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Carbon Fibers and Films Based on Biomass Resins W. M. Qiao,* M. Huda, Y. Song, S.-H. Yoon, Y. Korai, and I. Mochida* Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga, Fukuoka 816-8580, Japan
O. Katou Nippon Oil Company, Yokohama, Kanagawa 231-0815, Japan
H. Hayashi Kansai Electric Power Co., Ltd., Amagasaki, Hyogo 661-0974, Japan
K. Kawamoto General Environmental Technos Co., Ltd., Osaka 541-0052, Japan Received February 22, 2005. Revised Manuscript Received May 31, 2005
The analysis of biomass tar by 1H NMR, FT-IR, TOF-MS, GC-MS, and TG-MS indicated that it is different from tar or pitch from fossil fuels because of the remarkable oxygen content in biomass tar, originating from the botanical precursor, and it consists basically of phenols with one to three aromatic rings. It was polymerized in formaldehyde solution and yielded 70% of a tar-derived resin. The polymerization increased its molecular weights and improved its softening point. Although biomass tar gave a very small amount of char when carbonized at 600 °C, biomass resin exhibited twice the carbonization yield, about 30%. The resin was successfully spun, stabilized, and carbonized to carbon fibers at yields of 40-50%. The biomass resin-based carbon fibers showed mechanical properties (tensile strength and modulus up to 632 MPa and 44 GPa, respectively) comparable with commercially available samples. Further activation of carbon fibers provided activated carbon fibers with surface areas of 450-1600 m2/g, yields of 84-40%. The resin could also be spread over an aluminum foil, to be stabilized and carbonized into carbon films with thicknesses of 1-6 µm. Such a resin can be expected to be a useful precursor to prepare carbon materials.
Introduction Compared with fossil fuels such as coal and petroleum, wood is a renewable resource that can provide an available energy source when the cycle of its plantation and use is properly scheduled, although its calorific value is rather low. Pyrolysis of wood in the absence of air produces gas and liquid tar as well as solid carbon called charcoal. The charcoal can be used as a convenient solid fuel of higher calorific value by fixing carbon into a stable form and also used as an effective adsorbent. The byproduct tar contains a mixture of phenols,1-7 some of which can be valuable chemicals, * Corresponding authors. Tel: +81-92-5837801, Fax: +81-925837798, E-mail:
[email protected] (Qiao). E-mail:
[email protected] (Mochida). (1) Churin, E.; Maggi, R.; Grange, P.; Delmon, B. In Research in thermochemical biomass conversion; Bridgwater, A. V., Kuester, J. L., Eds.; Elsevier Applied Science: Barking, Essex, U.K., 1988; p 896. (2) Ogan, K.; Katz, E. Anal. Chem. 1981, 53 (2), 160-163. (3) Elder, T. J.; Soltes, E. J. Wood Fiber 1980, 12, 217-226. (4) Achladas, G. E. J. Chromatogr., A 1991, 542, 263-275. (5) Amen-Chen, C.; Pakdel, H.; Roy, C. Biomass Bioenergy 1997, 13 (1-2), 25-37.
although their separation8,9 and purification are complicated and costly because their contents are very limited. Hence, its application as a carbon source may be more feasible since phenols are the major components. The pretreatment and carbonization properties of eucalyptus pitch have been reported to prepare a potential precursor for carbon materials.10-13 In the references, Prauchner et al. produced eucalyptus tar (6) Williams, P. T.; Horne, P. A. J. Anal. Appl. Pyrol. 1995, 31, 1537. (7) Chandal, P.; Kaliaguine, S.; Grandmaison, J. L.; Mahay, A. Appl. Catal. 1984, 10 (3), 317-332. (8) Pakdel, H.; Roy, C.; Zeidan, K. In Research in thermochemical biomass conversion; Bridgwater, A. V., Kuester, J. L., Eds.; Elsevier Applied Science: Barking, Essex, U.K., 1987; p 572. (9) Meier, D.; Larimer, R. D.; Faix, O. Fuel 1986, 65 (7), 916-921. (10) Prauchner, M. J.; Pasa, V. M. D.; Otani, C.; Otani, S. Energy Fuels 2001, 15 (2), 449-454. (11) Prauchner, M. J.; Pasa, V. M. D.; Otani, C.; Otani, S.; Menezes, S. M. C. J. Appl. Polym. Sci. 2004, 91 (3), 1604-1611. (12) Prauchner, M. J.; Pasa, V. M. D.; Molhallem, N. D. S.; Otani, C.; Otani, S.; Pardini, L. C. Biomass Bioenergy 2005, 28, 53-61. (13) Prauchner, M. J.; Pasa, V. M. D.; Otani, S.; Otani, C. Carbon 2005, 43, 591-597.
