Anal. Chem. 1998, 70, 1943-1948
Factors Affecting High-Pressure Solvent Extraction (Accelerated Solvent Extraction) of Additives from Polymers Harold J. Vandenburg, Anthony A. Clifford,* Keith D. Bartle, and Shuang A. Zhu
School of Chemistry, University of Leeds, LS2 9JT, U.K. John Carroll, Ian D. Newton, and Louise M. Garden
Research and Technology Centre, ICI Technology, P.O. Box 90 Wilton, Middlesbrough, Cleveland TS90 8JE, U.K.
Irganox 1010 (pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)] propionate) is successfully extracted from polypropylene using solvents at high temperatures and pressures in a homemade accelerated solvent extraction system. For example, using freeze-ground polymer, 90% extraction is possible within 5 min with 2-propanol at 150 °C. Extraction curves for 2-propanol and acetone fit well to the “hot ball” model, previously developed for supercritical fluid extraction. Diffusion coefficients are determined for extractions with 2-propanol, acetone, and cyclohexane over a range of temperatures, and the activation energies for the diffusion are 134, 107, and 61 kJ mol-1, respectively. The lower figure for acetone and cyclohexane indicates that these solvents swell the polymer more than does 2-propanol. The polymer dissolves in the solvent at too high a temperature, which causes blockage of the transfer lines. For maximum extraction rates, the highest temperature for each solvent that avoids dissolution of the polymer should be used. The use of mixed solvents is investigated and shows advantages in some cases, with the aim of producing a solvent that will swell the polymer but not dissolve it. The additive content of polymers needs to be known for quality and regulatory reasons. The additive is usually extracted from the polymer before analysis. Traditional methods such as Soxhlet extraction, boiling under reflux, and dissolution followed by reprecipitation of the polymer are often very time-consuming. Soxhlet extraction, for example, can take as long as 48 h.1,2 Dissolution can give rapid extraction,3 but without high-pressure, nonvolatile high boiling solvents such as Decalin must be used to dissolve polymers such as polyolefins. These solvents cannot be easily removed, which makes reversed-phase liquid chromatography difficult. Subsequent cleanup of the solution by removal of “waxes” has also been considered too time-consuming.4 There are several new techniques of extraction that have been widely applied to environmental samples which reduce both extraction (1) Haney, M. A.; Dark, W. A. J. Chromatogr. Sci. 1980, 18, 655-659. (2) Majors, R. E. J. Chromatogr. Sci. 1970, 8, 339-345. (3) Scabron, J. F.; Fenska, L. E. Anal. Chem. 1980, 52, 1411-1415. (4) Spell, H. L.; Eddy, R. D. Anal. Chem. 1960, 32, 1811-1814. S0003-2700(97)01090-1 CCC: $15.00 Published on Web 03/24/1998
© 1998 American Chemical Society
time and solvent usage. These are supercritical fluid extraction, microwave-assisted extraction (MAE), and accelerated solvent extraction (ASE), which have been described in recent articles.5-8 (ASE is a trademark of the Dionex Corp. but will be used here, by permission, to refer to high-pressure solvent extraction, using any experimental system.) These techniques can all be used above atmospheric pressure and could therefore be called highpressure extraction methods. Of these techniques, supercritical fluid extraction (SFE) has been most widely applied to polymers, resulting in rapid extractions, summarized in a recent review.9 MODELS OF EXTRACTION Various models have been published, which aim at understanding the kinetics of the SFE processes. Bartle et al.10 proposed the “hot ball” model to describe the processes occurring during SFE for diffusion-limited extractions from spherical particles. In the model, the ratio of mass remaining (mt) in the particle of radius r at time t to the initial amount (m0) is given by ∞
mt/m0 ) (6/π2)
∑(1/n ) exp(-n π Dt/r ) 2
2 2
2
(1)
n)1
where n is an integer and D is the diffusion coefficient. The origin and underlying assumptions of this model have been fully described previously.10 The right-hand side of the equation comprises a series of diminishing exponential terms. When t is small, all terms of the right-hand side significantly contribute to the summation. However, if t is sufficiently large, the first term becomes dominant. Therefore, when ln(mt/m0) is plotted against time, the line falls steeply initially, becoming linear eventually. (5) Majors, R. E. LC-GC 1995, 8, 128-133. (6) Majors, R. E. LC-GC Int. 1996, 14, 638-648. (7) Renoe, B. W. Am. Lab. 1994, (Aug), 34-40. (8) Richter, B. E.; Jones, B. A.; Ezzell, J. L.; Porter, N. L.; Avdalovic, N.; Pohl, C. Anal. Chem. 1996, 68, 1033-1039. (9) Vandenburg, H. J.; Clifford, A. A.; Bartle, K. D.; Carroll, J.; Newton, I. D.; Garden, L. M.; Dean, J. R.; Costley, C. T. Analyst 1997, 123, 101R-115R. (10) Bartle; K. D.; Clifford, A. A.; Hawthorne, S. B.; Lagenfield, J. J.; Miller, D. J.; Robinson, R. J. Supercrit. Fluids 1990, 3, 143.
