Method of improving surface cure in digital flexographic printing plates

1. A method if producing a relief image printing element from a photocurable printing blank, the method comprising the steps of:

a) providing a photocurable printing blank, the photocurable printing blank comprising:

i) a backing or support layer

ii) one or more photocurable layers disposed on the backing or support layer, wherein the one or more photocurable layers comprise:

1) a binder;

2) one or more monomers;

3) a photoinitiator;

4) an additive selected from the group consisting of phosphites, thioether amine compounds, and combinations or one or more of the foregoing; and

5) an antioxidant selected from the group consisting of 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene, alkylated phenols, alkylated bis-phenols, polymerized trimethyldihydroquinone, dilauryl thiopropionate, and combinations of one or more of the foregoing;

iii) a laser ablatable mask layer disposed on the one or more photocurable layers, the laser ablatable mask layer comprising a radiation opaque material;

b) selectively laser ablating the laser ablating the laser ablatable mask layer to create an in situ negative of a desired relief image in the laser ablatable mask layer;

c) exposing the one or more photocurable layers to actinic radiation through the in situ negative to selectively crosslink and cure portions of the at least one photocurable layer; and

d) developing the exposed one or more photocurable layers of the photocurable printing blank to reveal the relief image therein, said relief image comprising a plurality of relief printing dots;

wherein the edge sharpness of the relief printing dots, defined as the ratio of the radius of curvature r e at the intersection of a shoulder and the top of the relief printing dot to the width of the dot's top printing surface p is less than 5%; and

wherein the average shoulder angle, measured as the angle between a shoulder of the dot across the width of the dot's top printing surface, is less than 30°; and

wherein the relief image printing element has a Shore A hardness of between 45 and 70.

2. The method according to claim 1 , wherein the additive is a phosphite selected from the group consisting of tris(nonylphenyl)phosphite (TNPP), diphenyl phosphite, tridecyl phosphite, triisodecyl phosphite, tris(tridecyl)phosphite, trilauryl phosphite, disteraryl pentaerythriol diphosphite, diisodecyl phenyl phosphite, diphenyl isodecyl phosphite, diphenyl octyl phosphite, diphenyl isooctyl phosphite, diphenyl tri isodecyl monophenyl dipropyleneglycol diphosphite, alkyl bisphenol A phosphite, tetraphenyl dipropyleneglycol diphosphite, poly(dipropyleneglycol) phenyl phosphite, tris(dipropylene glycol) phosphite, and dioleyl hydrogen phosphite and combinations of one or more of the foregoing.

