Polypropylene-rubber blends: 1. the effect of matrix properties on the impact behaviour

The effect of matrix properties, i.e. crystallinity and molecular weight, on the impact behaviour of polypropylene–EPDM blends was studied. The blends were made on a twin-screw extruder. The impact strength was determined as a function of temperature, using a notched Izod impact test. The matrix crystallinity was varied by varying the matrix isotacticity, and ranged from 33 to 50 wt%. With increasing temperature the polymers show a sharp brittle–ductile transition. This brittle–ductile transition temperature (Tbd) shifts to higher temperatures with increasing crystallinity of the polypropylene. However, the balance of properties and the modulus–Tbd relationship were better with blends made with higher crystalline PP. The matrix molecular weight was decreased by treating a high molecular weight PP–EPDM (85/15 vol%) master blend with peroxide. In this way blends were obtained with a high MFI and a small rubber particle size. The matrix MFI of the blends thus obtained ranged from 2 to 30 dg min−1. With decreasing matrix molecular weight the Tbd increased. The peroxide treated blends exhibited a considerably lower Tbd than comparable blends made in the standard way with a similarly small particle size. Peroxide treatment of a master blend is an effective method of preparing blends with a high MFI, small particle size and good ductility.