Specialty pigments

Lake pigments consist of a dye associated with an inorganic support such as alumina hydrate. They are used in packaging to obtain special visual effects. 

Metallic pigments are generally based on fine flakes of aluminum. They impart a shiny look to the plastic. 

This chapter deals with pigments that produce special effects magnetic pigments (Section 5.1), anticorrosive pigments (Section 5.2), luster pigments (Section 5.3), transparent pigments (Section 5.4), and luminescent pigments (Section 5.5). 

Since the pigments are subjected to considerable thermal stress during this conversion, the FeOOH particles are stabilized with a protective coating of silicates [5.5], phosphates [5.6], chromates [5.7], or organic compounds such as fatty acids [5.8]. 

Finely divided stoichiometric Fe304 pigments are not stable to atmospheric oxidation. They are therefore stabilized by partial oxidation or by complete oxidation to y-Fe203 below 500 °C. 

In an alternative process, the starting material consists of needle-shaped particles of a-Fe203 instead of FeOOH pigments [5.9], [5.10]. The synthesis is carried out in a hydrothermal reactor, starting from a suspension of Fe(OH)3, and crystal growth is controlled by means of organic modifiers. 

The magnetic properties may be determined by measurement of hysteresis curves on the powder or magnetic tape. 

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They may not provide superior hiding or coverage because these properties are not as important for primers as they are for topcoat paints. Primers may also contain specialty pigments and additives to help resist corrosion or prevent stain bleed-through. 

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