Printing ink

1. A colourless inkjet ink composition comprising at least 30% by weight of monofunctional acrylate monomer based on the total weight of the ink and at least 3% by weight of a photoinitiator based on the total weight of the ink, wherein the photoinitiator comprises an a-hydroxy ketone photoinitiator and wherein the ink comprises 0 to 6% by weight of acyl phosphine photoinitiator based on the total weight of the ink, wherein the colourless inkjet ink comprises 3 to 40% by weight of N-vinyl amide and/or N-acryloyl amine based on the total weight of the ink, and wherein the a-hydroxy ketone photoinitiator is present in an amount of 25 to 100% by weight based on the total weight of photoinitiator, wherein the monofunctional acrylate monomer is selected from the group consisting of phenoxyethyl acrylate, cyclic TMP formal acrylate, isobornyl acrylate, tetrahydrofurfuryl acrylate, dicyclopentenyl oxyethyl acrylate and mixtures thereof, and wherein the n-acyloyl amine is n-acryloylmorpholine.

2. The colourless inkjet ink of claim 1 comprising one or more other radiation curable monomers selected from multifunctional (meth)acrylate monomers, N-vinyl amides, N-(meth)acryloyl amines and unsaturated ether monomers.

3. The colourless inkjet ink of claim 1 comprising 15 to 50% by weight of multifunctional (meth)acrylate monomer, based on the total weight of the ink.

4. The colourless inkjet ink of claim 1 wherein the monofunctional acrylate monomer comprises cyclic TMP formal acrylate.

5. The colourless inkjet ink of claim 1 comprising one or more monomers that includes a C6 to C14 linear alkyl group.

6. The colourless inkjet ink of claim 1 comprising 3-methylpentanediol diacrylate.

7. The colourless inkjet ink of claim 1 wherein the monofunctional acrylate monomer includes 0 to 75% by weight of monofunctional acrylate monomer that includes a C6 to C14 linear alkyl group and 25 to 100% by weight of a monofunctional acrylate monomer that includes an aliphatic or aromatic cyclic group, based on the total weight of monofunctional acrylate monomer present in the ink.

8. The colourless inkjet ink of claim 1 further comprising an oligomer or inert resin.

9. The colourless inkjet ink of claim 1 comprising at least one monofunctional acrylate monomer; a functional oligomer; and, optionally, an N-vinyl amide monomer or N-acryloyl amine monomer.

10. The colourless inkjet ink of claim 1 comprising at least one monofunctional acrylate monomer; a functional oligomer; a multifunctional (meth)acrylate monomer; and, optionally, an N-vinyl amide monomer.

11. The colourless inkjet ink of claim 1 that is substantially free of water and volatile organic solvents.

12. The colourless inkjet ink of claim 1 comprising one or more additional free radical photo initiators.

13. The colourless inkjet ink of claim 1 comprising 3 to 20% by weight of photoinitiator, based on the total weight of the ink.

14. A method of inkjet printing comprising printing the colourless inkjet ink as claimed in claim 1 on to a substrate and exposing the ink to actinic radiation.

15. An inkjet cartridge comprising the ink as claimed in claim 1 .

16. A substrate having the ink as claimed in claim 1 .

17. The colorless inkjet ink of claim 1 comprising 20 to 40% by weight of multifunctional (meth)acrylate monomer, based on the total weight of the ink.

18. The colorless inkjet ink of claim 1 comprising 3 to 15% by weight of photoinitiator, based on the total weight of the ink.

19. The colourless inkjet ink of claim 1 wherein the a-hydroxy ketone photoinitiator is present in an amount of 50 to 100% by weight based on the total weight of photoinitiator.