Isotactic polypropylene/ethylene-co-propylene blends: effects of the copolymer microstructure and content on rheology, morphology and properties of injection moulded samples

Melt rheology, phase morphology and properties of (60/40) isotactic polypropylene/ethylene-co-propylene (iPP/EPR) blends containing EPR copolymer synthesized by means of a titanium based catalyst with very high stereospecific activity (EPRTi) were compared to that of (60/40) iPP/EPR blends containing EPR copolymer synthesized by using a traditional vanadium based catalyst (EPRV). It was found that both the melts are to be classified 'negative deviation blends' and that for such a composition the copolymer represents the dispersed phase. SEM investigations showed in fact that the EPR phase segregate in to irregular shaped domains localized in the core of the bars, the dispersion coarseness increasing with an increase in the ratio between the EPR viscosity and iPP viscosity. At low temperatures the iPP impact strength was improved, comparatively better properties being shown by the blends containing the EPRV copolymer. The different behaviour of EPRTi and EPRV phases as impact modifiers was ascribed to their both different dispersion degree and microstructure. For test temperatures higher than EPR T-g the ethylenic crystallinity exhibited by the EPRTi phase was considered to induce toughening deterioration by decreasing tie molecules density in the EPRTi domains, notwithstanding the beneficial effect of the ethylenic build-up. The reduction in iPP crystallinity index (X-c) value found for iPP/EPRTi blends indicated a strong interference of the crystallization process of EPRTi ethylenic sequences with the iPP crystallization process. SAXS investigations revealed in addition that the iPP/EPRTi materials are characterized by an amorphous interlamellar layer (L-a) higher than that shown by plain iPP and iPP phase crystallized in presence of EPRV copolymer. Taking also into account results obtained studying blends containing lower content of such EPR phases composition effects were investigated. WAXS studies showed that with increasing copolymer content the ratio between the apparent crystal size of iPP phase grown in a direction perpendicular to the (110) crystallographic plane [D-(110)] and that grown in direction perpendicular to the (040) crystallographic plane [D-(040)] increases. Such an effect was related to an increased iPP nucleation density according to DSC results. (C) 1999 Elsevier Science Ltd. All rights reserved.