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Effect of annealing on the separation of resin from CFRP cross-ply laminate via electrical treatment
Composite Structures xxx (xxxx) xxxx
Contents lists available at ScienceDirect
Composite Structures journal homepage: www.elsevier.com/locate/compstruct
Effect of annealing on the separation of resin from CFRP cross-ply laminate via electrical treatment ⁎
⁎
Shinya Matsudaa, , Kazumasa Oshimab, , Masaki Hosakab, Shigeo Satokawab a Area in Advanced Materials Science, Department of Engineering and Design, Faculty of Engineering and Design, Kagawa University, 2217-20 Hayashicho, Takamatsu-shi, Kagawa 761-0396, Japan b Department of Materials and Life Science, Faculty of Science and Technology, Seikei University, 3-3-1 Kichijoji Kitamachi, Musashino-shi, Tokyo 180-8633, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: CFRP laminate Annealing Electrical treatment
Carbon fiber reinforced plastics (CFRP) are desirable owing to their high specific strength and rigidity. Traditional recycling methods for these products such as thermal decomposition cause considerable damage to the carbon fibers. In search for better recycling methods, we explored the use of electrical treatment for the separation of resin. Herein, we investigated the effects of annealing on the separation of resin from a CFRP crossply laminate molded from unidirectional prepreg (carbon fiber/epoxy) using electrical treatment. The annealing of the CFRP cross-ply laminate [0°/90°]2s was performed at temperatures ranging from 60 °C to 450 °C, followed by the electrical treatment. Experimental results demonstrated that the separation of resin, without the carbon fiber damage, could be achieved by annealing at temperatures close to the epoxy decomposition temperature. For the non-annealed CFRP specimens, the separation of resin was achieved with considerable damage to the carbon fibers.
1. Introduction Carbon fiber reinforced plastics (CFRP) have high specific strength and rigidity. They are in high demand in the aviation and automotive industries. Although disposal risk is widely discussed with increasing demand for CFRP [1–4], the incineration and landfill of CFRP are listed as the least expensive disposal methods; however, they are not desirable owing to the high cost of CFRP products and environmental effects [1]. Despite various proposals for recycling methods, no industrially established process is available for these products [2]. The primary recycling method is thermal decomposition, which decomposes only resin in CFRP at high temperatures (≥500 °C). The thermal decomposition method is established as a baseline recycling method for these plastics due to its high recycling capacity [3–8]. However, it has been reported that high temperature results in the damage of the carbon fiber, which degrades to ~80% strength of the virgin carbon fiber [7,8]. To prevent the thermal damage of carbon fiber, a resin extraction method using supercritical or subcritical fluid as a solvent has been investigated [3,4,9–11]. Carbon fiber damage can be reduced by using supercritical or subcritical solvents. However, this method presents significant economic hurdles for industrial application because supercritical and subcritical fluids are expensive. To overcome these problems, the recovery of carbon fiber via
⁎
selective treatments has been rapidly attracting attention in recent years. It was reported that carbon fiber recovery could be achieved by selectively decomposing and eluting the resin via a chemical reaction at ~80 °C [12,13]. Xing et al. [14–17] reported that carbon fiber could be recovered at room temperature by supplying electrical energy to the CFRP woven material. The chemical reaction and electrical treatments can be performed under milder conditions compared to thermal decomposition and other resin extraction methods, which can significantly reduce the environmental risks. However, for utilization and validation of these methods, extensive investigations of various CFRP materials and treatment conditions are required. In this study, an electrical treatment on a cross-ply laminate molded from unidirectional prepreg (carbon fiber/epoxy) was performed, and the effect of annealing on the separation of epoxy resin before the electrical treatment was investigated. It is speculated that annealing denatured the resin to prevent the damage of carbon fiber by the electrical treatment, resulting in an improvement in the efficiency of the electrical treatment. Herein, we report the effect of the annealing temperature on the separation of resin via the electrical treatment. 2. Experimental The material used in the study was carbon/epoxy composite
Corresponding authors. E-mail addresses: [email protected] (S. Matsuda), [email protected] (K. Oshima).
https://doi.org/10.1016/j.compstruct.2019.111665 Received 12 May 2019; Accepted 3 November 2019 0263-8223/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Shinya Matsuda, et al., Composite Structures, https://doi.org/10.1016/j.compstruct.2019.111665
Composite Structures xxx (xxxx) xxxx
S. Matsuda, et al.
Fig. 1. Schematic of the experimental setup for electrical treatment of CFRP cross-ply laminate.
