Process for Bromine Surface Treatment of Photosensitive Elastomeric Flexographic Printing Plates

1. Technical Field

This invention relates to a process for surface treatment of photosensitive flexographic printing plates. More particularly this invention relates to the surface treatment of flexographic printing plates wherein the photosensitive layer contains a polymer of a conjugated diolefin hydrocarbon. Still more particularly this invention relates to the surface treatment with bromine of a photosensitive elastomeric layer containing a polymer of a conjugated diolefin hydrocarbon.

2. Background Art

Photosensitive flexographic printing plates are becoming more important in the printing trade. Particularly useful compositions for preparing such flexographic printing plates include: compositions containing addition-polymerizable, ethylenically unsaturated compounds, photoinitiators and as polymeric binders either block copolymers such as styrene-butadiene-styrene and styrene-isoprene-styrene polymers, or butadiene/acrylonitrile polymers with optional carboxyl groups. The former compositions which are in commercial use are particularly useful with alcohol or alcohol-acetate based inks. The latter compositions which are new to the trade are particularly useful with hydrocarbon based inks.

The flexographic printing plates described above as well as those that are prepared from other natural or synthetic polymers of conjugated diolefin hydrocarbons, after imagewise exposure and development, generally require a treatment to reduce tackiness of the relief printing surface. Both chlorine and bromine treatments are known to reduce the layer tackiness. The bromine treatment has been found to be effective with flexographic printing plates and particularly with those flexographic printing plates containing a butadiene/acrylonitrile polymer with optional carboxyl groups. The bromine treatment solutions generally require at some point in their preparation the handling of acid, acid fumes and/or free elementary bromine in the liquid or gaseous state. For operator safety it is desirable to eliminate the handling of such aggressive and dangerous reagents.

It is an object of this invention that a process for treating flexographic printing plates be provided that eliminates the use of a separate acid or free bromine components. Another object is to provide such a process which is operable over a pH range from near neutral to acid conditions. Still another object is to provide such a process wherein dry, solid components of the treatment solution are stable, readily available, and are capable of being combined to give a stable mixture which can be stored in the dry state prior to mixing with water and subsequent use as a bromine treating agent.

In accordance with this invention there is provided a process for modifying the surfaces of relief flexographic printing plates wherein the reliefs are prepared by imagewise exposure with actinic radiation and liquid development of the unexposed areas of a layer of a photosensitive, elastomeric composition comprising a polymer of a conjugated diolefin hydrocarbon and a nongaseous ethylenically unsaturated compound containing at least one terminal ethylenic group, the improvement, wherein after drying, the developed surface is, in either order,

(1) postexposed to a source of actinic radiation, and

(2) contacted with an aqueous treatment solution of an alkali monopersulfate and a bromide salt for about 15 seconds to 40 minutes.

The photosensitive elastomeric compositions used in the process of this invention comprise a polymer of a conjugated diolefin hydrocarbon, a nongaseous, ethylenically unsaturated compound, an organic, radiation-sensitive, free-radical generating photoinitiator or system as well as other additives discussed below. The compositions may be used in layer form or the layer may be adhered to flexible supports, or a temporary support may be used. Another configuration may have a cover sheet and polymeric layer on each side of the photosensitive layer. Useful flexible supports include: plastic films and metal sheets, open or closed cell foams, compressible rubber layers, or a combination of foams and rubber layers with plastic films. When the combination support is used the plastic film is generally adjacent to the photosensitive layer. A polyamide coated film, e.g., polyester such as polyethylene terephthalate, provides a suitable temporary support. Examples of such supports include but are not limited to polymeric film supports such as polyethylene terephthalate, flame-treated polyethylene terephthalate, electron-discharge treated polyethylene terephthalate, polyimide, e.g., film as disclosed in U.S. Pat. No. 3,179,634 which is incorporated by reference, etc., and thin metal supports such as aluminum, tin-plated steel (dull or shiny). The polymeric supports generally have a thickness in the range of 0.001 inch to 0.007 inch (0.025 to 0.18 mm). The metal supports generally have a thickness in the range of 0.010 to 0.0115 inch (0.25 to 0.29 mm) for aluminum and 0.008 to 0.010 inch (0.20 to 0.25 mm) for tin plated steel. Examples of foam supports include open or closed cell foams, e.g., polyvinyl chloride, polyurethane, ethylenepolylenediene rubber, neoprene, etc. Examples of compressible rubbers include: styrene/isoprene block copolymers, butadiene/acrylonitrile copolymers, natural rubber, etc.

The photosensitive compositions disclosed herein are developable in their unexposed state in aqueous, semiaqueous basic, or solvent solutions depending on the particular polymeric binder or binders present in the photosensitive composition. The photosensitive layers are in the thickness range of 0.005 to 0.250 inch (about 0.13 to 6.35 mm) or more.

One essential ingredient of the photosensitive, elastomeric composition is an elastomeric polymeric binder. Suitable binders include natural or synthetic polymers of conjugated diolefin hydrocarbons. Examples of the binders include: 1,2-polybutadiene, 1,4-polybutadiene, butadiene/acrylonitrile, butadiene/styrene, block copolymers of the A-B-A type, e.g., styrene-butadiene-styrene block copolymer, styrene-isoprene-styrene block copolymer, etc., and copolymers of the binders. The block copolymers may be of the linear, radial, or sequential type. The preferred binders are the styrene-isoprene-styrene or styrene-butadiene-styrene block copolymers and butadiene/acrylonitrile copolymers optionally having a carboxyl content.

Useful linear block copolymers of this invention have at least one unit of the general formula, A-B-A, wherein each A is an independently selected nonelastomeric polymer block having a number average molecular weight (Mn) of 2000-100,000 and a glass transition temperature above 25.degree. C. and B is an elastomeric polymer block having an average molecular weight between about 25,000 and 1,000,000 and a glass transition temperature below about 10.degree. C. The nonelastomeric blocks, A, having between them an elastomeric block, B, together comprise the unit A-B-A which represents the copolymers uniquely suitable for use in combination with the photopolymerizable components in the compositions of the invention. This unit may comprise the entire general formula of the copolymer; it may be appended to another polymer chain; or it may be repeating. It is, of course, possible to vary the precise nature of the unit within the scope of the invention, e.g., by using two different nonelastomeric terminal blocks, A, or by creating block or graft polymeric substitutions in blocks A and B. Preferably, the elastomeric mid-section blocks, B, are polymers of aliphatic conjugated diolefins while the nonelastomeric blocks, A, are those formed by polymerizing alkenyl aromatic hydrocarbons, preferably vinyl substituted aromatic hydrocarbons, and still more preferably vinyl monocyclic aromatic hydrocarbons. The block copolymers are disclosed in Holden et al. U.S. Pat. No. 3,265,765 and counterpart British Pat. No. 1,000,090, hereby incorporated by reference. Particularly preferred species of the subject copolymers comprise block copolymers of polystyrene terminal groups connected by a mid-section of polyisoprene or polybutadiene, e.g., polystyrene-polyisoprene-polystyrene or polystyrene-polybutadiene-polystyrene, the polydiene block being 70 to 90% by weight of the block copolymer. Other typical block copolymers useful in this invention are polystyrene-polybutadiene-polystyrene and polystyrene-polyisoprene-polystyrene block copolymers which have been hydrogenated according to the teachings of Jones, U.S. Pat. No. 3,431,323 and Hefele et al., U.S. Pat. No. 3,333,024. The hydrogenated block copolymers have the additional advantage of improved thermal and oxidative resistance. However, some residual unsaturation in hydrogenated block copolymers is desirable, since only very small concentrations of monomer are then needed in the photosensitive compositions to reduce solvent solubility upon exposure to actinic radiation. Still other typical block-copolymers useful in this invention are those wherein the terminal blocks are polyalkyl styrenes, e.g., poly(.alpha.-methyl styrene)-polyisoprene-poly (.alpha.-methyl styrene), and those composed of a plurality of polymer blocks, e.g., polyisoprene-polystyrene-polybutadiene-polystyrene-polyisoprene.

