Low Temperature O-ring Materials:
- Fluorosilicone (FMQ, FVMQ) down to -100°F (-73°C)
- Butyl (IIR) down to -75°F (-59°C)
- Silicone (VMQ) down to -65°F (-54°C)
- Specialty Compounds
Many compounds support a sub degree service temperature. These materials are the most often used in extreme low temperature applications extending to under -65°F. Elastomers can be compounded with different properties for different low temperature gradients. Temperature ranges are recommendations with a margin of safety when setting the general operating temperature of common elastomers.
O-rings have an advantage of holding a strong seal through a wide range of temperatures. Considering the minimum temperature will help narrow down material choices. Range of withstood temperature is important for applications that will swing from hot to cold and vice versa, in addition to the products that will be used to seal multiple types of projects.
How Is Low Temperature Resistance Measured?
Low temperature resistance is measured by determining the flexibility of an elastomer at a given low temperature.
- The lowest temperature at which the seal is expected to function should be determined.
- The low temperature test method that most nearly simulates the actual service requirement should be chosen to give the best possible assurance that the seal which passes this test will function in the application. Parker believes that the Temperature Retraction Test (TR-10) is the best method for determining a compound’s ability to seal at low temperatures. Most low temperature tests are designed to indicate the brittle point of a material. This only tells at what low temperature the compound is most likely to be completely useless as a seal in a standard design, but very little about the temperature at which it is useful. This is not the case with TR-10 that consists of stretching 3 or 4 samples 50%, freezing them, then warming them gradually at a constant rate, and finally recording the temperature at which the samples have returned to 9/10 of the original stretch (1/10 return). This temperature (TR-10) then is the lowest temperature at which the compound exhibits rubber-like properties and therefore relates to low temperature sealing capabilities. Functional tests indicate that O-rings will usually provide reliable dynamic sealing at or below the TR-10 value. Static O-rings normally function satisfactorily to about -8°C (15°F) below this.
Temperature at the TR-10 point should be taken for all elastomers to determine a minimum functional temperature. In practice, a static seal may have a minimum functional temperature of about 15°C (-8°F) lower than the TR-10 point, assuming a correctly designed gland.
If the permeability rate of silicones is thought to be too high for the application, bear in mind that the rate decreases as the temperature goes down. For applications requiring moderately high temperatures as well as low, it is sometimes feasible to use two O-rings, to maintain the seal at the extreme low temperature plus a butyl or fluorocarbon to reduce permeability when the seal is warmer.
If a low temperature seal must have resistance to a fluid that attacks silicone, the answer may be a fluorosilicone. This material has excellent resistance to a wide range of fluids, is usable up to 177°C (350°F) or higher in many applications, and will often seal at temperatures as low as -73°C (-100°F). Its primary disadvantage is its lack of toughness, giving it limited usefulness as a dynamic seal, yet in certain dynamic applications, fluorosilicone O-rings have served well as springs to activate a U-type shell of fluorocarbon elastomer or other wear resistant material.
Minimum temperature recommendations are based on long term functional service. If it is subjected to this temperature continuously, it should perform reliably for 1,000 hours. Since the low limit for any compound varies with the medium, the low-temperature limit for many compounds is shown as a range rather than a single figure. It is important that you analyze all aspects of your application. Due to the variety of operating conditions and applications for these products or systems, the user, through his or her own analysis and testing, is solely responsible for making the final selection of the products and systems. Temperature should not be the only consideration when selecting an o-ring material. Media, (fluid and gas), wear, and application requirements must also be considered.
What Causes O-ring Failure At Low Temperatures?
When an o-ring is subject to low temperature cooling, it causes the elastomer to lose its elasticity - measured as the compression set. The compression set is an integral part of o-ring performance necessary to provide a reliable seal, defined as the percentage of the material that fails to return to its original size after compression. The worsening of compression set in low temperatures leads to seal leakage and ultimately o-ring failure.