Introduction

Tanning, in simple terms, refers to the treatment of rawhides or skins with tanning materials to render the material immune to microbial degradation (1). There are a large variety of chemicals used in the production of many different leather types. However, the major chemicals are the tanning agents as they define the process of leather manufacture as a whole (2). In the tanning process, tanning agent penetrates into the collagen matrix and is distributed evenly through its cross-section. It is then bound irreversibly to the collagen reactive sites (3). It has been accepted that the tanning ability of a substance is related to the type of interaction that occurs between the tanning agent and collagen (4). The tanning efficiency is conventionally defined by the T_s which is a measure of the resistance of leather to heat in aqueous medium. Fibre bundles of collagen can be induced to undergo an abrupt decrease in length at a characteristic T_s when subjected to slow heating in aqueous medium. The factors affecting shrinking include intramolecular interactions and superimposed intermolecular interactions. The latter is brought about by tanning and the sites available for tanning vary depending on the tanning agent. If the tanning agent forms strong bonds, such as covalent or coordinative, the leather has high hydrothermal stability i.e. high T_s values (5). The introduction of these crosslinks produces a more regular structure, decreases the entropy and so more energy is required to denature the collagen, hence the T_s rises.

Today, tanners choose tanning chemicals based on their performance, price, ease of use, environmental issues and their aesthetic properties (grain, colour, touch) (6). Chromium salts (commonly basic chromium sulfate) are the most widely used tanning agents with a global utilisation rate of 85% owing to their low cost, high versatility and quality of the final product obtained (7). Chromium compounds give high hydrothermal stability to leathers up to 100°C with light weight and soft touch. Besides these advantages, chromium also has disadvantages such as low exhaustion rate from floats (70% in 24 h), its blue-greenish colour, too much elasticity in some cases and a risk of forming carcinogenic Cr⁶⁺ species from unbound chromium (8). In conventional chromium tanning the low exhaustion rate results in discharging 30% of the chrome tanning agent into wastewater (9). These disadvantages motivate a move towards more environmental friendly tanning alternatives (10). For this purpose other inorganic tanning agents such as zirconium, aluminium, titanium or zinc compounds or organic tanning agents, i.e. vegetable tannins, syntans, polyaldehydes or phosphonium salts, are commonly used alone or in combination as chromium-free or metal-free tanning systems (11). It is worth noting that metal-free tanning agents employed for the production of ‘wet-white’ leather have limited application compared to chrome tanned leather (‘wet-blue’) since the physical and mechanical characteristics of wet-white leather are generally lower compared to wet-blue leather (12). Moreover, consumers’ anxieties about the possible effects of metals on human health as well as European Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) restrictions on heavy metals mean that metal-free tanning systems are increasingly attractive (13, 14).

Vegetable tannins, syntans and aldehydes are some alternative tanning agents for metal-free leather goods (13, 15, 16). Some chemical companies have developed new tanning systems for chrome-free and metal-free leathers for more sustainable leather making. The investigation of alternative tanning systems to basic chromium sulfate for the leather industry, together with detailed analysis of their tanning abilities, is extremely important.

As mentioned above hydrothermal stability of leather is generally measured by observing the temperature at which the leather specimen shrinks when heated in water at 2°C min^–1. This phenomenon is called as T_s defined by the standard method ISO 3380:2002 [IULTCS/IUP 16] (17). On the other hand, there are also alternative analytical techniques providing information about the thermal behaviours of tanned leathers (18–20). Fully hydrated (>200% water/collagen) native collagen undergoes denaturation when heated to approximately 62°C, as observed by shrinkage of the samples to a third of the original length, and by the peak in measurements taken by means of differential scanning calorimetry/differential thermoanalysis (DSC/DTA). The peak of the first endothermic event observed in the DSC thermograms usually refers to the T_s and the area below this peak corresponds to the heat requirement of the endothermal melting process. Thermal behaviour of the tanned collagen can be accurately measured using much smaller samples by thermogravimetry/derivative thermogravimetry (TG/DTG) and DSC methods (21). These thermal analyses are useful for fast evaluation of thermal stability and behaviour, degradation temperature, absorbed moisture content, crystallised water content, melting point and thermal decomposition kinetics in a closed measurement atmosphere (22). Application of these sensitive techniques provides more realistic evidence of denaturation or deterioration degree by the phase transitions of dry biomaterial in a short time using milligram quantities (23, 24), especially when the daily use conditions of leather materials are considered. Leather materials applied in automotive upholstery, furniture, military shoes, gloves and aircraft seating require high thermal resistance and must be analysed under interior and exterior extreme conditions (25). To our knowledge, there are only a few reports on the comparative thermal behaviour of dry and wet collagen (26) and less is known about the degradation mechanism of tanned leathers (27).

