US3272121A

US3272121A - Lithographic printing plate prepared by photoelectrostatic reproduction, a method for its production and a method for lithographic printing

Info

Publication number: US3272121A
Application number: US258405A
Authority: US
Inventors: Jay J Uber; Hsia Shwang
Original assignee: Plastic Coating Corp
Current assignee: Plastic Coating Corp
Priority date: 1963-02-14
Filing date: 1963-02-14
Publication date: 1966-09-13
Anticipated expiration: 1983-09-13

Description

fl 3, 1966 J. .J. UBER ETAL 3,272,121

LITHOGRAPHIC PRINTING PLATE PREPARED BY PHQTOELECTROSTATIC REPRODUCTION, A METHOD FOR ITS PRODUCTION AND A METHOD FOR LITHOGRAPHIC PRINTING Filed Feb. 14, 1963 ,5 5 I, s a v 2] HA) cm Kora JAY J. USER SWANG HS/A (Emma Pu. bckmaszrwg United States Patent Filed Feb. 14, 1963, Ser. No. 258,405 12 Claims. (Cl. 101-1492) This invention relates to a lithographic printing plate which has been prepared for photoelectrostatic reproduction, to a method for its production and to a method for lithographic printing utilizing this printing plate.

Lithographic printing is now a highly-developed art. Such printing utilizes a planographic printing plate which carries a hydrophobic image, which is wettable by an oleaginous printing ink and has background areas which are hydrophilic in character and, when wet with water, will not accept printing ink. During a lithographic printing operation, the background areas of the plate are kept wet with water or with a dilute, aqueous, fountain solution which contains dissolved chemicals, which facilitate the wetting of the background areas of the printing plate.

Photoelectrostatic reproduction is a recent development which permits the rapid reproduction of copy. In one type of such reproduction, co is produced directly on a sheet of paper carrying, on its surface, a photoelectrostatic coating comprising a photoconductive pigment, such as a photocond-uctive grade of zinc oxide, dispersed in an insulating matrix of a resinous, film-forming material by a procedure consisting of three essential steps. These steps are; the application of an electrostatic charge to the surface of the photoelectrostatic coating in the absence of light, the exposure of the charged surface to an optical image to produce an electrostatic image on the surface of the sheet, and the toning of the surface to convert the electrostatic image to a permanent, visible image.

The latent electrostatic image carried by the photoelectrostatic coating on the sheet may be developed by either of two procedures. In one of these procedures, which is usually termed dry toning, a suitably-pigmented, resinous powder, hereinafter referred to as a dry toner, carrying an electrostatic charge, is applied to the latent electrostatic image. Powdered resins such as copal, sandarac, Vinsol resin, rosin and various synthetic resins, as well as hard waxes carrying a suitable pigment dispersed therein, may be used for this purpose. The powdered toner is retained by the image areas to form a direct image, or by the background areas to form a reversed image, depending upon its polarity with respect to the electrostatic image being developed. The electrostatic image is usually negative in polarity. The application of a positively charged toner powder to a negative electrostatic image produces a direct image, while the use of a negatively charged toner powder produces a reversed image. Following the application of the toner powder to form either a direct or a reversed image, the toner powder is fused by subjecting it to heat, thereby rendering the image permanent in nature.

The other of the procedures for developing the latent electrostatic image, usually termed liquid toning, consists of wetting the photoelectrostatic coating carrying a latent image with a liquid toner which comprises a dispersion of a pigmented, resinous material in a volatile liquid having a high-dielectric strength and a high-volume resistivity. The dispersed particles may carry either a positive or a negative electrical charge, depending upon their chemical composition. As in the case of a dry toner powder, a liquid toner produces a direct or a reversed image, depending upon the charge carried by its pigmented particles.

In the liquid toning procedure, the excess toner liquid is removed from the sheet and the volatile solvent permitted to evaporate. No fusion of the toner particles is required, since the image is permanent after the solvent evaporates.

There has been a strong incentive to develop a method by which photoelectrostatic copies, produced as described in the foregoing, can be converted into planographic printing plates which are suitable for use in oifset printing, since the photoelectrostatic copies can be made in a short time and are inexpensive. The image areas of such photoelectrostatic copies are resinous and, hence, inherently hydrophobic in nature and capable of accepting an oleaginous printing ink. However, the exposed areas of the photoelectrostatic coating are also resinous, since the continuous matrix of the coating is a resin and, hence, are also hydrophobic in nature and capable of accepting an oleaginous printing ink. Such copies cannot be used as lithographic printing plates, since both the image areas and the background areas of the copy will accept printing ink.

Much effort has been spent on the development of procedures for selectively converting the background areas of the exposed photoelectrostatic coatings of such copies from their inherently hydrophobic nature to a hydrophilic character, without destroying the hydrophobic character of the image areas. Some of the procedures which have been developed have produced printing plates which are capable of reproducing limited numbers of copies by a lithographic printing technique, but none of the suggested procedures have provided a fully-satisfactory solution to the problem.

It is an object of this invention to provide a method for the production of a lithographic printing plate, by which a durable printing plate can be produced in a matter of minutes at a relatively low cost.

Another object is to provbide a lithographic printing plate which is inexpensive to produce, and capable of reproducing hundreds of copies utilizing the usual officetype offset duplicating apparatus or commercial offset printing presses, with water as a fountain solution.

A further object is to provide a method for offset printing by which several thousand clean and acceptable copies can be reproduced from a single printing plate.

Other objects of this invention and its various advantageous features, will become apparent from the detailed description of this invention which follows.

This application is a continuation-in-part of our application Serial No. 821,287, filed June 18, 1959, entitled, Lithographic Printing Plate Prepared by Electrophotographic Reproduction and a Method for Its Production, now abandoned.

