Photothermographic material containing particular hydrophilic polymer

1. A photothermographic material, comprising a support and an image-forming layer containing a binder, a photosensitive silver halide, a non-photosensitive organic silver salt, and a reducer for the organic silver salt on at least one surface of the support, wherein the image-forming layer contains in addition to the binder, a hydrophilic polymer which has a vinyl monomer unit containing a quaternary nitrogen atom-containing group or a nitrogen-containing heterocyclic group.

2. The photothermographic material according to claim 1 , wherein the vinyl monomer unit containing a nitrogen-containing heterocyclic group comprises a repeating unit represented by Formula (N):

wherein: R represents a hydrogen atom or an alkyl group; L represents a bivalent connecting group or a simple bond; k denotes 0 or 1; X represents a nitrogen atom or a —CH— or —C═ group; Z represents a bivalent group containing a nitrogen or carbon atom; and at least one of X and Z contains a nitrogen atom.

3. The photothermographic material according to claim 1 , wherein the vinyl monomer unit containing a quaternary nitrogen atom comprises a repeating unit represented by Formula (M):

wherein: R represents a hydrogen atom or an alkyl group; L represents a bivalent connecting group or a simple bond; R 1 , R 2 , and R 3 each represent a substituted or unsubstituted alkyl group; Y − represents an anionic atom or group; k denotes 0 or 1; m is the number of Y − needed for neutralizing a cationic charge; if any one of R 1 to R 3 has an anionic charge neutralizing the cationic charge, m denotes 0; and R 1 , R 2 , and R 3 may bind to each other to form a ring.

4. The photothermographic material according to claim 3 , wherein the repeating unit represented by Formula (M) contains a ring formed by binding at least two of R 1 , R 2 , and R 3 .

5. The photothermographic material according to claim 3 , wherein the repeating unit represented by Formula (M) contains a repeating unit represented by the following Formula (NB):

wherein: R, L, k, R 1 , and R 2 are the same as in Formula (M); R 4 and R 5 each represent a bivalent connecting group; m denotes 0 or 1; and A − represents an anionic residue containing an anionic group selected from sulfonate, carboxylate, and phosphate groups.

6. The photothermographic material according to claim 4 , wherein the repeating unit represented by Formula (M) contains a repeating unit represented by the following Formula (NB):

wherein: R, L, k, R 1 and R 2 are the same as in Formula (M); R 4 and R 5 each represent a bivalent connecting group; m denotes 0 or 1; and A − represents an anionic residue containing an anionic group selected from sulfonate, carboxylate, and phosphate groups.

7. The photothermographic material according to claim 2 , wherein the vinyl monomer unit containing a nitrogen-containing heterocyclic group is one selected from the group consisting of vinylpyrrolidone, vinylimidazole, vinylpyridine, vinylpyrazine, vinylimidazoline, and vinylimidazolidine.

8. The photothermographic material according to claim 3 , wherein the vinyl monomer unit containing a quaternary nitrogen atom is one selected from the group consisting of vinyl-N-substituted-quaternary ammonium, vinyl-N-substituted-imidazole, vinylpyridinium, vinyl-N-substituted-pyridine, vinyl-N-substituted-pyrazine, vinyl-N-substituted-imidazoline, and vinyl-N-substituted-imidazolidine.

9. The photothermographic material according to claim 4 , wherein the vinyl monomer unit having a quaternary nitrogen atom is one selected from the group consisting of vinyl-N-substituted-quaternary ammonium, vinyl-N-substituted-imidazole, vinylpyridinium, vinyl-N-substituted-pyridine, vinyl-N-substituted-pyrazine, vinyl-N-substituted-imidazoline, and vinyl-N-substituted-imidazolidine.

10. The photothermographic material according to claim 5 , wherein the vinyl monomer unit having a repeating unit represented by Formula (NB) is one selected from the group consisting of vinyl quaternary ammonium salt, vinyl-N-substituted-imidazole, vinyl-N-substituted-pyridine, vinyl-N-substituted-pyrazine, and vinyl-N-substituted-imidazoline, each having a sulfonate, carboxylate or phosphate group.

11. The photothermographic material according to claim 6 , wherein the vinyl monomer unit having a repeating unit represented by Formula (NB) is one selected from the group consisting of vinyl quaternary ammonium salt, vinyl-N-substituted-imidazole, vinyl-N-substituted-pyridine, vinyl-N-substituted-pyrazine, and vinyl-N-substituted-imidazoline, each having a sulfonate, carboxylate or phosphate group.

12. The photothermographic material according to claim 1 , wherein the hydrophilic polymer contains the vinyl monomer unit in an amount of 10 mole % to 80 mole %.

13. The photothermographic material according to claim 1 , wherein the hydrophilic polymer is contained in an amount of 0.1 mass % to 30 mass % with respect to the binder in the image-forming layer.

14. The photothermographic material according to claim 1 , wherein the binder in the image-forming layer is an aqueous latex.

15. The photothermographic material according to claim 1 , wherein the binder in the image-forming layer is a hydrophilic, animal protein-derived polymer or polyvinyl alcohol.

16. The photothermographic material according to claim 15 , wherein the hydrophilic animal protein-derived polymer is gelatin.

17. The photothermographic material according to claim 1 , wherein the image-forming layer contains a water-soluble dye.

18. The photothermographic material according to claim 2 , wherein the nitrogen atom contained in at least one of X and Z comprises a carbon-nitrogen double bond.

19. The photothermographic material according to claim 1 , wherein the heterocyclic ring of the nitrogen-containing heterocyclic group is an imidazole, triazole, pyrazole, or pyrimidine ring.