The right stuff -- choosing the best elastomer

Part 1 of a 2-Part Series

While there are countless factors that can affect your choice of elastomer -- more than can be covered here -- the following provides some general guidelines that can help you with your specifications

Friction

Standard methods employed for minimizing the effects of O-ring friction include reducing seal squeeze; increasing compound hardness; specifying a low friction compound, such as Teflon^®; surface treatment with a low friction coating; and reducing the O-ring's cross section to reduce the amount of contact area (being conscious of avoiding spiral failure).

Best Choice(s)

To date, homogeneously dispersed lubrication in the form of graphite, molybdenum disulfide, Teflon, or oils has been successfully incorporated into Ethylene Propylene, Nitrile, Neoprene™, Fluorocarbon, and Silicone.

Rotary Applications

In rotary applications, a turning shaft protrudes through the I.D. of the O-ring, continuously exposing the inside surface of the O-ring to friction-generated heat from the rotating shaft. Elastomers are poor thermal conductors: if heat is generated faster than it can be dissipated, O-ring failure may result.

Best Choice(s)

In rotary applications, polymer selection is based upon abrasion resistance, heat resistance and other environmental conditions. Our material selection section of our Seal Design Guide can help you choose the best polymer for your rotary application.

Extreme High Temperature Situations

Exposure of O-ring elastomers to extreme high temperatures can cause physical and/or chemical deterioration. When exposed to extremely high temperatures, the O-ring will initially soften and swell within the gland, which causes increased friction in dynamic applications.

Best Choice(s)

A number of special compounds have been developed to provide dependable o-ring sealing performance in high temperature situations. These include Viton^®, Aflas™, Chemraz^®, Kalrez^®, Silicone, Fluorosilicone and Teflon^®. All of these compounds feature heat resistance to at least 400°F with Kalrez rated to 600°F (for short periods of time).

Extreme Low Temperature Situations

Exposure to low temperature contracts elastomeric materials, resulting in decreased compression and possible leakage. Extreme cold also affects seals by making them less flexible and brittle. However, once the seal is returned to room temperature, the changes induced by the cold are physical and usually reversible, unlike extreme high temperatures, which are chemical and irreversible. To utilize seals for low temperature service, use lower durometer compounds, especially when sealing at low pressures.

Best Choice(s)

Silicone is one elastomer that is outstanding in retaining its flexibility to a low limit Silicone and Fluorosilicone are two of the most common shielding elastomeric of -75°F.

Drawbacks with silicone use, however, may be encountered with excessive swelling in aliphatic and aromatic hydrocarbon fuels and many lubricating oils.

Fluorosilicone may be an alternative compound in applications where a particular fluid severely attacks silicone. The low temperature limit of Fluorosilicone is -75°F, with significant improvements over silicone in resistance to swelling in lubricating oils, aliphatic and aromatic hydrocarbon fuels.

Teflon is another outstanding low temperature limit material. With good resistance to gas permeation, Teflon is capable of maintaining its flexibility to -300°F.

NOTE: In regards to low temperature performance it is important to note that many other compounds may seal at temperatures below their normal low temperature limits by increasing O-ring squeeze.

FDA Food Applications

Seals proposed for use by the food processing field are often required by law to be comprised of only the compound ingredients determined by the U.S. Food and Drug Administration (FDA) to be non-toxic and non-carcinogenic.

Such O-ring compounds must consist exclusively of the ingredients listed in the FDA's "White List" located in the Code of Federal Regulations (title 21) section number 177.2600. It is the responsibility of the O-ring manufacturer to utilize food grade materials only from the white list of FDA sanctioned ingredients.

Best Choice(s)

Food service O-rings that have thus far met FDA white list requirements have been produced primarily from the elastomers Ethylene Propylene, Fluorocarbon, Neoprene, Nitrile and Silicone.

These are just a few examples of some applications requiring special elastomers. We look at some others in our next e-newsletter in Part 2 of our series. In the meantime, Apple Rubber's Seal Design Guide provides much more detail on special elastomer applications.

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