Compression set is a test method that is often used for short durations to study how much of its elastic properties an elastomeric material can retain after strain in heat. It is often used to investigate how well vulcanized a material is, due to its relatively fast and easy procedure.

Related Standards

ISO 815, ASTM D395

Introduction

Compression set is a test method that is often used for short durations to study how much of its elastic properties an elastomeric material can retain after strain in heat. It is often used to investigate how well vulcanized a material is, due to its relatively fast and easy procedure. This is done at short testing times, usually 24 hours. For longer hours, 168 hours and above, compression set instead gives information on the chemical degradation of the material. Such a test can be a good way to study a material aimed at sealing applications.

Compression set can also be performed in low temperature conditions and is also closely related to tests of tension set. Both these tests will be discussed in separate articles.

Performance

A sample body is ideally prepared specifically with the test in mind and with the shape of a button with 13 mm diameter and 6,5 mm height. The thickness is measured at the center of the button and it is then compressed to 75% of its initial thickness. While compressed, the sample is stored at a desired temperature which depends on the material. After the desired time has passed, often 24, 72 or 168 hours, the sample is released and let recover for 30 minutes before measuring the thickness again. The remaining difference between initial and final thickness is called the compression set and is expressed as a percentage of the compressed thickness.

The performance of a compression set test with standardized samples are quite simple and reliable. If the sample shape deviates from the standard, however, compression set can still be a useful test for batch-to-batch comparisons. It is also important to lubricate the sample before compression with an oil that doesn’t affect the material. For common rubber materials that swell in mineral oils, a silicone oil can be used instead.

In depth considerations

Compression set gives a simple value on several physical- and chemical processes in combination, making the choice of test conditions very important. Consider three different materials and what happens to them in a high temperature compression set test at different times:

1. An EPDM-rubber without sufficient vulcanization.
2. An EPDM-rubber with sufficient vulcanization.
3. A TPE-material like SBS or SEBS.

The compression set of these three materials might develop something like in figure 1 below.

Figure 1. Illustration of the three materials' compression set development.

The first material will quickly undergo post-vulcanization in the high heat, meaning the shape will stabilize in the compressed shape giving a large compression set. As time goes on, the chemical degradation of the material will also gradually increase the compression set due to loss of elastic properties.

The second material will see some compression set over the first 24 hours but far less than the first material. Some physical relaxation would take place where the polymer chains and filler materials would rearrange to a more favorable position. Afterwards, a similar gradual degradation would be observed afterward increasing compression set.

The third material would likely show a large initial compression set, but not due to lacking vulcanization like material 1. TPE-materials are not chemically vulcanized, physical interaction between the polymer chains is used to a similar effect. This is often achieved with styrene groups in materials like SBS/SEBS. It has been observed that these TPE-materials show a very large degree of initial compression set, likely due to a larger degree of physical relaxation.

These processes vary significantly but are all confounded in the single test result in the end: Compression Set, expressed as a percentage of remaining compression after release. For this reason, it is very important to know what you want to test and choose test conditions accordingly.

Conclusion

Compression set according to ISO 815 or ASTM 395 is a useful and fast way to determine if a material is sufficiently vulcanized. Alternatively, with longer testing times, it can give information about how well the material would perform in a sealing application where the material would be under compression for extended times.

Figure 2. Elastocon EB 20 Cell Oven for ageing according to ISO 188 or ISO 815.

Figure 3. Elastocon Compression Set Rig EV 03 (Left) and Elastocon Thickness Gauge EV 01 (Right) for testing according to ISO 815.

Elastocon performs compression set tests in our test laboratory using our in-house developed compression set rigs with matching cell ovens as well as thickness measurement. For more information, please contact us at This email address is being protected from spambots. You need JavaScript enabled to view it.

And for further reading, please find the referenced standards at:

ISO 815: https://www.iso.org/standard/74943.html

ASTM D395: https://www.astm.org/Standards/D395.htm

Abbreviations:

EPDM – Ethylene propylene diene monomer rubber

TPE – Thermoplastic Elastomer

SEBS – Styrene-Ethylene-Butylene-Styrene

 

Kim Bini