The effect of the acid/dithionite/peroxide treatments on reactively dyed cotton and indigo dyed denim and the implications for waste cellulosic recycling

Previous studies have established that the application of crosslinking dyes and easy care finishes to cotton can significantly reduce the dissolution of waste cotton in swelling solvents and limit the potential for recycling of cellulosic materials through the Lyocell fibre regeneration process. In this study a sequential acid hydrolysis-dithionite reduction-peroxide oxidative treatment was investigated as a potential method to completely strip all types of reactive dye chromophores from cotton. It was established that using the sequential treatment the reactively dyed fabrics and post-consumer denim jeans fabric could be completely stripped of their colour resulting in a white, NMMO dissolvable cellulosic material, which was used as a feedstock for Lyocell fibre regeneration. The white cellulose had an acceptable degree of polymerisation and the fibres regenerated through the Lyocell process had structural and mechanical properties similar to those of fibres regenerated from conventional wood pulp. Blending conventional wood pulp and recycled cotton pulp resulted into fibres with improved properties.