Ind. Eng. Chem. Res. 2003, 42, 6431-6456
6431
A Review of CO2 Applications in the Processing of Polymers David L. Tomasko,* Hongbo Li, Dehua Liu, Xiangmin Han, Maxwell J. Wingert, L. James Lee, and Kurt W. Koelling Department of Chemical Engineering, The Ohio State University, 140 W. 19th Avenue, Columbus, Ohio 43210
The use of supercritical carbon dioxide as a processing solvent for the physical processing of polymeric materials is reviewed. Fundamental properties of CO2/polymer systems are discussed with an emphasis on available data and measurement techniques, the development of theory or models for a particular property, and an evaluation of the current state of understanding for that property. Applications such as impregnation, particle formation, foaming, blending, and injection molding are described in detail including practical operating information for selected topics. The review concludes with some forward-looking discussion on the future of CO2 in polymer processing. Introduction Supercritical fluid technology has made tremendous strides in the past decade in terms of commercial application and fundamental understanding of solution behavior. Much of the success can be attributed to the hard work of clever chemists and engineers recognizing opportunities to use these tunable solvents to solve a particular problem or fill a processing niche. We are happy to dedicate this review to one of the cleverest chemist/engineers, Chuck Eckert, whose creativity has been an inspiration to the field. Another significant contribution has come from the continuing environmental pressures on industry to move away from volatile organic compounds (VOCs) and ozone-depleting substances (ODS) as processing solvents. However, using supercritical fluids and supercritical carbon dioxide in particular cannot always be economically justified based solely on replacing environmentally harmful solvents. There must be an additional process advantage arising from simplified reaction/separation schemes, lower energy requirements, improved product quality, or some combination of these. While all of these characteristics are often touted as “potential” advantages of a supercritical fluid process, there are hurdles remaining to instill it as a viable choice in process design. The existence of several small and growing companies (Phasex, Thar Technologies, Micell, Trexel, and Novasep) confirms that the recent successes are starting to carve out a market segment. Specific examples of recent successes are the new DuPont facility for producing fluoropolymers in a supercritical carbon dioxide-based solvent. Dry cleaning technology based on liquid CO2 is competing in the textile market with both Washpoint (ICI/Linde) and Micare (Cool Clean Technologies) technologies, representing viable alternatives to chlorinated solvents.1 And there are several developments underway to commercialize cleaning technologies in the microelectronics industry. Many new developments have arisen out of the use of CO2 as a novel reaction solvent. Novel construction of CO2-phillic catalysts and surfactants have allowed both traditional and new reaction pathways to be * To whom correspondence should be addressed. Tel.: 614292-4249. Fax: 614-292-3769. E-mail:
[email protected].
explored. Studies have shown that CO2 as a solvent can offer a “green” alternative for carrying out many types of chemistry often with significant process improvements in selectivity, conversion, or rates. In fact, the development of novel polymers and polymeric surfactants for use with CO2 has resulted in two Presidential Green Chemistry Awards. Much of this work was recently reviewed.2-5 In this work we focus on nonreactive processes with carbon dioxide. Like reaction systems, most processing using supercritical or compressed CO2 as a solvent is carried out in a fluid phase. Extraction is the most advanced application and there exists a tremendous amount of literature on extraction using supercritical fluids. It is fair to say that the design of a high-pressure extraction process is now facile, given carefully planned bench-scale results. The food and pharmaceutical industries use this technology more often than others since the nontoxic nature of CO2 provides a strong impetus. Other applications being carried out in a fluid phase include several of the particle generation technologies wherein material is dissolved in CO2 or an organic solvent and precipitated from that solution via a pressure or solvent composition change. It is notable that a variety of morphologies are attainable over a wide size range. Particle generation is certainly an area of intense inquiry and is receiving attention primarily from the pharmaceutical and biotechnology industries, although there are significant polymer and inorganic material applications of these techniques. In the applications addressed so far, the common feature has been a fluid phase solvent in which compressed CO2 is a major (if not the predominant) component. It is in these applications that one may take advantage of proximity to critical points and the divergent thermodynamic properties in those regions. There is also another range of composition that is an active area of research and this arises from the dissolution of compressed CO2 into a condensed phase. One example is the injection of CO2 into a solute-laden organic solvent to precipitate the solute, although this technique often still requires CO2 compositions upward of 50 mol %. When the condensed phase is a polymer, the dissolution of CO2 is almost always less than 30 wt % and more typically less than 10 wt %. The system is not near any mixture critical points so the divergent thermodynamic
10.1021/ie030199z CCC: $25.00 © 2003 American Chemical Society Published on Web 09/06/2003
6432 Ind. Eng. Chem. Res., Vol. 42, No. 25, 2003
assisted extrusion to provide a balanced viewpoint of the technology. Fundamentals
Figure 1. Topics covered in this review.
