No abstract

In recent years, there has been extensive research into the development of cheaper and more sustainable carbon fiber (CF) precursors, and air-gap-spun cellulose-lignin precursors have gained considerable attention where ionic liquids have been used for the co-dissolution of cellulose and lignin. However, ionic liquids are expensive and difficult to recycle. In the present work, an aqueous solvent system, cold alkali, was used to prepare cellulose-lignin CF precursors by wet spinning solutions containing co-dissolved dissolving-grade kraft pulp and softwood kraft lignin. Precursors containing up to 30 wt% lignin were successfully spun using two different coagulation bath compositions, where one of them introduced a flame retardant into the precursor to increase the CF conversion yield. The precursors were converted to CFs via batchwise and continuous conversion. The precursor and conversion conditions had a significant effect on the conversion yield (12–44 wt%), the Young’s modulus (33–77 GPa), and the tensile strength (0.48–1.17 GPa), while the precursor morphology was preserved. Structural characterization of the precursors and CFs showed that a more oriented and crystalline precursor gave a more ordered CF structure with higher tensile properties. The continuous conversion trials highlighted the importance of tension control to increase the mechanical properties of the CFs.

No abstract

In washing of softwood pulp, pH is reduced to such low levels that re-precipitation of dissolved lignin may result. It was hypothesized that the lower the residual alkali after cooking, the higher the risk of lignin re-precipitation, which in turn would affect the subsequent oxygen delignification stage negatively. To test the hypothesis, kraft cooks were performed to different residual alkali levels, ranging from 5 to 15 g/l and the delignified chips were subjected to different washing strategies and then oxygen delignified. However, the results show that even at low residual alkali, lignin stays in solution and only minor amounts of lignin re-precipitate during washing. The amount was irrespective of the residual alkali level in the cook and amounted to 1-2 kappa number units. No effect of residual alkali level was observed on the performance of the oxygen delignification stage. The residual alkali level affected the lignin concentration in the lumen after cooking. At the low residual alkali level, the lignin concentration in the lumen liquor was equal to the free liquor and as the residual alkali level increased, the lignin concentration in the lumen increased, while the lignin concentration in the free black liquor remained on the same level at all residual alkali levels.