by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

Finding the right low-temperature o-ring solution can be critical to both the success and safety of a design — and FEP encapsulated o-rings are an excellent solution.

Low Temperature and Cryogenic Applications

Cryogenic refers to temperatures below freezing and extending to absolute zero (-460°F / -273°C), while low-temperature environments are typically defined as below -25°F. Common chemicals that are stored or transported at cryogenic temperature include 

  • Liquid Oxygen (LOX), -297°F
  • Liquid Natural Gas (LNG), −265°F
  • Liquid Hydrogen (LH2), -423°F
  • Liquid Nitrogen (LN2), –130°F 
  • Liquid Helium, -452°F 

The industries that involve low temperatures include aerospace, energy, electronics, chemical processing, food, pharmaceutical, and medicine. Quantum computing, rockets, and MRI machines are just a few specific examples where cryogenic o-rings are needed. 

Four Critical Low-Temperature O-Ring Issues

At low and cryogenic temperatures, the design and specification of o-rings can be challenging.

One of the most critical issues for o-rings used at cryogenic temperature lies in the tendency of polymeric and elastomeric temperatures to assume brittle behavior below their glass transition temperature. In fact, below this temperature, an o-ring will easily crack or even shatter. Therefore, O-rings for cryogenic applications must not exhibit brittle behavior at the operating temperature.

A key characteristic of o-rings for extremely low temperatures is their compression set, which measures how much elasticity a material loses at low temperatures. Compression set is defined as what percentage of the material fails to return to its original size after being compressed. Low compression set values are associated with better resistance to continuous strain and changes in pressure. 

Another issue with o-rings for cryogenic applications involves the media. In some instances, the media may not be compatible with more traditional elastomeric materials. Therefore, when specifying an effective o-ring, it is essential to find a material that not only possesses suitable mechanical properties but is fully compatible with the media being sealed.

 Finally, lubrication at cryogenic temperatures is exceptionally problematic and runs the risk of any lubricant sealing the contact surfaces together. Therefore, low-temperature applications require low-friction o-ring materials that are preferably self-lubricating and do not exhibit stick-slip.

Encapsulated O-Rings

The use of encapsulated o-rings for cryogenic applications is rapidly rising. An encapsulated o-ring is engineered to combine critical features (e.g., cryogenic performance and chemical resistance) of a polymer such as FEP (Fluorinated ethylene propylene) with the resilience and elasticity of a core made from a stainless steel spring energizer, silicone, or FKM. 

 The energizer at the core of the o-ring is key to maintaining its resiliency at cryogenic temperatures and enables the o-ring to retain its dimensional stability once installed. On the other hand, the encapsulating material protects the energizer from corrosive media that adversely impacts its performance.

FEP Encapsulated O-Rings: A Low-Temperature O-Ring Solution

FEP is an excellent encapsulating material for low-temperature o-rings:

  • The operating temperature range for FEP is -420F through 400F (-251C through 200F), which means it does not exhibit brittle behavior within the cryogenic range.
  • FEP has a low compression set, which means it can quickly recover from continuous strain and pressure changes.
  • FEP not only has the mechanical properties needed for a successful cryogenic o-ring, but it is highly resistant to attack from a wide range of chemicals.
  • FEP is a self-lubricating material with extremely low friction and no stick-slip behavior.

FEP is also available in FDA-approved grades and is considered a high purity material. In addition, it is corrosion-resistant, non-flammable, and melt-processable (i.e., compatible with extrusion, vacuum forming, and injection molding). Furthermore, swelling and chemical attack are the most common issues behind o-ring failures– and FEP encapsulated o-rings can resist both. 

FEP Encapsulated O-Ring Cores for Cryogenic Applications

When FEP is the material of choice for low-temperature applications, three specific types of cores are generally used: an FKM core, a hollow silicone core, or a helical spring core.

FKM Core

FKM (Fluoroelastomer) cores are a standard solution that offers good rubber-elastic properties and a low compression set. Combining their elastomeric properties with a low compression set means they can quickly return to their original shape after deformation.

Silicone Hollow Core

Silicone is a softer core than FKM that exhibits excellent flexibility at cold temperatures. In addition, a hollow core provides even more elasticity and very low contact pressure (which may be needed for particular delicate operations or fragile equipment).

Helical Spring Core

Not all encapsulated o-rings have an elastomeric core. Another option is using a helical spring (also known as a canted spring) as the inner energizer. These cores have outstanding resilience, exhibit practically no compression set, and provide a very consistent spring force that can be customized. Spring cores are typically manufactured from a 300 Series Stainless Steel (e.g., SS 301 or 302).

Conclusion

FEP encapsulated o-rings combined with the suitable core can provide low-temperature elasticity, low compression set, chemical resistance, and self-lubricating properties necessary for cryogenic environments. 

If you need a reliable, effective low-temperature o-ring solution, contact the sealing experts at Advanced EMC. Our team of specialists has the knowledge and experience to assist you in finding the correct encapsulated o-ring for even the harshest of cryogenic environments. 

 

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