Viton (FKM, fluorocarbon rubber) and Buna-N (NBR, nitrile rubber) together cover the majority of industrial O-ring applications. Viton is specified when temperature exceeds +120°C, when fuel chemistry includes aromatic or alcohol components, or when chemical exposure would degrade Buna-N; Buna-N is the cost-effective baseline for petroleum fluid and hydraulic service below +120°C. The price differential is 3–5× (Viton costs more), but in most cases where Viton is correctly specified, Buna-N would fail in service — the cost comparison is irrelevant when a cheaper material produces field failures. This guide gives engineers the technical data to make the decision correctly.
Quick answer: Viton (FKM): −20°C to +200°C continuous (standard grade); aromatic fuels, E85, biodiesel, ketone-free chemical processing; cost index 5–10×. Buna-N (NBR): −40°C to +120°C continuous; mineral hydraulic oil, pneumatics, water, general industrial; 70 Shore A standard; cost index 1×. Phosphate ester hydraulic fluids (Skydrol) require EPDM, butyl or specialty aerospace FKM grades — standard FKM is not Skydrol-compatible. The decision threshold is approximately +120°C — below this in petroleum oil service, Buna-N is equivalent in performance and lower in cost. Above +120°C or with aromatic/alcohol fuels, Viton prevents the failures that make Buna-N replacement frequent enough to exceed Viton's unit price premium. For steam, hot water, or amine service: neither Buna-N nor standard Viton — specify EPDM or FFKM.
Material Chemistry: Why the Differences Exist
NBR (Nitrile Butadiene Rubber)
NBR is a copolymer of acrylonitrile (ACN) and butadiene. The acrylonitrile content is the most important single parameter in NBR selection — it governs the trade-off between oil resistance and low-temperature flexibility:
| ACN Content | Low-Temp Limit (TR10) | Oil/Fuel Resistance | Common Application |
|---|---|---|---|
| 18–24% (low ACN) | −50 to −45°C | Moderate | Arctic hydraulics; low-temp sealing |
| 28–34% (medium-low ACN) | −35 to −30°C | Good | Refrigeration; cold-climate general use |
| 34–38% (medium ACN) | −25 to −20°C | Very good | General-purpose hydraulics — most common |
| 38–45% (medium-high ACN) | −20 to −15°C | Excellent | Fuel injection; aromatic hydrocarbon service |
| 45–50% (high ACN) | −10 to −5°C | Best in class | Concentrated aromatic solvents; jet fuel |
The ACN trade-off: Each 5% increase in ACN content improves oil resistance (swell in ASTM Oil 3 decreases by approximately 3–5% per 5% ACN increase) but raises the TR10 (temperature at which elastic recovery drops to 10% of original) by approximately 5–8°C. Most general-purpose NBR is compounded at 33–36% ACN — the midpoint that serves the largest number of hydraulic and pneumatic applications.
The butadiene backbone limitation: The butadiene comonomer provides elasticity but introduces C=C double bonds into the main chain. These unsaturated sites are vulnerable to: ozone attack (even trace ozone concentrations crack NBR over time), oxidative degradation above +100°C (the O₂ attacks the double bonds, forming crosslinks that harden and embrittle the material), and UV degradation in outdoor exposure. These limitations are not present in saturated backbone elastomers (EPDM, HNBR, FKM).
HNBR (Hydrogenated NBR): Hydrogenating the NBR backbone converts most C=C double bonds to C-C single bonds, producing a saturated main chain. HNBR retains the oil resistance of medium-to-high ACN NBR (because acrylonitrile groups are preserved) while eliminating the oxidation and ozone vulnerabilities. HNBR extends the continuous service temperature to +150°C and is the correct specification for oil and gas sour service, automotive timing cover seals, and high-temperature hydraulics where NBR fails but FKM is cost-prohibitive.
