GB2032124A - Lithographic Printing Plate Having Two Photosensitive Layers - Google Patents

Abstract

A method for preparing metal based e.g. aluminum based lithographic printing plates which comprises treating at least one surface of a metal base sheet with a lithographic photosensitive, preferably water-soluble, diazo composition and then applying to said surface a lithographic photopolymerizable composition. The photopolymerisable layer comprises a) an acrylonitrile butadiene styrene resin b) a cellulosic resin, c) an acrylic resin polyurethane or nylon, d) an ethylenically unsaturated compound and e) a photoinitiator.

Description

SPECIFICATION Lithographic Printing Plates Having Dual Photosensitive Layering This invention relates to a novel method for the production of presensitized lithographic printing plates and to the novel lithographic printing plates produced thereby. More particularly, this invention relates to a novel method of producing aluminum based photosensitive lithographic printing plates which comprises coating at least one surface of an aluminum base sheet with a lithographic, lightsensitive diazo composition, and finally applying to said surface a lithographic photopolymerizable coating to yield the desired photosensitive lithographic printing plate.

It is well recognized that the substantial majority of photosensitive lithographic printing plates now produced are made from an aluminum based sheeting. This aluminum sheeting has, over the years, been found to suffer from certain disadvantages which prevent the direct application of the lightsensitive coating to the aluminum base sheet. It has been found that upon direct application of the light-sensitive composition to the aluminum base sheet, and subsequent exposure thereof to light, and the removal of the non-imaged areas for the preparation of a lithographic printing plate, the resultant printing plate has many undesirable characteristics which render it unacceptable for commercial use in the printing industry.Among the disadvantages suffered by such plates can be included the fact that the removed non-image areas do not possess sufficient hydrophilic and oleophobic characteristics, and therefore on printing will pick up background imperfections which will be imprinted on the final copy. In addition, since the aluminum is a rather soft metal, it does not have the resistance properties to sustain long press runs where more than about 100,000 copies are required. Further, the characteristic of the aluminum surface is such that a problem is encountered in achieving a strong bond between the light sensitive composition and the aluminum base sheet thus causing the image area of the plate to be dislodged from the surface, and renders the printed copy imperfect.

Heretofore, in the production of metal presensitized lithographic printing plates, it had been found beneficial to treat the surface of the metal substrate sheet, with a protective interlayer substance which imparts beneficial characteristics to the final lithographic printing plate thus produced. The prior art teaches that it is desirable to treat the metal sheet substrate surface receiving the light sensitive coating material, which when exposed to light and developed becomes the printing surface of the printing plate, with an undercoating substance that forms a strong bond with the metal sheet substrate and with the light sensitive coating material.

Many such undercoating treatments are known in the art for manufacturing longer running lithographic plates, and can be used on the sheets of this invention. U.S. Patents No. 3,160,506, No.

3,136,636, No. 2,946,683, No. 2,992,71 5, disclose a variety of suitable materials for undercoating bonding substances onto plates and methods for applying them. Alkali metal silicate, silicic acid, alkali zirconium fluoride and hydrofluozirconic acid solutions presently are the most important commercial bonding substances. Those materials substantially improve the bonding of the light sensitive coating to the underlying metallic base which otherwise generally tends to have inadequate affinity for the coating. Of the various known bonding materials, the Group IV-B metal fluorides, the alkali metal salts and the acids thereof are preferred. In particular, the alkali zirconium fluorides, such as potassium zirconium hexafluoride, and hydrofluozirconic acid disclosed in Patents No. 3,160,506 and No.

2,946,683 are used for preparing aluminum bases to receive a light-sensitive coating.

One method in particular which is disclosed in the Jewett et al. U.S. Patent 2,714,066 issued July 26, 1955, involves the use of a silicious overcoating on the aluminum sheet surface which acts as an interlayer bonding the photosensitive composition to the aluminum sheet surface. However, it has been found that this method does not completely solve the problem of bonding strength in that the bonding between the photosensitive composition and the aluminum base sheet has been found not to be strong enough to withstand long press runs and, in addition, tends to wear out or fail over an extended period of time.

The instant invention provides a novel method of manufacturing presens7tized printing plates wherein a photosensitive, lithographically suitable, preferably water-soluble diazo type composition is applied directly to the surface of a metal sheet substrate, preferably aluminum although other metals such as zinc or copper may also be employed. Said metal sheet substrate may have been previously treated in a variety of ways known in the art, such as graining, etching or anodizing treatments or may have been provided with an interlayer composition as hereinbefore mentioned. The diazo coated substrate is then subsequently coated with a photosensitive, solvent soluble, lithographic photopolymerizable composition.

The resulting lithographic plate demonstrates a substantially extended shelf life, a protracted press life and improved clarity of the resultant printed image.

