Carbon fibers (CFs) possess outstanding specific strength and modulus. Therefore, they are used commercially in structural composite applications (viz. aerospace) for enhanced performance and fuel efficiency, resulting from superior strength-to-weight ratio. The vast majority of commercial CFs are currently produced from a synthetic precursor, polyacrylonitrile (PAN), via wet-spinning. The cost of PAN precursor not only contributes to about 50% of CF costs, but the cyanide groups generate toxic by-products (viz. hydrogen cyanide) during heat-treatment. Thus, bio-derived CF precursors are of increasing interest from an environmental perspective.<br/><br/>Among renewable feedstocks, lignin is an important potential precursor for carbon fibers. Although lignin is readily available as an inexpensive by-product, it is too high in metal impurities. Therefore, a renewable solvent system, Aqueous Lignin Purification with Hot Acids (ALPHA), consisting of hot-acetic acid-water mixture has been developed to purify, fractionate, and solvate kraft lignins. The ash content of the kraft lignin purified by this continuous flow ALPHA process was below 0.1%, which was lower than unfractionated kraft lignin and other organosolv lignins. Using different molecular weight fractions of the solvated lignin, direct dry-spinning of lignin precursor fibers was accomplished at temperatures ranging up to 325K. The precursor fibers were converted to carbon fibers by thermal oxidative stabilization and carbonization under tension to improve molecular orientation. The resulting CFs possessed a tensile strength of 1.3±0.3 GPa and modulus of 86.0±8.8 GPa, the highest combination of strength and modulus of CFs derived from lignin. The fractionated ultra-pure lignin and higher molecular weight precursor lead to better carbon structure formation, which in turn leads to the improved carbon fiber properties.

Carbon fibers derived from fractionated-solvated lignin

255th National Meeting (2018)

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