Petroleum Derived Carbons

22 High Modulus Carbon Fiber Processes

Downloaded by UNIV OF ARIZONA on January 16, 2013 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0021.ch022

R. J. DIEFENDORF Rensselaer Polytechnic Institute, Materials Engineering Department, Troy, N.Y. 12181

Although carbon fibers have been made inadvertently from natural fibers for thousands of years, it was Edison (1) in 1878, who purposely took cotton fibers, and later bamboo, to make carbon lamp filaments. Interest in carbon fibers was renewed in the late 1950's, when synthetic cellulosics (rayons) in textile forms were converted to carbon fibers. Although other precursor fibers were studied, cellulosics remained the main source for carbon fibers until the mid 1960's. A l l these fibers had low stiffness, although some obscure reports and related technology indicated that high modulus carbon fibers could be made. In early 1964, Bacon (2) made the first high modulus carbon fiber by hot stretching rayon precursor carbon fibers. This was followed by Watt (3), who made a high modulus carbon fiber by retaining some of the original preferred orientation of polyacrylonitrile in the carbon fiber. Although other fiber types have been investigated for precursors, the low cost of commercially available fibers produced for textile uses has made rayon and particularly polyacrylonitrile the main precursors for carbon fiber production. More recently, in the late 1960's, the realization that truly widespread use of carbon fibers for reinforcement would require lower cost than that possible from the rayon or PAN processes caused initiation of research on using petroleum pitches to make carbon fibers. In addition to hot stretching, two other processes (4,5) have been developed using pitch precursors to make high modulus carbon fibers. In the following sections, the rayon, PAN, and two pitch 315

In Petroleum Derived Carbons; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

0

1

0

PETROLEUM DERIVED CARBONS

processes

for

making carbon f i b e r

HIGH MODULUS CARBON F I B E R H i g h modulus planes

of

fiber

axis.

Commercial processes

made can

by p l a s t i c

be


precursors,

introduced

i n the

a l s o may b e and the

will

in

result

tation.

case

will

attempt

to

polymer p a r a l l e l basal

planes

or

to

the

the

formed

under

ferred

orientation,

tension.

stress,

plastic

fiber,

carbon f i b e r .

h i g h modulus

A.

allowing

Graphite

can only

graphite

like

temperature, a

structure

Deformation tures

to

a

that of

either

diffusion

to

structures,

single pins

these allow

occur.

the

deformation

when

under h i g h

load.

high

depre-

applying occur

given the

in

the

carbon

for

this

amounts

fiber, differ-

processes. Carbon F i b e r s

by b a s a l

crystal

-

plane

a highly c a n be

slip

at

oriented

bent

at

room

random g r a i n o r i e n t a t i o n ,

basal

plane

materials other of

slip

for

At lower

in

high

temperaoperate,

randomly

is

short

or

brittle.

systems t o

2800°C

can occur

is

requires

slip

Practically,

a temperature

that

to

make

can

of

accounts

while

a carbon with

tend

Fibers

yield

Hence,

fiber

the

high

stretch

High Temperature Deformation of

room t e m p e r a t u r e .

of

substantial

during processing

carbon f i b e r

as

the

the

by

The d e f o r m a t i o n

Rayon P r e c u r s o r Carbon

to

of

required to

This

orien-

such

when p l a s t i c a l l y

and p o s t - s p i n n i n g

or

the

Similarly,graphite

accomplished

and t h e n

precursor ent

is

is

cause

backbone

fiber. on

stress

compounds w i l l

axis,

hopefully

carbon

applied

axis.

ring

which

or

to

The development

deformation.

spinnerette i n the

fiber

the

polymer,

zig-zag

fiber

carbon fibers,

orienting the

the

and

a tensile

load

are

orientation

anisotropic

try

linear

a tensile

aromatic

align p a r a l l e l to

modulus

a

orien-

The p r e f e r r e d

in

of

which w i l l

align

fibers

are

stiff the

this

preferred

introduced

of

the

carbon fibers

carbon.

application

forces

In the

polyacrylonitrile,

of

to

that

develop

the

p r e c u r s o r and c a r b o n f i b e r s

ultrascale,

described.

p a r a l l e l to

Since

precursor

upon c o n v e r s i o n

orientation The

aligned

deformation.

from o r g a n i c

preserved

be

require

layer tation

be

PROCESSES

carbon fibers

graphite

will

required times

temperatures,

or

oriented such

(seconds) (2100


-

22.

DIEFENDORF

2400°C),

the

filament

in colder

diffusion tures,

the

the


to

slow.

