The production of high-quality waterproof leather is a challenge to both leather chemists and applications technologists – especially if long-lasting waterproofness is required and no mineral salts are to be used for fixation.

This system, which is extremely easy to use, preserves the breathability of natural leather, while at the same time giving excellent, long-lasting waterproofness under dynamic stress. Since no chrome salts are used as fixing agents, the quality of the surface and the colour intensity of the treated leathers are noticeably improved.

Where would the shoe and clothing industries be without leather? No other product combines tenacity, robustness, wear comfort and wear climate better than this versatile and popular material.

Leather also has the advantage that it can be adapted to suit varying fashion requirements by applying different types of finish. It is not surprising, therefore, that attempts are being made to expand the applications of this natural product to include fields which were previously the domain of other materials. A good example is waterproofing.

Three ways to waterproof leather

For some time – and especially during the past few years – scientists have been trying to find a way of making leather not just water-repellent, but properly waterproof and therefore suitable for such applications as hiking boots and rainproof motor cyclists’ clothing. It is relatively simple to make clothing leather, for example, fast to water spotting by applying an appropriate finish to the surface. It is a different story, however, if not just the surface, but the entire, basically water-permeable leather is to be made waterproof, ie impermeable to water.

There are three possible ways of doing this: either the leather can be coated with a waterproof layer of polymer (at the expense of the wear comfort), or the interstices between the leather fibres can be filled with water-repellent substances such as greases, paraffins or synthetic resins (closed waterproofing), or the fibre bundles can be impregnated with special substances which prevent them from being able to absorb and transport water (open waterproofing).

Up until now, scientists had not managed to produce a satisfactory, long-lasting waterproofing system which preserved the high level of wear comfort and, more importantly, was also capable of withstanding the dynamic stress associated, for example, with the deformation of shoe upper leather during walking.

In addition to the numerous factors which have to be taken into account in waterproofing – such as the fact that the waterproofness of the treated leather depends among other things on the drying method used, the type of retannage and even the salt content of the leather – there were various fundamental obstacles to be overcome in trying to develop new waterproofing processes.

The problem with closed waterproofing is that it (partially) seals the pores and therefore frequently impairs the water vapour permeability and water vapour absorption of the leather and also noticeably increases the thermal conductivity.

It is not easy to find the right balance between the water absorption and the water vapour absorption of the treated leather. A high storage capacity for water vapour makes for a pleasant wear climate. The water absorption, on the other hand, should be as low as possible, so that the material dries quickly and therefore does not feel damp. Good-quality leather with an open waterproofing finish can absorb up to 20% moisture at 95% atmospheric humidity without feeling wet.

In many cases, both closed and open waterproofing systems proved unable to withstand repeated mechanical stress: sooner or later, even strongly waterproofed leathers allow moisture to pass through if they are subjected to repeated stress as simulated, for example, by the Bally penetrometer test or the (more stringent) Maeser test, in which leather specimens are repeatedly flexed in a water bath (Figure 1).

Furthermore, efficient waterproofing agents have to be fixed with chrome salts or other mineral salts – a considerable disadvantage at a time when the industry, being driven by the consumer, is striving to minimise the amount of these salts in residual tanning and retanning floats.

There are obvious reasons for these problems. The development of waterproofing agents presents a real challenge to leather chemists and applications technologists, because the chemicals whose job it is to make the leather waterproof are by nature hydrophobic and therefore tend not to be emulsifiable in water.

But the waterproofing process is carried out in an aqueous medium so as not to make the retannage even more complicated. So the waterproofing agents have to be masked, ie designed in such a way that they do not form a water-repellent surface until they come into contact with the collagen fibre.

Alternatively, they can be transported to the fibre by water-soluble chemical ‘vehicles’ (emulsifiers), but these then have to be removed afterwards or fixed with metal salts.

There is also a risk of the waterproofing agents gradually losing their effect when shoes, for example, are subjected to repeated mechanical stress. This may happen if the leather fibres are only waterproof on the surface due to inadequate distribution of the waterproofing agent or because the emulsifier system is not ideally suited to the float conditions and leather properties.

Now, a team from Bayer AG in Leverkusen consisting of polymer chemists from the Central Research Division, leather experts, process engineers and applications technologists has succeeded in developing a waterproofing system which makes it possible for the first time to produce waterproof leathers capable of withstanding extreme mechanical stress and whose properties and wear comfort are equal to those of top-quality standard leathers.

