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

Investigation into the removal of pigment, sulphur and vat colourants from cotton textiles and implications for waste cellulosic recycling

Cotton can be coloured by dyeing and printing using either dye or pigment colourants. Pigments are insoluble in water but can be dispersed in a polymer print binder and heat-fixed to the fabric. Vat and sulphur dyes similarly have low solubility in water but through alkali/reduction can be solubilised and exhausted onto the cotton fibre. Following their reoxidation, the dyes are insolubilised in the fibre and, like pigment prints, in general exhibit good wash fastness. In this study, sequential acid/alkali/peroxide or acid/dithionite/peroxide treatments were investigated as a means of removing pigment/polymer prints and common sulphur and vat dyes from coloured cotton fabric. Using the sequential acid/dithionite/peroxide treatment, up to 97% of both sulphur and vat colourants could be “stripped” from dyed cotton producing a white cellulosic feedstock material that could be used for “new” fibre regeneration. By contrast, the “stripping” of the pigment/polymer binder colourant system was less predictable and less complete and is probably a reflection of the different chemical natures of the polymer binder and pigments. This contrasting behaviour highlights the range of chemistries applied to cotton and that developing a universal single treatment to strip out all finishes may be problematic

Surface chemical and colorimetric analysis of reactively dyed cellulosic fabric. The effect of ISO 105 CO9 laundering 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 solvents, such as N-Methylmorpholine oxide, and limit the potential recycling of cellulosic materials through the Lyocell fibre regeneration process. In this investigation the surface chemical compositions of three reactive dyed Tencel fabrics were studied using X-ray Photoelectron Spectroscopy (XPS) and the presence of the dye at the fibre surface demonstrated. The effect of the ISO 105 C09 oxidative-bleach fading test on the azo and anthraquinone chromophoric species was established by both surface chemical and colorimetric analyses. At low dye application levels the C. I. Reactive Black 5 and C. I. Reactive Red 228 dyed fabrics (azo chromophore) exhibited obvious colour fade while the anthraquinone-based C. I. Reactive Blue 19 dyed fabric was resistant to colour fade. However it is apparent that although some of the covalently bound dye will be removed during “first life” usage, most of the reactive colorant will remain bound to the cotton and will therefore need to be stripped from the waste garments to produce a white cellulosic feedstock prior to reprocessing through Lyocell fibre regeneration. A sequential acid, alkali and peroxide treatment completely removed the azo-based C. I. Reactive Black 5 and C. I. Reactive Red 228 colorants from the dyed cotton, however, the anthraquinone-based C. I. Reactive Blue 19 was highly resistant to removal and will require alternative chemical processing to remove the colorant