1 Carbon fiber: black gold, king of high performance fibers
1.1 Carbon fiber is a new strategic material in the field of modern high technologies
Carbon fiber is a globally recognized strategic new material in the field of modern high technologies, known as "black gold". Carbon fiber (CF) is a fibrous carbonized product with a carbon content above 90%, undergoes pyrolysis and carbonization with organic fiber precursors (precursors) at high temperatures (1000-3000°C) under the protection of inert gases. a series of physical and chemical changes in the system. From the point of view of the molecular structure, carbon fiber can be considered as composed of flake graphite crystallites along the axial direction of the fiber, but real carbon fiber cannot achieve the ideal state of graphite, and the plane of the graphite layer is wavy, and the distance between the planes is clearly larger than that of graphite 0.335 nm graphite. Carbon fiber has significant anisotropy, high modulus along the fiber axis, and high strength. It is a high-performance reinforcing fiber with good electrical conductivity, thermal conductivity, corrosion resistance, and ultra-high temperature resistance. It also has the characteristics of a textile material. soft weave fibers.
Compared with other metals and alloys, carbon fiber and its composite materials mainly have the following advantages: (1) Generally, the specific modulus of the unidirectional material of high modulus carbon fiber composite materials is 5 to 7 times that of aluminum. alloys, so prepared structural parts can meet high rigidity requirements; (2) With high modulus carbon fiber as the reinforcing material, a material with almost zero thermal expansion coefficient can be obtained through reasonable design of the composite material structure, which is consistent with dimensional stability. in the field of alternating high and low temperature applications (3) The specific gravity of carbon fiber is less than 1/4 of the weight of steel, which can meet the requirements for lightness of structural parts.
The vigorous development of carbon fiber plays a more important role in national defense, economy and people's livelihood. Carbon fiber not only has the excellent comprehensive properties of carbon materials, such as light weight, high strength, high elastic modulus, corrosion resistance, fatigue resistance, high temperature resistance, thermal conductivity and electrical conductivity, but also has the softness and processability of textile fibers. These are internationally recognized modern high-tech strategic new materials in the field of science and technology, known as "black gold". Due to the growing demand of people for quality of lifeand the continuous development of science and technology, especially in high-tech areas such as aerospace and military industries, as well as the demand for advanced materials in civilian areas such as the automotive industry and construction. sports, traditional materials and their composite materials are gradually becoming unsatisfactory, the emergence and development of new materials represented by carbon fiber have contributed to the development of these industries. place in the field of new energy and light demand.
PAN-based carbon fiber accounts for more than 90% of the total carbon fiber. At present, carbon fiber is generally understood to mean PAN-based carbon fiber. Carbon fibers can be classified according to various parameters such as condition, mechanical properties, tow characteristics, and types of raw materials. Carbon fibers can be divided into filaments, short fibers and chopped fibers according to their condition; they can be divided into general purpose types and high performance types according to their mechanical properties; and they can be divided into aerospace grade small fibers. industrial grade large tow carbon fibers and carbon fibers according to the specifications of the tow. According to the type of raw silk, it can be divided into polyacrylonitrile-based carbon fiber, resin-based carbon fiber, viscose-based carbon fiber and phenolic fiber. based on carbon fiber.
Due to the long production chain of carbon fiber and the many key control points, defects that occur at each stage of the manufacturing process are passed on to the next stage and affect the performance of the final carbon fiber. Thus, precise control of each process and precise coordination between them is the key to producing stable, high performance carbon fibers, and it is especially important to understand and be familiar with the carbon fiber preparation process.
1.2 PAN-based carbon fiber: the most widely used type of carbon fiber
PAN-based carbon fiber is made from acrylonitrile as a precursor through a series of complex processes such as polymerization, spinning, oxidation stabilization, carbonization and graphitization. Each process includes fluid mechanics, heat transfer, mass transfer, structuring. Multi-stage processes such as aggregation and aggregation are carried out simultaneously and are interconnected, and the influencing factors are more complex.
In the 18th century, British Swan and American Edison used bamboo and cellulose to make the earliest carbon fiber through a series of post-processing, used it as a thread, and applied for a patent. In the 1950s, research into rayon-based carbon fibers began in the United States, and rayon-based carbon fibers were produced in 1959 under the name "Tormei-25". In the same year, Shindo Akio of Japan pioneered the invention of carbon fiber based on polyacrylonitrile (PAN). In 1962, Japan's Toray Corporation began to develop and produce high-quality carbon fiber raw silk, and in 1967 successfully developed T300 polyacrylonitrile-based carbon fiber. In 1966, Watt and others at the Royal Institute of Aeronautical Research improved the technology and created a new way to produce high-strength, high-modulus PAN-based carbon fibers. In 1969, Toray Corporation of Japan successfully developed a carbon fiber precursor made from polyacrylonitrile copolymerized with special monomers, combined with Union Carbide Corporation's carbonization technology in the United States to produce high strength and high modulus carbon fiber. Since then, the United States, France, and Germany have also introduced technology or independently developed and produced PAN-based raw silk and carbon fiber, but Japan's Toray's carbon fiber R&D and production technology has always maintained a world-class level. According to a WeChat public account article on carbon fiber and its composite material technology on August 11, 2021, it can be seen that Toray has released more than 30 models of carbon fiber products in 2021, and the coverage has expanded from aerospace to transportation routes, oceans, pressure vessels , medical treatment, civil engineering, electronics and other fields.
The production process for PAN-based carbon fiber is cumbersome and involves many complex chemical reaction processes. It needs to go through several steps such as polymerization, spinning, pre-oxidation, carbonization, graphitization and surface treatment. Process, each process parameter will have a certain effect on the structure and characteristics of the final carbon fiber. The manufacturing process includes various disciplines and technologies, such as polymer chemistry, polymer physics, physical chemistry, inorganic chemistry, polymer processing, and automatic control. It is a complex systems engineering, and the final structure and performance of PAN-based carbon fiber is highly dependent on process control and structure regulation in each process.
1.2.1 Preparation of polyacrylonitrile copolymer: participants and equipment are the basis of the polymerization reaction
Polymerization is a process in which acrylonitrile (AN) monomers undergo a free radical chainpolymerization to obtain PAN with a long chain. The polymerization process is roughly divided into raw material preparation, polymerization reaction, etc. according to the process sequence. The process of preparation of raw materials, raw materials for the production of PAN copolymers includes monomers, comonomers, initiators, chain transfer agents and solvents.
In terms of monomers, Acrylonitrile (AN) is the main monomer for producing PAN copolymers. Polyacrylonitrile fiber made from acrylonitrile, that is, acrylic fiber, has properties very similar to wool, which is why it is also called synthetic wool. Acrylonitrile and butadiene can be copolymerized to produce nitrile rubber, which has good oil resistance, cold resistance, abrasion resistance and electrical insulation properties, and is relatively resistant to most chemical solvents, sunlight and heat.
For the comonomers, it is difficult to control the preoxidation process due to the high temperature of the PAN homopolymer in the initial pre-oxidation stage and the concentrated heat generation. In addition, concentrated heat release will cause PAN molecular chain breakage in the precursor and the formation of a macroporous defect structure, which will affect the stability of the production process and the quality of carbon fibers. In actual production, acrylonitrile and comonomers are usually copolymerized to effectively control the exothermic reaction during pre-oxidation, and the total comonomer content is typically around 5%. To obtain carbon fibers based on PAN, methyl acrylate (MA), methyl methacrylate (MMA), methacrylic acid (MAA) and itaconic acid (IA) are mainly used as comonomers.
In terms of initiators and chain transfer agents, carbon fiber manufacturers at home and abroad mainly use azo type initiators, among which azobisisobutyronitrile (AIBN) is a widely used initiator, and its main function is to provide free radicals and molecular AN. the action generates monomer free radicals to complete chain growth. According to the book "Carbon fiber based on polyacrylonitrile", when using the DAK initiator, the polymerization temperature is usually maintained within 55-65 ° C, and the amount of initiator in relation to the mass concentration of the monomer does not exceed 0.5. %, preferably below 0.3%. A chain transfer agent, also known as a molecular weight regulator, is a substance that can regulate and control a polymer's molecular weight, molecular weight distribution, and reduce chain branching. When AN is polymerized, alcohols or mercaptans are used as chain transfer agents, and the added amount is controlled in the range of 0.1% to 0.2% with respect to the mass concentration of AN monomer, which can greatly control the molecular weight and branching degree of the PAN polymer and improve torque ability.
In terms of solvents, PAN-based homogeneous solution polymerization can be divided into inorganic and organic solvents. Inorganic solvents include sodium thiocyanate, zinc chloride, nitric acid, etc., and organic solvents include dimethylamide, dimethylacetamide, and dimethyl sulfoxide. Among them, dimethyl sulfoxide has relatively low corrosion, small chain transfer constant, low toxicity, and no metal residue, so it has become the most commonly used solvent for solution polymerization.
The quality of the precursor is critical to the performance of carbon fiber, and obtaining a suitable spinning solution is a prerequisite for preparing a high-quality precursor, so obtaining a PAN solution with excellent characteristics becomes a source ofactions. subsequent process. At present, methods for synthesizing PAN copolymers mainly include solution polymerization (also known as homogeneous solution polymerization), water precipitation polymerization (also known as heterogeneous solution polymerization), aqueous suspension polymerization, and emulsion polymerization.
PAN polymerization products for carbon fiber are mainly required as follows: (1) higher average molecular weight (need to reach 10 5 classes), molecular weight distribution from 2 to 3 and as small as possible; (2) the presence of ideal copolymerization Suitable content of monomers and comonomers (mole fraction of about 2%); (3) Contains as few impurities as possible and a minimum of defects in the molecular structure at every level. Therefore, monomer concentration, initiator concentration, polymerization temperature, polymerization time, polymerization agitation method, etc. are factors influencing the polymerization reaction.
In the polymerization reaction, the polymerization equipment is the key equipment for carrying out the polymerization reaction. Advanced and intelligent reactors are critical to improving product quality.
Solution reaction, because the resulting polymerization solution can be directly used for spinning after one-time removal and defoaming, so it is also called the one-step method. Almost all polymerization reactors are stirred reactors. In addition, when mixing different batches of stock solutions prepared by the polymerization reaction, they should be mixed evenly so as to reduce the difference between batches of stock solutions and ensure the stability of the quality of stock solutions.
Compared to the solution polymerization reaction, since the aqueous phase precipitation polymerization process produces powdered or granular PAN polymers instead of the PAN stock solution that can be directly centrifuged, there is an additional "dissolution" process, so the aqueous phase precipitation The polymerization process is also known as a two step process. At present, acrylic fibers obtained by the method of precipitation polymerization in water phase are of good quality, so they are more suitable for industrial use.
1.2.2 Preparation of polyacrylonitrile precursor: the key is to effectively avoid the disadvantages of various spinning processes
Carbon fiber precursor is a chemical fiber made from a specific polymer compound (such as acrylonitrile) as a raw material during dope preparation, spinning and post-processing. In the carbon fiber preparation process, the PAN precursor structure has a very close relationship with its performance, and is the main source of the final carbon fiber structure and high performance. Various levels of precursor structure will be selectively inherited from the subsequent pre-oxidation step to the carbonization step, so the precursor structure must be well controlled in order to obtain excellent performance PAN-based carbon fibers.
The common production methods of PAN carbon fiber precursors include wet method, dry wet spinning (dry-wet) method, etc. Wet method, dry jet wet spinning (dry-wet) method is mainly suitable for high molecular weight polymers, which must be dissolved in a solvent before being extruded into filaments.
Wet spinning is the earliest developed and most widely used technique. During the spinning process, the spinning solution ejected from the spinneret opening enters the coagulation bath, and physical changes such as mass transfer, heat transfer, and phase separation occur, resulting in the precipitation of PAN to form filaments with a gel-like structure.
Wet spinning with dryjet is suitable for highly concentrated and highly viscous PAN solutions, and the maximum viscosity can be more than ten times higher than that of PAN solutions used in the conventional wet spinning process.
In terms of raw materials, it mainly includes four parts: polyacrylonitrile spinning solution, solvent, precipitating agent and oil agent.
PAN precursors are produced by solution spinning of PAN, and solution spinning is currently the only process for producing highly efficient PAN-based carbon fiber precursors. Generally speaking, the larger the molecular weight, the stronger the ability to withstand external impact and the lower the likelihood of defects.
