Lubricant facilitated thermo-mechanical stretching of PTFE and morphology of the resulting membranes

Abstract

Poly(tetrafluoroethylene) (PTFE) membranes were prepared using thermo-mechanical methods of billet formation, cold extrusion, rolling, uniaxial stretching at 100–250°C and heat treatment. In contrast to conventional PTFE membrane preparation approaches, the oligomeric lubricant used in this study remained throughout the processing steps. SEM images of the expanded PTFE (ePTFE) membranes showed that the node domains were bridged with fibrils. The effects of lubricant, length of heat treatment, and the stretching temperature, rate, and ratio on the morphology and mechanical properties of ePTFE membranes were investigated. The presence of lubricant during stretching allowed membrane formation at a markedly lower temperature, increased its mechanical strength, porosity, and pore size, and decreased shrinkage. Increasing the stretching temperature and ratio and decreasing the stretching rate, produced membranes with larger fibril domains. The results indicate that an increase in heat treatment time at a temperature just above the PTFE melting point increased the mechanical strength and decreased porosity of the membranes. The dead end filtration test results revealed that water permeability increased as stretching increased in the presence of lubricant.