Why do domestic carbon fiber card neck? Epoxy resin toughness is lacking

"The core of the carbon fiber industry chain is many, including upstream raw silk production, midstream carbonization, downstream composite materials and their applications. After more than ten years of research and development and breakthrough, the current 'carbon neck' problem of China's carbon fiber is mainly in the downstream application, namely Composite materials and products," said He Yanli, vice president of China Chemical Fiber Industry Association.

Carbon fiber is a high-strength new fiber material with a carbon content of more than 95%. Its quality is lighter than that of metal aluminum, but its strength is higher than that of steel. It can also withstand high temperature, corrosion resistance, fatigue resistance and creep resistance. One of the key composite materials is epoxy. Epoxy resin has excellent physical and mechanical insulation properties and strong adhesion, which can bond carbon fibers together. However, the high-end carbon fiber currently produced in China is all imported .

Vulnerable epoxy resin modification

According to the mechanical properties, carbon fiber can be divided into high-strength, ultra-high-strength, high-modulus and ultra-high modulus. In the Toray product code of Japan, T refers to the number of tonnages of this type of carbon fiber that can be withstood by a unit area of ​​1 square centimeter, that is, the higher the T number, the better the carbon fiber quality; the modulus refers to the external tension or pressure. The tensile modulus of the original shape is restored. At present, China has been able to produce higher-end carbon fiber such as T800, but Japan Toray has mastered this technology in the 1990s.

Zhang Dingjin, chairman of China Composites Group Co., Ltd. said that compared with carbon fiber, China's high-end epoxy resin industry lags behind the international situation more seriously. Especially used in high-end carbon fiber in the fields of aircraft, aerospace and other fields.

Polymer compounds containing epoxy groups in the molecular structure are collectively referred to as epoxy resins. In addition to carbon fibers, they are also widely used in machinery, electronics, home appliances, and civil engineering. The dependence of high-end epoxy resin on imports is related to the weak foundation of China's chemical industry, and on the other hand, it is related to the characteristics of epoxy resin. There are two or more polyfunctional molecules on one molecular chain, which can be cross-linked to form a polymer which is insoluble and infusible and has a three-dimensional network structure. The use of aerospace structural parts is extremely demanding. Carbon fiber composites must be able to withstand high temperatures of hundreds of degrees Celsius and low temperatures of tens of degrees Celsius for a long time. At the same time, glass transition temperature, modulus of elasticity and compressive strength under hot and humid conditions. It cannot be significantly reduced, which requires related products with higher functionality, epoxy value and viscosity.

The number of functional groups that can participate in the reaction in a molecule is called a functionality, and it has been shown by the industry that the higher the functionality, the better. The energy is too high and the composite material will be too hard and tough. Therefore, it must be specific to the strength, modulus, toughness, high and low temperature, fatigue, etc. under different conditions of use, from the formulation system, molecular structure analysis, which is a very complex system work, and high technology content, research difficulty Big.

The weather resistance of epoxy resin is directly related to the glass transition temperature. When the composite material is applied in the aviation field, it is generally required that the glass transition temperature of the epoxy resin should not be lower than 180 °C, and most enterprises in the domestic resin industry still do not have it. Related technology.

Lack of intelligent automation equipment

The biggest threat to the performance of continuous carbon fiber reinforced composites is the low-speed impact delamination of composites, which is the core of high-performance composites that can be applied in aircraft structures. One of the main reasons for the sensitivity of composite materials to impact delamination damage is the insufficient toughness of epoxy resin itself.

In order to meet the requirements, the toughened composite material has a compressive strength (CAI value) of at least 200-300 MPa after impact. At present, the internationally popular resin toughening methods include in-situ particle toughening or off-site intercalation toughening. "The combination relationship between the various molecules is very complicated. It is difficult to develop a long-term research foundation and many years of experimentation without the long-term research foundation," said Kong Zhenwu, secretary general of the epoxy branch of the Resin Association.

“The modification of epoxy resin is also closely related to intelligent automation equipment.” Yu Muhuo, a professor at the School of Materials Science and Engineering at Donghua University, said that China’s carbon fiber production time is short and lacks low-cost complete sets of automated production equipment, resulting in low production efficiency and product stability. Insufficient issues.

“There is also a need to increase R&D and productivity for such intelligent devices,” said He Yanli.

Application of traction is not progressing slowly

Epoxy resin is special, "different uses, its structure and performance are different." Kong Zhenwu said that China's carbon fiber materials production and application are out of touch with each other, the application is not enough traction, no feedback correction, epoxy resin and other technological progress naturally slow.

At present, the most used high-end carbon fiber is on the aircraft. For example, on the Boeing B787 model, the carbon fiber composite material produced by Toray has accounted for 50% of the total material. In 2016, Toray's carbon fiber production was about 40,000 tons. China's carbon fiber enterprises have more than 30, with a total production capacity of 20,000 tons and actual production of about 7,000 tons.

The use of Toray carbon fiber in Boeing is by no means an overnight success. Since the 1980s, Toray has been cooperating with Boeing in an all-round way. Dongli people even lived in Boeing and designed and produced carbon fiber according to Boeing requirements. Until 2011-2012, aircraft using carbon fiber began to test flight, running for nearly 30 years, and based on Boeing's use requirements and feedback, constantly correcting and correcting products.

In addition, once a leading effect is formed in an industry, it is difficult to surpass it. The carbon fiber epoxy resins currently used on Boeing aircraft, the US F-22 and F-35 fighters are all from Huntsman, USA. Yu Muhuo said that Huntsman's products have already passed the material and process certification. If you want to use epoxy resin produced by other companies, it will take a long and complicated demonstration process. Carbon fiber manufacturers are naturally willing to use Huntsman. This is also not conducive to the domestic high-end epoxy resin products to catch up.