Dechlorination of poly(vinyl chloride) using NaOH in ethylene glycol under atmospheric pressure

Polymer Degradation and Stability 93 (2008) 1138–1141

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Dechlorination of poly(vinyl chloride) using NaOH in ethylene glycol under atmospheric pressure Toshiaki Yoshioka *, Tomohito Kameda, Shogo Imai, Akitsugu Okuwaki Graduate School of Environmental Studies, Tohoku University, 6-6-07 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 February 2008 Received in revised form 21 February 2008 Accepted 6 March 2008 Available online 12 March 2008

A solution of NaOH dissolved in ethylene glycol (EG) was effective in the dechlorination of poly(vinyl chloride) (PVC) at atmospheric pressure. The degree of dechlorination increased with increasing temperature, reaching a maximum of 97.8% at 190  C. The dechlorination proceeded under chemical control and exhibited first-order kinetics with an apparent activation energy of 170 kJ mol1. The apparent rate constant for dechlorination in 1.0 M NaOH/EG was approximately 150 times greater than that in 1.0 M NaOH/H2O. In addition, dechlorination was faster at atmospheric pressure in NaOH/EG than under high pressure in NaOH/H2O. The dechlorination reaction occurs via a combination of E2 and SN2 mechanisms. Ó 2008 Elsevier Ltd. All rights reserved.

Keywords: Poly(vinyl chloride) Dechlorination Reaction kinetics Ethylene glycol Atmospheric pressure NaOH

1. Introduction Japan began chemical recycling of vessel-wrapping waste plastics (VWWPs) in 2000. Pyrolysis of non-halogenated plastics such as polyethylene, polypropylene, and polystyrene is simple, and the liquid products obtained by thermal degradation can be used as fuel oil or as a feedstock [1]. However, VWWPs often contain chlorinated plastics such as poly(vinyl chloride) (PVC) and poly (vinylidene chloride) (PVDC). The liquid products formed as a result of pyrolysis of these chlorine-containing VWWPs may contain HCl and chlorinated organic compounds, making them unsuitable for use as fuel or feedstock materials. In addition, liquefaction of PVC residues via dry-dehydrochlorination using thermal or catalytic decomposition is energy intensive. Therefore, dechlorination treatment is necessary for chemically recycling chlorine-containing waste plastics. Bhaskar and Sakata [2] reviewed the decomposition of PVC, with particular focus on pyrolysis treatments. Furthermore, the thermal decomposition of PVC has been postulated with one, two, or three stages [3–10]. Simultaneous thermogravimetric-mass spectrometry (TG-MS) has shown, however, that this degradation can be divided into three stages: two dehydrochlorination reactions and a breakdown of the polyene–aromatic network [11]. Wet treatment processes for the dechlorination of PVC have also been reported [12–14]. Recently, degradation of PVC in high temperature and * Corresponding author. Tel./fax: þ81 22 795 7211. E-mail address: [email protected] (T. Yoshioka). 0141-3910/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2008.03.007

supercritical water was studied using a hydrothermal, diamond anvil cell to determine phase change characteristics of the polymer with respect to water density [15]. Extensive dehydrochlorination of PVC in dioxane using a KOH/poly(ethylene glycol) system has been reported [16], and wet-processing of PVC in aqueous NaOH has been extensively studied. Products such as oxalic acid and benzenecarboxylic acid were produced by oxidation of pure PVC powder [17], vinylidene chloride–vinyl chloride copolymer powder [18], rigid PVC pellets [19], and flexible PVC pellets [20] with molecular oxygen. The dechlorination progressed as a zero-order reaction, with an apparent activation energy of 41.0 kJ mol1. The dechlorination of flexible PVC films [21] and rigid PVC pellets [22] in the absence of oxygen was reported as a first-order reaction, with apparent activation energies of 125.4 and 96.0 kJ mol1, respectively. Processes that use aqueous solutions of NaOH, however, typically require an autoclave to obtain high temperatures and pressures necessary for dechlorination. A process that can be performed at atmospheric pressure would be much easier to implement. Therefore, we investigated the use of ethylene glycol (EG), which has a boiling point of 196  C, as a replacement for water in the wetprocess dechlorination of PVC.

