Aqueous polymer dispersions for clear coats

Details Aqueous polymer dispersions for clear coats Aqueous polymer dispersions for clear coats

1995-06-07

1997-07-15

Short, Patricia A.

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Processes of preparing a desired or intentional composition...

525 28, 525 30, 525 39, 525 48, 525 63, 525 69, 525131, 525162, 525163, 525176, 525440, 525443, C08L 5108

Patent

active

056484100

DESCRIPTION:

BRIEF SUMMARY
The invention relates to aqueous polymer dispersions of polyester- and acrylate-based graft polymers, to processes for the production thereof, to aqueous clear-coat compositions containing said dispersions, and to the use of same.
A reduction of the contents of organic solvents in coating compositions is generally desired. There is great interest in aqueous lacquer systems in the area of coating and painting, especially so in the field of automotive painting, but also in other areas.
In the production-line painting of automotive bodies, multi-layer coatings according to the so-called "base coating/clear coating" process are preferred. Therein, first a pigmented base coating is applied which, after some short exposition to air without a baking step, is covered with a clear-coat composition. Then the base coating and clear coating compositions in combination are subjected to baking. This procedure results in coatings having an optically and technologically very high quality.
As far as primers and base coatings are concerned, aqueous systems for a long time have belonged to the state of the art, and in part the use thereof has become a standard operation in the automobile industry. Thus, it is generally desirable to develop and appropriately use a suitable aqueous system for the topmost layer, i.e. the clear coat.
To serve this purpose, such an aqueous clear-coat composition with respect to its profile of properties must at least be a match for the conventional clear-coat compositions dissolved in organic solvents. The solid contents at the spray viscosity must be high such as to provide sufficient layer thicknesses by one or two spraying steps. Furthermore, the clear coat films when baked must be well levelled and exhibit high gloss. In addition, the film should be clear and be "full" and have a good "standing".
Also important is reparability. Repair painting to form a well-adhering gravel-resistant coating must be possible, most desirably without grinding by using any conventional base coating, on a first paint already formed by final baking. Furthermore, there must be constituted a good resistance against chemical attacks such as, e.g., against fuel, acids (sulfuric acid) or lyes. Very important is resistance to condensed water.
DE-OS 31 28 062 describes aqueous dispersions of copolymers preparable from styrene, (meth-)acrylic acid esters, vinyl esters, monomers containing hydroxyl groups, (meth-)acrylic acid, optionally acryl- and/or methacrylamide and addition compounds of glycidyl esters of branched monocarboxylic esters by polymerization with unsaturated carboxylic acids in the presence of an anionic emulsifier. Therefrom, heat-curable coatings are obtained in combination with aminoplastic resins. In accordance with the Application Examples 3 and 4 provided therein, these emulsion polymers were said to be suitable in combination with conventional metallic base coatings for producing weather-resistant two-layer coatings. The reparability and resistance to condensed water during 240 h at 40.degree. C. constant climate (DIN 50017), however, are not attained in combination with aqueous base coatings.
EP-A-0 365 775, DE-OS 38 34 738, DE-OS 40 09 931 and DE-OS 40 09 932 describe solution polymers which are prepared by polymerizing carboxyl group-free and carboxyl group-containing monomers in sequence or alternatingly in partial amounts in an organic solvent by way of a free radical-initiated polymerization. The stability of the clear coat films made therefrom, especially towards sulfuric acid and sodium hydroxide solution, is by far not sufficient.
In EP-A-0 391 271 there have been described aqueous coating compositions which contain, as the main component, a polyester oligomer polyacrylate produced by a free radical-initiated polymerization of ethylenically unsaturated monomers in a hydroxy-functional polyester oligomer which is not polymerizable via a radical-initiated polymerization. The resistance to sulfuric acid of the clear-coat films made therefrom is very poor.
DE-OS 40 27 594 describes aqueous clea

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