US3508924A - Lithographic plate and method of making same - Google Patents

Description

United States Patent 3,508,924 LITHOGRAPHIC PLATE AND METHOD OF MAKING SAME John E. Pickard, Selma, Ind., assignor to Ball Corporation, Muncie, Ind., a corporation of Indiana No Drawing. Continuation of application Ser. No. 480,496, Aug. 17, 1965. This application Apr. 9, 1969, Ser. No. 817,253

Int. Cl. G03c 1/94 U.S. Cl. 96-86 11 Claims ABSTRACT OF THE DISCLOSURE This invention pertains to a method of preparing zinc surfaced lithographic plates and particularly zinc plated grained aluminum plates, by coating the zinc surface with a photosensitive vinyl cinnamate polymer solution, and immediately heating the coated surface to adhere the polymer to the zinc surface without adversely alfecting the photosensitivity of the polymer, and to plates so prepared.

This is a continuation of application Ser. No. 480,496, filed Aug. 17, 1965, now abandoned.

This invention relates to a lithographic plate and to a method of making the same. More particularly, this invention relates to a metallically coated, grained lithographic plate and to the method by which such a plate is produced.

The planographic process (lithography) is well known in the graphic arts field as a process of printing from a flat surface. To enable printing from such a surface, the desired copy to be printed is placed on the surface in the form of a greasy material and then water and printing ink are successively applied to the surface. Since grease repels water, only the non-greasy areas of the plate are wetted during water application, and, since water will not absorb printing ink, the ink is accepted only in the greasy areas on the fiat surface during the subsequent ink application.

Therefore, when the inked surface is th :reafter brought into contact with paper, ink is transfer ed to the paper only in those areas corresnonding to gr asy areas of the surface and a lithographic print is thuz achieved. If desired, the lithographic print may be transferred first to a blanket and the paper brought into contact with the blanket, as is also well known in the art.

Smooth, porous stones having a flat surface were first used in the planographic process. Later, it was found, however, that such a stone could be replaced by a plate made of aluminum or zinc, if the metal plate was first grained, that is, if the plate had its surface dulled or roughened (sometimes referred to as mat-finish), to increase the surface area for the necessary water and ink retention.

Although zinc sheets were first more commonly used for lithographic plates, the use of aluminum lithographic plates has more recently become widespread. Aluminum plates enjoy an advantage over zinc plates, in that they are lighter in weight, are cheaper to produce, and avoid a scumming problem commonly found when using zinc plates. The use of aluminum lithographic plates, however, has presented new problems, principally due to the fact that the aluminum plates lose sharpness and clarity rapidly and therefore have a relatively short press life, and hence the aluminum lithographic plate has also not proven to be completely satisfactory.

It is therefore an object of this invention to provide a lithographic plate and method of making the same that combines advantages of both the zinc lithographic plate and the aluminum lithographic plate, while at the same time overcoming disadvantages of both such plates.

It is another object of this invention to provide a novel lithographic plate and method for making the same that 3,508,924 Patented Apr. 28, 1970 has long press life, avoids scumming problems, is lighter in weight, and is of higher quality than prior plates.

It is yet another object of this invention to provide a novel method for producing a lithographic plate which is simple, convenient and yet provides a plate that is relatively low in cost per printed copy produced.

It is still another object of this invention to provide a novel lithographic plate that can be sensitized at time of manufacture and which has a substantially longer press life than is obtainable for lithographic plates of this type heretofore available.

In accordance with the present invention, a lithographic plate is achieved wherein a base plate is grained before a substantially uniform thin zinc coating is applied to the grained surface of the base plate, said zinc coating adhering to the contours of the grained base plate surface so that the zinc coating itself has a mat-finish, i.e., a grained or roughened surface as is necessary for lithographic printing.

The base plate of the lithographic plate of this invention may be grained by any of the known methods, mechanically or chemically, and while mechanical graining of the base plate surface is more fully described hereinafter, this invention is not meant to be limited thereto.

The dimensions chosen for the base plate will depend in large measure upon the copy, size and type of press, etc. In most cases, the base plate should, however, have sufficient thickness so that the plate can be easily handled, yet not be completely rigid. The base plate may either be metallic (such as steel or aluminum, for example) or non-metallic (such as plastic, for example). The plate may be cut from coiled or sheet stock and if aluminum (pure or alloy) is utilized, for example, the preferred thickness of the base plate has been found to be between about 0.006 inch and 0.025 inch.

