13 - Metal-complex dyes

Abstract

Metal-complex dyes are synthesised through coordination of bi- or polyvalent transition metal ions with selective acid dyes. They are mainly applied to wool, silk and nylon to achieve better wash fastness for dyed fabrics, compared to those obtained with the parent acid dye. These dyes may be produced on the substrate, during dyeing, through a one- or two-step process, which is known as mordant dye. Alternatively the metal is chelated in the dye structure by the manufacturer before application, which is called premetallised dye. Overall increase in the size of the acid dye molecules after coordination with metal is at the root of improved wash fastness. The extent of exhaustion, diffusion and levelling through migration, is utilised to promote build-up of shade. Chromium salt, the most commonly used mordant, increases waste-water load. This has necessitated development of eco-friendly formazan and other dyes through complex formation with iron and aluminium. In spite of this, metal-complexed dyes, especially 1:2 chromium premetallised dyes, are commercially important to produce bright, wash-fast shades on natural protein and polyamide fibres without the risk of overdyeing and degradation of fibre.