UNIAXIAL DRAWING OF POLYTETRAFLUOROETHYLENE VIRGIN POWDER BY EXTRUSION PLUS COLD TENSILE DRAW

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 extrusio n temperature of 325 degrees C, near the T-m of 335 degrees C. These e xtrudates from first draw were found to exhibit the highest ductility at 45-100 degrees C for the second-stage tensile draw, depending on th e initial EDR and draw rate. The maximum achievable total draw ratio ( DRt,max) was 36-48. Such high ductility of PTFE, far below the T-g (12 5 degrees C) and T-m, is in sharp contrast to other crystalline polyme rs that generally exhibit the highest ductility above their T-g and ne ar T-m. The unusual draw characteristics of PTFE was ascribed to the e xistence of the reversible crystal/crystal transitions around room tem perature and the low intermolecular force of this polymer, which leads to a rapid decrease in tensile strength with temperature. The structu re and tensile properties of drawn products were sensitive to the init ial EDR, although this had no significant influence on DRt,max. The mo st efficient and highest draw was achieved by the second-stage tensile draw of an extrudate with the highest EDR 20 at 100 degrees C, as eva luated by the morphological and tensile properties as a function of DR t. The efficiency of draw for the cold tensile draw at 100 degrees C w as a little lower than that for solid-state coextrusion near the T-m. However, significantly higher tensile modulus and strength along the f iber axis at 24 degrees C of 60 +/- 2 GPa and 380 +/- 20 MPa, respecti vely, were achieved by the two-stage draw, because the DRt,max was rem arkably higher for this technique than for solid-state coextrusion (DR t,max = 48 vs. 25). The increase in the crystallite size along the fib er axis (D-0015), determined by X-ray diffraction, is found to be a us eful measure for the development of the morphological continuity along the fiber axis of drawn products. (C) 1998 John Wiley & Sons, Inc.