FAILURE MECHANICS IN ELASTOMER TOUGHENED POLYPROPYLENE

Dynamic mechanical, tensile and impact properties and modes of failure of binary blends of polypropylene and ethylene-propylene random copolymer rubber (PP/EPR) were studied over a wide interval of EPR volume fractions. The effect of grafting maleic anhydride on both PP and EPR on the mechanical properties was also investigated. The effects of elastomer inclusions on the failure processes were analysed in terms of critical strain energy release rate and Charpy notched impact strength (NIS). Phase morphology of the blends was examined using dynamic mechanical spectroscopy and SEM. PP remains as a continuous phase even as a minor component. This is reflected by the experimental values of elastic moduli significantly higher than those predicted using the Kerner model. The plane strain and plane stress limits of the critical strain energy release rate for initiation of cracks in the blends were the same as those for neat PP. On the other hand, the presence of EPR decreased the yield strength of the material due to the overlapping stress fields of adjacent particles, thereby increasing significantly the critical strain energy release rate for crack propagation. This was reflected by an increase in NIS. The results are explained in terms of a mixed mode of fracture with macroscopic transition from the state of plane strain to plane stress.