Recycling of Superbase-Based Ionic Liquid Solvents for the Production of Textile-Grade Regenerated Cellulose Fibers in the Lyocell Process

Ioncell is a Lyocell based technology for the production of man-made cellulose fibers. This technology exploits the intrinsic dissolution power of superbase-based ionic liquids (ILs) toward cellulose and the ability to form spinnable cellulose solutions. The regenerated fibers are produced via a dry-jet wet spinning process in which the cellulose filaments are stretched in an air gap before regenerating in an aqueous coagulation medium. For the commercialization of this process, it is essential to demonstrate the quantitative recovery of the solvent from the coagulation bath without impairing its solvation power. This study reports on the spinnability and recyclability of the IL 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-enium acetate ([mTBDH][OAc]) over five cycles in comparison to 1,5-diaza-bicyclo[4.3.0]non-5-enium acetate ([DBNH][OAc]). The aqueous IL solutions were recovered from the coagulation bath by successive thermal treatments under reduced pressure. Accordingly, the recycled ILs were utilized to dissolve 13 wt % cellulose pulp in each cycle without the addition of make-up IL. While using [mTBDH][OAc], the pulp was completely dissolved and processed into easily spinnable cellulose solutions during all five cycles, whereas the ability to dissolve pulp was completely lost after the first recovery cycle when using [DBNH][OAc]. The composition of the recovered ILs and extent of side-products generated in the adopted process was analyzed in detail. This includes characterization of the rheological properties of the solutions as well as the macromolecular and mechanical properties of the regenerated fibers. In addition, we review the toxicity of both solvents using Vibrio fischeri bacteria. Finally, the spun fibers from all [mTBDH][OAc] spinning trials were combined to produce a demonstration dress (Paju), designed and sewn by Marimekko Design House in Finland.