GB2171713A - Radiation-polymerizable composition - Google Patents

Abstract

A radiation polymerizable composition for use in a photographic element, such as that for producing a lithographic printing plate, comprises a polymeric binder, a photoinitiator, a diazonium salt, a polyfunctional acrylic monomer having from 3 to 6 unsaturated groups and a monofunctional acrylic monomer having 1 unsaturated group.

Description

SPECIFICATION Radiation-polymerizable composition The present invention relates to radiation-polymerizable compositions and photographic elements and printing plates formed therefrom.

Many approaches in the formulation of lithographic printing plates from radiation-polymerizable compositions have been adopted in the search for a system which may be prepared from relatively inexpensive ingredients, which does not require prolonged imaging cycles (has a high quantum efficiency), which is able to undergo an increased number of press runs and which results in the creation of high resolution images within the composition which can be manifested without prolonged and elaborate development.

Most such lithographic printing plates comprise a metal substrate, usually of metal, for example aluminum, coated with a light sensitive material, for example a diazonium compound, in admixture with suitable binding resins, photoinitiators, photopolymerizable compositions, colorants, stabilizers, exposure indicators, surfactants and the like.

Although the art is replete with photosensitive compositions which may be used for lithographic printing plates, the prior art compositions' serviceability is restricted by their limited stability and number of press runs.

This invention provides a radiation polymerizable composition, especially one suitable for use in forming a photographic element, for example, a lithographic printing plate, comprising a) a polymeric binder; b) a photoinitiator; c) a diazonium salt; and d) a polymerizable mixture of i) a polyfunctional acrylic monomer having from 3 to 6 unsaturated groups; and ii) a monofunctional acrylic monomer having 1 unsaturated group.

As used herein, the terms "acrylic" and "acrylate" include, where the context permits, acrylic acid itself and its derivatives, and methacrylic acid and derivatives thereof.

The polymeric binder suitable for use in the composition of this invention is preferably a polymer which does not react with itself or other compounds when exposed to actinic radiation.

The preferred binder is a resin which is substantially organic solvent-soluble and is preferably one which is not substantially soluble in aqueous alkali, acid or water. The binder should have a molecular weight sufficient to provide a tack-free surface when in admixture with the photopolymerizable mixture and to provide a tough cohesive matrix which when used to provide a lithographic printing plate is capable of providing numerous impressions having good quality. The molecular weight must be low enough, though, to permit solubility in formulation as well as during development. The molecular weight of the binder is accordingly advantageously greater than about 20,000, preferably greater than about 30,000 and most preferably greater than about 40,000.

Examples of binders suitable for use in this invention include epoxy resins, polyvinyl acetate, polyvinyl acetals, polyesters, polyamides, polyethers, polyurethanes and polyacrylic resins which are either homo- or copolymers of acrylates (or methacrylates) and/or acrylic acid (or methacrylic acid). In the preferred embodiment, the polymeric binder is a terpolymer of polyvinyl acetate, polyvinyl alcohol and polyvinyl formal available as Formvar 12/85 from Monsanto of St. Louis, Missouri, U.S.A.

Preferably, the polymeric binder is present in the composition at a percent solids level of from 20% to 75% by weight. A more preferred range is from 30% to 65% by weight and, most preferably, the polymeric binder is present at a percent solids level of from 35% to 50% by weight.

Among suitable photoinitiators which may be used in this invention are preferably those freeradical photoinitiators having an absorption maximum in the range of from 320 to 400 nm.

Examples include the acetophenones, benzophenones, triazines, benzoins, benzoin ethers, xanthones, thioxanthones, acridines and benzoquinones. More preferred of these are the triazines having the formula

wherein Ar represents

in which R1, R2, R3, and R4 independently represent hydrogen, chlorine, bromine, alkoxy, or alkyl.

The most preferred photoinitiator is 2-stilbenyl-4,6-di(trichloromethyl) triazine.

