Characterization of the Liquid Products from Hydrolyzed Epoxy and

Article pubs.acs.org/IECR

Characterization of the Liquid Products from Hydrolyzed Epoxy and Polyester Resin Composites Using Solid-Phase Microextraction and Recovery of the Monomer Phthalic Acid Hülya U. Sokoli,*,† Morten E. Simonsen,† Rudi P. Nielsen,† Jens Henriksen,‡ Marianne L. Madsen,‡ Niels H. Pedersen,‡ and Erik G. Søgaard† †

Department Chemistry & Bioscience, Aalborg University, 6700 Esbjerg, Denmark FORCE Technology, 2605 Brøndby, Denmark

‡

S Supporting Information *

ABSTRACT: A method to recover the fibers and monomers from epoxy-based and unsaturated polyester-based composites and characterize the liquid products using solid-phase microextraction has been investigated in this study. Experiments were performed using near-critical water in a batch reactor (1 L) at temperatures and pressures up to 350 °C and 170 bar. In-situ sampling was performed. The samples collected at temperatures in the range of 300 to 350 °C were divided into an aqueous phase and an oil phase. It was possible to identify various phenolic and aromatic compounds in both phases. In the samples collected during degradation of the polyester composites, the monomer phthalic acid precipitated nearly purified at temperatures in the range of 250−325 °C. By the recovery of fibers, an oil product, and the possibility to recover phthalic acid, the polymer composite end-of-life cycle is a step closer to completion.

1. INTRODUCTION Fiber reinforced polymer (FRP) composites are a group of materials that are being increasingly employed in industries such as automobiles, aeronautics, and constructions.1 The increased interest for FRP composites can be attributed to their unique properties, such as high strength to weight ratio, corrosion resistance, and design flexibility and stability.2 Despite the successful application of FRP composites, recycling at the end-of-life is associated with great difficulties. Once the thermosetting polymer material is cured, it is not possible to remelt or remold it. Although some thermosetting polymers, such as polyurethane, can easily be converted back into the original monomers, the more common thermosetting polymers such as unsaturated polyester (UP) and epoxy are difficult to depolymerize into the original constituents.3 Therefore, the majority of FRP composites are disposed at landfills or incinerated for energy recovery without further recycling attempts.3 These disposal routes are not sustainable in the long term and can lead to negative impact on the environment.4 Consequently, legislation has been enacted in order to protect the environment (Directive 91/556/EEC; Directive 99/31/EC; EU 2000/53/EC), leading to strict control of composite disposal. Legal landfilling of composites is limited in many EU countries and for instance, it is illegal to landfill composite waste in Sweden and Germany.5 Therefore, development of efficient recycling technologies is required. The most optimal recycling approach is degradation of the polymer matrix into usable monomers and recovery of the fibers with retained mechanical properties. To fulfill such © XXXX American Chemical Society

requirements, the following recycling technologies have been investigated the most: mechanical recycling, thermal recycling, and chemical recycling.6 Chemical recycling at elevated temperatures and pressures can provide carbon fibers with retained mechanical properties and solubilized resin containing monomers and smaller degradation products. The technology is referred to as solvolysis and was found to be the most promising recycling technology in the review paper by Morin et al.7 Chemical recycling involves the application of solvents such as water, alcohols, and glycols to decompose the polymer matrix in the composite. Temperatures and pressures range between 200 and 450 °C and 50−300 bar, meaning that the solvents are operated under near-or supercritical conditions. Near-critical or supercritical water has been employed as solvent in several studies as it is harmless, inexpensive, and highly reactive with organic materials.8 Oliveux et al.9 degraded glass fiber reinforced (GFR) UP resin using near-critical water at temperatures in the range of 250−300 °C and pressures in the range of 40−180 bar. They found that it was possible almost completely to remove the resin from the surface of the fibers at temperatures greater than or equal to 300 °C. In the process, the recovered glass fibers lost more than 50% of their strength, due to surface etching caused by the water.10 The Received: June 1, 2016 Revised: August 3, 2016 Accepted: August 9, 2016

A

DOI: 10.1021/acs.iecr.6b02111 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 1. PI diagram of the reactor system.

in the field of polymer composite degradation to perform the characterization. The second objective of this study was to hydrolyze epoxyand UP-based composites in a 1 L reactor to investigate the feasibility of recovering the monomers. This includes monomers such as phthalic acid, styrene, and bisphenol A. Thus, the focus of this work will exclusively be on the liquid products and the recovery of monomers, as several studies have already been conducted with focus on the recovered fibers.8,9,17−24

