavelength.

After the analysis of the spectroelectrochemical curves, it can be observed that PEDOT and copolymers films presented an absorption band in the high-energy side of the spectrum when in their reduced state. As we apply the shifts of potential, the energetic states of the polymer change that is corroborated by the quenching of the transition between the valence band (VB) and conducting band (CB), also to the appearance of a band near to the infrared region. This band appearance is linked to the generation of new energy levels inside the band gap during the doping process, which leads to polaron and bipolaron states generation[41].

Also, all the films presented color changing in function of the conditioning potential, going from a colored neutral state to a colored oxidized state. Indeed, PEDOT-co- PhTAc-2a film presented a brownish neutral state and a grayish oxidized state, PEDOT-co-PPhTAc-2b changed from dark-purple to light-blue, and PEDOT-co-PPhTCb changed from purple to grayish-green.

In general, all the spectroelectrochemical curves from the copolymers seemed to be similar to PEDOT curves, and this may be explained by the higher amount of EDOT monomer in the copolymer backbone so that it could be expected a more significant influence of EDOT over the spectroelectrochemistry.

The maximum absorption value to the films at reduced state, as well as the onset energy and the molecular orbital energies for each polymer, are presented in Table 1 below.

The onset energy was extracted from the Tauc curves (Figure S1 at the Supplementary Material section) and the HOMO energy was calculated through the onset of the anodic peak potential (extracted from the voltammogram).

The band gap and the molecular orbital levels (HOMO and LUMO) for PEDOT film are corroborated with the literature[42]. This table shows that the copolymers presented a shift towards the high-energy side of the spectrum (hypsochromic shift) in comparison to PEDOT, which may be explained by the presence of substituted groups on the polymeric structure, rising the possible electronic transitions.

This shift was lower for PEDOT-co-PPhTCb since there is more electron delocalization due to the resonance effect of the group –COOPh, directly attached to the thiophene ring[36].

On the other hand, PEDOT-co-PPhTAc-2b presented the opposite effect: the band shifting to the low-energy spectrum region (bathochromic shift), which corroborates the influence of the electron-withdrawing effect of the nitro group[36].

When we observe the band gap values (Eg), it is evident a bathochromic shift found to copolymer films, referring to a decrease on the transition energy value, which may be linked, 3/10

Araujo, M. H., Matencio, T., Donnici, C. L., & Calado, H. D. R.

Figure 1. Cyclic voltammograms and chemical structure for PEDOT (a), PEDOT-co-PPhTAc-2a (b), PEDOT-co-PPhTAc-2b (c) and PEDOT-co-PPhTCb (d); E/V= potential/volts, Ag = silver, µA = microampere.

Figure 2. Spectroelectrochemical curves and digital photographs to the films of PEDOT (a), PEDOT-co-PPhTAc-2a (b), PEDOT-co-PPhTAc-2b (c) and PEDOT-co-PPhTCb (d); E/V= potential/volts, Ag = silver, λ/nm = wavelength/nanometers.

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Polímeros, 30(1), e2020001, 2020

Electrical and spectroelectrochemical investigation of thiophene-based donor-acceptor copolymers with 3,4-ethylenedioxythiophene Table 1. Data of maximum of absorption (λmax), transition onset energy (Eg), highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels for PEDOT and the D-A copolymers; determined under reduction potential (E).

E/V PEDOT P(EDOT-co-PhTAc-2a) P(EDOT-co-PhTAc-2b) P(EDOT-co-PhTCb)

-0.9 -0.8 -0.6 -0.5

Absorption

Onset

λmax/nm

Eg/eV

HOMO/eV

Molecular Orbitals LUMO/eV

596 544 602 591

1.53 1.45 1.61 1.40

-3.67 -4.51 -4.41 -4.34

-2.14 -3.06 -2.80 -2.94

Table 2. Data of optical contrast (ΔT), coloration efficiency (η), oxidation switching time (τox), and reduction switching time (τred) for PEDOT and copolymer films.

PEDOT P(EDOT-co-PhTAc-2a) P(EDOT-co-PhTAc-2b) P(EDOT-co-PhTCb)

Thickness/nm

ΔT

η/cm2 C-1

τox/s

τred/s

24.9 10.5 15.5 65.5

27.63 10.18 19.01 12.70

40.55 43.79 304.7 37.61

1.10 1.38 1.81 1.15

1.97 1.54 2.58 1.21

Figure 3. Transmittance spectra (dashed line) and chronoamperograms (solid line) to the films of PEDOT (a), PEDOT-co-PPhTAc-2a (b), PEDOT-co-PPhTAc-2b (c) and PEDOT-co-PPhTCb (d); t/s = time/seconds.

again, to the D-A groups effect, meaning a larger electronic density conjugation, and in turn, decreasing the transition energy value[36]. On the contrary, it was observed a slight increase in the band gap energy for PEDOT-co-PPhTAc-2b, which may probably be linked to the electron-withdrawing effect of the nitro group.

The chronoabsorptometry curves (Figure 3) show the transmittance spectra on the UV-Vis region versus time and the chronoamperograms. Then, it was possible to measure the switching times for coloration and bleaching processes beyond calculating the electrochromic parameters using Equations 1 and 2. These results are presented in Table 2.

By taking a look into Table 2, PEDOT-co-PPhTAc-2b film has presented coloration efficiency almost 7.5 times higher than neat PEDOT. This behavior may be related to the Polímeros, 30(1), e2020001, 2020

nitro group presence, acting as a strong electron-withdrawing group, fortifying the electron-acceptor effect of the PhTAc-2b block[36]. All copolymer films have presented moderately fast switching times located on the scale of a few seconds, as observed in other works[43-46]. Besides, we have noticed that the copolymers, based on electron-acceptor monomers that possess a methylene group (PhTAc-2a and PhTAc-2b), presented the highest switching times. On the contrary, PEDOT-co-PPhTCb film, which does not have this methylene group, has presented faster switching times, which might be associated with a direct conjugation path among the D-A blocks, increasing the conjugation length and, therefore, decreasing the response times.

It is well-known from the literature that the thickness of the film directly interferes in the electrochromic properties, 5/10

Araujo, M. H., Matencio, T., Donnici, C. L., & Calado, H. D. R. where the higher thickness is supposed to lead to greater performances[4]. The thickness found for the copolymers reported on Table 2, even neat PEDOT, was much lower when compared with other D-A copolymers and, therefore, the values of optical contrast and coloration efficiency were quite lower[47-49]. Nevertheless, with thickness, almost twice lower than PEDOT, P(EDOT-co-PhTAc-2a) and P(EDOT-co-PhTAc-2b) films have presented electrochromic properties similar or even higher than neat PEDOT, which is an improvement after all.

Another fact that may influence over the electrochromic properties of the polymer is the methodology used during the electropolymerization process. Gu et al.[50] have employed chronoamperometry method to prepare an isoindigo-based donor-acceptor polymer with EDOT and exhibiting an outstanding electrochromic performance with coloration efficiency around 360 cm2 C-1, fast switching time of 0.5 s, and high optical contrast of almost 60%. Indeed, the chronoamperometry is a faster technique to grow thicker polymeric films when compared to the cyclic voltammetry.

Once the polymers of this work have been electropolymerized through cyclic voltammetry, this explains the lower thickness and, consequently, the apparent poor values of electrochromic parameters.

3.2 Electrochemical Impedance Spectroscopy (EIS) The impedance experiments took place by applying two potentials: Edc = -0.5 V – reduced polymer and Edc = 1.5 V – oxidized polymer. Figure 4a below shows, on the shape of the Nyquist diagram, the impedance results, at low frequencies, obtained for PEDOT and its several

copolymers at the oxidized state (Edc = 1.5 V). On the other hand, Figure 4b shows the impedance results at high frequencies.

As it can be observed, at higher frequencies, the PEDOT behavior is in agreement with the literature results[38], pointing the presence of an almost vertical line, Figure 3b, typical of a capacitive behavior, and indicating a fast electrical charge transfer at the interface metal | PEDOT | electrolyte. However, to the copolymers, this electronic transference appears to be slower and it is translated by line with a slope < 90°.

The profile observed at low frequencies, Figure 3a, it was also observed by Bobacka et al.[38], and it is assigned to a reaction that happens in parallel to the electrode doping that might be related to the presence of oxygen traces in the electrolyte solution. This behavior fits with the model proposed by Vorotyntsev et al.[51], which counts a charge transfer involving the ionic and electronic shape between the polymer and the solution. The intersection at high frequencies of the Nyquist diagrams with Z´ axis is mainly related to the electrolyte resistance, amount near to 150 Ω for PEDOT and 200 Ω for copolymers. This variation might be since this resistance would also exist a resistive contribution due to the polymer | platinum interface. We have neglected effects due to the ohmic resistance (electronic) of the oxidized films since we will see in the following part that in the reduced state, the behavior of this high-frequency resistance is the same.

The Nyquist diagrams recorded at reduced state (Edc = -0.5 V) are presented in Figure 5. At lower frequencies (Figure 5a), the behaviors are similar and show a vertical

Figure 4. Nyquist diagrams to ■ PEDOT, Δ PEDOT-co-PPhTAc-2a, □ PEDOT-co-PPhTAc-2b, and ▲ PEDOT-co-PPhTCb films; Edc = 1.5V; (a) low frequencies; (b) high frequencies; Z = impedance.

Figure 5. Nyquist diagrams to ■ PEDOT, Δ PEDOT-co-PPhTAc-2a, □ PEDOT-co-PPhTAc-2b, and ▲ PEDOT-co-PPhTCb films; Edc = -0.5 V; (a) low frequencies; (b) high frequencies; Z = impedance.

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Polímeros, 30(1), e2020001, 2020

Electrical and spectroelectrochemical investigation of thiophene-based donor-acceptor copolymers with 3,4-ethylenedioxythiophene Table 3. Data of resistance (Rs), charge transfer resistance (Rct), double-layer capacitance (Cdl), and low-frequency capacitance (Clf) to PEDOT and copolymers films.

Rs/(Ω cm-2)

Rct/(Ω cm-2)

Cdl/(μF cm-2)

Clf/(F g-1 cm-2)

16,900 20,000 21,900 22,200

20,000 19,600 36,000

190 86 110

488,400 1,355,800 920,300 87,300

PEDOT PEDOT-co-PPhTAc-2a PEDOT-co-PPhTAc-2b PEDOT-co-PPhTCb

line with a slope near to 90° (capacitive profile). Figure 5b illustrates the behavior at higher frequencies.

4. Conclusions

The four copolymers have shown similar diagrams, which allows seeing at higher frequencies, in the diagrams intersection with the Z´ axis, a resistance near to 200 Ω and, as in the oxidized state, which can be assigned to the electrolyte resistance associated in series with the polymer | platinum interface resistance. At intermediate frequencies, it is observed a characteristic semicircle of the double-layer resistance in parallel with the charge transfer resistance. Furthermore, at lower frequencies, the behavior seems to be purely capacitive, with an almost vertical line crossing over the Z´ axis. Peculiarly, PEDOT-co-PPhTAc-2b shows, at intermediate frequencies, a Warburg behavior, translated by a line with a slope near to 45° – related to the observation of the diffusive process during the polymer redox reactions. At the PEDOT diagram, it is only observed the capacitive process and it was no possible to see the semicircle presented in the copolymers diagrams. In Table 3, the impedance values are presented for each process seen in Figure 4a and Figure 4b diagrams.

Additionally, through the experiments of chronoabsorptometry, it was possible to determine some electrochromic parameters indispensable to study the device building viability, like the optical contrast and the coloration efficiency, beyond the switching times to electrochemical stimulation, in which those D-A-like copolymers have demonstrated a potential application on electrochromic devices building.

Differently from PEDOT, the copolymers have presented a Rct value quite high, indicating a greater difficulty for charge transfer. The electron-acceptor effect of the substituent influences at position 3 on the thiophene ring over the data of resistance obtained is also notable. In the same way, as presented out in the switching times analysis, the PhTCb copolymer has shown a more significant variation of Rct value, perhaps also due to the direct conjugation between the thiophene ring and the carboxylic substituent, which may worsen even more the charge transfer.

On the other hand, the Cdl values are compatible with the double-layer capacitance values usually expected for platinum in contact with the electrolyte[52,53], confirming the porosity of the films.

It is noteworthy that the Clf values were quite high, indicating such materials as promising candidates for supercapacitors devices building, and this might be due to the efficient electron conjugation between the electron-donor monomer (EDOT) and electron-acceptor monomers (PhTAc-2a, PhTAc-2b, and PhTCb) from the D-A system, proposed in this work.

If we make a comparison between spectroelectrochemical and impedance results, it is possible to observe that when the polymer remains reduced or neutral, it presents a higher resistance. Instead, when the polymer is oxidized, it is noticed the arising of new energetic states within the band gap, and the resistance became lowest.

Polímeros, 30(1), e2020001, 2020

In this work, the spectroelectrochemical experiments have shown that D-A-like copolymers prepared by electropolymerization of EDOT and electron-acceptor thiophene derivatives have presented electrochromism.

Such color changing was visually noticed, beyond observing the arising of new bands at the absorption spectrum as the potential changes.

The impedance results show that in the oxidized state, there is a fast electrical charge transfer at the interface metal | polymer | electrolyte and in the reduced state, the charge transfer is much slower and there is a purely capacitive behavior at low frequencies. Finally, the Cdl values have confirmed the film porosity and capacitive profile at low frequencies indicating that those materials also have potential applicability to supercapacitors building.

5. Acknowledgements This work was partially supported by the Brazilian Institute of Science and Technology in Carbon Nanomaterials (INCT) and the Brazilian agencies CAPES, CNPq [457586/2014-1 and 4076186/2013-1], FAPEMIG [CEX - PPM-281-17, CEX - PPM-00916-15, and TEC - APQ-02715-14], and Rede Mineira de Química (RQ-MG). Marcus Henrique de Araujo gratefully thanks the scholarship received from CAPES. The authors also thank LaMPaC/UFMG for providing infrastructure to run the EIS experiments.

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36. Araujo, M. H., Matencio, T., Donnici, C. L., & Calado, H. D. R. (2016). Synthesis and electrochemical investigation of betasubstituted thiophene-based donor-acceptor copolymers with 3,4-ethylenedioxythiophene (EDOT). Journal of Solid State Electrochemistry, 20(9), 2541-2550. http://dx.doi.org/10.1007/ s10008-016-3297-1.

37. Bechinger, C., Burdis, M. S., & Zhang, J. G. (1997). Comparison between electrochromic and photochromic coloration efficiency of tungsten oxide thin films. Solid State Communications, 101(10), 753-756. http://dx.doi.org/10.1016/S0038-1098(96)00703-X. 38. Bobacka, J., Lewenstam, A., & Ivaska, A. (2000).

Electrochemical impedance spectroscopy of oxidized poly(3,4ethylenedioxythiophene) film electrodes in aqueous solutions.

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42. Spencer, H. J., Skabara, P. J., Giles, M., McCulloch, I., Coles, S. J., & Hursthouse, M. B. (2005). The first direct experimental comparison between the hugely contrasting properties of PEDOT and the all-sulfur analogue PEDOT by analogy with well-defined EDTT-EDOT copolymers. Journal of Materials Chemistry, 15(45), 4783-4792. http://dx.doi.org/10.1039/b511075k.

43. Zhao, H., Tang, D. D., Zhao, J. S., Wang, M., & Dou, J. M. (2014). Two novel ambipolar donor-acceptor type electrochromic

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polymers with the realization of RGB (red-green-blue) display in one polymer. RSC Advances, 4(106), 61537-61547. http:// dx.doi.org/10.1039/C4RA11628C. 44. Data, P., Zassowski, P., Lapkowski, M., Domagala, W., Krompiec, S., Flak, T., Penkala, M., Swist, A., Soloducho, J., & Danikiewicz, W. (2014). Electrochemical and spectroelectrochemical comparison of alternated monomers and their copolymers based on carbazole and thiophene derivatives. Electrochimica Acta, 122, 118-129. http://dx.doi. org/10.1016/j.electacta.2013.11.167.

45. Yigit, D., Udum, Y. A., Gullu, M., & Toppare, L. (2014).

Electrochemical and spectroelectrochemical studies of poly(2,5-di-2,3-dihydrothieno 3,4-b 1,4 dioxin-5-ylthienyl) derivatives bearing azobenzene, coumarine and fluorescein dyes: effect of chromophore groups on electrochromic properties.

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46. Wang, Z., Xu, J. K., Lu, B. Y., Zhang, S. M., Qin, L. Q., Mo, D. Z., & Zhen, S. J. (2014). Poly(thieno[3,4-b]-1,4-oxathiane): medium effect on electropolymerization and electrochromic performance. Langmuir, 30(51), 15581-15589. http://dx.doi. org/10.1021/la503948f. PMid:25469424.

47. Lu, B. Y., Zhen, S. J., Zhang, S. M., Xu, J. K., & Zhao, G. Q. (2014).

Highly stable hybrid selenophene-3,4-ethylenedioxythiophene as electrically conducting and electrochromic polymers. Polymer Chemistry, 5(17), 4896-4908. http://dx.doi.org/10.1039/ C4PY00529E. 48. Ming, S. L., Zhen, S. J., Liu, X. M., Lin, K. W., Liu, H. T., Zhao, Y., Lu, B. Y., & Xu, J. K. (2015). Chalcogenodiazolo [3,4-c]pyridine based donor-acceptor-donor polymers for green and nearinfrared electrochromics. Polymer Chemistry, 6(48), 8248-8258. http://dx.doi.org/10.1039/C5PY01321F. 49. Ming, S. L., Zhen, S. J., Lin, K. W., Zhao, L., Xu, J. K., & Lu, B. Y. (2015). Thiadiazolo[3,4-c]pyridine as an acceptor toward fast-switching green donor-acceptor-type electrochromic polymer with low bandgap. ACS Applied Materials & Interfaces, 7(21), 11089-11098. http://dx.doi.org/10.1021/acsami.5b01188.

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Electrochimica Acta, 260, 772-782. http://dx.doi.org/10.1016/j. electacta.2017.12.033.

51. Vorotyntsev, M. A., Deslouis, C., Musiani, M. M., Tribollet, B., & Aoki, K. (1999). Transport across an electroactive polymer film in contact with media allowing both ionic and electronic interfacial exchange. Electrochimica Acta, 44(12), 2105-2115. http://dx.doi.org/10.1016/S0013-4686(98)00318-1.

52. Pajkossy, T., & Kolb, D. M. (2007). Double layer capacitance of the platinum group metals in the double layer region.

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53. Pajkossy, T., & Kolb, D. M. (2001). Double layer capacitance of Pt(111) single crystal electrodes. Electrochimica Acta, 46(20-21), 3063-3071. http://dx.doi.org/10.1016/S00134686(01)00597-7.

Received: May 12, 2019 Revised: Feb. 27, 2020 Accepted: Mar. 04, 2020

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Supplementary Material Supplementary material accompanies this paper.

Figure S1. Tauc curves for PEDOT (a), PEDOT-co-PPhTAc-2a (b), PEDOT-co-PPhTAc-2b (c), and PEDOT-co-PPhTCb (d).

This material is available as part of the online article from http://www.scielo.br/po

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PolĂ­meros, 30(1), e2020001, 2020

ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.06819

Synergistic effect of adding lignin and carbon black in poly(lactic acid) Thaís Ferreira da Silva1* , Fernanda Menezes1, Larissa Stieven Montagna1, Ana Paula Lemes1 and Fabio Roberto Passador1 1 Laboratório de Tecnologia em Polímeros e Biopolímeros – TecPBio, Instituto de Ciência e Tecnologia – ICT, Universidade Federal de São Paulo – UNIFESP, São José dos Campos, SP, Brasil

*thais.ferret@hotmail.com

Abstract Antistatic packaging is a very important sector since the electrostatic discharge of electronic devices can damage and/or disable these products. In addition, it is essential to dispose of this packaging correctly. In this work, the synergistic effect of the addition of lignin and carbon black on the development of antistatic and biodegradable packaging was verified. In this study, PLA was mixed with lignin and carbon black and the composites were prepared using a high-speed thermokinetic homogenizer where the melting of the PLA and the blend with fillers occurred by friction. The composites were characterized by Izod impact tests, scanning electron microscopy, thermal properties, electrical characterization and biodegradation tests in garden soil. The results show that lignin is a great option to accelerate the biodegradation of PLA in the garden soil and the carbon black acts as an antistatic agent reducing the electrical resistivity of the composites.

Keywords: antistatic, biodegradable, poly(lactic acid), lignin, carbon black.

How to cite: Silva, T. F., Menezes, F., Montagna, L. S., Lemes, A. P., & Passador, F. B. (2020). Synergistic effect of adding lignin and carbon black in poly(lactic acid). Polímeros: Ciência e Tecnologia, 30(1), e2020002. https://doi. org/10.1590/0104-1428.06819

1. Introduction Antistatic packaging is used for the protection and storage of electronic boards and sensitive electronic components[1,2].

Conductive carbon black is the most commonly used antistatic agent for the production of antistatic packaging and is intended to increase the electrical conductivity (and consequently reduce electrical resistivity) of the packaging so that the electronic devices are not subjected to electrical shocks and are damaged during their transport and storage[3-5]. The conductive carbon black has typical values of electrical conductivity of 100 S.cm-1[6].

Antistatic packaging is generally composed of polyethylenes[1], polystyrene[7] or polyethylene terephthalate[2].

However, after using these packaging, there is a serious problem of disposal, which generates waste and damages the environment. Several strategies are being carried out to reduce the amount of plastic waste, including the biodegradation process. The biodegradation process consists of physical or chemical modifications, caused by the action of microorganisms, under certain conditions of heat, humidity, light, oxygen and suitable materials nutrients and minerals[8].

Another way to help reduce the amount of plastic waste in the environment is the use of biodegradable polymers.

Biodegradable polymers are considered a generation of environmentally sustainable polymers. To be considered a biodegradable polymer, it must present, after significant degradation, final products compatible with the environment, such as carbon dioxide, water and microbial biomass[9].

Polímeros, 30(1), e2020002, 2020

Among the biodegradable polymers, the poly(lactic acid) (PLA) is one of those that has attracted researchers attention due possesses a variety of desirable properties, such as biodegradability, biocompatibility and exhibiting excellent mechanical properties under tensile tests[10,11].

As limitations, PLA has a high cost, low mechanical toughness, and degradation that ranges from 6 months to 2 years, depending on the conditions under which the material is submitted[12,13].

In our previous work[3], the authors have shown that the PLA/carbon black composite can be an excellent alternative for the preparation of antistatic packages, but the biodegradation time of these packages is still long (Composites of PLA with 15 wt% carbon blacks lost approximately 0.52% residual mass in 6 months). PLA/carbon black with 5, 10 and 15 wt% carbon black were prepared in a thermokinetic homogenizer. The addition of 15 wt% of carbon black in the PLA matrix increased the electrical conductivity and decreased the electrical resistivity of the composites by 11 and 8 orders of magnitude, respectively.

The composites of PLA with the addition of 10 and 15 wt% of carbon black presented values of electrical resistivity that allow its use as antistatic packaging. Furthermore, the authors confirm that the addition of carbon black does not change the time of biodegradation of these composites, but nevertheless are still too long.

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Silva, T. F., Menezes, F., Montagna, L. S., Lemes, A. P., & Passador, F. R. Rane et al.[14] made a comparative analysis of processing techniques’ effect on the strength of carbon black filled poly (lactic acid) composites. Two techniques were used to obtain the specimens, the first was the dispersion of carbon black was prepared by using chloroform in water bath sonication at room temperature and the second using a Haake mixer, followed by pressing. Hardness and Tensile Tests tests were performed. They concluded that both methods were efficient to homogenize the mixtures and there was no significant difference between mechanical property results of the samples made by sonicator and mixer.

One way to reduce the time of biodegradation of PLA/carbon black antistatic packages is through the addition of another phase in the composite. This phase can be composed of organic material of renewable origin, such as lignin[15]. Lignin is a compound of plant origin with a high molecular weight and can contribute to the reduction of PLA biodegradation time, as reported by Gordobil et al.

[16] that prepared PLA/lignin composites with different contents of lignin (0.5, 1, 5, 10 and 20 wt%) by extrusion and demonstrated that the addition of 5 wt% lignin decrease the degree of crystallinity of the PLA and facilitate the degradation of the composite.

Silva et al.[9] studied the addition of lignin in PLA. The composites were prepared using a high speed rotary mixer at 3000 rpm, followed by hot pressing. The thermal characterization, biodegradation test and impact resistance test was performed. The results show that lignin is a great option to accelerate PLA biodegradation in garden soil. PLA/lignin compound with a 10% by weight addition of lignin is the best option for the manufacture of biodegradable packaging as it has similar thermal and mechanical properties to clean the PLA and has a higher biodegradation rate.

Thus, this work seeks a synergistic effect of the addition of carbon black and lignin in a PLA matrix. In order to obtain antistatic packaging for storage and transportation of electronic devices it is necessary to add the antistatic agent, in this case, the carbon black, and in order to decrease the biodegradation time of the PLA matrix the lignin was used.

The combination of these two fillers (lignin and carbon black) may contribute to accelerate the biodegradation process and to reduce the electrical resistivity of PLA for the preparation of a new technological and sustainable packaging. Another goal is the use of a high-speed mixer to provide a very homogeneous mixture between PLA and the fillers. In this process, the mixing, melting, and homogenization occur by friction between the mixing components and the equipment rotor with a rotation of 3000 rpm that may lead to better dispersion and distribution of lignin and carbon black in PLA matrix. The effect of different contents of lignin and carbon black was also investigated.

2. Materials and Methods 2.1 Materials Poly(lactic acid) (PLA) - PLI 005 produced by Nature Plast (France) with a density of 1.25 g/cm3 and melt flow index of 5-10 g/10 min (235 °C/2.16 kg). Hardwood Kraft lignin was kindly provided in powder by Suzano Papel e Celulose S.A. Corp. (Brazil) with a surface area of 1.56 m2 g-1, a pore 2/10

diameter of 47 Å (BET, Quantachrome Instruments, model Nova 4200e) and particle size of 3.76 μm (CILAS analyzer, model 1190 L), and was used as received. Conducted carbon black (VULCAN XC72R) supplied by CABOT (USA) as a fine powder with a mean particle size of 50 nm and a density of 0.264 g/cm3. The electrical conductivity found for this material was 4.5 S cm−1[3].

2.2 Preparation of PLA/lignin/carbon black composites All materials were dried for a minimum of 24 h in an oven at 80 °C prior to melting processing. PLA/lignin/carbon black composites were prepared using a high-speed mixer (thermokinetic mixer produced by MH Equipamentos Ltda, model MH50-H) rotating at 3000 rpm and mixing chamber with a capacity of 70 g of material. The mixing temperature was monitored using a thermocouple and reached 200 °C in 40 seconds of mixing. This technique were chosen because promote great homogeneity between the polymer matrix (PLA) and the fillers (lignin and carbon black) as observed in our previously works[1,3,9]. Composites with 5, 10 and 15 wt% of lignin and 5, 10 and 15 wt% of carbon black were prepared.

Table 1 shows the nomenclature and compositions prepared.

After 1 min of mixing, the homogenized composites were collected and pressed into 3.2 mm thick plates with standard dimensions for the Izod impact tests in a hydropneumatic press (MH Equipamentos Ltda, model PR8HP) at 200 °C with a pressure of 5 bar for 3 min. All the composition presented great homogeneity and the fillers were completly incorporated in the polymer matrix.

2.3 Characterization of PLA/lignin/carbon black composites 2.3.1 Thermal characterization of PLA/lignin/carbon black composites Thermal characterization of PLA/lignin/carbon black composites was evaluated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC).

Thermogravimetric analysis (TGA) of the composites were performed using a Netzsch equipment model TG 209 F1 Iris, from room temperature to 600 °C at a rate of 20 °C/min, under N2 atmosphere.

The DSC analyses were performed using a TA Instruments equipment, QS100 model under an N2 atmosphere with a flow rate of 50 mL.min-1. Samples were sealed in an Table 1. Nomenclature and composition of the PLA/lignin/carbon black composites.

Samples Neat PLA 90/5/5 85/10/5 80/15/5 85/5/10 80/10/10 75/15/10 80/5/15 75/10/15 70/15/15

PLA (wt%) 100 90 85 80 85 80 75 80 75 70

Lignin (wt%) -5 10 15 5 10 15 5 10 15

Carbon Black (wt%) -5 5 5 10 10 10 15 15 15

Polímeros, 30(1), e2020002, 2020

Synergistic effect of adding lignin and carbon black in poly(lactic acid) aluminum DSC pan and heated from room temperature to 200 °C at 10 °C/min. They were kept at 200 °C during 5 min to eliminate the heat history; subsequently cooled to 0 °C at 10 °C/min to determinate the crystallization temperature (Tc). After this, they were heated to 200 °C at 10 °C/min.

The degree of crystallinity (Xc) was calculated according to Equation 1: Xc ( % ) =

∆Hm − ∆Hc . 100 ( % ) ∆Hmº

(1)

where Xc(%) is the degree of crystallinity; ∆Hm is the melting enthalpy obtained by DSC; ∆Hc is the crystallization enthalpy during heating and ∆Hmº is the theoretical melting heat value for 100% crystalline material (93.7 J/g for PLA[17]).

The ∆Hmº value for the composites was calculated for each composition considering the mass fraction of the PLA in the composite.

2.3.2 Electrical characterization of PLA/lignin/carbon black composites The surface electrical characterization of all composites was performed by impedance spectroscopy and AC electrical resistivity (alternating current) according Silva et al.3]. The values of electrical conductivity (σ) were calculated from the inverse of the resistivity (ρ) (Equation 2), which were obtained from the relation between the impedance values (Z) and the electrical contact area dimensions of the samples (“A”, area and “l”, thickness), Equation 3. A thin layer of gold/palladium alloy was deposited by a metallizer (MED020 Bal-tec) on both sides of the samples, in order to form the electrical contact, producing a metal-composite‑metal structure. σ=

ρ=

1

(2)

Z .A

l

(3)

ρ

Impedance measurements were performed on an impedance analyzer (Solartron SI 1260, Impedance/Gain‑phase Analyzer) which sends the data to a computer. The measurements were performed at room temperature at a frequency of 1 Hz and voltage amplitude of 0.5 V. 2.3.3 Biodegradation test of PLA/lignin/carbon black composites The soil biodegradation test was carried out according to ASTM G160-98[18]. An aquarium apparatus was used containing garden soil (pH 5.92). Figure 1 shows the

apparatus used. Test specimens for Izod impact strength tests of all compositions were buried in the garden soil and kept at room temperature (25 °C) and humidity controlled between 20 and 30%. Samples of all compositions were removed at 0, 30, 60, 90 and 180 days. The samples were removed from the soil at the end of each period and were gently washed with distilled water to remove soil particles.

Then the samples were dried at 30 º C for 6 h. After each time interval, the samples were characterized by Izod impact strength, and scanning electron microscopy (SEM) to verify the effect of biodegradation in the samples. The residual mass (Mr) in %, was calculated according to Equation 4:  Mf  Mr ( % ) =   .100  Mi 

(4)

where Mf is the final mass of the sample after soil exposure and Mi is the initial mass before soil exposure.

2.3.4 Mechanical characterization of PLA/lignin/carbon black composites Impact strength tests were performed on a CEAST/Instron Izod Impactor Test Machine (model 950). The test method adopted was carried out according to ASTM D256-78[19].

All the test specimens were notched using a manual notched machine (CEAST/Instron, model 9050) and the tests were performed using a 0.5 J hammer. A minimum of five samples of each composition was tested.

2.3.5 Morphological characteristics of PLA/lignin/carbon black composites SEM images of neat PLA and PLA/lignin/carbon black composites before and after of exposure in garden soil were analyzed using a scanning electron microscope FEI Inspect S50, operating at 7.5 keV. The samples were placed on aluminum stubs and coated with gold.

3. Results and Discussions 3.1 Thermal characterization of PLA/lignin/carbon black composites: TGA and DSC Figure 2 shows the TGA curves and Table 2 shows the Tonset values for the neat PLA and the PLA/lignin/carbon black composites. It is possible to observe that the temperature of the irreversible thermal degradation (Tonset) of neat PLA at 324 °C with a mass loss of 98.5% at 600 °C. This event was also observed in the studies of Zhao et al.[20].

Figure 1. (A) Apparatus with garden soil used in the biodegradation test; (B) specimens buried in garden soil.

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Silva, T. F., Menezes, F., Montagna, L. S., Lemes, A. P., & Passador, F. R.

Figure 2. TGA curves: (A) neat PLA and PLA/lignin/carbon black composites with 5, 10 and 15 wt% of lignin and 5 wt% of carbon black; (B) PLA/lignin/carbon black composites with 5, 10 and 15 wt% of lignin and 10 wt% of carbon black; and (C) PLA/lignin/carbon black composites with 5, 10 and 15 wt% of lignin and 15 wt% of carbon black.

Table 2. Values of Tg1, TCC1, ΔHCC1, Tm1, ΔHm1, and Xc1 obtained during first scans of heating, Tc obtained during cooling and Tg2, TCC2, ΔHCC2, Tm2, ΔHm2 and Xc2 obtained during second heating (DSC) and (Tonset) values obtained by TGA for the neat PLA and PLA/lignin/carbon black composites.

Neat PLA 90/5/5 85/10/5 80/15/5 85/5/10 80/10/10 75/15/10 80/5/15 75/10/15 70/15/15

TGA Tonset (°C) 324 347 359 364 350 365 350 355 354 349

Tg1 (°C) 59 57 59 58 56 57 58 56 58 57

TCC1 (°C) 88 85 90 87 86 88

First heating ΔHCC1 Tm1 (J/g) (°C) 9.3 175 175 175 174 175 9.0 175 18.0 173 19.2 173 15.7 174 18.7 174

ΔHm1 (J/g) 56.1 47.5 53.1 52.1 49.3 50.5 51.8 52.4 58.7 54.8

Xc1 (%) 49.9 47.8 53.7 53.1 49.6 37.2 26.1 27.2 36.9 25.3

Cooling Tc (°C) 113 111 109 106 111 109 108 110 110 107

Tg2 (°C) 62 63 64 64 65 66 65 65 65 65

TCC2 (°C) -----------

Second heating ΔHCC2 Tm2 (J/g) (°C) -177 -176 -176 -175 -176 -176 -175 -176 -176 -175

ΔHm2 (J/g) 50.9 51.8 52.4 53.4 52.0 48.1 53.5 52.7 54.4 52.6

Xc2 (%) 54.3 55.3 55.9 57.0 55.5 51.3 57.1 56.2 58.1 56.2

Tg1 is the glass transition temperature obtained during heating scans, TCC1 is cold crystallization temperature obtained during heating scans, ΔHCC1 is the cold crystallization enthalpy obtained during heating scans, Tm1 is crystalline melting temperature obtained during heating scans, ΔHm1 is the melting enthalpy obtained during heating scans, Xc1 is degree of crystallinity obtained during heating scans, Tc is crystallization temperature, Tg2 is the glass transition temperature obtained during second heating scans, TCC2 is cold crystallization temperature obtained during second heating scans, ΔHCC2 is the cold crystallization enthalpy obtained during second heating scans, Tm2 is crystalline melting temperature obtained during second heating scans, ΔHm2 is the melting enthalpy during second heating scans,, Xc2 is degree of crystallinity obtained during second heating scans, Tonset is start temperature of irreversible thermal degradation, and TGA is thermogravimetric analysis.

The initial temperature of degradation (or initial thermal stability) was altered with the incorporation of lignin and carbon black, just as it is possible to observe a single thermal event. The addition of lignin and carbon black increased the Tonset of the composites. There was an increase of about 41 °C in Tonset for the PLA/lignin/carbon black composite (80/10/10) compared to the neat PLA. Overall, this was the formulation that obtained a larger increase in the Tonset. Thus, a synergism is observed when the two fillers (lignin and carbon black) are added, reaching values higher than neat PLA and PLA/lignin composites[9,16] and PLA/carbon black[3].

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Figure 3 shows the DSC thermograms of the neat PLA and the PLA/lignin /carbon black composites. For neat PLA, three thermal transitions can be observed in the first heating: the glass transition temperature (Tg) at 59 °C, an exothermic event at 88 °C corresponding to the cold crystallization and, the crystalline melting temperature (Tm) at 175 °C, results close to those found by Kumar Singla et al.[21] and Mosnáčková et al.[22]. All thermal parameters obtained by DSC are shown in Table 2. Qin et al.[23] showed that PLA exhibits an exothermic event that represents a rate of cold crystallization, with heat gain without phase change, during its Tg, energy is released with increasing temperature, reorganization occurs in structure until the polymer melts.

Polímeros, 30(1), e2020002, 2020

Synergistic effect of adding lignin and carbon black in poly(lactic acid)

Figure 3. DSC thermogram for the first (A) heating cycle; (B) cooling; and (C) the second heating cycle for the neat PLA and for the PLA/lignin/carbon black composites with different contents of lignin and carbon black.

The degree of crystallinity (Xc) of the compositions was calculated from the enthalpy in the DSC in the first and second heating cycles[24,25]. For the first heating, there was a significant change in Tg and Tm (Figure 3A) and in the degree of crystallinity for all composites when compared to neat PLA (Table 2). Xc is strongly affected by the addition of lignin and carbon black. By analyzing the compositions with 5 wt% of carbon black, it is noted that the increase in the lignin content increases the value of the Xc of the composite. On the other hand, increasing the carbon black values (10 and 15 wt%) shows a decrease in the value of Xc for all the composites. Thus, it can be inferred that the carbon black is one of the main responsible for the changes in the crystallization of the composites. The reason for the decrease in the degree of crystallinity with increasing of carbon black content in PLA may be that PLA chains have less mobility and crystallize with great difficulty at high temperatures in the presence of carbon black[26-28].

The composite 75/15/15 showed Xc (in the first heating) of 25.3% lower than neat PLA (49.9%).

In Figure 3B it can be seen that there was a decrease in the Tm during cooling for all PLA/lignin/carbon black composites compared to neat PLA. In the second heating (Figure 3C), it is possible to observe an increase in Tg and a reduction in Tm, the PLA/lignin/ carbon black composites also presented a double peak endotherm (melt temperature) between 154 °C and 156 °C, values close to that found by Arruda et al.[29] in their study of PLA/PBAT blends. This double peak is common in polyesters, and the literature Polímeros, 30(1), e2020002, 2020

justifies its presence by a melt/recrystallization/remelt mechanism[30,31]. However, in the second heating, all values of Xc of the composites were higher than neat PLA, but these values are very close to each other.

Thus, it can be concluded that the effect of processing conditions greatly influence the degree of crystallinity of the composites. The controlled cooling rate facilitated the crystallization of the PLA and, in the second heating, the increase of the carbon black content contributed to a slight increase in the degree of crystallinity of the composites. It is worth mentioning that in order to be applied as antistatic packaging the effect of the first heating is more significant since the packaging will not undergo heat treatments after its production.

3.2 Electrical characterization of PLA/lignin/carbon black composites The results of electrical conductivity and electrical resistivity of the neat PLA and PLA/lignin/carbon black composites were shown in Table 3. It is possible to observe that most of the composites exhibit characteristics of insulating materials (electrical resistivity greater than 1 x 1011 Ω.cm) [2] , except compositions 80/10/10 and 75/15/10. These compositions exhibit characteristics of dissipative materials, that is, their electrical resistivity is between 1 x 104 Ω.cm and 1 x 1011 Ω.cm.

Regarding the electrical resistivity of the composites, the addition of lignin significantly reduces this property.

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Silva, T. F., Menezes, F., Montagna, L. S., Lemes, A. P., & Passador, F. R. The lignin makes it difficult to form the percolative path between the carbon black particles, which resulted in a significant increase in the electrical resistivity (or decrease in the electrical conductivity) of the composites. Thus, compositions with the addition of 5 wt% carbon black cannot be used for the preparation of antistatic packages.

With the increase in black carbon content to 10 wt%, it can be noticed that two compositions (80/10/10 and 75/15/10) presented a reduction in electrical resistivity in 5 decades of magnitude, making possible its use as antistatic packaging.

In this case, the decrease in the electrical resistivity was due to the formation of conductive paths between the carbon black in the polymer matrix. As the content of the carbon black is increased the distance between this filler black is reduced and particles began to contact each other to form a continuous conductive pathway.

When 15 wt% of carbon black was used in the composites, an increase in the electrical resistivity of the composites was again noted. In this case, the large content of carbon black and lignin used possibly helped to form agglomerates and aggregates of these fillers in the polymer matrix, contributing to a low dispersion and distribution of these fillers and preventing the formation of electric percolation path. Therefore, it is feasible to use the compositions 80/10/10 and 75/15/10 for the preparation of antistatic packages, since in these compositions the polymer has become less resistive, framing it as dissipative and reaching the values similar to materials already used for the manufacture of these packaging. This feature makes it suitable for packaging and uses in protected ESD areas in order to avoid electric discharges and to avoid damaging electronic components.

Table 3. Electrical conductivity (S/cm) and electrical resistivity (Ω.cm) for neat PLA and PLA/lignin/carbon black composites with different contents of lignin and c carbon black.

Neat PLA 90/5/5 85/10/5 80/15/5 85/5/10 80/10/10 75/15/10 80/5/15 75/10/15 70/15/15

Electrical Conductivity (S/cm) 3.32 x 10-12 3.88 x 10-16 5.50 x 10-16 4.12 x 10-16 1.14 x 10-15 8.29 x 10-07 3.75 x 10-07 8.14 x 10-15 8.52 x 10-15 8.59 x 10-16

Electrical Resistivity (Ω.cm) 3.01 x 1011 2.58 x 1015 1.82 x 1015 2.43 x 1015 8.79 x 1014 1.20 x 106 2.66 x 10 6 1.23 x 1014 1.17 x 1014 1.16 x 1015

3.3 Evaluation of biodegradation tests of PLA/lignin/ carbon black composites The degradation of polymers is associated with changes in characteristics such as shape, color, surface morphology and mechanical properties of the materials[32]. Some works in the literature related that PLA takes a long time to degrade in other media such as photodegradation, laser exposure and accelerated aging test[33,34]. Grigull et al.[34] performed the accelerated aging test on PLA films. The films were stored in the aging chamber, following the ASTM G154‑06 standard, kept at 45 °C, 65% air humidity, and under direct incidence of UV lamp rays. Samples were taken at 0, 30, 60, 90 and 120 days. The results showed that the start of biodegradation occur after 60 days, and only after 120 days did the authors observe more significant changes in mass loss.

