Chemical Recycling of Carbon Fiber Reinforced Epoxy Resin

Research Article pubs.acs.org/journal/ascecg

Chemical Recycling of Carbon Fiber Reinforced Epoxy Resin Composites via Selective Cleavage of the Carbon−Nitrogen Bond Yuqi Wang,†,‡ Xiaojing Cui,† Hui Ge,† Yongxing Yang,† Yingxiong Wang,† Ce Zhang,‡,§ Jingjing Li,§ Tiansheng Deng,*,† Zhangfeng Qin,† and Xianglin Hou*,† †

The Biorefinery Research and Engineering Center, Institute of Coal Chemistry, Chinese Academy of Sciences, 27 South Taoyuan Road, Taiyuan 030001, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100049, People’ Republic of China § Institute of Coal Chemistry, Chinese Academy of Sciences, 27 South Taoyuan Road, Taiyuan 030001, People’s Republic of China S Supporting Information *

ABSTRACT: An efficient strategy is developed for chemical recycling of cured epoxy resin (CEP) from its carbon fiber reinforced polymer composites (CFREP) using AlCl3/ CH3COOH as the degradation system. Acetic acid swells the dense structures of CEP, facilitating the penetration of the aluminum ion catalyst into the polymer matrix. The weakly coordinating aluminum ions in CH3COOH solution selectively cleave the C−N bond while leaving the C−C, C−O (aryl alkyl ether) bonds intact. This process recovers valuable oligomers and carbon fibers from CFREP. KEYWORDS: Chemical recycling, Epoxy resin, Carbon fiber, Selective C−N bond cleavage

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surface. The CEP can be dissolved in supercritical fluids. For example, thermosetting resin composites have been degraded in supercritical water9−15 and supercritical alcohols16 including methanol,17 ethanol, n-propanol,18,19 and i-propanol.20,21 Weirong Dang et al. recycled glass fiber reinforced epoxy resin composites (GFREP) using a solution nitric acid.22,23 Juan Li et al. recycled CFREP using hydrogen peroxide.24,25 However, both high temperatures and strong oxidation led to nonselective cleavage of the chemical bonds of CEP and produced a large amount of toxic gases. In addition to high temperature and strong oxidation, other researchers degraded fiber reinforced epoxy resin composites using expensive ionic liquid, which limits their large-scale application.26,27 Ideally, the recycling of CFs requires a selective cleavage of specific bonds in CEP using an economical, high yielding, and scalable system. The complexity of products and emission of toxic gases should be minimized. CEP contains several different functionalities with different reactivities, i.e., benzene rings, C− C bonds, C−O bonds, and C−N bonds. The selective cleavage of C−N or C−O bond could not only facilitate recycling of CFs but also retain the valuable carbon skeleton of CEP. In our previous work, we demonstrated that metal ions in incompletely coordinating or weakly coordinating state enable the cleavage of C−N and C−O bonds in organic

INTRODUCTION In the past few decades, carbon fiber reinforced epoxy resin composites (CFREP) have been widely used in the aeronautical field, traffic transformation industry, wind turbine field, electronic industry, and sports or entertainment products because of their excellent physical and chemical properties. With the extensive application of CFREP, the leftover materials in the manufacturing process and end-of-life waste products have caused environmental problems.1 The recycling of waste CFREP is highly desirable but challenging because cured epoxy resin (CEP) with its three-dimensional network is both insoluble and infusible.2 CFREP composites are composed of two parts, i.e., carbon fibers (CFs) as core and CEP as coating. Traditional disposal routes such as landfill and incineration are becoming increasingly restricted because of environmental protection and legal restriction.3 The recycling of CFREP is usually available by degrading the CEP coating, with as limited damage of CFs as possible. In recent years, many recycling methods have been investigated, including mechanical, thermal, and chemical recycling methods.4−7 Among them, chemical recycling is the most promising since CFs can be recycled from CFREP together with valuable building blocks of CEP all in the same process. Conventionally, chemical recycling of CFREP utilizes high temperature, special solvents (e.g., supercritical fluid8), or strong oxidation (e.g., nitric acid or hydrogen peroxide) in order to cleave the chemical bonds of CEP from the CFs © 2015 American Chemical Society

