A Brief Discussion on Mesophase Pitch

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This post was last edited by Qingse Nianhua on 2018-4-9 08:04. Asphalt-based carbon fibers are classified into general-purpose carbon fibers and high-performance carbon fibers based on their properties. General-purpose asphalt carbon fibers, due to their low strength and modulus, are mainly used in insulation materials. High-performance asphalt-based carbon fibers possess excellent properties such as high modulus, high strength, high thermal conductivity, heat resistance, and corrosion resistance, making them indispensable engineering materials in the aerospace industry. High-performance asphalt-based carbon fibers are special fibers produced from mesophase pitch through processes such as melt spinning, pre-oxidation, carbonization, and graphitization. Defects in the carbon fiber manufacturing process tend to be hereditary; therefore, the quality of mesophase pitch determines to the greatest extent the properties of the final carbon fiber. 1. What is intermediate-phase pitch: Since the 1960s, when anisotropic spheres were discovered during the liquid-phase carbonization of pitch-like materials, extensive research has been conducted on the process of liquid-phase carbonization of pitch. Research has found that when bituminous substances transition to solid semi-coke, there is an intermediate state in which the bitumen exhibits anisotropy. Drawing on terms from liquid crystal science, bitumen in this state is referred to as mesophase bitumen. Intermediate-phase pitch is abundant, inexpensive, and possesses excellent properties; it serves as a precursor for many high-performance carbon materials. In addition to being used to produce high-performance pitch-based carbon fibers, it can also be used to manufacture products such as intermediate-phase carbon microspheres and pitch-based foam carbon. Among these, pitch-based carbon fibers are the most promising high-performance structural materials. 2. Preparation of mesophase pitch Mesophase pitch is produced through a thermal polycondensation reaction using raw materials such as ordinary asphalt, heavy oil, coal tar, or pure aromatic compounds. Compared to ordinary asphalt, it has a higher molecular weight (several hundred to several thousand), a high C/H ratio, and a softening point generally ranging from 230 to 300. Intermediate-phase pitch used for spinning requires a low solute viscosity at temperatures above its softening point, as well as stable internal structure (no decomposition over time), in order to facilitate further processing of the intermediate-phase pitch. 2.1 ? ?Using petroleum asphalt as a raw material, petroleum asphalt is abundant and inexpensive; it is currently primarily used in road construction and as a waterproofing material. Research shows that petroleum asphalt contains a large amount of olefins with a flaky, highly fused-ring molecular structure, characterized by high relative molecular mass, high degree of aromatization, and good thermal stability. These olefins can, through simple cyclization and polycondensation reactions, form the flaky structural units of intermediate-phase asphalt molecules. However, petroleum asphalt also contains many small molecules with lower molecular weights and lower aromaticity; therefore, the preparation of intermediate-phase asphalt involves removing these small molecules while simultaneously undergoing further polycondensation and cyclization of the larger molecules to form even larger and more stable sheet-like macromolecules. The most direct method is high-temperature nitrogen purging, for example, purging petroleum asphalt with nitrogen at 400°C for 8–10 hours. This method can effectively remove low-molecular-weight substances from petroleum asphalt, and it can also promote dehydrogenation polymerization among large molecules, thereby enabling the production of intermediate-phase asphalt with excellent properties; however, the yield of intermediate-phase asphalt using this method is relatively low. If pressurized high-temperature treatment is carried out followed by purging, the yield of mesophase pitch can be effectively increased. In addition to the direct purge method, many people also use the solvent extraction method to remove light components from asphalt. For example, petroleum asphalt is extracted using solvents such as benzene, toluene, and quinoline, and the insoluble portion is then heat-treated to produce intermediate-phase asphalt. The intermediate-phase pitch content obtained by this method is high, but the organic solvents remaining within the pitch have an adverse effect on subsequent spinning. 2.2 ? ?Using coal tar pitch as a raw material, coal tar is one of the important products of the coking industry. The approach to producing mesophase asphalt from coal tar is similar to that used for petroleum asphalt; the key step is to remove the light components and retain the heavy ones. Unlike petroleum asphalt, the asphalt molecules in coal tar contain less hydrogen; therefore, a hydrogen source (such as **naphthalene**) often needs to be added during the asphalt modification process to adjust the reactivity of coal tar asphalt, and this also helps to improve the softening point of mesophase asphalt. Mitsubishi Chemical Corporation’s “DIALEAD” series are high-performance pitch-based carbon fibers manufactured using coal tar pitch as raw material. 2.3 Using pure aromatic hydrocarbons as raw materials, high-performance intermediate-phase asphalt can be produced through heat treatment and catalytic methods. Commonly used pure aromatic hydrocarbons include naphthalene, methylnaphthalene, anthracene, phenanthrene, etc., while the catalysts are generally HF/BF3 and AlCl3. The mesophase pitch prepared by this method has a narrow molecular weight distribution, high yield, and good experimental reproducibility. However, it is often difficult to completely remove the catalyst from asphalt, which to some extent affects the reprocessability of intermediate-phase asphalt. 3. Simple characterization of mesophase pitch ??? Before conducting formal experiments, it is possible to make a preliminary assessment of the properties of mesophase pitch to determine whether it is suitable for further processing. 