Date of Award

12-2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical and Biomolecular Engineering

Committee Member

Amod A. Ogale, Committee Chair

Committee Member

Douglas E. Hirt

Committee Member

Mark C. Thies

Committee Member

Igor Luzinov

Abstract

Carbon fibers are known for their outstanding specific strength and modulus, and they have been commercially used in structural and lightweight composites. However, the main barriers to the large-scale commercial application of carbon fibers are the high cost of petroleum-based precursor polyacrylonitrile (PAN) and the environmental concerns associated with its conversion to carbon fibers. Toxic by-products such as hydrogen cyanide (HCN) are generated during the stabilization step of PAN-based carbon fibers. Other precursors being used today, carbonaceous pitch and rayon, either are too expensive or produce carbon fibers with low strength. Among bio-derived feedstocks, lignin is of increasing interest as a precursor for producing carbon fibers. Most of early research studies utilizing lignin focused on the melt-spinning of hardwood lignins because of their better spinnability. Softwood lignin is difficult to melt-spin, but can be dry-spun with partial acetylation. Tensile strength for most of lignin-based carbon fibers was below 1.0 GPa, which is much lower than that needed for high-performance composites. In this dissertation, the primary research goal was to develop lignin precursors to produce carbon fibers with improved mechanical properties. In the first part of this study, lignin/PAN (L/P) polymer blend was used as the precursor to produce carbon fibers. As-received softwood kraft lignin and PAN were blended to form precursor with lignin content as high as 50 wt%. Rheological measurements established that increasing lignin content in the spinning solution reduced shear viscosity and normal stress, indicating a decrease of viscoelastic behavior. The lignin/PAN blend solutions were wet-spun into fibers, but lignin leaching occurred during coagulation that resulted in macro-void formation within initial L/P fibers. To eliminate the macro-voids within L/P fibers, out-diffusion of lignin during wet-spinning was reduced by controlling the coagulant composition. It was confirmed by UV–vis spectroscopy that an insignificant amount of lignin leached out from fibers into coagulant when 0.2% initial lignin content was incorporated into the coagulant. Thus, void-free, equi-component, L/P- derived carbon fibers were successfully produced. Using Raman spectroscopy and wide-angle X-ray diffraction, it was observed that higher lignin content led to a lower degree of graphitic crystallinity. For lignin contents of 25 to 50 wt%, the tensile strength of resulting carbon fibers was 1.2 GPa and not significantly affected. However, the modulus decreased from 148 GPa to 106 GPa, because the higher lignin content disturbed the formation of a well-layered turbostratic carbon structure. The second major component of this research study focused on producing carbon fibers using fractionated-solvated lignin precursors (FSLPs) by dry-spinning. The FSLPs were generated via a process, Aqueous Lignin Purification using Hot Acids (ALPHA), recently developed by Thies and co-workers (1). Three purified lignin fractions of increasing molecular weight were dry-spun into fibers. The spinning conditions were systematically investigated with the goal of enhancing draw-down of the precursor fibers. The solution viscosity was estimated at about 2 Pa∙s (shear rate of 2×105 s-1). Larger draw-down ratios could be obtained for the higher MW FSLPs. Within the range of conditions investigated in this study, a solution concentration of 50-55 wt% and a temperature range of 30-45 °C were identified as the window of optimal spinnability. Thermo-oxidative stabilization of lignin fibers was successfully performed under constant load conditions to obtain about 400% extension within fibers, whereas carbonization was performed under constant-length conditions. Finally, the microstructure and properties of FSLPs-based carbon fibers were studied. From Raman spectroscopy and X-ray diffraction analysis, it was observed that an increase in the molecular weight of lignin fractions led to a better layered microstructure within the resulting carbon fibers. At low carbonization temperatures (1000-1600°C), the lignin-based carbon fibers displayed a low degree of graphitic crystallinity, as measured by Raman spectroscopy, XRD, and TEM. Carbonization temperatures above 2000 °C improved the ordering of the layer planes, but FSLP-based carbon fibers still displayed a low degree of graphitization. The mechanical and electrical properties of FSLP-based carbon fibers were also investigated. Carbon fibers produced at 1000°C from the highest MW FSLP possessed the highest average tensile strength of 1.39±0.23 GPa and a modulus of 98±5 GPa, representing the best-quality lignin-based carbon fibers reported in the literature to date. When the fibers were carbonized at higher temperatures (>1300°C), a reduction in tensile strength was observed due to the generation of surface defects. However, the tensile modulus and electrical conductivity increased as the carbonization temperature increased because of the improved graphitic crystalline structure. In summary, the results from this study established a route for wet-spinning void-free lignin/PAN fibers and dry-spinning fractionated lignin precursors with increasing molecular weight, leading to carbon fibers with enhanced properties.

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