Process for preparing an effect pigment

1. A process for preparing a coloured effect pigment, the process comprising

(a) preparing in an aqueous coating medium at least one layer of a hydroxyl-containing metal oxide on a substrate, thereby obtaining in the aqueous coating medium a first coloured pigment material CPM1 comprising the substrate coated with the hydroxyl-containing metal oxide, wherein the substrate is made of aluminium or aluminium alloy which optionally comprises a passivating layer, and wherein the hydroxyl-containing metal oxide is a hydroxyl-containing iron oxide or a hydroxyl-containing titanium oxide and/or a mixture thereof,

(b) providing the first coloured pigment material CPM1 in a liquid post-treatment medium comprising a high boiling organic liquid, and

(c) heating the liquid post-treatment medium to a temperature of at least 90° C. so as to convert the first coloured pigment material CPM1 to a second coloured pigment material CPM2.

2. The process according to claim 1 , wherein the passivating layer is a metal phosphate layer and/or an inorganic oxide layer.

3. The process according to claim 1 , wherein the hydroxyl-containing iron or titanium oxide layer has a thickness which results in a colour according to 1 st order or 2 nd order interference series.

4. The process according to claim 1 , wherein

an inorganic non-metallic solid is added to the aqueous coating medium containing the first coloured pigment material CPM1 , and

the inorganic non-metallic solid optionally is sheet or layered silicates, aluminium oxides, aluminosilicates, glass, or any combination thereof.

5. The process according to claim 1 , wherein the first coloured pigment material CPM1 is provided in the liquid post-treatment medium by partially removing the aqueous coating medium, thereby obtaining the first coloured pigment material CPM1 as a wet material, and suspending the wet CPM1 in the liquid post-treatment medium.

6. The process according to claim 5 , wherein the aqueous coating medium is partially removed by filtration and/or thermal treatment.

7. The process according to claim 1 , wherein the first coloured pigment material CPM1 is provided in the liquid post-treatment medium by

using in said preparing (a) an aqueous coating medium which already contains one or more high boiling organic liquids, and/or adding one or more high boiling organic liquids to the aqueous coating medium during or after formation of the first coloured pigment material CPM1 in said preparing (a), and

subsequently increasing, optionally maximizing, an amount of the one or more high boiling organic liquids, relative to a total amount of liquid components, by thermal treatment.

8. The process according to claim 1 , wherein

the high boiling organic liquid has a boiling point of at least 90° C., and

the high boiling organic liquid is optionally selected from the group consisting of an alcohol, a glycol ether, a polyglycol ether, an aldehyde, an ester, an organic acid, a ketone, an ether, an alkane, a halide-substituted alkane, an alkene, an alkane, a liquid polymer, a mineral oil, or an mixture thereof.

9. The process according to claim 1 , wherein the liquid post-treatment medium comprises the high-boiling organic liquid in an amount of at least 70 wt %.

10. The process according to claim 1 , wherein the liquid post-treatment medium is heated in said heating (c) for at least 0.5 h.

11. The process according to claim 1 , wherein the liquid post-treatment medium is heated in said heating (c) at least for a time period which is sufficient for maximizing an amount of the high boiling organic liquid, relative to a total amount of liquid components.

12. The process according to claim 1 , further comprising

(d) carrying out a surface modification of the second coloured pigment material CPM2 in the liquid post-treatment medium.

13. The process according to claim 1 , wherein, prior to step said preparing (a),

the passivating layer is applied onto the aluminium or aluminium alloy in a liquid passivation medium, thereby obtaining a passivated substrate,

the passivated substrate is provided in a liquid pre-treatment medium comprising a high-boiling organic liquid, and

the liquid pre-treatment medium is heated to a temperature of at least 90° C. so as to obtain a pre-treated passivated substrate.

14. The process according to claim 13 , wherein the passivated substrate is provided in the liquid pre-treatment medium by partially removing the liquid passivation medium, optionally by filtration and/or thermal treatment, thereby obtaining the passivated substrate as a wet material, and suspending the wet substrate material in the liquid pre-treatment medium.

15. The process according to claim 13 , wherein the passivated substrate is provided in the liquid pre-treatment medium by

using a liquid passivation medium which already contains one or more high boiling organic liquids, and/or adding one or more high boiling organic liquids to the liquid passivation medium during or after formation of the passivated substrate, and

subsequently increasing, optionally maximizing, a relative amount of the one or more high boiling organic liquids by thermal treatment.

16. The process according to claim 1 , wherein the process does not include any drying operations which result in a dry or substantially dry first coloured pigment material CPM1.

17. A wet second coloured pigment material CPM2 obtained by the process according to claim 1 .

18. The wet second coloured pigment material CPM2 according to claim 17 , having a solids content of from 10 wt % to 80 wt %.

19. The wet second coloured pigment material CPM2 according to claim 17 , having a water content of less than 10 wt %.

20. The wet second coloured pigment material CPM2 according to claim 17 , comprising a high boiling organic liquid, which has a boiling point of at least 90° C.