Carbon fibers also known as graphite fibers, are about 5-10 micrometers in diameter and contain about 92 wt % carbon. Several carbon atoms join to produce a carbon fiber.  Carbon fiber structure is same as that of a graphite, which consists of layers of carbon atoms forming a regular hexagonal pattern.

Based on manufacturing processes and the precursors used, the structure of carbon fibers can be turbostratic, graphitic, or a hybrid. 

History

For the first time, the carbon fibers were developed in the year 1950s, which were manufactured by heating rayon strands until they were carbonized. This method of producing the carbon fibers was inefficient because it contains only 20% carbon with low strength and stiffness. In 1960s, polyacrylonitrile was used as a raw material to develop a process which resulted in the production of carbon fiber with 55% carbon and much better strength. Therefore, this method of polyacrylonitrile conversion became the primary method to produce carbon fibers. During 1970s, experiments were carried out to find alternative raw materials which led to the introduction of petroleum pitch derived carbon fiber from oil processing. This carbon fiber had 85% carbon with outstanding flexural strength. But unfortunately, these carbon fibers had only limited compression strength, therefore, were not accepted widely.

Today, carbon fibers are of extensive use, therefore, new applications are being developed every year. The United States, Japan, and Western Europe are the leading producers of carbon fibers.

Applications

Carbon fiber industry has been growing substantially until 2018, which can be attributed to the increasing demand from various fast-growing industries, with strongest demand coming from aircraft and aerospace, wind energy and from automotive industry.

In addition, carbon fibers are widely used in body parts (hoods, doors, front end, deck lids bumpers, etc.), chassis and suspension systems (e.g., leaf springs), drive shafts and so on.

Carbon fiber is versatile as it can be molded into various forms. Mixing with different materials such as plastic, metal, concrete, wood and other fabrics, the carbon fiber can be augmented to meet the requirements of any application.

Carbon Fiber Manufacturing

Raw Materials

The raw materials which are used to produce carbon fiber is known as the precursor. Most of the carbon fibers produced (around 90%) are made from polyacrylonitrile (PAN). The remaining percentage of carbon fibers are developed using rayon or petroleum pitch as a raw material. The precursors are organic polymers, having long strings of molecules that are bounded together by thousands of carbon atoms. The composition of each raw material used in the production process varies from company to company and is generally considered a trade secret.

The Manufacturing Process

The manufacturing of carbon fiber involves both chemical and mechanical processes. The precursors are assembled into long strands of fibers that are heated at a very high temperature preventing its contact with the oxygen. The fiber cannot be burned without oxygen. Such high temperature causes the atoms that are present in the fiber to vibrate intensely until most of the non-carbon atoms are drum out.

To prevent the fibers from getting damaged they are coated during winding or weaving process. Then the coated fibers are wound onto cylinders typically known as bobbins.

Here is a typical sequence of operations used to form carbon fibers from polyacrylonitrile.

Polymerization

The polymerization process starts with a precursor, which is the backbone of molecular fiber as the quality of finished fiber is directly dependent on the precursor used. The acrylonitrile polymer is placed in a reactor where it is combined with plasticized acrylic comonomers along with a catalyst such as itaconic acid, sulfur dioxide acid, sulfuric acid or methyl acrylic acid. Continuous stirring is done to blend the ingredients, ensuring consistency and purity. This leads to the formation of free radicals within the acrylonitrile’s molecular structure.  

After washing, acrylonitrile is dried to obtain it in a powder form which is dissolved in either organic solvents like dimethyl sulfoxide (DMSO), dimethyl acetamide (DMAC) or dimethyl formamide (DMF), to avoid contamination by trace metal ions or in aqueous solvents, like zinc chloride and rhodan salt. During this stage, the powder and solvent slurry forms a maple syrup.

Spinning

The obtained polyacrylonitrile plastic is then spun into fibers using different methods. In one such method, the plastic is mixed with chemicals and pumped through tiny jets into a chemical bath where the plastic coagulates and solidifies into fibers. Another method includes the heating of plastic mixture and then pumping through tiny jets in a chamber in which solvents evaporate, leaving behind a solid fiber. The spinning process plays a vital role in the formation of atomic structure of the fiber.

At the end, the fibers are washed and stretched to obtain the desired fiber diameter. During the stretching the molecules are aligned within the fiber, thus providing the basis for the formation of the tightly bonded carbon crystals after carbonization.

