The elastic behaviour during capillary extrusion of LDPE/LLDPE blend melts

摘要

A low-density polyethylene (LDPE) and a linear low-density polyethylene (LLDPE), with different melt flow index, were melt-blended in a screw extruder. The elastic behaviour, such as end pressure losses and die-swell, and the factors affecting it for the sample melts were investigated by means of a capillary rheometer at shear rates varying from 10 to 10(3) s(-1) and a temperature range of 240-260 degreesC. At lower wall shear stress (tau (w)), the entry pressure drop (DeltaP(en)) for LDPE and the blend melts increased quickly with increasing tau (w), while for the LLDPE melt the entry effect was relatively significant, and the dependence of DeltaP(en) on tau (w) was insignificant. The melt die-swell ratio (B) increased approximately linearly with increasing tau (w) and DeltaP(en), while it decreased with increase of the die length/diameter ratio and with a rise of temperature. It was found that B achieved maximum value at a blending ratio of 50:50. This phenomenon can be attributed to the amount of elastic deformation energy stored in the melt flow and its transition due to the viscoelastic difference between the two phases. (C) 2001 Elsevier Science Ltd. All rights reserved. [References: 12]