Ind. Eng. Chem. Res. 2002, 41, 2617-2622
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KINETICS, CATALYSIS, AND REACTION ENGINEERING Reduction of Residual Monomer in Latex Products by Enhanced Polymerization and Extraction in Supercritical Carbon Dioxide Maartje Kemmere,*,† Marcus van Schilt,† Mascha Cleven,† Alex van Herk,‡ and Jos Keurentjes† Process Development Group and Polymer Chemistry Research Group, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
One of the most important environmental incentives for the polymer industry is to reduce the residual monomer content in polymer products. In addition to the conventional techniques for the reduction of residual monomer, a process based on supercritical carbon dioxide (scCO2) can be a viable alternative as both further polymerization and extraction of the monomer can occur. In this work, the reduction of methyl methacrylate (MMA) in a PMMA latex was chosen as a representative model system. Pulsed electron beam experiments were performed to study the effect of scCO2 on the monomer concentration inside the polymer particles during the polymerization reaction. The partitioning behavior of MMA between water and scCO2 was measured in a high-pressure extraction unit as a function of pressure and temperature. The obtained results show that the extraction of MMA is the predominant effect as compared to the enhancement of polymerization through plasticization. 1. Introduction Emulsion polymerization is widely used in industry for the production of water-based paints, synthetic rubbers, coatings, and adhesives.1 In general, conventional emulsion polymerization achieves relatively high reaction rates. At the end of the polymerization, however, the reaction rate decreases significantly because of the limiting diffusion of the monomer in the polymer particles.2 This results in a significant amount of residual monomer in the product latex. At this time, one of the most important environmental incentives for the polymer industry is to reduce the residual monomer content in latex products. In practice, the residual monomer is often converted at increased temperatures in a postprocessing tank. As reaction rates are low, this implies an energy-intensive and time-consuming operation. The application of supercritical fluids can be a viable alternative for the reduction of residual monomer in latex particles. In the supercritical phase, the density can be varied continuously from gaslike to liquidlike values with relatively small changes in pressure and temperature.3 Because of this tunability, density-related properties such as viscosity, diffusivity, and solubility can easily be influenced. Moreover, the tunability of the solvent strength and the hydrodynamic characteristics make supercritical fluids interesting extraction and reaction media. Moreover, supercritical carbon dioxide (scCO2), which has its critical point at 73.8 bar and 304.3 K, allows extreme operating pressures and tem* Corresponding author. Tel.: +31 40 2473673. Fax: +31 40 2446104. E-mail address:
[email protected]. † Process Development Group. ‡ Polymer Chemistry Research Group.
peratures to be avoided and, consequently, investment costs to be reduced. In addition, scCO2 is an inexpensive solvent that is chemically inert, nonflammable, and nontoxic and that is considered to be environmentally benign. Therefore, scCO2 has already found numerous industrial applications. For over four decades, scCO2 has successfully been used to extract caffeine on an industrial scale. Additionally, scCO2 has been used for a variety of extraction processes to obtain specific compounds from natural products such as tea, hops, and spices.4 The application of scCO2 in polymer production and processing creates an additional advantage because of the plasticizing effect, resulting in an increase in the mobility and a decrease in the glass transition temperature of the polymer.5-9 In the past decade, a substantial number of promising developments in polymer processes based on scCO2 have been identified,10 e.g., in the synthesis of fluoropolymers,11 catalytic polymerization of olefins,12 and dispersion polymerization of MMA in supercritical carbon dioxide.13 The strong plasticization of a number of polymers makes scCO2 a promising ingredient for facilitating polymer shaping, blending, impregnation, and chemical modification.14-18 In addition to these applications of scCO2 in polymer synthesis and processing, scCO2 potentially forms an interesting alternative for reducing residual monomer as it enables additional conversion as well as extraction of the monomer. First, the plasticizing effect is expected to enhance the diffusivity of monomer inside the polymer particles. This could increase the diffusion-controlled propagation rate of the polymerization and, thus, increase the conversion of monomer within an acceptable time of operation. Second, the enhanced diffusivity is expected to facilitate the transport of monomer through the polymer particles to the aqueous phase.
