Low migration free radical radiation curable inkjet inks

1. A free radical radiation curable inkjet ink for food packaging comprising:

a polymeric or polymerizable thioxanthone photoinitiator;

an acylphosphine oxide-based polymerization photoinitiator;

a thiol compound; and

a vitrification control monomer; wherein

the vitrification control monomer includes a (meth)acrylate group and an ethylenically unsaturated functional group selected from the group consisting of a vinyl ether group, an allyl ether group, and an allyl ester group;

the thiol compound includes two or three thiol groups;

the free radical radiation curable inkjet ink has a viscosity smaller than 30 mPa·s at 40° C. and at a shear rate of 1,000 s −1 ; and

the free radical radiation curable inkjet ink does not include an organic acid.

2. The free radical radiation curable inkjet ink according to claim 1 , wherein the thiol compound is represented by Formula (I):

wherein

n represents an integer from 1 to 4;

m represents an integer from 1 to 6; and

R represents an m-valent linking group including at most 20 carbon atoms.

3. The free radical radiation curable inkjet ink according to claim 1 , wherein the vitrification control monomer is represented by a compound according to Formula (II):

wherein

R 1 represents hydrogen, or a substituted or unsubstituted alkyl group;

L represents a linking group including at least one carbon atom; and

n and m independently represent a value from 1 to 5.

4. The free radical radiation curable inkjet ink according to claim 2 , wherein the vitrification control monomer is represented by a compound according to Formula (II):

wherein

R 1 represents hydrogen, or a substituted or unsubstituted alkyl group;

L represents a linking group including at least one carbon atom; and

n and m independently represent a value from 1 to 5.

5. The free radical radiation curable inkjet ink according to claim 1 , wherein the vitrification control monomer represents at least 50% by weight of a total monomer composition in the free radical radiation curable inkjet ink.

6. The free radical radiation curable inkjet ink according to claim 1 , wherein the thiol compound has a molecular weight smaller than 1,000 Dalton.

7. The free radical radiation curable inkjet ink according to claim 4 , further comprising a colorant.

8. A free radical radiation curable ink jet ink set comprising at least one free radical radiation curable inkjet ink as defined by claim 1 .

9. A substrate comprising a cured layer of the free radical radiation curable inkjet ink as defined by claim 1 .

10. The substrate according to claim 9 , wherein the substrate is a food packaging material.

11. A method of inkjet printing on a food packaging material, the method comprising the steps of:

applying on a substrate a free radical radiation curable inkjet ink for food packaging including a polymeric or polymerizable thioxanthone photoinitiator, an acylphosphine oxide-based polymerization photoinitiator, a thiol compound, and a vitrification control monomer; and

at least partially curing the free radical radiation curable inkjet ink; wherein

the vitrification control monomer includes a (meth)acrylate group and an ethylenically unsaturated functional group selected from the group consisting of a vinyl ether group, an allyl ether group, and an allyl ester group;

the thiol compound includes two or three thiol groups;

the free radical radiation curable inkjet ink has a viscosity smaller than 30 mPa·s at 40° C. and at a shear rate of 1,000 s −1 ; and

the free radical radiation curable inkjet ink does not include an organic acid.

12. The method of inkjet printing according to claim 11 , wherein the free radical radiation curable inkjet ink is applied on a surface of the substrate, and the surface includes a polymer selected from the group consisting of polyethylene, polypropylene, polycarbonate, polyvinyl chloride, polyethylene terephthalate, polyethylene naphthalate, polylactide, and polyimide.

13. The method of inkjet printing according to claim 11 , wherein the step of applying the free radical radiation curable inkjet ink on the substrate includes jetting the free radical radiation curable inkjet ink on the substrate.

14. The method of inkjet printing according to claim 11 , wherein the substrate is the food packaging material.

15. The free radical radiation curable inkjet ink according to claim 3 , wherein the substituted or unsubstituted alkyl group is a methyl group.

16. The free radical radiation curable inkjet ink according to claim 4 , wherein the substituted or unsubstituted alkyl group is a methyl group.