Carbon Fibers in Plastic Materials
The use of carbon fibers in plastic materials has a long history. As early as 1879, Thomas Edison was experimenting with carbon fibers made from cotton threads and bamboo slivers. In fact, the first incandescent light bulb heated by electricity contained carbon fibers.
In the 1960s, Dr. Akio Shindo at the Agency of Industrial Science and Technology in Japan developed a carbon fiber based on polyacrylonitrile (PAN). The resulting fiber contained 55% carbon.
The PAN-based conversion process quickly became the primary method for producing carbon fiber. Ninety percent of carbon fibers today are made from polyacrylonitrile (C3H3N)n or PAN a synthetic, semi-crystalline organic polymer resin. The remaining 10% are made from rayon or petroleum pitch. Fibers made from PAN are extremely strong and light. These fibers are bound by thermoset or thermoplastic polymers such as polyester, vinyl ester, or nylon to make carbon fiber-reinforced plastic or carbon FRP.
Adding Carbon Fiber to a Polymer Has Many Benefits
Tensile strength and flexural modulus are increased as is the heat deflection temperature or HDT. Additionally, adding carbon fiber reinforcement diminishes shrinkage and warping.
Each carbon fiber is a long thin strand made up of thousands of carbon filaments. A single fiber is about 5-10 μm in diameter and composed mostly of carbon. Microscopic crystals in the carbon bond together in a structure that is more or less aligned parallel to the long axis of the fiber. It is this alignment of crystals that makes the fibers so strong.
Classified by Tensile Modulus
Carbon fibers are classified by the tensile modulus* of the fiber. The tensile modulus may range from 34.8 million psi to 72.5-145.0 million psi. Steel has a tensile modulus of 29 million psi thus the strongest carbon fiber is five times stronger than steel.
“Low” modulus fibers have a tensile modulus below 34.8 million psi (240 million kPa). Fibers are also classified in ascending order of tensile modulus as “standard modulus,” “intermediate modulus,” “high modulus,” and “ultrahigh modulus.” Carbon fibers with a classification of ultrahigh modulus have a tensile modulus of 72.5-145.0 million psi (500 million-1.0 billion kPa).
Spinning, Stabilizing, Carbonizing, Surface Treatment, and Sizing
The manufacturing process for carbon fiber is partly chemical and partly mechanical.
- Spinning: The PAN is spun using one of a few spinning processes. This step is important because it forms the internal atomic structure of the fiber. The fibers are then washed and stretched to the required diameter. The stretching also helps align the molecules to aid in the formation of the carbon crystals created by carbonization.
- Stabilizing: In this step, the fibers are treated with chemicals to change their linear bonding to a thermally stable ladder-bonding structure. The filaments are then heated in the air, so they pick up oxygen molecules and change their atomic bonding pattern.
- Carbonizing: The fibers are then exposed to very high heat without oxygen present so the fiber cannot burn. The atoms in the fiber vibrate violently expelling most of the non-carbon atoms in the precursor.
- Surface Treatment: After carbonizing, the surface of the fibers does not bond well with the materials used in making composite materials. In this step, the surface of the fibers is slightly oxidized by immersion in various gases or liquids.
- Sizing: In this process, the fibers are coated to protect them from damage during winding or weaving.
A few products made from carbon fibers are fishing rods, bicycles, golf equipment, tennis rackets, and parts for aircraft, bridges, and automobiles.
*Tensile modulus is how much pulling force a fiber of a certain diameter can exert without breaking. Tensile modulus is described by pound per square inch or psi.
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