FKM (Fluorocarbon Rubber)
FKM is a family of fluorinated elastomers based on vinylidene fluoride (VDF) copolymerized with hexafluoropropylene (HFP) and optionally other fluoromonomers. Fluorine content by weight determines chemical resistance level:
| FKM Type | Fluorine Content | Key Improvement Over Previous | Representative Trade Name |
|---|---|---|---|
| Type 1 (VDF/HFP dipolymer) | 66% | Baseline; broad oil and chemical resistance | Viton A, FKM Type A |
| Type 2 (VDF/HFP/TFE terpolymer) | 68% | Better fuel resistance; reduced swell in ASTM Fuel C | Viton B, FKM Type B |
| Type GF (with cure-site monomer) | 70% | Better base and amine resistance (reduced dehydrofluorination) | Viton GF |
| Type GFLT (low-temp grade) | 67% | Low-temperature flexibility to −40°C | Viton GFLT |
| FFKM (perfluoroelastomer) | > 70% | Chemical resistance of PTFE; highest temperature | Kalrez, Chemraz |
For this comparison, Type 1 (66% fluorine, standard FKM) represents approximately 80% of industrial FKM O-ring consumption. The chemistry basis is the same across types — higher fluorine content increases chemical inertness by reducing the proportion of C-H bonds available for chemical attack.
Dehydrofluorination vulnerability: Standard FKM still contains some C-H bonds (at the VDF units). These C-H sites can be attacked by strong bases, primary and secondary amines, and hot steam — mechanisms that progressively remove HF from the backbone, breaking crosslinks and causing softening and disintegration. This is the primary FKM failure mode in alkaline or amine environments. Type GF was developed specifically to reduce VDF content and limit dehydrofluorination; FFKM eliminates C-H bonds entirely.
Temperature Performance Comparison
| Condition | NBR Behavior | FKM Behavior | Correct Choice |
|---|---|---|---|
| Below −40°C (cryogenic) | Below TR10 — loses all sealing force | Fails below −20°C (standard) | Low-temp NBR (special compound) or VMQ |
| −40°C to −20°C | Marginal — stiffens, may leak at startup | Fails (standard FKM); GLT/GFLT grades viable | Low-temp NBR or FKM GLT |
| −20°C to +100°C | Excellent — standard service range | Excellent — well within FKM range | NBR (lower cost) |
| +100°C to +120°C | Marginal — compression set increasing rapidly | Excellent | FKM if maintenance interval is long |
| +120°C to +150°C | Failing — oxidative degradation accelerates | Excellent | FKM; HNBR if petroleum fluid |
| +150°C to +200°C | Failed — hardened and embrittled | Excellent — within standard FKM range | FKM |
| Above +200°C | N/A | Approaching thermal limit; FFKM required | FFKM |
Compression set at elevated temperature (ASTM D395 Method B):
| Temperature / Duration | NBR 70A | FKM 70A | Significance |
|---|---|---|---|
| 70°C / 70h | 20–35% | 12–20% | Both adequate for general service |
| 100°C / 70h | 35–55% | 15–25% | NBR approaching inadequate for long intervals |
| 150°C / 70h | Not meaningful — degrading | 18–28% | FKM the only elastomeric option |
| 200°C / 70h | N/A | 20–30% | FKM long-term static sealing |
Compression set > 40% means the O-ring has permanently deformed to approximately 40% of its original CS deformation — at this point, contact stress is significantly reduced, and a system that was sealed at installation now has inadequate sealing force for the same pressure. For maintenance intervals > 6 months at elevated temperature, NBR's compression set behavior is the limiting factor.
Chemical Resistance: Specific Data Points
Petroleum-based fluids (both adequate; NBR preferred on cost)
Both NBR and FKM perform well in mineral hydraulic oils (ISO VG 32–68), petroleum-based greases, and petroleum lubricants. In IRM 903 oil immersion at +100°C (ASTM D471 test conditions commonly used for hydraulic fluid compatibility):
- NBR (medium ACN, 34%): Volume change +3 to +8%, hardness change −3 to −8 Shore A
- FKM (Type A): Volume change +1 to +3%, hardness change −1 to −3 Shore A
Both perform within acceptable limits for standard mineral oil hydraulic service. NBR is the economically correct choice for this application.
