A possible way to reduce the solvent content of the coating formulation is the use of waterbased polyurethane systems. These systems have been developed to a point where now they fullfill the requirements of the Deco-Paint-Directive 2004/42/EC with properties comparable to those of solventborne systems, in some cases they even exceed them.
Two-component polyurethane primer fillers in general
Unlike in OEM coating, for an automotive refinishing coating the substrate has to be sanded down to bare steel at first, any damaged parts being leveled out, polished and evened out with a filler. Film properties such as rapid drying and good corrosion inhibition play a particularly important role here. These requirements are broadly met by waterborne two-component polyurethane primer fillers like those based on Bayhydrol® A 2546 or A XP 2695.
Waterborne two-component polyurethane clearcoats in general
When using waterborne two-component polyurethane clearcoats for automotive refinishing coating, the coating result, e.g. the bubble-free dry film thickness, depends to a large degree on the mixing quality, the temperature and the ambient moisture. With a combination of suitable binders and hardeners it is possible to formulate waterborne two-component hydro-clearcoats that fullfill the high quality standards required in this application. OH-functional polyacrylate dispersions (for fast drying, hardness and resistance) and OH-functional polyurethane dispersions (for good film optics and ease of application), combined with isocyanate crosslinkers meet these requirements. For automotive refinishing clearcoat too, an excellent property profile can be obtained by combining highly reactive, internally activated polyacrylate dispersions like Bayhydrol® A 2601 and/or OH-functional polyurethane dispersions like Bayhydrol® U XP 2750 with a hydrophilized polyisocyanate hardener of the latest generation (e.g. Bayhydur® XP 2655).
Waterborne base coats in general
The base coat provides the color and effect in the final coating build-up. A major criteria for the binder in a base coats is a good pigment wetting, fixation of the bronze and obviously a good color retention over a long period of time. Usually polyacrylates and polyurethane binders are employed in this application. The polyurethanes provide the color brilliance, adhesion, leveling and elasticity of the system. Their pronounced elasticity being responsible for a superior stone chip resistance. Some lightfast polyurethane dispersions particularly suitable for automotive refinish base coats are Bayhydrol® UH 2621 and UH 2952/1. Also, a moderate addition of a hydrophilized poly-isocyanate crosslinker of the Bayhydur® range substantially improves the adhesion to the underlying coating layer.
Products and formulations
Waterborne polyurethane dispersions and hardeners for low temperature systems
OEM – 2K-metallic base coat for low temperature applications (Bayhydrol® UH 2621 / Bayhydur® 3100 / Desmodur® 3900)
UV-curing coating systems
Beyond the options described above efforts are made to reduce solvents and to accelerate the crosslinking process. UV-curing polyurethane systems offer promising alternatives, when fast drying processes, i.e. short down times are important considerations. When more complex geometries need to be coating, "Dual-Cure Systems" can be a solution. They combine UV-curing with conventional crosslinking reactions. Read more on this in the "UV/EB-Technologies" section.
