US3508923A

US3508923A - Bimetal planographic plate and method of preparation

Info

Publication number: US3508923A
Application number: US581117A
Authority: US
Inventors: John E Pickard
Original assignee: Ball Corp
Current assignee: Ball Corp
Priority date: 1966-09-21
Filing date: 1966-09-21
Publication date: 1970-04-28
Anticipated expiration: 1987-04-28

Description

April 28, 39% J. E. PICKARD 3,508,923

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ATTORNEYS United States Patent 3,508,923 BIMETAL PLANOGRAPHIC PLATE AND METHOD OF PREPARATION John E. Pickard, Selma, Ind., assignor to Ball Corporation,

Muncie, Ind., a corporation of Indiana Filed Sept. 21, 1966, Ser. No. 581,117

Int. Cl. G03c 1/94 US. Cl. 9686 7 Claims ABSTRACT OF THE DISCLOSURE A method for producing a bimetal lithographic plate by coating a bimetal substrate with a solution of polyvinyl cinnamate photosensitive resin, immediately drying the resin at an elevated temperature, and, after imagewise development of the dried photosensitive layer, printing images directly from the polymer coating, and a plate so produced.

This invention relates to a bimetal type lithographic plate and to a method of producing such a plate. More particularly, this invention relates to a presensitized bimetal lithographic plate and to the method by which such a plate is prepared.

The planographic process (lithography) is well known in the graphic arts field as a process of printing from a flat surface. To permit printing from such a surface, the indicia, or other matter, to be printed is reproduced imagewise on the lithographic surface in the form of a greasy material. Then water and printing ink are successively applied to the surface. Since the oleophilic grease repels water, only the hydrophilic non-greasy areas of the plate are wetted during the water application. Conversely, since water repels oleophilic printing ink, the ink is accepted only in the greasy areas on the surface during the subsequent ink application. Therefore, when the inked surface is thereafter brought into contact with the surface upon which the printing is to appear, ink is transferred to such printed surface only in those areas corresponding to the greasy areas of the lithographic surface. A lithographic image is produced by means of this mechanism. If desired, the image may be transferred to an intermediate roller or blanket and from the intermediate blanket or roller to the surface to be printed. This alternative is well known in the art as offset lithography.

Smooth, porous stones having a fiat surface were first used'in the lithographic process. Later it was discovered that the stone could be replaced by a plate made of a single metal, such as zinc or aluminum, if the metal plate was provided with a dull, roughened or matte finish. The matte surface provides increased surface area for necessary water and ink retention.

Aluminum or zinc plates are quite satisfactory lithographic plate materials for printing on more conventional materials, such as paper, or for press runs of moderate duration. However, when the printed surface is a metal, as is common in printing of closures or metal containers, the above-described conventional lithographic surfaces are Worn at a greatly accelerated rate and rendered useless before a reasonable number of images have been produced. For this reason, a bimetal lithographic plate is conventionally employed for printing on metal surfaces, or for runs of unusual length.

Conventional bimetal lithographic plates are usually produced by electroplating between about .00005- to .004-inch of an oleophic metal such as copper, on a hydrophilic textured base or support plate of, for instance, stainless steel. A lithographic printing surface is produced by etching away the thin oleophilic metal coating in the 3,508,923 Patented Apr. 28, 1970 ice non-image areas. Water is applied to the exposed hydrophilic support areas and ink is thereafter applied to the remaining areas of oleophilic metal. As will be apparent, the principle is the same as that of the conventional lithographic plate but the image areas are of metal and thus much more resistant to wear.

While conventional bimetal lithographic plates have decided advantages with regard to wear resistance and durability, such plates also have certain drawbacks. For instance, the oleophilic metal is normally protected during etching by a water-soluble photosensitive resist of poly vinyl alcohol and dichromate. Even after burn-in at an elevated temperature, this protective coating is attacked to a noticeable degree by the aqueous etching compound. In a 133 line screen, 3% to 5% dots are normally lost. Often this loss resulting from attack of the stencil by the etchant extends to 10% dots. Conventional bimetal lithographic plates are, as a general rule, useful for only about 500,000 images. Preparation of conventional bimetal plates for use involves a complicated, timeconsuming and expensive series of steps which involve long exposure, burn-in and removal of the hydrophilic resist from the oleophilic metal after etching.

