US3275441A - Printing plate and method of making same - Google Patents

Description

Sept. 27, 1966 STEP (e) B. COHN 3,275,441

PRINTING PLATE AND METHOD OF MAKING SAME Filed March '7, 1963 STEP (0) 1 PAL l MOLYBDATE TREATMENT STEP(b) 3 BEFORE REDUCTlON MOLYBDATE TREATMENT AFTER REDUCTION PHOTOSENSITIVE LAYER STEP(C) 5 BERNARD COHN I-NVENTOR.

AGENT T United States Patent 3,275,441 PRINTING PLATE AND METHOD OF MAKING SAME Bernard Cohn, Elmsford, N.Y., assignor to Technilrth Inc., Mount Vernon, N.Y., a corporation of New York Filed Mar. 7, 1963, Ser. No. 263,479

8.Claims. .(Cl. 96 -75) The present invention relates to planograph'ic printing plates and like lithographic printing surfaces as well as to a method of producing such plates.

In the lithographic printing art, it is known to provide a metallic substrate (e.g. an aluminum plate or aluminumfoil support with a hydrophilic layer) and thereafter to coat this layer with a photosensitive material. After exposure to light, nonsensitized portions of this layer may then be rubbed oif or otherwise removed to prepare the plate for use upon a lithographic press. Prior to. this, however, an image developer may be employed to render visible the image formed during sensitization. I

Many techniques have been proposed heretofore for the preparation of such plates but none have met with much success. is the plate proposed by Jewett et al. (US. Patent No. 2,714,066) which makes use of a silicate layer as the hydrophilic coating. While this plate represents a vast improvement over earlier printing layers, it was nevertheless characterized by a relatively short press life and a shorter shelf life than may be desirable. The problem of shelf life is of particular importance since modern lithographic processes make use, to a large degree, of sO-called presensitized plates, one form of which can be produced in accordance with commonly assigned copending application Ser. No. 183,817, filed March 30, 1962. In such presensitized plates, the hydrophilic surface of the substrate is provided with the photosensitive material and stored after drying but prior to exposure. In the latter application there is disclosed a system for making use of essentially nonmetallic substrates and a method of providing such substrates with a suitable hydrophilic layer. As previously mentioned, however, it is desirable to attain longer shelf life and press life with metal-based lithographic plates as well.

It is another object of the present invention to provide an improved lithographic printing plate of this nature.

These objects are attained, in accordance with the invention, by a method which comprises the steps of treating a metal substrate and preferably one composed of an amphoteric metal, especially aluminum, with an acidified solution of a molybdate salt to produce a hydrophilic coating thereof. -It has, however, been discovered that the mere use of a molybdate treatment does not permit extended shelf life although it does provide a suitable hydrophilic printing surface. It has been found, surprisingly, that both the shelf life and the press life of a molybdate-treated aluminum plate can be improved manyfold if, after the molybdate treatment, the plate is exposed to a reducing agent capable of reducing molybdate ion so that excessive quantities thereof are removed from the plate surface. The latter is presumably penetrated by the molybdate solution which forms bonds therewith with the excess being substantially in a non-bonded state and possibly available for affecting the photosensitive layer deposited upon the hydrophilic surface. This layer is, advantageously, formed by polymeric condensation of a diazodiphenylaminosulfate and paraformaldehyde. Other substances of the general type disclosed in US. Patent No. 2,063,631 to Schmidt and Zahn (Example I) are also suitable. It has been found that best results are obtained when the molybdate solution is derived from an alkali metal molybdate salt and preferably sodium molybdate.

An exception insofar as shelf life is concerned 3,275,441 Patented Sept. 27, 1966 The acid employed in conjunction with the sodium molybdate may be any of the protonic acids although, in some cases, Lewis acids (eg; aluminum chloride) are suitable in addition to carboxylic acids. The acid, when of the protonic variety, should contain a large anion having a central atom from Groups 5 or 6 of the Periodic Table; thus arsenious and phosphoric acids are suitable along with sodium biphosphate and sodium bisulfite.

