Coating processes – Direct application of electrical – magnetic – wave – or... – Polymerization of coating utilizing direct application of...
Reexamination Certificate
2001-03-02
2002-11-26
Cameron, Erma (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Polymerization of coating utilizing direct application of...
C427S517000, C427S375000
Reexamination Certificate
active
06485793
ABSTRACT:
The present invention relates to an aqueous dispersion of UV-curable powder coating materials which are applied to substrates and then, following partial or complete evaporation of the water and heating to form a film of the powder particles, are cured by means of high-energy radiation, preferably UV light, to give crosslinked coating films. They are particularly suitable for auto bodies made of sheet metal and/or plastic, applied by means of electrostatically assisted high-speed rotation or pneumatically, as a coating for car bodies coated with waterborne basecoat.
Liquid coating materials are presently used with preference for the coating of car bodies. Such materials give rise to numerous environmental problems owing to their solvent content. This also applies to the use of waterborne coating materials.
It is for this reason that increased efforts have been made in recent years to use powder coating materials for the coating. The result to date, however, have not been satisfactory; in particular, increased coat thicknesses are necessary in order to obtain a uniform appearance.
Further serious problems of powder coating materials for thermal curing arise from the requirement for blocking resistance on storage and storability even in summer temperatures. In order to ensure that this requirement is met, the softening point of the coating powders must be high. Because of the high softening point of the coating materials, however, the heat-activated crosslinking reaction begins prematurely during the melting of the powders on the substrate, before the film surface has achieved optimum leveling. To solve this problems, UV-curable powder coating materials are proposed in which it is possible to separate the melting operation from the crosslinking. The UV powder coating materials which have been disclosed to date are all based on substances containing acrylic or vinylic unsaturation, which owing to the high melting temperature required for good blocking resistance can also undergo thermal polymerization prior to UV irradiation. In order to guarantee blocking resistance, the binders employed for the UV powder coating materials must be absolutely solvent-free polymers, which are highly problematic to obtain owing to their tendency to undergo thermal polymerization.
A further problem with the use of powder coating materials is that they cannot be used on existing plants designed for liquid coating materials, because of the different application technology. This was the spur to the development of powder coating materials in the form of aqueous dispersions which can be processed using liquid coating technologies. Examples of the publications in this field are DE 196 13 547.8-43 and DE 195 18 392.4.
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 process, a conventional powder coat is first applied to the bodywork, and the transparent coating slurry is applied as a second coat. This transparent coating slurry, based on acrylate resins, uses ionic thickeners which result in relatively high sensitivity of the applied coating film to moisture, especially to condensation. Furthermore, in one of the examples these contain from 0.5 to 30% of glycidyl-containing monomers. Moreover, it is necessary to operate with high stoving temperatures (more than 160° C.).
However, the general problem of heat-activated crosslinking when the water is evaporated off and the resultant covering of powder coating material is melted is still not always solved using this technology, since crosslinking does not start at a sharply defined temperature but instead starts gradually, before the water has fully evaporated and an optimum surface has formed. Following the onset of the crosslinking reaction, water still emerging as a result of the high temperatures required is the cause of bubbles and craters.
In the text below, the terms transparent powder coating dispersion and transparent powder coating material are used synonymously.
It is an object of the present invention to provide an aqueous dispersion, comprising a solid pulverulent component A and an aqueous component B, which can be applied to car bodies by means of the conventional liquid coating technology and which is UV-curable.
We have found that this object is achieved when component A comprises a UV-curable powder coating material.
The technical advantages of the dispersion of the invention lie in the possibility of using UV powder coating materials having a markedly lower softening point than in the case of dry application, since the particles in the dispersion are unable to cake together. This lower softening temperature results in surfaces which exhibit good flow and are retained since it is not necessary to heat them to high “stoving” temperatures for the purpose of crosslinking. It has in fact been found, completely surprisingly, that UV curing, with a minimal residual water content of the melted film, takes place particularly quickly and completely. Accordingly, the natural equilibrium between the water content in the film and in the ambient air, which is dependent on the hydrophilicity of the crosslinking coating films, is quickly established as early as during cooling. A further important advantage is the energy saving due to the lower drying and melting temperatures.
Since the UV powder coating dispersions are self-crosslinking systems, the homogenization of binder, crosslinker and additives by means, for example, of extrusion is not absolutely necessary, in contradistinction to thermally crosslinking powder coating materials.
Suitable UV-curable powder coating materials are the binders known from the prior art. Particularly suitable are acrylate resins having lateral functional groups such as, for example, epoxy groups or hydroxyl groups, with molecular weights in the range from Mn 1000 to 10,000 with molecular weight distributions <4, as are described, for example, in DE 4203278, which are subsequently reacted with acrylic acid or acrylic acid derivatives, such as acryloyl chloride, to give the corresponding acrylated acrylates (EP 650979).
Functional resins which can be employed are preferably those containing aliphatic compounds. For example, epoxy-containing binders with a glycidyl-containing monomer content of from 5 to 45% by weight, preferably from 25 to 40% by weight, are suitable.
Copolymers containing epoxy groups can be prepared either with acrylic acid, methacrylic acid or mixtures of these. It is likewise possible to use mixtures of acrylic acid, methacrylic acid and (meth)acrylic anhydride.
Particularly suitable are the acrylate resins that are described in DE 203278 and have lateral functional groups, e.g. epoxy groups or hydroxyl groups, which are preferably reacted with acrylic acid or acrylic acid derivatives, e.g. acryloyl chloride, to give the corresponding acrylated acrylates (cf. EP 650 979).
Examples of suitable epoxy-functional binders for the solid transparent powder coating material are polyacrylate resins which contain epoxy groups and are preparable by copolymerization of 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. Polyacrylate resins of this kind, containing epoxy groups, 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
Bendix Maximilian
Binder Horst
Blum Rainer
Königer Rainer
Meisenburg Uwe
BASF Coatings AG
Cameron Erma
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