Low temperature performance is one of the most overlooked properties in seal performance. Exposure to low temperature can contract elastomeric materials, resulting in decreased compression and possible leakage. When seal materials are exposed to a lower temperature than their designed limit, seals become less flexible and brittle. Seals can fail by two modes under low temperature:
- The seal material will harden when the low temperature limit is reached and resist deformation to pressure causing leak paths.
- The seal will undergo a compression set, so when heated above the low temperature, this allows for leak. Material selection is key for the low temperature seal performance.
Low Temperature Testing
Three standard low temperature tests are performed to measure material performance. Brittleness (ASTM D2137) measures the ability of a material to withstand breaking when bent at a given temperature for a period of time. Temperature Retraction (ASTM D1329) measures the temperature at which a material returns from an elongated state. Torsional Stiffness Ratio (ASTM D1053) measures the ratio from when a material is twisted, first at room temperature and then at a given low temperature. These tests give some idea of low temperature performance but have limited value for seal applications. One example is that a material might not break at -40°C, but the material could be stiff enough to allow leak paths.
A good indicator for seal performance is Compression Set at Low Temperature (ASTM D1229). This test measures set at 3 min. after exposure to a given temperature and at 30 min. This gives a clear indication of what will happen to a material when exposed to low temperature and allowed to return to higher temperatures. A quick indicator is the Glass Transition Temperature (Tg). This shows the temperature at which the material becomes hard.
- Vinyl Silicone (VMQ) is considered the general polymer. These types of polymers have brittle points to -80°F as tested in A-A-59588 (ZZ-R-765 Class 2B). Phenyl Silicone (PVMQ) is an extreme low temperature polymer with brittle points down to -130°F as tested in A-A-59588 Class 1. Drawbacks with Silicone use are excessive swelling in aliphatic and aromatic hydrocarbon fuels and many lubricating oils.
- Fluorosilicone (FVMQ) can be used to -104°F in oil and fuel applications. The addition of the Fluorine group to the polymer chain gives the polymer swell resistance. Many aircraft applications use Fluorosilicone because of low temperatures at higher altitudes and contact with JP-4, de-icing agents and hydraulic oils.
- General polymers can be used, but service temperatures need to be watched. Many EP polymers work at -85°F. Nitrile (Buna) compounds when formulated correctly can withstand -40°F to -85°F. With Nitrile, the better the low temperature performance, the more swell you will have in oils and fuels. Fluorocarbons (Viton™) can be used from -13°F to -40°F. To reach the lower limit, low temperature polymers must be used. This normally costs more than standard polymers but must be used for this type of service.
- Teflon™ is outstanding in low temperature service. With good resistance to gas permeation, Teflon™ is capable of sealing to -300°F. Teflon's drawback is poor elastic memory and a tendency to “creep” when not confined. Addition of fillers and energizers can help limit some of these drawbacks.