Pearlescent pigments obtained by wet oxidation

Abstract

The present invention relates to pearlescent pigments, to a process of manufacturing such pearlescent pigments based on a wet oxidation step as well as to the use of such pearlescent pigments.

Claims

1. A process of manufacturing pearlescent pigments, comprising the steps of: providing substrates of monolithically structured aluminum platelets having an average thickness of from 1 to 150 nm, and having an aspect ratio expressed by the ratio of the average size to the average thickness of at least 80; coating the above aluminum platelets with a coating layer A of silicon dioxide; coating the such coated aluminum platelets with a coating layer B of a metal oxide as an interference layer, with the metal oxide being other than silicon dioxide and having a refractive index of at least 1.8; wet-oxidizing under basic conditions the above aluminum platelets, after coating with the coating layers A and B, whereby the aluminum platelets are partially or entirely converted into aluminum oxide or aluminum hydroxide; and subsequently isolating the partially or entirely wet-oxidized aluminum platelets coated with the coating layers A and B.

2. The process of manufacturing pearlescent pigments according to claim 1, wherein the step of coating with the coating layer A comprises a hydrolytic decomposition of an organic silicon compound.

3. The process of manufacturing pearlescent pigments according to claim 1, wherein the step of coating with the coating layer B comprises a precipitation of a dissolved metal salt, followed by annealing.

4. The process of manufacturing pearlescent pigments according to claim 1, wherein the monolithically structured aluminum platelets are entirely converted into aluminum oxide or aluminum hydroxide in the wet oxidation step.

5. The process of manufacturing pearlescent pigments according to claim 1, wherein the coating layer B is coated with one or more layer(s) of a metal oxide having a refractive index of at least 1.8.

6. The process of manufacturing pearlescent pigments according to claim 1, wherein the monolithically structured aluminum platelets have an average thickness of from 2.5 to 80 nm.

7. The process of manufacturing pearlescent pigments according to claim 1, wherein the monolithically structured aluminum platelets have an average size of from 5 to 100 m.

8. The process of manufacturing pearlescent pigments according to claim 1, wherein the wet oxidation step is carried out in a temperature range of from 25 to 100 C. for 10 minutes to 48 hours.

9. The process of manufacturing pearlescent pigments according to claim 1, wherein the wet oxidation step is carried out at a pH in the range of from 8 to 12, using a basic solution selected from the group consisting of sodium hydroxide, potassium hydroxide, and ammonia.

10. Pearlescent pigments, obtainable by the process of manufacturing pearlescent pigments according to claim 1, comprising: monolithically structured, partially or entirely wet-oxidized aluminum platelets having an average thickness of from 1 to 150 nm, and having an aspect ratio expressed by the ratio of the average size to the average thickness of at least 80; a coating layer A of silicon dioxide; and a coating layer B of a metal oxide as an interference layer, with the metal oxide being other than silicon dioxide and having a refractive index of at least 1.8, wherein the coating layer A and the coating layer B coat the wet-oxidized aluminum platelets in this order, wherein the coating layer B is coated with one or more layer(s) of a metal oxide having a refractive index of at least 1.8.

11. The pearlescent pigments according to claim 10, wherein the monolithically structured, wet-oxidized aluminum platelets are entirely oxidized.

12. The pearlescent pigments according to claim 10, wherein the monolithically structured, wet-oxidized aluminum platelets have an average thickness of from 2.5 to 80 nm.

13. The pearlescent pigments according to claim 10, wherein the monolithically structured, wet-oxidized aluminum platelets have an average size of from 5 to 100 m.

14. A composition comprising the pearlescent pigments according to claim 10.

15. The composition of claim 14, wherein the composition is chosen from a varnish, an ink, a printing ink, a plastic, a glass, a ceramic, or a cosmetic composition.

Description

(1) The figures show:

(2) FIG. 1 shows a transmission electron microscopy (TEM) image in combination with energy dispersive X-ray analysis of an aluminum platelet coated with the coating layers A and B before carrying out the wet oxidation step. As can be taken from FIG. 1, there is no oxygen present in the aluminum platelet.

(3) FIG. 2 shows a transmission electron microscopy (TEM) image in combination with energy dispersive X-ray analysis of an aluminum platelet coated with the coating layers A and B after carrying out the wet oxidation step. As can be taken from FIG. 2, oxygen is present throughout the aluminum platelet, i.e. the aluminum platelet has been entirely converted into aluminum oxide. In addition, FIG. 2 illustrates that aluminum atoms have migrated into the coating layer B as a result of the wet oxidation step.

