US3247791A - Surface treated lithographic plates and production thereof - Google Patents

Description

United States Patent 3,247,791 SURFACE TREATED HTHOGRAPHEC PLATES AND PRODUCTION THEREUF Robert F. Leonard, East Rockaway, N.Y., assignor to Litho Chemical and Supply (In. line, Lynhrook, N.Y., a corporation of New York g No Drawing. Original application May 6, 1960, Ser. No. 27,240. Divided and this application May 4, 1962,

Ser. No. 192,329 (Cl. 1i)1149.2)

dustry for printing plates have recognized disadvantages.

One of the serious disadvantages is that they become oxidized and hence the chemical properties of their surfaces continually undergo change. They thus present a storage problem due to oxidation and change undernormal storage conditions. Another disadvantage of known plates is that they require counteretching prior to the application of the light-sensitive coating. Where albumin or equivalent sensitizer is left on non-image areas, scrumming occurs as these areas tend to pick up ink during use of the plates and such in most undesirable as defective prints results despite the use of desensitizers for removing undeveloped sensitizer from the non-image areas. Such plates have a distinct tendency to accumulate sensitizer scrum or to tone up during use. Conventional plates are also subject to corrosion during prolonged periods of storage. Prior plates, other than silicated plates, which have been presensitized are apt to deteriorate during storage so that they are not fit for use merely upon exposure to light through a negative or stencil and then washing away the unexposed light-sensitive material.

Such plates, in particular, especially if they have undergone change or deterioration do not receive and hold well the light-sensitive diazo resin or other light-sensitive coating or maintain substantially constant permanency of condition. Prior plates are not characterized by long press life or freedom from halation. These and other disadvantages are overcome by the present invention.

It has now been found that by treating metal sheets with phospho-molybdate solutions and subsequently subjecting the thus-treated sheets to scaling operations, there results a conversion coating on the surfaces of the metal sheets and the so-obtained phospho-molybdate treated metal sheets combine the desirable physical properties of the original sheets with the below-stated and other advantages of a phospho-molybdate surface. The sheets so produced are ideally suited for printing plates.

A phospho-molybdate treated metal sheet responding to this invention as compared with conventional aluminum and zinc sheets now used in the lithographic industry for printing plates has a number of important advantages and considerably greater versatility.

A phospho-molybdate treated plate does not oxidize under normal storage conditions. The chemical properies of the surfaces are therefore substantially constant and the storage problem is eliminated. In contrast, conventional zinc and aluminum sheets do oxidize and hence continually change with respect to the chemical properties of their surfaces and they thus present a storage problem.

A phospho-molybdate treated sheet has a permanent, durable, hydrophilic surface. A printing plate which does not have a hydrophilic surface in the non-printing 3,247,791 Patented Apr. 26, 1966 areas has a tendency to scum because the non-image areas of the printing plate becomes ink-respective. The phospho-molybdate treated plate with a hydrophilic image greatly reduces or eliminates scurnming.

A phospho-molybdate treated sheet does not require counteretching prior to the application of the light-sensitive coating to the sheet whereas counteretching is standard procedure in the processing of zinc and aluminum plates of conventional character. Counteretching involves scrubbing the plates with an acidic solution to remove loose soil and oxide neither of which is present on the surface of a phospho-molybdate treated plate or sheet.

The phospho-molybdate treated plate has the further advantages over a conventional lithographic plate in that all traces of casein, albumin, etc. can be positively removed from the plate in the first instance without dependence upon any separate chemical desensitizing procedure.

