Liquid electrophotographic varnish composition

1. A liquid electrophotographic varnish composition comprising:

a carrier liquid;

a polymer resin suspended as particles in the carrier liquid;

an epoxy-based cross-linking agent dispersed or dissolved directly in the carrier liquid; and

a metal catalyst and/or a photo-initiator for catalysing cross-linking of the polymer resin;

wherein the liquid electrophotographic varnish composition is substantially colorless.

2. The liquid electrophotographic varnish composition according to claim 1 , wherein the epoxy-based cross-linking agent is present in an amount of less than 10 wt %.

3. The liquid electrophotographic varnish composition according to claim 1 , wherein the epoxy-based cross-linking agent is present in an amount of 6 wt % or less.

4. The liquid electrophotographic varnish composition according to claim 1 , wherein the epoxy-based cross-linking agent has a molecular weight of 5000 Daltons or less.

5. A liquid electrophotographic varnish composition, comprising:

a polymer resin;

an epoxy-based cross-linking agent;

a metal catalyst and/or a photo-initiator for catalysing cross-linking of the polymer resin; and

a carrier liquid;

wherein the epoxy-based cross-linking agent is of formula (I):

(X)—(Y—[Z—F] m ) n   formula (I)

wherein, in each (Y—[Z—F] m ) n , Y, Z and F are each independently selected, such that:

F is an epoxide of the formula CH(O)CR 1 H, wherein R 1 is selected from H and alkyl;

Z is alkylene;

Y is selected from (i) a single bond, —O—, —C(═O)—O—, or —O—C(═O)— and m is 1 or (ii) Y is NH 2-m and m is 1 or 2;

n is at least 1;

and X is an organic group.

6. A liquid electrophotographic varnish composition, comprising:

a polymer resin;

an epoxy-based cross-linking agent;

a metal catalyst and/or a photo-initiator for catalysing cross-linking of the polymer resin; and

a carrier liquid;

wherein the epoxy-based cross-linking agent is selected from the group consisting of 1,2,7,8-diepoxy octane, trimethylolpropane triglycidyl ether, resorcinol diglycidyl ether, N,N-diglycidyl-4-glycidyloxyaniline, 4,4′-Methylenebis(N,N-diglycidylaniline), tris(4-hydroxyphenyl)methane triglycidyl ether, diglycidyl 1,2-cyclohexanedicarboxylate, 1,4-cyclohexanedimethanol diglycidyl ether, tris(2,3-epoxypropyl) isocyanurate, neopentyl glycol diglycidyl ether, bisphenol A diglycidyl ether, bisphenol A propoxylate diglycidyl ether, 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate, poly[(o-cresyl glycidyl ether)-co-formaldehyde], poly(ethylene-co-glycidyl methacrylate), poly(ethylene-co-methyl acrylate-co-glycidyl methacrylate), poly(bisphenol A-co-epichlorohydrin) glycidyl end-capped, poly(ethylene glycol) diglycidyl ether, and poly(propylene glycol) diglycidyl ether.

7. The liquid electrophotographic varnish composition according to claim 1 , wherein the liquid electrophotographic varnish composition includes the metal catalyst, and the metal catalyst comprises a chromium (III) complex or a zinc complex.

8. The liquid electrophotographic varnish composition according to claim 1 , wherein the metal catalyst is present in an amount of less than 2 wt %.

9. The liquid electrophotographic varnish composition according to claim 1 , wherein the liquid electrophotographic varnish composition includes the photo-initiator, and the photo-initiator comprises a cationic photo-initiator.

10. The liquid electrophotographic varnish composition according to claim 1 , wherein the photo-initiator is present in an amount of less than 5 wt %.

11. The liquid electrophotographic varnish composition according to claim 1 , wherein the polymer resin comprises a polymer having acidic side groups.

12. The liquid electrophotographic varnish composition according to claim 1 , wherein the polymer resin comprises a polymer selected from the group consisting of (i) ethylene or propylene acrylic acid co-polymers and (ii) ethylene or propylene methacrylic acid co-polymers.

