Magnetic pigments

Abstract

The present invention is related to magnetic pigments comprising a transparent flaky homogeneously composed substrate having two parallel major surfaces and a coating comprising a layered structure composed of a hematite and a magnetite layer, to a process for the production of said pigments as well as to their use.

Claims

1. A magnetic pigment, comprising a transparent flaky homogeneous substrate having two parallel major surfaces wherein said substrate is a synthetically produced material of Al.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 and a coating comprising a layered structure, said layered structure consisting of a first layer comprising hematite and/or goethite having a thickness, and of a second layer comprising magnetite, the second layer having a thickness and being located on the first layer, in this sequence, at least on the two major surfaces of the substrate, wherein the thickness of the second layer of magnetite is larger than the thickness of the first layer of hematite and/or goethite and wherein the magnetic pigments further comprise one or more dielectric layers on top of the magnetite layer wherein said dielectric layer comprises a silicon oxide hydrate layer being located directly on the magnetite layer and a high refractive index layer on the silicon oxide hydrate layer.

2. A magnetic pigment according to claim 1, wherein the substrate has a mean thickness between 100 and 1000 nm.

3. A magnetic pigment according to claim 1, wherein the layer composed of hematite and/or goethite is located directly onto the substrate.

4. A magnetic pigment according to claim 1, wherein there is at least one dielectric coating located between the substrate and the layer composed of hematite and/or goethite.

5. A magnetic pigment according to claim 1, wherein the magnetite layer is doped with an aluminum compound.

6. A magnetic pigment according to claim 5, wherein the aluminum compound is an oxide and/or an oxide hydrate.

7. A magnetic pigment according to claim 5, wherein the content of the aluminum compound is between 0.1 and less than 5% by weight, based on the weight of the magnetite layer.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1: exhibits a magnetically induced pattern, prepared by contacting a black and white lacquer card coated with a lacquer containing pigments according to example 1 with a permanent magnetic sheet exhibiting such a pattern

(2) FIG. 2 depicts depict the sequence of the claimed layers (20first layer, 30second layer) and their relative locations on the major surfaces (10) of the pigment substrate.

(3) FIG. 3 depicts the sequence of the claimed layers (20first layer, 30second layer, 40silicon oxide hydrate, 50high refractive index layer) and their relative locations on the major surfaces (10) of the pigment substrate.

EXAMPLE 1

(4) 200 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 17.5 wt. %). While still keeping the pH value constant, a Fe(NO.sub.3).sub.3 solution (100 ml, 10.12 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 then 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 an Al component and of a Fe(II) and Fe(III) component (800 ml, 288 g FeSO.sub.4*7 H.sub.2O, 1.5 g Al.sub.2(SO.sub.4).sub.3*16 H.sub.2O and 18 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. Thereafter, a water glass solution (about 190 g, 26% as SiO.sub.2) is added while still 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.

(5) Thereafter, the pigments are resuspended in another 2000 ml of deionized water, heated to a temperature of about 75 C., while adjusting a pH of about 2.0. A water soluble tin compound (124 ml, 6.95 g SnCl.sub.4) is added to the pigment suspension while keeping the pH constant. Next, a water soluble titanium compound (800 ml, 100 g TiCl.sub.4 in 1000 ml of deionized water) is added to the pigment suspension. The suspension is kept for another 10 minutes, then filtered and washed with deionized water. Eventually, the resulting pigments are dried at a temperature of about 120 C. and sieved.

(6) The resulting pigment exhibits a brilliant silver powder colour with vivid luster and high magnetism as well as a high hiding power.

(7) The powder colour is measured by using a Minolta CR-300 apparatus (product of Konica Minolta Holdings, Inc.). The resulting L value is 69.4, the a value is 1.9, the b value is 8.7 and chroma is 8.9.

(8) A lacquer card is prepared as follows: 0.5 g of the pigment according to example 1 is mixed with 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. The resulting colouristic characteristics are measured by using the Minolta apparatus as mentioned above. The L value is determined to be 48.5, the a value is 2.3, the b value is 9.2 and chroma is determined to be 9.5.

(9) The magnetic characteristics are measured by means of a vibrating sample magnetometer (Model VSM-5, made by Toei Industry Co., Ltd.) and the standardized procedure (in a 10 kOe magnetic field at room temperature). The saturation magnetization (M.sub.s) is 14.21 emu/g, the residual magnetization (M.sub.r) 9.0 emu/g and the coercive force 444 Oe.

(10) Furthermore, a further black and white lacquer card is prepared as mentioned before. While the coating is still wet, a permanent magnetic sheet carrying a pattern is brought into contact with the reverse side of the lacquer card, facing away from the wet coating layer. The magnetic pigments contained in the coating lacquer orient themselves according to the magnetic lines of the magnetic field generated by the permanent magnetic sheet. The resulting magnetically induced pattern, achieved after drying of the pigment containing coating layer, is shown in FIG. 1.