The number average molecular weights for the block copolymers can be determined by membrane osmometry utilizing a gel cellophane 600 W membrane manufactured by Arro Laboratories, Inc., Joliet, IL and toluene as the solvent at 29.degree. C. The Mn for the nonelastomeric polymer blocks and elastomeric polymer blocks are preferably determined as follows:

A. The molecular weight of the first block (polystyrene) can be measured by gel permeation chromatography (GPC) of a terminated sample removed immediately after the polymerization. The chromatograph is calibrated using commercially available polystyrene molecular weight standards.

B. The Mn of the second block (polyisoprene or polybutadiene) can be determined in the following manner:

(1) measuring by suitably calibrated GPC the Mn of a sample of polystyrene-polyisoprene (or polybutadiene) diblock polymer terminated and removed immediately after its polymerization, and (2) subtracting from this value the Mn of the first block as determined in (A) above.

C. The Mn of the third block (polystyrene) can be determined in the same general manner:

(1) measuring by suitably calibrated GPC the Mn of the sample of polystyrene-polyisoprene (or polybutadiene)-polystyrene triblock polymer, and

(2) subtracting from this value the Mn of the diblock polymer obtained in (B) above. The block copolymers are manufactured by Shell Chemical Company and sold under the trademark "Kraton.RTM.".

Useful butadiene/acrylonitrile copolymers are high molecular weight butadiene/acrylonitrile copolymers (a) having a number average molecular weight (Mn) in the range of about 20,000 to about 75,000, preferably in the range of about 25,000 to about 50,000. The Mn for the polymers described herein can be determined by gel permeation chromatography employing a polybutadiene standard. The acrylonitrile content of the polymers varies from about 10 to about 50% by weight, preferably from about 15 to about 40%. Optionally, the copolymer also has a carboxyl content of 0 to about 15% by weight. When the copolymer contains carboxyl groups, the carboxyl content is preferably in the range of about 1 to about 15%, most preferably in the range of about 2 to about 10% by weight. The copolymer is present in an amount of about 55 to 90% by weight, based on the total weight of the composition, and preferably about 60 to about 75% by weight. At least about 55% by weight of the copolymer is necessary to give adequate flexibility and physical integrity to the photosensitive elements, particularly for flexographic plates.

Carboxyl groups may be incorporated into the high molecular weight copolymer by addition to the polymerization process of a carboxyl containing monomer, e.g., acrylic or methacrylic acid, or a monomer which is convertible to a carboxyl containing group, e.g., maleic anhydride or methyl methacrylate. Such polymers are available commercially from several sources, e.g., from the B. F. Goodrich Chemical Company under the trade name Hycar.RTM..

Another essential ingredient of the photosensitive compositions of this invention is a nongaseous, ethylenically unsaturated compound containing at least one terminal ethylenic group (b). This compound should be capable of forming a high polymer by free-radical initiated chain-propagating addition polymerization and be compatible with the high molecular weight polymers (a) described above. Ethylenically unsaturated compounds which are compatible with the A-B-A type block copolymers set forth above are disclosed in British Pat. No. 1,366,769 incorporated by reference. Many of these monomers are specifically disclosed below. One class of suitable ethylenically unsaturated compounds includes unsaturated esters of alcohols, especially such esters of .alpha.-methylene carboxylic acids and substituted .alpha.-methylene carboxylic acids, more especially such esters of alkylene polyols and polyalkylene polyols, and most especially alkylene polyol di- and tri-acrylates and polyalkylene polyol di- and tri-acrylates prepared from alkylene polyols of 2-15 carbon atoms or polyalkylene ether polyols or glycols of 1-10 ether linkages.

The following specific compounds are further illustrative of this class: ethylene glycol diacrylate, diethylene glycol diacrylate, glycerol diacrylate, glycerol triacrylate, trimethylolpropane triacrylate, ethylene glycol dimethacrylate, 1,3-propanediol dimethacrylate, 1,2,4-butanetriol trimethacrylate, 1,4-cyclohexanediol diacrylate, 1,4-benzenediol dimethacrylate, 1,2-benzenedimethanol diacrylate, pentaerythritol triacrylate, pentaerythritol tetramethacrylate, 1,3-propanediol diacrylate, 1,3-pentanediol dimethacrylate, p-.alpha., .alpha.-dimethylbenzylphenyl acrylate, t-butyl acrylate, N,N-diethylaminoethyl acrylate, diethylaminoethyl methacrylate, 1,4-butanediol diacrylate, hexamethylene glycol diacrylate, decamethylene glycol diacrylate, 2,2-dimethylolpropane diacrylate, tripropylene glycol diacrylate, 2,2-di(p-hydroxy-phenyl)propane diacrylate, 2,2-di(p-hydroxyphenyl)propane dimethacrylate, polyoxyethyl-2,2-di(p-hydroxyphenyl)propane triacrylate (molecular weight of 462), 1,4-butanediol dimethacrylate, hexamethylene glycol dimethacrylate, 2,2,4-trimethyl-1,3-pentanediol dimethacrylate, 1-phenylethylene-1,2-dimethacrylate, trimethylolpropane trimethacrylate, triethylene glycol diacrylate, ethylene glycol acrylate phthalate, polyoxyethyltrimethylolpropane triacrylate, diacrylate and dimethacrylate esters of diepoxy polyethers derived from aromatic polyhydroxy compounds such as bisphenols, novolaks and similar compounds such as those described by Crary in U.S. Pat. No. 3,661,576, which is incorporated by reference, the bis-acrylates and methacrylates of polyethylene glycols of molecular weight 200-500, etc.

Another class of suitable ethylenically unsaturated compounds includes the compounds disclosed by Martin and Barney in U.S. Pat. No. 2,927,022 which is incorporated by reference, e.g., those having a plurality of addition polymerizable ethylenic linkages, particularly when present as terminal linkages, and especially those wherein at least one and preferably most of such linkages are conjugated with a double bonded carbon, including carbon double bonded to carbon and to heteroatoms such as nitrogen, oxygen and sulfur. Preferred are such materials wherein the ethylenically unsaturated groups, especially the vinylidene groups, are conjugated with ester or amide structures. Specific examples of such compounds include unsaturated amides, particularly those of .alpha.-methylene carboxylic acids, especially with .alpha.-omega-diamines and oxygen-interrupted omega-diamines, such as methylene bis-acrylamide, methylene bis-methacrylamide, ethylene bis-methacrylamide, 1,6-hexamethylene bis-acrylamide, diethylene triamine trismethacrylamide, bis(methacrylamidopropoxy)ethane, .beta.-methacrylamidoethyl methacrylate, N-(.beta.-hydroxyethyl)-.beta.-(methacrylamido)ethyl acrylate, and N,N-bis(.beta.-methacryloxyethyl) acrylamide; vinyl esters such as divinyl succinate, divinyl adipate, divinyl phthalate, divinyl terephthalate, divinyl benzene-1,3-disulfonate, and divinyl butane-1,4-disulfonate; diallyl fumarate; etc.