Analytical Chemistry, Vol. 70, No. 9, May 1, 1998 1943
The equation of the linear portion of the logarithmic plot is given by11
ln(mt/m0) ) -0.4977 - (π2Dt/r2)
(2)
The physical explanation of the shape of the curve is that the additive near the surface is rapidly extracted until a smooth falling concentration gradient is established accross the particles. The extraction rate is then completely controlled by the rate at which the additive diffuses to the surface. During SFE, solubility limitations have been observed, particularly at high temperatures and at the start of extractions where the concentration of additive in the fluid is high. The model has been extended to account for solubility and flow rate effects.12,13 As the extraction progresses and concentration of additive in the fluid falls, the rate-limiting step in most cases is the diffusion of the additive to the surface of the polymer. For this reason it is usually necessary to grind the polymer before extraction to provide a high surface area and a short distance for the analyte to diffuse. These models are successful for describing SFE of polymers but have not been applied to liquid extraction. High-Pressure Liquid/Solid Extractions. There is little reported work on extractions with liquid/solid extractions with solvents at high pressure. MAE has been used successfully to extract oligomers from poly(ethylene terephthalate) (PET).14 Using dicloromethane as solvent, 120 °C was the highest temperature that could be used before the polymer fused. Additives from polyolefins have also been successfuly extracted using microwave extraction.15,16 Lou et al.17 extracted monomers and oligomers from nylon and poly(1,4-butylene terephthalate) (PBT) using hexane as the extraction solvent in a homemade ASE. They investigated the effect of temperature, pressure, and flow rate on the extraction. Pressure was found to have no effect other than to keep the solvents liquid at high temperature and flow rate had little effect between 0.4 and 2 mL min-1. Extraction efficiencies increased in all cases as temperature was raised from 50 to 170 °C, which was attributed to faster diffusion rates. The authors observed that solvents that are good swelling agents, and hence give fastest extractions during Soxhlet extraction, tend to dissolve the polymer at the high temperatures used during ASE. This was exploited by Macko et al.,18,19 who used high pressures to dissolve polyethylene in heptane at 160 °C. The polymer is not soluble in heptane at room temperature and therefore precipitates on cooling. The hexane can be readily removed by evaporation and the sample redissolved in reversed-phase liquid chromatography solvents. However, for ASE, dissolved polymer reprecipitating on cooling (11) Cotton, N. J.; Bartle, K. D.; Clifford, A. A.; Dowle, C. J. J. Appl. Polym. Sci. 1993, 48, 1607-1619. (12) Bartle, K. D.; Boddington, T.; Clifford, A. A.; Hawthorne, S. B. J. Supercrit. Fluids 1992, 5, 207-212. (13) Clifford, A. A.; Bartle, K. D.; Zhu, S. A. Anal. Proc. 1995, 32, 227-230. (14) Costley, C. T.; Dean, J. R.; Newton, I. D.; Carroll, J. Anal. Commun. 1997, 34, 89-91. (15) Freitag, W.; John, O. Angew. Makromol. Chem. 1990, 175, 181-185. (16) Nielson, R. C. J. Liq. Chromatogr. 1991, 14, 503-519. (17) Lou, X.; Janssen, H.; Cramers, C. A. Anal. Chem. 1997, 69, 1598-1603. (18) Macko, T.; Siegl, R.; Lederer, K. Angew. Makromol. Chem. 1995, 227, 179191. (19) Macko, T.; Furtner, B.; Lederer, K. J. Appl. Polym. Sci. 1996, 62, 22012207.
