Silica flake pigments

Instead of mica, SiOj flakes can be used as a substrate for pearlescent pigments. The SiOj flakes are produced by a web-coating process. Synthetic SiOj flakes offer three advantages over the use of natural mica (1) the thickness of the SiOj substrate can be controlled in the preparation so that at the end a pigment with a true 

Pearlescent Pigment on Mica Pearlescent Pigment on Silica Flakes 

SiOj can be coated not only with TiOj but also with other metal oxides that are deposited from hydrolyzable salts. Iron(III) oxide is deposited as hydrated iron oxide on the transparent silica flakes. Subsequent drying and calcining produces a-Fe203 (hematite) layers on the flakes. The high refractive index of iron oxide generates strong interference effects and, in conjunction with the mass tone, brilliant red pigments. 

Improved color strength and very high luster are produced by the combination of precise thickness silica flake substrate materials, selected for their interference chromaticity, and by deposition of titania or iron oxide coating layers of the precise thickness required to generate the same interference color. These pearlescent pigments show stronger chromaticity L a b -values than can be produced with mica-based designs. 

Improved Interference Colors such as Gold, Red, Blue and Green 

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The SEM micrograph in Figure 5.25 shows a cross section through a Ti02 silica flake pigment. It shows clearly that both the Si02 flake and the Ti02 layer thickness are precisely controlled. 

Figure 7-7 shows a cross-sectional diagram of metal oxide coated on mica and metal oxide deposited on silica flake pigments. The new properties available with silica flake pigments can be summarized as follows. 

On the other hand, new pigments based on transparent silica flakes show extremely strong optical effects, which are different to mica pigments [5.244]. Angle dependent colors and other effects, achieved by the combination of these Si02-flakes with... 

Other materials coated in this manner were clays, asbestos fibers, aluminum flake pigment, iron powder, and a titanium dioxide pigment which showed tenfold improvement in photodurability in an organic polymer. By using mordants, the individual fibers in cotton and nylon fabrics were coated with silica which amounted to over 5% by weight. 

Figure 7.12 Properties of new pearl lustre pigments on silica flakes.

The first interferential pigments were created on the basis of natural muscovite mica as a substrate and coated with titanium or iron oxide. Other substrates have subsequently been used in this type of pigment, notably the silica flake and the alumina flake. The substrate provides the necessary platelet shape, while the interference colour is determined by the thickness of the metal oxide layer and its refractive index. 

The dominant class of lustre pigments is based upon oxide coatings of mica platelets, but newer materials have appeared e.g. silica and alumina flakes from Merck, with better performance characteristics than traditional mica." " There has also been developments in the technology for the coating of reflective metal platelets, e.g. aluminium, with oxides using chemical vapour deposition that has led to exciting new products. ... 

Amorphous silica n. Si02. A naturally occurring or synthetically produced pigment, characterized by the absence of pronounced crystalline structure, and which has no sharp peaks in its X-ray diffraction pattern. It may contain water of hydration or be an anhydrous type. It is used as an extender pigment, fatting agent, and as a desiccant in metal flake and metal powder coatings. 

In practice, platelet crystals are synthesized with a layer thickness d calculated to produce the desired interference colors (iridescence). Most pearlescent pigments now consist of at least three layers of two materials with different refractive indices. Thin flakes (thickness ca. 500 nm) of a material with a low refractive index (mica, silica, alumina, glass) are coated with a highly refractive metal oxide (Ti 2, FejOj, layer thickness ca. 50-150 nm). This results in particles with four... 

Mastercolor are flake like aluminum pigments, which are encapsulated in silica that is colored with organic pigments. 

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