CARBON FIBERPRODUCTION METHODS, PROPERTIES AND COMPOSITE APPLICATIONS

INTRODUCTION

• Carbon fibres have been under continuous development for the last 50 years.

• There has been a progression of feedstocks, starting with rayon, proceeding to polyacrylonitrile (PAN), on to isotropic and mesophase pitches, to hydrocarbon gases, to ablated graphite and finally back to carbon containing gases.

CHEMICAL STRUCTURE

• The word ‘graphite’ is much misused in carbon fibre literature. The word refers to a very specific structure, in which adjacent aromatic sheets overlap with one carbon atom at the centre of each hexagon

• The high-performance carbon fibres are made up of large aromatic sheets, these are randomly oriented to each other, and are described as ‘turbostratic’

Irregular stacking of aromatic sheets; turbostratic carbon

Regular stacking of aromatic sheets in graphite

Some allotropes of carbon: a) diamond; b) graphite; c) lonsdaleite; d–f) fullerenes (C60, C540, C70); g) amorphous carbon; h) carbon nanotube.

MECHANICAL PROPERTIES OF SELECTED CARBON FIBRES

METHODS OF PRODUCING CARBON FIBER• All commercial C.F.’s from 3 processes:

• Rayon (cellulose)• Polyacrylonitrile (PAN)

• Accounts for 90% of commercial C.F.’s• 93-95% acrylonitrile units

• Pitch Based• Isotropic pitch• Mesophase pitch

• New method for C.F.• Vapor Growth (high performance application)

PAN PROCESS

• Polyacrylonitrile (PAN) fibres are made by a variety of methods.

• The polymer is made by free-radical polymerisation, either in solution or in a solvent–water suspension.

• The polymer is then dried and redissolved in another solvent for spinning, either by wet-spinning or dry-spinning.

• Wet spinning: the spin dope is forced through a spinneret into a coagulating liquid and stretched

• Dry spinning: the dope is spun into a hot gas chamber, and stretched

CHEMISTRY OF CARBON FIBER PRODUCTION

The strength of fibers spun in this way and subsequently heat treated was found to improve by >80% over conventionally spun fibres. The mechanism is presumed to be removal of small impurities which can act as crack initiators. This technology is believed to be critical for production of high strength fibres suchas Toray’s T800 and T1000.

PRODUCTION OF CARBON FIBER FROM PAN• Heating/Stretching• Pre-carbonization• Carbonization• Surface Treatment

HEATING/STRETCHING

• Stretching (500-1300%)• 220-270oC for 30min to 7hrs

• Temp./Time dependent on composition/diameter of Precursor

• Chemical changes• Cyclization of nitrile groups• Dehydration of saturated C-C bonds• Oxidation• Generates CO2 and HCN

• Large furnace + Drive rollers• Controlled tension essential for alignment

• PAN carbon content: 54%

The first critical step in making carbon fibre from PAN fibre is causing the pendant nitrile groups to cyclise. This processis thermally activated and is highly exothermic.

The next step is to make the fibre infusible: this is accomplished by adding oxygen atoms to the polymer, again by heatingin air

CARBON FIBERS

PAN based Carbon FiberStabilization Process The stabilization process is highly

exothermic, the heat released has adverse effect onfinal properties of fibers.

The process should be modified in such a way that either the heat released during stabilization should be reduced or it should be dissipated properly

The heat of stabilization can be reduced by using comonomers during PAN synthesis.

PRE-CARBONIZATION

• After diffusion (tens of minutes), about 8% oxygen by weight has been added, the fibre can be heated above 600 °C without melting.

• At such temperatures, the processes of de-nitrogenation and dehydrogenation take place, and above 1000 °C large aromatic sheets start to form,

CARBONIZATION

• 1300oC – 2800OC• 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, 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.

• This process is called carbonization and leaves a fiber composed of long, tightly inter-locked chains of carbon atoms with only a few non-carbon atom remaining.

• 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.

• Final C content: 80% to >99%

CARBON FIBERS

PAN based Carbon FiberGraphitization ProcessThis step involves heating the carbonized fiber under

tension at about 1200- 3000 °C in an inert atmosphere.This leads to an increase in the size crystals resulting

in enhanced mechanical properties.

SURFACE TREATMENT

• 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.

PAN PRECURSOR FIBER PROCESS

C.F.’S FROM PAN PRECURSOR FIBERS

CARBON FIBERS

Type ModulusUltra-high-modulus (UHM) >450 GpaHigh-modulus (HM) 350-450 GpaIntermediate-modulus (IM) 200-350 GpaLow modulus and high-tensile (HT) modulus < 100 GPa, tensile

strength >3.0 GPa

Super high-tensile (SHT) tensile strength > 4.5 GPa

PAN based Carbon Fiber PAN based carbon fibers are classified according to the tensile

properties and the heat treatment temperature as follows.

