Lithographic printing plate precursor

1. A lithographic printing plate precursor comprising a support and a coating comprising (i) a photopolymerisable layer comprising a polymerisable compound and a photoinitiator, and (ii) a toplayer above the photopolymerisable layer;

wherein the toplayer has a thickness between 0.1 g/m 2 and 0.552 g/m 2 and comprises an infrared absorbing compound comprising 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, and the infrared absorbing compound forms a print-out image upon exposure to IR radiation between 10 mJ/cm 2 and 150 mJ/cm 2 .

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 print-out image is characterized by a CIE 1976 colour distance ΔE of at least 2.

4. 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.

5. The printing plate precursor of claim 4 , wherein the infrared absorbing dye is of Formula I, II or III:

wherein

Ar 1 , Ar 2 and Ar 3 independently are an optionally substituted aromatic hydrocarbon group or an aromatic hydrocarbon group comprising an optionally substituted annulated benzene ring;

W 1 and W 2 independently are a sulphur atom, an oxygen atom, NR″ wherein R″ is an optionally substituted alkyl group, NH, a —CM 10 M 11 group wherein M 10 and M 11 are independently an optionally substituted aliphatic hydrocarbon group or an optionally substituted (hetero)aryl group, or M 10 and M 11 together comprise the necessary atoms to form a cyclic structure;

W 3 is a sulphur atom or a —C(A 3 )=C(A 4 )-group;

W 4 is a sulphur atom or a —C(A 7 )=C(A′)-group;

M 1 and M 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 3 and M 4 independently are an optionally substituted aliphatic hydrocarbon group;

M 5 , M 6 , M 7 , M 8 , M 16 and M 17 independently are hydrogen, a halogen, or an optionally substituted aliphatic hydrocarbon group;

A 1 , A 2 , A 3 , A 4 , A 5 , A 6 , A 7 , and A 8 independently are hydrogen, a halogen atom, an optionally substituted aliphatic hydrocarbon group, an optionally substituted (hetero)aryl group, or A 1 and A 2 , A 3 and A 4 , A 5 and A 6 , or A 7 and A 8 together comprise the necessary atoms to form a cyclic structure;

M 12 , M 13 , M 14 , and M 15 independently are an optionally substituted aliphatic hydrocarbon group; an optionally substituted (hetero)aryl group; two of M 14 , M 15 , A 5 , and A 7 together comprise the necessary atoms to form at least one cyclic structure; or two of M 12 , M 13 , A 2 , and A 4 together comprise the necessary atoms to form at least one cyclic structure; and

M 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.

6. The printing plate precursor of claim 5 , wherein M 1 and M 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.

7. The printing plate precursor of claim 5 , wherein the infrared absorbing dye is of Formula IV

wherein

Ar 1 , Ar 2 , W 1 , W 2 , M 1 , M 2 , M 3 , M 4 , M 5 , M 6 , M 7 , M 8 , and M 9 are as defined in claim 5 and wherein the dye optionally comprises one or more counter ions in order to obtain an electrically neutral compound.

8. The printing plate precursor of claim 5 , wherein M 9 is —(N═CR 17 ) a —NR 5 —CO—R 4 , —(N═CR 17 ) b —NR 5 —SO 2 —R 6 , —(N═CR 17 ) c —NR 11 —SO—R 12 , —SO 2 NR 15 R 16 , or —S—CH 2 —CR 7 (H) 1-d (R 8 ) d —NR 9 —COOR 18 ;

wherein

a, b, c and d independently are 0 or 1;

R 17 is hydrogen, an optionally substituted aliphatic hydrocarbon group, an optionally substituted (hetero)aryl group, or R 17 and R 5 or R 17 and R 11 together comprise the necessary atoms to form a cyclic structure;

R 4 is —OR 10 , —NR 13 R 14 , or —CF 3 ;

R 10 is an optionally substituted (hetero)aryl group or an optionally branched aliphatic hydrocarbon group;

