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Dispersion of organic pigments using supercritical carbon dioxide
Journal of Colloid and Interface Science 270 (2004) 106–112 www.elsevier.com/locate/jcis
Dispersion of organic pigments using supercritical carbon dioxide W.T. Cheng,∗ C.W. Hsu, and Y.W. Chih Department of Chemical Engineering, National Chung-Hsing University, 250 Kuo-Kuang Rd., Taichung 402, Taiwan, Republic of China Received 22 August 2002; accepted 20 August 2003
Abstract This research describes dispersion of organic pigments using supercritical fluids. With low surface tension and high diffusivity of fluids in supercritical states, aggregated particles may be effectively wetted and swelled to form the primary constituent of the dispersing solution by volume. In this paper, the conditions of temperature and pressure are used to control the density of supercritical carbon dioxide subject to PGMEA as cosolvent for dispersing organic powder in a solution. As shown from measurement with a laser scattering particle analyzer, the average diameter of phthalocyanine green 36 with the haloid structure can be significantly reduced to 93.5 nm; for aminoanthraquinone red containing and amino group (–NH2 ) and phthalocyanine blue 15:6 with symmetry benzene and inner hydrogen bond, the mean particle sizes are 178.5 and 188.7 nm, respectively, using supercritical CO2 . Additionally, the transmittance of UV light is used to confirm the dispersing performance in this study. 2003 Elsevier Inc. All rights reserved. Keywords: Supercritical carbon dioxide; Organic pigment; Dispersion; Mean particle size; Laser scattering particle analyzer; UV light
1. Introduction Organic pigments are usually used as colorants. The typical example is the paint and printing industries [1]. Recently, they have become increasingly important in the areas of electronics and communication [2]. For example, with the properties of resistance to heat and chemistry, organic pigments have been successfully applied to fabricate color filters for LCDs [3]. Therefore, the dispersion of organic pigments is the key to their continuing application. Some factors such as particle or crystal shape, molecular arrangement in particles or crystals, particle size and distribution, surface character, and interaction of pigments and medium [4,5] determine the chemical structure and physical properties of organic pigments. The primary diameter of organic pigments generally ranges from 10 to 50 nm, so that ease of aggregation into larger particles is found due to the high surface energy. Hence, the dispersion method is important for advanced applications [6,7]. In mechanical dispersion methods, shearing forces are usually applied to disperse the aggregated particles into a separate and fine particle, but result in a long processing time * Corresponding author.
E-mail address: [email protected] (W.T. Cheng). 0021-9797/$ – see front matter 2003 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2003.08.029
and contamination problems when the dispersing apparatus is washed after completion of the dispersion process. In order to reduce these drawbacks, a dispersion method [8] has been proposed as follows: the dispersoid is mixed in a supercritical state and the dispersoid including the supercritical fluid is then rapidly expanded to finely divide the dispersoid; and finally the fine particles are dispersed into a solvent, as illustrated in Fig. 1. Under the influence of pressure and temperature, pure substances can assume a gas, liquid, and solid state of matter except where the equilibrium saturation curve converges such that all three phases co-exit at the triple point. Extension of the liquid–gas phase line ends at the critical point and represents the maximum temperature and pressure at which the liquid and vapor phases coexist in equilibrium, after which gas and liquid have the same density and appear as a single phase. A fluid is said to be a supercritical fluid (SCF) when its temperature and pressure are in a state above its critical temperature and critical pressure, permitting the gaseous and liquid phases to coexist [9]. The solubility of supercritical fluids discussed by Hannay and Hogarth [10,11]. Supercritical fluids possess a liquidlike density and solubility and exhibits gaslike transport properties of diffusivity and viscosity [12]. Additionally, supercritical fluids have low surface tension, permeate faster than
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Fig. 1. The concept of dispersion of aggregate pigments using supercritical carbon dioxide.
liquids and have solubility that gases do not possess [13]. Therefore, high efficiency of dispersion may be achieved using SCF. In this paper, taking supercritical carbon dioxide and PGMEA as cosolvent and dispersion medium produces a system that disperses the organic pigments into the primary size in the dispersing solution. This concept is different from that of a supercritical antisolvent (SAS) that precipitates a substrate dissolved in a solvent as dried particles. In the dispersion method, as illustrated in Fig. 1, a liquefied carbon dioxide–PGMEA–organic pigment mixture is expressed from conditions of high pressure to ambient pressure, whereupon the gas undergoes an adiabatic expansion and a fine dispersoid (that is, a mixture of dispersed pigment with PGMEA) is immediately obtained. It is noted that the cosolvent PGMEA is partially miscible with CO2 and can be used for preparing an ultrafine color resist to fabricate a color filter in an advanced LCD. The effects of temperature and pressure on the dispersion process are to be investigated and discussed in detail. In addition, laser scattering particle analysis and UV light are applied to evaluate the dispersion performance.