The photoinitiator is preferably present in the composition at a percent solids level of from 1.5% to 8.0% by weight, more preferably from 2.0% to 6.0% by weight, and is most preferably present at a percent solids level of from 3.0% to 4.0% by weight.

The diazonium salt which is useful in the practice of this invention may be any suitable lightsensitive diazonium polymeric or monomeric compound. Many such compounds are well known to the skilled artisan; the polymeric diazonium compounds are preferred. Suitable diazonium compounds include those condensed with formaldehyde such, for example, as disclosed in U.S.

Patents Nos. 2,063,631 and 2,667,415, the polycondensation products such, for example, as disclosed in U.S. Patents Nos. 3,849,392 and 3,867,147, and the high speed diazos such, for example, as disclosed in U.S. Patent No. 4,436,804, the disclosures of which are incorporated herein by reference.

Preferably, the diazonium salt comprises the 1:1 polycondensation product of 3-methoxydiphenylamine-4-diazonium sulfate and 4,4'-bis-methoxymethyl-diphenyl ether, advantageously isolated as by precipitation as the mesitylene sulfonate, as described in U.S. Patent No. 3,849,392.

If desired the ether may first be precondensed with itself to an oligomer with from 2 to 7 repeating units.

The diazonium salt is preferably present in the composition of the subject invention at a percent solids level of from 3% to 20% by weight. More preferably it is present at from 5% to 18% by weight and most preferably the diazonium salt is present at a percent solids level of from 10% to 15% by weight.

The polymerizable composition of the present invention comprises both a polyfunctional acrylic monomer which has from 3 to 6 unsaturated groups and a monofunctional acrylic monomer which has 1 unsaturated group.

The polyfunctional acrylic monomer is an ethylenically unsaturated compound having from three to six unsaturated groups and capable of reacting with the monofunctional acrylic monomer upon exposure to imaging radiation. The polyfunctional monomer is characterized as having at least some, and preferably all, of the.unsaturated groups in the form of acrylate or methacrylate ester groups. The preferred monomer is either a solid or liquid having a viscosity of greater than 0.7 Pa.s at 25 C, preferably greater than 2 Pa.s at 25 C. Most preferably, the monomer has a viscosity of greater than about 4 Pa.s at 25 C.

Examples of compounds which are suitable for use as the polyfunctional acrylic monomer include trimethylol propane triacrylate and the ethoxylated or propoxylated homologs thereof, trimethylol propane trimethacrylate and the ethoxylated or propoxylated homologs thereof, pentaerythritol triacrylate, pentaerythritol trimethacrylate, dipentaerythritol monohydroxy pentaacrylate, dipentaerythritol monohydroxy pentamethacryiate, dipentaerythritol hexaacrylate, dipentaerythritol hexamethacrylate, pentaerythritol tetraacrylate and pentaerythritol tetramethacrylate. Preferably, the monomer is pentaerythritol tetraacrylate, although a combination of suitable monomers is also advantageous.

The monofunctional monomer is an ethylenically unsaturated compound having one saturated group and is capable of reacting with the polyfunctional monomer upon exposure to imaging radiation. The monofunctional monomer is characterized as having its unsaturated group in the form of an acrylate or methacrylate ester group. Preferably, the monofunctional monomer is a liquid having a viscosity in the range of from 1 to 25 mPa.s at 25 C.

Examples of compounds which are suitable for use as the monofunctional monomer include trimethyloi propane monoacrylate and the ethoxylated or propoxylated homologs thereof, trimethylol propane monomethacrylate and the ethoxylated or propoxylated homologs thereof, pentaerythritol acrylate, pentaerythritol methacrylate, tetrahydrofurfuryl acrylate, tetrahydrofurfuryl methacrylate, cyclohexyl acrylate, cyclohexyl methacrylate, n-hexyl acrylate; n-hexyl methacrylate, glycidyl acrylate and glycidyl methacrylate. Preferably, the monomer is pentaerythritol monoacrylate, although a combination of suitable monomers is also advantageous.