The phospho-molybdate treated plate is characterized by being utilizable for all three types of lithographic processes, namely the Deep Etch Process, the Surface Process and the Wipe-On Process. In the Deep Etch Process, 21 bichromated-gum arabic solution is dried onto the plate surface, exposed through a photographic positive to a light source, developed with an acidic aqueous salt solution, thereby washing away unexposed sensitizer,

etched with an acidic ferric chloride solution, optionally plated with electroless copper, lacquered and inked. The light hardened stencil is then washed off the plate. The areas of the plate which contained the stencil are now the non-printing areas while the area which have been developed, etched, copperized, lacquered and inked are the printing areas. I In the Surface Process, a bieromated casein solution is dried onto the plate surface, exposed through a photographic negative to a light source, coated with a layer of lacquer and then ink, and developed with a slightly ammoniacal solution. The areas of the plate which contained the unexposed, developed (washed away) sensitizer become the non-printing areas while the lightexposed, lacquered and inked areas are the printing areas. In the Wipe-On Process, a diazo resin solution or a diazo resin combined with a colloid solution is hand coated onto the plate surface, the plate is next exposed through a photographic negative, coated with a layer of ink and developed with an acidic gum arabic solution or the exposed plate may be lacquered and developed in one operation with the use of a lacquer emulsion developer. The unexposed areas of the plate which were developed and washed away become the non-printing areas of the plate while the exposed areas of the plate which were lacquered or inked become the printing areas of the plate.

While conventional zinc and aluminum plates can be used in the Deep Etch and Surface Processes, they have a tendency to scum and must first be counteretched.

Light-sensitive diazo resins and similar light-sensitive materials are notably sensitive to metals and thus a conventional zinc or aluminum plate cannot be used for wipeon platemaking, whereas a phospho-molybdate treated plate is ideally suited for the Wipe-On Process,

A silicated plate can be used for wipe-on platemaking, but cannot be penetrated with the etches commonly used in deep etch platemaking whereas a phospho-molybdate treated plate is suitable for deep etch platemaking since it can be etched with conventional deep etch etches.

A lithographic plant could, if so desired, use and store a single type of lithographic printing plate for all three lithographic platemaking processes referred to above by using a phospho-molybdate treated zinc or aluminum plate according to the present invention which, however, also includes the use of phospho-molybdate treated copper sheets or plates.

The phospho-molybdate treated plate can be presensitized with light-sensitive diazo resins and other like lightsensitive organic materials and can then be stored for periods up to several months without loss of photographic sensitivity.

Phospho-molybdate treated plates according to this invention have been produced with two different types of surface roughness, namely smooth surface" plates produced by chemical etching and grained plates produced by metchanical surface treatment. On a theoretical basis, the smoother the surface of a lithographic plate the greater is the resolution of halftone dots; on the other hand, the greater the surface roughness of the plate the greater is the capacity of the plate to carry ink and water. There are, consequently, two types of plates in general use, the smooth surface plate exemplified by the chemically etched plate and the grained plate exemplified by the mechanically surfaced plate.

This invention is applicable both to smooth surface" plates and to grained plates as illustrated by the following non-limitative examples:

EXAMPLE I Production of a phospho-molybdate treated sheet with a chemically etched surface A x x 0.006 sheet of Alcoa 28 aluminum was immersed for 2 minutes at 140 F. in a combination cleaner and etch prepared by mixing the following ingredients in the order listed:

Water ml 3785 Sodium phosphate tribasic grams 28.35 Sodium hydroxide do 141.75 Wetting agent (Tergitol non-ionic NPX) do .38

(Alkyl phenyl ether of polyethylene glycol) sold by Union Carbide The sheet was rinsed thoroughly with deionized water at 68 F., then immersed in a desumitting bath prepared as follows:

rnl. Nitric acid (70%) 1500 Hydrofiuoric acid (5255%) 500 at room temperature for 30 seconds. The slightly grained aluminum was thoroughly rinsed with deionized water at 68 F. before being immersed in the phospho-molybdate bath at 165 F. for 2 minutes. The bath was prepared by mixing the fol owing in the order listed:

Water rnl 7000 Molybdic acid grams 175.2 Sodium phosphate tribasic do 34.9

After rinsing thoroughly with deionized water at 68 F., the treated aluminum was immersed in a sealing bath for 2 minutes at 210 F. The bath is composed of the following:

Water ml 9900 Sodium acetate grarns 100 The sheet was thoroughly rinsed with deionized water at 68 F. and finally forced air dried.

