Coating processes – With post-treatment of coating or coating material – Heating or drying
Reexamination Certificate
1996-12-20
2001-10-30
Short, Patricia A. (Department: 1712)
Coating processes
With post-treatment of coating or coating material
Heating or drying
C427S385500, C427S393500, C523S402000, C523S412000, C523S415000, C523S416000, C523S417000, C524S507000, C524S512000, C524S513000, C524S539000, C524S542000, C524S558000, C524S591000, C524S598000, C524S604000
Reexamination Certificate
active
06309707
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a coating composition comprising at least three components and based on a hydroxyl- and carboxyl-containing polymer which is dissolved or dispersed in one or more organic solvents, and on an isocyanato-containing crosslinking agent.
The present invention relates, furthermore, to a process for the preparation of these coating compositions and to the use of the coating compositions in refinishing, for the coating of plastics, and as topcoat or filler.
BACKGROUND AND SUMMARY OF THE INVENTION
Coating compositions based on hydroxyl-containing binders and isocyanato-containing crosslinking agents, which are employed in the field of automotive refinishing and are conventional, ie. are dissolved in organic solvents, customarily represent multicomponent systems. One component contains the binder, if desired pigments and fillers, and conventional auxiliaries and additives, and the other component contains the crosslinking agent. In addition, an adjustment additive consisting essentially of a mixture of different solvents is prepared in order to adjust the coating composition to the desired viscosity.
However, it is the aim of the coatings industry, on ecological and economic grounds, to replace as great as possible a proportion of the organic solvents employed in coating materials by water. For the coating of motor vehicles, in particular, there is a great need for aqueous coating materials. This applies both to the area of automotive production-line finishing and to the area of automotive refinishing.
In this context aqueous coating compositions are employed in particular in the area of topcoats. Topcoats are understood here as being coating materials which are used to produce the topmost coat. This topmost coat may comprise one or more layers, especially two layers. Two-layer topcoats are composed of a pigmented basecoat layer and of a clearcoat layer which is pigmented either not at all or only with transparent pigments and is applied over the basecoat layer.
Two-layer coatings are currently produced by the wet-on-wet method, in which a pigmented basecoat is initially applied and the resulting basecoat layer, without a baking step, is covered with a clearcoat, and then basecoat layer and clearcoat layer are cured together. This method is very advantageous in economic terms, but places stringent requirements on the basecoat and the clearcoat. The clearcoat which is applied to the as yet uncured basecoat must neither partially dissolve nor otherwise disturb the basecoat layer, lest coatings of poor appearance be obtained. This applies in particular to coatings in which basecoats containing special-effect pigments (eg. metallic pigments, especially aluminum flakes or pearlescent pigments) are employed. Furthermore, the topcoat compositions must be capable of being applied by spraying using automatic spraying units. For this purpose their solids content at spray viscosity must be high enough for coating films of adequate thickness to be obtained with 1 to 2 spray passes (cross-passes), and they must give baked coating films of good appearance (good evenness, high gloss, low tendency to solvent boil, good topcoat appearance and a high degree of hardness) and good weathering resistance.
In the area of automotive refinishing there is the additional requirement that the coating compositions employed are able to cure fully at low temperatures (generally<80° C.) and lead, even when fully cured at these low temperatures, to films having the good mechanical properties required. polyurethane coating compositions which comprise a hydroxyl-containing polyacrylate resin, dispersed in water, and a polyisocyanate component. However, these coating materials described in EP-B-358 979 exhibit great disadvantages with respect to gloss, relaxation (low graininess of the coating), tendency to solvent boil and, because of this, spray safety, and in respect of weathering resistance, in particular with respect to the resistance in a constant humid climate (40° C., 240 h), processability (fall in viscosity and too short a pot life) and hardness.
Furthermore, DE-B 25 07 884 discloses a process for the preparation of aqueous coating compositions, in which first of all a solution of a carboxyl-containing polyacrylate and/or polyester in an organic solvent is prepared, if desired crosslinking agents, and also pigments and fillers, are dispersed in the organic solution, and the resulting dispersion, following neutralization of the carboxyl groups of the binder, is dispersed in water.
A disadvantage of this process is that the organic solvent has to be removed by azeotropic distillation after preparation of the aqueous dispersions. This additional process step results in additional cost. Moreover, this distillative removal of the solvent can only be carried out at the premises of the manufacturer of the coating compositions, and not at those of the customer. It is therefore necessary to transport and store the finished, aqueous coating materials, which frequently leads to problems especially in the area of automotive refinishing, where in fact very long shelf-lives of at least 24 months are required.
The crosslinking agents employed to prepare the coating compositions described in DE-B 25 07 884 are blocked polyisocyanates, epoxy resins and/or amino resins. These coating compositions described therein therefore cure only at elevated temperatures of between 100 and 300° C., and are therefore unsuitable for the area of refinishing.
Finally, EP-A-368 499 discloses aqueous coating compositions which contain, as hydroxyl-containing binders, polyethers or ether group-containing polyesters, and, as crosslinking agents, amine/formaldehyde resins. These coating compositions are prepared by first of all preparing the binder and the crosslinking agent in an organic solvent. This mixture is admixed shortly before the application of the coating composition with a curing catalyst, and is adjusted with water to the desired viscosity.
These aqueous coating compositions known from EP-A-368 499, however, have the disadvantage of completely inadequate compatibility of the individual components of the coating composition. They are therefore not suitable for the production of clearcoats. In addition, these coating compositions cure only at elevated temperatures and are therefore unsuitable for the area of refinishing. A final, additional disadvantage is the inadequate weathering stability of the resulting coatings.
The object of the present invention is therefore to provide an aqueous coating composition which, in relation to the aqueous two-component polyurethane coating compositions known from EP-B-358 979, has improved properties and/or gives improved coating films. The novel coating composition should above all ensure, in comparison with aqueous coating compositions based on aqueous acrylate copolymer dispersions, improved evenness, improved gloss, improved relaxation (low graininess), lower tendency to solvent boil, enhanced spray safety, improved fullness and an improved weathering resistance of the resulting coatings.
Furthermore, the novel coating composition should be suitable for the area of automotive refinishing, ie. should be fully curable at low temperatures (generally <~80° C.) and should lead to coatings which at the very least meet the requirements which are customarily placed on an automotive refinish. The coating composition should therefore, for example, exhibit good gloss (≧85 E at 20° ), a high degree of hardness (≧110 impacts) and good weathering resistance (≦m1g1 after 10 days in a constant climate).
This object has surprisingly been achieved by the coating composition of the type mentioned at the outset, which is characterized in that
1.) component (I) contains as binder (A)
(A1) at least one hydroxyl- and carboxyl-containing acrylate copolymer (A1) which is dissolved or dispersed in one or more organic, water-dilutable solvents and has a number-average molecular weight of between 1000 and 30,000, an OH number of from 40 to 200 mg of KOH/g an
Mayer Bernd
Meisenburg Uwe
Nienhaus Egbert
BASF Coatings AG
Short Patricia A.
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