Ind. Eng. Chem. Res. 2008, 47, 8619–8624
8619
Ball Mill-Assisted Dechlorination of Flexible and Rigid Poly(vinyl chloride) in NaOH/EG Solution Tomohito Kameda,† Masahiko Ono,‡ Guido Grause,† Tadaaki Mizoguchi,‡ and Toshiaki Yoshioka*,† Graduate School of EnVironmental Studies and EnVironment ConserVation Research Institute, Tohoku UniVersity, Aramaki, Aoba-ku, Sendai 980-8579, Japan
Both flexible and rigid forms of poly(vinyl chloride) (PVC) were effectively dechlorinated in NaOH/ethylene glycol (EG) solution during ball mill pulverization. The high degree of dechlorination obtained was attributed to the increased surface area of the crushed PVC particles and the resulting enhancement of contact between the PVC and dissolved hydroxide ions. The common additives diisononyl phthalate and CaCO3 were easily separated from the PVC bulk during the dechlorination reaction. The reaction proceeded under chemical control, with degrees of dechlorination for both flexible and rigid PVC increasing with temperature with apparent activation energies of 110 and 80 kJ/mol, respectively. This reaction was accurately represented by a modified shrinking-core model. Introduction Waste plastic feedstocks may contain several different materials, such as polyethylene (PE), polypropylene (PP), polystyrene (PS), poly(vinyl chloride) (PVC), poly(vinylidene chloride) (PVDC), and polyethylene terephthalate (PET).1 The quality of products and materials produced from waste plastics is strongly dependent on the purity of the recycled feedstock. However, chlorine-containing plastics such as PVC and PVDC can be problematic, particularly during pyrolysis procedures, because of the production of HCl and consequent corrosion of recycling plant equipment. Furthermore, liquid products manufactured by the pyrolysis of chloropolymers contain chlorinated organic compounds, making them undesirable for use as fuel or feedstock. Accordingly, dechlorination treatment prior to pyrolysis is essential for feedstock recycling of waste plastics. Dry processes for the dechlorination of PVC have been reviewed by Bhaskar et al.2 and Montaudo et al.3 Recently, PVC was ground in air together with CaO and quartz powder (a grinding medium) with a small-scale planetary ball mill, resulting in mechanochemical dechlorination.4 Several reports also described wet processes for the dechlorination of PVC. For example, a satisfactory degree of dechlorination was obtained by hydrolysis of PVC in mildly alkaline organic solvents, such as tetrahydrofuran (THF) and dimethylsulfoxide (DMSO).5 Kise6 reported that reaction of PVC powder with aqueous sodium hydroxide in the presence of quaternary ammonium or phosphonium halides yielded several dehydrochlorinated products with a conjugated polyene structure. Guo et al.7 performed extensive dechlorination of PVC using a homogeneous solution of KOH in THF and poly(ethylene glycol) at room temperature. Wet treatment processes have also been developed in aqueous NaOH at high temperature and pressure using an autoclave.8-11 However, to more easily dechlorinate PVC in large-scale operations, a process that can be performed at atmospheric pressure is highly desirable. Recently, the dechlorination of pure PVC in NaOH/ethylene glycol (EG) solution under atmospheric pressure has been demonstrated, due primarily to the high boiling point of EG (196 °C).12 * To whom correspondence should be addressed. E-mail: yoshioka@ env.che.tohoku.ac.jp. Tel.: +81-22-795-7211. Fax: +81-22-795-7211. † Graduate School of Environmental Studies. ‡ Environment Conservation Research Institute.
PVC is currently available in flexible and rigid varieties and contains various types of modifiers, plasticizers, and stabilizers. Because of these additives, both flexible and rigid PVC may be difficult to dechlorinate efficiently as a result of hindered contact between the PVC and dissolved OH-. Here, we report the reaction of both flexible and rigid PVC in NaOH/EG solution using a ball mill to mechanically digest the polymer as a means to increase the active surface area of PVC and thereby accelerate dechlorination. The dechlorination reaction was modeled and analyzed kinetically. The effects of temperature, NaOH concentration, and polymer additives on the dechlorination efficiency were also examined. Experimental Details Materials. Tables 1 and 2 show the compositions of flexible and rigid PVC pellets (diameter 4 mm × height 2 mm). In addition to the primary PVC constituent, flexible PVC also contains CaCO3 as a filler, and diisononyl phthalate (DINP) Table 1. Chemical Composition of Flexible PVC Pellets wt % PVC CaCO3 DINPa chlorinated paraffin alkylbenzene Pb stabilizer calcium stearate wax a
36.8 28.3 23.9 6.99 1.84 1.10 0.74 0.37
DINP: C6H4(COOC9H20)2.
