Effects of crystalline morphology on the impact behavior of polypropylene

A series of notched impact tests were made to polypropylene (PP) materials of different crystalline morphology through a certain effective filled nucleating agent (0.1-2 wt.%) and subsequent heat treatment. A maximum of impact strength exists at the 0.4 wt.% filled quantity of the nucleating agent. It was shown that when the filled quantity of the nucleating agent was below 0.4 wt.%. the spherulite size decreased and the crystallinity increased. The impact strength was controlled mainly by the spherulite size, but when it went beyond 0.4 wt.%, the decline tendency of the spherulite size decreased and tended towards progressive stabilization, the crystallinity still increased and the rising gradient increased. Therefore, crystallinity is the decisive factor in strength property. This illustrates that the appearance of a maximum of impact strength is due to the comprehensive result of the spherulite size and the crystallinity. The succeeding heat-treatment only affected the crystallinity. The spherulite size showed basically little change. The impact strength can be described by a quantitative formula. The fracture surface was composed of the expanding zone, the are stripe zone and the last fracture zone. The fractal dimension of the expanding zone exists at a maximum at the 0.4 wt.% filled quantity of the nucleating agent. The size of the are stripe has nothing to do with the quantity of nucleating agent. (C) 2000 Elsevier Science Ltd. All rights reserved.