Manufacture and care of water vapour permeable leather18 September 2001
This paper is divided into several sections and to start with it is important to understand basic concepts and general observations when dealing with water vapour permeability. It is also useful to explain the difference between the water storage capacity, the water vapour absorption and the water vapour permeability. Basic concepts * Water storage capacity: Leather contains water in liquid form. It is only when the water content is more than 25% that the leather stops being dry to the touch and feels wet. Wet leather is difficult to dry * Water vapour absorption: The percentage of water in the leather as a function of the relative ambient atmospheric humidity. Leather can absorb, store and transport water vapour very efficiently (water vapour permeability) * Water vapour permeability: Ability to transport gaseous H2O, in other words, the 'breathing properties' of the leather The first concept, the water storage capacity, describes the capability of leather to absorb water in liquid form. Leather is capable of absorbing up to 25% of its dry weight in liquid water without it feeling damp. If it does become wet, however, it is difficult to dry again. The water vapour absorption refers to the binding of gaseous water in the leather. This naturally depends on the ambient atmospheric humidity, which may be very high - for example in footwear. Water vapour can also be transported through the leather. This diffusion process is known as the water vapour permeability or WVP for short. The high water storage capacity of leather and its ability to absorb up to 25% water vapour, calculated on the dry weight, without it feeling wet are what make it ideal for shoes. No other material is as comfortable to wear as leather. It is also capable of absorbing far more water but then it feels wet and the leather loses its good thermal insulation properties. In most cases we want to take action to stop the leather from being wet through. The answer is to make the leather itself water repellent or hydrophobic. Unfortunately, treatment to enhance water repellence nearly always has an adverse effect on the water storage capacity. Indeed, it may even reduce it to zero, as the polar positions are then occupied. In such cases, it is necessary to compensate for the poorer water storage capacity by ensuring good water vapour permeability. Only then can the wear comfort be guaranteed. However, care has to be taken here, as the ability of the leather to transport water vapour may be significantly impaired if the waterproofing is not carried out properly. To explain why this is we have to look at the following. There are two types of waterproofing - open and closed In closed waterproofing, the fibre interstices are filled with a water-repellent substance, such as a modified fat or oil. The disadvantage of closed waterproofing is that it drastically reduces not only the water storage capacity and the water vapour absorption but also the water vapour permeability of the leather. You simply create a massive wall impermeable to liquids and gases. In open waterproofing, the individual collagen fibre bundles are impregnated, so to speak, creating a porous, hydrophobic net. The leather thus loses its ability to absorb and store liquid water. However, it is still capable of absorbing and transporting water vapour as well as other gases. The need for optimum quality of the leather is much higher for open waterproofing, since every hole in the net will let the liquid water pass as well as the vapour, and cannot be blocked as with the closed waterproofing system. So much for the absorption of water and water vapour. Next it is important to understand the definition of the water vapour permeability of leather. The best index to define the capacity of the leather to 'keep your feet dry' is probably the Water Vapour Permeability Rate (WVPR) which includes Water Vapour Permeability (WVP) and Water Vapour Absorption (WVA) according to the formula: WVPR = 8 x WVP + WVA. Definition and measurement of the water vapour permeability of leather DIN Standards: 1) WVP (water vapour permeability) in mg/cm² x h, measured according to DIN 53 333 (DIN EN 12991, draft) 2) WVA (water vapour absorption) in mg/cm², measured according to DIN 4843 (EN 344) 3) WVPR (water vapour permeability rate) in mg/cm² (DIN 4843) Definition WVPR = 8 x WVP + WVA The WVPR is the better parameter for waterproofed leather as this type of leather sometimes has a very low WVA value. * Customer demands re WVP vary and depend on the type of leather * Minimum value required for a leather to be described as water vapour permeable: WVPR 20 at WVP 0.8 * The WVP of the finished article should be as high as possible; the WVA can then be very low. The water vapour permeability rate is very important for characterising waterproofed leathers, because these leathers have very low water vapour absorption - it may be virtually zero - and so the water vapour permeability obviously has to be high in order to obtain an adequate water vapour permeability rate. Consequently, the demands on the water vapour permeability vary considerably. Leather can generally be considered having good water vapour permeability if it has a water vapour permeability rate of at least 20 coupled with a water vapour permeability of more than 0.8. Using the equation mentioned earlier, it is then easy to calculate how high the water vapour absorption has to be (at least 13.6 mg/cm²) in this case. However, it is generally recommended to aim for as high a water vapour permeability as possible. The water vapour permeability can vary considerably, depending on the particular type of leather concerned. As table 1 illustrates, an unfinished crust can easily be made to fulfil the demand for a value of 20 in the WVPR, but finished grain leathers or transfer coated leathers have to show optimum values to have a chance to pass this test. And this means that we not only have to employ an optimum finish but the crust leather, defined by tanning, retanning and fatliquoring, also has to play its role. It has been shown that for comfort and performance, an open waterproofing is superior to a closed one, and that the hydrophobic net which is formed has to be as regular and free of holes as possible. But there is one more important factor determining the wear and comfort of leather products - the density of the leather. Lightly finished chrome leather with a density of approximately 0.6 g/cm³ has a water vapour permeability of around 12 mg/cm²/h. This decreases drastically as the density increases until, at a density of 1.2 g/cm³ - the density of rolled sole leather, for example - the permeability is more or less zero. Thermal conductivity, the parameter to measure the insulation properties, decreases in the opposite direction. Open leathers have very low thermal conductivity - only about twice that of air - but in the case of rolled sole leather the value is approximately five times higher. In other words: the denser a leather is, the poorer the WVP and the thermal insulation properties. This is why Bayer have spent considerable time and energy in developing the 'Feel Fine' system for the production of leathers with high WVP. It consists of an optimised process for retanning, fatliquoring and finishing of leathers for different applications which have been finely tuned to result in the best WVPR values available. Water vapour permeability of finishing systems Different types of finish have vastly differing WVP values. There is still considerable room for improvement with regard to finishing systems with good water vapour permeability. The WVP depends on numerous parameters throughout the manufacturing process and cannot be attributed to any one particular operation. 