AcidDyes

Acid dyes are a class of water-soluble, anionic dyes used primarily for protein fibers such as wool and silk, and for some synthetic fibers like nylon (polyamide). They are applied in an acidic bath, which promotes dye uptake; acidity is typically achieved with acetic or formic acid, or by the inherent acidity of the fiber. The dyes contain sulfonic acid groups that maintain water solubility and enable binding to amino groups in proteins or, in the case of nylon, electrostatic interactions and sometimes covalent bonding under heat.

Chemistry and application: Acid dyes are often azo- or anthraquinone-based and bear sulfonic acid groups. They are applied in an aqueous bath at elevated temperature, with the color developing as the bath pH is controlled. Binding relies on electrostatic attraction between the negatively charged sulfonate groups and the positively charged sites on protein fibers that are protonated in acidic conditions. Some acid dyes are formulated as metal-complex dyes to improve washfastness and, in some cases, lightfastness.

Fiber compatibility and performance: Acid dyes are favored for bright, clear hues on wool and silk and are also used for nylon. They generally provide good washfastness and a wide color range, though lightfastness can vary by dye and fiber. They are not suitable for most cellulose fibers (cotton, viscose, etc.), which require other dye classes such as direct, sulfur, or reactive dyes.

Environmental and processing considerations: Dye baths often require salts and buffering to control exhaustion and uptake. Wastewater management is an environmental consideration due to dye concentrations and sulfonated groups.