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Synthesis and characterization of eco-friendly carboxymethyl cellulose based carbon foam using electron beam irradiation
Composites Part B 151 (2018) 154–160
Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Synthesis and characterization of eco-friendly carboxymethyl cellulose based carbon foam using electron beam irradiation
T
Hong Gun Kima, Lee Ku Kwaca, Yong-Sun Kima, Hye Kyoung Shina,∗, Kyong-Yop Rheeb,∗∗ a b
Institute of Carbon Technology, Jeonju University, 303 Cheonjam-ro, Wansan-gu, Jeonju-si, Jeollabuk-do 55069, Republic of Korea School of Mechanical and Industrial Systems Engineering, Kyunghee University, Yongin 449-701, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Carbon foam Eco-friendly composites Carboxymethyl cellulose Electron beam irradiation
Carbon foams were prepared by carbonization process without additional stabilization process after manufacturing carboxymethyl cellulose (CMC) composites using a facile electron beam irradiation (EBI) method. A gel fractionation technique was used to study cross-linking degree of infusible structure in the produced CMC composite materials. We observed an increase in cross-linking with increasing CA concentration and EBI doses between 20 kGy and 80 kGy. The CMC composite prepared using 4 wt% CA and 80 kGy EBI represents the highest gel fraction value of ∼98%, showing the highest carbon yields and compressive strength due to the increase of cross-linked parts in carbon foam obtained from these CMC composites, which lowers break defects after carbonization. In addition, available surface area was estimated via Brunauer-Emmett-Teller analysis of the carbon foam samples. The carbon foam produced from the CMC composite treated with 4 wt% CA via 80 kGy resulted in highest specific surface area of 372.06 m2/g and adsorption pore size of 2.20 nm indicating greater interaction between gas and the carbon atoms.
1. Introduction Carbonaceous materials have good thermal stabilities, chemical stabilities, and non-corrosive properties [1–15]. Carbon foams, one of various carbonaceous materials, have 3-dimensional (3-D) reticular structures of various shapes and sizes. Their high porosity makes them applicable as additives of phase change materials, in sound absorption, physical adsorption, and as catalyst supports. Thus, many researchers have reported on carbon foam-based materials due to their tunable and unique surface functionality and their well-defined macro- and microporous 3-D structures [16–25]. Various carbon foam manufacturing processes have been proposed and studied with diverse precursors such as phenolic resin, coal, and pitch obtained from fossil fuel, but utilization of these industrial side products in carbon foam synthesis generated large amounts of toxic gases and shrinkage during carbonization [26–35]. In addition, these processes for manufacturing carbon foam are multi-step, complex, and require large amounts of capital. Therefore, it is highly desirable to develop simple processes for carbon foam manufacturing that use biopolymers as precursors instead of toxic precursors derived from fossil fuel. Generally, biopolymers are biodegradable, biocompatible, and non-toxic. Among them, CMC is a cellulose derivative which has been substituted with sodium carboxylate
∗
Corresponding author. Corresponding author. E-mail addresses: [email protected] (H.K. Shin), [email protected] (K.-Y. Rhee).
∗∗
https://doi.org/10.1016/j.compositesb.2018.06.013 Received 9 April 2018; Received in revised form 21 May 2018; Accepted 12 June 2018 1359-8368/ © 2018 Elsevier Ltd. All rights reserved.
groups from the anhydroglucose unit of cellulose, which is one of the most abundant organic polymers obtained from wood or non-wood precursors. Therefore, CMC is classified as a non-toxic natural biopolymer that has the advantages of being biodegradable, biocompatible, and water-soluble. CMC has widely been used as a hydrogel due to its unique porosity [36–41] and still provide eco-friendly, light-weight, and low-cost materials. In addition, electron beam irradiation (EBI) can modify various substances without solvents or additives through chain scission, polymerization and cross-linking [42–44]. Therefore, using the above novel merits of CMC composites and EBI, we prepared echofriendly carbon foams through facile and simple method via EBI. However, during carbonization over 1000 °C, to survive without decomposing and thus to obtain a high carbon yield, cross-linking degrees in CMC foam composites is critical for commercial applications. In this study, we aimed to prepare 3-dimensional carbon foams from CMC of biopolymer without fossil fuel polymer for various applications. The influences of EBI doses and CA concentrations on the cross-linking of CMC foam composites for the preparation of carbon foams are measured by gel fraction, and the structural, and mechanical properties of the products are characterized by SEM, compressive strength, and BET.