Additional ethylenically unsaturated compounds which may be used include styrene and derivatives thereof: 1,4-diisopropenylbenzene, 1,3,5-triisopropenyl-benzene; itaconic anhydride adducts with hydroxyethyl acrylate (1:1), itaconic anhydride adducts with liquid butadiene/acrylonitrile polymers containing terminal amino groups, and itaconic anhydride adducts with the diacrylate and dimethacrylate esters of diepoxy polyethers described in Crary U.S. Pat. No. 3,661,576; polybutadiene and butadiene/acrylonitrile copolymers containing terminal and pendant vinyl groups; and unsaturated aldehydes, such as sorbaldehyde (2,4-hexadienal).

Ethylenically unsaturated compounds which are water soluble or contain carboxyl or other alkali-reactive groups are especially suitable when aqueous basic-developable systems are involved. In addition, the polymerizable, ethylenically unsaturated polymers of Burg, U.S. Pat. No. 3,043,805; Martin, U.S. Pat. No. 2,929,710; and similar materials may be used alone or mixed with other materials. Acrylic and methacrylic esters of adducts of ethylene oxide and polyhydroxy compounds such as those desribed by Cohem and Schoenthaler in U.S. Pat. No. 3,380,831 are also useful. The photocrosslinkable polymers disclosed in Schoenthaler, U.S. Pat. No. 3,418,295, and Celeste, U.S. Pat. No. 3,448,089, may also be used. All of these patents are incorporated by reference.

The ratio of the weight of block copolymer used to the weight of addition polymerizable ethylenically unsaturated compound use is 99:1 to about 1:1.

The amount of unsaturated compound present in a photosensitive composition containing a butadiene/acrylonitrile copolymer should be in the range of about 2 to about 40% by weight, based on the total weight of composition. The specific amount for optimum results will vary depending on the particular polymers used. Preferably the amount of unsaturated compound is in the range of about 5 to about 25%.

The ethylenically unsaturated compound preferably has a boiling point at normal pressure of over about 100.degree. C. The most preferred ethylenically unsaturated compounds are triethylene glycol diacrylate, triethylene glycol dimethacrylate, hexamethylene glycol dimethacrylate and hexamethylene glycol diacrylate.

The photosensitive compositions of this invention substantially do not scatter actinic radiation when in the form of thin layers, as described above. In order to secure a substantially transparent mixture, i.e., a nonradiation-scattering mixture, the polymer binder components should be compatible with, and preferably soluble in, the ethylenically unsaturated compound in the proportions used.

By "compatible" is meant the ability of two or more constituents to remain dispersed in one another without causing any significant amount of scattering of actinic radiation. Compatibility is often limited by the relative proportions of the constituents, and incompatibility is evidenced by formation of haze in the photosensitive composition. Some slight haze can be tolerated from such compositions before or during exposure in the preparation of printing reliefs, but when fine detail is desired, haze should be completely avoided. The amount of ethylenically unsaturated compound, or any other constituent, used is therefore limited to those concentrations which do not produce undesired light scatter or haze.

Another essential ingredient of the photosensitive compositions of this invention is an organic, radiation-sensitive, free-radical generating system (c). Practically any organic, radiation-sensitive, free-radical generating system which initiates polymerization of the unsaturated compound and does not excessively terminate the polymerization can be used in the photosensitive compositions of this invention. The term "organic" is used here and in the claims to designate compounds which contain carbon, and one or more of oxygen, hydrogen, nitrogen, sulfur and halogen, but no metal. Because process transparencies transmit heat originating from conventional sources of actinic radiation, and since the photosensitive compositions are usually prepared under conditions resulting in elevated temperatures, the preferred free-radical generating compounds are inactive thermally below 85.degree. C. and more preferably below 185.degree. C. They should be dispersible in the composition to the extent necessary for initiating the desired polymerization or crosslinking under the influence of the amount of radiation absorbed in relatively short term exposures. These initiators are useful in amounts from about 0.001 to about 10% by weight, and preferably from about 0.1 to about 5% based on the total weight of the solvent-free photosensitive composition.

The free-radical generating system absorbs radiation within the range of about 2000 to about 8000 A and has at least one component that has an active radiation absorption band with molar extinction coefficient of at least about 50 within the range of about 2500 to about 8000 A, and preferably about 2500 to about 5000 A. The term "active radiation absorption band" means a band of radiation which is active to produce the free radicals necessary to initiate polymerization or crosslinking of the unsaturated material.

The free-radical generating system can comprise one or more compounds which directly furnish free radicals when activated by radiation. It can also comprise a plurality of compounds, one of which yields the free radicals after having been caused to do so by a sensitizer which is activated by the radiation.

A large number of such free-radical generating compounds can be utilized in the practice of the invention and include aromatic ketones such as benzophenone, Michler's ketone [4,4'-bis(dimethylamino)benzophenone], 4,4'-bis(diethylamino)benzophenone, 4-acryloxy-4'-dimethylaminobenzophenone, 4-acryloxy-4'-diethylamonobenzophenone, 4-methoxy-4'-dimethylaminobenzophenone, 2-phenyl-2,2-dimethoxyacetophenone(2,2-dimethoxy-1,2-diphenyl ethanone), 2-ethylanthraquinone, phenanthraquinone, 2-t-butylanthraquinone, 1,2-benzanthraquinone, 2,3-benzathraquinone, 2,3-dichloronaphthoquinone, benzil dimethyl acetal, and other aromatic ketones; benzoin, benzoin ethers such as benzoin methyl ether, benzoin ethyl ether, benzoin isobutyl ether, and benzoin phenyl ether, methylbenzoin, ethylbenzoin and other benzoins; and 2,4,5-triarylimidazolyl dimers such as 2-(o-chlorophenyl)-4,5-diphenylimidazolyl dimer, 2-(o-chlorophenyl-4,5-di(m-methoxyphenyl)imidazolyl dimer, 2-(o-fluorophenyl)-4,5-diphenylimidazolyl dimer, 2-(o-methoxyphenyl(-4,5-diphenylimidazolyl dimer, 2-(p-methoxyphenyl(-4,5-diphenylimidazolyl dimer, 2,4-di(p-methoxyphenyl)-5-phenylimidazolyl dimer, 2-(2,4-dimethoxyphenyl)-4,5-diphenylimidazolyl dimer, 2-(p-methylmercaptophenyl)-4,5-diphenylimidazolyl dimer, etc., as disclosed in U.S. Pat. Nos. 3,479,185 and 3,784,557 and in British Pat. Nos. 997,396 and 1,047,569. These patents are incorporated herein by reference.

The imidazolyl dimers can be used with a free-radical producing electron donor such as 2-mercaptobenzoxazole, Leuco Crystal Violet or tri(4-diethylamino-2-methylphenyl)methane. Such sensitizers as Michler's ketone may be added. Various energy transfer dyes such as Rose Bengal and Eosin Y can also be used. Additional examples of suitable initiators are disclosed by Plambeck in U.S. Pat. No. 2,760,863 which is incorporated by reference.

