Study on two‐stage stretching strategy for microstructure improvement of polytetrafluoroethylene hollow fiber membrane

Abstract

AbstractPolytetrafluoroethylene (PTFE) has excellent chemical resistance, thermal stability as well as hydrophobicity, and is regarded as a good material for membrane fabrication. PTFE hollow fiber membrane is commonly prepared by the paste extrusion–stretching–sintering process, where the stretching temperature is usually fixed in existing reports. In this paper, a new two‐stage stretching method (first at 50°C and then at 200°C) was proposed to optimize the microstructure of PTFE hollow fiber membranes. Meanwhile, the stretching experiments at low‐temperature (50°C) and high‐temperature (200°C) were also conducted for comparison. The chemical composition, surface morphology, surface porosity, overall porosity, maximum pore size, water contact angle, and mechanical property were discussed. Moreover, the ethanol permeation flux and bovine serum albumin (BSA) rejection of PTFE hollow fiber membranes prepared from different stretching manners were tested. The results showed that with the increase of stretching ratio, the porosity, and pore size gradually increased for all three membranes. However, for a specific stretching ratio, the membrane fabricated by two‐stage stretching process exhibited better comprehensive properties in terms of porosity and pore size, that is, higher porosity than the 50°C method and smaller pore size than the 200°C method. As stretching ratio was 2.0, the ethanol permeation flux and BSA rejection was 2467 L/m2 h and 95%, respectively. Overall, the PTFE hollow fiber prepared from “two‐stage” stretching exhibited comparable BSA rejection (both around 95%) and much higher permeation flux (increased by 43.1%) compared to the sample prepared by low‐temperature stretching at 50°C, and meanwhile, comparable permeation flux (decreased by 10.8%) and higher BSA rejection (95% against 81%) compared to the sample prepared by high‐temperature stretching at 200°C. The proposed method provided a good reference for the microstructure improvement of PTFE hollow fiber membrane fabricated by paste extrusion–stretching–sintering technology.