Dyes and Pigments

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Dyes and Pigments

Chemists and other scientists use the term colorant for the entire spectrum of coloring materials, including dyes and pigments. While both dyes and pigments are sources of color, they are different from one another. Pigments are particles of color that are insoluble in water, oils, and resins. They need a binder or to be suspended in a dispersing agent to impart or spread their color. Dyes are usually water soluble and depend on physical and/or chemical reactions to impart their color. Generally, soluble colorants are used for coloring textiles, paper, and other substances while pigments are used for coloring paints, inks, cosmetics and plastics. Dyes are also called dyestuffs. The source of all colorants is either organic or inorganic.

Colorants are classified according to their chemical structure or composition (organic or inorganic), method of application, hue, origin (natural or synthetic), dyeing properties, utilization, and, sometimes, the name of the manufacturer and place of origin. The Society of Dyers and Colourists, based in England, and the American Association of Textile Chemists and Colorists, based in the United States, have devised a classification system, called the Color Index, that consists of the common name for the color, and a five-digit identification number.

Organic and inorganic colorants

Organic colorants are made of carbon (C) atoms and carbon-based molecules. Most organic colors are soluble dyes. If an organic soluble dye is to be used as a pigment, it must be made into particle form. Some dyes are insoluble and must be chemically treated to become soluble.

Vegetable-based organic colorants are produced by obtaining certain extracts from the plants. An example of a dye that is not water soluble is indigo. Indigo is derived from plants of the genus Indigofera. By an oxidation process where the plant is soaked and allowed to ferment, a blue-colored, insoluble solid is obtained. To get the indigo dye into solution, a reducing agent (usually an alkaline substance such as caustic soda) is used. The blue dye, after reduction, turns a pale yellow. Objects dyed with indigo react with the air, oxidize, and turn blue. The imparted color is not always that of the dye itself. Animals are another, rather interesting, source of organic colorants. Royal purple, once worn only by royalty as the name suggests, is obtained from the Murex snail. Sepia is obtained from cuttlefish, and Indian yellow is obtained from the urine of cows that have been force-fed mango leaves.

Organic sources of color often have bright, vivid hues, but are not particularly stable or durable. Dyes that are not affected by light exposure and washing are called colorfast, while those that are easily faded are called fugitive. Most organic natural dyes need a fixing agent (mordant) to impart their color.

Inorganic colorants are insoluble, so by definition, they are pigments. This group of colorants is of mineral origin—elements, oxides, gemstones, salts, and complex salts. The minerals are pulverized and mixed with a dispersing or spreading agent. Sometimes heating the minerals produces different hues.

Synthetic colorants

Organic and inorganic colorants can be produced synthetically. Synthetic organic and inorganic colorants are copies of vegetable, animal, and mineral-based colorants, and are made in a laboratory. Until the nineteenth century, all colorants were of natural origin. The first synthetically made commercial colorant, mauve, was developed from aniline, a coal tar derivative, by English chemist Sir William Henry Perkin (1838–1907) in 1856. Today, chemists arrange and manipulate complex organic compounds to make dyes of all colors. Synthetic dyes, made in a controlled atmosphere, are without impurities and the colors are more consistent from batch to batch. Natural dyes still have some commercial value to craftspeople, but synthetic colorants dominate the manufacturing industry.

Pigments

The color of a pigment is deposited when the spreading agent dries or hardens. The physical property of a pigment does not change when it is mixed with the agent. Some organic dyes can be converted into pigments. For example, dyes that have salt groups in their chemical structure can be made into an insoluble salt by replacing the sodiummolecule with a calcium molecule. Dyes that depend on chemical treatment to become soluble, such as indigo, can also be used as pigments. Pigments are also classified, in addition to classification mentioned above, by their color—white, transparent, or colored.

Pigments are also used for other purposes than just coloring a medium. Anticorrosive pigments, such oxides of lead, are added to paint to prevent the rusting of objects made of iron. Metallic pigments such as aluminum, bronze, and nickel are added to paints and plastics for decorative, glittery effects. Pulverized mica produces a sparkle effect and bismuth oxychloride gives a pearlescent appearance to paints and cosmetics.

Luminous pigments have the ability to radiate visible light when exposed to various energy sources. The luminous pigments that emit light after exposure to a light source and placed in the dark are called phosphorescent or commonly, glow-in-the-dark. Phosphorescent pigments are made from zinc or calcium sulfides and other mineral additives that produce the effect. Another good example of how dyes are made into pigments are some of the fluorescent pigments. Fluorescent pigments are those that are so intense that they have a glowing effect in daylight. These pigments are added to various resins, ground up, and used as a pigment. Some fluorescent pigments are illuminated by an ultraviolet light source (black light).

Dyes

Dyes are dissolved in a solution and impart their color by staining or being absorbed. What makes one organic source a dye and another not a dye depends on a particular group of atoms called chromophores. Chromophores include the azo group, thio group, nitroso group, carbonyl group, nitro group, and azoxy group. Other groups of atoms called auxochromes donate or accept electrons and attach to the dye molecule, enhance the color and increase solubility. Auxochrome groups include amino, hydroxyl, sulfonic, and substituted amino groups.

Other than chemical structure, dyes are classified by their dyeing properties. There are a great number of dyes and a greater number of fibers and materials that incorporate colorants in their manufacture. Certain dyes are used for specific materials depending on the chemical properties of the dye and the physical properties of the material to be dyed, or dyeing properties. Dyeing properties are categorized as basic or cationic, acid and premetalized, chrome and mordant, direct, sulfur, disperse, vat, azoic, and reactive dyes.

Utilization

Every manufactured object is colored by a dye or pigment. There are about 7, 000 dyes and pigments, and new ones are patented every year. Dyes are used extensively in the textile industry and paper industry. Leather and wood are colored with dyes. Food is often colored with natural dyes or with a synthetic dye approved by a federal agency. Petroleum-based products such as waxes, lubricating oils, polishes, and gasoline are colored with dyes. Pigments usually color plastics, resins, and rubbers. Dyes are used to stain biological samples, fur, and hair. Special dyes are added to photographic emulsions for color photographs.