Oxide-Mica Pigments

The dominant class of nacreous pigments is based on platelets of natural mica coated with thin films of transparent metal oxides (see Fig. 73). Mica minerals are sheet silicates (laminar). Nacreous pigments are usually based on transparent muscovite [9941-63-5] but some are based on natural or synthetic phlogopite [110710-26-4]. Although muscovite occurs worldwide, few deposits are suitable for pigments it is biologically inert and approved for use as a filler and colorant [5.220], [5.221], 

Increasing layer thickness of metal oxide causes different interference colors in reflection. Combination with absorption colorants (e.g., Fe203) produces metallic effects. 

A) Interference colors B) Combination pigments Q Metallic colors 

The development of the mica-based pigments started with pearlescent colors (Fig. 76 A, TiOz - mica). It was followed by brilliant, mass-tone-colored combination pigments (i.e., mica, Ti02, and another metal oxide) with one color (interference color same as mass tone) or two colors (interference and mass tone different) that depend on composition and viewing angle (Fig. 76 B). In the 1980s further development was made by coating mica particles with transparent layers of iron(III) oxide (Fig. 76 C) [5.222]. 

Titanium Dioxide - Mica (Fig. 77). The first multilayer pigments were marketed in the 1960s as Ti02-coated muscovite micas [5.193], [5.201]. Two different processes are used for coating mica in aqueous suspension on a commercial basis  

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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...

It is also possible to produce iron oxide mica pigments by a direct CVD fluidized bed process where iron pentacarbonyl is oxidized and Fe203 is deposited on the mica surface [5.227] ... 

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. 

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

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