Composites Part B 151 (2018) 154–160
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Fig. 1. Schematic diagram of the preparation of CMC carbon foam.
2. Experimental
adsorption-desorption isotherms specific surface areas, and adsorptionaverage pore size, obtained using a gas adsorption analyzer (BarrettJoyner-Halenda model).
2.1. Preparation of CMC composites and their carbon foams CMC (average MW: ≈ 250,000) and CA (C6H8O7·H2O) used in this research were purchased from Sigma-Aldrich Co. As shown in Fig. 1, for preparation of CMC composites, 13 wt% CMC was dissolved in distilled water by stirring at room temperature. 2 wt% and 4 wt% CA were respectively added to the 13 wt% CMC paste. The homogeneous CMC and CA paste mixtures were transferred into plastic molds. Carbonization reaction conditions were determined for optimal cross-linking of CMC and CA, to minimize sample decomposition at high temperature during the carbonization process. The samples were then irradiated at various absorbed doses of 20 kGy, 40 kGy, 60 kGy, 80 kGy, and 100 kGy (an accelerating voltage of 1.14 MeV, a beam current of 7.6 mA, irradiation width of 110 cm, distance between samples and window of 20 cm, dose rate of 6.67 kGy/sec) in air at room temperature. Dosimetry was carried out using cellulose tri-acetate (CTA; ISO/ASTM 51,650 (2013)). All irradiated samples were freeze-dried and CMC foam composites were obtained. The obtained CMC composites were carbonized for 1 h at 1000 °C (heating rate = 2 °C/min) in a tubular furnace under an atmosphere of high-purity nitrogen (99.999%) without stabilization.
3. Results and discussion 3.1. Gel fraction and carbon yields Cross-linking reaction between CMC and CA generally happens through esterification by hydroxyl groups of CMC and carboxyl groups of CA during EBI (Fig. 2). The gel fraction exhibit the degree of conversion of the linear structure of precursors to cross-linking structure in composites materials. As shown in Fig. 3, the gel fraction values obtained in CMC composites increased for EBI dose up to 80 kGy, followed by a slight decrease at 100 kGy. This trend is obseved for both CA concentrations of 2 wt% and 4 wt%. These results are because excessive radiation over a certain range of EBI dose can occurre decomposition by chain scission and then decrese gel fraction. Among CMC composites obtained in this study, CMC composite containing 4 wt% CA via 80 kGy of EBI exhibited the highest gel fraction value of around 97%. Table 1 shows the carbon yields of CMC composites, obtained according to various EBI doses and CA concentration. As exhibited in Table 1, we can see that the carbon yields for the CMC composites mostly increase with increasing EBI dosage up to 80 kGy and CA concentrations from 2 to 4 wt%. The slight decrease in the gel fraction as obseved in samples dosed at 100 kGy can be rationalized by the decomposition of the noncross-linked fraction in CMC composites at 1000 °C during carbonization. Therefore, it is concluded that CMC composites with high gel fraction values directly result in higher carbon yields.