At higher

is

l i m i t e d by the

of

the

was

developed

fiber to

in

oxidation fiber

highly

chemistry

is

i n the

formed

to

to

has

of

sembles

a bowl o f

planes

of

oriented rayon,

the

to

the

(9) .

tangled

provements

high

has

is

treatment

are

the to

is

orien-

Q/4) .

also

lower.

tangled

with

applied as

to

strain

un-

well

widest

treatment in

as

range

of

of

of

ori-

significant

and

to

basal

The a p p l i c a t i o n heat

are

observed

the

(6)

re-

fibers

observed

been

can r e s u l t

and the

Upon

the

are

produced the

120Msi).

fiber. the

and w h i c h

of

low

carbon

trans-

im-

temperature

The main d i s a d v a n t a g e

s t r u c t u r a l changes which occur

of upon

the heat

2800°C.

Deformation of Fibers

has

temperature

i n moduli,

requirements

be

of

ribbons

axis

in Fig.

and

pyrolysis

When t h e s e

alignment

fiber

process

carbon fibers

process

consist 30A wide

The p r o c e s s

(6Msi t o

during

the

is

schematic

shown

carbon

cannot

p i t c h precursor carbon fibers

moduli

with-

increase

structure

to

about

spaghetti

180%.

and the

stress

structure

Almost p e r f e c t

parallel

strains

it

can

agents

to

zone.

modulus

but A

is

rayon fiber,

observed

2800°C,

straighten.

it

original preferred

graphite

layer planes

at

hot

shrunken idiomorph of the

the

strength

2800°C,("graphitization"),

been

ribbons strained

that

an o r i e n t e d

structure

a

cellulosic

heat-treatment

charring

(carbonization)

is

the

tempera-

the

(,2,(3) .

(stabilization)

crenulated

such

tation

provided

stress

but

of and

produce h i g h

rayon precursor process

upon p y r o l y s i s

original

B.

apparatus,

stress

a thermoset,

Although the

ented

strength

the

any c a r b o n f i b e r

the

temperature

fiber

is

required orienting

for

Rayon i s

yield

creep

of

from r a y o n p r e c u r s o r c a r b o n f i b e r s ,

diagram 1.

l i m i t e d by the

rupture)

process

applicable stand

is

portions

applied stress

This fibers

stress

activated

(perhaps,

317

Carbon Fiber Process

Aromatic

Pitches

-

Pitch Precursor

(3,10)

The i n t r o d u c t i o n o f p r e f e r r e d o r i e n t a t i o n i n a n a r o m a t i c p i t c h a t low t e m p e r a t u r e , and t h e m a i n t e n a n c e of at l e a s t p a r t o f t h i s o r i e n t a t i o n during processing w o u l d seem t o o f f e r a d v a n t a g e s o v e r d e f o r m a t i o n a t 2800°C. In a d d i t i o n , p e t r o l e u m p i t c h e s are o f interest because o f low c o s t , h i g h c a r b o n y i e l d , and, a t l e a s t i n the p a s t , a v a i l a b i l i t y . I n one p r o c e s s (4)(Fig.2),


318


FIIEI

STAIILIZE

CARIONIZE

6RA PHITIZE


ORIENT

Figure 1. Rayon precursor process for carbon fibers. Rayon fibers are oxidized to increase carbon yield, slowly heated to 950°C to convert to carbon, and then strained at 2800°C to give a high modulus carbon fiber.

TtntiRR Figure 2. Isotropic pitch process for carbon fibers. An aromatic isotropic pitch is meltspun at high strain rate to give an oriented fiber. Thefiberis thermoset by oxidation and then heated under tension to maintain the preferred orientation.



22.