In the polymerizable mixture the polyfunctional monomer is advantageously present in an amount of at least 50% and, more preferably, should be present in an amount of from 65% to 80%, based on the total weight of the monofunctional and polyfunctional monomers. The monofunctional monomer advantageously comprises the balance of the polymerizable mixture.

The polymerizable mixture is advantageously present at a percent solids level which is in the range of from 20% to 70% by weight. Preferably, the polymerizable mixture is present at a percent solids level of from 30% to 60% by weight and more preferably from 30% to 50% by weight.

One of the advantages in having a polyfunctional monomer in the polymerizable mixture in combination with a monofunctional monomer is that the 3-dimensional structure provided by the polyfunctional monomer provides good matrix integrity allowing for a tough image and the linear propagation provided by the monofunctional monomer provides good photospeed. If the polymerizable mixture were composed of two polyfunctional acrylic monomers, without a monofunctional monomer, the resulting image would have good toughness but poor photospeed. If the mixture were composed of two monofunctional acrylic monomers, without a polyfunctional monomer, there would be good photospeed but poor image toughness.

One of the significant aspects of this invention is that the unique combination of photoinitiator, diazonium salt and polymerizable mixture reduces the need for use of an oxygen barrier layer or the necessity for processing in a nitrogen environment, although the exact mechanism for this is unclear. Among the possible advantages derived thereby are elimination of (1) the inconvenience of applying a second coating, (2) the concern over the refractive index and solubility of the oxygen barrier layer, (3) the concern over blinding on a printing press because of residue from the oxygen barrier layer, and (4) the concern over potential image gain because of the oxygen barrier layer.

Other components which may be included in the radiation-polymerizable composition of this invention include, for example, acid stabilizers, exposure indicators, plasticizers, photoactivators and colorants.

Suitable acid stabilizers include phosphoric, citric, benzoic, m-nitro benzoic, p(p-anilino phenylazo) benzene sulfonic, 4,4'-dinitro-2,2'-stilbene disulfonic, itaconic, tartaric and p-toluene sulfonic acid, and mixtures thereof. Preferably, the acid stabilizer is phosphoric acid. When used, the acid stabilizer is preferably present in the radiation-polymerizable composition in the amount of from 0.3% to 2.0%, and more preferably from 0.75% to 1.5%, although the skilled artisan may use more or less as desired.

Exposure indicators (or photoimagers) which may be useful in conjunction with the present invention include 4-phenylazodiphenylamine, eosin, azobenzene, Calcozine Fuchsine dyes and Crystal Violet and Methylene Blue dyes. Preferably, the exposure indicator is 4-phenylazodiphenylamine. The exposure indicator, when one is used, is preferably present in the composition in an amount of from 0.001% to 0.35% by weight. A more preferred range is from 0.002% to 0.30% and, most preferably, the exposure indicator is present in an amount of from 0.005% to 0.20%, although the skilled artisan may use more or less as desired.

The photoactivator which may be included in the composition of this invention should be an amine-containing photoactivator which combines synergistically with the free-radical photoinitiator in order to extend the effective half-life of the photoinitiator, which is normally in the approximate range of about 10 9 to 10 15 seconds. Suitable photoactivators include 2-(n-butoxy) ethyl4-dimethylamino benzoate, 2-(dimethylamino) amino benzoate and acrylated amines. Preferably the photoactivator is ethyl-4-dimethylamino benzoate. The photoactivator is preferably present in the composition of this invention in an amount of from 1.0% to 4.0% by weight, although the skilled artisan may use more or less as desired.

A plasticizer may also be included in the composition of this invention to prevent coating brittleness and to keep the composition pliable if desired. Suitable plasticizers include dibutyl phthalate, triaryl phosphate and, preferably, dioctyl phthalate. The plasticizer is preferably present in the composition of this invention in an amount of from 0.5% to 1.25% by weight, although the skilled artisan may use more or less as desired.

