Method Of Preparing Lithographic Printing Plate

Abstract

The printing plate and process for preparing the same described herein comprises a printing plate suitable for dry lithography having in the non-image producing areas an ink repellent coating such as a cured elastomeric organopolysiloxane and in the ink receptive image producing areas the same cured elastomeric organopolysiloxane coated with a fused toner, and having under said organopolysiloxane layer a base which may be of various materials. In a preferred process for preparing this printing plate, the base is an electrophotographic sheet which is given an electrostatic charge which serves as the means for attracting and holding the toner imposed on the polysiloxane layer until the toner image is fused onto the polysiloxane layer. In another modification an image of toner is imprinted on one base and then the toner image is superimposed and transferred to the organopolysiloxane coating and the toner fused to make it ink receptive.

Description

BACKGROUND OF THE INVENTION

This invention relates to planographic printing. More specifically, it relates to lithographic printing which does not require water or dampening solutions applied in the non-image areas to prevent the printing ink from depositing in these non-image areas. Still more particularly, it relates to planographic plates and printing in which the non-image areas comprise a layer of material which itself repels the printing ink and the image producing areas comprise a layer of fused toner which has been deposited and retained on the otherwise repellent surface by means of a charge retained by an electrophotographic base positioned between the layer of repellent material, with the toner being subsequently fused and permanently affixed to the repellent layer.

RELATED PRIOR ART

Lithographic printing has been a widely accepted and well recognized part of the printing art for many years. This is based on the principle that water is coated over the non-image areas of a printing surface and when ink is imposed on the printing surface it is retained only in the areas which have not been wet with water, whereas the water-wet areas reject the oil-based printing ink. There are many problems associated with this type of printing, such as back-flowing, the requirement for high quality paper stock, control of the delicate balance between ink and fountain solution in order to maintain image fidelity and uniformity, the need for constant attention to guard against changes in printing as conditions change on the press during the course of a printing run.

The desire and need for improvements in this art are evidenced by the number of patents issued in this field, many of which are directed to reducing the amount of water needed and otherwise controlling the quality of printing produced by such processes. Typical of these patents are U.S. Pat. Nos. 1,406,837, 1,817,522, 2,090,704, 2,381,704 and 3,025,789.

The Curtin Patent No. 3,511,178 discloses the use of a cured elastomeric organopolysiloxane layer in the non-image areas to repel the printing ink and having the organopolysiloxane etched away in the image areas, or altered in these areas by electrical discharge to remove from the image areas the material or the property of the material which will repel printing ink. The etching operation is found to be rather cumbersome and requires very careful control to maintain fidelity and accuracy in reproducing the image. The electrical discharge method is found to be much less effective than the etching operation.

STATEMENT OF THE INVENTION

In accordance with the present invention, it has been found that the image area of the organopolysiloxane layer can be more easily and more accurately reproduced by having under this layer an electrophotographic base which will allow the composite sheet to be charged electrostatically and to hold onto the cured elastomeric organopolysiloxane layer a toner reproduced image which can be made permanently receptive to printing ink by fusing the toner into position. Most surprisingly it has been found that coating of the electrophotographic base with the polysiloxane elastomer does not destroy or interfere with the ability of the electrostatic base to retain the electrostatic charge.

In one modification, the desired image, such as a photographic positive, is superimposed on the base on which an electrostatic charge has been imposed by corona discharge, and upon exposure of the image to light the electrostatic charge is dissipated in the non-image areas and retained only in the desired image areas. The toner powder is then applied to the entire surface of the printing area. The toner is rejected in the non-image areas and retained in the image areas. Then the toner is fused permanently in place in accordance with conditions normally used for the particular toner and the fused toner thereby becomes ink receptive and capable of printing the desired image.

In a second modification, the toner is not directly imposed on the repellent polysiloxane surface, but the image is placed on a separate, independent electrostatically charged master, exposed to light and the toner applied to such separate master to reproduce the image with toner particles positioned in the image areas. Then the electrostatic base which is coated with the ink repellent organopolysiloxane coating is given an electrostatic charge over its entire surface. The toner image on the separate master is thereafter superimposed on the organopolysiloxane surface with the toner image in contact with the repellent surface, and by hand applied to roller applied pressure is transferred to that surface. The electrostatic charge on the siloxane-coated base attracts and holds the toner so that it is transferred to the polysiloxane surface. Then the image toner is fused and permanently affixed to the polysiloxane surface so that the image areas now become receptive to printing ink.

In a third modification, the second modification is repeated except that the base on which the silicone coating is applied does not have an electrostatic charge and the toner image is transferred from the transfer sheet and its toner image are superimposed on the ink repellent surface.

In still another modification the silicone coating can comprise a mixture of the polysiloxane and zinc oxide. The amount of zinc oxide in range from 0 percent to 60 percent by weight with the balance of the coating material comprising the silicone or polysiloxane material. The preferred mixture is about 50 percent by weight of each of the two components.

