Optical Materials 26 (2004) 297–300 www.elsevier.com/locate/optmat
Preparation of a kind of red encapsulated electrophoretic ink H.L. Guo, X.P. Zhao
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Department of Applied Physics, Institute of Electrorheological Technology 141 #, Northwestern Polytechnical University, 710072 Xi’an, People’s Republic of China Received 8 October 2003; received in revised form 5 January 2004; accepted 20 January 2004 Available online 5 March 2004
Abstract This paper presents a kind of red encapsulated electronic ink prepared by in situ polymerization utilizing urea and formaldehyde as wall materials. Pigment scarlet powders with bright tone, small size and low special gravity were modified with polyethylene (PE) to have superior affinity for tetrachloroethylene. The modifying pigments were characterized by IR spectrometry and microphotography, and the microcapsules were also investigated in the same way. Experiments revealed that the pigments in the microcapsule moved toward negative electrode, while the field revised the pigments were pulled back reversible. And the response time of the particles was approximately 3.2 s. Ó 2004 Elsevier B.V. All rights reserved. PACS: 85.60.Pg; 87.15.Tt; 42.70.)a Keywords: Electronic ink; Display materials; Electric response
1. Introduction It has for many years been an ambition of researchers in display media to create a flexible low-cost system that is paper-like display. One of the most appealing applications of flexible display is electronic paper, which combines the desirable viewing characteristics of conventional printed paper with the ability to manipulate the displayed information electronically. In this context, a number of different technologies have been proposed for use in electronic paper, such as twisting ball display [1–4], electrochemical reaction display, electronic ink display [5–10] and electrowetting display [11]. Electronic ink display, also called microencapsulated electrophoretic image display, is a non-emissive device based on the electrophoretic phenomenon of charged pigment particles suspended in a solvent and the display images can be electrically written or erased repeatedly. It was developed to solve the lateral drift and agglomeration [12] of particles by enclosing the suspension in microcapsules, which resulted in improved reliability. Addi*
Corresponding author. Tel.: +86-29-88495950; fax: +86-02988491000. E-mail address:
[email protected] (X.P. Zhao). 0925-3467/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2004.01.010
tionally, electronic ink display possesses exceptional portability, wide viewing angle and nil electromagnetic radiation [13,14]. In view of the above advantages of electronic ink display, many companies and institutes devote to the research of materials of electronic ink display and other related technologies [15–18]. Barrett Comiskey et al. report a sort of microcapsules of electronic ink with white particles dispersing in a blue dyed fluid that microencapsulated by means of an in situ polycondensation of urea and formaldehyde [5]. Rutile titanium dioxide (specific gravity ¼ 4.2) is used as white particles because its reflectivity is very high and its color is pure. A coating of polyethylene (PE) is covered on the surface of TiO2 to reduce the specific gravity of the particles and present a modified surface chemistry for charging purposes to respond under applying an electric field. Research reveals that the respond time is about 0.1 s. Satoshi Inoue et al. also fabricated a type of microcapsules using a complex coacervation method with gelatin and gum arabic as materials. And the respond time is on the order of several hundred milliseconds [19]. In addition, color displays can be realized by splitting each picture element into RGB (red–green–blue) or superimposing the CMY (cyan–magenta–yellow) layers
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vertically. And the E Ink Corporation and its strategic partners exhibited a prototype of full-color electronic ink display at ‘‘Society for Information Display (SID) Symposium, Seminar and Exhibition’’ 2002, Boston, Massachusetts. Although electronic paper based on electronic ink display has been demonstrated and exhibited in some occasions, it has not been commercialized genuinely because there are still many problems in displaying high contrast ratio, high resolution, videospeed image and manufacturing cost. By now, the research of color electronic display is just at the outset and the exhibited prototypes are still images. The authors try to prepare microcapsules of full-color electronic ink which containing red, green and blue pigments dispersed in a clear fluid and provide full-color displays by RGB methods. In this paper, we report the synthesis of red electrophoretic ink with pigment scarlet powders as the red pigments. Pigment scarlet powders were chose because of not only its bright tone, but also small size and low special gravity, and that can make the suspension stable. To improve its affinity for the solvent, the pigment was modified by polyethylene. Experiments demonstrated the particles moved in the capsules reversibly while applying an electric field. 2. Experiments 2.1. Surface treatment of particles 1.0 g of PE (specific gravity ¼ 0.94) is dissolved in 500 ml cyclohexane (specific gravity ¼ 0.78, 20 °C) with stirring at 80 °C. Fifty milliliter of a 0.02 wt% solution of PE and 0.2 g of pigment scarlet powders (specific gravity ¼ 1.91, approximately 1 lm in diameter) are sonicated for several minutes. After placed for 24 h, the mixture centrifuged at about 3000 rpm for 5 min. The particles then are washed by anhydrous ethanol (specific gravity ¼ 0.79, 20 °C) and dried. Pigment scarlet powders modified by PE are obtained. 2.2. Preparation of internal phase (electrophoretic suspension) The internal phase is a suspending fluid containing pigment scarlet powders modified by PE suspended in tetrachloroethylene (specific gravity ¼ 1.62, 20 °C) using sorbitan monooleate (Span 80) as the stabilizer. Thirty milligram of modified pigments and 6 ml of tetrachloroethylene with some Span 80 are sonicated for several minutes to mixture completely and coefficient of viscosities of the mixture is 0.86 mPa s (20 °C). 2.3. Microencapsulation In this paper, the microcapsules were prepared by in situ polymerization of urea and formaldehyde as wall
