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CARBON FIBERS

91

CARBON FIBERS

Introduction

Carbon fibers continue to be the main reinforcement materials in advanced com-
posites (qv). The ability to manipulate their physical, chemical, electrical, and
thermal properties makes carbon fibers suitable across a wide range of commer-
cial applications, including military [aircraft and missiles (1)], structural [con-
crete reinforcement (2,3) and automobile body panels], sports equipment (golf

Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.

92

CARBON FIBERS

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Fig. 1.

Tensile strength versus modulus for some commercial carbon fibers. From Ref. 8.

To convert GPa to psi, multiply by 145,000.

clubs, skis, surfboards, and softball bats), energy storage [batteries and capaci-
tors (4,5)], and activated carbons (catalysis, gas storage, and purification) (6,7).
The ultimate properties of the carbon fiber directly depend on the selection and
processing of the precursor materials, the formation of the fiber, and subsequent
processing of the fiber. Figure 1 shows a comparison of carbon fiber strengths and
moduli as a function of precursor selection and treatment temperature.

Carbon fibers are defined by the International Union of Pure and Applied

Chemistry (IUPAC) as “fibers (filaments, tows, yarns, rovings) consisting of at least
92% (mass fraction) carbon, usually in a non-graphitic state (9).” Carbon fibers are
produced either through carbonization (the controlled pyrolysis of a fiber precur-
sor in an inert atmosphere) or through growth from gaseous hydrocarbons. The
carbonization process is preceded by a stabilization step in which precursor fibers
are converted to thermally stable forms that will not melt during carbonization.
During the carbonization process, inorganics and aliphatic carbons are driven off,
leaving a fiber consisting of graphene layer planes. An idealized carbon fiber is
shown in Figure 2.

Real carbon fibers cannot achieve the graphitic ideal. Instead the graphene

planes are uneven and wavy, with a mean interplanar spacing significantly greater
than the 0.335 nm of graphite. This turbostratic structure of carbon fibers is shown
in Figure 3.

The carbon fiber production process involves a series of steps including spin-

ning of the precursor fiber, stabilization, carbonization, and post-carbonization
treatments that include sizing and surface treatments. The specifics of these steps
vary greatly, depending on the precursor material and the desired properties of
the carbon fiber. Figure 4 shows a schematic of the carbon fiber production process.

In the United States, carbon fibers are classified into two broad categories:

high performance (HP) and general performance (GP) (12). High performance
fibers are further classified into high tensile (HT) and high modulus (HM).
Table 1 summarizes the tensile properties of HT and HM fibers.

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93

Fig. 2.

Idealized carbon fiber structure. Adapted from Ref. 10.

Fig. 3.

The turbostratic structure of carbon fibers. Adapted from Ref. 11.

Fig. 4.

Schematic of carbon fiber production. Adapted from Ref. 11.

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CARBON FIBERS

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Table 1. Properties of High Performance Carbon Fibers

a

Fiber type

Tensile strength, GPa

Young’s modulus, GPa

High tensile (HT)

3.0–7.1

150–300

High modulus (HM)

0.2–3.0

400–900

a

Ref. 12.

This categorization of carbon fibers is not uniform. In the United Kingdom,

the fiber categories are Type I (HM), Type II (HS), and Type III (GP) (13). Some
researchers refer to fibers at the higher end of the Young’s modulus range as ultra-
high modulus (UHM) fibers (14). Fibers are also distinguished as being graphi-
tizable or nongraphitizable. Graphitizable fibers can, upon heat treatment, be
converted to graphene layer planes with a mean interlayer spacing of less than
0.34 nm (9), compared to the 0.335-nm spacing for pure graphite. Although there
is an objective standard for graphitizable fibers, many researchers incorrectly use
the term to describe any carbon fiber that develops measurable three-dimensional
order. The Inorganic Chemistry Division Commission on High Temperature and
Solid State Chemistry Subcommittee has developed recommended terminology to
alleviate the confusion in nomenclature (9), but the multiple terminologies remain
embedded in the culture of carbon fiber research.

The majority of commercial carbon fibers are produced from polyacryloni-

trile (PAN) fibers. In fact, HTA-12K PAN-based carbon fibers are the most com-
monly used commercial carbon fiber (15). PAN-based fibers are the strongest
commercially available carbon fibers and dominate structural applications.
Mesophase pitch-based carbon fibers represent a smaller but significant market
niche. These fibers develop exceptional moduli and excel in lattice-based prop-
erties, including stiffness and thermal conductivity (1). Rayon-based fibers are
used in heat shielding and in missile nosecones (16). Carbon fibers made from
high performance polymers (17–19) or from chemical vapor deposition of hydro-
carbons, such as benzene or methane, display unique properties that make them
potentially attractive future alternatives (20–22).

