Weathering resistance of waterbased UV-cured polyurethane-acrylate coatings

The light stability of waterbased UV-cured polyurethane-acrylate (PUA) coatings has been tested in an accelerated QUV-A weatherometer. Infrared spectroscopy was used to monitor both the ultrafast polymerization of the acrylate double bond upon intense illumination, and the chemical changes occurring upon photoageing of 30 mum thick clearcoats. The UV-curing reaction was hardly affected by the addition of the HALS radical scavengers and UV absorbers needed to improve the light stability of waterbased UV-cured PUA coatings. The alpha-hydroxyphenyl ketone used as photoinitiator was shown to disappear rapidly upon UV-curing and early QUV exposure, thus having no detrimental effect on the weathering resistance. The urethane linkage (C-NH) is the most sensitive to photodegradation. Such waterbased PUA coatings were found to be as resistant to weathering as typical UV-cured PUA coatings. In the presence of light stabilizers, they undergo only minor chemical changes after as much as 4800 h QUV-A ageing, the surface erosion being restricted to a few micron thick layer. The observed permanency of the UV-absorber after such heavy exposure ensures a longlasting UV-screen effect of the protective coating. Waterbased UV-cured PUA coatings proved to be more resistant to hydrolysis than melamine-acrylate thermosets. (C) 2003 Elsevier Ltd. All rights reserved.