Nitrile butadiene rubber (NBR): Basics, properties, material comparison
Acrylonitrile butadiene rubber, or NBR (nitrile butadiene rubber) for short, is one of the most important technical synthetic rubbers for applications involving contact with oils, fuels and greases. According to ISO 1629, the NBR elastomer is assigned to the so-called R group. This includes elastomers with a carbon-based polymer backbone and unsaturated double bonds that enable chemical cross-linking. Elastomers often have either good mechanical properties or good media resistance. NBR combines good mechanical properties with high
1 What is NBR?
NBR is a synthetic rubber that is used for applications with high requirements for resistance to oils, fuels and greases. Compared to many other elastomers, the material is characterised by increased media resistance combined with good mechanical properties. The property profile of NBR rubber can be customised over a wide range of applications by adjusting its chemical structure or adding additives. This allows the material to be used in both static and dynamic sealing and moulded part applications. At the same time, NBR is cost-effective. This combination of mechanical performance, media resistance and cost-effectiveness makes NBR one of the most frequently used elastomers in technical applications.
What is the difference between NBR, AR, Perbunan and Europrene N?
The designations NBR, AR, Perbunan and Europrene N refer to the same type of material, but differ in their meaning. NBR (nitrile butadiene rubber) is the internationally used abbreviation for acrylonitrile butadiene rubber, AR (acrylonitrile rubber) is an older designation for the same polymer material that has not become established in general use.
Perbunan and Europrene N are trade names for NBR base polymers from different manufacturers and do not represent independent material classes. Differences between these designations result exclusively from manufacturer-specific productions, in particular from the acrylonitrile content and the polymer architecture.
How did NBR develop historically?
The development of NBR dates back to the 1930s and 1940s. The increasing use of mineral oils and fuels, particularly in the automotive and mechanical engineering industries, highlighted the weaknesses of natural rubber. When it came into contact with oil-containing media, natural rubber showed strong swelling and a rapid loss of mechanical properties. The targeted copolymerisation of butadiene with acrylonitrile provided an oil- and fuel-resistant rubber for the first time. This laid the foundation for a material that is still one of the most important technical elastomers today.
The raw materials for NBR mainly come from the petrochemical industry. Butadiene is typically a by-product of ethylene production in steam cracking, while acrylonitrile is obtained industrially through the ammoxidation of propene.
What is the chemical structure of NBR?
NBR is a copolymer of acrylonitrile (ACN) and 1,3-butadiene. The two monomers - acrylonitrile with its characteristic nitrile group (- C≡N) and butadiene as a conjugated diene - together form a polymer main chain in which the individual building blocks are present as sequences or statistically distributed. In industrial practice, NBR is generally a random copolymer whose properties are adjusted via the formulation, polymerisation conditions and, in particular, the ACN content.
Figure 1: Chemical composition of NBR, consisting of an acrylonitrile monomer (left) and a butadiene monomer (centre). Polymerisation produces the NBR copolymer (right)
The central structural influencing factor is the nitrile group of the acrylonitrile. It is bound to the polymer chains as a side function and gives NBR a significantly higher polarity than many other favourable elastomers. This increased polarity has a significant influence on the interaction with media: non-polar liquids such as mineral oils or many fuel components have a lower thermodynamic affinity to polar polymers, which reduces dissolution and swelling. In practice, this means that as the ACN content increases, swelling in oil and fuel generally decreases and the volume and property stability in contact with these media improves.
However, this improvement is accompanied by a conflict of objectives. The increasing polar content typically increases the glass transition temperature (Tg) and restricts the segment mobility of the polymer chains. As a result, the low-temperature behaviour deteriorates with a high ACN content, as the flexibility decreases at low temperatures. A low ACN content, on the other hand, leads to better low-temperature flexibility, but at the expense of oil and fuel resistance.
The butadiene component introduces unsaturated double bonds into the polymer structure, which are decisive for elasticity and processability on the one hand and represent the chemical points of attack for cross-linking (vulcanisation) on the other.
