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An Efficient Method of Recycling of CFRP Waste Using Peracetic Acid Mohan Das, Rinu Chacko, and Susy Varughese ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b01456 • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 3, 2018

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ACS Sustainable Chemistry & Engineering

An Efficient Method of Recycling of CFRP Waste Using Peracetic Acid Mohan Das, Rinu Chacko and Susy Varughese* Department of Chemical Engineering, Indian Institute of Technology Madras, Chennai, India - 600036 Email address: [email protected] KEYWORDS: Recycling, Composites, CFRP, Peracetic acid, Oxidative degradation

ABSTRACT: We report a one-step oxidative method for the recycling of Carbon Fiber Reinforced Polymer (CFRP) composite waste and the recovery of carbon fiber and the epoxy resin by treatment with peracetic acid (PAA) formed in-situ from a mixture of acetic acid and hydrogen peroxide. Surface of the recovered carbon fibers were clean and the tensile strength was comparable to that of the virgin fibers. A higher resin decomposition ratio of 97% could be achieved for the epoxy matrix under mild reaction conditions in comparison to other chemical recycling processes. ATR-FTIR, MALDI/TOF–MS, GPC, GC–MS, 1H-NMR and Pyro-GC/MS analysis

showed the

formation of low molecular weight oxidation products of amine cured epoxy resin along with high molecular weight compounds of high viscosity. A possible reaction mechanism for the degradation of the epoxy matrix is proposed. All the solvents used were recovered in pure and re-usable form with more than 90% recovery efficiency. The recovered epoxy was re-used along with an adhesive grade epoxy (2 wt%) with no significant loss of 1 ACS Paragon Plus Environment

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tensile strength. Almost complete recovery of the recycled products as well as the solvent along with no gaseous emissions and mild reaction conditions make this process more environment-friendly.

Introduction

Carbon fiber reinforced polymer composites (CFRP) are known for their high mechanical strength, low weight, good corrosion and thermal resistance as well as their ease of making into complex shapes which make them ideal for applications in industries such as aerospace,

automotive,

construction,

sports,

marine,

and

consumer

goods

manufacturing.1-4Based on the annual global output of carbon fibers,5 the amount of CFRP produced could be close to 180,000 metric tonnes annually. It is estimated that 30% of carbon fibers produced are discarded as production waste, along with solid waste generated during CFRP manufacturing and at the end-of-life for various CFRP components. If recycled efficiently, the recyclates (especially, the carbon fibers) can be re-used in various engineering and automotive components.6 Thus, CFRP waste recycling has an advantage from both economic and environmental perspective.7

Various recycling processes such as mechanical8-11, thermal12 and chemical processes13-18 have been proposed for the same. Unlike mechanical processes, thermal and chemical processes are able to recover long continuous fibers with a higher potential for re-use. However, the use of high temperatures (450–550 °C) in thermal processes result in fibers with only 70-75% strength retention and leads to harmful gas emissions and fiber surface contamination due to char deposition.12 The past decade has also seen supercritical fluids as green alternatives for chemical recycling of CFRP waste.19-21 2 ACS Paragon Plus Environment

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However, these processes require extremely high temperatures (400–500 °C) and pressures (10–30 MPa) for the fluids to attain supercritical state and are thus energy intensive. Chemical recycling methods ensure complete resin removal and recovery of long and continuous fibers with better strength. However, the use of harmful chemicals (e.g. nitric and sulfuric acid)13-18 and/or the use of extreme processing conditions (supercritical fluids) limits their scope for large scale application from an environmental and economic viewpoint. Considering these issues, we previously reported a sonochemical method for CFRP recycling using mild nitric acid–H2O2 mixture and high frequency ultrasound that reduced the consumption of nitric acid drastically.22 However, the process could still not be considered eco-friendly due to release of small quantities of N2O during the reaction and the difficulty in the recovery of the acids used. Recently there has been some success in developing eco-friendly chemical recycling processes that can recover both the fibers and the polymer matrix with minimal damage to the environment. This was achieved by using a combination of mild chemicals (acetic acidacetone mixture, H2O2 and N,N-dimethylformamide mixture, etc.) and/or catalysts(AlCl3, ZnCl2–ethanol, etc.).23-27 These processes although comparatively mild, involved the use of catalysts (prone to loss and/or reduced activity after recovery), use of high reaction temperatures (220 °C) and do not emphasize on the complete recycling/recovery of all the materials used. Another recent approach is a fully recyclable CFRP composite based on polyimine with the possibility of malleability and complete chemical recovery.28

