The production of asphalt-based carbon fiber is a complex process involving multiple steps, including the selection and refinement of raw pitch, the preparation of spinning pitch,pitch spinning, non-melting treatment, and carbonization. If the carbon fiber is further subjected to high-temperature heat treatment (graphitization) at 2500-3000°C, pitch-based graphite fiber will be obtained.
Petroleum asphalt or coal tar asphalt serves as the raw material for producing pitch-based carbon fiber. Due to the wide variety of available raw materials, such as thermal cracking residue oil, catalytic cracking residue, and ethylene cracking tar, the composition is complex. Since the sources vary and the compositions of different raw materials are inconsistent, it is necessary to undergo impurity removal and refining processes. For coal tar pitch, methods such as solvent extraction, thermal filtration, centrifugation, and modification can be used for selective refining. For petroleum pitch, methods such as modification, rotary scraping evaporation, vacuum distillation, and supercritical extraction can be employed for refining. The preparation of spinning asphalt involves subjecting refined asphalt to thermal treatment at a certain temperature, causing it to undergo oxidation reactions with air while simultaneously undergoing polymerization, dehydrogenation, and aromatization reactions, ultimately yielding spinning asphalt. The purpose of this step is to adjust the molecular structure, size, and composition of spinning asphalt, increase its softening point, and remove light components. Asphalt spinning generally employs the melt spinning method, which is the process of forming carbon fibers from asphalt. The primary spinning methods include melt spinning, centrifugal spinning, vortex spinning, and melt blowing. Key process conditions to note in this step include spinning temperature, spinning pressure, spinning speed, and spinning draw ratio. The non-melting treatment of pitch carbon fibers is to prevent deformation, adhesion, and melting of the pitch fibers during high-temperature carbonization, while fixing the oriented arrangement of the asphalt molecules formed during spinning. This step is achieved through oxidation, also known as pre-oxidation, typically conducted at 200–400°C. Non-melting treatment can be performed in the gas phase or liquid phase. Oxidizing agents can be oxidizing gases or liquids, and inert gases may be introduced into the environment to prevent excessive or accelerated oxidation of the pitch fibers. Carbonization involves subjecting the oxidized asphalt fibers treated by the non-melting process to further thermal treatment at 1000–1600°C in a chemically inert atmosphere. This process causes non-carbon elements such as hydrogen and oxygen in the fibers to escape as gases, resulting in the formation of graphite microcrystalline structures and high-carbon-content fibers. This endows the fibers with excellent mechanical, electrical, and thermal properties.
This process can be simply described as follows: starting at approximately 300°C, the oxidation of asphalt molecules begins to undergo condensation reactions, with carbonyl groups being released; When the temperature reaches approximately 400°C, methane gas begins to form; at around 500°C, the polymerization reaction between molecules accelerates, and hydrogen atoms on the aromatic rings begin to form hydrogen gas and be released; at approximately 600°C, a preliminary graphite microcrystalline structure begins to form, after which hydrogen gas is released more rapidly. As the temperature continues to rise, the content of non-carbon atoms in the fiber gradually decreases, and at around 1000°C, the fiber is primarily composed of graphite microcrystals.