13. A method of manufacturing a liquid electrophotographic varnish composition, comprising:

mixing a carrier liquid and a polymer resin to form varnish solids;

mixing the varnish solids with additional carrier liquid and a charge director to form a working dispersion;

dispersing or dissolving an epoxy-based cross-linking agent in the working dispersion; and

dispersing, in the working dispersion, a metal catalyst and/or a photo-initiator for catalysing cross-linking of the polymer resin, to form the liquid electrophotographic composition, wherein the liquid electrophotographic varnish composition is substantially colorless.

14. A method of electrophotographic printing, comprising printing the liquid electrophotographic varnish composition of claim 1 onto a substrate using a liquid electrophotographic printer.

15. A print substrate, having printed thereon the liquid electrophotographic varnish composition of claim 5 .

16. The liquid electrophotographic varnish composition according to claim 1 wherein the epoxy-based cross-linking agent is selected from the group consisting of 1,2,7,8-diepoxy octane, trimethylolpropane triglycidyl ether, resorcinol diglycidyl ether, N,N-diglycidyl-4-glycidyloxyaniline, 4,4′-Methylenebis(N,N-diglycidylaniline), tris(4-hydroxyphenyl)methane triglycidyl ether, diglycidyl 1,2-cyclohexanedicarboxylate, 1,4-cyclohexanedimethanol diglycidyl ether, tris(2,3-epoxypropyl) isocyanurate, neopentyl glycol diglycidyl ether, bisphenol A diglycidyl ether, bisphenol A propoxylate diglycidyl ether, 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate, poly(phenyl glycidyl ether)-co-formaldehyde, poly[(o-cresyl glycidyl ether)-co-formaldehyde], poly(ethylene-co-glycidyl methacrylate), poly(ethylene-co-methyl acrylate-co-glycidyl methacrylate), poly(dimethyl siloxane)diglycidyl ether terminated), poly(bisphenol A-co-epichlorohydrin) glycidyl end-capped, poly(ethylene glycol) diglycidyl ether, poly(propylene glycol) diglycidyl ether, and poly[dimethylsiloxane-co-(2-(3,4-epoxycyclohexyl)ethyl)methylsiloxane].

17. The method according to claim 15 wherein:

the epoxy-based cross-linking agent is selected from the group consisting of 3,4-Epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate, neopentyl glycol diglycidyl ether, 1,2,7,8-diepoxyoctane, resorcinol diglycidyl ether, trimethylolpropane triglycidyl ether, tris(4-hydroxyphenyl)methane triglycidyl ether, diglycidyl 1,2-cyclohexanedicarboxylate, 1,4-cyclohexanedimethanol diglycidyl ether (mixture of cis- and trans-), tris(2,3-epoxypropyl) isocyanurate, and bisphenol A propoxylate diglycidyl ether; and

dispersing the epoxy-based cross-linking agent in the working dispersion involves adding the epoxy-based cross-linking agent to the working dispersion and mixing.

18. The method according to claim 15 wherein:

the epoxy-based cross-linking agent is selected from the group consisting of 4,4′-methylenebis(N,N-diglycidylaniline), N,N-Diglycidyl-4-glycidyloxyaniline, and bisphenol A diglycidyl ether; and

dispersing the epoxy-based cross-linking agent in the working dispersion involves:

dispersing the epoxy-based cross-linking agent in a solution including a polymeric dispersant; and

then adding the solution to the working dispersion and mixing.

19. The method according to claim 15 wherein:

the epoxy-based cross-linking agent is selected from the group consisting of poly(ethylene-co-methyl acrylate-co-glycidyl methacrylate), poly(dimethyl siloxane) diglycidyl ether terminated, poly(ethylene glycol) diglycidyl ether, poly(propylene glycol) diglycidyl ether, and poly[dimethylsiloxane-co-(2-(3,4-epoxycyclohexyl)ethyl)methylsiloxane]; and

dispersing the epoxy-based cross-linking agent in the working dispersion involves adding the epoxy-based cross-linking agent to the working dispersion and mixing.