Planographic printing plate precursor Number:7,074,542 from the United States Patent and Trademark Office (PTO) owispatent

Title: Planographic printing plate precursor

Abstract: A planographic printing plate precursor comprising: a substrate; a photosensitive layer disposed on the substrate, the photosensitive layer including a light-to-heat conversion agent and a compound, which is at least one of crosslinkable and polymerizable, with solubility of the photosensitive layer in an alkali developing solution being decreased by the effect of at least one of light and heat; and an overcoat layer including a polymer, which is hydrophobic and soluble in an aqueous alkali solution.

Patent Number: 7,074,542 Issued on 07/11/2006 to Aoshima

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Inventors:	Aoshima; Keitaro (Shizuoka-ken, JP)
Assignee:	Fuji Photo Film Co., Ltd. (Minami-Ashigara, JP)
Appl. No.:	087834
Filed:	March 5, 2002

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Foreign Application Priority Data

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Mar 06, 2001 [JP]			2001-061475

Current U.S. Class:	430/162 ; 101/463.1; 101/467; 430/273.1; 430/278.1
Current International Class:	B41N 1/00 (20060101); B41N 3/00 (20060101); G03F 7/016 (20060101); G03F 7/11 (20060101)
Field of Search:	430/162,273.1 101/467,463.1

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References Cited [Referenced By]

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U.S. Patent Documents

	3729313		April 1973		Smith
	4058400		November 1977		Crivello
	4058401		November 1977		Crivello
	4228232		October 1980		Rousseau
	4445998		May 1984		Kanda et al.
	5340699		August 1994		Haley et al.
	5506090		April 1996		Gardner, Jr. et al.
	6153356		November 2000		Urano et al.
	6555291		April 2003		Hauck

Foreign Patent Documents

	WO 97/00777		Jan., 1997		WO
	WO-02/14071		Feb., 2002		WO

Primary Examiner: Hamilton; Cynthia
Attorney, Agent or Firm: Buchanan Ingersoll PC

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Claims

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What is claimed is:

1. A negative planographic printing plate precursor which is capable of directly making a printing plate using an infrared laser comprising: a substrate; a photosensitive layer disposed on the substrate, the photosensitive layer including a light-to-heat conversion agent and a compound, which is at least one of crosslinkable and polymerizable, with solubility of the photosensitive layer in an alkali developing solution being decreased by the effect of at least one of light and heat; and an overcoat layer including a polymer, which is hydrophobic and soluble in an aqueous alkali solution and which does not have sensitivity to an infrared laser, wherein the substrate, photosensitive layer and overcoat layer are provided in that order; the photosensitive layer is a photopolymerization layer comprising an infrared ray absorbing agent, a radical generating agent, and a radical polymerizable compound which polymerizes with the generated radicals and cures; and the radical generating agent is at least one onium salt.

2. The negative planographic printing plate precursor according to claim 1, wherein the overcoat layer is formed on the photosensitive layer.

3. The negative planographic printing plate precursor according to claim 1, wherein the hydrophobicity is expressed by a contact angle with water of at least 50 degrees.

4. The negative planographic printing plate precursor according to claim 1, wherein the aqueous alkali solution-soluble polymer comprises at least one kind of minimum constituent unit including at least one acidic group selected from a group consisting of a phenolic hydroxyl group, sulfonamide group, substituted sulfonamide-based acidic group, carboxylic acid group, sulfonic acid group, and phosphoric acid group.

5. The negative planographic printing plate precursor according to claim 4, wherein the acidic group has a pKa of no more than 14 and the acidic group is contained in the aqueous alkali solution-soluble polymer in an amount of 0.1 to 12 milliequivalents based on 1 gram of the aqueous alkali solution-soluble polymer.

6. The negative planographic printing plate precursor according to claim 1, wherein the aqueous alkali solution-soluble polymer is a copolymer copolymerized from a compound including at least one acidic group selected from the group consisting of a phenolic hydroxyl group, sulfonamide group, substituted sulfonamide-based acidic group, carboxylic acid group, sulfonic acid group, and phosphoric acid group, and the copolymer is copolymerized with the compound being present in an amount of 10 mol % or more.

7. The negative planographic printing plate precursor according to claim 1, wherein the aqueous alkali solution-soluble polymer has a weight-average molecular weight of 5,000 to 300,000, a number-average molecular weight of 800 to 250,000 and a dispersion degree of from 1.1 to 10.

8. The negative planographic printing plate precursor according to claim 1, wherein the overcoat layer comprises 30 to 99% by weight of the aqueous alkali solution-soluble polymer.

9. The negative planographic printing plate precursor according to claim 1, wherein the onium salt is at least one onium salt selected from a group consisting of an iodonium salt, diazonium salt, and sulfonium salt, and has a maximum absorption of 400 nm or less.

10. The negative planographic printing plate precursor according to claim 1, wherein the light-to-heat conversion agent is an infrared ray absorbing agent having maximum absorption at a wavelength of from 760 nm to 1200 nm.

11. The negative planographic printing plate precursor according to claim 1, comprising a cyanine dye represented by general formula (I) as the light-heat conversion agent: ##STR00013## wherein, X.sup.1 represents one of a halogen atom, X.sup.2-L.sup.1 and NL.sup.2L.sup.3; X.sup.2 represents one of an oxygen atom and a sulfur atom; L.sup.1 represents a hydrocarbon group having from 1 to 12 carbon atoms; L.sup.2 and L.sup.3 each independently represents a hydrocarbon group having from 1 to 12 carbon atoms; and R.sup.1 and R.sup.2 each independently represents a hydrocarbon group having from 1 to 12 carbon atoms; Ar.sup.1 and Ar.sup.2 each independently represents an aromatic hydrocarbon group which may have a substituent; Y.sup.1 and Y.sup.2 each independently represents a sulfur atom or dialkylmethylene group having from 1 to 12 carbon atoms; R.sup.3 and R.sup.4 each independently represents a hydrocarbon group which may have a substituent and have from 1 to 20 carbon atoms; R.sup.5, R.sup.6, R.sup.7 and R.sup.8 each independently represents a hydrogen atom or a hydrocarbon group having from 1 to 12 carbon atoms; and Z.sup.1- represents a counter anion.

12. The negative planographic printing plate precursor according to claim 1, wherein a substrate is selected from a polyester film and an aluminum plate.

