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Mesophase pitches as matrix precursor of carbon fiber reinforced carbon: I. Mesophase pitch preparation and characterization
Carbon Vol. 34, No. 9, pp. lOS7-1066,1996 Copyright 0 1996 Elsevier Science Ltd
Pergamon
Printed in Great Britain. All rights reserved 0008-6223/96 $15.00 + 0.00
60008-6223 (96) 00055-3
MESOPHASE PITCHES AS MATRIX PRECURSOR OF CARBON FIBER REINFORCED CARBON: I. MESOPHASE PITCH PREPARATION AND CHARACTERIZATION Institut
fur Chemische
Technik,
V. LIEDTKE and K. J. H~TTINGER Universitat Karlsruhe, KaiserstraDe 12, D-76128
Karlsruhe,
Germany
(Received 26 October 1995; accepted in revised form 5 March 1996)
Abstract-Mesogenic and mesophase pitches were produced from Ashland A 240 pitch in a stirred tank reactor under various pyrolysis conditions. Isothermal experiments showed strongly different properties of the products as compared to results with an earlier lot of the same pitch. The products were characterized by their solubility in tetrahydrofuran and N-methyl-pyrrolidone, their mesophase content, glass transition temperature, coke yield, and aromaticity. Pitch reactivities with oxygen in view of a stabilization treatment were analyzed by FT-IR. Copyright 0 1996 Elsevier Science Ltd
Key Words-Mesogenic reactivity.
pitch,
mesophase
pitch,
matrix
1. INTRODUCTION
Mesophase pitch was first used as a matrix precursor of carbon fiber reinforced carbon more than 15 years ago [l,Z]. In these studies, a fiber preform was impregnated with a standard coal tar binder pitch which was then transformed into mesophase pitch by thermal treatment. To overcome swelling of the composites by the bloating effect of pyrolysis gases, mechanical pressure was applied during carbonization up to 550°C. Later, White et al. showed that swelling of the composites during carbonization can be avoided by an oxygen treatment of the infiltrated pitch [3]. White et al. [4] additionally succeeded in infiltrating a fiber preform with an anisotropic pitch produced from alkylbenzenes by polymerisation and pyrolysis [5]; the structure of such pitches is discussed controversially in the literature [6-lo]. Some authors conclude that they are best described as anisotropic gels, which are different from the classic Brooks-Taylor mesophase as obtained by thermal pyrolysis [ 6-83, and some other authors support the opposite point of view [9,10]. In any case, these anisotropic pitches soften at much lower temperatures and exhibit a much lower viscosity than the classic Brooks-Taylor mesophase [ 111. Infiltration of a fiber preform with a thermally produced mesophase pitch containing about 40 ~01% mesophase spheres has recently been reported [ 12,131. A filter effect was not observed, but the green composites had to be treated with oxygen in order to prevent swelling during carbonization. Coke yields in the range of 85% were achieved. The present paper represents a continuation of this latter work [ 12,133. It is concerned with the development of mesophase and especially mesogenic pitches* *A mesogenic pitch is defined as a pitch which is still optically isotropic but converted to such a degree that it can form polyaromatic mesophase by any further thermal treatment immediately.
precursors,
property
relationships,
oxygen
with improved properties in respect to infiltration of a carbon fiber preform. Pitches with improved properties should (1) be homogeneous, i.e. single-phase materials such as a mesogenic pitch [ 14-171, or exhibit a very low content of mesophase spheres only, (2) soften at as low a temperature as possible, (3) produce a coke yield of at least 80%, and (4) form a graphitizable carbon matrix after pyrolysis. A major reason for doing this research was the fact that the Ashland A 240 petroleum pitch (pitch I) used in the previous studies [12,13,15-191 formed spherulitic mesophase in a unique manner as coalescence of mesophase spherulites by formation of bulk mesophase did not occur up to mesophase contents of at least 40 ~01%. With all later batches of Ashland A 240 (pitch II) coalescence of spherulites was already found to occur at mesophase contents as low as 20 ~01%. Such a material was shown not to be favourable for infiltration of a fiber preform [ 12,131. Mesophase pitches with high contents of spherulites, even from pitch II, can be produced by an iron-catalyzed pyrolysis [20, 211. Unfortunately, those pitches are less suitable for impregnation of a fiber preform; they show a severe filter effect. For development of more suitable pitches for the infiltration of a fiber preform, some special pyrolysis treatments of the A 240 petroleum pitch (pitch II) were performed: (1) pyrolysis under mild conditions, i.e. at low temperature, and (2) pyrolysis by fast heating to high temperature and opening of the reactor at this temperature with zero residence time. In the following, the synthesis and the properties of the synthesized pitches will be treated. Results on the stabilization chemistry of the pitches produced will also be presented. Stabilization treatments of the infiltrated pitches at elevated oxygen pressure and the usefulness of the pitches in the fabrication of carbon fiber reinforced carbons will be shown in two following parts of this paper [22,23].
1057
V. LIEDTKEand K. J. HUTTINCER
1058 2. EXPERIMENTAL
2.1 Pitch The studies are based on the petroleum pitch Ashland A 240. Typical properties of this pitch II are compiled in Table 1 and compared with those of the earlier batch pitch I [ 12,13,15-191. Some properties are identical, but some others, such as the glass transition temperature and the coke yield, are not. For a more detailed analysis of the pitches, FT-IR spectroscopy was used (see Section 2.3). The spectra of pitch I and pitch II are nearly identical (Fig. 1). This conclusion is supported by the ratios of the peak heights at 3045 cm-‘, corresponding to aromatic hydrogen, and (1) 2962 cm-‘, corresponding to methyl groups, and (2) 2916 cm-‘, corresponding to methylene groups: (1) 1.06 (pitch I) and 1.05 (pitch II), (2) 1.16 (pitch I) and 1.13 (pitch II) [25,26]. 2.2 Pyrolysis apparatus andprocedure For the pyrolysis treatments, a stirred tank reactor was used; it is described in an earlier paper 1151. A pitch sample of 300 g was heated under a pressure of 1 MPa argon with a rate of 10 K/min to the desired temperature. Isothermal experiments were performed by variation of the residence time; in these experiments the reactor was depressurized and opened at reaction temperature. In further experiments with
zero residence time, the reactor was depressurized and opened immediately after reaching the desired maximum temperature, or it was depressurized and opened after cooling to a lower temperature. 2.3 Analytical methods The pyrolysis residues were analyzed by extraction with tetrahydrofuran (THF) and N-methyl-pyrrolidone (NMP), determination of the mesophase content (MP) from polarized-light micrographs, the glass transition temperature (T,) by thermomechanical analysis (TMA), and the coke yield (CY) according to the method used in our laboratory. All analytical methods have been described in earlier papers [lS,lS]. The extractions with the solvents used yielded nearly identical results to those carried out with toluene instead of THF and quinoline instead of NMP, but the solvents used are less toxic. For further characterization of the mesophase pitches, FT-IR spectroscopy was used. Attempts to study the reactivity of the produced pitches with oxygen by differential scanning calorimetry (DSC) failed; the powdered materials fused before the reaction started. Therefore, thin layers of the produced pitches of about 1 mm thickness were treated with oxygen and analyzed by FT-IR spectroscopy. For all FT-IR studies a Nicolet spectrometer
Table 1. Properties of Ashland A 240 petroleum pitches
Pitch I Pitch II
C (%)
H (%)
S (%)
THF-IS (%)
NMP-I (%)
CY (%)
Tg(“C)
P (g/em3)
89.9 90.6
5.5 5.5
3.0 2.7
5.1 3.2
0 0
44.0 38.5
73.0 83.5
1.24 1.24
The content of carbon, hydrogen, and sulfur was analyzed by Mikroanalytisches Labor Paschen, D-53424 Remagen. THF-IS - insolubles in tetrahydrofuran (Soxhlet extraction). NMP-I - insolubles in N-methyl-pyrrolidone (determined according to Q-I, DIN 51 921 1251). CY - coke yield (determined with the method used in our laboratory: open crucible at 600°C). Tg- glass transition temperature (determined with Mettler TA 4000 thermomechanical analyzer) p ~~ powder density (determined with pycnometer method according to DIN 51 907 25.
