Buna O-Rings (NBR): Properties, Uses, and Limitations
When it comes to reliable and cost-effective sealing solutions, Buna O-rings — also known as Nitrile O-rings or NBR O-rings — remain one of the most widely used elastomer compounds across industries. From hydraulic systems to fuel handling and general industrial applications, Buna continues to be a go-to material for engineers and procurement teams alike.
But while Buna O-rings are versatile and affordable, they are not the right fit for every environment. Understanding their properties, temperature range, chemical compatibility, and limitations is critical to selecting the right sealing solution.
In this guide, we’ll break down everything you need to know about Buna O-rings and where they perform best.
What Are Buna O-Rings?
Buna is a synthetic rubber compound scientifically known as Acrylonitrile-Butadiene Rubber (NBR). It is commonly referred to as:
- Buna
- Nitrile
- NBR
Among elastomer sealing materials, Buna is one of the “big four,” alongside:
- EPDM (Ethylene Propylene Rubber)
- FKM (Fluorocarbon/Viton®)
- Silicone (VMQ)
What sets Buna apart is its excellent balance of performance and cost, making it one of the most widely stocked and specified O-ring materials in the sealing industry.
Key Properties of Buna (NBR) O-Rings
Buna O-rings are valued for their well-rounded mechanical and physical properties. These include:
- Good wear resistance – Suitable for dynamic applications
- Strong compression set resistance – Maintains sealing force over time
- Good resilience – Recovers shape after deformation
- Low permeability – Effective at sealing gases and fluids
- High tensile strength – Withstands mechanical stress
Standard Buna O-rings are typically available in:
- 70 durometer (Shore A) – Standard, more flexible
- 90 durometer (Shore A) – Harder, better for high-pressure environments
They are most commonly manufactured in black and are available in both standard (AS568) and metric sizes, making them easy to source and replace.
Temperature Range of Buna O-Rings
One of the most important factors in O-ring selection is temperature capability.
Buna O-rings typically operate within a temperature range of -30°F to +250°F (-34°C to +121°C)
This makes them suitable for a wide range of industrial environments, but they are not ideal for extreme temperatures.
- At low temperatures, Buna can become stiff and lose elasticity
- At high temperatures, it may degrade or lose sealing performance over time
For applications outside this range, materials like silicone (for low temps) or FKM (for high temps) may be better suited.
Chemical Compatibility of Buna O-Rings
Buna’s popularity is largely driven by its strong resistance to petroleum-based fluids, which makes it a top choice in automotive, oil & gas, and hydraulic systems.
Recommended Uses for Buna O-Rings
Buna performs well in contact with:
- Petroleum oils and fuels
- Hydraulic fluids
- Water
- Silicone oils and greases
- Air and inert gases
Because of this, Buna is commonly used in:
- Hydraulic cylinders and pumps
- Fuel systems
- Pneumatic systems
- General industrial sealing
Where Buna O-Rings Fall Short
Despite its versatility, Buna is not universally compatible.
Buna O-rings are NOT recommended for:
- Ketones (e.g., acetone)
- Halogenated hydrocarbons
- Strong acids
- Glycol-based brake fluids (automotive and aircraft)
In addition, Buna has poor resistance to environmental factors, including:
- Ozone exposure
- Direct sunlight (UV)
- Weathering and outdoor conditions
For outdoor or ozone-heavy environments, EPDM or FKM are typically better alternatives.
Advantages of Buna O-Rings
There’s a reason Buna remains one of the most commonly used sealing materials.
1. Cost-Effectiveness
Buna O-rings are among the least expensive elastomer options, making them ideal for high-volume applications and cost-sensitive projects.
2. Wide Availability
They are readily available in standard sizes and durometers, reducing lead times and simplifying maintenance.
3. Versatility
With compatibility across many oils, fuels, and industrial fluids, Buna is suitable for a wide range of applications.
4. Good Mechanical Performance
Buna offers a strong combination of durability, flexibility, and sealing capability in both static and dynamic environments.
Limitations of Buna O-Rings
While Buna is a reliable general-purpose material, it does have clear limitations that must be considered.
1. Limited Temperature Range
Not suitable for extreme heat or deep cryogenic conditions.
2. Poor Ozone and UV Resistance
Buna degrades when exposed to outdoor environments over time.
