Differences in melt extrusion behaviour between ultra-high molecular weight polyethylene powder and a dried gel

Abstract

Ultra-high molecular weight polyethylene (UHMWPE) is known to be unextrudable at least under conventional conditions of capillary extrusion [1]. However, it was observed as long ago as 1974 [2] that UHMWPE melts would extrude or extend (draw) smoothly in a " t empera tu re window!' between 145 and 155 °C. More recently, Waddon and Keller [3], using a range of high molecular weight (Mw) polyethylenes (Mw = 3.5 x 105 to 106) observed a narrow temperature window of 150-152 °C within the above-defined broader window of smooth extrudability where the extrusion pressure, and hence the melt viscosity, had a pronounced minimum. This effect was attributed to a transient orientation-induced melt---> mesophase 4o transformation. During continuing studies of this effect [4, 5], amongst others, significant variability was noticed in the highest Mw (UHMWPE) material [5] dependent on its hstory before the extrusion 30 experiment. In particular, there is a difference between the nascent powder as received and the same material after dissolution, precipitation and ,~ z0 drying (the sample to be referred to as dried gel on if_ account of its prior gel consistency). The latter is also more readily extrudable and provides better 10 reproducibility for the study of the novel pressure minimum. It is this difference that we shall report and describe in the present letter, prompted by growing interest in issues relating to the nascent morphology processabitity of UHMWPE. For the preparation of the gel sample the conventional procedure was used [6]. Briefly, about 6% (w/w) UHMWPE powder was dissolved in Dekalin containing antioxidant, at 160 °C. On cooling, a gel 24 was formed which synerized. The solvent was then squeezed out and the remaining solvent extracted in a vacuum oven at 60 °C for up to 24 h. 19 The extrusion procedure followed that described earlier [3-5], using the same Davenport extrusion Z rheometer. To recapitulate, the material was melted ~ 14 first at around 180 °C and cooled subsequently to below 149 °C while in the stationary state. This was if_ followed by extruding the melt at a constant piston speed and temperature until the pressure has 9 reached a plateau. The temperature was then raised at 0.5-1 °C rain -I while extruding, until the temperature has risen to about 172 °C. Our main ,a~2 concern now is the difference in flow behaviour between the two samples (i.e. nascent powder and the dried gel), which to the best of our knowledge has not been published elsewhere.