FEPM, the TFE/propylene copolymer sold under several trade names, is a specialist for steam, hot water, amines and strong bases. A part made from it may be mistaken for FKM at first glance.
Telling FEPM from FKM
| Evidence | FKM | FEPM |
|---|---|---|
| Density (compound) | about 1.8 to 2.0 g/cm³ | about 1.5 to 1.7 g/cm³ |
| Fluorine content | about 66 to 70 % and up | about 55 to 58 % (lower, because of propylene) |
| IR | strong C–F, with VDF bands | strong C–F plus clear CH₂ and CH₃ bands from propylene |
| Glass transition | about −40 to −15 °C | close to 0 °C |
| Response to a hot amine or base | attacked, hardens | resistant |
| Cure | bisphenol or peroxide | peroxide with coagent |
Fluorine content lower than any FKM, together with methyl bands in IR and a Tg near zero, is the combination to look for.
Fillers and cure
Typical FEPM seals use carbon black (often a medium thermal or a furnace grade), sometimes silica or other fillers, a peroxide such as a dialkyl type, and a coagent such as TAIC. Some grades include a peroxide-curable bromine or iodine cure site.
Matching for duty
- Steam and amine duty: match polymer, peroxide-coagent system and filler purity. Strongly acidic fillers or metal oxides can matter.
- Compression set at high temperature: depends on cure state and post-cure. Match the rheometer curve and post-cure schedule.
- Cold flexibility: FEPM is stiff in the cold. If the duty includes low temperature, the match must be checked there, and a different polymer might be a better choice.
Report wording
State clearly that identification is based on several independent measurements and give the confidence level. Reports that say "Aflas" are naming a trademark. Use "FEPM (TFE/propylene copolymer)".