Stabilizing (Oxidation)

Before carrying out the carbonization process, the fibers need to be chemically altered in need to convert their liner atomic bonding to a thermally stable ladder bonding. This is achieved by heating the fibers in the presence of air at a temperature of about 200-300°C for around 30-120 minutes. During this process, the fibers pick the oxygen molecules which causes the polymer chains to start crosslinking to arrange their atomic bonding pattern. The heat is generated during the chemical reactions which needs to be controlled to avoid overheating of the fibers. At the end of the process, the oxidized PAN fiber contains about 50 to 65 percent carbon molecules along with a mixture of hydrogen, nitrogen and oxygen.

Commercially, this stabilizing process uses variety of equipment for the heating purpose. Some method uses the series of heated chambers through which the fibers are drawn. Another method allows the fiber to pass over hot rollers and through beds of loose materials which are held in suspension by a hot flow of air. Furthermore, some other processes use heated air intermixed with gases which accelerates the process of stabilization.

Carbonizing

After the stabilization of fibers, they are heated in a furnace filled with a gas mixture without oxygen at a temperature of about 1,000-3,000° C for few minutes. Absence of oxygen prevents the carbon fiber from burning at a very high temperature, this is because every oxygen molecule that is carried through the oven removes a portion of the fiber. The pressure of the gas inside the furnace is kept higher than the outside air pressure and the entrance and exit are sealed to prevent oxygen intrusion. As the fibers are heated in the absence of oxygen, only the noncarbon atoms with few carbon atoms are removed from the furnace in the form of various gases including ammonia, water vapor, carbon monoxide, hydrogen. carbon dioxide, nitrogen, and others. Throughout the production process, as the non-carbon molecules are expelled, the remaining carbon atoms continue to form tightly bonded carbon crystals. Ultimately, carbon fiber tensioning can be optimized to produce a finished fiber containing more than 90 percent carbon.

Treating the surface and Sizing

The next step is critical to fiber performance. After carbonizing, the fibers have a surface that remains unbonded with matrix resin, therefore the surface treatment is performed to enhance this adhesion. The surface of the fibers is slightly oxidized to enhance their binding properties. The adhesion of oxygen molecules to the surface intensify chemical as well as mechanical bonding properties. It can be achieved by immersing the fibers in varied gases such as carbon dioxide, air and ozone or in different liquids like sodium hypochlorite or nitric acid. This step of treating the surface should be controlled to avoid any kind of surface defects such as pits which could lead to fiber failure.

Once the surface treatment is done, the fibers are coated during winding or weaving to prevent any damage. This process is referred as sizing. The coating material must be compatible with the adhesive used to form composite materials. Epoxy, polyester, nylon, urethane, and others are typically used as coating materials. Then at the next step, coated fibers are wound onto cylinders known as bobbins. Lastly, the bobbins are loaded into a spinning machine and the fibers are twisted into yarns of various sizes.

Health and Safety Concerns

Some areas of concern during the production of carbon fibers includes skin irritation, dust inhalation and the effect of fibers on electrical equipment.

ü  During fiber processing, the pieces can shatter and circulate in air in the form of a fine dust, that can be an irritant when inhaled. Therefore, people working in the production area must wear protective masks.

ü  Moreover, the carbon fibers can lead to skin irritation, especially on the back of hands and wrists. As a protective measure, use of barrier skin creams or a protective clothing is recommended for people working in the zone.

ü  The coating materials used during sizing contains chemicals which causes skin reactions, and therefore adequate care must be taken while handling.

Conclusion:

 The future of the carbon fiber market looks attractive with opportunities in the aerospace, industrial and sporting goods industries. The global carbon fiber market is anticipated to grow at a formidable rate over the coming years which can be attributed to rising demand for high performance and lightweight composite materials coupled with escalating performance requirements in the end use industries. In addition, increasing use of carbon in 3D printing is positively influencing the growth of carbon fiber market, globally.

Some of the key players operating in the market are Kemrock Industries, Toray Industries, Hexcel Corporation, Formosa Plastic Corporation, Kureha Corporation, and Mitsubishi. Major companies are developing new products in order to stay competitive in the market. Other competitive strategies include mergers & acquisitions in order to increase their customer bases and expand their geographic reach.

According to TechSci Research report, Global Carbon Fiber Prepreg Market By Manufacturing Process (Solvent Dip & Hot Melt), By Resin Type (Epoxy, Phenolic, Thermoplastic & Others), By End Use Industry (Aerospace & Defense, Wind Energy & Others), Competition, Forecast & Opportunities, 2024,  the global carbon fiber prepreg market is projected to grow from $ 7 billion in 2018 to approximately $ 11.5 billion by 2024, exhibiting a CAGR of 10.6% during forecast period. The market is growing on account of rising demand for carbon fiber prepreg from various industries, including automotive, aerospace & defense, etc. However, the high cost of both processing and manufacturing is hampering the growth of the market.