Additives pearlescent pigments

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 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. 

On the other hand, the uses of nonorganic compounds of bismuth in alloys, metallurgical additives and chemicals are still increasing, especially in pearlescent pigments for cosmetics and plastics, and in varistors and ceramic capacitors. 

By combining metallic pigments with pearlescent pigments, unique optieal effects can be achieved. Small additions of metallic pigments to material colored with pearlescent pigments will improve the hiding substantially. 

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 spectrophotometric technique is very useful for comparative purposes for a color classification system in a database and for the identification of individual organic or pearlescent (or interference) pigments or additives such as ultraviolet (UV)-absorbers batch-to-batch discrimination is also possible for analyzing the basecoats. MSP permits to measure absorption spectra in a given spectral range comprising typically 250-780 nm and to calculate... 

Mica platelets can be coated with a variety of other compounds to produce further pigments. Solid-state reactions and CVD processes extend the possibilities for the synthesis of mica pigments. In addition, the calcination of the materials in the presence of inert (e.g., N2, Ar) or reactive gases (e.g., NH3, H2, hydrocarbons) allows the formation of phases which are not producible by working in air. Table 7-5 contains a summary of pearlescent mica pigments with special coloristic properties. 

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