2. Materials and methods Table 1 lists basic information on organic pigments (three kinds) used in this study, for which the chemical structure is shown in Fig. 2, from commercial grades suitable for color resists in manufacturing color liquid crystal display. Aminoanthraquinone red (Red A3B manufactured by the Ciba–Geigy Company), phthalocyanine green 36 (ECG 403 manufactured by the Dainipon Precision Chemical Com-
Table 1 The basic information on the examined organic pigments Color Product name Red Green Blue
Red A3B ECG 403 B6700
Chemical type Aminoanthraquinone Phthalocyanine Phthalocyanine
C.I. No. C.I. generic name 65300 74265 74160
Pigment red 177 Pigment green 36 Pigment blue 15:6
pany), and phthalocyanine blue 15:6 (B6700 manufactured by the BASF Company), were charged into PGMEA (propylene glycol monomethyl ether acetate, manufactured by Lancaster), stirred for 20 min using ultrasonic waves (with a Branson 5510 ultrasonic cleaner at 40 kHz), and then transport into a supercritical vessel. Liquid-type carbon dioxide was input for 20 min to reach the supercritical state under conditions of temperature and pressure ranging from 25 ◦ C and 1 atm to 55 ◦ C and 150 atm. Finally the dispersoid with CO2 was released into a collecting bottle under atmospheric pressure. The experimental apparatus of dispersion using SCF is shown in Fig. 3. This is a batch process, so the flow rates of liquid-type CO2 and the mixture of PGMEA with organic pigment transported to the dispersion column are unspecified. Additionally, the cosolvent PGMEA is mainly considered for the safety and compatibility of formulating color resists in the future. To identify the effect of dispersion on the supercritical process, the transmittance spectra of the dispersing solution is measured by a UV spectrometer (Hitachi U-2000) and the mean diameter size of the dispersoid is measured using a laser scattering particle analyzer (Brookhaven 90Plus Particle Sizer). Additionally, a microscope (Olympus BXP-50) is used to observe the dispersion solution on a glass substrate.
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3. Results and discussion 3.1. Temperature and pressure effects
(a)
(b)
(c) Fig. 2. The chemical structure of organic pigments used in this investigation: (a) red pigment 177, (b) green pigment 36, (c) blue pigment 15:6.
Fig. 3. Schematic diagram of dispersion apparatus using supercritical fluid.
The crystal structure of an organic pigment is critical in determining its stability to solvents, heat, and light. In addition, the stability, flow, and dispersion properties of a pigment material [14] depend on the morphology and nature of the major surfaces of the crystal, and these properties again depend on the crystal structure. In the supercritical state, the density can be increased by elevating pressure to enhance the wetting ability of SCF. Furthermore, the diffusivity of SCF may be promoted by increasing temperature. With a concentration of 0.002% (w/w) to PGMEA, for aminoanthraquinone red, phthalocyanine green 36, and phthalocyanine blue 15:6, the effect of pressure on the mean particle sizes of dispersing pigments is demonstrated in Fig. 4 under specified temperatures. The effect of temperature on supercritical state for the mean particle size of dispersing pigments is indicated in Fig. 5. As viewed in the deviation of mean particle size from these figures, for phthalocyanine green 36 with the haloid structure, the dispersing ability of supercritical carbon dioxide with PGMEA is significantly enhanced with increasing pressure and temperature, whereas aminoanthraquinone red, containing an amino group (–NH2), and phthalocyanine blue 15:6, with symmetry benzene and inner hydrogen bond, are not sensitive to the solution of supercritical CO2 with PGMEA as cosolvent. This is because amino groups (–NH2) and symmetry benzene with inner hydrogen bonds inhibit the diffusion of supercritical CO2 into the aggregated particles for wetting and swelling [15]. The conditions of 300 atm and 55 ◦ C for the supercritical CO2 have reached the limitation of the experimental equipment in this work.
Fig. 4. The effect of pressure on mean particle size of dispersed organic pigments with 0.002% (w/w) into the solvent PGMEA using supercritical CO2 at fixed temperature.
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(a) Fig. 5. The effect of temperature on mean particle size of dispersed organic pigments with 0.002% (w/w) into the solvent PGMEA using supercritical CO2 at fixed pressure.
