J OURNAL OF FOREST PRODUCTS & INDUSTRIES , 2013, 2(1), 5-12 ISSN:2325 – 4513(PRINT) ISSN 2325 - 453X (ONLINE ) 5
Eco-friendly Vegetable Combination Tanning System for Production of Hair-on Shoe Upper Leather A.E Musa1 and G.A Gasmelseed2 Department of Leather Technology, College of Applied and Industrial Sciences, University of Bahri, Khartoum – Sudan, P.O.Box 12327 E mail: ali206w@hotmail.com Telephone: +249919440560 (2) Department of Leather Technology, College of Applied and Industrial Sciences, University of Bahri, Khartoum – Sudan, P.O.Box 12327 E mail: gurashigar@hotmail.com Telephone: +249919634134 (Received November 10, 2012; Accepted December 12, 2012) (1)
Abstract— Leather has remained a unique material for a long time. Visco-elasticity and pore size distributions are two important properties of leather that have rendered it a unique material. The ability to breathe and readjust to volume fluctuations of the foot has made leather a unique material of choice in footwear industry. The environmental pollution problems pressures on chromium and forced the leather industry to find the possible alternatives. In the present study, a combination tanning system based on a garad –aluminum tannage for the production of hair on upper leathers as a cleaner alternative is presented. Acacia nilotica spp nilotica pods (garad) from Sudan have been evaluated for its tanning characteristics in a combination tanning system based on garad and aluminum sulfate. Aluminum-garad (Al-garad) leathers tanned using 2% Al2O3; followed by 20% garad resulted in shrinkage temperature above 96°C. The uptake of garad tannin in garad-Al tanning system with garad (20%) followed by aluminum (2% Al2O3) has been found to be better than the tanning system of Al-garad. The combination tanning system provides significant reduction in the discharge of COD, BOD5 and TDS in the wastewater. Garad-Al combination tanning system resulted in leathers with good organoleptic and strength properties. The work presented in this paper established the use of garad and aluminum combination tanning system as an effective alternative cleaner tanning methodology. Index Terms— Garad; Combination tanning; Aluminum; Hair-on leather
I. INTRODUCTION
T
anning is a process in which the leather-making protein is permanently stabilized against heat, enzymatic biodegradation, and thermo mechanical stress [1]. In commercial practice, vegetable and chrome tanning methods are widely used. The vegetable tanning method does not need
* Corresponding author.
the prior preparation stage of pickling and therefore the contributions to pollution load from sulfate salts are lower. Vegetable tannins, however, are known to be hard to biodegrade [2], and hence wastes bearing vegetable tannins degrade slowly. Among the various tanning systems, chrome tanning is the most commonly used tanning system in commercial practice due to its ability to produce softer, weightless, bright-shade leathers with high wet heat resistance in a shorter time frame. More than 90% of the leathers processed globally contain chromium [3,4,5]. In chrome tanning, the cationic matrix (pickled pelt) is treated with BCS (Basic Chromium Sulphate). Diffusion of chromium (III) salts into the skin matrix at pH < 3.0 initially leads to ionic interactions [6]. Over longer durations of tanning, readjustments in pH of the medium around 3.8–4.0 lead to irreversible binding of Cr(III) salts to the protein through coordinate covalent bonding [7]. The addition of an alkali, sodium bicarbonate, not only increases the pH to the pKa value of carboxy amino acids but also increases the basicity (Schorlemmer basicity, defined as the percentage of number of hydroxyl groups combined with one atom of chromium [8] of Cr(III) species [9]. Even though chrome tanned leathers have many significant advantages, they suffer from the serious disadvantage due to the constraints of its discharge norms of 2ppm [10]. It is well established that hexavalent chromium is carcinogenic [11]. It has also been shown that there is a possibility for the formation of chromium(VI) in leather during the processing conditions [12,13]. Some reports suggest that at higher levels and under certain ligand environments, chromium(III) also is toxic[14]. The problem is provoked by the fact that the conventional chrome tanning procedures results in an uptake of only 60-65% of the chromium offered to the leather and hence a substantial amount of chrome is discharged into the effluent [15]. Leather is a unique commodity that links the rural farmer to the fashion world. Leather as a natural material offers numerous advantages over synthetics, namely, aesthetic
