Dyeing

Dyeing

The application of color-producing agents to material, usually fibrous or film, in order to impart a degree of color permanence demanded by the projected end use. True dyeing covers mechanisms in which molecules of material to be dyed become involved by various means with the molecules of the coloring matter, or small aggregates thereof. There is some overlapping between true dyeing and other methods of coloring, which are called dyeing in the industry. Products which are commonly dyed include textile fibers, plastic films, anodized aluminum, fur, wood, paper, leather, and some foodstuffs.

Dyeing is accomplished by dissolving or dispersing the colorant in a suitable vehicle (usually water) and bringing this system into contact with the material to be dyed. Although many dye molecules or aggregates may adhere to the material surface when they meet, dyeing does not occur until the adhering dye particles migrate within the fibers or films. All dyeing processes are designed to accomplish ultimately penetration of the undyed substance by the colorant.

Assistants are materials which do not impart color to the product to be dyed but promote or retard dyeing. Usually, they affect the dye molecule.

Swelling agents are assistants which open up the structure of the fiber temporarily so that dye molecules or aggegates may enter more freely and reach otherwise inaccessible dye sites.

Carriers are agents (often solvents of low water solubility) which accelerate dyeing by breaking up or dissolving dye aggregates and bringing them to the fiber-water interface in a size small enough to be absorbed by the material.

Dye retarders are a class of dyeing assistants, usually inorganic or organic salts, which slow up the dyeing process by forming evanescent compounds with the dye, by buffering or depressing the ionization of an acid assistant, or by temporarily occupying the more active or more accessible dye sites on the fiber, later to be dislodged therefrom by the dye.

Aftertreating agents are salts, resins, or other products (more frequently applied to cellulosic fibers) to render the colored fabric more resistant to the effects of washing, perspiration, or fading by ozone or combustion gases. More often than not, their application causes a loss in light fastness of the dyed material.

Textiles

Cellulose fibers, such as cotton and rayon, are most commonly dyed by immersion of the fibers in a solution of direct dyes using an electrolyte such as common salt as assistant and then boiling this dyebath. Such dyeings usually exhibit only commercial (minimum) resistance to washing. Treatment of the properly dyed fibers with resins and copper, for example, increases the resistance to washing with minimum loss of light resistance.

Synthetic fibers, such as cellulose acetates and triacetate (Arnel), are dyed in a supension of solvent-soluble dyes by immersion. Polyamide synthetic fibers are dyed like wool with acid, metallized acid, neutral metallized, or fiber-reactive mordant dyes, azoics, and selected direct dyes from an acid bath. Special processes have also been developed for acrylic, polyester, and propylene fibers. See Textile chemistry

Nontextile materials

Anodized aluminum is readily dyed by many textile dyes. Light and weather resistance undreamed of in textile applications of some of these same dyes is achieved.

Paper pulp is usually dyed in the paper beater by dyes normally employed for cotton; on occasion, it is tinted by wool dyes, and it is frequently tinted by addition of pigments to the beater. Finished paper is also colored by passing it over rollers which supply dye or colored coatings to its surface (calender staining).

Leather is dyed at low temperatures with the classes of dyes normally used for wool and cotton. Formic acid is normally used to exhaust the dye. For dress gloves, leather is usually colored by applying the dye on the grain surface, leaving the flesh side undyed. Leather is also dyed with natural dyes such as logwood, fustic, and quercitron. Leather fresh from tanning and containing considerable moisture is dyed in Europe by tumbling with dry water-soluble dye.

Most food products which are artificially colored are not actually dyed. Maraschino cherries, however, are dyed for several hours with food dyes, then washed and placed in flavored syrup.

Many plastic materials may be dyed by processes similar to those employed for textiles. Nylon, cellulose acetate, polyethylene, polypropylene, and polyester resins are dyeable with dyes which color these materials in yarn form.