Lithographic printing plate precursor

Abstract

A lithographic printing plate precursor is disclosed including a support and a coating comprising (i) a photopolymerisable layer including a polymerisable compound and an optionally substituted trihaloalkyl sulfone photoinitiator, and (ii) a toplayer provided above the photopolymerisable layer; characterized in that the toplayer includes an infrared absorbing dye capable of forming a print-out image upon exposure to heat and/or IR radiation.

Claims

1. A lithographic printing plate precursor comprising a support and a coating comprising (i) a photopolymerisable layer comprising a polymerisable compound and an optionally substituted trihaloalkyl sulfone photoinitiator, and (ii) a toplayer above the photopolymerisable layer; wherein the toplayer has a thickness between 0.1 g/m.sup.2 and 0.54 g/m.sup.2 and comprises an infrared absorbing compound capable of forming a print-out image upon exposure to heat and/or IR radiation.

2. The printing plate precursor of claim 1, wherein the coating is capable of being developed on-press with dampening liquid and/or ink.

3. The printing plate precursor of claim 1, wherein the infrared absorbing compound comprises a thermocleavable group which, upon exposure to heat and/or IR radiation, transforms into an electron-donor group such that the infrared absorbing compound undergoes a hypsochromic shift.

4. The printing plate precursor of claim 1, wherein the print-out image is characterized by a CIE 1976 colour distance ΔE of at least 2.

5. The printing plate precursor of claim 1, wherein the infrared absorbing compound is an infrared absorbing dye which has a main absorption in the infrared wavelength range of the electromagnetic spectrum before exposure to heat and/or IR radiation, and the infrared absorbing dye absorbs more light in the visible wavelength range of the electromagnetic spectrum after exposure to heat and/or IR radiation.

6. The printing plate precursor of claim 5, wherein the infrared absorbing dye is of Formula I, II or III: ##STR00010## wherein Ar.sup.1, Ar.sup.2 and Ar.sup.3 independently are an optionally substituted aromatic hydrocarbon group or an aromatic hydrocarbon group comprising an optionally substituted annulated benzene ring; W.sup.1 and W.sup.2 independently are a sulphur atom, an oxygen atom, NR wherein R is an optionally substituted alkyl group, NH, a —CM.sup.10M.sup.11 group wherein M.sup.10 and M.sup.11 are independently an optionally substituted aliphatic hydrocarbon group or an optionally substituted (hetero)aryl group, or M.sup.10 and M.sup.11 together comprise the necessary atoms to form a cyclic structure; W.sup.3 is a sulphur atom or a —C(A.sup.3)=C(A.sup.4)-group; W.sup.4 is a sulphur atom or a —C(A.sup.7)=C(A.sup.8)-group; M.sup.1 and M.sup.2 independently are hydrogen, an optionally substituted aliphatic hydrocarbon group, or together comprise the necessary atoms to form an optionally substituted cyclic structure which may comprise an optionally substituted annulated benzene ring; M.sup.3 and M.sup.4 independently are an optionally substituted aliphatic hydrocarbon group; M.sup.5, A.sup.6, M.sup.7, M.sup.8, M.sup.16 and M.sup.17 independently are hydrogen, a halogen, or an optionally substituted aliphatic hydrocarbon group; A.sup.1, A.sup.2, A.sup.3, A.sup.4, A.sup.5, A.sup.6, A.sup.7, and A.sup.8 independently are hydrogen, a halogen atom, an optionally substituted aliphatic hydrocarbon group, an optionally substituted (hetero)aryl group, or A.sup.1 and A.sup.2, A.sup.3 and A.sup.4, A.sup.5 and A.sup.6, or A.sup.7 and A.sup.8 together comprise the necessary atoms to form a cyclic structure; M.sup.12, M.sup.13, M.sup.14, and M.sup.15 independently are an optionally substituted aliphatic hydrocarbon group; an optionally substituted (hetero)aryl group; two of M.sup.14, M.sup.15, A.sup.5, and A.sup.7 together comprise the necessary atoms to form at least one cyclic structure; or two of M.sup.12, M.sup.13, A.sup.2, and A.sup.4 together comprise the necessary atoms to form at least one cyclic structure; and M.sup.9 is a group which, upon exposure to heat and/or IR radiation, transforms into an electron-donor group such that an integrated light absorption of the dye between 350 to 750 nm increases; wherein the dye optionally comprises one or more counter ions in order to obtain an electrically neutral compound.

7. The printing plate precursor of claim 6, wherein M.sup.1 and M.sup.2 together comprise the necessary atoms to form an optionally substituted 5- or 6-membered ring, which may comprise an optionally substituted annulated benzene ring.

8. The printing plate precursor of claim 6, wherein the infrared absorbing dye is of Formula I.

9. The printing plate precursor of claim 6, wherein M.sup.9 is —(N═CR.sup.17).sub.a—NR.sup.5—CO—R.sup.4, —(N═CR.sup.17).sub.b—NR.sup.5—SO.sub.2—R.sup.6, —(N═CR.sup.17).sub.c—NR.sup.11—SO—R.sup.12, —SO.sub.2—NR.sup.15R.sup.16, or —S—CH.sub.2—CR.sup.7(H).sub.1-d(R.sup.8).sub.d—NR.sup.9—COOR.sup.18; wherein a, b, c and d independently are 0 or 1; R.sup.17 is hydrogen, an optionally substituted aliphatic hydrocarbon group, an optionally substituted (hetero)aryl group, or R.sup.17 and R.sup.5 or R.sup.17 and R.sup.11 together comprise the necessary atoms to form a cyclic structure; R.sup.4 is —OR.sup.10, —NR.sup.13R.sup.14, or —CF.sub.3; R.sup.10 is an optionally substituted (hetero)aryl group or an optionally branched aliphatic hydrocarbon group; R.sup.13 and R.sup.14 independently are hydrogen, an optionally substituted aliphatic hydrocarbon group, an optionally substituted (hetero)aryl group, or R.sup.13 and R.sup.14 together comprise the necessary atoms to form a cyclic structure; R.sup.6 is an optionally substituted aliphatic hydrocarbon group, an optionally substituted (hetero)aryl group, —OR′—NR.sup.13R.sup.14, or —CF.sub.3; R.sup.5 is hydrogen, an optionally substituted aliphatic hydrocarbon group, SO.sub.3, a —COOR.sup.18 group, an optionally substituted (hetero)aryl group, or R.sup.5 together with at least one of R.sup.10, R.sup.13, and R.sup.14 comprise the necessary atoms to form a cyclic structure; R.sup.11, R.sup.15 and R.sup.16 independently are hydrogen, an optionally substituted aliphatic hydrocarbon group, or an optionally substituted (hetero)aryl group, or R.sup.15 and R.sup.16 together comprise the necessary atoms to form a cyclic structure; R.sup.12 is an optionally substituted aliphatic hydrocarbon group or an optionally substituted (hetero)aryl group; R.sup.7 and R.sup.9 independently are hydrogen or an optionally substituted aliphatic hydrocarbon group; R.sup.8 is —COO— or —COOR.sup.8′ wherein R.sup.8′ is hydrogen, an alkali metal cation, an ammonium ion, or a mono-, di-, tri- or tetra-alkyl ammonium ion; and R.sup.18 is an optionally substituted (hetero)aryl group or an alpha-branched aliphatic hydrocarbon group.

10. A method for making a printing plate precursor comprising the steps of: coating a support with: (i) a photopolymerisable layer comprising a polymerisable compound and an optionally substituted trihaloalkyl sulfone photoinitiator, and (ii) a toplayer provided above the photopolymerisable layer, wherein the toplayer has a thickness between 0.1 g/m.sup.2 and 0.54 g/m.sup.2 and includes an infrared absorbing dye which has a main absorption in the infrared wavelength range of the electromagnetic spectrum before exposure to heat and/or IR radiation, and the infrared absorbing dye absorbs more light in the visible wavelength range of the electromagnetic spectrum after exposure to heat and/or IR radiation, and drying the precursor.

11. A method for making a printing plate including the steps of: image-wise exposing a lithographic printing plate precursor to heat and/or IR radiation to form a lithographic image comprising image areas and non-image areas and to induce a colour change in the image areas; and developing the exposed precursor, wherein the lithographic printing plate precursor comprises a support and a coating comprising (i) a photopolymerisable layer comprising a polymerisable compound and an optionally substituted trihaloalkyl sulfone photoinitiator, and (ii) a toplayer above the photopolymerisable layer, and wherein the toplayer has a thickness between 0.1 g/m.sup.2 and 0.54 g/m.sup.2 and comprises an infrared absorbing compound capable of forming a print-out image upon exposure to heat and/or IR radiation.

12. The method of claim 11, wherein the precursor is developed by mounting the precursor on a plate cylinder of a lithographic printing press and rotating the plate cylinder while feeding dampening liquid and/or ink to the precursor.

13. The method of claim 11, wherein the colour change is characterized by a CIE 1976 colour distance ΔE between the image and non-image areas of at least 2.