Table 2. Chemical Composition of Rigid PVC Pellets wt % PVC MBSa Sn stabilizer monoglycelide processing aid LDPEb pigment
82.4 13.2 2.47 0.99 0.82 0.082 0.012
a MBS: Methyl methacrylate/butadiene/styrene ) 15/70/15. low density polyethylene.
10.1021/ie8006819 CCC: $40.75 2008 American Chemical Society Published on Web 10/22/2008
b
LDPE:
8620 Ind. Eng. Chem. Res., Vol. 47, No. 22, 2008 Scheme 1. Alkalysis of DINP
Figure 1. Experimental apparatus.
Figure 2. The effects of ball milling on the degree of dechlorination of flexible PVC in 1.0 M NaOH/EG solution at 190 °C.
plasticizer. Rigid PVC contains a methyl methacrylate/butadiene/ styrene (MBS) modifier. Both types of pellet were cooled in a nitrogen atmosphere and sieved to obtain particle sizes 150-250 µm in diameter. The chlorine contents of the flexible and rigid PVC powders were 20.6 and 46.5 wt %, respectively. Methods. NaOH/EG solution was prepared by dissolving NaOH pellets in EG. The experimental apparatus is shown in Figure 1. A 1 L ball mill reactor (inner diameter 150 mm; width 60 mm) containing Y2O3-ZrO2 balls as a grinding medium, was charged with 0.5 g of the flexible and rigid PVC powders and 50 mL of 0 to 1.0 M NaOH/EG solution. The numbers of grinding balls used were 800 and 50 for balls 2 and 5 mm in diameter, respectively. The rotation speed of the reactor was set to 60 rpm at 150-190 °C under N2 gas at flow rate of 200 mL/min. The reaction progressed for 0-6 h. After the reaction was complete, the reactor was cooled with water, and the reactant was filtered, washed with deionized water and methanol, and dried under reduced pressure. Characterization. Chloride and phthalate concentrations of the filtrate were determined using a Dionex DX-100 ion chromatograph and a Dionex model AS-16A column (eluent: 35 mM NaOH). The residue was measured by X-ray diffraction (XRD) and observed by scanning electron microscopy (SEM). Results and Discussion Dechlorination of Flexible PVC. Figure 2 shows the effects of ball milling on dechlorination of flexible PVC in 1.0 M NaOH/EG solution at 190 °C. Both the degree of dechlorination, defined as the mole percentage of chlorine in the filtrate to that in the PVC, and the degree of dechlorination increased as a
function of time with and without the Y2O3-ZrO2 grinding medium. However, sufficient dechlorination was not observed without the use of grinding medium. Figure 3 shows SEM images of flexible PVC residues following ball-assisted dechlorination in 1.0 M NaOH/EG solution at 190 °C. The mean PVC particle size decreased as a function of time due to the action of the ball mill. The high degree of dechlorination observed in samples containing grinding medium was therefore attributed to the increased surface area of the PVC and the resulting contact enhancement between the PVC and dissolved OH-. Figure 4 shows that the degree of flexible PVC dechlorination increased as a function of both time and temperature. At a reaction time of 2 h, the degree of dechlorination was 50% at 150 °C, 80% at 170 °C, and 97% at 190 °C. The accelerated rate of dechlorination at higher temperatures implies that the reaction proceeded under chemical control. The effects of NaOH concentration are shown in Figure 5. At a NaOH concentration of 0 M, the degree of dechlorination was only 3% after 2 h. At any given time, the degree of dechlorination increased with increasing NaOH concentration from 0.1 to 0.5 M, but decreased slightly at higher concentrations. This decrease was attributed to the hindrance of OH- penetration into the interior of the PVC particles due to the increased viscosity of the surrounding solution. Figure 6 shows the effects of NaOH concentration on the alkalysis of DINP during the ball-assisted dechlorination of flexible PVC at 190 °C. The yield of phthalic acid was defined as the percentage of phthalic acid in the filtrate relative to the theoretical amount obtained by decomposition of DINP in the flexible PVC, shown in Scheme 1. The yield of phthalic acid in 0 M NaOH was 0%, but increased with both time and NaOH concentration between 0.1 and 1.0 M, obtaining an optimal value of 99% after 1.0 h in 1.0 M NaOH. These results confirmed that the decomposition of DINP requires hydroxyl ions, as illustrated in Scheme 1. DINP was easily separated from flexible PVC in NaOH/EG solution. Figure 7 shows XRD patterns of flexible PVC and residues obtained following dechlorination. In samples that were processed without grinding medium, the XRD peaks indicate the presence of CaCO3 (see Table 1) before and after dechlorination treatment. Conversely, XRD peaks corresponding to CaCO3 were not observed in samples that were subjected to ball-assisted milling. These results showed that it is possible to separate CaCO3 from the flexible PVC by Y2O3-ZrO2-assisted ball milling. Dechlorination of Rigid PVC. Figure 8 shows the effects of ball milling on the dechlorination of rigid PVC. The degree of dechlorination increased with time, and both the rate and degree of dechlorination were enhanced by the use of grinding medium, particularly at reaction times greater than 0.5 h. After 6 h, the dechlorination rates were 84% and 43% for samples with and without grinding medium, respectively. Figure 9 shows SEM images of rigid PVC pellets and residues following
Ind. Eng. Chem. Res., Vol. 47, No. 22, 2008 8621
Figure 3. SEM images of flexible PVC residues obtained following dechlorination in a ball mill with Y2O3-ZrO2 grinding medium in 1.0 M NaOH/EG solution at 190 °C. The values in brackets represent the degrees of dechlorination.
