Preparation methods of asphalt-based carbon fibers and research on their industrialization

1 Properties and Applications of Asphalt-Based Carbon Fibers. Asphalt-based carbon fibers can be divided into two types based on their properties and applications: general-purpose carbon fibers and high-performance carbon fibers. General-purpose carbon fibers are made from isotropic asphalt, whereas high-performance carbon fibers are produced from mesophase asphalt. 1.1 General-purpose asphalt-based carbon fibers: Due to their low cost, general-purpose asphalt-based carbon fibers can be utilized in a wide range of applications. For example: ① Taking advantage of their low density, high strength, and light weight, they can be used as reinforcing materials in concrete, thereby increasing the strength of concrete by 6 to 10 times and its bending toughness by more than 60 times; this allows for thinner structures, reduced weight of components, savings in steel usage, and lower construction costs. ② Thanks to their high friction coefficient and strength, they can be used as friction materials in car and airplane brake pads, as well as in other braking components. ③ Their excellent electrical conductivity enables their use in manufacturing conductive paper, electric heating plates, conductive surface felts, needle-punched felts, and anti-static materials. ④ Their radiation-resistant properties allow them to be used in creating shielding materials such as smoke shields, shielding curtains, and various types of electromagnetic and static electricity shields. ⑤ Their resistance to high and low temperatures, acidity, and their low thermal expansion coefficient enable them to be used as performance enhancers in materials such as nylon (PA), polypropylene (PP), polycarbonate (PC), phenolic resin (PF), polytetrafluoroethylene (PTFE), and polyimide (PI), thereby improving their properties. ⑥ Their ability to withstand high temperatures and exhibit excellent ductility makes them suitable for use as sealing fillers, C/C composite materials resistant to wear and high temperatures, fireproof materials, adsorption materials, as well as in pipelines for transporting gas and oil in oil fields.

1.2 High-performance asphalt-based carbon fibers High-performance asphalt-based carbon fibers are produced from mesophase pitch through spinning, non-fusing treatment, and carbonization. Due to differences in their asphalt raw materials and processing methods, particularly the structure of the spinnerets, the cross-sectional structures of the asphalt-based carbon fibers produced vary, such as irregular structures, onion-shaped structures, and folded structures. During the preparation of high-performance asphalt-based carbon fibers, it is necessary to avoid the formation of radial structures, so as to prevent cracks from arising during subsequent heat treatment and resulting in low tensile strength. Carbon fibers made from structures other than radial structures possess numerous excellent properties. The properties of pitch-based carbon fibers depend on their internal structure, and the formation of this fiber structure is influenced not only by its molecular structure but also by the spinning process of the pitch and the heat treatment conditions. During the spinning process, key parameters include spinning temperature, die structure, shear force, and drawing ratio. In the heat treatment process, the main factors are temperature, time, rate, residence time, axial tension, and carbonization temperature. Therefore, the preparation of high-performance asphalt-based carbon fibers requires a perfect molecular structure, along with optimal fiber-forming and heat-treatment processes.