14. The method of claim 11, wherein the energy density of the IR radiation is between 80 mJ/m.sup.2 and 120 mJ/m.sup.2.

Description

DESCRIPTION OF EMBODIMENTS

(1) The Lithographic Printing Plate Precursor

(2) The lithographic printing plate precursor according to the present invention is negative-working, i.e. after exposure and development the non-exposed areas of the coating are removed from the support and define hydrophilic (non-printing) areas, whereas the exposed coating is not removed from the support and defines oleophilic (printing) areas. The hydrophilic areas are defined by the support which has a hydrophilic surface or is provided with a hydrophilic layer. The hydrophobic areas are defined by the coating, hardened upon exposing, optionally followed by a heating step. Areas having hydrophilic properties means areas having a higher affinity for an aqueous solution than for an oleophilic ink; areas having hydrophobic properties means areas having a higher affinity for an oleophilic ink than for an aqueous solution.

(3) “Hardened” means that the coating becomes insoluble or non-dispersible for the developing solution and may be achieved through polymerization and/or crosslinking of the photosensitive coating, optionally followed by a heating step to enhance or to speed-up the polymerization and/or crosslinking reaction. In this optional heating step, hereinafter also referred to as “pre-heat”, the plate precursor is heated, preferably at a temperature of about 80° C. to 150° C. and preferably during a dwell time of about 5 seconds to 1 minute.

(4) The coating contains a toplayer and at least one layer including a photopolymerisable composition, said layer is also referred to as the “photopolymerisable layer”. The toplayer is provided on top of the photopolymerisable layer. The coating may further include other layers such as for example an intermediate layer, located between the support and the photopolymerisable layer and/or between the top layer and the photopolymerisable layer, an adhesion improving layer and/or other layers.

(5) The coating of the printing plate precursor is most preferably capable of being developed on-press with dampening liquid and/or ink.

(6) The printing plate of the present invention is characterized in that it can be exposed at a low energy density, i.e. below 190 mJ/m.sup.2; preferably between 70 and 190 mJ/m.sup.2; more preferably between 75 and 150 mJ/m.sup.2 and most preferably between 80 and 120 mJ/m.sup.2.

(7) Toplayer

(8) The coating includes a toplayer or protective overcoat layer which preferably acts as an oxygen barrier layer. Low molecular weight substances present in the air may deteriorate or even inhibit image formation and therefore a toplayer is applied to the coating. A toplayer should preferably be easily removable during development, adhere sufficiently to the photopolymerisable layer or optional other layers of the coating and should preferably not inhibit the transmission of light during exposure. The toplayer is provided on top of the photopolymerisable layer.

(9) The toplayer includes an infrared absorbing compound which is capable of forming a coloured compound—whereby a print-out image is formed—upon exposure to infrared light and/or heat. The infrared absorbing compound which is capable of forming a coloured compound is preferably an infrared absorbing dye, also referred to as IR dye. This colour-forming IR dye is also referred to herein as thermochromic infrared absorbing dye or thermochromic IR dye. The thermochromic IR dye preferably has a main absorption in the infrared wavelength range of the electromagnetic spectrum—i.e. a wavelength range between about 750 and 1500 nm—and does preferably not have a substantial light absorption in the visible wavelength range of the electromagnetic spectrum—i.e. a wavelength range between 390 and 700 nm. The thermochromic compound, preferably a dye, preferably includes at least one thermocleavable group which is transformed by a chemical reaction, induced by exposure to IR radiation or heat, into a group which is a stronger electron-donor. As a result, the exposed thermochromic IR dye absorbs substantially more light in the visible wavelength range of the electromagnetic spectrum, or in other words, the thermochromic IR dye undergoes a hypsochromic shift whereby a visible image is formed, also referred to as print-out image. The formation of this print-out image is clearly different from a process of the prior art where a compound changes from an essentially colourless compound into a pale-coloured to coloured compound. These compounds typically change absorption from the UV wavelength range of the electromagnetic spectrum to the visible wavelength range of the electromagnetic spectrum, i.e. these compounds typically have a batochromic shift. The contrast of the print-out image obtained by such a process is much weaker compared to the colour forming process described above of the thermochromic IR dyes of the present invention.

(10) The contrast of the print-out image may be defined as the difference between the optical density at the exposed area and the optical density at the non-exposed area, and is preferably as high as possible. This enables the end-user to establish immediately whether or not the precursor has already been exposed and processed, to distinguish the different color selections and to inspect the quality of the image on the plate precursor. The contrast of the print-out image preferably increases with increasing optical density in the exposed areas and can be measured in reflectance using an optical densitometer, equipped with several filters (e.g. cyan, magenta, yellow).

(11) The concentration of the thermocromic IR dyes with respect to the total dry weight of the coating, may be from 0.1% wt to 20.0% wt, more preferably from 0.5% wt to 15.0% wt, most preferred from 1.0% wt to 10.0% wt.

(12) The thermocromic IR dye is preferably represented by Formulae I, II or III:

(13) ##STR00001##

(14) wherein Ar.sup.1, Ar.sup.2 and Ar.sup.3 independently represent an optionally substituted aromatic hydrocarbon group or an aromatic hydrocarbon group with an annulated benzene ring which is optionally substituted, W.sup.1 and W.sup.2 independently represent a sulphur atom, an oxygen atom, NR″ wherein R″ represents an optionally substituted alkyl group, NH, or a —CM.sup.10M.sup.11 group wherein M.sup.10 and M.sup.11 are independently an optionally substituted aliphatic hydrocarbon group or an optionally substituted (hetero)aryl group, or wherein M.sup.10 and M.sup.11 together comprise the necessary atoms to form a cyclic structure, preferably a 5- or 6-membered ring; W.sup.3 represent a sulphur atom or a —C(A.sup.3)=C(A.sup.4)-group, W.sup.4 represents a sulphur atom or a —C(A.sup.7)=C(A.sup.8)-group, M.sup.1 and M.sup.2 independently represent hydrogen, an optionally substituted aliphatic hydrocarbon group or together comprise the necessary atoms to form an optionally substituted cyclic structure, preferably M.sup.1 and M.sup.2 together comprise the necessary atoms to form an optionally substituted cyclic structure which may comprise an optionally substituted annulated benzene ring, preferably a 5- or 6-membered ring, more preferably a 5-membered ring, most preferably a 5-membered ring having a cyclic structure of 5 carbon atoms; M.sup.3 and M.sup.4 independently represent an optionally substituted aliphatic hydrocarbon group; M.sup.5, M.sup.6, M.sup.7 and M.sup.8, M.sup.16 and M.sup.17 independently represent hydrogen, a halogen or an optionally substituted aliphatic hydrocarbon group, A.sup.1 to A.sup.8 independently represent hydrogen, a halogen atom, an optionally substituted aliphatic hydrocarbon group or an optionally substituted (hetero)aryl group, or wherein each of A.sup.1 and A.sup.2, A.sup.3 and A.sup.4, A.sup.5 and A.sup.6, or, A.sup.7 and A.sup.8, together comprise the necessary atoms to form a cyclic structure, preferably 5- or 6-membered ring; M.sup.12 and M.sup.13 and M.sup.14 and M.sup.15 independently represent an optionally substituted aliphatic hydrocarbon group or an optionally substituted (hetero)aryl group, or wherein, two of said M.sup.14, M.sup.15, A.sup.5 or A.sup.7 together comprise the necessary atoms to form at least one cyclic structure, preferably 5- or 6-membered ring; two of said M.sup.12, M.sup.13, A.sup.2 or A.sup.4 together comprise the necessary atoms to form at least one cyclic structure preferably 5- or 6-membered ring; M.sup.9 is a group which is transformed by a chemical reaction, induced by exposure to IR radiation or heat, into a group which is a stronger electron-donor than said M.sup.9; and said transformation provides an increase of the integrated light absorption of said dye between 350 and 700 nm; and optionally one or more counter ions in order to obtain an electrically neutral compound.

(15) The thermocromic IR dye can be a neutral, an anionic or a cationic dye depending on the type of the substituting groups and the number of each of the substituting groups. In a preferred embodiment, the dye of formula I, II or III comprises at least one anionic or acid group such as —CO.sub.2H, —CONHSO.sub.2R.sup.h, —SO.sub.2NHCOR.sup.i, —SO.sub.2NHSO.sub.2R.sup.j, —PO.sub.3H.sub.2, —OPO.sub.3H.sub.2, —OSO.sub.3H, —S—SO.sub.3H or —SO.sub.3H groups or their corresponding salts, wherein R.sup.h, R.sup.i and R.sup.j are independently an aryl or an alkyl group, preferably a methyl group, and wherein the salts are preferably alkali metal salts or ammonium salts, including mono- or di- or tri- or tetra-alkyl ammonium salts. These anionic or acid groups may be present on the aromatic hydrocarbon group or the annulated benzene ring of Ar.sup.1, Ar.sup.2 or Ar.sup.3, or on the aliphatic hydrocarbon group of M.sup.3, M.sup.4 or M.sup.12 to M.sup.15, or on the (hetero)aryl group of M.sup.12 to M.sup.15. Other substituting groups can be selected from a halogen atom, a cyano group, a sulphone group, a carbonyl group or a carboxylic ester group.

