Formation of Nylon Particles and Fibers Using Precipitation with a

were obtained. However, under certain circumstances, oblong particles (short fibers), fibers, and thin film morphologies were observed by manipulating...
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Formation of Nylon Particles and Fibers Using Precipitation with a Compressed Antisolvent YoonKook Park, Christine W. Curtis, and Christopher B. Roberts* Department of Chemical Engineering, Auburn University, Auburn, Alabama 36849

Micrometer-sized particles and fibers of nylon 6/6 were produced by the means of precipitation with a compressed antisolvent (PCA). Specifically, nylon 6/6 micron-sized particles were produced by expanding a nylon/formic acid solution through various capillary nozzles into compressed carbon dioxide antisolvent. The effect of various operating parameters, such as the nylon/formic acid solution concentration, nozzle length and diameter, and nylon solution injection velocity, were examined, and no significant effect of varying operating conditions in the mean particle size, and particle size distribution was observed. In almost all of the cases, spherical particles were obtained. However, under certain circumstances, oblong particles (short fibers), fibers, and thin film morphologies were observed by manipulating various process parameters, including nylon solution concentration and the manner of spraying the solvent/nylon solution into the carbon dioxide antisolvent. The lack of significant influence of the varying operating conditions on the mean size may be explained in that the surface tensions of the sprayed droplets diminish at very short distances from the nozzle outlet as compared to the jet breakup length. In other words, the disappearance of surface tension stimulates gaslike mixing, without the formations of discrete droplets. Introduction Supercritical fluid (SCF) solvents have recently received a great deal of attention in the production of micron-sized materials. Environmentally benign SCF solvents, such as carbon dioxide, can be used as replacement solvents for traditional toxic solvents in many polymer and materials processing applications.1-6 Because of the high volatility of the SCF solvents used, a high-purity material with low residual solvent content material can be recovered from SCF solutions.7 One such application involves the precipitation of a polymer (e.g., solute) from an organic solution using a compressed or supercritical fluid as an antisolvent. This method is known as the PCA (precipitation with a compressed fluid antisolvent) or SAS (supercritical antisolvent) process. The formation of polymer particles and fibers using PCA is attractive because it allows for the efficient production of micron-sized particles,2-4,7 provides low residual solvent levels in the polymer products,7 and through the use of CO2 solvent, is an environmentally friendly process.8 In addition to the supercritical CO2’s high solubilization capacity for the liquid solvent, which enables the rapid extraction of the solvent from the polymer, large quantities of CO2 can also diffuse and dissolve into the polymer.9 In the PCA process, a polymer/solvent solution is sprayed through a nozzle or capillary into a pressurized chamber that contains a compressed gas or SCF antisolvent. The SCF (or the compressed gas) rapidly diffuses or dissolves into the polymer/solvent solution, thereby rapidly decreasing the local solvent strength of the solution, while the solvent is rapidly extracted and dissolved into the gas or the SCF antisolvent. The polymer (or organic), which is insoluble in SCF, rapidly precipitates out of solution in the PCA process. * To whom correspondence should be addressed. Fax: 334-844-2063. Phone: 334-844-2036. E-mail: croberts@ eng.auburn.edu.

Many studies have shown that, typically, microspherical particles are obtained in cases where very dilute concentrations of polymer in liquid solvent were sprayed into SCF CO2. However, small variations in process conditions, such as the concentration of the solute (polymer)/solvent solution, can produce a variety of different morphology types from the PCA process. To address the change of morphology as well as the particle size and particle size distribution obtained from the PCA process, several fundamental studies have been performed. In 1993, Dixon et al.3 focused their attention on the dimensionless Weber and Ohnesorge numbers to estimate particle size trends in the PCA process. The Weber number (We ) FΑV2D/σ) is a ratio of inertial forces to surface tension forces (where FΑ is the antisolvent density, V is the relative velocity, D is the drop diameter, and σ is the interfacial tension), and the Ohnesorge number (Oh ) We1/2/Re), relates viscosity to surface tension forces, where Re is the Reynolds number. This work suggested that the low interfacial tensions and high injection velocity (due to pressure difference across the nozzle) would lead to an extremely large Weber number relative to that for a spray in a low-pressure gas. A large Weber number indicates that the deforming external forces are large as compared to the reforming surface forces, thus leading to drop breakup into smaller droplets. Thus, according to this theory, smaller particles should be obtained at a higher Weber number in the PCA process. This trend of increasing particle size (distribution) with increasing pressure difference (injection velocity) is not always observed. For example, Randolph et al.2 reported that particle sizes tend to increase modestly with increasing upstream pressure for poly (L-lactic acid) particles precipitated from methylene chloride in SCF CO2. They suggest that particle nucleation and growth rather than initial droplet size are the major determining factors of particle size.2 In this theory,

