When the engineering community needs a seal that can handle heat that would melt standard nitrile, resist fuels that would swell EPDM into uselessness, and deliver reliable service across a wide range of demanding applications, the material of choice has been FKM (fluorocarbon rubber) for over six decades. Since its commercial introduction by DuPont under the famous Viton® trademark in the late 1950s, FKM has become the gold standard for elevated-temperature sealing, finding its way into everything from turbocharged engine fuel systems to aircraft hydraulic circuits to industrial valve stem packings. Its unique combination of heat resistance, chemical compatibility, and mechanical durability fills a critical performance gap between commodity elastomers like NBR and the ultra-premium FFKM materials at the pinnacle of the market.

 

 

 

FKM is a family of copolymers and terpolymers built from combinations of vinylidene fluoride (VDF), hexafluoropropylene (HFP), tetrafluoroethylene (TFE), and perfluoromethyl vinyl ether (PMVE), among other monomers. By varying the ratio of these building blocks, polymer chemists can tune the properties of the final elastomer across a remarkably broad spectrum. The defining characteristic of all FKM compounds is their fluorine content, typically ranging from 66% to 71% by weight.

 

Thermal Oxidative Stability: FKM can operate continuously at temperatures of 200°C (392°F), with intermittent excursions to 230°C (446°F) for some grades. At these temperatures, most hydrocarbon-based elastomers undergo rapid chain scission and oxidative crosslinking, becoming brittle and losing all elasticity. FKM, by contrast, maintains its mechanical properties because the carbon-fluorine backbone is far less susceptible to attack by atmospheric oxygen.

 

Chemical and Fuel Resistance: The dense, electronegative fluorine sheath around the polymer backbone repels most organic fluids. FKM exhibits minimal volume swell and property loss when exposed to petroleum-based fuels, diesel, lubricating oils, transmission fluids, and many industrial solvents. This makes it the default choice for fuel system seals in modern vehicles, where exposure to gasoline-ethanol blends (including aggressive E85) would destroy standard NBR seals within months.

 

Low Volatility and Permeability: FKM compounds have inherently low outgassing characteristics and very low permeability to gases, making them suitable for vacuum applications where elastomer outgassing could contaminate sensitive processes, and for air conditioning systems where refrigerant retention is critical.

 

 

 

 

Copolymer FKM (Type 1, VDF/HFP): This is the original FKM chemistry, containing approximately 66% fluorine. It offers the best overall balance of properties, including excellent tensile strength and compression set resistance, and is the most cost-effective FKM grade. Its limitation is relatively poor resistance to steam, hot water, and strongly basic environments. Copolymer FKM is the standard choice for general industrial sealing, automotive fuel systems, and moderate-temperature oil exposure.

 

Terpolymer FKM (Type 2, VDF/HFP/TFE): The addition of TFE monomer raises the fluorine content to approximately 68-69.5%, which translates into significantly improved resistance to aggressive fuels and solvents. Terpolymer FKM swells substantially less than copolymer grades in aromatic hydrocarbons, oxygenated fuel blends, and aggressive engine oils with high additive packages. The trade-off is a slight reduction in low-temperature flexibility and a modest increase in compound cost. Terpolymer FKM dominates in modern automotive fuel systems and is rapidly becoming the baseline for oilfield chemical exposure.

 

Low-Temperature FKM (Type 3, VDF/PMVE/TFE): By incorporating perfluoromethyl vinyl ether, polymer chemists can dramatically improve the low-temperature flexibility of FKM, achieving glass transition temperatures (Tg) as low as -40°C (-40°F) while maintaining fluorine content above 67%. These compounds seal reliably in Arctic pipeline valves, aerospace hydraulic systems subjected to stratospheric cold, and automotive applications in extreme winter climates. The penalty is higher cost and somewhat reduced mechanical strength at elevated temperatures compared to standard copolymer grades.

 

Peroxide-Cured FKM: Traditional FKM compounds are cured using bisphenol-based systems that leave residual unsaturation in the polymer network. Peroxide cure systems create carbon-carbon crosslinks that are inherently more stable. Peroxide-cured FKMs exhibit superior resistance to steam, hot water, acids, and amine-containing oil additives. They are increasingly specified for engine head gaskets, oilfield completion equipment, and any application involving prolonged exposure to hot aqueous fluids.

 

 

The global market for FKM seals is plagued by a chronic problem: material substitution. Unscrupulous manufacturers offer seals labeled and sold as FKM that are actually blended with lower-cost polymers like NBR or EPDM, or that use FKM with such a high loading of cheap fillers that performance is severely compromised. A simple specific gravity test or a thermal gravimetric analysis (TGA) can often reveal the fraud, but many end-users lack the analytical resources to verify every batch of seals entering their facility.

 

Our solution is absolute transparency. Every lot of FKM material we process is certified to its specific compound formulation, with physical property test data provided on request. We maintain long-term relationships with the world's leading FKM polymer manufacturers—Chemours (Viton®), Solvay (Tecnoflon®), Daikin (Dai-El™), and 3M (Dyneon™)—and we compound and mold under ISO-certified quality systems that ensure batch-to-batch consistency. When you order an FKM seal from us, you receive exactly the material you specified, with full traceability back to the polymer manufacturer's lot number.