Suspension for use in electrophoretic image display systems

A process for forming an electrophoretic display composition comprising the steps of:

• treating a first plurality of discrete organic pigment particles with a surface active agent, each of said first plurality of pigment particles defining a surface;
• providing a second plurality of said discrete organic pigment particles which has not been treated with a surfactant, each of said second plurality of pigment particles defining a surface;
• dispersing said first and second plurality of discrete organic pigment particles in a weight ratio of approximately 50:50 in a suspension medium of tetrachloroethylene, substantially each surface having at least one acidic hydrogen site and said first and second plurality of particles being selected from the group consisting of Pigment Yellow 14 and Pigment Yellow 55;
• providing a charge control agent having at least one basic anchoring group in said suspension medium;
• adsorbing said charge control agent on the surfaces of said first and second plurality of particles and reacting said acidic hydrogen sites with said basic anchoring groups, thereby forming charge pairs and polymer chains extending from each surface to provide a stearic barrier between the particles of both said first and second plurality of particles;
• dissolving a solvent dye in said suspension medium;
• and
• adding an effective amount of secondary-butyl benzene to said suspension medium to obtain a specific gravity of said suspension medium which is substantially the same as that of said first and second plurality of pigment particles.