Carbon Fiber Molding Process and Bottleneck Discussion | Carbon Fiber Industry Investment Strategy Report 2

 

1. Introduction

Carbon fiber (CF) is a new type of fiber material with high strength and high modulus fiber with a carbon content of more than 95%. It is a microcrystalline graphite material obtained by stacking organic fibers such as flake graphite microcrystals along the axial direction of the fiber and undergoing carbonization and graphitization treatment.

 

Carbon fiber is “soft on the outside and hard on the inside”. It is lighter than metal aluminum, but stronger than steel. It also has the characteristics of corrosion resistance and high modulus. It is an important material in national defense and civilian use. It not only has the inherent intrinsic properties of carbon materials, but also has the softness and processability of textile fibers. It is a new generation of reinforcing fibers. Carbon fiber has many excellent properties. It has high axial strength and modulus, low density, ultra-high temperature resistance in non-oxidizing environment, good fatigue resistance, specific heat and conductivity between non-metal and metal, small thermal expansion coefficient and anisotropy, good corrosion resistance, and good X-ray transmittance.

 

Compared with traditional glass fiber, the Young’s modulus of carbon fiber is more than three times that of traditional glass fiber; compared with Kevlar fiber, the Young’s modulus of carbon fiber is about twice that of traditional glass fiber. It is insoluble and non-swelling in organic solvents, acids, and alkalis, and has outstanding corrosion resistance.

 

Due to the differences in performance, morphology, manufacturing methods, and starting raw materials, carbon fiber not only has industrial products, but also types that are still in the experimental stage and whose prospects are still difficult to predict. The product range is very wide, and the technical development of any product in different production and application stages is in continuous progress. This article mainly introduces the common carbon fiber and its related production technology on the market. The classification models in the laboratory are not discussed here. In addition, this article only analyzes the domestic carbon fiber production technology and its related development. For information on the development history of domestic and foreign carbon fiber, the global carbon fiber market structure, the strength and profitability of domestic carbon fiber companies, please refer to the Carbon Fiber Industry Investment Strategy Report 1: “The downstream application field is broad and the domestic growth space is huge” (published on 2020/02/17, full text 28 pages).

 

2. Introduction To Carbon Fiber Classification

 

2.1 Classification By Raw Material System

 

Carbon fiber is mainly divided into three types: viscose-based, asphalt-based and polyacrylonitrile (PAN)-based, each with different usage scenarios and production methods. Among them, asphalt-based carbon fiber has the highest carbon yield, which can reach 80%-90%. However, in actual production, in order to obtain high-quality and high-performance carbon fiber from asphalt, the asphalt must be refined and modulated.

 

This process will greatly increase the production cost. Even if the asphalt raw material is abundant and cheap, it is difficult to apply it to large-scale industrial applications.

 

PAN-based carbon fiber has the best comprehensive performance, mature and simple production process, the widest application, the highest output, and the most varieties. It is the mainstream carbon fiber product in the global carbon fiber market, and its output accounts for more than 90% of the global carbon fiber total output.

 

 

2.2 Classification By Product Form

Common carbon fiber products on the market are continuous fiber bundles curled around paper tubes, which contain 1,000 to tens of thousands of carbon fiber filaments with a diameter of 5-8μm and a circular or elliptical cross section.

 

At present, the basic forms of carbon fiber are continuous long fibers and short fibers (carbon fibers with a length of 1-100mm). In actual use, they can be divided into various forms according to the processing method and the shape of the final product. That is, through various processing of continuous long fibers and short fibers, fabrics, woven fabrics, paper, felt and other forms can be obtained.

 

 

2.3 Classification By Mechanical Properties

The mechanical properties of carbon fiber will vary within a wide range depending on the specific model and grade. The most important performance indicators are tensile strength, elastic modulus and density.

 

The higher the tensile strength of carbon fiber, the higher the load that the fiber axis can bear and the greater the material strength; the larger the elastic modulus, the smaller the deformation of the fiber under a certain load, that is, the better the rigidity of the fiber; the smaller the density, the lower the weight of the fiber of the same volume, and the better the weight reduction effect of the related composite materials.

 

According to the difference in the mechanical properties of carbon fiber, my country has promulgated the “National Standard for Polyacrylonitrile (PAN)-based Carbon Fiber (GB/T26752-2011)” on November 13, 2011, which divides carbon fiber into four types: high strength, high strength medium modulus, high modulus and high strength high modulus. Since Toray of Japan has an absolute leading advantage in the global carbon fiber industry, Toray’s T series and M series standards of Japan are also used for classification in some relevant reports in China.

 

 

2.4 Classification By Tow Size

In the relevant technical standards of carbon fiber, K represents the number of carbon fiber filaments, such as 1K represents a bundle of fiber filaments containing 1000 filaments. Generally speaking, 1K, 3K, 6K, 12K and 24K are called small tows; 48K, 60K, 80K, 120K and above are called large tows.

 

Small tow products are standard products of carbon fiber and are the basic materials for the development of finished carbon fiber composite materials. Small tow carbon fiber has stricter requirements on process control, and the cost of carbonization equipment is high. It is mainly used in high-tech fields such as national defense and military industry, as well as sports goods such as aircraft, missiles, rockets, satellites and fishing gear, golf clubs, tennis rackets, etc.

 

Large tow carbon fiber has relatively low cost and higher cost performance, but in the early stage of product development, there are problems such as difficulty in improving performance and difficulty in processing operation.

 

Currently, it is mainly used in industrial fields such as medical equipment, electromechanical, civil engineering, transportation and energy.

 

 

3. Analysis Of The Production Process of PAN-based Carbon Fiber

 

We take the acrylonitrile (PAN)-based carbon fiber industry chain, which is the mainstream in the current market, as an example. The complete carbon fiber industry chain includes the upstream crude oil chemical industry, the midstream raw silk processing, carbon fiber related products and carbon fiber composite material production and processing, core machinery manufacturing and the downstream application market.

 

The preparation process of PAN-based carbon fiber starts with PAN raw silk, which is obtained by polymerizing acrylonitrile (AN) monomer and then spinning it by wet or dry wet method; after pre-oxidation (200-300℃), carbonization (1000-1500℃), and graphitization (2500-3000℃), the linear polyacrylonitrile polymer undergoes a series of chemical reactions such as oxidation, pyrolysis, cross-linking, and cyclization, and removes atoms such as hydrogen, nitrogen, and oxygen to form graphite carbon fiber; then the fiber is given chemical activity through surface treatment such as gas phase or liquid phase oxidation, and sizing agent is applied for sizing treatment to protect the fiber and further improve the affinity with the resin; finally, it is rolled and packaged to form a carbon fiber unidirectional tape, or it is knitted to form a carbon fiber fabric and exported to downstream sales.

 

 

3.1 PAN Precursor Manufacturing Process

In the early stage of carbon fiber industry research and development, the main product was ordinary acrylic carbon fiber, but this manufacturing process is difficult to obtain high mechanical properties of carbon fiber products. Only by using specially optimized PAN fiber can the performance of carbon fiber be improved.

 

This PAN fiber specially optimized for high-performance carbon fiber is called precursor. PAN precursor is the raw material for manufacturing carbon fiber. The performance of the precursor can largely determine the performance of the carbon fiber. In other words, if you want to obtain high-performance carbon fiber, you must first have high-performance PAN precursor.

 

The performance of PAN precursor, in essence, depends mainly on the structure and arrangement of the PAN molecules. The control of the PAN molecular structure is mainly concentrated in the polymerization process, while the arrangement of PAN molecules is mainly formed in the spinning process.

 

3.1.1 Polymerization Process

The PAN polymer used to prepare carbon fiber must be specially optimized and designed, and the key is the design of the polymerization process, because this will directly affect the structure of the PAN molecules in the precursor.

 

Acrylonitrile polymerization is a free radical addition reaction and is an exothermic process. Each addition polymerization of acrylonitrile monomer requires opening a C=C double bond and generating two σ single bonds, thereby releasing heat. The PAN molecular chain in the obtained PAN fiber has good regularity and high crystallinity, but the fiber lacks flexibility, which is not conducive to the subsequent process.

 

In addition, the initial pre-oxidation temperature of PAN homopolymer is high. Since an exothermic reaction will occur in the initial stage of pre-oxidation, concentrated heat release will cause the PAN molecular chain in the precursor to break and form a macroporous defect structure, which affects the stability of the production process and the quality of carbon fiber, and is one of the difficulties in production.

 

Therefore, in the actual production process, acrylonitrile is usually copolymerized with some comonomers, which can effectively control the exothermic reaction in the pre-oxidation process and obtain higher quality carbon fiber in the subsequent steps. Itaconic acid (IA), methyl acrylate (AA), methyl methacrylate (MAA), etc. are commonly used comonomers, which can adjust the spinnability of the spinning solution. And improve the phase separation process in the coagulation bath. Obtain a PAN precursor with a denser structure.