monomers were degraded into smaller degradation products. Other studies performed using near-critical water has also shown that the highest degradation of the resin is obtained at elevated temperatures, hence resulting in degradation of the monomers.11−13 The balance between recovery of the highest possible amounts of the monomers and complete removal of resin from the surface of the fibers is challenging; nevertheless it is crucial to obtain an efficient recycling process with usable end-products. The process of converting polymer composite waste into valuable products can be compared to the process of converting biomass into bio-oil. Sokoli et al.14 solvolytically converted UP resin-based composites into a high heating value fuel oil with properties close to that of crude oil and with better heating value than bio-oil from Aspen wood. Although it is possible to recover a valuable fuel oil from both the biomass and the polymer conversion process, there are still major challenges associated with the further development and up-scaling of both techniques. This includes characterization of the aqueous phase, which is considered a byproduct. Owing to the complexity and polarity of the degradation products in the aqueous phase, characterization is associated with considerable analytical challenges. A recent study by Arturi et al.15 reported that the characterization of liquid products from the biomass conversion process was significantly improved by the use of solid-phase microextraction (SPME) techniques. It was possible by this method to characterize both very polar and volatile degradation products in the aqueous phase among other reaction products, revealing that the composition of the aqueous phase is far more complex than previously known. One of the objectives of this study concerns the characterization of the aqueous and oil phases obtained from the hydrolysis of epoxy and unsaturated polyester (UP)-based composites. SPME techniques will be applied for the first time

2. MATERIALS AND METHODS 2.1. Materials. Epoxy- and UP resin-based composites were used in this study. The UP-based composite was reinforced with glass fibers (58.2 wt % ± 0.2) and based on an orthophthalic acid resin consisting of 41 wt % styrene and 1 wt % methylethylketone peroxide (MEKP). The epoxy resin composites were reinforced with glass (74.7 wt % ± 0.2) or carbon fibers (60 wt % ± 0.3) and based on bisphenol A/F and diglycidylether,1,6-bis(2,3epoxypropoxy)hexan) cured with polyoxyalkylene-diamine and 3-aminomethyl-3,5,5-trimethylcyclohexylamine. The fiber content was evaluated by calcination following the recommendation of the standard DS/EN ISO1172. An average of the value measured on three samples was calculated. Phenol (purity ≥90.0%) was purchased from VWR Bie & Berntsen. Phthalic acid (for synthesis, ≥ 98.0%), terephthalic acid (for synthesis, 99.0%), and benzoic acid (for analysis, 99.9%) were purchased from MERCK. Bisphenol A (≥99.0%,), P (analytical standard, 99.0%), F (analytical standard, ≥ 98.0%), and trimethylsilyl trifluoromethanesulfonate (TMSOTf, purum, 98.0%) were purchased from Sigma-Aldrich. 2.2. Near-Critical Water Treatment. The experiments were performed in a 1 L nonstirred high pressured batch reactor constructed in 316 stainless steel (series 4650, model B


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Industrial & Engineering Chemistry Research Table 1. Experimental Details

a

exp no.

matrix/fiber

initial comp mass [g]

initial water mass [g]

resin/solvent ratio [g/g]

1 2 3 4 5

epoxy/glass epoxy/carbon epoxy/glass epoxy/glass polyester/glass

200 156 195 195 202

413 480 400 400 419

0.13 0.13 0.13 0.13 0.13

phenol [mL]

tempa [°C]

pressureb [bar]

8

350 350 300 300 350

170 162 100 85 112

Operating temperature. bOperation pressure reached in the stationary phase at the operating temperature.

method, a fused-silica fiber coated with a polymer on the outside is used to extract different analytes from volatile and nonvolatile medias. The fiber applied in this study was coated with 65 μm polydimethylsiloxane−divinylbenzene (PDMS-DVB) and purchased from Supelco (Bellefonte, PA, USA). Prior to the analyses, the fiber was conditioned in the GC injector for 30 min at 250 °C to remove any pollutants. A blank run was performed to check for any carry-over. The fiber was immersed in the headspace (HS) in both the aqueous and the oil phases by inserting the SPME needle through a septum in the lid (polytetrafluoroethylene-coated silicone rubber) and into the headspace. The fiber was then extended through the needle and exposed to the environment. The depth of immersion (reading 4.0 on the holder) was constant throughout all analyses. After extraction, the fiber was withdrawn into the needle, removed from the septum, and inserted directly into the gas chromatography−mass spectrometry (GC−MS) injection port. The extraction process depends on several parameters, such as extraction temperature, extraction time, and agitation as some of the most important parameters (cf. section 2.3.1 and 2.3.2). The samples were analyzed with a PerkinElmer Clarus GC 580 and MS SQ 8 S with EI ionization and quadrupole ion analyzer. A cross-bond column (30 m × 0.25 mm inner diameter (ID) coated with 0.25 mm stationary phase (95% polydimethylsiloxane and 5% diphenyl) was used for the analysis with helium as carrier gas at a constant flow of 1 cm3 min−1. Electron impact mass spectra were recorded at 70 eV ionization energy and the injector was used in split mode (10:1). The SPME needle was desorpted at 200 °C (Tinj) in the GC-injection port. Initial temperature of the oven was 40 °C (Tstart) with a hold time of 2 min. The oven temperature was then increased to 200 °C (hold for 2 min) following a heating ramp of 10 °C min−1. A solvent delay time of 2 min was used, to avoid overloading the mass spectrometer with organic solvents. Identification of the decomposition products was based on comparison of the mass spectra with NIST library data. 2.3.1. SPME on the Aqueous Phase. An automatic pipet was used to collect 5 mL of the aqueous phase from the in situ samples and loaded into a 22 mL glass vial containing a magnetic stirrer (stirring at 360 rpm); 50 μL of bromobenzene (BB, concn, 1.5 g/L methanol) was added as internal standard, corresponding to 0.0298 wt % BB in the 5 mL aqueous sample. The glass vial was placed in a thermostatic water bath and adjusted to 50 °C with an extraction time of 20 min. 2.3.2. SPME on the Oil Phase. Preparation of the oil phase was conducted by dissolving approximately 50 mg of the oil in 1−2 mL of acetone. This was necessary, as the oil was sticky and therefore stuck to the glassware and spatula during sampling. Acetone was used as solvent, due to the high