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Our earlier study using dilute nitric acid and H2O2 mixture with ultrasound showed the usefulness of H2O2 in the oxidative degradation of epoxy matrix in CFRP composite. This led us to explore the use of acetic acid along with H2O2. Glacial acetic acid has a pKa value of 4.75 which is much higher to that of nitric acid (pKa< 1). Hence, nitric acid is capable of decomposing crosslinked epoxy13 whereas pure acetic acid is less effective for such applications. Although H2O2 is a strong oxidizing agent, it is incapable of oxidizing the chemically resistant, crosslinked epoxy matrix. However, mixing H2O2 with pure acetic acid in equimolar concentrations leads to the formation of peracetic acid (PAA) which is a very strong oxidising agent and is expected to break down the crosslinked epoxy matrix.

CH3COOH + H2O2 ⇌ CH3CO3H + H2O

(1)

Formation of the PAA is reversible and an equilibrium mixture of reactants and products is obtained. Even though PAA is a weaker acid (pKa = 8.2) than acetic acid, it is a strong oxidizer.29 Thus, PAA finds extensive application as a disinfectant for food and medical supplies, water purification, as a bleaching agent in textile and pulp industry, and for epoxidation of olefins. Such applications also indicate that PAA is not considered toxic.30-31

In the present work, we show that the in-situ formation of PAA can oxidatively degrade crosslinked epoxy in a single stage process and recover clean carbon fibers with good mechanical properties. This process is different from the already reported two stage process involving acetic acid,24 where CFRP was first swollen in acetic acid at 120 °C before being decomposed in acetone–H2O2 mixture at 50-120 °C. Also, the possibility of 4 ACS Paragon Plus Environment

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full recovery and re-use of the polymeric matrix as well as all the chemicals used are demonstrated, making it an efficient and sustainable process for the CFRP waste recycling. We also elucidate the possible reaction mechanism of the oxidative degradation of the crosslinked epoxy resins through various characterizations.

Materials and methods

Post fabrication and post-test aerospace CFRP waste and virgin carbon fabric in mat form were provided by National Aerospace Laboratories, Bengaluru, India. The composition of the resin, precursor for the fibers and the wt% of fiber and resin in the composite were unknown. 99.7 wt% (or 17.4 M) glacial acetic acid and 99.5 wt% ethyl acetate were supplied by Thermo Fisher Scientific India Pvt. Ltd., 30 wt% (or 9 M) hydrogen peroxide was supplied by Merck Specialities Pvt. Ltd., India and were used as received. Composite samples of dimensions 30 x 25 x 2 mm3 were cut using a band saw cutter. These samples were immersed in an aqueous mixture of 14 M glacial acetic acid and 9 M hydrogen peroxide of varying compositions (Table 1) in a closed Pyrex glass bottle in oil bath at constant temperature. The bottle was removed every hour from the commencement of the reaction, cooled to room temperature to collect sample for UV-Vis spectroscopy (Jasco V630), to monitor the progress of the reaction and then placed back. This process was repeated for different time periods. After this the fibers were separated from the solution by filtering using Whatman Grade 1 filter paper and washed with distilled water until neutral pH was reached. The recovered fibers were washed with