10.1021/ie0109408 CCC: $22.00 © 2002 American Chemical Society Published on Web 05/02/2002
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Table 1. Characteristics of PMMA Latex Used for the Electron Beam Experiments recipe
MWDsol fraction (g/mol)
MMA (g) water (g) sodium dodecyl sulfate (g) sodium persulfate (g) EGDM (g) sodium carbonate (g)
60.0 140.3 1.27 0.374 1.2 0.32
cross-linking (%) Treaction (°C) final particle size (nm)
95 60 165
Moreover, scCO2 is an excellent extraction medium for a variety of monomers, as very little water and virtually no polymer dissolves in it whereas most monomers have relatively high solubilities in scCO2.19 In this study, both the enhanced polymerization and the extraction of residual monomer have been studied in the presence of scCO2. The emulsion polymerization of MMA was taken as an example because MMA is an extensively used monomer in industrial emulsion polymerization. Pulsed electron beam experiments were performed to investigate the effect of scCO2 on the polymerization reaction. Concerning monomer extraction to the CO2 phase, the partitioning behavior of MMA between water and scCO2 was measured in a highpressure extraction unit as a function of pressure and temperature. 2. Experimental Section Pulsed Electron Beam Experiments. To study the effect of scCO2 on the emulsion polymerization process as well as to determine the amount of monomer inside the polymer particles, electron beam experiments were performed.20 Pulsed electron beam polymerization involves the generation of radicals in the aqueous phase, activated by an electron beam. These radicals initiate the polymerization of the residual monomer inside the latex particles. From the molecular weight of the newly formed polymer chains, the local monomer concentration in the polymer particles can be calculated. The growth time of a polymer chain is directly determined by the time between two pulses (tp). The chain length (Li) of the polymer produced in this time tp is given by
Li ) ikp[M]tp
(1)
where kp and [M] are the propagation rate coefficient and monomer concentration in the polymer particles, respectively. Higher-order peaks in molecular weight (i ) 2, 3, ...) can occur when growing chains survive termination by one or more subsequent pulses. In this case, only the primary peak (i ) 1) has been observed. Between two pulses, bimolecular termination or transfer can occur, resulting in so-called background polymer. In this study, electron beam experiments were performed using a LINAC SL75-5 electron accelerator (MEL, Sussex, England). During the electron beam experiments, the energy of the electrons was adjusted to 5 MeV, resulting in pulse energies of 0.8 J. A highpressure view cell of 2.9-mL volume was used for irradiating the sample, a cross-linked (0.6 wt % ethylene glycol dimethacrylate, EGDM) PMMA latex that was swollen to its maximum with MMA; see Table 1 for latex characteristics. The cell was equipped with a stainless steel window for the electron beam following Wishart
and van Eldik21 and one sapphire window. The electron beam experiments were performed at 75 °C with a pulse frequency of 10 Hz. The number of pulses radiated on the samples varied from approximately 300 to 600 pulses. Before the view cell was filled and the electron beam experiments were performed, the latex-scCO2 mixture was stabilized for 1 h at 160 bar in a highpressure 80-mL vessel (160 bar, 40 vol % CO2). As a reference, an electron beam experiment was performed at atmospheric conditions without carbon dioxide. In all cases, the molecular weight distribution (MWD) of the sol fraction of the cross-linked polymer was determined using gel permeation chromatography (GPC). The inflection point was taken as a measure of the local monomer concentration experienced by the newly formed polymer chains.22 Taking the propagation rate coefficient based on the Arrhenius parameters for MMA emulsion polymerization as recommended by IUPAC [kp ) 1.179 × 103 L/(mol s), the intrinsic value), the electron pulse frequency, and i ) 1, the local monomer concentration in the polymer particle could be calculated according to eq 1. Measuring Partitioning Behavior of MMA between Water and CO2. Figure 1 shows a schematic setup of the high-pressure extraction unit and a description of the relevant components and features. With this equipment, the partition coefficient K, given by eq 2, was measured as a function of pressure and temperature.
