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Preparation of general purpose carbon fibers from coal tar pitches with low softening point
Carbon Vol. 35,No.&pp. 1079%1087,1997 0 1997ElsevierScienceLtd
Pergamon
Printed in Great Britain. All rights reserved 000%6223197 $17.00+ 0.00 PII: SOOOS-6223(97)00064-X
PREPARATION OF GENERAL PURPOSE CARBON FIBERS FROM COAL TAR PITCHES WITH LOW SOFTENING POINT J. ALCA~IZ-MONGE,~
D. CAZORLA-AMOR~S,~
A. LINARES-SOLANO,~‘* A. OYA,~
A. SAKAMOTO~ and IS. HOSHI~ aDepartamento de Quimica Inorginica, Universidad de Alicante, Alicante, Spain bDepartment of Materials Science, Gunma University, Gunma, Japan (Received 22 November 1996; accepted in revisedform
6 March 1991)
Abstract-This paper describes the different procedures applied for the preparation of general purpose carbon fibers from four coal tar pitches. The four raw materials have a low softening point (of about 373 K) for this application and, hence, must be subjected to a treatment before spinning to increase this temperature. The treatments applied are the following: (i) heating in N,, (ii) heating in air, (iii) consecutive heating in N, and air, and (iv) blending of the coal tar pitch with a petroleum one and further treatment in air. The changes in chemical composition, softening point and yield of the treatments used have been followed by different techniques. Only treatments (iii) and (iv) produce materials with an adequate viscosity for spinning and a sufficiently high softening point to be transformed in carbon fibers. The carbon fibers obtained have similar mechanical properties to those prepared in a previous work from a suitable petroleum pitch with the same experimental system. However, the mechanical properties of these fibers are inferior to those of a commercial carbon fiber, the differences being due to the lower diameter of the commercial fibers. 0 1997 Elsevier Science Ltd Key Words-A.
Carbon
fiber, A. coal tar pitch, C. infrared
spectroscopy,
properties.
them to an additional treatment before spinning to increase their SP. In this paper, the different treatments used to obtain adequate starting materials for carbon fiber preparation (i.e. materials with a SP sufficiently high and a good spinnability) are discussed. Thus, several techniques, some conventional and other novel, have been applied for the development of spinnable pitches. In this way, GPCF of acceptable properties have been finally prepared from the four starting pitches.
I. INTRODUCTION
As it is well known, carbon fibers are obtained from different raw materials such as polyacrylonitrile, rayon, pitches, resins [ 11, and gases such as methane and benzene [2]. The first pitch-based carbon fibers were prepared in 1963 by Otani and coworkers [3]. Nowadays, it is expected that these materials will gain considerable industrial importance [ 11. The pitches used as starting materials for the preparation of carbon fibers are byproducts of the coke-making and petrochemical industries. For this reason, these materials have the advantage of being cheap precursors of carbon fibers. Two types of carbon fibers based on pitches can be differentiated: general purpose carbon fibers (GPCF) and high performance carbon fibers (HPCF) [4]. The first are obtained from conventional isotropic pitches, while the second are prepared from mesophase pitches. The HPCF are quite expensive compared with the GPCF and their applications are mainly limited to aerospace and sport goods industries. On the other hand, the GPCF have many potential applications due to their much lower price
2. EXPERIMENTAL
2.1 Materials Four commercially available coal tar pitches from two Spanish companies (Bilbaina de Alquitranes S.A. and Industrial Quimica de1 Nalon) and a commercial isotropic petroleum pitch (Showa-Shell Co. Run-18) have been used as raw materials. Table 1 summarizes
Table 1. Some
Elemental
151.
In this work, the study carried out on the preparation of GPCF from commercially available coal tar pitches is presented. All the pitches used have a low softening point (SP) that impedes the stabilization process of the pitch fiber necessary to obtain the carbon fiber. Therefore, it is necessary to submit *Corresponding
D. mechanical
A B C D Pa
properties of the coal tar pitches petroleum pitch analysis
(wt%)
1079
Insolubles
(wt%)
the
C
H
N
S+Odi,
H/C
Tl
QI
SP (K)b
92.13 91.85 92.86 92.90 93.30
4.46 4.79 4.32 4.33 6.60
0.77 0.79 0.70 0.68 0.00
2.64 2.65 2.13 2.19 0.1
0.58 0.62 0.56 0.56 0.81
44 19 40 41 0.3
22 3 27 22 0
383 346 393 378 352
aP means petroleum pitch. bMeasured by DTA or TMA.
author.
and
J. ALCA~IZ-MONGE et ul.
1080
some of their properties (elemental analysis, softening point (SP), insoluble in quinoline (QI) and insoluble in toluene (TI )). It can be observed that the SP’s of the coal tar pitches are very low as to be used in the preparation of carbon fibers, due to the difficulty of the stabilization stage of the pitch fibers that would be obtained. On the other hand, the coal tar pitches contain an important amount of infusible matter (i.e. primary QI ) that distorts seriously a continuous spinning. Because, pitch B is the coal tar pitch which has the smallest content in QI (see Table I), the treatments studied in this work are mainly focused on this pitch. Then, the most promising methods are applied to the remaining pitches (A, C and D).
Treatments carbonjbers
2.2
to trunsjkm
the pitches into
The infusible matter was removed by filtration of the fraction of the pitch insoluble in quinoline or by hot filtration of the melted pitch. In this last case, a steel mesh (50 pm) was used and the filtration was done between 3733423 K. The resulting material was employed as the starting one for the treatments that are detailed below. Pitch B was subjected to the following heat treatments: i) heating in air; ii) heating in N,; iii) consecutive heating in N, and air, and iv) blending of pitch B with a petroleum pitch (ratio 1 : 1 in weight) and subsequent treatment in air. The heat treatments were performed at different temperatures (between 423 and 773 K) and times (no longer than 2.5 hours), using air and N, flows of 100 ml mini i. In all cases, 5 g of pitch were used and the reproducibility of the treatments, from the point of view of yield and spinnability, was good. Once the pitches were treated, the general procedure used for the preparation of carbon fibers was the following. Only the samples with SP higher than 453 K were spun; the samples with SP smaller were not used because of the difficulties that would involve their subsequent stabilization. A monofilament spinning apparatus was used to obtain the pitch fibers. The diameter of the nozzle of the spinning apparatus, that determines the diameter of the fibers, was 1.5 mm. The spinning temperature was 50-100 K higher than the SP of the spinnable pitch. After that, the pitch fibers obtained were stabilized in a flow of air (500 ml min-‘, STP) using the following stabilization programme that lasted 6 hours: ( 1) Heating at 5 K min- ’ to 403 K. to 493 K, maintaining this (2) Heating at 1 K mini temperature for 1 hour. (3) Heating at 1 K min-’ to 573 K, maintaining this temperature for 2 hours. In the carbonization stage, the stabilized pitch fibers were heated at 10 K mini to 1273 K in a N, atmosphere (80 ml min-‘, STP), maintaining this temperature for 30 minutes.
