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Formation of Perfluoropolyether Coatings by the Rapid Expansion of Supercritical Solutions (RESS) Process. Part 1: Experimental Results Yury Chernyak,† Florence Henon,† Robert B. Harris,‡ Richard D. Gould,‡ Randall K. Franklin,‡ Jack R. Edwards,‡ Joseph M. DeSimone,†,§ and Ruben G. Carbonell*,† Department of Chemical Engineering and Department of Mechanical and Aerospace Engineering, North Carolina State University, Raleigh, North Carolina 27695, and Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599
The rapid expansion of supercritical solutions (RESS) process is a promising environmentally benign technology for fine droplet or particle formation. The absence of organic solvents and narrow size distribution of RESS precipitates make this process attractive for polymer coating applications. In our work, this technique has been used to produce droplets of perfluoropolyethers from CO2 solutions without the aid of cosolvents for the coating of porous materials applied in monumental and civil infrastructures. The present work is aimed at gaining an understanding of the relationship between droplet and spray characteristics and RESS process conditions. As such, a combined experimental/computational approach is applied to a representative binary system consisting of a low-molecular-weight perfluoropolyether diamide (PFD) dissolved in supercritical CO2. Part 1 of this work presents phase equilibria measurements and polymer droplet size characterizations under different operating conditions. The effects of temperature, solute concentration, and nozzle configuration on droplet and spray characterization and transfer efficiency are discussed. Part 2 of this work presents a multidimensional computational fluid dynamics model of the RESS expansion process and describes the use of the model in further analyzing and interpreting experimental data. 1. Introduction Highly fluorinated polymers have been shown to be ideal coatings for the protection of historical buildings and monumental civil infrastructure from stone degradation processes.1 Low-molecular-weight perfluoropolyethers and the amides of their carboxylic acids, such as isobutyl amide and ethylene or hexamethylene diamides, are liquids at room temperature and have many physical properties that are favorable for the protection of porous surfaces.2 These polymers are highly water repellent; have low surface energies; and are stable to corrosive acids, high temperatures, UV radiation, and oxidizing agents.3 Perfluoropolyethers are insoluble in water, transparent, and colorless. Moreover, they have refractive indices near that of water, meaning that the natural appearance of the stone can be maintained after coating.4 Perfluoropolymers are generally insoluble in most common solvents except for volatile organic compounds (VOCs) and fluorinated solvents. Ironically, VOCs and fluorinated solvents required to deliver coating materials to protect substrates from environmental degradation themselves can have a contaminating effect on the environment. A number of recent studies have focused * Author to whom correspondence should be addressed. E-mail:
[email protected]. Fax: (919)-515-5118. † Department of Chemical Engineering, North Carolina State University. ‡ Department of Mechanical and Aerospace Engineering, North Carolina State University. § Department of Chemistry, University of North Carolina at Chapel Hill.
on finding an alternative to the use of conventional organic and fluorinated solvents in order to give perfluoropolyethers a viable future as large-scale protective agents. It has been shown experimentally that fluorinated polymers and perfluoropolyethers in particular are highly soluble in CO2.5 Carbon dioxide as an alternative solvent has a number of advantages. It is environmentally benign, nontoxic, nonflammable, and easily recyclable and is a low-cost, widely available material. CO2 has a relatively low critical temperature and moderate critical pressure (Tc ≈ 31 °C, Pc ≈ 72 bar). Thus, its supercritical state (1 < T/Tc < 1.1, 1 < P/Pc < 1.5)6 can be easily reached by conventional spraying equipment. In the supercritical state, the solvent properties of carbon dioxide are very sensitive to changes in the temperature and pressure. This provides access to a wide range of solvating strengths through modulations of the process conditions. This versatility, coupled with environmental benefits, has led many industries to consider CO2 as an alternative solvent. The rapid expansion of supercritical solutions (RESS) process7-22 is an environmentally benign alternative to conventional processes for producing fine droplets or powders. In this process, a dilute solution of a solute in a supercritical fluid (usually carbon dioxide) is expanded through a capillary or pinhole nozzle from a high upstream pressure to a low downstream pressure. The resulting decompression leads to a high degree of supersaturation of the solution, with characteristic times for phase separation on the order of 10-5-10-6 s.7-9 The precipitation of solute from the solution is driven by nucleation, condensation, and particle coagulation. The process can produce very small, nearly