'Feel Fine' leather processing For the wet-end process a combination of different 'classical' fatliquors is used, which varies according to the type of leather that is required. Since the overall effect is still not perfect, Bayer have been working to develop special products of their own to improve the process and the properties of the leather. The result of this research will be put forward in the near future. The situation is basically the same with coatings, only in most cases it is even more problematic since, up until now, the emphasis here has always been on parameters such as the fastness properties, as considerably more product is applied than in a finish. However, normally this is inconsistent with good water vapour permeability. Some leathers, eg leather for safety shoes and boots require a robust finish in order to meet the extremely high demands. This can be achieved with coating systems. Recently, there has been an increasing call for good water vapour permeability as well. Bayer is therefore trying to incorporate this in the further development of their Levacast coating technology. The Levacast system is a special transfer process in which the pu coating is first applied on a mould or release paper and then transferred to the leather in a second operation. The coating is produced in situ on the release carrier by reacting an NCO prepolymer with an amine hardener. The advantage of this system is that it is possible to use splits as the basic material, which have high water vapour permeability (WVP 10 mg/m² x h at a normal shaving substance). Also, because the leather is not compressed during the coating process, the physical density is not increased. So far, Bayer have introduced two generations of Levacast products onto the market, and are currently in the process of introducing the third. The first generation was concentrated exclusively on high physical fastness properties. However, as new prepolymers were developed, Bayer began to look for ways of increasing the water vapour permeability of the resultant coating as well. Bayer have also been able to reduce the solvent content of the coating system considerably. Thus, as we have advanced from the original aromatic polyester/NCO prepolymer (WVP 0.3) through an aromatic polyether/NCO prepolymer (WVP 1.6) to the latest system, an aliphatic polyether/NCO prepolymer (WVP 2.3), Bayer have been able to improve the performance continuously and at the same time to reduce the amount of solvent required drastically. With the new aliphatic system it is now possible to produce finishes which have the least possible influence on the water vapour permeability. All in all, this development may represent a significant innovation, not only from the technical point of view, but also from the point of view of responsible care. It is also important to mention the care of leather products with good water vapour permeability. There is no point in producing top-quality leather with good breathability if it loses these special properties the first time it is cleaned. This is a particularly important aspect given that water vapour permeable leather in general has no finish, or only a very light finish, and therefore needs a lot of care. Care of water vapour permeable leather Repeated application of care products must not: * Impair the water vapour permeability * Alter the appearance and handle The leather should: * Remain water, oil and dirt-repellent * Retain its gloss and polishing properties * Be re-fatliquored to a certain extent without a closed water-repellent effect being produced Suitable leather care products must: * Not form a film * Not form a crust * Produce an open water-proofing effect * Be binder free * Contain small amounts of conditioning fats, oils and waxes This is the basis of Bayer's 'Keep Fine' collection. Care products for water vapour permeable leathers have to meet very high standards. Most importantly, they must not impair the water vapour permeability or alter the appearance and handle of the leather even on repeated application. They should, however, make the leather water, oil and dirt-repellent. They are also often required to preserve the gloss, or to make the leather shine again on polishing. Care products should also replace some of the fats removed during cleaning, although there is a risk of this resulting in a closed waterproofing effect if unsuitable fatliquoring components are used. It is a question of reconciling the expectations with the technical possibilities, and very few care products satisfy all the demands made on water vapour permeable leathers. Their composition must be carefully tailored to the particular field of application. The following criteria applies: * Care products for water vapour permeable leather should be free of film-forming components (polymer lattices) * The waterproofing component should not alter the aspect of the leather and should produce an open waterproofing effect * The products should not contain too many re-fatliquoring components; the components themselves should be liquid, or at least have a very low melting point Bayer have been approached on several occasions by customers seeking advice on this complex subject. This led Bayer to develop the 'Keep Fine' collection. 'Keep Fine' is a specific line of care products created to satisfy the many different demands in terms of waterproofing and transpirability of the leather. The paper was presented by Dr Martin Kleban, Bayer AG speciality products business group - R&D, at the First Unione Nazionale Industria Conceria (UNIC) International Workshop held at the Bologna Fiere following the last edition of Lineapelle. Presentations at the International Workshop were based on 'Chemicals and technologies for tanneries incorporating innovations and research topics'.