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In situ ESR measurement of mesophase formation during the heat treatment and cooling processes of pitches
Carbon Vol. 27. No. 6. pp. X69-875. Printed in Great Britain
19X9
IN SITU ESR MEASUREMENT OF MESOPHASE FORMATION DURING THE HEAT TREATMENT AND COOLING PROCESSES OF PITCHES OSAMU ITO, TOMOKI KAKUTA, and MASASHI IINO Chemical Research Institute of Nonaqueous Solutions, Tohoku University. Katahira, Sendai-980, Japan (Received 3 January 1989; accepted in revked form 21 March 1989) Abstract-Both heating and cooling processes for mesophase formation from various pitches have been followed by in situ ESR measurements. In addition to signal intensity and linewidth, the saturation phenomenon of the signal intensity with changing microwave power was measured during heat treatment and cooling processes. As a convenient measure of the saturation factor, l/P,,, (P,,,,, is the microwave power at which the signal shows maximum intensity) was employed. In general, l/P,,, decreased with an increase in heat-treatment temperature; this change seems to correspond to a decrease in the viscosity of pitch. When pitches were heat treated at 500°C for 2 h, the carbonization reactions and the rearrangement of molecules may occur, which irreversibly changes 1iP,& Pronounced differences in the behavior of l/P,,,,, measured during cooling of the heat-treated pitches were found to depend upon pitch optical texture (isotropic, flow, and mosaic). Key Words-Mesophase,
pitch, ESR.
2. EXPERIMENTAL
1. INTRODUCTION
2.1 Materials and carbonization process Elemental analysis and softening point of four pitches investigated in this study are summarized in Table 1. The primary quinoline insoluble fraction was removed from a coal tar pitch (Mistubishi Kasei Co., Ltd.) with a mixed solvent consisting of CS2 and pyridine, which is as efficient at room temperature as is quinoline at elevated temperature[ll]. Under N,-gas flow, 5 g of each pitch was heated with a heating rate of 10”Cimin to a desired temperature, held at the temperature for a given time. and then cooled to room temperature. The microscopic observation of mesophase was performed by an Olympas BH-2 microscope with crossed Nicol polarizers.
Heat-treatment processes of pitches leading to the formation of mesophase and coke have been investigated by various methods[l,2]. To understand the weak interactions binding the polycondensed aromatic hydrocarbons in liquid crystal state, it has been pointed out that magnetic relaxation methods were quite useful[3-51. Some relationships between proton nuclear relaxation time and optical texture were revealed by Miyazawa et al.[3]. Electron spin relaxation (ESR) phenomena at room temperature were also used to assess the mesophase formation by Singer et al.[4] and by Doetschman et al.[5]. Also, it has been frequently pointed out that in situ measurements afford important information about mesophase formation processes. Microscopic observation[6] and X-ray diffraction[7,8] have been applied to follow the mesophase formation during heat treatment. For the carbonization process of acenaphthylene. in situ ‘H-NMR relaxation measurements were successfully applied by Bacon et al. [9]. In the previous paper, we reported that the in situ ESR measurements were useful to follow the carbonization process of coals; the saturation phenomenon of the signal intensity that was closely related to electron spin relaxation times was especially useful to reveal the molecular motions around the carbon radicals during heat treatments of coals[lO]. In this study, we applied this method to the mesophase formation process of coal tar pitch and petroleum pitches. On cooling down the heat-treated pitches, some differences in the in situ ESR parameters were found to depend upon the isotropic, flow, and mosaic textures. CM 27:6-F
2.2 In situ ESR measurements ESR spectra were measured with a JEOL FE3X ESR spectrometer equipped with a high-temperature ESR cavity. In a quartz ESR tube of inner diameter of 3 mm, 15 mg of pitch was placed and heat treated in the cavity under N, gas flow with a heating rate of 1OWmin. The cooling rate was also controlled at lO”C/min. In situ ESR parameters were measured keeping temperature for 5 min; the measurement intervals were 2040°C. For each pitch, ESR spectrum was seemingly a single signal at heat-treatment temperature (HTT) below 500°C. The saturation phenomena were analyzed by the continuous-wave-saturation method. Figure 1 shows an example for the saturation of ESR signal with microwave power (P). From the plot of the peak-peak signal height (h,,) against log P, the microwave power showing a maximum signal inten869
0.
870
IT0 et al.
Kubelka-Munk function, f(RJ, which is proportional to the absorbance in the ordinary transmittance method. By the diffuse reflectance method, the electronic transition in the near IR region can be measured without a background correction procedure, which is required for the usual transmittance measurements of carbonaceous materials[l4].
Table 1. Elemental analysist and softening point (sp) for four pitches Pitch (abbrev.)
C%
H%
N%
S%
sp”C
Ashland 240 (A-240) Coal tar pitch (CTP) Petroleum no. 4 (P4) Eureka pitch (Eureka)
91.4 90.7 84.2 88.1
5.7 4.2 8.5 5.5
0.2 0.1 1.3
0.5 4.7 5.1
120 97 190 100
tDifference from 100% may be attributed to both 0% and error in the measurements.