It is known in the prior art to produce lithographic printing plates by use of a diazo resin applied on a sheet of aluminum. It is also known in the prior art to produce lithographic printing plates by coating an aluminum sheet substrate with a layer of a photopolymerizable layer.

U.S. Patent 3,905,815 issued to Bonham further discloses the production of a lithographic printing plate by means of a dual layer coating on an aluminum sheet. This dual layer consist of a first coating of a diazo resin and a second over-coating of a photopolymer resin.

The present invention provides a lithographic printing plate comprising two photosensitive layers.

The construction comprises a layer of a diazo resin applied to at least one surface of a substrate, said diazo resin having coated thereon a second layer comprising the photopolymerizable composition as hereinbefore defined. The diazo compositions usable in the context of the present invention for the lower layer, may be comprised of any of the standard photosensitive diazo compositions which are well known in the art. A number of such compositions will be hereinafter described. The photopolymerizable layer is comprised of at least one ethlenically unsaturated monomeric or oligomeric compound having a functionality above one and preferably above two, capable of forming a high polymer by free radical initiated additional polymerization, a photoinitiator, acrylonitrile butadiene styrene resin, a cellulosic resin and an Acryloid resin.

It has been found that by producing a lithographic printing plate in this manner, an extremely durable, long lasting, high resolution plate is achieved.

As hereinbefore mentioned, the present invention provides for a commercially acceptable, extremely long running lithographic printing plate.

As the first step in the production of the lithographic printing plate, a sheet metal substrate such as zinc, copper or most preferably aluminum and the alloys thereof, especially those aluminum compositions suitable for the manufacture of lithographic printing plates such as Alcoa 3003 and Alcoa 1100, which may or may not have been pretreated by standard pretreatments such as graining and/or etching and/or anodizing techniques as are well known in the art, and also may or may not have been treated with a composition suitable for use as an interlayer for lithographic plates is first coated with a layer of a lithographic light-sensitive, preferably water-soluble diazo composition. Said coated substrate is then treated with a stratum of a composition which comprises a lithographic, photopolymerizable composition as above defined.Interlayer compositions employable in the practice in this invention include aqueous solutions of alkali silicate, silicic acid, the Group IV-B metal fluorides, the alkali metal salts or the acids thereof, polyacrylic acid, the alkali zirconium fluorides, such as potassium zirconium hexafluoride, or hydrofluozirconic acid in concentrations of 0.5% to 20% by volume coated by spraying, brushing, dipping or other equivalent means.

Preferred diazo resins which may be employed within the context of the present invention to form the coating layer closest to the base substrate include the following which are prepared in a method well known in the art: The aromatic diazo compounds such as the reaction product of paradiazo diphenyl amine with para-formaldehyde, the azido-pyrenes, for example, 1 -azido-pyrene, 6-nitro-1 -azidopyrene, 1 ,6- diazidopyrene, 1 ,8-diazido-pyrene, 1 -propionyl-6-azidopyrene, 1 -acetyl-6-azidopyrene, 1 -n-butyryl-6- azidopyrene, 1 -n-propionyl-8-bromo-6-azidopyrene; and 8-n-propionyl- 1 ,6-diazidopyrene.

4-diazo-diphenylamine sulfate 1 -diazo-4-N,N-dimethylamino-benzene zinc chloride 1 -diazo-4-N,N-diethylamino-benzene zinc chloride 1 -diazo-4-N-ethyl-N-hydroxyethylamino-benzene, 1/2 zinc chloride 1 -diazo-4-N-Methyl-N-hydroxyethylamino-benzene, 1/2 zinc chloride 1 -diazo-2,5-diethoxy-4-benzoylamino-benzene, 1/2 zinc chloride 1-diazo-4-N-benzylamino-benzene, 1/2 zinc chloride 1 -diazo-4-N,N-dimethylamino-benzene borofluoride 1-diazo-4-morpholino-benzene, zinc chloride 1 -diazo-4-morpholino-benzene-borofluoride 1 -diazo-2,5-dimethoxy-4-p-tolylmercaptobenzene, 1/2 zinc chloride 1 -diazo-2-ethoxy-4N,N-dimethylaminobenzene, 1/2 zinc chloride p-diazo-dimethyl aniline, 1/2 zinc chloride 1-diazo-4-N,N-diethylamino-benzene,1/2 zinc chloride 1-diazo-2,5-dibutoxy-4-morpholino-benzene sulfate 1 -diazo-2,5-diethoxy-4-morpholino-benzene, 1/2 zinc chloride 1 -diazo-2,5-diethoxy-4-morpholino-benzene, 1/2 zinc chloride 1 -diazo-2,5-dimethoxy-4-morpholino-benzene, zinc chloride 1 -diazo-2,5-diethoxy-4-morpholino-benzene, 1/2 zinc chloride 1 -diazo-2,5-diethoxy-4-morpholino-benzene-borofluoride 2-diazo-1-naphthol-5-sulfonic acid, sodium salt 1 -diazo-4-N,N-diethylamino-benzene, borofluoride 1 -diazo-2,5-diethoxy-4-p-tolylmercapto-benzene, 1/2 zinc chloride 1 -diazo-3-ethoxy-4-N-methyl-N-benzylamino-benzene, 1/2 zinc chloride 1 -diazo-3-chloro-4-N,N-diethylamino-benzene, 1/2 zinc chloride 1 -diazo-3-methyl-4-pyrrolidino-benzene chloride, zinc chloride 1 -diazo-3-methyl-4-pyrrolidino-benzene-borofluoride 1 -diazo-2-chloro-4-N,N-dimethylamino-5-methoxy-benzene, borofluoride 1-diazo-3-methoxy-4-pyrrolidino benzene, zinc chloride Condensation product of 4-diazo-diphenylamine sulfate and formaldehyde zinc chloride; and the diazo compositions listed on pages 201-214 of the reference volume, Light-Sensitive Systems, by Jaromir Kosar, John Wiley and Sons, New York 1965.

