Carbon fiber, known as the king of new materials in the 21st century, is a bright pearl in materials. Carbon fiber (CF) is a kind of inorganic fiber with more than 90% carbon content. Organic fibers (viscose based, pitch based, polyacrylonitrile based fibers, etc.) are pyrolyzed and carbonized at high temperature to form carbon backbone.
As a new generation of reinforced fiber, carbon fiber has excellent mechanical and chemical properties. It not only has the inherent characteristics of carbon materials, but also has the softness and processability of textile fiber. Therefore, it is widely used in aerospace, energy equipment, transportation, sports and leisure fields
Light weight: as a strategic new material with excellent performance, the density of carbon fiber is almost the same as that of magnesium and beryllium, less than 1 / 4 of that of steel. Using carbon fiber composite as structural material can reduce the structural weight by 30% – 40%.
High strength and high modulus: the specific strength of carbon fiber is 5 times higher than that of steel and 4 times higher than that of aluminum alloy; The specific modulus is 1.3-12.3 times of other structural materials.
Small expansion coefficient: the thermal expansion coefficient of most carbon fibers is negative at room temperature, 0 at 200-400 ℃, and only 1.5 at less than 1000 ℃ × 10-6 / K, not easy to expand and deform due to high working temperature.
Good chemical corrosion resistance: carbon fiber has high pure carbon content, and carbon is one of the most stable chemical elements, resulting in its very stable performance in acid and alkali environment, which can be made into all kinds of chemical anti-corrosion products.
Strong fatigue resistance: the structure of carbon fiber is stable. According to the statistics of polymer network, after millions of cycles of stress fatigue test, the strength retention rate of the composite is still 60%, while that of steel is 40%, aluminum is 30%, and glass fiber reinforced plastic is only 20% – 25%.
Carbon fiber composite is the re strengthening of carbon fiber. Although carbon fiber can be used alone and play a specific function, it is a brittle material after all. Only when it is combined with the matrix material to form carbon fiber composite can it give better play to its mechanical properties and carry more loads.
Carbon fibers can be classified according to different dimensions such as precursor type, manufacturing method and performance
According to the type of precursor: polyacrylonitrile (Pan) based, pitch based (isotropic, mesophase); Viscose base (cellulose base, rayon base). Among them, polyacrylonitrile (Pan) based carbon fiber occupies the mainstream position, and its output accounts for more than 90% of the total carbon fiber, while viscose based carbon fiber accounts for less than 1%.
According to the manufacturing conditions and methods: carbon fiber (800-1600 ℃), graphite fiber (2000-3000 ℃), activated carbon fiber, vapor grown carbon fiber.
According to the mechanical properties, it can be divided into general type and high-performance type: the strength of general type carbon fiber is about 1000MPa, and the modulus is about 100GPa; High performance type can be divided into high strength type (strength 2000mPa, modulus 250gpa) and high model (modulus 300gpa or more), among which the strength greater than 4000mpa is also called ultra-high strength type, and the modulus greater than 450gpa is called ultra-high model.
According to the size of tow, it can be divided into small tow and large tow: small tow carbon fiber is mainly 1K, 3K and 6K at the initial stage, and gradually developed into 12K and 24K, which is mainly used in aerospace, sports and leisure fields. Carbon fibers above 48K are usually called large tow carbon fibers, including 48K, 60K, 80K, etc., which are mainly used in industrial fields.
Tensile strength and tensile modulus are two main indexes to evaluate the properties of carbon fiber. Based on this, China promulgated the national standard for PAN based carbon fiber (GB / t26752-2011) in 2011. At the same time, due to Toray’s absolute leading advantage in the global carbon fiber industry, most domestic manufacturers also adopt Toray’s classification standard as a reference.
1.2 high barriers bring high added value. Improving process and realizing mass production can significantly reduce cost and increase efficiency
1.2.1 the technical barrier of the industry is high, the precursor production is the core, and the carbonization and oxidation is the key
The production process of carbon fiber is complex, which requires high equipment and technology. The control of precision, temperature and time of each link will greatly affect the quality of the final product. Polyacrylonitrile carbon fiber has become the most widely used and the highest output carbon fiber at present due to its relatively simple preparation process, low production cost and convenient disposal of three wastes. The main raw material propane can be made from crude oil, and the PAN carbon fiber industry chain includes a complete manufacturing process from primary energy to terminal application.
