How Do Dyes Work? Colour That Will Not Wash Out

A substance looks coloured because it absorbs some wavelengths of visible light and reflects or transmits the rest, and the colour seen is what is left. Absorption at visible wavelengths requires a molecule whose electrons can be lifted between energy levels by relatively low energy photons, which in practice means an extended system of alternating double and single bonds, a conjugated system, usually including aromatic rings. The part of the structure responsible is called the chromophore, and common ones include the azo group, two nitrogen atoms double bonded to each other and joining two aromatic systems, which underlies the majority of commercial dyes, along with anthraquinone, indigoid and phthalocyanine systems. Additional groups called auxochromes, such as hydroxyl and amino groups, shift the absorption and, importantly, supply the sites that attach the dye to the fibre, so the same skeleton can be tuned both in colour and in behaviour.

Fastness depends on the kind of bond formed, and the classes of dye are effectively a list of attachment strategies:

• •Direct dyes, which are large flat molecules held to cotton by weak hydrogen bonding and van der Waals forces, simple to apply and mediocre in wash fastness
• •Reactive dyes, which form an actual covalent bond to hydroxyl groups in cellulose, giving excellent wash fastness on cotton and dominating that market since the 1950s
• •Acid dyes, anionic dyes that bond ionically to protonated amino groups in wool, silk and nylon, which is why these fibres dye well in acidic baths and cotton does not take acid dyes at all
• •Disperse dyes, small non-ionic molecules with almost no water solubility, applied as a fine dispersion at high temperature so they dissolve into the polymer itself, which is the only practical way to colour polyester
• •Vat dyes, including indigo, which are insoluble and must be chemically reduced to a soluble form, absorbed, then oxidised back to the insoluble pigment trapped inside the fibre
• •Mordant dyes, which need a metal salt, historically alum, to form a complex bridging the dye and the fibre, the basis of most natural dyeing

The distinction is about solubility and it determines everything about how colour is applied. A dye is soluble in the medium at some stage and penetrates the material, becoming part of it, so a dyed fabric is coloured through and a dyed fibre feels no different. A pigment is an insoluble solid that must be ground fine and held onto a surface with a binder, so paints, printing inks and most plastics are pigmented rather than dyed, and pigment-printed fabric has colour sitting on the surface with a resin. Pigments generally have better light fastness because the crystal structure protects the molecules, while dyes usually give brighter, deeper and more transparent colour and better handle on textiles. Several substances can be either depending on treatment, and indigo is the classic case, working as a vat dye on cotton and as a pigment in paint.

Until the middle of the nineteenth century every dye came from a plant, an insect or a mineral, and the expensive ones shaped trade: indigo from India, cochineal from Mexican insects, madder, woad, and Tyrian purple from sea snails at a cost that made it an imperial monopoly. In 1856 William Perkin, aged eighteen and attempting to synthesise quinine from coal tar derivatives, produced instead a purple substance he marketed as mauveine, and the demonstration that brilliant colour could be made from industrial waste created the modern chemical industry, with German firms including BASF and Bayer founded on dye manufacture and building the research laboratory model that later produced pharmaceuticals. The consequences were not only economic: dyeing is among the most water-intensive and polluting industrial processes, consuming enormous volumes of water and discharging salts, alkali and unfixed dye, since reactive dyeing commonly fixes only seventy to eighty percent of the dye applied. Some azo dyes can break down to release aromatic amines classed as carcinogenic and are restricted in many markets. Responses include low liquor ratio machines, supercritical carbon dioxide dyeing that uses no water at all, digital printing that applies dye only where required, and enzymatic effluent treatment.

A dye is coloured because a conjugated chromophore absorbs part of the visible spectrum, and it lasts because of how it attaches: covalently to cotton for reactive dyes, ionically to wool and nylon for acid dyes, dissolved into the polymer for disperse dyes on polyester, or trapped as an insoluble form inside the fibre for vat dyes like indigo. Pigments differ by being insoluble and needing a binder. Perkin's accidental mauveine in 1856 created the synthetic dye industry, which remains one of the most water-polluting processes in manufacturing.