Car manufacturers and their suppliers impose particularly stringent demands on surface coating systems. The fact that the latest environmentally compatible, waterborne polyurethane coatings are able to comply with these strict specifications is illustrated by the example of the cardan shafts that are being built into some of the top models. They are coated with an aqueous one-component system containing Bayhydrol® B 130, a binder supplied by Bayer MaterialScience AG.
The Bayhydrol® based system provides the cardan shafts with outstanding corrosion protection. It can withstand 240 hours in a salt spray environment and six cycles of the alternating climate test (VDA 621-415). The coating also has excellent stone-chip resistance, due mainly to the fact that it adheres very well on the substrate - but then that is not really surprising as Bayhydrol® B 130 is also frequently used as a waterborne adhesion promoter. "It has to be said that this particular application is especially challenging for the coating system because of the production process: the components of the cardan shaft first undergo an alkaline degreasing operation, are then welded together and subsequently coated without any additional pretreatment," says Dr. Ulrich Freudenberg, an expert for polyurethane coatings at Bayer MaterialScience.
Almost as soon as the coating operation has finished, the cardan shafts are packed into transport frames, where they are secured with the aid of rubber-covered clamps. It is only because Bayhydrol® B 130 is such a fast-drying binder that the paint film can withstand this mechanical stress undamaged.
Until now, components for the automotive industry like cardan shafts, shock absorbers, axles etc. were usually coated with conventional solvent-based systems. The changeover to waterborne coatings makes it possible to comply with planned VOC guidelines, which aim to further reduce emissions of volatile organic compounds.
Information about polyurethane coatings, including detailed descriptions of applications, and contact details can be found on the Internet at www.bayercoatings.com.