Method for preparing high surface area and high activity stable silica sol

摘要

Colloidal silicas which do not require surface treatment with surface treatment agents to achieve stability. Preparing a stable colloidal silica having a surface area greater than 700 m 2/g and an enhanced stability value (S-value) of 20-50 comprises: (a) forming an initial composition containing water, an alkali metal silicate having a mole ratio of silicon dioxide (SiO 2) to alkali metal oxide of 15:1-1:1 and a pH of at least 10, and a compound containing an acid, a corresponding salt and its mixture, where the alkali metal silicate and the compound are initially present in a ratio by weight of at least 63:1, while maintaining the temperature of the initial composition below 90 [deg] F; (b) slowly and continuously adding to the initial composition an aqueous silicic acid composition having a SiO 2 content of 4.0-8.5 wt.% while maintaining the temperature below 90 [deg]F until one-half to three-quarters of the silicic acid composition has been added to the initial composition; (c) slowly increasing the temperature of the composition to 115-125 [deg]F and maintaining the temperature until the addition of the silicic acid composition is complete; (d) optionally, maintaining the temperature below 125 [deg]F for an hour; and (e) discontinuing the heating and optionally removing water from the composition until the solids content based on SiO 2 is at least 7 wt.%. Independent claims are also included for: (A) making a cellulosic sheet, comprising: (a) forming a cellulosic furnish containing 0.01-1.5 wt.% cellulosic fiber; (b) adding to the furnish: (i) 0.00005-1.25 wt.%, based on the dry weight of fiber in the furnish, of a stable colloidal silica; (ii) 0.001-0.5 wt.%, based on the dry weight of fiber in the furnish of a water soluble polymeric flocculant having a molecular weight greater than 500000 Dalton; and (c) dewatering the furnish to obtain a cellulosic sheet; and (B) increasing drainage of a papermaking furnish on a papermaking machine, comprising: (a) adding to a papermaking furnish before placing the furnish on a machine a stable colloidal silica; (b) placing the furnish on the machine; and (c) subjecting the furnish to papermaking conditions. The rate of drainage of water from the furnish on the machine is increased.