EXAMPLE 11 Production of a phospho-molybdate treated sheet with a mechanical grain A x 22" x 0.12" sheet of Alcoa 25 aluminum was cleaned and degreased by immersion at 68 F. for 15 minutes in the following:

Water liters 4.8 Ethylene glycol rnonoethyl ether do 3.2 Sodium carbonate grams 86 Sodium phosphate tribasic do 28.4

The sheet was rinsed with water at 65 F. for 2 minutes before being grained. The graining consisted of passing the sheet in a horizontal plane under a set of revolving brass brushes which are located on the perimeter of a rotating circular support. During this operation, the plate is flushed with a slurry of water and pumice powder. The aluminum sheet was rinsed thoroughly with water at 65 F. and then immersed in the above water, ethylene glycol monoethyl ether, sodium carbonate, sodium phosphate tribasic cleaner for 5 minutes at 68 F. The plate was then rinsed thoroughly with water at 65 F. and then immersed in the phospho-molybdate solution at 160 F. for 2 minutes. The bath consisted of the following:

Water "gallons" 30 Molybdic acid grams 2480 Sodiurnphosphate tribasic do 566 The sheet was rinsed for 2 minutes with tap water-at 65 F. before being immersed in the sealing bath at 210 F. for 2 minutes. Sealing bath consisted of:

Water gallons 30 Sodium acetate grarns 1225 A The sheet was rinsed with water at 65 F. for 2 minutes,

then forced air dried. Finally, the sheet was passed at a distance of six inches over a series of lighted gas burners for 4-0 seconds to completely seal the phospho-molybdate surface.

The molybdic acid referred to in Examples I and II is molybdic acid which is a commercially known product composed of ammonium paramolybdate with added molybdic oxide. The molybdic oxide amounts to 85% of the weight of the product which produces molybdic oxide when the product is dissolved in water. The molybdic oxide combines with phosphate ions present in the treating bath to form phosphomolybdate ions in solution. The sodium and ammonium ions present in the phosphomolybdate solution, resulting from the combining of the molybdic acid 85% and the tribasic sodium phosphate, remain as free ions as long as they are in solution. The phosphomolybdate ions can, however, be produced in other ways without adversely affecting the working characteristics of the treatment bath. Either of the following baths A or B can be used in place of those of Examples I and II, viz.:

Water ml 1000 Sodium phosphomolybdate (Na PO -l2MoO grarns 20 Sodium hydroxide do 5 Bath A was used on a chemically grained aluminum plate at to F. for 2 minutes. The remainder of the treatment was the same as Example I.

Water ml 1000 Ammonium molybdate [(NH4)6MO7O24'4H2O] "grams" 50 Phosphoric acid (85%) ml 1.25 Sulfuric acid (98%) ml 1.00

Bath B was used on a chemically grained aluminum plate at F. for 2 minutes. The remainder of the treatment was the same as Example I.

It has been found that, in regard to treating metallic sheets, the best phosphomolybdate solutions are those which contain 12 to 16 moles of molybdic oxide to 1 mole of phosphate. It appears, therefore, that either phospho- 12-molybdate ions or phospho-l8-molybdate ions, or a combination of both, are formed in solution and ultimate- 1y react with the aluminum, zinc or copper to form the corresponding metallic phosphomolybdate.

It has further been found that the above reaction is best promoted, Without attacking the metal sheet, at a pH in the range of 4- to 6.

weight) aqueous solution of sodium acetate.

