ASTM D 1418 Designation: NBR, XNBR

ASTM D 2000, SAE J200 Type / Class: BF, BG, BK, CH

RELATIVE COST: Low

GENERAL TEMPERATURE RANGE: -25° to +225° F

Nitrile rubber is the most commonly used elastomer in the manufacture of shaft seals and other sealing devices. Also known as Buna N, nitrile (see Figure 35) 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 the polymerization of butadiene). The letter “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 is also improved through increased ACN content, which typically ranges from 18% to 45%. A standard, general-purpose nitrile compound usually contains 34% ACN. Low temperature performance is reduced as ACN level increases.

The relationship between the ACN content, volume swell in IRM 903 oil, and the brittle point of the elastomer is illustrated in Table 5.

General-purpose nitrile compounds with a 34% ACN content have a recommended temperature range of -25° F to +225° F (-32° C to +107° C). The low temperature capability can be extended to -40° F (-40° C) by reducing the ACN content to about 30%. Nitrile compounds with an even lower ACN content of 18% to 20% remain flexible at temperatures down to -65° F (-54° C). Be aware that reducing ACN content improves low temperature properties but reduces high temperature properties, increases material swell, and reduces fluid resistance.

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 +250° F, +121° C) and improved compression set characteristics, but they are also more difficult to process and more expensive than sulfur-cured compounds.

Nitrile compounds have excellent tensile strength, as well as excellent abrasion, tear, and compression set resistance. Because of the double bonds present in the polybutadiene segments of the chemical backbone, nitrile compounds have poor resistance to ozone, sunlight, and weathering. They should not be stored near ozone-generating electric motors, welding equipment, or in UV light.

NBR PERFORMS WELL IN:
• Petroleum oils & fuels
• Silicone oils & greases
• Ethylene glycol
• Dilute acids
• Water (below 212° F)

NBR DOES NOT PERFORM WELL IN:
• Aromatic hydrocarbons (benzene, toluene, xylene)
• Halogen derivatives (carbon tetrachloride, trichloroethylene)
• Ketones (MEK, acetone)
• Phosphate ester hydraulic fluids (Skydrol®, Pydraul®)
• Strong acids
• Extreme pressure (EP) lubes at elevated temperatures

Carboxylated nitrile rubber compounds (XNBR) provide even better strength properties, especially abrasion resistance. Carboxylated nitriles are produced by the inclusion of carboxylic acid groups on the polymer during polymerization. These carboxylic acid groups provide extra “pseudo” crosslinks, producing harder, tougher compounds with higher abrasion resistance, modulus, and tensile strength than standard nitriles. Carboxylated nitriles are, however, less flexible at low temperatures and less resilient than non-carboxylated compounds. Also, the “pseudo” crosslinks (being ionic in nature) are thermally sensitive. As temperatures increase, the ionic bonds lose strength.

Other nitrile variations are 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.

“Nitrile rubber is the most commonly used elastomer in the manufacture of shaft seals and other sealing devices.”

Figure 35

Table 5