Waterless tanning: chrome tanning in ethanol and its derivatives

Abstract

An approach towards waterless tanning is crucial to address present challenges faced by humanity such as global warming and the depletion of water resources. Here, green solvent alternatives to water such as ethanol, ethyl acetate and ethyl lactate were employed for both pickle-based and pickle-less chrome tanning. The results show that an ethanol medium appears to be the best solvent for chrome tanning in terms of color, chromium uptake and other bulk properties of tanned leathers. Extensive studies indicate that chrome tanning in the ethanol medium leads to a higher exhaustion (87% for pickle-based and 95% for pickle-less), better chromium content, distribution and shrinkage temperature, and low chromium leaching in tanned leathers compared to water mediated tanning. Both visual and electron microscopic analyses demonstrate a comparable grain structure and fiber architecture in tanned and crust leathers. Similarly, the strength and organoleptic properties of crust leathers are also comparable between ethanol and water mediated tanning. The process enables the reduction of COD, BOD and TS loads in the composite liquor by 14–26, 21–28 and 42–46%, respectively. The leather properties are not altered upon recycling of the chromium containing ethanol liquor up to two times. These results suggest that it is possible to replace water with ethanol for chrome tanning, which offers great potential for sustainable leather manufacture with solvent recycling.

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Introduction

Sustainable production and consumption is gaining importance in all spheres of human civilization, including industry.^1,2 The challenges of global warming such as environmental pollution, resource depletion and threats to food, water and energy securities require a paradigm shift in production and consumption patterns.³ Leather making is one such sector wherein the aforementioned factors are being scrutinized globally to ensure sustainable production.⁴ Chrome tanning is a popularly employed process to convert skins and hides into leather by stabilizing their collagen matrix.⁴ Currently the process is carried out in aqueous medium. The aqueous medium is helpful in solubilizing the tanning agent, basic chromium sulphate, and aiding its penetration into the collagen matrix. Furthermore, water is essential for the hydrolysis and olation of chromium molecules and in the formation of various oligomeric species.⁵ Nevertheless, the conventional chrome tanning process suffers from poor chromium uptake in the skin matrix and therefore a significant amount of chromium molecules remain in large amounts of water, which is released as effluent.⁶ This large amount of wastewater contains potentially toxic chromium molecules and the disposal of such remains as a major problem in leather industry. To address this issue, a number of researchers have developed various chromium management technologies.^4,7 Chrome recovery and reuse,⁸ direct chrome liquor recycling,⁹ high exhaust chrome tanning and tanning salts,^6,10 closed loop aluminium–chrome combination tanning, pickle-basification free chrome tanning⁵ and two-stage tanning¹¹ are some examples of recent developments in this area by our group as well as others. Some advanced and futuristic clean chrome tanning systems have also been developed by our group such as the three-step tanning^12,13 and reverse tanning processes,^14,15 which are now being further investigated globally.¹⁶ In addition to reducing the emission of chromium, these advanced processes also possess other benefits such as reduced water consumption and discharge and reductions in the amount of chemicals, power, time and cost compared to a conventional tanning process. However, in view of the increased demand and reduced supply of water coupled with a projected global water scarcity, the development of a waterless tanning technology is the need of the hour.^17–19 Solvent based chrome tanning techniques have already been reported in the literature.^20–23 Chagne et al. and Silvestre et al. employed a 200% trichlorotrifluoroethane or Forane 113 solvent along with 250% of water based on the dry weight of the pickled skins.^20,21 Manfred et al. and Renner et al. carried out chrome tanning using carbon dioxide as the medium at high pressure (one of few experiments in the supercritical fluid state) with considerable savings in time and chromium usage.^22,23

