US3106158A - Method of preparing lithographic printing plates - Google Patents

Description

'\image areas transfers thereto.

United States Patent f 3,106,158 METHOD OF PREPARING LITHOGRAPHIC PRINTING PLATES viichael Michalchik, Levittown, N.J., assignor to Radio Corporation of America, a corporation of Delaware No Drawing. Filed Jan. 22, 1962, Ser. No. 167,908

5 Claims. (Cl. 101-1492) This invention relates to improved lithographic print ing plates and, more particularly, to the use of improved conversion solutions for preparing lithographic printing plates from prints made by an electrostatic duplicating process.

Lithographic printing is a Well known form of planographic printing. In general, the process includes printing from a flat (or planographic) plate and it depends upon different properties of the image and non-image areas for printability. In the ordinary lithographic printing process a printing plate is prepared by afiixing to a Water-attractive, hydrophilic surface, a water-repellent hydrophobic image, usually greasy, resinous or waxy in nature. An aqueous wet-out liquid is then applied to the surface so imaged. This liquid wets all portions of the surface not occupied by the hydrophobic image matetrial but does not wet the hydrophobic image.

An inking roll coated with a grease-base lithographic printing ink is passed across the surface of the printing plate, leaving a film of ink upon the ink-receptive hydrophobic irnage areas. No ink is left on the non-image areas which are covered by a film of ink-repelling wetout liquid.

When the inked plate is brought into contact with another ink-receptive surface, some of the ink from the be transferred directly to a paper sheet but generally is transferred, first, to a rubber offset blanket which, in .turn, is used to transfer the print to a final paper sheet. For each print made during a printing run, the printing plate is dampened with the wet-on or, as it is usually called, the fountain solution, for the purpose of keeping the non-image areas wet. I

In the past, plano graphic printing plates have been made of zinc or aluminum sheets having surfaces especially treated to render them hydrophilic. For shorter printing ,runs, paper base plates coated with a pigment dispersed in a hydrophilic adhesive, have been used.

Images have generally been placed on the metal plates photographically with the aid of a special photographically sensitized master, or, non-photographically, as with a grease pencil. Images have generally been placed on the paper base plates by typing using a special ribbon.

More recently, electrostatic methods have been used for applying images to lithographic printing plates. This type of process, in general, comprises placing an over-all, uniform, electrostatic charge on a plate having a surface made of a photoconductive material, such as selenium, exposing the plate to an image of light and shadow to be reproduced, thereby removing the electrostatic charge in those areas of the plate receiving light, and thus producing an electrostatic charge pattern on the plate surface exactly corresponding to the dark areas of the original to be reproduced. The charge pattern is made visible by contacting the surface heating the electrostatic charge pattern The ink image may thus zinc oxide, and a resin binder.

3,106,158 Patented Oct. 8, 1963 IQC 2 with a finely divided electroscopic developer material (also referred to herein as a toner) bearing a charge opposite to that of the electrostatic image, whereby the developer material deposits on the plate in image configuration.

The powder image must then be transferred to the printing plate surface and stabilized as by heating to a temperature sufiiciently high to fuse the developer material and cause it to adhere to the plate surface.

Still more recently, the above-described electrostatic method has been improved by eliminating the transfer step necessitated by using a selenium plate to first record the light image. In the improved method the image is recorded and fixed directly upon an improved type of printing plate. The improved type of printing plate has a printing surface composed of a powdered photoconductive material, preferably a particular grade of photoconducting The resin binder must have good electrical insulating properties since the plate must be able to hold an electrostatic charge for an appreciable length of time in darkness. It has further been found that resins which are hydrophilic are unsuitable since they apparently attract sufficient Water to lower the surface resistance to an unsuitable level. This permits too much lateral leakage of the electrostatic-charge and prevents storage of an electrostatic charge pattern long enough to permit satisfactory developing. It has therefore been found necessary to limit the resin binders in the above described recording composition to those which are hydrophobic.