10.1021/ie010566b CCC: $22.00 © 2002 American Chemical Society Published on Web 02/13/2002

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within each droplet formed from atomization of the solvent jet, the rapid mass transfer of antisolvent and solvent results in high supersaturations for the solutes. High supersaturation causes rapid nucleation and results in the growth of more than one particle per primary droplet.9 However, most studies have shown that only modest changes of particle size and particle size distribution are observed over a significant range of operating conditions. For example, Griffith et al.7 showed very little influence of large changes in operating conditions on the mean particle size and particle size distribution of nylon precipitated from formic acid in SCF CO2. This nylon system was cited as an example in a recent paper by Lengsfeld et al.9 that addresses the fact that most studies of the PCA process have demonstrated modest control over the mean particle size and distribution. Lengsfeld et al.9 reported the linear jet breakup length along with the dynamic surface tension in immiscible, partially miscible, and miscible systems to explain the limited dependence of the PCA process on various process parameters. They demonstrated that the surface tensions of the solute (polymer)/solution diminish at very short distances from the nozzle outlet, which causes no distinct droplets to form. Consequently, a rather uniform particle size and particle size distribution upon changing operating conditions was observed. Werling and Debenedetti10,11 presented a mathematical model for the mass transfer between an isolated solvent droplet and an antisolvent continuum ranging from the subcritical10 to the supercritical regime.11 They investigated the effect of process conditions on the timescales for mass transfer showing that lower pressures and larger initial radii result in longer droplet lifetimes in two-way diffusion of solvent and antisolvent. However, the time for a droplet center to reach saturation does not correlate with droplet lifetime, meaning that droplets with short lifetimes are not necessarily the fastest to reach saturation.10 Although the masstransfer study done by Werling and Debenedetti10 is performed for isothermal processes, they provided valuable insight into the environment to which a solute is exposed within the precipitation process. As the concentration of solute (polymer)/solvent is increased, the injection velocity at a given pressure drop across a nozzle will be decreased along with the increase of the viscosity of the solution, and a dramatic change of morphology can be observed. Luna-Barcenas et al.,12 for example, reported that a transition to fiber formation from discrete (spherical) particles may be explained in terms of a scaling law. Further, the transition concentration depends strongly on the type of polymer and molecular weight.12 Our previous work,7 in which we observed little influence of changing operating conditions on particle size and distribution, mainly focused on the effect of upstream pressure (upstream of the nozzle), downstream (antisolvent) pressure, and the concentration of the polymer/solvent solution (nylon 6/6 in formic acid) in a process for recovering nylon 6/6 particles from waste carpet. The focus of the work presented here is to test the generality of these results and to further explore the influence of various process parameters on particle size and morphology from this PCA process. We examine, here, pure nylon 6/6 (as opposed to the nylon material recovered from waste carpet which may have contained various other species including dyes, pig-

Figure 1. Experimental apparatus. (CP: collection plate; N: 50, 100, and 154 µm spray nozzle; P: pressure transducer; SP: Isco syringe pump.)

