Carbon fiber (CF) is a new type of fiber material with high strength and high modulus fiber with a carbon content of more than 95%. It is a microcrystalline graphite material obtained by stacking organic fibers such as flake graphite microcrystals along the axial direction of the fiber and undergoing carbonization and graphitization treatments. Carbon fiber is "flexible on the outside and rigid on the inside". It is lighter than metallic aluminum but stronger than steel. It is corrosion-resistant and has high modulus. It is an important material in national defense and civilian applications. 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 fiber. [1-4]
Carbon fiber has many excellent properties. Carbon fiber has high axial strength and modulus, low density, high specific performance, no creep, ultra-high temperature resistance and fatigue resistance in non-oxidizing environment. Good, the specific heat and electrical conductivity are between non-metals and metals, the thermal expansion coefficient is small and anisotropic, the corrosion resistance is good, and the X-ray transmittance is good. Good electrical and thermal conductivity, good electromagnetic shielding, etc.
Compared with traditional glass fiber, the Young's modulus of carbon fiber is more than 3 times; compared with Kevlar fiber, the Young's modulus is about 2 times, and it can be used in organic solvents and acids. , does not dissolve or swell in alkali, and has outstanding corrosion resistance.
(1) Structure
Carbon fiber is an inorganic polymer fiber with a carbon content higher than 90%. Those with a carbon content higher than 99 are called graphite fibers. The microstructure of carbon fiber is similar to artificial graphite, which is a turbostratic graphite structure. [5] The distance between the layers of carbon fiber is about 3.39 to 3.42A. The arrangement of carbon atoms between the parallel layers is not as regular as that of graphite, and the layers are connected together by van der Waals force. [6]
The structure of carbon fiber is usually regarded as consisting of two-dimensional ordered crystals and holes. The content, size and distribution of holes have a greater impact on the performance 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, flexural strength and tensile strength of carbon fiber composite materials. Some studies have pointed out that the critical porosity that causes the mechanical properties of materials to decrease is 1-4. When the pore volume content is in the range of 0-4, for every increase in pore volume content by 1, the interlaminar shear strength decreases by approximately 7. Through research on carbon fiber epoxy resin and carbon fiber bismaleimide resin laminates, it can be seen that when the porosity exceeds 0.9, the interlaminar shear strength begins to decrease. It is known from experiments that pores are mainly distributed between fiber bundles and at the interface between layers. And the higher the pore content, the larger the size of the pores and significantly reduces the area of ??the interlayer interface in the laminate. When the material is stressed, it is easy to be damaged along the layers, which is why the interlayer shear strength is relatively sensitive to pores. In addition, the pores are stress concentration areas with weak load-bearing capacity. When stressed, the pores expand to form long cracks and are destroyed. [8]
Even two laminates with the same porosity (using different prepreg methods and manufacturing methods during the same curing cycle) exhibit completely different mechanical behaviors. The specific numerical value of the decrease of mechanical properties with the increase of porosity is different, which is manifested in that the influence of porosity on mechanical properties is highly discrete and has poor repeatability. The effect of pores on the mechanical properties of composite laminates is a complex issue due to the large number of variables involved. These factors include: shape, size, and location of pores; mechanical properties of fibers, matrix, and interfaces; static or dynamic loads. [8]
Compared to porosity and pore aspect ratio, pore size and distribution have a greater impact on mechanical properties. It was also found that large pores (area gt; 0.03mm2) have an adverse effect on mechanical properties, which is attributed to the effect of pores on crack propagation in the glue-rich region between layers. [8]
(2)
Physical Properties
Carbon fiber has both the strong tensile strength of carbon materials and the soft processability of the fiber.
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Carbon fiber
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 1.5 to 2.0 grams per cubic centimeter. In addition to being related to the structure of the original fiber, it mainly depends on the temperature of the carbonization treatment.
