ASTM D1418, ISO 1629 Designation:HNBR

ASTM D2000, SAE J200 Type/Class: DH

Standard Colors: Black, Green

Relative Cost: High

General Temperature Range: -40° to +300° F

Though the double bonds within nitrile’s butadiene segments are needed for cross-linking, they are also the main attack sites for heat, chemicals, and oxidation. As part of an ongoing effort to engineer elastomers that are less susceptible to such attacks, a new class of nitrile was developed in the 1980s. Initially known as highly saturated nitrile (HSN), this class is now more commonly called hydrogenated nitrile butadiene rubber (HNBR), or simply hydrogenated nitrile.

Hydrogenated nitrile results from the hydrogenation of standard nitrile. Hydrogenation is the process of adding hydrogen atoms to the butadiene segments; this is also known as "saturating" the material. Adding hydrogen causes many of the carbon-to-carbon double bonds (C=C) in the polymer backbone to become single bonds (C-C). Single bonds are desirable because they have greater stability and require more energy to break than double bonds. The higher the percent of saturation, the greater the number of carbon-to-carbon single bonds, and the greater the chemical and heat resistance.

The hydrogenation process is closely controlled, thus allowing saturation rates ranging from 85% to 99.9%. The few remaining unsaturated butadiene segments (15% or less) serve as cure sites for either peroxide or sulfur curing. Peroxide cured HNBR retains the best thermal and chemical properties, and it will not continue to vulcanize like the sulfur-cured nitriles. As with standard nitrile, the acrylonitrile (ACN) content of HNBR imparts toughness, as well as fuel and oil resistance. The ACN level can be modified for specific uses.

In addition to good processibility, compounded HNBR yields high tensile properties, low compression set, good low temperature properties, heat resistance, excellent ozone resistance, good resistance to aggressive oils, resistance to crude oil in the presence of hydrogen sulfide and amines, and resistance to alkaline and oxidizing media.

Since its introduction, HNBR has proven itself in a variety of areas, including oilfield, automotive, and industrial applications. Deeper and deeper oil wells require materials that can resist higher pressures, heat, hydrogen sulfide (H2S), amine-based corrosion inhibitors, steam, and the detrimental effects of explosive decompression. HNBR meets these needs and is used for a variety of products, including O-rings, packings, wellhead seals, drill bit seals, blowout preventors, and drill pipe protectors.

HNBR is used in fuel parts due to its increased resistance to sour gasoline and ozone. It is used in oil line parts thanks to its resistance to elevated temperatures, oil additives, and copper-containing metal sludge. HNBR is also used in automotive air conditioning systems where R134a refrigerant gas has replaced the chlorofluorocarbon (CFC)-containing R12 refrigerant.

HNBR is also finding wider use as an alternative to fluorocarbon rubber (FKM) in shaft seals. Why the switch? The hardness of the mineral fillers - primarily calcium sulfate (CaSO4) and barium sulfite (BaSO3) - used to improve fluorocarbon’s wear properties can cause grooving of the metal shaft, eventually providing a leak path that leads to seal failure. With other materials, carbon black (which is not as abrasive as the mineral fillers) might be substituted, but carbon black is not sufficient to give fluorocarbon good abrasion resistance. On the other hand, proper compounding of HNBR will yield superior abrasion resistance, making it a viable alternative to FKM. HNBR also has better low temperature properties and tear resistance than fluorocarbon.

HNBR performs well in:

  • Automotive applications (as O-rings, timing belts, fuel injector seals, fuel hose, rotating shaft seals, diaphragms, air conditioning systems, lip seals)
  • Oil field applications (as O-rings, well-head seals, packers, ram and annular blowout preventors, drill-bit seals, drill-pipe protectors, valve seals, stators).

HNBR does not perform well in:

  • Esters
  • Ethers
  • Hydrocarbons (chlorinated)
  • Ketones

Keep in mind that increased hydrogenation and heat resistance make HNBR more likely to creep (cold flow). Increased hydrogenation also leads to decreased low temperature elasticity.