LITHOGRAPHIC PRINTING PLATE PRECURSOR  PLATE MAKING METHOD THEREOF AND LITHOGRAPHIC PRINTING METHOD THEREOF

FIELD OF THE INVENTION

The present invention relates to a lithographic printing plate precursor  a plate making method thereof and a lithographic printing method thereof. More particularly  it relates to a lithographic printing plate precursor which is capable of undergoing a so-called direct plate making in which the plate making is directly conducted based on digital signals  for example  from a computer using various kinds of lasers  a plate making method thereof and a lithographic printing method thereof.

BACKGROUND OF THE INVENTION

In general  a lithographic printing plate is composed of an oleophilic image area accepting ink and a hydrophilic non-image area accepting dampening water in the process of printing. Lithographic printing is a printing method utilizing the nature of water and oily ink to repel with each other and comprising rendering the oleophilic image area of the lithographic printing plate to an ink-receptive area and the hydrophilic non-image area thereof to a dampening water-receptive area (ink-unreceptive area)  thereby making a difference in adherence of the ink on the surface of the lithographic printing plate  depositing the ink only to the image area  and then transferring the ink to a printing material  for example  paper.
In order to produce the lithographic printing plate  a lithographic printing plate precursor (PS plate) comprising a hydrophilic support having provided thereon an oleophilic photosensitive resin layer (image-recording layer) is used. Specifically  the PS plate is exposed through a mask  for example  a lith film  and then subjected to development processing  for example  with an alkaline developer to remove the unnecessary image-recording layer corresponding to the non-image area by dissolving while leaving the image-recording layer corresponding to the image area  thereby obtaining the lithographic printing plate.
Due to the recent progress in the technical field  nowadays the lithographic printing plate can be obtained by a CTP (computer-to-plate) technology. Specifically  a lithographic printing plate precursor is directly subjected to scanning exposure using laser or laser diode without using a lith film and developed to obtain a lithographic printing plate.
With the progress described above  the issue on the lithographic printing plate precursor has transferred to improvements  for example  in image-forming property corresponding to the CTP technology  printing property or physical property. Also  with the increasing concern about global environment  as another issue on the lithographic printing plate precursor  an environmental problem on waste liquid discharged accompanying the wet treatment  for example  development processing comes to the front.
In response to the environmental problem  simplification of development or plate making or non-processing has been pursued. As one method of simple plate making  a method referred to as “on-press development” is practiced. Specifically  according to the method after exposure of a lithographic printing plate precursor  the lithographic printing plate precursor is mounted as it is on a printing machine without conducting conventional development and removal of the unnecessary area of image-recording layer is performed at an early stage of printing step.
Also  as a method of simple development  a method referred to as “gum development” is practiced wherein the removal of the unnecessary area of image-recording layer is performed using not a conventional high alkaline developer but a finisher or gum solution of near-neutral pH.
In the simplification of plate making operation as described above  a system using a lithographic printing plate precursor capable of being handled in a bright room or under a yellow lump and a light source is preferred from the standpoint of workability. Thus  as the light source  a semiconductor laser emitting an infrared ray having a wavelength from 760 to 1 200 or a solid laser  for example  YAG laser  is used.
As the lithographic printing plate precursor capable of undergoing on-press development or gum development  a lithographic printing plate precursor using a thermoplastic polymer fine particle in an image-forming layer and a plate making method are described in JP-A-9-123387 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) and JP-A-2003-255527. Also  a lithographic printing plate precursor using a thermoplastic polymer fine particle having a reactive group in an image-forming layer and a plate making method are described in JP-A-2001-260554 and JP-A-2003-316021. However  these techniques are still insufficient for obtaining both good development property and high printing durability.

SUMMARY OF THE INVENTION
An object of the present invention is to provide a lithographic printing plate precursor which can achieve both printing durability and development property  a plate making method using the lithographic printing plate precursor and a lithographic printing method using the lithographic printing plate precursor.
As a result of the intensive investigations  the inventor has achieved the above-described objects by incorporating a polymer compound (hereinafter  also referred to as a star polymer) having a star-like shape in which a main chain is branched to three or more branches and the branched main chains have a hydrophilic side chain into an image-recording layer containing a thermoplastic polymer fine particle.
The invention provides the following items 1 to 10.
1. A lithographic printing plate precursor comprising: an image-recording layer containing a thermoplastic polymer fine particle  an infrared absorbing agent and a polymer compound; and a support having a hydrophilic surface  wherein the polymer compound has a star-like shape in which a main chain is branched to three or more branches and the branched main chains have a hydrophilic group in a side chain of the branched main chain.
2. The lithographic printing plate precursor as described in the above item 1  wherein the hydrophilic group is at least any one of a carboxyl group or a salt thereof  a sulfo group or a salt thereof and a polyethylene oxy group.
3. The lithographic printing plate precursor as described in the above item 1 or 2  wherein the thermoplastic polymer fine particle has a crosslinkable group.
4. The lithographic printing plate precursor as described in the above item 3  wherein the polymer compound having the star-like shape has  in a side chain of the branched main chain  a group capable of reacting with the crosslinkable group of the thermoplastic polymer fine particle.
5. The lithographic printing plate precursor as described in the above item 4  wherein both the crosslinkable group of the thermoplastic polymer fine particle and the group capable of reacting with the crosslinkable group of the thermoplastic polymer fine particle are ethylenically unsaturated groups.
6. The lithographic printing plate precursor as described in any one of the above items 1 to 5  wherein the image-recording layer contains a polymerization initiator.
7. The lithographic printing plate precursor as described in any one of the above items 1 to 6  wherein the image-recording layer is capable of being removed with at least any of printing ink and dampening water.
8. A lithographic printing method comprising after either mounting the lithographic printing plate precursor as described in the above item 7 on a printing machine and exposing imagewise the lithographic printing plate precursor with laser or exposing imagewise the lithographic printing plate precursor as described in the above item 7 with laser and mounting the exposed lithographic printing plate precursor on a printing machine  supplying at least any of printing ink and dampening water on the exposed lithographic printing plate precursor to remove an unexposed area of the image-recording layer.
9. A plate making method of a lithographic printing plate precursor comprising after exposing imagewise the lithographic printing plate precursor as described in any one of the above items 1 to 6 with laser  developing the exposed lithographic printing plate precursor with an aqueous solution having pH from 2 to 12.
10. The plate making method of a lithographic printing plate precursor as described in the above item 9  wherein after the development of the exposed lithographic printing plate precursor with an aqueous solution having pH from 2 to 12  the developed lithographic printing plate precursor is subjected to a heat treatment.
The functional mechanism of the invention is not quite clear but it is estimated that due to using the star polymer as the polymer compound dispersibility of the thermoplastic polymer fine particle in the image-recording layer is improved in comparison with a conventional strait-chain polymer and the thermoplastic polymer fine particle is uniformly dispersed so that the development property is more improved and a heat fusion efficiency of the thermoplastic polymer fine particle increases to improve the printing durability.
According to the present invention  a lithographic printing plate precursor which can achieve both printing durability and development property  a plate making method using the lithographic printing plate precursor and a lithographic printing method using the lithographic printing plate precursor can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a view schematically showing a main chain structure of a star polymer.
Fig. 2 is a view explaining a structure of an automatic development processor.
[Description of reference numerals and signs]
4: Lithographic printing plate precursor (PS plate)
6: Developing unit
10: Drying unit
16: Carrying-in roller
20: Developing tank
22: Transport roller
24: Brush roller
26: Squeeze roller (carrying-out roller)
28: Backup roller
36: Guide roller
38: Skewer roller