The present investigation aims to develop a chrome tanning process using green solvents chosen from a set of GSK green chemistry principles such as LD50, environmental and health hazards.²⁴ This approach relies on the fact that the free water present in the pelt is sufficient for the diffusion and fixation of chromium in the skin matrix, which was recently established based on a systematic investigation using a lyophilization technique.¹⁹ Based on green chemistry principles, we have shortlisted three solvents, namely ethanol, ethyl acetate and ethyl lactate.²⁴ Furthermore, Capello et al. assessed substance-specific hazards with the quantification of emissions and resource use over the full life-cycle of a solvent and found that ethanol was “greener” in comparison to other solvents.²⁵ Besides, ethanol and its derivatives are produced from the fermentation of renewable resources such as sugar-containing feeds, in comparison to solvents obtained from petrochemical routes, leading to the avoidance of fossil fuel resource use and CO₂ emission into the environment. In this work, pickle-based and pickle-less chrome tanning⁵ processes have been examined in non-aqueous media such as ethanol, ethyl acetate and ethyl lactate. Based on the results of these preliminary tanning experiments, the ethanol medium was chosen for carrying out chrome tanning in matched pair experiments along with a control followed by a conventional post tanning process in aqueous medium. A detailed analysis of the quality of the wet blue leathers as well as effluent characteristics and emission factors was carried out. Crust leathers were examined for their physical, chemical and organoleptic properties.

Materials and methods

Materials

Wet salted goat skins with an average area and weight of 4.5 ± 1 sq. ft were procured from local suppliers at Chennai and conventionally processed into pickled/delimed pelts at the Pilot Tannery, Central Leather Research Institute. For all of the tanning trials, commercial grade chemicals such as sodium formate, sodium bicarbonate, acetic acid, basic chromium sulphate, etc. were employed. Analytical grade ethanol was procured from M/s Hayman and used without further purification. Analytical grade ethyl acetate and ethyl lactate were procured from M/s Loba Chemicals Pvt. Ltd and used without further purification. Tanning trials were carried out in a stainless steel drum.

Preliminary chrome tanning trials in ethanol and its derivatives

Three solvents, namely ethanol, ethyl lactate and ethyl acetate were chosen for the preliminary tanning trials. Pickled/delimed goat pelts were cut into small pieces measuring 1 ± 0.2 sq. ft and used for the preliminary tanning trials. In the case of pickle-based chrome tanning, two pickled pelt cut pieces (each measuring 1 ± 0.2 sq. ft) were treated with 7% basic chromium sulphate (BCS) salt (percentage based on pelt weight) in stainless steel drums for 1 h. Then 100% v/w of the chosen solvents (individually) was added as a medium and drummed for 30 min. After ascertaining the penetration of chromium, the pH was found to be 2.9 ± 0.2. In the subsequent basification step, the pH of the tanning bath was increased to 3.7 ± 0.2 using 1% sodium formate with 10 min of drumming followed by the addition of 1% sodium bicarbonate in 3 installments at 10 min intervals and finally drummed for 1 h. In the case of pickle-less chrome tanning, the pH of two delimed pelt cut pieces (each measuring 1 ± 0.2 sq. ft) was adjusted to 5.5 ± 0.5 using 0.5% acetic acid based on the weight of the pelt. The pH adjusted delimed pelt was drummed with 7% BCS for 1 h. Then 100% v/w of the chosen solvents (individually) was added as a medium and drummed for 2 h. After ascertaining the penetration of chromium, the pH was found to be 4.0 ± 0.2. The total duration of both pickle-based and pickle-less chrome tanning was around 3 h. The tanned leathers were assessed by an experienced leather technologist for their quality through organoleptic examination. The chrome content in the tanned leathers was also analyzed as described in the forthcoming section.

Chrome tanning in an ethanol medium followed by a conventional post tanning process

The ethanol medium that was employed for tanning yielded good chrome tanned leather, compared to its counterparts such as ethyl lactate and ethyl acetate. Hence, ethanol was selected for further experiments. Two pickled/delimed goat pelts were used for each experiment employing pickle-based and pickle-less chrome tanning in an ethanol medium as described in the previous section along with control trials in a water medium. The control and experimental chrome tanned leathers were converted into crust upper leathers employing a conventional post tanning process in an aqueous medium.

