Carbon fiber has many excellent properties, such as high axial strength and modulus, low density, high specific performance, no creep, ultra-high temperature resistance in non-oxidizing environment, good fatigue resistance, small thermal expansion coefficient and anisotropy, good corrosion resistance and good X-ray permeability. Good electrical and thermal conductivity, good electromagnetic shielding, etc.
Compared with traditional glass fiber, the Young's modulus of carbon fiber is more than three times that of traditional glass fiber. Compared with Kevlar fiber, its Young's modulus is about twice that of Kevlar fiber. It is insoluble and swollen in organic solvents, acids and alkalis, and has outstanding corrosion resistance.
(1) composition structure
Carbon fiber is an inorganic polymer fiber with carbon content higher than 90%. Among them, graphite fibers with carbon content higher than 99% are called graphite fibers. The microstructure of carbon fiber is similar to artificial graphite, which is a disordered graphite structure. [5] The interlayer spacing of carbon fiber is about 3.39 ~ 3.42a. The arrangement of carbon atoms between parallel layers is not as regular as graphite, and the layers are connected by van der Waals force. [6]
The structure of carbon fiber is usually considered to be composed of two-dimensional ordered crystals and holes, in which the content, size and distribution of holes have great influence on the properties of carbon fiber. [7]
When the porosity is lower than a certain critical value, the porosity has no obvious effect on the interlaminar shear strength, bending strength and tensile strength of carbon fiber composites. It has been pointed out that the critical porosity that causes the decrease of mechanical properties of materials is 1%-4%. When the pore volume content is in the range of 0-4%, the interlaminar shear strength decreases by about 7% for every increase of pore volume content of 65438 0%. Through the study of carbon fiber epoxy resin and carbon fiber bismaleimide resin laminate, it is found that when the porosity exceeds 0.9%, the interlaminar shear strength begins to decrease. According to experiments, pores are mainly distributed between fiber bundles and interlayer interfaces. And the higher the pore content, the larger the pore size, and significantly reduce the interlayer interface area in the laminate. Materials are easy to break along the interlayer when stressed, which is also the reason why the interlayer shear strength is relatively sensitive to pores. In addition, the pore is a stress concentration area, and its bearing capacity is weak. When subjected to pressure, the pore expands to form a long crack, and the crack is destroyed. [8]
Even two laminates with the same porosity (using different prepreg methods and manufacturing methods in the same curing cycle) show completely different mechanical behaviors. The specific values of mechanical properties decreasing with the increase of porosity are different, which shows that the influence of porosity on mechanical properties is discrete and repetitive. Because it contains many variable factors, the influence of porosity on the mechanical properties of composite laminates is a very complicated problem. These factors include: the shape, size and location of pores; Mechanical properties of fiber, matrix and interface; Static or dynamic load. [8]
Compared with porosity and pore length-width ratio, pore size and distribution have greater influence on mechanical properties. And found a big hole (area >; 0.03mm2) has an adverse effect on the mechanical properties, which is attributed to the influence of porosity on the crack propagation in the rubber-rich area between layers. [8]
(2)
physical features
Carbon fib has two characteristics of strong tensile strength of carbon material and softness and processability of fib,
carbon fibre
It is a new material with excellent mechanical properties. The tensile strength of carbon fiber is about 2 to 7GPa, and the tensile modulus is about 200 to 700GPa. The density is about 65438 0.5 ~ 2.0g/cm3, which is not only related to the structure of the precursor, but also depends on the carbonization temperature. Generally, after graphitization at 3000℃, the density can reach 2.0g/cm3. Plus light weight, the specific gravity is lighter than aluminum, less than 1/4 of steel, and the specific strength is 20 times that of iron. The thermal expansion coefficient of carbon fiber is different from other fibers and has anisotropic characteristics. The specific heat capacity of carbon fiber is generally 7. 12. The thermal conductivity decreases with the increase of temperature, which is negative (0.72 ~ 0.90) parallel to the fiber direction and positive (32 ~ 22) perpendicular to the fiber direction. The specific resistance of carbon fiber is related to the type of fiber. At 25℃, the high modulus is 775, and the high strength carbon fiber is 1500 per cm. This makes carbon fiber have the highest specific strength and modulus among all high-performance fibers. Compared with titanium, steel, aluminum and other metal materials, carbon fiber has the characteristics of high strength, high modulus, low density and small linear expansion coefficient in physical properties, and is called the king of new materials. [3] [9- 1 1]
