DISSOLUTION AND REGENERATION OF DYED CELLULOSE FIBERS

One element in the circular economy strategy for textiles is to recover fibers from used articles and employ them to create new products. A direct reuse of fibers is often not feasible, and thus it is of interest to explore avenues of manufacturing fibers afresh from the polymeric material. With cellulosic fibers (e.g., cotton, viscose), the process of conversion into new fibers involves dissolution of the polymer in solvent, and then reprecipitating it in fiber form, in non-solvent (i.e., regeneration). The technologies available for cellulose fiber regeneration include the viscose process, lyocell process and the use of ionic liquids, among others.

Fibers in clothing are typically colored, and thus the question arises on the fate of dyestuff(s) in conversion of the polymer into new fibers. The options are to either remove the dyestuff prior to processing or attempt to retain it in the polymer and produce colored fibers. The latter option has the advantage that energy and material inputs required for both removal of dyestuff, and later dyeing of fresh fibers, are avoided. Efforts towards treatment of dyeing process wastewater are also avoided. However, for this to be viable with cellulosic fibers, the dyestuff should not be degraded, leach into the solvent and non-solvent during the dissolution/regeneration process, or interfere in the process of fiber spinning.

The measurements performed in this study suggest that there is no loss of SB1 dye from cellulose, when dyed fibers were subjected to dissolution and regeneration through the viscose process. This was observed with fibers containing SB1 alone as well as combinations of indigo and SB1. The redox potential developed during the xanthation and dissolution stages did not reach levels required for solubilization and desorption of dye molecules. There was also no difference of dye content (as measured indirectly through nitrogen content) between the original dyed fibers and their regenerates from the viscose process, and this held true over a wide range of CS2 contents during xanthation (30–75% of the mass of cellulose). Although firm evidence of no change in dye content was difficult to obtain with fibers containing both indigo and SB1, here too, all indications pointed to no dye loss. When undyed fibers were mixed with dyed fibers for dissolution and regeneration, in imitation of mixing virgin pulp along with colored cellulose recovered from waste, the dyestuff was observed to be uniformly distributed through the regenerated mass. PXRD analyses of regenerates yielded evidence of indigo when present in the fiber mass, and while no direct evidence of SB1 dye could be discerned, there were indications that some components from the SB1 dye formulation may have been entrapped in the regenerates. The color appearance of regenerates was proportional to dyestuff levels in the cellulose, and did not change between the different levels of CS2 in the xanthation stage. With fibers containing both indigo and SB1, a color difference was noted between the original fibers and regenerates (ΔE*ab = 3.29; ΔH*ab = 0.33), which is attributed to a redistribution of dyestuff molecules.

The focus in this work was on the fate of dyestuff. Apart from this, the quality of cellulose xanthation (degree and uniformity), polymer dissolution levels, and properties of the regenerated fibers are also important to be considered.

Publication:

Mueller SM, Braun DE, Bechtold T, Pham T, Manian AP Regeneration of CI Sulfur Black 1 dyed cellulose through the viscose process. Text Res J 0:00405175251345478. https://doi.org/10.1177/00405175251345478