 

In addition, intramolecular cyclization can be initiated during pre-oxidation, converting the cyclization reaction from free radical type to ionic type, and increasing the oxygen permeability of the precursor, which is beneficial to the process control of the pre-oxidation process.

 

 

Neutral acrylate comonomers have a plasticizing effect, which improves the solubility of PAN and the rheological properties of the solution, making it spinnable, and improving the penetration of oxygen into the precursor during the pre-oxidation process. The presence of comonomers containing carboxylic acid groups such as itaconic acid can improve the permeability of the coagulation medium into the fiber during the coagulation of PAN precursor, improve the coagulation process of PAN precursor, and improve the coagulation uniformity.

 

In addition, the carboxylic acid group affects the ease of pre-oxidation, exothermic performance and carbon yield of PAN precursor. It should be pointed out that the presence of comonomers will also affect the ring formation process during the preparation of PAN-based carbon fibers, thereby affecting the structure and properties of carbon fibers.

 

Therefore, the comonomer content in PAN resin used to prepare carbon fibers is usually <5%. In addition to the content, the sequence distribution of comonomers on the PAN molecular chain will have an important impact on the uniformity of the precursor structure, the stability of the pre-oxidation process, and even the performance of the final carbon fiber.

 

Therefore, it is necessary to achieve the uniform distribution of comonomers on the PAN molecular chain as much as possible according to the characteristics of the comonomers, combined with process control and adjustment of polymer equipment, to lay a vital material foundation for the preparation of high-performance carbon fibers.

 

There are mainly one-step and two-step methods for the preparation of PAN polymerization solution: the one-step method is usually acrylonitrile polymerization in dimethyl sulfoxide (DMSO), which is directly used for the preparation of PAN precursor after desing and degassing; the two-step method usually uses PAN aqueous phase precipitation polymerization, and the obtained PAN powder is washed with water, dried, and then dissolved in solvents such as DMSO and dimethylacetamide (DMAC) to prepare spinning solution.

 

Most carbon fiber manufacturers in China use the one-step method to prepare PAN precursor, while Jilin Chemical Fiber Group uses the two-step method to produce PAN precursor. The two-step method is more difficult and more expensive than the one-step method. It is easy to introduce impurities, resulting in larger polymer particle size and difficult to produce high-quality PAN precursor. It is difficult to use, so there are currently fewer companies using it.

 

In the widely used DMSO solution polymerization one-step method for preparing PAN precursor, based on the polymerization equipment and technical traditions, most of my country’s carbon fiber manufacturers use intermittent or semi-continuous polymerization process flows. Because the continuous polymerization reactor is always full of materials and uses a full mixing method, it is difficult to avoid the appearance of molecular chains with ultra-long residence time. If the intermittent polymerization method is used instead, this drawback can be eliminated. Intermittent polymerization means that the main polymerization process is completed once in an independent device and time period, and the input and output are intermittent processes, strictly operated in batches.

 

The difference between its production process and continuous polymerization is that the prepared raw material auxiliary agent solution is intermittently fed into the first polymerization kettle in batches, without mixing with any reacted materials, and the whole process from monomer to polymer long chain is completed in it, achieving the conversion rate required by the process (about 90%); the subsequent processes are not much different from continuous polymerization, but storage equipment needs to be added at appropriate locations to connect the intermittent and continuous processes.

 

Compared with the continuous polymerization process, intermittent polymerization is a single-pot polymerization, which can change fewer conditions and has less operating flexibility. The process is short, and various problems are easy to solve. Ultra-high molecular weight acrylonitrile chains are eliminated, and the quality of the spinning solution obtained is more reliable, which is more suitable for the production of carbon fiber in my country.

 

 

3.1.2 Preparation Of Spinning Solution

Spinning solution is the raw material for spinning. Its performance is directly related to the performance of the original fiber, so it has relatively strict restrictions. The so-called spinning solution refers to the polymer solution adjusted to a certain concentration of polymer after the solution is polymerized through a certain process to remove the unreacted monomers and tiny bubbles in the system.

 

There are two issues that need to be paid attention to in spinning solution:

The first is gelation. PAN polymer solution is prone to gelation. Generally, the higher the storage temperature, the greater the concentration of the polymer, and the faster the gel is produced. Therefore, preventing gelation is the main consideration when determining the storage conditions of the spinning solution;

 

The second is the filtration of the spinning solution. Before spinning, solid impurities, undissolved polymers, polymer gels, etc. in the spinning solution must be removed as much as possible, otherwise it will greatly increase the frequency of broken fibers in the manufacturing process of original fibers and carbon fibers. In severe cases, it may cause the spinneret to be blocked, which has a great impact on the stability of production.

 

In industry, two-stage filtration is usually used to improve the efficiency and life of the filter element. The minimum pore size of the filter element is 5μm or even 2μm.

 

3.1.3 Spinning Process

 

During the preparation of PAN precursor fibers, the spinning solution is ejected from the spinneret assembly and solidified into a fibrous solid after entering the coagulation bath. For the newly formed precursor fibers, the PAN macromolecules inside are almost disordered. This disordered arrangement is not conducive to the improvement of the tensile strength of the precursor, which directly affects the performance of the carbon fiber.

 

In order to obtain densely structured PAN precursor fibers, the fibers must be drawn. The higher the drawing ratio applied to the precursor fibers, the higher the regularity of the arrangement of the PAN macromolecular chains in the precursor fibers, the denser the fiber structure, and the more likely it is to obtain high-performance carbon fibers. The spinning methods of PAN-based carbon fibers are usually melt, dry, and wet.

 

Since the decomposition temperature of PAN polymers is close to their melting temperature, melt spinning is generally not possible in industrial production. Dry spinning is the earliest industrialized PAN spinning method. Using dry spinning, dense precursor fibers can be obtained, which is very beneficial for obtaining high-performance carbon fibers. However, due to its poor production capacity, it has not been industrially applied in the field of carbon fiber precursor fibers.

 

Considering productivity and equipment complexity, wet spinning is currently the most commonly used spinning method in industry. The mainstream wet spinning on the market refers to the spinning method in which the coagulation process of the polymer occurs in the liquid phase.

 

Therefore, the spinneret is immersed in the coagulation bath, and the spinning solution directly enters the coagulation bath through the spinneret. Wet jet wet spinning and the spinneret is not in direct contact with the coagulation bath. After the spinning solution is ejected from the spinneret, it first passes through a certain distance of air segment and then enters the coagulation bath for coagulation. This method is called dry jet wet spinning. Both of the above methods belong to wet spinning.

 

In China, wet spinning is usually referred to as wet jet wet spinning, and dry jet wet spinning is sometimes referred to as dry-wet spinning. Compared with the wet method, dry jet wet spinning technology can significantly increase the draft ratio in the spinning process, thereby increasing the overall spinning speed, making it easier to regulate the fiber structure formation process and its physical and mechanical properties, and in some cases it is also beneficial to solvent recovery and improve the operating environment.

 

Both spinning methods are used in the industrial production of PAN-based carbon fiber precursor, and each has its own advantages and disadvantages.

 

 

At present, the production of T300 carbon fiber in China mainly adopts wet spinning, that is, the raw silk liquid is sprayed out from the spinneret and directly enters the coagulation liquid. In this way, the pores and defects generated inside the fiber increase accordingly. At the same time, due to the obstruction of the diffusion of the solvent outward, the solvent molecules evaporate in the pre-oxidation carbonization stage, leaving many defects. These defects will eventually be inherited by the carbon fiber, resulting in low strength of the carbon fiber.

 

The production of T700 carbon fiber takes a different route. Most of them use dry-jet wet spinning technology, that is, the raw silk liquid comes out of the spinneret hole and does not directly enter the coagulation liquid, but first passes through a section of air before entering the coagulation liquid. Because the viscosity of polyacrylonitrile solution is high, it needs to be under a certain pressure to spray out of the spinning hole. The raw silk liquid will expand when it comes out of the hole. At this time, under the action of stretching, the diameter of the raw silk liquid gradually becomes thinner.

 

At the same time, since the surface layer has not yet contacted water, the shrinkage rate of the surface layer and the core is the same, and stretching will not cause surface collapse. This will make the cross-section of the spun raw silk more regular, the surface layer and the core phase are uniform, and the defects produced are relatively few. Therefore, there will be fewer defects in subsequent pre-oxidation and carbonization, so the strength of T700 carbon fiber will be higher than that of T300.

 

In practical applications, these two methods have their own advantages and disadvantages. After optimizing the corresponding production process, the wet method can also produce carbon fiber precursors with T700 and T800 strengths.

 

The surface structure of wet products is relatively more conducive to compounding with matrix materials such as resins and then processing them into composite components through molding and manufacturing, but the relatively low production efficiency will make the production cost relatively high, so it is more suitable for application in high-end equipment fields with relatively high requirements for performance and stability; the dry-wet process has relatively high production efficiency and lower production costs. The products are more suitable for general industrial and civil fields that do not have high performance requirements but are more concerned about economy, as well as application fields with winding as the molding process.

