Carbon Fibers Derived from Fractionated–Solvated Lignin Precursors for Enhanced Mechanical Performance

The sustainable solvent system acetic acid + water was used to simultaneously fractionate, solvate, and clean a softwood Kraft lignin for conversion to carbon fibers. By exploiting the novel liquid–liquid equilibrium (LLE) phase behavior exhibited by this pseudoternary system, three fractionated–solvated lignin precursors (FSLPs) of increasing molecular weight (7200, 13 800, and 28 600) were obtained via the continuous Aqueous Lignin Purification using Hot Acids (ALPHA) process. It is noteworthy that all three FSLPs, isolated as the lignin-rich liquid phase, had very low metals/ash content (230 ppm of Na and 0.07 wt % ash), in contrast to that of the bulk feed lignin (1400 ppm of Na and 0.60 wt % ash). Lignin fibers were successfully spun from the FSLPs by dry-spinning. Subsequently, the lignin fibers could be rapidly stabilized and carbonized at 1000 °C to produce carbon fibers with equivalent diameters less than 7 μm. Carbon fibers obtained from the highest molecular weight FSLP possessed an average tensile strength and modulus of 1.39 and 98 GPa, respectively, representing the highest mechanical properties ever obtained for carbon fibers derived from low-cost, chemically unmodified lignin.