Preparation and properties of swelling-resistant polyperfluoroethylene propylene/polytetrafluoroethylene composite film

Nanofiltration membranes with polytetrafluoroethylene (PTFE) fiber membranes as the support layer are commonly used for non-polar organic solvent filtration. The intermolecular forces among PTFE macromolecules are weak, which makes them prone to slip and creep under stress. This leads to swelling of the fiber network during filtration in non-polar solvents, resulting in poor dimensional stability and reduced filtering precision. To ensure the long-term stability of the separation membranes in harsh non-polar organic solvent systems, the support layer of the nanofiltration membrane must have good anti-swelling properties.

This study employs a post-treatment process using polyperfluoroethylene propylene (FEP) emulsion as the finishing agent. The strategy proposed is to enhance the anti-swelling performance of PTFE fiber membranes by combining FEP with PTFE. FEP is applied to PTFE fiber membranes through a dipping method, resulting in an FEP/PTFE composite membrane upon drying. By soaking the FEP/PTFE composite membrane in n-hexane for seven days, we can analyze the principle by which FEP post-treatment improves the anti-swelling properties of PTFE fiber membranes. The influence of FEP mass fraction and drying temperature on the swelling performance of the FEP/PTFE composite membranes is investigated, along with the bond strength between FEP and the PTFE fiber membranes and the separation performance and reusability of the FEP/PTFE composite membranes.

The PTFE fiber membrane treated with FEP emulsion undergoes melting during drying, adhering to the surface of the PTFE fibers, creating a "vine and tree" effect, where PTFE acts as the "trunk" and FEP as the "vine," thus ensuring the dimensional stability of the PTFE fibers. The tensile strength of the PTFE base membrane is 21.1 MPa, with a fracture elongation rate of 126.62% and Young's modulus of 1.68 GPa. After a seven-day soaking experiment, the fracture strength significantly decreases to 16.31 MPa. In contrast, the fracture elongation rate increases to 139.97%, and Young's modulus decreases to 1.16 GPa, indicating a clear decline in mechanical strength due to the swelling phenomenon, as well as reduced elastic modulus and increased flexibility. From the FESEM images, we can see that the fibers noticeably thicken after soaking, and the fiber network becomes disorganized and fluffy, making the fibers prone to deformation. Under a FEP mass fraction of 3 wt%, the FEP/PTFE composite membrane shows no significant changes in appearance even after soaking in n-hexane for seven days. The tensile fracture strengths are 23.17 and 22.41 MPa, with a change rate of 3.28%; the fracture elongation rates are 108.45% and 111.51%, with a change rate of 2.82%. The solvent fluxes are 7946.78 and 8350.55 L m-2 h-1, with a change rate of 5.01%, and the pore size distribution remains uniform. Under a drying temperature of 320℃, FEP melts and permeates into the pores of the PTFE fibers, enhancing the dimensional stability of the FEP/PTFE composite membranes. The change rates after seven days of soaking in n-hexane for the composite membrane's fracture strength, fracture elongation rate, Young's modulus, and solvent flux are 1.43%, 2.67%, 4.07%, and 4.04%, respectively, showing significant stability improvement. The tight fusion between FEP and PTFE ensures that the FEP/PTFE composite membranes do not experience delamination after six hours of ultrasonic cleaning, with no significant changes in surface or cross-section. The mass loss rate is ±0.27%. Tests show that the FEP/PTFE composite membrane retains 99% of the 300 nm SiO contaminants after being soaked in n-hexane for seven days, and the flux recovery ratios (FRR) from five cyclic tests remain above 95%. After seven days in four different non-polar organic solvent systems, the retention rate against contaminants also remains largely unchanged.Conclusions: The intermolecular interactions in the PTFE fiber network are weak. The fibers can easily slide and swell in organic solvent separation systems as a support layer for nanofiltration membranes. After drying, FEP, a thermoplastic fluoropolymer, melts and ensures the morphological stability of the PTFE fiber membrane in a "vine-wrapping tree" configuration. The interface compatibility of the FEP/PTFE composite membrane is good, with excellent interlayer adhesion strength. It demonstrates outstanding filtration performance and reusability. The results provide strategic support for popularizing and applying PTFE-based organic solvent-resistant separation membranes.