Solvents can be divided into inorganic and organic. Inorganic solvents include sodium thiocyanate, zinc chloride, nitric acid, etc., and organic solvents include dimethylamide, dimethylacetamide, and dimethyl sulfoxide.
The PAN solution is converted into fibers by a coagulation process that requires a non-PAN solvent precipitant. Commonly used precipitants typically include water and other low molecular weight substances.
The surface defect of carbon fiber is one of the important factors affecting its mechanical properties. The purpose of using an oil agent is to reduce the surface defect caused by the carbon fiber precursor and the pre-oxidation process, so as to improve the performance of the carbon fiber.
The common production methods of PAN carbon fiber precursors include wet method, dry wet spinning (dry-wet) method, etc. Wet method, dry jet wet spinning (dry-wet) method is mainly suitable for high molecular weight polymers, which must be dissolved in a solvent before being extruded into filaments.
Wet spinning is the earliest developed and most widely used technique. During the spinning process, the spinning solution ejected from the spinneret opening enters the coagulation bath, and physical changes such as mass transfer, heat transfer, and phase separation occur, resulting in the precipitation of PAN to form filaments with a gel-like structure.
Dry jet wet spinning is suitable for highly concentrated and highly viscous PAN solutions, and the maximum viscosity can be more than ten times higher than that of PAN solutions used in the conventional wet spinning process.
In terms of equipment, spinning devices, washing devices, stretching devices and auxiliary devices are the main equipment.
In terms of the spinning device, the spinning machine is the key equipment of the entire spinning production line. According to the different spinning methods, the appropriate spinning system is selected, and the corresponding mechanical and electrical automatic control also changes according to the process. Although different spinning methods use different spinning systems, spinning machines, coagulation baths, dosing pumps and temperature controllers are important components.
In terms of laundry equipment, natural silk contains a certain amount of solvents. If these solvents are not removed, the fibers can become coarse and hard, and the color will turn gray and sticky, and the fibers will easily turn yellow when dried and heated.
With regard to the drawing device, the primary fibers formed in the coagulation bath are not hardened enough, and a series of post-treatments must be applied to the PAN fibers to a certain extent of the drawing process in order to remove the solvent from the surface. and inside the fibers and ensure the quality of the fibers. Typically, the exhaust device includes pre-stretching before washing with water and subsequent stretching with waste water and steam stretching.
In terms of auxiliary devices, oiling device, drying and sealing device, fusing device and yarn collecting device are relatively common auxiliary devices. The oiling device is mainly designed to improve the anti-static ability of the fiber and withstand high temperature during pre-treatment. -oxidation Drying and compacting device eliminates internal stress and defects in the fiber; heat fixing device can improve the supramolecular structure of the fiber, improve the stability and mechanical properties of the fiber; the spinning device packs the PAN precursor into a roll.
1.2.3 Production of pre-oxidized polyacrylonitrile fiber: improving the efficiency of pre-oxidation presents a challenge to reduce costs
PAN precursors are heated in air at 200-300℃, and the process of converting the linear PAN molecular chain into a nitrogen-containing ladder structure capable of high temperature is pre-oxidation.
During the pre-oxidation process, the precursor is subjected to a pulling force, and the linear molecular structure of polyacrylonitrile is transformed into a heat-resistant non-plastic ladder structure, so it can withstand thousands of degrees of high temperature. temperature during carbonization without melting or burning. The pre-oxidation process mainly proceeds in reactions such as cyclization, dehydeRating, oxidation and cracking to remove non-carbon elements. Preoxidation degree characterization parameters include aromatization index, also known as carbonization index, moisture content, density, and limiting oxygen index of the preoxidized fiber. During the pre-oxidation process, the linear molecular structure of polyacrylonitrile changes dramatically, and defects are easily formed. If the process parameters are not properly controlled, the mechanical properties of the carbon fibers will be greatly affected. - The oxidation process is crucial for improving the structure of carbon fibers and improving the mechanical properties is of great importance. The parameters controlling the pre-oxidation step are processing time, processing temperature and drawing force.
Pre-oxidation temperature, pre-oxidation time, pre-oxidation voltage, pre-oxidation atmosphere and equivalent pre-oxidation time are the main factors affecting the pre-oxidation process.
Preoxidation temperature and its gradient are the most important control factors in the PAN precursor preoxidation process, which largely determine the degree of fiber preoxidation and the characteristics of the final carbon fiber. . In the actual preparation process, the exothermic curve of differential scanning calorimetry (DSC) analysis of the PAN precursor can be used to set the pre-oxidation temperature of the PAN precursor.
The formation of the PAN ladder structure during pre-oxidation is a complex chemical reaction process. Like ordinary chemical reactions, the cyclization, dehydrogenation and oxidation reactions in the pre-oxidation process have a strong influence of time, and the completion of the reaction requires a certain amount of time, a certain amount of time. Reasonable control of pre-oxidation time is extremely important to improve production efficiency and reduce production costs.
During the pre-oxidation of PAN precursors, the fibers will shrink to a certain extent, which is caused by the deorientation of the precursors and the cyclization reaction of the polymer macromolecular chains under the action of heat, and appropriate stresses will be created. Oxide pre-treatment at a certain tension is a necessary condition for obtaining high-performance carbon fibers, the purpose of which is to prevent the relaxation of polymer molecular chains in order to preserve the orientational structure.
PAN precursor pre-oxidation is usually carried out in air, but air humidity affects its pre-oxidation effect. On the one hand, air moisture can plasticize the precursor, which contributes to the preferential orientation of the fibers during stretching., on the other hand, air moisture can passivate the precursor, hindering the conformational rearrangement of macromolecular segments; therefore, in industrial production, it is necessary to control the atmospheric humidity and keep it stable.
In terms of equipment, wire feeder and preoxidizer are the main equipment. The wire feeder is equipped with a tension control device to ensure stable wire tension, prevent uneven feeding of the original single wire, and improve the quality of the PAN-based carbon fiber production process. According to the design capacity, the large-scale carbon fiber production line usually has 6-8 large-scale pre-oxidation plants to meet the process requirements of various tows.
1.2.4 Obtaining polyacrylonitrile carbon fiber: choose the appropriate method wisely according to the needs in the subsequent steps
The carbonization process takes place in a special carbonization furnace and requires an inert atmosphere. The pre-oxidized wire is gradually heated from 400°C to 1600°C and goes through two stages of low-temperature carbonization (400-1000°C) and high-temperature carbonization (1100-1600°C). During the carbonization process, a certain tension must be applied during drawing so that the crystallization of carbon molecules can be optimized to produce carbon fibers with a carbon content of more than 90%.
This link mainly includes carbonization, high temperature graphitization, surface treatment, sizing and other steps. After the PAN precursor undergoes a pre-oxidation reaction, the formed pre-oxidized fiber has a trapezoidal structure resistant to high temperatures, but at this time, the fiber is still composed of organic substances, the carbon content is low, and heteroatoms such as N, H and O occupy a large number of ratio, the role of the carbonization process is to remove non-carbon elements in the fiber, convert from organic matter to inorganic matter, and finally form PAN-based carbon fiber with a hexagonal network graphite-like structure. This process is usually divided into two stages, namely low-temperature carbonization and high temperature carbonization is carried out under the protection of an inert atmosphere (usually high purity nitrogen) to prevent side reactions with air at high temperatures. Carbonization temperature, carbonization time and carbonization voltage are the main influencing factors.
Graphitization of carbon fibers refers to the process of heat treatment of carbon fibers in an argon atmosphere at a temperature of 2200–3000°C. During the graphitization process, carbon in the disordered structure of the carbon fiber is transformed into a three-dimensional graphite structure.y, which is accompanied by high-temperature pyrolysis. At such a high temperature, nitrogen can react with carbon to form nitrides, so nitrogen cannot be used as a protective gas. The carbon content of carbon fiber obtained by graphitization is more than 99%. Compared with carbon fiber, the density of graphite fiber is increased, the elongation is reduced, the tensile modulus is increased, and the electrical conductivity, thermal stability, and thermal conductivity are further improved. Increasing the heat treatment temperature is useful to improve the degree of order, thickness and area of the graphite structure, which usually shows that the crystal structure of graphite increases along the axial direction of the fiber, the interplanar distance of graphite crystals decreases, and the content of micropores in the fiber increases, the modulus of the fiber increases. The increase in the modulus is mainly associated with an increase in the degree of graphitization of the fiber during high-temperature processing and the formation of a more ordered graphite structure in the fiber.
The purpose of surface treatment is to increase the surface activity of the fiber after treatment, thereby increasing the surface area available for fiber/matrix interfacial bonding, and adding reactive chemical groups to enhance the bond between carbon fiber and matrix.
Sizing is the last process before winding the carbon fiber into a roll after surface treatment to obtain the finished carbon fiber. Its main function is to bind the carbon fibers, which is like glue to hold the carbon fibers together, which improves process performance and makes processing easier. Reduces fiber wastage.
In terms of equipment, the main equipments are carbonization device, graphitization device, surface treatment device and calibration device. The carbonization process is the same as the pre-oxidation process, and often uses multi-stage carbonization and multi-stage traction. At present, two-stage carbonization and two-stage stretching processes are commonly used in China. There are certain differences. between low temperature carbonization furnaces and high temperature carbonization furnaces. The key factors affecting carbon fiber graphitization are graphitization equipment and high temperature heat treatment technology. At present, most of the carbon fiber graphitization equipment commonly used by large manufacturers at home and abroad are high-temperature graphitization resistance tube furnaces. There are many surface treatment methods for carbon fiber, mainly including gas phase oxidation, liquid phase oxidation, electrolytic oxidation, plasma treatment, and coating treatment. Carbon fiber sizing methods mainly include transfer method, dip methodI and the spray method, among which the immersion method is commonly used.
1.3 Other types of matrix carbon fiber: specific areas meet specific needs
1.3.1 Resin-based carbon fiber: compensation for the high-tech area that PAN-based carbon fiber cannot achieve
Pitch based carbon fiber uses petroleum pitch or coal tar pitch as raw material. After pitch refining, spinning, pre-oxidation, carbonization or graphitization, etc., pitch-based carbon fiber has high elastic modulus, excellent thermal conductivity. , and not easy to heat.Carbon fiber resin based with properties such as expansion. In general, pitch-based carbon fibers are divided into general-purpose carbon fibers and high-performance pitch-based carbon fibers, however, general-purpose pitch-based carbon fibers do not require high pretreatment of raw materials, so they are mainly used in civil applications. such as sports and adsorbents. The raw materials for high-performance pitch-based carbon fiber are various, mainly mesophase pitch, and graphite crystals have a high degree of order, so they have very outstanding thermal conductivity and modulus. They are excellent thermal management materials and are used in the aerospace industry. , satellites and radars. and other military areas.
The highest tensile strength of PAN based carbon fibers on the market is 6500MPa, the tensile modulus of the standard type ranges from 230 to 300GPa, and the high modulus type can reach 600GPa. For pitch-based carbon fibers, the modulus can vary from 50 GPa to over 900 GPa. The carbon fiber obtained from the mesophase pitch as the raw material has an axis-oriented graphite layer fiber, and the graphite crystal has extremely high strength and rigidity due to the carbon-carbon double bond located in the direction of the graphite layer (called "a" structure is reflected in the strength and stiffness of the final carbon fiber, in addition to extremely low thermal expansion coefficient and high thermal conductivity, which are also characteristics reflected in the "a" direction.In contrast, isotropic pitch in the "a" direction does not have sufficient crystallinity, so it can only show low modulus and low thermal conductivity.These properties of pitch-based carbon fibers are very different from those of traditional PAN-based carbon fibers.The characteristics of the final pitch fiber can be controlled by controlling the characteristics of the pitch raw material and the spinning process conditions to obtain fiber kna pitch with different characteristics.
Because the coefficient of thermal expansion of pitch-based carbon fiber is negative, the material with zero coefficient of thermal expansion can be easily pImplemented by combining with another matrix. As a new application using high thermal conductivity and negative coefficient of thermal expansion, pitch-based carbon fiber is widely used in components such as satellite dish reflectors and solar panels. Its high thermal conductivity is also widely used in the field of electronic equipment, such as thermal interfaces, high thermal conductivity PCBs, etc.
General purpose carbon fiber is made from isotropic pitch, while high performance carbon fiber is made from mesophase pitch. Its preparation process is slightly different from the production process of PAN-based fibers, mainly including raw material preparation, melt spinning, pre-oxidation, carbonization, graphitization and surface treatment.
