Monika Kannadaguli & Haoming Rong
Carbon fibers are a new breed of high-strength materials.
Carbon fiber has been described as a fiber containing at least 90% carbon
obtained by the controlled pyrolysis of appropriate fibers. The existence of
carbon fiber came into being in 1879 when Edison took out a patent for the
manufacture of carbon filaments suitable for use in electric lamps. However,
it was in the early 1960s when successful commercial production was started,
as the requirements of the aerospace industry - especially for military
aircraft - for better and lightweight materials became of paramount
importance. In recent decades, carbon fibers have found wide application in
commercial and civilian aircraft, recreational, industrial, and
transportation markets. Carbon fibers are used in composites with a
lightweight matrix. Carbon fiber composites are ideally suited to
applications where strength, stiffness, lower weight, and outstanding
fatigue characteristics are critical requirements. They also can be used in
the occasion where high temperature, chemical inertness and high damping are
important. The suppliers of Advanced Composites Materials Association
released 1997 industry statistics on worldwide shipments of carbon fibers
for composites[1,2] (Table 1). However, from 1997 to 1999 there
was a global slowing of carbon fiber demand.
Table 1 Worldwide shipments of carbon fibers for
Currently, the United States of America uses nearly 60%
of the world production of carbon fibers and the Japanese account for almost
50% of the world capacity for production. The largest producer of this fiber
is Toray Industries of Japan. The world production capacity of pitch
based carbon fiber is almost totally based in Japan.
CLASSIFICATION AND TYPES:
On the basis of modulus, strength, and final heat
treatment temperature, carbon fibers can be classified into the following
Based on carbon fiber properties, carbon fibers can be
Ultra-high-modulus, type UHM (modulus >450Gpa)
High-modulus, type HM ( modulus between 350-450Gpa )
Intermediate-modulus, type IM (modulus between 200-350Gpa)
Low modulus and high-tensile , type HT ( modulus < 100Gpa, tensile
strength > 3.0Gpa)
- Superhigh-tensile, type SHT (tensile strength > 4.5Gpa)
Based on precursor fiber materials, carbon fibers are
PAN-based carbon fibers
Pitch-based carbon fibers
Mesophase pitch-based carbon fibers
Isotropic pitch-based carbon fibers
Rayon-based carbon fibers
Gas-phase-grown carbon fibers
Based on final heat treatment temperature, carbon
fibers are classified into:
Type-I, high-heat-treatment carbon fibers (HTT), where final heat
treatment temperature should be above 2000EC
and can be associated with high-modulus type fiber.
Type-II, intermediate-heat-treatment carbon fibers (IHT), where final
heat treatment temperature should be around or above 1500EC
and can be associated with high-strength type fiber.
Type-III, low-heat-treatment carbon fibers, where final heat treatment
temperatures not greater than 1000EC. These
are basically low modulus and low strength materials.
In Textile Terms and Definitions, carbon fiber has
been described as a fiber containing at least 90% carbon obtained by the
controlled pyrolysis of appropriate fibers. The term "graphite fiber" is
used to describe fibers that have carbon in excess of 99%. A large variety
of fibers called precursors are used to produce carbon fibers of different
morphologies and different specific characteristics. The most prevalent
precursors are polyacrylonitrile(PAN), cellulosic fibers (viscose rayon,
cotton), petroleum or coal tar pitch and certain phenolic fibers.
Carbon fibers are manufactured by the controlled
pyrolysis of organic precursors in fibrous form. It is basically a heat
treatment of the precursor that removes the oxygen, nitrogen and hydrogen to
form carbon fibers. It is well established in carbon fiber literature that
the mechanical properties of the carbon fibers are improved by increasing
the crystallinity and orientation, and by reducing defects in the fiber. The
best way to achieve this is to start with a highly oriented precursor and
then maintain the initial high orientation during the process of
stabilization and carbonization through tension.
Carbon fibers from polyacrylonitrile(PAN):
There are three successive stages in the conversion of
PAN precursor into high-performance carbon fibers.
$Oxidative stabilization: The polyacrylonitrile
precursor is first stretched and simultaneously oxidised in a temperature
range of 200-300EC. This treatment converts
thermoplastic PAN to a non-plastic cyclic or ladder compound.
$Carbonization: After oxidation, the fibers are
carbonized at about 1000EC without tension in
an inert atmosphere (normally nitrogen) for a few hours. During this process
the non-carbon elements are removed as volatiles to give carbon fibers with
a yield of about 50% of the mass of the original PAN.