Wavenumbers, cm-’
Fig. 1. FT-IR spectra of two different lots of Ashland A 240 petroleum pitch.
Mesophase
pitches as matrix
precursor
of carbon
model Magna IR 550 was used, the spectra were taken by diffuse reflectance (DRIFT). The absorption wavenumbers of relevant groups are compiled in Table 2. For characterization of a material, the ratios of the peak heights of aromatic hydrocarbon at 3045 cm-‘, methyl groups at 2962 cm-‘, and methylene groups at 2916 cm-’ were used.
Functional
I
1059
k, -MA=MP
and corresponding
FT-IR
absorption
frequencies
[26,27]
Absorption
methyl
CH2 (mostly
methylene
substituents)
hydrocarbons rings containing rings containing rings containing
linkages
between
aromatic
(1)
A + a = original pitch; a = fraction of volatiles escaping during heating to reaction temperature; PA = polyaromatics; MA = mesogenic aromatics; MP = mesophase. According to refs [ 16,18,19], the rate constants were considered as k, = k, = k. For the results with pitch I, k =0.426 h-l, for pitch II, k =0.263 h-‘. These rate constants differ almost by a factor of 2. In all isothermal experiments at 440°C the pyrolysis yields with pitch I and II were very similar and varied between 87 and 84% only. Thus, it can clearly be concluded that the different mesophase contents are a pure effect of pitch reactivities, at least in respect to formation of mesogenic aromatics and mesophase.
groups
CH3 (mostly
Aromatic Aromatic Aromatic Aromatic
k, pA
A+aZA-
3.1 Pyrolysis studies at 440” C Results of isothermal pyrolysis studies will mainly be shown in order to demonstrate the different reactivities of pitch I and pitch II in pyrolysis. Results with pitch I were taken from earlier publications [ 12,13,18,19]. The mesophase contents of the mesophase pitches produced at 440°C and various reaction times are presented in Fig. 2. The filled symbols (A, n) describe the results with pitch I found by two different authors [ 12,13,18,19]. As compared to these mesophase contents, significant lower mesophase
groups
carbon:
yields were found in the studies with pitch II (open points). The curves drawn in Fig. 2 were calculated using the reaction scheme proposed by Wang and Htittinger [ 161, eqn (1):
3. RESULTS
Table 2. Functional
fiber reinforced
frequencies 2962, 2866 1444, 1377 2916, - 2850 1478 3078-3035 750 815 875
rings)
4 adjacent hydrogen atoms 2 adjacent hydrogen atoms isolated hydrogen atoms
C=O (aldehydes, ketones, carboxyl groups) C=O (aromatic/aliphatic esters) -C = 0 of aromatic cyclic anhydrides
U-c=0 “X=0 U-c=0
- 1700 - 1735* 1840 and 1775
-C=C
"-c=c
1600
(aromatic
rings)
OH (free) OH (hydrogen-bonded)
- 3550 -3400-3450
“OH “Ml
0
1
2
3
4
5
t, h Fig. 2. Mesophase
content
as a function
of residence
time at 44o”C, 1 MPa Ar. A,
n , pitch
pitch II.
(cm-‘)
V. LIED~KE and K. J. HUTTINGER
1060
The different pyrolysis behaviour of the pitches from different lots also follows from the content of insolubles of the pyrolysis residues in THF (THF-IS) and NMP (NMP-I), which are shown in Fig. 3 (a) and (b). The continuous increase of THF-IS (Fig. 3 (a)) and the sigmoid increase of NMP-I (Fig. 3 (b)) with increasing residence time as found with pitch I have been reproduced in all studies with this pitch [12,13,15-191. Completely different dependencies exist for pitch II. At short residence time, the content of insolubles in both solvents is higher than that of pitch I. This result and the delayed increase of insolubles with progressing pyrolysis time have never been observed with other pitches, not even with coal tar pitch [21]. Figure 4 shows a plot of the NMP-I values vs mesophase contents of the pitches obtained with pitch I and pitch II. With pitch I, a linear correlation exists over the total range of mesophase contents. The slope of the curve is nearly identical to that of the diagonal. It indicates that about 10% of mesophase of all pitches are soluble in NMP. With pitch II, the mesophase and NMP-I contents are nearly identical. At low mesophase contents, the NMP-I content is
Fig. 3. Content
of insolubles
in THF (a) and NMP
even slightly higher. This result is more surprising because a plot of the coke yield as a function of the mesophase content yields an inverse correlation. Figure 5 (a) and (b) shows the coke yield and the glass transition temperature of the mesophase pitches. The coke yields of the mesophase pitches from pitch I determined by two different authors [ 12,13,18,19] are nearly identical (Fig. 5 (a)), but not the glass transition temperatures (Fig. 5 (b)). At short reaction times, i.e. at low conversion of the pitch, they differ by about 10 K. The reason is known: Christ [ 12,131 (A) opened the reactor at 44O”C, Bernhauer [ 18,191 ( n) only after cooling to 400°C. One or two per cent more volatiles, which can escape at 44O”C, but not at 4OO”C, obviously have a strong influence on the glass transition temperature, but they do not have a measurable effect on the further properties analyzed. Figure 5 (a) and (b) also shows that the coke yield and the glass transition temperature of the mesophase pitches from pitch II are lower than those found with mesophase pitches from pitch I. With increasing pyrolysis time, the differences increase. In view of the high content of insolubles, and especially of NMP-I
(b) as a function 0, pitch II.
of residence
time at 44o”C, 1 MPa Ar. A, W, pitch I;
Mesophase
pitches as matrix
-0
precursor
of carbon
25
fiber reinforced carbon: I
50
1061
75
MP, ~01% Fig. 4. Content
of NMP-I
as a function
45 c
35
of mesophase content; products synthesized W, pitch I; 0, pitch II.
-----pitch
I--
L----
60 0
residence
times.
