Swelling behavior and mechanism of perfluoro rubber in methanol environments

Objective With the advancement of the “dual carbon” strategy, the demand for pipeline transportation of methanol as an alternative fuel is gradually increasing. However, as a highly polar organic solvent, methanol may cause swelling of fluororubber-based sealing materials, leading to deterioration of material properties and affecting the sealing safety of pipeline transportation. Currently, the understanding of the swelling behavior and mechanism of perfluoro rubber (FFKM) in methanol environments needs further research.

Methods To reveal the swelling behavior and mechanism of FFKM in anhydrous and water-containing methanol, and to provide a theoretical basis for the selection and modification of sealing materials, FFKM tensile and compression specimens, processed according to relevant standards, were immersed respectively in anhydrous methanol and 8％ water-containing methanol solution for 7 days on a high-temperature and high-pressure dynamic reactor test platform. The swelling behavior of FFKM in methanol solution was systematically evaluated by analyzing the volume, mass, surface, and fracture morphology, as well as mechanical properties and functional group structure changes of the specimens through scanning electron microscope, Fourier transform infrared spectroscopy and universal material testing machine.

Results In anhydrous methanol, FFKM exhibited significant volume expansion (18.18％) and mass increase (1.94％). Meanwhile, the Shore hardness decreased by 7.77％ and the tensile strength dropped by 18.7％, indicating severe damage to its sealing performance and structural integrity. In an 8％ water-containing methanol environment, the volume change rate was only 4.5％, the tensile strength decreased by 12.5％, and the hardness declined by 2.88％. In anhydrous methanol solution, protrusions, pits and fibrous structures appeared on the material surface, and the fracture became a porous structure, with obvious damage to the microstructure. The results of infrared spectroscopic analysis further revealed that the characteristic peaks of C=O and CH₂ in the TAIC cross-linking point were significantly weakened or even disappeared in anhydrous methanol, while the fluorine structures such as —CF₃ and —CF₂ in the main chain were relatively stable. In the water-containing methanol environment, the retention rate of C=O peak increased and the degree of swelling damage decreased. The swelling mechanism of FFKM in a methanol environment involves the diffusion of methanol molecules, the formation of hydrogen bonds with polymer chains, and subsequent segmental relaxation and structural destruction. Water molecules can form a solvation layer, weakening the destructive effect of methanol and protecting the structure.

Conclusion The research results have clarified the swelling behavior of FFKM in methanol polar medium, offering scientific guidance for the anti-swelling design of methanol pipeline sealing materials. However, this study does not involve factors such as high temperature and long-term aging. The evolution of material behavior under multi-factor coupling conditions can be considered in future studies.