PAN-Based Carbon Fibers

PAN-based carbon fibers develop exceptional tensile strength [in excess of
7000 MPa (23)] and are more resistant to compressive failure than high perfor-
mance polymers (24). These factors combine to make PAN-based carbon fibers
the ideal choice for applications requiring significant fiber strength. However,
PAN-based fibers are less appropriate than mesophase pitch-based fibers for ap-
plications in which molecular order-dependent properties are key.

Commercial PAN is actually a copolymer of acrylonitrile and another

monomer (vinyl acetate, methyl acrylate, or acrylic acid) that is added to lower
the glass-transition temperature of the material and control its oxidation resis-
tance (see A

CRYLONITRILE AND

A

CRYLONITRILE

P

OLYMERS

). The repeat unit of PAN

is shown in Figure 5. The copolymer is formed through a wet-spinning process
in which the polymer solution is extruded directly into a liquid bath. A blend of

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CARBON FIBERS

95

Fig. 5.

The molecular structure of PAN.

phase separation and chemical reaction in the liquid results in solidification of the
polymer. A concentrated copolymer solution containing 93–95 wt% acrylonitrile is
dissolved in a solvent (typically dimethylacetamide) to form a concentrated poly-
mer solution that is fed to a storage tank (25). The solution is filtered to minimize
impurities and passed through a spinnerette.

The fibers emerge through the small capillary holes directly into a coagula-

tion bath containing ethylene glycol that extracts the solvent from the fiber. The
solvent extraction rate is governed by complex thermodynamic and kinetic rela-
tionships but can be influenced by the selection of coagulation liquid, temperature,
and the circulation rate of the fluid within the bath. Figure 6 shows a schematic
representation of the wet spinning of PAN fibers.

The development of internal voids or flaws in the fiber as well as the shape

and texture of the fiber are controlled by the solvent removal rate. Concentration
gradients can occur during the solvent extraction process, resulting in a warping
of the fiber. For example, dog-bone–shaped fibers result from removing the sol-
vent too rapidly (23). The fiber that emerges from the coagulation bath undergoes
a series of post-spinning steps including washing, drawing, and drying during
which the fiber solidifies into its final form. These processes play a crucial role in
developing the internal morphology of the fibers (26).

Before the acrylic fibers (qv) produced in the wet-spinning process can be

subjected to the elevated temperatures of carbonization, they must be converted
to a thermally stable form that will not melt through oxidative stabilization. Sta-
bilization is usually performed in air at temperatures between 200 and 300

◦

C (27)

and under tension to prevent fiber shrinkage (28).

During the stabilization process, dehydrogenation, oxidation, and nitrile cy-

clization reactions result in the formation of a condensed pyridine ring “ladder”
structure that is suitable for subsequent carbonization (29). The general mecha-
nism is shown in Figure 7. The details of the stabilization mechanism are highly

Fig. 6.

Wet spinning of PAN fibers. Adapted from Ref. 24.

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CARBON FIBERS

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Fig. 7.

General mechanism for the stabilization of PAN. Adapted from Ref. 12.

complex and have been the subject of many studies (30–33). All studies agree that
control of the heating rate is essential because the reaction is highly exothermic.
The stabilization time varies greatly with the size of the fiber bundle, the tem-
perature, and the oxidizing environment. Studies of stabilization at elevated tem-
peratures (350–400

◦

C) show the development of additional intermolecular cross-

linking, resulting in improved strengths (34,35). The stabilized fiber is carbonized
in an inert atmosphere at temperatures ranging from 1000 to 3000

◦

C, depending

on the desired properties. During the pyrolysis, nearly all inorganic and nonaro-
matic carbons are driven off. Primary off-gases include hydrogen cyanide, carbon
dioxide, water, ammonia, methane, and hydrogen (35). The carbon yield during
the process is generally in the range of 40–45% (36).

Heating rate plays a major role in controlling the release of gases. A decrease

in tensile strength is generally observed at temperatures above 1500

◦

C (37), which

corresponds to the final major release of nitrogen (38). As a result, many of the
highest strength PAN-based carbon fibers contain residual nitrogen. Although
heat treatments of PAN-based carbon fibers above 1700

◦

C are often referred to

as graphitization, these fibers are not graphitizable and do not develop highly
ordered graphene planes.

