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In a typical run in the presence of O2, oxygen-saturated Milli-Q water (30 mL) and 90 mg of the ..... First, a hydrogen atom is abstracted from a −C...
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Efficient-Oxygen Induced Mineralization of Melt-Processable Fluoropolymers in Subcritical and Supercritical Water Hisao Hori,†,* Takehiko Sakamoto,† Kenta Ohmura,† Haruka Yoshikawa,† Tomohisa Seita,† Tomoyuki Fujita,‡ and Yoshitomi Morizawa‡ †

Department of Chemistry, Faculty of Science, Kanagawa University, 2946 Tsuchiya, Hiratsuka, Kanagawa 259-1293, Japan Research Center, Asahi Glass Co., 1150 Hazawa-cho, Kanagawa-ku, Yokohama, Kanagawa 221-8755, Japan

‡

ABSTRACT: The decomposition of poly(vinylidene fluoride) (PVDF) and ethylene−tetrafluoroethylene copolymer (ETFE) in subcritical and supercritical water was investigated. Heating PVDF in supercritical water at 380 °C for 6 h with an approximately 5.8-fold molar excess of O2 relative to the fluorine or carbon content in the polymer afforded F− and CO2 in 96.9% and 99.3% yields, respectively. ETFE was also efficiently mineralized to F− (97.6%) and CO2 (98.2%) with an 11-fold molar excess of O2 relative to the fluorine or carbon content of the polymer under the same reaction conditions. The PVDF and ETFE reactivities differed markedly under argon: PVDF formed mainly F−, CO2 formation was suppressed, and a carbon-rich residue formed; in contrast, ETFE was unreactive. This difference suggests that PVDF decomposed via dehydrofluorination in the absence of O2, whereas ETFE did not. Adding stoichiometric Ca(OH)2 to the reaction in the presence of O2 afforded X-ray spectrometrically pure CaF2.

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col for treatment of F− ions could be used, whereby Ca2+ is added to the system to form CaF2, which is a raw material for hydrofluoric acid. Thus, the development of such a method would allow the recycling of fluorine, the global demand for which is increasing. Several studies have focused on the degradation of PVDF and ETFE.6−13 However, most of the previous studies examined the thermal stability or aging characteristics of the polymers; none focused on their decomposition to obtain F− ions for waste treatment. Only one report, a patent13 describing monomer production for numerous polymers in supercritical water, noted PVDF as an applicable species, but no specific results were shown. Reactions in subcritical or supercritical water are recognized as an innovative and environmentally benign waste-treatment technique, owing to the high diffusivity and low viscosity of these media, as well as their ability to hydrolyze many organic compounds.14 Subcritical water is defined as hot water with sufficient pressure to maintain the liquid state, and supercritical water is defined as water at temperatures and pressures higher than the critical point, 374 °C and 22.1 MPa. Recently, supercritical water was used for pilot- and practical-plant-scale decomposition of trinitrotoluene15 and polychlorinated biphenyls.16 Depolymerization of common polymers such as poly(ethylene telephtalate) in subcritical water was also investigated.17−19 We previously demonstrated that a perfluoroalkyl sulfonic acid membrane polymer for fuel cells can be efficiently decomposed in subcritical water in the presence of metals.20 Herein, we report on the decomposition of PVDF and ETFE in subcritical and supercritical water, and we present an effective

INTRODUCTION Owing to their high chemical and thermal stability, fluoropolymers (olefinic polymers in which some or all of the hydrogen atoms are replaced by fluorine atoms) are used in industrial equipment to impact corrosion resistance. Poly(tetrafluoroethylene) (PTFE, −(CF2CF2)n−) is the most frequently used fluoropolymer.1 However, PTFE cannot be processed by melt molding, a conventional technique for fabricating thermoplastic polymers, because the viscosity of the PTFE melt (109−1011 Pa s) is about 6 orders of magnitude higher than that of common thermoplastic polymers.1 To overcome this weakness, melt-processable fluoropolymers, such as poly(vinylidene fluoride) (PVDF, −(CF2CH2)n−) and ethylene−tetrafluoroethylene copolymer (ETFE, −(CH2CH2)m(CF2CF2)n−), which can be fabricated by melt processes including extrusion, injection, compression, and blow molding, have been developed and introduced in industry.1−5 Melt-processable fluoropolymers show high resistance to temperature, chemicals, ignition, mechanical stresses, UV irradiation, and weather and have been used for various applications, including piping, tubing, valves, sinks, cables, films, and lithium ion battery electrode binders. It is estimated that the proportion of PTFE to the total global fluoropolymer demand will decrease gradually to 52% in 2016 (from 61% in 1996), during which time total fluoropolymer demand is expected to increase from 115 000 to 305 000 tons.1 Wider use of melt-processable polymers will require the establishment of waste treatment. Some of these polymers are recycled, and they can also be incinerated. However, incineration requires high temperatures to break the strong C−F bonds, and the hydrogen fluoride gas that forms can damage the firebrick of an incinerator. Thus, in many cases, the wastes of these polymers are disposed of in landfills. If the polymers could be decomposed to F− ions (that is, mineralized) by means of environmentally benign techniques, the well-established proto© 2014 American Chemical Society

