21 сентября, 2015 
Pokraskin Simple blending of transparent absorption pigments with pearlescent pigments is only one way to attain new coloristic effects. It is possible to produce pearlescent pigments coated with a layer of transparent absorption colorant to realize more pronounced brilliant colors with a sharper color flop. An additional advantage of such pigments is the elimination of dispersion problems associated with transparent absorption pigments due to their small particle size and high surface area.
One possibility for attractive combination pigments is the coating of TiO2-mica pigments with an additional layer of an inorganic or organic colorant. The thickness of the TiO2 layer is decisive for the brilliance or interference effect under regular viewing conditions, whereas the transparent colorant dominates at all other viewing angles. A deep, rich color with a luster flop at all angles is attained when the colorant and interference color are matched. If the interference color and the mass tone of the colorant are different, a color flop (two-tone pigments) is seen in addition to the luster flop.
Iron(III) oxide is the most important metal oxide for combination with titanium dioxide on mica flakes. Brilliant golden pigments result which can be applied for several purposes. Two routes are used to synthesize these pigments, and different structures are formed. In the first, a thin layer of Fe2O3 is coated onto the surface of a TiO2- mica pigment. The overall interference color is the result of both metal oxide layers. The mass tone is determined by the Fe2O3 layer, and interesting gold pigments (e. g., reddish gold) are possible. In the second case, coprecipitation of iron and titanium oxide hydroxides on mica particles followed by calcination leads to greenish gold pig-
ments. Interference and mass tone can be explained as above. The mass tone in this case is, however, further modified because of the additional formation of the highly refractive yellowish iron titanate phase Fe2TiO5 (pseudobrookite).
Other inorganic colorants used instead ofiron oxide for combination pigments are Cr2O3 (green), iron blue, cobalt blue, Fe3O4 (black), and FeTiO3. In the case of black colorants, the interference color is seen as the mass tone. There is an analogy to blends with black pigments in a color formulation where the transmitted part of the light is absorbed. Coating of TiO2-mica with an organic colorant for a mass-tone or two-tone pigment is performed by precipitation or deposition on the mica pigment surface in aqueous suspension, assisted by complexing agents or surfactants. Another method is to fix the colorant as a mechanically stable layer by using proprietary additives.
Mica platelets can be coated with a variety of other compounds to produce further pigments. Solid-state reactions and CVD processes extend the possibilities for the synthesis of mica pigments. In addition, calcination of the materials in the presence of inert (e. g., N2, Ar) or reactive gases (e. g., NH3, H2, hydrocarbons) allows the formation of phases which are not producible by working in air. Table 7.5 contains a summary of pearlescent mica pigments with special coloristic properties.
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Table 7.5 Examples of mica-based pearlescent pigments with special coloristic properties.
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Pigment composition |
Preparation |
Remarks |
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TiO2/C/mica [22] |
TiOCl2 + C + mica |
Silver-gray, interference |
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(precipitation) |
colors (carbon inclusion |
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|
calcination under N2 |
pigments) |
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BaSO4/TiO2/mica [7, 23] |
Ba2+ + SO42- + TiOCl2 + mica (precipitation) |
Low luster pigments |
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Fe3O4/mica |
Fe2+ + O2 + mica |
Transparent colors |
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(mica surface only partially coated) [7, 24] |
(precipitation) |
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Table 7.5 Continued |
Initially, metal oxide-mica pigments were developed purely for their excellent coloristic properties. Since then, they have also become of interest for functional uses. In coatings with a high content of platelet fillers, an advantageous overlapping roof-tile arrangement is possible that provides close interparticle contact, barrier effects, and dense covering. The composition and thickness of the oxide layer on the mica surface are always responsible for the physical properties like electrical conductivity, magnetism, IR reflectivity, and laser markability. Table 7.6 lists data on some functional metal oxide-mica pigments.
Table 7.6 Functional metal oxide-mica pigments.
Pigment composition Property Application
(Sn, Sb)O2/mica Electrically conductive Conductive flooring, antistatic packaging
Sn(O, F)2/mica [22, 25, 26] materials, light colored primed plastic
surfaces which can be electrostatically painted in further coating process, light colored conducting surfaces in clean room conditions for dust reduction
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