The above and other objects, features and advantages will become more readily apparent from the following description and examples, reference being made to the appended drawing whose sole figure represents a flow diagram in cross-sectional views illustrating the successive steps a-e of manufacturing a printing plate in accordance with the present invention, some of the layers being exaggera-ted in thickness.

In the drawing, there is shown an aluminum substrate 2 whose surface 1 is to be rendered hydrophilic. The substrate can be a sheet of aluminum or an aluminum foil having a base of other material although an aluminum coating on a support will also sufiice for the present purposes. As indicated at step b the surface 1 is treated with an acidic molybdate solution to provide a hydrophilic layer 3 which, at the interface 4 between this layer and the substrate, apparently bonds to the aluminum. Excess or nonbonded molybdate is removed by treatment with a reducing agent to yield a surface zone 3 whose molybdate content renders the surface 5 highly hydrophilic without substantial long-term effect upon a presensitizing di-azo layer 6 (step d). This latter photosensitive layer can then be exposed to actinic light through a stencil 'or negative whereupon sponging removes the noninsolubilized portions of the layer to produce channels or grooves 7 while the insolubilized portions form lands 8. 'The grooves 7 extend to the hydrophilic surface 5 which is receptive to water and thus repels the lithographink when moistened while the lands 8' are hydropholic and inkreceptive.

Example I A plate is produced in the aforementioned manner by immersing a sheet of aluminum in a cleaning or pick-ling bath such as hot trisodium phosphate until all surface Oils and grease films have been removed. Satisfactory cleaning is attained at a temperature of about C. with a 20% solution of trisodium phosphate after a period of about 5 minutes. The sheet is then rinsed in water and immersed in concentrated nitric acid to eliminate any scum which may have been formed during the cleaning step. In the absence of such a scum, the nitric-acid scouring can be eliminated. Y e

The rinsed plate is then treated by immersion with an aqueous solution containing about 0.1 weight percent sodium molybdate and 0.2 volume percent phosphoric acid at a temperature of about 60 C. fora period of 2 minutes. The hydrophilic surface is then rinsed in water and dried and is found to have a coating of the type illustrated in step b with some of the molybdate at the surface beipg nonbonded to the aluminum but adhering to that portion of the coating which is so bonded. A photosensitive layer consisting of a condensation polymer of p-diazodiphenylamine-sulfate and formaldehyde can then be deposited upon the hydrophilic coating. While the resulting plate is found to have .a relatively long press life, its shelf life is somewhat diminished apparently by reason of the excess molybdate.

The photosensitive compound can be produced in the manner disclosed in the Jewett et al. patent, e.g. by mixing 34 parts of the sulfate salt of p-diazodiphenylamine with 3.25 parts of paraformaldehyde in the presence of 4.5 parts of anhydrous zinc chloride. The mixture is introduced into sulfuric acid at a temperature below 6 C. with the resulting solution poured on ice to, produce a tarry material which is dissolved in water, treated with zinc chloride and precipitated. A water solution of the yellow precipitate forms the light-sensitive coating. The resulting plate has a press life which is greater than that of a silicate-treated plate.

Example ll The method of Example I is followed with the exception that, after the treatment with molybdate solution and prior to coating with the light-sensitive compound, the hydrophilic surface is subjected to the action of a reducing agent. The reducing agent can be an aqueous solution, containing weight percent thiosulfate, the.

treatment time being about 1 minute at a temperature of 60 C. When the plate is then rinsed, dried and coated 4 t and 5 weight percent sodium:bisulfite, the, latter acting as a protonic acid.

Example XI The method of Example 11 is followed except that the molybdate bath contains 1% by weight sodium molybdate and 3% by volume'arsenious acid.

In all thesecases long-wearing lithographici plates are produced whose shelf life in a presensitizedstate far exceeds that possible with earlier treatments.

I claim:

1. A method of making lithographic printing plates,"

comprising the steps of reacting an acidic, molybdate solution with a surface of an aluminum substrate to with the diazo polymer, it has a shelf life much greater than thatof the plate of Example I. Comparative tests indicated that the plate produced according to the instant example had a press life almost four times as great as the silicate-treated plate. The press life of the plates whose preparation is described hereunder is of a similar order of magnitude.

