Mechanism of photostabilization of polystyrene film by dihydroxyphenyl‐pirazoles

A new family of ultraviolet absorbers, the dihydroxyphenyl‐pyrazoles, has shown to be able to act as photostabilizers of polymers. In the present work the efficiency of the derivative 2,5 bis(pyrazol‐1′‐yl)‐1,4‐dihydroxybenzene (BPHB) in preventing the photooxidative degradation of polystyrene films have been measured. The effect of light screening on polymer photostabilization was examined by using external filter solutions of BPHB and it was found that it does not function simply as a light screener. Phosphorescence quenching experiments show that BPHB behaves also as an efficient triplet‐state quencher of the polymer in‐chain carbonyl impurity groups, which play an important part in the overall mechanism of light stabilization.