Date of Award

5-2016

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Chemical Engineering

Committee Member

Dr. Amod A. Ogale, Committee Chair

Committee Member

Dr. Douglas Hirt

Committee Member

Dr. Christopher Kitchens

Committee Member

Dr. Rajendra Kumar Bordia

Abstract

Cost and environmental concerns arise from the manufacture of carbon fibersusing petroleum-based precursors such as polyacrylonitrile (PAN). Toxic by-productssuch as hydrogen cyanide (HCN) are generated during stabilization and carbonization ofPAN-based carbon fibers. These concerns have promoted increasing interest in biomass-based carbon fibers. As the second most abundant biomass material on the earth, lignin isbeing investigated as a potential carbon fiber precursor. Therefore, this research wasfocused on converting lignin materials into carbon fibers with enhanced performanceproperties. Since the 1960s, various types of lignin have been investigated as carbon fiber precursors. Hardwood kraft lignin and organosolv lignin could be converted into carbon fibers without chemical modification, whereas softwood kraft lignin was very difficult to convert without suitable modification or plasticization. Strength of most of the carbon fibers produced from the above lignin precursors were below 800 MPa, which is much lower than that of commercial carbon fibers derived from PAN precursors. Thus, the overall goal of this study was to produce lignin-based carbon fibers with enhanced mechanical properties by a scalable process. The specific objectives were to: (i) identify different types of lignin precursors for their potential of being carbon fiber precursors; (ii) study the modified lignin-acetone solutions to establish a range of suitable combinations of solution concentrations and spinning temperatures; (iii) establish thermal stabilization and carbonization conditions for lignin-based precursor fibers to enhance the performance of resulting carbon fibers; and (iv) to develop a UV/thermal dual stabilization route to increase the speed of stabilization. The lignin precursors investigated in this study included an organosolv lignin, a soda lignin, and a softwood kraft lignin. The organosolv lignin was successfully melt-spun into fibers without any modification of the precursor material. However, it took more than 200 hours for the thermo-oxidative stabilization step. The infusible soda lignin was chemically modified by acetylation into a fusible material, but it could not be cross-linked. The softwood kraft lignin was modified by a similar acetylation reaction and fractionation method, and the resulting material could be melt-spun into fibers, as the melt possessed significant thermal stability. The large extent of acetylation of hydroxyl groups that led to thermal stability also hindered the thermo-oxidative stabilization. Consequently, the melt-spinning approach was abandoned. Instead, to preserve more hydroxyl groups within the precursor material, the acetic anhydride amount used in acetylation of the softwood kraft lignin was reduced from 15 to 0.66 ml per gram lignin. As indicated by FTIR spectroscopy, the hydroxyl peak was significantly increased. In addition, the weight gain of lignin after reaction was reduced from 18% to 5%, indicating a partial acetylation of the hydroxyl groups in softwood kraft lignin. The resulting acetylated softwood kraft (Ace-SKL) lignin could be dry-spun using acetone as solvent, and the fibers could be thermo-oxidatively stabilized. The rheology of Ace-SKL/acetone solutions prepared with different solid contents was investigated for the purpose of dry-spinning into precursor fibers. The solution viscosity was investigated at high shear rates encountered during fiber spinning. The solutions displayed a significant shear-thinning behavior at various temperatures studied with power-law exponents ranging from 0.33 to 0.82, confirming the macromolecular nature of the Ace-SKL lignin/acetone solutions. As expected, elevated temperatures led to lower viscosities and facilitated extrusion at moderate pressures. Dry-spinning was performed over a range of concentrations (1.85 to 2.15 g/ml acetone) and appropriate temperatures (25-50°C). It was observed that all of the resulting dry-spun lignin fibers displayed a crenulated surface pattern, with increased crenulation achieved for fibers spun at higher temperatures. Presence of some doubly-convex and sharp crevices was found on fibers produced from solutions containing lower concentrations (1.85 and 2.00 g lignin/mL solvent). In contrast, no crevices were found on the fibers obtained from the concentrated solution (2.15 g/mL), likely due to the reduced extent of solvent out-diffusion. Dry-spinning at room temperature was also performed to obtained fibers with relatively smooth surface, but the pressure drop was excessive. The results above have established temperature/concentration combinations for dry-spinning of Ace-SKL. About 30% larger surface area could be achieved in the crenulated lignin fibers (as compared with equivalent circular fibers), indicating the potential advantage of such biomass-derived fibers in providing larger fiber/matrix bonding area when used in composites. During thermo-oxidative stabilization, tension of about 2000-2500 g/(g/cm) was applied on fiber tows that led up to 800% extension. In the carbonization step, tension was also applied using a customized graphite rack and tungsten weights, and stabilized Ace-SKL fibers were successfully carbonized at 1000°C. It was found that the load needed to be above 20150 g/(g/cm) to prevent shrinkage of fiber tows. Both tensile strength and modulus were measured as a function of extension during carbonization (EDC). As expected, Ace-SKL carbon fibers with larger EDC had better mechanical properties due to preserved molecular orientation. Carbon fibers derived from lignin precursor fibers obtained from 2.15 g/ml Ace-SKL solution (6 μm diameter) displayed a tensile modulus, strength, and strain-to-failure values of 52 ± 2 GPa, 1050 ± 70 GPa, and 2.0 ± 0.2%, respectively. These values are amongst the best reported for lignin-based carbon fibers. In contrast, the carbon fibers spun from 2.00 g/ml solution with sharp crevices displayed a reduced tensile strength of 790 ± 80 MPa due to occlusion-type defects formed by sharp crevices during spinning. The Ace-SKL carbon fibers displayed low crystallinity as investigated by Wide Angle X-ray Diffraction (WAXD) and Raman spectroscopy. The crenulated surface from dry-spinning was preserved, which can provide a larger specific interfacial area for enhanced fiber/matrix bonding in composite applications. Above results elucidate the importance of precursor composition and processing conditions on microstructure and properties of resulting precursor and carbon fibers. A limitation of the partially acetylated lignin (i.e., with a fraction of hydroxyl moieties converted to acetyl groups) is the slow heating rate during thermal stabilization, which required up to 40 hours due to the slow heating rate needed to prevent the fibers from becoming tacky and sticking to each other. Therefore, a rapid strategy of dual UV-thermoxidative stabilization was developed. The fibers undergo UV-induced reaction close to the surface in a short duration (15 min) such that they can be subsequently stabilized at a rapid heating rate without fibers fusing together. The glass transition temperature of UV irradiated fibers was about 15°C higher than that of fibers without UV treatment. This strategy reduces the total stabilization time significantly from 40 to 4 hour. Stabilized fibers were successfully carbonized at 1000°C and resulting carbon fibers displayed a tensile strength of 900 ± 100 MPa, which is amongst the highest reported for carbon fibers derived from rapidly stabilized lignin precursors. In summary, the results from this study established a route for dry-spinning of partially acetylated softwood kraft lignin into precursor fibers and successful stabilization and carbonization. This precursor could be dissolved in acetone for dry-spinning. The lignin/acetone solutions were investigated to establish suitable concentration/temperature combinations for dry-spinning. The dry-spun precursor fibers were thermally treated under tension to convert into a carbon fiber with tensile strength of more than 1 GPa. Those carbon fibers possessed crenulated surface which could provide larger fiber/matrix interfacial bonding area for composite applications. Furthermore, UV/thermal dual stabilization was developed to reduce the time duration of stabilization.

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