K)
[MMA]CO2 [MMA]w
(2)
The experimental procedure was started by adding a carefully weighed amount of aqueous MMA solution into the equilibrium cell, filling it to approximately 50%. The cell was immediately sealed and heated to the desired temperature. The liquid phase was stirred vigorously, creating a deep vortex. At the desired temperature, CO2 was introduced and pressurized by the high-pressure syringe pump. Equilibrium was assumed if the pressure and temperature did not vary by more than 0.2 bar and 0.2 K, respectively, within a period of 30 min. Then, sample valve 3 was slowly opened, and by the careful opening of valve 4, the CO2 was vented from the sample lines and replaced with the aqueous sample. Then, valve 4 was closed, the HPLC valve was turned around, and the contents of the sample lines were flushed back into the equilibrium cell together with fresh CO2 to repressurize the cell. By slowly opening needle valve 11, the contents of the sample loop were depressurized and led through the first tube of chilled methanol (MeOH, 0 °C) containing 0.3 wt % 1-butanol as an internal standard, thus removing the MMA from the sample. The second
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Figure 1. Schematic view of the high-pressure laboratory-scale extraction equipment (sample valve in sampling position): (1) stainless steel high-pressure equilibrium cell (64.33 mL), (2) stainless steel heating jacket, (3) sample line for extracting fluidphase samples and venting valve, (4) sample line for extracting light-phase samples and venting valve, (5) PT 100 temperature sensor and miniaturized high-pressure transducer, (6) magnetic stirrer for fluid-phase agitation, (7) equilibrium cell outlet, (8) sixport HPLC sample valve, (9) calibrated 500-µL sample loop, (10) open connection for syringe, (11) needle valve, (12) chilled beaker containing glass vials for sample collection, (13) display of temperature and pressure, (14) thermostatic water heater, (15) pulsefree 150-mL high-pressure syringe pump, (16) cryostat for cooling the syringe pump.
Figure 2. Deviation between calculated and true values of partition coefficient K in the case of a 3% error (e) in the determination of the MMA concentration in the water and CO2 phases.
tube was used only to verify that no significant amounts of MMA passed through the first tube. The sample loop was flushed twice with air and twice with 0.8 mL of MeOH, which was collected in the first sample tube. Then, the contents of the first tube were analyzed by GC/FID to determine the concentration of MMA. In the proposed method, the presence of only two phases was assumed and checked in a series of highpressure view-cell experiments. Moreover, any significant change in volume of the phases was neglected. From the measured MMA concentration in the aqueous or supercritical phase, the partition coefficient can be calculated using eq 3 or 4. Here, [MMA]w,0 stands for the initial MMA concentration in the aqueous phase.
K)
(
K)
[MMA]w,0
[MMA]exp w
-1
1 [MMA]w,0 exp [MMA]CO 2
-
)
Vw VCO2
VCO2
(3)
(4)
Vw
Note that the initial MMA concentration is limited by the solubility of MMA in water, whereas the experimental setup restricts the VCO2/Vw ratio. Figure 2 demonstrates the effect of a 3% error in the determination of the MMA concentration in terms of the discrepancy between the calculated and true values of the partition coefficient K. This graph clearly shows that, if the MMA concentration in the CO2 phase is used, the difference between the calculated and true K values increases rapidly as soon as K becomes sufficiently large. This also happens at low values of K if the MMA concentration in the aqueous phase is used for the calculation, whereas, at higher values the deviation in the calculation using the aqueous phase composition approaches 3%. Because the value of the partition
Figure 3. Molecular weight distribution of a PMMA latex, swollen to its maximum with MMA, irradiated with an electron accelerator: (A) at atmospheric conditions without CO2, 512 pulses; (B) in the presence of scCO2 at 160 bar, 310 pulses.