The photosensitive compositions may also contain a small amount of thermal addition polymerization inhibitor, e.g., 0.001% to 2.0%, based on the weight of the total solvent-free photosensitive composition. Suitable inhibitors include hydroquinone and alkyl and aryl-substituted hydroquinones, 2,6-di-tert-butyl-4-methylphenol, p-methoxyphenol, tert-butylpyrocatechol, pyrogallol, .beta.-naphthol, 2,6-di-tert-butyl-p-cresol, phenothiazine, pyridine, nitrobenzene, dinitrobenzene and the nitroso dimer inhibitor systems described in U.S. Pat. No. 4,168,982 which is incorporated by reference. Other useful inhibitors include p-toluquinone, chloranil and thiazine dyes, e.g., Thionine Blue G (CI 52025), Methylene Blue B (CI 52015) and Toluidine Blue (CI 52040). Such compositions can be photopolymerized or photocrosslinked without removal of the inhibitor. The preferred inhibitors are 2,6-di-tert-butyl-4-methylphenol and p-methoxyphenol.

The oxygen and ozone resistance of the printing reliefs can be improved by incorporating into the photosensitive composition a suitable amount of compatible well known antioxidants and/or antiozonants. Antioxidants useful in this invention include: alkylated phenols, e.g., 2-6-di-tert-butyl-4-methylphenol; alkylated bis-phenols, e.g., 2,2-methylene-bis-(4-methyl-6-tert-butylphenol); 1,3,5-trimethyl-2,4,6-tris-(3,5-di-tert-butyl-4-hydroxybenzyl)benzene; 2-(4-hydroxy-3,5-di-tert-butylanilino)-4,6-bis-(n-octyl thio)-1,3,5-triazine; polymerized trimethyldihydroquinone; and dilauryl thiopropionate.

Antiozonants useful in this invention include: microcrystalline wax and paraffin wax; dibutylthiourea; 1,1,3,3-tetramethyl-2-thiourea; Antiozonant AFD, a product of Nafton Co.; norbornenes, e.g., di-5-norbornene-2-methyl adipate, di-5-norbornene-2-methyl terephthalate; Ozone Protector 80, a product of Reichhold Chemical Co.; N-phenyl-2-naphthylamine; unsaturated vegetable oils, e.g., rapeseed oil, linseed oil, safflower oil; polymers and resins, e.g., ethylene/vinyl acetate copolymer resin, chlorinated polyethylene, chlorosulfonated polyethylene, chlorinated ethylene/methacrylic acid copolymer, polyurethanes, polypentadienes, polybutadiene, furfural-derived resins, ethylene/propylene/diene rubber, diethylene glycol ester of rosin and .alpha.-methylstyrene/vinyltoluene copolymer. Ozone resistance of the printing relief produced can also be improved by annealing it at elevated temperatures prior to use.

If desired, the photosensitive compositions can also contain immiscible, polymeric or nonpolymeric organic or inorganic fillers or reinforcing agents which are essentially transparent at the wavelengths used for exposure of the layer of the photosensitive composition and which do not scatter actinic radiation, e.g., polystyrene, the organophilic silicas, bentonites, silica, powdered glass, colloidal carbon, as well as various types of dyes and pigments. Such materials are used in amounts varying with the desired properties of the elastomeric compositions. The fillers are useful in improving the strength of the elastomeric layer, reducing tack and, in addition, as coloring agents.

The photosensitive layer preferably contains a compatible plasticizer to lower the glass transition temperature of the binder and facilitate selective development. The plasticizer may be any of the common plasticizers compatible with the polymeric binders. Among the common plasticizers are dialkyl phthalates, alkyl phosphates, polyethylene glycol, polyethylene glycol esters, and polyethylene glycol ethers. Other useful plasticizers include oleic acid, lauric acid, etc. Polyesters are preferred plasticizers, e.g., polyethylene sebacate, etc. The plasticizer is generally present in an amount of 1 to 15% by weight based on weight of total solids of the photosensitive composition.

The photosensitive compositions of this invention can be prepared in any suitable way by mixing the ingredients, i.e., (a) the polymeric binder, (b) the compatible ethylenically unsaturated compound, and (c) the free-radical generating system. For example, flowable compositions can be made by mixing them and other desired adjuvants in any order and, if desired, with the aid of a solvent such as chlorinated hydrocarbons, e.g., methylene chloride, chloroform, methyl chloroform, chlorobenzene, trichloroethylene, tetrachloroethylene and chlorotoluene; ketones, e.g., methyl ethyl ketone, diethyl ketone and methyl isobutyl ketone; aromatic hydrocarbons, e.g., benzene, toluene and xylene; and tetrahydrofuran. The above solvents can contain as diluents acetone; lower molecular weight alcohols, e.g., methyl, ethyl and propyl alcohol; and esters, e.g., methyl, ethyl and butyl acetate. The solvent can be removed later by heating the admixture or extruded layer.

Conventional milling, mixing, and solution techniques can be used in making these compositions, the particular technique varying with the differences in properties of the respective components. The homogeneous, substantially nonradiation-scattering compositions, are formed into sheets in any desired manner. For example, solvent-casting, hot pressing, calendering, or extrusion are suitable methods for preparing layers of the desired thickness.

The photosensitive elements of this invention can be made by solvent casting or by extruding, calendering or pressing at an elevated temperature of the photosensitive composition into the form of a layer or self-supporting sheet on a suitable casting wheel, belt or platen. The layer or sheet may be laminated to the surface of the flexible support described above and may be adhered by means of an adhesive blend as described below. When a solution is used the coating may be made on an adhesive-bearing support.

The thickness of the photosensitive layer is a direct function of the thickness desired in the relief image and this will depend on the subject being reproduced and the ultimate use of the relief, e.g., thick soft reliefs are useful for flexographic printing and thin hard reliefs are useful for planographic printing. In general, the thickness of the polymerizable layer will be less than about 0.250 inch, e.g., it will vary from about 0.005 to about 0.250 inch (0.0127-0.635 cm) and layers within this range of thickness will be used for the majority of the printing plates.

Between the photosensitive layer butadiene/acrylonitrile printing plate and flexible support is preferably placed a layer of an adhesive blend which comprises at least two polymers taken from a group of the following four polymers:

(1) Polyester resin, a condensation polymer of ethylene glycol, terephthalic acid, isophthalic acid and azelaic acid in a molar ratio of about 6:2:1:3 have a Mn of 19,000 and Mw of 37,000;

(2) Polyether polyurethane resin, a crystalline, thermoplastic resin having a Brookfield viscosity of 100-1200 centipoises using 15% by weight solids in methyl ethyl ketone and a Brookfield spindle #3 at 12 rpm, and an adhesive activation temperature in the range of 54.degree. to 63.degree. C. The polyether polyurethane has an elongation at yield of 15%, elongation at break of 615%, modulus at 400% elongation of 600 psi (42.18 kg/cm.sup.2), decrystallization temperature of about 49.degree. C.;

(3) Polyamide resin, a translucent light amber color, Ball and Ring softening point of 132.degree.-145.degree. C., melt viscosity of 40 to 60 poises at 210.degree. C., flash point greater than 299.degree. C., percent water absorption for 1 day is 0.7, for 7 days is 1.6, tensile yield of 460 psi (32.34 kg/cm.sup.2), tensile break of 450 psi (31.64 kg/cm.sup.2) and elongation of 560%. (The tensile yield, tensile break and elongation are determined at 24.degree. C. according to ASTM Procedure D-1708); and

(4) Polyamide resin, a translucent light amber color, Ball and Ring softening point of 150.degree.-160.degree. C.; viscosity of 28-38 poises at 210.degree. C., percent water absorption for 1 day is 1.5, for 7 days is 2.8, and

(The tensile yield, tensile break and elongation are determined at the stated temperatures according to ASTM Procedure D-1708).