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monodisperse particles or droplets, depending on the nature of the solute and the operating conditions. The RESS process has attracted researchers hoping to overcome many of the issues facing the modern coatings industry. Initially, reduction of VOC emissions was the goal, but as the process developed, a number of other benefits were recognized. The RESS process is unique among spraying techniques because of its capacity for a wide range of organic, inorganic, polymeric, and organometallic materials; its applicability to various substrates; and its ability to produce nearly uniform precipitates with different morphologies.10 One disadvantage is that the process is limited to materials that can be dissolved in the supercritical fluid. The possibility of controlling RESS product characteristics from submicron powders to supermicron fibers by varying operating conditions has been an active area of investigation since the late 1980s.7,8,12-20 Most of this research is experimental in nature and more descriptive than predictive. Conclusions are generally limited to the specific material and process conditions studied. Some studies have focused on the modeling of the RESS process.21-23 Results tend to be rather qualitative in nature as a result of uncertainties in the modeling of different particle growth mechanisms and simplifications made in modeling the influences of nozzle and jet hydrodynamics on the process. The present work focuses both experimental and computational efforts toward an analysis of the RESS processing of a perfluoropolyether diamide (PFD) from its supercritical solution with carbon dioxide. The intent is to achieve a more complete understanding of the processes that govern PFD droplet growth, with an eventual view toward predicting the effects of process conditions on droplet sizes and size distributions. Part 1 of this study summarizes experimental results, including PFD/CO2 phase equilibria and the effects of operating conditions on on-line droplet and spray characterization. Experimentally observed spray characteristics, such as droplet size distribution and transfer efficiency measurements, are correlated to the thermodynamics of the PFD/CO2 system and to the RESS fluid expansion path determined through the modeling of the flow expansion process in the nozzle. The effects of polymer concentration, preexpansion temperature and pressure, and nozzle geometry on the spray characteristics are also described. The companion paper24 describes a computational fluid dynamics approach for modeling RESS hydrodynamics as well as PFD droplet precipitation and growth. Factors that influence droplet growth are studied in detail, and predicted results are compared with experimental measurements. 2. Physical Principles of Droplet Formation in RESS Process vs Conventional Atomization Mechanisms The physical basis of droplet formation by the RESS technique differs from those of conventional spraying technologies25 and the UNICARB process.26,27 Conventional atomization of a binary (solute plus VOC) liquid coating material results from the shearing of droplets away from the liquid core using aerodynamic forces. Dissolving CO2 in the spray solution, as is done in the UNICARB process, reduces the required concentration of organic solvent, as CO2 can reduce the solution viscosity. In the UNICARB process, atomization is also enhanced by the expansion of the carbon dioxide from
Figure 1. RESS process path: F, single-phase state; LL, twophase state; 1, conditions at the nozzle entrance (stable homogeneous state); 2, conditions in the nozzle (metastable homogeneous state); 3, conditions at the nozzle exit (unstable heterogeneous state).
the solution upon its decompression. In contrast, carbon dioxide functions as the main solvent in most applications of the RESS process. Precipitation results from a reduction of the solvent power of CO2 through gasification of the fluid during rapid expansion. Thus, solute precipitation in RESS is driven by other physical mechanisms, such as solute nucleation, condensation, and coagulation. Because RESS can result in smaller droplets than normally achieved through the UNICARB process, it might be particularly suited to the formation of thin, uniform coatings on porous or structured surfaces. The physical principles that govern droplet or particle formation during RESS have been reviewed by several authors,7,14,21 with most efforts focusing specifically on the precipitation of solids. From a thermodynamic viewpoint, the expansion of the solution in the nozzle is the process of transferring the solute/CO2 system from a single- to a two-phase state. An example of a typical RESS thermodynamic process path of a polymer/CO2 solution representing low critical solution temperature (LCST) phase behavior is illustrated schematically in Figure 1. Point 1 on the P-T diagram corresponds to the thermodynamic parameters at the nozzle entrance. Because of the pressure and temperature drop in the nozzle, the process path crosses the equilibrium or cloud-point curve (binodal) and enters the two-phase metastable region (point 2). The solution becomes supersaturated, initiating the growth of a dilute, polymerrich phase. In general, crossing the binodal from a single- to a two-phase region is a necessary condition for nucleation to begin, whereas crossing the stability limit (spinodal) from a metastable to an unstable region is a sufficient condition for irreversible and spontaneous phase decomposition to occur. Classical nucleation theory38 predicts the rate at which nuclei first exceed a threshold size and begin to grow spontaneously. It also establishes the dependence between the supersaturation of the solution (or the depth of penetration into the metastable phase) and the time characteristics of the initial process of phase separation. The modeling results presented in part 2 of this work24 indicate that the local supersaturation (defined as the ratio of the mass fraction of polymer in solution to the equilibrium mass fraction at a particular pressure and temperature)