2.2. Analysis Gel fraction values were obtained using distilled water, and the soluble fraction was extracted for 24 h at room temperature. The insoluble fraction was completely dried in a vacuum oven at 80 °C. The gel fraction values were calculated according to the following equation; Gel fraction (%) = (W2/W1) × 100
3.2. SEM images
where W2 is the weight of the insoluble fraction after extraction with distilled water, W1 is the known weight of CMC composites before extraction in distilled water. Scanning electron microscopy (SEM) images were obtained using a Jeol JSM 5910 LV microscope to study the morphology of CMC composites and their carbon foams. Compressive strength was measured by an Instron 5050 tester (Instron USA) by method of ASTM standard C365. In the Brunauer-Emmertt-Teller (BET) method, the textural properties were observed in terms of nitrogen
Digital photos of Fig. 4 show CMC composites ((a) and (c)) and their carbon foams ((b) and (d)). Carbon foam was prepared through carbonization at 1000 °C without stabilization process. As shown in Fig. 4, cross-linking in composites generally plays an important role in preparing of carbon foam. After carbonization, CMC composite containing 4 wt% CA via 40 kGy of EBI can be seen that most of them burned due to low gel fraction. However, we can see that the carbon foam obtained
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Fig. 2. Proposed mechanism for cross-linking of CMC and CA by EBI.
Table 1 Carbon yields of CMC composites obtained according to various EBI dose and CA concentration. CA concentration
2 4
EBI dose (kGy) 20
40
60
80
100
10.15 12.29
11.94 13.38
18.06 20.52
23.38 25.74
21.56 23.04
from CMC composites containing 4 wt% CA via 80 kGy of EBI maintains its shape stability of 3-dimensional structure because of high gel fraction. The apparent size shrinkage of CMC composite containing 4 wt% CA via 80 kGy of EBI is also clearly observed and its size reduction was estimated to be ∼34% due to high thermal decomposition of noncrosslinked fraction in CMC composites. Fig. 5 displays SEM images of surface morphologies for CMC composites and its carbon foams. CMC composites have non-uniform porosity with the pore size range of around 100–200 μm. After carbonization, the obtained carbon foams were also observed to have non-uniform porosity, while pore size
Fig. 3. Gel fraction of CMC composites obtained according to CA concentration and EBI doses.
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Fig. 4. Digital photos of CMC composites obtained by each condition and their carbon foams and their carbon foams.
distribution was measured to be 50–100 μm. This corresponds to a decrease of 34% of the pre-carbonization samples.
carbon foams obtained from each condition. The specific surface area and average pore size of adsorption were highest for the carbon foam obtained from CMC composite containing 4 wt% CA via 80 kGy EBI, implying that the carbon atoms in this carbon foam interacted with the more gas than the other carbon foams. The carbon foam obtained from CMC composite containing 4 wt% via 80 kGy presented the highest specific surface area of 372.06 m2/g and adsorption pore size of 2.20 nm among carbon foams.
3.3. Compressive strength Fig. 6 shows the compressive strength curves for CMC composites derived from 2 wt% and 4 wt% CA obtained according to EBI dosages and its carbon foams. Generally, compressive strengths of CMC composites were lower (0.31 and 0.67 MPa) than those of carbon foams. However, after carbonization, compressive strength increases from 0.47 MPa to 0.88 MPa. It could be attributed to the increase of crystalline carbon present in the carbon foams. In addition, higher crosslinked chemical species during carbonization would result in lower partial break and lower defects during carbon formation, and higher density in carbon foam. Therefore, carbon foam obtained from CMC composites containing 4 wt% CA via 80 kGy of EBI dose with the highest gel fraction value resulted in highest compressive strength value (0.88 MPa).
4. Conclusions CMC composites of biopolymer were used as carbon foams precursor and were prepared according to various EBI doses and CA concentration. This study was conceived and executed to optimize and increase cross-linked molecules in CMC composites that can withstand temperatures over 1000 °C during carbonization reaction. Gel fraction analysis for the obtained CMC composites increased with the increase of CA concentration and EBI doses between 20 kGy and 80 kGy. Among samples, CMC composite with 4 wt% CA via 80 kGy represents the highest gel fraction value of about 97% the highest carbon yields, and highest compressive strength. These enhanced materials properties result from higher cross-linking conversion in the carbon foam obtained from CMC composites with 4 wt% CA via 80 kGy of EBI dose reduce more break defects after carbonization. In BET analysis, the carbon foam obtained from CMC composite with 4 wt% via 80 kGy also was observed the highest specific surface area of 372.06 m2/g and adsorption pore size of 2.20 nm due to interaction of the more gas with further carbon atoms.