The number average molecular weights (Mn) of the resins can be determined by gel permeation chromatography (GPC) employing a known standard, e.g., polybutadiene, as known to those skilled in the art. The weight average molecular weights (Mw) of the resins can be determined by using a light scattering technique using known standard samples, e.g., polystyrene, polymethacrylic acid, polymethylmethacrylate, etc., as known to those skilled in the art.

The particular polymers can be present in the adhesive blend in the following ranges based on the total weight of resin in the adhesive composition: (1) 0 to 78% by weight, (2) 0 to 78% by weight, (3) 0 to 94% by weight, and (4) 0 to 97% by weight. Preferred adhesive blends containing four, three and two resin components are set forth below wherein the percentages are by weight based on the total resin content.

Percentage ranges for the quaternary adhesive blend are:

(1) 25 to 31%, preferably 25%,

(2) 25 to 31%, preferably 25%,

(3) 25 to 19%, preferably 25%, and

(4) 25 to 19%, preferably 25%.

Percentage ranges for two tertiary adhesive blends, A and B, are:

(1) 1 to 78%, preferably 1 to 65%,

(2) 1 to 78%, preferably 1 to 65%, and

(3) 1 to 94%, preferably 1 to 90%; and

(1) 1 to 63%, preferably 1 to 45%,

(3) 1 to 93%, preferably 1 to 85%, and

(4) 1 to 97%, preferably 1 to 90%.

Percentage ranges for five binary adhesive blends, C to G, are:

(1) 7 to 77%, preferably 15 to 50%, most preferably 30% and

(3) 93 to 23%, preferably 85 to 50%, most preferably 70%.

(This adhesive blend is particularly preferred.)

(1) 3 to 60%, more preferably 5 to 30%, and

(4) 97 to 40%, more preferably 95 to 70%,

(1) 23 to 77%, more preferably 35 to 45%, and

(2) 77 to 23%, more preferably 65 to 55%.

(2) 10 to 60%, preferably 25 to 30%, and

(4) 90 to 40%, preferably 75 to 70%; and

(2) 7 to 72%, preferably 15 to 50%, and

(3) 93 to 28%, preferably 85 to 50%.

The adhesive blends of this invention provide an adhesion value for the photosensitive layer to the support of at least 3 lbs/inch (53.57 kg/m) and generally much greater adhesion, e.g., in the range of 8 lbs/inch (142.86 kg/m) or more. These adhesion values are sufficient when the elements of the invention are used as printing plates, particularly flexographic printing plates.

The adhesive blends preferably contain additives such as antiblocking agents, colorants, e.g., dye, etc. Useful antiblocking agents include: preferably polyolefin particles or beads but also other hard particles or beads such as silicon dioxide, etc. Dioctyl sodium sulfosuccinate, a surfactant, can be used. A preferred polyolefin material is described in the examples. The bead size of the antiblocking agents can be greater than the thickness of the adhesive layer resulting in beads partially protruding out of the layer of the adhesive blend. Such a configuration appears to have little or no effect on the degree of adhesion. Many types of colorants or dyes are also useful in the adhesive layer. A preferred dye is Du Pont Milling Blue BL (CI Acid Blue 59). Other useful dyes include: Methylene Violet (CI Basic Violet 5), "Luxol" Fast Blue MBSN (CI Solvent Blue 38), "Pontacyl" Wool Blue BL (CI Acid Blue 59 or CI 50315), "Pontacyl" Wool Blue GL (CI Acid Blue 102 or CI 50320), Victoria Pure Blue BO (CI Basic Blue 7 or CI 42595), CI 109 Red Dye, Rhodamine 3 GO (CI Basic Red 4), Rhodamine 6 GDN (CI Basic Red 1 or CI 45160), Fuchsine dye (CI 42510), Calcocid Green S (CI 44090) and Anthraquinone Blue 2 GA (CI Acid Blue 58).

An adhesive solution is prepared by adding the ingredients generally in the following order to the solvent with continuous stirring: polymers, polyolefin antiblocking agent, colorants. Useful solvents include mixtures, e.g., methylene chloride/ethyl acetate, methylene chloride/n-butyl acetate, methylene chloride/cyclohexanone, methylene chloride/methanol/Cellosolve.RTM., etc., and preferably a mixture of methylene chloride/Cellosolve.RTM., 90/10 parts. Additional solvent can be added to make up any weight loss. The choice of solvents is governed by the need to provide the fastest practical drying rates without blistering the coating and without leaving behind small amounts of solvent. The solvents should also have a solubilizing effect on the dyes that may be present.

The adhesive solution is then applied to the flexible support by known means, e.g., coated by use of a doctor blade or in a commercially available continuous web coater-drier to provide a dry coating weight in the range of about 80 to 500 mg/dm.sup.2 preferably about 260 to 300 mg/dm.sup.2. A most preferred coating weight of the adhesive layer is about 260 mg/dm.sup.2. Generally, the adhesive layer has a dry thickness of 0.0008 to 0.001 inch (0.020 to 0.025 mm). In a continuous coating the web speed can vary, e.g., 15 to 150 feet/minute (4.57 to 45.72 m/minute). The drying temperature ranges from 60.degree. to 130.degree. C., preferably 80.degree.-90.degree. C.

A preferred flexible support is flame-treated polyethylene terephthalate, 0.001 to 0.007 inch (0.025 to 0.18 mm) thick, preferably 0.005 inch (0.13 mm) in thickness. Flame-treatment of polymeric films is known. The following U.S. patents describe useful procedures and apparatus for flame treating polymeric films: Bryan U.S. Pat. No. 3,145,242, and Thompson U.S. Pat. Nos. 3,360,029 and 3,391,912 which are incorporated by reference. The fuel equivalence ratio of the combustible gas mixture, .phi., is 1.4 which is equal to 5(propane flow rate)/[(oxygen flow rate)+(0.21 air flow rate)]. All flow rates are in standard cubic feed or cubic liters/minute. The web speed is 175 lineal feet/minute (53.34 m/minute).

