modifications to carbonization of mesophase pitch by addition of carbon blacks

Pergamon

PII: SOOO8-6223(97)00123-1

Carbon Vol. 35, No. 10-11, 1627-1637, 1997 pp. 0 1997 Elsevier Science Ltd

Printed in Great Britain. All rights reserved 0008-6223/97 $17.00 + 0.00

MODIFICATIONS TO CARBONIZATION OF MESOPHASE PITCH BY ADDITION OF CARBON BLACKS

K. KANNO,~ J. J. FERNANDEZ,~ F. FORTIN,~ Y. KORAI~ and I. MOCHIDA~** “Institute of Advanced Material Study, Kyushu University, Kasuga, Fukuoka 816, Japan

bCorporate Research Laboratory, Mitsubishi Gas Chemical Company Inc., 22 Wadai, Tsukuba, Ibaraki 300-42, Japan

“Institute National de1 Carbon, Oviedo, Spain

(Received 5 August 1996; accepted in revisedform 12 June 1997)

Abstract-The addition of carbon blacks with large surface area or high oil absorption ability such as Ketjen Black (KB) was found very effective in suppress the swelling of mesophase pitch during carbonization. A good correlation between the swelling suppression and the dibutylphthalate (DBP) absorption ability indicates that a large effective volume of carbon black in the pitch system is a key factor in suppressing the swelling. The hollow structure and developed chain-like aggregate of carbon black particles are the origins of such large effective volume. Carbon black with large effective volume is well dispersed in mesophase pitch to initiate the pyrolysis of mesophase pitch at the lower temperature range, avoiding the intense pyrolysis and gas evolution in the narrow temperature range which leads to severe swelling of mesophase pitch. The carbon black dispersed in mesophase pitch is also effective to release the pyrolyzed gas smoothly from almost solidifying mesophase pitch of very high viscosity. Air- and CO,-gasification of a carbon black (MA600) improved its ability of swelling suppression comparable to that of KB. The air-gasification introduced a graphitic shell and hollow core in the particles of MA600 as observed with KB, significantly increasing the surface area at the low burn-off rate. The CO,-gasification decreased the nodule size, increasing the surface area proportionally with the burn-off rate. Such a structural change appeared to improve the ability for suppressing the swelling of mesophase pitch. 0 1997 Elsevier Science Ltd

Key Words-A. Carbon black, A. mesophase pitch, B. carbonization, B. gasification, D. textures.

1. INTRODUCTION

Mesophase pitch has been expected to be an excellent binder for advanced composites because of its high melt fluidity, coke yield, and graphitizability [ 11, however it suffers severe swelling during carbonization. Such swelling is its major disadvantage, which restricts its wide application for advanced carbon composites. The present authors found that addition of particular carbon black by 1-2 wt% with high surface area and oil absorption ability such as Ketjen Black (KB) was very effective in suppressing the swelling [2]. The KB chain aggregate of hollow shell particles, which are believed to originate its high surface area and oil absorption ability. KB particles were found well dispersed in the whole area of the pitch, breaking the molecular stacking into smaller optical units (fine mosaic), and hindered the graphitization at the calcined and graphitized stages of the mesophase pitch. Such characteristics are considered to be key factors for suppressing the swelling.

In this study, the correlation between the properties of carbon black and the swelling suppressing ability, and the intermediate stage of the carbonization of mesophase pitch/carbon black blends was further investigated to clarify key factors and the mechanism for suppression of the swelling. Gasification of a conventional less effective carbon black (MA600) by

*Corresponding author.

1627

air or CO, was also studied to improve its ability to suppress the swelling. The gasified carbon black was observed under TEM to further clarify the structural factors for swelling suppression. The scheme to pro- duce hollow shell spheres is also discussed.

2. EXPERIMENTAL

Table 1 summarizes the naphthalene derived mesophase pitch (MP) which was used in this study. Table 2 shows the properties of various carbon blacks used in this study. The densities of carbon blacks were measured by a pycnometer using n-propanol. Several kinds of carbon blacks (#3250, #3750, #3950, KB-1, and KB-2) are conductive grades with high surface area and high dibutylphthalate (DBP) absorption ability [3] according to their catalogs.

MA600 was gasified in a glass capsule under atmospheric air at 450-550°C for l-2 hours, and in the quartz tube under atmospheric CO2 flow at 750-900°C for 5-40 hours, respectively.

