Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
2000-10-23
2002-02-05
Dawson, Robert (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C523S415000, C525S123000, C525S327300, C525S386000
Reexamination Certificate
active
06344501
ABSTRACT:
The present invention relates to a nonionically stabilized powder clearcoat dispersion particularly suitable as a coating for automobile bodies coated with aqueous basecoat.
For the coating of automobile bodies, preference is currently given to the use of liquid coating materials. These cause numerous environmental problems owing to their solvent content. The same applies to cases where aqueous coating materials are employed.
For this reason, increased efforts have been made in recent years to use powder coating materials for the coating operation. The results so far, however, have not been satisfactory; in particular, increased coat thicknesses are necessary in order to achieve a uniform appearance. On the other hand, the use of pulverulent coating materials entails a different application technology. The plants designed for liquid coating materials cannot, therefore, be used for this purpose. Consequently, attempts are being made to develop powder coating materials in the form of aqueous dispersions which may be processed using liquid coating technologies.
U.S. Pat. No. 4,268,542, for example, discloses a process using a powder coating slurry which is suitable for coating automobiles. In this process, a conventional powder coat is first applied to the body, and the clearcoat slurry is applied as the second coat. With this clearcoat slurry, based on acrylate resins, ionic thickeners are used. Moreover, in one of the examples, they include from 0.5 to 30% of glycidyl-containing monomers. Moreover, it is necessary to operate at high baking temperatures (more than 160° C.).
DE-A 196 13 547 discloses an aqueous powder coating dispersion which meets the specified requirements. However, following application and crosslinking, the powder clearcoat dispersion described therein, like the solid transparent powder coating materials known to date, exhibits poor values in respect of its incipient etch stability with respect to water, tree resin, and sulfuric acid. Moreover, the system displays a tendency to yellow.
Unpublished German Patent Application 19518392.4, furthermore, discloses a powder clearcoat dispersion composed of two components. The first component comprises epoxy binder, crosslinking agents, and also catalysts, auxiliaries, and additives. The second component is a nonionic thickener, which is present if desired in a mixture with catalysts, auxiliaries, defoamers, wetting agents, antioxidants, UV absorbers, free-radical scavengers, biocides, solvents, leveling agents, and neutralizing agents. Dispersion auxiliaries used are preferably carboxy-functional dispersants.
The transparent powder coating materials to date have generally contained dispersants based on ionically stabilized polymers. In the case of wet-on-wet application, these lead to cracking in the clearcoat. Additional difficulties are the sedimentation tendency and the pumpability of the paint dispersion.
In the text below, the term powder clearcoat dispersion is used synonymously with powder clearcoat slurry.
The present invention has now set itself the object of providing an aqueous powder clearcoat dispersion which may be applied to automobile bodies with the existing liquid coating technology, which in particular may be baked at temperatures of just 130° C., and which does not lead, especially after wet-on-wet application, to cracking in the clearcoat.
This object is achieved by means of an aqueous powder clearcoat 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 binder containing from 25 to 45%, preferably from 30 to 35%, of glycidyl-containing monomers, with or without a fraction of vinylaromatic compounds, preferably styrene,
b) at least one crosslinking agent, preferably straight-chain, aliphatic dicarboxylic acids, carboxy-functional polyesters and/or tris(alkoxycarbonylamino)triazine, and
c) if desired, catalysts, auxiliaries, additives typical for transparent powder coating materials, such as devolatilizers, leveling agents, UV absorbers, free-radical scavengers, and antioxidants, and
component B is an aqueous dispersion comprising
a) at least one nonionic thickener and
b) if desired, catalysts, auxiliaries, defoamers, dispersion auxiliaries, wetting agents, antioxidants, UV absorbers, free-radical scavengers, small amounts of solvent, leveling agents, biocides and/or water retention agents, and
c) a dispersant medium in the form of a nonionic polyurethane dispersion.
Preference is given in accordance with the invention to the following proportions:
Component A
a) 60-80 parts
b) 15≅30 parts
c) 3-10 parts
Component B
20-50 parts of component a
80-50 parts of component b
20-50 parts of component c
1000-5000 parts of distilled water
The dispersion preferably contains
25-100 parts of component A
100 parts of component B
Suitable epoxy-functional binders for the solid transparent powder coating material used to prepare the dispersion are, for example, polyacrylate resins which contain epoxide groups and are preparable by copolymerizing at least one ethylenically unsaturated monomer containing at least one epoxide group in the molecule with at least one further ethylenically unsaturated monomer containing no epoxide group in the molecule, at least one of the monomers being an ester of acrylic acid or methacrylic acid. Polyacrylate resins of this kind containing epoxide groups are known, for example, from EP-A-299 420, DE-B-22 14 650, DE-B-27 49 576, U.S. Pat. Nos. 4,091,048 and 3,781,379.
Examples of ethylenically unsaturated monomers which contain no epoxide 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 epoxide groups in the molecule are acid amides, such as acrylamide and methacrylamide, vinylaromatic compounds, such as styrene, methylstyrene and vinyltoluene, nitriles, such as acrylonitrile and methacrylonitrile, vinyl 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 polyacrylate resin containing epoxide groups normally has an epoxide equivalent weight of from 400 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, with particular preference from 40 to 60° C. (measured by means of differential scanning calorimetry (DSC)). Very particular preference is given to around 50° C. Mixtures of two or more acrylate resins may also be employed.
The polyacrylate resin containing epoxide groups may be prepared by addition polymerization in accordance with widely known methods.
Suitable crosslinkers are carboxylic acids, especially saturated, straight-chain, aliphatic dicarboxylic acids having 3 to 20 carbon atoms in the molecule. Very particular preference is given to the use of 1,12-dodecanedioic acid. To modify the properties of the finished powder clearcoats it is possible if desired to use other crosslinkers containing carboxyl groups. Examples thereof that may be mentioned include branched or unsaturated straight-chain dicarboxylic and polycarboxylic acids, and also polymers containing carboxyl groups.
As crosslinkers it is also possible to employ tris(alkoxycarbonylamino)triazines in accordance with U.S. Pat. Nos. 4,939,213, 5,084,541, and EP 0 624 577.
These are tris(alkoxycarbonylamino)triazines of the formula
where R is methyl, butyl or ethylhexyl. It is also possible to use derivatives of said compounds.
Preference is given in accordance with the invention to
Frey Thomas
Grosse-Brinkhaus Karl-Heinz
Niemann Jörg
Sierakowski Claudia
Wegner Egon
Aylward D.
BASF Coatings AG
Dawson Robert
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