A study on the exhaust dyeing of various synthetic fibres with indigo

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Various synthetic fibres, namely, polyethylene terephthalate, nylon 6.6, acrylic and spandex fabrics, were dyed with indigo blue using an exhaust technique. The factors studied were sodium dithionite and sodium hydroxide concentrations, liquor/material ratio, dyeing temperatures and process times. The pH levels of the dyebaths were achieved by controlling the ratios of sodium dithionite and sodium hydroxide concentrations. Dyebath analysis at the end of dyeing showed that, in different pH regions, different forms of the indigo existed and each one exhibited different degrees of substantivity for synthetic fibres. The nonionic form exhibited substantivity for all the synthetic fibres, while the mono‐sodium phenolate form showed substantivity only in the acidic range. Synthetic fibres could best be dyed in the pH range of 5.5–6. Reflections spectra showed that indigo in synthetic fibres and when dissolved in dimethyl formamide absorbed maximally at 610 nm, while in cotton indigo absorbed at 660 nm. The fastness properties were measured and the results showed that washing and rubbing were good, while light and sublimation were moderate to poor.