EP2889138A1

EP2889138A1 - Resin composition for laser engraving, flexographic printing plate precursor for laser engraving and process for producing same, and process for making flexographic printing plate

Info

Publication number: EP2889138A1
Application number: EP14196934.5A
Authority: EP
Inventors: Atsushi Sugasaki
Original assignee: Fujifilm Corp
Current assignee: Fujifilm Corp
Priority date: 2013-12-27
Filing date: 2014-12-09
Publication date: 2015-07-01
Also published as: JP2015123714A; CN104744923A

Abstract

A resin composition for laser engraving that comprises (Component A) a binder polymer comprising an ethylenically unsaturated bond in at least one part selected from the group consisting of a main chain interior, a main chain terminal, and a side chain, (Component B) a polymerization initiator, and (Component C) a foaming agent, Component A comprising a plastomer selected from the group consisting of a polybutadiene structure-containing polymer, a polyisoprene structure-containing polymer, and a urethane (meth)acrylate, and Component A having a content of at least 50 mass% relative to the total mass of the composition.

Description

The present invention relates to a resin composition for laser engraving, a flexographic printing plate precursor for laser engraving and a process for producing the same, and a process for making a flexographic printing plate.
The so-called 'direct engraving CTP method', in which a printing plate is made by directly engraving a relief-forming layer by means of a laser, has often been proposed. In this method, a flexographic plate precursor is directly irradiated with a laser, thermal decomposition and evaporation are caused in the relief-forming layer as a result of photothermal conversion, and a concave part is thus formed. Unlike relief formation using an original image film, the direct engraving CTP method enables the relief shape to be freely controlled. Because of this, when forming an image such as an outline character, it is possible to engrave that region more deeply than another region, or in a fine halftone image, engraving with a shoulder can be carried out while taking into consideration resistance to printing pressure. As the laser used in this method, a high-output carbon dioxide laser is generally used. In the case of a carbon dioxide laser, all organic compounds can absorb irradiation energy to thus convert them to heat. On the other hand, inexpensive and small-sized semiconductor lasers have been developed, but since they employ visible and near-infrared light, it is necessary for the laser light to be absorbed and converted to heat.
As conventional flexographic printing plates, for example, those described in Published Japanese translation 2007-520379 of a PCT application, JP-A-2002-3665 (JP-A denotes a Japanese unexamined patent application publication) and JP-A-2002-103539 are known.
It is an object of the present invention to provide a laser-engravable flexographic printing plate precursor that can give a flexographic printing plate that is excellent in terms of rinsing properties for engraving residue and is excellent in terms of image quality and ink transfer properties, a process for producing same, and a process for making a flexographic printing plate.
It is another object of the present invention to provide a resin composition for laser engraving that can be suitably used in such a printing plate precursor.
The objects of the present invention have been attained by means described in <1>, <8>, <9>, <16>, or <18>. They are described together with <2> to <7>, <10> to <15>, and <17>, which are preferred embodiments.

<1> A resin composition for laser engraving comprising (Component A) a binder polymer comprising an ethylenically unsaturated bond in at least one part selected from the group consisting of a main chain interior, a main chain terminal, and a side chain, (Component B) a polymerization initiator, and (Component C) a foaming agent, Component A comprising a plastomer selected from the group consisting of a polybutadiene structure-containing polymer, a polyisoprene structure-containing polymer, and a urethane (meth)acrylate, and Component A having a content of at least 50 mass% relative to the total mass of the composition,
<2> the resin composition for laser engraving according to <1>, wherein Component A is polybutadiene, polyisoprene, or a copolymer formed by copolymerization of at least butadiene and/or isoprene,
<3> the resin composition for laser engraving according to <1> or <2>, wherein Component A is polybutadiene or polyisoprene,
<4> the resin composition for laser engraving according to any one of <1> to <3>, wherein Component B is an organic peroxide,
<5> the resin composition for laser engraving according to any one of <1> to <4>, wherein Component C is a chemical foaming agent,
<6> the resin composition for laser engraving according to any one of <1> to <5>, wherein it further comprises (Component D) a polymerizable compound,
<7> the resin composition for laser engraving according to any one of <1> to <6>, wherein it further comprises (Component E) a photothermal conversion agent,
<8> a laser-engravable flexographic printing plate precursor comprising a crosslinked relief-forming layer formed by foaming and crosslinking a layer comprising the resin composition for laser engraving according to any one of <1> to <7>,
<9> a laser-engravable flexographic printing plate precursor comprising a crosslinked relief-forming layer formed by foaming and crosslinking a layer comprising a resin composition for laser engraving comprising (Component A) a binder polymer comprising an ethylenically unsaturated bond in at least one part selected from the group consisting of a main chain interior, a main chain terminal, and a side chain, (Component B) a polymerization initiator, and (Component C) a foaming agent, Component A comprising a plastomer selected from the group consisting of a polybutadiene structure-containing polymer, a polyisoprene structure-containing polymer, and a urethane (meth)acrylate, and Component A having a content of at least 50 mass% relative to the total mass of the composition,
<10> the laser-engravable flexographic printing plate precursor according to <9>, wherein Component A is polybutadiene, polyisoprene, or a copolymer formed by copolymerization of at least butadiene and/or isoprene,
<11> the laser-engravable flexographic printing plate precursor according to <9> or <10>, wherein Component A is polybutadiene or polyisoprene,
<12> the laser-engravable flexographic printing plate precursor according to any one of <9> to <11>, wherein Component B is an organic peroxide,
<13> the laser-engravable flexographic printing plate precursor according to any one of <9> to <12>, wherein Component C is a chemical foaming agent or a physical foaming agent,
<14> the laser-engravable flexographic printing plate precursor according to any one of <9> to <13>, wherein the resin composition for laser engraving further comprises (Component D) a polymerizable compound,
<15> the laser-engravable flexographic printing plate precursor according to any one of <9> to <14>, wherein the resin composition for laser engraving further comprises (Component E) a photothermal conversion agent,
<16> a process for producing the laser-engravable flexographic printing plate precursor according to any one of <8> to <15>, comprising a step of forming a layer comprising a resin composition for laser engraving comprising (Component A) a binder polymer comprising an ethylenically unsaturated bond in at least one part selected from the group consisting of a main chain interior, a main chain terminal, and a side chain, (Component B) a polymerization initiator, and (Component C) a foaming agent, Component A comprising a plastomer selected from the group consisting of a polybutadiene structure-containing polymer, a polyisoprene structure-containing polymer, and a urethane (meth)acrylate, and Component A having a content of at least 50 mass% relative to the total mass of the composition, and a step of forming a crosslinked relief-forming layer by foaming and crosslinking the layer comprising the resin composition for laser engraving,
<17> the process for producing the laser-engravable flexographic printing plate precursor according to <16>, wherein foaming and crosslinking are carried out at least partially at the same time, and
<18> a process for making a flexographic printing plate, comprising a step of preparing the laser-engravable flexographic printing plate precursor according to any one of <8> to <15>, and an engraving step of laser-engraving the crosslinked relief-forming layer to thus form a relief layer.

In accordance with the present invention, there can be provided a laser-engravable flexographic printing plate precursor that can give a flexographic printing plate that is excellent in terms of rinsing properties for engraving residue and is excellent in terms of image quality and ink transfer properties, a process for producing same, and a process for making a flexographic printing plate.
Furthermore, in accordance with the present invention, there can be provided a resin composition for laser engraving that can be suitably used in such a printing plate precursor.
The contents of the present invention are explained in detail below. The explanation of the constituent features given below might be based on representative embodiments of the present invention, but the present invention should not be construed as being limited to such embodiments.
In the present invention, when 'to' is used, it is intended to include numerical values given before and after it as a lower limit value and an upper limit value.
Furthermore, in the present invention, '(Component A) a binder polymer comprising an ethylenically unsaturated bond in at least one part selected from the group consisting of a main chain interior, a main chain terminal, and a side chain', etc. may also be called simply 'Component A', etc.
Moreover, with regard to the notation for groups (groups of atoms) in the present specification, a notation without indication as to whether it is substituted or unsubstituted includes both one without a substituent and one with a substituent. For example, an 'alkyl group' includes not only an alkyl group without a substituent (unsubstituted alkyl group) but also an alkyl group with a substituent (substituted alkyl group).
In the present invention, 'parts by mass' and 'mass%' have the same meanings as 'parts by weight' and 'wt%' respectively.
Furthermore, in the present invention a combination of preferred embodiments is a more preferred embodiment.

(Resin composition for laser engraving)

The resin composition for laser engraving of the present invention (hereinafter, also called simply a 'resin composition' or 'composition') comprises (Component A) a binder polymer comprising an ethylenically unsaturated bond in at least one part selected from the group consisting of a main chain interior, a main chain terminal, and a side chain, (Component B) a polymerization initiator, and (Component C) a foaming agent, Component A comprising a plastomer selected from the group consisting of a polybutadiene structure-containing polymer, a polyisoprene structure-containing polymer, and a urethane (meth)acrylate, and Component A having a content of at least 50 mass% relative to the total mass of the composition.
Furthermore, the resin composition for laser engraving of the present invention preferably has foamability and crosslinkability.
It has been found as a result of an intensive investigation by the present inventors that due to the resin composition for laser engraving comprising Component A to Component C there can be provided a resin composition for laser engraving that is excellent in terms of rinsing properties for engraving residue generated when laser engraving and that is excellent in terms of image quality and ink transfer properties of a resulting flexographic printing plate.
Although the detailed mechanism is unclear, it is surmised that by carrying out foaming and crosslinking of a resin composition comprising (Component A) a binder polymer comprising an ethylenically unsaturated bond in at least one part selected from the group consisting of a main chain interior, a main chain terminal, and a side chain by means of (Component B) a polymerization initiator and (Component C) a foaming agent, it becomes possible to form a relief layer containing bubbles in a specific resin structure, and a bubble-derived concave part is also formed on the relief layer surface, thus improving ink transfer properties.
Furthermore, the present inventors have found that, if the Young's modulus of the relief layer of the flexographic printing plate is too high, although the image quality is high, the ink transfer properties are degraded, whereas if the Young's modulus is too low, although the ink transfer properties are good, the image quality is degraded, there thus being a choice between mutually exclusive alternatives. However, in the present invention, sufficient ink transfer properties can be ensured by the above effects and, moreover, it is surmised that, since bubbles give flexibility, a matrix part other than the bubbles has a specific structure, the relief layer thus having a high Young's modulus and flexibility due to bubbles, thereby achieving ink transfer properties and image quality at the same time. When hollow particles having a shell as described in Published Japanese translation 2007-520379 of a PCT application are used, both the ink transfer properties and the image quality are poor, and this is surmised to be because a concave part and flexibility of the relief layer do not fully occur.
Moreover, although the detailed mechanism is unclear, it is surmised that since a flexographic printing plate obtained using the resin composition for laser engraving of the present invention comprises a relief layer containing bubbles in a specific resin structure, the amount of engraving residue generated is small, and the rinsing properties are excellent.
In the present specification, with regard to explanation of the flexographic printing plate precursor, a layer that comprises Component A and Component B above, that is an image formation layer subjected to laser engraving, that has a flat surface, and that is an uncrosslinked crosslinkable layer is called a relief-forming layer, a layer formed by crosslinking the relief-forming layer is called a crosslinked relief-forming layer, and a layer formed by laser-engraving the crosslinked relief-layer to impart asperities to the surface is called a relief layer. When a physical foaming agent is used as Component C, the foaming agent may be included in the relief-forming layer or the crosslinked relief-forming layer by impregnation, etc. after the relief-forming layer or the crosslinked relief-forming layer is formed.
Components contained in the resin composition for laser engraving of the present invention are now explained.

