Effect of dynamic crosslinking on tensile yield behavior of polypropylene/ethylene-propylene-diene rubber blends

Tensile yield behavior of the blends of polypropylene (PP) with ethylene-pr opylene-diene rubber (EPDM) is studied in blend composition range 0-40 wt % EPDM rubber. These blends were prepared in a laboratory internal mixer by simultaneous blending and dynamic vulcanization. Vulcanization was performe d with dimethylol phenolic resin. For comparison, unvulcanized PP/EPDM blen ds were also prepared. In comparison to the unvulcanized blends, dynamicall y vulcanized blends showed higher yield stress and modulus. The increase of interfacial adhesion caused by production of three-dimensional network is considered to be the most important factor in the improvement. It permits t he interaction of the stress concentrate zone developed at the rubber parti cles and causes shear yielding of the PP matrix. Systematic changes with va rying blend composition were found in stress-strain behavior in the yield r egion, viz., in yield stress, yield strain, width of yield peak, and work o f yield. Analysis of yield stress data on the basis of the various expressi ons of first power and two-thirds power laws of blend compositions dependen ce and the porosity model led to consistent results from all expression abo ut the variation of stress concentration effect in both unvulcanized and vu lcanized blend systems. Shapes and sizes of dispersed rubber phase (EPDM) d omains at various blend compositions were studied by scanning electron micr oscopy. (C) 2000 John Wiley & Sons, Inc.