Fuel chemical resistance (FKM significantly better)
Modern fuels create a different challenge than straight mineral oil. In ASTM Fuel B (30% toluene, 70% isooctane — simulating aromatic fuel content):
| Material | Volume Swell in ASTM Fuel B (70h, 23°C) | Volume Swell in ASTM Fuel C (50% toluene, 50% isooctane) |
|---|---|---|
| NBR, 34% ACN | 25–40% | 40–65% |
| NBR, 45% ACN | 10–18% | 20–35% |
| FKM, Type A | 2–5% | 3–7% |
| FKM, Type B | 1–3% | 2–5% |
For E10 gasoline (10% ethanol), NBR 34% ACN shows 15–25% volume swell in concentrated ethanol exposure; FKM shows < 3%. For E85 (85% ethanol), NBR at any ACN content is inadequate; FKM is the standard material.
For biodiesel (fatty acid methyl esters, FAME), the methyl ester groups penetrate NBR aggressively — 15–30% volume swell in B100 biodiesel. FKM in B100 biodiesel shows < 5% swell. All automotive fuel system O-rings in biodiesel-compatible designs specify FKM or a specialty FKM/HNBR compound.
Chemical families where FKM fails (and NBR may also fail)
| Chemical | NBR Behavior | FKM Behavior | Correct Material |
|---|---|---|---|
| Steam > +100°C | Degrades — hydrolysis and oxidation | Dehydrofluorination attack | EPDM (platinum-cured) |
| Strong bases (NaOH > 10%, > 60°C) | Moderate degradation | Dehydrofluorination — same failure | EPDM or FFKM |
| Amines (MEA, DEA) | Moderate degradation | Rapid dehydrofluorination | FFKM or EPDM |
| Ketones (MEK, acetone) | 15–30% swell | 30–80% swell — worse than NBR | PTFE (static) or FFKM |
| Glycol brake fluid (DOT 3/4) | 20–40% swell — marginal | Very poor — unusable | EPDM (brake system standard) |
| Phosphate-ester hydraulic fluid | Very poor — 40–100% swell | Poor — standard FKM swells 20–40%; use EPDM, butyl or specialty FKM | EPDM / butyl / specialty FKM |
| Ozone (atmospheric) | Poor — surface cracking | Excellent — fully ozone resistant | FKM (or EPDM) |
Mechanical Properties Comparison
| Property | NBR (34% ACN, 70A) | FKM (Type A, 70A) | Notes |
|---|---|---|---|
| Tensile strength | 10–20 MPa | 8–17 MPa | NBR slightly higher; both adequate |
| Elongation at break | 250–600% | 150–300% | NBR more extensible — installs more easily |
| Tear resistance | Good (25–40 kN/m) | Moderate (18–25 kN/m) | NBR preferred for rough surfaces and dynamic |
| Abrasion resistance | Excellent | Good | NBR better for abrasive media |
| Compression set (100°C/70h) | 35–55% | 15–25% | FKM significantly better at elevated temperature |
| Compression set (150°C/70h) | Not serviceable | 18–28% | FKM only option above 120°C |
| Gas permeability | Moderate | Low | FKM better for gas-tight applications |
| Density | 1.10–1.25 g/cm³ | 1.80–1.90 g/cm³ | FKM ~60% heavier — relevant for weight-sensitive applications |
NBR abrasion advantage in dynamic service: NBR's higher tear resistance and better abrasion performance make it preferable for reciprocating dynamic seals in environments where gland surface finish may be imperfect (field-repaired cylinders, older equipment with scratched rod surfaces). FKM's lower tear resistance means it is more sensitive to sharp edges during installation and to surface defects during dynamic cycling — the same surface scratch that an NBR O-ring seals around may initiate a crack in FKM.
FKM density implication: FKM is approximately 60% denser than NBR. For the same geometric O-ring size, an FKM O-ring weighs significantly more. In aerospace applications where mass budget is a constraint, this difference is a design consideration — though for standard industrial sizes the mass difference is measured in grams and rarely matters.