(Component A) Binder polymer comprising ethylenically unsaturated bond in at least one part selected from the group consisting of main chain interior, main chain terminal, and side chain

The resin composition for laser engraving of the present invention comprises (Component A) a binder polymer comprising an ethylenically unsaturated bond in at least one part selected from the group consisting of a main chain interior, a main chain terminal, and a side chain, Component A comprising a plastomer selected from the group consisting of a polybutadiene structure-containing polymer, a polyisoprene structure-containing polymer, and a urethane (meth)acrylate, and Component A having a content of at least 50 mass% relative to the total mass of the composition.
Furthermore, Component A is preferably a water-insoluble binder polymer.
Component A comprises an ethylenically unsaturated bond in at least one part selected from the group consisting of a main chain interior, a main chain terminal, and a side chain, preferably comprises an ethylenically unsaturated bond in at least one part selected from a main chain interior and a main chain terminal, and more preferably comprises an ethylenically unsaturated bond in at least one part of a main chain interior.
In the present invention, a 'main chain' means the longest bonded chain in the molecule of a polymer compound forming a resin, a 'side chain' means a carbon chain branching from the main chain, and the side chain may comprise a heteroatom.
The ethylenically unsaturated group that Component A may comprise is not particularly limited, but is preferably a butadiene- or isoprene-derived ethylenically unsaturated bond or a (meth)acrylic group, more preferably a butadiene- or isoprene-derived ethylenically unsaturated bond or a (meth)acryloxy group, and yet more preferably a butadiene or isoprene-derived ethylenically unsaturated bond.
Furthermore, the butadiene or isoprene-derived ethylenically unsaturated bond may be an ethylenically unsaturated bond in the interior of a main chain formed by 1,4-addition, an ethylenically unsaturated bond of a side chain formed by 1,2-addition- or 3,4-addition, or an ethylenically unsaturated bond that is a positional isomer of the above ethylenically unsaturated bond; Component A preferably comprises an ethylenically unsaturated bond in the interior of a main chain formed by 1,4-addition of butadiene or isoprene.
Component A is a plastomer selected from the group consisting of a polybutadiene structure-containing polymer, a polyisoprene structure-containing polymer, and a urethane (meth)acrylate; Component A is preferably a plastomer selected from the group consisting of polybutadiene, polyisoprene, a copolymer formed by copolymerization of at least butadiene and/or isoprene, and a urethane (meth)acrylate, more preferably a plastomer selected from the group consisting of polybutadiene, polyisoprene, and a copolymer formed by copolymerization of at least butadiene and/or isoprene, yet more preferably a plastomer selected from the group consisting of polybutadiene and polyisoprene, and particularly preferably a polybutadiene that is a plastomer. With this embodiment, a flexographic printing plate is obtained that is excellent in terms of printing durability and ink transfer properties, and it is also excellent in terms of rinsing properties for engraving residue.
Component A is a plastomer at 20°C.
When as Component A an elastomer such as a thermoplastic elastomer or a rubber is used, a balance between strength and flexibility cannot be achieved, when carrying out crosslinking sufficient crosslinking cannot be carried out due to lack of flowability, and the flexographic printing plate will have poor printing durability and image quality and poor rinsing properties for engraving residue.
In the present invention a 'plastomer' means a polymer having the properties of easily being made to flow and deform by heating and being capable of solidifying into a deformed shape by cooling, as described in 'New Polymer Dictionary' edited by the Society of Polymer Science, Japan (published in 1988, Asakura Shoten, Japan). A plastomer is the opposite of an elastomer (having the properties of instantaneously deforming in response to an external force when it is applied and of recovering its original shape in a short time when the external force is removed), and is one that does not exhibit elastic deformation as in an elastomer and that easily undergoes plastic deformation.
In the present invention, the plastomer means one that, when the original size is defined as 100%, can be deformed up to 200% at room temperature (20°C) by a small external force and that does not return to a size of 130% or less when the external force is removed. The small external force specifically means an external force having a tensile strength of 1 to 100 MPa. More specifically, the plastomer here means a polymer that can be stretched twice the distance between reference lines of a test piece having a No. 4 dumbbell shape without being broken in accordance with JIS K 6251-1993 in a tensile test at 20°C based on a tensile set test in accordance with JIS K 6262-1997, and that exhibits a tensile set of at least 30% 5 minutes after removing the external tensile force after being held for 60 minutes in the stretched state with a distance twice the distance between the reference lines prior to stretching. In the present invention, all of the procedures were in accordance with the tensile set test method of JIS K 6262-1997 except that the test piece had a dumbbell shape defined by JIS K 6251, the holding time was 60 minutes, and the temperature of the test chamber was 20°C.
A polymer that cannot be measured as above, that is, a polymer that deforms without applying an external tensile force in the tensile test and does not return to its original shape or a polymer that breaks when a small external force is applied during measurement, corresponds to a plastomer. For example, a polymer that is a liquid at 20°C is a plastomer.
Furthermore, a polymer may be determined as being a plastomer from the glass transition temperature (Tg) of the polymer being less than 20°C. In the case of a polymer having two or more Tgs, all of the Tgs are less than 20°C. The Tg of a polymer may be measured by a differential scanning calorimetry (DSC) method.
The polybutadiene structure-containing polymer or the polyisoprene structure-containing polymer may be a homopolymer of butadiene or isoprene, a copolymer formed by copolymerization of butadiene and/or isoprene, or a polymer formed by modifying the above.
Furthermore, the polybutadiene structure-containing polymer or the polyisoprene structure-containing polymer may be a polymer comprising mainly an isoprene or butadiene monomer unit in the main chain, and part thereof may be hydrogenated and converted into a saturated bond. Moreover, the main chain or a terminal of the polymer may be modified with an amide, a carboxy group, a hydroxy group, a (meth)acryloyl group, etc., or may be epoxidized.
Examples of copolymerizable monomers include, but are not particularly limited to, styrene, α-methylstyrene, o-methylstyrene, p-methylstyrene, acrylonitrile, methacrylonitrile, vinyl chloride, vinylidene chloride, acrylamide, methacrylamide, vinyl methacrylamidoacetate, an acrylic acid ester, a methacrylic acid ester, acrylic acid, and methacrylic acid.
Specific examples of the polymer formed by copolymerization of at least butadiene and/or isoprene include a styrene-butadiene copolymer, a styrene-isoprene polymer, an acrylonitrile-butadiene copolymer, an acrylonitrile-isoprene copolymer, an acrylic acid ester-butadiene copolymer, an acrylic acid ester-isoprene copolymer, an acrylic acid ester-chloroprene copolymer, an acrylonitrile-butadiene-styrene copolymer, a styrene-isoprene-styrene block copolymer, a styrene-butadiene-styrene block copolymer, a terminal (meth)acrylate-modified polybutadiene, a terminal (meth)acrylate-modified polyisoprene, a terminal (meth)acrylate-modified hydrogenated polybutadiene, and a terminal (meth)acrylate-modified hydrogenated polyisoprene.
The polybutadiene structure-containing polymer and the polyisoprene structure-containing polymer may be produced by emulsion polymerization or solution polymerization.
In the present invention, the polybutadiene structure-containing polymer and the polyisoprene structure-containing polymer preferably have a proportion of aliphatic hydrocarbon (isoprene, butadiene, or hydrogenated derivative thereof)-derived monomer unit in the main chain of at least 80 mol%. It is preferable for the proportion of an aliphatic hydrocarbon-derived monomer unit in the main chain to be at least 80 mol% since the rinsing properties are good. The aliphatic hydrocarbon-derived monomer unit content is more preferably at least 90 mol% of the total monomer units forming the main chain of Component A, yet more preferably at least 95 mol%, and particularly preferably at least 99 mol%.
That is, for example, the proportion in total of isoprene and isoprene hydrogenated derivative-derived monomer units of the polyisoprene structure-containing polymer is preferably at least 80 mol%, more preferably at least 90 mol%, yet more preferably at least 95 mol%, and particularly preferably at least 99 mol%.
Similarly, the proportion in total of butadiene and butadiene hydrogenated derivative-derived monomer units in the polybutadiene structure-containing polymer is preferably at least 80 mol%, more preferably at least 90 mol%, yet more preferably at least 95 mol%, and particularly preferably at least 99 mol%.
When an isoprene or butadiene copolymer is used as Component A, it preferably comprises isoprene, butadiene, and their hydrogenated derivative-derived monomer units in a total amount of at least 80 mol%, more preferably at least 90 mol%, yet more preferably at least 95 mol%, and particularly preferably at least 99 mol%.
It is known that isoprene can be made to undergo polymerization via 1,2-, 3,4-, or 1,4-addition by means of a catalyst or reaction conditions, and the present invention may employ polyisoprene that has been polymerized by any of the above addition modes. Among them, from the viewpoint of obtaining a desired Mooney viscosity, it is preferable for cis-1,4-polyisoprene to be contained as a main component. The content of cis-1,4-polyisoprene is preferably at least 50 mass%, more preferably at least 65 mass%, yet more preferably at least 80 mass%, and particularly preferably at least 90 mass%.
Furthermore, as the polyisoprene a commercial polyisoprene may be used, and examples include the NIPOL IR series (Nippon Zeon Corporation).
It is known that butadiene can be made to undergo polymerization via 1,2- or 1,4-addition by means of a catalyst or reaction conditions, and the present invention may employ polybutadiene that has been polymerized by any of the above addition modes. Among them, from the viewpoint of obtaining a desired Mooney viscosity, it is preferable for 1,4-polybutadiene to be the main component.
The content of 1,4-polybutadiene is preferably at least 50 mass%, more preferably at least 65 mass%, yet more preferably at least 80 mass%, and particularly preferably at least 90 mass%.
The cis-isomer and trans-isomer contents are not particularly limited, and they may be selected as appropriate within a desired range of viscosity or Mooney viscosity, but from the viewpoint of rubber elasticity being exhibited after crosslinking and foaming, the cis-isomer is preferable, and the content of cis-1,4-polybutadiene is preferably at least 50 mass%, more preferably at least 65 mass%, yet more preferably at least 80 mass%, and particularly preferably at least 90 mass%.
As the polybutadiene, a commercially available product may be used, and examples thereof include the NIPOL BR series (Nippon Zeon Corporation) and the UBEPOL BR series (Ube Industries, Ltd.).
As liquid polybutadiene, commercially available products such as Kuraprene LBR-305 (Kuraray Co., Ltd.) and Ricon (Sartomer) can be cited as examples.
Examples of liquid polyisoprene include Kuraprene LIR-30 and LIR-50 (Kuraray Co., Ltd.).
Examples of a (meth)acrylate group-containing polyisoprene include a polyisoprene into which methacrylate has been introduced (Kuraprene US-203 and UC-102, Kuraray Co., Ltd.).
Examples of a terminal ethylenically unsaturated group-containing polybutadiene include a polybutadiene into which a (meth)acrylate group has been introduced (NISSO-PB TEAI-1000 and EA-3000, Nippon Soda Co., Ltd.).
Among them, the polybutadiene structure-containing polymer and the polyisoprene structure-containing polymer are more preferably polyisoprene, polybutadiene, a styrene-isoprene-styrene block copolymer, or a styrene-butadiene-styrene block copolymer, and particularly preferably polyisoprene or polybutadiene.
From the viewpoint of printing durability of a flexographic printing plate, rinsing properties for engraving residue, and ease of handling, Component A is preferably polybutadiene or polyisoprene, and particularly preferably polybutadiene.
The urethane (meth)acrylate is a polyurethane resin comprising a (meth)acryloxy group in a main chain interior, a main chain terminal, or a side chain; examples include an aliphatic urethane (meth)acrylate and an aromatic urethane (meth)acrylate. Details may be referred to in the Oligomer Handbook (Ed. by Junji Furukawa, The Chemical Daily Co., Ltd.).
The urethane (meth)acrylate preferably comprises in the molecule at least one type of bond selected from a carbonate bond and an ester bond. Due to the urethane (meth)acrylate comprising the above bond, resistance of a printing plate toward an ink washing liquid containing an ester-based solvent or an ink washing liquid containing a hydrocarbon-based solvent used in printing is improved.
Examples of carbonate bond-containing diol compounds used in the production of a hydroxy group-containing polyurethane resin include an aliphatic polycarbonate diol such as a 4,6-polyalkylene carbonate diol, an 8,9-polyalkylene carbonate diol, or a 5,6-polyalkylene carbonate diol. Furthermore, an aromatic ring-containing polycarbonate diol may be used.
A urethane bond may be introduced by a condensation reaction between a terminal hydroxy group of these compounds and a polyisocyanate compound such as a diisocyanate compound like tolylene diisocyanate, diphenylmethane diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, dicyclohexylmethane diisocyanate, tetramethylxylene diisocyanate, xylene diisocyanate, naphthalene diisocyanate, trimethylhexamethylene diisocyanate, p-phenylene diisocyanate, cyclohexylene diisocyanate, lysine diisocyanate, or triphenylmethane diisocyanate, or a triisocyanate compound such as triphenylmethane triisocyanate, 1-methylbenzene-2,4,6-triisocyanate, naphthalene-1,3,7-triisocyanate, or biphenyl-2,4,4'-triisocyanate.
The urethane (meth)acrylate may be obtained as a commercial product such as for example UV-3200, UV-3000B, UV-3700B, UV-3210EA, UV-2000B, or UV-3630ID80 of the Shikoh series (all from The Nippon Synthetic Chemical Industry Co., Ltd.), EBECRYL 230 or EBECRYL 9227EA (both from Daicel-Cytec Company Ltd.), or AU-3040, AU-3050, AU-3090, AU-3110, or AU-3120 of the Hi-Coap AU series (all from TOKUSHIKI Co., Ltd.).
As an alternative method for obtaining a urethane (meth)acrylate, etc., there is a method in which a polyurethane is formed by a polyaddition reaction between the above polyisocyanate compound and a (meth)acryloxy group-containing diol compound.
Preferred examples of the (meth)acryloxy group-containing diol compound used in this case include Blemmer GLM manufactured by NOF Corporation, and DA-212, DA-250, DA-721, DA-722, DA-911M, DA-920, DA-931, DM-201, DM-811, DM-832, and DM-851 of the 'Denacol acrylate' series manufactured by Nagase ChemteX Corporation.
The molecular weight of Component A, as a weight-average molecular weight (GPC, polystyrene basis), is preferably at least 1,000, more preferably at least 3,000, yet more preferably at least 10,000, particularly preferably at least 25,000, and most preferably at least 80,000. The molecular weight of Component A, as a weight-average molecular weight (GPC, polystyrene basis), is preferably no greater than 3,000,000, more preferably no greater than 2,000,000, and yet more preferably no greater than 1,500,000. When in this range, it is easy to process a resin composition for laser engraving, and a resulting flexographic printing plate has excellent printing durability.
In the present invention, weight-average molecular weight and number-average molecular weight may be measured by a gel permeation chromatographic (GPC) method, and determined by conversion using a reference polystyrene. Specifically, for example, an HLC-8220 GPC (Tosoh Corporation) is used as the GPC with three columns, that is, TSKgeL SuperHZM-H, TSKgeL SuperHZ4000, and TSKgeL SuperHZ2000 (Tosoh Corporation, 4.6 mm ID × 15 cm) as columns, and THF (tetrahydrofuran) as eluent. The conditions are such that the sample concentration is 0.35 mass%, the flow rate is 0.35 mL/min, the amount of sample injected is 10 µL, the measurement temperature is 40°C, and an IR detector is used. Furthermore, a calibration curve is generated from eight samples of 'reference sample TSK standard, polystyrene', that is, 'F-40', 'F-20', 'F-4', 'F-1', 'A-5000', 'A-2500', 'A-1000', and 'n-propylbenzene' manufactured by Tosoh Corporation.
In the present invention, the Mooney viscosity of Component A is not particularly limited, but is preferably no greater than 90 from the viewpoint of solvent solubility and ease of handling when mixing, more preferably no greater than 70, and yet more preferably no greater than 60. Furthermore, the lower limit value of the Mooney viscosity is not particularly limited, and Component A may be in for example a liquid, syrup, etc. state.
In the present invention, the Mooney viscosity is a value measured in accordance with JIS K 6300-1. Specifically, a cylindrical space is formed between temperature-controllable dies as a sample chamber, a rotor is disposed in a central part of the sample chamber, the sample chamber is charged with a sample to be measured, in a state in which the temperature thereof is maintained at a predetermined temperature the rotor is rotated at a defined rotational speed, and the counter-torque of the rotor caused by viscous resistance of the molten sample is detected using a load cell. In addition, the value for Mooney viscosity used in the present invention is obtained using an L type rotor for a preheating time of 1 minute and a rotor rotational time of 4 minutes, and the Mooney viscosity of a rubber sample is the Mooney viscosity (ML₁₊₄) measured after preheating at 100°C for 1 minute and rotating the rotor for 4 minutes.
The content of Component A is at least 50 mass% relative to the total mass of the resin composition for laser engraving, preferably 50 to 95 mass%, more preferably 55 to 90 mass%, yet more preferably 65 to 90 mass%, and particularly preferably 70 to 85 mass%. When the content of Component A is in this range, a relief layer that is excellent in terms of rinsing properties for engraving residue and ink transfer properties is obtained.
The resin composition for laser engraving of the present invention may comprise a binder polymer (resin component) other than Component A. Examples of the binder polymer other than Component A include non-elastomers described in JP-A-2011-136455 , unsaturated group-containing polymers described in JP-A-2010-208326 , and binder polymers described in paragraphs 0009 to 0030 of JP-A-2012-45801 .
When the resin composition for laser engraving of the present invention comprises a binder polymer other than Component A, it is preferably no greater than 30 mass% relative to the total mass of the resin composition for laser engraving, more preferably no greater than 10 mass%, yet more preferably no greater than 5 mass%, particularly preferably no greater than 2 mass%, and most preferably 0 mass%, that is, none being contained. When in this range, a relief layer that is excellent in terms of rinsing properties for engraving residue and ink transfer properties is obtained.
Preferred examples of the binder polymer other than Component A include an ethylene-propylene-diene copolymer, and more preferred examples include an ethylene-propylene-5-ethylidene-2-norbornene copolymer.