The printing plate in accordance with this invention is the result of our discovery that the wetting of the surface of an electrophotographic master with an aqueous prewetting solution which comprises, as essential ingredients, a water-soluble, polymeric, organic acid selected from the group consisting of polyacrylic acid and polymethacrylic acid, and an inorganic salt selected from the group consisting of alkali metal ferrocyanides and alkali metal ferricyanides, selectively converts the exposed areas of the photoelectrostatic coating from a hydrophobic nature to a hydrophilic characteristic, without altering the hydrophobic nature of the image areas formed by the toner. The aqueous solution is maintained at a pH below about 7.25 and, preferably, within the range of about 3.0 to about 7.25. We have found that the best results are obtained by maintaining the pH within the slightly acidic range below a pH of about 6.5.

The photoelectrostatic master may be converted to a lithographic printing plate by uniformly moistening its surface with the said aqueous solution, and using it immediately for the reproduction of copies, without permitting the surface to dry. Alternatively, the surface may be treated with the solution and allowed to dry before it is used in lithographic printing. In either case, the printing plate is durable and capable of producing many clean copies. Such a plate is capable of reproducing as many as two thousand copies, when using water as a fountain solution, and will produce in excess of thirty thousand clean copies when using, as a fountain solution, a dilute version of the said prewetting solution.

Extensive research has left no doubt that the organic polymeric material and the alkali metal ferrocyanide or alkali metal ferricyanide in the aqueous prewetting solution, both enter into the change of the exposed surface of the photoelectrostatic coating from a hydrophobic to a hydrophilic character and become firmly bonded to the exposed areas of the photoelectrostatic coating. The firm bonding of these materials to the photoelectrostatic coating is demonstrated by the fact that the printing plates are capable of producing in excess of one thousand copies, while being constantly rewet with water. Aqueous solutions of polyacrylic acid, of polymethacrylic acid, of the alkali metal ferrocyanides and of the alkali metal ferricyanides, respectively, as well as many other watersoluble, organic compounds and water-soluble, inorganic salts, have been demonstrated to show detectable action in changing the hydrophobic nature of the photoelectrostatic coating to a hydrophilic nature. However, none of these materials, alone, were sufficiently effective to be of commercial value. The combination of polyacrylic acid or polymethacrylic acid with an alkali metal ferrocyanide or an alkali metal ferricyanide in an aqueous solution is synergistic, in that the various combinations are definitely superior to either of the components of the binary combination in producing the described selective action. The phenomenon involved is specific, since the combinations of polyacrylic acid or polymethacrylic acid with other complex inorganic salts which, alone, show detectable effect, do not produce improved results. Those combinations are usually less effective than either component alone.

This research has failed to furnish conclusive explanations of the nature of the action of the combination of the polyacrylic acid or the polymethacrylic acid and an alkali metal ferrocyanide or an alkali metal ferricyanide on the exposed areas of the photoelectrostatic coating, or the nature of the bond between these materials and the photoelectrostatic coating. This research has demonstrated that it is essential to keep the pH of the pretreating solution within, or very near, the acidic range, indicating that the carboxylic groups of the polymeric organic compound must be completely, or substantially completely, unneutralized and free to react with the photoconductive pigment of the photoelectrostatic coating. A hypothesis that the carboxylic groups of the polyacrylic acid or the polymethacrylic acid reacts chemically with the photoconductive pigment, for example zinc oxide, ignores the fact that the zinc oxide is in a matrix of a resinous binder which can be expected to protect the zinc oxide from any such reaction. Further, this hypothesis fails completely to explain the essential role of the alkali metal ferrocyanide or the alkali metal ferricyanide in the conversion of the photoelectrostatic coating.

As already noted, the pH of the prewetting solution is important to its reaction with the exposed surface areas of the photoelectrostatic coating. For this reason, it is usually desirable and, with some combinations of the essential ingredients, necessary to include a buffer in the prewetting solution to obtain the desired pH, and to assure its retention. The various alkali metal phosphates are suitable for this purpose.

The method in accordance with this invention, based on the discoveries described in the foregoing, comprises first preparing a copy on a photoelectrostatic master carrying an image which is to be reproduced by lithographic printing and, second, the conversion of this photoelectrostatic master to a lithographic printing plate.

The desired image is reproduced on a photoelectrostatic master consisting of a base sheet which has at least a limited electrical conductivity and a shape and dimensions, together with any marginal perforations necessary to adapt it for attachment to the cylinder of an offset ofiice duplicating machine or of a commercial offset printing press, and which carries on one of its surfaces a photoelectrostatic coating comprising a photoconductive grade of zinc oxide dispersed in a matrix of an electrically-insulating binder. The image is reproduced on the surface of the photoelectrostatic coating of the master by following the three procedural steps described hereinbefore for the production of a photoelectrostatic image, i.e., the imposition of an electrostatic charge on the surface of the photoelectrostatic coating in the absence of light, the exposure of the charged surface to produce a latent electrostatic image thereon, and the development of this image by the application of a resinous hydrophobic toner thereto by a dry toning step, followed by the fusion of the resinous toner, or by a liquid toning step followed by the evaporation of the volatile liquid of the liquid toner. It is not necessary that the resinous toner carry a pigment in either the dry or the liquid toning, although the presence of a pigment is not detrimental to the performance of the printing plate and is a convenience in the visual inspection of the master. These steps produce a permanent photoelectrostatic copy on the master. It may be immediately converted to a lithographic printing plate, or stored for an indefinite period and then converted to a printing plate.

The photoelectrostatic master produced as described in the foregoing, carrying a resinous image, is converted into the lithographic printing plate by the single step of uniformly wetting its image-bearing surface with an aqueous prewetting solution of polyacrylic acid or polymethacrylic acid, and an alkali metal ferrocyanide or an alkali metal ferricyanide, which has a pH within the range of about 3.0 to about 7.25. This step may be conveniently carried out, for example, by swabbing the entire surface of the mater which carries the resinous image with a cotton pad which is wet with the said aqueous solution. The resulting printing plate may be immediately attached to the cylinder of an office offset duplicating machine or of a commercial offset printing press, for reproduction of copies.