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can block transfer lines in the instrument. Melting or softening of the polymer causes the particles to coalesce, reduces the surface area, and hence slows down the extraction. Solvents therefore cannot be selected for ASE on the basis of those used for atmospheric pressure extractions. Lou et al.17 used hexane in the extractions from nylon and PBT, even though it gives poor recoveries during Soxhlet extraction. They pointed out that selection of a suitable extraction solvent is probably the most difficult step in optimizing ASE, as there is little data on the solubility of polymers in solvents at high temperatures. Solubility of Polymers. The solubility of polymers in solvents can be broadly predicted by the use of solubility parameters. The Hildebrand parameter, δ, is defined as the square root of the internal energy of vaporization divided by the molar volume (cohesive energy density). Broadly speaking, the closer the solubility parameters, the more polymer will dissolve in a solvent. There are published data for the empirically determined solubility parameter range over which polymers will dissolve or swell.20 However, these are generally determined at or near room temperature and certainly not above the boiling points of the solvents. As the temperature rises, the miscibility range of the polymer will increase. Flory and Huggins derived an interaction parameter, χ, whereby polymer and solvent will be completely miscible if
χ e 0.5(1 + 1/(m1/2))2
(3)
where m is the ratio of molar volume of polymer to that of the solvent. For polymers of large molecular weight, m . 1 and thus the critical Flory parameter χc . 0.5. Since χ is a Gibbs energy parameter, it consists of an entropy and an enthalpy term:
χ ) χS + χ H
(4)
The Flory-Huggins interaction parameter can be combined with the solubility parameter to estimate the maximum solubility parameter difference over which the polymer and solvent will be miscible:21
(δ1 - δ2)2 ) [0.5(1 + 1/(m1/2)2) - χS]RT/V2
(5)
where 1 and 2 refer to polymer and solvent, respectively, and V is the molar volume. Thus the solubility parameter range increases with rising temperature, falling solvent molar volume, and falling χS. The relationship works best for nonpolar, nonhydrogen-bonded systems. This paper describes the extraction of Irganox 1010 from polypropylene using a dynamic extraction technique. A variety of solvents and temperatures are used in order to determine the optimum extraction conditions. The relationship between the solubility parameter and extraction rate and the method of cell packing are examined. The “hot ball” model derived for SFE is applied to ASE. (20) Barton, A. F. M. Handbook of Polymer-Liquid Interaction Parameters and Solubility Parameters; CRC Press: Boca Raton, FL, 1990. (21) Kumar, R.; Prausnitz, J. M., Solvents in Chemical Technology. In Solutions and Solubilities; Dack, M. R., Ed.; John Wiley and Sons: New York, 1975.
