Surface Treatment Effects on Humidity Resistance for Coated Mica Pigments in Waterborne Coatings
Color generated by thin film interference can be likened to the colors observed from a soap bubble. Analogous to the film of a soap bubble, a metal-oxide film interacts with light, causing reflection of certain wavelengths. By carefully controlling the thickness of the film, the observed specular reflection is of a particular color. For TiO2, which itself absorbs no visible light, a white reflection is observed from the thinnest film, followed by gold, red, blue and, finally, green reflection from the thickest film. For iron oxide, which has a reddish orange color due to light absorption, the observed reflections begin with gold. At non-specular angles, the absorption color is observable and becomes stronger as the film thickness increases.
If metal oxide films are applied to a very small and/or irregularly shaped substrate, reflections will effectively be "scattered" over a range of viewing angles. The observed color at any one angle will then be too weak to observe. This is why such pigments inherently require a large flat (platy) surface - 5-50 mm, and commonly about 20 mm across. In addition, application methods that align platelets in a common direction maximize the special effect obtainable from a pearlescent pigment.