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acetone and dried at 100 °C in an air oven for 4 h. The filtrate was distilled at 120 °C using a simple distillation apparatus. The distillate was collected and analysed. The remaining viscous liquid was mixed with ethyl acetate and distilled water to carry out liquid-liquid extraction.13 The organic phase containing decomposed resin and ethyl acetate was neutralized using 10 M NaOH solution after separation from the aqueous phase. This was followed by another stage of liquid-liquid extraction using ethyl acetate to separate neutralized decomposed resin. The organic phase was again distilled at 80 °C for recovering ethyl acetate and the decomposed resin was collected as a viscous liquid. All the above steps were repeated on a known composition of commercially available DGEBA epoxy (Araldite GY257) cured with polyamidoamine based hardener (Aradur 140), mixed in 1:1 ratio to elucidate the mechanism of composite matrix degradation.

The efficiency of matrix dissolution for different solvents was compared using the values of resin decomposition ratio, DR,17

( ) × × )

= (

100

(1)

where, m1 is the mass of the untreated sample, m2 is the mass of the dried sample remaining after treatment and Wm is the mass fraction of resin in the initial sample, determined using ASTM D 3171.

Surface of the virgin carbon fibers (as received) was compared with the recovered fibers using High Resolution Scanning Electron Microscope (HR–SEM, Hitachi S–4800) at an operating voltage of 1-3 kV along with Energy Dispersive X-Ray Spectroscopy (EDS) analysis. X-ray photoelectron spectroscopy using ESCA Probe 125, (Omicron 6 ACS Paragon Plus Environment

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Nano Technology) was carried out to analyse changes in their surface elemental composition. Quantity of resin remaining on the fiber surface after treatment was determined by the thermogravimetric analysis of the fibers Tensile strength of single fibers of virgin as well as that of the recovered fibers was determined using Universal Testing Machine (Zwick/Roell) with a 1 kN load cell (ASTM C1557–03) for a minimum of 20 fibers. Chemical analysis of the untreated matrix, the recovered polymer and the solvent was carried out using ATR-FTIR analysis (Cary 630 FT–IR Spectrophotometer, Agilent Technologies). The structure of the recovered polymer was predicted using Bruker NMR spectrometer operating at 500 MHz with deuterated chloroform (CDCl3) as the solvent and tetramethylsilane as internal standard Fast pyrolysis experiments on a known composition of DGEBA epoxy in pure, crosslinked and decomposed form were performed in a single shot micropyrolyzer (PY-3030 S, Frontier Laboratories, Japan) interfaced with a Shimadzu QP2010 Plus GC/MS. Epoxy samples weighing 390 ± 10 µg were taken in a stainless steel cup, purged with helium for 10 min and dropped into the furnace preset at 600 °C. Ultra high pure helium (99.9995%) was used as the carrier gas at a flow rate of 1.59 mL min−1 in splitless mode. The pyrolysates were analyzed by gas chromatography using an alloy capillary column (UA-5; 30 m × 0.25 mm; 0.25 µm film thickness). The column oven was initially held at 45 °C for 2 min, followed by heating at a rate of 10 °C min−1 to 300 °C, and finally held at 300 °C for 15 min. The pyrolysate mass spectra were acquired in the m/z range of 50–500 at an electron ionization voltage of 70 eV. The organic compounds were identified by comparing the mass spectra with NIST database using a percentage match factor cut-off of 85%.



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The rheology of the recovered polymer was studied on a stress controlled rheometer (Anton Paar MCR 301, 25 mm parallel plate geometry, 0.4 mm gap) at shear rates varying from 0.1 to 100 s-1. This was compared with an adhesive grade epoxy resin (ROTEX resin EP415-AF, Roto Polymers and Chemicals, Chennai) and polyamide resin hardener (ROTEX hardener EH400). To study the re-usability of the recovered polymer,2 wt% of the same was mixed with the adhesive grade epoxy and 40 parts of hardener. Tensile strength was compared with crosslinked neat epoxy samples as control. Cured epoxy sheets of 160 x 100 x 3 mm3 size were prepared by moulding in a hydraulic press at 60 °C for 2 h at 0.2 MPa pressure from which rectangular tensile specimens of dimensions 110 x 20 x 3 mm3 were cut. Tensile properties (ASTM D3039) were determined using a Universal Testing Machine (Zwick/Roell) with a 50 kN load cell and wedge grip. The crosshead speed was maintained at 2 mm/min.