I (= A)
- - - - - pitch II 1
I
at 440°C and different
pitch
( o )
I
I
I
4
I (mu)
I
1
4
I
I
1
2
3
4
5
I
t, h Fig. 5. Coke yield (a) and glass transition
temperature
(b) as a function I; 0, pitch II.
at short residence time (Fig. 3 (a) and (b)), the low glass transition temperatures and the low coke yields are difficult to understand. Unfortunately, mesophase pitch samples produced with pitch I and sufficient
of residence
time at 44o”C, 1 MPa Ar. A,
n , pitch
quantities of pitch I to reproduce the samples were no longer available. Therefore, their molecular structure as compared to mesophase pitches produced with pitch II could not be analyzed (see Table 1).
V. LIEDTKE and K.J.
1062
3.2 Pyrolysis studies under varied conditions In Fig. 6 (a), the dependence of the glass transition temperature on the mesophase content of various pitches is shown. The full symbols represent again results of Christ et al. [ 12,131 and Bernhauer et ul. [1X,19] with pitch I. These results reflect the typical correlation between glass transition temperature and mesophase content found in all earlier studies [ 12,13,15-191. The glass transition temperature increases slightly with the mesophase content. The bend in the curve is ascribed to the phase inversion after which the continuous phase is formed by the mesophase [ 12,13,15- 191. At equal mesophase content, mesophase pitches obtained with pitch II have lower glass transition temperatures than mesophase pitches produced from pitch I. This property relationship should be advantageous. In fact it is a disadvantage because the mesophase spheres coalesce at lower contents. To underline this fact, polarized light optical micrographs will be presented later. The optimum pitch used by Christ [ 12,131 is marked by an arrow (la); it only exhibits spherulitic mesophase. The pitch produced at the same temperature of 440°C from pitch II and used for the fabrication of compos-
75
H~~TTINGEX
ites in this study is marked by an arrow (lb); it already contains bulk mesophase (see Fig. 7). Further materials which were used for the fabrication of composites are also marked by arrows. The materials (2), (3), and (4a) are designated as mesogenie pitches because their mesophase content is lower than 5 ~01%. They were produced at 440°C with 0.5 hours residence time (2), at 400°C with 3 hours residence time (3), and at 460°C without residence time (4a). In all three cases, the reactor was depressurized and opened at pyrolysis temperature. Material (4b) was synthesized in a similar way as material (4a), but at 490°C. Pitches (4a) and (4b) are of interest because of their low mesophase content as compared to their high glass transition temperature. The reason for this unusual property relationship has to be seen in an unusual high content of highmolecular, i.e. mesogenic aromatics in the isotropic pitch phase. The different glass transition temperatures not only of the pitches (4a) and (4b), but also of the pitches (2) and (3) are a result of the synthesis conditions, which lead to very different pyrolysis yields. The yield values and the properties of all pitches used for further studies are summarized in
(b)
-.
U
I
I
25
50
J 75
MP, ~01% Fig. 6. Glass transition temperature (a) and coke yield (b) as a function of mesophase content of various pyrolysis products. Full symbols: pitch I, open symbols: pitch II. Arrows indicate pitches selected for the fabrication of composites.
Mesophase
Fig. I. Polarized-light
optical
pitches as matrix
micrographs
precursor
of pyrolysis
of carbon
products
fiber reinforced
carbon:
used for the fabrication
I
1063
of composites.
The pyrol lysis
conditions are compiled in Table 3. 3; polarized-light optical micrographs of these ials are presented in Fig. 7. Ma terials produced with a high yield exhibit a low
Table mater
Table 3. Properties
of the pitches
glass transition temperature. Opening of the reac :tor at a high or very high temperature allows volat .iles to escape. The residue is relatively enriched vvith used for fabrication
of composites
Pitch no.
Pyrolysis temperature (“C)
Residence time (h)
Residue yield (%)
THF-IS (wt%)
NMP-I (wt%)
(la) (lb) (2) (3) (4a) (4b)
440 440 440 400 460 490
2.0 3.5 0.5 3.0 0 0
85.0 84.3 84.8 92.3 80.8 74.4
47.9 53.8 32.6 4.4 22.1 49.6
31.3 7.8 3.6 8.7 31.9
*
T, (“C)
CY
(TMA)
MP (vol%)
(wt%)
120.0 97.0 85.9 74.6 104.5 119.7
30 32.5 3 3 5 20
64.1 60.3 50.6 46.9 53.8 63.8
1064
V. LIEDTKE and K. J. H~JTTINGER
aromatics of high molecular weight, but mesophase could not be formed because of rapid cooling down of the pitch. This suggests that especially the materials (4a) and (4b) have a substantial amount of “dormant” mesophase. “Dormant” mesophase means mesogenic polyaromatics dissolved in the isotropic pitch phase in a concentration beyond the saturation solubility [ 241. Such materials should be favourable for infiltration of fiber preforms and give a high coke yield. Coke yields as a function of the mesophase content are presented in Fig. 6(b). Identical results of Bernhauer et al. [ 18,191 and Christ et al. [ 12,131 with pitch I show that the coke yield is less sensitive to the opening of the reactor than the glass transition temperature. On the other hand, the more drastic procedure in producing pitches (4a) and (4b), namely opening of the reactor at 460 and 490°C respectively, can also be seen in relatively higher coke yields. The effect of the temperature at which the reactor was opened was analyzed as follows: the pitch was generally heated to 490°C and the reactor was opened (a) at 490°C; (b) after cooling down to 440°C and (c) to 360°C. The results are shown in Fig. 8. The residue yield decreases strongly with increasing opening temperature, the effect on the increase of the glass transition temperature is even more pronounced. This corresponds to the results presented above. The coke yield increases as the residue yield decreases, in such a way that the product of coke yield and residue yield is nearly independent of the opening temperature.
3.3 Characterization ofpitches with infrared spectroscopy 3.3.1 Pitches used for fabrication of composites FT-IR transmission spectra of pitches (lb), (2), (3) (4a), and (4b) are presented in Fig. 9. The similarity is obvious. For a quantitative characterization, the ratios between the peak heights of aromatic
Opening
hydrogen and (1) methyl groups, and (2) methylene groups are shown in Table 4. All ratios are in the range of 1, but clearly lower than those found for the starting pitch (see Section 2.1). This leads to the first conclusion that the ratio between the concentrations of methyl and methylene groups is nearly constant, independent of the pyrolysis treatment or the conversion of the starting pitch. The mesogenic pitches (2) (3) and (4a) exhibit similar ratios between the content of aromatic hydrogen and the content of methyl or methylene groups. As compared to the starting pitch the ratios are only slightly decreased. Clearly lower values of these ratios were found with pitch (lb) and especially with pitch (4b). Therefore, it can further be concluded that the aromaticity of mesogenic pitches is only slightly higher than that of the starting pitch, but pitches (lb) and (4b) exhibit a clearly higher aromaticity.
