Coating formulation for use in aqueous multicoat paint systems

Details Coating formulation for use in aqueous multicoat paint systems Coating formulation for use in aqueous multicoat paint systems

1997-06-20

2001-04-24

Gorr, Rachel (Department: 1711)

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...

C528S071000, C524S591000, C525S457000, C427S532000, C427S541000, C427S385500, C427S388400, C427S407100, C427S409000, C427S419500, C427S419300

Reexamination Certificate

active

06221949

ABSTRACT:

The present invention relates to a novel coating formulation for use in aqueous multicoat paint systems and which can be employed in particular for stone-chip protection coats.
The prior art discloses processes for the coating of motor-vehicle bodies, especially auto bodies, in which an electrodeposition coating material is applied and baked, an intermediate stone-chip primer is optionally applied, and the coats are baked separately or together with the filler coat to be applied, a filler is applied and baked, and a single—or multicoat topcoat system is applied and baked.
The third coat has the function in particular of filling out and covering instances of unevenness in order to level the substrate for the subsequent topcoat. The better this filling-out and covering of instances of unevenness in the substrate, the better the optical quality of the overall paint system. Not only the optical quality but also important mechano-technological properties of the overall paint system, such as protection from corrosion and, in particular, resistance to stone chipping and other mechanical attacks depend critically on the quality of the filler coat and—if present—of the intermediate stone-chip primer coat.
A corresponding process for the coating of motor-vehicle bodies in particular is known, for example, from EP-A-238 037.
Formerly, stoving enamels based on organic solvents were employed predominantly in the paint industry in order to produce the filler coats and intermediate stone-chip primer coats. For economic and ecological reasons, the paint industry has been attempting for some years to replace paint inorganic solvents by aqueous coating materials.
The preparation of such aqueous coating materials is known, inter alia, from DE-A-40 05 961. The coating materials described therein are suitable for the initially described process for producing filler coats and/or intermediate stone-chip primer coats. The DE-A document relates essentially to a coating material which comprises as binder a combination of water-dilutable polyurethane resins, water-dilutable polyester resins and amino resins.
However, these coating materials are not suitable for use in so-called wet-on-wet processes. Apart from this, the coating thicknesses obtained are still not satisfactory. This is because, for reasons of cost and the environment, the automobile industry has been attempting for some years to apply paint films which are as thin as possible without a loss in quality in comparison with the thicker coats which have been customary to date.
Hitherto, the coat applied in the abovementioned process step
3
was applied with a coat thickness of approximately 35 &mgr;m. In order to save on raw materials and energy in the production of multicoat paint systems, especially in the finishing of auto bodies, attempts have been made in particular to reduce the thickness of this coat. When using the aqueous coating systems known to date for these intended applications, however, such attempts were associated with a drastic deterioration in the properties of the overall paint system.
The object of the present invention is therefore to provide a coating formulation for use in aqueous paint systems, which formulation can be employed in step
3
of the process described initially and with which high-quality paint systems can be produced even when the coat thickness of the coat applied in step
3
is below 35 &mgr;m.
This object is achieved by the coating formulation comprising a) as binder a water-dilutable polyurethane resin which has an acid number of from 10 to 60 and a number-average molecular weight of from 4000 to 25,000, preferably from 8000 to 25,000, and can be prepared by reacting with one another aa) a polyester—and/or polyether-polyol having a number-molecular weight of from 400 to 5000, or a mixture of such polyester—and/or polyether-polyols, bb) a polyisocyanate or a mixture of polyisocyanates, cc) a compound which has in the molecule at least one group which is reactive toward isocyanate groups and at least one group which is capable of forming anions, or a mixture of such compounds, and optionally dd) a hydroxyl—and/or amino-containing organic compound having a molecular weight of from 40 to 400, or a mixture of such compounds, and at least partially neutralizing the resulting reaction product, and which b) comprises pigments and/or fillers, the ratio of binder to pigment being 0.5:1 and 1.5:1.
It is essential to the invention that, in contrast to the prior art, a coating formulation based on a physically drying polyurethane dispersion can be prepared without polyesters and amino resins. Surprisingly, the use of a coating material which comprises only polyurethane resins and pigments leads to a material which can be used particularly well as a filler coat and intermediate stone-chip primer coat, since it is unexpectedly stable to mechanical loads, especially stone chipping and impacts.
The component (a) can be prepared from aa), bb), cc) and optionally dd) by the methods of polyurethane chemistry which are well known to the person skilled in the art (cf. e.g. U.S. Pat. No. 4,719,132, DE-A 36 28 124, EP-A-89 497, EP-A-256 540 and WO 87/03829). As component (aa) it is possible to employ saturated and unsaturated polyester-—and/or polyether-polyols, especially polyester—and/or polyether-diols having a number-average molecular weight of from 400 to 5000. Examples of suitable polyether-diols are polyether-diols of the general formula H(—O—(CHR
1
)
n
-)
m
OH, where R
1
is hydrogen or a substituted or unsubstituted lower alkyl radical, n is from 2 to 6, preferably from 3 to 4, and m is from 2 to 100, preferably from 5 to 50. Examples are linear or branched polyether-diols, such as poly(oxyethylene) glycols, poly(oxypropylene) glycols and poly(oxybutylene) glycols. The selected polyether-diols should not introduce excessive quantities of ether groups, since otherwise the polymers formed swell in water. The preferred polyether-diols are poly(oxypropylene) glycols in the molecular mass range M
n
from 400 to 3000.
Polyester-diols are prepared by esterifying organic dicarboxylic acids or their anhydrides with organic diols, or are derived from a hydroxycarboxylic acid or from a lactone. In order to prepare branched polyester-polyols, it is possible to employ a minor proportion of polyols or polycarboxylic acid with a higher functionality. The dicarboxylic acids and diols can be linear or branched aliphatic, cycloaliphatic or aromatic dicarboxylic acids or diols.
The diols used to prepare the polyesters consist, for example, of alkylene glycols such as ethylene glycol, propylene glycol, butylene glycol, 1,4-butanediol, 1,6-hexanediol, neopentylglycol and other diols, such as imethylolcyclohexane. It is also possible, however, to add small quantities of polyols, such as trimethylol-propane, glycerol and/or pentaerythritol. The acid components of the polyester consist primarily of low molecular weight dicarboxylic acids or their anhydrides having 2 to 30, preferably 4 to 18, carbon atoms in the molecule. Examples of suitable acids are o-phthalic acid, isophthalic acid, terephthalic acid, tetrahydrophthalic acid, cyclohexanedicarboxylic acid, succinic acid, adipic acid, azelaic acid, sebacic acid, maleic acid, fumaric acid, glutaric acid, hexachloroheptanedicarboxylic acid, tetrachlorophthalic acid and/or dimerized fatty acids. Instead of these acids it is also possible to use their anhydrides, where these exist. When forming polyester-polyols, it is also possible for relatively small quantities of carboxylic acids having 3 or more carboxyl groups to be present, for example trimellitic anhydride or the adduct of maleic anhydride with unsaturated fatty acids.
It is also possible to employ polyester-diols which are obtained by reacting a lactone with a diol. These polyester-diols are distinguished by the presence of terminal hydroxyl groups and repeating polyester components of the formula (—CO—(CHR
2
)
n
—CH
2
—O). In this formula, n is preferably 4 to 6 and the substituent R
2
is hydrogen or an alkyl, cycloalkyl or alko

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