Alternatively, the moistened, image-bearing surface of the printing plate may be permitted to dry by the evaporation of the water of the prewetting solution, the printing plate stored and, at some future time, utilized for the lithographic reproduction of the image which it carries on its surface.

The aqueous compositions which we utilize in our method to produce the improved lithographic printing plate described in the foregoing, may contain about an amount of polyacrylic acid or polymethacrylic acid within the range of about 0.5%, to about 2.5%, by weight, and an amount of an alkali metal ferrocyanide or an alkali metal ferricyanide within the range of about 0.5% to about 2.5%, by weight, with the remainder of the composition being water. Preferred aqueous compositions contain an amount of the polyacrylic acid or the polymethacrylic acid within the range of about 1.0%, by weight, to about 2.0%, by weight, and an amount of an alkali metal ferrocyanide or an alkali metal ferricyanide within the range of about 1.0% to about 2.0%, by weight.

The pH of these aqueous solutions should be within the range of about 3.0 to about 7.25, and we prefer that the solution have a pH within the range of about 5.0

to about 6.0. Some of the aqueous compositions inherently have pH values within the range of 3.0 to 7.25 and require no buffer to maintain the desired pH, depending upon the proportions of the two essential ingredients included in the composition and the particular alkali metal ferrocyanide or ferricyanide which is used. Thus, equal proportions, by weight, of polyacrylic acid and potassium ferrocyanide give a pH somewhat above 3.0, while equal proportions, by weight, of polyacrylic acid and sodium ferrocyanide give a solution having a pH below 3.0. Higher proportions of the alkali metal ferrocyanides give solutions having higher pH values.

The pH of a solution of the two essential ingredients of this aqueous composition can readily be adjusted by the addition of a buffer salt. In general, we prefer to include a buffer in the composition to assure that the pH is within the range of pH values which we have found essential to the best performance of these compositions. We have found that .the alkali metal phosphates are particularly suitable for use as buffers in these compositions to establish and maintain the desired pH in these solutions.

We have found that combinations of sodium ferrocyanide with polyacrylic acid, when buffered with trisodium phosphate, produce somewhat more durable printing plates than combinations of potassium ferrocyanide with polyacrylic acid, buffered either with trisodium phosphate or tripotassiu-m phosphate. Thus, the alkali metal cation included in the composition appears to have an influence on the bonding of the ingredients of the composition to the photoelectrostatic coating. This is a surprising result which we are unable to explain, since it could be assumed that the anions of the composition are entirely responsible for its selective action in prewetting the photoelectrostatic coating, without destroying hydrophobic nature of the resinous toner image.

Specific aqueous compositions which are suitable for use in the method of this invention to produce our improved planographic printing plate, are specifically illustrated by the examples given in Table I:

With the single exception of Example 2, each of the compositions illustrated by Table I are buffered by the addition of a tri-alkali rnetal phosphate. Example 2, consisting of an aqueous solution of equal parts, by Weight, of polyacrylic acid and potassium ferrocyanide, as shown by Table I, had a pH of 3.2. within the pH range of 3.0 to 7.25 which we have found to give improved results. The substitution of sodium ferrocyanide for potassium ferrocyanide in the composition of Example 2, or stated in another way, the omission of trisodium phosphate from the composition of Example 3 (pH 4.5) gave a solution having a pH of 3.2, which was noticeably inferior to either the solution of Example 2 or 3, as to the durability of the printing plate produced by its use.

The printing plate in accordance with this invention is suitable for use in lithographic printing, using water as a fountain solution. A paper-base printing plate, prepared as described in the foregoing, will reproduce in excess of one thousand copies which are clear and have clean backgrounds which are free of residual ink.

We have discovered that the useful life of a printing plate in accordance with this invention can be increased by severalfold by the use, as a fountain solution, of a dilute counterpart of the same solution which is applied to a photoelectrostatic master to produce the printing plate. By this method of printing, we have produced in excess of thirty thousand clean copies utilizing a paperbase printing plate, with no evidence of failure by the plate.

The method of lithographic printing in accordance with this invention may utilize any of the usual offset oflice duplicating machines or commercial offset printing presses. In this printing operation, we keep the surface of the printing plate wet with a fountain solution which is identical with the solution with which the plate has been prewet, or with such a solution which has been diluted by the addition of water. It is desirable, from the standpoint of operating cost, to dilute the prewetting solution with water to a fraction of its original concentration. We prefer to use a fountain solution prepared by the dilution of the prewetting solution with water to an extent which falls within the range of one volume of the prewetting solution TABLE I.EXAMPLES OF AQUEOUS PREWET- TING COMPOSITIONS USED TO PRODUCE THE LITHOGRAPHIC PRINTING PLATE Example Number 1 2 3 4 5 6 7 Polyacrylic Acid (pts. by wt.). 1. 67 15 15 15 15 Polymethacrylic Acid -(pts.

Potassium Ferrocyanide (pts. Sodium Ferrocyanide (pts. Lithium Ferrocyanide (pts. Sodium Ferricyanide (pts. 'Irisodinm Phosphate s(pts. Tripotassium Phosphate (pts. 'lrilithium Ph0sphate (pts. Water (pts.

Both the polyacrylic acid and the polymethacrylic acid used in the compositions exemplified by Table I are commercial products. We have been unable to detect differences between different lots of polyacrylic acid or polymethacrylic acid, and it is evident that the exact degree of polymerization of either of these acids is immaterial, provided that they are freely water soluble.

