JP2010234589A - Original plate of on-machine development type lithographic printing plate and lithographic printing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an original plate of an on-machine development type lithographic printing plate which can record an image by means of a laser and has excellent plate detectability and resistance to plate wear, and a lithographic printing method. <P>SOLUTION: The original plate of the lithographic printing plate has an image recording layer from which an unexposed part can be removed by supplying a printing ink and damping water on a support. The image recording layer contains (a) an infrared absorbent represented by a formula (1), (b) at least, one polymerization initiator selected from a group composed of a diaryliodonium salt compound and a sulfonium salt compound both of which have different substituting groups, respectively, and (c) a polymerizable compound. In the formula, R<SP>1a</SP>and R<SP>2a</SP>are alkyl groups with oxygen atoms etc., m is an integer of 1 or 2, Y is a dialkylmethylene group etc., Z<SP>1</SP>and Z<SP>2</SP>are aromatic hydrocarbon rings etc., B is an n-value organic group, n is an integer of 2 to 6, and A<SP>-</SP>is a counter anion. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a lithographic printing plate precursor and a lithographic printing method using the same. More specifically, the present invention relates to an on-press development type lithographic printing plate precursor capable of image exposure with a laser, and a lithographic printing method for on-press development of the lithographic printing plate precursor.

In general, a lithographic printing plate comprises an oleophilic image area that receives ink in the printing process and a hydrophilic non-image area that receives dampening water. Lithographic printing utilizes the property that water and oil-based inks repel each other, so that the oleophilic image area of the lithographic printing plate is the ink receiving area, and the hydrophilic non-image area is dampened with the water receiving area (ink non-receiving area). In this method, a difference in ink adhesion is caused on the surface of the lithographic printing plate, and after ink is applied only to the image area, the ink is transferred to a printing medium such as paper and printed.
In order to produce this lithographic printing plate, conventionally, a lithographic printing plate precursor (PS plate) in which an oleophilic photosensitive resin layer (image recording layer) is provided on a hydrophilic support is used. After exposure through a mask such as a film, development with an alkaline developer is performed to leave the image recording layer corresponding to the image area, and dissolve and remove the unnecessary image recording layer corresponding to the non-image area. And obtained a lithographic printing plate.

  Due to recent advances in this field, lithographic printing plates are now available with CTP (computer to plate) technology. That is, a lithographic printing plate can be obtained by scanning and exposing a lithographic printing plate precursor directly using a laser or a laser diode without a lith film, and developing it.

  Along with the above progress, problems related to the lithographic printing plate precursor have been changed to improvements in image formation characteristics, printing characteristics, physical characteristics and the like corresponding to the CTP technology. In addition, due to increasing interest in the global environment, environmental issues related to waste liquids associated with wet processing such as development processing have been highlighted as another issue related to lithographic printing plate precursors.

For the above environmental problems, simplification and no processing of development or plate making are directed. As one simple plate making method, a method called “on-press development” is performed. That is, after the exposure of the lithographic printing plate precursor, conventional development is not performed, but it is mounted on a printing machine as it is, and unnecessary portions of the image recording layer are removed at the initial stage of a normal printing process.
In addition, as a simple development method, a method called “gum development” in which unnecessary portions of the image recording layer are removed with a finisher or gum solution having a pH close to neutral instead of a conventional highly alkaline developer is also performed. ing.

  In the simplification of the plate making operation as described above, a system using a lithographic printing plate precursor and a light source that can be handled in a bright room or under a yellow light is preferable from the viewpoint of ease of work. A solid-state laser such as a semiconductor laser or a YAG laser emitting an infrared ray of 1200 nm is used. Further, a UV laser can be used.

  As a lithographic printing plate capable of on-press development, for example, in Patent Documents 1 and 2, a lithographic printing plate precursor having an image recording layer (thermosensitive layer) containing a microcapsule containing a polymerizable compound on a hydrophilic support. Is described. Patent Document 3 describes a lithographic printing plate precursor in which an image recording layer (photosensitive layer) containing an infrared absorber, a radical polymerization initiator, and a polymerizable compound is provided on a support. Furthermore, in Patent Document 4, on-press development is possible in which an image recording layer containing a polymerizable compound and a graft polymer having a polyethylene oxide chain in the side chain or a block polymer having a polyethylene oxide block is provided on a support. A planographic printing plate precursor is described.

In general, as a pre-process for attaching the printing plate to the printing machine, an operation (inspection) for inspecting and identifying the image on the printing plate is performed to check whether the image is recorded on the printing plate as intended. In general lithographic printing plate precursors with development processing steps, colored images can be obtained by development processing if the image recording layer is colored, so it is easy to check the image before attaching the printing plate to the printing press. is there.
However, with an on-press development type or no processing (no development) type lithographic printing plate precursor that does not undergo development processing, there is no image on the printing plate at the stage of attaching the printing plate to the printing press, and plate inspection cannot be performed. In particular, whether or not it is possible to determine that a register mark (register mark) that serves as a registration mark in multi-color printing is drawn is important for the printing operation. Therefore, the on-press development type or non-processed (no development) type lithographic printing plate precursor is required to produce a so-called print-out image in which the exposed area is colored or decolored, that is, means for confirming the image at the stage of exposure. Has been.

A lithographic printing plate precursor using a compound that generates an acid, base, or radical by light or heat and a compound that changes color by interacting with the generated acid, base, or radical has been proposed (for example, , See Patent Document 5). It has also been proposed to use the color change of a thermally decomposable compound as a printing-out agent for a direct-drawing lithographic printing plate precursor having a heat-sensitive layer (see, for example, Patent Document 6). Furthermore, it has also been proposed to use a thermally decomposable dye having a thermal decomposition temperature of 250 ° C. or lower as a print-out agent (see, for example, Patent Document 7).
According to these, coloring or decoloring occurs in the exposed area, and the plate inspection performance of the image is improved to some extent, but it is still not sufficient.

Patent Document 8 discloses a print-out image at a level that has good visibility and can be inspected by a system containing a cyanine dye-based infrared absorber having a methine chain and a radical generator. It is described that it is obtained. However, there is a demand from the market for further excellent plate inspection.
Patent Document 9 contains an infrared absorber having two or more chromophores that can absorb infrared rays in the same molecule, and these chromophores are bonded by a covalent bond, and image formation is performed by infrared exposure. A lithographic printing plate precursor provided with possible recording layers is described. However, the development method of the lithographic printing plate precursor described in Patent Document 9 is alkali development, and the lithographic printing plate precursor of the system that is directly attached to a printing machine and printed without performing development processing is described in Patent Document 9 On-machine developability deteriorates when an infrared absorbing dye is contained.

JP 2001-277740 A JP 2001-277742 A JP 2002-287334 A US Patent Application Publication No. 2003/0064318 JP-A-11-277927 JP 2000-335129 A JP 2003-191657 A JP 2007-090850 A JP 2001-242613 A

  An object of the present invention is to provide an on-press development type lithographic printing plate precursor and a lithographic printing method which are capable of image recording with a laser and are excellent in plate inspection and printing durability.

  The inventors have two or more chromophores that can absorb infrared rays in the same molecule, and by using an infrared absorber in which the chromophores are bonded by a covalent bond, plate inspection and printing durability can be improved. It has been found that an excellent on-press development type lithographic printing plate precursor can be obtained. The present invention is as follows.

  1. A lithographic printing plate precursor having an image recording layer capable of removing an unexposed portion by supplying a printing ink and a fountain solution on a support, and (a) an infrared ray represented by the general formula (1) A lithographic printing comprising an image recording layer containing an absorber, (b) at least one polymerization initiator selected from a sulfonium salt compound and an asymmetric diaryliodonium salt compound, and (c) a polymerizable compound Version original edition.

In the general formula (1), R ^1a and R ^2a each independently represents an alkyl group having at least one atom selected from an oxygen atom, a sulfur atom and a nitrogen atom. m represents an integer of 1 or 2. Y may be the same or different and each represents a sulfur atom, an oxygen atom or a dialkylmethylene group having 12 or less carbon atoms. Z ¹ and Z ² each independently represents an aromatic hydrocarbon ring or a heteroaromatic ring. B represents an n-valent organic group, and n represents an integer of 2 to 6. A ⁻ represents a counter anion selected from halide ion, perchlorate ion, tetrafluoroborate ion, hexafluorophosphate ion and sulfonate ion.

2. R ^1a and R ^2a in the general formula (1) are alkyl groups having at least one group selected from an ether group, an ester group, an amide group, a thioether group, and a ketone group, 1. The lithographic printing plate precursor as described in 1.
3. 3. The lithographic printing plate precursor as described in 1 or 2 above, wherein R ^1a and R ^2a in the general formula (1) are alkyl groups having at least one group selected from an ether group and an ester group .

  4). B in the general formula (1) is represented by the following general formula (2) and n is 2, or B in the general formula (1) is represented by the general formula (3), and n 3. The lithographic printing plate precursor as described in any one of 1 to 3 above, wherein

In general formula (2) and general formula (3), D represents a sulfur atom, an oxygen atom, or NR each independently. R represents a hydrogen atom or an alkyl group. X ¹ represents a single bond, a sulfonyl group, a carbonyl group, an azo group, a phenylene group, or a group represented by the following general formula (4). X ² represents a nitrogen atom, a methine group (—CH <), or an aromatic hydrocarbon group. R ^1b and R ^2b each independently represent a hydrogen atom or an alkyl group. k represents an integer of 1 or 2.

