Introduction
Carbon fiber is made from acrylic, viscose or asphalt fiber raw materials through a series of processes such as oxidation, carbonization, graphitization, and surface treatment. Carbon fiber is resistant to high temperatures, has high strength, and is lightweight. It is an excellent material for manufacturing high-tech equipment such as aerospace. With the continuous maturity and development of production technology, carbon fiber has also begun to be widely used as an alternative material in the fields of automobiles, medical treatment, wearable equipment, sports equipment, etc. This article will introduce the production process of carbon fiber in detail.
Producing
1、Selection and preparation of raw fibers
There are three main types of raw fiber materials for making carbon fiber: viscose fiber, polyacrylonitrile fiber, and asphalt fiber. The following table shows the parameters of these three raw materials:
| Fiber name | Chemical composition | Carbon content | Carbonization yield | Carbonization temperature |
| Viscose fiber | (C6H10O5)n | 21-35% | ＜50% | 1000℃-1600℃ |
| Polyacrylonitrile fiber | (C3H3N)n | 68% | 50%-60% | 1000℃-1600℃ |
| Asphalt fiber | C,H | 95% | ＞80% | 1000℃-2500℃- |
Polyacrylonitrile (PAN) is the main raw material for manufacturing carbon fiber, accounting for more than 90%. It has a high carbon content and its molecular structure is easily graphitized. Asphalt/viscose fiber accounts for less than 5% and is mostly used in special scenarios (such as ultra-high modulus requirements).
PAN is mixed with a solvent and extruded through a spinneret into filaments with a diameter of 5-10 microns. This PAN filament is the raw material for producing carbon fiber, also called the precursor of carbon fiber.
2、Pre-oxidation of raw silk
The PAN fiber precursor is heated to 200-300℃ in air for about 30 minutes to 2 hours, so that the carbon atoms combine with the oxygen atoms in the air and change their linear atomic bonds into ladder bonds, which can greatly improve the thermal stability of the fiber material and facilitate subsequent carbonization. The pre-oxidized precursor will turn black.
3、High temperature carbonization
After the PAN fiber is thermally stabilized, it is heated to 1,000-3,000℃ in the absence of oxygen for a few minutes. Under this high temperature and oxygen-free condition, the fiber expels its non-carbon atoms, and the remaining carbon atoms form tightly bound carbon crystals. These carbon crystals are arranged parallel to the long axis of the carbon fiber, which greatly increases its strength.
4、Surface treatment
The surface of carbon fiber after carbonization is very smooth, which is not conducive to bonding with epoxy resin and other materials. Therefore, it is necessary to slightly oxidize the surface of carbon fiber to improve its surface properties. The main methods of surface oxidation include gas phase oxidation, liquid phase oxidation, electrochemical oxidation, etc. The specific methods are shown in the table below:
| Method Name | Specific Process | Oxidizing Agent
|
| Gas phase oxidation | Oxidizing gas is used to oxidize the fiber surface to make the fiber surface have a certain degree of roughness. In the later treatment, the bonding degree between the fiber and the resin matrix can be improved, thereby improving the interface performance of the carbon fiber. | Carbon dioxide, air or ozone |
| Liquid phase oxidation | Liquid oxidants are used to oxidize the carbon fiber surface. Compared with the gas phase oxidation method, it is less likely to cause excessive etching and cracking of the carbon fiber. | Concentrated nitric acid, mixed acid and strong oxidants, etc. |
| electrolytic oxidation | Carbon fiber is used as anode, carbon graphite, nickel and copper are used as cathode, and an aqueous solution of acid, alkali, salt or a concentrated mixture is used as an electrolyte. By electrolyzing water, oxygen is generated at the anode to oxidize the surface of the carbon fiber. | Oxygen produced by electrolysis |
5、Sizing
After the surface oxidation treatment, the carbon fiber is coated with polyester, nylon, urethane or epoxy resin. This process is called sizing. Sizing can help stabilize the structure of carbon fiber, reduce the brittleness of carbon fiber, avoid fuzzing and broken fibers during later processing, and protect them from damage during transportation and production.
Finally, the finished carbon fiber filaments are wound into reels for easy transportation and subsequent production of other carbon fiber products.
