The tanning process is one of the important unit operations in leather processing where the tanned leather is obtained by the treatment of tanning agents. This operation provides permanent stability to the skin material through the form of leather. Chrome tanning process is widely employed and accepted method to achieve desirable properties in the final leather. The exhaustion property in tanning is enhanced by using protein based product in the tanning process. The protein based product that provides various functional groups enhance the crosslinking effect was characterised and used in the chrome tanning process. The chromium uptake in the tanning process and the exhaustion levels achieved by using the product for various experiments are discussed in the present investigation.
3.1. Characterisation of the product
The protein based product synthesised from skin trimming waste was characterised. The Table 2 and Figure 1 shows the characteristic features of the product. The pH of the product was 4.0, particle size was1538 nm, relative viscosity was 1.006 cP and % solids were of 6.85. In overall the product showed that the low-molecular-weight polymer with the ability to interact with collagen molecules at molecular level provides the necessary functional groups for the crosslinking of chromium with collagen in the tanning process.
3.2. Chromium uptake in the tanning process
The newer chrome tanning experiment carried out using the product prepared from skin trimming wastes was evaluated and found out. Initially, the waste was hydrolysed and then modified to contain more functional groups responsible for enhanced uptake of chromium in the tanning experiments. The influence of the product in the chrome tanning process was thoroughly studied. Three sets of the experiments were carried out using product at the level of 2, 4 and 6% in the tanning experiments. The uptake of chromium in the tanning bath was estimated and presented in the Table 3. It is seen from the table that the experiment carried out using the product at the level of 2% gave the chromium uptake (as Cr2O3) of 0.55%. Similarly, the other chrome tanning experiments carried out with the product at the level of 4 and 6% exhibited the chromium (%Cr2O3) presence of 0.58 and 0.60% respectively against the conventional control sample of 1.90% and another control sample using commercial product of 0.59%. It is inferred from the results that the experiment carried out using the product at the level of 2% gave exhaustion of chromium of 90% in the tanning process. Similarly, the other experiments carried out using the product at the level of 4 and 6% showed the exhaustion of chromium of 90%, 93% respectively. The conventional chrome tanning experiment showed the exhaustion of chromium at the level of 79% and another one using equivalent commercial product exhibited exhaustion of 88%. The maximum chromium uptake was found in the experimental sample carried out using the product at the level of 6%. The reason for enhancing chromium uptake in the tanning process in the experimental samples may be due to the free functional carboxylic groups, aldehyde groups, OH groups and amide groups involved in cross-linking of chromium with collagen moieties. Several researchers reported that auxiliaries exhibiting functional groups such as carboxylic acid, amide groups, OH groups favour the uptake of chromium in chrome tanning process.
3.3. Fourier Infrared (FT-IR) spectroscopic analysis
The control sample and experimental samples of tanning process with application of product (2%, 4% and 6% product) were subjected to FTIR analysis. Separate FTIR spectrum for each sample with characteristic wavenumbers may be viewed in Figure 2. In case of experimental sample (2%), a broad and strong band can be seen in Figure 2(a) at 3388 cm-1 which appeared due to presence of stretching frequency of –OH group of product. The peak visible at 1712 cm-1 is due to stretching of C=O which (aldehyde) mostly enhances adsorption of chromium and is responsible for improved uptake. The peak at 1638 cm-1 has appeared due to presence of N-H bending of amide groups of product from the active sites. The peak at 1104 cm-1 is due to C-N stretching of amide groups observed in the 2% sample and strong broad peak at 2926 cm-1 is because of the presence of O-H stretching of carboxylic acid group. Thus, the spectra give evidences of existence of –OH, -C=O, -NH-, -COOH and amide active groups in the sample. A medium peak (in case of 4% sample, Figure 2b near 832 cm-1 shows the presence of C-O-H bending at out-of-plane. Similarly, the experimental sample carried out using the product at 6% showed the presence at 2929 cm-1 is because of the presence of O-H stretching of carboxylic acid group at 1712 cm-1 is due to stretching of C=O which represents aldehyde group. It can be observed that –COOH group is absent in the control sample (Figure 2e). In the control sample (treating with commercial equivalent product at 2%), aldehyde moieties are almost saturated while peaks corresponding to –COOH groups are still available to take part in further adsorption of chromium. It is inferred from the FT-IR investigation that product prepared from skin trimmings contained the functional groups of aldehyde which may be due to the conversion of amino acids in treating with sodium perborate in oxidation process. Besides, the product contained carboxylic acids, amide groups that favour the exhaustion of chromium in the tanning process. On the contrary, the control sample that was carried out using only BCS has not shown any indication of carboxylic acid groups and other control sample carried out using commercial equivalent product has clearly shown the presence of carboxylic acid group and not aldehyde group. Several researchers have made an attempts to increase the exhaustion of chromium in tanning process. Application of akovite−alumino silicate nano-composite (Ciuffi et al., 2012), sulphonic aromatic acid (Bcardit et al., 2008), oxazolidine derivatives (Covington, 1997), complexing agents such as sodium formate and disodium phthalate with nanoclay (Zhang et al., 2019), silk hydrolysate (Aslan et al., 2013), polycarboxylic material (Anggriyani and Nugroho, 2018), fleshing hydrolyzate (Kanagaraj et al., 2002), complexing agents such as sodium formate and disodium phthalate with nanoclay (Zhang et al., 2019), keratin hydrolyzate (Ramamurthy et al., 2015), methanosulfonic acid (MSA) (Zhang et al., 2018), gallic acid (Ramamurthy et al., 2014) in tanning process enhanced the exhaustion level of chromium to higher value.
