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Heavy duty equipment moves industry forward in all climates, from the sunny Caribbean to icy Greenland. Effective, reliable sealing is what allows hydraulic systems in heavy duty equipment to do work, no matter the temperature. Reliable sealing solutions allow cylinders on dump trucks and excavators to move icy, frozen tundra, and allow actuators on subsea valves to operate 5,000 - 20,000 feet below the surface of the ocean. We depend on these seals for our safety and productivity, so a little chilly weather is no reason to call it quits.
BRRR! What happens to seals at cold temperatures?
Most objects shrink as they get cold, with few exceptions (water, I’m looking at you). This applies to all matter in the universe. Materials shrink at different rates, and this is a measurable property called the Coefficient of Thermal Expansion (CoTE). Thermoset elastomers and thermoplastics shrink roughly 5 times more than metals1 for a given temperature change. This means at cold temperatures, seals shrink more than their housings, and thus have less “squeeze” to make a tight seal.
To make matters worse, elastomers also harden as the temperature drops. At some temperatures, known for each material as its Glass Transition Temperature (abbreviated ‘Tg’), seals become rock hard and brittle … like glass. We don’t make seals out of glass for a reason; they wouldn’t work. In order to keep seals springy and resilient we need to specify materials with a Tg below the coldest temperature a system will see.
In very high pressure, low temperature applications, there is one additional concern. Applying pressure to seals effectively raises the Tg of the material by about +1°C per 750 PSI. This is called Pressure-Induced Glass Transition and is the reason high pressure seals fail slightly above their measured Tg.
That’s not cold!
So, there’s low temp, and then there’s looooow temp. I work at Parker Hannifin Engineered Polymer System (“EPS”) Division’s headquarters in Salt Lake City, Utah. Winter low temperatures in downtown Salt Lake are typically just below freezing. In this climate, most seal materials function just fine.
Siberia purportedly has the coldest inhabited villages in the world, with temperatures down to -60°C (-76°F). Northern Canada isn’t much warmer. In these regions, an operating hydraulic system can generate enough heat from friction to keep the seals sufficiently warm. However, if hydraulic equipment is unused or stored overnight in such frigid environments, the use of cold-rated seals with a low Tg is critical to preventing leaks in equipment.
For rotating equipment that is idled or shut off in extreme cold temperatures, the lip on a rotary shaft seal can freeze to the shaft when moisture is present at the seal lip/shaft interface. When the shaft starts, the tip of the lip can be ripped or sheared off, leaving a small band of rubber on the shaft (cold temperature seal fracture).
And then there are cryogenic systems, which are entirely different beasts. Liquid nitrogen tanks require seals that can handle the -320°F fluid. At this temperature, all elastomers will be rock solid. So what seal material can handle that? Polytetrafluoroethylene (PTFE).
Pure (unfilled) PTFE, while not considered an elastomer, remains flexible down to -425°F. That’s 35 degrees above Absolute Zero – the coldest possible temperature. The chart in Figure 1 shows the effective ranges for seal materials offered by Parker 2, 3.
Typical material temperature ratings
After looking at this chart, you might think, “Why doesn’t Parker make all seals out of PTFE and dump the rest?” PTFE is a great seal material, but it comes with its own set of tradeoffs. Hardware manufacturing and seal installation tend to be more complicated with PTFE than with elastomer seals.
Fluorocarbon rubber (FKM), and more recently perfluorinated rubber (FFKM), have traditionally been selected to seal hot temperatures and nasty chemicals. However, they perform poorly in cold. Parker offers special low-temp blends, spanning -40 to 400°F and -40 to 600°F respectively. “Do-everything” materials such as these tend to be more costly than traditional FKM rubber.
Highly saturated nitriles (HNBR) offer higher temperature capability with better wear resistance compared to standard nitrile (NBR). To improve low temperature resilience, Parker has developed HNBR compounds that perform better at low temperatures than most general HNBR compounds.
