Deep bluish-black effect pigments

Abstract

The present invention is related to effect pigments exhibiting a deep black body color as well as a blue interference color, to a process for the production of such pigments as well as to the use thereof, especially in coating compositions.

Claims

1. Deep bluish-black effect pigments, each pigment comprising a synthetically produced transparent dielectric flake-form substrate which has a green interference color and a refractive index n of greater than 1.5, and at least a layered structure consisting of a first layer composed of hematite and/or goethite, and a second layer composed of magnetite, on the flake-form substrate, wherein the second layer is located on top of the first layer and the first layer is located directly on the substrate; wherein the transparent dielectric flake-form substrate is a glass flake having an SiO.sub.2 proportion of at most 70% by weight, or wherein the transparent dielectric flake-form substrate consists of Al.sub.2O.sub.3, of Al.sub.2O.sub.3 with a content of up to 5% by weight of TiO.sub.2, based on the weight of the substrate, or of ZrO.sub.2 or of TiO.sub.2, or wherein the transparent dielectric flake-form substrate comprises Al.sub.2O.sub.3, ZrO.sub.2 or TiO.sub.2 with a proportion of at least 90% by weight, based on the weight of the substrate; wherein the flake-form substrate has a particle size of 5 m to 200 m; wherein the first layer has a geometrical thickness of 8 to 10 nm; wherein the second layer has a geometrical thickness of 80 to 230 nm; and wherein the effect pigments are deep bluish-black effect pigments.

2. The effect pigments according to claim 1, wherein the transparent dielectric substrate consists of Al.sub.2O.sub.3 or of A.sub.2O.sub.3 with a content of up to 5% by weight of TiO.sub.2, based on the weight of the substrate, and has a geometrical thickness of 50 to 110 nm.

3. The effect pigments according to claim 1, wherein the flake-form substrate has a particle size of 7 m to 50 m.

4. The effect pigments according to claim 1, wherein the second layer has a geometrical thickness of 80 to 150 nm.

5. The effect pigments according to claim 1, wherein the second layer comprises an aluminium compound.

6. The effect pigments according to claim 1 comprising furthermore a colorless dielectric layer on top of the second layer.

7. The effect pigments according to claim 6, wherein the colorless dielectric layer is a layer of silicon dioxide and/or silicon oxide hydrate.

8. The effect pigments according to claim 1, comprising an outermost inorganic and/or organic aftercoating.

9. The effect pigments according to claim 1, wherein the transparent dielectric substrate consists of Al.sub.2O.sub.3 or of Al.sub.2O.sub.3 with a content of up to 5% by weight of TiO.sub.2, based on the weight of the substrate, and has a geometrical thickness of 180 to 260 nm.

10. The effect pigments according to claim 1, wherein the transparent dielectric substrate consists of Al.sub.2O.sub.3 or of Al.sub.2O.sub.3 with a content of up to 5% by weight of TiO.sub.2, based on the weight of the substrate, and has a geometrical thickness of 350 to 450 nm.

11. A product selected from the group consisting of pigmenting inks, lacquers, paints, varnishes, coating compositions, plastics, foils, paper, ceramics, glasses, and laser marking pigments, comprising the effect pigments of claim 1.

12. The product according to claim 11, which is an automotive lacquer, automotive paint or automotive coating composition.

13. A process for preparing the deep bluish-black effect pigments according to claim 1, comprising the following steps: (a) dispersing a synthetically produced transparent dielectric flake-form substrate which has a green interference color and a refractive index n of greater than 1.5, in water, (b) adding a water-soluble iron (III) compound at a pH of between 2 and 4 and keeping the pH value constant, and precipitating a layer composed of hematite and/or goethite onto the surface of the substrate particles, (c) raising the pH to 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, and directly precipitating a magnetite layer, which does optionally comprise 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 of >100 C. to 260 C.

14. The process according to claim 13, wherein the transparent dielectric substrate consists of Al.sub.2O.sub.3 or of Al.sub.2O.sub.3 with a content of up to 5% by weight of TiO.sub.2, based on the weight of the substrate, and has a geometrical thickness of 50 to 110 nm, of 180 to 250 nm or of 350 to 450 nm.

15. The process according to claim 13, which is executed in an inert gas atmosphere.

16. The process according to claim 13, wherein after performing step (c) and prior to performing step (e), in an additional step a colorless dielectric layer is coated onto the magnetite layer.

17. The process according to claim 16, wherein the colorless dielectric layer coated onto the magnetite layer is a layer of silicon dioxide and/or silicon oxide hydrate.

18. The process according to claim 13, 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.

19. Deep bluish-black effect pigments, each pigment comprising a synthetically produced transparent dielectric flake-form substrate which has a green interference color and a refractive index n of greater than 1.5, and at least a layered structure consisting of a first layer composed of hematite and/or goethite, and a second layer composed of magnetite, on the flake-form substrate, wherein the second layer is located on top of the first layer and the first layer is located directly on the substrate; wherein the transparent dielectric flake-form substrate is a glass flake having an SiO.sub.2 proportion of at most 70% by weight, or wherein the transparent dielectric flake-form substrate consists of Al.sub.2O.sub.3, of Al.sub.2O.sub.3 with a content of up to 5% by weight of TiO2, based on the weight of the substrate, or of ZrO.sub.2 or of TiO.sub.2, or wherein the transparent dielectric flake-form substrate comprises Al.sub.2O.sub.3, ZrO.sub.2 or TiO.sub.2 with a proportion of at least 90% by weight. based on the weight of the substrate; wherein the flake-form substrate has a particle size of 5 m to 200 m; wherein the first layer has a geometrical thickness of 6 to 10 nm; wherein the second layer has a geometrical thickness of 80 to 230 nm; and wherein the effect pigments are deep bluish-black effect pigments.

