Sealing and Shielding

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:

• Many conductive elastomers, with a variety of conductive fillers, such as: silver, silver-plated copper, silver-plated aluminum, and nickel-plated aluminum pass NASA outgassing requirements (Parker Chomerics Conductive Elastomer Handbook for more information)
• Nearly all metal mesh EMI gaskets including those made of Monel®, aluminum, and Ferrex pass outgassing
• Copper foil tapes with conductive and non-conductive adhesive meet NASA requirements
• EMI shielding heat shrinkable tubing meets outgassing levels 

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.

 

 

 

 

 

Related content

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EMI Shielding Gaskets: Spliced Gaskets vs. Molded Gaskets

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We’re thrilled to announce the Chomerics Division of Parker Hannifin Corporation, the global leader in motion and control technologies, has been awarded the prestigious 2019 GM Global Supplier Quality Excellence Award for its Engineered Plastic Solutions business unit in Fairport, NY.

This award recognizes the suppliers who have demonstrated consistent quality performance throughout the year and is only given to the top performing supplier manufacturing locations. GM suppliers who receive this award recognition have met or exceeded a very stringent set of quality performance criteria and have achieved the cross-functional support of the entire GM organization.

“I am proud of all of our Chomerics team members at Fairport for their contributions to earn this award three years running,” said John Beswick, global business unit manager, Chomerics Division.

"We know that maintaining a focus on the details to deliver defect-free product[s] to our assembly plants consistently is not an easy task, so much that only a fraction of our suppliers has earned this prestigious recognition," noted Richard Demuynck, General Motors' executive director, global supplier quality and development. "You are a critical part of the team who helps ensure the customer is delighted with their product purchase and we want to recognize and thank you for that."

“Congratulations to the Fairport team,” said Dave Hill, global general manager, Chomerics Division, “It’s a direct reflection of GM's confidence in our operation, and of the hard work and dedication of our remarkable team members.”

Parker Chomerics Engineered Plastic Solutions business unit in Fairport, NY, located 10 miles outside of Rochester, NY, heavily utilizes robotics and automation to achieve high quality and delivery standards across many industries.

For more information on Parker Chomerics products, visit our website or download our Engineered Custom Injection Molded Plastics Solutions brochure. 
 

 

 

This blog was contributed by Jarrod Cohen, marketing communications manager, Parker Chomerics Division.

 

 

 

 

Related content:

Parker Chomerics Earns 2018 Outstanding Quality Supplier Award from FCA

Chomerics Awarded Prestigious Ford Q1 Certification

Chomerics Division Honored with Boeing Award

 

Parts 1 and 2 of this series discussed the theory behind CSR testing and what to look for in a CSR result curve.  This 3rd and final section will focus on how to use CSR data and apply it to real world applications and how to incorporate it into a material specification.

  CSR curve

For the reasons discussed previously, it is important to view a full CSR curve, rather than a single data point, and to resist the urge to draw conclusions from incomplete data.  For example, Figure 1 compares a FKM to an HNBR material.  Because the fluorocarbon material has a larger viscoelastic loss within the first 24 hours of the test, it appears to be worse (less retained seal load) than the HNBR for most of the test duration.  However, the slope of the HNBR curve is steeper than that of the fluorocarbon, and the curves of retained load force cross at about the 2,300 hour point.  If these curves are extrapolated, the HNBR is predicted to reach the point of zero residual load force at 4,262 hours, whereas the fluorocarbon is not expected to reach the same point until 8,996 hours have elapsed.  Had the HNBR material been selected for this application based solely on the higher percent retained load force observed at 1,008 hours, the end user would have achieved roughly half of the service life they could have enjoyed had they selected the FKM compound instead.

 

Figure 1: An HNBR and a fluorocarbon in engine oil.

