BASF launched the first aqueous acrylic binders for base coats on the market in the 1930s. Lacquer emulsions for top coats first appeared on the market at the beginning of the 1950s. These products typically consisted of solutions of cellulose nitrate or cellulose acetobutyrate in organic solvents, dispersed in water.

Solvent-based polyurethane lacquers were introduced in the mid-1970s, and they have gradually been displaced by aqueous polyurethane dispersions over the past 10-15 years.

The success of polyurethane is easy to appreciate if the far superior performance that can be achieved with polyurethane-based finishes is taken into account. Also, various additives can be used very effectively to obtain the required aesthetic impression and visual appearance. Modern aqueous polyurethane binders are capable of fulfilling current ecological demands.

The chemistry of polyurethanes

Chemically speaking, the polyurethane molecule is similar to the collagen fibre of leather because they both incorporate a large number of functional groups that contain oxygen and nitrogen. This enables polyurethane films to adhere very tightly to the collagen fibres.

Further characteristics of polyurethane films can be explained by the segmented structure of the polyurethane molecule. It consists of a mixture of hard components (isocyanates, dimethylolpropionic acid, short-chain diols), and soft components (long-chain diols, usually based on polyethers or polyesters) (see Figure 1).

The hardness of the film is determined by the ratio of hard to soft components. The great advantage of polyurethanes is that this ratio can be manipulated in order to obtain a molecule that is perfectly tailored to the application of the binder, ie in adhesion coats, base coats or top coats.

Due to their segmented structure, polyurethane films are generally highly elastic, and even very soft films have a high tensile strength. This ensures that the leather has a soft, pleasant handle and that the finish itself is highly flexible.

Another remarkable feature of these polyurethane binders is that they remain very flexible at very low temperatures, even in the case of versions which form a relatively hard film.

Aqueous polyurethane dispersions

Water-dispersible polyurethanes can be produced by building ionic groups such as dimethylolpropionic acid (DMPA) into the polymer chain. The arrival of aqueous polyurethane dispersions was an important milestone in the development of environmentally friendly finishing, because this allowed a large proportion of the solvents to be eliminated.

There are two basic methods of synthesising polyurethane dispersions. In the acetone process (see Figure 2), the polyurethane is manufactured by polymerising the diisocyanate and diol components in acetone as a solvent. The dissolved polymer is then dispersed in water and the acetone is distilled off. With this process, it is possible to manufacture polyurethane dispersions that are completely free of solvents.

Another method that can be used is the prepolymer mixing process (see Figure 3). The diisocyanates are first of all caused to react with the corresponding polyether diols or polyester diols and DMPA. The next step is to disperse the prepolymers in water and to extend the chain length. Solvents have to be used here in order to reduce the viscosity of the reaction mixture during the process. N-methylpyrrolidone is normally added, and it remains at a rate of 5-10% in the finished dispersion.

However, BASF’s research staff have succeeded in modifying the prepolymer mixing process to make it possible to produce polyurethane binders that are completely free of solvents. These binders are Astacin Finish PUMN TF and Astacin Finish SUSI TF.

Film formation

Finishes consist of several different ingredients such as pigment preparations, various fillers, waxes and matting agents, along with polyurethane, polyacrylate and polybutadiene binders as the film-forming components. It is important that the individual ingredients in the finish are fully compatible with each other in order to ensure that the finish remains stable over the life of the leather and to allow the finish to form an optimum film when it is dried. The non-film-forming components are embedded in the film-forming matrix of the binder mixture.

The most important factor here is that the complete finish formulation is able to form a film that is as homogeneous as possible. The film-forming behaviour of the individual ingredients is of secondary importance.

It is usually very easy to obtain an homogeneous finish if the polymer is dissolved in a solvent, as is the case with solvent-based polyurethane lacquers. This is because the dissolved polymer molecules and the non-film-forming components are already homogenised in the form in which they are applied. When the finish is sprayed onto the leather, the solvents evaporate and an homogeneous film is left behind.

The mechanism by which aqueous systems form films is rather more complicated (see Figure 4). In aqueous polyurethane dispersions, the polymer is dispersed in the form of spherical droplets with diameters in the 10-100nm range. In acrylic dispersions, the polymer has a particle size of between 50nm and 200nm.

Finishes consist of an aqueous dispersion of spherical polymer particles and non-film-forming components. The water is evaporated off during the drying process, which causes the dispersed polymer particles to move closer together until the point is reached at which they touch each other. As the drying process proceeds, the soft binder particles deform to fill the spaces between them and the non-film-forming components. For an homogeneous film to be obtained, polymer molecules from different particles have to be able to permeate each other in such a way that the interfaces between the particles disappear.

