Brilliant black pigments

Abstract

The present invention is related to brilliant black pigments comprising a flaky aluminum oxide substrate having a coating comprising a layered structure composed of a hematite and a magnetite layer, to a process for the production of said pigments, as well as to their use.

Claims

1. Brilliant black pigments, each pigment comprising flaky aluminum oxide substrate particles exhibiting an aspect ratio of at least 85, and a coating comprising a layered structure consisting of a first layer composed of hematite and/or goethite and a second layer composed of magnetite, in this sequence, on the substrate, wherein the thickness of the layer composed of magnetite is 50 to 250 nm, and wherein the substrate particles have a mean particle diameter of lower than 20 m and a D.sub.50 value of 10 to 15 m, measured by the laser diffraction method, wherein the pigments are brilliant black pigments.

2. The brilliant black pigments according to claim 1, wherein the substrate particles are of Al.sub.2O.sub.3 or of Al.sub.2O.sub.3 containing up to 5% by weight TiO.sub.2.

3. The brilliant black pigments according to claim 1, wherein the substrate particles have a mean thickness between 50 and 200 nm.

4. The brilliant black pigments according to claim 1, wherein the substrate particles have a mean particle diameter of 5 and 15 m m.

5. The brilliant black pigments according to claim 1, wherein the mean particle diameter is lower than 16 m.

6. The brilliant black pigments according to claim 1, wherein the thickness of the layer composed of magnetite is higher than the thickness of the layer composed of hematite and/or goethite.

7. The brilliant black pigments according to claim 1, wherein the layer composed of hematite and/or goethite is located directly on the substrate.

8. The brilliant black pigments according to claim 1, wherein there is at least one dielectric coating located between the substrate and the layer composed of hematite and/or goethite.

9. The brilliant black pigments according to claim 1, comprising furthermore a colourless dielectric layer on top of the magnetite layer.

10. The brilliant black pigments according to claim 9, wherein the colourless dielectric layer is a silicon oxide hydrate layer which is located directly on top of the magnetite layer.

11. The brilliant black pigments according to claim 1, wherein the magnetite layer is doped with an aluminum compound.

12. The brilliant black pigments according to claim 11, wherein the aluminum compound is an oxide and/or an oxide hydrate.

13. The brilliant black pigments according to claim 11, wherein the content of the aluminum compound is between 0.1 and less than 5% by weight, based on the weight of the magnetite layer.

14. The brilliant black pigments according to claim 9, exhibiting a deep black body colour and, optionally, a blue interference colour.

15. A product selected from the group consisting of pigmenting inks, paints, varnishes, coating compositions, plastics, foils, paper, ceramics, glasses, cosmetic formulations, pharmaceutic formulations, laser marking and pigment preparations of diverse solvent contents, comprising pigments according to claim 1.

16. The product according to claim 15, wherein the ink is a printing ink.

17. The product according to claim 15, wherein the paint or coating composition is an automotive paint or automotive coating composition.

18. The product according to claim 15, wherein the cosmetic formulation is a mascara formulation, an eyeliner formulation or a kohl eye pencil formulation.

19. A product, comprising a pigment according to claim 1.

20. A process for preparing the pigments according to claim 1, comprising the following steps: (a) dispersing flaky aluminum oxide substrate particles exhibiting an aspect ratio of at least 85, optionally being coated with at least one dielectric coating, in water, (b) adding a water soluble iron (III) compound at a pH of between 2 and 4 and keeping the pH value constant, thereby precipitating a layer composed of hematite and/or goethite onto the surface of the substrate particles, (c) raising the pH to a value between 5.5 and 7.5 and adding a water soluble iron (II) compound and a water soluble iron (III) compound, and optionally also adding an aqueous solution of an aluminum compound, while keeping the pH value constant, thereby directly precipitating a magnetite layer, which is optionally doped with an aluminum compound, onto the surface of the substrate particles precoated in step (b), (d) optionally washing and filtering the resulting product and (e) drying at a temperature in the range of from >100 C. to <180 C.

21. The process according to claim 20 which is executed in an inert gas atmosphere.

22. The process according to claim 20, wherein after performing step (c) and prior to performing step (e), in an additional step at least one colourless dielectric layer is coated onto the magnetite layer.

23. The process according to claim 22, wherein the dielectric layer coated onto the magnetite layer is a silicon oxide hydrate layer.