3.3.2 Pitches
after
treatment
with
oxygen
Analogous FT-IR studies were performed after oxygen treatment at ambient pressure and temperatures of 200 and 250°C with 30 minutes of residence time (see Section 2.3). The results are shown in Fig. 10 and Fig. 11. As far as the ratios used for characterization of the pitches before oxygen treatment are concerned, no significant changes could be found. The effect of the oxygen treatment is reflected by the carbonyl peak at about 1700cm~’ after both treatments at 200 and 250°C. The appearance of very small peaks at 1840 cm-’ and 1775 cm-’ corresponding to anhydrides was only detected after treatment at 250°C. Ester peaks appearing at about 1740 cm-’ are not a consequence of the oxygen treatment, they were already found with the starting pitch (see Fig. 1). Because of the shoulder resulting from the esters, a quantitative evaluation of the carbonyl peak at 1700cm-’ is not possible. A rough estimate of the carbonyl peak heights leads to the conclusion that
temperature,
“C
Fig. 8. Effect of opening temperature of the reactor on the residue yield (*), glass transition temperature (+), coke yield (A), and the product of residue yield and coke yield (A) of the mesophase pitches. Pyrolysis at 49O”C, 1 MPa Ar, zero residence time.
Mesophase
pitches as matrix precursor
2500
of carbon
fiber reinforced carbon: I
1065
2000
Wavenumbers, cm-’
Fig. 9. FT-IR
spectra
of mesogenic
and mesophase
pitches
2500
selected for the fabrication
of composites
(compare
Table 3).
2000
Wavenumbers, cm-’
Fig. 10. FT-IR
spectra
of mesogenic and mesophase pitches selected for the fabrication of composites after oxygen treatment at 2OO”C, 30 minutes residence time.
Table 4. FT-IR spectroscopy: peak ratios derived from Figs 1 and 9 3045 cm-‘/2962 cm-’ aromatic H/ methyl H
Pitch
3045 cm-r/2916 cm-r aromatic H/ methylene H
Pitch II
1.05
1.13
Pitch Pitch Pitch Pitch Pitch
0.82 0.95 1.01 0.94 0.79
0.93 1.03 1.08 1.05 0.91
(lb) (2) (3) (4a) (4b)
the mesogenic pitches (2), (3), and (4a) have formed more carbonyl groups than the mesophase pitches (lb) and (4b). As already mentioned, this cannot be seen in the ratios used above for characterization of
(compare
Table 3)
the aromaticity of a pitch. Nevertheless, it can be concluded that the reactivity of a mesogenic or a mesophase pitch towards oxygen decreases with increasing aromaticity. This conclusion is in accordance with observations made in the stabilization treatment of the green composites. For a similar oxygen uptake, mesophase pitches as compared to mesogenic pitches require extended oxygen treatments, but optimum stabilization in view of reduced bloating is achieved with lower oxygen uptake. These results will be presented in subsequent parts of this paper [ 22,231 4. DISCUSSION
The study has shown severe problems in the synthesis of a well-defined product from a technical raw
V. LIEDTKEand K. J. H~~TTINGER
1066
_ _._~.
_pYm-
4000
3500
3000
_~.
2000 2500 Wavenumbers, cm“
-~
--~
1500
1000
500
Fig. 11. FT-IR spectra of mesogenic and mesophase pitches selected for the fabrication of composites (compare Table 3) after oxygen treatment at 250°C. 30 minutes residence time. material. Attempts to identify the reaction behaviour of a pitch in pyrolysis by careful analysis of the properties were not very successful. Influences resulting from different operational procedures, such as opening of the reactor at different temperatures, could be clarified (Fig. 8). On the other hand, this experience was employed to synthesize a mesogenic and a mesophase pitch (pitches (4a) and (4b)) with property relationships that cannot be obtained otherwise. A ranking of the pitches in view of the usefulness in the fabrication of composites cannot be given at the current stage of the research. In view of the glass transition temperature, pitch (3) should be most promising, provided that the coke yield can be increased substantially by oxygen treatment. In view of the coke yield, pitch (4b) should be the most attractive one. The attractiveness of this pitch also results from the fact that it should contain a rather high percentage of “dormant” mesophase, which can form mesophase during infiltration of the fiber preform. According to its glass transition temperature (Fig. 6 (a)), a mesophase content of about 40 ~01% can be expected. With such a mesophase content the coke yield should be nearly 70%. The real potential of the pitches can only be evaluated after infiltration into a fiber preform and after optimized stabilization and carbonization treatment of the composites [ 271. Acknowledgement-Financial support of this study by the German Research Foundation (DFG) is gratefully acknowledged.
REFERENCES
1. K. J. Htittinger and H. Briickmann, BMFT-FBTSl-001 (1981)
Forschungshericht
2 H. Brhckmann, Doctoral Thesis, University of Karlsruhe ( 1979) 3. J. L. White and P. M. Shaeffer, Curbon 27, 697 (1989). and B. Fathollahi, 4. J. L. White, M. K. Gopalakrishnan Carbon 32, 201 (1994). Y. Korai, I. Mochida, K. Yanagida, M. 5. A. Sakanishi, Noda, I. Thunori and K. Tate, Carbon 30, 459 (1992). 6 K. Lafdi, S. Bonnamy and A. Oberlin, Carbon 29, 831 (1991). 7. K. Lafdi, S. Bonnamy and A. Oberlin, Carbon 29, 849 (1991). 8. K. Lafdi, S. Bonnamy and A. Oberlin, Carbon 29, 857 (1991).
9. M. Hamagushi International
10. 1 I. 12. 13. 14.
15. 16. 17. 18. 19. 20.
and T. Nishizawa, Carbon
Conference,
Proceedings
p. 45,
of the 5th
Essen,
Germany (1992) T. Nishizawa and M. Sakata, Carbon 30, 147 (1992). J. D. Brooks and G. H. Taylor, Carbon 3, 185 (1965). K. Christ and K. J. Hiittinger, Carbon 31, 731 (1993). K. Christ, Doctoral Thesis, University of Karlsruhe (1992) D. D. Edie. In Cnrbon Fibers Filaments and Composites (edited by J. L. Figueireido, C. A. Bernardo, R. T. K. Baker, and K. J. Hiittinger) p. 43. Kluwer Academic Publishers, Dordrecht, The Netherlands (1989) K. J. Hiittinger and J. P. Wang, Carbon 29, 439 (1991). K. J. Hiittinger and J. P. Wang, Carbon 30, 1 (1992). K. J. Hiittinger and J. P. Wang, Carbon 30, 9 (1992). M. Bernhauer, K. Christ, A. Gschwindt and K. J. Hiittinger, Carbon 30, 931 (1992). M. Bernhauer, Doctoral Thesis, University of Karlsruhe (1994) M. Bernhauer, M. Braun and K. J. Hiittinger, Carbon 32, 1073 (1994).
M. Braun and K. J. Hiitttinger, Carbon 33, 1359 (1995). K. J. Hiittinger and V. Liedtke, Carbon 34, 1067 (1996) K. J. Htittinger and V. Liedtke, Carbon 34, 108 1 (1996) S. Otani, U.S. Patent 4,472,265 (1985) DIN 51 921, in DIN Taschenbuch 221, Kohlenstoffmaterialien, pp. 76, 123, Beuth-Verlag, Berlin, Kiiln (1986) Meth26. M. Hesse, H. Meier and B. Zeeh, Spektroskopische oden in der organischen Chemie, 3rd. edition. Georg Thieme Verlag, Stuttgart, Germany (1987) 21. P. Chen, W.-J. Wang and P. R. Griffiths, Fuel 64, 307 (1985). 21. 22. 23. 24. 25.
