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

University of British Columbia

Resin processability depends heavily on its rheological properties. The molecular structure of the resin, in turn, influences its rheological behavior. In this work, experiments were conducted using capillary and extensional rheometers, a melt indexer and a blow molder unit to determine the rheological properties and processability of high density polyethylene blow molding resins. Twenty four commercial resins were analyzed in terms of their shear flow properties, extensional flow properties, extrudate swell characteristics, and melt strength. The studied samples had varying molecular weight characteristics and were produced using a variety of technologies. Using the experimental results, correlations between rheological properties and molecular structures were determined. Furthermore, to assess resin processability, pillow mold (blow molding) experiments were performed. The implications of rheology on processability (parison sag and weight swell) were then discussed. Additional experiments were also conducted to assess the usefulness of melt index (MI), stress exponent (S.Ex.) and melt flow ratio (MFR) in characterizing rheological properties. It was found that shear viscosity is technology dependent and that it is influenced by the weight average molecular weight (Mw) and polydispersity index (PI). Increasing Mw was found to increase the shear viscosity, while increasing PI by increasing the concentration of smaller molecules increases the tendency of the resin to shear thin. The extensional viscosity was also affected by Mw in the same manner. The influence of PI on extensional viscosity, however, was not apparent. In order to relate the melt strength and temperature sensitivity of shear viscosity to molecular parameters, resins had to be grouped according to the polydispersity index 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. With regard to extrudate swell, it was found that the Z-average molecular weight (Mz) and PI are useful for determining the sensitivity of the swell to changes in shear rate. Extrudate swell behavior and melt strength are important parameters to be considered during parison formation, as observed during blow molding experiments. Finally, MI, S.Ex., and MFR were found to be technology dependent and are useful only for resin comparisons.

Thesis/Dissertation

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2009-05-25

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http://hdl.handle.net/2429/8116

Chemical and Biological Engineering

University of British Columbia

UBCV

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