Viton (FKM) and Buna-N (NBR) are the two most widely used O-ring elastomers in industrial and commercial sealing. Both are typically black or brown, both seal petroleum fluids effectively at moderate temperatures, and both are available in standard AS568 and metric sizes — yet they fail in entirely different environments and serve fundamentally different performance requirements. NBR is the correct default for petroleum oil, hydraulic fluid, and water at temperatures below +120°C, at a fraction of the cost of FKM. FKM is necessary when temperatures exceed +120°C, when the fluid contains aromatic hydrocarbons, or when long-term chemical and oxidative resistance is required. Substituting one for the other without verifying compatibility causes predictable, avoidable seal failure.
Quick answer: NBR (Buna-N): −40°C to +120°C, petroleum oil, hydraulic fluid, water, glycol — cost baseline 1×. FKM (Viton): −20°C to +200°C (standard), aromatic fuels, high-temperature petroleum, oxidizing acids, ozone — cost 5–10×. Both fail in steam above +100°C and in ketones/esters. NBR fails in aromatic fuel (30–50% swell in ASTM Fuel C); FKM fails in amines (dehydrofluorination) and steam. For temperatures between +120–150°C in petroleum service, HNBR is the cost-effective intermediate (2–4× NBR). Distinguish black NBR from black FKM by specific gravity: NBR ~1.10–1.25, FKM ~1.85.
What is Buna-N (NBR)?
Buna-N is the tradename for nitrile butadiene rubber (NBR), a copolymer of acrylonitrile (ACN) and butadiene. It is the highest-volume O-ring elastomer in the world, used as the default material for hydraulic cylinders, automotive engines, fuel handling, and general industrial machinery.
- Temperature range: −40°C to +120°C (standard grades); −50°C (low-temperature grades)
- Best for: Mineral oils, petroleum hydraulic fluids, aliphatic fuels (low aromatic content), water, glycol antifreeze
- Avoid: Aromatic hydrocarbons, ketones, esters, strong oxidizing acids, ozone, UV exposure above +120°C
- Cost index: Lowest among all O-ring elastomers (baseline = 1×)
ACN Content and Its Effect on NBR Properties
The acrylonitrile (ACN) content of NBR is the primary formulation variable. It directly controls the balance between oil resistance and low-temperature flexibility:
| ACN Content | Grade Type | Oil/Fuel Resistance | TR10 (°C) | Typical Application |
|---|---|---|---|---|
| 18–24% | Low NBR | Moderate | −45 to −50°C | Low-temperature hydraulics, arctic service |
| 28–32% | Medium NBR | Good | −35 to −40°C | General-purpose; most commercial O-rings |
| 33–36% | Medium-high NBR | Very good | −25 to −30°C | Automotive fuel systems |
| 36–42% | High NBR | Excellent | −20 to −25°C | Petroleum oil, fuel injection seals |
| 43–50% | Very high NBR | Superior | −15 to −20°C | Maximum oil resistance, sacrifices cold flexibility |
TR10 is the temperature at which rubber torsional stiffness increases by 10× over its room-temperature value — a practical low-temperature limit. Low-ACN NBR remains flexible at arctic temperatures where high-ACN NBR becomes stiff and loses sealing force.
Key constraint: Increasing ACN content decreases low-temperature performance. An engineer who selects 40% ACN NBR for superior fuel resistance must verify that the seal does not need to function below −20°C. For arctic applications requiring both oil resistance and cold flexibility, HNBR or low-temperature FKM grades are alternatives.
What is Viton (FKM)?
Viton is the Chemours (formerly DuPont) trademark for fluorocarbon rubber (FKM). FKM is a copolymer or terpolymer of vinylidene fluoride (VF2) with hexafluoropropylene (HFP) and/or tetrafluoroethylene (TFE). The high fluorine content — 65–71% by weight depending on grade — gives FKM its characteristic resistance to heat, chemicals, and oxidation.