13. The negative planographic printing plate precursor according to claim 1, comprising at least one layer selected from a group consisting of an undercoat layer, an intermediate layer, and a backcoat layer.

14. The negative planographic printing plate precursor according to claim 1, wherein the onium salt is a sulfonium salt.

15. A negative planographic printing plate precursor which is capable of directly making a printing plate using an infrared laser comprising: a substrate; a photosensitive layer disposed on the substrate, the photosensitive layer including a light-to-heat conversion agent and a compound which is at least one of crosslinkable and polymerizable, with solubility of the photosensitive layer in an alkali developing solution being decreased by the effect of at least one of light and heat; and an overcoat layer including a polymer, which is hydrophobic and soluble in an aqueous alkali solution and which does not have sensitivity to an infrared laser, wherein the substrate, photosensitive layer and overcoat layer are provided in that order; the photosensitive layer is an acid crosslinking layer comprising: a compound, which generates acid by being exposed by at least one of light and heat; a crosslinking compound, which crosslinks by the generated acid; and an alkali-soluble polymer, which reacts with the crosslinking agent in the presence of an acid; and the compound which generates acid by at least one of being irradiated with light having a wavelength of 200 to 500 nm and by being heated at least 100.degree. C.

16. The negative planographic printing plate precursor according to claim 15, wherein the overcoat layer is formed on the photosensitive layer.

17. The negative planographic printing plate precursor according to claim 15, wherein the hydrophobicity is expressed by a contact angle with water of at least 50 degrees.

18. The negative planographic printing plate precursor according to claim 15, wherein the aqueous alkali solution-soluble polymer comprises at least one kind of minimum constituent unit including at least one acidic group selected from a group consisting of a phenolic hydroxyl group, sulfonamide group, substituted sulfonamide-based acidic group, carboxylic acid group, sulfonic acid group, and phosphoric acid group.

19. The negative planographic printing plate precursor according to claim 18, wherein the acidic group has a pKa of no more than 14 and the acidic group is contained in the aqueous alkali solution-soluble polymer in an amount of 0.1 to 12 milliequivalents based on 1 gram of the aqueous alkali solution-soluble polymer.

20. The negative planographic printing plate precursor according to claim 15, wherein the aqueous alkali solution-soluble polymer is a copolymer copolymerized from a compound including at least one acidic group selected from the group consisting of a phenolic hydroxyl group, sulfonamide group, substituted sulfonamide-based acidic group, carboxylic acid group, sulfonic acid group, and phosphoric acid group, and the copolymer is copolymerized with the compound being present in an amount of 10 mol % or more.

21. The negative planographic printing plate precursor according to claim 15, wherein the aqueous alkali solution-soluble polymer has a weight-average molecular weight of 5,000 to 300,000, a number-average molecular weight of 800 to 250,000 and a dispersion degree of from 1.1 to 10.

22. The negative planographic printing plate precursor according to claim 15, wherein the overcoat layer comprises 30 to 99% by weight of the aqueous alkali solution-soluble polymer.

23. The negative planographic printing plate precursor according to claim 15, wherein the light-to-heat conversion agent is an infrared ray absorbing agent having maximum absorption at a wavelength of from 760 nm to 1200 nm.

24. The negative planographic printing plate precursor according to claim 15, comprising a cyanine dye represented by general formula (I) as the light-heat conversion agent: ##STR00014## wherein X.sup.1 represents one of a halogen atom, X.sup.2-L.sup.1 and NL.sup.2L.sup.3; X.sup.2 represents one of an oxygen atom and a sulfur atom; L.sup.1 represents a hydrocarbon group having from 1 to 12 carbon atoms; L.sup.2 and L.sup.3 each independently represents a hydrocarbon group having from 1 to 12 carbon atoms; R.sup.1 and R.sup.2each independently represents a hydrocarbon group having from 1 to 12 carbon atoms; Ar.sup.1 and Ar.sup.2 each independently represents an aromatic hydrocarbon group which may have a substituent; Y.sup.1 and Y.sup.2 each independently represents a sulfur atom or a dialkylmethylene group having from 1 to 12 carbon atoms; R.sup.3 and R.sup.4 each independently represents a hydrocarbon group which may have a substituent and have from 1 to 20 carbon atoms; R.sup.5, R.sup.6, R.sup.7 and R.sup.8 each independently represents a hydrogen atom or a hydrocarbon group having from 1 to 12 carbon atoms; and Z.sup.1- represents a counter anion.

25. The negative planographic printing plate precursor according to claim 15, wherein a substrate is selected from a polyester film and an aluminum plate.

26. The negative planographic printing plate precursor according to claim 15, comprising at least one layer selected from a group consisting of an undercoat layer, an intermediate layer, and a backcoat layer.

27. A planographic printing plate precursor comprising: a substrate; a photosensitive layer disposed on the substrate, the photosensitive layer including a light-to-heat conversion agent and a compound, which is at least one of crosslinkable and polymerizable, with solubility of the photosensitive layer in an alkali developing solution being decreased by the effect of at least one of light and heat; and an overcoat layer including a polymer, which is hydrophobic and soluble in an aqueous alkali solution, wherein the substrate, photosensitive layer and overcoat layer are provided in that order; the photosensitive layer is an acid crosslinking layer comprising: a compound, which generates acid by being exposed by at least one of light and heat; a crosslinking compound, which crosslinks by the generated acid; and an alkali-soluble polymer, which reacts with the crosslinking agent in the presence of an acid; and the crosslinking compound is at least one crosslinking compound selected from a group consisting of: aromatic compounds substituted with at least one of hydroxymethyl group and alkoxymethyl group; compounds comprising at least one of N-hydroxymethyl group, N-alkoxymethyl group, and N-acyloxymethyl group: and, epoxy compounds.

28. The planographic printing plate precursor according to claim 27, wherein the overcoat layer is formed on the photosensitive layer.

29. The planographic printing plate precursor according to claim 27, wherein the overcoat layer does not have sensitivity to an infrared laser.

30. The planographic printing plate precursor according to claim 27, wherein the hydrophobicity is expressed by a contact angle with water of at least 50 degrees.

31. The planographic printing plate precursor according to claim 27, wherein the aqueous alkali solution-soluble polymer comprises at least one kind of minimum constituent unit including at least one acidic group selected from a group consisting of a phenolic hydroxyl group, sulfonamide group, substituted sulfonamide-based acidic group, carboxylic acid group, sulfonic acid group, and phosphoric acid group.

