FEPM is the ASTM D1418 designation for rubbers made from tetrafluoroethylene (TFE) and propylene. AFLAS is the best-known commercial family. FEPM has about 57% fluorine, less than most FKM, but it behaves differently because the polymer contains propylene units and not vinylidene fluoride. That difference is the reason for its strengths.
What FEPM does well
- Steam and hot water. Good resistance at temperatures where bisphenol-cured FKM degrades.
- Amines and strong bases. FEPM is not attacked by amines or caustic solutions in the way that standard FKM can be.
- Sour service. Resistant to hydrogen sulfide and many corrosion inhibitors, which is why it appears in oil and gas qualifications.
- Broad chemical resistance. Acids, bases, oils, many solvents and some hydraulic and brake fluids.
- Heat. Continuous service to about 200 °C is quoted, with grade-dependent limits.
Where it falls short
- Low temperature. The glass transition of standard AFLAS grades is around -3 °C according to manufacturer data (published values differ by grade, so check the producer's datasheet), so low-temperature sealing is poor. A VDF-containing grade such as AFLAS 200P improves this.
- Aromatic fluids. Swelling is higher than FKM in fuels and aromatic solvents.
- Compression set. Often higher than FKM at similar temperatures, which limits use in static seals that must hold force for years.
- Price and processing. The polymer costs more than HNBR and the cure needs attention.
Cure
FEPM is almost always peroxide cured with a coagent, typically TAIC. Without the coagent the network is weak. Carbon black, usually N990, is common. Some modern grades incorporate a cure site to give faster cure and better compression set.
Typical applications
Steam seals, geothermal and downhole tools, oilfield chemicals with amines or caustic, wire and cable jackets, and some automotive coolant or transmission applications. It is chosen when the fluid rules out standard FKM and the temperature rules out HNBR.