IL32501A - Desensitizer for photolithographic printing plate - Google Patents

Description

DESENSITIZER FOR PHOTOLITHOGRAPHIC PRINTING PLATE m noain mm* nay niwii i»oa FMC 5358 This invention relates to photolithographic printing plates. More particularly, the invention is concerned with an improved acid gum desensitizer for such plates.

The production of lithographic printing plates by photographic means is a known printing technique, being referred to in the art as photolithography. In this system of photoreproduction, a printing surface is derived by the action of light on a photosensitive layer.

The first stage of the process consists of preparing a presensitized printing plate by applying a layer of photosensitive material on a base sheet, commonly a metal, such as aluminum or zinc. The presensitized plate is then subjected to a light pattern, the effect of which is to cause a hardening or resinification of the photosensitive layer in the lightstruck area. Next, the exposed plate is developed by dissolving out the unhardened portion of the photosensitive layer leaving bonded to the base a hardened image corresponding to the light pattern. The plate is treated with ink which adheres to the hardened image, but not to the uncovered metal areas of the plate which are oleophobic. A gum etch is normally required to clean the non-image metal areas and to maintain them in an ink repellent state. The aforesaid treatment is known as desensitization and the gum etch as a desensitizer. Finally, the plate is inserted in a press and printed copies run off.

The first presensitized printing plates were formulated using a bichromated colloid such as gelatin or albumin for the light sensitive layer, which becomes insolubilized in the presence of light. Later, developments were based on of polarity on exposure to actinic radiation.

Recently, a great deal of attention has centered on photopolymer systems in which a photopolymerlzable layer containing a photosensitizer and a monomer compound is exposed to actinic light, whereby the monomer undergoes polymerization in the exposed areas, which then constitute the printing surfaces. An especially effective printing plate based on photopolymerization is mentioned in U. S. in greater detail in U. S. light sensitive layer is a sensitized prepolymer of an allyl ester which undergoes cross-linking in the presence of actinic radiation, thereby forming a hardened image of remarkable toughness and durability. In fact, when utilized as a printing surface, such an image is capable of providing from 100,000 to 500,000 impressions without appreciable deterioration.

Although a decided advance in the art, printing plates based on photopolymerlzable processes may be troublesome to desensitize. That is to say, it is often difficult to effect a clean removal of the non-hardened photopolymerlzable coating in the unexposed areas of the plate. Apparently, even after thorough scrubbing during development, some of the unexposed layer tends to remain adsorbed to the base plate. When this occurs, it is difficult to obtain clean non-image areas. As a consequence, the plate may produce printed copies having streaked or smudgy background areas.

Manifestly, it would be desirable to obliterate such streaking and/or spotting tendencies of lithographic printing plates derived from photopolymerlzable coatings. printing plates based on photopolymerizable layers and free of streaking and spotting tendencies can be obtained by using an acid gum desensitizer in which at least part of the acid is hydrofluoric acid and containing a minor amount of hydrogen peroxide.

In carrying out the invention, a photopolymerizable element is first prepared by applying a photosensitive polymer coating onto a metallic base plate. The element is then exposed to a light pattern to form a hardened polymerized image, the unhardened portions in the unexposed areas dissolved out and the developed plate then desensitized with the hydrofluoric acid/hydrogen peroxide desensitizer composition of the invention. The resulting plate is then placed in a press and printed copies run off. The so-obtained copies are remarkably free of background staining or smudging.

Any normally liquid or solid photopolymerizable unsaturated organic compound is applicable in the practice of the invention. Preferably, such compounds should be ethylenically unsaturated, i.e., containing at least one non-aromatic double bond between adjacent carbon atoms.