The properties of the pitch-based carbon fibers produced by Amoco are highly representative; the pitch-based carbon fibers in the P series of commercial products exhibit high modulus, high density, and excellent thermal conductivity. In particular, the Thor-nel K-1100 type of pitch-based carbon fiber has a thermal conductivity of 1170 W/(m·K), which is 58% of the theoretical value. The structural parameters of carbon fibers are related to their tensile strength and Young’s modulus; the higher the Young’s modulus, the lower the compressive strength. For example, the graphite fiber Thornel P-100 has a modulus as high as 765 GPa, but its tensile strength is only 0.14 GPa, which limits its applications in certain areas. The main way to increase tensile strength is by controlling the size of the graphite microcrystals; larger microcrystals result in lower tensile strength. Typically, ion implantation is used to adjust the size of these microcrystals and promote surface crystallization, thereby enhancing the tensile strength of the fibers. Therefore, the ultra-high performance inherent in high-performance asphalt-based carbon fibers enables them to be combined with metals, carbon, and resins to form high-performance composite materials, which has led to their wide application. In recent years, they have been extensively used in fields such as aviation, aerospace, and nuclear energy. Due to its excellent electrical and thermal conductivity, it can also be used as a high-temperature ablation material, a high-thermal-conductivity material, and an enhancer for composite materials in wind turbine blades. Currently, regarding the total annual demand for carbon fiber, the approximate distribution across various applications is as follows: 20% for aerospace, 50% for industrial use, and 30% for sports applications. High-performance asphalt-based carbon fibers are widely used in fields such as aerospace, aviation, sports, and industry due to their superior properties, and they have been a focus of research in recent years. However, with the continuous advancement of industrialization, an increasing number of civilian enterprises require carbon fiber materials. Although high-performance asphalt-based carbon fibers can also be used in such enterprises, their production cost is extremely high, the manufacturing process is very complex, and their price is likewise very high. General-purpose asphalt-based carbon fibers have advantages such as inexpensive and readily available raw materials, high carbonization yields, and low product costs. Their performance is superior to that of traditional materials, enabling them to better meet the needs of general industry, which requires high-performance materials while also demanding that prices remain low.

2 Research status of domestically and internationally standard asphalt-based carbon fibers: In 1963, Saburo Ota from Gunma University in Japan prepared carbon fibers by subjecting organic materials to heat treatment, using their molten state as a raw material, and then through spinning and carbonization processes. Subsequently, the Japanese company Wuho Chemical utilized this technology to produce general-purpose asphalt-based carbon fibers with a strength of 1×10^4 kg/cm2 and a modulus of 0.7×10^6 kg/cm2, thereby opening up the use of asphalt as a new raw material for carbon fibers. In the 1980s, China also began research on general-purpose carbon fibers. In 1978, the Shanxi Coal Chemical Research Institute started working on the development of general-purpose pitch-based carbon fibers, and by 1985 it had passed the laboratory evaluation tests. During the “Seventh Five-Year Plan” period, a production line for general-purpose continuous filament pitch-based carbon fibers with an annual output of 10 tons was established. Starting in 1984, the Ministry of Metallurgical Industry began developing pitch-based carbon fibers, and during the Seventh Five-Year Plan period, a production line for general-purpose pitch-based carbon fibers was established in the Yantai Economic Development Zone, with an annual output of 150–200 tons. In 1985, the Petrochemical Science Research Institute began to research and explore spinning techniques for petroleum asphalt; spinning asphalt was successfully produced using a medium-scale facility, and its properties met the quality standards required for carbon fibers of international standard grade. Subsequently, efforts were started to modify mesophase asphalt.

Due to the lack of communication among the research institutions in China that are involved in the production of general-purpose asphalt-based carbon fibers, there is no effective pooling of human and financial resources, and a lot of redundant work is carried out. This has, to some extent, slowed down the development of carbon fiber technology in the country. To date, China’s annual production of general-purpose asphalt-based carbon fibers does not exceed 100 tons; although this amount is sufficient to meet domestic demand, the demand from rapidly developing industries for such carbon fibers is increasing year by year. The current annual production level is far from meeting the needs of these industries. Compared to other types of carbon fibers, general-purpose asphalt-based carbon fibers are relatively inexpensive, and their demand in areas such as household items, sports equipment, and building materials is much higher than in other fields. Therefore, it is an extremely urgent task to produce and develop general-purpose asphalt-based carbon fibers. The spinning of asphalt is the key to producing asphalt-based carbon fibers. Typically, the asphalt structures that can be spun are quite complex and highly heat-sensitive. Industrial production of extrusion spinning and centrifugal spinning for isotropic asphalt has already been achieved abroad. As for the production of pitch-based carbon fibers via melt spinning, it is quite technically challenging. Although there are reports from abroad, production remains on a small-scale experimental level to date, and foreign countries keep this technology highly confidential. There are almost no reports in China on related technical fields. Therefore, research on the melt spinning process for asphalt in China is limited to laboratory studies. Yet melt spinning of asphalt is the most practical and industrially feasible technique for producing asphalt-based carbon fibers; as such, there is still a long way to go before a production model for asphalt-based carbon fibers that features high output, low costs, and high profitability can be achieved.