32. The planographic printing plate precursor according to claim 31, wherein the acidic group has a pKa of no more than 14 and the acidic group is contained in the aqueous alkali solution-soluble polymer in an amount of 0.1 to 12 milliequivalents based on 1 gram of the aqueous alkali solution-soluble polymer.

33. The planographic printing plate precursor according to claim 27, wherein the aqueous alkali solution-soluble polymer has a weight-average molecular weight of 5,000 to 300,000, a number-average molecular weight of 800 to 250,000 and a dispersion degree of from 1.1 to 10.

34. The planographic printing plate precursor according to claim 27, wherein the overcoat layer comprises 30 to 99% by weight of the aqueous alkali solution-soluble polymer.

35. The planographic printing plate precursor according to claim 27, wherein the light-to-heat conversion agent is an infrared ray absorbing agent having maximum absorption at a wavelength of from 760 nm to 1200 nm.

36. The planographic printing plate precursor according to claim 27, comprising a cyanine dye represented by general formula (1) as the light-heat conversion agent: ##STR00015## wherein X.sup.1 represents one of a halogen atom, X.sup.2-L.sup.1 and NL.sup.2L.sup.3; X.sup.2 represents one of an oxygen atom and a sulfur atom; L.sup.1 represents a hydrocarbon group having from 1 to 12 carbon atoms; L.sup.2 and L.sup.3 each independently represents a hydrocarbon group having from 1 to 12 carbon atoms; R.sup.1 and R.sup.2 each independently represents a hydrocarbon group having from 1 to 12 carbon atoms; Ar.sup.1 and Ar.sup.2 each independently represents an aromatic hydrocarbon group which may have a substituent; Y.sup.1 and Y.sup.2 each independently represents a sulfur atom or a dialkylmethylene group having from 1 to 12 carbon atoms; R.sup.3 R.sup.4 each independently represents a hydrocarbon group which may have a substituent and have from 1 to 20 carbon atoms; R.sup.5, R.sup.6, R.sup.7 and R.sup.8 each independently represents a hydrogen atom or a hydrocarbon group having from 1 to 12 carbon atoms; and Z.sup.1- represents a counter anion.

37. The planographic printing plate precursor according to claim 27, wherein a substrate is selected from a polyester film and an aluminum plate.

38. The planographic printing plate precursor according to claim 27, comprising at least one layer selected from a group consisting of an undercoat layer, an intermediate layer, and a backcoat layer.

39. The planographic printing plate precursor according to claim 27, wherein the aqueous alkali solution-soluble polymer is a copolymer copolymerized from a compound including at least one acidic group selected from the group consisting of a phenolic hydroxyl group, sulfonamide group, substituted sulfonamide-based acidic group, carboxylic acid group, sulfonic acid group, and phosphoric acid group, and the copolymer is copolymerized with the compound being present in an amount of 10 mol % or more.

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Description

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BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a planographic printing plate precursor having sensitivity in the infrared wavelength range, and more specifically, to a negative type planographic printing plate precursor which can be used in so-called direct plate making and is capable of directly making a printing plate using an infrared laser based on digital signals outputted from a computer or the like.

2. Description of the Related Art

There have been remarkable developments in lasers in recent years. In particular, solid lasers or semiconductor lasers (hereinafter, sometimes referred to as "infrared lasers") emitting infrared rays having wavelengths of 760 nm to 1200 nm, and having a compact size and high output are now readily available. These infrared lasers are very useful as a recording light source when direct plate making is carried out with digital data outputted from a computer or the like. Therefore, recent years have seen an increasing desire for an image recording material having high sensitivity to the above-described infrared ray recording light sources, namely, an image recording material whose solubility in a developing solution changes significantly upon irradiation with infrared rays.

As an example of the above-mentioned negative type image recording material capable of recording images by the use of the infrared lasers, U.S. Pat. No. 5,340,699 discloses a recording material comprising an infrared ray absorbing agent, an acid generating agent, a resol resin, and a novolak resin. However, the above type of negative type recording material needs to be heat-treated after being exposed with lasers to form images. Therefore, negative type image recording materials, which do not require the heat treatment after exposure, are desired.

Further, in a method in which a high-power density exposure using a high-power laser is employed, the exposure areas are intensively and briefly irradiated so as to accumulate with a large amount of light energy during an exposure time, and the light energy is efficiently converted into a heat energy. Due to the heat derived from the light energy, thermal changes such as a chemical change, a phase change, a formation change, or a structural change are caused, and these changes are used to image record. However, in photosensitive layers of conventional planographic printing plate precursors, if an amount to be added of an infrared ray absorbing agent increased to improve recording sensitivity, laser exposing devices and light sources might be contaminated by materials scattered by ablation of the photosensitive layers.

As an example of a negative type image forming material for accomplishing two objectives of improving an image formability and suppressing the ablation, International Publication (WO) No. 97/00777 discloses a negative type image forming material having a photosensitive layer structured by two layers. In this structure, a surface layer, which is a surface being exposed, has photosensitivity so a problem of ablation being easily generated at the surface layer occurs. Further, a post-exposure is required to obtain a rigid image. In view of the suppression of the ablation, a method, in which an overcoat layer, which comprises a water-soluble polymer and can be removed at the time of development, is provided, may also be employed. The aforementioned overcoat layer can easily be removed together with the photosensitive layer in non-image portions. However, there is a concern that, in a case in which the overcoat layer remains on image portions, a large amount of paper loss caused by insufficient ink application will occur.

The object of the present invention is to provide a negative type planographic printing plate precursor which: can be used for direct plate making by being recorded with digital data from a computer, or the like, using a solid laser or a semiconductor laser emitting infrared rays; has high sensitivity to an infrared laser; suppresses ablation of the photosensitive layer during recording; and in addition, is to suppress paper loss, which is caused by the insufficient ink application during printing.

SUMMARY OF THE INVENTION

The present inventors focused on layer structures and characteristics of surface layers of negative type planographic printing plate precursors which can be used for direct plate making by irradiation thereof with an infrared ray. As a result of intensive research thereof, the present inventors found that the above-described problems can be solved by providing an overcoat layer which does not have sensitivity to an laser exposure, but is hydrophobic and has an excellent solubility in an alkali developing solution, thus completing the present invention.