The dried adhesive coated support can be adhered immediately to the photosensitive layer or can be stored for subsequent adherence. The adhesive-coated support can be laminated to the photosensitive layer in a press, e.g., at 140.degree. to 160.degree. C. at a pressure of 20,000 to 30,000 psi (1406 to 2109 kg/cm.sup.2) for up to about three minutes, followed by cooling in the press to less than 60.degree. C. Preferably the photosensitive element is prepared by calendering. The photosensitive layer, which preferably is formed by extruding through a die, has present on the side remote from the side adjacent the adhesive layer a 0.005 inch (0.13 mm) thick polyethylene terephthalate film which subsequently acts as a protective cover sheet. Other films can be used such as polystyrene, polyethylene, polypropylene or other strippable material. There preferably is present between the photosensitive layer and the film cover sheet a thin hard, flexible, solvent-soluble layer, such as a flexible, polymeric film or layer, e.g., a polyamide or a copolymer of ethylene and vinyl acetate. The flexible polymeric film remains on the photosensitive layer after removal of the film cover sheet described above. The flexible polymer film protects for reuse an image-bearing negative or transparency superposed thereon or improves contact or alignment with the photosensitive surface. Prior to the imagewise exposure using the sources described below, the element is exposed through the support to polymerize a predetermined thickness of the photosensitive layer adjacent the adhered support. This polymerized portion of the photosensitive layer is designated a floor. The floor thickness varies with the time of exposure, exposure source, etc. The exposure is generally for 1 to 30 minutes.

Printing reliefs can be made in accordance with this invention by exposing to actinic radiation selected portions of a photosensitive layer of an element described above, for example, through a process transparency, i.e., an image-bearing transparency or stencil having areas substantially transparent to actinic radiation and of substantially uniform optical density and areas opaque to actinic radiation and of substantially uniform optical density until substantial addition-polymerization or photocrosslinking takes place. During the addition-polymerization or crosslinking, the polymeric binder/ethylenically unsaturated compound composition is converted to the insoluble state in the radiation-exposed portions of the layer, with no significant polymerization or crosslinking taking place in the unexposed portions or areas of the layer. The unexposed portions of the layer are removed by means of a liquid developer for the polymeric binders. The process transparency may be constructed of any suitable material including cellulose acetate film and oriented polyester film.

Actinic radiation from any source and of any type can be used in the photopolymerization process. The radiation may emanate from point sources or be in the form of parallel rays or divergent beams. By using a broad radiation source relatively close to the image-bearing transparency, the radiation passing through the clear areas of the transparency enters as divergent beams and thus irradiates a continually diverging area in the photosensitive layer underneath the clear portions of the transparency. This results in a polymeric relief having its greatest width at the bottom of the photosensitive layer, i.e., a frustum, the top surface of the relief being the dimensions of the clear area.

Inasmuch as the free-radical generating systems activatable by actinic radiation generally exhibit their maximum sensitivity in the ultraviolet range, the radiation source should furnish an effective amount of this radiation, preferably having a wavelength range between about 2500 A and 5000 A. Suitable sources of such radiation, in addition to sunlight, include carbon arcs, mercury-vapor arcs, fluorescent lamps with ultraviolet radiation-emitting phosphors, argon glow lamps, electron flash units and photographic flood lamps. Electron accelerators and electron beam sources through an appropriate mask may also be used. Of these, the mercury-vapor lamps, particularly the sun lamp or "black light" type, and the fluorescent sun lamps, are most suitable.

The radiation exposure time may vary from fractions of a second to minutes, depending upon the intensity and spectral energy distribution of the radiation, its distance from the composition and the nature and amount of the composition available. Customarily, a mercury vapor arc, a sunlamp or high ultraviolet output fluorescent tubes are used at a distance of about 1.5 to about 60 inches (3.8 to 152 cm) from the photosensitive composition. Exposure temperatures are not particularly critical, but it is preferred to operate at about ambient temperatures or slightly higher, i.e., about 20.degree. to about 35.degree. C.

After exposure, the image may be developed by washing with a suitable developer. The developer liquid should have good solvent or swelling action on the polymeric binder/ethylenically unsaturated compound composition and little action on the insolubilized image or upon the support or adhesive layer in the period required to remove the nonpolymerized or noncrosslinked portions. Suitable developers include: organic solvents, e.g., 2-butanone, benzene, toluene, xylene, trichloroethane, trichloroethylene, methyl chloroform, tetrachloroethylene, and solvent mixtures, e.g., tetrachloroethylene/n-butanol, etc. When the high molecular weight butadiene polymer component contains carboxyl groups, suitable developers include: aqueous base to which a water-insoluble organic solvent may be added. Suitable specific developer mixtures include sodium hydroxide/isopropyl alcohol/water, sodium carbonate/water, sodium carbonate/2-butoxyethanol/water, sodium borate/2-butoxyethanol/water, sodium silicate/2-butoxyethanol/glycerol/water, sodium carbonate/2-(2-butoxyethoxy)ethanol/water and sodium hydroxide (0.5 weight percent) in 16.6 volume percent 2-(2-butoxyethoxy) ethanol in water, which is preferred. The particular developer combination chosen will depend upon the carboxyl content of the photosensitive composition and the properties and amounts of the binders employed. Other aqueous developer combinations which may be employed are described in U.S. Pat. No. 3,796,602 which is incorporated by reference. These aqueous base/water-soluble organic solvent combinations may be preferred in some cases because of their low cost, nonflammability and reduced toxicity.

Development may be carried out at about 25.degree. C., but best results are sometimes obtained when the solvent is warm, e.g., 30.degree.-60.degree. C. Development time can range from 1 to 120 minutes, preferably 1 to 25 minutes.

In the development step where the relief is formed, the developer may be applied in any convenient manner, as by pouring, immersion, spraying, or roller application. Brushing aids in the removal of the unpolymerized or uncrosslinked portions of the composition. When the printing plate undergoes aqueous development a water rinse is subsequently applied, e.g., for about 5 to 300 seconds, to the developed plate to remove tracves of developer from the plate surface. The term "aqueous development" includes the water rinse.

After solvent development, the printing plate is dried at a temperature in the range of room temperature to about 125.degree. C., preferably 60.degree. C. for one hour. After aqueous development the printing plate may be dried, but it has been found that the rinsed aqueous developed plate while still wet can be contacted with the aqueous bromine-containing solution. Hot air drying can be accomplished by use of a forced hot air drier or other suitable dryer.

The plate is then contacted with an aqueous treatment solution prepared as described below. The plate can either be flooded with the aqueous treatment solution or can be dipped in the treatment solution, the latter being preferred. The treatment solution is maintained in contact with the printing plate for from 15 seconds to 40 minutes, more preferably from 1 minute to 10 minutes. The treatment solution is preferably used at ambient temperature but can be heated up to about 35.degree. C. The treatment solution is prepared by dissolving an alkali monopersulfate, e.g., potassium monopersulfate triple salt (2KHSO.sub.5.KHSO.sub.4.K.sub.2 SO.sub.4) and a bromide salt, e.g., potassium bromide, in aqueous solution. Since both of the components are solids and are nonreactive with one another in the absence of moisture, they can be mixed dry and stored in moisture proof containers for an extended period prior to use. The proportions of dry ingredients in the mixture is from 98 to 10 alkali monopersulfate and from 2 to 90 bromide salt. The potassium salts are particularly preferred since they are significantly less moisture sensitive. However, other alkali salts, e.g., sodium, lithium, etc., are useful if maintained separately from each other until dissolution. No acid is required to be added to the treatment solution which is advantageous. The treatment solution is operable at higher pH ranges than previously known bromine or chlorine treatment solutions including pH values in the neutral region, and above, e.g., 7 to 8.5 or more. The overall pH range is 0.7 to 8.5 or more.

