Liquid Photopolymer Resin Compositions for Flexographic Printing

Abstract

A liquid photopolymer resin composition comprising: a) at least one ethylenically unsaturated prepolymer; b) at least one ethylenically unsaturated monomer; c) at least one photoinitiator; and d) at least one polythiol and a method of using the same to make soft relief image printing plates that have good tensile strength and elongation.

Claims

1. A liquid photopolymer resin composition comprising: a) at least one ethylenically unsaturated prepolymer; b) at least one ethylenically unsaturated monomer; c) at least one photoinitiator; and d) at least one polythiol.

2. The liquid photopolymer resin according to claim 1, wherein the ethylenically unsaturated prepolymer comprises one or more of an unsaturated polyester resin, an unsaturated polyurethane resin, an unsaturated polyamide resins and an unsaturated poly(meth)acrylate resin.

3. The liquid photopolymer resin composition according to claim 1, wherein the ethylenically unsaturated monomer comprises a mixture of monomers having one ethylenically unsaturated group and monomers having two or more ethylenically unsaturated groups.

4. The liquid photopolymer resin composition according to claim 1, wherein the at least one polythiol is a compound comprising molecules having two or more thiol groups per molecule.

5. The liquid photopolymer resin composition according to claim 4, wherein the at least one polythiol is selected from the group consisting of esters of thioglycolic acid, -mercaptopropionic acid and -mercaptopropionic acid with polyhydroxy compounds.

6. The liquid photopolymer resin composition according to claim 5, wherein the at least one polythiol is selected from the group consisting of ethylene glycol bis(thioglycolate), ethylene glycol bis(-mercaptopropionate), trimethylolpropane tris(thioglycolate), trimethylolpropane tris(-mercaptopropionate), pentaerythritol tetrakis(-mercaptopropionate), and combinations of one or more of the foregoing.

7. The liquid photopolymer resin composition according to claim 1, wherein the at least one polythiol is present in the liquid photopolymer resin composition in an amount of between about 0.10 and about 3.0 percent by weight.

8. The liquid photopolymer resin composition according to claim 7, wherein the at least one polythiol is present in the liquid photopolymer resin composition in an amount of between about 0.25 and about 2.0 percent by weight.

9. The liquid photopolymer resin composition according to claim 1, further comprising an additive selected from the group consisting of antioxidants, accelerators, dyes, inhibitors, activators, fillers, pigments, antistatic agents, flame-retardant agents, thickeners, thixotropic agents, surface active agents, light scattering agents, viscosity modifiers, extending oils, plasticizers, detackifiers and combinations of one or more of the foregoing.

10. The liquid photopolymer resin composition according to claim 9, wherein the additive comprises an antioxidant.

11. The liquid photopolymer resin composition according to claim 10, wherein the antioxidant is selected from the group consisting of sterically hindered monophenols, alkylated thiobisphenols and alkylidene bisphenols, hydroxybenzyls, triazines, polymerized trimethyldihydroquinone, dibutylzinc dithiocarbamate, dilauryl thiodipropionate, phosphites, and combinations of one or more of the foregoing.

12. The liquid photopolymer resin composition according to claim 1, further comprising a performance enhancing additive.

13. The liquid photopolymer resin composition according to claim 12, wherein the performance enhancing additive comprises a high molecular weight fatty acid.

14. A flexographic relief image printing plate comprising the photochemical reaction product of claim 1.

15. The flexographic relief image printing plate according to claim 14, wherein the printing plate has a Shore A hardness of less than about 30.

16. The flexographic relief image printing plate according to claim 15, wherein the printing plate has a Shore A hardness of less than about 25.

17. The flexographic relief image printing plate according to claim 16, wherein the printing plate has a Shore A hardness of less than about 20.

18. A method of making a relief image printing plate from a liquid photoresin, the method comprising the steps of: a) casting a liquid photoresin composition on top of a coverfilm to a predetermined thickness, wherein the liquid photoresin composition comprises: i) at least one ethylenically unsaturated prepolymer; ii) at least one ethylenically unsaturated monomer; iii) at least one photoinitiator; and iv) at least one polythiol; b) laminating a backing sheet over the cast liquid photopolymer; c) exposing the photopolymer to actinic radiation to selectively crosslink and cure the liquid photopolymer, wherein the liquid photopolymer that is not exposed to actinic radiation remains in a liquid state; and d) removing the liquid photopolymer; wherein a relief image of cured photopolymer is obtained.