3.2. Dispersing effect To reduce the concentration of organic pigment in the solvent of PGMEA from 0.002 to 0.001% (w/w), according to the results from Figs. 4 and 5, it is expected that the phthalocyanine green may be obtained as nanoscale particles in the solution by supercritical fluid dispersion under conditions of 300 atm and 55 ◦ C. Figure 6b displays a microscopic view of the dispersion of phthalocyanine green 36 at a magnitude of 500×, with mean particle size 93.5 nm from measurement with a laser scattering particle analyzer. Figure 6a corresponds to the dispersion of green pigments by preprocessing of SCF. For aminoanthraquinone red and phthalocyanine blue 15:6, the minimum average particle sizes of dispersion are 178.5 and 188.7 nm, respectively, with the microscopy of dispersion shown in Figs. 7b and 8b in the optimal supercritical state. Compared with phthalocyanine green 36, the supercritical carbon dioxide with PGMEA as cosolvent may be not suitable for organic pigments containing amino groups (–NH2 ) and symmetry benzene with inner hydrogen bonds. As shown in Fig. 2a, pigment red 177 has the chemical structure of 4,4 -diamino-1,1-dianthraquinonyl. X-ray diffraction analysis discloses a twisting of the two anthraquinone units by 75◦ relative to each other. This allows optimum formation of intermolecular hydrogen bonds [16] that inhibit nonpolarity of SCF to penetrate and swell. Polyhalogenated green copper pigment with 8 bromine and 2 chlorine atoms (as shown in Fig. 2b), which has the potential for dramatically improving solubility in supercritical CO2 , with a cosolvent like PGMEA is not polymorphous and thus is exempt from modification [1]. In contrast to the organic pigment blue 15:6, which displays more than one crystal modification, copper phthalocyanine is the copper(II) complex of tetraazaterabenzoporphine. As shown in Fig. 2c, the mesomeric structures indicate that all of the pyrrole rings simultaneously contribute to the aromatic system. Therefore, the molecule adopts a planar and completely con-
(b) Fig. 6. Microscopy of phthalocyanine green 36 with 0.001% (w/w) in the solvent of PGMEA in (a) predispersion state and (b) postdispersion using supercritical CO2 under condition of 300 atm and 55 ◦ C at a magnitude of 500×.
jugated structure that exhibits exceptional stability [17]. In other words, it is difficult to be wetted and swelled using supercritical carbon dioxide for dispersion of ultrafine powder with mesomeric structures. In short, the polar groups of –NH2 and symmetry benzene with inner hydrogen bonds are not compatible with the supercritical CO2 and PGMEA. This suggests the need to change the cosolvent from PGMEA to disperse copper phthalocyanine pigment blue 15:6 and pigment red 177 at the nanometer level by means of supercritical CO2 . As observed from Figs. 4 and 5, nevertheless, the dispersing effect of supercritical fluid on organic powder is positive.
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(a)
(b) Fig. 7. Microscopy of aminoanthraquinone red pigment with 0.001% (w/w) in the solvent PGMEA in (a) predispersion state and (b) postdispersion state using carbon dioxide under conditions of 100 atm and 35 ◦ C at a magnitude of 500×.
Microscopy and gross observation of the pigment in processing and nonprocessing of SCF may be applied to show that supercritical carbon dioxide swells the aggregated organic pigment. In the present study, UV spectrometer is also employed to make sure of the dispersing ability of supercritical carbon dioxide for the aggregates of organic pigments. This is based on the theory that if the particle size is smaller, the transmittance of UV light is higher [18]. Taking the concentration of 0.002% (w/w) to PGMEA as an example, Fig. 9 shows the transmittance of UV light as 61%, 70%, and 82% in a dispersing solution of phthalocyanine green 36 at wavelength 550 nm under three states of (a) 25 ◦ C and 50 atm, (b) 55 ◦ C and 100 atm, and (c) 55 ◦ C and 150 atm for the mean particle sizes of 264.1, 185.4, and 103.1 nm, respectively. This clearly indicates that the
(a)
(b) Fig. 8. Microscopy of phthalocyanine blue 15:6 at 0.001% (w/w) in the solvent PGMEA in (a) predispersion state and (b) postdispersion state using carbon dioxide under conditions of 50 atm and 35 ◦ C at a magnitude of 500×.
wetting and diffusion ability of CO2 can be enhanced by controlling pressure and temperature to swell the aggregated pigment and obtain fine phthalocyanine green 36 particles in the solvent PGMEA. Particularly, the conditions of 55 ◦ C and 150 atm can nearly be used to disperse the aggregated phthalocyanine green 36 pigment as nanometer scale particles in the solvent PGMEA. For the dispersing solutions of aminoanthraquinone red and phthalocyanine blue 15:6, the results are the same as in the case of phthalocyanine green 36. Figures 10 and 11 show that the dispersion of fine particles can attain high transmittance of UV light. This means that decreasing the mean diameter increases the transmittance of UV light, but the large and unstable particles of the dispersing solution decrease transmittance of UV light due to scattering effect.