The preparation process differs from PAN-based carbon fiber by the pitch modulation process. The chemistry of asphalt is relatively complex, so a number of cleaning and cleaning procedures are required to improve its rheological properties and adjust the molecular weight to meet spinning requirements. Among them, the hydrogenation method of hydrogen donating solvents has problems such as low degree of hydrogenation, inability to efficiently remove heteroatoms, and high cost of hydrogen donating solvents, resulting in extremely low industrialization scale.
Isotropic asphalt preparation methods include vacuum stirring thermocondensation polymerization method, film evaporator method, air oxidation method, vulcanization method, etc. The essence of isotropic asphalt preparation is that dehydrogenation, crosslinking reactions occur in the system during the pyrolysis reaction , condensation and other reactions with the removal of light components from asphalt and the formation of condensate with a high softening point, while the formation of an anisotropic structure is suppressed.
The key to producing high-performance pitch-based carbon fibers lies in the preparation of spinnable mesophase pitch, which requires both high anisotropy and good spinnability. Therefore, it is necessary to choose a suitable mesophase pitch preparation. The essence of the preparation of mesophase pitch is that during the pyrolysis process, a number of reactions occur in the system, such as cracking, dehydrogenation and condensation, with the formation of a nematic liquid crystalline material of an anisotropic structure with a relative molecular weight from 370 to 2000.
Prepared pitch before spinning must be completely filtered and defoamed to remove all impurities and air bubbles, since their presence seriously affects spinning and the mechanical properties of carbon fibers. Pitch spinning can use the melt spinning method commonly used in the general synthetic fiber industry, such as blowing method, extrusion type, centrifugal type, vortex type, etc.
Pressure and spinning speed are also important parameters influencing the spinning process. If the spinning pressure is too low, the melt flow rate is difficult to ensure continuity of spinning, and thread breaks are possible; if the spinning pressure is too high, the resulting fiber diameter will be too thick, which will greatly reduce the physical properties of the fiber and greatly reduce the spinning speed. The diameter and the degree of orientation are affected, the higher the spinning speed, the greater the pull force on the fiber and the higher the degree of orientation. The solidification rate of the pitch melt is very fast, and after solidification, it is difficult to draw again due to the brittleness of the fiber itself, so in order to obtain high performance carbon fibers, it is necessary to control the spinning pressure and spinning speed in the spinning stage to obtain carbon fibers of suitable diameter fiber filaments.
Resin fibers must be carbonized to completely remove non-carbon atoms from them and finally develop the properties inherent in carbon elements; but due to the solubility and viscosity of the pitch, they will be bound together at the beginning of heating, and cannot form a single fiber. Silk carbon fiber, so the carbon fiber pre-oxidation must be done first. In addition, pre-oxidation can also improve the mechanical properties of the pitch fibers and increase the tensile strength before carbonization. During the oxidation process, pitch fiber undergoes significant chemical and physical changes, among which the most important change is the formation of cross-links between molecules, which makes the fiber insoluble and non-melting. At present, there are two kinds of pre-oxidation methods: gas phase method and liquid phase method. Gas phase method oxidizers generally use air, NO2, SO3, ozone and oxygen enriched gas, etc.; liquid-phase method oxidizers use nitric acid, sulfuric acid, potassium permanganate and hydrogen peroxide, etc. solution.
Unmelted pitch should be sent to an inert atmosphere for carbonization or graphitization to improve the final mechanical properties. Carbonization refers to processing at a temperature of about 1200°C, while graphitization is carried out at a temperature close to 3000°C. During carbonization, polycondensation occurs between single molecules, accompanied by dehydrogenation, demethanation and dehydration reactions. Due to the continuous removal of non-carbon atoms, the carbon content of carbonized fiber can reach more than 92%, and the inherent characteristics of carbon increase the tensile strength and elastic modulus of silk. With the expansion of the use of carbon fiber, for example, its assembly into lithium-ion batteries and supercapacitors, the requirements for its properties are increasing, so further graphitization and a further increase in carbon content become necessary.
1.3.2 Viscose-based carbon fiber: a specialty fiber for aerospace and advanced military technology
Viscose-based carbon fiber refers to carbon fiber obtained by pre-oxidizing and carbonizing viscose fiber. Viscose fiber is a regenerated cellulose fiber, also known as rayon, which is made from natural fibers such as wood, hemp, and cotton through pulping, sulphurization, aging, and spinning. Compared with polyacrylonitrile (PAN) based carbon fiber and pitch based carbon fiber, viscose based carbon fiber has unique properties. Viscose-based carbon fibers have a lower density and allow the production of lighter components. Viscose-based carbon fiber has a low degree of graphitization and low thermal conductivity, making it an ideal thermal insulation material. Viscose-based carbon fiber has a low content of alkali metal and alkaline earth metal, so the sodium light generated by combustion during flight is weak and difficult to detect with radar. Viscose-based carbon fiber has good biocompatibility and can be used to make medical biomaterials such as medical motors, ligaments, bone splints, and artificial bones.
However, viscose-based carbon fibers also have some disadvantages. The most important of these is that in a real production process, the actual production of L-glucose and other by-products is often difficult to control due to the fact that the actual carbon yield is low and the strength of the carbon fiber is not ideal. Therefore, this material is currently only used in aerospace non-structural parts and civil applications. The University of Alabama in the United States has developed a new method for preparing green carbon fiber material that can be used to ablate and thermally insulate rocket nozzles, and the method has been patented. This new green ion processing process was developed at the Reliability and Failure Analysis Laboratory. For the manufacture of solid rocket nozzles, carbon fiber rayon-based carbonized laminated fabrics can be coated with pitch and mandrel winding, after heat treatment, the pitch on the surface can be converted into solid carbon, and finally carbon fiber nozzle. you can get reinforced composites. The amount of carbon fiber used in the same type of solid-propellant engine for rocket boosters can reach 35 tons.
The transformation of viscose fiber into carbon fiber is a very complex physical and chemical reaction, and the process of its dehydration and thermal cracking can be divided into four stages:
First stage (25-150°C): basically remove physically adsorbed water. Moisture, fiviscose fiber is about 10%-14%, and the removal of this moisture at low temperature contributes to the removal of structural water at high temperature.
Second stage (150-240°C): mainly dehydration within the molecular structure with the formation of chain segments (pieces) containing hydroxyl, ketone, enol or carboxyl groups.
Third stage (240-400°C): This is a violent reaction zone, in which two competing reactions mainly take place. First, the 1,4-glucose bond is thermally cleaved to form an anhydrous ring, and the 1,6-unit is dehydrated to form L-glucose, which at higher temperature turns into a resin. the cellulose ring undergoes further dehydration. Dehydrated cellulose is formed, and the C-C bond and the C-O bond with poor thermal stability in the ring are thermally cleaved to form a residual chain of four carbon atoms.
Fourth stage (400-700°С): Aromatization of carbon is carried out by four residual bonds, polycondensation into graphite sheets of six carbon atoms. At temperatures above 700°C, the polycondensation layer rapidly increases, and the arrangement gradually becomes ordered, transforming into a turbostratic graphite structure.
Enhanced removal of hydroxyl groups is the key to structural transformation. The chemical structure of cellulose contains three hydroxyl groups, an oxygen atom in the oxygen heterocycle, and the oxygen content in the glycosidic bond reaches 49.39%. During the actual conversion, the non-dehydrated portion is converted to L-glucose and tar, and the volatile products of small molecules such as CO, CO2, HCOOH, CH4, etc., which are thermally cracked and released, also take up carbon. atoms. In particular, the formation of L-glucose and resin not only reduces the yield of carbon, but also causes serious contamination of the fibers, which leads to sticking between individual fibers, hardening, brittleness and breakage of the threads after carbonization. Therefore, how to effectively suppress the production of L-glucose is the technical key.
The key to suppressing L-glucose production is adequate dehydration. For complete dehydration, an effective method is to use a catalytic dehydrating agent, which is also widely used in the production of rayon-based carbon fibers. The function of introducing the catalytic dehydrating agent is mainly to reduce the heat generation of pyrolysis and bring the structural dehydration and pyrolysis reaction to low temperatures, thereby facilitating the pyrolysis and dehydration reaction and making it easier to control the parameters of the production process. In this sense, the introduction of a catalytic dehydrating agent for viscose fiber is analogous to the introduction of a copolymerization initiating monomer (such as itaconic acid, etc.) of PAN resin.
Compared to the PAN-based carbon fiber manufacturing process, the production process of viscose fiber for carbon fiber production includes two steps, namely water washing and catalytic impregnation. Undoubtedly, this will increase the cost of production of carbon fiber based on viscose. Viscose fiber can be converted to carbon fiber through pre-oxidation and carbonization processes after washing and catalytic impregnation. Catalytic impregnation and oxidation pre-treatment are important processes in the production of viscose-based carbon fibers and are the key to converting organic viscose fibers into inorganic carbon fibers.
Catalytic impregnation. Catalytic impregnation is mainly for impregnation with a catalytic dehydrating agent. The amount of impregnation with the inorganic catalytic dehydrating agent is usually 20%-62%, and the amount of impregnation with the organic catalytic dehydrating agent is usually 2%-20%.
Pre-oxidation process. The pre-oxidation process is mainly to carry out dehydration, thermal cracking and structural transformation under the action of a catalyst, so that the white viscose fiber turns into a black pre-oxidized fiber and has fire resistance. The temperature distribution gradient of the preoxidizer furnace is very important and must be determined from the TG and DTG spectra of the fibers. The analysis and test data show that the tensile strength drops by 74%-84% after the viscose fiber turns into pre-oxidized silk, and the oxygen content drops to 40%-50%. This indicates a fundamental change in structure and performance.
Reactions in the low-temperature carbonization process are mainly deep dehydration, thermal cracking and aromatization. At this time, quite a lot of waste gases and tar are generated. Therefore, an important design parameter is the method of instantaneous removal of exhaust gases and resin.
The off-gas and tar produced by the high-temperature carbonization process are much smaller. The main components of the off-gas are polycondensation products and flavored low molecular weight volatile substances such as H2, CH4, C2H6, C2H4, CO, CO2, etc. .2 Application side: the ultimate cost-effectiveness is the eternal goal
2.1 In terms of performance. Complementing with other materials to achieve additional performance is a general trend
Composite materials are materials with new properties, consisting of two or more materials with different properties, using physical or chemical methods. Different materials learn from each other in terms of performance and produce synergistic effects, so that the comprehensive performance of the composite material is better than that of the original constituent materials to meet different requirements. MatrixComposite materials are divided into two categories: metallic and non-metallic. Commonly used metal substrates are aluminum, magnesium, copper, titanium and their alloys. Non-metal substrates mainly include synthetic resin, rubber, ceramics, graphite, carbon, etc. Reinforcing materials mainly include glass fiber, carbon fiber, boron fiber, aramid fiber, silicon carbide fiber, asbestos fiber, filament, metal wire and solid particles. Among composite materials, fiber-reinforced materials are the most widely used. It is characterized by low specific gravity, high specific strength and specific modulus. Carbon fiber composite materials are composed of two or more materials with different properties that are macroscopically composed of materials with new properties through physical or chemical methods. The carbon fiber composite material is a two-phase composite material obtained from organic fibers, which has excellent mechanical properties, and also has the original characteristics of carbon materials, and is a new type of reinforcing fiber.
2.1.1 Carbon fiber + polymer: the best substitute for metal materials at medium and high temperatures
Carbon fiber composite materials based on various polymer materials are collectively referred to as carbon fiber polymer matrix composite materials, and may also be referred to as carbon fiber resin matrix composite materials. According to the properties of resin matrix materials, composite materials can be divided into general resin composite materials, heat-resistant resin composite materials, chemical corrosion-resistant resin composite materials, and flame-retardant resin composite materials. materials. According to the structural form, polymeric materials can be divided into thermoset resin composite materials, thermoplastic resin composite materials, and rubber-based composite materials. High specific strength, high specific modulus. The specific strength of high strength high modulus carbon fiber/epoxy resin is 5 times that of steel and 4 times that of aluminum alloy. Good resistance to fatigue. The endurance limit of polymer matrix composites can reach 70-80% of their tensile strength. Good ablation resistance. The resin composite material interface has good damping effect and high shock absorption capacity. Good processing technology. Resin-based composites can be made by various molding methods, and the manufacturing process is relatively simple.
