INFLUENCE OF RUBBER PARTICLE-SIZE ON MECHANICAL-PROPERTIES OF POLYPROPYLENE-SEBS BLENDS

Isotactic polypropylene blends with 0-20 vol % thermoplastic elastomer s were prepared to study the influence of elastomer particle size on m echanical properties. Polystyrene-bloch-poly( ethene-co-but-l-ene)-blo ck-polystyrene (SEBS) was used as thermoplastic elastomer. SEES partic le size, determined by means of transmission electron and atomic force microscopy, was varied by using polypropylene and SEES of different m olecular weight. With increasing polypropylene molecular weight and, c onsequently, melt viscosity and decreasing SEES molecular weight, SEES particle size decreases. Impact strength of pure polypropylene is alm ost independent of molecular weight, whereas impact strength of polypr opylene blends increases strongly with increasing polypropylene molecu lar weight. The observed sharp brittle-tough transition is caused by m icromechanical processes, mostly shear yielding, especially occurring below a critical interparticle distance. The interparticle distance is decreasing with decreasing SEES particle size and increasing volume f raction. If the polypropylene matrix ligament between the SEES particl es is thinner than 0.27 mu m, the blends become ductile. Stiffness and yield stress of polypropylene and polypropylene blends increase with increasing polypropylene molecular weight in the same extent, and are consequently only dependent on matrix properties and not on SEES parti cle size. (C) 1998 John Wiley & Sons, Inc.