Optical properties metallic pigments

In many inorganic pigments, lanthanides and transition elements are responsible for color. Metal oxides and oxide hydroxides are, however, also important as colored pigments because of their optical properties, low price, and ready availability. Colored pigments based on oxides and oxide hydroxides are either composed of a single component or mixed phases. In the latter, color is obtained by incorporation of appropriate cations. 

There are as yet no national or international (ISO) standards for mixed metal oxide pigments, and none are expected in the foreseeable future. However, the general tests of chemical, physical, and optical properties described in ASTM, DIN, or ISO standards for pigments may be utilized, such as DIN 6174, DIN 55 986, and ISO 787 Part 16 for the optical properties, and ISO 787 Parts 1-20 for the physical properties (see also Table 1). 

Figure 15.2. Optical properties of conventional pigments, metallic pigments, pearl pigments and pearls.

If you ever had the opportunity to use almninum pigments in plastics, you are familiar with the fact that you are not dealing with spherical pigments, but with flakes. Because of their shape, metallic pigments appear to behave in regard to their optical properties in almost every way opposite to non-metallic pigments. Here are some examples ... 

Figure 4. Optical properties of aluminum pigments as a function of particles size distribution and flake type. ME - metallic...

Metallic pigment n. Particles or flakes of non-oxidized metals or alloys used as pigments to modify the optical characteristics of a paint, to hide the substrate, modify the color or adjust other properties. The metals most commonly used as aluminum, bronze, and zinc. 

Apart from optical properties such as color and hiding, certain pigments are used in protective coatings to increase their corrosion resistance properties. The anticorrosive pigments used in conjunction with suitable binder in protective coatings systems play a major role in protection of metals from corrosion. 

Alumina trihydrate (ATH) is made from waste generated by the almninum metal purification process. It has high brightness but low refi active index. At low levels of substitution for Ti02, 25% or less, optical properties can be maintained. Like the synthetic silicas, ATH can be used as a paper brightning pigment when opacity is not as important. ATH is used in NCR (no carbon required) papers where it favorably affects the special dyes used in the paper. ATH is also known for its flame retardant properties. ATH retards bmming by the release of water at rather low temperatures. It is added to impart flame retardance to certain specialty papers. 

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