Received: August 27, 2015 Revised: October 9, 2015 Published: October 26, 2015 3332

DOI: 10.1021/acssuschemeng.5b00949 ACS Sustainable Chem. Eng. 2015, 3, 3332−3337

Research Article

ACS Sustainable Chemistry & Engineering Scheme 1. Selective C−N Bond Cleavage of CEP in the Degradation Reaction

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polymers.28−30 Besides the catalyst, the selective degradation of thermosetting resin is strongly dependent on the solvent. The solvent should efficiently swell the thermosetting resin and favor the transferring of the catalyst into the resin matrix. Moreover, the catalytic cations should coordinate weakly with the solvent molecules. In this way, the weakly coordinating ions that enter the matrix of the swelled resin can coordinate with the lone pair electrons of the heteroatom (O or N), facilitating the selective cleavage of carbon−heteroatom bonds (C−O or C−N). In this article, an economical and efficient catalytic system to selectively cleave the C−N bond rather than the C−O and C− C bond is developed. In this way not only CFs can be recycled but also the carbon skeleton structure of CEP, all in one process (as shown in Scheme 1). CEP and its composites CFREP were decomposed under mild conditions (180 °C, 1−3 atm) using AlCl3/CH3COOH as the degradation system. The compositions of degradation products were investigated by FTIR, NMR, and elemental analysis, from which a possible route for decomposing CEP and its composites was proposed. The selected epoxy resin is bisphenol A diglycidyl ether type epoxy resin (DGEBA), which is a versatile epoxy resin. The selected curing agent is 3,3′-dimethyl-4,4′-diaminodicyclohexylmethane (DMDC), which is an intermediate temperature curing agent. The curing process of CEP is shown in SI Scheme 1. To selectively break the C−N bond, a Lewis acid is required to coordinate with the nitrogen lone electron pair. In this work, AlCl3 is used as the Lewis acid catalyst for the selective cleavage of the C−N bond in CEP.

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RESULTS AND DISCUSSION Initially, the swelling behaviors of CEP were investigated using various solvents. As shown in Figure 1, the CEP swelled best in

Figure 1. Swelling property of CEP in various solvents at 180 °C for 6 h.

acetic acid, where its weight increase reached more than 56%. On the contrary, the swelling ability of water was the least, and the weight increase of CEP was only about 3.8%. The swelling abilities of alcohols were relatively low, and the swelling ratio of methanol, ethanol, n-propanol, and i-propanol is 18.79%, 20.85%, 16.16%, and 11.06%, respectively. The good swelling ability facilitates the transferring of catalyst into the matrix of epoxy resin. The solvent should, however, also dissolve the catalyst and form weakly coordinating metal ions. Thus, the acetic acid was selected as the solvent. The degradation of CEP was performed in acetic acid with various metal salts as the catalyst. The metal cations including Al3+, Fe3+, Zn2+, Cu2+, and Mg2+ (Table 1, entries 1−5). AlCl3 showed superior ability for decomposing CEP relative to that of the other tested metal ions. FeCl3, ZnCl2, CuCl2, and MgCl2 only had little catalytic effect for decomposing CEP. The investigated anions include Cl−, NO3−, SO42−, and CH3COO− (Table 1, entries 6−8). Cl− was found to be better than other anions for the catalytic effect of Al3+. Therefore, AlCl3 was chosen as the catalyst.

