HNBR O-rings: stronger than NBR

What are HNBR O-rings?

HNBR (hydrogenated acrylonitrile butadiene rubber) is one of the elastomers with the widest range of applications in industry. Not only sealing rings, but also timing belts and hydraulic hoses are made from this material. This already illustrates the potential of HNBR for use as an O-ring.

The combination of good mechanical load capacity and a high maximum operating temperature is what distinguishes HNBR sealing elements. Added to this is the resistance to mineral-oil-based media.

HNBR is produced by the partial or complete hydrogenation of the O-ring material NBR (acrylonitrile butadiene rubber). This step enables key property improvements over NBR, above all resistance to weathering and ozone as well as higher temperature resistance.

Many heavily loaded NBR O-rings are increasingly being replaced by variants made from the more capable, but also more expensive, HNBR. In practice, HNBR is also known under the trade names Therban and Zetpol.

The 5 most important advantages of HNBR O-rings

The fact that HNBR also serves as a material for timing belts in the automotive industry makes it clear: O-rings made from this material are suitable for use in dynamic seals.

When O-rings seal components against each other that are in motion, they are subjected to particular stress. Under these conditions, friction arises that can cause material loss on the O-ring. Added to this is the increased heat input into the material, which can alter its internal structures. Thanks to their properties, HNBR O-rings withstand both stresses and thus achieve sufficient durability for dynamic applications.

The wide temperature range, combined with resistance to mineral-oil-based products, makes HNBR O-rings suitable for seals in automotive drivetrains. In add-on components of combustion engines, the O-rings come into contact with hot media such as coolant, fuels, air or oil, which can also occur as a mixture. In addition to these high-volume applications, HNBR O-rings also qualify for special industries such as gas exploration. This is due to the material's particularly low permeability to gas and vapour.

The 3 most common areas of application

Automotive industry: In automotive combustion engines, motor oils classified according to SAE serve to lubricate moving parts. HNBR O-rings are resistant to these media over a temperature range of −30 °C to +150 °C and thus cover the full application-specific temperature spectrum.

Thanks to their stable elastic properties and freedom from fatigue, they ensure a durable seal of the engine components against each other and against the environment. This also applies after long downtimes in extreme cold or under the high temperatures of continuous operation.

Drinking water systems: Systems for the storage, treatment and distribution of drinking water are subject to intensive controls because they are critical to the health of humans and animals. The components must therefore meet numerous requirements. Many, mostly national, regulations ensure the safety of the components, including the sealing elements.

O-rings made from HNBR pass these special physical and microbiological tests, making them suitable for the safe operation of drinking water systems. In Germany, the KTW guideline is the leading standard here. Corresponding grades are available as standard. They are also available in an FDA-compliant version and with approvals under numerous other national guidelines, such as KIWA for the Netherlands or WRAS for the United Kingdom.

Gas supply and gas consumption: For use in connection with gas, HNBR brings both key properties. Its good chemical resistance together with its low gas permeability make HNBR O-rings a valuable option for sealing gas-carrying systems. The specific requirements for this area of application are set out in the DIN EN 549, VP 406 and VP 614 guidelines.

What is the media resistance of HNBR O-rings?

In addition to the increased temperature range, HNBR is superior to its base material NBR above all through its improved resistance to oxidative attack. This is an extension of the already broad resistance profile that is important for practical use.

HNBR is resistant to:

• Aliphatic hydrocarbons
• Greases and oils of animal and vegetable origin
• Water and steam within the upper continuous temperature of 150 °C
• Flame-resistant hydraulic fluids: HFA, HFB, HFC fluids
• Light heating oil and diesel fuel
• At low temperature, dilute acids, bases and salt solutions
• Ozone and weathering

HNBR swells strongly in aromatic hydrocarbons such as benzene or trichloroethylene. In HNBR O-rings, this swelling causes a significant reduction in mechanical properties. However, these return once the material has dried out again.

The oil resistance of HNBR O-rings depends significantly on the acrylonitrile content (ACN) of the material compound. A higher blend proportion increases resistance, but at the same time causes a deterioration in low-temperature behaviour. The selection of the right HNBR material should therefore be based on the specific application. This makes it possible to exploit the maximum performance of HNBR O-rings.

What is the temperature resistance of HNBR O-rings?

HNBR O-rings have a temperature resistance from −30 °C to +150 °C. The temperature range of common grades extends from −30 °C to +150 °C.

Depending on the composition, however, HNBR is also suitable for the low-temperature range down to −40 °C. This means HNBR covers similarly low temperatures as the alternative sealing materials NBR, FKM or FFKM. In terms of maximum continuous temperature, HNBR is comparable to ACM or EPDM. With FKM or silicone (VMQ), however, the market also offers materials with a significantly higher upper temperature limit.

In tests, thermally aged HNBR O-rings show significantly less swelling on contact with media. The loss of mechanical properties is also less pronounced. This is due to a higher cross-linking density compared to the base material.

In general, the temperature and media resistance of O-ring materials must always be assessed in context. The temperature ranges of HNBR O-rings on contact with certain media can therefore differ significantly from the general figures. Material selection must therefore always be based on the specific application.

What are the mechanical properties of HNBR O-rings?

HNBR O-rings stand out from the large number of O-ring materials above all thanks to their excellent abrasion resistance. Together with their temperature suitability, this makes HNBR O-rings insensitive to the friction of dynamic sealing. The material's high tensile strength also contributes to its suitability for this application. Other materials with good abrasion resistance worth mentioning are EPDM, NBR, CR and FKM.

In addition, thanks to its high mechanical strength, HNBR is particularly resistant to explosive decompression. This phenomenon occurs when there is a rapid pressure drop in a gas-carrying sealing system. In response, the gas contained in the O-ring expands sharply.

If the natural gas permeability of the O-ring material is not sufficient for this, the gas forces its own way through the structure of the material. This results in permanent damage to the O-ring, which manifests itself as cracks and blistering.

Especially for long-term use in the oil and gas industry, low gas permeability and insensitivity to explosive decompression play an important role. The industry has therefore developed its own standard with the NORSOK M-710 norm. As with FFKM and FKM, compliant HNBR O-rings are also available.

What do I need to consider for groove design?

Groove design for HNBR O-rings:

1. Compression in the groove should be around 20 % for static use
2. Choose lower compression for dynamic sealing
3. 20 % of the groove should still be free after compression
4. Always design grooves according to the standard tables common in the market
5. Permanent stretch after installation should be below 6 %
6. Permanent compression after installation should be below 6 %

Installation of HNBR O-rings:

1. Thoroughly clean the groove space before installation
2. Do not stretch elastic O-rings by more than 50 % during installation
3. Provide lead-in chamfers of between 15 and 20 degrees
4. Deburr bores, even if they are only passed over during installation
5. Round off sharp edges with a minimum radius of 0.1 to 0.3 mm
6. Use silicone grease to ease installation
7. Avoid twisting the O-ring

Frequently asked questions