Nowadays, mobility plays a very important role in every aspect of life in industrialised countries. People have never spent as much time in their cars as they do today, and they expect to feel just as at home in their cars as they do in their living room.

The interior design of motor vehicles plays a very large part in determining whether or not we feel comfortable in them. For example, one of the factors is the colour scheme of the interior.

In the past, upholstery leather was mainly supplied in the standard black and brown shades, but pale shades such as cream, beige and grey are becoming increasingly popular and they create a much more attractive environment.

The overall experience that we gain depends on a variety of different sensory impressions. The materials used in the interior of vehicles have to be pleasing to the eye and to touch.

They are not expected to make any squeaking and grinding noises or cause any unpleasant odours or emissions. The interiors of motor vehicles have to be easy to maintain and keep clean, and they are ideally expected to look as good as new after years of service.

Visual appearance of matt leather

The various components that make up the interior of motor vehicles are made from a wide variety of different materials.The roof lining, dashboard, seats, door panels and foot well have to be functional and also have to present a harmonious appearance. Dashboards must have a very matt surface to ensure that the driver’s view of the road is not obscured by reflections in the windscreen.

The steering wheel and the seating upholstery are required to match the dashboard and this is the reason why only leather with a very matt finish is used here as well.

Physically, the matt appearance of a surface originates from its high roughness, which causes the incident light to be reflected diffusely in all directions. Smooth surfaces with a heavy gloss reflect all the incident light in one direction. This is shown schematically in Figure 1.

Aqueous leather finishes are usually matted by mixing a binder with an inorganic or organic matting agent.

The binders typically consist of aqueous dispersions of acrylic polymers and polyurethanes. When the finish is dried, it forms a polymer film, which physically holds the matting agent in place on the surface and leads to a surface roughness in the micron range.

Therefore, the finished leather appears matt, as can be seen from the scanning electron micrograph in Figure 2.

When the leather is used, in practice the matt particles on the surface are slowly worn off or rubbed into the binder layer as the result of abrasion.

This occurs because they are only held in place physically and do not form any chemical bonds with the adjacent polymer molecules.

The roughness of the surface gradually decreases as the result of mechanical action. The surface, therefore, becomes increasingly smoother and glossier and takes on a worn appearance.

This can usually be seen most clearly on the areas of the interior that are subjected to the most wear, such as the driver’s seat or the steering wheel.

Little by little, they become shiny and take on a very different appearance to components such as the dashboard that are subjected to less wear.

The differences stand out and spoil the overall visual impression. The effects of wear on matt finishes can be tested very effectively by subjecting a piece of finished leather to intense mechanical action, such as in the VESLIC dry rubfastness test.

The degree of matting and the surface roughness before and after testing are then compared (see Figure 3).

Tactile properties of leather

Comfortable seats

One of the most important factors for the occupants of motor vehicles is that the seats are comfortable. Leather seats have to feel soft, warm and supple.

The handle of leather is largely determined by the type of finish that is applied to it.

Handle improvers such as waxes and silicone compounds can be added to the finish, but the handle depends, to a very large extent, on the selection of the binder polymers and the surface structure on the finish.

A wide variety of subjective factors determine whether or not a particular type of leather is perceived as being pleasant and attractive to the touch, but a variety of empirical factors can also be used to characterise and optimise the handle.

Examples include the surface roughness, the hardness of the leather and the finish, the heat conductivity, and the co-efficients of static friction and kinetic friction.

Noise reduction

Mechanical noises such as squeaking door hinges are not the only source of irritation for motorists.

The driver’s comfort also depends on the level of noise from the road and from the vehicle itself while it is in motion.

In recent years, car manufacturers have greatly improved their sound-proofing techniques, and advances in engineering have reduced engine and transmission noise to a minimum.

However, the downside of this is that the squeaks and other noises from the interior trim and fittings of vehicles are much more audible and irritating nowadays.

These unpleasant noises are caused whenever two surfaces, such as the backrest and seat cushions of car seats, rub against each other during the journey. Surfaces rubbing against each other constantly oscillate between a state of static friction and sliding friction, and this generates the vibrations that cause squeaking noises. This phenomenon can be measured with a stick-slip tester (see Figure 4). A wide variety of components can be tested, including leather seats.

The noise generated by surfaces rubbing together depends on many different factors such as the relative humidity, the pressure applied to the specimen and the speed with which it is rubbed.

Normally, the probability of noise being generated by leather is greater at higher pressures and lower speeds.

However, the structure and roughness of the surface and the chemical composition of the finish itself have a decisive influence on the tendency of leather to generate noise. A significant reduction in the level of unpleasant noise can be achieved by careful selection of the ingredients used in the finish.