The printing plate, after development, can be postexposed to a source of actinic radiation, either prior to or subsequent to the above-described treatment. The postexposure is generally for 5 to 15 minutes duration preferably to the actinic radiation source used for the imagewise exposure. After both treatments the printing plate is ready for use.

The best modes are illustrated in Example 7 wherein the treatment solutions are neutral (about pH 6.8) and low pH (about 2.0). At the higher pH the treatment times preferably are 2 to 10 minutes. At the lower pH the preferred treatment time range is 1 to 2 minutes.

The printing reliefs prepared from the photosensitive flexographic elements of the invention can be used in all classes of printing but are most applicable to those classes of printing wherein a distinct difference of height between printing and nonprinting areas is required and particularly to the flexographic printing class wherein a resilient print area is required, e.g., for printing on deformable printing surfaces. These classes include those wherein the ink is carried by the raised portion of the relief such as in dry-offset printing, ordinary letterpress printing, the latter requiring greater height differences between printing and nonprinting areas, and those wherein the ink is carried by the recessed portions of the relief such as in intaglio printing, e.g., line and inverted halftone. The plates are particularly useful for multicolor printing.

The relief and printed images obtained show fidelity to the original transparency both in small detail and in overall dimensions even when the element is imagewise exposed on a cylindrical support. The reliefs have high impact strength, are tough and abrasion-resistant, have broad ink compatibility, and those reliefs of a plate based on acrylonitrile/butadiene binders have improved solvent resistance, particularly to the alcohol solvents of alcohol based inks, when treated according to the teaching of this invention.

The following examples illustrate the invention wherein the parts and percentages are by weight.

The following procedure is used to determine the compatibility and usefulness of printing inks and solvent with flexographic printing plates having a layer of the photosensitive, elastomeric composition described herein. An exposed, developed printing plate, after determination of its thickness, Shore.RTM.A hardness (ANSI/ASTM D2240-75) and weight, is immersed for 24 hours in a particular solvent or ink. Upon removal from the solvent or ink, and patting dry, the thickness, Shore.RTM.A hardness and weight are redetermined. If the changes in thickness (.DELTA.T) expressed in inches (millimeters), Shore.RTM.A hardness (.DELTA.H), and weight (.DELTA.%W) meet the following standards, the printing plate is deemed to be compatible or excellent (E) with respect to the solvent and/or ink:

.DELTA.T: 0.003 inch (0.076 mm)

.DELTA.H: -5 Shore.RTM.A units

.DELTA.%W: 3.5%.

When the following standards are achieved or exceeded the printing plate is deemed to be incompatible or unsatisfactory (U) with respect to the solvent and/or ink:

.DELTA.T: 0.012 inch (0.305 mm)

.DELTA.H: -20 Shore.RTM.A units

.DELTA.%W: 7.0%

In the intermediate range between the above two standards the treated printing plates are of variable or intermediate (I) utility with respect to the solvent and/or ink. The overall ratings are set forth in the Examples below.

The exposed, developed printing plate of this invention can be tested for tackiness as follows:

(1) The surface is wiped clean with isopropanol,

(2) Tissue such as Scott.RTM. brand 510 toilet tissue is pressed by means of a 500 g weight against the plate surface (1 inch by 2 inches; 2.54 cm by 5.08 cm) for 30 seconds,

(3) The tissue is removed noting the results:

(A) No tack-free from sticking

(B) Slight tack-sticks but peels from surface

(C) Tacky-peels but leaves a few fibers on the plate

(D) Very tacky-sticks and tissue rips upon peeling.

Skilled artisans learn to identify nontacky or (A) grade printing plate surfaces by touch, i.e., finger-tip touch methods. Both methods have been used. All treated samples below are tack-free.

An adhesive solution is prepared from the following ingredients:

The above ingredients are added in order to a 90/10 parts mixture of methylene chloride/Cellosolve.RTM. to give a solution of about 16% solids. The polyolefin beads do not dissolve. The mixture is stirred continuously during and after the addition of the ingredients to effect solution. Any weight loss during mixing is made up by addition of methylene chloride.

The adhesive solution is applied to the flame-treated surface of a polyethylene terephthalate film support, 0.005 inch (0.13 mm) in thickness using a continuous web coater-drier to provide a dry coating weight of about 260 mg/dm.sup.2. The web speed is 45 feet/minute (13.72 m/minute) and the drying temperature is 86.degree. C. (187.degree. F.).

The adhesive-coated polyethylene terephthalate support is placed adhesive side up in a steel platen dammed to a thickness of (0.080 inch (2.03 mm), the thickness of the finished printing plate. The adhesive-coated support and platen are placed on a press and an extruded sheet, 0.090 inch (about 2.29 mm) thick, of a photopolymerizable composition on a 0.005 inch (0.13 mm) thick polyethylene terephthalate cover sheet bearing a layer of a polyamide resin having a dry thickness of 0.00017 inch (0.004 mm) is placed thereon with the cover sheet side up, and is covered with a steel plate.

The cover sheet bearing the polyamide layer is prepared by coating the polyethylene terephthalate film with the following solution using an extrusion die coater:

The extruded sheet of the photopolymerizable composition is prepared from the following ingredients which are blended and the blend is extruded at 170.degree. C. through a die.

The temperature is raised and pressure is gradually applied which spreads the photopolymerizable sheet throughout the dammed area of the platen. After the sheet is evenly distributed the temperature is raised to 160.degree. C. and pressure in the range of 20,000 to 30,000 psi (1406 to 2109 kg/cm.sup.2) is applied and held for three minutes. The assembly is cooled in the press to less than 60.degree. C. by flowing water through the press platens. The laminated element formed is removed from the press and is placed support side up in the exposure unit noted below. The element is given an overall exposure in air at atmospheric pressure for 4 minutes through the support to polymerize a predetermined thickness of the photopolymerizable layer adjacent the adhered support. This polymerized portion of the element is designated a floor.

After removal of the cover sheet, the element is then placed polyamide resin layer side up in a Cyrel.RTM.3040 Exposure Unit (registered trademark of E. I. du Pont de Nemours and Company) fitted with Sylvania BL-VHO fluorescent lamps. An image-bearing transparency (negative) is placed on the element surface, and the element is exposed for 15 minutes while under vacuum. The duration of exposure is a function of the photopolymer sheet thickness, thickness of the polymerized floor and the type of image-bearing transparency used.

After exposure the transparency is removed, and the exposed element is placed in a rotary drum-brush type Cyrel.RTM.3040 Processor. The unpolymerized areas of the element and the entire polyamide resin layer are removed in the processor by washing for 15 minutes perchloroethylene (75 vol. %/n-butanol 25 vol. %) solvent. A 0.035 inch (0.89 mm) relief image is obtained. The developed element (printing plate) is placed in a forced hot air drier or other suitable drier and is dried at 60.degree. C. for 15 minutes. The dry plate is postexposed in air for 10 minutes using the same exposure source used for the imagewise exposure described above. The plate has a Shore.RTM.A2 hardness in the range of 56 to 60.

The dry printing plate is immersed in either the monopersulfate-bromide treatment solution or the chlorine solution at room temperature as set forth in Table 1 below. The changes in plate thickness, hardness, and weight percentage of the exposed, developed and dried printing plate are determined for a variety of solvent mixtures and/or other common printing ink components. The values set forth in Table 1 are determined using the ink/solvent compatibility test described above. A rating is set forth for each sample.