- Temperature range: −20°C to +200°C (standard grades); −40°C (GFLT grades) to +225°C (specialty peroxide-cured)
- Best for: High temperatures, aromatic fuels, strong acids, hydraulic fluids at elevated temperature, aggressive chemical environments, ozone and UV exposure
- Avoid: Steam above +100°C, hot water, ketones (MEK, acetone), esters, amines, strong alkalis, low-temperature service below −20°C with standard grades
- Cost index: 5–10× NBR for standard grades; 10–20× for specialty low-temperature or high-fluorine grades
FKM Grade Types
FKM is not a single material — it encompasses several distinct grade families with different fluorine content and monomer composition:
| FKM Grade Type | Fluorine Content | Key Property | Typical Application |
|---|---|---|---|
| Type 1 (VF2/HFP copolymer) | ~65% | General purpose; standard fuel and oil resistance | Automotive seals, general industrial |
| Type 2 (VF2/HFP/TFE terpolymer) | ~68% | Improved chemical resistance over Type 1 | Chemical processing, aggressive fuels |
| GF (VF2/TFE/PVMQ terpolymer) | ~70% | Methanol/ethanol fuel resistance (E10–E85) | Flex-fuel automotive; biofuel systems |
| GFLT / GLT (low-temperature grade) | ~65–68% | Extends low temperature to −30°C to −40°C | Aerospace, cold-climate equipment |
| Peroxide-cured FKM | Same as base type | Better steam resistance, improved compression set | Static seals in hot fluid exposure |
| Bisphenol-cured FKM | Same as base type | Better dynamic performance | Reciprocating seals |
GF grade significance: Standard FKM (Type 1 and 2) swells 20–40% in methanol and is not recommended for E85 or methanol-containing fuels. GF grades reduce methanol swell to 5–10%, making them the required choice for flex-fuel and biofuel applications.
Cure system significance: The curing agent (bisphenol vs. peroxide) affects compression set and steam resistance but not chemical resistance. Peroxide-cured FKM typically has 10–20% lower compression set at +150°C compared to bisphenol-cured grades, and somewhat better resistance to steam/hot water — though FKM is still not recommended for steam above +100°C regardless of cure system.
Direct Comparison Table
| Property | Buna-N (NBR) | Viton (FKM) |
|---|---|---|
| Max continuous service temperature | +120°C | +200°C |
| Max short-term peak temperature | +135°C | +225°C |
| Min temperature (standard grade) | −40°C (medium ACN) | −20°C |
| Min temperature (specialty grade) | −50°C (low-ACN) | −40°C (GFLT) |
| Petroleum oil resistance | Excellent | Excellent |
| Aromatic fuel swell (ASTM Fuel C) | 15–30% | 2–5% |
| Methanol swell (standard grade) | 20–40% | 20–40% (Type 1); 5–10% (GF grade) |
| Ozone resistance | Poor (cracks at 0.05 ppm) | Excellent |
| Compression set at +120°C (ASTM D395 Method B, 70h) | 35–55% | 15–30% |
| Compression set at +150°C (70h) | Not applicable (thermal limit) | 20–35% |
| Tensile strength | 15–25 MPa | 15–22 MPa |
| Tear resistance | 25–50 kN/m | 20–35 kN/m |
| Steam resistance | Poor above +100°C | Poor above +100°C |
| Hot water (+80°C) resistance | Fair | Fair–poor |
| Ketone/ester compatibility | Not compatible | Not compatible |
| Amine compatibility | Fair | Poor (FKM dehydrofluorinates) |
| Strong acid resistance | Poor | Excellent (except oxidizing acids) |
| Oxidation/UV resistance | Poor | Excellent |
| Cost index | 1× | 5–10× |
| Standard lead time (stock sizes) | 3–7 days | 7–15 days |
Chemical Resistance in Detail: Swell and Degradation Data
Aromatic Hydrocarbons
Aromatic compounds (benzene, toluene, xylene rings) strongly swell NBR because the polar aromatic electrons interact with the polar ACN groups in the NBR chain. The swell effect is proportional to aromatic content:
| Fluid | NBR Swell (volume %) | FKM Swell (volume %) | Verdict |
|---|---|---|---|
| Aliphatic mineral oil | 5–12% | 2–5% | Both acceptable |
| ASTM Reference Fuel B (70% iso-octane, 30% toluene) | 20–35% | 3–6% | FKM required for high aromatic |
| ASTM Reference Fuel C (50% iso-octane, 50% toluene) | 30–50% | 4–8% | FKM only |
| Biodiesel (B20) | 10–20% | 2–6% | FKM preferred |
| E10 gasoline | 12–25% | 3–8% | NBR borderline; FKM preferred |
| E85 gasoline | 30–60% | 20–40% (GF grade: 5–10%) | FKM GF grade required |
| Benzene | 60–100%+ | 5–15% | FKM only |
NBR swollen 20%+ in aromatic fuel loses its dimensional stability, extrudes from grooves, and eventually loses elastic recovery — it cannot return to its original cross-section after removal from the pressurized groove.
Compression Set Comparison at Elevated Temperature
Compression set measures the permanent deformation retained after sustained compression. A seal with high compression set loses sealing force as the elastomer "relaxes" permanently toward the groove. Lower values indicate better long-term sealing:
| Temperature | Test Duration (ASTM D395B) | NBR (70 Shore A, standard) | FKM (70 Shore A, peroxide) |
|---|---|---|---|
| +100°C | 70 hours | 20–35% | 10–20% |
| +120°C | 70 hours | 35–55% | 15–25% |
| +150°C | 70 hours | Material limit exceeded | 20–35% |
| +175°C | 70 hours | — | 25–45% |
A compression set above 40–50% indicates the seal has lost substantial sealing force and is approaching failure for static seals. NBR at +120°C approaches or exceeds this limit over sustained service — explaining why FKM is the required material for any application with continuous temperatures above +120°C.