DEEFENDORF


319

an i s o t r o p i c p i t c h i s m e l t spun a t v e r y h i g h s t r a i n r a t e s t o a l i g n the aromatic molecules p a r a l l e l to the f i b e r a x i s and t h e n t h e f i b e r i s quench c o o l e d t o r e t a i n the p r e f e r r e d o r i e n t a t i o n . This thermoplastic f i b e r i s c a r e f u l l y o x i d i z e d a t low t e m p e r a t u r e t o c r o s s l i n k the s t r u c t u r e t o an i n f u s i b l e f i b e r . The o x i d a t i o n i s r a t h e r slow a t low temperature b u t t o o h i g h a temperature causes the aromatic molecules to r e l a x to the i s o t r o p i c s t a t e . Even with oxidative s t a b i l i z a t i o n , r e l a x a t i o n c a n o c c u r when t h e oxidative bonds are b r o k e n d u r i n g c a r b o n i z a t i o n . Hence, the p r e f e r r e d o r i e n t a t i o n i s l o s t , u n l e s s an o r i e n t i n g s t r e s s is applied during this c r i t i c a l period. Similarly, a s t r e s s i s found advantageous d u r i n g the h i g h e r h e a t treatment temperatures ( 1 7 0 0 ° C - 2 2 0 0 ° C ) t o o b t a i n commercially practical moduli. T h i s p h e n o m e n a i s t h e same as d e s c r i b e d f o r a l i g n e d f i b e r s i n S e c t i o n A . At p r e s e n t , n o f i b e r s a r e c o m m e r c i a l l y made b y t h i s p r o c e s s , the major problem p r o b a b l y b e i n g the lengthy o x i dation step. Higher molecular weight aromatic pitches often form a n i s o t r o p i c l i q u i d s (mesophase, l i q u i d c r y s t a l s ) . Mesophase i s a t h e r m o d y n a m i c a l l y s t a b l e s t r u c t u r e , and once formed w i l l n o t r e l a x t o an i s o t r o p i c l i q u i d , u n l e s s h e a t e d above the m e s o p h a s e - l i q u i d t r a n s i t i o n t e m perature. In g e n e r a l , t h i s t r a n s i t i o n temperature is above t h e d e c o m p o s i t i o n t e m p e r a t u r e i n p e t r o l e u m pitches, so t h a t the h i g h p r e f e r r e d o r i e n t a t i o n i n the mesophase i s r e t a i n e d ( p a r t i a l l y ) upon c o n v e r s i o n t o carbon. I n one c o m m e r c i a l p r o c e s s ( 5 J where the p i t c h i s p a r t i a l l y m e s o p h i t i c and p a r t i a l l y i s o t r o p i c , the p i t c h i s spun a t s u f f i c i e n t l y h i g h temperature t h a t b o t h the mesophase and i s o t r o p i c p i t c h e s a r e d e f o r m e d . The p i t c h d o e s n o t need t o be s p u n a t t h e h i g h s t r a i n r a t e s and t h e n quenched as i n t h e p r i o r example, since the o r i e n t a t i o n i n the mesophase i s s t a b l e . The o n l y t h i n g t h a t i s r e q u i r e d i s t h a t the mesophase be deformed s u f f i c i e n t l y so t h a t the a r o m a t i c m o l e c u l e s a r e o r i e n t e d p a r a l l e l to the f i b e r a x i s . S i n c e mesophase i s o f t e n s p h e r i c a l a t lower volume f r a c t i o n s i n p i t c h e s , the d e f o r m a t i o n r e s u l t s i n l o n g c y l i n d e r s o f mesophase i n t h e more i s o t r o p i c m a t r i x o f t h e f i b e r s . At higher volume f r a c t i o n s o f mesophase, where i t i s a c o n t i n u o u s phase, or at h i g h e r s t r a i n r a t e s , the f i b e r s w i l l a p p e a r more h o m o g e n e o u s . Higher s t r a i n rates w i l l cause the i s o t r o p i c component t o o r i e n t , especially


320


because

of

the

so

it

may n o t

that

more

isotropic

thermal cause

the

is

lost

heat-treatment,


of

(single

The r e s u l t i n g

modulus C.

as

embedded

in

Although

in

60

radial

organic

orientation

carbonization

-

been

a glass

in orientation,

backbone

the

The

axial

parallel

to

cyclized

the

chains

are

During

this

oxidation,

to

maintain the

tension

relaxation

occurs,

disoriented

with

stabilization,

the a

temperature, tension nitrogen

in

polymer, the to

fiber

has

it

is

not

and t h e s e

that

are

of

high

d u r i n g the

1000°C)(12).

be

are

of

t h e PAN molecules polymer

kept

polymer.

under

PAN, o t h e r w i s e

axis.

transition

to

maintain

(11).

basic

carbonization

Considerable

i n the

units

largely

is

After

ladder

glass

present

un-

attractive

ladder polymer

necessary

form the

elements

sufficiently

the

fiber

to

para-

considered

an o r i e n t e d

s t i l l

easily

Then the

must

remaining processing

rings

structure

the

Fibers

a

polymer

resulting

to

fiber

to form a l a d d e r

alignment

is

more

on h e a t i n g

an

axis.

fibers

and t h e

and hydrogen

ridene-type

the

sufficiently

that

the

fiber

the

planes

A

c a n be

is

the

from (which

may b e

stretching

of

by o x i d a t i o n

respect

structure

with

initial

alignment

mini-

produced.

transition

Polyacrylonitrile offers

(Fig. 4).

a

Alternatively,

loss

increases

orien-

basal

lost

with

(Upon

further

PAN P r e c u r s o r

ladder polymer,

solution

it

either

high

spinnable.

tempera-

matrix

molecules

temperatures.

prevent

for

or

lost

enough

(resulting

w o u l d be

be

perfect

structure.