Received: Revised: Accepted: Published: 6934

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method for complete mineralization of the fluorine and carbon in these polymers to F− and CO2, respectively. Furthermore, we report that CaF2 forms upon addition of a stoichiometric amount of Ca(OH)2 to the reaction system.

(2.7 mM), NaHCO3 (0.3 mM), and H2O2 (26.5 mM) and was then transferred to the ion-chromatograph unit. The F− concentrations were measured with an ionchromatography system (IC-2001, Tosoh, Tokyo, Japan) consisting of an automatic sample injector (30 μL injection volume), a degasser, a pump, a guard column (TSKguard column Super IC-A, 4.6 mm i.d., 1.0 cm length, Tosoh), a separation column (TSKgel Super IC-Anion, 4.6 mm i.d., 15 cm length, Tosoh), a column oven (40 °C), and a conductivity detector with a suppressor device. The mobile phase was an aqueous solution containing Na2B4O7 (6 mM), H3BO3 (15 mM), and NaHCO3 (0.2 mM); the flow rate was 0.8 mL min−1. An ion-chromatography system (IC-2001) with a separation column (TSKgel Super IC-AP, 4.6 mm id, 7.5 cm length, Tosoh) was also used to quantify the organic acids (HOOCCH2COOH and HOOCCOOH). The mobile phase was an aqueous solution containing NaHCO3 (1.7 mM), Na2CO3 (1.8 mM), and acetonitrile (23 vol %). ATR-IR spectra were recorded with a FTIR spectrophotometer (Spectrum 100, PerkinElmer, Waltham, MA) with a diamond ATR cell. The reaction solution was dropped into the ATR cell, concentrated to dryness with a gentle N2 stream, and then subjected to measurement. A GC system (GC 323, GL Sciences) consisting of an injector (150 °C), a column oven (110 °C), and a thermal conductivity detector (130 °C) was used to quantify CO2. The column was an active carbon column (60/80 mesh, 2.17 mm i.d., 2 m length), and the carrier gas was argon. The products in the gas phase were also analyzed with a GC/MS (QP2010 SE, Shimadzu) system with a fused-silica capillary column (Rt-QBOND, Restek, Bellefonte, PA). The carrier gas was helium, and the injection temperature was held constant at 120 °C. The sample gas was introduced into the GC/MS system in split mode (ratio, 20/1) and analyses were conducted in full-scan mode (m/z 2.0−200). The oven temperature was held constant at 30 °C for 30 min or kept at 30 °C for 5 min, raised to 200 °C at a rate of 20 °C min−1, and held at that temperature for 20 min. XRD patterns were measured with Cu Kα radiation (Multiflex, Rigaku, Tokyo, Japan).

