Change of volume (of elastomers). The secondary seals of the mechanical seal are always in contact with the process medium and so are always exposed to the medium's chemical and physical action. In a chemical reaction between the medium and the elastomer, the structure of the molecular bond changes – there is either further crosslinking or degradation of the molecule chains. This results in a change of physical properties, mostly in the form of embrittlement or softening, which almost always leads to failure of the sealing element. Chemical reactions can be prevented by selecting resistant materials.
The physical action of the medium entails two processes which take place simultaneously and overlap:

(1) soluble constituents (e.g. plasticizers) of the elastomer are extracted
(2) the medium penetrates the elastomer. Both processes bring about a change of volume.
Swelling occurs when , shrinking when . Every change of volume affects the mechanical properties of the elastomer. A change of hardness, elasticity, strength and temperature characteristic can impair the component's function so much that it fails. All elastomers that swell less than 10% in the process medium and do not react chemically with the medium are suitable as material for a mechanical seal's secondary seals.