Colorants useful herein include dyes such as Rhodamine, Calcozine, Victoria Blue and methyl violet, and such pigments as the anthraquinone and phthalocyanine types. Generally, the colorant is present in the form of a pigment dispersion which may comprise a mixture of one or more pigments and/or one or more dyes dispersed in a suitable solvent or mixture of solvents. When a a colorant is used, it is preferably present in the composition of this invention in an amount of from 1.5% to 4.0% by weight, more preferably from 1.75% to 3.0% and most preferably from 2.0% to 2.75%, although the skilled artisan may use more or less as desired.

In order to form a coating composition for the production of lithographic printing plates, the composition of this invention may be dispersed in admixture in a solvent or mixture of solvents to facilitate application of the composition to the substrate. Suitable solvents for this purpose include tetrahydrofuran, propylene glycol monomethyl ether, butyrolactone, methyl Cellosolve-, ethylene glycol ethers, alcohols such, for example, as ethyl alcohol and n-propanol, and ketones such, for example, as methyl ethyl ketone, or mixtures thereof. Preferably, the solvent comprises a mixture of tetrahydrofuran, propylene glycol monomethyl ether and butyrolactone. In general, the solvent system is evaporated from the coating composition once it is applied to an appropri ate substrate, however, some insignificant amount of solvent may remain as residue.

Substrates useful for coating with the composition of this invention to form a lithographic printing plate include sheets of transparent films such, for example, as polyester, aluminum and its alloys and other metals, silicon and similar materials which are well known in the art.

Preferably, the substrate comprises aluminum. The substrate may first be pretreated by standard graining and/or etching and/or anodizing techniques as are well known in the art, and also may optionally have been treated with a composition comprising a hydrophilizing agent such, for example, as polyvinyl phosphonic acid or sodium silicate.

In the production of photographic elements such, for example, as lithographic printing plates, a substrate,advantageously aluminum, is first preferably grained as by art recognized methods such as by means of a wire brush, a slurry of particulates or by chemical or electrochemical means, for example in an electrolyte solution comprising hydrochloric acid. The grained plate is preferably then anodized for example in sulfuric or phosphoric acid in a manner well known in the art.

The grained and anodized surface is preferably then rendered hydrophilic by treatment with polyvinyl phosphonic acid by means which are also known to the skilled artisan. The plate is then coated with the composition of the present invention, advantageously at a coating weight of from 0.6 g/m2 to 2.5 g/m2, preferably from 0.8 g/m2 to 2.0 g/m2 and more preferably from 1.2 g/m2 to 1.5 g/m2, although these coating weights are not critical to the practice of this invention. It is then dried.

Trade Mark The exposure required by the plate may readily be determined; for example and preferably the lithographic printing plate is exposed to actinic radiation through a negative test flat so as to yield 6 solid steps on a 21 step Stouffer exposure wedge after development. The exposed plate is then developed with a suitable developer composition, preferably an organic solvent based developer, such as one comprising 2-propoxy-ethanol, a nonionic surfactant and an inorganic salt such as is disclosed in U.S. Patents Nos. 4,308,340 and 4,381,340. In conventional use, the developed plate is finished with a subtractive finisher such as a hydrophilic polymer. Examples include cold water-soluble dextrin and/or polyvinyl pyrrolidone, a nonionic surfactant, a humectant, an inorganic salt and water, as taught by U.S. Patent No. 4,213,887.

For the purpose of improving the press performance of a plate prepared as described above, baking of the exposed and developed plate may be carried out; this can result in an increase in the number of quality impressions over that otherwise obtainable. To bake the plate properly, it is first treated with a solution designed to prevent loss of hydrophilicity of the background during baking. An example of an effective solution is disclosed in U.S. Patent No. 4,355,096, the disclosure of which is hereby incorporated herein by reference. The plate is then heat treated by baking at a temperature of from about 180 C up to the annealing temperature of the substrate, most preferably about 240 C. The effective baking time is inversely proportional to the temperature and averages in the range of from 2 to 15 minutes.At 2400C the time is about 7 minutes.