In addition to the characteristics of ordinary carbon materials,
Carbon fiber woven cloth [12]
Its appearance has obvious anisotropy and softness, and it can be processed into various fabrics. Because of its small specific gravity, it shows high strength along the fiber axis. The comprehensive indexes of specific strength and specific modulus of carbon fiber reinforced epoxy resin composites are the highest among the existing structural materials. [1 1] The tensile strength of carbon fiber resin composites is generally above 3500 MPa, which is 7 to 9 times that of steel, and the tensile elastic modulus is 230 to 430G Pa, which is also higher than steel. Therefore, the specific strength of CFRP, that is, the ratio of material strength to density, can reach more than 2000 MPa, while the specific strength of A3 steel is only about 59 MPa, and its specific modulus is also higher than that of steel. Compared with traditional glass fiber, Young's modulus (the physical quantity representing the tensile or compressive strength of materials within the elastic limit) is more than three times that of glass fiber. Compared with Kevlar fiber, not only the Young's modulus is about 2 times. The test of carbon fiber epoxy resin laminate shows that the strength and modulus decrease with the increase of porosity. Porosity has great influence on interlaminar shear strength, flexural strength and flexural modulus. With the increase of porosity, the tensile strength decreases relatively slowly; The tensile modulus is less affected by porosity. [8]
Carbon fiber also has excellent fineness (one of the manifestations of fineness is the grams of 9000-meter-long fiber), generally only about 19 grams, and the tensile force is as high as 300kg per micron. Almost no other material has so many excellent properties as carbon fiber, so it has strict requirements in size, stiffness, weight, fatigue characteristics and so on. Carbon fiber is not in contact with air and oxidant, and can withstand high temperature above 3000 degrees, with outstanding heat resistance. Compared with other materials, when the temperature is higher than 1500℃, the strength of carbon fiber begins to decrease, and the higher the temperature, the greater the fiber strength. The radial strength of carbon fiber is not as good as the axial strength, so the radial strength of carbon fiber (that is, it cannot be knotted) and the whisker performance of other materials have been greatly reduced. In addition, carbon fiber also has good low temperature resistance, such as not embrittlement at liquid nitrogen temperature. [3] [9] [ 13]
chemical property
The chemical properties of carbon fiber are similar to that of carbon, except that it can be oxidized by strong oxidant, and it is inert to general alkalinity. When the temperature in the air is higher than 400℃, obvious oxidation occurs, producing CO and CO2. [6-7] Carbon fiber has good corrosion resistance to common organic solvents, acids and alkalis, is insoluble and does not swell, has outstanding corrosion resistance, and has no rust problem at all. [1 1] Some scholars soaked polyacrylonitrile-based carbon fiber in strong alkali sodium hydroxide solution at 198 1, and it has been more than 30 years, and it still maintains the fiber shape. However, it has poor impact resistance and is easy to be damaged. Under the action of strong acid, the electromotive force of carbon fiber is positive, while that of aluminum alloy is negative. When carbon fiber composites are combined with aluminum alloy, metal carbonization, carburization and electrochemical corrosion will occur. Therefore, carbon fiber must be surface treated before use. [4] Carbon fiber also has the characteristics of oil resistance, radiation resistance, radiation resistance, absorption of toxic gases and neutron deceleration [3] [9] [13].
(3) Classification
Carbon fiber can be divided into polyacrylonitrile-based carbon fiber according to the source of raw materials,
1K carbon fiber pipe
Pitch-based carbon fiber, viscose-based carbon fiber, phenolic-based carbon fiber and vapor-grown carbon fiber; According to the performance, it can be divided into ordinary carbon fiber, high strength carbon fiber, medium strength carbon fiber, high modulus carbon fiber and ultra-high modulus carbon fiber. According to the state, it is divided into filament, short fiber and short fiber; According to mechanical properties, it can be divided into general type and high performance type. Generally, the strength of carbon fiber is 1000 MPa, and the modulus is about 100G Pa. High-performance carbon fiber can be divided into high-strength type (strength 2000 MPa, modulus 250G Pa) and high-modulus type (modulus above 300G Pa). Strength greater than 4000 MPa is also called ultra-high strength type; Modulus greater than 450G Pa is called superelevation model. With the development of aerospace industry, carbon fiber with high strength and high elongation has appeared, and its elongation is more than 2%. Polyacrylonitrile-based carbon fiber is the most widely used. [14] More than 90% of carbon fibers in the market are mainly polyacrylonitrile-based carbon fibers. Because the mystery of carbon fiber has not been completely unveiled, people can't directly use carbon or graphite to make carbon fiber. They can only use some carbon-containing organic fibers (such as nylon, acrylic fiber and rayon) as raw materials, and combine organic fibers with plastic resin to make carbon fibers. [4] [ 15- 17]
