The Accelerated Photo-Degradation of Low Density Polyethylene

Abstract

It has been reported previously that transitional metal-di-thiocarbamates act as powerful activators. The present work concerns with the investigation of compounds of various structural types having uv-absorption characteristics in the range 280 - 340 nm as possible photo-activators to low density polyethylene (LDPE). They included benzophenone, substituted benzophenones, metal-di-thiocarbamates, transitional metal-complexes containing ligands, acetyl-acetone substituted salicylaldoximes, disalicylaldehyde-ethylene-diamine mercaptobenzimidazole(MBI) and mercaptobenzthiazole(MBT). Evaluation of the photo-degradation of the polymer initiated by these additives was carried out by physical, mechanical and chemical test methods. The relative efficiency as photo-activators, of the additives was determined and mechanisms of photo-oxidation have been proposed. A good correlation between different methods of measuring uv-degradation was found. A preliminary investigation of the mechanism using model systems has been initiated. It has been observed by IR spectrophotometry that in general photo-oxidation of LDPE resulted in the formation of carbonyl, vinyl, hydroxyl, carboxyl groups and the decay of vinylidene group. This was accompanied by change in the morphology and in the degree of crystallinity of the polymer. The photo-degradation of LDPE was found to differ with different additive. Benzophenones showed an auto-retarding rate of oxidation after an initial acceleration. Iron complexes of dithiocarbamic acid and substituted oximes, on the other hand, were powerful activators proceeded by an induction period. They were also thermal stabilizers during processing. This is consistent with the view previously put forward that believed as peroxide-decomposing anti-oxidants during processing and initially, on uv-irradiation. The high rate of degradation after the induction period is attributed by the fact that free metal ion, in addition to the other chain propagating radicals, liberated during the photo-decomposition of the complex acts as a powerful redox initiator in the presence of u.v light. Metal acetyl-acetonates were also found to accelerate degradation but they showed thermal pro-oxidant activity which increases with higher concentration. Two component uv-activator systems gave a synergistic effect in photo-activation of LDPE.