2.3 Techniques oj’charactertation Information about the softening point was assessed by differential thermal analysis, DTA (Setaram TG-DCS92), or by thermomechanical analysis, TMA (Rigaku TM ~ 8140). In the second case, the penetration method was used. The diameter of the pin and the load were 1 mm and 500 mg, respectively. A heating rate of 5 K mini and a N, flow of 60 ml mini 1 (STP) were used in both techniques. The elemental analysis of the samples was performed by a conventional method. The FTIR spectra of the powdered samples were obtained using diffuse reflectance (Nicolet 510P). The molecular weight of the fraction of the pitch soluble in THF was determined by an osmometric method. The fibers were observed by scanning electron microscopy, SEM. (JEOL JSM-35C). The mechanical properties of the fibers were measured in a Tensilon equipment U-II. The load-elongation curves were obtained by applying a stretching speed of 2 mm min 1 until fiber failure. The tensile strength (G). fracture elongation (E) and Young’s modulus (E) were estimated from the following equations g= P/A t = SL/L E=CT/E where P means the load at fracture, A is the crosssectional area of the fiber, 6L is the elongation at fracture and L is the gauge length. Thirty measurements of each carbon fiber material were done. In each measurement, the gauge length used was 2.5 cm and the diameter of the fiber after failure was obtained by SEM. 3. RESULTS AND DISCUSSION
3.1 Heat treatment
in N,, air and N, + air.
Heat treatments of the pitches in N, or air are used for the preparation of suitable precursors of GPCF [6-I 11. An excessive polymerization of the pitch during the treatment with N2, as a consequence of a long reaction time, may lead to the formation of mesophase spheres and, finally, to a mesophase pitch [ 121 which is not adequate for the preparation of GPCF. A treatment in air produces a considerable increase in the softening point and viscosity compared with the starting material. Barr and Lewis [ 131 have observed that a heat treatment in air results in the oxidation of the pitch molecules through a dehydrogenative polymerization without the formation of oxygenated products. Additionally, Brooks and Taylor [ 141 have confirmed that oxidation of pitch strongly affects the subsequent development of mesophase spheres. In this way, the formation of mesophase could be avoided after some degree of oxidation. Considering the results mentioned above obtained
General purpose carbon fibers from the literature, both treatments in air or N, have been applied to transform the coal tar pitches in materials with sufficiently high SP and adequate spinnability for the preparation of GPCF. The results obtained are commented on in the following discussion.
1081
5oo SP (K)
3.1 .l Yield of the process, evolution of SP and extraction with solvents.. Figure 1 shows the evolution of yield and SP with the treatment temperature for samples treated in N, or air for 1 hour. In both cases, the weight loss starts at about 473 K and there are no important differences between both treatments in the evolution of the yield. On the contrary, the change of SP with treatment temperature of pitch is quite different in air and N,. Thus, a heating in N, results in a small increase of SP only after a treatment at temperatures higher than 623 K, while a heating in air produces this effect at lower temperatures (at about 523 K). It must be noted that only the treatment in air produces samples with SP values sufficiently high (453-473 K) for subsequent stabilization in air of the resulting pitch fiber in a period of reasonably short time. The content in QI increases with the treatment temperature and is related to the SP of the samples [ 151. Because the fraction insoluble in quinoline is constituted by the part of the pitch formed by molecular structures with a high degree of condensation, the larger the amount of QI, the higher the SP of the pitch (see Fig. 2). This result is in concordance with what was previously observed by other authors [ 9,131. 3.1.2 Elemental analysis and FTIR spectroscopy. Elemental analysis and FTIR spectroscopy provide further information about the effect of the N2 and air treatments. Figure 3 shows the evolution of the ratio H/C after heat treatment in air and nitrogen, together with the change of the SP for comparison purposes. It is observed that the evolution of the ratio H/C with temperature is, in both cases, related with that followed by the SP. The fact that the samples heated in air exhibit a more important decrease in the ratio H/C than those Yield (%)
SF’ (K) Y
n
rSO0
Nitrogen blowing
703.
3oo Temperature
(K)
Fig. 1. Evolution of yield and SF’ of the residue obtained from pitch B, with treatment temperature in N, ( n ) and air (0) (all points correspond to treatments for 1 hour).
Fig. 2. Evolution of .SP of samples obtained after air-blowing for 1 hour at different temperatures versus the content in quinoline
insolubles.
500
600
0.64 0.62 0.60 0.58 0.56 0.54 0.52 0.50
I 300
’ 300 400
Temperature
700
(K)
Fig. 3. Evolution of H/C ratio and SP of the residues obtained from pitch B with treatment temperature in N, (m) and air (0) (all points correspond to treatments for 1 hour). treated in nitrogen at a similar temperature, is indicative of an oxidative effect. In spite of the interesting information that provides the H/C ratio, from these results no conclusion about the origin of the differences between N, and air can be extracted. In this sense, it is worth mentioning that two samples with the same H/C ratio, but obtained with different treatment (air or N,), present a large difference in their SP (see Fig. 3). Thus, for example, the sample obtained after heating in air at 623 K has a H/C ratio of 0.53 and a SP of 461 K. However, the sample treated with N2 having the same H/C ratio, obtained after heating at 723 K, has a SP of 387 K. The interpretation of these aspects can be carried out more efficiently by using a spectroscopic technique like FTIR. Figure 4 shows the FTIR spectra obtained for the original pitch, for the material obtained after heat treatment in air at 623 K for 1 hour and for the sample prepared after a similar treatment in N,. The FTIR spectra show that the treatment in air does not result in the formation of oxygen groups, in agreement with previous results [9,13]; the bands between 1650 and 1860 cm-’ (characteristics of carbonyl and carboxyl groups), between 3500 and 3300cm-’ (characteristic of O-H bonds) and between 1340 and 1110 cm-’ (related with C-O-H
J.
1082
3500
3000
ALCAGIZ-MONGE
2500
2000 Wavenumbers
Fig. 4. FTIR spectra of pitch B and the samples obtained and C-O-C structures) remain nearly unchanged or, even, decrease. The zone of the spectra obtained for the different samples corresponding to the C-H stretching bands (between 3100 and 2800 cm-i), has been analyzed in more detail by calculating the change of the ratio between the aromatic and the aliphatic hydrogens (Har/Hal). This ratio has been obtained by dividing the area of the band at 3050 cm-’ and that for the bands in the region between 2960 to 2800 cm- ‘. An increase in this ratio is indicative of an increase in the degree of aromatization of the sample. Figure 5 presents the evolution of the ratio Har/Hal calculated from the spectra, for the samples obtained after heating the original pitch B in air or N,. The figure contains experiments done
10
HarolHali
I
n
Nitrogen
0
693
n 673
K
I
I
30
45
ef ul
0
K 55
Time (min) Fig. 5. Evolution of Har!Hal ratio of the samples obtained after heating in N, ( n ) and air (0) for different periods of time and temperatures.
