Magnetic pigments comprising a flaky substrate and layer of maghemite

Abstract

Magnetic pigments containing a transparent flaky homogeneously composed substrate having two parallel major surfaces and a coating containing maghemite, a process for the production of said pigments as well as their use.

Claims

1. Magnetic pigments, comprising a substrate and a coating encapsulating the substrate wherein the substrate is a transparent flaky homogeneously composed substrate having two parallel major surfaces and wherein the coating comprises maghemite, wherein the maghemite coating is doped with at least one alkaline earth metal oxide.

2. Magnetic pigments according to claim 1, wherein the substrate is of a synthetically produced material.

3. Magnetic pigments according to claim 1, wherein the substrate is of AI.sub.2O.sub.3, of Al.sub.2O.sub.3 containing up to 5% by weight TiO.sub.2, of SiO.sub.2, of SiO.sub.2 containing up to 20% by weight silicon hydroxide, of glass or of a borosilicate.

4. Magnetic pigments according to claim 1, wherein the substrate has a mean thickness between 100 nm and 1000 nm.

5. Magnetic pigments according to claim 1, wherein the maghemite coating is located directly onto the substrate.

6. Magnetic pigments according to claim 1, wherein there is at least one dielectric coating located between the substrate and the maghemite coating.

7. Magnetic pigments according to claim 5, comprising furthermore one or more dielectric coatings on top of the maghemite coating.

8. Magnetic pigments according to claim 1, wherein the alkaline earth metal oxide is chosen from the group of oxides of Mg, Ca, Sr and Ba or mixtures thereof.

9. Magnetic pigments according to claim 1, wherein the alkaline earth metal oxide is MgO.

10. Magnetic pigments according to claim 1, wherein the content of the alkaline earth metal oxide is between 0.01 and less than 0.1% by weight, based on the weight of the maghemite coating.

11. Magnetic pigments according to claim 1, exhibiting a golden interference color.

12. Process for the production of pigments according to claim 1, comprising: (a) dispersing particles being composed of transparent flaky homogeneously composed substrates having two parallel major surfaces, optionally being coated with at least one dielectric layer, in water, (b) adjusting a pH value of between 2 and 4 and keeping the pH value constant, (c) adding a water soluble iron compound while still keeping the pH value constant, (d) raising the pH to a value between 5.5 and 7.5, (e) adding an aqueous solution of an alkaline earth metal compound while keeping the pH value constant, (f) optionally washing and filtering the resulting product and (g) drying at a temperature between 100 C. and 250 C. for a time period between 1 and 10 hours or, alternatively, calcining at a temperature between 350 C. and 450 C. for a time period between 5 and 30 minutes.

13. Process according to claim 12 which is executed in an inert gas atmosphere.

14. Process according to claim 12, wherein the alkaline earth metal compound in step (e) is added while simultaneously adding a further water soluble iron compound.

15. Process according to claim 12, wherein the iron compound added in step (c) is a iron (III) compound and the iron compound added in step (e) comprises at least an iron (II) compound.

16. Process according to claim 12, wherein the alkaline earth metal compound is a magnesium compound.

17. A composition for pigmenting inks, paints, varnishes, coating compositions, plastics, foils, paper, ceramics, glasses, cosmetic and pharmaceutic formulations, for laser marking and for pigmenting pigment preparations of diverse solvent contents comprising a pigment of claim 1.

18. Composition according to claim 17, wherein the ink is a printing ink.

19. Product, comprising a pigment according to claim 1.

20. Product according to claim 19, which is a security product.

21. Security product according to claim 20, which is a bank note, a cheque, a passport, an identification document, a smart card, a drivers license, a share certificate, a bond, a cheque card, a tax banderol, a postage stamp, a ticket, a credit card, a debit card, a telephone card, a lottery ticket, a gift voucher, a packing material, a decorative material, a brand product or any other product which has to be secured.

22. Magnetic pigments according to claim 1, wherein there is no gradient or particular zones of different materials within a single substrate particle.

23. Magnetic pigments, comprising a transparent flaky homogeneously composed substrate having two parallel major surfaces and a coating comprising maghemite, wherein the substrate is of Al.sub.2O.sub.3, of Al.sub.2O.sub.3 containing up to 5% by weight TiO.sub.2, SiO.sub.2, SiO.sub.2 containing up to 20% by weight silicon hydroxide, glass or a borosilicate, and wherein the maghemite coating is a single layer containing maghemite as sole iron compound and is located directly on the substrate.

24. Magnetic pigments according to claim 23, comprising furthermore one or more dielectric coatings on top of the maghemite coating.

25. Magnetic pigments according to claim 23, wherein the maghemite coating is doped with at least one alkaline earth metal oxide.

26. Magnetic pigments according to claim 25, wherein the alkaline earth metal oxide is an oxide of Mg, Ca, Sr, Ba or mixtures thereof.

