Wet-white paper – industry research

Concerns about ecological issues and their effects on human health seem to govern anthropogenic activities causing the increasing total buoyancy that is a well-known dilemma. Having become conscious of all the parties of producers and consumers in the world, recent news released in the international media and attempts by the non-governmental organisations make the environmental and human health issues more important than ever. This is the key evidence that the chemical characteristics of process wastes and the traces in final products should be proved analytically as well as by physical characteristics such as durability or flexibility.

A closer monitoring of the tanning industry has revealed that it possibly has a negative impact on the environment and its products sometimes contain hazardous substances like chromium VI (CrVI), although only trivalent forms of chromium of entire valences are used in the tanning process. With the increased interest in the avoidance of certain chemicals and industrial products that are particularly harmful to our environment, chromium-tanning technology is not able to meet requests including a ‘green’ approach and, also, it is not surprising that many brands are pointing out attributes that play to this script – so one hears the claims of ‘chrome-free’ leathers.

The stringent norms set by regulations as well as the interest in clean technologies have led tanners to increase their efforts to develop chrome-free tanning agents. Chromium-free leathers have advantages, like the lack of chromium in the effluents, obtaining fully recyclable shavings and end-products for agricultural applications, no risk of CrVI formation, metal-free leathers, improved sorting in the pre-tanned stage, and white and light-coloured brilliant leathers. Therefore, different markets require the manufacture of chrome-free leather to have comparable properties to chrome-tanned leather such as feel, fullness, softness and hydrothermal stability.

Due to the lack of predictability of the future in chromium tanning, wet-white technology seems to be ecologically better without any withdrawal from the distinct properties of mineral tannage. A wide variety of different types of leather can successfully be produced using a wet-white system, including automotive leathers, upholstery leather, garment leather and shoe-upper leather.

This technology also suggests that the environmental impact of chromium could be alleviated or eliminated by substituting the other metal-tanning salts, such as Al(III), Ti(III)/(IV), Fe(II)/(III), Zr(IV) and La(III) due to the proportion of the offering. Other alternative tanning options can be polyphenols, oxazolidine, formaldehyde, polymers or carbohydrates. Protein products that have the cross-linking ability to collagen are also among these options.

Because the cross-linking agents play an important role in the physical properties of collagen, wet-white processing is carried out by certain methods using the agents capable of reacting with functional groups to enhance the properties. In this study, collagen-based biopolymers with a strong reactive moieties were used in combination with mineral tannages such as aluminium and zirconium salts, and organic alternatives such as aldehydes, phosphonium, THPS and syntans to enhance wet-white leather properties.

How it was done

In this study, collagen-based biopolymer (hydrolysates) obtained from gelatine production by enzymatic hydrolysis were used, and hydrolisate and low-grade by-products were intended to valorise for reuse in wet-white processing. The water-soluble collagen-based biopolymer proportions were added directly into the drums and dissolved during the process. The trials were carried out on first-quality pickled hides with 1.2–1.5mm thickness and 38–40ft² surface areas on average. The hides were cut into two halves along the backbone before the trials to obtain comparable results.

The process design was based on the improvement of the properties obtained by the collagen-based biopolymers by using some supplemental tannages such as mineral (aluminium and zirconium salts), and organic basis (aldehydes, THPS and syntans). So, as a fundamental additive, the biopolymer needed to have a capability to convert the collagen to leather far beyond the estimations. In order to determine the sufficiency, the self-tanning action tests were carried out with constantly increasing proportions of biopolymer.

Considering the self-tanning action tests, the ten hide parts were treated with a standard tanning procedure, starting with a depickle to the pH value of 3.5–4.0 and running with the increased proportions of 5, 7, 10 and 15%. The time was four hours and overnight, and, subsequently, 10% synthetic oil was added and the final pH value was adjusted to 3.2. After rinsing the hide parts three times, they were taken out for sammying and horsing-up overnight, and dried properly. The self-tanning actions were assessed according to the hydrothermal stability tests, physicomechanical test results and organoleptic evaluations.

Experimental process design

After the optimisation of the biopolymer proportion for acceptable tanning properties with the self-tanning action tests, experimental process design was composed over the properties obtained by biopolymer by using some supplemental tannages. The amount of biopolymer was 0, 5, 10 and 15% in each of the four procedures. Each procedure indicated herewith was considered to be easily applicable and appropriate for the conventional process equipment and infrastructures. Full cattle hides with the same characteristics were used in process design trials.

The leathers processed with the optimised proportions of biopolymer and the supplemental different tannages according to the four different procedures were compared with the results obtained from the tests such as hydrothermal stability (ISO 3380 and DSC), tensile strength (ISO 3376), moisture contents (ISO 4684) and nitrogen contents (TS 4134).

Results and discussion

Today, chrome tanning among the mineral-tanning practices is largely common and a comprehensively applicable method for the global industry. The reason for versatility of the method is based on the high stability and ability to provide superior skin characteristics. While this preference is sustained by hydrothermal, heat and light stability, and also good dyeability, some drawbacks of the method include the lack of durability and stability of alternative methods; heavy metal accumulation and pollution; and high discharge parameters in terms of chloride, sulphate, total suspended solids, chromium and other metal salts to the sewage; and are considered to be the main obstacles for the sustainability of the versatile method. In particular, the negativity around the amount of effluents on the sludge quality and the possibility of the formation of hexavalent chromium compounds in products and sludge wastes over time are the proof that the sustainability properties of the chromium-tanning method could no longer be foreseen. As a result of the problems on the agenda, leather production is governed by an eco-friendly industrial process.

