First, Second and Third Generation Carbon Fibers

Compared with traditional metal and other fiber composites, carbon fiber composites have the characteristics of light weight, high strength and high elastic modulus, and can reduce the weight by 30% compared with traditional aluminum alloy structures, it is widely used in manufacturing aircraft body, engine, missile Shell and so on. US F-22 and F-35 fighters use 24% and 36% of carbon fiber composites respectively, and the new large civil aircraft, represented by the A350 and the Boeing 787, use more than 50% of carbon fiber composites. The application of carbon fiber composite material has become one of the symbols to measure the advancement of weapon equipment. The tensile strength and elastic modulus of carbon fiber, which bear about 90% of the load, are the key factors to achieve the structural properties of composites.

• First-generation carbon fiber

Carbon Fiber takes tensile strength and elastic modulus as its main index. At present, commercial products have developed to the second generation, and the performance of the second generation carbon fiber products widely used in Japan and America is similar. The first, represented by the Toray T300 in the 1960s and the hearst AS4 low strength, low modulus carbon fiber, was used primarily as a secondary load bearing member for Boeing 737 and other models, and the AS4 was used in the flat tail section of early F-14 fighters.

• Second-generation carbon fiber

The second generation of high strength and medium modulus carbon fiber is represented by Toray's T800 and Hershey's IM7 series in the 1980s, as well as Toray’s T700, T1000, Hershey’s IM8 and IM9.

The strength of the T800 is 68% higher than the strength of the T300, and the modulus is 28% higher. The T800 is widely used in the main supporting structure of the airframe with wings, such as the A350 and the Boeing 787. IM7 is 37% stronger than AS4 and 21% higher in modulus. It is widely used in American Trident II submarine-launched missiles and F-22 and F-35 fighters.

At present, the second generation of high strength and medium modulus carbon fiber is the most widely used in the fields of aeronautics and Astronautics. Due to its low modulus and High Brittleness, it is easy to cause fatigue damage to composite structural components, and even to cause catastrophic failure, it limits the improvement of the performance of aviation weapon equipment, and it is difficult to meet the performance requirements of the new generation of aviation weapon equipment. As the United States launches the development of the sixth generation fighter, the new generation of long-range bombers and the first generation of unmanned carrier-based combat aircraft, aviation Weapons and equipment put forward higher requirements for cruise speed, range, maneuverability, stealth performance, protective ability and maintainability, etc. , therefore, the carbon fiber with higher tensile strength, fracture toughness and impact strength is needed. In order to obtain high comprehensive performance carbon fiber, it is necessary to make a breakthrough in strength and Modulus, and the main technical feature of the third generation carbon fiber is to achieve high tensile strength and high elastic modulus.

It is difficult to achieve high tensile strength and elastic modulus at the same time. The preparation and carbonization of Pan Precursorare the two core processes of carbon fiber preparation: High Quality Pan Precursor is the key to achieve high performance and mass production of carbon fiber, and the carbonization process is directly related to tensile strength and elastic modulus of carbon fiber. Many years’ experience of carbon fiber development shows that the tensile strength of carbon fiber will decrease when the elastic modulus of carbon fiber is greatly increased, but it is difficult to increase the elastic modulus of carbon fiber when the high tensile strength of carbon fiber is maintained. The reason is that carbon fiber is an anisotropic material composed of a large number of graphite crystallites. High Strength Carbon fibers usually require small crystallite size, while high modulus carbon fibers usually require large crystallite size. How to solve this contradiction is the most difficult problem in the development of carbon fibers.

• Third-generation carbon fiber

Japan and the United States have obtained high strength and high modulus carbon fibers from two different technical approaches.

According to the current research results, Toray’s third-generation carbon fiber products have higher strength and are more suitable for structural components with high tensile strength design values; US products have higher elastic modulus, more suitable for bending, impact resistance, high fatigue strength design value components. Japanese and American related enterprises and institutions have stated clearly that the application goal of the third-generation carbon fiber is to replace the current T800 and IM7 second-generation carbon fiber products in the high-end aerospace market, and to improve the strength and stiffness of the structural components of military aircraft. Toray is the pioneer of the traditional Pan Solution Spinning technology. The PROTONEMA technology is highly mature and has strong industrialization ability. From the first and second generation products, the third generation products are expected to be industrialized and put on the market in the next 5 ~ 10 years. The United States abandoned the traditional solution precursor preparation process, the use of Gel spinning technology, there is more room for process optimization, carbon fiber performance also has more room for improvement. The sixth generation fighters, the new long range bombers and the first generation unmanned carrier based combat aircraft that the United States plans to launch around 2030 are likely to significantly improve operational performance through the application of third generation carbon fiber technology.

Toray Company, Japan through breakthrough carbonization process, carbon fiber strength and Modulus at the same time increase more than 10% , the first to achieve the third generation of carbon fiber technology requirements. Toray believes that the key to achieving both high tensile strength and high modulus of elasticity lies in the heat treatment technology and high temperature equipment used in the carbonization process. In Heat Treatment Technology, temperature, drafting, catalysis, magnetic field and many other factors will affect the properties of carbonized fiber. In March 2014, Toray announced the successful development of T1100G carbon fiber. Dongli used the traditional Pan Solution Spinning technology to fine control the carbonization process, improve the microstructure of carbon fiber on the nanometer scale, and control the orientation, size and defects of graphite crystallites in the carbonized fiber, so that the strength and Modulus of elasticity are greatly improved. The tensile strength of T1100G is 6.6 GPA, which is 12% higher than that of T800, and the Modulus of elasticity is 324 GPA, which is 10% higher.

The Georgia Institute of Technology Team improved the strength of the carbon fiber by more than 28 percent by breaking the precursor process. Hearst carbon fiber products for 30 years has been at the level of medium Modulus of elasticity, performance is difficult to break. In 2006, the Defense Research Agency (Darpa) launched the advanced structural fibers project, which aims to bring together leading national research forces to develop the next generation of carbon-based structural fibers. As one of the research institutions, Georgia Institute of Technology has improved the elastic modulus of carbon fiber through the preparation process of precursor. In July 2015, the team used innovative PAN-based Carbon Fiber Gel spinning technology to increase the tensile strength of carbon fibers to 5.5-5.8 GPA, tensile modulus of elasticity up to 354-375 GPA. Although the tensile strength is similar to that of IM7, the modulus of elasticity increases by 28% ~ 36% . This is currently reported as the combination of high strength and highest modulus of carbon fiber. The mechanism is that the GEL links the polymer chains together, resulting in strong internal force and orientation of Crystallites, which ensures high strength under the condition of large crystallites required for high elastic modulus. This shows that the United States has a third generation of carbon fiber products of its own R & D strength