Metal oxide-mica pigments

Pigments Formed by Coating of Substrates 7.2.3.1 Metal Oxide-Mica Pigments 

The dominant class of pearlescent pigments is based on platelets of natural mica coated with thin films of transparent metal oxides [2-8, 16]. The mica substrate acts as a template for the synthesis and as a mechanical support for the deposited thin optical layers of the pearlescent pigments. Mica minerals are sheet layer silicates. Pearlescent pigments are usually based on transparent muscovite but some are based on natural or synthetic phlogopite. Although muscovite occurs worldwide, few deposits are suitable for pigments. Mica is biologically inert and approved for use as a filler and colorant. 

The first multilayer pigments were marketed in the 1960s as TiOj-coated muscovite micas. Two different processes are used for coating mica in aqueous suspension on a commercial basis  

TiOCl2 -I- 2 NaOH -1- mica TiOj-mica -1- 2 NaCl -1- H2O 

The pigments are then dried and calcined at 700-900 °C. The titration (chloride) process is preferred for interference pigments with thick Ti02 layers because it is easier to control. Chemical vapor deposition in a fluidized bed has also been proposed  

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Figure 73. Simplified scheme of light reflection at the phase boundaries of a metal oxide-mica pigment...

Figure 76. Upper half of metal oxide - mica pigments...

Practically applied platelet-like crystalline materials are produced with layer thickness (d) which are necessary to obtain the desired interference colors (iridescence) [11], Most of the pearlescent pigments synthesized today consist of at least three layers of two materials with different refractive indices. A simplified structure of such pigments and their interaction with light is shown in Fig. 15.4. For the case of metal oxide-mica pigments, thin flakes of mica (thickness ca. 500 nm, compara-... 

Figure 15.6. Schematic illustration of different metal oxide-mica pigments.

Table 15.4. Functional Metal Oxide—Mica Pigments...

Table 15.4 contains data about the functional properties of some metal oxide-mica pigments. 

Fig. 5.18 Schematic illustration of different metal oxide mica pigments. A) interference colors B) combination pigments C) metallic colors. Only the upper half of the pigments is shown.

Initially, metal oxide-mica pigments were developed purely for their excellent coloristic properties. Since then, they have also become of interest for functional uses. 

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