As previously pointed out, a lithographic printing plate surface must have hydrophilic properties in its non-printing areas. Thus, in order to use a conventional zinc oxide-resin coated electrostatic recording surface as a lithographic printing plate, the hydrophobic surface must first'ebe converted to hydrophilic. Solutions for accomplishing this conversion have been devised and have been used with more or less success. Some of these, however, require an unduly long treating time and do not work satisfactorily unless the plate is completely immersed in the solntionfor an appreciable period of time. It is desirable to be able to accomplish the conversion step in a highly effective manner but in as short a time as possible and with assimple a treatingtechnique as possible.

In the case of zinc oxide-resincoated plates, the conversion solution reacts with the -zinc oxide to produce an insoluble compound that attracts water to its surface. A

further requirement for a satisfactorily acting conversion solution is that it shall not undercut the fused toner deposit on the printing surface or weaken the deposit in any other manner.

One object of the present invention is to provide an improved conversion solution for converting a zinc oxidehydrophobic resin surface from hydrophobic to hydrophilic Example I A planographic plate is prepared by spray-coating an aluminum sheet to a thickness of about 1.0 (after drying) with the following composition:

Pliolite S-7 (butadiene-styrene copolymer of the Goodyear Tire and Rubber Company -gm 270 SR-82 (silicone resin of the General Electric Company) gm 135 Zinc oxide (Green Seal #8, New Jersey Zinc Company) gm 800 Xylene ml 2,000 Dye (2% solution of sodium fluorescein in methanol) ml 11 The plate is permitted to dry in air until the xylene has evaporated.

A printing image is applied to the plate as follows:

First, the surface of the plate is given a uniform electrostatic charge (in the dark) by passing a high voltage corona discharge unit across it. Next, the charged suriiace is exposed to a positive image pattern of the subject matter is to be printed so that a charge image of the subject matter remains on the surface of the printing plate.

Then the charge image (again in the dark) is developed by sweeping across the surface a mixture of iron particles free from grease and other contaminants and having a greatest dimension between about 0.002" and 0.008", and an easily fusible powder or toner above iron in the triboelectric series. The iron merely serves as a carrier for the toner particles. Particles of toner are attracted to the electrostatic charge image and deposit in the charged areas.

A suitable toner can be made by, first, melting then cooling and ball milling together 200 g. of Piccolastic resin 4358A (Pennsylvania Industrial Chemical Co.), 12 g. carbon black G, 12 g. Nigrosine SSB dye and 8 g. Isol Black dye. The ground powder is finally screened through a 200 mesh screen.

The toner particles are fused to the zinc oxide-resin coated surface by heating with an infra-red lamp for about 20 seconds at 190 C. This completes the formation of the printing image.

Using the printing plate as described'above, a planegraphic printing run is carried out as follows.

A conversion solution in accordance with the present invention is made up with the composition:

Potassium cobalt'icyanide g 5 Glacial acetic acid ml 2 Ammonium dichromate g 0.10 Water ml 100 The solution is swabbed on the image-containing surface of the plate so that the entire surface is thoroughly and uniformly wetted. The solution is allowed to react with the zinc oxide-resin surface for 20-30 seconds. The cobalticyanide reacts with at least part of the zinc oxide in the non-image areas forming an insoluble zinc cobalticyanide which is hydrophilic.

V In order to increase contrast in the printed product, the plate is next treated with a solution of Platex (Addres-sograph-Multigraph Corp, Cleveland, Ohio). P-la'tex is believed to comprise a nickel salt, ammonium acid phosphate, butanal, cellulose gum, diethylene glycol, formaldehyde, and water. The solution is applied in the form of one volume Platex to 3 volumes water. This solution reduces any tendency of the conversion solution to wet the black areas of the printing surface.

The next step in the process is to swab the printing surface of the plate with the fountain solution to be used on the press during the printing run. Sufiicient fountain solution is applied in this manner to wash off the Platex.

The plate is mounted on a lithographic press and wet with a fountain solution each time a print is made. A suitable fountain solution comprises:

Sodium ferrocyanide 0.1% byweight. Glacial acetic acid 0.5% by volume. Glycerine 1.0% by weight. Water balance.