Compounds particularly advantageous are the photopolymerizable vinyl or vinylidene compounds containing a CH2=C group activated by direct attachment to a negative group, such as halogen, C=0, C≡N, CONH2 , C≡C, 0 or aryl. The ethylenically unsaturated organic compounds, including both monomers and prepolymers with residual unsaturated linkages, may be used either alone or in admixture in order to vary the physical properties, such as molecular weight and cognized practice in order to produce a vinyl polymer of the desired physical properties, to polymerize it in the presence of a small amount of an unsaturated compound containing at least two terminal vinyl groups, each linked to a carbon atom in a straight chain or in a ring. The function of such compounds is to cross-link the polyvinyl chain. This technique, as used in polymerization, is described by Kropa and Bradley in vol. 31, No. 12 of "Industrial and Engineering Chemistry," 1939· Typical of such cross--^Q linking agents, for the purpose described herein, are triallyl cyanurate, divinylbenzene , divinylketone and similar polyunsaturated compounds. Generally speaking, increasing the quantity of cross-linking agents increases the hardness of the polymer obtained in the range wherein the ratio of monomer to cross-linking agent varies from 10 to 1 to 50 to 1.

The invention is particularly effective wherein the light sensitive polymerizable material is a partially polymerized monomer still containing residual centers of 2Q unsaturation . A remarkably effective class of materials, in this respect, are the cross-linkable allyl resins obtained by the addition polymerization of an addition polymerizable allyl carboxylic ester having a plurality of aliphatic ethy- lenically unsaturated linkages, preferably 2 to 4, at least one of which is an allyl ester group. Polymerization is carried out under such condition, given elsewhere herein in greater detail, that at least one of the ethylenically unsaturated linkages, preferably one of the allyl ester groups attached to the monomer, remains intact and unreacted. This I i units each of which bears an aliphatic ethylenically unsaturated group, preferably an allyl ester. It is these residual centers of unsaturation which enable such resins to undergo photopolymerization when formulated and used in accordance with the invention.

The polymerization of allyl ester monomers in the aforedescribed manner is a known technique and is based on the more rapid polymerization between allyl monomers than cross-linking of the allyl ester groups or other unsaturated groups in the resin polymer. It is this differential in reactivity which makes it possible to prepare and isolate the non-cross-linked allylic resin. The latter type of resins are solvent-soluble solids at ambient temperatures and contain at most very small percentages of the parent monomers so that very little shrinkage occurs on being cross-linked or photopolymerized by actinic radiation. The aforementioned physical properties are among those which make non-cross-lir^ed allylic resins excellent materials for producing the herein presensitized printing plates.

The cross-linkable allyl ester resins used in practicing the invention fall generally into the following classes: (a) Prepolymers derived from allyl esters of unsaturated monobasic acids having either the general formula or C H„ Ί X COOR. such as allyl acrylate, allyl chloro- n 2n-l-y y acrylate, allyl methacrylate , allyl crotonate, allyl cinnamatej allyl cinnamalacetate , allyl furoate and allyl furfuryl-acrylate. It is to be understood that in all formulas used herein, R is an allyl group, n can be any integer from 1 to 17 inclusive, except where the acid is unsaturated in which phenyl, substituted phenyl, or furfuryl group or an alkyl or alkoxy group having 1-4 carbon atoms. (b) Precopolymers of allyl esters of unsaturated monobasic acids, such as allyl methacrylate with butadiene, allyl methacrylate with methyl methacrylate, allyl methacrylate with styrene, allyl methacrylate with vinylidene chloride, allyl crotonate, with methyl methacrylate, allyl crotonate with styrene, allyl crotonate with vinyl chloride, allyl crotonate with vinyl acetate, allyl crotonate with vinylidene chloride, allyl crotonate with diethyleneglycol maleate, allyl cinnamate with vinylidene chloride, allyl cinnamate with styrene, allyl cinnamate with cinnamyl cinnamate, allyl furoate with styrene, and allyl furoate with vinylidene chloride. (c) Prepolymers derived from allyl esters of aliphatic carboxylic acids having two or more allyl groups and having one of the following general formulas: C H„ (COOR)„, & & n 2n 2 * C Η» , (COOR)-, C H X (C00R)„, C H„ Ί X (COOR), n 2n-l 3 n 2n-y y 2* n 2n-l-y y ' CnH2n_2(COOR)2, cnH2n_2yXy(COOR)2i or R00C0R> such as diallyl oxalate, diallyl malonate, diallyl succinate, diallyl sebacate, diallyl maleate, diallyl fumarate, diallyl itaconate, diallyl tartrate, diallyl carbonate, diallyl adipate, triallyl citrate, triallyl carballylate , diallyl malate, and diallyl citraconate. (d) Precopolymers of allyl esters of aliphatic carboxylic acids having two or more allyl groups, such as diallyl oxalate with vinylidene chloride, diallyl oxalate with styrene, diallyl malonate with vinylidene chloride, diallyl succinate with vinylacetate , diallyl succinate with vinylidene chloride, diallyl succinate with polyvinyl acetate, diallyl adipate with diallyl maleate with methyl methacrylate, diallyl maleate with styrene, diallyl maleate with vinylidene chloride, and diallyl carbonate with methyl methacrylate.

