Cryogenic sealing applications are challenging, to say the least. One popular solution is the use of spring-energized seals, and in this blog post we are going to talk about the 3 post popular materials used for spring-energized seals for cryogenic applications: PTFE, modified PTFE, and UHMW PE.
Cryogenic Seal Challenges
Applications with temperature that fall in the range of -238° F to -460° F (absolute zero) are considered cryogenic. This poses a significant challenge because of the tendency of polymers to exhibit brittle behavior below certain temperatures. In addition, these types of seals usually are lubricant prohibitive because of the danger of the lubricant freezing, thus allowing the contacting surfaces to freeze to each other. Another major issue with cryogenic seals is the fact that most polymers have a rather high coefficient of thermal expansion, which means they can contract significantly when exposed to cryogenic temperatures. Therefore spring-energized seals are a popular choice for cryogenic applications.
PTFE
PTFE (polytetrafluoroethylene) is usually recognized by its tradename, Teflon. It is an incredible polymer that combines extremely low friction, outstanding chemical compatibility, low thermal conductivity, and excellent wear characteristics. In addition, PTFE is self-lubricating, thus eliminating the need for a lubricant. It also remains quite flexible at low temperatures. Most importantly, perhaps, is the fact that its temperature range includes cryogenic temperatures: the range for virgin PTFE begins at -420°F and -450°F for glass-filled PTFE.
Modified PTFE
Modified PTFE, also known as TFM, retains the best characteristics of PTFE and improves on them through chemical modification. It includes an improved flex-life, better stiffness, improved thermal conduction, and better creep resistance. Its temperature range is comparable to that of PTFE, and it is also self-lubricating. Modified PTFE also offers an even smoother surface finish, fewer microscopic voids, and lower gas permeability because it has a much more dense molecular structure than standard PTFE.
UHMW PE
Another popular choice for spring-energized cryogenic seals is UHMW PE (ultra-high molecular weight polyethylene). Its temperature range starts at -300°F, well within the range for cryogenic applications. Like PTFE and modified PTFE, it is self-lubricating with low friction and good chemical compatibility (although not at the same level as PTFE). One of the benefits of UHMW PE over PTFE is its outstanding wear resistance, as well as its more economical cost.
One of the benefits of UHMW PE over PTFE is its outstanding wear resistance, as well as its more economical cost.
Conclusion
When designing a spring-energized seal for a cryogenic application, remember to consider the three most popular sealing materials for such applications: PTFE, modified PTFE, and UHMW PE. All three can handle cryogenic-level temperatures, are self-lubricating, provide outstanding chemical compatibility, and offer good wear characteristics.
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