J OURNAL OF FOREST PRODUCTS & INDUSTRIES , 2013, 2(1), 5-12 ISSN:2325 â&#x20AC;&#x201C; 4513(PRINT) ISSN 2325 - 453X (ONLINE ) 6 appeal, feel, texture, and breathability. Major product applications for leather are leather goods, garments, and footwear. More than 60% of the leathers produced are being converted to footwear [16]. There has been a continuous development in the understanding of technology of tanning. The search for alternatives to chromium has been both intensive and extensive. Viable alternatives to chromium have not yet been forthcoming. Many metal ions exhibit a degree of tanning activity. On application of objective criteria for the selection of technically as well as commercially viable mineral tanning material, metal salts suitable for tanning is restricted to those of Al(III), Si(VI), Fe(III), Ti(IV), Zr(IV) and Ce(III) or (IV) other than Cr(III) [17]. The Romans employed aluminium for tanning of skins and fur some 2000 years ago. It is considered to be an incomplete tannage [18]. Unlike chromium(III), aluminium(III) does not form stable coordination complexes. The interactions of Al(III) with protein are predominantly electrovalent [19]. Bound aluminium(III) salt is easily reversed. Formate or citrate masked basic aluminium sulfate salt has found better application in tanning [19, 20, 21]. With such masked salts, reversibility of the tannage is minimized. Sudan has various indigenous tanning materials. Some of these, such as Garad pods (Acacia nilotica sub. sp. nilotica) and Talh bark (Acacia seyal) are used extensively in the Sudan by rural tanners. The tannin content of garad pods is fairly high and amounts to approximately 30% of the total weight, soluble nontans are nearly 20%, while moisture and insolubles make up the remainder. The main constituent of the garad tannin is presumably leucocyanidin gallate i.e. gallic acid esterified with a flavanoid (Fig. 1). Garad tannin is reported to contain chebulinic acid, gallic acid and to have a high sugar content, factors which are common in hydrolysable tanning materials. Garad tannins are therefore mixed tannins i.e. containing condensed tannins as well as hydrolysable tannins containing gallic acid esterified with glucose [22]. When garad pods are crushed, they disintegrate into three parts, the husk with about 12% pure tannins, the seeds with no tannin content and the grain powder with approximately 55% tannins. The seeds and husk form about 63.6% of the weight of the pod, the remainder being the grain powder [22].The dressing of calfskins and other similar skins with hair on for upper leather purposes in one of the fashion requirements which occur regularly from time to time. Only skins free from bacterial damage should be used, as loose hair and hair slip during processing causes a big drop in quality. The main problem is to get the leather soft, as one cannot use the conventional tannery processes which cause loosening of hair [23].
O
A
C
B
Figure 1. Flavonoid
II. MATERIALS AND METHODS Preparation of basic Aluminum sulphate solution A known amount of Aluminum sulphate has been taken in a beaker and 150% of water (% based on the weight of Aluminum sulphate) has been added and the solution stirred for 15-20 minutes, subsequently required amount of ligand (sodium citrate and sodium tartrate) have been added and stirring has been continued for 45 min followed by slow addition of sodium carbonate until the pH has been raised to 3.5. For 0.5M of Aluminum sulphate 0.1M of ligand has been added. Garad and Aluminum Combination Tannage Sheep skins with hair on were employed for combination tanning trials; garad-Al and Al-garad tanning processes are given in Table 1 and 2, respectively. Four Sheep skins were used for each trial. The amount of aluminum sulfate used for the tanning trials was 2% Al2O3 in both the experimental processes. A control tanning process was carried out using garad only as given in Table 3. The post tanning process as mentioned in Table 4 was followed for experimental and control leathers. All the tanning and post tanning experiments were carried out using smaller drums with a capacity to process five skins. Measurement of hydrothermal stability of leathers The shrinkage temperature of control and experimental leathers has been determined using Theis shrinkage tester [24]. 2X0.5 cm piece of tanned leather cut from the official sampling position has been clamped between the jaws of the clamp and has been immersed in solution containing 3:1 glycerol: water mixture. The solution has been continuously stirred using mechanical stirrer attached to the shrinkage tester. The temperature of the solution has been gradually increased and the temperature at which the sample shrinks has been measured as the shrinkage temperature of the leathers.
