Electrophoretic particles, and processes for the production thereof

1. An electrophoretic medium comprising at least one electrically charged particle disposed in a fluid and capable of moving through the fluid on application of an electrical field to the fluid, the at least one electrically charged particle comprising an inorganic black pigment having a surface area of at least about 7 m 2 /g.

2. An electrophoretic medium according to claim 1 wherein the inorganic black pigment has a surface area of at least about 10 m 2 /g.

3. An electrophoretic medium according to claim 2 wherein the inorganic black pigment has a surface area of at least about 15 m 2 /g.

4. An electrophoretic medium according to claim 1 wherein the inorganic black pigment comprises magnetite or a mixed metal oxide containing two or more of iron, chromium, nickel, manganese, copper and cobalt.

5. An electrophoretic medium according to claim 4 wherein the inorganic black pigment comprises a copper iron manganese oxide spinel.

6. An electrophoretic medium according to claim 4 wherein the inorganic black pigment comprises a copper chromium manganese oxide spinel.

7. An electrophoretic medium according to claim 1 wherein the inorganic black pigment has a polymer chemically bonded to, or cross-linked around the pigment particles.

8. An electrophoretic medium according to claim 7 wherein the polymer comprises from about 1 to about 15 per cent by weight of the pigment particles.

9. An electrophoretic medium according to claim 7 wherein the polymer comprises a main chain and a plurality of side chains extending from the main chain, each of the side chains comprising at least about four carbon atoms.

10. An electrophoretic medium according to claim 1 further comprising at least one second particle having at least one optical characteristic differing from that of the inorganic black pigment particles, the second particles also having an electrophoretic mobility differing from that of the inorganic black pigment particles.

11. An electrophoretic medium according to claim 10 wherein the two sets of particles bear charges of opposite polarity.

12. An electrophoretic medium according to claim 1 comprising a capsule wall within which the fluid and the inorganic black pigment particles are retained.

13. An electrophoretic medium according to claim 12 further comprising a polymer binder surrounding the capsules.

14. An electrophoretic display comprising an electrophoretic medium according to claim 1 and at least one electrode disposed adjacent the electrophoretic medium for applying an electric field to the electrophoretic medium.

15. An electronic book reader, portable computer, tablet computer, cellular telephone, smart card, sign, watch, shelf label or flash drive comprising a display according to claim 14 .

16. A process for producing a polymer-coated pigment particle; this process comprises:

(a) reacting an inorganic black pigment particle having a surface area of at least about 7 m 2 /g with a reagent having a functional group capable of reacting with, and bonding to, the particle, and also having a polymerizable or polymerization-initiating group, thereby causing the functional group to react with the particle surface and attach the polymerizable group thereto; and

(b) reacting the product of step (a) with at least one monomer or oligomer under conditions effective to cause reaction between the polymerizable or polymerization-initiating group on the particle and the at least one monomer or oligomer, thereby causing the formation of polymer bonded to the particle.

17. A process according to claim 16 wherein the pigment particles is coated with silica prior to step (a).

18. A process according to claim 16 wherein the reagent used in step (a) comprises a silane coupling group.

19. A process according to claim 18 wherein the reagent further comprises an amino group.

20. A process according to claim 16 wherein the monomer or oligomer comprises at least one acrylate or methacrylate.