O-Ring Seals

O-rings are circular or doughnut-shaped rings generally molded from an elastomer.1 They are used mainly for sealing purposes. To ensure a tight seal over a wide range of pressure, temperature, and tolerance, the elastomer has to be thermally stable over the intended service temperature range, incompressible, but very flexible (deformable) to ensure a tight seal. It also has to withstand the mechanical pressure from the surrounding structure or by pressure transmitted through hydraulic fluids and, depending on the applications, it also has to have good oil, fuel and chemical resistance. Thermally stable rubbers2 are special polymers that are very elastic. They are lightly cross-linked to suppress irreversible flow and amorphous with a glass transition temperature well below the intended lower service temperature to avoid embrittlement during service. They can be envisaged as one very large molecule of macroscopic size. O-rings are made from many different elastomers, but three dominate the market and
account for the majority of O-rings produced, namely Nitrile, EPDM and Chloroprene. 


Nitriles (NBR)

Acrylonitrile butadiene rubber (NBR), usually shortened to nitrile, is the most widely used elastomer in the seal industry due to its good physical properties, mineral oil resistance and useful temperature range. NBR grades with high acrylonitrile content have better oil and abrasion resistance, whereas grades with low acrylonitrile content have better low temperature flexibility and resilience. In general, nitrile rubbers have only moderate physical properties but good abrasion and excellent oil and hydrocarbon solvent resistance. They also have low gas permeability particularly those with high nitrile content but poor ozone and modest heat resistance. To improve the physical properties, nitrile rubbers are sometimes carboxylated (XNBR) which improves their temperature resistance.
NBR o-rings are mainly used for general purpose sealing. They have good chemical resistance to petroleum oils and fluids, silicone greases, water, di-ester and ethylene glycol based lubricants.


Chloroprene (Neoprene®, CR)

Neoprene is another widely used elastomer in the sealing industry. It is an unique elastomer in having often acceptable or good resistance to both petroleum lubricants/fuels and oxygen/ozone. It is one of the few rubbers that are self-extinguishing. However, chloroprene tends to harden over time and degrades in the presence of some fairly common chemicals such as hydrochloric acid, acetone, xylene, chlorinated solvents, acetic acid, and hydrogen peroxide. Its mechanical properties are generally inferior to those of natural rubber but it has superior chemical resistance.
Common uses of Neoprene seals include refrigeration, freon/air conditioning, and engine coolants among many other applications.

Ethylene Propylene (EPDM)

Ethylene propylene diene monomer (EPDM) is a synthetic elastomer consisting of ethylene and propylene. As a saturated non-polar rubber, it has outstanding heat, ozone and weather resistance, good electrical resistivity as well as outstanding resistance to many polar solvents such as phosphate esters, silicone oils, hot water and steam, diluted acids and alkalies, ketones (MEK, acetone) and alcohols. EPDMs are probably the most water resistant elastomers available. However, they are not compatible with mineral and synthetic di-ester lubricants, and hydrocarbon oils, fuels and solvents. They also have poor flame resistance.
EPDM o-rings are often used in high temperature water, steam and brake fluid applications. However, contact with mineral oils and grease should be avoided.



  1. In some cases, O-rings are made from thermoplastic materials such as PTFE, as well as from soft hollow or solid metals.

  2. The term rubber is often used interchangeably with the term elastomer.

  • Summary

  • Heat Resistance

    Elastomers can be classified according to their heat resistance. Fluorocarbon, fluorosilicone and silicone elastomers are recommended for elevated temperature use, wheras Neoprene and nitrile rubber are not recommended for continuous use above 100°C / 212°F.

  • Oil Resistance

    Because many applications involve hydrocarbon oils, elastomers (seals) are classified according to their heat and oil resistance. In the ASTM D 2000 system, elastomers are ranked by heat resistance (type) and by oil resistance (class).
    The highest oil resistance have fluorosilicone and fluorocarbon elastomers.

  • Tear and Abrasion Resistance

    Most elastomers with good abrasion resistance have also good tear resistance, and elastomers with poor abrasion resistance have usually poor tear resistance.

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