Effects of molecular structure on the rheology and processability of blow‐molding high‐density polyethylene resins

Abstract

The influence of molecular structure on the rheology and processability of blow-molding grade high-density polyethylene (HDPE) resins is studied using capillary and extensional rheometers, a melt indexer, and a blow-molder unit. Twenty-four commercial HDPE resins were analyzed in terms of their shear and extensional flow properties, extrudate swell characteristics, and melt strength. The resins had varying molecular weight characteristics and were produced using a variety of polymerization technologies (gas, slurry, and solution phase). It was found that shear viscosity is not only influenced by the weight average molecular weight (Mw) and polydispersity index (PI), but also technology dependent, irrespective of molecular characteristics. Increasing Mw was found to increase both shear and extensional viscosity, while increasing PI by increasing the concentration of smaller molecules increases the tendency of the resin to shear thin. In relating melt strength and temperature sensitivity of shear viscosity to molecular parameters, resins had to be grouped according to ranges of PI < 8, 8. < PI < 10, and PI > 10. Moreover, it was possible to relate melt strength to the Hencky strain obtained from creep experiments and to the melt index of the resins. Finally, it was found that extrudate swell behavior and melt strength are important parameters to be considered during parison formation, as observed during blow-molding experiments. © 2001 John Wiley & Sons, Inc. Adv Polym Techn 20: 1–13, 2001