3.4. BET surface area analysis Fig. 7 shows the N2 adsorption isotherms for carbon foams obtained from CMC composites via 80 kGy and 100 kGy. As can be observed in Fig. 7, we could find that most of carbon foams drastically increased at P/P0 below 0.1 due to micro-pore structures but slowly increased at P/ P0 over 0.1 owing to meso- and macro-pore structures. These isotherms for the obtained carbon foams can be sorted as the isotherms of middle foam between Types I and II according to BET classification. Table 2 shows specific surface area and average pore size of adsorption for the
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Fig. 5. SEM images of CMC composites obtained by each condition and their carbon foams.
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Table 2 Textural properties for the obtained carbon foams obtained from each condition. EBI dose
80 kGy
100 kGy
CA concentration
2 wt%
4 wt%
2 wt%
4 wt%
Specific surface area (m2/g) Ads. average of pore size (nm)
329.29 2.13
372.06 2.20
311.07 2.11
344.11 2.05
Acknowledgement This research was supported by Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education (No. 2016R1A6A1A03012069). This work was also supported by the National Research Foundation of Korea grant funded by the Korea government (MSIP) (No. 2017R1A2B1008753). References [1] Chand S. Carbon fibers for composites. J Mater Sci 2000;35:1303–13. [2] Yadav M, Rhee KY, Park SJ, Hui D. Mechanical properties of Fe3O4/GO/chitosan composites. Compos Part B 2014;66:89–96. [3] Mittal G, Rhee KY, Park SJ, Hui D. Generation of the pores on grapheme surface and their reinforcement effects on the thermal and mechanical properties of chitosanbased composites. Compos Part B 2017;114:348–55. [4] Hu R, Chu L, Zhang J, Li X, Huang W. Carbon materials for enhancing charge transport in the advancements of perovskite solar cells. J Power Sources 2017;361:259–75. [5] Lu J, Guo K, Song Q, Li Y, Zhang L, Li H. In-situ synthesis silicon nitride nanowires in carbon fiber felts and their effect on the mechanical properties of carbon/carbon composites. Mater Des 2016;99:389–95. [6] Dhami TL, Bahl OP, Jain PK. Carbon-carbon composites made with oxidized PAN (Panex) fibers. Carbon 1995;33:1517–24. [7] Hambach M, Möller H, Neumann T, Volkmer D. Carbon fibre reinforced cementbased composites as smart floor heating materials. Compos Part B 2016;20:465–70. [8] Kang WS, Rhee KY, Park SJ. Thermal, impact and toughness behaviors of expanded graphite/graphite oxide-filled epoxy composites. Compos Part B 2016;94:238–44. [9] Shin HK, Park M, Kim HY, Park SJ. An overview of new oxidation methods for polyacrylonitrile-based carbon fibers. Carbon Lett 2015;16:11–8. [10] Thang PT, Nguyen TT, Lee J. Anew approach for nonlinear buckling analysis of imperfect functionally graded carbon nanotube-reinforced composite plates. Compos Part B 2017;127:166–74. [11] Chandra Y, Scarpa F, Adhikari S, Zhang J, Saavedra FEI, Peng HX. Pullout strength of graphene and carbon nanotube/epoxy composites. Compos Part B 2016;102:1–8. [12] Yao X, Gao X, Jiang J, Xu C, Deng C, Wang J. Comparison of carbon nanotubes and grapheme oxide coated carbon fiber for improving the interfacial properties of carbon fiber/epoxy composites. Compos Part B 2018;132:170–7. [13] Ozkan C, Karsli NG, Ayse A, Deniz V. Short carbon fiber reinforced polycarbonate composite: effects of different sizing materials. Compos Part B 2014;62:230–5. [14] Lu J, Guo K, Song Q, Li Y, Zhang L, Li H. In-situ synthesis silicon nitride nanowires in carbon fiber felts and their effect on the mechanical properties of carbon/carbon composites. Mater Des 2016;99:389–95. [15] Shin HK, Rhee KY, Park SJ. Effects of exfoliated graphite on the