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Supercritical fluid-assisted dispersion of ultra-fine pigment red 177 particles with blended dispersants
J. of Supercritical Fluids 39 (2006) 127–134
Supercritical fluid-assisted dispersion of ultra-fine pigment red 177 particles with blended dispersants Hsien-Tsung Wu, Ming-Jer Lee ∗ , Ho-mu Lin Department of Chemical Engineering, National Taiwan University of Science and Technology, 43 Keelung Road, Section 4, Taipei 106-07, Taiwan Received 25 November 2005; received in revised form 6 January 2006; accepted 10 January 2006
Abstract A series of experiments was conducted to investigate the dispersion of 1 wt.% of pigment red 177 particles in propylene glycol monomethyl ether acetate (PGMEA). It was found that 40 wt.% of Hypermer PS3 dispersant could provide better dispersion and could stabilize the pigment particles in PGMEA at ambient condition. With the aid of supercritical carbon dioxide (CO2 ), additional 1 wt.% of CO2 -philic dispersant, Zonyl FSO-100, could further enhance the dispersion. Holding the mixtures in dispersion chamber at higher temperatures and pressures for a sufficient period of time was also favorable to improve the dispersion. The size of the dispersed particles appeared to have a correlation with the phase behavior of the mixtures in the dispersion chamber. The preferable operating conditions are at 348.2 K and 20 MPa, and 20 min of holding time was long enough before rapidly releasing chamber’s pressure. Under the recommended operating conditions, the mean sizes of the dispersoids could be as small as 148 nm, which met the required range of 100–200 nm in industrial applications. The TEM images have proven that the supercritical carbon dioxide-assisted dispersion method could efficiently disperse the pigment particles in PGMEA. © 2006 Elsevier B.V. All rights reserved. Keywords: Dispersion; Supercritical carbon dioxide; Pigment red 177
1. Introduction Pigments are the essential materials in the preparation of photo resists for fabricating color filters, which is one of key components to assembly color liquid crystal displays (colorLCD). For industrial applications, the mean sizes of pigment dispersoids are required to be in a range of 100–200 nm. Using smaller sizes of pigment particles is capable of increasing the color strength, contrast, and transmittance of color-LCD products [1–3]. Wu et al. [4] systematically studied the preparation of nanometric particles of pigment red 177 via a continuous supercritical anti-solvent (SAS) method. The study found the favorable operating conditions to produce the particles as small as 46 nm. However, it deems necessary to further disperse these ultra-fine particles in suitable liquid media for practical use. Conventional dispersion apparatuses, such as a roll mill, a mediumdispersion machine, or a homogenizer, are often adopted to disperse fine particles into liquids, but it is a time-consuming process [5]. Alternatively supercritical fluid technique could be
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applied to efficient dispersion of fine particles into liquid media. Kamiwano et al. [5] developed a supercritical dispersing method, which has been successfully utilized to disperse micrometric carbon black particles into pure water without using any dispersants. With the assistance of supercritical carbon dioxide, pure water and 2 wt.% of carbon black powder were pressurized and heated stepwise from ambient condition up to 10 MPa and 333 K. The mixture was then diverted to an explosion-crashing tank. The resulting sample was still uniformly dispersed after 100 h and the sizes of the dispersed particles were less than 5 m. Cheng et al. [6] used a similar method to disperse the ultra-fine particles of pigment red 177, green 36 and blue 15:6 in PGMEA, respectively. In their series of experiments, the pigment concentration was as dilute as 0.002 wt.% and no dispersants were applied in the dispersion process. Among several others, those previous experimental results revealed that supercritical carbon dioxide was an excellent agent, which could quickly penetrate into the spaces of the aggregates of fine particles and then effectively disrupt the agglomerated particles via a rapid expansion. The stabilization of dispersed particles could be achieved by using some proper dispersants in the dispersing process. It is favorable if dispersants contain the surfactant molecules having both pigment- and solvent-affinity
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ends. Applying additional CO2 -philic surfactants should also be helpful in the dispersion process, since the surfactants are possibly carried by supercritical carbon dioxide and then penetrate into the vacancies of the aggregated particles. Consani and Smith [7] measured the solubility of 130 surfactants in carbon dioxide at 323 K and pressures from 10 to 50 MPa. Among these investigated surfactants, fluorinated hydrocarbons were found to be highly CO2 -philic. Such surfactants are potential candidates for the supercritical dispersion process. In the present study, a supercritical carbon dioxide-assisted dispersion process with blended dispersants was developed to produce the pigment dispersoids whose mean sizes were ranging from 100 to 200 nm. This size range is generally acceptable for preparing photo resists. A series of dispersion experiments was conducted to find the favorable formulations of the blended dispersants and the preferable operating conditions for dispersing 1 wt.% of ultra-fine pigment red 177 particles into PGMEA. The pigment concentration of this study is higher than that of Cheng et al. [6] by about 500 folds. The supercritical carbon dioxideassisted dispersion runs were made at temperatures from 318.2 to 348.2 K and pressures up to 35 MPa. The mean sizes of dispersoid samples were determined with a dynamic laser scattering (DLS) particle analyzer, and the transmittances of dispersoid samples were measured with an UV–vis spectrophotometer. 2. Experimental 2.1. Materials Pigment red 177 (aminoanthraquinone red) was supplied by Ciba Special Chemicals Co., Hong Kong. Its molecular structure has been presented in Wu et al. [4]. Two commercially available dispersants were used in this study. One is a polyester/polyamine copolymeric dispersant, Hypermer PS3, weakly cationic, 100% copolymer (ICI Americas Inc., USA) and the other is a CO2 -philic dispersant, Zonyl FSO-100, F(CF2 CF2 )1–7 CH2 CH2 O(CH2 CH2 O)0–15 H, molecular weight about 725, nonionic, 100% fluorosurfactant (du Pont de Nemours and Co., USA). Propylene glycol monomethyl ether acetate (PGMEA) (purity of 99.9 mass%), a suitable chemical for preparation of photo resists, was purchased from Aldrich, USA, and carbon dioxide (purity of 99.8 mass%) from LiuHsiang Gas Co., Taiwan. These chemicals were used without further purification. 2.2. Apparatus and procedure Fig. 1 is the schematic diagram of the dispersion apparatus. It consists of three main parts: feeding, dispersion, and receiver. The feeding unit includes a carbon dioxide source (1), a filter (2), a high-pressure metering liquid pump (3) (model: NPL-5000, Nihon Seimitsu Kagaku Co., Japan), and a disposable syringe (4) (internal volume of 20 cm3 ). The metering liquid pump was used for delivery of carbon dioxide and the syringe for injection of feeding mixtures of pigment red 177 + PGMEA + dispersants. A stainless steel high-pressure chamber (6) (P/N: CNLX1608316, operable up to 75 MPa, Autoclave Engineering, USA) with
Fig. 1. Schematic diagram of dispersion apparatus.
