Dynamic strain rate and relative density effect on compression behavior of PP and PP/PE copolymers foamed by microcellular injection molding

Isotactic polypropylene-polyethylene copolymer microcellular foams of various relative densities were prepared using nitrogen (N-2) in supercritical solid state. Also, series of copolymers with different polyethylene weight percentages were prepared. Mechanical properties (compression and impact conditions) dependence on relative density, PE content, and strain rate was studied. Results showed that yield stress gradually decreased with lower density. The yield stress of PP copolymer foams decreased with the relative densities, and was always smaller than that of the solid PP copolymer. Nevertheless, the sensitivity to strain rate effect decreases at impact velocities. Analytical models were developed including the effects of both relative density and strain rate.