5). 5. The lithographic printing plate precursor as described in 4 above, wherein D in the general formula (2) or the general formula (3) is a sulfur atom or an oxygen atom.
6). The lithographic printing plate precursor as described in any one of 1 to 5 above, wherein the iodonium salt compound has an electron donating group.
7). The lithographic printing plate precursor as described in any one of 1 to 5 above, wherein the sulfonium salt compound has an electron-withdrawing group.
8). The lithographic printing plate precursor as described in any one of 1 to 7 above, wherein the image recording layer contains a binder polymer having a hydrophilic group.
9. The lithographic printing plate precursor as described in any one of 1 to 8 above, wherein the image recording layer contains a hydrophobized precursor.
10. Any of 1 to 9 above, wherein the lithographic printing plate precursor has an undercoat layer between the support and the image recording layer, and the undercoat layer contains a polymer having a substrate-adsorbing group and a polymerizable group. The lithographic printing plate precursor as described in 1 above.
11. The lithographic printing plate precursor as described in any one of 1 to 10 above, which further comprises a protective layer containing an inorganic stratiform compound on the image recording layer.
12 The lithographic printing plate precursor as described in 1 to 11 above is imagewise exposed with an infrared laser, and printing ink and fountain solution are supplied without performing any development treatment on the exposed lithographic printing plate precursor. And a printing process for printing, and the infrared laser unexposed portion of the image recording layer is removed during the printing process.

In the present invention, an on-press development type lithographic printing plate precursor excellent in plate inspection and printing durability was obtained by using the specific infrared absorber as described above.
The coloring mechanism of this lithographic printing plate precursor is considered as follows. By exposing infrared light to a lithographic printing plate precursor, the infrared absorber is excited, electron transfer occurs in the specific radical generator, and at that time, the structure of the infrared absorber changes (generation of radical cations), causing a color change. I think because. By using a specific radical initiator, it is considered that electron transfer efficiently occurs and proceeds from the infrared absorber, resulting in a large color change and excellent plate inspection.
The function expression of the present invention is considered as follows. Other molecules that have two or more chromophores that can absorb infrared rays in the same molecule and have a multi-layered structure due to the intermolecular interaction between the chromophores linked by covalent bonds. It is considered that the radical cation, which is the main color former, has a longer lifetime and the color stability over time is improved. In addition, by including an alkyl group having at least one atom selected from an oxygen atom, a sulfur atom and a nitrogen atom in the infrared absorber, the image recording layer can be applied without depositing a solid under a high concentration condition. Since the dye and the initiator are in the vicinity, electron transfer is likely to occur, and high printing durability can be obtained. Furthermore, it has been found that the on-press developability is improved by incorporating the infrared absorbent of the present invention in the image recording layer.

  According to the present invention, it is possible to provide an on-press development type lithographic printing plate precursor capable of recording an image with a laser and having excellent plate inspection and printing durability.

[Lithographic printing plate precursor]
The lithographic printing plate precursor according to the invention has an image recording layer on a support and can be developed on-press by supplying printing ink and / or fountain solution. In some cases, a protective layer may be provided on the image recording layer, and an undercoat layer may be provided between the support and the image recording layer.
The components and components of the lithographic printing plate precursor according to the invention will be described below.

(Image recording layer)
The image recording layer used in the present invention is an image recording layer capable of forming an image after exposure by supplying printing ink and fountain solution on a printing machine to remove unexposed portions. Typical image forming mechanisms that can be developed on-press contained in the image recording layer include (1) (A) an infrared absorber, (B) a polymerization initiator, and (C) a polymerizable compound. A mode in which an image portion is cured by using a polymerization reaction, and (2) (A) an infrared absorber and (D) a hydrophobized precursor are used, and heat fusion or thermal reaction of the hydrophobized precursor is utilized. Thus, a mode in which a hydrophobic region (image portion) is formed can be exemplified. Further, a mixture of the above two embodiments may be used. For example, (1) the polymerization type image recording layer may contain (D) a hydrophobized precursor, or (2) the hydrophobized precursor type may contain a polymerizable compound. Especially, the aspect of the polymerization type containing (A) infrared absorber, (B) polymerization initiator, and (C) polymeric compound is preferable.
Below, each component used for an image recording layer is demonstrated one by one.

<(A) Infrared absorber represented by general formula (1)>
The image recording layer of the present invention contains an infrared absorber represented by the general formula (1). The infrared absorber represented by the general formula (1) is a cyanine dye that contains a plurality of cyanine dye skeletons in the molecule and has a maximum absorption in the infrared wavelength region, and is hereinafter also referred to as a specific cyanine dye.

  Since the specific cyanine dye has a function as an infrared absorber, the lithographic printing plate precursor according to the present invention can form an image using a laser emitting an infrared ray of 760 to 1200 nm as a light source. In the present invention, the infrared absorbent refers to a compound having a function of converting absorbed infrared light into heat and a function of being excited by infrared light and transferring electrons and / or energy to a polymerization initiator (radical generator) described later. . In addition to the specific cyanine dye, another infrared absorber, that is, a dye or pigment having an absorption maximum at a wavelength of 760 to 1200 nm can be used in combination with the image recording layer according to the present invention.

In general formula (1), R ^1a and R ^2a each independently represents an alkyl group having at least one atom selected from a sulfur atom, an oxygen atom and a nitrogen atom. R ^1a and R ^2a are preferably an alkyl group having an ether group, an ester group, an amide group, a thioether group, a ketone group, a sulfonyl acid group, an amino group, a sulfonylamide group or a hydroxy group, more preferably , An alkyl group having an ether group, an ester group, an amide group, a thioether group or a ketone group. Of these, an alkyl group having an ether group, an ester group or an amide group is more preferred, and an alkyl group having an ether group or an ester group is most preferred.

  B represents an n-valent organic group, and n represents an integer of 2 to 6. Preferable examples include those in which B contains two or more aromatic rings. More preferably, n = 2 or 3, and most preferably n = 2.

  Preferred organic groups represented by B include organic groups represented by the following general formula (2) or (3).

  In general formula (2) and general formula (3), D represents a sulfur atom, an oxygen atom, or NR each independently. R represents a hydrogen atom or an alkyl group. Of these, D is preferably a sulfur atom or an oxygen atom.

X ¹ represents a single bond, a sulfonyl group, a carbonyl group, an azo group, a phenylene group, or a group represented by the following general formula (4). In General Formula (4), R ^1b and R ^2b each independently represent a hydrogen atom or an alkyl group. The alkyl group preferably has 1 to 10 carbon atoms. k represents an integer of 1 or 2.
X ² represents a nitrogen atom, a methine group (—CH <) or an aromatic hydrocarbon group.

Of these, sulfonyl is most preferred as X ¹ .

  In general formula (1), m represents an integer of 1 or 2. m is most preferably m = 1 (that is, a 5-membered ring is formed) from the viewpoint of improving the visibility of the color image.

Z ¹ and Z ² each independently represents an aromatic hydrocarbon ring or a heteroaromatic ring. An aromatic hydrocarbon ring which may have a substituent is preferable, and specific examples include a benzene ring and a naphthalene ring. Moreover, as a preferable substituent, a C12 or less hydrocarbon group and a halogen atom are mentioned.

  Y may be the same or different and each represents a sulfur atom, an oxygen atom or a dialkylmethylene group having 12 or less carbon atoms.

A ⁻ represents a counter anion selected from halide ion, perchlorate ion, tetrafluoroborate ion, hexafluorophosphate ion and sulfonate ion. These anions are preferable from the viewpoint of storage stability of the image recording layer coating solution. Particularly preferred are perchlorate ion, hexafluorophosphate ion, and aryl sulfonate ion. Further, from the viewpoint of improving visibility, A ⁻ is preferably an inorganic anion or a counter anion of a strong acid. From such a viewpoint, tetrafluoroborate ion, hexafluorophosphate ion, CF ₃ SO ₃ ⁻ , C ₄ F ₉ SO ₃ ^— , and hexafluorophosphate ion is most preferable.

  Specific examples of the specific cyanine dye that can be suitably used in the present invention include the following compounds (IR-1) to (IR-32), but the present invention is not limited thereto.

  The specific cyanine dye of the present invention can be synthesized, for example, by the method specifically shown in Synthesis Example 1 described later. In addition, various cyanine dyes having the above-described structure can be synthesized in the same manner by changing starting materials, solvents and the like according to the scheme.

The specific cyanine dye is contained in the image recording layer. Although it may be added to another layer, when a negative lithographic printing plate precursor is produced, the absorbance at the maximum absorption wavelength in the wavelength range of 760 nm to 1200 nm of the image recording layer is 0. Add in a range of 3 to 1.2. Preferably, it is the range of 0.4-1.1. Within this range, a uniform polymerization reaction proceeds in the depth direction of the image recording layer, and good film strength of the image area and adhesion to the support can be obtained.
The absorbance of the image recording layer can be adjusted by the amount of infrared absorber added to the image recording layer and the thickness of the image recording layer. The absorbance can be measured by a conventional method. As a measuring method, for example, on a reflective support such as aluminum, an image recording layer having a thickness appropriately determined in a range in which the coating amount after drying is necessary as a lithographic printing plate is formed, and the reflection density is set to the optical density. And a method of measuring with a spectrophotometer by a reflection method using an integrating sphere.

  The amount of the specific cyanine dye added to the image recording layer is preferably 0.1 to 30% by mass, more preferably 0.5 to 20% by mass, more preferably 1 to 10% by mass based on the total solid content. % Addition is more preferable.

