Compression Stress Relaxation (CSR) is an important factor in evaluating elastomer materials and seal configurations for their ability to provide sealing force retention under a variety of conditions. Increasing emphasis on the performance and longevity of elastomer seals, used most often in aerospace and automotive applications, is strengthening the need for more accurate CSR testing procedures.

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Figure 1: Compression Stress Relaxation samples and test jigs.

CSR data is only valuable if different samples and test jigs provide consistent results. Obtaining reproducible, scaleable CSR results is now the primary emphasis for predicting long-term sealing performance.

In the past, compression set resistance was relied upon to judge the performance of different materials, as a means of quality control, and for specification purposes. CSR and compression set resistance can be used to estimate sealing capability; however, problems arise when different materials can be shown to have the same compression set resistance but different percentage of retained sealing force as measured with CSR. When using CSR methods, the sealing force will provide a direct correlation to sealing capability.

CSR testing equipment and methods can have advantages, as well as limitations, that can include cost, ease of use, jig size, test capability, and reproducibility. Since variables in each test method and differences in equipment can determine how accurate CSR test results are, it is important to differentiate among equipment, procedures, and responses seen with each test configuration.

Equipment used to help collect sealing force retention data can provide benefits and challenges, depending on how each piece functions. A modified tensometer can provide the most information about sealing force, but only when a precise test configuration is used. Similarly, the success with a Shawbury Wallace tester that uses a contact break point method is also dependent upon a specific test configuration because certain fluids or environmental conditions affect the electrical contact conductivity during testing and can introduce erroneous test results.

A third method of testing utilizing the Elastocon Relaxation tester is most useful for measuring sealing force at elevated temperatures, but does not reflect where the sealing force is lost first, which is at lower temperatures.

Test results indicate the most effective way to collect data is with smaller samples and jigs. Smaller samples generally are more representative of gasketing profiles or cross sections. Smaller test jigs also allow more samples to be evaluated in a smaller volume of liquid, making them easier to handle, drain fluid from, and cool.

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Figure 2: Loading Jig and Pin Indentor: Dyneon test configurations.

Dyneon LLC, a 3M company, has designed a small CSR test jig that offers easier handling for CSR testing. Dyneon also has developed a centering jig and pin indentor to help limit the rocking effects of shims. This equipment yields better-defined load deflection curves for the determination of sealing force.

Test results show how frictional effects on jigs and samples can affect the variability of initial sealing force measurements. The use of lubricants and polished test surfaces will provide the best results. The advantage of the Dyneon jigs and others that use removable shims is that uniform frictional properties can be obtained by either repolishing or replacing the shims for a minimal cost.

Shape factor effects also are important to control during testing. Samples with high shape factors show more variability due to the large change in sealing force with small changes in deflection. Also, their high compressive modulus makes it difficult to define the sealing force from changes in the slope of the load deflection curve. Using samples with lower shape factors appears to provide more consistent data. Samples with higher shape factors also appear to experience increased sealing force loss.

Sealing force is affected, in part, by changes in temperature — decreasing at lower temperatures and increasing at higher ones. Because of this, it is important to measure and control both the sample and jig temperatures when measuring sealing force. Dyneon's test jig provides a thermocouple well to allow for this measurement.

It is important to differentiate the sealing force responses that occur during testing. Initial stress decay takes place when the compressed sample reaches an equilibrium-relaxed value at room temperature. Relaxation occurs when the sample is heated to a temperature above that at which it was compressed. This response is a result of thermal expansion, increased stress, and higher molecular motion that forces the polymer to relieve stress through molecular rearrangement. Aging effects, which are time-dependent, result from molecular bonds being formed or broken. This response is of most concern for predicting long-term durability or service life.

For more information, contact Doug Chirhart at Dyneon, a 3M Company; Tel: 651-736-9241; or visit the Web site at: www.dyneon.com