NMP-free polyurethane products for the leather industry: a new challenge for finishing

Polyurethanes have played a key role in numerous fields of application for many years now. With their high quality and excellent fastness properties, they are the ideal product whenever lacquers or coatings are required. And their property profile can be adjusted to suit the particular application by selecting the appropriate components. The outstanding film-forming properties of polyurethane dispersions are particularly valuable in the leather finishing process, giving homogeneous, long-lasting coatings with good aging resistance. Leathers coated with polyurethane dispersions are easy-care, have good adhesion and flex resistance and a high level of wear comfort. Even better physical fastness properties are obtained if the finish is crosslinked. Polyurethanes can be effectively combined with other materials such as acrylics. Polyurethane dispersions generally have a particle size of between 50 and 400nm, which is considerably larger than that of acrylic dispersions, for example. As I said before, polyurethanes can be formulated to order. For example, soft or hard segments can be incorporated into the polymer chains, depending on the application, making the film hardness and the adhesion properties more or less infinitely adjustable. Polyurethane coatings have good flex resistance, even at low temperatures, and generally also have a relatively high melting point. With all these advantages, the range of applications is almost unlimited, from extremely thin finishes which preserve the grain pattern to thick coats and from dull finishes to a high gloss. In 2001, the use of numerous polyurethane dispersions containing the co-solvent N-methylpyrrolidone (NMP) was suddenly questioned. The reason for this was that on June 15, 2001, NMP was added to 'California Proposition 65'. This is a list of chemicals that are known to the US state of California to be harmful. All products containing these substances must carry a warning to this effect when sold in California. Since to this day there are no safe-harbor values, even products which contain only minimal quantities of NMP must be labeled. Although in the first instance this compulsory labeling only affected a very limited geographical area, it nevertheless had far-reaching consequences for PU formulations containing NMP in the leather industry of the United States and worldwide. After all, who can predict which leather is going to be made into a bag, car seat cover or shoe which will end up being sold in California? Throughout the industry, therefore, there was an immediate call for NMP-free dispersions which would allow manufacturers to produce NMP-free leather. Bayer was quick to take up the challenge and now offers a range of new NMP-free dispersions in addition to the numerous products which were always NMP-free. Why is NMP used in PU dispersions in the first place? The viscosity of the polymers must be such that they remain easy to handle throughout the production process, and that calls for suitable diluents. As the graph shows, this is particularly difficult in the case of crosslinked polymers, since their viscosity rises sharply as the molecular weight increases. Here, NMP is often used successfully to dilute the polymers. The NMP remains in the final dispersion and improves the flow and film formation of the polyurethane during the finishing process. Not all PU dispersions contain NMP. It depends on the particular manufacturing process used. First of all, let us take a quick look at polyurethane synthesis in general: Polyurethanes are usually produced by a two-stage polyaddition reaction process. In the first stage, diisocyanates and polyols react to form prepolymers, which still contain reactive terminal groups and are reacted in the second stage to produce the final product. There are various possible ways of carrying out this second stage. The first possibility is the melt process, which involves reacting low-molecular-weight prepolymers in the presence of chain terminators such as urea to produce similarly low-molecular-weight polyurethanes. As a rule, the viscosity is not crucial here and so no NMP is required. It is a different story with the inverse process, where high-molecular-weight prepolymers are formed. In this case, NMP has to be added to reduce the viscosity and is then still present in the high-molecular-weight final product. In the solvent process, which is widely used at Bayer, a solvent is added to limit the viscosity during the formation of high-molecular-weight polyurethanes. This solvent - generally a low-boiling solvent - can be easily distilled out of the final product, so that the final dispersion is solvent-free. In the second part of my presentation I would like to focus on certain product groups based on polyurethane dispersions which are important to the leather industry. The table shows a list of the aqueous dullers marketed by Bayer and Rohm & Haas in 2001. It contains only one NMP-free product, and even that was not universally applicable. So the companies had to respond quickly when NMP was included in California Proposition 65. The need for action becomes even more obvious when we look at a typical top coat formulation. Here, the matting agent is the only component which contains NMP. Although the entire formulation contains only around 1.4% by weight NMP, and the quantity remaining in the corresponding leather is only about 0.06%, the leather would have to be labeled accordingly in California. To make matters worse, it is more or less impossible to remove NMP from a finish at the temperatures which are permissible. Of the 627mg NMP originally applied per kilogram leather, around 558mg, i.e. about 89%, is still contained in the finished product. The NMP is then released very gradually from the finish into the ambient air. A model trial carried out under standardised conditions showed that this leads to a virtually constant concentration of about 100g NMP per cubic meter ambient air over days and weeks. It was therefore clear what we had to do. We had to develop a new, universally applicable duller that was NMP-free. The result of our work is AQUADERM Matting Agent HPM - HPM standing for High Performance Matting Agent. This new product meant working out completely new formulations or optimising the relevant formulations in order to obtain totally predictable and reproducible results, in other words: to guarantee reliability in application. AQUADERM Matting Agent HPM represents a genuine alternative to formulations containing NMP. And we intend to replace other dullers by NMP-free products and add more NMP-free brands to our range in the future. The situation with regard to polyurethane binders for base coats is less dramatic. Only two of the 14 BAYDERM products contain NMP, and in most cases these can, if necessary, be replaced by other, NMP-free products. The situation with regard to binders for top coats is more problematic. The Bayer/Rohm & Haas range contains six NMP-free binders, but also eight products which still contain NMP. We shall therefore concentrate our efforts on this sector so that we can offer more NMP-free binders in the near future. There are two things we can deduce from the case of NMP-free polyurethane dispersions: - Firstly, it is clear that even established products have to move with the times and adapt to meet constantly changing requirements. - Secondly, we can see that an efficient, market-oriented research and development team can respond to such challenges in a comparatively short time. By the time Tanning Tech comes round again, we will no doubt have made some more progress with regard to NMP-free products. And perhaps by then there will also have been some changes to California Proposition 65 such as the introduction of safe-harbor values. Biography of Dr Wolfgang Wenzel, head of R&D for the Leather Business Unit of Bayer Chemicals, a Division of Bayer AG Dr Wolfgang Wenzel has been head of R&D for the Leather Business Unit of Bayer Chemicals, a Division of Bayer AG, since February 2002. Wenzel was born in Bad Nauheim, Germany, on March 16, 1946. After graduating with a doctorate in chemistry from the University of Cologne, he joined the Polyurethanes Research Department at Bayer's Dormagen site in 1973. From 1982 to 1985 he worked for Bayer's US subsidiary Mobay, now Bayer Corporation, in New Martinsville, where he was responsible for developing products and processes in the polyurethanes dispersions sector. On returning to headquarters in Leverkusen, he was transferred to the Applications Development Department in what was then the Dyestuffs Business Group, a department he headed until the beginning of 1996. He was then placed in charge of global product management in the Leather Business Unit. He has been head of research for the Leather Business Unit of Bayer Chemicals since the beginning of 2002. Wenzel is married and has two grown-up children.