Лако-красочные материалы — производство

Adhesion promoters containing chromium fall into two main classes: inorganic and organic complexes. Examples of the former are the chromate conversion coatings used extensively in the aerospace industries for pretreatment of aluminum and its alloys. Although acting as anticorrosion coatings in their own right, they improve paint adhesion substantially [56]. Chromium conversion coatings that may be of chromium phosphate (amorphous, accelerated, or nonaccelerated) may be applied by brush dip or spray. The crystalline chromium phosphate type is normally restricted to steel surfaces. In a simulated sterilization test using epoxide can coatings on an aluminum surface, Paramonov and co­workers showed that the use of a chromate conversion coating was essential for satisfac­tory adhesion [57].

Examples of the organic type are the coordination complexes of trivalent chro­mium chloride with carboxylic acids (Volan manufactured by Du Pont). The metha­crylate-chrome complex is well known in fiberglass technology. In water solution the chromium chlorides hydrolyze to form basic salts that form oligomeric salts through solvation of hydroxyl groups on the adjacent chromium molecules. Hydroxyl groups also bond to silanol groups on the glass surface via hydrogen bonding and possibly covalent oxane bonds. The organic acid group develops a fairly stable bond to chromium by being coordinated to adjacent chromium atoms [8]. Other chromium complexes of functional carboxylic acids have been proposed [58]. Following is a typical structure:

A trivalent chromium fumarato-coordination compound, Volan 82, has been claimed to be an effective adhesion promoter for polyethylene coatings on aluminum. The toxicity of chromium compounds must place a question mark against their continued use as adhesion promoters.