NBR vs FKM O-Ring Comparison: Material Properties, Chemical Resistance, and Temperature Limits

Selecting the correct elastomer for an O-ring seal directly impacts system reliability, maintenance intervals, and total cost of ownership. For engineers working in fluid power, automotive, aerospace, and chemical processing, the choice between NBR (Nitrile Butadiene Rubber, commonly called Buna-N) and FKM (Fluorocarbon Rubber, commonly sold under the Viton trademark) is one of the most frequent material decisions. Both compounds are widely available and cost-effective, but their performance diverges significantly under thermal stress, aggressive chemicals, and dynamic cycling. This guide provides a side-by-side technical comparison to support informed specification.

Material Chemistry and Structure

NBR (Nitrile Butadiene Rubber)

NBR is a copolymer of acrylonitrile (ACN) and butadiene. The ACN content determines the material's balance between oil resistance and low-temperature flexibility. Standard grades range from 18% to 50% ACN:

• Low ACN (18–24%): Excellent low-temperature flexibility, poor aromatics resistance.
• Medium ACN (32–36%): General-purpose compromise, most common for hydraulic and pneumatic seals.
• High ACN (38–50%): Superior fuel and aromatic resistance, reduced low-temperature capability.

The butadiene backbone provides elasticity but contains unsaturated double bonds, making NBR susceptible to oxidation, ozone attack, and thermal degradation above 100°C continuous service.

FKM (Fluorocarbon Rubber)

FKM is a family of fluorinated hydrocarbon elastomers based on the copolymerization of vinylidene fluoride (VDF) and hexafluoropropylene (HFP). The high electronegativity of the carbon-fluorine bond creates exceptional chemical stability and heat resistance. Standard dipolymers (Type A, ~66% fluorine) offer broad chemical resistance. Specialty terpolymers (Type B, Type F, ~68–70% fluorine) improve base resistance, while perfluoroelastomers (FFKM) push fluorine content above 70% for extreme service.

For this comparison, we focus on standard FKM Type A (66% fluorine), which represents approximately 80% of industrial FKM O-ring consumption.

Temperature Performance

Temperature capability is typically the primary differentiator between NBR and FKM.

NBR Temperature Range

• Standard NBR: −40°C to +100°C (−40°F to +212°F)
• Low-temperature NBR: −55°C to +90°C
• High-temperature hydrogenated NBR (HNBR): −30°C to +150°C

Above 100°C, NBR hardens rapidly due to oxidative crosslinking. Compression set at 100°C for 70 hours typically exceeds 40% for standard grades, making NBR unsuitable for high-temperature static seals with infrequent maintenance access.

FKM Temperature Range

• Standard FKM Type A: −26°C to +204°C (−15°F to +400°F)
• Low-temperature FKM (GLT/GFLT): −45°C to +204°C
• High-temperature specialty grades: Up to +316°C short-term

FKM maintains elastomeric properties at temperatures where NBR has already embrittled. Compression set at 200°C for 70 hours is typically 15–25% for standard FKM, allowing long-term static sealing in engine compartments, chemical reactors, and exhaust systems.

Chemical Resistance Profiles

Petroleum-Based Fluids and Lubricants

Both NBR and FKM perform well in mineral oils, hydraulic fluids (ISO VG 32–68), and petroleum-based greases. NBR is the default choice for general hydraulic systems because it offers adequate performance at the lowest material cost. FKM is specified when the operating temperature exceeds 100°C or when synthetic ester-based fire-resistant hydraulic fluids are present.

Fuels

NBR with medium-to-high ACN content resists aliphatic hydrocarbon fuels such as gasoline and diesel. However, modern fuels with high aromatic content, alcohol blends (E85, E10), and biodiesel accelerate NBR swelling and extract plasticizers. FKM demonstrates superior resistance to aromatics, alcohol-blended fuels, and aviation jet fuels. Swell in ASTM Fuel B after 70 hours at 23°C is typically <5% for FKM versus 15–30% for medium-ACN NBR.

Chemicals and Solvents

FKM is the clear winner for exposure to acids, bases, ketones, and chlorinated solvents. NBR is attacked by ozone, weathering, polar solvents (acetone, MEK), and strong oxidizing acids. FKM resists concentrated sulfuric acid, hydraulic phosphate ester fluids, and silicone-based oils that severely swell NBR.

Notable exceptions: FKM is not recommended for strong bases (pH >11), hot water/steam above 100°C, or certain polar solvents such as low-molecular-weight ketones and amines. For these environments, EPDM or FFKM may be required.

Mechanical and Physical Properties

NBR generally offers better tear and abrasion resistance, making it preferable for dynamic reciprocating seals where the gland surface finish is less than ideal. FKM's lower tear strength and higher modulus require sharper edge radii and better surface finishes to avoid nicking during assembly.

Cost and Availability

NBR is the lowest-cost elastomer for oil-sealing applications. Standard NBR O-rings cost 30–50% less than equivalent FKM parts. For high-volume hydraulic equipment, the cost delta alone drives material selection unless temperature or chemistry demands FKM.

FKM commands a premium due to raw material fluoropolymer costs and more demanding curing chemistry. However, in applications where NBR failure causes unplanned downtime—such as downhole tools, aircraft engines, or chemical processing—the incremental material cost of FKM is negligible compared to maintenance and safety economics.

Application Selection Matrix

Manufacturing and Quality Considerations

At O-Ring Supply Co., we mold both NBR and FKM compounds to AS568, ISO 3601-1, and custom dimensions. Our standard NBR is formulated with 34% ACN for balanced hydraulic and fuel resistance. Standard FKM Type A uses a bisphenol cure system for optimum compression set and metal adhesion. For FDA or aerospace specifications, we offer peroxide-cured FKM and specialized NBR grades with certified batch records.

Both materials are available from stock in standard AS568 sizes with no mold fees and MOQ of one piece. Custom colors (black, brown, green, blue) and hardnesses are available with 7–15 day lead time.

FAQ

Q1: Can I substitute FKM for NBR in a hydraulic system to improve durability?

You can, but it is rarely necessary if the system operates below 100°C with standard mineral oil. FKM provides no meaningful advantage under these conditions and increases cost. Specify FKM only when temperature, fuel content, or synthetic fluid chemistry demands it.

Q2: What is the lowest temperature at which FKM remains flexible?

Standard FKM Type A reaches its glass transition point around −20°C to −26°C. Below this temperature, the material stiffens and loses sealing force. For cryogenic or Arctic service, specify a low-temperature FKM (GLT/GFLT) rated to −45°C, or switch to low-temperature NBR or HNBR.

Q3: Does NBR swell in biodiesel?

Yes. Biodiesel (FAME) contains methyl esters that penetrate NBR and cause significant volume swell, typically 15–25%. High-ACN NBR reduces swell somewhat, but FKM is the preferred material for biodiesel and modern low-sulfur diesel blends.

Q4: Why does FKM fail in steam or hot water applications?

The VDF-HFP backbone of standard FKM is susceptible to dehydrofluorination in the presence of hot water and strong bases. This chemical reaction breaks crosslinks and causes embrittlement. For steam service above 100°C, specify EPDM, HNBR, or a specialty base-resistant FKM with reduced VDF content.

Q5: How do I specify the correct hardness for NBR and FKM O-rings?

For static seals in well-machined metal glands, 70–90 Shore A is typical. For dynamic reciprocating seals, 70 Shore A is the most common compromise between extrusion resistance and friction. For low-pressure pneumatic or vacuum seals, softer compounds (50–60 Shore A) improve conformability. Avoid FKM below 50 Shore A due to its inherently lower tear strength.