properties are not an issue. The proximity to the critical point of CO2 impacts primarily the solubility. However, the addition of small amounts of compressed gases to polymer phases results in substantial and sometimes dramatic changes in the physical properties that dictate processing. These include viscosity, permeability, interfacial tension, and glass transition temperature. By understanding the effects of CO2 on these properties and developing techniques for incorporating CO2 in continuous processes, a wide range of opportunities open up for impacting the plastics industry. The products range from foam board insulation and high-impact polymer blends to surface-modified biomedical implants and biological micro-electromechanical system (bio-MEMs) devices. This review will cover the fundamental properties of CO2-polymer systems, describe a wide range of applications under development, and finish with some comments on the future outlook for CO2 processing of polymers. The section on fundamental properties is divided into phase equilibrium (solubility, Tg and Tm reduction, and interfacial tension), transport phenomena (mass and heat transfer), rheology, and dynamic processes (crystallization and nucleation). The applications are discussed in an approximate decreasing order of maturity. Since extraction and fractionation are welldeveloped, our discussion begins with impregnation followed by particle formation, foaming, blending, and injection molding. Within the applications, methods for continuous extrusion with CO2 are discussed in some detail. A schematic illustration of the review structure is shown in Figure 1. In general, we attempt to not only assess the literature data but also provide some sense of how welldeveloped the models and theories are for each particular topic. Where possible, we point out any semipredictive methods or rules of thumb for practitioners. The review also includes several developed and potential applications of CO2 in polymer processing. Some practical operating information is discussed with respect to CO2-
(A) Phase Equilibrium. The phase behavior of compressed gases and polymers is rich and complex. It has been the research subject of thermodynamicists and polymer scientists for many years and has formed the basis of several stand-alone reviews.6-12 For studying pure thermodynamics and polymer physics, CO2 is not the molecule of choice due to its unusually low cohesive energy density and high quadrupole moment that add ambiguity to interpretations of results. Most often, a monomer (e.g., ethylene) or other simple molecule (alkane) is chosen as the compressed gas to provide more direct interpretations of polymer-gas and gas-gas interactions. While the gas-rich region of the phase diagram is often devoid of polymer, the polymer-rich region may contain upward of 10 wt % gas at typical processing pressures ( 1-10, depending on the viscosity ratio). When the capillary number is below 0.1 or so, coalescence will dominate. In the region 0.1 < Ca < 10, both the breakup and coalescence will take place simultaneously.84 The pendant drop is the most commonly used method to measure the interfacial tension for polymer melts and is the most promising for simultaneous high-pressure and high-temperature application. In a study of highviscocity liquid-crystalline polymers (LCPS),85 the pendant drop was demonstrated to be the only suitable method. De Marquette and Kamal studied the transient behavior of interfacial tension using this method by following the exponential decay curve and thus greatly reducing the experimental time and the risk of polymer degradation.86 The pendant drop method has been applied to SC-CO2 systems for the interfacial tension, σ, between oligomers and CO2. While γ is often used for interfacial or surface tension, we use σ in this review to distinguish these properties from shear rate in polymer rheology discussed below. Harrison et al. measured interfacial tensions for PEG(Mw ) 600)/ CO2 87 at 45 °C up to 300 bar wherein σ decreases from 40.4 dyn/cm at 1 bar to 3.1 dyn/cm at 300 bar, and PS(Mw ) 1850)/CO2 88 at 45 °C up to 310 bar where σ decreases from 37.4 dyn/cm at 1 bar to 1.5 dyn/cm at 310 bar. Although it would be very useful for polymer blending, no literature has yet been reported for the effect of CO2 on the interfacial tension between immiscible polymers. Theories and advances in predicting interfacial tension of polymer melts were recently reviewed.89 Two primary theoretical approaches have been developed to predict the interfacial tension between polymer melts. Helfand and Tagami90-92 formulated a statistical mechanical theory of the interface between immiscible polymers for symmetric systems, which is based on selfconsistent field theory. The theory has since been extended to nonsymmetric polymer systems.93 The other common approach is based on square gradient theories94-96 combined with the Flory-Huggins expression for the free energy density, or with equations of state, such as the Flory-Orwoll-Vrij model (FOV)97 or the Sanchez and Lacombe lattice gas model (LF).59,60,98,99 The latter have had considerable success modeling compressible systems.100-103 Although conceptually different, the results by Poser and Sanchez103 give comparable predictions to those of Hefand and Sapse.93 Sanchez has shown that the gradient theory is “in harmony with the microscopic theory of Helfand and co-workers, although the latter treats the polymer interfaces from a different point of view”.90-93,104
(B) Transport Phenomena. (1) Mass Transfer in Polymer/Supercritical CO2 Systems. All industrial applications of supercritical CO2 such as foaming,61,105 extrusion,106 and impregnation48,107,108 require an accurate understanding of mass transfer of carbon dioxide and additives in a polymer matrix to optimize the process. Therefore, in this section, kinetic studies of supercritical CO2 transport are reviewed along with some comments regarding heat transfer. Experimental techniques and models are discussed first, and then transport phenomena in CO2/solid polymer, CO2/molten polymer, and CO2/additive/solid polymer systems are addressed individually. (a) Experimental Techniques for Binary Systems (Polymer/CO2). Diffusivity or mass transfer in polymer phases can be obtained using several of the same techniques for solubility by following the dynamic approach to equilibrium. This has been accomplished with barometric methods61,106 and gravimetric methods.29,38,43,48,109 Other techniques that are suited to transient measurements but not quantitative solubility measurements include the following. (i) Optical Observation. With optical recording of the swelling behavior of polymer melts under high pressure with a CCD camera and then correlation of the degree of swelling to mass uptake, the diffusion coefficient is obtained.105,110 The method features a highpressure view cell which constrains the diffusion of CO2 and polymer swelling to one dimension. This technique is useful in monitoring the liquid level for one-dimensional swelling of polymer melt, but does not work well for solid polymers because of the inaccurate real time estimation of volume change. (ii) Spectroscopic Technique. With measurement of the transient absorbance data in the near-IR region, the relative concentration of CO2 in the polymer as a function of time could be determined according to the Beer-Lambert law.25,33 (b) Experimental Techniques for Ternary Systems (Polymer/CO2/Additive). Some techniques follow similar procedures as for binary systems with slight modifications, such as gravimetric measurement111 and spectroscopic technique by UV-vis108 and FTIR.112 The techniques below are specifically used in determining transport properties of additives in a supercritical atmosphere. (i) Film Roll Method. In this technique, a roll with multiple layers of polymer film is impregnated with dyes in supercritical CO2. The films are unrolled after dyeing and the dye concentration in each layer is determined by UV-vis. With construction of the relationship between concentration and distance, the diffusivity of the dye is calculated.107 (ii) Forced Rayleigh Scattering (FRS). This method is significant because it directly determines diffusivity of additives in a polymer matrix without reduction from absorption data as in other techniques. FRS detects the rate of relaxation of a “forced” gradient in concentration of cis and trans azobenzene isomers. However, this technique requires the polymer to be transparent since the cis and trans transition is photochemically driven. 113
(iii) Fluorescence Nonradiative Energy Transfer Technique. This technique is based on the principle that energy transfer from a donor chromophere to an acceptor induces a continuous change in both molecules’
Ind. Eng. Chem. Res., Vol. 42, No. 25, 2003 6439
emission intensity. Diffusivity of the additive molecules is calculated from real time intensity data.114 (c) Diffusion Modeling. The transport mechanism in a polymer matrix under supercritical environments is very complicated because of the significant effects of CO2 on the polymer, such as state transitions, chain mobility, and swelling. When a polymer is exposed to temperatures above Tg, the quick response of the polymer chains to the presence of CO2 makes it behave as a homogeneous phase and then the transport of CO2 simply obeys the classic Fick’s law. As a result, many researchers deliberately set up the experimental conditions to lower the polymer Tg and reasonably employ the Fickian diffusion model to analyze the kinetic sorption (desorption) data.29,48,109,115,116 The analytical solutions for one-dimensional simple geometries (cylinder and thin film) are well-known117 and expressed in terms of the mass uptake ratio, Mt/M∞, which can be correlated to other measurable variables such as the swelling ratio,110 pressure,106 and infrared absorbance.26 Balik118 summarized commonly used mathematical methods to deduce the diffusivity from available kinetic data and proposed a new nonlinear regression method (hybrid method) with the advantage of using all the mesured data and not requiring the M∞ initially. The transport of CO2 in glassy polymers below Tg can be described by a dual-mode model or a modified partialimmobilization dual-mode model.119 These approaches assume that some of the gas molecules follow Henry’s law and are completely free to diffuse while others are sorbed in the microvoids and partially immobilized. A Langmuir adsorption model characterizes the latter. The transport observed is the net contribution from both types of diffusion and the differences in the microscopic mechanism complicate the real case. The effective diffusivity is correlated to solution diffusion in the Henry’s Law region (DH) and Langmuir adsorption (DD) by the following equation.