Since in the present work impact strength specimens were used for the biodegradation test, the soil biodegradation test was performed. Figure 4 shows the images of the neat PLA and the PLA/lignin/carbon black composites before and after the biodegradation test (180 days). It is possible to observe that neat PLA prior to the biodegradation test has a very smooth and shiny surface and is almost transparent.

After 180 days submitted to degradation conditions in garden soil, there are no major modifications to the surface of the samples. The loss of brightness and yellowing being the main changes observed. Our results with neat PLA were consistent with those of other researchers[27,32] and show that the biodegradation rate of neat PLA is very slow under garden (natural) soil conditions and room temperature. In the Figure 4, it is also possible to observe that all samples were dark with the addition of lignin and carbon black, predominating the black color of carbon black. After 180 days of exposure in garden soil, the samples lost their brightness and showed no other significant visual changes.

Figure 5 shows the loss of mass obtained in the biodegradation test in garden soil. An increase in the mass of all samples was observed during the first 30 days of the test. This behavior may be due to the hydrophobicity of the PLA. According to several authors[3,13,35,36], the degradation of PLA occurs in two stages: the hydrolysis of the material (to the ester group occurs by the penetration of water in the material that attacks the amorphous phase), followed by the attack of microorganisms to the oligomers of lactic acid.

In general, the degradation time in the environment may vary from 6 months to 2 years, depending on the conditions under which the material is subjected[13]. As a result of the degradation process, the chains are reduced in smaller and soluble fragments, which leads to a reduction in the mass of the material[35,36].

Figure 4. Samples of neat PLA and PLA/lignin/carbon black composites (A) before and (B) after the biodegradation test for 180 days.

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Polímeros, 30(1), e2020002, 2020

Synergistic effect of adding lignin and carbon black in poly(lactic acid)

Figure 5. Residual mass (%) after the biodegradation test for 30, 60, 90 and 180 days of exposure: (A) neat PLA and PLA/lignin/carbon black composites with 5, 10 and 15 wt% of lignin and 5 wt% of carbon black; (B) PLA/lignin/carbon black composites with 5, 10 and 15 wt% of lignin and 10 wt% of carbon black; and (C) PLA/lignin/carbon black composites with 5, 10 and 15 wt% of lignin and 15 wt% of carbon black.

It is possible to observe that all the composites developed had a mass increase in the first 30 days of exposure in garden soil, may be related to the adhesion of the microorganisms in the polymeric material surface. Because, the contact with the microorganism for 30 days leads to the polymer integrity loss, which is proportional to the surface area since the biodegradation, is usually initiated on the polymer surface, ie first adhesion, biofilm formation, followed by biodegradation, resulting in mass loss. Thus, the beginning of mass loss was observed after 60 days[37], of exposure which was more significant for the composition 80/15/5 (2.7%) and after 180 days the largest loss of mass was for composition 90/5/5 (6.4%).

According to the results of mass loss and electrical characterization, it is possible to conclude that PLA/lignin/carbon black composites (80/10/10) and (75/15/10) would be the best options for the manufacture of antistatic and biodegradable packaging. These composites presented characteristics of dissipative materials, which is an option for the production of antistatic packaging and presenting a significant mass loss. The composites 80/10/10 and 75/15/10 lost about 4.6% and 4.8% of the mass, respectively, after 180 days of soil biodegradation test, being a good option for the production of biodegradable packaging.

In our previous work, we studied the biodegradation and electrical characterization of PLA/lignin[9] and PLA/carbon black[3] composites. We observed that the PLA/lignin composition with 10% lignin showed higher biodegradation, while the PLA/carbon black compounds had a minor loss, PolĂ­meros, 30(1), e2020002, 2020

showing that lignin aids degradation and carbon black does not accelerate, but also does not interfere with PLA degradation. However, the PLA/carbon black composition 15% increased the electrical conductivity and decreased the electrical resistivity of the composites by 11 and 8 orders of magnitude, respectively, allowing their use as antistatic packaging. And the PLA/lignin composites, as expected, presented characteristics of insulating materials. Thus, this work shows the synergistic effect of the addition of lignin and carbon black in the PLA matrix. The compositions 80/10/10 and 75/15/10 presented characteristics of dissipative materials, which is an option for the production of antistatic packaging and present a significant mass loss after 180 days of exposure in garden soil, is a good option for the production of biodegradable packaging.

3.4 Mechanical characterization of PLA/lignin/carbon black composites Figure 6 presents the results of Izod impact strength tests of notched specimens for all compositions, before and after biodegradation tests in garden soil. It is possible to observe that the Izod impact strength decreased after the exposure of the samples in garden soil, including neat PLA, and this fact may be indicative of the biological degradation of the material. Neat PLA lost about 4.7% of its impact strength after the biodegradation test on garden soil after 180 days.

Recalling that lignin and carbon black have not been added to improve impact strength, they are not reinforcing agents. Lignin was added to aid in the acceleration of the 7/10

Silva, T. F., Menezes, F., Montagna, L. S., Lemes, A. P., & Passador, F. R. biodegradation process of PLA and carbon black to decrease the electrical resistivity of PLA. When lignin and carbon black were added to the PLA, the composite 75/15/10 decreased 10.8% impact strength, while the composite 90/5/5 had a 15.7% increase in impact strength. Lignin actually improved the biodegradation, and thus notably had a reduction in impact resistance. It is noted that after 180 days of exposure

in garden soil, the PLA/lignin/carbon black composite in composition 90/5/5 was the most lost in impact strength, about 66% and composition 80/10/10 was the one that lost the least impact resistance, about 44%.

3.5 Morphological characterization using scanning electron microscopy (SEM) of PLA/lignin/carbon black composites

Figure 6. Impact resistance of PLA/lignin/carbon black compositions according to the biodegradation time: (A) neat PLA and PLA/lignin/carbon black composites with 5, 10 and 15 wt% of lignin and 5 wt% of carbon black; (B) PLA/lignin/carbon black composites with 5, 10 and 15 wt% of lignin and 10 wt% of carbon black; and (C) PLA/lignin/carbon black composites with 5, 10 and 15 wt% of lignin and 15 wt% of carbon black.

Figure 7. SEM images of the PLA/lignin/carbon black 75/15/10 after (A) 0 day, (B) 90 days and (C) 180 days exposure on garden soil and PLA/lignin/carbon black 80/10/10 after (D) 0 day, (E) 90 days and (F) 180 days exposure on garden soil.

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PolĂ­meros, 30(1), e2020002, 2020

Synergistic effect of adding lignin and carbon black in poly(lactic acid) Figure 7 shows the surface morphology of the 75/15/10 and 80/10/10 composites after different time intervals (0, 90 and 180 days) of exposure in garden soil. These compositions were chosen because they were the most biodegradable characteristics. In Figures 7A and 7D it is possible to observe a surface with marks resulting from the pressing process. In Figures 7B and 7E, there are the presence of granules on the surface of the samples, these may be biofilm residues that have rested after washing, or some other material adhered or deposited during the analysis[3,38].

Figures 7C and 7F show the formation of large spots and some small surface cracks, results similar to those found by previous searches[27,39,40], which may be associated with the formation of biofilms, once the PLA begins to degrade with 180 days. It is possible to observe similar structures of the compositions 75/15/10 and 80/10/10.

4. Conclusions Antistatic and biodegradable packages based on the poly(lactic acid) matrix were successfully obtained due to the synergy of the addition of lignin that acted as a biodegradation agent and carbon black that acted as an antistatic agent. The addition of 10 wt% of carbon black reduced the electrical resistivity of the composites, allowing the application as antistatic packaging. The addition of lignin was crucial to shortening the biodegradation time of the samples compared to neat PLA. PLA/lignin/carbon black composites (80/10/10) and (75/15/10) are the best alternatives for the manufacture of an antistatic and biodegradable packaging, since these composites presented characteristics of dissipative materials, which is an option for the production of antistatic packaging and present a significant mass loss after 180 days of exposure in garden soil. The composite of PLA/lignin/carbon black (80/10/10) lost about 4.6% of its mass and the composite (75/10/15) lost about 4.8% of its mass after 180 days of exposure in garden soil, is a good option for the production of biodegradable packaging.

5. Acknowledgements The authors are grateful to FAPESP (2014/04900-9) the financial support. This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001.

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Received: Aug. 28, 2019 Revised: Feb. 26, 2020 Accepted: Mar. 20, 2020

Polímeros, 30(1), e2020002, 2020

ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.09819

Thermal treatment of açaí (Euterpe oleracea) fiber for composite reinforcement Felipe Fernando da Costa Tavares1* , Marcos Danilo Costa de Almeida1, João Antonio Pessoa da Silva2, Ludmila Leite Araújo1, Nilo Sérgio Medeiros Cardozo2 and Ruth Marlene Campomanes Santana1 Laboratório de Materiais Poliméricos – LAPOL, Departamento de Materiais, Escola de Engenharia, Universidade Federal do Rio Grande do Sul – UFRGS, Porto Alegre, RS, Brasil 2 Laboratório de Processamento e Tecnologia dos Polímeros – LATEP, Departamento de Engenharia Química, Escola de Engenharia, Universidade Federal do Rio Grande do Sul – UFRGS, Porto Alegre, RS, Brasil

1

*felipe.tavares@ueap.edu.br

Abstract This work investigated the effect of thermal treatment in an autoclave on the chemical, physical, and morphological properties of lignocellulosic fibers from açaí (Euterpe oleracea Mart), and the behavior of this treated fiber in polypropylene (PP) matrix composites with polypropylene-graft-maleic anhydride (PPgMA) as the coupling agent.

The treated and untreated fibers were characterized by chemical composition, x-ray diffraction, FTIR spectroscopy, and thermogravimetry, scanning electron microscopy and tensile tests were carried out for the composites. The results showed that the thermal treatment modified the hemicellulose and lignin content and increased the fiber surface roughness, without compromising the thermal stability. The composite prepared with thermally treated fibers and PPgMA exhibited an increase in tensile strength but a reduction in tensile modulus. In conclusion, the thermal treatment of vegetable fiber is a promising technique for improving the performance of composites.

Keywords: açai, autoclave, fibers, heat treatment, polypropylene.

How to cite: Tavares, F. F. C., Almeida, M. D. C., Silva, J. A. P., Araújo, L. L., Cardozo, N. S. M., & Santana, R. M. C. (2020). Thermal treatment of açaí (Euterpe oleracea) fiber for composite reinforcement. Polímeros: Ciência e Tecnologia, 30(1), e2020003. https://doi.org/10.1590/0104-1428.09819

1. Introduction Açaí fibers, Figure 1, have great potential to be used in composites because of their remarkable thermal stability and availability[1]. They are found in the external layer of the fruit seed. The seed, which is generally discarded as a residue after separation from the edible pericarp, represents 83% (wt.) of the fruit[2]. However, the fibers can be separated from the internal part of the seed though processing in hammer mills.

Composites with vegetable fibers have attracted attention from the scientific community due to some advantages in relation to conventional fibers, including non-abrasiveness, low density, and relatively low cost[3-7].

Vegetable fibers are natural fibers mainly composed of cellulose, hemicellulose, and lignin. Cellulose constitutes a semi-crystalline structure with thousands of glucose units that confers high mechanical strength to the plant[8-10].

Hemicellulose is structurally similar to cellulose, consisting of pentoses and hexoses, and has a great interaction with cellulose[8,9]. This compound is responsible for the stability and flexibility of the lignocellulosic system and acts by joining the semi-crystalline bundles of cellulose and maintaining the structure as regularly spaced and organized[9]. Lignin is an amorphous polymer with a complex structure based on

Polímeros, 30(1), e2020003, 2020

hydroxyphenylpropane units. This combination of hydroxyl and aromatic groups confers amphipathic properties to lignin and allows its use as a polypropylene fiber coupling agent[10,11]. However, Akin[10] reported that lignin has a greater hydrophobic character than cellulose. Lignin is responsible for mechanical rigidity and resistance against microbial pathogens in plant cell walls.

The use of vegetable fibers in combination with thermoplastics polymers for composite preparation presents some drawbacks due to the poor interaction between the fiber/matrix interface, resulting in materials with low-mechanical properties. An approach to improve the performance of this system is using coupling agents[12] or some physicochemical fiber process, such as alkaline and heat treatments.

Coupling agents are macromolecules with polar and non-polar regions and are able to promote physical or chemical linkage between the vegetable fiber and polymer, respectively. Maleic anhydride[13], polyethylene grafted with maleic anhydride (PEgMA)[14], polypropylene grafted with maleic anhydride (PPgMA)[12], diisocyanates[15] and silanes[16] are examples of coupling agents reported in the literature.

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O O O O O O O O O O O O O O O O

Tavares, F. F. C., Almeida, M. D. C., Silva, J. A. P., Araújo, L. L., Cardozo, N. S. M., & Santana, R. M. C.

Figure 1. (a) Açaí fruit (average diameter from 1 to 2cm and average weight from 0.8 to 2.3g); (b) açaí seeds after removal of the pulp; and (c) internal part of the seed and the fibers obtained after separation in a hammer mill.

Alkaline treatment causes a decrease of the lignin and hemicellulose contents and increases the fiber surface roughness, improving tensile strength and Young’s modulus[17-19].

mentioned decrease in compressive strength in heat-treated woods, reductions in wood fiber density and surface hardness were observed by Gunduz et al.[31].

Heat treatment is conducted under air or nitrogen atmosphere or using steam or hot water, representing a chemical-free alternative. It can enhance the dimensional stability, thermal stability, and biological durability of vegetable fibers or wood-based materials[20-22], besides being considered a simple process for obtaining cellulose[23].

Hakkou[24], for example, carried out the heat treatment of beech wood under inert atmosphere and reported a significant increase in wood hydrophobicity for treatment temperatures from 130 to 160 °C.

To the extent of our knowledge, the use of açaí fibers submitted to the referred thermal treatment for preparing composites of PP matrix has not yet been carried out.

Therefore, based on the promising results obtained, we believe that this investigation is useful to the development of alternative methods for improving the performance of vegetable fibers in composites.

It is reported that the thermal treatment can improve the crystallinity of cellulose and remove impurities of Sinal and Kenal fibes[25,26] and provide better thermal stability for Agave fiber[27]. Arwinfar et al.[28] noted good compatibility in polypropylene composites with thermally treated beech wood flour and PPgMA, which resulted in higher tensile strength. Unsal and Ayrilmis[29] and Ayrilmis et al.[30] conducted studies on the effects of heat treatment with autoclaved saturated steam at 120, 150 and 180 °C. They describe that hemicellulose is generally thermodegraded by hydrolysis, leading to some modifications in wood properties, such as decreased swelling and shrinkage due to reduced water‑absorbing capacity, and improved thermal stability. However, other properties are impaired, such as the rupture modulus, elastic modulus, internal bond strength, and compressive strength. In addition to the previously 2/9

Therefore, in this work, açaí fibers were subjected to an autoclave thermal treatment and used in preparing polypropylene composites containing PPgMA and treated fibers. The effect of the heat treatment on the fiber characteristics and the properties of the obtained composites were evaluated.

2. Materials and Methods 2.1 Materials Açaí seeds were collected in Curiaú district (Macapá – Brazil) as a residue of the pulp extraction. At this stage, the fibers and residual pulp are attached to the seed surface. The collected seeds were first cleaned under tap water to remove pulp residues and then dried in an oven at 70 °C for 24 h. Açaí fibers, which constitute the pericarp of the açaí seeds, were separated from the seeds in a hammer mill (SOLAB apparatus, model SL-034).

Polímeros, 30(1), e2020003, 2020

Thermal treatment of açaí (Euterpe oleracea) fiber for composite reinforcement Heterophasic copolymer of polypropylene CP 141 was acquired from BRASKEM (Triunfo, Brazil). Polypropylene grafted with maleic anhydride, PPgAM (PolyBond 3200, Addivant – Molecular weight of 42000 g/mol and acid number of 11 mg KOH/g) was provided by Clariant (Novo Hamburgo, Brazil) and used as a compatibilizer agent.

Thermogravimetric analyses of AF, TAF, PP/AF, PP/TAF, PP/AF/PPgAM, and PP/TAF/PPgAM were performed in a NETZSCH TG 209F1 Libra under nitrogen atmosphere flowing at 100 mL/min, using 10 mg samples, 10 °C/min heating rate, and a temperature range from 25 to 800 °C.

2.2 Methods

The statistical analysis of the data was performed with Action Stat, using a t-test with a significance level of 95% (p<5%).

2.2.1 Thermal treatment and physico-chemical characterization of the açaí fibers The açaí fibers were put into a fabric bag and thermally treated in a PRISMATEC autoclave for 1h at 121 °C and 1 atm. In natura (AF) and treated açaí fibers (TAF) were analyzed for moisture, ash, cellulose, hemicellulose, and lignin contents according to the methodology reported by TAPPI T222 standard[32].

2.2.2 Production of the açaí fiber composites The fibers, both AF and TAF, were previously sieved (mesh sizes: 250 to 1000 µm) and only the fraction of fibers passing the 250 μm sieve was employed for composite preparation. PP/fiber (mass proportion of 70:30), and of PP/fiber/PPgMA (mass proportion of 67:30:3) composites were prepared using an internal mixer (Thermo Scientific Haake Rheomix OS) at 180 °C, 60 rpm and 5 min residence time. After mixing, the resulting composite was comminuted in a mill, and subsequently, tensile test specimens were prepared in an injection molding machine (Thermo Scientific Haake, MiniJet II), at 185 °C and 400 bar, with a 40 °C mold temperature. The composites produced were designated as PP/AF, PP/TAF, PP/AF/PPgAM and PP/TAF/PPgAM, according to their components.

2.2.3 Characterization tests The mechanical properties were evaluated in a universal testing machine (INSTRON, model 3382). The tensile tests were performed according to ASTM D638, using a type V specimens and 5 mm/min crosshead speed.

AF, TAF, PP/AF, PP/TAF, PP/AF/PPgAM, and PP/TAF/PPgAM were characterized by scanning electron microscopy (SEM) in a Hitachi TM3030 Plus Tabletop Microscope, with a 10 kV voltage and 200x magnification.

The samples were fixed on stubs with double-sided tape and were not metalized.

XRD analyses of AF and TAF were performed on a Bruker D2 Phaser, using Ni filter and Cu-α radiation (λ = 1.54 Å) at 30 kV, and 2Θ scan from 5 to 70°. The crystallinity index (CI) was determined according to Segal et al.[33].

FTIR spectra of AF and TAF were obtained in a Perkin Elmer Spectrometer, model Spectrum Two-Ft-IR, with a lithium tantalate detector, using attenuated total reflection analysis (ATR) mode, a wavenumber range of 4000 to 700 cm-1, 16 scans, and 4 cm-1 resolution.

2.2.4 Statistical analysis

3. Results and Discussion 3.1 Effect of treatment on fiber properties Table 1 presents the composition for AF and TAF fibers obtained by chemical analysis. Heat treatment decreased the hemicellulose and lignin contents and increased the moisture and cellulose contents. The most pronounced reduction occurred in the hemicellulose content, which is in agreement with results reported for other vegetal fibers[34].

The decrease of the hemicellulose and lignin contents can be attributed to the thermal degradation and lixiviation of these compounds. The thermal degradation of these compounds can be attributed to their structural heterogeneity and lack of crystallinity. In the specific case of lignin, Kim et al.[35] observed that heat treatment promotes rearrangement of the molecular structure and confirmed the cleavage of oxygen bonds in the chain by 13C NMR analysis. The increase of moisture retention for TAF fibers can be explained by the cleavage glycosidic linkages, which expose OH groups.

The apparent increase in cellulose content is probably a direct result of the mass balance due to the reduction of the other components.

The XRD spectra of the AF and TAF fibers are presented in Figure 2. The peaks at 2Θ = 15° and 22° are characteristic of cellulose[36], and it is visible that they presented an increase in the intensity for TAF. The CI % calculated for AF and TAF fibers were 31.3% and 35.3%, respectively. This result is due to a crystallinity growth of the fiber after the thermal treatment since TAF had a cellulose content higher than AF fiber, and cellulose is the most crystalline component of the lignocellulosic fiber. Additionally, the extraction of the amorphous components (lignin, hemicellulose, and extractives) by the thermal treatment also contributed to that effect.

The FTIR spectra of the AF and TAF fibers are showed in Figure 3. The bands in the 3200 to 2500 cm-1 range of, related to OH, are assigned to many molecules, such as water, cellulose, hemicellulose, lignin, and extractives[37].

At 1722 cm-1, a C=O stretching characteristic of carboxyl groups of lignin or groups of hemicelluloses appears[18,38,39], and a reduced intensity of this group is observed for TAF fiber, probably related to the removal of these compounds by the

Table 1. Chemical composition of AF and TAF fibers.

Açaí fiber AF TAF

Moisture (%) 8.88 ± 0.16a 9.57 ± 0.12b

Cellulose (%) 41.37 ± 1.25a 45.13 ± 1.94b

Hemicel. (%) 11.54 ± 0.83a 6.60 ± 1.08b

Lignin (%) 40.25 ± 1.35a 32.61 ± 1.56b

Ash (%) 1.96 ± 0.15a 1.62 ± 0.18a

Different letters in the same column indicate statistically different mean values (p < 0.05).

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Tavares, F. F. C., Almeida, M. D. C., Silva, J. A. P., Araújo, L. L., Cardozo, N. S. M., & Santana, R. M. C. autoclaving treatment. Two peaks at 1600 (aromatic skeletal vibration breathing with C=O stretching) and 1508 cm-1 (aromatic skeletal vibration) are characteristics of lignin[40,41].

The band at 1453 cm-1 is for the C-H deformation (methyl and methylene) of lignin[38,40,42,43]. The peak at 1272 cm-1

is related to the C-O of the guaiacyl ring of lignin[38,43].

The peak at 1240 cm-1 refers to the C–O stretching vibration of acetyl groups present in lignin and hemicellulose[38,39].

The bands at 1112 and 1030 cm-1 are attributed to C-H in‑plane deformation of the syringyl unit of lignin and C-H deformation in guaiacyl with C-O deformation in the primary alcohol, respectively[42]. The results can be correlated to the changes in the chemical composition of AF and TAF fibers since a general reduction in the intensity of the groups is observed, mainly those referring to hemicellulose and lignin, and possibly to extractives. Thus, FTIR also confirms the removal of components by the autoclaving treatment.

The thermogravimetric curves (TGA) of the evaluated samples and their respective first derivative (DTGs) are exhibited in Figure 4, while the parameters of the DTG peaks are presented in Table 2. The considered parameters were: Ti, the temperature at 3% of the total weight loss, used to compare the thermal stability between composites; the maximum temperature (Tmax) and weight loss percentage (Δmi) of each peak; and the residue content. The pure fibers (AF and TAF) exhibited an initial weight loss at low temperature (25-100 °C), attributed to the loss of moisture and volatile compounds, such as extractives[37]. Due to the character of the initial loss, the thermal stability of the fibers is better represented by the onset temperature of the second thermal event peak, which was designated as Peak 1 because it represents the first thermal degradation process. This temperature was 256 °C for AF and 255 °C for TAF fiber, indicating that the autoclave treatment did not compromise the thermal stability of the fibers.

Figure 2. X-ray diffractograms of the AF and TAF fibers.

Three main characteristic events of thermal degradation kinetics were observed on the peak temperatures obtained

by DTG. The first one, around 290 °C, is probably related to the thermal degradation of hemicellulose and low-molecular lignin. The second event, near 352 °C, is attributed to cellulose decomposition[44]. For AF and TAF fibers, the third event appears as a small shoulder at 400-450 °C and is due to the decomposition of lignin.

Figure 3. FTIR spectra of AF and TAF fibers.

Regarding the residual material at the end of the TGA analysis for the pure fibers, TAF fiber presented a smaller percentage than AF fiber, probably due to the removal of inorganic compounds, including silica and extractives by the thermal treatment. Another possibility is that reported

by Dorez et al.[45], who suggest that an interaction between cellulose and lignin during decomposition can affect the ash content due because a decrease in the lignin content leads to decrease in the activation energy of vegetable fiber pyrolysis reactions, thus reducing the amount of residue.

Figure 5 presents surface micrographs of AF and TAF fibers obtained by SEM. AF fiber exhibits an irregular surface on which small spherical particles (highlighted with arrows in

Figure 4. TGA and first derivative of TGA curves for AF and TAF fibers.

Table 2. Thermal degradation temperatures, DTG peaks and% residue at 800 °C for AF and TAF fibers.

Sample AF TAF a

Ti (°C)

DTG

3 wt% loss

Tpeak1 (°C)

Δm1 (%)

Tpeak2 (°C)

45 44

292 297

23.1 24.6

353 364

Δm2 (%) 35.3 36.3

Tpeak3 (°C)

Δm3 (%)

422 a 426 a

11.8 11.1

Residue 800 °C (%) 20.4 17.0

Appears as a shoulder for AF and TAF

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Polímeros, 30(1), e2020003, 2020

Thermal treatment of açaí (Euterpe oleracea) fiber for composite reinforcement Figure 5a) are deposited. According to the results presented

by Mesquita et al.[46], these particles are composed of silica.

Regarding TAF fiber, the following changes are noticed in comparison to the non-treated fibers: partial removal of the silica particles, diameter reduction, and increased surface irregularity. The removal of the silica particles is evidenced by the presence of some globular cavities on the surface of the treated fibers (arrows in Figure 5b).

3.2 Composites properties The thermogravimetric curves (TGA) of the evaluated samples and their respective first derivative (DTGs) are exhibited in Figure 6, the DTG peaks are presented in Table 3. PP/AF composite presented the lowest Ti value, while the others were closer together. However, the increased

Ti for PP/TAF composite may not be related to the treated fiber, since AF and TAF fibers do not exhibit a significant difference, and it is attributed to fluctuations of the thermal analysis. For composites with PPgAM, a better fiber/matrix interaction supposedly led to Ti increased.

The neat PP had higher thermal stability at 3% of weight loss than composites. For the composites, the observed third event (420 to 480 C), with the highest area, is mainly related to the decomposition of PP matrix with Tpeak at 454 °C, where is maximum the kinetics of decomposition, although it must also include a small contribution corresponding to the thermal degradation of lignin. Negligible variations for the mass of hemicellulose and lignin were observed (as shown by the values of Δm1 and Δm3 in Table 3) when compared to the chemical analysis results presented in Table 1. However,

Figure 5. SEM images of the surface of the fibers used: (a) AF; (b) TAF.

Figure 6. Thermal curves: (a) TGA and (b) The first derivative of TGA for PP/AF, PP/TAF, PP/AF/PPgAM and PP/TAF/PPGAM composites.

Table 3. Thermal degradation temperatures, DTG peaks and % residue at 800 oC for composites.

Sample PP PP/AF PP/TAF PP/AF/PPgMA PP/TAF/PPgMA

Ti (°C) 3 wt% loss 375 219 262 262 263

Polímeros, 30(1), e2020003, 2020

DTG Tpeak1 (°C)

Δm1 (%)

Tpeak2 (°C)

284 288 290 303

9.8 10.3 7.9 5.3

355 361 352 368

Δm2

(%) 9.4 10.2 7.9 6.6

Tpeak3 (°C)

Δm3 (%)

454 462 461 463 463

100 72.8 74.1 73.5 79.5

Residue a 800 °C (%) 0 7.9 5.4 8.5 8.6

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Tavares, F. F. C., Almeida, M. D. C., Silva, J. A. P., Araújo, L. L., Cardozo, N. S. M., & Santana, R. M. C. overlapping responses related to the degradation of cellulose, which takes place in an intermediary region (250 to 380 °C), hemicellulose, and lignin were likely[1,17,35,37], which may have masked the results. Analyzing the values of Tpeak 1 and Tpeak 2 in Table 3, a displacement to higher temperatures is noted when comparing PP/TAF and PP/TAF/PPgAM to PP/AF and PP/AF/PPgAM, respectively. Similar to the increase of Ti discussed previously, this probably reflects an effective improvement in the fiber/matrix interaction due to the combined action of the treated fibers and the presence of PPgMA. Similar behavior was reported by Redighieri and Costa[47] for composites of recycled polyethylene and wood particles with polyethylene functionalized with maleic anhydride as a compatibilizer agent.

In relation to the composites, the difference of residue content between PP/TAF and PP/AF is also significant and it can be explained on the same basis as for the fibers. On the other hand, the composites with PPgMA presented higher residue contents compared to the respective composites without the presence of this agent, which may be attributed to the coupling action of PPgMA. SEM micrographs of the fracture surface of the composites are shown in Figure 7. SEM micrographs of PP/AF and PP/TAF show that thermal treatment alone was not able to improve the fiber/matrix coupling, since both micrographs present similar patterns with voids between the fiber surface and polymeric matrix around it and smooth surface holes due to fiber extraction during the tensile test (indicated by red arrows in Figure 7a and 7b). The reduction of these voids and holes in the micrographs of the composites with a coupling agent (Figure 7c and 7d) indicates that PPgMA developed some level of coupling between the açaí fibers and the PP matrix. Additionally, the more compact structure observed for PP/TAF/PPgMA composite (Figure 7d) suggests that the thermal treatment also contributed to a higher level of fiber/matrix interaction, probably because the surface changes it promotes leads to higher mechanical contact with the coupling agent and the polymeric matrix.

Figure 8 presents the stress vs. strain curves for the evaluated composites; the main results of these curves are summarized in Table 4. In general, when comparing the composites with neat PP, the addition of fibers provided a molecular interlock, increasing modulus and tensile stress, but decreased strain, as reported in the literature[4,7,9,14].

The mechanical behaviors of the composites prepared without the addition of PPgMA are quite similar, presenting statistically equal values of modulus and small differences in terms of tensile strength and elongation at rupture. The small values of these last two parameters can be attributed to the defects related to the poor interaction between the fibers and polymeric matrix discussed in the previous section.

However, the slight improvement in the tensile stress of PP/TAF when compared to PP/AF may be attributed to the higher roughness of the treated fiber. This roughness effect was also observed by Bulut and Aksit[48] for composites of jute fiber and propylene matrix. Regarding the composite PP/AF/PPgMA, the change of morphology observed in comparing Figure 7c with Figure 7a, 7b is also reflected in the mechanical properties. An increase of tensile strength and a decrease of elongation at break is observed in comparison to PP/AF composite, which may be ascribed to a limited action coupling effect exerted by PPgMA with the non-treated açaí fibers due to the lower number of OH groups in their surface available to react. Finally, PP/TAF/PPgAM composite presented differentiated properties compared to the other three composites, with a reduction of the tensile modulus and simultaneous increase of the tensile strength and elongation at break, indicating a higher coupling action of PPgMA with the presence of treated fibers and, consequently, a higher level of interaction between the fibers and the matrix. This is in agreement with the morphology changes observed in Figure 7d and with the results reported by Lopes and Sousa[49] for composites of polypropylene with glass fiber using PPgMA as a compatibilizer. They attributed the effects observed in the mechanical properties to plastic deformation and high shear resistance of the compatibilized interface.

Table 4. Results of the mechanical properties of the PP matrix and composites.

Sample PP PP/AF PP/TAF PP/AF/PPgMA PP/TAF/PPgMA

Young’s modulus (MPa) 71.36 ± 5 c 1293 ± 65 a 1306 ± 113 a 1282 ± 48 a 887 ± 16 b

Tensile stress at Maximum / Yield (MPa) 19.4 ± 1.2 d 19.1 ± 0.2 d 21.3 ± 0.4 c 25.6 ± 0.4 b 27.5 ± 1.0 a

Tensile stress at break (MPa) 16.9 ± 0.8 c 14.2 ± 1.1 d 16.6 ± 0.8 c 21.7 ± 0.7 b 25.9 ± 0.6 a

Strain at break (%) 120.8 ± 15.8 a 6.5 ± 0.4 c 5.3 ± 0.2 d 4.6 ± 0.1 d 9.3 ± 1.2 b

Different letters in the same column represent statistically different means (p < 0.05).

Figure 7. SEM of the fracture impact surface of the composites: (a) PP/AF; (b) PP/TAF; (c) PP/AF/PPgMA; and (d) PP/TAF/PPgMA. 6/9

Polímeros, 30(1), e2020003, 2020

Thermal treatment of açaí (Euterpe oleracea) fiber for composite reinforcement

Figure 8. Stress vs. strain curves for the PP matrix and composites.

4. Conclusions Açaí fibers were submitted to thermal treatment in an autoclave at 121 °C and 1 atm and polypropylene composites were prepared using treated and non-treated fibers. The thermal treatment of the açaí fibers promoted a reduction in the hemicellulose and lignin content, which contributed to an increase of the fiber crystallinity, partial removal of surface silica particles, and an increase in the fiber surface roughness, without compromising the thermal stability of the fibers.

The morphology and mechanical properties of the PP/fiber composites produced with the treated and non‑treated fibers were quite similar. In the case of the PP/PPgMA/fiber composites, the use of the treated fibers has effectively improved the interaction between the fibers and the PP/matrix, reducing the occurrence of the volumetric defects (voids between the fibers and the matrix). The use of thermally treated fibers together with PPgMA led to a composite with significantly improved tensile strength but with a reduction in the tensile modulus.

The effectiveness of the proposed water vapor based thermal treatment in improving the coupling action in the ternary composite and the good mechanical properties of this composite indicates the potential of this process as an environment-friendly alternative for the destination of the fibers generated as by-products in açaí fruit processing.

5. Acknowledgements We would like to thank the technicians at the Laboratory of Polymer Materials (LAPOL) of the Federal University of Rio Grande do Sul (UFRGS), in the State University of Amapá (UEAP), and Rede de Saneamento e Abastecimento de Água – Sistema Brasileiro de Tecnologia (RESAG‑SIBRATEC).

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36. Rambo, M. K. D., Schmidt, F. L., & Ferreira, M. M. C. (2015).

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38. Huang, L., Mu, B., Yi, X., Li, S., & Wang, Q. (2016). Sustainable use of coffee husks for reinforcing polyethylene composites.

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Natural rubber composites filled with cereals straw modified with acetic and maleic anhydride: preparation and properties.

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40. Rashid, T., Kait, C. F., & Murugesan, T. (2016). A “Fourier Transformed infrared” compound study of lignin recovered from a formic acid process. Procedia Engineering, 148, 13121319. http://dx.doi.org/10.1016/j.proeng.2016.06.547.

41. Kaparaju, P., & Felby, C. (2010). Characterization of lignin during oxidative and hydrothermal pre-treatment processes of wheat straw and corn stover. Bioresource Technology, 101(9), 3175-3181. http://dx.doi.org/10.1016/j.biortech.2009.12.008.

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44. Poletto, M., Zeni, M., & Zattera, A. J. (2012). Effects of wood flour addition and coupling agent content on mechanical properties of recycled polystyrene/wood flour composites.

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45. Dorez, G., Ferry, L., Sonnier, R., Taguet, A., & Lopez-Cuesta, J. M. (2014). Effect of cellulose, hemicellulose and lignin contents on pyrolysis and combustion of natural fibers. Journal of Analytical and Applied Pyrolysis, 107, 323-331. http://dx.doi. org/10.1016/j.jaap.2014.03.017.

46. Mesquita, A. L., Barrero, N. G., Fiorelli, J., Christoforo, A. L., De Faria, L. J. G., & Lahr, F. A. R. (2018). Eco-particleboard Polímeros, 30(1), e2020003, 2020

Thermal treatment of açaí (Euterpe oleracea) fiber for composite reinforcement manufactured from chemically treated fibrous vascular tissue of acai (Euterpe oleracea Mart.) Fruit: A new alternative for the particleboard industry with its potential application in civil construction and furniture. Industrial Crops and Products, 112, 644-651. http://dx.doi.org/10.1016/j.indcrop.2017.12.074.

47. Redighieri, K. I., & Costa, D. A. (2008). Composites of recycled polyethylene and reforestation wood particles treated with modified polyethylene. Polímeros: Ciência e Tecnologia, 18(1), 5-11. http://dx.doi.org/10.1590/S0104-14282008000100006.

48. Bulut, Y., & Aksit, A. (2013). A comparative study on chemical treatment of jute fiber: potassium dichromate, potassium

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permanganate, and sodium perborate trihydrate. Cellulose, 20(6), 3155-3164. http://dx.doi.org/10.1007/s10570-013-0049-6.

49. Lopes, P. E., & Sousa, J. A. (1999). Influence of interface/ interphase characteristics on mechanical properties of polypropylene/glass fiber composites with PP-g-MAH intefacial compatibilizer. Polímeros: Ciência e Tecnologia, 9(4), 98-103. http://dx.doi.org/10.1590/S0104-14281999000400017.

Received: Dec. 04, 2019 Revised: Mar. 22, 2020 Accepted: Apr. 06, 2020

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ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.01020

Preparation and analysis of melamine and melamine-silica as clarifying agents of waste lubricating oil Mirna Sales Loiola Rosa1* , Timm Knoerzer2, Francisco Cardoso Figueiredo1 and José Ribeiro dos Santos Júnior1 Laboratório de Bioeletroquímica – LAB, Departamento de Química, Universidade Federal do Piauí – UFPI, Teresina, PI, Brasil 2 Department of Chemistry, United States Air Force Academy – USAFA, Colorado Springs, CO, United States

1

*mirnasales01@hotmail.com

Abstract Melamine is a key compound used as a clarifying agent for waste lubricating oil primarily due to its excellent adsorbent properties. Moreover, considerable interest exists for the further modification of melamine in order to provide a remediation agent with improved clarification capacity. In this study, hexamethylolmelamine was prepared using a solution of formaldehyde, which provided an agent capable of incorporation into a silicate polymer framework. Subsequently, the resultant monomer was added to a solution of silicate to produce the melamine-silica polymer. The melamine and melamine-silica polymer were characterized using the techniques of XRD, FTIR, SSA and thermal analysis to confirm structural and morphological characteristics. These characterizations indicated that the increase in the surface area of the 0.315 m2/g to melamine to 26.71 m2/g of melamine-silica suggests the effective introduction of silanols groups to hexamethylolmelamine and, therefore, corresponds to thehigh performance in relation to melamine as clarifying of waste lubricant oil.

Keywords: clarifying, melamine, melamine silica, waste lubricating oil.

How to cite: Rosa, M. S. L., Knoerzer, T., Figueiredo, F. C., & Santos Júnior, J. R. (2020). Preparation and analysis of melamine and melamine-silica as clarifying agents of waste lubricating oil. Polímeros: Ciência e Tecnologia, 30(1), e2020004. https://doi.org/10.1590/0104-1428.01020

1. Introduction Waste lubricating oil remains an important target for environmental remediation. The challenge is that after certain end-use time recommended by the manufacturer, waste lubricating oil suffers from various modes of contamination including the formation of low molecular weight products (ketones, acids and alcohols), toxic organic compounds (polycyclic aromatic hydrocarbons and dioxins) and metals, (Fe, Pb, Ni, Cu, Zn)[1-5].

Typical remediation processes for waste lubricating oil are comprised of incineration, landfill disposal, and re-refining of base oil. Among these options, the process of re-refining or recovering the base oil is the most desirable from a green chemistry perspective. Therefore, current efforts have shown that a two-step re-refinement process involving an organic solvent-based extraction followed by materials adsorption provides a more rapid and economical approach to solve this problem[3,4].

Melamine (2,4,6-triamino-1,3,5-triazine) is a functionalized heterocyclic organic molecule that has been widely used in industry for the production of laminates, glues, adhesives and plastics. In the form of melamine-formaldehyde thermoset resins, these compounds are attractive because they resist organic solvents and oils as well as some weak

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acids and bases[5-10]. However, for the evolution of use of these resins as sorbent materials is due to the presence of of the pendant amino groups that potentially bind to cationic dyes by coordinate-covalent bonding. In addition, the amino groups also possess the capability to bind to anionic dyes by an ionic bonding model[11-16]. Therefore, melamine-based compounds provide the key functionalty needed to remove organic contaminants. The adsorption reaction between melamine and cationic dye can be seen in Figure 1 and the adsorption reaction between melamine and anionic dye can be seen in Figure 2.

In order to extend the reach of melamine applications, several studies have shown that the synthesis reaction of melamine and formaldehyde can result in the synthesis of hexamethylolmelamine (Figure 3), which may play an important role in increasing the capability of the adsorption process.

In addition, silicate incorporation leads to chemical modification and ultimately to surface coordination of the silicate to hexametylolmelamine via a process called organofunctionalization[17-19]. The resulting melaminesilica polymer achieves a specific surface area greater than melamine, which renders these resins capable of

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O O O O O O O O O O O O O O O O

Rosa, M. S. L., Knoerzer, T., Figueiredo, F. C., & Santos Júnior, J. R. 2.3 X-ray Diffraction (XRD) Melamine samples before and after the synthesis were characterized by X-ray diffraction (XRD) using an EMPYREAN diffractometer in range 2θ between 5.4 and 90°.

The CoKα radiation source was used, with a wavelength equal to 1.79290 Å nm and scanning speed was from 5° per min.

Figure 1. Potential adsorption sites between melamine and cationic dye (D+).

2.4 Fourier Transform Infrared (FTIR) The analyzes were performed on KBr pellet samples using a Perkin Elmer Template Espectrum 100 KBr (in the region of 4000 to 400 cm-1) with 16 scans and resolution of 4 cm-1.

2.5 Specific Surface Area (SSA)

Figure 2. Potential adsorption sites between melamine and anionic dye (D-SO3-) – NOTE: other basic amine sites in melamine can be protonated leading to anionic dye adsorption.