materials [20]. First, urea was dissolved in equal mol of 37 wt% formaldehyde aqueous solution, and the mixture was adjusted to a pH of about 7–8 with triethanolamine and stirred at 70–85 °C for 1 h. Then it was cooled and diluted with double volume of distilled water to obtain the prepolymer. The internal phase described above was poured into the mixture of the prepolymer and 3–5 volumes of distilled water under rapid stirring condition for about 5 min. The emulsion reacted at ambient temperature at moderate stirring condition for about 1– 1.5 h with maintaining the pH at about 2–4 adjusted by drop-wise addition of hydrochloric acid (2 wt% HCl). Lastly, the slurry were heated to about 60 °C for another 1 h to obtain the microcapsules containing the pigments and tetrachloroethylene. The resultant capsule slurry was then filtered, washed, dried and sieved, and the microcapsules (specific gravity ¼ 1.47, 20 °C) in definite size were obtained. 2.4. Measurement The surface morphology of microcapsules was observed under an optical microscope (Alphaphot-2 YS2-H, Nikon, Japan) with CCD (Fijitsu, Japan) and collected photographs dealt with grey scale through a seizing card of video capture. The response behavior of the capsules under electronic field was also investigated in the same way. The surface functional groups of the pigments and microcapsules were obtained with a FTIR (scan 400–4000 cm1 , Equinox55, Bruker, Germany) spectrophotometer. 3. Results and discussion 3.1. Investigation of surface treatment of pigment scarlet powders The pigment scarlet powders were modified because of their poor dispersibility in tetrachloroethylene. The powders departed from tetrachloroethylene during the microencapsulated process. Fig. 1 presents the IR spectra of the pigment scarlet powders. According to the
b
a
4000 3500 3000 2500 2000 1500 1000 Wavenumber (cm-1)
500
Fig. 1. IR spectra of pigment scarlet powders ((a) non-modified, (b) modified by PE).
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IR spectra in Fig. 1, two peaks of CAH band at 2917 and 2848 cm1 are strength and have moved 5 cm1 toward long wave. The peak of CAH band at 1740 cm1 is also strength and has moved 4 cm1 toward short wave. So it is found that PE is coated at the surface of the powders. After modified with PE, the powders have presented superior dispersity and can remain in internal phase while they are encapsulated. Fig. 2 indicates the micrograph of the powders suspended in tetrachloroethylene. It is observed that the powders modified with PE made the size small. 3.2. Characterization of microcapsules of electronic ink 3.2.1. IR spectra of microcapsules Fig. 3 presents the IR spectra of microcapsules of red electronic ink. From the IR spectra, it is shown that three peaks of a NAH stretching vibration at 1550 cm1 , a C@O stretching vibration at 1645 cm1 , and a NAH stretching vibration at 1251 cm1 are observed. CAH stretching vibrations are showed at 1251 and 1153 cm1 . The OAH peak is showed as a broad absorption peak at 3300–3500 cm1 . So it is found that urea–formaldehyde capsule is formed, while the specific absorption bands of tetrachloroethylene are not observed in the microcapsule due to the sealing of internal phase in the microcapsule.
Fig. 2. Micrograph of pigment scarlet powders ((a) non-modified, (b) modified by PE).
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a
b
4000 3500 3000 2500 2000 1500 1000 500 Wavenumber ( cm-1 )
Fig. 3. IR spectra of urea–formaldehyde microcapsules ((a) tetrachloroethylene, (b) microcapsules).
3.2.2. Micrograph of microcapsules The image photographs of the microcapsules of red electronic ink are showed in Fig. 4. The urea–formaldehyde capsule prepared by in situ polymerization has a clear, firm and regular appearance. The average diameter of the microcapsules is 80 lm. 3.3. Electric response of microcapsule To investigate the movement of the particles in the microcapsule, we have made a microelectrophoretic cell. It is made up of two parallel copper electrodes with 1mm apart. And each electrode was welded a wire to
Fig. 4. Microcapsules of red electronic ink.
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4. Conclusions A kind of red encapsulated electronic ink was prepared by in situ polymerization utilizing urea and formaldehyde as wall materials. Pigment scarlet powders modified with PE were found to have superior affinity for tetrachloroethylene, and they had superior dispersibility and stability while they were dispersed in tetrachloroethylene. The particles moved in the capsules toward negative electrode attached to the capsule by applying a DC electric field (E ¼ 120 V/mm). When the field was reversed, the particles were pulled back reversible. And the response time of the particles was approximately 3.2 s.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 90101005) and the National Natural Science Foundation of China for Distinguished Young Scholar (Grant No. 50025207).
References [1] [2] [3] [4] Fig. 5. Microcapsules in the electric field ((a) E ¼ 0, (b) E ¼ 120 V/mm, (c) E ¼ 120 V/mm).
provide a DC electric field. Fig. 5a is the microphotograph of microcapsules under no electronic field, and the pigments are distributed randomly in the capsule. The particles moved fast in the capsule toward the negative electrode under a DC electric field (E ¼ 120 V/mm) (Fig. 5b). The respond time is about 3.2 s. When the field was revised, the particles were pulled back reversible (Fig. 5c). From the investigation, we also found that the degree of dryness of capsules could influence the pigments’ electric response behavior deeply. If the microcapsules were not dried completely, there would be no response under the electric field.
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