Analysis of chrome tanned leathers

Shrinkage tester apparatus (SATRA STD 114, SATRA Technology Centre) was used for determining the shrinkage temperature of chrome tanned leathers. The tanned leathers were analysed for moisture content in a hot air-oven for 5 h at 104 ± 2 °C.²⁶ The chrome tanned leathers were analysed for chromium content employing the standard procedure²⁷ using a UV-visible spectrophotometer (UV 1800, Shimadzu) and the chrome content was expressed as a moisture-free basis. Triplicate measurements were carried out for each analysis and the average values were calculated. Bulk properties of the chrome tanned leathers such as fullness, grain smoothness, color and wrinkles were assessed by an experienced leather technologist through organoleptic examination.

Analysis of leachable chromium

A known weight of experimental chrome tanned leather sample was placed in a beaker with 50 mL of distilled water and ethanol separately. In the case of the control leathers (water mediated), a known weight of a tanned leather sample was placed in a beaker with 50 mL of distilled water. It was agitated in a shaker at low rpm for 3 h. The chromium content in the solution was analyzed using an alkaline peroxide procedure and the concentration of chromium was determined at 372 nm using a UV-visible spectrophotometer (UV 1800, Shimadzu).²⁸ The amount of chromium was then calculated using a molar absorption coefficient (ε) value of 4.8 × 10³ M⁻¹ cm⁻¹ based on the Beer–Lambert law. Triplicate measurements were carried out and the average values were calculated. Chromium leached from Cr-tanned leather samples (Cr₂O₃%) was calculated by dividing the amount of chromium in the leachate by the amount of chromium in the chrome tanned leather sample and multiplying by 100.

Analysis of the spent chrome liquor

The spent chrome liquor was collected from both the control and experimental pickle-based and pickle-less chrome tanning processes. Liquors were acid digested and analyzed for chromium using the alkaline peroxide procedure as described above.²⁸ The percentage exhaustion of chromium was calculated from the amount of spent liquor collected. Triplicate measurements were carried out and the average values were calculated.

Composite liquor analysis

Composite liquors from control and experimental leather processing were collected from the chrome tanning up to post-tanning. The liquors were analyzed for chemical oxygen demand (COD), biological oxygen demand (BOD), and total solids (TS) as per the standard procedures.²⁶ Emission loads were calculated by multiplying concentration (mg L⁻¹) with the volume of effluent (L) per ton of raw skins processed.

Scanning electron microscopy analysis

The chrome tanned leather samples were dehydrated gradually using acetone and methanol as per the standard procedure.²⁹ Excess solvent was removed from the samples by placing them between filter papers. Crust leathers were directly analyzed without dehydration. Samples were then cut into specimens of uniform thickness and coated with gold using an ion-sputtering device. The samples were mounted on aluminium stubs and analyzed using scanning electron microscopy (SEM, Tescan Vega 3 SB) at an accelerating voltage of 3 kV at different magnifications.

Analysis of crust leathers

Samples for various physical tests were obtained from control and experimental crust leathers as per the IUP method.³⁰ Samples were conditioned at 80 ± 4 °F and 65 ± 2% R.H. over a period of 48 h. Physical properties such as tensile strength, % elongation at break, tear strength and grain crack strength were then investigated employing standard procedures.^31–33 Control and experimental crust leathers were assessed for softness, fullness, grain smoothness and general appearance by organoleptic examination. The leathers were rated on a scale of 0–10 points for each property by an experienced leather technologist, where higher points indicate better properties.

Studies on the recycling of the spent ethanol liquor

In order to determine the feasibility of recovering and recycling the ethanol used in the chrome tanning, a limited study was conducted employing the pickle-based chrome tanning. One pickled goat pelt was used for each experiment employing pickle-based chrome tanning in an ethanol medium as described above. At the end of the chrome tanning process for the first batch, the spent ethanol liquor was collected, filtered and analyzed for the chromium content as described above. In the 1^st recycling process, BCS equivalent to the balance amount of the spent ethanol liquor was added to the pelt and drummed for 1 h. Then the chromium containing spent ethanol liquor was added and the total volume was adjusted to 100% v/w by the addition of fresh ethanol and drummed for 30 min. After confirming the penetration of chromium, the pH was found to be 3.2 ± 0.2. During basification, 0.5% sodium formate was added and drummed for 10 min followed by the addition of 0.8% sodium bicarbonate in 3 installments at 10 min intervals and finally drummed for 1 h. The pH of the tanning bath was found to be 3.8 ± 0.2. At the end of the chrome tanning process for the second batch, the spent ethanol liquor was collected, filtered and analyzed for the chromium content. The collected spent ethanol liquor was reused in the third batch (2^nd recycling process) as described above.

