Direct dyeing

Direct dyes (also substantive dyes) are water-soluble anionic dyes, mostly azo compounds with long, coplanar molecular chains and conjugated double bonds. They bind to cellulose, cellulose regenerate (viscose, modal, lyocell), jute and paper without chemical reaction or mordant — hence the name.

The dyeing process is particularly simple: the goods are placed in an aqueous dye bath with sodium chloride or sulfate as electrolyte and slowly heated to 80-100 degrees. High-quality direct dyes reach exhaustion rates above 99 percent. Binding is via hydrogen bonds and van der Waals forces between the coplanar dye molecules and the cellulose.

Disadvantage is comparatively poor wet fastness — direct-dyed goods can bleed during washing and stain other textiles. To improve fastness, aftertreatments with cationic fixing agents or copper aftertreatment are used. Direct dyes are significantly cheaper than reactive dyes and are used for price-sensitive goods, voile curtains and paper printing.