Melt Drawability of Ultrahigh Molecular Weight Polyethylene

Abstract

Some ultrahigh molecular weight polyethylenes have been shown to be highly ductile but only under specific conditions. When the initial starting state is reactor powder, the ductility is presumed to be due to the low initial entanglement. Samples were prepared for such draw by compression molding of reactor powder that had been held at different temperatures in the melt for only brief periods (5 min), followed by cooling to room temperature. These films were then quickly heated and drawn at a constant temperature of 140−155 °C, above the static melting temperature (T m ≈ 135 °C). The stress/strain behavior from this state was found to be significantly influenced by both the prior melt temperature and the time needed for preparation of the initial morphologies. X-ray diffraction showed that the strain-induced crystallization from the melt had occurred during draw above a critical strain, corresponding to the draw ratio of ∼10. Differential scanning calorimetric data clarified that the rate of this crystallization is accelerated by the higher draw stress for the samples prepared at the higher prior-melt temperature. Combination of X-ray and calorimetric results also indicated that the samples drawn from the melt contain two kinds of crystals, i.e., highly chain-extended and oriented crystals having a higher T m and chain-folded ones having a lower T m. With increasing draw ratio, the relative amount of the former crystals, formed on draw, gradually increased. The efficiency of draw, evaluated from tensile tests and thermal shrinkage measurements, was also interpreted from the differences of the prior-melt conditions, which increase entanglement. The samples could be successfully drawn from melt up to a maximum draw ratio of 45−50 at the optimum temperature of 150 °C. Such highly drawn films exhibited a tensile modulus of 55 and a strength of 0.95 GPa, respectively. For the solid-state drawing below T m, the stress/strain behavior of the samples molded at different temperatures were all identical and had a low draw ratio of ∼6. This suggests a difference in entanglements as they affect draw above and below the static melting temperature.

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

“…As reported in a previous article, 6 the drawing behavior of MGC films from its molten state was significantly affected by several factors, including draw temperature, T d , and the initial strain rate, ␥ . The influence of these parameters on draw from the melts of Ziegler-and metallocene samples were evaluated from their stress/strain curves.…”

“…Here, Meta-40 could be drawn up to a strain around 2,500%. However, no increase in draw stress indicates that the strain-induced crystallization during melt-draw 6 did not occur at such a higher T d , as confirmed by WAXD patterns.…”

“…This indicates the development of crystalline size with draw, due to some disentanglements during elongation. 6 In contrast, DR dependence of the low temperature melting peaks showed different trends for both UHMW-PE samples drawn from their melts. A single endothermic peak alone, corresponding to the high temperature melting around 142°C, was finally obtained around its DR max of 12 for Meta-40 (see Fig.…”

“…As reported, 6 UHMW-PE melts are also drawable, due to its high melt viscosity which can transmit the applied draw stress within the sample. In contrast, normal MW-PE can not be effectively drawn above the T m (melting temperature).…”

“…The results also test the influence of differences in original reactor powder as reported in our related study. 6 …”