Pigments, pearlescent

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. 

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The brittle, silvery, shiny metal was long considered the last stable element of the Periodic Table. In 2003 it was unmasked as an extremely weak alpha emitter (half-life 20 billion years). Like thulium, there is only one isotope. Bismuth alloys have low melting points (fuses, fire sprinklers). As an additive in tiny amounts, it imparts special properties on a range of metals. Applied in electronics and optoelectronics. The oxichloride (BiOCl) gives rise to pearlescent pigments (cosmetics). As bismuth is practically nontoxic, its compounds have medical applications. The basic oxide neutralizes stomach acids. A multitalented element. Crystallizes with an impressive layering effect (see right). 

Pearlescent lusters are commonly seen in many paints, inks, and cosmetics. The pearlescent pigment technology that brings us these unusual effects relies on a common mineral to achieve these opalescent qualities. 

Pearlescent pigments contain small flakes or platelets of the mineral mica that are additionally coated with a very thin layer of titanium dioxide. The simultaneous reflection of light from many layers of small platelets creates an impression of luster and sheen. By varying the thickness of the coating on the surface of the mica particles, pigment manufacturers can achieve a range of colors for the pearlescent effect. 

The presence of mica in pearlescent pigments only partly accounts for the appearance of the pigment. A very thin layer of the inorganic oxide titanium dioxide (TiC>2) or iron oxide (Fe2C>3) or both is coated on the mica platelets. The various colors and pearlescent effects are created as light is both refracted and reflected from the titanium dioxide layers. The very thin platelets are highly reflective and transparent. With their plate-like shape, the platelets are easily oriented into parallel layers as the paint medium is applied. Some of the incident light is reflected... 

Lustre/pearlescent pigments have found wide application in automotive coatings, coloured plastics, synthetic leather, printing inks, ceramic products and cosmetics, e.g. nail polish, lipstick and mascara. 

The majority of pearlescent coatings in the automotive industry are based on a two-stage process, in which pearlescent pigments with good opacity are included in... 

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... 

Pearlescent pigment Advantages Disadvantages Main application field... 

Pearlescent pigment Shape Particle size (pm) Thickness (nm) Density (g/cm3)... 

Table 54. Examples of mica-based pearlescent pigments with special coloristic properties...

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. 

Merck in Japan has recently patented [16] a process for the production of water and weather-resistant pearlescent pigments produced by coating mica with hydrous zirconia. This is in many ways similar to processes operated in the titanium oxide industry and mentioned previously. The zirconium hydroxide aids dispersion and gives better compatibility with the polymer matrix that it is incorporated in. 

The optical principles of pearlescent pigments are shown in Fig. 15.3 in a simplified manner where nearly normal incidence without multiple reflection and absorption occurs. At the interface P, between two optically different materials with the refractive indices n (e.g. lacquer film) and n2 (e.g. single layer of a thin crystalline platelet), a part of the light beam L, is reflected (L,) and a part is transmitted (i.e., refracted L2). The intensity ratio between Li and L2 depends on ni and n2. In multilayer arrangements like in pearl or pearlescent and iridescent materials, each interface... 

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-... 

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