Interference/, pearlescent pigments

Pearlescent pigments give rise to a white pearl effect often accompanied by a coloured iridescence. The most important pearlescent pigments consist of thin platelets of mica coated with titanium dioxide which partly reflect and partly transmit incident light. Simultaneous reflection from many layers of oriented platelets creates the sense of depth which is characteristic of pearlescent lustre and, where the particles are of an appropriate thickness, colours are produced by interference phenomena. Pearlescent pigments are used in automotive finishes, plastics and cosmetics. 

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... 

For known refractive indices, the maximum and minimum intensities of the reflected light, which are seen as interference colors, can so be calculated. The results are in good agreement with experimental data. Refractive indices of materials playing an important role for pearlescent pigments are listed below ... 

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-... 

This difference in refractive indices, arising equally on the interface between air/ oil film or oil film/water, is a prerequisite for the well-known iridescent color images in these media. Small highly refractive platelets of pearlescent pigments align themselves parallel in optically thin systems such as paints, printing inks, or plastics. Interference effects develop when the distances of the various layers or the thicknesses of the platelets have the right values. 

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. 

Improved color strength and very high luster are produced by the combination of precise thickness silica flake substrate materials, selected for their interference chro-matidty, 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 chromatidty L a b -values than can be produced with mica-based designs. 

Figure 3.8 Schematic presentation of interference by pearlescent pigments...

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,... 

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]. 

Everyone by now is aware of pearlescent and interference pigments. Although not a household word, they are used extensively in every imaginable application. They are incorporated in plastics, coated on substrates, used in printing inks, in cosmetics and used extensively in... 

In order to understand how three distinct colors from one pigment are possible, it is first necessary to understand the optical properties of pearlescent and interference pigments and also the optical properties of absorption colorants. The manner in which these reflect light differs and this forms the basis of the three color effect. 

Pearlescent and interference pigments reflect light in a specular manner similar to a mirror. The angle at which most of the light is reflected is equal to the angle of the incident light. The... 

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