Fig. 2. Images obtained after electrical treatment of specimens annealed at temperatures of (a) 60 °C, (b) 250 °C, and (c) 450 °C.
Fig. 3. Images of the electrolyte after electrical treatment of specimens annealed at various temperatures. 2
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Fig. 7. Raman spectra of electrolyte residue after electrolytic treatment of specimens annealed at 60, 250, and 450 °C.
laminate (T700SC/#2592, carbon fiber volume content of 0.6, Toray Industries). Stacked prepreg sheets were cured at 130 °C for 2 h in a vacuum chamber. Cross-ply laminate [0°/90°]2s with a rectangular shape and thickness of 1.1 mm was fabricated using a diamond cutter. The specimens had a length of 50 mm parallel, and a width of 15 mm perpendicular, to the fiber direction of the surface, respectively. Glasstransition temperature of the specimen measured using differential scanning calorimeter was determined as 91 °C [18]. The specimen was annealed using an electrical furnace at a wide temperature range of 60 °C–450 °C in air. The electrical treatment was performed using a two-electrode cell (Fig. 1), with the specimen as the anode and a carbon rod (ϕ2 mm) as the cathode. The specimen was immersed up to 20 mm in the longitudinal direction in the electrolyte. The specimens were treated at a constant voltage of 15 V for 20 h. Neutral phosphate pH standard solutions (pH = 6.86) were used as electrolytes, which were diffused with a stirrer during the voltage application.
Fig. 4. Relationship between weight loss and annealing temperature of specimens after the annealing and the electrical treatments.
3. Results and discussion Fig. 5. Raman spectra of specimens annealed at temperatures of 60, 250, and 350 °C.
3.1. CFRP laminate and electrolyte after electrical treatment Fig. 2 shows the images after the electrical treatment of the specimens annealed at 60, 250, and 450 °C. For the specimens annealed at 60 °C and 250 °C, the resin is separated on the surface of the side of electrical treatment. Conversely, for the specimens annealed at 450 °C, although the carbon fiber and resin can be separated, many carbon fibers are found broken into pieces after the separation. These result confirm that the resin can be separated from the cross-ply laminates molded from unidirectional CFRP using the electrical treatment. Fig. 3 shows an image of the electrolyte after the electrical treatment of the annealed specimens. The colors of the electrolytes after the electrical treatment of the specimens annealed at 60–200 °C are brown. The electrolytes for samples annealed at 200–350 °C are also brown, but the color becomes lighter with an increase in the annealing temperature; the electrolyte for samples annealed at 350 °C is almost colorless. Conversely, the electrolytes for samples annealed at 400–450 °C are black owing to the dispersion of severely damaged micro carbon fibers in the electrolyte, together with the separation of the resin, as shown in Fig. 2. Fig. 4 shows the relationship between weight loss and the annealing temperature of specimens subjected to only annealing and electrical treatment. These weight losses are estimated as
Fig. 6. Raman spectra after electrolytic treatment of specimens annealed at 60 °C and 250 °C.
3
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Fig. 8. SEM images of specimens after electrolytic treatment of specimens annealed at (a) 60 °C and (b) 250 °C.
W − Wanneal × 100 [wt%] W
(1)
Wanneal − Wanneal - el. × 100 [wt%] Wanneal
(2)
fiber is already damaged by the annealing at high temperature. In the below sections, we discuss the effect of annealing on the electrical treatment of the specimens, which were annealed at temperatures lower than the epoxy resin degradation temperature of 300 °C.
