Photo-oxidation of thermoplastics in bending and in uniaxial compression

Abstract

Injection-moulded bars of polypropylene, glass fibre-reinforced polypropylene and polystyrene have been exposed to ultraviolet irradiation (UV) while held in four-point bending. In some cases the UV was applied to the tensile surface while in others the UV was applied to the compressive surface. Measurements of molecular weight using gel permeation chromatography showed that compressive stress retarded the rate of chain scission and confirmed that tensile stress accelerated it. Confirmation of the effect of stress on chain scission was obtained from samples exposed in uniaxial tension and uniaxial compression. The deterioration of mechanical properties of samples exposed under different loading modes did not always follow the molecular degradation. This is believed to be a consequence of different distributions of molecular damage: tensile stress favours the diffusion of small radicals, causing the molecular degradation to be more uniformly distributed and making the nucleation and growth of submicrocracks less favourable.