An alkali metal salt of either the polyacrylic acid or of the polymethacrylic acid may be used instead of the free acids in the preparation of these aqueous compositions, and the pH of the resulting solution adjusted accordingly by the substitution of a dior mono-alkali metal phosphate for the tri-alkali metal phosphates illustrated by Table I. At any given pH, the resulting aqueous compositions are identical regardless of whether the free polymeric organic acid or its alkali metal salt is used as a starting material.

diluted with about five volumes of water to one volume of prewetting solution diluted with twelve volumes of water. We have found that the effectiveness of the fountain solution in extending the press life of a plate, drops rapidly to that secured using water as a fountain solution at dilutions greater than 112. An optimum fountain solution is one consisting of one volume of the prewetting solution diluted with about eight volumes of water.

Stated in another way, the fountain solution which we use in the method of lithographic printing in accordance with this invention comprises, essentially, an aqueous solution having dissolved therein an amount of polyacrylic acid or polymethacrylic acid within the range of about 0.04%, by weight, to about 2.5% by weight, and an alkali metal ferrocyanide or an alkali metal ferricyanide within the range of about 0.04%, by weight, to about 2.5%, by

weight, and preferably an amount of polyacrylic acid or polymethacrylic acid within the range of about 0.4%, by weight, to about 0.50%, by weight, and an amount of an alkali metal ferrocyanide or an alkali metal ferricyanide within the range of about 0.4% to about 0.50%, by weight. Solutions containing an amount of polyacrylic acid or polymethacrylic acid within the range of about 0.14%, by weigth, to about 0.35%, by weight, and an amount of alkali metal ferrocyanide or of an alkali metal ferricyanide within the range of about 0.14%, by weight, to about 0.35%, by weight, have been particularly satisfactory for use as aqueous fountain solutions in this method of printing, in providing an optimum balance between the cost of the solution and the number of clean copies which can be reproduced from a single plate. These fountain solutions should have a pH within the range of about 3.0 to about 7.25. A pH range of about 5.0 to about 6.0 has been found to be particularly suitable.

A fountain solution comprising an aqueous solution of about 0.2%, by weight, of polyacrylic acid and about 0.2%, by weight, of an alkali metal ferrocyanide, having a pH within the range of about 3.0 to about 7.25, and preferably, a pH adjusted to a value within the range of about 5.0 to about 6.0 by the addition of a tri-alkali metal phosphate, is representative of an optimum fountain solution for use in this printing method. This solution illustrates our preference for the use of proportions, by weight, of the polymeric carboxylic acid and of the alkali metal ferrocyanide or ferricyanide, which are relatively close to 1: 1, to secure the full benefit of the synergistic action of these two essential ingredients of our composition.

In this method of printing, it is possible to interrupt the printing run and to permit the printing plate to dry out, without taking special precautions to protect the printing plate from contact with air. nite advantage over the metal-base lithographic plates of the prior art. As is well known, the metal base plates must be protected from air both before the printing its started and during any interruption by a protective coating which must be removed to permit the utilization of the plate in printing. The application of the protective coating to the prior art metal based plates and its removal therefrom is time consuming and expensive in that it delays the printing operation and wastes valuable press time.

As pointed out in the foregoing, the method of printing in accordance with this invention. gives a plate life, in terms of useful, clean copies, manyfold that secured by the use of the plate with water as a fountain solution. The failure of the printing plate, when using water as a fountain solution, is by a progressive darkening of the background areas, giving them an increasingly unsightly, dirty appearance and, ultimately, a complete inking of the copy which renders the image invisible.

The ultimate failure of a fully-equivalent plate, when used by the printing method of this invention, is by a wearing out of the toner image, a break-up of the photoelectrostatic ooating and a deterioration of the base sheet. In the case of a paper-base plate, the failure of the image and of the photoelectrostatic coating which supports the image is caused primarily by the disintegration of the paper base after thirty to forty thousand copies have been produced. In the case of a plate which has a plastic base which is largely or completely unaffected by water, the failure of the plate is due primarily to the continued abrasion of the image and of the photoelectrostatic coating. Such a plate will reproduce a materially greater number of copies than a paper-base plate.

The different types of failure of the printing plate in accordance with this invention when, on one hand, using water as a fountain solution and, on the other, when using a diluted version of the prewetting solution as a fountain solution suggests that the bonding This is a defiof the soluble elements of the prewetting solution to the photoelectrostatic coating to render it hydrophilic is an adsorption phenomona, with the ions adsorbed by the surface being desorbed when using water as a fountain solution, but with such desorbtion being suppressed when the same ions are present in the fountain solution, This suggestion is not conclusive in ruling out a superficial or a loosely-bonded chemical reaction. It is recognized that adsorption and superficial chemical reaction differ in degree, rather than in kind. In any event, it will be fully appreciated that the mechanism involved in the bonding of the ions of the prewetting solution is immaterial to the improved result which we obtain. It will be fully understood that the foregoing comments .are solely for the purpose of fully explaining our invention, and that our invention is not limited to any particular mechanism by which the hydrophilic characteristics are produced and maintained during our printing operation.

The offset printing plate in accordance with this invention and the photoelectrostatic masters from which it is prepared are illustrated by the accompanying drawing in which like reference characters are used to represent like parts wherever they may occur.

In the drawing:

FIGURE 1 is a broken cross-section of a light-sensitizable photoelectrostatic master which is used to prepare the offset printing plate in accordance with this invention;

FIGURE 2 is a broken cross-section of a modified form of a light-sensitizable master which is suitable for the preparation of the offset printing plate of this invention;

FIGURE 3 is a broken cross-section of an offset printing plate in accordance with this invention, and

FIGURE 4 is a plan view of the offset printing plate in accordance with this invention.

Referring to FIGURES 1 and 3, the numeral 1 designates a base sheet which may be a sheet of paper or a self-sustaining plastic film, which carries on its surface a thin film 2, which in turn, carries on its surface a film or layer 3 of a finelydivided, photoconductive, inorganic pigment embedded in a matrix of an organic film-forming solid.

