Liquid electrophotographic ink composition
A method for coating pigment particles is provided, the method comprising heating a polymer resin in a carrier fluid to dissolve the polymer resin, suspending in the carrier fluid the pigment particles to be coated; and effecting precipitation of the polymer resin from the carrier fluid, such that a coating of the resin is formed on the pigment particles.
1. A method of producing a white liquid electrophotographic ink composition, the method comprising:
heating a polymer resin in a carrier fluid to dissolve the polymer resin;
suspending in the carrier fluid white pigment particles to be coated; and
controlling cooling of the carrier fluid to effect precipitation of the polymer resin from the carrier fluid such that a coating of the resin is formed on the pigment particles, thereby producing the white liquid electrophotographic ink composition, wherein the precipitation is effected by controlled addition of extra carrier fluid at a rate of less than 10 cm 3 /min.
2. The method according to claim 1 , wherein the carrier fluid is or comprises an isoparaffinic carrier fluid.
3. The method according to claim 1 , wherein the carrier fluid is heated to at least 100° C. to dissolve the polymer resin.
4. The method according to claim 1 , further comprising grinding the polymer resin coated white pigment particles.
5. The method according to claim 4 wherein:
the white liquid electrophotographic ink composition has 18% non-volatile solids content; and
the grinding is accomplished at 40° C. and 250 rpm for up to 3 hours.
6. The method according to claim 5 wherein the grinding is accomplished for 45 minutes.
7. The method according to claim 5 wherein the polymer resin coated white pigment particles have a median particle size (d 50 ) of less than 20 μm after the grinding.
8. The method according to claim 1 , further comprising adding a charge director:
to the carrier liquid before precipitation of the polymer resin; or
to the carrier liquid during precipitation of the polymer resin; or
to the carrier liquid after precipitation of the polymer resin.
9. The method according to claim 1 wherein the white pigment particle comprises surface modifying groups to increase hydrophobicity.
10. The method according to claim 1 wherein the white pigment particles are selected from the group consisting of TiO 2 , calcium carbonate, zinc oxide, and mixtures thereof.
11. A method of producing a white liquid electrophotographic ink composition, the method comprising:
heating a polymer resin in a carrier fluid to dissolve the polymer resin;
suspending in the carrier fluid white pigment particles to be coated; and
controlling cooling of the carrier fluid to effect precipitation of the polymer resin from the carrier fluid such that a coating of the resin is formed on the pigment particles, thereby producing the white liquid electrophotographic ink composition, wherein precipitation is effected by cooling the carrier fluid containing dissolved polymer resin and suspended pigment particles at a rate of less than 5° C. per hour.
12. The method according to claim 11 , wherein the precipitation is effected by cooling the carrier fluid containing dissolved polymer resin and suspended pigment particles at a rate of less than 3° C. per hour.