(16) In another preferred embodiment, at least one of M.sup.3, M.sup.4 or M.sup.12 to M.sup.15 is terminally substituted with at least one of these groups, more preferably with —CO.sub.2H, —CONHSO.sub.2-Me, —SO.sub.2NHCO-Me, —SO.sub.2NHSO.sub.2-Me, —PO.sub.3H.sub.2 or —SO.sub.3H groups or their corresponding salt, wherein Me represents a methyl group.

(17) In a preferred embodiment, the thermocromic IR dye represented by Formulae I, II or III above includes M.sup.9 represented by one of the following groups:
—(N═CR.sup.17)a—NR.sup.5—CO—R.sup.4,
—(N═CR.sup.17)b—NR.sup.5—SO.sub.2—R.sup.6,
—(N═CR.sup.17)c—NR.sup.11—SO—R.sup.12,
—SO.sub.2—NR.sup.15R.sup.16 and
—S—CH.sub.2—CR.sup.7(H).sub.1-d(R.sup.8).sub.d—NR.sup.9—COOR.sup.18, wherein a, b, c and d independently are 0 or 1; R.sup.17 represents hydrogen, an optionally substituted aliphatic hydrocarbon group or an optionally substituted (hetero)aryl group, or wherein R.sup.17 and R.sup.5 or R.sup.17 and R.sup.11 together comprise the necessary atoms to form a cyclic structure; R.sup.4 represents —OR.sup.10, —NR.sup.13R.sup.14 or —CF.sub.3; wherein R.sup.10 represents an optionally substituted (hetero)aryl group or an optionally branched aliphatic hydrocarbon group; R.sup.13 and R.sup.14 independently represent hydrogen, an optionally substituted aliphatic hydrocarbon group or an optionally substituted (hetero)aryl group, or wherein R.sup.13 and R.sup.14 together comprise the necessary atoms to form a cyclic structure; R.sup.6 represents an optionally substituted aliphatic hydrocarbon group or an optionally substituted (hetero)aryl group, —OR.sup.10, —NR.sup.13R.sup.14 or —CF.sub.3; R.sup.5 represents hydrogen, an optionally substituted aliphatic hydrocarbon group, a SO.sub.3— group, a —COOR.sup.18 group or an optionally substituted (hetero)aryl group, or wherein R.sup.5 together with at feast one of R.sup.10, R.sup.13 and R.sup.14 comprise the necessary atoms to form a cyclic structure; R.sup.11, R.sup.15 and R.sup.16 independently represent hydrogen, an optionally substituted aliphatic hydrocarbon group or an optionally substituted (hetero)aryl group, or wherein R.sup.15 and R.sup.16 together comprise the necessary atoms to form a cyclic structure; R.sup.12 represents an optionally substituted aliphatic hydrocarbon group or an optionally substituted (hetero)aryl group; R.sup.7 and R.sup.9 independently represent hydrogen or an optionally substituted aliphatic hydrocarbon group; R.sup.8 represents —COO— or —COOR.sup.8′ wherein R.sup.8′ represents hydrogen, an alkali metal cation, an ammonium ion or a mono-, di-, tri- or tetra-alkyl ammonium ion; R.sup.18 represents an optionally substituted (hetero)aryl group or an alpha-branched aliphatic hydrocarbon group.

(18) Suitable examples of thermocromic IR dyes used in the present invention are described in EP 1 910 082 pages 4 to 8, IRD-001 to IRD-101, and incorporated herein by reference.

(19) In a highly preferred embodiment, the thermocromic IR dye is represented by Formula IV

(20) ##STR00002##

(21) wherein Ar.sup.1, Ar.sup.2, W.sup.1, W.sup.2 and M.sup.1 to M.sup.9 are as defined above.

(22) Most preferably the thermocromic IR dye is represented by Formula IV wherein Ar.sup.1 and Ar.sup.2 independently represent an optionally substituted aryl group; optionally annulated with an optionally substituted benzene ring, W.sup.1 and W.sup.2 represent —C(CH.sub.3).sub.2; M.sup.1 and M.sup.2 together comprise the necessary atoms to form an optionally substituted 5-membered ring which may comprise an optionally substituted annulated benzene ring; M.sup.3 and M.sup.4 independently represent an optionally substituted aliphatic hydrocarbon group, M.sup.5, M.sup.6, M.sup.7 and M.sup.8 represent hydrogen; M.sup.9 represents
—NR.sup.5—CO—R.sup.4
—NR.sup.5—SO.sub.2—R.sup.6
—NR.sup.11—SO—R.sup.12
—SO.sub.2—NR.sup.18R.sup.16

(23) wherein R.sup.4, R.sup.5, R.sup.6, R.sup.11, R.sup.12, R.sup.15, and R.sup.16 are as defined above;

(24) and optionally one or more counter ions in order to obtain an electrically neutral compound. Preferably the IR dye comprises at least one anionic group or an acid group, such as —CO.sub.2H, —CONHSO.sub.2R.sup.h, —SO.sub.2NHCOR.sup.i, —SO.sub.2NHSO.sub.2R—PO.sub.3H.sub.2, —OPO.sub.3H.sub.2, —OSO.sub.3H, —SO.sub.3H or —S—SO.sub.3H groups or their corresponding salts, wherein R.sup.h, R.sup.i and R.sup.j are independently an aryl or an alkyl group. More preferably, at least one of the aliphatic hydrocarbon groups of M.sup.3 or M.sup.4 is terminally substituted with at least one of said anionic groups or acid groups.

(25) In a highly preferred embodiment the thermocromic IR dye is represented by Formula IV wherein Ar.sup.1 and Ar.sup.2 independently represent an optionally substituted aryl group; W.sup.1 and W.sup.2 represent —C(CH.sub.3).sub.2; M.sup.1 and M.sup.2 together comprise the necessary atoms to form an optionally substituted 5-membered ring which may comprise an optionally substituted annulated benzene ring; M.sup.3 and M.sup.4 independently represent an optionally substituted aliphatic hydrocarbon group, M.sup.5, M.sup.6, M.sup.7 and M.sup.8 represent hydrogen; M.sup.9 represents
—NR.sup.5—CO—R.sup.4
—NR.sup.5—SO.sub.2—R.sup.6

(26) wherein R.sup.4 is —OR.sup.10, wherein R.sup.10 is an optionally branched aliphatic hydrocarbon group; R.sup.5 represents hydrogen, an optionally substituted aliphatic hydrocarbon group or an optionally substituted (hetero)aryl group, R.sup.6 represents an optionally substituted aliphatic hydrocarbon group or an optionally substituted (hetero)aryl group; and optionally one or more counter ions in order to obtain an electrically neutral compound.

(27) Preferably the thermocromic IR dye comprises at least one anionic group or an acid group, such as —CO.sub.2H, —CONHSO.sub.2R.sup.h, —SO.sub.2NHCOR.sup.i, —SO.sub.2NHSO.sub.2R.sup.j, —PO.sub.3H.sub.2, —OPO.sub.3H.sub.2, —OSO.sub.3H, —SO.sub.3H or —S—SO.sub.3H groups or their corresponding salts, wherein R.sup.h, R.sup.i and R.sup.j are independently an aryl or an alkyl group. More preferably, at least one of the aliphatic hydrocarbon groups of M.sup.3 or M.sup.4 is terminally substituted with at least one of said anionic groups or acid groups. The salts are preferably alkali metal salts or ammonium salts, including mono- or di- or tri- or tetra-alkyl ammonium salts.

(28) The optional counter ions in order to obtain an electrically neutral compound may be selected from for example a halogen, a sulphonate, a perfluorosulphonate, a tosylate, a tetrafluoroborate, a hexafluorophosphate, an arylborate, an arylsulphonate; or a cation such as alkali metal salts or ammonium salts, including mono- or di- or tri- or tetra-alkyl ammonium salts.

