Mechanical properties and morphology of isotactic polypropylene/ethylene-propylene copolymer blends

Abstract

Mechanical tensile and dynamic mechanical tests have been performed at different temperatures on specimens of polypropylene (iPP)/ethylene-propylene copolymer (EPR) blends, obtained under various crystallization conditions. The initial morphology was observed by optical transmission and scanning electron microscopy. Furthermore the influence of the rubber composition on the neck formation was analysed and the tensile mechanical properties of the fibres evaluated as well. Up to 10% of EPR in the blend, the morphology of iPP was only slightly changed. At higher EPR content, a decrease in spherulite size was observed. The curves of modulus E versus drawing temperature T d exhibited a change in the drawing mechanism at a temperature T i. This transition temperature increased on enhancing the crystallization temperature T c and is almost independent of the EPR content. The change in the flowing mechanism was also visible by direct inspection of the specimen fibres which were opaque for T d < T i and transparent for T d > T i. These features were indicative of a dependence of the drawing mechanisms on the initial morphology and on the testing temperature. The fibre modulus depended strongly on the drawing temperature T d but not on the crystallization temperature T c. This suggested that an almost complete cancellation of the initial morphology occurred after fibre formation. The effect of the rubber addition was a decrease of the Young's modulus of the fibres in all the blends. A tentative interpretation of the above illustrated features has been provided.