Water in the wet processes of leather manufacture acts as a solvent, transportation medium, support system and is an integral component of the collagen matrix stability at the macro and molecular levels. There have been proposals as to what extent water can be substituted with non-aqueous media, partly due to its role in collagen and processing. As the water content increases from bulk to float water, the system acts as a transport medium for chemicals as well as a support system in processing vessels. Here, deep eutectic solvents (DESs) are investigated as an alternative solvent system to conventional tanning systems.

The novel solvent is non-aqueous, has a high solubility for metal salts, low toxicity, is readily biodegradable and operates at variable temperatures without the release of harmful vapours. These properties make the DES system a potential alternative in leather processing, improving reagent uptake with the potential of lowering effluent discharge. This article focuses on the role of the solvent associated with stabilising collagen, such as the influence of inter and intra-fibrillar, as well as bulk water, using type 2 DES systems. Thermal, mechanical analysis and electron microscopy were undertaken to determine the effects of the solvent system. The studies indicated that the solvent used, as well as the choice of counter ion, have an important effect on the thermal and mechanical stability of collagen.

In the century and a half of the history of chromium tanning, the industrial process has been conducted in water. Conventionally, leather production needs an average of 40m3 of water per ton of raw material during processing. The water in the wet processes of leather manufacture acts as a solvent, transport medium, support system and is an integral part of the collagen. There are three types of water that can be identified in the collagenic structure: intramolecularly bound water; intra and interfibrillary bound water, which forms a sheath between fibrils and fibres by hydrogen bonding; and bulk water, which acts as a solvent for the chemical modification of native collagen to be transformed into leather. Furthermore, the system acts as a transport medium for chemicals as well as a support system for the material in processing vessels.

Over the years, several attempts have been made to modify the solvent system, either by mixing water with miscible organic solvents or by substituting water by an immiscible organic solvent. In this research report, an entirely different approach is adopted wherein the reagent to be fixed in the tanning reaction is actually part of the solvent system, which is referred to as an ionic liquid or, more strictly, a DES. Clearly, one of the benefits of this new way of leather processing is the savings that can be made in the requirements for fresh water.

There are four types of DES and can be generally represented as:

  • type 1: metal salt plus organic salt
  • type 2: metal salt hydrate plus organic salt
  • type 3: organic salt plus hydrogen bond dono
  • type 4: metal salt (hydrate) plus hydrogen bond donor.

A DES is a type of ionic solvent composed of a mixture of two or more different chemical compounds, held together in a defined spatial arrangement by chemical bonds and has a melting point much lower than either of the individual components involved in the chemical reaction. The two solid compounds (A and B) shown in Figure 1 are mixed together to obtain an ionic liquid at the eutectic point.

The melting point of the mixture is dependent upon the interaction between the components at a defined pressure. Ionic compounds generally have a high melting point but the ionic liquid melting point is lower due to the weaker electrostatic forces between oppositely charged ions. If the eutectic-based systems of known chemical composition are dissolved in water, they revert into their constituent components.

Compared with the currently used solvents, ionic liquids are distinctively beneficial for operating at variable temperatures without the release of harmful vapours. Ionic liquids allow a high solubility of metal salts, low toxicity and are readily biodegradable. These properties make ionic liquids the most promising solvents for the reduction of water in leather processing, improving uptake and possibly lowering the discharge to effluent treatment.

This preliminary study focuses on type 2 DES, in which the metal salt is chromium III chloride hydrate and the organic salt is choline chloride (although other quaternary ammonium salts can be used). Here, results are reported from experiments that begin to define the impact of the new reactions and the limits to applicability in the leather industry.

Materials and method

The materials used for the experiments were wet-salted goatskins preserved in accordance with the European standard (EN 16055:2012) and sourced from the Institute of Creative Leather Technologies, University of Northampton, UK. The DES were synthesised with analytical-grade chemicals from Sigma-Aldrich, in collaboration with the University of Leicester, UK. The distilled water used for processes complied with the International Organisation for Standardisation (ISO 3696:1995).

Type 2 DESs were prepared by heating an organic salt and a metal salt hydrate at 70°C (molar ratio 2:1 of choline
chloride: chromium chloride hexahydrate respectively). Initial stirring using a glass rod, shortly after liquid begins to form a magnetic stirrer was used to maintain efficient combination of components.

The goatskins were sampled in accordance with the ‘Chemical, Physical and Fastness Test Sampling Location’ method (ISO 2418:2002). The pieces were processed using a standard recipe (up to and including the deliming step) to remove unnecessary components such as hair, fats, proteoglycans and globular protein with a final adjustment to a range of pH values: 2, 4, 6 and 8.

An acetone dehydration process was implemented to extract residual water from the goatskin. The goatskin samples were immersed in a bath of acetone that was replaced periodically until there was no observable change in the specific gravity of the solvent in a 24-hour period. Skins were then removed from the acetone bath and dried in a 40°C oven overnight. The fully dried skin was cut into 1cm2 pieces and ground in a cutting mill in accordance to the ‘Preparation of Chemical Test Samples’ (ISO 4044:2008).

All samples were conditioned for 24 hours at 23°C in conditioning cabinets, each with a fixed relative humidity (0, 33, 50, 76 and 98%) controlled by saturated salt solution (see Table 1). The pH of the conditioned goatskin powder was processed with DES in vials with varying conditions. All the experimental trials were carried out in triplicate in a water bath. The parameters investigated were moisture, water content, pH, temperature, concentration of the chromium in DES and time.

