HYDROPEROXIDE FORMATION IN THE EARLY STAGES OF POLYPROPYLENE PHOTOOXIDATION

The total chemiluminescence from polypropylene films heated in an inert atmosphere is a sensitive measure of the hydroperoxide content in the induction period of photo-oxidation. The examination of the hydroperoxide content in unstabilised film melt-pressed from the powder, stabilised melt-pressed film and a commercial stabilised film all show a very rapid increase in the hydroperoxide content in the early stages of photo-oxidation which could not be detected by ATR-IR or XPS analysis. This appears as a peak in all of the films which increases in area with film thickness, indicating that it is a bulk property. While stabilisers may inhibit the photo-oxidation as measured by carbonyl index, they cannot prevent this early buildup and decay of hydroperoxides. These are believed to form by the rapid oxidation of the polypropylene in localised zones of high photoinstability. The peak in hydroperoxide content is a consequence of the kinetics of the consecutive reaction of hydroperoxide formation and photolysis which leads to secondary oxidation products and subsequent slower photo-oxidation.