Photostabilising action of ortho-hydroxy benzophenones in polypropylene film: Influence of processing and wavelength of irradiation

Abstract

The effect of processing on the rates of photodecomposition and photostablising action of five different para-substituted ortho-hydroxy benzophenones in polypropylene film under both photolysis and photosensitised oxidation conditions has been examined. Under the latter conditions rates of photodecomposition and photostabilising effects of the compounds are significantly reduced in polymer films containing higher initial concentrations of hydroperoxides indicating that a photosensitised radical attack on the ortho hydrogen atom is occurring. Enhanced protection and much slower photodecomposition rates are observed for compounds containing long para-substituted n-octoxy and n-dodecyloxy groups compared with smaller para-substituents, suggesting that these groups may also have some antioxidant function. The higher photoprotective effect of the longer n-alkyl substituents is associated with cage effects in the polymer reducing their mobility and encouraging radical recombination. The smaller para-alkyl substituents are able to diffuse out of the polymer cage more rapidly. Under photolysis the effect of a photosensitised radical attack still predominates but only with the compounds containing long para-alkyl substituents. No evidence for migration or absorption could be found under the experimental conditions used here.