Generally, after graphitization treatment at high temperature of 3000℃, the density can reach 2.0 grams per cubic centimeter. In addition, its weight is very light, its specific gravity is lighter than aluminum, less than 1/4 of steel, and its specific strength is 20 times that of iron. The thermal expansion coefficient of carbon fiber is different from other fibers in that it has anisotropic characteristics. The specific heat capacity of carbon fiber is generally 7.12. The thermal conductivity decreases with increasing temperature with negative values ??parallel to the fiber direction (0.72 to 0.90) and positive values ??perpendicular to the fiber direction (32 to 22). The specific resistance of carbon fiber is related to the type of fiber. At 25°C, the high modulus is 775 and the high strength carbon fiber is 1500 per centimeter. This gives carbon fiber the highest specific strength and specific modulus of all high-performance fibers. Compared with metal materials such as titanium, steel, and aluminum, carbon fiber has the characteristics of high strength, high modulus, low density, and small linear expansion coefficient in terms of physical properties. It can be called the king of new materials. [3] [9-11]
In addition to the characteristics of general carbon materials, carbon fiber woven cloth [12]
its appearance has significant It is anisotropically soft and can be processed into various fabrics. Due to its small specific gravity, it shows high strength along the fiber axis. The comprehensive indicators of specific strength and specific modulus of carbon fiber reinforced epoxy resin composite materials are among the best in existing materials. It is the highest among structural materials. [11] The tensile strength of carbon fiber resin composites is generally above 3500 MPa, which is 7 to 9 times that of steel. The tensile elastic modulus is 230 to 430 GPa, which is also higher than steel; therefore, the specific strength of CFRP is the strength of the material. The ratio to its 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 steel. Compared with traditional glass fiber, Young's modulus (referring to the physical quantity that characterizes the tensile or compressive resistance of a material within the elastic limit) is more than three times that of glass fiber; compared with Kevlar fiber, not only Young's modulus It is about 2 times that. Tests on carbon fiber epoxy laminates have shown that both strength and modulus decrease as porosity increases. Porosity has a great influence on interlayer shear strength, flexural strength, and flexural modulus; tensile strength decreases relatively slowly with the increase of porosity; tensile modulus is less affected by porosity. [8]
Carbon fiber also has excellent fineness (one of the expressions of fineness is the number of grams of 9000-meter-long fiber), which is generally only about 19 grams, and the tensile force is as high as 300kg per micron. Almost no other material has as many excellent properties as carbon fiber, so it has strict requirements in areas such as strength, stiffness, strength, fatigue properties, etc. When not in contact with air and oxidants, carbon fiber can withstand high temperatures of more than 3,000 degrees and has outstanding heat resistance. Compared with other materials, the strength of carbon fiber does not begin to decrease until the temperature is higher than 1,500 degrees Celsius, and the higher the temperature, the lower the fiber strength. The greater the intensity. The radial strength of carbon fiber is not as good as the axial strength, so carbon fiber avoids radial strength (that is, it cannot be knotted) and the whisker performance of other materials has also 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 properties
The chemical properties of carbon fiber are similar to those of carbon. In addition to being oxidized by strong oxidants, it is inert to general alkali. When the temperature in the air is higher than 400°C, significant oxidation occurs, generating CO and CO2. [6-7] Carbon fiber has good corrosion resistance to general organic solvents, acids, and alkali, does not dissolve or swell, has outstanding corrosion resistance, and has no rust problem at all. [11] Some scholars soaked PAN-based carbon fiber in a strong alkali sodium hydroxide solution in 1981. More than 30 years have passed, and it still maintains its fiber shape. However, its impact resistance is poor, easy to damage, and oxidized under the action of strong acid. The electromotive force of carbon fiber is positive, while the electromotive force of aluminum alloy is negative. When carbon fiber composite materials are used in combination with aluminum alloys, metal carbonization, carburization and electrochemical corrosion will occur. Therefore, carbon fiber must be surface treated before use. [4] Carbon fiber also has properties such as oil resistance, radiation resistance, radiation resistance, absorption of toxic gases, and deceleration of neutrons [3] [9] [13] .
(3) Classification
Carbon fiber can be divided into polyacrylonitrile-based carbon fiber and tubes made of 1K carbon fiber according to the source of raw materials.