Figure 1. Schematic of high-pressure solvent extraction apparatus. Table 1. Particle Size Distribution of Freeze-Ground Polymer size (µm)
% of polymer
size (µm)
% of polymer
1000 500 250
29.2 47.3 16.7
125 63 38
6.2 0.6 0.0
EXPERIMENTAL SECTION Apparatus. A schematic of the apparatus used is shown in Figure 1. The pump used was an Isco 100D (Jones Chromatography, Hengoed, U.K.). Extraction cells were SFE cells, supplied by Keystone Scientific. HPLC analysis was performed using a Merck-Hitachi pump with a Jasco 875-UV and a Merck Hitachi D2500 integrator. Separation was on an ODS2 column (25 × 4.6 mm) (Phase Separations Ltd., Deeside, U.K.). Materials and Reagents. Polypropylene (PP) was obtained as pellets (approximately 3 mm diameter, determined by measuring with a micrometer) from ICI plc (Wilton, Middlesbrough, U.K.) with a nominal Irganox 1010 content of 0.15%. Irganox 1010 and Irganox 1330 (1,3,5-tris(3,5-di-tert-butyl-4-hydroxybenzyl)-2,4,6trimethylbenzene) were supplied by Ciba-Geigy. Solvents were analytical grade or HPLC grade. The internal standard solution was prepared by dissolving Irganox 1330 (0.05 g) in methanol (50 mL) Freeze Grinding. Pellets of PP were freeze-ground under liquid nitrogen. A sample of the ground polymer was sieved; the particle size distribution of the ground polymer is given in Table 1. For experiments with sieved polymer, the fraction of particle size 500-1000 µm was used. Cell Packing. Samples of PP (0.2 g) were weighed into an extraction cell (3.75 mL) which was connected to the pump with Valco 1/16-in. stainless steel nuts and ferrules. Samples were either pellet, freeze-ground, or sieved ground material (500-1000 µm). Cells were packed by placing the ground polymer between glass wool plugs, sometimes mixing with sand before packing. In the latter case, it was important to fill the cell completely to prevent the polymer from separating from the sand during extractions. Dynamic Extraction. The loaded cell was preheated in the oven for 2 min before introduction of the solvent. Solvent was then pumped into the cell until the pressure was stable (2000 psi). The timing was started and valve 2 opened to allow solvent to pass into the collecting vessel at a flow rate of 1.5-2.5 mL min-1. The collecting vessel was changed at intervals. During extractions
Figure 2. Extraction of Irganox 1010 from PP using 2-propanol, acetone, acetonitrile at 120 °C and chloroform at 62 °C: ([) 2-propanol, (4) acetone, (0) acetonitrile, and (×) chloroform.
using cyclohexane, the cell frit tended to block if the cell was preheated but remained clear if the solvent was pumped through the sample before heating. Therefore, cyclohexane extractions were carried out without preheating the cell. Chromatographic Analysis. Internal standard solution (20100 µL) was added by microsyringe to the extracts. Cloudy solutions were centrifuged for 5 min and a subsample (100-300 µL) was transferred to a 2-mL vial and evaporated to dryness under a stream of nitrogen. The residue was redissolved in methanol (120 µL) and injected on to the HPLC column using a 100-µL sample loop. The mobile phase was methanol at a flow rate of 1 mL min-1. Detection was at 254 nm. A calibration curve was constructed by adding Irganox 1010 to the internal standard solution at concentrations covering the range found in the polymer extracts. The peak area ratio Irganox 1010/Irganox 1330 was plotted against the weight ratio. The resulting line was linear with a correlation coefficient of 0.9998. Recovery was checked by spiking Irganox 1010 onto glass wool in the extraction cell and extracting using chloroform at 70 °C. Recovery was 96% within 5 min and 102% within 10 min. RESULTS AND DISCUSSION Cell Packing. The ground polymer was mixed well to ensure homogeneity. This reduces the chances of obtaining a nonrepresentative sample. Mixing the ground polymer with sand had little effect when conditions did not lead to softening or swelling of the polymer (i.e., at low temperatures and with “poor” solvents) However, under swelling conditions (at higher temperatures and with stronger solvents), faster extractions were obtained when the polymer was dispersed in sand. If no sand was used, the particles partially coalesced and extraction rates were reduced. Effect of Different Solvents. Freeze-ground PP was extracted at 120 °C with 2-propanol, acetone, and acetonitrile and at 62 °C with chloroform. Chloroform could not be used at 120 °C as the polymer dissolved. Figure 2 shows the extraction curves for each extraction. The solubility parameters are given in Table 2. The greater the solubility parameter difference between solvent and PP, the slower the extraction. However, the relationship is not linear, with a modest change in δ from 2-propanol to acetonitrile resulting in a very large change in extraction rate. The hydrogenbonding character of 2-propanol is strong, and that of acetonitrile is poor, which may be a factor in the larger than expected Analytical Chemistry, Vol. 70, No. 9, May 1, 1998
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Table 2. Hildebrand Solubility Parameters (25 °C)
a
substance
solub param (MPa1/2)
cyclohexane24 PP20 chloroform24 acetone24 2-propanol24
16.8 18.8 19.0 20.3 23.5
substance
solub param (MPa1/2)
acetonitrile24 10% cyclohexane in 2-propanola 15% cyclohexane in 2-propanola 20% cyclohexane in 2-propanola
24.3 22.9 22.6 22.3
Calculated by using volumewise proportions.