where W, Wanneal, and Wanneal-el. are the weights of a fully dried virgin specimen, the specimen after annealing, and the specimen after electrical treatment, respectively. The weight losses (W–Wanneal)/W of specimens after annealing increase from temperatures of 300 °C (see circle plot in Fig. 4). It is well-known that the thermal degradation of epoxy resin occurs in the temperature range of 300 °C–450 °C [19]. Therefore, (W–Wanneal)/W value increases due to the thermal degradation of epoxy resin. Three behaviors are seen in relationship between the weight loss (Wanneal–Wanneal-el.)/Wanneal of specimens after electrical treatment (see square plot in Fig. 4) and annealing. In region I (60–200 °C), (Wanneal–Wanneal-el.)/Wanneal is ~ 15% and is independent of the temperature. These results suggest that the electrical treatment of the annealed specimen is independent of the glass-transition temperature. In region II (200–300 °C), (Wanneal–Wanneal-el.)/Wanneal decreases with an increase in the annealing temperature. As the thermal degradation of epoxy resin typically occurs at ~300 °C, the decrease in (Wanneal–Wanneal-el.)/Wanneal is speculated to be caused by the denaturation of the epoxy resin due to annealing. In region III (300–450 °C), (Wanneal–Wanneal-el.)/Wanneal again increases with an increase in the annealing temperature. On the basis of these results (see Fig. 2), it is speculated that an increase in the weight loss (Wanneal–Wanneal-el.)/ Wanneal in region III is mainly caused by the significant damage of the carbon fiber during separation due to the annealing at high temperature.
3.2.2. Raman and scanning electron microscopy (SEM) observations of electrolyte and CFRP laminates after electrical treatment Fig. 6 shows the Raman spectra of the electrically treated specimen after annealing at 60 °C and 250 °C. The ID/IG ratio for the specimens annealed at 60 °C and 250 °C are 0.98 and 0.99, respectively. These values are similar to those shown in Fig. 5. This result implies that the damage due to the electrical treatment cannot be evaluated by Raman spectroscopy. Fig. 7 shows the Raman spectra of the residue obtained by drying the electrolyte after electrical treatment of the specimens annealed at 60 °C and 250 °C. For comparison, the spectra after the electrical treatment of the specimen annealed at 450 °C are also shown in Fig. 7. Two peaks attributed to the carbon fiber are clearly observed at 450 °C. Minor peaks for carbon fiber are also observed at 60 °C and 250 °C. Consistent with these results, the color changes in the electrolytes shown in Fig. 3 are caused by the micro carbon fiber released from the specimen. Therefore, it is presumed that the damage caused by the electrical treatment of the specimen annealed at 250 °C is less compared to that annealed at 60 °C because the color of the electrolyte for the specimen annealed at 250 °C is very light compared to that annealed at 60 °C. Therefore, a detailed analysis of the micro surface area of the carbon fiber in the specimen, which could not be evaluated by Raman analysis, was performed using SEM. Fig. 8 shows the SEM images of the specimens after electrical treatment and annealing at temperatures of 60 °C and 250 °C. Carbon fiber damage is observed in the specimen annealed at a temperature of 60 °C as shown in Fig. 8(a). Conversely, cracking in the interface is observed in the specimen annealed at a temperature of 250 °C, but no damage to the carbon fiber is observed as shown in Fig. 8(b). Therefore, it is concluded that the weight loss due to the electrical treatment in the temperature range of region I (Fig. 4) includes not only the separation of the epoxy resin but also a minor amount of the damaged carbon fiber. Conversely, in the weight loss for region II (Fig. 4), the damage to the carbon fiber is minimal and the separation of epoxy resin is dominant. As a result, as shown in Fig. 3, the color of the electrolyte in region I is brown due to the floating carbon fibers, and in region II, it is almost colorless, with negligible damage to the carbon fibers.
3.2. Raman and SEM observations 3.2.1. Raman spectral observation of annealed CFRP laminates Fig. 5 shows the Raman spectra of the specimens annealed at 60, 250, and 450 °C using Raman spectrophotometer (NRS-3100: JASCO). The ID/IG ratios of specimens annealed at 60 °C and 250 °C are 0.94 and 0.98, respectively, which are almost equal to the values for non-annealed specimens. The ID/IG ratio for the specimen annealed at 450 °C is 1.08, and a broad peak is observed, which is different from the nonannealed specimen and the specimens annealed at 60 °C and 250 °C. An increase in the ID / IG ratio and the broadening of the peak suggest a decrease in the crystallinity of the carbon fiber. Therefore, the results for the specimen annealed at 450 °C suggest that the carbon fiber is degraded by annealing. It is speculated that more severe carbon fiber damage, as shown in Fig. 2, occurs by the electrical treatment as the 4
