Hot nitric acid diffusion in fluoroelastomer composite and its degradation

Fluoroelastomers (FKM) are vital sealing materials in acidic environment and their failure can cause severe safety problems. Therefore, investigation of the degradation behavior and mechanism of FKM materials is of great significant. Herein, we investigate a diffusion model of an acidic solution into an FKM composite and its degradation behavior upon immersion in hot nitric acid solution. The results indicate that the diffusion process of the HNO₃ solution into the FKM composite conforms to the Fick diffusion model at a low concentration of nitric acid solution. Besides, the concentration of HNO₃ solution affects the diffusion process of solvent molecules and the dissolution process of the filler particles to some extent. SEM showed that the surface topography of the FKM was significantly altered after it was immersed in HNO₃ solution. The structural and chemical changes of the FKM were studied using ATR-FTIR, SEM-EDS and MAS NMR, which demonstrated the occurrence of decrosslinking via hydrolysis of the crosslinks and backbone cleavage by dehydrofluorination. This was also manifested by the decrease in crosslinking degree and mechanical properties. The present study is helpful for revealing the chemical changes in FKM in hot HNO₃ solution.