Patent ID: 8105432

Claim:
A method for making titanium dioxide particles surface coated with silica and alumina, comprising the steps in order: A. (1) heating a slurry of raw titanium dioxide particles to a temperature of from 85 to 100° C., (2) adding citric acid as a solution in water to the slurry to form a mixture, (3) adjusting the pH of the mixture to 10 or more, (4) adding sufficient sodium silicate as a water solution to the mixture to deposit silica on the surface of the particles of from 1 to 6% based on the weight of the titanium dioxide particles in the slurry, (5) neutralizing the slurry by addition of a mineral acid over the course of one hour, thereby forming a slurry of silica coated titanium dioxide particles; B. (1) adjusting the temperature of the slurry of silica coated titanium dioxide particles to a temperature of from 55 to 90° C., (2) adding sufficient sodium aluminate as a water solution to the slurry in step B(1) and adjusting the pH of the mixture formed to from 5 to 9 by addition of a strong mineral acid to deposit alumina as Al 2 O 3 of from 1 to 4% by weight based on the weight of titanium dioxide particles present in the slurry of step A(1) on the surface of the silica coated particles, and digesting the resulting mixture for from 15 to 30 minutes to form the titanium dioxide particles surface coated sequentially with amorphous silica and amorphous alumina; and C. dispersing the surface coated titanium dioxide into a resin to form a coating formulation.