FKM vs Viton™: what lies behind each name and when the difference matters

In any engineering department dealing with sealing in chemically aggressive environments, the conversation inevitably reaches the same point: the drawing specifies "Viton" but the supplier offers "generic FKM", or vice versa. The confusion between FKM, FPM and Viton™ is not a trivial nomenclature issue. It affects tender specifications, material traceability arriving at the plant and, ultimately, seal performance in service.

This article dismantles the confusion with concrete data, explains precisely what lies behind each designation and, most importantly, provides technical criteria for deciding when you need to specify by brand and when a well-defined ASTM specification suffices.

Three names, one family: FKM, FPM and Viton™

FKM is the ASTM D1418 designation for fluorocarbon elastomers with a saturated main chain. FPM is exactly the same material, but named according to ISO 1629 and DIN nomenclature. There is no technical difference between these acronyms: FKM is the American convention, FPM the European one. In industrial practice, FKM has become the universal term in most markets.

Viton™ is the registered trademark of Chemours (formerly DuPont Performance Elastomers) for their FKM polymer range. It is the most recognised brand in the sector, to the extent that it has undergone lexicalisation: many engineers say "Viton" when they actually mean "any FKM". Similar to what occurs with Neoprene® and polychloroprene, or Teflon® and PTFE. DuPont/Chemours created both brands.

However, Chemours is not the sole manufacturer of FKM polymers. Solvay produces Tecnoflon™, Daikin markets DAI-EL™, 3M has Dyneon™, and other brands exist such as Noxtite® and Fluorel®. All are FKM fluoroelastomers. All share the chemical basis of vinylidene fluoride (VDF) and hexafluoropropylene (HFP) copolymers, with possible terpolymers incorporating tetrafluoroethylene (TFE) or perfluoromethyl vinyl ether (PMVE).

Is all FKM equal? What varies between compounds

Here lies the critical point that many articles omit. Stating "FKM" without qualification is akin to saying "steel": it defines a family, not a specific material. Within FKM there are significant variations that directly affect in-service performance.

Fluorine content

The fluorine content of an FKM compound typically ranges between 64% and 70%. Higher fluorine content yields greater chemical resistance, particularly against aromatic hydrocarbons, chlorinated solvents and concentrated acids. A standard Type A FKM with 66% fluorine offers good general resistance. A Type GF or GFLT compound with 69-70% fluorine withstands media that would rapidly degrade Type A. Chemours markets this as Viton™ Extreme in their highest fluorine grades.

Cure system

FKM compounds can be cured with bisphenol (ionic cure) or peroxides. Bisphenol curing is the most widespread and offers good general chemical resistance with excellent compression set properties. Peroxide curing improves resistance to steam, acids and bases, making it the preferred option for food, pharmaceutical and applications involving CIP/SIP cycles. This difference depends not on brand but on compound formulation.

Polymer families

Within the Viton™ range alone, Chemours offers more than 25 distinct polymers grouped into families: Type A (general purpose), Type B (enhanced chemical resistance), Type F (aggressive fluid resistance and low temperature), Type GLT and GFLT (low temperature flexibility) and Type Extreme ETP (maximum chemical resistance, capable of competing with FFKM in specific applications). Each family has a different performance profile. Specifying simply "Viton" on a drawing without indicating the type is a certain source of problems.

When to specify Viton™ by brand and when generic FKM suffices

This is the practical decision that matters at procurement and engineering level. There is no universal answer, but there are clear criteria to guide the decision.

Specifying Genuine Viton™ by brand makes sense when the application demands complete traceability of the base polymer, when OEM requirements explicitly name the brand in their specifications, or when seal criticality does not admit batch-to-batch variability. The Genuine Viton™ programme from Chemours guarantees that the compound uses exclusively 100% virgin Chemours FKM polymer, manufactured in ISO-certified facilities. This provides an additional layer of quality assurance that may be relevant in aerospace, Oil & Gas or medical equipment.

Specifying FKM by ASTM D2000 standard is sufficient, and often preferable, when what matters are the properties of the final vulcanised compound, not the polymer origin. A competent compounder can formulate an FKM with the mechanical properties, chemical resistance and thermal range you require, using polymers from any manufacturer. Moreover, restricting the specification to Viton™ on the drawing may limit the moulder or extruder, increase part cost and extend lead times without genuine functional benefit.

