Nitrile (Buna-N / NBR) O-Rings

Nitrile is the seal you reach for when nothing about the job argues against it. It is a copolymer of acrylonitrile (ACN) and butadiene, and those polar nitrile groups in the backbone are what give it its reason for existing: petroleum oils and fuels do not swell it, soften it, or wash it away. It is cheap, it is made in every size anyone has ever cut a groove for, and it has been in production longer than most of the equipment it seals. Freudenberg, which has worked with nitrile longer than any seal manufacturer in the world, still builds a large share of its catalog from it.

That is the whole pitch: the fluid is oil, the temperature is ordinary, and nitrile already solved it decades ago. Roughly speaking, if you pull an O-Ring out of a hydraulic cylinder, a fuel pump, a gearbox, or an air line and it is black and generic, it is nitrile. Every other compound on this site exists because of a specific thing nitrile cannot do.

Quick specs

Common names

Nitrile collects more aliases than any other compound, and they all mean the same polymer:

• Buna-N, the original trade name and still what most countermen say out loud
• NBR, the ASTM D1418 designation
• Nitrile rubber, nitrile butadiene rubber, or just nitrile
• Perbunan®, a common polymer trade name

Watch for the near-misses on drawings. HNBR and XNBR are relatives, not synonyms, and they are covered below.

Mechanical properties

• Excellent resistance to petroleum oils, greases, fuels, and hydraulic fluids, which improves as ACN content rises
• Good tensile strength, tear strength, and abrasion resistance for the price
• Good compression set resistance, best in the lower-ACN grades
• Good gas tightness, which improves as ACN content rises
• Good extrusion resistance in the 90 duro grades
• Works well in both static and dynamic service, including reciprocating rod and piston seals and rotary shaft seals
• Available in more sizes, durometers, and certifications than any other elastomer

The single most useful thing to understand about nitrile is that these properties are not fixed. They slide with ACN content, and they slide in opposite directions. More acrylonitrile buys oil resistance and gas tightness. It costs you elasticity, compression set, and cold flexibility. See the grades section below for the numbers.

Limitations

The honest list. Nitrile is the default, not a universal seal:

• Ozone, sunlight, and weathering are its classic weakness. Those butadiene double bonds in the backbone are what ozone attacks, and the result is surface cracking on a part that was never overheated and never touched a bad fluid. For outdoor or ozone-exposed static seals, use EPDM. If you need the oil resistance too, that is exactly what HNBR is for.
• Not for glycol brake fluid (DOT 3, 4, 5.1). Freudenberg lists nitrile as unstable in glycol ether-based brake fluids. Use EPDM.
• Not for ketones or esters. Use EPDM.
• Not for chlorinated or aromatic hydrocarbons, or polar solvents. Use FKM (Viton®).
• Only medium resistance in high-aromatic or high-alcohol fuels and in heavily additivated oils. Pump gas with a large ethanol fraction is not the same fluid as the gasoline nitrile was originally specified against, and a high-ACN grade or a jump to FKM may be the honest answer.
• Runs out of heat around +250°F. Above that, step up to HNBR at +300°F or FKM at +400°F. Nitrile does not fail loudly when it is too hot: it hardens, takes a set, and quietly stops sealing.

Chemical compatibility

Handles well:

• Mineral oils and greases, including most engine and gear oils
• Hydraulic fluids and lubricating oils
• Gasoline, diesel, and kerosene, with the caveat on aromatics and alcohol above
• Vegetable and animal oils and fats
• Water at moderate temperature
• Propane, butane, and many gases
• Some grades handle dilute inorganic acids and bases at moderate temperatures

Medium (check the grade and the temperature):

• Fuels with high aromatic or alcohol content
• Additivated oils

Attacks it:

• Aromatic and chlorinated hydrocarbons
• Esters, ketones, and polar solvents
• Glycol ether-based brake fluids
• Ozone, weather, and sunlight

Confirm your exact fluid on the Chemical Compatibility page or in the interactive Chemical Compatibility Chart. Concentration and temperature move these ratings, and a fluid that is fine at 100°F can be a different animal at 220°F.

A word on temperature ranges

Our published range for standard nitrile is -40°F to +250°F (-40°C to +121°C), and that stays the headline. But nitrile is a family, not a recipe, and the range moves with the compound:

• The low end is set almost entirely by ACN content. At about 18 percent ACN, nitrile stays flexible to roughly -40°F. At 50 percent ACN, it stiffens up at about +23°F. That is a 63-degree swing driven by one dial, and it is why “nitrile is good to -40” is only true for the grades that are actually built that way.
• Freudenberg publishes a thermal range to +212°F for its NBR compounds and says seals are made from nitrile when maximum operating temperatures do not significantly exceed that point and good oil resistance is required. That is more conservative than the commercial norm, and it is a fair reading for continuous duty rather than occasional peaks.
• Low-temperature mil-spec grades are qualified to -65°F. The MS29513 fuel series carries a -65°F to +160°F design range: colder at the bottom, and deliberately capped at the top, because the low-ACN polymer that buys the cold flexibility gives back heat life.

