Flexographic printing plate with improved storage stability

1. A method of making a relief image printing element comprising a plurality of relief printing dots, the method comprising the step of:

a) providing at least one photocurable layer disposed on a substrate, the at least one photocurable layer being capable of being selectively crosslinked and cured upon exposure to actinic radiation, the at least one photocurable layer comprising:

i) an ethylenically unsaturated monomer;

ii) a binder; and

iii) a photoinitiator, the photoinitiator exhibiting a quantum yield of initiation (Qi) of more than 0.05 at a 365 nm wavelength;

wherein the substrate comprises a flexible polymer sheet with an ultraviolet absorbing compound incorporated therein, and wherein the substrate has an optical density from 0.5 to 5 in the wavelength range of 365 nm to 450 nm but allows transmission of from 0.1% to 10% of incident light in the wavelength range of 365 nm to 450 nm;

b) imagewise exposing the at least one photocurable layer to actinic radiation to selectively crosslink and cure portions of the at least one photocurable layer; and

c) developing the relief image printing element to separate and remove uncrosslinked and uncured portions of the at least one photocurable layer to reveal a relief image therein, wherein the relief image comprises the plurality of relief printing dots, and

wherein the plurality of relief printing dots exhibit an edge sharpness of the dots such that the ratio of the radius of curvature at the intersection of the shoulder and the top surface of the dot, r e , to the width of the top of the dot, p, is less than 5%, and wherein the printing element exhibits improved storage stability and shelf life.

2. The method according to claim 1 , wherein the step of imagewise exposing the at least one photocurable layer to actinic radiation is conducted in the presence of normal atmospheric oxygen concentrations at the location where the method is being conducted.

3. The method according to claim 1 , wherein the photoinitiator exhibits a quantum yield of initiation (Qi) greater than 0.075 at the 365 nm wavelength.

4. The method according to claim 3 , wherein the photoinitiator exhibits a quantum yield of initiation (Qi) greater than 0.08 at the 365 nm wavelength.

5. The method according to claim 1 , wherein an extinction coefficient of the photoinitiator is greater than 300 1·cm −1 ·mol −1 at a 365 nm wavelength.

6. The method according to claim 5 , wherein the extinction coefficient of the photoinitiator is greater than 400 1·cm −1 ·mol −1 at the 365 nm wavelength.

7. The method according to claim 6 , wherein the extinction coefficient of the photoinitiator is greater than 500 1·cm −1 ·mol −1 at the 365 nm wavelength.

8. The method according to claim 1 , wherein the photoinitiator is selected from the group consisting of 1-butanone-2-(dimethylamino)-2-[(4-methylphenyl)methyl]-1-[4-(4-morpholinyl)phenyl], 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1, Diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, phenylbis(2,4,6-trimethylbenzoyl) phosphine oxide, and combinations of one or more of the foregoing.

9. The method according to claim 1 , wherein the photoinitiator is present in the at least one photocurable layer at a concentration of between 1.5 and 5.0 percent by weight.

10. The method according to claim 9 , wherein the photoinitiator is present in the at least one photocurable layer at a concentration of between 2.0 and 3.5 percent by weight.

11. The method according to claim 1 , wherein the substrate has an optical density from 1 to 5.

12. A printing element comprising:

a) at least one photocurable layer disposed on a substrate, the at least one photocurable layer being capable of being selectively crosslinked and cured upon exposure to actinic radiation, the at least one photocurable layer comprising:

i) an ethylenically unsaturated monomer;

ii) a binder; and

iii) a photoinitiator, the photoinitiator exhibiting a quantum yield of initiation (Qi) of more than 0.05 at a 365 nm wavelength;

wherein the substrate comprises a flexible polymer sheet with an ultraviolet absorbing compound incorporated therein and wherein the substrate has an optical density from 1 to 5 in the wavelength range of 365 nm to 450 nm but also allows transmission of 0.1% to 10% of incident light in the wavelength range of 365 nm to 450 nm, and wherein the printing element exhibits improved storage stability and shelf life.

13. The printing element according to claim 12 , wherein the photoinitiator exhibits a quantum yield of initiation (Qi) greater than 0.075 at the 365 nm wavelength.

14. The printing element according to claim 13 , wherein the photoinitiator exhibits a quantum yield of initiation (Qi) greater than 0.08 at the 365 nm wavelength.

15. The printing element according to claim 12 , wherein an extinction coefficient of the photoinitiator is greater than 300 1·cm −1 ·mol −1 at a 365 nm wavelength.

16. The printing element according to claim 15 , wherein the extinction coefficient of the photoinitiator is greater than 400 1·cm −1 ·mol −1 at the 365 nm wavelength.

17. The printing element according to claim 16 , wherein the extinction coefficient of the photoinitiator is greater than 500 1·cm −1 ·mol −1 at the 365 nm wavelength.

18. The printing element according to claim 12 , wherein the photoinitiator is selected from the group consisting of 1-butanone-2-(dimethylamino)-2-[(4-methylphenyl)methyl]-1-[4-(4-morpholinyl)phenyl], 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1, Diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, phenylbis(2,4,6-trimethylbenzoyl) phosphine oxide, and combinations of one or more of the foregoing.

19. The printing element according to claim 12 , wherein the photoinitiator is present in the at least one photocurable layer at a concentration of between 1.5 and 5.0 percent by weight.

20. The printing element according to claim 19 , wherein the photoinitiator is present in the at least one photocurable layer at a concentration of between 2.0 and 3.5 percent by weight.