Special grades of silicone can handle colder temperatures, but their wear properties are so poor Parker EPS does not recommend these for dynamic sealing. Ethylene propylene rubber (EPDM) compounds are also capable of remaining quite flexible at low temperatures, but care must be taken to ensure that the application is compatible. EPDM often has the look and feel of nitrile rubber but reacts to fluids much differently.
Parker polyurethanes (compounds start with a ‘P’ in the table in Fig. 1) are popular because they offer the best all-around balance between low and high temperature sealing, wear resistance, pressure rating, and cost. Parker’s compound P5065A88 is compounded specifically to be more resilient at low temperature than most other polyurethane compounds.
In all sealing applications where temperature ratings are a concern, it is important to know that sealing compounds perform their best when they stay well within their temperature range. Applications that push seal compounds to the end of their temperature range may only perform for a short period of time before damage to the seal occurs, or they become too stiff to effectively control leakage. A good rule of thumb for long lasting seals is to remain within 80% of the compound’s temperature range.
ConclusionNow that you’ve selected a seal material with a Tg low enough for the cold environment it will be used in, are you done? Make sure other properties such as pressure rating, wear resistance, fluid compatibility, and high temp capability are also adequate for the system. Be aware of tradeoffs when switching materials to avoid causing a problem in another area. If in doubt, send us an e-mail and we will be happy to help.
Stay cool! Much more information about seals can be found in our Fluid Power Seal Design Guide, Catalog EPS 5370.
Recommendations on application design and material selection are based on available technical data. They are offered as suggestions only. Each user should make their own tests to determine the suitability for their own specific use. Parker offers no express or implied warranties concerning the form, fit, or function of a product in any application.
This article was contributed by Nathan Wells, application engineer, Engineered Polymer Systems Division.
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Design Considerations
Hardware geometry and limitations are the first consideration. A traditional O-ring groove shape is rectangular and more wide than deep. This allows space for the seal to be compressed, about 25% (for static sealing), and still have some excess room for the seal to expand slightly from thermal expansion or swell from the fluid. Reference Figure 1 as an example. Once the available real estate on the hardware is established, then we look at options for the O-ring inner diameter and cross section.
20 May 2020
Design Considerations
Hardware geometry and limitations are the first consideration. A traditional O-ring groove shape is rectangular and more wide than deep. This allows space for the seal to be compressed, about 25% (for static sealing), and still have some excess room for the seal to expand slightly from thermal expansion or swell from the fluid. Reference Figure 1 as an example. Once the available real estate on the hardware is established, then we look at options for the O-ring inner diameter and cross-section.
20 May 2020
In the rush to massively increase the number of ventilators available to treat patients with severe cases of Covid-19, using the correct seal materials for those ventilators should never take a back seat to expediency.
Patient ventilators are mechanical devices that essentially breathe for a patient with damaged lungs. They force air into the lungs and draw it out, augmenting or even replacing the natural functions provided by the movement of the diaphragm and the inflation/deflation of the lungs themselves. These devices can supply room air, pure oxygen, or nearly any ratio of the two to the patient, depending on health needs.
What makes a good seal selection in this environment?
First, seals within the device must be compatible with air and pure oxygen. They should not harden or crack, nor should they contain a significant amount of volatile matter that can evaporate out of the seal where it could be inhaled by the patient or potentially catch fire in a concentrated oxygen environment. Further, it should be assumed that any air that contacts the seals will likely end up in the patient’s lungs. As a result, Parker strongly recommends using seal materials that have passed USP <87> Class VI testing for any seals used in a ventilator.
Parker O-Ring & Engineered Seals Division has already helped several customers ramp up production of critical medical equipment with supplying the right materials and O-rings for the application.
These application requirements limit the recommended compounds to only a small handful.