The surface area was determined by the SSA method in samples of melamine and melamine silica, in a NOVA physical nitrogen adsorption instrument (Quanta Chrome Instruments, version 10.01).

2.6 Thermal analysis Thermogravimetric (TG) analysis was conducted using a Build 20 SDT Q600 TGA model (V20.9), using alumina crucibles, inert nitrogen atmosphere, and heating ramp method.

The gas flow rate 100.0 mL/min, heating rate 20 °C/min, mass of sample 18.5080 mg, temperature range 20 to 600 °C.

2.7 Extraction and yield of base oil Figure 3. Synthesis reaction of melamine and formaldehyde resulting in hexamethylolmelamine.

being employed as adsorbent materials for the removal of metals, organic, and inorganic contaminants[20]. This strategy is critical to achieving the increased efficacy of the adsorption process and consequently clarification of the used oil, but a complete evaluation of the physical characteristics of the constructs remains limited[20].

Herein, we present a study to demonstrate the structural and the specific surface area characteristics of a silica melamine polymer in order to validate the efficacy of the macromolecular framework in the process of waste lubricating oil clarification.

2. Materials and Methods 2.1 Reagents Melamine PA was derived from Sigma-Aldrich, formaldehyde was obtained from Contemporary Chemical Dynamics and sodium silicate PA was derived from Sigma-Aldrich.

2.2 Preparation of melamine silica Melamine (2.0 g) was added to 2.0 mL of a 40% formaldehyde solution with stirring and constant temperature of 60 °C time 12 h. The resulting product was added to 5 mL of 40% silicate (aq) solution under stirring and constant temperature of 70 °C for 8 h.

The resulting solid hexamethylolmelamine-silicate was filtered and dried during 24 h to obtain the melamine silica (MS) hybrid.

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Semi-synthetic lubricating oil (SAE 15W-40) standard was purchased on the local market. The testc lubricating oils were supplied by Monobloco - Teresina - Piauí Garage, removed from specific vehicles in gasoline engines subject to use for 15,000 km. Lubricating oil was mixed with iso-amyl alcohol in a 3:1 ratio. The mixture was stirred for 30 min and a precipitate of additives, impurities, and carbon particles was formed. The precipitate was separated from the supernatant by filtration. The alcohol was then removed from the oil by rotary evaporation. The yield of base oil was determined from the oil previously isolated from the alcohol.

2.8 Application of the samples as a clarifying of the base oil The process of clarifying the used lubricating oil was determined by international standards ASTM D1500[21].

The lubricant clarification process was performed after extraction of the base oil with iso-amyl alcohol. Initially, a filtration column was used and eluted with hexane in the proportion of lubricant:hexane 1:1 (p/p). The system, lubricant, and hexane were collected and transferred to a vacuum distillation flask to recover the solvent and obtain the clarified lubricant at 50 °C. To promote the passage of the lubricant and hexane by the polymers, a filtration column was used. At the end, the clarified lubricant was stored in an amber container for later physico - chemical characterization.

2.9 Physical-chemical characterization of oil samples The lubricant samples were characterized by color (ASTM D1500-12)[21], kinematic viscosity at 40 °C and 100 °C (ASTM D445-18)[22], total number of acidity (ASTM D664-11)[23] and ash content (ASTM D482-13)[24].

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Preparation and analysis of melamine and melamine-silica as clarifying agents of waste lubricating oil 2.9.1 Colorimetric analysis

3.2 FTIR analysis

The color measurement of the samples the samples new lubricating oil (OLN), waste lubricating oil (OLW), clarified lubricating oil with melamine (OLCM), clarified lubricating oil with melamine silica (OLCMS) was performed using a model 15260-4 U Seta-Lovibond Colour, instrument, set at 24 V. Measurements were made by comparisons between the color of the analyzed sample and the Lovibond color scale, which discerns color measurements for red and yellow.

This technique involves the combination of the color of the light transmitted through a specific depth of the oil with the color of the light transmitted from the same source of the set of filters as the reference color[21].

FTIR analyses was conducted to confirm the effective preparation of the melamine and melamine-silica synthetic targets. Figure 5.a and 5.b shows the FTIR spectrum of the samples of melamine and melamine silica, respectively.

In the melamine spectrum several bands appear above 3000 cm-1 due to symmetrical and asymmetrical vibrations of the groups NH2. As in the chemical structure of melamine, there are 3 groups NH2, and six total N-H bonds, but the six vibrations can be superimposed on one another to produce two sharp bands at 3414 cm-1 and 3477 cm-1 and two wide bands at 3130 cm-1 e 3327 cm-1[30]. The band in 1659 cm-1 corresponds to the primary amine, the band in 1561 cm-1 corresponds to symmetrical elongation C=N, 1492 cm-1, 1445 cm-1 and 1202 cm-1 belong to the symmetrical elongation C – N. The band in 1028 cm-1, 815 cm-1, 768 cm-1 and 728 cm-1corresponds to the vibration of the triazine ring[30-36].The bands in 612 cm-1 and 577 cm-1 is due to ring bending and the band in 461 cm-1, it’s the vibration C – N. In the spectrum of the MS, there is a broadening of bands in 3477 cm-1, 3414 cm-1, 3327 cm-1 and 3130 cm-1, due to the

2.9.2 Kinematic viscosity The measurement of the kinematic viscosity was determined using a Quimis Aparelhos Científicos – LTDA, model Q383SR26 automatic viscometer. Values were derived by generating the product of the flow time of the constant of the capillary of Cannon-Fenske - 150 at 100 °C and Cannon-Fenske - 300 at 40 °C[25].

2.9.3 Total number of acidity The acidity of the samples of lubricants was measured by titration using the amount of potassium hydroxide required for neutralization (mg KOH/g lubricant), and resultant in total number of acidity[26].

2.9.4 Ash content The lubricating oil samples were transferred to the crucible, burned to ash and carbon. The carbonaceous residues of the samples were reduced to ashes by heating in a muffle furnace at approximately 775 °C, cooled and weighed[27].

2.10 UV-vis analysis The ability of chemically activated and natural samples to discolor the lubricating oil was also determined by the approximation of their absorption band to the new lubricating oil using a Thermo Fisher SCIENTIFIC, modelo GENESYS 10S UV-VIS digital spectrophotometer operating from 100-1000 nm. Hexane was used to dilute samples in the proportion of 1:10. All measurements were performed in triplicate[28]. The device can read above 4.

Figure 4. Diffractogram of samples. Melamine in 4.a and melamine silica 4.b.

3. Results and Discussions 3.1 XRD analysis Initial attempts at characterization were completed via XRD analysis (Figure 4). In Figure 4.a, the diffraction pattern of rays (XRD) of the melamine, show the crystallographic profile of melamine (C3H6N6) according to reference code 00-039-1950. After the chemical treatment of melamine silica (MS) in Figure 4.b, crystalline peaks of silica (SiO2) appeared in 34.87 (d= 3.02 Å) and 31.19 (d=3.29 Å) in the angle 2θ which correspond to the reference code 01-0810069. However, it was observed that the crystallinity of the material decreased in 25.21(d= 4.12Å) and 26.07 (d= 3.96 Å) in the angle 2θ with the chemical treatment, presumably due to the amorphous nature of the silica[29].

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Figure 5. FTIR of the samples. Melamine in 5.a and MS in 5.b.

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Rosa, M. S. L., Knoerzer, T., Figueiredo, F. C., & Santos Júnior, J. R. presence of groups Si –OH incorporated into the polymer with the band at 2919 cm-1, 2845 cm-1, 1459 cm-1 and 720 cm-1 are related to the different vibration mode of C-H bending[30].

In the spectrum of silica melamine there is a reduction of these bands. The main structural elucidation bands observed in the spectra are summarized in Table 1.

3.3 SSA analysis Further analysis by SSA can provided additional evidence for the formation of the melamine silica. Figure 6.a and 6.b shows adsorption–desorption isotherms of nitrogen and pore size distributions for OLCM and OLCMS. Each isotherm seems to indicate type IV behavior with the presence of both meso and micropores (IUPAC classification)[36]. As can be seen in Table 2, comparative analysis showed that melamine exhibited a surface area value of 0,315 m2/g, pore volume of 1,14.10-4 cm3/g and increased to 26,71 m2/g and 1,013.10-2, respectively when chemically treated with formaldehyde and silica. These results strongly suggest the covalent introduction of silanols groups to hexamethylolmelamine[11].

mass loss events appear throughout the TG analysis. The first event occurs in the temperature range of 32 °C to 92 °C with loss of mass (PM1) which corresponds with a 12% loss of water and formaldehyde[32]. The second event occurs in the temperature range of 92 °C to 296 °C with 14% mass loss (PM2), corresponding to the elimination of triazine[33-35].

The third event occurs in the temperature range of 296 ° C at 410 ° C with mass loss (PM3) of 26% is attributed to the decomposition of the organic groups of higher molecular mass[31-41]. The fourth event is from 410 °C to 505 °C with a loss of mass (PM4) of 10% correspond to the decomposition of the rest of the organic groups of greater molecular weight.

The total mass loss of the polymer was 62%. As the mass loss was more pronounced at the higher temperature, we

3.4 Thermogravimetric analysis (TG) Thermogravimetric analysis (TG) provide information about the interval of the mass losses and the chemical and the physical processes that involve energy variation and DTG provides the first derivative of the TG curves solid corroborative evidence that the melamine-silica construct was realized. Figure 7 shows the thermogravimetric curves (TG) of melamine in 7.a and MS in 7.b. With regard to melamine, there is only one event during the decomposition of melamine in nitrogen in the temperature range of 231 °C to 335 °C with loss of mass of 100% and there is no residue formation. On the contrary, for the melamine-silica, four

Figure 6. Adsorption–desorption isotherms of nitrogen and pore size distributions for MS e Melamine.

Table 1. Assignment of the vibration bands in the infrared spectrum of melamine and silica melamine.

Wavenumber (cm-1) 3477 3414 3327 3130 2919 1659 1561 1492 1445 1202 1028

Assignment melamine N – H primary amine N – H primary amine N – H primary amine N – H primary amine Symetrical elongation C-H N – H primary amine Symmetrical elongation C=N Symmetrical elongation C =N Symmetrical elongation C – N Symmetrical elongation C – N Vibration of the triazine ring

Assignment melamine silica group insertion Si – OH group insertion Si – OH group insertion Si–OH group insertion Si–OH Symetrical elongation C-H N – H primary amine Symmetrical elongation C=N Symmetrical elongation C=N Symmetrical elongation C=N Symmetrical elongation C=N Vibration of the triazine ring

815

Vibration of the triazine ring

Vibration of the triazine ring

768

Vibration of the triazine ring

Vibration of the triazine ring

728

Vibration of the triazine ring

Vibration of the triazine ring

612 577 461

Flexão do anel Flexão do anel Vibração C – N

Flexão do anel Flexão do anel Vibração C – N

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Reference Bal et al.[30] Bal et al.[30] Bal et al.[30] Bal et al.[30] Bal et al.[30] Isam et al.[31] Isam et al.[31] Isam et al.[31] Isam et al.[31] Isam et al.[31] Bal et al.[30], Isam et al.[31], Sangeetha et al.[32], Shahbazi et al.[33], Pevida et al.[34], Cheng et al.[35], Papoulis et al.[36] Bal et al.[30], Isam et al.[31], Sangeetha et al.[32], Shahbazi et al.[33], Pevida et al.[34], Cheng et al.[35], Papoulis et al.[36] Bal et al.[30], Isam et al.[31], Sangeetha et al.[32], Shahbazi et al.[33], Pevida et al.[34], Cheng et al.[35], Papoulis et al.[36] Bal et al.[30], Isam et al.[31], Sangeetha et al.[32], Shahbazi et al.[33], Pevida et al.[34], Cheng et al.[35], Papoulis et al.[36] Pevida et al.[34] Pevida et al.[34] Cheng et al.[35]

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Preparation and analysis of melamine and melamine-silica as clarifying agents of waste lubricating oil Table 2. Result of the specific surface area and pore volume of the samples melamine and silica melamine.

Samples Melamine Melamine silica

Surface area /m2.g-1 0,315 26,71

Pore volume/cm3.g-1 1,14.10-4 1,013.10-2

can conclude that MS is more stable than melamine and resists decomposition until achieving temperatures greater than 410 °C.

3.5 Extraction and yield of base oil Effective clarification analyses depend upon utilizing reliable methods to extract and recovery the base oil. In order to demonstrate reliability, the yield of base oil was calculated based on the ratio of the mass of oil recovered (MOrec), free of alcohol and water, and the mass of oil used (MOused). For the 15,000 km used lubricating oil, the yield obtained from iso-amyl alcohol extraction was 78% by mass.This finding shows that recovery can be reliably completed using our extraction protocol.

3.6 UV-vis analysis With material characterizations and extraction protocols established, we now turned our attention to the evaluation of the clarification process using our constructs. The effectiveness of clarification can be determined by UV-Vis spectroscopy in the 400 to 700 nm spectral window (Figure 8)[41]. In the region of 380 to 600 nm, a distinct spectral band can be seen that increases in intensity for OLN versus OLW. For the clarified oils OLCM and OLCMS, a decrease in absorbance can be seen in 450 nm is possible to see that used oils are influenced by red pigments in the region of 400 to 500 nm and only pure oil does not have this influence. Among the recovered oil samples, the OLCMS was the one that most approached the color of the new oil (Figure 8, green trace), presumably due to the capacity of chelating melamine silica to absorb metallic cations in both aqueous and non-aqueous solutions[11].

Figure 7. TG/DTG thermal analysis curves of the materials.

Melamine in 7.a and MS in 7.b.

Figure 8. Visible spectrum absorbance of new lubricating oil (OLN), clarified lubricating oil with melamine silica (OLCMS), clarified lubricating oil with melamine (OLCM) and waste lubricating oil (OLW).

3.7 Physico-chemical characterization of new lubricating oils (OLN), waste lubricating oil (OLW) and lubricating oil clarified with melamine and melamine silica (OLCM, OLCMS) Physico-chemical characterizations as shown in Table 3 provide additional evidence for the effective clarification of waste lubricating oils using MS. According to Table 3, it is observed that there was an increase in all characteristics analyzed for the sample of OLW compared to OLN. This observation is congruent with mechanical use of the oil. Pleasingly, after the process of recovering of OLW by extraction with iso-amyl alcohol and adsorption with melamine and melamine silica, the results for clarified lubricating oils have approached the limits established by the concierge ASTM and approached to the results of Lima et al.[41] for marketing. These results suggest that OLCMS also presented the best whitening result of the used lubricating oil due to the creation of sites meso and microporous[36].

Table 3. representation of results of the physical chemical characterization of OLN, OLW, OLCM and OLCMS and the limits established by the ASTM which establishes the specifications for the commercialization of basic lubricating oils recovered in the country.

Test Kinematic viscosity at 40°C (mm2/s2) Kinematic viscosity at 100°C (mm2/s2) Color ASTM Acidity (mgKOH/g) Ashes (massas, %) ND: not determined.

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OLN 100.2 16. 8 1.0 2.09 0.01

OLW 114.0 18. 7 8.0 5.97 2.0

OLCM 30.5 12.0 5.5 0.011 0.300

OLCMS 25.55 6.5 4.5 ND 0.09

Limit 26-32 ND 3, máx.

0.05, máx 0.02, máx.

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Rosa, M. S. L., Knoerzer, T., Figueiredo, F. C., & Santos Júnior, J. R.

4. Conclusion The polymerization reaction of hexamethylolmelamine and silicate resulted in the synthesis of a highly effective product for treating waste lubricating oil. Our results showed that MS contained the highest surface area measured in nitrogen. This finding corroborates the process of adsorption of contaminants of the waste lubricating oil and consequent clarification phenomena. The increase of the adsorption capacity of the silica melamine was confirmed by XRD, which showed change in the crystallinity of the material. Further evidence was shown via FTIR which demonstrated the broadening of bands in 3477 cm-1, 3414 cm-1, 3327 cm-1 and 3130 cm-1, and can be attributed to the presence of groups Si– OH incorporated in the polymer. These results suggests that the consequent adsorption capacity may be due to the high surface area presence of silica. Moreover, the TG/DTG curves of the polymer melamine-silica presented four distinct events at different temperature intervals that are related to mass losses of the water, formaldehyde, triazine and organic groups of higher molecular weight.

The synthesis of melamine favored the increase of adsorbed molecules which are responsible for the dark coloration of the waste lubricating oil which was confirmed by the UV-vis and by the colorimetric analyses. Although the lubricating oil used has been clarified with adsorbent materials, it has not met the standards established by the concierge ASTM.

5. References 1. Kuczenski, B., Geyer, R., Zink, T., & Henderson, A. (2014).

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36. Papoulis, D., Komarneni, S., Nikolopoulou, A., Tsolis-Katagas, P., Panagiotaras, D., Kacandes, H. G., Zhang, P., Yin, S., Sato, T., & Katsuki, H. (2010). Palygorskite- and Halloysite-TiO2 nanocomposites: synthesis and photocatalytic activity. Applied Clay Science, 50(1), 118-124. http://dx.doi.org/10.1016/j. clay.2010.07.013.

37. Ajdari, F. B., Kowsari, E., Ehsani, A., Chepyga, L., Schirowski, M., Jäger, S., Kasian, O., Hauke, F., & Ameri, T. (2018). Melaminefunctionalized graphene oxide: synthesis, characterization and considering as pseudocapacitor electrode material with intermixed POAP polymer. Applied Surface Science, 459, 874-883. http://dx.doi.org/10.1016/j.apsusc.2018.07.215.

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40. Scapin, A. M., Duarte, L. C., Bustillos, V. W. O. J., & Sato, M. I. (2009). Assessment of gamma radiolytic degradation in waste lubricating oil by GC/MS and UV/VIS. Radiation Physics and Chemistry, 78(7-8), 733-735. http://dx.doi.org/10.1016/j. radphyschem.2009.03.063.

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Received: Jan. 22, 2020 Revised: Feb. 18, 2020 Accepted: Mar. 23, 2020

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ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.08319

Heat transfer simulation for decision making in plastic injection mold design Piery Antonio Gruber1 and Diego Alves de Miranda1*  Departamento de Engenharia Mecânica, Universidade da Região de Joinville – UNIVILLE, São Bento do Sul, SC, Brasil

1

*diegoalves_klx@hotmail.com

Abstract The solidification of a thermoplastic during the injection process directly influences the productivity and quality of the final product. This paper presents a study of the solidification performance of parts produced by a thermoplastic injection process, verifying their dimensional, visual, and production behavior according to the variation of geometry, temperature, and design of the injection mold cooling system. SolidWorks Plastics software was used to perform the simulations. Experiments were performed with a plastic injection mold to confront and validate the simulations. Given the comparison of different cooling geometries, the simulations made it possible to obtain parts with a shorter mold cooling cycle time. Payback analysis has the primary objective of determining which cooling system is the most viable and has the highest return on invested capital. The results demonstrated a solution for engineers and designers to justify maintenance or modifications to existing injection molds through numerical simulation.

Keywords: numerical simulation, injection process, cooling system, payback.

How to cite: Gruber, P. A., & Miranda, D. A. (2020). Heat transfer simulation for decision making in plastic injection mold design. Polímeros: Ciência e Tecnologia, 30(1), e2020005. https://doi.org/10.1590/0104-1428.08319

1. Introduction Industry is increasingly seeking the use of polymers in its products, as it has numerous characteristics that allow versatility, low cost, lightness, and a multitude of applications, from the aeronautics industry to children’s toys. According to Alfrey and Gurnee[1], heat exchange by conduction is proportional to the volume of the injected part. That is, it occurs more slowly in thick parts; in thin parts, cooling will take place in less time.

To find a cooling system suitable for parts with complex geometries, Mercado-Colmenero et al.[2] developed a new algorithm capable of recognizing the discrete topology of the part, obtaining its depth map and detecting flat and concave regions and delicate details of cool down. The design of the cooling channel system is essential to achieve better control over cycle time. Clemente and Panão[3] state that the flow configuration is also extremely relevant in the optimization criteria for cooling. In the setting explored for cooling small scale mold inserts, the flow enters through a channel and returns through secondary pathways that are equally spaced and similar to an umbrella shape with smaller secondary channels and higher return angles, resulting in better thermal exchange of the coolant with the mold.

According to Jahan et al.[4], the cooling system for injection molds through shaped cooling channels can improve the thermal performance of an injection mold. An improvement in the heat exchange performance between the mold wall and the injected part can also be made by employing heat treatments on the steel surface[5].

Polímeros, 30(1), e2020005, 2020

Hassan et al.[6] state that shrinkage or shrinkage of the injected plastic is one of the many essential factors in determining the quality of injection molded products, through this rate it is possible to ensure proper dimensionality to the product, allowing perfect applicability concerning possible joint parts. According to Blass[7], for the mold cooling system to be effective, we must consider the proper distance of the cooling duct. If it is too close, it may cause cold spots, failures, or internal stress on the parts. Blass demonstrated that the distance between the cooling channel and the cavity should be between 25 and 40 mm. According to Oliaei et al.[8], parameters such as melting temperature, refrigerant temperature, mold temperature, and packing time have a significant influence on the shrinkage and warping of thermoplastic processed products. Hassan et al.[6], reported that the effect of the position of the mold cooling system channels and the cross-sectional geometry are directly related to the melt cooling process. The results indicate that for the same cross-sectional area and refrigerant flow as the channels, rectangular-shaped cooling channels provide the lowest time required to solidify the plastic product completely.

The authors further demonstrate that as cooling channels approach the product surface, cooling efficiency increases.

According to Steinko[9], at least 60% of apparent defects, such as shape distortion, dimensional variations, burr formation, and surface defects, are due to system design defects and/or improper mold cooling design caused by a thermal difference in the mold. Park and Dang[10] suggest shaped cooling channels, in which the cooling lines are

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O O O O O O O O O O O O O O O O

Gruber, P. A., & Miranda, D. A. spiral-shaped, to cool thick-walled parts. Park and Dang results show that shaped cooling channels reduce cycle time by approximately 30% compared to conventional cooling channels.

To improve a cooling system suitable for parts with complex geometries, Xiao et al.[11] developed a new algorithm capable of recognizing the discrete part topology; obtaining its depth map; and detecting flat, concave, and thin regions with complicated details to cool. The algorithm performs an automatic heat transfer analysis, considering functional parameters to ensure uniform part cooling. Wang et al.[12] studied the behavior of plastic parts and their post-cooling behavior using steam injection molding. It is an advanced technology for producing thermoplastic products with excellent appearance. According to Vieira and Lona[13], when considering polymer processing such as plastic injection molding, the mold cavity temperature profile is directly related to part quality and part rejection rates, which implies that the online approach can be used to accurately predict the transient temperature behavior of the mold cavity surface.

According to Kantor[14], analyzing the technical and economic feasibility of automating a grain warehouse is extremely important for determining the interventions and improvements that can be made to the project. For this, they evaluated the possibility of substituting the usual process that is done manually and analyzed the main results that would be achieved by implementing an automated system and its financial impact through the analysis of cost and return on investment. For this, they used several formulas and considerations about the returned time of the invested capital, called payback. Concern for the environment and sustainable development, as well as other responsible measures, are making companies look for renewable energy sources such as biogas. Domingos et al.[15] presented a study of simple payback for replacing liquefied petroleum gas with biogas by implementing a biodigester in a hospital unit in Minas Gerais, Brazil. The study aimed to realize the initial investment return time through simple payback.

Mendes and Miranda[16] carried out a study analyzing the possibility of verifying the acquisition of an ornamental plant pruning machine (Buxus), tested the financial viability, and reported how long it will take to recover the money spent on this purchase. The study was carried out evaluating the productivity and yield of both current processes and after equipment acquisition using Economic Engineering concepts. The results were achieved, because the investment

proved viable for the company that, consequently, will have a considerable productivity increase.

In this context, the SolidWorks Plastics injection simulation software (CAE) was used to enable the analysis of the solidification step efficiency of different thermoplastic injection mold cooling systems, thus allowing the simulation of a more efficient cooling system, making it possible to obtain an injected part with shorter cycle time and better visual and dimensional qualities. The simulations were validated with experimental tests to improve the accuracy of new simulations with changes to a new cooling system.

Calculations of return on invested capital for implementing the changes were applied and proved viable. Changes in the actual mold were made, and the return on invested capital was rapid compared to the literature.

2. Materials and Methods 2.1 Workpiece The part made employing the mold is a lid (Figure 1) that is made of thermoplastic material, Braskem Random Copolymer Polypropylene (DP180A).

The lid is esthetically and dimensionally accurate because it must be adequately produced to meet specifications.

2.2 Injection mold The injection mold used in this study had 8 productive cavities, as shown in Figure 2.

2.2.1 Gate The mold uses hot runner technology. The material does not solidify during the transition from the plasticizing cylinder to the mold cavity due to the presence of electrical resistances inside the matrix. For its sizing, some factors such as material fluidity, shear rate and stress, product thickness, injection pressure, and volume to be injected, among other determining factors, were taken into account.

The mold gate has a 1.0 mm diameter, as shown in Figure 3, a value used to allow adequate flow to fill the lid.

The gate must be correctly sized and balanced between the eight mold cavities, ensuring uniform and cohesive filling in all cavities. If the gate of one of the cavities is larger than the others, that cavity will fill before the others, so when the complete injection fill occurs, the larger diameter cavity will be saturated with material, i.e., causing excess material and burrs around it.

Figure 1. Polypropylene Lid. (a) Injected; (b) in CAD. 2/9

Polímeros, 30(1), e2020005, 2020

Heat transfer simulation for decision making in plastic injection mold design 2.2.2 Mold cooling system The original mold cooling system was a “U” type.

The circuit was distributed in more critical plates and/or mold components. Plate 2 contains two pairs of more centralized, concentrated cooling inputs and outputs on the sides of the plate, shown in Figure 2a, due to the high temperature caused by the composite electrical resistances

in the hot runner, which leads to needing a greater heat exchange rate with the coolant.

Plate 3 (Figure 4b) also has two pairs of fluid inlets and outlets. The plates are fixed in the male cavities because of this; it needs precise temperature control since it is directly related to the productive and dimensional capacities of the product. In Plate 6 (Figure 4c), the two pairs are arranged to

Figure 2. Lid injection mold: (a) isometric view; (b) demonstration of the mold males; (c) floating plate opening (plate numbering); (d) isometric view demonstrating mold opening; (e) assembled mold.

Figure 3. Gate location in lid mold.

Figure 4. Mold cooling system: (a) plate number 2; (b) plate number 3; (c) plate number 6.

Polímeros, 30(1), e2020005, 2020

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Gruber, P. A., & Miranda, D. A. homogeneously cool the cavity, ensuring an adequate thermal distribution to the product, as it is the hole responsible for the external shape of the cavity cover produced.

The Plate 6 cooling system is also a “U” type circuit; the differential of the plate cooling model is that the circuit returns the fluid through the hose to complete the circuit.

The distance from one channel to another is 10 mm, with 6 entrance walls and a 4.5 mm channel diameter.

2.3 Simulation procedure Injection process simulations were performed using SolidWorks Plastics software, which is an interface coupled with SolidWorks. This interface makes it easy for engineers to work by making changes to product design and resuming simulations in the same interface[17-19].

2.3.1 Cavity fill simulation SolidWorks Plastics calculates the cavity filling phase using the generalized Hele-Shaw model, which is used for flow into a thin cavity (midplane thickness two-dimensional (2D) formulation). This model considers a non-Newtonian fluid incompressible under non-isothermal conditions[20].

The relevant governing equations describing the flow of Hele-Shaw fused polymer are: ∂ρ ∂ ∂ + ( ρu ) + ( ρv ) = 0 ∂t ∂x ∂y

(1)

∂p ∂  ∂u  = η  ∂x ∂z  ∂z 

(2)

∂p ∂  ∂v  = η  ∂y ∂z  ∂z 

(3)

 ∂T ∂T ∂T  ∂ 2T 2 ρCp  +u + v=  k 2 + ηγ ∂x ∂y  ∂z  ∂t

(4)

where x and y indicate the Cartesian coordinates of the plane; z denotes thickness coordinates; (u, v) are the velocity components in the (x, y) directions for time t under pressure p; ρ is polymer specific mass; η is the shear viscosity; γ is the shear rate; T is the temperature; Cp is the specific heat; and k is the thermal conductivity. The z coordinate represents the direction of thickness, and no flow will occur in that direction. The shear rate is given by: = γ

2

 ∂u   ∂v    +   ∂z   ∂z 

2

η0 (T)

η ( γ , T, p ) =

1− n

 η ( T ) γ  1+  0  τ  

(9)

where: η0 ( T )

( (

 − A1 T − Tg   A + T − Tg = D1e  2

)  ) 

(10)

where D1 is the viscosity at a reference temperature (1.92686 e16 Pa.s); D2 = 236.15K; Tg is the glass transition temperature of the polymer (108 °C); and the other constants are A1 = 34.52, A 2 = 51.6K, τ = 63,836 Pa, and n = 0.19118.

2.3.2 Cooling simulation Only after the mold cavity fill phase does Solidworks Plastics perform the simulations of the mold cooling and solidification phase. The governing equation for the mid-plane cooling stage is the average steady-state cycle temperature governed by a second-order partial differential equation, the Laplace equation: ∂ 2T ∂x

2

+

∂ 2T ∂y 2

= 0

(11)

where T is the average cycle temperature. This equation can be solved with the appropriate boundary conditions imposed on the different mold boundaries, i.e., the cavity surface, cooling channel surface, and external surface[17]. Transient heat conduction gives the field temperature in the mold as:  ∂ 2Tm ∂ 2Tm  ∂T km = + = ρm C m m  2 2   ∂t ∂ x ∂ y  

(12)

where TM is the mold temperature; kM is the thermal conductivity of the mold; ρm is the density of the mold; and CM is the specific heat of the mold.

2.4 Simulation validation To validate the simulations, a qualitative and quantitative comparison of the lid injection process was performed.

The cover design was built in SolidWorks with the same proportions as the actual part. Additionally, the same “U” cooling channels were reproduced in the simulations for validation, as shown in Figure 5.

(5)

The threshold and initial conditions for the Hele-Shaw model are given by: u= v= 0, T= Tw a z= h

∂u ∂v ∂T = 0;= 0;= 0 and = z 0 ∂z ∂z ∂z p = 0 along the front flow

(6) (7) (8)

According to Guerrier et al.[21], the cross viscosity model is directly related to temperature and pressure dependence variables: 4/9

Figure 5. Validation simulation performed on the cover with a “U” type cooling channel.

Polímeros, 30(1), e2020005, 2020

Heat transfer simulation for decision making in plastic injection mold design For this type of validation, the simulation processing parameters must be equal to the experimental conditions.

The processing parameters of this validation are represented in Table 1.

In Table 1, P0 is the injection pressure, T0 is the injection temperature, and TL is the water temperature in the feed channels. For each condition in Table 1, ten experimental samples were performed to qualitatively and quantitatively validate the simulations.

2.5 Mold cooling system modifications With the validation of the simulations performed, new cooling channels were developed in CAD to analyze which geometry will provide a faster homogeneous solidification.

The geometries chosen for the cooling channels were the “Z”, “Rectangular”, and “Helical” geometries. A representation of these geometries can be seen in Figure 6.

The simulations were performed considering three different types of cooling channels to compare which one is more efficient, including the “U” channel itself. However, to make such modifications to the actual mold, it was necessary to modify the existing mold, which takes time and cost.

2.6 Cost analysis for modifications With the simulation results, as there is the possibility of improving the solidification efficiency, a payback analysis was performed to verify if the modification investment is viable for the company. This type of analysis can ratify the amount of time that the amount invested can be repaid and/or depreciated during the assessed period[22,23].

TR = ( GR + RV ) − d

(13)

where GR is gross revenue; RV is the residual value; and d is the depreciation rate.

2.6.2 Payback (Pb) To obtain the payback of the invested capital, use:  Inv MRA  −Log 1 −  AS   Pb = Log (1 + MRA )

(14)

where Pb is the payback; Inv is the value of the investment; MRA is the minimum rate of attractiveness; and AS is the annual savings.

2.6.3 Net present value For calculating the net present value (NPV), the following equation is used: NPV =

n

∑ Rj (1 + i )

−j

n

− ∑ Cj (1 + i )

=j 0=j 0

−j

(15)

2.6.4 Depreciation (d) To calculate how much the project will depreciate annually, use the equation: d=

( Inv − RV ) t

(16)

where d is the depreciation rate and t is the useful life of the investment. The calculations were performed considering the three different proposed and simulated modifications.

2.6.1 Taxed revenue

3. Results and Discussions

Taxed revenue (TR) is the calculated revenue value, which is added to the tax amounts inherent to the area of interest:

This stage presents the results obtained experimentally and by simulation. Based on these results, a case study was elaborated, verifying the obtained quality and the economic viability of altering the original mold cooling circuit.

Table 1. Processing parameters for simulation validation.

Step

P0 (bar)

T0 (°C)

TL (°C)

a b c d e f

30 50 60 70 85 95

210 210 210 210 210 210

35 35 35 35 35 35

3.1 Validation I: qualitative analysis With the intent to apply a qualitative analysis of the cap injection mold, ten samples were run on the injection molding machine using the processing parameters in Table 1.

The qualitative comparison of this process can be compared with simulations performed under the same processing conditions, as shown in Figure 7.

Figure 6. Proposed modifications to the lid mold cooling system. (a) “Z” type circuit; (b) “Rectangle” type circuit; (c) “Helical” type circuit.

Polímeros, 30(1), e2020005, 2020

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Gruber, P. A., & Miranda, D. A. According to the results, coherence is verified since the results found experimentally are similar to the value obtained in the simulation, as shown in Figure 7. Visually, it is possible to observe the similarity between the processes. Figure 7f illustrates the cap with 100% of its filled volume, so 9.5MPa (95bar) filler pressure is required. The fully filled lid has a volume of 15.52 cm3 and a mass of 14 grams. To verify the filling pressure phases, Figure 7a demonstrates the step with only 3MPa (30bar). This pressure allows the injection of only 4.9 cm3, which represents 31.57% of the total volume. Only 4.42 grams of plastic material is inserted into the cavity.

3.2 Validation II: quantitative analysis With the ten samples, it is possible to verify the variability presented between them, as shown in Table 2, and also becomes possible to calculate the standard deviation [σ (%)] among them resulting from the referred samples.

The samples have a proportional standard deviation; that is, the more filled the cavity is, the smaller its standard deviation due to the better stability provided by increased the part filling pressure. After performing the experiments and simulations, some parameters can be compared to validate the simulations quantitatively.

Simulations provide the parameters, which can also be collected experimentally, in a general analysis of results.

These parameters can be observed in Table 3.

Consistency between the results of experimental fill volume (Fv) and simulated fill volume (Fvs) is verified since they have the same tendency and proportionality of fill. As the injection pressure is gradually increased at the rate of fill, the volume cavity behaves evenly. The values of Fvs are incremented every five %. That is, there is no decimal precision in the simulated results, which means the filled volume values had little difference between the experimental and the simulated ones.

Figure 7. Qualitative validation under process conditions: (a) step a; (b) step b; (c) step c; (d) step d; (e) step e; (f) step f.

Table 2. The filling volume of the samples and the corresponding standard deviation.

Samples

a 31.57 31.33 31.27 31.67 31.72 31.76 31.82 31.28 31.25 31.15

1 2 3 4 5 6 7 8 9 10

0.226

σ (%) ±

Fill Volume (%) c d 63.14 73.64 63.24 73.54 63.04 73.56 63.00 73.74 62.97 73.70 62.94 73.58 63.35 73.44 63.19 73.84 62.98 73.82 63.25 73.81

b 52.64 52.58 52.48 52.78 52.85 52.50 52.45 52.35 52.78 52.63 0.132

0.124

e 89.42 89.28 89.26 89.22 89.52 89.48 89.52 89.6 89.48 89.50

0.114

f 100.0 99.90 99.83 99.82 100.0 99.85 100.0 99.81 99.92 99.75

0.106

0.080

Table 3. Qualitative validation under process conditions: (a) step a; (b) step b; (c) step c; (d) step d; (e) step e; (f) step f.

Step a b c d e f

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Fv (%) 31.57 52.64 63.14 73.64 89.42 100.0

σ (%) ± 0.226 ± 0.132 ± 0.124 ± 0.114 ± 0.106 ± 0.080

Fvs (%) 35.0 55.0 65.0 75.0 90.0 100.0

Eft (s) 0.70 1.20 1.30 1.60 1.75 1.90

tps (s) 0.47 1.09 1.18 1.48 1.63 1.79

Sft (s) 3.0 5.0 6.5 7.5 8.0 9.5

trs (s) 3.04 5.12 6.91 7.74 8.09 9.67

Efct (ºC) 90.55 93.75 96.55 97.70 98.62 99.80

Sfct (°C) 92.09 95.74 98.45 99.40 100.14 101.68

Polímeros, 30(1), e2020005, 2020

Heat transfer simulation for decision making in plastic injection mold design The experimental fill time (Eft), and simulated fill time (Sft) follow the same trend, which is proportional to volume. The time increases when the injection pressure value is high. The results of tpe and tps present similar values, demonstrating their consistency. Regarding the experimental final cooling temperature (Efct) and simulated cooling final temperature (Sfct), the obtained values also show cohesion since the rates of increase are linear. The temperature of the part rises in proportion to the increase in the filled volume of the cavity, i.e. the more material is injected into the mold cavity, the higher the temperature of the mold cavity.

3.3 Analysis of modifications with alternative cooling systems After validating the parameters and results, the solidification efficiencies and cost-effectiveness of various injection mold cooling systems were verified using SolidWorks Plastics.

3.3.1 Cooling time More excellent reliability of the values obtained for cooling is set in the software to ensure some parameters for all simulation steps, such as coolant temperature (water) at 30 °C and maximum melting temperature of polymer material at 210 °C. The maximum cooling time values obtained by the simulation ranged from 13.15 to 15.9 s for the helical and “U” type circuits, respectively, as can be seen in Figure 8.

Figure 8 shows the results obtained from simulations in which it is possible to verify coherence between the value obtained in the simulation of the “U” circuit, original mold, and the value obtained in the injection molding process standard sheet, 15.69 and 16 seconds respectively. Additionally, the most efficient cooling system is helical since it required a shorter cooling cycle time for solidifying the injected part, dramatically reducing the cycle time.

As shown in Figure 8, the system that presented the least efficient result was the “U” type system, followed by the “Z”, rectangular, and finally, the one that presented the best result, helical.

Figure 10 shows the results obtained in simulation, proving that the most efficient cooling system is the helical one, since it obtained a higher heat exchange rate between the injected mass and the cooling system refrigerant, maintaining a lower temperature than the injection mold.

The more stable and closer to the coolant temperature, the better the quality of the injected product will be and the less time the machine will need to cool down.

3.4 Payback analysis in constructing cooling systems The results found during the simulation stages showed that the helical cooling circuit presented higher efficiency because it required a shorter cooling time for the part.

The manufacturing costs of a new injection mold cavity cooling system were evaluated by checking the system cost and payback.

With the original “U” type mold cooling system, the total injection cycle is 22s. As the mold under study was composed of 8 productive cavities, every 22s, lids are produced. 1,300 caps are produced per hour, and the machine works 24 hours a day, so 31,200 units are produced per day.

Monthly, the machine works, on average, 17 days with the lid mold. Due to the high contingent of molds, there is a need to produce other products in the same injector. Adding maintenance stops and color changes, the average monthly

Figure 8. Cooling time variation.

3.3.2 Part temperature after solidification The results obtained from the simulation software for the lid temperature after the cooling step ranged from 101.68 to 93.66 °C for the “U” and helical circuits, respectively, as shown in Figure 9.

Figure 9 shows the results obtained in the simulation.

These results verify coherence between the value obtained in the simulation of the original “U” circuit of the mold and the value obtained by a laser thermometer, 101.68 and 99.8 °C, respectively.

The most efficient cooling system is the coil, because it obtained a higher heat exchange rate between the injected mass and the cooling system refrigerant, requiring less cooling time to reduce the lid temperature inside the mold, causing cycle loss and increasing productivity.

Figure 9. Cover temperature after solidification in each type of cooling system.

3.3.3 Mold temperature after cooling The results obtained by the simulation software of the mold temperature after the cooling step varied between 49.54 and 42.2 °C for the “U” and helical circuits, respectively, as shown in Figure 10.

Polímeros, 30(1), e2020005, 2020

Figure 10. Mold temperature after solidification in each type of cooling system.

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Gruber, P. A., & Miranda, D. A. Table 4. Estimated production for each cooling system.

Cooling system

Cooling Time (s)

Total cycle (s)

“U” “Z” Retangular Helical

15.7 15.6 14.4 13.2

22.0 21.9 20.7 19.5

Quantity of Parts/h 1,300 1,315 1,390 1,475

Quantity of Parts/day 31,200 31,560 33,360 35,400

Quantity of Parts/Month 487,500 493,000 521,250 553,000

Quantity of Parts/Year 5,600,000 5,624,000 5,910,000 6,200,000

Table 5. Results of values for payback calculations.

Cooling system Inv GR d TR Income Tax AS Present value NPV Pb

“Z” R$6,869.00 R$2,800.00 R$ 555,75 R$2,444.25 R$262.26 R$2,537.73 R$15,720.16 R$8,851.16 42 Months

Rectangular R$8,952.20 R$37,200.00 R$729.35 R$36,670.35 R$3,934.76 R$33,265.24 R$206,057.30 R$197,105.10 3.5 Months

Helical R$14,159.60 R$72,000.00 R$1,163.00 R$71,037.00 R$7,622.27 R$64,377.73 R$398,779.10 R$384,619.50 3 Months

production is 487,500 caps (375h). It is possible to make 5,600,000 lids annually, according to Table 4.

