The mechanical behaviour of high-impact polystyrene under pressure

Abstract

High-impact polystyrene [HIPS], a two-phase polymeric system, has been investigated studying the pressure dependence of stress-elongation behaviour in tension over the range from atmospheric pressure to 4 kbar at room temperature and constant strain-rate. A comparative study of polystyrene [PS] was also undertaken. HIPS sealed from the environment exhibited ductile behaviour at all pressures. Surprisingly, specimens exposed to silicone oil environment exhibited two transitions as the applied hydrostatic pressure was raised: a ductile-to-brittle followed by a brittle-to-ductile transition. Stress-whitening was suppressed at relatively low pressures. The dilational requirement for profuse crazing was restrained by the combined effect of fluid under pressure resulting in the suppression of the energy absorption mechanism.

Analysis of the stress-elongation curves for sealed specimens indicated that the pressure dependency of craze-initiation stress differs from that of shear band initiation stress. The brittle-to-ductile transition occurred when the initiation stresses of both processes became equal. The principal stress for craze initiation showed almost no pressure dependency, suggesting that crazes initiate when the principal stress level of the tensile specimen reaches a critical value irrespective of the applied hydrostatic pressure. A value for the proposed triaxial tension around the rubber particles was determined from the experimental results and found to be in good agreement with a calculated value. A general mechanics argument was used to explain the existence of the ductile-to-brittle and the brittle-to-ductile transition in HIPS, and also to predict the pressure dependencies of brittle-fracture stress and craze-initiation stress for sealed and non-sealed specimens.