J ) -Deff Deff )
dCD dCH dC ) -DD -DH , dx dx dx
DD[1 + FK/(1 + CDb/kD)2] [1 + K/(1 + CDb/kD)2]
F ) DH/DD
w
Deff[1 + K/(1 + CDb/kD)2] ) DD[1 + FK/(1 + CDb/kD)2] Hence, a plot of Deff[1 + K/(1 + CDb/kD)2] against K/(1 + CDb/kD)2 is expected to be linear, from which one can evaluate the effects of an individual diffusion mechanism. However, the real case is always more complex because of various factors, including concentrationdependent properties, Tg depression, and the relaxation time scale of the polymer chain.120 For example, Nikitin et al.121 observed a sharp diffusion front during the transport of CO2 in PMMA because of the distinct difference in diffusion rates of CO2 in the glassy and rubbery states. The plasticization clearly takes place in the interface region and moves with the concentration front of CO2. Furthermore, the front propagation dynamics reveals Fickian diffusion characteristics in data that were initially regarded as anomalous diffusion behavior.122 Besides plasticization, the relaxation time of the polymer affects the diffusion behavior significantly. This is normally described by the Deborah number (De), that is, the ratio of relaxation time to
characteristic diffusion time. For small or large De, the diffusion follows Fickian law, while for intermediate values, non-Fickian behavior will appear.123 Because of the pronounced effects of CO2 on polymers, the variation of relaxation properties is presumably expected. One foreseeable consequence is the effect of relaxation on the plasticization kinetics. This discussion just highlights some of the complex factors in CO2 diffusion through glassy polymers, thus indicating that more fundamental studies in CO2 and its effects on various types of polymers are required for future development. (d) Mass Transfer of CO2 in Solid Polymers. Here, we present several examples to illustrate the transport of CO2 and its complexity. In Berens’ work,29,115 the sorption of near-critical CO2 in a variety of polymers by the simple gravimetric method shows that the diffusivity of CO2 increases with concentration and ultimately enters the range of 10-6 to 10-7 cm2/s. These represent typical values of CO2 diffusivity in rubbery polymers and the results clearly demonstrate the plasticizing effects of CO2. More examples include the CO2/ PVC system116 and the CO2/PET system,48 both of which demonstrate increased diffusivity with an increase of CO2 pressure and temperature (for conditions above Tg). The characteristic S-bend shape of the sorption isotherms of CO2 in PET indicates the change from dualmode sorption at lower pressures to Fickian diffusion at higher pressures.48 As for the semicrystalline polymers, it is generally accepted that the sorption of gas occurs mainly in the amorphous regions. The almost identical sorption for both initially amorphous PET and partially crystalline PET samples in Brantley’s work26 confirms this viewpoint. (e) Mass Transfer of CO2 in Molten Polymers. The fundamental understanding and construction of models for CO2 behavior should be easier for molten polymer systems since they can be treated as pure liquids. Sato et al. measured the diffusivity of CO2 in a variety of molten polymers, including PS,38 HDPE,61 poly(butylene succinate) (PBS), and poly(butylene succinate-co-adipate).124 The authors provide basic diffusion data and demonstrate the importance of free volume in understanding gas transport. They correlated the free volume fraction of the polymer/gas solution with the measured diffusion coefficients and generally achieved good predictions with about ∼10% relative deviation. Free volume also helps explain the decrease of CO2 diffusion rates with an increase of pressure in some systems. For example, in the CO2/gelatinized starch system,106 as the result of high compressibility, the free volume available for CO2 transport would be reduced significantly under high hydrostatic pressure. Royer and co-workers also noticed this phenomena,105,110 though the decrease of diffusion coefficient is not very evident. Table 4 lists some typical values of CO2 diffusion coefficient in molten polymers from various sources. (f) Diffusion of Additives in Polymers under Supercritical Atmosphere. The diffusion of additives through polymers was significantly promoted by the presence of supercritical CO2. For example, the diffusivity of dimethyl phthalate (DMP) in PVC is 6 orders of magnitude higher under SC-CO2 conditions than without CO2.111 Other examples include the diffusion of azobenzene in glassy PS113 and decacyclene in PS.114 This particular ability of SC-CO2 has already led to several important industrial applications, such as dye-
6440 Ind. Eng. Chem. Res., Vol. 42, No. 25, 2003 Table 4. Diffusion Coefficient of CO2 in Molten Polymers sample
T (K)
P (MPa)
diffusivity (cm2/s) 10-6
source
343 343 343
2.6 9.2 11.8
7.5 × 1.9 × 10-6 0.9 × 10-6
106
39.2% (m.c) gelatinized starch polyamide 11
488 488
10.3 37.9
5.29 × 10-5 2.29 × 10-5
105
303
10.5
1.7 × 10-5
303 343
24 24
1.2 × 10-5 9 × 10-5
poly(dimethylsiloxane)
110 (data read from figure)
polystyrene
423.15 473.15
8.319 8.42
5.33 × 10-6 9.9 × 10-6
38
393.15 453.15 453.15
12.341 2.466 8.304
1.23 × 10-5 2.04 × 10-5 2.68 × 10-5
124
poly(butylenes succinate) (PBS)
393.15 453.15 453.15
12.229 2.34 8.616
0.95 × 10-5 2.06 × 10-5 2.05 × 10-5
124
poly(butylenes succinate-co-adipate)
ing, impregnating biological agents, and creating polymer composites or blends as discussed later. Furthermore, the environmental benefits and easy control by altering CO2 pressure offer distinct process advantages. The specific interaction between additives and polymer matrix can strongly influence the diffusion process, as exemplified by the faster diffusion rate of 4,4′(diethylamino)nitroazobenzene (DENAB) in PMMA than Disperse Red 1 (DR1).108 However, we are typically more interested in the role of SC-CO2 in enhancing the diffusion rather than the solute-substrate interactions that are dictated by the application. In this respect, CO2 acts as a tunable carrier fluid to alter the polymer matrix or a cosolvent is added to enhance the solventpolymer interactions. For example, the addition of ethanol in Sicardi’s study107 increased the diffusion coefficient of a dye over that with pure CO2. The combination of tuning CO2 and adding cosolvent provides more controllable process parameters and allows for the impregnation of thermally labile and metastable materials under lower temperature and pressure. Because of the higher chain mobility and more available free volume, the increased transport rate of solute as temperature and pressure increase is certainly expected, as demonstrated in the systems of dye/PET/ CO2,107 dye/PMMA/CO2,108 and DMP/PVC/CO2.111 In comparison, the influence of dye concentration on diffusion is more complex, as illustrated in Figure 3. The observed curve could be explained by the overall contribution from the opposite diffusion behavior of two types of dye species when penetrating the polymer.107 (2) Heat Transfer. Although heat transfer is an important property in continuous polymer processing operations such as extrusion and injection molding, there have been no reports in the literature for the measurement of heat-transfer coefficients in CO2 (or other SCF) saturated polymer melts. (C) Rheology of Polymer Melts with Dissolved CO2. Because of the pivotal role of rheological properties of CO2/polymer melt systems in equipment design and process simulation, increased attention has been given to understanding them. In general, the viscosity is observed to decrease as CO2 is dissolved into various polymer melts (as shown in Figure 4). This viscosity reduction is greatly favorable for processing high molecular weight polymers where high viscosity is the major obstacle. It also facilitates the processing of temperature-sensitive polymers at