(4) The following Examples are intended to further illustrate the present invention without being limited thereto.

EXAMPLE 1

(5) Thin aluminum oxide platelets with silicon dioxide coating (50 nm) and iron (III) oxide coating (90 nm)

(6) First, 10 g of aluminum platelets (thickness between 10 nm and 20 nm, d.sub.50=20 m) were coated by means of a sol gel process using tetraethyl orthosilicate (TEOS) to yield 60 g SiO.sub.2. In a round bottom flask equipped with a reflux condenser and a stirrer, these aluminum platelets were exposed to 500 mL of deionized water and warmed up to 75 C. under stirring. The pH was adjusted to a value of 3.2 by adding a NaOH solution (10 weight %). 700 g of a FeCl.sub.3 solution (40 weight %) were added to the reaction mixture. The pH was kept essentially constant at 3.2 by simultaneously adding a NaOH solution (10 weight %). After the FeCl.sub.3 solution had been completely added, the mixture was stirred for further 15 minutes in order to ensure a complete precipitation. Then, the pH was increased to a value of 7.0 by adding a NaOH solution (10 weight %) for 30 minutes in a dropwise manner. After 30 minutes of further stirring, the coated pigments were separated from the supernatant reaction solution by filtration and washed until the pigments were free of salt. Finally, the coated aluminum platelets were annealed in an air atmosphere at 350 C. for 5 hours.

(7) The such obtained pigments (Al: 5%, SiO.sub.2: 30%, Fe.sub.2O.sub.3: 65%) were then exposed to a wet oxidation step. 50 g of said pigments which had a golden color hue were dispersed in 500 g of water and heated up to 95 C., and kept for 1 hour at pH 10.5 using an aqueous ammonia solution (25 weight %). The slurry was then filtrated, washed by water, dried at 120 C., and sieved through a sieve with 32 m mesh size.

(8) The pearlescent pigments obtained after the wet oxidation step appeared orange in color, and were evaluated using a BYK-mac device in terms of their coloristic properties.

(9) When compared to a commercially available pigment with a comparable color, the pearlescent pigment of Example 1 exhibited a better coverage property in terms of its total color difference E45, and a higher chroma C*15, which can be taken from Table A, below.

(10) TABLE-US-00001 TABLE A coloristic properties of the pearlescent pigment of Example 1 and a commercially available pigment with a comparable color Sample L*15 a*15 b*15 C*15 E45 Example 1 91.7 50.3 72.9 88.6 17.7 Iriodin 9502 109.5 42.8 39.6 58.3 24.8

EXAMPLE 2

(11) Thicker aluminum oxide platelets with silicon dioxide coating (50 nm) and iron (III) oxide coating (90 nm)

(12) In contrast to Example 1 as described above, aluminum platelets having a thickness of 80 nm were used in Example 2. Apart from this, the coating of the aluminum platelets with the coating layer A (SiO.sub.2) and with the coating layer B (Fe.sub.2O.sub.3) was performed in the same way.

(13) The such obtained pigments (Al: 22.5%, SiO.sub.2: 5.5%, Fe.sub.2O.sub.3: 72%) were then exposed to a wet oxidation step. 50 g of said pigments which had a golden color hue were dispersed in 500 g of water and heated up to 30 C., and kept for 1 hour at pH 12 using a KOH solution (30 weight %). The slurry was then filtrated, washed by water, dried at 120 C., and sieved through a sieve with 32 m mesh size.

(14) The pearlescent pigments obtained after the wet oxidation step appeared orange in color, and were evaluated using a BYK-mac device in terms of their coloristic properties.

(15) When compared to a commercially available pigment with a comparable color, the pearlescent pigment of Example 2 exhibited a better coverage property in terms of its total color difference E45, and a higher chroma C*15, which can be taken from Table B, below.