27. Magnetic pigments according to claim 26, wherein the alkaline earth metal oxide is MgO.

28. A process for the production of pigments according to claim 23, comprising: (a) dispersing in water particles being composed of transparent flaky homogeneously composed substrates having two parallel major surfaces, wherein the substrate is Al.sub.2O.sub.3, Al.sub.2O.sub.3 containing up to 5% by weight of TiO.sub.2, SiO.sub.2, SiO.sub.2 containing up to 20% by weight silicon hydroxide, glass or a borosilicate, (b) adjusting a pH value of between 2 and 4 and keeping the pH value constant, (c) adding a water soluble iron compound while still keeping the pH value constant, (d) raising the pH to a value between 5.5 and 7.5, (e) optionally adding an aqueous solution of an alkaline earth metal compound while keeping the pH value constant, (h) optionally washing and filtering the resulting product and (i) drying at a temperature between 100 C. and 250 C. for a time period between 1 and 10 hours or, alternatively, calcining at a temperature between 350 C. and 450 C. for a time period between 5 and 30 minutes.

29. A composition for pigmenting inks, paints, varnishes, coating compositions, plastics, foils, paper, ceramics, glasses, cosmetic and pharmaceutic formulations, for laser marking and for pigmenting pigment preparations of diverse solvent contents comprising a pigment of claim 23.

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 220 nm, mean particle diameter 18 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 15 wt. %). While still keeping the pH value constant, a Fe(NO.sub.3).sub.3 solution (5.65 g of Fe(NO.sub.3).sub.3*9 H.sub.2O in 100 ml deionized water) is added to the suspension. The pH value is 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 a Mg component and at least a Fe component (243 g FeSO.sub.4*7 H.sub.2O, 108 g MgSO.sub.4*7 H.sub.2O and 16 g Fe(NO.sub.3).sub.3*9 H.sub.2O, in 1000 ml deionized water) is slowly metered into the suspension which is then kept for another 30 minutes while stirring. The resulting pigments are separated off by filtering, washed with deionized water and dried for about 4 hours at 200 C. in an oven.

(2) The resulting pigment exhibits a strong golden color with vivid lustre and high magnetism. The structure of the iron oxide layer on the substrate was confirmed to be gamma iron oxide (maghemite) by powder X-ray diffraction and thermal analysis (TG/DTA).

EXAMPLE 2

(3) The procedure of example 1 is repeated except that MgSO.sub.4*7 H.sub.2O is not added to the suspension.

(4) The resulting pigment exhibits a golden lustreous color and very good magnetic properties.

COMPARATIVE EXAMPLE 1

(5) The procedure of example 1 is repeated except that mica (mean particle diameter 20 m, mean thickness 450 nm) is used instead of the aluminum dioxide flake as transparent substrate.

(6) The resulting pigment exhibits a yellowish color with low chroma and a weak magnetism.

(7) Measurement of Coloristic Properties:

(8) 0.5 g of each of the pigments received in examples 1 and 2 as well as in comparative example 1 are mixed into 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 onto usual black/white paper strips and dried.

(9) The coloristic properties of the samples are measured on the black precoated areas of the paper strips using a Minolta CR-300 apparatus (product of Konica Minolta Holdings, Inc.). The coloristic results are as follows:

(10) TABLE-US-00001 TABLE 1 Sample L a b C Example 1 53.7 7.22 32.34 33.13 Example 2 51.69 5.76 29.67 30.22 Comp. Example 1 41.87 1.57 12.97 13.08

(11) From the results in Table 1 it is clearly detectable that the pigments according to example 1, having a doping of MgO within the maghemite layer, exhibit a very strong golden color with high lustre and high chroma. The coloristic results of the pigments according to example 2 are somewhat lower in each instance when compared to example 1, but still sufficient in order to show a distinct and vivid golden interference color with good chroma. In contrast, the sample of comparative example 1 does merely exhibit a weak yellowish color with low chroma and lustre.

(12) Measurement of Magnetic Properties:

(13) The magnetic properties of the samples resulting from examples 1, 2 and comparative example 1 were measured using a vibration magnetometer (Model VSM-5, product of Toei Industry Co., Ltd.) and the standardized procedure (in a 10 kOe magnetic field at room temperature).

(14) The corresponding results are as follows:

(15) TABLE-US-00002 TABLE 2 M.sub.s M.sub.r Hc SR Sample Substr. Doping emu/g emu/g Oe Ms/Mr ex. 1 alumina MgO 28.74 14.51 334 0.5048 ex. 2 alumina none 29.16 14.78 349 0.5069 comp. ex. 1 mica MgO 21.65 8.62 148 0.3083 M.sub.s Saturation magnetization M.sub.r Residual magnetization Hc Coercive force SR Ms/Mr

(16) The results as shown in Table 2 exhibit absolutely comparable magnetic properties of the sample according to example 1 in comparison to the sample according to example 2, whereas the magnetic behavior of the mica based magnetic pigments according to comparative example 1 are much weaker than those of the alumina based magnetic pigments according to the present invention.