Low-temperature fluorocarbon elastomers have been used for decades in aircraft fuel and turbine oil applications because of their useful temperature operating range and fuel resistance. However, until now, these materials have suffered from one performance Achilles Heel: compression set.
In O-ring sealing lingo, compression set refers to testing that indicates how much and how quickly a seal permanently flattens out and the joint begins to weep. To test this, the seal’s thickness is measured, and then the seal is compressed 25% before being placed in an oven for a given amount of time at a given temperature. It is then taken out and measured again.
For aerospace applications, the most commonly used seals and o-ring materials are GLT-type fluorocarbons (generically, ASTM D1418 Type 3 elastomers), which have a glass transition temperature of -30°C. Materials of this type that meet the new AMS 7287 and predecessor AMS-R-83485 specifications have a useful operating temperature down to -40°F and up to 400°F. They also boast outstanding fuel resistance—in particular, significantly better resistance to hot HTS turbine oil than traditional fluorocarbon compounds (AMS 7276).
But, then there is compression set.
Historically, the high temperature (392° F) compression-set values of low-temperature fluorocarbons have been about twice those of AMS 7276-type fluorocarbon compounds, meaning the service life is noticeably shorter than that of “traditional” fluorocarbons.
Parker Engineered Materials Group’s O-Ring Division applied its material science expertise to solve this problem and break the paradigm of poor compression set performance of low temperature fluorocarbon compounds.
New low-temperature fluorocarbon
VM125-75, our new low-temperature fluorocarbon for AMS 7287 and AMS-R-83485 specifications, maintains the low-temperature performance and HTS oil resistance of older technologies and beats (by quite a bit) the compression set performance of traditional AMS 7276 material.
In fact, after testing the material for only a week (168 hours) at 527°F, the compression set of VM125 rivals that of FFKM! While we don’t recommend using VM125 in applications that get this hot continuously, the performance described here does provide a significant safety factor for applications that involve short-term temperature spikes.
Our new VM125-75 material not only meets all common industry standards for aerospace fuel sealing (which means it can be retrofitted into existing applications with no engineering design changes), but also redefines the performance window of what a familiar technology can accomplish.
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This article contributed by Dan Ewing, Senior Chemical Engineer, Parker Hannifin O-Ring Division.