 

Therefore, the choice of wet products or dry-wet products is often not determined by the producer, but by the user in the end.

 

3.2 Carbon Fiber Manufacturing Process

The core of the carbon fiber manufacturing process is to transform the PAN raw yarn obtained by the aforementioned spinning process into carbon fiber through a series of high-temperature heat treatment processes. The production speed of carbon fiber is very different from that of raw yarn. The international dry-wet method has a maximum speed of 1000 m/min, and the domestic method has reached 500 m/min, but the carbon fiber is basically less than 20 m/min.

 

Therefore, these two processes cannot be organized into a continuous production line, but can only be divided into two independent parts. In the entire carbon fiber preparation process, high-temperature treatment equipment is the most core and key equipment in the carbon fiber production line. The stability and reliability of the equipment have a direct impact on the continuous operation of the carbon fiber production line and the product performance of the carbon fiber.

 

Overall, my country’s high-temperature technology and high-temperature equipment are still somewhat behind the international advanced level. Most of the newly built carbon fiber production lines in China use high-temperature equipment imported from abroad.

 

 

3.2.1 Pre-oxidation

Pre-oxidation refers to the slow and gentle oxidation of PAN raw fibers under a certain tension in an oxidizing atmosphere at a temperature of 200-300°C, and the formation of a large number of ring structures on the basis of PAN straight chains to achieve the purpose of high temperature resistance.

 

The density of the fiber obtained after pre-oxidation (generally pre-oxidized fiber) can be increased to more than 1.3g/m³. Usually, in order to achieve such density requirements, the fiber needs to stay in the oxidation furnace for more than 1h. Therefore, the pre-oxidation process is the most time-consuming and energy-consuming process in the entire process of carbon fiber manufacturing.

 

From an economic point of view, the oxidizing atmosphere used is naturally the best air. Other oxidizing gases such as oxygen, nitrogen dioxide, sulfur dioxide, ozone, etc. will also be used in industry or experiments.

 

 

The structural homogeneity of pre-oxidized fibers is a prerequisite for the preparation of high-performance carbon fibers, because the fiber structure and defects formed during the pre-oxidation of the precursor will be inherited to the carbonization stage, which will eventually affect the various properties of the carbon fibers.

 

The process parameters in the oxidation process mainly include temperature and its gradient distribution, pre-oxidation atmosphere, pre-oxidation time, drafting force, etc. The core modulus of carbon fibers is related to the density and orientation of the cortical structure in the fiber. Among them, the loose and disordered pre-oxidized fiber structure has a lower core modulus.

 

Generally speaking, the fiber pre-oxidation time is short and the cortical structure is thin; when the pre-oxidation time is long, the cortical structure of the generated carbon fiber is thicker. If the relevant parameters used in the pre-oxidation production process are low (such as traction, temperature, etc.) and the processing time is long, it is not easy to form an obvious skin-core structure, but the relative production efficiency is low.

 

In the whole process of producing carbon fibers, preventing the fiber skin-core structure from bringing two-phase phenomenon to the carbon fiber structure is an important factor in the preparation of homogeneous carbon fibers.

 

 

Among them, the hot air circulation system is the most technically advanced part of the industrial preoxidation furnace, and it is also the part with the greatest difference among the preoxidation furnaces provided by different preoxidation furnace manufacturers. The hot air circulation system directly forms the isothermal area inside the preoxidation furnace, so it plays a decisive role in the temperature uniformity of the working space inside the furnace.

 

The preoxidation process of PAN fiber is an exothermic process. A large amount of reaction heat will be generated during the preoxidation process. If this heat cannot be transferred and removed in time, it will cause heat storage and local overheating, thereby affecting the oxidation uniformity of the fiber. In severe cases, it may even cause the fiber to burn out or even catch fire.

 

Therefore, this point must be taken into consideration when designing the hot air circulation system. In the constant temperature zone, the temperature fluctuation is best controlled below ±2℃. It is necessary to achieve uniform temperature inside the furnace through strict calculation and ingenious design of air volume, wind speed, wind direction, etc.

 

In addition, the preoxidation time is also directly linked to the cost. Improving the preoxidation technology and reducing the preoxidation time of carbon fiber are also one of the current development directions of preoxidation-related processes. At present, few companies in my country can manufacture pre-oxidation furnaces that can meet all these relevant indicators. This is also one of the important gaps between my country’s carbon fiber companies and the world’s leading carbon fiber companies.

 

3.2.2 Carbonization And Graphitization

 

Pre-oxidized fibers must be followed by a carbonization process. The carbonization process is a process in which the pre-oxidized fibers that will not burn at high temperatures are treated at a high temperature of 300-1500℃ under nitrogen protection, and most of the non-carbon fiber elements are removed under the action of high temperature.

 

In the early stage of the carbonization process, in the range of 300-400℃, the PAN straight chain breaks and begins to cross-link; in the range of 400-900℃, the thermal decomposition reaction of PAN begins, releasing a large amount of small molecular gas, and the graphite structure begins to form; above 900℃, the remaining nitrogen atoms begin to fall off in the form of nitrogen, the carbon content increases rapidly, the graphite structure is developed, the fiber shrinks as a whole and forms a carbon fiber with good mechanical properties. The mass fraction of carbon in the treated fiber is at least 92%, and the total weight loss is 55%-56%.

 

 

The graphitization process is not a necessary process for the preparation of carbon fiber, it is an optional process.

 

In the traditional industrial preparation method, if you want to obtain carbon fiber with high elastic modulus, you will need to carry out the graphitization process; if you want to obtain high-strength carbon fiber, you usually do not need to carry out the graphitization process.

 

The processing temperature of the graphitization process is above 2000℃, and the time is very short, just a few seconds. In order to prevent nitrogen from reacting with carbon at this high temperature, the protective atmosphere needs to use more inert argon.

 

The first function of the protective atmosphere is to maintain the positive pressure in the low-temperature furnace, and the second function is to take away the toxic pyrolysis products.

 

After graphitization, the carbon content in carbon fiber can reach more than 99.9%, so some places call such carbon fiber graphite fiber.

 

High temperature forms a developed graphite mesh structure inside the fiber, and drawing regularizes these graphite structures. Both have an important impact on the performance of the final fiber.

 

3.2.3 Surface Oxidation Treatment

The surface of carbonized fiber is basically composed of carbon atoms, so it has strong chemical inertness. However, the fiber needs to be compounded with a substrate such as resin, and its surface is required to have appropriate activity. Therefore, the number of oxygen-containing active functional groups on the fiber surface should be increased through the surface oxidation process.

 

There are many oxidation methods, and electrochemical oxidation is mainly used in industry. Electrochemical oxidation treatment utilizes the conductivity of carbon fiber. The carbon fiber is placed in an electrolyte solution as an anode. The active oxygen generated by anode electrolysis oxidizes the surface of the carbon fiber, thereby introducing oxygen-containing functional groups to improve the interfacial bonding performance of the composite material.

 

The degree of surface oxidation of carbon fiber can be controlled by changing the reaction temperature, electrolyte concentration, treatment time and current size.

 

The electrolytes used for electrochemical oxidation include nitric acid, sulfuric acid, phosphoric acid, acetic acid, ammonium bicarbonate, sodium hydroxide, potassium nitrate, etc.

 

At present, ammonium electrolytes such as ammonium bicarbonate are most commonly used because they do not corrode equipment and have good electrolysis effects.

 

 

3.2.4 Sizing Agent Treatment

The surface of carbon fiber is an inert graphite-like structure. Although such a structure makes it have good corrosion resistance, it also reduces the wettability between the fiber and the resin.

 

Therefore, the presence of sizing agent can effectively make the carbon fiber fully wetted with resin, reduce the air content in the prepreg, and reduce the porosity of the composite material.

 

The sizing agent is a thin layer of resin evenly covering the surface of the carbon fiber. Its mass fraction in the fiber is 0.3% to 1.2%. Although its content is very low, it plays an important role in the performance of carbon fiber and its woven cloth, the preparation of prepreg, and the performance of composite materials.

 

According to different specifications, a bundle of carbon fiber contains tens of thousands of carbon fiber filaments. Therefore, the primary function of the sizing agent is to bundle a large number of filaments into a bundle to prevent the fiber from fluffing and loosening; in addition, the carbon fiber will rub against multiple rollers during the production and weaving process. If there is no protection of the sizing agent layer, the carbon fiber filaments are easily broken, thereby reducing the strength of the fiber body.

 

Currently, sizing agents are mainly divided into three categories: solution-type sizing agents, emulsion-type sizing agents and water-soluble sizing agents. The emulsion-type sizing agents are currently the main ones used in the market.

 

3.3 Carbon Fiber Preform Fabric Production

The definition of preform is a carbon fiber reinforcement that is pre-shaped according to the designed structural details before being placed in the mold for resin impregnation. The purpose is to prevent damage to the carbon fiber in subsequent industrial production and to improve the strength in the thickness direction.