(29) Especially preferred thermocromic IR dye are presented by one of the following formulae V to XII:

(30) ##STR00003##

(31) wherein X.sup.− represents halogen, sulphonate, perfluorosulphonate, tosylate, tetrafluoroborate, hexafluorophosphate, arylborate or arylsulphonate; and R.sup.3, R.sup.3′ independently represent an optionally substituted alkyl group, preferably a methyl or ethyl; or an ether group, preferably —CH.sub.2—CH.sub.2—O—CH.sub.3;

(32) ##STR00004##

(33) wherein M.sup.+=Li.sup.+, Na.sup.+, K.sup.+, NH.sub.4.sup.+, R′R″R′″NH.sup.+ wherein R′, R″, R′″ independently represent hydrogen, an optional substituted alkyl or aryl group;

(34) ##STR00005##

(35) The thermocromic IR dyes mentioned above may also be coupled to each other or to other IR dyes as to form IR dye dimers or oligomers. Besides a covalent coupling between two or more thermocromic IR dyes, supra-molecular complexes, comprising two or more IR dyes, may also be formed by ionic interactions. Dimers, consisting of two different IR dyes, may be formed for example by an interaction between a cationic and an anionic IR dye, as described in e.g. WO/2004069938 and EP 1 466 728. IR dyes may also be ionically bond to a polymer as e.g. described in EP 1 582 346 wherein IR dyes, comprising two to four sulphonate groups are ionically bonded to a polymer comprising covalently attached ammonium, phosphonium, and sulphonium groups.

(36) Supra-molecular complexes comprising two or more thermocromic IR dyes, may also be formed by hydrogen bonding or dipole-dipole interaction.

(37) The colour difference between the exposed and non-exposed areas of the coating calculated from the L*a*b* values of the image areas (exposed areas) of the coating and the L*a*b* values of non-image areas (non-exposed areas) of the coating, is denoted as ΔE. Upon exposure of the coating of the present invention even with a low energy density, for example between 70 and 190 mJ/m.sup.2, more preferably between 75 and 150 mJ/m.sup.2, most preferably between 80 and 120 mJ/m.sup.2, a print-out image is formed characterised by a CIE 1976 colour difference ΔE of at least 2, more preferably at least 2,5 and most preferably at least 3. According to the present invention, a CIE 1976 colour difference ΔE of at least 2 is obtained at very low exposure energies, for example below 150 mJ/m.sup.2. ΔE is the CIE 1976 colour distance Delta E that is defined by the pair wise Euclidean distance of the CIE L*a*b* colour coordinates. CIE L*a*b* colour coordinates are obtained from reflection measurement in 45/0 geometry (non-polarized), using CIE 2° observer and D50 as illuminant. More details are described in CIE S 014-4/E: 2007 Colourimetry—Part 4: CIE 1976 L*a*b* Colour Spaces and CIE publications and CIE S 014-1/E:2006, CIE Standard Colourimetric Observers.

(38) The CIE 1976 colour coordinates L*, a* and b* discussed herein are part of the well-known CIE (Commission Internationale de l'Eclairage) system of tristimulus colour coordinates, which also includes the additional chroma value C* defined as C*=[(a).sup.2+(b).sup.2].sup.1/2. The CIE 1976 colour system is described in e.g. “Colorimetry, CIE 116-1995: Industrial Colour Difference Evaluation”, or in “Measuring Colour” by R. W. G. Hunt, second edition, edited in 1992 by Ellis Horwood Limited, England.

(39) CIE L*a*b* values discussed and reported herein have been measured following the ASTM E308-85 method.

(40) The toplayer may further include a binder. Preferred binders which can be used in the toplayer are polyvinyl alcohol. The polyvinylalcohol has preferably a hydrolysis degree ranging between 74 mol % and 99 mol %, more preferably between 80-98%. The weight average molecular weight of the polyvinylalcohol can be measured by the viscosity of an aqueous solution, 4% by weight, at 20° C. as defined in DIN 53 015, and this viscosity number ranges preferably between 2 and 26, more preferably between 2 and 15, most preferably between 2 and 10.

(41) The overcoat layer may optionally include other ingredients such as inorganic or organic acids, matting agents, surfactants, (organic) waxes or wetting agents as disclosed in EP 2 916 171 and are incorporated herein by reference.

(42) The coating thickness of the toplayer is preferably between 0.10 and 1.75 g/m.sup.2, preferably between 0.20 and 1.3 g/m.sup.2, more preferably between 0.25 and 1.0 g/m.sup.2, most preferably between 0.30 and 0.80 g/m.sup.2. In a more preferred embodiment of the present invention, the toplayer has a coating thickness between 0.25 and 1.75 g/m.sup.2 and comprises a polyvinylalcohol having a hydrolysis degree ranging between 74 mol % and 99 mol % and a viscosity number as defined above ranging between 3 and 26.

(43) The hydrophilic polymers in the protective overcoat layer may result in a problematic viscosity increase of press chemicals such as for example fountain solution and/or developer solution. Therefore, the thickness of the protective overcoat layer should preferably not be too high e.g. above the ranges as given above.

Definitions

(44) An aliphatic hydrocarbon group preferably represents an alkyl, cycloalkyl, alkenyl, cyclo alkenyl or alkynyl group; suitable groups thereof are described below. An aromatic hydrocarbon group preferably represents a hetero(aryl) group; suitable hetero(aryl) groups—i.e. suitable aryl or heteroaryl groups—are described below.

(45) The term “alkyl” herein means all variants possible for each number of carbon atoms in the alkyl group i.e. methyl, ethyl, for three carbon atoms: n-propyl and isopropyl; for four carbon atoms: n-butyl, isobutyl and tertiary-butyl; for five carbon atoms: n-pentyl, 1,1-dimethyl-propyl, 2,2-dimethylpropyl and 2-methyl-butyl, etc. Examples of suitable alkyl groups are methyl, ethyl, n-propyl, isopropyl, n-butyl, 1-isobutyl, 2-isobutyl and tertiary-butyl, n-pentyl, n-hexyl, chloromethyl, trichloromethyl, iso-propyl, iso-butyl, iso-pentyl, neo-pentyl, 1-methylbutyl and iso-hexyl, 1,1-dimethyl-propyl, 2,2-dimethylpropyl and 2-methyl-butyl, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl and methylcyclohexyl groups. Preferably, the alkyl group is a C.sub.1 to C.sub.6-alkyl group.

(46) A suitable alkenyl group is preferably a C.sub.2 to C.sub.6-alkenyl group such as an ethenyl, n-propenyl, n-butenyl, n-pentenyl, n-hexenyl, iso-propenyl, iso-butenyl, iso-pentenyl, neo-pentenyl, 1-methylbutenyl, iso-hexenyl, cyclopentenyl, cyclohexenyl and methylcyclohexenyl group.

(47) A suitable alkynyl group is preferably a C.sub.2 to C.sub.6-alkynyl group; a suitable aralkyl group is preferably a phenyl group or naphthyl group including one, two, three or more C.sub.1 to C.sub.6-alkyl groups; a suitable alkaryl group is preferably a C.sub.1 to C.sub.6-alkyl group including an aryl group, preferably a phenyl group or naphthyl group.

(48) A cyclic group or cyclic structure includes at least one ring structure and may be a monocyclic- or polycyclic group, meaning one or more rings fused together.

(49) Examples of suitable aryl groups may be represented by for example an optionally substituted phenyl, benzyl, tolyl or an ortho-meta- or para-xylyl group, an optionally substituted naphtyl, anthracenyl, phenanthrenyl, and/or combinations thereof. The heteroaryl group is preferably a monocyclic or polycyclic aromatic ring comprising carbon atoms and one or more heteroatoms in the ring structure, preferably, 1 to 4 heteroatoms, independently selected from nitrogen, oxygen, selenium and sulphur. Preferred examples thereof include an optionally substituted furyl, pyridinyl, pyrimidyl, pyrazoyl, imidazoyl, oxazoyl, isoxazoyl, thienyl, tetrazoyl, thiazoyl, (1,2,3)triazoyl, (1,2,4)triazoyl, thiadiazoyl, thiofenyl group and/or combinations thereof.

(50) A cyclic group or cyclic structure includes at least one ring structure and may be a monocyclic- or polycyclic group, meaning one or more rings fused together.

(51) Halogens are selected from fluorine, chlorine, bromine or iodine.

(52) The term “substituted”, in e.g. substituted alkyl group means that the alkyl group may be substituted by other atoms than the atoms normally present in such a group, i.e. carbon and hydrogen. For example, a substituted alkyl group may include a halogen atom or a thiol group. An unsubstituted alkyl group contains only carbon and hydrogen atoms.

(53) The optional substituents on the alkyl, cycloalkyl, alkenyl, cycloalkenyl, alkynyl, aralkyl, alkaryl, aryl and heteroaryl group are preferably selected from, —Cl, —Br, —I, —OH, —SH, —CN, —NO.sub.2, an alkyl group such as a methyl or ethyl group, an alkoxy group such as a methoxy or an ethoxy group, an aryloxy group, a carboxylic acid group or an alkyl ester thereof, a sulphonic acid group or an alkyl ester thereof, a phosphonic acid group or an alkyl ester thereof, a phosphoric acid group or an ester such as an alkyl ester such as methyl ester or ethyl ester, a thioalkyl group, a thioaryl group, thioheteroaryl, —SH, a thioether such as a thioalkyl or thioaryl, ketone, aldehyde, sulfoxide, sulfone, sulfonate ester, sulphonamide, an amino, ethenyl, alkenyl, alkynyl, cycloalkyl, alkaryl, aralkyl, aryl, heteroaryl or heteroalicyclic group and/or combinations thereof.

