High strain deformation and ultimate failure of HIPS and ABS polymers

The role of the rubber particle in the ultimate failure of High Impact Polystyrene and Acrylonitrile Butadiene Styrene was investigated by modifying the rubber content and the shear modulus of the rubber phase in the materials. The rubber content in a commercial grade HIPS and a commercial grade ABS, both with 8 wt. % rubber, was varied by blending with general purpose polystyrene and general purpose poly(styrene-aciylonitrile) respectively. The shear modulus of the rubber phase was varied through blending the materials with sulfur or irradiating the materials with gamma irradiation. Dynamical mechanical thermal analysis confirmed that the Tg of the rubber phase increased with increasing sulfur content. It was found that with decreasing rubber content or increasing rubber shear modulus, the yield, flow and breaking stresses and the elastic modulus of the composite increased, while the failure strain decreased. In a similar experiment to Sjoerdsma and Boyens (1994), the statistics of failure of the materials were investigated with respect to rubber content and rubber shear modulus. Batches of specimens numbering not less than 20 were extended under a constant applied stress until failure occurred. A custom designed creep rig was built to carry out several long term creep tests simultaneously. From these tests it was concluded that the probability of failure increased as the stress on the rubber increased and underpinning this, is a novel discussion of the high strain deformation and the mechanism controlling failure in HIPS and ABS. This conclusion was discussed in terms of rubber content and rubber shear modulus and a model was developed which describes the maximum failure strain in terms of these variables. The level of applied stress was also found to have an effect on the probability of failure. It was found that the success achieved by Sjoerdsma and Boyens (1994), in correlating failure strain data for a single grade of HIPS, could not be repeated when their model was applied to another grade of HIPS. The tw oparameter Weibull equation gave an improved correlation between the failure of HIPS and the strain on the material. Analysis of the relationship between the experimental failure strain distribution and the Weibull distribution revealed that the mean stress on the rubber phase at failure may be a better basis for achieving a Weibull distribution.