Powdered mixtures of MP with carbon black were well blended in acetone under ultrasonic irradiation. After drying in vacuum, the blends were moulded at room temperature into a disk of 20 mm in diameter and 2 g weight. The disk was carbonized in a glass tube of 25 mm diameter and 300 mm height at 600°C for 1 hour with a heating rate of 1°C min-’ under nitrogen flow, and then further graphitized at 2500°C

1628 K. KANW (‘/ t/l.

ii600 MA600 #3250 #37SO #3950 KB“-I KBd-2

85 25 71 13 360 80 18 150 130 28 240 I70 28 800 210 16 I500 310 30 800 360 30 1050 495

Table I. Analysts and properties of mesophase pitch

Solubility (%)

Code SP” ( C) AC” ( C) H:C’ BS“ BI PS’ PI’ C.1” (Ye,)

MP 216 100 0.64 51 I9 30 87

a Softening point. h Anistropic content. ’ Hydrogen and carbon molar ratio. d Benzene soluble. e Benzene insoluble- pyridine soluble. r Pyridine insoluble. B Carbon value.

Table 2. Carbon blacks data

Carbon Black Diameter” (nm) Nz SAb (m’g ‘) DBP’ (ml:100 g) Volatiles (B) Density (g cm -‘)

0.4 I.84 I.8 1.84 I .o I.81 I.0 I .92 I .o I .99 1.0 0.5 I .99 0.7 2.1 I

* Average diameter. ’ Surface area by N, BET method. ’ Absorbed volume of DBP. d Ketjen Black.

100

Fig. I. Photograph of cokes from MP:KB-I blends (I Cmin ‘. 600 C. I hour).

for 30 minutes with a heating rate of 400 C h- ’ under argon flow.

The carbon disk produced was evaluated in terms of the height, optical texture of polished surface. and graphitizability of the carbon. The first term was a measure of swelling during carbonization.

The intermediate stage of carbonization of MP and MP with 5 wt% KB-1 blends was characterized in terms of swelling extent, H/C atomic ratio change, TG weight loss. and gas evolution.

Microtexture of carbon black particles was observed by transmission electron microscopy (JEOL 1OOCX).

0 5

Carbon Black(wt%)

Fig. 2. Efiect of carbon black content on the swelling of the mesophase pitch. Swelling (%) = MP/CB (cm:MP (cm):

I ‘C min’. 600 ‘C, I hour. N, Row.

3. RESULTS

3.1 Eff&ts of’carhon thck uddition on tfw carhonixztion qf MP

3.1 .1 S~cvlling oJ’ MP during curh~ix- tion. Figure I shows the photographs of carbons produced from MP alone and MP with 2 to IO wt% KB-1 at 600‘C. MP alone suffered severe swelling during carbonization, while the addition of KB-I suppressed the swelling as reported in a previous

Modifications to carbonization of mesophase pitch by addition of carbon blacks 1629

Fig. 3. Optical textures of carbons produced from MP/CB blends (5 wt%) (low magnification).

1630 K. KAXXO (‘I cd.

Fig. 4. Optical textures of carbons produced from MPCB blends (5 wt%) (high magnificatmn)


Table 3. Values of d(002) and Lc(OO2) of carbons graphitized at 2500°C (MPjCB =95/5)

Sample

&2600 MP/MA600 MP/#3250 MP/#3750 MP/#3950 MP/KB-1 KB/KB-2

W32) (nm) Lc(OO2) (nm)

0.337 30.3 0.338 24.1 0.338 23.9 0.338 21.3 0.339 20.9 0.339 18.6 0.340 16.3 0.342 13.0

paper [2]. Addition of 2 and 5 wt% KB-1 reduced the swelling extent to 40% and 10% of that of MP alone, respectively. No further reduction was obtained with more amount.

Figure 2 shows the swelling extents of carbons produced at 600°C from MP blended with some carbon blacks of various weight. KB-2 was most effective to suppress the swelling, Addition of 3 wt% completely suppressing the swelling. KB-1 was the second best, although its 5 wt% addition still allowed 5% swelling. Other carbon blacks (#3950, #3750, MA600, #3250, and #2600) of 5 wt% suppressed the swelling to 22%, 25%, 33%, 38%, and 55%, respectively. Although more addition was effective for suppression, 10 wt% addition still left a considerable swelling of 8% to 25%.

3.1.2 Optical texture and graphitizability of the produced carbons. Figures 3 and 4 show optical textures of carbons produced from MP with 5 wt% carbon black at 600°C at low and high magnifications, respectively. MP alone provided very porous carbon and flow domain texture. Only 5 wt% carbon black addition suppressed the swelling to reduce the porosity of carbon, decreasing the size of optical units significantly to provide mosaic texture. KB-2, which was most effective to suppress the swelling, most drastically reduced the size of the optical unit to ultra fine mosaic texture. The trend found was that the size of the optical unit increased with the extent of swelling.