(54) Support

(55) The lithographic printing plate used in the present invention comprises a support which has a hydrophilic surface or which is provided with a hydrophilic layer. The support is preferably a grained and anodized aluminium support, well known in the art. Suitable supports are for example disclosed in EP 1 843 203 (paragraphs [0066] to [0075]). The surface roughness, obtained after the graining step, is often expressed as arithmetical mean center-line roughness Ra (ISO 4287/1 or DIN 4762) and may vary between 0.05 and 1.5 μm. The aluminum substrate of the current invention has preferably an Ra value between 0.1 and 1.4 μm, more preferably between 0.3 and 1.0 μm and most preferably between 0.4 and 0.9 μm. The lower limit of the Ra value is preferably about 0.1 μm. More details concerning the preferred Ra values of the surface of the grained and anodized aluminum support are described in EP 1 356 926. By anodising the aluminum support, an Al.sub.2O.sub.3 layer is formed and the anodic weight (g/m.sup.2 Al.sub.2O.sub.3 formed on the aluminum surface) varies between 1 and 8 g/m.sup.2. The anodic weight is preferably ≥2.0 g/m.sup.2, more preferably ≥2.5 g/m.sup.2 and most preferably ≥3.0 g/m.sup.2

(56) The grained and anodized aluminium support may be subjected to so-called post-anodic treatments, for example a treatment with polyvinylphosphonic acid or derivatives thereof, a treatment with polyacrylic acid or derivatives thereof, a treatment with potassium fluorozirconate or a phosphate, a treatment with an alkali metal silicate, or combinations thereof. Enlargement or sealing of micropores of the amodized aluminum as disclosed in JP2001-253181A or JP2001-322365A may be performed. Alternatively, the support may be treated with an adhesion promoting compound such as those described in EP 1 788 434 in [0010] and in WO 2013/182328. However, for a precursor optimized to be used without a pre-heat step it is preferred to use a grained and anodized aluminium support without any post-anodic treatment.

(57) Besides an aluminium support, a plastic support, for example a polyester support, provided with one or more hydrophilic layers as disclosed in for example EP 1 025 992 may also be used.

(58) Photopolymer Coating

(59) Photopolymerisable Compound

(60) The coating has at least one layer including a photopolymerisable composition, said layer is also referred to as the “photopolymerisable layer”. The coating may include an intermediate layer, located between the support and the photopolymerisable layer.

(61) The photopolymerisable layer includes at least one polymerisable compound and optionally a binder. The photopolymerisable layer has a coating thickness preferably ranging between 0.2 and 5.0 g/m.sup.2, more preferably between 0.4 and 3.0 g/m.sup.2, most preferably between 0.6 and 2.2 g/m.sup.2.

(62) According to a preferred embodiment of the present invention, the polymerisable compound is a polymerisable monomer or oligomer including at least one terminal ethylenic unsaturated group, hereinafter also referred to as “free-radical polymerisable monomer”. The polymerisation involves the linking together of the free-radical polymerisable monomers. Suitable free-radical polymerisable monomers include, for example, multifunctional (meth)acrylate monomers (such as (meth)acrylate esters of ethylene glycol, trimethylolpropane, pentaerythritol, ethylene glycol, ethoxylated trimethylolpropane, urethane (meth)acrylate) and oligomeric amine di(meth)acrylates. The (meth)acrylic monomers may also have other ethylenically unsaturated groups or epoxide groups in addition to the (meth)acrylate group. The (meth)acrylate monomers may also contain an acidic (such as a carboxylic acid or phosphoric acid) or basic (such as an amine) functionality.

(63) Suitable free-radical polymerisable monomers are disclosed in [0042] and [0050] of EP 2 916 171 and are incorporated herein by reference.

(64) The Initiator

(65) The lithographic printing plate precursor of the current invention comprises a trihaloalkyl sulfone initiator, further also referred to as “TBM-initiator”.

(66) The TBM-initiator is a compound capable of generating free radicals upon exposure, optionally in the presence of a sensitizer. The TBM-initiator is preferably an optionally substituted trihaloalkyl sulfone compound wherein halo independently represents fluoro, bromo, chloro or iodo and sulfone is a chemical compound containing a sulfonyl functional group attached to two carbon atoms. Tribromomethyl phenyl sulfones are most preferred initiators. More details concerning this initiator can be found in unpublished copending application EP 18163285.2 paragraphs [0029] to [0040].

(67) It was found that a coating including the TBM initiator results in a significantly improved image sharpness (resolution) on printed sheets compared to coatings including initiators of the art. Also, it was found that by using the TBM initiator in the photopolymerisable layer in combination with the thermochromic dye in the toplayer, the image sharpness (resolution) on printed sheets is further improved and/or the contrast (i.e. the colour difference between the exposed areas and the non-exposed areas) remains surprisingly stable during handling and/or storage in for example regular white office light (800 lux).

(68) The TBM-initiator is an optionally substituted trihaloalkyl aryl or heteroaryl sulfone compound. The optionally substituted aryl is preferably an optionally substituted phenyl, benzyl, tolyl or an ortho- meta- or para-xylyl, naphtyl, anthracenyl, phenanthrenyl, and/or combinations thereof. The heteroaryl group is preferably a monocyclic or polycyclic aromatic ring comprising carbon atoms and one or more heteroatoms in the ring structure, preferably, 1 to 4 heteroatoms, independently selected from nitrogen, oxygen, selenium and sulphur. Preferred examples thereof include an optionally substituted furyl, pyridinyl, pyrimidyl, pyrazoyl, imidazoyl, oxazoyl, isoxazoyl, thienyl, tetrazoyl, thiazoyl, (1,2,3)triazoyl, (1,2,4)triazoyl, thiadiazoyl, thiofenyl group and/or combinations thereof. The optionally substituted heteroaryl is preferably a five- or six-membered ring substituted by one, two or three oxygen atoms, nitrogen atoms, sulphur atoms, selenium atoms or combinations thereof. Examples thereof include furan, thiophene, pyrrole, pyrazole, imidazole, 1,2,3-triazole, 1,2,4-triazole, tetrazole, oxazole, isoxazole, thiazole, isothiazole, thiadiazole, oxadiazole, pyridine, pyridazine, pyrimidine, pyrazine, 1,3,5-triazine, 1,2,4-triazine or 1,2,3-triazine, benzofuran, benzothiophene, indole, indazole, benzoxazole, quinoline, quinazoline, benzimidazole or benztriazole.

(69) Preferably the TBM-initiator is an optionally substituted trihalomethyl aryl sulfone; more preferably a tribromomethyl aryl sulfone, most preferably the TBM-initiator is an optionally substituted tribromomethyl phenyl sulfone.

(70) The term “alkyl” herein means all variants possible for each number of carbon atoms in the alkyl group i.e. methyl, ethyl, for three carbon atoms: n-propyl and isopropyl; for four carbon atoms: n-butyl, isobutyl and tertiary-butyl; for five carbon atoms: n-pentyl, 1,1-dimethyl-propyl, 2,2-dimethylpropyl and 2-methyl-butyl, etc. Preferably, the alkyl group is preferably a C.sub.1 to C.sub.6-alkyl group. Most preferably the alkyl is a methyl group.

(71) The term “substituted”, in e.g. substituted alkyl group means that the alkyl group may be substituted by other atoms than the atoms normally present in such a group, i.e. carbon and hydrogen. For example, a substituted alkyl group may include a halogen atom or a thiol group. An unsubstituted alkyl group contains only carbon and hydrogen atoms.

(72) The optional substituents represent an alkyl, cycloalkyl, alkenyl or cyclo alkenyl group, an alkynyl group, an aryl or heteroaryl group, an alkylaryl or arylalkyl group, an alkoxy or aryloxy group, a thio alkyl, thio aryl or thio heteroaryl group, a hydroxyl group, —SH, a carboxylic acid group or an alkyl ester thereof, a sulphonic acid group or an alkyl ester thereof, a phosphonic acid group or an alkyl ester thereof, a phosphoric acid group or an alkyl ester thereof, an amino group, a sulphonamide group, an amide group, a nitro group, a nitrile group, a halogen, or a combination thereof.

(73) A suitable alkenyl group is preferably a C.sub.2 to C.sub.6-alkenyl group such as an ethenyl, n-propenyl, n-butenyl, n-pentenyl, n-hexenyl, iso-propenyl, iso-butenyl, iso-pentenyl, neo-pentenyl, 1-methylbutenyl, iso-hexenyl, cyclopentenyl, cyclohexenyl and methylcyclohexenyl group.

(74) A suitable alkynyl group is preferably a C.sub.2 to C.sub.6-alkynyl group; a suitable aralkyl group is preferably a phenyl group or naphthyl group including one, two, three or more C.sub.1 to C.sub.6-alkyl groups; a suitable alkaryl group is preferably a C.sub.1 to C.sub.6-alkyl group including an aryl group, preferably a phenyl group or naphthyl group.