Changing the mold cooling circuit to type “Z” increases production by 24,000 pieces per year. Production capacity increases with the rectangular circuit, allowing the production of 310,000 more lids compared to the original system (“U”). The system that showed the highest efficiency is the helical type, where the simulation results showed that it is possible to reduce the cooling time by 2.5s, i.e., the injection cycle can be obtained with 19.5s. It would be possible to produce 1,475 pieces per hour, 35,400 injected caps per day, or 553,000 per month. 6,200,000 lids could be made annually. Changing the cooling system increases 600,000 caps over a year.

Comparing the most effective (helical) and second (rectangular) systems, the annual increase in production practically doubles, 600,000 and 310,000, respectively; that is, the helical circuit is the most efficient. The market value of the cap is $0.12. Annual expenses for electric power, machine maintenance, and molds are R$250,000.00.

According to the data shown in Table 5, the helical system is the one that demands the highest investment value. However, due to its efficiency, as demonstrated in the simulation stages, the return on invested capital comes in less time compared to other circuits. The helical system becomes more attractive due to the year-end gross revenue.

It is directly related to improved mold cooling efficiency and reduced injection machine cycle time.

Given the data obtained and elaborate calculations, the helical cooling system is the most attractive because the return on investment occurs in less time compared to the “Z” and rectangular systems.

that the helical geometry cooling system is more efficient.

The remarkable points are:

4. Conclusions

1. Alfrey, T., & Gurnee, E. (1971). Polímeros orgânicos: série de textos básicos de ciência dos materiais. São Paulo: Blucher.

2. Mercado-Colmenero, J. M., Rubio-Paramio, M. A., MarquezSevillano, J. J., & Martin-Doñate, C. (2018). A new method for the automated design of cooling systems in injection molds.

The simulations carried out make it possible to analyze the efficiency of the solidification steps for different thermoplastic injection mold cooling systems, demonstrating 8/9

1. The simulations were validated with experimental tests showing the accuracy of the simulations of the new cooling system; 2. With the validation, it was possible to numerically simulate different cooling channel geometries for the lid injection mold, to compare the different modifications proposed; 3. Calculations of return on invested capital demonstrated that the helical circuit obtained better performance, both in terms of efficiency and product quality, as well as in the shorter return time of the invested value; 4. Given this, it is highly recommended to change the injection mold cooling design. With the change, it is possible to increase productivity and product quality.

This demonstrates greater precision in decision making for changes and modifications of new designs; 5. The results showed that numerical simulation is a practical tool for engineers and designers to justify future maintenance or modifications to the injection molds already existing within the company.

5. Acknowledgements The authors gratefully acknowledge CAPES (Coordination for the Improvement of Higher Education Personnel, Brazil) for financial support.

6. References

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Heat transfer simulation for decision making in plastic injection mold design Computer Aided Design, 104, 60-86. http://dx.doi.org/10.1016/j. cad.2018.06.001.

3. Clemente, M. R., & Panão, M. R. O. (2018). Introducing flow architecture in the design and optimization of mold inserts cooling systems. International Journal of Thermal Sciences, 127, 288-293. http://dx.doi.org/10.1016/j.ijthermalsci.2018.01.035.

4. Jahan, S. A., Wu, T., Zhang, Y., Zhang, J., Tovar, A., & Elmounayri, H. (2017). Thermo-mechanical design optimization of conformal cooling channels using design of experiments approach. Procedia Manufacturing, 10, 898-911. http://dx.doi. org/10.1016/j.promfg.2017.07.078.

5. Corazza, E. J., Sacchelli, C. M., & Marangoni, C. (2012).

Cycle time reduction of thermoplastic injection using nitriding treatment surface molds. Información Tecnológica, 23(3), 5158. http://dx.doi.org/10.4067/S0718-07642012000300007.

6. Hassan, H., Regnier, N., Arquis, E., & Defaye, G. (2016).

Effect of cooling channels position on the shrinkage of plastic material during injection molding. In Proceedings of the 19th French Congress on Mechanics (pp. 1-6). Marseille: French Association of Mechanics.

7. Blass, A. (1988). Processamento de polímeros (2. ed.).

Florianópolis: Editora UFSC. 8. Oliaei, E., Heidari, B. S., Davachi, S. M., Bahrami, M., Davoodi, S., Hejazi, I., & Seyfi, J. (2016). Warpage and shrinkage optimization of injection-molded plastic spoon parts for biodegradable polymers using Taguchi, ANOVA and artificial neural network methods. Journal of Materials Science and Technology, 32(8), 710-720. http://dx.doi.org/10.1016/j. jmst.2016.05.010.

9. Steinko, W. (2004). Avaliação do projeto térmico do molde garante qualidade e redução de custos. Plástico Industrial, 6(1), 64-71.

10. Park, H. S., & Dang, X. P. (2017). Development of a smart plastic injection mold with conformal cooling channels.

Procedia Manufacturing, 10, 48-59. http://dx.doi.org/10.1016/j. promfg.2017.07.020.

11. Xiao, C. L., Huang, H. X., & Yang, X. (2016). Development and application of rapid thermal cycling molding with electric heating for improving surface quality of microcellular injection molded parts. Applied Thermal Engineering, 100(1), 478-489. http://dx.doi.org/10.1016/j.applthermaleng.2016.02.045.

12. Wang, W., Zhang, Y., Li, Y., Han, H., & Li, B. (2018). Numerical study on fully-developed turbulent flow and heat transfer in inward corrugated tubes with double-objective optimization.

International Journal of Heat and Mass Transfer, 120(1), 782792. http://dx.doi.org/10.1016/j.ijheatmasstransfer.2017.12.079.

13. Vieira, R. P., & Lona, L. M. F. (2016). Simulation of temperature effect on the structure control of polystyrene obtained by atom transfer radical polymerization. Polímeros: Ciência e Tecnologia, 26(4), 313-319. http://dx.doi.org/10.1590/0104-1428.2376.

14. Kantor, N. L. S. (2011). Análise da viabilidade técnica e econômica da automação de um armazém de grãos. In Anais do

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13º Congresso Nacional de Estudantes de Engenharia Mecânica (pp. 1-2). Erechim: Associação Brasileira de Engenharia e Ciências Mecânicas.

15. Domingos, B. S., Moreira, C. R., Resende, E. W. B. S., Moreira, C. R., Resende, E. W., Rodrigues, D. M. S., & Dornelas, J. O. (2017). Estudo de payback simples para a substituição do gás liquefeito de petróleo pelo biogás em uma unidade hospitalar em Minas Gerais. In Anais do 9º Simpósio de Engenharia de Produção de Sergipe (pp. 193-204). São Cristóvão: Departamento de Engenharia de Produção, Universidade Federal de Sergipe.

16. Mendes, E. C., & Miranda, D. A. (2018). Análise de payback aplicado no processo de automatização de podas na produção de Buxus. In Anais do 3º Congresso Nacional de Inovação e Tecnologia (pp. 1-10). São Bento do Sul: INOVA. 17. Miranda, D. A., & Nogueira, A. L. (2019). Simulation of an injection process using a CAE tool: assessment of Operational conditions and mold design on the process efficiency. Materials Research, 22(2), e20180564. http://dx.doi.org/10.1590/19805373-mr-2018-0564.

18. Sacchelli, C. M., Miranda, D. A., Drechsler, M., & Nogueira, A. L. (2017). Simulação computacional da injeção de termoplásticos: comparação de ferramentas tipo CAE. In Anais do 9º Congresso Brasileiro de Engenharia de Fabricação. Joinville: COBEF. http://dx.doi.org/10.26678/ABCM.COBEF2017.COF20171320.

19. Miranda, D. A., & Nogueira, A. L. (2017). Influência dos parâmetros de processo e da presença de saídas de gases na eficiência de moldes de injeção de peças em poliestireno cristal.

In Anais do 14º Congresso Brasileiro de Polímeros (pp. 1-5). Águas de Lindóia: Associação Brasileira de Polímeros.

20. Fernandes, C., Pontes, A. J., Viana, J. C., & Gaspar-Cunha, A. (2016). Modeling and optimization of the injection-molding process: a review. Advances in Polymer Technology, 17(2), 429-449. http://dx.doi.org/10.1002/adv.21683.

21. Guerrier, P., Tosello, G., & Hattel, J. H. (2017). Flow visualization and simulation of the filling process during injection molding.

CIRP Journal of Manufacturing Science and Technology, 16(1), 12-20. http://dx.doi.org/10.1016/j.cirpj.2016.08.002.

22. Hummel, P. V. R., & Taschner, M. R. B. (1995). Análise e decisão sobre investimentos e financiamentos: engenharia econômica: teoria e prática (4. ed.). São Paulo: Atlas.

23. Miranda, D. A., & Cristofolini, R. (2016). Análise de retorno financeiro aplicado a dois robôs autonômos manipuladores que atuam na descarga de peças no processo de injeção de termoplásticos. In Anais do 6º Congresso Brasileiro de Engenharia de Produção (pp. 1-12). Ponta Grossa: Associação Paranaense de Engenharia de Produção.

Received: Oct. 01, 2019 Revised: Mar. 27, 2020 Accepted: Apr. 13, 2020

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ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.07919

Nanofibers of gelatin and polivinyl-alcohol-chitosan for wound dressing application: fabrication and characterization Paola Campa-Siqueiros1, Tomás Jesús Madera-Santana1* , Jesús Fernando Ayala-Zavala1, Jaime López-Cervantes2, María Mónica Castillo-Ortega3 and Pedro Jesús Herrera-Franco4 1

Coordinación de Tecnología de Alimentos de Origen Vegetal, Centro de Investigación en Alimentación y Desarrollo, Hermosillo, Sonora, México 2 Laboratorio Biotecnología y Ciencias Alimentarias, Instituto Tecnológico de Sonora, Ciudad Obregón, Sonora, México 3 Laboratorio de Química de Polímeros, Universidad de Sonora, Hermosillo, Sonora, México 4 Unidad de Materiales, Centro de Investigación Científica de Yucatán, A.C., Mérida, Yucatán, México *madera@ciad.mx

Abstract Electrospun nanofibers from gelatin (G), chitosan (CS), and chitosan-polyvinyl alcohol (CS-PVA) were developed by electrospinning process. Mechanical properties were determined by the tensile test, the elastic modulus values of the nanofibers were G (15.418-34.34 MPa) and CS-PVA (17.44-126.427 MPa). The morphological characterization by SEM revealed that the systems with 15% G and 6% CS-PVA showed morphological homogeneity. Structural characterization

by FTIR indicated an interaction among some functional groups of the component. Thermal analysis by DSC and TGA showed degradation temperatures for G (330 °C), CS (210 °C to 370 °C), and PVA (310 °C to 420 °C). The contact angles values denoted the hydrophilic nature of the material. Finally, the antimicrobial assay proved that both 15% G and 7% PVA on the CS-PVA system presented the best antimicrobial effect. The results indicate that the electrospun nanofibers fabricated with G or CS-PVA can be used as wound healing dressings.

Keywords: chitosan, electrospinning, gelatin, PVA, wound dressing.

How to cite: Campa-Siqueiros, P., Madera-Santana, T. J., Ayala-Zavala, J. F., López-Cervantes, J., Castillo-Ortega, M. M., & Herrera-Franco, P. J. (2020). Nanofibers of gelatin and polivinyl-alcohol-chitosan for wound dressing application: fabrication and characterization. Polímeros: Ciência e Tecnologia, 30(1), e2020006. https://doi.org/10.1590/01041428.07919

1. Introduction Nowadays, diabetes is the second cause of mortality in México, only 4.5% less than cardiovascular diseases. This metabolic disease affects glucose levels in blood, increasing them in consequence of a lack of insulin secretion or resistance[1]. This prevalence is translated to a considerable economic impact in the country, where the National Health Sector has reported a cost of around 3,400 million dollars for the treatment of diabetes complications, above all, skin wounds[2].

The reason which the health sector manages such a high budget resides on the complexity of the skin wound healing process in a diabetic patient. In comparison to a healthy person, the wound healing process of a diabetic patient is compromised[3], consequence of multiple factors, such as immune system deficiency, poor circulation, metabolic disturbances, propensity of infection and loss of sensation because of neuropathy[1]. On tissue injury, a healthy person forms a fibrin plug for either the re-establishment of homeostasis or the aggregation of platelets for the secretion

Polímeros, 30(1), e2020006, 2020

of growth factors (such as transforming growth factor-beta (TGF-β)[4]. Subsequently, these inflammatory cells induce other growth factors, e.g. platelet-derived growth factor (PDFG) amongst others, all of this on the extracellular matrix (ECM)[5]. Nevertheless, on a diabetic patient, the expression of these growth and angiogenic factors is impaired, stalling the healing process[5,6].

Different alternatives for diabetic skin ulcer treatment, particularly on the polymeric material field, one of these alternatives is chitosan (CS), it is a natural polymer that has reported promising features for diabetic skin ulcers[6,7], in fact there are some commercial wound dressings based on chitosan (Table 1). As addressed before, CS is a natural biopolymer, obtained by the partial deacetylation of chitin under strong alkaline conditions, formed by β (1-4)-D-glucosamine and β (1,4) N-acetylD-glucosamine (NAGA)[8]. Aside from its antimicrobial, analgesic, antioxidant, and neuroprotective[9-11] effects, chitosan has presented an effect upon the wound healing

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O O O O O O O O O O O O O O O O

Campa-Siqueiros, P., Madera-Santana, T. J., Ayala-Zavala, J. F., López-Cervantes, J., Castillo-Ortega, M. M., & Herrera-Franco, P. J. Table 1. Commercial chitosan-based wound dressings.

Trademarks Tegasorb P 3M Chitoflex HemCom Chitopack C Eisai

Characteristics Gel. Contains chitosan particles that swell while absorbing exudate and forms the gel.

Gel. Seals and stabilizes the sound Cotton-like chitosan gel. Rebuilds body and subcutaneous tissue.

Modified from Liu et al.[3].

process[3], specifically on its effect of enhancing growth factor release[10,12,13]. However, most of the reports of the chitosan effect upon wound healing of diabetic patients are based on gels, with the disadvantage that the treatment must be applied by a professional as well as quite often.

From different ways for chitosan processing, electrospinning for the obtaining of nanofibers is a way to surpass the previously mentioned gel disadvantages. These ultrafine continuous fibers are the product of high electric potentials[14]. Apart from being a versatile (a great number of polymers can be processed), simple and the rather cheap process[15], electrospun nanofibers possess a couple of properties which give them a great spectra of applications on biomedical materials: high surface area to volume ratio (which translates on oxygen permeability, fluid exchange without accumulation and uniform adherence in situ[16]) accompanied with high porosity at the various pore size[15].

There are reports of their application as scaffolds for different tissue regeneration[17], cartilage[18], bone[19], drug delivery[20] and, as the type of material of interest in the present work, wound dressings[9]. This particular application is possible because of the nanofibers’ structural capacity to attract the appropriate cellular growth substrates[21]. As the principal barrier of protection for the wound, the scaffolds for wound dressing application have met some important criteria: They should facilitate gas permeation, present a controlled adhesion to the wound, as well as durability, flexibility and is capable of absorbing the wound exudates[22]. Even though electrospun chitosan nanofibers possess many desirable characteristics, the electrospinning process for chitosan tends to be difficult, being that chitosan is polycationic and electrospinning principle is passed on charge application[8].

Therefore, to process chitosan by electrospinning it requires the incorporation of other material to enable the process (electrospinning agent).

There are numerous electrospinning agents for chitosan, either natural or synthetic polymers. A natural polymeric alternative is a gelatin (G). This protein is prepared by collagen hydrolysis[23], resulting in a structure consisting of mainly hydroxyproline, glycine, and proline amino acids[24]. Aside from being biodegradable and electrospinning compatible, gelatin has the important characteristic of being affordable and giving electrospun fibers with controllable thickness and physical stability[25]. On the other hand, some examples of synthetic biopolymers are polyethylene oxide (PEO)[26], collagen[27], and polycaprolactone (PCL)[28]. However, all the previously mentioned electrospinning agents present disadvantages, such as the need for a crosslinking agent, difficult processing, and pearling formation. Another electrospinning agent for chitosan is polyvinyl alcohol (PVA) which is a 2/11

non-toxic and water-soluble synthetic biopolymer[29], it has been reported a significant improvement on the mechanical properties of chitosan electrospun nanofibers[30].

The aim of this study is to fabricate electrospun gelatin and chitosan-PVA nanofibers and evaluate their physicochemical properties (optical, mechanical, structural, thermal and morphological). An antimicrobial analysis was performed in order to explore the potential of the biomaterials produced for a possible application as wound dressings.

2. Materials and Methods 2.1 Materials Gelatin (type B), medium weight chitosan with 75 to 85% deacetylated grade, and polyvinyl alcohol (PVA) (high Mw) were purchased from Sigma Aldrich, (St. Louis, Missouri, USA).

Glacial acetic acid (GAA), CAS [64-19-7] with 2.5 pH and a density of 1.05 g/cm3 was obtained from Fagalab, MEX.

2.2 Preparation of G, CS, PVA and CS-PVA solutions G solution was prepared following the method described

by Okutan et al.[31] with some modifications. The gelatin was dissolved in acetic acid solution (20% w/v) at 15 and 20% w/v.

Solutions were stirred for 4 h, at 40 °C and 900 rpm on a magnetic stirrer until a clear and homogenous blend. CS solution was prepared at 1% w/v with the GAA and MiliQ water at 1:1 relation. The solution was stirred for 24 h at 25 °C. PVA solutions of 6%, 7%, and 8% w/v were prepared with MiliQ water as a solvent and were stirred at 900 rpm and 80 °C for 4 h. The CS-PVA solution was stirred at 900 rpm, 25 °C for 3 h at 1:1 relation.

2.3 Fabrication of G and CS-PVA nanofibers The electrospinning process was carried at room temperature on a system composed as follows: A dual syringe infusion withdrawal pump (KDS 2010, KDScientific, Holliston, USA), a high voltage power supply (CZE1000R, Spellman, USA), the tip of the needle where the voltage was applied and an aluminum collector. Table 2 shows the sample number and the biopolymer used gelatin (G) or polyvinyl alcohol (PVA), as well as, the electrospinning conditions for G and CS-PVA. It is important to point out that in the case of CS-PVA solutions, CS % was a constant 1% w/v. PVA percentages were chosen based on their best morphological characteristics. Whereas CS percentage was the only one that could form nanofibers at the moment of electrospinning.

2.4 Characterization of G and CS-PVA nanofibers 2.4.1 Thickness The thickness of the G and CS-PVA nanofibers were measured with a digital micrometer (Mitutoyo MDC -1”PX, Kawasaki-Shi, Kamagaya, Japan). To obtain the average thickness of the nanofibers, five measurements were performed, one measure on each corner and one in the middle of the resultant square nanofiber membranes.

Polímeros, 30(1), e2020006, 2020

Nanofibers of gelatin and polivinyl-alcohol-chitosan for wound dressing application: fabrication and characterization Table 2. Formulations and electrospinning conditions of G and CS:PVA solutions.

Sample 1 2 3 4 5 6 7 8

G

CS

PVA

(%) 15 20 -

(%) 1 1 1 1 1 1

(%) 6 6 7 7 8 8

2.5 Color The nanofibers color was evaluated with a colorimeter (Minolta CR-300, Osaka, Japan), calibrated with a standard (Y= 94.1, x= 0.3155, and y= 0.3319). Each nanofiber membrane was measured 5 times. The color change (ΔE), chromaticity (C*), and Hue angle (H*) were measured with Equations 1, 2, and 3, respectively.

∆E = = C*

( ∆L ) + ( ∆a ) + ( ∆b ) * 2

a*2 + b*2

 b*  H = artg  *  a  *

* 2

* 2

(1) (2)

Collector distance (cm)

Injection rate (mL/h)

Voltage (Kv)

Needle caliber (g)

10 10 10 15 15 15 15 15

1 1 1 1 1 1 2 2

-17 15 17 24 21 20 25 22

23 23 22 22 22 23 20 18

samples was focused on the fracture surfaces. In this case, an environmental scanning electron microscope (ESEM) FEI-Philips model XL30 ESEM (Tokio, Japan) was used, with a voltage of 20 kV and magnifications of 500X, 10000X, and 25000X.

2.8 Structural properties Structural properties were characterized by Fourier Transform Infrared- Attenuated Total Reflectance (FTIR‑ATR) spectroscopy (Thermo Scientific, with Nicolet iS5 with ATR, Waltham, USA) with a resolution of 4 cm-1 between 4000 and 400 cm-1.

2.9 Thermal properties

(3)

where L* is luminosity; a* is red/green coordinates; and b* yellow/blue coordinates.

2.6 Tensile strength properties The mechanical analysis of the nanofiber’s membranes was measured using an universal tensile unit Shimadzu (AGS-X Kyoto, Japan) following the ASTM D1708.

Rectangular membrane samples were cut with the following dimensions 5 mm wide and 24 mm long, the thickness of each membrane was measured in triplicate.

Between 5 and 10 probes for each treatment were measured.

The mechanical parameters, such as elastic modulus (EM), tensile strength (TS), and elongation at break (Eb) were calculated from the tensile test, the sample was clamped at the ends of the jaws of the equipment. The separation between clamps was 12 mm, 6 mm of headspace, at a head speed of 10 mm/min, and the load cell of 100 N was used for force measurements.

2.7 Morphological properties Morphological properties of the electrospun nanofibers were evaluated by scanning electron microscopy (SEM), using a JEOL (JSM 3000, Akishima, Tokio) with a previous sample coating of Au/Pd using a Quorum QI5OR. For better understanding, the sample number was delimited to 6 samples, 2 per PVA percentage, each one with different electrospinning conditions and one of 15% and 20% of gelatin each.

A morphological analysis of the micromechanical tension Polímeros, 30(1), e2020006, 2020

The thermal properties were analyzed by differential scanning calorimeter (DSC) and thermogravimetric analysis (TGA). DSC analysis was performed using DSC from TA Instruments Inc. (Discovery Series, Delaware, USA) measurements were carried out under a nitrogen flow.

Around 5 mg of sample was placed into aluminum cell and sealed, and an empty cell was used as reference. Once that the two cells were ready, these were placed inside the equipment, and heated at 10 °C/min from 25 °C to 300 °C. TGA analysis was carried out on a thermogravimetric analyzer (TGA 8000 PerkinElmer Inc., Massachusetts, U.S.A.), the temperature range was from 24 °C to 700 °C, at a heating rate of 10 °C/min. This analysis was used to record the decomposition temperature of the nanofiber materials.

2.10 Surface hydrophilicity The contact angle is a water contact angle of CS-PVA electrospun nanofibers was determined by Contact Angle Meter (CAM-Plus, ChemInstruments, Fairfield, USA).

Ten measurements per PVA percentage and each membrane were taken. The droplet of water on a flat nanofiber (solid surface), the balance on the three-phase interface is expressed

by Young’s equation:

γ= γ sl + γ l cosθ s

(4)

where the surface tension is γL, the contact angle between the interface liquid-air is θ, the interfacial tension γSL, and surface free energy of a solid is γS.

The sum of the interfacial tensions is given by γSL minus the work of adhesion, the work of adhesion can be expressed as: 3/11

Campa-Siqueiros, P., Madera-Santana, T. J., Ayala-Zavala, J. F., López-Cervantes, J., Castillo-Ortega, M. M., & Herrera-Franco, P. J.

W = γ l (1 + cosθ ) A

(5)

2.11 Antimicrobial analysis The nanofibers of G and CS-PVA were tested in their antimicrobial properties using a qualitative methodology reported by Ruiz-Ruiz et al.[32] with some modifications.

Müller-Hinton agar was inoculated with S. tiphy or S. aureus at 1 x 108 UFC and incubated at 37 °C for 12 h. Samples of 1.5 cm2 were deposited on the surface of the culture and incubated at 37 °C for 8 h. A paper disk with ciprofloxacin was considered as control. Photographs of the resultant Petri dishes were taken by the imaging system Gel Doc™ (EZ system, BIO-RAD, Hercules, USA).

2.12 Statistical analysis Statistical analyses of mechanical, optical, and contact angle characterization were processed by NCSS ver.

7 software (Kaysville, U.S.A). Data were presented by its mean ± standard deviation, at a significance level of P<0.05.

If a significant difference was observed a mean comparison

by Tukey-Kramer was processed.

3. Results and Discussions 3.1 Color The color parameters ΔE, C*, and H* of the samples are presented in Table 3. In the case of ΔE, the behavior of the samples showed changes between those of G and CS‑PVA. Samples 1 and 2 (G) showed higher ΔE values in comparison to samples 3 to 8 (CS-PVA). This behavior can be explained Table 3. Optical properties of color of G and CS-PVA nanofibers.

Sample 1 2 3 4 5 6 7 8

ΔE 3.40 ± 0.17 c 2.02 ± 0.20 b 1.19 ± 0.41 ab 1.42 ± 0.05 ab 0.68 ± 0.17 a 1.75 ± 0.10 b 1.28 ± 0.44 ab 1.88 ± 0.12 b

C* 1.33 ± 0.20 e 1.03 ± 0.08 e 1.36 ± 0.39 ab 1.33 ± 0.22 ab 1.68 ± 0.74 a 1.06 ± 0.18 bc 2.03 ± 0.31 ab 1.92 ± 0.32 cd

H* 66.23 ± 2.01 d 82.38 ± 1.87 d 53.56 ± 3.77 b 49.14 ± 6.33 bc 61.16 ± 1.24 a 61.33 ± 0.99 c 78.70 ± 1.45 bc 76.24 ± 0.82 c

Data is presented as mean ± standard deviation. Abbreviation signification is follows: ΔE color difference, C* chromaticity, and H* Hue angle.

with the reported by Horsfall[33], the author concluded that the color change observed in the material is produced by the chemical composition of itself. The ΔE values for samples with gelatin revealed that the chemical composition would be strongly depended with the gelatin content in the samples, therefore a significant difference was found between them (P<0.05). However, this behavior becomes to be more complex in the case of the CS-PVA samples.

Samples 5 and 6 are significantly different (P<0.05), even though they belong to the same PVA percentage. However, they differ in morphology, meanwhile sample 5 was fabricated with a needle caliber of 22 g, which has a higher diameter (0.7 mm) than the needle used on sample 6 which is caliber of 23 g and 0.6 mm of diameter. This effect can also be seen between samples 7 and 8.

Chromaticity can be defined as the color saturation.

When a sample presents a low C* value, it infers that they present a high interference of colors, whereas a C* value of 0 means that the samples show an achromatic stimulus[34].

The achromatic stimuli addressed previously represented itself by the visually white color of all the samples obtained.

In contrast to ΔE values, samples 1 and 2 did not show chromatic differences between them, although these showed significant difference (P<0.05) with samples formulated with CS-PVA. From previous optical parameters, H* is the most outwardly explaining. This parameter indicates the color of the sample within the color sphere, where tones are represented with the grades within the sphere. Being 90° represents yellow, 180° green, 270° blue and 360°, as well as 0°, represent red[35]. According to Table 3, the samples were between 49.14° and 82.38°. This, along with a luminosity of average of 95 (data not shown) and the C* obtained, the samples presented a light color, in the yellow region, therefore, the whiteness present visually on all the samples.

3.2 Thickness The thickness of G and CS-PVA nanofibers membranes are presented in Table 4, the values of these systems were found in the range from 0.219 to 0.297 nm and from 0.015 to 0.57 nm, respectively. The nanofibers of CS-PVA (samples 3 to 8) were obtained at different electrospinning conditions (Table 2), nevertheless, the thickness of these samples did not show significant differences (P>0.05) as it can be observed in Table 4. Although the electrospinning conditions experimented in this work were different, these allowed the fabrication of nanofibers at different percentages

Table 4. Thickness and tensile at strength properties of G and CS-PVA nanofibers.

Sample 1 2 3 4 5 6 7 8

Thickness (mm) 0.219 ± 0.05 b 0.297 ± 0.073 b 0.057 ± 0.014 a 0.032 ± 0.042 a 0.023 ± 0.003 a 0.035 ± 0.007 a 0.024 ± 0.0005 a 0.015 ± 0 a

EM (MPa) 34.342 ± 5.3 b,c,d 15.418 ± 2.205 b,d NA a NA b 116.49 ± 12.599 b,c, 17.444 ± 2.948 b,c,d 29.201 ± 6.225 b,c,d 126.427 ± 20.40 b,c,d

TS (MPa) 1.052 ± 0.318 b,d 0.409 ± 0.215 b,d 14.89 ± 1.726 a 4.378 ± 3.063 b,d 8.269 ± 2.252 a,b,c 4.237 ± 2.581 b,d 5.984 ± 3.426 b,d 14.310 ± 5.047 a,c

E (%) 4.178 ± 1.354 ª,c 4.344 ± 1.679 ª,c 16.202 ± 1.56 a 16.852 ± 8.266 a 15.818 ± 5.693 a 27.674 ± 9.698 ª,b 23.155 ± 8.281 ª,b 26.502 ± 9.442 ª,b

Data is presented as mean ± standard deviation. Different letters on each column indicate significant differences (P<0.05). Abbreviation signification is follows: EM: Elastic modulus; TS: Tensile strength; E: Elongation at brake.

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Polímeros, 30(1), e2020006, 2020

Nanofibers of gelatin and polivinyl-alcohol-chitosan for wound dressing application: fabrication and characterization of PVA. However, a significant difference was found between G and CS-PVA nanofibers, where G presented the highest values, a consequence of the different electrospinning conditions, added to the nature of the material, in this case, G can produce nanofiber with higher thickness. It is attributed that gelatin has the most complex and larger structure (formed by proteins, mineral salts, and water), samples 1 and 2 showed the higher nanofiber diameter with 111.66 and 286.33 nm (Table 5), respectively. It is difficult to compare our values with the literature because the references did not report the values and conditions that the nanofibers are produced.

3.3 Mechanical properties The values of mechanical parameters (elastic modulus, tensile strength, and elongation at brake) for all the samples tested are presented in Table 4. As we can be observed, the samples 3 and 4 have not reported the EM data, as well as they present the higher standard deviation among all the samples. It is related to physical aspect of the material instead of the chemical nature. G nanofibers showed the lowest values in tensile strength and elongation at break, although these samples did not showed significant difference (p>0.05) between them. The tensile strength values of samples of CS-PVA nanofibers is very fluctuating, it is attributed to Table 5. Nanofiber diameter and general morphological characteristics of G and CS-PVA nanofibers.

1 2 3 4

Fiber diameter (nm) 111.66 ± 7.57 a 286.33 ± 2.3 b 33 ± 2.5 c 33 ± 2.5

5

17.8 ± 0.9 d

6 7 8

17.8 ± 0.9 27.8 ± 1.06 e 27.8 ± 1.06 e

Sample

General morphological characteristics Fibrillar formation, no drops or fractures.

Fibrillar formation, no drops or fractures.

Fibrillar formation, drops and fiber fracture Fibrillar formation with fractures and pearling Fibrillar formation, pearling, sparse fiber fracture Fibrillar formation and sparse fiber fracture Fibrillar formation, drops and fiber fracture Fibrillar formation, drops and fiber fracture

Data are mean ± standard deviation (n=10). Significant difference (P<0.05).

sample’s thickness which is highly irregular (based on standard deviation). The elongation is a mechanical parameter that it is not related to the thickness. The elongation at break of CS-PVA samples increases as the PVA content is increased.

Due to the thickness of the materials, it is difficult to compare these results with literature, since there is no information available (as far as the present authors know).

3.4 SEM analysis SEM micrographs at 50000X of magnification of the electrospun CS:PVA nanofibers are presented on Figure 1, with their fiber diameter and general morphological characteristics presented on Table 5.

The samples of G had the highest fiber diameter value of 286.33 nm for 20% G, followed by 15% G with 111.66 nm. These results seem coherent since, as explained before, gelatin is a larger molecule. Also, G nanofibers of both concentrations (15% and 20% w/v) present the best morphological characteristics of all the materials since they do not present any drops or fractures. The last characteristics can be also observed in Figure 1a and 1b. In comparison with other studies[36], the morphology of G nanofibers in the present study is also more homogeneous than gelatin nanofibers obtained in other studies[31]. Following this comparison, it can be observed that gelatin concentration and voltage used in the electrospinning process were two times higher in the previous study than the used in the present study.

Samples 3 and 4 (6% PVA) showed small morphological improvement since fibrillar formation was observed, although they still presented fracture, pealing and drops.

However, samples 5 and 6 presented the best morphological characteristics, since apart from fibrillar formation, they presented sparse fiber fracture, especially sample 6, which in comparison with sample 5, did not present pearling.

Nevertheless, Samples 7 and 8 presented undesirable morphological characteristics again, with drops and fiber fracture, as well as an increase of the fiber diameter.

The proportional increase in the nanofibers diameter has been reported by Tarus et al.[37]. These authors reported the same effect on cellulose acetate nanofibers with a solvent system of acetone/dimethyl acetamide, where the diameter

Figure 1. SEM images of (a) sample 1; (b) sample 2; (c) sample 3; (d) sample 4; (e) sample 5; (f) sample 6; (g) sample 7; (h) sample 8.

Polímeros, 30(1), e2020006, 2020

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Campa-Siqueiros, P., Madera-Santana, T. J., Ayala-Zavala, J. F., López-Cervantes, J., Castillo-Ortega, M. M., & Herrera-Franco, P. J. of the nanofibers increased from 60 to 122 nm when the polymer concentration was increased from 10 to 16%.

3.5 FTIR analysis FTIR spectrum of 15 and 20% G is presented in Figure 2a.

As observed, there is a very slight peak at approximately 3300 cm-1 attributed to stretching vibration of amide group (N-H), as well as hydrogen bonding[24]. Following the signals that represent the digital fingerprint, such as the stretching vibration of: C=O of primary amide, bending and stretching of N-H of secondary amide and finally a bending of de N-H group at 1650 cm-1,1540 cm-1, and 1250 cm-1, respectively[38].

FTIR spectrum of neat CS and PVA powder, as well as CS-PVA is presented in Figure 2b as well. The FTIR spectra of neat CS showed the characteristic bands of the saccharide structure in the range of 880-1150 cm-1 [28], as well as the bands for amide I bending at 1660 cm-1 and amide II at 1560 cm-1, result of carbonyl stretching by the partial deacetylation of chitin[29,30]. The signal at 3455 cm-1 corresponds to -NH stretching is also present[31]; however, this band overlaps with the -OH vibrations[30]. Neat PVA spectra have three predominant signal: The band at 1760 cm-1 results of the C=O stretching present on the PVA backbone[29], the band at 2900 cm-1 for the CH2 asymmetric stretching vibration, and the band about 3500 cm-1 attributed to -OH is stretching[34].

In comparison with neat CS and neat PVA, CS-PVA presents a decrease in the wavenumber of the -OH and -NH signal.

This behavior may be credited to the interactions between CS and PVA within the nanofiber system[30,33]. This interaction is by intermolecular and intramolecular hydrogen bonds through hydrophobic side chain aggregation, as represented

by Alhosseini et al.[39].

The presence of the amino and NAGA signals on the sample is evidence of its biodisponibility, which translates on the theoretical capability of them to carry their antimicrobial and therapeutic effects, by the interaction of the protonated -NH with the microorganism’s cell walls[40] and the stimulation of growth factors by the interaction between the NAGA present in CS and the NAGA receptors in macrophages[7].

3.6 DSC analysis DSC data for neat CS, neat PVA, and CS-PVA nanofibers are presented in Table 6. Neat CS presented one endothermic peak at 91.3 °C and PVA an endothermic peak at 181.3 °C. However, the samples presented two endothermic peaks, a signal per compound, with a slight displacement of each temperature regarding the neat compounds. This effect denotes an interaction of the compounds, but also a non-miscibility between them. Therefore, it is important to differentiate the concepts of miscibility and interaction of the components.

Miscibility is when a single-phase system is formed in a polymer-polymer blend, whereas an interaction is a chemical approach between chemical groups[37].

3.7 TGA analysis

Figure 2. (a) FTIR spectra of G and samples 1 and 2; (b) FTIR spectra of CS-PVA nanofibers and neat CS and PVA.

TGA thermograms of G and CS-PVA electrospun nanofibers are shown in Figure 3. In general terms, the thermal stability of gelatin electronspun fibers was fairly similar to gelatin powder. Beginning with sample 1 (15% of G), it presented a first weight loss at 50 °C to 100 °C, which is most likely the result of moisture evaporation. However,

Table 6. Thermal characterization of neat CS and PVA and CS-PVA nanofibers.

Sample Neat 3 4 5 6 7 8

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Melting Temperature CS (°C) 91.3 101.0 82.9 90.3 84.1 90.0 92.4

Melting Temperature PVA (°C) 181.3 182.0 185.1 184.3 185.1 185.1 185.9

ΔH CS (J/g) 350.8 70.0 67.0 44.7 83.2 56.3 60.9

ΔH PVA (J/g) 102.9 11.5 15.0 12.0 12.8 16.3 18.5

Polímeros, 30(1), e2020006, 2020

Nanofibers of gelatin and polivinyl-alcohol-chitosan for wound dressing application: fabrication and characterization

Figure 3. Thermogravimetric curves of (a) TG and (b) DTG of G and CS-PVA nanofibers. G: gelatin, CS: chitosan, and PVA: polyvinyl alcohol.

around 330 °C the maximum weight loss occurs, which is due to protein degradation[23]. At high temperatures, above 400 °C corresponds to the thermal decomposition of gelatin networks and molecular arrangements. The changes on thermal stability (maximum temperature) of gelatin samples were corroborated from DTG thermograms (Figure 3b).

For CS-PVA samples, the samples showed a release of moisture around 100 °C, although all of these presented a significant weight loss of approximately 80%, from 210 °C to 370 °C. This loss mass is associated with amine unit, paired with -CH2OH group degradation[41]. As previously stated on DSC results, there is no apparent miscibility of the polymers, since the material present another twin degradation signal between 310 °C and 420 °C, which is the characteristic of PVA[42]. It is due to polymer dehydration and by the formation of structures similar to polyacetylene.

Moreover, during the thermal decomposition of PVA, it can release CO2 gas and to form oxides. CS showed the main degradation temperature between 280 to 375 °C, the DTG of Figure 3b indicate that the maximum temperature of CS was 308 °C. Compared with materials, electrospun nanofibers of CS-PVA did not show an improvement on thermal stability. Nevertheless, it is important to emphasize that these results determine a very important part of possible application of these biomaterials. Since it can be observed that the nanofiber can withhold temperatures higher than 100 °C, they can carry out as wound dressings as well as possibly be used as biomedical devices, since these materials generally do not involve a sterilization process with heating above 100 °C.

3.8 Surface hydrophilicity The contact angle measurement result of water droplets on electrospun CS-PVA nanofiber surfaces are presented in Figure 4. The contact angle of all the nanofibers was found to be <80°, which indicates the hydrophilic nature of the material. CS and PVA are hydrophilic materials, were CS hydrophilicity are due to the existence of -NH and -OH groups, as well as PVA hydrophilic nature, is due to its -OH groups[41,43]. Additionally, an inversely proportional behavior between PVA percentage and contact angle was observed, since at increasing PVA percentage Polímeros, 30(1), e2020006, 2020

Figure 4. Contact angle for CS-PVA nanofibers.

in the nanofibers, a lower contact angle was obtained.

This behavior is because of the hydrophilic nature of PVA explained before. This behavior was previously reported by Agrawal and Pramanik[44]. The importance of this analysis resides in the fact that, since the nanofiber mats would be for biomedical application. For this end, a hydrophilic material is desired, as previous studies have reported that materials with contact angles between 60 to 80° enhance cell adhesion capability[39], which is essential for the purpose of this work, that is to say, the application of electrospun CS-PVA nanofibers on diabetic skin ulcers.

3.9 Antimicrobial analysis Antimicrobial photographs are shown on Figure 5 and Figure 6. With the purpose of screening the possible antimicrobial effect of the nanofibers, bacteria of Gram negative (S. typhi) and Gram positive (S. aureus) were assayed. It is important to point out that as today, there is no clear activity mechanism for the antimicrobial effect presented by chitosan, and instead, there have been different theories, which will be discussed next. Concerning to the discussion on chitosan and its possible activity mechanism upon S. typhi, Verlee et al.[45], reported two mechanisms which share the characteristic of being developed upon the outer membrane of the bacteria, 7/11

Campa-Siqueiros, P., Madera-Santana, T. J., Ayala-Zavala, J. F., López-Cervantes, J., Castillo-Ortega, M. M., & Herrera-Franco, P. J.

Figure 5. Antimicrobial assay of G and CS-PVA nanofibers against S. aureus. 1: (a) control; (b) sample 2; (c) sample 3; (d) sample 4; (e) sample 5. 2: (a) control; (b) sample 1; (c) sample 7; (d) sample 2; (e) sample 8.

Figure 6. Antimicrobial assay of G and CS-PVA nanofibers against S. typhi. 1: (a) control; (b) sample 2; (c) sample 3; (d) sample 4; (e) sample 5. 2: (a) control; (b) sample 1; (c) sample 7; (d) sample 2; (e) sample 8.

chelation of cations (Mg2+, Ca2+, etc)[46] or electrostatic interaction of chitosan with the lipopolysaccharide present on the outer membrane (principal difference between gram negative and gram positive bacteria). Both effects result on a disruption of the inner membrane[43], propitiating the intercellular material leaking. All these effects are attributed to the polycationic nature of CS, since in acid media, the amino groups present on CS backbone get protonated. Since membrane configuration of Gram-positive bacteria differs from Gram negative bacteria (as mentioned above), chitosan action mechanism is said to also vary.

As observed on Figure 6, the sample of CS-PVA that presented the most activity was sample 6 and sample 1 for G nanofibers. In the case of these bacteria, such as 8/11

S. aureus, the membrane consists, among other things, of teichoic acids embedded into the cell surface. These acids are of the most important for bacterial physiology, since, they are responsible of controlling cationic concentration and enzyme activity, receptor and surfaces binding and protection against environmental stress[47]. According to Verlee et al.[45], CS can establish electrostatic interactions with teichoic acids, compromising the bacteria physiology and resulting on antimicrobial activity. The samples that presented the better antimicrobial activity against S. typhi were also sample 6 for CS-PVA and sample 1 (15% G). These effects agree with the expressed on the FTIR results, since there are amine group signals either in CS‑PVA and G, and both were in acidic media, producing amino protonation.