(Component B) Polymerization initiator

The resin composition for laser engraving of the present invention comprises (Component B) a polymerization initiator.
With regard to the polymerization initiator, one known to a person skilled in the art may be used without any limitations. A radical polymerization initiator, which is a preferred polymerization initiator, is explained in detail below, but the present invention should not be construed as being limited by these descriptions.
The polymerization initiator is preferably a radical polymerization initiator.
Moreover, the polymerization initiator is preferably a thermal polymerization initiator, and is more preferably a thermal radical polymerization initiator.
In the present invention, preferable polymerization initiators include (a) aromatic ketones, (b) onium salt compounds, (c) organic peroxides, (d) thio compounds, (e) hexaallylbiimidazole compounds, (f) ketoxime ester compounds, (g) borate compounds, (h) azinium compounds, (i) metallocene compounds, (j) active ester compounds, (k) compounds having a carbon halogen bond, and (I) azo compounds. Hereinafter, although specific examples of the (a) to (I) are cited, the present invention is not limited to these.
In the present invention, when applies to the relief-forming layer of the flexographic printing plate precursor, from the viewpoint of engraving sensitivity and making a favorable relief edge shape, (c) organic peroxides and (I) azo compounds are more preferable, and (c) organic peroxides are particularly preferable.
The (a) aromatic ketones, (b) onium salt compounds, (d) thio compounds, (e) hexaallylbiimidazole compounds, (f) ketoxime ester compounds, (g) borate compounds, (h) azinium compounds, (i) metallocene compounds, (j) active ester compounds, and (k) compounds having a carbon halogen bonding may preferably include compounds described in paragraphs 0074 to 0118 of JP-A-2008-63554 .
Moreover, (c) organic peroxides and (I) azo compounds preferably include the following compounds.

(c) organic peroxides

Preferred examples of the organic peroxide (c) as a radical polymerization initiator that can be used in the present invention include peroxyester-based ones such as 3,3',4,4'-tetra(t-butylperoxycarbonyl)benzophenone, 3,3',4,4'-tetra(t-amylperoxycarbonyl)benzophenone, 3,3',4,4'-tetra(t-hexylperoxycarbonyl)benzophenone, 3,3',4,4'-tetra(t-octylperoxycarbonyl)benzophenone, 3,3',4,4'-tetra(cumylperoxycarbonyl)benzophenone, 3,3',4,4'-tetra(p-isopropylcumylperoxycarbonyl)benzophenone, di-t-butylperoxyisophthalate, t-butylperoxybenzoate, t-butylperoxy-3-methylbenzoate, t-butylperoxylaurate, t-butylperoxypivalate, t-butylperoxy-2-ethylhexanoate, t-butylperoxy-3,5,5-trimethylhexanoate, t-butylperoxyneoheptanoate, t-butylperoxyneodecanoate, and t-butylperoxyacetate, α,α'-di(t-butylperoxy)diisopropyl benzene, t-butylcumylperoxide, di-t-butylperoxide, t-butylperoxyisopropylmonocarbonate, t-butylperoxy-2-ethylhexylmonocarbonate, and dicumyl peroxide.

(I) Azo compounds

Preferable (I) azo compounds as a radical polymerization initiator that can be used in the present invention include those such as 2,2'-azobisisobutyronitrile, 2,2'-azobispropionitrile, 1,1'-azobis(cyclohexane-1-carbonitrile), 2,2'-azobis(2-methylbutyronitrile), 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), 4,4'-azobis(4-cyanovaleric acid), dimethyl 2,2'-azobis(isobutyrate), 2,2'-azobis(2-methylpropionamideoxime), 2,2'-azobis[2-(2-imidazolin-2-yl)propane], 2,2'-azobis{2-methyl-N-[1,1-bis(hydroxymethyl)-2-hydroxyethyl]propionamide}, 2,2'-azobis[2-methyl-N-(2-hydroxyethyl)propionamide], 2,2'-azobis(N-butyl-2-methylpropionamide), 2,2'-azobis(N-cyclohexyl-2-methylpropionamide), 2,2'-azobis[N-(2-propenyl)-2-methyl-propionamide], and 2,2'-azobis(2,4,4-trimethylpentane).
It has been found that in the present invention the organic peroxide (c) above is preferable as a thermopolymerization initiator in the present invention from the viewpoint of the crosslinkablility of the film (relief-forming layer), and as an unexpected effect it is particularly preferable from the viewpoint of improvement of engraving sensitivity.
From the viewpoint of engraving sensitivity, combined use of an organic peroxide and (Component E) a photothermal conversion agent, which is described later, in combination is particularly preferable.
This is presumed as follows. When the relief-forming layer is cured by thermal crosslinking using an organic peroxide, an organic peroxide that did not play a part in radical generation and has not reacted remains, and the remaining organic peroxide works as an autoreactive additive and decomposes exothermally in laser engraving. As the result, energy of generated heat is added to the irradiated laser energy to thus raise the engraving sensitivity.
It will be described in detail in the explanation of photothermal converting agent, the effect thereof is remarkable when carbon black is used as the photothermal converting agent. It is considered that the heat generated from the carbon black is also transmitted to (c) an organic peroxide and, as the result, heat is generated not only from the carbon black but also from the organic peroxide, and that the generation of heat energy to be used for the decomposition of Component A etc. occurs synergistically.
With regard to Component B in the resin composition of the present invention, only one type thereof may be used or two or more types thereof may be used in combination.
The content of Component B in the resin composition for laser engraving of the present invention is preferably at least 0.01 mass%, more preferably at least 0.1 mass%, and yet more preferably at least 0.5 mass%, relative to the total mass content of the resin composition. The upper limit of the content of Component B in the resin composition for laser engraving of the present invention is not limited, but the content is preferably no greater than 30 mass%, more preferably no greater than 20 mass%, and yet more preferably no greater than 10 mass%, relative to the total mass content of the resin composition, from the viewpoint of contents of the other components.

