Photodegradation and photooxidation of thermoset and UV-cured acrylate polymers

Infrared spectroscopy has been used to monitor the chemical changes occurring during accelerated QUV-A weathering in thermoset acrylate (TSA) and UV-cured acrylate (PUA) coatings. In the acrylic melamine crosslinked polymer, UV irradiation causes a decrease of the TSA functional groups (ether, C-H, C-N) and the formation of oxidation products (hydroperoxide and carboxylic acid). In the W-cured polyurethane-acrylate network, the urethane linkage is the most sensitive to photodegradation. The UV-cured PUA coating was shown to be more resistant to accelerated weathering than the high-solid TSA clearcoats which are presently used as automotive finishes. The light-stability of both types of coatings was substantially increased by the addition of an hydroxyphenyl-s-triazine UV absorber and a HALS radical scavenger. The best stabilized W-cured PUA coating remained essentially unchanged after a 2000 h exposure, and proved to be as resistant to wet cycle accelerated QUV ageing as a thermoset acrylic urethane clearcoat. (C) 1999 Elsevier Science Ltd. All rights reserved.