<Other infrared absorbers>
Examples of infrared absorbers that can be used in combination include the compounds described in paragraph numbers [0058] to [0087] of JP-A-2008-195018.

<(B) Polymerization initiator>
The (B) polymerization initiator used in the present invention refers to a compound that generates radicals by light, heat, or both, and initiates and accelerates polymerization of the polymerizable compound (C). Examples of the polymerization initiator that can be used in the present invention include (1) an asymmetric diaryliodonium salt compound or (2) a sulfonium salt compound. An onium salt generally used as a polymerization initiator has sufficient polymerization sensitivity and color development sensitivity, but has insufficient stability as a light-sensitive material, and is difficult to use.
The asymmetric iodonium salt used in the present invention means that two aryl groups adjacent to the iodonium cation are different.
Among these, diaryl iodonium salts each having a different substituent on the aryl group are preferred. Furthermore, examples of the aryl group include an aryl group composed of a benzene ring or a naphthalene ring, but an aryl group composed of a benzene ring is particularly preferable, and a group represented by the formula (5) is particularly preferable.

  (1) The diaryl iodonium salt compound having different substituents used in the present invention refers to a compound represented by the following general formula (5).

In General Formula (5), Ar ₁₀₁ and Ar ₁₀₂ are benzene rings which may have a substituent, the substituents of the two benzene rings are different from each other, and at least one of the benzene rings is This represents a structure in which the sum of Hammett σ values of substituents is negative. Z ⁻ represents a counter anion.

  In the general formula (5), the substituents of the two benzene rings are different from each other, not only when the substituents of the two benzene rings are different, but also when one of the benzene rings has a substituent. In addition, the case where the other benzene ring does not have a substituent is also included.

  The iodonium salt represented by the general formula (5) is preferably represented by the following general formula (6).

In General Formula (6), Ar ₂₀₁ represents a benzene ring having a substituent, and the sum of Hammett σ values of the substituent is negative. Ar ₂₀₂ represents a benzene ring having a substituent, and the sum of Hammett σ values of the substituent is positive. Z ⁻ represents a counter anion.

In the general formulas (5) and (6), the Z ⁻ counter anion is preferably a halide ion, a perchlorate ion, a tetrafluoroborate ion, a hexafluorophosphate ion, a carboxylate ion, or a sulfonate ion. More preferred are tetrafluoroborate ion, hexafluorophosphate ion and perfluoroalkyl sulfonate ion.

The Hammett σ value is the effect of meta- and para-substituents on the acid dissociation equilibrium of benzoic acid in water at 25 ° C as described in the reference "Chemical Seminar 10 Hammett-Structure and Reactivity". This is the value shown. If it is negative, it is electron donating, and if it is positive, it is electron withdrawing. The ortho substituent is calculated by substituting the value of the p-position substituent.
When calculating the sum of Hammett σ values, the σm value is used for substituents in the m position relative to the C—I bond, and the σp value is used for substituents in the p position.
In the iodonium salt of the present invention, any iodonium salt can be suitably used as long as these Hammett σ values are summed in each benzene ring and the sum of at least one is negative.
In the iodonium salt in the present invention, it is preferable that the sum of Hammet α values of the substituents of the two benzene rings is 0 or less.

The iodonium salt of the present invention is any one as long as the substituents of the two benzene rings are different from each other in the general formula (5) and the sum of Hammett σ values of the substituents of at least one benzene ring is negative. The iodonium salt may be a diaryliodonium salt having two or more electron-withdrawing groups, more preferably three or more electron-donating groups.
Preferred examples of the electron donating group include an alkyl group, amino group, alkoxy group, urea group, alkoxyalkyl group, acyloxyamino group, cycloalkyl group, and allyl group. You may have substituents, such as an alkyl group, an alkenyl group, an aryl group, a hydroxyl group, an alkoxy group, a thiol group, a thioalkoxy group, an amino group, a halogen atom.
Among the above, particularly preferable electron donating groups include an alkyl group and an alkoxy group, and the most preferable substituent is an alkoxy group.

Preferable specific examples of the electron donating group include an alkyl group having 1 to 20 carbon atoms, such as a methyl group, an n-butyl group, a t-butyl group, an n-octyl group, and a dodecyl group. Examples of the alkoxy group include a methoxy group, an ethoxy group, a butoxy group, a heteroxy group, an octyloxy group, a dodecyloxy group, and a cyclohexyl group.
The number of substitution of the electron donating group is more preferably 3 or more, and most preferably a 4-substituted compound.
The electron donating group is introduced into the aryl group of the iodonium salt, and preferred substitution positions include para-position and ortho-position. One or more electron-donating groups may be introduced into both of the two aryl groups in the iodonium salt, or two or more electron-donating groups may be introduced into only one of them.
As a combination, a compound having an alkoxy group with 3 or more substitutions, more preferably 4 or more substitutions in the polymerization initiator molecule is preferable.

If the standard as a physical property value in introducing an electron donating group is given, it is preferable that the sum of Hammett values of substituents (electron donating groups introduced into an aryl group) is −0.27 or less, -0.54 or less is more preferable, and -0.84 or less is most preferable. The Hammett value represents the degree of electron withdrawing of the substituent in the diaryliodonium salt structure, and the Hammett value in the present invention is edited by the Chemical Society of Japan, Chemistry Handbook, Basic II (1984, Maruzen Co., Ltd.). (Issued) can be referred to.
The Hammett value is usually calculated with the values of the substitution positions at the m-th and p-th positions. This time, as an electronic effect, the value of the o-th time is calculated as the same value as the value of the p-th place.

The counter anion of the iodonium salt of the present invention can be used without limitation as long as it has an anion structure. Usable counter anions include carboxylate anion, sulfonate anion, sulfonamide anion, borate anion, phosphate anion, sulfinate anion, sulfate anion, PF ₆ ⁻ , BF ₄ ⁻ , SbF ₆ ⁻ , halide anion and the like. Can be mentioned. Among these, sulfonic acid anions, particularly sulfonic acid anions having a fluoroalkyl group, PF ₆ ⁻ and BF ₄ ⁻ are preferable from the viewpoints of stability and color developability.
Examples of iodonium salts (I-1) to (I-22) that can be used in the present invention will be given below, but the present invention is not limited thereto.

The (2) sulfonium salt used in the present invention refers to triarylsulfonium.
Triarylsulfonium is a monovalent cation having a molecular structure in which three aryl groups are bonded to a sulfur atom.
Salts consisting of triarylsulfonium and a counter anion are known to function as polymerization initiators (see, for example, J. Amer. Chem. Soc. 112 (16), 1990, p6004-6015; Chem., 1988, pp 5571-5573; WO 02/081439; described in EP-A-1113005).
The aryl group is preferably phenyl or naphthyl, and more preferably phenyl. The aryl group may have a substituent, and it is preferable that at least one aryl group is substituted with an electron-withdrawing group.

  The electron withdrawing group substituting the aryl group of the triarylsulfonium salt preferably has a sum of Hammett values greater than 0.46, and more preferably greater than 0.50. Most preferably, the value is larger than 0.60. Further, the total sum of Hammett values is preferably less than 4.0, more preferably less than 3.5, and most preferably less than 3.0.

The electron withdrawing group includes a halogen atom, a halogenated alkyl group, an acyl group, an acyloxy group, an alkanesulfinyl group, cyano, an amide group, and carboxyl.
The halogen atom includes a fluorine atom (the Hammett value is meta position (hereinafter, m): 0.34, para position (hereinafter, p): 0.06, chlorine atom (m: 0.37, p: 0.23). ), A bromine atom (m: 0.37, p: 0.23), and a chlorine atom (m: 0.35, p: 0.18). It is almost the same value (the same is true for other groups).
Halogenated alkyl groups include trifluoromethyl (m: 0.43, p: 0.54).
Acyl groups include acetyl (o: 0.37, p: 0.45) and formyl (m: 0.36, p: 0.43).
The alkanesulfinyl group includes a methanesulfinyl group (m: 0.52, p: 0.45).
The Hammett value of the cyano group is (m: 0.56, p: 0.66).
The amide group includes acetamide (m: 0.21, p: 0.00).
The Hammett value of the carboxy group is (m: 0.37, p: 0.45).

The electron withdrawing group is preferably a halogen atom or a halogenated alkyl group, more preferably a fluorine atom, a chlorine atom, a bromine atom or trifluoromethyl, and most preferably a chlorine atom.
Electron withdrawing groups can be introduced into multiple aryl groups of triarylsulfonium. One aryl group may be substituted with a plurality of electron withdrawing groups.
It is preferable that all of the aryl groups of the triarylsulfonium salt are substituted with halogen atoms, and it is particularly preferable that all of the aryl groups of the triarylsulfonium salt are substituted with chlorine atoms.
The following are examples of triarylsulfonium in which at least one aryl group is substituted with an electron withdrawing group.
The counter anion constituting the triarylsulfonium salt includes sulfonate ion, sulfinate ion, carboxylate ion (eg, benzoylformate ion), borate ion, halide ion (F ⁻ , Cl ⁻ , Br ⁻ I ^−). ), SO ₃ ²⁻ , HSO ₃ ⁻ , PF ₆ ⁻ , BF ₄ ⁻ and ClO ₄ ⁻ . Sulfonate ions and benzoylformate ions are more preferred, and sulfonate ions are most preferred.
Examples of triarylsulfonium salts are shown below by citing the exemplified numbers of the respective ions, but the sulfonium salts used in the present invention are not limited to these.