4653 high pressure reactor, Parr Instruments, USA). The reactor has an inside diameter of 6.35 cm and a depth of 33.35 cm and was heated by an electrical ceramic heater (model 4926, Parr Instruments, USA). The temperature inside the reactor was measured by a T316 thermocouple placed in a 34.29 cm deep thermowell from the top of the reactor and connected to an electrical temperature controller (model 4838, Parr Instrument USA). The pressure was measured manually by a manometer and digitally by a pressure transducer connected to the reactor controller. The sampling and gas inlet valves are shown in the PI diagram in Figure 1. Sampling from the bottom of the reactor was performed through valve G2 or G3 and from the headspace through valve G4. The composite, solvent, and additive (phenol) were loaded into the reactor prior to the experiments, see Table 1 for details. The reactor was subsequently closed with a split-ring closure with 8 cap screws and purged with nitrogen through valve G1 to remove oxygen. A ceramic heater with a temperature increase of 10 °C/min was used to heat the reactor. The experiments were conducted using the following temperature ramp: Heating from ambient conditions to 250 °C with a hold time of 20 min. After the hold time, in situ sampling through valve G2 was conducted. The reactor was then heated to the temperature set point of 300 °C (exp 3 and exp 4 with/without phenol, respectively) or 350 °C (exp 1, 2, and 5) without hold time, but with in situ sampling at 275, 300, 325, and 350 °C, in order to follow the polymer degradation and the produced degradation products. Sampling was also conducted at 350 °C after a reaction time of 15 min (exp 1, 2, and 5) and after 15, 30, and 45 min reaction time at 300 °C (exp 3 and exp 4). Phenol was added in exp 3, in order to observe the effect on the resin degradation efficiency and on the nature of the produced degradation products. Sampling was conducted in autoclave glass bottles, which were closed with gas-impermeable lids immediately after sampling. After the experiments, the reactor was cooled by natural convection inside the ceramic heater for approximately 15−20 h. The recovered fibers were then removed manually from the reactor and washed several times with acetone to remove residue resin. To estimate the percentage of degraded resin from the surface of the fibers, eq 1 was applied. degraded resin (%) ⎛ weight of composite − weight of solid residue ⎞ =⎜ ⎟100 weight of resin in composite ⎝ ⎠ (1)

2.3. Aqueous and Organic Phase Analyses by Solid Phase Microextraction. Solid-phase microextraction (SPME), a solventless sample preparation method, was used to analyze both the aqueous and the organic oil phase. In this C


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5, as the monomer phthalic acid precipitated in this experiment (cf. section 3.5.2). Approximately 5−10 mg of precipitate was weighted in an aluminum crucible and sealed. The precipitate was heated to 250 °C using a dynamic heating procedure (10 °C/min) in a nitrogen atmosphere with a flow of 50 mL/min. Duplicate determination was performed on the precipitate.

