Melting and crystallization behaviors of injection-molded polypropylene

The melting and crystallization behaviors of the skin layer in an injection molded isotactic polypropylene (PP) have been studied, mainly in compariso n with those of the core layer and subsidiarily in comparison with those of a compression-molded PP and a nucleator (talc)-added PP. The skin layer co ntains about 5% crystals, which have a high melting point of up to 184 degr ees C. They thermally vanish by melting once. The subsequent melting histor y will scarcely affect the melting behaviors. On the other hand, crystalliz ation behaviors are strongly affected by the melting history. The skin laye r crystallizes in a wide temperature range at high temperature. This tenden cy weakens with increasing melting temperature, approaching a constant and that of the core layer above 230 degrees C, which suggests that the memory effect of the residual structure of PP vanishes by melting above 230 degree s C. In explaining these experimental results, it is assumed that the resid ual structure substance is a melt orientation of molecular chains that work s as crystallization nuclei and that the vanishing of the residual structur e is nothing but a relaxation of the melt orientation. (C) 2000 John Wiley & Sons, Inc.