ANTHRAQUINONE DYE

any of a group of organic dyes having molecular structures based upon that of anthraquinone. The group is subdivided according to the methods best suited to their application to various fibres. Anthraquinone acid dyes contain sulfonic acid groups that render them soluble in water and substantive for wool and silk; that is, they have an affinity for these fibres without the aid of auxiliary binding agents (mordants). Anthraquinone disperse dyes lack the water-solubilizing groups of the acid dyes, but they are adsorbed by hydrophobic fibres such as nylon or acetate rayon with the aid of soap or other agents that keep the dye suspended in the application bath. In several mordant dyes, the anthraquinone structure contains hydroxyl groups that participate in binding the dye to fibres such as cotton, wool, or silk that have been previously impregnated with the oxide of a metal such as aluminum, iron, tin, or chromium. The anthraquinone vat dyes, valued for their brilliant colours and fastness to light and washing, are insoluble in water but become soluble upon treatment with a reducing agent, usually sodium hydrosulfite. The soluble vat or leuco form is adsorbed by the fibre and is then converted back to the insoluble form.