Photo-stabilising action of ortho-hydroxy aromatic compounds in polypropylene film: UV absorption versus radical scavenging

The relative importance of uv absorption and radical scavenging in the photo-stabilisation of polypropylene film by various concentrations of 4-methoxy and 4-n-octoxy substituted 2-hydroxybenzophenones and 2(2′-hydroxy-3′-t-butyl-5′-methyl phenyl)-5-chlorobenzotriazole is examined using infra-red and second-order derivative uv absorption spectroscopy and hydroperoxide analysis. Under both photolysis with polychromatic light wavelengths greater than 300 nm and photo-sensitised oxidation with monochromatic light of mainly 365 nm, the additives were considerably more effective photo-stabilisers when present in the polymer than when used as screens. Only under exposure to far uv light of 254 nm wavelength at 0·5 and 1·0% w/w of additives was any evidence of screening important. At 0·01% w/w concentration both the ortho-hydroxybenzophenones sensitised the photo-oxidation of the polymer and this correlated with higher concentration levels of hydroperoxide in the polymer films, confirming the pro-oxidant behaviour of these additives. Enhanced photo-protection by long n-alkyl groups in the 4-position of ortho-hydroxybenzophenones under photo-sensitised oxidation is associated with the lower mobility of the n-alkyl radical and cage effects in the polymer encouraging radical recombination. These conclusions are confirmed by solution photolysis experiments in the absence and presence of cumene hydroperoxide. The results clearly show that whilst reversible proton transfer may be important for protecting the additive itself, a mechanism involving uv absorption is of little or no importance in photo-stabilisation.