coefficient of MMA is typically larger than unity for this system, the aqueous phase is preferred for extracting samples. Reproducibility and validation experiments have demonstrated that the developed method is reliable as well as accurate.23 3. Results and Discussion Enhanced Polymerization of MMA in scCO2. Pulsed electron beam experiments were carried out to study the polymerization of a monomer swollen crosslinked PMMA latex at atmospheric and scCO2 conditions. At ambient pressure, Figure 3A clearly shows the additional peak of the newly formed polymer of molecular weight 79 800 g/mol, produced by the radicals in
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Table 2. Experimental Conditions for the Determination of Phase Equilibrium Compositions of MMA Using the High-Pressure Extraction Equipment T (K)
pressure range (bar)
number of measurements
initial conc of MMA (g/g of water)
initial Vw (mL)
298 313 323 333
52-95 51-86 50-100 49-83
11 6 6 5
0.012 07 0.012 07 0.012 07 0.012 07
31.7 32.3 32.1 31.4
Table 3. Determined Phase Equilibrium Compositions of MMA and Resulting Partition Coefficients between CO2 and Water T (K)
P [MMA]w [MMA]CO2 (bar) (mg/mL) (mg/mL)
K
xMMA yMMA (×10-3) (×10-3)
K′
298 298 298 298 298 298 298 298 298 298 298
48.4 62.7 65.0 79.8 52.6 63.6 70.3 79.6 79.1 84.6 95.3
4.46 2.50 0.87 0.23 3.77 1.23 0.49 0.32 0.66 0.38 0.28
7.62 9.40 10.79 11.22 8.24 10.56 11.08 11.08 11.33 11.42 11.35
1.71 3.76 12.37 49.31 2.19 8.61 22.42 34.44 17.19 29.96 40.42
0.831 0.467 0.162 0.042 0.702 0.229 0.092 0.060 0.122 0.071 0.052
27.1 19.2 19.5 6.4 25.3 20.4 6.6 6.3 6.9 6.4 6.2
32.6 41.2 119.8 151.8 36.1 89.2 71.9 105.7 56.4 90.2 118.4
313 313 313 313 313 313
50.7 59.6 71.3 75.4 80.6 85.6
6.50 5.23 3.20 2.95 1.98 1.44
5.63 6.79 8.61 8.71 9.46 9.78
0.87 1.30 2.69 2.95 4.79 6.79
1.180 0.949 0.581 0.358 0.261 0.535
21.6 20.3 18.6 16.5 14.9 12.1
18.3 21.4 32.0 30.9 41.5 46.1
323 49.7 323 59.8 323 71.5 323 81.1 323 91.0 323 100.1
6.03 5.45 3.98 3.56 3.07 2.28
6.12 6.48 7.75 8.02 8.32 8.89
1.02 1.19 1.95 2.25 2.71 3.89
1.096 0.991 0.724 0.647 0.558 0.415
25.9 21.2 19.3 15.9 12.8 10.6
23.6 21.4 26.6 24.5 22.9 25.7
333 333 333 333 333
6.47 6.33 4.55 4.25 3.56
5.46 5.51 7.06 7.22 7.71
0.84 0.87 1.55 1.70 2.16
1.183 1.157 0.832 0.776 0.651
24.5 19.4 19.6 18.5 16.8
20.7 16.8 23.5 23.8 25.8
49.9 60.0 71.2 75.0 82.9
the aqueous phase induced by the electron accelerator. According to eq 1, this results in a monomer concentration inside the polymer particles of 6.8 mol/L, which is in agreement with the maximum concentration of monomer in the PMMA particles at 75 °C.1 These results show that pulsed electron beam experiments can, indeed, reveal the local monomer concentration in this system. It should be noted that the position of the MWD of the sol fraction is somewhat shifted to higher molecular weight after irradiation as compared to the original MWD of the sol fraction of the latex, because the interaction of the polymer chains with the applied electron beam can result in some cross-linking as well as scission. It is expected that, in the presence of scCO2, a change in monomer concentration will be reflected in a shift of the molecular weight of the additional peak. However, experiments at scCO2 conditions (Figure 3B) do not show any newly formed polymer produced by the pulsed electron beam. Measured Partitioning Behavior of MMA between Water and CO2. The experimental conditions for the determination of phase equilibrium compositions are given in Table 2. In Table 3, the results are presented for the isothermal series. In addition to the regular K, a partition coefficient K′ based on MMA mole
Figure 4. Measured partition coefficients of MMA in water and CO2 at 298 K (1 and 2 denote different experimental series).