24. The process according to claim 20, wherein the layer composed of magnetite is applied onto the substrate particles at a higher thickness than the layer composed of hematite and/or goethite.

Description

EXAMPLE 1

(1) 100 g of an aluminum dioxide flake (Al.sub.2O.sub.3 with minor content of TiO.sub.2, mean thickness 150 nm, D.sub.50 14 m) is suspended in deionized water. The suspension is heated to 80 C. while stirring. Nitrogen gas is slowly added into the reaction vessel. The pH value is adjusted and kept constant to 3.0 by metering an acidic compound into the suspension (HCl, about 17.5 wt. %). While still keeping the pH value constant, a Fe(NO.sub.3).sub.3 solution (100 ml, 5.06 g of Fe(NO.sub.3).sub.3*9H.sub.2O in 100 ml deionized water) is added to the suspension. The pH value is then raised to about 7.0 by adding a basic composition (NaOH, about 30 wt. %) to the suspension. While keeping the pH value constant, an aqueous solution of an Al component and of a Fe(II) and Fe(III) component (2000 ml, 720.4 g FeSO.sub.4*7H.sub.2O, 4.5 g Al.sub.2(SO.sub.4).sub.3*16H.sub.2O and 45.1 g Fe(NO.sub.3).sub.3*9H.sub.2O, in 2000 ml deionized water) is slowly metered into the suspension which is then kept for another 30 minutes while stirring. Thereafter, a water glass solution (about 38.5 g, 26% as SiO.sub.2) is added while still keeping the pH constant. The suspension is kept for about 2 hours, then the resulting pigments are separated off by filtering and washed with deionized water.

(2) Eventually, the resulting pigments are dried at a temperature of about 120 C. and sieved.

(3) The resulting pigment exhibits a brillant black powder colour with vivid lustre as well as a high hiding power.

(4) The powder colour is measured by using a Minolta CR-300 apparatus (product of Konica Minolta Holdings, Inc.). The resulting L value is 47.03, the a value is 0.9, the b value is 1.02 and chroma is 1.36.

(5) A lacquer card is prepared as follows: 0.5 g of the pigment according to example 1 is mixed with 9.5 g of a standard NC-Acrylate lacquer (available from Merck KGaA per catalogue). The resulting mixture is coated by means of a bar coater (No. 20) onto usual black/white paper strips and dried. The resulting colouristic characteristics are measured by using the Minolta apparatus as mentioned above.

(6) The L value is determined to be 28.41, the a value is 0.59, the b value is 1.06 and chroma is determined to be 1.21.

EXAMPLE 2

(7) The procedure of example 1 is repeated except that no silicon oxide hydrate layer is applied onto the magnetite layer.

(8) The corresponding amounts used for examples 1 and 2 may be taken from table 1.

(9) TABLE-US-00001 TABLE 1 substrate Fe.sub.2O.sub.3 Fe.sub.3O.sub.4 SiO.sub.2 Sample g g g g Ex. 1 100 1 155 10 Ex 2 100 1 155 0

(10) The colouristic values of the pigments according to examples 1 and 2 measured by the Minolta 300 apparatus are listed in table 2 beneath. In addition, a commercially available black mica pigment [mica coated with TiO.sub.2 and Fe.sub.3O.sub.4 (Iriodin 600, Product of Merck KGaA)] is also measured for comparison.

(11) TABLE-US-00002 TABLE 2 Powder colour colour of lacquer card Sample L a b C L a b C Ex. 1 47.03 0.90 1.02 1.36 28.41 0.59 1.06 1.21 Ex. 2 48.85 2.80 3.45 4.44 28.02 2.22 3.75 4.36 Mica black 39.45 0.10 0.45 0.46 23.60 0.28 1.28 1.31

(12) Regarding blackness (C-value), the pigments according to the present invention are comparable to mica black, but exhibit a somewhat bluish interference colour, demonstrated by the a- and b-values. This interference colour is substantially diminished by the silicon oxide hydrate layer applied onto the magnetite layer in example 1. Since the L-value, measured by the Minolta 300 apparatus, refers to lightness rather than to lustre, further draw down lacquer cards are prepared as mentioned above and the lustre of each sample is evaluated in the Hunter Lab-system by using a Model D25 Optical Sensor, made by Hunter Associates laboratory, Inc.

