Photo-oxidation of ethylene-propylene copolymers in the solid state

Abstract

The photo-oxidation of various ethylene-propylene copolymers (EPM) (40–76 weight % ethylene) has been compared with the photo-oxidation of physical mixtures of polyethylene and polypropylene. The photo-chemical behaviour of the PE-PP mixtures changes gradually from pure LDPE to pure isotactic PP. The photo-chemical behaviour of EPM copolymers is quite different. In EPM copolymers, isolated hydroperoxides absorbing at 3550 cm −1 appear on the propylene units surrounded by polyethylene sequences. Associated hydroperoxides are formed on the polyethylene segments. The rate of hydroperoxidation of EPM is an increasing function of the propylene content; at the same propylene content, this rate is higher in EPM than in physical mixtures. Even at high propylene contents, the carbonyl region of the ir spectra of photo-oxidized EPM bears much more resemblance to that of photo-oxidized polyethylene. This results from the formation of isolated hydroperoxides on tertiary carbon atoms. The hydroxyl region shows a sharp band at 3615 cm −1, indicating the formation of isolated tertiary alcohols. In the EPM samples, the formation of vinyl groups is favoured in comparison with PE; in contrast to the ketones formed in PP, those which appear in EPM are converted into shorter ketones and vinyl groups through Norrish type II processes.