Metallic luster pigments

Abstract

The present invention relates to metallic luster pigments, to a process for production thereof and to the use of such metallic luster pigments.

Claims

1. A metallic luster pigment based on coated aluminum platelets, wherein the aluminum platelets are in monolithic form and are encased by at least one coating B of iron(III) oxide which is optionally encased in turn by a layer C selected from TiO.sub.2, SnO.sub.2 and Al.sub.2O.sub.3, and wherein the proportion by mass of iron(III) oxide in the coated substrate platelets, either alone or in combination with TiO.sub.2, SnO.sub.2 or Al.sub.2O.sub.3 from layer C, if present, is at least 65% by weight, the proportion by mass of aluminum metal in the coated substrate platelets is not more than 20% by weight, and the quantitative ratio of oxygen not bonded to aluminum relative to aluminum is at least 3, and between the surface of the aluminum platelets and coating B there is a further coating A of at least one metal oxide having a low refractive index of at most 1.8, selected from the group consisting of SiO.sub.2, B.sub.2O.sub.3, MnO.sub.2, MgO, GeO.sub.2 and Al.sub.2O.sub.3, and wherein the coating B and the coating A are in direct contact, wherein the aluminum platelets have a thickness of 1 to 30 nm and have optionally been passivated, and wherein coating B has a thickness in the range from 100 to 300 nm.

2. The metallic luster pigment as claimed in claim 1, wherein the coating A is composed of SiO.sub.2.

3. The metallic luster pigment as claimed in claim 1, wherein the coating A has a thickness of 1 to 100 nm.

4. The metallic luster pigment as claimed in claim 2, wherein the coating A has a thickness of 1 to 100 nm.

5. The metallic luster pigment as claimed in claim 1, wherein layer C is present in the aluminum platelets.

6. A process for producing metallic luster pigments as claimed in claim 1, comprising the steps of: providing optionally passivated aluminum substrate platelets, coating the aluminum substrate platelets by hydrolytic decomposition of one or more organic metal compounds and/or by precipitation of one or more dissolved metal salts.

7. A composition comprising the metallic luster pigments as claimed in claim 1.

8. The composition of claim 7, wherein the composition is chosen from a coloring paint, an ink, a plastic, a glass, a ceramic product, or a formulation for decorative cosmetics.

9. The metallic luster pigment as claimed in claim 1, wherein the monolithic aluminum platelets are each a single self-contained unit with no fractures, layering, or occlusions.

10. The metallic luster pigment as claimed in claim 1, wherein the monolithic aluminum platelets have a homogeneous structure.

11. The metallic luster pigment as claimed in claim 1, wherein the monolithic aluminum platelets do not have a layered structure and do not have any particles distributed therein.

12. The metallic luster pigment as claimed in claim 1, wherein the monolithic aluminum platelets do not have a core-shell structure where a shell comprises aluminum and a core comprises a different material.

Description

(1) FIGS. 1 and 2 show results from combustion tests on various coated aluminum platelets having a different proportion by weight of aluminum (FIG. 1) and iron(III) oxide (FIG. 2). The performance rating for the fire characteristics (y axis) as a function of the proportion of aluminum (FIG. 1) or iron(III) oxide (FIG. 2) in the coated substrate platelets is determined by assessing the behavior of the sample in the combustion test described hereinafter.

(2) FIG. 3 shows a coated aluminum platelet of the invention. The aluminum platelet has a very homogeneous thickness and is encased by an SiO.sub.2 layer (coating A, light) and an iron oxide layer (coating B, dark).

(3) FIG. 4 shows a coated aluminum platelet of the invention having an aluminum core, SiO.sub.2 layer (coating A, light) and iron oxide layer (coating B, dark).

(4) The examples which follow serve to further illustrate the present invention, without being restricted thereto.

EXAMPLE 1 (THIN ALUMINUM PLATELETS WITH THICK IRON OXIDE COATING)

(5) First of all, 50 g of Al platelets (thickness between 20 nm and 30 nm, d50=12 μm) were coated with 10 g of SiO.sub.2 by means of a sol-gel method using tetraethyl orthosilicate (TEOS). In a round bottom flask with reflux condenser and stirrer, these Al platelets were admixed with 500 mL of deionized water and heated to 75° C. while stirring. The pH was adjusted to a value of 3.2 by adding a 10% NaOH solution. 1016 g of a 20% FeCl.sub.3 solution were added to the reaction mixture, in the course of which the pH was kept essentially constant at 3.2 by simultaneous addition of a 10% NaOH solution. On completion of addition of the FeCl.sub.3 solution, the mixture was stirred for a further 15 minutes, in order to assure complete precipitation. The pH was then increased to a value of 7.0 by dropwise addition of a 10% NaOH solution over a period of 30 minutes. After stirring for a further 30 minutes, the coated pigment was separated from the supernatant reaction solution by filtering and washed until it was free of salts. The resultant coated aluminum platelets were dried at 250° C. for 215 minutes and sieved with a sieve (mesh size 25 μm). The resultant product was subjected to an assessment of its color properties and to a fire test as described below.

EXAMPLE 2 (THIN ALUMINUM PLATELETS WITH THIN IRON OXIDE COATING)

(6) In this example, analogously to the method of example 1, coated aluminum platelets were produced, with the difference that, rather than 1016 g, only 102 g of the 20% FeCl.sub.3 solution were used. The resultant product was subjected to an assessment of its color properties and to a fire test as described below.

