Ultradrawing of ultrahigh molecular weight polyethylene reactor powders prepared by highly active catalyst system

Abstract

Drawability of ultrahigh molecular weight polyethylene (UHMW-PE) reactor powders, prepared by using a highly active Ziegler type catalyst, has been studied as functions of catalyst activity, polymerization temperature, monomer pressure and sample molecular weight. Compacted powder films of UHMW-PE, prepared in a 1000-fold range of polymerization rates, were drawn by a two-stage technique; i.e. the initial solid-state coextrusion at 110°C followed by tensile drawing at 135°C, some 5°C below the melting point. It was found that an appropriate range of molecular weights (Mv=1.6–3.0×106) existed for achieving a high draw. The UHMW-PE prepared even at a high polymerization rate of 43,600 (g of PE)/(mmol of transition metal,h) could be drawn to draw ratios (DR) over 50, producing tapes with tensile modulus and strength over 100 and 1.0GPa, respectively. This level of catalyst activity is sufficient for the modern industrial production of UHMW-PE. The reduction of polymerization rate, by lowering monomer pressure and polymerization temperature, produced UHMW-PE with improved ductility. Furthermore, at the same polymerization rate, the ductility of reactor powder was improved more significantly by lowering the former parameter than the latter, reflecting the characteristic effect of each of these polymerization variables on the morphology formation during polymerization. Thus, the reactor powders prepared at 40–60°C and monomer pressure of 1atm could be drawn to DR's of 100–125. Such highly drawn products exhibited tensile moduli and strength up to 144 and 1.6GPa, respectively.