Pitch-based carbon fiber, viscose-based carbon fiber, phenolic-based carbon fiber, vapor-grown carbon fiber; according to performance, it can be divided into general type, high strength type, medium mold high strength type, high model and ultra high model carbon fiber; according to state, it can be divided into filament and short fiber and chopped fibers; divided into general-purpose and high-performance types according to mechanical properties. The strength of general-purpose carbon fiber is 1000 MPa and the modulus is about 100 GPa. High-performance carbon fiber is divided into high-strength type (strength 2000 MPa, modulus 250GPa) and high model (modulus 300GPa or above). Those with a strength greater than 4000 MPa are also called ultra-high strength models; those with a modulus greater than 450 GPa are called ultra-high models. With the development of the aerospace and aviation industry, high-strength and high-elongation carbon fibers have also appeared, with an elongation greater than 2. The largest amount of polyacrylonitrile PAN-based carbon fiber is used. [14] More than 90% of the carbon fibers on the market are mainly PAN-based carbon fibers. Since the mystery of carbon fiber has not yet been completely unveiled, people cannot directly use carbon or graphite to make it. They can only use some carbon-containing organic fibers (such as nylon filaments, acrylic filaments, rayon, etc.) as raw materials, and combine organic fibers with Plastic resins are bonded together and carbonized to create carbon fiber. [4] [15-17]
PAN-based carbon fiber
The production process of PAN-based carbon fiber mainly includes two processes: raw fiber production and raw fiber carbonization: first, through acrylonitrile polymerization and A series of processes such as spinning are processed into polyacrylonitrile fibers or raw filaments called "mother". These raw filaments are put into an oxidation furnace for oxidation at 200 to 300°C, and in a carbonization furnace at a temperature of Carbon fiber is made from carbonization and other processes at 1000 to 2000°C. [18] [19]
Pitch-based carbon fiber
The United States invented the base metal mesophase pitch for textile pitch-based carbon fiber. After the raw filaments are stabilized and carbonized, the carbon fiber becomes The tensile strength is 3.5G Pa and the modulus is 252 G Pa; France has developed heat-resistant and highly conductive mesophase pitch-based carbon fiber; Poland has developed a new metal-coated carbon fiber method. For example, copper-coated pitch-based carbon fiber is used It is made by the mixing method. First, copper salt and isotropic coal pitch are mixed, centrifugally spun, stabilized in air and treated in high-temperature hydrogen to obtain copper alloy carbon fiber. The production capacity of pitch-based carbon fiber in the world is small. The research and development of domestic pitch-based carbon fiber was earlier, but there is a large gap compared with foreign countries in terms of development, production and application. [19-20]
Carbon fiber is divided into aerospace grade and industrial grade according to different product specifications, also known as small tows and large tows. Carbon fibers above 48K are usually called large-tow carbon fibers, including 360K and 480K. Aerospace-grade carbon fiber was mainly 3K at the beginning, and gradually developed into 12K and 24K. It is mainly used in national defense and military industry, high technology, and sports and leisure products, such as aircraft, missiles, rockets, satellites, fishing rods, clubs and rackets
(4) Preparation method
Industrial production of carbon fiber can be divided into three categories according to the raw material route: polyacrylonitrile (PAN)-based carbon fiber
, pitch-based carbon fiber and viscose-based carbon fiber , but mainly produces the first two types of carbon fiber. Carbon fiber with high mechanical properties made from viscose fiber must be graphitized by high-temperature stretching. The carbonization yield is low, the technology is difficult, the equipment is complex, and the raw materials are abundant. The carbonization yield is high, but the raw material preparation is complex and the product performance is low. It has not been developed on a large scale; the production process of high-performance carbon fiber made from polyacrylonitrile fiber precursors is simpler than other methods, and its output accounts for more than 90% of the total global carbon fiber output. [18] [22-23]
Process flow
Carbon fiber can be produced by carbonization of polyacrylonitrile fiber, pitch fiber, viscose fiber or phenolic fiber. The most commonly used carbon fibers are polyacrylonitrile carbon fiber and pitch carbon fiber. The manufacturing of carbon fiber includes four processes: fiber spinning, thermal stabilization (pre-oxidation), carbonization, and graphitization. The accompanying chemical changes include dehydrogenation, cyclization, pre-oxidation, oxidation and deoxygenation, etc.
[22-23]
Preparing carbon fibers with high mechanical properties from viscose fibers must be graphitized through high-temperature stretching. The carbonization yield is low, the technology is difficult, and the equipment is complex. The products are mainly ablation-resistant materials. and used in thermal insulation materials; carbon fiber is produced from pitch, with rich sources of raw materials and high carbonization yield. However, due to the complex preparation of raw materials and low product performance, it has not been developed on a large scale; high-quality carbon fiber can be produced from polyacrylonitrile fiber precursors. The production process of high-performance carbon fiber is simpler than other methods and has excellent mechanical properties. It has developed well in the carbon fiber industry since the 1960s. [19]
The production of polyacrylonitrile-based carbon fiber mainly includes two processes: protofilament production and protofilament carbonization. [19] [21]
The raw silk production process mainly includes processes such as polymerization, degassing, metering, spinning, traction, washing, oiling, drying and collecting. [19] [21]
The carbonization process mainly includes wire drawing, pre-oxidation, low-temperature carbonization, high-temperature carbonization, surface treatment, sizing and drying, wire taking and winding and other processes. [19] [21]
Preparation of PAN-based carbon fiber