19. The method according to claim 18, wherein the at least one polythiol is a compound comprising molecules having two or more thiol groups per molecule.

20. The method according to claim 19, wherein the at least one polythiol is selected from the group consisting of esters of thioglycolic acid, -mercaptopropionic acid and -mercaptopropionic acid with polyhydroxy compounds.

21. The method according to claim 20, wherein the at least one polythiol is selected from the group consisting of ethylene glycol bis(thioglycolate), ethylene glycol bis(-mercaptopropionate), trimethylolpropane tris(thioglycolate), trimethylolpropane tris(-mercaptopropionate), pentaerythritol tetrakis(-mercaptopropionate), and combinations of one or more of the foregoing.

22. The method according to claim 19, wherein the at least one polythiol is present in the liquid photopolymer resin composition in an amount of between about 0.10 and about 3.0 percent by weight.

23. The method according to claim 22, wherein the at least one polythiol is present in the liquid photopolymer resin composition in an amount of between about 0.25 and about 2.0 percent by weight.

24. The method according to claim 19, further comprising an additive selected from the group consisting of antioxidants, accelerators, dyes, inhibitors, activators, fillers, pigments, antistatic agents, flame-retardant agents, thickeners, thixotropic agents, surface active agents, light scattering agents, viscosity modifiers, extending oils, plasticizers, detackifiers and combinations of one or more of the foregoing.

25. The method according to claim 24, wherein the additive comprises an antioxidant.

26. The method according to claim 19, further comprising a performance enhancing additive.

27. The method according to claim 19, wherein the printing plate has a Shore A hardness of less than about 30.

28. The method according to claim 27, wherein the printing plate has a Shore A hardness of less than 25.

29. The method according to claim 28, wherein the printing plate has a Shore A hardness of less than 20.

Description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0047] As described herein, in one embodiment, the present invention relates generally to a liquid photopolymer resin composition comprising: [0048] a) at least one ethylenically unsaturated prepolymer; [0049] b) at least one ethylenically unsaturated monomer; [0050] c) at least one photoinitiator; and [0051] d) at least one polythiol.

[0052] The present invention utilizes a typical formulating approach, which requires no special prepolymer structure and no intentional removal or dramatic reduction in crosslink density. As a result, there is also no dramatic reduction in the strength of the resulting photopolymer.

[0053] The use of thiols in the compositions results in an adjustment in the crosslink mechanism in the presence of acrylate/methacrylate monomers, which results in a lower Shore A material with little or no change in the physical toughness of the resulting photopolymer. The photoresins described herein are liquid photopolymer resins, meaning that the uncured photoresin is a liquid at room temperature. Thus, once portions of the liquid photoresin are crosslinked and cured, the remaining photoresin can be reclaimed and reused.

[0054] The ethylenically unsaturated prepolymer may include, for example, unsaturated polyester resins, unsaturated polyurethane resins, unsaturated polyamide resins and unsaturated poly(meth)acrylate resins, such as, for example polyether urethane polymers, or polyether polyester urethane copolymers such as polyether polyester urethane methacrylate photopolymers.

[0055] Typically, the ethylenically unsaturated prepolymer is present in the liquid photosensitive composition at a concentration of between about 60 to about 80 percent by weight of the photosensitive resin composition, more preferably from about 65 to about 75 percent by weight of the photosensitive resin composition.

[0056] The ethylenically unsaturated monomer may be any commonly available acrylate or methacrylate such as isobornylester, t-butylester, laurylester, monoesters or diesters of acrylic acid or methacrylic acid, and/or triesters of trimethylopropanol or propoxylated trimethylolpropanol. However, the use of a monomer having two or more ethylenically unsaturated groups in the monomer increases the hardness of the resultant printing plate. Thus, the amount of monomers with two or more ethylenically unsaturated groups must be controlled.