(16) TABLE-US-00002 TABLE B coloristic properties of the pearlescent pigment of Example 2 and a commercially available pigment with a comparable color Sample L*15 a*15 b*15 C*15 E45 Example 2 92.6 37.4 51.6 63.7 8.9 Iriodin 9502 109.5 42.8 39.6 58.3 24.8

EXAMPLE 3

(17) 76.34 g of aluminum paste (thickness 100 nm, d.sub.50=26 m, 65.5 weight % solids, stearin acid 0.8 weight % in solids, 34.5 weight % white spirit, equivalent to 50 g aluminum, dollarflakes) were dispersed in 500 g of isopropanol in a 1 L round flask, and stirred for 30 minutes with heating up to 70 C. 66.67 g of an aqueous ammonia solution (25 weight %) were added and kept for 8 hours at 70 C. with mixing. The such prepared sample was filtrated, washed with isopropanol, dried at 120 C., and sieved through a sieve with 32 m mesh size. In the wet-oxidized sample, metal aluminum was contained in an amount of 36.20% as determined by a TGA analyzer.

(18) Then, a silicon dioxide layer was applied on 68.4 g of the above wet-oxidized sample by means of a sol gel process using tetraethyl orthosilicate (TEOS) to yield 38 g SiO.sub.2.

(19) Finally, an iron (III) oxide layer was applied on the wet-oxidized sample coated with the silicon dioxide layer as follows: 600 mL of the above slurry (containing 20 g aluminum in total, calculated from aluminum and aluminum oxide) were heated up to 75 C., and 10 g of iron (III) oxide were coated on the sample under stirring at pH 3.2 using a FeCl.sub.3 solution (20 weight %) and an aqueous ammonia solution (18 weight %). The such prepared sample was filtrated, washed by water, dried at 120 C., and calcinated at 350 C.

(20) The pearlescent pigments obtained after the final coating step appeared bronze in color, and were evaluated using a BYK-mac device in terms of their coloristic properties.

(21) When compared to a commercially available pigment with a comparable color, the pearlescent pigment of Example 3 exhibited a better coverage property in terms of its total color difference E45, and a higher chroma C*15, which can be taken from Table C, below.

(22) TABLE-US-00003 TABLE C coloristic properties of the pearlescent pigment of Example 3 and a commercially available pigment with a comparable color Sample L*15 a*15 b*15 C*15 E45 Example 3 132.3 24.3 74.1 78.0 0.3 Iriodin 500 120.9 19.8 43.4 47.7 19.4

EXAMPLE 4

(23) 69.16 g of aluminum paste (thickness 100 nm, d.sub.50=22 m, 72.3 weight % solids, stearin acid 0.8 weight % in solids, 27.7 weight % white spirit, equivalent to 50 g aluminum, dollarflakes) were dispersed in 500 g of isopropanol in a 1 L round flask, and stirred for 30 minutes with heating up to 65 C. 80 g of an aqueous ammonia solution (25 weight %) were added and kept for 8 hours at 65 C. with mixing. The such prepared sample was filtrated, washed with isopropanol, dried at 120 C., and sieved through a sieve with 32 m mesh size. In the wet-oxidized sample, metal aluminum was contained in an amount of 30% as determined by a TGA analyzer.

(24) Then, a silicon dioxide layer was applied on 33 g of the above wet-oxidized sample by means of a sol gel process using tetraethyl orthosilicate (TEOS) to yield 30 g SiO.sub.2.

(25) Finally, a titanium dioxide layer was applied on the wet-oxidized sample coated with the silicon dioxide layer as follows: 600 mL of the above slurry (containing 20 g aluminum in total, calculated from aluminum and aluminum oxide) were heated up to 75 C., and 15 g of titanium dioxide were coated on the sample under stirring at pH 3.4 using a TiOCl.sub.2 solution (25.33 weight %) and an aqueous ammonia solution (13 weight %). In the coating process, 10 mL of the slurry were sampled to evaluate the colors. Each sample was filtrated, washed by water, dried at 120 C., and calcinated at 350 C.

(26) The pearlescent pigments obtained after the final coating step were evaluated using a BYK-mac device in terms of their coloristic properties.

(27) When compared to a commercially available Ti pigment, the pearlescent pigment of Example 4 exhibited a better coverage property in terms of its total color difference E45, and a higher chroma C*15, which can be taken from Table D, below.

(28) TABLE-US-00004 TABLE D coloristic properties of the pearlescent pigment of Example 4 and a commercially available Ti pigment Sample L*15 a*15 b*15 C*15 E45 Example 4 132.3 24.3 74.1 78.0 0.3 Iriodin 7235 97.0 47.6 20.8 52.0 53.9

(29) Despite the partially or entirely transparent nature of the wet-oxidized aluminum platelets, the total color difference E45 was comparatively small. Accordingly, the pearlescent pigments obtained in Examples 1 to 4 had an excellent coverage property. In addition, the pearlescent pigments had an excellent chroma C*15.