When to Choose Buna-N (NBR)
Choose NBR when:
- The fluid is petroleum oil, mineral hydraulic fluid (ISO VG 32–100), or aliphatic fuel with low aromatic content
- The continuous operating temperature stays below +120°C (short excursions to +135°C tolerable)
- The application is cost-sensitive and the operating environment is within NBR's range
- Low-temperature flexibility below −20°C is required (medium or low-ACN NBR)
- The fluid is water, glycol antifreeze, or aqueous solutions without strong oxidizers
Common NBR failure modes:
- Thermal hardening and cracking: Exposure above +120°C continuously causes oxidative crosslink increase, reducing elongation below 50% and causing surface cracks
- Aromatic fuel swelling: High-aromatic fuels cause 20–50% volume swell, extruding the seal from its groove and losing dimensional stability
- Ozone cracking: Ozone concentrations as low as 0.05 ppm cause surface cracking perpendicular to the stress direction — NBR stored near electric motors or stored outdoors develops ozone cracks within months
When to Choose Viton (FKM)
Choose FKM when:
- Continuous operating temperature exceeds +120°C
- The fluid contains aromatic hydrocarbons (high-octane gasoline, BTEX-containing process streams, aromatic solvents)
- The seal is exposed to ozone, UV, or outdoor weathering over extended service life
- Compression set resistance is critical for long-term static seals at elevated temperature
- Strong acid resistance is required (nitric, sulfuric, hydrochloric acid — excluding concentrated oxidizing acids)
- Long service intervals at elevated temperature justify the 5–10× material cost premium
Common FKM failure modes:
- Amine attack (dehydrofluorination): Primary and secondary amines attack FKM at elevated temperature, stripping fluorine from the polymer backbone. The result is surface softening, swelling, and eventually disintegration. For amine environments (MEA, DEA, production chemicals in oil and gas), use EPDM, AFLAS, or FFKM instead
- Steam/hot water hydrolysis: FKM degrades in steam above +100°C. The fluorine-carbon backbone is attacked by hydroxyl ions, causing delamination and surface blistering. EPDM or AFLAS are the correct steam-service materials
- Low-temperature seal loss: Standard FKM loses elastic recovery below −20°C. Cold-climate equipment that starts at −30°C will experience leak-on-start until the FKM warms — use GFLT or GLT grades for cold-start service
- Ketone/ester swelling: FKM is not compatible with MEK, acetone, ethyl acetate, or most polar organic solvents — it swells 30–80% in ketones, similar to NBR. No standard O-ring elastomer seals ketones reliably; use PTFE, FFKM, or encapsulated seals
Cost-Benefit Analysis
FKM costs 5–10× more than NBR per seal in equivalent size. The cost is justified when:
| Scenario | NBR | FKM | Justification |
|---|---|---|---|
| +150°C continuous oil seal | Fails in weeks | 2–5 year service | Replacement cost + downtime justifies FKM premium |
| Aromatic fuel seal (ASTM Fuel C) | Fails in 2–6 weeks | 2–5 year service | Mandatory — NBR will not survive |
| Room-temperature mineral oil, low pressure | 5–10 year service | 5–10 year service | NBR is correct choice — no reason for FKM |
| Outdoor, ozone-exposed static seal | Ozone cracking in 6–24 months | 5+ year service | FKM justified if replacement is costly |
| Cold-start hydraulic at −30°C | Leak-on-start risk; FKM worse | Specific grade required | Low-temp NBR or HNBR may be better than standard FKM |
Break-even calculation: If an NBR seal lasts 6 months at +150°C in oil service and requires $200 in labor to replace, the annual seal cost including labor is approximately $400/year. An FKM seal at 5× material cost but 3× service life (18 months) would save approximately $200/year in total cost — FKM pays back within the first replacement cycle.
Can You Substitute One for the Other?
For critical sealing applications: no. FKM will function in most NBR environments but provides no additional performance value in moderate-temperature, low-aromatic mineral oil service. Using FKM where NBR is adequate wastes 5–10× the material cost without benefit.
NBR cannot substitute for FKM in:
- High-temperature service (above +120°C continuous)
- Aromatic fuel service (FKM-grade only — NBR will fail)
- Long-term outdoor/ozone-exposed static sealing
FKM cannot substitute for NBR in:
- Steam service (neither is good above +100°C, but EPDM is correct for steam — not FKM)
- Amine-rich environments (FKM dehydrofluorinates; EPDM or AFLAS required)
- Low-temperature service below −20°C without specifying GFLT or GLT grades
FAQ
Q1: Is Viton always better than Buna-N?