Therefore, it is an object of the instant invention to provide a bimetal lithographic plate which displays resolution from at least 300% to 500% greater than that of conventional bimetal lithographic plates, and a method of preparing such a plate.

Another object of the instant invention is to provide a presensitized bimetal lithographic plate requiring only a relatively short exposure to actinic radiation to produce the desired image.

Yet another object of the instant invention is to provide a presensitized bimetal lithographic plate which is usable for about 50% longer press runs under given conditions than conventional bimetal plates.

A further object of the instant invention is to provide a bimetal lithographic plate and method of preparation which do not involve a high temperature burn-in step and in which it is not necessary to remove the stencil before use.

Still another object of the instant invention is to provide presensitized bimetal lithographic plates which may be prepared for use in a much simpler and therefore less expensive manner than conventional bimetal lithographic plates.

A still further object of the instant invention is to provide a bimetal lithographic plate which, when considered in cost terms of materials and labor, enables lithographic images to be produced at a significant reduction in cost.

Other objects of the instant invention will be apparent from the following discussion and claims.

According to the present invention, a superior bi-metal lithographic plate is produced by providing a resist coating of an active radiation cross-linkable, oleophilic poly-- mer on the oleophilic surface of a bimetal plate.

The oleophilic metal is preferably copper coated to a thickness of .00005- to .004-inch, and preferably .0001- inch, upon a textured hydrophilic support plate.

Oleophilic metals as referred to herein are merely those metals which display a greater relative aflinity for greasy inks. Other suitable oleophilic metals include lead, nickel and tin. Still other metals having this characteristic will be readily apparent to those skilled in the art.

As specified above, the oleophilic metals can be deposited in rather widely varying thicknesses. Of course, thicker deposits will withstand greater use before wearing away. However, it is also desirable that the oleophilic metals accurately reproduce the textured surface of the support plate. When the preferred electrolytic deposition method is used to produce the oleophilic metal layer, the

surface of the oleophilic metal tends to become smooth rather than reproducing the texture of the support plate as the thickness of the oleophilic metal increases. Further, etching of the oleophilic metal is complicated by greater thicknesses. Obviously, etching time will increase with greater thicknesses of the oleophilic metal. An even more deleterious effect of thicker oleophilic metal layers is the longitudinal etching or undercutting which results from deeper etching.

Thus, the thickness of the oleophilic metal, the metal used, and even the method of applying the metal, are not critical. Rather, the practicality of increasing the thickness of the oleophilic metal to increase wear diminishes as the etching becomes more diflicult and control of surface texture presents problems. In any event, the instant invention is operable with any bimetal combination useful with the water base dichromate masking and produces improved results and increased convenience. If anything, the instant invention minimizes the adverse results of going to an extreme to enhance one quality at the expense of others.

Stainless steel is an almost ideal support plate in that it displays great strength and resistance to wear while having a marked aifinity for water. Particularly useful compositions are the 304 and 316 series stainless steel. But again, the support plates operable in the instant invention are substantially the same as those which have been conventionally used for bimetal lithographic plate supports. Thus, those skilled in the art will readily recognize the operable and preferred materials such as the above-mentioned series 304 and 316 stainless steels, steel and aluminum.

Summarily, with regard to bimetal plates, the instant invention, as do conventional bimetal lithographic plates, only requires a textured support plate having adequate wear properties and a hydrophilic nature with a relatively thin layer of an etchable oleophilic metal be deposited thereon. The specific metals, thicknesses and method of formation are those conventionally used in the art.

In contradistinction to the conventional approach of using a water-base resist coating, the instant invention employs a Water-insoluble oleophilic polymer as the photosensitive resist coating. These materials have the advantage of being inert to the conventional water-based etchants while the conventional water-based resists are, to a significant extent, attacked by conventional etchants. Also, it is not necessary to remove the oleophilic resist of the instant invention before printing. In fact, the durability of the plate is greatly enhanced since a substantial number of images are printed from the resist before it is worn away by use. Not only is it necessary in the conventional method to go through an extra step of removing the hydrophilic resist from the oleophilic metal in order that an image may be produced, but the oleophilic metal is also subject to wear from the first image produced.