Prior to application of the zinc coating to the base plate, it is, of course, necessary for the plate to be clean and free from grease and other contaminants, in order to achieve good adherence of the zinc to the base plate. To clean the base plate, a conventional cleaning solution may be utilized and may be, for example, an acid, e.g., mineral acid; a base, e.g., caustic soda; a solvent, e.g., trichloroethylene; or a mixture of a solvent and an acid or a base.

When a metallic base plate is utilized, it has been found preferable for the cleaning solution to contain a base. A typical cleaning solution that has been successfully used for aluminum and steel base plates contained between about 5% and 30% and preferably between about 10% and 20% by weight of the base. If desired, the cleaning solution employed may be utilized at an elevated temperature to increase the effectiveness of the cleaning and degreasing step. It has been found advantageous, when elevating the temperature, to keep the cleaning solution at a temperature of between about F. and F.

The cleaning solution is used on the metallic base plate for as long as is necessary to achieve the desired purpose of cleaning and degreasing the same. Normally, for an aluminum plate that is not excessively dirty, this will take a relatively short time, usually not exceeding two minutes.

After the plate is cleaned with cleaning solution, the solution is washed from the surface, commonly with water. It has been found to be necessary to keep the surmechanical gra-ining procedures. If the plate is to be mechanically grained, for example, the graining can be accomplished with a brush graining machine or, a combination of a brush and abrasive slurry may be employed. The abrasive slurry is usually an aqueous slurry with a small amount of a wetting agent included to increase the efficiency of the abrasive, and the abrasive mixture con monly employed for graining a plate contains a substantial proportion of a brasive particles such as quartz, pumice, etc., for example. When using mechanical graining procedures, it has been found preferable that the proportion of the abrasive particles be between about and 30% by weight and more advantageously between about and by weight of the slurry. If desired, a small proportion of a chemical cleaning agent such as that employed in the cleaning solution may also be incorporated in the slurry. The aqueous slurry is commonly evenly distributed over the surface being grained to assure a uniform graining effect.

If chemical graining is to be utilized, the clean plate is subjected to a commercially available chemical graining solution suitable for the metal employed as the base plate. Chemical graining of an aluminum plate can be done, for example, by the use of hydrochloric acid and resin plate.

After the graining operation has been completed, the base plate is washed to remove any slurry and/or metal particles remaining on the surface of the plate, which washing may be done With water. It is important that the surface of the plate be completely cleaned to avoid uneveness and streaking of the plate, and if desired, this washing step may also include scrubbing or agitation and may be followed by exposure of the plate surface to a high pressure water jet to facilitate complete removal of slurry and/or metal particles. If desired, the surface of the plate may be dried in conventional fashion after washing, and drying may be accomplished with forced air to expedite the process and provide uniform drying to avoid streaking.

The clean-grained plate is now ready for the zinc coating, and while the coating to be applied may be varied depending on intended use, a coating thickness up to about 10 microns has been found to be preferable. To form the zinc coating, the base plate may be treated with various vinc compounds, such as, for example, various zinc salts including zinc phosphates, nitrates, sulfates with hydrofluoric acid added, and hydroxides, or the base plate may be conventionally electroplated, so long as a uniform coating is achieved that is sufficiently thin to allow the exposed surface of the coating to have a grained surface due to the grained surface of the base plate.

It has been found preferable, however, to treat the base plate with an alkaline aqueous solution of a zinc compound, preferably zinc oxide or zinc cyanide, in order to produce a superior lithographic plate. To provide for additional control of the coating operation and facilitate the formation of a tightly adherent coating, it has also been found desirable to add minor amounts of other materials to the coating solution, for example, cyanides, copper, and iron salts, sodium nitrate, potassium sodium tartrate, etc.

To achieve a thin zinc coating of uniform thickness on the plate, it is important that the coating operation be carefully controlled, Since the formation of the zinc coating on the plate is an exothermic reaction, it has been found preferable to maintain the plate at a relatively constant temperature during coating, within the range of 60 F. to 150 F. The base plate may be dipped vertically into a bath and maintained stationary during the coating operation, or the plate may be flow-coated in a substantially horizontal position. In either event, the base plate is coated in a relatively short period of time, about one-eighth to two minutes for a zinc coating of between 1 and 10 microns.

For coating, by means of a bath, the proportion of the caustic employed in an alkaline zinc coating bath will generally be substantially larger than the zinc concentration and, preferably several times the proportion of the zinc compound, e.'g., up to three times or more. It has been found preferable that such a bath contain between about 1% and 20% by Weight of the zinc compound and between about 10% and 40% by weight of caustic compound and, preferably between about 5% and 15 of the zinc compound and between about 15% and 30% caustic. Other salts may be present in minor amounts, if desired, for example, up to about 6% of the bath when relatively large concentrations of zinc and caustic are present.

