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 spe ctroscopy 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 dur ation (ti) decreases with increasing temperature. Arrhenius plots of i nduction 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 t he Arrhenius law. We tried to explain the observed phenomenon by using a 'unimolecular closed loop' mechanistic scheme of oxidation with sim ultaneous 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.