Chemistry: electrical and wave energy – Processes and products – Electrophoresis or electro-osmosis processes and electrolyte...
Reexamination Certificate
1998-09-11
2001-07-03
Gorgos, Kathryn (Department: 1741)
Chemistry: electrical and wave energy
Processes and products
Electrophoresis or electro-osmosis processes and electrolyte...
C204S487000, C204S488000, C205S120000
Reexamination Certificate
active
06254751
ABSTRACT:
The present invention relates to a process for the multilayer coating of substrates with a primer coat of electrodeposition coating material and with a topcoat of powder coating material.
The coating of first and foremost electrically conductive substrates with an electrodeposition coating material is a process which has been common for many years. The electrodeposition coating material in this process is present as an (aqueous) dispersion in a bath. The substrate to be coated is connected as one of two electrodes and is lowered into this bath. This is followed by the electrophoretic deposition of the electrodeposition coating material on the substrate. After a sufficiently thick coat of material has been obtained, the coating operation is ended and the coat of material is dried and, generally, baked.
Resins which can be electrodeposited at the cathode are described, for example, in U.S. Pat. No. 3,617,458. They comprise crosslinkable coating compositions which deposit themselves at the cathode. These coating compositions are derived from an unsaturated addition polymer which comprises amine groups and carboxyl groups and from an epoxidized material.
U.S. Pat. No. 3,663,389 describes cationically electrodepositable compositions which are mixtures of specific amine-aldehyde condensates and a large number of cationic resinous materials, one of these materials being preparable by reacting an organic polyepoxide with a secondary amine and solubilizing the product with acid.
U.S. Pat. No. 3,640,926 discloses aqueous dispersions which can be electrodeposited at the cathode and consist of an epoxy resin ester, water and tertiary amino salts. The epoxy ester is the reaction product of a glycidyl polyether and a basic unsaturated oleic acid. The amine salt is the reaction product of an aliphatic carboxylic acid and a tertiary amine.
Epoxy- and polyurethane-based binders for use in binder dispersions and pigment pastes are, moreover, known in numerous configurations. Reference may be made, for example, to DE-27 01 002, EP-A-261 385, EP-A-004 090 and DE-C 36 30 667.
The coating of substances with powder coating materials is also a common process. In this case, the dry, pulverulent coating material is applied uniformly to the substrate that is to be coated. Subsequently, through heating of the substrate, the coating material is melted and baked. The particular advantages of powder coating materials are, inter alia, that they manage without solvents and that the overspray losses which occur with conventional coating materials are avoided, since virtually all of the nonadhering powder coating material can be recycled. The powder coating is applied to the substrate preferably by electrostatic adhesion, generated through the application of high voltage or by frictional charging.
Combination coating with electrodeposition coating material and powder coating material is also known from the prior art. For example, in accordance with DE-C 4313762, a powder coat is first of all sintered on and then an electrodeposition coating material is applied. It is also known, from JP 63274800, to apply an electrodeposition coating material and to dry it at 110° C., to apply a powder coating material, and, finally, to jointly bake both coats. This two-coat or multicoat system enables the product properties to be optimized. Priming with electrodeposition coating material may also become necessary in the case of substrates which, for technical reasons related to their material or on geometric grounds, are relatively unaminable to powder coating material. A typical application of this multicoat system is the coating of heating-system radiators. The procedure here is such that, following the coating of the substrate with the electrodeposition coating material, said coating material is first baked in a drier. The temperatures in the drier typically reach more than 100° C., and the electrodeposition coating material sets. Following this baking operation, the primed substrate is cooled again before then being provided with the powder coat. A second baking operation is then necessary to cure the applied powder coating material. The disadvantage of this procedure is that the substrate has to be twice dried and heated during the coating operation. This is very energy-intensive, and entails considerable capital and operating costs.
Against the background of this prior art, the invention has set itself the object of developing a process for the multilayer coating of substrates with electrodeposition and powder coating materials which operates more simply, more cost-effectively and with greater energy savings while maintaining identical product qualities. This object is achieved in accordance with the invention by a process in which
a) to a substrate (
1
) made preferably of metal, especially iron or zinc, at least one coat (
2
) of liquid coating material, preferably electrodeposition coating material, is applied,
b) after deposition the substrate (
1
) is, if desired, wholly or partially dried,
c) at least one coat of powder coating material (
3
) is applied, and
d) electrodeposition coating material and powder coating material are jointly baked,
where drying takes place at temperatures of ≦100° C., preferably ≦40° C.
The process of the invention therefore omits a separate drying and baking step for the electrodeposition coating material before the powder coating material is applied. Instead, both coating materials are baked in a joint step. This approach represents a considerable simplification of the coating operation. The omission of one baking operation reduces both the capital costs and the operating costs. Only a single baking oven needs to be provided and operated. As a result, there is also a saving of heating energy. In addition, the overall processing time for the coating operation is shorter, and so the productivity of the unit is increased.
Since the substrate to be coated is preferably preprimed with an electrodeposition coat, said substrate is principally an electrically conductive substrate. In particular, it can be a metal, preferably iron or zinc.
In step a), in accordance with the invention, a liquid coating material is applied to the above-described substrate. This can be done using all coating techniques known in the prior art.
As the coating material it is possible to use all liquid coating materials which are known in the art. Suitable in particular are all customary aqueous electrodeposition coating materials. It is possible, for example, to use electrodeposition coating materials which comprise epoxy resins, which are preferably amine-modified, and/or blocked aliphatic polyisocyanate, pigment paste and, if desired, further additives.
In a preferred embodiment of the process of the invention the electrodeposition coat, following removal of the substrate from the bath, is predried, preferably by air drying with the aid, for example, of a fan. The air may preferably be dry air, e.g. compressed air.
Simultaneously with the drying operation, gentle heating of the substrate is performed in the course of which, however, flow or baking of the coating material must be avoided. The primary aim, rather, is—when using the customary aqueous electrodeposition coating materials—to remove the film of water remaining thereon. For this reason, temperatures of ≦100° C. are preferred. Preferably, temperatures of ≦80° C., with particular preference ≦60° C. and, most preferably, of ≦40° C. should be observed.
The drying operation extends over a period of not more than 60 minutes. The drying time is preferably ≦40 minutes, with particular preference ≦30 minutes and, most preferably, ≦20 minutes.
The predrying of the electrodeposition coat is preferably performed until its content of solvents has fallen such that on subsequent baking the substance of the coat decreases by less than 20%, preferably less than 13%, this is because, when baking an electrodeposition coat, there is always a loss of substance through the evaporation of residual solvents and through the emission of elimination produc
Boysen Rolf
Brucken Thomas
Rademacher Josef
Reiter Udo
BASF Coatings AG
Gorgos Kathryn
Maisano J.
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