Coating processes – Applying superposed diverse coating or coating a coated base – Synthetic resin coating
Patent
1997-07-03
1999-03-02
Cameron, Erma
Coating processes
Applying superposed diverse coating or coating a coated base
Synthetic resin coating
523406, 523412, 523415, 524507, 524513, 524560, 524591, 524839, B05D 136
Patent
active
058768025
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to an aqueous two-component polyurethane coating composition comprising carboxylate groups and having an OH number of from 40 to 200 mg of KOH/g, preferably from 60 to 140 mg of KOH/g, an acid number of from 20 to 100 mg of KOH/g, preferably from 25 to 50 mg of KOH/g, and
The present invention additionally relates to a process for the preparation of these aqueous coating compositions and to their use in methods of producing a multicoat paint system, as well as to coated articles in which at least one coat has been produced using these coating compositions.
For ecological and economic reasons, the paint industry is attempting to replace as much as possible of the organic solvents employed in coating materials by water. In automotive finishing in particular there is a great need for aqueous coating materials. This applies both to the sector of production-line automotive finishing and to the sector of automotive refinishing. In this context, aqueous coating compositions are employed in particular in the area of topcoats. The term topcoats refers here to coating materials which are used to produce the topmost layer of paint. This topmost layer of paint can comprise one or more coats, especially two coats. Two-coat topcoats consist of a pigmented basecoat and of an unpigmented clearcoat, or a clearcoat which is pigmented only with transparent pigments, which is applied over the basecoat.
Two-coat finishes are nowadays produced by the wet-on-wet method, in which a pigmented basecoat material is first applied and the resulting basecoat is coated over using a clearcoat material, without a baking step, and then basecoat and clearcoat are cured together. This method is very advantageous economically, but places high requirements on the basecoat and clearcoat materials. The clearcoat material applied to the as yet uncured basecoat must not start to dissolve the basecoat or otherwise disrupt it, since otherwise the resulting finishes are of poor appearance. This applies in particular to paint systems in which basecoats containing special-effect pigments (e.g. metal pigments, especially aluminum flakes or pearlescent pigments) are employed. In addition, the topcoat compositions must be able to be applied by spraying with the aid of automatic painting units. For this purpose, their solids content at spray viscosity must be such that films of adequate thickness are obtained with 1 to 2 spray passes (cross-passes), and they must produce baked films which have a good appearance (good evenness, high gloss, good topcoat holdout and high hardness) and good weathering resistance.
In the sector of automotive refinishing, there is the additional requirement that the coating compositions employed are fully curable at low temperatures (generally <80.degree. C.) and, even with curing at these low temperatures, lead to films having the good mechanical properties required.
EP-B-358 979 discloses aqueous two-component polyurethane coating compositions which comprise a hydroxyl-containing polyacrylate resin and a polyisocyanate component. However, when used as a transparent topcoat over a basecoat, these coating compositions are in need of improvement with respect, for example, to the adhesion to the basecoat. There are also problems in respect of other mechanical properties of the resulting coatings, for example in respect of their weathering resistance.
German Patent Application P 43 22 242.0 moreover, which is not a prior publication, discloses aqueous two-component polyurethane coating compositions which comprise a hydroxyl-containing polyacrylate resin and a polyisocyanate component. Hydroxyl-containing polyacrylate resins based on specific cycloaliphatic (meth)acrylic esters, however, are not described in this document.
Furthermore, coating compositions based on hydroxyl-containing polyacrylate resins and crosslinking agents are known, for example, from JP-A 4-1254, where the hydroxyl-containing polyacrylate resins employed as binders are obtainable from hydroxyl-containing monomers, alkyl acrylat
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Brunnemann Michael
Meisenburg Uwe
Nienhaus Egbert
Rink Heinz-Peter
BASF Coatings AG
Cameron Erma
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