[0057] Suitable monomers include, for example, the esters of acrylic acid and/or methacrylic acid with monohydric or polyhydric alcohols, such as, for example and without limitation, butyl acrylate, 2-ethylhexyl acrylate, isodecyl acrylate, lauryl acrylate, phenoethoxy acrylate, ethylene glycol diacrylate, 2-hydroxyethyl acrylate, hexane-1,6-diol diacrylate, 1,1,1-trimethylolpropane triacrylate, di, tri, and tetraethylene glycol diacrylate, tripropylene glycol diacrylate, pentaerythritol tetraacrylate, propoxylated trimethylolpropane mono- di- and tri-acrylate, ethoxylated trimethylolpropane triacrylate, and oligomeric polybutadienes with acrylic acid, i.e., oligomeric polybutadienes possessing activated, photopolymerizable olefinic double bonds, butyl methacrylate, 2-ethylhexyl methacrylate, isodecyl methacrylate, lauryl methacrylate, phenoethoxy methacrylate, ethylene glycol dimethacrylate, 2-hydroxyethyl methacrylate, hexane-1,6-diol dimethacrylate, 1,1,1-trimethylolpropane trimethacrylate, di, tri, and tetraethylene glycol dimethacrylate, tripropylene glycol dimethacrylate, pentaerythritol tetramethacrylate, propoxylated trimethylolpropane mono- di- and tri-methacrylate, ethoxylated trimethylolpropane trimethacrylate, and oligomeric polybutadienes with methacrylic acid, i.e., oligomeric polybutadienes possessing activated, photopolymerizable olefinic double bonds.

[0058] It is preferred that the ethylenically unsaturated monomer comprise a mixture of monomers with some having one ethylenically unsaturated group and some having two or more ethylenically unsaturated groups. The optimum ratio of the mixture will be determined in part by the desired hardness of the resulting printing plate. The amount of the monomer or monomer mixture will also have an effect on the viscosity of the photosensitive resin. The greater the amount of the monomer or monomer mixture, the lower the resultant viscosity of the photosensitive resin. The viscosity of the photosensitive resin is preferably between 10,000 cps and 100,000 cps at room temperature and more preferably between 20,000 cps and 50,000 cps.

[0059] Typically, the one or more ethylenically unsaturated monomers are present in the liquid photosensitive composition in a total amount from about 10 to about 40 percent by weight of the photosensitive resin composition, more preferably from about 15 to about 30 percent by weight of the photosensitive resin composition.

[0060] The photoinitiator may be any of a number of photoinitiators commonly used in photoresin compositions and combinations thereof. Examples of suitable photoinitiators include, for example, one or more of acenaphthenequinone, acylphosphine oxide, -aminoacetophenone, benzanthraquinone, benzoin methyl ether, benzoin isopropyl ether, benzoin n-butyl ether, benzoin isobutyl ether, benzophenone, benzoyl dimethyl ketal, benzophenone, benzil dimethyl acetal, benzil 1-methyl 1-ethyl acetal, camphorquinone, chloroacetophenone, 2-chlorothioxanethone, dibenzosuberone, 2,2-diethoxy-2-phenylacetophenone, 2,2-diethoxyacetophenone, 2-dimethoxybenzoyldiphenylphosphine oxide, 2,2-dimethoxy-2-phenylacetophenone (i.e., Irgacure 651), 4,4-bis(dimethylamino)benzophenone, 2-ethylanthraquinone, ethyl 2,4,6-trimethylbenzoylphenyl phosphinate, exanophenone, hydroxyacetophenone, 2-hydroxy-2-methylpropiophenone, 2-hydroxy-2-methyl-4-isopropylisopropiophenone, 1-hydroxycyclohexyl phenyl ketone, 3-ketocoumarin, o-methoxybenzophenone, (methyl)-benzoylbenzoate, methylbenzoyl formate, Michler's ketone, 4-morpholinodeoxybenzoin, 4-morpholinobenzophenone, -phenylbutyrophenone, sodium 2,4,6-trimethylbenzoylphenylphosphinate, thioxanone, thioxanethone, 10-thioxanthenone, thioxanthen-9-one, tetramethylthiuram monosulfide, 3,3,4,4-tetra(t-butylperoxicarbonyl)-benzophenone, trichloroacetophenone, 2,4,6-trimethylbenzoyldiphenylphosphine oxide, valerophenone, axanethone, p-diacetylbenzene, 4-aminobenzophenone, 4-methoxyacetophenone, benzaldehyde, alpha-tetralone, 9-acetylphenanthrene, 2-acetylphenanthrene, 3-acetylphenanthrene, 3-acetylindone, 9-fluorenone, 1-indanone, 1,3,5-triacetylbenzene, xanthene-9-one, 7-H-benz[de]anthracen-7-one, 1-naphthaldehyde, 4,4-bis(dimethylamino)-benzophenone, fluorene-9-one, 1-acetonaphthone, 2-acetonaphthone, 2,3-butanedione, acetonaphthene benz[a]anthracene 7.12 diene, among others. Phosphines such as triphenylphosphine and tri-o-tolylphosphine are also operable herein as photoinitiators.

