外文翻译原文：电泳显示器微胶囊有机颜料粒子的

Surface modifi cation of organic pigment particles for microencapsulated electrophoretic displays Zi-Qiang Wen, Ya-Qing Feng*, Xiang-Gao Li, Yu Bai, Xiao-Xu Li, Jing An, Min Lu School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China a r t i c l ei n f o Article history: Received 1 April 2011 Received in revised form 3 June 2011 Accepted 7 June 2011 Available online 21 June 2011 Keywords: Organic pigment Surface modifi cation Microcapsule Electrophoretic display Dispersion stability Chargeability a b s t r a c t The surface modifi cation of Pigment Yellow 13, Pigment Red 254 and Pigment Blue 15 to improve their electrophoretic properties in electrophoretic suspension were studied. The particle size distribution and the surface morphology of the modifi ed pigments were determined, the presence of the functional groups on the surface of pigments was confi rmed, and the dispersion stability and chargeability of pigments suspended in electrophoretic slurry were characterized. For the application of micro- encapsulated electrophoretic displays, the colored microcapsules containing the modifi ed organic pigments were prepared and spread on an ITO coated conducting fi lm. A working prototype of elec- trophoretic display was successfully fabricated in this approach. ? 2011 Elsevier Ltd. All rights reserved. 1. Introduction Microencapsulated electrophoretic display (EPD), which is also called electronic ink display, is made of microcapsules including charged particles and fl uid medium. It has been of great interest due to its advantages of ink-on-paper appearance, high refl ectance, good contrast ratio, wide-viewing angle, state bistability and low power consumption 1,2. Currently, blackewhite electronic books (e-books) based on microencapsulated EPD have been commer- cialized and have achieved mass market success. The paper-like displays designed by the E-Ink corp., are used by the majority of todays e-book readers. However, full-color e-books or displays are not realized, so the colorization will be the next target of e-reader market 3. The blackewhite EDPs comprise of a fl at, two-dimensional array of microcapsules containing a dual-particle system of titanium oxide white particles and black carbon pigments with opposite charges. When the backplane electronics apply voltage to the microcapsules, one set of particles is driven to the top of the microcapsules and the other set goes to the bottom to show images and texts. Based on this principle, several techniques of colorization are explored. The conventional method of color fi ltering will bring on the loss of light and resolution, leaving unsaturated colors as a result. The genuine realization of full-color display needs a sub- pixel comprising of tricolor unites such as red, green and blue or cyan, magenta and yellow. Pigment particles in electrophoretic fl uid must have properties such as an appropriate density, low solubility, good disperse stability and high chargeability. Many kinds of pigments can be used as colored electrophoretic particles. Tang et al. doped the hollow titania with Cr3, Fe3and Co2/Al3to fabricate colored inorganic pigment particles 4. Kang et al. prepared polymer nanoparticles by free-radical polymerization of styrene and 4- vinylpyridine, encapsulating organic dyes such as Acid Blue 25, Acid Red 8, and Acid Yellow 76 5. Kim et al. used laser printer toner particles modifi ed with wax materials to fabricate colored microcapsules containing cyanewhite, magentaewhite and yel- lowewhite pigment pairs 6. However, the weakness of inorganic 7,8 and polymer particles 9 in chroma and light refl ectance was known to infl uence the color saturation of display. The organic pigments, on the other hand, have merit of excellent optical properties such as good brilliance, color strength and transparence, and therefore can offer great potential in the context of electronic, optical and photoelectric devices. Tang et al. prepared pigment- based RGB tricolor ink particles via mini-emulsion polymeriza- tion, but the polystyrene coating in pigments decreased the refl ex light saturation and made the ink particles look albescent 10. In this article, we report the use of the typical organic pigments such as C. I. Pigment Yellow 13, C. I. Pigment Red 254 and C. I. * Corresponding author. Tel./fax: 86 22 27401824. E-mail address: (Y.-Q. Feng). Contents lists available at ScienceDirect Dyes and Pigments journal homepage: /locate/dyepig 0143-7208/$ e see front matter ? 