Process for preparing metal oxide coated aluminium effect pigments

Abstract

The present invention relates to a process for preparing a colored effect pigment, comprising: (i) coating aluminum-based substrate particles in an aqueous coating medium with at least one metal oxide layer, wherein the metal oxide is selected from a titanium oxide, an iron oxide, or any mixture thereof, (ii) providing a mixture of the coated aluminum-based substrate particles and a particulate inorganic non-metallic material in the aqueous coating medium by adding the particulate inorganic non-metallic material to the aqueous coating medium, and (iii) separating the mixture of the coated aluminum-based substrate particles and the particulate inorganic non-metallic material from the aqueous coating medium and subjecting the separated mixture to a thermal drying step so as to obtain a dry colored effect pigment material.

Claims

1. A process for preparing a coloured effect pigment having an aluminum-based substrate particle and an iron oxide or titanium oxide layer, the process comprising: (i) coating aluminium-based substrate particles in a liquid aqueous coating medium with at least one metal oxide layer comprising at least one of a titanium oxide and an iron oxide, (ii) adding a particulate inorganic non-metallic material to the liquid aqueous coating medium to obtain a mixture of the coated aluminium-based substrate particles and the particulate inorganic non-metallic material in the liquid aqueous coating medium, (iii) separating the particulate mixture of the coated aluminium-based substrate particles and the particulate inorganic non-metallic material from the liquid aqueous coating medium, and (iv) thermally drying the separated particulate mixture to obtain the coloured effect pigment as a dry material.

2. The process according to claim 1, wherein the aluminium-based substrate particles comprise an aluminium or aluminium alloy core which is optionally at least partly coated with one or more passivation layers.

3. The process according to claim 2, wherein the aluminium-based substrate particles comprise a passivation layer and the passivation layer is at least one of a metal phosphate layer and an inorganic oxide layer.

4. The process according to claim 1, wherein a thickness of the metal oxide layer is such that 1.sup.st order or 2.sup.nd order interference is obtained.

5. The process according to claim 1, wherein the particulate inorganic non-metallic material is at least one selected from the group consisting of a sheet silicate, a layered silicate, an aluminium oxide, an aluminosilicate, glass, perlite, synthetic mica, and borosilicate glass.

6. The process according to claim 1, wherein the particulate inorganic non-metallic material added to the liquid aqueous coating medium is not coated.

7. The process according to claim 1, wherein the particulate inorganic non-metallic material added to the liquid aqueous coating medium is coated with at least one metal oxide layer.

8. The process according to claim 1, wherein the particulate inorganic non-metallic material is added to the liquid aqueous coating medium during (i) and/or is already present in the liquid aqueous coating medium before (i) is carried out.

9. The process according to claim 1, wherein the particulate inorganic non-metallic material is added to the liquid aqueous coating medium after (i).

10. The process according to claim 1, wherein the particulate inorganic non-metallic material is added to the liquid aqueous coating medium in an amount of from 1 wt % to 50 wt %, based on the amount of the aluminium-based substrate particles.

11. The process according to claim 1, wherein an average particle diameter of the aluminium-based substrate particles and an average particle diameter of the particulate inorganic non-metallic material do not differ by more than 30%.

12. The process according to claim 1, wherein the mixture of the coated aluminium-based substrate particles and the particulate inorganic non-metallic material is separated from the liquid aqueous coating medium by filtration.

13. The process according to claim 1, wherein the thermal drying comprises calcination at a temperature of at least 150 C.

14. The process according to claim 1, further comprising a pigment surface modification wherein the dry coloured effect pigment material is brought into contact with a surface-modifying agent.

Description

EXAMPLES

(1) Preparation of Effect Pigment Samples E1 to E3

(2) Aluminium platelets having a SiO.sub.2 passivation layer (which was prepared according to step (a) of Example 1 of EP 0 708 154) are dispersed in water.

(3) The suspension of passivated aluminium in water is heated to 80 C. By adding iron nitrate over a period of about 12 to 48 hours, an iron(III) oxide coating is applied onto the passivated aluminium. The pH is adjusted to a range of 2.5 to 4 by adding a base (NaOH, NH.sub.3, NaHCO.sub.3). The iron oxide coating has a layer thickness which results in 2.sup.nd order interference.

(4) The suspension of iron oxide coated aluminium platelets is stirred for 30 minutes, followed by adjusting pH to a value of about 2.8 to 3.2 and adding in varying amounts, as indicated in Table 1, mica which is coated with iron oxide.

(5) TABLE-US-00001 TABLE 1 Weight ratio of Al-based pigment particles to mica in samples E1-E3 Weight ratio of Al-based pigment particles Sample (Al/SiO2/Fe2O3) to mica E1 No mica added E2 80/20 E3 70/30

(6) After having added the mica, the dispersion is stirred for about one hour so as to ensure a high degree of homogeneity, followed by filtration and washing the mixture of Al-based pigment particles and mica with water.

(7) Finally, the mixture of Al-based pigment particles and mica is subjected to a drying step at a temperature of about 300 C.

(8) Fire propagation rates of these mixtures were measured according to Transport of Dangerous Goods, Manual of Tests and Criteria, 2.sup.nd revised edition, Part III, Test N.1, Section 33.2.1.4.

(9) The results are shown below in Table 2.

(10) TABLE-US-00002 TABLE 2 Fire propagation rates Sample Fire propagation rates E1 1 sec E2 94 sec E3 Short lightening and then immediately extinguishing

(11) For evaluating the homogeneity of the mixture of iron oxide coated Al-based pigment particles and mica particles after separation from the aqueous coating medium, the filter cake of sample E2 was chemically analyzed for its content of Al, Si and Fe at three different locations; i.e. upper part, middle part and lower part of the filter cake. The results are shown in Table 3:

(12) TABLE-US-00003 TABLE 3 Chemical analysis of filter cake of E2 Location in filter cake chemically Amount Amount Amount analyzed Al (%) Si (%) Fe (%) Upper part 17.7 6.6 40 Middle part 17.6 6.6 39 Lower part 17.6 6.6 39

(13) The data of Table 3 clearly demonstrate that a very homogeneous mixture is obtained. However, due to this high homogeneity, the mica particles are evenly distributed throughout the mixture and can effectively suppress a thermite reaction during the thermal drying step of the wet filter cake.

(14) Furthermore, due to this high mixture homogeneity, there is also a high colour homogeneity throughout the effect pigment material.