Table 3 summarizes d(002) and Lc(OO2) values of carbons from MP/carbon black blends (5 wt%) graphitized at 2500°C. All carbon blacks hindered the graphitization of MP carbons, their crystallo- graphic parameters appearing to reflect the size of optical units and the swelling extent.

3.2 Intermediate stage during carbonization of MP

Figure 5 shows the H/C change and swelling extents at the intermediate stage during carbonization of MP and MP with 5 wt% KB-I with a heating rate of 1°C min-‘. The swelling of MP alone started at 400°C and became most vigorous around 460°C. The H/C value decreased rapidly in this temperature range. Such a decrease of MP with KB-1 started at a lower temperature, and continued in the broader

- 0.5

- 0.4

Temperature (‘C)

Fig. 5. Swelling extent and H/C value at the intermediate stage of carbonization of MP and MP/KB-1 blends (5 wt%)

with the heating rate of 1°C min-‘.

temperature range than that of MP alone. A more rapid heating of 10°C min-’ shifted the temperature range of the intense swelling and H/C decrease to 500°C.

Figure 6 shows TG weight loss of MP and MP/KB-I(5 wt%) blends at the intermediate stage of the carbonization. MP started its weight loss at 290°C and lost 28% by 480°C while MP/KB-1 started at 250°C and lost 25% by 480°C respectively. The slower weight loss at 440 to 480°C was more marked with MP/KB- 1.

Figure 7 shows relative volume of evolved gas including hydrogen and Cl to C4 hydrocarbons from MP and MP/KB-1 at 200-800°C with a heating rate of 10°C min-‘. MP/KB-1 evolved the larger amount of gas at a lower temperature range of 3504OO”C, indicating that KB-1 accelerated the pyrolysis of MP at the initial stage of carbonization. In the temperature range of extensive swelling of MP around 500°C the volume of gas evolution from MP/KB-1 was a little smaller than that of MP alone. The apparent solidification of MP/KB-1 took place at slightly lower temperature than that of MP. A larger amount of hydrogen evolved above 500°C after the solidification from MP/KB-1 than MP alone.

3.3 GasiJication of MA600 by air and CO, to modifv the carbonization properties of MPjMA600 blends

Figure 8 compares the swelling extents of carbons produced from MP alone, MP with KB-1, and

K. KANNO et al 1632

, 300 400 500 6

Temperature (“C )

Fig. 6. TG weight change of MP and MP/KB-I blends (5 wt%) during carbonization with the heating rate of

1 ‘C min-‘.

as-received and air-gasified MA600. The carbon black content was fixed at 5 wt%. The burn-off rates at the air-gasification of MA600 were also plotted in the figure. Although as-received MA600 suppressed the swelling of MP during carbonization, its ability for reducing the swelling was much lower than that of KB-1. Air-gasification of MA600 was very effective to improve the ability. MA600 gasified by air at 550°C for 1 hour was most effective to suppress the swelling, being comparable to KB-I, although the burn-off rate was as high as 90%. While MP/KB-I and MP/as-received MA600 provided fine and coarse mosaic textures, respectively, air-gasification of MA600 significantly reduced the size of optical units to give very fine mosaic texture.

Figure 9 shows the swelling extents of the carbons produced from MP with as-received and CO,-gasified MA600 with the burn-off rates, where gasification temperature and time at 75O’C were selected as the reaction parameters. The CO,-gasification was also effective to improve the ability for reducing the swelling, the same suppressing effect being achieved at the smaller burn-off rate than that by the air-gasification. The CO,-gasification for 40 hours at 75O’C improved the ability at a lower burn-off rate than that by CO,-gasification for 5

(MPMB-1(5wt%) h

200 360 4dO 450 6dO 650 760 7&O 8bO

Temperature (“C ) Fig. 7. Gas evolution profiles of MP and MPIKB-1 blends (5 wt%) during-carbonization (2OOXWO C).


a

.:.:.:.:: i::.:.:.. .::. ::::.:.:.

.:::. ._:_: .:::::: .:.:. ., :::. __:::. :::.:::.: :::::..:: ‘.

:::::.: .:. .:.I

j:.:::.:.

: ..‘.’

:. :::.:::

.:.:.:.,.

:.:.:.:.: .‘.. Z.‘. ::::_:. ,_:j:..::

:. :i:.:::..