Figure 3. Diffusivity versus dimensionless concentration in the polymer (disperse blue: 22 MPa, 110 °C) [Reprinted from: Sicardi et al. Diffusion of disperse dyes in PET films during impregnation with a supercritical fluid. J. Supercrit. Fluids 2000, 17 (2), 187194. Copyright 2000, with permission from Elsevier].107
Figure 4. Viscosity reduction of CO2/polystyrene solution with different CO2 content at 175 °C (fitted by Carreau model, experimental data are from Kwag et al.)130
lower temperatures to prevent thermal degradation and save energy. (1) Shear Viscosity Measurement. To measure the viscosity of CO2/polymer solutions, traditional rheometers are modified and two issues must be emphasized: one is to ensure the formation of a homogeneous solution before the measurement; the other is to prevent phase separation during the measurement. Early work from foaming research demonstrated many experimental
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devices capable of measuring viscosities of polymer/ blowing agent mixtures under high pressures, based on which new devices are specially developed for the CO2/ polymer solutions. These devices are classified into two categories: (1) pressure-driven and (2) drag-driven. Pressure-driven devices can be either a capillary or a slit extrusion rheometer controlled by a back-pressure regulator. This method was originally designed for viscosity measurement of polymers with fluorocarbon blowing agents.125,126 Recently, a capillary rheometer was used by Gerhardt et al.127-129 to measure the shear viscosity of polydimethyl siloxane (PDMS) containing dissolved CO2. A sealed loading apparatus helps the transfer of an equilibrated sample to the rheometer and a backpressure assembly at the exit holds the gas in the solution. Corrections for back-pressure, wall friction, and entrance/exit pressure drop were considered in the viscosity calculation. A similar rheometer was used by Kwag et al.130 to measure the shear viscosity of polystyrene (PS) with dissolved CO2 and several refrigerants. Lee et al.131,132 applied a foaming extrusion apparatus to determine the viscosity of PS/CO2 solutions. A capillary tube die with two pressure transducers mounted along the flow stream was used and phase separation was prevented by maintaining a high pressure in the die. With use of a positive displacement pump, a metered amount of gas was injected into the extrusion barrel and mixed with the polymer melts. Such a capillary rheometer was also applied by Areerat et al.133,134 to measure the viscosities of low-density polyethylene (LDPE) and polypropylene (PP) with CO2 concentration monitored on-line by near-infrared spectroscopy. When a similar extrusion apparatus is used, a slit die can also be used to measure the shear viscosity of polymer/CO2 solutions. The pressure drop along a flow streamline can be directly measured without worrying about the entrance/exit pressure correction or pressure measurement error created by the curvature of the capillary die. However, the total pressure drop may be lower than that in a capillary die, which may affect the measurement accuracy. Elkovitch et al.135-137 applied a slit die to measure the viscosity reduction for CO2/PS and CO2/poly(methyl methacrylate) (PMMA) without back-pressure control. Royer et al.138,139 attached nozzles of different sizes on the slit die to hold the high pressure and measured shear viscosities of PS, PMMA, PP, LDPE, and poly(vinylidene fluoride) (PVDF) with dissolved CO2. Lee et al.140 also applied a wedge die mounted on a twin-screw extruder to measure the viscosities of CO2/polymers. Gendron and coworkers141-143 examined the rheological behavior of PS and PP with dissolved CO2 by using a commercial online rheometer, basically a slit contraction, mounted on a twin-screw extruder. Overall, the pressure-driven devices have been found convenient to provide accurate rheological data. Their limitation is that the large pressure drop across the capillary or slit die limits the CO2 concentration dissolved in the polymer melts and as a result viscosities are usually measured at low CO2 concentration to ensure the formation of a single-phase solution. Also in the low shear rate region, the small pressure drops detected, regardless of the large absolute pressure, may cause uncertainties in the raw data. On the other hand, drag-driven devices can be used
to measure shear viscosities at low shear rates near the Newtonian plateau. They are usually operated at or near equilibrium CO2 concentration and a steady, uniform distribution of pressure, stress, and deformation rate can be created. However, such devices are usually difficult to design because the signals (for example, torque, force, and displacement) have to be transferred under pressure through a dynamic seal, although a magnetic sensor may improve the design. A magnetically levitated sphere rheometer was designed by Royer et al.144 to study the viscosity of PDMS/ CO2 solution. In this design, the sphere is held stationary at a fixed height through magnetic levitation while the cylindrical sample chamber is moved vertically to generate different shear rates. The device provides a nonhomogeneous flow field and needs to be calibrated against a known fluid viscosity. Recently, a Couette viscometer was designed by Oh et al.145 to measure the viscosity of PS/CO2. A magnetic transmission has been used to eliminate dynamic seals and reduce torque losses in the drive train. The viscosity near the Newtonian plateau was evaluated from the torque and the rate of rotation. (2) Shear Viscosity Prediction. As discussed above, the viscosity decreases as the dissolved CO2 concentration increases for various polymer melts. It is also found that the viscosity curves (i.e., shear viscosity versus shear rate) of the polymer/CO2 solution are usually similar in shape to that of pure polymer and are therefore analogous to the effect of increasing temperature or decreasing pressure. These analogies imply that carbon dioxide affects the viscosity of polymer melts following a similar mechanism to the temperature and pressure, predominantly through a change in free volume. Other factors, such as the improvement in polymer chain mobility, the dilution of polymer chains, and the reduction of chain entanglement upon CO2 plasticization, also contribute to the viscosity reduction. Consequently, the traditional scaling techniques can be used and a scaling factor ac, similar to the familiar temperature-dependent shift factor aT employed in time-temperature superposition, can be applied to represent the influence of the CO2 concentration.128-130,132,138,139,144 Although power law or Cross-Carreau models146 can fit the shear thinning behavior easily, extensive research has focused on how to predict the effect of CO2 concentration on viscosity of various polymer/CO2 solutions. Doolittle’s free volume theory147,148 is always used as the starting point,
η0 ) A exp
(
)
B f(T,P,ωg)
(2.4)
where η0 is the zero shear viscosity; A and B are unique constant parameters for the polymer. f(T,P,ωg) denotes the free volume fraction which is given as a function of temperature, pressure, and weight fraction of gas,
f(T,P,ωg) )
V(T,P,ωg) - V0(ωg) V(T,P,ωg)
(2.5)
where V(T,P,ωg) is the specific volume of the polymer at temperature T, pressure P, and gas concentration ωg, and V0 is the weight-average hard core specific volume of polymer/gas solution.
