Rates of photooxidation induced crosslinking and chain scission in thermoset polymer coatings .1.

Chemical stress relaxation was used to measure the rates of crosslinking and chain scission during accelerated weathering in two acrylic/melamine clearcoats that photooxidize at different rates. The more photooxidation-resistant clearcoat undergoes an initial increase in crosslink density followed by a steady decrease, while the more photooxidation-prone clearcoat undergoes primarily chain scission until approximately 800 h of accelerated weathering, after which the rate of crosslink formation begins to exceed the rate of chain scission. The net changes in crosslink density mirror those found using dynamic mechanical analysis. Clearcoat films weathered under no stress displayed the same amount and type of chemical composition change as the films weathered under stress. Chemical stress relaxation appears to be a ready means of assessing the influence of the fate of free radicals on the physical properties of the clearcoats. (C) 1997 Elsevier Science Limited.