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(a)
(a)
(b)
(b)
(c)
(c)
Fig. 9. The transmittance of UV light for dispersing phthalocyanine green 36 with 0.002% (w/w) in the solvent PGMEA using carbon dioxide under conditions of (a) 25 ◦ C and 50 atm, (b) 55 ◦ C and 100 atm, and (c) 55 ◦ C and 150 atm, respectively, at wavelength 550 nm.
Fig. 10. The transmittance of UV light for dispersing aminoanthraquinone red pigment with 0.002% (w/w) in the solvent PGMEA using carbon dioxide under conditions of (a) 25 ◦ C and 50 atm, (b) 55 ◦ C and 100 atm, and (c) 55 ◦ C and 150 atm, respectively, at wavelength 620 nm.
As displayed in this two figures, the averaged particle sizes of the dispersing pigments red 77 and phthalocyanine blue 15:6 are gradually reduced by increasing temperature and pressure of carbon dioxide from 25 ◦ C and 50 atm as a single phase to 55 ◦ C and 150 atm as a supercritical fluid from 457.5 to 180.7 nm and from 450.7 to 206.9 nm, respectively. Accordingly, the performance of supercritical CO2 is further to be evidenced to disperse organic pigment in the solution with cosolvent PGMEA for manufacturing fine color paste in application of advanced flat plate panel.
As validated by the measurement of laser scattering and UV light, phthalocyanine green 36 pigment with the haloid structure can be dispersed into nanometer-scale particles at a concentration of 0.001% (w/w) into the solvent PGMEA using supercritical carbon dioxide under processing conditions of 55 ◦ C and 150 atm for 20 min. Additionally, the dispersions of aminoanthraquinone red containing the polarity of amino groups (–NH2 ) and phthalocyanine blue 15:6 with symmetry benzene and inner hydrogen bonds are difficult to wet and swell by the supercritical fluid with PGMEA as cosolvent in the same conditions that disperse phthalocyanine green 36 pigment, and the mean particle sizes are 178.5 and 188.7 nm, respectively, in the dispersing solution. These results suggest that supercritical carbon dioxide swells the aggregated organic pigment and upon depressur-
4. Conclusions For organic pigments, dispersion using supercritical carbon dioxide has been successfully developed in this study.
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scattering observation give rise to our hypothesis that supercritical carbon dioxide processing technology is a novel approach to the dispersion of organic pigment. Hopefully, these results will generate sufficient interest so that future studies can ascertain the nanometer-scale dispersion at high concentrations of organic pigments to manufacture high-resolution color filters for application in advanced FPD. (a)
Acknowledgment The authors are grateful for the financial support of this work by Grant NSC 89-2216-E-005-017 from the National Science Council of the Republic of China.
References
(b)
(c) Fig. 11. The transmittance of UV light for dispersing phthalocyanine blue 15:6 with 0.002% (w/w) in the solvent PGMEA using carbon dioxide under conditions of (a) 25 ◦ C and 50 atm, (b) 55 ◦ C and 100 atm, and (c) 55 ◦ C and 150 atm, respectively, at wavelength 500 nm.