After the base plate has been immersed in a coating, as described hereinabove, the plate is removed and quickly flooded with water, such as immersing in a water bath, for example, to stop the chemical reaction. This is essential since it has been found that if the plate is not substantially instantaneously transferred from the coating bath to a water bath, sufficient heat is generated by the chemical reaction occurring on the surface of the plate during exposure to the atmosphere to destroy the uniform mat-finish necessary for lithographic printing.

Subsequent to immersion of the plate in a water bath to stop the chemical reaction, the plate is washed, preferable with Water, and during washing may be subjected to scrubbing action to remove anyloose surface deposits. After drying the plate, which may be done with warm air, if desired, to expedite the process, the plate commonly has a protective film applied (and is then ready for printing shop use as a wipe-on lithographic plate) or has applied thereto a sensitizinc coating (and is then ready for printing shop use as a presensitized lithographic plate).

If a wipe-on lithographic plate is desired, the zinccoated plate is conventionally treated with mild acid solution such as a mild solution of chromic acid or phosphoric acid, for example. When this plate is sub-' sequently used by the printer, the plate is first sensitized by applying a coating of a conventional photosensitive resin, such as a diazo resin, for example, and processed in the same manner as described hereinafter with respect to presensitized lithographic plates.

If the lithographic plate is to be sensitized at time of manufacture, a photosensitve polymer is applied to the zinc surface. Such polymers are well known in the art, and may, for example, consist of a coating of cinnamic acid polymer, as taught in United States Patent No. 2,- 640,120.

The presensitized lithographic plate may be conventionally processed in the printing shop by exposing the plate to ultraviolet light through a negative, which crosslinks and hardens portions of the polymer exposed to the light; and then developing the plate by treating with solvent to remove the unexposed polymer from the plate leaving only the exposed polymer (which has been crosslinked and hardened due to exposure to ultraviolet light). The plate is then desensitized in conventional fashion and gummed, such as with gum arabic, to protect the non-image areas of the plate and the plate is then ready for press use.

It has been found that, in tests run with lithographic plates made according to this invention, press runs were achieved that were several times as long as the maximum run obtained with heretofore available lithographic plates. It was also noted that lithographic plates, made according to this invention, had press runs that were more trouble-free, even under unfavorable press conditions, and that such plates have a substantially longer storage life, both before exposure and after initial press run, than did their predecessors.

The following examples represent specific embodiments of the invention and are not intended to restrict or limit the scope of the claims. In the examples, parts and percentages are by weight except where noted.

EXAMPLE I An aluminum plate made from an aluminum alloy (1100H27) -and a size of 48 inches by 60 inches and a thickness of 0.015 inch was processed as follows.

"The plate was first. cleaned by spraying the plate (which was on.a moving conveyor) for a period*of one-half minute with an acid-cleaning solution containing 15% by volume of an acid cleaner (sold as Oakite 33) and 85% by volume of water, the solution being maintained at a temperature of 150 F. After being exposed to the cleaning solution, the plate, while still wet with the solution, was sprayed-with water and brushed witih 'a soft bristle brush rotating against the travel of the plate on a conveyor at a speed of eight times the speed of the plate.

After washing, the clean plate was transferred to a commercially available brush graining machine (known as a Fuller Brush Grainer) to grain the-surface of the plate. A graining slurry for use in the Fuller Brush Gr'ainer was prepared by mixing fifteen pounds of 7/0 quartz, 25 pounds of PEP Italian pumice, 3 pounds of trisodium phosphate and 35 gallons of water. The brush pressure of the machine was adjusted to give one-halfinch wide pattern across the width of the machine when the brush was in contact with the surface of the plate. The slurry was uniformly distributed over the surface of the plate immediately preceding the contact with the brushes. The plate moved through the machine at a speed of eight feet per minute. "After the graining had been completed, the plate was sprayed with water and scrubbed according to the procedure employed for the previous washing operation above,-except that the brush was rotated at a somewhat higher-speed. 'A" high pressure water'jet was used to flush the plate free of dirt. After the second washing step; the plate was air-dried by dir ecting an air jet against the surface of the plate. Thedried plate was then dipped into a zinc coating bathwhich was maintainedat' a temperature of 70 F. The 'plat'eswere dip'ped vertically and kept motionless after immersion in the bathQThe bath was prepared by mixing 13.3 pounds of zinc oxide, 70 pounds of sodium hydroxide, 1.3 pounds of potassium sodium tartrate, 120 gramsof hydrated iron chloride and 60 grams of sodium nitrate, together withsufiicient 'water to prepare 40 gal- Ions of bath solution. Since the reaction of'the bath on the aluminum plate is exothermic, it was necessary to refrigerate the bath in order to maintain the temperature of the bath constant. w

