Refractive index pearlescent pigments

Colorants have been developed for special effects. Pearlescent or iridescent give an attractive appearance. They are often used for coating paper (e.g., cosmetic packaging). The pigment consists of thin platelets, less than 1 pm thickness, which have a high refractive index, e.g., mica coated with Ti02. 

Nacreous Pigments Nacreous, i.e.. pearlescent pigments are used for creating special decorative effects typical of natural pearls. Nacreous pigments are fine, thin, plate-like transparent particles having a high refractive index. Because of these physical characteristics, when dispersed in a transparent film, they produce a silky appearance. 

The newest form of pearlescent pigment is a coated mica. The mica provides a transparent platelet shape with its two parallel faces. These faces are coated with a high-refractive-index layer of titanium dioxide or iron oxide. An absorption color layer can be overlaid on the oxide layer, giving additional reflective surfaces. These coated micas have high operating temperatures and are nontoxic. They are supplied in a range of particle sizes, usually from 10 to 50 xm. Loadings of coated mica pearlescents usually run about 1.5-3 percent. 

The oxychloride BiOCl is utilized in an odd application of bismuth. The compound can be crystallized as small plates with a diameter of less than 10 pm. The small crystals have a very high refractive index and, dispersed in nail varnish or ointment, a pearlescent effect. For that reason, bismuth oxychloride has been an important pigment for cosmetics. Annually more than 500 tonnes of bismuth is used for this purpose. 

With given ni and n2 the maximum and minimum intensities of the reflected light - seen as interference colors - can be calculated and agree well with cstperi-mental results. Values for the refractive index of the most important materials for pearlescent pigments are shown in Table 7-2. 

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... 

Pearlescent por- le-s nt adj (1936) An appearance resembling that of natural pearls or mother-of-pearl it results from the specular reflectance of alternating thin layers of differing refractive index similar to nacreous, interference color the pigment particles are transparent, thin platelets of high refractive index which partially reflect and partially transmit incident light simultaneous reflection from many layers of oriented platelets creates a sense of depth that is characteristic of nacreous luster. Syn Nacreous. See Interference Color. (Bailey s industrial oil and fat products. Shahidi F,... 

Nacreous (pearlescent) pigments are thin transparent platelets of high refractive index which partially transmit and partially reflect light. The pearlescent effect is due to specular reflection from the broad surfaces of the transparent platelets, and the parallel orientation of those platelets at various depths within the binder film. Light transmitted through platelets near the film surface is partially reflected by deeper platelets. The dependence of reflection on viewing angle and the sense of depth created by reflection from many layers produces the pearly luster. 

A pearl effect can also be obtained by using appropriate pigments. They are the so-called pearlescents. Pearlescent pigments reflect and transmit light by their transparency. The pearlescent effect of a pigment is obtained by superpositioning plates of transparent material and different refractive indexes. This stmcture allows the reflection of part of the incident light and transmits the remainder to the plates below. 

Mica also has pearlescent properties. It gives a pearly aspect to skin, because it has a refractive index which is different to that of air. A new generation of pearlescent pigments appeared when refined (micronized) mica dust was coated with highly refrin-gent substances such as titanium dioxide and iron oxides. These are called interferential pigments. 

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