Results and discussion

Preliminary trials

Preliminary trials were carried out using three solvents, namely ethanol, ethyl acetate, and ethyl lactate employing both pickle-based and pickle-less chrome tanning processes with pickled and delimed pelts, respectively. Although chrome tanning is possible in all of the selected solvents, the quality of the tanned leather depended on the solvent used for the process as shown in Table 1. A visual assessment of chrome tanned leathers processed through pickle-based and pickle-less chrome tanning in ethyl acetate and ethyl lactate medium shows a slight swelling, coarse grain and moderate penetration of chromium. Chrome tanned leathers processed with ethyl acetate did not show a significant color change from the conventional wet blue, however the ethyl lactate medium yielded bluish leathers with a green tinge. On the other hand, chrome tanned leathers (pickle-based and pickle-less) prepared using the ethanol medium are bluish comparable to the conventional wet blue without any coarse grain and swelling effect. Furthermore, leathers prepared in the ethanol medium show a high uptake of chromium compared to those prepared in ethanol derivatives. This could be due to the fact that the dispersion and diffusion of BCS in the skin matrix is better in the ethanol medium compared to the other two solvents assuming that the internal moisture present in the pelt is constant for all the three systems. Therefore, ethanol was selected for further detailed analysis using matched pair experiments.

Table 1 Results of the preliminary trials using ethanol, ethyl acetate and ethyl lactate solvents employing pickle-based and pickle-less chrome tanning processes

Parameters	Ethanol	Ethyl acetate	Ethyl lactate
Chrome tanned leather (pickle-based)
Visual assessment	Comparable to conventional wet blue color, no swelling, good penetration	No color issues, slightly coarse grain, slight swelling, reasonable penetration	Greenish blue color, swelling effect, slightly coarse grain, reasonable penetration
Cr2O3% (dry weight basis)	5.47 ± 0.18	3.5 ± 0.12	3.92 ± 0.15
Chrome tanned leather (pickle-less)
Visual assessment	Comparable to conventional pickle-less processed wet blue color, no swelling, good penetration	No color issues, slightly coarse grain, slight swelling, reasonable penetration	Bluish green color, coarse grain, swelling effect, reasonable penetration
Cr2O3% (dry weight basis)	6.45 ± 0.21	4.35 ± 0.16	4.76 ± 0.14

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Ethanol mediated chrome tanning

Digital images of the control and experimental chrome tanned leathers and respective process liquors (insets) are shown in Fig. 1. It is evident that the color of pickle-based chrome tanned leathers is light blue for both ethanol and water mediated processes, whereas the pickle-less chrome tanning yields slightly dark blue leathers. The chromium exhaustion, distribution and shrinkage temperature of pickle-based and pickle-less chrome tanning systems in ethanol and water medium are shown in Table 2. The exhaustion levels of chromium for both pickle-based and pickle-less chrome tanning in the ethanol medium are higher compared to similar processes carried out in a water medium (control). This may be due to the limited solubility of BCS in the ethanol medium, which drives more chromium into the skin matrix containing internal water. The color variations in the process spent liquors (see insets of Fig. 1) also demonstrate a higher exhaustion in ethanol mediated processes. The improved uptake of chromium is also reflected in the chromium distribution data. It is seen that the leathers processed in the ethanol medium possess higher amounts of chromium in all the regions of leather, namely the butt, neck, and belly, compared to those processed in a water medium. Furthermore, both ethanol and water mediated chrome tanning processes yield leathers with a reasonably uniform chromium distribution in all regions of the leather. The shrinkage temperature of the leathers processed in the ethanol medium is slightly higher than that of the control leathers (water mediated). These results suggest that replacing water with ethanol for chrome tanning improves the chromium uptake and thermal stability of leathers.