thermal properties of erythritol-based composites used as phase-change materials. Compos Part B 2016;96:350–3. [16] Li J, Wang C, Zhan L, Qiao WM, Liang XY, Ling LC. Carbon foams prepared by supercritical foaming method. Carbon 2009;47:1204–6. [17] Gallego NC, Klett JW. Carbon foams for thermal management. Carbon 2003;41:1461–6. [18] Chen C, Kennel EB, Stiller AH, Stansberry PG, Zondlo JW. Carbon foam derived from various precursors. Carbon 2006;44:1535–43. [19] Lei S, Guo Q, Shin J, Liu L. Preparation of phenolic-based carbon foam with controllable pore structure and high compressive strength. Carbon 2010;48:2644–73. [20] Inagaki M. Pores in carbon materials – importance of their control. New Carbon Mater 2009;24:193–222. [21] Michio I, Jieshan Q, Quangui G. Carbon foam: preparation and application. Carbon 2015;87:128–52. [22] Amandine F, Marc B, Rénal B, Guido S, Hervé D. Preparation of hierarchical porous carbonaceous foams from Kraft black liquor. Mater Today Commun 2016;7:108–16. [23] Letellier M, Macutkevic J, Kuzhir P, Banys J, Fierro V, Celzard A. Eelctromagnetic properties of model vitreous carbon foams. Carbon 2017;122:217–27. [24] Wang Y, He Z, Zhan L, Liu X. Coal tar pitch based carbon foam for thermal insulating material. Mater Lett 2016;169:95–8. [25] Letellier M, Ghaffari Mosanenzadeh S, Naguib H, Fierro V, Celzard A. Acoustic properties of model cellular vitreous carbon foams. Carbon 2017;119:241–50. [26] Dang A, Li T, Xiong C, Zhao T, Shang Y, Liu H, Chen X, Li H, Zhuang Q, Zhang S. Long-life electrochemical supercapacitor based on a novel hierarchically carbon foam template carbon nanotube electrode. Compos Part B 2018;141:250–7. [27] Lei S, Guo Q, Shi J, Liu L. Preparation of phenolic-based carbon foam with
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Fig. 7. N2 adsorption isotherms of carbon foams obtained from CMC composites prepared according to EBI doses and CA concentration.
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Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Synthesis and characterization of eco-friendly carboxymethyl cellulose based carbon foam using electron beam irradiation
T
Hong Gun Kima, Lee Ku Kwaca, Yong-Sun Kima, Hye Kyoung Shina,∗, Kyong-Yop Rheeb,∗∗ a b
Institute of Carbon Technology, Jeonju University, 303 Cheonjam-ro, Wansan-gu, Jeonju-si, Jeollabuk-do 55069, Republic of Korea School of Mechanical and Industrial Systems Engineering, Kyunghee University, Yongin 449-701, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Carbon foam Eco-friendly composites Carboxymethyl cellulose Electron beam irradiation
Carbon foams were prepared by carbonization process without additional stabilization process after manufacturing carboxymethyl cellulose (CMC) composites using a facile electron beam irradiation (EBI) method. A gel fractionation technique was used to study cross-linking degree of infusible structure in the produced CMC composite materials. We observed an increase in cross-linking with increasing CA concentration and EBI doses between 20 kGy and 80 kGy. The CMC composite prepared using 4 wt% CA and 80 kGy EBI represents the highest gel fraction value of ∼98%, showing the highest carbon yields and compressive strength due to the increase of cross-linked parts in carbon foam obtained from these CMC composites, which lowers break defects after carbonization. In addition, available surface area was estimated via Brunauer-Emmett-Teller analysis of the carbon foam samples. The carbon foam produced from the CMC composite treated with 4 wt% CA via 80 kGy resulted in highest specific surface area of 372.06 m2/g and adsorption pore size of 2.20 nm indicating greater interaction between gas and the carbon atoms.