solubility of the produced oil in acetone. The solvent was then evaporated by natural convection and 5 mL of distilled water was added to the oil as a solvent. Organic solvents (including acetone) cannot be used as solvent for this purpose, because they will be adsorbed by the SPME fiber, making it impossible to characterize other reaction products. A similar procedure as for the aqueous phase was applied on the oil phase, except that the extraction temperature was 70 °C instead of 50 °C. At ambient conditions, the oil was insoluble in water. Throughout the extraction time, it was observed that some of the oil gradually dissolved in the water due to the higher temperature, facilitating identification of degradation products from the oil. 2.4. Precipitate Analysis. 2.4.1. Fourier Transform Infrared-Attenuated Total Reflectance (FT-IR-ATR) Spectroscopy. The FT-IR-ATR analysis was performed on the solid precipitate from all the samples using a Thermo Fisher Nicolet iS5 FT-IR spectrometer with an ID7 ATR. The software OMNIC version 9 was used for spectra acquisition. The spectra were recorded in absorbance mode with 32 scans at a resolution of 4 cm−1 in the range of 600−4000 cm−1. The FT-IR-ATR results presented in this paper are based on the average of the values measured from three recordings. The precipitate was sampled directly from the bottom of the glass bottles using a tweezer and dried at ambient conditions prior to the FT-IR-ATR analysis. 2.4.2. Gas Chromatography−Mass Spectrometry (GC− MS) Analysis. The solid precipitate from exp 5 was analyzed using a PerkinElmer Clarus model 500 gas chromatograph coupled with a PerkinElmer Clarus model 500 quadrupole mass spectrometer. The gas chromatograph was equipped with an analytical Elite-5 cross-bond column (30 m × 0.25 mm ID with 0.10 μm film thickness) coated with 0.25 mm stationary phase (95% polydimethylsiloxane and 5% diphenyl). The following method was used to analyze the precipitate: Initial column temperature of the GC was 75 °C (hold for 1.5 min), followed by increasing the temperature linearly to 275 °C at a rate of 20 °C/min. The temperature was held at 275 °C for 10.5 min. The flow rate of the carrier gas (helium) was maintained at 1.0 mL/ min. All MS analyses were conducted in scan mode (mass range of 75−600 amu) with electron impact ionization (EI) of 70 eV. A PerkinElmer Clarus model 500 autosampler was used for sample injection with an injection volume of 1.0 μL Vinj. The precipitates were chemically derivatized prior to analysis in order to increase the volatility of the compounds. Silylation was used as the derivatization procedure with trimethylsilyl trifluoromethanesulfonate (TMSOTf) as silylation reagent. Since silylation reactions require anhydrous reaction conditions, approximately 50 mg of precipitate was loaded into test tubes and completely dried with nitrogen for 5 min. Two drops of silylation agent was added, and the tubes were closed with plastic lids and tape. The test tubes were left in a heating block at 75° for 1 h. Subsequently, the residue silylation reagent was removed by evaporation with nitrogen for 2 min and the remaining solid residue dissolved in 1.5 mL of isooctane. Because of small undissolved particles, the solution was filtered through 0.45 μm filters before the GC−MS analyses. Identification of the decomposition products was based on comparison of the mass spectra with NIST library data. 2.4.3. Differential Scanning Calorimetry (DSC) Analysis. The differential scanning calorimetry (DSC 822 Mettler Toledo) analysis was performed on the precipitate from exp

3. RESULTS AND DISCUSSION 3.1. Resin Elimination Efficiency. The resin elimination efficiency was calculated using eq 1 and found to be ≥90% in the experiments (exp 1−5), meaning that the fibers were recovered by means of the hydrolysis process (Figure 2). There

Figure 2. Resin elimination efficiency, exp 1−5. The number above each bar indicates the percentage degraded resin.

was no difference between the experiments with (exp 3) or without phenol (exp 4) at 300 °C in terms of resin degradation efficiency, showing that a process temperature of 300 °C is enough to degrade the resin and recover the fibers using the batch reactor system independent of the presence of phenol (Figure 1). However, although no difference was obtained in resin elimination efficiency in the experiments with/without phenol, differences in chemical contents of products were observed (cf. section 3.2). 3.2. Investigation of Liquid Samples. The samples collected at 250−350 °C from exp 5 (polyester) appeared opaque directly after sampling, cf. Figure 3 right. However, after some rest at ambient conditions, an insoluble precipitate was observed at the bottom of the glass bottles and the aqueous phase became transparent with a yellow color. Similar observations were made by Oliveux et al.16 when only the aqueous phase was investigated with GC−MS. In the present work, both the precipitate obtained during degradation of the epoxy resin and UP resin was analyzed (cf. section 3.5). The samples collected during degradation of the epoxy resin (exp 1−4) appeared as three phases at temperatures greater than or equal to 300 °C; a dark/brown precipitate, an organic sticky oil phase at the bottom and the walls of the glass bottles, and an aqueous phase (cf. Figure 3 left). The temperature at which oil was produced in exp 1 and exp 2 with the different fibers were similar and included 325, 350, and 350 °C after 15 min reaction time. The experiments with an operation temperature of 300 °C with/without phenol (exp 3 and exp 4, respectively) were slightly different from each other in terms D


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Figure 3. Appearance of liquid product after sampling. To the left: Liquid product from epoxy-based composite degradation in the presence of phenol from 250 to 300 °C (exp 3). To the right: Liquid product from polyester-based composite degradation from 250 to 350 °C (exp 5).