3. RESULTS AND DISCUSSION
3.1 Microscopic observation
sity (P,.,) and that at half the maximum signal intensity (Pin) were evaluated. Although P,,, is closely related to spin relaxation times, we employed P,,,= as a convenient measure of the saturation factor[l2], since a wide range of P,, could be extracted. Furthermore, at higher temperatures, P,, seems to be influenced by inhomogeneous behavior of the saturation phenomena more strongly than P,,,,, does[4,13].
Figure 2 shows the microscopic observations for cooled samples after heat treatment at 500°C for 2 h. Ashland 240 (A-240) and coal tar pitch (CTP) showed mesophase having large flow texture, whereas petroleum pitch no. 4 (P4) and Eureka pitch showed coarse grain-mosaic texture. When these pitches were cooled immediately after reaching a HlT of 5OO”C,isotropic carbonaceous materials were obtained; these observations are summarized in Table 2 with the ESR parameters for cooled samples.
2.3 Diffuse reflectance FT-IR Diffuse reflectance FT-IR spectra were measured by a JEOL JIR-100 equipped with a diffuse reflectance apparatus. The spectrum is expressed by the
3.2 In situ ESR measurement Figure 3 shows variation of l/P,,,, , h,, and peakpeak linewidth (AH,) for A-240 during heat treatment. The g-factor (2.0023-2.0025) did not appre-
a
mW
mW
b Pmax
Microwave
power
(mW)
Fig. 1. (a) Dependence of ESR signal intensity on microwave power (P) for A-240 at room temperature; signals are depicted with changing the central magnetic field; (b) Plot of h, against log P; P,,,., and P,,, are shown by arrow.
ciably vary with the heat treatment. The AH,, increased slightly with HTT, showing a maximum at 150°C followed by a gradual decrease until HTT of 500°C; keeping HTT at 500°C AH,, did not vary. In general, AH,, increases with increasing hydrogen content around the carbon radicals; however, such chemical change could not be considered at 150°C. The softening point of this pitch is close to this HTT; thus, the observed maximum of AH,, may rather correspond to some phase changes such as softening point. On cooling process, no appreciable change in AH,, was observed. The h, increased with HTT up to 150°C followed by a decrease to the original height at 400°C. On cooling down after reaching 5Oo”C, h,, decreased slightly. On keeping at 500°C h,, slightly increased again. A similar tendency showing an increase in the spin concentration at 150-200°C during heat treatment of petroleum pitch was also reported by Yokono et af. [ 151. The reason for the increase in h,, at 150°C was not clear. The slight increase in h, on keeping at 500°C was attributed to some carbonization reactions, as Singer and Lewis indicated[ 161. Figure 4 shows the diffuse reflectance FT-IR spectra for heat-treated samples of A-240. The spectrum for pitch cooled immediately after reaching 500°C is similar to that before heat treatment. No appre-
ciable change was found for the spectrum after heat treatment at 200°C. An increase in h, in Fig. 3 in the temperature region of 150-200°C occurs without accompanying chemical changes. If some parts of the carbon radicals in polycondensed aromatic hydrocarbons aggregate at low temperature yielding the singlet state, the breakdown of such weak interactions between the carbon radicals yielding the doublet state may increase the signal intensity. Since the increases in both A HP, and h,, were not observed in the heat treatment process of coal[lO], these observations may be characteristic of pitches. Furthermore, such large changes were not observed for pitches exhibiting mosaic texture, as will be described in a later section in this article. A further prominent change was observed for l/P,,, among the in situ ESR parameters. It is presumed that l/P,,, is closely related to molecular motion and phase change. At 50°C a steep decrease in l/P,,, was observed; although large change in molecular motion below 100°C have not been reported for pitches, it is possible that this behavior is related to phase changes such as glass transition. After 60°C l/P,,, increases again showing a maximum at 120°C which is in good agreement with the softening point of this pitch. Above 120°C l/P,, decreases gradually; this decrease may correspond to a decrease in
872
0.
0
100
200
Ccooling
(500%
- 0
l:cooling
(500%
- 2h
300
400
HTT (‘C)
500 (Oh)
500 (lh)
IT0
500 (2h)
Fig. 3. In situ ESR parameters for A-240. AH,,p; peakpeak linewidth, h,,,,; peak-peak height, and l/P,,,;,,. (0) for increasing HTT, (a) cooling immediately after HTT of SOW’C,and (0) cooling after soaking at 500°C for 2 h; sp refers to softening point.