3 Preparation of general-purpose pitch-based carbon fibers. The preparation process for general-purpose pitch-based carbon fibers mainly includes the following steps: ① Preliminary treatment of raw materials; ② Preparation of spinning pitch; ③ Melting and spinning of pitch; ④ Non-melting treatment and carbonization treatment. 3.1 Pretreatment of asphalt raw materials Asphalt raw materials can be classified into coal tar asphalt, petroleum asphalt, and synthetic asphalt (such as naphthalene asphalt, polyvinyl chloride asphalt, etc.), depending on their source. Coal tar pitch is the residue remaining after the distillation and extraction of fractions from coal tar, which is a by-product of the high-temperature carbonization of coal used in the production of metallurgical coke for ironmaking. Coal tar pitch is a major product obtained during the processing of coal tar, and its yield varies depending on the distillation conditions used for coal tar. At room temperature, coal tar pitch exists as a black, highly viscous semi-solid or solid; it has no fixed melting point and softens and then melts when heated. Generally, coal tar pitch can be divided into three types based on its softening point (according to the Global method): low-temperature pitch (also known as soft pitch, with a softening point of 35–75°C), medium-temperature pitch (with a softening point of 75–95°C), and high-temperature pitch (also known as hard pitch, with a softening point of 95–120°C). Unlike petroleum asphalt, the composition of coal tar pitch is extremely complex. According to statistics, there are tens of thousands of organic compounds in coal tar pitch, with over 500 of them having been identified to date. Among these, there are approximately 63 acidic compounds (such as phenols, cresols, and xylene derivatives), around 174 neutral compounds (such as benzene, toluene, naphthalene, anthracene, acenaphthene, etc.), and about 113 basic compounds (such as pyridine, quinoline, and isoquinoline). In addition to these compounds, coal tar pitch also contains other polycyclic as well as oxygen- and sulfur-containing heterocyclic compounds.

Petroleum asphalt is a product of the crude oil processing process, mainly composed of aromatic compounds and alkyl-substituted aromatic compounds. It usually exists as a viscous liquid, semi-solid, or solid in black or dark brown color, and its properties and composition vary depending on the source of the crude oil and the production methods used. Synthetic asphalt is a poly-nuclear polycyclic aromatic resin obtained through chemical reactions of condensed polycyclic aromatic compounds; it contains long-chain aliphatic structures with two or more carbon atoms, which gives it high heat resistance. Compared to other types of asphalt such as coal tar asphalt and petroleum asphalt, coal tar asphalt has the following characteristics: ① It contains a high amount of aromatic components, has a complex composition, an unpleasant odor compared to petroleum asphalt, and contains many carcinogenic substances; ② It contains many components that tend to crystallize, resulting in poor temperature stability; ③ It has a high content of unsaturated aromatic components, which leads to poor resistance to oxygen and ultraviolet aging; ④ It exhibits good adhesion properties; ⑤ It has excellent anti-corrosion properties. Aromas are toxic and prevent the growth of microorganisms. It is precisely because of its high content of aromatic components, ease of crystallization, excellent rheological properties, and low cost that coal tar pitch makes coal pitch the best raw material for producing carbon fibers.

