Dependence of impact strength on the fracture propagation direction in dynamic packing injection molded PP/EPDM blends

In order to further understand the brittle-ductile (B-D) transition in PP/EPDM blends, a shear stress field achieved via dynamic packing injection molding was used to control the rubber particles as elongated and orientated in the PP matrix. The impact strength of the blends was measured in three fracture directions, namely, along the shear flow direction, perpendicular to and oblique (45°) with the flow direction. A definite B-D transition of impact strength was found at 20 wt% of EPDM content along the shear flow direction. About 10 times increase of impact strength was observed at the B-D transition. However, a B-D transition and then a decrease of impact strength brittle-ductile-brittle (B-D-B) was found as increasing of EPDM content in the impact direction perpendicular to and oblique with the flow direction. One observes a big increase of impact strength at 20-30 wt% of EPDM content (B-D transition) from 10-20 to 70-80 kJ/m², then a sharp decrease of impact strength is seen when EPDM content reaches to 30-45 wt% (D-B transition) from 70-80 to 40-50 kJ/m². Correspondingly, there exists a change of rubber particles from roughly spherical shape to highly elongated and oriented shape at D-B transition. SEM shows a very smooth fractured surface when fracture propagation is along or oblique with the shear flow direction, but a lay-by-layer fracture behavior when fracture propagation is perpendicular to the shear flow direction. Our results suggest that the impact fracture direction with respect to the orientation direction play an important role to determine the impact strength. Wu's theory holds true as long as the rubber particles are roughly spherical when viewed in the same direction with fracture propagation direction, but no longer valid when dispersed rubber particles are elongated and oriented when viewed in the direction perpendicular to or oblique with the fracture propagation direction.