(Component C) Foaming agent

The resin composition for laser engraving of the present invention comprises (Component C) a foaming agent.
A known foaming agent may be used; it may be a chemical foaming agent or a physical foaming agent, but it is preferably a chemical foaming agent from the viewpoint of ease of handling and storage stability of the resin composition.
The chemical foaming agent may be an inorganic compound or an organic compound, and two or more types thereof may be used.
Examples of organic chemical foaming agents include a nitrosamine compound such as dinitrosopentamethylenetetramine (DPT), an azo compound such as azodicarbonamide (ADCA), and a hydrazine compound such as 4,4'-oxybisbenzenesulfonylhydrazide (OBSH) or hydrazodicarbonamide (HDCA).
Examples of inorganic chemical foaming agents include a bicarbonate such as sodium bicarbonate, a carbonate, and a combination of a bicarbonate and an organic acid salt.
Among them, an organic chemical foaming agent is preferable, a nitrosamine compound, an azo compound, and a hydrazine compound are more preferable, an azo compound is yet more preferable, and azodicarbonamide is particularly preferable.
The azo compound used as the foaming agent is preferably a compound that, when decomposed, generates a radical that has low polymerization-initiating ability, such as an amino radical (·NH₂).
Examples of the physical foaming agent include an inert gas such as nitrogen or carbon dioxide and a volatile organic compound. Among them, it is preferable to use an inert gas, and it is preferable to use carbon dioxide or nitrogen in a supercritical state, or a mixture thereof.
When a chemical foaming agent is used, it is mixed into the resin composition to thus form a relief-forming layer or a crosslinked relief-forming layer, and it is preferable to carry out foaming by means of heat.
When a physical foaming agent is used, it is mixed into the resin composition under normal pressure or increased pressure, a relief-forming layer or a crosslinked relief-forming layer is produced, and foaming is carried out, or a relief-forming layer or a crosslinked relief-forming layer is produced, these layers are impregnated with a physical foaming agent, and foaming is carried out.
Furthermore, it is particularly preferable to produce a relief-forming layer and then carry out foaming and crosslinking by means of a polymerization initiator at least partially at the same time.
A foaming method is not particularly limited; specific examples include a batch foaming method, a press foaming method, a normal pressure foaming method, and a normal pressure secondary foaming method.
Furthermore, a foaming agent and a foaming method may be referred to in JP-A-11-106543 or 'New Edition Basics of Rubber Techniques, revised edition' edited by the Society of Rubber Science and Technology, Japan.
A combination of the polymerization initiator and the foaming agent in the resin composition for laser engraving of the present invention is preferably a combination that causes generation of a polymerization initiating species from the polymerization initiator and decomposition of the foaming agent at the same temperature.
Furthermore, the combination of the polymerization initiator and the foaming agent in the resin composition for laser engraving of the present invention is preferably a combination of an organic peroxide as the polymerization initiator and an organic foaming agent as the foaming agent, more preferably a combination of an organic peroxide as the polymerization initiator and a nitrosamine compound, an azo compound, or a hydrazine compound as the foaming agent, yet more preferably a combination of an organic peroxide as the polymerization initiator and a nitrosamine compound or an azo compound as the foaming agent, and particularly preferably a combination of an organic peroxide as the polymerization initiator and dinitrosopentamethylenetetramine or azodicarbonamide as the foaming agent. With this embodiment, it is easy to carry out foaming and crosslinking of a relief-forming layer at least partially at the same time, a flexographic printing plate that is excellent in terms of image quality and ink transfer properties can be obtained and, furthermore, rinsing properties for engraving residue are excellent.
Moreover, in addition to an organic peroxide and an organic foaming agent, it is preferable to further combine (Component F) a foaming adjuvant.
With regard to the foaming agent, one type may be used on its own or two or more types may be used in combination, and a chemical foaming agent and a physical foaming agent may be used in combination.
The content of Component C in the resin composition for laser engraving of the present invention is preferably 0.1 to 30 mass% relative to the total mass of the resin composition, more preferably 0.5 to 20 mass%, yet more preferably 1 to 15 mass%, and particularly preferably 2 to 10 mass%. When the content is in this range, a flexographic printing plate that is better in terms of image quality and ink transfer properties can be obtained and, moreover, engraving sensitivity and rinsing properties for engraving residue are better.

(Component D) Polymerizable compound

The resin composition for laser engraving of the present invention preferably comprises (Component D) a polymerizable compound in order to promote formation of a crosslinked structure. Due to it comprising Component D a film, such as a crosslinked relief layer or a relief layer, that has higher breaking strength is obtained.
Component D is a polymerizable compound other than Component A.
Component D is preferably a compound having a molecular weight of less than 3,000, more preferably a compound having a molecular weight of less than 2,000, and yet more preferably a compound having a molecular weight of less than 1,000.
Component D is preferably a radically polymerizable compound, and more preferably an ethylenically unsaturated compound.
The polymerizable compound that can be used in the present invention is preferably a polyfunctional polymerizable compound, and more preferably a polyfunctional ethylenically unsaturated compound. When in this mode, curability of the resin composition is excellent, and a flexographic printing plate having better printing durability can be obtained.
Furthermore, the polyfunctional ethylenically unsaturated compound that can be used in the present invention preferably has a molecular weight (or weight average molecular weight) of less than 2,000.
The ethylenically unsaturated compound is a compound having one or more ethylenically unsaturated groups. Regarding the ethylenically unsaturated compound, one kind may be used alone, or two or more kinds may be used in combination.
Furthermore, the compound group which belongs to ethylenically unsaturated compounds is widely known in the pertinent industrial fields, and in the present invention, these compounds can be used without particular limitations. These compounds have chemical forms such as, for example, monomer, prepolymer (namely, dimer, trimer and oligomer), or copolymer thereof, and mixture thereof.
As the ethylenically unsaturated compound, a polyfunctional monomer is preferably used. Molecular weights of these polyfunctional monomers are preferably at least 200 but less than 2,000, more preferably 200 to 1,000, and yet more preferably 200 to 700.
The polyfunctional monomer is preferably a compound having 2 to 20 terminal ethylenically unsaturated groups.
Examples of the polyfunctional monomer include an unsaturated carboxylic acid (e.g. acrylic acid, methacrylic acid, itaconic acid, crotonic acid, isocrotonic acid, maleic acid, etc.), and an ester or amide thereof. An ester between an unsaturated carboxylic acid and an aliphatic polyhydric alcohol compound and an amide between an unsaturated carboxylic acid and an aliphatic polyvalent amine compound are preferably used. Furthermore, an addition reaction product between a polyfunctional isocyanate or an epoxy and an unsaturated carboxylic acid ester or amide having a nucleophilic substituent such as a hydroxy group or an amino group and a dehydration-condensation reaction product with a polyfunctional carboxylic acid are also suitably used. Furthermore, an addition reaction product between a monofunctional or polyfunctional alcohol or amine and an unsaturated carboxylic acid ester or amide having an electrophilic substituent such as an isocyanate group or an epoxy group, and a substitution reaction product between a monofunctional or polyfunctional alcohol or amine and an unsaturated carboxylic acid ester or amide having a leaving substituent such as a halogeno group or a tosyloxy group are also desirable. As another example, a group of compounds formed by replacing the unsaturated carboxylic acid with a vinyl compound, an allyl compound, an unsaturated phosphonic acid, or styrene may also be used.
The ethylenically unsaturated group contained in the polyfunctional ethylenically unsaturated compound is preferably an acrylate, methacrylate, vinyl compound, or allyl compound residue, and particularly preferably an acrylate or methacrylate, from the viewpoint of reactivity.
Specific examples of ester monomers comprising an ester of an aliphatic polyhydric alcohol compound and an unsaturated carboxylic acid include acrylic acid esters such as ethylene glycol diacrylate, triethylene glycol diacrylate, 1,3-butanediol diacrylate, tetramethylene glycol diacrylate, propylene glycol diacrylate, neopentyl glycol diacrylate, trimethylolpropane triacrylate, trimethylolpropane tri(acryloyloxypropyl) ether, trimethylolethane triacrylate, hexanediol diacrylate, 1,4-cyclohexanediol diacrylate, tetraethylene glycol diacrylate, pentaerythritol diacrylate, pentaerythritol triacrylate, pentaerythritol tetraacrylate, dipentaerythritol diacrylate, dipentaerythritol hexaacrylate, sorbitol triacrylate, sorbitol tetraacrylate, sorbitol pentaacrylate, sorbitol hexaacrylate, tri(acryloyloxyethyl) isocyanurate, and a polyester acrylate oligomer.
Examples of methacrylic acid esters include tetramethylene glycol dimethacrylate, triethylene glycol dimethacrylate, neopentyl glycol dimethacrylate, trimethylolpropane trimethacrylate, trimethylolethane trimethacrylate, ethylene glycol dimethacrylate, 1,3-butanediol dimethacrylate, hexanediol dimethacrylate, pentaerythritol dimethacrylate, pentaerythritol trimethacrylate, pentaerythritol tetramethacrylate, dipentaerythritol dimethacrylate, dipentaerythritol hexamethacrylate, sorbitol trimethacrylate, sorbitol tetramethacrylate, bis[p-(3-methacryloxy-2-hydroxypropoxy)phenyl]dimethylmethane, and bis[p-(methacryloxyethoxy)phenyl]dimethy methane.
As examples of other esters, aliphatic alcohol-based esters described in JP-B-46-27926 (JP-B denotes a Japanese examined patent application publication), JP-B-51-47334 and JP-A-57-196231 , those having an aromatic skeleton described in JP-A-59-5240 , JP-A-59-5241 , and JP-A-2-226149 , those having an amino group described in JP-A-1-165613 , etc. may also be used preferably.
The above-mentioned ester monomers may be used as a mixture.
Furthermore, specific examples of amide monomers including an amide of an aliphatic polyamine compound and an unsaturated carboxylic acid include methylenebisacrylamide, methylenebismethacrylamide, 1,6-hexamethylenebisacrylamide, 1,6-hexamethylenebismethacrylamide, diethylenetriaminetrisacrylamide, xylylenebisacrylamide, and xylylenebismethacrylamide.
Preferred examples of other amide-based monomers include those having a cyclohexylene structure described in JP-B-54-21726 .
Furthermore, a urethane-based addition-polymerizable compound produced by an addition reaction of an isocyanate and a hydroxy group is also suitable, and specific examples thereof include a vinylurethane compound comprising two or more polymerizable vinyl groups per molecule in which a hydroxy group-containing vinyl monomer represented by Formula (i) below is added to a polyisocyanate compound having two or more isocyanate groups per molecule described in JP-B-48-41708 .