6442 Ind. Eng. Chem. Res., Vol. 42, No. 25, 2003
By applying an extension of Doolittle’s free volume theory, originally developed by Kelley and Bueche,149 Gerhardt et al.128,129 calculated the scaling factor ac ) {ηo(ωg)}/{ηo(purepolymer)} for PDMS/CO2 mixtures. Both Sanchez-Lacombe and Panayiotou-Vera equations of state, which model the P-V-T properties, were used to calculate the specific volumes of the pure polymer melt and polymer-gas mixture. The prediction was found to be in very good agreement with the experimental data. Areerat et al.133,134 approaches this problem by substituting Doolittle’s equation in the Cross-Carreau model to get a generalized equation to relate viscosity to shear rate, temperature, pressure, and CO2 concentration. All the model coefficients except the one corresponding to CO2 concentration can be determined from the P-V-T data and molten viscosity data of the pure polymer. The Sanchez-Lacombe equation of state was used to calculate the specific volume of the polymer/CO2 solution and then the free volume fraction. The viscosity of LDPE/CO2 and PP/CO2 solutions was successfully predicted by this model. Lee et al.132 also started from Doolittle’s equation and expressed the fractional free volume as a power law series in terms of temperature, pressure, and CO2 concentration. A seven-parameter model, a generalized Arrhenius equation, was built to predict the zero shear viscosity of the neat PS melt to accommodate the effects of temperature and pressure. Further, an eight-parameter model was built to include the effect of CO2 concentration. Royer et al.138,139 adopted the Williams-LandelFerry (WLF) equation,150 a direct descendant of Doolittle’s equation, to relate the viscosity scaling factors corresponding to pressure and CO2 concentration to the glass transition temperature (Tg) when the temperature is in the range from Tg to Tg + 100 °C. Then the effects of CO2 concentration and pressure can be directly incorporated as the Tg depression, which was predicted by Chow’s model.78,151 On the other hand, when the temperature is higher than Tg + 100 °C and beyond the effective range of the WLF equation, Arrhenius analogues were applied to build the relation between shift factors and Tg. Systems of PS, PP, LDPE, and PVDF with dissolved CO2 were studied by applying these models. (3) Extensional Viscosity Measurement. Extensional viscosity is important to help understand the entrance pressure drop, foam bubble growth, and other processes related to elongational deformation. Ladin et al.152 applied a contraction slit die attached to a foaming extrusion system to measure the entrance pressure drop, which was then converted to extensional viscosity according to Cogswell’s analysis153 for PBS/CO2 solutions. The extensional viscosity was found to significantly depend on the CO2 concentration and a large reduction was obtained with the dissolution of CO2. Tension-thinning behavior, where extensional viscosity decreases with an increase of extensional rate, was observed. The extensional viscosity was also observed to decrease as the temperature increased. By applying a similar design, Xue and Tzoganakis154,155 studied the entrance pressure drop and extensional viscosity of PS/ CO2 solution. Extensional viscosity reduction and tension-thinning behavior were also reported. The entrance pressure drop was found to be a strong function of
pressure. Of course, such a design can be used to measure the shear viscosity simultaneously. (D) Dynamic Processes. The processes of nucleation and growth of crystals or bubbles are important properties in a variety of applications. While growth can be accurately modeled according to diffusion-limited mass transfer from the bulk to the new phase, nucleation continues to confound researchers and practitioners. Below is a limited discussion of current topics in the use of CO2 for nucleation of crystalline domains in bulk polymers and nucleation of bubbles for producing foams. (1) CO2-Induced Polymer Crystallization. Relatively little work has been published on the Tm depression and crystallization kinetics of polymers under highpressure CO2. This gas-induced crystallization has been reported for poly(ethylene terephthalate) (PET),156-159 polypropylene,160 polycarbonate,161 poly(ether ether ketone) (PEEK),162 poly(p-phenylene sulfide) (PPS),163 methyl-substituted PEEK (MePEEK),70 syndiotactic polystyrene (sPS),159,164 and blends of poly(vinylidene fluoride) and poly(methyl methacrylate).157 Measurement methods include correlation of crystallization to the bulk density of the polymer,156,158 infrared spectroscopy,156 and high-pressure DSC.160,164 It appears that the first two techniques give comparable results and may be more amenable to very high pressures as commercial cells for high-pressure DSC investigations are currently limited to approximately 100 bar. The data are commonly expressed as Xt, the weight fraction of the material crystallized at time t, which is then fit to the Avrami equation:
Xt ) 1 - exp(-ktn) By plotting of ln(-ln(1 - Xt)) vs ln(t), the Avrami exponent n, and the logarithm of the kinetic constant, ln k can be determined. CO2 was found to accelerate the crystallization kinetics for both PET156 and syndiotactic polystyrene.164 The crystallization kinetics for sPS-CO2 solutions follow the Avrami equation, but the value of the exponent n is lower than that when crystallization is conducted under ambient pressure. Furthermore, the presence of CO2induced morphology changes in the sPS crystal, which do not occur upon treatment with liquids. For the crystallization of polypropylene, CO2 decreased the overall crystallization rate.160 This apparent conflict in the effect of CO2 was explained in terms of the proximity to the temperature of maximum crystallization rate (Tmax). The crystallization rate changes with temperature and reaches its maximum at Tmax, which is close to the mean of Tg and the equilibrium melt temperature Tm0: Tmax = (Tg + T0m)/2. Above Tmax the overall crystallization rate is controlled by the nucleation rate, and the temperature region between Tmax and T0m is called the nucleation region. Below Tmax the overall rate is controlled by the crystal growth rate. Alternatively, one can interpret Tmax as a competition between the thermodynamic driving force (∆T) and the decreasing mobility at lower T. Dissolved CO2 will depress T0m, and hence Tmax. This means that the dissolved CO2 accelerates the crystallization rate of an isothermally crystallized semicrystalline polymer within the crystallization growth region and reduces the rate within the nucleation controlled region. This explains why the crystallization rate of Mizoguchi et al.156 for the isothermally crystallized PET