I5Uil
IOO~1
(cm-l)
after air and N, treatments
at 623 K for 1 hour
at two temperatures (673 and 693 K) as a function of the treatment times (lower than 1 hour). It must be noted that the value of the Har/Hal ratio included in the figure for a treatment time of 0 minutes, corresponds to the pitch B without any treatment. Figure 5 clearly shows that the treatment in nitrogen produces a slight variation in the ratio Har/Hal. in the range of temperatures (4233773 K) and treatment times studied, in spite of the strong devolatilization observed in Fig. 2. On the contrary, the treatment in air produces a notable change in this ratio. It is also observed that the most important decrease in the fraction of aliphatic C-H bonds happens during the first 10 minutes of treatment. This result is indicative of the high reactivity of such CH bonds in air. As a summary, the results obtained show that the heat treatment in nitrogen mainly produces physical processes, such as volatilization of light fractions of the pitch, and chemical processes, such as cracking of aliphatic groups and/or aromatic rings with a lower degree of condensation. Additionally, the treatment in air results in an important reaction with aliphatic hydrogens and with a significant attack of aromatic hydrogens, if the reaction time is sufficient, favoring polycondensation reactions [ 131. This produces in the treated material a larger increase in QI and SP than N,. These conclusions agree with the work of other authors [9-l I]. 3.1.3 Spinnubility of‘ the treated coul tur pitches. The samples obtained with SP higher than 453 K, i.e. pitch treated in air, were spun to check the possibility of obtaining pitch fibers. The problem
General purpose carbon of these samples was their high viscosity, compared with those obtained after a treatment with N,, this made their spinning very difficult. Furthermore, though some samples were spun (this is the case for that treated at 623 K for 1 hour, that was successfully spun at 550 K), a considerable dispersion resulted in the diameter of the obtained fibers. On the other hand, the treatment with N,, that produces samples with an adequate viscosity for spinning, does not provide suitable materials to obtain finally carbon fibers because of the low increase in SP observed. Considering the different effect of N, and air on the properties of the resulting pitch, a novel combination of both treatments has been applied in this study to gain a better control of the spinnability and SP of the material. The method consists in combining both treatments in air and Nz: i) a heating in N,, that results in the evolution of the volatile matter with low molecular weight, and ii) a subsequent heating in air to favor the polycondensation reactions and to increase considerably the SP. Different temperatures and reaction times have been studied. This combination allows us to shorten the time of the treatment in air and, hence, to control the spinnability of the material. This treatment has been applied to the four coal tar pitches (A,B,C and D) and has resulted in samples with SP between 453-473 K with a good spinnability. The treatments have been done at 663 K in all the cases and for the following periods of time: pitch A - 1 hour in N,, 0.5 hours in air; pitch B - 2 hours in N2, 0.5 hours in air; pitch C 2 hours in N,, 0.5 hours in air; pitch D - 1.5 hours in N,, 0.5 hours in air. The pitch fibers obtained were stabilized and carbonized using the stabilization programme described in the experimental section.
of the coal tar pitches is lower than that of the petroleum pitches (see for example Table 1). According to the above comments, this difference makes more difficult the transformation of a coal tar pitch into pitch fibers than from a petroleum pitch. Hence, to obtain pitch fibers with an improved spinnability and stabilizability it should be necessary to increase the content of hydrogen of the starting coal tar pitches Several studies have been accomplished to introduce alkyl groups in coal tar pitches, especially in form of saturated rings, which confer to the pitch a high reactivity toward oxidation and a low viscosity [ 18-201. Considering that the petroleum pitch used in this study is easily spinnable and sufficiently reactive during the stabilization stage [21] and that it has a higher content in hydrogen (mainly of aliphatic type) than the coal tar pitches (see Table l), the following approach has been used. The petroleum pitch has been blended with the coal tar pitch B (ratio 1:l in weight) to obtain a material with improved characteristics to prepare GPCF that is novel from the point
300
3.2 Blending of a coal tar pitch with a petroleum pitch: treatment in air Considering the above study, the most important problem of the preparation of GPCF from these coal tar pitches is that the treatment in air, although produces samples with a high SP, their spinnability is bad due to their high viscosity. In this sense, it was necessary to shorten as much as possible the contact of the sample with air by applying a previous treatment in N,. An additional difficulty of the coal tar pitches is the lower reactivity of the pitch fibers prepared from coal pitches than those obtained from a petroleum pitch. For this reason, the stabilization of pitch fibers prepared from coal pitches requires more time than in the case of petroleum pitch fibers. It has been shown that the content of hydrogen determines the viscosity of the pitches because it is related with the content of aliphatic groups [ 161. In this sense, the viscosity decreases with increasing the content of hydrogen, this would improve the spinnability of the pitch. Also, it has been observed that the high reactivity of alkyl groups of pitches toward oxidation, is favorable for the stabilization of the pitch fibers [ 171. In general, the content of hydrogen
1083
fibers
400
500
608
700
Temperature (K) Fig. 6. Evolution of SP with treatment temperature of samples obtained from pitch B (0), petroleum pitch (M) and the mixture (+), after air-blowing for 1 hour.
100
80
400
580
608
700
Temperature (K) Fig. 7. Evolution of yield with treatment temperature of samples obtained from pitch B (O), petroleum pitch ( n ) and the mixture (+), after air-blowing for 1 hour.
J. ALCAGE-MONCE CI rrl
1084
of view of the materials used to prepare the mixture. After that, the blended material has been subjected to a treatment in air to increase its softening point. Figure 6 shows the change of softening point versus treatment temperature in air of: (i) the petroleum pitch, (ii) pitch B and (iii) a mixture of both materials. The three samples exhibit a similar behavior. The softening point increases with the treatment temperature, reaching a plateau for temperatures higher than 623 K. The blending technique produces, after a treatment in air at 673 K for 1 hour, samples with softening points of about 473 K that are adequate for stabilization after spinning. Figure 7 shows the evolution of the yield of the process with the treatment temperature in air for the three materials. In this case, interesting behaviors are observed. The coal tar pitch B shows a gradual decrease of the yield with increasing treatment temperature in air. The yield of the petroleum pitch experiences a much faster decrease for temperatures higher than 573 K. This is probably related with the elimination of volatile matter of low molecular weight formed by cracking processes. These differences between the two materials would be in agreement with the higher content in aliphatic structures of the petroleum pitch. Finally, it must be noted that the blended material presents a similar behavior to the coal pitch. This means that the molecules that compose both pitches (petroleum and coal pitches) interact efficiently giving a highly homogeneous mixture. Thus, during the treatment in air, reactions between molecules of both pitches occur that stabilize structures existing in the petroleum pitch that,
3xlo
3ono
2500
WC
0.85
I
0 coal n Petroleum
0.80 1 0.75 0.70 0.65 C> 0.60 0.55 -
,
0.50
500
400
300
700
600
Temperature (K)
Fig. 9. Evolution of H/C ratio of the residues obtained from pitch B (Cl), petroleum pitch ( n ) and the mixture (+), after air-blowing for 1 hour at different temperatures.
Table 2. SP and molecular weight of samples obtained from pitch B, petroleum pitch and the mixture after air-blowing at 623 K for I hour
Sample
Molecular weight (g mol
ST(K)
314 355 488
461 451 470
B + treatment B + P + treat. P” + treatment aP means petroleum pitch
otherwise, would be cracked and volatilized in the non-blended material. Consequently, these processes increase considerably the yield of the petroleum pitch.
2000
1500
low
Wavenumbers (cm-l) Fig. 8. FTIR
spectra
of pitch B, petroleum
‘)
pitch and their mixture
after air-blowing
at 623 K for 1 hour.
General purpose carbon fibers
1085
(4
(b) Fig. 10. SEM photographs
of carbon fibers obtained from pitch B free of primary Ql.
Figure 8 presents the FTIR spectra of the samples obtained after treatment in air at 623 K for 1 hour, for pitch B, the petroleum pitch and a mixture of both materials. The FTIR spectra reveal that the content of hydrogen of the blended sample is intermediate to that of the original materials separately (see zone between 3100&2800 cm-‘). The evolution of the H/C ratio with the treatment CARBD” 15-8-C
temperature in air is shown in Fig. 9. From a qualitative point of view, the evolution of this ratio is similar to that obtained for the yield (see Fig. 7). Thus, it is observed that the petroleum pitch suffers a considerable decrease in its H/C ratio for treatment iemperatures higher than 573 K and that the coal tar pitch experiences a smaller decrease in the whole range of treatment temperatures. On the other hand, for the
.I. ALCARIZ-MONGE rt ul.