In the alternative in which the base sheet 1 is a paper sheet, the film 2 may be a film of a conventional coating material such as, for example, a casein-clay film. In this alternative, the combination of the paper sheet 1 and film 2 is, in effect, a commercial coated paper of any of the various types sold under numerous brand names by different manufacturers. Alternatively, the film 2 may be a lacquer having suspended therein a finely divided, electrically-conductive solid. Still, again, the film may be a thin, flexible film of metal.

In the alternative in which the base sheet 1 is a paper sheet and the film 2 is a conventional coating material such as, for example, a casein-clay film, the paper sheet may be rendered electrically conductive by the presence of its natural moisture content. However, it is preferably impregnated with hygroscopic inorganic salt such as, for example, zinc chloride, lithium chloride, potassium acetate, calcium chloride, zinc nitrate or potassium carbonate to render it uniformly electrically conductive under different climatic conditions, In the alternative in which the film 2 is a lacquer containing a finely-divided, electrically conductive solid such as, for example, graphite, aluminum pigment, powdered copper, powdered bronze, litharge, tannic oxide, copper sulfite, silver sulfite or an electrically-conductive grade of carbon black, or is a thin, flexible, film of metal such as, for example, a film of aluminum applied by vacuum metalizing or by cathodic spattering, this film 2 provides the requisite electrical conductivity required to render the sheet electrophotosensitivc.

In an embodiment of our invention in which the base sheet 1 is an unsupported plastic film, it may be, for example, a polyvinyl chloride film, a vinyl chloride-vinyl acetate copolymer film, a vinylidene chloride, vinyl chloride copolymer film, a cellulose acetate film, a cellulose triacetate film, a cellulose acetobutyrate film, a cellulose nitrate film, an ethyl cellulose film, a polytetrafluoroethylene film, a polychlorotrifluoroethylene film, a polystyrene film, an acrylonitrile-styrene copolymer film, a polyamide (nylon) film, a polyethylene terephthalate film, a polymethyl methacrylate film, a polyethylene film, a rubber hydrochloride film, or a vinylnitrile rubber film or any one of the various commercially available types. Such films are, of course, plasticized in the case of the plastics which require a plasticizer for the formation of a strong, flexible, unsupported film. Of these various plastic films, we prefer to use the polyethylene terephthalate film because of its excellent dimensional stability, high tensile strength, high tearing strength and good fold endurance.

In an embodiment of our invention in which the base sheet 1 is an unsupported plastic film, it is necessary for the film 2 to be either a lacquer film containing a finelydivided, electrically conductive solid or to be a thin, flexible, metal film such as one supplied by vacuum metalizing or cathodic spattering to render the sheet photoelectrostatic.

The finely-divided, photoconductive, inorganic pigment carried by the film or layer 3 may be, for example, zinc oxide or lead oxide. A satisfactory Zinc oxide for our purpose is that sold under the trade name of Florence Green Seal No. 8. This zinc oxide meets both ASTM specification D79-44 and Federal specification TT-Z301 and has the following analysis:

Percent Insoluble in HCl .03 Loss at 110 C .25 Total sulfur as S .04 Lead oxide (PbO) .01 Total zinc oxide as 2110 2 99.50

1 Maximum. 2 Maximum.

In this sensitive photoelectrostatic coating, we prefer to use about one part, by weight, of the insulating, filmforming material to an amount of the zinc oxide within the range of about 2.6 parts, by weight, to about 4.5 parts, by weight, and prefer to use about one part of the film-forming material to an amount of the zinc oxide within the range of about 2.8 parts, by weight, to about 4.0 parts, by weight.

The examples which follow specifically illustrate coating compositions which are suitable for the deposition of the light sensitizable, photoelectrostatic coating 3 on the paper base coated with the film 2 in the production of the new products in accordance with this invention. It will be understood that the compositions of the lightsensitizable films which are deposited by these various compositions are identical with the compositions of the non-volatile components therein, i.e., all the ingredients except the volatile solvent or solvents in the composition.

Example 1 Parts by weight Silicone resin solution (G.ESR82) 26 Zinc oxide (Florence Green Seal No. 8) 39 Toluene 35 The silicone resin solution used in this composition contained 60% solids, by weight, and 40%, by weight, of xylene. The solution had a viscosity within the range of about to 30 centipoises at 25 C. and was straw in color.

10 Example 2 Parts by weight Copolymer of n-butyl methacrylate and iso-butyl methacrylate (Lucite 46) 12.25 Zinc oxide (Florence Green Seal No. 8) 43.00 Toluene 56.25

The copolymer of n-butyl methacrylate and iso-butyl methacrylate used in this composition was prepared by the use of equal parts, by weight, of the two monomers and has a density of 1.05 and a refractive index at 25 C. of 1.4778.

Example 3 Parts by weight Polymerized iso-butyl methacrylate (Lucite 45) 12.25 Zinc oxide (Florence Green Seal No. 8) 43.00 Toluene 56.25

The polymerized iso-butyl methacrylate used in this composition had a density of 1.05, a refractive index of 1.477 and a dielectric constant of 2.5.

Example 4 Parts by weight Styrene butadiene copolymer (Pliolite SSB) 10.00 Zinc oxide (Florence Green Seal No. 8) 34.00 Toluene 71.50

Example 5 Percent by weight Styrenated alkyd copolymer solution (Cycopol S-101-1) 24.00 Zinc oxide (Florence Green Seal No. 8) 42.50 Xylene 33.50

The styrenated alkyd copolymer resin solution used in this composition contained 50%i1%, by weight, of solids dissolved in petroleum spirits. The solution had a color of 5-8 (Gardner 1933), a viscosity at 25 C. of Zl-Z4, and weighed 7.6 pounds per gallon. The solid resin had an acid number of 3-8.

