Metallic pigments optical effect

Metallic, pearlescent and fluorescent pigments are grouped together in this section as three types of pigment used for their ability to produce unusual optical effects. 

Some of the metallic inorganic pigments are used for their non-optical effects. They can provide a complete optical barrier against visible, infrared (IR) and UV radiation. 

The metallic effect is not only influenced by the optical characteristics of the metallic pigment, but also by the clarity of the plastic. Resins like PS, PC and PVC give the best effect because of their high degree of transparency. While polyolefins, ABS and other milky resins will offer less brilliant metallic appearance. For these resins, the metallic effect can be enhanced by raising the amount of metallic pigment from 0.5-1 % to 2-3% aluminum pigment. 

There is an interesting and diverse range of pigment types which have the ability to produce novel and unusual optical effects in application. These products, which include metallic, pearlescent,... 

Figure 71. Optical principles of conventional and luster pigments A) Conventional pigment that absorbs and scatters light B) Metal effect pigment with complete regular reflection C) Natural pearl composed of alternating layers of protein and CaC03 D) Nacreous pigment the pearl is simulated by parallel orientation of the pigment platelets...

The metal layers frequently consist of chromium (semitransparent absorber metal) or aluminum (opaque reflector metal). Silicon dioxide or magnesium fluoride are the materials mostly used for the dielectric layers. In the case of pigment particles, there is a symmetrical arrangement of the layers, as shown in Figure 5.28, whereas optical coatings can also consist of a system of unsymmetrical layers. All these arrangements are the basis for an optical phenomenon called the Fabry-Perot effect... 

A CVD-process is known to produce iron-oxide-coated aluminum flakes which form high brilliance colored aluminum pigments with highly interesting interference effects [5.174]. Some of the very new optically variable pigments consist of multilayer flake pigments with a metallic core to obtain metaUic reflection and coverage ]5.175]. 

Figure 7-1 illustrates the various optical principles of conventional pigments (absorption pigments), metal effect pigments, and pearls and pear] luster pigments. In the case of absorption pigments, the interaction with light is based upon absorp-... 

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