M s i has

Polymers

linear

this

or

attain

modulus

section with

6 to

Deformation

to

not

higher

may r e s e m b l e

pitch),

by

orientation

p r o v i d e d enough

lower

The be-

orientation.

to

fibrils

isotropic

in cross of

try

fiber a

an o n i o n - s k i n range

at

a director

high-modulus

from the

oriented,

will

act

with

assuming

mesophase w i l l A l t h o u g h some

crystal),

to

composite

homogeneous

fiber.

infusible

temperatures,

a high preferred

growth.)

produced

high

heat-treatment

graphite

present

mesophase)

the

mesophase,

i n the

r a p i d l y made

relatively

further

redevelop

is

neighboring

mesophase d u r i n g c a r b o n i z a t i o n ,

that

orientation

the

c a n be

temperatures.

i n the

will

tation

at

of

distinguishable

orientation

oxidation

remains,

be

matrix

oxidation

at

ture

restraints

naphthy-

of

the

eliminated stage

from

(heating

The c a r b o n atoms w h i c h r e m a i n

are



22.

DIEFENDORF

Carbon Fiber

Process

321

Figure 3. Mesophase pitch process for carbon fibers. A pitch which is partially isotropic and partly liquid crystal is melt-spun to give a preferred orientation to the liquid crystal elongated domains. The fiber is oxidized to thermoset the isotropic pitch, and the fiber is heated to carbonize and develop the structure.

P O L Y A C R Y L O N I T R I L E

P R O C E S S

Figure 4. Polyacrylonitrile precursor for carbon fibers. PAN fibers are stretched, converted to ladder polymer, and converted to carbon fiber. Remnants of the preferred orientation of the ladder polymer are retained in the carbon fiber.


322


principally networks alignment relative low.

with to

the the

fiber

other

the

treatment

Moduli

form of

tend

axis,

their

degree

fiber

axis

preferred to

hexagonal

ribbons

and the

higher

from approximately

varying

extended

Although these

each

However,

by heat

ment

in

(13).

orientation temperatures

6 to

of

is

ordering

relatively

c a n be

improved

(14-16).

7 5 M s i c a n be

i n i t i a l precursor orientation

ribbon

towards

and

obtained

by

heat-treat-

temperature.


SUMMARY The p r o c e s s e s fibers the

all

depend

that

produce

on p l a s t i c

h i g h modulus

deformation

required preferred orientation

review

of

the

tation

may b e

carbon

fiber

w h i c h have compared

processes introduced itself.

shows t h a t into

These

the

development,

conventional promise

carbon introduce

for

high modulus.

the

preferred

precursor or

processes produce

much i m p r o v e d s t r u c t u r a l

to

to

materials.

substantially

A

orien-

the fibers,

efficiencies Processes, lower

fiber

now

in

costs.

LITERATURE CITED 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

Edison, T., "Electric Lamp", U.S. Patent 223,898, 27 Jan. 1880. Bacon, R., "Chemistry and Physics of Carbon", Vol. 9, pp.1-102, Marcel Dekker, Inc.,N.Y. (1973). Johnston, W., Phillips, L. and Watt, W., British Patent 1,110,791 (Appl.April 24 and Dec.29, 1964). Kureha Kagaku Kagyo KK, Patent DT-2027384 (Dec.17, 1970). Singer, L . , Netherlands Patent 239490 (April 4, 1972). Hawthorne, H., Paper #13, Int.Conf.Carbon Fibres, Plastics Industry, London (1971). Bacon, R., and Tang, M. M., Carbon, 2, 211 (1964). Bacon, R., and Tang, M. M., Carbon, 2, 220 (1964). Fourdeux, A., Perret, R., and Ruland, W., Paper #9 Int. Conf. Carbon Fibres, Plastics Industry, London (1971). Standege, A. E. and Prescott, R., British Appl. No. 49850/65 (Nov. 24, 1965). LeMaistre, C. W., Ph.D. thesis, Rensselaer Polytechnic Institute (1971). Watt, W., and Green, N., Paper #4, Int. Conf. Carbon Fibres, Plastics Industry, London (1971). In Petroleum Derived Carbons; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

22. 13.

14. 15.


16.

DEEFENDORF


Johnson, D. J., Crawford, D., and Oates, C., "The Fine Structure of a Range of PAN-Based Carbon Fibers", Tenth Biennial Conf. on Carbon, Bethlehem, Pa. (June 1971). Badami, D. V., Joiner, J . C . , and Jones, G. A., Nature, 215, 386 (1967). Ruland, W., "The Relationship Between Preferred Orientation and Young's Modulus of Carbon Fibers", presented at Am. Chem. Soc. Polym. Chem.Div. Conf., Atlantic City (Sept. 1968). Brydges, W. T., Badami, D. V., Joiner, J . C. and Jones, G. A., Am. Chem. Soc., Polym. Chem. Div., Preprint 9, No. 2, 1310 (1968).


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Petroleum Derived Carbons

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