Auto molding PTFE seals and seats offer a wide variety of benefits, especially for high-volume production runs. In this blog post, we cover some background on both PTFE and auto molding (also known as compression molding) and discuss why this particular manufacturing process is often preferred by engineers for both seals and ball valve seats.
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.
In general, thermoplastics are known for being corrosion resistant, robust, and usually easy to fabricate. However, the operating environment makes it challenging to find good polymer seals for nuclear applications. In this blog post, the focus will be on thermoplastic polymer sealing solutions as an alternative to elastomeric seals.
Nuclear Sealing Solutions
There are many different applications where seals are necessary for the nuclear industry, including transport flasks and port seals, waste containment packages, ventilation systems, repair systems, valve seats, robotics, manipulators, and hydraulic rams. Reliable seals are especially crucial to reactor coolant pumps (RCP) and self-actuated abeyance systems.
Harsh Environments for Nuclear Seals
One of the most challenging environments for nuclear sealing applications is a combination of halogenated material issues and high gamma radiation exposure levels.
Halogenated Material Complications
In many nuclear plant applications, the use of halogenated materials (particularly those containing fluorine or chlorine) is strictly prohibited because these materials can potentially leach aggressive chemicals such as halides. Halides in aqueous streams, for example, can cause such problems as stress corrosion cracking in stainless steel. This means that while a seal made from a halogenated material may perform well, it can cause severe damage downstream. In addition, halogenated materials may also release corrosive gasses resulting in serious safety issues.
Gamma radiation can cause polymers to degrade and lose critical properties rapidly. This degradation typically takes the form of chain scission and cross-linking, which leads to brittleness and fractures. This has proven to be a severe problem for commonly used seal polymers such as PTFE, PFA, PA, and UHMW PE. And even though seals may not be directly exposed to gamma radiation, this type of radiation can penetrate materials and lead to indirect exposure.
In addition, free radicals can also be trapped within the polymeric structure when exposed to radiation, leading to continued degradation after exposure in a phenomenon known as post-irradiation degradation. High gamma radiation dose rates can also generate heat to complicate matters further.
Nuclear applications can also involve very high temperatures and pressures and wide pressure variation. Exposure to corrosive chemicals can also be problematic. And for a sealing solution to be effective, there are vital considerations such as qualified life and ability to withstand station black-out (SBO) conditions. Therefore, the importance of reliable seals cannot be overestimated.
Polymer Seal Materials for Nuclear Environments
For harsh nuclear environments, three particular polymers can be used as a seal jacket (depending on the application and relevant operating conditions). These are PEEK, FEP, and ETFE.
Polyetheretherketone is often known by brand names such as Fluorolon by Advanced EMC Technologies, PEEK by Victrex, Ketron by Mitsubishi, and TECAPEEK by Ensinger. Of thermoplastic polymers suitable for nuclear applications, PEEK is the most commonly used material for rotary shaft seals. Among the key properties of PEEK are:
- Excellent performance at high temperatures up to 500°F
- Excellent chemical compatibility
- Very low friction
- Excellent resistance to high-energy radiation on the order of 109 rads
PEEK is, however, sensitive to certain acids, carbon sulfides, fluorine, and chlorine.
FEP (Fluorinated Ethylene Propylene)
Fluorinated Ethylene Propylene is often recognized by brand names such as Teflon FEP by Dupont, Neoflon FEP by Daikin, and Dyneon FEP from Dyneon/3M. Critical properties of FEP include:
- Excellent performance at high temperatures up to 400°F
- Very low friction
- Excellent chemical compatibility
- Good resistance to high-energy radiation on the order of 105 rads
In addition, FEP is melt-processable and therefore reasonably easy to use in the manufacture of seals. However, FEP’s limitations for nuclear applications are primarily its susceptibility to attack by acids.