: ::::‘_:,’

:, . . .:::.,

MPalone KB-1 Untreated 45OC, lh 45OC.2h 5OOC, lh SIOC, 2h SSOC, lh

L MA600 -

Condition of air-gasification

Fig. 8. Effect of air-gasification on the properties of carbon black. 1) Swelling extent of MP/carbon black (5 wt%) blends at 600°C. 2) Bum-off rate of MA600 at the air-gasification.

641 1

750 ‘C 750 “C. 750 “C- 750°C 850 “C 900 “C 5h 10h 15h 20h 5h 5h

COz-gasification

Fig. 9. Effect of CC&gasification on the properties of carbon black. 1) Swelling extent of MP/carbon black (5 wt%) blends at 600°C. 2) Bum-off rate of MA600 at the CO,-gasification.

I634 K. KANW <“t t/l

Air-gasification 0

COz-gasification @

I , I I 0 20 40 60 80 100

Burn off rate (%)

Fig. IO. BET surface area of gasified MAhOO.

hours at 850 C. The abilities of COz-gasified MA600 at lower temperature appear larger than those at higher temperatures when the burn-off rates were the same.

The CO,-gasification also reduced the size of optical units of the carbon to provide fine mosaic texture, however the size was certainly larger than that obtained with air-gasified MA600 at the same swelling extent.

Figure IO shows BET surface area of air and COz-gasified MA600 vs their burn-off rates. The air-

gasification significantly increased the surface area at the low burn-off rate to be saturated at 770 m’ g-’ at 90% burn-off. further burn-off rate failing to increase the area. In contrast, CO,-gasification increased the surface area linearly with the burn-off rate, providing 820 m3 g- ’ at 70% burn-ofY.

Figure I I shows oxygen contents of gasified carbon blacks. Air-gasification increased the oxygen contents, severer conditions providing slightly niore oxygen. CO,-gasification gave much the same oxygen contents regardless of the gasification conditions.

Table 4 summarizes average values of tl(002) and Lc( 002) of gasified MA6OOs. The tl( 002) value decreased through the air-gasification. while the I!X( 002) value became slightly larger than that of the as-received MA600. In contrast. the CO?- gasification monotonously reduced both crystallo- graphic parameters.

Table 4. (/( 001) and Lc( 002) values of gasilied MA600

Treatment

As-received Air 3.50 C. I! hours 500 c. I hOUl

SO0 C. 2 hours 550 c. I hour CO, 750 C. 5 hours 750 c. 40 hours 850 C, 5 hours 900 c, 5 hours

tl( 002) (nm)

0.37 0.360 0.358 0.353 0.352 0.36X 0.370 0.370

fk( 00’) (ml)

I.1 I .4 I.5 I.4 I .4 I .o 0.x

<0.5 10.5

Air-gasification 0

MA600 450°C 450°C 500°C 500°C 550°C lhrs Zhrs lhrs Zhrs lhrs

I I I I I I

I I I I I I

MA600 ,750”C 750°C 800 “C 850°C 900°C Shrs 1Ohrs 5hrs 5hrs Shrs

COz-gasification

Fig. I I. Oxygen contents of E&tied MA600.


Fig. 12. TEM bright field images of carbon blacks.

1636 K. KANNO et 01

3.4 TEA4 projiles of’ carbon blacks Figure I2 shows TEM bright fieldimages of carbon

black particles. KB-I and KB-2 carried well-developed chain-like aggregates of hollow particles with a graphite-like shell, KB-2 consisted of hollow particles of thinner shell as reported [4]. #3950 and #3750 carried less developed chains of hollow shell particles, the particles of #3950 being smaller. MA600 was a chain-like aggregate of rather smaller particles, which did not carry a hollow shell. #3250 was also a chain- like aggregate of particles without a definite hollow shell. #2600 was a small chain-like aggregate of the smallest particles. #5, which is the least effective for suppressing the swelling [2], consisted of rather large spheres without chain structure.

Figure 13 shows TEM lattice field images of as-received, air-gasified, and CO,-gasified MA600 at higher magnifications. As-received MA600 basically

Fig. 13. TEM bright field images of gasified MA600. (a) MA600, (b) air-gasified MA600 (55O‘C, 1 hour), (c)

CO,-gasified MA600 (9OO”C, 5 hours).

exhibited a turbostratic structure where the layers in the particles were arranged in a concentric way [5]. In the outer part of the particle, higher orientation of layer stacking was observed than that in the inner part, where the layers were highly distorted. Severe air-gasification completely removed the inner part of the highly distorted layers, leaving only the external skin of rather highly oriented layers as observed in high conductive carbon blacks such as KBs [6,7]. CO,-gasification burnt off to reduce the diameter of particles. Although no definite hollow structure appeared, some dense skin of slight orientation was observable.

