The historical cavalry boot shown is made of Russian leather, which was subsequently impregnated by chrome stearate. This boot is heavy and the foot might get soaked with sweat when worn for a long time. In contrast, the trekking boot is made of state-of-the-art waterproof leather.
Ramblers can walk quite long distances in these shoes and the feet stay comfortable in spite of perspiration even in the rain, even when walking in a meadow moist with dew, which is a very harsh test for the water resistance of shoes.
Demands on waterproof leathers
The aim of processing waterproof leather is the production of leather which has an appealing appearance and results in shoes or motorbike garments etc with high wearing comfort even under wet and cold conditions. Leather should act as a second breathing skin. The skin protects humans against external influences. However, it also allows the body to regulate its temperature by perspiration.
Waterproof leather literally stands for leather which water doesn’t penetrate. However, the leather should allow additionally high water vapour permeability and some reversible water up-take to remove perspiration from the foot. The leather should insulate against heat and cold and be lightweight.
The common testing procedure for waterproofness – the static tests of the life time of a water droplet, Kubelka water up-take and the soaking up test as requested by membrane shoe manufacturers, as well as the dynamic tests, Bally penetrometer and Maeser value, have to be seen in the context of the use of this leather. For example, a high Maeser value will be requested for an outdoor walking boot as the mechanical action during measurement simulates the mechanical action during walking in water.
The situation is completely different for outdoor boots which are equipped with a polymeric membrane. For this use excellent water vapour permeability is requested. Furthermore, the leather should not soak up water either from the grain side, the flesh side, or through the cut. Then, the static Wicking-test will be sufficient.
Likewise, the static water droplet test is sufficient for aniline upholstery leather which is required to be resistant against spilled liquids. Leather which has absorbed too much water, looses its ability to insulate against heat and cold. Therefore, waterproof leather should not take up more than 25-30% of water.
The dynamic tests Maeser flexometer and Bally penetrometer and their comparability are intricate because – depending upon the customer requirements – different modifications are done. Often the customers’ needs deviate from the official testing procedure.
The read-out can be done electronically – sometimes referred to as ‘con luce’ – or optically. Electron read-out has a higher degree of rigour, especially when thin leathers are processed. In addition, how demanding a requested value is depends on the processed wet-blue, on the storage time, the degree of olation of the wet-blue and on the thickness of wet-blue.
For example, 50,000 Maeser flexes are excellent when heavy hides are processed with a thickness higher than 2.5mm due to the strong mechanical action during measurement. This value can mostly be exceeded when leathers of 1.5mm are processed.
Sometimes certain requirements should be challenged, eg the request for leather which stands more than 100,000 Maeser flexes. A boot made out of this leather would allow a 100km walk in rain. However, it is unlikely that a walker would do that.
This request might come from the experience that waterproof values sometimes break down after buffing or finishing combined with the need for a reasonable 15-40,000 flexes in the final article. However, in such cases the overall leather production process is wrong and should be revised.
Some general thoughts about waterproofing
If we understand how the wetting of leather takes place, then we will understand more easily how we can slow down or completely prevent this process. The wetting of leather takes place in a four-step process. The water spreads over and wets the leather surface. Then the water penetrates the leather and, thereafter, the water wets the fibre network; in other words, it wets the internal surface of the leather.
Finally, due to attractive interactions between water and the leather network the leather gets soaked with water. The collagen backbone but also tanning agents, dye molecules, salts etc, which are present in the leather network, might be involved in these interactions.
This chain of process steps must be interrupted to prevent the wetting of leather. At least one process step must be stopped, which will be explained later.
A closed waterproof film can be applied in finishing. The spreading of water over the surface is prevented by the film and the leather cannot be wetted at least under static conditions. However, such films even with most modern technologies drastically reduce the water vapour permeability.
The gaps in the leather can be filled in two completely different ways: firstly, impregnation and secondly, hydrophilic waterproofing. Firstly, impregnation is a treatment of leather by molten waxes. The filling of the gaps with wax prevents the penetration of the water into the fibre network. The disadvantage is that the leather is extremely heavy and completely prevents any air and water vapour permeability.
Secondly, hydrophilic waterproofing is achieved by application of certain surfactants, eg sulphosuccinates which bind to the leather and make the leather absorb a certain quantity of water. The surfactants and the water form a water-in-oil emulsion, which fill the gaps in the fibre network. Additionally, these micelles are hydrophobic on their outer side and, therefore, the gaps are filled with a hydrophobic material.
