Patent GB-464836-A (GB)
Title: Blended pigments
IPC: C09C1/00
Filed: 1935-10-26
Abstract: A blended pigment, i.e. a purely mechanical blend or mixture, as distinct from a pigment the ingredients of which are coalesced as by co-precipitation and/or co-calcination, is prepared by intimately mixing a pigment titanium dioxide having a frequency particle size average of not more than ,5 micron with one or more alkaline earth metal sulphates having a frequency particle size average of not more than ,7 micron, 90 per cent of the particles of the titanium dioxide and of the alkaline earth metal sulphate being not greater than 1,5 microns in size.  The titanium dioxide used preferably has a hiding power of at least 90 cm.<2> per gram, and a reflectance in the blue region of at least 0,94, the alkaline earth metal sulphate having a reflectance in the blue region of at least 0,94 and preferably at least 0,975, and the blended pigments thus obtained are equivalent or superior in hiding power, tinting strength, reflectance and brightness to coalesced pigments of substantially the same chemical composition, and may have a greater oil absorption.  A suitable barium sulphate may be prepared by adding pure sulphuric acid, preferably of 60--60 DEG  B<\>ae., to a solution of purified barium chloride, 50--60 grams Ba per litre.  The precipitation may be conducted at a low temperature or at room temperature, but above 65 DEG  C. or with other concentrations, the precipitated barium sulphate would be too coarse.  Or a barium sulphide solution of 30 DEG  B<\>ae. is heated to 80 DEG  C. and a solution of pure sodium sulphate, 230 gms. per litre, is heated to 35 DEG  C. and the two solutions mixed, with agitation.  The precipitated barium sulphate is washed with water, repulped in water to a 33 per cent slurry, and sulphuric acid added to give a pH of about 2. The treated precipitate is washed and the slurry neutralized by dilute alkali, filtered, and the filter cake dried and calcined at 500 DEG  C. in an oxidizing atmosphere.  The calcined product is made into a 33 per cent slurry and given a second acid treatment, being p  finally neutralized. This gives a barium sulphate of suitable particle size, and one which is free from sulphides.  A suitable calcium sulphate in the form of insoluble anhydrite, may be prepared by adding a slurry of slaked lime, containing 150 gms. calcium oxide per litre, to 60 DEG  B<\>ae. sulphuric acid until about 85 per cent of the acid is neutralized. The temperature is held closely to boiling, with agitation.  The anhydrite slurry is filtered and washed, the filter cake repulped in water, made slightly alkaline, and filter pressed.  The product may be calcined before blending.  A specified method of blending is as follows:--A slurry of barium sulphate is agitated with a slurry of pigment titanium dioxide, and barium hydroxide is added to give a slight alkalinity.  The mixture is then agitated for 24 hours, filtered, dried, and the press cake crushed.  The titanium dioxide and the alkaline earth metal sulphate may, however, be dry blended, or may be blended directly in the presence of a paint vehicle. Specifications 180,089, [Class 94], 275,672, 310,949, [both in Class 1 (iii)], 354,799, 360,436, and 364,562 are referred to.