Melt fracture and wall slip of metallocene-catalyzed bimodal polyethylenes in capillary flow

Abstract

The melt fracture and wall slip behaviors of three metallocene-catalyzed bimodal polyethylene (PE) resins and one unimodal PE resin were investigated and critically compared. By comparing the flow curve observed by capillary rheology measurements with the linear viscoelastic (LVE) data, it was observed that the apparent slip increased with content of low molecular weight (MW) component. The bimodal resins that had higher content of low MW component first showed a matte surface at lower stresses. The matte surface remained until the extrudate converted to a wavy appearance before the stick-slip transition as shear rates were increased. The plateau modulus estimated from the LVE data decreased with the increase in low MW content. It was observed that the onset stress of the matte surface (or sharkskin) melt fracture was proportional to the plateau modulus. It was proposed that the distinct separation of the two modes of the bimodal PE resin and the high content of small chains could cause the significant wall slip and the unusual melt fracture behaviors.