10.1021/ef050046j CCC: $30.25 © 2005 American Chemical Society Published on Web 08/16/2005
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Table 1. Basic Data for Wood, Bamboo Tars, and Their Resins after Polymerization C
Elemental Analysis (wt %) H N O (by diff.)
C/H (atomic ratio)
C/O (atomic ratio)
yield (%)
ash (%)
SP (°C)
wood tar wood tar resin stabilized fibers
64.66 68.98 67.80
6.92 6.78 4.90
0.16 0.14 0.20
28.26 24.10 27.10
0.78 0.85 1.15
3.05 3.82 3.34
72
0.05 0.17
60
bamboo tar bamboo tar resin stabilized fibers
61.98 70.02 69.20
7.33 6.56 4.50
0.42 0.52 0.55
30.27 22.90 25.75
0.70 0.89 1.28
2.73 4.08 3.58
80
0.21 0.10
80
from pyrolysis of eucalyptus chips at 500 °C and obtained eucalyptus tar pitch with a yield of about 50% by vacuum distillation for 8 h (distillation cut temperature: 180 °C at 30-38 mmHg) as crude wood tar pitch. The pitch was further modified to develop a carbon precursor through heat-treatment at 250 °C. The modification improved its softening point from 76 to 134 °C and increased molecular weights from 2000 to 6000. The above authors further prepared carbon fibers by using pitch modified at 250 °C for 6 h, which showed a low yield of about 20% from tar to carbon fibers (CFs) and inferior mechanical properties (tensile: 130 MPa; modulus: 14 GPa) as compared to general-purpose carbon fibers.14 The above process does not appear so efficient because in which some compounds with small molecular weights may not be effectively polymerized, as a consequence, the carbon yield is low. Moreover, the modified pitch showed broad molecular weight distribution and rather high molecular weights, and appears not to be most suitable for the application. Generally, the polymerization of phenols with formaldehyde is employed in a commercial manufacture of phenolic resin, which is used as an important precursor for many carbon materials such as glasslike carbon, carbon fibers, activated carbon fibers, carbon films, and binder. To develop a more competitive carbon precursor from the renewable resource, the conversion of biomass tar into biomass resin through similar processing should be explored according to components in biomass tar. A simple and practical process is necessary to be considered to develop carbon materials with high carbon yield and high performances from the renewable resource. In the present study, biomass tars (wood tar and bamboo tar) were polymerized in formaldehyde by using acid catalyst to prepare biomass tar-based resins. The molecular compositions of biomass tars and their resins were analyzed by 1H NMR, FT-IR, TOF-MS, GC-MS, and TG-MS. The resultant resins were spun or spread on the surface of aluminum foil, stabilized, and carbonized to carbon fibers and carbon films, respectively. Carbon fibers (CFs) were further activated into activated carbon fibers (ACFs). Experimental Section Polymerization of Biomass Tar into Resin. Two kinds of biomass tars (wood tar and bamboo tar) were provided by Kansai Electric Power Co., Ltd, in which wood tar was collected during pyrolysis (highest temperature: 500 °C) of mixed woods from trees of beech, oak, and maple, and bamboo tar was collected during pyrolysis (highest temperature: 750 °C) of phyllostachys pubescens. (14) Mora, E.; Blanco, C.; Prada, V.; Santamarı´a, R.; Granda, M.; Mene´ndez, R. Carbon 2002, 40 (12), 2719-2725.
