Application of the crack resistance concept to the toughness characterization of high-impact thermoplastics

摘要

Heterophasic reactor-grade polypropylene-ethylene copolymers were diluted with a propylene-ethylene random copolymer to obtain materials with a constant EPR/PE-particle diameter but various interparticle distances. In addition to these copolymers, PP/EPR blends were investigated. According to the results of instrumented impact tests, brittle-to-tough transitions were found at different temperatures. In the materials with PP matrix, two transitions in the load-deflection behavior were seen. The first transition occurs from the elastic to the elastic-plastic materials behavior, the second from the unstable to stable crack growth. "Critical" interparticle distances could be determined in the region of predominantly unstable crack growth as well as in the region of predominantly stable crack growth. With increasing test temperature, both transitions shift to higher interparticle distances. From this theoretically similar behavior of fracture mechanical values quantifying different properties of the crack growth process, one can conclude that different processes must be occurring.