The effect of in-process ethylene incorporation on the evolution of particle morphology and molecular characteristics of commercial heterophasic ethylene propylene copolymers (HEPCs)

Abstract

The development of particle morphology, composition, crystallinity and microstructure were investigated for commercial heterophasic (high impact) propylene–ethylene copolymers (HEPCs). Samples from gas phase polymerization processes were collected at set intervals after the introduction of ethylene into the second reactor. TREF profiles show pronounced differences between these samples where the amount of the lower crystallinity fractions (30–80°C), increased with increasing ethylene content, with a corresponding decrease in the relative amount of SEM analysis of the external surfaces of all the samples has indicated very little porosity, whereas the internal porosity has been maintained (as indicated by FE-SEM). FE-SEM images have also shown the presence of thin threads of copolymer between polypropylene globules for the lowest ethylene content sample and this detail seemingly disappears with higher ethylene contents as larger parts of the surface are covered through copolymer pooling effects. Solution 13C NMR has shown a disproportionate microstructural development where continuous ethylene (E) sequences seem to increase at a higher rate than random-like mixed ethylene (E) and propylene (P) sequences. The overall length of continuous E sequences is however limited by the total ethylene content. Solid state NMR has shown that the sample with the highest ethylene content has at least some ethylene monomers incorporated in rigid amorphous areas within the polymer. From GPC analysis, it was observed that molecular weight and -distribution increased with ethylene content. SEC–FTIR results for bulk samples have shown that both ethylene content and crystallinity is prevalent in the high molecular weight area of the curves, whereas the same properties for propylene were found in the lower molecular weight portion.