Carbon Fiber Manufacturing Process

By Eman Abdallah Kamel

Eman is a writer and engineer. She received her bachelor’s degree in textile science from the Faculty of Applied Arts in Egypt.

This article covers the steps of carbon fiber production, including thermal fixing, carbonization, graphitization, surface treatment, and final processing.

Image source: istockphoto.com

Carbon Fiber

Carbon fibers are composed of long, tightly intertwined chains of carbon. These fibers are composed of strands ranging from 5 to 10 micrometers in diameter. Carbon fibers are notable for their rigidity, strength, and lightness. Reinforced carbon fibers are available in various forms, including woven, braided, and other structures, such as cross-linked and unidirectional fibers. These fibers are combined with multiple resins to form carbon fiber-reinforced composites in a variety of fiber shapes and patterns.

Let’s learn more about the production process of carbon fiber.

Manufacturing Process

The manufacturing process of carbon fiber involves the following stages:

Precursor
Heat Stabilisation
Carbonization
Graphitization
Surface Treatment
Final Treatment

1. Precursor

To produce carbon fibers, an organic polymer is required as a raw material. This raw material is treated with heat and chemicals to transform it into carbon fibers. The first high-performance carbon fibers were made from rayon.

Did You Know?

Currently, approximately 90% of carbon fibers are made from polyacrylonitrile, while the remaining 10% are made from rayon or petroleum tar.

2. Heat Stabilisation

The thermal fixation process takes place at temperatures between 200 and 300 degrees Celsius, typically in air for 30 minutes to three hours. This step, which involves oxidation, is essential to prevent the fibers from melting. It alters the spun fibers at the atomic level, breaking hydrogen bonds and capturing oxygen molecules. A stretching process is sometimes carried out concurrently to enhance the fibers’ mechanical properties.

Carbonization is a high-temperature heat treatment (1000-1500°C) that removes non-carbon elements.

3. Carbonization

Carbonization is a high-temperature heat treatment (1000-1500°C) that removes non-carbon elements. This process is carried out in an inert atmosphere and is very rapid (a few minutes). The heating rate is very slow to prevent fiber damage. This step is not performed under tension. At the end of this process, the product consists of approximately:

90% carbon,
8-9% nitrogen,
1% oxygen,
Less than 1% hydrogen.

This composition is characterized by high strength and a high Young’s modulus (3000-6000 MPa) on average.

4. Graphitization

In this process, the fibers are exposed to temperatures ranging from 2000°C to 3000°C for a few seconds. During this process, the semi-ordinary carbon transforms into a regular, three-dimensional graphite structure. Depending on the raw materials and processing parameters, different degrees of similarity to the ideal structure of single-crystal graphite are obtained.

Graphitization is an optional step; it is chosen due to the properties obtained at the end of the process.

5. Surface Treatment

Some surface treatments are performed to improve the mechanical properties of the finished carbon fibers. This step allows for:

Improved bonding between the fibers and the base material during the pre-impregnated composite fabrication process.
The interfacial bond between the carbon filaments and the resin base material can be strengthened by increasing the surface area, which results in more contact points between the fibers and the base material, or by improving the chemical reaction between the two. There are two types of treatments:

Oxidative treatments, such as gas-phase oxidation at low or high temperatures, liquid-phase oxidation, which can be performed chemically or electrochemically, or catalytic oxidation.
Non-oxidative treatments, such as filamentation, pyrolytic carbon deposition, or polymer grafting onto the surface.

6. Final Treatment

Carbon fibers must be treated, or coated, with a polymer before weaving to protect them during the process. The treatment material is chosen to complement the coating resin used. The fibers are then wound onto reels, spun, and processed to create various fabrics.

Let’s explore more about the methods of manufacturing carbon fibers from different raw materials.