DETAILED DESCRIPTION OF THE INVENTION

[Image-recording layer]
The image-recording layer of the lithographic printing plate precursor according to the invention contains a thermoplastic polymer fine particle  an infrared absorbing agent and a polymer compound having a star-like shape in which a main chain is branched to three or more branches and the branched main chains have a hydrophilic side chain.
[Thermoplastic polymer fine particle]
According to the invention  it is preferred to incorporate a thermoplastic polymer fine particle having Tg of 60?C or higher. In case of mixing two or more kinds of thermoplastic polymer fine particles having different Tg  it is preferred that Tg of at least one kind of the thermoplastic polymer fine particles is 60?C or higher.
As the thermoplastic polymer fine particle having Tg of 60?C or higher  thermoplastic polymer fine particles described  for example  in Research Disclosure  No. 33303  January (1992)  JP-A-9-123387  JP-A-9-131850  JP-A-9-171249  JP-A-9-171250 and European Patent 931 647 are preferably exemplified.
Specific examples of the polymer constituting the thermoplastic polymer fine particle include a homopolymer or copolymer of a monomer  for example  ethylene  styrene  vinyl chloride  methyl acrylate  ethyl acrylate  methyl methacrylate  ethyl methacrylate  vinylidene chloride  acrylonitrile or vinyl carbazole and a mixture thereof. Among them  polystyrene and polymethyl methacrylate are more preferred.
The thermoplastic polymer fine particle contained in the image-recording layer of the lithographic printing plate precursor according to the invention may comprise two or more kinds of the thermoplastic polymer fine particles having different particle sizes or two or more kinds of the thermoplastic polymer fine particles having different Tg.
By using two or more kinds of the thermoplastic polymer fine particles as described above  film curing property of the image area is more improved and when a lithographic printing plate is prepared  printing durability is more improved.
For instance  in case of using the thermoplastic polymer fine particles having the same particle size  since voids exist in some extent between the thermoplastic polymer fine particles  the film curing property may not reach the desired level in some cases even when the thermoplastic polymer fine particles are fused and solidified  for example  by heat mode exposure. On the contrary  in case of using the thermoplastic polymer fine particles having different particle sizes  a void ratio between the thermoplastic polymer fine particles can be reduced and as a result  the film curing property of the image area after the heat mode exposure can be improved.
Also  in case of using the thermoplastic polymer fine particles having the same Tg  when temperature elevation of the image-recording layer  for example  by heat mode exposure is insufficient  the thermoplastic polymer fine particles are not sufficiently fused and solidified and the film curing property may not reach the desired level in some cases. On the contrary  in case of using the thermoplastic polymer fine particles having different Tg  the film curing property of the image area can be improved even when the temperature elevation of the image-recording layer  for example  by heat mode exposure is insufficient.
The average particle size of the thermoplastic polymer fine particle is preferably from 0.005 to 2.0 µm. The value is also applied to the case in which two or more kinds of the thermoplastic polymer fine particles are used as a mixture. The average particle size thereof is more preferably from 0.01 to 1.5 µm  and particularly preferably from 0.05 to 1.0 µm. When the average particle size is extremely large  resolution may decrease in some cases  whereas when it is extremely small  time lapse stability may decrease in some cases. In case of mixing two or more kinds of the thermoplastic polymer fine particles  a polydispersity is preferably 0.2 or more. In case of using a mixture comprising the particle of a large particle size and the particle of a small particle size  the occurrence of void at the heat fusion decreases and high printing durability can be achieved. The polydispersity and average particle size are determined by a laser scattering method.
The Tg of at least one kind of the thermoplastic polymer fine particles is preferably 60?C or higher  more preferably from 70 to 140?C  and still more preferably from 80 to 120?C. In case of mixing two or more kinds of the thermoplastic polymer fine particles having different Tg  difference of the Tg is preferably 10?C or more  and more preferably 20?C or more. The content of the thermoplastic polymer fine particles having Tg of 60?C or higher is preferably 70% by weight or more based on the total thermoplastic polymer fine particles. When the content is 70% by weight or more  deterioration of the on-press development property due to the lapse of time is preferably more reduced.
The thermoplastic polymer fine particle contained in the image-recording layer of the lithographic printing plate precursor according to the invention may have a crosslinkable group. The crosslinkable group may be a functional group which undergoes any reaction as long as a chemical bond is formed and includes  for example  an ethylenically unsaturated group (for example  an acryloyl group  a methacryloyl group  a vinyl group or an allyl group) undergoing a polymerization reaction  an isocyanate group or a blocked form thereof undergoing an addition reaction and a functional group having an active hydrogen atom (for example  an amino group  a hydroxy group or a carboxyl group) as the reaction partner thereof  an epoxy group also undergoing an addition reaction and an amino group  a carboxyl group or a hydroxy group as the reaction partner thereof  a carboxyl group undergoing a condensation reaction and a hydroxyl group or an amino group as the reaction partner thereof  and an acid anhydride undergoing a ring opening addition reaction and an amino group or a hydroxyl group as the reaction partner thereof.
The thermoplastic polymer fine particle having a thermo-reactive functional group contained in the image-recording layer of the lithographic printing plate precursor according to the invention specifically includes that having an acryloyl group  a methacryloyl group  a vinyl group  an allyl group  an epoxy group  an amino group  a hydroxy group  a carboxyl group  an isocyanate group  an acid anhydride and a protected group thereof. The introduction of the functional group into the thermoplastic polymer fine particle may be conducted at the time of polymerization of the polymer fine particle or may be conducted by utilizing a polymer reaction after the polymerization of the polymer fine particle.
In the case of introducing the crosslinkable group at the time of polymerization of the polymer fine particle  a monomer having the crosslinkable group may preferably be subjected to emulsion polymerization or suspension polymerization.
Specific examples of the monomer having the crosslinkable group include allyl methacrylate  allyl acrylate  vinyl methacrylate  vinyl acrylate  glycidyl methacrylate  glycidyl acrylate  2-isocyanateethyl methacrylate or a blocked isocyanate thereof  for example  with an alcohol  2-isocyanateethyl acrylate or a blocked isocyanate thereof  for example  with an alcohol  2-aminoethyl methacrylate  2-aminoethyl acrylate  2-hydroxyethyl methacrylate  2-hydroxyethyl acrylate  acrylic acid  methacrylic acid  maleic anhydride  a divalent acrylate and a divalent methacrylate  but the invention should not be construed as being limited thereto.
The polymer reaction used in the case of conducting the introduction of the crosslinkable group after the polymerization of polymer fine particle includes  for example  a polymer reaction described in WO 96/34316.
The polymer fine particles may be reacted with each other via the crosslinkable groups or with a reactive group introduced into a side chain of the polymer having a star-like shape or a low molecular weight polymerizable compound added to the image-recording layer. Also  different kinds of crosslinkable groups capable of undergoing thermal reaction with each other are introduced into two or more kinds of polymer fine particles so that the thermoplastic polymer fine particles are reacted with each other. The reaction involved therein includes a polymerization reaction by an unsaturated group  an addition reaction between an isocyanate group or a blocked form thereof and a compound having an active hydrogen atom (for example  an amine  an alcohol or a carboxylic acid)  an addition reaction between an epoxy group and an amino group  a carboxyl group or a hydroxy group  a condensation reaction between a carboxyl group and a hydroxy or an amino group  and a ring opening addition reaction between an acid hydride and an amino group or a hydroxy group. However  any reaction may be used as long as a chemical bond can be formed.
The amount of the thermoplastic polymer fine particle for use in the invention to the image-recording layer is preferably from 40 to 95% by weight  more preferably from 50 to 90% by weight  particularly preferably from 60 to 85% by weight  based on the solid content of the image-recording layer.
[Star polymer]
The polymer compound for use in the invention is a star polymer in which a main chain is branched to three or more branches and a polymer compound which has a main chain structure as schematically shown in Fig. 1 and is represented buy formula (1) shown below.
Formula (1):

In formula (1)  A represents a branch unit (constituting unit including branch points) of the star polymer  Polymer represents a polymer chain constituting a main chain and has a hydrophilic group in its side chain  and n represents 3 or more.
[Polymer moiety]
The star polymer for use in the invention may be any polymer having the main chain structure as described above and a hydrophilic group in its side chain. The hydrophilic group is preferably contained in the Polymer moiety as a repeating unit having the hydrophilic group in its side chain.

The hydrophilic group includes –COOM1 (a carboxyl group or a salt thereof)  -SO3M1 (a sulfo group or a salt thereof)  -OH  -OSO3M1  -CONR1R2  -SO2NR1R2  -NR1SO3M1  -P(=O)(OM1)(OM2)  -OP(=O)(OM1)(OM2)  a group containing an ethylene oxide structure  for example  a hydroxyethyl group or a polyethylene oxy group  and a group containing an propylene oxide structure  for example  a hydroxypropyl group or a polypropylene oxy group. In the formulae above  M1 and M2 each represents a hydrogen ion  a metal ion  an ammonium ion or a phosphonium ion  and R1 and R2 each independently represents a hydrogen atom  an alkyl group  an alkenyl group or an aryl group.
In the case where any one of M1 and M2 represents the metal ion  specific examples of the metal ion include Li+  Na+  K+ and Cu+. Of the metal ions  Li+  Na+ and K+ are particularly preferred.
In the case where any one of M1 and M2 represents the ammonium ion  any ordinary ammonium ion can be preferably used and an ammonium ion having a number of carbon atoms of 24 or less per molecule is preferred and an ammonium ion having a number of carbon atoms of 16 or less per molecule is particularly preferred.
In the case where any one of M1 and M2 represents the phosphonium ion  any ordinary phosphonium ion can be preferably used and a phosphonium ion having a number of carbon atoms of 24 or less per molecule is preferred and a phosphonium ion having a number of carbon atoms of 16 or less per molecule is particularly preferred.
In the case where any one of R1 and R2 represents the alkyl group  any ordinary alkyl group can be used  and it may be a branched or cyclic form and may have a substituent  for example  a halogen atom  an ether group  a thioether group  a hydroxy group  a cyano group  a keto group  a carboxyl group  a carboxylate group  a carbonic acid ester group  a carbonic acid amido group  a sulfo group  a sulfonate group  a sulfonic acid ester group  a sulfonamido group  a sulfoxide group  a phenyl group  a phosphonic acid group  a phosphonate group  a phosphoric acid group  a phosphate group  an amino group  an aminocarbonyl group  an aminocarboxyl group or an aminosulfonyl group. As the alkyl group  an alkyl group having a total number of carbon atoms of 12 or less is particularly preferred.
In the case where any one of R1 and R2 represents the alkenyl group  any ordinary alkenyl group can be used  and it may be a branched or cyclic form and may have a substituent  for example  a halogen atom  an ether group  a thioether group  a hydroxy group  a cyano group  a keto group  a carboxyl group  a carboxylate group  a carbonic acid ester group  a carbonic acid amido group  a sulfo group  a sulfonate group  a sulfonic acid ester group  a sulfonamido group  a sulfoxide group  a phenyl group  a phosphonic acid group  a phosphonate group  a phosphoric acid group  a phosphate group  an amino group  an aminocarbonyl group  an aminocarboxyl group or an aminosulfonyl group. As the alkenyl group  an alkenyl group having a total number of carbon atoms of 12 or less is particularly preferred.
In the case where any one of R1 and R2 represents the aryl group  any ordinary aryl group can be preferably used. The aryl group may have a substituent  for example  a halogen atom  an ether group  a thioether group  a hydroxy group  a cyano group  a nitro group  a keto group  a carboxyl group  a carboxylate group  a carbonic acid ester group  a carbonic acid amido group  a sulfo group  a sulfonate group  a sulfonic acid ester group  a sulfonamido group  a sulfoxide group  a phenyl group  a phosphonic acid group  a phosphonate group  a phosphoric acid group  a phosphate group  an amino group  an aminocarbonyl group  an aminocarboxyl group or an aminosulfonyl group. As the aryl group  an aryl group having a total number of carbon atoms of 12 or less is particularly preferred.
Of the hydrophilic groups  –COOM1 (a carboxyl group or a salt thereof)  -SO3M1 (a sulfo group or a salt thereof) and a polyethylene oxy group are particularly preferred.
As the repeating unit having at least one hydrophilic group for forming the star polymer according to the invention  any repeating unit is preferably used as long as it is formed from a repeating unit having at least one of the hydrophilic groups. Specific examples of the repeating unit having a hydrophilic group which can be used in the invention are set forth below  but the invention should not be construed as being limited thereto.


n and m each independently represents an optional integer.

The star polymer according to the invention may contain only one kind of the repeating unit having the hydrophilic group as described above or two or more kinds of the repeating units having the hydrophilic group as described above.