Polímeros, 30(1), e2020006, 2020

Nanofibers of gelatin and polivinyl-alcohol-chitosan for wound dressing application: fabrication and characterization

4. Conclusions Nanofibers of gelatin and chitosan-polyvinyl alcohol were produced electrospinning process. The electrospun nanofibers produced showed a white color and did not showed chromatic differences between the samples. The gelatin nanofibers showed higher thickness in comparison to CS‑PVA nanofibers, it is attributed to complex structure of this biopolymer. The elongation at break of CS-PVA samples is directly related to PVA content. The morphological analysis of gelatin nanofibers and sample 6 showed a homogeneous fibrillar formation. Particular structural and thermal characteristics of the samples were identified by infrared and thermal analysis. The contact angles developed by the samples would be suitable for biomedical applications.

The sample that showed the best antimicrobial activity against S. thyphi. This sample is capable of integrating with typical small molecules (bioactive compounds) or grow factors to provide a sustained release behavior for an application on chronic patients of diabetes.

5. Acknowledgements MSc. Campa Siqueiros want to thank you for financial support provided by Consejo Nacional de Ciencia y Tecnología (CONACyT). The authors would like to thank to Laboratorio Nacional de Nano y Biomateriales (LANNBIO) at CINVESTAV-IPN. Unidad Mérida (under projects FOMIX-Yucatán 2008-108160, CONACYT LAB-2009-01123913, 292692, 294643, and 299083) and Laboratorio de Microbiología Molecular y Aplicada at ITMérida, for the facilities to carry out this research.

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25. Wang, X., Ding, B., & Li, B. (2013). Biomimetic electrospun nanofibrous structures for tissue engineering. Materials Today, 16(6), 229-241. http://dx.doi.org/10.1016/j.mattod.2013.06.005.

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29. Yousefi, A., Aliyeh, H. Z., Khorasani, S. N., & Abdolmaleki, A. (2017). Optimization and characterization of electrospun chitosan/poly(vinyl alcohol) nanofibers as a phenol adsorbent via response surface methodology: optimization of Cs/PVA electrospun nanofibers as adsorbent via RSM. Polymers for Advanced Technologies, 28(12), 1872-1878. http://dx.doi. org/10.1002/pat.4075.

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35. Maskan, M. (2001). Kinetics of color change of kiwifruits during hot air and microwave drying. Journal of Food Engineering, 48(2), 169-175. http://dx.doi.org/10.1016/S02608774(00)00154-0.

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37. Tarus, B., Fadel, N., Al-Oufy, A., & El-Messiry, M. (2016). Effect of polymer concentration on the morphology and mechanical characteristics of electrospun cellulose acetate and poly (vinyl chloride) nanofiber mats. Alexandria Engineering Journal, 55(3), 2975-2984. http://dx.doi.org/10.1016/j.aej.2016.04.025.

38. Ji, L., Qiao, W., Zhang, Y., Wu, H., Miao, S., Cheng, Z., Gong, Q., Liang, J., & Zhu, A. (2017). A gelatin composite scaffold strengthened by drug-loaded halloysite nanotubes.

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39. Alhosseini, S. N., Moztarzadeh, F., Mozafari, M., Asgari, S., Dodel, M., Samadikuchaksaraei, A., Kargozar, S., & Jalali, N. (2012). Synthesis and characterization of electrospun polyvinyl alcohol nanofibrous scaffolds modified by blending with chitosan for neural tissue engineering. International Journal of Nanomedicine, 7, 25-34. http://dx.doi.org/10.2147/IJN. S25376. PMid:22275820.

40. Martínez-Camacho, A. P., Cortez-Rocha, M. O., GracianoVerdugo, A. Z., Rodríguez-Félix, F., Castillo-Ortega, M. M., Burgos-Hernández, A., Ezquerra-Brauer, J. M., & PlascenciaJatomea, M. (2013). Extruded films of blended chitosan, low density polyethylene and ethylene acrylic acid. Carbohydrate Polymers, 91(2), 666-674. http://dx.doi.org/10.1016/j. carbpol.2012.08.076. PMid:23121962.

41. Bonilla, J., Fortunati, E., Atarés, L., Chiralt, A., & Kenny, J. M. (2014). Physical, structural and antimicrobial properties of poly vinyl alcohol-chitosan biodegradable films. Food Hydrocolloids, 35, 463-470. http://dx.doi.org/10.1016/j. foodhyd.2013.07.002.

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44. Agrawal, P., & Pramanik, K. (2016). Chitosan-poly(vinyl alcohol) nanofibers by free surface electrospinning for tissue engineering applications. Journal of Tissue Engineering and Regenerative Medicine, 13(5), 485-497. http://dx.doi. org/10.1007/s13770-016-9092-3. PMid:30603430.

45. Verlee, A., Mincke, S., & Stevens, C. V. (2017). Recent developments in antibacterial and antifungal chitosan and its derivatives. Carbohydrate Polymers, 164, 268-283. http:// dx.doi.org/10.1016/j.carbpol.2017.02.001. PMid:28325326.

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Nanofibers of gelatin and polivinyl-alcohol-chitosan for wound dressing application: fabrication and characterization 46. Goy, R. C., Britto, D. D., & Assis, O. B. G. (2009). A review of the antimicrobial activity of chitosan. Polímeros: Ciência e Tecnologia, 19(3), 241-247. http://dx.doi.org/10.1590/S010414282009000300013.

47. Xia, G., Kohler, T., & Peschel, A. (2010). The wall teichoic acid and lipoteichoic acid polymers of Staphylococcus aureus.

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Received: Feb. 04, 2020 Revised: Apr. 13, 2020 Accepted: Apr. 14, 2020

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ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.08419

Adsorption of terbium ion on Fc/dymethylacrylamide: application of Monte Carlo simulation Norma Aurea Rangel Vázquez1*  Departamento de Posgrado, Instituto Tecnológico de Aguascalientes – TECNM/ITA, Aguascalientes, Ags., México

1

*polymer1979@yahoo.com

Abstract The crosslinking of the Fc fragment (IgG antibody) on a polymer matrix about of dimethylacrylamide (DMA), melamide group (MG) and n-acryloxy succinimide (NAS) was analyzed through Monte Carlo simulation at 277.15K and pH 7, in where Gibs free energy and the dipole moment indicated the spontaneity of the reaction through van der Waals interactions. In addition, the QSAR properties determinated that both the surface area and the volume allow to carry out the physical adsorption of the Fc fragment that was verified through the electronic distribution of the electrostatic potential maps (MESP) where the nucleophilic zones (blue color) and electrophilic (red color) were observed, while the partition coefficient (Log P) indicated the solubility of the process. Subsequently, the analysis of the adsorption of the terbium ion (Tb+3) at 277.15K and a pH 7 in Fc/polymeric matrix was carried out, observing that the Fc fragment presented a flat-on optimization geometry attributed to the Tb+3 that generates electronic repulsions, as well as van der Waals forces, hydrogen bonds derived from the Cys aminoacids formed a polar structure and that was corroborated by the Log P negative. Finally, the surface area and volume determined that Tb+3 adsorption showed an increase in surface area and volume with temperature.

Keywords: Fc, polymer, QSAR, Gibbs, Monte Carlo.

How to cite: Rangel Vázquez, N. A. (2020). Adsorption of terbium ion on Fc/dymethylacrylamide: application of Monte Carlo simulation. Polímeros: Ciência e Tecnologia, 30(1), e2020007. https://doi.org/10.1590/0104-1428.08419

1. Introduction The development of contrast agents has produced an advance in the analysis of clinical images of biological processes in which the lanthanide elements are analyzed to obtain multimodal images, diagnosis and cancer therapies due to properties of density and magnetic susceptibility[1].

In addition, the lanthanides exhibit a low toxicity attributed to the trivalent state that allows are bonded to ligands that present an oxygen donor, besides are excellent indicators of degenerative diseases due to the antioxidant properties[1,2].

The lanthanide elements have become more important in the identification and treatment of cancer, where the terbium ion (Tb+3) is used in the recognition of protein structures to develop new cancer inhibitors[3]. However, the weak adsorption of these ions is attributed to the coordination numbers, the polarity of the transitions f-f and the restriction of rotation.

Nevertheless using polymer matrices the adsorption of lanthanides is carried out and therefore, the design of the controlled release systems are efficient and have no toxicity in the release of peptides, proteins and genes[4,5].

There is a diversity of polymers used in the adsorption of biological macromolecules such as polymethyl methacrylate used in the identification of antigen-antibody[6], while N-isopropylacrylamide has been studied in the adsorption of immunoglobulin IgG[7]. On the other hand, Polystyrene generates hydrophobic and electrostatic attractions during

Polímeros, 30(1), e2020007, 2020

the immobilization of antibodies in immunoassays[8].

Polyacrylamide is the polymer of greatest use in the synthesis of polymer matrices for the adsorption of biological macromolecules in delivery systems due to the characteristics of biocompatibility[1,9].

On the other hand, antibodies represent the most important biomolecules in the study of immune systems in the human body for applications in nanobiotechnology and biomedicine[10-12]. Antibodies are constituted by a series of aminoacids; however, performing the computational analysis of the interactions of these molecules with lanthanide elements represents important computational times[10].

The IgG (Immunoglobulin Gamma) antibody is constituted by glycoproteins of approximately 150 kDa and consists of two heavy chains of 50 kDa and two light chains of 25 kDa, respectively, which are bonded by internal disulfide bonds that have the function of generating the formation of loop to compact the antibody structure[12,13]. The IgG antibody is used in therapies against advanced cancers due to the efficiency of the antigen-binding fragment (Fab) and crystallizable fragment (Fc), which are bonded through a hinge zone that allows the movement of Fab fragments[12].

The Fc fragment is composed of the CH2 and CH3 domains that interact with the neonatal Fc receptor (FcRn) of the placenta, liver, mammary glands as well as the intestine in

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Vázquez, N. A. R. order to control homeostasis and the delivery of IgG to the fetus through of the placenta[14-19].

Although there are different computational models for the analysis of antibodies, the force fields of molecular mechanics are more accurate in the calculation of molecular properties mainly in the local reorganization as well as the analysis of the Fc receptor, the CH2 and CH3 domains in addition to the disulfide bonds where the free energy shows information of the molecular interactions and the electronic densities indicate the binding affinity[16,20].

Computational simulation allows the design and understanding of scaffolding behavior or determination of antibody from QSAR descriptors or properties (Quantitative structure-activity relationship) that allow obtaining information on physicochemical properties, molecular weight, electronic density, surface area, volume, mass and partition coefficient (Log P).

In addition the electrostatic potential maps (MESP) indicate the interactions present in the antibodies[21]. Besides, computational simulation allows the design of Fc derivatives for treatments of cancer, viral and immune diseases because they are used as binding media for the selection of functional binders or the obtaining of new proteins for clinical use by calculating the QSAR properties of new biopharmaceuticals[22].

QSAR properties is obtaining using two-stage computational simulations; firstly the polymeric matrix and the Fc fragment and subsequently the adsorption of the lanthanide ion[10]. These simulations are developed using Monte Carlo modeling to calculate the van der Waals interactions on the surface of the material in order to determine the electrostatic forces when the fragments have strong dipole moments because they predict the orientation on the surface, for example, if there is the influence of the van der Waals forces so the resulting geometry will be flat-on, these characteristics are of vital importance. In the addition of lanthanides to the antibodies in order to increase the efficiency of medical treatment[23-25].

Thus, the objective of the research was to use the Monte Carlo modeling to determine the structural properties, the electronic distribution and the Log P of the Tb+3 adsorption on the Fc fragment using a polyacrylamide/melaimide/N‑acryloxy succinimide polymeric matrix at 277.15K and a pH of 7.

2. Materials and Methods Optimization geometry was calculated using the Hyperchem 8v software on a DELL computer with i7 processor.

AMBER force field (Assisted Model Building with Energy Refinement) of molecular mechanics was used applying the conjugate gradient method with the Polak-Ribiere algorithm, 19,000 processing cycles, and an RMS (Root mean square)

of 0.001 Kcal/(Å-mol) as convergence criteria in order to obtain the minimum of the potential energy surface (PES) according to the Born-Oppenheimer approximation and the Schrodinger equation[26-29]. In the design of the polymeric matrix, the geometry was optimized using the model PM3 (Parameterized Model number 3) of quantum mechanics.

Table 1 shows the structural properties of DMA (Dimethylacrylamide), NAS (N acryloxy succinimide) and MG (Melaimide group) respectively, which were used for the design of the polymer matrix of 10 and 40 units.

The negative Gibbs free energy shows the thermodynamic stability of the individual molecules[27]. On the other hand, the positive Log P determined that the MG is hydrophobic while the negative Log P of the NAS and DMA is slightly hydrophilic, a characteristic that can be used in medical applications as a support in the synthesis of proteins and controlled release systems[28]. The dipole moment indicates that there is a difference in electronegativities attributed to the carbonyl bonds.

Figure 1 shows the mapping of the surface of the isoelectronic density, where the shape and size of the electronic density was appreciated, besides the red coloration represents the negative zone or electrophilic and blue-staining the nucleophilic or positive generated mixture of molecular orbitals[27,29-32]. The sequence design of the Fc fragment was carried out by means the amino acid and nucleic acid database in the Hyperchem software[27].

Table 2 shows the structural properties of the Fc fragment of the IgG antibody, where the AMBER force field was first used at 298.15K and then the Monte Carlo simulation was used at 277.15K and a pH of 7. The results show an increase in Gibbs free energy due to a structural rearrangement caused by the covalent interactions of the CH2 domain that generate a loss of thermodynamic stability while the CH3 domain remains constant due to a homo-interaction[19,23]. According to the dipole moment it was observed that at 298.15K it has a value of 1,729 D due to the electrostatic interactions while at 277.15K the dominant interactions are van der Waals that produces a null dipole. The negative partition coefficient demonstrates that the Fc fragment is soluble in water due to the amount of OH groups present in the structure.

Figure 2 shows the electronic distribution of the Fc fragment at 298.15 and 277.15K and a pH 7 where, the electrophilic zones are observed in red and the nucleophilic zones in blue.

It is also appreciated that the change in temperature increases the positive electronic distribution due to the molecular rearrangement caused by the hydrogen bonds and the van der Waals forces.

Table 1. Structural properties of DMA, NAS and MG obtained through the AMBER/PM3 hybrid model.

Properties Gibbs free energy (kcal/mol) Dipole moment (Debyes-D) Surface area (Å2) Volume (Å3) Mass (amu) Log P

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DMA - 27,362 3.7310 142.50 158.15 99.13 0.210

NAS - 52,401 2.5330 147.16 170.31 169.14 - 0.050

MG - 29,268 1.8280 305.08 458.49 154.24 1.870

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Adsorption of terbium ion on Fc/dymethylacrylamide: application of Monte Carlo simulation

Figure 1. MESP of (a) DMA, (b) NAS and (c) MG obtained through the AMBER/PM3 hybrid model. Where, cyan color: carbon, red color: oxygen, white color: hydrogen and purple color: nitrogen, respectively.

Figure 2. MESP of Fc fragment using Monte Carlo modeling. Where, (a) 298.15 and (b) 277.15K, respectively.

Table 2. Structural properties of Fc fragment by means AMBER force field.

Properties Gibbs free energy (kcal/mol) Dipole moment (Debyes-D) Surface area (Å2) Volume (Å3) Mass (amu) Log P

Fc

Fc

(298.15 K) 478.397 1,7290 25,877.80 40,016.99 24,483.99 - 130.790

(277.15K, pH 7) 5,846.13 0 25,775.73 40,074.38 24,594.85 - 166.650

3. Results and Discussion Table 3 shows the structural properties of the ratios 10/1:1 and 40/1:1 of the DMA/(NAS:MG) polymeric matrix calculated at 298K using the AMBER/PM3 hybrid model. It is appreciated that the free energy of Gibbs (ΔG) is spontaneous because the interactions between the molecules generate a rearrangement of charges caused by the heteroatoms, in addition to the difference of the ΔG that can be seen in Tables 1 and 3[28,33]. The addition of the MG to Polímeros, 30(1), e2020007, 2020

the polymer generated the Michael reaction (nucleophilic) in the double bond of the structure of the melamide allows a reaction with the functional groups of DMA and NAS attributed to the electron loss of the carbonyls[34].

Log P negative determined that the permeation of the hydroxyl group and the amount of carbonyl groups increased the solubility in the polymer chain, being observed in an electronic distribution in red color as shown in Figure 3[35].

Table 4 shows the structural properties of the crosslinking of the Fc with the 10 and 40 units of the polymeric matrix applying the Monte Carlo modeling at 277.15K and a pH of 7 were used to control the stability of the melaimide[36].

Gibbs free energy indicated that there is a crosslinking process between Fc and polymer matrix attributed to the nucleophilic and polar character of the Cys (Cysteine) aminoacid that allows acting as a precursor in the formation of disulfide bonds, hydrogen bonds and van der Waals forces present in the Fc fragment on the surface in the polymer matrix producing a molecular rearrangement of the nucleophilic zones first and then the electrophilic zones according to the Michael reaction of thiol groups and maleimides as 3/8

Vázquez, N. A. R. well as an increase in the surface area that allowed the crosslinking[37,38].

On the other hand, the aminoacids on Fc are bonded to the polymer by means amino and carboxylic groups producing an amide bond called alpha-aminoacid[39]. Finally, the sulfhydryl groups of the Cys aminoacids maintain the stability of the Fc fragment avoiding the oxidation process in addition to carrying out the bioconjugation reactions with the melaimide group of the polymer[40,41].

Figure 4 shows that the hydrogen bonding interactions were located in the CH2 domain, however it is appreciated that there is a decrease in the molecular distance that causes an increase in the dipole-dipole, hydrophilic, hydrophobic, and ionic interactions producing a change in the potential surface energy that generates the electrophilic zones in red coloration, while the nucleophilic zones in blue coloration[42,43].

Table 3. Structural properties of the polymer synthesized with DMA, NAS and MG. Polymer (DAM)/(NAS-MG) Gibbs free energy - ΔG(kcal/mol) Dipole moment (Debyes) Surface area (Å2) Volume (Å3) Mass (amu) Log P

10/(1:1) units 40/(1:1) units 93.2824 341.116 0 0 1,543.32 4,863.53 3,250.29 10,918.87 1,271.64 4,245.63 - 0.390 - 7.870

Table 5 shows that the adsorption of the terbium ion (Tb+3) at 277.15K and a pH of 7 presents a Gibbs free energy that indicates a rearrangement due to the change in surface area and volume, attributed to the transfer of intramolecular energy originated by the overlap of the resonance energy of the Tb+3 and the energy of the triplet of the aminoacids presents in the Fc fragment[44,45].

In addition, the presence of covalent interactions corresponding to Cys aminoacids that allow crosslinking with Tb+3[46] as well as van der Waals forces, hydrogen bonds and ionic bonds. On the other hand, the melaimide group in the polymeric matrix allows carrying out the adsorption of thiols by the Cys aminoacids due to the formation of a thioether bond[47,48].

Log P indicates the hydrophilic character attributed to the Cys aminoacid due to the sulfhydryl group and the hydroxyls present in the side chain of polar aminoacids such as Tyr (Tyrosine), Trp (Tryptophan) and Gln (Glutamine)[48-50].

The control of the temperature at 277.15K is carried out to avoid a total deployment of the CH2 domains that would generate a decrease in the adsorption of Tb+3 as well as a degradation of the polymeric matrix and conformational changes of the Fc fragment[44,51,52].

Figure 5 shows that the interactions with the Tb+3 ion produce α-helix and β-sheet structure of the Fc fragment attributed to the increase in structural stability that maintains the conformation[23,38]. In addition, the electron density

Table 4. Properties of crosslinking about of Fc/polymer matrix using Monte Carlo modeling.

Relation: Fc/DMA/(NAS:MG) Properties/Temperature Gibbs free energy (kcal/mol) Dipole moment (Debyes-D) Surface area (Å2) Volume (Å3) Mass (amu) Log P

298.15K 497.398 1,923 4,672.26 10,627.59 4,342.780 - 5.42

1/10/(1:1) 277.15K, pH7 5,309.23 0 34,403.61 36,775.80 25,965.62 - 201.53

298.15K 3,245.65 2,771 26,882.63 36,838.94 28,826.77 - 253.04

1/40/(1:1) 277.15K, pH7 9,686.59 0 39,803.36 39,085.50 28,948.71 - 298.48

Figure 3. MESP where (a)10/(1:1) and (b) 40/(1:1) ratios of DMA/(NAS:MG) obtained by the AMBER/PM3 hybrid model.

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Adsorption of terbium ion on Fc/dymethylacrylamide: application of Monte Carlo simulation

Figure 4. MESP of the Fc fragment using the Monte Carlo modeling where, (a) 10 and (b) 40 units of polymeric matrix (DMA/NAS:MG).

Figure 5. MESP of the adsorption of the Tb+3 using the Monte Carlo modeling where (a) 10 and (b) 40 units of polymeric matrix (DMA/NAS:MG). Terbium ion is denoted in black color.

Table 5. Structural properties of adsorption process of de Tb+3 using Monte Carlo modelling.

Properties Gibbs free energy (kcal/mol) Dipole moment (Debyes) Surface area (Å2) Volume (Å3) Mass (amu) Log P

Polímeros, 30(1), e2020007, 2020

Fc/Tb+3 7,643.43 0 33,274.34 40,080.84 28,974.39 - 1,047.15

distribution indicates the presence of an electronic repulsion due to the polarization effected of the Tb+3 ion obtaining a geometry of the Fc fragment of the flat-on type[25,53]. On the other hand, the carbonyl groups form bonds with the Tb+3 due to the transfer of electrons to the amino groups attributed to the delocalized electrons[54,55]. Furthermore, pH control at 7 is carried out because at acid levels the aminoacid chain are protonated, that is, an electrostatic repulsion is generated, thus producing the denaturation of Fc[56].

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VĂĄzquez, N. A. R.

4. Conclusions The Monte Carlo simulation allowed to analyze first the crosslinking between the polymer matrix and the crystallizable fragment (Fc) and then the adsorption of the terbium ion (Tb+3) at 277.15K and a pH of 7, observing that the Gibbs free energies indicated a spontaneous process of the adsorption of the terbium ion using the Cys aminoacid.

In addition, the partition coefficient (Log P) determined that there is solubility in the adsorption attributed to the sulfhydryl and hydroxyl groups present in the Fc and through the melaimide group it was established that there is greater adsorption of Fc in the polymeric matrix. Conformational surface changes were determined due to the van der Waals forces that originated a flat-on structure of the Fc as well as hydrophilic and hydrophobic interactions and hydrogen bonds that were verified by changes in the electronic distribution of the MESPs where the increase of the nucleophilic zones and subsequently the electrophilic zones is appreciated as established in the Michael reaction.

5. Acknowledgements I would like to say, thanks so much at Dr. Edgar Arriaga from School of Chemistry-University of Minnesota (Twin Cities) for all the support during the stay.

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Received: Jan. 09, 2020 Revised: Mar. 27, 2020 Accepted: Apr. 14, 2020

Polímeros, 30(1), e2020007, 2020

ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.02020

Electropolymerization of polyaniline nanowires on poly(2hydroxyethyl methacrylate) coated Platinum electrode Maria Fernanda Xavier Pinto Medeiros1, Maria Elena Leyva1,2* , Alvaro Antonio Alencar de Queiroz2 and Liliam Becheran Maron3 Instituto de Físico Química, Universidade Federal de Itajubá – UNIFEI, Itajubá, MG, Brasil Laboratório de Alta Tensão, Universidade Federal de Itajubá – UNIFEI, Itajubá, MG, Brasil 3 Instituto de Ciencia y Tecnología de Materiales, Universidad de la Habana – UH, Calle Zapata, Habana, Cuba 1

2

*mariae@unifei.edu.br

Abstract A platinum electrode (Pt) was coated with poly(2-hydroxyethyl methacrylate) (PHEMA) by electrochemical polymerization using chronopotentiometry. Electropolymerization of polyaniline nanowires doped with camphorsulfonic acid (PANI:CSA) was further performed on the surface of the Pt-PHEMA electrode by cyclic voltammetry. The coated Pt-PHEMA-PANI:CSA electrode was characterized by Fourier transform infrared spectroscopy (FTIR), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and scanning electron microscopy (SEM). According to EIS, the Pt-PHEMA electrode exhibits a charge transport resistance (Rct) of 169.19 kΩ. The EIS analysis of Pt-PHEMA-PANI:CSA electrode reveals a less resistive character (Rct=1.28 Ω) than the observed for the Pt electrode coated with PANI:CSA (Rct=0.47 kΩ).

As demonstrated by SEM, the Pt-PHEMA-PANI:CSA electrode has a high surface area due to the PANI:CSA nanowires embedded in Pt-PHEMA. The biocompatibility of PHEMA, allied to the electrochemical characteristics of PANI:CSA, could be useful to the development of implantable electrodes for biomedical applications.

Keywords: electroactive hydrogels, chronopotentiometry, cyclic voltammetry, polyaniline, poly(2-hydroxyethyl methacrylate).

How to cite: Medeiros, M. F. X. P., Leyva, M. E., Queiroz, A. A. A., & Maron, L. B. (2020). Electropolymerization of polyaniline nanowires on poly(2-hydroxyethyl methacrylate) coated Platinum electrode. Polímeros: Ciência e Tecnologia, 30(1), e2020008. https://doi.org/10.1590/0104-1428.02020

1. Introduction There has been recently an increasing interest in the development of soft implantable microelectrodes for medicine[1-3]. The long-term implementation of this technology, however, has not yet been achieved due to practical issues that can be mainly related to the biological tissue response at the microelectrode interface[4-7].

Electroconductive hydrogels (ECH) have shown significant potential for the design of implantable microelectrodes for medicine, bringing together the redox switching and the electrical properties of inherently conductive electroactive polymers with high hydration levels and biocompatibility properties, providing a soft and conductive interface[8-12].

When compared to conventional metal electrodes, implantable microelectrodes coated with ECH exhibit low interfacial impedance at the biological tissue interface and provide the necessary biocompatibility for long-term implantation[13-15].

Several research groups have strategically explored the electrochemical synthesis (ECS) of poly(2-hydroxyethyl methacrylate) (PHEMA) from the monomer 2-hydroxyethyl methacrylate (HEMA) at metallic surfaces to reduce inflammatory reactions at the biological interface tissue-titanium[16,17]. At this interface, however, leachable chemicals from PHEMA, such

Polímeros, 30(1), e2020008, 2020

as catalysts, chemical initiators, and organic solvents, were shown to cause cytotoxicity, resulting in severe biological effects ranging from the alteration of cellular transduction pathways and gene expression levels to cell transformation, mutagenesis, and cell death[18,19].

A key limitation for the preparation of effective implantable PHEMA microelectrodes capable of interfacing with living tissues and organs is related to the high resistance of this polymer. The incorporation of polymers containing a spatially extended π bonding system appears to represent a simple approach to overcome this limitation.

The combination of the conductive properties of polyaniline (PANI) with the hydrophilic nature of PHEMA makes PHEMA-PANI a dynamic and versatile ECH for biomedical purposes. The conductive emeraldine salt of PANI has attractive dedoping and redoping processes that increase conductivity, making it closer to that of metals or semiconductors, while offering an easy synthetic method and biocompatibility with biological tissues[20].

It is well known that the conductivity of PANI can be adjusted by doping with secondary agents such as camphorsulfonic

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O O O O O O O O O O O O O O O O

Medeiros, M. F. X. P., Leyva, M. E., Queiroz, A. A. A., & Maron, L. B. acid (CSA)[21]. Additionally, PANI nanofibers are obtained during the electrochemical polymerization of aniline in the presence of camphorsulfonic acid (CSA)[22]. It is believed that CSA anions have a “surfactant-like” property that induces the formation of aggregates in solution which act as supramolecular-templates for fibrillary PANI growth[22].

The present paper aims at the electropolymerization of PANI with fibrillary morphology by cyclic voltammetry and at its further embedding in PHEMA, synthesized through chronopotentiometric technique in an aqueous medium.

The PHEMA-PANI microstructure was characterized by various analytical techniques such as Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). Results on the electroactivity and electrical behavior of the PHEMA impregnated with fibrillary PANI molecules were respectively studied by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS).

2.Materials and Methods 2.1 Materials Monomer 2-hydroxyethyl methacrylate (HEMA), potassium chloride (KCl), camphorsulfonic acid (CSA), hydrochloric acid (HCl), and dimethylformamide (DMF) were obtained from Sigma-Aldrich (Brazil) and used as received. Aniline was purchased from Sigma-Aldrich (Brazil) and purified by vacuum distillation at 70 °C. Other chemicals were also of analytical grade and used without any further purification.

2.2 Electrodeposition of PHEMA hydrogel on Pt electrode (Pt-PHEMA) In the present study, the synthesis of PHEMA-PANI at the surface of platinum (Pt) electrodes was conducted in a two-step procedure. In the first step, PHEMA chains were electrodeposited at Pt electrodes (45.0 mm in length and 0.45 mm of diameter) by a chronopotentiometric technique using a Metrohm potentiostat/galvanostat model AUTOLAB PGSTAT302N (NOVA interface). The electrodeposition of PHEMA at the surface of Pt electrodes was conducted in an aqueous solution of 0.4 M HEMA monomer. The reactions were conducted at constant currents of 50, 125, and 200 mA for 1 hour at room temperature (25 ºC) and air atmosphere.

The PHEMA hydrogel polymerized on the Pt surface (PtPHEMA) was thoroughly washed with distilled water and dried under vacuum at 50 oC. The synthesis conditions for the first step were optimized to obtain PHEMA hydrogels with the best mechanical properties on the surface of Pt electrodes.

2.3 Electropolymerization of PANI on Pt-PHEMA electrode (Pt-PHEM-PANI:CSA) In the second step, the Pt-PHEMA electrode was immersed in an aqueous solution of 0.5 M aniline containing 1.0 M CSA. The electropolymerization of PANI was performed at a Metrohm potentiostat/galvanostat, model AUTOLAB PGSTAT302N (NOVA interface), via cyclic voltammetry in the potential range of 1.4 to 0.05 V and scan rate of 50 mV/s.

The Pt-PHEMA was used as the working electrode, a platinum wire as the counter electrode, and Ag/AgCl as the reference 2/7

electrode. After the reaction, the Pt-PHEMA-PANI:CSA was thoroughly washed with distilled water and dried in a vacuum oven at 50 ◦C until constant weight.

2.4 Characterizations Morphologies of Pt-PHEMA and Pt-PHEMA-PANI:CSA were studied by scanning electron microscopy (SEM).

SEM micrographs of the samples were performed using a scanning electron microscopy (Carl Zeiss Model EVO MA 15). The samples were previously coated with gold and analyzed using an applied tension of 10 to 15 kV. The microstructure of the PHEMA-PANI at the Pt interface was recorded in the range of 650–4000 cm−1 by Fourier transform infrared spectroscopy (FTIR) in a Perkin Elmer Spectrum 100 FTIR spectrometer equipped with an attenuated total reflectance accessory (ATR) and resolution of 4 cm-1.

Electrochemical characterization of Pt-HEMA-PANI:CSA electrodes was performed using a Metrohm Autolab PGSTAT302N potentiostat/galvanostat controlled by the NOVA software.

The Pt-PHEMA and Pt-PHEMA-PANI:CSA were used as working electrodes, Pt wire as the counter electrode, and Ag/AgCl as the reference electrode. The electrochemical behavior of Pt-PHEMA and Pt-PHEMA-PANI:CSA was studied by cyclic voltammetry from -0.2 to 1.0 V, at a scan rate of 50 mV/s considering 3 cycles.

The electrochemical impedance spectra of the samples were obtained potentiostatically; the AC signal was recorded in the frequency range from 0.1 to 103 Hz, and the DC potential was set to 0 V and used as input. Both cyclic voltammetry and electrochemical impedance spectroscopy (EIS) measurements were carried out in an aqueous solution of 1.0 M HCl. The electrochemical cell was kept in air atmosphere and at 25 oC during all experiments.

3. Results and Discussions 3.1 Chronopotentiometry deposition of PHEMA at Pt surface The polymerization of HEMA at Pt surface is an example of a free radical reaction in which water molecules undergo electrolysis to form hydroxyl radicals (HO•). The HO• radicals serve as initiators for the electropolymerization process by generating free radicals on the >C=C< bonds of HEMA[23-25]. During the propagation step, the crosslinking of PHEMA appears to form chains in a 3D framework due to the functional nature of HEMA monomers.

In this work, chronopotentiometric electrolysis of HEMA on Pt electrodes, at different applied currents, was investigated to search for the adequate hydrogen evolution (from water electrolysis) that allowed increasing the PHEMA coating thickness at the Pt surface. Figure 1 shows the potential waveform responses at different applied currents.

The experimental results (Figure 1) suggest that HEMA can be reduced at the cathode prior to the hydrogen reduction.

An organic deposit was rapidly formed at the cathodic surface after reduction of the monomer. The deposit appears to have low conductivity and causes a significant decrease in the Polímeros, 30(1), e2020008, 2020

Electropolymerization of polyaniline nanowires on poly(2-hydroxyethyl methacrylate) coated Platinum electrode electrolytic current (Figure 1). It was found that PHEMA is formed in appreciable yield at the Pt surface.

For comparison, PANI was also electrosynthesized using CSA at the surface of the Pt electrode in an aqueous medium. Figure 2 shows the cyclic voltammograms recorded during 14 cycles for the electropolymerization of PANI:CSA on the Pt electrode surface. The deposition of oxidized aniline starts from the first cycle at 0.9 V (Figure 2, peak B); in later cycles, the oxidation potential is seen at peak A (Figure 2, 0.34 V), suggesting the formation of a lowmolecular-mass cation radical. The cathodic peak (A*) is observed at 0.19 V. This A-A* redox couple corresponds to the redox pernigraniline-emeraldine process[26].

3.2 Electropolymerization of embedded PANI:CSA in Pt-PHEMA Incorporation of PANI:CSA into the framework of Pt-PHEMA is primarily driven by the osmotic pressure present across PHEMA on the Pt electrode surface and by the dipole-dipole interactions between CSA and carbonyl oxygen, giving an amphiphilic nature to PHEMA at the Pt electrode surface. Thus, during the electropolymerization process, PANI:CSA spreads into the interior and external surfaces of Pt-HEMA to form a channel for ionic conduction.

The ionic conduction channel suggests a “bottom-up” approach, in which nanowire PANI structures are built up from aniline molecules by self-assembly.

Figure 3 shows the cyclic voltammogram recorded during the electropolymerization of PANI:CSA embedded in Pt-PHEMA electrode. The oxidation of aniline starts from the first cycle at 0.9V but at a lower anodic current (1.4mA) if compared to that of aniline oxidized directly on the Pt electrode (3.2 mA, Figure 2). This difference can be explained considering that oxidation is directly proportional to the concentration of monomers next to the Pt-HEMA electrode surface.

Therefore, the diffusion of aniline through hydrogel in Pt-PHEMA decreases the monomer concentrations next to the electrode surface. It can be seen that the oxidation of aniline is also significant in the second cycle (Figure 3).

The second cycle shows the oxidation of cation-radicals at peak A (0.33 V), that exhibits a potential shift to more positive values during the electropolymerization process. The cathodic peak (A*, Figure 3) is observed at 0.19 V. The A-A* redox couple pernigraniline-emeraldine was observed at the same potential during the electropolymerization of PANI:CSA on Pt electrode. The only difference is seen at the lower current of both cathodic and anodic peaks (Figure 3). This observation could be due to the reduction in the aniline concentration that diffuses through Pt-PHEMA.

Figure 1. Potential waveform responses at 50 mA, 125 mA, and 200 mA vs. time for the chronopotentiometric deposition of PHEMA at Pt electrodes.

Figure 2. Cyclic voltammograms of electrochemical polymerization of aniline on Pt electrode in aqueous solution of 0.5 M aniline containing CSA at 1.0 M, in the potential range of 0.05 to 1.4 V, scan rate of 50 mV/s, and room temperature (25 oC). A – A* redox couple corresponds to pernigraniline – emeraldine process.

3.3 Structural analysis FTIR-ATR spectroscopy was used to characterize the chemical structure of Pt-PHEMA, Pt-PANI:CSA, and PtPHEMA-PANI:CSA. Figure 4 shows the FTIR spectra of Pt-PHEMA. The absorption bands at 1163 cm-1 (C-O stretch), 1500-1350 cm-1(C-H bend), 1731 cm-1 (C=O stretch), 2976 cm-1 (C-H stretch), and 3459 cm-1 (O-H stretch) are in good agreement with those given in previous works of Polímeros, 30(1), e2020008, 2020

Figure 3. Cyclic voltammograms of the growth of PANI:CSA on Pt-PHEMA electrode in aqueous solution of 0.5 M aniline containing CSA at 1.0 M, scan rate of 50mV/s, and scan limits from 0.05V to 1.4V. A – A* redox couple corresponds to the redox pernigraniline-emeraldine process.

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Medeiros, M. F. X. P., Leyva, M. E., Queiroz, A. A. A., & Maron, L. B.

Figure 4. FTIR spectra of Pt-PHEMA (a), Pt-PANI:CSA (b) and Pt-PHEMA-PANI:CSA (c).

PHEMA, confirming the presence of a thin layer of PHEMA at Pt electrode surface[27].

Figure 4b shows the FTIR spectra of Pt-PANI:CSA. The FTIR spectra show characteristic bands at 1700 cm-1 and 1160 cm-1 corresponding to the C=O and S=O stretching vibration, respectively. These bands confirm the presence of the camphorsulfonic acid and are in good consistency with the literature[28]. The quinoid and benzenoid absorption bands of PANI are seen at 1542 cm-1 and 1434 cm-1, respectively. Characteristic absorption bands of PANI were also observed (Figure 4b) at 1280 cm-1, attributed to C=N stretching of secondary amines. The characteristic bands observed in the FTIR spectra of electrodeposited PANI:CSA at the Pt electrode surface are in good agreement with the literature[29]. The FTIR spectra of Pt-PHEMA-PANI:CSA (Figure 4c) did not present significant changes in the PHEMA or PANI FTIR-spectra, suggesting that PANI:CSA nanowires could be dispersed in-situ in Pt-PHEMA electrode.

The SEM micrographs of the Pt-PHEMA, Pt-PANI:CSA, and Pt-PHEMA-PANI:CSA can be observed in Figure 5.

Morphology of Pt-PHEMA (Figure 5a) consists essentially of porous microstructures formed by aggregate granules of different shapes in the size of microns.

SEM micrographs of the Pt-PANI:CSA can be observed in Figure 5b. Morphology of PANI:CSA at Pt surface (Figure 5b) consists essentially of a heterogeneous microstructure formed by spheroidal granules in the range of microns alternating with fibrillary layers of PANI. Granules are organized in homogeneously distributed aggregates. The granular aggregates observed at the Pt-PANI:CSA surface suggest a characteristic morphological feature of PANI:CSA with a heterogeneous nucleation mechanism. The nanowire morphology of Pt-PHEMA-PANI:CSA can be clearly seen in Figure 5c.

These images depict that PANI:CSA has a strong effect on the Pt-PHEMA morphology. The PANI:CSA nanowires embedded in Pt-PHEMA were found to have average diameters of 40-60 nm. Although the growth of PANI 4/7

Figure 5. SEM micrographs of Pt-HEMA (a), Pt-PANI:CSA (b) and Pt-PHEMA-PANI:CSA (c) electrodes surfaces.

Polímeros, 30(1), e2020008, 2020

Electropolymerization of polyaniline nanowires on poly(2-hydroxyethyl methacrylate) coated Platinum electrode nanowires has no directional alignment, the fibrils are uniform, suggesting a growth by diffusion of aniline at the Pt-PHEMA interface into a less-dense aqueous phase containing the CSA doping acid.

3.4 Electrical and electrochemical behaviors

Figure 6. Cyclic voltammograms (CVs) of Pt-PANI:CSA and Pt-PHEMA-PANI:CSAin 1.0 M HCl aqueous solution at 20mV/s (from -0.2 V to 1.0 V). A - A* redox couple corresponds to emeraldine - leucoemeraldine process, B - B* redox couple corresponds to pernigraniline - emeraldine process.

Figure 6 shows the cyclic voltammetry (CV) curves of the modified Pt electrodes in the potential range of the two redox stages characteristic of polyaniline. The same redox couples were observed for both Pt-PANI:CSA and Pt-PHEMA-PANI:CSA electrodes. The first one (A*/A) was observed at lower potential values (0.05V/0.20V) and may be associated with the transition leucoemeraldine/emeraldine.

The second redox pair (B*/B) presented higher potentials values (0.73V/0.8V) and was associated with the transition emeraldine/pernigraniline. These two redox couples show a behavior similar to the observed in polyaniline films prepared by aniline electropolymerization in an aqueous solution containing inorganic acids[30]. CV of Pt-PHEMA-PANI:CSA confirmed that the hydrogel maintains the electroactive properties of pure PANI:CSA. It was found that the electrical conductivity of the Pt-PHEMA electrode was enhanced after the growth of PANI:CSA nanowires (Pt-HEMA-PANI).

To further understand the electron transfer resistance at the electrode/electrolyte interface, analyses of electrochemical impedance spectroscopy (EIS) were performed on Pt-PHEMA, Pt-PANI:CSA, and Pt-PHEMA-PANI:CSA. Nyquist plots obtained from EIS measurements are exhibited in Figure 7.

