NBR (acrylonitrile butadiene rubber)

The automotive industry is a major consumer of technical moldings and other rubber products. Its specific requirements, particularly regarding resistance to mineral oils, fuels and greases, have made NBR a vitally important raw material in this sector. Acrylonitrile butadiene rubber, developed by Bayer in the 1930s, in short: nitrile rubber, is an excellent solution for many applications due to its properties and therefore will be the focus of this blog post.

There are very few raw materials like NBR whose properties are directly dependent on its exact formulation.  Acrylonitrile content typically is between 10 and 50% and this is the major determinant in the behavior of NBR products in their ultimate application. During the course of this blog post, we will look in detail at how properties can vary according to the composition of the compound.

NBR is either peroxide or sulfur cross-linked.

Mechanical properties of NBR

The mechanical properties of nitrile rubber can be impressive, but depend strongly on the acrylonitrile content of the particular compound.

The quality of some of the mechanical properties important to many applications, such as compression set or absorption, decreases as the acrylonitrile content of NBR increases. However, a higher acrylonitrile percentage increases resistance to oils, greases and fuels – which is why getting the balance right can be so important.

It is possible to formulate NBR compounds that perform well with a good compression set at both high and low temperatures. However, the acrylonitrile content must be kept as low as possible. And this comes at the expense of oil resistance. Compression set is a very important selection criterion, particularly for seals. Should an application require a seal to be oil resistant, the engineer faces a trade-off decision. In this situation, an NBR composition with peroxide crosslinking can be useful. The compression set will be better than for a sulfur cross-linked NBR compression set.

In general, it is useful to bear in mind that with increasing acrylonitrile content, mechanical values may also be positively influenced. For example, the tensile strength of an NBR compound is enhanced the higher the acrylonitrile content.

Nitrile rubber also has very good abrasion resistance and therefore is not particularly susceptible to wear in comparison to other rubbers.

The typical hardness range of NBR is between 30 and 95 Shore A 

The gas permeability of NBR is highly dependent on the acrylonitrile content of a formulation. The higher the acrylonitrile content of an NBR compound, the lower its gas permeability. NBR can achieve values comparable to those of butyl rubber. As such, NBR rubber parts are also used in the LPG industry. However, low acrylonitrile content equates to greater gas permeability.

Thermal properties of NBR

Products and rubber molded parts made of NBR can tolerate sustained temperatures as high as 110 ° C; 130° C is also possible for a short time. Please note that particularly the tensile strength of the material NBR decreases sharply with an increase in temperature.

The acrylonitrile content of NBR is very significant for low-temperature flexibility. The lower the acrylonitrile content of the NBR compound, the better its cold flexibility.

In other words, the low-temperature flexibility of NBR runs counter to its oil, grease and fuel resistance. For an acrylonitrile content of 10%, NBR products tolerate temperatures down to -50° C. However, there will be a substantial increase in volume or swelling of parts made of nitrile rubber coming into contact with oil, and its resistance therefore is inadequate. Increasing the acrylonitrile content of NBR to 40-50% increases its chemical resistance, but lowers its cold temperature resistance range from to 0° to -10° C. Careful control of the compound formulation and the addition of certain softeners can alleviate this problem. However, incorporation of softeners has repercussions for mechanical properties, which are an important consideration in the design of a compound and something that needs to be tested on finished parts.

Media resistance of NBR

As already indicated in the previous sections, suitably formulated nitrile rubber NBR can have excellent resistance to oils, greases and fuels. This has led to the ubiquitousness of NBR products, especially in the automotive sector. NBR resistance is largely determined by its acrylonitrile content and improves the more acrylonitrile is added (up to 50%).

At that level NBR has excellent resistance to vegetable and animal fats and oils. Even silicone oil and silicone grease. Resistance to dilute acids, alkalis and aqueous, non-oxidizing salt solutions is also very good. Water is no problem either.

The same applies to the resistance of NBR to gasoline, diesel fuel, mineral oil, mineral fat and propane. Resistance to aliphatic hydrocarbons is generally very good

Weaker, but also satisfactory, is the resistance of NBR to fuels with high aromatic content (such as super-petrol).

There is no resistance to chlorinated or aromatic hydrocarbons such as benzene. NBR cannot tolerate strong acids, alkalis or polar solvents such as acetone. In general, nitrile rubber is not resistant to ketones.

NBR resistance to ozone, weather and aging is also poor.

Since NBR is popular in the automotive industry and in the hydraulics sector, it is important to look at its performance specifically with flame retardant hydraulic fluids. NBR has good resistance to the HFC group. Resistance to the HFA group is mediocre and there is no resistance to the HFD group of flame retardant fluids.

Application areas of NBR

Applications of nitrile rubber are determined primarily by its resistance to gasoline, mineral oil and grease.

The major industrial consumers of NBR products are the automotive, motor vehicles and mechanical engineering sectors. For example, NBR diaphragms, frame seals, profile gaskets, O-rings, flange gaskets, bearings, roller covers or hoses are common components.

Due to its fat resistance, NBR is also used for articles that come into contact with food. This likewise includes various hoses, seals and conveyor belts. There are special NBR compounds that meet the requirements of the German Federal Institute for Health Protection and Veterinary Medicine (BgVV) or the American Food and Drug Administration (FDA). Current regulations explicitly address the limits to be observed regarding free acrylonitrile content.

Overview of NBR properties

In conclusion, here is a recap of the properties of NBR (acrylonitrile 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.

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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 info@hepako.de   

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