Cost Comparison and Break-Even Analysis
FKM O-rings typically cost 3–5× more than equivalent NBR in standard commercial grades. The correct way to compare is total cost per year of service, not unit price:
| Scenario | NBR | FKM |
|---|---|---|
| Unit cost (AS568-214, 70A) | $0.15–0.30 | $0.60–1.20 |
| Expected life at +80°C, mineral oil | > 5 years | > 5 years |
| Expected life at +140°C, mineral oil | 3–6 months | > 5 years |
| Annual seal cost at +80°C (1 seal point) | $0.06–0.06/year | $0.12–0.24/year |
| Annual seal cost at +140°C (1 seal point) | $0.30–1.20/year (2–4× replacements) | $0.12–0.24/year |
| Break-even service temperature | ~+120°C | Same |
At +140°C service, FKM is actually less expensive annually than NBR due to longer replacement intervals. The 3–5× unit price premium converts to a cost advantage when replacement frequency is accounted for.
Non-economic factors: In applications where seal replacement requires process shutdown (chemical reactor, sealed gearbox, subsea equipment), the maintenance cost of replacement — not the seal unit cost — drives the specification. If each NBR replacement requires a $10,000 shutdown, one replacement per year costs $10,000 in maintenance versus the $1 unit cost difference. FKM is the obvious choice even if it lasts only 50% longer.
Application Selection Matrix
| Application | Recommended Material | Rationale |
|---|---|---|
| General hydraulic cylinder (< 100°C, mineral oil) | NBR 70A | Lowest cost; adequate performance |
| General hydraulic cylinder (> 100°C, mineral oil) | HNBR 70A | Improved temp vs. NBR; lower cost vs. FKM |
| Automotive fuel injection (E10, > 120°C) | FKM | Aromatic fuel + temperature combined |
| Automotive fuel injection (E85/biodiesel) | FKM | Biodiesel swell with NBR is unacceptable |
| Aerospace hydraulic (phosphate-ester fluid) | EPDM / specialty FKM | NBR incompatible with phosphate ester |
| Pneumatic cylinder (dry air, ambient) | NBR | Good abrasion; low cost; no chemical threat |
| Refrigeration (R134a, R410a, −40°C) | Low-temp NBR or HNBR | Low-temp flexibility; halogenated refrigerant compatible |
| Chemical processing (aromatics, > 100°C) | FKM | Chemical + temperature combined |
| Gas turbine fuel system (jet fuel, > 150°C) | FKM | Standard aerospace specification |
| Oilfield downhole (petroleum + H₂S + high temp) | HNBR or FKM | Petroleum + sour gas; HNBR to 150°C, FKM to 200°C |
| Outdoor weathering service | FKM or EPDM | NBR degrades in ozone/UV; FKM and EPDM do not |
| Steam/hot water (> 100°C) | EPDM | Neither NBR nor standard FKM for steam |
Viton vs Buna-N: Brand Names and Common Grades
In procurement documents and maintenance manuals, FKM is often called Viton (DuPont/Chemours trade name) and NBR is called Buna-N or Nitrile. These are material-family names, not exact compound specifications. The same brand name can cover multiple grades with very different properties.
| Common Name | Material Family | Typical Grades | Temperature Range |
|---|---|---|---|
| Viton A / FKM Type A | Standard FKM | 66% fluorine, dipolymer | −20°C to +200°C |
| Viton B / FKM Type B | Improved fuel resistance | 68% fluorine, terpolymer | −20°C to +200°C |
| Viton GLT / GFLT | Low-temperature FKM | −40°C to +200°C | Cold-climate fuel service |
| Viton GF / GFLT-S | Base/amine resistant | Reduced dehydrofluorination | Chemical processing |
| Buna-N / NBR | Nitrile | 18–50% ACN | −40°C to +120°C |
| HNBR / HSN | Hydrogenated nitrile | Saturated backbone | −35°C to +150°C |
If a drawing simply says "Viton" or "Buna-N," request the specific grade and hardness. "Viton" alone does not guarantee low-temperature GLT or base-resistant GF performance.