3. The method according to claim 1 , wherein the additive is a thioether amine compound selected from the group consisting of 2,6-di-tert-butyl-4-(4,6-bis(octylthio)-1,3,5-triazin-2-ylamino)phenol, 4-[[4,6-bis(nonylsulfanyl)-1,3,5-triazin-2-yl]amino]-2,6-ditert-butylphenol, 4-[[4,6-bis(octadecylsulfanyl)-1,3,5-triazin-2-yl]amino]-2,6-ditert-butylphenol, 4-[[4,6-octylsulfanyl)-1,3,5-triazin-2-yl]amino]-2,6-bis(2-methylnonan-2-yl)phenol, 4-[[4,6-bis(hexylsulfanyl)-1,3,5-triazin-2-yl]amino]-2,6-ditert-butylphenol, 4-[[4,6-bis(heptylsulfanyl)-1,3,5-triazin-2-yl]amino]-2,6-ditert-butylphenol, 4-[[4,6-bis(octylsulfanyl)-1,3,5-triazin-2-yl]amino]-2-tert-butyl-6-methylphenol, 4-[[4,6-bis(ethylsulfanyl)-1,3,5-triazin-2-yl]amino-2,6-ditert-butylphenol, 4-[[4,6-bis(2,4,4-trimethylpentan-2-ylsulfanyl-1,3,5-triazin-2-yl]smino]-2,6-ditert-butylphenol, 4-[[4,6-bis(2-octylsulfanylethylsulfanyl-1,3,5-triazin-2-yl]amino]-2,6-ditert-butylphenol, 4-[[4,6-bis(octylsulfanyl)-1,3,5-triazin-2-yl]amino]-2,6-dibutylphenol, 4-[[4,6-bis(octylsulfanyl)-1,3,5-triazin-2-yl]amino]-2,6-dimethylphenol, 2,6-ditert-buty-4-[[4-(3,5-ditert-butyl-4-hydroxyanilino)-6-octylsulfanyl-1,3,5-triazin-2-yl]amino]phenol, 4-[[4,6-bis(pentylsulfanyl)-1,3,5-triazin-2-yl]amino]-2,6-dimethylphenol, 4-[[4,6-bis(hexylsulfanyl)-1,3,5-triazin-2-yl]amino]-2-tert-butylphenol, 2,6-ditert-butyl-4-[(4-octylsulfanyl-1,3,5-triazin-2-yl)amino]phenol, 4-[[4,6-bis(ethylsulfanyl)-1,3,5-triazin-2-yl]amino]-2,6-dimethylphenol, 4-[[4,6-bis(octylsulfanyl)-1,3,5-triazin-2-yl]-butylamino]-2,6-ditert-butylphenol, 4-[[4,6-bis(octylsulfanyl)-1,3,5-triazin-2-yl]-cyclohexylamino]-2,6-ditert-butylphenol, 2-[[4,6-bis(octylsulfanyl)-1,3,5-triaxin-2-yl]amino]-6-tert-butylphenol, 2-tert-butyl-6-methyl-4-[[4-octylsulfanyl-6-[(2,2,6,6-tetramethylpiperidin-4-yflamino]-1,3,5-triazin-2-yl]amino]phenol, 4-[[4,6-bis(octylsulfanylmethyl)-1,3,5-triazin-2-yl)amino]-2,6-ditert-butylphenol, 4-[[4,6-bis(octylsulfanyl)-1,3,5-triazin-2-yl)methylamino]-2,6-ditert-butylphenol, 4-[(4-amino-6-chloro-1,3,5-triazin-2-yl)amino]-2,6-ditert-butylphenol, and 4-[(4-cyclohexyl-6-cyclohexylsulfanyl-1,3,5-triazin-2-yl)amino]-2,6-di(propan-2-yl)phenol.

4. The method according to claim 2 , wherein the additive comprises tris(nonylphenyl)phosphite (TNPP).

5. The method according to claim 4 , wherein the additive comprises 2,6-di-tert-butyl-4-(4,6-bis(octylthio)-1,3,5-triazin-2-ylamino)phenol.

6. The method according to claim 1 , wherein the antioxidant comprises 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene.

7. The method according to claim 1 , wherein the one or more photocurable layers further comprises one or more materials selected from the group consisting of plasticizers, antiozonants, fillers, reinforcing agents, thermal polymerization inhibitors, UV absorbers and combinations of one or more of the foregoing.

8. The method according to claim 7 , wherein the one or more materials comprise a plasticizer.

9. The method according to claim 1 , wherein the relief image printing element has a Shore A hardness of between 50 and 65.

10. The method according to claim 1 , wherein the ratio of r e :p is less than 2%.

11. The method according to claim 1 , wherein the planarity of the top surface of the relief printing dots, measured as the radius of curvature across the top surface of the relief printing dots and the radius of curvature across the top surface of the relief printing dots, is greater than the thickness of the one or more photocurable layers.

12. The method according to claim 1 , wherein an average shoulder angle, measured as the angle between a shoulder of the dot across the width of the dot's top printing surface, is less than 20°.

13. The method according to claim 1 , wherein the exposed one or more photocurable layers is developed using solvent development.

14. The method according to claim 1 , wherein the exposed one or more photocurable layers is developed using thermal development.