the dimensions of 1.75 cm inside diameter, 20.32 cm length and 49 cm3 internal volume was served as the dispersion unit. The dispersion chamber was immersed in a thermostatic air bath (5). Its temperature was regulated to within ±0.1 K. The bath temperature was measured with a precision platinum RTD sensor (8) (model: 1560, Hart Scientific Co., USA) to an accuracy of ±0.02 K. A pressure transducer (7) (model: PDCR 4070, ranging 0–35 MPa, Druck, UK) with a digital display (model: DPI-280, Druck, UK) monitored the operating pressure of the dispersion vessel, accurate to ±0.1%. A sample cylinder with an internal volume of 1000 cm3 (Swagelok, USA) was utilized as a receiver (9), which was connected to the bottom of the dispersion chamber for collecting the rapidly expanded dispersoid at atmospheric pressure. The experimental procedure is described as follows. A feeding mixture containing pigment red 177, PGMEA and the dispersants was prepared with an electronic balance. The concentration of pigment in the feeding mixture was kept at 1 wt.% through all the runs of this study. The mixture was vigorously mixed with ultrasonic wave (Branson 5510 ultrasonic wave at 40 KHz, USA) for about 2 h before loading to the dispersion chamber. At the beginning of dispersion process, about 15 cm3 of well-mixed feeding mixture was injected into the dispersion vessel via the syringe, and carbon dioxide was then charged into the dispersion chamber by the liquid pump, under a given flow rate, until the vessel reaches to a pre-specified pressure. The dispersion chamber was maintained at constant temperature and pressure for a period of time, denoted as a holding time. After that, the chamber was depressurized rapidly, and the dispersoid expanded simultaneously into the receiver at atmospheric pressure. The mean size of dispersoid samples was determined with a DLS particle analyzer (Model: LSR, Protein Solutions Ltd., USA). The outputs of the DLS analyzer were calibrated with two standard samples (Latex Microsphere Suspensions 5016A, mean diameter = 160 nm and NanosphereTM Size Standard
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3200A, 200 nm, mean diameter = 199 ± 6 nm, Duke Scientific Co., USA). All dispersion runs were repeated twice at each experimental condition. The mean sizes of the replicated samples could be reproduced to within about ±5%. Three representative samples were also analyzed with a transmission electron microscopy (TEM, Hitachi H-7100, Japan) by depositing the dispersoids on 400 mesh carbon-coated copper grids. The transmittances of four diluted dispersoid samples were measured with a UV–vis spectrophotometer (JASCO V-530, Japan). A volume-variable phase equilibrium analyzer has been installed to investigate the phase boundary of mixtures containing carbon dioxide + dimethyl sulfoxide (DMSO) with or without pigment blue 15:6 [8]. This apparatus was also employed in this study to measure the bubble and the dew points of carbon dioxide + PGMEA from which the critical points at the operating temperatures of dispersion process were estimated. The relationship between the dispersion of pigment particles and the phase behavior of mixtures in dispersion chamber will be discussed later. The volume-variable cell consists of a highpressure generator (Model 62-6-10, High Pressure Equipment Co., USA), a sapphire window (Part No.: 742.0106, Bridgman closure, SITEC, Switzerland), a rupture disk and a circulation jacket. The internal volume of the cell is adjustable manually with a piston screw pump and the cell is operable up to 50 MPa and 473.2 K. The cell’s temperature was maintained by circulated thermostatic water to within ±0.1 K. A pressure transducer (PDCR 407-01, Druck, UK) with a digital indicator (DPI 280, Druck, UK) displayed the cell’s pressure to within an accuracy of ±0.04%. Two precision syringe pumps (Model: 260D, Isco Inc., USA) were utilized to charge carbon dioxide and PGMEA, respectively, under constant-pressure mode. A magnetic stir bar was placed inside the cell to enhance the mixing of the loaded mixture. The weight of each loaded substance was calculated from the known charged volume (accurate to ±0.01 cm3 ) and the density value at the delivery pressure and temperature. While the densities of carbon dioxide were taken from the NIST Chemistry WebBook [9], the densities of PGMEA were determined experimentally by a high-pressure densitometer (DMA 512P, Anton Paar, Austria) with an oscillation period indicator (DMA 48, Anton Paar, Austria) to an accuracy of ±0.0001 g/cm3 . The uncertainty of the composition of the prepared mixtures was estimated to be ±0.003 in mole fraction. The phase behavior of the loaded sample in the cell was observed via a digital camera (Model: DSC-F88/S, Sony, Japan) and a television (Model: PV-C2062, Panasonic, Japan). The bubble point or dew point of the loaded mixture can be determined visually by manipulating manually the position of piston screw pump. The uncertainty of the observed phase transition pressures at a given temperature is about ±0.05 MPa.
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process include pressurization rate, the holding time of pigment mixtures at elevated pressures in the dispersion chamber, the temperature and pressure during the holding stage. 3.1. Dispersion with Hypermer PS3 at ambient condition Preliminary tests were conducted without supercritical carbon dioxide treatment to explore the favorable composition of dispersion agents for stabilizing pigment red 177 particles in PGMEA at ambient condition. The tested samples were prepared by adding different weight percents of dispersion agents, on the PGMEA-free basis, in the mixtures of pigment + PGMEA. The mean size of each sample, denoted as an initial mean size, was measured after well mixing with ultrasonic wave for 2 h. The evolution of the mean sizes of the dispersoid samples was observed every other day. Fig. 2 shows the histogram of the preliminary tests with or without using Hypermer PS3 dispersant up to 7 days after preparation. The sample without the presence of dispersant (denoted as a blank case, shown as a symbol of solid circle in Fig. 2) exhibited the smallest initial mean size, 260 nm, but its size drastically increased up to a micrometric scale on the third day. It was suggested that ultrasonic mixing could temporarily break the original agglomerated pigment particles in PGMEA. However, the particles re-aggregated quickly due to the absence of any dispersants to stabilize the distributed particles in PGMEA. As also seen from the graph, the initial mean sizes are larger than that of blank case while different amounts of Hypermer PS3 were added. It may have resulted from cluster formation of the pigment particles surrounded by the dispersion agent. The mean sizes grew much slower in the presence of dispersant, especially for the samples containing Hypermer PS3 greater than 10 wt.%. Their sizes are substantially smaller than that of the blank case on the third day, implying that the dispersant effectively provided steric barrier to keep the dispersed particles from re-agglomeration. The
3. Results and discussion The objective of this study is to find favorable formulations and operating conditions of the supercritical fluid-assisted dispersion process to produce the dispersoids whose mean sizes meet the required range of 100–200 nm in LCD industrial applications. The investigated operating variables of the dispersion
Fig. 2. Histogram of mean particle sizes variation for the dispersed samples prepared from the operations using different amounts of Hypermer PS3 dispersant at ambient condition.
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mechanism of steric stabilization of the hydrocarbon-based dispersant suggested that the pigment-affinity groups (segments of the polyamines) absorbed onto the pigment particles’ surface and the solvent-soluble chains (segments of the polyesters) stretching into the PGMEA liquid phase, and to establish a sufficient thickness to stabilize the dispersed pigment particles [3,10]. Under lower concentrations of Hypermer PS3 (e.g., less than 10 wt.%) the dispersant may not form enough thickness to prevent the particles from re-agglomeration, whereas high concentrations of Hypermer PS3 (e.g., equal to or greater than 60 wt.%) may result in inevitable flocculation [2]. The experimental results disclosed that using 40 wt.% of Hypermer PS3 (0.4 g Hypermer PS3 to 1 g pigment, on the PGMEA-free basis) yielded better dispersion. The mean size increased from about 300 to 390 nm after 1 day of preparation and maintained about the same size up to the 7th day. 3.2. Dispersion with addition of Zonyl FSO-100 and supercritical carbon dioxide A CO2 -philic dispersant Zonyl FSO-100 was tested in this study. It was found that the initial mean sizes increased monotonically from 335 to 750 nm while the concentrations of Zonyl FSO-100 increased from 3 to 40 wt.%. High concentrations of Zonyl FSO-100 appeared unfavorable to reduce the size of dispersed particles. Fig. 3 illustrates the results of the samples containing 10–60 wt.% of Hypermer PS3 and addition of Zonyl FSO-100 up to 3 wt.% (0.03 g dispersant to 1 g pigment). It also shows that the presence of Zonyl FSO-100 did not significantly affect the mean sizes of the dispersoid samples as the dispersion process was achieved only by ultrasonic mixing at ambient condition. However, there are certain benefits by the addition of this CO2 -philic fluorosurfactant, Zonyl FSO-100, when the dispersion was operated with the aid of supercritical carbon dioxide [7,11]. More dispersion experiments were thus implemented with the treatment of supercritical carbon dioxide. Before the dispersoids were rapidly released from the high-pressure dispersion chamber to the receiver at atmospheric pressure, the pigment mixtures were maintained at 328.2 K and 20 MPa for 20 min in this series of runs. Table 1 lists the experimental results. The initial mean size was reduced down to 250 nm for the sample containing 40 wt.% of Hypermer PS3 after supercritical carbon dioxide-assisted dispersion (run #1), whereas the size was about
Fig. 3. Histogram of mean particle sizes variation for the dispersed samples prepared from the operations using various blended dispersants at ambient condition.
