PHYSICAL SPREADING OF OXIDATION IN SOLID POLYPROPYLENE AS STUDIED BY CHEMILUMINESCENCE

Highly sensitive chemiluminescence equipment was applied to study the oxidative behaviour at 125-150-degrees-C of individual polypropylene powder particles (10-500 mum) and film sections pressed from the powder. The chemiluminescence curves from the oxidation of single isolated powder particles revealed a range of induction periods equivalent to different intrinsic stabilities of the individual particles. Physical contact of the particles, however, resulted in a relatively early oxidation of the combined sample. i.e. a single induction period, as a result of the apparent initiation by the weakest particle and a subsequent spreading of the oxidation through the ensemble of particles. Evidence of this oxidative spreading was also found in polypropylene film samples. The physical spreading of the oxidation from an initial highly reactive centre has therefore been established as being the predominant step in the oxidation of a larger sample, and thus the weakest centre can control the overall oxidation of a sample. This mechanism was found to be consistent with the sigmoidal curve shape of the chemiluminescence curve obtained from the oxidation of both powder and film. The observed spreading is believed to be a highly efficient process, as the oxidation can 'jump' from particle to particle over distances of approximately 100 mum. In stabilized film the oxidation at the end of the induction period shows features different from that of an unstabilized film, and it is proposed that the spreading of the oxidation is limited by residual stabilizer in zones of lower reactivity.