Any published range assumes the compound, the media, the pressure, the hardware, and the duty cycle all cooperate. Heat plus an aggressive fluid shortens life faster than heat alone, and a seal that cycles is not the same job as one that sits. Send us the conditions and we will pull the datasheet for the actual compound rather than guess from a chart.

Shop Nitrile (Buna-N) O-Rings

Every Nitrile variation we stock, by grade and size:

Inch (AS568):

• Buna-N Nitrile 50 Duro (Extra-Soft)
• Buna-N Nitrile 70 Duro (Standard)
• Buna-N Nitrile 90 Duro (Extra-Hard)
• Buna-N Nitrile 70 Duro FDA (White)
• Buna-N Nitrile 70 Duro FDA / NSF-61
• Buna-N Nitrile 70 Duro Low-Temp Mil-Spec

Metric:

• Buna-N Nitrile Metric 70 Duro
• Buna-N Nitrile Metric 90 Duro

Not sure of the size? Start at the Size Charts or the Master Size Chart.

Variations and grades explained

Nitrile is not one material. Understanding the variations is how you land on the right grade above.

By acrylonitrile (ACN) content

The ACN percentage is the biggest lever in the material. Common content runs 18 to 50 percent, and everything moves with it: oil and fuel resistance improves as ACN rises, gas tightness improves, and elasticity and compression set get worse. The clearest way to see the trade-off is the cold end.

That -40°F to +23°F span is the whole argument in one line. You cannot have maximum fuel resistance and maximum cold flexibility in the same part, and anyone who tells you otherwise is selling from a generic chart.

By hardness (durometer)

• 50 Shore A: soft, for low closing force, low pressure, or irregular sealing surfaces.
• 70 Shore A: the standard for most applications, and the bulk of what we ship.
• 90 Shore A: for high pressure and to resist extrusion through the gland gap. If you are nibbling 70 duro rings, this or a backup ring is the fix. See Groove Design.

Carboxylated nitrile (XNBR)

A terpolymer, made by bringing acrylic acid into the acrylonitrile and butadiene chemistry. The result has higher abrasion resistance and higher tear resistance than standard NBR, and it is used in dynamic applications where standard nitrile wears out.

Hydrogenated nitrile (HNBR / HSN)

The butadiene backbone is hydrogenated, saturating the double bonds that ozone and heat attack. You keep the oil resistance and gain heat (to about +300°F), weathering resistance, and strength. See the HNBR page, or the side-by-side in HNBR vs Buna-N Nitrile O-Rings.

FDA and potable-water grades

White 70 duro nitrile compounded to FDA 21 CFR 177.2600 for food contact, and grades certified to NSF/ANSI 61 for drinking water system components.

Low-temperature mil-spec grades

Built on low-ACN polymer to reach -65°F, for aerospace and defense work that calls it out. The MS29513 fuel series runs a -65°F to +160°F design range, and the MS28775 hydraulic series -65°F to +275°F: the same ACN dial, set for cold instead of heat. See the mil-spec reference for the full family, and note we stock this class as Low-Temp Mil-Spec Buna-N 70 Duro.

Standards and specifications

Nitrile O-Rings are supplied in the full AS568 inch size range and common metric sizes. Useful callouts:

• ASTM D1418: designates the polymer as NBR.
• ASTM D2000 / SAE J200: the line-callout system most commercial nitrile is bought to.
• Military and aerospace specs: nitrile has the richest mil-spec heritage of any elastomer, and the numbers still show up on drawings every day: MS28775, M83461/1, MS29513, MS9021, and AN6227 among them. The mil-spec reference sorts out which is which and what it supersedes.

If your drawing calls out a specific spec or compound, tell us and we will match it, with a Certificate of Conformance and lot traceability under our ISO 9001:2015 system.

Typical applications by industry

See O-Rings by Industry for the wider picture, or Design & Engineering for the gland around the seal.

Is Nitrile the right choice?

Three questions settle it:

1. Is the fluid petroleum oil, fuel, or hydraulic fluid? Then nitrile is almost certainly the answer, and the rest is durometer and size.
2. Does the seal stay under about 250°F and out of the weather? If yes, nothing more expensive buys you anything.
3. Is there a solvent, a brake fluid, sunlight, or heat in play? Then look at EPDM, FKM, or HNBR instead.

Nitrile is the default because it is right most of the time, not because it is right every time. We will tell you when it is not. Compare the whole lineup on the Materials page.

Not sure whether nitrile covers your fluid and temperature? Send us the conditions: the media, the temperature, the pressure, and whether the seal moves. We will spec the compound and the size with you. We make getting O-Rings easy.

Polymer chemistry, ACN content range and its effect on oil resistance, elasticity, compression set and gas tightness, the -40°F at 18% ACN to +23°F at 50% ACN low-temperature data, the +212°F thermal range, media stability ratings, and XNBR: Freudenberg FST 2025 Technical Manual, ch. 2 (Materials and Process Engineering), §3.4, p. 58. Commercial grade ranges reflect the wider nitrile market. The datasheet for your specific compound always governs; test parts in your actual service conditions before committing to a critical seal.