298 nm without supercritical carbon dioxide treatment. It was evident that the supercritical fluid could assist the dispersion of the pigment particles due to, at least partly, the rapid expansion of carbon dioxide from the spaces of the aggregated pigment particles during the process of pressure release. As also seen from runs #2 and #3 in Table 1, the higher concentrations of Zonyl FSO-100 resulted in the larger mean sizes of the dispersed particles with using a single dispersant. The mean size, however, could be reduce effectively by applying the dispersant blends of Hypermer PS3 and Zonyl FSO-100 as shown in runs #4 and #5. In the case of run #4, the mean size decreased down to 201 nm with 40 wt.% of Hypermer PS3 and addition of 1 wt.% of Zonyl FSO-100. The improvement of particles dispersion is attributable to the fact that supercritical carbon dioxide may carry the CO2 -philic Zonyl FSO-100 and penetrate into the inter-particles’ vacancies. During the course of rapid release of the applied CO2 pressure, the expansion of fluid disrupts the aggregated particles, and the dispersed particles are then isolated, mainly, by Hypermer PS3. The dispersant blend of 40 wt.% of Hypermer PS3 and 1 wt.% of Zonyl FSO-100 was used throughout all the following supercritical carbon dioxide-assisted dispersion experiments to
Table 1 Experimental results from the operations in the presence of various combinations of dispersants Run
Hypermer PS3 (g/g pigment)
Zonyl FSO-100 (g/g pigment)
Ambienta mean size (nm)
With CO2 b mean size (nm)
Size’s reduction with CO2 (nm)
1 2 3 4 5
0.4 (40%) 0 (0%) 0 (0%) 0.4 (40%) 0.4 (40%)
0 (0%) 0.40 (40%) 0.03 (3%) 0.01 (1%) 0.03 (3%)
298 750 335 360 363
250 545 265 201 210
−48 −205 −70 −159 −153
a
The samples were prepared with ultrasonic mixing at ambient condition. The samples were prepared with the aid of supercritical carbon dioxide; the charging rate of carbon dioxide was 2.0 cm3 /min at 278 K and 6 MPa, and holding time was 20 min. The holding temperature and pressure were at 328.2 K and 20 MPa, respectively. b
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Table 2 Experimental results from the operations under various charging rates of carbon dioxide and holding timesa Run
FCO2 b (cm3 /min)
Holding time (cm3 /min)
Mean size (nm)
Standard deviation (nm)
6 7 8 9 10 11 12 13
1.0 2.0 4.0 6.0 2.0 2.0 2.0 2.0
20 20 20 20 5 10 40 60
165 153 155 150 235 195 154 155
27.3 11.6 37.6 34.3 37.3 13.0 20.5 17.9
a The holding pressure was 30 MPa and the holding temperature was 338.2 K throughout all the runs. b The volumetric flow rate of carbon dioxide was at the conditions of 278 K and 6 MPa.
find suitable operating conditions for preparing the dispersoids whose sizes are within the acceptable range of 100–200 nm in industrial applications. 3.3. Effects of pressurization rate and holding time Table 2 lists the results of dispersion experiments (runs #6–13) operated at different pressurization rates (i.e., the volumetric charging rates of carbon dioxide at the conditions of 278 K and 6 MPa, FCO2 ) and holding times. All the feeding pigment mixtures have the same composition, the mass ratio of pigment:Hypermer PS3:Zonyl FSO100:PGMEA = 1 g:0.4 g:0.01 g:98.59 g, and all these runs were maintained at 338.2 K and 30 MPa during the holding stage. Fig. 4 presents the mean size varying with the charging rate of carbon dioxide, indicating that the dispersion is only slightly affected by the pressurization rate as the feed rates of carbon dioxide are greater than 1 cm3 /min. Fig. 5 depicts the mean size of dispersoids against the holding time. It shows that increas-
Fig. 5. The effect of holding time on the mean size of the dispersoids prepared at 338.2 K, 30 MPa, and 2.0 cm3 /min of carbon dioxide charging rate.
ing holding time from 5 to 20 min can substantially improve the dispersion of the pigment particles in PGMEA. However, no obvious influence was found when the holding times were longer than 20 min. It was suggested that the penetration process of carbon dioxide with the dispersants into the spaces of agglomerated pigment particles was almost completed at about 20 min. 3.4. Effects of holding temperature and pressure Table 3 reports the mean sizes of dispersoid samples prepared at different holding temperatures and pressures (runs #14–32). Table 3 Experimental results from the operations at various temperatures and pressures during the holding stagea Run 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
Fig. 4. The effect of charging rate of carbon dioxide on the mean size of the dispersoids prepared at 338.2 K, 30 MPa, and 20 min of holding time.
a
P (MPa) 5 10 20 30 35 5 10 20 30 35 5 10 20 35 5 10 20 30 35
T (K) 318.2 318.2 318.2 318.2 318.2 328.2 328.2 328.2 328.2 328.2 338.2 338.2 338.2 338.2 348.2 348.2 348.2 348.2 348.2
Mean size (nm) 332 255 235 225 220 302 230 200 187 182 269 189 160 151 233 165 148 147 145
Standard deviation (nm) 7.5 20.0 20.1 9.7 20.0 13.2 13.6 13.0 12.0 13.2 14.0 21.0 12.5 12.7 27.8 13.5 10.7 17.8 14.3
The charging rate of carbon dioxide was 2.0 cm3 /min at 278 K and 6 MPa, and the holding time was 20 min throughout all the runs.
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Fig. 6. The effects of pressure and temperature on mean size of dispersoids prepared with the aid of supercritical carbon dioxide. The charging rate of carbon dioxide and the holding time were 2.0 cm3 /min and 20 min, respectively.
Throughout these 19 runs, the loaded pigment mixtures have same composition as mentioned in the above section, and the charging rate of carbon dioxide and the holding time were 2.0 cm3 /min at 278 K/6 MPa and 20 min, respectively. Fig. 6 illustrates the effects of holding pressure and temperature on the mean sizes of dispersed samples. Higher holding temperatures are favorable to enhance dispersion. The temperature effect, however, becomes minor as temperatures higher than 338.2 K, especially at elevated pressures. At a fixed temperature the mean size decreases with increasing pressure, but the effect sharply leveled-off as pressures exceeded about 10 MPa. As shown in Table 3, the mean size of dispersoid could be as small as 145 nm as the dispersion chamber was maintained at 348.2 K and 35 MPa during the holding stage (run #32). While the dispersion processes were operated at 338.2–348.2 K and 20 MPa during the holding stage, the mean sizes of the resulting dispersoids were within a range of 148–160 nm, which satisfies the requirement (100–200 nm) for industrial applications.