Durability and resistance to soiling

One of the problems associated with the use of pale leather for the interior trim of motor vehicles is that the leather is susceptible to soiling. For instance, it can become dark and discoloured with age, and dark seat belts can cause unpleasant stains on the seats.

Soil can originate from a variety of different sources, and so its chemical composition can vary widely (eg fats, oil, carbohydrates, proteins, dyes, pigments, soot, salts).

Very stubborn stains can be caused, depending on the origin and composition of the soil and the structure and the chemical composition of the leather surface.

One of the greatest challenges facing leather finishers is to develop soil-repellent finishes that are resistant to all types of soil.

The Martindale Test according to ISO 12947-1, which was originally developed for testing the abrasion resistance of textiles, can be used to assess the soiling resistance of leather (see Figure 5).

What can BASF offer?

Great challenges are posed by the task of fulfiling current demands for leather with very high fastness that does not become shiny as the result of wear and that is pleasant to the touch, generates little noise and has high resistance to soiling.

In partnership with the leather industry, BASF have succeeded in developing a new finishing system based on Astacin Novomatt and Lepton Protector that enables all these expectations to be fulfiled.

Finishes that are formulated with Astacin Novomatt retain their matt appearance for much longer, because the matting is based on an innovative technology.

The matting particles that are used to obtain the required surface roughness are chemically bonded to the surrounding polymer matrix.

This ensures that they are not rubbed off or rubbed into the binder film. The surface of the leather stays rough, with the result that it retains its original matt appearance even if it is subjected to mechanical action (see Figure 6).

Finishes formulated with a combination of Astacin Novomatt and Lepton Protector types can be used to finish automotive leather with a warm, pleasant, slightly waxy handle and a characteristic soft touch.

Irritating noises in the interior of the vehicle can be minimised using finishes based on Astacin Novomatt and Lepton Protector.

This can be seen from the results of the stick-slip test: leathers with this innovative finish perform significantly better than leathers with a conventional finish (see Figure 7).

The soiling resistance of finishes formulated with Astacin Novomatt in combination with Lepton Protector types shows significant advantages compared with industry standard leathers as can be seen from the results of the Martindale Test in Figure 8.

More environmentally-friendly?

Nowadays, the finishes applied to leather have to fulfil the highest environmental standards. One of the most important issues is the level of volatile organic compounds (VOC), which make a significant contribution to the total emissions in the interior of motor vehicles.

Here, the main factor to be considered in the finishing of automotive leather is the organic solvent content of the products used to formulate finishes.

N-methylpyrrolidone (NMP) has, in particular, given rise to controversial discussion recently. NMP is a widely-used solvent that is employed in the synthesis of aqueous polyurethane dispersions.

It is distinguished by its excellent performance as a solvent, film-forming agent and levelling agent and it has a very high boiling point. Its low volatility has the effect that a significant proportion of the solvent is retained in the leather when it is dried, and it is then gradually emitted from the leather over long periods.

NMP is suspected of being teratogenic, and it is included in the California Proposition 65 list of carcinogenic, mutagenic and teratogenic substances.

Automotive leather marketed in California which contains NMP has to carry a warning label indicating that it contains substances on this list.

European Union legislation is being prepared that will require products which contain NMP to be labelled with T, R61 (may cause harm to an unborn child).

Although the teratogenicity of NMP has still not been conclusively proved, the leather department of BASF has, nevertheless, decided to market a completely NMP-free range of finishing products worldwide.

This conforms to the principles of ‘Responsible Care’ and sustainability in the field of environmental protection, and BASF are committed to a policy of continuous improvement.

Astacin Novomatt and the Lepton Protector types have been developed in line with this policy, and they are completely free of N-methylpyrrolidone and other organic solvents.

BASF provide tailor-made solutions to specific problems in the manufacture of automotive leather: a comprehensive range of products for the entire process from the beamhouse through to finishing, in line with ecological and economic benefits to leather manufacturers, automotive manufacturers, consumers and the environment.

The innovative products and processes supplied by BASF are the result of continuous research and development. They:

* create added-value for leather manufacturers;

* improve the quality of the leather;

* satisfy the demands of automotive leather manufacturers for high fastness properties;

* are environmentally-friendly; and

* fulfil the demands of consumers for comfort and serviceability.

BASF have a network of modern technical service centres worldwide and BASF’s experts are available locally to provide an individual service to customers.

The authors would like to thank Dr Karl Häberle, Reinhard Treiber, Lars Heckhoff and Dr Thomas Frechen for their valuable contributions to this project.