No. FKM outperforms NBR at high temperature (above +120°C) and in aromatic/aggressive chemical environments. NBR is the superior choice for low-temperature applications (below −20°C), cost-sensitive petroleum oil sealing, water and glycol systems, and applications where FKM's chemical weaknesses (amines, steam, hot water) are present. Match material to the specific fluid and temperature — "better" depends on the application.
Q2: What do the FKM grade types (Type 1, Type 2, GF) mean in practice?
Type 1 FKM (VF2/HFP copolymer, ~65% fluorine) is general-purpose — correct for most petroleum and aromatic fuel sealing. Type 2 FKM (VF2/HFP/TFE terpolymer, ~68% fluorine) provides better resistance to aggressive acids and some solvents at slightly higher cost. GF grade adds perfluoromethyl vinyl ether to the polymer chain, improving compatibility with oxygenated fuels (methanol, ethanol, E85) where standard FKM swells 20–40% but GF grade swells only 5–10%. Specify GF grade for any application with methanol, E15+, or B20+ biofuel.
Q3: What color are Viton and Buna-N O-rings?
NBR is almost always black (carbon black filler is standard). FKM is traditionally brown (from iron oxide in the polymerization process) but is also produced in black and green. Color alone is not a reliable material identifier — black O-rings of both materials are indistinguishable by appearance. Verify material by the supplier's certificate of conformance, or perform a specific gravity test (FKM ~1.85, NBR ~1.10–1.25).
Q4: Does FKM last longer than NBR?
In the correct application for each material, both can last years. FKM outlasts NBR in high-temperature oil service because NBR thermally degrades above +120°C while FKM remains stable to +200°C. In room-temperature mineral oil service within NBR's range, a properly specified NBR seal can last 5–10+ years. The comparison is only meaningful relative to the application — the question is whether either material is within its design envelope, not which material is inherently longer-lived.
Q5: Can I use NBR in E10 gasoline to save money?
E10 (10% ethanol, 90% gasoline) causes moderate swelling in NBR — typically 12–25% volume swell — depending on the aromatic content of the base gasoline. Low-aromatic E10 may be acceptable for NBR in static or slow-change seals. However, for dynamic seals or high-aromatic E10 blends, the swelling causes dimensional instability and extrusion. FKM or FKM GF grade is the reliable choice for gasoline-ethanol blends in automotive and fuel handling applications. If using NBR in E10, verify with a 168-hour immersion test per ASTM D471 at the actual fuel blend composition before specifying.
Q6: What hardness should I specify for NBR or FKM?
70 Shore A is the standard for most O-ring applications (static and dynamic). Softer grades (50–60 Shore A) improve sealing conformance to rough surfaces in low-pressure applications. Harder grades (80–90 Shore A) resist extrusion in high-pressure dynamic service (above 100–150 bar) but require tighter groove tolerances and produce higher friction in reciprocating seals. For FKM specifically, 70 Shore A is standard; 75–80 Shore A is preferred for high-pressure hydraulic service where compression set resistance is critical.
Q7: How do I tell if a failed O-ring was NBR or FKM?
The failure mode often differs by material and root cause:
- NBR failures in high-temperature service show uniform hardening, surface crazing, and reduced cross-section (thermal oxidation)
- NBR failures in aromatic fuel show uniform swelling and softness — the seal is larger and softer than original
- FKM failures in amine exposure show surface softening, swelling, and a brown-to-green color change
- FKM failures in steam show delamination and surface blistering
- Both NBR and FKM show spiral (helical) cracking when used in a dynamic groove that is too narrow
If material identification is needed for failure investigation, a specific gravity test (FKM ~1.85 vs. NBR ~1.1) or FTIR spectroscopy on a sample provides definitive material identification.
Q8: Is HNBR a better alternative to FKM for high-temperature oil?
HNBR (hydrogenated NBR) extends the upper temperature limit to +150°C — higher than standard NBR (+120°C) but below FKM (+200°C). HNBR costs approximately 2–4× NBR versus 5–10× for FKM, making it the cost-effective intermediate choice for applications in the +120–150°C range. HNBR also has superior ozone resistance over standard NBR and excellent sour gas (H₂S) resistance. For temperatures above +150°C or for aromatic fuel applications where FKM's superior swell resistance is needed, FKM remains the correct choice.
---
Unsure which material fits your application? Contact our engineering team with your fluid type, temperature range, pressure, and dynamic requirements — we provide free material selection guidance with all quote requests. MOQ as low as 1 piece; 7–15 day standard lead time; express 3–5 days for stocked compounds.