Nitrile rubber (NBR)
also called nitrile-butadiene rubber, or sometimes referred to as Buna for short. Is an oil-resistant synthetic rubber, with a elastomer composition of high molecular weight amorphous random copolymers of 1, 3-butadiene and acrylonitrile. The acrylonitrile portion can vary from a minimum of 15% to a maximum of 50%. As the level of Acrylonitrile increases, such properties as hardness, tensile, abrasion resistance, resistance to fuels, resistance to oils, and gas impermeability increases. Acrylonitrile content can be extremely important when trying to match performance with a specific application. Most standard Nitrile compounds are based on a 33 to 36 ACN polymer. This gives the most versatile compound performance for oil swell and low temperature properties. Higher acrylonitrile content would be considered critical in applications that are fully immersed in fuels or oils. This can help in dynamic application where a designer would not want excess compression from higher volume swell as the oil swells the rubber. Low ACN polymers are used to improve Low Temperature properties especially when designers need to have seals perform down to -50°C. Seal designers will then need to compensate for the larger volume swell that will be seen. The rubber compound will typically stay more flexible at the lower temperature and allow the seal to respond to the change in pressure.The chart below shows both the positive and negative side effects of increased ACN content.
|Effect of acrylonitrile (ACN) on NBR properties|
Effect of increasing ACN level
|Hardness (at room temperature)||+|
|Hardness (at 75 C)||0|
|Compatibility with polar materials||+|
|Low temperature flexibility||–|
|+ Property improves | 0 Property is unaffected | – Property is reduced|
One of the most common chemical modification for Nitrile rubber is hydrogenation (HNBR). Hydrogenation is done so that little of the unsaturation remains. This results in a product with much improved resistance to oxidation and weathering, but little or no sacrifice of other useful properties.
Chemically modified NBR containing pendant carboxylic acids are made by copolymerizing methacrylic or acrylic acid with the butadiene and acrylonitrile (XNBR). These polymers will give improved tensile strength, tear, modulus and abrasion resistance.
The permissible operating temperature range of nitrile rubber is –40 to +100 °C (–40 to +210 °F). For brief periods, temperatures of up to 120 °C (250 °F) can be tolerated. Increasing the ACN content can allow for higher operating temperatures, but for applications in Northern Alberta where the enviromental temperature can plummet to below -30°C. Its important to remember that any increase to the ACN content will result in sacrifices to the materials performance in low temperature applications.
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For more information on Nitrile, such as material specs, please visit our nitrile page