The preparation and molding process of polymer matrix composites have excellent performance compared to other materials. Molding compositionResin-based materials and product molding are completed at the same time. The material preparation process is also the production process of its products, so that large-scale products can be formed entirely at one time. , thereby simplifying the product structure and reducing the number of components and connections The number of parts, thus reducing product quality and reducing process consumption. Secondly, since the resin has a certain fluidity before curing, and the fiber is very soft, it is easy to mold the desired shape and size with a mold, so molding the resin composite is more convenient and one-time. Production of piece and small-scale production is possible. The production of resin matrix composites typically includes prepreg production, molding, post-processing, and part machining. Prepreg is a resin matrix (thermoset or thermoplastic) impregnated with continuous fibers or fabrics under strictly controlled conditions to obtain a resin matrix composition and reinforcement, is an intermediate material for the production of composite materials and can be widely used in hand laying. upward molding, automatic In the process of preparing composite materials such as layering or winding molding. Currently, most of the carbon fibers in the world are used in the form of prepregs, and the mechanical and chemical properties of composite products are largely dependent on the quality of the prepregs. However, prepregs are usually stored at low temperatures to ensure proper viscosity, styling and gel time in use. As for the molding process, hand molding, autoclave molding, compression molding, winding molding, pultrusion molding and resin transfer molding are more commonly used.
From the point of view of processing, due to the nature of the structure, carbon fiber reinforced resin matrix composites are usually directly molded into the shape of the product, but in many cases, the molded product cannot meet the requirements of precision or assembly, so it needs to be processed for secondary processing. At present, there are many mature methods for processing CFRP, and the most widely used method is mechanical processing. After a long period of development, CFRP machining already has mature processing technology and special processing equipment, which can basically meet the requirements of use. However, some shortcomings have been found: the structure of the material itself will be destroyed during processing, resulting in fiber breakage, accompanied by thermal damage in cutting. , and strong tool wear increases the cost of production. Gradually, ultrasonic vibration processing, electrical discharge processing, water jet processing and laser processing have been gradually developed and used.
2.1.2Carbon fiber + metal: can effectively improve the high temperature performance of metal materials
Composite material made from metal, alloy and intermetallic compound as matrix, carbon fiber as reinforcement, and made by impregnation and consolidation process is called carbon fiber metal matrix composite material. The matrix typically occupies 50-70% of the volume in continuous fiber reinforced metal matrix composites and over 70% of the volume in short fiber reinforced metal matrix composites. Typical carbon fiber reinforced metal composite materials are: carbon fiber silver composite, carbon fiber copper composite, carbon fiber lead composite, carbon fiber aluminum composite.
Resin matrix composites can only be used in various temperature ranges below 350°C, while metal matrix composites are suitable for temperature ranges from 350°C to 1200°C in between. The operating temperature of aluminum, magnesium and their alloys is below 450°C, titanium alloys - 450-650°C, intermetallic compounds, heat-resistant alloys based on nickel and iron - 450-650°C. between 650-1200°C. Methods for producing carbon fiber/metal matrix composites fall into two categories: the solid state method and the liquid state method, but both methods require surface treatment of the fibers or preparation of prefabricated fiber/matrix filaments. Fabrication of fiber-reinforced metal matrix composites is difficult, and many factors must be considered, such as the location of reinforcing materials, interface response, economy, and residual stress. For smooth final forming and manufacturing of fiber reinforced metal matrix composite products with excellent properties such as quality and strength, there are approximately four forming methods, namely: direct molding with fiber weaving, molding with fiber surface treatment, and fiber transformation. into prefabricated belts Composite molding with metal matrix, fiber and matrix to make a composite prefabricated belt, and then molding.
The surface properties of carbon fiber depend on the size of its roughness, the type and number of active functional groups, the size of the microcrystalline structure, and the strength of the acid-base interaction. Judging by the surface morphology of carbon fiber, there are many micropores, foreign matter, crystals, etc. on the surface, which are important factors affecting the bonding characteristics between the matrix and the reinforcement in the composite material. Raw carbon fiber has a smooth surface, high chemical inertness, low surface energy, very few active functionalgroups (polar functional groups such as hydroxyl and carbonyl groups) and poor matrix wettability in the composite, which reduces the interfacial bonding capacity of the composite and containment reinforcement. Maximum fiber performance. After surface treatment, the mechanical properties of carbon fiber reinforced materials (such as interlayer shear strength and interfacial shear strength) will be improved. At present, the following surface treatment methods are commonly used: gas phase oxidation (air, ozone, inert gas), liquid phase oxidation method (concentrated nitric acid, mixed acid and strong oxidizing agent), anodic oxidation method (electrochemical oxidation), plasma oxidation method (plasma oxygen, plasma ammonia), surface coating modification method, and the use of two or more methods Composite surface Carbon surface treatment method fibers.
Solid method means that the base metal is mostly in the solid state. Due to the low manufacturing temperature of the solid state method, the reaction at the interface between the metal matrix and the reinforcement is negligible. Commonly used methods include powder alloy, solid state hot pressing, hot isostatic pressing, rolling, hot extrusion, hot drawing, and explosion welding.
The fluid method refers to a manufacturing method in which the base metal is essentially in a molten state and is bonded to a solid rebar. The production temperature of the liquid method is high, and process parameters such as impregnation temperature, liquid matrix contact time and solid reinforcement must be strictly controlled during the production process. Liquid methods include vacuum pressure impregnation, injection molding, stir casting, liquid metal impregnation, co-spraying, thermal spraying, etc.
The main technical challenge in the production of carbon fiber-reinforced metal matrix composites is interfacial bonding. It is difficult to form a stable and effective interfacial connection, which leads to a decrease in the overall performance of composite materials and limits the popularization and application of carbon. fiber/metal matrix composites.
The interface types of fiber-reinforced metal matrix composites can be divided into five categories: Simple mechanical connection: no chemical reaction occurs between the matrix and the reinforcement, such as copper/tungsten wire, copper/carbon fiber, aluminum/silicon carbide, etc. .d. Purely mechanical connection. This is achieved solely by the friction created by the surface roughness. Combination of dissolution and infiltration: liquid metal penetrates the reinforcing fiber, diffuses, dissolves and combines, forming a fang-like dissolution-diffusion interface. For example, nickel/ugice fiber, etc. However, there is often an oxide film on the surface of the fiber, which often needs to be modified on the surface of the fiber to dissolve and increase the bonding strength. Reactive combination: that is, a chemical reaction occurs to form compounds at the interface. This type of interfacial tie layer is often non-compounded and is typically submicron thick. Combination of exchange reactions: the fibers chemically react with the metal matrix to form compounds, and the exchange of elements occurs through diffusion. Mixed Combination: The combination of fiber-reinforced metal matrix composites is often not just one combination method, but several of the four above.
2.1.3 Carbon fiber + ceramics: ideal high temperature construction materials and friction materials
Carbon fiber ceramic matrix composites combine the thermal and chemical resistance of monolithic ceramics with the mechanical strength of carbon materials. Inorganic materials such as ceramics have many outstanding advantages in terms of heat resistance, oxidation resistance, wear resistance, corrosion resistance, electrical properties, etc., but their resistance to mechanical and thermal shock is low, and "brittleness" is a fatal weakness. Composite materials composed of carbon fiber and ceramics can greatly improve the fracture work and heat resistance, and improve the brittleness of ceramics. Ceramic protects the carbon fiber from oxidation at high temperature, so it has high thermal stability and modulus of elasticity. For example, silicon nitride ceramics reinforced with carbon fiber can be used for a long time at a temperature of 1400°C, and also used as turbine blades for jet aircraft.
According to the different arrangement of fibers in the ceramic matrix, it can be divided into composite materials reinforced with fibers in one direction and composite materials reinforced with fibers in different directions. Long-fiber reinforced unidirectional composites are anisotropic and generally have good thermal stability; therefore, they have broad prospects for application in spacecraft fuel components. Multidirectional long fiber reinforced composites have high performance in various dimensions and can be used to fabricate flat plate elements or shell elements with large radii of curvature. Composite materials reinforced with short fibers have a certain anisotropy, which makes it possible to create high-performance composite materials.
The production of ceramic matrix composites is usually divided into two stages: the reinforcing material is mixed with the unconsolidated matrix material and the matrix is consolidated. Typically, the slurry impregnation technique is used, followed byby hot pressing and sintering. The main production methods for continuous fiber-reinforced ceramic matrix composites include suspension impregnation and hot pressing sintering, in-situ chemical reaction method, direct oxidation method, precursor pyrolysis method, hot-dip method, and reaction sintering method. The production process of short fiber reinforced ceramic matrix composites can include: dispersion of short fibers → mixing of fibers and matrix materials → molding → sintering → products. Molding methods often use pressure impregnation, reaction sintering, hot isobaric sintering, microwave sintering, etc. Carbon fiber ceramic matrix composites have two main disadvantages. Firstly, the thermal stability of most polycrystalline ceramic fibers is too low for use at temperatures above 1200 °C, and secondly, the low oxidation resistance of carbon fiber ceramic matrix composites made from carbon fibers or carbon interfacial layers. Regarding the first disadvantage, most of them are improved by die type and process, while the second disadvantage can be improved by carbon fiber modification, matrix anti-oxidation, boundary layer anti-oxidation and cutting. surface coatings.
2.1.4 Carbon fiber + carbon material: structural-functional integrated material at ultrahigh temperatures
C/C composite material is a kind of composite material with carbon fiber as the reinforcing skeleton and carbon as the matrix, prepared by liquid phase method or gas phase method. It not only has the characteristics of excellent abrasion resistance and high temperature resistance of carbon materials, but also has strong fiber-reinforced composite material design capabilities, excellent mechanical, thermophysical properties and thermal shock resistance, and is the only one that can maintain high mechanical properties. above 2500 ° C. Execution material. Since its first discovery in 1958, C/C composite materials have been considered the preferred high temperature thermal barrier materials in the aerospace field due to their outstanding all-round performance. They are widely used in strategic missile warheads, engine nozzles and spacecraft. shuttle nose Cone/leading edge, hypersonic nose cone and other key components of the hot end.
The preparation of C/C composites is divided into steps such as carbon fiber selection, preform molding, densification and graphitization.
Regarding the choice of carbon fiber, the appropriate CF should be selected according to the operating conditions and the requirements for the use of the material, and a reasonablebody space prefabricated for the manufacture of C/C composite materials. Different types of carbon fibers and different structures of prefabricated bodies (fiber bundle orientation, fiber volume content, etc.), material characteristics have significant differences. Therefore, it should be ensured that the selected carbon fiber can meet its specific requirements. If different requirements are to be met, different types of carbon fiber bundles can also be used in cross-blending.
The molding process of the prefabricated body should be determined according to the requirements for the use of the product. The most common spinning processes include: short fiber spinning, fabric lamination, 3D weaving, Novoltex technology, etc.
Condensation consists in filling the pores inside the preform with matrix carbon. Common methods include chemical vapor deposition and high pressure liquid phase. Among them, the high pressure liquid-phase method has received much attention because it can shorten the compaction cycle from months to hours.
C/C composites begin to oxidize at temperatures above 400°C, which seriously limits their long-term reliable use in high temperature aerobic environments. Currently widely used methods include matrix modification technology and anti-oxidation coating technology. Matrix modification technology, adding antioxidant components inside the C/C composite material, and using the "oxygen absorption + oxygen barrier" thermal protection mechanism can greatly improve the oxidation resistance and ablation resistance of the C/C composite material. Anti-oxidation coating technology is to form an effective protective coating against oxidation on the outer surface of the C/C composite material in order to isolate ambient oxygen from the material and avoid deterioration of the mechanical properties and change in the shape of the material due to oxidation. ablation. Due to the simplicity of the process and the short cycle, the anti-oxidation protection effect, the level of heat resistance and the self-healing ability of the anti-oxidation coating determine the temperature level and environment in which the C/C composite material can be used.
2.2 Price Perspective: Innovation in Process Equipment and Reducing Processing Costs is a Main Theme
2.2.1 Costs: Process equipment and scales are the basis of cost control
In the production process of carbon fiber products, according to the production process, according to the industry information network, the raw silk preparation cost is the highest, reaching 51%, followed by carbonization, which accounts for about 23%. In terms of cost elements, the cost of raw materials and fuel account for 30% each.
