Dyeing


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dyeing

[′dī·iŋ]
(chemical engineering)
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.

Dyeing

 

the application to textiles, leather, paper, and plastics of dyes that are sufficiently resistant to light, water and soap, and abrasion. Painting or staining, as opposed to dyeing, is the application to a specially prepared surface of a layer of liquid paint or varnish, which forms a continuous hard layer after drying. Dyeing has been known since remote antiquity; it became particularly common with the development of large-scale manufac-turing. The preparation in the second half of the 19th century of organic dyes, which have almost completely replaced the expensive and less durable natural dyes, was of great significance.

Dyeing methods are determined by the properties of the dye and the material being dyed. Inorganic pigments (ultramarine and ocher) and basic, acid, direct, sulfur, and vat dyes are used for dyeing paper, including the coloring of the pulp and the dyeing of the finished paper. The methods for using such dyes are discussed later in this article. For dyeing finished paper, the paper is immersed in a solution of the dye, usually an acid dye, or a thin layer of mineral substances, such as blanc fixe, with adhesive and coloring agents is applied to the surface.

Deep dyeing of leather, including tanned, chrome, kid, and suede leathers, is done with mordant, acid, basic, and direct dyes, as well as special leather dyes. For surface dyeing of leather, pigments are used in conjunction with binding and film-forming agents such as casein and nitrocellulose.

Oxidizing, vat, acid, and mordant dyes, as well as special fur dyes, are used for dyeing furs. The methods of dyeing plastics are determined by the production technology and the properties of the plastics. For polymerized thermoplastics, the dyes are introduced into the monomer before polymerization or into the finished polymer before molding and pressing. Heat-resistant fat-soluble and basic dyes are used to produce transparent dyed products. The coloring takes place through dissolution of the dyes in the plastic. Opaque dyed products are produced when using dyes with fillers or pigments that are insoluble in the plastic. For coloring condensing thermosetting plastics, such as phenoplastics and amino plastics, the dyes—with fillers and pigments—are introduced during production of the molding powder.

For fibers, a dyebath containing the dye, auxiliary agents, and water is prepared, and the process consists in the spontaneous transition of the dye from the bath to the fiber until an equilibrium is reached. The rate at which equilibrium is attained— which, in effect, is the rate of dyeing—is determined entirely by the diffusion coefficient of the dye in the fiber, which is thousands and even hundreds of thousands of times less than in the aqueous medium. The rate of diffusion of the dye in the fiber may be increased by an increase in temperature, introduction of a number of types of hydrophilic organic solvents into the dyebath, or creation of conditions conducive to swelling of the fiber. These methods are widely used to accelerate the dyeing process. Fibers and fiber products are dyed in batch or continuous-action devices. The latter type is more widely used because such devices provide high operating efficiency. A distinction is made between uniform dyeing, in which the material is evenly tinted in one color, and design or print dyeing, in which the material is dyed in a number of colors.

Dyeing with direct dyes. Direct dyes are readily soluble in water; aggregates of ions of greater or lesser size are present in the solutions, in addition to anions of the dyeing agents. This class includes azo dyes, which are used mainly for dyeing cellulose fibers, but also for silk, wool, and polyamide fibers. The coloring agent is held on the fiber by van der Waals forces and, apparently, as a result of the formation of hydrogen bonds. Dye consumption is 1-4 percent of the weight of the fibers, depending on the color intensity desired. Up to 20 percent sodium chloride or sulfate (relative to the weight of the material) is added to the dyebath to increase color intensity and facilitate removal of the dye from the bath. The optimum temperature in the case of batch dyeing is 70°-90°C. In continuous dyeing, the material to be dyed is soaked at 90°-95°C in a dye solution with a concentration of 1-20 grams per liter (g/l), steamed in a steam chamber at 102°C for 60-90 sec (to speed the penetration of the fiber by the dye), and washed. Improved fiber coloring and increased uniformity of color require the addition of organic solvents of the ethanolamine type, called equalizers (10 g/l). The color produced with direct dyes is not sufficiently durable; therefore, it is stabilized with special preparations.

