This invention relates to a preferably cathodically depositable aqueous electrodeposition lacquer coating composition containing an organic synthetic resin binder which is dilutable with water by protonization with acids and which contains an organic amine-containing synthetic resin component and crosslinking agents; and thermal crosslinking catalysts and, optionally, pigments, fillers, corrosion inhibitors, standard lacquer auxiliaries and, in a quantity of up to 20% by weight, based on the total weight of the coating composition, of organic solvents. The invention also relates to the use of this coating composition for coating objects having an electrically conductive surface. These objects are immersed in the bath of the electrodeposition lacquer coating composition and, in an electrical field, the coating composition is deposited on the object to be coated serving as cathode.
Electrodeposition lacquer coating compositions of the type in question are used on an industrial scale. They are also known as electrophoresis lacquers or EC lacquers. At the present time, aminoepoxy resins produced from epoxy resins based on bisphenol A and amines and/or aminoalcohols are used in practice as high-coverage EC lacquers affording effective protection against corrosion.
These binders are thermally crosslinked by various methods, for example by reactions with blocked polyisocyanates (DE-A-20 57 799, DE-A-21 31 060, DE-A-22 52 536, DE-A-22 65 195, DE-A-23 63 074, DE-A-26 34 211) or by addition of transesterifiable crosslinking agents (EP-A-0 004 090, EP-A-0 012 463, EP-A-0 040 867, EP-A-0 066 859, EP-A-0 082 291, DE-A-31 03 642, DE-A-32 24 864, DE-A-33 15 469). The properties of the lacquer are not satisfactory in every respect, depending on customer requirements. Resin systems containing blocked polyisocyanates are generally attended by the disadvantage of high stoving losses at the high hardening temperatures necessary, thereby polluting the environment. Low molecular weight alcohols, such as methanol or ethanol, cannot be used as blocking agents because their urethanes are stable at temperatures below about 250.degree. C. Resins crosslinked by transesterification show inadequate adhesion and poor resistance to detergents coupled with serious bubble formation.
In the production of color-stable two-coat lacquers of the type applied to domestic appliances, both systems have the unpleasant property of yellowing to a considerable extent under the effect of cracking products emanating from the primer during stoving of the surface lacquer at elevated stoving temperatures. In some cases, cloud-like changes in color are even produced in the surface lacquer, spoiling the optimal appearance thereof. Other state-of-the-art binder systems based on aminopoly(meth)acrylate resin or aminopolyurethane resin, which do not yellow to the same extent, have so far failed to show the high corrosion inhibition level achieved by aminoepoxy resins in the automotive industry It is not possible with the priming systems hitherto used, on account of their properties, to achieve good two-coat lacquering for this particular application without the filler.