Materials for use in electrophoretic displays

• US 7,679,814 B2
• Filed: 01/23/2007
• Issued: 03/16/2010
• Est. Priority Date: 04/02/2001
• Status: Expired due to Term

• Current Assignee: E Ink Holdings Incorporated
• Original Assignee: E Ink Holdings Incorporated
• Inventors: Webber, Richard M., Paolini, Richard J. Jr., Herb, Craig A., Harris, George G., Honeyman, Charles Howie, Whitesides, Thomas H.
• Primary Examiner(s): Ben; Loha
• Application Number:
  US11/625,907
• Publication Number:
  US 20070146310A1
• Time in Patent Office: 1,148 Days
• Field of Search: 359/296, 345/84, 345/85, 345/87, 345/95, 345/97, 345/107, 345/108, 264/4.1, 264/4.3, 264/4.33, 428/402.2, 428/402.21, 204/664, 204/665
• US Class Current: 359/296
• CPC Class Codes: E06B 2009/2464 featuring transparency control by applying voltage, e.g. LCD, electrochromic panels
• CPC Class Codes: E06B 9/24 Screens or other constructions affording protection against light, especially against sunshine; Similar screens for privacy or appearance; Slat blindsoperating, guiding or securing devices or arrangements for roll-type closures E06B9/56; free-hanging flexible screens A47H23/00
• CPC Class Codes: G02F 1/167 by electrophoresis
• CPC Class Codes: G02F 2001/1678 characterised by the composition or particle type
• CPC Class Codes: G02F 2203/11 involving infrared radiation
• CPC Class Codes: G09G 3/344 based on particles moving in a fluid or in a gas, e.g. electrophoretic devices electrophoretic devices per se G02F1/167

Abstract

The image stability of electrophoretic media, comprising a plurality of particles disposed in a fluid and capable of moving through the fluid upon application of an electric field to the medium, can be improved by including in the fluid either a polystyrene or an aggregating diblock copolymer which forms micelle-like structures in the fluid, the diblock copolymer having a first block soluble in the fluid and a second block not swellable by the fluid. In variable transmission electrophoretic media, haze can be reduced by using as the fluid a mixture of a partially hydrogenated aromatic hydrocarbon and a terpene.

Claims

1. An electrophoretic medium comprising a plurality of particles disposed in a fluid, the particles being capable of moving through the fluid upon application of an electric field to the medium, the fluid having dissolved or dispersed therein an aggregating polymer which forms micelle-like structures in the fluid, the aggregating polymer being essentially non-absorbing on the particles, the aggregating polymer comprising a diblock copolymer having a first block soluble in the fluid and a second block not swellable by the fluid.

2. An electrophoretic medium according to claim 1 wherein the diblock copolymer comprises a first polystyrene block and a second block of a polyalkene.

3. An electrophoretic medium according to claim 1 wherein the diblock copolymer comprises repeating units derived from styrene and isoprene.

4. An electrophoretic medium according to claim 1 wherein the diblock copolymer comprises from about 20 to about 40 weight per cent styrene.

5. An electrophoretic medium according to claim 1 wherein the block copolymer is present in an amount of from about 0.25 to about 2.5 per cent by weight of the fluid.

6. An electrophoretic medium according to claim 1 which is encapsulated, with the fluid and particles being retained within a plurality of capsules or microcells.

7. An electrophoretic medium according to claim 1 which is of a two-phase type, the fluid and particles forming a discontinuous phase comprising a plurality of droplets, the droplets being surrounded by a continuous phase.

8. An electrophoretic medium comprising a plurality of particles disposed in a fluid, the particles being capable of moving through the fluid upon application of an electric field to the medium, the fluid having dissolved or dispersed therein a polymer having an intrinsic viscosity of [η

] in the fluid and being substantially free from ionic or ionizable groups in the fluid, the polymer being present in the fluid in a concentration of from about 0.5 [η

]<sup>−

1</sup> to about 2.0 [η

]<sup>−

1</sup>, the polymer comprising an aggregating polymer which forms micelle-like structures in the fluid, the aggregating polymer being essentially non-absorbing on the particles, the aggregating polymer comprising a diblock copolymer having a first block soluble in the fluid and a second block not swellable by the fluid.

9. An electrophoretic medium according to claim 8 wherein the diblock copolymer comprises a first polystyrene block and a second block of a polyalkene.