EXPERIMENTAL SECTION

The degradation of CEP was carried out in a Teflon-lined autoclave with a content of 10 mL. In each experiment, a CEP strip (about 0.8 g, 10 mm × 10 mm × 2 mm of cuboid), AlCl3 (0.75 g), and acetic acid (4.25 g) were added, and the degradation reaction was carried out under the specified conditions. After the decomposition, the reaction products were dissolved in acetone with the aid of ultrasound, while the aluminum catalyst was separated by filtration, due to its insolubility in acetone. The degradation products were obtained by removing acetone using rotary evaporation at 45 °C. The compositions of the degradation products and recycled catalyst were characterized by FTIR, NMR, elemental analysis, and GPC. In the case of CFREP degradation, about 1.5 g of CFREP was degraded under the degradation condition described above. After the decomposition reaction, the CFs were collected and washed 3 times with 10 mL of acetone. The recycled CF properties and appearance were tested by a monofilament tensile machine, XPS, and SEM. 3333

DOI: 10.1021/acssuschemeng.5b00949 ACS Sustainable Chem. Eng. 2015, 3, 3332−3337

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ACS Sustainable Chemistry & Engineering Table 1. Catalytic Effects of Various Cations or Anions on the Degradation Reaction of CEPa entry

catalyst

degradation ratio/wt %b

1 2 3 4 5 6 7 8

AlCl3 FeCl3 ZnCl2 CuCl2 MgCl2 Al(NO3)3 Al2(SO4)3 Al(CH3COO)3

84.21 swollen swollen swollen swollen swollen swollen swollen

a

Acetic acid as solvent. Reaction conditions: catalyst mass fraction10 wt %, 180 °C, 6 h. bThe calculation formula of the degradation ratio is Dr = (m1 − m2)/m1 × 100%, where Dr is degradation ratio, m1 is the initial quality of CEP before degradation, and m2 is the remaining quality of CEP after degradation. Figure 2. FT-IR spectra of (a) CEP, (b) degradation products, and (c) recycled catalyst.

In the AlCl3/CH3COOH degradation system, the effects of AlCl3 concentration, reaction temperature, and reaction time on the degradation behaviors of CEP were investigated (Table 2). CEP decomposed completely with the minimum AlCl3 Table 2. Influence of Catalyst Mass Fraction, Temperature, and Time on Degradation of CEPa entry

mass fraction (wt %)b

temperature (°C)

time (h)

degradation ratio (wt %)c

1 2 3 4 5 6 7 8

15 10 5 15 15 15 15 15

180 180 180 170 160 180 180 180

6 6 6 6 6 5 4 3

100 84.21 swollen 79.17 37.75 92.74 74.18 22.13

a

React in the AlCl3/CH3COOH degradation system. bMass fraction of AlCl3 in the degradation system. cThe calculation formula of the degradation ratio is Dr = (m1 − m2)/m1 × 100%, where Dr is the degradation ratio, m1 is the initial quality of CEP before degradation, and m2 is the remaining quality of drying CEP after degradation.

Figure 3. Solid state NMR of (a) CEP, (b) degradation products, and (c) recycled catalyst.

Table 3. Elemental Analysis of CEP, Degradation Products, and Recycled Catalysta

mass fraction being 15 wt % at 180 °C for 6 h (Table 2, entries 1−3). CEP decomposition into oligomers increased from 37.75% to 100% in 15 wt % AlCl3/CH3COOH, when the reaction temperature was raised from 160 to 180 °C (Table 2, entries 4−5). The decomposition time had a significant effect on the degradation of CEP because the degradation process gradually takes places from the outer surface into the resin interior. It took 6 h to degrade CEP completely (entries 6−8). Thus, the optimal degrading conditions of CEP were determined to be 15 wt % AlCl3 in acetic acid at 180 °C for 6 h. After the degradation, the degradation products in the reaction mixture were separated and weighted (see SI Figure 1 and SI Table 1). The recovery ratio of degradation products was up to 97.43 wt %. The compositions of separated products were analyzed by FT-IR (Figure 2), solid state NMR (Figure 3), and elemental analysis (Table 3). The FT-IR spectra of raw materials (CEP) and degradation products are shown in Figure 2a and b. The peaks of 1607, 1510, and 1458 cm−1 in raw materials belong to the stretching vibration of benzene ring. The peaks of the benzene ring in degradation products appear in 1600, 1506, and 1458 cm−1, which indicates that the benzene ring structure remains intact.

a

sample

C (%)

H (%)

N (%)

O (%)

Al (%)

CEP degradation products recycled catalyst

73.99 59.75 18.12

8.79 7.38 4.80

2.85 2.27 0.11

14.21 17.96 27.87

0.0049 18.12

The content of Al is characterized by ICP.