Because PAN-based carbon fibers are commonly used in applications requir-

ing high tensile strength, great care must be taken to preserve the potential for
high tensile strength during the spinning of the precursor fibers. A fiber is often
viewed as a chain consisting of interlocking links. If there were no misalignments
or flaws, each link would fail at essentially the same tensile load. However, flaws
in the fiber act as weak links in the chain, lowering the observed tensile strength
of the fiber (39). As a result, the actual strength at which a given fiber will fail
depends on both the frequency and severity of these flaws (40,41). Macroscopic
flaws can result from inclusions, large voids, or damage from handling. Micro-
scopic flaws can also lead to premature failure but are more difficult to quantify.
Johnson (42) attributed many flaws to the misalignment of crystallites that in-
terlink and result in voids. Highly ordered fibers are far more sensitive to these
crystallite misalignments. As a result, fibers with high moduli tend to have lower
tensile strengths than less-ordered, less–flaw-sensitive lower modulus fibers.

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CARBON FIBERS

97

Fig. 8.

Schematic representation of a carbonized PAN fiber. Adapted from Ref. 43.

Diefendorf and Tokarsky (43) showed that carbonized PAN fibers have a fib-

rillar microstructure as shown in Figure 8. This microstructure may be viewed as
undulating ribbons and is highly resistant to premature tensile failure because
of its flexibility. Therefore, PAN-based carbon fibers are well suited to develop-
ing high tensile strengths, but are not likely to develop the high level of three-
dimensional order associated with high modulus precursor materials such as
mesophase pitch. As such, PAN-based carbon fibers are considered nongraphitiz-
ing and maintain a highly turbostratic organization of the graphene layer planes,
even when exposed to very high treatment temperatures (42).

Recent research in Poland is helping PAN-based carbon fibers find new appli-

cations as biological implants. Low temperature carbonized fibers have found use
in ligament and tendon prostheses and in surgical sutures (44,45). These fibers
are heat treated to temperatures below 1300

◦

C and are far less crystalline than

traditional carbonized PAN fibers (46,47). Despite the promise of these studies,
low temperature carbonized PAN fibers do not share the commercial success of
their high performance cousins.

Mesophase Pitch-Based Carbon Fibers

Pitch consists of a heavy fraction of predominantly aromatic hydrocarbons that is
the residue of petroleum or coal tar distillation following the removal of creosote
oil and anthracene (48). Table 2 provides a list of the major components in coal-
tar fractions. The pitch consists of hundreds of thousands of different species
with an average molecular weight of several hundreds. Many of the species are
heterocyclic and are formed by a complex variety of chemical reactions, including
thermal decompositions, hydrogen transfers, and oligomerization reactions (49).
Pitch is a glassy solid at room temperature, but softens upon heating to form a

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CARBON FIBERS

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Table 2. Major Components in Coal-Tar Fractions

a

Boiling Temperature,

◦

C

<200

200–250

250–300

300–350

>350

Benzene

Tar acids

Methyl-naphthalene

Phenathracene

Chrysene

Toluene

Tar bases

Acenapthene

Anthracene

Fluoranthene

Xylene

Naphthalene

Fluorene

Carbazole

Pyrene

Tar acids

Diphenylene oxide

Tar bases
Solvent
Naphtha

a

Ref. 50.

viscous liquid. Pitches are often characterized by determining their solubility in
different organic solvents. Solvent fractionation also serves as a critical method
of altering the composition of pitches prior to fiber spinning (48).

Certain pitches can be spun directly into isotropic pitch fibers with only

minor devolitization. Carbonized isotropic pitch fibers cannot be graphitized and
develop mechanical and thermal properties that are substantially inferior to those
produced by other precursors. However, isotropic pitch fibers are very inexpensive
and have found commercial applications in areas that do not require the excep-
tional mechanical and thermal properties of mesophase pitch-based carbon fibers.
Isotropic pitch-based carbon fibers are used in filters, brake pads, activated car-
bons, and as substrates for chemical vapor deposition (23). Table 3 summarizes
properties of isotropic pitch-based carbon fibers.

