FKM (Viton®) — the complete O-ring material guide

FKM — the fluorocarbon rubber most maintenance people know by the trade name Viton® — is the default premium elastomer for high temperatures and aggressive media. Where standard nitrile (NBR) gives up around +100 °C, FKM runs continuously to +200 °C and tolerates short peaks near +230 °C, while shrugging off hot oils, fuels and most mineral acids. You pay for that: on the order of five times the price of NBR. This guide is written to help a maintenance engineer or purchaser make one decision cleanly — is FKM the right material here, and if so, which grade and hardness — without over-specifying and without getting caught by one of FKM's sharp, well-known weaknesses.

What FKM is, and why people call it Viton

FKM is the ASTM D1418 designation for fluorocarbon rubber. Older European catalogues use the ISO 1629 equivalent FPM — it is the same material, not a substitute. Viton is a trade name (originally DuPont, now Chemours); Tecnoflon (Solvay) and Dai-el (Daikin) are others. In practice the names are interchangeable: if a supplier offers you an FPM ring instead of Viton, you are getting the same class of material, and the price should reflect that. The class is broad, though — dipolymers and terpolymers, 66 to 70 % fluorine, two different cure systems — so match the compound, not just the FKM/FPM label.

What makes FKM behave the way it does is fluorine. The polymer backbone is heavily fluorinated, and it is that dense shield of C–F bonds that buys the heat resistance, the oil and fuel resistance, and the excellent ageing in ozone and open air. The same chemistry also explains the weaknesses further down this page — the media that get past FKM are the ones that attack a fluorinated backbone directly.

Temperature range: -20 °C to +200 °C

Standard FKM has a continuous service window of -20 °C to +200 °C, with short-term peaks to roughly +230 °C. The top end is the main reason FKM gets bought:

• NBR stops near +100 °C (short-term ~+120 °C)
• HNBR extends the nitrile family to about +150 °C
• FKM takes over from +150 °C up to +200 °C continuous
• For higher temperatures combined with aggressive media, the step up is FFKM (perfluoroelastomer), with a ceiling around +260 °C and short-term capability into the +300s °C depending on grade — at 20 to 50 times the price of FKM. For non-aggressive or specific media, VMQ silicone (~+200–230 °C) and FEPM / Aflas (~+200 °C) reach comparable temperatures, but neither matches FKM's oil and fuel resistance.

Watch the bottom of the range as carefully as the top. At -20 °C standard A-type FKM stiffens and stops sealing before it suffers any mechanical damage. The classic symptom is a cold-start leak that disappears once everything warms up — the ring simply is not elastic enough to follow the groove when cold. This is a formulation limit, not a defect. If your duty cycle includes genuine cold starts or outdoor winter service, do not fight standard FKM: specify a GLT-type grade rated to about -40 °C, or for fuel service at even lower temperatures move to fluorosilicone (FVMQ), which runs from about -60 °C but costs several times as much as FKM.

Compound grades: one label, several materials

The single word "FKM" hides a family of compounds that differ mainly in fluorine content and cure system. Getting the grade right often solves a problem that would otherwise push you to a far more expensive material.

Fluorine content runs roughly 66 % to 70 % by weight across the common types, and it drives a direct trade-off: more fluorine means better chemical resistance but worse low-temperature flexibility.

• Type A (~66 % F) — a VDF/HFP copolymer; the standard, everyday grade
• Type B / Type F (~68–70 % F) — terpolymers (VDF/HFP/TFE) with better resistance to fuels, methanol blends and aggressive chemistry, at the cost of cold flexibility
• GLT / GFLT — low-temperature grades (peroxide-cured) to about -40 °C, with GFLT adding fuel resistance
• ETP — extended/extreme chemical resistance, including to some amines and polar solvents that defeat ordinary FKM
• Peroxide-cured grades — better resistance to hot water, steam and acids than the standard bisphenol cure

That last point matters more than most catalogues admit: standard bisphenol-cured FKM is poor in hot water and steam, while peroxide-cured compounds tolerate it considerably better. Bisphenol is the workhorse — easy to process, and the reason FKM holds such a low hot compression set — but the cure chemistry itself is what water, steam and bases attack. Peroxide curing buys that resistance back, and it is also the cure behind the GLT/GFLT low-temperature grades. So when you order, state not only "FKM" but the governing requirement — frost, fuel, steam, or a specific chemical — so the supplier can pick the cure and fluorine level correctly. Ordering bare "FKM 75" and hoping is how the wrong grade ends up in a groove.

Properties at a glance

Values are typical for general-purpose FKM. Always confirm the specific compound's datasheet and a chemical-compatibility chart for your exact medium.

Chemical resistance: where FKM excels

FKM's home turf is the combination that slowly destroys nitrile:

• Mineral oils and hot oils — the primary automotive and hydraulic use case
• Aliphatic and aromatic hydrocarbons — including aromatics that swell NBR
• Fuels — including biodiesel (FAME), but only with a suitable peroxide-cured, high-fluorine grade, confirmed on the datasheet; standard bisphenol-cured FKM is not recommended for biodiesel, especially at elevated temperature
• Mineral acids and many oxidising media
• Ozone, oxygen and weathering — FKM does not surface-crack in open air the way NBR does, and it has low gas permeability

The precise cocktail that ages nitrile in an engine bay — hot oil + fuel + ozone — is exactly where FKM is designed to live, which is why it is the OEM engine-seal default.

