Carbon Fiber

Carbon Fiber

Carbon Fiber: A Carbon Fiber is a long, thin strand material about 0.0002-0.0004 in (0.005-0.010 mm) in Diameter and Composed mostly of Carbon Atoms. The Carbon Atoms are bonded together in Microscopic Crystals aligned Parallel to the long axis of the Fiber. The Crystal alignment makes the Fiber incredibly strong for its size. Several thousand Carbon Fibers are twisted together to form a Yarn, which may be used by itself or woven into a Fabric.

Yarn or Fabric: Combined with epoxy and wound or Molded into Shape to form Various Composite Materials. Carbon Fiber Reinforced composite Materials are used to make Aircraft and Spacecraft Parts, Racing car bodies, Golf Club Shafts, Bicycle Frames, Fishing Rods, Automobile Springs, Sailboat Masts, and many other components where light weight and high strength are Needed.
Classification of Carbon Fibers: Carbon Fibers are classified by the Tensile Modulus of the Fiber. Tensile Modulus is a measure of how much Pulling Force a certain Diameter Fiber can Exert without Breaking. The English unit of Measurement is Pounds of Force per square inch of Cross Sectional Area, or psi. Carbon Fibers classified as "low modulus" have a Tensile Modulus below 34.8 Million psi (240 million kPa).

Raw Materials: The Raw Material used to make Carbon Fiber is called the Precursor. 90% of the Carbon Fibers produced are made from polyacrylonitrile. The remaining 10% are made from Rayon or Petroleum Pitch. These Materials are Organic Polymers, Characterized by Long Strings of Molecules Bound together by Carbon Atoms. Composition of each Precursor varies from one company to another and is Generally Considered a Trade Secret.
The Manufacturing Process: 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 with out Allowing it to come in Contact with Oxygen.

Without Oxygen: Fiber cannot 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 called Carbonization and Leaves a Fiber composed of Long, Tightly
Spinning: Acrylonitrile Plastic Powder is mixed with another Plastic, like Methyl Acrylate or Methyl Methacrylate, and is reacted with a Catalyst in a Conventional Suspension or Solution Polymerization Process to form a Polyacrylonitrile Plastic.

Fibers are then Washed and Stretched to the desired Fiber Diameter: The Stretching helps align the Molecules within the Fiber and Provides the basis for the formation of the Tightly Bonded Carbon Crystals after Carbonization.
Stabilizing: Before the Fibers are Carbonized, they need to be Chemically Altered to convert their linear Atomic Bonding to a more Thermally Stable Ladder Bonding. This is accomplished by heating the Fibers in air to about 390-590° F (200-300° C) for 30-120 minutes.

This causes the Fibers to pick up Oxygen Molecules from the Air and Rearrange their Atomic Bonding Pattern. The Stabilizing Chemical Reactions are Complex and Involve Several Steps, some of which occur Simultaneously. They also generate their own heat, which must be controlled to avoid Overheating the Fibers.
Carbonizing: Once the Fibers are Stabilized, they are Heated to a Temperature of about 1,830-5,500° F (1,000-3,000° C) for several minutes in a Furnace filled with a gas Mixture that does not contain Oxygen. The lack of Oxygen prevents the Fibers from Burning in the very high Temperatures.

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. As the Fibers are Heated, begin to lose their Non Carbon Atoms, Plus a few Carbon Atoms, in the form of Various Gases including Water Vapor, Ammonia, Carbon Monoxide, Carbon Dioxide, Hydrogen, Nitrogen, and others.
As the non Carbon Atoms are expelled: the remaining Carbon Atoms form tightly Bonded Carbon Crystals that are Aligned more or Less Parallel to the long axis of the Fiber. In some processes, two Furnaces operating at two different Temperatures are used to Better control the rate de Heating During Carbonization.

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. The addition of Oxygen Atoms to the surface provides better Chemical Bonding Properties and also Etches and Roughens the surface for better Mechanical Bonding Properties.
Oxidation can be Achieved: By Immersing the Fibers in Various Gases such as air, Carbon Dioxide, or Ozone; or in various Liquids such as Sodium Hypochlorite or Nitric Acid. The Fibers can also be Coated Electrolytically by making the Fibers the Positive Terminal in a bath Filled with various Electrically Conductive Materials. The Surface Treatment Process must be Carefully Controlled to avoid Forming tiny Surface Defects, such as Pits, which could Cause Fiber Failure.

Sizing: 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.

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.

Quality Control: The very small size of Carbon Fibers does not allow Visual Inspection as a quality Control Method. Instead, producing consistent precursor Fibers and closely Controlling the Manufacturing Process used to Turn them into Carbon Fibers Controls the Quality. Process Variables such as time, Temperature, Gas Flow, and Chemical Composition are Closely Monitored During each stage of the Production.
Health and Safety Concerns: There are three areas of Concern in the Production and Handling of Carbon Fibers: Dust Inhalation, Skin Irritation, and the Effect of Fibers on Electrical Equipment.

During Processing: Pieces of Carbon Fibers can Break Off and Circulate in the air in the Form of a Fine Dust. Industrial Health Studies have shown that, Unlike some Asbestos Fibers, Carbon Fibers are too Large to be a Health Hazard when Inhaled. They can be an Irritant, however, and People Working in the Area should wear Protective masks.


Wishing you all the best,
http://www.seeyourneeds.in