The physical explanation for why some precursors produced graphitizable

carbons while others did not was developed by Brooks and Taylor (51). During
thermal treatment of aromatic hydrocarbons between 400 and 550

◦

C, an interme-

diate liquid crystalline phase (mesophase) of spheroids with a mosaic structure
formed within the continuous isotropic phase. Selected-area electron diffraction
patterns show that each mesophase sphere has a single direction of preferred ori-
entation as shown in Figure 9. As the pyrolysis progresses, the spherules grow
and coalesce. Ultimately, a phase inversion occurs and the mesophase becomes the
continuous phase (52). There are no grain boundaries in the mesophase, though
disclinations do exist. The composition of each phase changes continuously during
the pyrolysis and when the entire material is mesophase, polymerization contin-
ues with the release of volatiles (48). The formation of the mesophase can be

Table 3. Properties of Isotropic Pitch-Based Carbon Fibers

a

Carbon

Density,

Tensile strength,

Young’s modulus,

Fiber type

wt%

g/cm

3

MPa

GPa

T-101F

>95

1.65

790

33

T-101S

>95

1.65

720

32

T-201F

>99

1.57

690

33

T-201S

>99

1.57

590

30

a

Ref. 12.

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CARBON FIBERS

99

Fig. 9.

Lamellar structure of mesophase spherule

influenced by numerous factors including the presence of insoluble particles, the
presence of oxygen or sulfur, the applied pressure, the nature of the pitch itself,
and the temperature of the heat-soak treatment. Partial oxidation of the pitch is
particularly troubling because it results in the presence of inclusions of isotropic
pitch which persist in the carbonized product and will not convert to mesophase
at any treatment temperature (51).

The two main processes for producing mesophase from pitch are heat soak-

ing and solvent extraction. The first heat-soaking process was developed at Union
Carbide in 1977 to convert 50 wt% of low cost Ashland 240 isotropic pitch into
a spinnable mesophase by heating the pitch for approximately 40 hours at 400–
410

◦

C (53). During this process, the higher density of the mesophase fraction

caused it to collect on the bottom of the vessel. Agitating the pitch during pyrol-
ysis led to a homogeneous emulsion of the mesophase and isotropic fraction that
resulted in a more uniform and spinnable product (54). Chwastiak and Lewis (55)
produced 100% bulk mesophase by using an inert gas to agitate the mixture and
remove volatile components. Subsequent research by Otani and Oya (56) produced
a mesophase pitch with a lower softening temperature by adding a hydrogenation
step either before or after the production of the mesophase. Figure 10 shows a
typical molecule of mesophase pitch formed during the heat-soaking process.

Mesophase is also produced through a solvent extraction technique. A por-

tion of an isotropic pitch is extracted with an organic solvent such as benzene,
hexane, or toluene. The remaining insoluble fraction is pyrolized for 10 min (as
opposed to 40 h in the Lewis heat-soaking process) in a temperature range from
230

◦

C to 400

◦

C, yielding a 75–100 wt% mesophase product (57). Both production

strategies convert inexpensive pitch feedstocks to spinnable mesophases, but nei-
ther can overcome some fundamental limitations of the feedstocks themselves.
Pitch is a mixture of components refined through a complex process of feedstocks
that are inherently variable, making batch-to-batch consistency extremely diffi-
cult. Further, during every step of pitch production and pyrolysis, impurities are
concentrated in the retained fraction. These problems have spurred interest in
alternative mesophase production strategies.

Mochida (58) developed a process in which a synthetic mesophase is produced

from the HF/BF

3

catalyzed polymerization of naphthalene or methyl naphthalene.

HF/BF

3

is a Bronsted acid that has been used to catalyze coal liquifaction and aro-

matic condensation. The aromatic resin (AR)-mesophase produced through this
process is both more spinnable and more easily oxidized than that produced from

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CARBON FIBERS

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Fig. 10.

A typical heat-soaked mesophase molecule. Adapted from Ref. 55.

traditional pitch precursors. Mochida (59) reports that the thermal and mechani-
cal properties of the carbonized AR-mesophase fibers are comparable with those of
the best commercial fibers. AR-mesophase has a much narrower molecular weight
distribution than a mesophase formed from natural pitch and is free from sulfur
and other impurities that present problems in traditional pitches. Figure 11 shows
representative benzene-soluble fractions of AR-mesophase.

Fig. 11.

Representative benzene-soluble fractions from AR-mesophase. Adapted from

Ref. 12.

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CARBON FIBERS

101

Fig. 12.

The stacking behavior of mesophase pitch. Adapted from Ref. 64.