In the case where the thermoplastic polymer fine particle has a crosslinkable group  the star polymer according to the invention preferably has a reactive group capable of reacting with the crosslinkable group in its side chain.
The reactive group may be a reactive group which undergoes any reaction as long as a chemical bond with the crosslinkable group of the thermoplastic polymer fine particle is formed. For instance  in the case where the thermoplastic polymer fine particle has an ethylenically unsaturated group (for example  an acryloyl group  a methacryloyl group  a vinyl group or an allyl group) undergoing a polymerization reaction  it is preferred to introduce also an ethylenically unsaturated group (for example  an acryloyl group  a methacryloyl group  a vinyl group or an allyl group) into the side chain of the star polymer.
In the case where the thermoplastic polymer fine particle has an isocyanate group or a blocked form thereof undergoing an addition reaction  it is preferred to introduce a functional group having an active hydrogen atom (for example  an amino group  a hydroxy group or a carboxyl group)  which is the reaction partner of the isocyanate group  into the side chain of the star polymer. In the case where the thermoplastic polymer fine particle has an epoxy group undergoing an addition reaction  it is preferred to introduce an amino group  a carboxyl group or a hydroxy group  which is the reaction partner of the epoxy group  into the side chain of the star polymer.
Also  in the case where the thermoplastic polymer fine particle has a crosslinkable group having an active hydrogen atom (for example  an amino group  a hydroxy group or a carboxyl group)  it is preferred to introduce an isocyanate group or a blocked form thereof undergoing an addition reaction or an epoxy group also undergoing an addition reaction into the side chain of the star polymer.
Similarly  with respect to a combination of a carboxyl group and a hydroxyl group or an amino group undergoing a condensation reaction  a combination of an acid anhydride and an amino group or a hydroxyl group undergoing a ring opening addition reaction or the like  it is preferred for the star polymer to have a functional group  which is a reaction partner of the functional group included in the thermoplastic polymer fine particle.
According to the invention  it is preferred to form crosslinkage between the polymer molecules by an ethylenically unsaturated group and to accelerate curing.
(1) Ethylenically unsaturated group
As the ethylenically unsaturated group  for example  a (meth)acryloyl group  a vinyl group or an allyl group is preferred. The ethylenically unsaturated group can be introduced into the polymer by a polymer reaction or copolymerization. For instance  a reaction between an acrylic polymer having a carboxyl group in its side chain and glycidyl methacrylate  a reaction between a polymer having an epoxy group and a carboxylic acid having an ethylenically unsaturated group  for example  methacrylic acid  or a reaction between a polymer having a hydroxy group and a methacrylate having an isocyanate group can be utilized.
From the standpoint of preservation stability of the polymer compound and film strength  a (meth)acryloyl group is preferred.
Specific examples of the repeating unit having a reactive group for use in the invention are set forth below  but the invention should not be construed as being limited thereto.

The content of the ethylenically unsaturated group in the polymer compound according to the invention is preferably from 0.1 to 10.0 mmol  more preferably from 0.25 to 7.0 mmol  most preferably from 0.5 to 5.5 mmol  per 1 g of the polymer compound.
(2) Other reactive group
Specific examples of the repeating unit having other reactive group include repeating units shown below.
? Epoxy group (which reacts with an amino group  a hydroxy group or a carboxyl group of the thermoplastic polymer fine particle)

? Isocyanate group or blocked isocyanate group (which reacts with an amino group  a hydroxy group or a carboxyl group of the thermoplastic polymer fine particle)

? Hydroxy group  carboxyl group or amino group (which reacts with an epoxy group  an isocyanate group or a blocked isocyanate group of the thermoplastic polymer fine particle)

? Compound having acid anhydride skeleton (which reacts with an amino group or a hydroxy group of the thermoplastic polymer fine particle)

The star polymer for use in the invention may contain other repeating unit. Specific examples of the other repeating unit are set forth below  but the invention should not be construed as being limited thereto.

Specific examples of the structure of the Polymer moiety in formula (1) are set forth below.

[Branch unit A]
The branch unit represented by A in formula (1) is not particularly restricted and is preferably a branch unit having a hub portion  which is a residue of a three or higher functional thiol. In the idealized structure  a main chain of an addition polymer extends from each thio part of the hub portion and thus  three or more main chains extend from the thio parts. Specifically  the branch unit A has preferably a structure represented by formula (2) shown below.
Formula (2):

In formula (2)  A1 represents a trivalent or higher valent organic group  and n represents an integer of 3 or more. Specific examples of A1 include structures shown below and organic groups composed of combination of two or more of these structures. n is preferably an integer from 3 to 6  and particularly preferably an integer from 4 to 6.

Polyvalent naphthalene  Polyvalent anthracene

The three or higher functional thiol residue is derived from an aromatic or aliphatic thiol. Examples of the aromatic thiol include benzene-1 3 5-trithiol  3 4 8 9-tetramercaptotetrathiafulvalene and 7-methyltrithiouric acid.
The thiol residue of the aliphatic thiol is preferably a residue of an ester formed from a three or higher functional alcohol and a mercaptoalkane acid having from 2 to 6 carbon atoms.
Examples of the appropriate alcohol include glycerine  sorbitol  an alcohol represented by formula (3) and an alcohol having a group represented by formula (4). In particular  the alcohol represented by formula (3) and alcohol having a group represented by formula (4) are preferred.

In formulae (3) and (4)  R1 represents a hydrogen atom  an alkyl group having from 1 to 4 carbon atoms or a hydroxy-substituted alkyl group having from 1 to 4 carbon atoms. In particular  R1 is preferably a methyl group  an ethyl group  a hydroxymethyl group or a hydroxyethyl group.
Examples of the mercaptoalkane acid having from 2 to 6 carbon atoms include 2-mercaptoacetic acid  2-mercaptopropionic acid  3-mercaptopropionic acid  4-mercaptobutyric acid  5-mercaptovaleric acid and 6-mercaptocaproic acid. Among them  2-mercaptoacetic acid and 3-mercaptopropionic acid are preferred.
Specific examples of the ester formed from a three or higher functional alcohol and a mercaptoalkane acid having from 2 to 6 carbon atoms include a compound having three mercapto groups  for example  1 2 6-hexanetrioltrithioglycolate  1 3 5-trithiocyanuric acid  1 3 5-tris(3-mercaptobutyryloxyethyl)-1 3 5-triazine-2 4 6(1H 3H 5H)-trione  trimethylolpropane tris(3-mercaptopropionate)  trimethylolpropane tristhioglycolate  trimethylolpropane tristhiopropionate  trihydroxyethyltriisocyanuric acid tristhiopropionate or tris-[(ethyl-3-mercaptopropionyloxy)ethyl] isocyanurate  and a compound having four mercapto groups  for example  pentaerythritol tetrakis(3-mercaptopropionate)  pentaerythritol tetrakis(3-mercaptobutyrate)  pentaerythritol tetrakisthioglycolate or dipentaerythritol hexakis-3-mercaptopropionate  but the invention should not be construed as being limited thereto.
Commercially available products of the multifunctional thiol compound include trimethylolpropane tristhiopropionate (TMTG) (trademark) and pentaerythritol tetrakisthiopropionate (PETG) (trademark) (each produced by Yodo Kagaku Co.  Ltd.)  pentaerythritol tetrakis(3-mercaptobutyrate) (KarenzMT PE1) (trademark) and 1 3 5-tris(3-mercaptobutyryloxyethyl)-1 3 5-triazine-2 4 6(1H 3H 5H)-trione (KarenzMT NR1) (trademark) (each produced by Showa Denko K.K.)  and trimethylolpropane tris(3-mercaptopropionate) (TMMP) (trademark)  pentaerythritol tetrakis(3-mercaptopropionate) (PEMP) (trademark)  dipentaerythritol hexakis-3-mercaptopropionate (DPMP) (trademark) and tris-[(ethyl-3-mercaptopropionyloxy)ethyl] isocyanurate (TEMPIC) (trademark) (each produced by Sakai Chemical Industry Co.  Ltd.)  but the multifunctional thiol compound according to the invention should not be construed as being limited thereto.
The branch unit represented by formula (2) includes structures shown below.


The weight average molecular weight (Mw) of the star polymer for use in the invention is preferably from 5 000 to 500 000  and more preferably from 10 000 to 250 000.
Specific examples of the star polymer for use in the invention are set forth below  but the invention should not be construed as being limited thereto.

TABLE 1: Specific Examples of Star Polymer
Star Polymer Branch Unit Polymer Chain Mw
P-1 A-1 B-1 2.6 x 104
P-2 A-1 B-2 2.6 x 104
P-3 A-1 B-3 2.6 x 104
P-4 A-1 B-4 2.5 x 104
P-5 A-1 B-5 2.6 x 104
P-6 A-1 B-6 3.4 x 104
P-7 A-1 B-7 3.4 x 104
P-8 A-1 B-8 3.5 x 104
P-9 A-1 B-13 3.6 x 104
P-10 A-1 B-17 4.6 x 104
P-11 A-1 B-18 4.6 x 104
P-12 A-1 B-19 4.7 x 104
P-13 A-1 B-20 4.8 x 104
P-14 A-1 B-21 4.9 x 104
P-15 A-1 B-24 4.6 x 104
P-16 A-1 B-25 4.6 x 104
P-17 A-1 B-26 4.7 x 104
P-18 A-1 B-27 4.6 x 104
P-19 A-1 B-28 4.7 x 104
P-20 A-1 B-4 3.6 x 104
P-21 A-2 B-4 2.7 x 104
P-22 A-3 B-4 3.6 x 104
P-23 A-4 B-4 3.6 x 104
P-24 A-5 B-4 5.4 x 104
P-25 A-6 B-4 2.7 x 104
P-26 A-7 B-4 3.6 x 104
P-27 A-8 B-4 2.7 x 104
P-28 A-9 B-4 5.4 x 104
P-29 A-10 B-4 3.6 x 104
P-30 A-11 B-4 2.7 x 104
P-31 A-5 B-19 6.3 x 104
P-32 A-5 B-25 6.3 x 104
P-33 A-5 B-28 6.3 x 104

