Development of Ultraviolet Light Stabilization Method for Transparent Polyarylsulfones

The Air Force polyarylsulfone polymer is an exceptionally photosensitive plastic the degradation of which is manifested by ultraviolent visible and infrared absorption spectral changes as well as haze development, changes in fluorescence, embrittlement and loss of impact strength. The wavelengths most effective in causing these events are in the short wavelength ultraviolent 310-330 nm region corresponding to onset of strong absorption by the polymer itself. As the photochemical processes proceed the rate of degradation slows but only after optical properties near the exposed surface have deteriorated to unacceptable levels. The photolysis is independent of moisture but requires oxygen, and generates hydroxylic and carbonyl compounds without an induction period. The degradation is not accelerated by singlet oxygen sensitizers or significantly quenched by singlet o quenchers, energy transfer agents, antioxidants or antioxidant synergists. It is accelerated by benzophenone and its rate is reduced with UV absorbers particularly hydroxybenzotriazoles which are most effectively competitive for the short wavelength ultraviolent radiation that is primarily responsible for the degradation. The damaging light is absorbed by the polymer itself or a very strongly absorbing impurity with similar solubility characteristics. The specific photochemical and chemical processes responsible for the degradation are unknown. Protective UV light absorbing additives are the only effective means of reducing the photodegradation of this material. The absorbers should be concentrated at the exposed surface for optimum effectiveness. At most they can provide no more than five-to-tenfold decrease in rate of photodegradation of the polymer.