Water-insoluble photosensitive polymers are well known in the art. A particularly useful photosensitive vinyl polymer is that based on cinnamic acid and described in detail in U.S. Patent 2,610,120. Preferably the polymer contains arylkenoxy groups such as cinnamoyl groups, for example cinnamic acid esters of polyvinyl alcohol, arvinylbenzalacetophenone polymers, ar-vinylcinnamalacetophenone polymers, hydroxyalkyl cellulose cinnamates, etc. Other suitable photosensitive polymers include the photosensitive polyurethane resins and photosensitive epoxy resins.

The photosensitive polymers useful in this invention are initially thermoplastic materials which include, either in the polymer chain proper or as an intimate admixture, an actinic radiation sensitive cross-linking initiator. lnitronaphthalene is a particularly advantageous initiator. A large number of organic-nitro compounds function satisfactorily. In addition to the preferred l-nitronaphthalene, other operable initiators are listed in the aforementioned U.S. Patent 2,610,120. Preferably, the initiator is sensitive primarily to ultraviolet radiation so that it may be exposed to substantial portions of the visible spectrum without cross-linking. When the polymer is exposed to suitable actinic radiation, the cross-linking initiator reacts and produces cross-linking between the polymeric chains. The polymer is thus changed from a thermoplastic, organic solvent-soluble material into an essentially thermosetting, or elastomeric, material which is relatively insoluble in most organic solvents.

When such photosensitive polymers are exposed to radiation in an imagewise manner, only the light struck areas are rendered insoluble. The exposed photosensitive resist can then be developed by exposure to organic solvents to remove the uncross-linked polymer. A particularly advantageous method of developing and apparatus for carrying out the method is described in U.S. patent application Ser. No. 543,675. However, development can be accomplished by merely spraying an organic solvent or solvents such as, but not limited to, methylene chloride, dimethylene formamide, trichloroethylene, 1,2-dichloroethylene, tetrahydrofuran, methyl ethyl ketone, dimethyl acetamide, methyl acetate, acetone, monochlorobenzene, Z-methoxyethanol acetate available as methyl Cellosolve acetate, dimethyl sulfoxide, ethyl acetate, benzene, furfuryl alcohol, 1,4-dioxane, Z-ethoxyethanol acetate, available as Cellosolve acetate, toluene, cyclohexanone, ylene, diglycol monoethyl ether acetate, available as Carbitol acetate, diethylene glycol diethyl ether available as diethyl Carbitol, N-butyl acetate, tetrahydrofurfuryl alcohol, 1,1,1-trichloroethane, and methyl isobutyl ketone. Yet another method of development is to simply immerse the exposed plate in a tray of liquid solvent.

After development, a stencil of the exposed image remains on the bimetal plate formed by the undissolved, light struck portion of the photosensitive resist.

It is a critical requirement of the instant invention that the photosensitive polymer be soluble in an organic solvent but not in a dielectric solvent, such as Water, as is the conventional polyvinyl alcohol-dichrornate waterbase system. Solubility in organic solvents, if at all soluble, is a characteristic of oleophilic polymers. Also, it is important that the resist not be soluble in dielectric solvents such as water so that the water-base etchant will not overdevelop the resist. Since the photosensitive polymer of the instant invention is developed only by organic solvents, the water-base etchant has no effect on the resist. However, conventional water-base resists are attacked, or overdeveloped, by the water-base etchant with resulting loss of highlight details. Water-base etchants are also subject to variation in performance as a result of changes in humidity. This drawback is also obviated by the bimetal plate of the instant invention.

Coating of the bimetal plate with the oleophilic photosensitive polymer may be accomplished by, for example, an airless or air spray, roller coating, dip coating, flow coating, or any of the other well known conventional coating systems. If an air spray is employed, a particularly advantageous spraying solution is one containing 12 parts by volume of a solvent of methylene chloride containing 2% to 5% methyl Cellosolve acetate, and one part by volume of a photosensitive polyvinyl cinnamate lacquer. A particularly useful airless spray mixture is one containing 4 parts by volume of methyl Cellosolve acetate to one part of the polyvinyl cinnamate lacquer. After spraying, the coating is baked or dried to remove the solvent. Drying can be accomplished at a temperature between about 225" F. to 360 F., and preferably between 230 F. to 300 F. It is particularly advantageous to dry at a temperature below about 250 F. The drying step should be completed within a period of about between 1 and 5 minutes, and preferably between 1.5 to 2.5 minutes.