CH₂=C(R)COOCH₂CH(R')OH (ii)

(R and R' independently denote H or CH₃.)
Furthermore, urethane acrylates described in JP-A-51-37193 , JP-B-2-32293 and JP-B-2-16765 , urethane compounds having an ethylene oxide-based skeleton described in JP-B-58-49860 , JP-B-56-17654 , JP-B-62-39417 and JP-B-62-39418 are also preferable.
Furthermore, by use of addition-polymerizable compounds having an amino structure in the molecule described in JP-A-63-277653 , JP-A-63-260909 , and JP-A-1-105238 , a resin composition for laser engraving which can crosslink in a short time can be obtained.
Other examples of the polyfunctional ethylenically unsaturated compound include polyester acrylates such as those described in JP-A-48-64183 , JP-B-49-43191 , and JP-B-52-30490 , and polyfunctional acrylates and methacrylates such as epoxy acrylates etc. formed by a reaction of an epoxy resin and (meth)acrylic acid. Examples also include specific unsaturated compounds described in JP-B-46-43946 , JP-B-1-40337 , and JP-B-1-40336 , and vinylphosphonic acid-based compounds described in JP-A-2-25493 . In some cases, perfluoroalkyl group-containing structures described in JP-A-61-22048 are suitably used. Moreover, those described as photocuring monomers or oligomers in the Journal of the Adhesion Society of Japan, Vol. 20, No. 7, pp. 300 to 308 (1984) may also be used.
Examples of the monofunctional ethylenically unsaturated compound having one ethylenically unsaturated bond in the molecule include esters of unsaturated carboxylic acids, for example, acrylic acid, methacrylic acid, itaconic acid, crotonic acid, isocrotonic acid, and maleic acid, and monohydric alcohol compounds, and amides of unsaturated carboxylic acids and monovalent amine compounds.
Furthermore, addition reaction products of an unsaturated carboxylic acid ester or amide having a nucleophilic substituent such as a hydroxyl group, an amino group or a mercapto group, and an isocyanate or an epoxide, and dehydration condensation reaction products with a monofunctional or polyfunctional carboxylic acid, are also suitably used.
Furthermore, addition reaction products of an unsaturated carboxylic acid ester or amide having an electrophilic substituent such as an isocyanato group or an epoxy group, and an alcohol, an amine or a thiol, and substitution reaction products of an unsaturated carboxylic acid ester or amide having a detachable substituent such as a halogeno group or a tosyloxy group, and an alcohol, an amine or a thiol, are also suitable.
Also, as other examples, a group of compounds substituted with unsaturated phosphonic acid, styrene, vinyl ether or the like instead of the unsaturated carboxylic acid described above, can also be used.
The polymerizable compound is not particularly limited, and various known compounds can be used in addition to the compounds exemplified above. For example, those compounds described in JP-A-2009-204962 and the like may also be used.
Examples of monofunctional ethylenically unsaturated compounds include (meth)acrylic acid derivatives such as methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, butoxyethyl (meth)acrylate, carbitol (meth)acrylate, cyclohexyl (meth)acrylate, benzyl (meth)acrylate, N-methylol(meth)acrylamide, and epoxy (meth)acrylate, as well as N-vinyl compounds such as N-vinylpyrrolidone and N-vinylcaprolactam, and allyl compounds such as allyl glycidyl ether, diallyl phthalate, and triallyl trimellitate.
Among them, Component D is preferably an acrylate (an acrylic acid ester compound) or a methacrylate (a methacrylic acid ester compound), and particularly preferably an ester of an aliphatic polyol and an acrylic acid or a methacrylic acid. Component D preferably has 2 to 20 (meth)acryloyloxy groups per one molecule, more preferably has 2 to 6, and yet more preferably has 2 to 4.
Among them, Component D includes a compound selected from the group consisting of hexanediol di(meth)acrylate, decanediol di(meth)acrylate, tricyclodecanedimethanol di(meta)acrylate, pentaerythritol tetra(meth)acrylate, and dipentaerythritol hexa(meth)acrylate.
The total content of Component D in the resin composition for laser engraving of the present invention is preferably 1 to 50 mass%, more preferably 3 to 40 mass%, and yet more preferably 5 to 30 mass%, relative to the total solids content of the resin composition. When the content is in this range, a relief-forming layer that is formed by the resin composition for laser engraving has excellent printing durability.
Furthermore, the total content of Component C in the resin composition for laser engraving of the present invention is preferably 10 to 50 mass% relative to 100 parts by mass of Component A, from the viewpoint of improvement of printing durability and decreasing of the content of engraving residue generated.

(Component E) Photothermal conversion

The resin composition for laser engraving of the present invention preferably comprises (Component E) a photothermal conversion agent. That is, it is considered that the photothermal conversion agent in the present invention can promote the thermal decomposition of a cured material during laser engraving by absorbing laser light and generating heat. Therefore, it is preferable that a photothermal conversion agent capable of absorbing light having a wavelength of laser used for graving be selected.
When a laser (a YAG laser, a semiconductor laser, a fiber laser, a surface emitting laser, etc.) emitting infrared at a wavelength of 700 to 1,300 nm is used as a light source for laser engraving, it is preferable for the relief printing plate precursor for laser engraving which is produced by using the resin composition for laser engraving of the present invention to comprise a photothermal conversion agent that has a maximun absorption wavelength at 700 to 1,300 nm.
As the photothermal conversion agent in the present invention, various types of dye or pigment are used.
With regard to the photothermal conversion agent, examples of dyes that can be used include commercial dyes and known dyes described in publications such as 'Senryo Binran' (Dye Handbook) (Ed. by The Society of Synthetic Organic Chemistry, Japan, 1970). Specific preferable examples include dyes having a maximum absorption wavelength from 700 nm to 1,300 nm, and such preferable examples include azo dyes, metal complex salt azo dyes, pyrazolone azo dyes, naphthoquinone dyes, anthraquinone dyes, phthalocyanine dyes, carbonium dyes, diimmonium compounds, quinone imine dyes, methine dyes, cyanine dyes, squarylium colorants, pyrylium salts, and metal thiolate complexes. Examples of dyes preferably used include cyanine-based colorants such as heptamethine cyanine colorants, oxonol-based colorants such as pentamethine oxonol colorants, phthalocyanine-based colorants and dyes described in paragraphs 0124 to 0137 of JP-A-2008-63554 .
With regard to the photothermal conversion agent used in the present invention, examples of pigments include commercial pigments and pigments described in the Color Index (C.I.) Handbook, 'Saishin Ganryo Binran' (Latest Pigments Handbook) (Ed. by Nippon Ganryo Gijutsu Kyokai, 1977), 'Saisin Ganryo Ouyogijutsu' (Latest Applications of Pigment Technology) (CMC Publishing, 1986), 'Insatsu Inki Gijutsu' (Printing Ink Technology) CMC Publishing, 1984). Furthermore, examples of pigments include pigments described in paragraphs 0122 to 0125 of JP-A-2009-178869 .
Among these pigments, carbon black is preferable.
Any carbon black, regardless of classification by ASTM and application (e.g. for coloring, for rubber, for dry cell, etc.), may be used as long as dispersibility, etc. in the composition is stable. Carbon black includes for example furnace black, thermal black, channel black, lamp black, and acetylene black. In order to make dispersion easy, a black colorant such as carbon black may be used as color chips or a color paste by dispersing it in nitrocellulose or a binder in advance using, as necessary, a dispersant, and such chips and paste are readily available as commercial products. Furthermore, examples of carbon black include carbon black described in paragraphs 0130 to 0134 of JP-A-2009-178869 .
The photothermal conversion agent in the resin composition for laser engraving of the present invention may be used singly or in a combination of two or more compounds.
The content of the photothermal conversion agent capable in the resin composition for laser engraving of the present invention largely depends on the size of the molecular extinction coefficient characteristic to the molecule, and is preferably 0.01 to 30 mass% relative to the total solids content of the resin composition, more preferably 0.05 to 20 mass%, and yet more preferably 0.1 to 10 mass%.

(Component F) Foaming adjuvant

The resin composition for laser engraving of the present invention preferably further comprises (Component F) a foaming adjuvant. Due to the use of a foaming adjuvant, it is possible to easily adjust the decomposition temperature of a foaming agent and the size, etc. of bubbles.
A known foaming adjuvant may be used, and the type and amount thereof may be selected as appropriate according to the type of foaming agent used and/or the temperature (decomposition temperature) at which a polymerization initiating species is generated from a polymerization initiator that is used.
Examples of the foaming adjuvant include a urea-based compound such as urea or a urea derivative, a zinc compound such as flowers of zinc (zinc oxide), zinc carbonate, or zinc stearate, an organic acid compound such as salicylic acid, phthalic acid, stearic acid, or oxalic acid, triethanolamine, and titanium white.
Among them, it is preferable to use a urea-based compound, a zinc compound, and/or an organic acid compound, it is more preferable to use a urea-based compound, and it is yet more preferable to use a urea-based compound, a zinc compound, and an organic acid compound.
With regard to the foaming adjuvant, one type may be used on its own or two or more types may be used in combination.
The content of the foaming adjuvant in the resin composition for laser engraving of the present invention is preferably 0.1 to 20 mass% relative to the total mass of the resin composition, more preferably 0.5 to 15 mass%, yet more preferably 1 to 10 mass%, and particularly preferably 2 to 10 mass%. When the content is in this range, a flexographic printing plate that is excellent in terms of image quality and ink transfer properties can be obtained, and engraving sensitivity and rinsing properties for engraving residue are better.
In addition to Component A to Component F, various types of components that can be contained in the resin composition for laser engraving of the present invention are explained below.

When preparing the resin composition for laser engraving of the present invention, a solvent may be used, but is preferably not used.
As the solvent, an organic solvent is preferably used.
Specific preferred examples of the aprotic organic solvent include acetonitrile, tetrahydrofuran, dioxane, toluene, propylene glycol monomethyl ether acetate, methyl ethyl ketone, acetone, methyl isobutyl ketone, ethyl acetate, butyl acetate, ethyl lactate, N,N-dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide.
Specific preferred examples of the protic organic solvent include methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 1-methoxy-2-propanol, ethylene glycol, diethylene glycol, and 1,3-propanediol.
Among these, propylene glycol monomethyl ether acetate is preferable.
The content of the solvent in the resin composition for laser engraving of the present invention is preferably no greater than 10 mass% relative to the total mass of the resin composition, more preferably no greater than 1 mass%, yet more preferably no greater than 0.1 mass%, and particularly preferably the solvent is not comprised.

The resin composition for laser engraving of the present invention may comprise as appropriate various types of known additives as long as the effects of the present invention are not inhibited. Examples include a plasticizer, a filler, a wax, a process oil, an a metal oxide, an antiozonant, an anti-aging agent, a polymerization inhibitor, and a colorant, and one type thereof may be used on its own or two more types may be used in combination.

(Laser engravable flexographic printing plate precursor and process for producing same)

A first embodiment of the laser-engravable flexographic printing plate precursor of the present invention comprises a relief-forming layer comprising the resin composition for laser engraving of the present invention.
Furthermore, a second embodiment of the laser-engravable flexographic printing plate precursor of the present invention comprises a crosslinked relief-forming layer that is formed by foaming and crosslinking a layer comprising the resin composition for laser engraving of the present invention.
In the present invention, the 'laser-engravable flexographic printing plate precursor' means a relief-forming layer that comprises a resin composition for laser engraving, that has foamability and crosslinkablility, but that is in both or either one of a state prior to foaming and crosslinking and a state in which it has been foamed and crosslinked by means of light and/or heat.
The 'relief-forming layer' referred to in the present invention means a layer in a state prior to foaming and crosslinking, that is, it is a layer that comprises the resin composition for laser engraving of the present invention and that may be dried as necessary.
A 'flexographic printing plate' is made by laser-engraving a printing plate precursor comprising a crosslinked relief-forming layer.
In the present invention, the 'crosslinked relief-forming layer' means a layer that is formed by foaming and crosslinking the relief-forming layer. The above foaming and crosslinking may be carried out by means of heat and/or light. Furthermore, the above crosslinking is not particularly limited as long as the resin composition is cured; examples include a crosslinked structure due to a reaction between Component A and Component B, etc. The above foaming is foaming due to Component C, and a crosslinked relief-forming layer that is obtained comprises bubbles.
Furthermore, in the present invention a 'relief layer' means a layer of a flexographic printing plate that has been engraved by laser, that is, the crosslinked relief-forming layer after laser engraving.
The laser-engravable flexographic printing plate precursor of the present invention comprises a relief-forming layer comprising a resin composition for laser engraving comprising the components as described above, and a crosslinked relief-forming layer that is formed by foaming and crosslinking a layer comprising a resin composition for laser engraving comprising the components as described above. The relief-forming layer and the crosslinked relief-forming layer are preferably provided above a support.
The laser-engravable flexographic printing plate precursor may further comprise, as necessary, an adhesive layer between a support and the relief-forming layer or the crosslinked relief-forming layer, and a slip coat layer and a protective film above the relief-forming layer or the crosslinked relief-forming layer.