Dyeing with reactive dyes. Reactive dyes are used for dyeing cellulose, protein, and polyamide fibers. Such dyeing consists of two stages: soaking the fiber material in the dye solution and fixation in a basic medium. In the fixation stage, the dye reacts with the fiber to form a new coloring compound—for example,

Part of the dye (up to 30 percent) reacts with water. If the dye hydrolyzed by water is not removed from the fiber in the washing stage, the resistance of the color to moisture treatments decreases sharply. Dyeing with reactive dyes is done in batch, semicontinuous, and continuous processes. The most widely used technique involves a single-vat impregnation and steaming, in which the fiber is steamed at 100°-103°C (with or without intermediate drying), after the material being dyed has been soaked in the dye solution in the presence of sodium bicarbonate. The dyed fiber is then washed and dried.

Dyeing with vat and sulfur dyes. Vat dyes are insoluble in water; therefore, they must first be reduced in basic mediums by sodium hydrosulfite, yielding soluble sodium salts of leuco compounds, which may be absorbed from aqueous solutions by cellulose fibers. Oxidation of the leuco compounds by atmospheric oxygen or other oxidizing agents leads to conversion to the original insoluble dyes, which are firmly fixed in the pores of the fibers. Among the vat dyes are polycyclic and indigoid dyes. The dyeing process is conducted at temperatures of 30°-80°C, depending on the type of dye used.

The application of insoluble sulfur dyes is similar. The difference between vat and sulfur dyes lies in the use of sodium sulfide, rather than sodium hydrosulfite, as the reducing agent. The coloring power of sulfur dyes is not great; thus, for dyeing with dark colors, up to 20 percent dye relative to the weight of the fiber is used. Dyeing is conducted at temperatures close to 100°C. Water-soluble sulfur dye preparations are also known. In using such preparations, the material is soaked in an aqueous solution of the dye, dried with hot air, and processed in a solution containing sodium sulfide and sodium hydroxide. Color development—that is, the conversion of the dye to its original insoluble form—takes 30 sec in this solution at 80°-90°C.

In addition to the reducing technique for dyeing with vat dyes, the suspension method is also widely used. In this method the fiber material is soaked with a highly disperse suspension of the vat dye, dried, and treated with a basic hydrosulfite solution. The insoluble dye is converted into the sodium salt of the leuco compound directly on the fiber; the salt is absorbed by the fiber and diffuses within it during subsequent steaming. The leuco compound is oxidized to the original dye and the dye is fixed on the fiber by treatment with solutions of such oxidizing agents as K2Cr2O7 or H2O2, after which the material is washed thoroughly.

Indigosols are a special group of vat dyes. Fibrous materials are soaked in aqueous solutions of indigosols, followed by hydrolysis and oxidation of the dyes in acid mediums by various means (with nitrite or bichromate or by steaming). As a result the original vat dye, from which the corresponding indigosol was produced, is fixed on the fiber.

Dyeing with insoluble azo dyes that form in the fiber (cold dyeing). During cold dyeing, the dyes are produced directly on the fiber as a result of coupling of the azo components with diazonium salts. In the dyeing process, the fiber is soaked in a basic solution of the azo component, dried, and treated in a cold aqueous solution of the diazonium salt at pH 7-9. The azo coupling reaction takes place on the fiber, which is thus dyed. Cellulose fibers are dyed in this manner, whereas insoluble azo dyes may be synthesized in polyamide, polyester, and polyacrylonitrile fibers by a well-known modification of the method.

In addition to insoluble azo dyes, other pigments, particularly aniline black, which is the most widely used oxidizing dye, may be synthesized on the fiber. Aniline black is produced by oxidizing condensation of aniline in an inorganic acid medium. Sodium chlorate and sodium or potassium bichromate are used as the oxidizing agent; copper sulfate and potassium ferrocyanide and ferricyanide are used as oxidation catalysts.

Prepared organic pigments are also used for dyeing fibers. Various synthetic binding agents, such as precondensates of urea- and melamine-formaldehyde resins, are used for fixing such pigments.