Figure 4. The ball mill-assisted dechlorination of flexible PVC with grinding medium as a function of temperature in 1.0 M NaOH/EG solution.
Figure 6. The effects of NaOH concentration on the yield of phthalic acid during the dechlorination of flexible PVC with grinding medium at 190 °C.
Figure 5. The ball mill-assisted dechlorination of flexible PVC with grinding medium as a function of NaOH concentration at 190 °C.
Figure 7. XRD patterns of flexible PVC before and after ball mill-assisted dechlorination with and without grinding medium in 1.0 M NaOH/EG solution at 190 °C. The values in brackets represent the degrees of dechlorination.
dechlorination treatment. The particle surfaces in Figure 9a were smooth, suggesting that the surface had been covered by MBS (see Table 2). This surface morphology remained unchanged following dechlorination in the absence of grinding medium,
as shown in Figure 9b. The MBS coating likely resulted in a decrease in the PVC reactive surface area that came into contact with dissolved OH-, thereby resulting in lower rates and degrees
8622 Ind. Eng. Chem. Res., Vol. 47, No. 22, 2008
Figure 8. The effects of ball milling on the degree of dechlorination in rigid PVC in 1.0 M NaOH/EG solution at 190 °C.
of dechlorination relative to those observed for flexible PVC. However, the particle size was much smaller when the Y2O3-ZrO2 grinding medium was used, as shown in Figure 9c. Thus, as noted above for flexible PVC, ball milling of the rigid PVC resulted in an increase in the reactive surface area of the PVC and improved contact with dissolved OH-, thereby contributing to the acceleration of dechlorination, as shown in Figure 8.
Figure 10 shows that the degree of dechlorination of rigid PVC increased with both temperature and time. After the reaction had progressed for 6 h, the degrees of dechlorination were 18% at 150 °C, 55% at 170 °C, and 84% at 190 °C. These results were similar to those observed for flexible PVC (Figure 4) in which the accelerated rate of dechlorination at higher temperatures indicated that the reaction proceeded under chemical control. Analysis of Reaction Kinetics. As mentioned earlier, the temperature dependence of the dechlorination of flexible and rigid PVC indicates that the reactions proceeded under chemical control. Ball milling of PVC in NaOH/EG solution resulted in simultaneous dechlorination and mechanical digestion of the PVC particles. Therefore, the reaction was likely accelerated because of the constantly increasing surface area of the PVC particles. Accordingly, a modified shrinking-core model for dechlorination of PVC particles in NaOH/EG using a ball mill is proposed, as shown in Figure 11. This model has been applied previously to the degradation of poly(ethylene terephthalate) particles in sulfuric acid under chemical control.13 On the basis of this model, the dechlorination kinetics of PVC are discussed below. The dechlorination of PVC may be derived from SN2 substitution and E2 elimination reactions.12 Therefore, the
Figure 9. SEM images of rigid PVC residues obtained following dechlorination in a ball mill with Y2O3-ZrO2 grinding medium in 1.0 M NaOH/EG solution at 190 °C. The values in brackets represent the degrees of dechlorination.
Ind. Eng. Chem. Res., Vol. 47, No. 22, 2008 8623
Figure 10. The ball mill-assisted dechlorination of rigid PVC with grinding medium as a function of temperature in 1.0 M NaOH/EG solution.