ETFE (Ethylene Tetrafluoroethylene)
Ethylene Tetrafluoroethylene, or ETFE, is often referred to by trade names such as Tefzel by DuPont, Neoflon ETFE by Daikin, and Texlon by Vector Foiltec. Important properties of ETFE include …
- High melting temperature
- Excellent chemical compatibility
- Very low friction
- Non-stick and self-lubricating
- Excellent resistance to high-energy radiation on the order of 107 rads
Its limitations in nuclear applications include the fact that it is rated to only 300°F and is susceptible to attack by esters and aromatics. In addition, it is often cost-prohibitive.
Seals are a critical component in the nuclear industry. While elastomeric seals may be commonly used, there are high-performance polymer alternatives such as PEEK, FEP, and ETFE that provide fundamental properties needed. In addition, such materials can provide the required performance even when subject to gamma radiation, intense heat, and extreme pressures.
If you are investigating polymer seals for nuclear applications, contact the sealing solution experts at Advanced EMC. We understand the specific challenges of the nuclear industry and will share our comprehensive knowledge of polymers to help you find the ideal sealing solution.
Finding the right sealing solution for aerospace applications involves a host of considerations, especially when it comes to the jacket material. However, there is one high-performance solution that rises to the top over and over: PTFE aerospace seals.
Aerospace Seal Challenges
Aerospace sealing solutions can face a wide range of harsh environments that can include exposure to extreme temperatures, high pressures, and corrosive chemicals such as de-icing liquids and aviation fuels. There is also the ever-present demand for energy-efficient solutions, critical requirements related to safety, and compliance with industry standards.
In addition, aerospace seals must often perform in potentially explosive or flammable environments. There are other constraints in aerospace applications as well, such as the need to minimize weight and promote energy efficiency. And looking to the future, there will also be demand for more environmentally friendly, sustainable solutions.
Kynar PVDF (property of Arkema) is a high purity polymer that combines extreme-temperature performance, easy manufacturability, and durability in some of the harshest environments.
What is PVDF?
PVDF (polyvinylidene difluoride or polyvinyl fluoride) is a fluorinated thermoplastic resin that is classified as a specialty polymer whose brand names include Kynar (Arkema), KF (Kureha), and Solef or Hylar (Solvay). This engineering polymer can often be found in environments that involve high purity, hot acid, extremely high temperatures, and/or radiation.
Where is PVDF Used?
PVDF is used extensively in a wide range of industries. Semiconductor manufacturing makes use of PVDF’s ultra-pure status and its ability to perform in harsh environments that may involve extreme temperatures and aggressive chemicals. Electronics and electricity applications depend on PVDF’s outstanding low smoke emission and fire-resistant properties along with electrical properties for use as wiring insulation.
PVDF’s ability to handle radiation makes it an excellent choice for nuclear waste handling, and its high-temperature performance and chemical compatibility lends itself readily to the oil and gas industry. Because PVDF has excellent high-temperature performance, high purity, and low permeability, excellent strength, and chemical compatibility, it is used extensively in chemical processing.
Purity and FDA approval have made it a popular choice in food and beverage packaging and processing as well as pharmaceutical processing. It is often used in connection with water and wastewater management for similar reasons. PVDF is also used extensively in the medical market and healthcare industry where it is used as a biomaterial for medical textiles, such as hernia meshes, as well as for medical sutures.
The transportation and energy market has begun using PVDF as a binder for cathodes and anodes in HEV/EVs (Hybrid Electric Vehicle/Electric Vehicle). Its chemical compatibility and anti-corrosion properties make it useful as a barrier liner for fuel lines and tanker trailer lines. Aviation also makes ample use of PVDF for wiring harnesses and general coatings
How is PVDF Used?
