Product Overview
Buna-N (also known as Nitrile Rubber or NBR) at a durometer of 70 Shore A is the go-to general-purpose elastomeric seal material for O-rings across numerous industries. With excellent resistance to petroleum-based oils and fuels, good mechanical strength, ease of manufacturing, and cost-effectiveness, the 70 duro NBR O-ring offers a balanced combination of performance and economy. The O-Ring Store LLC supplies this standard compound to meet the majority of typical hydraulic, pneumatic, industrial and fluid-system sealing requirements.
Link to AS568 Inch Sizes | Metric Sizes
Historical Background
The synthetic elastomer known as Buna-N (acrylonitrile-butadiene rubber, NBR) originated in the 1930s as a synthetic alternative to natural rubber — specifically designed to resist oils and fuels that rapidly degraded natural rubber. Over decades, NBR’s formulation was refined for better wear resistance, compression-set control, and compatibility with petroleum-based fluids. It became a standard material for O-rings in aircraft and automotive systems during WWII, and remains the default choice for many industrial sealing applications today.
Material Composition & Properties
Material Science
- NBR is a copolymer of acrylonitrile (ACN) and butadiene. The ACN content has a major effect on oil/fuel resistance, low-temperature flexibility, and compression set resistance.
- Typical formulations for general-purpose O-rings use medium ACN to optimize the balance between oil resistance and flexibility.
- The “70 duro” indicates a hardness of approximately 70 Shore A, which is a common hardness for O-ring sealing elastomers that must balance resilience, deformation resistance, and cost.
Key Physical & Mechanical Properties
While exact proprietary values vary by manufacturer, typical expected properties for a standard 70 Shore A NBR compound are:
- Tensile strength: on the order of > 14 MPa (≈ 2031 psi) (varies)
- Elongation at break: often ≥ 250 % for general NBR compounds.
- Compression set (e.g., 100 °C for 70 h): often ≤ 25 % for quality compounds.
- Specific gravity: ~1.20–1.30 typical for NBR compounds.
- Temperature range (approximate for general use): –40 °C to +121 °C (–40 °F to +250 °F) typical for many standard 70 duro NBR O-rings.
Temperature & Environmental Limits
- Lower usage limit: approximately –30 °C (–22 °F) to –40 °C depending on compound and application.
- Upper continuous service limit: approximately +121 °C (+250 °F) for many standard NBR formulations; beyond that point, life and properties may degrade.
- Note: Special NBR variants (e.g., hydrogenated nitrile HNBR) extend temperature or chemical performance — but this page covers standard NBR (70 duro).
Key Features & Advantages
- Excellent resistance to petroleum-based oils, fuels, lubricants, and many common hydraulic & pneumatic fluids.
- Good mechanical wear resistance: tear, abrasion and resilience make it suited for dynamic and static sealing.
- Good compression set resistance when properly maintained: ensures long seal life and minimal leakage.
- Cost effective: compared with specialty elastomers, NBR is economical and widely available.
- Widely manufactured in standard sizes (AS568, ISO etc) and in a range of cross-sections, enabling easy replacement and inventory.
Typical Applications
- Hydraulic cylinders, valves and actuators in industrial machinery.
- Pneumatic systems (air compression, actuators) where oil-contaminated air or lubricated systems exist.
- Fuel systems, fuel lines, oil pumps, transmissions (for petroleum-based fluids).
- Water and glycol-based antifreeze fluids (with proper formulation and specification).
- General industrial sealing tasks: pumps, compressors, gearboxes, fluid couplings, valves.
- Gas and propane sealing in general service (subject to proper design).
Industries
- Automotive & transportation.
- Oil & gas (non-extreme service, e.g., fuel and lubricant handling).
- General manufacturing & industrial equipment.
- Maintenance, repair and operations (MRO) environments.
Chemical & Media Compatibility
Compatible Media
The standard NBR 70 duro is well suited for:
- Mineral oils, petroleum oils, lubricants, fuel oil.
- Water (especially at moderate temperatures) and glycol-based fluids (with caveats).
- Some silicone oils/greases, air, inert gases.
Limitations / Not Recommended
However, there are several important limitations to consider. Standard NBR does not perform well in certain chemical or environmental exposures:
- Not suited for prolonged UV exposure, ozone, sunlight or outdoor weathering (unless a special variant is used).
- Poor resistance to ketones (e.g., MEK, acetone), strong acids, halogenated hydrocarbons (carbon tetrachloride, trichloroethylene), phosphate ester hydraulic fluids (such as Skydrol).
- Incompatible with certain automotive/aircraft brake fluids, steam, some hot water environments beyond the normal range.
- Exposure to low-molecular-weight organic acids and some solvents may lead to rapid swelling or degradation.
Notes on Selection
It is important to verify compatibility considering not just the chemical media, but also temperature, pressure, dynamic vs static motion, exposure time, and any cleaning or side-agent chemicals. Compatibility charts are helpful but not definitive — real-world testing is recommended.
Design & Sizing Considerations
- Standard sizes: Inch (AS568 series 000–400) and Metric (ISO 3601, DIN 3771) O-rings available. Custom cross-sections and non-standard sizes can be produced or special-ordered.
- Gland design: Proper groove dimensions (radial/axial clearance, squeeze, stretch) must be maintained to ensure seal life, especially under pressure or with dynamic motion.
- Durometer choice: 70 Shore A is a standard compromise – higher hardness (e.g., 90 A) means better extrusion resistance but less flexibility; lower hardness (e.g., 60 A) improves flexibility but may reduce extrusion resistance.