Figure 3. Extraction of Irganox 1010 from PP with 2-propanol at temperatures from 120 to 150 °C: ([) 120, (4) 130, (0) 140, and (+) 150 °C; solid line, curve fitted from hot ball model.
Figure 4. Extraction of Irganox 1010 from PP using acetone at temperatures from 110 to 140 °C: ([) 110, (4) 120, (0) 130, and (+) 140 °C; solid line, curve fitted from hot ball model.
difference in extraction rates. However, as PP is essentially nonpolar and non-hydrogen-bonding, it is difficult to see why a more strongly hydrogen-bonding solvent should produce faster extractions. In extraction from Nylon 6 (δ ) 27-32 MPa1/2), Lou et al.17 used hexane (δ ) 14.9 MPa1/2) as extraction solvent, as methanol (δ ) 29.7 MPa1/2) caused the nylon to dissolve. As the solubility parameters of nylon and methanol are much closer, methanol would be expected to dissolve the nylon at a much lower temperature than hexane. However, the authors did not use solvents of intermediate solubility parameter. Costley et al. found that PET (δ ) 19.9-22 MPa1/2) dissolved in dichloromethane at temperatures above 120 °C. The behavior of PET is not well described by a single solubility parameter, but better in term of “bimodal” properties, the aromatic residue (δ ) 20.5 MPa1/2), and the aliphatic ester residue (δ ) 25 MPa1/2).20 The solubility parameter of dichloromethane is quite close to those of PET at 19.9 MPa1/2 and would therefore be expected to dissolve the polymer at moderate temperatures. The solubility parameter may therefore act as a useful guide to the extraction rate and maximum temperature, but accurate predictions are not possible with current knowledge. Effect of Temperature. Extraction curves for sieved, ground PP with 2-propanol from 120 to 150 °C and with acetone from 110 to 140 °C are shown in Figure 3 and Figure 4. The temperature ranges were selected such that the extractions were complete within a reasonable time at the lowest temperatures, and the polymer did not dissolve at the highest temperature. The curves shown were obtained by using the “hot ball” model, i.e., by assuming the solubility of the Irganox 1010 is infinite and therefore the extraction is limited only by the rate of diffusion in the polymer. Although not likely to be infinite in reality, the solubility of Irganox 1010 in the solvents at the temperatures used
in the extractions is likely to be high enough that solubility will not limit the extractions, and therefore, diffusion-limited extraction is a reasonable assumption. The extraction rate increases with temperature and at 150 °C is 90% complete within 5 min with 2-propanol and within 6 min at 140 °C with acetone. The fit of the model is generally good, but there is evidence that the extraction is slower than predicted from the model at longer times. This apparent slowing down of the extraction may be due to the solvent not penetrating to the center of the particles or possibly an artifact of the flow through the extraction cells. The extracts at the higher temperatures were cloudy, presumably due to dissolved lower oligomers which precipitated out when the solvent cooled. Diffusion Coefficients and Activation Energies. The diffusion coefficients derived from the equations of the curves are shown in Table 3, assuming a particle size of 750 µm (the middle of the particle size range used). Cyclohexane extractions were started cold and then heated in order to prevent the cell frit blocking. It is therefore not possible to fit the extraction curve using the hot ball model as the temperature is not initially constant. However, the diffusion coefficient can be estimated from the linear portion of the ln(m/m0) plot, after a constant temperature has been reached. From eq 2 it can be seen that the slope is equal to (π2D/r2). As this method uses only data on the linear part of the plot, the errors will be higher than by fitting all the data to the model. The diffusion coefficients calculated for cyclohexane extractions are shown in Table 3. The extraction is expected to be diffusion-limited and therefore should follow the Arrhenius form, rate ) constant × exp(-E/RT), where E is the activation energy for the process, in this case diffusion of the Irganox 1010 through the polymer. The Arrhenius plots for cyclohexane, acetone, and
1946 Analytical Chemistry, Vol. 70, No. 9, May 1, 1998
Table 3. Diffusion Coefficients for Irganox 1010 in PP diffusion coeff (10-6 cm2 s-1) temp (°C)
2-propanol
acetone
cyclohexane
50 60 70 80 110 120 130 140 150
a a a a a 0.039 0.10 0.34 0.87
a a a a 0.048 0.12 0.27 0.58 a
0.052 0.095 0.23 0.31 a a a a a
a
Not determined.