ments, and other surface treatments) precipitated from a formic acid solution using SCF CO2 antisolvent. In this paper, we examine the influence of various operating parameters, such as nylon 6/6 solution concentration, nozzle configuration, and nylon solution injection velocity on the morphology and particle size of pure nylon 6/6 precipitated from formic acid solution by spraying into SCF CO2 antisolvent. 2. Experimental Section 2.1. Materials. Nylon 6/6 (Mv ) 20 000) was purchased from Scientific Polymer Products (Ontario, NY). Formic acid at 88% purity (major impurities were water at 11.5% and acetic acid at 0.4%) was purchased from Aldrich (St. Louis, MO) and used as received. Carbon dioxide (SFC grade) was obtained from BOC Gases, Inc. (Bessemer, AL). 2.2. Apparatus and Method. The experimental setup used for this study of the antisolvent precipitation of nylon 6/6 using SCF CO2 can be found elsewhere7 and is shown in Figure 1. A Jerguson sight gauge (model R-12) was used as the collection (precipitation) vessel. Nylon 6/6 was dissolved in formic acid at appropriate concentrations, and the liquid solution was placed in the sample side of a stainless steel piston/cylinder assembly. We have determined that the solubility of this nylon 6/6 material in the 88 wt % formic acid solution can be greater than 15 wt %. The piston/cylinder assembly was then compartmentalized with two valves, as shown in Figure 1, and the pressure on the piston was then maintained with a high-pressure nitrogen tank or a model 260D Isco high-pressure syringe pump. Tubing of varying inside diameter (50, 100, and 154 µm) was used as a spray nozzle and was attached to one end of the collection vessel. A glass plate was placed inside the vessel for particle collection. Antisolvent (CO2) was delivered to the collection vessel using another Isco 260D high-pressure syringe pump programmed to operate at a constant desired pressure. The system temperature was maintained during all experiments by use of a water bath fitted with an immersion circulator. The system pressure was measured using a pressure transducer (Omega, model PX945). The computer controlled syringe pump, jacketed for heating/cooling, was charged with CO2, which was subsequently brought to and kept at experimental

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Table 1. Experimental Conditionsa nozzle diameter (µm)

polymer concentrationb (wt %)

1 2 3g

154 154 154

0.6 1.2 2.5

622 593 404

2.7 ( 0.007 4.1 ( 0.005 7.5 ( 0.003

4 5 6g

100 100 100

0.6 1.2 2.5

1492 1001 688

7 8 9g

100 100 100

10i 11 12 13

100 50 50 50

run

5 10 15 0.6 0.6 1.2 2.5

relative velocityc (cm/s)

kinematic viscosityd (mm2/s)

mean particle sizee (µm)

Re

(Ref/ Re(conc ) 0))

(Wef/ We(conc ) 0))

(Ohf/ Oh(conc ) 0))

2.9 2.9 3.3

355 ( 0.5 218 ( 0.3 80 ( 0.05

0.3746 0.2305 0.0844

0.4951 0.4499 0.2089

1.9 2.9 5.4

2.7 ( 0.007 4.1 ( 0.005 7.5 ( 0.003

3.6 3.3 3.2

553 ( 0.8 244 ( 0.3 92 ( 0.06

0.4180 0.1847 0.0694

0.6497 0.2925 0.1382

1.9 2.9 5.4

498 ( 121 120 ( 48 88 ( 12

17.9 ( 0.013 85.9 ( 0.691 120.3 ( 0.190

2.8 N/Ah N/A

28 ( 6 1.4 ( 0.6 0.7 ( 0.1

0.0210 0.0011 0.0006

0.0724 0.0042 0.0023

12.8 61.4 85.9

N/A 401 365 184

2.7 ( 0.007 2.7 ( 0.007 4.1 ( 0.005 7.5 ( 0.003

3.1 2.0 2.3 2.9

N/A 75 ( 0.2 45 ( 0.05 12 ( 0.02

N/A 0.2435 0.1459 0.0402

N/A 0.2200 0.1822 0.0463

N/A 1.9 2.9 5.4

particle shape spherical spherical oblong/ spherical/ fiber spherical spherical oblong/ spherical spherical spherical spherical/ fibers spherical spherical spherical spherical

a Material, nylon 6/6; temperature, 313 K; F 3 CO2, 0.51 g/cm ; nozzle length, 20 mm; upstream pressure, 128 bar; down stream pressure, 88 bar. b Estimated error on concentration, (0.02 wt %. c Estimated error on relative velocity, (5%. d Mean values of kinematic viscosity and standard deviation from three repeated experiments. e Estimated error on the mean particle size, (0.5 µm. f Ratio of three dimensionless numbers of Reynolds (Re), Weber (We), and Ohnesorge (Oh), assuming constant surface tension (σ). g Nonspherical particles were obtained. h N/A, not available. i Nozzle length, 60 mm.

