Polypropylene-rubber blends: 5. Deformation mechanism during fracture

The deformation mechanism of polypropylene-EPDM rubber blends during fracture was studied by post-mortem SEM fractography. The deformation mechanism was determined for various blend morphologies and test conditions. Brittle fracture merely gives rise to voids, which are caused by voiding of the rubber particles. In the case of ductile fracture, voiding of rubber particles and strong shear yielding of the matrix takes place. In this yielding process these voids become elongated. As the fracture surface is approached the voids are more deformed. At high test speed, in ductile fracture, along the fracture surface a layer is formed without deformation. The thickness of this layer is 10-100 mu m. This layer without deformation indicates that during deformation relaxation of the matrix material in this layer has taken place. With the formation of the relaxation layer the impact energy increases. The relaxation layer has thus a blunting effect. If a blend with large EPDM particles (1.6 mu m) is deformed, in the rubber particles now several cavities are formed and these cavities are positioned near the interface with the matrix. It can be expected that it is an advantage to have several small cavities instead of one large cavity. The polypropylene matrix was found to deform by a shear yielding mechanism and multiple crazing was not observed. (C) 1999 Elsevier Science Ltd. All rights reserved.