Namely, the planographic printing plate precursor of the present invention is characterized by disposing, in the following order: a photosensitive layer, comprising a light-to-heat conversion agent and a compound having a crosslinking property or a polymerizing property, a solubility of the photosensitive layer in an alkali developing solution decreases by being affected by light or heat; and an overcoat layer, comprising a polymer, which is hydrophobic and soluble in an aqueous alkali solution; on a substrate.

First aspect of the present invention is a planographic printing plate precursor comprising: a substrate; a photosensitive layer disposed on the substrate, the photosensitive layer including a light-to-heat conversion agent and a compound, which is at least one of crosslinkable and polymerizable, with solubility of the photosensitive layer in an alkali developing solution being decreased by the effect of at least one of light and heat; and an overcoat layer including a polymer, which is hydrophobic and soluble in an aqueous alkali solution.

It should be noted that, in the present invention, a polymer having a hydrophobic property and solubility in an aqueous alkali solution is simply referred to as "an aqueous alkali solution-soluble polymer" with appropriateness. Further, a description of "by being effected by light or heat" implies that of "by being effected by both light and heat".

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, a detailed description of the present invention will be given.

The planographic printing plate precursor of the present invention is characterized by successively providing on a substrate a negative type photosensitive layer and an overcoat layer containing a polymer having a hydrophobic property and solubility in an aqueous alkali solution. It suffices that these layers are provided in that order, and known layers such as an intermediate layer, a backcoat layer or the like, may also be provided insofar as effects of the present invention are not impaired.

First, a description will be made of constituents of the overcoat layer, which is a distinctive structure of the present invention.

[Hydrophobic and Aqueous Alkali Solution-soluble Polymer]

The overcoat layer of the present invention contains a polymer having a hydrophobic property and solubility in an aqueous alkali solution as a primarily component. Since a compound containing this polymer has an excellent coated film formability, the polymer compound can form a layer by itself.

The hydrophobic aqueous alkali solution polymer used in the present invention is not particularly limited, and any of those having an alkali-soluble group in a molecule and the coated film formability can be used.

Further, in the prevent invention, the term "hydrophobic property" refers to a hydrophobicity of 30 degrees or greater, and preferably 50 degrees or greater, when converted into a contact angle with water (an angle of contact with atmospheric water droplets). Therefore, the polymer which forms a coated film with the surface having a value of a contact angle with water of 30 degrees or smaller may cause insufficient ink application when remaining on a surface of a photosensitive layer, which is not preferable as a component of the overcoat layer used in the present invention. Known methods can be applied as a measuring method of the contact angle. For example, a method which measures a contact angle (an angle of contact with atmospheric droplets) using a commercially available device such as CA-Z manufactured by Kyowa Surfactant Science, Ltd., can be applied.

Examples of the alkali-soluble polymer introduced in the aqueous alkali solution-soluble polymer used in the present invention include a carboxylic acid group, group of oxyacid of phosphorus, group of oxyacid of sulfur, imide group, sulfonimide group, sulfonylimide group, phenolic hydroxyl group and the like. In the alkali-soluble polymer, a homopolymer having the above-mentioned acidic group on the main chain and/or side chain in the polymer, a copolymer thereof or mixture thereof is included. Therefore, the overcoat layer relating to the present invention has a characteristic such that it dissolves when it contacts an alkaline developing solution.

Among these alkali-soluble polymers, those containing at least one acidic group selected from the following groups (1) to (6) on the main chain and/or side chain of a polymer are preferable from the standpoint of solubility in an alkaline developing solution.

(1) Phenolic hydroxyl group (--Ar--OH)

(2) Sulfonamide group (--SO.sub.2NH--R)

(3) Substituted sulfonamide-based acidic group (hereinafter, referred to as "an active imido group") [--SO.sub.2NHCOR, --SO.sub.2NHSO.sub.2R, --CONHSO.sub.2R]

(4) Carboxylic acid group (--CO.sub.2H)

(5) Sulfonic acid group (--SO.sub.3H)

(6) Phosphoric acid group (--OPO.sub.3H.sub.2)

In the above-described groups (1) to (6), Ar represents a divalent connecting aryl group, optionally having one or more substituents and R represents a hydrocarbon group which may be substituted.

Among aqueous alkali solution-soluble polymers having an acidic group selected from the above-described groups (1) to (6), aqueous alkali solution-soluble polymers containing (1) a phenol group, (2) a sulfonamide group and/or (3) an active imide group are preferable, and, particularly, aqueous alkali solution-soluble polymers containing (1) a phenol group or (2) a sulfonamide group are most preferable from the standpoints of ensuring sufficient solubility in an alkali developing solution and film strength.

Examples of the aqueous alkali solution-soluble polymer containing an acidic group selected from the above-described groups (1) to (6) include the following polymers.

Examples of the aqueous alkali solution-soluble polymer containing (1) a phenol group include a novolak resin, a polymer having a hydroxyaryl group on the side chain thereof, and the like. Examples of the novolak resin include a resin obtained by condensing phenols and aldehydes under the acidic condition.

Of the novolak resin, examples of the preferable novolak resins include a novolak resin obtained from phenol and formaldehyde, a novolak resin obtained from m-cresol and formaldehyde, a novolak resin obtained from p-cresol and formaldehyde, a novolak resin obtained from o-cresol and formaldehyde, a novolak resin obtained from octylphenol and formaldehyde, a novolak resin obtained from m-/p-mixed cresol and formaldehyde, a novolak resin obtained from a mixture of phenol/cresol (any of m-, p-, o-, or m-/p-, m-/o-, o-/p-mixture may be used) and formaldehyde, and the like.

These novolak resin preferably have a weight average molecular weight of 800 to 200,000 and a number average molecular weight of 400 to 60,000.

Further, as the aqueous alkali solution-soluble polymer containing a phenol group, a polymer having a hydroxyaryl group on the side chain thereof is also preferable. In this polymer, the hydroxyaryl group refers to an aryl group to which one or more --OH groups are bonded.

Although a phenyl group, naphthyl group, anthracenyl group, phenanthrenyl group and the like are exemplified as the aryl group, the phenyl group and the naphthyl group are preferable in view of their easy availability and physical properties.

Therefore, as the hydroxyaryl group, a hydroxyphenyl group, dihydroxyphenyl group, trihydroxyphenyl group, tetrahydroxyphenyl group, hydroxynaphthyl group, dihydroxynaphthyl group or the like are preferable.

These hydroxyaryl groups may further have one or more substituents such as a halogen atom, a hydrocarbon group having no more than 20 carbon atoms, an alkoxy group having no more than 20 carbon atoms, an aryloxy group having no more than 20 carbon atoms or the like.