A further important advantage for leather manufacturers is the fact that the new waterproofing agents do not need chrome salts or any other heavy metal salts for fixation; all they need is final acidification with a standard, unproblematic reagent – formic acid.

Simple application a priority

The task the Bayer team set itself was to produce waterproof leather which was capable of withstanding dynamic stress, yet still had good breathing properties, and could be subjected to over 50,000 flexes in the Maeser test or eight hours in the Bally penetrometer without allowing water to penetrate.

It was also to be suitable for producing very soft leathers. In addition, the system should be simple to meter, ecologically sound and, most importantly, would satisfy the highest demands with regard to the quality of the surface and the handle and dyeing properties of the treated leathers.

The new system took two years to develop and has passed the first practical tests, for example in the production of leather for army boots, sports shoes and fashion goods. It is currently marketed under the names Xeroderm P-AF and Xeroderm S-AF.

The new, two-component waterproofing system consists of a special polyacrylate (P-AF), which reduces the water absorption while at the same time preserving the full water vapour permeability of the leather, and a silicone (S-AF), which is specially tailored to leather chemistry and ensures exceptionally good waterproofness of the leather under dynamic stress.

Thanks to a finely balanced emulsifying technology, both products penetrate uniformly, completely and deeply into the leather structure during the wet finishing of the wet-blues. The development of the chemistry of the silicone component was particularly difficult as, up to now, these components had been considered difficult to emulsify.

No additional operation is needed when waterproofing with Xeroderm P-AF and S-AF. The process can be incorporated into the normal wet finishing operation without any problems and without adding to the time required. The waterproofing agents are simply added in place of the fatliquoring system (Table 1).

The amount of Xeroderm P-AF/S-AF required (calculated on the shaved weight) depends on the desired degree of waterproofness.

The new products are subsequently fixed with formic acid alone to give a long-lasting effect which cannot be washed out. Absolutely no mineral salts such as chrome, aluminum or zirconium are needed for fixation.

Well-balanced property profile

All in all, the halogen-free, two-component waterproofing system fixed with formic acid gives high-quality leathers with good water repellency and long-lasting waterproofness.

The static water absorption of leather treated with Xeroderm P-AF and S-AF is about 10%, which means that the material does not feel damp even on long walks across wet fields.

Provided the wet finishing is carried out properly, there is no adverse effect on the tensile strength, tear strength and buckle tear load of the leather. The water vapour permeability, one of the highly valued properties of non-waterproofed leather, is also preserved, as there is no sealing of the pores with the open Xeroderm P-AF/S-AF waterproofing system: the breathing properties remain intact.

As no green chrome salts are used for fixation when waterproofing with Xeroderm P-AF/S-AF, there are no differences in colour between the front and back of the leather (two-sidedness), and the changes in shade which occurred with chrome fixation are likewise a thing of the past.

Thus the new Xeroderm products can also be used in the production of very pale-coloured or brilliantly dyed waterproof leathers, and there is nothing to stop ambitious fashion designers from using these colours for outdoor clothing as well.

Pale or colourful, soft or firm

Leathers treated by the waterproofing system with formic acid fixation also have important advantages with regard to the quality of the surface.

They have a particularly smooth, supple and yet tight-grained surface, which does not feel silicone-like despite the silicone component, and a soft, wax-like handle. There is no shrinkage of the surface as there normally is with chrome fixation.

In addition, Xeroderm P-AF and S-AF solve the problems which previously arose when waterproofing firm leathers in particular. It is a well-known fact that large amounts of conventional waterproofing agents have a softening effect.

With the new Xeroderm products this is no longer a problem, as their high affinity to the leather ensures good penetration even of thick leathers.

Bayer has also developed special tanning materials (Tanigan F) for the retannage of particularly firm leathers which are to be waterproofed with Xeroderm. Thus the firmness and softness of the treated leather can be carefully adjusted by varying the retannage accordingly. The result is leather (upper leather for army boots) which combines a pleasant surface with good firmness.

Thus leathers treated with Xeroderm P-AF and S-AF are a valuable addition to the applications of this, the oldest material in the world. Thanks to their pleasant handle and smoothness, they can be used for the production of high-quality, waterproof articles.

With their waterproofness under dynamic stress, their good wear climate associated with high water vapour permeability and their good drying properties, they provide shoe manufacturers with an excellent material for boosting the popularity of traditional leather for the outdoor shoe sector, which, like textile leisure wear, has become increasingly the domain of products based on water-vapour-permeable membranes.