Figure 5. Arrhenius plot (ln(D) vs 1/T) for extraction of Irganox 1010 from PP with 2-propanol, acetone, and cyclohexane: (4) 2-propanol, (0) acetone, and ([) cyclohexane. Table 4. Activation Energy for Diffusion Through PP Additive
MW
activ energy (kJ mol-1)
ref
phenothiazine BHT 2-hydroxy-4-methoxybenzophenone n-octadecane 2-hydroxy-4-octyloxybenzophenone 2-hydroxy-4-dodecyloxybenzophenone bis(ethylhexyl) phthalate DLTPa Phenol Ab Irganox 1330c
199 220 228 254 326 382 391 514 538 774
141 97 80 86 101 114 105 83 94 117
22 22 22 22 22 22 22 22 22 25
a Didodecyl thiodipropopionate. b 1,1,3-Tris[2-methyl-4-hydroxy-5(tert-butylphenyl)]butane. c 1,3,5-Tris[3,5-di-tert-butyl-4-(hydroxybenzyl)]mesitylene.
2-propanol are shown in Figure 5. From the slope, the activation energy for the diffusion can be calculated to be 134 kJ mol-1 for 2-propanol, 112 kJ mol-1 for acetone, and 61kJ mol-1 for cyclohexane. Previously published data for activation energy of several additives diffusing through PP are shown in Table 4. The energies range from 83 to 141 kJ mol-1. Jackson et al.22 reported that, for rubbers and polyethylene, the activation energy for diffusion is independent of the additive, and the energy required for diffusion is used in overcoming polymer-polymer interactions. However, for PP, the additive does have a marked effect on the activation (22) Jackson, R. A.; Oldland, S. R. D.; Pajaczkowski, A. J. Appl. Polym. Sci. 1968, 12, 1297-1309.
energy. There is no simple correlation between molecular weight and activation energy, but within homologous series E increases with molecular weight. Moller and Gevert23 found that E increased with molecular weight of five substituted hindered phenols diffusing through low-density polyethylene. Therefore the figure of 134 kJ mol-1 is about what would be expected compared to the figure of 117 kJ mol-1 for Irganox 1330. However, E using acetone and cyclohexane as solvents are lower than would be expected. The different activation energies obtained using different solvents indicates that the process being measured is not simply diffusion through PP but diffusion through the polymer swelled by the solvent. Acetone and cyclohexane interact more strongly with PP than does 2-propanol and, therefore, swell the polymer more at lower temperatures. The swelling of the polymer will increase with temperature for all solvents. However, with 2-propanol, the polymer is less swollen at lower temperatures, and the amount of swelling can increase more as the temperature is raised. Therefore, the swelling with cyclohexane and acetone is less dependent on temperature than the swelling with 2-propanol, and the activation energy is lower for cyclohexane and acetone. The higher activation energy for the “weaker” solvent means that there is a greater rise in diffusion rate for a given temperature rise. At 110 °C the diffusion coefficient using acetone is 3.4 times that using 2-propanol whereas at 150 °C the ratio is only 1.9. Therefore, extraction at a high temperature is more important for “weaker” solvents. The extraction rate increases at higher temperatures because the diffusion coefficient increases with temperature, but also because the solvent interacts more strongly with the polymer, causing greater swelling. To achieve maximum extraction rates with a given solvent, the extraction should be carried out at a temperature that causes the maximum swelling without dissolving the polymer. This temperature will be lower for a “stronger” solvent (i.e., a solvent with a solubility parameter closer to the polymer). The upper temperature limit for an extraction may be set by the stability of the analyte or the melting point of the polymer. However, the best strategy for fast extractions is to select a solvent that can be used at as high a temperature as possible, that temperature being just below that at which the polymer dissolves in that solvent. Mixed