Fig. 7. Vapor–liquid phase diagram of carbon dioxide + PGMEA around critical regions.
blue 15:6. Fig. 7 is the VLE phase diagram of carbon dioxide + PGMEA in the vicinity of critical regions over temperatures from 318.2 to 358.2 K. It shows that the dew points are located in the region where the mole fractions of carbon dioxide are substantially high, e.g., greater than about 0.95 at 348.2 K. The critical pressure and the critical composition at each temperature were estimated from the smoothed isothermal phase boundary around the critical point as shown in Fig. 7. The determined critical locus was also mapped on Fig. 6. This critical locus divides the graph into two zones: subcritical region (left-hand side) and supercritical region (right-hand side). The mixtures of carbon dioxide + PGMEA formed a homogeneous phase in the supercritical region regardless of their compositions. In the subcritical region, the mixtures may exist as a
3.5. Effects of phase behavior The influences of temperature and pressure on the particles’ dispersion may be in accordance with the phase behavior of the mixtures in the dispersion chamber. To find this correlation, the vapor–liquid equilibrium (VLE) phase boundaries of carbon dioxide + PGMEA were measured with the volume-variable phase equilibrium analyzer. Since the concentrations of pigment and dispersants were very dilute in the dispersion mixtures during the holding stage (e.g., the mass ratio of pigment:dispersants:PGMEA:carbon dioxide was about 1 g:0.41 g:98.59 g:218 g), the phase boundaries of the mixtures in the dispersion chamber could be approximately represented by those of carbon dioxide + PGMEA, similar to the observations of Wu et al. [8] for carbon dioxide + DMSO + pigment
Fig. 8. UV–vis spectra from four representative samples.
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Fig. 9. TEM images from three tested samples: (a) Case 1, (b) Case 2, and (c) Case 4.
compressed liquid, vapor–liquid coexistence, or a superheated vapor, depending on the composition range. Because the mole fractions of carbon dioxide in the dispersion mixtures during the holding stage were well below 0.95, most of the subcritical cases were in the vapor–liquid coexistence region during the holding stage. As discussed in Section 3.4, the pressuredependences of the mean size of dispersoids were obviously different at lower and higher pressures, whose boundary appeared coincidentally at the critical locus as shown in Fig. 6. As a consequence, the phase behavior could be a crucial factor to govern the pigment particles dispersion. It suggests that the existence of liquid phase during the holding stage is unfavorable to the dispersion. From the economical point of view, the favorable dispersion process should be operated in the supercritical region during the holding stage, but not too far beyond the critical region.
3.6. The performance of dispersoids The performances of four representative samples were analyzed with a UV–vis spectrophotometer. These tested samples were prepared under the following dispersion conditions, respectively: (Case 1) The mass ratio of pigment red 177 to PGMEA = 1 g to 99 g, without any dispersants; dispersed with ultrasonic mixing; at ambient condition; after 1 day of preparation; mean size = 530 nm. (Case 2) The mass ratio of pigment red 177:Hypermer PS3: Zonyl FSO-100:PGMEA = 1 g:0.4 g:0.01 g:98.59 g (i.e., 40 wt.% of Hypermer PS3 and 1 wt.% of Zonyl FSO-100); dispersed with ultrasonic mixing; at ambient condition; mean size = 360 nm.
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4. Conclusions
Fig. 10. Histogram of mean particle sizes variation for the dispersed samples taken from runs #7, #26, #27, and #29–32.
A series of experiments has been conducted in a high-pressure dispersion apparatus to investigate the dispersion of 1 wt.% of pigment red 177 particles in PGMEA with the aid of supercritical carbon dioxide. It was found that 40 wt.% of Hypermer PS3 and 1 wt.% of Zonyl PSO-100 could be a favorable formulation of dispersants for the particles’ dispersion. The dispersion was only slightly affected by the pressurization rate. However, the holding time of pigment mixtures at elevated pressures is one of the crucial factors affecting the mean size of dispersoids. The suitable holding time is about 20 min. Higher holding temperatures and pressures are also favorable to yield smaller mean sizes of dispersoids. The preferable condition was found to be 348.2 K and 20 MPa. Under this operating condition, the mean size of the dispersoid can be as small as 148 nm. Within 16 h after preparation, the mean size of the dispersed particles can be maintained to be smaller than 200 nm, which is satisfactory for industrial applications. Acknowledgements
(Case 3) With 40 wt.% of Hypermer PS3 and 1 wt.% of Zonyl FSO-100; treated with carbon dioxide; holding at 318.2 K and 5.0 MPa (at subcritical conditions) for 20 min; mean size = 332 nm (i.e., run #14). (Case 4) With 40 wt.% of Hypermer PS3 and 1 wt.% of Zonyl FSO-100; treated with carbon dioxide; holding at 348.2 K and 35.0 MPa (at supercritical conditions) for 20 min; mean size = 145 nm (i.e., run #32). Each sample was diluted with PGMEA by 500 times before the analysis. Fig. 8 shows that the transmittances are 87.6, 91.3, 92.0 and 95.8% at the wavelength of 620 nm for the samples obtained from Cases (1) to (4), respectively. It again illustrates the smaller sizes of dispersed pigment particles resulting in the higher transmittances. Fig. 9(a–c) is the TEM images of dispersoid samples taken from Cases (1), (2) and (4), respectively. Serious aggregation of pigment particles is shown in Fig. 9(a), in the case of no dispersants and without treatment of supercritical carbon dioxide. Fig. 9(b) indicates that better dispersion was obtained by adding appropriate amounts of dispersants with ultrasonic mixing at ambient condition. As evidenced from Fig. 9(c), the supercritical carbon dioxide-assisted method can effectively improve the dispersion of pigment red 177 particles in PGMEA, and the mean size of the produced dispersoid meets the requirement (between 100 and 200 nm) for industrial applications. Fig. 10 presents the evolution of the mean sizes of samples obtained from the experiments, whose initial mean sizes were smaller than 200 nm. As shown from the graph, higher holding temperatures and pressures are favorable to stabilization of the dispersed particles. The mean sizes of dispersoids prepared from run #27, runs #29–32 can be maintained to be smaller than 200 nm until about 16–18 h after the preparation. Within this time period, the sizes of the dispersoids are acceptable for industrial applications.
The authors gratefully acknowledged the financial support from the National Science Council, Taiwan, through Grant No. NSC94-2214-E011-009. The authors also thank Dr. J.T. Chen, Department of Chemical Engineering, Ming-Hsin University of Science and Technology, for valuable discussions. Thanks also to Mr. H.Y. Chiu for phase boundaries measurements and Dr. K.T. Lee, Department of Chemical Engineering, Mingchi University of Science and Technology, for kindly providing the DLS particle analyzer. References [1] W. Herbst, K. Hunger, Industrial Organic Pigments: Production, Properties, Applications, VCH, New York, 1993. [2] T.C. Patton, Paint Flow and Pigment Dispersion, Wiley, New York, 1979. [3] G.D. Parfitt, Dispersion of Powders in Liquids, with Special Reference to Pigments, Applied Science Publishers, London, 1981. [4] H.T. Wu, M.J. Lee, H.M. Lin, Nano-particles formation for pigment red 177 via a continuous supercritical anti-solvent process, J. Supercrit. Fluids 33 (2005) 173–182. [5] M. Kamiwano, K. Nishi, Y. Inoue, U.S. Patent No. 5,921,478 (1999). [6] W.T. Cheng, C.W. Hsu, Y.W. Chih, Dispersion of organic pigments using supercritical carbon dioxide, J. Colloid Interf. Sci. 270 (2004) 106–112. [7] K.A. Consani, R.D. Smith, Observations on the solubility of surfactants and related molecules in carbon dioxide at 50◦ C, J. Supercrit. Fluids 3 (1990) 51–65. [8] H.T. Wu, M.J. Lee, H.M. Lin, Precipitation kinetics of pigment blue 15:6 sub-micro particles with a supercritical anti-solvent process, J. Supercrit. Fluids 37 (2006) 220–228. [9] National Institute of Standard and Technology, Isothermal properties for carbon dioxide, in: NIST Chemistry WebBook, NIST Standard Reference Database No. 69, National Institute of Standard and Technology, USA, 2003 (http://webbook.nist.gov/chemistry/). [10] D.H. Everett, Basic Principles of Colloid Science, The Royal Society of Chemistry, Cambridge, 1988. [11] K. Harrison, J. Goveas, K.P. Johnston, Water-in-carbon dioxide microemulsions with a fluorocarbon-hydrocarbon hybrid surfactant, Langmuir 10 (1994) 3536–3541.