1086
Fig.
I I. SEM photographs
of carbon
fibers obtained
of the mixture of both pitches a different behavior is observed. So, while the evolution in the yield follows the same tendency for the coal pitch (see Fig. 7), the ratio H/C resembles the petroleum pitch more (see Fig. 9). Moreover, the values of the H/C ratio for the blended material are between those of the petroleum and coal tar pitches for all the treatment temperatures in air. The measurement of the molecular weight of the different samples obtained after treatment in air at 623 K for 1 hour in the coal tar pitch B, petroleum pitch and the blended material (Table 2), reveals that the material obtained after blending, has a higher molecular weight than the coal pitch. The material obtained after treating the blended sample in air at 623 K for 1 hour, had an excellent spinnability and was easily stabilized. case
3.3 Prepurution of’carbonJibers: mechanicul properties qf the muteriuls obtained The coal tar pitches that, after any of the treatments given above, reached SP values higher than 453 K and were successfully spun, were stabilized Table 3. Mechanical
properties
of the carbon
Fiber diameter
(ilm) CF from mixture CF from pitch A CF from pitch C CF from pitch D Cl+’ from petroleum Kureha T-IOlT
pitch
35 30 36 40 30 I8
fibers (CF)
Tensile strength (MPa)
Young’s modulus (GPa)
230 230 330 170 2 I0 590
25 25 29 I’)
22 30
from pitch B without elimination of primary Qt and carbonized according to the procedure described in the experimental section. The carbon fibers obtained from the coal tar pitches were observed by SEM. In principle, no differences are apparent between the fibers obtained from the different starting pitches and with the different treatments. Figure 10 gives an example of the type of observations obtained for fibers prepared from pitch B. From the photographs it is clear that the carbon fibers obtained have a uniform size. between 20-40 /tm, a smooth surface, without any thickening and a smooth cross-section (indicating that the fibers are isotropic). Moreover, it is observed that the fibers are loose, indicating that the stabilization process has been adequate. Figure 11 shows a SEM photograph of the carbon fibers obtained from the same pitch but without extracting the infusible material they contains initially. In this case, the fibers are not uniform in diameter and contain an important number of thickenings. Table 3 collects the results of the measurement 01 the mechanical properties of the following carbon fibers: a) prepared with the blending of pitch B with the petroleum pitch and after treatment with air; b) obtained by consecutive heating with N1 and air of the pitches A. C and D; c) prepared from the petroleum pitch [21]; and d) a general purpose carbon fiber provided by Kureha Co. The results obtained indicate that the carbon fibers prepared from coal tar pitches, have mechanical properties comparable (and even better in the case of pitch C ) to the fibers prepared from a commercial petroleum pitch [21]. On the other hand, it is observed that the values of the Young’s modulus are
General
purpose
also similar to that of the commercial carbon fiber Kureha T-lOIT, while their tensile strengths are lower. These differences in tensile strength should be mainly related with the higher diameter of the carbon fibers prepared in this study, that can introduce a larger amount of structural defects. In any case, if only the fibers with diameters close to 20 pm are considered then similar mechanical properties to the commercial fibers are obtained. 4. CONCLUSIONS It must be emphasized that carbon fibers have been successfully prepared from all the coal tar pitches studied. For this purpose, the materials have been subjected to adequate treatments that permit to control the properties (viscosity and SP) that are essential to spin the pitches and to stabilize the pitch fibers successfully. In this sense, two novel methods have been developed to transform the pitches into spinnable materials. The first one consists of a consecutive treatment in nitrogen and air that allows us to control the viscosity and SP of the pitches. The second method consists of the blending of the coal tar pitch with a petroleum pitch. This procedure results in materials with an excellent spinnability that can be easily stabilized. The pitch fibers prepared have been stabilized and carbonized giving carbon fibers that are isotropic and belong to the group of general purpose carbon fibers (GPCF). The carbon fibers obtained have comparable mechanical properties or even better (case of fibers prepared from pitch C), to those of the fibers prepared from a commercial petroleum pitch using the same experimental system (that leads to a similar average diameter). However, the mechanical properties of these fibers are inferior to those of a commercial carbon due to their lower diameters. Arknowledgemenrs-The (project C23-353) and
authors thank DGICYT (project
OCICARBON PB93-0945) for
carbon
fibers
1087
financial support and IBERDROLA for the thesis grant of 1. A. M. We also wish to thank Bilbaina de Alquitranes S.A. and Industrial Quimica del Nalon for providing the coal tar pitches.
REFERENCES 1. The Economics of Carbon Fibre, 2nd edn. Roskill Information Services Ltd., 2 Clapham Road, London SW9 OJA, 1990. A., J. Mater. Sci., 1995, 30, 2061. 2. Madroiiero, 3. Otani. S., Mol. Crl.‘st. Lia. Cryst., 1981, 63, 249. 4. Otani, S. and Oya, A., in Pe;roleum Derived Carbons, ed. J. D. Bacha. J. W. Newman and J. L. White. ACS Symposium Series No. 303, 1986, p. 323. 5. Donnet, J. B. and Bansal, R. C., Carbon Fibers, International Fiber Science and Technology, Vol. 10. Marcel Dekker, New York, 1990. 6. Sawran, W. R., Turill, F. H., Newman, J. W. and Hall, N. W.. US Patent No. 4497789. 1985. I. Otani,‘S., Carbon, 1965, 3, 31. Y., Sekiyu Gakkaishi, 1987, 30, 291. 8. Matsumura, K., Mondori, J. 9. Maeda, T., Zeng, S. M., Tokumitsu, and Mochida, I., Carbon, 1993, 31, 407. K., Mondori, J. 10. Zeng, S. M., Maeda, T., Tokumitsu. and Mochida. 1.. Carbon. 1993. 31. 413. J., Tokumitsu, K. 11. Zeng, S. M., Maeda, T., Mondori, and Mochida, I., Carbon, 1993, 31, 421. S., British Patent Application GB 12. Chawatiak, 2005298A, 1979. 13. Barr, J. and Lewis, I. C., Carbon, 1978, 16, 439. 14. Brooks, J. D. and Taylor, G. H., in Chemistry and Physics of Carbon, Vol. 1, ed. P. L. Walker, Jr. Marcel Dekker, New York, 1968, p. 243. 15. Wagner, M. H., Jager, H. and Wilhelmi, G., in Exrended Abstracts 18th Biennial Conference on Carbon, Massachusetts, U.S.A., 1987, p. 391. 16. Eser, S. and Jenkins, R., Carbon, 1989, 27, 889. 17. Mochida, I., Toshima, H., Korai, Y. and Matsumoto, T., J. Materials Sci., 1988, 23, 670. I., Korai, Y., Nakamura, M., Zeng, S. M. 18. Mochida, and Kameyama, M., Carbon, 1989, 27, 498. I., Ando, T., Maeda, K., Fujitsu, H. and 19. Mochida, Takeshita, K., Carbon, 1980, 18, 318. H., Mochida, I., Korai, Y. and Hino, T., 20 Toshima, Carbon, 1992, 30, 773. J. A., Cazorla-Amoros, D., Linares21 Alcaiiiz-Monge, Solano, A. and Oya, A., Anales de Quimica, 1994, 90, 201.