Another approach to improving the quality of the mesophase precursor in-

volves supercritical fluid extraction, which has been shown to fractionate isotropic
petroleum pitches into fractions of different compositions, some of which contain
100% mesophase (60). When using supercritical fluids, both temperature and pres-
sure can be used to alter the strength of the solvent (61). Continuously varying the
temperature and pressure allows selective fractions of relatively narrow molec-
ular weight distributions to be isolated (62). Supercritical fluid extraction offers
several advantages in mesophase preparation including lower solvent require-
ments and the ability to use a single, cost-effective solvent to produce a variety of
pitch fractions (63).

Once the mesophase pitch has been prepared, the next step in processing

is spinning it into fibers. This process is particularly important because when
mesophase pitch is carbonized, the morphology of the pitch is the dominant fac-
tor in determining the microstructure of the resulting graphitic fiber. This re-
sults from the stacking behavior of the mesophase molecules (64), as shown in
Figure 12.

Mesophase pitch fibers are produced through melt spinning that is essen-

tially the same as that used to spin commercial polymers. Figure 13 shows this
process in which an extruder melts pitch particles and pumps the molten pitch
through a multiholed spinnerette. The high extensional shear orients the liquid
crystalline mesophase molecules as it approaches and flows through the spin-
nerette. The fibers emerging from the spinnerette are drawn by a windup spool.

Although there are considerable similarities between melt spinning pitch

and polymers, the rheological behavior of mesophase pitch is far more complex
than that of most polymers. First, the viscosity of mesophase pitch displays an
extreme sensitivity to temperature (65–67). This has direct impact on fiber spin-
ning because pitch fibers achieve their final diameter within a few millimeters
of exiting the spinnerette, unlike most polymeric materials. These data provide
no insight into the texture in the developing fibers, but provide a basis for deter-
mining appropriate spinning temperatures for pitches. Mesophase pitch exhibits
shear-thinning behavior at low shear rates and becomes increasingly Newtonian

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CARBON FIBERS

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Fig. 13.

Melt spinning of mesophase pitch fibers.

as shear rate increases (65). The pseudoplasticity of the mesophase likely relates
to the alignment of the liquid crystalline mesophase domains at elevated shear
(67). Mesophase pitch has been reported to display thixotropic behavior, but no
clear basis for this behavior has been developed (68,69).

McHugh and Edie (68) demonstrated that the microstructure of carbonized

mesophase pitch-based carbon fibers can be predicted through the application of
continuum theory. Leslie (69) and Ericksen (70) proposed a continuum theory that
applies to the flow of nematics. In addition to the standard conservation equations
(continuity and the equation of motion), a conservation of angular momentum
equation (known as the balance of torques) is also considered. Unfortunately, the
shear stress is not proportional to the shear rate. Instead, the viscous stress tensor
is the sum of six terms, each with its own unique viscosity coefficient. Although
these equations are quite complex, they can be solved for a unique analytical solu-
tion for a given geometry. When applied to fiber spinning, the Leslie and Ericksen
model can predict the flow-induced orientation of the aromatic planes in the as-
spun fiber. For example, the equations predict and explain the development of
the radial texture observed for fully developed flow of discotic mesophase through
a circular capillary (ie, a spinnerette hole). The balance of torques requires the
mesophase to flow with a radial orientation as shown in Figure 14.

As a result of the ability to predict fiber texture as a function of geometry,

improved spinning geometries can be evaluated. McHugh and Edie (68) showed
that a capillary with a tapered entry forms a more ordered fiber as a result of

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CARBON FIBERS

103

Fig. 14.

Origin of radial structure in mesophase pitch fibers. From Ref. 23.

creating less disruption to the fully organized flow assumed by the Leslie and
Ericksen model. Spinning fibers through a rectangular capillary generates fiber
with a highly linear texture that results in fibers with higher thermal conductiv-
ity (71). The line-origin texture of these “ribbon fibers” is shown in Figure 15.
Figure 16 shows the Leslie and Ericksen model solution to flow of discotic
mesophase flowing through a rectangular channel. The edges within the chan-
nel represent the edges of the aromatic planes (68).

Regardless of the shape and texture, the as-spun fibers must be stabilized

prior to carbonization. During this process, oxygen reacts with side groups in the
pitch fiber, creating cross-linkages and adding weight. Typically, a 6% weight gain
is needed to completely stabilize mesophase pitch fibers (72), while an 8% weight
gain is more typical for PAN (73). Lower heating rates and lower temperatures

Fig. 15.

The line-origin texture of ribbon fibers.

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CARBON FIBERS

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Fig. 16.

Orientation developed in discotic mesophase flow through a rectangular channel.

Adapted from Ref. 68.

during stabilization result in a more uniform stabilization, but also add to pro-
cessing time (74).