J OURNAL OF FOREST PRODUCTS & INDUSTRIES , 2013, 2(1), 5-12 ISSN:2325 – 4513(PRINT) ISSN 2325 - 453X (ONLINE ) 7
Table 1 Formulation of garad-aluminum tanning system (experimental) for fresh sheep skin to produce Hair-on Upper Leather Process
%
Product
Duration
Remarks
(min) Soaking
300
Water
Pickling
10 1.2
Salt H2SO4
Adjustment of the pH
100
Water
0.75 2
Tanning
10 120
pH 2.8 - 3
Sodium bicarbonate
3 15
pH 4.5 -4.7
Basyntan P
30
(phenolic syntan)
Basification
10
Garad powder
120
10
Garad powder
120
2
Al2O3 (prepared Aluminium sulphate solution))
90
0.75
Sodium bicarbonate
Visual assessment of the crust leather Experimental and control crust leathers were assessed for softness, fullness, grain smoothness, grain tightness (break), general appearance and dye uniformity by hand and visual examination. Three experienced tanners rated the leathers on a scale of 0-10 points for each functional property, where higher points indicate better property.
3 15
Check the pH to be 4. Drain the bath and pile overnight. Next day sammed and shaved to 1.2 mm. The shaved weight noted.
Samples for various physical tests from experimental and control crust leathers have been obtained as per IULTCS methods [25]. Specimens have been conditioned at 20 2 oC and 65 2 % R.H (relative humidity) over a period of 48 hrs. Physical properties such as tensile strength, percentage elongation at break [26], grain crack strength [27] and tear strength [28] have been measured as per standard procedures. Each value reported is an average of four samples (2 values along the backbone and 2 values across the back bone).
Physical testing and hand evaluation of leathers Analysis of spent liquors from tanning trials Experimental and control crust leathers have also been assessed for softness, fullness, grain smoothness, grain tightness (break), general appearance and dye uniformity by hand and visual examination. Three experienced tanners rated the leathers on a scale of 0-10 points for each functional property, where higher values indicate better property of leathers.
The spent tannin liquor from control and experimental tanning processes was collected, filtered and analyzed for chemical oxygen demand (COD), biochemical oxygen demand (BOD5), and total dissolved solids (TDS) as per standard procedures [29].
J OURNAL OF FOREST PRODUCTS & INDUSTRIES , 2013, 2(1), 5-12 ISSN:2325 – 4513(PRINT) ISSN 2325 - 453X (ONLINE ) 8 Table 2 Formulation of Aluminum-Garad combination tanning system (Experimental) for fresh sheep skin to produce Hair- on Upper Leather Process
%
Product
Duration
Remarks
(min) Soaking Pickling
300 10 1.2
Water Salt H2SO4
Aluminum tanning
2
Al2 O3 (prepared Aluminum sulphate solution)
Adjustment of pH
0.75
Sodium bicarbonate
Garad tanning
2
Basyntan P (phenolic syntan)
30
10
Garad powder
90
10
Garad powder
90
0.5
Formic acid
3 10 +30
Fixing
10 120
pH 2.8 - 3
120
3 15
pH 4.5 -4.7
Check the pH to be 3.5. Drain the bath and pile overnight. Next day sammed and shaved to 1.2 mm. The shaved weight noted.
Table 3 Formulation of control garad tanning process for fresh sheep skin to produce Hair- on Upper Leather Process
%
Product
Duration
Remarks
(min) Soaking
Water
300
Pickling
Salt H2SO4
10 1.2
Adjustment of the pH
100
Water
0.75
Sodium bicarbonate
3 15
2
Basyntan P (phenolic syntan)
30
10
Garad powder
120
10
Garad powder
120
Fixing
0.25
Formic acid
3 10 + 30
Washing
300
Water
10
Tanning
10 120
pH 2.8 - 3
pH 4.5 -4.7
Check the pH to be 3.5. Drain the bath and pile overnight. Next day sammed and shaved to 1.2 mm. The shaved weight noted.
