Pigment

In papermaking, pigments are fine, inorganic particles added to fill, color, or coat paper. Pigments such as clay, titanium dioxide, and calcium carbonate are used as fillers and coatings in papermaking. When used as a paper coating, the pigment is applied in the presence of a binder, which assists the pigment particles in adhering to the paper fibers. (See Coating.)

In ink manufacturing, pigments are fine, insoluble, organic or inorganic particles of colorant that are suspended in the vehicle (which carries the pigment in the ink-transfer process and helps the pigment adhere to the substrate) and provide not only the color of ink, but also such specific properties as transparency, opacity, specific gravity, and resistance to heat, chemicals, water, or oils. Pigments, therefore, help determine the suitability of the ink to a particular printing process or end use.

Pigments can be primarily divided into black, white, and colored pigments, and there are a variety of different types of each. Black pigments include the Carbon Blacks (including Furnace Black—the most common type of black pigment—and Channel Black), Vegetable Black, and Black Iron Oxides.

White pigments come in two varieties, opaque pigments and transparent pigments. Opaque pigments, as their name implies, reflect light from their surface, and are used to hide the background on which they are printed. Transparent pigments allow light to pass through them, and are commonly added to other, more expensive inks to "pad them out" and reduce the use of expensive pigments. (Transparent pigments are also called extenders.) They are also used to reduce the color strength of other inks and to produce tints.

Inorganic color pigments include pigments of varying colors derived from mineral, or inorganic, sources. Inorganic color pigments include "Chromes," "Cadmiums," and "Irons." (See Inorganic Color Pigments.) A type of mixture containing co-precipitates of titanium and mica (or other minerals) is used to make a pearlescent, which are transparent and highly light-refractive, imparting to the ink film the luster characteristic of mother-of-pearl. (See Pearlescent.)

Organic Color Pigments are the most widely used pigments and are derived primarily from derivatives of coal tar. Organic color pigments tend to be brighter and stronger than their inorganic counterparts, and the presence of lead in inorganic pigments results in the desire for less toxic substitutes. (See Organic Color Pigments.)

Metallic powders are, as their name implies, produced from flakes of metal, in varying degrees of fineness and grade. Despite some of their names (such as "Rich Gold" and "Silver"), however, few metallic powder pigments come from precious metals. (See Metallic Powders.)

Flushed colors are colored pigments produced in a manner (called flushing) different from typical pigment manufacture. Typically, pigments begin life as a water suspension of pigment particles, which are then filtered out of the suspension and dried into a presscake. In flushing, the particles are not dried after filtering, but are left with a water content of as much as 80%, and the resulting presscake is mixed with oil, which then disperses the water. Flushing is performed on pigment particles that, in the traditional method of drying, form hard to grind clumps. The paste generated by flushing produces finer-dispersed particles.

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