The operation of the bimetal lithographic plate, as well as the method of preparing and using it, will be more readily understood in view of the drawings, in which:

FIGURE 1 is an enlarged cross-sectional view;

FIGURE 2 is an enlarged cross-sectional view illustrating the effect of exposure to actinic radiation;

FIGURE 3 is an enlarged cross-sectional view illustrating the plate after solvent development; and

FIGURE 4 is an enlarged cross-sectional view illustrating the bimetal plate after etching.

The drawings are not to scale, but are instead shown with certain layers greatly out of proportion for purposes of illustration. In FIGURE 1, a bimetal plate according to the instant invention is illustrated in the state in which it is received by the craftsman. That is, a hydrophilic metal base 11 carries a layer of oleophilic metal 12 and the oleophilic metal 12 in turn carries an oleophilic, photosensitive polymer 13. Upon imagewise exposure to actinic radiation, as shown in FIGURE 2, a portion 14 of oleophilic, photosensitive polymer 13 is cross-linked in the actinic radiation-struck portions. Cross-linking renders cross-linked portions 14 less soluble in organic solvents. Thus, when the plate is developed in an organic solvent, the uncross-linked portions of oleophilic polymer 13 are removed, as in FIGURE 3, thereby exposing oleophilic metal 12. However, cross-linked portions 14 are not removed by the solvent to any substantial degree. Crosslinked portion 14 serves as a mask, or stencil, for etching. Thus, when the solvent-developed plate is exposed to the conventional aqueous-base etchant, oleophilic metal 12 is removed in areas not protected by cross-linked portions 14 thereby exposing hydrophilic metal 11 as shown in FIG- URE 4. However, in the portions of oleophilic metal 12 covered by cross-linked portions 14, the oleophilic metal 12 is protected from the etchant and remains as a facsimile of the exposure image.

As will be evident from FIGURE 4, water will be retained by hydrophilic metal 11. Oleophilic surfaces of oleophilic metal 12 and cross-linked portions 14 do not retain water. Thus, when Water is applied to hydrophilic metal 11 and greasy ink to cross-linked portions 14, an image can be printed from cross-linked portions 14. Further, as cross-linked portions 14 wear with use, the even more wear-resistant oleophilic metal 12 is exposed and excellent images continue to be produced.

The invention will be more readily appreciated upon consideration of the following examples:

EXAMPLE I A standard bimetal plate of series 304 stainless steel with a special 2D finish having electroplated thereon .00005 to .00007 inch of copper was subjected to a mild graining on the copper side of the plate. Graining was carried out on a commercially available brush graining machine Fuller brush grainer) using a slurry prepared by mixing 50 pounds of 7/0 quartz, 25 pounds of PEP size Italian Pumice, 3 pounds of trisodium phosphate and 35 gallons of water. The plate was then water-rinsed and rapidly dried with large volumes of cool air to prevent oxidation. The thus prepared plate was sprayed with a sensitizing solution by means of an airless spray. One part of a polyvinyl cinnamate resist lacquer to 4 parts of methyl Cellosolve acetate on a volume basis was used to prepare the sensitizing solution. Polyvinyl cinnamate lacquer resist is available commercially under the name of Kodak Photo Lacquer and is vended by Eastman Kodak Company. After being sprayed, the plate was dried by baking at 300 F. for 2 minutes. At this stage, the plate was packaged and stored for an extended length of time. During storage the plate was protected from ultraviolet light and elevated temperature.

After being stored in the dark for six weeks, the plate was exposed imagewise for one minute to radiation from an arc source. Development of the thus exposed plate was accomplished by exposure to trichloroethylene solvent in the manner described in detail in application Ser. No.

6 543,675. After development, the plate was etched with 42 Baum FeCl until exposure of the stainless steel and then water-rinsed. The stainless steel non-image area was desensitized by pouring a 2% solution of sulfuric acid over the plate. The non-image area of the plate was preserved with a preserver such as described in patent application Ser. No. 531,037 and stored. After three weeks storage, the plate was placed on an offset press and 750,000 satisfactory images were produced. This plate was then taken ofi and put on press 18 times during the course of the run. Similar plates have produced 1,000,000 satisfactory images.