The present study aims to investigate the thermal behaviour of leathers with various tanning systems via different techniques and to provide a better understanding of the relationship between tanning and thermal stability of leather composed of collagen fibres.

Experimental Setup

Thermogravimetric Analysis

Thermal analysis was carried out by the TGA method on different tanned leathers using a TGA 8000^TM instrument (PerkinElmer, USA). 3–5 mg of leather samples were weighed in ceramic pans and the flow rate of nitrogen gas (99.99% purity) in the system was set at 20 ml min^–1. Samples were analysed between 30–800°C with a heating rate of 10°C min^–1. The main degredation process of the samples was observed with the peak points obtained by thermogravimetric (TG) and DTG analysis.

Results and Discussions

Tanning means converting the rawhide or skin, a highly putrescible material, into leather, a stable material. In this process the different kinds of bonds are replaced with tanning agents like chromium, aluminium or other mineral salts, vegetable or syntan agents to stabilise the material and to protect it against microbial attack. In the tanning process the collagen fibre is stabilised by the crosslinking action of the tanning agents such that the hide (pelt) is no longer susceptible to heat increases. The level of susceptibility to heat changes with the tanning system. In this study leathers tanned with eight different widely used tanning agents were evaluated for their thermal behaviour using conventional T_s measurement, DSC and TGA. The results are given in Table X and Figures 1–5. Examining the relationship between T_d and T_s, it was clear that there was a correlation between the T_s and T_d values obtained from both methods as previously observed (3). Although there were small differences in the temperature values, the T_d and T_s had the same increasing tendency.

From the results, we can see that the highest T_s and T_d results were obtained from chromium tanned leathers. The T_s of chrome tanned leathers in the control group was measured as 103.5°C, and the T_d as 97.6°C. Similarly, ecol-tan^® tanning as a model of a chrome tanning process provided 96.5°C T_s and 97.4°C T_d to the leather. ecol-tan^® tanning is an innovative model providing higher chrome exhaustion rates and an ecological solution with its pickle and basification free chrome tanning process. The binding mechanism of these two tanning agents is through crosslinking at the carboxylate side chains of collagen with coordinated covalent bonds. The stability of the chrome-collagen complexes formed in this manner is characterised by the T_s, which is one of the most important criteria in determining the overall hydrothermal stability of leathers (29). Chrome tanned collagen is resistant to boiling water typically up to 95–100°C, thus indicating formation of highly hydrothermally stable crosslinks within the structure.

Following chromium, the highest T_s/T_d values among the metal-free tanning systems were obtained from phosponium-tanned leathers, as expected. Tetrakis(hydroxymethyl)phosphonium sulfate (THPS) can form short and strong cross-bonds mostly with amino groups and less with hydroxyl and carboxyl groups and peptide bonds of collagen in leather. It has also been reported that THPS is converted into tri-hydroxymethyl phosphonium (TrHP) and tri-hydroxymethyl phosphine oxide (TrHPO) during the tanning process. The nucleophilic substitution between formaldehyde and amino groups of collagen takes place during the reaction. The hydroxylmethylated amino groups of collagen combine with highly reactive phosphorus in TrHPO. The hydroxylmethyl groups of TrHPO combined with collagen dissociate continuously and nucleophilic reactions take place between formaldehyde and amino groups of collagen. Therefore, combination between hydroxylmethylated amino groups and phosphorus results in a large number of crosslinks in collagen fibres and accomplishes the tanning process which results in high thermal stability (30, 31).

The other T_s/T_d values obtained from metal-free tanning systems were in decreasing order: aldehyde, mimosa, tara, EasyWhite Tan^® and syntan. Giving the second highest shrinkage value, glutaraldehyde is the best-known aldehyde tanning agent. It is the most versatile and widely used, especially in automotive upholstery and upper leathers (32). The aldehyde functional group forms covalent bonds with nonionised amino sidechains of collagen. During this interaction Schiff bases can be synthesised from collagen amine sites and a carbonyl compound of aldehyde. It also forms semiacetal bonds with the hydroxyls of hydroxyproline, hydroxylysine and serine (33).