- Application environment: For dynamic sealing (reciprocating/rotary), ensure that lubrication, surface finish, and motion are understood. Temperature extremes or aggressive chemicals may call for specialty elastomers rather than standard NBR.
- Storage & shelf life: Store in a cool, dark environment (away from ozone/UV). Many standard NBR compounds have a recommended shelf life of ~10-15 years when stored properly.
Why Choose The O-Ring Store LLC’s Standard B70 NBR O-Rings
- Manufactured under ISO 9001:2015 certified processes, ensuring dimension accuracy, material traceability, and lot control.
- Standard “B70” compound represents the industry rule-of-thumb for general-purpose use — simplifying sourcing, inventory and specification for customers.
- Broad inventory of AS568 and metric sizes makes rapid fulfillment and distribution possible.
- Competitive pricing owing to high-volume standardized material and streamlined procurement.
- Technical support: As part of our sales and service team (sales coordinators, remote technical support), we assist in material selection, sizing advice, and full specification compliance — ideal for distributors, OEMs and maintenance operations alike.
Installation & Service Tips
- Before installation, inspect the gland surfaces for scratches, burrs or sharp edges — these can damage the O-ring and reduce service life.
- Use compatible lubricant (oil, grease) that does not degrade NBR. Avoid substances that NBR is incompatible with (see compatibility section).
- For static sealing, apply ~10-30% compression (squeeze) depending on gland design; for dynamic sealing, lower squeeze and proper dynamic clearance may be necessary.
- Avoid twisting the O-ring during installation; use installation tools if necessary to prevent damage.
- Periodic inspection is recommended in dynamic or high-temperature duty — check for extrusion, swelling, hardening, brittleness, cracking or compression set.
- For long-term storage, keep O-rings in sealed, opaque bags, in ambient temperature, away from ozone/UV sources, out of direct sunlight and away from electrical sources generating ozone (e.g., motors, welders). Standard NBR is sensitive to ozone and should not be used in ozone-rich environments unless specifically compounded.
Technical Specification Summary (Reference)
| Property | Typical Value* | Notes |
|---|---|---|
| Hardness (Shore A) | 70 ±5 | Standard “70 duro” designation |
| Temperature Range | –40 °C to +121 °C (–40 °F to +250 °F) | Dependent on formulation & service conditions |
| Tensile Strength | > 14 MPa (≈2031 psi) | Varies by compound |
| Elongation at Break | ≥ 250 % | Varies by compound |
| Compression Set @100 °C, 70 h | ≤ 25 % | Approximate |
| Common Media | Petroleum oils/fuels, water, glycol fluids, air/gas | See compatibility |
| Non-Compatible Media | Ketones, strong acids, ozone/UV, phosphate ester fluids | Important to evaluate |
*Values are typical for standard Buna-N 70 Shore A compounds; specific performance must be verified with manufacturer data for each lot or custom formulation.
How to Order & Available Options
- Select appropriate size: Inch (AS568) or Metric (ISO/DIN) cross-section.
- Confirm hardness (70 Shore A) and material (NBR, standard black) as default B70 compound.
- Specify any special requirements: FDA/USP certification, ADI-Free (amine dicyclopentadiene inhibitor-free) compound, Class VI (biocompatibility) rating (if applicable).
- For non-standard sizes, cross-sections or special compounds (e.g., low-temp, oil-extra resistance), contact our customer service for custom quoting.
- Typical lead times for standard sizes: immediate from stock; custom sizes/compounds may require manufacturing scheduling.
- Stocking & distribution: We offer bulk quantities, kit packs, distributor assortments or customer-specific lot control.
Frequently Asked Questions (FAQ)
Q: Why choose 70 Shore A instead of 60 or 90?
A: 70 duro provides an excellent balance of flexibility and mechanical strength for general use. 60 duro would yield improved flexibility but lower extrusion resistance; 90 duro yields higher strength but less conformability. For standard sealing tasks, 70 remains the industry standard.
Q: Can I use these O-rings outside in sunlight or in outdoor equipment?
A: Standard NBR compounds are not recommended for prolonged exposure to direct sunlight, ozone, UV or weathering unless specifically formulated for that service. If the application involves outdoor exposure, consider a weather/ozone-resistant variant or different material (e.g., EPDM, FKM).
Q: What about use with phosphate-ester hydraulic fluids (Skydrol, etc)?
A: Standard NBR is incompatible with many phosphate-ester fluids and other aggressive hydraulic fluids. For such service, select a compound specifically rated for those fluids (often FKM, HNBR or fluoroelastomers).
Q: Is the compound safe for potable water or food contact?
A: Standard NBR may not meet food/medical grade certifications (such as FDA/USP VI) unless specifically compounded and certified. Please specify if you require certification (e.g., FDA-compliant, ADI-Free, Class VI).
Q: What shelf-life should I expect in inventory?
A: If stored properly (cool, dark, sealed, away from ozone/UV), standard NBR O-rings can retain useful service life for ~10-15 years, though handling and service conditions will affect actual performance.
Summary
The standard Buna-N 70 duro (NBR) O-ring is the “work-horse” elastomer for sealing applications in industrial, pneumatic, hydraulic, fuel and general equipment. Its excellent balance of mechanical performance, oil/fuel resistance, cost and availability make it the first choice when no extreme environmental or chemical factors dictate more exotic materials. The O-Ring Store LLC’s supply of B70 compound ensures reliability, traceability, and inventory readiness for your customers.
If you are looking for a general-purpose O-ring solution, this is that solution. If your service conditions involve extreme temperatures, aggressive chemicals or ozone/UV exposure, please consult for a specialty material.