Chemical and Synthetic Dyes

 

Organic molecules are used to create synthetic dyes. Prior to the discovery of synthetic dyes in 1856, dyestuffs were made from natural materials such as flowers, roots, vegetables, insects, minerals, wood, and mollusks. Natural dye batches were never identical in hue and intensity, whereas synthetic dyestuffs can be manufactured consistently. Using computers and computer color matching (CCM), color is produced that is identical from batch to batch.

Organic molecules are used to create synthetic dyes. Prior to the discovery of synthetic dyes in 1856, dyestuffs were made from natural materials such as flowers, roots, vegetables, insects, minerals, wood, and mollusks. Natural dye batches were never identical in hue and intensity, whereas synthetic dyestuffs can be manufactured consistently. Using computers and computer color matching (CCM), color is produced that is identical from batch to batch.

Accidental Discovery

An eighteen-year-old English chemist named William Henry Perkin was looking for a malaria cure, a synthetic quinine, when he accidentally discovered the first synthetic dye. He discovered that aniline oxidation could color silk. He created a reddish purple dye from a coal tar derivative. Mauve was the name given to the brilliant purple. The dye was not resistant to sunlight or water and faded quickly to the color now known as mauve, a pale purple. This discovery prompted additional research with coal tar derivatives and other organic compounds, resulting in the birth of an entire new industry of synthetic dyes. Synthetic dyes are less expensive, have better colorfastness, and completely dominate the industry in the twenty-first century when compared to natural dyes. Thousands of distinct synthetic dyes are produced around the world.

Classification

Dyes are classified based on their chemical composition, the types of fibers to which they can be applied, their hue, or their application method. Dye molecules can attach to the surface of the fiber, be absorbed by it, or interact with the molecules of the fiber. Each fiber reacts to dyes differently. The same dye will react differently to different fiber modifications. Different hues within a dye classification will have varying colorfastness.

Acid Dyes

Water-soluble acid (anionic) dyes are used on wool, silk, nylon, modified rayon, certain modified acrylic, and polyester fibers. This dye family cannot be used on fibers that will be damaged by acids, such as cellulosics. The chemical composition of the dyes in this class varies, but they all use an acid bath. These dyes produce vibrant colors with a wide color range, but colorfastness varies.

Azoic Dyes

Azoic (naphthol) dyes are created within the cellulose fiber. The fiber is impregnated with one component of the dye, then treated with another component, resulting in the formation of the dye. A large, insoluble, colored molecule forms within the fiber when the two components are joined under suitable conditions (a low temperature water bath is used). Colorfastness is excellent because the color is contained within the fiber. If excess color on the outside of the fiber is not removed, it will rub off (crock).

Basic Dyes

Basic (cationic) dyes are very bright but have poor colorfastness; their use on cellulosic and protein fibers is restricted. Wool and silk can be dyed with basic dyes in an acidic dye bath. Cotton fibers can be dyed with basic dyes, but only if a mordant, usually a metallic salt, is present. The dye molecule's colored portion is positively charged. On acrylic fibers, basic dyes are relatively colorfast. Colorfastness is excellent in nylon and polyester fibers that have been modified to accept basic dyes. Mauveine, the first synthetic dye, falls into this category.

Direct Dyes

Direct dyes are soluble and have a preference for cellulose fibers. To control the rate of dye absorption by the fiber, an electrolyte, salt, is added to the dye bath. The dye is absorbed by the fiber; colorfastness to light is good but not to laundering. When wet cleaning is not possible, direct dyes are the best option. Direct dyes that have been developed on the fabric after dyeing are known as developed direct dyes. They generate an insoluble dye that forms a chemical bond with the molecules of the fiber. When compared to direct dyes, developed direct dyes have better wash fastness but poorer light fastness. Both are used on less expensive fabrics.

Disperse Dyes

Disperse dyes were first used to color acetate fibers. Water-soluble dyes have little affinity for hydrophobic fibers. A method for dyeing hydrophobic fibers was developed by dispersing colored organic substances in water with a surfactant. Finely colored particles are dispersed in water, and the color dissolves in the hydrophobic fiber. Disperse dyes are the most effective way to dye acetate and polyester. Dispersed dyes are used to color acrylic, aramid, modacrylic, nylon, olefin, and polyester; colorfastness is good to excellent.Disperse dyes were first used to color acetate fibers. Water-soluble dyes have little affinity for hydrophobic fibers. A method for dyeing hydrophobic fibers was developed by dispersing colored organic substances in water with a surfactant. Finely colored particles are dispersed in water, and the color dissolves in the hydrophobic fiber. Disperse dyes are the most effective way to dye acetate and polyester. Dispersed dyes are used to color acrylic, aramid, modacrylic, nylon, olefin, and polyester; colorfastness is good to excellent.

Pigment Dyes

Pigment dyes are insoluble coloring particles rather than dyes. Pigments are added to synthetic fiber spinning solutions (the liquid fiber before extrusion) and become an integral part of the fiber. The colorfastness is exceptional. Resin binders are also used to print pigments on fabric. The color is adhered to the fabric by the adhesive. Colorfastness is determined by the binder or adhesive used, not the pigment. Pigment printing is a cheap and easy way to add color to fabrics.

Reactive Dyes

Dyes that are reactive (fiber-reactive) combine with fiber molecules through addition or substitution. If properly applied, the color cannot be removed. Colors are bright and have excellent colorfastness, but they are susceptible to damage from chlorine bleaches. Cellulosics (cotton, flax, and viscose rayon), silk, wool, and nylon are all colored with reactive dyes. To dye polyester and cellulosic fiber blends, reactive dyes are combined with disperse dyes. They first appeared in the industry in 1956.

Sulfur Dyes

Sulfur dyes are insoluble in water but dissolve in sodium polysulphide. They are extremely water resistant. Another benefit is their low cost and ease of use. Sulfur dyes typically come in dark colors such as black, brown, and navy blue. Newer sulfur dyes come in brighter colors. They work well when used correctly. They are vulnerable to chlorine bleach damage. Sulfur dyes primarily color cellulosics like heavyweight cotton and viscose rayon.

Vat Dyes

When reduced in the presence of an alkali, vat dyes become soluble in water. When dyed fabric is oxidized, a water-insoluble dye is produced. The term "vat dyes" refers to the large containers used to apply the dye. This category includes the first synthetic indigo dye, which was introduced to the industry in 1896. Vat dyes have a limited color range but excellent colorfastness. They are most commonly used to color cotton work clothes, sportswear, prints, drapery fabrics, and cotton polyester blends. (1068)