NBR O-Rings (Buna-N / Nitrile Rubber)
The industry-standard sealing material for hydraulic systems, fuel lines and petroleum-based fluid applications.
Overview
NBR (Acrylonitrile Butadiene Rubber), also known as Buna-N, nitrile, or Buna-N rubber, is the most widely used O-ring material in industrial sealing. Buna-N = NBR = Nitrile — all three terms refer to the same acrylonitrile butadiene copolymer elastomer. Buna o rings, buna rubber o rings, and buna no ring are common buyer search terms for this material. Its exceptional resistance to petroleum-based oils, hydraulic fluids and fuels makes it the default choice for hydraulic cylinders, pneumatic systems, fuel handling equipment and general industrial machinery.
With an operating temperature range of -40°C to +120°C and hardness options from 40 to 90 Shore A, NBR O-rings provide reliable sealing across a broad range of static and dynamic applications. Compression set (ASTM D395 Method B, 70 h/100°C) of ≤ 25% ensures sustained sealing force in long-service static applications.
Acrylonitrile (ACN) content is the key compound variable: standard 33–34% ACN grades balance oil resistance with low-temperature flexibility (TR10 approximately -35°C to -40°C); high-nitrile grades (39–40% ACN) deliver improved resistance to aromatic fuels and hydrocarbon solvents but have reduced cold flexibility (TR10 approximately -25°C to -30°C). Specify ACN content for applications at temperature extremes or with high aromatic fuel content.
NBR is the most cost-effective elastomeric sealing material, making it the preferred option wherever oil resistance is required. The full AS568 range (AS568-001 through AS568-395) is stocked in 70 Shore A black NBR. Buna o rings and buna rubber o rings are available in the same full size range with identical specifications. ISO 3601 metric sizes, and 70/80/90 Shore A variants are available on short lead times.
Lead time: same-day to 3 days for stocked sizes; 5–10 days for non-standard compounds. MOQ: 1 piece. ISO 9001 certified.
From a molecular perspective, NBR is produced by the emulsion copolymerization of butadiene and acrylonitrile monomers. The butadiene units introduce unsaturated double bonds into the polymer backbone, which enables sulfur vulcanization but also makes NBR susceptible to ozone attack. The ACN content directly determines the polymer's polarity: each 1% increase in ACN raises the glass transition temperature (Tg) by approximately 1.5°C and reduces volume swell in IRM 903 oil by 2–3%. Sulfur vulcanization creates polysulfidic, disulfidic, and monosulfidic crosslinks — polysulfidic bonds provide better dynamic flexibility but poorer compression set resistance. Peroxide-cured NBR variants trade some tensile strength for improved heat resistance up to +135°C continuous.
Quantified comparison data reveals where NBR sits relative to its upgrade path, HNBR. In ASTM D412 tensile testing, standard NBR achieves 10–20 MPa versus HNBR's 15–25 MPa — a 25–67% deficit. Compression set at 100°C (ASTM D395 Method B, 70 h) for NBR is ≤ 25%, while HNBR achieves 15–25% at 150°C — meaning HNBR holds better sealing force at a temperature 50°C higher. In Taber abrasion testing (ASTM D4060), NBR loses approximately 200–400 mg per 1000 cycles, whereas HNBR loses 80–150 mg — HNBR is 2–3× more wear-resistant. However, NBR costs roughly one-third of HNBR, making it the rational choice whenever the +120°C temperature ceiling and ozone exposure limits are not exceeded.
A practical hardness selection decision tree for engineers: if your application is a static hydraulic flange seal at pressures below 10 MPa with minimal clearance gap variation, select 70 Shore A NBR — it provides the best conformity and sealing force balance. If your application is a reciprocating hydraulic rod seal at 15–25 MPa with potential bore wear increasing clearance gaps, select 80 Shore A for improved extrusion resistance. If your application is a high-pressure hydraulic accumulator or injection moulding press above 40 MPa, select 90 Shore A NBR or add PTFE backup rings to 70–80 Shore A. For pneumatic systems at ≤ 15 bar, always default to 70 Shore A — harder grades increase friction and wear in low-pressure air service. For fuel system O-rings in modern gasoline with 10% ethanol (E10), specify high-nitrile 39–40% ACN NBR; for pure diesel or mineral hydraulic oil, standard 33–34% ACN is sufficient.
