Material Comparison6 min read
FKM vs NBR: Which Rubber Is Right for Your Application?
Nitrile is the default for oil and it is right far more often than it is specified. Fluoroelastomer is the escalation, not the upgrade — and on both sides the grade matters as much as the polymer, because each family buys its chemical resistance with the same currency: performance in the cold.
Published 2 September 2026 · Updated
FKM or NBR: which should you specify?
A note on “Viton”
FKM (Viton®) is the material class; Viton® is a trademark of The Chemours Company. The two are used interchangeably in everyday specification and they should not be.
The trademark covers a family of grades with materially different properties — different fluorine contents, different cure systems, different low-temperature limits. A drawing calling for “Viton 75” has specified a hardness and a brand, and left the chemistry open. Two compounds that both answer that description can behave differently in the same fluid at the same temperature.
Specify FKM with the duty, or with an ASTM D2000 line callout. Section 07 shows how. We use FKM throughout the rest of this guide.
NBR: the number that is not on your drawing
Nitrile is a copolymer, and the proportion of acrylonitrile in it is what makes it oil resistant. Commercial grades run from 18 % to 50 % acrylonitrile, and the difference across that range is not marginal.
More acrylonitrile means more oil and fuel resistance, and worse flexibility in the cold. The two move in opposite directions, always. There is no grade that is best at both, which is why the question “is nitrile good in oil?” has no single answer.
| Grade | Acrylonitrile | Behaviour | Typical duty |
|---|---|---|---|
| Low ACN | ~15–25 % | Best low-temperature flexibility and resilience; limited oil resistance | Cold-climate service |
| Medium ACN | ~25–35 % | Balanced — the general-purpose middle | General hydraulics, around 31–35 % |
| High ACN | 35 % and above | Maximum oil and fuel resistance; the cold end suffers | Diesel and petrol systems, 36 % and up |
So “NBR 70 Shore A” on a drawing is a hardness, not a specification. A seal that hardens on a cold morning and one that swells in diesel can both be supplied against it, entirely correctly. Give the fluid and the cold end, and the acrylonitrile content follows.
FKM: the same trade-off, a different element
Fluoroelastomer works the same way, and it buys its performance with the same currency. Fluorine content sets the chemical and heat resistance, and the more of it there is, the worse the compound behaves cold.
| Type | Chemistry | Fluorine | Character |
|---|---|---|---|
| A | Copolymer of vinylidene fluoride and hexafluoropropylene | ~66 % | The general-purpose fluoroelastomer |
| F | Terpolymer adding tetrafluoroethylene | 68–69.5 % | Better chemical and heat resistance, at the cost of compression set and low-temperature flexibility |
| GLT | Low-temperature grade | ~65 % | Flexible to roughly −40 °C; not for flex fuels or aggressive solvents |
| GFLT | Low-temperature grade retaining chemical resistance | ~67–68 % | The compromise grade — good cold performance with fuel and chemical resistance retained |
This is why the low-temperature limit is the number to check on any FKM specification. A standard grade is commonly limited to around −20 °C. A compound chosen entirely for heat can fail on a cold start, and the customer will describe it as a seal that leaked in winter rather than as a specification error.
Low-temperature grades exist and reach roughly −40 °C. They cost more and, in the case of GLT, give up some chemical resistance to get there. Tell us the cold end and we will pick the grade; do not assume the standard one covers it.
Where FKM is the wrong answer
Fluoroelastomer is expensive, so it is treated as the safe choice. It is not universally better than nitrile, and there are duties where it is clearly worse.
Steam and hot water. FKM is a poor choice in steam; EPDM is the right answer there, and specifying a fluoroelastomer because it is the premium option is how that mistake happens.
Glycol-based brake fluid. DOT 3 and DOT 4 attack FKM. EPDM again.
Ketones, esters and ethers. Acetone and MEK are the familiar examples. Worth noting that biodiesel is a fatty-acid methyl ester — an ester — which catches people out on fuel-system parts.
Amines and ammonia. Common in refrigeration and in some process chemistry.
And the cold, as section 05 sets out. Where FKM genuinely runs out — on chemistry rather than temperature — the escalation is FFKM, at a further large step in cost.
FKM and NBR, side by side
| Property | NBR | FKM | Verdict |
|---|---|---|---|
| Temperature range | −30 to +120 °C | −20 to +250 °C continuous to +200 °C | FKM hot, NBR cold |
| Mineral oil, fuel, grease | Very good — set by ACN content | Excellent, including hot oil | NBR is enough for most duty |
| Solvents and aggressive chemicals | Limited | Excellent | FKM |
| Steam and hot water | Poor | Poor | Neither — use EPDM |
| Glycol brake fluid | Poor | Poor | Neither — use EPDM |
| Ozone, UV, weathering | Poor | Very good | FKM |
| Hardness range | 40–90 Shore A | 60–90 Shore A | NBR covers more, softer |
| Relative cost | Low | High | NBR, by a wide margin |
Cost-in-use, not unit price. A fluoroelastomer seal that outlasts three nitrile ones in a hot application is cheaper, once the downtime of two changeovers is counted. A fluoroelastomer seal specified for a duty nitrile would have handled is simply more expensive. The question is not which material is better — it is whether the duty justifies the escalation.
