Effect Pigments Formed by Coating of Metal Flakes

Effect Pigments Formed by Coating of Metal Flakes 

The aluminum flakes can be coated with aluminum oxide and/or siUcon dioxide in the Uquid phase before the CVD process to reduce the reactivity of the metal and to create multilayer color effects. These pigments show strong angle-dependent coloris-tic effects (color travel) [32]. 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 TiQi 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 techniques as those 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. 

Such multilayer interference configurations filter the spectrum into a sequence of high-reflectance regions surrounded by low-reflectance regions and thus lend themselves, in principle, to high-purity color production. The detailed spectral characteristics can be widely controlled by controlling the specific design parameters. 

It has been found that for flaky pigment particles suitable for strong color-shifting effects, a symmetrical arrangement of the layers is necessary. A minimum of five layers and, for some optical purposes, even more may be required. The need 

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