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at T ) 308.2-393.2 K increases (Tmax ) 448 K), and the crystallization rate of Handa et al.164 for the isothermally crystallized sPS at T ) 395.2 K increases (also below Tmax), while Takada et al.’s 160 crystallization rate of PP decreases as the crystallization was carried out in the nucleation region. Takada et al.160 also developed a model for the crystallization kinetics of polymers under CO2, which is based on Avrami’s theory for three-dimensional heterogeneous nucleation and the temperaure-dependent crystal growth models of Ishizuka and Koyama165 and Ito et al.166 (2) Bubble Nucleation. In the context of foaming, bubble nucleation (cell nucleation) is the formation of a new gas phase from a metastable melt phase, requiring an activation energy barrier to be surmounted (via density fluctuations) to induce the phase separation. In standard foaming applications, foaming occurs by nucleation rather than spinodal decomposition, which is the spontaneous phase separation from a thermodynamically unstable state. Concentration gradients, pressure gradients, or temperature gradients can drive nucleation. In most foaming applications either a pressure drop or a temperature increase is used to decrease the gas solubility and make the solution supersaturated. When clusters of gas molecules are greater than the critical size, the activation energy is overcome and nucleation occurs. The greater the supersaturation, the smaller the activation energy. Additives such as talc or nano-clay are commonly used to adjust the nucleation rate, presumably enhancing the rate by significantly decreasing the activation barrier. Classical homogeneous nucleation theory167,168 was derived for the formation of liquid drops from a pure vapor and one can argue the applicability of this theory in a foaming process. The latter is actually a binary or a cavitation-like process, but classical homogeneous nucleation theory is still widely used to explain phenomena qualitatively and/or quantitatively.169-178 The equations are exponential with respect to the activation energy barrier (∆G*), which must be overcome to phase separate the metastable solution. Two equations are commonly used, one for the rate (N) of homogeneous nucleation (bubble formation in bulk polymer phase)
N0 ) C0f0e(-∆G*hom/kT) ∆G*hom )
16πγbp 3∆P
and the other for heterogeneous nucleation (bubble formation at an interface between a polymer and an additive),
N1 ) C1f1e(-∆G*het/kT) 16πγbp3 1 (2 + cos θ)(1 - cos θ)2 ∆G*het ) 2 4 3∆P
()
where C is the concentration of gas molecules, f is the frequency factor of gas molecules joining the nucleus, k is Boltzman’s constant, T is the absolute temperature, γbp is the interfacial tension of the polymer-bubble interface, ∆P is the gas pressure, and θ is the contact angle of the polymer-additive-gas interface.170,171
To set up a technically correct experiment following the homogeneous nucleation equation, many restrictions are necessary. No pre-existing gas cavities can exist in the bulk, on the container walls, or in the form of microvoids (microbubbles). It is difficult because even very highly idealized cases of nucleation in boiling and nucleation in isothermal gas desorption still are not completely understood.179 Plus, all nucleation theories require nuclei as well as microvoids to be spherical in shape. It is not understood what effect this nonideality has on experimental results. Colton and Suh used classical nucleation theory as the basis to develop a model for the nucleation of microcellular foams in polystyrene with zinc stearate additive.170 They concluded that homogeneous nucleation occurs below the zinc stearate solubility limit since the nucleation rate increases for an increase of saturation pressure and increase of concentration of zinc stearate. Above the solubility limit, heterogeneous nucleation dominates and the nucleation rate increases with stearate concentration but is not affected by pressure. Around the solubility limit, each nucleation mechanism is significant with the nucleation rate assumed equal to the sum of both nucleation rates. Experimentally, injection molding is used to make samples and nucleation is measured by counting the number of cells using scanning electron microscopy (SEM) of the foamed polystyrene but no information about the experimental bubble sizes is given.169 Thus, it is not clear what the smallest bubble sizes are considered; this method assumes: each nucleation site produces a bubble, no bubble coalescence occurs, and all bubbles nucleate and grow to a certain detectable size. With counting of the number of bubbles formed, a large number of publications present intriguing conclusions about foaming behavior. These again assume each nucleation site produces a bubble, no bubble coalescence occurs, and all bubbles nucleate and grow to a certain detectable size. In many cases, the sample is quenched at cold temperatures in an effort to prevent cell coalescence. For physical blowing agent-assisted polymeric foaming with the use of nucleation agents, several authors present foaming data using heterogeneous nucleation.180-183 Park et al. present results where a limit on the effect of nucleation agent on cell density is reached.184 Ramesh et al. explains data for a rubberpolystyrene blend without any additives using microvoids as heterogeneous nucleation sites.185,186 Han and Han studied bubble nucleation in polymeric liquids using laser light scattering. The growth of bubbles is assumed to be monodisperse, thereby neglecting multiple scattering effects.174,175 A connection between pressure drop rate to nucleation rate was identified by Park et al.187 To lower the free energy, gas diffuses into the cells, resulting in a depleted region surrounding the cell. If the depleted regions from all cells are in contact, no further nucleation is possible, and this condition is called cell impingement. In terms of driving force, as the initial nucleation events occur, supersaturation decreases, decreasing the likelihood of ensuing nucleation. Therefore, due to this competition between nucleation and diffusion, a higher pressure drop rate results in higher supersaturation, which translates to a higher nucleation rate. The end result is that pressure drop rate, nucleation rate, and cell density can all be viewed equivalently. This has been
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verified experimentally using capillary nozzles where the smallest nozzle produced foam with the highest cell density (smaller nozzles had higher pressure drop rates).187 A lot of effort188-196 has been expended in modeling bubble growth. The procedure has evolved from models in which a single bubble is surrounded by an “infinite sea” of fluid with an infinite amount of gas available for growth, to the cell model in which the foam is divided into spherical microscopic unit cells of equal and constant mass, each consisting of a liquid envelope surrounding a single bubble, and thus the gas available for growth is limited. Overall, bubble growth is a process in which the heat, mass, momentum transport, and various constitutive equations need to be solved simultaneously. When these theories and models of both cell nucleation and growth are implemented, parameters such as the viscosity, solubility, surface tension, diffusivity, and glass transition temperature are found to play important roles in deciding the cell density and size. The situation becomes more complicated when we consider that these parameters are all functions of temperature, pressure, and CO2 concentration, by which cell nucleation and growth themselves are affected as discussed by Muller et al.197 Considering the advances being made in fundamental property measurement and modeling described earlier, there will soon be enough information to perform powerful simulations for studying the interaction of all these effects. Applications (A) Impregnation. Impregnation in this context can be described as the delivery of solutes to the desired sites inside the polymer matrix with the aid of SC-CO2. Basically, the impregnation process includes three major steps: first, to expose the polymer to SC-CO2 for a period of time; second, to introduce the SC-CO2 containing solutes to the polymer and conduct the subsequent solute transfer from SC-CO2 to polymer phase; third, release the CO2 in a controlled manner and trap the solutes in the polymer. When exposed to SC-CO2, polymers will exhibit various extents of swelling and enhanced chain mobility as described above, which significantly facilitate and accelerate the transport of components. Besides the environmental benefits, another advantage of the process is the control obtained by tuning the CO2 properties, which has been wellsummarized in prior reviews.4,198,199 Theoretically, any type of solute could be impregnated into polymers; however, the applicability of the process is determined by two main issues: (A) the transport rate of solutes that determines the possibility of developing such a process and (B) the compatibility and stability between solutes and polymer that determines if the process is worthwhile to develop. The second point refers to the possible subsequent changes of the impregnated product, for example, noticeable separation between solutes and polymer and a fast leach-out process. In this section, we summarize the practical issues of impregnating polymers and highlight recent applications appearing in the literature since the previous reviews. The swelling extent and alteration of microstructure of the polymer depends on the chemical nature of the polymer and its interaction with CO2. Lesser swelling always means relatively difficult impregnation. For example, compared with PVC and PC, PTFE demon-
