Repolymerization of pyrolytic lignin for producing carbon fiber with improved properties

Lignin is a promising precursor of low-cost carbon fibers. However, the mechanical properties of carbon fibers produced from melt-spinning of raw lignin are poor, restricted by the randomly cross-linked polymer structures of lignin. In the present study, carbon fibers were produced from lignin-derived phenolic oil. Pyrolytic lignin was isolated from pyrolysis oil of red oak and washed with toluene to remove volatile impurities. Upon repolymerizing with 0.5% of sulfuric acid, the toluene-washed pyrolytic lignin became solid with the glass transition temperature (Tg) of 101 °C and the average molecular weight of 1267 Da. The repolymerized pyrolytic lignin was further processed into carbon fibers through melt-spinning, oxidative stabilization and carbonization at 1000 °C. The average tensile strength and modulus of the fibers were 855 MPa and 85 GPa, while the highest values of individual fibers were 1014 MPa and 122 GPa, respectively. The present study suggests that the quality of the carbon fiber produced from pyrolytic lignin could be further improved by process optimization.