Fracture behavior of styrene-ethylene-propylene rubber-toughened polypropylene

The morphology and fracture behavior of isotactic polypropylene toughened by styrene-ethylene-propylene (PP/SEP) were investigated. The SEP rubber, having an average particle size of 0.2 mu m, is found to be well dispersed in the PP matrix. The fracture toughness of SEP-modified PP is greatly improved. The toughening mechanism investigation shows that a widespread crazing zone is generated in the crack tip damage zone. An intense narrow damage band in the center of crazed zone is formed. Crazing and shear yielding are found to be the dominant toughening mechanisms in PP/SEP. The crazes are initiated only by large SEP particles in the blend. The small SEP particles (< 0.3 mu m) can neither cavitate nor trigger crazing. As a result, large scale shear deformation is suppressed in this blend. These findings are consistent with the notion that the crack tip plane strain constraint has to be relieved in magnitude in order for the deviatoric stress to reach a critical value for widespread shear banding.