As the popularity of outdoor leisure activities increases, functional clothing is becoming the ‘in’ thing to wear. Originally conceived to meet the need to feel comfortable even when engaging in sports such as skiing or mountaineering where conditions are often extreme, high-tech materials such as Gore-Tex are now increasingly being used for everyday wear as well.

Leather has a high intrinsic level of wear comfort. It is durable, yet soft and flexible, has good insulating properties and high water vapour permeability. All this goes to explain why leather is still the number one material for shoes and protective clothing worldwide.

The disadvantage is that it consists of hydrophilic collagen molecules, which means that it absorbs water like a sponge, thereby losing a lot of its wear comfort.

The quick solution to this problem is to apply a polymer finish to the leather. This prevents water from penetrating, but also blocks a large proportion of the water vapour.

Another possibility is to integrate one of the highly sophisticated water-vapour permeable textile fabrics such Gore-Tex. However, this calls for complicated manufacturing processes which are not suitable for all types of leather.

So for some time now, experts have been trying to come up with a leather which is easy to produce and not just water-repellent but really waterproof. In the meantime, the demand for such leathers continues to increase.

Based on general demands with regard to wear comfort, specific requirements have been drawn up for waterproof leather:

* low static water uptake

* waterproofness under dynamic stress

* good water vapour permeability to ensure that the feet remain dry even under hot and sweaty conditions, and

* for aesthetic reasons, water should form droplets and run off the surface of waterproof leathers.

Most of the specifications for waterproof leather are based on four internationally recognised test methods.

To test the water absorption under static conditions, a piece of leather is immersed in water for a defined time of between 30 minutes and 24 hours.

These test conditions may appear to be the least severe, but this is the only test where the cut edge comes into contact with water.

The Permeometer test simulates the rolling movement of a shoe sole when walking and is, therefore, most suitable for testing sole leather.

The Bally penetrometer and the more stringent Maeser test (shown) simulate the dynamics to which a shoe upper leather is subjected at every step. The 50,000 Maeser flexes carried out over a period of about eight hours represent the equivalent of walking through water for over 70km.

Although these two test procedures appear very similar, the results are not transferable. The values obtained on the Bally penetrometer have better reproducibility and are less prone to irregularities, but a good performance in the Bally test does not necessarily mean that equally good results will be obtained in the Maeser test.

The first-generation waterproofing processes – ‘closed waterproofing’ – failed completely as regards to the water vapour permeability. The basic principle was to fill the interstices with a water-repellent substance such as fat or oil, thereby totally preventing both liquids and gases from penetrating the leather.

State-of-the-art systems use more advanced components, often polymer-based and combined with silicone or fluorinated resins. These combinations form a thin film on the individual leather fibres, creating a highly hydrophobic net that repels water droplets in the same way as a solid Teflon pan does.

At the same time, they remain highly permeable to individual water molecules in the water vapour and to gases. In addition, the aesthetic properties are considerably improved. The result is a soft, lightweight leather with a pleasant, dry touch.

Since the use of fluorinated compounds is restricted due to their ecotoxicological properties, a typical set of waterproofing agents consists of one or more fatliquoring agents, often polymer-based, combined with a silicone. When used in the retannage in place of classical fatliquoring agents, these result in a waterproof leather.

Experience has shown that even the best waterproofing systems are still highly sensitive to emulsifiers and surfactants, and producers of waterproof leather are generally aware of this fact. Using the wrong product for degreasing the raw hide, not washing the hide thoroughly enough or using a sulfonated pre-fat in the chrome tannage can have an adverse effect on the subsequent waterproofing.

During the retannage, too, different products can influence the waterproofing results. Over the past few years, a number of studies have been published which indicate that a high salt content in the leather, for example, or certain auxiliaries can impair the waterproofing effect.

Hydrophobic substances have to be combined with an emulsifier if they are to be suitable for use in an aqueous solution at all. Unfortunately, the emulsifier then nearly always severely impairs the waterproofness of the leather.