3. Chemical Restrictions
Incompatible with several aggressive chemicals and fluids.
4. Aging and Hardening
Over time, especially in harsh environments, Buna can harden and lose elasticity, reducing sealing effectiveness.
Common Applications for Buna O-Rings
Due to its balance of performance and cost, Buna is widely used across industries.
Hydraulic Systems
Buna O-rings are ideal for sealing hydraulic oils and fluids, making them common in cylinders, valves, and pumps.
Fuel Handling Systems
Their resistance to petroleum fuels makes them a staple in automotive and aerospace fuel systems.
Pneumatic Applications
Buna works well in air systems, including compressors and actuators.
General Industrial Equipment
From manufacturing equipment to fluid handling systems, Buna O-rings are used wherever a dependable, economical seal is needed.
How to Choose the Right Buna O-Ring
Selecting the right Buna O-ring goes beyond just material—it involves considering:
- Durometer (hardness) – 70 vs. 90, depending on pressure and motion
- Temperature range – Ensure compatibility with operating conditions
- Media exposure – Confirm chemical compatibility
- Application type – Static vs. dynamic sealing
- Size standards – AS568 vs. metric
Working with a trusted supplier like Rocket Seals ensures you get the right material, size, and specification for your application.
Buna vs. Other O-Ring Materials
Understanding how Buna compares to other elastomers can help guide material selection:
|
Material |
Strengths |
Weaknesses |
|
Buna (NBR) |
Cost-effective, oil-resistant |
Poor UV/ozone resistance |
|
EPDM |
Excellent weather and ozone resistance |
Poor with petroleum oils |
|
FKM (Viton®) |
High temperature, chemical resistance |
Higher cost |
|
Silicone (VMQ) |
Excellent low-temp flexibility |
Lower tear strength |
Each material has its place, but for oil-based systems at moderate temperatures, Buna is often the most practical choice.
Buna O-Ring Failure Modes and Troubleshooting
Even with proper material selection, Buna O-rings can fail if operating conditions or design factors are not properly accounted for. Understanding common failure modes can help extend seal life and improve system reliability.
1. Compression Set
One of the most common issues with Buna O-rings is compression set, where the material permanently deforms and loses its ability to rebound.
Causes:
- Prolonged exposure to high temperatures
- Excessive squeeze in the gland
- Long-term static applications
Solution: Select the correct durometer and ensure proper gland design to avoid over-compression.
- Swelling and Chemical Degradation
Exposure to incompatible chemicals can cause Buna to swell, soften, or break down.
Causes:
- Contact with strong acids or solvents
- Use in glycol-based fluids or ketones
Solution: Always verify chemical compatibility before selecting Buna. If exposure is unavoidable, consider alternative materials like FKM.
- Abrasion and Wear
In dynamic applications, Buna O-rings may experience surface wear or extrusion damage.
Causes:
- High friction or inadequate lubrication
- Rough sealing surfaces
- High-pressure environments without backup rings
Solution: Use proper lubrication and consider adding backup rings in high-pressure systems.
- Ozone and UV Cracking
Buna is particularly vulnerable to environmental degradation, which can lead to cracking and seal failure.
Causes:
- Outdoor exposure
- Ozone-rich environments (e.g., near electrical equipment)
Solution: Switch to EPDM or FKM O-rings for outdoor or ozone-exposed applications.
- Installation Damage
Improper installation can lead to nicks, cuts, or stretching damage, compromising the seal before it even enters service.
Causes:
- Sharp edges in the groove
- Improper tools or handling
- Overstretching during installation
Solution: Follow best practices for installation and use proper tools and lubrication.
Is Buna Right for Your Application?
Buna O-rings remain a reliable, economical, and widely used sealing solution for countless industrial applications. Their strong performance in oil and fuel environments, combined with broad availability and low cost, makes them a go-to option for engineers and maintenance teams.
However, they are not a one-size-fits-all solution. If your application involves extreme temperatures, aggressive chemicals, or outdoor exposure, alternative materials may be required.
At Rocket Seals, we help customers select the right O-ring material for their exact application, ensuring long-term performance and reliability.
Need help choosing the right Buna O-ring? Contact Rocket Seals today to speak with a sealing expert.