fractions in the aqueous (xMMA) and CO2-rich (yMMA) phases was calculated using a modified BenedictWebb-Rubin equation of state to estimate the densities of the phases24 assuming pure water and CO2 densities, respectively. First, high-pressure view-cell experiments were performed to study the phase behavior of the CO2-H2OMMA system at 298 K. Below 58 bar, a two-phase system [CO2(g), H2O(l)] was observed, whereas, between 58 and 63, bar a three-phase region [CO2(g), CO2(l), H2O(l)] exists. This region was omitted during the extraction experiments. At 63 bar, the beginning of the liquid-liquid phase system [CO2(l), H2O(l)] was observed. Similar results have been described in detail by Adrian et al.25 Considering the results of the extraction experiments performed at 298 K shown in Figure 4, the partition coefficient increases significantly above 63 bar. The increasing density of the CO2-rich phase at this point explains the sudden rise of the partition coefficient. Because the two phases in the liquid-liquid system are relatively incompressible, they cannot compensate for the increase of volume when a sample is extracted. As a consequence, the pressure drops until a third relatively light phase is formed, thus lowering the overall density of the system. The sudden formation of this third phase seriously disrupts the equilibrium. As a result, the compositions of both phases are expected to change within a short period of time. If, however, the initial pressure of the system is sufficiently high, the density of the CO2-rich phase can be lowered enough to compensate for the expansion of the system without forming a third phase. Unfortunately, such an adjustment of the liquid-phase density involves a pressure drop of close to 20 bar. Obviously, these effects are difficult to take into account for the interpretation of the experimental results at 298 K. Therefore, all experiments in which a pressure drop of 10 bar or more occurred when a sample was taken are disregarded in further discussions. These data points are printed in italics in Table 3. Figure 5 presents the measured partition coefficients at 313, 323, and 333 K. View-cell experiments at 313 K show that three phases resembling a boiling system can be distinguished close to 73-74 bar, marked by an open diamond in Figure 5. This effect was not observed at 323 or 333 K. In general, the system exhibits the same behavior at temperatures above the critical temperature of CO2. Around 50 bar, all partition coefficients are close to unity and increase with pressure. The rate of this
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Figure 5. Measured partition coefficients of MMA in water and CO2 at 313, 323, and 333 K.
Figure 6. Partition coefficients K′ based on mole fractions calculated from measured phase equilibrium compositions at 313, 323, and 333 K.
increase gradually rises with pressure and decreases with temperature. Similarly to the results at lower temperatures, the partition coefficient at 333 K is therefore expected to increase more significantly beyond the measured pressure range. In general, these results can be explained by the density of the CO2-rich phase as it increases continuously with pressure above 74 bar. This effect is most pronounced at temperatures just above the critical temperature of CO2. In Figure 6, the partition coefficient K′ is plotted against pressure. As K′ is expressed as a ratio of mole fractions, the changes in density of the CO2-rich phase are being taken into account. Figure 6 shows that the increase of the partition coefficients is significant only for the experiment performed at 313 K, and in this case, it only triples over the entire pressure range. It is therefore assumed that the density of the CO2-rich phase is the decisive factor in the behavior of the partition coefficient as a function of temperature and pressure. At relatively low pressures, the results in Figures 5 and 6 suggest an inversion of the temperature dependency of the partition coefficient. In this pressure region, the system consists of an aqueous liquid in equilibrium with a CO2-rich vapor. The partition coefficient in such a system is expected to be primarily determined by the vapor pressure of MMA and its solubility in water. As the latter is barely dependent on temperature, the vapor pressure is probably the determining factor for the behavior of the partition coefficient with respect to temperature. Therefore, it seems plausible that, at relatively low pressures, the partition coefficient increases with respect to temperature.
