GB1113093A - Coating pigments - Google Patents

Abstract

A metal oxide pigment is coated by adding, to an aqueous slurry of the metal oxide, a water-soluble titanium compound which yields a titanium oxide, a titanium oxide hydrate, a titanium hydrate condensate or a mixture thereof on hydrolysis and a water-soluble aluminium compound which yields aluminium hydroxide, an aluminium oxide hydrate, an aluminium hydrate condensate, or a mixture thereof on hydrolysis, the slurry being maintained at a pH above 8.5 but not above 10.5 during the addition of at least substantially all of the titanium and aluminium compounds. The metal oxide is preferably titanium dioxide produced by the vapour phase oxidation of titanium tetrachloride, tetrabromide or tetraiodide above 600 DEG C. After the addition of the titanium and aluminium compounds the slurry may be heated at 30-100 DEG C for from 5 minutes to 6 hours, the pH adjusted to from 5.3 to 7.3 and the slurry further digested for from 5 minutes to 3 hours until no pH shift is observed. The titanium compound is preferably introduced prior to the aluminium compound, each compound being introduced simultaneously with a base, e g NaOH, KOH, LiOH or NH4OH. A water-soluble silicon compound which forms a silicon oxide, a silicon hydrate, a silicon hydrate condensate or mixtures thereof on hydrolysis may be added to the slurry at a pH below 8.0 after the titanium and aluminium compounds, the slurry then being digested at 30-100 DEG C. for 5 minutes to 6 hours and the pH adjusted to 5.3-7.3. Alternatively the silicon compound may be added before the titanium and aluminium compounds and before the slurry is rendered alkaline. Suitable titanium compounds are TiCl4, TiI4, TiBr4, titanium sulphate, titanyl sulphate, tetraethyl titanate, tetra-2-chloroethyl titanate, tetraoctyl titanate, tetraphenoxy titanium, tetra-acetyl titanium and tetrabutyric titanium. The aluminium compound may be AlCl3, Al2(SO4)3, triethyl aluminium, tetrahexyl aluminium, triphenyl aluminium, triethoxy aluminium, tributoxy aluminium, triphenoxy aluminium, and acyloxy aluminiums such as triacetyl and cyclic aluminoxy trimers such as trimethoxy trialuminoxy. The silicon compound may be SiCl4, SiBr4, SiI4, SiH4, Si (OOCCH3)4, Si(OCH3)4, Si(NH2)2 Si(NH2)4, Si(OCH2CH3)4 and Na2SiO3. At least one dispersant may be introduced into the slurry prior to the addition of the titanium and aluminium compounds e.g. primary, secondary and tertiary amines, quaternary ammonium hydroxides and water-soluble sodium, potassium, ammonium and lithium phosphates. A flocculating agent, e.g. MgSO4.7H2O or polyacrylamide polymers may be added to the slurry prior to filtering. The pigment is filtered, washed (e.g. with water, alcohols, ethers, ketones or mixtures of these), dried at 90-C. and ground.