(13) The resulting Hunter-Lab-values are listed in the following table 3, whereby L refers to lustre and the a- and b-values are also measured at the lustre angle:

(14) TABLE-US-00003 TABLE 3 Lacquer card (Hunter) Sample L a b C Ex. 1 34.95 0.29 2.02 2.04 Ex 2 35.14 1.78 4.34 4.69 Mica black 28.96 0.55 1.74 1.82

(15) As can be seen in table 3, the lustre values for the pigments according to examples 1 and 2 of the present invention are much better than that for mica black. While the a-, b- and c-values of example 1 and of mica black are comparable, example 2 exhibits a somewhat bluish interference colour. Despite this slight interference, the compelling lustre is so impressive that pigments according of examples 1 and 2 are of high interest, in particular for cosmetic applications.

(16) Furthermore, for different applications, including cosmetic, automotive and printing applications, the hiding power of the pigments is of great interest, since it, inter alia, determines the necessary concentration of pigments for obtaining deep black colouristic results in application media.

(17) The hiding power of the present pigments may be evaluated by calculating the difference of the lightness L between the white and black part of the draw down cards. The higher the hiding power, the smaller the L-difference between white and black part of the draw down card, measured by Minolta CR-300. The results are listed in table 4 below:

(18) TABLE-US-00004 TABLE 4 L value (Minolta 300) Sample White card Black card Difference Ex. 1 30.53 28.41 2.12 Ex 2 30.18 28.02 2.16 Mica black 27.08 23.60 3.48

(19) As can be seen in table 4, the hiding power of the pigments according to examples 1 and 2 is higher than that of mica black.

COMPARATIVE EXAMPLES 2, 3 AND 4

(20) For further comparison, comparative examples 2 to 4 are carried out, using different substrate particles. To this end, the procedure of example 1 is repeated except that the coating amount for the Fe.sub.3O.sub.4 layer is adjusted to achieve at a dark bluish interference colour and, in addition, the presence of an outer silicon oxide hydrate layer is varied.

(21) Details are listed in the following table 5:

(22) TABLE-US-00005 TABLE 5 amount of substr. Fe.sub.2O.sub.3 Fe.sub.3O.sub.4 SiO.sub.2 Sample substrate g g g g Comp. Ex. 2 Al.sub.2O.sub.3-Flake 100 1 120 0 Comp. Ex. 3 Al.sub.2O.sub.3-Flake 100 1 120 12.5 Comp. Ex. 4 Mica 100 1 141 0 Al.sub.2O.sub.3-Flakes with average thickness 220 nm, D.sub.50: 18 m Mica-Flakes D.sub.50: 18 m

(23) The colouristic properties measured by Minolta 300 are as follows:

(24) TABLE-US-00006 TABLE 6 powder colour colour of lacquer card Sample L a b C L a b C Comp. Ex. 2 47.86 0.09 8.10 8.10 27.69 2.15 8.17 8.45 Comp. Ex. 3 47.49 0.57 3.71 3.75 28.20 0.64 6.92 6.95 Comp. Ex. 4 46.83 0.60 0.34 0.68 23.76 1.08 0.65 1.26

(25) Regarding the lustre measured using the Hunter-Lab-values as explained above, the following data is determined:

(26) TABLE-US-00007 TABLE 7 Lacquer card (Hunter) Sample L a b C Comp. Ex. 2 35.49 1.33 8.58 8.68 Comp. Ex 3 35.79 0.31 7.01 7.02 Comp. Ex. 4 30.44 0.72 0.22 0.75

(27) In table 8, the hiding power of each of the comparative examples is calculated according to the method mentioned above:

(28) TABLE-US-00008 TABLE 8 L value (Minolta 300) Sample White card Black card Difference Comp. Ex. 2 30.97 27.69 3.28 Comp. Ex. 3 33.78 28.20 5.58 Comp. Ex. 4 29.46 23.76 5.70

(29) As a result, none of the pigments according to the comparative examples fulfils all requirements for attractive blackness, low interference colour, high lustre and high hiding power, as the pigments of the present invention do.

(30) Pigments based on aluminum dioxide flakes, which do not fulfil the aspect ratio according to the present invention exhibit a higher lustre than mica pigments having the same particle size, but too strong interference colours, regardless on whether additionally coated with a silicon oxide hydrate layer or not. Black mica pigments, on the other side, provide good blackness and low interference colour, but not sufficient lustre. None of the pigments according to the comparative examples exhibits sufficient hiding power when compared with the pigments according to the present invention.