LAVIN, 1991

Prévia do material em texto

Carbon Vol. 30, No. 3. pp. 351-357. 1992 000%6223192 %S.(XI + .OO 
Printed in Great Britarn. Copyright 0 1992 Pergamon Press Ltd. 
CHEMICAL REACTIONS IN THE STABILIZATION 
OF MESOPHASE PITCH-BASED CARBON FIBER 
J. GERARD LAWN 
Du Pont Fibers, Experimental Station, Wilmington, DE 19880-0302, U.S.A. 
(Received 12 August 1991; accepted in revised form 25 November 1991) 
Absbnc&-Mode1 compounds based on anthracene were used to study the chemical reactions taking 
place during stabilization of mesophase pitch-based carbon fiber. Methyl- and hydro- groups were found 
to accelerate the oxidation reaction, and also to react with carbonyl groups. Linking reactions that 
produced high-molecular-weight compounds took place both in air and nitrogen. Similarities in heat- 
evolution profile were observed in the oxidation of a real pitch and a model pitch consisting of 80% 
anthracene, 10% dimethylanthracene, and 10% dihydroanthracene. 
Studies of off-bases produced in stabilization of a pitch-based fiber showed off-gases to be limited 
to H,O, CO,, and CO (H, would not have been detected). At a given temperature, H,O evolution 
eventually diminished to zero; however, it could be re-started by increasing temperature. The stabili- 
zation exotherm was largely associated with H,O production. Carbon-gas production was independent 
of HZ0 production, and appeared to be a surface reaction. 
Key Words-Pitch-based carbon fiber, stabilization, Fourier transform infra-red (FTIR), Differential 
scanning calorimetry (DSC), Thermo-gravimetric analysis (TGA). 
1. INTRODUCTION 2. EXPERIMENTAL 
The process for making carbon fiber from pitch typ- 
ically involves preparation of a mesophase (liquid 
crystalline) pitch from residues, converting it into 
fiber, stabilizing the fiber so that it can be heated 
above 500°C without melting, and carbonizing at 
temperatures above 1000°C. Stabilization is the rate- 
limiting step in the process. While many options exist 
for stabilization (e.g., reaction with HN03), the 
method practiced most broadly is thermal oxidation 
in air or a mixture of air and nitrogen. This reaction 
is believed to be diffusion limited[l,2]. Two stabi- 
lization mechanisms are possible: adding oxygen to 
an organic molecule raises the boiling or melting 
point (e.g., phenol boils at 182” vs. 80°C for benzene) 
or, alternatively, oxygen may promote cross-linking 
between molecules. 
The chemical and physical processes of stabiliza- 
tion are complex and poorly defined; the interested 
reader is referred to a recent survey article[3]. Ox- 
ygen uptake reactions are believed to occur first at 
aliphatic side chains to produce a variety of com- 
pounds by oxygen insertion and dehydrogena- 
tion[4,5], and further aromatization and cross-link- 
ing may take place. Gases such as H2, H20, CO, 
CO*, and CH, have been produced during stabili- 
zation[6]. Carbon-oxygen reactions have been the 
subject of extensive study[7,8]. The rate of stabili- 
zation has typically been studied via Thermo-grav- 
imetric analysis (TGA)[l]. 
This work seeks to add to the knowledge base by 
tracking the oxidation of model compounds by ther- 
mal analysis, and tracking the oxidation of a pitch 
fiber by a combination of thermal analysis and infra- 
red off-gas analysis. It was presented in part at the 
20th Carbon Conference[9]. 