  In addition to the above, the sulfonium salts described in paragraph numbers [0041] to [0052] of JP-A-2008-200891 are also suitable.

Among the above polymerization initiators, those having an inorganic anion such as PF ₆ ⁻ and BF ₄ ⁻ as a counter ion are preferable from the viewpoint of improving the visibility of the printout image. Furthermore, diaryliodonium is preferred as the onium salt because of its excellent color development.

  The polymerization initiator can be added at a ratio of 0.1 to 40% by mass, preferably 0.5 to 30% by mass, particularly preferably 1 to 20% by mass, based on the total solid content constituting the image recording layer. . Within this range, good sensitivity and good stain resistance of the non-image area during printing can be obtained. These polymerization initiators may be used alone or in combination of two or more. These polymerization initiators may be added to the same layer as other components, or another layer may be provided and added thereto.

  Moreover, in this invention, in addition to the said polymerization initiator, it is possible to use a well-known polymerization initiator together in the range which does not lose the effect of this invention. The amount of the polymerization initiator used in combination is preferably 50% by mass or less, preferably 25% by mass or less, and most preferably 10% by mass or less based on the total mass of the polymerization initiator.

(C) Polymerizable compound The polymerizable compound that can be used in the present invention is an addition polymerizable compound having at least one ethylenically unsaturated double bond, and preferably has at least one terminal ethylenically unsaturated bond, preferably It is preferably selected from compounds having two or more. Such a compound group is widely known in the industrial field, and can be used without any particular limitation in the present invention. These have chemical forms such as monomers, prepolymers, i.e. dimers, trimers and oligomers, or mixtures thereof and their (co) polymers.

  Specific examples include the compounds described in JP-A-2008-105018, paragraph numbers [0089] to [0098]. Among them, preferred are esters of aliphatic polyhydric alcohol compounds and unsaturated carboxylic acids (for example, acrylic acid, methacrylic acid, itaconic acid, crotonic acid, isocrotonic acid, maleic acid and the like). Another preferred polymerizable compound is a polymerizable compound having an isocyanuric acid structure described in JP-A-2005-329708.

  Among them, tris (acryloyloxyethyl) isocyanurate, bis (acryloyloxyethyl) hydroxyethyl isocyanurate, etc. are excellent in the balance between the hydrophilicity involved in on-press developability and the polymerization ability involved in printing durability. Isocyanuric acid ethylene oxide modified acrylates are particularly preferred.

  In the present invention, the polymerizable compound is used in an amount of preferably 5 to 80% by mass, more preferably 25 to 75% by mass, based on the total solid content of the image recording layer.

(D) Hydrophobized precursor In the present invention, a hydrophobized precursor can be used to improve on-press developability. The hydrophobized precursor in the present invention means fine particles capable of converting the image recording layer to hydrophobic when heat is applied. The fine particles are preferably at least one particle selected from hydrophobic thermoplastic polymer fine particles, heat-reactive polymer fine particles, microcapsules enclosing a hydrophobic compound, and microgel (crosslinked polymer fine particles). Among these, polymer fine particles and microgels having a polymerizable group are preferable.

Hydrophobic thermoplastic polymer fine particles include Research Disclosure No. 1 of January 1992. 33303, hydrophobic thermoplastic polymer fine particles described in JP-A-9-123387, JP-A-9-131850, JP-A-9-171249, JP-A-9-171250 and European Patent No. 931647 are suitable. Can be cited as
Specific examples of polymers constituting such polymer fine particles include ethylene, styrene, vinyl chloride, methyl acrylate, ethyl acrylate, methyl methacrylate, ethyl methacrylate, vinylidene chloride, acrylonitrile, vinyl carbazole, and polyalkylene structures. Mention may be made of homopolymers or copolymers of monomers such as acrylates or methacrylates or mixtures thereof. Among them, more preferred are polystyrene and polymethyl methacrylate.

  The average particle diameter of the hydrophobic thermoplastic polymer fine particles used in the present invention is preferably 0.01 to 2.0 μm.

  Examples of the heat-reactive polymer fine particles used in the present invention include polymer fine particles having a heat-reactive group, and these form a hydrophobized region by cross-linking by a heat reaction and a functional group change at that time.

  The thermally reactive group in the polymer fine particle having a thermally reactive group used in the present invention may be any functional group that performs a reaction as long as a chemical bond is formed, but is an ethylenically unsaturated group that performs a radical polymerization reaction. Group (for example, acryloyl group, methacryloyl group, vinyl group, allyl group, etc.), cationically polymerizable group (for example, vinyl group, vinyloxy group, etc.), isocyanate group that performs addition reaction or its block, epoxy group, vinyloxy group And a functional group having an active hydrogen atom as a reaction partner (for example, an amino group, a hydroxy group, a carboxy group, etc.), a carboxy group for performing a condensation reaction, a hydroxy group or an amino group as a reaction partner, and a ring-opening addition reaction. List preferred acid anhydrides and amino or hydroxy groups that are reaction partners It can be.

  As the microcapsules used in the present invention, for example, as described in JP-A Nos. 2001-277740 and 2001-277742, all or part of the constituent components of the image recording layer are encapsulated in the microcapsules. Is. The constituent components of the image recording layer can also be contained outside the microcapsules. Furthermore, the image recording layer containing microcapsules is preferably a mode in which hydrophobic constituent components are encapsulated in microcapsules and hydrophilic constituent components are contained outside the microcapsules.

  In this invention, the aspect containing a crosslinked resin particle, ie, a microgel, may be sufficient. This microgel can contain a part of the constituents of the image recording layer in and / or on the surface thereof. In particular, an embodiment in which a reactive microgel is formed by having a polymerizable compound on the surface is used for image formation. Particularly preferred from the viewpoint of sensitivity and printing durability.

  As a method for microencapsulating or microgelling the constituent components of the image recording layer, known methods can be applied.

  The average particle size of the microcapsules or microgel is preferably 0.01 to 3.0 μm. 0.05-2.0 micrometers is further more preferable, and 0.10-1.0 micrometer is especially preferable. Within this range, good resolution and stability over time can be obtained.

  The content of the hydrophobizing precursor is preferably in the range of 5 to 90% by mass in terms of the solid content concentration of the image recording layer.

(E) Other components The image recording layer in the invention may further contain other components as required.

(1) Binder polymer In the image recording layer of the present invention, a binder polymer can be used in order to improve the film strength of the image recording layer. As the binder polymer that can be used in the present invention, conventionally known binder polymers can be used without limitation, and polymers having film properties are preferred. Of these, acrylic resins, polyvinyl acetal resins, and polyurethane resins are preferable.

  Among them, as a binder polymer suitable for the present invention, as described in JP-A-2008-195018, a crosslinkable functional group for improving the film strength of an image portion is a main chain or a side chain, preferably a side chain. Have the following. Crosslinking is formed between the polymer molecules by the crosslinkable group, and curing is accelerated.

  The crosslinkable functional group is preferably an ethylenically unsaturated group such as a (meth) acryl group, a vinyl group or an allyl group, or an epoxy group, and these groups can be introduced into the polymer by polymer reaction or copolymerization. . For example, a reaction between an acrylic polymer or polyurethane having a carboxy group in the side chain and polyurethane and glycidyl methacrylate, or a reaction between a polymer having an epoxy group and an ethylenically unsaturated group-containing carboxylic acid such as methacrylic acid can be used.

  The content of the crosslinkable group in the binder polymer is preferably 0.1 to 10.0 mmol, more preferably 1.0 to 7.0 mmol, and most preferably 2.0 to 5.5 mmol per 1 g of the binder polymer. .

  Moreover, it is preferable that the binder polymer of this invention has a hydrophilic group further. The hydrophilic group contributes to imparting on-press developability to the image recording layer. In particular, the coexistence of the crosslinkable group and the hydrophilic group makes it possible to achieve both printing durability and developability.

  Examples of the hydrophilic group include a hydroxy group, a carboxy group, an alkylene oxide structure, an amino group, an ammonium group, an amide group, a sulfo group, and a phosphoric acid group. Among them, an alkylene oxide unit having 2 or 3 carbon atoms. An alkylene oxide structure having 1 to 9 is preferable. In order to impart a hydrophilic group to the binder polymer, a monomer having a hydrophilic group may be copolymerized.

  In addition, in the binder polymer of the present invention, an oleophilic group such as an alkyl group, an aryl group, an aralkyl group, or an alkenyl group can be introduced in order to control the inking property. Specifically, a lipophilic group-containing monomer such as an alkyl methacrylate may be copolymerized.

  Specific examples (1) to (11) of the binder polymer used in the present invention are shown below, but the present invention is not limited thereto.

  The binder polymer in the present invention preferably has a mass average molar mass (Mw) of 2,000 or more, more preferably 5,000 or more, and even more preferably 10,000 to 300,000.

  In the present invention, if necessary, hydrophilic polymers such as polyacrylic acid and polyvinyl alcohol described in JP-A-2008-195018 can be used. Further, a lipophilic binder polymer and a hydrophilic binder polymer can be used in combination.

  The content of the binder polymer is preferably 5 to 90% by mass, more preferably 5 to 80% by mass, and still more preferably 10 to 70% by mass with respect to the total solid content of the image recording layer.