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EXPERIMENTAL SECTION Materials. Powered PVDF was purchased from SynQuest Laboratories (Alachua, FL). Size-exclusion chromatography indicated that the weight-average molecular weight of this polymer was 6.47 × 105 with a polydispersity of 2.52. ETFE powder with an estimated weight-average molecular weight of 105−106 and a 50/50 ethylene/tetrafluoroethylene ratio (i.e., the m/n ratio in −(CH2CH2)m(CF2CF2)n−) was obtained from Asahi Glass (Tokyo, Japan).21 Combustion ion chromatography22 revealed that the fluorine contents in PVDF and ETFE were 60.7 and 60.6 wt %, respectively; these values are slightly higher than the corresponding ideal values (both 59.3 wt %). These analytical values were used to calculate the F− yields of the reactions. Argon (99.99%), O2 (99.999%), and CO2 (0.995%)/N2 gases were purchased from Taiyo Nippon Sanso (Tokyo, Japan). C2H4 (99.5%) and C2H6 (99.5%) were obtained from GL Sciences (Tokyo, Japan). HOOCCH2COOH (>98%), HOOCCOOH (>98%), 1,3,5trifluorobenzene, and other reagents were obtained from Wako Pure Chemical Industries (Osaka, Japan). Reaction Procedures. Reactions were carried out in a stainless steel high-pressure reactor fitted with a gold vessel to prevent contamination from the reactor material. The internal volume of the reactor was 96 mL. In a typical run in the presence of O2, oxygen-saturated Milli-Q water (30 mL) and 90 mg of the polymer (PVDF or ETFE) were introduced into the reactor, which was then pressurized to 0.60 MPa with O2, sealed, and heated to the desired temperature (150−380 °C). During the reactions, the mixture was stirred with a gold-plated impeller. After a specified time passed, the reactor was quickly cooled to room temperature, and the reaction solution was subjected to ion chromatography and attenuated total reflection infrared (ATR-IR) spectrometry. The gas phase was collected with a sampling bag and subjected to gas chromatography (GC) and gas chromatography−mass spectrometry (GC/MS). Control experiments were conducted with argon instead of O2. The reactions involving stoichiometric amount of Ca(OH)2 (the molar amount was half the molar amount of fluorine atoms in the polymer) were also performed: the white precipitate that formed during these reactions was collected by centrifugation, washed with pure water, and subjected to Xray diffractometry (XRD). Analysis. The molecular weight of PVDF was determined by means of size-exclusion chromatography with a refractive index detector (RID-10A, Shimadzu, Kyoto, Japan) and an analytical column (PLgel 10 μm Mixed-B × 2, Agilent Technologies, Santa Clara, CA). The mobile phase was N, Ndimethylformamide containing LiBr (10 mM), and the flow rate was 1.0 mL min−1. Molecular weight and molecular weight distribution are reported relative to polystyrene as a standard. The fluorine content in the polymers was quantified by combustion ion chromatography at Nissan Arc (Yokosuka, Japan) on an instrument consisting of a combustion unit (AQF-100, Dia Instruments, Chigasaki, Japan; matrix combustion temperature, 1100 °C) and an ion chromatograph unit (Dionex ICS-3000, Thermo Fisher Scientific, Waltham, MA). The fluorine content of the sample was converted to HF by combustion and absorbed into a solution containing Na2CO3

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RESULTS AND DISCUSSION Reactions of PVDF. Initially, we carried out the reactions of PVDF in the presence of argon (i.e., in the absence of O2) to investigate the reactivity of the polymer in pure subcritical and supercritical water. The temperature dependences of the amount of F− in the reaction solution and CO2 in the gas phase at a constant reaction time of 6 h are shown in Figure 1a. A solid residue was present after all these reactions. At 250 °C, almost no mineralization of the polymer occurred: the amount of F− in the aqueous phase after 6 h was 0.028 mmol, which corresponds to a F− yield [(moles of F− formed)/(moles of fluorine in the polymer)] of 0.97%. Similarly, the amount of CO2 was 0.014 mmol, which corresponds to a yield [(moles of CO2 formed)/(moles of carbon in the polymer)] of 0.50%. In contrast, F− clearly formed during the reaction at around 300 °C, and the amount of F− increased with increasing reaction temperature. At 380 °C, the temperature at which the water reached the supercritical state, the amount of F− reached 2.56 mmol (88.9%; Table 1, entry 1). Note that in this study, the temperature at which efficient decomposition of PVDF was observed in subcritical water (≥300 °C) was considerably lower than that (∼440 °C) reported for a pyrolysis experiment.6 6935

