Development and Studies of Isotropic Coal tar Pitch-based Carbon Fibers
Creators: name: Rani, Sonu
Publisher: name: RMIT University
Year: 2025
Subjects: classificationCode: 401602
subject: Composite and hybrid materials

description: Carbon fibers have emerged as a pivotal material due to their exceptional properties, including high mechanical strength, lightweight, chemical resistance, high-temperature tolerance, and low thermal expansion. These attributes make them invaluable in diverse applications such as aerospace, defence, sports equipment, wind turbines, construction, marine components, and medical devices, increasingly replacing metals and alloys. Currently, ~90% of carbon fibers in the market are produced from polyacrylonitrile (PAN) precursors, which are costly and limit their broader use in general-purpose applications where cost-effectiveness is crucial.
To address this issue, isotropic coal tar pitch-based carbon fibers present a promising alternative. Coal tar pitch, a by-product from the iron and steel industries, offers high carbon content, broad softening point, graphitizability, and melt-spinnability, making it suitable for general-purpose carbon fiber production. To synthesize carbon fibers from isotropic coal tar pitch, various steps are involved such as precursor synthesis, melt-spinning, stabilization, and carbonization. Each step significantly influences the processing and final properties of the carbon fibers. Therefore, this research is focused on modifying various processing steps to optimize the synthesis process and enhance the properties of carbon fibers.
In this study, various isotropic pitch precursors were synthesized from coal tar pitch and characterized for different properties. Subsequently, precursor was selected based on properties such as softening point, toluene insoluble content, and carbon content required for the synthesis of carbon fibers. During the synthesis of carbon fibers, carbonization plays a pivotal role in transforming the pitch structure into a carbonized form through the evolution of gases and molecular rearrangement via polymerization reactions. This arrangement significantly influences the final mechanical properties of carbon fibers. Therefore, the carbonization conditions were optimized by varying the heating rate and holding time at the carbonization temperature. It was found that a slower heating rate and shorter holding time at carbonization temperature improved crystalline alignment, promoted crystallite growth, and reduced defect concentrations. In contrast, longer holding time at carbonization temperature adversely affected microstructural and mechanical properties of carbon fibers. 
Furthermore, the stabilization step is crucial for converting the thermoplastic state of pitch to a thermosetting state, preventing fiber melting during carbonization. However, this step is time and energy-intensive, requiring long hours. To address this limitation, a new stabilization method was introduced which involve nitric acid treatment of pitch fibers prior to carbonization. In this study, various concentration of nitric acid including 5%, 10%, 15%, 30%, 40%, and 50% (v/v) were used for chemical treatment. The fibers were soaked for 15 min, followed by direct carbonization. The carbonization was performed using previously optimized conditions. The results revealed that optimal concentrations of HNO3 at 15% and 30% (v/v) yielded carbon fibers with superior microstructural, morphological, and mechanical properties. These concentrations enhance cross-linking among aromatic compounds, thereby increasing fiber stabilization. Lower HNO3 concentrations failed to retain fiber shape due to insufficient oxidation, while higher concentrations introduced defects through over-oxidation. This systematic study concludes that optimal HNO3 concentrations significantly enhance pitch fiber stabilization, improving the properties of resultant carbon fibers.
Next, the improvement in the spinnability of pitch precursors was addressed which is directly linked with the constitutional molecular structure of pitch. High molecular weight complex compounds contribute to high carbon content in pitch structures which are beneficial for the properties of carbon fibers. However, these structures exhibit poor spinnability due to their resistance to flow behaviour. Specifically, pitches with linear aliphatic and polycondensed chains of aromatic hydrocarbons are the most suitable precursors for good spinnability. Methods reported in the literature for synthesizing spinnable pitches involve expensive solvents, time-consuming processes, and corrosive solutions, highlighting the need for an easier and cost-effective approach. In this context, the spinnability of isotropic coal pitch was enhanced using a simple and economical process. Small amounts of petroleum pitch, a waste product of petroleum refineries, were utilized to modify the precursors through a ball-milling process. The isotropic coal tar pitch was blended with varying proportions of petroleum pitch including 5%, 10%, and 20% (w/w), and flow behaviour were analyzed using rheological measurements. During melt-spinning of these precursors, it was observed that increasing the amount of petroleum pitch remarkably improved their spinnability. However, the distinct constitutional properties of petroleum pitch also contributed to alterations in both the stabilization and carbonization processes of fibers. Carbon fibers derived from 10% petroleum pitch (10PP) exhibited the highest measured tensile strength of ~784 MPa and a modulus of ~47 GPa. The effect of petroleum pitch content on the stabilization and carbonization behaviour, as well as on the resultant microstructural and mechanical properties of carbon fibers, was thoroughly investigated. 
Subsequently, the potential of best performing isotropic pitch-based carbon fibers as skin materials for sandwich composites was thoroughly examined, with a focus on their mechanical performance and cost efficiency. For this, the sandwich composites were prepared through a two-step process involving vacuum-assisted resin infusion moulding. Initially, a middle syntactic foam core was fabricated using hollow glass microspheres, followed by the integration of syntactic foam as the middle core with carbon fiber layers as the upper and lower skins using the same resin infusion technique. Subsequently, the flexural and in-plane compressive properties of these composites were evaluated. In this study, a comparison with commercially available PAN-based carbon fibers and recycled carbon fibers was carried out. Results indicated that isotropic pitch-based carbon fibers showed approximately four times better material efficiency in compressive properties compared to commercial PAN-based carbon fibers and twice the performance index. Their flexural properties were also comparable to those of both commercial and recycled carbon fibers. The study also emphasized the relevance of recycled carbon fibers in promoting sustainability and reducing costs in composite manufacturing. Despite this, the superior performance of the pitch-based carbon fibers highlights their potential as a more viable option for various applications, especially where cost efficiency is essential.
In conclusion, various processing steps have been undertaken to simplify the overall carbon fiber production process and enhance the properties of carbon fibers. Finally, their practical applications have been explored, and the efficiency of isotropic coal tar pitch-based fibers has been compared with others.