Photo-stability of photo-cured organic coatings: Part III—Photo-degradation of organic coatings photo-cured with alkoxyacetophenone and hydroxy alkylacetophenone photo-initiators

Photo-degradation studies of polystyrene (as model) and coating resins were made with three different photo-fragmenting initiators: 2,2-dimethoxy-2-phenylacetophenone, 1-4-(2-propyl)-phenyl]-2-hydroxy-2-methyl-propanone-1 and 1-benzoyl-1-hydroxy-cyclohexane. Addition of the initiators to a benzene solution of polystyrene gave a rapid decrease in molecular weight when the solutions were uv irradiated. The degradation rate was increased by the presence of oxygen. Irradiation of polystyrene films, containing the same initiators, showed similar effects. In addition, the polymer became partially crosslinked. Addition of a free radical scavenger decreased the degradation rate. Both photo-oxidation rate and yellowing rate were affected by the concentration and the reactivity of the radicals. Photo-oxidation during photo-curing of a coating occurs in a thin surface layer. This is due to a combination of free radicals formed by the uv irradiation and the oxygen present. Oxidation during the photo-curing of a urethane resin corresponds to more than 50% of the total surface oxidation measured with ESCA.