3.4. Shrinkage temperature of the tanned leather
Shrinkage temperature of leather is one of the primary factor to be considered in tanning process because it gives the overall picture of the efficacy of tanning system in terms of type of cross-links formed, stability achieved and bonds formed, as simply, it can be defined as the measure of endothermic reaction resulting in the breakdown of hydrogen bonds in the polypeptide chain of collagen. The present work focuses on high exhaust tanning system with the protein based product developed from skin trimmings. The tanned leather was subjected to shrinkage temperature measurements and the results are given in Table 4. The results showed that the leather produced with various experiments showed better shrinkage temperature measurements. It can be seen form the table that experiment carried out with product at the level of 2% exhibited shrinkage temperature of 107 ± 0.5 oC. Similarly, the leather obtained from other experiments using the product at the level of 4 and 6% showed shrinkage temperatures of 110 ± 1.0 and 112 ± 0.7 oC respectively. The leather produced from control experiments using commercial product showed shrinkage temperature of the leather with the value of 107 ± 0.5 and 106 ± 0.5 oC respectively. It is evident from the results that all the experimental leather showed shrinkage temperature of more than 100 oC which was due to multipoint hydrogen bonds and covalent cross-links between the product, collagen and chromium linkages. Similar results were reported from the other researchers also (Fuchs et al., 1993).
Gallic acid-graphene hydrogel (GA-GH) adsorbent with 3D porous architectures fabricated using a facile in situ reducing-assembly design effectively removes Cr(iii) and organic dye from tannery wastewater. Besides, GA-GH can be regenerated at pH 2.0 with hydro chloric acid and has the tendency to retain high adsorption capacity after multiple adsorption–desorption cycles. In addition, modified GA on graphene sheets with benzene group also improves adsorption of organic dyes. This adsorption is due to the π–π interactions between GA-GH and aromatic dyes (Liu et al., 2019).
Literature also reports that a new product of chrome syntan has been developed that showed more than 90% uptake of chromium in tanning process. This product act as both tanning and retanning agent which can be applied to the delimed pelts preventing the important step in leather processing namely pickling process that employs huge quantity of salt. The product helps in reducing chemical oxygen demand (COD), total dissolved solids (TDS) and chlorides to the level of 51, 81 and 99%, respectively in the tanning process (Suresh et al., 2001).
Gallic acid (GA: Trihydroxybenzoic acid) assisted chrome tanning system has been developed. It has been found from the experiment that the chrome uptake at the level of 93% has been achieved through this method. In addition to that, this method produced the leather with better organoleptic properties such as softness and fullness. This chrome tanning system reduces the pollution load especially TDS in the spent liquor (Ramamurthy et al., 2014).
In another study, application of polyamidoamine hyperbranched polymer (HPAM) as a pretanning agent for the depickled hides increased the chromium uptake in the tanning process. The optimum concentration of HPAM (5.0 wt.%) when applied to tanning process led to the increased chromium exhaustion up to 99.8% for the residual chrome concentration in the tanning bath of 2.15 mg/L. The leather produced with the tanning system exhibited better softness and shrinkage temperature of the leather (Ibrahim et al., 2013).
Pickle-free chrome tanning system using polymeric syntan has been attempted. Application of this syntan increases chromium uptake to the level of more than 90%. This system also reduces COD, TDS, chlorides in the tanning process to the level of 50, 80, 90% respectively (Rao et al., 2004).