Fig. 1 Digital images of the control and experimental chrome tanned leathers and their respective process liquors: (a) pickle-less chrome tanning in an ethanol medium, (b) pickle-less chrome tanning in a water medium, (c) pickle-based chrome tanning in an ethanol medium and (d) pickle-based chrome tanning in a water medium.

Table 2 Exhaustion, chromium distribution and shrinkage temperature data of pickle-based and pickle-less chrome tanning in ethanol and water media^a

Sample	Exhaustion of chromium (%)	Cr2O3% (dry weight basis)			Shrinkage temperature (°C)
		Butt	Neck	Belly
a CP – control pickle-based; EP – experimental pickle-based; CPL – control pickle-less; EPL – experimental pickle-less.
CP	77 ± 2	5.12	5.09	5.10	104 ± 2
EP	87 ± 3	5.47	5.45	5.46	108 ± 1
CPL	91 ± 2	6.20	6.16	6.17	106 ± 1
EPL	95 ± 3	6.45	6.42	6.43	108 ± 2

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The extent of chromium leaching and the amount of chromium present before and after leaching in the tanned leathers processed through pickle-based and pickle-less chrome tanning in ethanol and water media are shown in Table 3. We observed a lower leaching of chromium from both the control and experimental leathers when water and ethanol were used as the leaching solvents. A maximum of 2% Cr₂O₃ was leached from the chrome tanned leather when water was used as the tanning as well as the leaching medium. This value is lower than those reported in the literature.¹⁹ This indicates that the remaining chromium (∼98%) is fixed chemically within the leather matrix. The experimental leather shows a fairly reduced chromium leaching when compared to the control leather. In addition, the amount of chromium present in the leathers after the leaching is comparable to the values obtained before the leaching.

Table 3 Analytical data on leaching studies in control (C) and experimental (E) chrome tanned leathers^a

Parameters	EPL	CPL	EP	CP
a CP – control pickle-based; EP – experimental pickle-based; CPL – control pickle-less; EPL – experimental pickle-less.
Cr2O3 (%) in chrome tanned leather before leaching (dry weight basis)	6.43 ± 0.20	6.17 ± 0.22	5.46 ± 0.19	5.10 ± 0.16

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Parameters	Solvent	Water	Water	Solvent	Water	Water
Leaching of Cr2O3 from chrome tanned leather (%)	1.57	1.87	2.17	0.98	1.49	1.3
Cr2O3 (%) in chrome tanned leather after leaching (dry weight basis)	6.32 ± 0.19		6.05 ± 0.18	5.41 ± 0.21		5.04 ± 0.20

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The organoleptic assessment data of both control and experimental chrome tanned leathers prepared through pickle-based and pickle-less chrome tanning processes using ethanol and water media is given in Table 4. It is seen that leathers tanned in the ethanol medium show either comparable or even better color, grain smoothness and fullness in comparison to leathers tanned in the water medium. However, leathers tanned in the ethanol medium through pickle-less chrome tanning show slightly more wrinkles in comparison to their counterparts processed in the water medium. This problem needs to be addressed separately, probably during a semi-commercial trial, to improve the appearance of tanned leathers.