1. Introduction Carbonaceous materials have good thermal stabilities, chemical stabilities, and non-corrosive properties [1–15]. Carbon foams, one of various carbonaceous materials, have 3-dimensional (3-D) reticular structures of various shapes and sizes. Their high porosity makes them applicable as additives of phase change materials, in sound absorption, physical adsorption, and as catalyst supports. Thus, many researchers have reported on carbon foam-based materials due to their tunable and unique surface functionality and their well-defined macro- and microporous 3-D structures [16–25]. Various carbon foam manufacturing processes have been proposed and studied with diverse precursors such as phenolic resin, coal, and pitch obtained from fossil fuel, but utilization of these industrial side products in carbon foam synthesis generated large amounts of toxic gases and shrinkage during carbonization [26–35]. In addition, these processes for manufacturing carbon foam are multi-step, complex, and require large amounts of capital. Therefore, it is highly desirable to develop simple processes for carbon foam manufacturing that use biopolymers as precursors instead of toxic precursors derived from fossil fuel. Generally, biopolymers are biodegradable, biocompatible, and non-toxic. Among them, CMC is a cellulose derivative which has been substituted with sodium carboxylate
∗
Corresponding author. Corresponding author. E-mail addresses: [email protected] (H.K. Shin), [email protected] (K.-Y. Rhee).
∗∗
https://doi.org/10.1016/j.compositesb.2018.06.013 Received 9 April 2018; Received in revised form 21 May 2018; Accepted 12 June 2018 1359-8368/ © 2018 Elsevier Ltd. All rights reserved.
groups from the anhydroglucose unit of cellulose, which is one of the most abundant organic polymers obtained from wood or non-wood precursors. Therefore, CMC is classified as a non-toxic natural biopolymer that has the advantages of being biodegradable, biocompatible, and water-soluble. CMC has widely been used as a hydrogel due to its unique porosity [36–41] and still provide eco-friendly, light-weight, and low-cost materials. In addition, electron beam irradiation (EBI) can modify various substances without solvents or additives through chain scission, polymerization and cross-linking [42–44]. Therefore, using the above novel merits of CMC composites and EBI, we prepared echofriendly carbon foams through facile and simple method via EBI. However, during carbonization over 1000 °C, to survive without decomposing and thus to obtain a high carbon yield, cross-linking degrees in CMC foam composites is critical for commercial applications. In this study, we aimed to prepare 3-dimensional carbon foams from CMC of biopolymer without fossil fuel polymer for various applications. The influences of EBI doses and CA concentrations on the cross-linking of CMC foam composites for the preparation of carbon foams are measured by gel fraction, and the structural, and mechanical properties of the products are characterized by SEM, compressive strength, and BET.
Composites Part B 151 (2018) 154–160
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Fig. 1. Schematic diagram of the preparation of CMC carbon foam.
2. Experimental
adsorption-desorption isotherms specific surface areas, and adsorptionaverage pore size, obtained using a gas adsorption analyzer (BarrettJoyner-Halenda model).