After propane was prepared from crude oil, propylene was obtained by selective catalytic dehydrogenation (PDH) of propane;
Acrylonitrile was obtained by ammoxidation of propylene. Polyacrylonitrile (Pan) precursor was obtained by polymerization and spinning of acrylonitrile;
Polyacrylonitrile is pre oxidized, carbonized at low and high temperature to obtain carbon fiber, which can be made into carbon fiber fabric and carbon fiber prepreg for the production of carbon fiber composites;
Carbon fiber is combined with resin, ceramics and other materials to form carbon fiber composites. Finally, the final products for downstream applications are obtained by various molding processes;
The quality and performance level of precursor directly determine the final performance of carbon fiber. Therefore, improving the quality of spinning solution and optimizing the factors of precursor forming become the key points of preparing high-quality carbon fiber.
According to “Research on production process of polyacrylonitrile based carbon fiber precursor”, spinning process mainly includes three categories: wet spinning, dry spinning and dry wet spinning. At present, wet spinning and dry wet spinning are mainly used to produce polyacrylonitrile precursor at home and abroad, among which wet spinning is the most widely used.
Wet spinning first extrudes the spinning solution from the spinneret hole, and the spinning solution enters the coagulation bath in the form of small flow. The spinning mechanism of polyacrylonitrile spinning solution is that there is a big gap between the concentration of DMSO in spinning solution and coagulation bath, and there is also a big gap between the concentration of water in coagulation bath and polyacrylonitrile solution. Under the interaction of the above two concentration differences, the liquid begins to diffuse in two directions, and finally condenses into filaments through mass transfer, heat transfer, phase equilibrium movement and other processes.
In the production of precursor, the residual amount of DMSO, fiber size, monofilament strength, modulus, elongation, oil content and boiling water shrinkage become the key factors affecting the quality of precursor. Taking the residual amount of DMSO as an example, it has influence on the apparent properties of precursor, cross-section state and CV value of the final carbon fiber product. The lower the residual amount of DMSO, the higher the performance of the product. In production, DMSO is mainly removed by washing, so how to control the washing temperature, time, the amount of desalted water and the amount of washing cycle becomes an important link.
High quality polyacrylonitrile precursor should have the following characteristics: high density, high crystallinity, appropriate strength, circular cross section, less physical defects, smooth surface and uniform and dense skin core structure.
Temperature control of carbonization and oxidation is the key. Carbonization and oxidation is an essential step in the production of carbon fiber final products from precursor. In this step, the accuracy and range of temperature should be accurately controlled, otherwise, the tensile strength of carbon fiber products will be significantly affected, and even lead to wire breakage
Preoxidation (200-300 ℃): in the preoxidation process, the PAN precursor is slowly and mildly oxidized by applying a certain tension in the oxidizing atmosphere, forming a large number of ring structures on the basis of the pan straight chain, so as to achieve the purpose of withstanding higher temperature treatment.
Carbonization (maximum temperature not lower than 1000 ℃): carbonization process should be carried out in inert atmosphere. In the early stage of carbonization, the pan chain breaks and the crosslinking reaction begins; With the increase of temperature, the thermal decomposition reaction begins to release a large number of small molecule gases, and the graphite structure begins to form; When the temperature increased further, the carbon content increased rapidly and the carbon fiber began to form.
Graphitization (treatment temperature above 2000 ℃): graphitization is not a necessary process for carbon fiber production, but an optional process. If high elastic modulus of carbon fiber is expected, graphitization is needed; If high strength of carbon fiber is expected, graphitization is not necessary. In the graphitization process, high temperature makes the fiber form a developed graphite mesh structure, and the structure is integrated by drawing to get the final product.
High technical barriers endow the downstream products with high added value, and the price of aviation composites is 200 times higher than that of raw silk. Due to the high difficulty of carbon fiber preparation and complex process, the more downstream the products, the higher the added value. Especially for the high-end carbon fiber composites used in the aerospace field, because the downstream customers have very strict requirements on its reliability and stability, the product price also shows a geometric multiple growth compared with the ordinary carbon fiber.
Post time: Jul-22-2021