(91b) Organosolv and Kraft Lignin: Fractionation and Conversion to Melt Spun and Electrospun Carbon Fibers

The global market for carbon fiber is growing rapidly as potential uses in energy and energy savings currently are expanding. The low cost, potential abundance, and highly aromatic carbon content of lignin, compared to polyacrylonitrile (PAN) which is the main precursor for making carbon fiber, make it an attractive precursor for making low-cost carbon fiber.

Melt spinning and electrospinning are two common methods for making fibers with different characteristic and applications. While melt spun fibers can be used in structural and composite applications, electrospun fibers have potential applications in energy storage and filtration. However, these spinning methods required lignins with different characteristics. For example, electrospinning requires high molecular weight and soluble lignin, while melt spinning requires low molecular weight and fusible lignin. Technical lignins, such as kraft and organosolv, are heterogeneous and contains different molecular weight fractions and functionality. Ultrafiltration and sequential solvent extraction are methods which have been employed for separation of different fractions from lignin.

We performed a sequential solvent extraction, using methanol and methanol/methylene chloride mixture to fractionate softwood kraft lignin and organosolv switchgrass lignin. The solvent extraction resulted in obtaining three lignin fractions, according to their solubility. Lignin samples were evaluated by thermal analysis (DSC and TGA), different spectroscopy method (FTIR and NMR), elemental and compositional analysis. Finally, the lignin fractions were used for making melt spun and electrospun carbon fibers. Softwood kraft lignin had the highest ash content and was not fusible. Although switchgrass lignin was fusible, continuous spinning was not possible. Both switchgrass and softwood kraft lignins were not completely soluble in electrospinning solvent (DMF/methanol). After solvent extraction, the methanol soluble fraction had low T_g and thermal decomposition temperature, while the methanol/methylene chloride soluble fraction had high T_g and thermal decomposition temperature (117 vs 181°C for softwodd kraft lignin and 107 vs 161°C for switchgrass lignin) which could indicate low and high molecular weight of these fractions. Methanol soluble fraction of both lignins was used for melt spinning. The methanol soluble lignin fraction was able to continuously spin fibers free of defects, which is a significant improvement over the raw lignin. The methanol/methylene chloride fractions from both lignin demonstrate good solubility in electrospinning solution and performed very well in electrospinning process. Continus meltspinning for about 40 min was achived with both lignins and meltspinnning was performed for about 8 h. Carbon fibers were evaluated by SEM and tensile properties of fibers were measured. Diameter of melt spun carbon fibers were in the range of 15-30 μm; while average diameter of elctrospun carbon fibers were in the range of 350-700 μm, depend of concentration of electrospinning solution. Based on tensile properties data tensile strnght of meltspun switchgrass carbon fibers were 378 MPa and it was 441 MPa for meltspun softwood kraft carbon fibers. Results indicate lignin fractionation can provide different fractions with different characteristics and applications and can play a role like refinery for oil industry.