The hydroxyaryl group is bonded to a main chain forming polymer as a pendant-like side chain and may have a linking group between the side chain and the main chain.

The polymers, having hydroxyaryl groups on the side chains, which are suitably used in the embodiment of the present invention include polymers having at least one of the structural units, which are represented by the following formulae (IX) to (XII) and described in detail in paragraph numbers [0016] through [0030] of Japanese Patent Application No. 2000-144732 previously proposed by the present inventors. The structural units represented by the general formulae (IX) to (XII) will be described below.

##STR00001##

In the general formulae (IX) through (XII), R.sup.11 represents a hydrogen atom or a methyl group. R.sup.12 and R.sup.13 may be the same or different and each represents a hydrogen atom, a halogen atom, a hydrocarbon group having no more than 10 carbon atoms, an alkoxy group having no more than 10 carbon atoms or an aryloxy group having no more than 10 carbon atoms. Further, R.sup.12 and R.sup.13 may be linked to each other and condensed to form a benzene ring or a cyclohexane ring. R.sup.14 represents a single bond or a divalent hydrocarbon group having no more than 20 carbon atoms. R.sup.15 represents a single bond or a divalent hydrocarbon group having no more than 20 carbon atoms. R.sup.16 represents a single bond or a divalent hydrocarbon group having no more than 10 carbon atoms. X.sup.1 represents a single bond, an ether bond, a thioether bond, an ester bond or an amide bond. p represents an integer with a value of from 1 to 4. q and r each independently represents an integer with a value of from 0 to 3.

Examples of the aqueous alkali solution-soluble polymer having (2) a sulfonamide group include polymers structured to have, as their main constituent, the minimum constituent unit derived from compounds having a sulfonamide group. As the above-described compound, compounds having one or more sulfonamide groups in which at least one hydrogen atom is bonded to a nitrogen atom and one or more polymerizable unsaturated groups are listed. Among these, compounds having a low molecular weight and an acryloyl group, allyl group or vinyloxy group and a substituted or mono-substituted aminosulfonyl group or a substituted sulfonylimino group are preferable, and specific examples thereof include the compounds represented by the general formulae (a) through (e) given below.

##STR00002##

wherein X.sup.1 and X.sup.2 each represents --O--or --NR.sup.7--; R.sup.1 and R.sup.4 each represents a hydrogen atom or --CH.sup.3; R.sup.2, R.sup.5, R.sup.9, R.sup.12 and R.sup.16 each represents an alkylene group, cycloalkylene group, arylene group or aralkylene group having 1 to 12 carbon atoms, and optionally having one or more substituents; R.sup.3, R.sup.7 and R.sup.13 each represents a hydrogen atom or an alkyl group, cycloalkyl group, aryl group or aralkyl group having 1 to 12 carbon atoms, and optionally having one or more substituents; R.sup.6 and R.sup.17 each represents an alkyl group, cycloalkyl group, aryl group or aralkyl group having 1 to 12 carbon atoms, and optionally having one or more substituents; R.sup.8, R.sup.10 and R.sup.14 each represents a hydrogen atom or --CH.sup.3; R.sup.11 and R.sup.15 each represents a single bond or an alkylene group, cycloalkylene group, arylene group or aralkylene group having 1 to 12 carbon atoms, which may have a substituent; and Y.sup.1 and Y.sup.2 each represents a single bond, or --CO--.

Among those compounds represented by the general formulae (a) through (e), compounds which can be favorably used in the negative type planographic printing plate precursor of the present invention include m-aminosulfonylphenyl methacrylate, N-(p-aminosulfonylphenyl) methacrylamide, N- (p-aminosulfonylphenyl) acrylamide, or the like.

Examples of the aqueous alkali solution-soluble polymer having (3) an active imide group include polymers structured to have the minimum constituent unit derived from a compound having an active imide group as their main constituent. As the above-described compound, compounds having in the molecule one or more active imide groups and one or more polymerizable unsaturated groups represented by the following structural formula can be listed.

##STR00003##

Specifically, N-(p-toluenesulfonyl) methacrylamide, N-(p-toluenesulfonyl) acrylamide or the like can be favorably used.

Examples of the aqueous alkali solution-soluble polymer having (4) a carboxyl group include polymers structured to have as their main constituent the minimum constituent unit derived from compounds having, in the molecule, one or more carboxylic acid groups and one or more polymerizable unsaturated groups.

Examples of the aqueous alkali solution-soluble polymer having (5) a sulfonic acid group include polymers structured to have, as their main constituent, the minimum constituent unit derived from compounds having one or more sulfonic acid groups and one or more polymerizable unsaturated groups.

Examples of the aqueous alkali solution-soluble polymer having (6) a phosphoric acid group include polymers structured to have, as their main constituent, a minimum constituent unit derived from compounds having one or more phosphoric acid groups and one or more polymerizable unsaturated groups.

The minimum constituent unit having an acidic group selected from the above-described groups (1) through (6), which constitutes the aqueous alkali solution-soluble polymer used for the material for the negative type planographic printing plate precursor of the present invention, does not need to be used singly. Polymers obtained by copolymerizing two or more minimum constituent units having the same acidic group or by copolymerizing two or more minimum constituent units each having different acidic groups can also be used.

As the copolymerization method, a conventionally known graft copolymerization method, block copolymerization method, random copolymerization method and the like, can be used.

The above-described copolymer preferably contains, in a copolymer, 10 mol % or more, and more preferably 20 mol % or more, of a compound containing at least one acidic group selected from the group consisting of (1) to (6) for copolymerization. If the content is less than 10 mol %, there is a tendency to be unable to sufficiently improve development latitude.

In the present invention, in a case in which a copolymer is formed by copolymerizing a compound, other compounds which do not contain an acidic group selected from the above-described groups (1) to (6) may also be used as the compound to form a copolymer. Examples of other compounds not containing acidic group selected from the above-described groups (1) to (6) include the compounds listed in the following (m1) to (m11).

(m1) Acrylic esters and methacrylic esters each of which have an aliphatic hydroxyl group and are exemplified by 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, or the like.

(m2) Alkyl acrylates such as methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, amyl acrylate, hexyl acrylate, octyl acrylate, benzyl acrylate, 2-chloroethyl acrylate, glycidyl acrylate, and N-dimethylaminoethyl acrylate, and the like.

