Date of Award

12-2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical Engineering

Committee Chair/Advisor

Prof Amod Ogale

Committee Member

Prof David Bruce

Committee Member

Prof Douglas Hirt

Committee Member

Prof Marek Urban

Abstract

Carbon fibers have the highest strength and modulus among all known fibers and are used as reinforcements in high-performance composites [1]. Carbon fibers also have a very low density relative to metals. Therefore, carbon fibers possess ultrahigh specific strength and modulus, which make them desirable for high-performance light-weight composites. A vast majority of commercial carbon fibers are produced from PAN precursors that are expensive, which limits the use of PAN-derived carbon fibers to aerospace applications (e.g., airplanes). However, for costsensitive applications, there is a need for low-cost, moderate performance carbon fibers. Lignin is a low-cost by-product of pulping and biorefining process that has been explored as a precursor for low-cost carbon fibers in several prior studies. However, most of the studies in the literature have focused on low molecular weight, melt-spinnable lignin grades or blending lignin with other polymers to obtain precursor fibers. To enable melt-spinning, glass transition temperature of lignin must be low, and that makes the stabilization process extremely slow or unsuccessful altogether. Wet-spinning, on the other hand, needs large amounts of strong solvents and coagulation bath. Dry-spinning offers an alternative path that uses relatively mild solvents. This approach was used by the Ogale group for dry-spinning of acetylated lignin precursor fibers and resulting carbon fibers [2]. However, this approach required chemical modification of lignin. In another development, Thies group developed a process (ALPHA) that can purify and fractionate lignin to obtain high molecular weight lignin fractions. The dry-spinning of lignin fractions separated using the ALPHA process was established in a prior study [3]. However, that study utilized acetic acid, a corrosive solvent. Therefore, the over-arching goal of the present study was to dry-spin precursor lignin fibers using a non-corrosive solvent in conjunction with non-edible bio-based precursors ii iii derived from renewable resources to produce carbon fibers. Lignins derived from renewable sources like corn stover, hardwood (Hybrid Poplar), and softwood (Southern Pine) all fit that criterion as bio-renewable precursors from by-product/waste streams, and ethanol is a noncorrosive solvent that is a product of biorefining. All of these precursors used in the present study were generated by the Thies group using the ALPHA process (or its variants). The specific objectives of the study presented in this dissertation were to: (i) Dry-spin lignin fibers from precursor materials produced using ecofriendly ethanol solvent and fractionated lignin sourced from bio-renewable softwood kraft lignin (SKL), corn stover (CS), and hybrid poplar (HP) hardwood; (ii) Investigate the effect of molecular weight of lignin and novel UV irradiation route to enhance stabilization kinetics for faster crosslinking of lignin fibers; (iii) Analyze the carbon-layer microstructure of resulting fibers carbonized at different temperatures to determine the role of processing conditions on resulting carbon fiber properties. Chapter 2 is devoted to the investigation of softwood kraft lignin-based precursors. Softwood kraft lignin (SKL) fibers were successfully dry-spun using bio-renewable ethanol-water as the mixed solvent. Precursor fibers could be further stretched as much as 800%. High molecular weight SKL fibers could be stabilized (crosslinked) in as little as 4 hrs, whereas low molecular SKL required 10 hrs. The carbon-layer microstructure of resulting SKL-based carbon fibers was found to be disordered with little graphitic content in fibers produced at 1000°C. The highest tensile strength of the SKL-based carbon fibers was measured at 1.1±0.1 GPa and compliancecorrected modulus was 51±7 GPa. This strength value is comparable with that of rayon-based iv carbon fibers and among the highest for carbon fibers derived from SKL lignin fractionated using ethanol (solvent). Chapter 3 of this dissertation explored the production of carbon fibers from corn stover (CS), which is waste byproduct from harvesting of corn kernels. Three different molecular weight fractions were dry-spun using ethanol/water mixed solvent. As molecular weight increased from 24 kDa to 56 kDa, the glass transition temperature increased by almost 20°C and stabilization could be performed at higher conditions, and so the stabilization time could be reduced to half. During thermo-oxidative stabilization under constant load, precursor fibers could be stretched about 200%. To reduce the stabilization time further, a UV irradiation method was used, which reduced the time of stabilization five-fold as compared with the thermo-oxidative stabilization method alone, while retaining the tensile strength of carbon fibers. The compliance-corrected tensile modulus for CS-derived carbon fibers was 82±8 GPa and the tensile strength achieved was 0.95±0.12 GPa. This is almost double of that reported in prior literature studies for CS-derived carbon fibers. Finally, the fourth chapter of this dissertation investigated the dry-spinning of precursor fibers using hardwood lignin sourced from hybrid poplar trees and subsequent conversion to carbon fibers. Hybrid poplar (HP) lignin fractions were dry-spun again using ethanol/water mixed solvent. Feed lignin (as-received) and three different molecular weight fractions were spun to assess the effect of the molecular weight of lignin on the processing and properties of the fibers. It was observed that feed lignin was not only hard to spin, but it was also very hard to stabilize. Subsequent improvements brought by ALPHA fractionation enabled better spinnability, but fibers produced using low molecular weight (11 kDa) fractions took more than 80 hrs for stabilization. When the molecular weight of fractionated lignin increased to 52 kDa, thermo-oxidative v stabilization time was decreased by a factor of 5. This was possible due to faster heating rate that could be used during thermo-oxidative stabilization owing to increased glass transition temperature (by almost 50°C). Furthermore, the novel UV-irradiation method of stabilization could reduce the stabilization time down to 3.5 hrs, i.e., sixteen-fold faster process. Carbon fibers obtained from HP lignin fibers stabilized using thermo-oxidative and/or UV irradiation methods have similar tensile strengths. The carbon-layer structure of the hardwood lignin-based carbon fibers was found to be more disordered as compared with even PAN-based carbon fibers. Fibers produced using 52 kDa lignin fractions resulted in carbon fibers with a tensile modulus of 78±8 GPa and tensile strength of 1.1±0.2 GPa, which is almost double of that previously reported for neat poplar and other hardwood lignin-derived carbon fibers. Thus, as summarized in Chapter 5, results from this study established that dry-spinning of softwood, hardwood, and grass lignin fibers using an ethanol/water solvent mixture is feasible. With an increase in the molecular weight of the precursor lignin, glass transition temperature increased and that resulted in a significant reduction in stabilization time. A novel UV-irradiation stabilization helped to reduce the stabilization time five-fold for grass lignin and sixteen-fold for hardwood lignin (as compared with thermo-oxidative stabilization alone). Carbon structure remained disordered in fibers derived from all three lignin sources. Finally, it is noted that although the strength of these carbon fibers derived from bio-based lignin did not reach the level found in commercial PAN-based carbon fibers, these fibers possess a specific strength of 0.6 GPa/(g/cm3), which is almost three times that of aluminum. Several suggestions are presented at the end of Chapter 5 for further modifications in precursors and carbon fiber processing conditions to enhance the microstructure and properties of the resulting carbon fibers

Comments

This is draft submitted to committee for the review but I would like to have the feedback on the formatting hence submitting here

Author ORCID Identifier

0000-0002-3829-5866

Available for download on Sunday, December 31, 2023

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