Shoes, which are made of this leather, might have an excellent wearing comfort directly after being put on because the leather is absorbing sweat. However, unfavourably, the leather weight increases drastically. In addition, the breathability of this leather will cease when water has been taken up. Afterwards the shoe will be dried and the water will be removed completely and the leather will return to its original state.
Open waterproofing
The so-called open waterproofing is the smartest approach to make waterproof leather. The internal surface of the leather is coated by a waterproof agent that binds to the fibres and fibrils through its functional groups. Waterproof agents are more efficient as the surface tension is lower.
Figure 1 shows the internal surface schematically coated with the waterproofing agent. Generally all molecules attract each other. The molecule 1, in the interior, is attracted equally from all sides. Overall, no resulting force acts on this molecule. In contrast, the molecule 2, at the surface, is attracted into the interior. As all molecules, which are on the surface, are drawn into the interior, surface energy must be spent to form a surface.
The larger the surface and the higher the surface tension is, more surface energy must be spent. A very thin coat of the internal surface is formed by waterproofing agents, which have a very low surface tension.
Glycerides, natural oils, have a surface tension of about 40mN/m and are not appropriate for waterproofing. However, this concept can be realised by waterproofing agents which show a surface tension of about 30mN/m, eg chrome stearates, and hydrophobic esters1 and so-called amphiphilic polymers2, which consist of hydrophilic and hydrophobic parts.
Silicone-based products allow even thinner coatings of the internal surface due to their low surface tension of 23mN/m. The use of chemically modified silicones materialised at the end of the last millennium. Modified silicones are linear polymers. Functional groups are attached to the backbone through a spacer group at the end of the molecule or randomly as a side chain.
Münzing modified silicones in their laboratory and noticed during their research that the efficiency of waterproofing agents depends on the molecular weight of the silicone, on the nature and number of functional groups per silicone molecule, on the position of the functional groups in the silicone molecule, but also on the spacer group between functional group and silicone. Although silicones act very efficiently, silicone-free products might be preferable for some articles.
Lastly, Fluorine chemicals could work even more efficiently, however, due to the extremely high costs, this approach is advisable only in cases when additional requirements are requested.
Openly waterproof treated leathers act like a membrane (Figure 2). Water vapour can penetrate into the fibre network; however, the ‘hydrophilic water’ droplets possess a high surface tension and cannot spread over the internal surface and, therefore, the fibres cannot be wetted (Figure 1).
Water cannot penetrate. Water vapour permeates always from the side with higher water vapour concentration to the reverse side with lower concentration, from the side with higher temperature to the side with lower temperature. Therefore, in hot and humid conditions, for example in a tropical rain forest, waterproof boots loose the ability to emit moisture.
The open waterproof effect can be visualised by an example from nature. Water striders can walk and jump over water. Their tarsus is covered with numerous fine hydrophobic hairs that cannot submerge allowing the water striders to stay on water. If we put soap into this water, the surface tension of the water would decrease and water striders would sink.
Similarly, waterproof leather cannot be wetted. However, surfactants cause the leather to be wetted quickly and should be strictly avoided. The lotus-effect, the self-cleaning of the bloom of the lotus in the rain which is commercially used in wall paints and in windscreen care, is based on the same physical fundamentals.
Actually, the presence of all kinds of hydrophilic substances within the leather might negatively influence the waterproof values of this leather by varying degrees. However, the use of dyes, synthetic and vegetable retanning agents is required to obtain the desired article with a pleasing appearance. Appropriate products and application processes ensure that the hydrophilicity, which is an inherent part of all kinds of leather treatment agents, will be masked in the final article.
Salts originating from the neutralisation are hydrophilic and, therefore, their presence will increase the water absorption of the final leather article. In addition, during the processing of waterproof leather, these salts might be harmful as they could cause the breaking of the emulsion of the waterproof agent and, hence, prevent even distribution through the cross-section. Consequently, waterproof leather should be washed well after neutralisation.
Likewise, retanning agents, which are huge and bulky molecules, might influence the waterproof values negatively. Therefore, vegetable tanning agents should be selected carefully. Sweetened vegetable tanning agents have to be strictly avoided because they are even more hydrophilic. Normally synthetic retanning agents cause fewer problems and can be applied in normal quantities. Polyacrylates are flexible molecules, which usually do not harm the waterproofness at all. The hydrophilic parts are supposed to bind to the internal surface, the hydrophobic parts are directed into the gaps of the fibre network.