Storage conditions significantly affect NBR shelf life. Store NBR O-rings in a cool, dry environment below 25°C and away from direct sunlight, ozone sources (electric motors, arc welding equipment), and heat radiators. NBR is susceptible to ozone cracking even in storage — concentrations as low as 50 ppb ozone cause visible surface cracking within weeks. Pack in opaque, airtight polyethylene bags with minimal tensile strain. When stored under ideal conditions, the usable shelf life of sulfur-cured NBR is approximately 5 years from the date of manufacture; peroxide-cured grades may extend to 7 years. A common error is storing NBR O-rings in a stretched state over mandrels or pegboards — this induces permanent set and reduces installed sealing force by 15–30% within 6 months.
Material Properties
| Temperature Range | -40°C to +120°C (-40°F to +248°F); short-term to +135°C |
| Hardness Range | 40–90 Shore A (ASTM D2240) |
| Tensile Strength | 10–20 MPa (ASTM D412) |
| Elongation at Break | 200–350% (ASTM D412) |
| Compression Set (70 h/100°C) | ≤ 25% (ASTM D395 Method B) |
| Acrylonitrile Content | 33–34% standard; 39–40% high-nitrile grade |
| Specific Gravity | 1.00–1.25 |
| Color (standard) | Black |
| Standards | AS568, ISO 3601, DIN 3771, ASTM D2000 BF/BG |
Typical Applications
Hydraulics
Cylinder seals, valve seals, piston rod seals in hydraulic systems using mineral oil-based fluids. Standard 70 Shore A NBR covers the majority of mobile and industrial hydraulic applications.
Automotive
Engine oil seals, fuel system O-rings (non-aromatic fuels), transmission seals and pneumatic system components in vehicles.
Pneumatics
Air cylinder seals, valve body seals and pneumatic fitting O-rings in compressed air systems at pressures to 15 bar.
Oil & Gas
Wellhead equipment seals, valve stem seals and pipeline flange gaskets in petroleum service at temperatures below +120°C.
Industrial Machinery
Pump seals, gearbox seals, compressor seals and general mechanical assembly sealing in mineral oil environments.
Chemical Compatibility Summary
Compatible With
- - Mineral oils
- - Hydraulic fluids
- - Gasoline and diesel fuels
- - Aliphatic hydrocarbons
- - Silicone oils and greases
Incompatible With
- - Ozone
- - Strong acids
- - Polar solvents (ketones, esters)
- - Aromatic hydrocarbons
- - Chlorinated hydrocarbons
Compare Nearby Materials
Further Reading
HNBR vs NBR O-Rings
When to stay with standard nitrile and when hydrogenated nitrile is worth the upgrade.
Read articleBest Material for Fuel Systems
Fuel compatibility, temperature, and when FKM displaces NBR in real systems.
Read articleKey Advantages
Excellent Oil and Fuel Resistance
Outstanding resistance to aliphatic hydrocarbons, petroleum oils, mineral hydraulic fluids and non-aromatic fuels. ASTM D471 volume swell in IRM 903 oil at 100°C is typically 10–20% — acceptable for most hydraulic and fuel applications.
Wide Hardness Range
Available from 40 Shore A (soft, low-friction static seals) to 90 Shore A (hard, high-pressure extrusion-resistant seals). 70 Shore A is the standard for most hydraulic and pneumatic applications.
Cost Effective
NBR is the most economical O-ring material — typically 3–8× less expensive than FKM and 50–100× less expensive than FFKM. For petroleum-fluid applications, NBR delivers strong performance at a fraction of the cost of fluorocarbon compounds.
Good Mechanical Properties
Tensile strength 10–20 MPa and abrasion resistance suitable for dynamic sealing including rotary shaft seals and reciprocating rod seals in hydraulic cylinders at standard operating speeds.
Superior Abrasion Resistance for General-Purpose Elastomer
In DIN 53516 abrasion testing, NBR exhibits wear losses of 200–400 mg per 1000 cycles — comparable to CR and significantly better than VMQ silicone (600–1200 mg). This makes NBR the default choice for reciprocating hydraulic rods and rotating shafts where moderate abrasive wear from contaminated fluid is expected.