 

There are many types of carbon fiber preforms, which can be classified into woven fabrics, braided fabrics, knitted fabrics, etc. by weaving methods. Woven fabrics are made of two or more mutually perpendicular fiber bundles, interwoven at a 90-degree angle. The longitudinal fiber bundles are called warp yarns, and the transverse fiber bundles are called weft yarns.

 

Braided fabrics are provided by the spindles that move in the circumferential direction while the cellulose is drawn out. The drawn cellulose is assembled in the vertical direction and continuously extended in the length direction to form a cross-structure. Knitted fabrics are the process of bending yarns into coils in sequence, and the coils are interlaced to form fabrics. It can be carried out horizontally or vertically. The horizontal weaving is called weft knitted fabrics, and the longitudinal weaving is called warp knitted fabrics.

 

Knitted fabrics have good stretchability and elasticity, their production process is highly automated, and require less manpower. It is one of the important weaving methods for 3D fabrics.

 

 

3.4 Carbon Fiber Intermediate Molding Products

 

The so-called carbon fiber intermediate molding products include prepreg, premix, CFRTP particles, SMC and BMC, etc., each of which is used for various purposes and uses. Among them, prepreg and SMC are the two most important and widely used intermediate products.

 

Prepreg is used to meet the pre-requirements of manufacturing high-precision and high-performance fiber-reinforced composite materials, usually using epoxy resin as the matrix resin. SMC and others pay more attention to the moldability during the product manufacturing process, and the reinforcing fibers used are all chopped fibers.

 

With the application of carbon fiber in general industrial fields, especially in the automotive industry, SMC has become a molding technology that has attracted much attention with its high efficiency and low cost advantages in combination with compression molding technology.

 

3.4.1 Prepreg

Prepreg is a secondary processing product that integrates reinforcing fibers and resin to improve quality and operating efficiency. It is mainly used as an intermediate base material for the molding of high-precision and high-performance fiber-reinforced resin materials.

 

More than half of the advanced composite materials represented by CFRP (carbon fiber composite material) are formed by prepreg. Prepreg, as the intermediate material of composite materials, is a prepreg sheet product made by impregnating reinforcing fibers into a matrix. The reinforcing materials used mainly include carbon fiber, glass fiber, aramid fiber, etc.

 

The matrix used mainly includes polyester resin, epoxy resin, thermoplastic resin, etc. The preparation of prepreg is to impregnate fibers or fabrics with resin. There are many production process methods, and different processes are used due to different resin matrices.

 

At present, thermosetting resins such as epoxy resin and phenolic resin are mostly used in the production of prepreg. The production processes mainly include solution method and hot melt method.

 

The solution method is difficult to control the resin content in the process, and there are problems such as environmental pollution. Therefore, the hot melt production process is mostly used in actual production. The advantages of hot melt prepreg are that the resin content is controllable, prepreg of specified gram weight can be produced, the control accuracy is high, the resin film is uniform, the prepreg appearance is good, the prepreg volatile content is low, and the process is safe.

 

In order to give full play to the reinforcing effect of carbon fiber, the resin material used in prepreg is generally epoxy resin with excellent comprehensive special effects. In some special fields, phenolic resin and bismaleimide resin are also used.

 

 

The Prepreg Process Has The Following Advantages:

(1) The content and orientation of the reinforcing fibers can be flexibly controlled. The resin content can be adjusted during the fiber impregnation process, so the amount of resin outflow during curing during the lamination process is very small, and a high-precision molded product can be obtained.

(2) It is a dry molding material, easy to laminate, and can partially reinforce the molded product. In addition, the thickness of the molded product can be adjusted by changing the number of laminates.

(3) Good surface condition. Through complete resin impregnation, degassing, etc., the surface condition of the molded flat is good.

(4) The operation process is also safe and hygienic.

 

However, The Following Disadvantages Should Also Be Noted:

(1) The range of resin selection is narrow, limited to semi-solid/solid resins at room temperature, mostly epoxy resins, and other resins are used in small amounts.

(2) The molding cycle is long. In order to improve the storage stability of prepregs, high-temperature curing curing agents are often used, so the molding time is long.

(3) Since some additional processes are required, the cost of the molded body is high.

 

3.4.2 SMC

Sheet molding compound (SMC) is a thin sheet intermediate molding material developed from fabric prepreg. The molding process of SMC is to press resin and short-cut carbon fiber (6-50mm) between two plastic films into a sheet (thickness of about 3mm) to obtain an intermediate molding material.

 

First, the resin is evenly coated on the plastic film, and then the cut fibers are spread on the coating surface. After calendering and degassing, the SMC finished product can be obtained. SMC products are currently widely used in the industrial field of enterprises.

 

Because SMC, as an intermediate product, can be stored for a long time, and SMC is a familiar material for designers, most existing automobile manufacturers already have the ability to manufacture and use SMC parts, so there is no need for a large amount of new investment, and it can be directly replaced.

 

 

3.5 Production of Carbon Fiber Composite Materials

Carbon fiber composite materials refer to composite materials in which at least one reinforcing material is carbon fiber. Regardless of the form, continuous fiber or short fiber, unidirectional or multi-directional, woven or non-woven, they are all carbon fiber composite reinforcement materials, and their excellent specific strength and specific modulus properties can bring obvious durability and weight reduction effects.

 

In addition, carbon fiber as an additive can also improve the electrical and thermal conductivity of composite materials, and because the thermal expansion coefficient of carbon fiber is very small, it can also be used to improve the dimensional stability of composite materials. Therefore, despite its inherent brittleness, carbon fiber has become one of the most important reinforcing fibers in the field of advanced composite materials.

 

Carbon fiber can be compounded with different matrix materials (such as resin, ceramic, etc.) to form various composite materials. The most common one is resin-based carbon fiber composite material (CFRP), which is widely used in general industrial fields such as aerospace, sports and leisure products, pressure vessels, wind blades, automobile manufacturing, and building reinforcement due to its obvious weight reduction and reinforcement effect.

 

Good thermal conductivity and near-zero thermal expansion coefficient make it also have incomparable advantages in the field of electrical appliances and space structures requiring dimensional stability.

 

 

For carbon fiber composite materials, the molding methods can be divided into three categories according to the use of the mold in the molding process: open mold molding (hand lay-up molding, injection molding, winding molding, autoclave molding, etc.), mold molding (resin transfer molding, compression molding, injection molding, vacuum bag molding) and other molding methods (sheet lamination, continuous pultrusion molding). There are many different molding methods under each category.

 

Different molding processes have their own advantages, disadvantages and limitations. If the selected molding process is not appropriate, the cost of the product may be greatly increased. Generally speaking, if the product size is small but the quantity is required, it is best to use molding methods such as compression molding that can be mechanized and continuously produced. If the product size is large and the shape is special, but the quantity is small, hand lay-up method, autoclave method, etc. can be used. The balance between the two can use resin transfer molding method. Pipes, high-pressure tanks and other rotating parts are particularly suitable for winding method.

 

3.5.1 Hand Lay-up and Spray Molding

 

As representatives of molding methods that do not actively perform wet lamination and heat and pressurization, hand lay-up molding and spray molding are currently the more basic carbon fiber composite molding methods.

 

Hand lay-up molding is one of the earliest molding methods used. It can be the starting point of all molding methods. It does not require special equipment and has a high degree of freedom in the size of the molded product. Therefore, it is still the preferred molding method for many composite products.

 

The spray molding method is to cut the fiber through a cutting device first, and then spray the cut fiber with resin through a spray gun to make it evenly deposited on the mold. It is a mechanized and labor-saving improvement of the hand lay-up molding method, but it is not suitable for applications such as aerospace that require high-performance materials.

 

 

3.5.2 Compression Molding

In order to improve the thickness accuracy and surface quality of the product in the hand lay-up method, a protective film is added to the laminate prepared with prepreg, etc., and then it is placed in a mold and hardened at a certain temperature and pressure. Compression molding is a molding method.

 

The most basic compression molding is stamping. By providing pressure through a stamping forming machine, high-performance CFRP sheets and products of various shapes can be obtained. As various extensions and new molding technologies developed from compression molding, there are vacuum bag molding, pressure bag molding, autoclave molding and other technologies.

 

Among them, autoclave molding is the earliest technology developed for the manufacture of composite materials for aviation structures and is still widely used, especially for some large-sized and complex-shaped parts.

 

The process flow of autoclave molding is: a composite material blank made of a single layer of prepreg laid in a predetermined direction is placed in an autoclave, and the curing process is completed at a certain temperature and pressure.

 

The raw material used in this molding process is also a carbon fiber prepreg intermediate, which has the advantages of being able to solidify laminates of different thicknesses, being able to manufacture complex curved parts, having a wide range of uses, and having stable and reliable processes. However, it also has disadvantages such as high equipment investment costs, high process production costs, and product size being limited by the size of the autoclave. It is suitable for manufacturing aircraft doors, fairings, airborne radar covers, brackets, wings, tail fins and other products.