Pergamon
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PREPARATION OF GENERAL PURPOSE CARBON FIBERS FROM COAL TAR PITCHES WITH LOW SOFTENING POINT J. ALCA~IZ-MONGE,~
D. CAZORLA-AMOR~S,~
A. LINARES-SOLANO,~‘* A. OYA,~
A. SAKAMOTO~ and IS. HOSHI~ aDepartamento de Quimica Inorginica, Universidad de Alicante, Alicante, Spain bDepartment of Materials Science, Gunma University, Gunma, Japan (Received 22 November 1996; accepted in revisedform
6 March 1991)
Abstract-This paper describes the different procedures applied for the preparation of general purpose carbon fibers from four coal tar pitches. The four raw materials have a low softening point (of about 373 K) for this application and, hence, must be subjected to a treatment before spinning to increase this temperature. The treatments applied are the following: (i) heating in N,, (ii) heating in air, (iii) consecutive heating in N, and air, and (iv) blending of the coal tar pitch with a petroleum one and further treatment in air. The changes in chemical composition, softening point and yield of the treatments used have been followed by different techniques. Only treatments (iii) and (iv) produce materials with an adequate viscosity for spinning and a sufficiently high softening point to be transformed in carbon fibers. The carbon fibers obtained have similar mechanical properties to those prepared in a previous work from a suitable petroleum pitch with the same experimental system. However, the mechanical properties of these fibers are inferior to those of a commercial carbon fiber, the differences being due to the lower diameter of the commercial fibers. 0 1997 Elsevier Science Ltd Key Words-A.
Carbon
fiber, A. coal tar pitch, C. infrared
spectroscopy,
properties.
them to an additional treatment before spinning to increase their SP. In this paper, the different treatments used to obtain adequate starting materials for carbon fiber preparation (i.e. materials with a SP sufficiently high and a good spinnability) are discussed. Thus, several techniques, some conventional and other novel, have been applied for the development of spinnable pitches. In this way, GPCF of acceptable properties have been finally prepared from the four starting pitches.
I. INTRODUCTION
As it is well known, carbon fibers are obtained from different raw materials such as polyacrylonitrile, rayon, pitches, resins [ 11, and gases such as methane and benzene [2]. The first pitch-based carbon fibers were prepared in 1963 by Otani and coworkers [3]. Nowadays, it is expected that these materials will gain considerable industrial importance [ 11. The pitches used as starting materials for the preparation of carbon fibers are byproducts of the coke-making and petrochemical industries. For this reason, these materials have the advantage of being cheap precursors of carbon fibers. Two types of carbon fibers based on pitches can be differentiated: general purpose carbon fibers (GPCF) and high performance carbon fibers (HPCF) [4]. The first are obtained from conventional isotropic pitches, while the second are prepared from mesophase pitches. The HPCF are quite expensive compared with the GPCF and their applications are mainly limited to aerospace and sport goods industries. On the other hand, the GPCF have many potential applications due to their much lower price
2. EXPERIMENTAL
2.1 Materials Four commercially available coal tar pitches from two Spanish companies (Bilbaina de Alquitranes S.A. and Industrial Quimica de1 Nalon) and a commercial isotropic petroleum pitch (Showa-Shell Co. Run-18) have been used as raw materials. Table 1 summarizes
Table 1. Some
Elemental
151.
In this work, the study carried out on the preparation of GPCF from commercially available coal tar pitches is presented. All the pitches used have a low softening point (SP) that impedes the stabilization process of the pitch fiber necessary to obtain the carbon fiber. Therefore, it is necessary to submit *Corresponding
D. mechanical
A B C D Pa
properties of the coal tar pitches petroleum pitch analysis
(wt%)
1079
Insolubles
(wt%)
the
C
H
N
S+Odi,
H/C
Tl
QI
SP (K)b
92.13 91.85 92.86 92.90 93.30
4.46 4.79 4.32 4.33 6.60
0.77 0.79 0.70 0.68 0.00
2.64 2.65 2.13 2.19 0.1
0.58 0.62 0.56 0.56 0.81
44 19 40 41 0.3
22 3 27 22 0
383 346 393 378 352
aP means petroleum pitch. bMeasured by DTA or TMA.
author.
and
J. ALCA~IZ-MONGE et ul.
1080
some of their properties (elemental analysis, softening point (SP), insoluble in quinoline (QI) and insoluble in toluene (TI )). It can be observed that the SP’s of the coal tar pitches are very low as to be used in the preparation of carbon fibers, due to the difficulty of the stabilization stage of the pitch fibers that would be obtained. On the other hand, the coal tar pitches contain an important amount of infusible matter (i.e. primary QI ) that distorts seriously a continuous spinning. Because, pitch B is the coal tar pitch which has the smallest content in QI (see Table I), the treatments studied in this work are mainly focused on this pitch. Then, the most promising methods are applied to the remaining pitches (A, C and D).
Treatments carbonjbers
2.2
to trunsjkm
the pitches into
The infusible matter was removed by filtration of the fraction of the pitch insoluble in quinoline or by hot filtration of the melted pitch. In this last case, a steel mesh (50 pm) was used and the filtration was done between 3733423 K. The resulting material was employed as the starting one for the treatments that are detailed below. Pitch B was subjected to the following heat treatments: i) heating in air; ii) heating in N,; iii) consecutive heating in N, and air, and iv) blending of pitch B with a petroleum pitch (ratio 1 : 1 in weight) and subsequent treatment in air. The heat treatments were performed at different temperatures (between 423 and 773 K) and times (no longer than 2.5 hours), using air and N, flows of 100 ml mini i. In all cases, 5 g of pitch were used and the reproducibility of the treatments, from the point of view of yield and spinnability, was good. Once the pitches were treated, the general procedure used for the preparation of carbon fibers was the following. Only the samples with SP higher than 453 K were spun; the samples with SP smaller were not used because of the difficulties that would involve their subsequent stabilization. A monofilament spinning apparatus was used to obtain the pitch fibers. The diameter of the nozzle of the spinning apparatus, that determines the diameter of the fibers, was 1.5 mm. The spinning temperature was 50-100 K higher than the SP of the spinnable pitch. After that, the pitch fibers obtained were stabilized in a flow of air (500 ml min-‘, STP) using the following stabilization programme that lasted 6 hours: ( 1) Heating at 5 K min- ’ to 403 K. to 493 K, maintaining this (2) Heating at 1 K mini temperature for 1 hour. (3) Heating at 1 K min-’ to 573 K, maintaining this temperature for 2 hours. In the carbonization stage, the stabilized pitch fibers were heated at 10 K mini to 1273 K in a N, atmosphere (80 ml min-‘, STP), maintaining this temperature for 30 minutes.
