Benefits of PTFE for Sealing Applications
by Jackie Johnson Jackie Johnson No Comments

Benefits of PTFE For Sealing Applications

PTFE (Polytetrafluoroethylene), also known by its trade name Teflon, is a polymer material commonly used in sealing applications that offers unparalleled stability and sealing characteristics across an extremely wide range of temperatures, from the extreme heat of a space shuttle engine to the cryogenically cold temperatures used to preserve

In this article, we will discuss how and why PTFE is one of the best materials to use for seals in a wide variety of applications.

Low Friction

PTFE has the highest melting point and lowest friction, and is the most inert of all the fluoropolymers. It has a continuous service temperature rating of 500 degrees Fahrenheit. Molding powders are excellent, fine cut granular resins, well suited for a variety of demanding chemical, mechanical, electrical and non-stick surface applications.

Such applications include:

  • Cookware
  • Outdoor Rain Gear
  • Medical Devices
  • And more!

Cryogenic Applications

Cryogenic seals are used with super-cooled media, like liquid hydrogen or compressed natural gas, at temperatures below -238°F and down to -460°F (absolute zero). Cold temperatures like this are rough on a seal because at these temperatures most materials begin to exhibit highly brittle behavior and lubricants typically cannot be used because they will freeze. PTFE seals, however, can handle temperatures all the way down to -450°F and are capable of dry running because of their extremely low friction. PTFE cryogenic seals are used in industries like oil & gas, pharmaceuticals, and aerospace.

High Temperature Applications

PTFE seals work well at the other end of the spectrum, too. They can continue to function in extreme temperatures up to 600°F, and continuous operating temperatures up to 600°F. Note that a filler may be required to enable the PTFE to dissipate heat more quickly. It’s not uncommon to see PTFE seals in petroleum or steam applications where temperatures greatly exceed 200°F.

PTFE is also non-flammable, making it ideal for use in applications such as jet propulsion engines. Where other materials would simply melt under the pressure of constant exposure to high temperature flames, PTFE is built to withstand even the hottest of environments.

The use of seals for high temperature applications include oil and gas industry and aerospace, to name a few.

Chemical Applications

The chemical resistance of PTFE is some of the best on the market. It is stable in most aggressive and corrosive media, including:

  • Acetone
  • Chloroform
  • Citric Acid
  • Hydrochloric Acid
  • Sulfuric Acid
  • Tallow
  • Sodium Peroxide
  • And more!

However, it should be pointed that that PTFE is not chemically resistive to liquid or dissolved alkali metals, fluorines and other extremely potent oxidizers, as well as fluorine gas and similar compounds. Outside of those, PTFE is an excellent choice for applications involving chemicals.

Oil and Gas Industry

Seals are critical for the safe and reliable operation of oil rigs across the globe. Not only do seals need to be able to withstand a wide variety of extreme temperatures, but they need to be able to handle extreme pressures as well. For well drilling, for example, seals need to handle pressures from 345 to 2070 bar (5000 to 30000 psi).

For those reasons, PTFE is an incredibly popular material to make oil and gas seals out of. Because of it’s resistance to heat, cold and high pressure, PTFE can withstand the rigors of oil and gas unlike any other material.

Spring-energized Seals

In order to retain sealing power under extreme temperatures, many engineers and designers go with spring-energized PTFE seals. The spring provides optimal sealing by forcing the lip of the seal against the mating surface and helps to account for dimensional changes as a result of temperature fluctuations.

A highly efficient seal is created as the system pressure increases enough to take over from the spring and engage the shaft or bore. The spring or energized seal assembly provides permanent resilience to the seal jacket and compensates for jacket wear, hardware misalignment and eccentricity. The jacket material is critical in design to assure proper seal performance.

Rotary Shaft Seals

Using PTFE in rotary shaft seals allows them to be able to run at higher pressures and velocities when compared to other materials. They are also able to have tighter sealing, often exceeding 35 BAR and can run at far more extreme temperatures ranging from -64 degrees Fahrenheit (-53 degrees Celsius) to 450 degrees Fahrenheit (232 degrees Celsius).

On top of that, they are:

  • Inert to most chemicals
  • Can withstand speeds up to 35 m/s
  • Compatible with most lubricants
  • Come in a wide range of sizes
  • And more!

Conclusion

PTFE is an ideal sealing material for both extremely high temperature applications and demanding cryogenic applications. It retains its key sealing properties: stiffness, strength, dimensional stability (may require spring energizer), low friction, and chemical compatibility- even in the most aggressive operating conditions.

Need PTFE sealing solutions? Advanced EMC Technologies is the leading provider of PTFE spring energized and rotary shaft seals in the US. Contact us today!

O-Rings in Spaceflight | Advanced EMC Technologies
by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

O-Rings in Spaceflight

Since the Challenger disaster, o-rings have come under close scrutiny in spaceflight designs and applications and they continue to play a vital role in modern spaceflight, including modern commercial spaceflight ventures such as SpaceX, Virgin Galactic, and Blue Origin.

In this week’s blog post, we will discuss o-rings in spaceflight, including problems that arise, the best materials, and more.

