Pearlescent pigment for water-borne surface-coating systems

Abstract

A pearlescent pigment based on a metal oxide-coated, platelet-shaped substrate and a top layer on the metal oxide layer, where the top layer comprises silicon dioxide, at least one further metal hydroxide or metal oxide hydrate and at least one organic coupling reagent. The further metal hydroxides or metal oxide hydrates are hydroxides or oxide hydrates of cerium, aluminum or zirconium, or mixtures thereof. The coupling reagents employed are organofunctional silanes, zirconium aluminates or metal acid esters.

1. A pearlescent pigment comprising a metal oxide  titanium dioxide-coated, platelet-shaped substrate and a top layer on the metal oxide  titanium dioxide coating, wherein the top layer comprises silicon dioxide, at least one metal hydroxide or metal oxide hydrate and at least one organic coupling reagent and is chromium free. 
     
     
       2. A pearlescent pigment according to  claim 1 , wherein the metal hydroxide or metal oxide hydrate is a hydroxide or oxyhydrate of cerium, aluminum or zirconium. 
     
     
       3. A pearlescent pigment according to  claim 1 , wherein the coupling reagent is a zirconium aluminate, a metal acid ester or an organofunctional silane. 
     
     
       4. A process for the preparation of a pearlescent pigment comprising a metal oxide  titanium dioxide-coated, platelet-shaped substrate and a top layer on the metal oxide  titanium dioxide coating, comprising suspending the metal oxide  titanium dioxide-coated substrate in water, depositing silicic acid by adding a water-soluble silicate at a pH from 6 to 9, then, after adding one or more water-soluble metal salts at a pH from 3 to 7 and heating to from 30° to 100° C., partly depositing the metal hydroxides or metal oxide hydrates, adding at least one coupling reagent and hydrolyzing at a pH from 3 to 4, and depositing the part of the metals remaining in solution as hydroxides or oxide hydrates together with the coupling reagent at a pH of from 4 to 8.5, and subsequently drying the pigment from 80° to 160° C. 
     
     
       5. The process according to  claim 4 , wherein the metal hydroxide or metal oxide hydrate is a hydroxide or oxyhydrate of aluminum, cerium or zirconium. 
     
     
       6. The process according to  claim 4 , wherein the coupling reagent is a zirconium aluminate, a metal acid ester or an organofunctional silane. 
     
     
       7. The process of  claim 4 , wherein  comprising, after adding one or more water-soluble metal salts, heating at from 40° to 75° C. 
     
     
       8. The process of  claim 4 , wherein subsequent drying of the pigment is at 120° C. to 160° C. 
     
     
       9. The pigment of  claim 1 , wherein the metal oxide  titanium dioxide-coated, platelet-shaped substrate is mica, kaolin or glass coated with titanium dioxide, titanium dioxide mixed with iron (III) oxide, iron (III) oxide, chromium oxide, zirconium dioxide, tin dioxide or zinc oxide . 
     
     
       10. The pigment of  claim 1 , wherein the top layer constitutes 4 to 20% by weight of the pigment. 
     
     
       11. The pigment of  claim 1 , wherein the silicon dioxide in the top layer constitutes 1 to 3% by weight of the pigment, the at least one metal hydroxide or metal oxide hydrate in the top layer constitutes 1 to 5% by weight of the pigment and the coupling reagent in the top layer constitutes 2 to 12% by weight of the pigment. 
     
     
       12. The pigment of  claim 1 , wherein the top layer is formed by depositing silicon dioxide on the substrate, subsequently depositing a portion of the metal hydroxide or metal oxide hydrate thereon and, then, depositing the remaining metal hydroxide or metal oxide hydrate and the coupling reagent, simultaneously, thereon. 
     
     
       13. The pigment of  claim 2 , wherein a maximum of 50% by weight of the metal hydroxide or metal oxide hydrate are  is deposited in the initial deposition without the coupling reagent. 
     
     
       14. The pigment of  claim 1 , which consists of said coated substrate and top layer and wherein the top layer consists essentially of silicon dioxide, at least one metal hydroxide or metal oxide hydrate and at least one organic coupling reagent. 
     
     
       15. The pigment of  claim 1 , wherein the top layer comprises silicon dioxide, at least one metal hydroxide or metal oxide hydrate of cerium, and at least one organofunctional silane. 
     
     
       16. A pearlescent pigment comprising a titanium dioxide- coated, platelet - shaped substrate and a top layer on the titanium dioxide coating, wherein the top layer consists of silicon dioxide, at least one metal hydroxide or metal oxide hydrate of cerium, aluminum or zirconium, and at least one organic coupling reagent which is zirconium aluminate, a metal acid ester or an organofunctional silane.   
     
     
       17. A pearlescent pigment comprising a titanium dioxide- coated, platelet - shaped substrate and a top layer on the titanium dioxide coating, wherein the top layer consists of silicon dioxide, a metal hydroxide or metal oxide hydrate of cerium, and at least one organofunctional silane.   
     
     
       18. A pearlescent pigment comprising a titanium dioxide- coated, platelet - shaped substrate and a top layer on the titanium dioxide coating, wherein the pearlescent pigment is chromium - free and wherein the top layer comprises silicon dioxide, at least one metal hydroxide or metal oxide hydrate and at least one organic coupling reagent.   
     
     
       19. A pearlescent pigment consisting of a titanium dioxide- coated, platelet - shaped substrate and a top layer on the titanium dioxide coating, wherein the top layer consists of silicon dioxide, at least one metal hydroxide or metal oxide hydrate and at least one organic coupling reagent.   
     
     
       20. The pigment of  claim 1  wherein the top layer comprises sublayers. 
     
     
       21. The pigment of  claim 15  wherein the top layer comprises sublayers. 
     
     
       22. The pigment of  claim 18  wherein the top layer comprises sublayers. 
     
     
       23. The pigment of  claim 19  wherein the top layer comprises sublayers. 
     
     
       24. A pearlescent pigment prepared by the process of  claim 4 .