  4 Limitations of CSR data

Several caveats remain in attempting to use CSR curve data to predict real-world performance.  First, a CSR procedure cannot mimic real-world application conditions.  In fact, it is not intended to.  CSR testing is meant to be an accelerated aging test, where specimens are exposed to constant temperatures hotter than anticipated in the actual application.  While some continuous CSR test machines are capable of temperature cycling, no CSR test can realistically replicate the actual temperature conditions, pressure fluctuations, or vibration seen in a real-world application.  Second, CSR testing cannot replicate actual fluid exposure conditions seen in typical seal applications.  CSR testing completely immerses a rubber specimen in a small amount of fluid that is kept at a high temperature – typically hot enough and with enough air exposure to cause the fluid to degrade.  In a real-world application, a seal is usually installed in a groove with fluid exposure on one side and air on the other.  In many applications, the fluid is contained within a sealed system that minimizes oxidative degradation of the fluid.  Third, there is as yet no definitive correlation between the onset of leakage and residual load force, regardless of whether that force is expressed in absolute terms (Newtons or pounds-force, for example) or as a percentage of the initial developed load force.  Finally, because low temperatures are more challenging for a seal, cycling the temperature to include load force measurements at -40°C would be considered a worst-case scenario.  Continuous measurement CSR devices are now commercially available with the capacity for running such a temperature cycle, but their use to date has been extremely limited due to the cost of the equipment.

  Other considerations

Several factors should be considered before incorporating CSR testing into a material specification.  CSR is a more expensive procedure than compression set, and it requires dedicated and expensive test equipment.  As a result, it is not always appropriate to replace compression set testing in a material specification with CSR testing.  Second, establishing limits for “what good looks like” should never be done arbitrarily.  It is extremely difficult to convert a real world application service life into a number of test hours at a set temperature.  Therefore, it is more practical to start with a material that is known to maintain a satisfactory service life in the application, test it for CSR at a relevant temperature, and establish meaningful limits based on the performance of that material.   Finally, resist the temptation to get too exotic with specification limits.  For example, it may seem valuable to impose specification limits related to the slope of the CSR curve or to the extrapolated “time to zero load force”.  These can be valuable tools for gathering engineering insight, but they are far too complicated to incorporate as specification limits.  CSR is a more complicated test than compression set, and it requires more “homework” to set proper limits and test conditions.

In conclusion, Compressive Stress Relaxation is a powerful tool for comparing the performance of two or more materials in a particular application, provided it is done on an apples-to-apples basis.  It also serves as a reasonable means of (roughly) estimating the long-term service life of a seal material in a given application.  There are too many assumptions for it to be used to guarantee a specific level of performance, and there remains much work to be done in correlating results to real-world observations, but it currently offers the best laboratory-scale means of evaluating a seal material for long-term use in a given application.

 

This article was contributed by Dan Ewing, senior chemical engineer, Parker Hannifin O-Ring & Engineered Seals Division.

 

 

 

 

How Much Do You Know About Compressive Stress Relaxation? CSR Part 1

How Much Do You Know About Compressive Stress Relaxation? CSR Part 2

How to Read a Rubber Test Report: The 4 Most Common Misunderstandings

Reduce Downtime and Costly Seal Replacements: Seal Failure Diagnosis Part 1

Electrically conductive elastomers are elastomeric polymers filled with metal particles. They can be grouped by filler type and elastomer type. Then within each of these classes, there are standard materials and specialty materials.

Parker Chomerics manufacturers electrically conductive elastomer gaskets, also known as EMI elastomer gaskets, under the CHO-SEAL brand. We won't get so much into gasket configurations and dimensions here, we'll just stick to classes of materials. So what is available? Let’s find out.

Conductive elastomers are metallic particle filled elastomeric polymers, the particles giving the shielding performance and the polymer making them “rubber." There are many materials within this generic material type, but we'll focus on the below.
 

Particle fillers

Setting up the grades of conductive elastomers by filler types involves six different particles:

 

 

Three types of elastomer material

• Silicone
  A polymer that has a large temperature range especially on the low end down to -55F. It is a very soft material with a low compression set. 
   