ization, disperses organic particles at a fine size in the solvent PGMEA. Microscopic, UV light transmittance, and laser
[1] W. Herbst, K. Hunger, Industrial Organic Pigments: Production, Properties, Applications, VCH, New York, 1993. [2] P. Gregory, High-Technology Applications of Organic Colorants, Plenum, New York, 1991. [3] M. Tsuboi, Jpn. Soc. Printing Sci. Technol. 33 (3) (1996) 155. [4] T.C. Patton, Paint Flow and Pigment Dispersion, Wiley, New York, 1978. [5] P. Bugnon, Prog. Org. Coat. 29 (1996) 39. [6] H. Saikatsu, H. Okamoto, M. Yamazaki, S. Sakamoto, S. Yamamiya, Y. Abe, M. Nakamura, U.S. Patent 5,961,711, 1999. [7] R.G. Gnerlich, A.M. Pearson, A.V. Gangal, U.S. Patent 5,891,231, 1999. [8] K. Mitsuo, N. Kazuhiko, I. Yoshitaka, U.S. Patent 5,921,478, 1999. [9] P. Subra, P. Jestin, Powder Technol. 103 (1999) 2. [10] J.B. Hannay, J. Hogarth, Proc. R. Soc. London 29 (1879) 324. [11] J.B. Hannay, J. Hogarth, Proc. R. Soc. London 30 (1880) 178. [12] G.M. Schneider, Angew. Chem. Int. Ed. Engl. 17 (1978) 716. [13] B. Guzel, et al., in: Proc. 5th Meeting on Supercritical Fluids, Nice, France, March 1998, p. 351. [14] Z. Hao, A. Iqbal, Chem. Soc. Rev. 26 (1997) 203. [15] T. Fujitani, Prog. Org. Coat. 29 (1996) 97. [16] K. Ogawa, H.J. Scheel, F. Laves, Naturwissenschaften 53 (1966) 700. [17] F.H. Moser, A.L. Thomas, The Phthalocyanines, Vols. I, II, CRC Press, Boca Raton, FL, 1983. [18] K. Petritsch, W. Graupner, G. Leising, U. Scherf, Synth. Met. 84 (1997) 625.
Dispersion of organic pigments using supercritical carbon dioxide W.T. Cheng,∗ C.W. Hsu, and Y.W. Chih Department of Chemical Engineering, National Chung-Hsing University, 250 Kuo-Kuang Rd., Taichung 402, Taiwan, Republic of China Received 22 August 2002; accepted 20 August 2003
Abstract This research describes dispersion of organic pigments using supercritical fluids. With low surface tension and high diffusivity of fluids in supercritical states, aggregated particles may be effectively wetted and swelled to form the primary constituent of the dispersing solution by volume. In this paper, the conditions of temperature and pressure are used to control the density of supercritical carbon dioxide subject to PGMEA as cosolvent for dispersing organic powder in a solution. As shown from measurement with a laser scattering particle analyzer, the average diameter of phthalocyanine green 36 with the haloid structure can be significantly reduced to 93.5 nm; for aminoanthraquinone red containing and amino group (–NH2 ) and phthalocyanine blue 15:6 with symmetry benzene and inner hydrogen bond, the mean particle sizes are 178.5 and 188.7 nm, respectively, using supercritical CO2 . Additionally, the transmittance of UV light is used to confirm the dispersing performance in this study. 2003 Elsevier Inc. All rights reserved. Keywords: Supercritical carbon dioxide; Organic pigment; Dispersion; Mean particle size; Laser scattering particle analyzer; UV light
1. Introduction Organic pigments are usually used as colorants. The typical example is the paint and printing industries [1]. Recently, they have become increasingly important in the areas of electronics and communication [2]. For example, with the properties of resistance to heat and chemistry, organic pigments have been successfully applied to fabricate color filters for LCDs [3]. Therefore, the dispersion of organic pigments is the key to their continuing application. Some factors such as particle or crystal shape, molecular arrangement in particles or crystals, particle size and distribution, surface character, and interaction of pigments and medium [4,5] determine the chemical structure and physical properties of organic pigments. The primary diameter of organic pigments generally ranges from 10 to 50 nm, so that ease of aggregation into larger particles is found due to the high surface energy. Hence, the dispersion method is important for advanced applications [6,7]. In mechanical dispersion methods, shearing forces are usually applied to disperse the aggregated particles into a separate and fine particle, but result in a long processing time * Corresponding author.
E-mail address: [email protected] (W.T. Cheng). 0021-9797/$ – see front matter 2003 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2003.08.029
and contamination problems when the dispersing apparatus is washed after completion of the dispersion process. In order to reduce these drawbacks, a dispersion method [8] has been proposed as follows: the dispersoid is mixed in a supercritical state and the dispersoid including the supercritical fluid is then rapidly expanded to finely divide the dispersoid; and finally the fine particles are dispersed into a solvent, as illustrated in Fig. 1. Under the influence of pressure and temperature, pure substances can assume a gas, liquid, and solid state of matter except where the equilibrium saturation curve converges such that all three phases co-exit at the triple point. Extension of the liquid–gas phase line ends at the critical point and represents the maximum temperature and pressure at which the liquid and vapor phases coexist in equilibrium, after which gas and liquid have the same density and appear as a single phase. A fluid is said to be a supercritical fluid (SCF) when its temperature and pressure are in a state above its critical temperature and critical pressure, permitting the gaseous and liquid phases to coexist [9]. The solubility of supercritical fluids discussed by Hannay and Hogarth [10,11]. Supercritical fluids possess a liquidlike density and solubility and exhibits gaslike transport properties of diffusivity and viscosity [12]. Additionally, supercritical fluids have low surface tension, permeate faster than
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107
Fig. 1. The concept of dispersion of aggregate pigments using supercritical carbon dioxide.