(m3) Alkyl methacrylates such as methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, amyl methacrylate, hexyl methacrylate, cyclohexyl methacrylate, benzyl methacrylate, 2-chloroethyl methacrylate, glycidyl methacrylate, N-dimethylaminoethyl methacrylate, and the like.

(m4) Acrylamides or methacrylamides such as acrylamide, methacrylamide, N-methylolacrylamide, N-ethylacrylamide, N-hexylmethacrylamide, N-cyclohexylacrylamide, N-hydroxyethylacrylamide, N-phenylacrylamide, N-nitrophenylacrylamide, N-ethyl-N-phenylacrylamide, and the like.

(m5) Vinyl ethers such as ethyl vinyl ether, 2-chloroethyl vinyl ether, hydroxyethyl vinyl ether, propyl vinyl ether, butyl vinyl ether, octyl vinyl ether, phenyl vinyl ether, and the like.

(m6) Vinyl esters such as vinyl acetate, vinyl chloroacetate, vinyl butyrate, vinyl benzoate, and the like.

(m7) Styrenes such as styrene, .alpha.-methylstyrene, methylstyrene, chloromethylstyrene, and the like.

(m8) Vinyl ketones such as methyl vinyl ketone, ethyl vinyl ketone, propyl vinyl ketone, phenyl vinyl ketone, and the like.

(m9) Olefins such as ethylene, propylene, isobutylene, butadiene, isoprene, and the like.

(m10) N-vinylpyrrolidone, N-vinylcarbazole, 4-vinylpyridine, acrylonitrile, methacrylonitrile, and the like.

(m11) Unsaturated imides such as maleimide, N-acryloylacrylamide, N-acetylmethacrylamide, N-propionylmethacrylamide, N-(p-chlorobenzoyl) methacrylamide, and the like.

In the present invention, as the hydrophobic aqueous alkali solution-soluble polymer, which is used as a constituent of the overcoat layer, a polymer having a weight-average molecular weight of 2,000 or more and a number-average molecular weight of 500 or more is preferable, and a polymer having a weight-average molecular weight of 5,000 to 300,000 and a number-average molecular weight of 800 to 250,000 is more preferable, in terms of the sensitivity and the development latitude, irrespective of whether the polymer is a homopolymer or a copolymer. Moreover, a polymer having a dispersion degree (weight-average molecular weight/number-average molecular weight) of 1.1 to 10 is preferable.

When a copolymer is used in the present invention, the weight ratio of the compound of the minimum constituent unit, which is derived from a compound having an acidic group selected from the above-described groups (1) to (6) and constitutes the main chain and/or the side chain of the copolymer, to another minimum constituent unit, which does not contain an acidic group selected from the above-described groups (1) to (6) and constitutes a part of the main chain and/or the side chain of the copolymer, is preferably in the range of from 50:50 to 5:95, and more preferably in the range of from 40:60 to 10:90, from the viewpoint of desired effects.

As the hydrophobic aqueous alkali solution-soluble polymer used in the present invention, a polymer having a carboxylic acid, oxyacid of phosphorus, oxyacid of sulfur, imide group, sulfonimide group, sulfonylimide group, or phenolic hydroxyl group as an alkali-soluble group is particularly preferable. In view of physical properties, an acid radical having a pKa (acid dissociation constant) of 14 or less is preferable.

These functional groups (acid radicals) allow the polymer to be alkali-soluble, while they act to enhance a hydrophilic property of the polymer. Therefore, it is preferable that the content of the functional group in the polymer be low to an extent which does not inhibit the alkali-solubility of the polymer. Preferably, an equivalence of the functional group content in a polymer (to 1 g of the polymer) is 0.1 to 12 mmg/g, and more preferably of 0.5 to 8 mmg/g.

The above-described aqueous alkali solution-soluble polymer may be used singly or in combinations of two or more.

The amount thereof may be 100% based on the total amount of solid contents in materials constituting the overcoat layer. However, with consideration of a possibility of using other components with the aqueous alkali solution-soluble polymer to improve a layer formability and a coated film characteristic, the amount is preferably in the range of from 30 to 99% by weight, more preferably in the range of from 45 to 95% by weight.

If the amount used of the aqueous alkali solution-soluble polymer is less than 30% by weight, the layer formability and the coated film characteristics of the overcoat layer tend to deteriorate, which is not desirable.

In the present invention, various additives can be used for the overcoat layer together with the above-described materials, as needed.

For example, heat decomposable compounds composed of an onium salt, aromatic surfonate or the like, which are described in paragraph number [0067], and thereafter, of Japanese Patent Application Laid-Open (JP-A) No. 11-174681 as "other components", which can be added to a positive type photosensitive composition, are suitable for adjusting an ability to suppress dissolution in image portions. Additives useful for improving sensitivity such as cyclic acid anhydrides, phenols, organic acids or the like, surfactants, printing-out agents, dyes and pigments serving as image coloring agents, or the like disclosed in JP-A No. 11-174681 as "other components" can be used as well in the present invention.

Further, phenol compounds, which have a hydroxymethyl group and which are described in Japanese Patent Application Laid-Open (JP-A) No. 8-276558, crosslinking compounds, which are able to suppress dissolution in alkali and are described in Japanese Patent Application Laid-Open (JP-A) No. 11-160860, epoxy compounds, vinyl ether compounds, or the like can be appropriately added to the overcoat layer of the present invention in response to the object.

Further, in the present invention, the overcoat layer is provided for the purpose of suppressing ablation generated at the photosensitive layer. Therefore, it is preferable that the overcoat layer does not contain components having sensitivity to light or heat, and that the overcoat layer itself does not have sensitivity to light or heat.

Although the weight of coating (solid content) obtained after drying varies depending on the application, the preferable amount of the dried overcoat layer coating is generally in the range of 0.01 to 2.0 g/M.sup.2, more preferably of 0.05 to 1.0 g/m.sup.2. Further, the preferable thickness of the coated overcoat layer is in the range of 0.01 to 2.0 .mu.m, and more preferably of 0.05 to 1.0 .mu.m. If the overcoat layer is too thin, the suppression of ablation becomes insufficient. If the overcoat layer is too thick, the sensitivity of the precursor to infrared rays tends to be reduced. Though the reason for the sensitivity reduction caused by the thickness of the overcoat layer which does not contain materials such as an infrared ray absorbing agent or the like and has an excellent light transmitting property is unclear, it is assumed that, because heat generated by exposure with an infrared ray laser is diffused into the thick overcoat layer, the temperature of the photosensitive layer does not sufficiently rise, and exposure energy is not efficiently used for image forming.