A 120 g sample of bamboo tar was polymerized with 120 g of an aqueous 37% formaldehyde solution in a flask with stirring by using oxalic acid and HCl as catalysts at 90 °C for 1 h and then for 35 min (two stages), respectively. Bamboo tar resin was obtained after water was removed. Wood tar was also polymerized into wood tar resin according to the above process after 0.1% Fe(NO3)3 was added to introduce mesopores in activated carbon fibers. Preparation of Carbon Fibers, Activated Carbon Fibers, and Carbon Films from Biomass Resin. The above resins were spun at 130 and 180 °C for wood tar-based resin and bamboo tar-based resin, respectively, under a nitrogen pressure of 0.2 MPa through a nozzle with a diameter of 0.3 mm and a length of 0.9 mm by a winding rate of 250 rpm, stabilized at 200 °C for 1 h with a heating rate of 10-20 °C/h, carbonized at 650-1000 °C for 1 h with a heating rate of 1-2 °C/min in a flow of argon to prepare carbon fibers (CFs), and further activated with CO2 at a rate of 100 mL/min at a temperature range from 900 to 1000 °C for 1 h to obtain activated carbon fibers (ACFs). In addition, as-prepared resin was dissolved in ethanol and spread over a piece of aluminum foil to prepare resin films. After the solvent was evaporated, resin films were stabilized at 200 °C in air for 1 h with a heating rate of 10 °C/h. To avoid the melting of aluminum foil at high temperature, stabilized films were carbonizd at 600 °C for 1 h with a heating rate of 2 °C/min into carbon films. Analysis of Biomass Tar and Resin. Elemental compositions of biomass tars and resins are summarized in Table 1. NMR (JEOL, JNM-LA 400) was used to obtain the information on hydrogen distribution and aromaticity (fa) of tar, in which CDCl3 was used as solvent. FT-IR was used to qualitatively verify the functional groups of tar and its resin. Dried spectrometric grade KBr (100 mg) and 0.5 mg of resin or its stabilized form were mixed and ground in a mortar with a pestle and pressed under a hydraulic pressure of 30 MPa. The transmittance of a KBr pellet was measured by using an FTIR spectrometer (JEOL 100) by averaging 100 times repeated scanning at 4 cm-1 resolution. (A baseline correction was not applied in this measurement.) In contrast, an FT-IR spectrum of tar was measured by painting tar on the surface of a KBr pellet. Molecular weight distributions of tar and its resin were measured by MALDI-TOF-MS (JEOL, JMS-ELITE-II) (matrixassisted laser desorption ionization time-of-flight mass spectrometer) under reflector mode without matrix assistance. GCMS (GC: HP6890; MS: HP5973) was employed to identify semiquantitatively organic species in tar and its resin, in which a DB-5MS column (length: 30 m; diameter: 0.25 mm; injecting temperature: 280 °C) was used, and acetone was used as solvent for diluting tar (500:1). TG-MS (Seiko Instruments, TG/DTA 220; ANELVA, M-200GA-DF) was employed to follow the carbonization property of tar and its resin (temperature: 30-1000 °C; heating rate: 10 °C/min; carrier gas: He of 100 mL/min; sample weight: 50 mg). Characterization of Carbon Fibers, Activated Carbon Fibers, and Carbon Films. The strength and modulus of carbon fibers were measured and calculated according to ref 15. The crystallographic data of carbon fibers and carbon films were collected with XRD (Rigaku Geigerflex, 40 kV, 30 mA,
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Figure 1. 1H NMR spectra of wood and bamboo tars: (a) wood tar; (b) bamboo tar. Table 2.
