Aqueous powder coat dispersions, process for their...

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...

Reexamination Certificate

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Reexamination Certificate

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06441078

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to aqueous powder coat dispersions, a process for their preparation and their use.
In more recent times aqueous powder coat dispersions and processes for their preparation have been the subject of a number of patent applications. By aqueous powder coat dispersions are meant powder coatings dispersed in an aqueous phase. Apart from water, the aqueous phase contains in general additives which, for example, have been used in the preparation of the aqueous powder coat dispersions and/or which are meant to give to the aqueous powder coat dispersions properties which are advantageous for their technical application.
Aqueous powder coat dispersions, for example, containing non-ionic polymer thickeners, in particular non-ionic polyurethane associative thickeners, are known from WO 96/32452 and WO 96/37561. The sedimentation stability of these aqueous powder coat dispersions requires improvement, they require a permanent thorough mixing in order to avoid sedimentation phenomena. A more advantageous sagging behavior of the coating layers prepared from the aqueous powder coat dispersions is desirable from a technical application point of view.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide aqueous powder coat dispersions having higher sedimentation stability and reduced sagging tendency.
The object can be achieved by providing aqueous powder coat dispersions, the aqueous phase of which contains a silica compound selected from the group consisting of (1) finely divided silicic acid. with large Brunauer-Emmett-Tellersurface (“BET-surface”) and (2) layered silicates.
It is therefore an object of the invention to provide an aqueous powder coat dispersion comprising (a) a powder coating dispersed in (b) an aqueous phase, wherein the aqueous phase contains a silicate compound selected from the group consisting of (1) 0.2 to 2% by weight, based on the weight of the powder coating, of finely divided silicic acid with a large BET-surface, (2) 0.1-1.5% by weight, based on the weight of the powder coating, of layered silicates and (3) combinations thereof.
DETAILED DESCRIPTION OF THE EMBODIMENTS
The aqueous powder coat dispersions according to the invention are aqueous coating agents which comprise a solid content of, for example, 20 to 60% by weight in the state in which they are ready for application. The solid content in the dispersion is comprised of the solid non-volatile powder coating and the silica compound, and optionally additional non-volatile components contained in the aqueous phase (e.g., pigments). The expression “non-volatile” includes for example volatile components which might appear under the particular conditions used for applying and curing the aqueous powder coat dispersions according to the invention. Examples of such volatile components are evaporation losses or cleavage products produced during application, flash-off, flow and optionally curing of the coating layers applied from the aqueous powder coat dispersions according to the invention. The cleavage products can for example form during cross-linking reactions taking place under condensation.
A preferred embodiment of the aqueous powder coat dispersions according to the invention is represented by aqueous powder clear coat dispersions with a solid content, for example, of 20 to 45% by weight, preferably of 25 to 40% by weight, and which are suited for the preparation of transparent (i.e., clear) coating layers.
The powder coating component of the dispersion consists of fine powder coating particles of an average particle diameter (d
50
) of, for example, 0.1 to 10 &mgr;m, preferably below 5 &mgr;m. Any of the conventional powder coatings known in the art can be used in the powder coating dispersions of this invention. Such powder coatings comprise binders, or binder/cross-linking agent combinations. Optionally, pigments, fillers and additives which are conventionally used in connection with powder coatings may be added to the powder coatings.
Examples of binders that may be used include thermoplastics. It is preferred that the powder coatings which are dispersed in the aqueous powder coat dispersions according to the invention are duroplastic powder coatings based on self cross-linking binders or, particularly preferred, based on external cross-linking binder/cross-linking agent combinations.
In the case of binders/cross-linking agent combinations, binding agents are understood to mean film-forming, higher molecular weight components of a duroplastic powder coating which, for example, represents at least 50% by weight of the basic binder/curing agent combination, whereas the curing agent component, for example, represents at the most 50% by weight of this combination. No limitations are in principle imposed on the binding base of the binder. Binders which are conventionally used for powder coatings, for example, are suitable. Examples are: polyester resins, (meth)acrylic copolymers, epoxy resins, phenolic resins, polyurethane resins and siloxane resins. In the preferred embodiment of aqueous powder clear coat dispersions, polyester resins, polyurethane resins and, in particular, (meth)acrylic copolymers are preferred. The binders, for example, have a glass transition temperature in the range of 30 to 120° C., preferably below 90° C., and, for example, have average molecular weights (Mn) in the range of 500 to 20000, preferably below 10000. The curing agents, for example, have average molecular weights (Mn) from 84 to 3000, preferably below 2000. Various binders and curing agents can be mixed together.
Binders and curing agents carry functional, mutually complementary groups which allow a chemical cross-linking, for example, through addition and/or condensation and/or radical polymerization. Examples of functional groups are carboxyl groups, epoxy groups, aliphatic or aromatic bonded OH groups, isocyanate groups, blocked isocyanate groups, anhydride groups, primary and secondary amino groups, blocked amino groups, N-heterocyclic groups capable of cycle-opening additions, such as, for example, oxazolin groups, olefinically unsaturated groups, such as, for example, (meth)acryloyl groups and CH-acidic groups, such as, for example, acetoacetate groups.
The selection of the mutually reacting groups is well known to the person skilled in the art. Different reactive groups can optionally be combined. This can be done by using binders which carry different reactive functional groups, or mixtures of various curing agents and/or binders are used.
The different functional groups can be present at the same time on the binder and/or curing agent. The binders as well as the curing agents on average contain at least two functional groups per molecule. The ratio of binder to curing agent in general lies in the range of, for example, 98:2 to 50:50. The preferred ratio lies in the range of 95:5 to 70:30. The mixture can also comprise several binders or several curing agents.
As previously mentioned, in a preferred embodiment the aqueous powder coat dispersions according to the invention are aqueous powder clear coat dispersions. The powder clear coat particles dispersed therein contain, for example, binder/cross-linker combinations known from powder clear coats. These are, for example, combinations of hydroxy-functional binders, in particular hydroxy-functional (meth)acrylic copolymers, with blocked polyisocyanate curing agents or, in particular, combinations of epoxy-functional binders, in particular epoxy-functional (meth)acrylic copolymers, with carboxylic- and/or carboxylic acid anhydride-functional curing agents and/or tris(alcoxycarbonylamino)triazine curing agents.
Preferred examples of combinations of epoxy-functional (meth)acrylic copolymers with acidic, particularly carboxylic- and/or carboxylic acid anhydride-functional curing agents are those which contain glycidyl(meth)acrylate copolymers with an epoxy-weight equivalent of between 250 and 1000 as binders and one or more low molecular or polymer compounds having on average two

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