US2559610A - Planographic printing plate and treatment thereof - Google Patents

Description

Patented July 10, 1951 PLAN OGRAPHIC PRINTING PLATE AND TREATMENT THEREOF Frederick H.

Frost, Portland, MaineQassignor to Warren Company, Boston, Mass., a corporation of Massachusetts No Drawing. Application December 12, 1947,

Serial No. 791,447

Claims. (Cl. 101149.2)

This invention relates to plates for planographic printing and to the preparation and treatment thereof.

Planographic or lithographic printing depends upon the immiscibility of the wet-out liquid and the lithographic printing ink. To print by lithography there is ailixed to a suitable surface or plate a water-repellent image, usually greasy, waxy, or resinous in nature. To the plate so imaged is then applied wet-out solution, usually a slightly acidified aqueous solution of glycerine or similar substance. The wet-out solution wets all portions of the plate not already covered .by imaging material. An inking-roll coated with lithographic printing ink now passed over the plate leaves a film of ink upon the imaged areas but leaves no ink on the unimaged areas which carry a film of the ink-repellent wet-out solution. The inked plate when brought into contact with another ink-receptive surface transfers thereto the ink in a pattern reverse to that on the plate. The surface so printed upon may be a paper sheet, but in most cases is an oifset blanket which in turn transfers the-image as av positive to a paper sheet which is the final printed matter. The plate is repeatedly dampened, inked and printed from until the desired number of prints have been made.

In recent years very substantial improvements have been made in the production of paper-base plates for lithographic or planographic printing. In general terms, such plates may be said to comprise a paper base, with or without wetstrength properties, and on at least one surface of said base a coating comprising hydrophilic film-forming material containing dispersed therein some filling material such as clay, blanc flxe, or the like. In some cases a. lightweight wash coating of substantially clear film-former, i. e. free of filling material, is applied over the before mentioned filled coatingin order to prevent excessive penetration by wet-out liquid, i. e. to afford improved hold-up of water or wet-out liquid. In the latter case, however, due care must be exercised to be sure that the top coating is not so impenetrable or non-porous that imaging material cannot be securely afllxed thereto.

Obviously it is essential that the image shall stick to the plate tenaciously. At the same time the unimaged areas must be very easily and completely wet by water or aqueous wet-out or dampening solution. Ideally the unimaged surface should be such that while it tenaciously holds water applied thereto it will permit no penetration of water. It has been found, however, that as hydrophilic surfaces improve in ability to 2 withstand penetration by water they also tend to I become less and less adhesive to imaging mateimage, and comprises a metal compound capable of reacting with a later to be applied reagent to form a relatively water-insoluble metal compound, the image is afilxed to said surface, and afterapplication of the image the surface is further treated with the reagent to increase the water-impenetrability of said surface in its unimaged areas. It appears probable that the resulting water-impermability of the treated hydrophilic surface is the result of the formation of a relatively insoluble substance which acts to fill or close.the pores or interstices of the hydropholic surface, so that water penetration is materially decreased without impairing the wettability of the surface and a film of water, i. e. wet-out liquid is held on the surface to repel printing ink and prevent the non-image areas from accepting ink.

In general in practicing the invention, it is preferred to have the metal salt in the hydrophilic coating which is to react with the later to be applied treating solution, at or near the surface of the hydrophilic coating. Soluble metal salt may be applied in aqueous solution to the surface of the hydrophilic coating and may be dried thereon whereupon the salt presumably deposits in the surface pores of the coating. In practice, however, instead of applying the solution of the metal salt in a separate step its application may usually be combined with some other step in the preparation of the plate. If, for example, the plate is of the type that has a wash coat of hydrophilic substance over an underlying filled hydrophilic coating, the metal salt advantageously may be included as an added ingredient in the said wash coat. If no wash coat is used, the metal salt may be included in the filled coating, though. this procedure may be somewhat less economical in consumption of the metal salt since in this case the latter will permeate the entire thickness of the coating layer rather than being confined at or near the top surface only as when either of the two former procedures is used.

Naturally the metal salt must be chosen with regard to the other constituents of the coating. That is, the metal salt must not be completely consumed by reaction with any ingredient of the coating, or if there is reaction between the metal alt and another ingredientof the coating sufficient of the metal salt must be used so that an excess or residue thereof remains to react with the reagent subsequently applied. Preferably the metal salt remains trapped in the pores of the hydrophilic surface without crystallizing to an appreciable extent on top of the exposed surface. Excessive crystallization of metal salt on top of the surface may make it difllcult or impossible to affix an image securely to the surface. In general such excessive crystallization on top of the surface can be avoided by limiting the quantity of solution of metal salt applied and controlling the conditions under which the material is dried. In a case when excessive metal salt for any reason is left exposed on the surface the undesired excess can be removed by subjecting the surface or plate to rubbing or brushing as disclosed in the pending applications of Frederick H. Frost, Serial Nos. 691,654, filed August 19, 1946 (now abandoned), and 760,939, filed July 14, 1947, which matured into Patent No. 2,534,588, December 19, 1950.

