US 5,700,318 A · Durable pigments for plastic

Abstract

Improved alumina-coated inorganic pigments having a first coating of boehmite alumina, a second coating of amorphous alumina, and a third coating of boehmite alumina which exhibit good durability, dispersibility, and optical properties in plastic compositions are disclosed. Also disclosed is a process whereby an aqueous slurry of an alumina-containing inorganic pigment is milled to achieve a particular particle size distribution, a first coating of boehmite alumina is precipitated onto the pigment material within a first pH range, a second amount of an alumina coating agent is added to the aqueous slurry within a second pH range to obtain a second coating of amorphous alumina over the first boehmite alumina coating, and then the pH of the slurry is adjusted to a third pH range thus forming a third boehmite alumina coating over the second amorphous alumina coating.

Claims48

Dependent claims

An alumina-coated inorganic pigment comprising an inorganic pigment material containing at least about 0.75 percent alumina by weight based upon the weight of said pigment material having a first coating of predominantly boehmite alumina on said inorganic pigment, a second coating of predominantly amorphous alumina over said first coating, and a third coating of predominantly boehmite alumina over said second coating;

said first coating being achieved in a discrete interval and said second and third coatings being achieved continuously.

A process for preparing alumina-coated inorganic pigments comprising the steps of:

(a) milling an aqueous slurry containing inorganic pigment material, wherein said pigment material contains at least about 0.75 percent alumina by weight based upon the weight of said pigment material, in the presence of a dispersing agent, to achieve a particle size distribution in said pigment material wherein no more than about 40 percent of said pigment particles are greater than 0.49 microns as measured by a Microtrac 9200 FRA;
(b) removing the milling media from said aqueous slurry;
(c) adding a first amount of an alumina coating agent to said slurry and adjusting the slurry pH to a level of at least about 1.5 to form a first coating of predominately boehmite alumina on said pigment;
(d) digesting the slurry at a temperature of from about 50.degree. C. to about 70.degree. C. for a time of from about 5 to about 60 minutes;
(e) increasing the pH of the slurry to a level within the range of from about 2.5 to about 5;
(f) adding a second aamount of an alumina coating agent to the slurry with concurrent addition of a pH adjusting agent to form a second coating of predominately amorphous alumina over said first coating while maintaining the pH of the slurry within a range of from about 5 to about 7;
(g) digesting the slurry at a temperature of from about 50.degree. C. to about 70.degree. C. for a time of from about 10 to about 90 minutes;
(h) adjusting the pH of the slurry to a level within the range of from about 6 to about 9 and digesting said slurry at a temperature of from about 50.degree. C. to about 70.degree. C. for a time of from about 5 to about 60 minutes to form a third coating of predominately boehmite alumina on said pigment and over said second coating;
(i) recovering said alumina coated pigment to provide a plastics grade pigment.

6 more elements

Dependencies are inferred from the claim text.

Durable pigments for plastic

Life of this patent

Durable pigments for plastic

Life of this patent

Abstract

Claims48