The signal at 1248 cm−1 in CEP is ascribed to the stretching vibration of C−O−C, while the signal of C−O−C in the degradation products appears at 1240 cm−1. The peak of aryl alkyl ethers (C−O−C) appears in the spectrum of recycled degradation products, indicating the conservation of aryl alkyl ethers. The peak at 1743 cm−1, found in the degradation products, belongs to the CO stretching vibration of ester bond. The ester bond forms from the esterification reaction between acetic acid and the C−OH of diglycidyl ether unit, rather than the cleavage of aryl alkyl ether bond because the carbonyl of aromatic ester will appear in higher wavenumber than 1743 cm−1, due to the conjugation with the benzene ring. The obvious decrease in peak intensity at 1045 cm−1 (C−OH of diglycidyl ether unit) in Figure 2b implies a condensation of 3334

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ACS Sustainable Chemistry & Engineering acetic acid and the C−OH in CEP (1034 cm−1) during the degradation process. By comparison with CEP, the remarkable change of degradation products is observed by a reduction in relative peak intensity of C−N stretching vibration at 1183 cm−1, which indicates the cleavage of the C−N bond during the degradation reaction. Moreover, in the FT-IR spectrum of the degradation products, the wide peak around 3226 cm−1 implies that substantial N−H bonds exist in the degradation products, which also proves the cleavage of the C−N bonds in CEP. By comparing with the ratio of the peak intensity of −CH2− (2927 cm−1) to −CH3 (2961 cm−1) in CEP, the decrease of this ratio in the degradation products indicates that the C−N bonds connected to the cyclohexyl in the curing agent unit were cleaved. The above results illustrate that the weakly coordinating aluminum ions enable the cleavage of the C−N bond without breaking the C−O−C bond of aryl alkyl ethers of CEP. In the spectrum of the recycled catalyst (Figure 2c), the signals of 1586 and 1474 cm−1 are the asymmetric and symmetric stretching vibrations of COO−, which indicate the catalysts are partially transformed into aluminum acetate during the degradation reaction. The carbon skeleton structures of CEP and degradation products were analyzed by solid state NMR (see Figure 3). In the NMR spectra, the peaks of CEP at 157, 143, 127, and 114 ppm in Figure 3a belong to the carbon signals of the benzene ring. The peaks of the benzene ring in the degradation product appear at 157, 143, 129, and 114 as seen in Figure 3b. This shows that the structure of the benzene ring remains intact in the recycled degradation products. Furthermore, the peak at 157 ppm remains unchanged, indicating the existence of the aryl alkyl ether (C−O−C) in the degradation products. For CEP, the peak at 42 ppm is designated to quaternary carbon of bisphenol A, and the peak at 70 ppm belongs to the carbons of glycidyl ether. Both of them remain unchanged in the degradation process. The peaks between 32 and 34 ppm in CEP are the signals of the cyclohexyl in the curing agent unit; however, these peaks in the degradation products narrow down to a limit range between 32 and 33 ppm. The peaks between 32 and 34 ppm decreased obviously in the degradation products, suggesting that the C−N bonds connected to the cyclohexyl group were cleaved. The structure of the degradation products were also explored by 1H NMR, 13C NMR, DEPT 135, and DEPT 90 (SI Figures 2 and 3). It was found that the signals from the benzene ring and the glycidyl ether unit are found in the degradation products, which illustrates that the cleavage takes place between carbon and heteroatom during degradation reaction. The 13C-NMR spectra of uncured epoxy resin (UPR) and curing agent after the degradation were also investigated (SI Figures 4 and 5). This shows that the carbon skeleton of UPR and curing agent remains intact. This result further indicates the cleavage of tertiary carbon nitrogen bonds. In the NMR spectrum of the recycled catalyst, the peak at 179 ppm is the signal of carbonyl of carboxylate, and the peak at 25 ppm is the signal of methyl. As shown in the 27Al-NMR spectra (SI Figure 6), the recycled catalyst is mainly five-coordinated aluminum or six-coordinated aluminum. These results indicate that the recycled catalyst is mainly aluminum acetate. Elemental analysis was employed to obtain the element components of degradation products and recycled catalyst (Table 3). The CEP is mainly made of the elements C, H, N, and O. These four elements account for 99.84 wt % of CEP. The degradation products are also mainly composed of C, H, N, and O. The aluminum content of degradation products is