(75) A cyclic group or cyclic structure includes at least one ring structure and may be a monocyclic- or polycyclic group, meaning one or more rings fused together.

(76) The amount of the TBM-initiator typically ranges from 0.1 to 30% by weight, preferably from 0.5 to 10% by weight, most preferably from 2 to 7% by weight relative to the total dry weight of the components of the photopolymerisable composition.

(77) Optionally, any free radical initiator capable of generating free radicals upon exposure directly or in the presence of a sensitizer, may further be used. Suitable examples thereof include onium salts, carbon-halogen bond-containing compounds such as [1,3,5] triazines having trihalomethyl groups, organic peroxides, aromatic ketones, thio compounds, azo based polymerization initiators, azide compounds, ketooxime esters, hexaarylbisimidazoles, metallocenes, active ester compounds, borates and quinonediazides. Of these, onium salts, especially iodonium and/or sulfonium salts are preferable in view of storage stability.

(78) More specific optional free-radical initiators include, for example, the derivatives of acetophenone (such as 2,2-dimethoxy-2-phenylacetophenone, and 2-methyl-I-[4-(methylthio) phenyl-2-morpholino propan-1-one); benzophenone; benzil; ketocoumarin (such as 3-benzoyl-7-methoxy coumarin and 7-methoxy coumarin); xanthone; thioxanthone; benzoin or an alkyl-substituted anthraquinone; onium salts (such as diaryliodonium hexafluoroantimonate, diaryliodonium triflate, (4-(2-hydroxytetradecyl-oxy)-phenyl) phenyliodonium hexafluoroantimonate, triarylsulfonium hexafluorophosphate, triarylsulfonium p-toluenesulfonate, (3-phenylpropan-2-onyl) triaryl phosphonium hexafluoroantimonate, and N-ethoxy(2-methyl)pyridinium hexafluorophosphate, and onium salts as described in U.S. Pat. Nos. 5,955,238, 6,037,098, and 5,629,354); borate salts (such as tetrabutylammonium triphenyl(n-butyl)borate, tetraethylammonium triphenyl(n-butyl)borate, diphenyliodonium tetraphenylborate, diphenyliodonium tetraphenylborate wherein the phenyl groups of the iodonium salt are substituted with a group including at least six carbon atoms, and triphenylsulfonium triphenyl(n-butyl)borate, and borate salts as described in U.S. Pat. Nos. 6,232,038 and 6,218,076); haloalkyl substituted s-triazines (such as 2,4-bis(trichioromethyl)-6-(p-methoxy-styryl)-s-triazine, 2,4-bis(trichioromethyl)-6-(4-methoxy-naphth-1-yl)-s-triazine, 2,4-bis(trichloromethyl)-6-piperonyl-s-triazine, and 2,4-bis(trichloromethyl)-6-[(4-ethoxy-ethylenoxy)-phen-1-yl]-s-triazine, and s-triazines as described in U.S. Pat. Nos. 5,955,238, 6,037,098, 6,010,824 and 5,629,354); and titanocene (bis(etha.9-2,4-cyclopentadien-1-yl) bis[2,6-difluoro-3-(IH-pyrrol-1-yl)phenyl) titanium). Onium salts, borate salts, and s-triazines are preferred free radical initiators. Diaryliodonium salts and triarylsulfonium salts are preferred onium salts. Triarylalkylborate salts are preferred borate salts. Trichloromethyl substituted s-triazines are preferred s-triazines. These initiators may be used alone or in combination.

(79) A very high sensitivity can be obtained by the combination of an optical brightener as sensitizer and a polymerisation initiator.

(80) The photopolymerisable layer may also comprise a co-initiator. Typically, a co-initiator is used in combination with a free radical initiator. Suitable co-initiators for use in the photopolymer coating are disclosed in U.S. Pat. Nos. 6,410,205; 5,049,479; EP 1 079 276, EP 1 369 232, EP 1 369 231, EP 1 341 040, US 2003/0124460, EP 1 241 002, EP 1 288 720 and in the reference book including the cited references: Chemistry & Technology UV & EB formulation for coatings, inks & paints—Volume 3—Photoinitiators for Free Radical and Cationic Polymerisation by K. K. Dietliker—Edited by P. K. T. Oldring—1991—ISBN 0 947798161. Specific co-initiators, as described in EP 107 792, may be present in the photopolymerizable layer to further increase the sensitivity. Preferred co-initiators are disclosed in EP 2 916 171 [0051] and are incorporated herein by reference.

(81) A very high sensitivity can be obtained by including a sensitizer such as for example an optical brightener in the coating. Suitable examples of optical brighteners as sensitizers are described in WO 2005/109103 page 24, line 20 to page 39. Useful sensitizers can be selected from the sensitizing dyes disclosed in U.S. Pat. Nos. 6,410,205; 5,049,479; EP 1 079 276, EP 1 389 232, EP 1 389 231, EP 1 341 040, US 2003/0124480, EP 1 241 002 and EP 1 288 720.

(82) Specific co-initiators, as described in EP 107 792, may be present in the photopolymerizable layer to further increase the sensitivity. Preferred co-initiators are sulfur-compounds, especially thiols like e.g. 2-mercaptobenzothiazole, 2-mercaptobenzoxazole, 2-mercapto-benzimidazole, 4-methyl-3-propyl-1,2,4-triazoline-S-thione, 4-methyl-3-n-heptyl-1,2,4-triazoline-5-thione, 4-phenyl-3-n-heptyl-1,2,4-triazoline-5-thione, 4-phenyl-3,5-dimercapto-1,2,4-triazole, 4-n-decyl-3,5-dimercapto-1,2,4-triazole, 5-phenyl-2-mercapto-1,3,4-oxadiazole, 5-methylthio-1,3,4-thiadiazoline-2-thione, 5-hexylthio-1,3,4-thiadiazoline-2-thione, mercaptophenyltetrazole, pentaerythritol mercaptopropionate, butyric acid-3-mercapto-neopentanetetrayl ester, pentaerythritol tetra(thioglycolate). Other preferred co-initiators are polythioles as disclosed in WO 2006/048443 and WO 2006/048445. These polythiols may be used in combination with the above described thiols, e.g. 2-mercaptobenzothiazole.

(83) The photopolymerizable layer may optionally include infrared light absorbing dyes as sensitizers absorbing light between 750 nm and 1300 nm, preferably between 780 nm and 1200 nm, more preferably between 800 nm and 1100 nm. Particular preferred sensitizers are heptamethinecyanine dyes disclosed in EP 1 359 008 paragraph [0030] to [0032].

(84) The Binder

(85) The photopolymerizable layer preferably includes a binder. The binder can be selected from a wide series of organic polymers. Compositions of different binders can also be used. Useful binders are described in for example EP 1 043 627 in paragraph [0013]. Also particulate shaped polymers including homopolymers or copolymers prepared from monomers such as ethylene, styrene, vinyl chloride, methyl acrylate, ethyl acrylate, methyl methacrylate, ethyl methacrylate, acrylonitrile, vinyl carbazole, acrylate or methacrylate, or mixtures thereof.

(86) Thermally reactive polymer fine particles including a thermally reactive group such as an ethylenically unsaturated group, a cationic polymerizable group, an isocyanate group, an epoxy group, a vinyloxy group, and a functional group having an active hydrogen atom, a carboxy group, a hydroxy group, an amino group or an acid anhydride.

(87) The average particle diameter of the polymer fine particle is preferably 0.01 mm to 3.0 mm. Particulate polymers in the form of microcapsules, microgels or reactive microgels are suitable as disclosed in EP 1 132 200; EP 1 724 112; US 2004/106060.

(88) Other Ingredients

(89) The photopolymerisable layer may also comprise particles which increase the resistance of the coating against manual or mechanical damage. The particles may be inorganic particles, organic particles or fillers such as described in for example U.S. Pat. No. 7,108,956. More details of suitable spacer particles described in EP 2 916 171 [0053] to [0056] are incorporated herein by reference.

(90) The photopolymerizable layer may also comprise an inhibitor. Particular inhibitors for use in the photopolymer coating are disclosed in U.S. Pat. No. 6,410,205, EP 1 288 720 and EP 1 749 240.

(91) The photopolymerizable layer may further comprise an adhesion promoting compound. The adhesion promoting compound is a compound capable of interacting with the support, preferably a compound having an addition-polymerizable ethylenically unsaturated bond and a functional group capable of interacting with the support. Under “interacting” is understood each type of physical and/or chemical reaction or process whereby, between the functional group and the support, a bond is formed which can be a covalent bond, an ionic bond, a complex bond, a coordinate bond or a hydrogen-bond, and which can be formed by an adsorption process, a chemical reaction, an acid-base reaction, a complex-forming reaction or a reaction of a chelating group or a ligand.

(92) The adhesion promoting compounds described in EP 2 916 171 [0058] are incorporated herein by reference.