PVDF is commonly used for several specific types of applications across industries:
- Pump assemblies
- Heat exchangers
- Tanks and vessels
- Sensors and actuators
- Fittings, pipes, tubing, and valves
- Membranes, including microfiltration membranes
- Filters and filter housings
- Liners and films
- Cable jacketing and harnessing
- Biocompatible materials
Key Properties of PVDF
As alluded to in previous sections, PVDF possesses several features of interest to engineers:
- Extremely high purity with low permeability
- FDA compliant and non-toxic
- Excellent heat resistance and thermal stability
- Good mechanical properties
- Resistant to a wide range of aggressive chemicals
- Resistant to UV exposure, ozone oxidation reactions, and radiation
- Resistant to the growth of microorganisms
- Excellent burn characteristics
- Good manufacturability
- One of the lowest melting points of commercial fluoropolymers
- Excellent electrical properties
- Excellent abrasion resistance
- Low density (1.78 gm/cm3)
In addition, PVDF offers excellent abrasion resistance, is lightweight, and can be recycled. Also, note that there are additives available for PVDF to enhance its properties and its melt processability.
Purity and FDA Compliance
In addition to being an extremely high purity polymer, PVDF is both FDA compliant and non-toxic while exhibiting very low gas and liquid permeability.
Heat Resistant and Thermal Stability
One of the outstanding features of PVDF lies in its excellent performance, chemical stability, and dimensional stability in high-temperature environments with a service temperature rating of up to 300 F.
Among the outstanding mechanical properties possessed by PVDF are good deflection, tension, compression, and torsion when compared to other fluorinated polymers. In addition, its low rate of water absorption (0.4%) means that it will remain dimensionally stable (not swell) when in a moisture-rich environment. In addition, PVDF has excellent impact strength.
PVDF is known for its excellent chemical compatibility that includes weak and strong acids (including mineral and organic); alcohols; aromatic and aliphatic solvents; weak bases; hydrocarbons; halogenated compounds; ionic and salt solutions; and oxidants. Its primary weaknesses are caustics, esters, strong bases, and ketones.
The surface of PVDF is highly resistant to the growth of microorganisms, including bacteria, fungi, and mold. It is also resistant to weathering, grime, and even graffiti (which is why it is often used in the architectural industry).
PVDF has excellent flame and smoke properties, including UL 94 V-0 rating indicating it is both non-flammable and self-extinguishing along, or more specifically “Burning stops within 10 seconds on a vertical specimen; drips of particles allowed as long as they are not inflamed.” In addition, certain grades of PVDF also possess an excellent flame spread/smoke developed rating of 25/50 (when tested in accordance with ASTM E 84).
PVDF is also highly manufacturable and melt-processable, lending itself to precision machining, rotomolding, compression molding, injection molding, and extrusion as well as subsequent welding and fabrication. Its ability to be used in molding is primarily due to its low melting point of 352 F, compared to PTFE at 621 F or FEP at 517 F.
In addition to electrochemical stability, PVDF also possesses a very high dielectric constant (280 volts per meter) and a high piezoelectric constant. In fact, it possesses both piezoelectric and pyroelectric properties.
One of the polymers we work with here at Advanced EMC is PVDF Kynar made by Arkema. If you are interested in Kynar, have questions about its usage and processing, or need a quote, feel free to contact us and we will have one of our experts respond right away.
O-rings are a circular seal that is seated within a groove and compressed between two or more parts during assembly to form a seal at the interface. While they may look simple, their importance cannot be overstated–especially when it comes to o-rings for semiconductor manufacturing applications.
Semiconductor Operating Environments
In the semiconductor manufacturing industry, it can be difficult to find an o-ring solution that can handle the harsh operating conditions that can involve factors such as aggressive media, extreme temperatures, and vacuum pressures. Chemicals such as bases, acids, solvents, amine-based strippers, and chlorinated gases may be involved depending on the application. Extended exposure to oxygen and fluorine plasmas are common
The performance requirements of o-rings for semiconductor manufacturing are challenging to meet as well, often requiring thermal, dimensional, and chemical stability at high temperatures as well as low outgassing and high purity. Requirements may also include extremely low levels of anionic and cationic impurities, low levels of TOC (Total Organic Carbon), reduced IR (Infrared Absorption), and low permeation rates.