temperature and pressure. The jacket was also connected to an immersion circulator operating at the desired temperature. After the collection vessel was allowed sufficient time to equilibrate and the polymer solution was pressurized, the valve restricting the flow of the polymer solution was opened briefly, causing a liquid jet to spray into the collection vessel. The precipitation of the nylon was noted by the formation of a very cloudy region inside the vessel. Following precipitation, the vessel was purged with more than three equivalent volumes of pure carbon dioxide at the experimental temperature and pressure to remove all of the formic acid solvent from the system. The collection vessel was then depressurized slowly while being maintained at the experimental temperature so as to avoid formation of liquid carbon dioxide within the cell. The glass collection plate was removed when atmospheric pressure was reached, and the particles collected were analyzed on a Zeiss DSM-940 scanning electron microscope (SEM). SEM micrographs of the recovered nylon were obtained and used for particle sizing (perimeter weighted average size distributions) and investigation of morphology. The viscosity of the nylon/formic acid solutions were obtained using an Ubbelohde calibrated viscometer tube (Fisher, Pittsburgh, PA) and are given in Table 1 over a wide concentration range at 298 K. The injection velocity was calculated by measuring the change in mass of a nylon 6/6 solution during a given injection time through a nozzle of given inside diameters. Given the mass flow rate, the density of the solution, and the nozzle diameter, the velocity of the solution through the nozzle can be determined (Table 1). A series of experiments were performed to investigate the influence of nozzle diameter, nozzle length, and nylon concentration on the recovered nylon’s morphology and particle size distribution. To investigate the influence of each process variable, experimental conditions were chosen such that a comparison of results could be made in which only one process variable was manipulated. Table 1 shows the experimental conditions used during each experimental run.

Figure 2. SEM micrograph of pure nylon particles recovered using a 154 µm nozzle diameter, a 20 mm nozzle length, a 1.2 wt % polymer solution, an upstream of 128 bar, and a downstream pressure of 88 bar at 313 K (run 2).

3. Results and Discussions The morphology and particle size distribution of nylon 6/6 that is obtained from the PCA process is potentially governed by a number of factors. As presented in the experimental conditions in Table 1, the effect of nozzle diameter and nozzle length as well as nylon solution concentration with constant pressure difference (same upstream pressure and antisolvent pressure) have been examined. Here, we discuss the trends in morphology, mean particle size, and particle size distribution resulting from the manipulation of these factors. 3.1. Particle Morphology. Figure 2 presents an electron micrograph of typical particles obtained from the precipitation of pure nylon 6/6 from formic acid using SCF CO2 with an upstream pressure of 128 bar and a downstream pressure of 88 bar (runs 1-13). The example presented in Figure 2 was obtained using a polymer concentration of 1.2 wt % and a nozzle diameter of 154 µm (run 2). Spherical particles in nature were typically obtained as shown in Figure 2. In addition,

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Figure 3. SEM micrograph of pure nylon particles obtained using a 154 µm nozzle diameter, a 20 mm nozzle length, a 2.5 wt % of polymer solution, an upstream of 128 bar, and a downstream pressure of 88 bar at 313 K (run 3). (a) The scale bar represents 10 µm, and (b) the scale bar represents 5 µm.

most of the particles are narrowly distributed in the range of 1 to around 10 µm. On random occasion, particles of unexpected morphology occurred. As the polymer concentration was increased to 2.5 wt % (runs 3 and 6), rather than obtaining spherical particles, oblong particles (short fibers) with length-to-diameter ratios of 3:1 or greater were obtained, as can be seen in Figure 3a. However, as can be seen at the higher magnification in Figure 3b, the product is composed of nearly homogeneous mono-dispersed oblong particles (short fibers). LunaBarcenas et al.12 mentioned that, beyond a certain critical concentration, a different morphology can be obtained, including fibers. Because of this unexpected morphology originally obtained at the conditions of both runs 3 and 6, the conditions of 3 and 6 were reperformed twice each, yielding nearly spherical particles at the conditions of runs 3 and 6 as reported in Table 1. Hence, only one out of the three experiments at both the 3 and 6 conditions (runs 3 and 6 were each run 3 times) produced the short fibers (oblong particles). When the nylon 6/6 concentration was further increased up to 15 wt % (run 9) the morphology was

Figure 4. SEM micrograph of pure nylon particles obtained using a 100 µm nozzle diameter, a 20 mm of nozzle length, a 15 wt % polymer solution, an upstream of 128 bar, and a downstream pressure of 88 bar at 313 K (run 9). (a) The scale bar represents 20 µm, and (b) the thickness of spinning fiber the scale bar represents 2 µm.