Supercritical fluid-assisted dispersion of ultra-fine pigment red 177 particles with blended dispersants Hsien-Tsung Wu, Ming-Jer Lee ∗ , Ho-mu Lin Department of Chemical Engineering, National Taiwan University of Science and Technology, 43 Keelung Road, Section 4, Taipei 106-07, Taiwan Received 25 November 2005; received in revised form 6 January 2006; accepted 10 January 2006
Abstract A series of experiments was conducted to investigate the dispersion of 1 wt.% of pigment red 177 particles in propylene glycol monomethyl ether acetate (PGMEA). It was found that 40 wt.% of Hypermer PS3 dispersant could provide better dispersion and could stabilize the pigment particles in PGMEA at ambient condition. With the aid of supercritical carbon dioxide (CO2 ), additional 1 wt.% of CO2 -philic dispersant, Zonyl FSO-100, could further enhance the dispersion. Holding the mixtures in dispersion chamber at higher temperatures and pressures for a sufficient period of time was also favorable to improve the dispersion. The size of the dispersed particles appeared to have a correlation with the phase behavior of the mixtures in the dispersion chamber. The preferable operating conditions are at 348.2 K and 20 MPa, and 20 min of holding time was long enough before rapidly releasing chamber’s pressure. Under the recommended operating conditions, the mean sizes of the dispersoids could be as small as 148 nm, which met the required range of 100–200 nm in industrial applications. The TEM images have proven that the supercritical carbon dioxide-assisted dispersion method could efficiently disperse the pigment particles in PGMEA. © 2006 Elsevier B.V. All rights reserved. Keywords: Dispersion; Supercritical carbon dioxide; Pigment red 177
1. Introduction Pigments are the essential materials in the preparation of photo resists for fabricating color filters, which is one of key components to assembly color liquid crystal displays (colorLCD). For industrial applications, the mean sizes of pigment dispersoids are required to be in a range of 100–200 nm. Using smaller sizes of pigment particles is capable of increasing the color strength, contrast, and transmittance of color-LCD products [1–3]. Wu et al. [4] systematically studied the preparation of nanometric particles of pigment red 177 via a continuous supercritical anti-solvent (SAS) method. The study found the favorable operating conditions to produce the particles as small as 46 nm. However, it deems necessary to further disperse these ultra-fine particles in suitable liquid media for practical use. Conventional dispersion apparatuses, such as a roll mill, a mediumdispersion machine, or a homogenizer, are often adopted to disperse fine particles into liquids, but it is a time-consuming process [5]. Alternatively supercritical fluid technique could be
∗
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applied to efficient dispersion of fine particles into liquid media. Kamiwano et al. [5] developed a supercritical dispersing method, which has been successfully utilized to disperse micrometric carbon black particles into pure water without using any dispersants. With the assistance of supercritical carbon dioxide, pure water and 2 wt.% of carbon black powder were pressurized and heated stepwise from ambient condition up to 10 MPa and 333 K. The mixture was then diverted to an explosion-crashing tank. The resulting sample was still uniformly dispersed after 100 h and the sizes of the dispersed particles were less than 5 m. Cheng et al. [6] used a similar method to disperse the ultra-fine particles of pigment red 177, green 36 and blue 15:6 in PGMEA, respectively. In their series of experiments, the pigment concentration was as dilute as 0.002 wt.% and no dispersants were applied in the dispersion process. Among several others, those previous experimental results revealed that supercritical carbon dioxide was an excellent agent, which could quickly penetrate into the spaces of the aggregates of fine particles and then effectively disrupt the agglomerated particles via a rapid expansion. The stabilization of dispersed particles could be achieved by using some proper dispersants in the dispersing process. It is favorable if dispersants contain the surfactant molecules having both pigment- and solvent-affinity
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ends. Applying additional CO2 -philic surfactants should also be helpful in the dispersion process, since the surfactants are possibly carried by supercritical carbon dioxide and then penetrate into the vacancies of the aggregated particles. Consani and Smith [7] measured the solubility of 130 surfactants in carbon dioxide at 323 K and pressures from 10 to 50 MPa. Among these investigated surfactants, fluorinated hydrocarbons were found to be highly CO2 -philic. Such surfactants are potential candidates for the supercritical dispersion process. In the present study, a supercritical carbon dioxide-assisted dispersion process with blended dispersants was developed to produce the pigment dispersoids whose mean sizes were ranging from 100 to 200 nm. This size range is generally acceptable for preparing photo resists. A series of dispersion experiments was conducted to find the favorable formulations of the blended dispersants and the preferable operating conditions for dispersing 1 wt.% of ultra-fine pigment red 177 particles into PGMEA. The pigment concentration of this study is higher than that of Cheng et al. [6] by about 500 folds. The supercritical carbon dioxideassisted dispersion runs were made at temperatures from 318.2 to 348.2 K and pressures up to 35 MPa. The mean sizes of dispersoid samples were determined with a dynamic laser scattering (DLS) particle analyzer, and the transmittances of dispersoid samples were measured with an UV–vis spectrophotometer. 2. Experimental 2.1. Materials Pigment red 177 (aminoanthraquinone red) was supplied by Ciba Special Chemicals Co., Hong Kong. Its molecular structure has been presented in Wu et al. [4]. Two commercially available dispersants were used in this study. One is a polyester/polyamine copolymeric dispersant, Hypermer PS3, weakly cationic, 100% copolymer (ICI Americas Inc., USA) and the other is a CO2 -philic dispersant, Zonyl FSO-100, F(CF2 CF2 )1–7 CH2 CH2 O(CH2 CH2 O)0–15 H, molecular weight about 725, nonionic, 100% fluorosurfactant (du Pont de Nemours and Co., USA). Propylene glycol monomethyl ether acetate (PGMEA) (purity of 99.9 mass%), a suitable chemical for preparation of photo resists, was purchased from Aldrich, USA, and carbon dioxide (purity of 99.8 mass%) from LiuHsiang Gas Co., Taiwan. These chemicals were used without further purification. 2.2. Apparatus and procedure Fig. 1 is the schematic diagram of the dispersion apparatus. It consists of three main parts: feeding, dispersion, and receiver. The feeding unit includes a carbon dioxide source (1), a filter (2), a high-pressure metering liquid pump (3) (model: NPL-5000, Nihon Seimitsu Kagaku Co., Japan), and a disposable syringe (4) (internal volume of 20 cm3 ). The metering liquid pump was used for delivery of carbon dioxide and the syringe for injection of feeding mixtures of pigment red 177 + PGMEA + dispersants. A stainless steel high-pressure chamber (6) (P/N: CNLX1608316, operable up to 75 MPa, Autoclave Engineering, USA) with
Fig. 1. Schematic diagram of dispersion apparatus.
