Typically, those are made from butadiene acrylonitrile rubber (i.e., BUNA) or fluoropolymer elastomer (i.e., VITON).
Viton and Buna are two of the most commonly used elastomers for sealing applications and for good reason. Both of these rubbers serve as great general purpose sealing options. These seals offer excellent compression set resistance, and both options are designed to resist most oils and lubricants, especially petroleum based lubricants. Additionally, moderate temperature applications between 5°F and 250°F are served by both seals, making either seal a perfect choice for general industrial use. However, for more specific applications the decision becomes much more important.
Viton and Buna seals have several very crucial differences that make them especially well suited to certain applications. While both Viton and Buna seals both serve as great sealing options at moderate temperatures, Viton is far superior to Buna for high temperature applications. Viton seals provide an indefinite seal for temperatures up to 400°F, and for temperatures up to 600°F they offer an excellent seal for more than 48 hours. Buna on the other hand is only effective up to 250°F. However, Buna seals provide a low temperature sealing option with effective sealing down to temperatures of –22°F, while temperatures below 5°F render Viton seals ineffective as they become quite hard and inflexible.
Along with temperature, other environmental conditions differentiate these two seals. An exceptionally broad range of chemical resistances make Viton seals a perfect option for most applications involving oils, fuels, and mineral acids, and these seals also boast excellent resistances to oxidation, ozone, UV exposure, weather, fungus, and mold. While also boasting some chemical resistances, Buna is much less universally resistant than its Viton counterpart suffering degradation from weather and ozone exposure. For most circumstances, however, this is not an issue, and Buna seals also offer the benefit of superior abrasion and tear resistance making them more suitable for more heavy duty industrial applications.
Both of these sealing options offer an extensive list of diverse applications, and they both serve as excellent general purpose seals. Nevertheless, to optimize your choice in seals be sure to consider the advantages and disadvantages of these two exceptional seals.