The compression set results may be confusing because there are several testing methods, test specimen dimensions and compression set test calculations. The compression set test may be performed by exposing the test specimen to a constant force (e.g. ASTM D-395, method A) or by compressing the test specimen by a certain percentage of its initial height (ASTM D-395, method B, which compresses the test specimen by 25 % in relation to its initial height). Irrespective of the test, the result of the compression set test should always be expressed as a percentage. The lower the percentage, the better is the material resistance to the compression set.
The compression set test measures the remaining or. residual deformation, which in other words means that it measures the percentage of deformation for which the test specimen failed to return to its original state or height. So if the test specimen completely returns to its original state, there was no deformation and so the percentage of the compression set is 0 %. On the other hand, if the test specimen remains completely compressed and does not return to its original state, the percentage of the compression set is 100 %. Thus, the material with good resistance to compression set has a low percentage and the material with poor resistance to compression set has a high percentage.
In static sealing, where the gasket is usually compressed, the reduction in resistance to compression set may cause poor sealing and thus leakage, therefore a low percentage of compression set is desirable for such applications. The percentage of compression set increases over time and with temperature.
Although a low percentage of compression set is desirable, it all depends on the final application. For example, an O-ring may continue to perform its sealing function even if the compression set percentage is 90 % and can therefore hardly be returned to its original state because the swelling of the material in contact with fluid compensates a high percentage of the compression set. This of course applies if the conditions in the system (e.g. pressure and forces) do not change (especially in the case of FKM seals at low temperatures). On the other hand, such a high percentage of compression set will cause the fluid to leak out of the system if the change in volume in contact with the fluid is negative and the material shrinks.