Properties of MP

Mesophase pitch (liquid crystal pitch, MP for short) is a liquid crystal substance formed during the heat treatment of petroleum pitch or coal pitch at 350~500℃, mainly composed of various flat disc-shaped condensed ring aromatic hydrocarbon molecules with a relative molecular mass of 370 ~ 2 000 mixture. MP displays the characteristics of easy graphitization.

   MP has a hydrogen-to-carbon atomic ratio of at most 0.35~0.5, a softening point of at most 205℃, a volatile content of 15%~20%, and a material density of 1.4~1.6g/cm3. The crystal structure of MP belongs to the hexagonal crystal system and has optical anisotropy. It is a high-quality precursor of advanced functional carbon materials.

Production technology of MP

   1. Coal-based

   The preparation process of MP using coal as raw material mainly includes three steps. First, extracting coal; the liquid is distilled and separated to obtain the extracted coal essential oil; and finally, MP is prepared through polymerization.

   2. Coal-based pitch

   Coal-based pitch is also known as coal tar pitch, which is a bituminous coal tar product obtained by further distillation of raw coal after further distillation. Coal asphalt material has a low viscosity and poor processability. Therefore, coal-based asphalt is not suitable as a precursor for the preparation of carbon materials, and raw asphalt needs to be pretreated.

   3.  Petroleum-based

   Petroleum-based pitch is the residue produced after the processing of natural petroleum to obtain the main product. These residues are low in price and rich in sources. Among them, asphaltenes (having a flaky fused ring aromatic hydrocarbon structure) with relatively high molecular weight, high aromatic ring ratio and thermal stability are suitable for preparing MP.

   4.  Heavy aromatics-based 

   Heavy aromatics contain condensed ring aromatics that are higher in purity than coal tar pitch and petroleum-based pitch, and also contain no ash and other impurities. In actual production, the raw materials of heavy aromatics mainly include tetrabenzophenazine, acenaphthene and naphthalene.

   5.  Coal-based pitch and petroleum-based pitch (or heavy aromatics) blends

   The process of synthesizing MP using a blend of coal-based pitch and petroleum-based pitch is a co-carbonization process. The co-carbonization reaction can obtain MP with optical anisotropy, good product liquidity and narrow molecular weight distribution.

Applications of MP

   1.  MP-based carbon fiber

   MP-based carbon fiber (MPCF), boasting the characteristics of ultra-high strength, ultra-high modulus, high conductivity and low coefficient of thermal expansion, has always been a research hotspot in the field of carbon materials, and its production technology is becoming more and more mature. It has important application value in the aerospace field, advanced electronic equipment and industrial equipment, and is an important supporting material for the development of high-end equipment manufacturing industry.

   The US and Japanese producers dominate the market, including Mitsubishi, Nippon Graphite Fiber and Cytec etc. In China, producers, such as Liaoning NOVCARB have successfully realized industrial production of MPCF products. 

   2. MP-based foamed carbon 

   MP-based foamed carbon has a low density, open cell structure and high mechanical strength. Such kind of materials are easy to bond (including phenolic or other binders), and are easy to compound with any reinforcing materials.

   MP-based foamed carbon finds potential applications in high-performance components in aerospace field, equipment for defense industry and energy storage equipment.    

   3. MP-based carbon/carbon composite materials

   Carbon/carbon composite materials have high mechanical strength and modulus, and can withstand harsh environments such as high temperature and corrosion. Even under a high temperature environment of 2 200 ° C, its mechanical strength increases, rather than decrease.

   MP-based carbon/carbon composites are usually synthesized by cyclic impregnation and carbonization. It has been widely used in various fields, especially in the field of aerospace and medical equipment.

   4.  Other applications

   Apart from being used as a high-quality precursor of the above materials, MP can also be used to prepare various carbon materials such as pitch coke, needle coke, mesophase carbon microspheres, carbonaceous refractory binders and fluorinated pitch.

Domestic MP production

   Global MP production is dominated by the US and Japan, with major producers including Exxon, Amoco and Mitsubishi. Mitsubishi takes up a large share in China’s MP market. 

   Domestic MP demand is around 5 kt, mainly used in the production of MPCF and MP foamed carbon. Domestic MP capacity totalled 730 t/a in 2019. Details are shown in Table 1.

 Table 1   China’s MP producers in 2019

   There are three new MP projects already disclosed in China. Details are shown in Table 2. 

Table 2   New MP projects in China

   China's asphalt industry is developing towards high performance and high added-value. At present, China’s MPCF capacity stands at around 730 t/a, accounting for 2.8% of the total carbon fiber capacity (26 kt/a). Domestic carbon fiber output reached around 10 kt and products are mostly produced through the acrylonitrile-based carbon fiber route. Further breakthroughs shall be made in MPCF technology, production and applications. The production of MP foamed carbon, carbon microspheres, needle coke and other products remains in the stage of small trial or research.