Viton® / FKM (Fluoroelastomer) O-Rings

FKM is the material people reach for when heat and chemistry have already beaten nitrile. It is built by polymerizing vinylidene fluoride (VDF) with variable proportions of other monomers: hexafluoropropylene (HFP), tetrafluoroethylene (TFE), and perfluoromethyl vinyl ether (PMVE). Change the mix and you change the seal. The resulting co-, ter-, and tetrapolymers run 65 to 71 percent fluorine, and that number is the dial that sets both media resistance and low-temperature flexibility.

Everything good about FKM traces back to one thing: the stability of the carbon-fluorine bond. Its high-temperature resistance and its chemical stability are two consequences of the same chemistry. That bond is also why FKM has low gas permeability, outstanding resistance to ozone, weathering, and light cracking, and why FKM vulcanizates are flame-retardant.

Viton® is Chemours’ trademark for their FKM. It is the name everyone says, the way people say Kleenex, but the ASTM designation for the whole class is FKM, and plenty of excellent FKM is not Viton®.

Quick specs

Common names

One material, a lot of names on a lot of drawings. All of these mean FKM:

• Viton®, the Chemours trademark, used generically far more often than it should be
• FKM, the ASTM D1418 designation, and the correct name for the class
• Fluoroelastomer, fluorocarbon rubber, or fluororubber
• FPM, the ISO/DIN designation for the same polymer family
• Tecnoflon® (Solvay), Dai-El® (Daikin), Fluorel® (3M), all FKM under other flags

Mechanical properties

• Excellent high-temperature resistance, continuous to about +400°F with short excursions higher
• Outstanding resistance to ozone, weathering, and light cracking, straight from the fluorinated backbone
• Low gas permeability, which is why it wins in vacuum and in gas-tight static seals
• Very low compression set, so a static seal set years ago is still pushing back
• Flame-retardant vulcanizates
• Good resistance to weathering, hot oils and greases, aliphatic hydrocarbons, mineral oils, and gasoline
• Fair to good abrasion resistance; usable dynamically, but not a wear champion the way HNBR is

Limitations

The honest list. FKM covers an enormous amount of ground, and then it stops cold:

• Amines are destructive to FKM. Not “reduced life,” destructive. Amine-inhibited fluids, amine gas-sweetening service, and amine-cured epoxy systems all point somewhere else. Go to Aflas® or FFKM.
• Ketones and esters swell standard FKM badly. MEK, acetone, ethyl acetate, and their relatives. Use EPDM or FFKM.
• Not for glycol brake fluid (DOT 3, 4, 5.1). Use EPDM.
• Hot water and steam are a real weakness for standard bisphenol-cured grades. Use EPDM, Aflas®, or a peroxide-cured FKM, which is a different animal in this service.
• Poor low-temperature flexibility compared to nitrile. See the temperature section below; this is the trade-off nobody mentions in the sales sheet.
• Costs much more than nitrile. On a warm seal in ordinary oil, that premium buys you nothing you can measure.
• If the chemistry is beyond FKM, the answer is Viton® Extreme ETP or FFKM, not a harder FKM.

Chemical compatibility

Handles well:

• Petroleum oils, hot oils, and greases
• Mineral oils and aliphatic hydrocarbons
• Gasoline and most fuels; high-fluorine grades handle blended and oxygenated fuels
• Aromatic and many chlorinated hydrocarbons, where nitrile fails outright
• Many acids, in the right grade and cure
• Ozone, weather, sunlight, and vacuum

Attacks it:

• Amines, destructively
• Ketones and esters, which swell standard FKM strongly
• Glycol brake fluids
• Hot water and steam, for standard bisphenol-cured grades
• Low-molecular-weight organic acids, hot hydrofluoric and chlorosulfonic acids
• Ammonia and certain strong bases

The compound and the cure matter more here than in almost any other elastomer family, so confirm your exact fluid on the Chemical Compatibility page or in the interactive Chemical Compatibility Chart, and tell us the grade you are quoting against.

A word on temperature ranges

Our published range is -20°F to +400°F (-29°C to +204°C), and the hot end is the easy half. FKM does not really have a heat problem until you are asking for FFKM territory.

The cold end is FKM’s real weakness, and it moves with the polymer type more than most people expect. Freudenberg publishes the honest spread for its fluororubber: standard types about -4°F, special grades about -40°F. That is a 36-degree swing driven purely by which monomers went into the polymer, and it is why “Viton® is good to -20°F” is a statement about a compound, not about a material.

If cold matters, the low-temperature types are real and they are worth asking for:

• GLT-type low-temperature FKM, using PMVE, aerospace-qualified under AMS7287 to roughly -22°F.
• GFLT-type, high fluorine plus PMVE, qualified under AMS7379 to roughly -40°F, which is the combination you want for cold fuel service. See the mil-spec reference.

As always, a published range assumes the compound, the media, the pressure, the hardware, and the duty cycle all cooperate. Heat plus an aggressive fluid ages a seal faster than heat alone, and a cold-start seal that also has to survive +350°F is a compounding problem, not a catalog pick. If your application sits near an edge, send us the conditions and we will pull the datasheet for the actual compound.

