Patent GB-753052-A (GB)
Title: Silicate pigments
Inventor: TORELLI ORLANDO LEONARD BER
IPC: C09C1/28
Filed: 1954-04-05
Abstract: Alkali metal-alumium silicate pigments are prepared by reacting aqueous solutions of an alkali metal silicate and an aluminium salt of a mineral acid of not more than 2 molar (with respect to Na2O) and 1 molar concentration respectively, the reaction being carried out under alkaline conditions with strong agitation; the average reacting concentrations should preferably be not greater than 0.1 molar.  In some embodiments of the invention (see below) pigments containing silicon or alkaline earth silicates may be obtained.  The solutions may be run at separate points (e.g. at the centre and the periphery, or at and below the surface, of a vortex produced by an agitating propellor) into a diluting aqueous medium, which should have at least half the combined volume of said solutions and preferably is greater than their combined volume; or a highly diluted aluminium salt solution may be run slowly with strong agitation into the whole of the alkali metal silicate solution; or very dilute streams of the reactants may be combined while strongly agitating, so that the water initially formed by reaction soon serves as a diluting medium for the remainder of the reaction.  In preferred embodiments the reacting concentrations are probably not more than .01 molar.  The product is a fine white precipitate, having particle sizes less than 1 micron usually less than 1/2 micron in diameter which may be separated, dried at e.g. 110 DEG C. to leave 9-11 per cent of water or a higher temperature (e.g. 500 DEG C.), and disintegrated to form a pigment having a specific gravity of 2.1 to 2.26 and a high brightness.  An aluminium salt solution containing also suspended silica may be prepared from kaolinitic clay by reaction with a mineral acid, e.g. sulphuric acid in an autoclave at 180 DEG C.: this may be used without filtration.  If desired, alkaline earth metal, e.g. calcium hydroxide in sufficient quantity to react with uncombined silica present may be added.  Other features referred to include: the maintenance of a pH between 8 and 12, preferably between 9 and 11 while combining the solutions, the pH finally being brought to 8 to 9.5; the use of aluminium sulphate (aluminium chloride and nitrate and ammonium alum are also mentioned); a SiO3/Na2O ratio of 1 to 3.5, preferably 1.5 to 3.36 in the sodium silicate, if used (potassium silicate is also mentioned); solutions of concentration less than 0.4 molar; molar ratios of Na2O/Al2O3 in the product of 0.8 to 1.2. Examples with full details of proportions, temperature &c. conditions and product proportions are given.  It is stated that pigments prepared from sodium meta-silicate are completely soluble in hot 20 per cent sodium bisulphate; those prepared from silicates with higher silica contents are only partly so.  The pigments are stated to be useful in natural and synthetic rubber compounds to increase the tear resistance; in paper coatings to impart high brightness; and in paints, plastics, inks, and protective coating compositions.ALSO:An alkali metal-aluminium silicate pigment (see Group III) in the form of a fine white precipitated powder having particle sizes less than 1 micron in diameter and a specific gravity, after drying at e.g. 110 or 500 DEG C., of 2.1 to 2.26 may be used to increase the tear resistance of rubber.  The pigment may also contain silica or, possibly, an alkali metal silicate, e.g. calcium silicate.  In an example the following mixture was vulcanized for 8 minutes at 287 DEG F.: smoked sheets (100 gms), zinc oxide (5.0 gms.), sulphur (3.5 gms), stearic acid (1.5 gms), dibenzo thiazyl dimethyl thiol urea (1 gm) diortho tolyl guanidine (0.5 gm) and pigment (52.5 gms).  The product had a tear resistance over three times that of a similar composition in which the pigment of the invention was replaced by Kaolin.