PARTICLE-SIZE EFFECT IN CRAZE PLASTICITY OF HIGH-IMPACT POLYSTYRENE

To explore the particle-size effect on craze plasticity and on the level of toughening of high-impact polystyrene (HIPS), a commercial grade HIPS (Mobil PS 4600) with a composite particle fraction of 0.217 and average particle size of 2.51 mu m was used. The matrix fractions of the material were dissolved in toluene and the unaffected particles were harvested as a gel fraction which was levitated in fresh toluene as a dilute suspension. This dilute particle suspension was centrifuged to separate the particles into two non-overlapping small and large particle populations of average size 1.03 and 3.97 mu m, respectively. With these separated particles, two new blends of HIPS-type material with narrow particle distributions were reconstituted using commercial grade Lustrex HH-104 PS, together with a reconstituted blend of HIPS made up of the original broad particle-size distribution to be used as a standard for comparison. All three blends had the same volume fraction as the original HIPS. Stress-strain experiments performed on the reconstituted blends showed that the mechanical properties and toughness levels of the reconstituted HIPS were nearly identical to the properties of the original HIPS. While the toughness of the reconstituted material with larger particles was roughly halved at the same flow stress level, the flow stress of the reconstituted blend with small particles had a craze flow stress 5% higher than that of the other two reconstituted blends, and a very severely reduced level of toughness. The behaviour of these blends was analysed with the aid of a theoretical model developed by Piorkowska et al. and furnished additional support for the correctness of the particle-size effect based on the principle of 'stress-induced displacement misfit' proposed by Argon et al. previously.