Printing ink

1. A radiation-curable ink-jet ink comprising at least two monofunctional (meth)acrylate monomers; a monofunctional monomer selected from an N-vinyl amide, an N-acryloyl amine, or a mixture thereof; a polymerisable (meth)acrylate oligomer; a radical photoinitiator; and a colouring agent; wherein the ink has a viscosity of less than 100 mPas at 25° C.; wherein the ink is substantially free of water, volatile organic solvents and multifunctional monomers; wherein the polymerisable (meth)acrylate oligomer has a degree of functionality from 2 to 6; and wherein the at least two monofunctional (meth)acrylate monomers include a combination of a cyclic monofunctional (meth)acrylate monomer and an acyclic-hydrocarbon monofunctional (meth)acrylate monomer.

2. An ink-jet ink as claimed in claim 1 , wherein the total amount of the at least two monofunctional (meth)acrylate monomers and the at least one monofunctional monomer selected from an N-vinyl amide, an N-acryloyl amine, or a mixture thereof is at least 60 wt % based on the total weight of the ink.

3. An ink-jet ink as claimed in claim 2 , wherein the total amount is at least 70wt %.

4. An ink-jet ink as claimed in claim 3 , wherein the total amount is at least 80wt %.

5. An ink-jet ink as claimed in claim 1 wherein the total amount of the monomer selected from an N-vinyl amide, an N-acryloyl amine, or a mixture thereof is from 15 to 40 wt % based on the total weight of the ink.

6. An ink-jet ink as claimed in claim 1 , wherein the total amount of the cyclic monofunctional (meth)acrylate monomer is from 15 to 70 wt %, based on the total weight of the ink.

7. An ink-jet ink as claimed in claim 1 , wherein the total amount of the acyclic-hydrocarbon monofunctional (meth)acrylate monomer is from 3 to 25 wt %, based on the total weight of the ink.

8. An ink-jet ink as claimed in claim 1 , wherein the cyclic monofunctional (meth)acrylate monomer is selected from phenoxyethyl acrylate (PEA), cyclic TMP formal acrylate (CTFA), isobornyl acrylate (IBOA), tetrahydrofurfuryl acrylate (THFA), and mixtures thereof.

9. An ink-jet ink as claimed in claim 1 , wherein acyclic-hydrocarbon monofunctional (meth)acrylate monomer contains a linear or branched C 6-20 alkyl group.

10. An ink-jet ink as claimed in claim 9 , wherein acyclic-hydrocarbon monofunctional (meth)acrylate monomer is selected from octadecyl acrylate (ODA), tridecyl acrylate (TDA), isodecyl acrylate (IDA), lauryl acrylate and mixtures thereof.

11. An ink-jet ink as claimed in claim 1 , wherein the polymerisable (meth)acrylate oligomer has a molecular weight from 450 to 5,000.

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

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

14. A method of ink-jet printing, comprising printing the ink-jet ink as claimed in claim 2 on to a substrate and curing the ink.

15. A method of ink-jet printing, comprising printing the ink-jet ink as claimed in claim 3 on to a substrate and curing the ink.

16. A method of ink-jet printing, comprising printing the ink-jet ink as claimed in claim 4 on to a substrate and curing the ink.

17. A method of ink-jet printing, comprising printing the ink-jet ink as claimed in claim 5 on to a substrate and curing the ink.

18. A method of ink-jet printing, comprising printing the ink-jet ink as claimed in claim 6 on to a substrate and curing the ink.

19. A method of ink-jet printing, comprising printing the ink-jet ink as claimed in claim 7 on to a substrate and curing the ink.