Thermomechanical degradation in the preparation of polyethylene blends

A Haake Rheocord 90 melt blender was used to prepare several types of commercial polyethylene samples intended for studies on blends as well as to condition the pure components. The objective was to assess whether thermomechanical degradation of polyethylenes was occurring during the "conditioning" process and, if so, to characterize its nature and to find ways to prevent the degradation. There was no significant evidence of molecular breakdown but, rather, abundant evidence for chain buildup and possible gel formation. Without mechanical stress, no degradation occurred. Melts were conditioned in the blender at temperatures in the range 190-220 degrees C. Samples were characterized by dynamic viscosity (eta') as well as by average molecular weight and MWD from GPC analysis, both before and after the blender experience. The rheological and GPC analyses were complemented by NMR; DSC, and TREF testing. The study included "conditioned" resins with and without additional antioxidants; results were compared to the properties of as-received polymers. The different techniques combine to explain: the modifications that can occur due to melt blending of polyethylenes; relationships to the polymer molecular structure; and possible means for detection and prevention of degradation. Addition of adequate amounts of antioxidant is successful in prevention of degradation. Ziegler-Natta and metallocene LLDPE mixed with LDPE at different temperatures showed different levels of degradation depending on blender conditions. Enhancement of eta' of "conditioned" resins over that of the as-received resins was the best indicator of degradation and correlated with branch content of the LLDPE, with metallocene LLDPE showing highest susceptibility to degradation. (C) 2000 Elsevier Science Ltd. All rights reserved.