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.dyepig.2011.06.003 Dyes and Pigments 92 (2011) 554e562 Pigment Blue 15 as colored electrophoretic particles. Organic pigment particles with smooth surface, small particle size and narrow size distribution were obtained after surface modifi cation. Good dispersion stability and surface charges of the modifi ed particles in solvent were achieved in the presence of hyper- dispersants and charge control agents, and the modifi ed particles offered good electrophoretic behavior in electrophoretic slurry. The microcapsules of dual-particle systems containing yellowewhite, redewhite and blueewhite pigments were prepared via coacer- vation of gelatin and gum arabic 11,12. The resulting particles exhibited bothgoodelectrophoretic movementand ideal bistability in the microcapsules. Therefore, these microcapsules containing modifi ed organic pigment particles could be used as a good candidate for the colored EPDs. 2. Experimental section 2.1. Materials The organic pigments of yellow, red and blue (commercial C. I. name: Pigment Yellow 13, Pigment Red 254, Pigment Blue 15) were obtained from Clariant Tianjin Co. Ltd (Tianjin, China), and their chemical structures were shown in Fig. 1. Sodium silicate, tetra- chloroethylene,polyethylene,stearicacid,cetyl- trimethylammoniumbromide,gelatinandgumarabicwere purchased from Guangfu Chemicals Co. Ltd (Tianjin, China). Silane coupling agent 3-aminopropyltriethoxysilane was obtained from Tianyang Assistant Co. Ltd (Yangzhou, China). Hyperdispersants CH-5 and CH-11B were purchased from Sanzheng Polymer Co. Ltd (Shanghai, China). All the chemicals used in this study were of reagent grade. 2.2. Modifi cation of yellow particles and preparation of yellow electrophoretic fl uid Pigment Yellow 13 (PY13) was modifi ed using the following process. PY13 (1.0 g) was emulsifi ed in 150 mL deionized water with 0.18 g cetyltrimethylammonium bromide (CTAB). The mixture was then transferred into a 500 mL round bottom fl ask and heated to 95 ?C. Sodium silicate solution (20 mL, 1 M) was dripped in 2 h and the pH was adjusted to 9e10 by addition of 2 M sulfate acid solution. The reactions were maintained for 5 h at pH 10, and continued foranother 2 h at pH 3. The solid product was collected bycentrifuge, and then dispersed into 200 mL mixture of water and ethanol(10:90,weightratio).Tothemixture,3- aminopropyltriethoxysilane(0.2g)wasaddedandpHwas adjusted to 3. The mixture was heated to refl ux with moderate stirring and the reaction was maintained for 10 h at 80 ?C. The resulting product was collected by centrifuge and washed several times with deionized water and absolute ethanol respectively. The yellow electrophoretic fl uid was prepared by ball milling method. The suspension was prepared by mixing 0.25 g modifi ed PY13 particles, 10 mL tetrachloroethylene, 0.025 g CH-5 as emul- sifi er, and a certain amount of CTAB as charge control additive. The mixture was milled for 24 h on a bead miller with 0.5 mm ZrO2 bead as milling medium at a stirring speed of 2000 rpm to obtain yellow electrophoretic slurry. 2.3. Modifi cation of red particles and preparation of red electrophoretic fl uid Pigment Red 254 (PR254) was modifi ed using the following process. Polyethylene (PE) 0.5 g was dissolved in 250 mL cyclo- hexane with stirring at 80 ?C. PR254 (1.0 g) was ultrasoniclly dispersed in the above solution for 30 min. After the suspension was placed for 24 h, the product was centrifuged and washed by absolute ethanol. Red pigment scarlet powders modifi ed by PE were obtained. The red electrophoretic slurry was prepared by the same ball milling method as that described for the yellow pigments. The red electrophoretic fl uid contained 0.25 g modifi ed PR254 particles, 10 mL tetrachloroethylene and 0.025 g CH-5 as stabilizer. 2.4. Modifi cation of blue particles and preparation of blue electrophoretic fl uid Pigment Blue 15 (PB15) 0.25 g, assistant-hyperdispersant CH- 11B 0.01 g and hyperdispersant CH-5 0.025 g were dispersed in 10 mL tetrachloroethylene, with a certain amount of stearic acid as charge control additive. The blue electrophoretic slurry was prepared by the ball milling method as described in Section 2.2. 