These diazos are applied at a coating weight of from about 5 to about 70 mg./sq. ft. and preferably from about 10 to about 30 mg./sq. ft.

The formulation of the novel photopolymerizable upper coating layer comprises the admixture of an ethylenically unsaturated monomeric or oligomeric compound having a functionality greater than one and preferably greater than two which is capable of forming a high polymer by free radical initiated addition polymerization, a photoinitiator capable of generating free radicals, an acrylonitrile butadiene styrene resin to provide a highly impact resistant printing surface, an Acryloid (TM of Rohm 8 Haas) resin which adds durability to the surface and promotes film uniformity, and a cellulosic resin which acts to permit use of aqueous developers after exposure.In addition to these ingredients, additional compounds may optionally be blended into the composition such as fillers, for example, fine silica, extenders, polymerization inhibitors and colorants as are well known in the art. For application to the printing plate all the constituents are mixed with a suitable solvent. Suitable solvents include Methyl Cellosolve (TM), methyl cellosolve acetate, methyl ethyl ketone, n-butanol, ethylene dichloride, methylene chloride and methanol in amounts of from about 1% to about 10% of the photopolymerizable composition.

Ethylenically unsaturated monomeric or oligomeric compounds useful for this invention may constitute from about 20 to about 60 per cent and preferably from 40% to 50% by weight of the upper coating composition. Typical of such monomers and oligomers include each of the following compositions or blends thereof: Polychrome Corporation's Uvimers 530, 545, 580 and 558, polyfunctional ethylenically unsaturated epoxies, urethanes, polyesters, and polyethers, allyl acrylate, acrylic anhydride, allyl methacrylate, butanediol diacrylate, butanediol dimethacrylate, diethylene glycol diacrylate, ethylene diacrylate, glycerol triacrylate, methacrylate anhydride, methallyl acrylate, methallyl methacrylate, pentaerythritol tetraacrylate, pentaerythritol tetramethacrylate, pentaerythritol triacrylate.These polyfunctional monomers and oligomers may optionally be blended with such monofunctional compositions as: glycidyl acrylate, glycidyl methacrylate, acrylonitrile, hydroxyethyl acrylate, methacrylic acid and methacryionitrile methylchloroacrylate.

Cellulosic resins useful for this invention may constitute from about 4% to about 50% and preferably from 10% to 20% by weight of the upper coating composition. Examples of such cellulosic resins include: cellulose, cellulose acetate, cellulose acetate butyrate, ethyl cellulose, hydroxy ethyl cellulose and cellulose acetate phthalate and cellulosic resins which are esters of dicarboxylic acids.

Acryloid (TM) resins employable within the context of the present invention may be present in an amount of from about 4% to about 50% and preferably from 10% to 25% of the upper coating composition. Particularly useful are such Acryloid (TM) resins as homopolymers and copolymers of methyl methacrylate (available as resin A-21 from Rhom s Haas). Nylon resins and polyurethanes may be substituted for the Acryloid resin.

Acrylonitrile butadiene styrene terpolymers such as are commercially available from the Dow Chemical Company, as Dow 300 and Dow 500, may be present in the upper coating composition in an amount of from about 2% to about 25% and most preferably from 3% to 1 Q% by weight.

Preferred photoinitiators include benzophenone, Michler's Ketone and O-chlornhexarylbiimidizole which may be used in the following percentages by weight of the coating composition either singly or in various admixed combinations: Operable Range Preferred Range Benzophenone 0.5 to 15% 1 to 3% Michler's Ketone 0.5 to 15% 1 to 3% O-chlorohexarylbiimidizole up to 15% 1 to 3% Other useful photoinitiators include benzil, benzoin ethers, benzoin substituted ethers as disclosed in U.S. Patent 3,905,815, Irgacure (available from Ciba-Geigy), Trigonal (available from the Noury Chemical Corp., although other photoinitiators as are well known in the art may also be employed.

