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.
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
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.
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 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
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.
In some cases, O-rings are made from thermoplastic materials such as PTFE, as well as from soft hollow or solid metals.
The term rubber is often used interchangeably with the term elastomer.