J OURNAL OF FOREST PRODUCTS & INDUSTRIES , 2013, 2(1), 5-12 ISSN:2325 – 4513(PRINT) ISSN 2325 - 453X (ONLINE ) 9 Table 4 Formulation of post-tanning process for control and experimental leathers Process % Product Duration (min) Washing 200 Water 10 Neutralization 0.75 Sodium bicarbonate 3 15 Pre-retannage 100 Water 2 Relugan RE (Acrylic syntan) 40 Pre-fatliquor 2 Lipoderm liquor SAF 40 (Synthetic fatliquor) 2 Basyntan DI 30 Fatliquoring 3 Lipoderm liquor SAF (Synthetic fatliquor) 4 LB II 40 Retanning 3 Basyntan DI 4 Basyntan FB6(phenolic 40 syntan) Fixing 1 Formic acid 3 10 + 30
Remarks
pH: 5-5.5
pH 3.5
* - % chemical offer is based on shaved weight of the tanned leather Analysis of exhaustion of tanning spent liquors Spent garad liquor from control and experimental tanning processes was collected and analyzed for the tannin concentration using a spectrophotometric method by measuring the absorbance value at λmax of the tannin used, after suitably diluting the spent tannin liquor, using UV-visible spectrophotometer (Hitachi, Japan). % Garad exhaustion = [(Co – Cs)/Co] x 100 Where Co is the concentration of garad offered and Cs is the concentration of garad in the spent liquor. Evaluation of chemical constituents in leathers The chemical constituents such as total ash content, % moisture, % oils and fats, % water soluble, % hide substance, % insoluble ash and degree of tannage have been carried out for control and experimental leathers according to standard procedures [30].
shrinkage temperature compared to garad control leathers. The garad–Al combination tanning provides shrinkage temperature of 100oC compared to 84oC for control. The exhaustion of garad for Al–garad and garad–Al and control (garad tanning) are given in Table 5. It is observed that there is an increase in the amount of garad fixed in the presence of Al and increased exhaustion of garad observed can be semi-quantitatively related to the increase in shrinkage temperature of combination tanning systems of garad. Organoleptic properties of Hair on-crust leathers for experimental and control The organoleptic properties (visual assessment) of crust leathers from experimental and control is given in Fig.2. From the figure, it is observed that experimental crust leathers from Garad-Al tanning exhibited good softness, fullness, smoothness, general appearance and dye uniformity compared to Al - Garad crust leathers and control leathers from Garad tannage.
III. RESULTS AND DISCUSSION Combination tanning using garad–aluminium with a Al2O3 offer of 2%, keeping the offer of garad constant at 20%, and changing the order of addition was carried out. Though the tanning system using garad and Al are eco-friendlier, it is essential to study the properties of the leathers whether it is comparable to that of chrome tanning system. The thermal stability of chrome tanned leathers is well known to be greater than 100oC. The shrinkage temperature data for various combinations are given in Table 5. It is seen from the table that just by the use of 2% of Al2O3 in combination with garad exhibited more than 10oC increase in
Physical strength characteristics of Hair on-crust leathers for Experimental and control The physical strength measurements of matched pair experimental (Al-Garad and Garad - Al) and control leathers (Garad) are given in Table 6. The physical strength measurements viz., tensile strength, elongation, tear strength, load at grain crack and distension at grain has been found to be comparable. The experimental tanning resulted in leathers with good physical strength characteristics compared to control leathers.
