Among the other rubber classes, fluoro-elastomers (FKM or FPM and FFKM) show the best properties in terms of resistance to fire and to chemical agents due to the high content of fluorine and to the intrinsic stability of the carbon-fluorine bond. Commercial grades of fluoro-elastomers are obtained from the radical polymerization of vinyl monomers. The most common are: difluoroethylene (VDF), tetrafluoroethylene (TFE), hexafluoropropylene (HFP), perfluoromethylvinylether (PMVE), 2-methoxyethylvinyl ether (MOVE), ethylene and propylene. The proper polymer in-chain dosing of groups with different steric hindrance and properties together with the employment of specific terminal chains allow modulating mechanical properties, their behaviour at low temperature, chemical resistance, the type of reticulation and processability. The rigidity of fluoro-elastomer polymer chains determines a peculiar plastic behaviour with a rather slow creep recovery and low resilience.
Fluroelastomer-based formulations are generally characterised by the use of non‑ or weak‑reinforcing fillers, both carbon blacks and white fillers, and by a low content of process aids.