2.3 Techniques oj’charactertation Information about the softening point was assessed by differential thermal analysis, DTA (Setaram TG-DCS92), or by thermomechanical analysis, TMA (Rigaku TM ~ 8140). In the second case, the penetration method was used. The diameter of the pin and the load were 1 mm and 500 mg, respectively. A heating rate of 5 K mini and a N, flow of 60 ml mini 1 (STP) were used in both techniques. The elemental analysis of the samples was performed by a conventional method. The FTIR spectra of the powdered samples were obtained using diffuse reflectance (Nicolet 510P). The molecular weight of the fraction of the pitch soluble in THF was determined by an osmometric method. The fibers were observed by scanning electron microscopy, SEM. (JEOL JSM-35C). The mechanical properties of the fibers were measured in a Tensilon equipment U-II. The load-elongation curves were obtained by applying a stretching speed of 2 mm min 1 until fiber failure. The tensile strength (G). fracture elongation (E) and Young’s modulus (E) were estimated from the following equations g= P/A t = SL/L E=CT/E where P means the load at fracture, A is the crosssectional area of the fiber, 6L is the elongation at fracture and L is the gauge length. Thirty measurements of each carbon fiber material were done. In each measurement, the gauge length used was 2.5 cm and the diameter of the fiber after failure was obtained by SEM. 3. RESULTS AND DISCUSSION
3.1 Heat treatment
in N,, air and N, + air.
Heat treatments of the pitches in N, or air are used for the preparation of suitable precursors of GPCF [6-I 11. An excessive polymerization of the pitch during the treatment with N2, as a consequence of a long reaction time, may lead to the formation of mesophase spheres and, finally, to a mesophase pitch [ 121 which is not adequate for the preparation of GPCF. A treatment in air produces a considerable increase in the softening point and viscosity compared with the starting material. Barr and Lewis [ 131 have observed that a heat treatment in air results in the oxidation of the pitch molecules through a dehydrogenative polymerization without the formation of oxygenated products. Additionally, Brooks and Taylor [ 141 have confirmed that oxidation of pitch strongly affects the subsequent development of mesophase spheres. In this way, the formation of mesophase could be avoided after some degree of oxidation. Considering the results mentioned above obtained
General purpose carbon fibers from the literature, both treatments in air or N, have been applied to transform the coal tar pitches in materials with sufficiently high SP and adequate spinnability for the preparation of GPCF. The results obtained are commented on in the following discussion.
1081
5oo SP (K)
3.1 .l Yield of the process, evolution of SP and extraction with solvents.. Figure 1 shows the evolution of yield and SP with the treatment temperature for samples treated in N, or air for 1 hour. In both cases, the weight loss starts at about 473 K and there are no important differences between both treatments in the evolution of the yield. On the contrary, the change of SP with treatment temperature of pitch is quite different in air and N,. Thus, a heating in N, results in a small increase of SP only after a treatment at temperatures higher than 623 K, while a heating in air produces this effect at lower temperatures (at about 523 K). It must be noted that only the treatment in air produces samples with SP values sufficiently high (453-473 K) for subsequent stabilization in air of the resulting pitch fiber in a period of reasonably short time. The content in QI increases with the treatment temperature and is related to the SP of the samples [ 151. Because the fraction insoluble in quinoline is constituted by the part of the pitch formed by molecular structures with a high degree of condensation, the larger the amount of QI, the higher the SP of the pitch (see Fig. 2). This result is in concordance with what was previously observed by other authors [ 9,131. 3.1.2 Elemental analysis and FTIR spectroscopy. Elemental analysis and FTIR spectroscopy provide further information about the effect of the N2 and air treatments. Figure 3 shows the evolution of the ratio H/C after heat treatment in air and nitrogen, together with the change of the SP for comparison purposes. It is observed that the evolution of the ratio H/C with temperature is, in both cases, related with that followed by the SP. The fact that the samples heated in air exhibit a more important decrease in the ratio H/C than those Yield (%)
SF’ (K) Y
n
rSO0
Nitrogen blowing
703.
3oo Temperature
(K)
Fig. 1. Evolution of yield and SF’ of the residue obtained from pitch B, with treatment temperature in N, ( n ) and air (0) (all points correspond to treatments for 1 hour).
Fig. 2. Evolution of .SP of samples obtained after air-blowing for 1 hour at different temperatures versus the content in quinoline
insolubles.
500
600
0.64 0.62 0.60 0.58 0.56 0.54 0.52 0.50
I 300
’ 300 400
Temperature
700
(K)
Fig. 3. Evolution of H/C ratio and SP of the residues obtained from pitch B with treatment temperature in N, (m) and air (0) (all points correspond to treatments for 1 hour). treated in nitrogen at a similar temperature, is indicative of an oxidative effect. In spite of the interesting information that provides the H/C ratio, from these results no conclusion about the origin of the differences between N, and air can be extracted. In this sense, it is worth mentioning that two samples with the same H/C ratio, but obtained with different treatment (air or N,), present a large difference in their SP (see Fig. 3). Thus, for example, the sample obtained after heating in air at 623 K has a H/C ratio of 0.53 and a SP of 461 K. However, the sample treated with N2 having the same H/C ratio, obtained after heating at 723 K, has a SP of 387 K. The interpretation of these aspects can be carried out more efficiently by using a spectroscopic technique like FTIR. Figure 4 shows the FTIR spectra obtained for the original pitch, for the material obtained after heat treatment in air at 623 K for 1 hour and for the sample prepared after a similar treatment in N,. The FTIR spectra show that the treatment in air does not result in the formation of oxygen groups, in agreement with previous results [9,13]; the bands between 1650 and 1860 cm-’ (characteristics of carbonyl and carboxyl groups), between 3500 and 3300cm-’ (characteristic of O-H bonds) and between 1340 and 1110 cm-’ (related with C-O-H
J.
1082
3500
3000
ALCAGIZ-MONGE
2500
2000 Wavenumbers
Fig. 4. FTIR spectra of pitch B and the samples obtained and C-O-C structures) remain nearly unchanged or, even, decrease. The zone of the spectra obtained for the different samples corresponding to the C-H stretching bands (between 3100 and 2800 cm-i), has been analyzed in more detail by calculating the change of the ratio between the aromatic and the aliphatic hydrogens (Har/Hal). This ratio has been obtained by dividing the area of the band at 3050 cm-’ and that for the bands in the region between 2960 to 2800 cm- ‘. An increase in this ratio is indicative of an increase in the degree of aromatization of the sample. Figure 5 presents the evolution of the ratio Har/Hal calculated from the spectra, for the samples obtained after heating the original pitch B in air or N,. The figure contains experiments done
10
HarolHali
I
n
Nitrogen
0
693
n 673
K
I
I
30
45
ef ul
0
K 55
Time (min) Fig. 5. Evolution of Har!Hal ratio of the samples obtained after heating in N, ( n ) and air (0) for different periods of time and temperatures.
