FKM is the ASTM D1418 abbreviation for fluorocarbon rubbers with fluorine on the polymer backbone. The family is built from vinylidene fluoride (VDF) combined with other fluorinated monomers. FKM is the standard choice for hot oil, fuels and many aggressive fluids up to about 200 °C, and its use is limited mainly by cost, low-temperature flexibility and a few chemicals that attack it.
Why fluorine content matters
The bond between carbon and fluorine is strong, and fluorine shields the backbone from attack. A higher fluorine content generally means better resistance to fuels, solvents and many chemicals. It also tends to mean poorer low-temperature flexibility. Manufacturers group FKM by fluorine content, often into bands around 66%, 68% to 69% and 70% or more.
- Copolymers (VDF/HFP), about 66% fluorine. The classic general-purpose FKM. Good heat and oil resistance with balanced cost and flexibility.
- Terpolymers (VDF/HFP/TFE), about 68% to 70% fluorine. Better resistance to fuels, aromatics and many chemicals than copolymers. Stiffer at low temperature.
- Specialty grades. Low-temperature grades that replace HFP with perfluoromethylvinyl ether (PMVE), and base-resistant grades built on different monomer mixes, such as ethylene and TFE with PMVE.
A "grade" also carries a viscosity: low-Mooney grades flow easily for injection moulding, and high-Mooney grades give higher green strength for compression moulding and extrusion.
Cure systems
How an FKM is crosslinked affects compression set, scorch safety, water and steam resistance, and the fluids it can survive.
Bisphenol cure. The most common system. Bisphenol AF is the crosslinker and a quaternary phosphonium salt accelerates it, with magnesium oxide and calcium hydroxide as acid acceptors. It gives low compression set, fast cure and good scorch safety. Many grades come with the curative already built in ("cure-incorporated"), which means the compounder adds only the acid acceptors and filler. The metal hydroxide in the recipe makes bisphenol-cured FKM weaker in hot water, steam and acids.
Peroxide cure. A peroxide with a coagent such as TAIC forms carbon-carbon crosslinks. These are more resistant to steam, hot water, acids and many chemicals, and peroxide-curable grades can reach better low-temperature performance. The trade-offs are more mould fouling, a need for a good coagent system, and sensitivity to air at the surface during cure.
Diamine cure. The oldest system. Diamine carbamate crosslinks the polymer but gives poorer compression set and faster scorch than bisphenol. It remains useful for bonded and fabric-reinforced parts.
What FKM does not like
Standard FKM is attacked by some chemical groups: low-molecular-weight ketones, esters and ethers, strong bases, many amines, and hot steam in bisphenol-cured grades. Amine-containing fluids, such as some corrosion inhibitors and drilling fluids, can attack the polymer and give hard, brittle seals. For these jobs, a base-resistant FKM, a peroxide-cured grade, FEPM or HNBR may serve better. See the page on choosing between FKM, HNBR and FEPM.
What to specify when you order FKM
- Fluorine class or grade family, and fluids to be sealed, with temperature and duration.
- Cure system, and whether the part must withstand water, steam or amines.
- Hardness, compression set condition (time, temperature, method) and any low-temperature requirement, such as TR10.
- Post-cure requirement and approvals needed.
Sources and further reading
- Manufacturer grade tables, for example the Syensqo Tecnoflon materials guide and the Chempoint Viton selector guide.
- P. A. Ciullo and N. Hewitt, The Rubber Formulary, 1999.