In the aromatic series are those cross-linkable copolymer resins derived from an allyl ester in which the acid is normally of the benzene-naphthalene series and cyanuric acid, typical monomers being diallyl isophthalate , diallyl terephthalate , diallyl orthophthalate , triallyl mellitate, tetraallyl pyromellitate and triallyl cyanurate. In the manufacture of cross-linkable allyl resins, also known as prepolymers, the monomeric materials are polymerized in conventional fashion to produce a solution of a soluble polymer in the monomer to the point short of gelation which occurs when cross-linking of the polymer approaches that point where it becomes insoluble in the monomer. These polymer solutions, or dopes, are then separated into a solvent-soluble prepolymer fraction and a monomer fraction. This is effected by treatment with a solvent which dissolves the monomer while precipitating the polymerized portion or by other means which will leave a soluble prepolymer substantially free of monomer. A typical method for separating such cross-linkable prepolymers is described in U. S. Patent 3,030,3^1.

Another important light-sensitive polymerizable system with which the sensitizer composition of the invention is effective includes the polyesters of alpha-beta ethylenic, alpha-beta dicarboxylic acids and ethylenically unsaturated compounds copolymerized therewith of the type described in U. S. Patent 2,673,151. compatible with the desensitizer compositions herein includes polyvinylaraphenones of the type disclosed in U. S. Patent 2,831j768} and cinnamic acid derivatives, particularly polyvinyl cinnamate. Such light-sensitive cinnamic acid derivatives are well known in the photolithographic art, and in this connection reference is made to "Light Sensitive Systems" by Jaromer Kosar, , published by John Wiley and Sons, Inc.

In order to obtain coatings which photopolymerize with sufficient speed to be commercially useful, it is usually necessary to add to the polymer a sensitizing agent, which absorbs actinic radiation, so as to dissociate into free radicals which accelerate complete polymerization of the polymerizable compound, whether it be a monomer or partial or prepolymer, or mixtures of these entities. The sensitizing agent can be ethers of benzoin, such as the methyl ether of benzoin; benzophenone ; p ,p ' -substituted benzo-phenones such as 4 ,M '-bis (dimethylamino) benzophenone and , '-bis ( diethylamino )benzophenone ; or bis ( 1-anthraquinonyl amino) anthraquinones such as 1 , 4-bis ( 1-anthraquinonyl amino) anthraquinone and l,5-bis( 1-anthraquinonyl amino) anthra-quinone, and combinations thereof. Other useful sensitizing agents include polynuclear quinones such as xanthone, 1,2-benzanthraquinone and 2-methyl anthraquinone; mononuclear quinones such as 2 , 5-diphenyl-p-quinone ; aromatic a-diketones such as benzil; substituted aryl methylene dioxy compounds such as piperonal, piperoin, 3 , 4-methylene dioxychalcone and 5 ,6-methylene dioxyhydrindone-1 ; substituted $-naph-thoselenazolines such as l-methyl-2-acetyl methylene-0- - 3-methyl-2-benzoyl methylene benzothiazoline ; substituted B-naphthothiazolines such as l-methyl-2-benzoyl methylene-β-naphthothiazoline ; anthrones such as anthrone; benzanthrones such as benz-2-ethyl-benzanthrone and 7-H-benz (de )-anthracen-7-one; and azabenzanthrones such as 2-keto-3-meth l-l , 3-dia-zabenzanthrone .

