Surface-modified pigment of natural kaolin material and a process of producing same

Abstract

A surface modified pigment formed from kaolin material and a process of producing same are described. The pigment consists of pigment particles of kaolin material, and on the surface of the particles a primary and a secondary layer are precipitated. The primary layer consists of an anionic Al.sub.2 O.sub.3 --SiO.sub.2 hydrate gel having a molar ratio of SiO.sub.2 :Al.sub.2 O.sub.3 of from 3 to 30 and is present in a quantity corresponding to from 2 to 15% by weight of SiO.sub.2, based upon the original quantity of kaolin material. The secondary layer consists of a polymeric binder which is bonded to the primary layer by hydrogen or ion bonding and which is included in a quantity of at least 2% by weight, based upon the original quantity of kaolin material. The pigment is prepared by dispersing the pigment particles in water and providing them, while they are dispersed, with precipitated primary and secondary layers. The primary layer of anionic Al.sub.2 O.sub.3 --SiO.sub.2 hydrate gel is precipitated by acidifying water glass with an aluminium salt and acid, and the secondary layer is formed by adding a polymeric binder, the primary and secondary layers being bonded to one another by hydrogen or ion bonding. The pigment is retained in aqueous dispersion until after application. The surface-modified pigment is intended for use in paper and coats of paint.

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