The carbonization of mesophase pitch-based fibers closely resembles that

which is previously described for PAN, but occurs in two stages. Typically, fibers
are heated to approximately 1000

◦

C and held there allowing for the evolution

of off-gases. The rate-limiting step in this low temperature carbonization is the
free-radical breaking of carbon–hydrogen bonds. Following the release of gases,
the fibers are heated to their final treatment temperature, which may be as high
as 3000

◦

C.

Unlike PAN-based carbon fibers, mesophase pitch-based fibers experience

significant graphitization during which dislocations in the turbostratic carbon
stacks are gradually annealed, resulting in the formation of a three-dimensional
lattice. Fischbach (75) presented a detailed study of graphitization which charac-
terizes the process as a combination of atomic diffusion and crystallite growth.
Table 4 provides a comparison of properties for some commercial pitch- and PAN-
based carbon fibers.

Table 4. Properties of Commercial Carbon Fibers

a

Fiber type

Precursor material

Tensile strength, GPa

Young’s modulus, GPa

T-300

PAN

3.66

231

T-650/35

PAN

4.28

241

T-650/42

Pitch

4.62

290

P-25

Pitch

1.38

159

P-30X

Pitch

2.76

201

P-55S

Pitch

1.90

379

P-75S

Pitch

2.07

517

P-100S

Pitch

2.41

759

P-120S

Pitch

2.41

828

K-1100

Pitch

3.10

931

a

Ref. 76.

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CARBON FIBERS

105

Fig. 17.

The repeat unit of Kevlar.

Fig. 18.

The repeat unit of PBO.

Carbon Fibers from High Performance Polymers

The primary factor limiting the increased commercial application of carbon fibers
is their high production cost, a significant portion of which is directly attributable
to the lengthy stabilization process required by both pitch and PAN (77). The
rigid-rod structure and high degree of aromaticity allow Kevlar (poly p-phenylene
terephthalamide) and PBO (poly p-phenylene benzobisoxazole) fibers to be con-
verted directly to carbon fiber without any stabilization (17–19). The repeat units
of Kevlar and PBO are shown in Figures 17 and 18 (see P

OLYAMIDES

, A

ROMATIC

;

R

IGID

-R

OD

P

OLYMERS

). Surprisingly, PBO-based carbon fibers also show some ca-

pacity to graphitize (19). However, the commercial development of high perfor-
mance polymer-based carbon fibers is impeded by their comparatively poor tensile
strength. Table 5 compares the properties of PBO- and Kevlar-based carbon fibers
with typical pitch- and PAN-based fibers.

In spite of the relatively poor tensile strengths of the fibers, carbonized

Kevlar possesses characteristics that make it an ideal candidate as a substrate
for chemical vapor deposition of silicon carbide including a compatible coefficient
of thermal expansion, sufficient tensile strength to survive the coating process,
availability from a reliable domestic supplier, and a smooth, even diameter of less
than 7

µm (78).

Table 5. Properties of High Performance Polymer Based Carbon Fibers

a

Tensile

Young’s

Electrical

Precursor

strength,

modulus,

Resistivity,

material

Product designation

GPa

GPa

µ-m

PBO

Batch, 1800

◦

C

0.52

245

13

PBO

Continuous, 1800

◦

C

0.85

183

10

Kevlar-29

1600

◦

C

0.94

143

23

PAN

T-300

3.66

231

18

Pitch

P-55

1.90

415

9

a

Refs. 18,78.

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CARBON FIBERS

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Rayon-Based Carbon Fibers

The first carbonization of cellulose-based fibers dates back to Thomas Edison, who
carbonized a natural cellulose filament for use as an incandescent lamp filament.
In the mid-1950s, the Carbon Wool Corporation introduced the first commercial
carbonized rayon fibers (79). PAN- and pitch-based carbon fibers have replaced
rayon-based fibers in most high performance applications; however, they continue
to find use as ablative materials in missile nosecones and heat shielding (16). Ad-
ditionally, the combination of low cost, ease of handling, and high natural porosity
makes rayon an attractive precursor for activated carbon fibers (see C

ELLULOSE

F

IBERS

, R

EGENERATED

).

Activated carbon fiber (ACF) offers several advantages over traditional gran-

ular and powdered carbons as an absorbent material. Rayon-based ACFs (80)
possess

(1) smaller fiber diameters, which encourages rapid adsorption and desorption

by minimizing diffusion limitation;

(2) more concentrated and controllable pore size distributions; and
(3) excellent adsorption capacity at low adsorbate concentrations.