J OURNAL OF FOREST PRODUCTS & INDUSTRIES , 2013, 2(1), 5-12 ISSN:2325 – 4513(PRINT) ISSN 2325 - 453X (ONLINE ) 10 Table 5 Shrinkage temperature and % exhaustion of control and experimental tanning processes Experiment Shrinkage temperature (oC) Exhaustion % Al- Garad (2% Al2 O3)
96±1
83±2
Garad -Al (2% Al2 O3)
101±1
87±2
Garad (Control)
84±1
76±2
10 Garad-Al Al-Garad Garad (Control)
9 8 7
Rating
6 5 4 3 2 1 0 Softness
Fullness
Grain tightness
Grain General smoothness appearance
Dye uniformity
Bulk properties Softness
Fullness Grain tighness Grain smoothness General appearance Dye uniformity
Figure 2: Graphical representation of organoleptic properties of the Experimental and Control leather Table 6 Physical strength characteristics of experimental and control Hair on - crust leathers Parameter
Al- Garad
Garad - Al
Control (Garad)
Tensile strength (Kg/cm )
230±2
250±2
210±3
Elongation at break (%)
46±0.61
56±0.61
41±1.48
Tear strength (Kg/cm)
41±061
44±0.61
40±0.61
Load at grain crack (Kg)
23±0.61
22±0.61
25±0.61
Distention at grain crack (mm)
10±0.61
11±0.61
10±0.61
2
Chemical Analysis of the Crust Leather The chemical analysis of crust leathers from control and experimental tanning trials are given in Table 7. The chemical analysis data for the experimental leathers is comparable to the control leathers. However, the water soluble matter for the control leathers (total ash content is high in control leather) is more than the experimental leathers. Spent Liquor Analysis The spent tan liquor contains highly organic matter in both control and experimental process which could lead to the contribution of high COD, dissolved and suspended solids.
Hence, it is vital to assess the environmental impact from control and experimental tanning process. The COD, BOD5 and TDS of the spent liquor for experimental and control trials have been determined and are given in Table 8. From the table, it is observed that the COD, BOD5 and TDS of the spent liquor processed using both the experimental tanning system are lower than the spent liquor from Garad tanning (control). The BOD5 and TDS of the spent liquor processed from Garad and aluminum combination tanning trials have significantly reduced compared to the spent liquor of control Garad tanning trial. This could be due to increased exhaustion of Garad during tanning, which is also observed from the exhaustion data of Garad given in Table 5.
J OURNAL OF FOREST PRODUCTS & INDUSTRIES , 2013, 2(1), 5-12 ISSN:2325 – 4513(PRINT) ISSN 2325 - 453X (ONLINE ) 11 Table 7 Chemical Analysis of Hair on- crust leather of experimental and control Parameter
Garad (control)
Al- Garad
Garad -Al
Moisture %
14.00
13.20
12.60
Total ash content %
2.50
2.30
2.50
Fats and oils %
3.40
3.30
2.80
Water soluble matter %
5.00
3.40
3.30
Hide substance %
52
51
53
Insoluble ash %
1.20
1.30
1.40
Degree of tannage %
48.85
54.51
50.76
Garad- 20%; Al2O3- 2%
Experiment Garad (Control) Al- Garad Garad -Al
Table 8 Characteristic of spent liquor for experimental and control COD % reduction BOD5 % reduction (mg\l) in COD (mg\l) in BOD 117600±2900 23000±900 103600±3100 12 14000±1200 39 102200±2750 13 11300±550 51
IV. CONCLUSIONS In the present study, an attempt has been made to produce hair-on upper leathers using a new eco-friendlier combination tanning process based on Garad and aluminum. It is seen that combination tanning using Garad (20%) followed by aluminum (2% Al2O3) resulted in leathers with shrinkage temperature of 101°C, which is 17°C more than the control (Garad tanned) leathers. Aluminum followed by Garad tanning resulted in leathers with shrinkage temperature 96°C. The exhaustion of Garad in this combination system was found to be greater than 80%. The physical and chemical characteristics of experimental leathers are comparable to control leathers. The experimental leathers are softer than the control leathers. The combination tanning using Garad and aluminum appears to be an eco-friendlier option and results in leathers with good thermal stability and organoleptic properties that is important for commercial viability of the tanning system. REFERENCES [1]. Ramasami, T. (2001). Approach towards a unified theory for tanning: Wilsons dream, J. Am. Leather Chem. Assoc. 96, 290–304.
TDS (mg\l) 92000±1500 42800±1050 40500±1000
% reduction in TDS 53.5 56
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