Two-component paints with good storage stability of polymers capable of forming amides or urethanes and blocked isocyanates are disclosed in U.S. Pat. No. 2,995,531. Further, according to DE-OS No. 20 57 799 such combinations may also be used for the formulation of cathodically depositable ED-paints. One-component systems in which a partially blocked polyisocyanate is linked to a polymer via a urethane grouping are disclosed in DE-OS No. 20 20 905 and DE-OS No. 22 52 536. Systems of this type have the disadvantages of requiring relatively high stoving temperatures and of significant losses of binder material on stoving. Low alkanols such as methanol or ethanol cannot be used as blocking agents. Also, the systems known from EP No. B1-0012463, including amino polyester polyols and .beta.-hydroxyalkylesters which cure through transesterification, require high stoving temperatures and have substantial losses on stoving.
It is further known from EP No.-B1-00 82 291 to use mixed esters of malonic acid, for example trimethylolpropanetriethylmalonate, as a crosslinker for basic resins capable of amide or ester formation. These crosslinkers have relatively low losses on stoving, and the product split-off is non-toxic ethanol. However, in aqueous systems for cathodic electrodeposition, these crosslinkers show defects with respect to stability to hydrolysis, and thus it is difficult to keep constant the pH-value and the MEQ-value of an electrodeposition bath.
DE-OS No. 33 15 569 discloses as crosslinkers for cathodic electrodeposition MICHAEL-addition products produced from CH-active mono- or dicarboxylic acid esters. These compounds cure at a minimum stoving temperature of 140.degree. to 160.degree. C. in the presence of catalysts, such as the lead salts.
It is known from Petersen, Taube; Liebigs Ann. Chem., 562, (1949) 205, that compounds formed from isocyanates and dialkylmalonates can be produced with a methanetricarboxylic acid dialkylester monoamide structure unit which will cross-link with polyols at temperatures of from about 130.degree. C. upwards in the presence of sodium and sodium alcoholates. This reaction has widely been used for the preparation of crosslinking components for liquid stoving paints or stoving paints dissolved in organic solvents. Thus, DE-OS No. 23 42 603 describes a process for producing dialkylestermalonate-blocked biuretpolyisocyanates. In this process, at first, in a rather complicated manner, a two-step reaction is carried out (preparation of a mixture of biuretpolyisocyanate and a monomeric diisocyanate and subsequent separation of the monomeric diisocyanate) to obtain a monomer-free biuretpolyisocyanate which is then reacted with a dialkylmalonate. A stoichiometric excess of dialkylmalonate is preferred. Thus, the reaction mixture contains unreacted dialkylmalonate which does not participate in the crosslinking reaction and may adversely influence the film performance.
A specially blocked isocyanate obtained by reaction of 3-isocyanatomethyl-3,5,5-trimethylcyclohexylisocyanate and two moles of dialkylestermalonate is described in DE-OS No. 24 36 877. DE-OS No. 25 50 156 discloses a process for polyisocyanate mixtures with blocked isocyanate groups easily soluble in paint solvents, where the 3-isocyanatomethyl-3,5,5-trimethylcyclohexylisocyanate is partially reacted with a polyol prior to the reaction with the CH-active blocking agent. DE-OS No. 27 23 117 describes the coemployment of malonate "blocked" diisocyanates in the formulation of stoving paints of aminoplast resins and polyols. The presence of the aminoplast resin is necessary to obtain a film with satisfactory hardness.
The "blocked" polyisocyanates described in DE-OSS Nos. 23 42 603, 24 36 877, and 25 50 156 may, in general, also be used for ED-compositions if they are combined with synthetic resins which are water-dilutable on partial neutralization with acids. At low stoving temperatures, for example 140.degree. C, however, the coatings show unsatisfactory hardness and solvent resistance. The co-employment of aminoplast resins, as described in DE-OS No. 27 23 117, are of no value. It appears that the catalysts necessary for the curing of the aminoplast resins, i.e., sulfonic acid, are neutralized and thus become ineffective.