Stock material or miscellaneous articles – Composite – Of aldehyde or ketone condensation product
Reexamination Certificate
2000-01-17
2002-01-08
Copenheaver, Blaine (Department: 1773)
Stock material or miscellaneous articles
Composite
Of aldehyde or ketone condensation product
C428S480000, C428S500000, C428S502000, C428S522000, C428S532000, C427S407100, C427S409000
Reexamination Certificate
active
06337139
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a process for producing a multicoat finish on a substrate surface by successive application of an aqueous basecoat and of an aqueous topcoat and the subsequent simultaneous baking of the basecoat film and of the topcoat film, said aqueous basecoat comprising hydrophobicized amino resins emulsified with hydroxyl-containing emulsifiers.
BACKGROUND AND SUMMARY OF THE INVENTION
In automotive finishing in particular, though also in other sectors where there is a desire for coatings having a good decorative effect and at the same time affording good protection against corrosion, it is known to provide substrates with two or more coating films arranged atop one another.
Multicoat paint systems are preferably applied in accordance with the so-called basecoat/clearcoat process; that is, a pigmented basecoat is applied first and following a short flash-off time without a baking step (wet-on-wet technique) is coated over with a clearcoat. Subsequently, basecoat and clearcoat are baked together. The basecoat/clearcoat process has acquired particular importance in connection with the application of automotive topcoats, especially in the case of metallic paints.
Economic and environmental considerations have resulted in attempts to use aqueous basecoats for the production of multicoat finishes.
The coating compositions for producing these basecoat films must be able to be processed by the nowadays customary, rational wet-on-wet technique; that is, following a short initial drying period without a baking step, they must be overcoated with a transparent topcoat film without exhibiting disruptive etching and “strike-in” phenomena.
DE-A-40 28 386 describes a process for producing a multicoat finish on a substrate surface, in which (1) as the basecoat, a water-thinnable basecoat is applied which comprises water, organic solvents if desired, a water-dilutable binder, pigment particles, and a polyamide as Theological assistant, (2) a polymer film is formed on the surface from the composition applied in stage (1), (3) a suitable transparent topcoat is applied to the resulting basecoat film, and subsequently (4) the basecoat film is baked together with the topcoat film. As the topcoat it is possible to use conventional solventborne clearcoats, water-thinnable clearcoats, or transparent powder coating materials.
DE-A-42 16 613 embraces a process for producing a two-coat paint system on a substrate surface, in which (1) a pigmented aqueous basecoat comprising as its binder a water-dilutable (meth)acrylate emulsion polymer prepared in the presence of specific emulsifiers is applied to the substrate surface, (2) a polymer film is formed from the basecoat applied in stage (1), (3) a transparent topcoat is applied to the resulting film, and subsequently basecoat film and topcoat film are baked together. As the topcoats, it is again possible to use conventional solventborne clearcoats, aqueous clearcoats, or transparent powder coating materials.
In the case where aqueous basecoats, in accordance, for example, with DE-A-40 28 386 or DE-A-42 16 613, are used simultaneously with aqueous topcoats in the basecoat/clearcoat process (wet-on-wet technique) great problems occur, especially when using aqueous slurries of transparent powder coating material as the topcoat, since the aqueous basecoat films, which have undergone only initial drying for a short period, when coated with the aqueous topcoat withdraw water from it and are themselves in turn partially dissolved. As a consequence of the partial dissolution of the aqueous basecoat film and of the attendant film softening, and as a consequence of the withdrawal of water from the drying aqueous topcoat, the initial drying and baking of the basecoat/topcoat system are accompanied by cracks over the entire surface—a phenomenon also known as “mud cracking”. A further severe problem is the tendency of aqueous basecoats toward popping in the course of baking.
DE-A-42 22 194 describes a process for producing two-coat paint systems on a substrate surface, in which (1) a pigmented, solvent- or waterborne basecoat is applied to the substrate surface, (2) a polymer film is formed from the paint applied in stage (1), (3) a transparent powder topcoat is applied to the resulting basecoat film, said topcoat comprising an epoxy-functional polyacrylate resin as its binder, and subsequently (4) the basecoat film is baked together with the transparent powder coating film.
A fundamental problem of DE-A-42 22 194 is the need for different application methods for the basecoat, which is generally applied by spraying or dipping, and for the transparent powder coating material, which is generally applied by means of electrostatic techniques. Furthermore, DE-A-42 22 194 allows solventborne basecoats, which again is objectionable from the environmental standpoint.
The object of the present invention, therefore, was to provide suitable basecoat compositions with which processes for multicoat finishing in accordance with the so-called basecoat/clearcoat process using the wet-on-wet technique, in which both basecoat and topcoat are aqueous systems and can be applied to the substrate using the same application method, can be conducted in such a way that the partial dissolution of the aqueous basecoat film and the attendant film softening by the aqueous topcoat play a minor part, that no cracks appear in the basecoat/topcoat system (mud cracking) in the course of initial drying and/or in the course of baking, and, in particular, that the tendency toward popping in the basecoat film during the baking step is reduced.
The invention accordingly provides a process for producing a multicoat protective and/or decorative finish on a substrate surface, comprising the following steps:
(1): applying an aqueous basecoat A, comprising a hydrophobicized amino resin as crosslinker, said amino resin being stably emulsified in the aqueous phase by means of a hydroxyl-containing emulsifier, and comprising at least one hydroxyl-containing binder present in aqueous solution or in aqueous emulsion, to the substrate surface,
(2): forming a polymer film from the aqueous basecoat A applied in stage (1),
(3): applying an aqueous transparent topcoat B to the resulting basecoat film, and finally
(4): baking the basecoat film and the topcoat film together.
In one preferred embodiment of the invention the hydroxyl-containing emulsifier for the hydrophobic amino resin, which is preferably a melamine-formaldehyde condensate etherified with C3 to C12 alcohols, is an oligomeric and/or polymeric diol and/or polyol having an emulsifying action, with particular preference a diol and/or polyol from the group consisting of polyacrylate diols and/or -polyols, polyesterdiols and/or -polyols and polyetherdiols and/or -polyols, and, with very particular preference, from the group consisting of polyurethanediols and/or -polyols and polycarbonatediols and/or -polyols.
The hydroxyl-containing binder in the basecoat A is preferably selected from the group consisting of polyacrylates, polyurethanes, polyesters, polyethers, alkyd resins and cellulose derivatives.
In a further preferred embodiment of the invention the aqueous topcoat B is an aqueous one-component and/or two-component clearcoat or, with particular preference, a powder slurry clearcoat.
DETAILED DESCRIPTION OF THE INVENTION
The Basecoat A
In accordance with the invention, the basecoat A comprises, as the crosslinking agents, hydrophobicized amino resins emulsified stably in the aqueous phase by means of hydroxyl-containing emulsifiers.
The hydrophobic amino resins, which are known per se, are preferably condensates of aldehydes, especially formaldehyde, and, for example, urea, guanamine, benzoguanamine, and preferably melamine.
The hydrophobic amino resins contain alcohol groups, preferably methylol groups, which are etherified preferably with C3 to C12 alcohols, examples being isopropanol, n-butanol, isobutanol, tert-butanol, amyl alcohol, 1-hexanol, 1-octanol, 2-ethylhexanol, 1-decanol, isodecanol, 1-dodecanol and isododeca
BASF Coatings AG
Copenheaver Blaine
Paulraj Christopher
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