Shop Viton® (FKM) O-Rings

Every Viton® / FKM variation we stock, by grade and size:

Inch (AS568):

• Viton® FKM 75 Duro (Black)
• Viton® FKM 75 Duro (Brown)
• Viton® FKM 90 Duro (Black)
• Viton® FKM 75 Duro (Dark Green)
• Viton® FKM 75 Duro FDA & 3A Sanitary
• Mil-Spec FKM 75 Duro (AS3209, M83248/1, AMS7276)
• Viton® / FKM Special Compounds

Metric:

• Viton® / FKM Metric 75/80 Duro
• Viton® / FKM Metric 90 Duro

Sizes are in the AS568 range and common metric; the Master Size Chart has the dimensions. Need a low-temperature grade, a peroxide cure, or a durometer we do not list? Ask us.

Variations and grades explained

FKM is a family, and the grade is chosen by duty. This is why two O-Rings that both say “Viton®” can behave completely differently in the same fluid.

By polymer type and fluorine content

More fluorine means better media resistance and worse cold behavior. That is the whole trade, and it never goes away.

By cure system

FKM is cross-linked with diamines, bisphenols, or organic peroxides depending on the type, and the choice has consequences your fluid will notice:

• Bisphenol cross-linked: more resistant to temperature and wear, but less resistant to acids and steam. This is the common commercial grade and the one most people mean when they say Viton®.
• Peroxide cross-linked: highly resistant to alcohols, hot water, steam, and alcohol-based fuels. If your service includes any of those, this is the grade to ask for by name, because the default will disappoint you.
• Diamine cross-linked: the original system, still used in specific bonded and adhesion-critical applications.

By hardness (durometer)

• 50 Shore A: soft, for low closing force or sealing against an imperfect surface.
• 75 Shore A: the standard for most FKM work, and what we stock deepest.
• 90 Shore A: for high pressure and to resist extrusion through the gland gap. Check your groove design before you assume harder is the fix.

Related and specialty grades

• Viton® Extreme ETP: a specialty fluoroelastomer with near-perfluoroelastomer chemical resistance across acids, bases, and amines. See Viton® Extreme ETP.
• FFKM: the step up. Broader chemical resistance and more heat, at a price that makes you certain you need it.
• FEPM (Aflas®): a related class, TFE plus propylene, about 57 percent fluorine. Excellent in hot water, steam, and amines, but only medium resistance to mineral oils. It is the answer to several of FKM’s limitations and the wrong answer for oil.
• Fluoroprene XP: Freudenberg’s special material range for the process and food industry, with properties that significantly exceed conventional FKM.
• FDA grades: available for food and beverage contact; see FDA 21 CFR 177.2600.
• Mil-spec FKM: compound M83248/1 and AMS7276, stocked and traceable. See the mil-spec reference.

Standards and specifications

FKM O-Rings are supplied in the full AS568 inch size range and common metric sizes. The callouts you will see:

• ASTM D1418: designates the polymer as FKM.
• ASTM D2000 / SAE J200: the line-callout system most commercial FKM is bought to.
• AS568: the inch O-Ring size standard. See Sizing.
• Mil-spec family: M83248/1 (75 duro) and M83248/2 (90 duro) under AMS-R-83248 / MIL-R-83248, plus AMS7276, AMS7259, and the AS3209 size standard. Low-temperature types fall under AMS7287 (GLT) and AMS7379 (GFLT). All covered in the mil-spec reference.

If your drawing calls out a specific spec or compound, tell us and we will match it, with a Certificate of Conformance and lot traceability under our ISO 9001:2015 system.

Typical applications by industry

See O-Rings by Industry for the wider picture, or Design & Engineering for the gland around the seal.

Is FKM the right choice?

Four questions settle it:

1. Does it run past +250°F? Nitrile is finished; FKM has room to +400°F.
2. Is the fluid fuel, hot oil, or an aromatic or chlorinated hydrocarbon? FKM is built for exactly that.
3. Is there an amine, a ketone, an ester, or steam anywhere in the system? Then FKM is the wrong answer no matter how hot it runs. Look at EPDM, Aflas®, or FFKM.
4. How cold does it get on the coldest day? Below about -20°F you are specifying a GLT or GFLT grade, not a catalog Viton®.

If the fluid is ordinary oil at ordinary temperature, nitrile already does the job for a fraction of the money, and we will tell you so. Browse the full materials guide if you are still narrowing it down.

Not sure whether FKM covers your fluid and temperature? Send us the conditions: the media, the temperature, the pressure, and whether the seal moves. We will spec the compound and the size with you. We make getting O-Rings easy.

Polymer chemistry, monomer composition, 65 to 71 percent fluorine range, cure-system behavior, the standard -4°F and special-grade -40°F low-temperature figures, FEPM composition, and the Fluoroprene XP range: Freudenberg FST 2025 Technical Manual, ch. 2 (Materials and Process Engineering), §3.4, p. 66. Commercial and aerospace grade ranges reflect the wider FKM market. The datasheet for your specific compound always governs; test parts in your actual service conditions before committing to a critical seal.