Structures and Properties of Injection-Molded Polypropylene Copolymers with Ethylene II. Distribution of Higher-Order Structures

Abstract

Flexural test specimens were injection-molded from random or block polypropylene copolymers with various ethylene contents at cylinder temperatures of 200∼320°C. Distributions, in the flow direction, of higher-order structural parameters such as thickness of skin layer (ST) and crystalline orientation functions and distributions, in the thickness direction, of higher-order structural parameters such as crystallinity (Xc), β-crystal content (K), and crystalline orientation functions were investigated.ST and crystalline c-axis orientation function fc decreased with increasing distance from the gate and with increasing ethylene content. Xc was low at the surface region and increased toward the interior. Xc was decreased throughout whole range in the thickness direction by copolymerization, in particular, by random copolymerization. K value was decreased by block copolymerization and was nearly zero for random copolymers. fc was high at the surface region and decreased toward the interior. The higher the cylinder temperature and ethylene content, the lower was fc at the surface region. There were cases at high cylinder temperatures where a shoulder or a peak of fc was observed at a middle point in the thickness direction, which is assumed to be caused by secondary flow during cooling and pressure holding process.