Manufacturing Carbon Fibres from Pitch

Pitch is the second most commonly used raw material worldwide. It is widely available and less expensive than other raw materials, and it yields high-performance carbon fibers. However, the refining and spinning processes increase manufacturing costs. In 1990, the most commonly used pitches were those made from petroleum, asphalt, coal tar, and polyvinyl chloride (PVC). Their composition varied depending on the source and processing conditions.

Did You Know?

Coal tar is more aromatic than petroleum tar. A study indicated that coal tar consists of two-thirds aromatic compounds. However, coal tar often has a high carbon content, which causes filament breakage during extrusion and heat treatment. As a result, petroleum tar is preferred for producing carbon fiber.

The main problem with this type of fiber is its homogeneous behavior before thermal decomposition. This phenomenon prevents the formation of oriented carbon fibers. Two solutions have been proposed:

Conversion to graphite under stress: This solution is applied under tension during heat treatment at high temperatures ranging from 2000 to 3000 degrees Celsius. This technique provides carbon fibers with excellent performance characteristics, but it is expensive.
The use of mesophase tar: Mesophase is a state of matter located between liquid and solid, and this process allows for the transformation of crystalline material into fibers. This state allows the fiber’s lamellar structure to be shaped during low-temperature spinning.

The manufacturing process for this type of fiber consists of four steps:

Pitch Preparation: This stage ensures precise control of the molecular weight, viscosity, and crystal orientation for spinning and further heating.
Spinning and Drawing: Spinning and drawing transform the bitumen into fibers with specific crystal alignment to achieve the desired directional properties.
Stabilization: The bitumen can be converted to a non-melting state. This transformation occurs during the stabilization stage, between 250°C and 400°C.
Carbonization and Graphitization: In general, carburizing and graphitization processes are carried out between 1000°C and 2500°C. Carbonization and graphitization play a crucial role in carbon fiber manufacturing. The temperature and rotation speed are determined by controlling the melt’s viscosity and melting range. These parameters can be controlled by adding specific materials such as

Tetramethylthiouram,
Polymers (such as polyethylene and polystyrene),
Plasticizers (such as castor oil or its alkyl and sulfur derivatives),
Extraction using specific solvents or by distillation.

Manufacturing Carbon Fibre from Polyacrylonitrile (PAN)

The oxidation process transforms polyacrylonitrile into a non-plastic cyclic or ladder compound that can withstand high temperatures. Oxidation is necessary to avoid fiber damage during carbonization. Polyacrylonitrile typically consists of 85% acrylonitrile and 15% comonomer, usually methyl acrylate. This comonomer acts as a plasticizer, aiding in the spinning process and converting polyacrylonitrile into a wire form. It also helps in fiber stretching during the oxidation process. Air plays a crucial role during heat treatment because it promotes the initiation of ring-shaped slitting by creating active centers. It is responsible for increasing the number of active centers and minimizing any side reactions.

The graphitization process aims to improve the arrangement and orientation of the crystals along the fiber axis, resulting in better mechanical properties for the final carbon fiber. The molecules tend to align in the same direction. This restructuring leads to the formation of extremely strong bonds at the molecular level.

Manufacturing Carbon Fibres from Rayon

The manufacturing steps for rayon carbon fibers are similar to those for polyacrylonitrile (PAN) carbon fibers, namely stabilization, carbonization, and graphitization. For rayon fibers, the stabilization process involves four stages:

First, physical water absorption occurs between 25°C and 150°C.
The cellulose unit then dries.
Finally, thermal cleavage of the cyclic bond and the breaking of ether bonds and some carbon-carbon bonds occur via a free radical reaction (240–400°C).
Then, aromatization takes place.

The carbonization of these fibers occurs between 400°C and 700°C, transforming the structure into graphite-like layers. Graphitization is typically carried out under pressure at temperatures between 700°C and 2700°C to obtain fibers with a high modulus of elasticity through longitudinal plane orientation.

Sources

Latest Posts

LongWayEducation

The Long Way Education Site path is based on self-learning. Here you will learn about science, education, and religion. You will also learn about fiber and its manufacture. And different engineering disciplines, such as mechanical and industrial engineering.