Emulsifiers can either be applied externally by adding a single, low-molecular-weight compound to the hydrophobic compound, or internally by chemically fixing an emulsifying group to the backbone of the hydrophobic substance.

In either case, the emulsifier must be destroyed during fixation, ie in the final stage of the process – not just to fix the product at all, but to boost the waterproofing effect.

For this reason, all the commercial waterproofing systems available so far have one common drawback – the need for fixation with mineral salt, in most cases chrome.

This not only increases the chrome load in the wastewater, but also causes the colour of the leather to shift during the very last stage of the retanning process, making it dark, green and dirty-looking.

Other metals such as aluminum and zirconium create similar ecotoxicological problems and in addition have an extremely high inorganic acidity which affects the long-term stability of the leather.

What is needed, therefore, is a waterproofing system that can be fixed without mineral salts, a system that requires only the addition of organic acid.

This means developing hydrophobic compounds which can be used in water without the need for any emulsifier, or the emulsifier must be sensitive to acids.

The new Bayer concept for the Xeroderm AF products is based on both mechanisms, combined in a two-component system:

The polymer-based Xeroderm P-AF contains no surfactant emulsifiers at all and can be regarded as a substitute for the classical fatliquoring agents. It can be added throughout the entire retanning process at pH levels of between 4 and 7. The silicone Xeroderm S-AF is tailormade for this system.

Its integrated anionic emulsifier ensures adequate emulsion stability at pH values of 7 and higher, but is completely destroyed at a pH of 3.5 to fix the compound and reinforce the waterproofing effect. Xeroderm S-AF is added more towards the end of the retannage.

Two characteristic features of a typical rapid process have proved particularly important. Firstly, the wet, chrome-tanned raw hide (wet-blue) has to be carefully neutralised, preferably overnight.

If the hide is not completely neutralised, because the process has been carried out too quickly, the waterproofing agents cannot penetrate properly, leaving areas within the leather which are not waterproof. The leather is then capable of absorbing too much water.

The second, more obvious peculiarity is the high pH needed for the application of the silicone in the final stage to ensure optimum waterproofing results. As already mentioned, the emulsifier integrated in the silicone is pH-sensitive and requires a pH of ideally 7 if it is to penetrate effectively and uniformly through the entire cross-section of the leather.

After adequate time for penetration, the products are then fixed by the stepwise addition of formic acid until a pH of 3.5 is reached. This method does not cause the colour to shift, and pastel shades can be produced just as successfully as brilliant colours.

It is widely believed that a neutral pH during retannage has an adverse effect, especially on the tightness of the grain. However, as we use ammonium bicarbonate, a mild basifying agent with excellent penetration, to increase the pH, we have not had any problems with this. We have even succeeded in producing waterproof leather from Brazilian raw stock which was just as good as that obtained with conventional systems, or even better.

With the new two-component system, characteristics such as the softness or touch can be easily adapted to meet the particular requirements by adjusting the ratio of polymer to silicone. Properties such as the fullness or firmness, on the other hand, are best adjusted by selecting the appropriate retanning materials.

It has also been found that the process conditions in combination with the Xeroderm products facilitate dyeing and make it possible to obtain more intensive shades with improved dye penetration, even with smaller amounts of dyestuff.

The advantages of this new waterproofing system over state-of-the-art methods can be summarised as follows:

* fixation without mineral salts, especially chrome salts

* lower wastewater load

* fixation by acidification to pH3.5 with formic acid

* suitable for both brilliant colours and pastel shades and no shifting of shade

Leather treated by the new system has the following properties:

* very low static water uptake

* high dynamic water resistance: 50,000 Maeser flexes

* no difference in shade between the grain

and the flesh side (twosidedness)

* good lightfastness, little tendency to heat yellowing

* high water vapour permeability (open waterproofing)

* good levelness (no staining)

* suitable for high-quality shoe upper leather, leathergoods, garment leather etc.

With the new products for Bayer’s Feel Fine system, Xeroderm S-AF and Xeroderm P-AF, even the toughest requirements can be met.