1,3,4-trimethyl-cyclohexene-1-carboxaldehyde is considered as a reaction product from acetone aldol reactions, as reported in another study.14 The wt % of p-cumenol (cf. Table S1) was higher than that of BB at 250 °C but decreased with increasing temperature up to 325 °C. At 350 °C, 0 min reaction time and 15 min reaction time, the amount increased considerably again. A similar tendency was obtained for the compounds 2-ethylphenol, 4ethylphenol, and p-thymol, which also are degradation products from the bisphenol A. This tendency could be explained by following: (1) The resin is decomposed layer by layer until reaching the fibers, as reported by Hernanz et al.31 On the basis of the results from this work, the first layers might be degraded when reaching 325 °C and the remaining layers when reaching 350 °C, 0 and 15 min reaction time. (2) The resin might be almost hydrolyzed and removed from the surface of the fibers when reaching 325 °C. However, the majority of the hydrolyzed resin could be high molecular weight oligomers, which start to decompose into smaller degradation products at 350 °C, 0 and 15 min reaction time. The presence of high molecular weight oligomers was reported by Oliveux et al.9 and Tagaya et al.32 A combination of both considerations could also explain the results. The total wt % of the reaction products shown in Table S1 and of the entire reaction products identified in the chromatogram including those not indicated is also calculated. Both increased with increasing temperature due to the production of more water-soluble degradation products. The shown degradation products in Table S1 explained approximately 50% of all the identified water-soluble degradation products from the degraded epoxy composite. 3.3.2. Aqueous Phase from Polyester. The major reaction products in the aqueous phase from the polyester experiment were ketones, phenolics, and aromatics (Table S2). The ketones with highest wt % included compounds such as 2methyl, 2-pentenal, 2,3,4,5-tetramethyl-2-cyclopentenone, and 4,7-dimethyl-1-indanone, and the major aromatics included styrene, 2-methylnaphthalene, 2-methyl-3-phenylacrolein, and 1,3-diphenylpropane, which are degradation products from the styrene chain.14 Phenol and p-cumenol was also identified, but with decreasing wt % when temperature increased. The total wt % of all the identified reaction products in the aqueous phase from polyester were higher compared to epoxy in the temperature range 200−325 °C, indicating that more watersoluble compounds are produced at lower reaction temperatures by the degradation of UP resin. The presence of the monomer styrene decreased with increasing temperature. Since recovery of the monomers is considered valuable in a material-to-material recycling perspective, the estimated amount of styrene was calculated in mg/L and shown graphically in Figure 4 (styrene in black). As it can be observed, the highest amount of styrene is present

of oil production. In the experiment without phenol (exp 4), the oil phase was observed in the samples after 15, 30, and 45 min reaction time, whereas in the presence of phenol, the oil phase was already observed at 0 min reaction time (at 300 °C). This indicates that the presence of phenol accelerates the production of oil. The fact that the resin degradation efficiencies were similar (Figure 2) at 300 °C with and without phenol, can possibly be explained by the long cooling time 15− 20 h (cf. section 2.2 for details about the process). The efficiency of using phenol as catalyzing additive for the degradation of epoxy resin is also supported by the investigations conducted by Liu et al.25 The samples collected after a reaction time of 15, 30, and 45 min resulted in bigger quantities of oil, showing that the reaction time is a key factor in terms of resin degradation and oil production, (Figure 3 left). A similar oil phase was not observed in the samples collected during degradation of polyester (Figure 3 right). The images were taken right after sampling. 3.3. Semiquantitative SPME-GC−MS Investigations of the Aqueous Phase. SPME-GC−MS investigations of the aqueous phases at 250−350 °C revealed the presence of more than 150 reaction products identified with the NIST library database. The abundance of the compounds at each temperature stage was estimated by eq 2, which follows the single point standardization method.26 A similar semiquantitative procedure is applied in the field of biomass conversion, where the liquid products are composed of a complex mixture.26−28

W=

A × WIS AIS

(2)

where A is the chromatographic peak area of the generic analyte, AIS is the chromatographic peak area of the internal standard, and WIS is the weight percent of internal standard 3.3.1. Aqueous Phase from Epoxy. The majority of the reaction products present in the aqueous phases, attained from the hydrolysis of epoxy resin (exp 1−4) were similar. In general, the abundance of the majority of the reaction products increased with increasing temperature and reaction time both in the absence and presence of phenol. Compared to all present compounds, the highest weight percents were found of phenol, 2-N-methylaminobornane, N,N-dimethyl 2-bornanamine, and p-cumenol in all four experiments. A systematic evaluation of reaction products attained from exp 1 is presented in Table S1 together with the wt % of the identified compounds at every temperature stage from 250 to 350 °C. The major constituents of the aqueous phase were phenolics, particularly phenol and p-cumenol, which are considered degradation products from the bisphenol A, as also reported in the literature elsewhere,25,29,30 amines, which are considered as degradation products from the amine curing agents in the epoxy resin, and finally ketones. The compound E