In the process of preparing carbon fiber composite materials, the price of products at different stages meansflax increases. Below 1,000 RMB/kg, about 3,000 RMB/kg for automotive composites, and about 8,000 RMB/kg for aircraft composites, each level of deep processing has significant added value. In terms of reports from local listed companies, take Jilin Carbon Valley, which is mainly engaged in carbon fiber precursors, as an example. Jilin Carbon Valley pointed out in the prospectus that direct materials account for the largest share in the production process of the company's main carbon fiber precursor product, and the main materials are acrylonitrile and oil agents. The cost of acrylonitrile and oil agents accounts for about 90% of the total material cost.
An example is Zhongfu Shenying, which is mainly engaged in carbon fiber. Zhongfu Shenying pointed out in the prospectus that the production of carbon fiber requires more raw materials and energy consumption, and has the characteristics of large area and high equipment cost, resulting in high wear and tear costs. Therefore, the main commercial costs are mainly related to the direct composition of materials and production costs.
An example of this is Hengshen, a company that manufactures carbon fiber composite materials. Hengshen pointed out in the prospectus that when analyzing the cost results of carbon fiber composite materials, the largest cost space is in the design and production process of composite structural parts.
From the point of view of each link, the main cost items are the price of acrylonitrile, fiber production, product design and manufacturing, and energy consumption. With regard to acrylonitrile, according to the prospectus of Hengshen Co., Ltd., acrylonitrile is a petrochemical product and its purchase price is affected by fluctuations in world oil prices. Oil is a mass commodity, and many factors influence its price, such as the relationship between supply and demand, the economic and political situation in the world, and so on. Therefore, it is difficult for carbon fiber enterprises to control the cost of acrylonitrile.
In terms of production costs, according to Zhongfu Shenying prospectus, the main reason for high production costs is that the equipment required for carbon fiber production is expensive and the depreciation amount is large. As a rule, the cost is reduced. as far as possible through process improvement and economies of scale. From the point of view of economies of scale, the production of 1 kg of carbon fiber requires the consumption of 2.2 kg of PAN precursor, that is, the production of 500 tons of carbon fiber requires a production capacity of 1100 tons of PAN precursor. In the process of production and preparation of carbon fiber, even if the scale is small, its auxiliary engineering and production equipmentThe analysis must be complete, so the advantage of scale is extremely noticeable in the production and preparation of carbon fiber.
From the point of view of product design and production, the structural molding of carbon fiber composite materials and the molding of materials are carried out at the same time, and the performance of subsequent products is largely affected by the matching of fibers and resins, as well as processability. . The development of subsequent products should form a process of benign interaction with carbon fiber research and development. Find and solve problems in the production → application → improvement → application expansion → improvement → stabilization cycle, from instability to gradual stabilization. Costs are generally reduced through process improvements.
In terms of energy consumption, the production process requires high-temperature heating, which consumes a lot of fuel and electricity. Given that natural gas, electricity and steam prices are regulated by the government, prices are relatively stable. Therefore, carbon fiber companies typically reduce costs by improving processes.
2.2.2 Fiber end: a thorough understanding of process equipment and power reduction is the main entry point
From the point of view of technological equipment, there is no absolute difference between good and bad processes. Only by fully understanding the principles of the process and equipment can we control the price initiative. There are various methods for preparing carbon fiber precursor and producing carbon fiber, and each process has its own advantages and disadvantages. The key becomes how to intelligently avoid the disadvantages of processes and equipment. Taking the recently registered Zhongfu Shenying as an example, in terms of technology, the prospectus reveals that "the company's key technology and the application of dry inkjet wet spinning of 1000t high strength / 100t medium shaped carbon fiber" won the 2017 National Science and Technology Progress Award .First Prize is the only enterprise in the field of domestic carbon fiber to win this award, which has made the company a leader in the field of domestic carbon fiber.
In terms of equipment, the prospectus discloses that, for reasons of protecting the company's core interests and other factors, the company should not purchase key equipment, such as polymerization boilers and spinning machines, from other parties that have the core technology. At the same time, Jiangsu Yingyou Devices' carbon fiber machines are only sold to companies.
Looking back at global companies, it can be seen from patent applications that carbon fiber companies have always focused on innovation in process equipment. Since 1980 foreign techThe science of carbon fiber entered a period of development, with more than 60 patent applications filed annually, and 341 in 1989. In the 1990s, Japan was in an economic crisis and the United States was in an economic crisis. in this field, the number showed a downward trend, after 2000, foreign carbon fiber technology gradually resumed development, and gradually entered the stage of rapid development. China started to create a patent system in 1985, so there were no patent applications until 1985; From 1985 to 2000, carbon fiber patent technology in my country was in its infancy, and the number of patent applications per year did not exceed 5. After 2000, the number of carbon fiber technology patent applications worldwide began to show an upward trend, especially after 2010, the number of patent applications in my country increased rapidly, reaching more than 300 patent applications per year, indicating that my country's carbon fiber technology has entered a period of rapid development.
Foreign companies take Mitsubishi Rayon as an example. Mitsubishi Rayon's patent applications since 2007 include all aspects of the preparation process, among which the quality of raw silk and carbon fiber make up a very large share; carbon fiber equipment includes spinning/wire drawing equipment, heat treatment equipment (carbonization furnace, graphitization furnace), pre-oxidation furnace, packaging equipment, etc. are the key elements to realize carbon fiber production and further improve the quality of carbon fibers. to the spinning ability of the spinning solution For example, comonomer modification, selection of initiators in the polymerization system, mixing modification, selection of polymerization process parameters, molecular weight control/PDI of polyacrylonitrile products, selection of polymerization solvents and specific polymerization processes are gradually becoming mature. , but there are still some patents related to the improvement of related processes.
In terms of energy consumption, the pre-oxidation time is more than 90% of the total production time. Therefore, the pre-oxidation process is the key to controlling yield and production costs. Therefore, from the point of view of quality assurance, how to shorten the pre-oxidation time is the current problem. In the process of obtaining carbon fiber based on polyacrylonitrile, pre-oxidation is one of the key steps in determining the performance of carbon fiber. Temperature and time are the two most important parameters in the pre-oxidation process. The effect of temperature and time must be comprehensively considered during process design. Pre-oxidation not only controls the yield of carbon fibers, but also controls the quality of carbon fibers.
In terms of technology, how to shorten the pre-oxidation time and reduce the formation of the shell-core structure is of great importance to reduce the cost of carbon fiber production and improve performance.
In terms of equipment, most of the pre-oxidation processing equipment in industrial production is a hot air circulation pre-oxidation furnace. The hot air circulation system can reuse hot air to reduce heat energy loss. It is known that in order to prevent fusion between individual fibers during pre-oxidation treatment, the method of applying an oil agent to the tow of precursor fibers is used, but the oil agent is volatile during the pre-oxidation heat treatment, and the resulting impurities remain in the hot air circulation system for a long time. The core is difficult to remove and sticks easily to the fibrous filaments, resulting in fluffing and breaking of individual fibers in the subsequent carbonization process. If the pre-oxidation furnace is not cleaned in time, the perforated plate used to straighten the air outlet wind speed may be blocked, preventing hot air from circulating, causing yarn breakage in mild cases and fire in severe cases. But to clean the pre-oxidation furnace, it is necessary to stop the operation of the equipment, which greatly affects the production efficiency, and the cleaning cost is huge.
2.2.3 End product: integration of new technologies, design and production can be expected in the future
The current development direction of low-cost composite materials technologies mainly includes out-of-autoclave forming process (Out-of autoclave, abbreviated as OOA), wet forming process, fast curing forming technology, low temperature curing forming technology and integrated design and manufacturing technology, among which are fast curing molding technology is currently being developed to effectively reduce costs and increase efficiency. role, significance of an engineering application. It is estimated that the production cost of composite materials is more than 50% of the total cost. The main reasons for the high cost are expensive raw materials, long molding cycles, high energy consumption, low production efficiency, and expensive processing accessories. According to the article "Application of low-cost non-autoclave technology in aircraft composite structures", for advanced autoclave-cured composite structure, the material cost is about 15% of the total cost, and the design cost is only 10% of the total cost of 5%, and most of the remaining cost is are manufacturing costs such as composite laying, curing, trimming and assembly, and manufacturing costs are usually70-80% of the total cost of composite parts. Statistics from American aircraft manufacturers show that, in terms of cost of composite products, laying accounts for 25%, assembly - 45%, curing - 10%. part closely related to work.
In terms of out-of-autoclave (OOA) technology, low-cost out-of-autoclave technology has become a major challenge in the field of composite materials. Among many non-autoclave molding technologies, the laying and cladding process of the non-autoclave prepreg technology is similar to that of the autoclave process, but the cure site is transferred to a cheaper and smaller process. or curing oven, the advantages of the autoclave molding process are mainly inherited, and it is believed that non-autoclave molding technology is likely to be implemented on a large scale.
Compared to autoclave resins, OOA resins should have a lower volatile content and more precise and controlled rheological behavior. Therefore, in the resin preparation process, try to use vacuum conditions in order to prepare as much as possible to remove air, water vapor and volatiles mixed in the resin system. If this must be maintained, pay attention to sealing.
OA-prepregs must impose increased requirements on the degree of impregnation and shelf life at room temperature. In addition, OOA molding technology overcomes the autoclave size limit and is suitable for manufacturing large and extra-large structural parts. Considering the production period, prepregs should have a longer storage period at room temperature. Currently, OOA basic prepregs are stored at room temperature. , not less than 30 days.
The steps of the vacuum forming process are basically the same as the autoclave process, but it is necessary to minimize the inclusion and sealing of gas when laying the prepreg. Cover several layers, that is, vacuum and seal for a certain period of time and remove as much as possible more gas trapped in the layer. However, the pre-compaction process is not a necessary step, and a short-term constant temperature at a lower temperature can be used to replace the long pre-compaction process.
Liquid Composite Molding (LCM) consists of injecting liquid resin (or a heated and melted film of resin) into a fiber preform laid on a mold, and the resin completes the infiltration of the fiber and solidifies as it flows. To become a finished product, the three processes of prepreg processing, prepreg low temperature storage and expensive autoclave can be eliminated. The LCM process mainly includes three categories: Resin Transfer Molded (RTM), Vacuum Assisted Resin Infusion (VARI) and Resin Film Infusion (RFI). RTM characteriterizable using closed molds, VARI is characterized by only one-sided open mold and only vacuum bag pressure is required, RFI is characterized by the fact that the resin is laid as a film between the layers of the intermediate body, the open mold is only on one side, only vacuum bag pressure is required (some of them also cured in an autoclave).
The wet molding process has also been applied to aerospace structures. For example, Lockheed Martin (Lockheed Martin) from the United States used a liquid molding process to manufacture the design of the UGM-133A Trident-2 ballistic missile, making the original 61 parts reduced to 1, which greatly reduces the cost of production; Hercules uses mixed fiberglass and carbon fiber blanks to make rocket wings and other parts, and its cost is only about 30% of the cost of the continuous fiber winding process. The Institute of Aerospace Materials and Technology of my country has applied this process to the processing of hats, cylinders, cable casings, fairing and other components, and has also used the VARI process to make large-sized launch boxes for storing and transporting composite materials. that meet the requirements.
In terms of fast curing molding technology, the key technology of fast curing molding is that the resin system has a certain storage stability at room temperature. When it rises to a certain temperature, it quickly reacts and solidifies. The higher the temperature, the faster the curing speed. Less than 20 minutes, the fastest about 2 minutes to achieve curing. The use of fast curing composite technology has led to a consensus to reduce costs and increase efficiency. Several major composite manufacturers in the world have developed their own fast curing prepreg systems, mainly Hexcel's M77 in the USA. Cure at 150°C for 2 minutes, VelinoxTM100 from Gurit can be cured at 130°C for 10 minutes, and P3831C from Japan's Toray can be cured at 150°C for 10 minutes. On May 10, 2021, the Japanese Mitsubishi Chemical Corporation announced the development of 1960s quick-curing carbon fiber prepregs and officially began sales.
Given that prepregs need to be refrigerated for shipping, latent, fast curing epoxy based systems are gradually becoming a trend. There are also many types of epoxy resin hardeners. According to the storage stability at normal temperature and normal condition after mixing with the resin, they can be divided into two types: conventional hardeners and latent hardeners. Common hardeners mainly include imidazoles, mercaptans, aliphatic polyamines and alicyclic polymers.oliamines. It can cure epoxy at room temperature, so it must be stored separately from epoxy before use, and it is very prone to uneven mixing due to poor fluidity during cooking. The hidden hardener can effectively solve the above problems. The latent hardener remains inert in the reaction with the epoxy resin system under normal temperature and normal conditions. Under certain conditions, the reaction of the epoxy group will be launched on a large scale. Therefore, the mixed epoxy resin system has a long storage time expect.
