Effect of pH on the coloration of synthetic fibres with indigo blue

Various synthetic fibres, viz. PET (polyethylene terephthalate), nylon 6.6, acrylic and lycra (polyurethane), have been dyed with indigo blue at various pH. The pH values are achieved by controlling the relative amounts of sodium dithionite and sodium hydroxide in the dyebath. PET, nylon 6.6, acrylic and lycra are dyed at 120, 100, 100 and 80 degrees C respectively for 30 min through an exhaust technique. The results show that leuco vat acid molecules exhaust well on all of these synthetic fibres in the appropriate acidic region (pH being 5.5 - 6). Lycra produces the highest shade depth followed by acrylic, nylon and then PET. On either side of this pH region, substantivity decreases. In the highly acidic region indigo molecules are precipitated while in the alkaline region dye molecules are ionized, and the fibre polymeric material is hydrolyzed. Reflection spectra show that the absorption maxima of indigo molecules are at 610 nm for PET, nylon and lycra while for acrylic it is 560 nm. It is also suggested that dye molecules exist as single molecules in the fibre matrix because when indigo is dissolved in dimethyl formamide it absorbs at 610 nm as well.