(93) Various surfactants may be added into the photopolymerisable layer to allow or enhance the developability of the precursor; especially developing with a gum solution. Both polymeric and small molecule surfactants for example nonionic surfactants are preferred. More details are described in EP 2 916 171 [0059] and are incorporated herein by reference.

(94) According to the present invention there is also provided a method for making a negative-working lithographic printing plate comprising the steps of imagewise exposing a printing plate precursor followed by developing the imagewise exposed precursor so that the non-exposed areas are dissolved in the developer solution. Optionally, after the imaging step, a heating step is carried out to enhance or to speed-up the polymerization and/or crosslinking reaction. The lithographic printing plate precursor can be prepared by (i) applying on a support the coating as described above and (ii) drying the precursor.

(95) Exposure Step

(96) The printing plate precursor is preferably image-wise exposed by a laser emitting IR light. Preferably, the image-wise exposing step is carried out off-press in a platesetter, i.e. an exposure apparatus suitable for image-wise exposing the precursor with a laser such as a laser diode, emitting around 830 nm or a Nd YAG laser emitting around 1060 nm, or by a conventional exposure in contact with a mask. In a preferred embodiment of the present invention, the precursor is image-wise exposed by a laser emitting IR light.

(97) Preheat Step

(98) After the exposing step, the precursor may be pre-heated in a preheating unit, preferably at a temperature of about 80° C. to 150° C. and preferably during a dwell time of about 5 seconds to 1 minute. This preheating unit may comprise a heating element, preferably an IR-lamp, an UV-lamp, heated air or a heated roll. Such a preheat step can be used for printing plate precursors comprising a photopolymerisable composition to enhance or to speed-up the polymerization and/or crosslinking reaction.

(99) Development Step

(100) Subsequently to the exposing step or the preheat step, when a preheat step is present, the plate precursor may be processed (developed). Before developing the imaged precursor, a pre-rinse step might be carried out especially for the negative-working lithographic printing precursors having a protective oxygen barrier or topcoat. This pre-rinse step can be carried out in a stand-alone apparatus or by manually rinsing the imaged precursor with water or the pre-rinse step can be carried out in a washing unit that is integrated in a processor used for developing the imaged precursor. The washing liquid is preferably water, more preferably tap water. More details concerning the wash step are described in EP 1 788 434 in [0026].

(101) During the development step, the non-exposed areas of the image-recording layer are at least partially removed without essentially removing the exposed areas. The processing liquid, also referred to as developer, can be applied to the plate e.g. by rubbing with an impregnated pad, by dipping, immersing, coating, spincoating, spraying, pouring-on, either by hand or in an automatic processing apparatus. The treatment with a processing liquid may be combined with mechanical rubbing, e.g. by a rotating brush. During the development step, any water-soluble protective layer present is preferably also removed. The development is preferably carried out at temperatures between 20 and 40° C. in automated processing units.

(102) In a highly preferred embodiment, the processing step as described above is replaced by an on-press processing whereby the imaged precursor is mounted on a press and processed on-press by rotating said plate cylinder while feeding dampening liquid and/or ink to the coating of the precursor to remove the unexposed areas from the support. In a preferred embodiment, only dampening liquid is supplied to the plate during start-up of the press and after a number of revolutions of the plate cylinder also the ink supply is switched on. In an alternative embodiment, supply of dampening liquid and ink is started simultaneously, or only ink can be supplied during a number of revolutions before switching on the supply of dampening liquid.

(103) The processing step may also be performed by combining embodiments described above, e.g. combining development with a processing liquid with development on-press by applying ink and/or fountain.

(104) Processing Liquid

(105) The processing liquid may be an alkaline developer or solvent-based developer. Suitable alkaline developers have been described in US2005/0162505. An alkaline developer is an aqueous solution which has a pH of at least 11, more typically at least 12, preferably from 12 to 14. Alkaline developers typically contain alkaline agents to obtain high pH values can be inorganic or organic alkaline agents. The developers can comprise anionic, non-ionic and amphoteric surfactants (up to 3% on the total composition weight); biocides (antimicrobial and/or antifungal agents), antifoaming agents or chelating agents (such as alkali gluconates), and thickening agents (water soluble or water dispersible polyhydroxy compounds such as glycerine or polyethylene glycol).

(106) Preferably, the processing liquid is a gum solution whereby during the development step the non-exposed areas of the photopolymerisable layer are removed from the support and the plate is gummed in a single step. The development with a gum solution has the additional benefit that, due to the remaining gum on the plate in the non-exposed areas, an additional gumming step is not required to protect the surface of the support in the non-printing areas. As a result, the precursor is processed and gummed in one single step which involves a less complex developing apparatus than a developing apparatus comprising a developer tank, a rinsing section and a gumming section. The gumming section may comprise at least one gumming unit or may comprise two or more gumming units. These gumming units may have the configuration of a cascade system, i.e. the gum solution, used in the second gumming unit and present in the second tank, overflows from the second tank to the first tank when gum replenishing solution is added in the second gumming unit or when the gum solution in the second gumming unit is used once-only, i.e. only starting gum solution is used to develop the precursor in this second gumming unit by preferably a spraying or jetting technique. More details concerning such gum development is described in EP1 788 444.

(107) A gum solution is typically an aqueous liquid which comprises one or more surface protective compounds that are capable of protecting the lithographic image of a printing plate against contamination, e.g. by oxidation, fingerprints, fats, oils or dust, or damaging, e.g. by scratches during handling of the plate. Suitable examples of such surface protective compounds are film-forming hydrophilic polymers or surfactants. The layer that remains on the plate after treatment with the gum solution preferably comprises between 0.005 and 20 g/m.sup.2 of the surface protective compound, more preferably between 0.010 and 10 g/m.sup.2, most preferably between 0.020 and 5 g/m.sup.2. More details concerning the surface protective compounds in the gum solution can be found in WO 2007/057348 page 9 line 3 to page 11 line 6. As the developed plate precursor is developed and gummed in one step, there is no need to post-treat the processed plate.

(108) The gum solution preferably has a pH value between 3 and 11, more preferably between 4 and 10, even more preferably between 5 and 9, and most preferably between 6 and 8. A suitable gum solution is described in for example EP 1 342 568 in [0008] to [0022] and WO2005/111727. The gum solution may further comprise an inorganic salt, an anionic surfactant, a wetting agent, a chelate compound, an antiseptic compound, an anti-foaming compound and/or an ink receptivity agent and/or combinations thereof. More details about these additional ingredients are described in WO 2007/057348 page 11 line 22 to page 14 line 19.

(109) Drying and Baking Step

(110) After the processing step the plate may be dried in a drying unit. In a preferred embodiment the plate is dried by heating the plate in the drying unit which may contain at least one heating element selected from an IR-lamp, an UV-lamp, a heated metal roller or heated air.

(111) After drying the plate can optionally be heated in a baking unit. More details concerning the heating in a baking unit can be found in WO 2007/057348 page 44 line 26 to page 45 line 20.

(112) The printing plate thus obtained can be used for conventional, so-called wet offset printing, in which ink and an aqueous dampening liquid is supplied to the plate. Another suitable printing method uses a so-called single-fluid ink without a dampening liquid. Suitable single-fluid inks have been described in U.S. Pat. Nos. 4,045,232; 4,981,517 and 6,140,392. In a most preferred embodiment, the single-fluid ink comprises an ink phase, also called the hydrophobic or oleophilic phase, and a polyol phase as described in WO 00/32705.

EXAMPLES

Example 1

(113) 1. Preparation of the Printing Plate Precursors

(114) Preparation of the Aluminium Support S-01

(115) A 0.3 mm thick aluminium foil was degreased by spraying with an aqueous solution containing 26 g/l NaOH at 65° C. for 2 seconds and rinsed with demineralised water for 1.5 seconds. The foil was then electrochemically grained during 10 seconds using an alternating current in an aqueous solution containing 15 g/l HCl, 15 g/l SO42− ions and 5 g/l Al3+ ions at a temperature of 37° C. and a current density of about 100 A/dm2. Afterwards, the aluminium foil was then desmutted by etching with an aqueous solution containing 5.5 g/l of NaOH at 36° C. for 2 seconds and rinsed with demineralised water for 2 seconds. The foil was subsequently subjected to anodic oxidation during 15 seconds in an aqueous solution containing 145 g/l of sulfuric acid at a temperature of 50° C. and a current density of 17 A/dm2, then washed with demineralised water for 11 seconds and dried at 120° C. for 5 seconds.

(116) The support thus obtained was characterized by a surface roughness Ra of 0.35-0.4 μm (measured with interferometer NT1100) and had an oxide weight of 3.0 g/m.sup.2.

(117) Photopolymerisable Layer

(118) The photosensitive compositions as defined in Table 1 were coated onto the above described support S-01. The components were dissolved in a mixture of 35% by volume of MEK and 65% by volume of Dowanol PM (1-methoxy-2-propanol, commercially available from DOW CHEMICAL Company). The coating solution was applied at a wet coating thickness of 30 μm and then dried at 120° C. for 1 minute in a circulation oven.