It should be noted that the reported extraction results comprise the partitioning behavior of MMA between water and scCO2 without polymer particles present. However, it is expected that the latex-scCO2 system also exhibits a strong extraction rate toward the CO2 phase, as the rate-limiting step for mass transfer of MMA appears to occur in the water phase at the scCO2 side whereas the mass transfer inside the polymer particles appears to be very fast. This is mainly due to a relatively small water-CO2 surface area as compared to the overall polymer-water interfacial area.26 Polymerization versus Extraction. Considering the results of the electron beam experiments with respect to the relatively high partition coefficients obtained with the extraction unit, it is very likely that the scCO2 phase extracted the monomer during the electron beam experiments to such an extent that hardly any residual monomer was left in the polymer particles to polymerize. As a consequence, parameters such as the contact time and the volume ratio of the water and the scCO2 phases determine the amount of monomer left in the polymer particles in this kind of experiment. Previously, the effect of CO2 on the emulsion polymerization of MMA was investigated (75 °C, 1-350 bar),27 and a decrease in average molecular weight and a much steeper change in the logarithm of the number distribution at higher pressure were observed. According to the authors, the reason for this effect is the swelling of the polymer by CO2, which becomes more significant at higher CO2 pressures. This swelling results in a reduction of the internal viscosity and a delay of the gel effect. However, to draw these conclusions, the effect of scCO2 on the polymerization rate and the monomer concentration in the polymer particles should be known. Our results complicate the interpretation of these experiments, as the observed position of the MWD is mainly determined by experimental factors such as the contact time and the interfacial area at nonequilibrium conditions. This determines the extent of extraction and does not provide any information on the plasticizing effect. However, it can be expected from diffusion coefficient measurements of monomer in polymer particles8,28 that it is possible to perform electron beam experiments under conditions where monomer is still present in the particles at scCO2 conditions. Nevertheless, extraction will be the dominating mechanism for the scCO2 method for the reduction of residual monomer. Concluding Remarks The potential of a novel process for the removal of residual monomer from latex particles based on scCO2 has been studied. Typically, the method comprises a counterflow process in which part of the residual monomer is converted by the increased diffusion inside the polymer particles due to swelling by scCO2. Moreover, the amount of residual monomer is further reduced by the extraction capacity of scCO2. In this study, pulsed electron beam experiments were performed to study the effect of scCO2 on the polymerization reaction. In addition, the extraction capacity of CO2 was measured in a laboratory-scale high-pressure extraction unit. The results show that the CO2 extraction of MMA is the predominant effect as compared to the enhanced polymerization due to plasticization. Acknowledgment Part of this work was supported by a grant from the Foundation of Emulsion Polymerization (SEP) awarded
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to Mascha Cleven. The authors thank Earl Goetheer and Tjerk de Vries for their useful comments during the development of the extraction unit. Literature Cited (1) Gilbert, R. G. Emulsion Polymerization: A Mechanistic Approach; Academic Press: London, 1995. (2) Maxwell, I. A.; Verdurmen, E. M. F. J.; German, A. L. Highconversion emulsion polymerization. Makromol. Chem. 1992, 193, 2677. (3) Kendall, J. L.; Canelas, D. A.; Young, J. L.; DeSimone, J. M. Polymerizations in supercritical carbon dioxide. Chem. Rev. 1999, 543. (4) McHugh, M. A.; Krukonis, V. J. Supercritical Fluid Extraction; Butterworth-Heinemann: Woburn, MA, 1994. (5) Wissinger, R. G.; Paulaitis, M. E. Swelling and sorption in polymer-CO2 mixtures at elevated pressures. J. Polym. Sci. B 1987, 25, 2497. (6) Kazarian, S. G.; Brantly, N. H.; West, B. L.; Vincent, M. F.; Eckert, C. A. In situ spectroscopy of polymers subjected to supercritical CO2: Plasticizing and dye impregnation. Appl. Spectrosc. 