Process for multicoat cathodic wet-on-wet painting of metallic conductive substrates

Claim 1

A process for coating an electrical conductive substrate, which comprises: immersing the substrate in a first acidic aqueous bath comprising a first cationic synthetic resin binder having amino groups or a mixture of amino groups and hydroxyl groups, a first cross-liner mixture of a pair of crosslinking agents, and additive agents selected from the group consisting of a pigment, a filler, a corrosion inhibitor, a surface coating additive, a catalyst and an organic solvent in an amount of up to 15% by weight, based upon the total weight of said first bath ingredients excluding water and acid, making the substrate the cathode for electrodeposition, depositing a first coat on the substrate by means of direct current to produce a sigle coated substrate, removing the single coated substrate form the bath and rinsing it, immersing the single coated substrate in a second acidic aqueous bath comprising a second cationic synthetic resin binder having amino groups or a mixture of amino groups and hydroxyl groups, a second cross-linker mixture of a pair of crosslinking agents, and additive agents selected from the group consisting of a pigment, a filler, a corrosion inhibitor, a surface coating additive, a catalyst and an organic solvent in an amount of up to 15% by weight based upon the total weight of said second bath ingredients excluding water and acid, making the single coated substrate the cathode for electrodeposition, depositing a second coat on the single coated substrate by means of direct current to produce a double coated substrate, removing the double coated substrate from the bath and rinsing it, and curing the double coated substrate; said pairs of crosslinking agents of the first and second cross-linker mixtures each being (a) a pair of carboxylic esters having the differing activated alcohol residual moieties wherein the alkyl group thereof has a substituent selected from the group consisting of beta ester, beta-alkyl ether, beta, gammadihydroxy, and beta hydroxy, (b) a pair of polyisocyanates blocked with different blocking agents, or (c) a combination of said carboxylic ester and said blocked polyisocyanate, each pair of cross-linking agents having different reactivities such that their crosslinking reactions are initiated at temperature which are at least 10° C. apart and the crosslinking agent which reacts first is present in an amount of from 15 to 60 mol %, based upon the mixture of the crosslinking agents.