R 13 and R 14 independently are hydrogen, an optionally substituted aliphatic hydrocarbon group, an optionally substituted (hetero)aryl group, or R 13 and R 14 together comprise the necessary atoms to form a cyclic structure;

R 6 is a optionally substituted aliphatic hydrocarbon group, an optionally substituted (hetero)aryl group, —OR 10 , —NR 13 R 14 or —CF 3 ;

R 5 is hydrogen, an optionally substituted aliphatic hydrocarbon group, SO 3 , a —COOR 18 group, an optionally substituted (hetero)aryl group, or R 5 together with at least one of R 10 , R 13 , and R 14 comprise the necessary atoms to form a cyclic structure;

R 11 , R 15 and R 16 independently are hydrogen, an optionally substituted aliphatic hydrocarbon group, or an optionally substituted (hetero)aryl group, or R 15 and R 16 together comprise the necessary atoms to form a cyclic structure;

R 12 is an optionally substituted aliphatic hydrocarbon group or an optionally substituted (hetero)aryl group;

R 7 and R 9 independently are hydrogen or an optionally substituted aliphatic hydrocarbon group;

R 8 is —COO— or —COOR 8′ , wherein R 8′ is hydrogen, an alkali metal cation, an ammonium ion, or a mono-, di-, tri-, or tetra-alkyl ammonium ion; and

R 18 is an optionally substituted (hetero)aryl group or an alpha-branched aliphatic hydrocarbon group.

9. The printing plate precursor of claim 5 wherein M 9 is —NR 5 —CO—R 4 or —NR 5 —SO 2 —R 6

wherein

R 4 is —OR 10 , wherein R 10 is an optionally branched aliphatic hydrocarbon group;

R 5 is hydrogen, an optionally substituted aliphatic hydrocarbon group, or an optionally substituted (hetero)aryl group; and

R 6 is an optionally substituted aliphatic hydrocarbon group or an optionally substituted (hetero)aryl group.

10. The printing plate precursor of claim 4 wherein the infrared absorbing dye is of Formula V, Formula VI, Formula VII, Formula VIII, Formula IX, Formula X, Formula XI, or Formula XII:

wherein

X − is halogen, sulphonate, perfluorosulphonate, tosylate, tetrafluoroborate, hexafluorophosphate, arylborate or arylsulphonate;

R is hydrogen, an optionally substituted aliphatic hydrocarbon group, or an optionally substituted (hetero)aryl group;

R 3 and R 3′ are independently an optionally substituted alkyl group or an ether group; and

M + is Li + , Na + , K + , NH 4 + , an optionally substituted aryl group, or R′R″R′″NH + wherein R′, R″, and R′″ independently are hydrogen or an optionally substituted alkyl group.

11. 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 a photoinitiator, and

(ii) a toplayer above the photopolymerisable layer, wherein the toplayer has a thickness between 0.1 g/m 2 and 0.552 g/m 2 and comprises an infrared absorbing compound comprising 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, and the infrared absorbing compound forms a print-out image upon exposure to IR radiation between 10 mJ/cm 2 and 150 mJ/cm 2 , to provide a precursor, and

drying the precursor.

12. 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 a photoinitiator, and (ii) a toplayer above the photopolymerisable layer, wherein the toplayer has a thickness between 0.1 g/m 2 and 0.552 g/m 2 and comprises an infrared absorbing compound comprising 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, and the infrared absorbing compound forms a print-out image upon exposure to IR radiation between 10 mJ/cm 2 and 150 mJ/cm 2 .

13. The method of claim 12 , wherein the colour change is characterized by a CIE 1976 colour distance ΔE between the image areas and non-image areas of at least 2.

14. The method of claim 12 , wherein the printing plate precursor is exposed to IR radiation and the IR radiation has an energy density between 75 mJ/cm 2 and 150 mJ/cm 2 .

15. The method of claim 12 , 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.