EXAMPLE 3 (THIN ALUMINUM PLATELETS WITH THICK IRON OXIDE COATING AND TITANIUM OXIDE COATING)

(7) This example was conducted analogously to example 1 up to and including the stirring for fifteen minutes after the addition of the FeCl.sub.3 solution had ended. Thereafter, the pH was adjusted to 2.0 by adding 10% HCl solution. 412 g of a 30% TiCl.sub.4 solution were added to the reaction mixture, in the course of which the pH was kept essentially constant at 2.0 by simultaneously adding a 10% NaOH solution. On completion of addition of the TiCl.sub.4 solution, the mixture was stirred for a further 15 minutes, in order to assure complete precipitation. The pH was then increased to a value of 7.0 by dropwise addition of a 10% NaOH solution over a period of 30 minutes. After stirring for a further 30 minutes, the coated pigment was separated from the supernatant reaction solution by filtering and washed until it was free of salts. The resultant coated aluminum platelets were dried at 250° C. and sieved with a sieve (mesh size 25 μm). The resultant product was subjected to an assessment of its color properties and to a fire test as described below.

COMPARATIVE EXAMPLE (THICK ALUMINUM PLATELETS WITH IRON OXIDE COATING)

(8) First of all, 50 g of Al platelets (thickness between 150 nm and 300 nm, d50=18 μm) were coated with 8.8 g of SiO.sub.2 by means of a sol-gel method using tetraethyl orthosilicate (TEOS). In a round bottom flask with reflux condenser and stirrer, these Al platelets were admixed with 500 mL of deionized water and heated to 75° C. while stirring. The pH was adjusted to a value of 3.2 by adding a 10% NaOH solution. 660 g of a 20% FeCl.sub.3 solution were added to the reaction mixture, in the course of which the pH was kept essentially constant at 3.2 by simultaneously adding a 10% NaOH solution. On completion of addition of the FeCl.sub.3 solution, the mixture was stirred for a further 15 minutes in order to assure complete precipitation. The pH was then increased to a value of 7.0 by dropwise addition of a 10% NaOH solution over a period of 30 minutes. After stirring for a further 30 minutes, the coated pigment was separated from the supernatant reaction solution by filtration and washed until it was free of salts. The resultant coated aluminum platelets were dried at 250° C. and sieved with a sieve (mesh size 40 μm). The resultant product was subjected to an assessment of its color properties and to a fire test as described below.

(9) Combustion Test

(10) 20 g in each case of the pigments produced were mixed thoroughly with 13.3 g of white spirit. 2 g of this mixture were applied to a glass plate and set on fire. This combustion test is recorded as a video. The fire characteristics are rated on a scale from 0 to 5, 0 meaning that there is merely gentle burnoff of the white spirit without occurrence of any further reaction. 1: isolated sparks during the solvent fire 2: slight evolution of sparks during the solvent fire 3: moderate evolution of sparks during the solvent fire 4: significant evolution of sparks and small explosions or crackling during the solvent fire, glowing of the sample after the solvent has burnt off 5: significant evolution of sparks and explosions or crackling during the solvent fire, and complete conversion of the sample after the solvent has burnt off

(11) To measure the total color difference ΔE, a paint layer which comprised the metallic luster pigment of the invention to be examined in a proportion by mass of 18% by weight (dry weight) was applied to a black surface and to a white surface. The layer thickness of the dried coat of paint was 15 μm. Thereafter, the total color difference ΔE between the coats of paint on white and black backgrounds was determined. The results of the measurements are shown in table 1.

(12) Table 1 shows results for various metallic luster pigments. The uncoated substrate platelets consist of aluminum metal. Test numbers 1 to 12 were produced in accordance with the methods from examples 1, 2 and 3 with the necessary modifications for establishment of the individually specific parameters (for example thickness of coatings A, B and, if present, C). Test numbers 5 to 7 and 12 are examples of the present invention; tests 1 to 4 and 8 to 11 are comparative examples. In addition to test numbers 1 to 12, table 1 shows values for the commercially available products Paliochrom L2800 (from BASF) and Meoxal Orange (from Merck) as comparative examples.

(13) It is apparent from table 1 and FIG. 1 that, with a content of aluminum metal in the coated substrate platelets of, in particular, equal to or less than 20% by weight, it is possible to provide a pigment which is noncombustible and is not an explosion hazard. This corresponds to a result of 1 or 0 in the combustion test.

(14) It is apparent from table 1 and FIG. 2 that, with an Fe.sub.2O.sub.3 content in the coated substrate platelets of, in particular, equal to or greater than 65% by weight, it is possible to provide a pigment which is noncombustible and is not an explosion hazard. This corresponds to a result of 1 or 0 in the combustion test.

(15) In addition, table 1 shows that the metallic luster pigments of the invention have a particularly small color difference ΔE and hence a particularly high hiding capacity. The proportions reported in the table are based on % by weight.

(16) TABLE-US-00001 TABLE 1 Perfor- mance rating Use of Al SiO.sub.2 Fe.sub.2O.sub.3 TiO.sub.2 for fire substrate con- con- con- con- charac- of the No. tent tent tent tent teristics ΔE invention 1 37 14 39 0 5 0.8 yes 2 34 21 35 0 5 0.6 yes 3 26 30 30 0 5 0.8 yes 4 22 25 45 0 4 0.2 yes 5 11 13 68 0 1 0.7 yes 6 6 8 73 0 0 0.4 yes 7 4 5 72 0 0 3.3 yes 8 4 6 45 30 0 15.4 yes 9 37 7 49 0 5 0.4 yes 10 51 6 32 0 4 5.5 no 11 64 4 20 0 2 11.2 no 12 8 3 76 0 0 0.8 yes Paliocrom 69 1 26.6 nd 3 12.0 no L2800 Meoxal 27 11 50.9 nd 5 25.0 no Orange