Procurement Checklist Before Choosing NBR or FKM
Before sending an RFQ, collect the service details that actually change the material recommendation. This prevents the common mistake of buying a cheaper NBR seal for a system that silently requires FKM, or over-specifying FKM where NBR would last just as long.
| RFQ detail | Why it matters | Practical rule |
|---|---|---|
| Fluid or gas name | Chemical swell is the first failure mode in fuel and solvent service | Use FKM for aromatic fuel, ethanol blends and biodiesel; use EPDM or specialty FKM for phosphate ester fluids |
| Continuous and peak temperature | NBR loses life quickly above about +120 C | Use FKM above +120 C unless HNBR is a better petroleum-oil option |
| Static or dynamic sealing | Dynamic seals need tear and abrasion resistance | NBR often wins in cool hydraulic cylinders; FKM wins in hot or fuel-exposed systems |
| Pressure and clearance gap | High pressure can extrude softer compounds | Consider 80A or 90A hardness, or add PTFE backup rings |
| Outdoor or ozone exposure | NBR cracks under ozone and UV | Use FKM or EPDM for weathered equipment |
| Required compliance | Food, drinking water, aerospace, or oilfield rules may limit compounds | Ask for compound-level approval data, not only material family |
Useful next steps:
- For petroleum hydraulic service below +120 C, start with NBR O-rings.
- For fuel, high heat, or aggressive chemical exposure, compare FKM O-rings and FFKM O-rings.
- If the fluid is uncertain, check the chemical compatibility tool before quoting.
- If pressure is high or the clearance gap is large, review backup rings with the O-ring selection.
Ozone, Weather and Shelf Life
NBR has poor ozone resistance: outdoor exposure, electric motors nearby, or long storage in uncontrolled conditions will crack it. FKM is effectively immune to ozone and weather. If the seal sits outdoors, in sunlight, or near ozone-generating equipment, FKM (or EPDM) is the safe default regardless of temperature.
When to Choose Neither
- Ketones, esters, glycol brake fluid → EPDM
- Hot water and steam → EPDM or AFLAS
- Amines, mixed solvents, > +200°C combined with chemicals → FFKM (see FKM vs FFKM)
- Cold below -40°C → VMQ (silicone) or specialty low-temp compounds
FAQ
Q1: Can I substitute FKM for NBR in a hydraulic system to improve durability?
Substituting FKM for NBR in a standard mineral oil hydraulic system below +100°C provides no meaningful performance benefit and adds 3–5× cost per seal. Both materials perform similarly in standard hydraulic oil at moderate temperature. Specify FKM only when the operating temperature exceeds +120°C, when the fluid includes aromatic hydrocarbons or alcohol blends at significant concentration, or when the maintenance interval justification requires lower compression set than NBR provides at the service temperature.
Q2: What is the lowest temperature at which standard FKM remains flexible?
Standard FKM (Type A, Type B) has a TR10 of approximately −20°C — the temperature at which elastic recovery drops to 10% of original. Below −20°C, standard FKM stiffens significantly and loses sealing force, particularly during cold-start conditions when pressure is applied before the seal has warmed. FKM GLT (low-temp dipolymer) and GFLT (terpolymer) grades extend this to approximately −40°C by modifying the comonomer ratio. For service that must provide a cold-start seal at −40°C, specify GLT or GFLT grade explicitly — standard FKM will not work.
Q3: Why does NBR fail faster than FKM in motor fuel applications?
Fuel chemistry has changed significantly over the past 30 years. Modern gasoline contains 10–50% aromatics (benzene, toluene, xylene) plus ethanol in E10/E85 blends. Aromatic hydrocarbons are non-polar solvents that dissolve into NBR's partially non-polar polymer matrix — the more aromatic the fuel, the more NBR swells. NBR at 34% ACN swells 40–65% in ASTM Fuel C (50% toluene), causing the O-ring to become dimensionally oversized and eventually extrude from the groove or lose mechanical integrity. FKM's fully fluorinated surface has near-zero solubility for aromatic and aliphatic hydrocarbons — it is chemically inert to all fuel components that degrade NBR.
Q4: Which material handles ozone exposure better?
