Effect of temperature on the photooxidation of polypropylene films

Photochemically unstabilised polypropylene films of similar to 100 mu m thickness were exposed in reactors equipped with a fluorescent lamp (300 < lambda < 450nm, lambda(max) = 365 nm) at 40, 55 and 70 degrees C. The kinetics of photooxidation were followed by using infra-red spectroscopy to determine the carbonyl (at 1720 cm(-1)) and hydroxyl (at 3400 cm(-1)) build-up, and by using gravimetry to determine film mass changes All kinetic curves present a pseudo-induction period whose duration (ti) decreases with increasing temperature. Arrhenius plots of induction periods and pseudo-stationary oxidation rates reveal that the apparent activation energy is an increasing function of temperature, i.e. the global kinetics of polypropylene photooxidation do not obey the Arrhenius law. We tried to explain the observed phenomenon by using a 'unimolecular closed loop' mechanistic scheme of oxidation with simultaneous thermal and photochemical decomposition of hydroperoxides as initiating reactions, and we propose a way for developing a lifetime model for the photooxidation of polypropylene films. (C) 1998 Elsevier Science Limited. All rights reserved.