Results

CFRP waste degradation and carbon fiber recovery studies were carried out in an aqueous mixture of acetic acid and H2O2 under different reaction conditions. The reaction mixture containing the decomposed polymer and the fibers was filtered for fiber recovery and the filtrate was then distilled for solvent recovery. The remaining decomposed polymer was recovered using liquid-liquid extraction, distillation and neutralization. The resultant liquid products as well as the recovered carbon fibers were analysed using techniques such as ATR-FTIR, UV-Vis, HR-SEM, XPS, TGA, 1H-NMR spectroscopy, GPC, and Pyro-GC/MS.

Effect of reaction conditions 8 ACS Paragon Plus Environment

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All solvents containing H2O2 showed considerable resin decomposition (Table 1) indicating the significance of H2O2 addition.

This can be attributed to the in-situ

formation of peracetic acid (PAA). The duration of the reaction was much shorter (5 h) compared to the reaction time for the sonochemical process (8 h), previously reported by us.22 Figure 1 shows the change in UV-Vis peak absorbance with reaction time of the solution, clearly highlighting the advantage of H2O2 addition to the solvent. Using pure 14 M acetic acid as the solvent shows only a small peak at 340 nm (Figure S2) and does not show large difference with increasing reaction time (Figure 1(a)). However, addition of H2O2 shifts the absorbance peak from 340 to 280 nm which is more prominent and shows considerable increase with reaction time (Figure 1(b)). Absorbance peak at 280 nm corresponds to phenol and phenolic derivatives32. The chemical species responsible for a peak at 340 nm could not be ascertained. Peak values for solutions with solvents Ac50H50 and Ac20H80 were not plotted in Figure 1 due to absence of a clear peak. These solutions showed infinite absorbance in the 250-280 nm range indicating high yield of phenolics (Figure S3). The physical effect of H2O2 addition was also clearly visible on the samples (Figure 2).



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Figure 1. Change in the UV-Vis absorbance peak with reaction time at (a) 340 nm for solvent Ac100 and (b) at 280 nm for solvents Ac95H5, Ac90H10 and Ac80H20. Reactions carried out at room temperature (28 °C) took 84 h for complete matrix dissolution. At 80 °C, the reaction time was similar to that at 65 °C (Table 1). Higher reaction temperatures in the presence of oxidizing agents can lead to excessive surface oxidation and reduction in strength of the carbon fibers and hence were not attempted.

The amount of solvent consumed was optimized by carrying out reactions at three different solvent volume/sample mass ratios: 20 ml/g, 40 ml/g and 60 ml/g. Higher solvent volume resulted in higher resin decomposition ratio, DR (Table 1) with shorter 10 ACS Paragon Plus Environment

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reaction times. However; this can lead to the use of large quantities of the solvent for a large scale process and will need further optimization. Only Ac95H5 was used for both the above mentioned studies. The reason for this will be explained in the forthcoming sections.

Table 1 Change in the resin decomposition ratio, DR with H2O2 content and the reaction conditions

Solution

vol% of 14 vol% of vol. of solvent/ M acetic 14 M mass of sample acid H2O2 (ml/g)

Temperature (°C)

DR (%)

reaction time (h)

Ac100

100

-

60

65

25.6

> 10

Ac95H5

95

5

60

65

97.2

4

Ac90H10

90

10

60

65

98.3

4

Ac80H20

80

20

60

65

98.8

4

Ac50H50

50

50

60

65

96.5

5

Ac20H80

20

80

60

65

98.2

5

H100

-

100

60

65

-

-

Ac95H5

95

5

60

28

98.3

84

Ac95H5

95

5

60

80

98

4

Ac95H5

95

5

20

65

67.3

8

Ac95H5

95

5

40

65

94.4

8



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Figure 2. Microscopic images of the CFRP sample surface undergoing reaction in solvents Ac100 and Ac95H5.