Recommended compounds suitable for use in ventilators Silicone
Parker’s silicone compound S1138-70 is an excellent first choice for many ventilator seal applications. It has low outgassing, it has been tested to and passes USP Class VI, and it’s completely compatible with air and pure oxygen. In addition, silicone is also naturally non-flammable – even self-extinguishing – so it offers no fire concerns. However, abrasion resistance is poor, so it should not be used in a dynamic application. In addition, permeation resistance is also poor, but pressures in a ventilator are low enough that this small amount of air or oxygen loss is usually negligible.
Ethylene PropyleneParker’s EPDM compound E3609-70 offers similar compatibility and outgassing with much better abrasion resistance and improved permeation resistance when compared to silicone. It is also less prone to pinching and tearing upon assembly than silicone. E3609-70 has also passed USP Class VI and performs well in air and oxygen. As a result, it is recommended for dynamic applications such as seals on moving shafts. EPDM polymer will burn under the wrong conditions, but this is not usually a real-world concern for ventilator applications.
Caution should be exercised with Parker’s E1244-70 compound. While the combination of passing USP Class VI and internal lubrication may seem attractive for reducing friction in dynamic applications, these internal lubricants are organic molecules which can be flammable. When deposited in a thin film on mating surfaces as in a dynamic application, they could ignite when exposed to oxygen. This should be tested prior to use to ensure it does not cause a problem in application. In general, it is safer to use a non-flammable liquid lubricant such as silicone or perfluorinated polyether (PFPE) oil or grease to reduce friction in dynamic applications. Parker’s Super O-Lube is a pure silicone lubricant, but it has not been tested or approved to any medical device specifications.
FluorocarbonFinally, Parker’s fluorocarbon compound V0680-70 offers an interesting balance of properties. While it is not as good for abrasion resistance as an EPDM, it is still much better than any silicone. Like silicone, it is self-extinguishing and fully compatible with all combinations of room air and oxygen. Permeation resistance and outgassing properties are outstanding. V0680-70 has been tested and passed USP Class VI and is the same rust color as most silicone rubber compounds.
Prioritizing medical equipmentDuring this time of crisis, Parker is prioritizing production of seals needed for critical medical equipment, and these four compounds in particular are strongly recommended for use in ventilators. They offer excellent long-term compatibility with room air and oxygen, have low outgassing, and have all passed USP Class VI testing. Learn more about the solutions available.
This article was contributed by Dan Ewing, senior chemical engineer, Parker Hannifin O-Ring & Engineered Seals Division
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Over the years, we've taken many questions from customers just like you about the measurement and testing of our thermal interface materials. How are they used? How are measurements determined? What's the best way to characterize performance?
So we asked our engineers for help answering these questions. Read on to find out their answers.
What test method does Parker Chomerics use to characterize thermal interface material (TIM) performance?
Parker Chomerics’ standard test method of characterizing TIM performance is by ASTM D5470.
ASTM D5470 measures thermal impedance (resistance) of a flat disk-shaped specimen or controlled volume of a liquid TIM between two flat polished calorimeter surfaces under controlled load.
Apparent thermal conductivity is a calculated value that uses the thermal impedance (resistance) measured from ASTM D5470 and the sample thickness to calculate a thermal conductivity value. This value is influenced by how effectively the sample contacts (or “wets out”, if a dispensable) the calorimeter surfaces.
The thermal resistance at the interface between the sample and the probes is called contact resistance. Contact resistance adds to the overall thermal impedance (resistance) and may produce a lower measurement than bulk thermal conductivity.
Bulk thermal conductivity is an intrinsic property of any homogenous material. To measure bulk thermal conductivity, we must subtract the contact resistance from the individual ASTM D5470 thermal resistance measurements.
This is achieved by measuring thermal impedance (resistance) of the material at multiple thicknesses (at least three) and generating a straight-line plot. The y-intercept of that plot is the total contact resistance and the slope can be converted to bulk thermal conductivity.
A material can have a very high intrinsic bulk thermal conductivity but be outperformed by a material of lower bulk conductivity that is softer and conformable. Measuring apparent (effective) thermal conductivity can help better identify real world performance of a thermal interface material in many cases.