1H
NMR Analysis of Wood and Bamboo Tars in CDCl3 Hydrogen Distributiona (%)
wood tar bamboo tar
Ha
HR
Hβ + Hγ
fa
19.0 25.8
66.5 56.3
14.5 17.8
0.48 0.47
a H ) aromatic hydrogen, δ(1H) ) 6-9 ppm; H ) R-hydrogen, a R δ(1H) ) 1.7-4 ppm; Hβ ) β-hydrogen, δ(1H) ) 1-1.7 ppm; and Hγ 1 ) γ-hydrogen, δ( H) ) 0.5-1 ppm.
Cu KR target), and the crystallographic parameter (d002) was calculated according to the JSPS procedure.16 A powdered sample was mixed with standard silicon at a weight ratio of sample to silicon of 98:2, prior to the analysis. Raman spectroscopy (JASCO, NRS-2000B) of carbon fibers and carbon films was also measured to examine their degree of graphitization, in which an excitation source was an Ar-ion laser with a wavelength of 514.5 nm. Morphologies of spun, stabilized carbon fibers and carbon films were observed under SEM (JEOL, JSM 6320F) after Os-coating through an Osmium Plasma Coater OPC80. Nitrogen adsorption isotherms of activated carbon fibers were measured with a Sorptomatic 1990 adsorption analyzer (FISONS Instruments) at liquid nitrogen temperature. All samples were degassed at 150 °C for 2 h prior to the measurements. The specific surface area was calculated in a relative pressure interval of 0.05-0.35 using a BET method. Total pore volume (at relative pressure of 0.98) of ACFs was calculated through a nitrogen adsorption isotherm. The micropore volume of ACFs was evaluated by using Alpha s-plot.
Results and Discussion Compositional Analysis of Biomass Tar. Table 1 summarizes some properties of biomass tars and their resins. Wood and bamboo tars all show very high oxygen contents of 28 and 30%, respectively. Wood tar exhibits slightly higher carbon content than bamboo tar, 65 and 62%, respectively. Figure 1 shows 1H NMR spectra of biomass tars. Hydrogen distributions are summarized in Table 2. Their 1H NMR spectra exhibit peaks at chemical shifts of 1-4 ppm and 6-9 ppm, respectively, which are attributed to aliphatic and aromatic hydrogens, respectively. Aromaticities (fa) of wood tar and bamboo tar are 0.48 and 0.47, respectively, lower than those of fossil pitches.17-19 (15) Mochida, I.; Ling, L. C.; Korai, Y. J. Mater. Sci. 1994, 29, 30503056. (16) Japan Society for the Promotion of Science (117th committee). Tanso 1963, 36, 25-34. (17) Ollivier, P.; Gerstein, B. C. Carbon 1984, 22, 409. (18) Dickinson, E. M. Fuel 1980, 59, 290.
Figure 2 shows FT-IR spectra of bamboo tar, its resin, and stabilized form (wood tar exhibits an IR spectrum similar to that of bamboo tar). The figure exhibits a lot of oxygen-containing functional groups, especially -OH groups in biomass tar. Strong absorption is observed at 3200-3600 cm-1, attributed to free phenolic OH and COOH groups. Two carbonyl bands demonstrate the presence of carboxylic acids and ketones at 1700 cm-1 as well as ether and phenolic OH at 1100-1200 cm-1. The peaks at 1400-1600 cm-1 are certainly ascribed to aromatic structures. Figure 3 shows TOF-MS spectra of tars and their resins. Wood tar shows a distribution of molecular weights from 200 to 600, while bamboo tar exhibits a slightly smaller molecular range of 160 to 450. Their average molecular weights are calculated as 310 and 315, respectively. Table 3 summarizes molecular compositions of tars derived from their GC-MS spectra (they are not shown here). Biomass tars contain many organic species, over 30. Bamboo tar carries more phenol, methyl phenol, 4-ethyl-4-methoxy phenol, and 2,6-dimethoxy phenol than wood tar, while wood tar has more benzenediol, dimethyl phenol, 2-methoxy-4-propyl phenol, and 2-methoxy-4-(1-propenyl) phenol. Relative phenolic contents of bamboo and wood tars are 71 and 50%, respectively. Figure 4 shows TG profiles of tars and their resins. The carbonization yields at 900 °C are low: 15 and 12% for bamboo