Pearlescent pigments, process for producing them, and use of such pigments

Abstract

The present invention relates to semitransparent pearlescent pigments, to processes for producing them, and to the use of such pearlescent pigments, where the pearlescent pigments comprise monolithically constructed substrate platelets composed of a metal oxide having an average thickness of 1 to 40 nm and a form factor, expressed by the ratio of the mean size to the average thickness, of at least 80, which are enveloped by at least one substantially transparent coating A composed of at least one low-index metal oxide and/or metal oxide hydrate, having a refractive index of less than 1.8, and at least one interference layer in the form of a coating B composed of at least one high-index metal oxide, having a refractive index of at least 1.8.

Claims

1. A process for producing semitransparent pearlescent pigments, comprising the steps of providing monolithically constructed metallic substrate platelets having an average thickness of 1 to 40 nm and a form factor, expressed by the ratio of the mean size to the average thickness, of at least 80, coating the metallic substrate platelets with an enveloping, substantially transparent coating A composed of at least one low-index metal oxide and/or metal oxide hydrate, having a refractive index of less than 1.8, coating the thus-coated metallic substrate platelets with at least one interference layer in the form of a coating B composed of at least one high-index metal oxide, having a refractive index of at least 1.8, the coating steps taking place by hydrolytic decomposition of one or more organometallic compounds and/or by precipitation of one or more dissolved metal salts, and subsequently calcining the thus-coated substrate platelets at 550 to 1200° C. for a period of 4 hours to 12 hours, to convert the metallic substrate platelets into the corresponding metal oxide where the metallic substrate platelets consist of aluminum, which is then converted completely, in the course of calcining, into aluminum oxide.

2. The process as claimed in claim 1, wherein before the calcining step, in coated substrate platelets, the amount-of-substance ratio α between oxygen not bonded to aluminum and aluminum is at least 3.

3. The process as claimed in claim 1, wherein the coating B is constructed substantially of a high-index metal oxide selected from at least one of iron(III) oxide, cobalt(II) oxide, chromium(III) oxide, nickel(II) oxide, copper(I/II) oxide, vanadium(V) oxide, titanium(III) oxide, titanium dioxide, antimony(III) oxide, zinc(II) oxide and/or zirconium dioxide.

4. The process as claimed in claim 1, wherein the coating A is constructed of at least one coating selected from silicon (di)oxide, silicon oxide hydrate, aluminum oxide, aluminum oxide hydrate, cerium oxide, tin oxide, and mixtures thereof.

5. The process as claimed in claim 1, wherein the metallic substrate platelets used have an average thickness of 25 nm or less.

6. The process as claimed in claim 1, wherein the metallic substrate platelets have a mean size d.sub.50 of 5 to 100 μm.

7. The process as claimed in claim 1, wherein the average thickness of the coating A is 5 to 50 nm.

8. The process as claimed in claim 1, wherein the average thickness of the coating B is 1 to 250 nm.

9. The process as claimed in claim 1, further comprising the step of providing a further coating C on the coating B, selected from metal oxides, metal oxide hydrates and/or organic silane compounds.

10. The process as claimed in claim 1, wherein the average thickness of the coating A is more than 50 nm.

11. The process for producing semitransparent pearlescent pigments of claim 1, wherein subsequently calcining the thus-coated substrate platelets occurs at 600 to 1200° C. for a period of 4 hours to 12 hours.

Description

(1) FIG. 1 shows a transmission electron micrograph (TEM) of a coated aluminum oxide platelet of the invention (cf. example 2). The aluminum oxide platelet (1) has a very uniform thickness and is enveloped by an SiO.sub.2 layer (2) (coating A, light) and by an iron oxide layer (3) (coating B, dark).

(2) FIG. 2 shows different TEM micrographs of a coated aluminum oxide platelet of the invention in accordance with example 4, extended with energy-dispersive x-ray analysis (EDX, also called energy-dispersive x-ray spectroscopy, EDS). In evidence are (from top to bottom) the scans of aluminum, silicon, iron, and oxygen. It is clearly apparent that the oxygen is also present in the aluminum layer.

(3) FIG. 3 shows transmitted-light micrographs of an (Al—SiO2-Fe2O3) pigment treated at a temperature of 400° and at 600°. It is clearly apparent that the higher temperature treatment produces a transparency.

(4) The examples below serve as further illustration of the present invention, without being confined thereto.