Generally, there is no “go-to” correction factor or simple equation to “convert” from apparent to bulk conductivity. The contact resistance can vary widely across different thermal interface materials and there are also many other factors to consider including pressure during test, flatness and thickness uniformity of sample, contact area, etc.
Both apparent and bulk conductivity are useful values for fully understanding a thermal interface material’s performance and expected behavior in application. It is useful to consider the bulk thermal conductivity as the maximum attainable thermal transfer efficiency parameter while apparent thermal conductivity values can offer an indication of how well the material performs in real world application where contact resistance cannot be ignored.
It is always difficult to compare values since it is unlikely that reported values from varying sources were generated using the same test method and parameters. There are many test instruments and methods used in the marketplace. Parker Chomerics relies on ASTM D5470 for accuracy and reliability.
The selector must be sure to consider test method used as well as any parameters used in the test that would influence outcomes (temperature, pressure, etc.). In addition, it is important to be aware of any “modified” methods reported. Without knowing the nature of the modifications, one can fall victim to overstatements of product performance.
Parker Chomerics reports bulk thermal conductivity for most TIM products on technical data sheets. Thin bond line products (such as phase change materials and thermal greases) data sheets will show thermal impedance at fixed pressure instead of bulk thermal conductivity as this is more practical and useful to the designer.
For lot-to-lot conformance testing, Parker Chomerics measures and retains apparent thermal conductivity and thermal impedance for every manufacturing lot of product.
This blog was contributed by Dana Drew, quality manager, Parker Chomerics Division.
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Design factors for satellite, high altitude, and space-based applications vary dramatically from those of land or ship-based programs. These factors include: low payload capacity, low operating temperature, and meeting material limits.
Outgassing standards, established by NASA, set a limit on the release of gasses that can possibly interfere with sensitive technology within vacuum environments such as Low Earth Orbit (LEO). This is especially important for sensitive optical systems or camera lenses where the smallest bit of vapor or gaseous components can dramatically reduce performance.
Test and metrics
NASA outgassing requirements are often used interchangeably with ASTM E595 which establishes the test method for determining outgassing levels. During the E595 test, small samples of material are kept under vacuum and heated to 125°C for a 24-hour period. While the samples are heated, all gasses are channeled through a single release port where a chromium-plated disk is used to collect the volatile materials.
After the test, there are two key metrics that are collected and used in certifying a material to NASA Outgassing Standards.
While NASA does keep an extensive database of all materials that they have tested in house, certified labs often run ASTM E595 testing as well. For a complete list of Parker Chomerics products that pass traditional outgassing requirements (and associated NASA Data Reference Numbers), please see: Parker Chomerics NASA Outgassing Information.
Outgassing in EMI shielding solutions
The most common materials to release stored vapors and gasses are sealants, adhesives, and less heavily crosslinked elastomers and polymers. Conversely, metals and glasses with few impurities tend to have a very low level of outgassing.
While often true, many conductive elastomers made of silicone and fluorosilicone can meet these standards due to high quality raw materials and efficient processing.
General trends for outgassing in EMI Shielding:
Steps can be taken to reduce the amount of vapor or gasses that are released by materials. One such example is known as post-baking, sometimes referred to bake-off or bake-out. This process involves baking materials at elevated temperatures (and sometimes in vacuum environments) after they have been manufactured in order to lower vapor and/or volatile compounds.
It is important to note that some materials that pass NASA outgassing standards are only able to do so after post-baking for some amount of time. It is possible that some NASA post-baking occurred at temperatures above the maximum recommended operating temperature of these materials. This elevated temperature exposure can change the physical, thermal, or electrical properties of tested materials.
Parker Chomerics has a long history of supplying manufacturers with outgassing-compliant solutions to EMI Shielding problems for vacuum and space-based. For a complete list of Parker Chomerics products that pass NASA outgassing requirements (and associated NASA Data Reference Numbers) and more information, please see: Parker Chomerics NASA Outgassing Information.
This blog contributed by Ben Nudelman, market development engineer, Chomerics Division.
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