A Guide to Nitrile O-Rings and Seals

Nitrile o-rings and seals are the most commonly used o-rings and seals on the market. The popularity of the material is due in large part to its excellent mechanical properties. O-ring seals are a necessity, an integral component found in mechanical devices, equipment, and machines. The familiar doughnut-shaped gaskets are used in a variety of static or dynamic applications whose parts require fluids, oils, or lubrication to function. Those composed of nitrile offer greater functionality and versatility than other materials.

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Though nitrile is not a universal sealant, as a synthetic rubber material it is unusual in that its mechanical properties do make it compatible with petroleum-based oils, greases, lubricants, and fuels, water, and varying types of chemicals and hydraulic fluids.

What is Nitrile?

Nitrile is an elastomeric synthetic rubber compound, a copolymer of acrylonitrile and butadiene, from which it derives its other common names&#;nitrile butadiene rubber or NBR, acrylonitrile-butadiene rubber, or simply, Buna-N.  Its many trade names include Krynac, Nipol, Perbunan, and Europrene.

With further development, the material&#;s elastic polymer content can be enhanced to be utilized in numerous applications for a wide range of environmental conditions or temperatures, with exceptional chemical compatibility. For example, the amount of acrylonitrile can vary in commercial products from 18% to 50%. This variation in the polymer composition of nitrile will affect the material&#;s physical and chemical properties. High nitrile content within the polymer provides higher resistance to oils but lowers its flexibility. Meaning, nitrile can offer superior strength and provide more resistance than natural rubber to oils and acids. However, as the resistance to petroleum-based oils and hydrocarbon fuels increases with the increase in NBR content, the flexibility of the material decreases.

Mechanical Properties

As an elastomeric material, nitrile has many mechanical properties that make it well-suited for O-rings and sealing applications. Nitrile holds high fuel and oil resistant properties. It is known for its tensile strength. It combines resistance with a low compression set and high abrasion resistance to make it the seal industry&#;s most widely used economical elastomer. This also makes it a cost-effective alternative to fluoroelastomers. In addition to its excellent fuel and oil resistant properties, nitrile rubber displays excellent resistance to moisture and water, various alcohols, silicone greases, and hydraulic fluids. Other advantages are its good tear resistance and non-polar solvent resistance.

Advantages and Disadvantages

Because nitrile can be compounded for various applications, it offers stability for service over a temperature range of approximately of -35°F to 250°F (-37°C to 120°C), providing excellent compression set, tear, and abrasion resistance at those working temperatures. With such wide working temperatures, nitrile o-rings and seals are used extensively throughout the automotive and aeronautical industry, and in the manufacture of other products like fuel and oil handling hoses, grommets, and self-sealing fuel tanks. It is such stability at low temperatures that make nitrile ideal for many aeronautical applications.

Though nitrile is a well-suited material with many general purpose applications, it does have its limitations. It is incompatible with different types of fluids, including automotive brake fluid, ketones, phosphate ester hydraulic fluids, or nitro and halogenated hydrocarbons. When compounding, in order to obtain good resistance and functionality for low-temperature applications, it is often necessary to sacrifice some high-temperature resistance and functionality. It also suffers from poor ozone, sunlight, and weather resistance, flame resistance, and cannot be used in high heat applications. Though its ozone and weather resistance can be improved upon through further compounding, it&#;s worth noting that nitrile o-rings and seals should not be stored near electric motors or other ozone-generating equipment.

Nitrile compounds are superior to most elastomers, offering superior strength and more resistance than any natural rubber to oils and acids. They perform well and are compatible with petroleum-based oils and fuels, vegetable oils, silicone oils and greases, aliphatic hydrocarbons, ethylene glycol, dilute acids, and water to temperatures up to 212ºF (100ºC). It is incompatible with brake fluids, ketones, ethers, and phosphate ester hydraulic fluids, aromatic and chlorinated hydrocarbons, strong acids, and adversely affected by ozone, weathering, and sunlight.

When faced with different types of rubber seals, which do you choose? If there is only one type of O-ring available, is it the right choice? Sadly, one size does not fit all. Each rubber type has is strengths and weaknesses which dictate where a seal can or can't be used.

Meet the Players

For the purposes of this article, we will focus on just three common rubber types found in O-rings and similar automotive seals: Buna N, Viton, and EPDM.