and wood tars, respectively. Polymerization of Biomass Tar into Resin. The polymerization of wood/bamboo tars with formaldehyde provided resins with yields of 72 and 80%, respectively. The higher yield of bamboo tar resin may reflect its higher contents of phenols, or bamboo tar may contain more such groups that are easier to be accessed and reacted, as a consequence, and the yield is higher. Two resins show higher carbon contents of about 70% and lower oxygen contents of 22-24% than their precursors. Compared with their original tars, bamboo/wood tarbased resins give higher carbonization yields, about 32 and 28%, respectively, as shown in Figure 4. TOF-MS spectra of tar resins demonstrate molecular weights of 200-800 by polymerization of tars with formaldehyde, as shown in Figure 3. Their average molecular weights are calculated as 386 and 406 for wood/bamboo tar resins, respectively. The softening points of wood/bamboo tar resins are 60 and 80 °C, respectively. IR spectra of resin indicate abundant functional groups, especially -OH groups, in the resin. Compared with bamboo tar, its resin still exhibits strong absorption peaks as observed at 3200-3600 cm-1 of phenolic OH groups as well as more remarkable absorption peaks at 1200 and 1264 cm-1 of phenolic OH and ether groups, as shown in Figure 2. Elemental compositions of wood/bamboo tars indicate that they contain high oxygen contents originating from wood/bamboo precursors. Low aromaticity of biomass tar demonstrates its different molecular structures with other tars from fossil fuels. Biomass tar exhibits rich -OH groups as shown in Figure 2, attributed to (19) Zhang, H. P.; Lu, X. C.; Li, K. X.; Liu, C. L.; Ling, L. C. New Carbon Mater. 2001, 16 (2), 49-53.
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Figure 2. FT-IR spectra of bamboo tar (a), its resin (b), and stabilized form (c).
Figure 3. TOF-MS spectra of tars and their resins: (a) wood tar (Mn ) 311); (b) wood tar resin (Mn ) 386); (c) bamboo tar (Mn ) 315); (d) bamboo tar resin (Mn ) 406).
structures of phenols. Biomass tar is composed of a complex mixture of organic species as shown in Table 5, in which phenols are major components. It is certain that molecular compositions of biomass tar depend on pyrolysis of wood/bamboo and kind of wood/bamboo. So, its compositions can be adjusted through changing pyrolysis conditions of wood/bamboo and selecting precursors. Biomass resin can be prepared through a reaction of phenols in biomass tar with formaldehyde by acid catalyst. The resin provides significant evolutions of CH4 and H2O as shown in Figure 5. The former is ascribed to the decomposition of methylene bridges among phenols, while the latter is attributed to the intermolecular dehydration of phenolic groups to form ether bridges. These phenols react with formaldehyde to improve average molecular weights through the formation of methylene bridges converted from methylol group (-CH2-OH) in phenols by further promoting the reaction among phenols. Preparation parameters such as polymerization temperature and time, kind of catalyst,
and amounts should be optimized in further research to obtain biomass resin with high performance. Compared with phenol, biomass tar obviously contains complicated compositional structures. On the basis of the characteristics of precursors, biomass resin is expected to exhibit not only some similarity to phenolic resin but also different properties. Both factors of biomass resin should be considered for its effective application. Preparation of Carbon Fibers, Activated Carbon Fibers, and Carbon Films. Biomass resin exhibited good spinning properties at a temperature lower than 200 °C. After spun fibers were stabilized and carbonized, resin-based carbon fibers (diameter: about 15 µm) were successfully prepared