Compositions: coating or plastic – Materials or ingredients – Pigment – filler – or aggregate compositions – e.g. – stone,...
Reexamination Certificate
2000-02-22
2001-05-22
Klemanski, Helene (Department: 1755)
Compositions: coating or plastic
Materials or ingredients
Pigment, filler, or aggregate compositions, e.g., stone,...
C423S409000
Reexamination Certificate
active
06235103
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to tantalum(V) nitride pigments having improved red pigment colour values. The present invention also relates to the production of the tantalum(V) pigments on the basis of nitriding tantalum(V) oxide with ammonia. The invention finally relates to the use of the tantalum(V) nitride pigments which are improved with regard to colour.
2. Description of the Related Art
Both during use and on disposal, toxicologically questionable constituents may be released from articles coloured or decorated with oxide, sulfide or selenide heavy metal compounds, such as for example cadmium and selenium from cadmium sulfoselenide red pigments. There is accordingly particular interest in using pigments having toxicologically less questionable constituents. Tantalum(V) nitride (Ta
3
N
5
) is an innocuous alternative, but the colour values, in particular brightness, of the pigments obtainable using hitherto known processes do not always meet expectations. Hitherto known production processes either give rise to unsatisfactory colour values or entail the use of special tantalum starting compounds.
According to H. Moureau and C. H. Hamblet (J. Amer. Chem. Soc. 59, 33-40 (1937)), the nitride of pentavalent tantalum may be produced by ammonolysing TaCl
5
. The nitride obtained in this manner is not pure as it contains oxide constituents. Reaction times are moreover very long. H. Funk and H. Böhland (Z. anorg. allg. Chem. 334, 155-62 (1964)) were able to shorten reaction times by nitriding (NH
4
)TaF
6
. Due to the volatility of TaF
5
and NH
4
F, yields of Ta
3
N
5
are only low and/or the products contain fluorine. The stated document also disclosed the nitriding of tantalum(V) oxide (Ta
2
O
5
) at 800° C. with ammonia; a nitride containing oxide (Ta
3
N
5
(O)) is formed at 800° C., the colour of which was not stated.
On the basis of Funk and Böhland's investigations, G. Brauer and J. R. Weidlein (Angew. Chem. 77, 218-9 (1965)) reexamined the nitriding of pure tantalum pentoxide with purified ammonia: in the presence of titanium chips as oxygen getter, the reaction requires a reaction time of 36 to 120 hours at 860 to 920° C. These reaction times could not be confirmed when these investigations were replicated (c.f. EP-A 0 592 867): colour formation was not complete, i.e. no further colour shift occurred, until a reaction time of 150 hours had elapsed. However, the tantalum(V) nitride obtained in this manner was red-brown and thus of no interest for colouring purposes.
According to the process described in EP-A 0 592 867, it is possible to obtain tantalum(V) nitride having elevated colour intensity and simultaneously shorten the reaction time. In this process, tantalum(V) oxide is replaced by a tantalum(V) oxide hydrate of the formula Ta
2
O
5
aq. having a hydrate content (aq. content) in the range from 14 to 17 wt. %. The duration of the nitriding reaction is shortened by adding fluxes to the tantalum oxide hydrate to be nitrided. The pigment obtained in this manner exhibited a red shift in comparison with prior art products and substantially greater colour intensity than products as were obtainable using tantalum(V) oxide.
One disadvantage of the latter-stated process is that commercially obtainable, low cost tantalum(V) oxide could not be used, but only a specially produced tantalum(V) oxide hydrate. As already explained, one disadvantage of prior processes using tantalum(V) oxide was that they yielded products, the colour values of which prevented the use thereof as a red pigment.
