Process for producing decorative multiple-layer coatings

Details Process for producing decorative multiple-layer coatings Process for producing decorative multiple-layer coatings

1996-02-13

1997-10-28

Dudash, Diana

Coating processes

Applying superposed diverse coating or coating a coated base

Synthetic resin coating

4273884, 427409, 524588, 524591, 524839, 524840, B05D 136, C08L 8300

Patent

active

056816227

DESCRIPTION:

BRIEF SUMMARY
This invention relates to a process for producing decorative multilayer coatings of the aqueous base lacquer/clear lacquer type.
A multiplicity of aqueous base lacquers has become known. These lacquers contain aqueous polyurethane dispersions as binder vehicles, on account of the outstanding properties of the latter. The chains of the polyurethane resins forming the basis of the polyurethane dispersions can be extended in a wide variety of different ways.
Thus, for example, EP-A-0 089 497, EP-A-0 228 003, DE-A-36 28 124 and EP-A-0 512 524 describe aqueous base lacquers which contain, as binder vehicles, aqueous polyurethane dispersions which can be produced by extending the chains of isocyanate-functional prepolymers with a polyamine and/or a polyol.
DE-A-39 15 459 and DE-A-42 24 617 provide examples of aqueous base lacquers which contain, as binder vehicles, aqueous polyurethane dispersions which can be produced by extending the chains of isocyanate-functional prepolymers with water.
Examples of aqueous base lacquers based on aqueous polyurethane dispersions which can be produced by extending, with polyisocyanates, the chains of PU prepolymers which react with polyisocyanates and which contain active hydrogen, are given in DE-A-39 03 804 and DE-A-40 01 841.
Multilayer coatings which are produced using aqueous base lacquers which are formulated based on known polyurethane dispersions are in need of improvement as regards their resistance to water of condensation. If the known polyurethane dispersions are used as the sole binder vehicle in aqueous effect base lacquers, the formation of the effect is less favourable compared with aqueous effect base lacquers containing at least one additional co-binder vehicle.
EP-A-0 163 214 describes aqueous, self-crosslinking polyurethane dispersions which preferably contain lateral siliconate groups. The PU dispersions are prepared and stored under conditions such that reaction of the siliconate groups with each other is deliberately prevented. After application, the binder vehicles crosslink by condensation of the siliconate groups with the formation of siloxane bridges.
Other aqueous solutions or dispersions of polyurethanes containing reactive siliconate groups which are suitable for crosslinking are known from EP-A-0 315 006. The binder vehicles described therein are synthesised by the reaction of the isocyanate groups of isocyanate prepolymers with amino groups, i.e. chain extension is effected by way of amino groups. During the preparation of the binder vehicles a large amount of water is added, so that practically no siloxane bridges are formed, due to the law of mass action.
Both EP-A-0 163 214 and EP-A-0 315 006 make no mention of the fact that the binder vehicles which they describe are suitable for the production of aqueous base lacquers.
The object of the present invention is to provide a process for the production of decorative multilayer coatings of the aqueous base lacquer/clear lacquer type having an improved resistance to water of condensation, wherein aqueous polyurethane resins are used as binder vehicles in the aqueous base lacquers.
This object is achieved by means of the process for producing a decorative multilayer coating to which the invention firstly relates, wherein a colour- and/or effect-imparting lacquer coat of an aqueous base lacquer is applied to an optionally pre-coated substrate and is overpainted with a transparent lacquer coat, characterised in that a self-crosslinking coating medium is used as the aqueous base lacquer, which contains, as a binder vehicle, an aqueous polyurethane resin dispersion based on a polyurethane resin of linear or branched structure having a number average molecular weight (Mn) of 2500 to 120,000, a content of 10 to 400 mmole, preferably 20 to 300 mmole, most preferably 40 to 200 mmole, of lateral and/or terminal hydroxyl and/or R'O groups bonded to silicon, where greater than 1 and less than 100 mg KOH/g, with respect to the solid resin, wherein hydroxyl groups bonded to silicon are not included in the calcul

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