Coating processes – With post-treatment of coating or coating material – Heating or drying
Reexamination Certificate
2000-04-18
2003-08-19
Beck, Shrive P. (Department: 1762)
Coating processes
With post-treatment of coating or coating material
Heating or drying
C427S385500, C427S388100, C427S407100, C427S409000, C524S457000, C524S458000, C524S460000, C524S461000
Reexamination Certificate
active
06607788
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to aqueous basecoat compositions which comprise as binder constituents an aqueous acrylate dispersion (i), an aqueous polyurethane resin dispersion (ii) and a rheological assistant (iii).
PRIOR ART
The prior art discloses processes for coating motor vehicle bodies, especially car bodies, in which the substrate is generally first coated with an electrodeposition coating material and/or intermediate stone-chip primer or with a surfacer coat and subsequently, using a coating material comprising at least one pigment, a basecoat film is applied and this basecoat film is overcoated if desired with a transparent coating material. The resulting single-coat or multicoat paint system is subsequently baked. The basecoats used to produce the basecoat film are normally water- or solvent-based systems. As principal binder they comprise, in general, polyurethane dispersions or acrylate dispersions in combination with water-miscible, crosslinkable polyesters and with water-miscible melamine resins.
DE-A 43 39 870 describes basecoats comprising as binder a polymer which is obtainable by subjecting an ethylenically unsaturated monomer or a mixture of ethylenically unsaturated monomers to free-radical polymerization in the presence of a water-insoluble initiator or of a mixture of water-insoluble initiators and in an aqueous dispersion of a polyurethane resin which has a number-average molecular weight Mn of from 1000 to 30,000 daltons and comprises on average from 0.05 to 1.1 polymerizable double bonds per molecule, the weight ratio between the polyurethane resin and the ethylenically unsaturated monomer or the mixture of ethylenically unsaturated monomers lying between 1:10 and 10:1.
Basecoats in accordance with DE-A 43 39 870 feature enhanced stability on storage, very good adhesion between basecoat film and substrate, and very good resistance to atmospheric humidity.
The as yet unpublished German Patent Application P 195 47 944.0 describes basecoats which are additionally compatible with customary clearcoats, such as, for example, aqueous or solvent-containing clearcoats or transparent powder coating materials, as a result of which such clearcoats can be applied to the basecoat without any accompanying deterioration in the appearance of the paint system.
The subject of German Patent Application P 195 47 944.0 is a process for producing a multicoat coating system on a substrate surface, wherein
(A) an aqueous coating composition comprising an aqueous polymer dispersion as film former is applied as basecoat to a substrate surface coated with a customary surfacer,
(B) a suitable transparent topcoating composition is applied to the resulting basecoat film, and
(C) the basecoat film is baked together with the topcoat film, the basecoat comprising an aqueous polymer dispersion, which comprises
(i) an acrylate dispersion having a content of from 30 to 60% by weight of C
1
-C
8
-alkyl (meth)acrylate-containing monomers, from 30 to 60% by weight of vinylaromatic monomers and from 0.5 to 10% by weight of (meth)acrylic acid and
(ii)a rheological assistant which is a synthetic polymer having ionic and/or associative groups.
A further subject of P 195 47 944.0 is a process for repairing multicoat paint systems, in which
(D) a basecoat film applied to a customary surfacer is repaired with an aqueous coating composition comprising an aqueous polymer dispersion as film former,
(E) the coating obtained in stage (D) is coated with a suitable transparent topcoating composition, and
(F) the basecoat film is baked together with the topcoat film, the basecoating composition comprising an aqueous polymer dispersion which comprises
(i) an acrylate dispersion having a content of from 30 to 60% by weight of C
1
-C
8
-alkyl (meth)acrylate-containing monomers, from 30 to 60% by weight of vinylaromatic monomers and from 0.5 to 10% by weight of (meth)acrylic acid and
(ii)a Theological assistant which is a synthetic polymer having ionic and/or associative groups.
To basecoats in accordance with P 195 47 944.0 it is possible to apply any desired transparent coating materials, such as, for example, aqueous or solvent-containing coating materials or else powder coating materials and also powder slurry coating materials, which is not possible in the processes known from the prior art. Coatings which comprise the basecoat film used in accordance with the invention can be repaired without problems in cases of damage.
Problem and Solution
It has been found that the basecoats in accordance with German Patent Application P 195 47 944.0, despite their excellent spectrum of properties, still exhibit relatively poor adhesion to the transparent topcoats, especially to 2-component clearcoats and to powder slurry clearcoats as topcoat.
Although this effect can be ameliorated slightly by replacing all of the binder employed in the basecoat in accordance with P 195 47 944.0 by the binder known from DE-A 43 39 870, the adhesion of basecoats modified in such a manner, following exposure in the condensation test, is still unsatisfactory.
It has surprisingly been found that basecoat compositions comprising as binder constituents
(i) a conventional acrylate dispersion having a content of from 30 to 60% by weight of C
1
-C
8
-alkyl (meth)acrylate-containing monomers, from 30 to 60% by weight of vinylaromatic monomers and from 0.5 to 10% by weight of (meth)acrylic acid, based in each case on the acrylate copolymer, and
(ii) a dispersion of a polymer which is obtainable conventionally by subjecting an ethylenically unsaturated monomer or a mixture of ethylenically unsaturated monomers to free-radical polymerization in the presence of a water-insoluble initiator or of a mixture of water-insoluble initiators and in an aqueous dispersion of a polyurethane resin which has a number-average molecular weight Mn of from 1000 to 30,000 daltons and comprises on average from 0.05 to 1.1 polymerizable double bonds per molecule, the weight ratio between the polyurethane resin and the ethylenically unsaturated monomer or the mixture of ethylenically unsaturated monomers lying between 1:10 and 10:1, and
(iii) a Theological assistant which preferably comprises a synthetic polymer having ionic and/or associative groups, and, if desired, an inorganic Theological assistant as well,
provide an outstanding solution to this problem, the proportion of component (i) lying between 10 and 80% by weight, with particular preference between 20 and 60% by weight, and the proportion of component (ii) lying between 20 and 90% by weight, with particular preference between 40 and 80% by weight, based in each case on the sum of the proportions by weight of components (i) and (ii).
The invention additionally embraces processes for producing a multicoat coating system on a substrate surface, by
(A) applying to a substrate surface coated beforehand with a customary surfacer or, if desired, with a known aqueous water-based coating material, for example a thin-film surfacer in accordance with DE-A 44 38 504, the aqueous basecoat composition of the invention,
(B) applying a suitable transparent topcoating composition to the resulting basecoat film, and
(C) baking the basecoat film and also, if present, the surfacer coat and/or the first basecoat film together with the topcoat film.
Implementation of the Invention
Component (i) consists of an aqueous dispersion of a copolymer of from 30 to 60% by weight, based on the acrylate copolymer, of C
1
-C
8
-alkyl (meth)acrylates, it being possible to employ the linear and branched-chain derivatives. As examples, mention may be made in particular of methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl and isopropyl (meth)acrylate, n-butyl and isobutyl (meth)acrylate and 2-ethylhexyl (meth)acrylate, or mixtures of these monomers. As further monomers it is also possible to employ (meth)acrylamide monomers and their derivatives and also (meth)acrylonitrile.
As vinylaromatic monomers it is possible as constituents of component (i) to employ, in proportions of from 30 to 60% by weight, based on th
Angermüller Harald
Sapper Ekkehard
Wegner Egon
Wiemann Gudrun
BASF Coatings AG
Beck Shrive P.
Jolley Kirsten Crockford
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