Polymer additives used in color electrophoretic display medium

Abstract

An electrophoretic medium may be incorporated into an electrophoretic display that includes a dispersion containing a non-polar fluid, a plurality of first charged colored particles, polyisobutylene, and an additive selected from co-polymers of vinyl aromatics and at least one hydrocarbon having 2 to 5 carbons and at least one double bond. The co-polymer and the ratio of the co-polymer to polyisobutylene is selected, such that bistability performance is maintained while improving the color state performance of the display.

We claim: 
     
         1 . An electrophoretic medium comprises:
 (a) a non-polar fluid;   (b) a plurality of first charged particles;   (c) polyisobutylene; and   (d) an additive selected from co-polymers of a vinyl aromatic and at least one substituted or unsubstituted hydrocarbon having 2 to 5 carbons and at least one double bond, wherein the weight percent of styrene in the copolymer is 20% to 40%.   
     
     
         2 . The electrophoretic medium of  claim 1 , wherein the vinyl aromatic is selected from the group consisting of styrene, alpha-methyl styrene, 2-methyl styrene, 3-methyl styrene, 4-methyl styrene, 44-butyl styrene, 4-cyclohexyl styrene, 4-dodecyl styrene, 2,4-diisopropyl styrene, 2,4,6-trimethyl styrene, 2-ethyl-4-benzyl styrene, 4-(phenyl butyl)styrene, 1-vinyl naphthalene, 2-vinyl naphthalene, vinyl anthracene, 4-methoxy styrene, monochlorostyrene, dichlorostyrene, divinyl benzene, indene, methyl indene, and combinations thereof. 
     
     
         3 . The electrophoretic medium of  claim 1 , wherein the vinyl aromatic comprises styrene. 
     
     
         4 . The electrophoretic medium of  claim 1 , wherein the at least one substituted or unsubstituted hydrocarbon is selected from the group consisting of ethylene, propylene, isoprene, butylene, butadiene, and combinations thereof. 
     
     
         5 . The electrophoretic medium of  claim 1 , wherein the additive is a co-polymer of styrene, ethylene, and propylene. 
     
     
         6 . The electrophoretic medium of  claim 1 , wherein the weight ratio of polyisobutylene to additive in the dispersion is 1:5 to 5:1. 
     
     
         7 . The electrophoretic medium of  claim 1 , wherein the first charged particles are red, green, blue, cyan, yellow, or magenta. 
     
     
         8 . The electrophoretic medium of  claim 7  further comprising a plurality of second charged particles dispersed in the non-polar fluid. 
     
     
         9 . The electrophoretic medium of  claim 8 , wherein the second charged particles are white or black. 
     
     
         10 . The electrophoretic medium of  claim 9 , wherein the second charged particles are white and the first and second charged particles have opposite charge polarities. 
     
     
         11 . The electrophoretic medium of  claim 10 , wherein the second charged particles comprise titania. 
     
     
         12 . The electrophoretic medium of  claim 10  further comprising a plurality of third charged particles dispersed in the non-polar fluid. 
     
     
         13 . The electrophoretic medium of  claim 12 , wherein the third charged particles are black and the first and third charged particles have the same charge polarity. 
     
     
         14 . The electrophoretic medium of  claim 13 , wherein the third charged particles comprise, carbon black or copper chromite. 
     
     
         15 . The electrophoretic medium of  claim 1 , further comprising an ionic surfactant. 
     
     
         16 . The electrophoretic medium of  claim 1 , wherein the combined weight percent of polyisobutylene and additive is 0.1 to 2.5% based on the total weight of the dispersion. 
     
     
         17 . The electrophoretic medium of  claim 1 , wherein the combined weight percent of polyisobutylene and additive is 0.5 to 1% based on the total weight of the dispersion. 
     
     
         18 . An electrophoretic display comprising an electrophoretic medium according to  claim 1 .