Where FKM fails

No elastomer is universal, and FKM's weaknesses are unusually sharp because they trace back to the same fluorinated backbone that gives it its strengths:

1. Ketones (acetone, MEK) — swell and soften FKM badly
2. Esters and ethers — similar attack; biodiesel is a fatty-acid methyl ester, which is why it needs the qualified grade noted above
3. Amines and ammonia — chemically degrade standard FKM
4. Glycol-based brake fluid (DOT 3/4) — degrades FKM; this is EPDM's job
5. Hot water and steam — standard bisphenol-cured FKM is poor here; peroxide-cured grades help but EPDM is still the correct choice for dedicated steam service
6. Low temperature — covered above; a physical, not chemical, limit

Putting Viton on a brake system or a steam circuit is an expensive mistake — that is EPDM territory, and EPDM is considerably cheaper too. Note the mirror image: EPDM does not belong in mineral oils. For amines, strong bases, steam and sour gas in oil & gas service, the right specialist is FEPM (Aflas), which also works to about +200 °C. And for the genuinely universal chemical envelope, FFKM.

Before ordering, always verify the specific material–medium pair in a chemical-compatibility chart. With blends and additive-laden fluids the exact formulation decides, not the media category — a "10 % acid" and a "10 % acid with surfactant and glycol" can behave very differently.

FKM vs NBR at a glance

The most common real-world question is not "FKM or FFKM" but "do I actually need FKM, or is NBR enough?" Work upward from the cheapest material that fits — over-specifying FKM where NBR would do is wasted budget, and it is a surprisingly common way to double a seal bill for no benefit.

If you are sealing mineral oil up to +100 °C with no aggressive chemistry, NBR is the correct and cheapest choice. FKM earns its price only when one of three things pushes past what nitrile can take: temperature above roughly +100 °C, fuel or aromatic content in the medium, or ozone and open-air ageing. For a full side-by-side with worked selection examples, see the dedicated article NBR vs FKM (Viton) — which O-ring material to choose.

Shore hardness and colours

FKM is commonly supplied in 60 to 90 Shore A, with 75 Shore A the everyday default — it gives the best balance of seal force and easy assembly, soft enough to seat and follow the groove yet firm enough to resist gap extrusion at normal pressures. Choose the higher hardness (90 Shore A) at high pressures, where a softer ring risks being extruded into the clearance gap — a hard compound (or a back-up ring) resists that gap extrusion. Drop to 70 Shore A only where a lower seating force or a rougher counter-surface justifies the softer compound. Our guide to O-ring Shore A hardness covers what a durometer figure does and does not promise.

The standard colour is black, but brown, green, white and blue are all commonly available. One warning worth repeating on the shop floor: colour is not a reliable material identifier. A green ring can be FKM or HNBR; a brown ring can be FKM or something else entirely. When receiving a delivery, go by the material marking and paperwork, not the shade — our article on O-ring colours and markings sets out which conventions actually hold.

Typical applications and certifications

FKM is the OEM standard for automotive engine seals — hot-oil circuits, fuel-system O-rings, and radial shaft seals around the engine. Beyond automotive, its natural domains are:

• Aerospace — fuel, hydraulic and high-temperature seals
• Chemical processing — acids, hydrocarbons, solvents (within the limits above)
• Oil & gas — downhole and surface equipment exposed to hydrocarbons and heat
• General high-temperature service — wherever heat meets an aggressive medium

Two certifications come up repeatedly and neither is standard on an ordinary ring:

• FDA-compliant grades for food and beverage contact
• Explosive-decompression-resistant (ED / RGD) grades for oil & gas, where rapid pressure release would otherwise blister a normal compound

Both are properties of the compound, not the material class. FDA compliance turns on every ingredient — fillers and process aids included — coming from the permitted list for repeated food contact, and it does not imply EU food-contact conformity. ED/RGD resistance is demonstrated by a qualification test on that specific compound, so check the test pressure, temperature and gas match your service.

If you need either, request it explicitly and get the documentation — do not assume a generic FKM ring carries it.

Frequently asked questions

Are Viton and FKM the same thing?

Yes. FKM is the technical designation per ASTM D1418, FPM is the older ISO 1629 designation for the same rubber, and Viton® is one manufacturer's trade name. Tecnoflon and Dai-el are further trade names for the same material class.

Can I use FKM with brake fluid?

No. Glycol-based (DOT 3/4) brake fluid degrades FKM. The correct material for brake systems is EPDM — and it works the other way too: EPDM does not belong in mineral oils.

Why does my FKM O-ring leak on cold mornings?

Standard A-type FKM stiffens near -20 °C and stops following the groove until it warms. It is a low-temperature elasticity limit, not damage. Specify a GLT-type grade (to ~-40 °C) or fluorosilicone (FVMQ, from ~-60 °C) for genuine cold service.

Is standard FKM OK for steam?

Not the standard bisphenol-cured grade — it is poor in hot water and steam. Peroxide-cured FKM tolerates it better, but for dedicated steam or hot-water service EPDM is the correct and cheaper choice.

When is FFKM worth the extra money?

When the medium or temperature beats even FKM: ketones, esters, amines, steam, or continuous operation above +200 °C. FFKM handles roughly +260 °C continuously but costs 20 to 50 times as much as FKM — deploy it only where downtime costs far more than the seal, typically in semiconductors, pharma and extreme chemistry.