• Fluorosilicone
  Close to silicone, but will not swell and degrade when exposed to solvents, fuels hydraulic fluids and other organic fluids. Although slightly harder than silicone, it is still relatively soft with low compression set properties.
   
• Ethylene propylene diene monomer (EPDM)
  Does not have the temperature range nor the softness of silicone, but is resistant to highly chlorinated solvents used for compliance with NBC decontamination and is only used for applications with those needs.

All of these materials are cured or cross linked when the gasket is made. The cure either happens with heat or atmospheric moisture.
 

Three main processes used to process the material into a gasket

• Compression molding - uncured material is placed into a mold that has a cavity machined into it. The mold is closed, put into a press, then with heat and pressure the material is formed into the gasket shape mirroring the cavity and cured with heat. This process is used for sheet stock also.
   
• Injection molding - instead of hand loading the mold with approximately the proper amount of material, the mold is closed, then the proper amount of material is injected into the cavity. The process is much faster and because the right amount of material is injected tighter size tolerances can be obtained. Injection molding tooling is more expensive than compression molding.
   
• Extrusion - raw material is pushed through a die with the cross section machined into it to create extruded cord stock. Heat is applied on the die and a finished continuous gasket strip is made. These gasket can be supplied in lengths or cut to size and then corners or intersections glued or spliced together to make a ready to use gasket.

In general, for small cross sectional electrically conductive elastomer shapes, the smaller the particle the more producible the part will be. Particle fillers have little to do with mechanical properties of the gasket except that they simply increase hardness.

Now that you have more information about how to select particle and polymers for electrically conductive gaskets, download or Electrically Conductive Elastomer Handbook and get started today!

 

 

 

 

 

 

 

 

 

 

 

This blog contributed by Jarrod Cohen, marketing communications manager, Chomerics Divison.

 

 

 

 

Related content

New Essential Handbook for EMI Shielding Applications

EMI Shielding Gaskets: Spliced Gaskets vs. Molded Gaskets

Can Electrical Resistance Be Used to Predict Shielding Effectiveness?

 

 

 

In Part 1 of this series, the theory behind Compressive Stress Relaxation (CSR) testing was discussed, as well as a brief discussion of the fixtures used to measure it. In Part 2, we will explore what to look for in a CSR result. Significant understanding of how a rubber seal material responds to a particular environment can be gleaned if one knows what to look for in a CSR curve.

 

  The end point

The first and most basic point of understanding is the end point. Does the material continue to maintain contact pressure throughout the test, or does it fall to zero (below the detectable limit of the load cell) before the end of the test? While there is no definitive correlation from residual load force to onset of leakage, it should be intuitive that a material that completely relaxes and loses all contact force is likely to leak in application. Anecdotally, multiple customers have reported that the load force must drop to very close to zero for leakage to occur in their particular test apparatus. While this is good guidance, these anecdotal reports should not be taken as a definitive answer that applies in all circumstances.

Specifications are often written such that a minimum of 10% of the initial contact load force must remain for a passing result. In practice, there is nothing special about 10%. This is a semi-arbitrary value that ensures a material continues to apply some non-zero load force to the mating surfaces, with some safety factor to ensure that it does so even after all normal test variations are considered. In practice, this appears to be a conservative limit, there is nothing magical about the 10% number.

The loss of compressive load force can be broken down into three different types of phenomena, each with its own time frame. All rubber materials relax viscoelastically when initially compressed, and this loss stabilizes within the first 24 hours. That initial drop seldom has much direct impact on real-world applications. However, in the specific case of an assembly having neither a compression limiter nor solid-to-solid contact, meaning the assembly torque of the fasteners is controlled solely by compression of the seal, this will be observed as “torque fade” if the fastener torque is rechecked a day or two after assembly. In such a case, Parker recommends against retorquing the fasteners unless leakage is observed as this retorquing can easily result in damage to the seal from excessive compression.

