Characterization of Modified Mesophase Pitch-Based Carbon Fibers

Carbon fiber (CF) is the premier building material in multiple, important industries and continues to grow in both its usage and demand. However, the cost of the premier CF, produced from polyacrylonitrile (PAN), has limited the CF market. Therefore, alternative CF precursor research remains an important topic with the goal of driving down cost while limiting environmental impacts.
The aim of this dissertation is to explore the capabilities of one possible alternative precursor material: mesophase pitch. The benefit of mesophase pitch lies in the production of the feedstock and precursor fibers. Since mesophase pitch is a byproduct produced from petroleum distillation and coal cracking, large cost saving opportunities exist. Additionally, it can be melt-spun, a more cost-effective extrusion method which removes the need for any environmentally harmful solvents required with PAN wet spinning. While the achievable strengths of mesophase pitch-based CF (MPCF) cannot compete with PAN-CF, MPCF has found uses in industries which are more concerned with stiffness-to-weight ratio, electrical and thermal conductivity, and low thermal expansion. For MPCF to enter additional markets, such as general car manufacturing, the cost must be further reduced while maintaining quality properties. This is possible through a better understanding of CF structure-property-relationship.
Prior to extrusion, mesophase pitch can be modified through the introduction of waste polymers. The addition of such polymers provides two benefits: 1) cost reductions and 2) environmental sustainability. Such hybrid fibers have been previously produced, but these fibers could not be converted into CF leaving the need for such exploration. By blending with linear low-density polyethylene and polyethylene terephthalate, these experiments revealed that such fibers could be produced successfully, but special considerations of the polymer and its effect on the fiber’s microstructure must be taken into account.
The focus on the fiber’s microstructure provided the basis for the remainder of this dissertation. The production of MPCF involves extrusion, oxidation, and carbonization, with all three steps playing a significant role in the CF development. Changes in the extrusion procedure create different texture shapes, each with unique properties. However, the microstructure of these different fibers has yet to be explored in detail and represents an opportunity to gain additional understanding. The fibers must be thermally stabilized in oxygen, such that the fibers can survive the final heat treatment process. A successful oxidation involves an even diffusion of oxygen throughout the fiber. If heated to quickly in an effort to reduce cost, a core-shell structure is developed resulting in depreciated tensile properties. However, if held at temperature for too long to ensure complete diffusion, over-oxidation as a result of the removal of carbon atoms sets in. Finding the balance of temperature and time for oxidation is vital and must be explored in more depth. Finally, carbonization, or high temperature heat treatment, is the process of removing noncarbon atoms resulting in carbon crystallite growth which are responsible for the desired CF properties. Given the complicated reactions, relevant rates for industrial production (> 50 °C/min) were examined, and a rate of 23.3 °C/min was found to produce the strongest fiber.
The work presented in this dissertation contributes to the understanding of the affects each production step has on the structure and properties of MPCF. Each alteration offers an opportunity to provide cost-reductions, decrease environmental impact, and control the CF macro-properties through the microstructure. Additionally, recommendations for future work are offered in the final chapter of this dissertation.