Polyacrylonitrile carbon fiber is a carbon fiber made from polyacrylonitrile fiber as raw material, and is mainly used as a reinforcement for composite materials. Carbon fibers can be produced from homopolymerized or polymerized polyacrylonitrile fibers. In order to produce high-performance carbon fibers and improve productivity, polyacrylonitrile fibers are often used as raw materials in industry. The requirements for raw materials are: few impurities and defects; uniform fineness, and the finer the better; high strength and less wool; the higher the orientation of chain molecules in the fiber along the fiber axis, the better, usually greater than 80; thermal conversion performance good. [6] [24]
The process of preparing polyacrylonitrile fiber in production is: first, acrylonitrile and other small amounts of second and third monomers (methyl acrylate, methylene butadiene, etc.) ) is polymerized to form polyacrylonitrile resin (molecular weight higher than 60,000 to 80,000), and then the resin is dissolved in solvents (sodium thiocyanate, dimethyl sulfoxide, nitric acid and zinc chloride, etc.) to form The spinning liquid with suitable viscosity is spun by wet method, dry method or dry-wet method, and then washed, drawn, dried and heat-set to make polyacrylonitrile fiber. If polyacrylonitrile fiber is directly heated, it will easily melt and cannot maintain its original fiber state. When preparing carbon fiber, the polyacrylonitrile fiber must first be placed in the air or other oxidizing atmosphere for low-temperature heat treatment, that is, pre-oxidation treatment. Pre-oxidation treatment is the preliminary stage for fiber carbonization. Generally, the fiber is heated to about 270°C in the air and kept warm for 0.5h to 3h. The color of the polyacrylonitrile fiber gradually changes from white to yellow or brown, and finally forms black pre-oxidized fiber. It is the result of a series of chemical reactions such as oxidation, pyrolysis, cross-linking, and cyclization that occur after polyacrylonitrile linear polymer is thermally oxidized to form a heat-resistant ladder polymer. The pre-oxidized fiber is then subjected to a high-temperature carbonization treatment of 1600°C in nitrogen, and the fiber further undergoes cross-linking cyclization, aromatization and polycondensation reactions, and removes hydrogen, nitrogen, and oxygen atoms, and finally forms a two-dimensional carbon ring. Carbon fiber with planar network structure and rough parallel layers of turbostratic graphite structure. [7] [24]
The process flow of preparing carbon fiber from PAN precursor is as follows: PAN precursor → pre-oxidation → carbonization → graphitization → surface treatment → coiling → carbon fiber. [7] [24]
First, raw yarn preparation. Polyacrylonitrile and viscose raw yarns are mainly produced by wet spinning, while asphalt and phenolic raw yarns are produced by melt spinning. . The preparation of high-performance polyacrylonitrile-based carbon fibers requires the use of polyacrylonitrile raw filaments of high purity, high strength and uniform quality. The polymer monomer used to prepare the raw filaments is itaconic acid, etc. To prepare anisotropic high-performance pitch-based carbon fibers, the pitch must first be pretreated into mesophase, pre-mesophase (benzene-soluble anisotropic asphalt) and potential mesophase (quinoline-soluble anisotropic asphalt). As viscose-based carbon fiber used as ablation materials, its raw filaments are required to contain no alkali metal ions. [22] [25]
Second, pre-oxidation (polyacrylonitrile fiber 200 to 300℃), non-melting (asphalt 200 to 400℃) or heat treatment (viscose fiber 240℃) to obtain Heat-resistant and infusible fiber, phenolic-based carbon fiber does not have this process.
[22] [25]
Third, carbonization, the temperature is: polyacrylonitrile fiber 1000 to 1500℃, asphalt 1500 to 1700℃, viscose fiber 400 to 2000℃. [22] [25]
Fourth, graphitization, polyacrylonitrile fiber is 2500 to 3000℃, asphalt is 2500 to 2800℃, and viscose fiber is 3000 to 3200℃. [22] [25]
Fifth, surface treatment, gas phase or liquid phase oxidation, etc., to impart chemical activity to the fiber to increase the affinity for the resin. [22] [25]
Sixth, sizing treatment to prevent fiber damage and improve affinity with the resin matrix. The resulting fibers have various cross-sectional structures. [22] [25]
Technical points
To obtain good quality carbon fiber, you need to pay attention to the following technical points:
(1) Achieve high purification of raw silk, High reinforcement, densification and smooth surface are the primary tasks for preparing high-performance carbon fibers. Carbon fiber system engineering begins with the polymerization of monomers. The quality of raw silk not only determines the properties of carbon fiber, but also restricts its production cost. High-quality PAN precursor is the primary prerequisite for manufacturing high-performance carbon fiber. [22]
(2) Minimizing impurities and defects, this is the fundamental measure to improve the tensile strength of carbon fiber, and it is also a hot topic for scientific and technological workers. In a sense, the process of increasing strength is essentially the process of reducing and minimizing defects. [22]
(3) During the pre-oxidation process, the pre-oxidation time should be shortened as much as possible while ensuring homogenization. This is a directional issue to reduce production costs.
(4) Research high-temperature technology, high-temperature equipment and related important components. The high-temperature carbonization temperature is generally between 1300 and 1800°C, and the graphitization temperature is generally between 2500 and 3000°C. When operating at such a high temperature, it is necessary to operate continuously and improve the service life of the equipment. Therefore, it is particularly important to study a new generation of high-temperature technology and high-temperature equipment. Such as microwave, plasma and induction heating technologies under inert gas protection and oxygen-free conditions. [22]