Uniaxial drawing of polytetrafluoroethylene virgin powder by extrusion plus cold tensile draw

Abstract

Polytetrafluoroethylene (PTFE) virgin powder was ultradrawn uniaxially by a two-stage draw. A film, compression molded from powder below the melting temperature (T m ), was initially solid-state coextruded to an extrudate draw ratio (EDR) of 6-20 at an established optimum extrusion temperature of 325°C, near the T m of 335°C. These extrudates from first draw were found to exhibit the highest ductility at 45-100°C for the second-stage tensile draw, depending on the initial EDR and draw rate. The maximum achievable total draw ratio (DR t x ) was 36-48. Such high ductility of PTFE, far below the T g (125°C) and T is in sharp contrast to other crystalline polymers that generally exhibit the highest ductility above their T g and near T The unusual draw characteristics of PTFE was ascribed to the existence of the reversible crystal/crystal transitions around room temperature and the low intermolecular force of this polymer, which leads to a rapid decrease in tensile strength with temperature. The structure and tensile properties of drawn products were sensitive to the initial EDR, although this had no significant influence on DR t,max . The most efficient and highest draw was achieved by the second-stage tensile draw of an extrudate with the highest EDR 20 at 100°C, as evaluated by the morphological and tensile properties as a function of DR t . The efficiency of draw for the cold tensile draw at 100°C was a little lower than that for solid-state coextrusion near the T m . However, significantly higher tensile modulus and strength along the fiber axis at 24°C of 60 ± 2 GPa and 380 ± 20 MPa, respectively, were achieved by the two-stage draw, because the DR t,max was remarkably higher for this technique than for solid-state coextrusion (DR t,max = 48 vs. 25). The increase in the crystallite size along the fiber axis (D 0015 ), determined by X-ray diffraction, is found to be a useful measure for the development of the morphological continuity along the fiber axis of drawn products.