METHOD FOR THE PREPARATION OF A MULTILAYER COATING AND AQUEOUS COATING MATERIAL SUITABLE FOR THIS

摘要

The present invention is directed to a method of producing a multi-layer coating in which a saturated base coating comprising an aqueous dispersion of a non-crosslinked polymer is applied to a substrate, wherein the aqueous dispersion has a polymer-phase particle diameter of 10 nm to 500 nm, the polymer has a numerical average molecular weight of 10,000 to 500,000 and an acid number of 12 to 40 and the dispersion is obtained by polymerisation, without adding an emulsifier and in the presence of water-insoluble initiators, of a) 0.65 - 9 parts by weight of a mixture of 28 to 90 wt. % of unsaturated monomers free from carboxyl groups and, apart from the unsaturated bond, not containing any groups which are reactive under the polymerisation and curing conditions, 10 - 65 wt. % of polymerisable monomers containing hydroxyl groups and having the general formula R"-CH=CR'-X-R'" in which R' = H or -C nH2n+1; R" = -R' or -COOC nH2n+1, n = 1 to 6, R'" = a straightchain or branched C1-6 alkyl group with 1 to 3 OH groups, and X = -COO-, -CONH-, -CH2-O- or -O-; and/or 0 - 7 wt. % of ethylenically polyunsaturated monomers, in b) 1 part by weight, relative to the resin content, of an aqueous dispersion of a polyurethane containing urea groups and prepared by prolonging the chain of a polyester-based prepolymer containing NCO groups and with an NCO group content of 1.0 to 10% with at least two urethane groups per molecule, with carboxyl groups corresponding to an acid number of 20 - 50 and a numerical average molecular weight of 600 - 6000, with a polyamine with primary and/or secondary amino groups and/or with hydrazine in an aqueous medium without addition of emulsifier, if required, followed by addition of pigments, fillers, solvents, thickening agents and/or lacquer adjuvants and additives, the resulting base coating is vented, a transparent coating agent which is dilutable in water or dissolved in organic solvents is applied and the thus-coated substrate is then heated at temperatures up to 140.degree.C. The invention is further directed to a water-dilutable coating material obtained by the method. The coating material is suitable for use in the automobile industry.