The expression "toner" as used herein is well known in the electrostatic printing art and refers to a pigment, either organic or inorganic, coated with a binder, generally a resin, and carrying an electrostatic charge so that it is attracted to a sheet or master having an opposite electrostatic charge in the image area, or less frequently in the non-image area, to which the toner particles are to be deposited. When toner particles have been appropriately applied in the desired areas, heat is applied to fuse the binder material and thereby permanently affix the toner in position.

Particularly effective materials found to function as ink repellent surfaces are cured, solid, rubbery organopolysiloxanes. There are also known as "silicone elastomers" formed by continued polymerization or curing of silicone "gums" achieved by heating in the temperature range of 30.degree.C to 200.degree.C. A particularly useful commercial product is General Electric's RTV-108.

Specific silicone gums can be used having the linear repeating unit ##SPC1##

There may be as few as two such repeating units or as many as 20,000 or more. The R groups may be identical or may be different, but they are selected from the class consisting of monovalent alkyl or aryl radicals, halogenated alkyl or aryl radicals, cyanolkyls. A small proportion of these may be vinyl, phenyl, halogenated vinyl, halogenated phenyl and the like. While other alkyl groups, up to 10 carbon atoms, may be used, and preferably a methyl group. The end groups in the polymers may be ##SPC2##

wherein Ac is a saturated aliphatic monoacyl radical having no more than 10 carbon atoms, preferably no more than four carbon atoms.

Silicone elastomers, formed by continuing the polymerization of the gums just referred to, are characterized generally as very sparsely crosslinked dimethylpolysiloxanes of high molecular weights, that is about 400,000-800,00 average molecular weight. These sparsely crosslinked polymers are indicated by R/Si ratios very close to 2, generally above 1.95, or even above 1.99, but generally below 2.1, or even below 2.01. Generally there are 200-500 dimethyl units between crosslink sites. In contrast, the much more densely crosslinked silicone resins found suitable for commercial use have R/Si ratios in the range of 1.2 - 1.5.

The silicone gums are preferably compounded with catalysts for promotion of cure, and also with fillers, such as silica, to improve the chemical properties. Available commercial silicone gums generally provide satisfactory repellency on curing. These also have superior mechanical properties which minimize wear caused by physical handling and by the abrasive conditions encountered on the press. The gum may be filled or unfilled. A particularly effective non-filled silicone gum is General Electric's silicone gum SE-76 having an average molecular weight of 400,000 - 500,000 and curing to an elastomeric satisfactorily repellent surface.

In evaluating materials as to being sufficiently ink repellent, such as the silicone elastomers described herein, it has been found that the repellency increases directly as the "release value" decreases. The effectiveness of a release agent surface is measured by recognized test by applying a piece of adhesive tape, such as surgical tape, to the surface and thereafter measuring the amount of force required to strip the adhesive tape from the surface.

In determining the "release value," an "Instron" Model TM is employed, which operates at a crosshead speed of 12 inches per minute and chart speed of 2 inches per minute. One inch Johnson & Johnson "Red Cross" brand, waterproof adhesive tape is used, selecting only a roll having a retention force of about 450 grams (425-475) as measured at 80.degree. F. on a 24-gauge, No. 4 finish stainless steel test panel. In determining either the retentive force of the tape to be used or the release value of a sample, a 10-inch strip of tape is applied to a 6-inch by 1 1/2 inch panel by passing a 4 1/2 pound rubber-faced tape roller twice over the tape, using only the weight of the roller. The sample is immediately placed in the Instron and the force in grams determined necesary to strip the tape at an angle of 180.degree..

The amount of force required to strip the tape is referred to as the "release value," and the larger the release value, the more adhesion there is between the adhesive tape and the surface. A small release value indicates a very effective release coating or a high repellency, and a large release value indicates a very ineffective release coating or low repellency. Standard tests for release value are described in TAPPI (Technical Association for the Pulp and Paper Industry), Vol. 43, No. 8, Pages 164A and 165A (August, 1960) and TAPPI Routine Control Method (not a TAPPI standard) RC-283 "Quality of Release Coatings" issued in 1960.

The General Electric RTV-108 described herein has a release value of 12 grams per inch and the SE 76 has a value of 1 gram per inch. Suitable silicone elastomer surfaces have been found to have a release value of only 1 gram per inch but no greater than 30 grams per inch. These materials are adequately non-adhesive or repellent to reject ink from an inking roll to serve as non-image background. Silicone resins, on the other hand, have release values in the range of 250-550 grams per inch and are unsatisfactory as ink repellent materials. For this purpose of this invention, a repellent material should have a release value of 30 or less. Materials with release values up to about 100 grams per inch have some utility and those having a value in excess of 100 definitely unsatisfactory for even the poorest equality of duplicator copy.

The base, or base sheet, may be of any suitable material such as paper, plastic, metal sheet or foil, with the properties such as strength, flexibility, etc., being determined according to the use to which the ultimate plate is to be put. Where an electrophotographic base is specified, means one capable of being electrostatically charged by a corona discharge or other electrostatic charging means, this refers to a paper base or other type of base coated with zinc oxide or other materials known in the art as being capable of accepting and retaining an electrostatic charge. These are well known in the art and numerous types are available commercially.