FKM is fully resistant to ozone at any atmospheric concentration — the fully saturated C-F bonds are not attacked by ozone. NBR is susceptible to ozone even at trace concentrations (< 1 ppm) — ozone attacks the C=C double bonds in the butadiene backbone, causing surface cracking that propagates under tensile stress. For outdoor applications, seals in electrical equipment (where corona discharge generates ozone), or any environment near ozone-generating equipment, FKM or EPDM is required — NBR will crack and fail within months of ozone exposure.
Q5: Does NBR swell in biodiesel?
Yes — significantly. Standard NBR (34% ACN) in B100 biodiesel swells 15–30% in volume at room temperature and up to 40% at +60°C. Biodiesel's methyl ester chemistry is highly aggressive toward NBR. Even B20 blends (20% biodiesel) cause NBR swell of 5–10% over extended exposure. FKM is the correct material for any application with routine biodiesel contact — FKM swell in B100 is < 5% at room temperature and < 8% at +80°C. High-ACN NBR (> 42%) reduces but does not eliminate swell in biodiesel.
Q6: How do I specify the correct hardness for NBR and FKM O-rings?
Start with 70 Shore A for both materials as the general-purpose baseline. Increase to 80 Shore A for pressures of 150–300 bar; 90 Shore A for > 300 bar or with large clearance gaps. Lower hardness (50–60 Shore A) for vacuum service or fragile plastic mating surfaces. Note that FKM at 50 Shore A has inherently lower tear resistance than NBR at the same hardness — FKM below 60 Shore A should be used only in static applications with smooth, well-chamfered grooves. For dynamic FKM seals, 70 Shore A minimum is strongly recommended.
Q7: What is HNBR and when should I choose it over both NBR and FKM?
HNBR (Hydrogenated Nitrile) is produced by selectively hydrogenating the double bonds in the NBR backbone, converting most C=C to C-C bonds. This saturated backbone retains NBR's oil resistance (the acrylonitrile groups are unchanged) while eliminating the oxidation, ozone, and elevated-temperature limitations. HNBR is the correct specification when: (1) temperature is +120–150°C with petroleum fluid and FKM is too expensive; (2) H₂S is present (NACE MR0175/ISO 15156 qualified HNBR compounds are available); (3) ozone resistance is needed but FKM price premium is not justified; (4) dynamic sealing with improved abrasion resistance over FKM at moderate temperature is required. HNBR costs approximately 1.5–2× NBR and is significantly less expensive than FKM for medium-temperature applications.
Q8: How should I read a chemical compatibility chart when selecting between NBR and FKM for an unfamiliar fluid?
Chemical compatibility charts express immersion test results qualitatively (A/B/C ratings or Excellent/Good/Fair/Poor) derived from ASTM D471 quantitative data. When reading a chart, verify three things before trusting the rating. First, identify the test temperature — a rating valid at +23°C may not hold at +80°C; many compatibility charts default to room temperature, which is non-conservative for hot-service applications. Second, look for the test duration — 7-day immersion data and 70-hour immersion data can give different ratings for the same material/fluid pair; 70-hour (ASTM D471 standard) is more conservative and more predictive of long-term service. Third, understand what the rating covers: "Good" typically means volume change 10–25%, hardness change ±5–10 Shore A, tensile retention 75–90% — borderline performance, not the same as "Excellent" (< 5% volume change, < ±5 Shore A, > 90% tensile retention). For borderline ratings between NBR and FKM on an unfamiliar fluid (e.g., a proprietary hydraulic additive package or a bio-based lubricant), the conservative approach is to order five test pieces of each candidate material and conduct a 70-hour immersion at operating temperature, measuring volume change and Shore A before and after — the $50 test cost is negligible compared to field failure risk.
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Need NBR or FKM O-rings for your application? Contact our engineering team with your fluid type, temperature range, pressure, and dynamic vs. static requirement — we recommend the correct material grade (including ACN content for NBR or grade type for FKM) and supply from MOQ 1 piece with 7–15 day lead time. Standard NBR 70A and FKM 70A available in AS568 and ISO 3601 sizes from stock for 3–5 day express shipping.
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Related engineering resources: NBR vs EPDM O-Rings | HNBR vs NBR O-Rings | FKM vs FFKM O-Rings | Chemical Compatibility Tool | NBR O-rings | FKM O-rings