Coating processes – Solid particles or fibers applied – Uniting particles to form continuous coating with...
Reexamination Certificate
2000-04-28
2002-03-12
Parker, Fred J. (Department: 1762)
Coating processes
Solid particles or fibers applied
Uniting particles to form continuous coating with...
C427S202000, C427S409000, C428S425800, C428S480000
Reexamination Certificate
active
06355303
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a process for the preparation of multi-layer coatings of the water-borne base coat/powder clear coat type.
BACKGROUND OF THE INVENTION
In the automotive coating sector environmentally friendly, low-solvent coating processes are becoming increasingly established. A process which causes particularly low solvent emissions is the preparation of colour- and/or effect-imparting base coat/clear coat two-layer coatings of water-borne base coat and powder clear coat. A problem during the preparation of such water-borne base coat/powder clear coat two-layer coatings is the formation of optically troublesome inclusions of gas bubbles in the powder clear coat layer and burst gas bubbles discernable in the powder clear coat surface as pinholes, hereinafter collectively referred to as blisters.
SUMMARY OF THE INVENTION
The present invention solves this problem by the process for multi-layer coating of a substrate, comprising forming a base coat layer of a water-borne base coat on said substrate, applying a clear coat layer of a powder clear coat on said base coat layer, and jointly baking both layers, said water-borne base coat containing an effective amount of at least one water-insoluble cellulose ester to reduce blistering in said clear coat layer during said joint baking.
The base coat layer is formed on the substrate by applying the water-borne base coat to the substrate and drying the resultant wet layer. The water-borne base coat contains resin solids which include one or more binder resins. Preferably, the amount of water-insoluble cellulose ester present in the water-borne base coat is 0.5 to 10 wt %, based on the resin solids in the water-borne base coat.
The blistering problem described above arises with increasing thickness of the base coat layer and/or of the clear coat layer, which thickness arises either from the need for a thick coat to produce the desired coating effect or from thin coat(s) which inevitably become thick in certain areas during application because of the shape of the substrate being multi-layer coated. The process of the present invention enables relatively thick base coat and clear coat layers to be formed without blistering, as compared to when the water-insoluble cellulose ester is not present in the water-borne base coat.
DETAILED DESCRIPTION OF THE EMBODIMENTS
The water-borne base coats used in the process according to the invention contain anionically and/or nonionically stabilised binder resin. Anionic stabilisation is achieved preferably by means of at least partially neutralised carboxyl groups in the binder resin, whereas nonionic stabilisation is achieved preferably by means of lateral and/or terminal polyethylene oxide units in the binder resin. Carboxyl groups are contained in the binder resin, for example, equivalent to an acid value from 5 to 40mg KOH/g of solid resin. The water-borne base coats may be of a physically drying nature or cross-linkable with the formation of covalent bonds. The water-borne base coats cross-linking with the formation of covalent bonds may be self- or externally cross-linking systems. In the latter case they may be one- or multi-component water-borne base coats.
Neither the binder resin contained in the water-borne base coat(s) nor the optionally contained cross-linking agents are subject to any restriction. The film-forming binder resin includes at least one film-forming, water-dilutable resin (water soluble or water dispersible) for water-borne base coats, well known to the skilled person, such as water-dilutable polyester resin, (meth)acrylic copolymer resin, polyurethane resin, (meth)acrylated polyester resin and/or (meth)acrylated polyurethane resin. Each of these resins can be present as a plurality of resins, e.g. a combination of polyester resins, or as a combination among the different resins, e.g. polyurethane resin and (meth)acrylated polyurethane. The (meth)acrylated polyester resins or (meth)acrylated polyurethane resins are binders in which (meth)acrylic copolymer and polyester resin or (meth)acrylic copolymer and polyurethane resin are linked together by covalent bonding and/or in the form of interpenetrating resin molecules. The preparation of the above-mentioned binders is well known to the skilled person and may be derived, for example, from the extensive patent literature in the field of water-borne base coats and water-borne base coat binders and does not, therefore, require any further explanation. Water-dilutable polyurethane resins and/or water-dilutable (meth)acrylated polyurethane resins preferably account for a proportion from 10 to 100 wt. % of the resin solid content of the water-borne base coats.
The water-dilutable binders may be non-functional or reactive resins having, for example, hydroxyl functions corresponding to a hydroxyl value from 20 to 140 mg KOH/g solid resin. Examples of cross-linking agents which may be contained include amine-formaldehyde condensation resins, e.g., melamine resins, and free or blocked polyisocyanates. If cross-linking agents are present, the mixing ratio of cross-linking agents to binders is preferably 10:90 to 40:60, particularly preferably 20:80 to 30:70, in each case based on the solids weight.
The water-borne base coats may also contain paste resins. Paste resins are used to grind pigments and may therefore be introduced into the water-borne base coats by means of pigment pastes.
The resin solids of the water-borne base coats are composed of the sum of the solids contributions of the binder resins, the optionally present cross-linking agents and the optionally present paste resins. If the binder resin is water soluble, it is nevertheless counted as a resin solid, because it becomes solid in the layer formed from the base coat. The proportion of the solids content of the water-borne base coats formed by water-insoluble cellulose esters is not counted as belonging to the resin solid content.
Most preferably, the water-borne base coats contain 0.5 to 7 wt % of at least one water-insoluble cellulose ester, based on their resin solid content. The water-insoluble cellulose esters are, for example, cellulose acetopropionate or cellulose acetobutyrate, which are both sold commercially, for example, by the company Eastman, in various water-insoluble variants with different hydroxyl, acetyl and propionyl or butyryl contents. Cellulose acetobutyrate is preferred, particularly with high butyryl contents from 35 to 55 wt. % based on the weight of the cellulose ester.
The water-insoluble cellulose ester is preferably introduced into the water-borne base coat by means of one or more water-dilutable binder resins, i.e. as a pre-mixture therewith. For example, the above-mentioned water-dilutable polyester resins, (meth)acrylic copolymer resins, (meth)acrylated polyester resins, polyurethane resins and/or (meth)acrylated polyurethane resins are suitable and can be used for this purpose. Water-dilutable polyurethane resins and (meth)acrylated polyurethane resins are particularly preferred. In order to introduce the water-insoluble cellulose esters into the water-borne base coats, the binder resins are mixed, before adding substantial amounts of water, with an appropriate amount of water-insoluble cellulose ester, for example, in a binder:water-insoluble cellulose ester solids weight ratio from 99.5:0.5 to 60:40 and then converted to the disperse phase by adding water, optionally followed by mixing with additional binder resin to obtain the proportion of celluose ester desired. If the water-insoluble cellulose esters are insufficiently miscible with the binders still present in the water-free form or insufficiently soluble therein, it is expedient to mix the water-insoluble cellulose esters in the form of an organic solution with the binder resin still present in the water-free form. Examples of organic solutions of the water-insoluble cellulose esters which may be used include those containing from 20 to 60 wt. % of cellulose esters in polar organic, preferably water-miscible solvents. Examples of suitable solvents are es
Bosch Werner
Goebel Armin
Mueller Frank
Schmidt Holger
Vogt-Birnbrich Bettina
Benjamin Steven C.
E. I. du Pont de Nemours and Company
Parker Fred J.
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