In terms of product application, fast curing prepregs are mainly used in the fields of automobile, sports, general aviation, etc., especially in the automotive industry. Fast curing prepregs/composites are rapidly spreading. For example, HEXCEL fast curing composite material system is widely used in automobile and sporting goods industry, GURIT quick curing prepreg/composite material is applied to car hoods, front and rear bumpers, etc. Japanese Mitsubishi Rayon Company Carbon Fiber Prepreg R02 is used to make the hood of the Subaru Impreza, which after use is 63% lighter than the original steel hood, and the weight is reduced by 9.2 kg; Inner fast curing carbon fiber prepreg for the production of LEA sports car. Other companies, such as Mitsubishi, who have mastered the fast cure prepreg technology, have also applied the fast cure prepreg technology to automobiles as an important research and development component, fully understanding that this technology will play an important role in composite materials in the automotive and other industries. industrial areas. The role is actively promoted and applied.
In China, AVIC Composites Co., Ltd.'s fast-curing ACTECH® 1201 composite material has been tested and applied in the main load-bearing structures of electric buses. It has cooperated with many domestic carbon fiber manufacturers and automobile manufacturers to jointly promote carbon fiber composites. Application of materials in the field of auto parts. Fast curing prepregs have been evaluated and applied in automotive and civil aviation, such as electric vehicle batteries, new energy vehicle hoods, and drone propellers, and have been praised by users: “ACTECH 1201 series prepregs can achieve cure quickly. 130°C-150°C for 3-15 minutes The prepreg has good styling properties, long shelf life at room temperature and good heat resistance (Tg≥140°C). minutes. Cycle fmolding can be further reduced by modifying the mold and curing process to meet the needs of high-volume and fast-growing production.” - molding technology at temperature, most of today's high-performance composites based on epoxy resin are made using In the form of a prepreg, it is molded at medium and high temperature curing (120-180 ° C). Higher curing temperature not only increases the internal stress of the composite material, affects the dimensional accuracy of the part, but also increases the manufacturing cost, even if it is made in small batches, the cost is 60%-70%. them, high power consumption, long molding cycle times, heat-resistant tools and accessories all increase production costs. Medium and low temperature curing is usually carried out at 70-90°C, and non-autoclave curing can be used to effectively reduce curing stress and significantly reduce the cost of manufacturing composite materials.
Low temperature curing composite materials have the advantages of low curing temperature, small internal stress generated during curing, and low preparation cost, and the mechanical properties and heat resistance of composite materials can be improved with a free high temperature post-treatment process after stripping is one of the important directions in the development of low-cost technology for the manufacture of composite materials. Conventional low temperature composite materials are mainly made by wet winding processes, RTM process and vacuum infusion process, and the resin system used has a shelf life of several hours. It is rare to prepare low temperature curable prepreg composite materials, mainly because the service life of ordinary low temperature curing resin is only a few hours, and it will be cured shortly after being converted into prepreg, which cannot meet certain requirements of the time requirements for prepreg, cutting, laying and other processes where components are made from prepreg. Therefore, low temperature cure prepregs must resolve the conflict between low temperature cure and pot life at room temperature, that is, the resin system must meet the requirements of low temperature cure and long shelf life at room temperature. , it must meet the requirements of the prepreg preparation process, that is, the resin film can be prepared at low temperature, and the film and fiber prepreg can be made. Therefore, there are very few low temperature hot melt prepregs on the market.
In terms of the integrated rapid curing process in the production of materials, in order to adapt to the charactereristics and needs of the automotive industry, it is necessary to implement inexpensive rapid prototyping of composite materials. In order to reduce material and production costs, the production of automotive composite parts needs to realize the comprehensive automation of material production and low-cost production.
The compression molding process, the high pressure RTM process (HP-RTM) and other processes are currently the most widely used. In terms of resin, fast curing resins such as polyurethane and epoxy resin are current research hotspots. Epoxy resin is suitable for prepreg molding and HP-RTM processes and others. HP-RTM equipment manufacturers in the world are DIEFFENBACHER, SCHULER, TECNALIA and other businesses.
The GIM (Gap Impregnation System) process was developed by the German company BREYER in cooperation with the University of Aachen for the rapid prototyping of automotive composite parts. The fiber braid is placed vertically on the mould, the resin is injected under vacuum and both ends are molded and molded to cure. The main features of this molding process are short curing time (5-15 min), low injection pressure (<1MPa) and sandwich structure (base material is closed cell foam). At present, the GIM casting production line is applied in the hood of the Ford model.
Compared to traditional crafts, 3D printing technology has great advantages in structural design. 3D printing can more easily and conveniently produce complex structures that are difficult to process and manufacture with traditional methods. Although the design space for structures is wider, it also has the advantage of customizable structures. With an integrated design and fabrication process for structures, everyone can design and create the structure you need. . Although 3D printing technology has outstanding advantages in structural design, in terms of materials, 3D printing resin base materials cannot meet model requirements in terms of mechanical properties, while fiber-reinforced 3D printing composite materials can compensate for mechanical properties. 3D printing technology.
The widely used 3D printing fiber-reinforced composite molding processes are mainly divided into material extrusion molding, liquid deposition, photocuring, powder sintering, and multi-layer solid production molding according to different implementation methods.
Material extrusion molding is a molding method that uses a nozzle to extrude molten wire. This method is also known as deposition forming or fused wire forming (FFF). FFF is currently the most widely used 3D printing method. It has the advantages of lowcost, high printing speed and a wide choice of materials. However, the dimensional stability of its molding is generally low, and the interlayer performance of printed products is relatively weak. Using the FFF method, thermoplastic resin wire containing short fibers can be used instead of pure resin wire for printing and making thermoplastic composites reinforced with short fibers. The preparation process of FFF continuous fiber reinforced thermoplastic composites is relatively complex and is generally divided into two types: the real-time impregnation method and the prepreg method. The currently widespread equipment for printing fiber-reinforced composite materials is developed based on this method.
In 2014, car 3D printing company Local Motors used carbon fiber 3D printing technology to print a car at the International Manufacturing Technology Show. The 3D-printed car made from carbon-fiber-reinforced composites has a high specific gravity and rigidity, which reduces printing distortion. In 2016, Local Motors 3D printed an Olli self-driving electric bus made partly from recycled carbon fiber reinforced plastic.
2.2.4 Finishing recycling: protecting the environment and reducing costs can take recycling to the next level
Energy saving, environmental protection and economic benefits are the main directions of carbon fiber recycling.
In carbon fiber-reinforced thermoset composites, the polymer matrix, after curing, forms a three-dimensional cross-linked network structure that does not dissolve in solvents and cannot decompose naturally under normal conditions. If it is not recycled, it will cause environmental pollution, and as the amount of carbon fiber increases, the pollution will become more and more serious.
It is estimated that 40% waste will be generated during the production of carbon fiber-reinforced polymer matrix composites. For every 100 kg of carbon fiber composite waste, there are about 60–70 kg of carbon fibers. Based on 200 yuan/kg of carbon fiber reinforced waste, the cost of carbon fiber in polymer matrix composites is expected to exceed 10 billion yuan. In addition, Fraser Barnes, CEO of ELG Carbon Fibre, a UK-based recycled materials company, believes that using recycled carbon fibers can reduce the cost of fibers by around 40%.
The carbon fiber waste recycling methods are mainly divided into thermal decomposition method, physical recycling method, solvent decomposition method and combination recycling method.
Physical recycling, also known as recyclingrecycling, refers to the use of mechanical forces such as cutting, rolling and grinding to grind the waste CFRP, break the interfacial adhesion between carbon fibers and the polymer matrix, so that the carbon fibers are separated from the resin matrix, followed by sieving to obtain a fine powder rich in resin matrix, and recycled materials from shredded carbon fiber. The physical recycling method is simple, easy to implement, cheap and does not create new pollution during the recycling process. It is an efficient and environmentally friendly recycling method. Degradation and recovery of CFRP waste by mechanical recycling can lead to some new materials for use, such as reducing agent for iron production, raw material for paving, etc. However, only chopped carbon fiber can be obtained by this method, and still remains on the surface resin residue, fiber performance is greatly reduced, and the cost of recycled products is greatly reduced.
Thermal degradation refers to the degradation of polymer chain breaking under high temperature heating conditions. Resin-based composite materials use many types of matrix resins, including various auxiliary components. These substances can be decomposed into low molecular weight compounds under heating conditions. Therefore, thermal decomposition is currently the main method of polymer composite materials. The main commercialization is the base polymer method. According to different reaction media, reactors and heating methods, thermal decomposition methods are divided into thermal cracking, fluidized bed cracking, vacuum cracking and microwave cracking.
The solvent decomposition method is to use various chemical solvents to decompose the cured resin matrix or solutes and extract the remaining carbon fibers. This method is also known as the solvent method. According to the state of solvent use, it is divided into ordinary solvent method, supercritical fluid method and subcritical fluid method.
Combined recycling is a great option for mixed waste that cannot be recycled in one method.
Recycled carbon fibers are fluffy short fibers that have been reduced in size and randomly distributed, while retaining the superior mechanical and electromagnetic properties of the original fibers. Waste segregation and recycling yields valuable carbon fibers, but requires large investments, and it is still difficult to industrialize in a short period of time Environmentally sound technologies for reusing materials at lower cost and with lower energy consumption are needed. At present, recycled fibers are generally recycled and recycled. It is mainlyThe owl is used in a secondary structure or low-level field.
Recycled carbon fiber is used in the same way as ordinary commercial carbon fiber, and can be used as a reinforcing material for resin in the manufacture of composite materials. At present, the processing of recycled carbon fiber mainly includes two technologies: direct molding and post-forming processing of carbon fiber. The article "Recycling and Reuse Technology of Carbon Composite Materials" pointed out that the application of resin-based carbon fiber in the automotive, aerospace, wind power and other industries is becoming more extensive, and the recycling and reuse of its waste will become a necessary problem, the corresponding research results. In recent years, my country has issued a number of policy papers to support the recycling of carbon fiber composite materials, but there is still a certain gap between general and foreign countries. In practical application, it faces many problems: at present, it is difficult to recycle waste parts, and complete the supply chain is not yet formed. In order to provide a source of waste, it is necessary to strengthen mutual cooperation between developers, manufacturers and processors, most of the current recycling technologies require high pressure, high temperature, and high corrosive conditions, and there are certain risks. Further research is needed to improve recycling and recycling. Technology safety and stability. China lacks recycling standards for relevant industries, as well as waste classification standards and recycling classification standards. have not yet been formulated.3 Market side: strong demand, domestic enterprises are gradually making efforts
According to the "2021 Global Carbon Fiber Composite Market Report" released by the well-known domestic company Guangzhou Sail Carbon Fiber Technology Co., Ltd., wind power is the main demand growth driver in the global carbon fiber market. , sports equipment, carbon-carbon composite materials and high pressure cylinder.
China's total demand for carbon fiber in 2021 is 62,379 tons, up from 48,851 tons in 2020, up 27.7% year-over-year, of which imports are 33,129 tons (which is 53.1% of the volume of imports). total demand, up 9.2% compared to 2020), the supply of fiber in the domestic market was 29,250 tons (46.9% of total demand, up 58.1% compared to 2020). The general situation in the Chinese market in 2021 is that supply exceeds demand, whether it is imported or domestic fiber. As for the price, in 2021, due to the shortage of the entire carbon fiber market, the price market will continue to rise.you compared to 2020. The quantity of imported carbon fiber and its products increased by only 9.15% year on year, while the value increased by 26.02% year on year. Domestic carbon fiber "does not show weakness", and price growth is not inferior compared to imported products .The large tug market is also facing a shortage. The price increased slightly, at the level of 13.5-14.5 USD/kg. This price is the purchase price for large quantities. For low volume customers, these producers often quote a price of $17-18 per kg.
3.1 Demand: Replacing Traditional Materials to Generate 10,000 Ton Demand
3.1.1 Aerospace: partially replaceable metals and alloys
Combined with extreme conditions (high vacuum, highly corrosive environment), variable load and variable temperature, light weight, high strength, extreme temperature resistance and corrosion resistance are important determining factors for the design and selection of aircraft body materials.
