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Rheological change by shear history is known as “shear modification” for long chain branch polymers. In most reports, low density polyethylene (LDPE) samples were studied. It was said that melt flow index increased, swell ratio or elongational viscosity decreased with shear history in spite of no change of the original molecular characteristics. Rheological properties were gradually recovered by heat treatment when the samples were annealed in molten quiescent state, or they were fully reverted by solution treatment when the samples were re-precipitated from solution in hot xylene. Hanson mentioned that shear modification depressed environmental stress crack resistance of LDPE, while it was recovered by solution-treatment. Munstedt reported similar changes in steady state elongational viscosity of LDPE samples, though uniaxial elongation seemed to be more effective in creating the viscosity decrease than simple shear. Yamaguchi et al. discussed the role of elongational flow to the modification by comparing rheological properties of LDPE samples and various kinds of conventional processing machines such as a two-roll mill, an internal mixer and a twinscrew extruder with two types of screw configurations. In this study, we studied the rheological properties of high density polyethylene (HDPE) samples which were polymerized by a standard Phillips Cr/silica catalyst and prepared by several kneading or extruding machines and their combinations. We found that the rheological behaviour of the shear modified HDPE was dramatically recovered by the tworoll milling. We considered the key points which affected the rheological changes.

Two Kinds of Flow Modification in HDPE Polymerized by Standard Phillips Cr/Silica Catalyst