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4. Summary
[5] Pickering SJ. Recycling technologies for thermoset composite materials—current status. Compos A Appl Sci Manuf 2006;37(8):1206–15. [6] Pimenta S, Pinho ST. Recycling carbon fibre reinforced polymers for structural applications: technology review and market outlook. Waste Manage 2011;31(2):378–92. [7] Feraboli P, Kawakami H, Wade B, Gasco F, DeOto L, Masini A. Recyclability and reutilization of carbon fiber fabric/epoxy composites. J Compos Mater 2011;46(12):1459–73. [8] Kim K-W, Jeong J-S, An K-H, Kim B-J. A low energy recycling technique of carbon fibers-reinforced epoxy matrix composites. Ind Eng Chem Res 2018;58(2):618–24. [9] Piñero-Hernanz R, Dodds C, Hyde J, García-Serna J, Poliakoff M, Lester E, et al. Chemical recycling of carbon fibre reinforced composites in nearcritical and supercritical water. Compos A Appl Sci Manuf 2008;39(3):454–61. [10] Goto M. Chemical recycling of plastics using sub- and supercritical fluids. J Supercrit Fluids 2009;47(3):500–7. [11] Okajima I, Watanabe K, Haramiishi S, Nakamura M, Shimamura Y, Sako T. Recycling of carbon fiber reinforced plastic containing amine-cured epoxy resin using supercritical and subcritical fluids. J Supercrit Fluids 2017;119:44–51. [12] Das M, Chacko R, Varughese S. An efficient method of recycling of CFRP waste using peracetic acid. ACS Sustain Chem Eng 2018;6(2):1564–71. [13] Lo JN, Nutt SR, Williams TR. Recycling benzoxazine–epoxy composites via catalytic oxidation. ACS Sustain Chem Eng 2018;6(6):7227–31. [14] Sun H, Guo G, Memon SA, Xu W, Zhang Q, Zhu J-H, et al. Recycling of carbon fibers from carbon fiber reinforced polymer using electrochemical method. Compos A Appl Sci Manuf 2015;78:10–7. [15] Sun H, Memon SA, Gu Y, Zhu M, Zhu J-H, Xing F. Degradation of carbon fiber reinforced polymer from cathodic protection process on exposure to NaOH and simulated pore water solutions. Mater Struct 2016;49(12):5273–83. [16] Sun H, Wei L, Zhu M, Han N, Zhu J-H, Xing F. Corrosion behavior of carbon fiber reinforced polymer anode in simulated impressed current cathodic protection system with 3% NaCl solution. Constr Build Mater 2016;112:538–46. [17] Zhu J-H, Chen P-y, Su M-n, Pei C, Xing F. Recycling of carbon fibre reinforced plastics by electrically driven heterogeneous catalytic degradation of epoxy resin. Green Chem 2019;21(7):1635–47. [18] Matsuda S, Ogi K, Yashiro S. Machining quality and characteristics of circular piercing hole produced by punch press in thermosetting CFRP laminates. J Jap Soc Compos Mater 2016;42(1):13–22. [19] Neǐman MB, Kovarskaya BM, Golubenkova LI, Strizhkova AS, Levantovskaya II, Akutin MS. The thermal degradation of some epoxy resins. J Polym Sci Part A 1962;56(164):383–9.
Electrical treatment typically results in the separation of resin from a CFRP cross-ply laminate molded from a unidirectional prepreg, but also causes damage to carbon fibers. In this study, it is experimentally confirmed that the separation of resin without any significant damage to the carbon fiber can be achieved by prior annealing at temperatures close to the decomposition temperature of the epoxy resin. We speculate that under these conditions, the separation results from the peeling of the resin from the CFRP laminate by the electrolysis of water. Therefore, annealing is beneficial for efficient separation of the resin from the carbon fibers. Acknowledgements Funding: This work was supported by a research grant from Mukai Science and Technology Foundation of FY2017 and Iketani Science and Technology Foundation [0301075-A]. References [1] Vo Dong PA, Azzaro-Pantel C, Cadene A-L. Economic and environmental assessment of recovery and disposal pathways for CFRP waste management. Resour Conserv Recycl 2018;133:63–75. [2] Oliveux G, Dandy LO, Leeke GO. Current status of recycling of fibre reinforced polymers: Review of technologies, reuse and resulting properties. Prog Mater Sci 2015;72:61–99. [3] Naqvi SR, Prabhakara HM, Bramer EA, Dierkes W, Akkerman R, Brem G. A critical review on recycling of end-of-life carbon fibre/glass fibre reinforced composites waste using pyrolysis towards a circular economy. Resour Conserv Recycl 2018;136:118–29. [4] Khalil YF. Comparative environmental and human health evaluations of thermolysis and solvolysis recycling technologies of carbon fiber reinforced polymer waste. Waste Manage 2018;76:767–78.
