Optimized microporous structure of ePTFE membranes by controlling the particle size of PTFE fine powders for achieving high oil-water separation performances

Advanced functional membranes consisting of low surface energy and tailored micropores are promising materials for oil-water separation applications. However, obtaining a high porosity yet controllable pore sizes to achieve high separation efficiency and flux remains challenging. Herein, four kinds of polytetrafluoroethylene (PTFE) powders with different particle sizes were simply sieved from a commercial resin and corresponding microporous membranes were fabricated through paste extrusion and thermal stretching processes. The microporous structure exhibited that choosing smaller powders caused the decrease of pore size but can still maintain a high porosity in stretched membranes, which was difficult to be realized by controlling processing conditions. The increased contact interfaces among small powders were regarded to promote the fibrillation during extrusion, thereby more entangled sites of fibrils were generated and only required a small amount of extension to get the target deformation during stretching. Consequently, more micropores with a smaller size were owned in the membrane by using smaller powders, which gave rise to superhydrophobicity with an ultrahigh water contact angle of 152° and superfast oil infiltration. As expected, the oil-water separation experiment obtained high separation efficiency closing 100% and the flux of 7500 L/m²·h, which were competitive in comparison to most of previously-reported PTFE microporous membranes. More significantly, such an outstanding separation capacity was suitable for many kinds of oils with different densities and almost stable even after 15 cycles. Meanwhile, the microporous structure of the membrane prepared by using smaller powders was also proven to be deformed more slowly when subjected to a large strain. Accordingly, this work provides a new and practical approach for fabricating PTFE microporous membranes suitable for applying in a wide variety of oil-water separation fields.