1. Field of the Invention
This invention relates to water soluble or water dispersible, urethane-modified polyisocyanate mixtures having blocked isocyanate groups, to their use in combination with water soluble or water dispersible polyhydroxyl compounds in stoving lacquers and to such an aqueous stoving lacquer.
2. Description of the Prior Art
In recent years, the significance of aqueous lacquers and coating compositions has greatly increased due to ever stricter emission guidelines concerning the solvents released after lacquer application. Although aqueous lacquer systems are already available for many applications, these systems often still cannot achieve the high levels of quality of conventional lacquers containing solvents in terms of resistance to solvents and chemicals or in terms of elasticity and resistance to mechanical stress. In particular, no polyurethane-based coating compositions which may be applied from the aqueous phase are known which fulfil the high practical demands placed on elastic fillers for coating motor vehicle bodywork in terms of impact resistance, resistance to stone impact and resistance to water and chemicals.
This statement applies both to GB-PS 1,444,933, EP-A-0,061,628 and DE-AS 2,359,613, which are directed to the hydrophilic modification of aromatic polyisocyanates; to DE-OS 4,001,783, which is directed to special anionically modified aliphatic polyisocyanates; and to the systems disclosed in DE-OS 2,456,469, DE-OS 2,814,815, EP-A-0,012,348 and EP-A-0,424,697, which are directed to aqueous stoving binders based on blocked polyisocyanates and organic polyhydroxyl compounds. The systems based on polyurethane prepolymers containing carboxyl groups with capped isocyanate groups according to DE-OS 2,708,611 or the highly functional (and therefore largely unsuitable for the production of elastic coatings), blocked, water-soluble urethane prepolymers according to DE-OS 3,234,590 are also largely unusable for the stated purpose.
Stoving lacquers, particularly stoving fillers, which optimally fulfil the previously stated requirements and which may be applied from the aqueous phase may successfully be produced when the polyisocyanate mixtures according to the invention and described below in more detail are used as hardeners for the water dispersible or water soluble polyhydroxyl compounds. Particularly good results are achieved when these polyisocyanate mixtures are combined with certain urethane-modified polyester resins, which are also described below in more detail, to form one-component binder compositions which may be applied from the aqueous phase.