Carbon fiber material and its production process

Carbon fiber material and its production process

Text/Meite polymer milanonly

Carbon fiber is actually a fibrous material with over 95% carbon content. That is, main element of carbon fiber is carbon. It is a microcrystalline graphite material obtained by stacking organic fibers such as flake graphite microcrystals along axial direction of fibers and undergoing carbonization and graphitization. Carbon fiber is not a new type of material, but it is not currently widely used in cars, more often people stick carbon fiber imitation film on car body. The current carbon fiber body is mainly used in high-end cars and race cars. A typical BMW i3 uses all-carbon fiber. In addition, many race cars also use carbon fiber materials for a large number of components in order to reduce weight. At moment, cost of carbon fiber is a big limitation for application of carbon fiber. Let's talk about characteristics of a carbon fiber body. ①Light weight

Studies have shown that for every 10% reduction in body weight, fuel consumption is reduced by 6-8%, emissions are reduced by 5-6%, acceleration from 0-100 km/h is increased by 8-10%. , and stopping distance is reduced from 2 to 7 meters. In addition, light weight of body can also improve handling of car. The biggest advantage of carbon fiber is that it weighs less than 1/4 weight of steel and is lighter than metallic aluminum, making it perfect material for "lightness". 30% lighter than aluminum and 50% lighter than steel. If all steel parts of car are replaced with carbon fiber composite materials, weight of car body can be reduced by 300 kg.

②High strength

Carbon fiber is 20 times stronger than iron and is only material that does not lose strength at a high temperature of 2000°C. Its excellent shock absorbing ability is 4-5 times that of ordinary metal materials. . For materials with same cross-sectional area, carbon fiber can withstand several times more force than steel. Carbon fiber is applied to axle shafts and drive shafts to improve transmission efficiency. In addition, carbon fiber is different from metal and has characteristics of fatigue and corrosion resistance. Carbon fiber car components have a longer service life under normal use.


However, main disadvantage of carbon fiber materials is their low ductility. This is mainly due to fact that metallic materials such as steel are ductile and can undergo large deformation without breaking, while carbon fiber composite materials do not have this characteristic. Therefore, it will break after a strong impact, and itscannot be repaired but replaced, and cost is relatively high. That's why only supercars and F1 cars use carbon fiber bodies.

In general, performance of carbon fiber material is much better than that of steel material. Under same conditions, a carbon fiber body can greatly improve a vehicle in terms of safety, handling, fuel economy and durability. We're just waiting for cost of carbon fiber materials to drop and family cars can also "afford" this top-tier material. The BMW i3 is said to be cheapest carbon fiber car out there, but don't smash it.

Carbon fiber material and its production process
Carbon fiber manufacturing process

There are many raw materials that can be used to make carbon fiber. Depending on source, it can be divided into two categories. One is man-made fibers such as viscose silk, viscose, lignin fiber, etc., and other is synthetic fibers, which are raw materials refined from oil and other natural resources, and then processed into silk after processing, such as acrylic fiber, pitch fiber, polyacrylonitrile (PAN) fiber, etc.

After many years of development, only viscose (cellulose) based carbon fibers, resin fibers and polyacrylonitrile (PAN) fibers are currently available. >. industrially mastered process of obtaining carbon fiber from three types of raw materials.

1. Viscose (cellulose) based carbon fiber

Abrasion-resistant materials reinforced with viscose-based carbon fibers can be used to make large area ablative shielding materials for nose cones and warheads of rockets, rockets and space shuttles, solid engine nozzles, etc., which is key to Decisive Aerospace and Rocket Technologies Material. Viscose-based carbon fiber can also be used as aircraft brake pads, automobile brake pads, radioisotope energy blocks, and reinforced resins as corrosion-resistant pump casings, vanes, pipes, containers, catalyst frame materials, conductive wires, and surface heating elements, seals. materials and medical absorbent materials, etc.

Although it is earliest precursor used in production of carbon fiber, since theoretical total carbon content of viscose fiber is only 44.5%, in actual production process, pyrolysis reaction often results in formation of cracked products such as L-glucose from - for misuse. cracking The actual yield of carbon is less than 30%. Therefore, cost of manufacturing viscose (cellulose) based carbon fiber is relatively high, currently, its production is less than 1% of total fiber in world. However, it has its unique advantages as an ablation-resistant material in aerospace vehicles. Because it contains less alkali and alkaline earth metal ions, sodium light generated during flight is weak and difficult to detect with radar, so it is still used in spacecraft. military industry. A small amount of production is reserved.

2. Carbon Fiber Resin

In 1965, Sugiro Otani of Gunma University in Japan successfully developed resin-based carbon fiber. Since then, pack a hundredl new raw material for carbon fiber production, and it is second largest raw material route in carbon fiber field after PAN. Sugiro Otani started heating polyvinyl chloride (PVC) to 400°C under protection of an inert gas, then melt-molded PVC bitumen, and then heated it to 260°C in air for non-melting processing, i.e. pre-oxidation, and then after a series of subsequent treatments, such as carbonization, pitch-based carbon fibers are obtained.

