Rubber phase dispersion in polypropylene

Abstract

Impact properties of rubber-modified blends are significantly dependent on particle size and size distribution (dispersion). Past work has shown strong variations in these particle parameters with different processing conditions and these variations reflect, in part, different shear fields developed during melt extrusion and moulding. A major problem ensues involving property variability because of process variability. It is neither practical nor economical to place stringent controls on process variability in a plant. Instead, our current research has indicated that lightly crosslinking the rubber reduced drastically the sensitivity of dispersion to processing conditions, and, with the correct rheology between the rubber and the resin, the same dispersion could be maintained through both the extrusion and moulding stages resulting in finished parts with increased chance of dispersion and property consistency. Four rubbers of different degrees of crosslinking and three polypropylenes of different melt flow and structure were blended at six rubber levels. Mixing was carried out with two twin-screw configurations and the blends were cast or injection moulded using a ram or a screw moulder representing six different processing shear histories. Dispersion was quantitatively characterized by scanning electron microscopy (SEM). Spherical rubber particles revealed on the fracture surface were counted and measured to determine particle size and size distribution for the individual blends. Correlation of the results indicated how particle size and size distribution was influenced by crosslinking and the rubber/resin rheology. This provided some understanding of the mechanics of soft-phase dispersion in these blends.