$Graphitization: Depending on the type of fiber
required, the fibers are treated at temperatures between 1500-3000EC
which improves the ordering and orientation of the crystallites in the
direction of the fiber axis.
Figure 1 Schematic representation of carbon fiber preparation from PAN
Carbon fibers from rayon:
The conversion of rayon fibers into carbon fibers is a
$Stabilization: Stabilization is basically an
oxidative process that occurs through steps. In the first step, between
25-150EC, there is physical desorption of
water. The next step is a dehydration of the cellulosic unit between 150-240EC.
Finally, thermal cleavage of the cyclosidic linkage and scission of ether
bonds and some C-C bonds via free radical reaction (240-400E
C) and, thereafter, aromatization takes place.
$Carbonization: Between 400 and 700EC,
the carbonaceous residue is converted into a graphite-like layers.
$Graphitization: Graphitization is carried out
under strain at 700-2700EC to obtain high
modulus fiber through longitudinal orientation of the planes.
Figure 2 Reactions involved in the conversion of cellulose into carbon
(Click image to enlarge)
Carbon fibers from pitch:
The carbon fiber fabrication from pitch generally
consists of the following four steps:
$Pitch preparation: It is an adjustment in the
molecular weight, viscosity, and crystal orientation for spinning and
$Spinning and drawing: In this stage, pitch is
converted into filaments, with some alignment in the crystallites to achieve
the directional characteristics.
$Stabilization: In this step, some kind of
thermosetting to maintain the filament shape during pyrolysis. The
stabilization temperature is between 250 and 400 EC.
$Carbonization: The carbonization temperature is
between 1000-1500 EC.
Figure 3 Manufacturing process schematic for pitch-based carbon fibers
CARBON FIBERS IN MELTBLOWN NONWOVENS
Carbon fibers made from the spinning of molten pitches
are of interest because of the high carbon yield from the precursors and the
relatively low cost of the starting materials. Stabilization in air and
carbonization in nitrogen can follow the formation of melt-blown pitch webs.
Processes have been developed with isotropic pitches and with anisotropic
mesophase pitches. The mesophase pitch based and melt blown discontinuous
carbon fibers have a peculiar structure. These fibers are characterized in
that a large number of small domains, each domain having an average
equivalent diameter from 0.03mm to 1mm and a nearly unidirectional
orientation of folded carbon layers, assemble to form a mosaic structure on
the cross-section of the carbon fibers. The folded carbon layers of each
domain are oriented at an angle to the direction of the folded carbon layers
of the neighboring domains on the boundary.
Carbon fibers from isotropic pitch:
The isotropic pitch or pitch-like material, i.e., molten
polyvinyl chloride, is melt spun at high strain rates to align the molecules
parallel to the fiber axis. The thermoplastic fiber is then rapidly cooled
and carefully oxidized at a low temperature (<100EC).
The oxidation process is rather slow, to ensure stabilization of the fiber
by cross-linking and rendering it infusible. However upon carbonization,
relaxation of the molecules takes place, producing fibers with no
significant preferred orientation. This process is not industrially
attractive due to the lengthy oxidation step, and only low-quality carbon
fibers with no graphitization are produced. These are used as fillers with
various plastics as thermal insulation materials.
Carbon fibers from anisotropic mesophase pitch:
High molecular weight aromatic pitches, mainly
anisotropic in nature, are referred to as mesophase pitches. The pitch
precursor is thermally treated above 350EC to
convert it to mesophase pitch, which contains both isotropic and anisotropic
phases. Due to the shear stress occurring during spinning, the mesophase
molecules orient parallel to the fiber axis. After spinning, the isotropic
part of the pitch is made infusible by thermosetting in air at a temperature
below it's softening point. The fiber is then carbonized at temperatures up
to 1000EC. The main advantage of this process
is that no tension is required during the stabilization or the
graphitizition, unlike the case of rayon or PAN precursors.
The characterization of carbon fiber microstructure has
been mainly been performed by x-ray scattering and electron microscopy
techniques. In contrast to graphite, the structure of carbon fiber lacks any
three dimensional order. In PAN-based fibers, the linear chain structure is
transformed to a planar structure during oxidative stabilization and
subsequent carbonization. Basal planes oriented along the fiber axis are
formed during the carbonization stage. Wide angle x-ray data suggests an
increase in stack height and orientation of basal planes with an increase in
heat treatment temperature. A difference in structure between the sheath and
the core was noticed in a fully stabilized fiber. The skin has a high axial
preferred orientation and thick crystallite stacking. However, the core
shows a lower preferred orientation and a lower crystallite height.
