Types of dyes

Textile dyes include acid dyes, used mainly for dyeing wool, silk, and nylon; and direct or substantive dyes, which have a strong affinity for cellulose fibres (see table). Mordant dyes require the addition of chemical substances, such as salts, to give them an affinity for the material being dyed. They are applied to cellulosic fibres, wool, or silk after such materials have been treated with metal salts. Sulfur dyes, used to dye cellulose, are inexpensive but produce colours lacking brilliance. Azoic dyes are insoluble pigments formed within the fibre by padding, first with a soluble coupling compound and then with a diazotized base. Vat dyes, insoluble in water, are converted into soluble colourless compounds by means of alkaline sodium hydrosulfite. Cellulose absorbs these colourless compounds, which are subsequently oxidized to an insoluble pigment. Such dyes are colourfast. Disperse dyes are suspensions of finely divided insoluble, organic pigments used to dye such hydrophobic fibres as polyesters, nylon, and cellulose acetates.

Reactive dyes combine directly with the fibre, resulting in excellent colourfastness. The first ranges of reactive dyes for cellulose fibres were introduced in the mid-1950s. A wide variety is now available.

Application process

The dyeing of a textile fibre is carried out in a solution, generally aqueous, known as the dye liquor or dyebath. For true dyeing (as opposed to mere staining) to have taken place, the coloration must be relatively permanent—that is, not readily removed by rinsing in water or by normal washing procedures. Moreover, the dyeing must not fade rapidly on exposure to light. The process of attachment of the dye molecule to the fibre is one of absorption; that is, the dye molecules concentrate on the fibre surface.

There are four kinds of forces by which dye molecules are bound to fibre: (1) ionic forces, (2) hydrogen bonding, (3) van der Waals forces, and (4) covalent chemical linkages. In the dyeing of wool, which is a complex protein containing about 20 different α-amino acids, the sulfuric acid added to the dyebath forms ionic linkages with the amino groups of the protein. In the process of dyeing, the sulfate anion (negative ion) is replaced by a dye anion. In the dyeing of wool, silk, and synthetic fibres, hydrogen bonds are probably set up between the azo, amino, alkylamino, and other groups, and the amido -CO-NH-, groups. Van der Waals forces (the attractive forces between the atoms or molecules of all substances) are thought to act in the dyeing of cotton between the molecular units of the fibre and the linear, extended molecules of direct dyes. Covalent chemical links are brought about in the dyebath by chemical reaction between a fibre-reactive dye molecule, one containing a chemically reactive centre, and a hydroxy group of a cotton fibre, in the presence of alkali.

In any dyeing process, whatever the chemical class of dye being used, heat must be supplied to the dyebath; energy is used in transferring dye molecules from the solution to the fibre as well as in swelling the fibre to render it more receptive. The technical term for the transfer process is exhaustion. Evenness of dyeing, known as levelness, is an important quality in the dyeing of all forms of natural and synthetic fibres. It may be attained by control of dyeing conditions—that is, by agitation to ensure proper contact between dye liquor and substance being dyed and by use of restraining agents to control rate of dyeing, or strike.

Serious consideration has recently been given to methods of dyeing in which water as the medium is replaced by solvents such as the chlorinated hydrocarbons used in dry cleaning. There are a number of technical advantages in solvent dyeing, apart from the elimination of effluent (pollution) problems associated with conventional methods of dyeing and finishing. Advantages include more rapid wetting of textiles, less swelling, increased speed of dyeing per given amount of material, and savings in energy, because less heat is required to heat or evaporate perchloroethylene, for example, than is needed for water.

For each application the dyer selects the combination of dyes best suited to the particular fibre or blend he plans to dye and best able to withstand the conditions the textile will encounter in further processing and in use in the finished article. In general, the higher the standard of fastness, the more expensive the dye, and the final choice may be a compromise between the desired fastness standards and the cost of the dyes. Fastness tests and standards have been the subject of work by the American Association of Textile Chemists and Colorists (AATCC), Europäisch-Continentale Echtheitsconvention (ECE), and the Society of Dyers and Colourists (SDC), Bradford, West Yorkshire. Efforts have been made to set up a unified system by the International Organization for Standardization (ISO). Lightfastness is assessed on a scale of 8; 1 represents the poorest fastness and 8 the best. Fastness to other agents, among them water, bleach, acid, alkali, detergent solution, and perspiration, is measured on a scale of 5.