Aircraft body design: the constant pursuit of weight reduction
According to the International Air Transport Association, fuel costs account for about 26% of the total cost of aviation, and for some domestic airlines, fuel costs are as high as 40%. Every pound of airframe structural material saved can generate a cost-effectiveness of nearly one million dollars, so low density has become an important consideration when choosing aircraft structural materials. In addition, the aircraft is operated in the atmosphere or space for a long time, and extremely high reliability and excellent performance in extreme conditions are required from it, therefore, when designing an aircraft, it is necessary to increase the efficiency of the design, because. as much as possible and not overpay. Characteristics such as specific strength and high specific modulus become key factors when choosing a material. In a comprehensive comparison, carbon fiber composite materials with low density, high specific modulus and high specific strength are currently the best choice. Its share in military aircraft and civil aircraft also increases significantly year by year, and it has replaced the original structural steel and aluminum, becoming the first choice for aviation structural materials.
Carbon fiber reinforced composite material is one of the most widely used composite materials in the aviation industry. Since its density is only 60% that of aluminum alloy, its widespread use in aircraft construction can reduce the weight of the structure by 20-25%. The most commonly used carbon fiber reinforced composite material in aircraft is polymer matrix composite (CFRP). As for military aircraft, compoCarbon fiber reinforced composite materials are mainly used in non-structural parts of aircraft, such as aircraft fairings, hatches, fairings, vertical tails, horizontal tails and aircraft tail rudders, etc., such as the tail structure of the French Mirage 2000 fighter. materials. For civil aircraft, the carbon-fiber-reinforced composite materials and glass-fiber-reinforced materials in the Boeing B787 airliner account for 50% of the weight of the entire aircraft structure, resulting in fuel savings of 20%.
Compared to traditional aircraft, helicopters have lower flight speeds and lower altitudes, and they often operate in harsh environmental conditions such as humidity, heat, drought and dust. Corrosion makes higher demands. The American CH-53E heavy multipurpose helicopter that first flew in 1974 used only about 5% composite materials, while its successor, the V-22 rotary propeller, used AS4/3501-6 composite materials for the fuselage. and tail use share is 41%, which reduces the overall weight by 13%, the number of parts by 35% and the cost by 22%. In 2020, the H-160 helicopter, which received the type certificate of the European Aviation Safety Agency, became the world's first civil helicopter, all-composite.
The need for lightweight drones has led to continuous improvements in body materials. After lightweight metal materials such as aluminum alloys and titanium alloys, carbon fiber composite materials have become the "mainstream" material for drones. It can be said that with the UAV structure design and production technology using carbon fiber composite materials as the core has become a key technology for the development of UAVs. The wing, tail, engine nacelles and rear fuselage of the U.S. RQ-4 Global Hawk Advanced Reconnaissance Unmanned Aircraft are made of carbon fiber reinforced composite materials. The fuselage of the AAI Shadow UAV is made of carbon fiber reinforced epoxy resin composite material, the empennage is made of carbon fiber or aramid fiber reinforced epoxy resin composite material, and the wing is made of carbon fiber reinforced epoxy resin composite material. honeycomb sandwich structure.
Missiles, rockets, satellites: high temperature resistance, low weight and high strength - hard targets
Aerospace technology such as rockets and missiles usually fly at high speeds. According to the data, when a rocket flies at Mach 4-10, the surface temperature range can reach 445-3173°C, which is difficult for conventional aluminum alloys. and even titanium alloys. meet the requirements. In a comprehensive review and comparison of carbon fiber composites that havethe density of only 1/4-1/3 superalloys and can maintain excellent mechanical properties at high temperatures, are undoubtedly the best choice for the future. On large launch vehicles, carbon fiber is mainly used in fairings, engine shrouds and other structures, especially the upper-level structure, carbon fiber is widely used, effectively reducing the weight of the upper-level structure and is very important. to improve the ability of the launch vehicle to launch payloads. Obvious effect. The US Hercules rocket uses carbon fiber as the load-bearing structural material of the radome, interstage section cockpit and conical tail cockpit, and the interstage section skin and conical tail cockpit shell are made of IM7/8552. composite materials. In addition, the United States, Japan and France solid-cast engine cases mainly use carbon fiber, such as the American Trident-2 missile, Tomahawk cruise missile, Hercules-4 missile, French Arianna-2 missile. , the Japanese M-5 rocket and other engine casings, among which the most commonly used medium-form high-strength carbon fiber IM-7 manufactured by Hexcel USA, with a tensile strength of 5.3 GPa. .
The use of a large amount of carbon fiber can reduce the mass of the rocket, increase the range of the rocket and improve the accuracy of the landing point. Therefore, carbon fiber composite materials are often used in rocket projectiles, launch tubes and other structures. Viscose-based phenolic composite materials reinforced with carbon fiber are used in the engine nozzles and heat-resistant layers of a large area of the Russian Krugliy Molot submarine missiles and Topol-M missiles. The American PAC-3 uses IM-7 carbon fiber for the motor housing and T300 carbon fiber for the warhead housing. The THAAD missile uses a high-strength, mid-modulus carbon fiber composite matrix material as the engine housing material, and high-modulus, high-strength carbon fiber is used in the interceptor cockpit structure. The French M51 ballistic missile uses an engine casing woven from carbon fiber. High modulus carbon fiber reinforced composites are commonly used in medicine. In satellite building structures, solar panels and antennas.
Engine: weight reduction and high temperature resistance
Increasing thrust-to-weight ratio and reducing maintenance costs have always been the focus of military engine development. At present, the air intake temperature of the most advanced F119 gas turbine engine has reached 1700 ° C. At present, the operating temperature of the nickel-based superalloy material with the best heat resistance is about 1100 ° C. Only the structure can meet the requirements of thermal structural materialsof modern aircraft engines, and the search for next-generation materials with superior high temperature resistance has become the key to aircraft engine design.
In terms of market size, according to the 2021 Global Carbon Fiber Composite Market Report, the demand for carbon fiber in the aerospace market in 2021 will be 16,450 tons, and demand is expected to grow to 20,635 tons in 2021 2025.
3.1.2 Wind energy field: some fibers and metals can be replaced
As one of the clean and renewable energy sources, wind power has the unique advantages of large power density, small footprint and relatively mature technology. Compared to nuclear power, wind power has higher safety, compared to water power, wind power has a greater development potential, and compared to solar power, wind power has a lower development cost.
Wind energy has a number of remarkable advantages such as fast development speed, low cost, low pollution and strong sustainable development, so it has become one of the energy conversion technologies with the largest development scale and commercial development prospects. In the 21st century, thanks to the consensus and active actions of many countries in the world, wind power technology has developed rapidly, and the installed capacity of wind power has also increased rapidly, which further contributes to the transformation of wind power into a fast-growing emerging industry. worldwide. . The principle of wind energy is that a wind turbine converts the kinetic energy of air into mechanical energy of rotation. As the oncoming flow approaches the wind wheel, the speed gradually decreases, the flow area gradually increases, and the air pressure also gradually increases, which converts kinetic energy into pressure energy; when the air flow passes through the wind wheel, the pressure energy is converted into rotational mechanical energy, the pressure drops, and the air speed decreases. Less area increases, and at the same time, due to the presence of wake loss, part of the kinetic energy is converted into pressure energy to make the pressure equal to remote atmospheric pressure.
According to the wind energy density formula, the ratio between fan power and blade length is P=1/2ρπr 2 V 3 Cp (ρ – air density; V – wind speed; Cp – wind energy utilization factor; r – blade length), that is, the length of the blade is proportional to the power.
The shape of the blades of wind farms has not changed much, but the material of the blades has undergone a qualitative change. The blade material development process has gone through about three stages: the stage of wooden blades, the stage of metal blades, and the stage of composite materials. Blades for both onshore and offshore wind power are developingflow in longer and larger directions, which is accompanied by the requirement that the quality of the blades does not increase too much, and the modulus must be higher, and the material is a key factor. Blade materials must satisfy the balance of density, performance and cost. The development process of blade materials is actually a process of developing light weight, high performance and low cost materials.
The early use of wooden blades was mainly due to two reasons: firstly, the power of wind turbines produced according to the technology of that time was small, so the size of the blades should not have been too large. Compared to metal, it is better to use wood to make blades. Secondly, the density of wood is obviously less than that of metal, so wood blades are lighter. But the main disadvantage of the wooden blade is that it is difficult to process and shape, and its strength is low, and it is greatly affected by the environment. With the development of technology, the size of the wooden blades gradually cannot match the power of the generator set, and the wooden blades are gradually being replaced by metal blades.
The increase in wind turbine power has led to the gradual development of metal blades. Compared with wooden blades, metal blades are easier to process and shape, and the size of metal blades is larger than Kimura blades, and the power generation capacity is further increased, making metal blades the most ideal. wind turbine blade material. However, metal blades have inherent disadvantages such as low corrosion resistance and high density. After the metal blade, the blade did not develop for a long time because the blade material was not developed.
With the advent of fiber-reinforced composite materials in the 1950s, composite materials began to be used instead of metal materials for the manufacture of blades, and wind energy began to be used in large-scale production. Composite blades are virtually devoid of the disadvantages of wood and metal blades. First, the density of composite materials is low, and the density of composite materials reinforced with glass fiber or carbon fiber is much lower than that of metal. Secondly, although the modulus of composites is somewhat lower than that of metals. Thirdly, composite materials are more resistant to corrosion than metals and have a longer service life. Fourth, the design capabilities of composite materials are strong.
The carbon fiber molding method in the field of wind power is mainly the pultrusion method. At present, there are three main processes for molding composite main beams: prepreg process, infusion process and pultrusion process. The use of pultrusion process to produce main beams has been recently developed in recent years. Because the length loThe mouth is from tens of meters to hundreds of meters, and the length of the main beam is similar to the length of the blade, when the main beam is produced in the prepreg process, due to energy consumption and equipment limitations, only low-temperature curing prepreg can be used The process is complicated, and the production efficiency is low. In addition, the process of transportation and storage of the prepreg must be carried out at a low temperature. Compared with the above two methods, composite material parts made by pultrusion have high fiber content, stable quality, and are suitable for mass production.
In terms of market size, according to the 2021 Global Carbon Fiber Composite Market Report, the demand for carbon fiber for wind power will be 33,000 tons in 2021, and is expected to grow to 80,566 tons in 2025.
3.1.3 Hydrogen energy field: replacement of some high-strength steel materials
Environmental protection and energy strategic security are gradually promoting the development of hydrogen energy. The average global temperature has increased by 1.7°C per century since 1970 and has decreased by 0.01°C per century over the past 7,000 years. Currently, over 25 billion tons of CO2 are emitted annually in the world, and the concentration of CO2 in the air has increased from 280 ppm to 380 ppm since industrialization in more than 150 years. Greenhouse gases will have catastrophic consequences for the earth, and various measures are being taken around the world to reduce CO2 emissions. According to the IPPC and the IEA, in 2020 carbon dioxide emissions from oil, coal and natural gas accounted for 31%, 42% and 23% of total fuel combustion emissions, respectively. Currently, global warming has brought a large number of extreme natural disasters, especially in the high latitudes of the northern hemisphere. All countries have now reached a consensus and signed the "Paris Agreement" in December 2015. The main goal is to limit global temperature rise to 1.5°C.
Hydrogen energy is considered an important part of the energy architecture of the future with low dependence on minerals and zero carbon emissions.
The mining of some metal minerals in my country is low and in the future it may not be able to provide large-scale application of some renewable energy, and the metal minerals on which hydrogen energy depends can be recycled, which is expected to be will become an important part of the future energy mix.
Clean and low carbon. Regardless of the combustion of hydrogen or the electrochemical reaction in fuel cells, the product is only water without the pollutants and carbon emissions generated by conventional energy use. In addition, the generated water can continue to produce hydrogen, which can be recycled many times, deindeed achieving low or even zero carbon emissions and effectively mitigating the greenhouse effect and environmental pollution.
At present, the use of carbon fiber in the field of hydrogen energy is mainly associated with hydrogen storage tanks and gas diffusion layers of fuel cells.
Hydrogen gas storage under high pressure has the advantage of high hydrogen loading and unloading speed and simple container design. The development and application of high-pressure carbon fiber hydrogen cylinder winding technology has brought about the transformation of high-pressure hydrogen gas storage from a stationary application to a vehicle.
