Viton (FKM) vs Nitrile O-Rings & Seals
Nitrile and FKM both seal against oil and fuel, so the choice between them is not about whether they work, but about how hot and how aggressive the environment gets, and how much you want to spend. Nitrile (Buna-N) is the low-cost workhorse for standard petroleum service to about 120°C. FKM, the fluoroelastomer sold under trade names like Viton, adds high-temperature and broad chemical resistance to roughly 200°C, at several times the cost.
This is the most common o-ring and seal material decision in industry. This guide compares the two so you specify the right one, using FKM only where nitrile genuinely cannot cope. Wayne Rubber molds both materials at our Wixom, Michigan plant.
A Note on Names
Viton is a registered trade name for a family of fluoroelastomers, now owned by Chemours; the generic ASTM designation is FKM. Nitrile, NBR, and Buna-N are three names for the same acrylonitrile-butadiene copolymer. Because FKM comes in several grade types (A, B, F, GF, GLT) with different chemical profiles, and nitrile behavior is set by acrylonitrile (ACN) content, the correct way to specify either is by ASTM designation and grade rather than by trade name. This page uses “FKM” and “nitrile,” with “Viton” where it helps match how the decision is searched.
Quick Comparison
| Property | Nitrile (NBR / Buna-N) | FKM (Viton type) |
|---|---|---|
| Petroleum oil and fuel resistance | Very good | Excellent |
| Aggressive chemical / solvent resistance | Limited | Excellent |
| Continuous service temperature | -40°C to +120°C | -20°C to +200°C |
| Low temperature flexibility | Good (low-ACN grades) | Fair (standard grades) |
| Ozone, UV and weathering | Poor. Cracks outdoors | Excellent |
| Aromatic and chlorinated solvents | Poor | Very good |
| Water, steam and brake fluid | Fair to poor | Fair. Poor in steam |
| Post-cure required | No | Yes, to reach full properties |
| Relative compound cost | Low | High (several times nitrile) |
| Typical hardness range | 40 to 90 Shore A | 60 to 90 Shore A |
The Two Materials
What Is Nitrile (NBR / Buna-N)?
Nitrile is a copolymer of acrylonitrile and butadiene and the most widely used sealing elastomer in the world, thanks to its strong oil and fuel resistance at low cost. Its key variable is ACN content (about 18 to 50 percent): higher ACN improves oil and fuel resistance, lower ACN improves cold flexibility, with most grades near 33 percent. Its limits are heat, above roughly 120°C it hardens, and the outdoors, where ozone cracks it. Full detail is on the nitrile (NBR) material page.
What Is FKM (Viton)?
FKM is a fluoroelastomer built on carbon-fluorine bonds, giving it thermal stability to about 200°C and near-inertness to fuels, oils, acids, and many aggressive solvents that nitrile cannot handle. It is the premium sealing material, priced accordingly, and standard grades are post-cured to develop full properties. Grade type is chosen to match the specific chemistry. See the FKM material page.
Property by Property
Oil and Fuel
Both resist petroleum oils and fuels well, which is why both are common in fuel and hydraulic systems. FKM has the edge with aromatic and high-ethanol fuels that cause excessive swell in even high-ACN nitrile, but for standard petroleum service nitrile seals reliably at a fraction of the cost. If the fuel is ordinary and temperatures are moderate, nitrile is enough.
Temperature
This is a primary divider. Nitrile is limited to about 120°C continuous before it hardens; FKM runs to roughly 200°C and holds its properties through long heat aging. Any hot-oil or under-hood application beyond nitrile’s ceiling points to FKM, or to HNBR as an intermediate step.
Aggressive Chemicals and Solvents
FKM wins decisively. It resists acids and aromatic and chlorinated solvents such as toluene and trichloroethylene that rapidly degrade nitrile. For chemical processing and solvent exposure, nitrile is not a candidate and FKM is the answer, with grade matched to the fluid.
Weather, Ozone and Temperature Extremes
FKM resists ozone and UV; standard nitrile does not and cracks outdoors. On the cold end, standard nitrile stays flexible slightly better than standard FKM, though low-temperature FKM grades close that gap. For outdoor petroleum service, FKM or a protected nitrile blend is the durable choice.
Cost and Processing
Nitrile is one of the most economical sealing elastomers; FKM costs several times more per pound and requires a post-cure step. The engineering discipline is to use nitrile wherever temperature and chemistry allow, and reserve FKM for the heat, fuel, and chemical conditions that genuinely require it. Matching material to the real environment is where custom formulation earns its keep.
Typical Applications
Choose nitrile for: standard fuel and hydraulic o-rings and gaskets, engine oil seals, pneumatic seals, and cost-sensitive petroleum service at moderate temperatures.
Choose FKM for: high-temperature fuel-system seals and o-rings, chemical processing gaskets, aromatic and high-ethanol fuel service, turbocharger and under-hood parts, and aerospace and oil-and-gas sealing.
How to Choose in 30 Seconds
- Is the temperature above about 120°C? Yes: FKM (or HNBR as an intermediate).
- Are aggressive chemicals or solvents present? Yes: FKM.
- Is it aromatic or high-ethanol fuel? FKM handles swell better.
- Is it standard oil or fuel at moderate temperature? Nitrile, at a fraction of the cost.
- Is it a low-temperature petroleum seal? Nitrile, or a low-temperature FKM grade if chemicals also apply.
- Unsure how hot or aggressive it gets? Send us the fluid, temperature, and o-ring or seal spec. We will recommend the material and grade. Contact Wayne Rubber.
Frequently Asked Questions
Is Viton better than nitrile for o-rings?
Only where its strengths are needed. FKM (Viton) o-rings handle higher temperatures, aggressive chemicals, and aromatic fuels far better than nitrile. But nitrile seals standard oil and fuel reliably at a fraction of the cost, so for moderate-temperature petroleum service nitrile is usually the better value.
What temperature can nitrile and FKM o-rings handle?
Standard nitrile is limited to about 120°C continuous before it hardens. FKM runs to roughly 200°C and retains its properties through long heat aging, which is the main reason to step up from nitrile to FKM.
Do both nitrile and FKM resist fuel?
Yes, both resist petroleum fuels. FKM has the advantage with aromatic and high-ethanol fuels that cause excessive swell in nitrile, so for modern ethanol-blended or aromatic fuels FKM is the more durable choice.
Why is nitrile so much cheaper than Viton?
Nitrile uses common, low-cost monomers, while FKM is a fluoroelastomer with expensive fluorinated chemistry and a required post-cure step. FKM typically costs several times more per pound, so it should be specified only where temperature or chemistry require it.
Is there a middle option between nitrile and FKM?
Yes. HNBR (hydrogenated nitrile) keeps nitrile’s oil and fuel resistance while raising the temperature ceiling and adding ozone resistance, at a cost between nitrile and FKM. It is the common upgrade when nitrile runs too hot but full FKM is not required.
Nitrile or FKM O-Rings? We Will Confirm the Right Spec
Send us the fluid, temperature, and o-ring or seal dimensions, and our engineers will recommend the material and grade, and quote your parts. In-house nitrile compounding and molding from our Wixom, Michigan plant.
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