The physical relaxation predominates during the early stage of the deformation process, while the chemical relaxation
is perceived at medium/long term, depending on the material sensitivity to heat aging.
Understanding the difference between the part of a material's response that might be a result of its viscous response (physical relaxation), as compared to the portion due its aging response (chemical relaxation), is an important exercise.
The physical relaxation occurs more rapidly than the chemical relaxation, but unlike the latter, can provide a certain level of reversible recovery.
Since all of the testing for CSR and compression set begin with an initial deflection and involve both physical and chemical relaxation that could be affected by internal stresses and their distribution, it would be good to know the extent that each test reflects a material's response and what range of configurations might skew this response.
This occurs in applications where elastomer seals are assembled at room temperature and then exposed to elevated temperatures for a period of time, short enough to not produce any chemical relaxation or aging, but long enough to reach the elevated temperature and undergo physical relaxation.
This same relaxation test could be run at higher temperatures for longer periods of time to provide an aged response that results in chemical relaxation and permanent set.
This temperature was used to try to generate more chemical relaxation in these shorter time intervals.