The photoelectrostatic master illustrated by FIGURE 2, may have, like that illustrated by FIGURE 1, a base sheet 1 which may be either a paper sheet or a self-supporting plastic film. In the alternative in which the base sheet 1 is a paper sheet, the numeral 4 designates a film of a paper coating material such as, for example, a casein-clay film, While the film 2 is either a lacquer film having suspended therein a finely-divided, electrically conductive solid dispersed in a matrix of an organic filmforming solid, or a flexible film of metal. In the alternative in which the base sheet 1 is a flexible, self-supporting, plastic film and the film 2 is a thin, flexible, metal film, the film 4 is a resinous film which functions as a sub-coating to promote the adhesion between the surface of the plastic film 1 and the metal film 2. In this alternative, the film 4 may be, for example, a thin film of a copolyester of ethylene terephthalate and ethylene sebacate or a similar polyester.

Referring specifically to FIGURE 3, which shows a broken cross-section of an offset printing plate in accordance with this invention, which has been prepared from the photoelectrostatic master illustrated by FIGURE 1, the numerals 5, designate fused toner which has been deposited on the surface of the film 3 of the photoconductive pigment in the film-forming, insulating matrix of the styrenated alkyd copolymer resin in the photoelectrostatic reproduction process. The areas of the fused

toner

5, 5 may form either a positive or negative image and in the case of a lithographic printing plate is ordinarily a positive image, since these are the hydrophobic areas of its surface which are receptive to lithographic printing ink. The areas 6, 6 are exposed areas of the surface of the film 3 which have been treated with the aqueous prewetting composition in accordance with this invention to render them hydrophilic.

The particular embodiment of our offset printing plate illustrated by FIGURE 4 is provided with punched attachment edges for afiixing it to the cylinder of a lithographic printing press. Both its length between the attachment ends and its width are predetermined to permit its attachment to the particular printing press on which it is intended to be used. This printing plate may have the cross-section illustrated by FIGURE 3 and will, of course, carry on one of its surfaces an image formed by the

areas

5, 5 on which the fused toner is located, which is to be reproduced by the printing operation.

A printing plate in accordance with this invention prepared from the photoelectrostatic master illustrated by FIGURE 2, has a cross-section identical with that illustrated by FIGURE 3, except that it will have the film 4 of the master between its base sheet 1 and the film 2.

The prior art printing plates which have heretofore been prepared by photoelectrostatic methods have offered definite advantages over printing plates prepared by other methods in the rapidity and ease with which they may be prepared. However, after a short period of use in a printing operation, the background areas of these prior art printing plates tend to lose their hydrophilic character and they pick up traces of printing ink which causes the background areas of the printed copies to have a dirty, unsightly appearance which is highly undesirable.

The printing plates in accordance with this invention have all of the advantageous features of these prior printing plates prepared by photoelectrostatic methods. In addition to these inherent advantages, they are definitely superior to the printing plates which have heretofore been prepared by photoelectrostatic methods, due to the fact that their background areas tenaciously retain their hydrophilic character of being non-wettable by printing ink and, even after prolonged use, do produce printed copies with clean, ink-free background areas.

It will be understood from the foregoing that the term photoelectrostatic master, as used in this specification and in the appended claims, shall mean a sheet of paper or of a free plastic film carrying an outer surface coating which is adapted to receive a hydrophobic resinous toner image by photoelectrostatic reproduction, but which does not carry such an image, and which is adapted by form and dimensions for attachment to the roll of a lithographic printing press or of a lithographic office duplicating apparatus.

The term lithographic printing plate is used in the commonly accepted sense to mean a printing plate having hydrophobic image areas which are receptive to lithographic ink and hydrophilic background areas which, when wet with water, will not accept lithographic ink, including both plates adapted for use on a lithographic printing press and plates adapted for use on a lithographic ofiice duplicating machine.

The term lithographic printing is used in a broad sense to mean the reproduction of copy by the use of a lithographic printing press or a lithographic olfice copy apparatus.

The term lithographic printing machine is used in a broad sense to include both a lithographic printing press and a lithographic office copy apparatus.

In the foregoing, details and specific illustrations of the printing plates and of the method in accordance with this invention have been given for the purpose of fully explaining the nature of the invention. However, it will be understood that many variations can be made in the details which have been given, without departing from the spirit of this invention or the scope of the following claims.

We claim:

1. A lithographic printing plate comprising:

a base sheet which has dimensions and two opposite edges which adapt the sheet for attachment to the plate cylinder of a lithographic printing press;

a photoelectric film comprising a finely-divided, photoconductive, zinc oxide pigment dispersed in a matrix of a resinous, film-forming material which is inherently hydrophobic in character, covering one surface of the said base sheet;

a hydrophobic, resinous material covering predetermined areas of the said photoelectrostatic film in the form of an image to be reproduced by lithographic printing, leaving exposed areas of the photoelectrostatic film corresponding to the background areas of the image to be reproduced; and

a synergistic combination of a polymeric carboxylic acid selected from the group consisting of polyacrylic acid and polymethacrylic acid, with an inorganic salt selected from the group consisting of alkali metal ferrocyanides and alkali metal ferricyanides bonded to the exposed areas of the photoelectrostatic film, causing the said exposed areas to be hydrophilic in character, while the said resinous material forming the image areas remains hydrophobic in character.

2. A lithographic printing plate comprising:

a photoelectrostatic film comprising a finely-divided, photoconductive, zinc oxide pigment dispersed in a matrix of a resinous, film-forming material which is inherently hydrophobic in character, covering one surface of the said base sheet;

a synergistic combination of polyacrylic acid and an alkali metal ferrocyanide selected from the group consisting of sodium ferrocyanide and potassium ferrocyanide bonded to the exposed areas of the photoelectrostatic film, causing the said exposed areas to be hydrophilic in character, while the said resinous material forming the image areas remains hydrophobic in character.