Rayon-based ACFs are used in the adsorption of many volatile organic com-

pounds including formaldehyde (80), methyl ethyl ketones (81), and benzene (81).
ACFs are also finding uses in natural gas storage (82), electrodes for batteries
(83), catalyst supports (84), and NO

x

removal (85). Stabilized rayon fibers are

carbonized and then activated with air (80), steam (86), or carbon dioxide (87),
much as in granular carbon activation. The extent of pyrolysis governs the pore
structure, carbon yield, and surface area of the fiber, while activation impacts
the presence of functional groups on the pore surface (12). Properties of some
commercial ACFs are summarized in Table 6.

ACFs develop a pore structure with micropores (

<60 nm) open directly to

the surface, unlike granular carbons where extensive surface oxidation results in
macropores on the surface (88) with microporosity only in inner zone, as shown
in Figure 19. The presence of extensive surface microporosity has a significant
impact on the adsorption of volatile organic compounds. The overlap of attractive
forces on opposite micropore walls are primarily responsible for the adsorption of
gases at low concentrations (89).

Carbonized rayon fibers present one drawback as an ACF precursor. Nearly

82% of the mass of the original fiber is evolved during pyrolysis, resulting in a

Table 6. Properties of Commercial Active Carbon Fibers

a

Surface

Mean pore

Benzene

Tensile

Fiber

Precursor

area, m

2

/g

size, nm

adsorption, wt%

strength, MPa

Toyobo KF Series

Rayon

1000–1600

0.5–10

30–60

60–130

Toho Finegard

PAN

500–1200

2–4

20–50

196–490

Adol A Series

Pitch

1000–2000

1–2.5

20–45

60–130

a

Ref. 12.

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CARBON FIBERS

107

Fig. 19.

Pore models for ACFs and granular carbon. Adapted from Ref. 88.

loss of both strength and flexibility (90). Rayon carbonization consists of a blend
of depolymerization and dehydration reactions. The depolymerization involves
the formation of l-glucosan and volatile products, while the dehydration reaction
inhibits l-glucosan and volatile production, resulting in a higher carbon yield (91–
93). The inclusion of Lewis acids such as AlCl

3

and ZnCl

2

, during low temperature

carbonization enhances the carbon yield by favoring the dehydration reactions
(94).

Vapor-Grown Carbon Fibers

Vapor-grown carbon fibers (VGCFs) have been produced by decomposing benzene
or methane over transition metal catalyst particles within a temperature range
of 1000–1300

◦

C (95–97). Fibers produced in this manner develop an annular ring

texture (often referred to as a tree trunk structure) with graphene planes running
parallel to the fiber axis (98), as shown in Figure 20. This microtexture facilitates
graphitization and leads to excellent electrical, thermal, and mechanical proper-
ties (99,100).

Fig. 20.

Cross-section of VGCFs with octagons representing graphene planes and circular

lines representing circular carbon layers. Adapted from Ref. 98.

108

CARBON FIBERS

Vol. 9

Table 7. Properties of Submicrometer Vapor-Grown Carbon Fibers

a

Carbonized

Graphitized

Measurement

Property

s-VCGFs

s-VCGFs

method

Lattice parameter, nm

0.6900

0.6775

XRD

b

Diameter,

µm

0.2

0.2

SEM

c

Length of fiber,

µm

10–20

10–20

SEM

c

Volume Density, g/cm

3

0.02–0.07

0.02–0.07

Tapping

Real Density, g/cm

3

1.9

2.1

Pycnometer

BET surface area, m

2

/g

37

15

Nitrogen adsorption

Ash content, %

1.5

0.03

SDK

d

pH

5

7

JIS K 6621

e

Starting temp. of oxidation,

◦

C

550

650

TGA

f

a

Ref. 101.

b

X-ray diffraction.

c

Scanning electron microscopy.

d

Scintillation detection.

e

Japan Industrial Standard.

f

Thermogravimetric analysis.

The production strategy with the greatest potential for scale-up into mass

production may be the “floating reactant method.” This technique disperses cat-
alyst particles with the reactive hydrocarbon in a reaction chamber, resulting in
a high yield of relatively uniform diameter fibers of 0.1 to 0.2

µm (101). Fibers

produced in this fashion are considered submicrometer VGCFs (s-VGCFs) and are
much smaller than VGCFs produced by the traditional seeding techniques. Typi-
cal diameters range from 10 to 20

µm, about the size of carbonized PAN fibers. The

high surface-to-volume ratio and electrical conductivity make the submicrometer
fibers exceptional candidates to be anode materials in lithium batteries. Table 7
summarizes the properties of s-VGCFs.