3.1 ? ?Polarizing microscope observation method: Due to the anisotropy of the asphalt mesophase, it is possible to directly observe the mesophase asphalt using a polarizing microscope. The properties of intermediate-phase asphalt can be evaluated based on the size of the homogeneous areas in the field of view and the degree of order in their arrangement; this method is the most economical and practical way for characterization. Intermediate-phase pitch, which has smaller regions of the same color and a chaotic arrangement of flow lines, exhibits poor order in the arrangement of its internal layers; as a result, it is difficult to melt and spin into fibers, making it hard to produce high-performance carbon fiber products. Conversely, if the overlapping areas of the homogeneous regions in the mesophase structure are large and the arrangement is more orderly, the reprocessability of that mesophase asphalt is better. In the figure below, for the mesophase pitch used in spinning, the left side is clearly superior to the right side. Furthermore, this method is the simplest way to determine the content of intermediate phases in asphalt; the ratio of optically anisotropic asphalt to the total amount of asphalt can be used as an indicator of the content of intermediate phases in asphalt. Compared to the solvent method, this method is simpler and more accurate (using the quinoline-insoluble content as an indicator of the intermediate phase content). 3.2 ? ?Softening point The softening point is one of the most important technical parameters for evaluating intermediate-phase asphalt. The softening point is too low, the molecular structure is unstable, which is not conducive to spinning and subsequent heat treatment. If the softening point is too high, the asphalt tends to \"burn,\" its flowability is restricted, which also hinders the spinning process. The softening point of mesophase pitch used for spinning is generally controlled between 230–300°C. This is of great significance for subsequent spinning as well as pre-oxidation. 3.3 ? ? Content of insoluble substances in various solvents: Asphalt contains a large number of aromatic polymers with multiple rings, and there are numerous different isomers; therefore, it is not feasible to separate each compound individually. When studying asphalt, different solvents (cyclohexane, toluene, **furan, quinoline, etc.) are typically used to gradually dissolve and extract the components of asphalt. The properties of these components are then correlated with those of the asphalt as a whole, and their impact on the manufacturing processes and product quality of carbon products is examined in order to establish certain relationships. For example, the quinoline-insoluble content in mesophase pitch used for spinning should not exceed 30%, otherwise the spinning properties will be very poor. 4. Other applications of mesophase pitch Besides being used to produce high-performance asphalt-based carbon fibers, mesophase pitch has many other applications: 4.1 Using mesophase pitch as a binder Mesophase pitch possesses good sintering properties, allowing it to be used directly in powder pressing without the need for additional binders. This simplifies processes such as mixing, impregnation, and calcination during the production of graphite products. Intermediate-phase pitch possesses advantages such as a high carbon residue content, high density, and easy graphitization; moreover, compared to similar products made from conventional pitch, it exhibits higher strength, density, as well as electrical and thermal conductivity. 4.2 ? ? Preparation of foam carbon materials using mesophase pitch Mesophase pitch-based foam carbon is a novel porous material obtained by subjecting mesophase pitch to a foaming process. Due to its low density, open pore structure, excellent mechanical properties, good thermal stability, and adjustable electrical and thermal conductivity, this carbon material holds promise for use in rocket engine nozzles, as well as in shock-resistant and noise-reducing launch platforms for rockets. It can also be utilized in engine components, fire-resistant doors and windows for aircraft and ships, high-performance heat conduction and dissipation systems, and as electrodes and catalyst carriers for energy storage. 4.3 ? ?Intermediate-phase pitch-based electrode materials Carbon materials are important materials for manufacturing various batteries. As a graphitizable carbon material, mesophase pitch, when subjected to high-temperature treatment, tends to transform into a crystalline graphite structure with a regular three-dimensional stacking arrangement. This structure allows for lower energy requirements for the insertion of lithium ions, which facilitates deep lithium intercalation and enhances the reversible capacity; as such, it can be used in the preparation of electrode materials. Carbon electrodes with high charge-discharge capacity and good cycle performance were obtained by using surface-modified mesophase pitch as a material for lithium-ion electrodes ； Using petroleum-based mesophase pitch that has been carbonized and graphitized as an anode material for lithium-ion batteries not only simplifies the material preparation process and reduces costs, but also results in a high specific capacity; its performance is comparable to that of commercially available mesophase carbon microspheres. 4.4 ? ? Mesophase pitch-based carbon/carbon composites: Mesophase pitch-based carbon/carbon composites possess advantages such as a high char yield, high density, and easy graphitization, making it an ideal precursor for carbon/carbon composites. Intermediate-phase pitch-based carbon/carbon composites are typically synthesized through cyclic impregnation and carbonization, and have found wide applications in various fields, mainly due to their many excellent properties such as low bulk density, high mechanical strength, good thermal conductivity, low coefficient of thermal expansion, and good friction resistance in an inert atmosphere. 4.5 ? ?Separation by solvent selection of mesophase carbon microballs: Mesophase carbon microballs (MCMB) can be prepared from mesophase pitch. Due to their high reactivity, mesophase carbon microballs can be activated under a wide range of conditions. Their high packing density, ease of graphitization, good thermal stability, and excellent electrical and thermal conductivity make them an attractive and high-quality precursor for the preparation of high-performance carbon materials. Activated carbon prepared from carbon microspheres possesses ultra-high surface area activity, with a specific surface area that far exceeds that of activated carbon fibers and pitch-based spherical activated carbon. By changing the preparation conditions, mesophase carbon microball activated carbons with different pore volumes and particle sizes can be obtained, which can be used as molecular sieves to separate various gases. Using carbon materials as electrodes in lithium batteries has become one of the hot topics in carbon material research. Studies on different carbon materials have shown that treated mesophase carbon microballs are the best choice as ideal electrode materials. 5. Conclusion: With the advancement of technology, there is an increasing demand for new types of carbon materials. Carbon materials have become a large and diverse range of materials used in various applications, from everyday life to high-tech aerospace industries, where carbon materials based on mesophase pitch play a crucial role.