Compression set is generally determined in air aging and reported as the percent of deflection by which the elastomer fails to recover after a fixed time under specified squeeze and temperature. Zero percent (0%) indicates no relaxation has occurred whereas 100% indicates total relaxation; the seal just contacts mating surfaces but no longer exerts a force against those surfaces.
Compression set may also be stated as a percent of original thickness. However, percent of original deflection is more common.
Although it is generally desirable to have low compression set properties in a seal material, this is not so critical as it might appear from a practical design standpoint, because of actual service variables. It is easy to go overboard on this property from a theoretical standpoint. Remember that a good balance of all physical properties is usually necessary for optimum seal performance. This is the eternal sealing compromise the seal designer always faces.
For instance, a seal may continue to seal after taking a 100% compression set provided temperature and system pressure remain steady and no motion or force causes a break in the line of seal contact. Also, as mentioned previously, swelling caused by contact with the service fluid may compensate for compression set. The table below shows the results of a laboratory test that illustrates this phenomenon.
|Compression Set vs. Volume Change|
|Parker Compound: Butyl||Temperature: 74°C (165°F)|
|Time: 168 hrs||Deflection used: 25%|
|Volume Change %||0||+19.5||-0.4|
|Set % of Original Deflection||25.4||0||20.7|
Note that in air and in the fluid that caused slight shrinkage, the compound took a set of approximately 20 to 25%. In the fluid that caused a 20% swell, there was no measurable compression set. The condition most to be feared is the combination of high compression set and shrinkage. This will always lead to seal failure unless exceptionally high squeeze is employed.