The development of soot as an unwanted byproduct is a problem in the “float-

ing reactant method” of producing s-VCGFs (102). The addition of gaseous am-
monia to the hydrocarbon feed along with hydrogen sulfide and iron particles
improves the length and uniformity of the fibers while reducing soot production
(103).

While substantial research has focused on developing submicron VGCFs,

significant interest also exists in developing large, rapidly-produced, inexpensive
VCGFs for use as filler materials in carbon composites. Koyama and co-workers
(104) first reported achieving vapor grown carbon fibers of 25 cm in length from
the thermal decomposition of benzene in a hydrogren atmosphere in 1972. More
recently, Masuda and co-workers (105,106) have developed a liquid pulse injection
technique that produces VCGFs with lengths of up to 5 cm in less than 30 s. By
adding phenyl silane to the benzene feedstock, the oxidation resistance of the
resultant fibers increased without reducing the mechanical or electrical properties
of the fibers (107).

During heat treatment, VCGFs (both traditional and submicrometer) un-

dergo significant topological changes. During the growth process, the hydrocarbon
feed molecules decompose and the carbon would diffuse into the catalyst particle.
This diffusion of atomic carbon into the metal particle is generally considered to

Vol. 9

CARBON FIBERS

109

be the rate-limiting step of the process (108). In fact, fiber yield has been shown to
increase greatly near the melting point of the metal catalyst (108). Once the car-
bon has diffused to the active site, the central core growth of the new fiber would
occur in the first seconds, terminate, and then the fiber would begin thickening
(109). This thickening process is primarily responsible for determining the surface
texture of VGCFs. When heat-treated to 2000

◦

C, the smooth surface containing

small pores transforms into a rope-like texture as a result of the sudden density
increase in the material corresponding to the decrease in mean interlayer spacing.
As the graphitizing fiber becomes less turbostratic, the contraction results in an
anisotropic shrinkage. By 2800

◦

C, well-developed microdomains form throughout

the fiber and appear to be limited primarily by the diameter of the fiber (109).

Summary

Carbon fibers have advanced greatly from Edison’s early experiments with car-
bonizing cellulose as an iridescent light bulb fiber. PAN-based carbon fibers domi-
nate commercial high performance applications where the tensile strength of the
fiber is paramount, but the same fibrillar microtexture that enables carbonized
PAN to achieve exceptional tensile strengths limits its long-range order. For ap-
plications requiring exceptional molecular-order–dependent properties such as
stiffness and thermal conductivity, mesophase pitch-based fibers are preferred.
Pitch-based fibers are made from the high molecular weight remains of petroleum
or coal-tar distillation, though synthetic aromatic resins have been developed that
can be converted to graphitized fibers while avoiding the problems inherent in
conventional pitches. Application of transport phenomena principles to the liquid
crystalline flow of mesophase have enabled researchers to explain the develop-
ment of structure in the fiber and to suggest improved spinnerette designs.

For applications that do not require exceptional mechanical properties, car-

bon fibers made from high performance aramid polymers show considerable poten-
tial. These aramid fibers do not require stabilization prior to carbonization, which
substantially simplifies the production process. Rayon-based carbon fibers con-
tinue to appear in some composite applications, but have become key substrates
for the development of activated carbon fibers. These ACFs develop a microp-
orous surface structure that is ideal for adsorption of low levels of volatile organic
compounds.

Carbon fibers are also produced from the catalytic decomposition of hydrocar-

bon gases. These vapor-grown fibers can be produced at submicrometer diameters
and show great potential in battery applications. Larger vapor-grown fibers can
be produced rapidly by pulse injection and show promise as filler material in com-
posites.

Despite the continued commercial dominance of pitch- and PAN-based fibers,

the carbon fiber field is continuously changing. Significant current research is
focused on the production of carbon nanotubes. Other researchers continue to
enhance the understanding of the processes and mechanisms governing the fiber
production today. As researchers are better able to understand and manipulate
fiber microstructures, the commercial applications for carbon fibers will continue
to expand.

110

CARBON FIBERS

Vol. 9

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J

AMES

A. N

EWELL

Rowan University

CASTING.

See P

LASTICS

, P

ROCESSING

.

CELLULAR MATERIALS.

See Volume 5.

CELLULOSE.

See Volume 5.