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MESOPHASE PITCHES AS MATRIX PRECURSOR OF CARBON FIBER REINFORCED CARBON: I. MESOPHASE PITCH PREPARATION AND CHARACTERIZATION Institut
fur Chemische
Technik,
V. LIEDTKE and K. J. H~TTINGER Universitat Karlsruhe, KaiserstraDe 12, D-76128
Karlsruhe,
Germany
(Received 26 October 1995; accepted in revised form 5 March 1996)
Abstract-Mesogenic and mesophase pitches were produced from Ashland A 240 pitch in a stirred tank reactor under various pyrolysis conditions. Isothermal experiments showed strongly different properties of the products as compared to results with an earlier lot of the same pitch. The products were characterized by their solubility in tetrahydrofuran and N-methyl-pyrrolidone, their mesophase content, glass transition temperature, coke yield, and aromaticity. Pitch reactivities with oxygen in view of a stabilization treatment were analyzed by FT-IR. Copyright 0 1996 Elsevier Science Ltd
Key Words-Mesogenic reactivity.
pitch,
mesophase
pitch,
matrix
1. INTRODUCTION
Mesophase pitch was first used as a matrix precursor of carbon fiber reinforced carbon more than 15 years ago [l,Z]. In these studies, a fiber preform was impregnated with a standard coal tar binder pitch which was then transformed into mesophase pitch by thermal treatment. To overcome swelling of the composites by the bloating effect of pyrolysis gases, mechanical pressure was applied during carbonization up to 550°C. Later, White et al. showed that swelling of the composites during carbonization can be avoided by an oxygen treatment of the infiltrated pitch [3]. White et al. [4] additionally succeeded in infiltrating a fiber preform with an anisotropic pitch produced from alkylbenzenes by polymerisation and pyrolysis [5]; the structure of such pitches is discussed controversially in the literature [6-lo]. Some authors conclude that they are best described as anisotropic gels, which are different from the classic Brooks-Taylor mesophase as obtained by thermal pyrolysis [ 6-83, and some other authors support the opposite point of view [9,10]. In any case, these anisotropic pitches soften at much lower temperatures and exhibit a much lower viscosity than the classic Brooks-Taylor mesophase [ 111. Infiltration of a fiber preform with a thermally produced mesophase pitch containing about 40 ~01% mesophase spheres has recently been reported [ 12,131. A filter effect was not observed, but the green composites had to be treated with oxygen in order to prevent swelling during carbonization. Coke yields in the range of 85% were achieved. The present paper represents a continuation of this latter work [ 12,133. It is concerned with the development of mesophase and especially mesogenic pitches* *A mesogenic pitch is defined as a pitch which is still optically isotropic but converted to such a degree that it can form polyaromatic mesophase by any further thermal treatment immediately.
precursors,
property
relationships,
oxygen
with improved properties in respect to infiltration of a carbon fiber preform. Pitches with improved properties should (1) be homogeneous, i.e. single-phase materials such as a mesogenic pitch [ 14-171, or exhibit a very low content of mesophase spheres only, (2) soften at as low a temperature as possible, (3) produce a coke yield of at least 80%, and (4) form a graphitizable carbon matrix after pyrolysis. A major reason for doing this research was the fact that the Ashland A 240 petroleum pitch (pitch I) used in the previous studies [12,13,15-191 formed spherulitic mesophase in a unique manner as coalescence of mesophase spherulites by formation of bulk mesophase did not occur up to mesophase contents of at least 40 ~01%. With all later batches of Ashland A 240 (pitch II) coalescence of spherulites was already found to occur at mesophase contents as low as 20 ~01%. Such a material was shown not to be favourable for infiltration of a fiber preform [ 12,131. Mesophase pitches with high contents of spherulites, even from pitch II, can be produced by an iron-catalyzed pyrolysis [20, 211. Unfortunately, those pitches are less suitable for impregnation of a fiber preform; they show a severe filter effect. For development of more suitable pitches for the infiltration of a fiber preform, some special pyrolysis treatments of the A 240 petroleum pitch (pitch II) were performed: (1) pyrolysis under mild conditions, i.e. at low temperature, and (2) pyrolysis by fast heating to high temperature and opening of the reactor at this temperature with zero residence time. In the following, the synthesis and the properties of the synthesized pitches will be treated. Results on the stabilization chemistry of the pitches produced will also be presented. Stabilization treatments of the infiltrated pitches at elevated oxygen pressure and the usefulness of the pitches in the fabrication of carbon fiber reinforced carbons will be shown in two following parts of this paper [22,23].
1057
V. LIEDTKEand K. J. HUTTINCER
1058 2. EXPERIMENTAL
2.1 Pitch The studies are based on the petroleum pitch Ashland A 240. Typical properties of this pitch II are compiled in Table 1 and compared with those of the earlier batch pitch I [ 12,13,15-191. Some properties are identical, but some others, such as the glass transition temperature and the coke yield, are not. For a more detailed analysis of the pitches, FT-IR spectroscopy was used (see Section 2.3). The spectra of pitch I and pitch II are nearly identical (Fig. 1). This conclusion is supported by the ratios of the peak heights at 3045 cm-‘, corresponding to aromatic hydrogen, and (1) 2962 cm-‘, corresponding to methyl groups, and (2) 2916 cm-‘, corresponding to methylene groups: (1) 1.06 (pitch I) and 1.05 (pitch II), (2) 1.16 (pitch I) and 1.13 (pitch II) [25,26]. 2.2 Pyrolysis apparatus andprocedure For the pyrolysis treatments, a stirred tank reactor was used; it is described in an earlier paper 1151. A pitch sample of 300 g was heated under a pressure of 1 MPa argon with a rate of 10 K/min to the desired temperature. Isothermal experiments were performed by variation of the residence time; in these experiments the reactor was depressurized and opened at reaction temperature. In further experiments with
zero residence time, the reactor was depressurized and opened immediately after reaching the desired maximum temperature, or it was depressurized and opened after cooling to a lower temperature. 2.3 Analytical methods The pyrolysis residues were analyzed by extraction with tetrahydrofuran (THF) and N-methyl-pyrrolidone (NMP), determination of the mesophase content (MP) from polarized-light micrographs, the glass transition temperature (T,) by thermomechanical analysis (TMA), and the coke yield (CY) according to the method used in our laboratory. All analytical methods have been described in earlier papers [lS,lS]. The extractions with the solvents used yielded nearly identical results to those carried out with toluene instead of THF and quinoline instead of NMP, but the solvents used are less toxic. For further characterization of the mesophase pitches, FT-IR spectroscopy was used. Attempts to study the reactivity of the produced pitches with oxygen by differential scanning calorimetry (DSC) failed; the powdered materials fused before the reaction started. Therefore, thin layers of the produced pitches of about 1 mm thickness were treated with oxygen and analyzed by FT-IR spectroscopy. For all FT-IR studies a Nicolet spectrometer
Table 1. Properties of Ashland A 240 petroleum pitches
Pitch I Pitch II
C (%)
H (%)
S (%)
THF-IS (%)
NMP-I (%)
CY (%)
Tg(“C)
P (g/em3)
89.9 90.6
5.5 5.5
3.0 2.7
5.1 3.2
0 0
44.0 38.5
73.0 83.5
1.24 1.24
The content of carbon, hydrogen, and sulfur was analyzed by Mikroanalytisches Labor Paschen, D-53424 Remagen. THF-IS - insolubles in tetrahydrofuran (Soxhlet extraction). NMP-I - insolubles in N-methyl-pyrrolidone (determined according to Q-I, DIN 51 921 1251). CY - coke yield (determined with the method used in our laboratory: open crucible at 600°C). Tg- glass transition temperature (determined with Mettler TA 4000 thermomechanical analyzer) p ~~ powder density (determined with pycnometer method according to DIN 51 907 25.