Note that each type is actually a family of rubber compounds. The term "Buna N" is really not much more specific than the term "steel". Just as there are many different alloys of steel with different physical properties, the chemistry of each of these rubber types can be tweaked to enhance resistance to a certain chemical, or for more desirable physical properties.

That means that we must deal in broad generalizations here. You may come across a specific manufacturer whose recommendations differ. That is because a manufacturer may specify a certain rubber formulation whose properties are somewhat different than what is typical for that type of rubber. The good news is that static applications (such as an O-ring between a threaded fitting and a female-threaded port) are more tolerant of material choice than dynamic applications (such as crankshaft seals or master cylinder seals). This article focuses on static seals only.

Buna N / NBR / Nitrile

Buna N (sometimes shortened to just "Buna") is a trade name for nitrile rubber, known internationally as NBR. It is normally the least expensive of the rubber types we will discuss here. NBR is compatible with petroleum fluids (including dinosaur and synthetic lubricating oils, pure gasoline, and diesel fuel). This makes it a good choice for seals in fuel systems running pure gasoline (no ethanol blends) or diesel (not biodiesel). NBR is also fully compatible with water and ethylene glycol coolants. On the other hand, NBR has a relatively low maximum operating temperature at 212F (250F can be tolerated in fully static seals, but seal life will diminish). It is not a great choice for seals in transmission or differential oil systems when higher temperatures are expected. (Note that some NBR blends have good resistance to alcohol, or higher temperature ratings.)

Viton / FKM / FPM / Fluorosilicone

Viton is a trade name for a type of fluorocarbon rubber developed by DuPont specifically for the aerospace industry. The ASTM in the USA uses FKM as the generic term, but DIN/ISO in Europe and the rest of the world prefer FPM. It is often the most expensive of the three rubber types discussed here. FKM is compatible with petroleum fluids (including dino and synthetic lubricants, pure gasoline, and diesel). FKM is also fully compatible with water and ethylene glycol coolants. Most FKM grades are also a little better than NBR at handling biodiesel and ethanol-blended fuels, although FKM is still not the best choice for ethanol -- and it is a downright poor choice for nitromethane. Its high 400F operating temperature makes FKM a better choice for sealing engine, transmission, and differential oil systems.

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EPDM

While NBR and FKM have similar chemical resistance, EPDM is completely different. It is incompatible with petroleum (oil and gasoline), but it is compatible with alcohols (ethanol, methanol, and nitromethane), water, coolants (both ethylene and propylene glycol), and glycol-based brake fluid. Of all three rubber types, EPDM is the only one that should be considered for use anywhere in a braking system. (Of course there have to be exceptions -- the Citroen and Rolls-Royce high-pressure brake/suspension systems that use green hydraulic oil must avoid EPDM and should use FKM instead.)

Location, Location, Location

So, what is the best choice where?

Brake system

Of these three rubber types, EPDM is the only choice for seals in braking systems using a glycol-based or silicone-based DOT-approved or pure racing brake fluid. (If you buy your brake fluid from Pegasus Auto Racing Supplies, use EPDM seals in that system.) Don't panic, most companies that make master cylinder and caliper seals only offer them in an EPDM version. If a seal is made for automotive brake systems and the material is not specified, you can safely assume it is EPDM. FKM is an appropriate seal material only for those few who find themselves undertaking the unique adventure of rebuilding a Citroen or Rolls-Royce with hydraulic oil instead of brake fluid.

Engine oil systems

Regardless of the type of lubricant, FKM is the preferred seal choice for dynamic seals (sliding or rotating, such as crankshaft seals). NBR is acceptable for static seals such as O-rings between threaded fittings.

Transmissions and differentials

Manual or automatic, transmission or transaxle, go with FKM. Low-temperature applications (such as street cars) may be able to use NBR for static seals.

Fuel systems

If you are running pure gasoline or petroleum diesel in a non-return fuel system, NBR seals will be perfectly acceptable. But change the fuel to an ethanol blend or biodiesel and you should be looking at FKM. If you have a recircualting system (i.e., return line to the fuel tank), FKM seals will generally be more resistant to the oxidized fuel. Nitromethane systems require EPDM.

Cooling systems

All three rubber types are compatible with water and ethylene glycol coolant. All three will handle 212F and are acceptable to 250F in static applications. If you are running propylene glycol coolant, then EPDM is the best choice.

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