 

 

3.5.3 Winding Technology

Filament winding (FW) can maximize the strength of reinforcing fibers. The basic operation process is to impregnate the continuous fiber bundle in liquid resin, wind it on the mold core, heat and harden it at room temperature or in a furnace, and then demold it to obtain the product (wet winding).

 

In contrast, dry winding is to use the corresponding prepreg and then heat it while winding. Winding is mostly used for general-grade cylindrical products such as pipes and containers, such as fishing rods, golf flagpoles, various industrial pipes, pressure vessel products, rocket nozzles, etc. It can also be used for the molding of polygonal aircraft parts, windmill impellers and other complex cross-section objects.

 

 

3.5.4 RTM Molding Technology

Resin transfer molding (RTM) technology is a low-cost composite material manufacturing method. It was originally used mainly for aircraft secondary load-bearing structural parts, such as doors and inspection covers. It has now become one of the most active research directions in the fields of aerospace material processing and automotive component assembly in recent years.

 

RTM technology has the advantages of high efficiency, low cost, good part quality, high dimensional accuracy, and low environmental impact. It can be used for the molding of large, complex, and high-strength composite parts.

 

The main principle of the RTM process is to lay a reinforced material preform designed according to performance and structural requirements in a mold cavity (the mold cavity needs to be pre-made into a specific size), and use an injection device to inject a special resin system into the closed mold cavity within a certain pressure range. The resin and the reinforcement are infiltrated and cured to form the mold.

 

It is a molding method that does not use prepreg or autoclave.

 

3.5.4 RTM Molding Technology

Resin transfer molding (RTM) technology is a low-cost composite material manufacturing method. It was originally used mainly for aircraft secondary load-bearing structural parts, such as doors and inspection covers. It has now become one of the most active research directions in the fields of aerospace material processing and automotive component assembly in recent years.

RTM technology has the advantages of high efficiency, low cost, good part quality, high dimensional accuracy, and low environmental impact. It can be used for the molding of large, complex, and high-strength composite parts.

The main principle of the RTM process is to lay a reinforced material preform designed according to performance and structural requirements in a mold cavity (the mold cavity needs to be pre-made into a specific size), and use an injection device to inject a special resin system into the closed mold cavity within a certain pressure range. The resin and the reinforcement are infiltrated and cured to form the mold.

It is a molding method that does not use prepreg or autoclave.

The main derivative technologies of RTM include vacuum induction molding process, flexible assisted RTM, co-injection RTM and high-pressure RTM (HPRTM).

Among them, HP-RTM uses preforms, steel molds, vacuum-assisted exhaust, high-pressure injection and high-pressure to complete the impregnation and curing process of high-performance thermosetting composite materials, achieving low-cost, short-cycle (large-volume) and high-quality production. BMW’s carbon fiber body production at the Landshut plant in Germany uses this process.

HP-RTM can produce high-quality, high-precision, low-porosity, high-fiber content complex composite components, with the advantages of high production efficiency, curing within minutes, CAD design of mold products, easy manufacturing and multiple uses.

 

3.5.5 Pultrusion Technology

Pultrusion is a method of shaping by continuous drawing, that is, after the fiber bundle is impregnated in the resin, it is passed through a mold with a predetermined inner cavity profile, and is formed by microwave heating and other means to quickly enter a gel state, thereby achieving the purpose of curing and shaping. Its typical profiles are round, square, I-shaped, etc.

 

 

The advanced pultrusion (ADP) technology developed by JAMCO, an aviation equipment manufacturer, is a continuous pultrusion molding of carbon fiber prepreg, which can theoretically obtain CFRP materials of any length.

 

This technology can achieve automated continuous molding, so it has high processing efficiency and low cost, and can obtain products with excellent quality, extremely low internal porosity and precise dimensions. ADP technology is particularly suitable for the molding of parts with certain cross-sectional shapes (such as C, H, L, Ω, etc.) and very straight length directions, such as the main wings, vertical and horizontal tails of passenger aircraft.

 

At present, its products have been used in the vertical tails of various models of Airbus A300 series aircraft and in the A380 as the second floor of the load-bearing structure.

 

3.5.6 Injection Molding

 

Injection molding is mainly used for short fiber reinforced thermoplastic resins. The prepared resin/short fiber mixture is stored in a storage tank and enters the sleeve under the action of gravity. The heating device on the outer wall of the sleeve heats the temperature to above the melting point of the resin, and the high-speed shearing of the screw also generates a large amount of heat to accelerate the softening and melting of the resin.

 

Under the action of the screw, the material is concentrated at the front end of the sleeve and is taken into the mold through the nozzle. The finished product can be obtained after cooling and demolding.

 

Currently, injection molding is easier to cope with complex shape molding than stamping and other technologies, and the dimensional accuracy is also good, and it is easy to realize automation.

 

However, due to the high-speed rotation of the screw, it will cause certain damage to the fiber and it is difficult to control the orientation of the fiber, so only isotropic products can be obtained.

 

Even so, injection molding is still very suitable for the molding of CFRP parts with large-scale production requirements and not very strict requirements on mechanical properties, such as the processing of automobile front end panels.

 

 

3.5.7 Laying-down Molding Technology

For a long time, most CFRP components used in the aerospace field have used the prepreg process, but the cost of the prepreg process is relatively high, especially the cutting and stacking process of the prepreg, which is the link with the highest labor cost and labor time consumption.

 

In developed countries such as Europe and the United States, this problem is particularly prominent due to the high labor cost of skilled workers. In addition, manual stacking and cutting are difficult to meet the requirements of large-scale and integrated aerospace composite components in terms of both construction period guarantee and quality. The laying-down molding process is a fully automatic manufacturing technology developed on the basis of the fiber winding molding process. It is a general term for the automated fiber placement (AFP) technology and the automated tape layer (ATL) technology.

 

The automated laying-down technology is a molding technology jointly researched and developed by aircraft manufacturers and material suppliers. Its main purpose is to complete the molding of large composite components by realizing automation and high speed, improve production efficiency and reduce production costs.

 

So far, the largest single composite component in the aerospace field is produced by ATL technology. Since large components can be molded in one go, this also reduces the assembly cost of the components. It is precisely because of the emergence of automatic placement technology that the large-scale application of CFRP composite materials in commercial aircraft has become a reality.

 

 

Composite material technology is not static. On the contrary, it will change with the development of technology and the expansion of the market.

 

The earliest wind power carbon fiber molding technology used classic prepreg laying. This method is expensive and has problems such as low production efficiency and poor product performance.

 

Later, the process of glass fiber was borrowed, and multi-layer fabrics were vacuum infused. However, carbon fiber has better wettability than glass fiber, which leads to the need to leave a resin flow channel for carbon fiber fabrics during the production process. This requires special technology for fabrics and brings expensive costs. In addition, it is difficult for fabrics to ensure the straightness of fibers under the impact of resin, which directly affects the performance of composite materials.

 

When VESTAS adopted pultruded plates that are easy to mass produce, the amount of carbon fiber used in wind turbine blades increased rapidly, because this technical route reflects the cost-effectiveness that previous processes did not have.

 

At present, as the entire wind power industry develops towards large-scale wind turbines, countries around the world have begun to develop corresponding pultruded carbon beam technologies to meet the growing market demand.

 

4. Current Development of China’s Carbon Fiber Production Technology

In 2019, China’s total demand for carbon fiber was 37,840 tons, compared with 31,000 tons in 2018, an increase of 22% year-on-year. Among them, the import volume was 25,840 tons (accounting for 68% of the total demand, an increase of 17.5% over 2018). The supply of domestic fibers has improved, and the domestic carbon fiber production capacity has been gradually released. It has exceeded 30% for two consecutive years. The high-speed growth rate, the substitution trend is obvious. It is expected that around 2025, the domestic carbon fiber market share in China is expected to exceed imports.

 

This substantial increase in domestic share is inseparable from the progress of domestic carbon fiber manufacturing technology and related application technologies.

 

4.1 Technological breakthroughs have been made, and domestic carbon fibers have been put into production one after another

 

4.1.1 Breakthrough In Small Tow Technology

The development of small tow carbon fiber materials in China began with military use, and currently aerospace is an important application field. Carbon fiber composite materials are ideal materials for large integrated structures. Compared with conventional materials, carbon fiber composite materials can reduce the weight of aircraft by 20%-40%, and have the ability to overcome the shortcomings of metal materials that are prone to fatigue and corrosion, thereby enhancing the durability of aircraft; the good formability of composite materials can significantly reduce the cost of structural design and manufacturing. It has been widely used and rapidly developed in the field of military aviation, and the penetration rate of carbon fiber composites has continued to rise.

