PHOTODEGRADATION OF UV SCREENERS

The UV screeners that are used to help photostabilize polymers and coa tings are themselves subject to photodegradation over the course of a few years of exposure. A simple computer program has been devised to m odel this process. All of the chromophores of the commercially availab le screeners degrade, but at varying rates. The rates are highly depen dent on the matrix since hydrogen bonding effects and free radicals ca n lead to decomposition of the screeners. Concentration seems to play no role, and hindered amine light stabilizers (HALS) stabilize the scr eeners only to the extent that they reduce free radical degradation of the matrix.