Rheological Analysis of the Degradation of HDPE During Consecutive Processing Steps and for Different Processing Conditions

Using a twin-screw extruder, HDPE has been processed six times consecutively under a range of processing conditions (changing barrel temperature, screw speed, and feed rate). After each pass, the product has been analyzed in terms of the melt flow index (MFI) and G(C)(omega(C)), the crossover point of the viscoelastic moduli as a function of the angular velocity at which it occurs. MFI data show changes in the structure of the HDPE after each processing step, but this information is limited in quality and quantity. G(C) data show the mechanism for degradation (side-chain branching and chain scission) and allow us to track relative changes in mean molecular weight (MMW) and molecular weight distribution (MWD). MMW and MWD both increase as a result of continued reprocessing. The apparent changes in MWD are substantial indicating significant chain scission initially, accompanied and followed during subsequent processing by a combination of side-chain branching and further chain scission. A relative measure of the polydispersity index (PI) of the melt is calculated and the PI increases as the HDPE is further reprocessed. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 639-648, 2009