Analysis of the recovered fiber and the polymer Clean and long carbon fibers were recovered only when the reaction medium containing both acetic acid and H2O2 were used and not when acetic acid alone is used. HR–SEM images of the fibers show that their surfaces are completely free of any polymer residues (Figure 3). EDS analysis of the fiber surface shows only carbon except in the case of fibers recovered using solvents Ac50H50 and Ac20H80, where, small amounts of oxygen was detected (Table S1). C-OH and COOH functional groups were detected also in the XPS analysis of the recovered fibers (Figure S4). As mentioned earlier, higher amounts


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of oxidizing agent in an acidic medium can oxidize the carbon fiber surface as well. This was observed in a previous study as well.22

Figure 3. HR-SEM images of (a) virgin fiber and fibers recovered from; (b) Ac95H5, (c) Ac90H10, (d) Ac80H20, (e) Ac50H50 and (f) Ac20H80 as solvents. Thermogravimetric analysis of the virgin and the recovered fibers further confirmed the absence of any resin on the fiber surface (Figure S5). The small amount of weight loss in the case of virgin fibers (5 wt%) could be due to the loss of the sizing material from their surface. Single fiber tensile strength tests performed on the virgin and recovered fibers show comparable values (Figure 4) indicating that the exposure of fibers to a mixture of H2O2, PAA and acetic acid does not affect the fiber strength adversely. Thus, within a single stage of mild chemical treatment, fibers with good mechanical properties were recovered unlike the solvolysis processes carried out at very high temperatures and pressures or the multi – stage eco-friendly chemical treatment processes.



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Figure 4. Single fiber tensile strength values for virgin fibers and fibers recovered from different reaction mixtures. One of the highlights of the present process is the possibility to extract the degraded polymer matrix and to recover the solvent using a simple distillation process. Since the epoxy matrix undergoes decomposition at various recycling stages, ATR – FTIR analysis was used to analyze the changes in the chemical composition of the decomposed epoxy at each stage (Figure 5). It was shown in our previous study22 that the matrix of the CFRP sample is made up of amine crosslinked DGEBA epoxy, where, the major mechanism for degradation was the scission of the amine crosslinks and the polymer chain by oxidation due to the sonochemical reactions. In the present study as well, the IR analysis of the recovered polymers (prior to the solvent recovery step) shows that the absorption peaks for the amine crosslinks represented by C–N bond stretch at 1110 cm-1 are absent (Figure 5(a)) indicating scission of the crosslinking points. The primary chemical changes taking place on the network apart from the C-N bond cleavage is the oxidation of C-OH groups (1034 cm-1) to C=O (1743 cm-1). The peak intensity for 14 ACS Paragon Plus Environment

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C=O bond increases with increasing H2O2 for Ac95H5 to Ac90H10 and remains constant thereafter for solvents with higher H2O2 content. This is an indication that epoxy chain oxidation does not increase linearly with H2O2 concentration in the solvent and as a result, the reaction time does not reduce for solvents with higher H2O2 concentration (Table 1). Other peaks corresponding to the aromatic ring vibrations (1508 and 1452 cm1

), aryl alkyl ether, C-O-C (1240 cm-1) and O-H stretch (3350 cm-1) were also detected.

