What is Carbon Fiber
Carbon fiber is a new kind of high strength and high modulus fiber material with more than 95% carbon content. It is made of flake graphite microcrystalline and other organic fibers stacked along the axial direction of the fibers. The microcrystalline graphite material is obtained by carbonization and graphitization treatment. Carbon fibers are soft and rigid, lighter in weight than aluminium, but higher in strength than iron and steel, and have the characteristics of corrosion resistance and high modulus. They are important materials in defense, military and civil fields. It not only has the inherent characteristics of carbon materials, but also has the soft processability of textile fibers. It is a new generation of reinforcing fibers.
Carbon fibers have many excellent properties, such as high axial strength and modulus, low density, high specific properties, no creep, super high temperature resistance under non-oxidation environment, good fatigue resistance, low thermal expansion coefficient and anisotropy, good corrosion resistance, and good X-ray transmission. Good conductivity and thermal conductivity, good electromagnetic shielding, etc
Carbon Fiber Process
Carbon fibers can be prepared by carbonization of polyacrylonitrile fibers, asphalt fibers, viscose fibers or phenolic fibers, respectively. The most common carbon fibers are polyacrylonitrile carbon fibers and asphalt carbon fibers. The fabrication of carbon fibers includes four processes: fiber spinning, thermal stabilization (pre-oxidation), carbonization and graphitization. The accompanying chemical changes include dehydrogenation, cyclization, pre-oxidation, oxidation and deoxidation.
The carbon fibers with high mechanical properties from viscose fibers must be graphitized by high temperature stretching, with low carbonization yield, difficult technology and complicated equipment. The products are mainly used for ablative resistant materials and heat insulation materials. The carbon fibers made from asphalt have abundant sources of raw materials and high carbonization yield, but they have not been developed on a large scale due to the complex preparation of raw materials and low product performance. Vinylon precursor can produce high performance carbon fibers. Its production process is simpler than other methods, and its mechanical properties are better. It has developed well in the carbon fiber industry since the 1960s.
The production of PAN-based carbon fibers mainly includes two processes: precursor production and precursor carbonization.
The production process of precursor mainly includes polymerization, defoaming, metering, spinning, drawing, washing, oiling, drying and winding.
The carbonization process mainly includes filament drawing, pre-oxidation, low temperature carbonization, high temperature carbonization, surface treatment, sizing, drying, winding and other processes.
PREPARATION OF PAN-BASED CARBON FIBER
Polyacrylonitrile carbon fibers are carbon fibers made from polyacrylonitrile fibers, mainly used as reinforcing materials for composite materials. Carbon fibers can be prepared from homopolymer or copolymerized polyacrylonitrile fibers. In order to produce high performance carbon fibers and improve productivity, copolymerized polyacrylonitrile fibers are often used as raw materials in industry. The requirements for raw materials are: less impurities and defects; uniform fineness, and finer, better; high strength, less wool; higher orientation of chain molecules along the fiber axis, the better, usually more than 80%; good thermal conversion performance.
The process of producing polyacrylonitrile fibers is as follows: first, the copolymerization of acrylonitrile and a small number of other second and third monomers (methyl acrylate, methylene butadiene, etc.) to produce copolymerized polyacrylonitrile resin (molecular weight is higher than 60,000 to 80,000), then the resin is dissolved by solvents (sodium thiocyanate, dimethyl alumina, nitric acid, zinc chloride, etc.) to form a spinning solution with appropriate viscosity, which is wet-processed, dry-processed Polyacrylonitrile fibers are prepared by wet or dry spinning, followed by washing, drawing, drying and heat setting. If polyacrylonitrile fibers were heated directly, they would melt easily, and could not keep their original fiber state. In the preparation of carbon fibers, polyacrylonitrile fibers should be first placed in air or other oxidative atmosphere for low temperature heat treatment, that is, pre-oxidation treatment. Pre-oxidation treatment is the preparation stage of fibre carbonization. Generally, the fibers are heated to about 270 C in air for 0.5 h to 3 h. The color of polyacrylonitrile fibers gradually changes from white to yellow and brown, and finally to black pre-oxidized fibers. It is the result of a series of chemical reactions, such as oxidation, pyrolysis, crosslinking, cyclization and so on, to form heat-resistant ladder polymer after thermal oxidation of polyacrylonitrile linear polymer. The pre-oxidized fibers were carbonized at 1600 C in nitrogen, and the cross-linking, cyclization, aromatization and condensation reactions were further produced. The hydrogen, nitrogen and oxygen atoms were removed. Finally, two-dimensional carbon ring planar network structure and rough parallel layered graphite carbon fibers were formed.
The technological process of preparing carbon fibers from PAN precursor is as follows：
PAN precursor pre-oxidation carbonization graphitization surface treatment coiling carbon fiber。
First, precursor preparation, polyacrylonitrile and viscose collagen yarn are mainly produced by wet spinning, while asphalt and phenolic precursor yarn are produced by melt spinning. The preparation of high performance PAN-based carbon fibers requires high purity, high strength and uniform quality PAN precursors, and itaconic acid as the copolymer for the preparation of precursors. To prepare anisotropic high performance asphalt-based carbon fibers, asphalt should be pretreated into mesophase, pre-mesophase (benzene-soluble anisotropic asphalt) and potential mesophase (quinoline-soluble anisotropic asphalt). The precursor of viscose-based carbon fibers used as ablative materials should not contain alkali metal ions.
Secondly, preoxidation (polyacrylonitrile fibers 200-300 C), non-melting (asphalt 200-400 C) or heat treatment (viscose fibers 240 C) to obtain heat-resistant and non-melting fibers. Phenolic-based carbon fibers do not have this process.
Third, carbonization, its temperature is: polyacrylonitrile fibers 1000 to 1500 degrees C, asphalt 1500 to 1700 degrees C, viscose fibers 400 to 2000 degrees C.
Fourth, graphitization, polyacrylonitrile fiber 2500 to 3000 C, asphalt 2500 to 2800 C, viscose fiber 3000 to 3200 C.
Fifth, surface treatment, gas-phase or liquid-phase oxidation and so on, give the fibrosis chemical activity, in order to increase the affinity of the resin.
Sixth, sizing treatment can prevent fiber damage and improve affinity with resin matrix. The obtained fibers have various cross-sectional structures.
Application Industry of Carbon Fiber
Carbon fiber materials have been widely used in military and civil industries. From civil industries such as aerospace, aviation, automobile, electronics, machinery, chemical industry and textile industry to sports equipment and leisure goods. Carbon fiber reinforced composites can be used in military fields such as aircraft manufacturing, wind turbine blades and other industrial fields, electromagnetic shielding materials, artificial ligaments and other body substitutes, as well as in the manufacture of rocket shells, motor ships, industrial robots, automotive leaf springs and driving axles. Sports fields such as baseball and bats. Carbon fiber is a new type of industrial material in the typical high-tech field.
