MELT-RECRYSTALLIZATION OF TALC-FILLED POLYPROPYLENE INJECTION MOLDINGS

Talc-filled polypropylene injection moldings show a peculiar crystal orientation, in which the plate planes of talc particles are aligned parallel to the molding surface, the c- and a*-axes of polypropylene crystals are bimodally oriented to the flow direction, and the b-axes are oriented to the thickness direction. If they are melt-recrystallized, although the orientation of talc and the b-axis orientation of polypropylene crystals are restored, the c- and a*-axes become unoriented in a plane parallel to the molding surface. This peculiar crystal orientation cannot be explained by any effect of an epitaxy caused by lattice matching between the cleavage plane of talc and (040) plane of polypropylene crystal, a crystallization nucleation effect, a concentration of stress or melt orientation, or their synergisms. From the facts that, even if talc-filled polypropylene injection moldings are melt-recrystallized, the b-axis orientation to the thickness direction is restored and that an injection molding of polypropylene filled with only 0.5 wt.-% talc already shows the peculiar crystal orientation, it is assumed that talc influences a remote interaction on the crystallization of polypropylene so that the (040) planes of polypropylene crystals pile on or the b-axes are oriented perpendicular to the plate planes of talc particles.