However, during solvent recovery by distillation, the decomposed epoxy undergoes further thermal and chemical degradation due to elevated temperatures (120 °C). The IR spectra of epoxy recovered after this stage (Figure 5 (b)) shows retention of aromatic structures, aryl alkyl ether linkage and small peaks of the C=O functional group. The oxidative decomposition by PAA during reaction stage and thermal decomposition (at 120 °C, 6–8 h) during solvent distillation stage yield functional groups similar to that reported in the literature for epoxy oxidation.33 Similar groups were detected in the decomposed epoxy obtained after recycling a known composition of cured DGEBA epoxy (Figure S15).



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Figure 5. IR analysis of the decomposed epoxy matrix recovered after drying from different solutions (a) before solvent distillation and (b) after solvent distillation. Another way of understanding the molecular composition of the degraded epoxy matrix would be to look at the molecular weight distribution. The molecules recovered before the solvent distillation step were soluble only in acetone. Hence, the molecular


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weight was determined using MALDI/TOF-MS after evaporating the original solvent and dissolving them in acetone. The analysis showed that the decomposed matrix contained species of low molecular weight (m/z < 300) and no other significant peaks were observed for m/z < 105 (Figure S6). This leads to a speculation whether the decomposition reaction was able to break down the main chain of the epoxy into very small compounds (m/z < 300) or if compounds had m/z values above 105 and were not detected. Molecular weight distribution of the decomposed epoxy recovered after the solvent distillation stage was carried out using GPC (Figure S7). The results indicated the presence of species having both extremely low as well as extremely high molecular weights with mostly low molecular weight compounds. At this stage, the recovered polymer was a highly viscous material indicating the presence of high molecular weight molecules. However, the GPC results may not be showing the presence of these high molecular weight molecules due to their partial solubility in the solvent used (THF). GC-MS analysis of the low molecular weight fractions of the decomposed epoxy recovered from solvent Ac95H5 shows the presence of long chain hydrocarbons and alcohols (Figure S8). 1

H-NMR analysis of the recovered resin (Figure 6) shows that it contains a

mixture of aromatic and aliphatic components. However, it was difficult to predict the structure of the final decomposition products at this stage.



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Figure 6. 1H NMR spectrum of the decomposed epoxy matrix recovered from solvent Ac95H5 indicating the presence of both aliphatic and aromatic species. A better understanding of the epoxy decomposition in CFRP was obtained by decomposing a cured DGEBA epoxy of known composition using Ac95H5 as solvent. The results from this study are included in the supporting information (Figure S13 –16). Pyrolysis – GC/MS analysis was used to determine the structure of the final decomposition products (Figure 7). The pyrogram of decomposed epoxy indicated the presence of mainly phenols and phenolic derivatives which are indicators of the DGEBA structure.34 This is a strong indication of the partial retention of the primary chemical structure of the DGEBA epoxy even after extensive degradation.


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Figure 7. Pyrograms of DGEBA epoxy (uncrosslinked, crosslinked and bottoms recovered from the distillation process). This analysis also showed that the highly viscous nature of the recovered epoxy is due to the presence of high molecular weight compounds consisting of aromatic structures, especially phenolics. A key observation from the FTIR (Figure S15) and PyGC/MS analysis is that the reaction with PAA does not affect the epoxy main chain significantly, but the solvent distillation stage degrades the epoxy main chain. The solubility of the resultant compounds decreases since there are possibilities of cyclisation, de-polymerization and crosslink formation by the free radicals formed at high temperatures. On the other hand, the free radical reactions by the PAA and H2O2 specifically targets the C-N bonds and oxidizes the C-OH functional groups to C=O groups leading to the complete dissolution of the epoxy matrix.

