REACTION OF BENZOTRIAZOLE ULTRAVIOLET-LIGHT ABSORBERS WITH FREE-RADICALS

The photolysis of benzotriazole ultraviolet light absorber additives (Bz-UVAs) in acrylic/melamine clearcoats suggests that Bz-UVAs are destroyed by clearcoat-driven photooxidation chemistry. Tinuvin 900 is converted to parent benzotriazole and phenol ring oxidation products. Bz-UVAs are not destroyed by photolysis in acrylic/melamine clearcoats in the absence of oxygen. Solution photolysis experiments reveal that Bz-UVAs are not attacked by free radicals generated by thermal decomposition of azoisobutyronitrile (AIBN) or cumene hydroperoxide (CHP) in ethanol or cyclohexane. Bz-UVAs are also not attacked by free radicals generated by photolytic decompostion of AIBN or CHP in cyclohexane. Bz-UVAs, however, are destroyed when AIBN or CHP are photolytically decomposed in ethanol. Photolysis without oxygen leads to Bz-UVA phenol ring solvent and initiator adducts. Photolysis with oxygen leads to cleavage to form free benzotriazole and to destruction of the phenol ring. It is suggested that an excited state of the non-planar Bz-UVA form may be susceptible to free radical attack. Detailed liquid chromatography tandem mass spectrometry (LC/MS/MS) product analysis results are reported in a companion paper.