Planorgraphic printing plate precursor

1. A positive planographic printing plate precursor comprising a support having successively disposed thereon

a lower thermosensitive layer containing a water-insoluble but alkali-soluble polymer compound, and

an upper thermosensitive layer containing a water-insoluble but alkali-soluble polymer compound, with solubility in an aqueous alkali solution increasing under heat,

wherein both the upper thermosensitive layer and the lower thermosensitive layer contain an IR absorbing dye, wherein the ratio of the IR absorbing dye concentration in the upper thermosensitive layer to the IR absorbing dye concentration in the lower thermosensitive layer is 1.6 to 10.0, and the coating amount of the upper thermosensitive layer is at most 0.4 g/m 2 .

2. The positive planographic printing plate precursor according to claim 1 , wherein the upper thermosensitive layer and the lower thermosensitive layer contain different IR absorbing dyes, and in which the amount of the IR absorbing dye added to the upper thermosensitive layer is 3 to 50% by weight of the overall weight of the layer, and the amount of the IR absorbing dye added to the lower thermosensitive layer is 0.1 to 10% by weight of the overall weight of the layer.

3. The photosensitive planographic printing plate precursor according to claim 1 , wherein at least one of the upper thermosensitive layer and the lower thermosensitive layer contains an IR absorbent having, in one molecule, at least two chromophoric groups that absorb IR light, with the chromophoric groups in the IR absorbent bonding to each other via a covalent bond, and in which the IR absorbent is represented by the following general formula (I):

(D) n -A  (I)

wherein A represents a divalent or more polyvalent organic group; n indicates an integer of 2 or more; D represents at least one chromophoric group having an absorption in infrared ray range; two or more chromophoric groups may be the same or different, and when the chromophoric group has a charged partial structure, it may have an ionic group charged oppositely to the charge of the chromophoric group so as to neutralize the charge of the chromophoric group.

4. The positive planographic printing plate precursor according to claim 1 , in which the ratio of the IR absorbing dye concentration in the upper thermosensitive layer to the IR absorbing dye concentration in the lower thermosensitive layer is 3.0 to 6.0.

5. The positive planographic printing plate precursor according to claim 1 , wherein the amount of the IR absorbing dye added to each thermosensitive layer is 0.01 to 60% by weight of the total solid content of each layer.

6. The positive planographic printing plate precursor according to claim 1 , wherein the amount of the water-insoluble but alkali-soluble polymer compound in the thermosensitive layers is 30 to 99% by weight of the total solid content of the layers.

7. The positive planographic printing plate precursor according to claim 2 , wherein the surface hardness of the upper thermosensitive layer is at least 0.50 GPa.

8. The positive planographic printing plate precursor according to claim 3 , wherein the amount of the IR absorbing dye in each thermosensitive layer is 0.01 to 50% by weight of the total solid content of each layer.

9. The positive planographic printing plate precursor according to claim 3 , wherein the amount of the water-insoluble but alkali-soluble polymer compound in the thermosensitive layers is 1 to 90% by weight of the total solid content of the layers.

10. The positive planographic printing plate precursor according to claim 1 , wherein the upper thermosensitive layer further contains a substance that substantially lowers the solubility of the alkali-soluble polymer compound.

11. The positive planographic printing plate precursor according to claim 10 , wherein the upper thermosensitive layer further contains, as the substance that substantially lowers the solubility of the alkali-soluble polymer compound, at least an onium dissolution inhibitor.

12. The positive planographic printing plate precursor according to claim 1 , wherein the water-insoluble but alkali-soluble polymer compound has, in the molecule, a functional group of any of phenolic hydroxyl groups, sulfonamido groups or active imido groups.

13. The positive planographic printing plate precursor according to claim 12 , wherein the water-insoluble but alkali-soluble polymer compound is a polymer compound prepared through polymerization of at least two of phenolic hydroxyl group-containing polymerizable monomers, sulfonamido group-containing polymerizable monomers, and active imido group-containing polymerizable monomers, or a polymer compound prepared through copolymerization of at least two such polymerizable monomers with any other polymerizable monomer.

14. The positive planographic printing plate precursor according to claim 13 , wherein the water-insoluble but alkali-soluble polymer compound is a copolymer prepared through copolymerization of a phenolic hydroxyl group-containing polymerizable monomer with a sulfonamido group-containing polymerizable monomer and/or an active imido group-containing polymerizable monomer, and the mixing ratio by weight of these components is 50/50 to 5/95.

15. The positive planographic printing plate precursor according to claim 14 , wherein the water-insoluble but alkali-soluble polymer compound is a copolymer prepared through copolymerization of a phenolic hydroxyl group-containing polymerizable monomer, a sulfonamido group-containing polymerizable monomer or an active imido group-containing polymerizable monomer with any other polymerizable monomer, and the copolymer contains at least 10 mol % of any of the phenolic hydroxyl group-containing polymerizable monomer, the sulfonamido group-containing polymerizable monomer and the active imido group-containing polymerizable monomer that render the copolymer soluble in alkali.