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S3). The peak area representing each classification was then calculated and divided by the total chromatographic peak area to find the composition in mass fraction (%). The composition of the oils from exp 1 and exp 2 was very similar, as both experiments were performed under similar process conditions. The proportion of acetone-derived compounds decreased with increasing temperature and longer residence time, possibly due to depleting amounts of acetone. Production of these compounds, such as dihydroisophorone and 2,6,6-trimethyl-2-cyclohexene-1-carboxaldehyde (cf. Table S3) is caused by acetone aldol reactions, as shown in a previous investigation.14 Phenolics accounted for the majority of the identified compounds and are considered as degradation products from the epoxy resin. The composition of the oils produced at a process temperature of 300 °C with/without phenol (exp 3 and exp 4), was also similar in oil composition compared to exp 1 and exp 2. However, the alcohol 1-dodecanol was identified with relatively high mass fraction (%) in the presence of phenol, particularly at 300 °C and 15 min reaction time. The phenol standard was analyzed for impurities, without any trace of 1dodecanol, and a standard of 1-dodecanol (RT = 15.28 min) was purchased, confirming the presence. This indicates that the aliphatic alcohol could be produced from either the resin or phenol. 3.5. FT-IR-ATR Analysis of Precipitate. 3.5.1. Precipitate from Epoxy Composite. FT-IR-ATR spectra of the precipitate observed in the samples collected from exp 1 at 250 to 350 °C are shown in Figure 6. Comparison of the spectra shows that the precipitates from 250 to 300 °C are similar, but with a transition into other degradation products from 325 to 350 °C. The spectra of the precipitates from 250 to 300 °C indicate the presence of aromatic groups. The peaks at 1609 and 1509 cm−1 are assigned to the stretching CC vibrations of the aromatic ring in the epoxy resin, and the absorption peaks at 1282, 1246, and 1179 cm−1 are assigned to the C−O−C stretching vibrations from the ester bands in the epoxy resin structure.35−37 The peaks at 728 and 830 cm−1 are characteristic for a 1,4-substituted benzene ring, indicating the presence of bisphenol A/F in the precipitate. However, the peaks at 2700−2500 and 1677 cm−1 do not originate from the epoxy resin. On the basis of the FT-IR-ATR spectra of phthalic acid,

Figure 4. Semiquantitative assessment of styrene and the degradation products in exp 5.

at 250 °C. Higher temperatures and longer residence time seem to accelerate the degradation of styrene. According to Jiang et al.,33 styrene degrades into benzaldehyde and acetophenone, both identified in the aqueous phase, cf. Table S2. The amount of these degradation products were also calculated and plotted in Figure 4 (benzaldehyde in dark gray, acetophenone in light gray). The amount of benzaldehyde was three times higher than acetophenone, indicating that styrene is mainly decomposed into benzaldehyde at the process conditions used in this experiment. Another consideration about the recovery of styrene is the relative volatility value of 1.4.34 Therefore, despite closing the glass bottles immediately after sampling, large amounts of styrene might have been lost in the meantime. To optimize the process in terms of recovering styrene, a closed sampling system with a cooling device could be integrated. 3.4. SPME-GC−MS Investigations of the Oil Phase. The composition of the oil phases were evaluated by dividing the identified compounds into five classifications which included (1) acetone-derived compounds (ACD), (2) phenolics, (3) aromatics, (4) alcohols, and (5) other (cf. Figure 5 and Table

Figure 5. Composition of the oils attained from exp 1, 2, and 3 at 300−350 °C. Acetone derived compounds are abbreviated ADC, and the retention times in exp 3 is indicated above the columns. F


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Figure 6. FT-IR-ATR spectra of the precipitate from exp 1 at 250−350 °C (left) and spectra of standards bisphenol A, P, and F (right).

cleavage at position 1 (Scheme 1), thus producing bisphenol A. The precipitate might thus contain both larger fragments of DGEBA epoxy resin and bisphenol A. The broad band at 1040 cm−1, which were not present in the precipitate at 250−300 °C, corresponds to the C−O stretching vibrations of alcohols. The characteristic CC and C−H (1605, 1511, and 831 cm−1) bands from the benzene rings in epoxy resin are also found in the precipitate at temperatures greater than or equal to 325 °C. 3.5.2. Precipitate from Polyester. As described previously, a predominant amount of homogeneous precipitate was already observed after rest for several hours. The samples were kept in the dark at ambient conditions for an additional 2 months without any sample preparation or separation of the aqueous phase and the precipitate, leading to an interesting observation. At temperatures greater than or equal to 300 °C, the precipitate was no longer homogeneous, but appeared as a white and brown precipitate (see Figure 7 right). The white precipitate formed at 300−325 °C was a solid material and therefore easily separated from the brown precipitate, whereas the white precipitate at 350 °C was a powder and hence more difficult to separate from the brown precipitate. The precipitate observed

(Figure 8), these peaks could arrive from O−H and CO stretching vibrations from aromatic carboxylic acids (phthalic acid). These observations could indicate the presence of phthalic acid in the epoxy resin residue. The increase in temperature to above 325 °C might have resulted in degradation of the phthalic acid, due to the disappearance of the peaks at 1677 cm−1 and 2700−2500 cm−1. Instead, a broad peak around 3283 cm−1 and a peak around 2930−2870 cm−1 were observed. The peaks at 2930−2870 cm−1 correspond to CH2−CH3 stretching vibrations of bisphenol A diglycidyl ether (DGEBA) epoxy resin,38 and the peak at 3283 cm−1 corresponds to the characteristic Ph−OH stretching vibration from bisphenol A (Scheme 1), found by running a spectra of bisphenol A/P/F (Figure 6, right). From literature, it has been found that OH stretching vibrations from DGEBA epoxy resin is found at 3500 cm−1 (Scheme 1).37−39 These observations indicate that some of the ether bonds between bisphenol A and diglycidyl ether were broken through Scheme 1. Bisphenol A Diglycidyl Ether Curing, Bond Breakage, and Production of Bisphenol A