<Relief-forming layer>

The relief-forming layer is a layer formed from the resin composition for laser engraving of the present invention, and is preferably crosslinkable by heat.
As a mode in which a flexographic printing plate is prepared using the flexographic printing plate precursor for laser engraving, a mode in which a flexographic printing plate is prepared by crosslinking a relief-forming layer to thus form a flexographic printing plate precursor having a crosslinked relief-forming layer, and the crosslinked relief-forming layer (hard relief-forming layer) is then laser-engraved to thus form a relief layer is preferable. By crosslinking the relief-forming layer, it is possible to prevent abrasion of the relief layer during printing, and it is possible to obtain a flexographic printing plate having a relief layer with a sharp shape after laser engraving.
The relief-forming layer may be formed by molding the resin composition for laser engraving that has the above-mentioned components for a relief-forming layer into a sheet shape or a sleeve shape. The relief-forming layer is usually provided above a support, which is described later, but it may be formed directly on the surface of a member such as a cylinder of equipment for plate producing or printing or may be placed and immobilized thereon, and a support is not always required.
A case in which the relief-forming layer is mainly formed in a sheet shape is explained as an example below.

A material used for the support of the flexographic printing plate precursor for laser engraving is not particularly limited, but one having high dimensional stability is preferably used, and examples thereof include metals such as steel, stainless steel, or aluminum, plastic resins such as a polyester (e.g. polyethylene terephthalate (PET), polybutylene terephthalate (PBT), or polyacrylonitrile (PAN)) or polyvinyl chloride, synthetic rubbers such as styrene-butadiene rubber, and glass fiber-reinforced plastic resins (epoxy resin, phenolic resin, etc.). As the support, a PET film or a steel substrate is preferably used. The configuration of the support depends on whether the relief-forming layer is in a sheet shape or a sleeve shape.

An adhesive layer may be provided between the relief-forming layer and the support for the purpose of strengthening the adhesion between the two layers. Examples of materials (adhesives) that can be used in the adhesive layer include those described in 'Handbook of Adhesives', Second Edition, Ed by I. Skeist, (1977).

For the purpose of preventing scratches or dents in the relief-forming layer surface or the crosslinked relief-forming layer surface, a protection film may be provided on the relief-forming layer surface or the crosslinked relief-forming layer surface. The thickness of the protection film is preferably 25 to 500 µm, and more preferably 50 to 200 µm. The protection film may employ, for example, a polyester-based film such as PET or a polyolefin-based film such as PE (polyethylene) or PP (polypropylene). The surface of the film may be made matte. The protection film is preferably peelable.
When the protection film is not peelable or conversely has poor adhesion to the relief-forming layer, a slip coat layer may be provided between the two layers. The material used in the slip coat layer preferably employs as a main component a resin that is soluble or dispersible in water and has little tackiness, such as polyvinyl alcohol, polyvinyl acetate, partially saponified polyvinyl alcohol, a hydroxyalkylcellulose, an alkylcellulose, or a polyamide resin.
A process for producing a flexographic printing plate precursor for laser engraving is not particularly limited, and examples thereof include a method in which a coating solution of a resin composition for laser engraving is prepared, solvent is removed from this coating solution composition for laser engraving, and it is then melt-extruded onto a support. Alternatively, a method may be employed in which a resin composition for laser engraving is cast onto a support, and this is dried in an oven to thus remove solvent from the resin composition.
Among them, the process for producing a flexographic printing plate precursor for laser engraving of the present invention is preferably a production process comprising a layer formation step of forming a relief-forming layer from the resin composition for laser engraving of the present invention and a crosslinking step of forming a crosslinked relief-forming layer by foaming and crosslinking the layer comprising the resin composition for laser engraving.
Subsequently, as necessary, a protection film may be laminated on the relief-forming layer. Laminating may be carried out by compression-bonding the protection film and the relief-forming layer by means of heated calendar rollers, etc. or putting a protection film into intimate contact with a relief-forming layer whose surface is impregnated with a small amount of solvent.
When a protection film is used, a method in which a relief-forming layer is first layered on a protection film and a support is then laminated may be employed.
When an adhesive layer is provided, it may be dealt with by use of a support coated with an adhesive layer. When a slip coat layer is provided, it may be dealt with by use of a protection film coated with a slip coat layer.

The process for producing a laser-engravable flexographic printing plate precursor of the present invention preferably comprises a layer formation step of forming a relief-forming layer using the resin composition for laser engraving of the present invention.
As a method for forming a relief-forming layer, a method in which the resin composition for laser engraving of the present invention is prepared and rubber-extruded above a support, a method in which the resin composition for laser engraving of the present invention is prepared, the solvent is removed from the resin composition for laser engraving as necessary, and melt-extrusion above a support is then carried out, and a method in which the resin composition for laser engraving of the present invention is prepared, the resin composition for laser engraving of the present invention is cast above a support, and this is dried in an oven so as to remove the solvent can be cited as preferred examples.
The resin composition for laser engraving may be produced by for example a method in which Component A to Component C are mixed, and Component D, etc. are optionally added in sequence, a method in which Component A and Component B are mixed, Component D, etc. are optionally added in sequence so as to adjust the resin composition, and Component C is subsequently mixed or impregnated, or a method in which Component A to Component C and an optional component, etc. are dissolved or dispersed in an appropriate solvent, and Component D, etc. are optionally added later, but a method that does not use a solvent is preferred. Since it is preferable to remove most of a solvent component in a stage in which a flexographic printing plate precursor is produced, it is preferable to use a volatile low molecular weight alcohol (e.g. methanol, ethanol, n-propanol, isopropanol, propylene glycol monomethyl ether), etc. as the solvent, and adjust the temperature, etc. so that the total amount of solvent added is as little as possible.
The thickness before and after crosslinking of the (crosslinked) relief-forming layer of the laser-engravable flexographic printing plate precursor is preferably at least 0.05 mm but no greater than 10 mm, more preferably at least 0.05 mm but no greater than 7 mm, and yet more preferably at least 0.05 mm but no greater than 3 mm.

The process for producing a laser-engravable flexographic printing plate precursor of the present invention is a production process comprising a crosslinking step of foaming and crosslinking a layer comprising the resin composition for laser engraving to thus form a crosslinked relief-forming layer.
With regard to the crosslinking step, it is preferable to carry out foaming and crosslinking by means of heat or carry out foaming by means of heat and crosslinking by means of light, and it is more preferable to carry out foaming and crosslinking by means of heat.
Furthermore, foaming and crosslinking in the crosslinking step may be carried out at the same time or separately; it is preferable to carry out crosslinking and then carry out foaming, or carry out foaming and crosslinking at least partially at the same time, and it is more preferable to carry out foaming and crosslinking at least partially at the same time. When carrying out foaming and crosslinking at least partially at the same time by means of heat, there can be cited a method in which they are carried out at a temperature at which both a thermopolymerization initiator and a foaming agent that are contained decompose. When foaming by means of heat and crosslinking by means of light are carried out at least partially at the same time, there can be cited a method in which, while heating to a temperature at which a foaming agent decomposes, irradiation with light by which a photopolymerization initiator is decomposed is carried out.
The heating temperature in the crosslinking step may be set freely according to the decomposition temperature of the polymerization initiator and the foaming agent; it is preferably 90°C to 250°C, more preferably 100°C to 180°C, yet more preferably 100°C to 160°C, and particularly preferably 120°C to 160°C. When in this mode, it is possible to easily carry out foaming and crosslinking by means of heat at the same time.
As heating means in the crosslinking step, there can be cited a method in which a printing plate precursor is heated in a hot air oven or a far infrared oven for a predetermined time or a method in which it is put in contact with a heated roll for a predetermined time.
Furthermore, irradiation with light in the crosslinking step is generally carried out over the entire face of the relief-forming layer. Examples of light (also called 'actinic radiation') include visible light, UV light, and an electron beam; UV light is the most preferable. When the side of a relief-forming layer on which there is a support, etc., that is, a substrate for immobilizing the relief-forming layer, is defined as the reverse face, only the front face may be irradiated with light, but if the support is a transparent film through which actinic radiation passes, it is preferable to also irradiate the reverse face with light. When a protective film is present, irradiation of the front face may be carried out with the protective film provided as it is or after peeling off the protective film. When oxygen is present, since there is a possibility that inhibition of polymerization will be caused, actinic radiation may be applied after covering the relief-forming layer with a polyvinyl chloride sheet and evacuating.
The crosslinked relief-forming layer obtained after the crosslinking step comprises bubbles in the interior and the surface due to foaming.
Bubbles in the crosslinked relief-forming layer may have a closed cell foam structure or an open cell foam structure; from the viewpoint of printing durability and image quality a closed cell foam structure is preferable.
The volume foaming ratio in each of the relief-forming layer, the crosslinked relief-forming layer, and the relief layer in the present invention is preferably 10% to 200% from the viewpoint of printing durability and image quality, more preferably 50% to 180%, and yet more preferably 70% to 140%.
The volume foaming ratio may be determined by measuring the change in volume between that before and that after foaming or may be determined by calculating the total volume of bubble portions in the crosslinked relief-forming layer or the relief layer after foaming.
Furthermore, the volume-average diameter of bubbles in the crosslinked relief-forming layer is preferably 0.1 to 20 µm, more preferably 0.5 to 15 µm, and yet more preferably 1 to 10 µm. When in this range, ink transfer properties and image quality are better.
As a method for measuring the volume-average diameter of bubbles in the crosslinked relief-forming layer, there can be cited a method in which a cross-section of the crosslinked relief-forming layer is examined and measured using an optical or electron microscope and a method in which a picture of a cross-section is taken and subjected to image processing for calculation.
Furthermore, in the crosslinking step, foaming may be carried out in a state in which opposite faces of a relief-forming layer are sandwiched between supports or between a support and a substrate such as a heating device surface from the viewpoint of smoothness or adjustment of the foaming ratio of the resulting crosslinked relief-forming layer. It is also preferable, from the viewpoint of smoothness or adjustment of the foaming ratio of the resulting crosslinked relief-forming layer, for foaming in the sandwiched state to be carried out while applying pressure from at least one of the faces.
Moreover, the support may be peeled off from the crosslinked relief-forming layer after the crosslinking step, or from the viewpoint of strength and smoothness the support provided during the crosslinking step may be peeled off from the crosslinked relief-forming layer and subsequently another support may be provided on the crosslinked relief-forming layer on the side where the above support has been provided or the side opposite thereto.

(Flexographic printing plate and process for making same)

The process for making a flexographic printing plate of the present invention preferably comprises a step of preparing the laser-engravable flexographic printing plate precursor of the present invention, and an engraving step of laser-engraving the crosslinked relief-forming layer to thus form a relief layer.
The flexographic printing plate of the present invention is a flexographic printing plate having a relief layer obtained by foaming and crosslinking and laser-engraving a layer formed from the resin composition for laser engraving of the present invention, and is preferably a flexographic printing plate made by the process for producing a flexographic printing plate of the present invention.
The layer formation step and the crosslinking step in the process for producing a flexographic printing plate of the present invention mean the same as the layer formation step and the crosslinking step in the above-mentioned process for producing a flexographic printing plate precursor for laser engraving, and preferred ranges are also the same.

