NBR vs FKM (Viton): which O-ring material to choose

NBR or FKM? This is the single most common material decision a maintenance engineer or purchaser faces, because these two elastomers cover the large majority of industrial O-ring applications between them. Get it right and the seal outlives the equipment; get it wrong and you either pay five times too much or you replace a cracked ring every few months. This NBR vs Viton guide compares FKM and nitrile head-to-head on the four things that actually decide the choice — temperature, the medium being sealed, environmental exposure, and cost — and tells you exactly when each one is the correct answer, and when neither is.

The short answer

Choose NBR (nitrile) when you are sealing mineral oils, hydraulic fluid, fuels or compressed air at temperatures up to about +100 °C, and the seal is not continuously exposed to weather, ozone or sunlight. Nitrile is the workhorse: cheap, tough, good with everything petroleum-based.

Choose FKM (Viton) when the temperature runs hotter than nitrile can survive, when the medium is aggressive — hot oils, aromatic or oxygenated fuels, many acids — or when the seal lives outdoors exposed to ozone and UV. FKM is the specialist: far more capable, several times more expensive, and worse than a properly selected nitrile in very cold conditions. The dedicated FKM (Viton) material guide goes deeper on grades, cure systems and limits.

A useful mental model: NBR is the default. FKM is what you reach for when NBR's temperature ceiling, chemical resistance or weathering resistance is not enough. Neither is "better" in the abstract — they solve different problems.

The decision at a glance

The table below is the core of the comparison. Read across each row and the choice usually declares itself within two or three rows.

Two rows in that table surprise people. First, FKM is not chemically superior across the board — it fails on exactly the same polar media that defeat NBR (ketones, esters, glycol brake fluid, amines). Second, NBR wins in the cold — a correctly chosen nitrile stays flexible below the point where standard FKM has already turned glassy.

Temperature: the first thing that decides it

Temperature is usually the row that settles the argument, so start here.

Standard NBR operates continuously from roughly −30 °C to +100 °C, with short-term peaks to about +120 °C. Push a nitrile ring past +100 °C for any length of time and it hardens, cracks and takes a permanent compression set — it stops springing back against the gland, and the seal leaks. This degradation is cumulative and irreversible; the ring does not recover when it cools down.

Standard FKM runs continuously from about −20 °C to +200 °C, with short-term excursions to around +230 °C. That +200 °C ceiling is the single biggest reason to specify Viton. Any application where a nitrile seal would cook — engine bays, hydraulics running hot, high-temperature zones near (but not sealing) steam or exhaust lines, process equipment — is FKM territory.

The cold end is where intuition misleads people:

• NBR's −30 °C floor depends on the compound. Nitrile is a copolymer of butadiene and acrylonitrile, and the acrylonitrile (ACN) content is a lever. High-ACN grades resist oil and fuel swell better but get stiff sooner in the cold; low-ACN grades stay flexible to lower temperatures but swell more in oil. You trade cold flexibility against oil resistance.
• Standard FKM stiffens somewhere around −15 to −20 °C and stops sealing before it is mechanically damaged — it goes glassy, loses its elastic recovery, and leaks. It is not destroyed, but it is not sealing either. The exact point is grade-dependent, which is why the flat "−20 °C" figure should be treated as approximate.
• For cold service that still needs FKM chemistry, low-temperature grades such as FKM GLT extend the floor to roughly −40 °C. If you need both very low temperature and fuel resistance, fluorosilicone (FVMQ) reaches about −60 °C, though with weaker mechanical properties.

Practical rule: if your continuous operating temperature sits comfortably inside −30 °C to +100 °C, temperature alone does not force FKM. If you routinely exceed +100 °C, temperature alone rules NBR out.

Media: where each one wins and where it fails

Once temperature is compatible, the medium being sealed is the next filter. This is where the two materials genuinely diverge in character.

What NBR handles well

• Mineral oils and greases
• Hydraulic oil (HLP and similar mineral-based fluids)
• Petrol and diesel
• Propane / LPG and many aliphatic hydrocarbons
• Cold water and air (pneumatics)

Where NBR fails

• Ozone, sunlight and weathering — nitrile surfaces craze and crack over time outdoors. This is a real limitation for any exposed seal.
• Ketones (acetone, MEK), esters, and aromatic hydrocarbons in concentration
• Glycol-based (DOT) brake fluid
• Strong acids and oxidisers

What FKM handles well

• Mineral oils and hot oils
• Aromatic hydrocarbons and oxygenated / ethanol-blended fuels
• Biodiesel — but only with a biodiesel-rated, peroxide-cured FKM grade (see the caveat below); standard bisphenol-cured Viton is not a safe default for FAME fuel
• Many mineral acids and oxidising media
• Ozone, oxygen and UV — excellent weathering resistance, which is a headline FKM advantage
• Broad chemical resistance at elevated temperature

A word on biodiesel, because it is the one place where "FKM resists fuels" is routinely overstated. Standard bisphenol-cured FKM is actually degraded by biodiesel (FAME) — especially hot fuel, or fuel that has aged to a high acid value — because the bisphenol cure site is attacked by the basic constituents of FAME fuel, with dehydrofluorination of the polymer backbone following on. This includes typical dipolymer grades such as Viton A, which are bisphenol-cured. Reliable long-term biodiesel service needs a specialised peroxide-cured grade (e.g. GBL-S / GFLT-S). Do not assume a generic Viton ring will survive continuous FAME exposure; specify a biodiesel-rated compound.

