What is the production process of carbon fiber yarn?

Carbon fiber yarn production is a complex process that involves multiple steps to transform raw materials into high - performance carbon fiber. Here is a general overview of the production process:

1. Raw material selection

Polyacrylonitrile (PAN) fiber: It is the most commonly used raw material for carbon fiber production. PAN fibers offer high strength and modulus potential after carbonization. The quality of the PAN fiber, including its molecular weight distribution, linear density, and uniformity, significantly affects the final carbon fiber properties.

Pitch - based fibers: Pitch can be derived from coal tar or petroleum residues. Pitch - based carbon fibers are often used in applications where high thermal conductivity is required, such as in heat - resistant materials. However, the production process is more complex compared to PAN - based fibers.

Cellulose - based fibers: Although less common, cellulose - based materials like rayon can also be used. But they generally require more extensive pre - treatment due to their high oxygen content.

2. Pre - oxidation

Oxidation environment: The selected raw fiber (usually PAN fiber) is placed in an oxidizing atmosphere, typically air, at a temperature range of 200 - 300 °C. This is a crucial step as it stabilizes the fiber structure and makes it non - melting, which is necessary for the subsequent high - temperature carbonization process.

Chemical reactions: During pre - oxidation, various chemical reactions occur within the fiber. The nitrile groups in PAN fiber start to cyclize, forming ladder - like structures. Oxygen from the air is incorporated into the fiber, leading to the formation of carbon - oxygen bonds. This process gradually increases the fiber's density and changes its color from white to brown.

3. Carbonization

High - temperature furnace: After pre - oxidation, the fiber is transferred to a high - temperature carbonization furnace. The furnace is usually filled with an inert gas such as nitrogen to prevent oxidation. The temperature in the carbonization furnace is raised to around 1000 - 1500 °C.

Carbon removal: At this high temperature, non - carbon elements such as hydrogen, nitrogen, and oxygen are removed from the fiber in the form of gases (e.g., H₂O, CO, CO₂, N₂). The fiber's carbon content increases significantly, typically reaching over 90%. As a result, the fiber's mechanical properties, such as tensile strength and modulus, are greatly enhanced.

4. Graphitization (optional)

Ultra - high - temperature treatment: For high - performance carbon fibers, an additional graphitization step may be carried out. The carbonized fiber is further heated to extremely high temperatures, usually between 2000 - 3000 °C, in a graphitization furnace under an inert gas atmosphere.

Graphite structure formation: At these high temperatures, the carbon atoms in the fiber rearrange themselves into a more ordered graphite - like structure. This leads to a significant increase in the fiber's electrical and thermal conductivity, as well as further improvement in its mechanical properties, especially the modulus.

5. Surface treatment

Oxidation treatment: The surface of the carbon fiber is often treated to improve its adhesion with matrices in composite materials. One common method is to expose the fiber to an oxidizing agent, such as a mixture of nitric acid and sulfuric acid, or to an electrochemical oxidation process. This treatment creates oxygen - containing functional groups on the fiber surface, such as hydroxyl (-OH), carboxyl (-COOH), and carbonyl (C = O) groups, which enhance the fiber - matrix bonding.

Sizing application: After oxidation treatment, a sizing agent is applied to the fiber surface. The sizing agent is a polymer - based material that not only protects the fiber from damage during handling but also further improves its compatibility with the matrix material in composites. The sizing agent forms a thin film around the fiber, typically a few nanometers to a few micrometers thick.

6. Yarn formation

Spinning and winding: The individual carbon fibers are then grouped together and spun into yarns. The spinning process involves carefully controlling the tension and alignment of the fibers to ensure uniform yarn quality. The yarns are wound onto bobbins or spools for further processing or use in the manufacturing of carbon fiber - reinforced composite materials.

The entire production process of carbon fiber yarn requires strict control of temperature, time, and environmental conditions at each stage to produce high - quality carbon fiber with consistent properties.