351 
Mixtures of model compounds were made by dis- 
solving the pure materials in toluene, mixing solu- 
tions, and evaporating to dryness at room temper- 
ature. The as-spun carbon fibers used in this study 
were made from 100% mesophase pitch prepared 
by the solvent-extraction method[lO]. Differential 
scanning calorimetry (DSC) was carried out on a Du 
Pont Model 2100 Thermal Analysis System with a 
model 910 DSC cell. DSC baseline shift was tracked 
by alternating periodically between an inert gas (NJ 
and the oxidizing gas (air), as shown on Fig. 1. Mass 
spectrometry was carried out on a Vacuum Gener- 
ator Model 7070 instrument, and Fourier transform 
infra-red measurements on a Perkin Elmer Model 
1800 instrument interfaced with a Du Pont Model 
9.51 Thermo-Gravimetric Analyser. 
3. RESULTS AND DISCUSSION 
3.1 Model compound studies: oxidation reactions 
Oxidation chemistry was explored using several 
model compounds based on anthracene: anthracene 
itself (AC), dimethylanthracene (DMAC), dihy- 
droanthracene (DHAC), and anthraquinone (AQ). 
Previous FITR studies showed these moieties to rep- 
resent the most abundant substituents on the pitch 
molecule during stabilization. The structure of these 
compounds is illustrated in Fig. 2. 
First, the oxidation of pure materials was studied 
by DSC, at 850 psi pressure, to limit sublimation. 
DSC profiles of pure compounds ramped at 5°C in 
air are shown in Fig. 3. AC melted at 215”C, then 
showed little activity until -3OO”C, where an exo- 
therm occurred. AQ melted at 282°C and sublimed. 
DHAC melted at 103”C, then showed three large 
352 J. G. LAWN 
Temperature 
Time, mins 
Fig. 1. Stabilization of pitch-based carbon fiber baseline 
tracking method. 
exotherms. Finally, DMAC melted at 172°C and a 
large exotherm followed immediately. 
Next, the stabilization process was simulated by 
ramping the samples to 240°C in air and holding for 
one hour under pressure. AC and AQ melted and 
sublimed, but did not show an exothermic reaction. 
Strong exothermic reactions were seen from DHAC 
and DMAC, almost from the time that heating 
started (Fig. 4). 
Mixtures of these compounds were studied next. 
AC and AQ were treated as base compounds, and 
DMAC and DHAC as additives, at the 10% level. 
This level was chosen arbitrarily; however, it rep- 
resents a 3% level of methyl or aromatic hydrogen 
groups, which is more than would be expected for 
coal-tar pitches and less than expected for petroleum 
pitches. The compositions studied are shown in 
Table 1. 
DSC curves for the anthracene-based model com- 
pounds are shown in Fig. 5. The AC/DHAC curve 
is very steep, and the reaction is essentially over two 
minutes after the isothermal temperature was 
reached. The peak in the AC/DMAC curve was not 
COMPOUND STRUCTURE CODE 
Anthracene m AC 
0 
Anthraqulnone 
Dlhydroanthracene DHAC 
Dimethylanthracene DMAC 
Fig. 2. Structure of model compounds. Fig. 4. Isothermal DSC curves for pure compounds. 
10 
5 
6 
P4 
3. 
s 
ii 
1 
=2 
0 
-2 
-4 I 
50 loo 154 200 250 300 350 400 
Temperature, “C 
Fig. 3. Ramped DSC curves for pure compounds. 
reached for five minutes after reaching 240°C. The 
AC/DMAC/DHAC curve was intermediate be- 
tween the two-not additive! Note that the curve 
for AC alone is essentially flat; no self-reaction oc- 
curs at this temperature. 
DSC curves for anthraquinone-based pitches are 
shown in Fig. 6. There was no exotherm when the 
AC/DHAC was heated. There was an exotherm 
with AQ/DMAC, and a larger one with AQ/ 
DMAUDHAC. Comparisons with exotherm for 
the additives alone and the model compounds are 
shown in Figs. 7 through 9. DMAC alone is com- 
pared with AC/DMAC and AQ/DMAC in Fig. 7 
(the DMAC curve is scaled down from Fig. 4 by a 
factor of 10). The reaction in DMAC alone was 
essentially over when the 240°C isotherm was 
Temp 
\ 
\ 
\ 
‘A - 0 --_-___ ._______-_-_-_-_-____ 
- 140% 
- E 
I- 
Stabilization of a carbon fiber 353 
Table 1. Composition of model compounds 
Antrhacene-based Antrhaquinone-based Ratio 
AUDMAC AQ/DMAC 90110 
AUDHAC AQ/DHAC 90/10 
AUDMAUDHAC AQlDMAClDHAC 80/10/10 
reached. The AUDMAC exotherm was large and 
sustained, while the AQ/DMAC exotherm was 
much smaller in peak value and duration. The AC/ 
DHAC exotherm is compared with the DHAC exo- 
therm (scaled by 10) in Fig. 8; the AC/DHAC peak 
is much larger. The model compounds containing 
both DMAC and DHAC are compared with a com- 
posite DMAC + DHAC curve in Fig. 9. Exotherms 
for the model compounds were approximately twice 
those for the additives alone. See Table 2. 
