Temperature is one of many factors to consider when selecting the best o-ring material for a given application. Rising or falling temperatures can have a variety of negative impacts on the efficacy of a seal. For example, prolonged exposure to heat degrades o-ring materials physically and chemically. Excessive heat is also known to cause o-ring materials to swell and become inflexible. Conversely, cold temperatures may cause o-ring shrinkage that results in leaks.
Falling Temperatures
Performance in low temperature environments is an often overlooked characteristic in rubber seals. Low temperature can cause elastomeric materials to shrink, increasing the likelihood of leaks. Seal failure caused by low temperatures occurs in one of two ways:
- The seal hardens as it reaches its low temperature limit, causing it to resist deformation to pressure and creating leak paths.
- The seal undergoes compression set, so if the seal is then heated above its usual low temperature, it allows leaks to occur.
Coolest Choices
- Vinyl Silicone (VMQ) has brittle points reaching -85°F. Phenyl Silicone (PVMQ) is an even more extreme low temperature polymer with brittle points reaching down to -155°F. The drawbacks with these types of Silicone are excessive swelling in aliphatic and aromatic hydrocarbon fuels and lubricating oils.
- Fluorosilicone (FVMQ) can be used to -75°F in oil and fuel applications. Adding Fluorine to the polymer results in swell resistance. Fluorosilicone is often used in aerospace applications due to exposure to low temperatures at high altitudes and frequent contact with de-icing agents and hydraulic oils.
- General polymers can be used for low temperature applications in some circumstances. Many EP polymers work at -76°F. Some Nitrile compounds can withstand temperatures between -40°F to -76°F. However, when using Nitrile, the better the low temperature performance, the more swell in oils and fuels. Fluorocarbons can be used for applications between -13°F to -40°F. In order for Fluorocarbons to work at their lower limit, more expensive low temperature polymers must be used.
Rising Temperatures
Exposing o-ring elastomers to high temperatures can cause physical and chemical deterioration. When exposed to extremely high temperatures, o-rings soften and swell within the gland, causing increased friction in dynamic applications.
High pressure applications are especially prone to failure because room temperature tests in such applications may be unreliable. Over time, irreversible chemical changes occur, increasing seal hardness and causing compression set and volumetric changes.
Hottest Choices
A number of special compounds have been developed to provide dependable o-ring sealing performance in high temperature situations. These include:
- Perfluoroelastomers
- Silicone
- Fluorosilicone
These compounds feature heat resistance to at least 400°F, with some reaching up to 600°F for short periods. An additional number of o-ring materials feature temperature resistance to 300°F, with special resistances to particular fluids or environmental factors. For example, ethylene propylene offers high resistance to steam.
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