| 摘要 |
<p>1494639 Coating TiO 2 pigment E I DU PONT DE NEMOURS & CO 20 Dec 1974 [20 Dec 1973] 55163/74 Heading C1N Pigmentary TiO 2 is coated by (a) gradually adding to an aqueous slurry of the pigment an alkaline solution of a soluble silicate in an amount to provide 1-4% SiO 2 , the pH of the slurry being maintained between 5 and 9 during silicate addition by introduction of acid to thereby precipitate SiO 2 , (b) gradually adding to the resultant slurry an alkaline solution of a soluble aluminium salt in an amount to provide 4-9% Al 2 O 3 , the pH being maintained in the range 5-9 during salt addition by introduction of acid to thereby precipitate Al 2 O 3 , (c) repeating step (a) and (d) repeating step (b). The initial slurry, which is preferably of rutile TiO 2 , suitably has a solids content of 100-900 g./l. and a temperature of 20-90‹ C., preferably 50-70‹ C. Suitable silicates are sodium or potassium silicates, particularly water-soluble sodium silicates with SiO 2 /Na 2 O ratios of 1À6-3À75. Suitable aluminium salts are aluminates of sodium or potassium. Steps (a) and (c) preferably precipitate 2-3% SiO 2 and steps (b) and (d) precipitate 5-8% Al 2 O 3 . The process may be applied to pigments which have been previously treated with oxides, e.g. TiO 2 and further coatings may also be applied. Preferably the pH is maintained between 6-8 throughout the process and mineral acids, e.g. HCI, H 2 SO 4 , HNO 3 and H 3 PO 4 may be used to precipitate the coatings. The coated pigments have improved hiding power.</p> |