(2) Low molecular weight hydrophilic compound The image recording layer in the invention may contain a low molecular weight hydrophilic compound in order to improve the on-press developability without reducing the printing durability.
Examples of the low molecular weight hydrophilic compound include, for example, water-soluble organic compounds such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol and the like and ether or ester derivatives thereof, glycerin, Polyhydroxys such as pentaerythritol and tris (2-hydroxyethyl) isocyanurate, organic amines such as triethanolamine and diethanolamine monoethanolamine and salts thereof, organic sulfones such as alkylsulfonic acid, toluenesulfonic acid and benzenesulfonic acid Acids and salts thereof, organic sulfamic acids such as alkylsulfamic acid and salts thereof, organic sulfuric acids such as alkylsulfuric acid and alkylethersulfuric acid and salts thereof, phenylphospho Organic phosphonic acids and salts thereof such as an acid, tartaric acid, oxalic acid, citric acid, malic acid, lactic acid, gluconic acid, organic carboxylic acids and salts thereof such as amino acids, betaines, and the like.

  Among these, in the present invention, it is preferable to contain at least one selected from the group consisting of polyols, organic sulfates, organic sulfonates, and betaines.

  Specific examples of organic sulfonates include alkyl sulfonates such as sodium n-butyl sulfonate, sodium n-hexyl sulfonate, sodium 2-ethylhexyl sulfonate, sodium cyclohexyl sulfonate, and sodium n-octyl sulfonate. Sodium 5,8,11-trioxapentadecane-1-sulfonate, sodium 5,8,11-trioxaheptadecane-1-sulfonate, 13-ethyl-5,8,11-trioxaheptadecane-1; Alkyl sulfonates containing ethylene oxide chains such as sodium sulfonate, 5,8,11,14-tetraoxatetradecosane-1-sodium sulfonate; sodium benzenesulfonate, sodium p-toluenesulfonate, p-hydroxy Benzenesulfo Acid sodium, sodium p-styrenesulfonate, sodium dimethyl-5-sulfonate isophthalate, sodium 1-naphthylsulfonate, sodium 4-hydroxynaphthylsulfonate, disodium 1,5-naphthalenedisulfonate, 1,3,6 -Aryl sulfonates such as trisodium naphthalene trisulfonate. The salt may be a potassium salt or a lithium salt.

  Organic sulfates include sulfates of alkyl, alkenyl, alkynyl, aryl or heterocyclic monoethers of polyethylene oxide. The ethylene oxide unit is preferably 1 to 4, and the salt is preferably a sodium salt, potassium salt or lithium salt.

  As the betaines, compounds having 1 to 5 carbon atoms of the hydrocarbon substituent on the nitrogen atom are preferable. Specific examples include trimethylammonium acetate, dimethylpropylammonium acetate, 3-hydroxy-4-trimethyl. Ammonioobylate, 4- (1-pyridinio) butyrate, 1-hydroxyethyl-1-imidazolioacetate, trimethylammonium methanesulfonate, dimethylpropylammonium methanesulfonate, 3-trimethylammonio-1-propanesulfonate, Examples include 3- (1-pyridinio) -1-propanesulfonate.

  The above low molecular weight hydrophilic compound has a small hydrophobic part structure and almost no surface-active action, so that dampening water penetrates into the exposed part of the image recording layer (image part) and the hydrophobicity and film strength of the image part. Ink acceptability and printing durability of the image recording layer can be maintained satisfactorily.

The amount of these low molecular weight hydrophilic compounds added to the image recording layer is preferably 0.5% by mass or more and 20% by mass or less of the total solid content of the image recording layer. More preferably, they are 1 mass% or more and 10 mass% or less, More preferably, they are 2 mass% or more and 8 mass% or less. In this range, good on-press developability and printing durability can be obtained.
These compounds may be used alone or in combination of two or more.

(3) Grease Sensitizer In the image recording layer of the present invention, a lipid sensitizer such as a phosphonium compound, a nitrogen-containing low molecular weight compound, or an ammonium group-containing polymer is used in the image recording layer in order to improve the inking property. be able to. In particular, when an inorganic layered compound is contained in the protective layer, these compounds function as a surface coating agent for the inorganic layered compound, and prevent a decrease in the inking property during printing by the inorganic layered compound.

  Suitable phosphonium compounds include phosphonium compounds described in JP-A-2006-297907 and JP-A-2007-50660. Specific examples include tetrabutylphosphonium iodide, butyltriphenylphosphonium bromide, tetraphenylphosphonium bromide, 1,4-bis (triphenylphosphonio) butanedi (hexafluorophosphate), 1,7-bis (tri Phenylphosphonio) heptane sulfate, 1,9-bis (triphenylphosphonio) nonane = naphthalene-2,7-disulfonate, and the like.

  Examples of the nitrogen-containing low molecular weight compound include amine salts and quaternary ammonium salts. Also included are imidazolinium salts, benzoimidazolinium salts, pyridinium salts, and quinolinium salts. Of these, quaternary ammonium salts and pyridinium salts are preferable. Specific examples include tetramethylammonium = hexafluorophosphate, tetrabutylammonium = hexafluorophosphate, dodecyltrimethylammonium = p-toluenesulfonate, benzyltriethylammonium = hexafluorophosphate, benzyldimethyloctylammonium = hexafluorophosphate. Examples thereof include fert and benzyldimethyldodecyl ammonium = hexafluorophosphate.

  The ammonium group-containing polymer may be any polymer as long as it has an ammonium group in its structure, but a polymer containing 5 to 80 mol% of a (meth) acrylate having an ammonium group in the side chain as a copolymerization component is preferable. .

  The ammonium salt-containing polymer has a reduced specific viscosity (unit: cSt / g / ml) determined by the following measurement method, preferably in the range of 5 to 120, more preferably in the range of 10 to 110. The range of 15-100 is especially preferable.

<Measurement method of reduced specific viscosity>
Weigh 3.33 g of 30% polymer solution (1 g as solid content) into a 20 ml volumetric flask and make up with N-methylpyrrolidone. This solution is put in an Ubbelohde reduced viscosity tube (viscosity constant = 0.010 cSt / s), and the time to flow down at 30 ° C. is measured. The calculation formula (“kinematic viscosity” = “viscosity constant” × “ Time of passage (second) ") was calculated by a conventional method.

Specific examples of the ammonium group-containing polymer are shown below.
(1) 2- (trimethylammonio) ethyl methacrylate = p-toluenesulfonate / 3,6-dioxaheptyl methacrylate copolymer (molar ratio 10/90)
(2) 2- (Trimethylammonio) ethyl methacrylate = hexafluorophosphate / 3,6-dioxaheptyl methacrylate copolymer (molar ratio 20/80)
(3) 2- (Ethyldimethylammonio) ethyl methacrylate = p-toluenesulfonate / hexyl methacrylate copolymer (molar ratio 30/70)
(4) 2- (Trimethylammonio) ethyl methacrylate = hexafluorophosphate / 2-ethylhexyl methacrylate copolymer (molar ratio 20/80)
(5) 2- (Trimethylammonio) ethyl methacrylate = methyl sulfate / hexyl methacrylate copolymer (molar ratio 40/60)
(6) 2- (butyldimethylammonio) ethyl methacrylate = hexafluorophosphate / 3,6-dioxaheptyl methacrylate copolymer (molar ratio 20/80)
(7) 2- (Butyldimethylammonio) ethyl acrylate = hexafluorophosphate / 3,6-dioxaheptyl methacrylate copolymer (molar ratio 20/80)
(8) 2- (butyldimethylammonio) ethyl methacrylate = 13-ethyl-5,8,11-trioxa-1-heptadecanesulfonate / 3,6-dioxaheptyl methacrylate copolymer (molar ratio 20/80 )
(9) 2- (butyldimethylammonio) ethyl methacrylate = hexafluorophosphate / 3,6-dioxaheptyl methacrylate / 2-hydroxy-3-methacryloyloxypropyl methacrylate copolymer (molar ratio 15/80/5 )

  The content of the sensitizer is preferably 0.01 to 30.0% by mass, more preferably 0.1 to 15.0% by mass and 1 to 5% by mass with respect to the total solid content of the image recording layer. Is more preferable.

(4) Others As other components, surfactants, colorants, bake-out agents, polymerization inhibitors, higher fatty acid derivatives, plasticizers, inorganic fine particles, inorganic layered compounds, and co-sensitizers or chain transfer agents are included. Can be added. Specifically, paragraph numbers [0114] to [0159] of Japanese Patent Application Laid-Open No. 2008-284817, paragraph numbers [0023] to [0027] of Japanese Patent Application Laid-Open No. 2006-091479, and US Patent Publication No. 2008/0311520. [0060] The compounds and addition amounts described in [0060] are preferred.

(G) Formation of Image Recording Layer The image recording layer in the present invention may be prepared by using the necessary components described above as described in paragraphs [0142] to [0143] of JP-A-2008-195018, for example. A coating solution is prepared by dispersing or dissolving in a coating solution, which is coated on the support by a known method such as bar coater coating and dried. The image recording layer coating amount (solid content) on the support obtained after coating and drying varies depending on the application, but is generally preferably 0.3 to 3.0 g / m ² . Within this range, good sensitivity and good film properties of the image recording layer can be obtained.

(Undercoat layer)
In the lithographic printing plate precursor according to the invention, an undercoat layer (sometimes referred to as an intermediate layer) is preferably provided between the image recording layer and the support. The undercoat layer enhances the adhesion between the support and the image recording layer in the exposed area, and easily peels off the image recording layer from the support in the unexposed area. Contributes to improvement. In the case of infrared laser exposure, the undercoat layer functions as a heat insulating layer, thereby preventing the heat generated by the exposure from diffusing to the support and reducing the sensitivity.