Figure 7. Precipitate from exp 5 sampled at 275 and 300 °C after rest for 2 months. G


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Figure 8. Comparison of FT-IR-ATR spectra of the white precipitate separated from the samples at 300−350 °C (black spectra). The red spectrum represents phthalic acid as a reference.

Figure 9. FT-IR-ATR spectra of brown precipitate obtained from the samples at 250−350 °C (black spectra). Spectra for benzoic acid (blue spectrum) and phthalic acid (red spectrum) are included as references.

at 250−275 °C was homogeneous and appeared as a brown granulate, as shown in Figure 7 left. The FT-IR-ATR spectra of the white precipitates are presented in Figure 8. A standard of phthalic acid (red spectra) was included for comparison with the precipitate. Absorption peaks at 1674 and 1584 cm−1 correspond to the characteristic carboxylic acid group (CO) of phthalic acid, and the CC bands correspond to the benzene ring. The peaks at 1400 cm−1 (O−H) and 1262 cm−1 (C−O) can also be assigned to the carboxylic acid group of phthalic acid. The remaining peaks are assigned to the stretching and bending vibrations of C−H bands in the benzene ring. It is evident that the characteristic peaks of phthalic acid are observed in the spectra from the precipitate at all temperatures. Particularly the solid (white) precipitate at 300−325 °C indicates nearly pure phthalic acid,

based on comparisons between the spectra (cf. section 3.6 for further analyses of the precipitate). However, the spectra of the white precipitate obtained at 350 °C, both 0 min reaction time and 15 min reaction time, were slightly changed in the entire spectra, particularly in the region at 600−800 cm−1. This indicates degradation, impurities, or changes in the structure of the phthalic acid. The residue brown precipitate after separation of the white precipitate was also analyzed with FT-IR-ATR. The results are shown in Figure 9 with comparison to spectra of phthalic acid (red spectrum) and benzoic acid (blue spectrum). Comparison of the precipitate with phthalic acid shows similarities for the samples at 250−325 °C. However, the spectrum of the precipitate at 350 °C shows closer similarities with benzoic acid, especially in the marked regions. These observations indicate H


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Industrial & Engineering Chemistry Research Table 2. Compounds in the White Precipitate Identified by GC−MS

Table 3. Compounds in the Brown Precipitate Identified by GC−MS peak area [%] RT

compound

T/°C = 250

T/°C = 275

T/°C = 300

T/°C = 325

T/°C = 350

T/°C = 350, 15 min

14.0 14.9 9.4 8.3

phthalic acid terephthalic acid phthalic anhydride benzoic acid explained

91.84 7.18 0.98 0 100

98.24 0.65 1.11 0 100

96.94 0.50 0.8 1.76 100

86.02 6.84 0.8 6.34 100

36.68 55.14 0 8.18 100

5.72 20.76 0 73.52 100

Figure 10. DSC curves of recovered phthalic acid (black and blue) and reference of phthalic acid (red) recorded in a dynamic nitrogen atmosphere.

= 14.0 min) and terephthalic acid (RT = 14.9 min) were silylated along with the precipitates, in order to confirm the presence of the compounds. As interpreted from the FT-IR-ATR spectra (Figure 8), the white precipitate is primarily composed of phthalic acid, with the highest purity at 300−325 °C. The area of phthalic acid decreases with increasing temperature simultaneously with the increase in the area of benzoic acid, explaining the observed changes in the FT-IR-ATR spectra in Figure 8. This confirms that phthalic acid is degraded into benzoic acid, as is also reported by other authors.40,41 The amounts of terephthalic acid and phthalic anhydride (produced through dehydration of

that phthalic acid begins to degrade into benzoic acid at temperatures greater than or equal to 325 °C (cf. section 3.6). On the basis of the comparisons between the spectra, the brown precipitate is also characterized as primarily phthalic acid for the samples collected at 250−325 °C. 3.6. GC−MS Analysis of Phthalic Acid. 3.6.1. GC−MS Silylation Analysis. Silylated compounds found in the white and brown precipitate are presented in Table 2 and Table 3 with the peak area, at temperatures from 250 to 350 °C. The total chromatographic peak area is explained by the four compounds phthalic acid, terephthalic acid, phthalic anhydride, and benzoic acid in both the white precipitate (Table 2) and the brown precipitate (Table 3). Standards of phthalic acid (RT I


