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

Heterophasic reactor-grade polypropylene-ethylene copolymers were diluted w ith a propylene-ethylene random copolymer to obtain materials with a consta nt EPR/PE-particle diameter but various interparticle distances. In additio n to these copolymers, PP/EPR blends were investigated. According to the re sults of instrumented impact tests, brittle-to-tough transitions were found at different temperatures. In the materials with PP matrix, two transition s in the load-deflection behaviour were seen. The first transition occurs f rom the elastic to the elastic-plastic material behaviour, the second from the unstable to stable crack growth. "Critical" interparticle distances cou ld be determined in the region of predominantly unstable crack growth as we ll as in the region of predominantly stable crack growth. With increasing t est temperature, both transitions shift to higher interparticle distances. From this theoretically similar behaviour of fracture mechanical values qua ntifying different proportions of the crack growth process, one can conclud e that different processes must be occurring.