Viton vs Buna-N O-Rings: What is the Difference?

If you are searching for O-ring materials in North America, you have almost certainly encountered the terms "Viton" and "Buna-N." These are the two most common brand names for fluorocarbon rubber (FKM) and nitrile rubber (NBR) respectively. Despite their similar appearance — both are typically molded into black or brown rings — they occupy very different positions in the performance and cost spectrum.

This article explains the practical differences between Viton (FKM) and Buna-N (NBR) O-rings, including specific compound grades, hardness options, real-world failure modes, and a decision framework for selecting the right material without over-specifying or under-specifying.

What is Buna-N (NBR)?

Buna-N is the generic tradename for nitrile butadiene rubber (NBR), the most widely used O-ring elastomer in the world. It is the default choice for sealing petroleum-based oils, hydraulic fluids, and fuels because of its excellent compatibility with aliphatic hydrocarbons.

• Temperature range: -40°C to +120°C (standard grades)
• Best for: Mineral oils, hydraulic fluids, diesel, gasoline (low aromatic), water, glycol mixtures
• Avoid: Ozone, ketones, strong acids, aromatic fuels, high temperatures above +120°C
• Cost: Low (the most economical elastomer)

NBR is available in a wide hardness range (40 to 90 Shore A) and is the standard material for hydraulic cylinders, automotive fuel systems, and general industrial machinery.

Compound grades: Standard NBR (acrylonitrile content 18-50%) balances oil resistance and low-temperature flexibility. High-nitrile NBR (36-50% ACN) offers superior oil and fuel resistance but becomes stiffer at low temperatures. Hydrogenated NBR (HNBR) is an upgraded version with better heat, ozone, and sour-gas resistance.

What is Viton (FKM)?

Viton is the DuPont/Chemours brand name for fluorocarbon rubber (FKM). It is specified when the application temperature or chemical environment exceeds what NBR can handle. FKM contains significantly more fluorine than NBR, which gives it superior heat and chemical resistance.

• Temperature range: -20°C to +200°C (standard grades); specialty grades to +225°C
• Best for: High temperatures, aggressive fuels, aromatic hydrocarbons, strong acids, ozone exposure
• Avoid: Steam, hot water above +100°C, polar solvents (ketones, esters), amines, strong bases
• Cost: High (typically 5-10x the cost of NBR)

FKM is the standard material for aerospace fuel systems, automotive transmission seals, chemical processing equipment, and any application where continuous temperatures exceed +120°C.

Compound grades: Type A FKM (66% fluorine) is the general-purpose grade. Type B (68% fluorine) offers improved chemical resistance. Specialty low-temperature FKM grades (e.g., GLT, GFLT) extend the minimum temperature to -30°C to -40°C for aerospace applications.

Direct Comparison Table

When to Choose Buna-N

Choose Buna-N when:

• The fluid is petroleum oil, mineral hydraulic fluid, or aliphatic fuel
• The operating temperature stays below +120°C
• The application is cost-sensitive and high performance is not required
• There is no significant ozone or weather exposure
• You need low-temperature flexibility below -20°C

Common NBR mistake: Using NBR in aromatic fuel applications (high-octane gasoline, biodiesel blends). Aromatic hydrocarbons cause NBR to swell and soften, leading to extrusion and leakage within weeks.

When to Choose Viton

Choose Viton when:

• Continuous temperatures exceed +120°C
• The fluid contains aromatics, strong acids, or aggressive chemicals
• The seal is exposed to ozone or outdoor weathering
• Long service life at high temperature justifies the higher material cost
• Compression set resistance is critical (e.g., long-term static seals in hot oil)

Common FKM mistake: Using FKM in steam or hot water service. Steam above +100°C causes FKM to hydrolyze and degrade rapidly. For steam applications, use EPDM or AFLAS instead.

Can You Substitute One for the Other?

No. NBR and FKM are not interchangeable in critical applications. FKM will work in many NBR applications, but it is usually overkill from a cost perspective. NBR will fail rapidly in FKM applications due to thermal degradation or chemical attack.

Interchangeability exceptions: In low-pressure, non-critical applications where an O-ring is used as a dust seal or backup seal, an FKM O-ring can replace an NBR O-ring without functional risk. However, the cost increase is rarely justified for such applications.

If you are unsure which material is correct for your fluid and temperature, use our Material Selector or request a free engineering review.

Frequently Asked Questions

Q1: Is Viton always better than Buna-N? No. Viton is better for high temperature and aggressive chemicals, but Buna-N is the superior choice for low-temperature flexibility, cost-sensitive oil sealing, and applications involving water or glycol mixtures.

Q2: What color are Viton and Buna-N O-rings? Buna-N is almost always black. Viton is commonly brown or green, though black FKM is also available. Color alone is not a reliable material identification method — always verify with the supplier's material certification.

Q3: Does Viton last longer than Buna-N? In the correct application, both materials last for years. Viton outlasts NBR in high-temperature service because NBR thermally degrades above +120°C. At room temperature in mineral oil, a properly specified NBR seal can last 10+ years.

Q4: Can I use Buna-N in a Viton application to save money? Only if you are willing to accept frequent seal replacement and potential fluid leakage. Using NBR above its temperature limit or in aromatic fuels causes rapid hardening, cracking, and leakage. The downtime cost usually exceeds the material savings.

Q5: What hardness should I specify for NBR or FKM O-rings? 70 Shore A is the standard hardness for most O-ring applications. Softer grades (50-60 Shore A) improve sealing in low-pressure or rough-surface applications. Harder grades (80-90 Shore A) resist extrusion in high-pressure dynamic service but require tighter groove tolerances.

Q6: How do I tell if a failed O-ring was NBR or FKM? NBR failures in high-temperature service typically show hardening, surface cracking, and a reduction in cross-section. FKM failures in incompatible chemicals (e.g., amines, strong bases) show swelling, softening, and surface blistering. Send failed seals to your supplier for forensic analysis — many suppliers offer free failure-mode identification.

Request a Custom Quote

O-Ring Supply Co. stocks both NBR and FKM O-rings in AS568 and metric sizes from 40 to 90 Shore A. We offer MOQ as low as 1 piece, standard lead times of 7-15 days, and free material selection guidance based on your fluid, temperature, and pressure requirements. Custom compounds are available on request. All orders include batch material certification and are produced under ISO 9001, RoHS, and REACH quality management.

Request a Quote or use our Material Selector to find the right compound for your application.