Despite their hydrophilicity, certain polymers3 and nitrogen containing aromatic syntans4 support the waterproof effect for several reasons. Firstly, these chemicals are anionic and, therefore, the charge of the cationic wet-blue will be changed into a weakly charged or even anionic substrate.
In other words the isoelectric point of the leather will be reduced by the presence of syntans and polymers. This is a precondition for the penetration of the anionic waterproofing agents into the inner section of the leather. In addition, specially designed nitrogen containing functional syntans4 support the even distribution of the waterproofing agent through the cross-section due to their dispersing power without negatively affecting the tightness of the wet-blue.
The use of such products is highly recommended when heavy substance wet-blue is processed, when wet-blue which is not uniform over the hide, when wet-blue from different origins are processed together, or when wet-blue is processed which cannot be neutralised too strongly because the tightness would be negatively affected by strong neutralisation. Likewise, special polymers3 improve the waterproofness as they disperse the waterproofing agents and support the penetration through the cross-section. The use of such polymers in the beginning of the waterproofing step is almost always recommended.
Figure 3 shows the cross-section of three leathers. All leathers were dyed and waterproof treated according to process 1. The straight curves show the distribution of the silicone-based waterproof agent through the cross-section. The lower leather exhibits the best waterproof performance in terms of water resistance and water vapour permeability because the leather was retanned strongly and the waterproof agent is evenly distributed through the cross-section. The content of silicone is higher in the inner section because here the leather is less anionic than in the outer section and, therefore, there are more potential binding sites for the anionic waterproofing agents. However, this leather might be too soft for shoe upper leather and the grain may be on the loose side depending on the wet-blue origin.
The upper leather was weakly retanned. Therefore there is a strong cationic zone in the middle of the cross-section. The waterproofing agents were fixed where the penetrating anionic agents met the cationic zone. This leather is tight, however, it is not waterproof and dyed through the cross-section.
Leathers treated in this way fail the Wicking-test which is of a high importance when leather is foreseen to be a component in membrane lined boots. Water is absorbed through the cut of the test strip up to the arrow when this type of leather is tested. The left strip failed, the right one passed.
The leather in the middle was retanned in a balanced way and just a thin cationic zone remained. Here, both good waterproofing, water vapour permeability and a tight leather character can be achieved. Due to the higher mechanical action, the dye penetration will be complete in a tannery-scale production.
The dotted line displays the distribution of silicone-based waterproof agents in leathers that were processed by the modified process 2 which involves two additions of waterproof agent.
As expected, the achieved waterproofness was even better. It was observed that openly waterproof treated leathers are relaxed and lay flat. Here, the waterproof treatment doesn’t cause an area loss, which often takes place when waterproof leathers are processed.
Conclusion
The production of waterproof leathers showing excellent leather properties and wearing comfort requires both appropriate products and application know-how. Both products and application process must be adapted to the requirements for the final article and to the processed wet-blue. Here, the character of the provenance, the charge and the degree of olation has to be considered.
Acknowledgement
The author thanks his colleagues at Münzing Chemie, and mentions Stefan Huster and Wolfgang Eberhardt for their highly committed work and Andreas Waechter for providing the photograph of the water strider.
Products
1. Ombrellon WR
2. Ombrellon WD
3. METOLAT WP
4. Development product Ombrellon TAN
5. Development products Ombrellon 072, Ombrellon 073, Ombrellon 2770
Usage requirements
No water penetration
Controllable water up-take
High water vapour permeability
Heat and cold insulation
Lightweight
Wearing comfort
Second breathing skin
Testing requirements
Water droplet test (IUP/420, EN ISO 15700)
Kubelka water up-take (IUP/7, EN ISO 2417)
Soaking-up test (Wicking-test)
Bally penetrometer (IUP/10, EN IS O 5403)
Maeser (ASTM D 2099)
Water vapour permeability (EN ISO 14268)
Process 1
Wet-blue, 1.8-2.0mm, US origin
Neutralisation pH, 5.0, wash well
Treatment with acrylic polymer3
Retannage and dyeing4
Addition silicone based waterproofing agent5
Fixation/hot water
Process 2
Wet-blue, 1.8-2.0mm, US origin
Neutralisation pH, 5.0, wash well
Treatment with acrylic polymer3
First addition silicone based waterproofing agent5
Retannage and dyeing4
Second addition silicone based waterproofing agent5
Fixation/hot water/acid/chrome
