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The Skin‐core Morphology And Structure–property Relationships In Injection‐molded Polypropylene

M. R. Kantz, H. Newman, F. H. Stigale
Published 1972 · Materials Science

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Tensile bars of an isotactic propylene homopolymer and an ethylene–propylene copolymer were prepared by injection molding under a variety of melt temperatures and injection pressures. The effects of these processing variables on morphology and crystalline orientation were studied using optical microscopy and x-ray diffraction. Microscopy of microtomed thin sections of the tensile bars revealed the presence of three distinct crystalline zones, namely, a highly oriented nonspherulitic skin, a row or shear-nucleated spherulitic intermediate layer, and a typically spherulitic core. The thickness of the oriented skin layer is a function of the polymer melt temperature and varies inversely with temperature. The thickness of the intermediate layer varies with injection pressure, but in a complex manner. Preferred crystallite orientation in the skin and intermediate layers exerts profound effects on mechanical properties. Tensile yield strength, impact strength, and shrinkage increase with increasing combined thickness of the two oriented outer layers.

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