In a fuel cell, the material of the gas diffusion layer is mainly carbon paper, which is located at both ends of the membrane electrode. Fuel cell carbon paper is made of crushed carbon fiber, which has good mechanical strength, light weight, high porosity, excellent air permeability, excellent electrical conductivity, high temperature resistance, oxidation resistance, corrosion resistance, etc.
In terms of market size, according to the "2021 Global Carbon Fiber Composite Market Report", the consumption of hydrogen storage cylinders in 2021 will be about 1,900 tons, and China is expected to add at least 10,000 vehicles powered by hydrogen in 2022. , mainly in logistics vehicles. In the field of heavy trucks and buses, the number of heavy trucks is 6000. The hydrogen storage tank for heavy duty vehicles is 210L-385L, the amount of carbon fiber in one bottle is 40 to 45kg, and the bicycle is generally equipped with 6-8 groups of bottles. fiber in the field of heavy trucks is about 1700-1900 tons, plus the amount in other logistics vehicles and cars Hydrogen fuel cell vehicles It is estimated that the total amount of carbon fiber can be used up to more than 2500 tons.
3.1.4 Automotive industry: plastic and metal replacement parts
According to the US Department of Energy and Lightweight Agencies, for every 10% reduction in overall vehicle quality, fuel consumption can be reduced by 6%-8%, and emissions can be reduced by 5-6%. This data may show that lightening vehicles is an effective way to reduce fuel consumption and protect the environment. Carbon fiber composite materials are widely used in the automotive industry due to their various benefits. The potential application rate of this material in automobiles is: 35% for body structure, 20% for chassis components, 17% for steering and suspension systems, 10% for upholstery, 18% for interior details.
Carbon fiber composite materials in automobiles are most often used in body components. The body components mainly include hood, frame, roof frame and A/B/C/D pillars, etc., each part uses carbon fiber composite material as the main production material. Comparing carbon fiber composite materials with traditional metal materials, it can be seen that the specific strength and specific stiffness of composite materials are significantly higher than those of conventional metal materials. Assuming the same structural strength, the use of carbon fiber composite materials for the manufacture of auto parts can reduce weight by 50%, which plays an important role in improving the dynamic performance of the car.
The wheel hub is one of the key components to keep the car running smoothly, which can withstand the overall quality and load of the car. Therefore, in order to ensure the stable operation of the car, it must have good impact resistance, durability, heat resistance and safety. Carbon fiber composite materials can effectively meet the actual needs of the wheel hub, which is conducive to the realization of the car's lightness. The special RX-X wheel hub for full-fledged vehicles manufactured by Kahm of the United Kingdom can fully reflect the advantages of this material, its weight is only 6 kg, it has the functions of high-speed driving and reducing the radial inertia of the wheel.
In order to ensure the stable operation of the car, the transmission shaft must have good carrying capacity, fatigue resistance and other advantages. Due to the unique anisotropy and high specific strength of carbon composite materials, the use of this material in the transmission shaft makes it possible to reduce the weight of the transmission shaft by more than two times compared to the original one. All else being equal, the torque of this material can reach more than 170% of the torque of steel compared to conventional metal materials. And the use of carbon fiber composite materials can significantly improve the durability and fatigue resistance of the vehicle driveshaft.
Short carbon fiber brake lining has stable coefficient of friction, low wear rate, good toughness and hardness, good braking without noise, and conducive to environmental protection. By increasing the coefficient of friction, the car's brakes are more stable, safer and more comfortable when driving fast.
With the good plasticity, modularity and high integration of composite materials, as well as the development of molding technology and automation, the advantages of composite materials in terms of technology will be further revealed. The four main processes realize disruptive innovation, process investment is reduced, and the same eeconomic scale can reduce the number of staff by 2/3-4/5 compared to traditional processes. In particular, the material has high anti-corrosion properties, which makes it possible to realize environmentally friendly production, and it is also easier to realize SKD production, centralized parts production, assembly in different places, and open up overseas markets.
Fiber composite materials, whether thermoset or thermoplastic, can be recycled and reused in a variety of ways. At the same time, there are appropriate measures for the care of damaged body parts in order to meet the requirements of the development of a green economy and a circular economy.
In terms of market size, according to the 2021 Global Carbon Fiber Composite Market Report, the demand for carbon fiber in the automotive applications market will be 9,500 tons in 2021, and is expected to grow to 12,645 tons in 2021 . 2025.
3.1.5 Other areas: new markets expected in the future
Sports, construction, electronics and electrical engineering are also areas with great potential for carbon fiber in the future.
Traditional sports facilities and sports equipment are usually designed and crafted using metal, wood, etc. combined with plastic, rubber, and other materials. However, with the continuous progress in sports equipment design and the continuous improvement of national requirements for sports facilities, traditional materials have been criticized for their excessive quality, backward performance, environmental protection, and high risk. With the advantages of high temperature resistance, light weight, no deformation, environmental protection and safe use, carbon fiber materials have achieved rapid development since they entered the field of sports equipment.
In addition, in competitive sports, sports competitions in the modern world are essentially a competition of science and technology, and advanced and new materials are one of the important conditions for raising the level of sports science and technology. At present, in many sports, people's physical and mental stimulation has almost reached the limit, and victory or defeat is often within reach. It is difficult to win only with strong muscles and good competitive condition. equipment. People expect athletes to challenge their abilities by upgrading their sports equipment.
In terms of market size, according to the "2021 Global Carbon Fiber Composite Market Report", the demand for carbon fiber in the sports and leisure applications market in 2021 will be 18,500 tons, and demand is expected to grow to 22,487 tons. in 2025.
Build the sphereThe coverage is relatively large and covers construction machinery, bridges, tunnels and industrial pipelines. Composite materials are mainly applied in the five main areas of building and bridge reinforcement, main structures of art buildings, construction machinery, new large-span/space structures, and pipeline reinforcement.
In terms of market size, according to the 2021 Global Carbon Fiber Composite Market Report, the demand for carbon fiber in the building materials market will be 4,200 tons in 2021 and 2025 in 2021. < /p>
The application of carbon fiber in the field of electrical and electronics is mainly divided into functional application and application for mechanical reinforcement. In terms of functional application, carbon fiber reinforced plastic has antistatic, electromagnetic shielding and other functions, and can be used in copiers, printers, digital cameras, data cable connectors and other products. In terms of mechanical reinforcement, long carbon fiber reinforced (LFT) plastics and continuous carbon fiber reinforced materials consider both cost and performance and can be used in products such as laptops.
In terms of market size, according to the "2021 Global Carbon Fiber Composite Market Report", the demand for carbon fiber in the electrical and electronic applications market in 2021 will be 2000 tons, and demand is expected to grow to 2928 tons in 2025. In addition, carbon fiber and carbon fiber products can also be used in railway transportation, silicon monocrystalline thermal equipment in the field of semiconductors and in the field of airgel.
3.2. Suggestion: China started late, but quickly catches up
By studying the development history and patent structure of carbon fiber, overseas companies will still have significant advantages in the field of carbon fiber in the short term. The "2019 Global Carbon Fiber Composite Market Report" summarizes the development history of the carbon fiber industry for more than 60 years. It can be seen that foreign companies have contributed the most to carbon fiber, while domestic companies have started relatively late.
The article "Carbon Fiber Technology Development Study Based on Patent Analysis" is based on the Patsnap database and with reference to the search items set by the technical experts of Hengshen Carbon Fiber Co. China's development in the field of carbon fiber technology is clearly more recent than in foreign countries, but in recent years, the patent filing trend in China is about the same as in the world.
Since 1980, foreign carbon fiber technology has entered a period of development. The number of patent applications per year exceeds 60 and reached 341 in 1989. toThe number of patent applications in China has shown a downward trend, after 2000, foreign carbon fiber technology gradually resumed development and gradually entered the stage of rapid development.
China started creating a patent system in 1985, so there were no patent applications until 1985; From 1985 to 2000, carbon fiber patent technology in my country was in its infancy, and the number of patent applications per year did not exceed 5 pieces. After 2000, the number of patent applications in the field of carbon fiber technology around the world began to show an upward trend, especially after 2010, the number of patent applications in my country increased rapidly, reaching more than 300 patent applications per year, which indicates that my country carbon fiber technology has entered a period of rapid development.
In terms of the top ten patent applications, China Petrochemical Corporation, Beijing University of Chemistry and Technology, Donghua University, and Harbin Institute of Technology ranked fifth, seventh, ninth, and tenth respectively. It can be seen that the relevant research institutions have begun to show their strength in related technical fields. From the point of view of patent policy, the quality of patents in the carbon fiber industry in my country still needs to be improved, and there is still a certain gap between the patenting models in Japan and the United States.
Therefore, in the short term, domestic companies still need to work hard to catch up. The main reason is that domestic related technologies appeared later and the patent system was established later than abroad. In terms of production capacity, China has become the world's largest carbon fiber producer for the first time in five years. According to the "2021 Global Carbon Fiber Composite Market Report", in 2021, the world's main production capacity increased: Jilin Chemical Fiber Group increased production by nearly 16,000 tons; The new production capacity of Changzhou Xinchuang Carbon Valley was 6,000 tons; Zoltek has increased by 3,000 tons. tons, Zhongfu Shenying increased by 8000 tons, Baojing increased by 2000 tons, Dongbang increased by 1900 tons.
China's dependence on foreign carbon fiber is declining every year, and the competitiveness of related enterprises will be further enhanced after the completion of production expansion in the future. Indoor carbon fiber will continue to improve in 2021, with its market share rising from 47% in 2019 to 58% in 2020. At present, China's leading carbon fiber companies are Sinopec (Shanghai Petrochemical), China National Building Materials Group (Zhongfu Shenying), China Baowu (Baojing Carbon Fiber and Taigang Steel), Sinochem Group (Blue Star), Shaanxi Coal(God State Company), Jilin Chemical Fiber (State Enterprise).
By the end of 2021, domestic Jilin Chemical Fiber, Zhongfu Shenying, Shanghai Petrochemical and other enterprises also announced their expansion plans: Jilin Chemical Fiber Group plans to complete the production of 200,000 tons of raw silk, 60,000 tons of carbon fiber and 10,000 tons composite materials expansion plan. In 2021, the expansion of carbon fiber production by 16,000 tons will be completed, and 27,000 tons of new carbon fiber production capacity is expected to be added in 2022; Zhongfu Shenying implements its 20,000 ton expansion plan in Xining; Baowan Raw Silk Construction Plan. In 2021, Xinchuang Carbon Valley will build a large towing complex with a capacity of 18,000 tons and complete the construction of a production capacity of 6,000 tons, and in 2022 will activate the installation and commissioning of another 12,000 tons of production capacity. Shanghai Petrochemical's 12,000t large towing project is under construction and is expected to reach a capacity of 6,000t in 2022; In 2021, Zhongjian Technology announced a plan to produce 1,500 tons of carbon fiber and fixed-growth products with a 1,867 expansion. billion yuan.
Looking to the future, establishing the development model of the entire industry chain is the top priority, and sincere cooperation between manufacturers and manufacturers can gradually promote the long-term and healthy development of the carbon fiber industry. The article "Building a Development Model of the Whole Carbon Fiber Production Chain in Beijing" combines patent data and industry research data, indicating that Japan is a traditional carbon fiber manufacturer and has formed a complete industrial chain with automobiles. as the main field of application, and already includes carbon fiber. The industry has expanded to overseas markets such as Europe, the United States, South Korea and China. Relying on its complete industrial structure and huge demand in overseas markets, it has gradually realized the industrial development model expansion of the world market. In the early stage of the development of the carbon fiber industry, Europe and the United States did not have a technological advantage over Japan, but Europe and the United States, driven by demand from the aerospace industry, introduced the advanced technology and products of the Japanese company Toray, stimulating the development of domestic companies such as Hexcel and Cytec. The carbon fiber industry is gradually improving to form a complete structure of the carbon fiber industry. The article also puts forward relevant opinions about the Beijing carbon fiber industry. The article says that in the field of carbon fiber, Beijing has obvious advantages in researchresearch and development, and related technologies are widely used in the aerospace industry. However, there are also disadvantages. in the supply of resources and increased pressure on environmental protection for the development of carbon fiber production and production. Through the coordinated development inside and outside the city, promote the industrial chain to a deeper horizontal and vertical division of labor, increase the density and expansion of the network of the industrial chain, and help increase the added the value of the entire production chain, while fully revealing Beijing's role in the production chain. The advantages of the middle and lower segment contribute to a more refined and professional development of enterprises in the city. .