Gel spun polyacrylonitrile based carbon fibers containing lignin and carbon nanotubes
Liu, Hsiang-Hao Clive
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Forestry bioproduct lignin has been long proposed as an ideal material for carbon fiber precursor due to its abundance, renewability, high carbon yield and cost-effectiveness as the biorefinery by-product. However, little success of lignin based commercial carbon fibers had been reported due to the barriers such as varying chemical structure, complex thermal stabilization process, poor miscibility with other polymers that resulted in unmanageable processing and limited mechanical performance. In this research, systematic investigations are conducted on the lignin/polyacrylonitrile (PAN) blends in solution, fiber structure, and during thermal stabilization and carbonization process to valorize biorefinery waste lignin for its potential towards green manufacturing and renewability of carbon fibers. Dynamic shear rheology study of PAN with various lignin content in solutions was carried out to investigate the potential boundary conditions for lignin incorporation, and to characterize the interactions of PAN and lignin with respect to fiber processing. The findings have shown that increasing incorporation of lignin in the PAN solution promotes fluid-solid transition during coagulation, and reduces solution viscosity, yield stress, relaxation time, and thermo-reversibility. The rheology findings along with gel spinning technology been successfully leveraged to address un-favored porous structure of PAN/lignin blends reported in the literature. Single-component PAN/lignin and PAN/lignin/carbon nanotubes (CNT) blend fibers, as well as PAN-sheath and PAN/lignin-core bi-component fiber are manufactured. The presence of lignin in spinning dope is shown to reduce the fiber maximum draw ratio. Since fibers manufactured with higher draw ratio can render smaller diameter to reduce number of defects per unit volume in the fiber structure, the decrease in maximum draw ratio limits the improvements on the derived carbon fiber mechanical properties. With the aid of bi-component fiber spinning, PAN sheath provides protection and endurance to fibers towards higher draw ratios. The presence of lignin and CNT is shown to affect the precursor fiber structure and to alter the structural reordering process during carbonization. In thermal stabilization process, a critical step in carbon fiber conversion, lignin was shown to reduce PAN cyclization, oxidation, and crosslinking reaction activation energies and increase the corresponding reaction rates. This renders a potential to reduce time and energy consumption during carbon fiber manufacturing. With ≥ 30 wt.% of lignin incorporation, batch-processed PAN/lignin carbon fibers exhibit tensile strength of 2.11 GPa and tensile modulus of 260 GPa, exceeding low-cost carbon fiber performance target (1.72 GPa, 172 GPa) set by the U.S. Department of Energy and the best PAN/APL carbon fiber mechanical performance reported in the literature to-date. With proper processing parameters, PAN sheath, and PAN/APL core bi-component carbon fibers with porous core (translates to ~24% overall porosity) can be manufactured as an approach for low-density carbon fibers.