Pearlescent pigments are widely used in a variety of applications, from cosmetics to plastics and inks to automotive topcoats. The goal of pearlescent pigments is to mimic the inherent gleam and luster of natural pearls. This is achieved by utilizing the light reflection characteristics of microscopic platelets and the optimization of their diffraction and diffusion properties. There are several types of pearl pigments commercially available, including natural pearl essence, bismuth oxychloride crystals and oxide-coated micas. There are also several other types of substrates that can be oxide coated, such as glass flake and platy alumina.
The most commonly used type is the oxide-coated micas; this also includes synthetic mica or fluorphlogopite. They are plate-like flakes of mica that are coated with thin layers of titanium dioxide (TIO2) and/or iron oxide, and potentially other metal oxides. The broad face of the platelets can range from 4 microns to 1,000 microns across and are approximately 0.5 micron thick, although synthetic micas can achieve thicknesses of less than 0.25 micron. The platelets of coated mica are very smooth, so they are light reflective while maintaining a high degree of transparency, which means only a portion of the light is reflected. The portion that is not reflected is transmitted through the platelet to the next layer where it can be further reflected (Figure 1A).