Mechanism of reinforcement in rubber‐modified polystyrene systems studied by use of a miniature dart drop test

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Abstract
A miniature dart drop test was used in a study of the mechanism of reinforcement in impact resistant rubber‐modified polystyrene. A typical SBR‐polystyrene system was diluted with varying amounts of polystyrene, and thin compression moldings were made from each blend. The impact‐whitened areas were observed directly under the microscope and photomicrographs are presented. The whitening is caused by scattering of light from interfacial separation of portions of many rubber particles from polystyrene and by formation of a multitude of microcracks and/or crazes, starting at the partially separated particles. The absorption of energy by this mechanism can be very large. The driving force for the initial separation appears to be the triaxial tension under which the rubber particles exist, and this results from the higher volume shrinkage rate of rubber compared to that of polystyrene. Included also are photomicrographs taken of specimens after tensile stretching, and the mechanism derived from the impact case is extended to explain the increased elongation and corresponding whitening.

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