In general, it is seen that the higher the tensile
strength of the precursor the higher is the tenacity of the carbon fiber.
Tensile strength and modulus are significantly improved by carbonization
under strain when moderate stabilization is used. X-ray and electron
diffraction studies have shown that in high modulus type fibers, the
crystallites are arranged around the longitudinal axis of the fiber with
layer planes highly oriented parallel to the axis. On the whole, the
strength of a carbon fiber depends on the type of precursor, the processing
conditions, heat treatment temperature and the presence of flaws and
defects. With PAN based carbon fibers, the strength increases up to a
maximum of 1300oC and then gradually decreases. The modulus has
been shown to increase with increasing temperature.
PAN based fibers typically buckle on compression and form
kink bands at the innermost surface of the fiber. However, similar high
modulus type pitch-based fibers deform by a shear mechanism with kink bands
formed at 45E to the fiber axis.
Carbon fibers are very brittle. The layers in the fibers
are formed by strong covalent bonds. The sheet-like aggregations allow easy
crack propagation. On bending, the fiber fails at very low strain.
The two main applications of carbon fibers are in
specialized technology, which includes aerospace and nuclear engineering,
and in general engineering and transportation, which includes engineering
components such as bearings, gears, cams, fan blades and automobile bodies.
Recently, some new applications of carbon fibers have been found. Such as
rehabilitation of a bridge in building and construction
industry. Others include: decoration in automotive, marine, general aviation
interiors, general entertainment and musical instruments and after-market
transportation products. Conductivity in electronics
technology provides additional new application. Table 2
illustrates some of the characteristics and applications of carbon fibers.
Table 2 Characteristics and Applications of Carbon Fibers
Table 2. Characteristics and Applications of
|1. Physical strength, specific toughness, light weight
||Aerospace, road and marine transport, sporting goods
|2. High dimensional stability, low coefficient of thermal expansion,
and low abrasion
||Missiles, aircraft brakes, aerospace antenna and support structure,
large telescopes, optical benches, waveguides for stable high-frequency
(Ghz) precision measurement frames
|3. Good vibration damping, strength, and toughness
||Audio equipment, loudspeakers for Hi-fi equipment, pickup arms,
|4. Electrical conductivity
||Automobile hoods, novel tooling, casings and bases for electronic
equipments, EMI and RF shielding, brushes
|5. Biological inertness and x-ray permeability
||Medical applications in prostheses, surgery and x-ray equipment,
implants, tendon/ligament repair
|6. Fatigue resistance, self-lubrication, high damping
||Textile machinary, genera engineering
|7. Chemical inertness, high corrosion resistance
||Chemical industry; nuclear field; valves, seals, and pump components
in process plants
|8. Electromagnetic properties
||Large generator retaining rings, radiological equipment
The production of highly effective fibrous carbon
adsorbents with low diameter, excluding or minimizing external and intra-diffusional
resistance to mass transfer, and therefore, exhibiting high sorption rates
is a challenging task. These carbon adsorbents can be converted into a wide
variety of textile forms and nonwoven materials .
Cheaper and newer versions of carbon fibers are being
produced from new raw materials. Newer applications are also being developed
for protective clothing(used in various chemical industries for work in
extremely hostile environments), electromagnetic shielding and various other
novel applications. The use of carbon fibers in nonwovens is in a new
possible application for high temperature fire-retardant insulation (eg:
SACMA Releases Carbon Fiber Industry Statistics, Composites News,
No. 1, 1998
SAMPE Plenary Describes Carbon Fiber Capacity, Trends, Composites
News, No. 6, 1998
Carbon fibers Seen as Having Big Long Term Growth Infrastructure is
Next Big Trend Driver, Advanced Materials & Composites News, No. 3, 1999
US5536486, Carbon fibers and Nonwoven Fabrics
Rehabilitation Bridges: Carbon Fiber-reinforced Polymer Shows
Promise for Repairing Structures, Advanced Materials & Composites News,
No. 2, 1999
New Company Launches Carbon Fiber Fabrics for Decorative
Applications, Advanced Materials & Composites News, No. 8, 1998
Carbon fibers Electrical Conductivity Found to Offer New Uses,
Composites News, No. 3, 1998
Jean-Baptiste Donnet, Roop Chand Bansal, Carbon Fibers, publicated
by Marcel Dekker Inc., 1990, p370
- Composites Edge; 1992.
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