Polypropylene/ethylene- co-propylene blends: influence of molecular structure and composition of EPR on melt rheology, morphology and impact properties of injection-moulded samples

Abstract

Polypropylene (PP) based blends containing as second non-crystallizable component commercial ethylene-propylene copolymers (EPR) having different C 3 contents (wt/wt) and molecular-weight distributions ( MWD), and, for fixed C 3 content and MWD almost constant, different average molecular mass, were investigated. The study was undertaken to establish the influence of EPR molecular structure and composition on the melt rheology, on the mode and state of dispersion of the minor component in the melt as well as in the solid state and on the impact properties of injection-moulded samples. An attempt to correlate the molecular structure of EPR with rheological behaviour, size of rubbery particles and impact resistance was also accomplished. It was found that the values of the blend zero-shear viscosity ( η o) and the α(blend) parameter, obtained by applying the Cross equation, decrease with increasing dispersion coarseness of the minor component according to the results already obtained while studying the different incompatible blend system made by polyamide-6 and ethylene-vinyl acetate copolymers. The size and size distribution of EPR domains are determined by the value of the phase viscosity ratio (μ) irrespective of C 3 content along the EPR chain. The trend of the number-average particle diameter ( D ̄ n ) versus log μ showed that D ̄ n decreases with decreasing log μ, in agreement with expectation on the basis of Taylor-Tomotika theory. The range of particle size effective for PP toughening was dependent on test temperature; for a temperature higher than EPR T g and close to PP T g, such range is narrower than that effective at room temperature.