Polyacrylonitrile based carbon fiber
The production process of polyacrylonitrile-based carbon fiber mainly includes two processes: precursor production and precursor carbonization: firstly, polyacrylonitrile fiber or precursor undergoes a series of processes such as acrylonitrile polymerization and spinning, and these precursors are oxidized in an oxidation furnace at 200-300℃, and then carbon fiber is made in a carbonizer at 1000-2000℃. [ 18] [ 19]
Asphalt-based carbon fiber
The United States invented pitch containing metal mesophase, which was used to spin pitch-based carbon fiber. After the precursor was stabilized and carbonized, the tensile strength of the carbon fiber was 3.5G Pa and the modulus was 252G Pa. France developed mesophase pitch-based carbon fiber with heat resistance and high conductivity; Poland has developed a new method of metal coating carbon fiber. For example, pitch-based carbon fibers coated with copper are manufactured by a hybrid method. Firstly, copper salt and isotropic coal tar pitch were mixed evenly, then spun by centrifugal spinning, stabilized in air and treated in high temperature hydrogen to obtain alloy copper carbon fiber. The production capacity of pitch-based carbon fiber is small in the world, and the research and development of pitch-based carbon fiber in China is earlier, but there is a big gap in development, production and application compared with foreign countries. [ 19-20]
Carbon fiber can be divided into aviation grade and industrial grade according to different product specifications, also known as small tow and large tow. Generally, carbon fibers above 48K are called large tow carbon fibers, including 360K and 480K. Aviation-grade carbon fiber was mainly 3K at first, and gradually developed to 12K and 24K. It is mainly used in national defense and high technology, as well as sports and leisure products, such as airplanes, missiles, rockets, satellites, fishing rods and clubs.
(4) preparation method
Industrial carbon fiber can be divided into polyacrylonitrile (PAN)-based carbon fiber according to the raw material route.
, pitch-based carbon fiber and viscose-based carbon fiber, but mainly produce the first two kinds of carbon fibers. Carbon fiber with high mechanical properties made of viscose fiber must be stretched and graphitized at high temperature, which has low carbonization yield, great technical difficulty, complex equipment and rich raw materials. However, due to the complicated preparation of raw materials and low product performance, it has not been developed on a large scale. Compared with other methods, the production process of polyacrylonitrile fiber precursor to produce high-performance carbon fiber is simpler, and the output accounts for more than 90% of the total global carbon fiber production. [ 18] [22-23]
process flow
Carbon fiber can be carbonized from polyacrylonitrile fiber, asphalt fiber, viscose filament or phenolic fiber. The widely used carbon fibers are mainly polyacrylonitrile carbon fiber and pitch carbon fiber. The manufacture of carbon fiber includes four processes: fiber spinning, thermal stabilization (pre-oxidation), carbonization and graphitization. Accompanying chemical changes include dehydrogenation, cyclization, preoxidation, oxidation and deoxidation. [22-23]
The preparation of carbon fiber with high mechanical properties from viscose fiber must be graphitized by high temperature stretching, which has low carbonization yield, great technical difficulty and complicated equipment. This product is mainly used for ablative materials and thermal insulation materials. Carbon fiber prepared from asphalt is rich in raw materials and has high carbonization yield, but it has not been developed on a large scale because of the complex preparation of raw materials and low product performance. PAN fiber precursor can be made into high performance carbon fiber. Compared with other methods, it has simple production process and excellent mechanical properties, and has developed well in the carbon fiber industry since the 1960s. [ 19]
The production of polyacrylonitrile-based carbon fiber mainly includes two processes: precursor production and precursor carbonization. [ 19] [2 1]
The production process of precursor mainly includes polymerization, defoaming, metering, spinning, traction, washing, oiling, drying and silk collection. [ 19] [2 1]
Carbonization process mainly includes pay-off, pre-oxidation, low-temperature carbonization, high-temperature carbonization, surface treatment, setting drying, winding and other processes. [ 19] [2 1]
Preparation of polyacrylonitrile-based carbon fiber
Polyacrylonitrile carbon fiber is a carbon fiber made of polyacrylonitrile fiber, which is mainly used as a reinforcement of composite materials. Carbon fibers can be prepared from homopolymerized or polymerized polyacrylonitrile fibers. In order to produce high-performance carbon fiber and improve productivity, polyacrylonitrile fiber is often used as raw material in industry. The requirements for raw materials are: less impurities and defects; Uniform fineness, the finer the better; High strength, less hair; The higher the orientation degree of chain molecules along the fiber axis, the better, usually greater than 80%; Good thermal conversion performance. [6] [24]