strated the least ability to be modified because of its limited swelling.200 Generally speaking, highly crystalline polymers are not suitable as an impregnating matrix because of the regular structure and the much slower transport process. As a result, current studies have been mainly limited to polymers with large amorphous fractions and/or specific interactions with CO2. For example, PET,24,107,201-206 PMMA,23,108,198,207 polycarbonate,111,200 polystyrene,111 and poly(vinyl chloride).200 However, the incorporation of molecules into inexpensive polymers, such as polyolefins, also holds great potential for applications. Wang et al.208 showed that a uniform distribution of a dye molecule (NBD) in polypropylene could be achieved even though the solubility is low and modifier/polymer interactions are not favorable. The impregnation of biodegradable polymers has also attracted much attention because of the potential for controlled drug-release systems or biosurface modification. Poly-DL-lactide-co-glycolide (PLGA) and its derivatives are subject to the most active research. Because of their relatively low glass transition temperature, it proves difficult to impregnate PLGA without observable deformation and foaming. In the process of delivering 5-fluorouracil and β-estradiol into PLGA, the final form of product could appear as foam with large pores or microporous particles by controlling the depressurization.209 In some applications, foaming may be an advantage and there is a degree of control over the product morphology when combining impregnation with anitsolvent precipitation methods. The final appearance of polymers after impregnation can be an important issue that is generally affected by the intrinsic Tg of the polymer, CO2 conditions, CO2 release rate, and geometry. When the pressure is released, CO2 near the surface sees the largest gradient and escapes from the polymer quickly and the surface reverts to its original morphology. Hence, the rate of CO2 release and polymer relaxation are interactive but only the former is a controllable process variable. Wellcontrolled slow depressurization allows for the escape of CO2 from the polymer matrix before the polymer recovers from the swollen state; otherwise, CO2 entrapped inside the polymer leads to foaming. In other ways CO2-induced plasticization or crystallization could be employed to improve the physical properties of the polymer during supercritical treatment, for example, the increase in the crystallinity of the polymer204,206 and the modification of ill-developed polymer structure.202 At present, the solutes studied in the process of impregnation range from dyes, to metal complexes, and to biological molecules. Obviously, solutes with high solubility in SC-CO2 can be easily delivered to the polymer matrix. Meanwhile, studies show that, for those with low solubility in the supercritical phase, the stronger affinity for the polymer matrix leads to favorable partitioning toward the polymer phase.198,207,210 In fact, distribution coefficients (ratio of polymer to fluid concentration) can be 100-1000 times higher than that with liquid solvents. A recent study demonstrates that, despite being thermodynamically unfavorable, the distribution of a large surfactant-like molecule inside polypropylene could be controlled and achieved.208 Dyeing using SC-CO2, in which the solutes transported are organic dyes, is perhaps the most common application of impregnation to date. Examples include the original work of Berens,120 dyeing of textile accessories,203 and a number of papers from Chuck Eckert’s
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group.23,25,108,198,207,211,212 Besides organic materials, inorganic metal complexes have also been incorporated into polymers to fulfill specific functions. For instance, the infusion of silver-containing additives leads to the formation of metalized polymer film with high reflectivity.213-215 Polymer/metal composites, even at a nanoscale level, can be produced by infusing proper metal complexes into a polymer matrix, such as nanoscale platinum clusters into PTFE,216 copper nanoparticles in polyacrylate,217 and chelate complexes of copper and iron into polyacrylate.218 Similarly, if the solute impregnated is a polymerizable monomer, polymer blends or surface modification can be accomplished, depending on the depth of penetration. The general procedure for producing polymer blends is to infuse the monomer and initiator together and subsequently initiate the polymerization, as shown by the extensive work of Watkins and McCarthy.216,219,220 Clearly, the final performance of the product is determined by the amount and distribution of components inside the polymer matrix. In some cases, because of the low diffusion rate, the impregnated components only penetrate a limited distance beneath the surface. For example, Muth et al. found that the depth of penetration of methacrylic acid into PVC is only about 180 µm after 4 h of impregnation.200 Interestingly, sometimes one wants to take advantage of this property and confine the functional monomers to the surface region of polymers. As a result, the bulk properties of the polymer could be maintained while the surface has been grafted with a certain type of modifier to fulfill specific functions.221-225 In comparison with the direct infusion of polymers, the advantages of impregnating functional monomers is the relatively more favorable diffusion of small molecules compared to larger chains, while the disadvantages come from control of distribution and subsequent reaction. Radiation is a common choice for initiating reaction in these systems as in the grafting of MA into PP.226 The dynamic and reactive extrusion of polypropylene with grafted maleic anhydride is facilitated by the supercritical CO2 because of the reduced viscosity and then improved mixing of the reactants.227 The impregnation of pharmaceuticals, proteins, or other bioactive molecules is a new, difficult, and very promising research area. Kazarian and Martirosyan investigated the formation of ibuprofen/PVP composites via in situ ATR-IR and Raman spectroscopy,20 and the impregnation of 5-fluorouracil and β-estradiol into PLGA has also been reported.209 For pharmaceuticals, an important issue is achieving a molecular level dispersion within the polymer substrate,20 while for more complicated biological agents, maintaining bioactivity and spatial structure after treatment is very important. The relatively large size and hydrophilic properties of biological agents imposes additional difficulties for the impregnation process. (B) Particle Formation. In recent years, CO2 techniques have emerged as a promising method for precipitating particles from solution with the traditional environmental and tunable solvent advantages while also leaving particles solvent-free. In particular, these novel methods provide a feasible and clean way to process thermal-labile or unstable biological compounds, such as the promising application in the development of drug-delivery systems.228 This section will mainly focus on the recent developments in this area as