the viscosity of the melted pitch. On keeping HTT at 500°C no appreciable change was observed. A minimum of l/P,,, (Pm,* = 5 mW; log l/P,,, = -0.7) was observed at 5OO”C-2 h. When the heat-treated sample was cooled down immediately after reaching 5OO”C, l/P,,, increased again along the heating line until 150°C. Below 150°C although two clear maxima at 150°C and at 50°C on the heating line disappeared on cooling, both lines substantially overlapped. By this heat treatment isotropic carbonaceous material was obtained; thus, the overlap of the cooling line with the heating line indicates insufficient soaking time. Similarity of the FT-IR spectrum for the heat-treated pitch at 500°C for 0 h (middle spectrum in Fig. 4) to that before the heat treatment supports this interpretation; however, it should be remarked that the slight physical changes observed in l/P,,, were undetected by the FT-IR spectra only. On the other hand, when the sample was cooled
er
al.
after keeping at 500°C for 2 h, a slight increase of l/P,,, reproducing the heating line down to 350°C was observed. Below this temperature, log l/P,,,, remained around 0, which is far smaller than those of the immediate cooling process. With this heat treatment, A-240 exhibited large flow textures as shown in Fig. 2. This cooling line of l/P,,, therefore seems to be a measure for assessing flow texture. Some carbonization reactions during heat treatment at 500°C for 2 h were suggested from the FTIR spectra (upper spectrum in Fig. 4); instead of the decreases, in the aliphatic C-H bands at 2,916 and 1,443 cm-‘, the increases in the aromatic C-H band at 3,045 cm-’ and in the near-IR band due to the electronic transition of polycondensed aromatic hydrocarbons were observed. By the carbonization reactions accompanying the polycondensation of aromatic hydrocarbons, l/P,,, of cooled samples tends to decreaseI 17,181. Since 1/Pm,, for the cooling process of a sample soaked at 500°C for 2 h did not recover to l/P,,, of the heating process, the rearrangement of molecules in pitch may occur during the soak period or cooling process. The importance of soaking and cooling processes was recognizable in the in situ l/P,,, measurement. Figure 5 shows variation of ESR parameters for CTP. The AHPP changed in a similar manner to that of A-240; i.e. a slight broadening was observed at 200°C. In h,, a clear maximum was observed at 200°C. After passing the maximum, h, decreased; above 350°C it became less than that at room temperature. On keeping the pitch at 500°C h, slightly increased again. The increases in both AH,, and h, at 200°C seem to be characteristic of pitches exhibiting flow texture. For l/P,,,,, in Fig. 5, a rapid decrease was observed at lOO”C, which may also correspond to softening of CTP. A rapid decrease in l/P,,,,, after the softening point may be correlated with a decrease in viscosity. A minimum of l/P,,,,, was observed at 400-500°C. When the sample was cooled down after keeping at 500°C for 2 h, l/P,,,,x increased over the heating line; this suggests that the cooling sample from 500°C shows more mobile molecular motion than that along the heating sample. It is noticeable that the cooling line of l/P,,, for CTP exhibiting flow texture overlaps with that of A-240 (5OO”C-2 h). Thus, this type of the cooling line can be considered characteristic of flow texture. Figure 6 shows the in situ ESR parameters for P4 which exhibits a coarse grain-mosaic texture after heat treatment at 500°C for 2 h. No appreciable change in AH,,,, was observed over the entire temperature range. Although h, increased slightly around 2OO”C, the extent is smaller than those for A-240 and CTP. On soaking at 500°C for 2 h, h,, increased. On cooling h, continues to increase. In the heating line of l/P,,,, a sharp maximum was found at 160°C that corresponds approximately to the softening point. On heating up to 5OO”C,
ESR measurement of mesophase formation
0.8
873
allphetic C-H
n
heating
23 Kicm’
1443cm-l ‘i 87&r;;’
5000
4000
I, 3000
2000
1000
wavenumber (ctil) Fig. 4. Diffuse reflectance FT-IR spectra of heat-treated A-240; 1 mg of sample in 150 mg KBr.
l/P,,, decreased. The minimum of log l/P,,, at 500°C for 0.5 h was - 1.2, which was smaller than those of A-240 and CTP at the same temperature (-0.7--0.9). On cooling after soaking at 500°C for 2 h, log l/P,,, was recovered slightly but did not exceed -0.6. Such small l/P,,,, on the cooling line is a prominent difference from those observed for A-240 and CTP, which exhibits flow texture. In the case of Eureka pitch, which also exhibited a coarse grain-mosaic, h,, and AHpp did not change appreciably (Fig. 7). II P,,, of the original pitch before the heat treatment was smaller than those of other pitches, suggesting less mobility of molecules in the pitch; i.e. three-dimensional cross-linkages may exist. By heat treatment, log l/P,,, gradually decreased down to - 1.3 at 500°C. On cooling immediately after 5OO”C,l/P,,, increased and overlapped with the heating line, showing the heat treatment was insufficient; thus, isotropic carbonaceous materials were obtained. After keeping 2 h at 500% by which a coarse grain-mosaic was produced, l/P,,, remains below those of the heating line. This cooling line was quite similar to that of P4 showing a coarse grain-mosaic. Thus, we can use the cooling lines to
distinguish either the flow texture formation (Figs. 3 and 5) or the coarse grain-mosaic texture (Figs. 6 and 7). In h, and AH,, , prominent increases observed at GO-200°C also seem to be characteristic of mesophase pitches that are capable of producing a flow texture after appropriate heat treatment. On the other hand, pitches without these appreciable increases had difficulty producing flow texture with various heat treatment methods.