Wavenumbers, cm-’
Fig. 1. FT-IR spectra of two different lots of Ashland A 240 petroleum pitch.
Mesophase
pitches as matrix
precursor
of carbon
model Magna IR 550 was used, the spectra were taken by diffuse reflectance (DRIFT). The absorption wavenumbers of relevant groups are compiled in Table 2. For characterization of a material, the ratios of the peak heights of aromatic hydrocarbon at 3045 cm-‘, methyl groups at 2962 cm-‘, and methylene groups at 2916 cm-’ were used.
Functional
I
1059
k, -MA=MP
and corresponding
FT-IR
absorption
frequencies
[26,27]
Absorption
methyl
CH2 (mostly
methylene
substituents)
hydrocarbons rings containing rings containing rings containing
linkages
between
aromatic
(1)
A + a = original pitch; a = fraction of volatiles escaping during heating to reaction temperature; PA = polyaromatics; MA = mesogenic aromatics; MP = mesophase. According to refs [ 16,18,19], the rate constants were considered as k, = k, = k. For the results with pitch I, k =0.426 h-l, for pitch II, k =0.263 h-‘. These rate constants differ almost by a factor of 2. In all isothermal experiments at 440°C the pyrolysis yields with pitch I and II were very similar and varied between 87 and 84% only. Thus, it can clearly be concluded that the different mesophase contents are a pure effect of pitch reactivities, at least in respect to formation of mesogenic aromatics and mesophase.
groups
CH3 (mostly
Aromatic Aromatic Aromatic Aromatic
k, pA
A+aZA-
3.1 Pyrolysis studies at 440” C Results of isothermal pyrolysis studies will mainly be shown in order to demonstrate the different reactivities of pitch I and pitch II in pyrolysis. Results with pitch I were taken from earlier publications [ 12,13,18,19]. The mesophase contents of the mesophase pitches produced at 440°C and various reaction times are presented in Fig. 2. The filled symbols (A, n) describe the results with pitch I found by two different authors [ 12,13,18,19]. As compared to these mesophase contents, significant lower mesophase
groups
carbon:
yields were found in the studies with pitch II (open points). The curves drawn in Fig. 2 were calculated using the reaction scheme proposed by Wang and Htittinger [ 161, eqn (1):
3. RESULTS
Table 2. Functional
fiber reinforced
frequencies 2962, 2866 1444, 1377 2916, - 2850 1478 3078-3035 750 815 875
rings)
4 adjacent hydrogen atoms 2 adjacent hydrogen atoms isolated hydrogen atoms
C=O (aldehydes, ketones, carboxyl groups) C=O (aromatic/aliphatic esters) -C = 0 of aromatic cyclic anhydrides
U-c=0 “X=0 U-c=0
- 1700 - 1735* 1840 and 1775
-C=C
"-c=c
1600
(aromatic
rings)
OH (free) OH (hydrogen-bonded)
- 3550 -3400-3450
“OH “Ml
0
1
2
3
4
5
t, h Fig. 2. Mesophase
content
as a function
of residence
time at 44o”C, 1 MPa Ar. A,
n , pitch
pitch II.
(cm-‘)
V. LIED~KE and K. J. HUTTINGER
1060
The different pyrolysis behaviour of the pitches from different lots also follows from the content of insolubles of the pyrolysis residues in THF (THF-IS) and NMP (NMP-I), which are shown in Fig. 3 (a) and (b). The continuous increase of THF-IS (Fig. 3 (a)) and the sigmoid increase of NMP-I (Fig. 3 (b)) with increasing residence time as found with pitch I have been reproduced in all studies with this pitch [12,13,15-191. Completely different dependencies exist for pitch II. At short residence time, the content of insolubles in both solvents is higher than that of pitch I. This result and the delayed increase of insolubles with progressing pyrolysis time have never been observed with other pitches, not even with coal tar pitch [21]. Figure 4 shows a plot of the NMP-I values vs mesophase contents of the pitches obtained with pitch I and pitch II. With pitch I, a linear correlation exists over the total range of mesophase contents. The slope of the curve is nearly identical to that of the diagonal. It indicates that about 10% of mesophase of all pitches are soluble in NMP. With pitch II, the mesophase and NMP-I contents are nearly identical. At low mesophase contents, the NMP-I content is
Fig. 3. Content
of insolubles
in THF (a) and NMP
even slightly higher. This result is more surprising because a plot of the coke yield as a function of the mesophase content yields an inverse correlation. Figure 5 (a) and (b) shows the coke yield and the glass transition temperature of the mesophase pitches. The coke yields of the mesophase pitches from pitch I determined by two different authors [ 12,13,18,19] are nearly identical (Fig. 5 (a)), but not the glass transition temperatures (Fig. 5 (b)). At short reaction times, i.e. at low conversion of the pitch, they differ by about 10 K. The reason is known: Christ [ 12,131 (A) opened the reactor at 44O”C, Bernhauer [ 18,191 ( n) only after cooling to 400°C. One or two per cent more volatiles, which can escape at 44O”C, but not at 4OO”C, obviously have a strong influence on the glass transition temperature, but they do not have a measurable effect on the further properties analyzed. Figure 5 (a) and (b) also shows that the coke yield and the glass transition temperature of the mesophase pitches from pitch II are lower than those found with mesophase pitches from pitch I. With increasing pyrolysis time, the differences increase. In view of the high content of insolubles, and especially of NMP-I
(b) as a function 0, pitch II.
of residence
time at 44o”C, 1 MPa Ar. A, W, pitch I;
Mesophase
pitches as matrix
-0
precursor
of carbon
25
fiber reinforced carbon: I
50
1061
75
MP, ~01% Fig. 4. Content
of NMP-I
as a function
45 c
35
of mesophase content; products synthesized W, pitch I; 0, pitch II.
-----pitch
I--
L----
60 0
residence
times.