Recycling and reusing of tanning floats by adopting masked chromium salts instead of commonly used chromium ones has been attempted. This process skips/ eliminates basification process and overcomes the change of pH in the process system thereby reducing the generation of considerable amount of neutral salts. The exhaust bath can be recycled and reused for several times. The present method helps in saving the water (90%) and also produces leather of better quality (Morera et al., 2011).
3.5. SEM-EDX analyses
The experimental samples using the product at 2%, 4% and 6% levels in tanning processes were collected; specimen was prepared and imaged for SEM-EDX in JEOL JSM-5300 scanning electron microscope. The images were obtained at magnification of 100 µmX315, 20µmX1.150, 20µmX1.360K, 20µmX1.59K respectively to study morphology of the final product. The results of SEM-EDX investigation are presented in Figure 3. The SEM micrographs with different resolutions show two types of grains. One type consists of thick conjugated fibrous and the other is of globular crystalline particles. The first one represents the presence of protein based product and the second one shows the presence of chromium (Cr). In case of pure or product sample, the first type consists of longer fibrous structures representing free carboxylic acid moieties/ groups which functionally crosslinks chromium. The experimental sample with 4% product shows that chromium is adsorbed on fibrous particles; the active sites of the product are responsible for improved exhaustion as evident by the presence of globular grain particles on the fibrous structures having some unoccupied sites left also. The experimental sample with 6% product shows that reactive sites of the product are almost occupied by chromium. This image also reveals that chromium particles are adsorbed by active sites of product; almost complete occupation of active sites leaving no un-occupied structures representing complete exhaustion of chromium.
Chromium uptake by the product has also been confirmed and estimated by SEM-EDX studies. The Figure shows stratified layers of fibrous structure containing carbon and oxygen with an equivalent amount of chromium as weight % of 56.5 and 43.4 corresponding to atomic % of 63.4 and 36.5 respectively. Experimental sample with 2% product shows an adsorption of equivalent chromium of 4.48 weight % corresponding to an atomic % of 2.34. This indicates higher amount of un-adsorption leaving lower amount of chromium for exhaustion. Sample with 4% of product shows exhaustion of 2.31 weight % corresponding to atomic % of 0.75 indicating moderate amount of chromium present in the sample indicating higher amount of adsorption by the substrate. The images with 6% of product reveal exhaustion of 5.24 weight % corresponding to atomic % of 1.67 showing better exhaustion of chromium by the substrate. This confirms that the product is capable of increasing the exhaustion of chromium using its active binding sites such as carboxylic acid groups. The SEM figures 4 & 5 indicating the control samples containing globular crystalline particles and representing more amount of chromium in the control samples and the protein based product sample representing thick conjugated fibrous particles representing free carboxylic acid moieties / groups which functionally crosslinks chromium.
3.6. AFM analyses
Experimental and control samples were subjected to AFM study using different magnifications (2, 3, 8, 25 µm and 800 nm) to analyse the topography and deflections of samples. The images obtained from AFM analyses of the experimental samples using product at the level of 2, 4 and 6% are provided in Figure 4. It can be seen that the product sample presented in Figure 4 portrays randomly scattered crystalline particles of the product.
It is seen from the figure that crystalline particles, spike, junk or fibrous particles are the protein based product (pbp) containing the functional groups. In other words, AFM images show two types of structures (i) smaller darker globular structures indicating presence of Cr(III) and other spots of linear fragments represent the protein based product. The non-uniformly distributed particles are ready for adsorption. This sample contains free functional groups of carboxylic acid which can adsorb chromium. However, deflection of mean of particles as represented by line graph indicates uniform distribution of chromium except at higher sizes (near 25 µm) where few spikes are visible with less spread of higher sizes of particles. Isometric views of product as observed in Figure also confirm similar results/ findings. Generally during tanning, product combines with collagen fibres leaving less numbers of grooves of product – particles. From the deflection image it is evident that the product helped in enhancing the exhaustion of chromium from aqueous solution leading to better crosslink with collagen fibres giving rise to a complex structure.
Figure 4a shows AFM image for the case of adding the prepared product at the level of 2% in chrome tanning process. It can be observed from the Figure that there are two types of patches, one is due to the product-particles; whereas the other is due to chromium species appearing like small-darker globular structures. Presence of lesser number of functional groups (compared to other images) confirms that most of the product has been exhausted by the collagen fibres. Deflection image also shows that product has adsorbed the chromium from bath resulting into consequent binding with collagen matrix and forming a complex to stabilize the putrefaction of skin/ hide proteins further. Corresponding line graph of the experimental sample (with 2% product) also reveals uniform distribution over mean fit. Unadsorbed moieties give rise to few spikes; however, surface fit-line of deflection image describe overall uniform exhaustion.
