Effects of expansion rate and sintering time on the morphology and mechanical properties of double-expanded polytetrafluoroethylene tubes

A novel manufacturing technique for expanded polytetrafluoroethylene (ePTFE) has been previously developed. Through modifications to the processing conditions by adding one additional expansion step at lower temperatures, one can produce highly compliant double-expanded PTFE (dePTFE) with enhanced mechanical properties. The superior mechanical properties of dePTFE stem from the unique presence of wavy fibrils, which can be tuned to meet the specific requirements of various applications. This study investigated two main processing parameters: the expansion rate during the first expansion step (50, 75, and 112.5 mm/min) and the sintering time between the two expansion steps (67.5, 90, and 135 min). The mechanical properties in both the longitudinal and circumferential directions were measured, and the microstructure of dePTFE was analyzed through scanning electron microscopy (SEM) and image processing. The dePTFE exhibited smaller pore structure and higher leakage (water entry) pressures when produced with a higher expansion rate. By increasing the sintering time, the circumferential strength of dePTFE could be improved by 50% per time interval increase. Higher sintering temperatures also increased material stiffness in the circumferential direction. The compliant properties of dePTFE were not compromised with changes in the microstructure, still attaining 30%–50% increased elastic strain, while maintaining its characteristic “toe region” in the modulus. Highlights: Relation between processing, morphology, and properties of dePTFE was studied. Wavy fibrils increased flexibility without reducing the strength of dePTFE. Expansion rate and sintering time are the two key processing parameters. Higher expansion yielded better mechanical strength and smaller pore sizes. Higher sintering temp increased material stiffness in the circ. direction.