The observed semicircle indicates that the electron transfer at the surface of the electrode is a kinetically controlled process[28]. The diameter of the semicircle gives the resistance to the charge transfer (Rct) at the electrode surface. This resistance controls the kinetics of the electron transfer at the interface electrode/electrolyte[29]. The Rct for Pt-PHEMA was 169.19 kΩ and indicates that PHEMA hinders the charge transfer at the Pt electrode surface. After modifying the Pt electrode with PANI:CSA, the Rct decreased to 0.47 kΩ.; while, when PANI:CSA nanowires were growth in Pt-PHEMA, the Rct dramatically decreased to 1.28 Ω, which indicates that PANI:CSA plays a role similar to a conductive Pt wire and thus makes electron transfer easier.

The differences of Rct among Pt-PHEMA, Pt-PANI:CSA, and Pt-PHEMA-PANI:CSA indicate that the electroactive PHEMA-PANI:CSA hydrogel has been effectively attached on the Pt surface. Lower CPE capacitances were obtained for the PANI film on Pt electrode, which might be due to a thick PHEMA film formed at the Pt electrode surface (0.54 μF) if compared to Pt-PHEMA-PANI:CSA (9.54µF) and Pt-PANI:CSA (95.66 µF). The drop in the capacitance of Pt-PHEMA-PANI:CSA relatively to Pt-PANI:CSA appears to be related with the higher ionic diffusion in the microporous surface of Pt-PHEMA-PANI:CSA caused by the presence of larger pores, which lead to a decrease of active sites on the electrode surface.

4. Conclusions Figure 7. Nyquist diagrams for pHEMA-Pt (a), PANI.CSA-Pt (b) and PANI.CSA/pHEMA-Pt (c) in 1.0 M of HCl at frequency range 10 kHz - 100 MHz. Polímeros, 30(1), e2020008, 2020

In the present study PHEMA, PANI:CSA, and PHEMA-PANI:CSA were successfully synthesized by employing chronoamperometric and cyclic voltammetry techniques on Pt electrodes and characterized by FTIR, cyclic 5/7

Medeiros, M. F. X. P., Leyva, M. E., Queiroz, A. A. A., & Maron, L. B. voltammetry, electrochemical impedance spectroscopy, and SEM. FTIR results showed that PHEMA can be efficiently electropolymerized by chronopotentiometry on the Pt electrode. In such conditions, Pt-PHEMA showed high values of Rct resistance, exhibiting a highly porous structure.

The PANI:CSA electropolymerized on Pt electrode presented a heterogeneous morphology formed by a dense network.

EIS analyses showed a low Rct resistance in PANI.CSA-Pt. The Pt-PHEMA-PANI:CSA showed a less resistive character (lower Rct) than PANI:CSA. The cyclic voltammetry results indicate that Pt-PHEMA-PANI:CSA keeps the electroactive character of Pt-PANI:CSA.

5. Acknowledgements We would like to acknowledge the Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG), Conselho Nacional de Desenvolvimento Cientifico e Tecnológico (CNPq), Coordenacão de Aperfeiçoamento de Pessoal de Ensino Superior (CAPES), for the financial support of this project.

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Received: Feb. 13, 2020 Revised: Apr. 24, 2020 Accepted: Apr. 29, 2020

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ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.05518

Disposable coffee capsules as a source of recycled polypropylene Michel Lincoln Bueno Domingues1, Jean Rodrigo Bocca1, Silvia Luciana Fávaro1 and Eduardo Radovanovic2*  Departamento de Engenharia Mecânica – DEM, Universidade Estadual de Maringá - UEM, Maringá, PR, Brasil 2 Departamento de Química – DQI, Universidade Estadual de Maringá – UEM, Maringá, PR, Brasil

1

*eradovanovic@uem.br

Abstract In this paper was investigated the chemical, physical, thermal, mechanical and morphological characteristics of the recyclable materials obtained of the NESCAFÉ DOLCE GUSTO branded beverage capsules, characterizing different resulting compositions. The characterization was made by following techniques: Fourier-transform infrared spectroscopy (FTIR), X-ray Diffraction (XRD), Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), Scanning Electron Microscopy (SEM), Water Absorption and techniques for the analysis of mechanical properties (tensile and impact test). The results showed that the body of the capsule and the inner filter, both made of polypropylene, are the most interesting materials to be reused, having good properties, while the materials resulting from the mixtures of all the constituents in the beverage capsules presented decreased mechanical properties.

Keywords: disposable capsules, polypropylene, recycling, mechanical properties.

How to cite: Domingues, M. L. B., Bocca, J. R., Fávaro, S. L., & Radovanovic, E. (2020). Disposable coffee capsules as a source of recycled polypropylene. Polímeros: Ciência e Tecnologia, 30(1), e2020009. https://doi.org/10.1590/01041428.05518

1. Introduction Since 2010, when the patents which regulated coffee capsules and their machines, especially Nespresso ones, began to expire[1,2], the capsules coffee consumption grew a lot in Brazil. This growth further increased when, in 2015, some manufacturers of coffee capsules settled in Brazil[2]; therewith, the products were no longer imported and became more accessible[3]. Because of the rush of daily life, the arising of espresso capsules made daily life easier. With a very simple system to use, providing individual portions with a variety of flavors, quality and affordability, the Brazilian Coffee Industry Association (ABIC)[4] estimates that, up to 2020, the consumption of the beverage will grow 17.5 percent a year. This number represented 12000 tons of capsules only in 2017, and reached 16000 tons in 2019.

The growth was so fast that it seems that the manufacturers did not have time to prepare themselves for the environmental problem caused by the incorrect disposal of the capsules.

The small capsules, which typically take in their composition plastic, aluminum, and organic material (coffee, milk, chocolate, etc), are not easily recycled. Technically they would be, if people who own these machines had the time and predisposition to separate the parts of the capsules and collect the aluminum, the plastic and the organic material separately, before throwing them into the trash. But, given the idea that those who buy this type of product does it precisely to save time, it is highly unlikely for it to happen.

Polímeros, 30(1), e2020009, 2020

Companies claim that their products are recyclable, but the problem is that, for this to happen, the capsules need to be collected and the recyclable materials need to be treated separately, and, even having ability to recycle 60 % to 75 % of the capsules sold in Brazil, the rate of return is only 11 %, according to the Cafeicultura magazine[5].

The problem is so serious that, in the year 2016, the German city of Hamburg banned the consumption of coffee in capsules in public agencies, citing the difficulty of recycling the waste as the reason of the decision.

In Brazil, the National Policy on Solid Waste[6] says that the manufacturer is responsible for the destination of the waste originated from its products, and the companies, in compliance with the legislation, must have collection points, but those are concentrated only in the main cities of the country.

At present, the mechanical recycling is the most used one[7] and it consists of the selective collection, screening of the polymers with the best purity, grinding, washing with water (containing detergent or not), drying and reprocessing[8], but due to the complexity of the capsules, often a mixture of plastic and aluminum, combined with the organic residues of coffee or other compounds according to the product, it becomes difficult to have the standard recycling process present in municipal organs. In this way, even though they are made of totally recyclable materials, the destination of the capsules are the common trash cans, most of the times,

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Domingues, M. L. B., Bocca, J. R., Fávaro, S. L., & Radovanovic, E. since there are not specific collection points throughout the country[9].

Polypropylene is one major plastic that is used to produce a lot of single-use products. PET and polyethylene must be included in this category also[10]. In case of PP, the main strategy to reduce its presence in landfills is the mechanical recycling[11]. In general, the products made with recycled PP present downgrade properties, and must be used in less demanding applications. The decrease in properties is not only due to heating process of mechanical recycling but based on the source of PP. PP containing other plastics and contaminants, is the main drawback to its properties[12]. To minimize this fact a lot of researches were developed producing composites, nanocomposites, blends with other polymers and with raw PP, changing the melt-flow index (MFI) characteristics with additives, etc.

[13,14] In this case, mechanical properties, thermal properties, morphology, durability and process ability are studied and compared with raw materials. Another way to improve the characteristics of products made from recycled PP is to ensure its origin. Three questions are important: is the PP a only one component, is the PP clean or contaminated with inorganic components, and, if PP came from a blend, is the blend composition well known?[15] Kozderka et al.[16] studied the recycling of polypropylene in the automotive industry. The authors concluded that the main difference between virgin and recycled high impact polypropylene used in this industry is based mainly in the production process itself and higher electricity consumption for recycling, with no deterioration in properties, even before the 6th reprocessing. Luna et al.[17] studied blends of copolymer polypropylene and recycled copolymer polypropylene obtained from industrial containers. They concluded that blends could contain up to 60 % of recycled material by

weight, maintaining quality and reducing the product final cost. Take this into account, this paper presents a proposal for separating all the components of a single-use coffee capsule.

Moreover, will be presented the production of mixtures containing these components through the extrusion process and the characterization of materials formed by different combinations of polypropylene, PET and aluminum, in order to enable final materials with different compositions and properties, with new possibilities for reusing this material that currently piles up in landfills worldwide.

2. Experimental Section 2.1 Materials To the characterization of the materials, polymeric specimens were obtained after disassembling the Dolce Gusto NESCAFÉ capsules, and these specimens were produced through the extrusion processes followed by injection.

The different parts of the capsules (body, lid, aluminum filter and plastic filter), which are shown in Figure 1, were separated and constructed six different materials with different compositions of the constituent materials.

2.2 Methodology The components of the capsules were grounded in a knife mill to particle size of about 2 mm and proportionally blended according to Table 1. The samples were processed in a Thermo Scientific MiniLab II HAAKE Rheomex CTW 5 twin-screw extruder, with conical screws, operating in the corrotation mode, using a screw temperature of 190 °C, at a speed of 70 rpm in a 5 min recirculation time.

The test specimens for the mechanical testing were processed in a Thermo Scientific HAAKE MiniJet II injection

Figure 1. (a) Body, (b) Lid, (c) Aluminum filter and (d) Plastic filter of the disposable capsules.

Table 1. Composition of the test specimens concerning their mass. sample A1 A2 A3 A4 A5 A6

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Composition Body of the capsule body + plastic filter body + plastic filter + aluminium filter body + plastic filter + aluminium filter + lid body + plastic filter + lid plastic filter

Body 96.4% 72.0% 67.9% 65.4% 66.6% -

Lid 2.5% 1.9% 7.4% 9.1% 9.3% -

Aluminium filter 1.1% 0.8% 1.8% 2.6% 0.8% -

Plastic filter 25.3% 22.9% 22.9% 23.3% 100%

Polímeros, 30(1), e2020009, 2020

Disposable coffee capsules as a source of recycled polypropylene molding machine, with a cannon temperature of 210 °C, a mold temperature of 40 °C, an injection pressure of 450 bar, an injection time of 15 s, holding pressure of 300 bar and holding pressure time of 30 s. The dimensions have followed the standard test method ASTM D638-14 for the tension, the ASTM D256 for the Izod Impact and the ASTM D570-98 for the water absorption specimens. 7 test specimens were prepared for the tensile testing, 5 test specimens for the impact test and 3 specimens for the water absorption test for each sample.

The materials characterization was performed through the following techniques: Fourier-transform infrared spectroscopy (FTIR) in a HATR accessory in a Thermo Fisher Scientific Nicolet iS10 FTIR equipment, in the spectral window of 4000-400 cm-1, in a resolution of 2 cm-1.

X-ray diffraction (XRD) in a Shimadzu D6000 equipment using Cu source, 40 kV voltage, a current of 30 mA, 2 °/min speed and angle of incidence, 2ϴ, between 10 ° and 40 °.

Differential scanning calorimetry (DSC), to determine the crystalline melting temperature (Tm) and the degree of crystallinity (χc) in the Thermal Analyzer Q20(TA Instruments) equipment under an atmosphere of nitrogen at 50 mL/min, with a heating rate of 10 °C/min, at the temperature range from 30 to 300 °C. Scanning electron microscopy (SEM), in which the fractured samples by the IZOD impact test were previously metallized with gold and observed in a Quanta FEI equipment, operating at 10 kV. Thermogravimetric analysis (TGA) was performed with the Thermal Analyzer Q50 (TA Instruments) equipment, operating at a heating rate of 10 °C/min from room temperature to 430 °C, with compressed air flow at 50 mL/min. Water absorption according to the standard test method ASTM D570 and Mechanical Analysis, in which tension tests were performed in a universal testing machine EMIC DL 10000 with 5000 N load cell, in accordance with ASTM D638, at a speed of 10 mm/min and impact resistance tests, carried out in a CEAST Resil Impactor Junior equipment with a 2.75 J pendulum, according to ASTM D256.

3. Results and Discussion After separating the materials present in the disposable capsules, six different sample compositions, named herein A1, A2, A3, A4, A5 and A6 (Table 1), were prepared.

The samples were not produced randomly; they were constituted by the possibilities of mixture among the beverage capsules materials, having samples composed only by the body of the capsule (A1), the body plus the plastic filter (A2), the body plus the plastic filter and the aluminum filter (A3), by the complete capsule (A4), the body plus the plastic filter and the lid (A5) and, finally, only by the plastic filter (A6).

that the main polymer that forms the parts of the capsule is the polypropylene, a fact confirmed by the presence of the characteristic peaks of asymmetric and symmetric stretch of the C-H bond of CH3 groups (asymmetric at 2950 cm-1 and symmetric at 2865 cm-1), CH2 (asymmetric at 2916 cm-1 and symmetric at 2837 cm-1) and CH (shoulder at ~2900 cm-1), as well as characteristic peaks of deformation vibrations in the CH3 groups at 1454 cm-1, CH2 at 1375 cm-1 and CH at 1358 cm-1, as described in the literature[18].

Thereby, both the capsule, the inner filter and the inner part of the lid are made of polypropylene. The external part of the lidding film is composed of PET, through the comparison of its spectrum with spectra of the literature[19].

This formation of the PP/PET double layer film has the function of sealing the capsule through the fusion and adhesion of the lidding film of PP to the PP of the capsule, as well as the function of a gas barrier, acting on sealing the coffee on the inside of the capsule, generated by PET. A detail to be noted in the FTIR analysis of the polypropylene (PP) of the capsule is the presence of small peaks in the region of 1742 cm-1. Peaks in this region are typical of C=O groups, and are not expected to PP. The PP of the filter and the lidding film does not exhibit these peaks. One possibility of allocation of these peaks is that

Figure 2. FTIR-HATR spectra of the body of the disposable capsules.

3.1 FTIR-HATR analysis The FTIR-HATR analyses were performed on the surface of the constituent parts of the Dolce Gusto NESCAFÉ disposable capsule, namely the body of the capsule, the lidding film and the inner plastic filter. Both the body and the lidding film were analyzed on both sides, internal and external.

Figure 2 and 3 show the FTIR spectra of these parts present in the disposable capsule. Through these analyses, it is clear Polímeros, 30(1), e2020009, 2020

Figure 3. FTIR-HATR spectra of the lidding film - internal and external side - of the disposable capsules.

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Domingues, M. L. B., Bocca, J. R., Fávaro, S. L., & Radovanovic, E. they belong to a second phase[20], probably an elastomeric phase. No other characteristic peaks of this component were identified in the spectrum, probably because they are less intense or in coincidence with PP peaks. Typical elastomeric phases that present C=O groups are thermoplastic copolyester elastomers and thermoplastic polyurethanes[21].

3.2 X-ray diffraction (XRD) analyses The results of the X-ray diffraction analyses of the samples (A1, A2, A3, A4, A5 and A6) are shown in Figure 4. It is possible to observe the presence of the peaks corresponding

to the crystallographic plans (110), (040), (130), (111), (131) and (041) very close to the Bragg angles (2θ): 14.2 °, 17.0 °, 18.5 °, 21.3 ° and 22.0 °, which are typical of the isotactic polypropylene[22,23]. Table 2 shows the exact angles presented by each sample.

The variation of the diffraction angles may be related to changes in the crystalline structure of the material, but it may also be related to the difference in the positioning and thickness of the samples in the XRD sample holder, which would cause a displacement in the acquisition angle. It was only possible to identify a very small peak at ~38 º related to the presence of metallic aluminium[24], probably due to the low amount of this material in the mixture. The peaks relative to the planes (110) and (040) presented a relation of similar intensities i​​ n the samples from A1 to A5, however the sample A6 presented a much more intense peak relative to the plane (040), what indicates a difference in the crystalline organization of this material in relation to the other parts of the capsule, concerning the polypropylene. The sample A6 consists only of PP of the internal filter, and it should be remembered that this material presented FTIR spectrum with no signals related to the C=O groups.

3.3 Differential Scanning Calorimetry (DSC) analyses

Figure 4. XRD analyses of samples A1 to A6.

Figure 5. DSC analyses of A1, A2, A3, A4, A5 and A6 samples.

The results of the DSC analyses are shown in Figure 5 and Table 3. Through them, it is possible to verify that there were no significant changes in the melting temperature of the samples A1 to A5, however, the sample A6 presented a significantly higher melting temperature, ranging from 2 to 3 ºC, in the comparison with the other samples.

As to the degree of crystallinity, samples A1, A2, A3 and A4 presented close values, showing that the presence of the aluminum filter, the plastic lid and the polypropylene filter does not change drastically the characteristics of crystallinity of these samples. It should be noted that both the plastic lid and the filter are also mostly made of polypropylene.

However, the sample A6 presented a high degree of crystallinity, which agrees with what was expected, because it was pure PP, as shown by the FTIR results. In addition, samples from A1 to A5 presented a small endothermic peak between 175 and 185 ºC, a little higher than the melting temperature of the PP, what may be related to the presence of second phase material on these samples. As elastomers

Table 2. Angles of the crystalline planes of the samples A1 to A6.

Plans (110) (040) (130) (111) (131) (041)

Isotactic PP 14.20º 17.00º 18.50º 21.30º 22.00º

A1 14.48º 17.26º 18.90º 21.50º 22.12º

A2 14.32º 17.12º 18.78º 21.38º 22.02º

A3 14.38º 17.18º 18.78º 21.40º 22.06º

A4 14.28º 17.08º 18.76º 21.42º 22.02

A5 14.80º 17.56º 19.22º 21.80º 22.52

A6 14.28º 17.04º 17.86º 21.32º 22.00º

Table 3. Sample melting temperatures.

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Sample

Tm(ºC)

ΔH (J/g)

Crystallinity (%)

A1 A2 A3 A4 A5 A6

164.3 165.6 165.9 164.3 164.8 167.5

67 65 62 70 63 108

40 39 38 42 38 65

Polímeros, 30(1), e2020009, 2020

Disposable coffee capsules as a source of recycled polypropylene for plastics are generally another polymer, this peak can be related to the melting of the second phase. Generally, the presence of a dispersed elastomeric phase in a PP matrix contributes to toughening[25,26]. The samples A2 and A5 showed exothermic peaks around 225 ºC, and they may be related to the process of cold crystallization of amorphous PET[16].

PET can be present in all samples (except A6) by adding the lids containing this polymer, or even the small amount of PET remaining adhered to the capsule after cutting the central part of the lid, separating it from the body of the capsule. As the DSC analysis involves a small fraction of the sample (~10 mg), the presence or absence of PET, which is found in small quantities, is random. As already reported, the presence of a second phase or other contaminants in PP decrease the crystallinity to values close to 40 %[13,19,23], as observed here to samples A1 to A5 (Table 3). The pure PP (A6) has the higher crystallinity, 65 %, in accordance with other recycled PP[17].

3.4 Scanning electron microscopy Scanning electron micrographs of sample A1 (body of the capsule) are shown in Figure 6A and 6B. These images were acquired from the broken face of the specimens submitted to the Izod test procedure. Figure 6A shows a kind of fracture typical of a brittle-fracture plastic material. This result is expected for the Izod rupture test, a test in which the process of rupture of the specimen occurs in a truly short time, with no possibility of elastic deformation of PP. In addition, in Figure 6B, it is observed the presence of a second phase, formed by small wired particles not adhered to the PP matrix. This material may be related to the phase attributed to the presence of the elastomer, previously identified in the FTIR and DSC techniques. Second phase particles were pulled out during the Izod test, then is possible to observe

well-adhered particles in the PP matrix and multiple voids can be identified in the SEM images in samples A1 to A5.

The scanning electron micrographs of samples A2, A3, A4 and A5 showed a behavior that is similar to the one presented by sample A1, that is, the separation of phase with a fibrillar and ball shaped second phase, however, some differences were observed, such as the presence of aluminum in the sample A3 and PET in A4 and A5, as observed in the Figures 7A and 7B, respectively. It may be noted that the amount of balls or wires has decreased.

The scanning electron micrograph of sample A6 (plastic filter of the capsule) is presented in Figure 8. It was observed, in this in case, the complete absence of a second phase (elastomeric), and a typical fracture of PP submitted to the impact Izod.

In Figure 9, an image (a) and a spectrum (b) of analysis

by EDS are presented, and a typical second phase particle present in the PP matrix was analyzed. It can be observed through the EDS spectrum that this material is full of oxygen atoms, as well as carbon. This analysis comes together and corroborates the possibility that this second phase is an elastomeric material. Other parts that presented the second phase were also analyzed, and, in all analyzes, it was possible to identify the presence of oxygen, what did not occur in the analysis of the PP matrix.

3.5 Thermogravimetric Analysis (TGA) The thermal analyzes of samples A1, A2, A3, A4, A5 and A6 are shown in Figure 10. It was found that the samples composed of the mixtures, A1 to A5, showed an increase in the thermal degradation temperature of around 20 to 30 ºC at 50 % of mass loss, as compared to the sample containing PP only (A6). The presence of PET and aluminum may have acted to retard the thermal degradation

Figure 6. Scanning electron microscopy of the fractured region of sample A1.

Polímeros, 30(1), e2020009, 2020

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Domingues, M. L. B., Bocca, J. R., FĂĄvaro, S. L., & Radovanovic, E.

Figure 7. Scanning electron microscopy of the fractured region of sample A3 (A) and A4 (B).

Figure 8. Scanning electron microscopy of the fractured region of sample A6.

Figure 9. EDS analysis of sample A1. (a) image and (b) spectrum.

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PolĂ­meros, 30(1), e2020009, 2020

Disposable coffee capsules as a source of recycled polypropylene process of PP. The amount of final waste, not degraded, gives an indication of the amount of aluminum present in composites, and it is always less than 5 % concerning the initial mass. The curves relative to TGA analysis of A2 and A5 samples were suppressed from the Figure 10.

They are in coincidence with results to A1 sample. Some researches stated that the enhancement of thermal stability of the PP in nanocomposites can be attributed to their higher degree of crystallinity[27,28], however, in samples A1 to A5, the crystallinity is lower compared to A6 sample. A reasonable explanation to the higher thermal stability of impure PP is that the presence of aluminium in contact with PP could inhibit the degradation of polypropylene and catalyze the formation of carbonaceous char on the surface, significantly improving the thermal stability.

are evident in Table 5, which presents values obtained from the tensile test versus strain for the six samples.

The Young’s Modulus values of ​​ the six samples are below the expected for recycled PP[13,16], probably due to the use of elastomeric phase that cause some changes in the final properties of the products, among them the reduction of stiffness, or the poor adhesion between PP and aluminium or PET. Samples A3 and A4 showed the largest differences in relation to the average Young’s Modulus values of all samples, what may be a consequence of the poor interaction between the matrix phase, polypropylene and the dispersed phase, aluminum, in the case of sample A3. However, for the

3.6 Water absorption In accordance with the standard test method ASTM D570, for the analysis of the water absorption in the samples, a first weighing of the samples was performed after a period of one week and put again in a thermostatic bath for another week, in which the new weighing presented an increase of less than 1 % in its weight, characterizing its saturation.

As observed in Table 4, there was a small gain of water weight in all the samples, not exceeding 0.3 % in any case.

The sample which presented a lower water absorption was A6, followed by A5 and A1, a fact that was already expected for the samples A6 and A1, since A6 is composed exclusively of PP and A1 presents mostly PP, and the process of water absorption is probably related to the presence of aluminum, PET and defects created by these materials in the PP matrix.

Figure 10. Thermogravimetric analysis of samples A1, A3, A4 and A6.

3.7 Mechanical analysis The stress-strain curves (typical) of the six samples analyzed by the tensile testing are shown in Figure 11.

Similar behavior is clearly observed in relation to the tensile strength in the six samples; however, the maximum strain varies greatly.

As it deals with blends or even composites, if elastomeric, aluminum and PET phases are considered as materials that interfere with the mechanical properties of the PP matrix, the interface and the adhesion between the phases and reinforcement mechanisms influenced the mechanical properties causing variations in those ones. These variations

Figure 11. Stress and Strain typical-curves for A1 to A6 samples.

Table 4. Water absorption analysis of samples A1 to A6.

Sample Average absorption after 1 week Average absorption after 2 weeks

A1 0.20% 0.25%

A2 0.26% 0.30%

A3 0.24% 0.28%

A4 0.24% 0.28%

A5 0.20% 0.24%

A6 0.15% 0.19%

Table 5. Values of ​​ tensile strength (σ), Young’s Modulus (E), Yield point (σf), ductility in terms of strain (ε), toughness (ut) and resilience (ur).

Sample

σ máx(MPa)

E (MPa)

A1 A2 A3 A4 A5 A6

30.2 (±0.7) 30.9 (±0.7) 32.7 (±0.4) 29.1 (±1.4) 29.5 (±1.4) 34.3 (±0.4)

792 (±33) 801 (±12) 401 (±17) 1157 (±11) 655 (±9) 866 (±36)

Polímeros, 30(1), e2020009, 2020

σf(MPa)

ε(%)

ut(J/m3)

ur(J/m3)

24.4 (±1.4) 11.4 (±3.4) 8.6 (±2.9) 23.0 (±1.4) 25.8 (±1.5) 31.5 (±1.7)

497.3 (±106.4) 201.6 (±75.0) 222.1 (±23.0) 162.9 (±32.3) 120.3 (±20.6) 611.2 (±146.0)

102.0 (±30.8) 27.6 (±10.9) 30.2 (±3.9) 23.6 (±4.5) 17.1 (±2.9) 117.7 (±30.5)

3.8 (±0.5) 0.9 (±0.5) 1.4 (±1.3) 2.2 (±0.6) 5.2 (±0.7) 5.9 (±0.5)

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Domingues, M. L. B., Bocca, J. R., Fávaro, S. L., & Radovanovic, E. A4 sample, the increase in the amount of PET and aluminum was predominant in the Young’s Modulus properties, with a significant increasing in the value, even when compared to the sample with pure PP. The sample A6 was the one that presented the smallest discrepancy among the values​​ measured in the tests of the multiple specimens, what was already expected, since this sample is made of pure PP, not having the presence of aluminum or PET that can cause defects in the structure of the specimen.

It is possible to observe that all the samples had similar behavior in relation to the limit of tensile strength, showing that both the variation of the amount of the dispersed phase and the type of materials present did not significantly have an influence on this characteristic. This uniformity was not repeated in the other parameters. It is possible to verify different yield points, in which sample A6 presents the highest index, a consequence of the higher degree of crystallinity, and samples A2 and A3 with the worst indexes, a consequence of the presence of aluminum and PET. To the ductility and toughness properties, it can be observed that samples A1 and A6 stand out positively, because they present larger relative quantities on polypropylene.

The other samples were as expected for ductility, since polypropylene-based materials may show elongation between 100 to 700 %[29], but both the toughness and ductility decrease due to the presence of aluminium and PET that tend to make defects in the specimens and cause a decrease in strength and a rupture at lower deformation values.

Regarding the resilience, the variations in the presented values can ​​ be explained by the thermomechanical effect that occurs during the process of preparation of the samples, generating possible processes of degradation in the polymer. Variables, such as the length and diameter of the extruder screw, thread profile and temperature used in the processing, may cause changes in material behavior.

In this case, the presence of the aluminum and PET, may have had an influence on this characteristic; however, its effect was not clear.

The results of the Izod impact strength of samples A1, A2, A3, A4, A5 and A6 are shown in Figure 12.

All samples presented increased impact resistance when compared to pure PP (sample A6). In this case, both the presence of elastomeric phase, aluminium and PET may have contributed to the absorption of impact energy and blocking of the process of crack propagation, improving the characteristics of impact strength of the materials, comparing to the expected results for polypropylene[12].

Figure 12. Izod impact strength values of ​​ samples A1 to A6.

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4. Conclusion The results of the chemical, physical, thermal, and morphological characterizations showed that the main constituent material of the body of the disposable capsules is the polypropylene. The mechanical characterization of the material obtained by mixing the recyclable materials of the disposable capsules showed that the polypropylene filter and the body of the capsule are the most interesting materials to be recycled, and those present good properties. The materials resulting from the mixture of all the constituent parts of the disposable capsule presented decreased mechanical properties compared to the samples composed only of the capsule or the PP filter, what is probably a consequence of the poor interactions between the polymer matrix and aluminum and PET. However, depending on the final application, all the material can be used in a process which reduces time and labor for the separation of the aluminum and the lid from the rest of the capsule parts.

5. Acknowledgements The authors acknowledge to COMCAP – UEM for the FTIR, XRD, DSC and SEM analyses, and to Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES and Conselho Nacional de Desenvolvimento Científico e Tecnológico - CNPq by scholarship.

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Uma Introdução. Rio de Janeiro: LTC. Received: Feb. 26, 2020 Revised: Apr. 07, 2020 Accepted: Apr. 30, 2020

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ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.07418

Physicochemical and drug release properties of microcrystalline cellulose derived from Musa balbisiana Martins Emeje1,2* , Marlene Ekpo1, Olubunmi Olayemi1, Christianah Isimi1 and Alak Buraghoin2 Department of Pharmaceutical Technology and Raw Materials Development, National Institute for Pharmaceutical Research and development – NIPRD, Abuja, Federal Capital City, Nigeria 2 Tezpur University, Tezpur, Assam, India

1

*martinsemeje@yahoo.com

Abstract Microcrystalline cellulose synthesized from the waste of Musa balbisiana (BMCC) was characterized to explore the possibility of application in the pharmaceutical industry especially as a drug delivery vehicle. The SEM, XRD and FTIR investigations revealed that the predominantly short, non-aggregated and irregular MCC rods were highly crystalline.

The moisture sorption value for BMCC was 5.65%, while total ash was 0.39%. Flow of BMCC was poor, but the product exhibited high hydration (11.7%) and swelling (277.0%) capacities. Preliminary investigation of BMCC tablets containing ascorbic acid carried out in simulated intestinal fluid, showed a concentration dependent retardation of drug release. No cytotoxicity of BMCC was observed in the hemolytic assay. Overall, the study revealed that BMCC can be prepared from an inexpensive and abundant agricultural waste and possesses properties advantageous for application in the pharmaceutical industry and may be explored further in drug delivery research.

Keywords: drug release, microcrystalline cellulose, Musa balbisiana, physicochemical characteristics.

How to cite: Emeje, M., Ekpo, M., Olayemi, O., Isimi, C., & Buraghoin, A. (2020). Physicochemical and drug release properties of microcrystalline cellulose derived from Musa balbisiana. Polímeros: Ciência e Tecnologia, 30(1), e2020010. https://doi.org/10.1590/0104-1428.07418

1. Introduction Microcrystalline cellulose, (MCC), is known as a purified, partially depolymerized cellulose which is prepared by treating alpha cellulose obtained as ‘pulp’ from a fibrous plant with mineral acids. One of the most commonly used direct compression filler-binders is MCC. Due to its excellent binding properties, it is used widely in tablet formulation especially as direct compression excipient. It has been reported[1] that MCC’s dilution potential is high and performs well in direct compression formulations. It also works well as disintegrants and lubricant as well as diluent or filler in formulation of tablets prepared by wet granulation and as filler in capsules and spheres[2-6]. MCC is derived from several sources including gymnosperms (generally conifers) and softwoods, as well as from hardwood dicotyledons.

The botanical or biological source has been reported to affect the chemical composition (amounts of lignin, cellulose and hemicellulose) as well as the structural arrangement. These invariably affect the composition of the alpha cellulose and consequently the crystallinity and composition of the final product which is MCC[1,7]. Several researchers[8-10] opine that strong hydrogen bonding exists between the cellulose crystals consequently boosting re-aggregation, a precursor to the formation of microcrystalline cellulose.

Musa balbisiana is among the two species (along with M. acuminata) that are wild progenitors of the complex hybrids that make up modern bananas and plantains.

The cultivated hybrids are tropical monocot tree-like plants

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grown in wet tropical areas worldwide, and are the fourth most cultivated food crop in the world, with 2009 global production of 97.4 million tons, harvested from 4.9 million hectares. M. balbisiana is indigenous to Southeast Asia, including China, India, Indonesia (Java), Malaysia, Myanmar, Nepal, New Guinea, Philippines, Sikkim, Sri Lanka, and Thailand, where it majorly grows in ravines in tropical evergreen forests at altitudes of up to 1,100 meters (3,575 feet). In some of these areas, including parts of New Guinea and Thailand, it may have naturalized following cultivation. It is not clear when the first hybrids were made, but archaeological evidence suggests that bananas have been cultivated for at least 7,000 years[11,12].

M. balbisiana is a perennial herbaceous plant with a hard, fibrous “trunk”. It often grows with several pseudo-stems in a cluster. The primary stem bears a single large terminal inflorescence, a spike with pistillate (female) flowers below, and staminate (male) flowers above. This develops into a bunch of bananas, consisting of 8 clusters of 15 or 16 bananas (technically, berries) arranged in two rows. Hybrids have greatly reduced and usually sterile seeds, but in wild types, seeds occupy up to 25% of the fruit. There are Bananas and plantains from hybrids of M. balbisiana, but vary in proportion of sugar to starch[12].

The novelty of this work lies on the premise that, Banana stem constitutes a waste challenge because they are dumped

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Emeje, M., Ekpo, M., Olayemi, O., Isimi, C., & Buraghoin A. on the roadsides or into lakes and rivers after the banana fruit has been consumed, thus, constituting environmental hazard.

The stem is a very good source of cellulose and its use as a biomass for development of excipients like microcrystalline will not only increase the pool of pharmaceutical excipients available, curbing the environmental menace it constitutes, but equally provide a cheaper alternative to the currently available but expensive microcrystalline cellulose.

There is scanty information on the characterization of the microcrystalline cellulose obtained from Musa balbisiana.

Hence this study aims to extract and characterize the microcrystalline cellulose from the waste of Musa balbisiana.

Also, the release profile of tablet formulated with this new microcrystalline cellulose was evaluated to determine the suitability of BMCC as an excipient in biopolymer industries.

2. Materials and Methods 2.1 Isolation of α- cellulose

The process outlined by Ohwoavworhoa and Adelakun[1] was followed; essentially, the banana was peeled and the waste was cut into pieces and dried in a hot air oven.

An 80 g quantity of this material was placed in a stainless steel container to which was added 4.0 L of 2% w/v sodium hydroxide and digestion effected for 4 h at 80 °C in a water bath (FGL 1083. Karl Kolb Scientific, West Germany). Following thorough washing and filtration, it was bleached with 2.0 L of a 1:1 aqueous dilution of sodium hypochlorite for 15 min at 80 °C. The material was then washed sufficiently with water and treated with 2.0 L of 17.5% w/v sodium hydroxide at 80 °C for 1 h. The resulting α-cellulose was washed thoroughly with water. The extraction process was then completed by whitening with a 1:2 aqueous dilution of sodium hypochlorite for 15 min at 80 °C and subsequent washing with water until neutral to litmus. The cellulose material was filtered, and the water manually squeezed out using calico cloth to obtain small lumps, which were dried in a fluidized-bed dryer at an inlet air temperature of about 60 °C for 1 h.

2.2 Production of microcrystalline cellulose (MCC) Again, the process outlined Ohwoavworhoa and Adelakun[1] was followed; a 50 g quantity of the α- cellulose obtained from the process described above was placed in a glass container and hydrolyzed with 0.8 L of 2.5 N hydrochloric acid at a boiling temperature of 105 °C for 15 min. The hot acid mixture was poured into 2.5 L of cold tap water which was followed by vigorous stirring with a stainless steel spatula and allowed to stand overnight. The microcrystalline cellulose obtained by this process was filtered, washed with water until neutral to litmus, filtered, pressed and dried in a fluidized-bed dryer at an inlet air temperature of about 60 °C for 1 h. Following further milling and sieving, the fraction passing through 650 µm sieve aperture was used for the characterization described below.

2.3 Total ash determination The total Ash content was estimated by the measurement of the residue left after combustion in a furnace at 550 °C. 2/6

2.4 Bulk and tap densities A 20 g quantity of powder sample was placed in a 50 mL graduated cylinder and the volume, Vo, occupied by the powder without tapping was noted. Using a Stampfvolumeter model STAV 2003 (JEF, Germany), the powder was tapped 100 times, and the volume occupied by the powder after tapping noted as, V100. The bulk, (Db), and tapped densities were then calculated.

2.5 Swelling capacity The method reported by Ohwoavworhoa, F. O. Adelakun[6] was used and computation was done using the equation below:

S = (V2 – V1 ) / V1 x 100 (1) V2 is the volume of the hydrated or swollen material and V1 is the tapped volume of the material prior to hydration.

2.6 Hydration capacity A 1.0 g quantity of each sample was placed in each of four 15 mL plastic centrifuge tubes and 10 mL distilled water was added from a 10 mL measuring cylinder and then stoppered. The contents were mixed on a vortex mixer (Vortex Gennie Scientific Industry, USA) for 2 min. The mixture was allowed to stand for 10 min and immediately centrifuged at 1000 rpm for 10 min on a bench centrifuge (Gallenkamp, England). The supernatant was carefully decanted and the sediment weighed. The hydration capacity was taken as the ratio of the weight of the sediment to the dry sample weight.

2.7 Moisture sorption capacity A 2 g quantity of the cellulose material was accurately weighed and evenly distributed over the surface of a 70 mm tarred Petri dish. The samples were placed in a large desiccator containing distilled water in its reservoir (RH = 100%) at room temperature and the weight gained by the exposed samples at the end of a five-day period was recorded and the amount of water absorbed was calculated from the weight difference.

2.8 Scanning Electron Microscopy (SEM) Using a Hitachi S5200 field emission scanning electron microscope (Hitachi High- Technologies Canada, Inc., Ontario, Canada), micrographs of microcrystalline cellulose from Musa balbisiana was obtained. The imaging was done at 1.0 kV accelerating voltage on platinum-coated samples.

2.9 X-Ray Diffraction Structural characterization of the microcrystalline cellulose molecules was done using a Siemens D5000 X-ray diffractometer (Siemens, Munich, Germany). Powder samples, packed in rectangular aluminum cells were illuminated using CuKa radiation (l = 1.54056 Å) at 45 kV and 40 mA. Samples were scanned between diffraction angles of 5 to 80°. Scan steps of 0.1 were used and the dwell time was 15 s. To reduce the size (kb) contribution to the X-ray signal a nickel filter was used at ambient temperature and three measurements were made.

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Physicochemical and drug release properties of microcrystalline cellulose derived from Musa balbisiana 2.10 Fourier Transform Infrared Spectroscopy (FT-IR) The IR spectra of BMCC sample were run as KBr pellets on impact 410 Nicolet FTIR spectrometer in the frequency range 4000-500 cm–1.

2.11 Hemolysis test Hemolysis test was carried out according to the method of Kalita et al.[9]. Mammalian blood sample from goat was collected in a vial containing 4% trisodium citrate. It was centrifuged at 750 × g. The supernatant was discarded and the precipitate containing the erythrocytes was washed with PBS (pH 7.4) twice at 750 ×g for 10 min. 48.5 mL of PBS was added to 1.5 ml of the erythrocyte suspension and was equally divided in 2 mL tubes. Samples containing three different composition of MCC weighing 10 mg each was added to tubes containing 2 ml of the erythrocyte suspension and incubated for 2 h at 37 °C. After the incubation, the tubes were centrifuged at 750 × g for 10 min. Now, 200 mL of the supernatant was collected in a fresh tube and to it 2.8 mL of PBS was added. The absorbance was taken at 415 nm.

PBS was taken as the negative control and TritonX 100 was taken as the positive control.

2.12 Preparation of ascorbic acid tablets Batches of ascorbic tablets containing 200 mg ascorbic acid per tablet, and BMCC at 25, 50 and 65% were prepared using the direct compression method. Tablet weights; 250, 300 and 330 mg corresponding to BMCC 25, 50 and 65% respectively were produced. The powders were mixed thoroughly with no extra excipients such as diluents, lubricants and glidants prior to compression using a single punch tableting machine (Erweka AR 400, Germany) at uniform compression pressure (8.0 metric tons). Fifty tablets were produced per batch and stored in an air tight container for 24 h preceding release studies.

indices increases, the flow character decreases. In general, however, Hausners ratio greater than 1.25, indicates poor flow; Carr’s compressibility index less than 10% indicates excellent flowability and values between 11-15 indicates good flowability while values greater than 38% indicates very poor flow[6,8,13]. In this study, the derived flow parameters (Car’s compressibility index and Hausner’s ratio) show that the powder does not possess excellent flow. Therefore, there is need for inclusion of a glidant or lubricant during tablet compression using this cellulose.

3.2 Total ash content Total ash value which is an index for purity is found in Table 1, the lower the value, the better the material as it establishes that extreme care was observed during production.

In this study, low values were obtained indicating relatively low impurities in the final material.

3.3 Swelling capacity One of the mechanisms of tablet disintegration is swelling.

This swelling can be evaluated by obtaining the hydration and swelling capacities as well as moisture sorption profile of the material. Swelling capacity is indicative of water absorption of the cellulose and this can be seen clearly in Table 2 to be very high. The swelling capacity value shows that the new cellulose increased in volume up to 277% of its original volume. This show that BMCC may be useful in developing a super disintegrant for incorporation in tablet formulation. The mechanism will presumably be by

2.13 Release studies Release (dissolution) tests were performed on the tablets using an Erweka dissolution test apparatus. The medium used was simulated intestinal fluid, thermostatically maintained at 37 °C at a rotational speed of 50 rpm. Samples of 5 mL were withdrawn at 5 min intervals and replaced with fresh 5 mL of the dissolution medium. The withdrawn samples were analyzed spectrophotometrically at a pre-determined wavelength of 265 nm and the amount of drug released was calculated with reference to the calibration curve (Figure 1).