After removal of the plate from the coating bath, the plate was immediately immersed for one minute in a waterbath to stop the chemical reaction. The plate was then removed from the water bath, sprayed with water and scrubbed withasoft. bristle-brush as employed for the/previous washing steps. The plate was dried by blowing'hOt 'air at a temperature of 250 F. against the surface of the plate for one minute. The surface of the driedzinc-coated plate was sprayed with a sensitizing solution using a traverse spraying machine equipped with three automatic spray guns. The spray guns were Nos AGA-502 with an air cap No. AV- 1239 and a fluid nozzle No. 6AV643-FX. The atomizing air pressure was 40 p.s.i. and the fluid pressure 3 p.s.i. The sensitizing'solution'consisted of one part by volume of a polyvinyl cinnamate resist lacquer-and 19 parts by volume of benzene. The lacquer is sold under the name of Kodak Photo Lacquer by Eastman Kodak Company. The plate was advanced under the spray nozzles at a rate of 2 /2 feet per minute. After being coated, the plate was passed through an oven and baked with infrared heat at a temperature of 325 F. for one to two minutes.

The sensitized plate was exposed to ultraviolet light through a flat of negatives and developed in trichloroethylene. After being developed, the plate was examined and found to have sharp, very durable images. The non-image areas of the plate were then desensitized by application of gum-free etch, inked and then gummed with gum arabic for protection of the non-image area.

The plate was then used to print on salmon-colored, uncoated stock using green ink. The plate was run for 234,000 impressions on a sheet-fed press without difficulty. After the run, the plate was examined and found to be in good condition.

Other lithographic plates made according to the proce dure of Example I were used under the following conditions and showed the noted results:

A press run of 180,000 impressions was obtained with linty newsprint as compared with a press run of only 10,000 impressions for a commercially available, presensitized plate run under the same conditions.

Press runs for four plates of the invention on a perfecting press using newsprint each exceeded 350,000 impressions compared with about 20,000 for commercial presensitized plates.

Press runs with sixteen plates of the invention on coated and uncoated stock exceeded 100,000 impressions for each plate and were as high as 500,000 impressions for one of the plates.

EXAMPLE II The procedure of this example was the same as that of' Example I, except that the zinc coating bath contained the following: 25 pounds of zinc cyanide, 66 pounds of sodium hydroxide, 13 pounds of potassium cyanide and sufiicient water to make 40 gallons of solution. A presensitized plate made according to the procedure of this example had a press run of 190,000 impressions on a multilith press, which was substantially longer than would normally be expected from commercially available lithographic plates.

" EXAMPLE III The procedure of this example was the same as that of Example I,,except that the zinc coating bath contained by the following: 25 pounds of Zinc oxide, 66 pounds of sodium hydroxide, 20 pounds of potassium cyanide and suificient water to make 40 gallons of solution.

A plate made according to the procedure of this example had a press run which was similar to the superior results shown by the plate of Example I, and substantially longer than would normally be expected from commercially available lithographic plates.

, EXAMPLE IV The procedure of this example was the same as that of Example I, except that the zinc coating bath contained 7 25 pounds of zinc cyanide, 66.7 pounds of sodium hydroxide, and sufficient water to make 40 gallons of solution, and'the zinc coating was applied by flow-coating while the base plate was in a horizontal position.

A plate made according to the procedure of this example had a press run which was similar to the superior resultsshown by the plate of Example I and substantially longer than would normally be expected from commercially available lithographic plates.

EXAMPLE V The procedure of this example was the same as that of Example I, except that the sensitizing solution contained methylene chloride as the solvent instead of benzene.

The plate produced by the procedure of this example showed the same superior length of press run shown by the plates of Examples I, II, III and IV.

EXAMPLE VI The procedure of this example was the same as that of Example I, except that the sensitizing solution was applied by flowing a solution having a viscosity of 100450 centipoises over the surface of the plate with a flow or curtain coater.

The plate made according to this procedure showed the same superior length of press run as did the plates of the preceding examples.

EXAMPLE VII The procedure of this example was the same as that of Example I, except that the zinc-coated base plate was a cold-rolled steel plate of a size of 0.010 inch by 26 inches by 39 inches. The zinc coating was applied by electroplating for 30 seconds in the bath of Example II using a current density of 30 amps per square foot at one volt.

The plate made according to the above procedure showed the same superior length of press run as did the plates of the earlier examples.