2. Experimental PVC resin powder containing 56.7 wt% Cl and other reagents were purchased from Kanto Chemical (Tokyo, Japan). NaOH/EG

T. Yoshioka et al. / Polymer Degradation and Stability 93 (2008) 1138–1141

1139

2.5

Pipette glass N2: 50ml/min

-ln(1-x)

2.0

Water

1.5

1.0

0.5

Silicone oil

0M 0.2 M 0.5 M

1.0 M 1.5 M 2.0 M

0

Iced methanol trap

0

20

40

60

80

100

120

Time / min Fig. 1. Experimental set-up.

Fig. 3. The first-order plot of the dechlorination of PVC in NaOH/EG at 170  C.

solutions, ranging in the concentration from 0 to 2.0 M, were prepared by dissolving the required amount of NaOH in EG. The experimental set-up is shown in Fig. 1. A 100-ml, three-necked, Pyrex flask was purged with N2 at a rate of 50 ml min1 to totally remove the air, and PVC powder and 50 ml of NaOH/EG solution was placed into the purged flask. For NaOH concentrations greater than 1.0 M, 0.1 g of PVC powder was used, and 0.05 g of PVC powder was used for solutions containing less than 0.1 M NaOH. The flask was then heated for 15 min to a final temperature of 130–190  C in a silicone oil bath. The reaction start time (i.e., 0 min) was defined as the time at which this final temperature was obtained. The resultant reaction mixture was held at 130–190  C and mildly agitated by a flow of nitrogen. Samples of the reaction mixture were withdrawn at defined intervals using a glass pipette, immediately filtered through a glass-fibre filter, and washed repeatedly with deionized water and methanol. The chloride concentration of the filtrate was determined using an ion chromatograph (DX-100; Dionex, Sunnyvale, CA) equipped with a Dionex model AS4A column (eluent: 1.8 mM Na2CO3 and 1.7 mM NaHCO3; flow rate: 1.5 ml min1). Residual C, H, and Cl were determined by elemental analyses. The residue was also measured by diffuse reflectance Fourier transform-infrared spectroscopy (FT-IR).

3. Results and discussion Fig. 2 shows the effect of NaOH concentration on the dechlorination of PVC in NaOH/EG at 170  C. The degree of dechlorination was defined as the mole percentage of Cl in the filtrate to that in PVC. The degree of dechlorination increased as a function of reaction time for NaOH concentrations ranging from 0.2 to 2.0 M. In the absence of NaOH, the degree of dechlorination was extremely low, 0.3% after 105 min, implying that dechlorination of PVC under these circumstances requires a source of OH. These results demonstrated that the dechlorination of PVC can be obtained at atmospheric pressure using NaOH dissolved in EG instead of aqueous solution. In the absence of oxygen, the dechlorination of flexible PVC films and rigid PVC pellets in aqueous NaOH proceeds via a firstorder reaction mechanism [21,22]. Therefore, a similar first-order mechanism was hypothesized for the dechlorination of PVC in NaOH/EG. To confirm this hypothesis, the experimental data shown in Fig. 2 were fitted to the equation: ln ð1  xÞ ¼ kt;

(1)

where x is the degree of dechlorination, t is the reaction time, and k (min1) is the apparent rate constant. In this way, the increasing degree of dechlorination was pseudo-ascribed to a homogeneous

3.0

100

k× × 10-2 / min-1

Dechlorination /

80

60

40

0

1.0

1.0 M 1.5 M 2.0 M

0M 0.2 M 0.5 M

20

2.0

0 0

20

40

60

80

100

120

Time / min Fig. 2. The effect of NaOH concentration on the dechlorination of PVC in NaOH/EG at 170  C.

0

0.5

1.0

1.5

2.0

NaOH concentration / M Fig. 4. The effect of NaOH concentration on the apparent rate constant, k, for the dechlorination of PVC in NaOH/EG at 170  C.

