Superdrawing of polytetrafluoroethylene virgin powder above the static melting temperature

A new two-stage draw technique was successfully applied to the super-drawing of polytetrafluoroethylene (PTFE) virgin powder. A film, compression-molded from powder below the melting temperature (T-m = 335 degreesC), was initially solid-state coextruded to an extrusion draw ratio (EDR) of 6-20 at 325 degreesC, about 10 degreesC below the Tm. These extrudates from the first-stage draw were further drawn by a second-stage pin draw in the temperature (T-d) range of 300-370 degreesC that covers the static T-m. The maximum achievable total draw ratio was similar to 60 at a T-d = 300 degreesC and increased rapidly with increasing Td, reaching a maximum of 100-160 at a temperature window between 340 and 360 degreesC, depending on the initial EDRs. At yet higher T-d'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 (x(c) less than or equal to 87%) and showed high chain orientation (f(c) less than or equal to 0.997) and a large crystallite size along the chain axis (D-0015 less than or equal to 160 nm). The molecular draw ratio, estimated from the entropic shrinkage above the T-m, 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 degreesC. These tensile properties are among the highest ever reported on oriented PTFE. (C) 2001 John Wiley & Sons, Inc.