2.5. Preparation of dual-particle systems electrophoretic fl uid White electrophoretic slurry (0.1 M) containing TiO2/PMMA composite particles (made in our laboratory) 13,14 was also prepared by ball milling. Yellowewhite or redewhite or bluee white electrophoretic fl uid of dual-particle system was obtained by mixingtwokindsofelectrophoretic fl uidandultrasoniclly dispersing for 0.5 h. The electrophoretic slurry volume ratio of yellow/white,red/white and blue/whitewas set at 1/1,1/10 and 1/2, respectively. 2.6. Preparation and coating of colored microcapsules The microcapsules were prepared bycoacervation of gelatin and gum arabic used as wall materials. First gum Arabic (0.4 g) was dissolved in 20 mL deionized water with stirring at 50 ?C for 30 min. Colored electrophoretic slurry (3 mL), prepared as described in Section 2.5, was introduced into the aqueous phase. Fig. 1. Chemical structures of pigment raw materials. Z.-Q. Wen et al. / Dyes and Pigments 92 (2011) 554e562555 Fig. 2. (a) Schematic representation of preparation of coreeshell composite particles, (b) FT-IR spectra and (c) TEM micrographs of original and modifi ed yellow pigments, (d) XPS spectra of silica coating and modifi ed yellow pigments. (For interpretation of the references to color in this fi gure legend, the reader is referred to the web version of this article.) Z.-Q. Wen et al. / Dyes and Pigments 92 (2011) 554e562556 The emulsionwas agitatedat 900 rpm forabout 2 min, and then the agitation was reduced to 300 rpm. Gelatin water solution (20 mL, 0.2 M) was drop-wise added in 5 min. The pH was then regulated to 3.8 by addition of 10 wt.% acetic acid solution. The emulsion was slowly cooled to 10 ?C and maintained for 1 h. After adding 5 wt.% glutaricdialdehyde solution (5 mL) as crosslinker for wall material, the polymerization was held for another 1 h below 0 ?C. The precipitated microcapsules were then rinsed off with deionized water several times, and the colored electric ink was obtained. The colored microcapsules were coated on indium tin oxide (ITO) fi lm with 5 wt.% polyvinyl alcohol (PVA) as binder by using a home-made coating machine. The weight ratio of microcapsule/ PVA was 3/1, the coating rate was 60 mm/min and the coating height was 0.75 mm. After natural drying, the coating microcap- suleswerebondedwithpolyurethaneadhesiveL758on a patterned electrode. The prototype of colored microencapsulated EPDs was obtained. 2.7. Characterization Chemical constitution of the product particles was identifi ed by fourier transform infrared spectroscopy (FT-IR, BID-RAD3000, scan 400e4000 cm?1) and X-ray photoelectron spectroscopy (XPS, PHI- 1600, PerkineElmer, USA). The surface morphology and structures were examined by transmission electron microscopy (TEM, JEM- 2001F, Jeol, Japan) and scanning electron microscopy (SEM, S4800, Hitachi, Japan). The particle size distribution, zeta potential and electrophoretic mobility of pigment particles were measured by zeta potential and submicron particle size analyzer (Delsa Nano C, Beckman Coulter, USA). In addition, the microcapsules were Fig. 3. (a) FT-IR spectra and (b) TEM micrographs of original and modifi ed red pigments. Fig. 4. (a) FT-IR spectra and (b) TEM micrographs of original and modifi ed blue pigments. Z.-Q. Wen et al. / Dyes and Pigments 92 (2011) 554e562557 observed under an optical microscope with CCD (BX51, Olympus, Japan). 3. Result and discussion 3.1. Modifi cation of organic pigment particles Pigment Yellow 13 is a traditional disazo pigment. However, the poor solvent resistance and light fastness limit its applications in EPDs 15. A nano-silica layer coating on the surfaces of the organic pigment particles to improve the solvent resistance and weather- ability 16, was reported in our previous work 17. Fig. 2a is the schematicrepresentationofthepreparationofcoreeshell composite particles. First, the yellow pigments were dispersed in CTAB/water solution, and SiO2shell was precipitated onto the surface of the pigments via the hydrolysis of sodium silicate at92 ?C andatpH9e10.Thesilanecouplingagent3- aminopropyltriethoxysilane was then grafted onto the surface of the composite particles, and a lipophilic and highly-charged surface was resulted. Fig. 2b shows the FT-IR spectra of the pigment before andafter modifi cation.TheSieOstretchingvibrationat 1080e1100 cm?1and SieOeSi bending at 460e480 cm?1of the modifi ed pigment confi rmed the presence of the silica. The NeH stretching vibration due to the amino group can be found at 3420e3440cm?1.Inorderto confi rmthegraftingof3- aminopropyltriethoxysilane, XPS analysis was carried out. Fig. 2d