Specific examples of the compound used for the undercoat layer include silane coupling agents having an addition-polymerizable ethylenic double bond reactive group described in JP-A-10-282679, and JP-A-2- Examples thereof include phosphorus compounds having an ethylenic double bond reactive group described in Japanese Patent No. 304441. More preferable is a polymer resin having an adsorptive group, a hydrophilic group, and a crosslinkable group that can be adsorbed on the surface of the support, as described in JP-A Nos. 2005-125749 and 2006-188038. It is done. The polymer resin is preferably a copolymer of a monomer having an adsorptive group, a monomer having a hydrophilic group, and a monomer having a crosslinkable group. More specifically, a phenolic hydroxyl group, a carboxyl _{group, -PO 3 H 2, -OPO 3} H 2, -CONHSO 2 -, - SO 2 NHSO 2 -, - having an adsorptive group such as COCH ₂ COCH ₃ Examples thereof include a polymer resin that is a copolymer of a monomer, a monomer having a hydrophilic sulfo group, and a monomer having a polymerizable crosslinkable group such as a methacryl group or an allyl group. This polymer resin may have a crosslinkable group introduced by salt formation between a polar substituent of the polymer resin, a substituent having a counter charge and a compound having an ethylenically unsaturated bond, Other monomers, preferably hydrophilic monomers may be further copolymerized.

The content of unsaturated double bonds in the polymer resin for the undercoat layer is preferably 0.1 to 10.0 mmol, and most preferably 2.0 to 5.5 mmol, per 1 g of the polymer resin.
The polymer resin for the undercoat layer preferably has a mass average molar mass of 5000 or more, more preferably 10,000 to 300,000.

  The undercoat layer of the present invention comprises a chelating agent, a secondary or tertiary amine, a polymerization inhibitor, an amino group, or a functional group having a polymerization inhibiting ability, in addition to the above-mentioned undercoat layer compound, to prevent contamination with time. Compounds having a group that interacts with the surface of an aluminum support (for example, 1,4-diazabicyclo [2,2,2] octane (DABCO), 2,3,5,6-tetrahydroxy-p-quinone, chloranil , Sulfophthalic acid, hydroxyethylethylenediaminetriacetic acid, dihydroxyethylethylenediaminediacetic acid, hydroxyethyliminodiacetic acid, and the like.

The undercoat layer is applied by a known method. The coating amount (solid content) of the undercoat layer is preferably from 0.1-100 mg / m ^2, and more preferably 1 to 30 mg / m ^2.

(Support)
As the support used in the lithographic printing plate precursor according to the invention, a known support is used. Of these, an aluminum plate that has been roughened and anodized by a known method is preferred.
In addition, the above aluminum plate is optionally subjected to micropore enlargement treatment or sealing treatment of an anodized film described in JP-A Nos. 2001-253181 and 2001-322365, and US Pat. 714,066, 3,181,461, 3,280,734 and 3,902,734, or alkali metal silicates as described in U.S. Pat. Surface hydrophilization treatment with polyvinylphosphonic acid or the like as described in each specification of 3,276,868, 4,153,461 and 4,689,272 is appropriately performed and performed. be able to.
The support preferably has a center line average roughness of 0.10 to 1.2 μm.

  If necessary, the support of the present invention includes an organic polymer compound described in JP-A-5-45885 and a silicon alkoxy compound described in JP-A-5-45885 on the back surface. A backcoat layer can be provided.

(Protective layer)
In the lithographic printing plate precursor according to the invention, a protective layer (overcoat layer) is preferably provided on the image recording layer. In addition to the function of suppressing the image formation inhibition reaction by blocking oxygen, the protective layer has a function of preventing scratches in the image recording layer and preventing ablation during high-illuminance laser exposure.

  The protective layer having such characteristics is described in, for example, US Pat. No. 3,458,311 and Japanese Patent Publication No. 55-49729. As the polymer having low oxygen permeability used for the protective layer, either a water-soluble polymer or a water-insoluble polymer can be appropriately selected and used. Specific examples include polyvinyl alcohol, modified polyvinyl alcohol, polyvinyl pyrrolidone, water-soluble cellulose derivatives, poly (meth) acrylonitrile, and the like.

Further, in order to enhance the oxygen barrier property, the protective layer preferably contains an inorganic layered compound such as natural mica and synthetic mica as described in JP-A-2005-119273.
Further, the protective layer may contain known additives such as a plasticizer for imparting flexibility, a surfactant for improving coating properties, and inorganic fine particles for controlling surface slipperiness. Further, the protective layer can contain the sensitizer described in the description of the image recording layer.

The protective layer is applied by a known method. The coating amount of the protective layer, the coating amount after drying is preferably in the range of 0.01 to 10 g / ^{m 2,} more preferably in the range of 0.02 to 3 g / ^{m 2,} and most preferably 0. in the range of 02~1g / ^{m 2.}

[Plate making method]
The plate making of the lithographic printing plate precursor according to the invention is preferably carried out by an on-press development method. The on-press development method includes a step of image-exposing a lithographic printing plate precursor, a printing step of supplying an oil-based ink and an aqueous component and printing without performing any development treatment on the lithographic printing plate precursor after exposure, and And the unexposed portion of the lithographic printing plate precursor is removed during the printing process. Imagewise exposure may be performed on the printing machine after the lithographic printing plate precursor is mounted on the printing machine, or may be separately performed with a plate setter or the like. In the latter case, the exposed lithographic printing plate precursor is mounted on a printing machine without undergoing a development process. Thereafter, by using the printing machine and supplying the oil-based ink and the aqueous component and printing as it is, an on-press development process, that is, an image recording layer in an unexposed area is removed at an early stage of printing, Accordingly, the surface of the hydrophilic support is exposed to form a non-image part. As the oil-based ink and the aqueous component, ordinary lithographic printing ink and fountain solution are used.
This will be described in more detail below.

In the present invention, the light source used for image exposure is preferably a laser. Although the laser used for this invention is not specifically limited, The solid laser and semiconductor laser etc. which irradiate infrared rays with a wavelength of 760-1200 nm are mentioned suitably.
Regarding the infrared laser, the output is preferably 100 mW or more, the exposure time per pixel is preferably within 20 microseconds, and the irradiation energy amount is preferably 10 to 300 mJ / cm ² . In the laser, it is preferable to use a multi-beam laser device in order to shorten the exposure time.

  The exposed lithographic printing plate precursor is mounted on a plate cylinder of a printing press. In the case of a printing press equipped with a laser exposure device, image exposure is performed after a lithographic printing plate precursor is mounted on a plate cylinder of the printing press.

  When printing is performed by supplying dampening water and printing ink to the lithographic printing plate precursor exposed imagewise, the image recording layer cured by exposure has a printing ink acceptance having an oleophilic surface in the exposed portion of the image recording layer. Forming part. On the other hand, in the unexposed area, the uncured image recording layer is removed by dissolution or dispersion with the supplied fountain solution and / or printing ink, and a hydrophilic surface is exposed in that area. As a result, the fountain solution adheres to the exposed hydrophilic surface, and the printing ink is deposited on the image recording layer in the exposed area and printing is started.

Here, dampening water or printing ink may be supplied to the printing plate first, but printing is first performed in order to prevent the dampening water from being contaminated by the removed image recording layer components. It is preferable to supply ink.
In this way, the lithographic printing plate precursor according to the invention is developed on the machine on an offset printing machine and used as it is for printing a large number of sheets.

  EXAMPLES Hereinafter, although an Example demonstrates this invention in detail, this invention is not limited to these.

[Synthesis Example 1: Synthesis of Specific Cyanine Dye (Exemplary Compound (IR-1))]
47 g of 3-methoxypropyl tosylate and 39.0 g of 2,3,3,5-tetramethyl-3H-indole were stirred at 120 ° C. for 3 hours and allowed to cool to room temperature. Washing with ethyl yielded 60 g of intermediate (1). 1,9.8 g of 2,5-bis [(phenylamino) methylene] cyclopentylidenechloroaminium chloride, 14.7 g of acetic anhydride, 36.4 g of triethylamine, 2-propanol with respect to 60 g of the obtained intermediate (1) 436 ml was added, and the mixture was further stirred at 80 ° C. for 3 hours. After completion of the reaction, the mixture was allowed to cool to room temperature, and the precipitated crystals were filtered and washed thoroughly with water to obtain 53.0 g of (A) (yield 47%).

  A solution of 2.4 g of bis (4-hydroxyphenyl) sulfone and 15 ml of N-methylpyrrolidone (hereinafter referred to as NMP) is cooled to 0 ° C., and sodium hydride (manufactured by Kanto Chemical Co., dispersion in paraffin liquid, 60% by mass) is 0. .38 g was added in small portions and stirred for 30 minutes. After stirring, a solution of 5.0 g of NMP (15 ml) of (A) was added dropwise, and the mixture was further stirred for 30 minutes and warmed to room temperature. Thereafter, the reaction solution was added dropwise to an aqueous solution of 2.5 g of potassium hexafluorophosphate and 150 ml of water, followed by stirring at room temperature for 30 minutes. The precipitated crystals were filtered, washed with water and dried to obtain 4.0 g of exemplified cyanine dye (IR-1) (yield 37%). Identification was performed by 1H-NMR (solvent DMSO).

