Molecular Orientation in Injection-molded Polypropylene Copolymers with Ethylene

Abstract

Abstract Flexural test specimens were injection-molded from random and block polypropylene copolymers with various ethylene contents at cylinder temperatures of 200 to 320°C. Properties such as flexural modulus (FM), flexural strength (FS), heat distortion temperature (HDT), Izod impact strength (IIS), and mold shrinkage (MS), and degrees of molecular orientation such as thickness of skin layer (ST) and crystalline orientation functions and their distributions in the flow and thickness directions were measured. The relationships between these properties and the degrees of molecular orientation were investigated. These properties decreased with increasing cylinder temperature. FM, FS and HDT were decreased by copolymerization with ethylene, in particular, by random copolymerization. IIS was increased by copolymerization with ethylene. MS was decreased by random copolymerization and slightly increased by block copolymerization. ST and crystalline c-axis orientation function fc decreased with increasing cylinder temperature and ethylene content and on increasing distance from the gate. fc is high at the surface region and decreases toward the interior. The higher the cylinder temperature and ethylene content, the lower is fc at the surface region. There are cases at high cylinder temperatures where a shoulder or a peak of fc is observed at a middle point in the thickness direction, which is assumed to be caused by secondary flow during cooling and pressure holding process. Rectilinear relationships between the properties and ST or fc were obtained for each sample. This means that the properties cannot be unified only with degree of molecular orientation and the character of sample still remains. The character of the sample is assumed to be originated from higher-order structures beside molecular orientation, for example, crystallinity, spherulite size, and thickness of lamellae, etc.