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remains greater than 1 until all polymer is exhausted from solution. This implies that the solution remains in a metastable state during precipitation. Further droplet growth might occur through condensation and through coagulative mechanisms. The possibility of droplet coagulation and the presence of additional components in the droplet phase are issues that are particularly pertinent to the modeling of liquid-liquid phase separation during RESS. These considerations are developed further in part 2 of this work.24 3. Experimental Section 3.1. Materials. Solutions of a 2500 Mw perfluoropolyether diamide (PFD) in CO2 were used in this work for the experimental study and theoretical modeling of the RESS process. This polymer has the following chemical structure
PFD is a liquid at ambient conditions, and it has known pressure-volume-temperature properties.28 Carbon dioxide of bone-dry grade (99.8% minimum purity) was supplied by National Welders Co. The physical properties of CO2 are well-known.29 All materials were used as received. 3.2. Phase Equilibrium Apparatus and Experimental Procedure. The experimental study of phase boundaries (cloud points) of the PFD/CO2 solution was performed using the static method.30 Cloud-point pressures corresponding to fixed solution compositions were measured along isotherms to generate a set of P-T-X data that would be sufficient for modeling PFD/CO2 phase behavior using an equation of state. The experimental setup is shown schematically in Figure 2. It is built around a 6-cm3 stainless steel cell (K) equipped with sapphire windows. The cell, loaded with a known amount of polymer, is connected to a CO2 supply system consisting of a cylinder with compressed CO2 (P), gas connections, valves (V1-V4), a HIP pressure generator (M), and an ISCO high-pressure syringe pump (N). This system allows for the purging and filling of the cell with known amounts of CO2 and for the pressurization of the cell. The composition of the polymer/CO2 solution is determined to within 0.002 wt % and is maintained constant during the measurements. The contents of the cell are mixed with a stir bar activated by a magnet (R) located below the cell. The cell is immersed in a thermostated air-bath (Q). The temperature of the bath is regulated to within (1 °C by an Omega temperature controller (D) connected to a heater (H). The temperature of the polymer/CO2 solution is measured using a K-type thermocouple placed inside the cell and an Omega temperature readout (F) with a systematic uncertainty of 0.8 K. The HIP pressure generator (M) is used to change the pressure in the cell. The pressure in the cell is measured using an Omega pressure
Figure 2. Phase equilibrium apparatus: A, thermostated bath; B, video camera; C, television monitor; D, temperature controller; E, pressure readout; F, temperature readout; G, fan; H, heater; I, rupture disk; J, pressure transducer; K, view cell; L, check valve; M, pressure generator; N, syringe pump; P, CO2 cylinder; Q, air bath; R, stirrer; V1-V5, valves.
transducer (J) with an accuracy of 2 bar. The image of the solution in the cell is projected onto a television monitor (C) using a video camera (B) placed against the sapphire window. Cloud-point pressures were typically measured three times at constant temperature and composition to minimize operator error. For a given concentration and temperature, the pressure in the cell is raised until the polymer/CO2 solution begins to form a single phase. At this point, the pressure is further increased until a completely homogeneous phase is present. Once this state has been achieved, the pressure is gradually lowered until phase separation begins. The equilibrium condition is defined to be the pressure at which opalescence (cloudiness) in the solution is first observed as the pressure is lowered. These measurements were reproducible to within (3% (i.e., (90 psia at 3000 psia) at each temperature and composition. Experimental data were taken first at a lower temperature and then at a higher temperature. 3.3. RESS Experimental Procedure. The RESS experimental apparatus is shown schematically in Figure 3. It consists of three major units: a 360-cm3 high-pressure stainless steel view cell (F) with sapphire windows, an ISCO syringe pump (C), and a preexpansion unit (J). A solution of polymer in CO2 is prepared gravimetrically at ambient temperature in the view cell. The pressure of CO2 in the cell is set above the solubility limit. When the polymer is dissolved in the cell, the solution is transferred into the syringe pump. The polymer/CO2 solution is then pressurized by the syringe
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Figure 3. RESS setup: A, CO2 cylinder; B, check valve; C, ISCO syringe pump; D and L, Omega temperature controllers; E, temperature readout; F, view cell; G, thermostated bath; H, computer; I, analog-digital interface; J, preexpansion unit; K and M, Omega pressure transducers; N and O, rupture disks, P, heater, V1-V6, valves.
