Dyes for textile materials

Dyes are derived from five basic raw materials: benzene, toluene, xylene, naphthalene and anthracene, the vast majority of which are petroleum-based. Synthetic dyes used in the textile industry are therefore essentially derived from aromatic organic molecules with conjugated systems. From these raw materials, a limited number of fundamental reactions (around six) lead to a very large number of intermediates. Textile dyes and pigments are also extracted from these intermediates. For dyes and pigments alone, there are now thousands of distinct molecular structures, and tens of thousands of trade names. Dyes are organic compounds capable of absorbing certain light rays and reflecting or scattering complementary rays. This property results from the introduction into dye molecules of certain groups of atoms called "chromophores", which are responsible for the color. The molecules thus transformed acquire dyeing properties by association with other groups of atoms known as "auxochromes".

The most rational chemical classification of dyes is based on chromophores. A more practical classification consists of sorting dyes according to the dyeing properties of textile substrates (dyes for plant fibers – dyes for animal or protein fibers – dyes for artificial or synthetic fibers). The essential characteristics of these three dye families are the ability to be applied in solution or aqueous dispersion, the rate of rise of the dye (dyeing yield), the kinetics of rise (dyeing affinity), the power of migration to obtain good unison and the rate of dye fixation.

Dyeing results depend on a large number of factors, such as the properties of the substrate to be dyed (maturity of plant and animal fibers, crystallinity of synthetic polymers, etc.), the pH of the dye bath, the temperature and temperature gradient, the presence of electrolytes, the quality of the water, and the addition of numerous auxiliary products.

The use of synthetic colorants is not insignificant in terms of human and environmental toxicity. Indeed, synthetic dyes are derived from petroleum products, which are not without effect on human health, given that residual synthetic products remain partially present in manufacturers' dye formulations. In addition, dyeing operations currently require the use of very large quantities of water, which must be purified before being released into the environment. The problem lies mainly in the fixation rate of dyes on fibers (varying between 75 and 95%), which means that textile industry effluents are heavily soiled with residual dyes, to which must be added all the chemicals and auxiliaries used during the various finishing operations.