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

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