3.3 ESR parametersfor cuoled pitches The ESR parameters for cooled pitches before and after heat treatment are summarized in Table 2. The linewidth of Eureka original pitch is wider than those of other pitches; but the linewidth can not be used to distinguish between optical textures. Some relationships’ between the spin concentration and the anisotropic development in heat-treated pitches and coals have been reported[l6,19]. Although it is difficult to find a general tendency for different pitches used in this study, the spin concentrations for the pitches with anisotropic optical texture are higher than that of isotropic carbonaceous material. Fur-
874
0.
IT0 et a/.
1.
P4
CTP 0: heating
l:cooling
(600
1
J ‘C - 2h)
0.
0
0
2 ::0
100
200
300 I-ITT (‘Cl
400
’
600 (Oh)
500 (lh)
500 (2h)
ii % G
Fig. 5. In situ ESR parameters for CTP; same notation as in Fig. 3.
9) 2 0I 0
I
I
I
I
100
200
300
400
I
500
500
500
(Oh)
(lh)
(2h)
HTT (C)
thermore, the spin concentrations for pitches with mosaic texture seem to be higher than those with flow texture. A clear trend was found for Pm, of cooled samples. Heat-treated pitches with similar Pm,, to that of isotropic original pitches remained as isotropic. Pitches with P,,,,, larger than 3.8 mW (or with log l/P,,,, smaller than -0.58) exhibited mosaic texture. Table 2. Optical texture of heat-treated
Fig. 6. In situ ESR parameters for P4. Pitches that exhibited flow texture showed an intermediate P,,,= value of 0.8-1.2 mW (log l/P,,,,, of -0.1-0.1). To further confirm this trend, it will be necessary to measure in situ ESR parameters for many other pitches; a further study is in progress.
samples and ESR parameters of cooled samples ESR parameters
Pitch
HHT (“C)
A-240
500 500 500 500
Cl-P P4
Time (h)
Optical texture
0 2
Isotropic Flow
2
Flow
2
Coarse mosaic
Eureka 5% 500
Isotropic Coarse mosaic
Spin cont. t (X 10’”spin/g) 2.4 1.6 4.5 9”:: 4.4 15.0 19.3 15.2 26.0
A%
(G)
5.4 6.0 4.5 4.8 4.2 4.8 5.1 6.0 6.2 6.8
P,,. (mw) 0.07 0.13 1.13 0.21 0.84 0.21 3.83 1.57 2.07 4.53
tThe spin concentrations were calculated by double integration of the derivative signal comparing with a standard 2,2-diphenyl-1-picrylhydrazyl (DPPH) in KBr.
ESR measurement
0: heating (500
l:cooilng
(5OO’c
x75
which is closely related to the spin relaxation times, l/P,,, varied drastically with heat-treatment temperature. 1/Pmaxwas useful in distinguishing the isotropic, flow, and mosaic textures. The signal intensity and linewidth also afford some clues for the assessment of mesophase pitches.
Eureka
Q: cooling
of mesophase formation
‘C - 0 h -
2h
Acknowledgements-The authors are grateful to Dr. T. Iwasaki of Government Industrial Research of Tohoku, Japan, for the use of the high-temperature ESR cavity. We express our thanks to T. Hino of Toa Nenryou Kogyo K. K. and S. Saito of Mitsubishi Oil Co. Ltd. for their kind donations of pitches. We are also indebted to Dr. N. Tsuchiya and Dr. M. Shishido of Tohoku University for their useful discussions. REFERENCES
0 96 % a4 $2 0 0
100
200
300
400
I-ITT (‘C)
500
500
500
(Oh)
(lh)
(2h)
Fig. 7. In situ ESR parameters for Eureka pitch
4. CONCLUSION For the mesophase formation process of various pitches, in situ ESR measurements have been em-
ployed. Among the ESR parameters such as signal intensity, linewidth, and saturation factor (l/P,,,),
1. H. Marsh and P. L. Walker, Jr., In Chemistry and Physics of Carbon (Edited by P. L. Walker, Jr., and P. A. Thrower), Vol. 15, p. 167, Marcel Dekker, New York (1978). 2. H. Honda, Carbon 26, 139 (1988). 3. K. Miyazawa, T. Yokono, Y. Sanada, and H. Marsh, Fuel 58, 691 (1979). 4. L. S. Singer, I. C. Lewis, D. M. Riffle, and D. C. Doetschman, J. Phys. Chem. 91, 2,408 (1987). 5. D. C. Doetschman, D. Mustafi, and L. S. Singer, .I. Phys. Chem. 92, 3,663 (1988). 6. I. C. Lewis, Carbon 18, 191 (1980). 7. J. N. Guet and D. Tchoubar, Carbon 22, 543 (1984). 8. J. M. Guet and D. Tchoubar, Carbon 23.273 (1985). 9. N. A. Bacon, W. A. Barton, L. J. Lynch, and D. S. Webster. Carbon 25. 669 (1987). 10. H. Seki. 0. Ito. and M. Iino, Energy & Fuels 2. 321 (1988). 11. M. Iino and M. Matsuda, Fuel 62. 744 (1983). 12. A. M. Portis, Phys. Rev. 91, 1,071 (1953). 13. T. G. Castner. Jr., Phys. Rev 115, 1.506 (1959). 14. 0. Ito. H. Seki, and M. Iino, Fuel 67. 573 (1988). 15. T. Yokono, T. Obara. and Y. Sanada. Carbon 24, 29 (1986). 16. L. S. Singer and I. C. Lewis, Carbon 16. 417 (1978). 17. 0. Ito, H. Seki, and M. Iino, Bull. Chem Sot. Jpn. 60. 2,967 (1987).