Description

EXAMPLE 1

(1) 140 g of an aluminum dioxide flake (Al.sub.2O.sub.3 with minor content of TiO.sub.2, mean thickness 220 nm, mean particle diameter 18 m, greenish inherent interference color) 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 20 wt. %). While keeping the pH value constant, a Fe(NO.sub.3).sub.3 solution (100 ml, 7.87 g of Fe(NO.sub.3).sub.3*9 H.sub.2O in 140 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 32 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, 768.9 g FeSO.sub.4*7 H.sub.2O, 0.66 g AlCl.sub.3*6 H.sub.2O and 24.3 g Fe(NO.sub.3).sub.3*9 H.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 5.9 g, 29% as SiO.sub.2) is added while 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 deep bluish-black powder color with vivid luster as well as high hiding power.

EXAMPLE 2

(4) For demonstrating the influence of the thickness (and, thus, an inherent interference color) of the flake-form pigment substrates, aluminium oxide substrate particles of different thickness are coated with a layered system of hematite/goethite-magnetite according to the procedure disclosed in example 1 to give different interference pigments. The respective substrates have a mean thickness of 300, 220 and 150 nm, respectively, where only the substrate particles having a mean thickness of 220 nm exhibit an inherent greenish interference color.

(5) Three polymer plates which are pre-coated with a 15 m thick coating containing carbon black are spray coated with a coating composition containing a mixture of 10 parts by weight of acrylic-melamine-resins as binder, 1 part by weight of the respective interference pigments exhibiting the different substrate thickness as described above, and 13 parts by weight of a solvent mixture in each case. If necessary, the viscosity of the coating composition is further adjusted by adding additional solvents for spray application. The coating composition is applied to the pre-coated polymer plates at a dry thickness of 15 m by means of a commercially available spray gun. Afterwards, a clear top coat substantially composed of an acrylic-melamine resin (applied by spray application in dissolved form) is applied on the layer containing the interference pigments at a dry thickness of 30 m. The respective coated test plates are thermally treated at 140 C. for 20 minutes.

(6) The test plates are evaluated visually and by measuring the coloristic data. The respective L*a*b* data measured by a BYK-mac i (spectrophotometer of BYK-Gardner GmbH) are disclosed in table 1.

(7) TABLE-US-00001 TABLE 1 Flake thickness Measuring (nm) angle L* a* b* C* h 300 15 42.8 1.8 4.3 4.7 254.7 300 25 19.5 0.2 4.1 4.2 266.8 300 45 5.4 0.1 4.9 4.9 270.6 220 15 43.4 0.2 11.3 11.3 268.9 220 25 20.6 0.1 7.4 7.4 270.8 220 45 4.9 0.2 5.1 5.1 272.6 150 15 45.2 3.4 4.3 5.5 231.7 150 25 24.9 1.9 3.7 4.1 243.1 150 45 6.5 0.5 4.0 4.0 263.3

(8) The color characteristics according to table 1 reveal that only for the deep bluish-black effect pigment according to the present invention, having flake-form substrates with a green inherent interference color, the color angle h remains stable at different measuring angles, the bluish interference is strong (negative b*-value) and a reddish or greenish tinge of the interference color (tiny a*-value) is not observable.

EXAMPLE 3

(9) Temperature Stability Test

(10) Pigment samples are prepared according to example 1, with the proviso that the following conditions are met:

(11) Ex. 3a: only with hematite/goethite-magnetite layered structure, no dielectric layer;

(12) Ex. 3b: with hematite/goethite-magnetite layered structure, dielectric layer of SiO.sub.2 and/or silicon oxide hydrate;

(13) Ex. 3c: with hematite/goethite-magnetite layered structure, dielectric layer of SiO.sub.2 and/or silicon oxide hydrate plus standard aftercoating;

(14) Ex. 3d: mica with magnetite layer, comparative example

(15) 2 g of each pigment are stored at 180 C. for 14 hours

(16) 2 g of each pigment are stored at 250 C. for 14 hours.

(17) 1.2 g of each pigment according to ex.3a to ex. 3d after storage are dry blended with 30.0 g of a commercial powder clear coat composition. The pigmented powder coating composition is then applied onto black/white metal test panels by means of a corona charging powder gun. The resulting coating layer is cured at 180 C. for 15 min. in each case.

(18) After curing, the panels are characterized visually and by measuring the corresponding L*a*b* values using a BYK-mac i spectophotometer. Using the data achieved, mDE*-values are determined over the black and white surface of the test panels. The results are disclosed in table 2.

(19) TABLE-US-00002 TABLE 2 mDE* over mDE* over mDE* over mDE* over white white black black 180 C. 250 C. 180 C. 250 C. Ex. 3a 7 28.5 4 16 Ex. 3b 2 14.5 1 3.5 Ex. 3c 2 3 2.5 2.5 Ex. 3d 9 27 7 22

(20) The results disclosed in table 2 show that a dielectric SiO.sub.2 and/or silicon oxide hydrate layer on top of the hematite/goethite-magnetite layered structure of the effect pigments according to the present invention enlarges the temperature stability of the pigments to a remarkable extent. The temperature stability may be enlarged even further in case a standard aftercoating is applied in addition to the SiO.sub.2 and/or silicon oxide hydrate dielectric layer.