Introduction to acrylonitrile-butadiene rubber
Nitrile rubber was first produced in 1930 by copolymerising acrylonitrile (ACN) and butadiene (vinyl ethylene). Industrial production of the synthetic rubber, which very quickly came to be regarded as vital to the build-up of arms, began in 1934. The biggest difference between NBR and natural rubber (NR), styrene-butadiene rubber (SBR) and other rubber types known up until that time is that it is resistant to fuels.
Like SBR, acrylonitrile-butadiene rubber is produced by emulsion polymerisation. Polymerisation temperatures of around +50°C produce "hot rubber", which has a high degree of branching, making onward processing more difficult. Polymerisation at lower temperatures of around +5 to 15°C leads to "cold rubber", which is easier to process because of its lower degree of branching.
As with SBR, most NBR today is produced as cold rubber. The acrylonitrile content of common nitrile rubber grades is between 18 and 50 percent.
Properties of acrylonitrile-butadiene rubber
The properties of NBR, including the oil resistance, are largely determined by the acrylonitrile content. Owing to the differing glass transition temperatures of acrylonitrile (+90°C) and butadiene (-90°), the glass transition temperature rises as the acrylonitrile content increases, while the elasticity drops. Swelling resistance to fuels, fats and oils also increases with the acrylonitrile content.
Nitrile rubber materials containing 18 percent acrylonitrile have very good low-temperature flexibility down to about -48°C along with moderate oil and fuel resistance. With an acrylonitrile content of 50 percent, low-temperature flexibility is only maintained down to about -3°C, whereas the oil and fuel resistance is optimum. Gas permeability falls as the acrylonitrile content increases, and compression set deteriorates.
As with most synthetic rubbers, good strength values can only be achieved in acrylonitrile-butadiene rubber by adding active fillers, such as carbon blacks. In addition, the following properties are characteristic of NBR:
- Media resistance/chemical resistance to mineral oils, fuels, lubricants, alcohols and vegetable and animal fats and oils
- Abrasion resistance
- Good mechanical properties
- Low gas permeability
- Relatively low elasticity (lower than that of SBR or NR)
- Thermal application range, depending on compound formulation: -50 to +100°C (up to 130°C for short periods)
- Good electrical conductivity because of its polarity
- Swells in mixtures of ketones, esters and other polar solvents
The thermal stability and ozone resistance are limited by the double bonds in the polymer backbone of the NBR. If these are converted into single bonds by hydrogenation, the thermal stability and ozone resistance increase significantly. The resulting material is known as hydrogenated acrylonitrile-butadiene rubber (HBNR). Nitrile rubber can also be blended with PVC to improve the ozone resistance. Typically, a 70-percent NBR is combined with a 30-percent PVC.
Acrylonitrile-butadiene rubber also swells or breaks down in pure chlorinated or polar hydrocarbons such as acetone but also in strong oxidising acids such as concentrated nitric acid. However, NBR is resistant to swelling in mixtures of aliphatic and aromatic hydrocarbons.
Both sulfur and organic peroxides can be used to cure acrylonitrile-butadiene rubber. The latter result in especially heat-resistant nitrile rubber grades. However, a lower tear propagation resistance has to be accepted as a consequence.
Applications for nitrile rubber
Acrylonitrile-butadiene rubber is used primarily in areas where in addition to good mechanical properties, importance is also attached to high swelling resistance to mineral oils and fuels along with ageing, heat and abrasion resistance. Because of their good technological characteristics, hoses, seals and other products based on this synthetic rubber are suitable for a wide variety of applications.
The bulk of the nitrile rubber manufactured worldwide is used by the automotive supply sector and in machine building, to produce media-carrying pipes, hydraulic hoses, seals, anti-corrosion coatings for metal surfaces and dirt-trapping mats. Pneumatic and hydraulic seals can be made from this rubber, as can O-rings and radial shaft seal rings. Other uses for NBR include drive and tensioning belts and technical rollers.
In the occupational safety sector, acrylonitrile-butadiene rubber is used as a base material for laboratory and surgical gloves and for the chemical proofing of textiles. The shoe industry uses NBR as a rugged sole material, while it is used for stators in industrial installations and for hoses in flexible media lines for gases and liquids, in oil and gas exploration for example.