Trends in non-chromium tannages

Introduction

Whilst there has been, and still is, considerable research into new tanning methods, essentially three systems dominate: aldehyde, chrome and vegetable tanning. Each of these tannages produces leathers with different properties but, in general, there seems to be a move towards non-chromium tannages, especially within the automotive sector.

It is of interest to evaluate the differences between two of the main types of tannages and the properties of the resulting leathers.

Chrome tanning

Chromium now forms the basis of virtually all light leather manufacture. It is relatively cheap, has a well-established technology and most auxiliary chemicals used to enhance leather performance have been developed on the basis of a chrome-tanned substrate. The unique characteristic of chrome-tanned leather is a shrinkage temperature greater than 100°C, which allows it to ‘withstand the boil’ for a period of time (1-3 minutes).

The key properties of chrome leather are as follows:

* Lightweight leather with an attractive appearance

* High tensile strength

* Good chemical stability (eg stable over a range of pH3-8)

* Versatile in physical properties (eg softness, stretch)

* Dyes and finishes in brilliant colours

* A certain degree of inherent water resistance

* Good sueding properties

* Good permeability to water vapour and air

* Rapid controllable tannage

* Good setting and other properties for upper material for footwear

* High hydrothermal and dimensional stability

The process of tanning occurs when tanning salts, such as chromium, are able to crosslink collagen protein molecules. This crosslinking action makes the hide less susceptible to the effects of heat and putrefaction.

The key components in the reaction are the chromium III ion, basifying agent or base, and the collagen molecule. The key reactive molecular group on the collagen molecule is the carboxylic acid group (-COOH).

When chrome powder is added to the acidified pelt, reactions are established between the chromium, the acid, and the pelt. Collagen molecules are crosslinked via the chromium salt, either by one large chromium complex reacting with two collagen carboxyl groups or by two bound chromium complexes reacting together.

Chrome tannages are cationic and have good affinity for most chemical products added in the post-tanning stages (retanning, dyeing and fatliquoring). This means that they also respond well to any finish or surface treatments that are applied and, consequently, the fastness properties are usually better than other tannage types.

There are some issues currently with regards to the use of chromium-tanned leathers. One specific area is their use for toys and children’s shoes. Currently chromium-tanned leather is not able to meet the testing requirements of EN71-3 which is a legal requirement for toys.

Aldehyde tanning (wet-white)

In some cases, it is preferable to avoid the use of chromium in tannage and other options for wet-white production exist.

One method for producing wet-white is to stabilise the hide sufficiently to withstand splitting and, more specifically, the shaving operation. In this way, only the preferred portion of the hide receives all processing treatments, and the split and shaving by-products may be disposed of to waste or be differently processed into value-added products such as fertiliser or animal feed. Wet-white is also sufficiently stable that it could be sold as a commodity or stored and transported in this state.

Properties of wet-white include:

* An adequate shrinkage temperature (~75°C), which allows splitting and shaving (but is poor for lasting)

* The shaving and trimmings are chrome-free

* Possesses a high degree of flexibility so the fibre structure can relax after the splitting process and is set out leading to significant area yield

Advantages also include:

* No disposal problems of shavings and trimmings, coupled with the possibility to produce a chrome-free leather

* Considerable savings in chemicals, as the usage of tanning materials is based on sammed/shaved weight and, furthermore, the process is more efficient, leading to less total chemicals being used

* With vegetable tanning, the shaving is carried out before the tannage, reducing the problem of iron staining

* The quality of the leather, particularly in regard to grain tightness, can be improved

Aldehydes react with the amino groups of collagen (the same groups that react with dyes and fatliquors). This means that it is often the case that more of these post-tanning chemicals are required to achieve a given depth of shade and softness. This type of tanning is now quite widely used in the production of automotive upholstery leathers. It should be noted, however, that aldehyde tanning is not suitable as a sole tannage. It needs to be carried out in conjunction with other tanning agents. Also there needs to be consideration of the potential for formaldehyde release that can occur in some cases. Specifically this is an issue for the automotive industry and again for the production of leather that is used for toys.

New tanning systems

The BLC, in collaboration with Loughborough University, is working on a DTI funded project ‘Development of a High Stability Chromium Free Tannage’. The project is intending to develop a high-performance, thermally-stable epoxy tannage to be used to produce mineral-free leathers, primarily for the automotive sector within the leather industry. One of the forces behind the push towards mineral-free tanning is the automotive industry. In this sector, there is a definite trend towards an increased use of leather for the internal furnishings of motor vehicles. Other leather sectors are also beginning to examine the potential of chrome-free alternatives.

Currently, many car manufacturers impose a very strict limit for formaldehyde concentration of 10-ppm for automotive leather which is difficult to achieve. Furthermore, glutaraldehyde-based tanning agents can cause problems within biological effluent treatment plants where the aldehyde can act as an effective biocide. This leads to demands for a mineral-free tanning agent, which can crosslink the carboxyl groups rather than the amine side chains of the skin’s collagen, to give the thermal and dimensional stability currently offered by chromium.

Epoxy resin tannage is not new to leather. It is well known that epoxide reacts with nucleophilic substances which have amino, carboxyl and hydroxy groups and it has been suggested that epoxide reacts with such residues on the collagen fibre. In fact, some investigations on the epoxy treatment of proteins can be found in literature which dates back to 1944.

Epoxy resins are important industrial polymers widely used in many major industrial applications such as coating, adhesive, civil engineering and casting. There are two characters of these polymers, namely high reactivity and high versatility, which make them suitable for various industrial applications.

It was found in the earliest studies that epoxy tannages required a high reaction temperature, a long reaction time and a large epoxy resin offer for a shrinkage temperature of 80°C to be obtained. Nevertheless, there were still many positive results worthy to note, including:

* The leather obtained upon tanning with an epoxy resin has good properties of chemical stability, including resistance to acid, alkali, boiling water and organic solvent treatment. The high stability is due to the introduction of covalent crosslinking bonds by the di- or multi-functional epoxides.

* Most epoxide tannages are carried out under basic conditions of pH8-9 followed by neutralisation

* Shrinkage temperatures (Ts) of over 80°C have been obtained within 10 hours at 35°C. Epoxy resins with lower molecular weight or higher functionality result in higher hydrothermal stability. A maximum shrinkage temperature of 90ºC has been achieved by using tetra-functional epoxy resins

Current research has focused on the use of commercially-available epoxy resins. Initial work concentrated on model systems of individual amino acids (lysine and glutamic acid). This illustrated that the reactivity was appropriate for further trials.

When one epoxy resin was applied to sheepskin, a shrinkage temperature of 89°C was obtained. Various re-tanning agents are currently being investigated to further optimise the Ts and currently 95°C can be achieved. In addition, a considerable improvement in dye uptake and fixation was shown in comparison with vegetable tanned leathers. This was attributed to the epoxy resin providing additional fixation sites for the dye/collagen interactions.

At present the main limitation to the industrial application of these resins is their poor solubility in water. Further research is currently underway with the project industrial partners to determine solutions to this issue.

The project has undoubtedly shown epoxy resins to have huge potential as alternative crosslinkers for use in the leather industry. Large-scale trials are underway to enable a full assessment of the physical performance of the leather produced and in addition the environmental fate of the resins will be evaluated.

Acknowledgements

The authors would like to acknowledge the input of the Dr R Heath and Dr Y Di of the University of Loughborough to this project along with the assistance of the other industrial partners. This project work has been part funded by the UK Department of Trade and Industry (DTI).