as shown in Figure 6. Stabilized resin fibers show a decrease of phenolic OH as shown in Figure 2, even though other bands at 1000-1200 cm-1 reduce, too. The FTIR spectrum of stabilized resin appears not to show clearly oxidative reactions (crosslink with oxygen) during the stabilization of fibers because both resin and its stabilized form exhibit relatively high oxygen contents. However, C/O ratios of resins decrease after stabilization, from 3.8 to 3.3 and from 4.1 to 3.8 for wood and bamboo tar resins, respectively, as shown in Table 1. The decrease confirms those oxidative reactions in resin during its stabilization in air. Some properties of fibers derived from resins are summarized in Table 4. The stabilization resulted in a slight decrease of weight by about 10% due to the loss of the volatile components with small molecular weights. A slow heating rate is necessary for stabilized fibers to be carbonized without melting and deforming. The stabilized fibers derived from bamboo and wood tar resins show carbonization yields of 48 and 40%, respectively. The interlayer spacings by XRD and intensity ratios of I1350/I1580 in Raman spectroscopy of two kinds of carbon fibers are 0.38-0.42 nm, 0.72-0.97 and 0.38 nm, 1.1 for bamboo and wood tar resins, respectively. Table 5 summarizes mechanical properties of two kinds of carbon fibers. Carbon fibers derived from bamboo tar resin display a tensile strength range from 340 to 620 MPa and a modulus from 27 to 44 GPa, depending upon carbonization conditions. Carbonization temperature is very influential on mechanical proper-
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Table 3. Compound Contents of Wood and Bamboo Tars Followed by GC-MS Analysis GC Peak Area (%) No.
RT (min)
compound
MW
formula
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
5.68 8.18 9.64 10.74 11.29 11.77 12.72 12.99 13.43 13.52 14.63 14.70 14.78 14.86 15.09 15.52 16.47 16.95 17.75 18.03 19.36 19.46 20.06 20.54 20.67 21.71 23.20
2-furancarboxaldehyde butyrolactone 5-methyl-2-furancarboxaldehyde tetrahydro-2-furanmethanol 3-methyl-1,2-cyclopentanedione phenol methoxy phenol methyl phenol methyl phenol methyl phenol dimethyl phenol dimethyl phenol 2-methoxy-4-methyl phenol 2-methoxy-4-methyl phenol 2-ethyl phenol dimethyl phenol 4-ethyl-4-methoxy phenol benzenediol 2,6-dimethoxy phenol 2-methoxy-4-propyl phenol 4-hydroxy-3-methoxybenzoic acid 2-methoxy-4-(1-propenyl) phenol 1-(4-hydroxy-4-methoxyphenyl)-ethanone 1-(2,6-dihydroxy-4-methoxyphenyl)-ethanone 1-(4-hydroxy-3-methoxyphenyl)-2-propanone 2,6-dimethoxy-4-(2-propeny) phenol 2,6-dimethoxy-4-(2-propeny) phenol
96 86 110 102 112 94 124 108 108 108 122 122 138 138 122 122 152 110 154 166 168 164 166 182 180 194 194
C5H4O2 C4H6O2 C6H6O2 C5H10O2 C6H8O2 C6H6O C7H8O2 C7H8O C7H8O C7H8O C8H10O C8H10O C8H10O2 C8H10O2 C8H10O C8H10O C9H12O2 C6H6O2 C8H10O3 C10H14O2 C8H8O4 C10H12O2 C9H10O3 C9H10O4 C10H12O3 C11H14O3 C11H14O3
peak area of phenol-containing group (%)
wood tar
bamboo tar 2.697
0.822 0.461 1.260 2.358 1.195 8.315 2.106 1.355 2.544 1.185 0.870 1.081 11.606
5.420 7.080 2.403 2.926 2.417
1.869 0.768 2.747
7.222
4.210
11.339 1.993 6.491
5.407 9.025
3.846 1.328 5.508 4.572 1.200 2.672 49.60
71.10
Table 4. Properties of Carbon Fibers Derived from Tar Resins stabilization conditions
stabilization yielda (%)
bamboo tar resin
200 °C-1 h-10 °C/h 200 °C-1 h-10 °C/h 200 °C-1 h-20 °C/h
90 90 87
wood tar resin
200 °C-1 h-10 °C/h 200 °C-1 h-10 °C/h
87 87
fiber
a
carbonization conditions
carbonization yieldb (%)
total yielda (%)
d002 (nm)
R, I1350/ I1580
650 °C-1 h-60 °C/h 1000 °C-1 h-60 °C/h 1000 °C-1 h-60 °C/h
47 49 47
42 44 41
0.417 0.388 0.385
0.72 0.91 0.96
1000 °C-1 h-60 °C/h 1000 °C-1 h-12 0 °C/h
40 40
35 35
0.386 0.385
1.09 1.09
Yield base: resin. b Yield base: stabilized resin.