3. Results and Discussions 3.1 Flow properties The flow of powdered material is highly important in ascertaining its usage as an excipient in direct compression of tablets. An indirect measure of powder flowability is Hausner index which shows friction between the particles and Carr’s compressibility index which is indicative of the propensity of a volume of a material to decrease on tapping[1,7]. The values are indirectly proportional to the flow character, as the Polímeros, 30(1), e2020010, 2020

Figure 1. Calibration curve for ascorbic acid in simulated intestinal fluid.

Table 1. Physicochemical parameters of BMCC. Property Yield (%) Moisture sorption (%) Total ash content (%) Swelling capacity (%) Hydration capacity (%) Bulk density (g/ml) Tapped density (g/ml) Carr’s Index (%) Hausners ratio

BMCC 6.57 5.65 ± 1.10 0.39 ± 0.08 277 ± 4.01 11.7 ± 0.10 0.609 ± 0.05 0.762 ± 0.01 20.08 1.25

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Emeje, M., Ekpo, M., Olayemi, O., Isimi, C., & Buraghoin A. swelling. This high swelling capacity may equally be due to the high porous nature of the extracted cellulose[1]. High swelling capacity has also been implicated in sustained release formulations[14-16].

3.4 Hydration capacity The hydration capacity value obtained for the extracted cellulose is shown in the Table 1. The very high hydration capacity shows its capability of absorbing water more than five times its weight. According to Ohwoavworhoa and Adelakun[1], the hydration capacity of microcrystalline cellulose they obtained from raw cotton of Cochlospermum planchonii was 4.73, the authors concluded that the cellulose was capable of absorbing more water than the standard microcrystalline cellulose, Avicel. In our current report, BMCC hydrates about 11 times more than its original volume. It is therefore predicted that, BMCC may be a good disintegrant in solid dosage preparations, although, it could also be employed as a binder in formulation of controlled delivery systems[16].

nature of the material is evident from the sharp intensity observed and the value shows the amount of the crystalline nature; crystallinity is an indication of BMCC’s ordered compact structure.

Whether BMCC had any cell damaging affect was investigated in terms of its hemolytic activity in erythrocytes.

Table 2. X-ray diffraction data for BMCC. Sample name BMCC

Angle 2θ 21.90 21.85 22.10 21.75 22.00

d value 2.06 2.07 2.05 2.08 2.06

Intensity (%) 137 143 146 153 166

3.5 Moisture sorption Moisture sorption capacity measures how sensitive a material is to moisture, the values for the moisture sorption capacity is found in the Table 1. The crystallite part of the cellulose has been said not to adsorb water invariably water adsorption by cellulose is dependent on the amount of amorphous cellulose present in the material[1,7,8]. Thus, the high moisture sorption capacity value recorded indicates the high amount of amorphous cellulose present in BMCC. Moisture sorption capacity also tells us the extent of the stability of any formulation made from the cellulose when stored under humid condition. This property generally gives an insight on the storage condition. Since it is sensitive to moisture, the material should always be stored in air-tight container[1,7,8].

Figure 2. Scanning electron micrograph of BMCC.

3.6 Scanning Electron Microscopy (SEM) The scanning electron microscopy of the extracted cellulose is predominantly uniform in shape. Its surface is equally smooth with predominantly rectangular shape (Figure 2). Materials with uniform-sized particles are have found wide use in the pharmaceutical industries[17] we therefore opine that, the potential relevance of BMCC as an excipient in pharmaceutical industry is high.

3.7 Fourier-Transform Infrared Spectroscopy (FTIR)

Figure 3. FTIR spectrum of BMCC.

The FTIR spectra identifies the functional groups present in the material. From Figure 3, the IR peak observed at 3475 is due to cellulose –OH stretching vibrations. The peak at 1436.44 and 1375.87 is attributed to the ‘bending vibrations of –CH2, C-H and C-O of cellulose. The peaks at 1161.78, 1115.30 and 1061.05 is attributed to the ‘deformation of the C-H rocking vibrations[8,9,18].

3.8 X-ray diffractometry (XRD) The X-ray pattern of indicates the crystallinity of a material[9,19,20]. The pronounced peak of BMCC observed at 22.1 (Figure 4) indicates that delignification and depolymerization has been done successfully. The crystalline 4/6

Figure 4. X-ray diffraction of BMCC. Polímeros, 30(1), e2020010, 2020

Physicochemical and drug release properties of microcrystalline cellulose derived from Musa balbisiana

Figure 5. Haemolytic activity of BMCC.

Figure 6. Hemolytic activity of BMCC before (a) and after incubation (b).

3.9 Release profile The release of the drug from the formulation was observed to decrease with increase in the concentration of BMCC (Figure 7). The release from tablets containing 25% BMCC was the highest, while tablets containing 65% BMCC had the lowest release. These results corroborate the high swelling capacity of BMCC which is known to decrease drug release as a result of elongated diffusion path length.

Therefore, microcrystalline cellulose extracted from Musa balbisiana may be used in formulations meant for sustained or extended drug release..

4. Conclusions Figure 7. Effect of concentration of BMCC on the release profile of ascorbic acid.

It was found that BMCC did not cause any damage to the blood cells (Figures 5 and 6). Its activity even at the highest polymer concentration of 65% was the same as that of the PBS buffer (pH 7.4) after 2 h of incubation which indicated no cytotoxic effect on the cells. This shows the compatibility of BMCC with blood cells and suggests its safety in development of delivery systems to tissues.

Polímeros, 30(1), e2020010, 2020

Sequential alkaline and acid treatments of the waste stem of Musa balbisiana led to the production of microcrystalline cellulose (BMCC) with desirable and unique physicochemical properties for potential applications as excipients in pharmaceutical formulations. In general, it was found to be a good direct compression excipient for ascorbic acid and was without cytotoxic effect as evident from the hemolytic assay. The investigation also revealed that BMCC supported a concentration dependent retardation of ascorbic acid release. Thus, the MCC prepared from this agricultural waste may be further explored for application in the pharma industry.

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5. References 1. Ohwoavworhoa, F. O., & Adelakun, T. A. (2005). Some physical characteristics of microcrystalline cellulose obtained from raw cotton of cochlospermum planchonii. Tropical Journal of Pharmaceutical Research, 4(2), 501-507. http://dx.doi. org/10.4314/tjpr.v4i2.14626.

2. Höckerfelt, M. H., & Alderborn, G. (2014). The crystallinity of cellulose controls the physical distribution of sorbed water and the capacity to present water for chemical degradation of a solid drug. International Journal of Pharmaceutics, 477(12), 326-333. http://dx.doi.org/10.1016/j.ijpharm.2014.10.034.

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3. Kranz, H., Jürgens, K., Pinier, M., & Siepmann, J. (2009).

Drug release from MCC- and carrageenan-based pellets: experiment and theory. European Journal of Pharmaceutics and Biopharmaceutics, 73(2), 302-309. http://dx.doi.org/10.1016/j. ejpb.2009.05.007. PMid:19465119.

4. Luukkonen, P., Schæfer, T., Podczeck, F., Newton, M., Hellén, L., & Yliruusi, J. (2001). Characterization of microcrystalline cellulose and silicified microcrystalline cellulose wet masses using a powder rheometer. European Journal of Pharmaceutical Sciences, 13(2), 143-149. http://dx.doi.org/10.1016/S09280987(00)00197-4. PMid:11297898.

5. Mallick, S., Pradhan, S. K., & Mohapatra, R. (2013). Effects of microcrystalline cellulose based comilled powder on the compression and dissolution of ibuprofen. International Journal of Biological Macromolecules, 60(0), 148-155. http://dx.doi. org/10.1016/j.ijbiomac.2013.05.021. PMid:23732329.

6. Ohwoavworhua, F. O., & Adelakun, T. A. (2010). Non-wood fibre production of microcrystalline cellulose from Sorghum caudatum: characterization and tableting properties. Indian Journal of Pharmaceutical Sciences, 72(3), 295-301. http:// dx.doi.org/10.4103/0250-474X.70473. PMid:21188036.

7. Ohwoavworhua, F. O., & Adelakun, T. A. (2005). Phosphoric acid-mediated depolymerization and decrystallization of α-cellulose obtained from corn cob: preparation of low crystallinity cellulose and some physicochemical properties.

Tropical Journal of Pharmaceutical Research, 4(2), 509-516. http://dx.doi.org/10.4314/tjpr.v4i2.14627.

8. Ohwoavworhua, F., Adelakun, T., & Okhamafe, A. (2009).

Processing pharmaceutical grade microcrystalline cellulose from groundnut husk: extraction methods and characterization.

International Journal of Green Pharmacy, 3(2), 97-104. http:// dx.doi.org/10.4103/0973-8258.54895.

9. Kalita, R. D., Nath, Y., Ochubiojo, M. E., & Buragohain, A. K. (2013). Extraction and characterization of microcrystalline cellulose from fodder grass; Setaria glauca (L) P. Beauv, and its potential as a drug delivery vehicle for isoniazid, a first line antituberculosis drug. Colloids and Surfaces. B, Biointerfaces, 108(0), 85-89. http://dx.doi.org/10.1016/j.colsurfb.2013.02.016.

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10. Thoorens, G., Krier, F., Leclercq, B., Carlin, B., & Evrard, B. (2014). Microcrystalline cellulose, a direct compression binder in a quality by design environment: a review. International Journal of Pharmaceutics, 473(1-2), 64-72. http://dx.doi. org/10.1016/j.ijpharm.2014.06.055. PMid:24993785.

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11. Carlos-Amaya, F., Osorio-Diaz, P., Agama-Acevedo, E., YeeMadeira, H., & Bello-Pérez, L. A. (2011). Physicochemical and digestibility properties of double-modified banana (Musa paradisiaca L.) starches. Journal of Agricultural and Food Chemistry, 59(4), 1376-1382. http://dx.doi.org/10.1021/ jf1035004. PMid:21214175.

12. Gogoi, K., Saikia, J. P., & Konwar, B. K. (2013). Immobilizing silver nanoparticles (SNP) on Musa balbisiana cellulose.

Colloids and Surfaces. B, Biointerfaces, 102, 136-138. http:// dx.doi.org/10.1016/j.colsurfb.2012.07.031. PMid:23010111.

13. Ohwoavworhua, F. O., Adelakun, T. A., & Kunle, O. O. (2007). A comparative evaluation of the flow and compaction characteristics of a-cellulose obtained from waste paper.

Tropical Journal of Pharmaceutical Research, 6(1), 645-651. http://dx.doi.org/10.4314/tjpr.v6i1.14642.

14. Emeje, M., John-Africa, L., Isimi, Y., Kunle, O., & Ofoefule, S. (2012). Eudraginated polymer blends: a potential oral controlled drug delivery system for theophylline. Acta Pharmaceutica, 62(1), 71-82. http://dx.doi.org/10.2478/v10007-012-0001-6.

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15. Emeje, M., Nwabunike, P., Isimi, C., Kunle, O., & Ofoefule, S. (2008). Hydro-alcoholic media: am emerging in vitro tool for predicting dose dumping from a controlled release matrices.

Journal of Pharmacology and Toxicology, 3(2), 84-92. http:// dx.doi.org/10.3923/jpt.2008.84.92.

16. Emeje, M. O., Kunle, O. O., & Ofoefule, S. I. (2006). Effect of the molecular size of carboxymethylcellulose and some polymers on the sustained release of theophylline from a hydrophilic matrix. Acta Pharmaceutica, 56(3), 325-335.

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17. Nwajiobi, C. C., Otaigbe, J. O. E., & Oriji, O. (2019). Isolation and characterization of microcrystalline cellulose from papaya stem. Der Pharma Chemica, 11(3), 19-26. Retrieved in 2018, November 6, from https://www.derpharmachemica.com/pharmachemica/isolation-and-characterization-of-microcrystallinecellulose-from-empapayaem-stem-18356.html 18. Pawar, H., & Varkhade, C. (2014). Isolation, characterization and investigation of Plantago ovata husk polysaccharide as superdisintegrant. International Journal of Biological Macromolecules, 69, 52-58. http://dx.doi.org/10.1016/j. ijbiomac.2014.05.019. PMid:24854213.

19. Atef, M., Rezaei, M., & Behrooz, R. (2014). Preparation and characterization agar-based nanocomposite film reinforced by nanocrystalline cellulose. International Journal of Biological Macromolecules, 70, 537-544. http://dx.doi.org/10.1016/j. ijbiomac.2014.07.013. PMid:25036597.

20. Panda, B., Parihar, A. S., & Mallick, S. (2014). Effect of plasticizer on drug crystallinity of hydroxypropyl methylcellulose matrix film. International Journal of Biological Macromolecules, 67, 295-302. http://dx.doi.org/10.1016/j.ijbiomac.2014.03.033.

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Received: Nov. 06, 2018 Revised: May 01, 2020 Accepted: May 04, 2020

Polímeros, 30(1), e2020010, 2020

ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.04919

Effects of weathering on mechanical and morphological properties cork filled green polyethylene eco-composites Gabriela Celso Melo Soares de Vasconcelos1, Laura Hecker de Carvalho2, Renata Barbosa3, Rita de Cássia de Lima Idalino4 and Tatianny Soares Alves3*  Programa de Pós-graduação em Ciência e Engenharia de Materiais, Universidade Federal do Piauí – UFPI, Teresina, PI, Brasil 2 Programa de Pós-graduação em Ciência e Engenharia de Materiais, Universidade Federal de Campina Grande – UFCG, Campina Grande, PB, Brasil 3 Curso de Engenharia de Materiais, Programa de Pós-graduação em Ciência e Engenharia de Materiais, Universidade Federal do Piauí – UFPI, Teresina, PI, Brasil 4 Curso de Estatística, Universidade Federal do Piauí – UFPI, Teresina, PI, Brasil 1

*tsaeng3@yahoo.com.br

Abstract This study aims to evaluate the effects of natural weathering in the city of Teresina, State of Piauí, Brazil, on the morphology and mechanical properties of eco-composites based on high-density green polyethylene, powdered cork and compatibilizer processed in a twin-screw extruder and injection molded. The analyses revealed that although weathering induced surface bleaching of eco-composites and cracking, these effects were not intense in the compatibilized samples.

The tensile properties of the investigated materials were affected by abiotic degradation, which led to a reduction of the tensile strength and elastic deformation of the eco-composites, however, the incorporation of PEgMA was fundamental for the maintenance of mechanical performance after natural aging. In general, the results obtained were satisfactory for external applications of the compatibilized eco-composite with 15% cork in the proposed weathering range, which indicates its possible use in temporary constructions.

Keywords: composites, compatibilizer, extrusion, colorimetry, natural aging.

How to cite: Vasconcelos, G. C. M. S., Carvalho, L. H., Barbosa, R., Idalino, R. C. L., & Alves, T. S. (2020). Effects of weathering on mechanical and morphological properties cork filled green polyethylene eco-composites. Polímeros: Ciência e Tecnologia, 30(1), e2020011. https://doi.org/10.1590/0104-1428.04919

1. Introduction The construction industry is the second-largest consumer of plastics in the world[1], as it exploits and takes into account the lightness and durability of these materials[2]. Plastics, however, decompose fairly slowly and end up harming the environment after use. An alternative to polymer applications in the construction industry in a sustainable way is through the production of composites reinforced by plant material, as they are advantageous in terms of weight, cost, strength, recyclability, and ease of maintenance[3,4].

However, many of these applications are intended for external use and the stability of the polymeric composites, when exposed to weathering, is reduced. The combined effects ultraviolet solar radiation, moisture, and temperature oscillation can lead to matrix photodegradation and embrittlement as well as the fast degradation of vegetable fiber components (cellulose, hemicellulose, lignin, and extractives). These combined effects are also deleterious to fiber/matrix adhesion and, consequently, to effective filler/matrix load transfer, thus reducing composite mechanical properties as well as changing product appearance[5-7]. Contributing factors

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for this reduction include changes in matrix crystallinity, composite surface oxidation and interfacial degradation[8-10].

Mechanical properties stand out among the properties most studied in polymer composites subjected to ultraviolet, natural or accelerated weathering as these conditions not only directly affect material suitability for a given application but can significantly alter the physical properties and aesthetics of the product. Matuana, Jin & Stark[7] studied the influence of accelerated ultraviolet radiation exposure on high-density polyethylene (HDPE) composites reinforced with wood flour, coextruded or not with a clear HDPE cap layer, and their results showed discoloration, loss of wood components and increase of surface roughness after exposure. Bajwa, Bajwa & Holt[3] evaluated the performance of eco-composites consisting of HDPE, three types of biofibers (oak, cotton, and guayule), two coupling agents (NE 556-004 and TPW 243) and one lubricant (TPW 113), after accelerated aging. They observed that aging led to porous surfaces, color changes and surfaces rich in exposed biofibers, as well as a reduction in mechanical properties of the composites. Their data also indicated that the coupling agent was responsible for

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Vasconcelos, G. C. M. S., Carvalho, L. H., Barbosa, R., Idalino, R. C. L. & Alves, T. S. maintaining the mechanical properties of the composites after exposure. Badji et al.[11] investigated changes in the visual appearance, surface topography and mechanical properties of polypropylene/wood composites exposed to natural and artificial UV radiation. Aging caused surface bleaching, exposure of wood particles and numerous microcracks. After exposure, matrix behavior evolved from ductile to brittle with greater intensity in natural weathering. The elongation of the eco-composites after natural weathering, when compared to the matrix, was higher due to a stabilizing role of the lignin contained in wood flour. The stiffness and strength are not impacted by artificial aging in contrast to natural aging but the elongation values are lower for artificial weathering than for natural. These authors suggest that artificial weathering conditions cause much less degradation of mechanical properties than natural weathering.

An alternative material to use in polymer composites is cork, an industry by-product used in small quantities and in low value-added applications[12-14], such as feed the boilers in industrial processes[14,15]. The material has a similar appearance to wood and can be applied in composites with thermal, acoustic and vibration insulation functions (walls, ceilings, and floors), false ceilings, cladding, baseboards, mortars, insulating joints, and expansion or compression joints, among others[14,16].

The raw material is relevant for incorporation in composites[17] due to its low density, low permeability to liquids and gases, good compressibility and elasticity, low coefficient of thermal conductivity, wear resistance, mechanical characteristics, corrosion and fire resistance[15,17,18].

The sustainable character of the composite can be increased with the use of high-density green polyethylene, a polymer produced from ethylene monomer obtained by the dehydration of ethanol from sugarcane. Its composition is exactly the same as that of synthetic, non-renewable polyethylene, and has the same performance and characteristics[19].

The major challenge in the production of polymer-cork composites is to promote good interfacial bonding between the components[16]. Since cork is a polar material and most polymer matrices are nonpolar, compatibility with these materials is low. As a result, load transfer from the matrix to the reinforcing agent, which takes place at the interface, is hindered and so are the product´s mechanical properties[20,21].

The addition of functionalized polymers, such as those containing maleic anhydride groups in their composition, is considered to be an effective strategy to improve interfacial adhesion in these systems[4,13,16].

Thus, the aim of this work is to evaluate the morphology and mechanical performance of high-density green polyethylene/cork eco-composites with and without the addition of a polar compatibilizer based on maleic anhydride, before and after exposure to abiotic degradation. This is an eco-friendly and sustainable product being intended for application in civil construction.

2. Materials and Methods 2.1 Materials High-density green polyethylene, hereinafter referred to as GPE, supplied by Braskem, grade SHA7260, density 0.955 g.cm-3 and flow rate 20 g/10 min-1 (temperature 190°C 2/11

and 2.16kg mass) was used as the matrix. Powdered cork (PC) (#74 μm), used as reinforcement, was supplied by Corticeira Paulista / SP, density 65 an 85 g/l, humidity 7%, and particle size distribution as per Table 1. Maleic anhydride grafted high-density polyethylene, labeled PEgMA, commercial-grade Orevac 18507, density 0.954 g.cm-3 and flow rate 5 g.10 min-1 (temperature 190 °C and 2.16 kg), purchased from Arkema Innovative Chemistry was used as a coupling agent.

2.2 Composites preparation The PC was previously dried at 80 °C for 24 hours in an air-circulating oven and then tumble mixed with high-density GPE and polar compatibilizer (PEgMA) in the proportions indicated in Table 2. The compositions were processed in a modular twin-screw co-rotating extruder, NZ brand, SJ-20 model, with a diameter of 22 mm, L/D=38 and a shape factor of 1.48. The screw used is composed of two sections of intensive mixing formed by mixing blocks. The feed rate was 5 kg/h and the temperatures in the heating zones of the extruder Z1, Z2, Z3, Z4, Z5, and Z6 and the matrix were, respectively: 160, 170, 170, 180, 180, 180 and 230 ºC, and the screw speed was 250 rpm.

After extrusion, the composites were ground and dried in an oven at 80 °C for 24 hours. Tensile test samples (ASTM D 638 standard) were injection molded in a FLUIDMEC injection molding equipment operating with pressure 55%, a temperature profile of 210 and 200 °C at heating zones Z1 and Z2, injection time of 4 s, the cooling time of 35 s and die temperature 41,8 ºC.

2.3 Abiotic degradation The samples were exposed to natural weathering in the city of Teresina-Piauí, at the Federal University of Piauí (UFPI) campus, in 2017, during 13 weeks (2184 h), from September 4th to December 3rd, following ASTM D5272-08, ASTM D1435-12, and G7/G7M-13 standards.

The experiment was carried out in the driest season where the highest values of temperature and ultraviolet index are regionaly expected. Morphological and mechanical tests were performed after 45 (10/18/2017) and 90 (12/03/2017) days of exposure and the results were compared to the Table 1. Cork particle size distribution.

Characteristics Mesh 50 - (0.30 mm) (%) Mesh 60 - (0.25 mm) (%) Mesh 80 - (0.18 mm) (%)

Specification 20 to 50 10 to 45 5 to 30

Table 2. Sample compositions.

Sample GPE GPE/5PC GPE/10PC GPE/15PC GPE/5PC /5PEgMA GPE/10PC/5PEgMA GPE/15PC/5PEgMA

GPE PC PEgMA (% by weight) (% by weight) (% by weight) 100 95 5 90 10 85 15 90 5 5 85 80

10 15

5 5

Polímeros, 30(1), e2020011, 2020

Effects of weathering on mechanical and morphological properties cork filled green polyethylene eco-composites non-weathered samples. Climatic conditions data were collected at the National Institute of Meteorology (Inmet) meteorological station A312, located in Teresina/PI. 2.3.1 Climatic conditions during abiotic degradation Climate data collected during the experimental period showed high temperatures and high ultraviolet index (UVI) (Table 3), factors responsible to intensify the speed of the photooxidative reactions which cause degradation and, hence, reduce the durability of the polymer products[22-24].

Mean temparature ranged between 21.9 to 38.2 °C, mean relative humidity was of 51% and total precipitation of 16mm. The city of Teresina is located within the tropical area and are exposed to high solar radiation intensities, a parameter responsible for the initiation of abiotic degradation in polymer materials and in the vegetal raw material due to its potential to break chemical bonds such as CC, CH, CO, OH, among others present in these materials[7,25]. One way to measure the intensity of ultraviolet radiation is by Ultraviolet Index (UVI), an integer that is presented in categories of intensities: if less than 2 intensity is low, between 3 and 5 is moderate, between 6 and 7 is high, between 8 and 10 is very high and greater than 11 is extreme (CPTEC-INPE, 2017).

Our data indicate that the samples suffered an intense incidence of UV radiation, as 76% of the time UVI was above 11 (Table 3). Also, UVI showed the highest value in October (weeks 5 to 8) and started to decrease slowly in November (week 9). This reduction is attributed to season transition as it corresponds to the end of spring to early summer which, in the region, is characterized as the rainy season.

Moisture, present in the environment, may also be responsible for physical or chemical degradation, as it can accelerate oxidation reactions and facilitate light penetration in the composites[10,26,27]. Water can reach the surface of samples exposed to weathering in several ways: as precipitation, relative humidity or water formed on the surface of the material as dew or condensation[25]. The results of precipitation levels showed rainfall in September and October with only 0.6 mm and a concentrated total of 15.4 mm in the month of November. This indicates that, until the

first removal on 20/10/2017, there was minimal action of water by precipitation.

The relative air humidity varies considerably during the year. The first semester is characterized by higher humidity and the second semester is the driest. The variation of the relative humidity of the air during the period chosen for exposure to abiotic degradation (dry period) was quite low, with an average value of 51% in the 90 days of the test.

2.4 Colorimetry The changes in color were determined by the Instrutherm ACR-1023 color meter equipment using the scales RGB (red, green and blue color components). The measurements were done directly on the surface of the samples.

2.5 Optical Microscopy – OM The surface of the samples, prior and after abiotic degradation, were analyzed using an optical microscopy in a Leica Microsystems MD500 with ICC 50E capture camera operating in the reflection mode and at 40× magnification (500 μm).

2.6 Scanning Electron Microscopy – SEM SEM micrographs were obtained on an FEI Quanta FEG 250 equipment coupled with EDS Apollo XSDD, under 10 kV. The fracture surfaces of tensile tested eco-composite samples and of impact tested matrix were coated by a thin layer Gold with a Quorum Q150R ES metallizer and were analyzed with 300 X magnification.

2.7 Mechanical performance The mechanical performance of the eco-composites was ascertained by tensile testing conducted according to ASTM D 638 standards in an EMIC DL 30000 N universal testing machine operating with a 50 kN load cell, 50 mm.min-1 the cross-head strain rate at room temperature. Mean and standard deviation values of elastic modulus, breaking and yield stress were calculated from 5 samples per composition.

Table 3. Summary of climatic conditions during the abiotic degradation test in the city of Teresina-PI. Week 1 2 3 4 5 6 7 8 9 10 11 12 13

Average temperature (°C) Máx Mín 37.80 18.10 37.50 19.50 37.90 20.10 39.00 20.50 38.40 22.30 38.40 23.80 39.10 22.40 39.00 22.80 38.30 22.00 37.60 23.80 37.80 22.80 37.80 23.70 37.40 22.50

UVI máx

Precipitation (mm)

Average relative humidity

11.50 12.00 12.40 12.00 12.30 12.50 12.40 12.50 12.10 12.00 11.80 11.90 11.60

0.00 0.00 0.00 0.00 0.00 0.00 0.60 0.00 0.00 0.60 1.40 13.40 0.00

48.50 49.50 51.50 49.00 49.50 52.00 49.50 48.50 48.50 54.00 51.00 55.50 50.00

Source: INMET, 2017; Division of Satellites and Environmental Systems (DIDSA) of the Center for Weather Forecasting and Climate Studies (CPTEC) of the National Institute for Space Research (INPE), 2017.

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Vasconcelos, G. C. M. S., Carvalho, L. H., Barbosa, R., Idalino, R. C. L. & Alves, T. S. 2.8 Statistical analysis Variations in mechanical properties before and at the end of natural aging were analyzed using the R statistical computing platform (version 3.4.4, 2018) with scatter plots considering the results of non-aged and 90-day-aged samples. The properties that underwent major changes over the exposure periods were studied: maximum strength and deformation.

3. Results and Discussions 3.1 Morphology Before OM and SEM analysis, the polymer and the eco-composites were visually inspected and showed that there a good dispersion of the cork particles in the matrix.

The eco-composites displayed homogeneous tonality and good surface finish, the cork caused color change (Figure 1).

Chemically modified samples with PEgMA showed a glossy appearance, better surface finish, and a darker shade, suggesting that the coupling agent altered filler wettability and promoted better interfacial bonding[4].

by different mechanisms, after exposure to ultraviolet light[5,7,28]. The main factor is the presence of chromophore groups in both polymer and lignin, which are responsible for the absorption of ultraviolet light (UV), thus intensifying sample photodegradation[8,22].

Cork has an average lignin content of 27%[14] which, when degraded, produces water-soluble products, leading to the formation of functional chromophore groups, such as carboxylic acids, quinones, and hydroperoxide radicals[5,11].

Thus, the absorption of UV light initiates photochemical reactions on cork surfaces generating aromatic and other free radicals, which cause degradation of lignin and photooxidation of cellulose and hemicelluloses, effectively causing discoloration[5,29].

3.1.1 Colorimetry

The polymer, containing chromophore groups such as catalytic residues, hydroperoxide and carbonyl groups introduced during its manufacture, processing, and storage, absorb UV energy and initiate photochemical reactions that cause degradation and lead to cracking [30]. Oxygen diffusion into polyethylene controls its photodegradation, causing stresses that, combined with chain scission, can initiate and propagate cracks on the surface, leading to bleaching and loss of mechanical properties[31].

The measurements of color parameters and the effects of UV exposure were measured (Figure 2). Abiotic degradation caused bleaching of the eco-composites and this effect was more evident after 90 days of exposure. The discoloration is attributed to photodegradation of both cork and GPE,

In eco-composites without compatibilizers, the cork content alone did not significantly interfere in the results, since at the end of the test the parameters were very close.

The color changes of chemically modified test samples were less intense than those observed with test samples without

Figure 1. Test samples of the matrix and eco-composites before (0 days) and after natural aging (45 and 90 days).

Figure 2. RGB values of the matrix and eco-composites before (0 days) and after exposure (45 and 90 days).

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PolĂ­meros, 30(1), e2020011, 2020

Effects of weathering on mechanical and morphological properties cork filled green polyethylene eco-composites PEgMA. According to Ndiaye et al.[32] on their study on the durability of wood composites based on HDPE/PP and coupling agent, compatibilizer addition led to improved dispersion of the vegetal filler in the matrix and to reduced oxidation rate of the composites.

3.1.2 Optical Microscopy – OM Optical micrographs of the composites illustrate the aspects observed during a visual inspection (Figure 3).

Analysis of the non-aged sample surfaces confirms homogeneous distribution of the particles of cork powder, homogeneous tonality, bright appearance and good surface finish free of bubbles.

After exposure to UV radiation, although no loose cork particles were identified on the surface of the composite, cracks appeared on the surface of all samples investigated, thus confirming polymer degradation. The appearance of cracks upon weathering of vegetable fiber reinforced polymer composites is a common phenomenon attributed to the cleavage of the polymer chain, which allows the polymer to rapidly crystallize and leading to the formation of cracks in the composite surface[11,33]. The number of cracks appears to increase with the incorporation of cork, confirming that the presence of the filler weakens the material´s resistance to natural weather conditions[11]. Spontaneous cracking of

polymer may also occur due to secondary crystallization, also known as chemical crystallization[34].

3.1.3 Scanning Electron Microscopy – SEM To analyze the morphological changes before and after exposure to abiotic degradation of the systems, SEM images of the fracture surface of samples were taken (Figure 4).

It is important to note that, even before natural weathering, filler incorporation into the polymer matrix reduced the ability of the matrix to deform rendering the composite less ductile (more brittle) than the pure matrix.

The increase in cork concentration was also responsible for the decrease of this characteristic, as can be seen when comparing the fracture surface of the composite GPE/5PC (Figure 4b) and GPE/15PC (Figure 4d) and the composite GPE/5PC/5PEgMA (Figure 4e) and GPE/15PC/5PEgMA (Figure 4g). Similar results were reported by Fernandes et al.[21] on 1:1 polyethylene/cork composites. According to these authors, the system displayed brittle fracture, a result attributed to the high volume of cork powder content. In the present work, a comparison of samples of the same cork content with and without coupling agent (Fig. 4be, cf, dg) indicates that the incorporation of PEgMA increased the ability of the composite to deform, confirming that the addition of a polar compatibilizer is an effective strategy to improve interfacial adhesion[4,13,16].

Figure 3. Images by OM on the surface of the GPE sample and its eco-composites before (0 days) and after the end of exposure to abiotic degradation (90 days), with 40X magnifications: (a) GPE; (b) GPE/5PC; (c) GPE/10PC; (d) GPE/15PC; (e) GPE/5PC/5PEgMA; (f) GPE/10PC/5PEgMA; e (g) GPE/15PC/5PEgMA. Polímeros, 30(1), e2020011, 2020

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Vasconcelos, G. C. M. S., Carvalho, L. H., Barbosa, R., Idalino, R. C. L. & Alves, T. S.

Figure 4. SEM micrographs of the GPE and their eco-composites on the fracture surface before (0 days) and after exposure to abiotic degradation (45 days and 90 days), with 300X magnifications: (a) GPE; (b) GPE/5PC; (c) GPE/10PC; (d) GPE/15PC; (e) GPE/5PC/5PEgMA; (f) GPE/10PC/5PEgMA; e (g) GPE/15PC/5PEgMA. 6/11

PolĂ­meros, 30(1), e2020011, 2020

Effects of weathering on mechanical and morphological properties cork filled green polyethylene eco-composites In general, after natural weathering, there were large changes in the morphology of the GPE samples (Figure 4a), allowing significant change to be observed as soon as after 45 days of exposure. The fracture surfaces of weathered samples were brittle, rough with the appearance of flaky structures, suggesting the existence of internal fractures. According to Fabiyi et al.[8], the cracks can be caused by polymer chain scission, which results in highly crystallized polymer zones that crack and/or differentially contract (resulting from wetting and drying cycles) between the surface and interior sections.

Similar findings were observed in studies by Yang et al.[24] when analyzing HDPE composites reinforced with different inorganic fillers. In this work, particles and whitish regions were also observed. According to Yang et al.[23], these structures are attributed to matrix spherulites, formed in the amorphous region of the polymer and exposed on the fracture surface and confirm that photooxidative degradation of the polymer matrix starts preferentially in the amorphous region of the material, since the crystalline region has a higher density, reducing the diffusion of oxygen.

In the microscopies of the eco-composites, before abiotic degradation, it was possible to see the elongation

of the GPE between the cork cell walls (Figure 4b), suggesting good interaction between the phases[13].

However, despite the good affinity, some filler clusters were identified (Figure 4b, c, f, g). Bledzki, Reihmane & Gassan[35] point out that one of the main problems in the processing of wood/thermoplastic composites is the tendency of the natural filler, when untreated, to form large agglomerates due to the high intermolecular bond between the fibers, a feature that should be considered when choosing processing conditions. Since cork is a raw material similar to wood, this justifies the formation of the observed agglomerates. After 45 days of weathering, the surface of the composites became rougher, as shown by a flaky appearance identified in the micrographs of the pure polymer and more apparent in the samples without compatibilizer. The samples taken at the end of the exposure time (90 days) did not show significant changes when compared to those with 45 days of exposure.

3.2 Tensile Testing The mechanical properties of the eco-composites are shown in Table 4 and Figure 5.

Figure 5. Stress-strain graphs of GHDPE and the eco-composites (a) before and (b) after weathering.

Table 4. Elastic of Modulus, Breaking stress and Yield stress of the GPE and composites before and after weathering.

Sample

Exposure time (days)

GPE

GPE/5PC

GPE/10PC

GPE/15PC

GPE/5PC/PEgMA

GPE/10PC/PEgMA

GPE/15PC/PEgMA

Polímeros, 30(1), e2020011, 2020

0 45 90 0 45 90 0 45 90 0 45 90 0 45 90 0 45 90 0 45 90

Elastic of Modulus (MPa) 379.00 ± 12.32 362.77 ±25.60 359.17 ±10.78 407.57 ± 21.07 305.98 ± 1.13 321.80 ± 6.88 336.90 ± 29.27 248.23 ± 5.09 297.78 ± 5.18 371.08 ± 28.84 243.70 ± 2.68 309.72 ± 9.23 329.90 ± 8.47 242.96 ± 8.06 297.57 ± 4.89 299.80 ± 22.54 279.20 ±17.87 293.73 ± 5.96 351.00 ± 15.43 280.57 ± 1.40 315.93 ± 24.2

Period: 9/4/2017 e 12/3/2017 Breaking stress (MPa) 4.19 ± 1.38 17.93 ± 1.00 12.63 ± 1.00 16.52 ± 0.88 16.80 ± 0.65 17.30 ± 0.17 11.27 ± 1.07 12.30 ± 1.44 14.34 ± 0.82 13.58 ± 1.44 14.07 ± 0.86 15.08 ± 0.82 11.82 ± 0.90 16.16 ± 1.23 17.63 ± 1.16 13.03 ± 1.72 16.10 ± 0.90 14.70 ± 0.78 10.00 ± 1.22 12.00 ± 1.49 17.25 ± 2.81

Yield stress (MPa) 9.89 ± 0.65 12.73 ±0.96 9.50 ± 0.96 9.68 ± 0.17 8.25 ± 1.29 10.13 ± 0.80 8.02 ± 0.56 8.18 ± 1.17 7.16 ± 2.34 7.93 ± 0.73 7.97 ± 1.68 9.50 ± 0.99 9.22 ± 0.68 10.02 ± 1.10 9.40 ± 0.61 9.09 ± 0.62 9.50 ± 1.47 8.43 ± 0.92 9.70 ± 0.47 10.40 ± 0.60 8.98 ± 0.74

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Vasconcelos, G. C. M. S., Carvalho, L. H., Barbosa, R., Idalino, R. C. L. & Alves, T. S. The data indicates that, prior to weathering, the elastic modulus of the composites did not significantly change with cork incorporation, as a slight increase of 7.50% for GPE/5PC and a decrease of 11.1% and 2.1% for the GPE/10PC and GPE/15PC samples, respectively, relative to the pure matrix were observed (Table 4). Similar behavior was reported by Fernandes et al.[17] and Brites et al.[36], which identified an improvement in the stiffness of the material with increasing load when analyzing cork/polyolefin composites. In this work, the use of the PEgMA did not significantly change elastic modulus values of the composites when compared to the untreated samples with the same cork content, except for GPE/5PC/5PEgMA, which presented a reduced value compared to the GPE/5PC. Fernandes et al.[17] found similar results when producing polypropylene/cork powder (15%) composites. According to those autors, the introduction of a coupling agent functionalized with maleic anhydride – PP-g-MA (2% and 4%) – reduced composite stiffness compared to the untreated composite. The other samples treated with the coupling agent, GHDPE/10CP/5PE-g-MA and GHDPE/15CP/5PE-g-MA, showed no significant changes in this property.

After abiotic degradation, Young’s modulus of the GPE did not change significantly when compared to the non-exposed polymer, which is attributed to the fact that GPE has a higher percentage of crystalline phase[37] impermeable to O2 and H2O, thus limiting and slowing degradation reactions as these predominantly occur in the amorphous phases[27,38,39].

Regarding the eco-composites, it was clearly observed that the modulus of all compositions, with and without PEgMA, were reduced after 45 days of exposure. When compared to non-exposed samples of the same composition, the decreases in modulus observed were approximately 20-34% for GPE/5PC, GPE/10PC, GPE/15PC, GPE/5PC/5PEgMA, and GPE/15PC/5PEgMA. The composition GPE/10PC/5PEgMA had less loss of stiffness (~7%) when compared to non-aged samples. Some studies suggest that part of this loss in modulus can be attributed to the appearance of cracks in the surface due to photodegradation, which promotes a decrease in the interfacial bond between the cork and the polymer matrix and, consequently, the embrittlement of the material[11,31]. In this study, the appearance of surface cracks is confirmed by optical microscopy images (Figure 3).

The tests performed at the end of 90 days showed a tendency to recover the modulus, which could be associated with a chemical crystallization process, in which radicals formed by molecular chain cleavages in the amorphous phase on the sample surface during degradation are recombined[31]. It is an interesting feature for the application of these materials in provisional constructions that have a useful life within the studied period.

Breaking stress data (Table 4) clearly demonstrated cork addition effects on this property. Before natural aging, the highest value for this property was obtained with GPE/5PC eco-composite which was at least 122% higher and up to 165% higher than all other systems developed. With high polymer content, cork can be fully encapsulated by the matrix, which helps to disperse the reinforcing particles, improves the stress distribution and increases breaking stress[9]. Except for GPE/5PC/5PEgMA which showed a 28% decrease in breaking stress compared to the GPE/5PC sample, all breaking 8/11

stress values of coupling agent-treated eco-composites were similar to those containing the untreated filler.

After the first 45 days of natural weathering, the GPE samples showed an increase in this property followed by a reduction at the end of 90 days. Results showed a fragile behavior of the degraded sample, which requires a lower breaking force.

SEM images (Figure 4a) show a rough, flaky degraded surface of weathered samples, suggesting the existence of internal fractures. The breaking stress values of non-compatibilized eco-composites did not change significantly with natural aging when compared with compatibilized samples of equivalent composition. Actually, significant growth in this property was obtained for the samples formulated with PEgMA aged for 45 days, which may be associated with secondary crystallization processes, induced by short macromolecular chains resulting from an amorphous chain cleavage of the polymer during UV weathering[40].

The yield stress was also used to monitor changes in the mechanical behavior of the eco-composites as a function of filler content, coupling agent addition, and weathering.

Before exposure, when compared to GPE, results indicated similar yield stress values for GPE/5PC and decreases for GPE/10PC and GPE/15PC, 19% and 20% respectively, which was interpreted as cork particles having low strain (smaller than the matrix) and acting as stress concentrators, accelerating the propagation of cracks[41-43]. SEM images confirm this change since it is noticeable that the 5% cork composites had a higher yield than the composites with 15% (Figure 4b, d, Figure 4e, g). The yield stress of the systems increases upon PEgMA addition and with higher cork concentrations, which is attributed to better interfacial adhesion, filler distribution and stress transfer to the particles which promote the efficient distribution of the applied stress, thereby maintaining similar flow stress values to the neat polymer. No significant changes in this property were observed after weathering. Our data indicate that the yield stress was not affected by the weather in the proposed period of 90 days, thus presenting values of yield stress similar to those of the eco-composites, with and without compatibilizer.

The tensile strength and the maximum strain for the GPE and eco-composites were analyzed and showed the deformation profile characteristic of ductile material for the pure polymer whereas the composites were typical of lower ductility materials (Figure 5).

