Nitrile rubber O-rings
What Are Buna-N O-Rings?
Buna-N O-rings, also known as Nitrile Rubber O-rings or NBR O-rings, are among the most widely used sealing solutions in industrial equipment, hydraulic systems, fuel handling applications, and general machinery.
Manufactured from a synthetic copolymer of acrylonitrile (ACN) and butadiene, NBR offers an exceptional balance of:
- Oil resistance
- Mechanical strength
- Abrasion resistance
- Cost efficiency
- Processing flexibility
Because of this combination, Buna-N remains the default sealing material for countless OEM and maintenance applications worldwide. NBR is especially valued for its resistance to petroleum-based oils and fuels while remaining relatively economical compared with premium elastomers such as FKM or FFKM.
Why Buna-N Is the Most Commonly Used O-Ring Material
Many sealing materials excel in one specific area.
NBR succeeds because it provides balanced performance across multiple operating conditions.
Key Benefits
✓ Excellent resistance to mineral oils
✓ Good fuel compatibility
✓ Strong wear resistance
✓ Reliable compression set performance
✓ Wide availability
✓ Cost-effective production
✓ Suitable for both static and dynamic seals
For hydraulic equipment, gearboxes, pumps, pneumatic systems, and industrial machinery, NBR is often the first material engineers evaluate.
Understanding NBR Polymer Chemistry
The performance of a Buna-N O-ring is largely determined by the amount of acrylonitrile (ACN) incorporated into the polymer chain.
Typical ACN content ranges from approximately 18% to 50%. Lower ACN improves flexibility at low temperatures, while higher ACN increases resistance to fuels and petroleum-based fluids. Standard industrial NBR compounds commonly contain around 34% ACN.
Low ACN NBR (18–25%)
Advantages:
- Better flexibility
- Improved low-temperature sealing
- Reduced brittleness
Recommended for:
- Outdoor equipment
- Cold-weather hydraulic systems
- Refrigeration applications
Medium ACN NBR (30–36%)
Advantages:
- Balanced performance
- Good oil resistance
- Versatile operating range
Recommended for:
- General industrial equipment
- Hydraulic systems
- Pneumatic machinery
High ACN NBR (40–50%)
Advantages:
- Improved fuel resistance
- Better solvent resistance
- Lower fluid permeation
Recommended for:
- Fuel systems
- Oil processing equipment
- Petroleum handling applications
Standard Temperature Range of NBR O-Rings
Typical operating range:
-40°C to +120°C
Special formulations can extend performance beyond standard limits.
However, prolonged exposure above 120°C generally accelerates:
- Hardening
- Compression set
- Loss of elasticity
For temperatures above 150°C, FKM or FFKM materials are usually preferred.
Sulfur-Cured vs Peroxide-Cured NBR
One of the most overlooked factors in O-ring selection is the curing system.
Global O-Ring highlights two primary cure technologies used in NBR compounds.
Sulfur-Cured NBR
Characteristics:
- Lower cost
- Better low-temperature flexibility
- Easier manufacturing
Best for:
- General industrial applications
- Standard hydraulic systems
- Pneumatic equipment
Peroxide-Cured NBR
Characteristics:
- Better heat resistance
- Lower compression set
- Improved aging resistance
Best for:
- Long service life requirements
- Elevated temperatures
- Demanding industrial environments
Although peroxide-cured compounds typically cost more, they often deliver longer maintenance intervals and improved sealing reliability.
Butyronitrile compounds
| Compounds | Durometer | Color | Temp Range (F) | Description |
|---|---|---|---|---|
| N90-B103 | 90 | Black | -40 to +257 | Peroxide-Cured |
| N90-C901 | 90 | Green | -4 to +248 | High visibility |
| N90-B102 | 90 | Black | -65 to +300 | Low Temperature |
| N90-A101 | 90 | Black | -13 to +250 | General purpose |
| N80-B102 | 80 | Black | -40 to +212 | UL 157 |
| N75-F701 | 75 | Green | -40 to +212 | Metal/X-ray detectable, FDA-Grade |
| N70-B502 | 70 | White | -40 to +212 | General purpose, FDA-Grade |
| N70-B701 | 70 | Green | -40 to +212 | General Purpose |
| N70-C401 | 70 | Blue | -40 to +212 | Metal/X-ray detectable, FDA-Grade |
| N70-B102 | 70 | Black | -35 to +250 | General purpose |
| N70-C102 | 70 | Black | -40 to +221 | General purpose, low temp resistance |
| N70-B106 | 70 | Black | -40 to +212 | NSF 61, FDA |
| N70-B103 | 70 | Black | -40 to +212 | UL 157 |
| N70-A101 | 70 | Black | -22 to +250 | General purpose |
| N50-B102 | 50 | Black | -40 to +212 | UL 157 |
| N50-B101 | 50 | Black | -40 to +212 | General purpose |
Chemical Compatibility of Buna-N O-Rings
Excellent Compatibility
NBR performs exceptionally well with:
- Petroleum oils
- Diesel fuel
- Gasoline
- Hydraulic oils
- Lubricating greases
- Vegetable oils
- Silicone oils
- Ethylene glycol solutions
These fluid compatibility characteristics make NBR one of the most common sealing materials in automotive and industrial applications.
Limited Compatibility
NBR is generally not recommended for:
- Ketones
- Esters
- Ethers
- Chlorinated solvents
- Aromatic hydrocarbons
- Phosphate ester hydraulic fluids
Using NBR in incompatible media can lead to:
- Swelling
- Softening
- Cracking
- Premature seal failure
Recommended Hardness Selection
Hardness is measured using Shore A durometer.
70 Shore A
Most common industrial hardness.
Suitable for:
- Hydraulic systems
- Pneumatic systems
- General industrial sealing
80 Shore A
Recommended for:
- Higher pressure environments
- Increased extrusion resistance
90 Shore A
Recommended for:
- Heavy-duty equipment
- High-pressure hydraulic systems
- Severe mechanical loads
Engineers often increase hardness when extrusion becomes a concern rather than changing the entire seal design.
Preferred application environment for nitrile rubber O-rings
Performs Well In:
- Petroleum-based oils and fuels
- Aliphatic hydrocarbons
- Vegetable oils
- Silicone oils and greases
- Ethylene glycol
- Dilute acids
- Water up to 100ºC (212ºF)
Doesn't Perform Well In:
- Aromatic hydrocarbons
- Automotive brake fluid
- Chlorinated hydrocarbons
- Ketones
- Ethers
- Esters
- Phosphate ester hydraulic fluids
- Strong acids
- Ozone, weathering, and sunlight
How to Select the Right Buna-N O-Ring
Before ordering, gather the following information:
Operating Fluid
Examples:
- Hydraulic oil
- Diesel fuel
- Gasoline
- Grease
Temperature Range
Minimum and maximum operating temperatures.
Pressure Conditions
Determine whether backup rings are necessary.
Dynamic or Static Service
Movement directly affects wear and material selection.
O-Ring Size
Provide:
- Inside Diameter (ID)
- Cross Section (CS)
- Groove dimensions
Regulatory Requirements
Consider:
- FDA
- NSF
- Drinking water approvals
- Food-contact compliance
Why Many Engineers Still Choose NBR First
Although advanced materials such as FKM, FFKM, AFLAS, and HNBR offer superior performance in specific environments, NBR remains the industry’s workhorse elastomer.
Its combination of:
- Reliable oil resistance
- Good mechanical properties
- Broad size availability
- Cost effectiveness
- Proven field performance
makes it one of the most practical sealing materials for everyday industrial applications.