Thermomechanical degradation in the preparation of polyethylene blends Ibnelwaleed A. Hussein a , Kam Ho b , Shivendra K. Goyal b , Elizabeth Karbashewski b , Michael C. Williams a, * a Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Canada T6G 2G6 b Nova Chemicals Ltd., 2928-16th St. NE, Calgary, AB, Canada T2E 7K7 Received 31 August 1999; received in revised form 29 November 1999; accepted 20 December 1999 Abstract 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 poly- ethylenes was occurring during the ``conditioning'' process and, if so, to characterize its nature and to ®nd ways to prevent the degradation. There was no signi®cant 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  C. Samples were characterized by dynamic viscosity ( 0 ) 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 dierent techniques combine to explain: the modi®cations 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 metallo- cene LLDPE mixed with LDPE at dierent temperatures showed dierent levels of degradation depending on blender conditions. Enhancement of  0 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. # 2000 Elsevier Sci- ence Ltd. All rights reserved. Keywords: Degradation; Polyethylene; Melt blender; Cross-linking; Viscosity enhancement 1. Introduction The conditions for melt processing of polymers, namely high temperature, the presence of oxygen, and substantial mechanical stresses, can cause chemical reactions to occur. Even a small extent of reaction can have an enormous eect on the physical properties of the polymer [1]. The buildup or breakdown of polymer chains can considerably in¯uence the liquid and solid- state properties of the polymer. Degradation is often a complex process involving combinations of dierent mechanisms [2,3]. The rates of oxidation of various polyole®ns such as linear poly- ethylene (HDPE), LLDPE, and polypropylene depend on details of their chemical structure. The susceptibility to oxidation increases with the increase in the number of branch points. At those points, weakly bound hydro- gens are likely points for initiation of oxidative degra- dation, and consequently LLDPE is less stable to oxidation than is HDPE [2] (see Fig. 108 of Ref. [2]). The extent to which hydrogen at branch sites in LDPE promotes oxidation, however, decreases at higher tem- peratures [4]. These dierences in resistance to oxidation are re¯ected in the amounts of stabilizers required to provide protection to these polymers. In general, LDPE needs less antioxidant than LLDPE under the same conditions and HDPE needs very little. The mechanisms of polyethylene oxidation have been reviewed and described as free radical chain reactions [5,6]. Various analytical techniques have been employed to detect and quantify degradation, but among those dierential scanning calorimetry (DSC) may not be useful. DSC is not expected to show any signi®cant 0141-3910/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(00)00022-7 Polymer Degradation and Stability 68 (2000) 381±392 * Corresponding author. Tel.: +1-780-492-3962; fax: +1-780-492- 2881.