The process for producing a flexographic printing plate of the present invention preferably comprises an engraving step of laser-engraving the flexographic printing plate precursor having a crosslinked relief-forming layer.
The engraving step is a step of laser-engraving a crosslinked relief-forming layer that has been crosslinked in the crosslinking step to thus form a relief layer. Specifically, it is preferable to engrave a crosslinked relief-forming layer that has been crosslinked with laser light according to a desired image, thus forming a relief layer. Furthermore, a step in which a crosslinked relief-forming layer is subjected to scanning irradiation by controlling a laser head using a computer in accordance with digital data of a desired image can preferably be cited.
This engraving step preferably employs an infrared laser (an IR laser). When irradiated with an infrared laser, molecules in the crosslinked relief-forming layer undergo molecular vibration, thus generating heat. When a high power laser such as a carbon dioxide laser or a YAG laser is used as the infrared laser, a large quantity of heat is generated in the laser-irradiated area, and molecules in the crosslinked relief-forming layer undergo molecular scission or ionization, thus being selectively removed, that is, engraved. The advantage of laser engraving is that, since the depth of engraving can be set freely, it is possible to control the structure three-dimensionally. For example, for an area where fine halftone dots are printed, carrying out engraving shallowly or with a shoulder prevents the relief from collapsing due to printing pressure, and for a groove area where a fine outline character is printed, carrying out engraving deeply makes it difficult for ink the groove to be blocked with ink, thus enabling breakup of an outline character to be suppressed.
In particular, when engraving is carried out using an infrared laser that corresponds to the absorption wavelength of the photothermal conversion agent, it becomes possible to selectively remove the crosslinked relief-forming layer at higher sensitivity, thus giving a relief layer having a sharp image.
As the infrared laser used in the engraving step, from the viewpoint of productivity, cost, etc., a carbon dioxide laser (a CO₂ laser) or a semiconductor laser is preferable. In particular, a fiber-coupled semiconductor infrared laser (FC-LD) is preferably used. In general, compared with a CO₂ laser, a semiconductor laser has higher efficiency laser oscillation, is less expensive, and can be made smaller. Furthermore, it is easy to form an array due to the small size. Moreover, the shape of the beam can be controlled by treatment of the fiber.
With regard to the semiconductor laser, one having a wavelength of 700 to 1,300 nm is preferable, one having a wavelength of 800 to 1,200 nm is more preferable, one having a wavelength of 860 to 1,200 nm is yet more preferable, and one having a wavelength of 900 to 1,100 nm is particularly preferable.
Furthermore, the fiber-coupled semiconductor laser can output laser light efficiently by being equipped with optical fiber, and this is effective in the engraving step in the present invention. Moreover, the shape of the beam can be controlled by treatment of the fiber. For example, the beam profile may be a top hat shape, and energy can be applied stably to the plate face. Details of semiconductor lasers are described in 'Laser Handbook 2^nd Edition' The Laser Society of Japan, Applied Laser Technology, The Institute of Electronics and Communication Engineers, etc.
Moreover, as plate making equipment comprising a fiber-coupled semiconductor laser that can be used suitably in the process for making a flexographic printing plate employing the flexographic printing plate precursor of the present invention, those described in detail in JP-A-2009-172658 and JP-A-2009-214334 can be cited. Such equipment comprising a fiber-coupled semiconductor laser can be used to produce a flexographic printing plate of the present invention.
The process for producing a flexographic printing plate of the present invention may as necessary further comprise, subsequent to the engraving step, a rinsing step, a drying step, and/or a post-crosslinking step, which are shown below.
Rinsing step: a step of rinsing the engraved surface by rinsing the engraved relief layer surface with water or a liquid comprising water as a main component.

Drying step: a step of drying the engraved relief layer.

Post-crosslinking step: a step of further crosslinking the relief layer by applying energy to the engraved relief layer.
After the above-mentioned step, since engraved residue is attached to the engraved surface, a rinsing step of washing off engraved residue by rinsing the engraved surface with water or a liquid comprising water as a main component may be added. Examples of rinsing means include a method in which washing is carried out with tap water, a method in which high pressure water is spray-jetted, and a method in which the engraved surface is brushed in the presence of mainly water using a batch or conveyor brush type washout machine known as a photosensitive resin letterpress plate processor, and when slime due to engraved residue cannot be eliminated, a rinsing liquid to which a soap or a surfactant is added may be used.
When the rinsing step of rinsing the engraved surface is carried out, it is preferable to add a drying step of drying an engraved relief-forming layer so as to evaporate rinsing liquid.
Furthermore, as necessary, a post-crosslinking step for further crosslinking the relief-forming layer may be added. By carrying out a post-crosslinking step, which is an additional crosslinking step, it is possible to further strengthen the relief formed by engraving.
The rinsing liquid that can be used in the present invention is preferably a rinsing liquid having the pH of 8 to 14. When in this mode, rinsing properties for engraving residue generated when laser engraving is better.
The pH of the rinsing liquid that can be used in the present invention is preferably at least 9, more preferably at least 10, and yet more preferably at least 11. The pH of the rinsing liquid is preferably no greater than 14, more preferably no greater than 13.5, and yet more preferably no greater than 13.2. When in the above-mentioned range, handling is easy, and rinsing properties for engraving residue generated when laser engraving is better.
In order to set the pH of the rinsing liquid in the above-mentioned range, the pH may be adjusted using an acid and/or a base as appropriate, and the acid or base used is not particularly limited.
The rinsing liquid that can be used in the present invention preferably comprises water as a main component.
The rinsing liquid may contain as a solvent other than water a water-miscible solvent such as an alcohol, acetone, or tetrahydrofuran.
The rinsing liquid preferably comprises a surfactant.
From the viewpoint of removability of engraved residue and little influence on a flexographic printing plate, preferred examples of the surfactant that can be used in the present invention include betaine compounds (amphoteric surfactants) such as a carboxybetaine compound, a sulfobetaine compound, a phosphobetaine compound, an amine oxide compound, and a phosphine oxide compound.
Furthermore, examples of the surfactant also include known anionic surfactants, cationic surfactants, and nonionic surfactants. Moreover, a fluorine-based or silicone-based nonionic surfactant may also be used in the same manner.
With regard to the surfactant, one type may be used on its own or two or more types may be used in combination.
It is not necessary to particularly limit the amount of surfactant used, but it is preferably 0.01 to 20 mass% relative to the total mass of the rinsing liquid, and more preferably 0.05 to 10 mass%.
The flexographic printing plate of the present invention having a relief layer above the surface of an optional substrate such as a support may be produced as described above.
From the viewpoint of satisfying suitability for various aspects of printing, such as abrasion resistance and ink transfer properties, the thickness of the relief layer of the flexographic printing plate is preferably at least 0.05 mm but no greater than 10 mm, more preferably at least 0.05 mm but no greater than 7 mm, and yet more preferably at least 0.05 mm but no greater than 3 mm.
Furthermore, the Shore A hardness of the relief layer of the flexographic printing plate is preferably at least 50° but no greater than 90°. When the Shore A hardness of the relief layer is at least 50°, even if fine halftone dots formed by engraving receive a strong printing pressure from a letterpress printer, they do not collapse and close up, and normal printing can be carried out. Furthermore, when the Shore A hardness of the relief layer is no greater than 90°, even for flexographic printing with kiss touch printing pressure it is possible to prevent patchy printing in a solid printed part.
The Shore A hardness in the present specification is a value measured by a durometer (a spring type rubber hardness meter) that presses an indenter (called a pressing needle or indenter) into the surface of a measurement target at 25°C so as to deform it, measures the amount of deformation (indentation depth), and converts it into a numerical value.
The flexographic printing plate of the present invention is particularly suitable for printing in a flexographic printer using an aqueous ink, but printing is also possible when using any ink such as an aqueous ink, an oil-based ink, or a UV ink in a letterpress printer, and printing is also possible in a flexographic printer using a UV ink. The flexographic printing plate of the present invention is excellent in terms of rinsing properties and leaves no engraving residue, and since the relief layer that is obtained has excellent elasticity the aqueous ink transfer properties and durability are excellent, printing can be carried out without worries about plastic deformation of the relief layer or degradation of durability over long time periods.

Examples

The present invention is explained more specifically below by reference to Examples and Comparative Examples. However, the present invention should not be construed as being limited by these Examples. In the description below, 'parts' means 'parts by mass' and '%' means 'mass%' unless otherwise specified. Furthermore, the units of each of the numerical values in Table 1 below are parts by mass.
The number-average molecular weight (Mn) and weight-average molecular weight (Mw) of polymers in the Examples are values measured by the GPC method unless otherwise specified.
Details of components used in each of the Examples and Comparative Examples are as follows.

UV-3000B: urethane acrylate containing acryloxy group at main chain terminal (Mw: 18,000, Tg: -39°C, Shikoh UV-3000B, manufactured by The Nippon Synthetic Chemical Industry Co., Ltd.; liquid polymer, therefore plastomer)
UV-3630ID80: urethane acrylate containing acryloxy group at main chain terminal (Shikoh UV-3630ID80, manufactured by The Nippon Synthetic Chemical Industry Co., Ltd.; liquid polymer, therefore plastomer)
IR2200L: polyisoprene (Mooney viscosity (ML₁₊₄, 100°C): 70, Mw: 1,350,000, Mn: 472,000, Tg: about -60°C, tensile set: 53%, NIPOL IR2200L, manufactured by Nippon Zeon Corporation)
BR150L: polybutadiene (Mooney viscosity (ML₁₊₄, 100°C): 43, Mw: 467,000, Mn: 243,000, Tg: about -103°C, tensile set: 63%, UBEPOL BR150L, manufactured by Ube Industries, Ltd.)
LBR305: liquid polybutadiene (Mn: 26,000, Mw: 28,000, Kuraprene LBR-305, manufactured by Kuraray Co., Ltd.; liquid polymer, therefore plastomer)
EPDM505: ethylene-propylene-5-ethylidene-2-norbornene copolymer (binder polymer not categorized as Component A, Mooney viscosity (ML₁₊₄, 125°C): 59, Esprene EPDM505, manufactured by Sumitomo Chemical Co., Ltd.)
D-1102: styrene-butadiene-styrene block copolymer (SBS resin, binder polymer not categorized as Component A, Mw: 100,000, tensile set: 0%, Kraton D-1102, manufactured by Kraton)

PCD-40: dicumyl peroxide (organic peroxide, Percumyl D-40, manufactured by NOF Corporation, dicumyl peroxide content 40%, 10 hours half-life temperature: 116.4°C)
Irgacure 651: photopolymerization initiator, 2-phenyl-2,2-dimethoxyacetophenone (IRGACURE 651, Ciba-Geigy Ltd.)

ADCA (azodicarbonamide, Tokyo Chemical Industry Co., Ltd.)
DPT (dinitrosopentamethylenetetramine, Tokyo Chemical Industry Co., Ltd.)

Urea (Wako Pure Chemical Industries, Ltd.)

Zinc oxide (Wako Pure Chemical Industries, Ltd.)
Stearic acid (Wako Pure Chemical Industries, Ltd.)

HDDA: hexanediol diacrylate (Tokyo Chemical Industry Co., Ltd.)

#45L: carbon black #45L (Mitsubishi Chemical Corporation, particle size: 24 nm, specific surface area: 125 m²/g, DBP oil adsorption: 45 mL/100g)

Expancel DU930-170 (thermally expandable microcapsules, particle size 28 to 38 µm, Japan Fillite Co., Ltd.)

(Examples 1 to 7)

The materials described in Table 1 and Table 2 below other than the foaming agent, urea, zinc oxide, stearic acid, and polymerization initiator in the constitution described in Table 1 below were kneaded in a Labo Plastomill (Toyo Seiki Co., Ltd.) (60°C for 20 minutes). Subsequently, the foaming agent, urea, zinc oxide, stearic acid, and polymerization initiator were added, and kneading (80°C for 5 minutes) was further carried out to thus give a resin composition for laser engraving. The resin composition thus obtained was molded into a 1 mm thick sheet using a rubber extruder, and this sheet was placed in a heating oven and heated at 140°C for 80 minutes to thus carry out crosslinking and foaming, thereby giving a laser-engravable flexographic printing plate precursor.