The above description and examples show that the present invention provides a new and improved lithographic plate which has substantially longer press life than was heretofore attainable. Moreover, the new lithographic plates of the invention have a longer storage life both prior to initial use and between press runs. Also, the new plates of the invention have good resistance to scumming and perform successfully even under unfavorable press conditions, such as pressure difierences due to improper roll alignment and variations in the quantities and formulations of fountain solutions. Furthermore, the present invention provides a novel method for producing such plates, which method is simple and convenient and yet provides a plate that is relatively low in cost per sheet produced.

From the above description, it will be apparent that various modifications and changes in the procedures and formulations may be made within the scope of the invention. Therefore, the invention is not intended to be limited to the specific examples and detailed procedures described except as may be restricted by the following claims.

What is claimed is:

1. A method of forming a presensitized, zinc-surfaced lithographic plate of improved press life comprising the steps of: applying a photosensitive vinyl cinnamate polymer to a zinc surface; and heating the vinyl polymercoated-surface immediately after coating until the polymer adheres firmly to the zinc surface.

2. A method as set forth in claim 1 wherein the surface is heated at a temperature of about 325 F.

3. A method of forming a presensitized lithographic plate of improved press life comprising the steps of: graining a surface of a base plate; forming a substantially uniform zinc coating thereon that is relatively thin and presents an exposed surface that is grained due to the graining of said base plate; applying a photosensitive vinyl cinnamate polymer to said zinc-coated surface; and heating the vinyl polymer-coated surface immediately after coating until the polymer adheres firmly to the zinc surface.

4. A method of forming a presensitized lithographic plate of improved press life comprising the steps of: cleaning a surface of a base plate; graining said cleaned surface; treating said grained surface with an aqueous solution of a decomposable zinc compound to form a substantially uniform, thin, zinc coating thereon that has a grained surface due to the graining of the base plate; applying a photosensitive vinyl cinnamate polymer to said coated surface; and promptly heating the base plate to an elevated temperature.

*5. A method of forming a lithographic plate of improved press life comprising the steps of: cleaning and graining a surface of a base plate; applying to said plate an aqueous solution of a zinc compound selected from the group consisting of zinc oxide and zinc cyanide to form a substantially uniform, thin, zinc coating on said grained surface that is grained due to the graining of said base plate; removing said coated plate from said bath and substantially immediately thereafter immersing said plate in a water bath; applying a solution comprising a photosensitive vinyl cinnamate polymer to the zinc-coated surface of said plate; and promptly heating the polymer-coated plate to an elevated temperature sufficient to dry said polymer without causing cross-linkmg.

6. A method of forming a lithographic plate of improved press life comprising the steps of: cleaning a surface of an aluminum plate with a basic cleaning solution; removing said solution from said surface; graining said cleaned surface; cleaning said grained surface; treating said plate with an aqueous solution of a zinc compound selected from the group consisting of zinc oxide and zinc cyanide to form a substantially uniform, thin, zinc coating on said grained surface that presents a grained surface due to the graining of said aluminum base plate, said zinc coating bath containing between about 1% and 20% by weight of the zinc compound and between about 10% and 40% by weight of sodium hydroxide and being maintained at a temperature between about 60 F. and F. during treatment; removing said coated plate from said bath and substantially immediately thereafter immersing said plate in a Water bath; cleaning said plate and drying the same; applying a solution comprising a photosensitive vinyl cinnamate polymer onto the coated surface of said plate; and heating the polymercoated plate to an elevated temperature to evaporate the solvent and firmly adhere the vinyl polymer to the plate.

7. The method according to claim 6 in which the zinc compound utilized is zinc oxide.

8. The method according to claim 6 in which the zinc compound utilized is zinc cyanide.

9. A method of forming a presensitized lithographic plate comprising applying a solution of photosensitive vinyl cinnamate polymer to a grained zinc-plated aluminum substrate and promptly baking the coated substrate to remove the solvent and adhere the polymer to the substrate while maintaining the photosensitivity of the polymer.

10. A method as set forth in claim 9 in which the baking takes place at a temperature of about 325 F.

11. A lithographic plate prepared according to the method of claim 9.

References Cited UNITED STATES PATENTS 637,554 11/1899 Rosell 101459 865,719 9/1907 Murray 101-458 1,163,166 12/19'15 Niemeyer 101459 2,670,286 2/1954 Minsk et a1. 9686 X 2,676,916 4/ 1954 Zelley 117-50 X 2,697,039 12/1954 Martinson 96-33 3,202,529 8/1965 Dunlap et a1 11750 OTHER REFERENCES Kodak Photosensitive Resists for Industry, P-7, 1962, pp. 21-22 and 55.

DONALD LEVY, Primary Examiner R. E. MARTIN, Assistant Examiner US. Cl. X.R. 11734