changed dramatically and inconsistently from spherical particles to fiber formation. The experiment at the conditions of 9 at 15 wt % was performed 4 times, with two experiments yielding spherical particles in the range of 10 µm, with one run yielding long fibers as shown in Figure 4, and with one run yielding shorter fibers. Figure 4 shows long nylon 6/6 fibers formed by spraying a 15 wt % solution into CO2 at the conditions of run 9. There are a number of reasons that may cause fibers to be formed. As Dixon et al.3 pointed out, increasing the viscosity decreases the Reynolds number and also hinders the development of any natural jet instabilities, thus delaying jet disintegration. It is likely that the timescale for droplet formation is longer than the timescale for polymer precipitation, resulting in fibers because of the high viscosity. However, the reproducibility of this result could be problematic because only one of the four experiments at the condition of 9 produced the long fibers; spherical particles were formed in two cases, and a different type of fibers were formed in one case. In these PCA experiments, if one were to spray in too much liquid solution (nylon/formic acid), a phase split would occur, resulting in a liquid phase that is rich in formic acid and a fluid phase that is rich in carbon

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Figure 5. Thin porous film of nylon produced by drying of the liquid solution using a 154 µm nozzle diameter, a 20 mm of nozzle length, a 2.5 wt % polymer solution, an upstream of 128 bar, and a downstream pressure of 88 bar at 313 K (conditions of run 3).

Figure 6. Comparison of particle size distributions by varying nozzle diameters (an upstream pressure of 128 bar, a downstream pressure of 88 bar, a 20 mm of nozzle length, and a 2.5 wt % of polymer solution at 313 K; runs 3, 6, and 13).

dioxide. Figure 5 presents the results from an experiment performed at the same conditions of run 3, where too much liquid solution was sprayed into the optical cell and a definite phase split was visually observed. When this happens, the nylon remains dissolved in the formic acid-rich phase, and as the solution is dried through the CO2 flushing stage and depressurization, the formic acid is removed from the nylon, resulting in a porous thin film of nylon.8 This can be seen in the micrograph shown in Figure 5. Therefore, if the amount of polymer solution that is loaded into a given mass of CO2 antisolvent does not result in a miscible solvent/ antisolvent mixture, a porous film of the type in Figure 5 can be formed. 3.2. Effect of Nozzle Diameter and Length. Figure 6 illustrates how the nozzle diameter influences the particle size distribution of nylon 6/6 precipitation from a formic acid solution using SCF CO2. The experimental conditions of a constant upstream pressure of 128 bar, a downstream pressure of 88 bar, and a pure nylon 6/6 concentration of 2.5 wt % in formic acid were maintained, while three nozzle diameters of 50, 100, and 154

Figure 7. Comparison of particle size distributions by varying concentration of nylon 6/6 (an upstream pressure of 128 bar, a downstream pressure of 88 bar, a 100 µm of nozzle diameter, and a 20 mm of nozzle length at 313 K; runs 4-7).

µm were employed. The particle size analysis was done by hand from the SEM micrographs. The maximum estimated error in the sizing of the particles is determined to be (0.5 µm. As can be seen in Figure 6, the vast majority of the particles are less than 6 µm in size at each nozzle diameter condition. Moreover, the results in Figure 6 suggest that nozzle diameter is not an important factor in controlling the particle size distribution. The influence of the nozzle length on the particle size distribution at a fixed nozzle diameter of 100 µm, a constant upstream pressure of 128 bar, a downstream pressure of 88 bar, and a pure nylon 6/6 concentration of 2.5 wt % in formic acid was also investigated. Changing the nozzle length from 20 to 60 mm does not significantly alter the particle size distribution. Interestingly, there are only slight effects of changing the nozzle diameter and length on the particle size and particle size distribution of the nylon 6/6 particles obtained. 3.3. Nylon Concentration. In a previous section in this paper, we showed that the polymer concentration is an important variable in influencing the polymer morphology obtained. To investigate the effect of nylon concentration on the particle size distribution, a set of experiments were performed with the polymer concentrations of 0.6, 1.2, 2.5, and 5 wt % in formic acid at a 128 bar upstream pressure and an 88 bar downstream pressure and with a 100 µm diameter and a 20 mm length nozzle. The condition of 10 wt % was also examined, but unfortunately, SEM images of too few particles were obtained to determine reliable size distributions. As can be seen in Figure 7, the nylon 6/6 particles obtained from the PCA process with nylon 6/6 concentrations from 0.6 to 5 wt % were within the same particle size range with very similar particle size distributions. The mean particle size obtained only ranged from 2.8 to 3.6 µm in these experiments. This result is consistent with that of the Yeo and co-workers13 study in which there was no significant influence of concentration on the particle size distributions in the production of insulin particles using PCA. Overall, the effect of the nylon 6/6 concentration (in the case of pure nylon 6/6 in formic acid) on the