The star polymers according to the invention may be used only one kind or two or more kinds thereof in combination. The content of the star polymer according to the invention in the image-recording layer is preferably from 3 to 50% by weight  more preferably from 5 to 40% by weight  particularly preferably from 10 to 30% by weight  based on the total solid content of the image-recording layer.
The star polymer according to the invention can be synthesized by a known method  for example  radical polymerization of the monomer constituting the polymer chain described above in the presence of the multifunctional thiol compound described above.
[Infrared absorbing agent]
The lithographic printing plate precursor according to the invention contains an infrared absorbing agent in the image-recording layer thereof. The infrared absorbing agent may also be incorporated into a layer adjacent to the image-recording layer (an undercoat layer or an overcoat layer described hereinafter). The infrared absorbing agent is a substance which absorbs an infrared laser beam and various pigments  dyes and metal particles can be used. In particular  a light-absorbing substance having an absorption band at least partially in a wavelength range from 700 to 1 200 nm is preferred.
Examples of the kind of pigment include black pigments  brown pigments  red pigments  purple pigments  blue pigments  green pigments  fluorescent pigments  metal powder pigments and polymer-bonded dyes. Specific examples of usable pigment include insoluble azo pigments  azo lake pigments  condensed azo pigments  chelate azo pigments  phthalocyanine pigments  anthraquinone pigments  perylene and perynone pigments  thioindigo pigments  quinacridone pigments  dioxazine pigments  isoindolinone pigments  quinophthalone pigments  dying lake pigments  azine pigments  nitroso pigments  nitro pigments  natural pigments  fluorescent pigments  inorganic pigments and carbon black.
The pigment may be used without undergoing surface treatment or may be used after the surface treatment. For the surface treatment  there are  for example  a method of coating a hydrophilic resin or an oleophilic resin on the surface  a method of attaching a surfactant on the surface and a method of bonding a reactive substance (for example  a silica sol  an alumina sol  a silane coupling agent  an epoxy compound or an isocyanate compound) to the pigment surface. The surface treatment methods are described in Kinzoku Sekken no Seishitsu to Oyo (Properties and Applications of Metal Soap)  Saiwai Shobo  Insatsu Ink Gijutsu (Printing Ink Technology)  CMC Publishing Co.  Ltd. (1984)  and Saishin Ganryo Oyo Gijutsu (Newest Application on Technologies for Pigments)  CMC Publishing Co.  Ltd. (1986). Of the pigments  those absorbing an infrared ray are preferred because they are suitably utilized with a laser emitting an infrared ray. As the pigment absorbing an infrared ray  carbon black is preferred. The particle size of the pigment is preferably in a range from 0.01 to 1 µm  and more preferably in a range from 0.01 to 0.5 µm.
As the dye  commercially available dyes and known dyes described in literatures (for example  Senryo Binran (Dye Handbook) compiled by The Society of Synthetic Organic Chemistry  Japan (1970)  Kinsekigai Kyushu Shikiso (Near Infrared Absorbing Dyes) on pages 45 to 51 of Kagaku Kogyo (Chemical Industry)  May  1986 and 90 Nen Dai Kinosei Shikiso no Kaihatsu to Shijo Doko (Development and Movement on Market of Functional Dyes in 90s)  Chap. 2  Item 2.3  CMC Publishing Co.  Ltd. (1990)) or patents can be used. Specifically  infrared absorbing dyes  for example  azo dyes  metal complex azo dyes  pyrazolone azo dyes  anthraquinone dyes  phthalocyanine dyes  carbonium dyes  quinoneimine dyes  polymethine dyes or cyanine dyes are preferred.
Further  cyanine dyes described  for example  in JP-A-58-125246  JP-A-59-84356 and JP-A-60-78787  methine dyes described  for example  in JP-A-58-173696  JP-A-58-181690 and JP-A-58-194595  naphthoquinone dyes described  for example  in JP-A-58-112793  JP-A-58-224793  JP-A-59-48187  JP-A-59-73996  JP-A-60-52940 and JP-A-60-63744  squarylium dyes described  for example  in JP-A-58-112792  cyanine dyes described in British Patent 434 875  dyes described in U.S. Patent 4 756 993  cyanine dyes described in U.S. Patent 4 973 572  dyes described in JP-A-10-268512  and phthalocyanine compounds described in JP-A-11-235883 are exemplified.
Also  near infrared absorbing sensitizers described in U.S. Patent 5 156 938 are preferably used as the dye. Further  substituted arylbenzo(thio)pyrylium salts described in U.S. Patent 3 881 924  trimethinethiapyrylium salts described in JP-A-57-142645  pyrylium compounds described in JP-A-58-181051  JP-A-58-220143  JP-A-59-41363  JP-A-59-84248  JP-A-59-84249  JP-A-59-146063 and JP-A-59-146061  cyanine dyes described in JP-A-59-216146  pentamethinethiopyrylium salts described in U.S. Patent 4 283 475  pyrylium compounds described in JP-B-5-13514 (the term “JP-B” as used herein means an “examined Japanese patent publication”) and JP-B-5-19702  and EPOLIGHT III-178  EPOLIGHT III-130 and EPOLIGHT III-125 produced by Epolin  Inc. are also preferably used. Of the dyes  water-soluble dyes are preferred. In particular  water-soluble cyanine dyes are preferred  and cyanine dyes which become water-soluble by a sulfonate are more preferred. Specific examples thereof are set forth below.

The amount of the infrared absorbing agent added is preferably in a range from 0.5 to 30 parts by weight  more preferably from 1.0 to 20 parts by weight  most preferably from 2.0 to 10 parts by weight per 100 parts by weight of the total solid content of the image-recording layer.
[Polymerization initiator]
The polymerization initiator for use in the invention is a compound which initiates or accelerates polymerization of the polymerizable compound. The polymerization initiator for use in the invention is preferably a radical polymerizable compound  and includes  for example  known thermal polymerization initiators  compounds containing a bond having small bond dissociation energy and photopolymerization initiators.
The polymerization initiator according to the invention include  for example  (a) an organic halide  (b) a carbonyl compound  (c) an azo compound  (d) an organic peroxide  (e) a metallocene compound  (f) an azido compound  (g) a hexaarylbiimidazole compound  (h) an organic borate compound  (i) a disulfone compound  (j) an oxime ester compound and (k) an onium salt compound.
As the organic halide (a)  compounds described in Paragraph Nos. [0022] to [0023] of JP-A-2008-195018 are preferred.
As the carbonyl compound (b)  compounds described in Paragraph No. [0024] of JP-A-2008-195018 are preferred.
As the azo compound (c)  for example  azo compounds described in JP-A-8-108621 are used.
As the organic peroxide (d)  for example  compounds described in Paragraph No. [0025] of JP-A-2008-195018 are preferred.
As the metallocene compound (e)  for example  compounds described in Paragraph No. [0026] of JP-A-2008-195018 are preferred.
As the azido compound (f)  a compound  for example  2 6-bis(4-azidobenzylidene)-4-methylcyclohexanone is exemplified.
As the hexaarylbiimidazole compound (g)  for example  compounds described in Paragraph No. [0027] of JP-A-2008-195018 are preferred.
As the organic borate compound (h)  for example  compounds described in Paragraph No. [0028] of JP-A-2008-195018 are preferred.
As the disulfone compound (i)  for example  compounds described in JP-A-61-166544 and JP-A-2002-328465 are exemplified.
As the oxime ester compound (j)  for example  compounds described in Paragraph Nos. [0028] to [0030] of JP-A-2008-195018 are preferred.
As the onium salt compound (k)  onium salts  for example  diazonium salts described in S. I. Schlesinger  Photogr. Sci. Eng.  18  387 (1974)  T. S. Bal et al.  Polymer  21  423 (1980) and JP-A-5-158230 (corresponding to diazonium of NI3)  ammonium salts described in U.S. Patent 4 069 055 and JP-A-4-365049  phosphonium salts described in U.S. Patents 4 069 055 and 4 069 056  iodonium salts described in European Patent 104 143  U. S. Patent Publication No. 2008/0311520  JP-A-2-150848  JP-A-2008-195018 and J. V. Crivello et al.  Macromolecules  10 (6)  1307 (1977)  sulfonium salts described in European Patents 370 693  233 567  297 443 and 297 442  U.S. Patents 4 933 377  4 760 013  4 734 444 and 2 833 827 and German Patents 2 904 626  3 604 580 and 3 604 581  selenonium salts described in J. V. Crivello et al.  J. Polymer Sci.  Polymer Chem. Ed.  17  1047 (1979)  arsonium salts described in C. S. Wen et al.  Teh  Proc. Conf. Rad. Curing ASIA  p. 478  Tokyo  Oct. (1988)  and azinium salts described in JP-A-2008-195018 are exemplified.
Of the polymerization initiators described above  the onium salt is preferred. Of the onium salts  the iodonium salt  sulfonium salt and azinium salt are preferred. Specific examples of these compounds are set forth below  but the invention should not be construed as being limited thereto.
Of the iodonium salts  a diphenyliodonium salt is preferred. In particular  a diphenyliodonium salt substituted with an electron donating group  for example  an alkyl group or an alkoxy group is preferred  and an asymmetric diphenyliodonium salt is more preferred. Specific examples of the iodonium salt include diphenyliodonium hexafluorophosphate  4-methoxyphenyl-4-(2-methylpropyl)phenyliodonium hexafluorophosphate  4-(2-methylpropyl)phenyl-p-tolyliodonium hexafluorophosphate  4-hexyloxyphenyl-2 4 6-trimethoxyphenyliodonium hexafluorophosphate  4-hexyloxyphenyl-2 4-diethoxyphenyliodonium tetrafluoroborate  4-octyloxyphenyl-2 4 6-trimethoxyphenyliodonium 1-perfluorobutanesulfonate  4-octyloxyphenyl-2 4 6-trimethoxyphenyliodonium hexafluorophosphate and bis(4-tert-butylphenyl)iodonium tetraphenylborate.
Examples of the sulfonium salt include triphenylsulfonium hexafluorophosphate  triphenylsulfonium benzoylformate  bis(4-chlorophenyl)phenylsulfonium benzoylformate  bis(4-chlorophenyl)-4-methylphenylsulfonium tetrafluoroborate  tris(4-chlorophenyl)sulfonium 3 5-bis(methoxycarbonyl)benzenesulfonate and tris(4-chlorophenyl)sulfonium hexafluorophosphate.
Examples of the azinium salt include 1-cyclohexylmethyloxypyridinium hexafluorophosphate  1-cyclohexyloxy-4-phenylpyridinium hexafluorophosphate  1-ethoxy-4-phenylpyridinium hexafluorophosphate  1-(2-ethylhexyloxy)-4-phenylpyridinium hexafluorophosphate  4-chloro-1-cyclohexylmethyloxypyridinium hexafluorophosphate  1-ethoxy-4-cyanopyridinium hexafluorophosphate  3 4-dichloro-1-(2-ethylhexyloxy)pyridinium hexafluorophosphate  1-benzyloxy-4-phenylpyridinium hexafluorophosphate  1-phenethyloxy-4-phenylpyridinium hexafluorophosphate  1-(2-ethylhexyloxy)-4-phenylpyridinium p-toluenesulfonate  1-(2-ethylhexyloxy)-4-phenylpyridinium perfluorobutanesulfonate  1-(2-ethylhexyloxy)-4-phenylpyridinium bromide and 1-(2-ethylhexyloxy)-4-phenylpyridinium tetrafluoroborate.
Of the polymerization initiators  water-soluble or water-dispersible polymerization initiators are preferred. Specific examples of water-soluble azo compound are set forth below.

