A generic term for substances made up of simple molecules. As a rule, they are highly active. Their molecules are either unsaturated, have a ring structure or possess one or several reactive groups. Monomers (in pure form or "swimming" in a carrier liquid) are sealed mainly by double mechanical seals. It is possible to use a single mechanical seal provided there is no danger of the monomers reacting under the influence of frictional heat in the sealing gap to form polymers.

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