What to Look for in an O-Ring for Semiconductor Applications
The key properties of an o-ring material for the semiconductor industry vary with the type of application involved. For example, track and lithography equipment and processes often require an o-ring that is very resistant to solvents, while CVD (Chemical Vapor Deposition) needs thermal stability and excellent performance in the presence of vacuum pressures.
Other applications, such as CMP (Chemical Mechanical Polishing), must have o-rings made from a material that is both abrasion resistant and resistant to high pH chemical exposure. Wet etch demands an o-ring made from a high purity material that will cause no elemental contamination (i.e., low particle generation) and dry etch requires that the material be resistant to plasma. Resist stripping not only requires general chemical resistance but outstanding performance in the presence of ozone.
O-ring materials may have to meet other requirements as well, such as resistance to poisonous doping agents and reactive fluids, low outgassing, and low trace metal content. Almost all semiconductor o-rings involve a low compression set, excellent dimensional stability, and a wide range of operating temperatures.
Is there a material that can handle the operating environments just described? Yes, there is: FFKM, which provides the resiliency and sealing force of an elastomer with the thermal stability and chemical compatibility of PTFE (trade name Teflon).
With the success of commercial spaceflight companies such as SpaceX, Blue Origin, and Virgin Galactic, there is an increasing demand for high performance, dependable seals. Rockets are one of the areas where harsh environment seals are needed, but also pose extremely challenging issues for success. Spring energized seals are one solution, but why?
What Makes a Modern Rocket
Successful spaceflight involves rockets, and the primary sections of a modern two-stage rocket are the first stage engine bay, first stage, second stage engine bay, second stage, and, last of all, the payload. This constitutes the most common configuration for today’s NewSpace companies.
Such a configuration features an expendable or reusable first stage that contains 4 to 9 engines (the number of engines varies based on company design) and an expendable second stage that typically contains a single vacuum-optimized engine. The goal of the first and second stages is to produce enough thrust to achieve a targeted orbital velocity–usually around 17,500 mph– for the payload that sits on top of the rocket.
Propellants and Pressurants
Most rockets use either solid or liquid propellant. In this blog post, the focus will be on bi-propellant rockets, which are most commonly being used or developed in the United States commercial market. Bi-propellant rockets, as the name implies, use a combination of propellants. Common propellant configurations include:
- RP-1 (Highly refined kerosene)/Liquid Oxygen (LOX) (aka, Kero-Lox)
- Liquid Methane/LOX (aka, Metha-Lox or Lox-meth)
- Liquid Hydrogen/LOX (Hydro-Lox)
Pressurants and support fluids include:
- GN2 (Gaseous Nitrogen)
- Helium (He)
- GOX (Gaseous Oxygen)
- GCH4 (Gaseous Methane)
How Modern Rocket Propulsion Systems Work
For a pump-fed system, the propellants are fed from low pressure tanks into a turbopump assembly (TPA). This significantly raises the pressures to be injected into the main combustion chamber (MCC). In most cases, a small portion of the propellants are scavenged from the high-pressure side to feed a separate small combustion chamber known as a gas-generator or pre-burner and used to drive the turbine. These fuel or oxygen rich gases can then either be vented to the atmosphere or re-injected into the MCC.
Operating Conditions of a Rocket Propulsion System
Consideration of the operating conditions within a rocket propulsion system provides insight into the challenges faced by the seals.
- State 1 – Tank to Turbopump Assembly (TPA) inlet: propellants (oxygen + methane) are usually around 50 -150 psi and RP1 will be between 20 F and 80 F while the cryogenics will be between -450 F to -260 F.
- State 2 – TPA outlet: depending on the engine, pumps will raise these pressures to somewhere between 1,500 and 16,000 PSI.
- State 3 – Pre-burner: pressure will have dropped across the lines and injector – usually 8-15%, however temperatures will be between 800 -1,500 F.
- State 4: depending on the engine cycle, propellants may be in a liquid-liquid state, gas-liquid state, or gas-gas state at an array of temperatures and pressures before mixing in the MC; note that in most cases the fluids will be supercritical.