(119) TABLE-US-00001 TABLE 1 composition of the photosensitive layer (g/m.sup.2) Ingredients g/m.sup.2 PL-1 PL-2 FST 510 (1) 0.192 0.250 CN 104 (2) 0.192 0.250 Initiator-01 (3) 0.060 0.060 S2539 (4) 0.020 0.020 Ruco coat EC4811 (5) 0.384 0.250 Tegoglide 410 (6) 0.0015 0.0015 Sipomer PAM 100 (7) 0.130 0.130 Albritect CP 30 (8) 0.024 0.024 1) FST 510 is a reaction product from 1 mole of 2,2,4-trimethylhexamethylenediisocyanate and 2 moles of hydroxyethyl-methacrylate commercially available from AZ Electronics as a 82 wt. % solution in MEK; 2) CN 104 is an epoxy acrylate oligomer commercially available from Arkema; 3) Initiator-01 is 4-hydroxyphenyl-tribromomethyl-sulfone; 4) S2539 is an infrared absorbing dye commercially available from FEW Chemicals having the following structure:

(120) ##STR00006## 5) Ruco coat EC4811 is a polyether polyurethane commercially available from Rudolf Chemistry; 6) Tegoglide 410 is a surfactant commercially available from Evonik Tego Chemie GmbH; 7) Sipomer PAM 100 is a methacrylate phosphonic ester commercially available from Rhodia; 8) Albritect CP 30, is a copolymer of vinylphosphonic acid and acrylic acid commercially available as a 20 wt. % aqueous dispersion from Rhodia.

(121) Protective Top Layer

(122) On top of the photosensitive layer, a solution in water with the composition as defined in Table 2 was coated (40 μm), and dried at 110° C. for 2 minutes.

(123) TABLE-US-00002 TABLE 2 composition of the protective layer (g/m.sup.2) Ingredients g/m.sup.2 OC-1 OC-2 Mowiol 4-88 (1) 0.30 0.30 Mowiol 4-98 (2) 0.18 0.18 IR-01 (3) 0.05 0.03 Lutensol A8 (4) 0.01 0.01 1) Mowiol 4-88TM and Mowiol 4-98TM are partially hydrolyzed polyvinylalcohols commercially available from Kuraray; 2) Exceval AQ4104 is a fully hydrolysed ethylene-vinyl alcohol copolymer commercially available from Kuraray; 3) IR-01 is an infrared absorbing dye having the following formula:

(124) ##STR00007## 4) Lutensol A8TM is a surface active agent commercially available from BASF

(125) PPP-01 to PPP-04 were obtained (Table 3).

(126) 2. Imaging

(127) The printing plate precursors PPP-01 to PPP-04 were imaged at 2400 dpi with a High Power Creo 40W TE38 thermal Platesetter™ (200 Ipi Agfa Balanced Screening (ABS)), commercially available from Kodak and equipped with a 830 nm IR laser diode, at energy densities between 50 and 200 mJ/cm.sup.2. Printing plates PP-01 to PP-04 were obtained.

(128) 3. ΔE Measurement

(129) Lab measurement executed with a GretagMacBeth Spectra Eye reflection spectrophotometer with the settings: D50 (illuminant), 2° (Observer), No filter; commercially available from GretagMacBeth. The total colour difference ΔE is a single value that takes into account the difference between the L, a* and b* values of the image areas and the non-image areas:
ΔE=√{square root over (ΔL.sup.2+Δa.sup.2+Δb.sup.2)}

(130) The higher the total colour difference ΔE, the better the obtained contrast. The contrast between image and non-image areas results in the occurrence of a print-out image.

(131) 4. Results

(132) TABLE-US-00003 TABLE 3 Results of the total colour difference ΔE Exposure Printing Photopolymer Overcoat energy plate layer layer mJ/cm.sup.2 ΔE PP-01 PL-1 OC-1 90 2.7 Inventive PP-02 PL-1 OC-1 120 4.8 Inventive PP-03 PL-1 OC-2 150 7.8 Inventive PP-04 PL-2 OC-2 150 3.7 Inventive

(133) The results in Table 3 demonstrate that the print-out image is strong (the total colour difference ΔE is equal or higher than 2) for the inventive printing plates including an overcoat layer including the dye according to the present invention.

Example 2

(134) 1. Preparation of Comparative Printing Plate PP-05

(135) Photopolymerisable Layer

(136) The photopolymerisable layer PL-01 as defined in Table 4 was coated onto the above described support S-01. The components were dissolved in a mixture of 35% by volume of MEK and 65% by volume of Dowanol PM (1-methoxy-2-propanol, commercially available from DOW CHEMICAL Company). The coating solution was applied at a wet coating thickness of 30 μm and then dried at 120° C. for 1 minute in a circulation oven.

(137) TABLE-US-00004 TABLE 4 composition of the photosensitive layer (g/m.sup.2) Ingredients g/m.sup.2 PL-01 FST 510 (1) 0.250 CN 104 (2) 0.250 Initiator-02 (3) 0.060 S0094 (4) 0.020 Ruco coat EC4811 (5) 0.250 Tegoglide 410 (6) 0.0015 Sipomer PAM 100 (7) 0.130 Albritect CP 30 (8) 0.024 1) FST 510 is a reaction product from 1 mole of 2,2,4-trimethylhexamethylenediisocyanate and 2 moles of hydroxyethyl-methacrylate commercially available from AZ Electronics as a 82 wt. % solution in MEK; 2) CN 104 is an epoxy acrylate oligomer commercially available from Arkema; 3) Initiator-02 is 4-hydroxyphenyl-tribromomethyl-sulfone 4) S0094 is an infrared absorbing dye commercially available from FEW Chemicals having the following structure:

(138) ##STR00008## 5) Ruco coat EC4811 is a polyether polyurethane commercially available from Rudolf Chemistry 6) Tegoglide 410 is a surfactant commercially available from Evonik Tego Chemie GmbH; 7) Sipomer PAM 100 is a methacrylate phosphonic ester commercially available from Rhodia; 8) Albritect CP 30, is a copolymer of vinylphosphonic acid and acrylic acid commercially available as a 20 wt. % aqueous dispersion from Rhodia.

(139) Protective Top Layer

(140) On top of the photosensitive layer, a solution in water with the composition as defined in Table 5 was coated on the photosensitive layer, and dried at 110° C. for 2 minutes. Printing plate precursor PPP-05 having a coating thickness of 2 g/m.sup.2 was obtained.

(141) TABLE-US-00005 TABLE 5 composition of the protective layer (g/m.sup.2) Ingredients OC-03 g/m.sup.2 comp Mowiol 4-88 (1) 0.785 Mowiol 8-88 (1) 0.343 Mowiol 6-98 (1) 0.685 IR-02 (2) 0.158 Acticide LA1206 (3) 0.012 Metolat FC 355 (4) 0.018 Lutensol A8 (5) 0.0015 1) Mowiol 4-88TM, Mowiol 8-88TM are partially hydrolyzed polyvinylalcohols and Mowiol 6-98TM is a fully hydrolysed polyvinylalcohol commercially available from Kuraray; 2) IR-02 is an infrared absorbing dye having the following formula:

(142) ##STR00009## 3) Acticide LA1206 is a biocide, commercially available from Thor. 4) Metolat FC 355 is an ethoxylated ethylenediamine, commercially available from Munzing Chemie. 5) Lutensol ASTM is a surface active agent, commercially available from BASF.
2. Imaging

(143) The printing plate precursor PPP-05 was imaged at 2400 dpi with a High Power Creo 40W TE38 thermal Platesetter™ (200 Ipi Agfa Balanced Screening (ABS)), commercially available from Kodak and equipped with a 830 nm IR laser diode, at energy densities between 80 and 300 mJ/cm.sup.2. Printing plate PP-05 was obtained.

(144) 3. ΔE Measurement

(145) See example 1

(146) 4. Results

(147) The total colour difference ΔE was measured and the results are summarized in the Table 6 below.

(148) A total colour difference ΔE above or equal to 2 is defined as a clear print out image. A total colour difference ΔE below 2 is defined as an unsufficient print out image.

(149) TABLE-US-00006 TABLE 6 ΔE of printing plate PP-05 Printing plate PP-05 Comparative Concentration OC Exposure IR dye thickness energy mg/m.sup.2 g/m.sup.2 mJ/cm.sup.2 ΔE 0.157 2 g/m.sup.2 90 0.6 0.157 2 g/m.sup.2 120 0.4 0.157 2 g/m.sup.2 150 0.5

(150) The comparative samples clearly demonstrate that the formation of a printout image is suppressed when the total dry weight of the protective topcoat containing the thermochromic dye is too high, i.e. 2 g/m.sup.2: At a significantly elevated dye concentration (157 mg/m.sup.2), the printout image at exposure energies of 90 to 150 mJ/cm.sup.2 remains very weak.