The Influence of Copolymerization Condition on Rheology, Morphology and Thermal Behavior of Polypropylene Heterophasic Copolymers

In this work, the polypropylene impact copolymers were synthesized by a modified sequential polymerization process. The copolymerization of ethylene and propylene was carried out between two homopolymerization stages at two different pressures and temperatures and the rheology, morphology and thermal properties of reactor alloys were studied. It is found that the ethylene propylene rubber (EPR) content increased up to 32wt% by increasing the copolymerization time to 20min. At a fixed copolymerization time of 10min, the addition of 50ppm hydrogen (H-2), increased the EPR content from 9.7 to 12.8wt%. By doubling H-2 concentration, no considerable change in EPR wt% was observed. It is found that the zero shear viscosity of the alloys is significantly under the influence of EPR wt%, not the molecular weight of matrix. The molecular weight of PP matrices determined by rheological data, mildly decreased from 463000 to 458000g/mol by increasing the copolymerization time from 10min to 15min. At high copolymerization time/high H-2 concentration, a melting peak in the differential scanning calorimetry test around 165 degrees C for isotactic PP and also an endothermic peak around 127 degrees C for the block copolymer with long ethylene segments, is observed. The study of interfacial strength by theoretical emulsion models showed that 15min copolymerization time is optimum considering EPR wt%.