The film-forming behaviour of different binders can be illustrated by examining thin films under an electron microscope. Figure 5 shows a film formed by an acrylic dispersion. Here, it can be seen quite clearly that the interfaces are still intact.

The polyurethane dispersion shown in Figure 6 forms an homogeneous film in which no interfaces between particles are visible. The film-formation is much more homogeneous and, therefore, better than in the case of the example (polyacrylic dispersion) shown in Figure 5.

The electron micrograph shown in Figure 7 again confirms that the interfaces between individual particles can be broken down in films formed by polyurethane dispersions.

In this example, polyurethane crystallites have formed which are very much larger than the discrete polymer particles of the dispersion. These crystallites can only form if the original interfaces between the former existing discrete particles disappear completely.


One method of enhancing the mechanical stability of finishes is to crosslink them with reactive substances such as oligomers of aziridine or isocyanate. The reactive groups contained in crosslinking agents react with the polyurethane molecules and, ideally, form a three-dimensional network with high tensile strength. Isocyanate crosslinking agents are supplied by BASF in the form of a 70% preparation which contains solvents.

Usually, only a relatively small amount of crosslinking agent needs to be added to modern polyurethane binders in order to obtain the level of fastness demanded by the type of leather in question. This ensures that the total level of solvents in the finish is low. Finishing techniques can be modified in order to eliminate this source of solvents completely or, at least, to reduce the amount to vanishing levels. For instance, in the US, it is quite common to use spray guns with dual inlets supplying the finish and the 90-100% crosslinking agent respectively. The finish and the crosslinking agent are mixed in the head of the spray gun. This eliminates all problems with the pot life of mixtures of binders and crosslinking agents, as well as solvent emissions.

Ecological leather finishes

The discussion concerning the ecological aspects of leather finishing is mainly concentrated on the following issues:

* The harm to the environment caused by the emission of volatile organic compounds (VOC)

* The energy consumed in drying leather finishes

* The toxic heavy metal contents – chromium (VI), lead, cobalt, nickel – of pigmented finishes from the point of view of recycling and disposing of leathergoods

Recently, the discussion concerning pollutants has been extended to cover alkyl tin compounds in polyurethanes. It is for this reason that BASF have developed a range of polyurethane binders that contain no traces of alkyl tin compounds at all. These products carry the additional suffix ‘TF’ after the product name (see Table 1).

The range of products marketed by BASF encompasses a large number of pigment preparations that are free of heavy metals and solvent-free binders. They enable high-performance leather finishes to be formulated without causing any unnecessary harm to the environment. Nowadays, all of the binders supplied by BASF for use in base coats and top coats are solvent-free or have a low solvent content. However, some small amounts of volatile organic compounds still have to be added to some finishes as levelling agents, film-forming agents or wetting agents because of the decisive contribution they make to the performance of the system.

The latest generation of polyurethane binders from BASF dispenses with solvents completely. Examples of these new products are Astacin Finish PUMN TF and Astacin Finish SUSI TF, which have recently been included in the global BASF product range. Astacin Finish PUMN TF (see Figure 8) is a completely solvent-free binder that is designed for use in base coats; it consists of a finely divided, aqueous dispersion with a solids content of ca. 37%. It is used in soft finishes with high coverage which respond very well to embossing, have a low tack and fulfil the highest standards of fastness.

Astacin Finish SUSI TF (see Figure 9) gives very elegant finishes and the leather is distinguished by its attractive break after milling. It is mainly used in base coats applied to semi-aniline leather and milled leathers. It is supplied in the form of a finely divided 30% dispersion and is completely free of solvents.

Astacin Finish PF TF (Figure 10) has a residual NMP content of 4%, which is unavoidable, but its performance as a binder in base coats is outstanding. It gives finishes with high lightfastness, high heat resistance and high resistance to hydrolysis. Its high Shore hardness also makes it a very useful ingredient for polyurethane top coats if the highest standards of fastness have to be met.

Although it is still not possible to eliminate all traces of solvents from even the most modern finishes, it is possible to reduce solvent contents to the absolute minimum without impairing the quality of the leather. The development of ecologically optimised finishing systems at BASF is proceeding at a rapid pace, and this represents a valuable contribution to protecting consumers and the environment. Our aim is to help our customers to fulfil the requirements of environmental regulations and to assist them in bringing environmentally friendly leathers onto the market.

The authors are grateful to Dr Ulrike Licht and Dr Walter Heckmann for providing the electron micrographs