the dimensions of 1.75 cm inside diameter, 20.32 cm length and 49 cm3 internal volume was served as the dispersion unit. The dispersion chamber was immersed in a thermostatic air bath (5). Its temperature was regulated to within ±0.1 K. The bath temperature was measured with a precision platinum RTD sensor (8) (model: 1560, Hart Scientific Co., USA) to an accuracy of ±0.02 K. A pressure transducer (7) (model: PDCR 4070, ranging 0–35 MPa, Druck, UK) with a digital display (model: DPI-280, Druck, UK) monitored the operating pressure of the dispersion vessel, accurate to ±0.1%. A sample cylinder with an internal volume of 1000 cm3 (Swagelok, USA) was utilized as a receiver (9), which was connected to the bottom of the dispersion chamber for collecting the rapidly expanded dispersoid at atmospheric pressure. The experimental procedure is described as follows. A feeding mixture containing pigment red 177, PGMEA and the dispersants was prepared with an electronic balance. The concentration of pigment in the feeding mixture was kept at 1 wt.% through all the runs of this study. The mixture was vigorously mixed with ultrasonic wave (Branson 5510 ultrasonic wave at 40 KHz, USA) for about 2 h before loading to the dispersion chamber. At the beginning of dispersion process, about 15 cm3 of well-mixed feeding mixture was injected into the dispersion vessel via the syringe, and carbon dioxide was then charged into the dispersion chamber by the liquid pump, under a given flow rate, until the vessel reaches to a pre-specified pressure. The dispersion chamber was maintained at constant temperature and pressure for a period of time, denoted as a holding time. After that, the chamber was depressurized rapidly, and the dispersoid expanded simultaneously into the receiver at atmospheric pressure. The mean size of dispersoid samples was determined with a DLS particle analyzer (Model: LSR, Protein Solutions Ltd., USA). The outputs of the DLS analyzer were calibrated with two standard samples (Latex Microsphere Suspensions 5016A, mean diameter = 160 nm and NanosphereTM Size Standard
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3200A, 200 nm, mean diameter = 199 ± 6 nm, Duke Scientific Co., USA). All dispersion runs were repeated twice at each experimental condition. The mean sizes of the replicated samples could be reproduced to within about ±5%. Three representative samples were also analyzed with a transmission electron microscopy (TEM, Hitachi H-7100, Japan) by depositing the dispersoids on 400 mesh carbon-coated copper grids. The transmittances of four diluted dispersoid samples were measured with a UV–vis spectrophotometer (JASCO V-530, Japan). A volume-variable phase equilibrium analyzer has been installed to investigate the phase boundary of mixtures containing carbon dioxide + dimethyl sulfoxide (DMSO) with or without pigment blue 15:6 [8]. This apparatus was also employed in this study to measure the bubble and the dew points of carbon dioxide + PGMEA from which the critical points at the operating temperatures of dispersion process were estimated. The relationship between the dispersion of pigment particles and the phase behavior of mixtures in dispersion chamber will be discussed later. The volume-variable cell consists of a highpressure generator (Model 62-6-10, High Pressure Equipment Co., USA), a sapphire window (Part No.: 742.0106, Bridgman closure, SITEC, Switzerland), a rupture disk and a circulation jacket. The internal volume of the cell is adjustable manually with a piston screw pump and the cell is operable up to 50 MPa and 473.2 K. The cell’s temperature was maintained by circulated thermostatic water to within ±0.1 K. A pressure transducer (PDCR 407-01, Druck, UK) with a digital indicator (DPI 280, Druck, UK) displayed the cell’s pressure to within an accuracy of ±0.04%. Two precision syringe pumps (Model: 260D, Isco Inc., USA) were utilized to charge carbon dioxide and PGMEA, respectively, under constant-pressure mode. A magnetic stir bar was placed inside the cell to enhance the mixing of the loaded mixture. The weight of each loaded substance was calculated from the known charged volume (accurate to ±0.01 cm3 ) and the density value at the delivery pressure and temperature. While the densities of carbon dioxide were taken from the NIST Chemistry WebBook [9], the densities of PGMEA were determined experimentally by a high-pressure densitometer (DMA 512P, Anton Paar, Austria) with an oscillation period indicator (DMA 48, Anton Paar, Austria) to an accuracy of ±0.0001 g/cm3 . The uncertainty of the composition of the prepared mixtures was estimated to be ±0.003 in mole fraction. The phase behavior of the loaded sample in the cell was observed via a digital camera (Model: DSC-F88/S, Sony, Japan) and a television (Model: PV-C2062, Panasonic, Japan). The bubble point or dew point of the loaded mixture can be determined visually by manipulating manually the position of piston screw pump. The uncertainty of the observed phase transition pressures at a given temperature is about ±0.05 MPa.
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process include pressurization rate, the holding time of pigment mixtures at elevated pressures in the dispersion chamber, the temperature and pressure during the holding stage. 3.1. Dispersion with Hypermer PS3 at ambient condition Preliminary tests were conducted without supercritical carbon dioxide treatment to explore the favorable composition of dispersion agents for stabilizing pigment red 177 particles in PGMEA at ambient condition. The tested samples were prepared by adding different weight percents of dispersion agents, on the PGMEA-free basis, in the mixtures of pigment + PGMEA. The mean size of each sample, denoted as an initial mean size, was measured after well mixing with ultrasonic wave for 2 h. The evolution of the mean sizes of the dispersoid samples was observed every other day. Fig. 2 shows the histogram of the preliminary tests with or without using Hypermer PS3 dispersant up to 7 days after preparation. The sample without the presence of dispersant (denoted as a blank case, shown as a symbol of solid circle in Fig. 2) exhibited the smallest initial mean size, 260 nm, but its size drastically increased up to a micrometric scale on the third day. It was suggested that ultrasonic mixing could temporarily break the original agglomerated pigment particles in PGMEA. However, the particles re-aggregated quickly due to the absence of any dispersants to stabilize the distributed particles in PGMEA. As also seen from the graph, the initial mean sizes are larger than that of blank case while different amounts of Hypermer PS3 were added. It may have resulted from cluster formation of the pigment particles surrounded by the dispersion agent. The mean sizes grew much slower in the presence of dispersant, especially for the samples containing Hypermer PS3 greater than 10 wt.%. Their sizes are substantially smaller than that of the blank case on the third day, implying that the dispersant effectively provided steric barrier to keep the dispersed particles from re-agglomeration. The
3. Results and discussion The objective of this study is to find favorable formulations and operating conditions of the supercritical fluid-assisted dispersion process to produce the dispersoids whose mean sizes meet the required range of 100–200 nm in LCD industrial applications. The investigated operating variables of the dispersion
Fig. 2. Histogram of mean particle sizes variation for the dispersed samples prepared from the operations using different amounts of Hypermer PS3 dispersant at ambient condition.
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mechanism of steric stabilization of the hydrocarbon-based dispersant suggested that the pigment-affinity groups (segments of the polyamines) absorbed onto the pigment particles’ surface and the solvent-soluble chains (segments of the polyesters) stretching into the PGMEA liquid phase, and to establish a sufficient thickness to stabilize the dispersed pigment particles [3,10]. Under lower concentrations of Hypermer PS3 (e.g., less than 10 wt.%) the dispersant may not form enough thickness to prevent the particles from re-agglomeration, whereas high concentrations of Hypermer PS3 (e.g., equal to or greater than 60 wt.%) may result in inevitable flocculation [2]. The experimental results disclosed that using 40 wt.% of Hypermer PS3 (0.4 g Hypermer PS3 to 1 g pigment, on the PGMEA-free basis) yielded better dispersion. The mean size increased from about 300 to 390 nm after 1 day of preparation and maintained about the same size up to the 7th day. 3.2. Dispersion with addition of Zonyl FSO-100 and supercritical carbon dioxide A CO2 -philic dispersant Zonyl FSO-100 was tested in this study. It was found that the initial mean sizes increased monotonically from 335 to 750 nm while the concentrations of Zonyl FSO-100 increased from 3 to 40 wt.%. High concentrations of Zonyl FSO-100 appeared unfavorable to reduce the size of dispersed particles. Fig. 3 illustrates the results of the samples containing 10–60 wt.% of Hypermer PS3 and addition of Zonyl FSO-100 up to 3 wt.% (0.03 g dispersant to 1 g pigment). It also shows that the presence of Zonyl FSO-100 did not significantly affect the mean sizes of the dispersoid samples as the dispersion process was achieved only by ultrasonic mixing at ambient condition. However, there are certain benefits by the addition of this CO2 -philic fluorosurfactant, Zonyl FSO-100, when the dispersion was operated with the aid of supercritical carbon dioxide [7,11]. More dispersion experiments were thus implemented with the treatment of supercritical carbon dioxide. Before the dispersoids were rapidly released from the high-pressure dispersion chamber to the receiver at atmospheric pressure, the pigment mixtures were maintained at 328.2 K and 20 MPa for 20 min in this series of runs. Table 1 lists the experimental results. The initial mean size was reduced down to 250 nm for the sample containing 40 wt.% of Hypermer PS3 after supercritical carbon dioxide-assisted dispersion (run #1), whereas the size was about
Fig. 3. Histogram of mean particle sizes variation for the dispersed samples prepared from the operations using various blended dispersants at ambient condition.
