Structures and Properties of Injection-Molded Polypropylene Copolymers with Ethylene I. Structure-Property Relationships

Abstract

Flexural test specimens were injection-molded from random or block polypropylene copolymers with various ethylene contents at cylinder temperatures of 200∼320°C. Properties such as flexural modulus (FM), flexural strength (FS), heat distortion temperature (HDT), Izod impact strength (IIS), and mold shrinkage (MS), and higher-order structures such as crystallinity (Xc), thickness of skin layer (ST), a*-axis-oriented component fraction ([A*]), and crystalline orientation functions were measured. The relationships between these properties and structural parameters 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.Xc was decreased and [A*] was increased by copolymerization, in particular, by random copolymerization. ST and crystalline c-axis orientation function fc decreased with increasing cylinder temperature and ethylene content.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 sample is assumed to originate from higher-order structures beside molecular orientation, for example, crystallinity, spherulite size, and thickness of lamellae.