Abstract:

Objective Polytetrafluoroethylene (PTFE) has garnered significant attention due to its exceptional high-temperature resistance and chemical stability. While the structure of PTFE flat-sheet membranes is relatively controllable, PTFE empty tube fiber membranes prepared by uniaxial stretching face challenges in simultaneously controlling pore size and porosity.

Method By wrapping the outer surface of PTFE tubular membranes with flat-sheet membranes of varying pore sizes and layers, an asymmetric pore structure was constructed, resulting in PTFE empty tube fiber membranes with suitable pore sizes and high porosity. After hydrophilic modification using materials containing hydrophilic groups.

Results The membrane with the highest separation efficiency for oil-in-water emulsions was the modified membrane wrapped with a 0.2 μm flat-sheet membrane and three wrapping layers, achieving a separation efficiency of 99.3%, a pure water flux of 38 710.02 L/(m²·h·MPa), and a permeation flux of 13 010.6 L/(m²·h·MPa). For small-aperture membrane wrapping(0.1 μm,0.2 μm), the gradual increase in flux with additional wrapping layers can be attributed to the expanded separation channels resulting from greater membrane thickness, which provides more porous pathways and a larger effective filtration area, thereby enhancing separation efficiency. Conversely, when wrapping large-aperture membranes(0.45 μm), nodule overlap occurs after multilayer wrapping, and the intrusion of hydrophilic coatings further contributes to pore blockage. The chemical stability of the membrane with the optimal configuration was tested by immersing it in 6% sodium hypochlorite solution, 0.1 mol/L NaOH solution, and 0.1 mol/L H₂SO₄ solution for 12 h. The fluxes were 37 640.33, and 34 950.57 L/(m²·h·MPa), respectively, indicating that the modified membrane exhibited excellent oxidation resistance and strong acid/alkali resistance. Analysis of oil-in-water emulsion separation under different pressures showed that the membrane remained stable at 0.24 MPa.

Conclusion By wrapping different flat-sheet membranes on the outer surface of PTFE empty tube fiber membranes, the maximum pore size was reduced while retaining the original porosity of the tubular membranes. The modified membrane achieved a maximum pure water flux of 45 540.1 L/(m²·h·MPa), while the optimal performance was observed in the tubular membrane wrapped with a 0.2 μm flat-sheet membrane and three layers, exhibiting a water flux of 38 710.02 L/(m²·h·MPa) and a contact angle of 45.2°. At this condition, the membrane demonstrated a separation efficiency of 99.3%, along with excellent resistance to acids, alkalis, and strong oxidants, making it suitable for most wastewater treatment environments. The membrane maintained stable operation under a maximum pressure of 0.22 MPa, indicating broad prospects for applications in oil-water separation.

Key words: polytetrafluoroethylene, empty tube fiber membrane, asymmetric structure, wrapping, oil-water separation, wastewater treatment, membrane separation technology