EXAMPLE 2

(11) The procedure of example 1 is repeated except that the amounts for the different coating layers are altered as follows:

(12) TABLE-US-00001 TABLE 1 substrate Fe.sub.2O.sub.3 Fe.sub.3O.sub.4 SiO.sub.2 SnO.sub.2 TiO.sub.2 Sample g g g g g g Ex. 1 200 2 70 50 4 80 Ex 2 200 2 90 50 4 100

(13) The colouristic values measured by the Minolta 300 apparatus are as follows:

(14) TABLE-US-00002 TABLE 2 powder colour lacquer card Sample L a b C H L a b C H Ex. 1 69.4 1.9 8.7 8.9 257.6 48.5 2.3 9.2 9.5 256.0 Ex 2 70.7 2.1 10.2 10.4 258.7 47.9 2.4 10.1 10.4 257.0

(15) The magnetic properties are determined as follows:

(16) TABLE-US-00003 TABLE 3 Magnetism M.sub.s M.sub.r H.sub.c SR Sample emu/g emu/g Oe M.sub.r/M.sub.s Ex. 1 14.27 9.03 444 0.633 Ex 2 16.92 10.00 312 0.591 M.sub.s: Saturation magnetization M.sub.r: Residual magnetization H.sub.c: Coercive force SR: M.sub.r/M.sub.s

EXAMPLES 3 TO 8

(17) Example 1 is repeated except that the amounts of the starting compounds are altered as demonstrated in table 4.

(18) TABLE-US-00004 TABLE 4 substrate Fe.sub.2O.sub.3 Fe.sub.3O.sub.4 SiO.sub.2 SnO.sub.2 TiO.sub.2 Sample g g g g g g Ex. 3 100 1 35 25 2 80 Ex. 4 100 1 35 25 2 100 Ex. 5 100 1 35 25 2 110 Ex. 6 100 1 35 25 2 130 Ex. 7 200 1 90 100 2 160 Ex. 8 200 1 90 100 2 200

(19) The powder colours and the interference colours on the corresponding lacquer cards of the pigments according to examples 3 to 8 are as follows:

(20) TABLE-US-00005 Example 3: powder: brilliant copper lacquer card: copper/bronze flop Example 4: powder: brilliant purple lacquer card: bluish violet/magenta flop Example 5: powder: brilliant blue lacquer card: steel blue/dark cobalt blue flop Example 6: powder: brilliant green lacquer card: yellowish green/dark bluish green flop Example 7: powder: brilliant lacquer card: copper/red violet flop red-violet Example 8: powder: brilliant lacquer card: bluish green/ turquoise turquoise flop

(21) The pigments exhibit an excellent hiding power.

(22) The colouristic properties measured by Minolta 300 are as follows:

(23) TABLE-US-00006 TABLE 5 powder colour lacquer card Sample L a b C H L a b C H Ex. 3 54.57 7.59 13.4 15.4 60.6 35.83 1.11 6.27 6.36 260 Ex. 4 46.48 19.24 26.75 32.95 305.6 31.04 8.63 23.31 24.85 290.2 Ex. 5 51.36 0.44 24.19 24.19 271 33.61 2.34 19.36 19.51 262.9 Ex. 6 63.98 16.4 3.97 16.87 166.5 38.29 8.99 0.24 8.99 181.5 Ex. 7 56.65 18.51 0.95 18.53 356.1 48.24 8.42 2.61 8.81 17.2 Ex. 8 55.04 8.82 17.78 19.84 243.7 48.9 12.04 4.96 13.02 202.3

COMPARATIVE EXAMPLES 1 TO 3

(24) Example 1 is repeated except that the substrate particles are changed to be mica flakes having a particle size of from 10-60 m and that the amounts for each coating layer are altered as set out in table 6:

(25) TABLE-US-00007 TABLE 6 substrate Fe.sub.2O.sub.3 Fe.sub.3O.sub.4 SiO.sub.2 SnO.sub.2 TiO.sub.2 Sample g g g g g g Comp. Ex. 1 200 2 70 50 4 40 Comp. Ex. 2 200 2 70 50 4 50 Comp. Ex. 3 200 2 70 50 4 80

(26) The colouristic properties of these samples are measured as follows:

(27) TABLE-US-00008 TABLE 7 powder colour lacquer card Sample L a b C H L a b C H Comp. Ex. 1 47.8 2.7 1.0 2.9 20.8 33.5 0.9 3.4 3.4 255.6 Comp. Ex. 2 47.6 1.4 1.9 2.3 305.8 34.4 1.6 4.8 5.0 251.4 Comp. Ex. 3 54.5 1.0 2.7 2.9 249.7 35.0 2.7 4.3 5.1 238.0

(28) The powder and interference colour of the resulting pigments is silvery, but they exhibit a very low luster and do not possess attractive colouristic properties.