shows the XPS spectra of silica coating and modifi ed yellow pigments. A comparison of the silica coating pigments and modifi ed pigments indicates the shift of the Si2p XP signal from 103.68 eV to 103.40 eV and the shift of the O1s from 532.93 eV to 532.65 eV, corresponding to the conversion of the SieOeHon thesurfaceto SieOeSiafter graftingwith3- aminopropyltriethoxysilane, which increases the shielding effect of electrons. The N1s XP signal appearing at 401.65 eV is charac- teristic for NH3 which confi rms that the amino groups are intro- duced on the surface of composite particles. Fig. 2c is the TEM photographs of the original and modifi ed yellow pigments. The Fig. 5. Particle size distributions and SEM microphotographs of modifi ed (a) yellow, (b) red and (c) blue pigment particles in electrophoretic slurry. Table 1 Zeta potentials and electrophoretic mobilities of modifi ed organic pigment particles in tetrachloroethylene. Electrophoretic particlesZeta potential (mV)Electrophoretic mobility (cm2/V s) Modifi ed yellow particles?3.45?8.269e-7 Modifi ed red particles?31.83?7.631e-6 Modifi ed blue particles?0.45?1.076e-7 Z.-Q. Wen et al. / Dyes and Pigments 92 (2011) 554e562558 strong contrast between the gray edge and white center of the modifi ed pigments confi rmed the coreeshell structure, and the composite form was found to enhance the solvent resistance and light fastness. Many red organic pigments have been used in EPD application 18,19. In this paper, we chose Pigment Red 254, a high- performance pigment of diketopyrrolopyrrole (DPP), to function as the red electrophoretic particles. Despite the low molecular masses, the DPP pigments are highly insoluble, and very chemically and thermally stable. Such behavior may be attributed to the presence of strong intermolecular forces in the pigment crystal lattice, for instance,pepinteraction between two planar molecules and H-bonds between the eNH group and oxygen 20,21. To improve theiraffi nity tothe solvent, the pigmentswere coatedwith polyethylene. The PE membrane covered on pigments presents a chemically modifi ed surface for the purposes of charging and providing a steric barrier between pigment particles to prevent agglomeration. Fig. 3a presents the FT-IR spectra of the pigment scarletpowders. The peaks ofCeHstretching vibration at 2918 cm?1and 2849 cm?1 of the modifi ed particles confi rmed that the red pigments were coated with PE. TEM micrographs in Fig. 3b show that the powders modifi ed with PE have a spheroidal shape and ideal dispersity in solvent. Copper phthalocyanine, which has excellent all-round fastness properties, is known to occur in no less than fi ve crystal modifi - cations 22. Pigment Blue 15 exists as thea-form copper phtha- locyanine. It is known that thea-form is not stable and tends to convert tob-form. Therefore, in this paper, we used assistant- dispersant CH-11B (4 wt.% relative to the pigments) to improve the stability of thea-form copper phthalocyanine in solvent. In the preparation of blue electrophoretic fl uid, the solubility of copper phthalocyanine in tetrachloroethylene played a decisive role in the phase transformation. As a derivative of phthalocyanine, CH-11B has physical and chemical properties similar to those of copper phthalocyanine 23. It participates in the crystal of copper phthalocyanine, which can be effectively stabilized against solu- bility and recrystallization to restrain the phase conversion process ofa-form tob -form copper phthalocyanine. This can be confi rmed fromthe FT-IR result as shown in Fig. 4a. The original blue pigments which were milled for 24 h in tetrachloroethylene, have theb-form characteristic absorption at 727.88 cm?1. The FT-IR spectra of the CH-11B modifi ed pigments milled for 24 h show thea-forms characteristic absorption at 721.15 cm?1. CH-11B is absorbed easily on the surface of PB15 to provide polar anchoring positions for stabilizers, and improves PB15 dispersibility in tetrachloroethylene. Fig. 4b is the TEM micrographs of the original and modifi ed blue pigments. It can be seen that the original particles without CH-11B have stronger fl occulation effect than those of modifi ed particles. This is probably because CH-11B enhances the absorption of stabilizer on the particle surface, and inhibits the aggregation of particles. 3.2. Dispersion stability and chargeability of particles in electrophoretic slurry Normally, the low molecule-weight surfactants are rega