Table 4 Organoleptic assessment data of the control (C) and experimental (E) chrome tanned leathers^a,b

Parameters	EPL	CPL	EP	CP
a CP – control pickle-based; EP – experimental pickle-based; CPL – control pickle-less; EPL – experimental pickle-less.b On a scale of 0 to 10, the qualitative descriptors such as excellent, good, average and poor are weighed as ≥9 excellent; ≥7 and ≤8 good; ≥4 and ≤6 average and ≤3 poor, respectively. While the terms such as fuller, more and less are weighed as ≥9, ≥7 more ≤8 and ≥4 less ≤6, respectively. Higher numbers indicate better properties except wrinkles.
Color	Good	Good	Excellent	Good
Grain smoothness	Excellent	Good	Excellent	Good
Fullness	Fuller	Good	Fuller	Good
Wrinkles	More	Less	Less	Less

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Scanning electron micrographs showing the grain surface of the control and experimental chrome tanned leathers are given in Fig. 2. The grain surface of the experimental leathers shows clear hair pores without any deposition of chromium compared to the control leather. The cross-sectional images of the control and experimental chrome tanned leather samples (Fig. 3) show fiber bundles with comparable porosity and architecture. A similar observation has been made on the surface and cross section of the control and experimental crust leathers (Fig. S1 and S2†). These results suggest that the microstructure of the leather is not altered significantly upon tanning in the ethanol medium.

Fig. 2 Scanning electron microscopy images showing the surfaces of the control and experimental chrome tanned leathers: (a) pickle-based, (b) pickle-less chrome tanning in an ethanol medium and (c) pickled-based chrome tanning in a water medium.

Fig. 3 Scanning electron microscopy images showing the cross section of control and experimental chrome tanned leathers: (a) pickle-based, (b) pickle-less chrome tanning in an ethanol medium and (c) pickle-based chrome tanning in a water medium.

Composite liquor analysis

The chrome tanned leathers processed through pickle-based and pickle-less chrome tanning in the ethanol medium were post tanned in a water medium. Composite liquors collected by mixing spent liquors from chrome tanning, neutralization, washing, retanning, fatliquoring, dyeing and fixing process steps for both control and experimental processes, were analyzed for COD, BOD, and TS and the values are given in Table 5 along with emission loads. It is seen that both pickle-based and pickle-less chrome tanning in the ethanol medium lead to a significant reduction in the COD/BOD/TS loads by 14–26, 21–28 and 42–46%, respectively compared to the control processes carried out in water. The observed reduction in pollution loads could be due to the improved uptake of chromium, syntans, fatliquors and dyes.

Table 5 Composite liquor analysis for control (C) and experimental (E) chrome tanning^a,b

Samples	COD (mg L^−1)	BOD (mg L^−1)	TS (mg L^−1)	Volume of effluent (L per ton of raw material)	Emission load (kg per ton of raw material processed)
					COD	BOD	TS
a CP – control pickle-based; EP – experimental pickle-based; CPL – control pickle-less; EPL – experimental pickle-less.b Composite liquors were collected from the chrome tanning up to the post tanning.
CP	12 950	2950	7378	2463	31.8	7.2	18.2
EP	11 150	2320	4296	2450	27.3	5.7	10.5
CPL	17 855	4580	8622	1882	33.6	8.6	16.2
EPL	10 650	2650	3784	2336	24.8	6.2	8.8

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Evaluation of crust leathers

The physical properties such as tensile, tear and grain crack strength of the control and experimental chrome tanned leathers processed through pickle-based and pickle-less chrome tanning were analyzed and the values are given in Table 6. It is seen that the strength characteristics of the experimental leathers are comparable to those of the control leathers. Hence, it is seen that the chrome tanning process in the ethanol medium did not critically affect the strength properties of the leathers in comparison to the conventional water-mediated tanning process.

Table 6 Physical properties of the crust leathers^a

Properties	EP	CP	EPL	CPL
a CP – control pickle-based; EP – experimental pickle-based; CPL – control pickle-less; EPL – experimental pickle-less.
Tensile strength (N mm^−2)	20.5 ± 1.5	21.1 ± 2.0	21.9 ± 1.8	20.2 ± 1.6
Elongation at break (%)	45.3 ± 1.4	46.3 ± 1.8	47.7 ± 2.5	45.3 ± 2.8
Tear strength (N mm^−1)	33.0 ± 1.2	29.4 ± 1.5	30.2 ± 1.4	30.9 ± 2.0
Grain crack strength (kg)	22 ± 1	24 ± 1	20 ± 0.5	22 ± 2
Distention at grain crack (mm)	6.9 ± 0.3	7.7 ± 0.4	7.0 ± 0.1	7.4 ± 0.2