The crosslinked resin composition layer (crosslinked relief-forming layer) was subjected to laser engraving using the two types of laser below.
For engraving by irradiation with a laser, as a carbon dioxide laser engraving machine an ML-9100 series high quality CO₂ laser marker (Keyence Corporation) was used. A 1 cm square solid printed area was raster-engraved using the carbon dioxide laser engraving machine under conditions of an output of 12 W, a head speed of 200 mm/sec, and a pitch setting of 2,400 DPI.
As a semiconductor laser engraving machine, laser recording equipment provided with an SDL-6390 fiber-coupled semiconductor laser (FC-LD) (JDSU, wavelength 915 nm) with a maximum power of 8.0 W was used. A 1 cm square solid printed area was raster-engraved using the semiconductor laser engraving machine under conditions of a laser output of 7.5 W, a head speed of 409 mm/sec, and a pitch setting of 2,400 DPI.
The engraving machines used are shown in Table 2. Here, 'FC-LD' means use of the semiconductor laser engraving machine and 'CO₂' means use of the carbon dioxide laser engraving machine.

(Example 8)

The materials described in Table 1 and Table 2 below other than the foaming agent, urea, zinc oxide, stearic acid, and polymerization initiator in the constitution described in Table 1 below were kneaded in a Labo Plastomill (Toyo Seiki Co., Ltd.) (60°C for 20 minutes). Subsequently, the foaming agent, urea, zinc oxide, stearic acid, and polymerization initiator were added, and kneading (80°C for 5 minutes) was further carried out to thus give a resin composition for laser engraving. The resin composition thus obtained was molded into a 1 mm thick sheet using a rubber extruder, and this sheet was subjected to UV exposure using a UV (UV) exposure machine equipped with a heating plate while heating at 100°C for 80 minutes to thus carry out crosslinking and foaming, thereby giving a laser-engravable flexographic printing plate precursor.
The laser-engravable flexographic printing plate precursor thus obtained was subjected to laser engraving as in Example 1, thus giving a flexographic printing plate.

(Comparative Examples 1 to 3)

Laser engravable flexographic printing plate precursors were produced by the same procedure as in Example 1 except that the materials and constitutions were as in Table 1 and Table 2 below and foaming was not carried out.
The laser-engravable flexographic printing plate precursors thus obtained were subjected to laser engraving as in Example 1, thereby giving flexographic printing plates.

-Evaluation of laser-engravable flexographic printing plate precursor and flexographic printing plate-

The performance of the laser-engravable flexographic printing plate precursors and the flexographic printing plates obtained in the Examples and Comparative Examples was evaluated with respect to the items below. The evaluation results are also given in Table 2.

The 'engraving depth' of the relief layer obtained by laser engraving a crosslinked relief-forming layer of the flexographic printing plate precursor was measured as follows. The 'engraving depth' referred to here means the difference between an engraved position (height) and an unengraved position (height) when a cross-section of the relief layer was examined. The 'engraving depth' in the present Examples was measured by examining a cross-section of a relief layer using a VK9510 Ultradepth Color 3D profile measurement microscope (Keyence Corporation). A large engraving depth means a high engraving sensitivity.

The presence/absence of residue on the surface of the relief layer of the laser-engraved plate was examined using an optical microscope, and comparative evaluation was carried out.

A laser-engraved plate was immersed in water and an engraved part was rubbed with a toothbrush (Clinica Toothbrush Flat, Lion Corporation) 10 times. Subsequently, the presence/absence of residue on the surface of the relief layer was ascertained with an optical microscope. When there was no residue the evaluation was A, when there was almost no residue the evaluation was B, when there was a little residue but there was no practical problem the evaluation was C, and when the residue could not be removed sufficiently the evaluation was D.

A flexographic printing plate that had been obtained was set in a printer (Model ITM-4, IYO KIKAI SEISAKUSHO Co., Ltd.), as the ink Aqua SPZ16 Red aqueous ink (Toyo Ink Manufacturing Co., Ltd.) was used without dilution, printing was carried out continuously using Full Color Form M 70 (Nippon Paper Industries Co., Ltd., thickness 100 µm) as the printing paper, and a highlight of 1% to 10% was confirmed for a printed material.
The degree of ink attachment in a solid printed part on the printed material at 1,000 m from the start of printing was compared by visual inspection.
With regard to the evaluation criteria, when there was no unevenness in density and there was uniform and slight gloss (gloss is an indicator that a considerable thickness (amount) of ink has been reliably transferred) the evaluation was A, when it was uniform without unevenness in density the evaluation was B, when there was unevenness over the whole area the evaluation was D, and when there was partial unevenness in density the evaluation was C. Evaluations of B and above are levels without problems in practice.

Three engraved relief images at 130, 150, and 175 lpi were produced on a printing plate using an FC-LD, UV ink printing (UV ink flexographic 500CF, T & K TOKA) was carried out for 10 km, at the point in time at which 10 km of the material had been printed it was examined using an optical microscope, and the limit for the number of lines that could be reproduced on the engraved image (line number) as a printed material was defined as the image quality in the Examples. For example, an engraved relief image was prepared with three types, that is, 130, 150, and 175 lpi, and in a printed material when 130 lpi and 150lpi were reproduced clearly but 175 lpi could not be reproduced as in the engraved relief, the image quality evaluation was 150 lpi.

A flexographic printing plate that was obtained was set in a printer (Model ITM-4, lyo Kikai Seisakusho). As the ink a solvent ink (XS-716 507 Blue (DIC GRAPHICS CORPORATION)) was used. Printing was continued using as print paper Full Color Form M 70 (Nippon Paper Group, Inc., thickness 100 µm), 1% to 10% highlights being checked for the printed material. Completion of printing was defined as being when halftone dots were not printed, and the length (km) of paper printed up to the completion of printing was used as an index. The larger the value, the better the evaluation of printing durability.

(Table 1)

	Binder polymer	Polymerization initiator	Polymerizable compound	Photothermal conversion agent	Foaming agent	Urea	Zinc oxide	Stearic acid	Additive (Expancel)	Total (parts by mass)
Ex. 1	83	1	0	5	5	1	2	3	0	100
Ex. 2	88	1	0	5	5	1	0	0	0	100
Ex. 3	83	1	0	5	5	1	2	3	0	100
Ex. 4	83	1	0	5	5	1	2	3	0	100
Ex. 5	83	1	0	5	5	1	2	3	0	100
Ex. 6	83	1	0	5	5	1	2	3	0	100
Ex. 7	73	1	10	5	5	1	2	3	0	100
Ex. 8	74	5	10	0	5	1	2	3	0	100
Comp. Ex. 1	94	1	0	5	0	0	0	0	0	100
Comp. Ex. 2	79	1	0	5	0	0	0	0	15	100
Comp. Ex. 3	45	1	38	5	5	1	2	3	0	100

(Table 2)

	Binder polymer	Polymerization initiator	Polymerizable compound	Photothermal conversion agent	Additive	Engraving depth (CO₂) (µm)	Engraving depth (FC-LD) (µm)	Amount of engraving residue remaining on plate	Rinsing properties	Ink transfer properties	Image quality (lpi)	Printing durability (km)
Ex. 1	UV-3000B	PCD-40	None	#45L	None	560	660	Little	B	B	175	35
Ex. 2	UV-3630ID80	PCD-40	None	#45L	None	570	670	Little	B	B	175	50
Ex. 3	IR2200L	PCD-40	None	#45L	None	650	750	Little	B	B	175	75
Ex. 4	BR150L	PCD-40	None	#45L	None	600	700	Very little	A	A	175	80
Ex. 5	BR150L: EPDM505 = 5:2	PCD-40	None	#45L	None	570	670	Little	B	B	150	60
Ex. 6	LBR305	PCD-40	None	#45L	None	600	700	Very little	A	A	175	40
Ex. 7	LBR305	PCD-40	HDDA	#45L	None	600	700	Very little	A	A	175	82
Ex. 8	LBR305	Irgacure 651	HDDA	None	None	590	690	Very little	B	B	175	60
Comp. Ex. 1	BR150L	PCD-40	None	#45L	None	280	380	Some	C	D	150	70
Comp. Ex. 2	D-1102	Irgacure 651	HDDA	None	Expancel DU930-170	470	570	Some	C	D	130	30
Comp. Ex. 3	BR150L	PCD-40	HDDA	#45L	None	480	560	Some	D	D	150	20

Claims

A resin composition for laser engraving comprising
(Component A) a binder polymer comprising an ethylenically unsaturated bond in at least one part selected from the group consisting of a main chain interior, a main chain terminal, and a side chain,

(Component B) a polymerization initiator, and

(Component C) a foaming agent,

Component A comprising a plastomer selected from the group consisting of a polybutadiene structure-containing polymer, a polyisoprene structure-containing polymer, and a urethane (meth)acrylate, and

Component A having a content of at least 50 mass% relative to the total mass of the composition.
The resin composition for laser engraving according to Claim 1, wherein Component A is polybutadiene, polyisoprene, or a copolymer formed by copolymerization of at least butadiene and/or isoprene.
The resin composition for laser engraving according to Claim 1 or 2, wherein Component B is an organic peroxide.
The resin composition for laser engraving according to any one of Claims 1 to 3, wherein Component C is a chemical foaming agent.
The resin composition for laser engraving according to any one of Claims 1 to 4, wherein it further comprises (Component D) a polymerizable compound.
The resin composition for laser engraving according to any one of Claims 1 to 5, wherein it further comprises (Component E) a photothermal conversion agent.
A laser-engravable flexographic printing plate precursor comprising a crosslinked relief-forming layer formed by foaming and crosslinking a layer comprising the resin composition for laser engraving according to any one of Claims 1 to 6.
A laser-engravable flexographic printing plate precursor comprising
a crosslinked relief-forming layer formed by foaming and crosslinking a layer comprising a resin composition for laser engraving comprising (Component A) a binder polymer comprising an ethylenically unsaturated bond in at least one part selected from the group consisting of a main chain interior, a main chain terminal, and a side chain, (Component B) a polymerization initiator, and (Component C) a foaming agent, Component A comprising a plastomer selected from the group consisting of a polybutadiene structure-containing polymer, a polyisoprene structure-containing polymer, and a urethane (meth)acrylate, and Component A having a content of at least 50 mass% relative to the total mass of the composition.
The laser-engravable flexographic printing plate precursor according to Claim 8, wherein Component A is polybutadiene, polyisoprene, or a copolymer formed by copolymerization of at least butadiene and/or isoprene.
The laser-engravable flexographic printing plate precursor according to Claim 8 or 9, wherein Component B is an organic peroxide.
The laser-engravable flexographic printing plate precursor according to any one of Claims 8 to 10, wherein Component C is a chemical foaming agent or a physical foaming agent.
The laser-engravable flexographic printing plate precursor according to any one of Claims 8 to 11, wherein the resin composition for laser engraving further comprises (Component E) a photothermal conversion agent.
A process for producing the laser-engravable flexographic printing plate precursor according to any one of Claims 7 to 12, comprising
a step of forming a layer comprising a resin composition for laser engraving comprising (Component A) a binder polymer comprising an ethylenically unsaturated bond in at least one part selected from the group consisting of a main chain interior, a main chain terminal, and a side chain, (Component B) a polymerization initiator, and (Component C) a foaming agent, Component A comprising a plastomer selected from the group consisting of a polybutadiene structure-containing polymer, a polyisoprene structure-containing polymer, and a urethane (meth)acrylate, and Component A having a content of at least 50 mass% relative to the total mass of the composition, and
a step of forming a crosslinked relief-forming layer by foaming and crosslinking the layer comprising the resin composition for laser engraving.
The process for producing the laser-engravable flexographic printing plate precursor according to Claim 13, wherein foaming and crosslinking are carried out at least partially at the same time.
A process for making a flexographic printing plate, comprising
a step of preparing the laser-engravable flexographic printing plate precursor according to any one of Claims 7 to 12, and
an engraving step of laser-engraving the crosslinked relief-forming layer to thus form a relief layer.