Figure 4. Interior of RESS nozzle geometries.
pump and is heated to the desired starting process conditions (Ppump and Tpump). The temperature of the solution in the syringe pump is regulated by circulating a water/ethylene glycol mixture from the thermostated bath (G). Before the solution leaves the nozzle, it is pumped at pressure Ppump to the preexpansion unit and is heated isobarically to the preexpansion temperature Texp by being passed through the heated coil leading to the nozzle. The supercritical solution is allowed to expand to ambient pressure through the nozzle. Temperatures in the preexpansion unit and in the nozzle are controlled by Omega temperature controllers D and L and are maintained constant to within 3 K during the solution expansion. The pressure and temperature of the polymer/CO2 solution are measured upstream of the nozzle/pinhole orifice by Omega pressure transducer K and K-type thermocouple with uncertainties of 1 bar and 0.8 K, respectively. All measured parameters (i.e., pressure and temperature of the flow and flow rate) are recorded during the spraying process. The typical amount of polymer solution in the pump is between 100 and 150 mL, which is sufficient for up to 3-4 min of spraying time at the selected operating conditions. A feature of the apparatus is its capacity for independent control of the temperatures in the bulk part of the solution (Tpump) and in the region before the nozzle (Texp) with simultaneous maintenance of a constant solution concentration. This allows a desired process path to be set and RESS product characteristics to be correlated with the operating conditions. Figure 4 presents the interior dimensions of two of the nozzle geometries tested in this work. The majority of the results were obtained from a geometry that
terminated with an 11-mm-long, 150-µm-i.d. capillary tube. This nozzle smoothly blends the reservoir section with the outer edge of the capillary tube. Other results were obtained using nozzles terminating with a 25-mmlong, 150-µm-i.d. capillary tube (Figure 4) and a 70-µmi.d. pinhole (not shown). The blended nozzle design was selected first, as it provides a smooth acceleration of the flow into the capillary tube. This design, though attractive, was difficult to fabricate and was replaced with the configuration shown in the top of Figure 4 when it became clogged. Most RESS spray characterizations described in the literature have been performed via off-line techniques. This is because most materials processed by RESS to date are solids at ambient conditions. In our experiments, droplet characterization has been performed online using a nonintrusive laser technique based on Fraunhofer diffraction. A Malvern Spraytec model RTS 5006 droplet sizing system capable of measuring particle diameters between 0.5 and 200 µm was used. The instrument consists of a transmitter unit, which produces a horizontally oriented collimated laser beam, and an in-line receiver unit separated by approximately 300 mm. The spray is directed downward through this beam between the transmitter and receiver units. The instrument measures the light scattered from the droplets passing through the laser beam at various angles using a solid-state ring detector located in the forward scattering direction. The accompanying software calculates the droplet size distribution necessary to produce the measured scattered light energy. The droplet size distribution (volume frequency vs diameter), the diameters at 10% [Dv(10)], 50% [Dv(50)], and 90% [Dv(90)] of the cumulative volume, the Sauter mean diameter (D[3][2]), the distribution span, the volume concentration (Cv), and the percent light transmission through the spray are output. Measurements reported in this work were taken 152 mm downstream of the nozzle exit plane. 3.4. Polymer Transfer Efficiency Measurements. Polymer transfer efficiency (TE) measurements were performed by spraying a PFD/CO2 solution through the 150-µm-i.d., 11-mm-long capillary nozzle onto a 5 × 5 cm sandstone substrate placed at a distance of 5 cm from the nozzle exit. The measurements were performed for a solution of 2 wt % of PFD at 165 bar (2400 psia) at various preexpansion temperatures. The transfer efficiency is defined as the mass of PFD present on the substrate after coating divided by the initial mass of PFD in the solution and is expressed as a percentage value. The mass of PFD on the substrate was measured by weighing the stone sample before and after coating. 4. Results and Discussion 4.1. Equilibrium Thermodynamics of PFD/CO2 Solution. Measured cloud-point pressures of PFD/CO2 solutions at six compositions (0.92, 1.42, 2.06, 2.64, 4.12, and 5.29 wt % of PFD) along seven isotherms (35, 40, 50, 60, 70, 85, and 100 °C) are presented in Figure 5. Polymer solutions of this type have been successfully described by equations of state based on lattice fluid theory, such as that developed by Sanchez and Lacombe.31 Reasonably good agreement between measured cloud-point data and predictions using the SanchezLacombe equation of state was obtained by using the nonlattice version of the lattice fluid model developed by Sanchez, Panayiotou, and co-workers.32 Details of
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Figure 7. Transfer efficiency of the RESS process at different normalized temperatures Tn ) Texp/Ts (165 bar, 2 wt % of PFD /CO2 solution, and L/D ) 150 µm/11 mm). Figure 5. Phase diagram of PFD/CO2 solution.