In case of experiment with 4% product, the rate of exhaustion was found along with AFM images where the topography and deflections can be visible in Figure 4b. It can be found that whitish particles in the image are due to unused product moiety. Chromium particles are almost adsorbed at a slightly higher rate than the earlier case (with 4% product). The deflection pattern can be visualized as uniformly distributed around the mean value as it is evident from line graph of corresponding experimental sample. Improved exhaustion was further visualized in case of experimental sample with 6% product, as can be seen from AFM image as shown in Figure 4C. The line graph for the deflection image shows absence of chromium particles/ spots confirming complete exhaustion of chromium by active functional groups of protein and amino acids of collagen. The deflection image shows few left over particles of product in the experimental sample.
From the topography image, it can be stated that the exhaustion of chromium created some roughness in surface. The faded lighter portions reveal about scope for more exhaustion of chromium; while the darker spots hint about adsorbed particles of chromium. The experimental sample made-up with 6% product showed a line graph of mean fit to represent deflections which can be seen to be distributed evenly around the mean. This indicates improved exhaustion of chromium and smoothness of surface compared to cases with 2 and 4% product. Thus AFM studies conclude that increase in the amount of product improves exhaustion of chromium and smoothness surface of the film.
The control sample Figure 4d shows the presence of small darker globular structures indicating presence of chromium and the raw protein based product revealed the presence of more amount of crystalline structure representing functional groups of the product. Hence the AFM analysis pours some in-depth view of the interaction of product-chromium-collagen compounds favouring the enhanced uptake of chromium.
3.7. XRD analyses
The samples were subjected to XRD analyses to assess the morphology and distance among functional moieties of the product. The results can be seen in Figure 5 where the pattern of images for the product during tanning at different levels (2, 4 and 6%) can be viewed. In case of the experimental samples, two broad peaks at 2θ= 18o, 22 o, 32o and 40o are observed which slowly flatten out. These are similar to collagen samples collected from scrap leather of pure-collagen. The diffraction peaks at 2θ= 10o- 80o are due to functional groups of protein based product in tanning process. However, peaks were not visible at 32o in case of experimental sample which reveals that functional groups have cross-linked with Cr(III) and collagen increasing the uptake property of tannin. This also proves good compatibility among molecules of collagen, co-polymeric nature of the product and reagents. The increase in the intensity of peaks in case of the experimental sample shows that functional moieties are displaced by large angles decreasing the crystalline structure of –NH2 groups. Also, agglomeration of amide groups tries to extend the amorphous region due to exhaustion of chromium and its fixation with functional moieties.
3.8. Circular dichroism (CD) spectroscopy analyses
Circular dichroism spectroscopy is used to find chirality in the molecules of sample by measuring the difference in absorption between left and right circularly polarised lights in optically active substances. The results on CD spectroscopy are shown in Figure 6. It can be seen that mean residue ellipticity for the control sample (marked as O) is in the negative domain, indicating contamination in the sample during following standard protocol of the prepared sample. All other samples show chirality near by 210 nm revealing presence of other species around 210 nm with triple helical & denatured spectra of collagen matrix.
3.9 Other schemes of increasing uptake
Literature reports that a newer chrome tanning system using methanosulfonic acid (MSA) was developed. The chrome tanning was conducted at pH 5.0. The results showed that chromium uptake was enhanced from 81.0% to 95.8% in the tanning process. The chromium dosage was decreased by 26.7% and the residual chrome concentration was also decreased by 44-85% with various operations. Besides, the experiment also gave comparable organoleptic properties, area yields and mechanical properties (Zhang et al., 2016).
Literature report was available to enhance the exhaustion of chromium to more than 90% in tanning process. Sometime intrinsically modified chrome salts improve the exhaustion level of the chromium with rational pickling and tanning have been attempted by various researchers (Thanikaivelan et al., 2002). The mechanism behind high exhaust tanning system using modified tanning salt is that preparation of tanning salts sans low-affinity species for increasing the exhaustion levels or affinity levels in the chrome tanning process.