[0061] The photoinitiators described herein can be used alone or in combination with coinitiators, e.g., ethylanthraquinone with 4,4,-bis(dimethylamino)benzophenone, benzoin methyl ether with triphenylphosphine, diacylphosphine oxides with tertiary amines or acyldiarylphosphine oxides with benzil dimethyl acetal.

[0062] The amount of the photoinitiator can be any effective concentration that will allow the formation of a floor layer to the flexographic printing plate via a back exposure of a reasonable length of time and the formation of the relief image with the required image resolution. This time is related to the type of image being formed, as well as the thickness of the desired flexographic printing plate. The effective amount of photoinitiator is dependent on the type of initiator chosen. However, a concentration range of about 0.1 to about 10 percent by weight, more preferably about 0.5 to about 5 percent by weight, of the photoinitiator is generally preferred.

[0063] Particularly preferred photoinitiators include one or more of 2,2-dimethoxy-2-phenylacetophenone and benzophenone.

[0064] To protect the photopolymer mixtures from decomposition by thermal oxidation and oxidation by atmospheric oxygen, effective amounts of antioxidants may also be added to the photopolymer mixture, including, for example, sterically hindered monophenols, such as butylated hydroxytoluene (BHT), alkylated thiobisphenols and alkylidene bisphenols, such as 2,2-methylenebis-(4-methyl-6-tert-butylphenol) and 2,2-bis (1-hydroxy-4-methyl-6-tert-butylphenyl) sulfide, hydroxybenzyls, such as 1,3,5-trimethyl-2,4,6-tris-(3,5-di-tert-butyl-4-hydroxybenzyl)benzene, triazines, such as 2-(4-hydroxy-3,5-tert-butylanilino)-4,6-bis-(n-octylthio)-1,3,5-triazine, polymerized trimethyldihydroquinone, dibutylzinc dithiocarbamate, dilauryl thiodipropionate and phosphites, such as tris (nonylphenyl) phosphite. In one embodiment, the antioxidant is BHT.

[0065] As described herein, the composition of the present invention also includes a polythiol, which enables the development of soft liquid photopolymer resins that are capable of producing relief image printing plates having a Shore A durometer of less than about 30, more preferably less than about 25 and most preferably less than about 20, at ambient conditions, while maintaining the strength of the resin.

[0066] In a preferred embodiment, the polythiols is a simple or complex organic compound having multiple pendant or terminally positioned SH functional groups per average molecule. Suitable polythiols have molecular weights of from about 100 to about 20,000 or more, more preferably from about 100 to about 10,000.

[0067] The polythiol can generally be any compound that comprises molecules having two or more thiol groups per molecule. Examples of preferred polythiol compounds because of their relatively low odor level include but are not limited to esters of thioglycolic acid (HSCH.sub.2COOH), -mercaptopropionic acid (HSCH(CH.sub.3)COOH and -mercaptopropionic acid (HSCH.sub.2CH.sub.2COOH) with polyhydroxy compounds such as glycols, triols, tetraols, pentaols, hexaols, and the like. Specific examples of the preferred polythiols include but are not limited to ethylene glycol bis(thioglycolate), ethylene glycol bis(-mercaptopropionate), trimethylolpropane tris(thioglycolate), trimethylolpropane tris(-mercaptopropionate), pentaerythritol tetrakis(-mercaptopropionate), all of which are commercially available. Poly--mercaptoacetate or poly--mercaptopropionate esters, particularly the trimethylopropane triesters or pentaerythritol tetra esters are preferred. Other polythiols which can be suitably employed include alkyl thiol functional compounds such as 1,2-dimercapthoethane, 1,6-dimercaptohexane and the like. Thiol terminated polysulfide resins may also be employed.

