Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Patent
1996-11-04
1998-11-10
Niland, Patrick D.
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
4284231, 524539, 524839, 524840, 525123, 525455, C08J 300, C08K 320, C08L 7500, C08F 830
Patent
active
058345558
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to an aqueous multicomponent polyurethane coating composition, comprising (A2) containing hydroxyl and carboxylate and/or sulfonate 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 15 to 100 mg of KOH/g, preferably from 25 to 50 mg of KOH/g, and a glass transition temperature of from -40.degree. C. to +60.degree. C., preferably from -20.degree. C. to +40.degree. C.,
The present invention further relates to a process for the preparation of these aqueous coating compositions and to their use in methods of producing a multicoat finish, and 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 making efforts to replace as great as possible a proportion of the organic solvents which are employed in paints by water. Especially in automotive finishing there is a great requirement for aqueous coatings. This applies both to the sector of production-line (OEM) automotive finishing and to the sector of automotive refinishing.
In this context aqueous coating compositions are employed in particular in the area of top coats. Top coats are understood here as being coating materials which are used to produce the topmost coat. This topmost coat may comprise one or more coats, especially two coats. Two-coat top coats are composed of a pigmented basecoat and of a clearcoat which is pigmented either not at all or only with transparent pigments and is applied over the basecoat.
Two-coat finishes are currently produced by the wet-on-wet method, in which a pigmented basecoat is initially applied and the resulting basecoat layer, without a baking step, is covered with a clearcoat, and then basecoat layer and clearcoat layer are cured together. This method is very advantageous in economic terms, but places stringent requirements on the basecoat and the clearcoat.
The clearcoat which is applied to the as yet uncured basecoat must neither partially dissolve nor otherwise perturb the basecoat layer, lest finishes of poor appearance be obtained. This applies in particular to finishes in which basecoats containing special-effect pigments (e.g. metallic pigments, especially aluminum flakes, or pearlescent pigments) are employed. Furthermore, the top coat compositions must be capable of being applied by spraying using automatic coating units. For this purpose their solids content at spray viscosity must be high enough for coating films of adequate thickness to be obtained with 1 to 2 spray passes (cross passes), and they must give baked coating films of good appearance (good evenness, high gloss, good top coat holdout and a high degree of hardness) and good weathering resistance. that the coating compositions employed are able to cure fully at low temperatures (generally <80.degree. C.) and lead, even when fully cured at these low temperatures, to films having the good mechanical properties required.
EP-B-358 979 discloses aqueous two-component polyurethane coating compositions which comprise a hydroxyl group-containing polyacrylate resin and a polyisocyanate component. However, these coatings described in EP-B-358 979 exhibit great disadvantages with respect to weathering resistance, in particular with respect to their resistance in a constant humid climate (40.degree. C., 240 h), and processability (fall in viscosity and too short pot life).
It is furthermore known, from EP-B-198 343, to add isocyanate derivatives containing carbodiimide groups to aqueous solutions of polymers which contain carboxyl groups, to improve the mechanical properties, especially the wet strength. However, when the carbodiimides described in EP-B-198 343 are used in aqueous polyurethane coating compositions, problems of compatibility with the binders occur and the resulting coatings have an inadequate weathering stability.
Moreover, EP-A-121 083 describes crosslinking agents based on carbodiimides, which are employed to crosslink aqueous resins containin
REFERENCES:
patent: 5276096 (1994-01-01), Serdiuk et al.
patent: 5574083 (1996-11-01), Brown et al.
patent: 5614584 (1997-03-01), Schwan et al.
Mayer Bernd
Meisenburg Uwe
Nienhaus Egbert
Seidemann Rolf
Tye Anthony J.
BASF Lacke & Farben AG
Niland Patrick D.
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