The concentration of sensitizer In the photopolymerizable composition depends upon the inherent sensitivity of the cross-linkable allyl ester resin present. In a typical case where the resin is the prepolymer of diallyl isophthalate from about 1 to 20%, preferably 1.5%, by weight of prepolymer, of 1 , -bis ( 1-anthraquinonyl amino) anthraquinone is recommended Some sensitizers, such as p ,p ' -substituted benzophenones , e.g. 4 , 4 '-bis- (dimethyl amino) benzophenone , in an amount less than 1.5% cause a considerable increase in sensitivity to actinic light. The sensitizer is excited by the actinic radiation, and, in turn, initiates the polymerization. The mechanism of the reaction is believed to be that the sensitizer dissociates into free radicals due to actinic radiation or energy that it absorbs, and the resulting free radicals initiate cross-linking of the prepolymer to render it insoluble .

The addition of the sensitizer to the polymerizable . material increases its response to actinic rays by a factor of 100 to 200 times. On exposure, the film coating polymerizes sufficiently rapidly whereby it can be used in conventional plate-making procedures.

The base plate metals used herein are ink repellent or oleophobic since such metal provides the non-printing surfaces used base metals are aluminum, zinc or stainless steel.

The presensitized plates aforesaid are converted into lithographic printing plates by exposure to a light pattern whereby the photocurable or cross-linkable polymeric compound is further polymerized or cured in the exposed areas. The plate is then developed by washing out the non-cured or unpolymerized compound corresponding to the unexposed portion of the plate thereby uncovering the oleophobic metallic base layer.

The liquid used to remove or dissolve out the non-polymerized coating in the exposed areas of the photographic element are advantageously relatively high boiling organic solvents of the type commonly employed in most lithographic shops. One preferred solvent is tetrahydrofurfurylphosphate although excellent solvents include aromatic hydrocarbons and their chlorinated derivatives such as toluene, xylene, chlorinated xylene and chlorinated toluene; and chlorinated aliphatic hydrocarbons, e.g. trichloroethy lne and tri-chloroethane ; higher boiling ketones; glycol monoethers such as ethylene glycol monoethyl ether, diethyleneglycol monoethyl ether and the carboxylic esters of such glycol monoethers. Such solvn|ets can be used alone or in admixture or as emulsions in water. The solvents are commonly applied in combination with a scrubbing action.

At this point the plate is desensitized in the usual manner, but using the new desensitizer of the invention, inserted in a printing press and copies run off. The copies so obtained are remarkable clean, exhibiting a marked absence of scum or tone in the background areas. desensitizer was used.

The desensitizer compositions of this invention are prepared by dissolving a hydrophilic colloid, of the type commonly used to prepare gum desensitizers , in a dilute aqueous acid solution in which at least part of the acid is provided by hydrofluoric acid and containing a minor amount of hydrogen peroxide. The presence of the hydrofluoric acid has been found necessary to effect removal of all the unhardened prepolymer in the exposed areas and thereby provide clean background areas free of streaks and smudges. Other acids normally used in preparing desensitizer gum solutions, such as phosphoric acid, are not capable of removing all of the non-hardened prepolymer.

The hydrogen peroxide can be formed in situ, or added directly to the sensitizer formulation. In some instances it may be desirable to use hydrogen peroxide stabilizers in order to prolong the storage stability of the desensitizer composition or to prevent its decomposition while in use. Such stabilization of hydrogen peroxide is well known in the art .

The presence of the hydrogen peroxide has been found necessary to prevent excessive foaming which occurs when the hydrofluoric acid is applied.

The hydrogen fluoride can serve as the sole acid component or it can be used in combination with phosphoric acid, or other acids known to the trade.

Although the amounts of ingredients in the desensitize composition of the invention are not particularly critical, we have found that the numerical ratios for the various of 30% hydrogen peroxide; 0.1% to 2% of 50% hydrofluoric acid; 3% to 12% of 85% phosphoric acid; 50% to 90% of a 28% hydrophilic gum and 0 to 10% water. Normally the amount of hydrogen peroxide will be that required to prevent foaming, whereas the amount of hydrofluoric acid is that which will effectively remove all of the unhardened prepolymer in the non-printing areas of the plate. The strengths of the ingredients are expressed on a weight basis.

