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Abstract

The invention provides a working fluid for an electrophoretic image display device including a dispersion of finely divided diarylide yellow pigment particles each having a charge of a given polarity and acidic hydrogen surface sites in a suspension medium. Each of the pigment particles are transportable within the suspension medium, under the influence of an electric field, toward or away from at least one transparent plate. Adsorbed on the surface of each pigment particle is a thin layer of a charge control agent having a polyisobutylene chain and a basic anchoring group which reacts with the acidic surface sites to form charge pairs therewith. The polyisobutylene chains extend from the pigment surfaces on which the charge control agent is adsorbed, thereby providing a steric barrier between pigments and preventing agglomeration. The specific gravity of the suspension medium is matched with that of the pigment particles by combining effective amounts of secondary butyl benzene and tetrachloroethylene. A blue solvent dye is added to absorb substantially all light scattered by the pigments when the pigments are transported moved away from the transparent plate being viewed.

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First Claim