They start off with another polymer, called polyacrylonitrile. They take this polymer, and heat it up. They are not sure just exactly what happens when they do this, but we do know that the end result is carbon fiber. They think the reaction happens something like this: when they heat the polyacrylonitrile, the heat causes the cyano repeat units to form cycles!
Then they heat it again! This time they turn the heat up higher, and our carbon atoms kick off their hydrogens, and the rings become aromatic. This polymer is a series of fused pyridine rings.
Then they heat it...AGAIN! Slow roasting the polymer something more, at around 400-600°C causes adjacent chains to join together.
This expels hydrogen gas, and gives us a ribbon-like fused ring polymer.
But they are not done yet! Next they crank up the heat, anywhere from 600 all the way up to 1300oC. When this happens, our newly formed ribbons will join together to form even wider ribbons.
When this happens, they expel nitrogen gas. On the polymer we get, it has nitrogen atoms along its edges, and these new wide ribbons can then merge to form even wider ribbons.
As this happens, more and more nitrogen is expelled. When we're through, the ribbons are really wide, and most of the nitrogen is gone, leaving us with ribbons that are almost pure carbon in the graphite form. That's why we call these things carbon fibers.
Spools of row Carbon fiber
Carbon fibers, under industrial production now, are classified into PAN-based, pitch-based and rayon-based. Among them, PAN-based carbon fiber is in the largest production and best used in volume. In the beginning of 1970's, commercial production of PAN-based and isotropic pitch-based carbon fibers was started on a large scale. In the latter half of 1980's, anisotropic pitch-based carbon fiber manufacturers broke into the market.
Usage of carbon fiber by itself is not the rule. Commonly, customers apply carbon fibers for reinforcement and / or functionality of composite materials, made with resin, ceramic or metal as matrix. Carbon fibers are extensively applied to a large variety of applications with supreme mechanical characteristics (specific tensile strength, specific modulus) and other characteristics due to carbon matter (low density, low coefficient of thermal expansion, heat resistance, chemical stability, self-lubricity, etc.).
Carbon Fibers, having supreme characteristics, are adopted in wide varieties of uses. Suppliers are able to provide, by using different raw material and applying divergent production processes, wide diversity of the fibers having different specifications.
PAN Type Carbon Fiber A type of the fiber, produced by carbonization of PAN precursor (PAN: Polyacrylonitrile), having high tensile strength and high elastic modulus, extensively applied for structural material composites in aerospace and industrial field and sporting / recreational goods.
PAN Type Carbon Fiber is an aggregation of continuous fiber (filaments), 5 – 7 micron meter in diameter with 1.74 - 1.95 g/cm3 of density, generally. Products with various filaments, such as 1K (1000 filaments), 3K (3000 filaments), 6K (6000 filaments), 12K (12000 filaments) and 24K (24000 filaments), referred to as "Regular Tow" or "Small Tow", have been used in large quantity for aircrafts and sports/recreational fields, making good use of low density, high specific tensile strength and high specific elastic modulus. PAN fibers have been undertaken a role for market expansion of carbon fibers.
Large Tow, extra-40K, even though slightly lower tensile strength is mainly used for industrial fields as a relatively inexpensive material, along with Regular Tow. PAN type carbon fibers are classified into Standard Elastic Modulus Type (- 240 GPa), Intermediate Elastic Modulus Type (- 300 GPa) and High Elastic Modulus Type (350 GPa -).
Pitch Type Carbon Fiber Another type of the fiber, produced by carbonization of oil/coal pitch precursor, having extensive properties from low elastic modulus to ultra high elastic modulus. Fibers with ultra high elastic modulus are extensively adopted in high stiffness components and various uses as utilizing high thermal conductivity and / or electric conductivity.
Regarding Pitch Type Carbon Fiber, there are continuous type and discontinuous type, based on respective spinning process.
Pitch Type Carbon Fiber is also classified into Isotropic Type (hardly-graphitizability) and Un-isotropic Type (easy-graphitizability), based on respective raw pitch. Isotropic Pitch Type Carbon Fiber is commonly a discontinuous fiber of 12 - 18 micron meter in diameter with 1.6 g/cm3 of density and has the properties of low modulus (- 40 GPa), strength and thermal conductivity due to its weak structural orientation of carbon atoms and underdeveloped graphite crystallinity. With its competitive cost, Isotropic Pitch Type Carbon Fiber is extensively applied for industrial fields due to light weight, chemical stability, heat resistance and abrasion characteristic.
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