Coating processes – Direct application of electrical – magnetic – wave – or... – Electrostatic charge – field – or force utilized
Reexamination Certificate
2000-05-08
2002-03-19
Dawson, Robert (Department: 1712)
Coating processes
Direct application of electrical, magnetic, wave, or...
Electrostatic charge, field, or force utilized
C427S485000, C523S406000, C523S407000, C523S410000, C525S327300, C525S375000, C525S381000
Reexamination Certificate
active
06358568
ABSTRACT:
The present invention relates to a transparent powder coating material and to an aqueous transparent powder coating dispersion which is particularly suitable as a coating for car bodies that are coated with waterborne coating material.
At present, liquid coating materials are preferably used for the coating of car bodies. These coating materials cause numerous environmental problems owing to their solvent content. This is true even when aqueous coating materials are employed.
For this reason, increased efforts have been made in recent years to use powder coating materials for the coating. The results to date, however, have been unsatisfactory; in particular, transparent powder coating materials still exhibit weaknesses in terms of chemical resistance and yellowing. Epoxy/carboxy-crosslinked transparent powder coatings exhibit a markedly poorer etch resistance relative to water, tree resin and sulphuric acid.
In the meantime, there have been numerous developments with the aim of developing powder coating materials in the form of aqueous dispersions which can be processed using liquid coating technologies. U.S. Pat. No. 4,268,542, for example, discloses a process which uses a powder coating slurry that is suitable for the coating of cars. In this case, a conventional powder layer is first of all applied to the body and the clearcoat slurry is applied as second layer. In this clearcoat slurry, which is based on acrylate resins, ionic thickeners are used, leading to relatively high sensitivity of the applied coating layer to moisture, especially to condensation. In addition, these thickeners have a content of glycidyl-containing monomers which in one of the examples is from 0.5 to 30%. Moreover, it is necessary to operate with high stoving temperatures (above 160° C.).
In the text below, the term transparent powder coating dispersion is used synonymously with transparent powder coating slurry.
DE-A 196 13 547 discloses an aqueous powder coating dispersion which meets the specified requirements. Following application and crosslinking, however, the transparent powder coating dispersion described therein, just like the solid transparent powder coating materials known to date, exhibits low values in terms of etch resistance to water, tree resin and sulphuric acid. In addition, the system shows a tendency towards yellowing.
The object of the present invention is to provide an aqueous powder coating dispersion comprising
a solid, pulverulent component A and an aqueous component B, where
component A is a transparent powder coating material comprising
a) at least one epoxy-containing binder having a content of from 30 to 45%, preferably from 30 to 35%, of glycidyl-containing monomers, with or without a content of vinylaromatic compounds, preferably styrene,
b) tris(alkoxycarbonylamino)triazine and polycarboxylic acids, preferably straight-chain aliphatic dicarboxylic acids and/or carboxy-functional polyesters, as crosslinking agents, and
c) if desired, catalysts, auxiliaries, additives typical for transparent powder coating materials, such as degassing agents, levelling agents, UV absorbers, free-radical scavengers, antioxidants,
and
component B is an aqueous dispersion comprising
a) at least one nonionic thickener and
b) if desired, catalysts, auxiliaries, defoamers, dispersing auxiliaries, wetting agents, preferably carboxy-functional dispersants, antioxidants, UV absorbers, free-radical scavengers, small amounts of solvent, levelling agents, biocides and/or water retention agents.
The content of glycidyl-containing monomers is preferably from 25 to 45%. Particular preference is given to from 30 to 45% by weight, in particular from 30 to 40%. From 30 to 35% are very particularly preferred. Utmost preference is given to 26-35%, especially from 27 to 33%.