The mechanism of chrome tanning process is a chemical process that involves formation of complex with salts and protein matrices at the molecular level by both inter and intra-chain crosslinks with triple helices, penta-fibril and fibrillary assemblies and fibre level hierarchies of skin structure. These crosslinks at the long range order of the collagen matrix provides functional stabilities such as shrinkage temperature and strength properties of the resultant leather (Gayathri et al., 1999).
In a similar work for enhancing the exhaustion of chromium in tanning process, reducing agents such as sodium bisulphite, sodium thiosulphate in conjuction with hydroxy carboxylic acids were employed. These reducing agents gave chrome exhaustion at the level of 93.51% and shrinkage temperature at the point of 121 ºC in the tanned leather (EL-Shahat et al., 2019).
The other way to enhance the chromium uptake of the leather in tanning process is by carrying out tanning at an elevated pH using ethanolamine–formic acid auxiliaries. This experiment or study gave fruitful results mainly by achieving uniform distribution of chromium throughout the entire layers of the leather. However, the leather industries are not accepting the technologies because of the common problems encountered such as swelling and case hardening (Thanikaivelan et al., 2002).
In an another study of using polycarboxylic material was used during repickling and then chrome tanning was carried out that yielded better chrome exhaustion as compared to the conventional chrome tanning system. In addition, the shrinkage of the resultant leather showed higher value than the leather produced from the conventional tanning system (Anggriyani and Nugroho, 2018).
Utilisation of horn for making horn-meal yielded low molecular weight peptides with average particle size of 3,838 nm (3.84 lm) and mean intensity of 5,243.7 nm (5.24 lm) and mean volume of 6,228.1 nm (6.23 lm). Application of these peptides in chrome tanning resulted in enhanced chromium exhaustion of more than 92%. The experimental leather showed expected level of softness, fullness, shrinkage temperature and strength properties with that of the conventionally produced leathers (Karthikeyan et al., 2008).
Chrome exhaustion was enhanced by increasing the cross-linking behaviour of chromium and collagen through pre-treating with complexing agents such as sodium formate and disodium phthalate with nanoclay (sodium montmorillonite) auxiliary. This complexing agent increases the exhaustion by distributing the chromium thoroughly in the various layers of collagen matrices. This technology directs the way forward for sustainable cleaner technology (Zhang et al., 2019; Yao et al., 2018).
Deviating from the conventional method, salt-less pickling using pre-tanning agent, Q 16, prepared by reacting cyanuric chloride and p-aminobenzoic acid was carried out. The results showed improvement in chromium uptake in the spent liquor, shrinkage temperature, mechanical properties in the resultant leather. This method also reduced COD of the chrome tanning process paving the strategy for sustainability in leather production (Cui et al., 2017).
An innovative idea of enhancing the chromium uptake in tanning process was attempted using auxiliary containing sulfonic acid groups. Induction of this auxiliary in tanning process enhanced chromium uptake level from 71.6% to 98.6%. The process also resulted in cost savings up to 46.9% in the processing. The other advantages of the methods were, comparable strength and organoleptic properties of the leather. This method also led to cleaner tanning process by generating lesser loads of Cr & Cl in the spent liquor to make sustainable leather processing (Zhang et al., 2017).
Chrome tanning supported by nano-composite gave increased chromium uptake in tanning process with a shrinkage temperature of the leather above 100 oC. The analytical findings using energy dispersive spectrometer study revealed that this novel type of system helped in distributing the chromium uniformly in the final leather. The specific system reduced the pollution loads namely BOD, COD of the effluent (Liu et al., 2016).
The environmental impact of the water used by leather industries could be minimised by recycling and reusing of waste water for the tanning operations. The experiment carried out at pilot and industrial scales reduced the risk of chromium and disposal of the wastewater in the tanning process (Kanagaraj et al., 2008; De Aquim et al., 2019).
Copolymer synthesised from collagen hydrolyzate and polyvinyl alcohol has been explored to attain high exhaust chrome tanning system. Application of copolymer at the level of 6% in chrome tanning process enhanced chromium uptake at the level of 94%. Analytical findings confirmed carboxylic acids, amide I and amide II, ester groups were responsible for increasing the chromium uptake level in the chrome tanning process (Kanagaraj et al., 2015b).
In a similar attempt, graft copolymer prepared from collagen hydrolysate and polyethylene glycol improved the uptake of chromium in the tanning process. Molecular modelling by gencollagen package study confirmed the reason for higher uptake of chromium to the level of 98%. The main reason for the higher uptake of chromium was the presence of functional group of carboxylic acids (Kanagaraj et al., 2015c).