liquids and have solubility that gases do not possess [13]. Therefore, high efficiency of dispersion may be achieved using SCF. In this paper, taking supercritical carbon dioxide and PGMEA as cosolvent and dispersion medium produces a system that disperses the organic pigments into the primary size in the dispersing solution. This concept is different from that of a supercritical antisolvent (SAS) that precipitates a substrate dissolved in a solvent as dried particles. In the dispersion method, as illustrated in Fig. 1, a liquefied carbon dioxide–PGMEA–organic pigment mixture is expressed from conditions of high pressure to ambient pressure, whereupon the gas undergoes an adiabatic expansion and a fine dispersoid (that is, a mixture of dispersed pigment with PGMEA) is immediately obtained. It is noted that the cosolvent PGMEA is partially miscible with CO2 and can be used for preparing an ultrafine color resist to fabricate a color filter in an advanced LCD. The effects of temperature and pressure on the dispersion process are to be investigated and discussed in detail. In addition, laser scattering particle analysis and UV light are applied to evaluate the dispersion performance.
2. Materials and methods Table 1 lists basic information on organic pigments (three kinds) used in this study, for which the chemical structure is shown in Fig. 2, from commercial grades suitable for color resists in manufacturing color liquid crystal display. Aminoanthraquinone red (Red A3B manufactured by the Ciba–Geigy Company), phthalocyanine green 36 (ECG 403 manufactured by the Dainipon Precision Chemical Com-
Table 1 The basic information on the examined organic pigments Color Product name Red Green Blue
Red A3B ECG 403 B6700
Chemical type Aminoanthraquinone Phthalocyanine Phthalocyanine
C.I. No. C.I. generic name 65300 74265 74160
Pigment red 177 Pigment green 36 Pigment blue 15:6
pany), and phthalocyanine blue 15:6 (B6700 manufactured by the BASF Company), were charged into PGMEA (propylene glycol monomethyl ether acetate, manufactured by Lancaster), stirred for 20 min using ultrasonic waves (with a Branson 5510 ultrasonic cleaner at 40 kHz), and then transport into a supercritical vessel. Liquid-type carbon dioxide was input for 20 min to reach the supercritical state under conditions of temperature and pressure ranging from 25 ◦ C and 1 atm to 55 ◦ C and 150 atm. Finally the dispersoid with CO2 was released into a collecting bottle under atmospheric pressure. The experimental apparatus of dispersion using SCF is shown in Fig. 3. This is a batch process, so the flow rates of liquid-type CO2 and the mixture of PGMEA with organic pigment transported to the dispersion column are unspecified. Additionally, the cosolvent PGMEA is mainly considered for the safety and compatibility of formulating color resists in the future. To identify the effect of dispersion on the supercritical process, the transmittance spectra of the dispersing solution is measured by a UV spectrometer (Hitachi U-2000) and the mean diameter size of the dispersoid is measured using a laser scattering particle analyzer (Brookhaven 90Plus Particle Sizer). Additionally, a microscope (Olympus BXP-50) is used to observe the dispersion solution on a glass substrate.
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3. Results and discussion 3.1. Temperature and pressure effects
(a)
(b)
(c) Fig. 2. The chemical structure of organic pigments used in this investigation: (a) red pigment 177, (b) green pigment 36, (c) blue pigment 15:6.
Fig. 3. Schematic diagram of dispersion apparatus using supercritical fluid.
The crystal structure of an organic pigment is critical in determining its stability to solvents, heat, and light. In addition, the stability, flow, and dispersion properties of a pigment material [14] depend on the morphology and nature of the major surfaces of the crystal, and these properties again depend on the crystal structure. In the supercritical state, the density can be increased by elevating pressure to enhance the wetting ability of SCF. Furthermore, the diffusivity of SCF may be promoted by increasing temperature. With a concentration of 0.002% (w/w) to PGMEA, for aminoanthraquinone red, phthalocyanine green 36, and phthalocyanine blue 15:6, the effect of pressure on the mean particle sizes of dispersing pigments is demonstrated in Fig. 4 under specified temperatures. The effect of temperature on supercritical state for the mean particle size of dispersing pigments is indicated in Fig. 5. As viewed in the deviation of mean particle size from these figures, for phthalocyanine green 36 with the haloid structure, the dispersing ability of supercritical carbon dioxide with PGMEA is significantly enhanced with increasing pressure and temperature, whereas aminoanthraquinone red, containing an amino group (–NH2), and phthalocyanine blue 15:6, with symmetry benzene and inner hydrogen bond, are not sensitive to the solution of supercritical CO2 with PGMEA as cosolvent. This is because amino groups (–NH2) and symmetry benzene with inner hydrogen bonds inhibit the diffusion of supercritical CO2 into the aggregated particles for wetting and swelling [15]. The conditions of 300 atm and 55 ◦ C for the supercritical CO2 have reached the limitation of the experimental equipment in this work.