only 0.0049 wt %, which indicates that the aluminum salt was effectively separated and removed in the process. However, the total content of C, H, N, and O is only 87.36 wt %, so the remainder 13 wt % is attributed to Cl according to element balance. The recycled catalyst is mainly composed of C, H, O, and Al. There is a small amount of N. The Al content reaches up to 18.12 wt %. The total content of the five elements (C, H, O, N, Al) is 69.02 wt %, suggesting that the remaining 31 wt % is the Cl element. These data show that the recycled catalysts contain a mixture of aluminum acetate and aluminum chloride. In order to make the proportion of elements more intuitive, the original data are transformed into mole ratio (see SI Table 2). The ratio of C to N of degradation products remains unchanged relative to raw material, but the content of H and O increases after degradation. The unchanged carbon nitrogen ratio indicates the cleavage of the C−N bond, which connects with the cyclohexyl group of the curing agent because cleavage of the other C−N bond will lead to nitrogen loss. There exist some Cl ions left in degradation products, but the Al content is close to 0, which suggests that the Cl atom may substitute the H atom of the phenyl group during the degradation reaction. As shown in Table 3, the degradation products are mainly composed of organic components. Thus, the molecular weight and its distribution were investigated using the GPC method (see SI Table 3). The weight-average molecular weight (Mw) of degradation products is 2396 g/mol, which is nearly twice that of the uncured epoxy resin (UEP). The number-average molecular weight (Mn) of degradation products is 1635 g/mol, and the polydispersity index (PDI) of degradation products is 1.46. The GPC data show that the Mn and Mw of degradation products are larger relative to UEP, which indicates that the degradation products should be the oligomers (UEP unit which is connected by the N atom). The main destination of this work is selective cleavage of the carbon heteroatom bond so that the degradation products can be reused in resin manufacturing. The FT-IR data show that the C−O−C bond keeps intact, whereas the C−N bond is cleaved. Furthermore, the FI-IR data also show that the C−N bond of the curing agent was cleaved in the degradation process, which can be verified by the relative decrease of the −CH2− peak intensity. The NMR data, including solid state NMR, 1H NMR, 13 C NMR, DEPT 135, and DEPT 90 indicate that the carbon skeleton structure of the resin remains intact. On the basis of these results, we propose a possible decomposition catalytic mechanism of CEP in the AlCl3/CH3COOH degradation system (see Scheme 2). The weakly coordinating aluminum ions coordinate the isolated electron pair of the nitrogen atoms. The carboxylate oxygen of acetic acid, then attack the now more electrophilic tertiary carbon, which leads to the cleavage of the bond between the tertiary carbon and the nitrogen. This results in the formation of the ester and the secondary amine. However, the degradation of CFREP may be different from that of CEP because of the existence of CFs. The effect of reaction time on recycling of CFREP was studied, and the morphology of the recycled CFs was observed using SEM (see SI Figure 7). The surface of CFs was still covered by CEP after reacting for 1 h in the 15 wt % AlCl3/CH3COOH degradation system at 180 °C, but cracks appeared on the surface. When the reaction time extended to 2 h, most of the surface of CFs was observed. Cleaner CFs were obtained after reaction for 3 h. To guarantee the complete degradation, the chemical recycling of CFREP was carried out in the optimal conditions for CEP (15 wt % AlCl3/CH3COOH, 180 °C, 6 h). Figure 4 shows that the 3335