3. Carbon fiber based on polyacrylonitrile (PAN)

The carbonization yield of PAN-based carbon fiber is higher than that of viscose fiber, which can reach more than 45%, and because production process, solvent recovery and three-waste processing are simpler than viscose fiber, cost is low and source of raw materials is abundant .The mechanical properties of polyacrylonitrile carbon fiber, especially tensile strength and tensile modulus, are first of three carbon fibers. Thus, it is currently variety of carbon fiber with widest scope and highest performance.

Polyacrylonitrile based The production of carbon fiber mainly involves two processes: production of a precursor and its carbonization.

The production process of raw silk mainly includes processes such as polymerization, defoaming, dosing, spinning, drawing, washing, oiling, drying and collecting.

The carbonization process mainly includes processes of unwinding, pre-oxidation, low temperature carbonization, high temperature carbonization, surface treatment, sizing and drying, and winding.

Carbon fiber material and its production process

Under certain polymerization conditions, double bonds of acrylonitrile (AN) are opened under action of free radicals of initiator and are interconnected into linear macromolecular chains of polyacrylonitrile (PAN), releasing 17.5 kcal. /mole of heat, that is, resulting solution of polyacrylonitrile (PAN) for spinning can be obtained after spinning processes such as wet spinning or dry jet wet spinning. After deformation, PAN raw silk is sent to 1# pre-oxidation furnace and 2# pre-oxidation furnace to obtain pre-oxidized fiber (commonly known as pre-oxidized fiber); The pre-oxidized fiber enters low-temperature carbonization furnace and high-temperature furnace. -temperature carbonization for production of carbon fiber; after processing and sizing, carbon fiber products can be obtained. The whole process is carried out continuously, and any problem in any process will affect stable production and quality of carbon fiber products. The whole process is lengthy and consists of many procedures that represent integration of several disciplines and technologies.

The glass transition temperature (Tg) of homopolymerized PAN is 104°C, there is no softening point and decomposes at 317°C, and Tg of copolymerized PAN is about 85-100°C. Tg changes accordingly. The higher copolymer content, lower Tg. The preoxidation temperature is controlled between glass transition temperature and cracking temperature, ie between 200 and 300°C. The purpose of pre-oxidation is to transform linear macromolecular chain of thermoplastic PAN into a non-plastic heat-resistant trapezoid structure, making it refractory and incombustible at high carbonization temperature, maintaining fiber shape and maintaining a stable thermodynamic state. The pre-oxidized trapezoidal structure greatly improves carbonization efficiency and greatly reduces production cost. At same time, pre-oxidized silk (OF pre-oxidized fiber) is also an important intermediate product, which, after deep processing, can be made into various products, directly enter market, and is practically applied in many fields.

PAN precursors are processed into heat-resistant trapezoidal structures after pre-oxidation treatment, and then converted into carbon fibers with a graphite turbostratic structure by low-temperature carbonization (300-1000°C) and high-temperature carbonization (1000-1800°C). °C). During this structural transformation process, smaller ladder-shaped structural units are further cross-linked, polycondensed, and accompanied by pyrolysis, releasing many small molecular by-products upon transformation into a turbostratic graphite structure. In this case, non-carbon elements O, N and H are gradually removed, and C postprogressively enriched, eventually forming carbon fibers with a carbon content of more than 90%.

In addition, high-modulus graphite fibers or high-strength, high-performance high-modulus carbon fibers can also be obtained by further graphitization of carbon fibers. That is, graphitization is carried out at a high heat treatment temperature of 2000~3000°C, so that carbon fiber is transformed from an amorphous and turbostratic graphite structure into a three-dimensional graphite structure.

For carbon fiber, pre-oxidation time is almost one hundred minutes, carbonization time is several minutes, and graphitization time is relatively short, usually from a few seconds to tens of seconds.

Development of international carbon fiber industry

Japan is largest producer of carbon fiber in world. Three Japanese companies: Toray, Toho and Rayon currently own over 50% of world market for acrylonitrile based carbon fiber. At present, world's carbon fiber technology is mainly in hands of Japanese companies. The carbon fiber they produce is world-leading in quality and quantity. Japan's Toray is "leader" in research and production of high-performance carbon fibers in world. The most mature carbon fiber technology is in Japan.

The United States is one of few developed countries to have mastered carbon fiber technology after Japan, and is world's largest consumer of acrylonitrile-based carbon fiber, accounting for about 1/3 of global consumption.

The production of carbon fiber in world is mainly concentrated in a few developed countries such as Japan, United States, Germany and Taiwan Province in my country. Among them, largest carbon fiber manufacturers Toray (Japan), Toho (Japan) and Mitsubishi Rayon (Japan) account for more than half of global production.