Figure 4. TG profiles of tars and their resins: (a) wood tar; (b) bamboo tar; (c) wood tar resin; (d) bamboo tar resin.
ties, as shown in Table 5. The carbonization at 800 °C provided the highest strength and maximum modulus. Carbon fibers from wood tar resin exhibit a low strength of 250 MPa and a relatively high modulus of 37 GPa. Carbon fibers from bamboo tar resin show a strength comparable with those of commercially available carbon
Figure 5. CH4 and H2O evolved from bamboo tar and its resin in TG-MS experiments: (a) tar-CH4; (b) resin-CH4; (c) tarH2O; (d) resin-H2O.
fibers from phenolic resin,20 while carbon fibers derived from wood tar resin show a low tensile strength because the resin contains more volatiles, whose evolving results in the formation of voids on the surface of carbon fibers, decreasing its strength. Salt additive may be another (20) http://www.kynol.com/NewFiles/kynol%20frameset.html.
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Table 5. Mechanical Properties of Carbon Fibers Derived from Bamboo and Wood Tar Resins carbonization (°C)
precursor
carbon content (%)
tensile strength (MPa)
modulus (GPa)
bamboo tar resin
650 800 1000
19.3 17.0 13.4
90.0 95.0 97.2
342 632 616
27 44 43
wood tar resin
1000
20.9
95.1
247
37
phenolic resina
800 2000 1000 1000
11.0-12.0 11.0-12.0 12.0-14.0 7.0-8.0
95.0 99.8 95.0 93.0
500-700 400-600 >500 >1500
20-30 15-20 >30 >150
pitcha PANa a
diameter of fiber (µm)
Commercially available carbon fibers from ref 20. Table 6. Properties of ACFs Derived from Biomass Tar Resins Activation Conditions temp (°C) time (h)
yielda (wt %)
SBET (m2/g)
pore volume (mL/g) total micro
micropore ratio (%)
precursor
sample
bamboo tar resin
ACF-1 ACF-2 ACF-3
900 950 1000
1 1 1
84 63 40
457 1048 1648
0.25 0.60 0.97
0.23 0.54 0.87
92 90 90
wood tar resin
ACF-4 ACF-5 ACF-6
900 950 1000
1 1 1
78 57 38
491 711 1020
0.35 0.61 0.89
0.24 0.32 0.46
69 53 48
phenolic resinb PANc pitchc cellulosec
ACF ACF ACF ACF
1500-2000 500-1200 1000-2000 1000-2000
0.63-0.75
a
0.50-1.10
Base: CF. b Commercially available carbon fibers from ref 20. c Commercially available activated carbon fibers from ref 21.
Figure 7. Nitrogen adsorption-desorption isotherms of ACF-3 and ACF-6.
Figure 6. Morphologies of spun, stabilized, and carbonized fibers from bamboo and wood tar resins.
factor responsible for the decrease. It should be noted that carbon fibers derived from biomass tar resins show much higher modulus than those from phenolic resin, ranging from 10 to 30 GPa. The diameter of carbon fibers prepared in the study is still larger than that of commercially available carbon fibers. Mechanical properties of carbon fibers from biomass tar resin can be expected to further improve through tailoring composition of original tar, resin, and carefully controlling preparation parameters for carbon fibers.
