H.O. Pierson. Handbook of carbon, graphite, diamond and fullerenes. Properties, processing and applications. 1993

176

Carbon,

Graphite,

Diamond,

and

Fullerenes

Of

the

two

fibers,

only the wet-spun

PAN

is

used

as precursor.

It

contains

a

co-polymer,

such

as

itaconic

acid

or other

proprietary

com-

pounds,

that

apparently

catalyzes

the

cyclization

in

air

and

helps

the

carbonization

process.r]

The

dry-spun fiber is

not

as

suitable

and

is

not

used.

Stretching.

The

spun

fiber

is

composed

of

a fibrillar

or

ribbon-like

network,

which

acquires

a

preferred

orientation

parallel

to

the

fiber

axis,

providing

that

the fiber

is stretched

either

while

it is still

in the coagulating

bath,

or subsequently

in boiling

water,

as shown

in Fig. 8.4.

This

stretching

results in an elongation

of 500 to 1300%,

and is an essential

step to obtain

a high-strength

fiber.

Stretching

Oxidation

(Steam Atmosphere)

(Air at 230°C)

Stabilization

and

Oxidation.

During the

carbonization

process,

the

elimination

of the

non-carbon

elements

(hydrogen

and nitrogen) is usually

accompanied by chain scission

and relaxation

of the fibrillar

structure.

This

isdetrimental

totheformation

of high-strength

and high-modulus fibers,

but

can be avoided by a stabilization

process

prior

to carbonization.

This

stabilization

consists

of slowly

heating

the

stretched

fiber to 200 -

280°C in an oxygen

atmosphere

(usually

air) under

tension

to

maintain

the

orientation

ofthe

polymer

skeleton

and stabilize

the structure

(Fig. 8.4). The

addition

of ammonia

to

oxygen

increases

the

rate

of stabilization.t13]

178

Carbon,

Graphite,

Diamond,

and

Fuiierenes

Carbonization

and

Graphitization.

Carbonization

takes

place

be-

tween

1000

and

1500°C.

These temperatures

are reached

slowly,

at a

heating

rate

of

-

20”C/min.

During

this

stage

a considerable

amount

of

volatile

by-products

is released.

These

include

H,O, CO,, CO, NH,, HCN,

CH,,

and other

hydrocarbons.

The

carbon yield

is between

50 and

55%.

The circular

morphology

of the

fiber

is maintained

and the

final

diameter

varies

from

5 to

10 pm,

which

is approximately

half that

of the

precursor

PAN

fiber.

The

removal

of

nitrogen

occurs

gradually

over

a range

of

temperatures

as shown

below:tll]

1300°C

-

0.3% nitrogen

left

The tensile modulus

of the fiber

can be further

increased

by graphiti-

zation.

It can

be argued

that

the term

“graphitization”

is not

correct since a

true graphite

structure

is not

obtained, and

“high-temperature

heat-treat-

ment”

would

be a better

term. This heat-treatment is usually

carried

out at

temperatures

up to 2500°C.

The final

carbon

content

is greater

than

99%.

Analytical

Techniques.

Analytical

techniques

to

determine

the

structure

of carbon

fibers

include:

wide-angle

and small-angle

x-ray diffrac-

tion,

electron

diffraction,

neutron

scattering,

Raman

spectroscopy,

electron

microscopy,

and

optical

microscopy.

Detailed

reviews

of these

techniques

are

found

in the

literature.t14]

Structure.

The

structure

of PAN-based

carbon

fibers

is still

conjec-

tural

to

some

degree.

Yet,

thanks

to

the

recent

advances

in

analytical

techniques

just

mentioned,

an

accurate

picture

is beginning

to emerge.

Unlike

the

well-ordered

parallel

planes

of

pyrolytic

graphite

which

closely

match

the

structure

of the graphite

crystal,

the structure

of

PAN-

based carbon

fibers

is essentially

turbostratic

and is composed

of small

two-

dimensional

fibrils

or ribbons.

These

are already

present

in the

precursor

and

are

preferentially

aligned

parallel

to the

axis

of the fiber.

The

structure

may

also

include

lamellas

(small,

flat

plates)

and

is probably

a combination

of both

fibrils

and lamellas.t1)t15]

Carbon

Fibers

179

Crystallite

Size.

Several

structural

models

have

been proposed

includ-

ing the one shown

in Fig.8.6.n1) The critical parameters

(as determined

by x-

ray diffraction)

are L,_,which

represents

the stack

height

of the ribbon and the

crystallite

size

l_

which in this

case can be considered

as the mean

length of

a straight

section

of the

fibril.nl]

This

alignment

(l_)

becomes

more

pro-

nounced

after high-temperature

heat-treatment

which

tends

to straighten

the

fibrils.