Analysis of the recovered solvent 19 ACS Paragon Plus Environment


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Unlike similar studies reported earlier, the present process enables the recovery and reuse of the solvent (acetic acid) used for decomposing the CFRP waste. A simple distillation process was used to recover more than 90 vol% of the acetic acid. The concentration and chemical composition analysis of the recovered solvent show its reusability. Acid-base titration showed that solvent Ac95H5 had the least change in concentration when compared to other solvents (Figure S9). The concentration of the solvent is reduced due to the dilution by water molecules formed during the reaction. With higher H2O2 content, more dilution occurs causing further loss of acid strength. IR analysis of the original and recovered solvents (Figure S10) shows identical compositions. IR analysis of the salt recovered after neutralizing the decomposed polymer with NaOH indicates formation of sodium acetate (Figure S11). As mentioned earlier, solvent Ac95H5 was considered for further studies since it retained its chemical composition and strength after one cycle of reaction indicating better potential for re-use. Mechanism of CFRP decomposition The major process for epoxy decomposition is the oxidation of C-OH groups to C=O and breaking of C-N bonds. The oxidation reaction proceeds by a free radical mechanism initiated by the hydroxyl radicals generated from H2O2 and acyloxy radicals generated from PAA.33 Thus, the following reaction mechanism is proposed for the decomposition of the crosslinked epoxy matrix during the reaction stage: Step 1: Generation of acyloxy and hydroxyl radicals


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O-O bond in peracetic acid (PAA) is unstable and decomposes PAA into acyloxy and hydroxyl radicals: CH COOOH → CH COO• + •OH

(2)

The generated hydroxyl radical can further react with PAA itself: CH COOOH + •OH → CH CO• + O + H O

(3)

CH COOOH + •OH → CH COOO• + H O

(4)

Acyloxy radicals also undergo further decomposition, CH COO• → •CH + CO

(5)

2CH COO• → 2 •CH + 2CO + O

(6)

Methyl radical is unstable and reacts with oxygen to form weak peroxy radical. •

CH + O → •CH OO

(7)

Recombination of the free radicals regenerates the PAA CH COO• + •OH → CH COOOH

(8)

Step 2: Reaction of the acyloxy radical with the crosslinked epoxy network The C–N bond is a weak bond compared to C-C bond and hence, the acyloxy and the hydroxyl radicals attack this bond leading to the breakup of crosslinks. Another reason for the C-N bond scission could be the competition between nitrogen, oxygen and the benzene ring in sharing the electrons due to their electronegative nature. This also makes the crosslink points weak and vulnerable. (Scheme 1):



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Scheme 1 Proposed reaction mechanism for the decomposition of amine crosslinked epoxy by acyloxy radical.

Step 3: Reaction of hydroxyl radical with crosslinked epoxy network (Scheme 2)


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Scheme 2 Proposed reaction mechanism of hydroxyl radical with amine crosslinked epoxy. Formation of phenols and phenolic derivatives as well as species with carbonyl groups takes place during the decomposition reaction. The products formed at this stage are a mixture of low and high molecular weight compounds. Further decomposition of these reaction products occurs during the solvent recovery stage using distillation. The high temperature and long duration of the distillation process degrades the reaction products further into a highly viscous mixture of both low molecular weight linear hydrocarbon compounds and high molecular weight aromatic compounds (Figure 6). No gases are released during the reaction and all the raw materials are recovered for re-use. Re-use of the recovered polymer 23 ACS Paragon Plus Environment


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Shear dependent viscosity measurements of the recovered compounds showed the viscosity to be very high (106 cP) and comparable to that of a commercially available adhesive grade epoxy resin (ROTEX resin EP415-AF) (Figure S12). A homogeneous mix of 2 wt% of the recovered epoxy with the adhesive grade epoxy could be easily obtained by manual mixing. This blend as well as the neat adhesive grade epoxy were cured using a hardener and the tensile strength was determined (Table S3). Comparable tensile properties indicate the potential for the re-use of the epoxy recovered. Conclusions This study reports a single stage oxidation process for recycling aerospace CFRP waste using an aqueous mixture of acetic acid and hydrogen peroxide. Clean and long carbon fibers were recovered with tensile strength comparable to that of virgin fibers. The solvent containing 95 vol% 14 M acetic acid and 5 vol% 9 M H2O2 (Ac95H5) showed an excellent resin decomposition ratio of 97%. More than 90% of the acetic acid could be recovered by simple distillation process with 94% of its original strength retained. Distillation process for solvent recovery leads to further decomposition of the polymer matrix resulting in the formation of both aliphatic and aromatic compounds, primarily phenolics. These were formed as a result of a combination of oxidative and thermal degradation of the epoxy matrix by acetic acid, PAA and H2O2 at high temperature. No hazardous solid, liquid or gaseous byproducts were formed during this process. The recovered polymer with good mechanical properties could be re-used along with an adhesive grade epoxy. Complete recycling of the recovered materials and the solvent under mild reaction conditions using non-hazardous chemicals, point to the effectiveness of this process as a more sustainable route to CFRP waste recycling.