The process of making polyacrylonitrile fiber in production is as follows: firstly, acrylonitrile and a small amount of second and third monomers (methyl acrylate, methylene butyl ester, etc. ) polymerization to produce * * * polyacrylonitrile resin (molecular weight is more than 60000 ~ 80000), and then the resin is dissolved in solvents (sodium thiocyanate, dimethyl alum, nitric acid, zinc chloride, etc.). ) to form a spinning solution with an appropriate viscosity, and then wetting, drying or wetting. If polyacrylonitrile fiber is directly heated, it is easy to melt and cannot maintain its original fiber state. When preparing carbon fiber, polyacrylonitrile fiber should be placed in air or other oxidizing atmosphere for low-temperature heat treatment, that is, pre-oxidation treatment. Pre-oxidation treatment is the initial stage of fiber carbonization. Generally, when the fiber is heated to about 270℃ in air and kept for 0.5h ~ 3h, the color of polyacrylonitrile fiber gradually changes from white to yellow and brown, and finally black preoxidized fiber is formed. It is the result of a series of chemical reactions, such as oxidation, pyrolysis, crosslinking, cyclization and so on, after polyacrylonitrile linear polymer is oxidized by heat to form heat-resistant ladder polymer. Then, the pre-oxidized fiber is treated at a high temperature of 1600℃ in nitrogen, so that the fiber further undergoes cross-linking cyclization, aromatization, polycondensation and other reactions, and hydrogen, nitrogen and oxygen atoms are removed, and finally the carbon fiber with two-dimensional carbon ring plane network structure and disordered graphite structure with rough parallel layers is formed. [7] [24]
The technological process of preparing carbon fiber from polyacrylonitrile precursor is: polyacrylonitrile precursor → pre-oxidation → carbonization → graphitization → surface treatment → winding → carbon fiber. [7] [24]
Firstly, precursor preparation: polyacrylonitrile and viscose precursor are mainly prepared by wet spinning, while asphalt and phenolic precursor are prepared by melt spinning. The preparation of high-performance polyacrylonitrile-based carbon fiber requires polyacrylonitrile precursor with high purity, high strength and uniform quality, and the monomer used to prepare the precursor is itaconic acid. In order to prepare anisotropic high-performance pitch-based carbon fibers, pitch must be pretreated into mesophase, pre-mesophase (benzene-soluble anisotropic pitch) and potential mesophase (quinoline-soluble anisotropic pitch). Viscose-based carbon fibers used as ablative materials require that their precursors do not contain alkali metal ions. [22] [25]
Second, pre-oxidation (polyacrylonitrile fiber at 200-300℃), non-melting (asphalt at 200-400℃) or heat treatment (viscose fiber at 240℃) to obtain heat-resistant non-melting fiber. Phenolic carbon fiber does not have this process. [22] [25]
3. Carbonization, the temperature is: polyacrylonitrile fiber 1000 ~ 1500℃, asphalt 1500 ~ 1700℃, viscose fiber 400 ~ 2000℃. [22] [25]
Fourth, graphitization: polyacrylonitrile fiber 2500-3000℃, asphalt 2500-2800℃, viscose fiber 3000-3200℃. [22] [25]
Fifth, surface treatment, such as gas phase or liquid phase oxidation, endows the fiber with chemical activity to increase its affinity for resin. [22] [25]
Sixth, sizing treatment to prevent fiber damage and improve affinity with resin matrix. The obtained fibers have various cross-sectional structures. [22] [25]
Technical points
In order to obtain good quality carbon fiber, we need to pay attention to the following technical points:
(1) The primary task of preparing high-performance carbon fiber is to realize the high purity, high strength, densification and surface defect-free of precursor. Carbon fiber system engineering needs to start with the polymerization monomer of precursor. The quality of precursor not only determines the properties of carbon fiber, but also restricts its production cost. High-quality polyacrylonitrile precursor is the first prerequisite for manufacturing high-performance carbon fiber. [22]
(2) Minimizing impurity defects is the fundamental measure to improve the tensile strength of carbon fiber, and it is also a hot research topic for scientific and technological workers. In a sense, the process of improving strength is essentially the process of reducing and reducing defects. [22]
(3) In the pre-oxidation process, the pre-oxidation time should be shortened as much as possible on the premise of ensuring homogeneity. This is a directional subject to reduce production costs.
(4) Research on high-temperature technology, high-temperature equipment and related important components. The high-temperature carbonization temperature is generally 1300 ~ 1800℃, and the graphitization temperature is generally 2500 ~ 3000℃. To run at such a high temperature, it is necessary to run continuously to improve the service life of the equipment, so it is particularly important to study a new generation of high-temperature technology and high-temperature equipment. Such as inert gas protection and microwave, plasma and induction heating in oxygen-free state. [22]