18. T. F. Yen and J. G. Erdman, Anal. Gem. 34. 694 (1962). 19. J. M. Rincon. R. Carvajal, and L. A. Pacheco. Fuel 64. 119 (1985).
19X9
IN SITU ESR MEASUREMENT OF MESOPHASE FORMATION DURING THE HEAT TREATMENT AND COOLING PROCESSES OF PITCHES OSAMU ITO, TOMOKI KAKUTA, and MASASHI IINO Chemical Research Institute of Nonaqueous Solutions, Tohoku University. Katahira, Sendai-980, Japan (Received 3 January 1989; accepted in revked form 21 March 1989) Abstract-Both heating and cooling processes for mesophase formation from various pitches have been followed by in situ ESR measurements. In addition to signal intensity and linewidth, the saturation phenomenon of the signal intensity with changing microwave power was measured during heat treatment and cooling processes. As a convenient measure of the saturation factor, l/P,,, (P,,,,, is the microwave power at which the signal shows maximum intensity) was employed. In general, l/P,,, decreased with an increase in heat-treatment temperature; this change seems to correspond to a decrease in the viscosity of pitch. When pitches were heat treated at 500°C for 2 h, the carbonization reactions and the rearrangement of molecules may occur, which irreversibly changes 1iP,& Pronounced differences in the behavior of l/P,,,,, measured during cooling of the heat-treated pitches were found to depend upon pitch optical texture (isotropic, flow, and mosaic). Key Words-Mesophase,
pitch, ESR.
2. EXPERIMENTAL
1. INTRODUCTION
2.1 Materials and carbonization process Elemental analysis and softening point of four pitches investigated in this study are summarized in Table 1. The primary quinoline insoluble fraction was removed from a coal tar pitch (Mistubishi Kasei Co., Ltd.) with a mixed solvent consisting of CS2 and pyridine, which is as efficient at room temperature as is quinoline at elevated temperature[ll]. Under N,-gas flow, 5 g of each pitch was heated with a heating rate of 10”Cimin to a desired temperature, held at the temperature for a given time. and then cooled to room temperature. The microscopic observation of mesophase was performed by an Olympas BH-2 microscope with crossed Nicol polarizers.
Heat-treatment processes of pitches leading to the formation of mesophase and coke have been investigated by various methods[l,2]. To understand the weak interactions binding the polycondensed aromatic hydrocarbons in liquid crystal state, it has been pointed out that magnetic relaxation methods were quite useful[3-51. Some relationships between proton nuclear relaxation time and optical texture were revealed by Miyazawa et al.[3]. Electron spin relaxation (ESR) phenomena at room temperature were also used to assess the mesophase formation by Singer et al.[4] and by Doetschman et al.[5]. Also, it has been frequently pointed out that in situ measurements afford important information about mesophase formation processes. Microscopic observation[6] and X-ray diffraction[7,8] have been applied to follow the mesophase formation during heat treatment. For the carbonization process of acenaphthylene. in situ ‘H-NMR relaxation measurements were successfully applied by Bacon et al. [9]. In the previous paper, we reported that the in situ ESR measurements were useful to follow the carbonization process of coals; the saturation phenomenon of the signal intensity that was closely related to electron spin relaxation times was especially useful to reveal the molecular motions around the carbon radicals during heat treatments of coals[lO]. In this study, we applied this method to the mesophase formation process of coal tar pitch and petroleum pitches. On cooling down the heat-treated pitches, some differences in the in situ ESR parameters were found to depend upon the isotropic, flow, and mosaic textures. CM 27:6-F
2.2 In situ ESR measurements ESR spectra were measured with a JEOL FE3X ESR spectrometer equipped with a high-temperature ESR cavity. In a quartz ESR tube of inner diameter of 3 mm, 15 mg of pitch was placed and heat treated in the cavity under N, gas flow with a heating rate of 1OWmin. The cooling rate was also controlled at lO”C/min. In situ ESR parameters were measured keeping temperature for 5 min; the measurement intervals were 2040°C. For each pitch, ESR spectrum was seemingly a single signal at heat-treatment temperature (HTT) below 500°C. The saturation phenomena were analyzed by the continuous-wave-saturation method. Figure 1 shows an example for the saturation of ESR signal with microwave power (P). From the plot of the peak-peak signal height (h,,) against log P, the microwave power showing a maximum signal inten869
0.