[0068] Suitable examples of aliphatic and cycloaliphatic dithiols include 1,2-ethanedithiol, butanedithiol, 1,3-propanedithiol, 1,5-pentanedithiol, 2,3-dimercapto-1-propanol, dithioerythritol, 3,6-dioxa-1,8-octanedithiol, 1,8-octanedithiol hexanedithiol, dithiodiglycol, pentanedithiol, decanedithiol, 2-methyl 1,4 butanedithiol, bis-mercaptoethylphenyl methane, 1,9-nonanedithiol(1,9-dimercaptononane), glycol dimercaptoacetate, 3-mercapto-,4-dimethyl-cyclohexaneethanethiol, cyclohexane dimethane dithiol, and 3,7-dithia-1,9-nonanedithiol.

[0069] Suitable examples of aromatic dithiols include 1,2-benzenedithiol, 1,3-benzenedithiol, 1,4-benzenedithiol, 2,4,6-trimethyl-1,3-benzenedimethanethiol, durene-.alpha.1,.alpha.2-dithiol, 3,4-dimercaptotoluene, 4-methyl-1,2-benzenedithiol, 2,5-dimercapto-1,3,4-thiadiazole, 4,4-thiobisbezenedithiol, bis(4-mercaptophenyl)-2,2-propane(bisphenol dithiol), and [1,1-biphenyl]-4,4-dithiol, and p-xylene-,-dithiol.

[0070] Suitable examples of oligomeric dithiols include difunctional mercapto functional urethane oligomers derived from end capping moieties of hydroxyethyl mercaptan, hydroxypropyl mercaptan, dimercaptopropane, dimercapto ethane. Examples of suitable trithiol functional compounds include, trimethylolethane tris-mercaptopropionate, trimethylolpropane tris-mercaptopropionate, trimethylolethane tris-mercaptoacetate, and trimethylolpropane tris-mercaptoaacetate glycerol tri(1,1-mercaptoundecate), trimethylol propane tri(1,1-mercaptoundecate). One preferred trithiol is trimethylolpropane tris(2-mercaptopropionate).

[0071] Examples of suitable tetrafunctional thiols include pentaerythritol tetramercapto propionate, pentaerythritol tetramercapto acetate, and pentaethritol tetra(1,1-mercaptoundecate)

[0072] Multi-functional thiols can be obtained by reacting thioalkyl carboxylic acids, e.g., thioglycolic acid, mercapto propionic acid with high functional alcohols, amines and thiols. Furthermore, multifunctional thiols can be obtained by reacting mercapto alkyl trialkoxy silanes with silanols that may be polymeric or silica based silanols.

[0073] Other preferred multifunctional thiols are obtained using thiol carboxylic acids (HSRCOOH) where R is an alkyl or aryl group, e.g., thioundecanoic acid of which the COOH groups are reacted with reactive enes, alcohols, thiols or amines that are multifunctional.

[0074] Particularly preferred polythiols for use in the compositions described herein include trimethylolpropane tris(3-mercaptopropionate) and pentaerythritol tetrakis(-mercaptopropionate).

[0075] The polythiol is preferably used in the liquid photosensitive composition in an amount of about 0.10 to about 3.0 percent by weight of the photosensitive resin composition, more preferably from about 0.25 to about 2.0 percent by weight of the photosensitive resin composition.

[0076] The liquid photoresin compositions may also, optionally, but preferably, comprise a variety of slip additives, dyes, stabilizers and other additives of a similar nature which are typically added to photosensitive resin compositions.

[0077] Thus, the liquid photopolymer resin may include, for example, one or more of antioxidants, accelerators, dyes, inhibitors, activators, fillers, pigments, antistatic agents, flame-retardant agents, thickeners, thixotropic agents, surface active agents, light scattering agents, viscosity modifiers, extending oils, plasticizers, and detackifiers, by way of example and not limitation. These additives may be pre-blended with one or monomers or other compounds to be polymerized. Various fillers, including for example, natural and synthetic resins, carbon black, glass fibers, wood flour, clay, silica, alumina, carbonates, oxides, hydroxides, silicates, glass flakes, glass beads, borates, phosphates, diatomaceous earth, talc, kaolin, barium sulfate, calcium sulfate, calcium carbonate, antimony oxide, etc. may also be included in the photopolymer composition in amounts that will not interfere with or otherwise inhibit the photocuring reaction or other steps in the platemaking process.