Fig. 4. The effect of pressure on mean particle size of dispersed organic pigments with 0.002% (w/w) into the solvent PGMEA using supercritical CO2 at fixed temperature.
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(a) Fig. 5. The effect of temperature on mean particle size of dispersed organic pigments with 0.002% (w/w) into the solvent PGMEA using supercritical CO2 at fixed pressure.
3.2. Dispersing effect To reduce the concentration of organic pigment in the solvent of PGMEA from 0.002 to 0.001% (w/w), according to the results from Figs. 4 and 5, it is expected that the phthalocyanine green may be obtained as nanoscale particles in the solution by supercritical fluid dispersion under conditions of 300 atm and 55 ◦ C. Figure 6b displays a microscopic view of the dispersion of phthalocyanine green 36 at a magnitude of 500×, with mean particle size 93.5 nm from measurement with a laser scattering particle analyzer. Figure 6a corresponds to the dispersion of green pigments by preprocessing of SCF. For aminoanthraquinone red and phthalocyanine blue 15:6, the minimum average particle sizes of dispersion are 178.5 and 188.7 nm, respectively, with the microscopy of dispersion shown in Figs. 7b and 8b in the optimal supercritical state. Compared with phthalocyanine green 36, the supercritical carbon dioxide with PGMEA as cosolvent may be not suitable for organic pigments containing amino groups (–NH2 ) and symmetry benzene with inner hydrogen bonds. As shown in Fig. 2a, pigment red 177 has the chemical structure of 4,4 -diamino-1,1-dianthraquinonyl. X-ray diffraction analysis discloses a twisting of the two anthraquinone units by 75◦ relative to each other. This allows optimum formation of intermolecular hydrogen bonds [16] that inhibit nonpolarity of SCF to penetrate and swell. Polyhalogenated green copper pigment with 8 bromine and 2 chlorine atoms (as shown in Fig. 2b), which has the potential for dramatically improving solubility in supercritical CO2 , with a cosolvent like PGMEA is not polymorphous and thus is exempt from modification [1]. In contrast to the organic pigment blue 15:6, which displays more than one crystal modification, copper phthalocyanine is the copper(II) complex of tetraazaterabenzoporphine. As shown in Fig. 2c, the mesomeric structures indicate that all of the pyrrole rings simultaneously contribute to the aromatic system. Therefore, the molecule adopts a planar and completely con-
(b) Fig. 6. Microscopy of phthalocyanine green 36 with 0.001% (w/w) in the solvent of PGMEA in (a) predispersion state and (b) postdispersion using supercritical CO2 under condition of 300 atm and 55 ◦ C at a magnitude of 500×.
jugated structure that exhibits exceptional stability [17]. In other words, it is difficult to be wetted and swelled using supercritical carbon dioxide for dispersion of ultrafine powder with mesomeric structures. In short, the polar groups of –NH2 and symmetry benzene with inner hydrogen bonds are not compatible with the supercritical CO2 and PGMEA. This suggests the need to change the cosolvent from PGMEA to disperse copper phthalocyanine pigment blue 15:6 and pigment red 177 at the nanometer level by means of supercritical CO2 . As observed from Figs. 4 and 5, nevertheless, the dispersing effect of supercritical fluid on organic powder is positive.
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(a)
(b) Fig. 7. Microscopy of aminoanthraquinone red pigment with 0.001% (w/w) in the solvent PGMEA in (a) predispersion state and (b) postdispersion state using carbon dioxide under conditions of 100 atm and 35 ◦ C at a magnitude of 500×.