The polymerization initiator can be added preferably in an amount from 0.1 to 50% by weight  more preferably from 0.5 to 30% by weight  particularly preferably from 0.8 to 20% by weight  based on the total solid content constituting the image-recording layer. In the range described above  good sensitivity and good stain resistance in the non-image area at the time of printing can be achieved.
[Other components]
The image-recording layer may contain other component  for example  a surfactant  a coloring agent  a development inhibitor  a development accelerator  a low molecular weight polymerizable compound capable of reacting with the crosslinkable group of the thermoplastic polymer fine particle  or a catalyst for increasing curing property or a precursor thereof.
[Formation of image-recording layer]
The image-recording layer according to the invention is formed by dispersing or dissolving each of the necessary components described above in a known solvent to prepare a coating solution and coating the solution on a support by a known method  for example  bar coater coating and drying as described  for example  in Paragraph Nos. [0142] to [0143] of JP-A-2008-195018. The coating amount (solid content) of the image-recording layer formed on the support after coating and drying may be varied according to the intended purpose but is in general preferably from 0.3 to 3.0 g/m2. In the range described above  good sensitivity and good film property of the image-recording layer can be obtained.
[Support]
As the support for use in the lithographic printing plate precursor according to the invention  a known support is used. Particularly  an aluminum plate subjected to roughening treatment and anodizing treatment according to a known method is preferred.
Also  an enlarging treatment or a sealing treatment of micropores of the anodized film described in JP-A-2001-253181 and JP-A-2001-322365 or a surface hydrophilizing treatment  for example  with an alkali metal silicate as described in U.S. Patents 2 714 066  3 181 461  3 280 734 and 3 902 734 or polyvinyl phosphonic acid as described in U.S. Patents 3 276 868  4 153 461 and 4 689 272 may be appropriately selected and applied to the aluminum plate  if desired.
The support preferably has a center line average roughness from 0.10 to 1.2 µm.
The support according to the invention may have a backcoat layer containing an organic polymer compound described in JP-A-5-45885 or an alkoxy compound of silicon described in JP-A-6-35174  provided on the back surface thereof  if desired.
[Protective layer]
In the lithographic printing plate precursor according to the invention  a protective layer (overcoat layer) may be provided on the image-recording layer. The protective layer has a function for preventing  for example  occurrence of scratch in the image-recording layer or ablation caused by exposure with a high illuminance laser beam  in addition to the function for restraining an inhibition reaction against the image formation by means of oxygen blocking.
With respect to the protective layer having such properties  there are described  for example  in U.S. Patent 3 458 311 and JP-B-55-49729. As a polymer having low oxygen permeability for use in the protective layer  any water-soluble polymer and water-insoluble polymer can be appropriately selected to use. The polymers may be used in mixture of two or more thereof  if desired. Specifically  for example  polyvinyl alcohol  a modified polyvinyl alcohol  polyvinyl pyrrolidone  a water-soluble cellulose derivative and poly(meth)acrylonitrile are exemplified.
As the modified polyvinyl alcohol  an acid-modified polyvinyl alcohol having a carboxyl group or a sulfo group is preferably used. Specifically  modified polyvinyl alcohols described in JP-A-2005-250216 and JP-A-2006-259137 are preferably exemplified.
It is also preferred for the protective layer to contain an inorganic stratiform compound  for example  natural mica or synthetic mica as described in JP-A-2005-119273 in order to increase the oxygen blocking property.
Further  the protective layer may contain a known additive  for example  a plasticizer for imparting flexibility  a surfactant for improving a coating property or a fine inorganic particle for controlling a surface slipping property. The oil-sensitizing agent described with respect to the image-recording layer may also be incorporated into the protective layer.
The protective layer is coated according to a known method. The coating amount of the protective layer is preferably in a range from 0.01 to 10 g/m2  more preferably in a range from 0.02 to 3 g/m2  most preferably in a range from 0.02 to 1 g/m2  in terms of the coating amount after drying.
[Plate making method]
The lithographic printing plate precursor according to the invention is subjected to plate making after image exposure by conducting development processing or development on a printing machine to use in printing.

The lithographic printing plate precursor is imagewise exposed with laser through a transparent original having a line image  a halftone dot image or the like  or imagewise exposed  for example  by scanning of laser beam based on digital data.
The wavelength of the exposure light source is preferably from 750 to 1 400 nm. As the light source of 750 to 1 400 nm  a solid laser or semiconductor laser emitting an infrared ray is preferably used. With respect to 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 is preferably from 10 to 300 mJ/cm2. Further  in order to shorten the exposure time  it is preferred to use a multibeam laser device. The exposure mechanism may be any of an internal drum system  an external drum system and a flat bed system.
The image exposure can be conducted in a conventional manner using a plate setter or the like. In the case of on-press development  after mounting the lithographic printing plate precursor on a printing machine  the image exposure may be conducted on the printing machine.

According to the on-press development  at least any of oily ink and an aqueous component is supplied on the imagewise exposed lithographic printing plate precursor on a printing machine to remove the image-recording layer in the non-image area  thereby preparing a lithographic printing plate.
Specifically  when the lithographic printing plate precursor after the image exposure is mounted as it is on a printing machine without undergoing any development processing or the lithographic printing plate precursor is mounted on a printing machine and imagewise exposed on the printing machine and then the oily ink and aqueous component are supplied to conduct printing  at an early stage of the printing in the unexposed area  the uncured image-recording layer is removed by dissolution or dispersion with the oily ink and/or aqueous component supplied to reveal the hydrophilic surface in the area. On the other hand  in the exposed area of the image-recording layer  the image-recording layer cured by the exposure forms the oily ink receptive area having the oleophilic surface. While either the oily ink or aqueous component may be supplied at first on the surface of lithographic printing plate precursor  it is preferred to supply the oily ink at first in view of preventing the aqueous component from contamination with the component of the image-recording layer removed. Thus  the lithographic printing plate precursor is subjected to the on-press development on the printing machine and used as it is for printing a large number of sheets. As the oily ink and aqueous component  conventional printing ink and dampening water for lithographic printing are preferably used  respectively.