 

In recent years, China’s T700 and T800 military carbon fiber material production technology has gradually matured. For example, the T700 production line put into production by Zhongfu Shenying Company is currently the third commercial production line in the world that uses a dry-jet wet spinning process system. In 2014, it won the first prize in the Progress Category of the Science and Technology Award of China Building Materials Group Co., Ltd., which is of great strategic significance.

 

Zhongfu Shenying’s wet spinning system includes a homogenization polymerization system suitable for dry-jet wet spinning, a low-disturbance air layer fiber forming system, and a high-speed high-multiple steam drafting system. The project independently developed and designed a fast heat exchange full-mix 60 cubic meter polymerization kettle, dry-jet wet spinning fiber forming equipment, steam drafting equipment, a full set of carbonization key equipment, and efficient solvent recovery and waste gas treatment systems.

 

At present, the domestic T700 has been successfully applied to emerging industrial fields such as wind blades, cable composite cores, pressure vessels and transportation, and has been initially used in certain aircraft models and aerospace equipment.

 

In January 2016, the Chinese Academy of Sciences took the lead in achieving a major breakthrough in the domestic M55J preparation technology. In September of the same year, the preparation technology was verified and high-strength and high-modulus carbon fiber with a tensile strength of 4.15GPa and a tensile modulus of 585GPa was obtained. Subsequent research further achieved the continuous and stable production of domestic M55J high-strength and high-modulus carbon fiber, and the batch-to-batch dispersion coefficient of the fiber main performance was <5%.

 

In 2018, high-strength and high-modulus carbon fibers with a tensile strength of 5.24 GPa and a tensile modulus of 593 GPa were successfully prepared. Compared with Japan’s Toray M60J high-strength and high-modulus carbon fibers (tensile strength of 3.92 GPa and tensile modulus of 588 GPa), they continued to maintain their advantages in tensile strength.

 

In September 2017, Zhongfu Shenying Carbonization Production Line No. 10, with a single-line SYT55 (T800) grade carbon fiber production capacity of 1,000 tons, was officially put into production. The official commissioning of Line 10 is a milestone in the history of China’s carbon fiber development, following the successful commissioning and continuous and stable operation of the 1,000-ton T800 precursor production line in May 2016, and a new step in technological improvement based on the existing 100-ton T800 carbon fiber production line.

 

From 2019 to early 2020, many companies also announced expansion plans. In January 2019, Zhang Shouchun’s team, a researcher at the Shanxi Coal Chemistry Research Institute of the Chinese Academy of Sciences, prepared T-1000-grade ultra-high-strength carbon fiber. The ultra-high-strength carbon fiber based on polyacrylonitrile, which was undertaken as a key project of the Chinese Academy of Sciences, passed the acceptance inspection and successfully developed a new type of hollow carbon fiber based on polyacrylonitrile. In February 2019, Zhongfu Shenying announced a major expansion project of 20,000 tons of carbon fiber in Xining with an investment of 5 billion yuan. In mid-2019, Jilin Jinggong Group expanded a 2,000-ton carbonization line, which is expected to be put into production in mid-2020. Jiangsu Hangke expects to build a 1,000-ton carbonization line in 2020. Ton-level T1000 carbon fiber production line and 100-ton-level MJ series high-strength and high-modulus carbon fiber production line, and carry out the research and industrialization of higher-performance fibers such as T1200 and M70J and special composite materials; In March 2020, China Baowu Strategic Planning Department and the People’s Government of Keqiao District, Shaoxing, Zhejiang signed the “Memorandum of Cooperation on Industrial Development”, which involves the fine work carbon fiber industry. Baowu Group may enter the carbon fiber industry in a big way; In March 2020, Lanzhou Bluestar Carbon Fiber signed a framework agreement with Yiyuan County, Shandong Province to start the construction of the second phase of the project, including 50,000 tons of raw silk and 25,000 tons of carbon fiber.

 

4.1.2 Large-tow Technology Breakthrough

Although the continuous performance of large-tow carbon fiber is not as good as that of aerospace-grade small-tow, its technical barriers are also quite high. As an industrial-grade carbon fiber, its core driving force lies in low cost. Therefore, it is extremely important to effectively control costs while ensuring large-tow.

 

The preparation of large-tow carbon fiber belongs to low-cost production technology, and its price is only 50%-60% of that of small-tow carbon fiber. Similarly, comparing PANEX35 and T300 carbon fibers, the current sales price of Toray T300 in the domestic market is about 1,000 yuan per kilogram, while the price of PANEX35 is less than 200 yuan per kilogram.

 

For decades, my country’s carbon fiber has followed the technical route of Japanese companies (especially Toray) and has achieved great results in the preparation of small tows. However, for the preparation of large tows, at the overall system technology level, my country is still basically a “layman”, and there is still a lot of equipment and engineering lack of experience, but more and more companies are beginning to devote themselves to the research and development of large tows.

 

Shanghai Petrochemical: In May 2016, the industrial research and test of 48K large-tow carbon fiber was carried out; in January 2018, the polymerization, spinning, and oxidation carbonization process technology of large-tow carbon fiber was successfully developed, forming a complete set of technical process packages for 1,000-ton PAN-based 48K large-tow carbon fiber; in March 2018, 48K large-tow carbon fiber was successfully trial-produced, and the entire process was connected, and the single-filament strength was higher than the level of T300-grade carbon fiber; in August 2018, the “Development of Polyacrylonitrile (PAN)-based Large-tow Precursor and Carbon Fiber Technology and Process Package” project passed the appraisal, marking a breakthrough in the domestic large-tow bottleneck.

 

Jilin Chemical Fiber: In July 2017, based on the experience in the research and development of 24k precursor, it began to study 48K large-tow carbon fiber precursor. Through the reengineering of the stock solution process and the upgrading of key equipment technology, it successfully developed the polymerization and spinning process technology of 48K carbon fiber precursor in July 2018, forming a thousand-ton 48K carbon fiber precursor technology process package; in August 2018, 100 bundles of 48K carbon fiber precursors passed the carbonization smoothly. After testing, the tensile strength of 48K carbon fiber reached 4000Mpa, the tensile modulus reached 240Gpa, and the interlaminar shear strength reached 60Mpa; in May 2019, it obtained the first batch of 120 tons of 48K large-tow carbon fiber precursor export orders, and domestically produced 48K large-tow carbon fiber precursor went abroad in batches for the first time. Lanzhou Fiber: In June 2019, Beijing Bluestar Cleaning, Lanzhou Fiber, Nantong Xingchen and other units used domestic large-tow carbon fiber (50K) and polyphenylene ether (PPE) resin to melt blend and successfully developed a continuous large-tow carbon fiber reinforced modified polyphenylene ether (MPPE) thermoplastic composite material with independent intellectual property rights, filling the gap in domestic products in this field; In March 2020, the person in charge of Lanzhou Fiber Company signed a strategic cooperation framework agreement for investment promotion with the People’s Government of Yiyuan County, and Lanzhou Bluestar’s 25,000-ton 50K large-tow carbon fiber project settled in Yiyuan County, Zibo, Shandong.

 

Guangwei Composites: In July 2019, Guangwei Composites, the People’s Government of Jiuyuan District, Baotou City, Inner Mongolia, the Jiuyuan Industrial Park Management Committee, and Vestas jointly signed the “10,000-ton Carbon Fiber Industrial Park Project Entry Agreement”, planning to invest 2 billion yuan to build a 10,000-ton carbon fiber industrialization project in the Jiuyuan Industrial Park in Baotou City in three phases; the project mainly relies on Inner Mongolia’s low-cost energy to build large-tow carbon fiber to further enhance the competitiveness of wind power carbon beam business.

 

4.2 The downstream market is expanding rapidly, and there are many technological breakthroughs in the application field.

The composite material industry often says that design is the leader, materials are the foundation, manufacturing is the key, application is the purpose, and maintenance is the guarantee. We believe that driven by the strong downstream demand, more and more companies will invest in the design and development of carbon fiber composite materials, thus setting off a new round of carbon fiber application technology revolution.

 

Globally, the four major application industries of carbon fiber are aerospace, wind turbine blades, sports and leisure, and automobiles, and the demand for carbon fiber in the four major industries accounts for 22.6%, 24.5%, 14.4%, and 11.3% respectively. The unit price of carbon fiber in the aerospace field is the highest. In terms of amount, the transaction amount of carbon fiber in the aerospace field accounts for 50% of the entire industry.

 

In China, the demand for carbon fiber mainly comes from the relatively low-end sports and leisure field. The two major application industries of carbon fiber in the Chinese market are sports and leisure and wind turbine blades, with carbon fiber demand accounting for 37% and 36.4% respectively. The consumption in the aerospace field accounts for only 3%, and the development potential is relatively large.

 

 

Modern carbon fiber materials originated from military use, and currently aerospace is an important application field. Carbon fiber composite materials are ideal materials for large integrated structures. Compared with conventional materials, carbon fiber composite materials can reduce the weight of aircraft by 20%-40%, and have the ability to overcome the shortcomings of metal materials that are prone to fatigue and corrosion, enhancing the durability of aircraft; the good formability of composite materials can significantly reduce the cost of structural design and manufacturing costs.