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precipitation experiments was negligible, as can be seen in Figure 7. However, the nylon particles obtained from the precipitation of carpet waste in formic acid using SCF CO2 were observed over the same size range for two different concentrations, 0.6 and 2.5 wt %, with an increase in frequency of the particles that were less than 2 µm in size from 48% to 85%.7 The differences in the particle size distributions obtained from experiments with nylon from carpet waste and those obtained from pure nylon 6/6 may result from impurities in the former, which generally include dyes, pigments, and other surface treatments. 3.4. Effect of Nylon Solution Injection Velocity. To investigate the effect of the injection velocity of the nylon solution on the mean particle size, the mass of the polymer solution injected through the nozzle over a given time duration was measured. Injection velocity was easily obtained by using the mass flow rate, the density of the solution, and the nozzle diameter. In general, as the polymer solution injection velocity is increased, there is a very slight increase in the mean particle size (figure not shown in this paper). However, the change of mean particle size appears to be negligible considering the maximum error of (0.5 µm, which is determined from the potential error incurred in counting and measuring the particles from the electron micrographs. The addition of polymers to a liquid is known to affect the surface tension-driven breakup of laminar jets. In this problem as well as spray atomization14 and splashing,15 the high viscosities of polymer solutions act to retard the breakup process and modify the formation of drops.16 Lengsfeld et al.9 reported that jet atomization behavior determined PCA particle morphology, at least in the limit of the slowest evaporation or mixing conditions. The jet breakup mechanism may be examined in terms of Re and Oh.3 The ratio of various dimensionless numbers (Re, Oh, and We) relative to those in the pure solvent is given in Table 1, assuming a constant surface tension (σ) of the different polymer solutions. As the concentration of polymer solution is increased and the relative velocity decreased, Oh changes a great deal, along with huge increases in Re as well as We. There appears to be no correlation between the mean particle size and Re, as can be seen in Table 1 and Figure 8. The observance of no correlation between increasing Re with decreasing particle size is unlike that studied by Dixon et al.3 A possible explanation for this lack of correlation that we observe is that, in this case, particle nucleation and growth, rather than initial droplet size, are the determining factors of particle size.2 Not to mention, mass transfer between solvent and antisolvent is important in terms of mean particle size and particle size distribution. In the case of nylon 6/6 particles produced here, there is no systematic influence of Re or injection velocity on the particle size distribution and the mean particle size. The event of these PCA experiments being largely independent of Re (and others) might be explained by the theory proposed by Lengsfeld et al.9 They developed a method for predicting the dynamic surface tension and combined this method with linear jet breakup equations to accurately predict jet breakup lengths in miscible solvent-antisolvent system (e.g., methylene chlorideCO2). They found that surface tension approaches 0.01 mN/m in 1 µm from the nozzle outlet for a 10 cm/s

Figure 8. Mean particle size by varying Reynolds number at an upstream pressure of 128 bar and a downstream pressure of 80 bar at 313 K.