According to a conventional developing process using a highly alkaline developer  a protective layer is removed in a pre-water washing step  the alkali development is conducted  the alkali is removed by washing with water in a post-water washing step  gum solution treatment is conducted and drying is conducted in a drying step. On the contrary  in the case of conducting the development of the lithographic printing plate precursor according to the invention using a developer having pH from 2 to 11  the protective layer and the unexposed area of the image-recording layer are together removed so that the resulting lithographic printing plate can be immediately mounted on a printing machine to perform printing. By incorporating a surfactant and/or a water-soluble polymer of oil-desensitization property into such a developer having pH from 2 to 11  the development and gum solution treatment are conducted at the same time so that the post-water washing step conducted after the alkali development is not particularly necessary and after conducting the development and gum solution treatment with one solution  the drying step can be performed. It is preferred that after the development and gum treatment  the excess developer is removed using a squeeze roller  followed by conducting the drying. Specifically  a considerably simplified processing process (gum development) composed of development and gum treatment with one solution and drying can be conducted.
The development according to the invention is performed in a conventional manner at liquid temperature ordinarily from 0 to 60oC  preferably from 15 to 40oC  using  for example  a method wherein the imagewise exposed lithographic printing plate precursor is immersed in the developer and rubbed with a brush or a method wherein the developer is sprayed to the imagewise exposed lithographic printing plate precursor and the exposed lithographic printing plate precursor is rubbed with a brush.
The developer having pH from 2 to 11 is preferably an aqueous solution containing water as a main component (containing 60% by weight or more of water based on weight of the developer). In particular  an aqueous solution containing a surfactant (for example  an anionic  nonionic  cationic or amphoteric surfactant) or an aqueous solution containing a water-soluble polymer is preferred. An aqueous solution containing both the surfactant and the water-soluble polymer is also preferred. The pH of the developer is more preferably from 5 to 10.7  still more preferably from 6 to 10.5  and most preferably from 7.5 to 10.3.
The anionic surfactant for use in the developer is not particularly limited and includes  for example  fatty acid salts  abietic acid salts  hydroxyalkanesulfonic acid salts  alkanesulfonic acid salts  dialkylsulfosuccinic acid salts  straight-chain alkylbenzenesulfonic acid salts  branched alkylbenzenesulfonic acid salts  alkylnaphthalenesulfonic acid salts  alkyldiphenylether (di)sulfonic acid salts  alkylphenoxy polyoxyethylene propylsulfonic acid salts  polyoxyethylene alkylsulfophenyl ether salts  N-alkyl-N-oleyltaurine sodium salt  N-alkylsulfosuccinic acid monoamide disodium salts  petroleum sulfonic acid salts  sulfated castor oil  sulfated beef tallow oil  sulfate ester slats of fatty acid alkyl ester  alkyl sulfate ester salts  polyoxyethylene alkyl ether sulfate ester salts  fatty acid monoglyceride sulfate ester salts  polyoxyethylene alkyl phenyl ether sulfate ester salts  polyoxyethylene styryl phenyl ether sulfate ester salts  alkyl phosphate ester salts  polyoxyethylene alkyl ether phosphate ester salts  polyoxyethylene alkyl phenyl ether phosphate ester salts  partially saponified products of styrene-maleic anhydride copolymer  partially saponified products of olefin-maleic anhydride copolymer and naphthalene sulfonate formalin condensates. Of the compounds  alkylbenzenesulfonic acid salts  alkylnaphthalenesulfonic acid salts and alkyldiphenylether (di)sulfonic acid salts are particularly preferably used.
The cationic surfactant for use in the developer is not particularly limited and hitherto known cationic surfactants may be used. For example  alkylamine salts  quaternary ammonium salts  alkylimidazolinium salts  polyoxyethylene alkyl amine salts and polyethylene polyamine derivatives are exemplified.
The nonionic surfactant for use in the developer is not particularly limited and includes  for example  polyethylene glycol type higher alcohol ethylene oxide adducts  alkylphenol ethylene oxide adducts  alkylnaphthol ethylene oxide adducts  phenol ethylene oxide adducts  naphthol ethylene oxide adducts  fatty acid ethylene oxide adducts  polyhydric alcohol fatty acid ester ethylene oxide adducts  higher alkylamine ethylene oxide adducts  fatty acid amide ethylene oxide adducts  ethylene oxide addacts of fat  polypropylene glycol ethylene oxide adducts  dimethylsiloxane-ethylene oxide block copolymers  dimethylsiloxane-(propylene oxide-ethylene oxide) block copolymers  fatty acid esters of polyhydric alcohol type glycerol  fatty acid esters of pentaerythritol  fatty acid esters of sorbitol and sorbitan  fatty acid esters of sucrose  alkyl ethers of polyhydric alcohols and fatty acid amides of alkanolamines. Of the compounds  those having an aromatic ring and an ethylene oxide chain are preferred  and alkyl-substituted or unsubstituted phenol ethylene oxide adducts and alkyl-substituted or unsubstituted naphthol ethylene oxide adducts are more preferred.
The amphoteric surfactant for use in the developer is not particularly limited and includes  for instance  amine oxide type  for example  alkyldimethylamine oxide  betaine type  for example  alkyl betaine and amino acid type  for example  sodium salt of alkylamino fatty acid. In particular  an alkyldimethylamine oxide which may have a substituent  an alkyl carboxy betaine which may have a substituent and an alkyl sulfo betaine which may have a substituent are preferably used. Specific examples of the compound are described  for example  in Paragraph Nos. [0255] to [0278] of JP-A-2008-203359 and Paragraph Nos. [0028] to [0052] of JP-A-2008-276166. Specific examples of the more preferable compound include a 2-alkyl-N-carboxymethyl-N-hydroxyethyl imidazolinium betaine  an alkyldiaminoethylglycine hydrochloride  lauryldimethylaminoacetic acid betaine  N-lauric acid amidopropyldimethyl betaine and N-lauric acid amidopropyldimethylamine oxide.
Two or more surfactants may be used in combination. The content of the surfactant in the developer is preferably from 0.01 to 20% by weight  and more preferably from 0.1 to 10% by weight.
The water-soluble polymer for use in the developer having pH from 2 to 11 includes  for example  soybean polysaccharide  modified starch  gum arabic  dextrin  a cellulose derivative (for example  carboxymethyl cellulose  carboxyethyl cellulose or methyl cellulose) or a modified product thereof  pllulan  polyvinyl alcohol or a derivative thereof  polyvinyl pyrrolidone  polyacrylamide  an acrylamide copolymer  a vinyl methyl ether/maleic anhydride copolymer  a vinyl acetate/maleic anhydride copolymer and a styrene/maleic anhydride copolymer.
As the soybean polysaccharide  known soybean polysaccharide can be used. For example  as a commercially available product  SOYAFIVE (trade name  produced by Fuji Oil Co.  Ltd.) is available and various grade products can be used. The soybean polysaccharide preferably used is that having viscosity in a range from 10 to 100 mPa/sec in the 10% by weight aqueous solution thereof.
As the modified starch  known modified starch can be used. The modified starch can be prepared  for example  by a method wherein starch  for example  of corn  potato  tapioca  rice or wheat is decomposed  for example  with an acid or an enzyme to an extent that the number of glucose residue per molecule is from 5 to 30 and then oxypropylene is added thereto in an alkali.
Two or more water-soluble polymers may be used in combination. The content of the water-soluble polymer in the developer is preferably from 0.1 to 20% by weight  and more preferably from 0.5 to 10% by weight.
Into the developer having pH from 2 to 11 for use in the invention  a pH buffer agent may further be incorporated.
As the pH buffer agent used in the invention  a pH buffer agent exhibiting a pH buffer function at pH from 2 to 11 is used without particular restriction. In the invention  a weak alkaline buffer agent is preferably used and includes  for example  (a) a carbonate ion and a hydrogen carbonate ion  (b) a borate ion  (c) a water-soluble amine compound and an ion of the water-soluble amine compound  and combinations thereof. Specifically  for example  (a) a combination of a carbonate ion and a hydrogen carbonate ion  (b) a borate ion  or (c) a combination of a water-soluble amine compound and an ion of the water-soluble amine compound exhibits a pH buffer function in the developer to prevent fluctuation of the pH even when the developer is used for a long period of time. As a result  for example  the deterioration of development property resulting from the fluctuation of pH and the occurrence of development scum are restrained. The combination of a carbonate ion and a hydrogen carbonate ion is particularly preferred.
In order for a carbonate ion and a hydrogen carbonate ion to be present in the developer  a carbonate and a hydrogen carbonate may be added to the developer or a carbonate ion and a hydrogen carbonate ion may be generated by adding a carbonate or a hydrogen carbonate to the developer and then adjusting the pH. The carbonate or hydrogen carbonate used is not particularly restricted and it is preferably an alkali metal salt thereof. Examples of the alkali metal include lithium  sodium and potassium and sodium is particularly preferable. The alkali metals may be used individually or in combination of two or more thereof.
When the combination of (a) a carbonate ion and a hydrogen carbonate ion is adopted as the pH buffer agent  the total amount of the carbonate ion and hydrogen carbonate ion is preferably from 0.05 to 5 mole/l  more preferably from 0.1 to 2 mole/l  particularly preferably from 0.2 to 1 mole/l  in the developer.
The developer may contain an organic solvent. As the organic solvent to be contained  for example  an aliphatic hydrocarbon (e.g.  hexane  heptane  ISOPAR E  ISOPAR H  ISOPAR G (produced by Esso Chemical Co.  Ltd.))  an aromatic hydrocarbon (e.g.  toluene or xylene)  a halogenated hydrocarbon (methylene dichloride  ethylene dichloride  trichlene or monochlorobenzene) or a polar solvent is exemplified. Examples of the polar solvent include an alcohol (e.g.  methanol  ethanol  propanol  isopropanol  1-butanol  1-pentanol  1-hexanol  1-heptanol  1-octanol  2-octanol  2-ethyl-1-hexanol  1-nonanol  1-decanol  benzyl alcohol  ethylene glycol monomethyl ether  2-ethyoxyethanol  diethylene glycol monoethyl ether  diethylene glycol monohexyl ether  triethylene glycol monomethyl ether  propylene glycol monoethyl ether  propylene glycol monomethyl ether  polyethylene glycol monomethyl ether  polypropylene glycol  tetraethylene glycol  ethylene glycol monobutyl ether  ethylene glycol monobenzyl ether  ethylene glycol monophenyl ether  propylene glycol monophenyl ether  methyl phenyl carbinol  n-amyl alcohol or methylamyl alcohol)  a ketone (e.g.  acetone  methyl ethyl ketone  ethyl butyl ketone  methyl isobutyl ketone or cyclohexanone)  an ester (e.g.  ethyl acetate  propyl acetate  butyl acetate  amyl acetate  benzyl acetate  methyl lactate  butyl lactate  ethylene glycol monobutyl acetate  polyethylene glycol monomethyl ether acetate  diethylene glycol acetate  diethyl phthalate or butyl levulinate) and others (e.g.  triethyl phosphate  tricresyl phosphate  N-phenylethanolamine  N-phenyldiethanolamine  N-methyldiethanolamine  N-ethyldiethanolamine  4-(2-hydroxyethyl)morpholine  N N-dimethylacetamide or N-methylpyrrolidone).
Two or more organic solvents may be used together in the developer.
Further  when the organic solvent is insoluble in water  it may be employed by being solubilized in water using a surfactant or the like. In the case where the developer contains an organic solvent  the concentration of the organic solvent is desirably less than 40% by weight in view of safety and inflammability.
The developer having pH from 2 to 11 may contain a preservative  a chelating agent  a defoaming agent  an organic acid  an inorganic acid  an inorganic salt or the like in addition the components described above. Specifically  compounds described in Paragraph Nos. [0266] to [0270] of JP-A-2007-206217 are preferably used.
The developer described above can be used as a developer and a development replenisher for the exposed lithographic printing plate precursor and it is preferably applied to an automatic processor described hereinafter. In the case of conducting the development processing using an automatic processor  the developer becomes fatigued in accordance with the processing amount  and hence the processing ability may be restored using a replenisher or a fresh developer.
The development processing using the developer having pH from 2 to 11 according to the invention is preferably performed by an automatic processor equipped with a supplying means for the developer and a rubbing member. An automatic processor using a rotating brush roll as the rubbing member is particularly preferred. Further  the automatic processor is preferably provided with a means for removing the excess developer  for example  a squeeze roller or a drying means  for example  a hot air apparatus  subsequently to the development processing means.
The lithographic printing plate precursor according to the invention may be subjected to heat treatment in a post heating step (baking process) after the development processing in order to improve printing durability.
The baking process is conducted at temperature higher than the coagulation temperature of the thermoplastic polymer fine particle  for example  from 100 to 230?C for 5 to 40 minutes. For example  the exposed and developed lithographic printing plate precursor is subjected to baking at temperature of 230?C for 5 minutes  at temperature of 150?C for 10 minutes or at temperature of 120?C for 30 minutes. The baking can be conducted in a conventional hot air oven or by irradiation using a lamp emitting an infrared or ultraviolet spectrum.