I (= A)
- - - - - pitch II 1
I
at 440°C and different
pitch
( o )
I
I
I
4
I (mu)
I
1
4
I
I
1
2
3
4
5
I
t, h Fig. 5. Coke yield (a) and glass transition
temperature
(b) as a function I; 0, pitch II.
at short residence time (Fig. 3 (a) and (b)), the low glass transition temperatures and the low coke yields are difficult to understand. Unfortunately, mesophase pitch samples produced with pitch I and sufficient
of residence
time at 44o”C, 1 MPa Ar. A,
n , pitch
quantities of pitch I to reproduce the samples were no longer available. Therefore, their molecular structure as compared to mesophase pitches produced with pitch II could not be analyzed (see Table 1).
V. LIEDTKE and K.J.
1062
3.2 Pyrolysis studies under varied conditions In Fig. 6 (a), the dependence of the glass transition temperature on the mesophase content of various pitches is shown. The full symbols represent again results of Christ et al. [ 12,131 and Bernhauer et ul. [1X,19] with pitch I. These results reflect the typical correlation between glass transition temperature and mesophase content found in all earlier studies [ 12,13,15-191. The glass transition temperature increases slightly with the mesophase content. The bend in the curve is ascribed to the phase inversion after which the continuous phase is formed by the mesophase [ 12,13,15- 191. At equal mesophase content, mesophase pitches obtained with pitch II have lower glass transition temperatures than mesophase pitches produced from pitch I. This property relationship should be advantageous. In fact it is a disadvantage because the mesophase spheres coalesce at lower contents. To underline this fact, polarized light optical micrographs will be presented later. The optimum pitch used by Christ [ 12,131 is marked by an arrow (la); it only exhibits spherulitic mesophase. The pitch produced at the same temperature of 440°C from pitch II and used for the fabrication of compos-
75
H~~TTINGEX
ites in this study is marked by an arrow (lb); it already contains bulk mesophase (see Fig. 7). Further materials which were used for the fabrication of composites are also marked by arrows. The materials (2), (3), and (4a) are designated as mesogenie pitches because their mesophase content is lower than 5 ~01%. They were produced at 440°C with 0.5 hours residence time (2), at 400°C with 3 hours residence time (3), and at 460°C without residence time (4a). In all three cases, the reactor was depressurized and opened at pyrolysis temperature. Material (4b) was synthesized in a similar way as material (4a), but at 490°C. Pitches (4a) and (4b) are of interest because of their low mesophase content as compared to their high glass transition temperature. The reason for this unusual property relationship has to be seen in an unusual high content of highmolecular, i.e. mesogenic aromatics in the isotropic pitch phase. The different glass transition temperatures not only of the pitches (4a) and (4b), but also of the pitches (2) and (3) are a result of the synthesis conditions, which lead to very different pyrolysis yields. The yield values and the properties of all pitches used for further studies are summarized in
(b)
-.
U
I
I
25
50
J 75
MP, ~01% Fig. 6. Glass transition temperature (a) and coke yield (b) as a function of mesophase content of various pyrolysis products. Full symbols: pitch I, open symbols: pitch II. Arrows indicate pitches selected for the fabrication of composites.
Mesophase
Fig. I. Polarized-light
optical
pitches as matrix
micrographs
precursor
of pyrolysis
of carbon
products
fiber reinforced
carbon:
used for the fabrication
I
1063
of composites.
The pyrol lysis
conditions are compiled in Table 3. 3; polarized-light optical micrographs of these ials are presented in Fig. 7. Ma terials produced with a high yield exhibit a low
Table mater
Table 3. Properties
of the pitches
glass transition temperature. Opening of the reac :tor at a high or very high temperature allows volat .iles to escape. The residue is relatively enriched vvith used for fabrication
of composites
Pitch no.
Pyrolysis temperature (“C)
Residence time (h)
Residue yield (%)
THF-IS (wt%)
NMP-I (wt%)
(la) (lb) (2) (3) (4a) (4b)
440 440 440 400 460 490
2.0 3.5 0.5 3.0 0 0
85.0 84.3 84.8 92.3 80.8 74.4
47.9 53.8 32.6 4.4 22.1 49.6
31.3 7.8 3.6 8.7 31.9
*
T, (“C)
CY
(TMA)
MP (vol%)
(wt%)
120.0 97.0 85.9 74.6 104.5 119.7
30 32.5 3 3 5 20
64.1 60.3 50.6 46.9 53.8 63.8
1064
V. LIEDTKE and K. J. H~JTTINGER
aromatics of high molecular weight, but mesophase could not be formed because of rapid cooling down of the pitch. This suggests that especially the materials (4a) and (4b) have a substantial amount of “dormant” mesophase. “Dormant” mesophase means mesogenic polyaromatics dissolved in the isotropic pitch phase in a concentration beyond the saturation solubility [ 241. Such materials should be favourable for infiltration of fiber preforms and give a high coke yield. Coke yields as a function of the mesophase content are presented in Fig. 6(b). Identical results of Bernhauer et al. [ 18,191 and Christ et al. [ 12,131 with pitch I show that the coke yield is less sensitive to the opening of the reactor than the glass transition temperature. On the other hand, the more drastic procedure in producing pitches (4a) and (4b), namely opening of the reactor at 460 and 490°C respectively, can also be seen in relatively higher coke yields. The effect of the temperature at which the reactor was opened was analyzed as follows: the pitch was generally heated to 490°C and the reactor was opened (a) at 490°C; (b) after cooling down to 440°C and (c) to 360°C. The results are shown in Fig. 8. The residue yield decreases strongly with increasing opening temperature, the effect on the increase of the glass transition temperature is even more pronounced. This corresponds to the results presented above. The coke yield increases as the residue yield decreases, in such a way that the product of coke yield and residue yield is nearly independent of the opening temperature.
3.3 Characterization ofpitches with infrared spectroscopy 3.3.1 Pitches used for fabrication of composites FT-IR transmission spectra of pitches (lb), (2), (3) (4a), and (4b) are presented in Fig. 9. The similarity is obvious. For a quantitative characterization, the ratios between the peak heights of aromatic
Opening
hydrogen and (1) methyl groups, and (2) methylene groups are shown in Table 4. All ratios are in the range of 1, but clearly lower than those found for the starting pitch (see Section 2.1). This leads to the first conclusion that the ratio between the concentrations of methyl and methylene groups is nearly constant, independent of the pyrolysis treatment or the conversion of the starting pitch. The mesogenic pitches (2) (3) and (4a) exhibit similar ratios between the content of aromatic hydrogen and the content of methyl or methylene groups. As compared to the starting pitch the ratios are only slightly decreased. Clearly lower values of these ratios were found with pitch (lb) and especially with pitch (4b). Therefore, it can further be concluded that the aromaticity of mesogenic pitches is only slightly higher than that of the starting pitch, but pitches (lb) and (4b) exhibit a clearly higher aromaticity.