5
Contents lists available at ScienceDirect
Composite Structures journal homepage: www.elsevier.com/locate/compstruct
Effect of annealing on the separation of resin from CFRP cross-ply laminate via electrical treatment ⁎
⁎
Shinya Matsudaa, , Kazumasa Oshimab, , Masaki Hosakab, Shigeo Satokawab a Area in Advanced Materials Science, Department of Engineering and Design, Faculty of Engineering and Design, Kagawa University, 2217-20 Hayashicho, Takamatsu-shi, Kagawa 761-0396, Japan b Department of Materials and Life Science, Faculty of Science and Technology, Seikei University, 3-3-1 Kichijoji Kitamachi, Musashino-shi, Tokyo 180-8633, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: CFRP laminate Annealing Electrical treatment
Carbon fiber reinforced plastics (CFRP) are desirable owing to their high specific strength and rigidity. Traditional recycling methods for these products such as thermal decomposition cause considerable damage to the carbon fibers. In search for better recycling methods, we explored the use of electrical treatment for the separation of resin. Herein, we investigated the effects of annealing on the separation of resin from a CFRP crossply laminate molded from unidirectional prepreg (carbon fiber/epoxy) using electrical treatment. The annealing of the CFRP cross-ply laminate [0°/90°]2s was performed at temperatures ranging from 60 °C to 450 °C, followed by the electrical treatment. Experimental results demonstrated that the separation of resin, without the carbon fiber damage, could be achieved by annealing at temperatures close to the epoxy decomposition temperature. For the non-annealed CFRP specimens, the separation of resin was achieved with considerable damage to the carbon fibers.
1. Introduction Carbon fiber reinforced plastics (CFRP) have high specific strength and rigidity. They are in high demand in the aviation and automotive industries. Although disposal risk is widely discussed with increasing demand for CFRP [1–4], the incineration and landfill of CFRP are listed as the least expensive disposal methods; however, they are not desirable owing to the high cost of CFRP products and environmental effects [1]. Despite various proposals for recycling methods, no industrially established process is available for these products [2]. The primary recycling method is thermal decomposition, which decomposes only resin in CFRP at high temperatures (≥500 °C). The thermal decomposition method is established as a baseline recycling method for these plastics due to its high recycling capacity [3–8]. However, it has been reported that high temperature results in the damage of the carbon fiber, which degrades to ~80% strength of the virgin carbon fiber [7,8]. To prevent the thermal damage of carbon fiber, a resin extraction method using supercritical or subcritical fluid as a solvent has been investigated [3,4,9–11]. Carbon fiber damage can be reduced by using supercritical or subcritical solvents. However, this method presents significant economic hurdles for industrial application because supercritical and subcritical fluids are expensive. To overcome these problems, the recovery of carbon fiber via
⁎
selective treatments has been rapidly attracting attention in recent years. It was reported that carbon fiber recovery could be achieved by selectively decomposing and eluting the resin via a chemical reaction at ~80 °C [12,13]. Xing et al. [14–17] reported that carbon fiber could be recovered at room temperature by supplying electrical energy to the CFRP woven material. The chemical reaction and electrical treatments can be performed under milder conditions compared to thermal decomposition and other resin extraction methods, which can significantly reduce the environmental risks. However, for utilization and validation of these methods, extensive investigations of various CFRP materials and treatment conditions are required. In this study, an electrical treatment on a cross-ply laminate molded from unidirectional prepreg (carbon fiber/epoxy) was performed, and the effect of annealing on the separation of epoxy resin before the electrical treatment was investigated. It is speculated that annealing denatured the resin to prevent the damage of carbon fiber by the electrical treatment, resulting in an improvement in the efficiency of the electrical treatment. Herein, we report the effect of the annealing temperature on the separation of resin via the electrical treatment. 2. Experimental The material used in the study was carbon/epoxy composite
Corresponding authors. E-mail addresses: [email protected] (S. Matsuda), [email protected] (K. Oshima).
https://doi.org/10.1016/j.compstruct.2019.111665 Received 12 May 2019; Accepted 3 November 2019 0263-8223/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Shinya Matsuda, et al., Composite Structures, https://doi.org/10.1016/j.compstruct.2019.111665
Composite Structures xxx (xxxx) xxxx
S. Matsuda, et al.
Fig. 1. Schematic of the experimental setup for electrical treatment of CFRP cross-ply laminate.
