Carbon fibers (CF) are fibers about 5–10 micrometers in diameter and composed mostly of carbon atoms.
There are several advantages of carbon fibers:
- high stiffness,
- high tensile strength,
- low weight and
- high chemical resistance.
After reading this post, you will know the main classification of carbon fibers, what raw materials are necessary and learn about its detailed manufacturing process.
With that being said, you can quickly distinguish the quality of carbon fiber products and get hold of the entire manufacturing process more efficiently.
1. The general classification of carbon fibers
A. Based on tensile modulus
Tensile modulus of the fiber is the key criterion to classify carbon fibers.
Basically, tensile modulus is short for “modulus of elasticity“, which means a measure of how stiff a certain diameter fiber is, or the resistance to stretching, the higher the number, the stiffer the fiber.
B. Based on ascending order of tensile modulus
Other classifications, in ascending order of tensile modulus, include “standard modulus,” “intermediate modulus,” “high modulus,” and “ultrahigh modulus.”
For example, ultrahigh modulus carbon fibers have a tensile modulus of 500 million-1.0 billion kPa. As comparison, steel has a tensile modulus of about 200 million kPa. Therefore, the strongest carbon fiber is about five times stronger than that of the steel.
2. What raw materials are necessary for manufacturing process?
The raw material used to make carbon fiber is the precursor.
A. Polyacrylonitrile
About 90% of the carbon fibers are made from polyacrylonitrile.
B. Rayon or petroleum pitch
Then, the remaining 10% are from rayon or petroleum pitch. All of these materials are organic polymers, characterized by long strings of molecules bound together by carbon atoms.
C. Gas and liquids
During the manufacturing process, a variety of gases and liquids are indispensable.
Some of these materials should react with the fiber to achieve a specific effect. However, other materials don’t need to react or to prevent certain reactions with the fiber.
As with the precursors, the exact compositions of many of these process materials are trade secrets.
3. The manufacturing process of carbon fiber
The process for making carbon fibers is part chemical and part mechanical.
The precursor is drawn into long strands or fibers and then heated to a very high temperature avoid allowing it to come in contact with oxygen.
Without the oxygen, the fiber can’t burn. Instead, the high temperature causes the atoms in the fiber to vibrate violently until most of the non-carbon atoms are expelled.
This process is carbonization and leaves a fiber composed of long, tightly inter-locked chains of carbon atoms with only a few non-carbon atoms remaining.
In the following paragraphs of the post, we will introduce to you a typical sequence of operations used to form carbon fibers from polyacrylonitrile.
3.1 Spinning
1) Mix acylonitrile plastic powder with another plastic (methyl acrylate or methyl methacrylate).
2) Reacted with a catalyst in a conventional suspension or solution polymerization process to form a polyacrylonitrile plastic.
3) The plastic is then spun into fibers, which are washed and stretched.
3.2 Stabilizing
1) Before carbonizing, fibers need to be chemically altered to convert their linear atomic bonding to a more thermally stable ladder bonding.
2) Accomplishing stabilizing by heating the fibers in air to about 200-300° C (390-590° F) for 30-120 minutes.
3.3 Carbonizing
1) Once the stabilizing process finishes, fibers are heated to a temperature of about 1,000-3,000° C (1,830-5,500° F) for several minutes in a furnace filled with a gas mixture that does not contain oxygen.
2) The gas pressure inside the furnace is kept higher than the outside air pressure and the points where the fibers enter and exit the furnace are sealed to keep oxygen from entering.
3.4 Treating the surface
After carbonizing, the fibers have a surface that does not bond well with the epoxies and other materials used in composite materials. To give the fibers better bonding properties, their surface is slightly oxidized.
3.5 Sizing
1) After the surface treatment, the fibers are coated to protect them from damage during winding or weaving. This process is called sizing.
Coating materials are chosen to be compatible with the adhesive used to form composite materials. Typical coating materials include epoxy, polyester, nylon, urethane, and others.
2) The coated fibers are wound onto cylinders called bobbins. The bobbins are loaded into a spinning machine and the fibers are twisted into yarns of various sizes.
That is the basic information about the manufacturing process of carbon fibers and we sincerely hope it’d be helpful. Don’t hesitate to leave you comment below if you have any questions.
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