The curves of the non-aged samples showed a deformation profile characteristic of ductile material for the pure polymer whereas those of the composites are typical of lower ductility materials. The results also indicate that the tensile strength and the maximum deformation of the composites were reduced with the incorporation of cork, especially for the non-compatibilizing and high-grade composites GPE/10PC and GPE/15CP. This reduction can be attributed to the incompatibility between cork particles and the polymer matrix. The lack of adhesion between the phases limits the load transfer between them, resulting in a decrease of the tensile strength since the stress cannot be transferred by the matrix[17,36]. Another factor that may have influenced this behavior is that cork has lower mechanical properties Polímeros, 30(1), e2020011, 2020

Effects of weathering on mechanical and morphological properties cork filled green polyethylene eco-composites

Figure 6. Average dispersion of (a) Maximum Force and (b) Deformation of samples before and after natural aging.

under tensile load when compared to the polyethylene matrix, contributing to the decrease in tensile strength. Some authors suggest incorporating coupling agents, for example, functionalized polymers containing maleic anhydride groups in the composition to improve interfacial adhesion[4,16].

In this study, the addition of PEgMA allowed the recovery of tensile strength and maximum deformation proving the efficiency of the compatibilizing agent in improving the adhesion between phases.

In the curves plotted after aging it was possible to notice a typical behavior of materials with low ductility for the matrix, with reduced tensile and deformation resistance, and a less intense variation in the eco-composite curves. Badji et al.

[11] found similar mechanical performance to natural and artificial aging of polypropylene and wood flour compound, who attributed the observed behavior to the stabilizing role of lignin contained in wood flour. The curves also showed that the presence of compatibilizer was fundamental in maintaining tensile strength, but that due to the load size, a reduction in deformation could not be avoided. Natural weatherin led to an intense reduction in plastic deformation of the matrix while that property was not affected for the composites, despite the observed decrease in their strain at break. The compatibilized composites GPE/15CP/PEgMA and GPE/10CP/PEgMA higher tensile strength and strain at break than the other composites after 90 days aging.

The spread on mechanical properties results were satisfactory, considering that they are below 10% and in accordance with the literature.

3.3 Statistical analysis The statistical analysis allowed us to investigate the mechanical properties found for non-aged and aged samples and to suggest which composition was more mechanically stable before and after the abiotic degradation. Figure 6 shows the variations of the maximum strength and strain at break (deformation capacity) throughout abiotic degradation.

These results confirm what was observed in Figure 5.

Prior to weathering, the incorporation of cork in higher contents (10 and 15%) was responsible for weakening the Polímeros, 30(1), e2020011, 2020

material and reducing its strain at break when submitted to a tensile test. The GPE/5PC composite had a similar deformation to GPE and the PEgMA compatibilizer improved the compatibility between the phases increasing their deformation capacity and consequently their maximum strength values. After weathering, GPE showed loss of strength and the compatible composites performed well even after exposure. The highlight is the GPE/15PC/5PEgMA, which presented the highest value of maximum force and deformation capacity, and similar values before and after the abiotic degradation period. The composites without PEgMA had lower values of these properties, however, they were constant under the studied aging conditions. In general, it can be stated that the 90-day exposure period did not significantly impact on the loss of mechanical properties of composites when compared to GPE.

4. Conclusions Aging caused by natural weathering promoted some aesthetic changes on the eco-composites such as surface bleaching and cracking. OM showed that the filler particles were coated by the polymer matrix as no loose cork particles were observed on the surface of the composites. SEM images of the fractured surfaces detected a rougher, flakier appearance, after weathering of all samples investigated.

Althougj natural aging led to a reduction in tensile strength and elastic deformation of the eco-composites, the incorporation of PEgMA was fundamental for the maintenance of mechanical performance. In general, the mechanical property results obtained were satisfactory for external applications of the GPE/15PC/5PEgMA eco-composite in the proposed weathering range, which indicates its possible use in temporary constructions.

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Vasconcelos, G. C. M. S., Carvalho, L. H., Barbosa, R., Idalino, R. C. L. & Alves, T. S. 2. Halliwell, S. M. (2002). Polymers in building and construction.

United Kingdom: Rapra Technology Limited.

3. Bajwa, D. S., Bajwa, S. G., & Holt, G. A. (2015). Impact of biofibers and coupling agents on the weathering characteristics of composites. Polymer Degradation and Stability Journal, 120, 212-219. http://dx.doi.org/10.1016/j.polymdegradstab.2015.06.015.

4. Fernandes, E. M., Mano, J. F., & Reis, R. L. (2013). Hybrid cork-polymer composites containing sisal fiber: Morphology, the effect of the fiber treatment on the mechanical properties and tensile failure prediction. Composite Structures, 105, 153162. http://dx.doi.org/10.1016/j.compstruct.2013.05.012.

5. Feist, W. C., & Hon, D. N.-S. (1984). Chemistry of Weathering and Protection. In R. Rowell. The chemistry of solid wood (pp.

401-451). Washington: American Chemical Society. http:// dx.doi.org/10.1021/ba-1984-0207.ch011.

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24. Yang, R., Yu, J., Liu, Y., & Wang, K. (2005). Effects of inorganic fillers on the natural photo-oxidation of high-density polyethylene. Polymer Degradation & Stability, 88(2), 333-340. http://dx.doi.org/10.1016/j.polymdegradstab.2004.11.011.

25. Jacques, L. F. E. (2000). Accelerated and outdoor/natural exposure testing of coatings. Progress in Polymer Science, 25(9), 1337-1362. http://dx.doi.org/10.1016/S0079-6700(00)00030-7.

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27. Stark, N. M., Matuana, L. M., & Clemons, C. M. (2004). Effect of processing method on surface and weathering characteristics of wood-flour/HDPE composites. Journal of Applied Polymer Science, 93(3), 1021-1030. http://dx.doi.org/10.1002/app.20529.

28. Pandey, P., Bajwa, S. G., Bajwa, D. S. & Englund, K. (2017).

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29. Pandey, K. (2005). Study of the effect of photo-irradiation on the surface chemistry of wood. Polymer Degradation & Stability, 90(1), 9-20. http://dx.doi.org/10.1016/j. polymdegradstab.2005.02.009.

30. Grum, J. (2008). Book Review: Plastics additives handbook, 5th Edition by H. Zweifel. International Journal of Microstructure and Materials Properties, 3(2-3), 451. http://dx.doi.org/10.1504/ ijmmp.2008.018747.

31. Yakimets, I., Lai, D., & Guigon, M. (2004). Effect of photooxidation cracks on behavior of thick polypropylene samples.

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Effects of weathering on mechanical and morphological properties cork filled green polyethylene eco-composites 32. Ndiaye, D., Fanton, E., Morlat-Therias, S., Vidal, L., Tidjani, A., & Gardette, J.-L. (2008). The durability of wood polymer composites: Part 1. Influence of wood on the photochemical properties. Composites Science and Technology, 68(13), 27792784. http://dx.doi.org/10.1016/j.compscitech.2008.06.014.

33. Rabello, M. S., & White, J. R. (1997). Crystallization and melting behavior of photodegraded polypropylene-I. Chemicrystallization. Polymer, 38(26), 6379-6387. http://dx.doi. org/10.1016/S0032-3861(97)00213-9.

34. Craig, I. H., & White, J. R. (2005). Crystallization and chemicrystallization of recycled photodegraded polyethylenes.

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35. Bledzki, A. K., Reihmane, S., & Gassan, J. (1998). Thermoplastics Reinforced with Wood Fillers: A Literature Review. PolymerPlastics Technology and Engineering, 37(4), 451-468. http:// dx.doi.org/10.1080/03602559808001373.

36. Brites, F., Malça, C., Gaspar, F., Horta, J. F., Franco, M. C., Biscaia, S., & Mateus, A. (2017). Cork plastic composite optimization for 3D Printing Applications. Procedia Manufacturing, 12, 156-165. http://dx.doi.org/10.1016/j.promfg.2017.08.020.

37. Visakh, P., & Martinez Morlanes, M. (2015). PolyethyleneBased Blends, Composites, and Nanocomposites: Stateof-the-Art, New Challenges, and Opportunities. In P. M. Visakh, M. J. Martínez Morlanes, Polyethylene Based Blends, Composites, and Nanocomposites (pp. 1-19). http://dx.doi. org/10.1002/9781118831328.ch1 38. Jakubowicz, I. (2003). Evaluation of degradability of biodegradable polyethylene (PE). Polymer Degradation & Stability, 80(1), 39-43. http://dx.doi.org/10.1016/S0141-3910(02)00380-4.

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39. Lucas, N., Bienaime, C., Belloy, C., Queneudec, M., Silvestre, F., & Nava-Saucedo, J. E. (2008). Polymer biodegradation: Mechanisms and estimation techniques - A review.

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40. Fayolle, B., Richaud, E., Verdu, J., & Farcas, F. (2008).

Embrittlement of polypropylene fiber during thermal oxidation.

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41. Essabir, H., Hilali, E., Elgharad, A., El Minor, H., Imad, A., Elamraoui, A., & Al Gaoudi, O. (2013). Mechanical and thermal properties of bio-composites based on polypropylene reinforced with Nut-shells of Argan particles. Materials & Design, 49, 442-448. http://dx.doi.org/10.1016/j.matdes.2013.01.025.

42. Essabir, H., Nekhlaoui, S., Malha, M., Bensalah, M. O., Arrakhiz, F. Z., Qaiss, A., & Bouhfid, R. (2013).

Bio-composites based on polypropylene reinforced with Almond Shell particles: mechanical and thermal properties.

Materials & Design, 51, 225-230. http://dx.doi.org/10.1016/j. matdes.2013.04.031.

43. Essabir, H., Bensalah, M. O., Rodrigue, D., Bouhfid, R., & Qais, A. E. K. (2016). Biocomposites based on Argan nutshell and a polymer matrix: effect of filler content and coupling agent. Carbohydrate Polymers, 143, 70-83. http://dx.doi. org/10.1016/j.carbpol.2016.02.002. PMid:27083345.

Received: Aug. 19, 2019 Revised: May 29, 2020 Accepted: June 16, 2020

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ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.00120

Influence of Prosopis Juliflora wood flour in Poly Lactic Acid – Developing a novel Bio-Wood Plastic Composite Sachin Sumathy Raj1* , Thanneerpanthalpalayam Kandasamy Kannan1 and Rathanasamy Rajasekar2 1

Department of Mechanical Engineering, Gnanamani College of Technology, Namakkal, Tamilnadu, India 2 Department of Mechanical Engineering, Kongu Engineering College, Erode, Tamilnadu, India *sachinsraj1991@gmail.com

Abstract A Bio composite comprising Prosopis Juliflora Fiber (PJF) and Poly Lactic Acid (PLA) was processed considering two particulate sized reinforcements, coarse PJF (avg. 15 µm) and fine PJF (10-50 nm). They were added individually at ratios of 10, 15, 20 and 25 wt% into PLA matrix. The composites were extruded and tested for mechanical properties.

The addition of PJF resulted with an increase in the tensile, flexural and impact strengths of the polymer. Adding PJF to PLA showed a decrease in the hardness of the polymer. Water Absorption test showed an increase in water uptake with increasing fiber content. The most optimum ratio of PLA to PJF was found to be 80:20. The fine PJF reinforced composites proved to be superior over the coarse PJF reinforced composites at all stages of the research. FESEM and TGA were used to study morphology and thermal characteristics respectively.

Keywords: biocomposite material, poly lactic acid, prosopis juliflora, wood flour, wood plastic composite.

How to cite: Raj, S. S., Kannan, T. K., & Rajasekar, R. (2020). Influence of Prosopis Juliflora wood flour in Poly Lactic Acid– Developing a novel Bio-Wood Plastic Composite. Polímeros: Ciência e Tecnologia, 30(1), e2020012. https://doi.org/10.1590/0104-1428.00120

1. Introduction Non biodegradable polymers and composites have always been a difficult task when it comes to waste management, they are not decomposable and pose a major threat towards land pollution. Biodegradable thermo plastics on the other hand, plastic materials derived commonly from agro products like cassava, sugarcane and beet[1]. These bioplastics undergo complete decomposition when they are buried, thereby helping to avoid land pollution. The rising demand for biomaterials as an alternative to non-biodegradable plastics has led to vast research to develop biopolymer composite materials that possess excellent mechanical properties and are suitable for various applications.

This research involves on such investigation, where Prosopis Juliflora Fiber (PJF) is reinforced in coarse and fine particle forms individually into Poly Lactic Acid (PLA) matrix to develop a novel bio-composite material for Wood Plastic Composite (WPC) applications, focusing on good flexural and impact properties along with low water absorptiona and good thermal stability.

PLA, a starch based biodegradable thermoplastic polymer is one of the most widely used bio-plastics in the world. Experiments with wood flour reinforced into different biopolymers[2,3] have been successfully executed among which PLA based composites have shown outstanding mechanical performance. PLA has also proven to have better mechanical properties over conventionally used petroleum based polymers like Polypropylene[4] & Polystyrene[5] and can also be processed by similar methods. PLA and its

Polímeros, 30(1), e2020012, 2020

composites are currently being used in versatile applications like aircraft and automobile interiors, medical implants, 3D printing and other biomedical equipment[4,5].

Prosopis Juliflora (PJ) a medium sized tree found in the tropical zones around the globe and abundantly available in South Asia, South America and Africa. It is considered as a weed to be eradicated due to its abnormal water absorbing tendency[6]. PJ is currently used as fire wood and its fruit as animal fodder. Saravanan et al.[7] did a detailed study on PJF, and has proven its superior chemical properties when compared to other commonly used plant fiber reinforcements like jute, flax, ramie, hemp, kenaf, and okra. PJF has one of the highest Lignin contents among plant fibers and this chemical acts as a thermal stability compound. Lignin also helps in providing higher stiffness and as a water proofing agent in the micro fibrils[8-10]. Plants with higher lignin content also have a natural capability of resistance towards brown and white rot fungus, which are common wood fungal attacks[8].

These features of PJF led to this approach of reinforcing this economically cheap source of wood fiber into PLA matrix to develop the novel PLA/PJF Bio-composite material, a combination that has not been taken up by any research till date to the author’s best knowledge. PJF also having one of the lowest wood fiber densities[7] ensures to provide a light weighing WPC. Literature reveals that PLA has been previously worked with a few saw dust reinforcements like rubber wood[11], Poplar wood[12,13] and

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O O O O O O O O O O O O O O O O

Raj, S. S., & Kannan, T. K., & Rajasekar, R. maple wood[14], which have showed improved mechanical properties. In few other cases, wood filler particles like bamboo[15] and pine[16] had shown reduction in the mechanical properties of PLA when reinforced. PJF when reinforced into Epoxy had produced improved mechanical properties of the polymer[17].

2. Materials and Methods 2.1 Matrix material PLA of grade 3052D is one of the most widely used bio-thermoplastic polymer and was obtained from Natur tek, Chennai, Tamilnadu, India. PLA was obtained in the form of granules having a density of 1.24 g.m-3, a glass transition temperature of 55-60 °C, a crystalline melt temperature of 150-160 °C and a melting temperature ranging between 170 and 180 °C. The granules were bright white in color in their virgin form.

2.2 Fiber extraction PJ wood was obtained from the trees at the waste lands of Namakkal district, Tamilnadu, India. The outer layers were peeled off and the inner solid bark was dried in an oven at 90 °C for 12 hours. The bark was then held in the chuck of a lathe machine and turning operation was performed (500 rpm and 2mm depth of cut) to obtain long continuous fiber strings as shown in Figure 1a.

The wood fiber strings were alkali treated for 12 hours in a solution containing 95% water and 5% NaOH[17,18] to enhance the adhesion of the fiber with the matrix. The soaked fiber was again dried in an oven at 90 °C for 12 hours to remove all the moisture as shown in Figure 1b. Finally the dried fibers were powdered in a pulverizer (an equipment commonly used to crush large wood pieces into powder).

The powdered PJF was sieved using a #400 mesh to obtain the Coarse PJ reinforcement as shown in Figure 1c. The average size (a particle between the smallest and largest size was considered) of the coarse powder was around 12 µm which was measured using FESEM at 4000 X magnification, (Model: Zeiss, Coimbatore Institute of Technology, Coimbatore) as shown in Figure 2a. A portion of the coarse powder was further ground into fine particles using a food processer at 10000 rpm approximately. After every two minutes of grinding, the lid of the processor was opened and the wood flour that had been accumulated on the lid’s inner surface due to centrifugal force, was collected. This procedure was followed to replicate the study carried out by Fan[19], where the author had used the principle of centrifugal force and additional setups in a pulverizer to achieve superfine wood particles. These fine particles were considered as the nano PJ reinforcement as shown in Figure 1d, having a size ranging from 10 to 50 nm (measured using Particle Size Analyzer, Model: Nanophox, Nano Lab, KSR Engg College, Erode, India). The graphical output of the nano particle size analysis is shown in Figure 2b.

Figure 1. a) Turning operation carried out on the lathe machine to extract the fibers from the bark, b) Alkali treated and dried fibers in the form of long strands, c) Coarse fiber particles (magnification 10 X), d) Fine fiber particles (magnification 10 X).

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Polímeros, 30(1), e2020012, 2020

Influence of Prosopis Juliflora wood flour in Poly Lactic Acid – Developing a novel Bio-Wood Plastic Composite

Figure 2. a) Size of the coarse particles, measured using FESEM and b) Size of the fine particles, measured using Nano particle size analyzer.

Table 1. Composition of PLA to PJF in wt% ratio for coarse and fine fiber reinforced composites.

Fiber form - collective name Plain Polymer Coarse/Micro sized PJF reinforced samples. – “C Samples”

Fine/Nano sized PJF reinforced samples. – “F Samples”

Name of the composite PLA C1 C2 C3 C4 F1 F2 F3 F4

2.3 Preparation of PJF reinforced PLA composites Measured quantity of PLA and wood flour as shown in Table 1 were processed using a mini twin screw extruder (Kongu Engineering College, R&D, Tamilnadu, India) which had four processing stages. PLA was fed into stage 1 (170 °C), through a hopper, wood flour was added at stage 2 (180 °C) through a side feeder. Stage 3 (190 °C) ensured thorough belnding of the matrix and filler. Stage 4 (170 °C) near the exit nozzle ensured that the molten composite was extruded through dies of ASTM dimensions. The extruded composite bars were then cut into 10 mm thick test specimens for tensile test (ASTM D256) and flexural test (ASTM D790). The impact test (ASTM D256) specimens were obtained from cutting the length of additional fleural specimens to 127 mm.

Huda et al.[14] has proved that the fiber reinforcement in a polymer matrix less than 10 wt% ratio reduces the tensile strength of the composite material due to insufficient fiber loading thereby resulting in flaws or plasticization effect of the composite. Considering the former statement, the amount of reinforcement for this research was set to start from 10 wt% with an increase of 5 wt% for each consecutive composite specimen that was fabricated. Fabrication of the composite was not successful beyond 25 wt%. At 30 wt% filler content (trial attempted), the physical quantity of the filler material was greater than that of the matrix material.

Extrusion, therefore could not be carried out due to the high melt viscocity. This occurrence coincides with the study carried out by Valentina[20] who had investigated the rheology of spruce flour reinforced PLA composite and proved that the melt flow index was very large at 30 wt%, Polímeros, 30(1), e2020012, 2020

wt% of PLA matrix 100 90 85 80 75 90 85 80 75

wt% of fiber reinforced 0 10 15 20 25 10 15 20 25

thereby preventing the easy processing of the composite.

This phenomenon is also justified by the fact that the density of PJ is very low when compared to other commonly used fibers like jute, ramie, flax, hemp and kenaf which is tabulated in detail by Saravanan et al.[7] who determined the density of PJF to be 580 kg.m-3 while that of Jute, Flax, Ramie, Hemp and Kenaf fibers were 1460, 1500, 1500, 1480 and 1400 kg.m-3 respectively. Due to the low density of PJF, the physical quantity of fiber during reinforcement was very high, thereby restricting the fabrication of the PLA/PJF composite to a maximum fiber loading limit of 25 wt%.

Chandramohan et al.[21], explained the detailed shape and dimensioning of polymer composite materials with respect to the ASTM standards that have been considered for this study. The composite materials were fabricated to ASTM D638 (Type I) for tensile test, depicted in Figure 3a, 3c and 3d. ASTM D790 was followed for flexural test specimens (Figure 3b, 3e) and ASTM D256 for impact test. These ASTM standards were also followed by various other researchers[14,16,17,22] to determine the mechanical characterization of polymer composite materials.

2.4 Mechanical characterization Mechanical testing was performed on all the speimens shown in Table 1. Tensile test was carried out using a Universal Testing Machine (Brand: Kalpauk, Model: KIC-2-1000-C) operating with a load cell of 10 KN and a cross head speed of 5 mm.min-1. Flexural test was also carried out on the same UTM, with the test specimen placed on a three point bending fixture, with a span of 120 mm. Izod impact test was carried out on unnotched specimens using a 15 kg hammer 3/11

Raj, S. S., & Kannan, T. K., & Rajasekar, R.

Figure 3. a) PLA and coarse fiber reinforced composites as per ASTM D638, b) PLA and composites fabricated to ASTM D790 and ASTM D256 for flexural and impact test respectively, (c) Width of tensile specimen at the end, (d) width of tensile specimen at the neck and (e) width of the flexural specimen.

head weight. Vickers micro hardness test was conducted (Make: Wilson hardness, Model: 402 MVD) and Vickers Hardness Number (HV) was calculated for each of the test specimens. A load of 100 kgf was maintained as constant for all the samples, with a dwell time of 10 seconds.

2.5 Morphological analysis The fractured surface of all the WPCs were scanned under FESEM. SEM micrographs of the flexural specimens were scanned at a magnifications of 2000 X to obtain clear images. The scale was kept constant at 20 µm. The specimens were gold sputtered prior to FESEM.

2.6 Degradation studies Focusing on constructional applications for which this composite is being developed, the composites were subjected to tests relating to environmental conditions like water and heat/thermal surroundings. Water Absorption Test (WAT) was carried out since a natural fiber is used as a filler, which has a natural tendency to absorb water.

Thermo Gravimetric Analysis (TGA) was conducted to analyze the thermal degradation and thermal stability.

Weight measurement for the WAT was carried out using a high sensitive digital weighing scale. The specimens were first measured in their dry condition (dry weight). Then they were immersed in water at room temperature for 48 hours.

The specimens were finally taken out, pat dry and measured again for the final weight (wet weight). Water Absorption 4/11

was calculated using the formula[23] [(Wet weight – Dry weight) / Dry weight] x 100.

TGA was used to study the thermal stability of PLA, PJF and the WPC that resulted in providing the best mechanical property. 50 mg of samples were place in the test pan in a nitrogen atmosphere and treated upto a maximum of 500 °C with variation of 20 °C/min.

3. Results and Discussions 3.1 Tensile properties The tensile strength in both the coarse and fine particle reinforcements increased with increasing fiber content upto 20 wt% which had a maximum tensile strength of 21.14 MPa for C3 and 24.69 MPa for F3 while plain PLA had a tensile strength of 10.05 MPa as shown in Figure 4. PLA had an increase in tensile strength by 110% and 145% with addition of 20 wt% coarse and fine filler cases respectively. At 25 wt% loading, the tensile strength of C4 had a negligible reduction to 20.78 Mpa and F4 had a reduction to 21.66 Mpa. This may be due to the insufficient wetting of the fiber by the matrix material due to the higher physical presence of the filler material at the higher reinforcement level which was clearly evident in the FESEM image 10d.

The tensile strength of F4 also had reduced when compared to the tensile strength of F3 due to the same reason. This study revealed that the F composites had superior tensile strength over the C composites in all the four cases of fiber loadings since the nano particles had better reinforcement Polímeros, 30(1), e2020012, 2020

Influence of Prosopis Juliflora wood flour in Poly Lactic Acid – Developing a novel Bio-Wood Plastic Composite capability than the micro particles[24]. The values of the tensile modulus followed similar fashion of the tensile strength values as shown in Figure 5a. The tensile modulus of both the C and F composites increased upto 20 wt% filler content and then reduced at 25 wt% reinfrocement.

The good bonding between the matrix and fiber due to the addition of PJ may have increased the stiffness.

Elongation of PLA increased with increasing PJ content in both the coarse and fine cases. This may be due to the nature of PJF, which when reinforced into PLA reduced its brittleness by giving it additional plasticity. C4 had the highest elongation among all the composites. The coarse PJ reinforced composites individually showed larger variation in elongation with each consecutive sample (C1, C2, C3 and C4), while the fine PJ reinforced composites showed smaller increase in elongation values with each consecutive sample (F1, F2, F3 and F4), as shown in Figure 5b. Nasrin et al.[25], carried out a study where chitin was added to PLA. An addition of chitin content from 1, 5 upto 10% into PLA matrix, showed an increase in the tensile strength, tensile modulus

and elongation parallely. A study with epoxy-bagasse also showed increase in the elongation of the composite with increment in the filler content from 15 to 30 wt%[26].

3.2 Flexural strength The flexural strength of all the C and F composites increased with upto 20 wt% filler content, where a flexural strength of 59.95 MPa and 67.73 MPa were observed for C3 and F3 respectively. PLA had a flexural strength of 20.61 MPa as shown in Figure 6.

The maximum increase in flexural strength was 190% and 228% with addition of 20 wt% coarse and fine PJ particles, respectively, into the PLA matrix. At 25 wt% loading, the flexural strength dropped down to 56.49 MPa for the C4 sample and 58.51 MPa for the F4 sample. This was due to the dominating physical quantity of wood particle content which had eventually led to the loose bonding between the matrix and fiber. The matrix therefore could not achieve complete wetting of the fiber which inturn could not producce good fexural strength as of the 20 wt% filled composite. This study revealed that the F composites had superior flexural strength over the C composites in all the four cases of fiber loadings since smaller sized reinforcements provide better mechanical properties over larger sized reinforcement by providing larger surface area and better impergnation with the matrix material[24]. The flexural modulus increased with increasing filler content as shown in Table 2.

3.3 Impact strength

Figure 4. Tensile strength for PLA, Coarse fiber reinforced composites and Fine fiber reinforced composites.

Impact strength of the composites were greater than that of the polymer (0.45 J.mm-2) in all reinforcement levels as shown in Figure 7. C1, C2, C3 and C4 had an impact strength of 0.55, 0.65, 0.89 and 0.67 J.mm-2 respectively while C4 sample had a decrease in impact strength which is justified by the SEM image studies from the number of delamination formed on the fractured surface which were responsible for the resistance towards the testing force.

The number/area of delamination was lesser for C4 when compared to the C3 composite. This may be due to the

Figure 5. (a) Tensile modulus of coarse and fine fiber reinforced composites and (b) Percentage Elongation at break of coarse and fine fiber reinforced composites.

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Raj, S. S., & Kannan, T. K., & Rajasekar, R. Table 2. Flexural modulus for PLA, Coarse fiber reinforced composites and Fine fiber reinforced composites.

Name of the composite PLA C1 C2 C3 C4

% of fiber reinforcement 0 10 15 20 25

Flexural Modulus (GPa) 17.70 40.51 48.16 51.47 48.39

Figure 6. Flexural strength for PLA, Coarse fiber reinforced composites and Fine fiber reinforced composites.

Name of the composite PLA F1 F2 F3 F4

% of fiber reinforcement 0 10 15 20 25

Flexural Modulus (GPa) 17.70 44.26 50.22 58.16 50.24

dominating fiber content which was similar in the case of the reduction of the tensile strength of the C4 sample.

In the case of fine fiber reinforced composite materials, F1, F2, F3 and F4 had impact strengths of 0.60, 0.74, 1.09 and 0.92 J.mm-2 respectively. The nano particulate fibers at the highest fiber loading of 25 wt% dominated over the plasticization effect of the matrix material to help in absorbing and transferring the energy created by the impact test to the polymer matrix effectively. This was also evident through the morphological studies which clearly showed the areas of resistance towards the force during testing.

Comparing the coarse and the fine fiber specimens, the Fine particle reinforced samples had better impact strength over the coarse reinforced samples at all the four different fiber loadings. This was due to the fact that nano particles have better distribution in a polymer matrix when compared to micro particles, which might have helped in the even energy transfer within the composite during the impact test[27]. The addition of nano particle improving the impact strength of a polymer composite was also proved

by Nagalingam et al.[28]. The impact resistance was found to be highest at 20 wt% fiber loading for both variants of composites.

3.4 Hardness

Figure 7. Impact strength for PLA, Coarse fiber reinforced composites and Fine fiber reinforced composites.

Figure 8. Hardness for PLA, Coarse fiber reinforced composites and Fine fiber reinforced composites.

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Figure 8 shows that the Vickers Micro hardness of all the composite samples was lower than PLA in both the micro and nano reinforcement studies due to the addition of the PJ. Increase in fiber content had a reduced pattern on hardness values, the C1 and F1 composite had the highest hardness while C4 and F4 composite had the lowest hardness value within their categories.

This factor is in contradiction with the fact that hardness increses with increase in elastic modulus[29]. The observed reduction in hardness can only be assumed that the nature of PJ, since every wood fiber has its own distinctive property[30]. PJF might have had the ability to enhance the rigidity of PLA by providing increase in modulus by possesing good bonding ability. On the other hand PJF had given a plasticizer effect on PLA (the latter being a very brittle material by nature[31]), thereby turning PLA into a softer compound thereby justifying the increase in the elongation as well as reduction in the hardness with increasing particle reinforcement. The hardness of the F samples was greater than the C samples at all reinforcement levels since smaller particles provide better mechanical properties when compared to the larger particles[32]. This factor was also substantiated in a study carried our by Sifat et al.[33], where nano paricle reinforcement had shown better mechanical performance than the micro particles.

Polímeros, 30(1), e2020012, 2020

Influence of Prosopis Juliflora wood flour in Poly Lactic Acid – Developing a novel Bio-Wood Plastic Composite 3.5 Water absorption analysis The PLA sample did not show any water uptake while in both the coarse and the fine reinforced samples, an increase in fiber content showed increase in water absorption. Comparing the coarse and the fine fiber specimens, sample F1 having the least water absorption tendency among the F samples also proved superior to C1 which had the least water absorption tendency among the C samples. The C samples at all reinforcement levels had greater amount of water uptake when compared to the F samples as tabulated in Table 3. The Standard Deviation for all the mechanical properties and water absorption test that were analyzed are elaborated in Table 4.

Generally NaOH treatment on natural fibers lead to a phenomenon known as Super-swelling[34] which is responsible for greater water absorption tendancy of the fiber. In this

research the micro PJ particles may have undergone larger super swelling than the nano particulates thereby the showing a larger physical presence of wood particles in the matrix at the similar reinforcement ratios. For better understanding a diagramatic explanation is shown in Figure 9 where PLA matrix is filled with 25 micro particles as C composites and 25 nano particles as F composites. The amount of PLA is greater in the F composites thereby leading to lower water uptake.

3.6 Morphological study Figure 10 a, b, c and d show the FESEM images of C1, C2, C3 and C4. The morphological image of C1 shows large areas with insufficient PJ loading which was the reason behind the low mechanical properties exhibited by C1 when

Table 3. Water Absorption test results for PLA, Coarse fiber reinforced composites and Fine fiber reinforced composites.

SAMPLE PLA C1 C2 C3 C4 F1 F2 F3 F4

INITIAL WEIGHT OF THE SAMPLE(grams) 3.488 2.720 2.696 2.650 2.629 3.146 3.115 3.057 3.024

FINAL WEIGHT OF THE SAMPLE(grams) 3.488 2.722 2.699 2.657 2.638 3.147 3.117 3.061 3.031

% of water absorbed 0 0.073 0.111 0.263 0.341 0.031 0.062 0.119 0.198

Table 4. Standard Deviations.

Property Tensile Strength

Tensile Modulus

Elongation

Flexural Strength

Sample 1 2 3 4 5 MEAN SD 1 2 3 4 5 MEAN SD 1 2 3 4 5 MEAN SD 1 2 3 4 5 MEAN SD

PLA 9.67 9.35 11.02 9.76 10.45 10.05 0.67 0.91 1.17 1.46 1.53 1.23 1.26 0.25 0.91 1.05 1.01 0.91 0.93 0.96 0.057 20.08 19.5 21.87 20.92 20.68 20.61 0.89

Polímeros, 30(1), e2020012, 2020

C1 13.58 13.38 12.98 12.54 13.46 13.19 0.43 2.11 1.35 1.81 1.61 1.56 1.69 0.28 1.18 1.23 1.11 1.09 1.14 1.15 0.05 47.12 47.52 46.86 46.5 47.91 47.18 0.55

C2 18.96 17.62 18.53 17.8 17.95 18.17 0.56 2.17 2.35 2.48 2.1 2.51 2.32 0.18 2.01 2 1.91 2.04 2.09 2 0.06 55.4 56 56.79 56.41 55.81 56.08 0.53

C3 20.67 20.78 21.56 21.22 21.48 21.14 0.4 2.56 2.68 2.44 2.96 2.81 2.69 0.2 2.11 2.14 2.17 2.2 2.13 2.15 0.03 60.21 60.54 59.66 59.23 60.08 59.95 0.51

C4 21.15 21.31 20.36 20.88 20.2 20.78 0.48 2.84 2.57 2.68 2.63 2.97 2.74 0.16 3.36 3.49 3.42 3.38 3.4 3.41 0.04 56.68 56.82 55.67 56.31 57.07 56.49 0.54

F1 17.61 17.82 16.98 18.06 17.33 17.56 0.42 1.94 2.31 2.11 2.45 2.4 2.24 0.21 2.1 2.13 1.95 1.89 2.04 2.02 0.09 50.87 51.46 51.3 51.91 52.21 51.55 0.52

F2 19.29 20.27 19.89 19.12 19.64 19.64 0.46 2.31 2.49 2.58 2.38 2.73 2.5 0.17 2.42 2.31 2.29 2.25 2.34 2.32 0.05 57.85 58.72 58.19 59.23 58.46 58.49 0.52

F3 24.6 24.91 25.52 24.18 24.25 24.69 0.49 3.22 3.18 3.27 2.88 3.1 3.13 0.15 2.49 2.54 2.55 2.59 2.61 2.56 0.04 66.96 67.93 67.71 68.68 67.39 67.73 0.57

F4 20.93 21.18 22.21 22.38 21.61 21.66 0.56 2.46 2.83 2.78 2.46 2.52 2.61 0.18 2.63 2.63 2.65 2.72 2.67 2.66 0.03 58.54 59.34 58.9 57.79 57.98 58.51 0.57

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Raj, S. S., & Kannan, T. K., & Rajasekar, R. Table 4. Continued...

Property Flexural Modulus

Impact Strength

Micro Hardness

Water Absorption

Sample 1 2 3 4 5 MEAN SD 1 2 3 4 5 MEAN SD 1 2 3 4 5 MEAN SD 1 2 3 MEAN SD

PLA 17.28 16.61 17.85 18.32 18.45 17.7 0.68 0.49 0.45 0.38 0.42 0.51 0.45 0.05 33.5 31.32 31.65 33.03 32.5 32.4 0.8 0 0 0 0 0

C1 39.98 40.34 39.75 40.86 41.6 40.51 0.66 0.5 0.53 0.6 0.57 0.56 0.55 0.03 20.95 21.36 21.61 20.6 21.15 21.13 0.3 0.073 0.073 0.073 0.073 0

C2 47.24 47.79 48.5 48.22 49.05 48.16 0.61 0.75 0.69 0.61 0.61 0.58 0.65 0.06 18.9 19.44 19.76 20.1 19.95 19.63 0.4 0.111 0.103 0.119 0.111 0.008

Figure 9. Diagrammatic explanation of lower micro and nano fiber distribution into PLA.

compared to the higher reinforcement content composites.

C2 had better fiber distribution when compared with C1 which improved the mechanical properties over C1. C3 being the best sample among the C samples, shows clear evidence of superior mechanical properties form the large delaminates formed on its fractured surface during the testing. C4 has largely dominating fiber content and reduced amount of laminates; hence the mechanical strengths were comparatively reduced when compared to C3. Figure 10 e, f, g and h show the FESEM images of F1, F2, F3 and F4. The morphological images of the fine fiber reinforced composites had large areas displaying clear resistance to the shear force during testing. F3 sample having the highest mechanical properties when compared to F1, F2 and F4 clearly exhibits the largest area of resistance to the flexural force.

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C3 50.6 51.47 50.85 52.45 51.98 51.47 0.68 0.79 0.84 0.94 0.99 0.88 0.89 0.07 19.18 19.25 18.95 19.48 19.64 19.3 0.2 0.247 0.284 0.258 0.263 0.019

C4 48.15 47.46 48.82 49.11 48.41 48.39 0.57 0.59 0.62 0.69 0.79 0.66 0.67 0.07 16.41 16.66 16.11 17 16.82 16.60 0.3 0.365 0.32 0.335 0.34 0.022

F1 44.4 43.02 43.82 44.85 45.21 44.26 0.77 0.69 0.61 0.51 0.57 0.62 0.6 0.06 22.66 21.91 22.4 21.71 22.12 22.16 0.3 0..031 0.031 0.031 0.031 0

F2 51.11 50.88 49.24 49.57 50.3 50.22 0.72 0.71 0.68 0.75 0.77 0.8 0.74 0.04 21.28 21.09 21.47 20.86 20.95 21.13 0.2 0.057 0.06 0.069 0.062 0.006

F3 57.05 57.65 58.8 59.1 58.2 58.16 0.74 1.02 1.08 0.98 1.21 1.16 1.09 0.08 20.65 21.15 21.03 20.03 20.87 20.8 0.3 0.112 0.137 0.108 0.119 0.015

F4 50.82 49.78 50.35 51.05 49.2 50.24 0.68 0.81 1.01 0.97 0.88 0.93 0.92 0.07 20.02 19.15 19.32 19.68 20.13 19.66 0.4 0.19 0.185 0.219 0.198 0.018

The FESEM images of the 10 and 15 wt% PJ reinforced composites in both the coarse and fine cases show better delamination when compared with plain PLA. This evidently proves that the addition of PJF incresed the mechanical properties of the polymer. The increase in the mechanical properties with increasing PJF reinforcement into PLA may be due to the excellent bonding of the PJF with PLA matrix, thereby resulting in a good resistance towards the testing forces. It is also visible that PJF has even particulate distribution within the matrix material with each consecutive fiber increament upto 20 wt% beyond which agglomeration of reinforced particlutes reduces the mechanical properties. Figure 11 shows the FESEM image of plain PLA sample that had been scanned on its fractured surface after the flexural test. The very small amount of delammination in it shows that it had lower mechanical strength when compared witht the micro annd nano PJF reinforced composites.

The composites in both the C and F cases with 20 wt% fiber loading had the best mechanical properties which is extensively in synchronization with a couple of researches who had used various wood fibers with PLA [12,35].

Ediga et al.[36] had analyzed the mechanical properties of Epoxy-PJF composite and also concluded that 20 wt% of PJF reinforced composite had the best strengths which strongly supports the fiber loading parameter of this study.

The mechanical properties of the PJF/Epoxy composite[36] and PJF/PLA composite (this study), when compared showed that PJF performed as a better reinforcement with PLA than with epoxy.

Polímeros, 30(1), e2020012, 2020

Influence of Prosopis Juliflora wood flour in Poly Lactic Acid – Developing a novel Bio-Wood Plastic Composite

Figure 10. (a) FESEM image of C1, (b) FESEM image of C2, (c) FESEM image of C3, (d) FESEM image of C4, (e) FESEM image of F1, (f) FESEM image of F2, (g) FESEM image of F3, (h) FESEM image of F4.

3.7 Thermo Gravimetric Analysis (TGA) Thermal stability of PLA was good upto 250 °C beyond which it had a sharp degradation rate and the polymer reduced to a mass of just 6% at 400 °C as shown in Figure 12. There were no residue remains at 500 °C, since PLA had turned Polímeros, 30(1), e2020012, 2020

volatile. PJF began to degrade around 220 °C losing 7% mass and had slow degradation to nearing 270 °C. Between 300 to 400 °C there was a sharp decrease in the mass to 9%.

The F3 specimen had equivalent thermal stability to that of the PJF and PLA, following the rule of mixtures. Majority 9/11

Raj, S. S., & Kannan, T. K., & Rajasekar, R. strength and increased thermal stability respectively. PJF/PLA composite can be sucessfully recomended for the primarily usage and as an alternative for conventional plywood in the construction field.

5. References

Figure 11. FESEM image of PLA.

Figure 12. TGA curves for PLA, PJF and F3.

of the plant fibers are thermally stable upto 200 °C, while PJF had superior thermal degradation comparing to other commonly used natural fiber reinforcements. This may be due to the high lignin content in PJF which is known to be responsible for the thermal stability of natural fibers[8].

The lignin content in PJF is 17%[7] while other common plant fiber reinforcements like Flax, kenaf, jute, hemp and Sisal have a lignin content of 2, 9, 12, 10 and 9 respectively[37].

4. Conclusions The addition of novel PJF in the form of both coarse and fine fiber reinforcements into PLA had significantly improved the tensile, flexural and impact properties of the polymer. The excellent bonding ability and even distribution of PJF with PLA matrix is the reason behind the improvization of the mechanical properties of the polymer.

The PJF PLA composites in both the C and F variants had reduced hardness when compared to the plain PLA sample.

The most optimum ratio of reinforcement of PLA to PJF irrespective of the size of particulate that was reinforced was concluded to be 80:20 wt% On comparing the coarse and fine fiber reinforcements, the fine fiber reinforced composites revealed better mechanical strengths in all the cases of this investigation.

The large lignin content in PJF had greatly supported in improving the stiffness and thermal resistance of the composite material which was proved by the high flexural 10/11

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Received: May 05, 2020 Revised: June 12, 2020 Accepted: June 16, 2020

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Polímeros VOLUME XXX - Issue I - Jan./Mar., 2020

Prof. Hermann Staudinger 23/Mar/1881

100 years of Polymer Science São Paulo 994 St. São Carlos, SP, Brazil, 13560-340 Phone: +55 16 3374-3949 Email: abpol@abpol.org.br 2020

08/Sep/1965