Molecular Orientation in Injection Molding of Particulate Filled Polypropylenes

Abstract

The purpose of this study is to clarify the relationships among raw material, molding conditions, supermolecular structure of the product and its mechanical property in the injection molding of particulate filled polypropylenes. Flexural specimens were prepared by injection-molding at the cylinder temperature of 200-320°C from polypropylenes filled with talc (flaky) and calcium carbonate (spherical) at the level of 0-60 wt%. We investigated supermolecular structures, such as the dispersion state of fillers, skin/core structure, and degree of crystal orientation and mechanical properties, such as flexural modulus, flexural strength, and mold shrinkage.The thickness of skin layer and the degree of crystal orientation decreased with increasing cylinder temperature, and increased with increasing filler content. For each sample a linear relationship between the thickness of skin layer and the degree of crystal orientation was found. The mechanical properties, such as flexural modulus, flexural strength, and mold shrinkage, also decreased with increasing cylinder temperature and increased with increasing filler content. In all cases studied, these properties correlated linearly with the thickness of skin layer or the degree of crystal orientation irrespective of filler content level. The results were interpreted as due to high degree of molecular orientation in the injection molded specimens of particulate filled polypropylenes.