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.
High pressure applications are especially prone to failure here because room temperature tests may provide inaccurate results. Over time, irreversible chemical changes occur that increase seal hardness as well as induce compression set and volumetric changes.
In the case of thermoplastic materials, prolonged exposure to high temperatures may cause partial reversion back to basic components. This occurs because thermoplastics are a class of polymers that are composed of individual molecules that are linear in structure and held together and crosslinked by weak intermolecular forces which can be broken by the heat and pressure. Any rubber compound has a point of heat failure which must be individually addressed. (See Temperature Graph in the Environmental Factors section)
Conversely, thermosets are cross-linked by stronger bonds more resistant to heat and pressure.
A number of special compounds have been developed to provide dependable o-ring sealing performance in high temperature situations. These include Viton™, AFLAS®, perfluoroelastomers, Silicone, Fluorosilicone, and Teflon™. These compounds feature heat resistance to at least 400°F, with FFKM rated to 600°F (for short periods of time).
An additional number of o-ring materials feature temperature resistance to 300°F, with special resistances to particular fluids or environmental factors. Ethylene propylene, for example, features excellent resistance to steam.
Thermoset rubbers can take short durations of high temperature beyond their service temperatures. The longer exposure will eventually cause seal failure and quicker service duration.