The plates disclosed in pending applications of Stephen V. Worthen, Serial No. 747,138, filed May 9, 1947 (now abandoned), and Serial No. 758,215, filed June 30, 1947 (now abandoned), said plates having a hydrophilic colloid surface containing a divalent metal salt are examples of plates of the type described above, i. e. having a hydrophilic colloid surface comprising a metal salt capable of reacting with a later applied reagent to form a pore filling or clogging relatively water-insoluble metal compound.

The hydrophilic surface containing the metal salt, rubbed or brushed if desired, is then marked with a lithographic imaging material, as by printing, typewriting, pen and ink, wax or resinous crayon, or the like.

After the image is applied the imaged surface is washed over with a reagent which will react with the metal salt already present in the pores of the hydrophilic printing surface to form an insoluble metal compound in the pores. The insoluble compound so deposited leaves the surface wettable by water but, I believe, to a large extent blocks the pores of the surface and prevents or at least greatly diminishes penetration of water or wet-out solution into or through the hydrophilic coated surface. The surface can then be kept wet with a minimum of wet-out solution, and while so wet is very resistant to soiling or scumming by printing ink when used as a plate on a lithographic or offset press.

A preferred class of insoluble metal compounds to form in the surface pores for enhancement of water hold-up and impenetrability comprises the various relativel insoluble metal phosphates. Many of these salts, in spite of their relative insolubility, crystallize with water in the molecule, and perhaps partly for this reason they are very easily wettable by water.

In order to form such desirable relatively insoluble metal phosphates in the surface pores of the unimaged areas of the plate the metal salt included in the hydrophilic surface as previously described may be a soluble salt of any metal which forms a relatively insoluble phosphate. Then after the plate has been imaged the surface is further treated with a solution containing phosphate ions, either simple or complex- 4 By the reaction of the phosphate ion with the soluble metal salt relatively insoluble metal phosphate is formed in situ in the pores of the coating.

The phosphate ion may be supplied by a solution of any soluble phosphate or polyphosphate. Suitable salts include monoand iii-ammonium phosphates, mono-, di-, and tri-sodium phosphates, sodium pyrophosphates, sodium hexametaphospbate, etc. For best results enough phosphate ion should be available to react with substantially all of the soluble metal salt available for reaction in the hydrophilic coated surface.

The application of phosphate ion can very conveniently be made at the time the plate is treated with wet-out solution preliminarily to running the plate on a printing press. In other words, the phosphate ion may be included in the wet-out solution. Since it is well-known that to avoid emulsification of lithographic ink it is advisable to keep all dampening and wet-out solutions non-alkaline, or preferably slightly acid, it is advantageous to use a phosphate which itself gives a non-alkaline solution, such as mono-ammonium phosphate. or to buffer the phosphate solution to yield a pH value of not over 7.0. A pH range of from about 3 to about 7 has been found to be satisfactory.

As has been previously indicated the soluble metallic ion used to react with the phosphate ion to cause precipitation of the metal compound which leaves the hydrophilic surface waterwettable but water-impenetrable may be an ion of an metallic element which readily forms a relatively insoluble phosphate. Operable compounds include soluble salts of practically all metals except the alkali metals, i. e. the divalent and multivalent metals. It has been found that when proteins are present in the hydrophilic coating of the plate the aging qualities thereof may be adversely affected by action of salts of certain metals, such as aluminum, chromic, ferric, manganic salts and the like. In such cases it is preferable to use soluble salts of divalent metals as the soluble material includedin the hydrophilic coating. If the hydrophilic coating is such that its aging properties are not harmed by metal salts other than those of divalent metals, e. g. the trivalent metals, such other salts of course may be used in the coating. Or to avoid all possibility of deleterious effects from aging the hydrophilic coating in contact with a metal salt the procedure may be altered as follows: a soluble phosphate may be included in the hydrophilic coating and then after the plate has been imaged it may be washed with a solution of an appropriate metal salt capable of reacting with the soluble phosphate to form a relatively insoluble compound. An insoluble phosphate will thus be formed in the pores to give substantially the same eifect as the alternate procedure. In this altered procedure the soluble phosphate is substituted for the metal salt and the metal salt is substituted for the soluble phosphate in the procedure described above.

Other insoluble compounds besides metal phosphates may be precipitated in the coating pores for the same purpose, e. g. metal oxalates and borates. Or compounds like barium sulfate may be deposited as by reaction of barium chloride and sodium sulfate, either of which may be used in the washing solution if the other is present in the hydrophilic coating.

The result of the practice of the invention is A auaum.

a, printing plate having a paper or similar base with on at least one side a hydrophilic coating,

the coated surface having securely afllxed there- Example 1 A sheet of paper body-stock having wetstrength properties derived from inclusion of melamine-formaldehyde resin in conventional manner, was coated on one side with about 5 pounds dry weight per'thousand sq. ft. of surface, of a coating containing 100 parts of clay and 18 parts of casein dispersed by ammonia. The coated paper was dried and then washed over with a aqueous solution of zinc acetate. The so-treated sheet was dried, calendered, brushed, and typed on by a typewriter. The typed sheet was swabbed over with a pad of cotton wet with 4% aqueous solution of di-ammonium phosphate and then was used as a plate in an offset duplicator where it proved to be very effective in holding water on its surface and very resistant to scum formation. Zinc phosphate or zinc-ammonium phosphate presumably was formed in the pores of the casein coating.

