A sealant must have ADHESION (chemical or physical) to the components to be sealed so that no medium, regardless of what it is, can penetrate between the component and the sealant.

Another important point to consider is the actual sealing material in terms of its properties, characteristics, compatibility, temperature range, toughness, ambient pressure, approval issues, etc.

This area is summarized under the term COHESION, i.e., the medium to be sealed must not be able to penetrate the sealant under any circumstances.

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https://www.ihb.ch/en/Products/products-for-sealing-1

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Source: ihb