[Examples 1 to 13 and Comparative Examples 1 to 11]

1. Production of lithographic printing plate precursors (1) to (13) and (21) to (31) (1) Production of support (part 1)
In order to remove rolling oil on the surface of an aluminum plate (material JIS A 1050) having a thickness of 0.3 mm, a degreasing treatment was performed at 50 ° C. for 30 seconds using a 10 mass% sodium aluminate aqueous solution, and then the hair diameter was adjusted to 0. The surface of the aluminum plate was grained using ^three 3 mm bundle-planted nylon brushes and a pumice-water suspension (specific gravity 1.1 g / cm ³ ) having a median diameter of 25 μm and thoroughly washed with water. This plate was etched by being immersed in a 25 mass% sodium hydroxide aqueous solution at 45 ° C for 9 seconds, washed with water, further immersed in 20 mass% nitric acid at 60 ° C for 20 seconds, and washed with water. At this time, the etching amount of the grained surface was about 3 g / m ² .

Next, an electrochemical roughening treatment was performed continuously using an alternating voltage of 60 Hz. The electrolytic solution at this time was a 1% by mass nitric acid aqueous solution (containing 0.5% by mass of aluminum ions) and a liquid temperature of 50 ° C. The AC power source waveform is electrochemical roughening treatment using a trapezoidal rectangular wave alternating current with a time ratio TP of 0.8 msec until the current value reaches a peak from zero, a duty ratio of 1: 1, and a trapezoidal rectangular wave alternating current. Went. Ferrite was used for the auxiliary anode. The current density was 30 A / dm ² at the peak current value, and 5% of the current flowing from the power source was shunted to the auxiliary anode. The amount of electricity in nitric acid electrolysis was 175 C / dm ² when the aluminum plate was the anode. Then, water washing by spraying was performed.

Subsequently, nitric acid electrolysis was performed with an aqueous solution of 0.5% by mass of hydrochloric acid (containing 0.5% by mass of aluminum ions) and an electrolytic solution having a liquid temperature of 50 ° C. under the condition of an electric quantity of 50 C / dm ² when the aluminum plate was the anode. In the same manner as above, an electrochemical surface roughening treatment was performed, followed by washing with water by spraying.
Next, the plate was provided with a direct current anodic oxide film having a current density of 15 A / dm ² and a current density of 15 g / m ² using 15% by mass sulfuric acid (containing 0.5% by mass of aluminum ions) as an electrolytic solution, followed by washing with water. It dried and produced the support body (1).
Thereafter, in order to ensure the hydrophilicity of the non-image area, the support (1) was subjected to a silicate treatment at 60 ° C. for 10 seconds using a 2.5 mass% No. 3 sodium silicate aqueous solution, and then washed with water. A support (2) was obtained. The adhesion amount of Si was 10 mg / m ² . The centerline average roughness (Ra) of this substrate was measured using a needle having a diameter of 2 μm and found to be 0.51 μm.

(2) Formation of undercoat layer Next, the following undercoat layer coating solution (1) is applied onto the support (2) so that the dry coating amount is 20 mg / m ² , and used in the following experiments. A support having a layer was prepared.

<Coating liquid for undercoat layer (1)>
-Undercoat layer compound (1) having the following structure: 0.18 g
・ Hydroxyethyliminodiacetic acid 0.10g
・ Methanol 55.24g
・ Water 6.15g

(3) Formation of image recording layer The image recording layer coating solution (1) having the following composition was bar-coated on the undercoat layer formed as described above, and then oven-dried at 100 ° C for 60 seconds to obtain a dry coating amount. An image recording layer of 1.0 g / m ² was formed.
The image recording layer coating solution (1) was obtained by mixing and stirring the following photosensitive solution (1) and microgel solution (1) immediately before coating.

<Photosensitive solution (1)>
-Binder polymer (1) [the following structure] 0.240 g
・ Infrared absorber (compound described in Table 1) 0.030 g
・ Polymerization initiator (compound described in Table 1) 0.162 g
Polymerizable compound tris (acryloyloxyethyl) isocyanurate (NK ester A-9300, manufactured by Shin-Nakamura Chemical Co., Ltd.) 0.192 g
・ Low molecular weight hydrophilic compound Tris (2-hydroxyethyl) isocyanurate 0.062g
・ Low molecular weight hydrophilic compound (1) [the following structure] 0.050 g
-Sensitizing agent Phosphonium compound (1) [the following structure] 0.055 g
・ Sensitizer Benzyl-dimethyl-octylammonium ・ PF ₆ salt 0.018g
・ Fluorosurfactant (1) [The following structure] 0.008g
・ 2-butanone 1.091g
・ 1-methoxy-2-propanol 8.609g

<Microgel solution (1)>
・ Microgel (1) 2.640 g
・ Distilled water 2.425g

  The structures of the binder polymer (1), the phosphonium compound (1), the low molecular weight hydrophilic compound (1), and the fluorosurfactant (1) are as shown below.

-Synthesis of microgel (1)-
As oil phase components, trimethylolpropane and xylene diisocyanate adduct (Mitsui Takeda Chemical Co., Ltd., Takenate D-110N) 10 g, pentaerythritol triacrylate (Nippon Kayaku Co., Ltd., SR444) 3.15 g, And 0.1 g of Pionein A-41C (manufactured by Takemoto Yushi Co., Ltd.) was dissolved in 17 g of ethyl acetate. As an aqueous phase component, 40 g of a 4% by mass aqueous solution of PVA-205 was prepared. The oil phase component and the aqueous phase component were mixed and emulsified for 10 minutes at 12,000 rpm using a homogenizer. The obtained emulsion was added to 25 g of distilled water, stirred at room temperature for 30 minutes, and then stirred at 50 ° C. for 3 hours. The microgel solution thus obtained was diluted with distilled water to a solid content concentration of 15% by mass, and this was designated as the microgel (1). When the average particle size of the microgel was measured by a light scattering method, the average particle size was 0.2 μm.

(3) Formation of Protective Layer After the protective layer coating solution (1) having the following composition was further bar-coated on the image recording layer, it was oven-dried at 120 ° C. for 60 seconds, and the dry coating amount was 0.15 g / m ^2. The lithographic printing plate precursors (1) to (13) [for Examples 1 to 13] and the lithographic printing plate precursors (21) to (31) [for Comparative Examples 1 to 11] were obtained. .

<Protective layer coating solution (1)>
・ Inorganic layered compound dispersion (1) 1.5 g
・ Polyvinyl alcohol (CKS50, Nippon Synthetic Chemical Industry Co., Ltd., sulfonic acid modification, saponification degree 99 mol% or more, polymerization degree 300) 6% by mass aqueous solution 0.55 g
Polyvinyl alcohol (PVA-405 manufactured by Kuraray Co., Ltd., saponification degree 81.5 mol%, polymerization degree 500) 6% by mass aqueous solution 0.03 g
・ Nippon Emulsion Co., Ltd. surfactant (Emalex 710) 1% by weight aqueous solution 0.86g
・ Ion-exchanged water 6.0g

(Preparation of inorganic layered compound dispersion (1))
6.4 g of synthetic mica Somasif ME-100 (manufactured by Coop Chemical Co., Ltd.) was added to 193.6 g of ion-exchanged water, and dispersed using an homogenizer until the average particle size (laser scattering method) became 3 μm. The aspect ratio of the obtained dispersed particles was 100 or more.

2. Preparation of lithographic printing plate precursors (14) to (16) The following image recording layer coating solution (2) is bar-coated on the above support having an undercoat layer, followed by oven drying at 70 ° C. for 60 seconds, followed by drying. An image recording layer having a coating amount of 0.6 g / m ² was prepared, and lithographic printing plate precursors (14) to (16) [for Examples 14 to 16] were obtained.

<Image recording layer coating solution (2)>
-Polymer fine particle aqueous dispersion (1) 20.0 g
・ Infrared absorbing dye (compound described in Table 1) 0.2g
・ Polymerization initiator Irgacure250 (Ciba Specialty Chemicals) 0.5g
-Polymerizable compound SR-399 (Sartomer) 1.50 g
・ Mercapto-3-triazole 0.2g
・ Byk336 (Byk Chimie) 0.4g
・ KlucelM (Hercules) 4.8g
・ ELVACITE4026 (Ineos Acrylica) 2.5g
・ N-propanol 55.0g
・ 2-butanone 17.0g

In addition, the compounds described by trade names in the above composition are as follows.
IRGACURE 250: (4-methoxyphenyl) [4- (2-methylpropyl) phenyl] iodonium = hexafluorophosphate (75% by mass propylene carbonate solution)
・ SR-399: Dipentaerythritol pentaacrylate
BYK 336: modified dimethylpolysiloxane copolymer (25% by mass xylene / methoxypropyl acetate solution)
・ KLUCEL M: Hydroxypropyl cellulose (2 mass% aqueous solution)
ELVACITE 4026: Hyperbranched polymethyl methacrylate (10% by mass 2-butanone solution)

(Production of polymer fine particle aqueous dispersion (1))
A 1000 ml four-necked flask is equipped with a stirrer, thermometer, dropping funnel, nitrogen inlet tube, reflux condenser, nitrogen gas is introduced for deoxygenation, 20 g of polyethylene glycol methyl ether methacrylate (PEGMA), distilled water 200 g and 200 g of n-propanol were added and heated until the internal temperature reached 70 ° C. Next, a mixture of 10 g of styrene (St), 80 g of acrylonitrile (AN), and 0.8 g of 2,2′-azobisisobutyronitrile was added dropwise over 1 hour. After the completion of the dropwise addition, the reaction was continued for 5 hours, and then 0.4 g of 2,2′-azobisisobutyronitrile was added to raise the internal temperature to 80 ° C. Subsequently, 0.5 g of 2,2′-azobisisobutyronitrile was added over 6 hours. Polymerization progressed 98% or more at the stage of reaction for a total of 20 hours, and a polymer fine particle water dispersion (1) having a mass ratio of PEGMA / St / AN = 10/10/80 was obtained. The particle size distribution of the polymer fine particles had a maximum value at a particle size of 150 nm.