It is further to be understood that the concentration of solids in the phosphomolybdate bath of Examples I and II can be varied materially as long as the molybdic oxidephosphate ion molar ratio is maintained constant. The bath can thus range in its components as follows:

Water rnl 1000 Molybdic acid, 85% "grams" 9.1 to 37.7 Sodium phosphate tribasic do 1.8 to 7.5

If the molybdic acid 85% and sodium phosphate tribasic concentrations exceed 37.7 and 7.5 grams, respectively, the phosphomolybdate bath will soon form a precipitate and become unusable; if, on the other hand, the molybdic acid 85% and sodium phosphate tribasic concentrations are below 9.1 and 1.8 grams, respectively, the metal sheet will require longer than 2 minutes at 155 to 180 F. and

hence the operation becomes uneconomical.

The optimum temperature range for use of the phosphornolybdate solutions is between 155 F. and 180 F. At a temperature below 155 F. the reaction is slowed down appreciably and the process becomes uneconomical and when the temperature is above 180 F. the bath gradually forms a precipitate and has only a limited storage life. a

The following phosphates have been successfully used in varying proportions to supply the required phosphate ions in the treatment bath:

Phosphoric acid (85%) Ammonium phosphate, monobasic Sodium phosphate, dibasic Sodium phosphate, tribasic No phosphate ion source has been found which did not operate satisfactorily as long as the pH of the resulting phosphomolybdate solution was adjusted to pH 4 to 6.

Molybdic acid 85 and ammonium molybdate are suitable sources of molybdic oxide and produce satisfactory results when combined with the proper amount of phosphate ion as long as the pH is in the range of 4 to 6. They are equally as good as the sodium phosphomolybdate mentioned above.

The sealing bath described above consists of a 1% (by Tests showed no advantage in using a sealing bath of concentration and hence a 1% solution is preferred. The sealing bath, is, however, not limited to a 1% solution of sodium acetate as any of the following chemicals can also be used in a 1% by weight concentration in water at 200 to 212 F. to form a satisfactory sealing bath: dibasic sodium phosphate, potassium acetate, magnesium acetate, barium acetate, calcium acetate, dibasic ammonium carbonate, barium carbonate, calcium carbonate, lithium carbonate, magnesium carbonate, zinc carbonate, ammonium hydroxide and urea.

The following chemicals when used in a 1% concentration at 200 to 212 F. did not produce a satisfactory sealing bath:

Ammonium phosphate, monobasic Sodium phosphate, monobasic Sodium. phosphate, tribasic Potassium phosphate, monobasic Magnesium phosphate, dibasic Ammonium acetate Zinc acetate Sodium carbonate, dibasic Potassium carbonate, dibasic Potassium oxalate Ammonium oxalate Sodium citrate Potassium citrate Ammonium citrate Calcium citrate Magnesium citrate Sodium nitrate Potassium nitrate Ammonium nitrate Calcium nitrate Magnesium nitrate Zinc nitrate Aluminum nitrate Barium nitrate Sodium chloride Potassium chloride Ammonium chloride Lithium chloride Calcium chloride Zinc chloride Barium chloride Sodium sulfate, dibasic Potassium sulfate, dibasic Calcium sulfate Zinc sulfate Magnesium sulfate From phospho-molybdate treated aluminum plates prepared according to Example I, there have been produced nylon-diazo presensitized plates in which the nylon-diazo sensitizer corresponds to the sensitizer described in US. Patent No. 2,826,501. The nylon-diazo sensitizer in conjunction with the phospho-molybdate treated aluminum plate produces a markedly superior plate as compared to the use of the same nylon-diazo sensitizer in conjunction with a silicated aluminum plate. The plate produced by the nylon-diazo sensitizer on the silicated surface is photographically too fast for lithographic purposes whereas the same sensitizer on a phospho-molybdate surface is ideal for lithographic purposes.

Phospho-molybdate treated aluminum plates prepared according to Example II have been very satisfactorily used to produce deep etch plates, surface plates and wipeon diazo plates.