3. A lithographic printing plate comprising:

a base sheet of coated paper which has dimensions and two opposite edges which adapt the sheet for attachment to the plate cylinder of a lithographic printing press;

a photoelectrostatic film comprising a finely-divided, photoconductive, zinc oxide pigment dispersed in a matrix of a resinous, film-forming material which is inherently hydrophobic in character, covering the coated surface of the said base sheet;

4. A lithographic printing plate comprising:

a plastic sheet having deposited on its surface an electrically conductive film, which has dimensions and two opposite edges which adapt the sheet for attachment to the plate cylinder of a lithographic printing press;

a photoelectrostatic film comprising a finely-divided, photoconductive, Zinc oxide pigment dispersed in a matrix of a resinous, film-forming material which is inherently hydrophobic in character, covering the electrically-conductive coating on the said base sheet;

5. A lithographic printing plate comprising:

a sheet of polyethylene terephthalate having deposited on its surface a metallic film, which has dimensions and two opposite edges which adapt the sheet for attachment to the plate cylinder of a lithographic printing press;

a photoelectrostatic film comprising a finely-divided, photoconductive, zinc oxide pigment dispersed in a matrix of a resinous, film-forming material which is inherently hydrophobic in character, covering the metallic coating on the said polyethylene terephthalate film;

6. A process for preparing a lithographic printing plate from a photoelectrostatic master comprising a relatively electrically-conductive backing sheet which carries, on one outer surface, a photoelectrostatic coating comprising a finely-divided, zinc oxide pigment dispersed in a matrix of a resinous, film-forming material which is inherently hydrophobic in nature, which comprises the steps ofi imposing a negative electrostatic charge on the photoelectrostatic coating carried by the said master, by exposing it to a corona discharge while protected from light;

exposing the charged surface of the photoelectrostatic coating to an optical image to produce a latent electrostatic image;

developing the latent electrostatic image by the selective deposition of charged particles of a resinous toner to form a toner image on the said photoelectrostatic surface which is inherently hydrophobic, while leaving exposed areas of the photoelectrostatic coating corresponding to the background areas of the resinous toner image; and

uniformly spreading over the resinous toner image and the exposed background areas of the photoelectrostatic coating an aqueous solution of a polymeric carboxylic acid selected from the group consisting of polyacrylic acid and polymethacrylic acid with an inorganic salt from the group consisting of alkali metal ferrocyanides and alkali metal ferricyanides, which has a pH within the range of about 3.0 to about 7.25, to render the exposed areas of the photoelectrostatic coating hydrophilic in character, without changing the hydrophobic nature of the toner image.

7. A process for preparing a lithographic printing plate from a photoelectrostatic master comprising a relatively electrically-conductive backing sheet which carries, on one outer surface, a photoelectrostatic coating comprising a finely-divided, zinc oxide pigment dispersed in a matrix of a resinous, film-forming material which is inherently hydrophobic in nature, which comprises the steps of:

imposing a negative electrostatic charge in the photoelectrostatic coating carried by the said master, by exposing it to a corona discharge while protected from light;

exposing the charged surface of the photoelectrostatic coating to an optical image to produce a latent electrostatic image; developing the latent electrostatic image by the selective deposition of charged particles of a resinous toner 'to form a toner image on the said photoelectrostatic surface which is inherently hydrophobic, while leaving exposed areas of the photoelectrostatic coating corresponding to the background areas of the resinous toner image; and

uniformly spreading over the resinous toner image and the exposed background areas of the photoelectrostatic coating an aqueous solution containing about 0.5 by weight, to about 2.5%, by weight, of a polymeric carboxylic acid selected from the group consisting of polyacrylic acid and polymethacrylic acid and about 0.5%, by weight, to about 2.5 by weight, of an inorganic salt selected from the group consisting of alkali metal ferrocyanides and alkali metal ferricyanides, which is bufiered to a pH within the range of about 3.0 to about 7.25 by an alkali metal phosphate.

8. A process for preparing a lithographic printing plate from a photoelectrostatic master comprising a relatively electrically-conductive backing sheet which carries, on one outer surface, a photoelectrostatic coating comprising a finely-divided, zinc oxide pigment dispersed in a matrix of a resinous, film-forming material which is inherently hydrophobic in nature, which comprises the steps of:

imposing a negative electrostatic charge on the photoelectrostatic coating carried by the said master, by exposing it to a corona discharge while protected from light;

developing the latent electrostatic image by the selective deposition of charged particles of a resinous toner to form a toner image on the said photoelectrostatic surface which is inherently hydrophobic, while leaving exposed areas of the photoelectrostatic of polyacrylic acid and polymethacrylic acid and an inorganic salt selected from the group consisting of alkali metal ferrocyanides and alkali metal ferricyanides, and has a pH within the range of about 3.0 to about 7.25.

11. A method for lithographic printing which comprises:

attaching to the plate cylinder of an offset printing macarboxylic acid selected from the group consisting of polyacrylic acid and polymethacrylic acid with an inorganic salt from the group consisting of alkali metal ferrocyanides and alkali metal ferricyanides, which has a pH .within the range of about 3.0 to about 7.25, to render the exposed areas of the photoelectrostatic coating hydrophilic in character, without changing the hydrophobic nature of the toner image,

and permitting the water of the said aqueous solution to evaporate from the surface of the plate.

