Process of preparing coating compositions comprising polyvinyl butyral and colloidal silica

Claim 1

A process for preparing a coating composition comprising a mixture of (1) an organic solvent solution of polyvinyl butyral resin, and (2) a dispersion, in alcohol, of substantially spherical colloidal silica particles having a mean particle diameter in the range of from about 5 to about 100 mμ, the solids of said composition consisting essentially of from 20 to 95% by weight of said polyvinyl butyral resin and correspondingly from 80 to 5% by weight of said substantially spherical colloidal silica particles, which comprises the steps of: preparing an aqueous dispersion of substantially pure, substantially spherical, colloidal silica particles which are of regular diameter in the range of 5 to 100 mμ and which have a high density and a reduced specific surface area by neutralizing and hydrolyzing a dilute aqueous solution of an alkali silicate with an acid, under alkaline conditions, to produce fine colloidal silica particles, then concentrating the solution under alkaline conditions effective to cause the fine colloidal silica particles to grow to form said substantially pure, substantially spherical, colloidal silica particles which have regular diameters in the range of 5 to 10 mμ and which are substantially free of alkali metal ions, hydroxyl groups and water within the particles, and then controlling the pH of the solution to prevent gelation of said substantially spherical colloidal silica particles; then adding a lower alkanol to said aqueous dispersion and removing the water from said aqueous dispersion to obtain an alkanol dispersion consisting essentially of said lower alkanol and from 5 to 30 wt.% of said substantially pure, substantially spherical, colloidal silica particles, and then mixing said alkanol dispersion with an organic solvent solution consisting essentially of an organic solvent which is compatible with said alkanol and from 10 to 20 wt.% of polyvinyl butyral resin.