The real risk lies in a third scenario: low-cost "FKM" that is actually a blend of fluoroelastomer with hydrocarbon rubbers (EPDM, CR, acrylics). These blends do not perform like pure FKM. They do not perform "almost as well". They perform like expensive hydrocarbon rubber. Protection against this is not demanding a brand, but requiring the compound to be 100% virgin FKM and verifying the vulcanised material properties against application requirements.

Technical properties of FKM/Viton™: what this elastomer delivers

FKM excels compared to other elastomers in a very defined performance profile. Its typical operating temperature range spans from –20°C to +200°C in continuous service, reaching +230°C in certain compounds and peaks of +300°C in special grades with high-temperature additives. At the cold end, standard Type A grades stiffen below –15/–20°C, limiting their use in cryogenic environments. GLT and GFLT grades extend flexibility to approximately –40°C.

Chemical resistance is where FKM most clearly differentiates itself from other conventional elastomers. It resists aliphatic and aromatic hydrocarbons, fuels, mineral and synthetic oils, hydraulic fluids, concentrated mineral acids and chlorinated solvents. This combination of resistances makes it irreplaceable in applications where standard silicone (VMQ) or EPDM would fail due to chemical incompatibility.

Where FKM is not the answer: ketones (acetone, MEK), esters, amines, high-temperature steam in bisphenol-cured grades, glycol-based brake fluids, and low molecular weight organic acids. For these exposures, other elastomers such as EPDM, FFKM (perfluoroelastomer) or fluorosilicone (FVMQ) may be more appropriate depending on the case.

FKM versus silicone and fluorosilicone: when to choose each

For an engineer specifying seals or gaskets, the choice between FKM, silicone (VMQ) and fluorosilicone (FVMQ) is typically the real project decision. Each material covers a different niche.

Standard VMQ silicone offers an exceptional temperature range (from –60°C to +200°C and up to +300°C with additives), excellent flexibility, certifiable biocompatibility and good ageing behaviour. However, its resistance to hydrocarbons, fuels and oils is poor. A silicone seal in contact with petrol, diesel or hydraulic oil will swell and degrade rapidly.

FVMQ fluorosilicone combines part of the thermal flexibility of silicone with moderate resistance to fuels and oils. It is a compromise: neither the full chemical resistance of FKM, nor the mechanical flexibility of pure silicone. It works well in aerospace and automotive applications where the seal encounters fuel but must also operate at low temperatures.

FKM is the choice when chemical exposure is the dominant factor: fuel circuits, hydraulic systems with aggressive fluids, sealing in chemical processes with solvents, or any application where the medium would degrade silicone. In return, it sacrifices low-temperature flexibility and does not offer the biocompatibility or food contact certifications that many silicone formulations possess.

Typical industrial applications of FKM

FKM seals and gaskets are routinely specified in automotive fuel and lubrication systems (injector O-rings, crankshaft seals, oil circuit seals), in chemical and petrochemical industry (flange gaskets, valve and pump seals exposed to solvents and acids), in aerospace (hydraulic and fuel system seals), and in Oil & Gas (wellhead seals, high-pressure valve packings).

In the food and pharmaceutical industries, peroxide-cured FKM is used for gaskets that must withstand CIP cycles with acids and bases at temperature. For these applications, compound traceability and FDA/CE certifications are critical, and here it may indeed make sense to specify particular Viton™ grades with Chemours documentation, or to request certificates of conformity from the compounder.

How to correctly specify FKM in your technical specification

The most robust practice is not to write "Viton" on the drawing, but to define the functional requirements of the compound. This means specifying the required Shore A hardness (with ±5 tolerance), the operating temperature range, contact fluids and their concentrations, working pressure, and compression set requirements if the seal is static. With this data, a qualified compounder can formulate or select the appropriate FKM, whether based on Chemours, Solvay, Daikin or any other polymer.

If your OEM or regulation requires Viton™ by name, ensure you specify "Genuine Viton™" and not simply "Viton", because the latter does not guarantee that the final compound uses exclusively Chemours polymer. And if what you need is assurance that you are receiving pure FKM and not an adulterated blend, request a compound composition certificate from the supplier and verify mechanical properties against expected values for 100% virgin FKM.

ProSilicones64: manufacture of FKM and Viton™ B components

At ProSilicones64 we manufacture flat gaskets, seals and custom-cut components in FKM and Viton™ B (a grade with enhanced chemical resistance compared to standard Type A). We work with both ASTM standard specifications and OEM requirements demanding brand traceability. Each project is technically evaluated to recommend the most suitable compound for the actual service environment of the part.