  Solvation effects

The second set of phenomena to resolve are solvation effects that occur when an elastomer is immersed in a liquid test media. The rubber material will absorb some amount of test fluid, causing some swelling of the rubber and a small increase in load force. When this happens, the test fluid may extract liquid constituents from the rubber, resulting in shrinkage of the seal material and a loss of load force.  These processes occur simultaneously, and both typically reach equilibrium within the first 72 hours. However, the net impact on the measured load force will only be noticeable if the volume change is significant.

  Degradation

The third set of phenomena are degradative effects caused by high temperatures and chemical reactions. These reactions are ongoing and cumulative.  If the test temperature remains constant, the rate of degradation will remain constant, as well. Unfortunately, there is no way to distinguish what percent of any degradation is due to thermal effects versus chemical effects from a CSR curve. Comparing a CSR output curve to one generated from testing in an inert control fluid at the same time and temperature may allow a user to isolate thermal effects from chemical ones, but it must be known that the control fluid does not also produce chemical degradation of the rubber material.

Ultimately, it is more important to consider the slope of the curve after the initial drop than the initial drop itself. The material should stabilize to a relatively flat line, and the slope of that line reflects how the material responds to thermal and chemical aging effects. A curve with very little slope (Figure 1) is extremely stable long-term, whereas as a CSR curve that shows a steeper negative slope (Figure 2) means the material is continuing to degrade due to chemical and/or thermal effects in that fluid and at that temperature. This does not mean the material is incompatible with that environment, but the continuing losses mean the end of service life point (onset of leakage) would be expected relatively soon after the end of the test.

 

Figure 1: A fluorocarbon in engine oil at 150°C shows very little change after the initial relaxation response.

 

Figure 2: An HNBR in engine oil at 150° shows ongoing degradation after the initial relaxation response. 

 

In summary, much knowledge about how a material will respond in an application can be gleaned from Compressive Stress Relaxation testing if one knows what to look for.  Watch for part 3 of this series, where we will focus on how to use the understanding gained from CSR testing and how to incorporate it into a material specification.

For more information or assistance with your application, contact our applications team at oesmailbox@parker.com or chat online by visiting Parker O-Ring & Engineered Seals Division website. 

 

 

 

This article was contributed by Dan Ewing, senior chemical engineer, Parker Hannifin O-Ring & Engineered Seals Division.

 

 

 

 

How Much Do You Know About Compressive Stress Relaxation? CSR Part 1

How to Read a Rubber Test Report: The 4 Most Common Misunderstandings

5 Factors to Consider When Determining Compressive Load of a Seal

Reduce Downtime and Costly Seal Replacements: Seal Failure Diagnosis Part 1

Reduce Downtime and Costly Seal Replacements: Seal Failure Diagnosis Part 2

 

Compressive Stress Relaxation (CSR) is a means of estimating the service life of a rubber seal over an extended period of time. As such, it can be thought of as the big brother of compression set testing. Rather than measuring the permanent loss of thickness of a compressed rubber specimen as is done in compression set, CSR testing directly measures the load force generated by a compressed specimen and how it drops over time. In part 1 of our blog series, we will explore the theory of CSR testing, common test methods, and how CSR differs from compression set testing.

 

Theory of CSR Testing

To understand the value of CSR testing and how it differs from compression set testing, it is helpful to return to the basic theory of how a rubber seal functions. In a standard compressed seal design, a rubber seal is deformed between two parallel surfaces to roughly 75% of its original thickness. Because the material is elastic in nature, the seal pushes back against the mating surfaces, and this contact force prevents fluid flow past the seal, thus achieving a leak-free joint. Over time, the material will slowly (or perhaps not so slowly) relax. The amount of force with which the seal pushes against the mating surfaces will drop, and the seal will become permanently deformed into the compressed shape. In compression set testing, the residual thickness of the specimen is measured, and it is assumed that this residual thickness is valid proxy for the amount of residual load force generated by the compressed seal. In CSR testing, the residual load force is measured directly.