A sensitive method of comparing the reactivityof 
pitches is to compare the derivative of their heat 
fluxes as a function of temperature. A comparison 
is made between a real pitch and the anthracene- 
based model pitch in Fig. 10. The similarities be- 
tween the curves are dramatic; the shoulder corre- 
sponds to the AC/DMAC curve, while the peak 
corresponds to the ACYDHAC curve. 
3.2 Model compound studies: the linking reaction 
Linking reactions are extremely difficult to study 
because they are overshadowed by the H,O and car- 
bon-gas formation reactions. The oxidation of the 
AQlDMAC mixture suggested that a linking reac- 
tion might be taking lace. A linking reaction was 
confirmed by a DSC experiment in which a 50/50 
mixture of AQ and DHAC was heated in 300 psi 
air to 25O”C, and held for an hour. Two reaction 
peaks were observed in DSC, (Fig. 11) and evidence 
of high-molecular-weight compounds was found by 
mass spectrometer (Fig. 12). 
In subsequent experiments, SO/SO mixtures of 
DHAC or DMAC were reacted with AC or AQ in 
nitrogen at 850 psi. Isothermal runs were made, in 
which the sample was heated to 300°C at SO”C/min., 
and held, and ramp runs were made in which the 
Fig. 5. Isothermal DSC curves for anthracene-based 
mixtures. 
Fig. 6. Isothermal DSC curves for anthraquinone-based 
mixtures. 
sample was heated at SUmin to 400°C. The first 
surprise came from the DHAC/AC mixture; an exo- 
thermic peak was observed in the isothermal run, 
indicating an addition reaction. A rough estimate of 
the heat of reaction was -10 kcal/mol; this was 
about 50% of the expected value, but since subli- 
mation was occurring simultaneously, the true value 
may be higher. High-molecular-weight material was 
found in mass spectroscopic analysis of the residue. 
DHAC reacted with AQ starting at 50°C although 
no high-MW compounds were found in the residue. 
DMAC/AC and DMAUAQ mixtures both showed 
exotherms and gave high-MW compounds; also, 
each showed 3 melting peaks in the ramped DSC 
runs. The size of the exotherms was of the order of 
10% of the normal stabilization exotherm. 
3.3 DSCIFTIR studies on as-spun fibers 
An experiment was devised to determine the na- 
ture of stabilization off-gases, and the exotherms 
associated with them. FTIR and DSC curves were 
obtained for each sample, using a heating profile 
which dried the sample at 120°C for one hour, then 
raised the temperature to 240°C at lO’C/minute and 
held for five hours. The temperature was next raised 
to 270°C and held for four hours, then raised to 
Fig. 7. Comparison of exotherms of mixtures containing 
DMAC with pure DMAC. 
354 J. G. LAWN 
300°C and held for three hours. The isothermal hold 
cycles were based on the author’s estimate of the 
time required to reduce H,O evolution to zero. Gas 
evolution is shown in graphic form in Fig. 13 and 
heat flux is added in Fig. 14. TGA data are shown 
in Fig. 15. Key observations were: 
1. Only CO, C02, and H,O were found in the 
off-gas (it is possible that Hz was present, but 
undetected). 
2. H,O generation peaked when the sample 
reached the isothermal temperature, and fell to zero 
by the end of each cycle, although it could be re- 
started by raising the temperature. 
