Abstract:

Objective Polytetrafluoroethylene (PTFE) fiber membranes are widely used in non-polar solvent filtration due to their excellent chemical inertness, superhydrophobicity, and broad resistance to solvent corrosion, including various strong polar solvents over extended periods. However, due to the weak intermolecular forces between PTFE chains, these membranes are prone to molecular chain slip under dynamic pressure or continuous solvent immersion conditions. This results in membrane creep, swelling, and structural deformation, ultimately leading to a gradual decline in filtration precision and a shortened service life. This limitation severely restricts the application of PTFE membranes in precision filtration scenarios that require long-term stable operation. To address the insufficient solvent swelling resistance of PTFE fiber membranes in non-polar solvent environments, a strategy of modifying PTFE fiber membranes with thermoplastic fluoropolymer polyperfluoroethylene propylene (FEP), which possesses thermoplastic properties, is proposed to enhance their stability and filtration performance.

Method The study uses PTFE fiber membranes pre-impregnated with FEP emulsion as a supporting matrix. An FEP micro-nanofiber membrane is further prepared on the surface of the PTFE fiber membrane using electrostatic centrifugal spinning technology. This results in a PTFE/FEP composite membrane with a more developed porosity and ultrafiltration characteristics. The preparation strategy of the PTFE/FEP composite membrane is analyzed, the FEP fiber membrane fabrication process is optimized, and the interface bonding strength, solvent swelling resistance, separation performance, and reuse properties of the PTFE/FEP composite membrane are investigated.

Results Introducing FEP significantly improves the solvent swelling resistance of pure PTFE fiber membranes. Under the optimal preparation process parameters, the FEP fibers formed by electrostatic centrifugal spinning are stacked layer by layer and combined with the supporting matrix. High-temperature sintering ensures chemical bonding between the fiber layers and the base membrane, as well as pore size refinement, with the average pore size reduced from 267 nm to 87.9 nm, reaching ultrafiltration-level pore dimensions. This structure maintains a porosity of 67.7% while forming a dense screening network that effectively inhibits swelling deformation caused by solvent penetration. The PTFE/FEP composite membrane showed no observable changes in its morphology after 7 days of swelling testing in n-hexane. The porosity and solvent flux variation rates were 1.63% and 3.37%, respectively. After five cycles of ultrasonic cleaning, the average mass loss rate was only 0.48%, and the tensile strength variation rate was only 0.76%, demonstrating excellent bonding strength and interface compatibility. The PTFE/FEP composite membrane exhibited a retention rate of 99.49% for 127 nm SiO₂ contaminants, with only minor variations in the retention rate of microspheres over seven days in four non-polar solvent environments, all remaining above 98%, confirming its excellent solvent resistance and filtration performance. After five cycles of solvent-emulsion circulation, the solvent flux of the PTFE/FEP ultrafiltration membrane showed a slight decrease, but its Flux Recovery Rate (FRR) was 95.36%, and the SiO₂ contaminant retention rate remained as high as 99.26%, demonstrating good reusability.

Conclusion In non-polar solvent separation applications, PTFE fiber membranes are prone to swelling due to the weak intermolecular forces between the molecular chains in their fibril network, which leads to performance degradation. This study proposes a strategy of optimizing membrane structure through electrostatic centrifugal spinning and sintering technology, aiming to maintain the solvent swelling resistance of the PTFE base membrane while enhancing its porosity. The successful development of a PTFE/FEP composite membrane with a more developed porosity and ultrafiltration characteristics, which is resistant to solvent swelling in non-polar solvents, provides a new approach to addressing the swelling and precision degradation problems of PTFE membranes in non-polar solvents. This research offers strategic support for the development of high-performance PTFE separation membranes for solvent separation and purification.

Key words: polytetrafluoroethylene fiber membrane, separation membrane, nonpolar solvent filtration, swelling resistance, polyperfluoroethylene propylene, electrostatic centrifugal spinning