 

It has been widely used and rapidly developed in the field of military aviation, and the penetration rate of carbon fiber composites has continued to rise. At present, my country has made significant technological progress from 3% of carbon fiber in the second-generation J-7 and 6% of carbon fiber in the J-10, to about 10% of carbon fiber in the third-generation J-11 and 20% of carbon fiber in the fourth-generation J-20. Although there is still a large gap with the average carbon fiber usage of 30% in the fourth-generation fighters of the United States, there is still huge room for improvement in the proportion of carbon fiber used in my country’s military aircraft.

 

The production line of the C919 large aircraft has also driven the growth of downstream carbon fiber technologies for aerospace, especially in the fields of large-size composite integrated molding and low-cost manufacturing.

 

In Made in China 2025, carbon fiber is listed as a strategic material. It is planned that the domestic carbon fiber consumption will reach more than 4,000 tons in 2020, meeting the technical requirements of large aircraft, and high-performance carbon fiber will basically achieve independent guarantee by 2025.

 

In the field of wind power, China is currently the world’s largest wind power market, with a large volume and rapid development. It is expected that China’s new installed capacity will remain at a high scale in the next few years, and the demand for wind power market is vast. Carbon fiber replacing glass fiber as blade material has become the mainstream trend in the market.

 

When the wind turbine exceeds 3MW and the blade length exceeds 40 meters, the advantages of carbon fiber blades are significantly higher than those of fan blades made of general fiberglass composite materials. The corresponding production cost increases little, but the efficiency is doubled.

 

At present, Vestas is the world’s main customer for the development of high-power wind power blades with carbon beams, and has multiple blade factories around the world.

 

In addition, in addition to Vestas, domestic downstream wind power blade manufacturers such as Goldwind Technology and Mingyang Smart Energy are also actively developing corresponding technologies. Envision Wind Power has begun to use pultruded plates to make prototypes.

 

At present, the proportion of domestic carbon fiber consumption in wind turbine blades is increasing rapidly. From 2015 to 2019, the compound growth rate of domestic carbon fiber consumption in wind power was as high as 97.8%.

 

 

The new energy vehicle market is also one of the markets with the largest potential for carbon fiber demand in my country. With the development of lightweighting in the automotive industry, the application of carbon fiber materials in the automotive field is also increasing.

 

At present, one of the biggest obstacles for consumers to pure electric vehicles should be the problem of mileage anxiety. In order to more effectively improve the driving range of pure electric vehicles, the lightweighting of the body is crucial. In addition to making lighter body structures, carbon fiber can also be used to make fuel cell stacks or high-pressure gas tanks for storing hydrogen in new energy vehicles, thereby better ensuring the safety of gas tanks.

 

We believe that the high-pressure gas tank market is one of the largest and fastest growing markets for advanced composite materials, especially carbon fiber wound composite materials.

 

It is expected that the carbon fiber consumed by composite compressed natural gas and hydrogen cylinders for fuel cell vehicles (FCVs) in 2025 will be almost the same as the expected consumption of wind turbine blades, 50% more than the predicted consumption of chassis and body parts for automobiles, railways and other ground transportation, and twice that of the aerospace field.

 

 

5. Current Problems in China’s Carbon Fiber Industry

 

5.1 Core technology has not yet made a fundamental breakthrough, and there is still a generation gap between high-end precursors and foreign ones

 

From the perspective of precursors, domestic carbon fibers generally have weak performance control capabilities, reflecting that the deep-level correlation between process-ingredient-structure-performance has not been thoroughly studied. For example, there have always been loopholes in the quality of high-performance PAN precursors, especially in the control of microscopic indicators such as molecular weight distribution.

 

Stable precursor production includes two major processes: chemical synthesis of polyacrylonitrile and spinning of polyacrylonitrile fibers. There are many process steps, high equipment requirements, and many influencing factors.

 

The key to the performance of carbon fiber lies in the quality of the precursor. The internal defects of the precursor are “inherited” to the carbon fiber almost unchanged in shape after carbonization. my country’s relevant technology accumulation is relatively weak. Some precursors have low yields in mass production, and there is still room for improvement in quality. In addition, for high-quality precursors, the precursor utilization rate is 2.1, that is, 2.1kg can produce 1.0kg of carbon fiber; while the quality of my country’s precursors is poor, and the average precursor utilization rate increases to 2.5, which increases production costs.

 

In addition, the domestic production process of carbon fiber precursor is single, and the DMSO-one-step-wet spinning process is generally adopted. The development of other precursor technologies is relatively lagging, resulting in product homogeneity. The products have the same advantages and disadvantages and cannot form a complementary effect, resulting in serious closed competition among my country’s carbon fiber companies.

 

Compared with the three major carbon fiber companies in Japan, the technical routes of the companies are different, and the product differentiation is obvious. For example, Toray Group adopts the dimethyl sulfoxide technical route, Mitsubishi Rayon adopts the dimethylformamide technical route, and Toho Corporation adopts the zinc chloride technical route. In terms of high-end precursor research and development, in July 2015, the research team of Georgia Institute of Technology in the United States used the innovative PAN-based carbon fiber gel spinning technology to increase the tensile strength of carbon fiber to 5.5-5.8GPa and the tensile elastic modulus to 354-375GPa. Although the tensile strength is equivalent to IM7, the elastic modulus has achieved a significant increase of 28%-36%.

 

This is the combination of high strength and highest modulus of carbon fiber reported so far. The mechanism is that the gel links the polymer chains together, generating strong intra-chain forces and directional orientation of the crystallites, ensuring high strength under the condition of large crystallites required for high elastic modulus.

 

This shows that the United States has the independent research and development capabilities of the third-generation carbon fiber products, while my country is still in the research and development and use stage of the second-generation carbon fiber.

 

 

5.2 Lack of key equipment manufacturing technology and heavy reliance on imported machinery from abroad

From the perspective of carbon fiber manufacturing technology, my country still has a large gap with the world’s leading companies in key carbonization furnace-related technologies and special equipment. The carbonization furnace is the place where the fiber is carbonized. According to the maximum working temperature, it can usually be divided into low-temperature carbonization furnace and high-temperature carbonization furnace. In order to obtain high modulus, ultra-high temperature carbonization furnace or graphitization furnace is also required.

 

High-temperature treatment equipment is the most core and key equipment in the carbon fiber production line. The stability and reliability of the equipment have a direct impact on the continuous operation of the carbon fiber production line and the performance of carbon fiber products.

 

Due to the monopoly of foreign countries on equipment production technology, only smaller domestic carbonization furnace equipment can be manufactured in China.

 

Due to the blockade of foreign high-end carbonization equipment and carbon fiber production equipment in China, it is difficult for domestic carbon fiber production enterprises to scale up, and mechanical equipment accounts for a large proportion of the cost, which increases the cost of enterprises and affects the survival and international competition of carbon fiber enterprises.

 

In addition, due to the lack of carbonization furnace technology, my country does not have the ability to mass-produce high-strength and high-modulus series carbon fibers. Only some enterprises can produce carbon fiber products with similar performance in small batches.

 

The preparation of high-strength and high-modulus carbon fiber requires a graphitization furnace, and the temperature of the graphitization furnace must reach above 2800 degrees. However, the performance of high-temperature equipment in my country is unstable due to the limitation of furnace raw materials, and these materials are embargoed by foreign countries, so production and research and development are difficult.

 

In comparison, the relevant carbonization technology abroad is more advanced and is in constant innovation. Toray successfully developed T1100G carbon fiber using traditional PAN solution spinning technology. By finely controlling the carbonization process, the microstructure of carbon fiber is improved at the nanoscale, and the orientation, crystallite size, and defects of graphite in the carbonized fiber are controlled, thereby greatly improving the strength and elastic modulus.

 

The tensile strength of T1100G is 6.6GPa, which is 12% higher than that of T800; the elastic modulus is 324GPa, which is 10% higher, and it is entering the industrialization stage. NEDO has also developed electromagnetic wave heating technology. Electromagnetic wave carbonization technology refers to the carbonization of fibers using electromagnetic wave heating technology under atmospheric pressure. The performance of the obtained carbon fiber is basically the same as that of the carbon fiber produced by high temperature heating, the elastic modulus can reach more than 240Gpa, and the elongation at break is also more than 1.5%.

 

Compared with the traditional oxidation furnace, the plasma oxidation furnace developed by RMX in the United States only needs 25 to 35 minutes, while the traditional oxidation furnace needs to stay for 80 to 120 minutes for the oxidation process, and at the same time achieves a 75% reduction in energy consumption; Composites World reported in 2016 that Toho of Japan is developing microwave-heated carbonization furnaces and plasma surface treatment technology. The gap between Chinese companies and these world-renowned companies is still large, and there is a clear generation gap in equipment.