Figure 13. The dechlorination rate of flexible PVC with grinding medium as a function of temperature in 1.0 M NaOH/EG solution.
rination, S is proportional to (1 - X )(1 + cX ) and may be expressed as: S ) S0(1 - X )(1 + cX ) ) 4πr20(1 - X )(1 + cX )
Figure 11. Modified shrinking-core model for the dechlorination reaction of PVC particles in NaOH/EG solution during ball milling.
reaction rate is proportional to the concentrations of PVC and dissolved OH- as follows: (1) V ) k[Cl][OH ] where V is the reaction rate, and k is the rate constant. As PVC is solid, the reactive surface area of the PVC particles may be used to represent its concentration. Accordingly, the reaction rate may be expressed as follows: -
V ) kSSCA
(2)
where kS is the rate constant per unit surface area, S is the reactive surface area of the PVC particle, and CA is the OHconcentration. SEM images of dechlorination residues are shown in Figure 12. In the absence of NaOH, shown in Figure 12a, the flexible PVC was not ground to a powder, and the degree of dechlorination reached 3%. When ground with 1.0 M NaOH/EG, shown in Figure 12b, the flexible PVC was ground to a powder with 97% dechlorination. From these results, it was concluded that the dechlorination reaction resulted in a physical change in the structure of the PVC pellet, making it more easily pulverized. Assuming that increases in the PVC reactive area due to ball milling are proportional to increases in the degree of dechlo-
(3)
where S0 is the initial surface area of the PVC, X is the degree of dechlorination, c is the proportional constant describing the increase of the PVC reactive surface area with increasing dechlorination, and r0 is the initial radius of the PVC particle. If the PVC is regarded as a sphere and its mass balance is considered to be solid, then: (4) V ) -d(4πr3F ⁄ 3) ⁄ dt where r is the radius of the PVC particle at any reaction time and F is the density of PVC. X can be represented from the volume ratio before and after the reaction as follows: X ) 1 -(4πr3F ⁄ 3) ⁄ (4πr30F ⁄ 3)
(5)
Equation 6 can be derived from eqs 2–5 -d(1 - X ) ⁄ dt ) (3 ⁄ r0F)(1 - X )(1 + cX )kSCA
(6)
and integrated to give (1 ⁄ (1 + c)) ln((1 + cX ) ⁄ (1 -X )) ) Kt (7) where K is the apparent rate constant equal to 3kSCA/(r0F). The rates of dechlorination for both flexible and rigid PVC were arranged according to eq 7 using the results shown in Figures 4 and 10. Figures 13 and 14 show the linear relationship for the dechlorination rates as a function of temperature. Arrhenius plots of K, determined from the slopes of the lines in Figures 13 and 14, are shown in Figure 15 and yielded apparent activation energies of 110 and 80 kJ/mol for flexible
Figure 12. SEM images of residues following ball mill-assisted dechlorination of flexible PVC with grinding medium in EG or 1.0 M NaOH/EG solution at 190 °C for 2 h. The values in brackets represent the degrees of dechlorination.
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hindrance of OH- penetration into the PVC particle. DINP, which is a typical constituent of flexible PVC, was easily decomposed to phthalic acid and separated from the PVC component. Likewise, CaCO3 was easily separated from PVC during ball mill treatment. The dechlorination reaction proceeded under chemical control, with degrees of dechlorination for both flexible and rigid PVC increasing with temperature with apparent activation energies of 110 and 80 kJ/mol, respectively. This reaction was accurately represented by a modified shrinkingcore model. The results of the present study indicated that treatment of flexible and rigid PVC varieties with a ball mill in NaOH/EG solution leads to efficient dechlorination, even if the PVC contains various additives. This process is therefore expected to become important in industrial processes. Figure 14. The dechlorination rate of rigid PVC with grinding medium as a function of temperature in 1.0 M NaOH/EG solution.
Figure 15. Arrhenius plots of the apparent rate constants for the dechlorination of flexible and rigid PVC in 1.0 M NaOH/EG.
and rigid PVC, respectively. These values confirmed that the dechlorination of both flexible and rigid PVC proceeded under chemical control. Furthermore, the modified shrinking-core model, illustrated in Figure 11, was demonstrated as an accurate representation of ball mill-assisted dechlorination of PVC in NaOH/EG. The apparent activation energy of dechlorination of flexible PVC was higher than that of rigid PVC. This suggests that the temperature dependence on the dechlorination of flexible PVC is larger than that of rigid PVC. Conclusions Flexible and rigid varieties of PVC were effectively dechlorinated in NaOH/EG during mechanical pulverization in a ball mill. At any given time during the reaction, the degree of dechlorination was higher with the use of a spherical Y2O3ZrO2 grinding medium. This was attributed to enhanced contact between the PVC and dissolved OH- as a result of the increased surface area of the PVC exposed during milling. In the case of flexible PVC, the degree of dechlorination increased with increasing NaOH concentration from 0.1 to 0.5 M, but decreased slightly from 0.5 to 1.0 M NaOH. This decrease was attributed to the increased viscosity of the solution and consequent
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ReceiVed for reView April 28, 2008 ReVised manuscript receiVed September 10, 2008 Accepted September 10, 2008 IE8006819
