HNBR (Hydrogenated Nitrile Butadiene Rubber)




A while ago in our blog, we wrote about NBR (nitrile rubber) describing in detail its properties and applications. One of the drawbacks of NBR we mentioned was its modest heat performance, making it unsuitable for a whole swathe of applications.

Hence, around the 1980s, HNBR was developed, a material that boosts the benefits of NBR with improved heat resistance.

Catalytic hydrogenation of NBR produces the nitrile rubber HNBR, which in many respects outperforms conventional nitrile rubber NBR. However, the catalytic hydration procedure is not straightforward, resulting in HNBR compounds being considerably more costly.

The process is based on a nitrile rubber generally with an acryl content of 33 to 49 percent.

During catalytic hydrogenation, the double bonds between the carbon atoms of the polymer chain - essential for crosslinking with sulfur and characteristic of NBR - are replaced with single bonds, in other words there is a reaction with hydrogen. HNBR is the result.

If all the double bonds are turned into single bonds, this is known as highly saturated rubber. If only some of the double bonds are converted, this is known as partially hydrogenated HNBR (unsaturated rubber).

The crosslinking system of HNBR depends substantially on whether the rubber is saturated or unsaturated. Unsaturated HNBR compounds can be crosslinked with sulfur. This is not an option for saturated HNBR compounds as the required double bonds have been turned into single bonds during the process of catalytic hydrogenation. Cross-linking in this case is achieved with peroxide, which benefits, for example, compression set and heat resistance, but in comparison to sulfur-crosslinked compounds can negatively affect the adhesion of rubber-metal compounds. If an HNBR compound is saturated, no double bonds remain, and crosslinking can only be accomplished with peroxides.

As many of the properties of HNBR are the same as the properties of NBR, we should like to refer you to our article about NBR – which you can find here. In that post, we talk about the importance of the acrylonitrile composition of NBR compounds, and this applies equally to HNBR compounds.

By varying the acrylonitrile component, target properties of the compound can be altered, such as media stability, gas permeability, low-temperature flexibility and compression set.

The mechanical properties of HNBR

HNBR has excellent tear resistance and very good abrasion resistance, and, as such, outperforms NBR which itself has very good values for these two properties.

HNBR is generally a good choice when the primary application requires high dynamic resilience.

Tear resistance is slightly less than the values for NBR but depends on the type of HNBR. Unsaturated, that is partially hydrogenated HNBR compounds, in which double bonding is still present, offer better tear resistance than the saturated types. The same applies to elongation at tear (elongation at break).

The compression set of HNBR is average. If an HNBR compound is peroxide crosslinked, then the compression set is likely to be good. You can expect about 20% for 70h/150° C. Similarly to NBR, there is a strong correlation between compression set and the acrylonitrile content of the HNBR compound. The lower the acrylonitrile content, the better the compression set. At the same time, its resilience to fuels is diminished. And here a quandary presents itself it to the user, particularly when HNBR (or the more conventional NBR) is going to be used for seals in the automotive sector. Seals need to have a good compression set, that is a low compression set, but also require chemical stability against any contact media.

HNBR compounds are generally available in 45 Shore A and 95 Shore A. Theoretically, harder compounds are possible up to 60 Shore D.  

The gas permeability of HNBR is very low. In this respect, HNBR can rival butyl compounds, which as a rule have extremely low gas permeability.

Thermal properties of HNBR

The moderate thermal properties of NBR can be attributed to the double bonds between the carbon atoms of the polymer chain that are required for crosslinking with sulfur.  Catalytic hydrogenation converts the double bonds to single bonds - and produces HNBR.

Single bonds react far slower than double bonds and, as such, improve the thermal properties of the material.

Saturated (therefore crosslinked with peroxides) HNBR compounds are good performers in elevated temperatures.

Molded components can be incorporated in applications exposed to temperatures up to 150° C, and with specialized compounds up to 170° C. Short exposure (max. 100 hours) is also possible to 180° C.

The thermal resistance of partially hydrogenated, and thus unsaturated, HNBR compounds is slightly better than for NBR, but not as good as saturated HNBR.