Classification based on Properties

CARBON FIBERS

PAN based Carbon Fiber

Classification based on Final Heat Treatment Temperature

Type Name Treatment Temperature

Modulus/Strength

Type-I High heat treated (HHT) above 2000 °C High-modulusType-II Intermediate heat treated

(IHT)above 1500 °C High-strength

Type-III Low heat treated 1000 °C Low modulus and low strength materials

CARBON FIBERS

Carbonization temperature& properties

High strength High modulus Ultra high modulus

Carbonization temperature (°C) 1200-1400 1800-2500 2800-3000

Carbon % in the fiber 92-96 99 99

Filament diameter (μm) 5.5- 8.0 5.4-7.0 8.4

Density (g/cm3) 1.75-1.80 1.78-1.81 1.96

Tensile strength (MPa) 3105-4555 2415-2555 1865

Tensile modulus (GPa) 228-262 359-393 (483-690), 577

Elongation at break (%) 1.3-1.8 0.6-0.7 0.38

PAN based Carbon Fiber

Classification and Typical Properties of PAN based Carbon Fibers

ISOTROPIC PITCHES

• These pitches are prepared from high-boiling fractions of petroleum feedstocks, usually heavy slurry oils produced in catalytic cracking of crude oil.

• A typical commercial pitch is Ashland Aerocarb 70, which has a softening temperature of 208 °C and a viscosity of 1Pa s at 278°C.

• Pitches may be subjected to additional treatments to reduce low-molecular weight components selectively

CENTRIFUGAL SPINNING

• Centrifugal spinning is practised commercially in the production of glass fibres. It was adapted for carbon fibre production by the Kureha Company in Japan in the 1970s.

• In this process, molten pitch is forced through small holes in a rotating bowl. The pitch stream is attenuated into a fibre by centrifugal forces, and is directed against a cutter by a stream of air

MELT BLOWING

• Melt blowing was originally developed for the manufacture of fibres from polypropylene, but was adapted for pitch by Ashland Oil Co in the 1970s.

• It is a very high productivity process, giving production rates per spinneret hole of the order of 10 times conventional melt spinning.

• In this process, a molten stream of pitch is extruded into a high velocity stream of forwarding gas, which rapidly attenuates the fibre

MESOPHASE PITCH PRECURSORS• Pitch is a viscoelastic material that is composed of aromatic

hydrocarbons. Pitch is produced via the distillation of carbon-based materials, such as plants, crude oil, and coal

• Pitch-based fibres satisfy the needs of niche markets, and show promise of reducing prices to make mass markets possible

• Same general process as PAN• Low tensile strength of precursor fibers• Melt-spin is used• Do not require stretching process to maintain preferred alignment• General purpose fibres are prepared by two different spinning

methods, centrifugal spinning and melt blowing.

PAN AND PITCH PROCESSING

CARBON FIBER PROPERTIES COMPARISON

FINAL CARBON FIBERS

• Woven, Braided, or Wound• Unidirectional lay-ups• Multi-directional weaves

• Strength limiting factors• Purity of precursor• Precursor void content• Temperature• Tension

C.F.’S FROM DIFFERING PROCESSES

CARBON VS. STEEL

Material Tensile Strength (GPa)

Tensile Modulus

(GPa)

Density (g/ccm)

Specific Strength

(GPa)

Standard Grade Carbon

Fiber

3.5 230.0 1.75 2.00

High Tensile Steel

1.3 210.0 7.87 0.17

CARBON FIBER MODULUS

http://www.carbonfiber.gr.jp/english/index.html

CARBON-CARBON COMPOSITES• Carbon substrate in Carbonaceous Matrix• Same element does not simplify composite behavior • each constituent has a wide range of forms

• Carbon fibers continuous and woven• Disadvantages• High fabrication cost• Poor oxidation resistance• Poor inter-laminar properties

FABRICATION OF C-C COMPOSITES• Liquid phase impregnation (LPI)• Hot isostatic pressure impregnation

carbonization (HIPIC)• Hot pressing• Chemical Vapor Infiltration (CVI)

APPLICATIONS OF C-C COMPOSITES• Aircraft Brakes• Heat Pipes• Reentry vehicles• Rocket motor nozzles• Hip replacements• Biomedical implants• Tools and dies• Engine pistons• Electronic heat sinks• Automotive and motorcycle bodies• Bicycles

FURTHER READING

• Buckley, John D. and Dan. D. Edie, eds. Carbon-Carbon Materials and Composites Noyes Publications: Park Ridge, NJ 1993• Chung, Deborah. Carbon Fiber Composites.

Butterworth-Heinemann, Boston 1994• Japan Carbon Fiber Manufacturer’s

Association http://www.carbonfiber.gr.jp/english/index.html