GB701245A - Process for producing stable silica sols - Google Patents

Abstract

An aqueous silica sol is produced by building up the size of particles in a substantially salt-free silica sol of spherical unaggregated dense particles by mixing with the sol a substantially salt-free sol of active silica, while maintaining the pH at 7.5-10.7, and heating the combined sols at above 60 DEG C. in the presence of a base having a monovalent cation M, the molecular ratio of SiO2 to M2O in the product being from 130:1 to 500:1. The freedom from salt is such that the sols have a specific conductance, measured at 2 per cent. SiO2 and 28 DEG C., of not more than 17 x 10-5 mho/cm.; a sol of active silica is defined as a sol in which, at pH 12 and 0.02 per cent. SiO2, at least 90 per cent. depolymerises to the monomer in not more than 100 minutes. Methods of preparing the active silica sol include hydrolysis of methyl silicate, silicon tetrachloride, or silicon sulphide, electro-osmosis or electrolysis of sodium silicate, or treatment of sodium silicate with cation-exchange resins as described in Specification 645,703; anions may be removed by contacting the sol with an anion-exchange material, for example of the metaphenylenediamine-formaldehyde or guanidine type. The sol to which the active silica is added may be prepared by adding a base of a monovalent cation to a sol of active silica to adjust the pH to 6-9 and heating to above 60 DEG C. to form a "heel" sol; the amount of silica added is at least equal to, and preferably 3-15 times, that present in the "heel" sol. The base may be derived from lithium, sodium, potassium, rubidium, caesium, ammonium, tetramethylammonium, trimethylamine, or ethanolamine, and may be added as hydroxide or soluble silicate, either before or during addition of the active silica to the "heel" sol. Preferably the mixture to which the active silica sol is added at its boiling point, and the volume is kept constant by means of evaporation. The sols may be concentrated to an SiO2 content of up to 45 per cent. and are stable against gelation, compatible with water-miscible organic solvents, and may be incorporated with polymeric materials. Specifications 663,013, 667,154, and 699,557 are referred to.