HNBR’s performance in low temperatures lags a little behind that of NBR. Molded parts for dynamically stressed applications can withstand a maximum of - 40° C. In general, performance at low temperatures is strongly dependent on the acrylonitrile content of the HNBR rubber compound. The lower the acrylonitrile content, the better the low temperature flexibility.  However, this may mean sacrificing other properties, such as fuel resilience.

Chemical compatibility of HNBR

Basically, HNBR has a similar chemical compatibility to NBR for compounds with the equivalent acrylonitrile content.

Neither plant nor animal fats and oils are a problem for HNBR.

In general, HNBR has excellent resilience to oils, fuels and greases, and retains resilience when exposed to hydrogen sulfides or amines. However, resilience to oils and fuels is strongly dependent on the acrylonitrile content of the specific compound. A high acrylonitrile content is required to achieve the best resilience. But this negatively affects low temperature performance. HNBR compounds with a high acrylonitrile content forfeit significant flexibility at low temperatures.

The same goes for lubricating and hydraulic oils of the groups H, H-L, H-LP, and mineral-based fats.

In this respect, it is important to emphasize the good resilience of HNBR to HFA, HFB and HFC fluids (fire-resistant fluids), this is equally true for NBR and makes the materials an interesting proposition for many applications.

There is no resilience to methanol fuels.  Nor does it resist acetone.

In comparison to standard NBR, HNBR does have improved steam resilience to about 140° C.

The single carbon-to-carbon bonds of the polymer chains in HNBR (replacing the double bonds characteristic of NBR) are responsible for superb resilience both to ozone and weathering - and, as such, its superiority to NBR. If you decide to use a partially hydrogenated, unsaturated grade of HNBR, its ozone resilience will be comparable to that of NBR.

Applications of HNBR

Hydrogenated nitrile rubber is often selected for strongly dynamically stressed applications when exposure to high temperatures is also anticipated and potential contact with media such as oils and fats.

This is a typical scenario, for example, in the automotive industry, mechanical engineering and oil production. HNBR’s excellent resilience to fats frequently makes it the best choice for products used in dairy plants or the drinks industry.

It is primarily the excellent tear and abrasion resistance that give elastomer products manufactured from HNBR such a good longevity.

Common HNBR molded parts are membranes, rod seals and piston seals for hydraulic systems, molded seals and O-rings, bellows, hoses, rotary shaft seals and V-belts.  

Timing belts in car engines are typically made with HNBR components as they are subjected to high dynamic stress, thermal stress and, in addition, are exposed to oils, fats and fuels.

Overview of HNBR properties

In conclusion, here is a recap of the properties of HNBR (hydrogenated nitrile butadiene rubber).

Remember, this is only a general guide and not to be used for your ultimate selection of materials. The individual properties of blends can be positively and negatively influenced by targeted formulation and as such may be different from the information presented here.

The rating ranges from ☆☆☆☆☆ (very poor) to ★★★★★ (very good).

Mechanical Properties  
Hardness range:  45 Shore A to 95 Shore A
Tear strength:  ★★★★★
Elongation at break:  ★★★☆☆ 
Tensile strength:  ★★☆☆☆
Compression set at high temperatures:  ★★☆☆☆
Compression set at low temperatures:  ★★☆☆☆
Rebound resilience:  ★★☆☆☆
Abrasion resistance:  ★★★★★
Thermal properties  
Low-temperature flexibility  ★★☆☆☆
High-temperature resistance  ★★★★☆
(Chemical) resistance  
Gasoline:  ★★★☆☆
Mineral oil (at 100° C):  ★★★★★
Acids:  ★★★☆☆
Alkalis:  ★★★☆☆
Water (at 100° C):  ★★★★☆
Weathering and ozone:  ★★★★★
UV/light:  ★★☆☆☆

For more details about properties or chemical resistance, or if you have a query about a particular application, please do not hesitate to contact us.  

If you have a question about this blog post or would like us to discuss a particular aspect of elastomers in an upcoming blog, please email us on   

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