Лако-красочные материалы — производство

Bi(NO3)3 + NaVO3 + 2 NaOH BiVO4 + 3 NaNO3 + H2O

Here bismuth vanadate is produced on combining an alkaline sodium or ammo­nium vanadate solution with an acidic bismuth nitrate solution. An amorphous product, of approximately bismuth-vanadium-oxide-hydroxide is formed in strongly acidic mediums (Figure 3.11). By heating the suspension to reflux and controlling pH, a transformation occurs producing a fine crystalline product (Figure 3.12). Se­lective formation of particular BiVO4 modifications is possible by controlling the co-precipitation conditions. Only two of the four known polymorphs, i. e. the clino — bisvanit (monoclinic) and the scheelit-like (tetragonal), are brilliant yellow in color. The tetragonal meta-stable modification is available for instance when doped with diverse elements, for example Ca and Mo [3.109]. The color tone and brilliance are strongly dependent upon the precipitation conditions; e. g. concentration, tempera­ture and pH.

Fig. 3.11 Amorphous bismuth — vanadium-oxide-hydroxide before trans­formation.

Further incorporation ofthermal processing into the manufacturing process leads to products with improved color properties. In the commercial paint sector, the use of inorganic stabilizers, for example calcium, aluminum or zinc phosphate or ox­ides like aluminum oxide, improves other pigment properties, e. g. photochromism, weathering and acid resistance.

For use in plastics the pigment is coated with a dense layer of silica (Figure 3.13) and other components to increase the stability in certain polymers, like polyamide, up to 300 °C [3.110].

To avoid any dust during handling, some suppliers offer the pigment in a fine granulated form (e. g. Sicopal® Yellow L 1100).

3.3.4