SUMMARY OF THE INVENTION
The object of the present invention is accordingly to provide a process for the production of tantalum(V) nitride if possible having improved colour values in comparison with prior art products, which process is based on nitriding tantalum(V) oxide with ammonia.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A process has been discovered for the production of tantalum(V) nitride pigment by nitriding pulverulent tantalum(V) oxide with ammonia at 700 to 1250° C., which process is characterised in that at least one pulverulent oxide from the series SiO
2
, GeO
2
, SnO
2
, TiO
2
, ZrO
2
and HfO
2
or a pulverulent precursor of such an oxide is added in a quantity of at least 0.1 wt. %, calculated as oxide, to the tantalum(V) oxide to be nitrided and the powder mixture is then nitrided. The dependent process claims relate to preferred embodiments.
It has utterly surprisingly been found that the nitriding of tantalum(V) oxide may be substantially improved if an inert oxide from the series SiO
2
, GeO
2
, SnO
2
, TiO
2
, ZrO
2
and HfO
2
or a pulverulent precursor of such an oxide is added to the pulverulent tantalum(V) oxide before nitriding. Red pigments are obtained when the homogeneous powder mixture is nitrided.
The term “inert” is taken to mean that the stated oxides or precursors thereof substantially do not themselves form any nitrides under the nitriding conditions.
The quantity of these oxides or precursors used may vary within wide limits: Using a quantity of less than 0.1 wt. %, relative to tantalum(V) oxide, is possible, but in such cases the effect is, of course, still slight. A quantity of at least 1 wt. % of oxide or a precursor thereof, calculated as oxide, is conventionally preferred. It is also possible to add more than 20 wt. % of one or more of the stated oxides or precursors thereof to the tantalum(V) oxide, but it must be noted that the resultant pigments then become increasingly lighter. The quantity used preferably amounts to 2 to 15 wt. %, particularly preferably 3 to approx. 10 wt. %.
The effect according to the invention of promoting nitriding and giving rise to products having better colour values may be achieved by using just one of the stated oxides or precursors thereof. It is, however, also possible to use a combination of two or more such compounds. The phrase “precursor of such an oxide” is taken to mean compounds which are converted into the oxide below the nitriding temperature, i.e. during heating of the powder mixture. Suitable precursors of the oxides are accordingly in particular oxide hydrates, nitrates, carbonates and salts of lower carboxylic acids.
According to particularly preferred embodiments of the process according to the invention, silica, zirconium dioxide or a precursor of these oxides, in particular basic zirconium carbonate, or mixtures of the stated compounds, is/are added to the tantalum(V) oxide.
It has been established that it is particularly advantageous to use one or more of the stated oxides or precursors thereof, as well as the tantalum(V) oxide to be nitrided, in the most finely divided form possible. Conventional commercial pigment grade products are suitable as the tantalum(V) oxide. The oxides or precursors thereof particularly preferably added to the tantalum(V) oxide to be nitrided are those having specific surface areas (measured by the BET method to DIN 66131 using N
2
) of at least 5 m
2
/g, preferably of greater than 50 m
2
/g and particularly preferably of around 200 m
2
/g. Oxides produced by grinding processes and by precipitation processes or by flame hydrolysis may, for example, be used. ZrO
2
, as is obtainable by thermal decomposition of zirconium silicate with subsequent leaching of the silica matrix, is also suitable.
Equipment known to person skilled in the art may be used to produce the powder mixture: the powder mixture may suitably be homogenised with any desired mixing and/or grinding apparatus by means of which the tantalum oxide to be nitrided is uniformly dispersed with the added oxide(s) or precursors thereof. Effective homogenisation is also made possible in nitriding reactors in which the powder mixture is kept in constant motion.
Nitriding conveniently proceeds by passing ammonia or an inert gas containing ammonia through a reactor containing the powder mixture at 700 to 1250° C., preferably at 850 to 950° C. According to a particularly preferred embodiment, nitriding is performed in a rotary tube reactor. It has been established that
Letschert Hans-Peter
Voigt Wilfried
Cerdec Aktiengesellschaft Keramische Farben
Faison Veronica F.
Klemanski Helene
Wenderoth , Lind & Ponack, L.L.P.
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