High Impact Polystyrene (HIPS)

High impact polystyrene (HIPS) or 'rubber-toughened polystyrene' is a two-phase mixture of polybutadiene (a synthetic rubber) in polystyrene. The two phases are immiscible, with polybutadiene crystallising out of the melt to form spheroidal particles. In fact, polystyrene is the first phase to crystallise, forming even an even finer dispersion of polystyrene particles which become trapped within the polybutadiene when a phase inversion occurs at around 10% solidification. In the particular region examined in this micrograph, a relatively high local concentration of polybutadiene has resulted in ellipsoid particles or cylinders rather than the more typical spheres