The following examples are illustrative of the invention but it is understood that the invention is not limited thereto. None of the plates prepared in Examples 1-5 have an oxygen barrier layer thereon nor were they processed in a nitrogen environment.

Example 1 An 8"X25" (203X636 mm) section of lithographic grade 1100 aluminum alloy is degreased with an aqueous alkaline degreasing solution and electrochemically grained using 900 coulombs of alternating current in an aqueous solution of nitric acid and aluminum nitrate. The grained plate is well rinsed and anodized in a sulfuric acid bath wherein the aluminum is made anodic.

Sufficient current and voltage are used to produce an oxide layer of 2.8 g/m2. The anodized plate is well rinsed and hydrophilized by immersing the plate in a solution of polyvinyl phosphonic acid. The plate is well rinsed, dried, and then whirler coated with a solution having the following composition:: % w/w A terpolymer of polyvinyl acetate, polyvinyl alcohol and polyvinyl formal commercially available as Formvar+ 12/85 4.54 Pentaerythritol tetraacrylate 2.81 Pentaerythritol monoacrylate 1.21 Polycondensation product of 3-methoxydiphenylamine-4-diazonium sulfate and 4,4-bis-methoxymethyl diphenyl ether isolated as the mesitylene sulfonate 1.22 2-Stilbenyl-4,6-di(trichloromethyl)triazine 0.22 Methyl Cellosolvet 90.00 + Trade Marks The coated and dried plate is exposed to actinic radiation through a negative exposure flat so as to yield seven solid steps on a 21 step Stouffer step wedge.The plate is developed using the following composition: % w/w 2-propoxy ethanol 14.85 n-propanol 11.15 MgSO4.7H20 8.00 NaH2PO4 (anhydrous) 1.50 Polyvinyl pyrrolidone 1.50 Trycol ' OP-407** 0.75 Polyethylene glycol 200 1.00 H20 Balance * MW=10,000 nonyl phenol polyoxyethylene ethanol (40 moles ethylene oxide) as a 70% solution in H20 and finished with the following composition:: % w/w Dextrin*** 5.52 sodium octyl sulfate 1.61 Triton X-100**** 1.00 Givguard' DXN 0.05 H3PO4 2.37 H20 Balance hydrolyzed tapioca dextrin isooctyl phenol polyoxyethylene ethanol (4.5 moles ethylene oxide) ***** 2,5-dimethyl-6-acetoxy-dioxane Trade Marks and run on a Solna' sheet-fed press using abrasive ink, over-packing, and a Dahlgren dampening system until image breakdown is achieved. Under these conditions the plate provides 540,000 acceptable impressions.

Trade Marks Example 2 A lithographic printing plate is prepared and processed as described in Example 1 except that the pentaerythritol tetraacrylate is omitted. Under these conditions the plate provides only 180,000 acceptable impressions.

Example 3 A lithographic printing plate is prepared and processed as described in Example 1 except that the pentaerythritol monoacrylate is omitted. Under these conditions the plate provides only 315,000 acceptable impressions.

Example 4 A lithographic printing plate is prepared and processed as described in Example 1 except that the 2-stilbenyl-4,6-di-(trichloromethyl)triazine is omitted. Under these conditions the plate provides only 335,000 acceptable impressions.

It can be readily observed that plates prepared according to this invention (Example 1) show substantially increased press runs.

Claims (26)

1. A radiation-polymerizable composition comprising in admixture a) a polymeric binder; b) a photoinitiator; c) a diazonium salt; and d) a polymerizable component comprising i) a polyfunctional acrylic monomer having from 3 to 6 unsaturated groups; and ii) a monofunctional acrylic monomer having 1 unsaturated group.

2. The composition of claim 1 wherein the unsaturated groups in the polyfunctional monomer are acrylate or methacrylate groups.