ASTM D2000 does this compactly, and it is the format to put on the drawing. Its Type letter is the heat-ageing temperature and its Class letter is the maximum oil swell, so nitriles typically fall in the BF, BG and BK classes as oil resistance increases, and fluoroelastomers in HK — Type H for 250 °C ageing, Class K for the tightest swell requirement. The full Type and Class tables are in our gasket material selection guide.
Or simply tell us the fluid, both temperatures and the hardness, and we will write the callout with you.
The compound most people skip past
The FKM-versus-NBR question is usually asked because nitrile has run out of temperature, not because the chemistry demands a fluoroelastomer. That is a different problem, and it has a different answer.
AEM sits between them — −40 to +175 °C, continuous to about +160 °C — with good hot-oil resistance and a low-temperature range that fluoroelastomer cannot match without a specialist grade. It costs substantially less than FKM.
For a hot under-bonnet part running in oil at +150 °C, AEM is frequently the correct answer and FKM is the one that gets specified, because AEM is not on most compound menus. It is on ours.
What we test, and what this guide does not cover
We test tensile strength and elongation to ASTM D412, and compression set to ASTM D395 Method B, in our own lab at Nashik, and those results ship with every consignment alongside a dimensional report and a certificate of conformance. Fluid immersion and heat-ageing tests to a D2000 Type and Class are arranged through an external laboratory where a drawing calls for them.
One exclusion, stated plainly. Fluoroelastomer is closely associated with oilfield service, and that market has its own qualifications — explosive decompression resistance to NORSOK M-710, and API specifications for downhole and wellhead equipment. We hold none of them, and nothing in this guide should be read as a claim to one. Our FKM work is industrial and automotive sealing.
We hold ISO 9001:2015, certificate 09111526A, Integral Certification Ltd, valid to 21 February 2027, scope: rubber moulded components — a quality-management certification, not a product approval. We are upgrading to IATF 16949, targeted by the end of 2026.
Five mistakes when choosing between FKM and NBR
Treating FKM as the safe upgrade
It fails in steam, in glycol brake fluid, in ketones and esters, and in the cold. It is an escalation for specific duties, not a better version of nitrile.
Writing “NBR” and a hardness
Acrylonitrile content runs from 18 % to 50 % and decides both oil resistance and cold flexibility. Give the fluid and the cold end instead.
Ignoring the cold end of an FKM spec
Standard grades are commonly limited to around −20 °C. Low-temperature grades reach roughly −40 °C and have to be asked for.
Jumping from nitrile straight to fluoroelastomer
AEM sits between them, continuous to about +160 °C, at a fraction of the cost. Most temperature escalations stop there.
Forgetting that biodiesel is an ester
FKM resists conventional diesel and is attacked by esters. A fuel-system part that was fine on mineral diesel may not be on a blend.
Frequently asked questions
Is FKM better than NBR?
Only where the duty demands it. FKM handles heat to about +250 °C and aggressive chemicals that nitrile cannot; it is worse than nitrile in the cold and it fails in steam and in glycol brake fluid. For ordinary oil, fuel and hydraulic duty up to about +120 °C, NBR does the job at a fraction of the price.
What is the lowest temperature FKM can handle?
Standard grades are commonly limited to around −20 °C, which surprises people who specify fluoroelastomer for heat. Low-temperature GLT and GFLT grades reach roughly −40 °C — GLT gives up some chemical resistance to get there, GFLT retains more. Always state the cold end of the duty.
What does the acrylonitrile percentage in nitrile mean?
It sets the oil resistance. Commercial grades run from 18 % to 50 %: more acrylonitrile means better oil and fuel resistance and worse flexibility in the cold. Around 31–35 % suits general hydraulics; 36 % and above suits diesel and petrol systems; 18–24 % suits cold-climate service.
Can I use FKM with brake fluid or steam?
No to both. Glycol-based DOT 3 and DOT 4 brake fluid attacks FKM, and FKM is a poor choice in steam. EPDM is the correct compound for both — and EPDM must never be used in mineral oil, which is the mirror image of the same mistake.
Do you supply oilfield-qualified seals?
No. We hold no NORSOK M-710 explosive decompression qualification and no API specification approval. Our fluoroelastomer work is industrial and automotive sealing. We hold ISO 9001:2015, certificate 09111526A, scope rubber moulded components — a quality-management certification, not a product approval. Minimum run 10,000 pieces per part number, per compound; we respond within 48 hours.
Tell us the fluid, the hot end, the cold end and the pressure — and we will tell you whether nitrile covers it, whether AEM is the cheaper way up, or whether the duty really does need a fluoroelastomer. We mix all three, so we have no reason to sell you the expensive one. We respond within 48 hours.