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

抄録

Modifications of rheological and mechanical properties by various kneading methods and histories were studied for high density polyethylene (HDPE) polymerized by a standard Phillips Cr/silica catalyst. The HDPE exhibits long-time relaxation in linear viscoelasticity and apparent strain-hardening in elongational viscosity due to long chain branches. These rheological features were depressed by single-screw extrusion. This phenomenon is known as “shear modification”. However, we found that the depressed property could be recovered quickly and easily by two-roll milling. This probably means that different modification being contrary to shear modification exists. The sample processed by an internal batch mixer with a low fill factor showed similar properties to the one kneaded by the two-roll mill, however, another sample processed with a high fill factor was close to the one prepared by the single-screw extruder. It is likely that the ratio influences the balance of convergent flow and shear flow which are generated between two counter-rotating blades and between each blade and the casing, respectively. Since convergent flow contains elongational flow, the modification which should be distinguished from shear modification may be called “elongational modification”. Furthermore, stress crack resistance changed as well as rheological properties in spite of no change of Charpy and tensile impact strength.