Carbon Fibres

Carbon Fibres

Snigdha Chakraborty

IndexIntroduction of carbon fibreProduction of carbon fibreProcess diagram of productionFibre morphologyProperties- physical, chemical, biological propertiesCarbon compositesUsesFuture scope

What is carbon fibre?

Carbon fiber, also called graphite fibre is basically very thin strands of carbon, consisting of strong crystalline regions within the fibre. It is very well oriented carbon atoms.

Diameter of carbon fibre is 5-10 m.

They can be either drawn as carbon filaments or cut at equal lengths to form carbon staple fibres.

They can also be twisted together to form carbon rovings.

The carbon rovings are measured by the no. of strands of carbon hair in each roving. The most commonly available ones are 3K (3000 strands in a roving), 5K, 10K, 12K.

Raw Materials:

The raw material used to make carbon fiber is called the precursor. About 90% of the carbon fibers produced are made from polyacrylonitrile (PAN). The remaining 10% are made from rayon or petroleum pitch.Zoltek, a hungarian company is worlds largest producer of carbon fibre

(T-B) 50K ribbon, 10K, 6K, 3K Carbon fibre staples

Carbon fibers from POLYACRYLONITRILE (PAN)

There are three successive stages in the conversion of PAN precursor into high-performance carbon fibers.

1) Oxidation stabilization: The PAN precursor is first stretched and drawn into long strands of fibers and simultaneously oxidized in a temperature range of 200-300C which results in stabilization, meaning the fibre changes from thermoplastic to non plastic.

2) CARBONIZATION: Then it is heated at 1000 C (without oxygen) in nitrogen for a few hours. Without oxygen the 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.

3) GRAPHITIZATION: Depending on the type of fiber required, the fibers are treated at temperatures between 1500-3000C, which improves the ordering, and orientation of the crystallites in the direction of the fiber axis.

Carbon fiber fabrication from pitch generally consists of the following four steps:

1) Pitch preparation: It is an adjustment in the molecular weight, viscosity, and crystal orientation by certain processes.

2) Spinning and drawing: In this stage, pitch is converted into filaments (by melt spinning process) some alignment in the crystallites to achieve the directional characteristics.

3) Stabilization: In this step, some kind of thermosetting to maintain the filament shape during pyrolysis. The stabilization temperature is between 250 and 400 C.

4) Carbonization: The carbonization temperature is between 1000-1500C.Carbon fibers from pitch

After carbonizing, the fibers have a surface that does not bond well with the epoxies and other materials. For better bonding properties, their surface is slightly oxidized.

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.

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 THEN loaded for spinning.SURFACE TREATMENT

Fibre MorphologyCarbon fibre under microscope.

It is entirely a mass of carbon atoms.

The small pores are created during carbonization when the other components present in PAN are vaporized.

Fibre morphology depends on:

1) Degree of polymerization:2) Temperature 3) Draw ratio4) Orientation5) Crystalline and amorphous region

And these factors are all interrelated

Here in the picture, we can see the atomic structure of carbon fibre,

By increasing the setting temperature gradually, the orientation of fibres can be bettered

The crystalline region is increasing in the fibre gradually and amorphous is decreasing.

Physical properties 1) Tensile Strength- is the maximum stress that a material can withstand while being stretched or pulled before breaking. Carbon fibre has a tensile strength of 4137 MPa compared to normal steels 2693 Mpa. And carbon nanotubes 11000-13000 Mpa. Carbon fiber (Toray T1000G)(the strongest man-made fibres) has 6370 Mpa /fibre .

2) Weight light weight

Carbon Fiber has High Strength to Weight Ratio (also known as specific strength) which is 2457 kN.m/kg, glass has 1307 and steel 254.

3) Elasticity- Elasticity is very poor as carbon is a very rigid fibre. Hence youngs modulus is high. For carbon its 180 compared to 0.01-0.1 of rubber.

4) Dimensional stability- Very stable dimensionally. Hence used as composites to make spare parts of missiles, automobiles, aircrafts, etc.

5) Fineness- of carbon fibre is 5-10 micrometer, where as that of carbon roving is given in the following chart.

6) Moisture regain - It is the ratio between the weight of water with the oven dry weight of the material express in percentage. For carbon fibres it is 0 whereas for jute it is 13.75.

7) Thermal property- a measure of how easily heat flows through a material. Carbon has fairly good thermal conductivity. Carbon Fiber Reinforced Epoxy has 24, carbon steel is 54. Air is 0.025, Aluminium has the highest 250.

NO. OF FILAMENTS TEX

3k300020012K1200080050K500003300

8)Electrical property- Carbon fibre is electrically conductive. Nanotubes of carbon are used as high conducting materials.

9) Fire retardance- As carbon is chemically inert, it does not get affected by flame. Can be use to make flame retardant jackets.

10)Affect of oxidation and chemicals- Carbon fiber is Corrosion Resistant and Chemically Stable.

11) Non Poisonous, Biologically Inert, X-Ray Permeable- These quality make Carbon fiber useful in Medical applications. Prosthesis use, implants and tendon repair, x-ray accessories surgical instruments, are all in development. Although not poisonous, the carbon fibers can be quite irritating and long term unprotected exposure needs to be limited.

12) Low Coefficient of Thermal Expansion- This is a measure of how much a material expands and contracts when the temperature goes up or down. Units are in Inch / inch degree F. For carbon fibre its