The effect of elastomer type and molding conditions on the properties of TPO blends

The effects of impact modifier type, crystallinity and molecular weight, and the injection velocity during molding on the properties of polypropylene blends (or TPOs) were examined. Two polypropylene homopolymers were melt blended at 30% modifier with a series of metallocene-catalyzed linear low density polyethylene (LLDPE) elastomers. The properties of molded TPOs exhibited a complex dependence on the polypropylene, the impact modifier, and the conditions under which the TPO was molded. Addition of impact modifiers to the higher flow polypropylene produced a 60% decrease in viscosity, but little enhancement of Izod impact strength, it increased the deflection temperature under load (DTUL), the coefficient of linear thermal expansion (CLTE) and the shear sensitivity of the viscosity in all TPOs. Increased injection velocity during molding produced a thinner surface layer(s) which decreased the tensile strength and flexural modulus of the TPOs. However, faster injection decreased both the tensile strength and flexural modulus.