EXAMPLES

The present invention will be described in more detail with reference to the following examples  but the invention should not be construed as being limited thereto. With respect to the polymer compounds used in the examples  unless otherwise particularly defined  a molecular weight means a weight average molecular weight (Mw) and a ratio of repeating unit is indicated in mole percent.
[Synthesis example of star polymer]
Synthesis of Star Polymer (P-4)
Under a nitrogen stream  a mixed solution composed of 283 g of acrylic acid  2.3 g of pentaerythritol tetrakis(mercaptoacetate)  2.4 g of V-601 (produced by Wako Pure Chemical Industries  Ltd.) and 330 g of 1-methoxy-2-propanol was dropwise added to 330 g of 1-methoxy-2-propanol heated at 80?C over a period of 2 hours and 30 minutes. After the completion of the dropwise addition  the reaction was further continued for 3 hours. A weight average molecular weight (Mw) of the resulting polymer was 5 x 104.
Synthesis of Star Polymer (P-16)
Under a nitrogen stream  a mixed solution composed of 144 g of acrylic acid  2.3 g of pentaerythritol tetrakis(mercaptoacetate)  2.4 g of V-601 (produced by Wako Pure Chemical Industries  Ltd.) and 100 g of 1-methoxy-2-propanol was dropwise added to 200 g of 1-methoxy-2-propanol heated at 80?C over a period of 2 hours and 30 minutes. After the completion of the dropwise addition  0.143 g of V-601 was added to the reaction mixture and the temperature was raised to 90?C to continue the reaction for 2 hours. After the reaction  the mixture was cooled to 50?C and a mixed solution composed of 0.971 g of 4-hydroxy-2 2 6 6-tetramethylpiperidine-1-oxide free radical  28.4 g of glycidyl methacrylate and 13.6 g of 1-methoxy-2-propanol was added thereto  followed by stirring for 5 minutes. The temperature of reaction mixture was raised to 90?C and after stirring for 10 minutes  2.21 g of tetraethylammonium bromide was added thereto. The tetraethylammonium bromide attached to the wall of the reaction vessel was washed with 30.47 g of 1-methoxy-2-propanol. After stirring at 90?C for 18 hours  1-methoxy-2-propanol was further added to dilute the reaction mixture so as to have a solid content concentration of 24.5%. A weight average molecular weight of the solution of Star Polymer (P-16) represented by the structural formula shown above measured by GPC was 8.0 x 104.
Examples 1 to 16 and Comparative Examples 1 to 3
[Preparation of Lithographic printing plate precursors (1) to (19)]
(1) Preparation of Support
An aluminum plate (material: JIS A 1050) having a thickness of 0.3 mm was subjected to a degreasing treatment at 50?C for 30 seconds using an aqueous 10% by weight sodium aluminate solution in order to remove rolling oil on the surface thereof and then neutralization and desmut treatment at 50?C for 30 seconds using an aqueous 30% by weight sulfuric acid solution.
Next  the aluminum plate was subjected to a so-called graining treatment in which the surface of support was roughened in order to improve adhesion property between the support and the image-recording layer and to impart water retentivity in the non-image area. Specifically  an aqueous solution containing 1% by weight of nitric acid and 0.5% by weight of aluminum nitrate was maintained at 45?C and the aluminum plate was run in the solution to conduct electrolytic graining by providing anode side electric amount of 240C/dm2 with alternating wave of current density of 20A/dm2 and a duty ratio of 1:1 from an indirect electric supplying cell. Then  an etching treatment was conducted in an aqueous 10% by weight sodium aluminate solution at 50?C for 30 seconds and neutralization and desmut treatment were conducted at 50?C for 30 seconds using an aqueous 30% by weight sulfuric acid solution.
Further  an oxide film was formed on the support by anodic oxidization in order to improve abrasion resistance  chemical resistance and water retentivity. As the electrolyte  an aqueous 20% by weight sulfuric acid solution was used and the aluminum web was transported in the electrolyte to conduct electrolytic treatment with direct current of 14A/dm2 from an indirect electric supplying cell to form an anodic oxide film of 2.5 g/m2.
Thereafter  a silicate treatment was conducted in order to ensure hydrophilicity in the non-image area of a lithographic printing plate. Specifically  the aluminum web was transported in an aqueous 1.5% by weight sodium silicate No.3 solution while maintaining at 70?C so as to have the contact time of the aluminum web of 15 seconds  followed by washing with water. The amount of Si adhered was 10 mg/m2. The center line surface roughness (Ra) of the support thus-prepared was 0.25 µm.
(2) Formation of image-recording layer
Coating solution (1) or (2) for image-recording layer shown below was coated as shown in Table 2 on the support described above using a bar and dried by an oven to from an image-recording layer having a dry coating amount of 0.6 g/m2  thereby preparing Lithographic printing plate precursors (1) to (19)  respectively.

Aqueous dispersion of polymer fine particle shown in Table 2 12.5 g (in terms of solid content)
Infrared absorbing dye (3) having structure shown below 1.0 g
Polymer compound represented by formula (1) shown in Table 2 2.0 g
Disodium 1 5-naphthalenedisulfonate 0.1 g
Pure water 10.0 g

Infrared absorbing dye (3)

Aqueous dispersion of polymer fine particle shown in Table 2 12.5 g (in terms of solid content)
Infrared absorbing dye (3) having structure shown above 1.0 g
Polymer compound represented by formula (1) shown in Table 2 2.0 g
Water-soluble azo compound shown below 0.5 g
Disodium 1 5-naphthalenedisulfonate 0.1 g
Pure water 10.0 g

Water-soluble azo compound

(Preparation of Aqueous dispersion of polymer fine particle (1))
A stirrer  a thermometer  a dropping funnel  a nitrogen inlet tube and a reflux condenser were attached to a 1 000 ml four-neck flask and while carrying out deoxygenation by introduction of nitrogen gas  350 ml of distilled water was charged therein and heated until the internal temperature reached 80?C. To the flask was added 1.5 g of sodium dodecylsufate as a dispersing agent  then was added 0.45 g of ammonium persulfate as an initiator  and thereafter was dropwise added 45.0 g of styrene through the dropping funnel over a period of about one hour. After the completion of the dropwise addition  the mixture was continued to react as it was for 5 hours  followed by removing the unreacted monomers by steam distillation. The mixture was cooled  adjusted the pH to 6 with aqueous ammonia and finally added pure water thereto so as to have the nonvolatile content of 15% by weight to obtain Aqueous dispersion of fine polymer particle (1). The particle size distribution of the fine polymer particle had the maximum value at the particle size of 60 nm.
The particle size distribution was determined by taking an electron microphotograph of the fine polymer particle  measuring particle sizes of 5 000 fine particles in total on the photograph  and dividing a range from the largest value of the particle size measured to 0 on a logarithmic scale into 50 parts to obtain occurrence frequency of each particle size by plotting. With respect to the aspherical particle  a particle size of a spherical particle having a particle area equivalent to the particle area of the aspherical particle on the photograph was defined as the particle size.
(Preparation of Polymer fine particle (2) having thermo-reactive functional group)
In a reaction vessel  7.5 g of allyl methacrylate  7.5 g of butyl methacrylate and 200 ml of an aqueous solution of polyoxyethylene nonylphenol (concentration: 9.84 x 10-3 mol/l) were placed and while stirring at 250 rpm  the air in the system was replaced by nitrogen gas. The solution was controlled at 25?C and then 10 ml of an aqueous solution of ammonium salt of cerium (IV) (concentration: 0.984 x 10-3 mol/l) was added thereto. At this time  an aqueous solution of ammonium nitrate (concentration: 58.8 x 10-3 mol/l)) was also added and pH of the reaction solution was adjusted to 1.3 to 1.4  followed by stirring for 8 hours. The resulting solution had the solid content concentration of 9.5% by weight and the average particle size of 0.4 µm.
(Preparation of Polymer fine particle (3) having thermo-reactive functional group)
In a reaction vessel  7.5 g of glycidyl methacrylate  7.5 g of butyl methacrylate and 200 ml of an aqueous solution of polyoxyethylene nonylphenol (concentration: 9.84 x 10-3 mol/l) were placed and while stirring at 250 rpm  the air in the system was replaced by nitrogen gas. The solution was controlled at 25?C and then 10 ml of an aqueous solution of ammonium salt of cerium (IV) (concentration: 0.984 x 10-3 mol/l) was added thereto. At this time  an aqueous solution of ammonium nitrate (concentration: 58.8 x 10-3 mol/l)) was also added and pH of the reaction solution was adjusted to 1.3 to 1.4  followed by stirring for 8 hours. The resulting solution had the solid content concentration of 9.5% by weight and the average particle size of 0.4 µm.
[Evaluation of Lithographic printing plate precursor]
(1) On-press development property
Each of the lithographic printing plate precursors thus-obtained was exposed by LUXEL PLATESETTER T-6000III equipped with an infrared semiconductor laser  produced by FUJIFILM Corp. under the conditions of a rotational number of an external drum of 1 000 rpm  laser output of 70% and resolution of 2 400 dpi. The exposed image contained a solid image and a 50% halftone dot chart of a 20 µm-dot FM screen.
The exposed lithographic printing plate precursor was mounted without undergoing development processing on a plate cylinder of a printing machine (LITHRONE 26  produced by Komori Corp.). Using dampening water (ECOLITY-2 (produced by FUJIFILM Corp.)/tap water = 2/98 (volume ratio)) and VALUES-G (N) Black Ink (produced by DIC Corp.)  the dampening water and ink were supplied according to the standard automatic printing start method of LITHRONE 26 to conduct printing on 100 sheets of TOKUBISHI art paper (76.5 kg) at a printing speed of 10 000 sheets per hour.
A number of the printing papers required until the on-press development of the unexposed area of the image-recording layer on the printing machine was completed to reach a state where the ink was not transferred to the printing paper in the non-image area was measured to evaluate the on-press development property. The results obtained are shown in Table 2.
(2) Printing durability
After performing the evaluation for the on-press development property described above  the printing was continued. As the increase in a number of printed materials  the image-recording layer was gradually abraded to cause decrease in the ink density on the printed material. A number of printed materials wherein a value obtained by measuring a halftone dot area rate of the 50% halftone dot of FM screen on the printed materials using a Gretag densitometer decreased by 5% from the value measured on the 100th paper of the printing was determined to evaluate the printing durability. The results obtained are shown in Table 2.

