Quantification of indigo on denim textiles as basis for jeans recycling

Abstract

Concepts for circularity of post-consumer textile wastes have to consider the presence of dyes in the respective feedstock. Denim textiles form an important group of products which almost exclusively have been dyed with indigo, thus fibre-to-fibre recycling will allow reuse of the dye. Such an approach will require an appropriate analytical method to quantify the indigo content on a textile material. The alkaline Fe(II)-complex with triethanolamine as ligand allows rapid and time stable reduction of the indigo dye into its soluble leuco-form. After desorption of the indigo leuco-dianion quantification of the dye concentration can be achieved by photometry at 410 nm. The measurement of the dye concentration in solution makes the method independent of the uneven dye distribution in denim, which consists of ring-dyed warp and white weft yarn. This uneven dye distribution in denim textiles makes common methods of colour measurement unsuitable for indigo quantification in denim wastes. The presented technique thus will be an essential tool for any approach to recycle denim textiles including valuable indigo dye therein.