Influence of the molecular weight of ethylene vinyl acetate copolymers on the flow and mechanical properties of uncompatibilized polystyrene/ethylene-vinyl acetate copolymer blends

Rubber-toughened polystyrene (PS) has been extensively studied and is a wel l-established material. However, the use of thermoplastic elastomers to tou ghen PS is new and not well understood. In this study, three types of ethyl ene vinyl acetate (EVA) copolymers with the same vinyl acetate (VA) content (27.2-28.8 wt%) but with different melt flow indexes (MFI; g (10 min)(-1)) of 365-440 (Elvax 210), 38.0-48.0 (Elvax 240) and 2.6-3.4 (Elvax 265) were used as impact modifiers for PS. The uncompatibilized blend systems at dif ferent compositions were prepared using a twin-screw extruder and injection moulding to produce the required test pieces. The viscosity of the dispers ed phase (EVA) has a significant effect on the mechanical properties of the blends. Rheological studies show that uncompatibilized PS/EVA265 blends ex hibit some degree of compatibility when the amount of EVA265 added is below 30wt%, These results indicate that EVA265 with the lowest melt flow index or highest molecular weight is the most effective impact modifier for PS, T he mechanism for such behaviour is still unclear. (C) 2001 Society of Chemi cal Industry.