Date of Award
Doctor of Philosophy (PhD)
Chemical and Biomolecular Engineering
Dr. Mark C. Thies
Dr. David A. Bruce
Dr. Igor A. Luzinov
Dr. Amod A. Ogale
As a category of materials, engineered carbons, specifically carbon fibers, are first-in-class for properties such as modulus, specific strength, and thermal resilience; however, the inability to directly process atomic carbon necessitates the development and optimization of carbonaceous precursors. Because the structure and properties of carbon are highly dependent on the precursors and requisite processing, numerous materials have been investigated as feedstocks for large-scale production. Although cellulose and rayon were among the first investigated, polyacrylonitrile (PAN) is the current hegemon of carbon fiber precursors. PAN feeds 90% of this market, but it is neither inexpensive nor renewable. Because a significant fraction of the cost of engineered carbon comes from the precursor, an inexpensive feedstock could open new sectors to the weight-saving, fatigue-resistant, and heat-resilient benefits of advanced carbons.
When examining existing renewable materials that could fill this low-cost, high-carbon niche, lignin stands out as a promising candidate. As a major constituent of biomass (in addition to cellulose and hemicellulose), lignin is an aromatic polymer synthesized by plants for structural rigidity and pathogenic resistance. Unfortunately, the combination of poor solubility, undesirable mechanical properties, abundant impurities, and high carbon content has led many to burn lignin for its heating value, while focusing on cellulose as the remunerative component in biomass. To change this paradigm, a process must be able to significantly remove impurities, while narrowing the molecular weight distribution to improve and unify the properties of a lignin stream. This must also be accomplished without abandoning the low-cost or renewable objectives of lignin utilization. To this end, this dissertation presents lignin/“renewable solvent”/water phase behavior with both kraft and biorefinery lignins. Specifically, the phase behavior for the lignin/ethanol/water system has been translated into a process that yields lignin-based carbon fibers, activated carbons, and polyurethane foams. Unique to these lignin fractionation processes, the phase behavior presented herein explores a region of liquid–liquid equilibrium, where the lignin partitions between a solvent-rich phase (resembling a dilute polymer solution) and a lignin-rich phase (resembling a polymer melt but plasticized with as much as 50 wt% solvent).
Contrary to the low-cost and renewably sourced objectives of lignin valorization, highly graphitic carbon materials instead prioritize performance. Due to the in-plane properties of graphite (versus disordered carbon), graphite fibers possess remarkably high modulus and specific thermal/electrical conductivity – higher than those of PAN-based carbon fibers. For these graphitic materials, mesophase (also called liquid crystalline) pitch is the premier precursor, due to its long-range molecular order and exceptionally high carbon content. However, the conversion of petroleum-based pitch into a mesogenic material is not a well-understood process. In this dissertation, a model pitch (of characterized chemical composition) is continuously processed via supercritical extraction, with the intent of better understanding the necessary conditions for an isotropic pitch to become mesogenic. Additionally, for both lignin and pitch, the phase behavior needed to generate fractions of controlled properties is presented, and then further applied to tune the properties of carbonaceous precursors.
Tindall, Graham, "Bridging the Gap Between Phase Behavior and Processing For Renewable and Refinable Carbonaceous Precursors" (2023). All Dissertations. 3340.
Author ORCID Identifier