[Photosensitive Layer Containing a Crosslinkable or Polymerizable Compound, and In Which an Ability to be Dissolved into an Alkaline Developing Solution Reduces by Being Affected by Light or Heat]

The planographic printing plate precursor of the present invention is formed by providing, on a substrate, a photosensitive layer having sensitivity to infrared rays, and the above-described overcoat layer in that order, and the description will be given of this photosensitive layer.

The photosensitive layer is provided directly or optionally through an undercoat layer and/or an intermediate layer on a substrate. The photosensitive layer of the present invention contains a compound which is crosslinkable or polymerizable. At the photosensitive layer, an infrared ray absorbing agent generates heat by being heated or exposed with infrared ray laser. Due to the heat generated thereby, a reaction to form covalent bonding takes place in the crosslinkable or polymerizable compound. Accordingly, only exposed (heated) portions of the photosensitive layer are cured, the ability of the exposed (heated) portions to be dissolved into an alkali developing solution deteriorates, and the exposed (heated) portions manifest a non-soluble characteristic to an alkali developing solution. On the other hand, in non-exposed portions of the photosensitive layer, solubility in an alkali developing solution is maintained, and high solubility in an alkali developing solution is exhibited therein. Therefore, after development, negative type images having a good image quality with no residual films can be formed on the photosensitive layer.

There are no specific limitations to the reaction which can form the above-described covalent bonding. If the photosensitive layer can reduce the solubility in the alkali developing solution and form images with a required intensity, then any of a known radical polymerization reaction, cationic polymerization reaction, anionic polymerization reaction, condensation polymerization reaction, addition polymerization reaction or the like can be applied. Further, a reaction, in which polymers having polymerizable functional groups are bonded by being crosslinked and cured, may also be applied.

A representative example of the photosensitive layers which form the covalent bonding and cure as described above, is a photopolymerization layer. The photopolymerization layer contains (A) a infrared ray absorbing agent, (B) a radical generating agent (radical polymerization initiator), and (C) a radical polymerizable compound which polymerizably reacts with the generated radicals and cures, and preferably, further contains (D) a binder polymer. Due to heat generated at the exposed or heated areas, the radical polymerization initiator such as onium salts or the like is decomposed and generates radicals. The radical polymerizable compound is selected from the compounds having at least one ethylenically unsaturated double bond, at least one terminal ethylenically unsaturated bond, and preferably, two or more terminal ethylenically unsaturated bonds. In the photopolymerization layer, a chained polymerization reaction is caused by the radicals generated therein, and the photopolymerization layer cures.

Another aspect of the photosensitive layer is an acid crosslinking layer. The acid crosslinking layer contains (E) a compound (hereinafter, referred to as an acid generating agent) which generates acid by being exposed with light or heat and (F) a crosslinking compound (hereinafter, referred to as a crosslinking agent) which crosslinks by the generated acid, and further contains (G) an alkali-soluble polymer which can react with the crosslinking agent under the existence of acid and is used for forming a layer containing (E) and (F). At this acid crosslinking layer, acid generated by the acid generating agent being decomposed by light irradiation or heating accelerates an effect of the crosslinking agent. Rigid crosslinking structures are formed between the crosslinking agents and/or between the crosslinking agent and the binder polymers, and the alkali-solubility of the acid crosslinking layer thereby deteriorates so that portions of the acid crosslinking layer, which were irradiated or heated, become insoluble in the developing solution. At this time, (A) an infrared ray absorbing agent is preferably blended in the photosensitive layer to efficiently use energy of an infrared laser.

Hereinafter, a description of each of the compounds used for the photosensitive layer of the negative type planographic printing plate precursor will be given.

[(A) Infrared Ray Absorbing Agent]

By containing the infrared ray absorbing agent, the photosensitive layer of the planographic printing plate precursor according to the present invention is formed in such a manner as to be capable of recording images using lasers emitting infrared rays. There is no specific limitation to the infrared ray absorbing agent used herein, and a known infrared ray absorbing agent having a function to convert the absorbed infrared rays to heat can be suitably selected. The infrared ray absorbing agent is exposed with infrared rays emitted from the infrared laser to generate heat, the radical generating agent and the acid generating agent decompose due to the heat generated and generates radicals and acids. Dyes and pigments having a wavelength absorption maximum in a range of 760 nm to 1200 nm are used in the present invention as the infrared ray absorbing agents.

The dyes which can be used may be any known dyes, such as commercially available dyes or dyes described in, for example, "Dye Handbook" (Senryo Binran) (edited by the Organic Synthetic Chemistry Association, published in 1970). Specifically, for example, those described in paragraph numbers [0050] to [0051] of Japanese Patent Application Laid-Open (JP-A) No. 10-39509 can be used.

Among these dyes, particularly preferable dyes are cyanine dyes, squalilium dyes, pyrylium salts, and nickel thiolate complexes. Cyanine dyes are preferable, and a cyanine dye represented by the general formula (I) described below is the most preferable.

##STR00004##

In the general formula (I), X.sup.1 represents a halogen atom, X.sup.2-L.sup.1 or NL.sup.2L.sup.3. Wherein, X.sup.2 represents an oxygen atom or a sulfur atom, L.sup.1 represents a hydrocarbon group having 1 to 12 carbon atoms, L.sup.2 and L.sup.3 each independently represents a hydrocarbon group having 1 to 12 carbon atoms. R.sup.1 and R.sup.2 each independently represents a hydrocarbon group having 1 to 12 carbon atoms. In terms of storage stability of the photosensitive layer coating solution, it is preferable that R.sup.1 and R.sup.2 each represents a hydrocarbon group having greater than 2 carbon atoms, and particularly preferably, R.sup.1 and R.sup.2 are bonded together and form 5- or 6-membered ring.