I5Uil
IOO~1
(cm-l)
after air and N, treatments
at 623 K for 1 hour
at two temperatures (673 and 693 K) as a function of the treatment times (lower than 1 hour). It must be noted that the value of the Har/Hal ratio included in the figure for a treatment time of 0 minutes, corresponds to the pitch B without any treatment. Figure 5 clearly shows that the treatment in nitrogen produces a slight variation in the ratio Har/Hal. in the range of temperatures (4233773 K) and treatment times studied, in spite of the strong devolatilization observed in Fig. 2. On the contrary, the treatment in air produces a notable change in this ratio. It is also observed that the most important decrease in the fraction of aliphatic C-H bonds happens during the first 10 minutes of treatment. This result is indicative of the high reactivity of such CH bonds in air. As a summary, the results obtained show that the heat treatment in nitrogen mainly produces physical processes, such as volatilization of light fractions of the pitch, and chemical processes, such as cracking of aliphatic groups and/or aromatic rings with a lower degree of condensation. Additionally, the treatment in air results in an important reaction with aliphatic hydrogens and with a significant attack of aromatic hydrogens, if the reaction time is sufficient, favoring polycondensation reactions [ 131. This produces in the treated material a larger increase in QI and SP than N,. These conclusions agree with the work of other authors [9-l I]. 3.1.3 Spinnubility of‘ the treated coul tur pitches. The samples obtained with SP higher than 453 K, i.e. pitch treated in air, were spun to check the possibility of obtaining pitch fibers. The problem
General purpose carbon of these samples was their high viscosity, compared with those obtained after a treatment with N,, this made their spinning very difficult. Furthermore, though some samples were spun (this is the case for that treated at 623 K for 1 hour, that was successfully spun at 550 K), a considerable dispersion resulted in the diameter of the obtained fibers. On the other hand, the treatment with N,, that produces samples with an adequate viscosity for spinning, does not provide suitable materials to obtain finally carbon fibers because of the low increase in SP observed. Considering the different effect of N, and air on the properties of the resulting pitch, a novel combination of both treatments has been applied in this study to gain a better control of the spinnability and SP of the material. The method consists in combining both treatments in air and Nz: i) a heating in N,, that results in the evolution of the volatile matter with low molecular weight, and ii) a subsequent heating in air to favor the polycondensation reactions and to increase considerably the SP. Different temperatures and reaction times have been studied. This combination allows us to shorten the time of the treatment in air and, hence, to control the spinnability of the material. This treatment has been applied to the four coal tar pitches (A,B,C and D) and has resulted in samples with SP between 453-473 K with a good spinnability. The treatments have been done at 663 K in all the cases and for the following periods of time: pitch A - 1 hour in N,, 0.5 hours in air; pitch B - 2 hours in N2, 0.5 hours in air; pitch C 2 hours in N,, 0.5 hours in air; pitch D - 1.5 hours in N,, 0.5 hours in air. The pitch fibers obtained were stabilized and carbonized using the stabilization programme described in the experimental section.
of the coal tar pitches is lower than that of the petroleum pitches (see for example Table 1). According to the above comments, this difference makes more difficult the transformation of a coal tar pitch into pitch fibers than from a petroleum pitch. Hence, to obtain pitch fibers with an improved spinnability and stabilizability it should be necessary to increase the content of hydrogen of the starting coal tar pitches Several studies have been accomplished to introduce alkyl groups in coal tar pitches, especially in form of saturated rings, which confer to the pitch a high reactivity toward oxidation and a low viscosity [ 18-201. Considering that the petroleum pitch used in this study is easily spinnable and sufficiently reactive during the stabilization stage [21] and that it has a higher content in hydrogen (mainly of aliphatic type) than the coal tar pitches (see Table l), the following approach has been used. The petroleum pitch has been blended with the coal tar pitch B (ratio 1:l in weight) to obtain a material with improved characteristics to prepare GPCF that is novel from the point
300
3.2 Blending of a coal tar pitch with a petroleum pitch: treatment in air Considering the above study, the most important problem of the preparation of GPCF from these coal tar pitches is that the treatment in air, although produces samples with a high SP, their spinnability is bad due to their high viscosity. In this sense, it was necessary to shorten as much as possible the contact of the sample with air by applying a previous treatment in N,. An additional difficulty of the coal tar pitches is the lower reactivity of the pitch fibers prepared from coal pitches than those obtained from a petroleum pitch. For this reason, the stabilization of pitch fibers prepared from coal pitches requires more time than in the case of petroleum pitch fibers. It has been shown that the content of hydrogen determines the viscosity of the pitches because it is related with the content of aliphatic groups [ 161. In this sense, the viscosity decreases with increasing the content of hydrogen, this would improve the spinnability of the pitch. Also, it has been observed that the high reactivity of alkyl groups of pitches toward oxidation, is favorable for the stabilization of the pitch fibers [ 171. In general, the content of hydrogen
1083
fibers
400
500
608
700
Temperature (K) Fig. 6. Evolution of SP with treatment temperature of samples obtained from pitch B (0), petroleum pitch (M) and the mixture (+), after air-blowing for 1 hour.
100
80
400
580
608
700
Temperature (K) Fig. 7. Evolution of yield with treatment temperature of samples obtained from pitch B (O), petroleum pitch ( n ) and the mixture (+), after air-blowing for 1 hour.
J. ALCAGE-MONCE CI rrl
1084
of view of the materials used to prepare the mixture. After that, the blended material has been subjected to a treatment in air to increase its softening point. Figure 6 shows the change of softening point versus treatment temperature in air of: (i) the petroleum pitch, (ii) pitch B and (iii) a mixture of both materials. The three samples exhibit a similar behavior. The softening point increases with the treatment temperature, reaching a plateau for temperatures higher than 623 K. The blending technique produces, after a treatment in air at 673 K for 1 hour, samples with softening points of about 473 K that are adequate for stabilization after spinning. Figure 7 shows the evolution of the yield of the process with the treatment temperature in air for the three materials. In this case, interesting behaviors are observed. The coal tar pitch B shows a gradual decrease of the yield with increasing treatment temperature in air. The yield of the petroleum pitch experiences a much faster decrease for temperatures higher than 573 K. This is probably related with the elimination of volatile matter of low molecular weight formed by cracking processes. These differences between the two materials would be in agreement with the higher content in aliphatic structures of the petroleum pitch. Finally, it must be noted that the blended material presents a similar behavior to the coal pitch. This means that the molecules that compose both pitches (petroleum and coal pitches) interact efficiently giving a highly homogeneous mixture. Thus, during the treatment in air, reactions between molecules of both pitches occur that stabilize structures existing in the petroleum pitch that,
3xlo
3ono
2500
WC
0.85
I
0 coal n Petroleum
0.80 1 0.75 0.70 0.65 C> 0.60 0.55 -
,
0.50
500
400
300
700
600
Temperature (K)
Fig. 9. Evolution of H/C ratio of the residues obtained from pitch B (Cl), petroleum pitch ( n ) and the mixture (+), after air-blowing for 1 hour at different temperatures.
Table 2. SP and molecular weight of samples obtained from pitch B, petroleum pitch and the mixture after air-blowing at 623 K for I hour
Sample
Molecular weight (g mol
ST(K)
314 355 488
461 451 470
B + treatment B + P + treat. P” + treatment aP means petroleum pitch
otherwise, would be cracked and volatilized in the non-blended material. Consequently, these processes increase considerably the yield of the petroleum pitch.
2000
1500
low
Wavenumbers (cm-l) Fig. 8. FTIR
spectra
of pitch B, petroleum
‘)
pitch and their mixture
after air-blowing
at 623 K for 1 hour.
General purpose carbon fibers
1085
(4
(b) Fig. 10. SEM photographs
of carbon fibers obtained from pitch B free of primary Ql.
Figure 8 presents the FTIR spectra of the samples obtained after treatment in air at 623 K for 1 hour, for pitch B, the petroleum pitch and a mixture of both materials. The FTIR spectra reveal that the content of hydrogen of the blended sample is intermediate to that of the original materials separately (see zone between 3100&2800 cm-‘). The evolution of the H/C ratio with the treatment CARBD” 15-8-C
temperature in air is shown in Fig. 9. From a qualitative point of view, the evolution of this ratio is similar to that obtained for the yield (see Fig. 7). Thus, it is observed that the petroleum pitch suffers a considerable decrease in its H/C ratio for treatment iemperatures higher than 573 K and that the coal tar pitch experiences a smaller decrease in the whole range of treatment temperatures. On the other hand, for the
.I. ALCARIZ-MONGE rt ul.
