Nitrile
ASTM D1418, ISO 1629 Designations: NBR, XNBR
ASTM D2000, SAE J200 Type/Class: BF, BG, BK, CH
Standard Color: Black
Trade Names:
- Krynac® (Polysar International, USA)
- Nipol® (Zeon Chemicals)
- Nysyn® (Copolymer Rubber & Chemical Corp.)
- Paracril® (Uniroyal, Inc.)
Relative Cost: Low
General Temperature Range: -40° to +225° F
Nitrile rubber is the most commonly used elastomer for O-rings and other sealing devices. Also known as Buna N, nitrile is a copolymer of butadiene and acrylonitrile (ACN). The name Buna N is derived from Butadiene and Natrium (the Latin name for Sodium, the catalyst used in polymerizing butadiene). The "N" stands for acrylonitrile.
The butadiene segment imparts elasticity and low temperature flexibility. It also contains the "unsaturated" double bond that is the site for crosslinking, or vulcanization. This unsaturated double bond is also the main attack site for heat, chemicals, and oxidation.
The acrylonitrile segment imparts hardness, tensile strength, and abrasion resistance, as well as fuel and oil resistance. Heat resistance and gas impermeability are also improved through increased ACN content, which typically ranges from 18% to 45%. A standard, general-purpose nitrile compound usually contains 34% ACN.
General-purpose nitrile compounds with a 34% ACN content have a recommended temperature range of -40° F to +225° F (-40° C to +107° C). The low temperature flexibility can be improved by reducing the ACN content. Nitrile compounds with an ACN content of 18% to 20% remain flexible at temperatures down to -65° F (-54° C).
Unfortunately, compounding ingredients and polymers that offer the best low temperature properties are usually adversely affected by high temperatures. A general-purpose compound is cured with sulfur, but as the ambient temperature in an application exceeds +225° F, free sulfur in the compound finds other unsaturated double bonds and forms additional crosslinks. This results in compression set and hardening of the compound. To improve high temperature properties, a peroxide cure system and/or mineral fillers must be used. Peroxide-cured compounds have both better high temperature properties (up to +275° F, +135° C) and improved compression set characteristics, but they are also more difficult to process and more expensive than sulfur-cured compounds.
Nitrile compounds are superior to most other elastomers with regard to high tensile strength, as well as excellent abrasion, tear, and compression set resistance. Nitriles also have very good aging properties under severe operating conditions. Because of the double bonds present in the polybutadiene portion of the chemical backbone, nitrile compounds do not have good resistance to ozone, sunlight, or weathering. They should not be stored near ozone-generating electric motors or equipment.
Nitrile performs well in:
- Dilute acids
- Ethylene glycol
- Petroleum oils and fuels
- Silicone oils and greases
- Water (below 212� F)
Nitrile does not perform well in:
- Aromatic hydrocarbons (benzene, toluene, xylene)
- Automotive brake fluid
- Halogen derivatives (carbon tetrachloride, trichloroethylene)
- Ketones (MEK, acetone)
- Phosphate ester hydraulic fluids (Skydrol®, Pydraul®)
- Strong acids
Carboxylated nitrile rubber compounds (XNBR) are available to provide even better strength properties, especially abrasion resistance. Carboxylated nitriles are produced by the inclusion of carboxylic acid groups on the polymer during the polymerization process. These carboxylic acid groups provide extra crosslinks in the polymer, thus producing harder, tougher compounds with higher abrasion resistance, modulus, and tensile strength than standard nitriles. On the down side, carboxylated nitriles are less flexible at low temperatures and less resilient than non-carboxylated compounds.
Other nitrile variations are also possible, including internally lubricated compounds with improved friction and wear properties, as well as Food and Drug Administration (FDA) and National Sanitation Foundation (NSF) formulations for food and potable water applications.
