How is carbon fiber made?

Carbon fiber is a remarkable material known for its high strength, low weight, and excellent chemical resistance. As a carbon fiber supplier, I am often asked about the process of making carbon fiber. In this blog post, I will take you through the intricate journey of how carbon fiber is produced, from the raw materials to the finished product.

Raw Materials

The production of carbon fiber begins with a precursor material, which is the starting point for the entire process. The most commonly used precursor is polyacrylonitrile (PAN), accounting for about 90% of all carbon fiber production. Other precursors include pitch and rayon, but they are less common due to various factors such as cost and the properties of the resulting carbon fiber.

PAN is a synthetic polymer that is chosen for its high carbon content and its ability to be easily spun into fibers. It is a long - chain molecule made up of repeating acrylonitrile units. The quality of the PAN precursor is crucial as it directly affects the quality of the final carbon fiber. High - purity PAN with a uniform molecular weight distribution is preferred to ensure consistent fiber properties.

Spinning

The first step in converting the precursor into carbon fiber is spinning. This process is similar to how natural fibers like cotton are spun into yarns. There are two main spinning methods used for PAN precursors: wet spinning and dry - jet wet spinning.

In wet spinning, the PAN polymer is dissolved in a solvent to form a viscous solution called a dope. This dope is then extruded through a spinneret, which is a small device with multiple tiny holes. As the dope emerges from the holes, it is immediately immersed in a coagulation bath. The solvent in the dope diffuses into the coagulation bath, while the non - solvent in the bath diffuses into the dope, causing the PAN to solidify into fine fibers.

Dry - jet wet spinning is a more advanced method. It is similar to wet spinning, but the dope is first extruded through a short air gap before entering the coagulation bath. This air gap allows for some initial stretching of the fibers, which helps to align the polymer chains in the fibers. This alignment is important as it contributes to the high strength and modulus of the final carbon fiber. The resulting PAN fibers from the spinning process are typically very fine, with diameters ranging from about 10 to 20 micrometers.

Stabilization

The spun PAN fibers are not yet carbon fiber. They need to undergo a stabilization process to make them thermally stable and prevent them from melting during the subsequent carbonization step. Stabilization is carried out in an oxygen - rich environment at temperatures ranging from about 200°C to 300°C.

During stabilization, a series of chemical reactions occur. The PAN fibers are oxidized, and the linear polymer chains start to form cross - links. This cross - linking makes the fibers more rigid and heat - resistant. The process is time - consuming and can take anywhere from 30 minutes to several hours, depending on the fiber diameter and the desired properties.

The stabilization process is carefully controlled because it has a significant impact on the final properties of the carbon fiber. If the stabilization is incomplete, the fibers may melt during carbonization. On the other hand, over - stabilization can lead to a decrease in the carbon yield and the mechanical properties of the final carbon fiber.

Carbonization

After stabilization, the fibers are ready for carbonization. This is a high - temperature process carried out in an inert atmosphere, usually nitrogen, at temperatures between 1000°C and 3000°C.

At these high temperatures, the non - carbon elements in the stabilized fibers, such as hydrogen, oxygen, and nitrogen, are removed as volatile gases. The remaining carbon atoms rearrange themselves into a graphite - like structure. The carbonization process further strengthens the fibers by increasing the degree of carbon - carbon bonding and improving the alignment of the carbon atoms along the fiber axis.

The temperature and the heating rate during carbonization are critical parameters. Higher carbonization temperatures generally result in carbon fibers with higher modulus but lower strength. The heating rate also affects the microstructure of the carbon fiber. A slow heating rate allows for more uniform carbonization and better - developed carbon structures.

Surface Treatment

Once the carbon fibers are carbonized, they undergo a surface treatment. The surface of the carbon fiber is relatively inert, which can make it difficult to bond with matrices such as resins in composite applications. Surface treatment is used to improve the adhesion between the carbon fiber and the matrix.

One common surface treatment method is oxidation. The carbon fibers are exposed to an oxidizing agent, such as air or a chemical oxidant, to create functional groups on the fiber surface. These functional groups can react with the matrix material, forming strong chemical bonds. Another method is plasma treatment, where the fibers are exposed to a low - temperature plasma. The plasma can etch the surface of the fibers, increasing the surface area and improving the mechanical interlocking between the fiber and the matrix.

Sizing

After surface treatment, the carbon fibers are coated with a sizing agent. The sizing is a thin layer of polymer that is applied to protect the fibers during handling, weaving, and composite manufacturing. It also helps to improve the processability of the fibers and provides additional protection against environmental damage.

The choice of sizing agent depends on the intended application of the carbon fiber. For example, if the carbon fiber is to be used in a epoxy - based composite, a sizing agent that is compatible with epoxy resins will be selected. The sizing agent should have good adhesion to the fiber surface and should not interfere with the bonding between the fiber and the matrix.

Finished Carbon Fiber Products

The resulting carbon fibers can be used to produce a wide range of products. For example, we offer Carbon Fiber Panel Sheet, which are flat sheets made from carbon fibers. These panels are widely used in aerospace, automotive, and sports equipment applications due to their high strength - to - weight ratio.

Carbon Ud Prepreg is another popular product. It consists of unidirectional carbon fibers pre - impregnated with a resin. Prepregs are convenient for composite manufacturing as they can be easily laid up and cured to form complex shapes.

Our 3k Carbon Fiber Fabrics are made by weaving carbon fibers together. The "3k" refers to the number of filaments in each tow, with 3k meaning 3000 filaments. These fabrics are commonly used in applications where a more aesthetically pleasing carbon fiber appearance is required, such as in high - end automotive interiors and consumer electronics.

The Champagne Gold Blend Carbon Fiber Cloth is a unique product that combines the strength of carbon fiber with an attractive champagne - gold color. It is often used in luxury applications where both performance and appearance are important.

We also supply Woven Glass Fibre, which is sometimes used in combination with carbon fiber to achieve a balance between cost and performance. Glass fiber is less expensive than carbon fiber but still offers good mechanical properties.

Conclusion

The production of carbon fiber is a complex and multi - step process that requires precise control at every stage. From the selection of the precursor to the final sizing of the fibers, each step contributes to the unique properties of carbon fiber. As a carbon fiber supplier, we are committed to providing high - quality carbon fiber products to meet the diverse needs of our customers.

If you are interested in our carbon fiber products or have any questions about carbon fiber, please do not hesitate to contact us. We are ready to discuss your specific requirements and provide you with the best solutions.

References

• "Carbon Fibers and Their Composites" by Lawrence T. Drzal, et al.
• "Handbook of Carbon Fibers" by Simon Watson