In addition to the ACN content, the microstructure of the butadiene units, such as the ratio of 1,4 to 1,2 additions, also influences the material behaviour. Among other things, this affects the glass transition temperature, dynamic properties and reactivity during crosslinking. In practice, this influence often takes a back seat to factors such as ACN content, filler system and type of crosslinking, but in demanding applications - for example with regard to compression set, dynamic seals or temperature resistance - it represents a relevant control lever.
2 What are the properties of NBR?
NBR rubber is characterised by good resistance to oils, greases and fuels and is suitable for use under moderate continuous thermal stress. The permissible temperature range of use is below that of high-performance elastomers such as FKM, but within the practical range of many industrial applications and well above that of natural rubber.
Another key property of NBR rubber is its excellent resistance to oil and fuel, which is largely determined by the acrylonitrile content. Media resistance improves with increasing acrylonitrile content, while elasticity and low-temperature behaviour decrease. NBR has only limited resistance to ozone, weathering and UV radiation, which is why its use in outdoor applications or under oxidative stress is limited. The gas and vapour permeability is in the medium range and depends heavily on the compound. Restrictions exist in particular at high continuous operating temperatures.
Standards & guidelines from NBR
The group of nitrile butadiene rubber materials (NBR) is categorised according to the international classification, testing and evaluation standards ISO 1629 and ASTM D1418. These standards serve to categorise the material within the elastomer families, but do not provide any information on the specific mechanical, thermal or chemical properties of individual compounds.
The evaluation of mechanical parameters is based on general elastomer testing standards. These include ISO 37 (tensile test), ISO 48 (Shore hardness), ISO 34 (tear propagation resistance) and ISO 815 (compression set). The long-term thermal stability of NBR compounds is usually assessed using heat ageing tests in accordance with ISO 188, whereby the change in mechanical characteristics after ageing is particularly relevant.
Tests in accordance with ISO 1817 are used to assess media resistance. This involves determining changes in volume, mass and properties after defined contact with liquids, in particular oils and fuels. These tests are of central importance for NBR, as the resistance is heavily dependent on the acrylonitrile content, the compound structure and the type of cross-linking.
For components with standardised geometry, in particular seals, geometric standards such as DIN ISO 3601 (O-rings) or DIN 7863 are also used. These standards define dimensional tolerances and shape requirements without specifying the elastomer material used or its properties.
Purity & cleanliness requirements for NBR
In addition to the classic mechanical and chemical properties, purity and cleanliness requirements play an increasingly important role for NBR compounds, depending on the application. These are generally not defined by independent standards, but result from application and process-specific requirements, such as industry-specific regulations or customer specifications. The focus is particularly on extractable and migratable components, low-molecular residues and particulate impurities. As the polymer backbone of NBR has a lower chemical inertness compared to highly fluorinated elastomers, extraction and migration tendencies can be more pronounced.
3. processing & production of NBR material
NBR rubber is used as a classic material in established elastomer-typical manufacturing processes. Moulding is carried out by Compression presses, Transfer moulding presses or Injection moulding, depending on the component geometry, quantity and required dimensional accuracy. NBR is also extruded, for example for profiles, hoses or semi-finished products that are then customised or further processed.
The elastic properties of NBR are formed by vulcanisation, during which a three-dimensional polymer network is created. Due to its unsaturated polymer structure, NBR is generally easy to crosslink and does not require any special reactive additive monomers.
Figure 2: Vulcanised form of NBR
Crosslinking is predominantly sulphur-based or peroxide-based, with the choice of crosslinking system having a significant influence on the property profile. Sulphur-crosslinked NBR compounds are characterised by good elasticity and dynamic properties, while peroxide-crosslinked systems offer greater thermal stability, better ageing resistance and lower compression set.