1140

T. Yoshioka et al. / Polymer Degradation and Stability 93 (2008) 1138–1141

0

80

-2.0

60

-4.0

1.0 M NaOH/EG 1.0 M NaOH/H2O*

lnk -6.0

40 130 ºC 150 ºC 160 ºC

60

120

180

-8.0

-10.0 1.8

240

2.0

2.2

2.4

2.6

1 / T / 10-3 / K

Time / min

C=C H

H

in Fig. 5. Fig. 6 shows the first-order plot of the dechlorination of PVC in 1.0 M NaOH/EG solution. The plot is linear for each reaction temperature although the plot for only 190  C does not pass through the origin at 0 min due to the high dechlorination rate during the preheat time of 15 min in this experimental system, suggesting a first-order reaction. An Arrhenius plot of k, determined from the slopes of the plots in Fig. 6, is shown in Fig. 7. The apparent activation energy (170 kJ mol1) was similar to that obtained in 1.0 M NaOH/H2O (190 kJ mol1 [24]), which confirms that the reaction proceeded under chemical control and suggests a similar mechanism for the two solutions. At 200  C, the apparent rate constant in 1.0 M NaOH/EG was approximately 150 times greater than that in 1.0 M NaOH/H2O. This may have been due to an increased solubility of PVC in EG versus water at these high temperatures. Note that the dechlorination of PVC in NaOH/EG at atmospheric pressure proceeded faster than the same reaction in NaOH/H2O under elevated pressure.

O

first-order reaction. Fig. 3 shows first-order plots of the dechlorination of PVC in NaOH/EG solution at 170  C. The plots exhibit good linearity. These data suggest that the dechlorination of PVC in NaOH/EG solution may be accurately expressed as a first-order reaction in accordance with previous studies [21,22]. Fig. 4 shows the effect of NaOH concentration on the apparent rate constant, k. Increasing k was observed with increasing NaOH concentration up to 1.0 M; above 1.0 M, k decreased with increasing NaOH. Similar results were obtained for the treatment of PVC materials in aqueous NaOH [22,23]. The dechlorination of PVC takes place at the solid–liquid interface between the PVC particle and dissolved OH. During the beginning stages of the reaction, OH reacts with both the surface and interior of the PVC particle. However, at high NaOH concentrations, a change likely occurs in the surface morphology of the PVC particle that prevents further penetration of OH. Since the largest apparent rate constant was observed in 1.0 M NaOH/EG (Fig. 4), the dechlorination of PVC in this solution was examined in greater detail. The effects of temperature are shown in Fig. 5. The degree of dechlorination increased as a function of both temperature and reaction time, with a maximum value of 97.8% after 45 min at 190  C. The accelerated rate of dechlorination at high temperatures implies that the reaction proceeds under chemical control. The rate of dechlorination between 130 and 190  C was determined according to Eq. (1) using the results shown

Fig. 7. Arrhenius plot of the apparent rate constant for the dechlorination of PVC in 1.0 M NaOH/EG and NaOH/H2O. *Cited from Ref. [24].

50

Fig. 5. The effect of temperature on the dechlorination of PVC in 1.0 M NaOH/EG.

Cl

0

190 ºC

C

0

170 ºC

C OH

20

C=C

Dechlorination /

100

a

b 130 ºC 150 ºC 160 ºC

2.5

Transmittance / T

3.0 170 ºC 190 ºC

-ln(1-x)

2.0

c

1.5

d

1.0

0.5

0

4000 0

60

120

180

240

3000

2000

1000

Wavenumber / cm-1

Time / min Fig. 6. First-order plot of the dechlorination of PVC in 1.0 M NaOH/EG.

Fig. 8. FT-IR spectra of (a) PVC resin and residues obtained by the dechlorination of PVC in 1.0 M NaOH/EG at 190  C after (b) 0, (c) 15, and (d) 45 min.

