Lithographic printing plate precursor

1. A lithographic printing plate precursor capable of forming an image upon irradiation with an infrared laser comprising in the order recited a support, a first layer containing as the main component an alkali-soluble resin and a second layer containing (a) as the main component, an alkali-soluble resin that is different from the alkali-soluble resin contained as the main component in the first layer and (b) a mixture comprising at least one kind of cyanine dye infrared absorbing agent having an absorption maximum wavelength of not shorter than 825 nm and at least one kind of cyanine dye infrared absorbing agent having an absorption maximum wavelength of shorter than 825 nm,

wherein the alkali-soluble resin contained in the second layer is an alkali-soluble resin having a phenol group or an alkali-soluble resin having a sulfonamido group, and

the difference of the absorption maximum wavelengths of the two kinds of cyanine dye infrared absorbing agents is not less than 15 nm and not more than 50 nm.

2. The lithographic printing plate precursor as claimed in claim 1 , wherein the cyanine dye infrared absorbing agents are cyanine dyes represented by the following formula (a):

wherein R 3 and R 4 each independently represent an alkyl group having from 1 to 12 carbon atoms, which may have a substituent selected from an alkoxy group, an aryl group, an amido group, an alkoxycarbonyl group, a hydroxy group, a sulfo group and a carboxy group; Y 1 and Y 2 each independently represent an oxygen atom, a sulfur atom, a selenium atom, a dialkylmethylene group or —CH═CH—; Ar 1 and Ar 2 each independently represent an aromatic hydrocarbon group, which may have a substituent selected from an alkyl group, an alkoxy group, a halogen atom and an alkoxycarbonyl group, or Ar 1 and Ar 2 each may form a condensed aromatic ring together with the adjacent two carbon atoms connected to Y 1 or Y 2 ; X − represents a counter ion necessary for neutralization of charge, and it is not always necessary in the case wherein the dye cation portion has an anionic substituent; and Q represents a polymethine group selected from a trimethine group, a pentamethine group, a heptamethine group, a nonamethine group and an undecamethine group.

3. The lithographic printing plate precursor as claimed in claim 1 , wherein the cyanine dye infrared absorbing agents are cyanine dyes represented by the following formula (a-1):

wherein X 1 a hydrogen atom or a halogen atom; R 1 and R 2 each independently represent a hydrocarbon group having from 1 to 12 carbon atoms, or R 1 and R 2 are connected with each other to form a 5-membered or 6-membered ring; Ar 1 and Ar 2 , which may be the same or different, each represent an aromatic hydrocarbon group, which may have a substituent; Y 1 and Y 2 , which may be the same or different, each represent a sulfur atom or a dialkylmethylene group having not more than 12 carbon atoms; R 3 and R 4 , which may be the same or different, each represent a hydrocarbon group having not more than 20 carbon atoms, which may have a substituent; R 5 , R 6 , R 7 and R 8 , which may be the same or different, each represent a hydrogen atom or a hydrocarbon group having not more than 12 carbon atoms; X − represents a counter anion necessary for neutralization of charge, and it is not necessary in the case wherein any one of R 1 to R 8 is substituted with an anionic substituent.

4. The lithographic printing plate precursor as claimed in claim 1 , wherein the alkali-soluble resin contained in the first layer is an acryl resin, a urethane resin or a butyral resin.

5. The lithographic printing plate precursor as claimed in claim 1 , wherein a mixing weight ratio of the cyanine dye infrared absorbing agent having an absorption maximum wavelength of not shorter than 825/the cyanine dye infrared absorbing agent having an absorption maximum wavelength of shorter than 825 is from 10/90 to 90/10.

6. A lithographic printing plate precursor capable of forming an image upon irradiation with an infrared laser comprising in the order recited a support, a first layer containing as the main component an alkali-soluble resin and a second layer containing (a) as the main component, an alkali-soluble resin that is different from the alkali-soluble resin contained as the main component in the first layer and (b) a mixture comprising at least one kind of cyanine dye infrared absorbing agent having an absorption maximum wavelength of not shorter than 825 nm and at least one kind of cyanine dye infrared absorbing agent having an absorption maximum wavelength of shorter than 825 nm,

wherein the alkali-soluble resin contained in the first layer is an acryl resin, a urethane resin or a butyral resin, and

the difference of the absorption maximum wavelengths of the two kinds of cyanine dye infrared absorbing agents is not less than 15 nm and not more than 50 nm.

7. The lithographic printing plate precursor as claimed in claim 6 , wherein the cyanine dye infrared absorbing agents are cyanine dyes represented by the following formula (a):

wherein R 3 and R 4 each independently represent an alkyl group having from 1 to 12 carbon atoms, which may have a substituent selected from an alkoxy group, an aryl group, an amido group, an alkoxycarbonyl group, a hydroxy group, a sulfo group and a carboxy group; Y 1 and Y 2 each independently represent an oxygen atom, a sulfur atom, a selenium atom, a dialkylmethylene group or —CH═CH—; Ar 1 and Ar 2 each independently represent an aromatic hydrocarbon group, which may have a substituent selected from an alkyl group, an alkoxy group, a halogen atom and an alkoxycarbonyl group, or Ar 1 and Ar 2 each may form a condensed aromatic ring together with the adjacent two carbon atoms connected to Y 1 or Y 2 ; X − represents a counter ion necessary for neutralization of charge, and it is not always necessary in the case wherein the dye cation portion has an anionic substituent; and Q represents a polymethine group selected from a trimethine group, a pentamethine group, a heptamethine group, a nonamethine group and an undecamethine group.

8. The lithographic printing plate precursor as claimed in claim 6 , wherein the cyanine dye infrared absorbing agents are cyanine dyes represented by the following formula (a-1):

wherein X 1 represents a hydrogen atom or a halogen atom; R 1 and R 2 each independently represent a hydrocarbon group having from 1 to 12 carbon atoms, or R 1 and R 2 are connected with each other to form a 5-membered or 6-membered ring; Ar 1 and Ar 2 , which may be the same or different, each represent an aromatic hydrocarbon group, which may have a substituent; Y 1 and Y 2 , which may be the same or different, each represent a sulfur atom or a dialkylmethylene group having not more than 12 carbon atoms; R 3 and R 4 , which may be the same or different, each represent a hydrocarbon group having not more than 20 carbon atoms, which may have a substituent; R 5 , R 6 , R 7 and R 8 , which may be the same or different, each represent a hydrogen atom or a hydrocarbon group having not more than 12 carbon atoms; X − represents a counter anion necessary for, neutralization of charge, and it is not necessary in the case wherein any one of R 1 to R 8 is substituted with an anionic substituent.

9. The lithographic printing plate precursor as claimed in claim 6 , wherein a mixing weight ratio of the cyanine dye infrared absorbing agent having an absorption maximum wavelength of not shorter than 825/the cyanine dye infrared absorbing agent having an absorption maximum wavelength of shorter than 825 is from 10/90 to 90/10.