Dyeing with cationic dyes. Cationic dyes are usually monoazo and polymethine dyes. They impart brilliant colors to polyaery lonitrile fibers, and the colors have high durability. The .dye is fixed on the fiber by intermolecular forces and as a result of the formation of ionic bonds:

where A- is an anion (for example, Ch3SO4—). The dyeing process takes place in an acetic-acid medium in the presence of Glauber’s salt and equalizers. The bath temperature is 95°- 100°C, and dyeing takes 60-90 min. Relative to the weight of the material to be dyed, the consumption of dye is 2-5 percent; of acetic acid, 1-5 percent; of Glauber’s salt, 5-15 percent; and of equalizer, 2-6 percent. Methods of continuous-flow dyeing of polyacrylonitrile fibers with cationic dyes also exist. In this case, resorcin and ethylene carbonate are used to speed the penetration of the fiber by the dye. The fiber is soaked with a solution of the dye (5-10 g/l) containing the accelerators mentioned above (10-50 g/l), steamed at 103°C for 60-90 sec, and thoroughly washed.

Dyeing with acid and mordant dyes. Acid dyes are soluble in water because of the presence of SO3M groups and, less frequently, COOM groups (where M is Na, K, or NH4), as well as SO2NH2 and SO2CH3 groups. Anthraquinone and azo dyes belong to this class. Acid dyes are used mainly for protein and polyamide fibers, as well as for leather, fur, paper, and wood pulp. In the dyeing process, the dye reacts with the fiber according to the scheme

Van der Waals forces also participate in the binding of the dye to the fiber. Dyeing is conducted at pH 3-5 in the presence of Glauber’s salt and an equalizer. The dyeing temperature is 85°-90°C, and the duration of the process is 45-60 min. Protein and polyamide fibers are also treated with mordant dyes, which color and bind to the fiber upon subsequent treatment with chemical substances called mordants. The most important mordant dyes are chrome dyes, which after soaking of the fiber are treated with a sodium bichromate solution; the chromium is reduced from the hexavalent to trivalent state with the formation of insoluble dye complexes of the fiber:

Such complexes may also bind to the fiber by van der Waals forces. Chrome dyes give colors that are highly resistant to the action of light and moisture treatments. The dyeing process is conducted in an acid or neutral medium, with the addition of equalizers, depending on the structure of the dye. Azo and anthraquinone dyes are most widely used as chrome dyes.

Dyeing with disperse dyes. In terms of chemical structure, disperse dyes belong to the azo or anthraquinone dye groups. They are usually used for polyamide, polyester, polyacrylonitrile, and acetate fibers. The molecules of disperse dyes are small and easily penetrate dense synthetic fibers. In addition, they lack anionic groups, which impede the reaction of the dye with the fiber (which has a negative charge in water). Disperse dyes are poorly soluble in hot water; therefore, they are ground (dispersed) to a particle size of less than 2 microns. Polyamide and diacetate fibers are dyed in a vat equipped with a disperser. The dyeing process is started at 40°-50°C and then conducted for 1 hr at 80°C. Under such conditions polyester and polyacrylonitrile fibers, which are distinguished by high density, are virtually untinted. Therefore, the dyeing process for such fibers must be carried out at temperatures of up to 130°-140°C or carriers, which break up the polymer structure, must be used. Substances used as carriers include o-hydroxydiphenyl, monochlorobenzene and dichlorobenzene, benzoic and salicylic acids, beta-naphthol, and cresols. The optimum carrier concentration in the dyebath is 3-8 g/l. Continuous-flow dyeing methods, such as the thermosol or vapokol processes, are very effective for polyester fibers. In these methods, the fiber is first soaked in a slightly thickened, highly disperse suspension of the dye and then, after drying, is heated for 60-90 sec at 200°-210°C (in the thermosol process) or treated with trichloroethylene vapor (in the vapokol process). After such treatment the fiber is washed and dried.

REFERENCES

Vickerstaff, T. Fizicheskaia khimiia krasheniia. Moscow, 1956. (Translated from English.)
Mel’nikov, B. N., and P. V. Moryganov. Teoriia ipraktika intensifikatsii protsessov krasheniia. Moscow, 1969.
Sadov, F. I., M. V. Korchagin, and A. I. Matetskii. Khimicheskaia tekhnologiia voloknistykh materialov, 3rd ed. Moscow, 1968.
Primenenie kubovykh krasitelei. Edited by V. Jacobi. Moscow, 1957. Pages 113-24. (Translated from English.)
Androsov, V. F., and V. S. Fel’. Krashenie sinteticheskikh volokon. Moscow, 1965.

B. N. MEL’NIKOV

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.

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