Microscopy and gross observation of the pigment in processing and nonprocessing of SCF may be applied to show that supercritical carbon dioxide swells the aggregated organic pigment. In the present study, UV spectrometer is also employed to make sure of the dispersing ability of supercritical carbon dioxide for the aggregates of organic pigments. This is based on the theory that if the particle size is smaller, the transmittance of UV light is higher [18]. Taking the concentration of 0.002% (w/w) to PGMEA as an example, Fig. 9 shows the transmittance of UV light as 61%, 70%, and 82% in a dispersing solution of phthalocyanine green 36 at wavelength 550 nm under three states of (a) 25 ◦ C and 50 atm, (b) 55 ◦ C and 100 atm, and (c) 55 ◦ C and 150 atm for the mean particle sizes of 264.1, 185.4, and 103.1 nm, respectively. This clearly indicates that the
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(b) Fig. 8. Microscopy of phthalocyanine blue 15:6 at 0.001% (w/w) in the solvent PGMEA in (a) predispersion state and (b) postdispersion state using carbon dioxide under conditions of 50 atm and 35 ◦ C at a magnitude of 500×.
wetting and diffusion ability of CO2 can be enhanced by controlling pressure and temperature to swell the aggregated pigment and obtain fine phthalocyanine green 36 particles in the solvent PGMEA. Particularly, the conditions of 55 ◦ C and 150 atm can nearly be used to disperse the aggregated phthalocyanine green 36 pigment as nanometer scale particles in the solvent PGMEA. For the dispersing solutions of aminoanthraquinone red and phthalocyanine blue 15:6, the results are the same as in the case of phthalocyanine green 36. Figures 10 and 11 show that the dispersion of fine particles can attain high transmittance of UV light. This means that decreasing the mean diameter increases the transmittance of UV light, but the large and unstable particles of the dispersing solution decrease transmittance of UV light due to scattering effect.
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Fig. 9. The transmittance of UV light for dispersing phthalocyanine green 36 with 0.002% (w/w) in the solvent PGMEA using carbon dioxide under conditions of (a) 25 ◦ C and 50 atm, (b) 55 ◦ C and 100 atm, and (c) 55 ◦ C and 150 atm, respectively, at wavelength 550 nm.
Fig. 10. The transmittance of UV light for dispersing aminoanthraquinone red pigment with 0.002% (w/w) in the solvent PGMEA using carbon dioxide under conditions of (a) 25 ◦ C and 50 atm, (b) 55 ◦ C and 100 atm, and (c) 55 ◦ C and 150 atm, respectively, at wavelength 620 nm.
As displayed in this two figures, the averaged particle sizes of the dispersing pigments red 77 and phthalocyanine blue 15:6 are gradually reduced by increasing temperature and pressure of carbon dioxide from 25 ◦ C and 50 atm as a single phase to 55 ◦ C and 150 atm as a supercritical fluid from 457.5 to 180.7 nm and from 450.7 to 206.9 nm, respectively. Accordingly, the performance of supercritical CO2 is further to be evidenced to disperse organic pigment in the solution with cosolvent PGMEA for manufacturing fine color paste in application of advanced flat plate panel.
As validated by the measurement of laser scattering and UV light, phthalocyanine green 36 pigment with the haloid structure can be dispersed into nanometer-scale particles at a concentration of 0.001% (w/w) into the solvent PGMEA using supercritical carbon dioxide under processing conditions of 55 ◦ C and 150 atm for 20 min. Additionally, the dispersions of aminoanthraquinone red containing the polarity of amino groups (–NH2 ) and phthalocyanine blue 15:6 with symmetry benzene and inner hydrogen bonds are difficult to wet and swell by the supercritical fluid with PGMEA as cosolvent in the same conditions that disperse phthalocyanine green 36 pigment, and the mean particle sizes are 178.5 and 188.7 nm, respectively, in the dispersing solution. These results suggest that supercritical carbon dioxide swells the aggregated organic pigment and upon depressur-
4. Conclusions For organic pigments, dispersion using supercritical carbon dioxide has been successfully developed in this study.
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scattering observation give rise to our hypothesis that supercritical carbon dioxide processing technology is a novel approach to the dispersion of organic pigment. Hopefully, these results will generate sufficient interest so that future studies can ascertain the nanometer-scale dispersion at high concentrations of organic pigments to manufacture high-resolution color filters for application in advanced FPD. (a)
Acknowledgment The authors are grateful for the financial support of this work by Grant NSC 89-2216-E-005-017 from the National Science Council of the Republic of China.
References
(b)
(c) Fig. 11. The transmittance of UV light for dispersing phthalocyanine blue 15:6 with 0.002% (w/w) in the solvent PGMEA using carbon dioxide under conditions of (a) 25 ◦ C and 50 atm, (b) 55 ◦ C and 100 atm, and (c) 55 ◦ C and 150 atm, respectively, at wavelength 500 nm.
ization, disperses organic particles at a fine size in the solvent PGMEA. Microscopic, UV light transmittance, and laser
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