Water-dispersible binders for cationic electropaints, and a process for their preparation

A process for preparing a water-dispersible binder which is intended for cationic electropaints and is based on reaction products which are formed from modified epoxy resins and primary and/or secondary amines, which comprises reacting

• (A) low molecular weight epoxy resins which contain aromatic groups and have an epoxide equivalent weight of less than 375 with
• (B) aliphatic and/or alicyclic polyfunctional alcohols or carboxylic acids having a molecular weight of less than 350 by addition to the epoxy group in such a way that the reaction products contain 10-45% of aromatic groups, calculated as phenylene group, and reacting the reaction products of A and B with
• (C) if desired up to 60% by weight, relative to total binder, of polyfunctional alcohols, carboxylic acids and/or SH compounds having a molecular weight of 300-5,000, the molar ratios of (A), (B) and, if desired, (C) being chosen in such a way that the resulting intermediate product has terminal epoxy groups,
• (D) by reacting the intermediate product by the addition of a phenolic hydroxyl group of a monophenol and/or diphenol to the epoxy groups, the monophenol or diphenol having as a substituent on the aromatic ring(s) at least one grouping of the formula ##STR14## in which R.sup.1 and R.sup.2 are identical to or different from one another and represent hydrogen or a methyl group, with the proviso that primary and/or secondary amines have been added, before or after the reaction of the phenolic hydroxyl groups with the epoxy groups, to at least some of the groupings of the formula (I).

A process for the electrophoretic coating of an electrically conductive substrate which is connected as the cathode, from an aqueous bath based on cationic heat-curable binders at least partially neutralized with acids, wherein the binders have been prepared by reacting

• (A) low molecular weight epoxy resins which contain aromatic groups and have an epoxide equivalent weight of less than 375 with
• (B) aliphatic and/or alicyclic polyfunctional alcohols or carboxylic acids having a molecular weight of less than 350 by addition to the epoxy group in such a way that the reaction products contain 10-45% of aromatic groups, calculated as phenylene group, and reacting the reaction products of A and B with
• (C) if desired up to 60% by weight, relative to total binder, of polyfunctional alcohols, carboxylic acids and/or SH compounds having a molecular weight of 300-5,000, the molar ratios of (A), (B) and, if desired, (C) being chosen in such a way that the resulting intermediate product has terminal epoxy groups,
• (D) by reacting the intermediate product by the addition of a phenolic hydroxyl group of a monophenol and/or diphenol to the epoxy groups, the monophenol or diphenol having as a substituent on the aromatic ring(s) at least one grouping of the formula ##STR15## in which R.sup.1 and R.sup.2 are identical to or different from one another and represent hydrogen or a methyl group, with the proviso that primary and/or secondary amines have been added, before or after the reaction of the phenolic hydroxyl groups with the epoxy groups, to at least some of the groupings of the formula (I).