Water and moisture contents were calculated based on conditioned goatskin powder weight and provided as percentages. The pH of the skin powder was altered (2, 4, 6 and 8) during the final process prior to drying. The temperature of each reaction was varied by altering the temperature of the water bath to the specified reaction temperature, allowing equilibration of the DES temperature before the addition of goatskin powder. Concentration was calculated based on chromium content and is expressed as grams per litre.

At the conclusion of each experiment, samples were rinsed with deionised water, vacuum filtered over a Buchner funnel with a Whatman number 541 filter paper. Consequently, the filtered samples were hydrated with distilled water for 24 hours and kept in vials for thermal analysis.

The blotted samples weighing 2–8mg were placed in 30μl gold-plated high-pressure crucibles for analysis using the differential scanning calorimetry (DSC). Measurement was undertaken at a heating rate of 5°C/min for the temperature range 25–120°C for all experiments.

Results and discussion

The thermal analysis of goatskin powder samples processed at different moisture contents (3–24%) showed they did not acquire additional stability when compared with the control Ts 60.4°C ±0.8°C (p≥0.05). The molecular water dominates the structure when the skin samples were fully hydrated.

The thermal analysis results in Figure 2 illustrate the necessity for water to act as a transport medium for the DES to interact with the active sites of the collagen triple-helix molecule. They show that the shrinkage temperature is dependent on the water content of the starting material, because there is a significant difference (p≤0.05) in shrinkage temperature as the water is increased; however, there is no significant difference in the water content region 200–500% (p≥0.05). The shrinkage temperature increases compared with the control sample from 60.4 to 74.2°C at 100% water content because of the requirement of the higher water content in the collagenous structure to facilitate the ionic mechanism of fixation.

The moderate shrinkage temperatures observed are consistent with the view that chromium III molecular ions are capable of conferring only moderate hydrothermal stability in the absence of a second agent for locking the ions together within the supramolecular matrix around the triple helices.

The thermal analysis results in Figure 3 illustrate that at pH 2, below the isoelectric point when the collagen is positively charged, the type 2 DES species interacted less with the collagen compared with pH 4 and above, more negatively charged carboxyl groups are available. There is a significant difference (p≤0.05) when the pH increases from 2, however there is no significant difference in shrinkage temperature above pH 4 (p≥0.05), although the observed trend in diminishing shrinkage temperature may be due to chromium species precipitating and interfering with the fixation reaction.

The thermal analysis results in Figure 4 illustrate that temperature from 25 to 55°C does not significantly change the interaction of the type 2 DES species with the collagen (p≥0.05). The rate of reaction at which the type 2 DES species interact with the collagen appears to not be dependent on the temperature. However, as referred to above, the reaction has reached its maximum effect at the lowest temperature. This means that the process may be more rapid than the conventional aqueous reaction, which is highly dependent on temperature.

The higher the concentration of a reactant, the more chance the molecules in the type 2 DES have in colliding with the active sites of the collagen, as indicated by Figure 5, so the shrinkage temperature increases with increasing concentration from 1.25 to 5.00g/L. There is a significant difference when the concentration increases, but further increase in the concentration does not show a significant difference.

As already discussed, these values are approaching the maximum possible value of shrinkage temperature with the concentration of 5g/l under these experimental conditions.

The thermal analysis of samples processed for different times illustrates the reaction period does not significantly change the interaction of the type 2 DES species with the collagen, so, under the given experimental conditions, maximum shrinkage temperature was achieved within two hours. This indicates that the fixation reaction is rapid and may be faster than the conventional process.

The thermal analysis results in Figure 6 demonstrate the effect of washing the type 2 DES chromium-tanned goatskin powder with a solution of sodium sulphate. This resulted in an elevated shrinkage temperature.

The rationale of the experiment relates to the link-lock theory of tanning. Moderate shrinkage temperature is achieved by chromium III molecular ions acting individually in the supramolecular matrix, the ‘polymer in the box’ merely interfering with the shrinking mechanism. The presence of ions such as sulphate, which can interact powerfully with the aquo ligands of the chromium III molecular ions via hydrogen bonding, creates a new matrix that resists shrinking strongly and is observed as high shrinkage temperature.

The resulting shrinkage temperatures are elevated in comparison with the untreated control, but there is no apparent effect due to sulphate offer, nor are the values as high as conventional chromium-tanned leather. However, this may merely be the influence of the experimental conditions and the nature of the substrate. But, the principle is demonstrated.

The results obtained in this preliminary study are consistent with what is already known about the conventional chromium tanning process. This might be considered surprising, because the chromium species in the ionic liquid are in a very different chemical state to solute chromium sulphate tanning salt in aqueous solution. Nevertheless, it has been demonstrated that chromium fixation can occur, and the reaction appears to be faster and less reliant on specific conditions than the water-based process.

However, the understanding of the new reaction must be tempered by the recognition that moderate shrinkage temperature is easy to achieve. What is less easy is the attaining of high hydrothermal stability, which is only possible by satisfying the requirements of the link-lock sequence of events. It is those elements of the mechanism that have to be incorporated into the new ionic liquid process if it is to rival conventional operations.

The big picture

The application of ionic liquids in the heterogeneous technology of leather-making is a paradigm shift. The delivery of chromium III molecular ions into the collagen matrix is fundamentally different to conventional processing and the results reflect the difference. However, it is clear that the mechanism of chromium fixation via the structural water around the collagen triple helix may not be significantly adversely affected. Therefore, it can be assumed that there is value in pursuing these studies; in particular, to investigate the novel chemistry options offered by myriad variations of ionic solvents possible. This is important in extending the properties and performance of leather and other biomaterials. 