Solvents. Pure solvents exist with discrete properties, and the “best” properties may not be available from a single substance. However, by mixing solvents, properties between those of the pure solvents are possible. The solubility parameter of a binary mixture is approximately volumewise proportional to the parameters of its components.24 We have examined the effect of mixing cyclohexane with 2-propanol on the rate of extraction from PP. Cyclohexane was added to 2-propanol to give a range from 10 to 20% cyclohexane. Freeze-ground PP of particle size between 500 and 1000 µm was extracted using these mixtures at 120 °C. Increasing cyclohexane proportion above 20% resulted in extensive polymer dissolution and blocking of the cell frit. Plots of ln(m/m0) vs time are shown in Figure 6. The extraction rate increases with increasing cyclohexane content, as would be (23) Moller, K.; Gevert, T. J. Appl. Polym. Sci. 1994, 51, 895-903. (24) Barton, A. F. M. Handbook of Solubility Parameters and Other Cohesion Parameters; CRC Press: Boca Raton, FL, 1983. (25) Schwarz, T.; Steiner, G.; Koppelmann, J. J. Appl. Polym. Sci. 1989, 37, 33353341.
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solvent was available with the correct properties to extract rapidly at that temperature. The initial aim of this part of the study was to find a mixture of a “poor” solvent with a small quantity of a “good” solvent such that the good solvent would preferentially be absorbed into the polymer. The latter component would then swell the polymer, resulting in fast extractions, but be insufficient to cause problems by dissolving large amounts of the polymer. The discussion above shows that this aim was achieved to a limited extent.
Figure 6. Extraction of Irganox 1010 from PP with 2-propanol/ cyclohexane mixtures and acetone: ln(m/m0) against time. (×) 20% cyclohexane, (4) 15% cyclohexane, (0) 10% cyclohexane, ([) 2-propanol, and (0) acetone.
expected. The calculated solubility parameters of the mixed solvents are shown in Table 2. The extraction rate is much faster for the mixtures than would be expected by comparison with the solubility parameter of acetone. The rate for 15% cyclohexane in 2-propanol is similar to that for acetone, but the calculated solubility parameter for the mixture is much higher than that for acetone. Addition of cyclohexane results in a small shift in solubility parameter but a large shift in extraction rate. This may be because the cyclohexane is selectively absorbed into the plastic, causing significant swelling, even at low concentrations. The ability to “fine-tune” the solvent by adding a small amount of another solvent would allow the optimization of an extraction at any temperature. This would be particularly useful if the analyte were thermally labile at fairly low temperatures and no suitable
1948 Analytical Chemistry, Vol. 70, No. 9, May 1, 1998
CONCLUSIONS The use of high temperatures above the boiling point of a solvent results in extraction rates many times faster than possible at atmospheric pressure. The kinetics appear to fit the “hot ball” model reasonably well. Maximum extraction rates are reached if (a) the temperature is as high as possible and (b) the temperature is just below that at which extensive dissolution of the polymer in the solvent occurs. The solvent should therefore be selected such that it dissolves the polymer below the melting point of the polymer or the temperature at which the analyte breaks down. If no pure solvent is available that meets these conditions, a mixed solvent can be prepared by addition of small quantities of a strong solvent to a weaker solvent. Solubility parameters can be a useful guide to solvent selection, but at present, precise quantitative relationships between solvent properties and extraction rates is not yet possible. Received for review October 2, 1997. Accepted January 28, 1998. AC9710902