Reference is made to the following examples: A typical formulation for photosensitized coating so lution used for making printing plates is prepared as follows : 12 g- diallylisophthalate prepolymer (Dapon®M) 55 g- xylene 33 g- Pentoxone ( 4-methoxy-4-methyl-penatanone-2 ) 0. 1 g- benzil 0. 1 g- Michler's ketone 0. g. xanthone The prepolymer is dissolved in the xylene and the solution refined to remove insoluble fractions by filtration or centrifuging . The photosensitizers , benzil, Michler's ketone, and xanthone, are dissolved in the Pentoxone and thoroughly mixed with the polymer solution. The coating solution is then ready for use.

This solution is applied to an aluminum sheet by whirl coating technique to produce a uniform coating 0.1 mil thick. After evaporation of some of the solvent, the plate Is heated o to 125 P. for minutes to remove most of the residual solvent colorless, dry film on the metal surface.

The coated plate is handled under reduced intensity illumination conventionally used in platemaking shops.

The plate is covered with a negative mask or film transparency and exposed through the mask to a carbon arc, mercury vapor, or other source of ultraviolet light.

After exposure, the plate is developed to remove the un-hardened polymer by covering the plate with xylene, 1,-1 , 1-trichloroethane , methyl ethyl ketone, trichloroethylene or a mixture of similar solvents. A contact time of about one minute is allowed before the developing solvent is flushed away with water. The image, composed of the light-hardened resin, is visible at this point.

After rinsing, and the surplus water removed, the plate is treated with the acid gum desensitizer of the invention. The desensitizer is applied in the usual manner which consists of flowing the desensitizer onto the plate, using an applicator pad to effect even distribution over the surface of the plate; time of treatment is about 1 to 1-1/2 minutes.

The results of processing exposed allyl ester pre-polymer coatings with various formulations of the desen-sitizers of the invention (Examples 1-3) and with typical gum etches normally used in the trade (Reference Examples A-E) are summarized in the following table: Composition in Percent by Volume Example A B 2 14 Be Gum Arabic 97 55 54 80 Phosphoric acid, 85% 12 Hydrofluoric acid, .2 Hydrogen peroxide, 30% 1.5 .5 Water 44 43.'5 7.3 scums clean clean clean plate ; plate ; plate ; I I—1 foams no foam, no foa 4=· badly , no streaks no stre I streaky , requires extreme care As is evident from examining the data of the table, it is necessary that the desensitizer composition contain at least some hydrofluoric acid in order to achieve a clean scum-free plate. It is also clear from the table that in order to achieve both a scum-clean plate without excessive foaming, it is necessary that the desensitizing composition contain sufficient hydrogen peroxide, normally a minor amount, to control such foaming. Example 2 is the preferred formulation.

Example The procedure of the previous examples was repeated but substituting polyvinyl cAnnamate for the diallyl iso-phthalate prepolymer. Generally speaking, the results paralleled those obtained with the previous examples.

Polyvinyl cinnamate is a. well-known commercial resin and was purchased on the chemical market as KPR, a product of Eastman Kodak Company, Rochester, New York.

Claims (8)

PMC 5358 r

1. A desensitizer composition for desensitizing presensitized printing plates in which the light sensitive element is a photopolymerizable layer, comprising an acidified aqueous hydrophilic gum colloid wherein at least part of the acid is hydrofluoric acid, and an amount of hydrogen peroxide sufficient to control foaming.

2. A composition according to claim 1 wherein all of the acid is hydrofluoric acid.

3. A composition according to claim 1 wherein the acid is a combination of phosphoric acid and hydrofluoric acid.

4. A composition according to claim 1 wherein the hydrophilic colloid gum is gum arabic.

5. A desensitizer composition according to claim 1 comprising approximately by volume 0.3% to 6% of 30% by weight hydrogen peroxide; 0.1% to 2% of 50% by weight hydrofluoric acid; 3% to 12% of 85% by weight phosphoric acid 50% to 90% of a 28% hydrophilic gum and 0 to 10% water.

6. A desensitizer composition according to claim 1 comprising by volume about 80% of l^ Be gum arabic; about 0.2% of 50% by weight hydrofluoric acid; about 12% of 85 % by weight phosphoric acid; about 0.5% of 30% by weight hydrogen peroxide and about 7-3% water.

7. A composition according to claim 1 wherein the polymer in the photopolymerizable layer is diallyliso-phthalate prepolymer.

8. A composition according to claim 1 wherein the polymer in the photopolymerizable layer