Recent Developments on Carbon Fibers from Rayon-Based Precursors

Gajanan Bhat, Sammy Akato, Nicholas Cross,

The University of Tennessee, Knoxville

West Hoffman, AFRL, Edwards, AFB, CA and

Farhad Mohammadi, Advanced Cerametrics, Lambertville, NJ

Carbon Fiber Precursors

Rayon (or regenerated cellulose)

PAN (Polyacrylonitrile)

Pitch (petroleum and coal tar based)

Natural materials such as lignin, wool, cotton,

ramie, and jute

High performance highly crystalline aramid

fibers such as Kevlar

Polymeric materials like phenolic resins

Rayon Fibers

Rayon is produced from naturally occurring cellulose polymers

Some properties of rayon fibers – Availability

– Low cost

– Non-melting character

– Ease of production

~ 44% carbon content

Repeat unit of cellulose

Earlier Research on Rayon-Based CF

Thomas Edison used CCF as filaments for

incandescent lamps (1880)

Bacon et al. patented a process using viscose rayon

fibers to produce high strength, high modulus RCF

Union Carbide used stress graphitization to produce

strong RCF (1959)

RCF were the first to be qualified by NASA

Later other precursors became more prominent

Why This Study?

Currently rayon fibers are not produced in

the US

Rayon-based CF are still of interest to DoD

Experimental rayon fibers (Advance

Cerametrics) are being evaluated as a

candidate for CF precursor

Precursors

Commercial rayon fibers from Lenzing, Austria

Experimental rayon fibers from Advanced

Cerametrics, Lambertville, NJ USA

X-ray patterns of the commercial rayon and experimental rayon fibers

Physical and Mechanical Properties of Rayon Fibers

Production of Carbon Fibers

Stabilization of the precursor at low

temperatures

– Oxidation in air at lower temperature (<400°C)

Carbonization in an inert atmosphere up to

~1500°C

– Longitudinal orientation

– Development of the crystalline ordering

Graphitization in an inert environment up to

3000°C

– Optional for high modulus

Set-up for Stabilization and Carbonization

0

200

400

600

800

1000

1200

0 2 4 6 8 10 12

Tem

pera

ture

(C

)

Distance (inches)

130C

600C

1200C

Production of Rayon Carbon Fibers (RCF)

Pretreatment

– Impregnated in 1 normal solution of phosphoric acid

for 5 hours

– Dried at room temperature overnight

Oxidation in air at lower temperature up to

380°C

Carbonization in an inert atmosphere (Nitrogen)

up to 1200°C under tension

HTT

Effect of HPA Pretreatment

Catalyzed the dehydration reactions

Lowered degradation temperature over a wider range

Suppressed the release of volatile organic substances by reacting with hydroxyl groups

DSC TGA

Stabilization Conditions

Carbonization Conditions

All samples were stabilized first from 110C to 380C in

air for a total time of 3 hours

Applied load for sample 6003 is 10g and applied load for

sample 6005 is 50g

SEM of Untreated and Treated rayon Fibers after Pyrolysis

TGA

TGA Scans of Fibers

Change in Crystalline Structure

The dehydration only changed the crystal structure partially

Fibers become amorphous at 300°C

Order develops after 380°C

0 10 20 30 40 50 60 70

2 Theta

380

350

300

250

200

150

Precursor

SEM

The fiber diameter changes with temperature

Smaller fibers

Smoother surface

The fibers appear to be of good quality

SEM micrographs of the surface and cross-section of the carbonized sample

Elemental Analysis

0

10

20

30

40

50

60

70

80

Precursor Stabilized 6002 6003

% C

on

ten

t

Sample ID

Carbon content

Hidrogen content

EDAX

EDXA results of the precursor, the stabilized fibers

and the carbonized fibers to 1200C respectively

Mechanical Properties

Effect of tension during carbonization

High temperature heat treatment

HT to 1700 C at ORNL

Change in Thermal Behavior (TGA)

Elemental Analysis

0

10

20

30

40

50

60

70

80

Precursor 502 506

% c

on

ten

t

Sample ID

Carbon content

Hidrogen content

SEM of CF

SEM micrographs at 2KX of the

carbonized sample

Comparison of the Produced CFs

SEM of HTT CF

Initial Observations

The use of phosphoric acid shifts the pyrolysis

reactions to lower temperatures

As the pyrolysis progresses, the structure changes

The mechanical properties of the obtained fibers

need to be improved

To improve the mechanical properties of the

obtained fibers:

– Lower fiber diameter

– Control the shrinkage

– Higher HTT

Studies with a Smaller Diameter Precursor

Precursor Diameter ~ 12 micron

Shrinkage and Fiber Diameter Change

0

2

4

6

8

10

12

14

16

18

20

100 150 200 250 300 350 400 450

Sh

rin

ka

ge

(%

)

Temperature (°C)

Shrinkage Percentages with Varying Tension

nc-17 (10 grams)

nc-16 (20 grams)

nc-14 (50 grams)

nc-18 (75 grams)

nc-19 (100 grams)

nc-110 (120 grams)

nc-30 (50 grams)

Shrinkage

0

5

10

15

20

25

30

35

40

0 200 400 600 800 1000 1200

Am

ou

nt

of

Sh

rin

ka

ge

(%

)

Temperature (Celsius)

50 grams

120 grams

Shrinkage

SEM of Carbon Fibers

Elemental Analysis

Future Work

Smaller diameter fiber stabilization

optimization

Tension is critical

Tension during Carbonization & HTT is also

important

Increase HTT to achieve higher ‘C’ content

Continuous processing

Acknowledgements

AFRL

Advanced Cerametrics

Lenzing

Harper International