Another novel way of carrying out chrome tanning was pickle free method by treating the pelt with sulphonic aromatic acid. This method completely sans use of formic and sulphuric acids in the pickling process that helps in reducing considerable amount of neutral salt and chrome tannin in the tanning process. Besides, this method has several advantages over conventional methods such as, reduction in float at the level of 75%, reduction in chromium at the level of 99% and chlorides at the level of 94% in the tanning process. The newer method was economical and cost saving up to 42% could be achieved (Bcardit et al., 2008).
The role of chromium in stabilising the collagen was investigated using real time small angle X-ray scattering, differential scanning calorimetry and natural cross-link analysis. The research indicated that the maximum stability of collagen can be achieved at concentrations as low as 1.8%. At lower concentrations of this, the active functional amino acids are saturated via covalent bonding with collagen matrices (Zhang et al., 2018).
pH-sensitive and chromium-loaded mineralized nanoparticles (Cr-PPA NPs) were developed by self-assembly of (poly(ethylene glycol) methyl ether acrylate-co-acrylic acid) poly(PEG-co-AA)) copolymers template Cr(OH)3 mineralisation. Cr-PPA NPs showed excellent colloidal stability in water with high salt concentration, protein, and pH above 3.0 and provided uniform distribution of chromium in leather during chrome tanning process (Li et al., 2019).
Keratin hydrolysate from poultry feathers along with chrome shavings have been utilised for high exhaust chrome tanning and dyeing process. Application of the above constituents has increased the chrome exhaustion to the maximum level. In addition to the present work of eliminating amount of liquid wastes of chromium, a leather-like flexible sheet is also made that can be employed in leather goods and footwear industries (Ramamurthy et al., 2015).
An eco-friendly chrome-less tanning methodology using nanocomposite was developed. The nanocomposite applied in tanning process gave better hydrothermal stability, biodegradability and softness. In addition, SEM and AFM results showed the leather treated with nanocomposites exhibited well-dispersed fibrils and uniform fluff. The pollution loads such as BOD, COD were reduced to the considerable level in the tanning process (Ma et al., 2014).
3.10. Color measurement study of leather
Color measurement study is one of the important analytical tool to know the effect of chrome tanning process. The main reason for carrying out color measurement study is that color properties reflect the rate of cross-linking phenomenon and chromium uptake in the tanning process. On the other hand, it is the indirect phenomenon to know the color chemistry behind the product-chromium-collagen interactions. It has been found from the several studies that the color properties were better in the case of enhanced chromium uptake in the tanning process. In other words, increased chrome uptake enhances the richness or brightness of the color of the leather. Hence the tanned leather was subjected to dyeing in the post-tanning process to manufacture the crust leather and the same was subjected to the color measurement property. The crust leathers were measured for color properties using CIELAB system. The Table 5 represents color measurement study of the crust leather. L, a, b, c are colour co-ordinates, ‘h’ is hue and ‘c’ is the chromaticity differences of the leather respectively. The experimental leather carried out with product at the level of 6% and control sample without product were subjected for colour studies. It is seen from the results that experimental leather showed lesser lightness of ‘L’ value indicating the increased trend of darker colour as that of control sample. The experimental samples also show relatively higher ‘a’ and ‘b’ value which is due to the presence of lesser greener and yellower component when compared with the control sample. It is also evident from that table that chromaticity and hue difference results were also lesser in the experimental sample compared to the control samples. It was confirmed form the results that the experimental sample showed better or increased colour uptake and produced darker shade as compared to the control sample. The better colour uptake of the leather was due to better uptake of chromium as compared to the control sample. The functional groups of product namely carboxylic acid, aldehyde, amide, OH groups enhances the crosslinks in the tanning process and this may be the reason for increased exhaustion of chromium in the tanned leather.
3.11. Physical strength properties of leather
Physical strength properties are the most important and decisive factor in choosing the correct type of tanning process for attaining the sustainability in leather sector. The leathers obtained from the experimental processes were subjected to various physical strength properties namely tensile strength, % elongation and tear strength properties. The results are presented in Table 6. The experimental leathers exhibited analogous strength properties and values for control samples. Tensile strength property of the experimental leather sample obtained by the treatment of product at the level of 2 and 4% had strength of 270±1.0, 275±3.0 (kg/cm2) in comparison with the control sample of 301 ±2.0 (kg/cm2). Similarly, the other properties namely, elongation at break (%) and tear strength properties of experimental leathers were also comparable with that of control sample.