1086
Fig.
I I. SEM photographs
of carbon
fibers obtained
of the mixture of both pitches a different behavior is observed. So, while the evolution in the yield follows the same tendency for the coal pitch (see Fig. 7), the ratio H/C resembles the petroleum pitch more (see Fig. 9). Moreover, the values of the H/C ratio for the blended material are between those of the petroleum and coal tar pitches for all the treatment temperatures in air. The measurement of the molecular weight of the different samples obtained after treatment in air at 623 K for 1 hour in the coal tar pitch B, petroleum pitch and the blended material (Table 2), reveals that the material obtained after blending, has a higher molecular weight than the coal pitch. The material obtained after treating the blended sample in air at 623 K for 1 hour, had an excellent spinnability and was easily stabilized. case
3.3 Prepurution of’carbonJibers: mechanicul properties qf the muteriuls obtained The coal tar pitches that, after any of the treatments given above, reached SP values higher than 453 K and were successfully spun, were stabilized Table 3. Mechanical
properties
of the carbon
Fiber diameter
(ilm) CF from mixture CF from pitch A CF from pitch C CF from pitch D Cl+’ from petroleum Kureha T-IOlT
pitch
35 30 36 40 30 I8
fibers (CF)
Tensile strength (MPa)
Young’s modulus (GPa)
230 230 330 170 2 I0 590
25 25 29 I’)
22 30
from pitch B without elimination of primary Qt and carbonized according to the procedure described in the experimental section. The carbon fibers obtained from the coal tar pitches were observed by SEM. In principle, no differences are apparent between the fibers obtained from the different starting pitches and with the different treatments. Figure 10 gives an example of the type of observations obtained for fibers prepared from pitch B. From the photographs it is clear that the carbon fibers obtained have a uniform size. between 20-40 /tm, a smooth surface, without any thickening and a smooth cross-section (indicating that the fibers are isotropic). Moreover, it is observed that the fibers are loose, indicating that the stabilization process has been adequate. Figure 11 shows a SEM photograph of the carbon fibers obtained from the same pitch but without extracting the infusible material they contains initially. In this case, the fibers are not uniform in diameter and contain an important number of thickenings. Table 3 collects the results of the measurement 01 the mechanical properties of the following carbon fibers: a) prepared with the blending of pitch B with the petroleum pitch and after treatment with air; b) obtained by consecutive heating with N1 and air of the pitches A. C and D; c) prepared from the petroleum pitch [21]; and d) a general purpose carbon fiber provided by Kureha Co. The results obtained indicate that the carbon fibers prepared from coal tar pitches, have mechanical properties comparable (and even better in the case of pitch C ) to the fibers prepared from a commercial petroleum pitch [21]. On the other hand, it is observed that the values of the Young’s modulus are
General
purpose
also similar to that of the commercial carbon fiber Kureha T-lOIT, while their tensile strengths are lower. These differences in tensile strength should be mainly related with the higher diameter of the carbon fibers prepared in this study, that can introduce a larger amount of structural defects. In any case, if only the fibers with diameters close to 20 pm are considered then similar mechanical properties to the commercial fibers are obtained. 4. CONCLUSIONS It must be emphasized that carbon fibers have been successfully prepared from all the coal tar pitches studied. For this purpose, the materials have been subjected to adequate treatments that permit to control the properties (viscosity and SP) that are essential to spin the pitches and to stabilize the pitch fibers successfully. In this sense, two novel methods have been developed to transform the pitches into spinnable materials. The first one consists of a consecutive treatment in nitrogen and air that allows us to control the viscosity and SP of the pitches. The second method consists of the blending of the coal tar pitch with a petroleum pitch. This procedure results in materials with an excellent spinnability that can be easily stabilized. The pitch fibers prepared have been stabilized and carbonized giving carbon fibers that are isotropic and belong to the group of general purpose carbon fibers (GPCF). The carbon fibers obtained have comparable mechanical properties or even better (case of fibers prepared from pitch C), to those of the fibers prepared from a commercial petroleum pitch using the same experimental system (that leads to a similar average diameter). However, the mechanical properties of these fibers are inferior to those of a commercial carbon due to their lower diameters. Arknowledgemenrs-The (project C23-353) and
authors thank DGICYT (project
OCICARBON PB93-0945) for
carbon
fibers
1087
financial support and IBERDROLA for the thesis grant of 1. A. M. We also wish to thank Bilbaina de Alquitranes S.A. and Industrial Quimica del Nalon for providing the coal tar pitches.
REFERENCES 1. The Economics of Carbon Fibre, 2nd edn. Roskill Information Services Ltd., 2 Clapham Road, London SW9 OJA, 1990. A., J. Mater. Sci., 1995, 30, 2061. 2. Madroiiero, 3. Otani. S., Mol. Crl.‘st. Lia. Cryst., 1981, 63, 249. 4. Otani, S. and Oya, A., in Pe;roleum Derived Carbons, ed. J. D. Bacha. J. W. Newman and J. L. White. ACS Symposium Series No. 303, 1986, p. 323. 5. Donnet, J. B. and Bansal, R. C., Carbon Fibers, International Fiber Science and Technology, Vol. 10. Marcel Dekker, New York, 1990. 6. Sawran, W. R., Turill, F. H., Newman, J. W. and Hall, N. W.. US Patent No. 4497789. 1985. I. Otani,‘S., Carbon, 1965, 3, 31. Y., Sekiyu Gakkaishi, 1987, 30, 291. 8. Matsumura, K., Mondori, J. 9. Maeda, T., Zeng, S. M., Tokumitsu, and Mochida, I., Carbon, 1993, 31, 407. K., Mondori, J. 10. Zeng, S. M., Maeda, T., Tokumitsu. and Mochida. 1.. Carbon. 1993. 31. 413. J., Tokumitsu, K. 11. Zeng, S. M., Maeda, T., Mondori, and Mochida, I., Carbon, 1993, 31, 421. S., British Patent Application GB 12. Chawatiak, 2005298A, 1979. 13. Barr, J. and Lewis, I. C., Carbon, 1978, 16, 439. 14. Brooks, J. D. and Taylor, G. H., in Chemistry and Physics of Carbon, Vol. 1, ed. P. L. Walker, Jr. Marcel Dekker, New York, 1968, p. 243. 15. Wagner, M. H., Jager, H. and Wilhelmi, G., in Exrended Abstracts 18th Biennial Conference on Carbon, Massachusetts, U.S.A., 1987, p. 391. 16. Eser, S. and Jenkins, R., Carbon, 1989, 27, 889. 17. Mochida, I., Toshima, H., Korai, Y. and Matsumoto, T., J. Materials Sci., 1988, 23, 670. I., Korai, Y., Nakamura, M., Zeng, S. M. 18. Mochida, and Kameyama, M., Carbon, 1989, 27, 498. I., Ando, T., Maeda, K., Fujitsu, H. and 19. Mochida, Takeshita, K., Carbon, 1980, 18, 318. H., Mochida, I., Korai, Y. and Hino, T., 20 Toshima, Carbon, 1992, 30, 773. J. A., Cazorla-Amoros, D., Linares21 Alcaiiiz-Monge, Solano, A. and Oya, A., Anales de Quimica, 1994, 90, 201.