EXAMPLE 1—THIN ALUMINUM PLATELETS WITH SIO.SUB.2 .(50 NM) AND IRON OXIDE COATING (110 NM)

(5) First of all 10 g of Al platelets (thickness (t.sub.50) between 10 nm and 20 nm, d.sub.50=15 μm) were coated with 60 g of SiO.sub.2 by means of a sol-gel process using tetraethyl orthosilicate (TEOS). In a round-bottomed flask with reflux condenser and stirrer, these Al platelets were admixed with 500 ml of deionized water and heated to 75° C. with stirring. The pH was adjusted to a level of 3.2 by addition of 10% NaOH solution. The reaction mixture was admixed with 700 g of 40% FeCl.sub.3 solution, the pH being kept substantially constant at 3.2 by simultaneous addition of 10% NaOH solution. Following complete addition of the FeCl.sub.3 solution, the mixture was stirred for 15 minutes more in order to ensure complete precipitation. The pH was then raised to a level of 7.0 by dropwise addition of 10% NaOH solution over a period of 30 minutes. After 30 minutes of further stirring, the coated pigment was separated from the supernatant reaction solution by filtration, and was washed until salt-free.

(6) The coated aluminum platelets obtained were calcined for 290 minutes at a heating rate of 2° C./minute up to about 600° and sieved with a sieve (mesh size 32 μm). The product obtained was evaluated for its color properties.

EXAMPLE 2—THIN ALUMINUM PLATELETS WITH SIO.SUB.2 .(50 NM) AND IRON OXIDE COATING (100 NM)

(7) First of all 10 g of Al platelets (thickness (t.sub.50) between 10 nm and 20 nm, d.sub.50=15 μm) were coated with 60 g of SiO.sub.2 by means of a sol-gel process using tetraethyl orthosilicate (TEOS). In a round-bottomed flask with reflux condenser and stirrer, these Al platelets were admixed with 500 ml of deionized water and heated to 75° C. with stirring. The pH was adjusted to a level of 3.2 by addition of 10% NaOH solution. The reaction mixture was admixed with 650 g of 40% FeCl.sub.3 solution, the pH being kept substantially constant at 3.2 by simultaneous addition of 10% NaOH solution. Following complete addition of the FeCl.sub.3 solution, the mixture was stirred for 15 minutes more in order to ensure complete precipitation. The pH was then raised to a level of 7.0 by dropwise addition of 10% NaOH solution over a period of 30 minutes. After 30 minutes of further stirring, the coated pigment was separated from the supernatant reaction solution by filtration, and was washed until salt-free.

(8) The coated aluminum platelets obtained were calcined for 290 minutes at a heating rate of 2° C./minute up to about 600° and sieved with a sieve (mesh size 32 μm). The product obtained was evaluated for its color properties.

EXAMPLE 3—THIN ALUMINUM PLATELETS WITH SIO.SUB.2 .(15 NM) AND IRON OXIDE COATING (100 NM)

(9) First of all 10 g of Al platelets (thickness (t.sub.50) between 20 nm and 30 nm, d.sub.50=20 μm) were coated with 10 g of SiO.sub.2 by means of a sol-gel process using tetraethyl orthosilicate (TEOS). In a round-bottomed flask with reflux condenser and stirrer, these Al platelets were admixed with 500 ml of deionized water and heated to 75° C. with stirring. The pH was adjusted to a level of 3.2 by addition of 10% NaOH solution. The reaction mixture was admixed with 400 g of 40% FeCl.sub.3 solution, the pH being kept substantially constant at 3.2 by simultaneous addition of 10% NaOH solution. Following complete addition of the FeCl.sub.3 solution, the mixture was stirred for 15 minutes more in order to ensure complete precipitation. The pH was then raised to a level of 7.0 by dropwise addition of 10% NaOH solution over a period of 30 minutes. After 30 minutes of further stirring, the coated pigment was separated from the supernatant reaction solution by filtration, and was washed until salt-free.

(10) The coated aluminum platelets obtained were calcined for 290 minutes at a heating rate of 2° C./minute up to about 600° and sieved with a sieve (mesh size 32 μm). The product obtained was evaluated for its color properties.

EXAMPLE 4—THIN ALUMINUM PLATELETS WITH SIO.SUB.2 .(25 NM) AND IRON OXIDE COATING (110 NM)

(11) First of all 10 g of Al platelets (thickness (t.sub.50) between 10 nm and 20 nm, d.sub.50=15 μm) were coated with 30 g of SiO.sub.2 by means of a sol-gel process using tetraethyl orthosilicate (TEOS). In a round-bottomed flask with reflux condenser and stirrer, these Al platelets were admixed with 500 ml of deionized water and heated to 75° C. with stirring. The pH was adjusted to a level of 3.2 by addition of 10% NaOH solution. The reaction mixture was admixed with 750 g of 40% FeCl.sub.3 solution, the pH being kept substantially constant at 3.2 by simultaneous addition of 10% NaOH solution. Following complete addition of the FeCl.sub.3 solution, the mixture was stirred for 15 minutes more in order to ensure complete precipitation. The pH was then raised to a level of 7.0 by dropwise addition of 10% NaOH solution over a period of 30 minutes. After 30 minutes of further stirring, the coated pigment was separated from the supernatant reaction solution by filtration, and was washed until salt-free.