2.1. Preparation of CMC composites and their carbon foams CMC (average MW: ≈ 250,000) and CA (C6H8O7·H2O) used in this research were purchased from Sigma-Aldrich Co. As shown in Fig. 1, for preparation of CMC composites, 13 wt% CMC was dissolved in distilled water by stirring at room temperature. 2 wt% and 4 wt% CA were respectively added to the 13 wt% CMC paste. The homogeneous CMC and CA paste mixtures were transferred into plastic molds. Carbonization reaction conditions were determined for optimal cross-linking of CMC and CA, to minimize sample decomposition at high temperature during the carbonization process. The samples were then irradiated at various absorbed doses of 20 kGy, 40 kGy, 60 kGy, 80 kGy, and 100 kGy (an accelerating voltage of 1.14 MeV, a beam current of 7.6 mA, irradiation width of 110 cm, distance between samples and window of 20 cm, dose rate of 6.67 kGy/sec) in air at room temperature. Dosimetry was carried out using cellulose tri-acetate (CTA; ISO/ASTM 51,650 (2013)). All irradiated samples were freeze-dried and CMC foam composites were obtained. The obtained CMC composites were carbonized for 1 h at 1000 °C (heating rate = 2 °C/min) in a tubular furnace under an atmosphere of high-purity nitrogen (99.999%) without stabilization.
3. Results and discussion 3.1. Gel fraction and carbon yields Cross-linking reaction between CMC and CA generally happens through esterification by hydroxyl groups of CMC and carboxyl groups of CA during EBI (Fig. 2). The gel fraction exhibit the degree of conversion of the linear structure of precursors to cross-linking structure in composites materials. As shown in Fig. 3, the gel fraction values obtained in CMC composites increased for EBI dose up to 80 kGy, followed by a slight decrease at 100 kGy. This trend is obseved for both CA concentrations of 2 wt% and 4 wt%. These results are because excessive radiation over a certain range of EBI dose can occurre decomposition by chain scission and then decrese gel fraction. Among CMC composites obtained in this study, CMC composite containing 4 wt% CA via 80 kGy of EBI exhibited the highest gel fraction value of around 97%. Table 1 shows the carbon yields of CMC composites, obtained according to various EBI doses and CA concentration. As exhibited in Table 1, we can see that the carbon yields for the CMC composites mostly increase with increasing EBI dosage up to 80 kGy and CA concentrations from 2 to 4 wt%. The slight decrease in the gel fraction as obseved in samples dosed at 100 kGy can be rationalized by the decomposition of the noncross-linked fraction in CMC composites at 1000 °C during carbonization. Therefore, it is concluded that CMC composites with high gel fraction values directly result in higher carbon yields.
2.2. Analysis Gel fraction values were obtained using distilled water, and the soluble fraction was extracted for 24 h at room temperature. The insoluble fraction was completely dried in a vacuum oven at 80 °C. The gel fraction values were calculated according to the following equation; Gel fraction (%) = (W2/W1) × 100
3.2. SEM images
where W2 is the weight of the insoluble fraction after extraction with distilled water, W1 is the known weight of CMC composites before extraction in distilled water. Scanning electron microscopy (SEM) images were obtained using a Jeol JSM 5910 LV microscope to study the morphology of CMC composites and their carbon foams. Compressive strength was measured by an Instron 5050 tester (Instron USA) by method of ASTM standard C365. In the Brunauer-Emmertt-Teller (BET) method, the textural properties were observed in terms of nitrogen
Digital photos of Fig. 4 show CMC composites ((a) and (c)) and their carbon foams ((b) and (d)). Carbon foam was prepared through carbonization at 1000 °C without stabilization process. As shown in Fig. 4, cross-linking in composites generally plays an important role in preparing of carbon foam. After carbonization, CMC composite containing 4 wt% CA via 40 kGy of EBI can be seen that most of them burned due to low gel fraction. However, we can see that the carbon foam obtained
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Fig. 2. Proposed mechanism for cross-linking of CMC and CA by EBI.
