Heat Treatment of Irradiated Polypropylene

Abstract

IT has been shown by Lawton^1,2 that during the high-energy irradiation of a polymer, the main effect produced is either intermolecular cross-linking or degradation of the molecular chain, and radicals produced during irradiation can be trapped in the crystalline regions. Crystalline ‘Marlex-50’, irradiated at 25° C. and then annealed at 150° C., gave more cross-links than ‘Marlex-50’ irradiated at 25° C. and not annealed, but less than that of the polymer irradiated at 150° C. for the same dose of irradiation. We have been able to demonstrate the delayed cross-linking effect by heat treatment of pre-irradiated isotactic polypropylene. We find that many trapped radicals produced during irradiation, which are stable at room temperature (as detected by electron spin resonance measurements), could contribute to delayed cross-linking on annealing the irradiated polymer after irradiation, and a big increase of gel content was observed. On the other hand, materials irradiated at 180° C. (molten state) gave no gel content for high dosage beyond 80 Mrads, indicating that in the molten state degradation of the molecular chain was predominant. The phenomenon is very different from the effect in polyethylene.