Fig. 2. Images obtained after electrical treatment of specimens annealed at temperatures of (a) 60 °C, (b) 250 °C, and (c) 450 °C.
Fig. 3. Images of the electrolyte after electrical treatment of specimens annealed at various temperatures. 2
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Fig. 7. Raman spectra of electrolyte residue after electrolytic treatment of specimens annealed at 60, 250, and 450 °C.
laminate (T700SC/#2592, carbon fiber volume content of 0.6, Toray Industries). Stacked prepreg sheets were cured at 130 °C for 2 h in a vacuum chamber. Cross-ply laminate [0°/90°]2s with a rectangular shape and thickness of 1.1 mm was fabricated using a diamond cutter. The specimens had a length of 50 mm parallel, and a width of 15 mm perpendicular, to the fiber direction of the surface, respectively. Glasstransition temperature of the specimen measured using differential scanning calorimeter was determined as 91 °C [18]. The specimen was annealed using an electrical furnace at a wide temperature range of 60 °C–450 °C in air. The electrical treatment was performed using a two-electrode cell (Fig. 1), with the specimen as the anode and a carbon rod (ϕ2 mm) as the cathode. The specimen was immersed up to 20 mm in the longitudinal direction in the electrolyte. The specimens were treated at a constant voltage of 15 V for 20 h. Neutral phosphate pH standard solutions (pH = 6.86) were used as electrolytes, which were diffused with a stirrer during the voltage application.
Fig. 4. Relationship between weight loss and annealing temperature of specimens after the annealing and the electrical treatments.
3. Results and discussion Fig. 5. Raman spectra of specimens annealed at temperatures of 60, 250, and 350 °C.
3.1. CFRP laminate and electrolyte after electrical treatment Fig. 2 shows the images after the electrical treatment of the specimens annealed at 60, 250, and 450 °C. For the specimens annealed at 60 °C and 250 °C, the resin is separated on the surface of the side of electrical treatment. Conversely, for the specimens annealed at 450 °C, although the carbon fiber and resin can be separated, many carbon fibers are found broken into pieces after the separation. These result confirm that the resin can be separated from the cross-ply laminates molded from unidirectional CFRP using the electrical treatment. Fig. 3 shows an image of the electrolyte after the electrical treatment of the annealed specimens. The colors of the electrolytes after the electrical treatment of the specimens annealed at 60–200 °C are brown. The electrolytes for samples annealed at 200–350 °C are also brown, but the color becomes lighter with an increase in the annealing temperature; the electrolyte for samples annealed at 350 °C is almost colorless. Conversely, the electrolytes for samples annealed at 400–450 °C are black owing to the dispersion of severely damaged micro carbon fibers in the electrolyte, together with the separation of the resin, as shown in Fig. 2. Fig. 4 shows the relationship between weight loss and the annealing temperature of specimens subjected to only annealing and electrical treatment. These weight losses are estimated as
Fig. 6. Raman spectra after electrolytic treatment of specimens annealed at 60 °C and 250 °C.
3
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Fig. 8. SEM images of specimens after electrolytic treatment of specimens annealed at (a) 60 °C and (b) 250 °C.
W − Wanneal × 100 [wt%] W
(1)
Wanneal − Wanneal - el. × 100 [wt%] Wanneal
(2)
fiber is already damaged by the annealing at high temperature. In the below sections, we discuss the effect of annealing on the electrical treatment of the specimens, which were annealed at temperatures lower than the epoxy resin degradation temperature of 300 °C.
where W, Wanneal, and Wanneal-el. are the weights of a fully dried virgin specimen, the specimen after annealing, and the specimen after electrical treatment, respectively. The weight losses (W–Wanneal)/W of specimens after annealing increase from temperatures of 300 °C (see circle plot in Fig. 4). It is well-known that the thermal degradation of epoxy resin occurs in the temperature range of 300 °C–450 °C [19]. Therefore, (W–Wanneal)/W value increases due to the thermal degradation of epoxy resin. Three behaviors are seen in relationship between the weight loss (Wanneal–Wanneal-el.)/Wanneal of specimens after electrical treatment (see square plot in Fig. 4) and annealing. In region I (60–200 °C), (Wanneal–Wanneal-el.)/Wanneal is ~ 15% and is independent of the temperature. These results suggest that the electrical treatment of the annealed specimen is independent of the glass-transition temperature. In region II (200–300 °C), (Wanneal–Wanneal-el.)/Wanneal decreases with an increase in the annealing temperature. As the thermal degradation of epoxy resin typically occurs at ~300 °C, the decrease in (Wanneal–Wanneal-el.)/Wanneal is speculated to be caused by the denaturation of the epoxy resin due to annealing. In region III (300–450 °C), (Wanneal–Wanneal-el.)/Wanneal again increases with an increase in the annealing temperature. On the basis of these results (see Fig. 2), it is speculated that an increase in the weight loss (Wanneal–Wanneal-el.)/ Wanneal in region III is mainly caused by the significant damage of the carbon fiber during separation due to the annealing at high temperature.