Example III The procedure of Example II is followed except that the molybdate solution is comprised of 1 weight percent sodium molybdate and 10 weight percent sodium bisulfate which acts as a protonic acid.

Example IV The processof Example IIiis followed with the exception thatthe molybdate bath thereof is replaced by an aqueous solution containing 0.1% by weight sodium molybdate and 1 'volume percent acetic acid. The molybdate treatment is carried out for 2 minutes at 80 C.

Example. V,

The method of Example H is followed with the exception that the molybdate bath contains 0.5 weight percent sodium molybdate and 5% by weight aluminum chloride, the latter acting as a Lewis acid.

Example VI The steps of Example H are followed withzthe exception that the reducing bath is replaced by an aqueous solutioncontaining 1% by weight ferrous sulfate at a temperature of 60 C., the treatmenttime being 1 minute.

Example VII Themethod of Example VI is followed with the molybdate bath beingoomposed of an aqueous solution containing 0.1% by weight sodium molybdate and 1% by volume nitric acid.

Example VIII The steps of Example II are followed with, however, the sodium molybdate replaced by an equal amount of ammonium molybdate.

Example IX The method of Example II is followed except thatthe molybdate solution is replaced by an 85% molybdic acid solution.

' Example X The method of Example H is followed with the molyb-' date bath containing 5% by weight sodium molybdate render said surface hydrophilic; thereafter; treating said surface with an aqueous solution of a reducing agent selected'from the group consisting of thiosulfate and ferrous salts; and coating :the hydrophilic surface 50 treated with a layer of a photosensitive polymeric diazo substance.

2., A method as defined in claim 1 wherein said molyb-- date solution includes a molybdate saltand an acid selected from the group which consists of sodium bisulfate,

7 sodium biphosphate, phosphoric acid, nitric acid, ar-- senious acid, acetic acid and aluminum chloride.

3. A method as defined in claim 2 wherein said molybdate salt is selected from the group whichconsists of sodium molybdate and ammonium molybdate.

4; A method as defined in claim 1 wherein said sol-u-- tion is an aqueous solution of molybdic acid.

5.v A method as defined in claim 1 wherein said reducing agent is an aqueous solution of: a salt" selected from the group which consists of ferrous sulfateand 8; A presensitized lithographic printing plate compris-- ing an aluminum substrate havinga surface initially treated .with an acidic molybdate solutionan-d thereafter with an aqueous reducing agentselected from the, group consisting of thiosulfate and ferroussalts, and a layer of a photosensitive polymeric diazo compound overlying said surface, said compound being a condensation polymer of diazodiphenylaminesulfate and paraformaldehyde.-

References Cited by theExaminer UNITED STATES PATENTS Strecker ,1486.15

597,366 1/ 1898 2,816,051 12/1957 Hal-ford 1486.14 2,899,368 8/ 1959 Spooner 204-58: 3,030,210 4/1962 Chebiniak 9633 X 3,052,575 9/1962 Crane 148-614 3,085,008 4/1963 Case. 96- 33 X 3,201,247 8/1965 Leonard 9633 FOREIGN PATENTS 5 815,471 6/1959 Great Britain.

NORMAN G. TORCHIN, Primary Examiner.

I. TRAVIS BROWN, Examiner. RQL. STONE, A. D. RICCI, .Assistant Examiners.

Claims (1)

1. 8. A PRESENTIZED LITHOGRAPHIC PRINTING PLATE COMPRISING AN ALUMINUM SUBSTRATE HAVING A SURFACE INITIALLY TREATED WITH AN ACIDIC MOLYBDATE SOLUTION AND THEREAFTER WITH AN AQUEOUS REDUCING AGENT SELECTED FROM THE GROUP CONSISTING OF THIOSULFATE AND FERROUS SALTS, AND A LAYER OF A PHOTOSENSITIVE POLYMERIC DIAZO COMPOUND OVERLYING SAID SURFACE, SAID COMPOUND BEING A CONDENSATION POLYMER OF DIAZODIPHENYLAMINESULFATE AND PARAFORMALDEHYDE.

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