10. An electrophoretic medium according to claim 8 wherein the diblock copolymer comprises repeating units derived from styrene and isoprene.

11. An electrophoretic medium according to claim 8 wherein the diblock copolymer comprises from about 20 to about 40 weight per cent styrene.

12. An electrophoretic medium comprising a plurality of particles disposed in a fluid, the particles being capable of moving through the fluid upon application of an electric field to the medium, the fluid comprising a partially hydrogenated aromatic hydrocarbon and a terpene.

13. An electrophoretic medium according to claim 12 wherein the fluid comprises a partially hydrogenated terphenyl.

14. An electrophoretic medium according to claim 12 wherein the fluid comprises d-limonene.

15. An electrophoretic medium according to claim 12 which is encapsulated, with the fluid and particles being retained within a plurality of capsules.

16. An electrophoretic medium according to claim 15 wherein the capsule walls are formed from a coacervate of gelatin and acacia, and wherein the difference in refractive index between the capsule walls and the fluid is not greater than about 0.005.

17. An electrophoretic medium according to claim 12 wherein the fluid and the particles are present as a plurality of discrete droplets in a continuous phase.

18. An electrophoretic medium according to claim 17 wherein the continuous phase comprises at least one of gelatin and a mixture of poly(vinyl alcohol) and poly(vinylpyrrolidone), and wherein the difference in refractive index between the continuous phase and the fluid is not greater than about 0.005.

19. An electrophoretic medium according to claim 12 wherein the fluid has dispersed therein a polystyrene having a weight average molecular weight in the range of about 10 kD to about 500 kD.

20. An electrophoretic medium according to claim 19 wherein the polystyrene has a weight average molecular weight in the range of about 12 kD to about 100 kD.

21. An electrophoretic display comprising an electrophoretic medium according to claim 12 and at least one electrode arranged to apply an electric field to the electrophoretic medium.

22. An electrophoretic display according to claim 21 further comprising voltage supply means arranged to supply voltages to the at least one electrode, the voltage supply means being arranged to drive the electrophoretic medium to a first optical state in which the medium is light transmissive and to a second optical state in which the medium is light absorbing.

23. An electrophoretic display according to claim 22 further comprising at least one sheet of light transmissive material disposed adjacent the electrophoretic medium, whereby the electrophoretic display forms a variable transmission window.

24. An electrophoretic medium comprising a plurality of particles disposed in a fluid, the particles being capable of moving through the fluid upon application of an electric field to the medium, the fluid having dissolved or dispersed therein a polystyrene having a weight average molecular weight in the range of about 10 kD to about 500 kD.

25. An electrophoretic medium according to claim 24 wherein the polystyrene has a weight average molecular weight in the range of about 12 kD to about 100 kD.

26. An electrophoretic medium according to claim 24 wherein the polystyrene has a weight average molecular weight in the range of about 15 kD to about 25 kD.

27. An electrophoretic medium according to claim 24 wherein the polystyrene is present in an amount of from about 1.0 to about 2.0 times its overlap concentration in the fluid.

28. An electrophoretic medium according to claim 24 which is encapsulated, with the fluid and particles being retained within a plurality of capsules or microcells.

29. An electrophoretic medium according to claim 24 wherein the fluid and the particles are present as a plurality of discrete droplets in a continuous phase.

30. An electrophoretic display comprising an electrophoretic medium according to claim 24 and at least one electrode arranged to apply an electric field to the electrophoretic medium.

31. An electrophoretic display according to claim 30 further comprising voltage supply means arranged to supply voltages to the at least one electrode, the voltage supply means being arranged to drive the electrophoretic medium to a first optical state in which the medium is light transmissive and to a second optical state in which the medium is light absorbing.

32. An electrophoretic medium comprising a plurality of particles disposed in a fluid, the particles being capable of moving through the fluid upon application of an electric field to the medium, the fluid having dissolved or dispersed therein a polymer having an intrinsic viscosity of [η

] in the fluid and being substantially free from ionic or ionizable groups in the fluid, the polymer being present in the fluid in a concentration of from about 0.5 [η

]<sup>−

1</sup> to about 2.0 [η

]<sup>−

1</sup>, the polymer comprising a polystyrene having a weight average molecular weight in the range of about 10 kD to about 500 kD.