Лако-красочные материалы — производство

Metal effect pigments consisting of flakes or lamellae of aluminum (aluminum bronzes), copper and copper-zinc alloys (“gold bronzes”), zinc, and other metals have been well known for a long time [5.157]. Some of these metal flakes, especially aluminum platelets, can be coated with iron oxide in a CVD process [5.126, 5.133, 5.140]. These are commercially the most important pigments based on CVD. The metal flakes are fluidized in nitrogen gas at temperatures around 450 °C. Then the reagents Fe(CO)5 and O2 are injected into the fluidized bed. For a proper coating these must be highly diluted in an inert gas. Sometimes water vapor is added to re­duce electrostatic charging. The thickness of the coating is controlled by the reaction time. The only side product is carbon monoxide, which is catalytically oxidized to CO2. The resulting pigments show golden, orange and reddish metal-like effects.

The aluminum flakes can be coated with aluminum oxide and/or silicon dioxide in the liquid phase before the CVD process to reduce the reactivity of the metal and to create multilayer color effects. These pigments show strong angle-dependent coloristic effects (color travel) [5.158]. Metal halogenides are difficult to use as gas — phase precursors for metal oxide coating because of the reactivity of the metal flakes and the corrosion of steel in chloride atmospheres. The use of TiCl4 and water vapor was described about ten years ago. However, no commercial products have, as yet, been introduced.

Mica flakes can also be coated in CVD processes using the same technique as discussed for metal flakes. In addition, one can also use metal chlorides as precursors for oxide coating on mica. However, essentially the same products can be obtained more easily by precipitating the oxides from the liquid phase.