Polypropylene-rubber blends: 1. The effect of the 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 (T-bd) shifts to higher temperatures with increasing crystallinity of the polypropylene. However, the balance of properties and the modulus-T-bd 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 T-bd increased. The peroxide treated blends exhibited a considerably lower T-bd 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. (C) 1998 Elsevier Science Ltd. All rights reserved.