However,

L,

still remains

small

and is generally

less than

20

nm, as

shown

in

Fig.8.7.m

This

figure

also

shows

the

much

greater

increase

of

crystallite

size

(La) of pitch-based

fibers

(see

Sec.

3.0 below).

The

straightening

of the

fibrils

occurs

preferentially,

the

outer

fibrils

being

more

oriented

(straightened)

than the inner ones as

shown

in

Fig.

8.8.f”)

This

has

an important

and favorable

consequence,

that

is, most of

the

load-bearing

capacity

is now

transferred

to the

outer portion

or “skin”of

the

fiber.

Interlayer

Spacing.

The change

in interlayer

spacing

(c spacing)

of

PAN-based

carbon

fibers

as

a function

of heat-treatment

temperature

is

shown

in Fig.8.9.p)

This

spacing

never

shrinks

to less than

0.344

nm, even

after a 3000°C heat-treatment,

indicating

a poor

alignment

of the

basal

planes

and the

presence

of defects,

stacking

faults,

and dislocations.

This

behavior

is characteristic

of carbons

produced

from

polymers

(see

Ch.

6).

Also shown

in Fig. 8.9 is the decrease

in interlayer

spacing

of a pitch-based

fiber.

It is far more

pronounced

than

that

of the

PAN-based

fiber

(see

Sec.

3.0

below).

Sp* and Sp3 Bonding.

Another

important

structural

characteristic

of

PAN-based

fibers

is

the

probable

existence

of

sp3

hybrid

bonding

as

indicated

by Raman

spectroscopy

and shown

in Fig. 8.10.

In thisfigure,

the

pitch-based graphitized fiber

(PI

00)

is the only

one

to

exhibit

a strong

sp*

line. All others show structural

disorders which

may be caused

by some

sp3

b0nding.p)

The

fibers

listed

in Fig. ‘8.10 are

identified

in Sets.

6.3 and

6.4

below.

Both sp* and sp3 hybrid

bonds

are

strong

covalent

bonds,

with

the

following

bond

energy

and

bond

lengths

(see

Ch. 2, Sets.

3 and

4):

sP3

-

370

kJ/mol

and

0.15

nm

sP*

-

680

kJ/mol

and

0.13

nm

These strong bonds within the crystallites

(or fibrils) and

the

preferred

orientation

of these

crystallites

account,

at least

in part, for the

high

stiffness

inherent

to

most

carbon

fibers.

1000

1400

1800

2200

2600

3000

Heat-Treatment Temperature, “C

Figure

8.7. Apparent

crystalline

size

(LJ

of PAN-based and

pitch-based carbon

fibers

as a function of

heat-treatment

temperature.rJ

Carbon Fibers

181

-

50 Inm

I

Figure

8.8.

Model

ofa

PAN-based carbon fiber

in cross-section

showing the “skin”

effect

(not

to

scale).[“l

I

E

0.355

Pan-Based

Carbon

Fibers

I=

03

OJ 0.350

.r

0

&

0.345

&

z?

0.340

2

(P-55)

5

Ideal Graphite

Crystal

1400

1800

2200

2600

Heat-Treatment Temperature, “C

Figure

8.9.

Mean

interlayer spacing

(c/2) of PAN-based and

pitch-based

carbon

fibers

as a function

of

heat-treatment

temperature.r]

IDjIG

T 300

1.74

(AMOCO)

1.81

G30.500

Pan-

(BASF)

1.49

GUO-500

(BASF)

M 40

0.53

TORAY

T 50

0.31

Rayon-

(UC)

0.19

Based

T75

(UC)

-J

P 55

0.44

Pitch-

(AMOCO)

Based

P 100

0.13

(AMOCO)

I

&

1

I

t

I

I

I

I

1800

1600

1400

1200

1000

Raman Shift, cm-’

Figure

8.10.

Laser

Raman

spectroscopy

of PAN-based,

rayon-based,

and

pitch-

based

carbon

fibers.~]

To summarize,

small

crystallite

size,

high

interlayer

spacing,

and

general

structural

disorder

are the

factors that contribute

to the unique

and

stable

turbostratic

structure

of PAN-based

carbon

fibers

(which

is likely

to

include

both

sp* and sp3 hybrid bonds),

and

explain

their

inability

to form

a

graphiticstructure

even after high-temperature

heat-treatment

(i.e., 3000°C).

