Carbon Fiber - Material Black Gold, With High Industry Barriers And High Product Added Value

1. Carbon fiber is rigid and flexible, and is widely used in downstream applications

The history of material development is closely related to the history of human development, and new materials are a powerful driving force for mankind to move from the "natural kingdom" to the "freedom". Materials are usually defined as substances used to make useful objects, and human's ability to recognize and utilize materials directly determines the social form and human living standards. In contemporary times, materials, energy and information have become the three pillars of social civilization and national economy, and among them, materials are the material basis and technological leader of the development of science and technology.

Throughout the entire history of material development, it can be summarized into six development periods of stone tools/bronze tools/iron tools/steel/silicon/new materials in terms of time. Among them, with the opening of the new technology revolution in the second half of the 20th century, new materials have become a booster for the development of various high-tech fields. For example, computer technology relies on the industrial production of semiconductor materials, and the aerospace industry requires a large number of high-temperature and high-strength structural materials. It is matched with it, and modern optical fiber communication is based on low-consumption optical fiber.

Carbon fiber, known as the king of new materials in the 21st century, is a shining jewel in the material crown. Carbon fiber (CF for short) is an inorganic fiber with a carbon content of more than 90%. It is prepared by cracking and carbonizing organic fibers (viscose-based, pitch-based, polyacrylonitrile-based fibers, etc.) in a high temperature environment to form a carbon backbone mechanism. As a new generation of reinforcing fibers, carbon fibers have excellent mechanical and chemical properties, not only have the inherent characteristics of carbon materials, but also have the soft processability of textile fibers, so they are widely used in aerospace, energy equipment, transportation, sports. Leisure and other fields:

Light weight: As a strategic new material with excellent performance, the density of carbon fiber is basically the same as that of magnesium and beryllium, and it is less than 1/4 of that of steel. Using carbon fiber composite material as a structural material can reduce the structural mass by 30%-40%.

High strength and high modulus: The specific strength of carbon fiber is 5 times higher than that of steel and 4 times higher than that of aluminum alloy; the specific modulus is 1.3-12.3 times that of other structural materials.

Small expansion coefficient: The thermal expansion coefficient of most carbon fibers is negative at room temperature, 0 at 200-400 °C, and only 1.5 × 10-6 /K when it is less than 1000 °C, so it is not easy to expand and deform due to high working temperature.

Good chemical corrosion resistance: Carbon fiber has a high content of pure carbon, and carbon is one of the most stable chemical elements, which makes it very stable in acid and alkali environments, and can be made into various chemical anti-corrosion products.

Strong fatigue resistance: The carbon fiber structure is stable. According to the statistics of the polymer network, the strength retention rate of the composite material is still 60% after millions of cycles of stress fatigue, compared with 40% for steel, 30% for aluminum, and 30% for glass fiber reinforced plastic. Then only 20%-25%.

Carbon fiber composite materials are re-strengthened on the basis of carbon fiber. Although carbon fiber can be used alone and perform specific functions, it is a brittle material after all. Only when it is combined with the matrix material to form a carbon fiber composite material can it exert better mechanical properties and carry more loads.

Carbon fibers can be classified according to different dimensions such as precursor type, manufacturing method, and performance:

According to the type of raw silk: polyacrylonitrile (PAN) based, pitch based (isotropic, mesophase); viscose based (cellulose based, rayon based). Among them, polyacrylonitrile (PAN)-based carbon fiber occupies the mainstream position, with the output accounting for more than 90% of the total carbon fiber, and viscose-based carbon fiber is less than 1%.

Classification according to manufacturing conditions and methods: carbon fiber (800-1600 ℃), graphite fiber (2000-3000 ℃), activated carbon fiber, vapor-grown carbon fiber.

According to the mechanical properties, it can be divided into general-purpose type and high-performance type: general-purpose carbon fiber has a strength of 1000MPa and a modulus of about 100GPa;), in which the strength greater than 4000MPa is also called the ultra-high strength type, and the modulus greater than 450GPa is called the ultra-high model.

According to the size of the tow, it can be divided into small tow and large tow: small tow carbon fiber is mainly 1K, 3K, 6K in the initial stage, and gradually developed into 12K and 24K, which is mainly used in aerospace, sports and leisure fields. Carbon fibers above 48K are usually called large tow carbon fibers, including 48K, 60K, 80K, etc., which are mainly used in industrial fields.

Tensile strength and tensile modulus are the two most important indicators to measure the performance of carbon fiber. Based on this, my country promulgated the "National Standard for Polyacrylonitrile (PAN)-based Carbon Fiber (GB/T26752-2011)" in 2011. At the same time, because Japan's Toray has an absolute leading advantage in the global carbon fiber industry, most domestic manufacturers also adopt the classification standard of Japan's Toray as a reference.

