Mechanism of the photo-oxidation of polyethylene

The mechanism of the photo-oxidation of both low density and high density polyethylenes has been studied kinetically. The kinetics are consistent with the view that when substantial quantities of carbonyl compounds are present in the polymers by photolysis of thermally formed hydroperoxides, they are intimately involved as initiators in the photo-oxidation by Norrish I photolysis of ketones. The latter process is also responsible for the ultimate end products of photo-oxidation, viz. carboxylic acids. The two polymers differ in that cross-linking occurs initially in LDPE with reduction in vinylidene unsaturation whereas chain scission predominates in HDPE. The effect of iron compounds is in accord with their behaviour in thermal oxidation. Iron carboxylates activate both types of polyethylene to photo-oxidation primarily by decreasing the amount of carbonyl compounds present, although initially they also generate radicals by photolysis. The iron dithiocarbamates on the other hand are initially antioxidants and initially prevent carbonyl formation in the polymer. They function as photo-activators primarily by photolysis to free radicals.