Ink composition, ink jet recording method, method for producing planographic printing plate, and planographic printing plate

1. An ink composition comprising (i) a polymerization initiator, (ii) a (meth)acrylate which includes a (meth)acryloyl group and an additional double bond having, at an α position, a carbon atom having an sp 3 hybrid orbital, (iii) an N-vinyllactam as another polymerizable compound other than the (meth)acrylate having a double bond with a carbon atom having an sp 3 hybrid orbital at an α position, and (iv) a colorant,

wherein the (meth)acrylate is represented by following formula (I):

wherein R 1 to R 4 each independently represents a hydrogen atom or an alkyl group, all of R 1 to R 4 are not hydrogen atoms, and at least one alkyl group represented by R 1 to R 4 is bonded to an acrylate group represented by formula (II), or a methacrylate group represented by formula (III), directly or via a bivalent linking group:

wherein in the formula (II) or (III), * represents a binding site for the alkyl group represented by any one of R 1 to R 4 , or for a linkage bound to the alkyl group,

wherein in the formula (I), two or more alkyl groups represented by R 1 to R 4 are not bonded to form a ring structure, the alkyl group represented by R 1 , R 2 , R 3 , or R 4 has two or more carbon atoms, and the other three of R 1 to R 4 are selected from the group consisting of a hydrogen atom and a methyl group, and wherein the bivalent linking group in formula (I) is an alkylene group, or an alkylene group wherein a methylene group in the alkylene group is replaced with a divalent group selected from —O—, —S—, or —NR 5 — wherein R 5 represents an alkyl group, and wherein the content of the colorant is from 1 to 10% by mass with respect to the total content of the ink composition.

2. The ink composition of claim 1 , wherein, in the (meth)acrylate, the number of the double bonds with carbon atoms having sp 3 hybrid orbitals at a positions is 1 to 8.

3. The ink composition of claim 1 , wherein the content of the (meth)acrylate having a double bond with a carbon atom having an sp 3 hybrid orbital at an α position is in the range of 3 to 45% by mass with respect to the total weight of the ink composition.

4. The ink composition of claim 1 , wherein the colorant is a pigment.

5. The ink composition of claim 1 , wherein the colorant is a dye.

6. The ink composition of claim 1 , wherein the N-vinyllactam is N-vinylcaprolactam.

7. The ink composition of claim 1 , wherein the polymerization initiator is a radical polymerization initiator.

8. The ink composition of claim 7 , wherein the radical polymerization initiator comprises an acylphosphine compound.

9. The ink composition of claim 8 , wherein the acylphosphine compound comprises diphenyl (2,4,6-trimethoxybenzoyl)phosphine oxide.

10. The ink composition of claim 1 , wherein the (meth)acrylate is selected from the group consisting of compounds (B-1) to (B-20):

11. The ink composition of claim 1 , further comprising a sensitizing dye.

12. The ink composition of claim 11 , wherein the sensitizing dye comprises a polynuclear aromatic compound, thioxanthone, xanthene, cyanine, merocyanine, thiazine, acridine, anthraquinone, squalium, or coumarin.

13. The ink composition of claim 12 , wherein the thioxanthone comprises isopropylthioxanthone.

14. An ink jet recording method comprising:

ejecting the ink composition of claim 1 onto a recording medium; and

irradiating the ejected ink composition with an active radiation ray to cure the ink composition.

15. A method for producing a planographic printing plate comprising:

ejecting the ink composition of claim 1 onto a hydrophilic support; and

irradiating the ejected ink composition with an active radiation ray to cure the ink composition to faun a hydrophobic region of the cured ink composition on the hydrophilic support.

16. A planographic printing plate having a hydrophobic region obtained by curing the ink composition of claim 1 on a hydrophilic support.