T. Yoshioka et al. / Polymer Degradation and Stability 93 (2008) 1138–1141

OH

Table 2 The relative contributions of elimination and substitution mechanisms on the dechlorination of PVC in 1.0 M NaOH/EG solution at 190  C

H CH CH

Elimination (E2)

CH CH

HCl n

n

Cl CH2 CH

Substitution (SN2)

OH

n

CH2 CH

Dechlorination (F )/%

Elimination (FE)/%

Substitution (FS)/%

5 15 45

60.0 83.3 97.8

32.2 48.8 57.8

27.8 34.5 40.0

Cl

OH

Cl

4. Conclusions

Fig. 8 shows the FT-IR spectra of PVC resin and residues obtained by the dechlorination of PVC in 1.0 M NaOH/EG at 190  C. As the dechlorination reaction progressed, the FT-IR peaks corresponding to the stretching vibrations of O–H (the large absorption band centered at 3500 cm1), C]C–H (ca. 3000 cm1), C]C (ca. 1700 cm1), and C–OH (1150 cm1) increased in intensity. Dechlorination was evidenced by the decreasing intensity of the C–Cl stretching vibration (700 cm1) with increased reaction time. The increasing intensity of the C]C–H and C]C stretching vibrations in the PVC residues support an E2 mechanism. Conversely, an SN2 component of the reaction mechanism was evidenced by the increasing intensity of the O–H and C–OH stretching vibrations. Thus, the overall dechlorination reaction proceeds via competitive pathways with the elimination of hydrogen chloride (E2) and hydroxyl group substitution (SN2), as shown in Scheme 1. Table 1 presents the chemical compositions of residues obtained by the dechlorination of PVC in 1.0 M NaOH/EG solution at 190  C. The amount of O was calculated by subtracting the amounts of C, H, and Cl from the total residual mass. As anticipated, the amount of Cl in the residue decreased with reaction time. The amount of O, however, increased. This was due to the SN2 component of the reaction mechanism and the formation of OH substituents. The O/ C molar ratios were calculated from the residual chemical composition and are shown in Table 1. The relative contributions of the E2 and SN2 mechanisms are given in Table 2 and were calculated according to the following equations: FS ¼ ½ðO=CÞ  2  100;

(2)

FE ¼ F  FS ;

(3)

where F, FE, and FS refer to the percentages of dechlorination, elimination, and substitution, respectively. According to the stoichiometry of the PVC repeat unit, –CH2–CHCl–, FS may be expressed as twice the O/C molar ratio, although after dechlorination, the Cl atom was substituted with an OH group. Five minutes into the dechlorination reaction, the two mechanisms were almost equally active. As the reaction progressed, however, the E2 mechanism became dominant, which was attributable to the inability of OH to penetrate the interior of the PVC particle as discussed above.

Table 1 Chemical compositions of residues obtained during the dechlorination of PVC in 1.0 M NaOH/EG solution at 190  C and the molar ratio of O/C

5 15 45

Time/min

n

Scheme 1. Proposed mechanism for the dechlorination of PVC in NaOH/EG.

Time/min

1141

wt%

Molar ratio

C

H

Cl

Oa

O/C

55.3 64.8 70.9

6.5 7.5 7.8

28.0 12.8 2.4

10.2 14.9 18.9

0.14 0.17 0.20

a The amount of O was calculated by subtracting the amounts of C, H, and Cl from the total residual mass.

The dechlorination of PVC was investigated in NaOH/EG solution at atmospheric pressure. The dechlorination reaction required OH and did not occur substantially in neat EG. However, the reaction was most effective at moderate NaOH concentration. At higher concentrations (i.e., greater than 1.0 M), the reaction was significantly slower due to changes at the surface of the PVC particle that prevented the penetration of OH. The dechlorination was expressed as a first-order reaction under chemical control with an apparent activation energy of 170 kJ mol1 at 190  C. At 200  C, the apparent rate constant in 1.0 M NaOH/EG was approximately 150 times larger than in 1.0 M NaOH/H2O. Dechlorination in NaOH/EG at atmospheric pressure was faster than that in NaOH/H2O solution at elevated pressure. Residual analyses indicated that the dechlorination pathway was represented by a combination of E2 and SN2 mechanisms. During the early stages of dechlorination, the E2 and SN2 mechanisms were equally active, with the E2 component becoming dominant as the reaction proceeded. Thus, the dechlorination of PVC was highly effective in NaOH/EG solution without high-pressure autoclaving as required during aqueous wet-processing, thereby providing a much more practical solution for the chemical recycling of VWWPs.

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