As

seen

in

Ch. 5,

Sec. 2.1, pitch

is a by-product of the destructive

distillation

of coal, crude

oil, or asphalt.

It meets the

conditions

for

carbon-

fiber

production

stated

above

(Sec. 2.1) and

is inexpensive

and

readily

available.

Its carbon yield can exceed

60%,

which

is appreciably

higher

than

the

yield

of PAN

(-

50%).

The

composition

of pitch

includes

four generic

fractions

with

variable

proportions!’

‘1 These

are:

. The polar aromatics, which are medium-molecular-

weight

compounds

with

some

heterocyclic

molecules

. The asphaltenes,

which

have

high

molecular

weight and

a high degree of aromaticity.

The higher

the

ratio of

asphaltene,

the higherthesoftening

point, thermalstability,

and carbon

yield.

Low-cost

carbon

fibers are produced from

an isotropic

pitch with a low-

softening

point.

The

precursor

is melt-spun,

thermoset

at relatively

low

temperature,

and

carbonized.

The

resulting

fibers generally have

low

strength

and

modulus

(-

35 - 70 GPa).

They are suitable for insulation

and

filler applications.

Their

cost dropped

to less

than $20/kg

in 1992.t6)

Carbon

fibers from

mesophase

pitch have high

modulus and

medium

strength. They

are presently

more

costly

than PAN-based

fibers ($65/kg in

1992).

Precursor

Pitch.

The

precursor

material is

a mesophase

pitch,

characterized

by a high

percentage of

asphaltene.

Table 8.5 shows the

approximate

composition

of three

common mesophase

compounds.

Table 8.5. Composition

of Mesophase

Pitch

Product

Composition,

%

Ashland

240*

Asphaltene

62

Polar aromatics

9

Naphthene

aromatics

29

Ashland

260*

Asphaltene

83

Polar aromatics

7

Naphthene

aromatics

10

CTP 240*

Asphaltene

68

Polar aromatics

11

Naphthene

aromatics

21

Processing.

The processing

of mesophase-pitch

fibers

is similar to

that of PAN

fibers,

except

that the

costly

stretching

step

during heat-

treatment

is

not

necessary,

making

the

process potentially

less

expen-

sive.t16)t1fl

The

processing

steps

can

be

summarized

as follows

and are

represented

schematically

in Fig.

8.11:

1.

Polymerization

of the

isotropic

pitch

to produce

mesophase

pitch.

3.

Thermosetting

the green fiber.

4.

Carbonization

and graphitization

of the thermoset fiber to

obtain a high-modulus carbon

fiber.

Polymerization.

The pitch is heated

to approximately 400°C and is

transformed

from

an

isotropic

to a mesophase

(or liquid

crystal)

structure

consisting

of large

polyaromatic

molecules

with

oriented

layers

in parallei

stacking.

This structure is similar to the

needle-coke

stage

of molded

carbons described

in Ch. 4, Sec. 2.3.

Figure

8.11. Schematic of the production steps in the manufacture of pitch-based

carbon

fibers.

Spinning and Thermosetting.[12] The mesophase

pitch ismelt-spun

in a mono-

or multifilament

spinneret,

heated

to 300 - 450°C and pressurized

with

inert

gas.

It

is

drawn

at

a

speed

>120

m/min

for

a

draw

ratio of

approximately

1000/l

to

a diameter

of

IO

- 15 pm.

The

draw

ratio

is an

important

factor

in the control of the

orientation

of the fiber

structure:

the

higher

the

draw,

the

greater the orientation

and

uniformity.

At this

stage

the

fiber

is thermoplastic

and

a thermosetting

operation

is needed

to avoid

the relaxation

of the structure

and

prevent

the

filaments

from

fusing

together.

This thermosetting

operation

is

carried

out

in an

oxygen

atmosphere

or

in

an

oxidizing

liquid

at approximately

300°C,

causing

oxidation

cross-linking

and stabilization

of the filament.

Tempera-

ture control

during

this thermosetting

step

is

critical

since

a temperature

which

is

too

high

would

relax

the

material

and

eliminate

its

oriented

structure.

Carbonization

and Heat-Treatment.

The thermoset

fibers

are then

carbonized

at temperatures

up to

1000°C.

This

is done

slowly to

prevent

rapid

gas

evolution

and the

formation

of bubbles

and

other

flaws.

Carbon-

ization

is followed

by a heat-treatment

from

1200 to 3OOO”C, at the end of

which

the

final

structure,

strength,

and modulus

are

established

(see

Sec.

6.4 below)

.t1°)t17)