Parker's O-Ring Division is committed to continuous improvement of elastomer materials, and recently we’ve been focusing on mitigating the weaknesses of aerospace fluorosilicone compounds.
For many years, fluorosilicone compounds have been the go-to material for Seals and O-rings in most jet fuel applications and even some low-temperature hydraulic fluid and polyalphaolefin (PAO) electronics coolant applications. There’s a good reason for the utility of these materials: they have outstanding low-temperature performance (functional temperature range between -73°C to +177°C [-100°F and +350°F,] with thermal stability necessary to accommodate almost all aerospace applications) and good chemical resistance to fuels and most petroleum-based oils and greases.
Traditional fluorosilicones have several well-known shortcomings. Our new generation LM100-70 sealing material offers an incremental (but notable) improvement over its predecessors, LM159-70 and L1120-70, which provided incrementally improved properties over the fluorosilicone compounds they replaced.
In particular, LM100-70 offers a 50 percent reduction in compression set compared with its predecessors. Result: significantly improved service life. O-rings constructed of traditional fluorosilicone and used in high-temperature jet fuel applications typically must be replaced about once a year due to compression set. LM100-70 can extend that by a factor of two or more. (Actual flight hours will vary depending on specific application.)
Historically, fluorosilicones also have mediocre rebound resilience, meaning that when a seal is stretched to fit into a male O-ring gland, the material tends to sag instead of snapping back tight against the groove. Improvements to this material quality—as Parker has achieved—decrease the incidence of O-rings tearing during installation, and make automated assembly faster and more feasible for more applications.
LM100-70 also has significantly higher original tensile strength and better tear resistance at elevated temperatures compared to traditional fluorosilicones. While LM100-70 is still not recommended for use in intentionally dynamic applications, it offers additional safety benefits in terms of mechanical robustness when vibration and unexpected movement do occur.
The other main issue with fluorosilicone is poor resistance to gas permeation (some gas permeates through the seal over long periods). Unfortunately, this property is inherent to the material and very difficult to improve significantly.
Like its predecessors, LM100-75 has a functional temperature operating range of -73°C to +177°C (-100°F to +350°F) and offers good resistance to jet fuel. It also meets AMS-R-25988 Type 1 Class 1 Grade 70 and MIL-STD-25988 Type 1 Class 1 Grade 70 (M25988/1 part numbers) standards.
These qualities mean that no spec or drawing changes are needed to use LM100-70 for seals in almost all aerospace applications. Put simply, these seals cost about the same as those using traditional fluorosilicone, but last longer. In the progression of “good, better, best,” LM100-70 unequivocally fits into the “better” category.
By the way, Parker EMG’s O-Ring Division is not content with achieving “better.” We also developed a new technology for jet fuel applications, VX065-75—to be described in another blog article soon—that raises the bar to “best.”
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This article contributed by Dan Ewing, Senior Chemical Engineer, Parker Hannifin O-Ring Division.