298 nm without supercritical carbon dioxide treatment. It was evident that the supercritical fluid could assist the dispersion of the pigment particles due to, at least partly, the rapid expansion of carbon dioxide from the spaces of the aggregated pigment particles during the process of pressure release. As also seen from runs #2 and #3 in Table 1, the higher concentrations of Zonyl FSO-100 resulted in the larger mean sizes of the dispersed particles with using a single dispersant. The mean size, however, could be reduce effectively by applying the dispersant blends of Hypermer PS3 and Zonyl FSO-100 as shown in runs #4 and #5. In the case of run #4, the mean size decreased down to 201 nm with 40 wt.% of Hypermer PS3 and addition of 1 wt.% of Zonyl FSO-100. The improvement of particles dispersion is attributable to the fact that supercritical carbon dioxide may carry the CO2 -philic Zonyl FSO-100 and penetrate into the inter-particles’ vacancies. During the course of rapid release of the applied CO2 pressure, the expansion of fluid disrupts the aggregated particles, and the dispersed particles are then isolated, mainly, by Hypermer PS3. The dispersant blend of 40 wt.% of Hypermer PS3 and 1 wt.% of Zonyl FSO-100 was used throughout all the following supercritical carbon dioxide-assisted dispersion experiments to
Table 1 Experimental results from the operations in the presence of various combinations of dispersants Run
Hypermer PS3 (g/g pigment)
Zonyl FSO-100 (g/g pigment)
Ambienta mean size (nm)
With CO2 b mean size (nm)
Size’s reduction with CO2 (nm)
1 2 3 4 5
0.4 (40%) 0 (0%) 0 (0%) 0.4 (40%) 0.4 (40%)
0 (0%) 0.40 (40%) 0.03 (3%) 0.01 (1%) 0.03 (3%)
298 750 335 360 363
250 545 265 201 210
−48 −205 −70 −159 −153
a
The samples were prepared with ultrasonic mixing at ambient condition. The samples were prepared with the aid of supercritical carbon dioxide; the charging rate of carbon dioxide was 2.0 cm3 /min at 278 K and 6 MPa, and holding time was 20 min. The holding temperature and pressure were at 328.2 K and 20 MPa, respectively. b
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Table 2 Experimental results from the operations under various charging rates of carbon dioxide and holding timesa Run
FCO2 b (cm3 /min)
Holding time (cm3 /min)
Mean size (nm)
Standard deviation (nm)
6 7 8 9 10 11 12 13
1.0 2.0 4.0 6.0 2.0 2.0 2.0 2.0
20 20 20 20 5 10 40 60
165 153 155 150 235 195 154 155
27.3 11.6 37.6 34.3 37.3 13.0 20.5 17.9
a The holding pressure was 30 MPa and the holding temperature was 338.2 K throughout all the runs. b The volumetric flow rate of carbon dioxide was at the conditions of 278 K and 6 MPa.
find suitable operating conditions for preparing the dispersoids whose sizes are within the acceptable range of 100–200 nm in industrial applications. 3.3. Effects of pressurization rate and holding time Table 2 lists the results of dispersion experiments (runs #6–13) operated at different pressurization rates (i.e., the volumetric charging rates of carbon dioxide at the conditions of 278 K and 6 MPa, FCO2 ) and holding times. All the feeding pigment mixtures have the same composition, the mass ratio of pigment:Hypermer PS3:Zonyl FSO100:PGMEA = 1 g:0.4 g:0.01 g:98.59 g, and all these runs were maintained at 338.2 K and 30 MPa during the holding stage. Fig. 4 presents the mean size varying with the charging rate of carbon dioxide, indicating that the dispersion is only slightly affected by the pressurization rate as the feed rates of carbon dioxide are greater than 1 cm3 /min. Fig. 5 depicts the mean size of dispersoids against the holding time. It shows that increas-
Fig. 5. The effect of holding time on the mean size of the dispersoids prepared at 338.2 K, 30 MPa, and 2.0 cm3 /min of carbon dioxide charging rate.
ing holding time from 5 to 20 min can substantially improve the dispersion of the pigment particles in PGMEA. However, no obvious influence was found when the holding times were longer than 20 min. It was suggested that the penetration process of carbon dioxide with the dispersants into the spaces of agglomerated pigment particles was almost completed at about 20 min. 3.4. Effects of holding temperature and pressure Table 3 reports the mean sizes of dispersoid samples prepared at different holding temperatures and pressures (runs #14–32). Table 3 Experimental results from the operations at various temperatures and pressures during the holding stagea Run 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
Fig. 4. The effect of charging rate of carbon dioxide on the mean size of the dispersoids prepared at 338.2 K, 30 MPa, and 20 min of holding time.
a
P (MPa) 5 10 20 30 35 5 10 20 30 35 5 10 20 35 5 10 20 30 35
T (K) 318.2 318.2 318.2 318.2 318.2 328.2 328.2 328.2 328.2 328.2 338.2 338.2 338.2 338.2 348.2 348.2 348.2 348.2 348.2
Mean size (nm) 332 255 235 225 220 302 230 200 187 182 269 189 160 151 233 165 148 147 145
Standard deviation (nm) 7.5 20.0 20.1 9.7 20.0 13.2 13.6 13.0 12.0 13.2 14.0 21.0 12.5 12.7 27.8 13.5 10.7 17.8 14.3
The charging rate of carbon dioxide was 2.0 cm3 /min at 278 K and 6 MPa, and the holding time was 20 min throughout all the runs.
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Fig. 6. The effects of pressure and temperature on mean size of dispersoids prepared with the aid of supercritical carbon dioxide. The charging rate of carbon dioxide and the holding time were 2.0 cm3 /min and 20 min, respectively.
Throughout these 19 runs, the loaded pigment mixtures have same composition as mentioned in the above section, and the charging rate of carbon dioxide and the holding time were 2.0 cm3 /min at 278 K/6 MPa and 20 min, respectively. Fig. 6 illustrates the effects of holding pressure and temperature on the mean sizes of dispersed samples. Higher holding temperatures are favorable to enhance dispersion. The temperature effect, however, becomes minor as temperatures higher than 338.2 K, especially at elevated pressures. At a fixed temperature the mean size decreases with increasing pressure, but the effect sharply leveled-off as pressures exceeded about 10 MPa. As shown in Table 3, the mean size of dispersoid could be as small as 145 nm as the dispersion chamber was maintained at 348.2 K and 35 MPa during the holding stage (run #32). While the dispersion processes were operated at 338.2–348.2 K and 20 MPa during the holding stage, the mean sizes of the resulting dispersoids were within a range of 148–160 nm, which satisfies the requirement (100–200 nm) for industrial applications.