870
IT0 et al.
Kubelka-Munk function, f(RJ, which is proportional to the absorbance in the ordinary transmittance method. By the diffuse reflectance method, the electronic transition in the near IR region can be measured without a background correction procedure, which is required for the usual transmittance measurements of carbonaceous materials[l4].
Table 1. Elemental analysist and softening point (sp) for four pitches Pitch (abbrev.)
C%
H%
N%
S%
sp”C
Ashland 240 (A-240) Coal tar pitch (CTP) Petroleum no. 4 (P4) Eureka pitch (Eureka)
91.4 90.7 84.2 88.1
5.7 4.2 8.5 5.5
0.2 0.1 1.3
0.5 4.7 5.1
120 97 190 100
tDifference from 100% may be attributed to both 0% and error in the measurements.
3. RESULTS AND DISCUSSION
3.1 Microscopic observation
sity (P,.,) and that at half the maximum signal intensity (Pin) were evaluated. Although P,,, is closely related to spin relaxation times, we employed P,,,= as a convenient measure of the saturation factor[l2], since a wide range of P,, could be extracted. Furthermore, at higher temperatures, P,, seems to be influenced by inhomogeneous behavior of the saturation phenomena more strongly than P,,,,, does[4,13].
Figure 2 shows the microscopic observations for cooled samples after heat treatment at 500°C for 2 h. Ashland 240 (A-240) and coal tar pitch (CTP) showed mesophase having large flow texture, whereas petroleum pitch no. 4 (P4) and Eureka pitch showed coarse grain-mosaic texture. When these pitches were cooled immediately after reaching a HlT of 5OO”C,isotropic carbonaceous materials were obtained; these observations are summarized in Table 2 with the ESR parameters for cooled samples.
2.3 Diffuse reflectance FT-IR Diffuse reflectance FT-IR spectra were measured by a JEOL JIR-100 equipped with a diffuse reflectance apparatus. The spectrum is expressed by the
3.2 In situ ESR measurement Figure 3 shows variation of l/P,,,, , h,, and peakpeak linewidth (AH,) for A-240 during heat treatment. The g-factor (2.0023-2.0025) did not appre-
a
mW
mW
b Pmax
Microwave
power
(mW)
Fig. 1. (a) Dependence of ESR signal intensity on microwave power (P) for A-240 at room temperature; signals are depicted with changing the central magnetic field; (b) Plot of h, against log P; P,,,., and P,,, are shown by arrow.
ciably vary with the heat treatment. The AH,, increased slightly with HTT, showing a maximum at 150°C followed by a gradual decrease until HTT of 500°C; keeping HTT at 500°C AH,, did not vary. In general, AH,, increases with increasing hydrogen content around the carbon radicals; however, such chemical change could not be considered at 150°C. The softening point of this pitch is close to this HTT; thus, the observed maximum of AH,, may rather correspond to some phase changes such as softening point. On cooling process, no appreciable change in AH,, was observed. The h, increased with HTT up to 150°C followed by a decrease to the original height at 400°C. On cooling down after reaching 5Oo”C, h,, decreased slightly. On keeping at 500°C h,, slightly increased again. A similar tendency showing an increase in the spin concentration at 150-200°C during heat treatment of petroleum pitch was also reported by Yokono et af. [ 151. The reason for the increase in h,, at 150°C was not clear. The slight increase in h, on keeping at 500°C was attributed to some carbonization reactions, as Singer and Lewis indicated[ 161. Figure 4 shows the diffuse reflectance FT-IR spectra for heat-treated samples of A-240. The spectrum for pitch cooled immediately after reaching 500°C is similar to that before heat treatment. No appre-
ciable change was found for the spectrum after heat treatment at 200°C. An increase in h, in Fig. 3 in the temperature region of 150-200°C occurs without accompanying chemical changes. If some parts of the carbon radicals in polycondensed aromatic hydrocarbons aggregate at low temperature yielding the singlet state, the breakdown of such weak interactions between the carbon radicals yielding the doublet state may increase the signal intensity. Since the increases in both A HP, and h,, were not observed in the heat treatment process of coal[lO], these observations may be characteristic of pitches. Furthermore, such large changes were not observed for pitches exhibiting mosaic texture, as will be described in a later section in this article. A further prominent change was observed for l/P,,, among the in situ ESR parameters. It is presumed that l/P,,, is closely related to molecular motion and phase change. At 50°C a steep decrease in l/P,,, was observed; although large change in molecular motion below 100°C have not been reported for pitches, it is possible that this behavior is related to phase changes such as glass transition. After 60°C l/P,,, increases again showing a maximum at 120°C which is in good agreement with the softening point of this pitch. Above 120°C l/P,, decreases gradually; this decrease may correspond to a decrease in
872
0.
