EXTRUDATE SWELL BEHAVIOR OF POLYETHYLENES - CAPILLARY-FLOW, WALL SLIP, ENTRY EXIT EFFECTS AND LOW-TEMPERATURE ANOMALIES/

This paper describes a comprehensive experimental study of the extruda te swell behavior of high-density (HDPE) and linear low-density polyet hylenes. The contributions from bulk melt elasticity, entry extensiona l flow, and exit boundary discontinuity are each explicitly investigat ed as a function of molecular weight and its distribution. The elusive apparent molecular weigh: dependence of the transient extrudate swell ratio (ESR) is elucidated to arise from the different molecular relax ation rates. A master curve is obtained at a fixed stress for the time -dependent ESR of different molecular weights when represented in term s of the rescaled time, t/tau, where t is the actual time allowed for growth of ESR and tau is related to the overall molecular relaxation t ime of each HDPE. The intricate influence of the exit boundary discont inuity on the extrudate swell is illustrated on the basis of the recen tly accumulated precise knowledge of wall slip behavior of linear poly ethylenes (LPE). Wall slip induced by fluoropolymer coatings localized at the die exit wall is demonstrated to reduce the ESR of LPE under a ll stresses below an interfacial stick-slip transition. The entry effe ct is characterized in two ways: by comparing the ESR values below and above the stick-slip transition of the HDPE at the critical stress, a nd by comparing the ESR from a long die of aspect ratio L/D = 15 with that from an orifice die of LID = 1/4. The HDPE samples are found to e xhibit an anomalously larger ESR above a critical stress at low temper atures (from 150 to 160 degrees C). (C) 1998 The Society of Rheology.