A Method of Measuring the Critical Shear Stress of Molten High-Density Polyethylene (HDPE) and the Relationship between the Critical Shear Stress and Molecular Weight or Molecular Weight Distribution

Abstract

The flow curve of molten high-density polyethylene (HDPE) has the discontinuous point characterized by the critical shear stress. In this paper we propose a method for measuring the critical shear stress and discusse the relationship between the critical shear stress and molecular weight or molecular weight distribution of HDPE.During the experiment at a constant shear rate, the pressure increases with maximum peak in the early stage of the extrusion. This peak appeares at initial flow and the critical shear stress is calculated from the peak. The critical shear stress decreases when increasing the molecular weight and molecular weight distribution. It is clarified by GPC measurements that molecular weight of strand surface is smaller than that of the whole strand and this tendency becomes remarkable in accordance with decreasing the critical shear stress. Therefore it is concluded that the decrease of the critical shear stress is caused by the low molecular weight ingredient at the surface.