O-Ring Failures in Modern Spaceflight

Few would argue the importance of seals and o-rings in space shuttles and rockets. From rocket engines to the International Space Station, the ability to retain media and prevent its contamination is of vital importance. This importance was first brought to public attention through the Challenger disaster where a stiff o-ring cost multiple lives. However, o-ring issues did not end there.

In 2005, orbiter tests prior to the space shuttle Discovery’s return to flight revealed a failure that traced back to Nitrile/Buna N o-rings. Six of nine flow control valve o-rings had suffered radial cracks, with one o-ring developing problematic leak paths as a result. The cause of the o-ring issue was found to be ozone attack of Nitrile/Buna N, which is one of its susceptibilities.

Back in 2016 a Blue Origin launch was delayed by o-ring issues. Jeff Bezos reported that the rubber o-rings in the New Shephard rocket’s nitrogen gas pressurization system were leaking and had to be replaced before the launch could continue. New Shephard is the same rocket used to take Star Trek legend William Shatner on his first real space flight.

Virgin Galactic, owned by Richard Branson, discovered a very dangerous issue with the flight vehicle SpaceShipTwo when it was returned to the hangar in 2019. A critical seal running along a stabilizer on one of the wings had “come undone.” While not an o-ring, this does reinforce the importance of seals on modern spacecraft.

Operating Environment Complications for O-Rings in Spaceflight

O-rings face a very hostile environment in space, including …

  • Extreme temperatures, ranging from cryogenic to high
  • Wide temperature variation
  • Extremely high pressures and vacuum pressures
  • Vibration during launch
  • Risk of permeation depending on the media involved
  • Chemical attack from media such as fuels and lubricants
  • Potential exposure to ozone, ultraviolet, and radiation

There are other potential issues as well. For rockets in particular, one of the challenges faced when specifying o-rings involves their ability to expand fast enough to maintain a seal even when joints (a common area of use for o-rings) move away from each other. Swelling when exposed to hydrocarbon-based greases used to protect components against corrosion can be problematic as well. 

O-Ring Materials in Spaceflight

O-rings are manufactured from a diverse group of materials, including EPDM, FEPM, FFKM, FKM, Fluorosilicone, HNBR, Hytrel, NBR, Neoprene, Polyurethane, and Silicone.

Any material used in spaceflight applications, however, would need to fall within the categories of high temperature service and/or chemical service, reducing the list to materials such as …

  • FEPM (trade name Aflas)
  • FFKM (trade names Kalrez, Chemraz, Markez, and Simriz)
  • FKM (trade names Viton, Technoflon, and Fluorel)
  • Silicone. 

Keep in mind, however, that other materials may be suitable that are not included in this list and the suitability of these materials is highly dependent on the application.

FEPM O-Rings

FEPM, perhaps better known by the trade name Aflas, is a copolymer of tetrafluoroethylene and propylene and often represented as TFE/P. In addition to chemical compatibility and a degree of high temperature performance, it offers excellent ozone resistance. It is known for providing excellent performance where traditional fluoroelastomers are known to fail.

FFKM O-Rings

FFKM, often referred to by trade names such as Kalrez or Chemraz, is an excellent option for applications that involve extreme pressures, extreme temperatures, and aggressive chemicals. FFKM, which is a perfluoro elastomer material, is available in various grades that offer key properties such as low permeation, low compression set, resistance to temperature cycling, and wide ranging chemical compatibility as well as resistance to explosive decompression and plasma resistance. 

FKM O-Rings

Fluoroelastomers such as FKM, known to most people as Viton, can provide excellent resistance to fuels, lubricants, and oils. Another key characteristic of is extremely permeability when exposed to a range of substances that include oxygenated aircraft fuels. They also offer reliable performance at extremely high temperatures where non-fluorinated elastomeric materials will start to degrade.

In addition, FKM comes in various grades focusing on features such as low temperature resistance, fuel resistance without sacrificing necessary elasticity, and chemical resistance that is unaffected by extremely high temperatures. Such features combined have already made them a common choice in aerospace applications, including o-rings.

Silicone O-Rings

Silicone rubber o-rings have been used extensively by NASA and remain a popular choice for o-rings used in spaceflight applications. In fact, here’s a direct quote from NASA that dates back to 2010:

“Silicone rubber is the only class of space flight-qualified elastomeric seal material that functions across the expected temperature range.”

It is considered by many to be the best in-class elastomer choice for extremely harsh environments involving high temperatures and among its key properties is its ability to maintain critical mechanical properties in the presence of extreme heat. A potential issue related to the use of silicone for o-rings lies in its gas permeability.

Conclusion

O-rings are just as important to modern spaceflight as ever, and so is the importance of choosing the right type of o-ring. A failed o-ring, no matter how tiny it may seem, can lead to serious disaster and potential loss of life. 

If you are looking for a reliable o-ring solution for an aerospace or spaceflight application, contact the sealing group here at Advanced EMC. Our team will work with you to explore all possible solutions, including materials beyond those discussed here. Give us a call today and let our team put their expertise to work for you.