Typically, asphalt has a carbon content of 91% to 95%, an average molecular weight of over 400, and possesses plasticity. The spinnability of asphalt and its ability to transition into a non-melting state are key factors determining whether it can be used to produce carbon fibers. Asphalt suitable as a raw material for carbon fibers must meet certain conditions: ① high carbon content and low impurity content; ② rheological properties that enable spinning; ③ chemical properties suitable for the non-melting and carbonization processes. Since the commonly used asphalt raw materials rarely meet the aforementioned conditions, it is necessary to pre-treat the raw asphalt to bring it up to the standards required for producing asphalt-based carbon fibers. During the spinning process, the free carbon, solid impurities, and fine particulate residues such as quinoline present in coal tar pitch can easily clog the spinning holes, becoming sources of fiber breakage. Therefore, the O in the raw asphalt must be removed during the preliminary treatment. The main methods for refined treatment include: hot-melt filtration, static sedimentation separation, centrifugal separation, solvent extraction, vacuum distillation, etc. After treatment, the content of insoluble matter should be below 2%, and it is preferable to reduce it to less than a few tenths of a percent. The asphalt raw materials are purified, with particular attention being paid to the removal of harmful compounds such as S, N, and O heterocyclic compounds, as their presence has a significant impact on subsequent processing steps. By using heat treatment methods or flux extraction methods to modify the molecular weight of asphalt, it is possible to keep the molecular weight of asphalt within a relatively narrow range and achieve a more uniform distribution of molecular weights.

3.2 Preparation of spinning asphalt The asphalt-based precursor for carbon fibers of general purpose is an isotropic asphalt. Isotropic asphalt refers to asphalt that, after being modified, contains crystalline regions and amorphous regions with varying degrees of ordered arrangement; it consists of a network structure formed by randomly oriented flaky microcrystals. Since the amorphous regions composed of amorphous carbon are embedded in the \"interstices\" between these microcrystals, its physical properties remain unchanged regardless of the direction of measurement, resulting in identical performance in all directions. Asphalt fibers must undergo processes of non-melting and carbonization in order to be converted into carbon fibers. The oxidation reaction occurs at high temperatures, and it is crucial to maintain the fibrous structure of the individual fibers while increasing production efficiency, preventing them from melting together. Therefore, it is necessary to raise the softening point of asphalt in order to improve its spinnability, ensuring that this softening point falls within the range of 260–290°C. The commonly used heat treatment methods to increase the softening point and spinnability of asphalt include: direct thermopolycondensation, oxidative thermopolycondensation, polymer copolymerization, and others. By separating and removing insoluble substances and thermally reactive components using an aromatic solvent under nitrogen pressure, an asphalt feedstock suitable for better spinning conditions can be obtained. By adding PVC resin in a mass fraction of 0.2% to 2% to coal tar pitch, and then subjecting the mixture to stirring and heating under nitrogen atmosphere, an asphalt with good spinnability is obtained; the strength of the fibers produced is significantly higher compared to that of fibers made without this additive.

BarrandLew- reported that air-blown asphalt undergoes reactions such as dehydrogenation, condensation, and aromatization, as well as cross-linking, which results in the formation of larger molecules, an increase in the BI content and the C/H ratio, and an elevation of the softening point to 175°C. They believe that during air oxidation, cross-linking reactions and condensation reactions occur simultaneously, which can also inhibit the formation of mesophase spheres. Osaka Gas Company used air-flushed oxidative thermopolymerization to treat asphalt at temperatures of 100–400°C; the resulting asphalt was isotropic and exhibited good spinnability. Therefore, the air oxidation method is highly feasible for modifying isotropically spinnable asphalt. Yang Jianmin from Yankuang Guohong Chemical Co., Ltd. developed a method for directly converting coal into spinnable coal tar pitch: the raw coal is processed through extraction, filtration, defoaming, hydrogenation polymerization, and vacuum evaporation to produce spinnable coal tar pitch. Although there are many modulation methods, the ultimate goal of modulation remains the same: to obtain isotropic asphalt with a high softening point, a narrow molecular weight distribution, good rheological properties, low viscosity, high aromaticity, and a certain amount of cycloalkyl and aliphatic side chains in its structure, so as to produce high-quality isotropic asphalt-based carbon fibers.