TABLE 2: Examples 1 to 16 and Comparative Examples 1 to 3
Lithographic Printing Plate Precursor Coating Solution for Image-recording Layer Polymer Fine Particle Star Polymer or Comparative Polymer Evaluation Result of Printing
Printing Durability (x 104 sheets) On-press Development Property (sheets)
Example 1 (1) (1) (1) P-1 3.0 20
Example 2 (2) (1) (1) P-2 3.0 20
Example 3 (3) (1) (1) P-3 3.0 20
Example 4 (4) (1) (1) P-4 3.0 20
Example 5 (5) (1) (1) P-5 3.0 20
Example 6 (6) (1) (1) P-6 3.0 20
Example 7 (7) (1) (1) P-7 2.9 19
Example 8 (8) (1) (1) P-8 2.8 18
Example 9 (9) (1) (1) P-19 3.0 22
Example 10 (10) (1) (1) P-27 3.0 22
Example 11 (11) (1) (2) P-4 3.2 21
Example 12 (12) (1) (2) P-16 3.5 20
Example 13 (13) (1) (3) P-8 2.8 18
Example 14 (14) (1) (3) P-4 3.5 18
Example 15 (15) (2) (2) P-4 3.5 22
Example 16 (16) (2) (2) P-16 4.0 21
Comparative Example 1 (17) (1) (1) R-1 2.5 30
Comparative Example 2 (18) (1) (2) R-2 2.5 30
Comparative Example 3 (19) (1) (3) R-1 2.5 30

Straight-chain polymer for comparison

Mw: 2.6 x 104 Mw: 4.6 x 104

Examples 17 to 32 and Comparative Examples 4 to 6
The lithographic printing plate precursor prepared as above was exposed and subjected to development processing to conduct plate making and then printing was performed.
(1) Exposure  Development and Printing
Each of the lithographic printing plate precursors was subjected to image exposure by Violet semiconductor laser plate setter Vx9600 (having InGaN semiconductor laser (emission wavelength: 405 nm ± 10 nm/output: 30 mW)) produced by FUJIFILM Electronic Imaging Ltd. (FFEI). The image drawing was performed at resolution of 2 438 dpi using FM screen (TAFFETA 20  produced by FUJIFILM Corp.) in a plate surface exposure amount of 0.05 mJ/cm2 so as to have a halftone dot area rate of 50%.
The exposed lithographic printing plate precursor was subjected to preheating at 100?C for 30 seconds and then subjected to development processing in an automatic development processor having a structure as shown in Fig. 2 using a developer 1 having the composition shown below.
The automatic development processor comprises a developing unit 6 for developing a lithographic printing plate precursor (hereinafter  also referred to as a “PS plate”) 4 and a drying unit 10 for drying the developed PS plate 4. An insertion slot is formed in a side plate of the automatic development processor (on the left side in Fig. 1) and the PS plate 4 inserted through the insertion slot is transported into the developing unit 6 by carrying-in rollers 16 provided inside the side plate of the automatic development processor. In a developing tank 20 of the developing unit 6  transport rollers 22  a brush roller 24 and squeeze rollers 26 are provided in order from the upstream side in the transporting direction and backup rollers 28 are disposed in appropriate positions therebetween. The PS plate 4 is immersed in the developer while being transported by the transport rollers 22 and the protective layer and the unexposed area of the image-recording layer of PS plate 4 were removed by rotation of the brush roller 24 to conduct development processing. The PS plate 4 subjected to the development processing is transported into the drying unit 10 by the squeeze rollers (carrying-out rollers) 26.
In the drying unit 10  a guide roller 36 and a pair of skewer rollers 38 are disposed in order from the upstream side in the transporting direction. In the drying unit 10  drying means  for example  hot air supply means or heat generating means (not shown) is also provided. A discharge slot is provided in the drying unit 10 and the PS plate 4 dried by the drying means is discharged through the discharge slot  whereby the processing of PS plate by the automatic development processor is completed. The automatic development processor used in the examples had one brush roller having an outer diameter of 50 mm and being implanted with fiber of polybutylene terephthalate (bristle diameter: 200 µm  bristle length: 17 mm)  and the brush roller was rotated at 200 rpm (peripheral velocity at the tip of brush: 0.52 m/sec) in the same direction as the transporting direction of the lithographic printing plate precursor. The temperature of the developer was 30?C. The transportation of the lithographic printing plate precursor was conducted at transporting speed of 82 cm/min (the transporting speed was varied in the evaluation of development property). After the development processing  the lithographic printing plate was dried in the drying unit. The drying temperature was 80?C.
The lithographic printing plate obtained was mounted on a printing machine (SOR-M  produced by Heidelberg) and printing was performed at a printing speed of 6 000 sheets per hour using dampening water (EU-3 (etching solution  produced by FUJIFILM Corp.))/water/isopropyl alcohol = 1/89/10 (by volume ratio)) and TRANS-G (N) Black Ink (produced by DIC Corp.).
(Developer)
The compositions of Developer 1 used in the examples and comparative examples are shown in Table 3 below. In the developer  NEWCOL B13 is polyoxyethylene ß-naphthyl ether (average number of oxyethylene: n=13  produced by Nippon Nyukazai Co.  Ltd.) and gum arabic used has a weight average molecular weight (Mw) of 20 x 104.
TABLE 3: Developer 1 (pH: 9.8)
0.2 M Aqueous boric acid solution 25.00 parts by weight

0.2 M Aqueous sodium chloride solution 25.00 parts by weight

0.1 M Aqueous sodium hydroxide solution 40.60 parts by weight

Water 9.40 parts by weight

NEWCOL B13 (produced by Nippon Nyukazai Co.  Ltd.)
5.00 parts by weight
Gum arabic 2.50 parts by weight

Hydroxy-alkylated starch (PENON JE66  produced by Nippon Starch Chemical Co.  Ltd.)
7.00 parts by weight

(2) Evaluation
Development property and printing durability were evaluated in the following manner. The results obtained are shown in Table 4.

With the lithographic printing plates obtained by performing the development while varying the transporting speed  cyan density of the non-image area was measured by a Macbeth densitometer (produced by Gretag Macbeth Co.  Ltd.). The transporting speed at which the cyan density of the non-image area became equivalent to cyan density of the aluminum support was determined and at the transporting speed  a time period from when the top of the lithographic printing plate precursor was immersed in the developer to when the top of the lithographic printing plate precursor came out from the developer was regarded as a developing time to evaluate the development property.

After performing the evaluation for the development property described above  the printing was continued. As the increase in a number of printed materials  the image-recording layer was gradually abraded to cause decrease in the ink density on the printed materials. A number of printed materials wherein a value obtained by measuring a halftone dot area rate of the 50% halftone dot of FM screen on the printed material using a Gretag densitometer decreased by 5% from the value measured on the 100th paper of the printing was determined to evaluate the printing durability. The results obtained are shown in Table 4.

TABLE 4: Examples 17 to 32 and Comparative Examples 4 to 6
Lithographic Printing Plate Precursor Coating Solution for Image-recording Layer Polymer Fine Particle Star Polymer or Comparative Polymer Evaluation Result of Printing
Printing Durability
(x 104 sheets) Development Property (sec)
Example 17 (1) (1) (1) P-1 2.8 25
Example 18 (2) (1) (1) P-2 2.8 25
Example 19 (3) (1) (1) P-3 2.8 25
Example 20 (4) (1) (1) P-4 2.8 25
Example 21 (5) (1) (1) P-5 2.8 25
Example 22 (6) (1) (1) P-6 2.8 25
Example 23 (7) (1) (1) P-7 2.7 24
Example 24 (8) (1) (1) P-8 2.6 23
Example 25 (9) (1) (1) P-19 2.8 27
Example 26 (10) (1) (1) P-27 2.8 27
Example 27 (11) (1) (2) P-4 3.0 26
Example 28 (12) (1) (2) P-16 3.3 25
Example 29 (13) (1) (3) P-8 2.6 23
Example 30 (14) (1) (3) P-4 3.3 23
Example 31 (15) (2) (2) P-4 3.5 27
Example 32 (16) (2) (2) P-16 4.0 26
Comparative Example 4 (17) (1) (1) R-1 2.3 33
Comparative Example 5 (18) (1) (2) R-2 2.3 33
Comparative Example 6 (19) (1) (3) R-1 2.3 33

Examples 33 to 39
Each of Lithographic printing plate precursors (4) and (11) to (16) was exposed and development as described above and then heated in an oven at 130?C for 2 minutes. The printing durability was evaluated under the same conditions as in Example 17 and the results shown in Table 5 were obtained.
TABLE 5: Examples 33 to 39
Lithographic Printing Plate Precursor Printing Durability (x 104 sheets)
Example 33 (4) 3.3
Example 34 (11) 3.8
Example 35 (12) 4.5
Example 36 (13) 3.0
Example 37 (14 ) 4.5
Example 38 (15 ) 5.0
Example 39 (16 ) 6.0

WHAT IS CLAIMED IS:
1. A lithographic printing plate precursor comprising: an image-recording layer containing a thermoplastic polymer fine particle  an infrared absorbing agent and a polymer compound; and a support having a hydrophilic surface  wherein the polymer compound has a star-like shape in which a main chain is branched to three or more branches and the branched main chains have a hydrophilic group in a side chain of the branched main chain.

2. The lithographic printing plate precursor as claimed in Claim 1  wherein the hydrophilic group is at least one of a carboxyl group or a salt thereof  a sulfo group or a salt thereof and a polyethylene oxy group.

3. The lithographic printing plate precursor as claimed in Claim 1 or 2  wherein the thermoplastic polymer fine particle has a crosslinkable group.

4. The lithographic printing plate precursor as claimed in Claim 3  wherein the polymer compound having the star-like shape has  in a side chain of the branched main chain  a group capable of reacting with the crosslinkable group of the thermoplastic polymer fine particle.

5. The lithographic printing plate precursor as claimed in Claim 4  wherein both the crosslinkable group of the thermoplastic polymer fine particle and the group capable of reacting with the crosslinkable group of the thermoplastic polymer fine particle are ethylenically unsaturated groups.

6. The lithographic printing plate precursor as claimed in any one of Claims 1 to 5  wherein the image-recording layer contains a polymerization initiator.

7. The lithographic printing plate precursor as claimed in any one of Claims 1 to 6  wherein the image-recording layer is capable of being removed with at least any of printing ink and dampening water.

8. A lithographic printing method comprising:
either mounting the lithographic printing plate precursor as claimed in Claim 7 on a printing machine and exposing imagewise the mounted lithographic printing plate precursor with laser or exposing imagewise the lithographic printing plate precursor as claimed in Claim 7 with laser and mounting the exposed lithographic printing plate precursor on a printing machine; and
supplying at least one of printing ink and dampening water onto the exposed lithographic printing plate precursor to remove an unexposed area of the image-recording layer.

9. A plate making method of a lithographic printing plate precursor comprising:
exposing imagewise the lithographic printing plate precursor as claimed in any one of Claims 1 to 6 with laser; and
developing the exposed lithographic printing plate precursor with an aqueous solution having pH of from 2 to 12.

10. The plate making method of a lithographic printing plate precursor as claimed in Claim 9  which further comprises  after the developing  subjecting the lithographic printing plate precursor to a heat treatment.

ABSTRACT