FKM is the ISO 1629 designation for fluorocarbon rubber; Viton is the best-known brand name (Chemours). Whatever the label, FKM is the premium sealing compound for heat, fuels and aggressive chemicals — but ‘FKM’ on a drawing is not enough. Different FKM grades behave like almost different materials, and specifying the wrong grade is an expensive way to learn that. This guide maps the grades buyers actually encounter.
FKM grades differ in fluorine content and cure chemistry. Higher fluorine means better chemical and fuel resistance; specialty monomers buy low-temperature flexibility. The families you will meet on quotation sheets:
| Type | Approx. fluorine | Key characteristics | Typical use |
|---|---|---|---|
| A-type (dipolymer, VDF/HFP) | ~66% | General-purpose, bisphenol-cured; the standard ‘Viton’ | Engine oils, fuels, general industrial seals |
| B-type (terpolymer, VDF/HFP/TFE) | ~68–69% | Better chemical and compression-set resistance | Automotive fuel systems, seals |
| F-type (high-fluorine terpolymer) | ~69–70% | Best resistance to alcohol fuels and aromatic content | Aggressive fuels, sour/aggressive media |
| GF-type | ~70% | Peroxide-cured high-fluorine grade; resists steam, acids, water-based fluids | Chemical processing, CIP/SIP environments |
| GLT / GFLT-type | ~64–67% | Special low-temperature grades using perfluoromethylvinylether | Cold-start service down to roughly −40°C |
| ETP / extreme grades | High | Near-FFKM chemical breadth at lower price than FFKM | Very aggressive mixed-media environments |
FFKM (perfluoroelastomer, e.g. Kalrez-type products) is a separate polymer class in which essentially all hydrogens are replaced by fluorine. It pushes service toward 300°C+ and near-universal chemical resistance at a price level one or two orders above FKM — use it when FKM has demonstrably failed.
| Grade family | Continuous service | Notes |
|---|---|---|
| Standard A/B/F | −15°C to +200°C | Short excursions to ~230°C are tolerated |
| GF (peroxide-cured) | −15°C to +210°C | Better in steam and hot aqueous media |
| GLT / GFLT | down to −35 / −40°C | Low-temperature sealing with most FKM chemical benefits |
| FFKM | to +300°C and beyond | Extreme chemical and thermal duty |
Low-temperature ratings are properly expressed as TR10 (the temperature at which the compound largely loses recovery), not as a single ‘minimum’ figure — ask for the TR10 value when cold starts are part of the duty cycle.
Most standard FKM is bisphenol-cured, giving low compression set in dry/oil service. Peroxide-cured compounds trade slightly higher set in some dry applications for markedly better resistance to steam, hot water, acids and sour service. If the failure mode is aqueous rather than oily, specify peroxide cure explicitly.
As a rule of thumb, FKM costs 5–15× an equivalent NBR ring. It pays back when any one of three conditions exists: sustained temperature above NBR's range, aggressive media or fuel blends, or outdoor/ozone exposure. For ordinary indoor hydraulics below 100°C, NBR remains the rational economic choice. For the complete material picture including EPDM, silicone and FFKM, see our elastomer materials comparison.
‘FKM’ is a family, not a compound. The three words on your drawing should be family + cure + hardness — e.g. ‘FKM GF-type, peroxide-cured, 75 Shore A’.
We mold FKM O-rings in standard and specialty grades across AS568, metric and custom sizes. Send us your media list and temperature envelope via the O-ring product page and our engineers will confirm the grade — or advise when EPDM or FFKM is the safer call.