Studying single-pass degradation of a high-density polyethylene in an injection molding process

This paper explores the potential for degradation of a Ziegler-Natta polymerized high-density polyethylene (HDPE) that may result from single-pass processing through a 55-tonne injection molding machine. The conducted experiments examined the influence of processing temperature (190-270 °C), shear rate (4000-7500 s−1), type of antioxidant, and concentration of antioxidant, on changes to the polymer structure. The extent of degradation was primarily evaluated using rheological measurements (i.e. melt index and zero-shear viscosity) which proved more sensitive to the small changes in the polymer structure rather than GPC and spectroscopic (ATR-FTIR and 1H NMR) techniques. Characterization of the molded samples showed that the cross-linking and potentially branching occurred with increased processing temperature, while the factors of injection flow rate and antioxidant concentration had no discernable influence on the extent of degradation. The inclusion of a radical scavenger in the stabilization package was found to be important, particularly towards the higher end of processing temperatures, in order to inhibit degradation reactions.