As the name, carbon fiber consists of mostly carbon atoms. They are about 5 to 10 micrometers. There are different types of carbon fiber. Each type is great for high stiffness, tensile strength, low weight, high chemical resistance.
Carbon fibers are generally used for composite materials. Carbon is combined with plastic resin and baked to make carbon-fiber reinforced composite while it can be combined with graphite to make reinforced carbon-carbon composite for higher strength.
Types of Carbon Fiber
There are different types of carbon fiber based on modulus, tensile strength (fiber materials) and final heat treatment temperature. In the carbonization process, temperature exposures range from 1000°C to 2000°C, each different level of exposure creating a different property for fiber. For example, high modulus type is processed at 2000°C, 1500°C for high strength type and 1000°C for low modulus and low strength type.
Based on Modulus
Ultra High modulus ( Modulus > 500GPa ) Ex: Pitch
High modulus ( Modulus 300-500GPa ) Ex: PAN
Intermediate modulus ( Modulus 100-300GPa ) Ex: PAN
Low modulus ( Modulus < 100GPa ) Ex: Isotropic Pitch, Rayon based carbon fiber
Based on Fiber Materials
PAN based carbon fiber
Pitch based carbon fiber
Mesophase pitch based carbon fiber
Isotropic pitch based carbon fiber
Rayon based carbon fiber
The main carbon fibers are made from Poly Acrylo Nitrile (PAN) based and pitch based formulation and they are well-known for their composite reinforcement and heat resistant and uses.
|Carbon fiber properties
|Continuous operation temperature °F
Based on Final Heat Treatment Temperature
Type I: In case of this carbon fiber final heat treatment temperature remains above 2000°C. This type of carbon fiber is high modulus type fiber.
Type II: Final heat treatment temperature for this type incorporates around or above 1500°C and this temperature yields high strength type carbon fiber.
Type III: Final heat treatment temperature for this type incorporates around or below 1000°C and this temperature yields low strength type carbon fiber.
How Carbon fiber is made?
Most of the carbon fiber is manufactured from poly acrylo nitrile (PAN), rayon or pitch based polymer. 90% carbon fiber is manufactured from PAN and the rest is from rayon or pitch based polymer. Polymers used to manufacture this fiber is called Precursor. These organic polymers are long chain molecules bounded by carbon atoms. For different manufactures, precursor composition and mechanical process used during spinning of the filament may vary.
Carbon fiber synthesis can be categorized into five phases
- Poly acrylo nitrile is formed from acrylonitrile plastic powder mixed with other plastic fibers such as, methyl acrylate or methyl methacrylate. This process incorporates suspension or solution polymerization where this mixture is reacted with catalyst.
- Then it is spun into filament using one of the spinning method. It may include, The plastic mixing with some chemicals can pumped through jets and passed through the quenching chamber to solidify into fiber or the plastic mixing with some chemicals is pumped through tiny jet where the solvents evaporate leaving solid fibers. The internal atomic structure of the fiber is formed in this process.
- Fibers are then washed off and stretched to the preferable diameter.
Stabilization (up to 300°C)
- Stabilization is required to chemically alter the fibers before it passes for carbonization process. Here the linear atomic bond is converted to a more thermally stable ladder bond.
- This is accomplished by heating the fibers in air to about 390-590° F (200-300° C) for 30-120 minutes. Here, fibers collect oxygen molecules from air and re-arrange their molecular patterns.
- In this process the fibers are drawing through series of heated rollers or chambers. Again, heated air mixed with certain gas can also be used to accelerate the stabilization process.
- After stabilization, the fibers are heated up to 1830 – 5000°F (up to 1000-3000°C) for several minutes in a furnace containing gas mixture that doesn’t contain oxygen. Inside the furnace air pressure is kept higher than the outsider atmospheric pressure.
- The entrance and exit option of the furnace are sealed so that the oxygen can’t enter into it. The absence of oxygen prevents the fibers from burning at higher temperature.
- Through this heating procedure, fibers lose their non-carbon atom and add a few new carbon atoms. This addition can be done from the gases like carbon mono-oxide, carbon di-oxide etc.
- Once the non-carbon atoms are lost, the remaining carbon atoms are tightly bonded to-gather to form carbon crystals that are more or less aligned parallel to the fibers long axis.
- Once the carbonizing is completed, the fibers have such a surface which can’t make bonds well with epoxies or other materials present in the composite. For better bonding properties, their surface is slightly oxidized.
- Oxygen atoms are added to the surface to provide better chemical bonding properties. It also etches and roughens the surface for better mechanical properties.
- By immersing the fibers in various gases such as air, carbon dioxide, ozone oxygen is achieved. The fibers can also be coated by electrical conductive materials.
- This treatment should be carefully controlled so that no tiny surface defects can form such as pits.
- Finally, the fibers are coated to protect them from damage during weaving or winding process. This is called sizing process.
- Coating materials should be compatible with the adhesive used in composite materials. Typical coating materials include epoxy, polyester, nylon, urethane etc.
The coated fibers are wound to bobbins that are loaded into spinning machine which are then twisted to form yarns.