PHOTODEGRADATION AND PHOTOSTABILIZATION OF URETHANE CROSS-LINKED COATINGS

Infrared spectroscopy has been used to monitor photodegradation chemistries in urethane cross-linked coatings as a function of copolymer (acrylate versus polyester), cross-linker (biuret versus triisocyanurate), and exposure condition. Results are compared with measurements of the photoinitiation rate of free radicals and with hydroperoxide concentrations. Cross-link scission and oxidation are the main degradation chemistries in all of the coatings studied. The rates of degradation correlate well with free radical photoinitiation rates and hydroperoxide concentrations, confirming that photodegradation is controlled by free radical chemistry. The addition of hindered amine light stabilizer to these coatings inhibits free radical oxidation and reduces the rate of degradation. The extent of inhibition depends on the coating composition and on the exposure condition.