Dyes are generally used in combination to achieve a desired hue or fashion shade. If the substance to be dyed consists of only one type of fibre, such as wool, the dye mixture will be made up solely of wool dyes. But if the fabric contains more than one kind of fibre and they differ in dyeing properties, then mixtures of different application classes of dyes are used.

Forms in which textiles are dyed

Loose stock consists of randomly distributed wool or cotton fibres; tow is the corresponding term for synthetic fibres. Sliver is a more orderly arrangement of fibres in a loosely connected, continuous form suitable for spinning. It is wound into either hanks or tops, loose balls about one foot in diameter. After spinning, the yarn is either made up into hanks or into packages weighing about two pounds each, by winding the yarn round perforated metal tubes. The packages are curiously named, some according to their shapes; for example, cones, cheeses, cakes, beams, and rockets. Piece goods, woven cloth or textiles knitted in rope form, and garments, a term that includes stockings, tights, hose, and half hose, are also dyed as such.

Machinery and equipment

Modern dyeing machines are made from stainless steels. Steels containing up to 4 percent molybdenum are favoured to withstand the acid conditions that are common. A dyeing machine consists essentially of a vessel to contain the dye liquor, provided with equipment for heating, cooling, and circulating the liquor into and around the goods to be dyed or moving the goods through the dye liquor. The kind of machine employed depends on the nature of the goods to be dyed. Labour and energy costs are high in relation to total dyeing costs; the dyer’s aim is to shorten dyeing times to save steam and electrical power and to avoid spoilage of goods.

A widely used machine is the conical-pan loose-stock machine; fibres are held in an inner truncated-conical vessel while the hot dye liquor is mechanically pumped through. The fibre mass tends to become compressed in the upper narrow half of the cone, assisting efficient circulation. Levelling problems are less important because uniformity may be achieved by blending the dyed fibres prior to spinning.

The Hussong machine is the traditional apparatus; it has a long, square-ended tank as dyebath into which a framework of poles carrying hanks can be lowered. The dye liquor is circulated by an impeller and moves through a perforated false bottom that also houses the open steam pipe for heating. In modern machines, circulation is improved especially at the point of contact between hank and pole. This leads to better levelling and elimination of irregularities caused by uneven cooling.

In package-dyeing machines dye liquor may be pumped in either of two directions: (1) through the perforated central spindle and outward through the package, or (2) by the reverse path into the outer layers of the package and out of the spindle. In either case levelness is important. In the case of soluble dyes the dye liquor must be free of suspended matter. In the case of disperse dyes, in which particles of dye are dispersed in, rather than dissolved in, the solution, no gross aggregates can be allowed; otherwise the packages would retain undesirable solids on the outer and inner surfaces. Some package-dyeing machines are capable of working under pressure at temperatures up to 130 °C.

The winch is the oldest piece-dyeing machine and takes its name from the slatted roller that moves an endless rope of cloth or endless belt of cloth at full width through the dye liquor. Pressurized-winch machines have been developed in the United States. In an entirely new concept, the Gaston County jet machine circulates fabric in rope form through a pipe by means of a high-pressure jet of dye liquor. The jet machine is increasingly important in high-temperature dyeing of synthetic fibres, especially polyester fabrics.

Another machine, the jig, has a V-shaped trough holding the dye liquor and guide rollers to carry the cloth at full width between two external, powered rollers. The cloth is wound onto each roller alternately; that is, the cloth is first moved forward, then backward, through the dye liquor until dyeing is complete. Modern machines, automatically controlled and programmed, can be built to work under pressure.

Solutions or suspensions of colorants or their precursors may be padded onto piece goods by passing the cloth through a trough containing the liquor and then between rollers under pressure. Development and fixation processes such as steaming or dry-heat treatment can be carried out in other apparatus. The method is used in semicontinuous and continuous operations.