Mature Global Supply Chain and Rapid Availability
NBR accounts for approximately 55% of all O-ring production by volume globally. The full AS568 standard range is maintained in stock by virtually every industrial distributor. Standard 70 Shore A black NBR O-rings from AS568-001 through AS568-395 are available with same-day shipping, and custom compounds can be mixed and moulded within 5–10 business days — the shortest lead time of any specialty elastomer.
Frequently Asked Questions - NBR
What is the maximum temperature for NBR O-rings?
Standard NBR O-rings operate reliably up to +120°C in continuous service. Short-term excursions to +135°C are possible without immediate failure, but compression set increases rapidly above +120°C, reducing sealing force over time. For continuous service above +120°C in petroleum oil, specify HNBR (to +150°C) or FKM (to +200°C). The specific temperature limit also depends on ACN content — high-ACN (39–40%) grades have slightly lower low-temperature flexibility but similar upper temperature limits.
Can NBR O-rings be used with water?
Yes, NBR O-rings can be used with fresh water and mild aqueous solutions at ambient and moderate temperatures below 60°C. For hot water above 60°C, or steam service, NBR degrades through hydrolysis and compression set increases rapidly. For these applications, EPDM is the correct material. For water-glycol hydraulic fluids (HFC type), verify compound compatibility — some NBR compounds resist water-glycol up to 60°C, but EPDM is preferred.
What fluids are NBR O-rings not compatible with?
NBR is incompatible with: ozone and UV exposure (causes surface cracking — NBR has an unsaturated backbone); strong oxidizing agents (hydrogen peroxide, nitric acid); ketones (acetone, MEK — severe swell); esters (ethyl acetate — moderate swell); chlorinated solvents (TCE, methylene chloride); aromatic hydrocarbons above 10–20% content (benzene, toluene, xylene — excessive swell); and steam or hot water above 60°C. For chemical resistance verification, request ASTM D471 immersion data for your specific fluid at operating temperature.
What hardness NBR O-ring should I use for hydraulic sealing?
For hydraulic cylinder seals at pressures up to 25 MPa with standard groove tolerances, 70 Shore A NBR is the industry standard. For pressures 25–40 MPa or clearance gaps at the larger end of tolerance, 80 Shore A provides better extrusion resistance. For pressures above 40 MPa or large clearance gaps, specify 90 Shore A NBR or add PTFE backup rings alongside 70–80 Shore A NBR. In pneumatic service (typically ≤ 15 bar), 70 Shore A is the default. Contact us with your system pressure and bore clearance for specific grade recommendation.
Do you supply FDA-grade NBR O-rings?
No, standard NBR is not FDA compliant for direct food contact. However, FDA-compliant NBR compounds formulated to FDA 21 CFR §177.2600 are available for food and beverage applications where oil resistance is also required — specify 'FDA NBR' when ordering. For pure water, dairy, and beverage contact without oil exposure, FDA-grade EPDM or VMQ (silicone) are the standard recommendations.
What AS568 sizes do you have in NBR?
We carry the full AS568 range (AS568-001 through AS568-395) in standard 70 Shore A black NBR from stock. 80 Shore A and 90 Shore A are available on 3–5 day lead times for most dash numbers. ISO 3601 metric sizes in NBR 70 Shore A are also stocked. Non-standard hardnesses (40, 50, 60, 85 Shore A) and high-nitrile (39–40% ACN) compounds are available to order. Same-day shipping for stocked sizes ordered before 2 PM.
How does acrylonitrile content affect NBR performance?
Acrylonitrile (ACN) content determines the balance between oil resistance and low-temperature flexibility. Standard 33–34% ACN: balanced properties for most industrial service — good oil resistance, TR10 retraction temperature (ASTM D1329) approximately -35°C to -40°C. High-nitrile 39–40% ACN: improved resistance to aromatic fuels (E10, E85, biodiesel) and hydrocarbon solvents, lower ASTM D471 volume swell in aromatic media — but TR10 is approximately -25°C to -30°C, unsuitable for cold-start below -25°C. Low-nitrile 18–26% ACN: best cold-temperature performance (TR10 to -50°C) but poor aromatic resistance — used in military cold-weather sealing applications.
