Melt drawing of ultra-high molecular weight polyethylene: Comparison of Ziegler- and metallocene-catalyzed reactor powders

The drawing behavior of the ultra-high molecular weight polyethylene (UHMW- PE) melts has been studied by comparing the stress/strain curves for two ty pes of samples as polymerized using conventional Ziegler and newer metalloc ene catalyst systems. Two UHMW-PE samples, having the same viscosity averag e molecular weight of 3.3 X 10(6), but different molecular weight distribut ion, have been drawn from melt at special conditions. The sample films for drawing were prepared by compression molding of reactor powders at 180 degr ees C in the melt. Differences in the structural changes during drawing and resultant properties, ascribable to their broad or narrow molecular weight distribution, were estimated from tensile tests, SEM observations, X-ray m easurements and thermal analyses. The metallocene-catalyzed sample having n arrower molecular weight distribution, could be effectively drawn from the melt up to a maximum draw ratio (DR) of 20, significantly lower than that o btained for the Ziegler-catalyzed sample, similar to 50. The stress/ strain curves on drawing were remarkably influenced by draw conditions, including draw temperature and rate. However, the most effective draw for both was a chieved at 150 degrees C and a strain rate of 5 min-l, independent of sampl e molecular weight distribution. The efficiency of drawing, as evaluated by the resultant tensile properties as a function of DR, was higher for the m etallocene-catalyzed sample having narrower molecular weight distribution. Nevertheless, the maximum achieved tensile modulus and strength for the Zie gler sample, 50-55 and 0.90 GPa, respectively, were significantly higher th an those for the metallocene sample, 20 and 0.65 GPa, respectively, reflect ing the markedly higher drawability for the former than the latter. The str ess/strain behavior indicated that the origin of differences during drawing from the melt could be attributed to the ease of chain relaxation for the lower molecular weight chains in the melt. (C) 1999 John Wiley & Sons, Inc.