  Here, the particle size distribution is obtained by taking an electron micrograph of polymer fine particles, measuring a total of 5000 fine particle sizes on the photograph, and a logarithmic scale between 0 and the maximum value of the obtained particle size measurement values. And the frequency of appearance of each particle size was plotted and obtained. For non-spherical particles, the particle size of spherical particles having the same particle area as that on the photograph was used as the particle size.

3. Preparation of lithographic printing plate precursors (17) to (19) The following image recording layer coating solution (3) is bar-coated on the above support having an undercoat layer, then oven-dried at 70 ° C. for 60 seconds, and dried. An image recording layer having a coating amount of 0.6 g / m ² was produced.

<Image recording layer coating solution (3)>
・ Polymer fine particle aqueous dispersion (2) 33.0 g
Infrared absorber (compound described in Table 1) 1.0 g
・ 1,5-Naphthalenedisulfonic acid disodium 0.1g
・ Polyacrylic acid (mass average molar mass 20000) 0.4 g
・ Methanol 16.0g

(Production of polymer fine particle aqueous dispersion (2))
A stirrer, thermometer, dropping funnel, nitrogen inlet tube, reflux condenser is applied to a 1000 ml four-necked flask, and 350 mL of distilled water is added while deoxygenating by introducing nitrogen gas, and the internal temperature becomes 80 ° C. Until heated. 1.5 g of sodium dodecyl sulfate was added as a dispersant, 0.45 g of ammonium persulfide was added as an initiator, and then 45.0 g of styrene was dropped from the dropping funnel over about 1 hour. After the completion of the dropwise addition, the reaction was continued as it was for 5 hours, and then the unreacted monomer was removed by steam distillation. Thereafter, the mixture was cooled and adjusted to pH 6 with ammonia water, and finally pure water was added so that the nonvolatile content was 15% by mass to obtain a polymer fine particle water dispersion (2). The particle size distribution of the polymer fine particles had a maximum value at a particle size of 60 nm.

  The image recording layer was formed as described above to obtain lithographic printing plate precursors (17) to (19) [for Examples 17 to 19].

4). Preparation of lithographic printing plate precursor (20) The lithographic printing plate precursor (1) was prepared except that the protective layer coating solution (3) in which the inorganic layered compound dispersion (1) was not added in the protective layer coating solution (1) was used. A lithographic printing plate precursor (20) [for Example 20] was obtained in the same manner as in the production.

5). Evaluation of lithographic printing plate precursor (1) Plate inspection properties The obtained lithographic printing plate precursors (1) to (31) were output by a Creo Trendsetter 3244VX equipped with a water-cooled 40W infrared semiconductor laser, the output was 11.7 W, and the external drum was rotated. The exposure was performed under conditions of several 250 rpm and resolution of 2400 dpi. The obtained exposed original plate was left as it was for 30 minutes in a dark place, at 25 ° C. and in an atmosphere of 50% relative humidity, and then the plate inspection property was evaluated. The ease of plate inspection was expressed as the difference ΔL between the L value of the exposed area and the L value of the unexposed area using the L value (brightness) of the L ^* a ^* b ^* color system. The larger the ΔL value, the better the plate inspection property. A ΔL value of about 2.0 is a level at which visual inspection can be managed, and if the ΔL value is 3.0 or more, the inspection is performed. It can be said that it has excellent plate quality. The measurement was performed by the SCE (regular reflection light removal) method using a spectrocolorimeter CM2600d manufactured by KONICA-MINOLTA and operation software CM-S100W. In the SCE method, specularly reflected light is removed and only diffused light is measured. Therefore, the color is evaluated visually, and correlates well with an actual human inspection. The results are shown in Table 1.

(2) On-machine developability Each lithographic printing plate precursor obtained as described above was subjected to an external drum rotation speed of 1000 rpm and a laser output of 70% on a Luxel PLASETTERT-6000III manufactured by Fuji Film Co., Ltd. equipped with an infrared semiconductor laser. The exposure was performed under the condition of a resolution of 2400 dpi. The exposure image includes a solid image and a 50% dot chart of a 20 μm dot FM screen.
The obtained exposed original plate was attached to a plate cylinder of a printing machine LITHRONE 26 manufactured by Komori Corporation without developing. Equality-2 (manufactured by FUJIFILM Corporation) / tap water = 2/98 (volume ratio) dampening water and Values-G (N) black ink (manufactured by Dainippon Ink & Chemicals, Inc.) After dampening water and ink were supplied by the standard automatic printing start method of LITHRONE 26 and developed on the machine, 100 sheets were printed on Tokuhishi Art (76.5 kg) paper at a printing speed of 10,000 sheets per hour.
The number of print sheets required until the on-press development on the printing press of the unexposed portion of the image recording layer was completed and the ink was not transferred to the non-image portion was measured as on-press developability.

(3) Printing durability On-press development was completed, and good printed matter was obtained. Printing continued in that state. As the number of printed sheets was increased, the image recording layer was gradually worn out, so that the ink density on the printed material decreased. Printing durability was evaluated using the number of printed copies when the value measured by the Gretag densitometer for the 50% halftone dot area ratio of the FM screen in the printed material was 5% lower than the measured value for the 100th printed sheet. . The results are shown in Table 1.

  Symbols in Table 1: Infrared absorber IR number and polymerization initiator symbol indicate exemplary compound numbers in this specification. The structures of comparative infrared absorbers R-1 and R-2 and comparative polymerization initiators R-3 and R-4 are shown below.

Claims (12)

A lithographic printing plate precursor having an image recording layer capable of removing an unexposed portion by supplying a printing ink and a fountain solution on a support, and (a) an infrared ray represented by the general formula (1) A lithographic printing comprising an image recording layer containing an absorber, (b) at least one polymerization initiator selected from a sulfonium salt compound and an asymmetric diaryliodonium salt compound, and (c) a polymerizable compound Version original edition.

In the general formula (1), R ^1a and R ^2a each independently represents an alkyl group having at least one atom selected from an oxygen atom, a sulfur atom and a nitrogen atom. m represents an integer of 1 or 2. Y may be the same or different and each represents a sulfur atom, an oxygen atom or a dialkylmethylene group having 12 or less carbon atoms. Z ¹ and Z ² each independently represents an aromatic hydrocarbon ring or a heteroaromatic ring. B represents an n-valent organic group, and n represents an integer of 2 to 6. A ⁻ represents a counter anion selected from a halide ion, a perchlorate ion, a tetrafluoroborate ion, a hexafluorophosphate ion, and a sulfonate ion. R ^1a and R ^2a in the general formula (1) are alkyl groups having at least one group selected from an ether group, an ester group, an amide group, a thioether group, and a ketone group. Item 2. The lithographic printing plate precursor as described in Item 1. The lithographic printing plate according to claim 1 or 2, wherein R ^1a and R ^2a in the general formula (1) are alkyl groups having at least one group selected from an ether group and an ester group. Original edition. B in the general formula (1) is represented by the following general formula (2) and n is 2, or B in the general formula (1) is represented by the general formula (3), and n The lithographic printing plate precursor as claimed in any one of claims 1 to 3, wherein is 3.

In general formula (2) and general formula (3), D represents a sulfur atom, an oxygen atom, or NR each independently. R represents a hydrogen atom or an alkyl group. X ¹ represents a single bond, a sulfonyl group, a carbonyl group, an azo group, a phenylene group, or a group represented by the following general formula (4). X ² represents a nitrogen atom, a methine group (—CH <), or an aromatic hydrocarbon group. In General Formula (4), R ^1b and R ^2b each independently represent a hydrogen atom or an alkyl group. k represents an integer of 1 or 2.

  The lithographic printing plate precursor as claimed in claim 4, wherein D in the general formula (2) or the general formula (3) is a sulfur atom or an oxygen atom.   The lithographic printing plate precursor as claimed in any one of claims 1 to 5, wherein the iodonium salt compound has an electron donating group.   The lithographic printing plate precursor as claimed in any one of claims 1 to 5, wherein the sulfonium salt compound has an electron withdrawing group.   The lithographic printing plate precursor as claimed in any one of claims 1 to 7, wherein the image recording layer comprises a binder polymer having a hydrophilic group.   The lithographic printing plate precursor as claimed in any one of claims 1 to 8, wherein the image recording layer contains a hydrophobizing precursor.   The lithographic printing plate precursor has an undercoat layer between the support and the image recording layer, and the undercoat layer contains a polymer having a substrate-adsorptive group and a polymerizable group. The lithographic printing plate precursor as described in any one of the above items.   The lithographic printing plate precursor as claimed in any one of claims 1 to 10, further comprising a protective layer containing an inorganic stratiform compound on the image recording layer.   The lithographic printing plate precursor according to any one of claims 1 to 11 is imagewise exposed with an infrared laser, and the printing ink and fountain solution are used without performing any development treatment on the exposed lithographic printing plate precursor. A lithographic printing method comprising: a printing step of supplying and printing, wherein an infrared laser unexposed portion of the image recording layer is removed during the printing step.