Compatibilisation of polypropylene–polystyrene blends: Part 1 – Effect of mixing intensity on morphology andrheological properties

Abstract

Propylene copolymer blends have been prepared using a proprietary, in reactor, grafting polymerisation technique, to give two phase materials composed of 70% polypropylene, 20% polystyrene, and 10%polypropylene- graft-polystyrene (PP-g-PS). The present study was carried out to determine the efficiency of PP-g-PS relative to block-copoly(styrene/ethylene–butylene/ styrene) (SEBS) as a compatibiliser for polypropylene–polystyrene blends by measuring rheological properties and examining the morphological structure. Mixing experiments were conducted on a twin screw mixing element evaluator and on a commercial ZSK-30 extruder. Dynamic mechanical analysis was used to characterise the viscoelastic properties of the extruded polymer pellets and transmission electron microscopy was used to examine the morphology of the polymer throughout its processing history, i.e. from carcasses at different stages of the mixing operations to injection moulded specimens. Both the graft copolymer, PP-g-PS, and the black terpolymer, SEBS, were found to enhance the compatibilisation of the blends, but the graft copolymer was found to be more effective. The results also show that the intensity of mixing affects the molecular and morphological structures of the graft and graft–rubber blends, as indicated by the intrinsic viscosity values of the extracted isotactic polypropylene fraction of the blends. This is supported by morphological examination of the graft blend, which showed a phase inversion from co-continuous to dispersed phase morphology during extrusion.