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

Abstract

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°C. At lower wall shear stress ( τ w), the entry pressure drop (Δ P en) for LDPE and the blend melts increased quickly with increasing τ w, while for the LLDPE melt the entry effect was relatively significant, and the dependence of Δ P en on τ w was insignificant. The melt die-swell ratio ( B) increased approximately linearly with increasing τ w and Δ P 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.