methylene cholride jet in 8.5 MPa, 35 °C CO2.9 They suggest that distinct droplets never form because this distance is shorter than characteristic breakup lengths.9 As illustrated in Table 1, variation of the relative velocity ranging from 90 to 1500 cm/s and the kinematic viscosity of polymer solution ranging from 3 to 120 mm2/s did not make an impact on the particle size distribution and mean particle size. The characteristics of the formic acid and CO2 mixture may provide significantly short lengths for the disappearance of the surface tension, resulting in no droplet formation at all. 4. Conclusions In this study, precipitation with a compressed antisolvent (PCA) turned out to be useful for producing micrometer-sized particles and fibers of nylon from pure nylon 6/6 starting material. Similar mean particle size and particle distributions were observed in each of the PCA runs, regardless of the operating conditions. At the higher concentrations, various morphologies were obtained, including spheres, oblong particles, fibers, and thin films. The recovered particle size does not appear to be dependent on the nozzle diameter, nozzle length, and polymer concentration in terms of mean particle size and particle size distribution. The morphology and mean particle size remained unchanged with concentrations ranging from 0.6 to 5 wt % of polymer solution. Literature Cited (1) Gallagher, P. M.; Krukonis, V. J.; Botsaris, G. D. Gas AntiSolvent (GAS) Recrystallization: Application to Particle Design. AIChE Symp. Ser. 1989, 82 (284), 96-103. (2) Randolph, T. W.; Randolph, A. J.; Mebes, M.; Yeung, S. SubMicrometer-Sized Biodegradable Particles of Poly(L-Lactic Acid) via the Gas Antisolvent Spray Precipitation Process. Biotechnol. Prog. 1993, 9, 429-435. (3) Dixon, D. J.; Johnston, K. P.; Bodmeier, R. P. Polymeric Materials Formed by Precipitation with a Compressed Fluid Antisolvent. AIChE J. 1993, 39 (1), 127-139. (4) Dixon, D. J.; Luna-Barcenas, G.; Johnston, K. P. Microcellular microspheres and microballons by precipitation with a vapour-liquid compressed fluid antisolvent. Polymer 1994, 35 (18), 3998-4005. (5) Powell, K. R.; McCleskey, T. M.; Tumas, W.; DeSimone, J. M. Polymers with Multiple Ligand Sites for Metal Extractions in Dense-Phase Carbon Dioxide. Ind. Eng. Chem. Res. 2001, 40 (5), 1301-1305.

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(6) Charpentier, P. A.; DeSimone, J. M.; Roberts, G. W. Continuous Precipitation Polymerization of Vinylidene Fluoride in Supercritical Carbon Dioxide: Modeling the Rate of Polymerization. Ind. Eng. Chem. Res. 2000, 39 (12), 4588-4596. (7) Griffith, A. T.; Park, Y.; Roberts, C. B. Separation and recovery of nylon from carpet waste using a supercritical fluid antisolvent technique. Polym.-Plast. Technol. Eng. 1999, 38 (3), 411-431. (8) Griffith, A. T. A Novel Process for The Separation and Recovery of Nylon from Carpet Waste. M.S. Thesis, Auburn University, Auburn, AL, 1998. (9) Lengsfeld, C. S.; Delplanque, J. P.; Barocas, V. H.; Randolph, T. W. Mechanism Governing Microparticle Morphology during Precipitation by a Compressed Antisolvent: Atomization vs Nucleation and Growth. J. Phys. Chem. B 2000, 104 (12), 27252735. (10) Werling, J. O.; Debenedetti, P. G. Numerical Modeling of Mass Transfer in the Supercritical Antisolvent Process. J. Supercrit. Fluids 1999, 16, 167-181. (11) Werling, J. O.; Debenedetti, P. G. Numerical Modeling of Mass Transfer in the Supercritical Antisolvent Process: Miscible Conditions. J. Supercrit. Fluids 2000, 18, 11-24.

(12) Luna-Barcenas, B.; Kanakia, S. K.; Sanchez, I. C.; Johnston, K. P. Semicrystalline microfibrils and hollow fibres by precipitation with a compressed-fluid antisolvent. Polymer 1995, 36 (16), 31733182. (13) Yeo, S. D.; Debenedetti, P. G.; Radosz, M.; Schmidt, H. W. Supercritical Antisolvent Process for Substituted Paralinked Aromatic Polyamides: Phase Equlibrium and Morphology Study. Macromolecules 1993, 26 (23), 6207-6210. (14) Ferguson, J.; Hudson, N. E.; Warten, B. C. H. The breakup of fluids in an extensional flow field. J. Non-Newtonian Fluid Mech. 1992, 44, 37-54. (15) Cheny, J. M.; Walters, K. Extravagant viscoelastic effects in the Worthington jet experiment. J. Non-Newtonian Fluid Mech. 1996, 67, 125-135. (16) Mun, R. P.; Byars, J. A.; Boger, D. V. The effect of polymer concentration and molecular weight on the breakup of laminar capillary jets. J. Non-Newtonian Fluid Mech. 1998, 74, 285-297.

Received for review July 2, 2001 Revised manuscript received November 26, 2001 Accepted November 26, 2001 IE010566B