Chemistry: electrical and wave energy – Processes and products – Electrophoresis or electro-osmosis processes and electrolyte...
Reexamination Certificate
2000-08-21
2002-02-26
Mayekar, Kishor (Department: 1741)
Chemistry: electrical and wave energy
Processes and products
Electrophoresis or electro-osmosis processes and electrolyte...
C204S496000
Reexamination Certificate
active
06350358
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method of producing an anodic electro-dip lacquer coating (ADL) using an electro-dip lacquer coating bath (ADL bath) which is low in solvents or free from solvents, wherein it is not necessary to perform electrodialysis in the EDL bath in order to maintain the bath and coating parameters. Therefore, it is also not necessary to discard ultrafiltrate on a regular basis.
The principle of anodic electro-dip lacquer coating (ADL) is described in the literature and has been proven in practice. Even after the introduction of cathodic electro-dip lacquer coating (CDL), anodic electro-dip coating is still a widely used coating method, particularly for the coating of industrial products. This is due firstly to the large number of existing anodic coating installations, and secondly to the good quality of anodic coating materials which is achieved nowadays. Moreover, certain materials, such as aluminium for example, can be coated more advantageously using anodic rather than cathodic electro-dip lacquer compositions. In anodic electro-dip lacquer coating a workpiece having an electrically conducting surface comprising a metal or an electrically conducting plastics material or comprising a substrate which is provided with an electrically conducting coating is placed in an aqueous ADL bath and is connected as an anode to a source of direct current.
The ADL bath consists of an aqueous dispersion, e.g. a suspension or emulsion, or of an aqueous solution of one or more binder vehicles which have been made at least partially dispersible or soluble in water by salt formation with organic or inorganic neutralizing agents, and of pigments, extenders, additives and other adjuvant substances which are dispersed therein.
When a DC electric current is applied, the polymer particles of the aqueous dispersion of the ADL bath migrate to the anode and react again there with the ions formed during the electrolysis of water, which proceeds simultaneously, to form a water-insoluble polymer which coagulates from the aqueous phase and is deposited, with the additives dispersed therein, as a lacquer film on the anode (Metalloberfläche 31(1977) 10, pages 455 to 459).
The usual ADL baths are operated continuously, i.e. the substrates described above are immersed and coated in an electro-dip lacquer tank filled with coating medium. Solids are thereby dragged out of the ADL bath and neutralizing agent is released in the ADL bath at the same time. In order to maintain the coating parameters and the quality of the coating constant, it is necessary to add replenishment material with an increased solids content to the ADL bath in order to compensate for the dragged-out solids and in order to compensate for the neutralizing agent released in the ADL bath so as to maintain the desired MEQ value.
In principle, there are two compensating procedures which are employed in order to compensate for the solids dragged out of the ADL bath and to compensate for the neutralizing agent released. The added replenishment material with an increased solids content is neutralized to a lesser extent than the ADL bath, and the neutralizing agent which is released is required for the dispersion and homogenisation of the replenishment material in the ADL bath and is thereby consumed. Compensation can also be effected using completely neutralized replenishment material. However, the equipment costs are then increased, since the neutralizing agent which is released has to be removed by means of (electro)dialysis (Glasurit-Handbuch 1984, page 377 and Willibald Machu “Elektrotauchlackierung”, Verlag Chemie GmbH Weinheim/BergstraBe, 1974, page 166). The neutralizing agent which is released during the coating operation can also be removed by discarding ultrafiltrate on a regular basis. When compensation for the neutralizing agent which is released during the coating operation is effected by a replenishment material with a lesser degree of neutralization, the latter requires a high content, of up to about 15% by weight, of organic solvents, since it is otherwise unstable and its viscosity is too high, and it cannot be incorporated in the coating material, which can contain more than 90% water. Coating media of this type are described, for example, in DE-A-32 47 756.
In Farbe und Lack 103, Number 6/97, page 26, there is a reference to a new, environmentally friendly anodic single-component system (1 C system) for electro-dip lacquer coating. However, the replenishment paste, which is the form in which the replenishment material is supplied, still always contains 6% of organic solvents, and the bath still always contains 0.5% of organic solvents when it is in operation.
However, high solvent contents are undesirable on account of the pollution of outgoing air and waste water, wherein the total usage of substances is calculated based on legal regulations. In order to remove the neutralizing agent which is released during the coating operation, the cathodes in the ADL bath can also be accommodated in re-flushable dialysis cells (electrodialysis) and the neutralizing agent which is formed there can be discarded, or the coating material can be subjected, continuously or discontinuously, to an ultrafiltration step, with the ultrafiltrate which is thus produced being discarded on a regular basis. Electrodialysis devices of this type are not used in most ADL baths, on account of increased capital costs and higher maintenance and inspection costs. Moreover, regularly discarding ultrafiltrate or dialysate results in an increased cost of waste water processing and is therefore undesirable. The make-up of electro-dip lacquer baths with completely neutralized material, consisting of one or two components, is known from the literature (Glasurit-Handbuch 1984, page 377) and is described there using cathodic electro-dip lacquer coating as an example. As mentioned above, however, the use of electrodialysis and the discarding of dialysate is absolutely necessary in the procedure described there.
The object of the present invention was therefore to provide a method of producing an aqueous coating composition, which is low in solvents or free from solvents, for anodic electro-dip lacquer coating, for which, when it is used for the coating of conductive substrates in an ADL bath, it is not necessary to remove neutralizing agent which is released during the coating operation by an electrodialysis device in order to maintain the bath and coating parameters, and a considerable amount of ultrafiltrate does not have to be discarded on a regular basis.
Surprisingly, this object has been achieved by the use of an anodic replenishment material consisting of a pigment-free aqueous binder vehicle component and a pigment-containing aqueous paste resin component in order to compensate for the coating material consumed during electro-dip lacquer coating and for the neutralizing agent which is released at the same time, which anodic replenishment material is under-neutralized to an extent such that when added to the ADL bath it compensates for the neutralizing agent released there, and which nevertheless only contains small amounts of organic solvents.
SUMMARY OF THE INVENTION
Therefore, the present invention firstly relates to a method for anodic electro-dip lacquer coating, wherein coating medium which is consumed in an anodic electro-dip bath is compensated for by an under-neutralized anodic replenishment material, which is characterised in that the replenishment material comprises
A) a pigment-free aqueous binder vehicle component with a solids content of 40 to 70% by weight, an MEQ value of 15 to 40 and a content of organic solvent of ≦0.5% by weight, and
B) a pigment-containing aqueous paste resin component with a solids content of 60 to 75% by weight, an MEQ value of 5 to 15 and a content of organic solvent of ≦1.0% by weight,
wherein A) and B) are present in a ratio by weight of 1:1 to 4:1 and the mixture of A) and B) has a solids content of 45 to 73% by weight, a solvent content of ≦0.75% by weight and a
Ehmann Eduard
Evers Manfred
Göbel Armin
Markou Konstadinos
Vogt-Birnbrich Bettina
Benjamin Steven C.
E. I. du Pont de Nemours and Company
Mayekar Kishor
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