High-Strength Carbon Fibers from Eco-Friendly Processing of Biomass

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Carbon fibers are among the strongest known reinforcing fibers used in a wide range of commercial composite materials. Because the element carbon does not melt, carbon fibers need to be obtained from organic precursors. Among renewable feedstocks, lignin is starting to play an important role as a potential precursor for carbon fibers, owing to its significant carbon content. However, the irregular chemical architecture of lignin makes the resulting carbon network very inefficient. Therefore, the resulting carbon-fiber properties are still significantly lower than those obtained from currently used synthetic polyacrylonitrile precursor, which involves the use of strong solvents and leads to toxic off-gases such as hydrogen cyanide. This research project is directed towards the development of a green precursor and an efficient processing route to obtain high strength carbon fibers.Lignin is readily available as an inexpensive by-product from Kraft paper mills, but this untreated form contains too many impurities to be used in high-performance products such as carbon fibers. Recently, Clemson researchers have discovered a renewable solvent that can purify Kraft lignin and isolate a high molecular weight lignin fraction in the solvated state. Clemson researchers (in a separate project) have also shown that dry-spinning of non-fractionated, acetylated lignin can lead to carbon fibers with enhanced strength. Therefore, the research team hypothesizes that the solvated lignin precursor will enable direct dry-spinning of precursor fibers, followed by their conversion to microstructure-controlled, high-performance carbon fibers. The specific research objectives are (i) control of the molecular architecture of fractionated-solvated lignin precursors (ii) rheological evaluation of suitable solvated lignin fractions to measure strain-hardening for efficient dry-spinning of precursor fibers, and (iii) microstructural control to obtain superior performance in the resulting carbon fibers. The intellectual significance of this research is that lignin fractions with controlled molecular weight and architecture will provide a unique materials platform for studying the rheological and microstructural (rheo-structural) behavior of these bio-derived fluids. Specifically, a long-linear architecture will be identified that leads to strain-hardening during extensional flow, a desirable phenomenon for stable dry-spinning of fibers. It is hypothesized that the control of molecular architecture of the fractionated-solvated lignin will lead to the efficient formation of graphitic carbon and superior properties for the resulting carbon fibers.