Thermal and photochemical oxidation of polypropylene. Influence of residual catalyst levels in unstabilised diluent and gas phase polymers

Abstract

The thermal and photochemical behaviour of several unstabilised polypropylene samples made experimentally by a gas phase (GP) process has been examined and compared with those of samples made commercially by the normal diluent phase (DP) process. Higher levels of residual catalysts are shown to have a marked effect on both the thermal and photochemical processes. The initial autocatalytic growth of hydroperoxides, observed during both oven-ageing and melt processing, is considerably suppressed in GP polymer when compared to DP polymers probably because higher levels of residual catalysts are ionically inducing the decomposition of hydroperoxides thereby accelerating the subsequent thermal and photochemical breakdown of the polymer. Pre-treatment of both types of polymers with alcoholic phosphoric acid solution or sulphur dioxide produces some interesting and markedly different effects on photostability. Of the catalyst residues present, variation in titanium levels seems to be important in controlling thermal and photochemical stability: the effect of aluminium is seen but is as yet undefined.