Study on Viscoelastic Behavior and Extrusion Blow Molding Processability of PP/PE Blends

Abstract

Abstract In this study, extrusion blow molding processability of polypropylene/polyethylene (PP/PE) blends was related to their viscoelastic behavior and morphology under shear flow. The propylene block-copolymer (PP-B) and blends comprising the PP-B and the linear low-density polyethylene (PE), weight fraction of the PE (ϕPE) was up to 20 wt.%, were mainly used for this study. Viscoelastic behavior under shear flow and swelling ratio of the extruded strands were measured by using a capillary type rheometer. Morphology of the strands was observed by TEM. In case of the PP-B, swelling ratio highly depended on shear rate. Coarse aggregated droplets were observed irrespective of shear rate under the measurement. The droplets were ethylene-propylene copolymers (EP) included in the PP-B aggregated at conversing section of the capillary. However, in case of the PP-B/PE blends, swelling ratio showed lower dependence on shear rate as the ϕPE was higher. When the shear rate was low, large droplets were observed and they were highly deformed in the flow direction. On the other hand, when shear rate was high, the droplets were smaller and showed no deformation in the flow direction because the droplets were broken up due to occurrence of high shear stress. Wall thickness of blow molded bottles produced from the PP-B/PE blends was not affected by the throughput rate. It was proportional to the die-core gap. On the other hand, wall thickness of bottles of the PP-B depended on the throughput rate and it was not proportional to the die-core gap. These results suggest that extrusion blow molding processability of polymer blends is good when the dependence of swelling ratio on shear rate is low.