For decades, choosing seals for low-temperature (-65°F) aircraft fuel applications in the aerospace industry has meant selecting a material with the most tolerable failure points. Low-temperature nitrile, fluorosilicone and fluorocarbon materials all have obvious weaknesses:
Sealing low-temperature fuels over the long-term shouldn’t have to mean choosing the lesser of several evils, though. Why should extended seal life come at the cost of appropriate temperature tolerance? What if there was a material with good resistance to aircraft fuels down to -65°F, durable enough to last the entire five years of a typical maintenance cycle?
There is now. Parker Engineered Materials Group’s O-Ring Division has developed VX065-75, a new extreme-low-temperature fluorocarbon (FKM) compound that solves the lifecycle/performance conundrum.
Reliable sealing at a specified temperature is a function of mechanical design (the seal and the mating gland) and material behavior. Standard O-ring and groove—the most practical design in most applications—relies on material temperature properties to achieve acceptable performance, usually defined as the TR-10 point (temperature at which the material maintains 10% of its elasticity).
Functional sealing in static applications usually can be attained at 15°F below the TR-10 point, as shown in Table 2. The new VX065-75 compound has an extremely low TR-10 point without the wear and tear problems that are inherent to fluorosilicone as noted Table 1.
The amount of swell an elastomer exhibits in a particular fluid is usually a good indicator of chemical degradation caused by the fluid upon a seal material. (Note: elevated temperatures are often used during testing to accelerate the process, but these methods tend to skew results for low-temperature nitrile materials.)
Tables 3 and 4 compare relative fuel resistance of various elastomers at room temperature and elevated temperature, demonstrating significant advantages offered by Parker’s VX065-75 compound. In terms of fuel compatibility, VX065-75 stacks up well with the best-in-class FKM materials, even those that do not offer improved low temperature performance.
An O-ring in a properly designed gland fails primarily from long-term loss of resiliency: the seal permanently flattens out and the joint begins to seep. Compression set is the most common way to test this material property change (over time, the extent to which a seal assumes the shape of a groove into which it is compressed).
Table 5 shows comparable compression set values for the different high-temperature elastomers. Testing was performed at elevated temperatures to heighten the effect. VX065-75 has the best performance and has a -65°F functional sealing temperature, unlike AMS-7287 (traditional low-temperature fluorocarbon) which is only rated to
-40°F (refer to Table 1).
Three other benefits of using Parker VX065-75 compound instead of fluorosilicone:
- Less prone to installation damage because it recovers faster from stretch; see Figure 1.
- Much better pressure resistance, eliminating need for backup rings to prevent extrusion
- Wears out much more slowly, so it can be used for intentionally dynamic applications
Figure 1: Various high-temperature O-rings were stretched 20%, released and photographed after 3 seconds against a shadowgraph of the original O-ring dimensions.
Parker fluorocarbon compound VX065-75 represents an opportunity to dramatically improve seal performance in aircraft fuel applications. It provides the low-temperature performance, compression resistance and overall durability required to dramatically extend service life in real-world applications, particularly where fluorosilicone is currently used. It also offers meaningful improvements over both low-temperature nitrile (e.g., AMS-P-5315) and traditional low-temperature fluorocarbon elastomers (e.g., AMS-R-83485,) so it’s an excellent candidate for use as a universal seal material throughout an aircraft fuel system.
If you are planning to attend the 2016 Farnborough International Air Show, please visit the Parker Hannifin Corporation booth - Hall 4, Stand E50, click here for more information about the show.
This article contributed by Dan Ewing, Senior Chemical Engineer, Parker Hannifin O-Ring Division