Towards predictive and sustainable indigo dyeing of denim: Linking redox chemistry, dye transport and yarn architecture

Abstract Indigo ring dyeing defines denim's characteristic appearance and fading behaviour by selectively colouring the yarn surface while preserving its white core. The key challenge is controlling radial indigo penetration to achieve the desired ring structure without unnecessary dye and chemical consumption. Existing research addresses indigo redox chemistry, dyeing technologies, reduction systems, sustainable colouration and process optimization, but these aspects remain insufficiently integrated around quantitative penetration and ring formation control. This review critically examines the mechanisms governing indigo reduction, adsorption, diffusion and oxidation, together with dye transport, pH, oxidation time, dip–nip sequences, reducing systems, fibre properties and yarn architecture. Conventional and emerging technologies, including rope and slasher dyeing, nitrogen‐assisted, electrochemical and foam dyeing, pretreatments and advanced reducing agents, are evaluated for penetration, ring quality, sustainability and industrial applicability. Digitalization through sensing, machine vision, automated redox control and artificial intelligence is also assessed. This review aims to establish an integrated framework for predictive, sustainable and intelligent control of indigo penetration.