(12) The coated aluminum platelets obtained were calcined for 290 minutes at a heating rate of 2° C./minute up to about 600° and sieved with a sieve (mesh size 32 μm). The product obtained was evaluated for its color properties. In FIG. 3 the corresponding TEM micrographs are extended with EDX analysis, demonstrating that the metallic Al substrate platelet is completely oxidized.

EXAMPLE 5—THIN ALUMINUM PLATELETS WITH TITANIUM DIOXIDE COATING

(13) First of all 10 g of Al platelets (thickness between 14 nm and 18 nm, d.sub.50=14 μm) were coated with 60 g of SiO.sub.2 by means of a sol-gel process using tetraethyl orthosilicate (TEOS).

(14) Subsequently 163 g of titanium dioxide were applied by means of TiOCl in 500 ml of water with stirring, at a pH of 2.0 and at 75° C. After the reaction, the pH was raised to 5.5. The coated aluminum flakes produced in this way were filtered and carefully washed and were dried at 120°.

(15) The coated aluminum flakes were subsequently heated to 800° C. at 2° C./minute. After cooling, the completed pigment was additionally sieved with a 25 μm sieve and then subjected to coloristic testing.

EXAMPLE 6—COLORISTIC INVESTIGATIONS

(16) For the coloristic testing, a paint having a pigment concentration of 12 wt % (in the case of masstone pearlescent pigments as per example 1 and example 4—cf. tables 1 and 2) or 6 wt % (in the case of transparent pearlescent pigments as per example 5—cf. table 3), based on the binder, was produced in each case. By means of a doctor blade, a black plate was coated with a film thickness of 12 μm. The color data were determined using a Byk-mac from Byk Additive and Instrumente.

(17) Employed for comparison were commercial pigments from Merck (Iriodin® 502 Red-brown) and from BASF (Mearlin® Exterior CFS Super Copper 3503Z), these being pearl pigments with inorganic substrate based on mica.

(18) The comparison was between pearlescent pigments with virtually the same interference color angle in each case, and the values were contrasted (cf. tables 1 and 2). Critical in this context are the chroma values of the interference (C), and also, in the case of masstone pearlescent pigments, the intensity of the masstone color and the hiding power expressed in the ΔE are important.

(19) Table 1 shows results for masstone pearlescent pigments, with the inventive pearlescent pigments from example 1 being compared with Mearlin® Exterior CFS Super Copper 3503Z from BASF.

(20) Table 2 shows results for masstone pearlescent pigments, with the inventive pearlescent pigments from example 4 being compared with Iriodin® 502 Red-brown from Merck and Mearlin® Exterior CFS Super Copper 3503Z from BASF.

(21) TABLE-US-00001 TABLE 1 Orange pigments L* a* b* C L* a* b* C No. 45°/15° 45°/15° 45°/15° 45°/15° 45°/45° 45°/45° 45°/45° 45°/45° ΔE 45° 1 80.8 80.8 61.4 81.7 25.1 27.8 28.5 39.8 18.2 Mearlin ® 3503Z 90.7 43.9 37.6 57.8 23.2 20.0 20.1 28.4 24.0

(22) TABLE-US-00002 TABLE 2 Orange-red pigments L* a* b* C L* a* b* C No. 45°/15° 45°/15° 45°/15° 45°/15° 45°/45° 45°/45° 45°/45° 45°/45° ΔE 45° 4 89.9 58.0 72.0 92.4 29.1 28.9 36.8 46.7 7.7 Iriodin ® 502 92.0 92.0 47.9 66.1 21.8 20.2 21.0 29.1 27.0

(23) From tables 1 and 2 it is evident that the inventive pearlescent pigments, in comparison to commercial pigments and at comparable interference color angles, display significantly higher chroma values.

(24) In particular, the chroma of the masstone color (45°/45°) of example 1 is much higher than the corresponding chroma of the commercial pigment Mearlin® 3503Z. The interference chroma (C 45°/15°) of example 1 as well is improved in comparison to Mearlin® 3503Z.

(25) From the comparison of example 4 with Iriodin® 502, there is again a distinct improvement apparent in the chroma of the masstone color (45°/45°) and also in the interference chroma (C 45°/15°).

(26) The figures in tables 1 and 2, furthermore, demonstrate that the inventive pearlescent pigments in the case of masstone pigments exhibit a particularly low color difference ΔE and hence a particularly high hiding power.