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ASSOCIATED CONTENT Supporting Information: Experimental steps, UV-Vis absorbance curves, XPS and TGA of the carbon fibers, MALDI-TOF/MS, GPC, GC-MS analysis of the recovered epoxy, ATR-FTIR analysis of the fresh and recovered solvents, viscosity and tensile strength measurements for the recovered compounds. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Funding Sources This study was funded by Aeronautics Research and Development Board (ARDB), India. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is funded by ARDB (Aeronautics Research and Development Board), India. We acknowledge the National Aerospace Laboratory, Bengaluru, India for providing us with the composite samples and carbon fibers, Environmental Engineering Lab, Department of Chemical Engineering, IIT Madras for the GC–MS analysis, Dr. Nandita Madhavan, Department of Chemistry, IIT Madras for discussions and Mr. Vivek, Department of Chemistry, IIT Madras for NMR data analysis, DST Unit of Nanoscience, IIT Madras for the XPS facility,

Complex Reaction Kinetics Lab, Department of

Chemical Engineering, IIT Madras for ATR – FTIR and Pyro-GC/MS facility and the DST-FIST grant for the HR-SEM facility. 25 ACS Paragon Plus Environment


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Synopsis: Oxidative recycling of CFRP waste using in situ formed peracetic acid to recover carbon fibers and resin without harmful emissions. 28 ACS Paragon Plus Environment

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Table of Content 254x190mm (300 x 300 DPI)

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Fig. 1Change in UV-Vis absorbance peak with reaction time at (a) 340 nm for solvent Ac100 and (b) at 280 nm for solvents Ac95H5, Ac90H10 and Ac80H20 105x173mm (300 x 300 DPI)


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Fig. 2Microscopic images of the CFRP sample surface undergoing reaction in solventsAc100 and Ac95H5 170x155mm (300 x 300 DPI)



Fig. 3 HR-SEM images of (a) virgin fiber and fibers recovered from solutions with (b) Ac95H5, (C) Ac90H10, (d) Ac80H20, (e) Ac50H50 and (f) Ac20H80 as solvents. 245x143mm (300 x 300 DPI)


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Fig. 4 Single fiber tensile strength values for virgin fibers and fibers recovered from different reaction mixtures. 64x49mm (600 x 600 DPI)



Fig. 5 IR analysis of the decomposed epoxy matrix recovered after drying from different solutions (a) before solvent distillation and (b) after solvent distillation. 95x173mm (300 x 300 DPI)


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Fig. 6 1H NMR spectrum of decomposed epoxy matrix recovered from solvent Ac95H5 indicating the presence of both aliphatic as well as aromatic species. 200x176mm (300 x 300 DPI)



Fig. 7 Pyrograms of DGEBA epoxy (uncrosslinked, crosslinked and bottoms recovered from the distillation process). 82x67mm (600 x 600 DPI)


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Scheme 1Proposed reaction mechanism for the decomposition of amine crosslinked epoxy by acyloxy radical. 48x62mm (600 x 600 DPI)



Scheme 2Proposed reaction mechanism of hydroxyl radical with amine crosslinked epoxy. 29x27mm (600 x 600 DPI)


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An Efficient Method of Recycling of CFRP Waste ... - ACS Publications

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