Process for preparing metal oxide coated aluminium effect pigments
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.
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 st order or 2 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.