Role of Elastomers in Impact-Resistant Polystyrene

Abstract

Incorporation of elastomers into polystyrene has been used in recent years to overcome its inherent brittleness. Mechanical mixing of rubber with polystyrene has been largely superseded by dissolving the elastomer in the styrene before polymerization. Impact-resistant polystyrene produced by either method is a two-phase system with polystyrene the continuous phase and elastomer the discrete phase. Polymerization of styrene in the presence of an unsaturated elastomer converts the elastomer to a graft copolymer. The graft copolymer is formed by reaction of the polymerization initiator with the rubber to form an active site thereon, which can initiate polymerization of styrene to form a side chain of polystyrene. The graft copolymer is crosslinked in the process. The size of the gel particles must not be too large for a satisfactory product, but can be controlled by shear and by chain transfer agents. The rubber chiefly used with polystyrene has been SBR, but use of polybutadiene is growing because of improved impact resistance at low temperatures. The resistance to impact increases with increasing amounts of elastomer, but surface gloss and hardness decrease. The chief advantage of graft copolymerization over poly-blending is more efficient utilization of the elastomer. The increase in resistance to impact has been attributed to the energy absorbing character of the elastomer. The two-phase system is necessary so as not to lose this basic property of elastomers. The rubber particles may also act to induce many small cracks rather than one large one, thus increasing the energy requirement for total failure. The greater efficiency obtained by a graft copolymer may be due to a more diffuse interface between rubber and polystyrene, leading to more absorption and less reflectance of a stress wave by the particle of rubber.

The effect of rubber particle size on the mechanical properties of high-impact polystyrene

“…The particle size distribution, especially in the resins with larger particles, is extremely broad. It is also interesting to note, in Figure Fig less than [1][2][3][4][5] l(a), that even in sample no. 1 there are a large number of very small (1 K r n or less) particles distributed among the very large particles.…”

The Theory of Rubber Toughening of Brittle Polymers

“…Kambour indicates that craze mate rial is 40-50% voids of 20 to 200 A in size (21). Matsuos electron (1) …”

Impact improvement mechanism of HIPS with bimodal distribution of rubber particle size

“…It is well-known that the toughness of glassy polymers can be improved by the dispersion of rubber particles,1'2 and the toughening mechanism of these systems has been studied extensively. [3][4][5][6] In high-impact polystyrene (HIPS), the absorption of impact energy is thought to increase mainly by the generation and growth of crazes in the polystyrene (PS) matrix.7 Effects of the rubber content and rubber particle size on the toughening of HIPS have been studied from both scientific and technical aspects.…”