DOI: 10.1021/acssuschemeng.5b00949 ACS Sustainable Chem. Eng. 2015, 3, 3332−3337

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ACS Sustainable Chemistry & Engineering

considerably larger compared with the C 1s peaks. The surface elemental composition is shown in SI Table 5. As shown in SI Table 5, the carbon content decreases about 10%, and the oxygen content increases about 10% after degradation in the AlCl3/CH3COOH system. To clarify the functional groups on the surface of the recycled carbon fibers, C 1s spectra are curvefitted (see SI Figure 8b, c, and d). The notable change of the recycled CF surface is the appearance of the CO bond (289.05 eV), which may be important for reuse of the recycled CFs. The O 1s peak-fitting curves are shown in SI Figure 9, and the percentage of oxygenated functional group is shown in SI Table 6. The C−OH group decreased about 38%, and the increase of the C−O group is about 26%. Especially, the new group OC−O appears, and its content reached up to about 11% in the recycled CFs surface. The above result shows that the AlCl3/CH3COOH degradation method is superior to that of other degradation methods, for example, thermal decomposition or supercritical liquids (see SI Table 7).

Scheme 2. Possible Decomposition Catalytic Mechanism of CEP in the AlCl3/CH3COOH Degradation System

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CONCLUSIONS In conclusion, chemical recycling of CFREP has successfully been realized via a selective cleavage of a tertiary carbon nitrogen bond using AlCl3/CH3COOH as the degradation system. The tertiary carbon nitrogen bond cleavage was evidenced by FT-IR, NMR, elemental analysis, and GPC. These characterizations also showed that the C−C and C−O bond remained intact. Under the optimal degradation conditions (15 wt %, 180 °C, 6 h), the recovery yield of CEP reaches up to 97.43 wt %, and the clean CFs recovered conserved about 97.77% of their tensile strength compared with that of virgin CFs. Especially, oligomers of CEP can be preserved and recycled, which facilitates their reuse in resin manufacturing. This promising novel strategy is an environmentally benign and economically feasible method for chemical recycling of the CFREP composite.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b00949. Synthesis process of CEP; effective separation of degradation products and catalyst; NMR characterizations; elemental analysis (mole ratio) and molecular weight of products; characterizations of recycled CFs; comparison of various recycling methods in recycling CFREP (PDF)

Figure 4. Pictures of (a) CFREP and (b) recycled CFs, and SEM images of (c) virgin CFs and (d) recycled CFs. Reaction conditions: degradation system AlCl3/CH3COOH and AlCl3 mass fraction 15 wt %, 180 °C, and 6 h.

CFREP decomposed completely, and the CFs were recycled. Clean CFs were obtained after removing the CEP from CFREP, as shown in Figure 4a and b. SEM was employed to further study the surface morphology of the recycled CFs (see Figure 4c and d). As compared with the very clean surface of virgin CFs, there existed some spots in the surface of recycled CFs. These spots are the incompletely degraded CEP. The tensile strength of the recycled CFs was investigated using a monofilament tensile test (see SI Table 4). The tensile strength of recycled CFs is 2871.96 MPa, which is 97.77% that of virgin CFs. The tensile modulus of recycled CFs is 168.46, which is 98.07% that of virgin CFs. These results confirm that the degradation system has insignificant impact on the tensile strength of the recycled CFs. The surface functional group compositions determined by XPS are shown in SI Figure 8. In the XPS analysis of the recovered carbon fiber ((a3) in SI Figure 8a), the O 1s peak is

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AUTHOR INFORMATION

Corresponding Authors

*(T.D.) E-mail: [email protected]. *(X.H.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Scientific and technological project of Shanxi Province (20130322005-01). We thank Associate Professor Christian Marcus Pedersen for help with preparing the manuscript. 3336

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