In addition to the type of crosslinking, the mechanical behaviour, media resistance and long-term stability of the material are influenced in particular by the acrylonitrile content, the additives and the process parameters. Crosslinking is therefore not an isolated material parameter, but a central component of the material design of NBR compounds.
4. material comparison: NBR vs. other elastomers
For NBR, comparisons with FKM, EPDM and HNBR are particularly meaningful, as these materials are often discussed as alternatives in material selection.
| Nitrile Rubber | Flour- Rubber | Ethylene- Propylene diene Rubber | Hydrogenated Nitrile- Rubber | ||
| International abbreviation | NBR | FKM | EPDM | HNBR | |
| Hardness range (in Shore) | 20A-75D | 50A-90A | 20A-95A | 50A-95A | |
| Mechanical Properties for Room temperatureerature | Tear resistance | 2 | 2 | 2 | 4 |
| Elongation at break | 3 | 2 | 3 | 3 | |
| Rebound resilience | 2 | 0 | 3 | 2 | |
| Tear propagation resistance | 2 | 1 | 3 | 1 | |
| Abrasion resistance | 2 | 2 | 1 | 1 | |
| Compression set | at max. continuous operating temperature | 0 | 0 | 0 | 2 |
| at room temperature | 0 | 1 | 0 | 1 | |
| Thermal behaviour | Cooling behaviour (Tg) up to °C | -30 | -30 | -30 | -30 |
| Max. Continuous operating temperature up to °C | 110 | 220 | 130 | 150 | |
| Resistance to | Petrol | 3 | 3 | 3 | 2 |
| Mineral oil (at 100 °C) | 3 | 3 | 1 | 3 | |
| Acids (aqueous inorganic acids at RT) | 2 | 3 | 3 | 2 | |
| Alkalis (aqueous inorganic alkalis at RT) | 2 | 3 | 3 | 2 | |
| Water (at 100 °C, distilled) | 2 | 3 | 3 | 2 | |
| Weather and ozone | 2 | 3 | 3 | 2 | |
Table 1: Material comparison of NBR (nitrile butadiene rubber) with FKM (fluororubber), EPDM (ethylene propylene diene rubber) and HNBR (hydrogenated nitrile rubber).
What does NBR cost?
The costs of NBR are in the lower range of technical elastomers (speciality compounds tend to be in the middle range) and are significantly lower than high-performance materials such as FKM. The specific price depends on the acrylonitrile content, the compound composition, the purity requirements and the purchase quantity.
| Material | EPDM | NBR | CR | TPE/TPU | Silicone (LSR) | Silicone (HTV) | HNBR | FKM |
|---|---|---|---|---|---|---|---|---|
| Cost factor | x 1,0 | x 1,0 | x 1,2 | x 1,3 | x 1,4 | x 1,8 | x 2,9 | x 3,7 |
Table 2: Cost comparison of NBR (nitrile butadiene rubber) with other common elastomers
What are the differences between NBR and EPDM?
NBR is very resistant to oils, greases and fuels, while EPDM is unsuitable for this, but has excellent resistance to water, vapour, ozone and weathering. Accordingly, NBR is primarily suitable for oil and fuel-carrying applications, while EPDM is preferably used for ageing and weather-resistant applications.
What are the differences between NBR and FKM?
NBR and FKM differ greatly in their performance level: NBR offers good oil and fuel resistance at moderate temperatures and comparatively low costs, while FKM is designed for significantly higher temperatures and aggressive chemical media. Accordingly, NBR is used in cost and volume-driven applications, while FKM is used for technically highly stressed and durable sealing applications.
What are the differences between NBR and HNBR?
NBR is an acrylonitrile-butadiene rubber with an unsaturated polymer structure. HNBR stands for hydrogenated nitrile butadiene rubber and is produced by subsequent hydrogenation of NBR, in which the double bonds in the polymer are largely saturated. This structural change makes the polymer significantly more resistant to heat, oxygen and ozone, while the oil and fuel resistance typical of NBR is largely retained.