3. The composition of claim 2 wherein the polyfunctional monomer is pentaerythritol tetraacrylate.

4. The composition of any one of claims 1 to 3 wherein the monofunctional monomer is an acrylate or methacrylate ester.

5. The composition of claim 4 wherein said monofunctional monomer is pentaerythritol monoacrylate.

6. The composition of any one of claims 1 to 5 wherein said polymerizable component constitutes from 20% to 70% by weight of the composition.

7. The composition of any one of claims 1 to 6 wherein said polyfunctional monomer constitutes from 65% to 80% by weight of said polymerizable component.

8. The composition of any one of claims 1 to 6 wherein said polyfunctional monomer constitutes more than 50% by weight of said polymerizable component.

9. The composition of any one of claims 1 to 7 wherein said monofunctional monomer constitutes less than 50% by weight of said polymerizable component.

10. The composition of claim 9 wherein said monofunctional monomer constitutes from 20% to 35% by weight of said polymerizable component.

11. The composition of any one of claims 1 to 10 wherein said polymeric binder is an epoxy resin, a polyvinyl acetate, a polyvinyl formal, a polyvinyl butyral, a polyester, a polyamide, a polyether, a polyurethane, a polyacrylic resin, or a copolymer comprising monomeric units of any one or more of these polymers with one or more copolymerizable monomers, or a copolymer comprising monomeric units of any two or more of these polymers.

12. The composition of claim 11 wherein said polymeric binder comprises a terpolymer of vinyl acetate, vinyl alcohol and vinyl acetal.

13. The composition of any one of claims 1 to 12, wherein the photoinitiator is a triazine.

14. The composition of claim 12 wherein the triazine has the formula

wherein Ar represents

and R > , R2, R3 and R4 independently represent hydrogen, chlorine, bromine, alkoxy or alkyl.

15. The composition of claim 13 wherein said photoinitiator is 2-stilbenyl-4,6-di(trichloromethyl)triazine.

16. The composition of any one of claims 1 to 15 wherein said diazonium salt comprises the condensation product of 3-methoxydiphenylamine-4-diazonium sulfate and 4,4'-bismethoxymethyldiphenyl ether.

17. The composition of any one of claims 1 to 15 wherein said diazonium salt comprises the condensation product of 4,4'-bismethoxymethyl-diphenyl ether, which has first been precondensed with itself to form an oligomer having from 2 to 7 repeating units, and 3-methoxy-4diazo-diphenylamine sulfate.

18. A radiation-polymerizable composition substantially as described in Example 1 herein.

19. A photographic element comprising a substrate having thereon a coating of the composition of any of claims 1 to 18.

20. The photographic element of claim 19 wherein said substrate is aluminum or an alloy thereof, a polyester, or silicon.

21. A photographic element comprising an aluminum-containing substrate upon which is coated a composition comprising in admixture a) a terpolymer of polyvinyl acetate, polyvinyl alcohol and polyvinyl formal; b) 2-stilbenyl-4,6-di(trichloromethyl)triazine; c) the condensation product of 3-methoxy-4-diazo-diphenylamine sulfate and 4,4'-bis-methoxymethyl diphenyl ether isolated as the mesitylene sulfonate; d) pentaerythritol tetraacrylate; and e) pentaerythritol monoacrylate.

22. A process for making and using a photographic element comprising coating a composition comprising a polymeric binder, a photoinitiator, a diazonium salt, a polyfunctional acrylic monomer having from 3 to 6 unsaturated groups and a monofunctional acrylic monomer having 1 unsaturated group on a suitable substrate to form a photographic element; drying said mixture; imagewise exposing said element to actinic light and developing said element with a suitable developer.

23. The process of claim 22 wherein said developer comprises 2-propoxyethanol.

24. A printing plate comprising a layer of a composition as claimed in any one of claims 1 to 18 on a substrate.

25. A printing forme obtained by imagewise exposure and subsequent development of a plate as claimed in claim 24.

26. Any new feature hereinbefore described or any new combination of hereinbefore described features.