work prior to 2001 is well-summarized in reviews by Bungert,229 Reverchon,230 Cooper,4 and Thiering et al.228 Generally, the techniques involved are categorized as rapid expansion from a supercritical solution (RESS) and supercritical fluids serving as antisolvents. In the RESS process, the materials are dissolved into supercritical CO2 and then forced to pass through a nozzle. As a result of rapid expansion caused by reduction of pressure, small and uniform polymer particles can be formed. Undoubtedly, the prerequisite for this technique is the proper solubility of the solute in the supercritical fluid. Therefore, most polymer research in RESS is confined to those polymers with high solubility in CO2, such as perflouroethers and siloxanes or polymers dissolved in SCFs other than CO2 such as HFCs or alkanes. The sudden reduction of pressure can sometimes lead to spongelike structures of particles and RESS cannot typically be applied to thermally labile substances,231 which would seem to limit its application for drug-delivery systems. The antisolvent precipitation techniques can be subdivided into GAS (gas antisolvent precipitation) and spray processes (ASES, aerosol solvent extraction system; PCA, precipitation with a compressed fluid antisolvent; SEDS, solution-enhanced dispersion by supercritical fluids; and SAS, supercritical antisolvent precipitation), depending on how the solvent containing the solute and the supercritical antisolvent are brought into contact.228,232 Antisolvent techniques by far have received the most attention for using CO2 in the pharmaceutical and biotechnology areas. As a result, the polymer component of this work emphasizes biocompatible and biodegradable polymers and their mixture with pharmaceutical compounds of interest. Although the differences between the various spray processes seem trivial at first, there is growing evidence that they operate in different hydrodynamic regimes and thus give different results for similar systems. More research is needed to control particle morphology but the benefits (both environmental and economical) are encouraging. (1) RESS (Rapid Expansion from Supercritical Solution). Recent developments in RESS include fluoropolymer coatings to protect historical buildings and monumental civil infrastructures.233,234 Others have worked with cosolvents to allow for the processing of more kinds of polymers. As might be expected, biopolymers are still of central interest and PLA remains the most popular.4,235-238 The cosolvents studied include acetone,235 CHClF2,236 and small alcohols.239,240 For example, the copolymer PS-b-(PMMA-co-PGMA), which is almost insoluble in either pure CO2 or pure ethanol, was dissolved into the mixture up to 20 wt %.239 Alcohol cosolvents are attracting attention because of the environmental benefits and their function as a nonsolvent after expansion, which could further help prevent the agglomeration of polymeric particles. A modified method, called RESS-N (RESS-nonsolvent), was applied for powder coating systems239 and protein-loaded microparticles.240 The proteins, that is, lysozyme and lipase, were suspended in supercritical solution and finally microencapsulated inside the polymer particles. Finally, efforts with an intention to overcome the limitation of homogeneous solutions in RESS were reported in Shim’s work.241 Heterogeneous suspensions of poly(2-ethylene acrylate) (PEHA) in liquid CO2 in the presence of surfactant were passed through an expansion nozzle to form uniform circular films. Viscosity reduction of the
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polymer suspension due to the dissolved CO2 plays a crucial role in the success of this process. (2) Antisolvent Processes. A detailed comparison of SAS with GAS could be found in Thiering’s review.228 Reverchon gives a good summary of the injectors used in the spray antisolvent processes (SAS, PCA, ASES, and SEDS), including nozzle, microcapillary, virbrating orifices and coaxial devices.230 Meanwhile, the effects of process parameters, pressure, and temperature are far from conclusive; even some contradictory results appear in the literature. Presently, biopolymers account for the largest part of antisolvent systems studied because of the great potential for the drug-delivery applications. Specific examples include PLA(poly(L-lactide))/PLGA(poly(D,Llactide-co-glycolide) and their derivatives,231,232,242-248 PHB (poly(β-hydroxybutyric acid)249], PCL (poly-caprolactone),244,250-252 and HYAFF-11 (poly(hyaluronic acid benzylic ester).238,253 Engwicht et al. studied PLA and PLGA and found that the crystallinity and thermal behavior play important roles in the preparation of particles with spherical shape.254,255 PLGA 50/50 is completely amorphous, and therefore the particles from PLGA 50/50 were soft and agglomerated significantly. To isolate polymer structure factors that affect the final morphology of particles, Breitenbach synthesized a series of biodegradable polyesters with a comb structure and conducted ASES experiments.249 They also found that crystallinity was far more important than solution viscosity, MW, Tg, or the density of supercritical gas in determining particle morphology. On the other hand, recent research demonstrates that by altering the property of the antisolvent, that is, introducing N2 to supercritical CO2, discrete particles of PGLA less than 10 µm could be prepared.250,251 Also, spherical particles of PCL (though still a mixture of discrete and agglomerated particles) were produced by the same modified SEDS for the first time, compared with the PCL films formed in other works.244,252 A major goal of the antisolvent techniques is to achieve the desired distribution of biologically active agent inside a polymer and maintain the biological activity. In some cases, the lack of proper solvent for polymer and drug limits the process development and meanwhile the deliberate selection of solvent could be an effective way to control the subsequent loaded particles. When incorporating insulin into PLA, Elvassore reported that better performance could be achieved by using a mixed solvent of dichloromethane and DMSO.256 To modify the low biodegradability and high hydrophobicity of PLA, insulin-loaded PLA/PEG composite particles were produced by incorporating poly(ethylene glycol) (PEG) into particles through the GAS process.257 Although only PEG with low molecular weight could be efficiently entrapped, its role in affecting the coprecipitation of polymer and protein, and in determining the release behavior, is evident. In addition to those biopolymers discussed above, Park also used PCA to recover nylon 6/6 from a formic acid solution.258 (C) Foaming. Foaming with CO2 is an active area of research and development due to the restrictions imposed by the Montreal Protocol on ozone-depleting substances. At present, three choicesshydrogen-containing chlorofluorocarbons/fluorocarbons (HCFC/HFCs), hydrocarbons, and inert gases (CO2, N2, argon, or water)shave the highest potential to replace the chlo-
rofluorocarbon (CFC or Freon) physical foaming agents, which were proven to be contributing to the destruction of the Earth’s ozone layer and are gradually being eliminated.259-262 Among these, CO2 is the most favorable foaming agent because of its unique properties. A nearly equivalent volume amount of CO2 (compared to CFC) can be dissolved in polymer melt at elevated pressures. The diffusivity of CO2 in polymer melt is large, which ensures a quick mixing process. Moreover, CO2 is environmentally benign and economically low cost. For low-value foam products (e.g., packaging), CO2 foaming is already a reality while high-value or highstrength applications are still in development. Of the latter, introducing CO2 as a foaming agent focuses on two large-scale applications: low-density insulation foams (