' Example 11 As in Example 1, a printing plate was made up by coating an aluminum sheet with the following composition:

Parts by Weight 'Piccolastic resin D 60 Piccolastic resin C-l25 40 Carbon black 7 Calco Black Dye (F-4610B, Allied Chemical and Dye Corp.) 2

The resin is melted and the carbon black added to the melt, followed by the dye. The ingredients are thoroughly mixed at a temperature of about 175 -200 C. The mixture isthen cooled and the hard cake is crushed and screened to 20 mesh. This powder is micronized to an average particle size of 5-10 microns.

After this toner is applied to the charge image, it can be fused to the zinc oxide-resin surface by heating to the relatively low temperature of C. lfOl' 30 seconds.

The remainder of the process is the same as in Example I except that the amount of potassium cob-alticyanide in the conversion solution is 2.5 g.

Example III A printing plate was made up by coating one side of a sheet of paper with the following composition:

Silicone resin solution (SR-'82, 60% solids content The silicone resin solution and toluene are mixed, then the zinc oxide is added with stirring. This mixture is dispersed in a blender until a smooth paint-like mixture is obtained. The mixture is applied uniformly to one side of the paper sheet with a doctor blade. sensitizing dyes such as Rose Bengal 01' Sodium Fluorescein may be included in the composition. As in the previous examples, the coating must be dried by driving off the solvents.

This plate is used for lithographic printing as in the previous examples.

The improved conversion solution of the present invention may be varied somewhat within the scope of the invention. Other alkali cobalticyanides, for example, sodium or lithium, may be used instead of the potassium salt given in the example. The concentration of this salt may be varied between about 0.5 and 5.0 g. per 100 ml. of Water. But the solution may be much more dilute if the plate is dipped instead of swabbed. The acetic acid may be varied between about one and four ml. per 10 0 ml. of water and any one of a number of equivalent acids, previously mentioned, may be used. Also, the dichromate may be any alkali metal dichromate instead of ammonium dichromate.

The hydrophobic resin used as the binder with the Zinc oxide of the printing plate is not critical. However, the a resin binder must not be hardened to the extent that the conversion solution cannot penetrate to act on the zinc oxide. What is claimed is: V 1. A planographic printing plate having non-image areas comprising zinc oxide and a hydrophobic resin binder, said areas having been rendered hydrophilic by converting at least part of said zinc oxide to Zinc cobalticyanide.

2. A planographic printing plate having non-image areas comprising zinc oxide and a hydrophobic resin binder, said areas having been rendered hydrophilic by treating with an aqueous solution comprising an alkali metal cobalticyanide, an acid and an alkali dichromate to convert at least part of said zinc oxide to zinc cobalticyanide.

3. A planographic printing plate having non-image areas comprising zinc oxide and a hydrophobic resin binder, said areas having been rendered hydrophilic by treating with an aqueous solution comprising potassium cobalticyanide, acetic acid and ammonium dichromate.

4. In the method of phanographic printing wherein the printing plate has non-image areas comprising zinc oxide and a hydrophobic resin binder, the step of rendering said non-image areas hydrophilic which comprises treating said areas with an aqueous solution comprising an alkali metal cobalticyanide, an acid and an alkali dichromate.

5. In the method of planographic printing wherein the printing plate has non-image areas comprising zinc oxide and a hydrophobic resin binder, the step of rendering said non-image areas hydrophilic which comprises treating said areas with an aqueous solution comprising an alkali metal cobalticyanide, acetic acid and ammonium dichromate.

References Cited in the file of this patent UNITED STATES PATENTS 2,957,765 Resetich Oct. 25, 1960 2,993,787 Sugarm-an July 25, 1961 3,001,872 Kurz Sept. 26, 1961

Claims (1)

1. A PLANOGRAPHIC PRINTING PLATE HAVING NON-IMAGE AREAS COMPRISING ZINC OXIDE AND A HYDROPHOBIC RESIN BINDER, SAID AREAS HAVING BEEN RENDERED HYDROPHILIC BY CONVERTING AT LEAST PART OF SAID ZINC OXIDE TO ZINC COBALTICYANIDE.