Printing ink

1. An ink-jet ink substantially free of water and volatile organic solvents comprising at least one monofunctional (meth)acrylate monomer; at least one monofunctional monomer selected from the group consisting of an N-vinyl amide, an N-acryloyl amine and mixtures thereof; at least one radical photoinitiator; and at least one colouring agent; wherein the ink has a viscosity of less than 100 mPas at 25° C., and contains no more than 15 wt % of multifunctional monomers based on the total weight of the ink; and wherein the total amount of the at least one monofunctional (meth)acrylate monomer and the at least one monomer selected from the group consisting of an N-vinyl amide, an N-acryloyl amine and mixtures thereof in combination is at least 60 wt % based on the total weight of the ink, and the molar ratio of the at least one monofunctional (meth)acrylate monomer to the at least one monomer selected from the group consisting of an N-vinyl amide monomer, an N-acryloyl amine and mixtures thereof is from 1.1 to 3.5, and further wherein said ink-jet ink has a higher cure speed than a cure speed of an ink-jet ink which comprises only one of the at least one monofunctional (meth)acrylate monomer and the at least one monofunctional N-vinyl amide and/or N-acryloyl amine monomer.

2. An ink-jet ink as claimed in claim 1 wherein the total amount of the at least one monofunctional (meth)acrylate monomer and the at least one one monomer selected from the group consisting of an N-vinyl amide, an N-acryloyl amine and mixtures thereof in combination is at least 70 wt %.

3. An ink-jet ink as claimed in claim 1 , wherein the ink contains at least one multifunctional monomer and one or more of the multifunctional monomers a multifunctional (meth)acrylate monomer.

4. An ink-jet ink as claimed in claim 1 , wherein the at least one monofunctional (meth)acrylate is selected from phenoxyethyl acrylate (PEA), cyclic TMP formal acrylate (CTFA), isobornyl acrylate (IBOA), tetrahydrofurfuryl acrylate (THFA), 2-(2-ethoxyethoxy)ethyl acrylate, octaldecyl acrylate (ODA), tridecyl acrylate (TDA), isodecyl acrylate (IDA) and lauryl acrylate.

5. An ink-jet ink as claimed in claim 1 , wherein the at least one monofunctional (meth)acrylate is a cyclic monofunctional (meth)acrylate.

6. An ink-jet ink as claimed in claim 1 , wherein the at least one monomer selected from the group consisting of an N-vinyl amide, an N-acryloyl amine and mixtures thereof is selected from the group consisting of N-vinyl caprolactam (NVC), N-vinyl pyrrolidone (NVP) and N-acryloyl morpholine (ACMO).

7. An ink-jet ink as claimed in claim 1 , wherein the at least one monomer selected from the group consisting of an N-vinyl amide, an N-acryloyl amine and mixtures thereof is N-vinyl caprolactam (NVC).

8. An ink-jet ink as claimed in claim 2 , wherein the total amount of the at least one monofunctional (meth)acrylate monomer and the at least one one monomer selected from the group consisting of an N-vinyl amide, an N-acryloyl amine and mixtures thereof in combination is at least 80 wt %.

9. An ink-jet ink as claimed in claim 4 , wherein the ink contains a combination of monomers selected from THFA (tetrahydrofurfuryl acrylate)/NVC (N-vinyl caprolactam), IBOA (isobornyl acrylate)/NVC (N-vinyl caprolactam), PEA(phenoxyethyl acrylate)/NVC(N-vinyl caprolactam), CTFA (cyclic TMP formal acrylate)/NVC(N-vinyl caprolactam), IBOA(isobornyl acrylate)/ACMO (N-acryloylmorpholine) and IBOA(isobornyl acrylate)/NVP(N-vinyl pyrrolidone).

10. An ink-jet ink as claimed in claim 5 , wherein the ink comprises at least one cyclic monofunctional (meth)acrylate as the sole monofunctional (meth)acrylate(s) present.

11. An ink-jet ink as claimed in claim 5 , wherein the ink contains a combination of monomers selected from THFA (tetrahydrofurfuryl acrylate) INVC (N-vinyl caprolactam), IBOA (isobornyl acrylate)/NVC (N-vinyl caprolactam), PEA(phenoxyethyl acrylate)/NVC(N-vinyl caprolactam), CTFA (cyclic TMP formal acrylate)/NVC (N-vinyl caprolactam), IBOA(isobornyl acrylate)/ACMO (N-acryloylmorpholine) and IBOA(isobornyl acrylate)/NVP(N-vinyl pyrrolidone).

12. An ink-jet ink as claimed in claim 5 , wherein the cyclic monofunctional (meth)acrylate is phenoxyethyl acrylate (PEA), cyclic TMP formal acrylate (CTFA), isobornyl acrylate (IBOA), tetrahydrofurfuryl acrylate (THFA), or mixtures thereof.

13. An ink-jet ink as claimed in claim 10 , wherein the cyclic monofunctional (meth)acrylate is phenoxyethyl acrylate (PEA), cyclic TMP formal acrylate (CTFA), isobornyl acrylate (IBOA), tetrahydrofurfuryl acrylate (THFA), or mixtures thereof.

14. An ink-jet ink as claimed in claim 10 , wherein the ink contains a combination of monomers selected from THFA (tetrahydrofurfuryl acrylate)/NVC (N-vinyl caprolactam), IBOA (isobornyl acrylate)/NVC (N-vinyl caprolactam), PEA(phenoxyethyl acrylate)/NVC(N-vinyl caprolactam), CTFA (cyclic TMP formal acrylate)/NVC(N-vinyl caprolactam), IBOA(isobornyl acrylate)/ACMO (N-acryloylmorpholine) and IBOA(isobornyl acrylate)/NVP(N-vinyl pyrrolidone).

15. An ink-jet ink as claimed in claim 12 , wherein the ink contains a combination of monomers selected from THFA (tetrahydrofurfuryl acrylate)/NVC (N-vinyl caprolactam), IBOA (isobornyl acrylate)/NVC (N-vinyl caprolactam), PEA(phenoxyethyl acrylate)/NVC(N-vinyl caprolactam), CTFA (cyclic TMP formal acrylate)/NVC(N-vinyl caprolactam), IBOA(isobornyl acrylate)/ACMO (N-acryloylmorpholine) and IBOA(isobornyl acrylate)/NVP(N-vinyl pyrrolidone).

16. A method of ink-jet printing, comprising printing the ink-jet ink as claimed in claim 1 on to a substrate and curing the ink.

17. A substrate having the ink-jet ink as claimed in claim 1 printed thereon.

18. An ink-jet ink cartridge containing the ink-jet ink as claimed in claim 1 .