As pointed out above, plates or sheets of zinc, aluminum and copper can all be beneficially phospho-molybdate treated and all are suitable for lithographic printing plates. i

The presensitized plates referred to above are prepared by making a chemically grained phosphomolybdate treated plate as already described, applying thereto the nylon-diazo resin solution of US. Patent No. 2,826,501 by wiping the same on by hand or by means of a roller and then air drying the coating thus produced. The pre-' sensitized plate is then processed by exposing the coating to a light source through a stencil or negative, developing or washing away the unexposed areas with the developer of US. Patent No. 2,826,501, applying a lacquer like that of US. Patent Nos. 2,754,279 or 2,865,873, and rinsing with water.

The nylon-diazo resin solution of Patent No. 2,826,501 is composed of the following constituents in approximately the following amounts, by weight:

0.11% of the water soluble condensation product of pdiazo diphenylamine and formaldehyde,

0.94% of soluble superpolyamide nylon resin,

0.01% of a non-ionic surface active agent 17.15% of water,

76.35% of denatured ethyl alcohol, and

5.44% of furfuryl alcohol.

The developer of Patent No. 2,826,501 is composed of the following constituents:

Citric acid grams 1.2 N,N-dimethylformamide milliliters 124.5 Furfuryl alcohol do 55.2 Methanol do 375.0

In a typical application of the deep-etch process, a mechanically grained plate produced as in Example II is centrifugally coated with a bicromated gum arabic solution according to the recommendations of the Lithographic Technical Foundation publication No. 806. Such solution contains water, gum arabic, ammonium bichromate, a wetting agent (surfactant), a blue dye and ammonium hydroxide. The basic coating solution contains 2840 ml. of 14 Baum gum arabic solution, 950 ml. of-

ammonium bichromate stock solution (758 grams of photo grade ammonium bichromate in enough water to make 1 gallon of 14.2 Baum at 77 F.), and 140 ml. of ammonium hydroxide (28% NH to which the other ingredients are added. The coating solution has a pH value of 8.8 to 9.0 and tests between 14.0 Baum at 77 F. Whirling is continued until the coating dries and the coated plate is then exposed to a light source through a suitable positive. The unexposed areas are developed or washed away with an aqueous acidic salt solution of calcium chloride in water to which lactic acid has been added according to the recommendations of Lithographic Technical Foundation publication No. 806, e. g.:

Zinc chloride (technical) grs 680 Calcium chloride (commercial) do 1360 Water -2 ml 1890 Lactic acid (85%) do 340 The bared metal is etched in acidic ferric chloride solution according to the recommendations of the above publication, e.g.:

Calcium chloride solution (40-41 B) ml 2630 Zinc chloride (technical) grs 1000 Iron perchloride solution (505l B) ml 750 Hydrochloric acid (3738.5%) ml 37 Cupric chloride grs 70 The plate is next washed with anhydrous alcohol to remove salts and water and a lacquer film applied by hand to the thus treated plate and allowed to dry, following which a developing ink is applied by hand and allowed to dry. The plate is now soaked in warm water which penetrates to the exposed coating leaving the plate with inked image areas. The non-printing areas are hydrophilic due to the previous treatment of the plate.

In a typical application of the wipe-on process, using diazo resin solution, a mechanically grained plate produccd as in Example II is wiped on with a pool of diazo resin solution and smoothed down with a soft-non-abrasive applicator and allowed to air dry. The diazo resin solution is a 1 to 5%, by weight, aqueous solution of the condensation product of p-diazo diphenylamine and formaldehyde. The plate is next exposed to actinic light rays through a photographic negative and is finished in either of the following ways:

A. A developing ink of known composition is applied in the same manner as the diazo resin solution and developed with a developer such as one composed of water, gum arabic and phosphoric acid. The developer penetrates through the ink and removes, by dissolving, the unexposed diazo sensitizer coating. The image areas are exposed water-insolubilized diazo resin covered by greasy ink and the non-printing areas are hydrophilic phosphomolybdated surfaced.