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reaction at 380 °C for 6 h indicated that most of the solid was carbon (C, 79.6 wt %; F, 4.3 wt %). The reported data for thermolysis of PVDF6−8 suggest two plausible mechanisms for PVDF decomposition: dehydrofluorination (Scheme 1a) and depolymerization (Scheme 1b). The former mechanism results in the formation of carbon-rich residue, whereas depolymerization produces CH2CF2 monomer, as has been reported for the thermolysis of PTFE.23−25 The fact that in our reaction, in the presence of argon, no CH2CF2 was detected (as indicated by GC/MS) and a carbonrich solid and a large amount of F− formed suggests that under these conditions, the decomposition of PVDF proceeded via the dehydrofluorination mechanism. Dehydrofluorination was followed by scission of the polymer chain, as reflected by the formation of 1,3,5-trifluorobenzene. Effect of O2. Mineralization of the fluorine in PVDF to F− was not complete (88.9% yield) in pure supercritical water in the presence of argon. Furthermore, the formation of the solid residue is not ideal for waste treatment, because the solid requires additional treatment. Therefore, to achieve complete mineralization, we performed reactions in the presence of O2. The addition of O2 (initial pressure 0.6 MPa, i.e., 16.5 mmol) to the reaction system dramatically changed the product distribution (Figure 2). Compared to the reactions in the presence of argon, the reaction in the presence of O2 resulted in a greater degree of mineralization to F− and efficient formation of CO2. When the reaction was carried out at 380 °C with O2, the amounts of both F− and CO2 reached 2.79 mmol (96.9% and 99.3% yields, respectively; Table 1, entry 2). In contrast to the reactions in the presence of argon, the reactions in the presence of O2 resulted in no quantifiable amounts of gaseous products other than CO2. The reaction atmosphere also affected the temperature dependence of the formation of organic acids. When the reaction was carried out in the presence of O2, the amount of HOOCCH2COOH increased with increasing temperature (Figure 2b), which indicates that scission of the polymer chain efficiently occurred. Increasing the initial amount of O2 to 30.5 mmol resulted in F− and CO2 yields that were almost the same as those when 16.5 mmol of O2 was used (compare entries 2 and 3 in Table 1). Thus, virtually complete mineralization of the polymer was achieved in the presence of at least 16.5 mmol of O2, which is approximately 5.8 times the molar amount of fluorine or carbon in the polymer. PVDF decomposition at 380 °C in the presence of 16.5 mmol of O2 with shorter reaction time was also investigated (Table 2). The mineralization of the polymer was not complete after 0.5 h, as indicated by the fact that F− and

Figure 1. Temperature dependence of PVDF decomposition in the presence of argon: detected amounts of (a) F− and CO2 and (b) organic acids. PVDF (90 mg; fluorine content, 2.88 mmol; carbon content, 2.81 mmol) and pure water (30 mL) were introduced into the reactor, which was pressurized with argon (0.60 MPa) and heated at 250−380 °C for 6 h.

Although a large amount of F− was detected, the amount of CO2 that formed remained low and was almost constant from 300 to 380 °C (Figure 1a); even when the reaction was carried out at 380 °C, only 0.27 mmol of CO2 formed (9.6%; Table 1, entry 1). This yield was considerably lower than F− yield (88.9%). GC/MS measurement revealed that the gas phase contained trace amounts of C2H6 (0.09 μmol) and 1,3,5trifluorobenzene (0.35 μmol). We used ion chromatography to quantify the organic acids that formed in the reaction solution: some of the solutions contained small amounts (on the order of micromoles) of HOOCCH2COOH and HOOCCOOH. The amounts of the organic acids reached maxima at around 300 °C and then decreased as the temperature was increased further (Figure 1b), suggesting that these species were reaction intermediates. Although organic acids were detected in the reaction solutions, the amounts were 3 orders of magnitude lower than the amount of F−. This result and the low yield of CO2 suggest that most of carbon was in the solid residue. Consistent with this suggestion, elemental analysis of the solid after the

Table 1. Decomposition of PVDF and ETFE in Supercritical Watera entry polymer 1 2 3 4 5 6 7

PVDF PVDF PVDF ETFE ETFE ETFE ETFE

initial O2 (mmol)

reaction pressure (MPa)

F− (mmol) [yield (%)]b

noned 16.5 30.5 none 9.95 16.5 30.2

24.2 23.8 24.9 23.7 23.3 23.3 23.6

2.56 [88.9] 2.79 [96.9] 2.83 [98.3] 0.24 ± 0.04 [8.4 ± 1.4] 1.55 [54.0] 2.49 [86.8] 2.80 [97.6]

CO2 (mmol) [yield (%)]c 0.27 2.79 2.70 0.05 1.79 2.37 2.76

[9.6] [99.3] [96.1] ± 0.01 [1.8 ± 0.3] [63.7] [84.3] [98.2]

HOOCCOOH (μmol)

HOOCCH2COOH (μmol)

n.d.e n.d. n.d. n.d. n.d. 0.12 n.d.

