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Impregnation is the opposite process of extraction. It is a process of imbuing, infusing, depositing or saturating certain materials with components. It intends to add a relatively pure compound to another raw material by first dissolving it in a solvent, like water. In supercritical processes this is achieved by dissolving the pure compound in CO2, transporting the CO2 to a substrate and exposing the substrate to the enriched CO2. With its high diffusivity and low surface tension, CO2 is an inert and easily removable solvent which scores high in impregnation of various materials. Depending on the affinity of the substrate to the pure compound an amount of the compound will physically attached to the substrate. This principle was applied to textile dyeing by E. Bach in 1994.
In this case the thermodynamics is ruled by the solubility of the pure compound and affinity of the substrate towards the pure compound. The same rules apply as for the extraction thermodynamics. The mass transfer is ruled by the substrate permeability of the CO2.
The engineering is largely how much CO2 is necessary for the transportation to the substrate. It is with Supercritical Impregnation utmost important to have virtually no pressure change between the pure compound and the substrate. This is due to the fact that in order to have a good end-product large amounts of CO2 will be required.
Supercritical Impregnation can be used in textile dyeing, coating of substrates, impregnations of actives on substrate for medical application, etc.