In practice, CSR results are typically presented very differently from compression set results. In CSR testing, it is common to see multiple time intervals over a long period of time (3,000 hours or more of testing), thus allowing a curve to be created (see Figure 1). In practice, however, specifications are written such that only the final data point has pass/fail limits. In compression set testing, it is common to see a single data point requirement with a single pass/fail limit. Multiple compression set tests can be performed to create a curve, but this is almost always down for research purposes rather than for specification requirements. In most cases, compounds that excel in compression set resistance also demonstrate good retention of compressive load force over time. However, there are exceptions.

 

 

 

 

 

 

 

 

 

 

 

Figure 1: Typical CSR curve.
These results display a fluorocarbon seal material immersed in engine oil at 150°C.

 

CSR Test Methods

CSR testing can be quite complicated, and caution is needed when comparing reports to ensure a valid apples-to-apples comparison can be made. As with compression set, CSR testing can be performed in air or immersed in a fluid. Because most seal materials will oxidize in the presence of hot air, the results for CSR in air can be strikingly different (worse) than the results for CSR in a fluid at the same temperature. In addition to time and temperature, the sample size (usually a button 12.7 mm in diameter and 6.35 mm thick) and amount of compression (typically 25%) must be the same to make a valid comparison.

There are multiple test procedures and fixture designs within the CSR test world that also have a significant impact on results. Numerous fixture designs exist, all of which produce different results. Regardless of Hornig’s1 conclusions regarding his preferred fixture, in practice, most CSR data for elastomer seal materials is gathered with a “Dyneon-modified Wykeham-Farrance” jig (see image).

Finally, CSR force measurements can be made intermittently at discreet time points using a standalone compressive load cell or continuously using a dedicated CSR testing device that incorporates a load cell for each test fixture and one or more integrated environmental chambers.  With the intermittent test method, fixtures are removed from the oven and/or oil bath, allowed to cool to room temperature (23°C), manually tested on a compressive load cell, and returned to the oven for additional aging. Continuous testing does not involve this repeated thermal cycling, which contributes to accelerated relaxation and worse results. As a result, intermittent CSR generally appears worse than continuous CSR for the same material under the same conditions. In addition, continuous CSR data points are gathered at the test temperature rather than at room temperature. Gathering load force data at elevated temperatures results in a higher measured load force; when compared to an initial load force data point taken at room temperature, as some procedures require. This can result in a counterintuitive situation whereby it appears that a material initially improves with thermal aging.  (See Figure 2.)  If this is observed, it should be considered an artifact of the difference in test temperature. When comparing CSR results, it is absolutely essential to confirm that the same test method and fixture were used for both tests. 

 

  Figure 2: Continuous method CSR testing above shows an initial increase in load force due and overall a higher percent retained load force than observed with the intermittent method.

 

Watch for part 2 of this series, where we will explore some of the powerful insights that can be gleaned through CSR curve interpretation. For more information or assistance with your sealing challenges, contact our applications team at oesmailbox@parker.com or chat with us online at the Parker O-Ring & Engineered Seals Division website.  

 

References:

1. Hornig, R.  Comparison of various CSR methods regarding the static long-term sealing behaviour of AEM, ACM and HNBR compounds, International Polymer Science and Technology, 37, No. 4, 2009.

 

 

 

 

 

 

 

 

 

This article contributed by Dan Ewing, senior chemical engineer, Parker Hannifin O-Ring Division.

 

How to Read a Rubber Test Report: The 4 Most Common Misunderstandings

5 Factors to Consider When Determining Compressive Load of a Seal

Reduce Downtime and Costly Seal Replacements: Seal Failure Diagnosis Part 1

Reduce Downtime and Costly Seal Replacements: Seal Failure Diagnosis Part 2