0.44 4.31 0.45 trace 0.21 0.88 0.49

The polymer (90 mg) and pure water (30 mL) were introduced into the reactor, which was pressurized with O2 and then heated at 380 °C for 6 h. F yield (%) = [(moles of F− formed)/(moles of fluorine in polymer)] × 100. cCO2 yield (%) = [(moles of CO2 formed)/(moles of carbon in polymer)] × 100. dReaction in the presence of argon. eNot detected. a

b −

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Scheme 1. Possible Mechanisms for the Decomposition of PVDF in the Presence of Argon, as Suggested by Thermolysis Data

CO2 yields were 89.9% and 89.3%, respectively (entry 1). After 1 h, the F− and CO2 yields reached 98.3% and 102%, respectively (entry 2). These values were similar to those after 6 h (entry 3), indicating that reaction time of 1−6 h was enough for complete mineralization. On the basis of our analysis of the products, we propose the reaction mechanism outlined in Scheme 2 for the decomposition of PVDF in the presence of O2. First, a hydrogen atom is abstracted from a −CH2− group: −CF2CH 2CF2CH 2CF2CH 2CF2− → −CF2CH 2CF2CH·CF2CH 2CF2 −

(1)

The resulting radical reacts with O2 in the presence of water to produce a hydroperoxide: −CF2CH 2CF2CH·CF2CH 2CF2− → −CF2CH 2CF2CH(OOH)CF2CH 2CF2 −

(2)

This product is unstable and can cause scission of the main chain, which results in a terminal −CF2• radical and an aldehyde:

Figure 2. Temperature dependence of PVDF decomposition in the presence of O2: (a) detected amounts of F− and CO2 and (b) organic acids. PVDF (90 mg; fluorine content, 2.88 mmol; carbon content, 2.81 mmol) and pure water (30 mL) were introduced into the reactor, which was pressurized with O2 (0.60 MPa; 16.5 mmol) and heated at 200−380 °C for 6 h.

−CF2CH 2CF2CH(OOH)CF2CH 2CF2− → −CF2CH 2CF2• + HC(O)CF2CH 2CF2 −

(3)

The −CF2• radical can be transformed to an acid fluoride (−COF) in the presence of O2, and the aldehyde can either

Table 2. Time Dependence of PVDF Decomposition in the Presence of O2 in Supercritical Watera entry

reaction time (h)

reaction pressure (MPa)

F− (mmol) [yield (%)]

CO2 (mmol) [yield (%)]

HOOCCOOH (μmol)

HOOCCH2COOH (μmol)

1 2 3

0.5 1 6

22.9 24.3 23.8

2.59 [89.9] 2.83 [98.3] 2.79 [96.9]

2.51 [89.3] 2.87 [102] 2.79 [99.3]

n.d. n.d. n.d.

0.69 1.83 4.31

a

The polymer (90 mg) and pure water (30 mL) were introduced into the reactor, which was pressurized with O2 (16.5 mmol) and then heated at 380 °C. 6937

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Scheme 2. Proposed Mechanism for the Decomposition of PVDF in the Presence of O2

undergo direct cleavage of the C−C bond to form a terminal −CF2• radical or can be oxidized to a carboxylic acid, which is subsequently cleaved as shown in Scheme 2. The acid fluoride is hydrolyzed to the corresponding carboxylic acid. This sequence of steps leads to complete mineralization of PVDF. Reactions of ETFE. Because PVDF was efficiently mineralized in the presence of O2, reactions of ETFE in subcritical and supercritical water were also carried out by means of the same approach (Figure 3). When the reaction of ETFE in the presence of O2 (16.5 mmol) was carried out at 150 °C for 6 h, almost no F− and CO2 formed (Figure 3a), and a solid residue formed. The formation of F− and CO2 was clearly observed at 200 °C, and the ATR-IR spectrum of the reaction mixture showed broad peaks around 1600−1700 cm−1, which can be assigned to carboxyl groups, indicating that the polymer had been oxidized. The solid residue did not form when the reaction temperature exceeded above 250 °C, and the amounts F− and CO2 increased monotonously with increasing temperature. When the reaction was carried out at 380 °C, the amounts of F− and CO2 reached 2.49 and 2.37 mmol (86.8% and 84.3% yields, respectively; Table 1, entry 6). These values are somewhat lower than those observed for PVDF under the same reaction conditions (96.9% and 99.3%; Table 1, entry 2). In the reaction solutions, small amounts of HOOCCH2COOH and HOOCCOOH were also detected (Figure 3b). In an attempt to achieve complete mineralization, we extended the reaction time of ETFE in supercritical water at 380 °C in the presence of O2 (16.5 mmol) (Figure 4). Although the amounts of F− and CO2 increased rapidly at the beginning of the reaction (