Reactive dyes are a class of highly coloured organic substances, primarily used for tinting textiles, that attach themselves to their substrates by a chemical reaction that forms a covalent bond between the molecule of dye and that of the fibre. The dyestuff thus becomes a part of the fibre and is much less likely to be removed by washing than are dyestuffs that adhere by adsorption.
The first fibre reactive dyes were designed for cellulose fibres, and they are still used mostly in this way. There are also commercially available fibre reactive dyes for protein and polyamide fibres. In theory, fibre reactive dyes have been developed for other fibres, but these are not yet practical commercially. The dyes contain a reactive group that, when applied to a fibre in a weakly alkaline dyebath, form a chemical bond with the fibre. Reactive dyes can also be used to dye wool and nylon, in the latter case they are applied under weakly acidic conditions.
Direct dyes are another class of dyes, one of the two types of dyes that are mixed in 'all purpose' dyes such as Rit. (The other type in the mixture is an acid dye, which will not stay in any cellulose fiber for long.) The colours of direct dyes are duller than those provided by fiber reactive dyes, and the washfastness is poor - expect anything dyed with them to 'bleed' forever. The one advantage is that direct dyes may be more lightfast, that is, resistant to fading in the light, than fiber reactive dyes.
The "direct dye" classification in the Colour Index system refers to various planar, highly conjugated molecular structures that also contain one or more anionic sulfonate group. It is because of these sulfonate groups that the molecules are soluble in water. Though most direct dyes still can be obtained in powder form, it is increasingly popular to receive them as liquid concentrates. The advantage of concentrates is that they are easy to handle and meter. The disadvantage is that the surfactants and co-solvents needed to keep the dye concentrates stable may interfere with retention and sizing in the case of very deeply coloured grades.
Acid dyes are water soluble anionic dyes that are applied to fibres such as silk, wool, nylon and modified acrylic fibres from neutral to acid dye baths. Attachment to the fibre is attributed, at least partly, to salt formation between anionic groups in the dyes and cationic groups in the fibre. Water soluable Acid dyes are not substantive to cellulosic fibres. Acid dyes are used both commercially and by the studio dyer to dye protein/animal fibers such as wool, silk, mohair, angora, alpaca and some nylons and synthetics. Acid dyes require the use of an acid such as vinegar, acetic or sulphuric acid to set the colour.