Lignocellulose solutions in ionic liquids

Cellulosic textiles are produced either from natural fibers, such as cotton, or from man-made fibers, such as viscose and Lyocell. Production of cotton cannot be increased, thus man-made fibers must supplant them when the overall demand grows. Strong fibers are produced by dry jet-wet spinning: a solution of dissolving pulp is extruded into a jet, which is drawn in air, coagulated in a water bath and the solvent is washed off. Ionic liquids could be even better spinning solvents than the current Lyocell solvent N-methyl-morpholine oxide monohydrate (NMMO). In this work, 1,5-diazabicyclo[4.3.0]non-5-enium acetate ([DBNH]OAc) is found to be a good solvent: the strength of the fibers spun from [DBNH]OAc is at par with Lyocell fibers. Water competes with cellulose for solvation and coagulates cellulose out. Even small amounts of water prevent the dissolution of cellulose in 1,1,3,3-tetramethylguanidium acetate ([TMGH]OAc) and propionate ([TMGH]EtCOO), whereas 1-ethyl-3-methylimidazolium acetate tolerates 10-15% water. The difference is explained by the Kamlet-Taft solvent parameters of the mixtures: the net basicity or the difference between basicity and acidity is already reduced with 0.5 equivalents of water. The mixture with 0.5 equivalents of water has a smaller rheological resilience than a 0 or 1 eqv. mixture. This prevents spinning from [emim]OAc with a 0.1 mm spinneret: in the water bath, the solidified surface of the jet is torn when the core yields. In contrast, [TMGH]OAc solutions are gelatinous and thus poorly spinnable. The diffusion constant of the ionic liquid depends on the water content of the cellulose solution-water mixture. The good solvents [DBNH]OAc and NMMO form a strong network structure during coagulation, so that the diffusion constant is strongly reduced from its initial value. For [emim]OAc the change is smaller. In [TMGH]OAc the diffusion constant does not change, because the gelatinous structure is already in place. The spinning of a [DBNH]OAc-cellulose solution is stable at the extrusion velocities of 0.01-0.045 ml/min. The most stable velocity is 0.02 ml/min, with which a draw ratio of 7.5 is reached. A higher extrusion velocity, a higher temperature and a smaller spinneret length-diameter ratio reduce the attained draw ratio. The excellent strength (40 cN/tex or 590 MPa) is explained by the orientation of amorphous domains along the fiber axis. The precipitation of cellulose from a solution of wood with an acetone-water mixture was also studied. The solubility of birchmeal (<0.125 mm) is only 93% and there is no delignification during the precipitation. An autohydrolysis pretreatment (P-factor 500, 170 °C) improves the solubility to 98% and delignification is improved to 63%. However, because there is still 13% residual lignin, the method cannot replace chemical lignin removal in pulping. The delignification is not improved by a higher P-factor (1500).