0
100
200
Ccooling
(500%
- 0
l:cooling
(500%
- 2h
300
400
HTT (‘C)
500 (Oh)
500 (lh)
IT0
500 (2h)
Fig. 3. In situ ESR parameters for A-240. AH,,p; peakpeak linewidth, h,,,,; peak-peak height, and l/P,,,;,,. (0) for increasing HTT, (a) cooling immediately after HTT of SOW’C,and (0) cooling after soaking at 500°C for 2 h; sp refers to softening point.
the viscosity of the melted pitch. On keeping HTT at 500°C no appreciable change was observed. A minimum of l/P,,, (Pm,* = 5 mW; log l/P,,, = -0.7) was observed at 5OO”C-2 h. When the heat-treated sample was cooled down immediately after reaching 5OO”C, l/P,,, increased again along the heating line until 150°C. Below 150°C although two clear maxima at 150°C and at 50°C on the heating line disappeared on cooling, both lines substantially overlapped. By this heat treatment isotropic carbonaceous material was obtained; thus, the overlap of the cooling line with the heating line indicates insufficient soaking time. Similarity of the FT-IR spectrum for the heat-treated pitch at 500°C for 0 h (middle spectrum in Fig. 4) to that before the heat treatment supports this interpretation; however, it should be remarked that the slight physical changes observed in l/P,,, were undetected by the FT-IR spectra only. On the other hand, when the sample was cooled
er
al.
after keeping at 500°C for 2 h, a slight increase of l/P,,, reproducing the heating line down to 350°C was observed. Below this temperature, log l/P,,,, remained around 0, which is far smaller than those of the immediate cooling process. With this heat treatment, A-240 exhibited large flow textures as shown in Fig. 2. This cooling line of l/P,,, therefore seems to be a measure for assessing flow texture. Some carbonization reactions during heat treatment at 500°C for 2 h were suggested from the FTIR spectra (upper spectrum in Fig. 4); instead of the decreases, in the aliphatic C-H bands at 2,916 and 1,443 cm-‘, the increases in the aromatic C-H band at 3,045 cm-’ and in the near-IR band due to the electronic transition of polycondensed aromatic hydrocarbons were observed. By the carbonization reactions accompanying the polycondensation of aromatic hydrocarbons, l/P,,, of cooled samples tends to decreaseI 17,181. Since 1/Pm,, for the cooling process of a sample soaked at 500°C for 2 h did not recover to l/P,,, of the heating process, the rearrangement of molecules in pitch may occur during the soak period or cooling process. The importance of soaking and cooling processes was recognizable in the in situ l/P,,, measurement. Figure 5 shows variation of ESR parameters for CTP. The AHPP changed in a similar manner to that of A-240; i.e. a slight broadening was observed at 200°C. In h,, a clear maximum was observed at 200°C. After passing the maximum, h, decreased; above 350°C it became less than that at room temperature. On keeping the pitch at 500°C h, slightly increased again. The increases in both AH,, and h, at 200°C seem to be characteristic of pitches exhibiting flow texture. For l/P,,,,, in Fig. 5, a rapid decrease was observed at lOO”C, which may also correspond to softening of CTP. A rapid decrease in l/P,,,,, after the softening point may be correlated with a decrease in viscosity. A minimum of l/P,,,,, was observed at 400-500°C. When the sample was cooled down after keeping at 500°C for 2 h, l/P,,,,x increased over the heating line; this suggests that the cooling sample from 500°C shows more mobile molecular motion than that along the heating sample. It is noticeable that the cooling line of l/P,,, for CTP exhibiting flow texture overlaps with that of A-240 (5OO”C-2 h). Thus, this type of the cooling line can be considered characteristic of flow texture. Figure 6 shows the in situ ESR parameters for P4 which exhibits a coarse grain-mosaic texture after heat treatment at 500°C for 2 h. No appreciable change in AH,,,, was observed over the entire temperature range. Although h, increased slightly around 2OO”C, the extent is smaller than those for A-240 and CTP. On soaking at 500°C for 2 h, h,, increased. On cooling h, continues to increase. In the heating line of l/P,,,, a sharp maximum was found at 160°C that corresponds approximately to the softening point. On heating up to 5OO”C,
ESR measurement of mesophase formation
0.8
873
allphetic C-H
n
heating
23 Kicm’
1443cm-l ‘i 87&r;;’
5000
4000
I, 3000
2000
1000
wavenumber (ctil) Fig. 4. Diffuse reflectance FT-IR spectra of heat-treated A-240; 1 mg of sample in 150 mg KBr.
