Method of forming a (rutile) titanium dioxide-coated platelet-like pigment
The invention relates to a method of forming (rutile) titanium dioxide-coated platelet-like pigments in which hydrous titanium dioxide is deposited on platelet-like particles followed by calcining, comprising (a) effecting the titanium deposit in the presence of an α-hydroxy carboxylic acid, especially glycolic acid, or oxalic acid, and an amino acid, especially glycine, alanine, valine, aspartic acid (α-, β- and γ-form); or an amino acid; to the pigments obtained by the method and the use thereof. The method can work in the absence of exogenous metal and Ti(III). The rutile TiO 2 -coated substrate of the present invention has at least the same advantages and characteristics of a tin containing product which includes luster, color, color homogeneity and few small particle formation during manufacture.
1. A method of forming a rutile titanium dioxide-coated platelet-like pigment in which hydrous titanium dioxide is deposited on platelet-like particles, comprising
(a) effecting the titanium deposit in the presence of an a-hydroxy carboxylic acid and an amino acid selected from the group consisting of alanine, phenylalanine, glycine, proline, cysteine, histidine, glutamine, leucine, isoleucine, arginine, lysine, serine, threonine, tryptophan, methionine, valine, tyrosine and asparagine,
wherein the platelet-like particles have two flat and parallel surfaces with an aspect ratio length to thickness of from about 2:1 to about 1000:1 and a length to width ratio of from 3:1 to 1:1.
2. The method according to claim 1 , comprising, in sequence,
(a) contacting the platelet-like particles with a titanium salt and an a-hydroxy carboxylic acid and an amino acid containing coating bath at a low pH and coating the platelet-like particles with titanium dioxide in the hydrous form at acid pH by deposition from the titanium salt containing coating bath at a low pH.
3. The method according to claim 2 , wherein the pH in step (a) is from 1.2to 2.5.
4. The method according to claim 3 , comprising in addition
(b) removing the titanium dioxide coated particles from the coating bath and washing said particles to remove excess acid and impurities therefrom; and
(c) calcining said particles at a temperature of from 500 to 1000° C. to produce a translucent coating of titanium dioxide in the rutile crystalline form thereon.
5. A method of forming a rutile titanium dioxide-coated platelet-like pigment according to claim 1 wherein the a-hydroxy carboxylic acid is selected from the group consisting of glycolic acid and oxalic acid, and the amino acid is selected from the group consisting of glycine, alanine, and valine.