Nacreous and Interference Pigments

Synthetic or natural pigments used to achieve lustrous, brilliant, or iridescent color effects by interference on thin optical films are called nacreous or pearlescent pigments [5.190]—[5.194]. They were originally used to simulate the appearance of 

Important nacreous pigments and producers are listed below  

Jaeger GmbH, Viechtach, Germany Poliperl, S. A., Argentina Semo Ltd., Seoul, South Korea 

Ludwigshafen, Germany Kemira OY, Pori, Finland Engelhard Corp./Mearl, N. J., USA 

Merck KGaA, Darmstadt, Germany (and overseas subsidiaries  

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Nacreous and Interference Pigments. The optical principles of nacreous (interference) pigments are shown in Figure 72 for a simplified case of nearly normal incidence without multiple reflection and absorption. At the interface P, between two materials with refractive indices n t and n2, part of the beam of light L, is reflected (L,) and partially transmitted (i.e., refracted) (L2). The intensity ratios depend on n, and n2. In a multilayer arrangement, as found in pearl or pearlescent and iridescent materials (Fig. 71 C), each interface produces partial reflection. After penetration through several layers, depending on the size and difference between n1 and n2,... 

Nacreous and interference pigments are used as colorants or part of color formulations for all applications where traditional pigments are used, but where additional color depth, brilliance, iridescence, color shift (flop), and other spectacular effects are desired [5.253]. Mica-based pigments dominate their combination of pearl and interference effects, brilliance, stability, and weather resistance is unsurpassed. Furthermore, they are non-toxic [5.221],... 

The term luster pigments includes metallic, nacreous and interference pigments. The luster effect is due to directed reflection on planar-shaped and ordered pigment particles. 

Interference pigments are nacreous pigments, which fulfill the interference criteria and hence exhibit color effects... 

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,... 

Thermal decomposition of iron pentacarbonyl. Very finely divided red iron oxide is obtained by atomizing iron pentacarbonyl, Fe(CO)5, and burning it in excess of air. The size of the particles depends on the temperature (580-800 °C) and the residence time in the reactor. The smallest particles are transparent and consist of 2-line ferri-hydrite, whereas the larger, semi-transparent particles consist of hematite (see Chap. 19). The only byproduct of the reaction is carbon dioxide, hence, the process has no undesirable environmental side effects. Magnetite can be produced by the same process if it is carried out at 100-400 °C. Thermal decomposition of iron pentacarbonyl is also used to coat aluminium powder (in a fluidized bed) and also mica platelets with iron oxides to produce interference or nacreous pigments. 

With the given n1 and n2 values, the maximum and minimum intensities of the reflected light - seen as interference colors - can be calculated and agree well with experimental results [5.206]. Refractive indices of materials commonly used in nacreous pigments follow ... 

Against a black background or in a blend with carbon black, the transmitted light is absorbed and the reflected interference color is seen as the mass tone (i.e., overall color) of the material. In blends of nacreous pigments with absorbing colorants, the particle size of the latter must be well below the scattering limit, i.e., they must be transparent. The nacreous effect or iridescent reflection is otherwise... 

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