Choosing the best elastomer

Part 2 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 additional general guidelines that can help you with your specifications.

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.

Silicone swelling in aliphatic, aromatic and oil-based fluids may also be a positive rather than a negative factor in helping to artificially maintain seal squeeze at low temperatures. As gases are less active at low temperatures, the high permeability of silicone may be acceptable, assuming that high operating temperatures will not also be encountered.

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. Inert to virtually all industrial chemicals, Teflon's only low temperature drawbacks are poor elastic memory, and a tendency to "creep" when not confined. Creep can be controlled, however, through the addition of fillers in compounding.

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.

Abrasion Resistance

Applications involving oscillation, reciprocation, or rotation induce friction and typically generate wear regions on one surface of the seal. This leads to premature seal failure, system contamination, and eventually system malfunction. When feasible, the use of lubricants, improved surface finishes, or system filtration reduces the effect from friction. However, proper compound selection is essential for extended seal utility.

Best Choice(s)

Carboxilated Nitrile (XNBR) has superior wear resistance properties, when compared to that of the base compound Nitrile, through the addition of carbon in the crosslinking organization. Polyurethane also has outstanding resistance to abrasion and is typically employed in high-pressure applications.

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. Apple Rubber's Seal Design Guide provides much more detail on special elastomer applications. Visit www.applerubber.com for more information

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