Superdrawing of polytetrafluoroethylene virgin powder above the static melting temperature

Abstract

A new two-stage draw technique was successfully applied to the superdrawing of polytetrafluoroethylene (PTFE) virgin powder. A film, compression-molded from powder below the melting temperature (Tm = 335 °C), was initially solid-state coextruded to an extrusion draw ratio (EDR) of 6–20 at 325 °C, about 10 °C below the Tm. These extrudates from the first-stage draw were further drawn by a second-stage pin draw in the temperature (Td) range of 300–370 °C that covers the static Tm. The maximum achievable total draw ratio was ∼60 at a Td = 300 °C and increased rapidly with increasing Td, reaching a maximum of 100–160 at a temperature window between 340 and 360 °C, depending on the initial EDRs. At yet higher Td's, the ductility was lost as a result of melting. The high ductility of the PTFE extrudates at such high temperatures was ascribed to the improvement of interfacial adhesion and bonding between the deformed powder particles upon the first-stage extrusion combined with the rapid heating of only a portion of the extrudate followed by the elongation at a high rate. The highly drawn fibers were highly crystalline (χc ≤ 87%) and showed high chain orientation (fc ≤ 0.997) and a large crystallite size along the chain axis (D0015 ≤ 160 nm). The molecular draw ratio, estimated from the entropic shrinkage above the Tm, was close to the macroscopic deformation ratio independently of the initial EDRs. These results indicate that the draw was highly efficient in terms of chain extension, orientation, and crystallization. Thus, the maximum tensile modulus and strength achieved in this work were 102 ± 5 and 1.4 ± 0.2 GPa, respectively, at 24 °C. These tensile properties are among the highest ever reported on oriented PTFE. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1995–2004, 2001