l/P,,, decreased. The minimum of log l/P,,, at 500°C for 0.5 h was - 1.2, which was smaller than those of A-240 and CTP at the same temperature (-0.7--0.9). On cooling after soaking at 500°C for 2 h, log l/P,,, was recovered slightly but did not exceed -0.6. Such small l/P,,,, on the cooling line is a prominent difference from those observed for A-240 and CTP, which exhibits flow texture. In the case of Eureka pitch, which also exhibited a coarse grain-mosaic, h,, and AHpp did not change appreciably (Fig. 7). II P,,, of the original pitch before the heat treatment was smaller than those of other pitches, suggesting less mobility of molecules in the pitch; i.e. three-dimensional cross-linkages may exist. By heat treatment, log l/P,,, gradually decreased down to - 1.3 at 500°C. On cooling immediately after 5OO”C,l/P,,, increased and overlapped with the heating line, showing the heat treatment was insufficient; thus, isotropic carbonaceous materials were obtained. After keeping 2 h at 500% by which a coarse grain-mosaic was produced, l/P,,, remains below those of the heating line. This cooling line was quite similar to that of P4 showing a coarse grain-mosaic. Thus, we can use the cooling lines to
distinguish either the flow texture formation (Figs. 3 and 5) or the coarse grain-mosaic texture (Figs. 6 and 7). In h, and AH,, , prominent increases observed at GO-200°C also seem to be characteristic of mesophase pitches that are capable of producing a flow texture after appropriate heat treatment. On the other hand, pitches without these appreciable increases had difficulty producing flow texture with various heat treatment methods.
3.3 ESR parametersfor cuoled pitches The ESR parameters for cooled pitches before and after heat treatment are summarized in Table 2. The linewidth of Eureka original pitch is wider than those of other pitches; but the linewidth can not be used to distinguish between optical textures. Some relationships’ between the spin concentration and the anisotropic development in heat-treated pitches and coals have been reported[l6,19]. Although it is difficult to find a general tendency for different pitches used in this study, the spin concentrations for the pitches with anisotropic optical texture are higher than that of isotropic carbonaceous material. Fur-
874
0.
IT0 et a/.
1.
P4
CTP 0: heating
l:cooling
(600
1
J ‘C - 2h)
0.
0
0
2 ::0
100
200
300 I-ITT (‘Cl
400
’
600 (Oh)
500 (lh)
500 (2h)
ii % G
Fig. 5. In situ ESR parameters for CTP; same notation as in Fig. 3.
9) 2 0I 0
I
I
I
I
100
200
300
400
I
500
500
500
(Oh)
(lh)
(2h)
HTT (C)
thermore, the spin concentrations for pitches with mosaic texture seem to be higher than those with flow texture. A clear trend was found for Pm, of cooled samples. Heat-treated pitches with similar Pm,, to that of isotropic original pitches remained as isotropic. Pitches with P,,,,, larger than 3.8 mW (or with log l/P,,,, smaller than -0.58) exhibited mosaic texture. Table 2. Optical texture of heat-treated
Fig. 6. In situ ESR parameters for P4. Pitches that exhibited flow texture showed an intermediate P,,,= value of 0.8-1.2 mW (log l/P,,,,, of -0.1-0.1). To further confirm this trend, it will be necessary to measure in situ ESR parameters for many other pitches; a further study is in progress.
samples and ESR parameters of cooled samples ESR parameters
Pitch
HHT (“C)
A-240
500 500 500 500
Cl-P P4
Time (h)
Optical texture
0 2
Isotropic Flow
2
Flow
2
Coarse mosaic
Eureka 5% 500
Isotropic Coarse mosaic
Spin cont. t (X 10’”spin/g) 2.4 1.6 4.5 9”:: 4.4 15.0 19.3 15.2 26.0
A%
(G)
5.4 6.0 4.5 4.8 4.2 4.8 5.1 6.0 6.2 6.8
P,,. (mw) 0.07 0.13 1.13 0.21 0.84 0.21 3.83 1.57 2.07 4.53
tThe spin concentrations were calculated by double integration of the derivative signal comparing with a standard 2,2-diphenyl-1-picrylhydrazyl (DPPH) in KBr.
ESR measurement
0: heating (500
l:cooilng
(5OO’c
x75
which is closely related to the spin relaxation times, l/P,,, varied drastically with heat-treatment temperature. 1/Pmaxwas useful in distinguishing the isotropic, flow, and mosaic textures. The signal intensity and linewidth also afford some clues for the assessment of mesophase pitches.
Eureka
Q: cooling
of mesophase formation
‘C - 0 h -
2h
Acknowledgements-The authors are grateful to Dr. T. Iwasaki of Government Industrial Research of Tohoku, Japan, for the use of the high-temperature ESR cavity. We express our thanks to T. Hino of Toa Nenryou Kogyo K. K. and S. Saito of Mitsubishi Oil Co. Ltd. for their kind donations of pitches. We are also indebted to Dr. N. Tsuchiya and Dr. M. Shishido of Tohoku University for their useful discussions. REFERENCES
0 96 % a4 $2 0 0
100
200
300
400
I-ITT (‘C)
500
500
500
(Oh)
(lh)
(2h)
Fig. 7. In situ ESR parameters for Eureka pitch
4. CONCLUSION For the mesophase formation process of various pitches, in situ ESR measurements have been em-
ployed. Among the ESR parameters such as signal intensity, linewidth, and saturation factor (l/P,,,),
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