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 acidadded 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 thechlorinated 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.