Progress in low-cost green carbonization technology of carbon fiber

Carbon fiber, with a carbon content of more than 90%, is fibrous, soft, and can be processed into various fabrics. It also has the characteristics of high temperature resistance, friction resistance, thermal conductivity and corrosion resistance. Because its graphite microcrystalline structure is preferentially oriented along the fiber axis, it has high strength and modulus along the fiber axis. The density of carbon fiber is small, so the specific strength and specific modulus are also high. The main use of carbon fiber is to be used as a reinforcing material and compounded with resins, metals, etc. to make advanced composite materials. For example, carbon fiber reinforced epoxy resin composite materials have the highest specific strength and specific modulus among existing engineering materials. Although its diameter is only 5 microns, which is equivalent to one tenth to one twelfth of a hair, its strength is more than 4 times that of aluminum alloy.

1. Types of carbon fiber

Polyacrylonitrile-based carbon fiber (PAN): It is the most widely used fiber in carbon fiber. Studies have found that more than 90% of commercial carbon fibers are obtained by carbonizing polyacrylonitrile precursor fibers. Its preparation process generally includes: polymerization, spinning, pre-oxidation, carbonization, surface treatment and carbon fiber formation. Polyacrylonitrile-based carbon fiber has broad application prospects. Currently, Japan's Toray, Mitsubishi Rayon and Toho Group are the main manufacturers of polyacrylonitrile carbon fiber.

Pitch-based carbon fiber (MPCF): The source of pitch is coal and petroleum. Processing pitch into high-value-added pitch-based carbon fiber can protect the environment well. Due to its good dimensional stability, the composite of pitch-based carbon fiber fabric and cyanate resin has a very small thermal expansion coefficient and can be used to make some precision materials; this type of carbon fiber also has the characteristics of wear resistance, high strength and fatigue resistance, and can be used in braking materials; and it has small deformation under stress, good thermal conductivity, and is not easy to expand during preheating, and is also widely used in adsorbents and other fields.

Viscose-based carbon fiber: Made of viscose fiber containing cellulose, compared with the above two types of carbon fibers, viscose-based carbon fiber has some unique properties, such as: viscose-based carbon fiber has a relatively low degree of graphitization and a low thermal conductivity, making it an ideal thermal insulation material; it has a relatively low density and can be made into relatively light components; it has good biocompatibility and can be made into ligaments, artificial bones and other medical biomaterials.

2. Carbonization Mechanism

The carbonization process generally includes two parts: low-temperature carbonization and high-temperature carbonization. Under the protection of inert gas, such as N2 or Ar, in the low-temperature carbonization stage (generally at 300~800℃), a large number of non-carbon atoms are removed in the form of CO2, CO, NH3, H2 and H2O, and molecular reorganization occurs to form a large network structure. The carbon aggregation structure inside the fiber changes from amorphous to graphite crystal, laying the foundation for the fiber to form the final graphite-like structure. During the high-temperature carbonization process (generally 800~1600℃), the cracking rate of non-carbon elements slows down, and the carbon structure continues to reorganize to generate a hexagonal carbon network plane. During the whole process, about 50% of the non-carbon elements are removed, and finally a carbon fiber material with a turbostratic graphite structure with a carbon content greater than 90% is formed.

3. Green and low-carbonization of carbon fiber precursor and its preparation process

In the production process of carbon fiber, the preparation cost of precursor accounts for a large proportion, about more than 50%. Therefore, it is the most critical step to achieve green and low-carbonization of carbon fiber precursor and its preparation process.

1. New polyacrylonitrile comonomer preparation technology

Introducing a second monomer into polyacrylonitrile is conducive to reducing the strong cyano effect between polyacrylonitrile homopolymer molecules, making polyacrylonitrile more soluble. Using methyl acrylate (MA) as a neutral comonomer and fumaric acid (MA) or maleic acid (MA) as an acidic comonomer can optimize the cyclization reaction to an ionic reaction, making the oxidation process exothermic and improving the oxidation efficiency, reducing the cyclization temperature and cyclization activation energy, and thus reducing the energy consumption of carbon fiber production.

2. Synthetic resin preparation of carbon fiber precursor technology

Synthetic resin preparation of carbon fiber precursor technology uses synthetic resin as raw material to prepare carbon fiber precursor, which can reduce the cost of carbon fiber and achieve low-carbon production; however, the thermal stability of synthetic resin is insufficient and usually needs to be modified [4]. Low-density polyethylene (LLDPE) fiber has the characteristics of low production energy consumption and high spinnability. LLDPE is mixed with photoinitiator 4-chlorobenzophenone (4-CBP), crosslinker triallyl isocyanurate (TAIC) and heat stabilizer antioxidant 1010 to make pellets and then melt-spinning can improve the shortcomings of LLDPE's low carbon yield and poor mechanical properties. The "ultraviolet crosslinking-vulcanization stabilization-carbonization" treatment is used to prepare carbon fiber, which can achieve a carbon yield of 62.9%; and the surface and cross section of the prepared carbon fiber are uniform and smooth, without obvious surface collapse or defects.

3. Polyacrylonitrile blended polymer precursor preparation technology

Polyacrylonitrile blended polymer precursor preparation technology blends polyacrylonitrile with plastic waste or bio-based polymers to make a new type of carbon fiber precursor, which not only solves the problem of waste plastic disposal, but also reduces carbon emissions in the carbon fiber manufacturing process. It is a new method for low-cost carbon fiber preparation [5]. Studies have shown that the carbon fiber structure made from polyacrylonitrile/cellulose blend precursor has the highest content of all-carbon ring structure and carbon yield. Adding cellulose to polyacrylonitrile can increase the content of O and N atoms in the system, accelerate the formation of graphite structure, increase the crystallinity of graphite crystals during carbonization, and ultimately improve the performance of carbon fiber. Introducing these biomass materials with abundant natural reserves and sustainable sources is one of the important ways to achieve green and low-carbonization of carbon fiber precursors and their preparation processes.

4. Summary and Future Outlook

Carbon fiber can replace traditional metal and glass fiber with its excellent mechanical properties and low density, and realize lightweight materials (such as lightweight materials for aircraft) and large-scale materials (such as wind turbine blades, new energy vehicles, etc.). It is favored by more and more industries. At the same time, reducing carbon emissions in the process of material use and improving energy utilization are becoming more and more important in promoting the green and low-carbon development of related industries. At present, the manufacturing technology of low-cost carbon fiber is still in the critical stage. It is believed that with the increasing demand for low-cost carbon fiber in the market in the future, the industrial manufacturing technology of low-cost carbon fiber will usher in new breakthroughs.