Nitrile Rubber (NBR)
Acrylonitrile Butadiene Rubber | Buna-N
Nitrile rubber is the most widely used elastomer in sealing applications worldwide. Its combination of petroleum oil resistance, mechanical durability, and cost-effectiveness has made it the standard compound for hydraulic seals, o-rings, fuel system gaskets, and oil-wetted industrial components across automotive, industrial, agricultural, and marine industries for over 80 years.
NBR’s properties are not fixed; they are tunable through the acrylonitrile content of the compound, which controls the balance between oil resistance and low-temperature flexibility. Selecting the right NBR grade for your application, and understanding where NBR’s limits require an upgrade to HNBR or FKM, is what this page covers.
Why NBR Resists Petroleum and What That Trade-Off Costs
Nitrile rubber is a copolymer of acrylonitrile (ACN) and butadiene. The acrylonitrile groups in the polymer chain are highly polar, containing a nitrile functional group (CN) with a large electronegativity difference between the nitrogen and carbon atoms. Non-polar hydrocarbon molecules (the basis of petroleum oils, fuels, and greases) are not chemically compatible with polar polymers, so they cannot penetrate the nitrile polymer chain efficiently. The result is that NBR swells very little in petroleum-based fluids compared to non-polar elastomers like natural rubber, SBR, and EPDM, which swell dramatically in the same fluids.
The higher the acrylonitrile content of the compound, the more polar the polymer chain becomes, and the better the oil resistance. However, polarity also raises the glass transition temperature of the polymer, which means the rubber becomes stiff and loses its sealing elasticity at lower temperatures. This is the central trade-off in NBR grade selection: oil resistance and low-temperature flexibility move in opposite directions as ACN content changes.
The same unsaturated butadiene groups in the NBR backbone that provide flexibility also make NBR vulnerable to ozone and UV attack, the same mechanism that degrades natural rubber and SBR outdoors. Ozone cleaves the double bonds in the butadiene segments, causing surface crazing and eventual crack propagation. This is NBR’s primary environmental limitation: it is not an outdoor material without protective coatings or shielding, and it will visibly crack in high-ozone environments within months.
NBR also has limited resistance to aromatic solvents, ketones, esters, and chlorinated hydrocarbons. For applications involving these fluid types alongside oil, HNBR or FKM should be evaluated. For applications requiring oil resistance combined with outdoor UV/ozone exposure, an NBR/PVC blend or neoprene is a more appropriate choice than standard NBR.
ACN Content: Selecting the Right NBR Grade
The acrylonitrile content of an NBR compound is the single most important specification variable after the fluid compatibility is confirmed. Standard NBR grades fall into three tiers based on ACN content, each representing a different balance between oil resistance and low-temperature performance. Most engineers encounter 33% ACN “medium nitrile” as the default and never question it, but for applications at temperature extremes or with aggressive fuels, the ACN content matters.
- Best low-temperature flexibility: useful to approximately -65°F (-54°C) depending on specific compound
- Moderate oil resistance, adequate for most mineral oils but not aggressive aromatic fuels
- Higher gas permeability than medium or high ACN grades
- Specified for arctic and cold-climate applications where -40°F is insufficient
- Refrigeration seals, cold-climate hydraulic systems, and cryogenic-adjacent applications
- The most widely used NBR grade; balances oil resistance and low-temperature flexibility for most industrial applications
- Good resistance to mineral oils, hydraulic fluids, and most fuel blends
- Continuous service temperature range: -40°F to +250°F (-40°C to +121°C)
- The default specification for hydraulic o-rings, fuel system gaskets, and industrial oil seals
- “70 Shore A, 33% ACN nitrile” is the most commonly stocked industrial seal compound in the world
- Maximum oil and fuel resistance; minimal swell in aggressive hydrocarbon fuels and oils
- Reduced low-temperature flexibility: service limit approximately -15°F (-26°C)
- Lower gas permeability; specified for pneumatic seal applications where gas leakage must be minimized
- Fuel-resistant seals for diesel, gasoline, and high-aromatic-content fuels where standard 33% ACN shows marginal swell
- Not appropriate for applications requiring flexibility below 0°F (-18°C)
NBR Chemical Resistance Summary
| Fluid / Environment | Resistance | Notes |
|---|---|---|
| Mineral Oils and Greases | Excellent | The defining property of NBR. Engine oil, gear oil, transmission fluid, and hydraulic mineral oil all produce minimal swell in medium and high ACN grades. NBR is the standard for any application where mineral oil contact is present. |
| Petroleum Fuels (gasoline, diesel) | Good to Excellent | Good resistance to standard mineral-base fuels. High ACN grades (40%+) provide better resistance to aromatic fuel fractions that cause moderate swell in standard 33% ACN compounds. See biofuel note below. |