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The organoleptic properties of the control and experimental crust leathers processed through pickle-based and pickle-less chrome tanning are given in Fig. 4. The properties of the leathers made using chrome tanning in the ethanol medium are comparable to those of leathers made using the conventional chrome tanning process in the water medium. The grain smoothness, softness and general appearance of the crust leathers processed through pickle-based and pickle-less chrome tanning in the ethanol medium are comparable to the control leathers tanned in the water medium. There seems to be a slight decrease in the fullness of the experimental leathers processed through pickle-less chrome tanning in the ethanol medium when compared to the control leathers. In general, chrome tanning in the ethanol medium did not adversely affect the organoleptic properties of the leathers in comparison to leathers tanned in the water medium.

Fig. 4 Organoleptic properties of the control and experimental crust leathers processed through (a) pickle-based and (b) pickle-less chrome tanning.

Recycling studies

A paramount requirement of solvent based processes is the ability to recover and recycle the used solvent in the subsequent processing, which would reduce the cost and environmental burden. Here, the results of recycling the spent ethanol liquor in the pickle-based chrome tanning up to two times are given in Table 7. It is seen that nearly 65% of the spent ethanol liquor can be collected during the recycling of up to three batches. Therefore, the fresh solvent requirement during recycling can be minimized significantly. As can be seen, the collected ethanol liquor contained chromium equivalent to about 0.7 to 0.9% BCS, which means that the BCS to be added can be reduced from 7% down to 6.1% (for the 4^th batch) leading to a cost saving to some extent. A lower amount of chromium in the spent ethanol liquor evidences a higher uptake of chromium in the tanned leathers as observed earlier. The color of the chrome tanned leathers produced from the recycled spent ethanol liquor is comparable to the first batch from the ethanol-mediated pickle-based chrome tanning process. The recycling of the spent ethanol liquor does not seem to affect the thermal stability of the tanned leathers. The results suggest that it is possible to recycle the used solvent up to two times without affecting the properties of the tanned leathers, which can be beneficial in cost effectiveness.

Table 7 Data on the recycling of the spent ethanol liquor in the pickle-based chrome tanning process

Parameters	First batch	Second batch (1^st recycle)	Third batch (2^nd recycle)
Volume of spent ethanol liquor collected (%)	64.7	60	63.2
BCS (%) present in the total volume of spent ethanol liquor	0.71	0.78	0.88
BCS (%) added	7	6.29	6.22
Fresh ethanol added (%)	100	35.3	40
Photographic images of chrome tanned leathers
Shrinkage temperature (°C)	106	108	109

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Conclusions

In search of alternatives to water based chrome tanning, an attempt has been made in the present study to perform chrome tanning in ethanol and its derivatives such as ethyl acetate and ethyl lactate employing both pickle-based and pickle-less methods. Although all three solvents enabled the chrome tanning process, the ethanol medium provided better chrome tanned leathers in terms of color, chrome content and organoleptic properties. Detailed studies on the use of the ethanol medium for chrome tanning show comparable shrinkage temperatures, grain and fiber architectures, strength and bulk properties and a reduction in chromium discharge in contrast to water mediated processing. The amount of leached chromium in the tanned leathers is also low compared to leathers tanned in a water medium. Tanning in the ethanol medium also helps in minimizing waste generation such as chromium, TS, COD, and BOD loads. Furthermore, we proved that the ethanol liquor used in the chrome tanning can be recovered and recycled up to 2 times without affecting the properties of the leathers. The results of this limited study show that ethanol mediated chrome tanning is feasible and helpful in preventing environmental pollution and improving the sustainability of the tanning industry.

Acknowledgements

Financial support from CSIR under the XII^th plan project “Research Initiatives for Waterless Tanning” (RIWT-CSC0202) is greatly appreciated. We also thank the Department of Mechanical Engineering, Anna University for providing the SEM facility. CSIR-CLRI Communication No. 1087.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra11740b

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