Microcapsule-based materials for electrophoretic displays
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Hecheng Li and Dongyuan Liu Lucky Huaguang Graphics Co., Ltd, Nanyang 473003, People’s Republic of China (Received 10 October 2011; accepted 5 December 2011)
Electrophoretic displays (EPDs) are attracting a great deal of academic and commercial interest due to the advantages of both electronic displays and conventional paper. The key materials for EPD application of microcapsules are the electrophoretic particles and the capsule wall enwrapping the electrophoretic suspension inside. Here, black and white electrophoretic particles with low density and good dispersity such as titanium dioxide, carbon black, and Cu2Cr2O3 were prepared by surface modification of the pigments. The preparation and properties of the gelatinbased microcapsules prepared by complex coacervation methods are also summarized. The microcapsules have transparent and elastic walls of compact structure, which endows them with good barrier properties and thermal stability for EPD application. EPD prototype devices based on the obtained microcapsules were prepared and could be driven at 9 V.
I. INTRODUCTION
Electrophoretic displays (EPDs), the most efficient solution for the reflective digital display, combine the advantages of electronic displays and conventional paper. EPD technology is based on the movement of charged pigments encapsulated in a liquid media of low dielectric constant when a voltage is applied.1–5 Charged pigment particles with low density, high mobility, and suspension stability are the basis of EPD. Though the first EPD device was introduced as early as 1969,6 it did not develop during the next 20 years until Joseph Jacobson first microencapsulated the electrophoretic suspension into individual microcapsules by in situ polymerization,7 which reduced undesirable particle clustering and agglomeration to a scale less than the microcapsule size. Although research on EPD has made rapid progress in the last decade, the efforts on improving the contrast and the response speed never stopped. For the white and black display, titanium dioxide (TiO2) is commonly selected to act as the white pigment due to its excellent whiteness. To avoid the sedimentation of the TiO2 in the suspension resulting from its high density (3.9–4.1 g/cm3), TiO2/polymer composite particles and hollow particles were prepared to decrease the density of TiO2 to match the media.8–13 Carbon black (CB) is applied as the black pigment. Because of its ultra-large surface area, CB easily
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Address all correspondence to this author. e-mail: [email protected] This article has been selected as an Invited Feature Paper. DOI: 10.1557/jmr.2011.427 J. Mater. Res., Vol. 27, No. 4, Feb 28, 2012
aggregates. Thus, surface modification is needed to prevent the aggregation of the CB particles.14,15 Also, modification of other black pigments were under research to meet the specific needs of EPD applications.16–18 Microencapsulation refers to the techniques using natural or synthetic polymer materials to enwrap solid, liquid, and/or gas materials. Since the development of the first carbon pa
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