3.2.2. Raman and scanning electron microscopy (SEM) observations of electrolyte and CFRP laminates after electrical treatment Fig. 6 shows the Raman spectra of the electrically treated specimen after annealing at 60 °C and 250 °C. The ID/IG ratio for the specimens annealed at 60 °C and 250 °C are 0.98 and 0.99, respectively. These values are similar to those shown in Fig. 5. This result implies that the damage due to the electrical treatment cannot be evaluated by Raman spectroscopy. Fig. 7 shows the Raman spectra of the residue obtained by drying the electrolyte after electrical treatment of the specimens annealed at 60 °C and 250 °C. For comparison, the spectra after the electrical treatment of the specimen annealed at 450 °C are also shown in Fig. 7. Two peaks attributed to the carbon fiber are clearly observed at 450 °C. Minor peaks for carbon fiber are also observed at 60 °C and 250 °C. Consistent with these results, the color changes in the electrolytes shown in Fig. 3 are caused by the micro carbon fiber released from the specimen. Therefore, it is presumed that the damage caused by the electrical treatment of the specimen annealed at 250 °C is less compared to that annealed at 60 °C because the color of the electrolyte for the specimen annealed at 250 °C is very light compared to that annealed at 60 °C. Therefore, a detailed analysis of the micro surface area of the carbon fiber in the specimen, which could not be evaluated by Raman analysis, was performed using SEM. Fig. 8 shows the SEM images of the specimens after electrical treatment and annealing at temperatures of 60 °C and 250 °C. Carbon fiber damage is observed in the specimen annealed at a temperature of 60 °C as shown in Fig. 8(a). Conversely, cracking in the interface is observed in the specimen annealed at a temperature of 250 °C, but no damage to the carbon fiber is observed as shown in Fig. 8(b). Therefore, it is concluded that the weight loss due to the electrical treatment in the temperature range of region I (Fig. 4) includes not only the separation of the epoxy resin but also a minor amount of the damaged carbon fiber. Conversely, in the weight loss for region II (Fig. 4), the damage to the carbon fiber is minimal and the separation of epoxy resin is dominant. As a result, as shown in Fig. 3, the color of the electrolyte in region I is brown due to the floating carbon fibers, and in region II, it is almost colorless, with negligible damage to the carbon fibers.
3.2. Raman and SEM observations 3.2.1. Raman spectral observation of annealed CFRP laminates Fig. 5 shows the Raman spectra of the specimens annealed at 60, 250, and 450 °C using Raman spectrophotometer (NRS-3100: JASCO). The ID/IG ratios of specimens annealed at 60 °C and 250 °C are 0.94 and 0.98, respectively, which are almost equal to the values for non-annealed specimens. The ID/IG ratio for the specimen annealed at 450 °C is 1.08, and a broad peak is observed, which is different from the nonannealed specimen and the specimens annealed at 60 °C and 250 °C. An increase in the ID / IG ratio and the broadening of the peak suggest a decrease in the crystallinity of the carbon fiber. Therefore, the results for the specimen annealed at 450 °C suggest that the carbon fiber is degraded by annealing. It is speculated that more severe carbon fiber damage, as shown in Fig. 2, occurs by the electrical treatment as the 4
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4. Summary