4. DISCUSSION

The addition of a series of carbon blacks was eflective to suppress the swelling of mesophase pitch during carbonization. Among the carbon blacks. KB-2 completely suppressed the swelling with only 3 wt% addition. The order of their effectiveness for tht suppression was KB-2 > KB- 1 > #3950 > #3750 > MA600 > #3250 > #2600.

Figure I4 correlates swelling extent of carbon disks produced from MP and carbon black blends at various weight ratios to their DBP absorption ability. A good correlation was found at the fixed mixing ratio. Absorption of DBP is believed to describe their effective volume dispersed in the polymer composite system. Effective volume is considered to be affected by porosity, surface area, size and anisotropy of the aggregate. and nodule size, however, there was a poor correlation between the swelling extent and BET surface area.

The swelling during carbonization is considered to take place through the gas evolution from the viscous liquid of the mesophase pitch at the last stage of the carbonization just before the solidification. Such a

Absorbed volume of DBP (ml/lOOg)

Fig. 14. EtTect of absorbed volume of DBP of carbon black on the swelling of the mesophase pitch.


stage is located in the narrow temperature range of 450 to 460°C with the heating rate of 1°C mm’. At the initial stage of carbonization up to 4Oo”C, MP with KB-1 lost its weight slightly but certainly more by gas evolution than MP alone, indicating that KB-1 accelerates the pyrolysis of MP at the initial stage of carbonization. At the temperature range of the most rapid weight loss and vigorous swelling of MP, the weight loss and gas evolution became much slower than those of MP alone. The apparent solidification of MP/KB-1 took place at slightly lower temperature than that of MP. Thus, carbon black with large effective volume in the pitch is well dispersed in MP to accelerate the pyrolysis of MP at the initial stage of the carbonization, and expanded the range of gas evolution to reduce its amount particularly at the solidification. Dispersed KB may smoothly release the gases over its surface through the aggregated chains dispersed in the viscous matrix at the final stage of carbonization. Such a carbonization scheme reduces the intense pyrolysis in the narrow temperature range of severe swelling of MP.

The carbon blacks highly dispersed in the pitch may interfere with coalescence of the anisotropic domains, and may even break the molecular stacking in the mesophase pitch to divide the domain optical texture to fine mosaic or even to convert it into isotropic texture. Such breakage of stacking and hinderance of the coalescence in the mesophase pitch reduced its graphitizability at the carbonization and graphitization simultaneously.

The gasification increased the surface area and oxygen content, reducing the particle size of MA600 with modifying the chain structure. Such structural change improved its ability to suppress the swelling and reduce the optical texture of carbon produced from MP.

A slight difference was found in the achievement by air and CO,- gasification. The swelling suppression was more effective by COZ- gasification, while the

reduction of the optical unit was by air- gasification. The smaller particle size obtained by CO,-gasification may be more effective for suppression of swelling, while more oxygen content by the air-gasification may be able to reduce the optical size through emphasized chemical interaction with mesophase pitch.

The gasification by air and COZ provided some contrasting features to MA600. Air oxidation provided definite hollow structure with rather graphitic skin. The gasification preferably burnt the inner portion of the carbon black where the carbon planes were poorly oriented, leaving the more graphitic skin. Such a gasification feature suggests the production scheme of an electroconductive grade of carbon black such as Ketjen Black and Black Pearl 2000, although the detailed procedures have not been disclosed [6]. CO,-gasification exhibited much less selective gasification to reduce the particle size although some densification of skin was also observed. The reactivity and molecular size of gasifying agents, and gasification temperature may delicately balance the gasification selectivity. The surface area vs bum-off rate is also influenced by the gasifying agent and temperature. CO2 monotonously increases the surface area with the bum-off rate. Micropores may be developed in the skin to effectively increase the surface area.

1.

2.

3.

4. 5.

Fujiura, R., Koiima, T., Kanno, K., Moehida, I. and Kdrai, Y., Curb&, 1993, 31, 97. Kanno. K.. Yoon. K. E.. Femandez. J. J., Moehida. I., Fortin,‘F. and Kdrai, Y., Carbon, 1994, 32, 801. Donnet, J. B. and Voet, A., in Carbon Black. Dekker, New York, 1976. Nelson, J. R. and Wissing, W. K., Curbon, 1985, 24,115. Biscoe, J. and Warren, B. E., J. Chem. Phys., 1942, 13,364.

6. Nelson, J. R. and Wissing, W. K., Carbon, 1985,24, 115. 7. Bourrat, X., Carbon, 1993, 31, 287.

REFERENCES

modifications to carbonization of mesophase pitch by addition of carbon blacks

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