Table 6 summarizes properties of activated carbon fibers. ACFs from bamboo tar resin carry surface areas of 460-1650 m2/g and pore volumes of 0.25-0.97 mL/g at yields of 84 to 40% (base: CFs), depending upon activation conditions, while ACFs from wood tar resin exhibit surface areas of 490-1020 m2/g and pore volumes of 0.35-0.89 mL/g at yields of 78 to 38%. ACF-6 from wood tar resin exhibits a hysteresis loop as shown in Figure 7, indicating its mesoporous structures. ACF-3 from bamboo tar resin exhibits dominantly micropores about 0.4-2 nm, while ACF-6 shows a broader pore distribution from 0.4 to 4 nm. ACFs with high surface area and different porous structure can be developed from biomass resin through generally physical activation. Especially, a small amount of iron salt greatly improves the mesopore ratio. The reason is that nanosized iron metal obtained by carbon reduction at high temperature exhibits an excellent catalytic gasification effect during activation of carbon materials.22,23 (21) Yoon, S.-H.; Korai, Y.; Mochida, I. In Sciences of carbon materials; Marsh, H., Rodriguez-Reinoso, F., Eds.; Universidad de Alicante, 2000; p 317.
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Qiao et al. Table 7. Properties of Carbon Films Derived from Tar Resins stabilization carbonization yielda yieldb thickness d002 R, raw material (%) (%) (µm) (nm) I1350/I1580 wood tar resin bamboo tar resin
85 90
38 44
6 1
0.393 0.395
1.09 0.72
a Stabilization conditions: 200 °C for 1 h with a heating rate of 10 °C/h in air. b Carbonization conditions: 600 °C for 1 h with a heating rate of 120 °C/h under argon flow.
Conclusion
Figure 8. Morphologies of carbon films derived from wood and bamboo tar resins. Bamboo tar resin: (a) surface, (b) cross section; Wood tar resin: (c) surface, (d) cross section.
Carbon films prepared from two resins exhibit smooth surfaces under SEM, as observed in Figure 8, whose thicknesses vary from 1 to 6 µm. Some properties of carbon films are summarized in Table 7. Their interlayer spacing by XRD and intensity ratio of I1350/I1580 in Raman spectroscopy is 0.39 nm and 0.72-1.1, respectively, indicating nongraphitic natures. It is wellknown that amorphous carbon films have many applications in metal coating, gas separation, catalyst support, field emission, and microelectronic devices. Compared with other carbon films, carbon films from biomass resin may be more competitive because of their cheap raw material and simple processing. (22) Rodriguez-Reinoso, F. In Instruction to carbon technologies; Marsh, H., Edward, A. H., Rodriguez-Reinoso, F., Eds.; Universidad de Alicante, 1988; pp 83-84. (23) Liu, Z. C.; Ling, L. C.; Qiao, W. M.; Liu, L. Carbon 1999, 37, 663-667.
Analyses of molecular compositions of wood/bamboo tars reveal that they contain a few phenol-group species. The components may be polymerized into a thermosetting resin. The resin can be stabilized and carbonized to develop carbon materials. In the present study, carbon fibers, activated carbon fibers, and carbon films have been successfully developed from biomass resin. The mechanical properties (maximum strength and modulus: 632 MPa and 44 GPa, respectively) of carbon fibers are found to be comparable and competitive with those of commercially available carbon fibers. The resultant activated carbon fibers exhibit high surface area of 1645 m2/g (pore volume: 0.97 mL/g). Carbon films can be also prepared from biomass resin. The resin can be expected to be a useful precursor to develop many carbon materials. Acknowledgment. The authors gratefully acknowledge Prof. Ikuo Komaki at Kitakyushu University for the GC-MS measurement and compositional analysis of biomass tars. EF050046J