[0078] Additionally, the liquid photopolymer resin composition may contain any one or more of a range of further performance-enhancing additives including, for example, esters of acrylic or methacrylic acid, stabilizers, defoamers, dyes and high molecular weight fatty acids, such as myristic acid, to ensure a dry, tack-free surface after post-curing of the washed plate.

[0079] The liquid photopolymer resin may be processed in a liquid platemaking process to produce a relief image printing element having the desired properties of Shore A hardness, elongation and tensile strength to produce a good printing result, especially when printing on corrugated substrates.

[0080] After processing the liquid photopolymer resin composition through the platemaking process, the resulting relief image printing plate preferably has a Shore A hardness of less than about 30, more preferably less than about 25 and most preferably less than about 20 under ambient conditions. The relief image printing plate also has an elongation in the 150 to 300% range, more preferably about 200 to about 250% and a tensile strength in the range of about 375 to about 700, more preferably about 450 to about 600 psi (as measured on an Instron system at 2 inches/minute sample speed).

Example 1

[0081] The following formulation provided below in Table 1 demonstrates one exemplary liquid photopolymer formulation for producing relief image printing elements in accordance with the present invention.

TABLE-US-00001 TABLE 1 Liquid photopolymer formulation Element % by Wt. EU1 Prepolymer 70.2 BHT 0.18 2,2-dimethyloxy-2pheyl-acetophenone 0.25 Benzophenone 0.72 Myristic Acid 1.51 Polypropylene glycol monomethacrylate 10.97 Lauryl methacrylate 8.40 Diethylene glycol dimethacrylate 3.38 N,N-dimethylaminoethyl methacrylate 1.90 Trimethylolpropane trimethacrylate 1.47

[0082] In addition to the above ingredients, trimethylolpropane tris(3-mercaptopropionate) was added to the photoresin composition at a level of 0.50 percent by weight, 1.0 percent by weight and 1.5 percent by weight. Blocks having a thickness of 0.25 inches were crosslinked and cured and the Shore A Value of each of the formulations was measured using a Shore Si digital durometer. The results are provided below in Table 2.

TABLE-US-00002 TABLE 2 Summary of Shore A Values Wt. % polythiol Shore A 0 35 0.5 28.1 1.0 23.5 1.5 20.3

[0083] As seen in Table 2, the printing plates produced from the liquid photoresin composition containing a polythiol exhibited the desired properties of Shore.

[0084] The present invention also relates generally to a printing plate having a Shore A hardness of less than about 30, the printing plate comprising the photochemical reaction product of: [0085] a) at least one ethylenically unsaturated prepolymer; [0086] b) at least one ethylenically unsaturated monomer; [0087] c) at least one photoinitiator; and [0088] d) at least one polythiol.

[0089] In another embodiment, the present invention relates generally to a method of making a relief image printing plate from a liquid photoresin, the method comprising the steps of: [0090] a) casting a liquid photoresin composition on top of a coverfilm to a predetermined thickness, wherein the liquid photoresin composition comprises: [0091] i) at least one ethylenically unsaturated prepolymer; [0092] ii) at least one ethylenically unsaturated monomer; [0093] iii) at least one photoinitiator; and [0094] iv) at least one polythiol; [0095] b) laminating a backing sheet over the cast liquid photopolymer; [0096] c) exposing the photopolymer to actinic radiation to selectively crosslink and cure the liquid photopolymer, wherein the liquid photopolymer that is not exposed to actinic radiation remains in a liquid state; and [0097] d) removing the liquid photopolymer; [0098] wherein a relief image of cured photopolymer is obtained.

[0099] The relief image printing plates produced in accordance with the present invention provide a good result when printing on corrugated substrates. The resulting printing plates have the desired softness to produce a good printing result but also have the necessary physical toughness and polymer strength necessary to reduce on press wear and damage.