Carbon Fiber



Carbon is the man-made refractory fiber manufactured from polyacrylonitrile. Commercially first carbon was invented in 1964, by Bacon and Wesley Schalamon produced from rayon using a new “hot stretching” process.


Classification of Carbon Fiber:

Based on carbon fiber properties.

  • Ultra-high-modulus – UHM (modulus greater than 450Gpa).

  • High-modulus – HM (modulus between 350-450Gpa).

  • Intermediate-modulus – IM (modulus between 200-350Gpa)

  • Low modulus and high-tensile, HT (modulus less than 100Gpa, tensile strength greater than 3.0 Gpa)

  • Super high-tensile, SHT (Tensile strength greater than 4.5Gpa)

Based on precursor fiber materials.

  • Isotropic pitch based.

  • PAN-based.

  • Mesophase pitch based.

  • Gas-phase-grown.

  • Pitch-based.

  • Rayon-based.



Based on final heat treatment temperature.

  • Type-I: High heat treatment carbon fibers – HTT, in this case the final heat temperature is above 2000oC and can be associated with high modulus type fiber.

  • Type-II: Intermediate heat treatment carbon fibers – IHT, in this case the final heat temperature is above 1500oC and can be associated with high strength type fiber.

  • Type-III: Low heat treatment carbon fibers, in this case the final heat temperature is less than 1000oC. These are low modulus and low strength materials.

Production Process:

Carbon fibers produced from polyacrylonitrile (PAN):

Raw materials:

The raw material used to produce carbon fiber is known as Precursor. Generally, about 90% of the carbon fibers are produced from polyacrylonitrile, the remaining 10% are produced from rayon or petroleum pitch. These materials are organic polymers which are characterized by long strings of molecules bonded by carbon atoms. The exact compositions of the raw materials may vary from one company to another and this is a trade secret. During the production process different gases and liquids are used. Some of these materials are designed to react with the fibers to obtain the specific effect/ property and some other materials are used to avoid certain reactions from the fibers. The exact compositions are trade secret.


Production process of PAN:

The image mentioned below will give information about production flow.


Spinning:

  • Acrylonitrile is mixed with another plastic, like methyl acrylate or methyl methacrylate, and is reacted with a catalyst in a conventional suspension or solution polymerization process to obtain polyacrylonitrile.

  • Now, this polyacrylonitrile is spun into fibers by using various methods, in some methods, this polyacrylonitrile is mixed with various chemicals and then pumped through tiny jets into a chemical bath or a quenching chamber where the polyacrylonitrile is coagulates and solidifies into fibers. This is like the production of polyacrylic fibers. In the other method this polyacrylonitrile is heated and them pumped through a spinneret where this solvent evaporates, and fibers are produced. This is an important process as the internal atomic structure of the fiber is formed during this process.

  • Then these fibers are washed and stretched to the desired diameter (count). This stretching helps for the alignment of the molecules within the fibers and provides the basis for the formation of the tightly bonded carbon crystals after carbonization.

Stabilizing:

Before the fibers are carbonized, they need to be chemically altered to convert their linear atomic bonding to a more thermally stable ladder bonding. Heating is carried out by hot air which is around 390-590oF (200-300oC) for 30-120 minutes, which will help to pick up oxygen molecules from the air and rearrange their atomic bonding pattern. The chemical reactions required for stabilization are complex and involves various steps, in which few of the reactions are occur simultaneously. The reactions are exothermic which must be controlled to avoid overheating of the fibers. Commercially this process uses different equipment and techniques. In some cases, the fibers are drawn through a series of heating chambers. In some other cases fibers pass over hot rollers and through beds of loose materials held in suspension by a flow of hot air. Some processes use heated air mixed with certain gases that chemically accelerate the stabilization.



Carbonization:

After the stabilization process these fibers are heated to a temperature of 1,830-5,500oF (1000-3000oC) for several minutes in special chamber which contain a mixture of gases (this mixture of gases does not contain oxygen). As we are heating in absence of oxygen which will prevents the fibers burning in the very high temperature. The gas pressure inside the special chamber is kept higher than the outside pressure and the point where the fibers enter, and exit are tightly sealed to avoid oxygen entering the chamber. When the fibers are heated, they will lose their non-carbon atoms and few carbon atoms, as various gases including water vapor, carbon monoxide, ammonia, carbon dioxide, nitrogen, hydrogen, and various other gases. As the non-carbon atoms are expelled the remaining carbon atoms are tightly bonded as carbon crystals, and all the aligned parallel to the long axis of the fiber. In some other processes, two chambers operating two different temperatures are used for better control over the temperature during carbonization.


Treating the Surface:

After carbonizing, the fiber surface will not bond with epoxies and other materials used in the composite materials. To improve the fiber bonding property these fibers are slightly oxidized. The addition of the oxygen atoms on the surface of the fiber will improve bonding property and etches and roughens the surface for better mechanical bonding properties. Oxidation can be done by immersing the fibers in various gases like air, carbon dioxide, ozone, or in various liquids like sodium hypochlorite or nitric acid. The fiber can also be coated electrolytically by making the fibers positive terminal in a bath filled with various electrically conductive materials. The surface treatment process can be carefully controlled to avoid surface defects such as pits, which may cause fiber failure.

Sizing:

  • After the surface modification process the fibers need to be coated to avoid damage during the winding and weaving process. This process is known as sizing. The coating material should be compatible with the adhesive used for the composite materials. Generally, the coating materials include epoxy, polyester, nylon, urethane, and others.

  • These coated fibers are wound onto a bobbin. These bobbins are loaded into a spinning machine and the fibers are twisted into yarns of various sizes.

Physical Properties:

  1. Tenacity: 1.8-2.4 (KN/mm2)

  2. Density: 1.95 gm/cc

  3. Elongation at break: 0.5%

  4. Elasticity: Not Good

  5. Moisture Regain (MR%): 0%

  6. Resiliency: Not Good

  7. Ability to protest friction: Good

  8. Color: Black

  9. Ability to Protest Heat: Good

  10. Luster: Like silky

Chemical Properties:

  1. Effect of Bleaching: Sodium Hypochlorite slightly oxidized carbon fiber.

  2. Effect of sun Light: Do not Change Carbon fiber.

  3. Protection against flame: Excellent

  4. Protection ability against insects: Do not harm carbon fiber

Applications:

  1. Sports Textile Applications.

  2. Automobile industry.

  3. Aerospace industry.

  4. Wind turbine blades.

  5. Military Applications.

  6. Medical Applications.



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