Where FKM still fails

FKM is not a universal solvent-proof rubber. It is attacked by:

• Amines (and amine-containing corrosion inhibitors / additives)
• Ketones such as acetone, and esters
• Ammonia
• Glycol-based brake fluid
• Hot water and steam (limited; standard grades degrade)

Notice that the last three bullets of NBR's failures and FKM's failures overlap heavily: for glycol brake fluid, ketones, esters and hot water/steam, neither NBR nor FKM is the right rubber. That is a signpost to EPDM or a specialty material — covered below.

Breaking the tie in the overlap zone

Both materials seal mineral oils and standard fuels well, so there is a genuine overlap where either would physically work. When you land there, let temperature and environmental exposure break the tie:

1. Cool oil, enclosed, indoors → NBR. Cheaper, and nothing about the duty needs FKM.
2. Hot oil, engine bay, or ozone/UV exposure → FKM. The temperature or the weathering demands it.
3. Ethanol-blended or aromatic fuel → FKM, because nitrile swells and degrades against aromatics.
4. Cost-critical, high volume, benign duty → NBR, unless a specific failure mode says otherwise.

Static vs dynamic duty, pressure and extrusion

Material and temperature are only half the specification. How the seal moves and how hard it is squeezed decides whether the ring survives mechanically, independent of chemistry.

• Static seals (a fixed face or gland, no relative motion) are the forgiving case. Almost any correctly sized NBR or FKM ring will do, and softer compounds are fine.
• Dynamic seals (reciprocating rods, rotating shafts) add abrasion and friction. Here the compound and hardness matter as much as the polymer family: you want good abrasion resistance and a surface finish matched to the ring. This is also why silicone (VMQ), despite its temperature range, is not a dynamic-duty material — it abrades and tears too easily.
• Pressure drives extrusion. Under high pressure, a soft ring is forced into the clearance gap between the mating parts and shaves off — classic extrusion failure. The two defences are higher hardness (75–90 Shore A instead of the default 70) and tighter gland clearances, sometimes with anti-extrusion back-up rings. Neither NBR nor FKM is inherently immune; the fix is durometer and gland design, not a change of elastomer.

The takeaway: choose the polymer for temperature, media and weathering, then choose hardness and gland fit for the mechanical duty. The two decisions are separate, and skipping the second one is a common cause of "the right material still leaked."

Cost: the FKM premium is real but not the whole story

FKM material costs roughly five times as much as NBR for an equivalent ring. That premium is real, and on a bill of materials with thousands of seals it is not trivial. But raw material price is the wrong number to optimise in isolation. The number that matters is the cost of failure:

• On high-downtime equipment — where a leaking seal means an unplanned stop, lost production, or a safety event — the FKM premium is negligible against one avoided failure.
• On low-duty, easily accessed, benign seals, paying 5× for capability you will never use is simply waste.

Do not "upgrade to Viton to be safe" reflexively. If NBR meets the temperature, media and exposure requirements with margin, NBR is the safe choice — and the cheaper one.

Choose NBR when…

Pick nitrile when all of these hold:

• The medium is mineral oil, hydraulic fluid, grease, petrol, diesel, LPG or compressed air
• Continuous temperature is at or below about +100 °C (and not below roughly −30 °C for standard grades)
• The seal is enclosed — not continuously exposed to ozone, UV or weather
• The specification is cost-sensitive and no specific chemical, thermal or weathering requirement demands more

Choose FKM (Viton) when…

Step up to fluoroelastomer when any one of these applies:

• Continuous temperature exceeds NBR's capability (roughly above +100 °C, up to about +200 °C)
• The medium is hot oil, aromatic or oxygenated fuel, a mineral acid, or biodiesel with a biodiesel-rated peroxide-cured grade
• The seal is exposed to ozone, oxygen, sunlight or general weathering
• You need better compression-set recovery at elevated temperature — the seal must keep pushing back after long hot service

If none of those triggers is present, you are almost certainly paying for FKM you do not need.

When the answer is neither: HNBR, EPDM, VMQ, FFKM

Quick decision flow: hot oil above +100 °C? → HNBR. Brake fluid, hot water, steam, or ketones and esters at ambient temperature? → EPDM. Widest temperature band in a static seal? → VMQ. Amines, hot ketones and esters, or above +200 °C continuous? → FFKM.

