Study of the relationship between shear modification and melt fracture in extrusion of LDPE

摘要

Successive passes through an extruder can modify the melt morphology of low-density polyethylene (LDPE) by applying a shearing process. The major effects of shear modification are to decrease the elastic properties, as evidenced by the decrease in extrudate swell at the exit of a capillary and in melt viscosity at a low-frequency region. The effect of shear modification is also shown by the delay in the onset of melt fracture upon extrusion. The critical shear stress resulted from extrudate appearance, and apparent discontinuity in the flow curve of LDPE shows a similar value with polypropylene rather than those of LLDPE and HDPE. The shearing histories experienced by these materials did not result in any measurable change in molecular weight, so that the chemical modification process such as degradation and crosslinking may be ruled out. These behaviors were also confirmed to the fact that the extrudate swell was fully reversible by annealing in a molten state. The effects of shear modification on rheological properties could be explained by the changes in melt morphology owing to the disentanglement of temporary couplings between long branches. Also, a reduction in melt elasticity by shear modification of LDPE can be used as an effective tool to improve the surface roughness of extrudates in the cable-making process. (C) 1998 John Wiley & Sons, Inc. [References: 16]