Titanium dioxide pigment particles with doped, dense SiO2 skin and methods for their manufacture

Abstract

A method of predicting photostability of coatings with various dopants on titanium dioxide pigment particles is disclosed. Calculations of the density of states show that a doped coating which reduces the density of states near the band edge or increases the density of states within the band gap of the pigment particles increases the photostability of the doped pigment.

Description

An example of the invention is described below with the help of FIGS. 1 to 18.

FIG. 1 shows the energy states at the transition from the atom to the solid (taken from: P. A. Cox, “The Electronic Structure and Chemistry of Solids”, Oxford Science Publications 1987, p. 13).

FIG. 2 shows the energy state density of the TiO₂ surface without and with SiO₂ coating.

FIG. 3 shows the energy state density of the TiO₂ surface with SiO₂ coating and with Sn-doped SiO₂ coating.

FIG. 4 shows the energy state density of the TiO₂ surface with SiO₂ coating and with Sb-doped SiO₂ coating.

FIG. 5 shows the energy state density of the TiO₂ surface with SiO₂ coating and with In-doped SiO₂ coating.

FIG. 6 shows the energy state density of the TiO₂ surface with SiO₂ coating and with Ge-doped SiO₂ coating.

FIG. 7 shows the energy state density of the TiO₂ surface with SiO₂ coating and with Y-doped SiO₂ coating.

FIG. 8 shows the energy state density of the TiO₂ surface with SiO₂ coating and with Nb-doped SiO₂ coating.

FIG. 9 shows the energy state density of the TiO₂ surface with SiO₂ coating and with F-doped SiO₂ coating.

FIG. 10 shows the energy state density of the TiO₂ surface with SiO₂ coating and with Mn-doped SiO₂ coating.

FIG. 11 shows the energy state density of the TiO₂ surface with SiO₂ coating and with Cu-doped SiO₂ coating.

FIG. 12 shows the energy state density of the TiO₂ surface with SiO₂ coating and with Mo-doped SiO₂ coating.

FIG. 13 shows the energy state density of the TiO₂ surface with SiO₂ coating and with Cd-doped SiO coating.

FIG. 14 shows the energy state density of the TiO₂ surface with coating and with Ce-doped SiO₂ coating.

FIG. 15 shows the energy state density of the TiO₂ surface with SiO₂ coating and with W-doped SiO₂ coating.

FIG. 16 shows the energy state density of the TiO₂ surface with SiO₂ coating and with Bi-doped SiO₂ coating.

FIG. 17 shows the energy state density of the TiO₂ surface with SiO₂ coating and with Mg-doped SiO₂ coating.

FIG. 18 shows the energy state density of the TiO₂ surface with SiO₂ coating and with Al-doped SiO₂ coating.

Claims

1. Titanium dioxide (TiO2) pigment particles, comprising: a) TiO2 core particles;b) a dense silicon dioxide (SiO2) skin covering the core particles, the dense skin produced in a dry process, the dense skin doped with at least one doping element, wherein the at least one doping element is selected from the group consisting of Sn, Sb, In, Y, Zn, F, Mn, Cu, Mo, Cd, Ce, W and Bi, as well as mixtures thereof.

2. The TiO2 pigment particles of claim 1, further comprising; c) a further coating of aluminium oxide or hydrous aluminium oxide upon the dense skin covering.

3. The TiO2 pigment particles of claim 2, wherein the aluminium content of the further coating is 0.5 to 6.0% by weight calculated as Al2O3 and referred to the total pigment.

4. The TiO2 pigment particles of claim 3, wherein the aluminium content of the further coating is 1.0 to 4.0% by weight.

5. The TiO2 pigment particles of claim 1, wherein the silicon content of the dense skin is 0.1 to 6.0% by weight calculated as SiO2 and referred to the total pigment.

6. The TiO2 pigment particles of claim 5, wherein the the silicon content of the dense skin is 0.2 to 4.0% by weight, calculated as SiO2 and referred to the total pigment.

7. The TiO2 pigment particles of claim 1, wherein the content of doping elements in the dense skin is 0.01 to 3.0% by weight calculated as oxide and in the case of F calculated as element.

8. The TiO2 pigment particles of claim 7, wherein the content of doping elements in the dense skin is 0.05 to 2.0% by weight.

9. A method for manufacturing TiO2 pigment particles whose surface is coated with a dense SiO2 skin doped with at least one doping element, comprising the steps: a) reacting titanium tetrachloride with an aluminium halide and a gas containing oxygen in a gas phase reactor at a temperature above 1,000° C., thereby creating a gas stream containing TiO2 particles;b) contacting the particle containing gas stream with at least two compounds, where a first compound is a silicon oxide precursor compound and a second compound is selected from the group consisting of oxide precursor compounds of Sn, Sb, In, Y, Zn, Mn, Cu, Mo, Cd, Ce, W, Bi and precursor compounds of F as well as mixtures thereof,c) cooling the particle stream, thereby creating pigment particles that are coated with a dense SiO2 skin deposited on the TiO2 core particles, the dense SiO2 skin doped with at least one doping element.

10. The method of claim 9, further comprising; d) adding a further layer of aluminium oxide or hydrous aluminium oxide on the dense SiO2 skin, the further layer added by either a dry process or a wet chemical process.

11. The method of claim 10, further comprising; e) adding further layer of organic material on the aluminium oxide or hydrous aluminium oxide layer in a wet-chemical process.

12. The method of claim 9, wherein the silicon content of the dense skin is 0.1 to 6.0% by weight calculated as SiO2 and referred to the total pigment.

13. The method of claim 12, wherein the the silicon content of the dense skin is 0.2 to 4.0% by weight, calculated as SiO2 and referred to the total pigment.

14. The method of claim 9, wherein the content of doping elements in the dense skin is 0.01 to 3.0% by weight calculated as oxide and in the case of F calculated as element.

15. The method of claim 14, wherein the content of doping elements in the dense skin is 0.05 to 2.0% by weight calculated as oxide and in the case of F calculated as element.

16. The method of claim 9, further comprising; d) adding the TiO2 pigment particles produced in step c) to a process for making plastics, paints, coatings or papers.

17. The method of claim 10, further comprising; e) adding the TiO2 pigment particles produced in step d) to a process for making plastics, paints, coatings or papers.

18. The method of claim 11, further comprising; f) adding the TiO2 pigment particles produced in step e) to a process for making plastics, paints, coatings or paper.

19. Titanium dioxide (TiO2) pigment particles, comprising: a) TiO2 core particles;b) a dense silicon dioxide (SiO2) skin covering the core, the dense skin produced in a dry process, the dense skin doped with at least one doping element, wherein the at least one doping element either reduces the density of states near the band edge or creates additional states in the band gap of the material of the dense skin, excluding doping elements selected from the group consisting of Al, B, Ge, Mg, Nb, P, and Zr.