Compositions: coating or plastic – Materials or ingredients – Pigment – filler – or aggregate compositions – e.g. – stone,...
Reexamination Certificate
1999-12-02
2001-09-25
Green, Anthony (Department: 1755)
Compositions: coating or plastic
Materials or ingredients
Pigment, filler, or aggregate compositions, e.g., stone,...
C106S400000, C423S409000, C524S408000, C501S017000
Reexamination Certificate
active
06294009
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a process for producing tantalum(V) nitride pigments which are notable for a higher a* color value in the CIE Lab system than predisclosed tantalum(V) nitride pigments. The invention furthermore relates to-tantalum(V) pigments having an a* color value according to CIE Lab of equal to/greater than +45; such pigments are obtainable by the process according to the invention. A further subject of the application relates to the use of the pigments.
2. Description of Related Art
There is a need to replace known pigments which may liberate, during use or during waste disposal, for instance in refuse combustion, toxic or ecologically harmful heavy metals by those pigments which are safe. Such a safe pigment is tantalum(V) nitride whose color is reddish-orange to red. The tantalum(V) nitrides obtainable by known processes have too low an a* color value in the CIE Lab system (DIN 5033, Part 3) and, in addition, appear slightly grayish or brownish. In accordance with H. Moureau and C. H. Hamplet (J. Amer. Chem. Soc. 59, 33-40 (1937)), tantalum(V) nitride can be produced by ammonolysis of TaCl
5
; the reaction times are very long and, in addition, a coloristically attractive product is not obtained. According to G. Brauer and J. R. Weidlein (Angew. Chem. 77, 218-219 (1965)), tantalum(V) oxide can be nitrided at 860 to 920° C. with ammonia. In copying the above-mentioned process, the inventors of the present application found that, despite prolonging the reaction time, no further color displacement from reddish brown to red takes place so that this product is also coloristically less attractive.
According to the process described in DE-OS 42 34 938, tantalum(V) nitride can be obtained by nitriding a tantalum(V) oxide hydrate at 750 to 950° C. with a shorter reaction time. The a* color values obtainable by this process on the basis of the test method described in said document was around +40. The a* color value was increased only by 2-3 units by thermal treatment of the Ta
3
N
5
at 300° C. From a coloristic point of view, on the other hand, as high as possible an a* color value is desired, in particular a value above 45 and, particularly preferably, above 50.
SUMMARY OF THE INVENTION
Accordingly, the object of the present invention is to provide tantalum(V) nitride pigments having an a* color value according to CIE Lab (DIN 5033, Part 3) measured on a PVC film gelled for 10 minutes at 140° C. and having a pigment content of 26% by weight. A further object relates to providing a process for producing such pigments. A further object is to improve the known process for producing tantalum(V) nitrides by nitriding a tantalum(V) oxide, tantalum(V) oxide hydrate or tantalum(V) oxide nitride with ammonia so that the resultant pigment has a higher a* color value.
It was found that the a* color value and the color intensity (chroma value) can be increased in a surprising way by a treatment of the tantalum(V) nitride in a salt melt. The pigment quality of the tantalum(V) nitride thereby increases in an unforeseeable way.
Accordingly, the invention relates to a process for producing a tantalum(V) nitride pigment, comprising nitriding a tantalum(V) oxide, tantalum(V) oxide hydrate or tantalum(V) oxide nitride with ammonia at 500 to 1100° C. in the presence or absence of a flux, which is characterized in that the nitriding is carried out either in the presence of one or more water-soluble salts from the series comprising alkali-metal and alkaline-earth-metal nitrates, chlorides and carbonates or/and in that the nitriding product is treated before or after washing out optionally present flux in a salt melt having a melting point in the range from 100 to 600° C. for 1 to 100 minutes at 100 to 600° C. and then extracting the pigment by dissolving the salts in water. The dependent process claims relate to preferred embodiments of the process according to the invention.
The invention furthermore relates to tantalum(V) nitride pigments having an a* color value of equal to/greater than +45, defined in the CIE Lab system according to DIN 5033, Part 3 and measured on a PVC film gelled for 10 minutes at 140° C. and having a content of 26% by weight of pigment. The pigments according to the invention are obtainable by the process according to the invention.
The nitriding of a tantalum(V) oxide, oxide hydrate or oxide nitride can be carried out in a manner known per se by passing ammonia at 500 to 1100° C., preferably 750 to about 1000° C., over the tantalum compound for a plurality of hours. Expediently, the nitriding can be carried out in a rotating tube or in another device with which the reaction mixture to be nitrided is agitated for the purpose of exposing new particle surfaces. The nitriding can be carried out in the presence or absence of one or more fluxes, for example boric acid, boric anhydride and/or ammonium salts of boric acid, carbonic acid or lower carboxylic acids in accordance with DE-OS 42 34 938. Although tantalum(V) nitrides produced in the presence of such fluxes have a higher color intensity than those which are produced in the absence of such fluxes, a substantially higher color intensity is achieved by the after-treatment according-to the invention of the nitriding product in a salt melt. It was furthermore found that the nitriding can be carried out at a higher temperature than was hitherto assumed in view of the limited temperature stability of tantalum(V) nitride if the reaction takes place in the presence of an adequate amount of ammonia. As a result of a temperature increase which has become possible in this way, the reaction time can be reduced and the space/time yield can be increased. In relation to the details of the nitriding, reference is made to the above-mentioned literature.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In accordance with one embodiment, the nitriding is carried out in the presence of one or more water-soluble salts from the series comprising the alkali-metal and alkaline-earth-metal nitrates, chlorides and carbonates, in particular alkali-metal nitrates. Such salts can be used in an amount of at least 0.1% by weight, based on the tantalum compound to be nitrided, up to a multiple of the tantalum compound. Small amounts of such salts act in a similar way to predisclosed fluxes. As the amount of said salts increases, the entire reaction mixture becomes pasty to liquid so that the state of aggregation of the mixture has to be borne in mind during the nitriding. Expediently, in the case last mentioned, the nitriding takes place in an amount of a salt melt which is such that the viscosity of the melt in which the tantalum compound to be nitrided is suspended makes possible an adequately intensive contact between the solid Ta phase and the gas phase (ammonia).
In accordance with a preferred embodiment, the nitriding is carried out in a manner known per se and the nitriding is followed by the after-treatment in a salt melt. The after-treatment comprises introducing the nitriding product into the salt melt; after the introduction of the tantalum compound into the melt, the mixture is allowed to stand or, if necessary, stirred. The after-treatment can be carried out in an oxygen-containing, but preferably in an inert atmosphere, for example, nitrogen or, preferably, ammonia. The reaction time necessary to improve the quality of the tantalum(V) nitride pigment by treatment in the salt melt is 1 to 100 minutes, preferably 10 to 60 minutes, at a reaction temperature in the range from 100 to 600° C., preferably 300 to 500° C. It goes without saying that, as the reaction temperature increases, the reaction time can be reduced. In accordance with a particularly preferred embodiment of the process according to the invention, the treatment in the salt melt takes place at a temperature in the range from 300 to 500° C. in the course of a reaction time of 5 to 60 minutes, in particular 10 to 30 minutes.
The salt melt to be used in the process according to the invention for after-treating th
Letschert Hans-Peter
Voigt Wilfried
dmc
Green Anthony
Wenderoth , Lind & Ponack, L.L.P.
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