Coiorants employable in the coating composition include all those listed in the Color Jndex which do not substantially interfere with the mechanism of the coating layer. Preferred colorants are dyes and most preferred are the Orasol dyes available from Ciba Geigy which may be used in amounts of from about 0.25 to about 3%, and preferably from 1 to 3% of the coating composition.

Exemplary polymerization inhibitors include p-phenol sulfate sodium salt, p-methoxy phenol hydroquinone, hydroxy butylated toluene and hydroxy butylated anisole which may be used in an amount of from about 0.05 to about 1% and preferably from 0.1% to 0.4% of the coating composition.

A typical solvent system for these ingredients is about 3 parts Methyl Cellosolve (TM) mixed with about 10 parts of methyl ethyl ketone, although a plethora of other solvent systems are employable within the context of the present invention.

The upper layer composition may preferably be coated at a weight of from about 100 to about 600 mg./sq. ft.

The formed lithographic printing plate may be exposed through a mask or transparency to ultraviolet or actinic light in a manner well known in the art, and the thus exposed plate may be developed by use of an aqueous alkaline developer, with or without wetting agents, in a manner well known to the skilled worker.

The following is an example which demonstrates the operation and intent of the invention.

Example A grained, anodized sheet of Alcoa grade 1100 alumina is first coated with a compound which is the reaction product of paradiazo diphenylamine with paraformaldehyde at a coating weight of 30 mg./sq. ft. The thus treated substrate is then coated with a composition which comprises: Uvimer 530 (ethylenically unsaturated oligomer available from Polychrome Corp.) 12 9 Cellulose acetate hydrogen phthalate 4g ABS-300 (available from Dow Chemical Co.) 29 Acryloid A-21 (available from Rohm a Haas) 69 Benzophenone .5 g Michler's Ketone .5 g Hexarylbiimidizole 5 g p-phenol sulfate sodium salt .1 g Orasol GN dye .4g in a solvent system of 60 ml of methyl cellosolve and 200 ml of methyl ethyl ketone at a coating weight of 500 mg./sq. ft. The thus produced lithographic printing plate was exposed through a photographic transparency on a 5KW Berkey ultraviolet light source for 40 seconds and developed with Polychrome's 982 developer. The thus produced printing plate provided 200,000 acceptable reproductions.

It is, of course, to be understood that the foregoing disclosure is intended to illustrate the invention and that numerous changes in the conditions and proportions can be set forth without departing from the scope of the invention as disclosed and defined in the claims appended hereinafter.

Claims (7)

Claims

1. A lithographic printing plate which comprises a metal sheet substrate having coated thereon a light-sensitive, lithographic, water-soluble diazo composition, said diazo composition having coated thereon a lithographic photopolymerizable composition, which photopolymerizable composition comprises a major proportion of an admixture of: a) an acrylonitrile butadiene styrene resin b) a resin selected from the group consisting of Acryloid resins, polyurethane and nylon resins c) a cellulosic resin d) an ethylenically unsaturated monomeric or oligomeric compound having a functionality above one capable of forming a high polymer by free radical addition polymerization e) a photoinitiator capable of generating free radicals.

2. A lithographic printing plate as claimed in claim 1 wherein the photopolymerizable composition further comprises ingredients, in minor amounts, selected from colorants, polymerization inhibitors, and fillers.

3. A lithographic printing plate as claimed in claim 1 or 2, wherein the photopolymerizable composition is comprised of, in per cent by weight of the photopolymerizable composition, a) an acrylonitrile butadiene styrene resin in an amount of from 2% to 25% b) a resin or resins selected from the group consisting of Acryloid resins, polyurethane and nylon resins, in an amount of from 4% to 50% c) a cellulosic resin in an amount of from 4% to 50% d) an ethylenically unsaturated monomeric or oligomeric compound having a functionality above one and capable of forming a high polymer by free radical addition polymerization in an amount of from 20% to 60% e) a photoinitiator capable of generating free radicals in an amount of from 0.5% to 45%.

4. A lithographic printing plate as claimed in any of claims 1 to 3, wherein the colorant is a dye, the polymerization inhibitor is p-phenol sulfate sodium salt and the filler is fine silica.

5. A lithographic printing plate as claimed in any of claims 1 to 4, wherein the photoinitiator is comprised of an ingredient or ingredients selected from benzophenone, Michler's Ketone and 0chlorohexarylbiimidizole.

6. A lithographic printing plate as claimed in claim 5 wherein the photoinitiator comprises equal parts of benzophenone, Michler's Ketone and O-chlorohexarylbiimidizole.

7. A lithographic printing plate as claimed in claim 1, substantially as hereinbefore described in the Example.