Pigments for special effects

New pigment technology (which is largely concerned with light-scattering techniques) offers a very wide range of special effects. 

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. 

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Colorants have been developed for special effects. Pearlescent or iridescent give an attractive appearance. They are often used for coating paper (e.g., cosmetic packaging). The pigment consists of thin platelets, less than 1 pm thickness, which have a high refractive index, e.g., mica coated with Ti02. 

Industrial paint applications for special effect pigments are e.g. furniture, bicycles, airplanes, rail wagons, cans and containers, building exteriors, interior decoration, artificial pearls and jewelry. The main reasons for their use are the achievable special effects, the attractiveness of the colors and the good application behavior [5.122]. 

Modifying Specific Properties Appearance - Colorants, Pigments, Dyes, Special Effects 69 6.10 Colorants for Food and Medicals... 

Green coloration, present in many vegetable oils, poses a particular problem in oil extracted from immature or damaged soybeans. Chlorophyll is the compound responsible for this defect. StmcturaHy, chlorophyll is composed of a porphyrin ring system, in which magnesium is the central metal atom, and a phytol side chain which imparts a hydrophobic character to the stmcture. Conventional bleaching clays are not as effective for removal of chlorophylls as for red pigments, and specialized acid-activated adsorbents or carbon are required. 

Another example is the influence of the electrical resistance of PVC cable insulation. This is caused not by the organic pigment itself but by ethoxylated surfactants, which are added as auxiliaries in the manufacture of these pigments, especially azo pigments. Contrary to a repeatedly expressed view, a possible electrolyte content, which laked azo pigments for example can have, has no effect on the dielectric properties of PVC [34]. Some pigment manufacturers offer special product ranges with verified dielectric properties for this purpose. 

From a practical point of view, the attainment of the specified dry film thickness is important. In many cases, a minimum thickness must be achieved or protection will fail in a short period. This aspect demands skill in application and close inspection and control. For metallic zinc paints, the thickness would normally be of the order of 60-100/im, for anticorrosive pigmented paints, together with top coats, 100-200 //m, and for inert barrier types 150-300/im. For special environments or projects, these thicknesses may be varied but it should not be presumed that protection will always be improved or be more effective over a longer period of time by increasing the film thickness. 

For the end user the appearance of the pigment is of interest as well as how it handles under specific conditions. For this reason for many years a variety of versions have been available that combine ease and safety of handling. Lately product cost has become more influential than in the past when selecting supply forms. However, special effects also play a very strong role which often makes cost secondary if it means gaining a market advantage. 

Bismuth-containing special effect pigments based on platelet-shaped crystals of bismuth oxide chloride (bismuth oxychloride, BiOCl) have been known for a long time (see Section 5.3.1.3). 

The total market for effect pigments can be estimated to be about 50,000 tons per year. Half of this amount can be calculated to be special effect pigments, the other half to be metal effect pigments. 

Special effect pigments suitable for outdoor applications must meet the highest standards for color fastness and weather resistance. These pigments are coated additionally with thin layers of transparent and colorless oxidic compounds. These layers increase the light resistance by reducing the photoactivity of the titanium dioxide surface. In addition, the interaction between pigment and binder is optimized. 

Mica platelets can be coated with a variety of other compounds to produce further pigments. Solid-state reactions and CVD processes extend the possibilities for the synthesis of mica pigments. In addition, the calcination of the materials in the presence of inert (e.g., N2, Ar) or reactive gases (e.g., NH3, H2, hydrocarbons) allows the formation of phases, which carmot be produced by working in air. Table 5.17 contains a summary of mica-based effect pigments with special coloristic properties. 

Special effect pigments are used as colorants or part of color formulations for all systems where traditional pigments are applied, but where additional color depth, brilliance, iridescence, color travel, and other spectacular effects are required [5.122, 5.123]. Mica-based pigments dominate their combination of pearl and interference effects, brilliance, stability and behavior in different application systems is as yet unsurpassed. Pearl luster pigments require transparent or at least translucent binders. Formulations with absorption pigments have to take their transparency and color mixing rules into account... 

Cosmetic applications require specific effect pigments that are approved for use according to cosmetic regulations. The applications of these pigments include color cosmetics (lipsticks, eyeshadows, blushers, eye pencils, make-up, mascaras, nail lacquers), personal care products (shampoos, body washes, lotions, creams, oral care products, hair gels), and special effect items (theatrical and costume make-up, temporary hair color sprays) [5.164]. 

Colorants can be subdivided into dyes, and dispersed organic, inorganic and special-effect pigments. The technique for applying the colour to the product usually involves precolour (material that is already compounded to the colour desired), dry colour (powdered colorant), liquid colour or colour concentrate (high loading colorant in a base resin). 

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