Optically Variable Pigments from Dielectric Layers and Their Applications

By Wei Liu, James B. Carroll, Jr.

Over the last decade, several new pigments made from optical layers of metals and dielectric layers, which fit the general term of optically variable p:1grnents (OVPs), have been commercialized. These pigments generate high chr0ma and color travel, dramatic changes in color at different angles of illu­mination, and viewing to produce fascinating color effects in coatings, plas­tics, and cosmetics. Advances in their development have resulted in high chroma color shifting pigments, with greater utility and impact.

This article will focus on dielectric layers for generating optically variable effects, present s0m!:’ basic structure appearance relationships, and cover some examples of application and performance in coatings. The mechanisms of color generation and color travel of dielectric opti­cally variable pigments are different from traditional pearlescent pigments that derive their color from the interference of light reflected from thin film of high refractive index materials.¹

These traditional effect pigments generate an immense specular, or mirror reflection color, and at a specular angles lose their color intensity. Viewing these pigments at different specular angles pro­duces only a small change in their hue and loss of color at near-planar angles. This is demonstrated in Figure 1, which shows changes in a* and b* relating to red-green and yellow-blue colors, respectively, with changes in angles of illumination and viewing. The color travel for a traditional blue inter­ference pigment is minimal, changing from a green shade to a red shade blue a shown in Figure 1.

Optically variable pigments, on the other hand, generate their color through the interference of light rays reflected from interfaces between mul­tiple layers of materials differing in refractive index. Most dielectric optically variable pigments use a high-low-high refractive index layer structure referred to as an etalon or “stack” structure.