How do I verify NBR O-ring quality on receipt?
For standard production: review the certificate of conformance (CoC) for compound designation, Shore A hardness, lot number, and date of manufacture. Spot-check hardness with a calibrated durometer per ASTM D2240 — accept ±5 Shore A from nominal. For critical applications: measure O-ring dimensions with a calibrated cone or optical comparator per ISO 3601-1 and compare to the dimensional standard. If material identity is uncertain, specific gravity measurement (approximately 1.0–1.25 for NBR vs 1.80–1.95 for FKM) provides a quick field check. FTIR spectroscopy ($80–200/sample) gives definitive polymer family identification.
Are your NBR compounds certified for automotive IATF 16949 or specific OEM specifications?
Yes, selected NBR compounds are produced at IATF 16949-certified facilities with full PPAP documentation (Level 1–5 on request), material test reports per AIAG standards, and lot traceability. Our standard industrial NBR compounds are manufactured under ISO 9001 quality management. Specific OEM approvals include VW 2.8.1 (for 70 Shore A fuel system NBR), GM 6031M, and Ford WSB-M2G341-A2 specifications. Specify the OEM material specification and required approval level when requesting a quote — not all stocked compounds carry every OEM approval.
How is material batch consistency guaranteed across different production lots?
Batch consistency is controlled through three layers: (1) Raw material qualification — every polymer bale, carbon black lot, and curative batch is tested for Mooney viscosity, ash content, and impurity levels against incoming specifications before release to mixing. (2) In-process control — each compound batch is tested on a moving die rheometer (MDR) for scorch time (tS2), cure rate, and torque development; compression-moulded slabs are tested for hardness (±3 Shore A), specific gravity (±0.02), and tensile strength (±15%). (3) Final O-ring testing — dimensional checks on 5 pieces per cavity per production lot, and ASTM D471 oil immersion on retention samples from every compound batch. Lot-specific CoCs document all test results.
Can I replace one supplier's NBR O-ring with another's without redesigning the groove?
For AS568 and ISO 3601 standard sizes, O-rings from different suppliers are dimensionally interchangeable provided both comply with the same tolerance class (AS568-001 to -475: ±0.08 to ±0.38 mm; ISO 3601-1 Class B). However, material equivalence requires verification: specify the same Shore A hardness (±5), the same ACN content range (33–34% or 39–40%), and the same cure system (sulfur vs peroxide). Compression set performance can vary by 5–10 percentage points between suppliers even within the same ASTM D2000 classification. For critical applications, request ASTM D395 and D471 test data from the alternate supplier and compare against your incumbent's baseline before switching.
What do O-ring color codes mean in industrial practice?
Color coding is not standardized across the sealing industry, but common conventions exist: black is the default for NBR 70 Shore A (approximately 80% of all NBR O-rings). Blue is frequently used for FDA-grade compounds to visually distinguish food-contact seals from industrial grades. Green sometimes denotes high-nitrile (39–40% ACN) NBR or HNBR in mixed-inventory environments. Brown or red-brown may indicate peroxide-cured NBR. Some facilities implement custom color systems: for example, yellow = 80 Shore A, red = 90 Shore A. Because colors are manufacturer-specific, never rely on color alone for material identification — always confirm the compound designation on the CoC or packaging label.
Are NBR O-rings recyclable, and what is the environmental impact?
NBR is a thermoset elastomer and cannot be remelted and reprocessed like thermoplastics. However, post-industrial NBR scrap (flash, reject parts, trimmings) is commonly ground into crumb rubber and used as filler in lower-grade rubber products, brake pads, and anti-vibration mats — achieving a recycling rate of 15–25% by weight in well-managed facilities. Post-consumer NBR O-rings are rarely recycled due to collection and contamination challenges. From a sustainability perspective, NBR's primary environmental advantage is its longevity in oil service: a properly specified NBR seal lasting 5 years in a hydraulic cylinder avoids multiple replacements and associated material consumption. All our NBR compounds are REACH and RoHS compliant, and we offer bio-based NBR options with 25–30% renewable carbon content for customers with Scope 3 emissions targets.