[5] Pickering SJ. Recycling technologies for thermoset composite materials—current status. Compos A Appl Sci Manuf 2006;37(8):1206–15. [6] Pimenta S, Pinho ST. Recycling carbon fibre reinforced polymers for structural applications: technology review and market outlook. Waste Manage 2011;31(2):378–92. [7] Feraboli P, Kawakami H, Wade B, Gasco F, DeOto L, Masini A. Recyclability and reutilization of carbon fiber fabric/epoxy composites. J Compos Mater 2011;46(12):1459–73. [8] Kim K-W, Jeong J-S, An K-H, Kim B-J. A low energy recycling technique of carbon fibers-reinforced epoxy matrix composites. Ind Eng Chem Res 2018;58(2):618–24. [9] Piñero-Hernanz R, Dodds C, Hyde J, García-Serna J, Poliakoff M, Lester E, et al. Chemical recycling of carbon fibre reinforced composites in nearcritical and supercritical water. Compos A Appl Sci Manuf 2008;39(3):454–61. [10] Goto M. Chemical recycling of plastics using sub- and supercritical fluids. J Supercrit Fluids 2009;47(3):500–7. [11] Okajima I, Watanabe K, Haramiishi S, Nakamura M, Shimamura Y, Sako T. Recycling of carbon fiber reinforced plastic containing amine-cured epoxy resin using supercritical and subcritical fluids. J Supercrit Fluids 2017;119:44–51. [12] Das M, Chacko R, Varughese S. An efficient method of recycling of CFRP waste using peracetic acid. ACS Sustain Chem Eng 2018;6(2):1564–71. [13] Lo JN, Nutt SR, Williams TR. Recycling benzoxazine–epoxy composites via catalytic oxidation. ACS Sustain Chem Eng 2018;6(6):7227–31. [14] Sun H, Guo G, Memon SA, Xu W, Zhang Q, Zhu J-H, et al. Recycling of carbon fibers from carbon fiber reinforced polymer using electrochemical method. Compos A Appl Sci Manuf 2015;78:10–7. [15] Sun H, Memon SA, Gu Y, Zhu M, Zhu J-H, Xing F. Degradation of carbon fiber reinforced polymer from cathodic protection process on exposure to NaOH and simulated pore water solutions. Mater Struct 2016;49(12):5273–83. [16] Sun H, Wei L, Zhu M, Han N, Zhu J-H, Xing F. Corrosion behavior of carbon fiber reinforced polymer anode in simulated impressed current cathodic protection system with 3% NaCl solution. Constr Build Mater 2016;112:538–46. [17] Zhu J-H, Chen P-y, Su M-n, Pei C, Xing F. Recycling of carbon fibre reinforced plastics by electrically driven heterogeneous catalytic degradation of epoxy resin. Green Chem 2019;21(7):1635–47. [18] Matsuda S, Ogi K, Yashiro S. Machining quality and characteristics of circular piercing hole produced by punch press in thermosetting CFRP laminates. J Jap Soc Compos Mater 2016;42(1):13–22. [19] Neǐman MB, Kovarskaya BM, Golubenkova LI, Strizhkova AS, Levantovskaya II, Akutin MS. The thermal degradation of some epoxy resins. J Polym Sci Part A 1962;56(164):383–9.
Electrical treatment typically results in the separation of resin from a CFRP cross-ply laminate molded from a unidirectional prepreg, but also causes damage to carbon fibers. In this study, it is experimentally confirmed that the separation of resin without any significant damage to the carbon fiber can be achieved by prior annealing at temperatures close to the decomposition temperature of the epoxy resin. We speculate that under these conditions, the separation results from the peeling of the resin from the CFRP laminate by the electrolysis of water. Therefore, annealing is beneficial for efficient separation of the resin from the carbon fibers. Acknowledgements Funding: This work was supported by a research grant from Mukai Science and Technology Foundation of FY2017 and Iketani Science and Technology Foundation [0301075-A]. References [1] Vo Dong PA, Azzaro-Pantel C, Cadene A-L. Economic and environmental assessment of recovery and disposal pathways for CFRP waste management. Resour Conserv Recycl 2018;133:63–75. [2] Oliveux G, Dandy LO, Leeke GO. Current status of recycling of fibre reinforced polymers: Review of technologies, reuse and resulting properties. Prog Mater Sci 2015;72:61–99. [3] Naqvi SR, Prabhakara HM, Bramer EA, Dierkes W, Akkerman R, Brem G. A critical review on recycling of end-of-life carbon fibre/glass fibre reinforced composites waste using pyrolysis towards a circular economy. Resour Conserv Recycl 2018;136:118–29. [4] Khalil YF. Comparative environmental and human health evaluations of thermolysis and solvolysis recycling technologies of carbon fiber reinforced polymer waste. Waste Manage 2018;76:767–78.
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