Figure 8. RESS process paths in the nozzle. Figure 6. Measured and predicted cloud-point curves.
this implementation can be found in part 2 of this work.24 Best-fit predictions are shown in Figure 6. Close agreement is evidenced at the lower temperatures of operation of our experiments. Larger deviations are observed at higher temperatures. 4.2. RESS Spray Characterization: Transfer Efficiency Measurements. Transfer efficiency (TE) is defined as the amount of polymer deposited on the substrate divided by the total amount of polymer in the solution. It is conjectured that the TE results are related to the size of the precipitate and, consequently, that they can be correlated to specific operating conditions. Figure 7 illustrates the effect of preexpansion temperature Texp on transfer efficiency of a 2 wt % solution of PFD at 165 bar (2400 psia). Overheating of the solution above its equilibrium temperature Ts for a given pressure and concentration (Ts ≈ 73 °C for 2 wt % at 2400 psia) results in solute precipitation prior to the flow entering the nozzle. The physics of the process change from the growth of droplets to the mechanical dragging of prematurely precipitated polymer through the nozzle. These process conditions result in high transfer efficiencies (up to 50%) but produce tendrils of polymer, rather than droplets. Further increases in the preexpansion temperature (Tn ) Texp/Ts ) 1.3) lead to substantial flow rate reduction as a result of deposition of polymer film on the inner surface of the nozzle, and
the nozzle eventually becomes plugged. These effects have been noted in earlier investigations.7 The transfer efficiency reaches its lowest values for preexpansion temperatures near the equilibrium temperature and increases as the preexpansion temperature is lowered further. Figure 8 shows a comparison of pressuretemperature process paths for a preexpansion temperature near the equilibrium temperature (60 °C) and a lower temperature (25 °C). The pressure-temperature traces are obtained from the fluid-dynamic model discussed in part 224 (assuming pure CO2) and indicate a transition from a supercritical fluid response to a more gas-dynamic response upon expansion. Experimental cloud-point data are also shown in the figure. The plot indicates that the process path at higher preexpansion temperatures crosses the cloud-point curve, initiating liquid-liquid phase separation within the nozzle. In contrast, the process path at lower temperatures bypasses the liquid-liquid equilibrium region, and liquidvapor phase separation is initiated at the very exit of the nozzle and in the free-jet region. Judging from these data, one would expect the higher preexpansion temperature to result in higher transfer efficiency, as more time is available for droplet growth as the fluid flows through the nozzle. Although experimental error cannot be discounted, the on-line measurements discussed subsequently also lend support to this counterintuitive observation of an increase in transfer efficiency with decreasing Texp.
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Figure 10. Experimental droplet diameters at 50% cumulative volume Dv(50). Figure 9. Transfer efficiency of the RESS process at different concentrations of the PFD/CO2 solution (165 bar, Tn ) Texp/Ts ) 0.8, and L/D ) 150 µm/11 mm).