The following proportions are preferred in this context:
a) 60-80 parts
b) 15-30 parts
c) 3-10 parts
A suitable epoxy-functional binder for the solid transparent powder coating material comprises, for example, epoxy-functional polyacrylate resins which can be prepared by copolymerizing at least one ethylenically unsaturated monomer which contains at least one epoxy group in the molecule with at least one further ethylenically unsaturated monomer which contains no epoxy group in the molecule, at least one of the monomers being an ester of acrylic acid or methacrylic acid. Epoxy-functional polyacrylate resins of this kind are known, for example, from EP-A-299 420, DE-B-22 14 650, DE-B-27 49 576, U.S. Pat. No. 4,091,048 and U.S. Pat. No. 3,781,379.
Examples of ethylenically unsaturated monomers which contain no epoxy group in the molecule are alkyl esters of acrylic and methacrylic acid containing 1 to 20 carbon atoms in the alkyl radical, especially methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, butyl methacrylate, 2-ethylhexyl acrylate and 2-ethylhexyl methacrylate. Further examples of ethylenically unsaturated monomers which contain no epoxy groups in the molecule are acid amides, such as acrylamide and methacrylamide, vinylaromatic compounds, such as styrene, methylstyrene and vinyltoluene, nitrites, such as acrylonitrile and methacrylonitrile, vinyl halides and vinylidene halides, such as vinyl chloride and vinylidene fluoride, vinyl esters, such as vinyl acetate, and hydroxyl-containing monomers, such as hydroxyethyl acrylate and hydroxyethyl methacrylate, for example.
The epoxy-functional monomers employed in the epoxy-functional binders are preferably glycidyl acrylate, glycidyl methacrylate and allyl glycidyl ether.
The epoxy-functional polyacrylate resin normally has an epoxide equivalent weight of from 300 to 2500, preferably from 420 to 700, a number-average molecular weight (determined by gel permeation chromatography using a polystyrene standard) of from 2000 to 20,000, preferably from 3000 to 10,000, and a glass transition temperature (T
g
) of from 30 to 80, preferably from 40 to 70 and, with particular preference, from 40 to 60° C. (measured with the aid of Differential Scanning Calorimetry (DSC)). Very particular preference is given to about 50° C. Mixtures of two or more acrylate resins can also be employed.
The epoxy-functional polyacrylate resin can be prepared by common and well-known methods, by addition polymerization.
As component (b), tris(alkoxycarbonylamino)triazines in accordance with U.S. Pat. No. 4,939,213, U.S. Pat. No. 5,084,541 and EP 0 624 577 are employed. Such compounds are also known from DE 2509561 and also Patent Abstracts of Japan, Publ. No. 09194769 A. However, the tris(alkoxycarbonylamino)triazines are employed therein only for cationic electrodeposition coating materials.
In accordance with the invention, the tris(alkoxycarbonylamino)triazines are of the formula
in which R=methyl, butyl, ethylhexyl groups. Derivatives of these compounds can also be employed.
The invention prefers the methyl-butyl mixed esters. These have the advantage over the plain methyl esters of better solubility in polymer melts, and butylethylhexyl mixed esters. The plain butyl esters are also preferred in accordance with the invention.
The tris(alkoxycarbonylamino)triazines and their derivatives can also, in accordance with the invention, be employed in a mixture with conventional crosslinking agents (component C). Here, blocked polyisocyanates different from the tris(alkoxycarbonylamino)triazines are particularly suitable. Similarly, it is possible to employ amino resins, e.g. melamines. The tris(alkoxycarbonylamino)triazines can be present in amounts of from 1 to 10, preferably from 2 to 10% by weight. In principle it is possible to employ any amino resin suitable for transparent topcoats, or a mixture of such amino resins.
Resins of this kind are well known to the person skilled in the art and are offered as commercial products by numerous companies. Amino resins are condensation products of aldehydes, especially formaldehyde, and, for example, urea, melamine, guanamine and benzoguanamine. The amino resins contain alcohol groups, preferably methylol groups, some or all
Baumgart Hubert
Schwarte Stephan
Woltering Joachim
Aylward D.
BASF Coatings AG
Dawson Robert
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