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Carbon Fiber

Carbon fibers have high tensile strength, and are very light and very stable. Carbon crystals are bonded together in a chain, creating a very strong material, which compared to steel is 5 times stronger on an equal weight basis. Carbon fiber diameter is very small, it ranges from 5-10 microns. Production and consumption of carbon fibers has grown recently because of their great mechanical properties. High manufacturing cost is balanced by its high strength in both tension and compression, and high resistance to corrosion, creep and fatigue, low weight and high performance.

Woven Carbon fabric is used in various applications like marine, sporting goods, defense and many others. The two most common weaving styles are «plain» and «twill». Both have an equal amount of carbon fiber going each direction, and their strengths are quite similar. Other styles are satin, unidirectional, biaxial. Alternatively, fabrics can be added to a resin system (like epoxy) that hardens, and this way structural composite parts are formed. Since resin systems are strong low density materials, the composite part is also very strong and light-weight at the same time.

The dominant raw material for carbon fiber manufacturing is polyacrylonitrile (PAN), pitch follows, and a very small amount of carbon fibers are derived from rayon.

Carbon fibers are usually grouped according to the modulus or strength band in which their properties belong. These bands are commonly referred as: high strength, intermediate modulus, high modulus and ultra high modulus etc. These references for carbon fiber quality are not very clear, as different companies that produce different qualities may consider or evaluate certain quality differently. PAN fiber density ranges from 1.75gr/cm3 to 1.90gr/cm3. PAN tensile strength can be as high as 1000Ksi.
Carbon was first invented near Cliveland, Ohio in 1958, but this process was inefficient as only 20% of the final fiber was carbon. Later another process using PAN as precursor was developed, with a resulting fiber of 55% carbon. But with the advent of a new manufacturing process in 1963, developed at a UK research center, carbon fiber’s high potential strength was realized, and the carbon manufacturing industry began to grow. In the 1970s research on alternative raw materials led to carbon fibers containing 85% of carbon, this time made from pitch.
Modern carbon fiber content is above 90% and is approaching 100%. The leading companies that make the highest qualities in our days are from Japan, and have many carbon yarn manufacturing sites around the world. In the early years, carbon composites have been very expensive, so were rarely used, and most exclusively for aerospace applications. However, as time passed, carbon fiber became affordable for more applications, manufacturing techniques have improved, and all this resulted in increased (and growing) consumption.
The atomic structure of carbon fiber is similar to that of graphite, consisting of carbon atom layers arranged in a hexagonal pattern. Depending on precursors and manufacturing process, the layers might be turbostratic, graphitic or in a hybrid structure. In graphitic structure the sheets are stacked parallel in a regular fashion. Bonding between planes is weak, giving graphite its soft characteristics. Carbon fibers made of PAN are turbostratic and can provide higher strength, while pitch can provide higher modulus.
PAN carbon fiber is produced by pyrolysis of a precursor fiber in an inert atmosphere at temperatures above 982°C. The most common precursor material used to manufacture carbon is polyacrylonitrile (or PAN), that is 90% of all carbon fiber production. The process consists of the five following steps:
  • Spinning (and Polymerization): PAN is mixed with other ingredients and spun into fibers, which are washed and stretched. The quality of finished fiber depends a lot on the quality of the precursor.
  • Stabilizing (or Oxidation): Chemical alteration to stabilize bonding at about 200°C-400°C.
  • Carbonizing: Stabilized fibers are heated to very high temperature (~1000°C) to remove hydrogen, oxygen, nitrogen and other non carbon elements, forming tightly bonded carbon crystals. In order to manufacture stiffer (high modulus) fibers, the pyrolizing process continues for longer, and at higher temperature, (up to 3000°C,) creating a smaller diameter round fiber. Higher modulus fibers are more expensive and more brittle. For this reason, they should be further processed (when weaving, winding, etc) with greater care.
  • Surface treatment: Surface of oxidized fibers is treated to improve bonding properties.
  • Sizing: Fibers are coated and wound onto bobbins.
Over other materials, carbon fiber offers many advantages. The main advantageous characteristics are:
  • High strength
  • Light in weight
  • Corrosion resistance
  • Excellent creep resistance
  • Good thermal and electrical conductivity
  • Compatible with most resin systems
  • Very high dimensional stability
  • Low thermal expansion coefficient
  • X-ray permeability
Carbon fiber is preferred to many applications, as it outperforms many other fiber materials. It is mainly used to high quality products to replace fiberglass, wood or alloys, as it offers lower weight, higher stiffness and better fatigue resistance. Also, as attention on environmental issues is increasing, carbon fiber use has grown. For instance, carbon fiber can reduce the vehicle weight and consequently fuel consumption. At the same time, composite product manufacturing has much less carbon footprint than metal (product) manufacturing, so this way there is an additional, (not so obvious to realize) positive impact on the environment.
Examples of carbon manufactured products are:
  • Sporting goods: Surf boards, bikes, fishing rods, tennis rackets, hokey sticks, running shoes.
  • Automotive – motor racing: Bodywork parts (like doors, hoods etc,) structural components (like chassis,) mechanical (like drive shafts) and protection (like helmets, shock absorbers.)
  • Marine: Manufacturing of boats, yachts and ships, structural and non structural parts.
  • Defense and aerospace: Aircrafts, vehicles, armor etc.
  • Musical instruments: Guitars (and other stringed,) drums, as well as wind instruments.
  • Wind industry: Turbine blades.
  • Electronic fields: Printed circuit, house electronic equipment, PCs, camera bodies.
  • Medical science: Wheel chairs, artificial body parts, x-ray transparent operation tables.
  • Construction: Bridge building, building close to sea and harsh weather conditions, old building rehabilitation.
  • Environment and energy fields: Fuel batteries, oil industry.
Due to carbon fibers great properties, a significant growth in the carbon fiber market is expected. For any application, in order to produce high quality products with carbon fiber, high skills and technical equipment are required.