B. A lacquer emulsion the same as or similar to that of United States Patent No. 2,865,873 is applied. The image areas are exposed water-insolubilized diazo resin covered by the lacquer composition and the background or non-printing areas are hydrophilic phosphomolybdated treated surfaces.

The surface process is carried out according to the recommendations of the Lithographic Technical Foundation publication No. 807, the contents of which are hereby made a part hereof. The plates are termed surface plates and are plates which have been exposed through negatives and on which the exposed coating serves as a base for the ink-receptive image. In general, an aluminum sheet is grained in known manner and cleaned with a counteretch such as acetic acid in water. A light-sensitive coating is applied, with or without (usually without) a prior pre-etch with the same plate etch used for desensitizing. The coating solution is a mixture of a solution of ammonium bichromate and a solution of a colloid such as albumin, casein, gum arabic, gelatin or cellulose gum. The coated plate is exposed to light through a negative to form the image or printing areas. Light passing through the clear portions of the negative hardens or tans the coating under these areas. Any suitable available commercial lacquer is applied to the exposed surface plate and serves as protection for the image areas. A developing ink is then applied to the expoesd plate to place a greasyink-receptive layer on the image areas and the plate is developed to remove unexposed coating from the non-image areas. The usual finishing operations are then carried out, all as described in the said publication #807.

What is claimed is:

1. A method of preparing a sealed phospho-molybdate treated lithographic plate which comprises reacting a cleaned metal plate selected from the group consisting of zinc, copper and aluminum plates in an aqueous bath consisting essentially of phospho-molybdate ions in solution to form on the plate surfaces the phospho-molybdate of the plate metal and then immersing the phosphomolybdated plate in an aqueous sodium acetate solution.

2. A method of preparing a phospho-molybdate treated lithographic plate which comprises immersing a cleaned metal plate selected from the group consisting of zinc, copper and aluminum plates in an aqueous bath consisting essentially of phospho-molybdate ions to form on the plate surfaces the phospho-molybdate of the plate metal and then immersing the phospho-molybdate coated plate thus formed in a 1% sodium acetate solution at a temperature of 200 to 212 F. followed by drying.

3. A phosphomolybdate treated plate utilizable for deep etch, surface and wipe-on lithographic processes, said plate being constituted of a metal selected from the group consisting of zinc, aluminum and copper the surfaces of which are composed of the corresponding zinc phosphomolybdate, aluminum phosphomolybdate and copper phosphomolybdate provided with a sealing coating of sodium acetate.

References Cited by the Examiner UNITED STATES PATENTS 597,366 1/1898 Strecker 1486.15 1,997,550 4/1935 OLeary 1486.15 X 2,008,733 7/1935 Tosterud 148-614 2,234,206 3/1941 Thompson 1486.5 2,328,540 9/1943 Hochwalt 1486.15 2,333,206 11/1943 Sloan 148-6.15 X 2,403,426 7/ 1946 Douty et al. 1486.15 2,502,441 4/1950 Dodd et al 148--6.15 2,557,509 6/1951 Miller 148-6.15 X 3,030,210 4/1962 Chebiniak 1486.15 3,060,066 10/1962 Ross et al 148-615 3,071,494 1/1963 Humphreys 1486.1 X

WILLIAM D. MARTIN, Primary Examiner,

Claims (1)

1. 3. A PHOSPHOMOLYBDATE TREATED PLATE UTILIZABLE FOR DEEP ETCH, SURFACE AND WIPE-ON LITHOGRAPHIC PROCESSES, SAID PLATE BEING CONSTITUTED OF A METAL SELECTED FROM THE GROUP CONSISTING OF ZINC, ALUMINUM AND COPPER THE SURFACES OF WHICH ARE COMPOSED OF THE CORRESPONDING ZINC PHOSPHOMOLYBDATE, ALUMINUM PHOSPHOMOLYBDATE AND COPPER PHOSPHOMOLYBDATE PROVIDED WITH A SEALING COATING OF SODIUM ACETATE.