3.3.2 Pitches
after
treatment
with
oxygen
Analogous FT-IR studies were performed after oxygen treatment at ambient pressure and temperatures of 200 and 250°C with 30 minutes of residence time (see Section 2.3). The results are shown in Fig. 10 and Fig. 11. As far as the ratios used for characterization of the pitches before oxygen treatment are concerned, no significant changes could be found. The effect of the oxygen treatment is reflected by the carbonyl peak at about 1700cm~’ after both treatments at 200 and 250°C. The appearance of very small peaks at 1840 cm-’ and 1775 cm-’ corresponding to anhydrides was only detected after treatment at 250°C. Ester peaks appearing at about 1740 cm-’ are not a consequence of the oxygen treatment, they were already found with the starting pitch (see Fig. 1). Because of the shoulder resulting from the esters, a quantitative evaluation of the carbonyl peak at 1700cm-’ is not possible. A rough estimate of the carbonyl peak heights leads to the conclusion that
temperature,
“C
Fig. 8. Effect of opening temperature of the reactor on the residue yield (*), glass transition temperature (+), coke yield (A), and the product of residue yield and coke yield (A) of the mesophase pitches. Pyrolysis at 49O”C, 1 MPa Ar, zero residence time.
Mesophase
pitches as matrix precursor
2500
of carbon
fiber reinforced carbon: I
1065
2000
Wavenumbers, cm-’
Fig. 9. FT-IR
spectra
of mesogenic
and mesophase
pitches
2500
selected for the fabrication
of composites
(compare
Table 3).
2000
Wavenumbers, cm-’
Fig. 10. FT-IR
spectra
of mesogenic and mesophase pitches selected for the fabrication of composites after oxygen treatment at 2OO”C, 30 minutes residence time.
Table 4. FT-IR spectroscopy: peak ratios derived from Figs 1 and 9 3045 cm-‘/2962 cm-’ aromatic H/ methyl H
Pitch
3045 cm-r/2916 cm-r aromatic H/ methylene H
Pitch II
1.05
1.13
Pitch Pitch Pitch Pitch Pitch
0.82 0.95 1.01 0.94 0.79
0.93 1.03 1.08 1.05 0.91
(lb) (2) (3) (4a) (4b)
the mesogenic pitches (2), (3), and (4a) have formed more carbonyl groups than the mesophase pitches (lb) and (4b). As already mentioned, this cannot be seen in the ratios used above for characterization of
(compare
Table 3)
the aromaticity of a pitch. Nevertheless, it can be concluded that the reactivity of a mesogenic or a mesophase pitch towards oxygen decreases with increasing aromaticity. This conclusion is in accordance with observations made in the stabilization treatment of the green composites. For a similar oxygen uptake, mesophase pitches as compared to mesogenic pitches require extended oxygen treatments, but optimum stabilization in view of reduced bloating is achieved with lower oxygen uptake. These results will be presented in subsequent parts of this paper [ 22,231 4. DISCUSSION
The study has shown severe problems in the synthesis of a well-defined product from a technical raw
V. LIEDTKEand K. J. H~~TTINGER
1066
_ _._~.
_pYm-
4000
3500
3000
_~.
2000 2500 Wavenumbers, cm“
-~
--~
1500
1000
500
Fig. 11. FT-IR spectra of mesogenic and mesophase pitches selected for the fabrication of composites (compare Table 3) after oxygen treatment at 250°C. 30 minutes residence time. material. Attempts to identify the reaction behaviour of a pitch in pyrolysis by careful analysis of the properties were not very successful. Influences resulting from different operational procedures, such as opening of the reactor at different temperatures, could be clarified (Fig. 8). On the other hand, this experience was employed to synthesize a mesogenic and a mesophase pitch (pitches (4a) and (4b)) with property relationships that cannot be obtained otherwise. A ranking of the pitches in view of the usefulness in the fabrication of composites cannot be given at the current stage of the research. In view of the glass transition temperature, pitch (3) should be most promising, provided that the coke yield can be increased substantially by oxygen treatment. In view of the coke yield, pitch (4b) should be the most attractive one. The attractiveness of this pitch also results from the fact that it should contain a rather high percentage of “dormant” mesophase, which can form mesophase during infiltration of the fiber preform. According to its glass transition temperature (Fig. 6 (a)), a mesophase content of about 40 ~01% can be expected. With such a mesophase content the coke yield should be nearly 70%. The real potential of the pitches can only be evaluated after infiltration into a fiber preform and after optimized stabilization and carbonization treatment of the composites [ 271. Acknowledgement-Financial support of this study by the German Research Foundation (DFG) is gratefully acknowledged.
REFERENCES
1. K. J. Htittinger and H. Briickmann, BMFT-FBTSl-001 (1981)
Forschungshericht
2 H. Brhckmann, Doctoral Thesis, University of Karlsruhe ( 1979) 3. J. L. White and P. M. Shaeffer, Curbon 27, 697 (1989). and B. Fathollahi, 4. J. L. White, M. K. Gopalakrishnan Carbon 32, 201 (1994). Y. Korai, I. Mochida, K. Yanagida, M. 5. A. Sakanishi, Noda, I. Thunori and K. Tate, Carbon 30, 459 (1992). 6 K. Lafdi, S. Bonnamy and A. Oberlin, Carbon 29, 831 (1991). 7. K. Lafdi, S. Bonnamy and A. Oberlin, Carbon 29, 849 (1991). 8. K. Lafdi, S. Bonnamy and A. Oberlin, Carbon 29, 857 (1991).
9. M. Hamagushi International
10. 1 I. 12. 13. 14.
15. 16. 17. 18. 19. 20.
and T. Nishizawa, Carbon
Conference,
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of the 5th
Essen,
Germany (1992) T. Nishizawa and M. Sakata, Carbon 30, 147 (1992). J. D. Brooks and G. H. Taylor, Carbon 3, 185 (1965). K. Christ and K. J. Hiittinger, Carbon 31, 731 (1993). K. Christ, Doctoral Thesis, University of Karlsruhe (1992) D. D. Edie. In Cnrbon Fibers Filaments and Composites (edited by J. L. Figueireido, C. A. Bernardo, R. T. K. Baker, and K. J. Hiittinger) p. 43. Kluwer Academic Publishers, Dordrecht, The Netherlands (1989) K. J. Hiittinger and J. P. Wang, Carbon 29, 439 (1991). K. J. Hiittinger and J. P. Wang, Carbon 30, 1 (1992). K. J. Hiittinger and J. P. Wang, Carbon 30, 9 (1992). M. Bernhauer, K. Christ, A. Gschwindt and K. J. Hiittinger, Carbon 30, 931 (1992). M. Bernhauer, Doctoral Thesis, University of Karlsruhe (1994) M. Bernhauer, M. Braun and K. J. Hiittinger, Carbon 32, 1073 (1994).
M. Braun and K. J. Hiitttinger, Carbon 33, 1359 (1995). K. J. Hiittinger and V. Liedtke, Carbon 34, 1067 (1996) K. J. Htittinger and V. Liedtke, Carbon 34, 108 1 (1996) S. Otani, U.S. Patent 4,472,265 (1985) DIN 51 921, in DIN Taschenbuch 221, Kohlenstoffmaterialien, pp. 76, 123, Beuth-Verlag, Berlin, Kiiln (1986) Meth26. M. Hesse, H. Meier and B. Zeeh, Spektroskopische oden in der organischen Chemie, 3rd. edition. Georg Thieme Verlag, Stuttgart, Germany (1987) 21. P. Chen, W.-J. Wang and P. R. Griffiths, Fuel 64, 307 (1985). 21. 22. 23. 24. 25.