A surprising number of "NBR vs Viton" questions are actually asking the wrong question — the correct material is a third one. Use this table to catch those cases:

A few notes on each:

• HNBR (hydrogenated nitrile) is the natural step between NBR and FKM for hot oil specifically. It keeps nitrile's oil and fuel resistance, but the reason you reach for it is heat: it pushes the oil temperature ceiling up to about +150 °C and adds meaningfully better ozone resistance, at roughly 3× NBR cost — cheaper than FKM. For aromatic-rich fuel alone, a high-ACN NBR is often just as good; HNBR earns its premium on hot oil and heat/ozone duty rather than on fuel resistance.
• EPDM is the correct choice for glycol brake fluid, hot water, steam and many polar solvents — including ketones and esters at ambient and moderate temperature — and it has excellent ozone and weather resistance. Critically, EPDM is not compatible with mineral oils and petroleum fuels — never substitute it into an oil seal. It is the standard material for brake systems and potable-water applications.
• VMQ (silicone) covers an extremely wide temperature band (about −60 °C to +200 °C) with outstanding flexibility, but it has poor abrasion resistance and tensile strength — treat it as a static-seal material, not for dynamic or high-pressure duty. (Silicone does see occasional low-friction dynamic use, but for a buyer's guide, specify it static.)
• FFKM (perfluoroelastomer) is the last resort for chemistry that defeats even FKM — amines, and ketones or esters hot enough to take EPDM out of the running as well — and for continuous service above +200 °C (grades reach well beyond +260 °C). It costs on the order of 20–50× FKM, so it is reserved for semiconductor, aggressive-chemical and critical high-temperature applications.

Practical selection notes

A few things that trip up buyers in the real world:

• Hardness (durometer) is a separate axis. Both NBR and FKM are commonly supplied around 70 Shore A, the general-purpose hardness default. Higher hardness (75–90 Shore A) resists extrusion at high pressure or with large gland gaps; softer compounds seal better on rough surfaces at low pressure. Choosing the elastomer does not choose the hardness — specify both.
• Swell and compression set are what actually fail. A seal rarely dies from a single dramatic event; it dies slowly from chemical swell (the medium is wrong) or compression set (the temperature is wrong and the ring has stopped springing back). The material choice is really a bet on keeping both of those under control for the service life.
• Confirm against the actual fluid, not the category. "Oil" and "fuel" and "coolant" are families with additives. When in doubt, check the specific fluid — especially fuels with ethanol or FAME/biodiesel content and brake/coolant fluids — against a compatibility chart before committing.

Frequently asked questions

Is FKM (Viton) always better than NBR?

No. FKM is more capable, not universally superior. In cool mineral-oil applications it costs roughly five times more and actually performs worse than a properly chosen nitrile at very low temperature. It is also attacked by the same polar media (ketones, esters, glycol brake fluid, amines) that defeat NBR, and standard grades are not a safe default for biodiesel. Capability you do not use is just cost.

NBR vs Viton — which lasts longer?

Whichever one matches the duty. Service life is set by how well the material tolerates the temperature and the medium, not by which polymer is "stronger." A correctly specified NBR ring in cool hydraulic oil will outlast an FKM ring dropped into hot glycol brake fluid, and vice versa. Match the material to the four filters — temperature, media, exposure, mechanical duty — and the longer-lasting choice is whichever one has margin on all four.

Can I swap an NBR ring for an FKM one of the same size?

Dimensionally, usually yes — the sizes are standardised. Functionally, only if the application genuinely needs FKM's properties. The riskier direction is the reverse: replacing an FKM ring with NBR "to save money." If the original spec called for FKM, it was almost certainly for a thermal, fuel or ozone reason that NBR cannot meet, and the substitution will fail.

Both rings are black — how do I tell NBR from FKM apart?

You cannot reliably tell by looking. Colour proves nothing — both are commonly black, and as the guide to O-ring colours and markings explains, colour is not a standardised material code. Rely on material markings, part numbers and supplier documentation, not visual inspection. (A density check can distinguish them in a lab — FKM is noticeably denser — but that is not a shop-floor method.)

Is Viton the same as FKM?

Yes, functionally. Viton is a trade name (originally DuPont, now Chemours) for fluoroelastomer. FKM is the ASTM D1418 code; FPM is the equivalent DIN/ISO 1629 code — the same material under two designation systems. Not every FKM is branded Viton, but every Viton is an FKM.

What about temperatures between +100 and +150 °C in oil?

That band is exactly where HNBR earns its place — oil-resistant like nitrile, but rated to about +150 °C, and cheaper than FKM. Consider it before defaulting to Viton for moderately hot oil.