Table 1 Carbon yields of CMC composites obtained according to various EBI dose and CA concentration. CA concentration
2 4
EBI dose (kGy) 20
40
60
80
100
10.15 12.29
11.94 13.38
18.06 20.52
23.38 25.74
21.56 23.04
from CMC composites containing 4 wt% CA via 80 kGy of EBI maintains its shape stability of 3-dimensional structure because of high gel fraction. The apparent size shrinkage of CMC composite containing 4 wt% CA via 80 kGy of EBI is also clearly observed and its size reduction was estimated to be ∼34% due to high thermal decomposition of noncrosslinked fraction in CMC composites. Fig. 5 displays SEM images of surface morphologies for CMC composites and its carbon foams. CMC composites have non-uniform porosity with the pore size range of around 100–200 μm. After carbonization, the obtained carbon foams were also observed to have non-uniform porosity, while pore size
Fig. 3. Gel fraction of CMC composites obtained according to CA concentration and EBI doses.
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Fig. 4. Digital photos of CMC composites obtained by each condition and their carbon foams and their carbon foams.
distribution was measured to be 50–100 μm. This corresponds to a decrease of 34% of the pre-carbonization samples.
carbon foams obtained from each condition. The specific surface area and average pore size of adsorption were highest for the carbon foam obtained from CMC composite containing 4 wt% CA via 80 kGy EBI, implying that the carbon atoms in this carbon foam interacted with the more gas than the other carbon foams. The carbon foam obtained from CMC composite containing 4 wt% via 80 kGy presented the highest specific surface area of 372.06 m2/g and adsorption pore size of 2.20 nm among carbon foams.
3.3. Compressive strength Fig. 6 shows the compressive strength curves for CMC composites derived from 2 wt% and 4 wt% CA obtained according to EBI dosages and its carbon foams. Generally, compressive strengths of CMC composites were lower (0.31 and 0.67 MPa) than those of carbon foams. However, after carbonization, compressive strength increases from 0.47 MPa to 0.88 MPa. It could be attributed to the increase of crystalline carbon present in the carbon foams. In addition, higher crosslinked chemical species during carbonization would result in lower partial break and lower defects during carbon formation, and higher density in carbon foam. Therefore, carbon foam obtained from CMC composites containing 4 wt% CA via 80 kGy of EBI dose with the highest gel fraction value resulted in highest compressive strength value (0.88 MPa).
4. Conclusions CMC composites of biopolymer were used as carbon foams precursor and were prepared according to various EBI doses and CA concentration. This study was conceived and executed to optimize and increase cross-linked molecules in CMC composites that can withstand temperatures over 1000 °C during carbonization reaction. Gel fraction analysis for the obtained CMC composites increased with the increase of CA concentration and EBI doses between 20 kGy and 80 kGy. Among samples, CMC composite with 4 wt% CA via 80 kGy represents the highest gel fraction value of about 97% the highest carbon yields, and highest compressive strength. These enhanced materials properties result from higher cross-linking conversion in the carbon foam obtained from CMC composites with 4 wt% CA via 80 kGy of EBI dose reduce more break defects after carbonization. In BET analysis, the carbon foam obtained from CMC composite with 4 wt% via 80 kGy also was observed the highest specific surface area of 372.06 m2/g and adsorption pore size of 2.20 nm due to interaction of the more gas with further carbon atoms.
3.4. BET surface area analysis Fig. 7 shows the N2 adsorption isotherms for carbon foams obtained from CMC composites via 80 kGy and 100 kGy. As can be observed in Fig. 7, we could find that most of carbon foams drastically increased at P/P0 below 0.1 due to micro-pore structures but slowly increased at P/ P0 over 0.1 owing to meso- and macro-pore structures. These isotherms for the obtained carbon foams can be sorted as the isotherms of middle foam between Types I and II according to BET classification. Table 2 shows specific surface area and average pore size of adsorption for the
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Fig. 5. SEM images of CMC composites obtained by each condition and their carbon foams.
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Table 2 Textural properties for the obtained carbon foams obtained from each condition. EBI dose
80 kGy
100 kGy
CA concentration
2 wt%
4 wt%
2 wt%
4 wt%
Specific surface area (m2/g) Ads. average of pore size (nm)
329.29 2.13
372.06 2.20
311.07 2.11
344.11 2.05
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