Article


4. CONCLUSION Fiber reinforced epoxy and polyester-based composites were depolymerized at 300−350 °C in a 1 L batch reactor. More than 90% of the resin was decomposed, recovering the glass and carbon fibers for further use. SPME-GC−MS analysis of the aqueous phase from the epoxy degraded composites revealed the presence of compounds such as phenol, 2-N-methylaminobornane, pcumenol, benzofuran, anisole, and primarily phenolic compounds in increasing or decreasing wt % depending on the sampling temperature. In general, the highest wt % of the identified degradation products were found at 350 °C 15 min reaction time, with phenol estimated to 0.088 wt % and pcumenol 0.0879 wt %. The majority of the oil was composed of phenolics, aromatics, and acetone derived compounds. The phenolics and aromatics were considered as degradation products from the resin, and the acetone derived compounds were derived from acetone undergoing aldol reactions. The SPME-GC−MS analyses of the aqueous phase from the polyester-degraded composites showed that the monomer styrene was produced in largest amounts at temperatures less than or equal to 250 °C and decreased in quantity with increasing temperature, with degradation into benzaldehyde and acetophenone. Because of the high volatility of styrene, the recovery of the monomer requires a closed sampling system with integrated cooling and sampling at temperatures less than or equal to 250 °C. Recovery of the monomer phthalic acid from polyester was considerably easier. By storing the collected samples in the dark at ambient conditions for two months, nearly pure phthalic acid was precipitated at the bottom of the glass bottles. From FTIR-ATR and GC−MS silylation techniques, it was found that temperatures at 275−325 °C recovered the most pure phthalic acid. With the recovery of an oil product, fibers, monomers and greater knowledge about the aqueous phase, the polymer composite end-of-life cycle is a step closer to be completed.

phthalic acid) are relatively low and completely degraded at 350 °C after 15 min reaction time. The brown precipitate was also primarily composed of phthalic acid at 250−325 °C. However, at temperatures greater than or equal to 350 °C, the main constituents in the precipitate were terephthalic acid at 0 min reaction time and benzoic acid at 15 min reaction time. These observations were also interpreted from the FT-IR-ATR spectra (Figure 9). 3.6.2. DSC Analysis. The DSC analysis of the recovered phthalic acid was conducted in order to attain greater knowledge about the purity compared to a reference of phthalic acid (Figure 10). The red thermogram represents the phthalic acid standard, the blue thermogram represents the phthalic acid precipitated in the sample collected at 275 °C, and the black thermogram represents the white fraction of the precipitate obtained in the sample at 300 °C (Figure 7). The exothermic peak around 200−215 °C represents the melting of phthalic acid. The recovered phthalic acid at both temperatures had melting points close to that of the reference, indicating high purity phthalic acid. The white precipitate at 300 °C seemed to have slightly higher purity than the precipitate obtained at 275 °C, indicating that the storage of the samples has provoked recrystallization of phthalic acid. The observations made in this research facilitate new and cost-effective possibilities to separate and purify monomers from polymer composites. Instead of separating the reaction products immediately after the hydrolysis process, storage for several weeks to months can be considered more beneficial since some expensive unit operations can be avoided, such as crystallization, filtration, drying, etc. A comparison of this study with a previous study also concerning purification of monomers from polymers by Piñero et al.,42 show that the separation and purification procedure in the present paper are considerably easier, less expensive, and more environmentally friendly than the procedures used by Piñero et al.42 They depolymerized polycarbonate using methanol or a mix of methanol/water under heat and pressure, to recover the monomer bisphenol A. In the present study, near-critical water without any cosolvents was enough to separate the monomer phthalic acid from the UP resin. Any extra purification steps after just leaving the product for precipitation in the dark were not necessary to obtain nearly pure fractions of phthalic acid. The phthalic acid fractions could hence be removed by a tweezer. Recovery of pure bisphenol A in the work of Pinero et al. involved both crystallization in water and filtration.42 With the recovery of the monomer phthalic acid, the last steps in terms of closing the composite end-of-life cycle can be considered accomplished. Fibers can be recovered, valuable monomers can be recovered with high purity, an oil product with potential as fuel can be recovered (based on a previous study), and the composition of the aqueous phase is now clearer. On the basis of the reaction products identified, processing the aqueous phase is considered necessary before being discharged. Alternatively, instead of considering the aqueous phase as an undesired byproduct, it can be utilized again in another composite degradation process. This will not only make the process more sustainable, but could also bring down the process temperature, due to the catalyzing effects of phenol and other phenolic compounds, as obtained in this work.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b02111. GC−MS and SPME analyses; information about the reaction products (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +45 9940 7622. Fax: +45 9940 7710. E-mail: ucar@bio. aau.dk. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the Danish Agency for Science, Technology and Innovation (Grant No. 11-118412) under the Ministry of Higher Education and Science for support of the GenVind Innovation Consortium.

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REFERENCES

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Characterization of the Liquid Products from Hydrolyzed Epoxy and

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