Figure 9 illustrates the effect of solution concentration on transfer efficiency. The result is as expected, in that increases in solution concentration lead directly to increases in the transfer efficiency. This result is also consistent with the physical picture of droplet growth due to nucleation, condensation, and coagulation, as developed in part 2.24 Droplet nucleation and condensation rates are directly proportional to the amount of solute in solution, meaning that more droplets occupying a larger amount of volume will be generated for higher concentrations. As shown in part 2,24 the average droplet size is directly related to the occupied volume fraction of the droplet phase. Coagulation rates are also proportional to the occupied volume fraction, meaning that larger droplets will be formed initially and will coalesce to form even larger droplets. 4.3. RESS Product Characterization: Droplet Size Distributions. The effects of preexpansion temperature, solution composition, and nozzle geometry on the RESS product characteristics are shown in Figures 10-14. The concentration of the polymer solutions varied from 0.7 to 5.1 wt %, and the preexpansion temperatures (Texp) ranged from 25 to 67 °C. The preexpansion pressure was set to 165 bar (2400 psia) for all measurements. The effect of process conditions on the droplet diameter at 50% cumulative volume [Dv(50)] is illustrated in Figure 10. The measured values range between 2 and 4 µm and appear to be relatively independent of both temperature and composition for a particular nozzle geometry. The Sauter means diameter (D[3][2]), a measure of the average droplet size, ranges between 1 and 4 µm for all process conditions (Figure 11). A general decrease in the Sauter mean diameter with increasing preexpansion temperature is evidenced, as is an increase with increasing concentration at fixed temperature. Although data scatter is significant, these trends provide some support for the conjecture that transfer efficiency increases with increasing average droplet size. The influence of nozzle geometry on the droplet size measurements is significant, with the 25mm capillary tube promoting the growth of larger particles than the other nozzles. Somewhat surprisingly, the pinhole nozzle produced larger particles than the 11-mm capillary tube.
Figure 11. Experimental Sauter mean droplet diameters (D[3][2]).
Droplet size distributions, such as those shown in Figures 12 and 13, reveal other effects of the process conditions. Low temperatures and high solution concentrations favor the formation of highly uniform droplets, whereas higher temperatures and low concentrations result in more dispersed droplet size distributions (Figure 12). The effect of the nozzle length alone on the uniformity of polymer droplets is less conclusive (Figure 13). The effects of the process conditions on the volume concentration (Cv) of polymer droplets in the spray are shown in Figure 14. For ease of comparison, Cv is normalized by its largest value (18.8), which occurs at a concentration of 5.1 wt % at a preexpansion temperature of 25 °C. Volume concentration is a measure of the detectable amount of precipitate volume per unit of total volume. As the detection limit of the Spraytec analyzer is around 0.5 µm, particles smaller than this size are not reflected in the volume concentration measurement. The Cv values generally mirror those discussed earlier, in that more detectable (larger) droplets are formed at higher polymer concentrations and at lower temperatures. The lowering of the Cv with increasing temperature and decreasing concentration most probably means that substantial numbers of undetectable, submicron droplets are being formed. For the higher temperatures, it is also possible that some polymer is being deposited on the capillary tube walls. The former explanation raises the possibility that the actual droplet size distribution is bimodal, with the first peak occurring somewhere within the submicron range
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Figure 12. Effect of preexpansion temperature and solute concentration on droplet size distribution at constant preexpansion pressure (165 bar) and nozzle dimensions (L ) 11 mm and i.d. ) 150 µm) (Tn ) Texp/Ts).
Figure 13. Effect of preexpansion temperature and nozzle dimensions on droplet size distribution at constant preexpansion pressure (165 bar) and solute concentration (2 wt %) (Tn ) Texp/Ts).
and the second at the detectable size of 2-3 µm. The process conditions (including structural changes in the
jet hydrodynamics) govern the redistribution of the precipitate mass between these peaks. The possibility
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A key factor in this scenario is the possibility of significant droplet and/or particle growth due to turbulent coagulation in the intense mixing regions of the jet, as developed further in part 2 of this work.24 5. Conclusion
Figure 14. Effect of preexpansion temperature, solute concentration, and nozzle dimensions on volume concentration of droplets larger than 0.5 µm in the spray.