From the results set forth in Table 1, it is shown that the bromine treatment results in improved printing plates over the printing plates treated with the chlorine solution. The intermediate and unsatisfactory ratings resulting after bromine treatment are observed for solvents or solvent mixtures that are outside the normal solvent concentration ranges known by those skilled in the art to be useful.

This example compares the ink/solvent compatibility of bromine and chlorine treated photosensitive flexographic printing plates described in Example 1 except that the unexposed image areas are removed in the processor by washing for 15 minutes with 0.5 weight percent sodium hydroxide in 16.6 volume percent 2-(2-butoxyethoxy) ethanol in water followed by a two minute water rinse. In Table 2 below a variety of solvent mixtures and/or other common ink components identified by the numbers set forth in Example 1 are compared with respect to .DELTA.T, .DELTA.H, .DELTA.%W and a rating is indicated for each sample.

From the results set forth in Table 2, it is noted that chlorine treatment has substantially no effect with regard to ink solvents. Improved results are achieved by the bromine treatment. Samples 5 to 8 show an unsatisfactory rating due to the solvent mixture being outside the known useful concentration range.

This example illustrates the interchangeability in the postexposure and aqueous treatment steps.

Three types of photosensitive flexographic printing plates are prepared. One plate designated "Plate A" is prepared as described in Example 2. A second plate designated "Plate B" is prepared as described in Example 1. A third plate designated "Plate C" is a flexographic printing plate, e.g., 0.112 inch (.about.2.85 mm) in thickness, containing a styrene-isoprene-styrene block copolymer and is known as Cyrel.RTM.II flexographic printing plate. Cyrel.RTM. is a registered trademark of E. I. du Pont de Nemours and Company, Wilmington, Del.

Plates A, B and C are placed support side up in a Cyrel.RTM.3040 Exposure Unit (registered trademark of E. I. du Pont de Nemours and Company) fitted with Sylvania BL-VHO fluorescent lamps and are exposed in air at atmospheric pressure for 240 seconds. The plates are then imagewise exposed (Plate C has cover sheet removed) as described in Example 1 for 10 minutes followed by either a 16-minute wash with perchloroethylene (75 vol. %)/n-butanol (25 vol. %) solvent (Plates B and C) or a 15-minute wash with 0.5 weight percent sodium hydroxide in 16 volume percent 2-(2-butoxyethoxy)ethanol in water (Plate A). Each developed plate is postexposed as described in Example 1 either preceded by or followed by a 2-minute aqueous treatment as described in Example 1. Each treated plate is soaked for 24 hours in the indicated solvent (see Example 1 for designation of solvent), and the .DELTA.%W of the dry plates are determined. The results are indicated in Table 3 wherein "Before" indicates aqueous treatment before postexposure.

From the results set forth in Table 3, there is no significant difference between the results obtained by the two sequences.

This example illustrates the effect of varying the time of aqueous treatment with the solution of the invention and a chlorine solution.

Photopolymerizable elements are prepared as described in Example 3 (Plates A, B and C) with the following variations:

Back exposure is 150 seconds,

Imagewise exposure is 10 minutes overall (without image-bearing transparency) with cover sheet retained,

Aqueous development is 15 minutes,

Forced hot air drying is 30 minutes,

Time of aqueous treatment according to the invention is 0 (Control) and 2 and 8 minutes (designated 1),

Chlorine treatment time (designated 2) is 0.5 and 2 minutes.

The changes in plate thickness, hardness and weight percentage are determined according to the Ink Solvent Compatibility Test for each element for the indicated time expressed in minutes yielding the results set forth in Table 4. The solvent/mixture numbers are the same as set forth in Example 1. The values for the changes in plate thickness (.DELTA.T) are expressed in millimeters.

From the results set forth in Table 4, there is no significant change in plate rating with increase in treatment time from 0.5 to 2.0 minutes for chlorine treatment (2) and from 2 to 8 minutes according to the treatment of the invention (1).

This example illustrates the stability of a mixture of potassium monopersulfate triple salt and potassium bromide used to prepare an aqueous treatment solution.

20.0 grams of 2KHSO.sub.5.KHSO.sub.4.K.sub.2 SO.sub.4 and 2.0 grams of potassium bromide dry powders are mixed together and stored in an air- and moisture-proof plastic bottle for nine months. No apparent change is observed in the appearance of the powder on storage. At the end of the period, 11.0 grams of the mixture is dissolved in 100 ml water and the solution (pH 1.7) is used to treat printing plates Plate A and Plate C, described in Examples 2 and 3, respectively. The rating of the printing plates is E. This Example is repeated except that in the preparation of the treatment solution the potassium salts are added to the water without any combined storage. These printing plates are rated E.

From the results of this example, there is no change in plate rating when the mixture of dry powders used to prepare the treatment solution of the invention is stored for an extended period of time as compared to the mixing of the salt components individually just prior to use.

This example illustrates the improvement obtained using the aqueous treatment solution of the invention as compared to a chlorine treatment.

Photosensitive flexographic printing plates are prepared as described in Example 1. In Table 5 below a variety of solvent mixtures and/or other common ink components identified by the numbers set forth in Example 1 are compared with respect to .DELTA.T, .DELTA.H, .DELTA.%W and a rating is indicated for each sample.

This example illustrates the use of preferred aqueous treatment solutions of the invention, one at a pH of about 6.8 and another at a pH of about 2.0, in several solvents or solvent mixtures.

Photosensitive flexographic printing plates are described in Example 2 (Plate A), Example 1 (Plate B) and Example 3 (Plate C). The procedure set forth in Example 3 is used to expose and develop these printing plates. The postexposure is prior to aqueous treatment as described in Example 1, and the aqueous treatment is for 2 minutes. The preferred treatment solutions are used to treat Plates A, B and C, and the plates are then tested in the stipulated solvents or solvent mixtures identified by the numbers set forth in Example 1. The changes in thickness, hardness and weight percentages that are determined and the rating of each treated plate are set forth in Table 6 below.

Treatment Solution A (pH 1.8.+-.0.2)

This solution is prepared as follows:

Into 1 liter of water is added 100 g of 2KHSO.sub.5.KHSO.sub.4.K.sub.2 SO.sub.4 with thorough stirring. 5 g Of KBr is then added followed by thorough stirring. Treatment solution A is particularly useful at a treatment time of 1 to 2 minutes.

Treatment Solution B (pH 6.6.+-.0.2)

This solution is prepared as follows:

Into 1 liter of water is added 10 g of 2KHSO.sub.5.KHSO.sub.4.K.sub.2 SO.sub.4 with thorough stirring. 10 g Of KBr is then added followed by thorough stirring. Treatment Solution B is particularly useful at a treatment time of 2 to 10 minutes.

From the results set forth in Table 6, it is shown that the plate rating does not change significantly when the pH of the treatment solution used on three types of plates is varied from about 2.0 to about 6.8.

A photosensitive printing plate is prepared as described in Example 2 except that after aqueous alkaline development and water rinse the printing plate is immersed in the treatment solution as indicated in Example 1. The printing plate is then dried and is postexposed as described in Example 1. Using 2-propanol as the solvent the printing plate is rated E.