| Hydraulic Fluids (mineral oil base) | Excellent | NBR is the standard compound for hydraulic system seals operating with mineral oil hydraulic fluid. Not suitable for water-glycol or phosphate ester hydraulic fluids; EPDM and FKM respectively are appropriate for those fluid types. |
| Water | Fair | NBR has moderate water resistance, adequate for short-term or splash exposure but not ideal for continuous water immersion. Prolonged water contact causes gradual softening and physical property reduction. For water service, EPDM is the correct material. |
| Ozone and UV | Poor | NBR has poor ozone and UV resistance due to its unsaturated butadiene backbone. Surface crazing and cracking occur in outdoor environments. NBR components with prolonged outdoor exposure should be shielded, coated, or replaced with neoprene or EPDM. This is the most common cause of premature NBR seal failure in outdoor equipment. |
| Dilute Acids | Fair | Moderate resistance to dilute inorganic acids. Not suitable for concentrated acid service. Verify specific acid and concentration against the Chemical Resistance Guide. |
| Ketones (acetone, MEK) and Esters | Poor | NBR is attacked by ketones and esters, a significant limitation in chemical processing applications. EPDM has good ketone resistance; FKM is poor in ketones. If the process fluid or cleaning agent contains ketones, neither NBR nor FKM is appropriate; evaluate EPDM or FFKM. |
| Glycol Brake Fluid | Poor | Glycol-based brake fluids attack NBR. EPDM is the standard compound for brake fluid sealing. This is a common specification error in automotive brake system components where the natural assumption (oil-adjacent environment = nitrile) leads to the wrong material. |
| Aromatic Solvents (toluene, xylene) | Poor | Aromatic hydrocarbons cause significant swell in NBR. FKM is required for aromatic solvent service. High ACN NBR (40%+) has marginally better aromatic resistance than standard grades but is not an adequate solution for concentrated aromatic solvent contact. |
| Animal and Vegetable Fats and Oils | Good | NBR has good resistance to animal fats and vegetable oils, useful in food processing equipment applications where meat-processing lubricants, cooking oils, or food-grade greases contact seals. FDA-compliant NBR grades are available for direct food contact. |
NBR Variants and When to Upgrade
Standard NBR covers the vast majority of oil-wetted sealing applications. When the application pushes beyond NBR’s temperature limit, ozone resistance, or chemical resistance, there are two natural upgrade paths and one blend option that extend NBR’s capabilities without moving to a fundamentally different elastomer family.
Hydrogenating NBR removes the double bonds from the butadiene segments, producing a saturated backbone. The result is dramatically improved heat resistance (continuous service to 300°F vs. 250°F for standard NBR), excellent ozone and UV resistance, and significantly better mechanical properties under heat aging. HNBR retains the oil resistance of high ACN NBR while adding the environmental resistance of a saturated elastomer. It is the correct upgrade when an application exceeds NBR’s temperature limit or requires outdoor exposure alongside oil resistance. Cost is approximately 3 to 5 times standard NBR. See the HNBR material page for full details.
Blending NBR with polyvinyl chloride (typically 70% NBR / 30% PVC) introduces ozone resistance without the full cost of HNBR. The PVC fraction is not attacked by ozone and masks the reactive double bonds in the NBR phase from ozone contact. NBR/PVC blends also have improved flame resistance and abrasion resistance compared to straight NBR. Oil resistance is slightly reduced relative to pure NBR. Used for cable jacketing, oil-resistant outdoor gaskets, and industrial flooring where ozone exposure is present but the temperature limit of standard NBR is adequate.
When an application requires continuous service above 250°F, resistance to aromatic fuels that cause unacceptable swell in even high ACN NBR, or resistance to aggressive chemicals alongside petroleum oil, FKM fluoroelastomer is the correct specification. FKM’s oil and fuel resistance meets or exceeds NBR’s in most petroleum applications while adding high-temperature and broad chemical resistance. Cost is substantially higher than NBR; the compound should only be specified where NBR genuinely cannot meet the requirements. See the FKM material page for full details.
Carboxylated nitrile introduces carboxylic acid groups into the polymer chain, allowing additional crosslinking and producing significantly higher tensile strength and abrasion resistance than standard NBR while retaining similar oil resistance. Used for high-wear sealing applications such as mechanical face seals, oil field equipment, and pump seals where abrasion at the seal face is the primary failure mechanism rather than chemical attack. Less commonly specified than standard NBR; requires the supplier to stock or compound the XNBR grade explicitly.
Nitrile Rubber Applications
Nitrile Rubber Products from Wayne Rubber
Wayne Rubber manufactures custom NBR components across the following product categories. Each page covers material-specific specifications, grade options, and design guidance for NBR in that product form.
Request Custom NBR Parts from Wayne Rubber
Provide your part drawing, the petroleum fluid type, operating temperature, and any low-temperature requirements. Wayne Rubber will recommend the correct ACN grade and quote.