| Property | Value |
|---|---|
| Temperature range | -20 °C to +205 °C, with excursions higher |
| Hardness | 55–90 Shore A |
| Compression set | The best available in general industrial use |
| Contains fluorine | Yes, and it falls inside the PFAS restrictions in progress |
Grade decides the envelope
FKM is not one material. The grades differ in fluorine content and in the monomers used, and that changes both what the part resists and how cold it can go. Specifying FKM without specifying which kind is how a part ends up technically correct and functionally wrong.
| Type | Roughly | Where it earns its place |
|---|---|---|
| A | 66 percent fluorine, the copolymer | The general-purpose grade, best low-temperature flexibility of the family, standard oils and fuels |
| B | 68–69 percent fluorine, the terpolymer | Better fluid resistance than A, the usual choice for aggressive chemical service |
| F | 70 percent fluorine | Maximum resistance to oxygenated fuels and methanol blends, at the cost of cold performance |
| GLT and similar | Low-temperature grades | Service down toward -40 °C where standard FKM would already be glass-hard |
The pattern is consistent: more fluorine buys chemical resistance and costs you low-temperature flexibility. A standard FKM part is effectively rigid below about -20 °C, so a seal that has to survive a cold start needs a low-temperature grade specified deliberately.
Where FKM fails
Three environments defeat it and they surprise people, because FKM is otherwise so capable. Hot water and steam attack it. Amines, including the amine inhibitors in many coolants and process fluids, attack it. Ketones and esters, brake fluid among them, swell it badly. The field failure we see most is an FKM seal specified on temperature alone, dropped into a steam or amine-inhibited system, and gone in weeks. That duty belongs to AFLAS or EPDM depending on the rest of the picture.
Mechanically it is also less tough than nitrile, with lower tear and abrasion resistance, so a dynamic seal taking rough installation or particulate wear is not automatically better in FKM.
The fluorine question
FKM contains fluorine and sits inside the scope of the PFAS restrictions moving through the EU and several US states. For most parts today that changes nothing. For programs that have committed to removing fluorine from the assembly, it decides the material before performance is discussed, and the work becomes finding how much of the FKM envelope you actually use.