3. CO/CO, ratios varied between 0.5 and 0.68, 
with a mean of 0.6, and evolution rates for these 
gases approximately doubled with each 30°C rise in 
temperature, falling off slowly with time in each 
cycle. 
4. The exotherm was largely associated with the 
H,O reaction. 
5. Sample weight increased with time until the 
temperature was raised to 3OO”C, when it decreased. 
The fact that the H,O evolution went to zero and 
restarted on heating suggests that at any given tem- 
Fig. 9. Comparison of exotherms of 3-component model Fig. 10. Comparison of real and model pitches: derivative 
compounds with theoretical DMAC + DHAC curve. DSC ramp curves. 
Table 2. Reactivity of the components and model 
compounds 
Compound or mixture Ratio Reactivity 
AC 
AQ 
DMAC 
DHAC 
loo 
loo 
100 
100 
AUDMAC 
ACIDMAC 
AUDMACYDHAC 
90/10 
90/10 
80/10/10 
AQ/DMAC 90/10 
AQ/DHAC 90/10 
AQ/DMAC/DHAC 80/10/10 
None 
None 
Strong 
Strong 
Strong 
Strong 
Strong 
Weak 
None 
Strone 
perature there are a certain number of reaction sites 
which eventually become saturated. Previous studies 
of carbon-oxygen reactions[7] proposed that the 
mechanism is desorption of carbon-oxygen surface 
complexes, as gases. The process is characterized by 
slow buildup of the surface complexes, followed by 
desorption. The process is illustrated in Fig. 16 by 
a diagram of gas evolution at the end of the 120”- 
240°C heating ramp. The CO, peak did not occur 
for approximately 10 minutes. By contrast, in later 
heating ramps, the peak occurred almost instanta- 
neously. The activation energy for the carbon gas 
reactions was 11 Kcal/gram mol, compared with a 
value of 6.4 reported previously[7]. The fraction of 
CO present in the off-gases is higher than would be 
expected from previous work[3]; however, temper- 
atures in this work are considerably lower (240”- 
300°C vs. 460”-900”(Z), and burn-off in this work was 
also very low. 
3.3.1 Chemical reactions. Weight losses are due 
to removal of carbon atoms as CO and CO, and 
hydrogen atoms as H,O. Weight gain is due to ad- 
dition of oxygen atoms, and is proportional to HZ0 
.05 , , , , , , , ( , , , , , , , , , , , , , ( , 
Peak 
Stabilization of a carbon fiber 355 
Temperature 
Reaction Peaks 
0.0,’ 
-HZ0 
_._.- CO* 
____CO 
240°C 270°C 300°C _ 
200 400 600 800 
Time, mins 
Fig. 11. Thermal oxidation of AQ/DHAC (50/50) 
mixture. Fig. 13. Gas evolution during stabilization of pitch-based 
carbon fibers. 
generated. The stochiometry of key reactions can be occurred at t = 613 minutes. At this point, gas gen- 
expressed as follows: eration was as follows: 
c+~o,-co (1) 
CO, = 0.477 mg/g/min 
2 CO = 0.277 mg/g/min 
c + 0, - co, (2) HI0 = 0.353 mg/g/min 
Ar-H + 02- Ar-0 + iH,O + to, From this data we can calculate the loss of carbon 
and hydrogen: 
(3) 
2 Ar-H + O2 - Ar-0-Ar + H,O (4) 
Reaction (3) adds one atom of oxygen for each hy- 
drogen atom produced, whereas reaction (4) adds 
one atom of oxygen for each two hydrogen atoms 
produced. 
w, = Wt(CO*) x $ + Wt(C0) x g 
2 
+ Wt(H,O) x Is 
= 0.288 mg/g/min (5) 
At one point in the cycle, the rate of weight gain Assuming reaction (3), oxygen gain is calculated as: 
was exactly equal to the rate of weight loss. This 
W, = Wt(H,O) x $ = 0.627 mg/g/min (6) 
20* AQ or assuming reaction (4): 
60 W, = Wt(H*O) x i = 0.313 mg/g/min (7) 
The result of eqn (7) is closest to (5) and suggests 
that two hydrogen atoms are liberated for each ox- 
150 200 250 300 350 400 450 500 
ygen added. 