Ar.sup.1 and Ar.sup.2 may be the same or different and each represents an aromatic hydrocarbon group which may have one or more substituents, Y.sup.1 and Y.sup.2 may be the same or different and each represents a sulfur atom or dialkylmethylene group having no more than 12 carbon atoms, R.sup.3 and R.sup.4 may the same or different and each represents a hydrocarbon group which may have substituents and have no more than 20 carbon atoms. Examples of the preferable substituents include an alkoxy group, carboxyl group, and sulfo group having no more than 12 carbon atoms. R.sup.5, R.sup.6, R.sup.7 and R.sup.8 may be the same or different, and each represents a hydrogen atom or a hydrocarbon group having no more than 12 carbon atoms; a hydrogen atom is preferable in terms of availability of the materials. Further, Z.sup.1-represents a counter anion. However, when any of R.sup.1 to R.sup.8 are substituted with a sulfo group, Z.sup.1-is unnecessary. Examples of the preferable Z.sup.1-include, in terms of the storage stability of the photosensitive layer coating solution, a halogen ion, perchloric acid ion, tetrafluoroborate ion, hexafluorophosphate ion, and sulfonic acid ion, and a perchloric acid ion, hexafluorophosphate ion, and arylsulfonic acid ion are particularly preferable.

Specific examples of the cyanine dyes suitably used in the present invention and represented by the general formula (I) include those described in paragraph numbers [0017] to [0019] of Japanese Patent Application No. 11-310623.

The pigments which can be used in the present invention include commercially available pigments, and pigments described in the Color Index (C. I.) Handbook, "Latest Pigment Handbook" (Saishin Ganryo Binran) edited by Japan Pigment Technical Association, published in 1977, "Latest Pigment Applied Technology" (Saishin Ganryo Oyo Gijutsu), CMC publications, published in 1986 and "Printing Ink Technology" (Insatsu Inki Gijutsu), CMC publications, published in 1984.

Examples of the pigments include black pigments, yellow pigments, orange pigments, brown pigments, red pigments, purple pigments, blue pigments, green pigments, fluorescent pigments, metal powder pigments, and polymer bonded pigments.

These pigments are described in detail in paragraph numbers [0052] to [0054] of Japanese Patent Application Laid-Open (JP-A) No. 10-39509, and those described therein can be applied in the present invention. Among these pigments, carbon black is preferable.

These infrared ray absorbing agents may be added to the same layer together with other components, or another layer may be provided to which the infrared ray absorbing agents are added. In other words, the photosensitive layer may be comprised of multiple layers. When a negative type planographic printing plate precursor is produced using these infrared ray absorbing agents, it is preferable for a photosensitive layer to have an optical density of 0.1 to 3.0 at an absorption maximum at a wavelength of from 760 nm to 1200 nm. In a case in which the optical density is out of the aforementioned range, sensitivity of the photosensitive layer tends to be low. Since the optical density is determined with an added amount of the infrared ray absorbing agent and a thickness of a recording layer, a predetermined optical density can be obtained by controlling the added amount of the infrared ray absorbing agent and the thickness of a recording layer. The optical density of the photosensitive layer can be determined by standard methods. Examples of the determination methods thereof include a method in which a photosensitive layer having a thickness which is suitably determined to have a dried coating weight within a range required for a planographic printing plate is formed on a transparent or white substrate and the optical density of the photosensitive layer is then determined by a transparent type optical densitometer, a method in which a photosensitive layer is formed on a substrate which is made of aluminum or the like and has a reflectivity and a reflection density is then determined, and the like.

[(B) Compounds Which Generate Radicals]

Examples of compounds suitably used in the present invention and generates radicals include onium salts such as iodonium salts, diazonium salts, and sulfonium salts. These onium salts can function as acid generating agents. Moreover, when these onium salts are used together with radical polymerizable compounds, which will be described later, they are function as initiators of radical polymerization. Examples of the onium salts suitably used in the present invention include the onium salts represented by general formulae (II) to (IV) given below.

##STR00005##

In the general formula (II), Ar.sup.11 and Ar.sup.12 each independently represents an aryl group having no more than 20 carbon atoms and optionally having one or more substituents. Examples of the preferable substituents used in a case in which the aryl group has one or more substituents include a halogen atom, nitro group, alkyl group having no more than 12 carbon atoms, alkoxy group having no more than 12 carbon atoms, or aryloxy group having no more than 12 carbon atoms. Z.sup.11-represents a counter ion selected from a group consisting of a halogen ion, perchloric acid ion, tetrafluoroborate ion, hexafluorophosphate ion, and a sulfonic acid ion, and preferably, a counter ion selected from a group consisting of a perchloric acid ion, hexafluorophosphate ion, and arylsulfonic acid ion.

In the general formula (III), Ar.sup.21 represents an aryl group having no more than 20 carbon atoms, and optionally having one or more substituents. Examples of the preferable substituents include a halogen atom, nitro group, alkyl group having no more than 12 carbon atoms, alkoxy group having no more than 12 carbon atoms, aryloxy group having no more than 12 carbon atoms, alkylamino group having no more than 12 carbon atoms, dialkylamino group having no more than 12 carbon atoms, arylamino group having no more than 12 carbon atoms, and diarylamino group having no more than 12 carbon atoms. Z.sup.21-represents a counter ion having the same meaning as the counter ion represented by Z.sup.11-.

In the general formula (IV), R.sup.31, R.sup.32 and R.sup.33 may be the same or different and each represents a hydrocarbon group having no more than 20 carbon atoms, and optionally having one or more substituents. Examples of the preferable substituents include a halogen atom, nitro group, alkyl group having no more than 12 carbon atoms, alkoxy group having no more than 12 carbon atoms, and aryloxy group having no more than 12 carbon atoms. Z.sup.31-represents a counter ion having the same meaning as the counter ion represented by Z.sup.11-.

Specific examples of onium salts which can be suitably used in the present invention include those described in paragraph numbers [0030] to [0033] of Japanese Patent Application No. 11-310623, which was previously proposed by the applicant of the present invention, and those described in paragraph numbers [0015] to [0046] of Japanese Patent Application No. 2000-160323.

Preferably, the onium salts used in the present invention have a maximum absorption wavelength of 400 nm or less, and more preferably, of 360 nm or less. By setting the absorption wavelength as described above, treatments of the planographic printing plate precursor can be carried out under a white light.

These onium salts can be added in a coating solution of a photosensitive layer in an amount of from 0.1 to 50% by weight, preferably from 0.5 to 30% by weight, and particularly preferably from 1 to 20% by weight, based on the total amount of solid contents in the coating solution of the photosensitive layer. An amount of the onium salts of less than 0.1% by weight causes low sensitivity, but an amount of more than 50% by weight produces smudges in non-image portions at the time of printing. The onium salts may be used singly or in combinations of two or more. These salts may be added to the same layer together with other components, or another la