The Viscoelastic Properties of Melts of Polypropylene and its Blends with High-Density Polyethylene
Using an indigenous rheometer set up for the lower range and the Brabender Plasticorder for the higher range, the rheological studies for polymer melts through circular dies of varying (Length/ Diameter) ratio were made in the shear rate range of 20–4000 sec-1. The samples used were three of polypropylene (MFI = 2.9, 0.62 and 0.3) and one of high-density polyethylene (MFI = 0.3) and blends of PP with hdPE. The blending characteristics of PP with hdPE were studied by means of a Brabender roller mixer where the concentration of hdPE in PP was varied from 0–20%. The torque absorption passes through a maximum as the concentration of hdPE varies in the blends. Viscosity average molecular weight and molecular weight distribution were determined by means of solution viscometry and melt flow indexer, respectively. The rheological measurements made were stress-strain behaviour and die swell characteristics of the virgin polymers, polymer blends and degraded polymers obtained after repeated extrusion. These were the interpreted to identify the effect of mol.wt. and mol.wt.distribution on the rheological properties of the polymer melts in the light of existing molecular viscoelastic theories. It was found that the polymer melt elasticity is greater for polymers having a broad mol.wt. distribution than the samples having higher average mol.wt. but lower mol.wt. distribution. The polymer degradation resulted in reduced average mol.wt. and mol.wt. distribution and also reduced melt elasticity.