of bimodal size distributions in RESS was raised earlier in the literature by Kwauk and Debenedetti21 and was attributed to the competition between nucleation and condensation processes for available solute. As shown in part 2 of this paper,24 droplet coagulation in regions of high shear and high turbulence intensity might play a key role in the formation of larger, detectable droplets, whereas nucleation/condensation processes might dominate the development of submicron droplets. The on-line spray characterization results, although self-consistent, do conflict with some other published results regarding the influence of preexpansion temperature on precipitate size and deserve further discussion. Mawson et al.,11 in particular, showed that a decrease in preexpansion temperature below the cloudpoint temperature (Tn < 1) leads directly to decreases in the size of precipitates of poly(1,1,2,2-tetrahydroperfluorodecyl acetate), a crystalline fluoropolymer that is highly soluble in CO2. Similarly, Lele and Shine7 report changes in the morphology of -polycaprolactone from supermicron fibers to submicron particles with a decrease in preexpansion temperature. In both cases, precipitate sizes were measured off-linesan impossibility in the present work, as PFD is liquid at room temperature. Theoretical work by Kwauk and Debenedetti,21 as well as that presented in part 2,24 also indicates that lower preexpansion temperatures, under idealized conditions, promote the formation of smaller particles. One possible explanation of the present results starts with the realization that, for preexpansion temperatures near room temperature, the temperatures encountered in free-jet expansion and beyond are low enough that formation of solid CO2 is a distinct possibility (see part 224). Solid CO2 particles, if formed in substantial numbers at micron sizes, could both bias the droplet sizing system toward larger particles and promote coagulative growth of polymer-solid CO2 aggregates that would maintain higher momentum than the polymer droplets alone. Transfer efficiencies might thus increase, even though the polymer droplets formed might actually be smaller than those at higher temperatures. One supporting factor for this conjecture is the presence, in some cases, of solid CO2 on the substrate after coating, although conversely, particle size measurements performed without polymer do not typically reveal the presence of particles larger than the detection limit. It is possible, though, that the presence of polymer droplets could trigger heterogeneous nucleation of solid CO2 particles, leading to the events mentioned above.
Experimental results on the formation of perfluoropolyether diamide droplets during the RESS process have been presented as part 1 of a two-part experimental/ computational study. The effects of process conditions on mean droplet size, droplet size distribution, and transfer efficiency have been discussed in detail. The data indicates that an increase in solution concentration results in an increase in mean droplet size, an increase in transfer efficiency, and a narrowing of the droplet size distribution. Decreases in the preexpansion temperature below its cloud-point value (for fixed pressure and composition) also result in an increase in mean droplet size, improved transfer efficiency, and a narrower size distribution. Precipitation in this case occurs at the nozzle exit or in the free-jet area, whereas, at temperatures near the cloud-point value, liquid-liquid phase separation and the initiation of droplet formation occur within the capillary tube. Indirect evidence based on spray volume concentration measurements indicates that substantial numbers of polymer droplets are being formed at submicron sizes, particularly for low solution concentrations and high temperatures. This raises the possibility of a bimodal droplet size distribution, with the growth of larger, detectable droplets influenced by coagulation mechanisms and the growth of submicron droplets governed by primarily by nucleation and condensation. The possibility of droplet coagulation as a key mechanism for droplet growth couples the process strongly with the hydrodynamics of the jet. These considerations and the modeling thereof are developed further in part 2 of this work.24 Acknowledgment This work was supported by U.S. Office of Naval Research; the DuPont Company; the Kenan Institute for Engineering, Technology, and Science; and the National Science Foundation (STC Program, Agreement CHE-9876674). 6. Nomenclature Cv ) volume concentration (PPM) D[3][2] ) Sauter mean diameter (µm) Dv(50) ) average particle size distribution (µm) Gm ) Gibbs free energy of mixture (J/mol) Pc ) critical pressure (bar) Ppump ) pressure of ISCO pump (psia) Tc ) critical temperature (°C) Texp ) expansion temperature (oC) Tn ) normalized temperature Tpump ) temperature of ISCO pump (K) Ts ) equilibrium temperature (oC) Xi ) mole fraction of component i µi′ ) chemical potential of component i stable phase (J/molecule) µi′′ ) chemical potential of component i metastable phase (J/molecule) Literature Cited (1) Castelvetro, V.; Aglietto, M.; Montagnini di Maribello, L.; Toniolo, L.; Peruzzi, R.; Chiantore, O. Adapting the Properties of New Fluorinated Acrylic Polymers to Suit the Conservation of Ancient Monuments. Surf. Coat. Int. 1998, 11, 551.
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Received for review March 22, 2001 Revised manuscript received October 3, 2001 Accepted October 5, 2001 IE010267M