2.High technical barriers in the industry, raw silk production is the core, carbonization and oxidation is the key

The carbon fiber production process is complex and requires extremely high equipment and technology. The control of the accuracy, temperature and time of each link will greatly affect the quality of the final product. Polyacrylonitrile carbon fiber has become the carbon fiber with the widest application field and the highest output at this stage due to its relatively simple preparation process, low production cost, and convenient disposal of three wastes. Its main raw material, propane, can be obtained from crude oil, and the polyacrylonitrile carbon fiber industry chain includes a complete manufacturing process from primary energy to end-use applications.

After preparing propane from crude oil, propane can be obtained by selective catalytic dehydrogenation (PDH);

Acrylonitrile is obtained after ammoxidation of propylene, and polyacrylonitrile (PAN) precursor is obtained after acrylonitrile polymerization and spinning;

Polyacrylonitrile is pre-oxidized, carbonized at low temperature and high temperature to obtain carbon fiber, and can be made into carbon fiber fabric and carbon fiber prepreg for the production of carbon fiber composite materials;

Carbon fiber is combined with resin, ceramics and other materials to form carbon fiber composite materials, and finally the final products required by downstream applications are obtained by various molding processes;

The quality and performance level of the precursor directly determine the final performance of carbon fiber. Therefore, improving the quality of the spinning solution and optimizing the various factors of the precursor fiber formation have become the key nodes in the preparation of high-quality carbon fibers.

According to "Research on the Production Process of Polyacrylonitrile-Based Carbon Fiber Precursor", the spinning process mainly includes three categories: wet spinning, dry spinning and dry-wet spinning. At present, the production process of polyacrylonitrile precursors at home and abroad mainly adopts wet spinning and dry-wet spinning, among which wet spinning is the most widely used.

In wet spinning, the spinning solution is first extruded from the spinneret hole, and the spinning solution enters the coagulation bath in the form of a fine stream. The spinning mechanism of polyacrylonitrile spinning solution is: there is a large gap between the concentration of DMSO (dimethyl sulfoxide) in the spinning solution and the coagulation bath, and the concentration of water in the coagulation bath and polyacrylonitrile solution is also huge. gap. Under the interaction of the above two concentration differences, the liquids begin to diffuse in both directions, and finally condense into filaments through processes such as mass transfer, heat transfer, and phase equilibrium movement.

Residual DMSO, fineness, monofilament strength, modulus, elongation, oil content, and boiling water shrinkage in the production of raw silk become the key factors affecting the quality of raw silk. Taking the residual amount of DMSO as an example, it has an effect on the apparent properties of the precursor, the cross-sectional state, and the CV value of the final carbon fiber product. The lower the residual amount of DMSO, the higher the performance of the product. In production, DMSO is mainly removed by washing, so how to control the washing temperature, time, the amount of desalinated water and the amount of washing circulation becomes an important link.

High-quality polyacrylonitrile precursors should have the following characteristics: high density, high crystallinity, appropriate strength, circular cross-section, less physical defects, and at the same time have a smooth surface and a uniform and dense skin-core structure.

The temperature control of carbonization and oxidation is the key. Carbonization and oxidation is an essential link in the production of raw silk into carbon fiber final products. In this link, the precision and range of temperature need to be accurately controlled, otherwise the tensile strength of carbon fiber products will be significantly affected, and even wire breakage will occur:

Pre-oxidation (200-300℃): In the pre-oxidation process, by applying a certain tension in an oxidizing atmosphere, the PAN precursor is slowly and mildly oxidized, and a large number of ring-packed structures are formed on the basis of the PAN straight chain, so that it can withstand The purpose of higher temperature processing.

Carbonization (maximum temperature not lower than 1000℃): The carbonization process needs to be carried out in an inert atmosphere. In the early stage of carbonization, the straight chain of PAN is broken, and the cross-linking reaction begins; as the temperature gradually rises, the thermal decomposition reaction begins, releasing a large amount of small molecular gas, and the graphite structure begins to form; after the temperature further rises, the carbon content increases rapidly, and the carbon fiber begins to form.

Graphitization (processing temperature above 2000°C): Graphitization is not a necessary process for carbon fiber production, but an optional link. If the carbon fiber is expected to have a high elastic modulus, graphitization is required; if the carbon fiber is expected to obtain high strength, graphitization is not required. In the graphitization process, the high temperature causes a developed graphite mesh structure to be formed inside the fiber, and the structure is normalized by drawing to obtain the final product.

High technical barriers endow downstream products with high added value, and the price of aerospace composite materials is 200 times higher than that of raw silk. Due to the high difficulty of carbon fiber preparation and the complex process, the more downstream its products are, the higher the added value, especially the high-end carbon fiber composite materials used in the aerospace field. Because downstream customers have very strict requirements on reliability and stability, the product price It is also geometrically increased compared to ordinary carbon fiber.