ASTM D 1418 Designation: HNBR

ASTM D 2000, SAE J200 Type / Class: DH

STANDARD COLOR: Black, Green

TRADE NAMES:
• Therban® (Bayer Corp.)
• Zetpol® (Zeon Chemicals, L.P.)

RELATIVE COST: High

GENERAL TEMPERATURE RANGE: -25° to +300° F

Though the double bonds within nitrile’s butadiene segments are needed for cross-linking (see Nitrile-Buna N), they are also the main attack sites for heat, chemicals, and oxidation. As part of an ongoing effort to engineer more resistant compounds, 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 just hydrogenated nitrile (see Figure 40).

As you might guess, hydrogenated nitrile results from the hydrogenation of standard nitrile. Hydrogenation is the process of adding hydrogen atoms to the butadiene segments. Adding hydrogen greatly reduces the number of carbon-to-carbon double bonds that would otherwise be weak links in the polymer chain. Why are double bonds weak? It stems from valence, or the ability of an atom to form one or more energy bonds with neighboring atoms. A carbon atom can form four distinct covalent bonds. Because carbon has this valence of four, it is most “satisfied” when it has actually formed four single bonds (a state known as saturation) rather than two single bonds and a double bond. A satisfied, saturated atom is more stable, so a compound composed largely of saturated carbons is less reactive and more resistant to chemical attack.

As shown in Figure 40, HNBR’s main chain is primarily composed of highly saturated hydrocarbons and acrylonitrile (ACN). Thanks to their saturation, the hydrocarbon segments impart heat, chemical, and ozone resistance. 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. As with standard nitrile, the ACN content of HNBR imparts toughness, as well as fuel and oil resistance. This ACN content can be modified for specific uses. There are also a few remaining unsaturated butadiene segments (typically well under 10%) to facilitate peroxide curing or, in some instances, sulfur vulcanization. Peroxide-cured HNBR has improved thermal properties and will not continue to vulcanize like sulfur-cured nitriles.

Since its introduction, HNBR has proven itself in a variety of applications. Deeper and deeper oil wells require materials that can resist heat, crude oil, 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 automotive air conditioning systems where R134a refrigerant gas has replaced the chlorofluorocarbon (CFC)-containing R12 refrigerant. HNBR is used in fuel parts due to its increased resistance to sour gasoline and ozone. It is used in oil line parts because of its resistance to elevated temperatures, oil additives, and copper-containing metal sludge.

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, HNBR has excellent 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, shaft seals, diaphragms, and in air conditioning systems)
• Oil field applications (as O-rings, well-head seals, drill-bit seals, packers, drill-pipe protectors)

HNBR DOES NOT PERFORM WELL IN:
• Esters
• Ethers
• Hydrocarbons (chlorinated)
• Ketones

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