Melt flow instabilities in polyethylene resins

The melt flow instabilities of four polyethylene (PE) pipe resins were investigated and critically compared. By comparing the flow curve obtained by capillary rheology measurements with the linear viscoelastic (LVE) data, the apparent slip was seen in the four polyethylenes. It was proposed that the high content of small chains could cause the significant wall slip, but the slip was not the origin mechanism of sharkskin melt fracture. The bimodal resins that had higher content of low molecular weight component first showed a matte surface at lower stresses. The matte surface remained until the extrudate converted to a wavy appearance as shear rates were increased. It was observed that the onset stress of sharkskin melt fracture was almost proportional to the plateau modulus. Furthermore, the chain relaxation time of the resins also influences the surface melt fracture behavior of the resin.