 

 

5.3 The application level of downstream is low, and the gap in high-end technology is large

From the perspective of downstream applications, the application level of carbon fiber composite materials in my country is low, and the main market application is still in the processing of sports and wind power. The upstream and downstream collaborative innovation is insufficient, and the situation that downstream composite material companies dare not use or do not know how to use it has not been significantly improved.

 

In recent years, my country’s large-scale investment in carbon fiber and composite materials in civil aviation, automobile and other transportation fields has not achieved the expected output and benefits, and enterprises are in trouble, only showing some light in the wind power industry.

 

The industrial maturity of large-scale application of advanced composite materials in my country in civil aviation, high-speed rail and automobile and other transportation fields is very low. Most products are still in the laboratory and engineering verification stage, new markets have not yet been formed, and the industry maturity is in its infancy; it is still necessary to continue to break through key technologies and various maturity levels down to earth.

 

This is also related to the fact that my country’s second-generation carbon fiber technology has not yet made a comprehensive breakthrough and has failed to keep up with the development of third-generation carbon fiber technology in a timely manner. This will widen the research and development gap between my country and foreign countries in the performance of next-generation aviation weapons and equipment and carbon fiber composite materials technology, including lightweight, low-cost large-tow carbon fiber preparation research for automobiles, building repairs, etc., additive manufacturing technology, recycling technology and rapid prototyping technology of carbon fiber composite materials, etc.

 

 

 

5.4 High Production Cost, Obvious Characteristics Of “Capacity But No Output”

Domestic carbon fiber has capacity but no output, mainly due to the low maturity of industrialization technology, low cost performance of products, and lack of application service capabilities of products. In 2019, China’s sales/capacity ratio was 45%, an increase from last year’s 33.6%. However, there is still a certain gap from the usual international capacity ratio of 65-85%.

 

Due to the lack of traction from the downstream market, domestic carbon fiber cannot be used, resulting in low domestic carbon fiber output. At this stage, domestic carbon fiber is still mainly produced in small tow products of 12K and below. The industrial production technology of high-quality, large tow, low-cost, and large-scale carbon fiber has not yet been completely broken through, while foreign countries have begun to integrate large tow low-cost with small tow high-quality carbon fiber industrial production technology, continuously improving product quality and reducing costs.

 

The main reason for the low technical maturity of China’s carbon fiber industry is that the industrialization construction of carbon fiber does not rely on the company’s own technology, but more on reference and imitation, and the company lacks the “key man” of technical background and core technology. Since the atmosphere of respecting intellectual property rights has yet to be established, effective cooperation between enterprises and scientific research institutes is difficult to establish, and the improvement of industrialization technology is restricted.

 

The lack of domestic equipment production capacity and the weak secondary transformation capacity of imported equipment have led to the industrialization process to cater to equipment conditions, losing the industrialization principle with process as the core, and the problems of low product quality stability and low capacity release rate are prominent.

 

In terms of engineering technology, due to the large number of problems that need to be solved in this regard, my country’s carbon fiber single line has a small scale and high cost. At present, the single line capacity of major foreign carbon fibers has reached 1500t/a, and the maximum exceeds 2000t/a. In addition to several leading enterprises in my country, the single line capacity of domestic carbon fiber is relatively small, and most of them are less than 1,000 tons. The small scale of single lines also leads to an increase in product production costs.

 

According to calculations, the cost of a production line with an annual output of 500t is about 159,000 yuan/t, and the cost of a production line with an annual output of 1500t is about 117,000 yuan/t. Compared with the former, the latter product cost is reduced by 27%.

 

 

At present, most of China’s carbon fiber production lines are low-level duplicate construction, concentrated in the T300-T700 level. The T800 military field is still being verified and is now in a small-scale mass production stage. The application level in other fields is currently low and the demand is small.

 

Moreover, since the cost cannot be reduced, it lacks competitiveness in the market. At present, domestic carbon fiber companies can only make profits in the aerospace field, and most other companies are in a loss-making state.

 

This is because aviation-grade carbon fiber is mainly supplied to the high-end market demand, and only needs to meet high performance. In the short term, profits can still be maintained at a high level.

 

Industrial-grade carbon fiber is usually used in the traditional low-end market, which needs to meet high performance under the premise of low cost. It is mainly price competition, and there is no strict performance requirement in performance. If a simple price war is fought, under the strategy of low-price dumping abroad, domestic carbon fiber will not be profitable.

 

 

5.5 The scale of talents is still small, and the development structure of the industrial chain is unbalanced.

After decades of hard work, China has made great progress in the research and development of carbon fiber and composite materials, and has cultivated a group of professional and technical talents. However, due to the fact that the overall scale and technical level of China’s carbon fiber and composite materials industry are far behind the world’s advanced countries, the scale of the talent team in the field of carbon fiber and composite materials is limited, and there is a serious shortage of talents who master key technologies. At the same time, the distribution of talents is uneven.

A large number of composite material design and process technology talents are mainly concentrated in the field of national defense, while the design and process technology personnel in the emerging industrial application field are seriously lacking, which directly affects the promotion and application of carbon fiber composite materials in the industrial field, and it is difficult to support the overall development of China’s carbon fiber and composite materials industry.

In recent years, although the industrialization level of enterprises has improved, the support for basic research is uneven, and due to the limitations of talents, professional foundations and production tasks, it is impossible to truly carry out basic research; while the research and development of universities and research institutions is often based on model products, with the goal of tracking and imitating foreign indicators, and insufficient investment in basic research.

There is a generation gap between China’s military high-performance fibers and composite materials and the advanced level of foreign countries. At the same time, the development of domestic fiber series is mainly based on the tracking and imitation model, and the independent innovation ability is insufficient, which is not suitable for the needs of high-end equipment to shoulder and lead the development. There is also an imbalance in the development of the domestic industrial chain structure.

 

The concentration of Chinese enterprises is low, and it is difficult to grow bigger and stronger in a short period of time; there are too many small enterprises in my country’s carbon fiber industry, with high repetitiveness and single varieties, and there are funds but no technical advantages; there are few high-end carbon fiber and composite material research and production units, while there are many low-end enterprises and fierce competition.

 

For example, a large number of carbon fiber companies have flocked to the defense and military fields, which not only has a serious impact on the limited military market, but also has uneven equipment and product technology levels, and it is difficult to unify the testing and standard systems. The survival of enterprises faces serious difficulties, which seriously affects the improvement of domestic carbon fiber technology level and has a dragging effect on the innovation and development of military carbon fiber composite materials.

 

At the same time, some local industry authorities have not strictly supervised, and there are many companies in the market that “fool” the national funds, which has also led to a waste of national resources and slowed down the overall development of my country’s carbon fiber industry.

 

 

6. Summary

In general, the production technology of carbon fiber is complex and the process is numerous. The manufacture of high-quality carbon fiber requires the interlocking of various technologies from raw silk to composite materials.

 

The core links of the carbon fiber industry chain include upstream raw silk production, midstream carbonization links, and downstream composite materials and applications; the industrial chain has very high consistency requirements from raw silk, carbonization, prepreg, and composite materials, which is reflected as a technology-intensive industry.

 

The manufacturing process of carbon fiber is complex. It is a system engineering that integrates multidisciplinary, refined, and high-end technologies. It involves multiple disciplines such as physics, chemistry, textiles, materials, precision machinery, and automation. The process includes high-precision control of thousands of parameters such as temperature and humidity, concentration, year, and flow. Comprehensive control can ultimately ensure the stability of carbon fiber performance and quality.

 

Therefore, the entire industry has high technical barriers, long R&D cycle, high investment, high viscosity, and strong first-mover advantage. my country’s carbon fiber production technology has made breakthroughs one after another, and downstream demand has grown steadily.

 

my country’s carbon fiber production enterprises have successively broken through the technical blockade. At present, they have stable production capacity of T300, T700, T800 and even a small amount of T1000 in small tows; in large tows, many industrial lines such as Jilin Chemical Fiber and Shanghai Petrochemical have been put into production, and many industrial lines are under construction by Guangwei Composites and Lanzhou Fiber. China’s large tow is expected to go abroad.

 

In addition, my country’s downstream demand for carbon fiber is also showing a rapid growth trend. In 2019, China’s total demand for carbon fiber was 37,840 tons, a year-on-year increase of 22% compared with 2018. The main growth point still comes from the wind power field. The demand potential related to aerospace and new energy vehicles is relatively large, and the downstream market growth rate is gratifying.

 

Although my country’s carbon fiber technology has made rapid progress in recent years, there is still a generation gap between high-end carbon fiber and foreign countries. Equipment, technology, production capacity, supporting industrial chain and talent reserves are still far behind those of the United States and Japan.

 

my country’s main gap still comes from the gap in high-end carbon fiber manufacturing technology caused by foreign technology blockade. The gap in core technology and core manufacturing equipment has also led to an increase in my country’s production costs. In addition, the current domestic talent pool is small and the unbalanced industrial chain structure has also restricted the development of my country’s carbon fiber industry to a certain extent.