9. A process for preparing a lithographic printing plate from a photoelectrostatic master comprising a relatively electrically-conductive backing sheet which carries, on one outer surface, a photoelcctrostatic coating comprising a finely-divided, zinc oxide pigment dispersed in a matrix of a resinous, film-forming material which is inherently hydrophobic in nature, which comprises the steps of:

chine a printing plate described by claim 1, which is adapted by its dimensions and two opposite edges for attachment to that particular cylinder; and

reproducing copies by the use of said machine, while 12. A method for lithographic printing which comprises:

attaching to the plate cylinder of an offset printing maimposing a negative electrostatic charge on the photoelectrostatic coating carried by the said master, by exposing it to a corona discharge while protected from light; exposing the charged surface of the photoelectrostatic chine a printing plate described by claim 1, which is adapted by its dimensions and two opposite edges coating to an optical image to produce a latent elecfor attachment to that particular cylinder; and trostatic image; reproducing copies by the use of the said machine while developing the latent electrostatic image by the seleckeeping the hydrophilic background areas of the said tive deposition of charged particles of a resinous printing plate wet with an aqueous fountain solutoner to form a toner image on the said photoelection having dissolved therein an amount of polytrostatic surface which is inherently hydrophobic, acrylic acid within the range of about 0.14%, by

while leaving exposed areas of the photoelectrostatic coating corresponding to the background areas of weight, to about 0.35%, by weight, and an amount of an alkali metal ferrocyanide selected from the group consisting of sodium ferrocyanide and potassium ferrocyanide within the range of about 0.14%, by weight, to about 0.35%, by weight, and having a pH Within the range of about 3.0 to about 7.25.

the resinous toner image; and

uniformly spreading over the resinous toner image and the exposed background areas of the photoelectro static coating an aqueous solution containing about 0.5%, by weight, to about 2.5%, by weight, of polyaerylic acid and about 0.5%, by weight, to about 2.5%, by weight, of an alkali metal ferrocyanide selected from the group consisting of sodium ferro- References Cited by the Examiner UNITED STATES PATENTS cyanide and potassium ferrocyanide which has a pH 2186945 1/1940 Wood 101149'2 2,993,787 7/1961 Sugarman. within the range of about 3.0 to about 7.25. 10 A method for litho ra hic rintin which com- 2997387 8/1961 Tanenbaum' prise; P P g 3,001,872 9/1961 Kurz 101 149.2 x

3,010,883 11/1961 Johnson et a1. attaching to the plate cylinder of an offset printing 3,107,169 V1963 Bomarth 101 149.2 X

machine a printing plate described by claim 1, which is adapted by its dimensions and two opposite edges for attachment to that particular cylinder; and reproducing copies by the use of the said machine while keeping the hydrophilic background areas of the said printing plate wet with an aqueous fountain solution which has dissolved therein a polymeric carboxylic acid selected from the group consisting OTHER REFERENCES Sugar: ElectrofaxA New Tool for the Graphic Arts,

reprinted (by RCA Laboratories) from Proceedings of the Seventh Annual Meeting of the Technical Association of the Graphic Arts, May 1955.

DAVID KLEIN, Primary Examiner.

Claims

1. A LLITHOGRAPH PRINTING PLATE COMPRISING: A BASE SHEET WHICH HAS DIMENSIONS AND TWO OPPOSITE EDGES WHICH ADAPT THE SHEET FOR ATTACHMENT TO THE PLATE CYLINDER OF A LITHOGRAPHIC PRINTING PRESS; A PHOTOELECTRIC FILM COMPRISING A FINELY-DIVIDED, PHOTOCONDUCTIVE, ZINC OXIDE PIGMENT DISPERSED IN A MATRIX OF A RESINOUS, FILM-FORMING MATERIAL WHICH IS INHERENTLY HYDROPHOBIC IN CHARACTER, COVERING ONE SURFACE OF THE SAID BASE SHEET; A HYDROPHOBIC, RESINOUS MATERIAL COVERING PREDETERMINED AREAS OF THE SAID PHOTOELECTROSTATIC FILM IN THE FORM OF AN IMAGE TO BE REPRODUCED BY LITHOGRAPHIC PRINTING, LEAVING EXPOSED AREAS OF THE PHOTOELECTROSTATIC FILM CORRESPONDING TO THE BACKGROUND AREAS OF THE IMAGE TO BE REPRODUCE; AND A SYNERGISTIC COMBINATION OF A POLYMERIC CARBOXYLIC ACID SELECTED FROM THE GROUP CONSISTING OF POLYACRYLIC ACID AND POLYMETHACRYLIC ACID, WITH AN INORGANIC SALT SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL FERROCYANIDES AND ALKALI METAL FERRICYANIDES BONDED TO THE EXPOSED AREAS OF THE PHOTOELECTROSTATIC FILM, CAUSING THE SAID EXPOSED AREAS TO BE HYDROPHILIC IN CHARACTER, WHILE THE SAID RESINOUS MATERIAL FORMING THE IMAGE AREAS REMAINS HYDROPHOBIC IN CHARACTER.

10. A METHOD FOR LITHOGRAPHIC PRINTING WHICH COMPRISES: ATTACHING TO THE PLATE CYLINDER OF AN OFFSET PRINTING MACHINE A PRINTING PLATE DESCRIBED BY CLAIM 1, WHICH IS ADAPTED BY ITS DIMENSIONS AND TWO OPPOSITE EDGES FOR ATTACHMENT TO THAT PARTICULAR CYLINDER; AND REPRODUCING COPIES BY THE USE OF THE SAID MACHINE WHILE KEEPING THE HYDROPHILIC BACKGROUND AREAS OF THE SAID PRINTING PLATE WET WITH AN AQUEOUS FOUNTAIN SOLUTION WHICH HAS DISSOLVED THEREIN A POLYMERIC CARBOXYLIC ACID SELECTED FROM THE GROUP CONSISTING OF POLYACRYLIC ACID AND POLYMETHACRYLIC ACID AND AN INORGANIC SALT SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL FERROCYANIDES AND ALKALI METAL FERRICYANIDES, AND HAS A PH WITHIN THE RANGE OF ABOUT 3.0 TO ABOUT 7.25.