1997, 51, 491. (7) Vincent, M. F.; Kazarian, S. G.; Eckert, C. A. Tunable diffusion of D2O in CO2 swollen poly(methyl methacrylate) films. AIChE J. 1997, 43, 1838. (8) Alsoy, S.; Duda, J. L. Supercritical devolization of polymers. AIChE J. 1998, 44, 582. (9) Banerjee, T.; Lipscomb, G. C. Direct measurement of the carbon dioxide induced glass transition depression in a family of substituted polycarbonates. J. Appl. Polym. Sci. 1998, 68, 1441. (10) Cooper, A. Polymer synthesis and processing using supercritical carbon dioxide. J. Mater. Chem. 2000, 10, 207. (11) DeSimone, J. M.; Guan, Z.; Elsbernd, C. S. Synthesis of fluoropolymers in supercritical carbon dioxide. Science 1992, 257, 945. (12) Kemmere, M. F.; de Vries, T. J.; Vorstman, M. A. G.; Keurentjes, J. T. F. A novel process for the catalytic polymerization of olefins in supercritical carbon dioxide. Chem. Eng. Sci. 2001, 56, 4197. (13) DeSimone, J. M.; Maury, E. E.; Menceloglu, Y. Z.; McClain, J. B.; Romack, T. J.; Combes, J. R. Dispersion polymerizations in supercritical carbon dioxide. Science 1994, 265, 356. (14) Mandel, F. S. Supercritical fluid assisted production of fine particles for life sciences and fine chemicals. In Proceedings of the 5th ISASF Meeting on Supercritical Fluids, Chemistry and Materials; ISASF: Nancy, France, 1998; Vol. T1, p 69. (15) West, B. L.; Kazarian, S. G.; Vincent, M. F.; Brantley, N. H.; Eckert, C. A. Supercritical fluid dyeing of PMMA films with
azo-dyes. J. Appl. Polym. Sci. 1998, 69, 911. (16) Yalpani, M. Supercritical fluids: Puissant media for the modification of polymers and biopolymers. Polymer 1993, 34, 1102. (17) Debenedetti, P. G.; Tom, J. W.; Kwauk, X.; Yeo, S. Rapid expansion of supercritical solutions (RESS): Fundamentals and Applications. Fluid Phase Equilib. 1993, 82, 311. (18) Goel, S. K.; Beckman, E. J. Generation of microcellular polymeric foams using supercritical carbon dioxide I. Effect of pressure and temperature on nucleation. Polym. Eng. Sci. 1994, 34, 1137. (19) Jiang, C.; Pan, Q.; Pan, Z. Solubility behavior of solids and liquids in compressed gases. J. Supercrit. Fluids 1998, 12, 1. (20) van Herk, A. M.; de Brouwer, H.; Manders, B. G.; Luthjens, L. H.; Hom, M. L.; Hummel, A. Pulsed electron beam polymerization of styrene in latex particles. Macromolecules 1996, 29, 1027. (21) Wishart, J. F.; van Eldik, R. High-pressure pulse radiolysis. Modification of an optical cell for 2-MeV electron pulse radiolysis at pressures up to 200 MPa. Rev. Sci. Instrum. 1992, 63, 3224. (22) Olaj, O. F.; Bitai, I.; Hinkelmann, F. The laser-initiated polymerization as a tool of evaluating (individual) kinetic constants of free-radical polymerization. 2a. The direct determination of the rate constant of chain propagation. Makromol. Chem. 1987, 188, 1689. (23) Kemmere, M. F.; van Schilt, M. A.; Jacobs, M. A.; Keurentjes, J. T. F. Measured and correlated partitioning coefficients of methyl methacrylate between water and supercritical dioxide., manuscript submitted. (24) Bush, D. Equations of State for Windows 95, version 1.01.14; Georgia Institute of Technology: Atlanta, GA, 1994. (25) Adrian, T.; Wendland, M.; Hasse, H.; Maurer, G. Highpressure multiphase behavior of ternary systems carbon dioxidewater-polar solvent: Review and modeling with the PengRobinson equation of state. J. Supercrit. Fluids 1998, 12, 185. (26) Kemmere, M. F.; Cleven, M. H. W.; van Schilt, M. A.; Keurentjes, J. T. F. Process design for the removal of residual monomer from latex products using supercritical carbon dioxide., manuscript submitted. (27) Quadir, M. A.; Snook, R.; Gilbert, R. G.; DeSimone, J. M. Emulsion polymerization in a hybrid carbon dioxide/aqueous medium. Macromolecules 1997, 30, 6015. (28) Chapman, B. R.; Gochanour, C. R.; Paulaitis, M. E. CO2enhanced diffusion of azobenzene in glassy polystyrene near the glass transition. Macromolecules 1996, 29, 5635.
Received for review November 26, 2001 Revised manuscript received March 18, 2002 Accepted March 18, 2002 IE0109408