Example 2 A wet-strength paper body-stock was coated on each side with about 5 pounds dry weight per thousand sq. ft. of a coating containing clay 100 parts, ammonium caseinate 18 parts, and di- 'methylolurea 1 parts, and dried. This baseton pad wet with 3% aqueous solution of mono- Example 3 A clay and casein base-coated sheet like that used in Example 2 was wash-coated on one side with an aqueous solution containing 1 of sodium alginate and 10% of diammonium phosphate.

This was dried and calendered, an image was fixed on the treated side, and the imaged sheet was swabbed with a 3% aqueous solution of zinc formate. Zinc-ammonium phosphate was prekept its unimaged areas satisfactorily free from scum during its useful life.

Example 4 The clay-casein base-coated sheet of Example 2 was treated with an aqueous solution of 2% 'of gum arabic and 10% of calcium acetate, dried,

calendered, brushed, imaged and then swabbed with 4% ammonium oxalate solution to form insoluble calcium oxalate and used as a printing plate with satisfactory results.

sumably formed. when used as a plate this sheet 6 Example 5 The clay-casein base-coated sheet of Example 2 was treated with an. aqueous solution containing 0.5% of carboxymethyl cellulose and 10% of ammonium sulfate, dried, calendered, brushed, imaged, swabbed with 3% solution of barium chloride to form insoluble barium sulfate and used as a printing plate with satisfactory results.

Example 6 Example 2 was repeated but a 4% solution of ammonium borate was substituted for the ammonium phosphate. Insoluble zinc borate presumably was formed. The plate gave satisfactory printing results.

Example 7 Example 2 was repeated but the ammonium Example 8' The clay-casein coated base of Example 2 was washed with an aqueous mixture containing 0.2% of guar gum and 12% of lead acetate, dried, calendered, brushed, imaged, swabbed with 5% solution of ammonium chromate to form insoluble lead chromate and the resulting plate printed with satisfactory results.

In practice of the invention as illustrated in the foregoing examples I have found it to be preferably generallyto first incorporate the soluble metal salt in the hydrophilic paper coating and to then, after imaging apply the reagent which is to react with the metal salt. My extensive experience indicates that zinc acetate and zinc formate are generally preferable to the metal salts of strong mineral acids. My present experience indicates that mono-ammonium phosphate is the preferred reagent and that a 3% solution thereof is preferred although stronger solutions thereof up to 10% appear to give identical results. A 2% solution does not appear to be quite as effective when applied in a thin coating with a cotton swab as a 3% solution. I have found it to be preferable generally to employ a final treating solution, whether the metal salt solution or the solution of a compound the anion of which is capable of forming an insoluble compound with the metal which is neither alkaline nor too strongly acid but has a pH value within the range from 3 to 7.

The invention has been described and illustrated above with reference to its preferred application for improving the resistance of the un-.

imaged portion of the plate to penetration by wetout liquid but the described treatment of the plate may be useful if the imaging step is made to follow the treatment of the plate with the metal salt and the reagent. Under certain circumstances an image applied subsequently may be suiliciently permanent and the unimaged plates are also useful for the production of photolithographic plates by a subsequent application of a photo-sensitive coating and for other purposes.

I claim:

1. Process for the production of a planographic printing plate having a hydrophilic printing surface which comprises incorporating into said surface prior to imaging a water-soluble compound capable of reacting with a water-soluble salt of a metal having a valence greater than 1 to deposit a water-insoluble compound of said metal.

2. Process as defined in claim 1 in which the water-soluble compound is a member of the group consisting of phosphates, oxalates, sulfates, b0- rates, citrates and chromates.

3. Process as defined in claim 1 in which the surface is imaged and a solution of a water-soluble salt of a metal having a valence greater than 1 and capable of reacting with the water-soluble compound to form a water-insoluble compound of said metal is applied to the imaged surface.

4. A planographic printing plate having a hydrophilic colloid printing surface which, prior to imaging, contains a water-soluble compound capable of reacting with a water-soluble salt of a metal having a valence greater than 1 to form 1;

an insoluble compound of said metal.

5. An imaged planographic printing plate having a hydrophilic colloid printing surface, said 8 surface containing in the imaged areas thereof only a water-soluble compound capable of react- .ing with a water-soluble salt of a metal having a valence greater than 1 to form an insoluble compound of said metal, andin the unimaged areas said insoluble compound of said metal.

FREDERICK H. FROST.

REFERENCES CITED The following references are of record the tile 0! this patent:

UNITED STATES PATENTS Number Name Date 2,003,268 Wescott May 28, 1935 2,154,219 Shepherd Apr. 11, 1939 2,373,287 Bassist. Apr. 10, 1945