- State 5: once across the injector, the remaining propellants will combust at temperatures higher than 4000 F while pressure in the MCC may be between 50-20% of State 2 depending on system losses; note that this pressure drops quickly as the gases are pushed toward the atmosphere.
Depending upon which stage is involved, seal requirements vary greatly but high pressures and extreme temperatures will always be involved.
Rocket Engine Seals
Rocket engine seals must perform in some of the most harsh environments imaginable and may involve wide operating temperature ranges (including cryogenic), extreme pressures, wide thermal cycling, and chemical compatibility with fuels, propellants, and pressurants. Most importantly, they must be extremely reliable. As an example, consider the just a rocket turbopump.
The image shown is a Hydro-Lox turbopump with a geared coupling used in the Aerojet Rocketdyne RL10 engine. Where it is labeled with a 1 indicates flange locations that likely use spring-energized face seals. Downstream of the outlets will be the main valves, and they too will most likely have additional flange connections that will require seals. Areas labeled with 2 indicate other flange locations that depend on face seals of unknown makeup but likely involve hot gas connections.
Spring Energized Seals: A Rocket Sealing Solution
One of the most reliable, harsh environment sealing solutions is the spring energized seal. Unlike conventional seals, a spring energized seal includes an energizer that enables the seal lip to stay in contact with the mating surface through extreme variations in pressure and temperature,and dimensional changes, as well as out of roundness, eccentricity, hardware misalignment, and some degree of wear. Vibration, cryogenic temperatures, and high temperatures are also an area where spring-energized seals offer outstanding performance.
They are highly durable in operating environments where other seals simply cannot survive. In fact, the performance of such seals has been well established in aviation and aerospace, including both NASA and commercial rockets.
A wide variety of jacket materials are available, with some of the most widely used aerospace options being PTFE (trade name Teflon) and Hytrel. Materials such as Teflon and Hytrel can handle extreme temperatures, are chemically compatible with media involved, are heat resilient, provide low friction, have excellent wear characteristics, and are typically self lubricating. In addition, both materials are available in grades that provide key characteristics such as improved wear, lower friction, additional stiffness, better strength, etc.
And the same is true for spring energizers, which vary in both geometry and material used. For example, vacuum pressure and cryogenic applications often utilize V-springs (also known as V ribbon springs), high pressure environments may use coil springs, and vacuum pressure operating conditions with medium speeds may utilize helical springs. Various materials can be used for the spring, which will be enclosed within the seal jacket; because of this, the spring material will be protected from whatever media is being sealed.
If you are in need of spring energized seals for space applications, allow the seal specialists at Advanced EMC help you. We have a long history of providing our customers with the seals they need, including custom engineered and manufactured solutions that not only meet their specifications but also the rigorous standards that may be involved. Advanced EMC has the design, manufacturing, and testing capabilities you need to make your design a success. Contact us today to learn more.
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.
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:
- Outdoor Rain Gear
- Medical Devices
- And more!
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.
The chemical resistance of PTFE is some of the best on the market. It is stable in most aggressive and corrosive media, including:
- Citric Acid
- Hydrochloric Acid
- Sulfuric Acid
- 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.
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!
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!
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)
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, 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, 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.
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 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.
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.
FEP encapsulated o-rings can survive corrosive chemicals and retain their sealing power in extreme temperatures, which is the main reason more and more engineers are choosing them for harsh environment applications. But what makes these particular o-rings special and what options are available for them?
What Makes Encapsulated O-Rings Different?
Unlike traditional o-rings, encapsulated o-rings contain a solid or hollow core that is typically made from a very elastomeric material. The exterior of the encapsulated o-ring is able to protect the encased elastomer from corrosive media that would adversely affect its performance. Together, the core and encapsulating polymer are able to provide a highly reliable seal even in extremely harsh conditions that may involve aggressive chemicals, extreme temperatures, and high pressures.