Fig. 7. Vapor–liquid phase diagram of carbon dioxide + PGMEA around critical regions.
blue 15:6. Fig. 7 is the VLE phase diagram of carbon dioxide + PGMEA in the vicinity of critical regions over temperatures from 318.2 to 358.2 K. It shows that the dew points are located in the region where the mole fractions of carbon dioxide are substantially high, e.g., greater than about 0.95 at 348.2 K. The critical pressure and the critical composition at each temperature were estimated from the smoothed isothermal phase boundary around the critical point as shown in Fig. 7. The determined critical locus was also mapped on Fig. 6. This critical locus divides the graph into two zones: subcritical region (left-hand side) and supercritical region (right-hand side). The mixtures of carbon dioxide + PGMEA formed a homogeneous phase in the supercritical region regardless of their compositions. In the subcritical region, the mixtures may exist as a
3.5. Effects of phase behavior The influences of temperature and pressure on the particles’ dispersion may be in accordance with the phase behavior of the mixtures in the dispersion chamber. To find this correlation, the vapor–liquid equilibrium (VLE) phase boundaries of carbon dioxide + PGMEA were measured with the volume-variable phase equilibrium analyzer. Since the concentrations of pigment and dispersants were very dilute in the dispersion mixtures during the holding stage (e.g., the mass ratio of pigment:dispersants:PGMEA:carbon dioxide was about 1 g:0.41 g:98.59 g:218 g), the phase boundaries of the mixtures in the dispersion chamber could be approximately represented by those of carbon dioxide + PGMEA, similar to the observations of Wu et al. [8] for carbon dioxide + DMSO + pigment
Fig. 8. UV–vis spectra from four representative samples.
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Fig. 9. TEM images from three tested samples: (a) Case 1, (b) Case 2, and (c) Case 4.
compressed liquid, vapor–liquid coexistence, or a superheated vapor, depending on the composition range. Because the mole fractions of carbon dioxide in the dispersion mixtures during the holding stage were well below 0.95, most of the subcritical cases were in the vapor–liquid coexistence region during the holding stage. As discussed in Section 3.4, the pressuredependences of the mean size of dispersoids were obviously different at lower and higher pressures, whose boundary appeared coincidentally at the critical locus as shown in Fig. 6. As a consequence, the phase behavior could be a crucial factor to govern the pigment particles dispersion. It suggests that the existence of liquid phase during the holding stage is unfavorable to the dispersion. From the economical point of view, the favorable dispersion process should be operated in the supercritical region during the holding stage, but not too far beyond the critical region.
3.6. The performance of dispersoids The performances of four representative samples were analyzed with a UV–vis spectrophotometer. These tested samples were prepared under the following dispersion conditions, respectively: (Case 1) The mass ratio of pigment red 177 to PGMEA = 1 g to 99 g, without any dispersants; dispersed with ultrasonic mixing; at ambient condition; after 1 day of preparation; mean size = 530 nm. (Case 2) The mass ratio of pigment red 177:Hypermer PS3: Zonyl FSO-100:PGMEA = 1 g:0.4 g:0.01 g:98.59 g (i.e., 40 wt.% of Hypermer PS3 and 1 wt.% of Zonyl FSO-100); dispersed with ultrasonic mixing; at ambient condition; mean size = 360 nm.
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4. Conclusions
Fig. 10. Histogram of mean particle sizes variation for the dispersed samples taken from runs #7, #26, #27, and #29–32.
A series of experiments has been conducted in a high-pressure dispersion apparatus to investigate the dispersion of 1 wt.% of pigment red 177 particles in PGMEA with the aid of supercritical carbon dioxide. It was found that 40 wt.% of Hypermer PS3 and 1 wt.% of Zonyl PSO-100 could be a favorable formulation of dispersants for the particles’ dispersion. The dispersion was only slightly affected by the pressurization rate. However, the holding time of pigment mixtures at elevated pressures is one of the crucial factors affecting the mean size of dispersoids. The suitable holding time is about 20 min. Higher holding temperatures and pressures are also favorable to yield smaller mean sizes of dispersoids. The preferable condition was found to be 348.2 K and 20 MPa. Under this operating condition, the mean size of the dispersoid can be as small as 148 nm. Within 16 h after preparation, the mean size of the dispersed particles can be maintained to be smaller than 200 nm, which is satisfactory for industrial applications. Acknowledgements
(Case 3) With 40 wt.% of Hypermer PS3 and 1 wt.% of Zonyl FSO-100; treated with carbon dioxide; holding at 318.2 K and 5.0 MPa (at subcritical conditions) for 20 min; mean size = 332 nm (i.e., run #14). (Case 4) With 40 wt.% of Hypermer PS3 and 1 wt.% of Zonyl FSO-100; treated with carbon dioxide; holding at 348.2 K and 35.0 MPa (at supercritical conditions) for 20 min; mean size = 145 nm (i.e., run #32). Each sample was diluted with PGMEA by 500 times before the analysis. Fig. 8 shows that the transmittances are 87.6, 91.3, 92.0 and 95.8% at the wavelength of 620 nm for the samples obtained from Cases (1) to (4), respectively. It again illustrates the smaller sizes of dispersed pigment particles resulting in the higher transmittances. Fig. 9(a–c) is the TEM images of dispersoid samples taken from Cases (1), (2) and (4), respectively. Serious aggregation of pigment particles is shown in Fig. 9(a), in the case of no dispersants and without treatment of supercritical carbon dioxide. Fig. 9(b) indicates that better dispersion was obtained by adding appropriate amounts of dispersants with ultrasonic mixing at ambient condition. As evidenced from Fig. 9(c), the supercritical carbon dioxide-assisted method can effectively improve the dispersion of pigment red 177 particles in PGMEA, and the mean size of the produced dispersoid meets the requirement (between 100 and 200 nm) for industrial applications. Fig. 10 presents the evolution of the mean sizes of samples obtained from the experiments, whose initial mean sizes were smaller than 200 nm. As shown from the graph, higher holding temperatures and pressures are favorable to stabilization of the dispersed particles. The mean sizes of dispersoids prepared from run #27, runs #29–32 can be maintained to be smaller than 200 nm until about 16–18 h after the preparation. Within this time period, the sizes of the dispersoids are acceptable for industrial applications.
The authors gratefully acknowledged the financial support from the National Science Council, Taiwan, through Grant No. NSC94-2214-E011-009. The authors also thank Dr. J.T. Chen, Department of Chemical Engineering, Ming-Hsin University of Science and Technology, for valuable discussions. Thanks also to Mr. H.Y. Chiu for phase boundaries measurements and Dr. K.T. Lee, Department of Chemical Engineering, Mingchi University of Science and Technology, for kindly providing the DLS particle analyzer. References [1] W. Herbst, K. Hunger, Industrial Organic Pigments: Production, Properties, Applications, VCH, New York, 1993. [2] T.C. Patton, Paint Flow and Pigment Dispersion, Wiley, New York, 1979. [3] G.D. Parfitt, Dispersion of Powders in Liquids, with Special Reference to Pigments, Applied Science Publishers, London, 1981. [4] H.T. Wu, M.J. Lee, H.M. Lin, Nano-particles formation for pigment red 177 via a continuous supercritical anti-solvent process, J. Supercrit. Fluids 33 (2005) 173–182. [5] M. Kamiwano, K. Nishi, Y. Inoue, U.S. Patent No. 5,921,478 (1999). [6] W.T. Cheng, C.W. Hsu, Y.W. Chih, Dispersion of organic pigments using supercritical carbon dioxide, J. Colloid Interf. Sci. 270 (2004) 106–112. [7] K.A. Consani, R.D. Smith, Observations on the solubility of surfactants and related molecules in carbon dioxide at 50◦ C, J. Supercrit. Fluids 3 (1990) 51–65. [8] H.T. Wu, M.J. Lee, H.M. Lin, Precipitation kinetics of pigment blue 15:6 sub-micro particles with a supercritical anti-solvent process, J. Supercrit. Fluids 37 (2006) 220–228. [9] National Institute of Standard and Technology, Isothermal properties for carbon dioxide, in: NIST Chemistry WebBook, NIST Standard Reference Database No. 69, National Institute of Standard and Technology, USA, 2003 (http://webbook.nist.gov/chemistry/). [10] D.H. Everett, Basic Principles of Colloid Science, The Royal Society of Chemistry, Cambridge, 1988. [11] K. Harrison, J. Goveas, K.P. Johnston, Water-in-carbon dioxide microemulsions with a fluorocarbon-hydrocarbon hybrid surfactant, Langmuir 10 (1994) 3536–3541.