The tanning mechanism of vegetable tannins or natural polyphenols is due to the formation of numerous hydrogen bonds with collagen basic groups, for example lysine, arginine and the peptide backbone. Due to the high number of hydrogen bonds they have high T_d/T_s values, following aldehydes. The tanning efficiency was higher for condensed tannin (mimosa) than hydrolysed (tara), as expected (21, 34, 35).

EasyWhite Tan^® tanning system is a new technology in leather processing as a completely chromium-free tanning method. The method offers numerous benefits by helping to meet the growing need for chromium-free leather processing. Although its tanning mechanism is not exactly explained it is assumed to be based on hydrogen bonding and the active chlorine of triazine ring in the molecule reacting with the amino groups of collagen fibre, since it gives T_d/T_s values close to syntans. On the other hand, replacement syntan tanned leather had the lowest shrinkage values, as expected. During the tanning process, the ionised sulfonic acid groups of the syntans have strong ionic attraction for the cationic amino functional groups on the collagen side chains, while the phenolic structures bind similarly to vegetable tannins via hydrogen bonds. However, bonding sites are lower in number (36–38).

DSC thermograms of different tanned leathers are shown in Figure 1. As can be seen in Figure 1 it is observed that the leathers processed with different tanning agents had similar thermograms having one endothermic peak.

TGA is one of the simplest and most practical techniques used to characterise the thermal stability of materials by monitoring the change in weight as a function of increasing temperature, or isothermally as a function of time, in a controlled atmosphere (nitrogen, oxygen, air). This information helps us identify the percent weight change and correlate chemical structure, processing and end‐use performance of a material (39). The mass evolutions with temperature of tanned leathers are shown in Figures 2–5. DTG curves in Figure 6 and Table XI indicate the peak temperatures of derivatives. Almost all samples displayed similar behaviour indicating two degredation steps within the temperature range of 20–800°C. The first step of mass loss observed up to 100°C was due to the loss of free and bound water within the samples. The main degradation step was observed between 230–450°C which indicates decomposition of proteinic material. However, the thermal behaviour of the leathers was different in the dry condition than in aqueous condition. Among the samples syntan and mimosa tanned leathers showed higher thermal stability than other leathers regardless of their T_s. Similarly, phosphonium tanned leather also showed high thermal stability. The reason for syntan and mimosa tanned leather may be due to the poor thermal conductivity and high thermal stability of phenolic and aromatic structures. This kind of substance is used in the composition of fire retardant materials and polymers. Similarly, phosponium based compounds are also well known as good fire retardants, thus increasing the thermal stability of materials. The chromium tanned leathers (chromium and ecol-tan^®) had high peak temperatures (337°C and 325°C) where the maximum degredation process took place. However, their degradation process seemed to be fast with high burn-off ratio and low ash amount. This may be explained due to the increased thermal conductivity of these leathers since chromium as a metal may dissipate heat efficiently through the proteinic material, resulting in a fast degradation process. Tara tanned leather showed a fast degradation with an early onset temperature possibly due to the hydrolysis of ester groups in its structure. Moreover, gluteraldehyde and tara tanned leathers also showed a third degradation step after 500°C leading to a high degree of degradation, possibly due to their high organic content. However this remains to be further investigated.

Conclusion

Thermal stability and decomposition kinetics of collagen-based materials are critical for quality control parameters of tanned leather products. TG-DTG and DSC techniques proved to be a straightforward experimental methodology to collect data on dry leather materials, giving more precision and sensitivity compared to conventional T_s which measures the hydrothermal stability of collagen.

There was a clear correlation between T_d and T_s according to the applied methods. There were small differences in the temperature values, while both T_d and T_s had the same increasing tendency. Unlike the shrinkage performance, the chromium and ecol-tan^® tanned leathers indicated lower thermal stability than the other leathers. This may be due to the increased thermal conductivity of these leathers since chromium as a metal may dissipate heat efficiently through the proteinic material, resulting in a fast degradation process. It is interesting to see that the leathers having lower T_s may have higher thermal stability, demonstrated by syntan and mimosa, due to their poor thermal conductivity. The findings have potential for the leather industry to comprehend the thermal behaviour of finished leather products.