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Improved dark state electrophoretic display

IP.com Disclosure Number: IPCOM000033723D
Publication Date: 2004-Dec-23
Document File: 3 page(s) / 56K

Publishing Venue

The IP.com Prior Art Database

Abstract

Electrophoretic displays switch due to particle migration into a colored fluid. They are a promising technology for development of high-resolution electronic (re-imageable) paper. This paper describes an electrophoretic device that is capable of showing both white and dark state reflectivities. This is achieved by dissolution of a photochromic material into the polymeric binder, which keep together the capsules containing the ink. The photochromic compound is switched with UV light to the colored state. The colored state of the photochromic compound is the same as the color of the dye in the fluid. The dark state of the electrophoretic device in improved (gets darker). Darker translates into increased contrast ration between dark and white states of the device, resulting in improved viewing of the document. The white state is obtained by switching electrophoretic particles on top and isomerisation of the photochromic compound to the colorless state (with Visible light).

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Improved dark state electrophoretic display

Electrophoretic displays switch due to particle migration into a colored fluid.  They are a promising technology for development of high-resolution electronic (re-imageable) paper.  One problem with electrophoretic devices is the difficulty of obtaining simultaneously high contrast, dark and white states with the same devices.  The brightness of the white state can be increased by using a less concentrated fluid (less colored), but this results in poor dark state quality because of reduced hiding power of a less colored fluid. In contrast, if the solution contains high concentration of the coloring material, the white state suffers because of hiding and partial coloration of the white particles by the colored fluid.

This paper describes an electrophoretic device that is capable of showing both white and dark state reflectivities.  This is achieved by dissolution of a photochromic material into the polymeric binder, which keep together the capsules containing the ink.  The photochromic compound is switched with UV light to the colored state.  The colored state of the photochromic compound is the same as the color of the dye in the fluid.  The dark state of the electrophoretic device in improved (gets darker).  Darker translates into increased contrast ration between dark and white states of the device, resulting in improved viewing of the document.  The white state is obtained by switching electrophoretic particles on top and isomerisation of the photochromic compound to the colorless state (with Visible light).

The Device

An electrophoretic display was fabricated by sandwiching capsules containing one particle electrophoretic ink into a polymeric matrix containing a small amount of photochromic material.  The device configuration is shown in Figure 1.

Figure 1. Device structure and working principle for improvement of the dark state of the colored state.  The device is shown in its colored state when viewed from top.  The areas illuminated with UV light are darker because both photochromic material and electrophoretic ink are in the colored state.  The unexposed areas (left) are blue only due to the ink, and their coloration is less intense when compared with the right side.

 

Ink and capsules preparation

5.5g of a white particle ink suspension (consisting of TiO2/Nucrel-Elvax blend composite particles) at 21.80% solids was dispersed in 19g of Isopar M and 16g of dyed Isopar M (containing 1% w/w Solvent Blue 58, an oil-soluble blue aminoanthraquinone dye).  Next, 0.20g of a solution of 10% w/w OLOA 371C in Isopar M was added (OLOA=polyisobutylene succinimide, a charge control agent).  40g of 1-particle ink suspension was prepared, with a final white particle solids loading of 3%, and a dye concentration of 0.8%, and a CCA concentration of 0.15%.

The prepared ink was encapsulated via complex coacervation with gelatin-polyphosphate.  The microcapsules were crosslinked with gluteraldeh...