Encapsulated o-rings can be used in a wide variety of applications, including flanges, swivels, joints, valve stems, pumps, and even rocket engines. They serve as an excellent replacement for solid PTFE o-rings that are just not flexible enough for sealing in the long term.
Characteristics of FEP
One of the most popular materials for the jacket of an encapsulated o-ring is FEP (fluorinated ethylene propylene), which has several trade names including Teflon FEP, Neoflon FEP, and Dyneon FEP. It is well known for its resistance to chemical attack, low friction, and a wide operating temperature range of -420°F through 400°F. FEP remains flexible even at cryogenic temperatures, as well. One of its key characteristics is a very low compression set, allowing it to return to its original shape after deformation. FEP is also non-flammable and easy to lubricate.
While FEP is often compared to PTFE (Teflon), there are several key differences to keep in mind. For example, it does have a low coefficient of friction but it is higher than PTFE; at the same time, it still possesses very low friction with minimal stick-slip behavior. In addition, FEP does exhibit better vapor and gas permeability, which could be key for some applications. It is also melt processable, which means it can be vacuum formed, injection molded, and extruded. And, like PTFE, it is easy to clean even viscous liquids from.
FEP is available in FDA-approved grades, is considered a high purity material, and is less expensive than PFA, another commonly used jacket material. Note that FEP is commonly used in applications such as pump housings, medical components, food processing, fluid handling, and chemical processing.
Recommended Cores for FEP
FEP encapsulated o-rings work especially well with FKM and silicone cores, but there are other options available. FKM, which is a fluro-elastomer, has rubber-elastic properties which allow it to reassume its original shape and form after deformation. This results in excellent properties related to compression set. Silicone cores are not as stiff or hard as FKM cores and exhibit very good flexibility, even in cold temperatures. When combined with a hollow core geometry, this additional flexibility means that less energy is needed to achieve a tight seal. They work best for applications that involve low compressive forces.
Cores made from stainless steel, such as SS 301 or 302, exhibit excellent performance at both cryogenic and high temperatures, ranging from -420°F to 500°F. These cores usually take the form of a spiral spring (not unlike spring-energized seals) and exhibit minimum compression set and good resilience. They are not commonly used with FEP, however. EPDM, which stands for ethylene propylene diene monomer, is a synthetic rubber that performs well in temperatures ranging from -58°F to 300°F. Again, this particular core material is not recommended for use with FEP.
Selecting an FEP Encapsulated O-Ring
First, there are limitations associated with FEP encapsulated o-rings. They should not be used with liquid alkali metals and some fluorine compounds, and should not be exposed to abrasive media such as slurries and some powders.
They are not suitable for applications that involve high pressures and are limited to static or slow moving applications. In addition, they are not recommended for applications where the o-ring will be highly elongated and end-users should be aware that installation forces will be higher for FEP encapsulated o-rings.
However, experts agree that chemical attack and swelling are among the most common causes of o-ring failure, and the use of FEP encapsulated o-rings can solve both of these issues. FEP with an FKM core is a standard solution with a low compression set, recommended for operating temperature ranges not exceeding -4°F to 401°F.
Use of a solid silicone core results in better low temperature performance, with an operating temperature range of -46°F to 401°F. A hollow core, on the other hand, involves lower contact pressures and is ideal for sensitive or fragile equipment.
FEP encapsulated o-rings involve several key advantages, starting with their excellent chemical resistance, which allows them to be used with corrosive chemicals. These o-rings can handle pressures up to 3,000 psi and provide both an excellent service life and reliable sealing, all at a cost effective price. Their reliability and durability also translate to less downtime and better M&O costs. If corrosive media or extreme temperatures are destroying your o-rings, it may be time to consider an FEP encapsulated solution.
Advanced-EMC will work with you to find the encapsulated o-ring solution your application needs, from FDA-approved solutions for use with food processing equipment or a reliable, cryogenically compatible solution for a rocket. Contact us today to learn more.