As per today’s consciousness eco-friendly methods and green approaches in production, it seems to be more effective to push for finding an appropriate way for subsidising sustainable life quality. Considering that chrome and other inorganic pollutants contaminate air and water, and vegetable and organic-tanned leathers are also known to be poorly biodegradable, which results in high biochemical oxygen demand (BOD) and chemical oxygen demand (COD), the combination tanning systems were considered as suitable tanning method to overcome these problems arising from single-tanning systems.

Many substances having various reactive sides or moieties exist and are capable of achieving this functionality. Gelatin is a product with partially fragmentised fibrous protein with some functional groups decomposed from collagen by enzymatic hydrolysis. It is an important functional biopolymer that has broad applications in the food industry. The functionality of collagen hydrolisate is useful for the transformation of hides and skins to leather by diminishing the environmental impact of mineral-tanning methods commonly used, and also by enhancing the final leather properties and aiding the biodegradability. In this study, some wet-white properties were aimed at recovering the functionality of biopolymers based on collagen denaturation products and their effects on the final products were evaluated.

Determination of self-tanning action

In these experiments, with the proportions of direct application to the pickling skin used without tanning agents based hydrolysates, the study tried to detect the changes in the physical properties, which depended on the chemical changes of leather.

The knowledge of the characteristics of cross-linking bonds is of great importance to the tanning chemistry. The formation of such bonds decreases the solubility of collagen, which increases the shrinkage temperature and influences many other properties.

The change from the soluble to the insoluble fibrous state is an essential biological requirement for cross-linking. Due to this, the shrinkage temperature remains important as an index, which reflects the quantity of new bonds formed in collagen, and the quality of tanning and tanned leather.

The following parameters were tested after the production trials: hydrothermal stability temperature, tensile strength, fibre insulation and total organic nitrogen content.

Following the evaluation of the results obtained by the two methods in the percentage use of tanning extract, collagen hydrolysate has been optimised. Accordingly, use of the hydrolysate in the concentration range 7–15%, skin collagen hydrothermal stability temperature strength was identified as being at the optimum level.

The increase in tensile strength properties was determined against the increased amount of collagen hydrolysate used, which has also been found to vary inversely with the use of the percent elongation at 100N when applied. The expansion of the skin is a parameter representing the skin firmness. The tanning process means the fixing of the collagen protein; after completing the process, a marked decrease in elongation is expected to occur. The results obtained from self-tanned leather, decrease in elongation amount and how it corresponds to the amount of collagen hydrolysate can be monitored.

A steady increase of the amount, depending on the percentage of nitrogen concentrations, was observed in the studies. The obtained results of the skin, which help increase the organic matter content of organic nitrogen, is increasing and this was used in the later stages of leather processing auxiliary substances, which increases the capacity of making a bond with the skin of the other chemical substances.

Fibre isolation by tanning agents (microscopic observations)

The isolation and uniformity in fibre bundles, which is important for good-quality leather and demanded performance, seem to be observable in the results. The physicomechanical properties are provided by avoiding sticking fibres together and opening up the fibrous network. In tanning and after tanning operations, the isolation is expected to improve with the additions and auxiliaries, which are capable of interacting with the reactive groups and filling up the gaps.

The effects of chemical modifications in collagen refer to the denaturation temperature, which is the main tanning effect. Recent studies on unmodified and chemically modified collagen indicate that the observed hydrothermal stability is dependent on the moisture content; reducing the water content causes the fibres to approach close up, preventing them from collapsing into the interstices that are correlated with an elevated denaturation temperature. Therefore, a reduced ability to shrink is the same as increased hydrothermal stability. Consequently, reducing the ability of collagen to shrink by chemical modification results in a higher observed denaturation temperature.

The hydrothermal stability of collagen can be modified by many different chemical reactions and thereby increases the performance as per the demands. Higher hydrothermal stability can be obtained by chromium tanning, which is up to 105–115°C and preferably achieved by other minerals to the sufficient temperature of up to 70–80°C.

The thermal denaturation of collagen was studied by using a differential scanning calorimeter (DSC 60, Shimadzu). The samples were fused in a DSC cell and the temperature was calibrated using indium as standard. The rate of heating was adjusted constantly to 5°C per minute. The peak temperature is associated with the phase change for the shrinkage process for self-tanned leathers.

According to the results, the use of collagen hydrolysate provided an increase in the hydrotermal stability temperature of the leather tanning process, and is realised at the level expected when the resistance value has shown improvement. According to the results of the self-tanning trials for wet-white leather, production process collagen hydrolisates optimum use amount was identified as 10–15%.

Future perspectives

The wet-white tanning procedure is a method developed for the avoidance of chromium tanning risks for human health and the environment. The method is applicable with some mineral and organic chemical additives capable of binding with collagen reactive moieties by using the equipments and infrastructure for conventional production. The method used and the tannages applied ascertain the characteristics. At the same time, the final properties would be able to be modified by retanning and fatliquoring supplements, with regard to the demands of the end users.

According to the results, wet-white leather qualities seem obviously convertable to the versatile demands required for many types of consumer goods. At the same time, the leather properties with ecologically compatibility were obtained by the increased proportion of biopolymer.

Full paper with references available upon request.