Electrophoretic medium

Abstract

An electrophoretic medium comprises a plurality of charged particles disposed in a fluid. The fluid comprises at least about 75, and preferably at least about 95, percent by weight of a hydrocarbon selected from monounsaturated nonenes, nonane and methyloctane. The electrophoretic medium is especially useful in microcell electrophoretic media comprising a substrate having a plurality of cavities, and a sealing layer closing the open ends of the cavities, the cavities being filled with the electrophoretic medium.

Claims

1. An electrophoretic medium comprising at least two types of charged particles disposed in a fluid, wherein the fluid comprises at least 75 percent by weight of a hydrocarbon mixture comprising monounsaturated nonenes, n-nonane and methyloctane.

2. The electrophoretic medium of claim 1 wherein the fluid comprises at least 90 percent by weight of the hydrocarbon mixture.

3. The electrophoretic medium of claim 2 wherein the fluid comprises at least 95 percent by weight of the hydrocarbon mixture.

4. The electrophoretic medium of claim 1 wherein the monounsaturated nonenes are selected from a group consisting of methyloctene, dimethylheptene, non-2-ene, and mixtures thereof.

5. The electrophoretic medium of claim 1 comprising two types of charged particles bearing charges of opposite polarity.

6. The electrophoretic medium of claim 1 comprising more than two types of charged particles.

7. The electrophoretic medium of claim 6 comprising six types of charged particles having different colors.

8. A microcell electrophoretic medium comprising a substrate having a plurality of cavities formed therein, and a sealing layer closing the open ends of the cavities, the cavities being filled with the electrophoretic medium of claim 1.

9. The microcell electrophoretic medium of claim 8, wherein the width of the plurality of the cavities is greater than thea height of the plurality of the cavities.

10. An electrophoretic display comprising a layer of an electrophoretic medium and at least one light-transmissive electrode disposed adjacent the layer of electrophoretic medium and arranged to apply an electric field thereto, wherein the electrophoretic medium is the electrophoretic medium of claim 1.

11. The electrophoretic display of claim 10, wherein the electrophoretic display comprises two light-transmissive electrodes, and wherein the electrophoretic medium is disposed in between the two light-transmissive electrodes.

12. The electrophoretic display of claim 10 wherein the electrophoretic medium is confined within a plurality of capsules or microcells.

13. The electrophoretic display of claim 12 wherein the capsules are held within a polymeric binder to form a coherent layer.

14. The electrophoretic display of claim 10 wherein the electrophoretic medium is present as a plurality of discrete droplets surrounded by a continuous phase of a polymeric material.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) As already indicated, FIG. 1A of the accompanying drawings is a schematic cross-section through a filled prior art microcell medium. FIG. 1B illustrates a schematic of an electrophoretic device according to the present invention.

(2) FIG. 2 is a graph plotting the vapor pressure of various hydrocarbons at 87? C. against their viscosity at 20? C.

(3) FIG. 3 is a graph similar to FIG. 3 but plotting the vapor pressure of various commercial solvents at 20? C. against their viscosity at the same temperature.

DETAILED DESCRIPTION OF THE INVENTION

(4) As indicated above, the present invention provides an electrophoretic medium comprising a plurality of charged particles disposed in a fluid, wherein the fluid comprises at least about 75 percent by weight of a hydrocarbon selected from the group comprising monounsaturated nonenes, nonane and methyloctane. It has been found that this relatively narrow class of hydrocarbon fluids provide a nearly ideal combination of low conductivity, low viscosity, and low vapor pressure. These fluids were chosen to give unexpected improvements to both vapor pressure and viscosity over electrophoretic fluids described in the prior art, and are particularly good choices for fluids in microcell media produced by a fill and seal process.

(5) As already indicated, fluids for use in microcell electrophoretic displays need to possess low conductivity, low viscosity, and low vapor pressure. Low conductivity can be ensured using a hydrocarbon fluid. However, because vapor pressure and viscosity are inversely correlated for most liquids, selection of a suitable fluid always involves some trade-off between these properties. FIG. 2 of the accompanying drawings shows the potential trade-offs by plotting calculated values for viscosity at 20? C. against vapor pressure at 87? C. for some common hydrocarbon fluids. Note that alkenes show a better vapor pressure-viscosity relationship than other classes of hydrocarbons, including linear, branched (di-methyl), and cyclic hydrocarbons. Monomethyl substituted linear alkanes also show some advantage.

(6) FIG. 3 is a plot similar to that of FIG. 2 but comparing the vapor pressure of various commercial hydrocarbon solvents at 20? C. against their viscosity at the same temperature. Commonly used electrophoretic suspending fluids are Isopar G, Isopar E and Isane IP 140 (a mixture of branched C.sub.9 hydrocarbons). Isane IP 140 gives vapor pressure-viscosity behavior intermediate between Isopar E and Isopar G. Tripropylene has an even better viscosity-vapor pressure relationships than Isane IP 140, and is thus predicted to be a superior solvent for use in fill-and-seal microcell media; it has lower vapor pressure and lower viscosity than Isane IP 140.

(7) From FIGS. 2 and 3, it may be expected that the previously specified group of nonene solvents, including tripropylene, methyloctene, dimethylheptene, non-1-ene, and the like, to be good choices for electrophoretic fluids. Two other solvents are included as part of the present invention, namely: n-nonane and methyloctane.

(8) From the foregoing, it will be seen that the present invention can provide electrophoretic media with excellent switching speeds, because of the low viscosity fluid, and improved manufacturability, because of the low vapor pressure of the fluid. Although this invention has primarily been described in its application to microcell media, the advantageous combination of properties provided by the fluids used in the present invention are useful in other types of electrophoretic media.

(9) It will be apparent to those skilled in the art that numerous changes and modifications can be made in the specific embodiments of the invention described above without departing from the scope of the invention. Accordingly, the whole of the foregoing description is to be interpreted in an illustrative and not in a limitative sense.