MASS 
1.5 I I I 
364 
EXPANDED SCALE 
MASS 
1 .o 
24O’C 270°C 3000 c 
200 400 600 800 
Time, mins 
Fig. 12. Mass spectrometer diagram: reaction products of Fig. 14. Heat and water evolution during stabilization of 
AQ and DHAC. pitch-based carbon fibers. 
356 J. G. 
112- 
110- 
Time (min) 
Fig. 15. Weight change during stabilization of pitch-based 
carbon fibers. 
"'60 62 64 66 66 70 72 74 76 76 60 62 64 66 68 90 
Time (min) 
Fig. 16. Gas evolution at the end of the first heating ramp. 
Time. mmr 
Fig. 17. Ratio of wt. loss/wt. gain during stabilization of 
pitch-based carbon fibers. 
Table 3. Average loss/gain ratio for 
each cycle 
Cycle temperature Average loss/ 
(“C) gain ratio 
240 0.3 
270 1.1 
300 2.1 
LAWN 
These results make possible an observation on the 
use of weight gain data to track stabilization rate[l]. 
Using eqns (5) and (7), weight loss/gain ratios were 
calculated for each minute of the cycle described 
in the previous section, and normalized so that 
WJW, = 1.0 where dWldt = 0 (the normalization 
factor was 1.087). The data is shown in Fig. 17 and 
Table 3. In any given experiment, the loss/gain ratio 
will depend on pitch type and thermal history. Ex- 
periments relying on weight gain alone will always 
understate the rateof oxygen uptake at any mean- 
ingful stabilization temperature. 
CONCLUSIONS 
The model compound work showed that: 
1. The oxidation reaction will proceed with an- 
thracene alone, provided the temperature is suffi- 
ciently high. 
2. The oxidation of anthracene is accelerated by 
dimethyl- and dihydroanthracene, also the dimethyl- 
appears to be responsible for the sustained oxida- 
tion reaction, and the dihydro- for the fast initial 
reaction. 
3. Reactions take place between anthraquinone 
and dimethylanthracene, and mixtures of dimethyl- 
and dihydroanthracene, in the presence of oxygen. 
4. The correlation between the oxidation of pitch 
and the model compound AC/DMAC/DHAC (80/ 
lO/lO) is good. 
5. There is a possible linking reaction between 
anthraquinone and dihydroanthracene. 
6. Linking reactions between the compounds take 
place even in nitrogen, although they are accelerated 
in the presence of oxygen. 
Clearly, the abundance of CH2 and CH3 groups in 
pitches will determine the response of the fiber to 
stabilization processes. 
The DSC/FTIR studies of pitch fibers showed 
that: 
1. HZ0 production is independent of CO, and 
CO production, contrasting with combustion reac- 
tions, where the gases are byproducts of the same 
reactions; 
2. The decay of H,O production to zero, with 
subsequent restart on heating indicates a bulk de- 
hydrogenation reaction, with a limited number of 
sites available at each temperature; 
3. The CO2 and CO production pattern indicates 
a surface reaction, and the mechanism appears to 
be desorption of surface oxygen complexes. 
4. Tracking weight gain via TGA alone is not a 
reliable method of determining stabilization kinetics. 
Acknowledgements-The author gratefully acknowledges 
the contribution of I. M. Sarasohn, who developed the DSC 
baseline tracking method and showed the value of com- 
bined DSCYFI’IR studies. Also acknowledged are stimu- 
lating mechanistic discussions with H. Kobsa, M. Katz, and 
Stabilization of a carbon fiber 357 
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Richardson in FTIR/TGA. 15th Japan Carbon, lA16, 35 (1988). 
5. Y. Korai, M. Nakamura, and I. Mochida, Carbon 29, 
561 (1991). 
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