EXAMPLE II A plate as described in Example I was prepared using a drying temperature of 250 F. for a period of 3 /2 minutes. The resulting plate was substantially identical to that of Example I.

' EXAMPLE III A plate as described in Example I was prepared using a drying temperature of 360 F. for a period of 1 /3 minutes. The resulting plate was substantially identical to that of Example I.

EXAMPLE IV A basic bimetal plate identical to that of Example I was sensitized in the conventional manner. That is, after hand scrubbing with pumice and water and an acid cleaning, the plate was placed in a conventional whirler and coated with engravers glue-dichromate water-base photosensitive material and dried. It thereafter was exposed for 6 minutes to an arc source. Development was accomplished by subjecting the exposed surface to water in a soak tank. After the non-exposed portions of the water-base resist were removed, the plate was dried and subjected to a temperature of 375 F. for 5 minutes under heat lamps to burn-in the unremoved water base stencil. After burning-in, the plate was etched with 42 Baum FeCl until the stainless steel was exposed. Etching was terminated by a water rinse and the plate was scrubbed with 2% sulfuric acid and pumice to remove the stencil, thereby permitting the copper to accept ink. Again, the plate was rinsed in water. The copper was activated with a 2% H 50 solution and inked. The plate then can be used on a press or gummed with gum arabic to protect the non-image area. If stored, the plate is coated with asphaltum to insure easy press roll-up. After storage, the plate was run on a press but displayed deterioration after producing only two-thirds of the images produced by the plate of Example I.

EXAMPLE V An ungrained support plate of series 304 stainless steel having a .001-inch copper layer thereon was sprayed with a sensitizing solution by means of an airless spray. The sensitizing solution included one part polyvinyl cinnamate to 12 parts methylene chloride on a volume basis. After spraying, the plate was dried at 300 F. for 2 minutes. The thus prepared plate was mounted on a press and removed 12 times during which 108,000 images were cumulatively run.

EXAMPLE VI A series 304 stainless steel support plate was grained asdescribed in Example I. Copper was then electro-deposited thereon in the amount of .0001-inch. Thereafter, the plate was provided with an oleophilic polymeric coat ing as described in Example V. The thus prepared plate satisfactorily produced 1,000,000 images.

From the above description and examples, it is evident that the instant invention provides the art with a lithographic plate which is simpler to process, which provides improved resolution, and which permits a greater number of images to be produced before wear deleteriously affects the image. As will also be evident from the above description and examples, various modifications in the methods and apparatus described may be made without varying from the scope of the invention. Therefore, the invention is not intended to be limited to the specific details herein described except as may be required by the following claims.

What is claimed is:

1. A method for producing a bimetal lithographic plate comprising coating a solution of a photosensitive vinyl cinnamate oleophilic polymer onto the oleophilic metal side of the hydrophilic support plate having a layer of etchable oleophilic metal thereon, and maintaining the thus coated plate at a temperature between about 225 F. to 360 F. until dry.

2. A method as set forth in claim 1 wherein the plate is dried at a temperature between about 230 F. and 300 F.

3. A method as set forth in claim 1 wherein the drying period is between 1 and 5 minutes.

4. A method for producing a bimetal lithographic plate graining a hydrophilic support plate, depositing a layer of between .OOOOS-inch and .004-inch thickness of etchable oleophilic metal on said support plate, spraying a solution of photosensitive oleophilic vinyl cinnamate polymer onto said oleophilic metal, and maintaining said plate at a temperature of 225 F. to 360 F. until dry.

5. A method for producing bimetal lithographic plates comprising coating a solution of photosensitive vinyl cinnamate oieophilic polymer onto the oleophilic metal side of a hydrophilic support plate having a layer of etchable References Cited UNITED STATES PATENTS 9/1952 Minsk et al 9633 X 2/1954 Minsk et a1 9633 X OTHER REFERENCES Hartsuch: Chemistry of Lithography, 1961, LTF, Inc., pp. 177-182.

Kodak Industrial Data Book P-7, Kodak Photosensitive Resists for Industry, 1962, pp. 21-22, 38.

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