Aqueous ruthenium chloride solution for blackening ceramic surfa

Details Aqueous ruthenium chloride solution for blackening ceramic surfa Aqueous ruthenium chloride solution for blackening ceramic surfa

1998-04-30

2000-03-28

Beck, Shrive

Coating processes

Heat decomposition of applied coating or base material

427299, 427336, B05D 302

Patent

active

060428848

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to mixtures for blackening ceramic products by using aqueous solutions of ruthenium compounds.
2. Description of the Related Art
It has been disclosed that ruthenium chloride can be used in an aqueous solution for dyeing porcelain gray-black. During this process, a strongly acid aqueous solution of the ruthenium chloride is applied to the ceramic body and is burnt in; during this process, the chloride decomposes and the ruthenium oxide is left in superfine form and combines with the silicic acid or alumina of the ceramic bodies while forming color pigments (see Ullmann, Enzyklopadie der technischen Chemie [Encyclopedia of Technological Chemistry], 2nd edition, volume 4, pages 837 and 838).
The Czech patent application CS 248 541 B1 discloses that ruthenium oxide in zirconium silicate crystal lattices results in a gray-black color, so that such compounds can be used as black pigments.
Furthermore, EP 0 704 411 A1 describes that ruthenium salt or ruthenium complexes with polycarboxylic acids result in a dark gray coloration (identified as "black") if the application takes place on the ceramic body by first pretreating (activating) the ceramic body with water, then dying it with the ruthenium solution and subsequently aftertreating it with water. But this method also leads only to comparatively dark but not genuinely black colorations.
Therefore, the problem arose to find means with which ceramic surfaces can be dyed jet black in a simple and economical manner.


SUMMARY OF THE INVENTION

This object is solved by the features of the main claim and is advanced by the features of the dependent claims.
The solutions according to the invention are comprised of an aqueous solution of commercially available ruthenium chloride, which solutions normally comprise a certain excess of the mineral acid for stabilization. This aqueous solution is buffered with sodium or potassium acetate or propionate to a pH of >1.5, in particular >2, while stable solutions are also obtained over rather long time periods.
Here, the solution is set to a ruthenium concentration of 0.1-10 weight %; depending on the excess of mineral acid, correspondingly large amounts of, for example, sodium acetate are required for buffering; normally, concentrations which are also in the range from 0.1-10 weight % are used.
The solutions according to the invention are applied to the ceramic bodies to be dyed in the usual manner by spraying, dipping, painting, printing, etc., with amounts of 0.1-5 g/m.sup.2 of surface being sufficient for the coloration.
After drying, the ceramic bodies are fired in a suitable kiln at a temperature between 1300 and 1400.degree. C. For the decomposition of the ruthenium compounds, temperatures of 800-1200.degree. C., in particular 1140.degree. C., are preferred. The firing duration amounts to between one half hour and 5 hours, preferably between 1 and 2 hours. During this time, the ruthenium compounds are pyrolyzed and ruthenium dioxide is formed in finely distributed form. The color effect can be promoted in a known manner through suitable mineralizers such as alkali fluorides and alkali chlorides, fluorosilicates, etc. as well as through the addition of molybdenum compounds or tungsten compounds.
The color effect obtained after firing is determined objectively with the assistance of an La*b* system. In this system, "L" stands for the luminosity, while "a*" and "b*" indicate the tint as well as the color saturation. In this context, "a*" designates the position on a red-green axis, and "b*" the position on a yellow-blue axis.
Up until now it has not been possible to determine what the substantial color intensification of the buffered systems according to the invention is based on compared to unbuffered systems. Possible explanations might be that the particle shape and/or particle size of the deposited ruthenium oxides from the buffered solution is different than in the unbuffered solution. Another possibility might be that a certain

REFERENCES:
patent: 4927671 (1990-05-01), Nawothnig
patent: 5032452 (1991-07-01), Nowitzki et al.
Ullmann, F., Enzyklopadie der technischen Chemie [Encl. of Tech. Chem.], 2nd ed., vol. 4, 1929, pp. 837-838.

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