Typical base materials capable of being electrostatically charged are described in U.S. Pat. Nos. 3,052,539, 3,052,540, 3,121,006, 3,128,204, 3,160,503 and 3,481,734. In addition to zinc oxide as photoconductive materials there may be used oxides and sulfoxides of cadmium, mercury, antimony, bismuth, thallium, molybdenum, aluminum, lead or zinc, as well as organic substances such as anthracene, benzidine, anthraquinone, acenaphthene, fluoranthene, naphthalene, etc. The resinous binding agent may be a heat-hardenable melamine-formaldehyde resin in an amount between 0.3 and 2 parts of resin per part of zinc oxide, urea-formaldehyde resins, xylene-formaldehyde resins, polyvinyl chloride, polyvinylidene chloride, polyvinyl ethers, polyacrylic esters, polymethacrylic esters, etc.

Since the invention does not reside in the particular composition of the coating used for accepting and retaining the electrostatic charge, and the various types available commercially are suited for present purposes, a description of the composition of these electrostatic bases is not considered necessary for an understanding and practice of the present invention. In any case, since zinc oxide coated paper is easily available, economical and very satisfactory for this purpose, it is preferred.

Toners generally suitable for use in the commercially available electrostatic "copying" machines are suitable for the purpose of this invention. These are generally pigments such as carbon black, aniline black, Prussian blue, phthalocyanine blue, red oxide, cadmium yellow, and various resin dyes such as nigrosine resin dye, etc., mixed with or coated with a binder material, generally a resin or an organic material which hardens, or fuses, and thereby binds the pigment to the base, such as zinc oxide coated paper, and the like, on which the copy image is being made. Typical toners are described in U.S. Pat. Nos. 3,320,169 and 3,554,946, although various others known in the art are also suitable for this purpose.

Various resins can be used as the binder material including such thermoplastic and thermosetting resins as polyamides, of both the nylon and aminoacid or polycaprolactam types, polystyrene, polyvinylchloride, vinyl chloride-vinyl acetate copolymers, ethylene-vinyl acetate copolymers, alkyd resins such as pentaerythrityl abietate, etc. These resins advantageously have a melting or curing temperature in the range of 80.degree.-150.degree. C., preferably 100.degree.-130.degree. C.

In applying an electrostatic charge where indicated in the practice of this invention, various types of corona discharge equipment as are present in commercial copy units are suitable for this purpose. In these commercial units a corona discharge is used to apply an electrostatic charge. Depending on the particular type of equipment used and the voltage and other conditions applying, the base material such as the zinc oxide coated paper may be given a single pass to develop the desired amount of electrostatic charge. In other cases, depending on the particular conditions, several passes may be desired for this purpose. In the example described hereinafter, a corona discharge is used with 7,000 volts and a metal ground plate beneath the paper. Two or three passes may be used to develop the desired charge. However, the paticular manner of applying an electrostatic charge is not a part of or critical to this invention.

Various methods of practicing the invention are illustrated by the following examples. These examples are intended merely to illustrate the invention and not in any sense to limit the manner in which the invention can be practiced. The parts and percentages recited therein and all through the specification, unless specifically provided otherwise, are by weight.

EXAMPLE I

The repellent coating is applied by preparing a solution of 20 parts of the higher molecular weight linear polydimethylsiloxane available commercially as "G.E. RTV-108" in 80 parts heptane, the gum being stirred into solution at room temperature. A coating is applied by passing a commercial zinc oxide coated paper (a ZnO resin binder coating having 70 percent ZnO and 30 percent pentaerythrityl abietate resin binder) with the zinc oxide coated side outward around a back-up roller positioned below a gravity feed extrusion coating head at a web speed of about 7 feet per minute, and applying a coating weight of polysiloxane on a dry basis of 300 mg/sq.ft. The coating is then cured by heating for 10 minutes at 95.degree. C. in an oven situated in aa room wherein the air at 22.degree. C., is at a relative humidity in the range of 10 - 40 percent, preferably about 30 percent relative humidity. The coating is cured and converted to a firm, tough, highly repellent surface.

The web is then passed through a second oven at 150.degree. C. for about 10 minutes, whereby the adhesion to the base is improved. The coated sheet is exposed to a corona discharge for 1.75 seconds so that the composite sheet is electrostatically charged. The image is superimposed on the electrostatically charged, highly ink repellent surface and exposed to actinic light for 0.5 seconds. The light transmitted through the non-image areas dissipates the charge in those areas, and the charge is retained in the unexposed image areas. Then the image is removed and a toner powder is applied to the outer surface of the plate. This toner comprises 35 percent of a nylon type polyamide having a melting point of about 110.degree. C., 10 percent of a copolymer of 72 percent ethylene and 28 percent vinyl acetate having a melting point of 255.degree. C., 45 percent of a polystyrene having an average number molecular weight of 22,000 and a melting point of about 170.degree. C., and 10 percent Nigrosine resin dye. The toner is not attracted by the exposed or non-image areas on the silicone surface, but is retained in the non-exposed or image areas, the toner being attracted and retained by the electrostatic charge. The toner is fused, preferably by exposure to radiant energy, in accordance with standard procedures.

The resultant plate is run on a standard offset lithographic press from which the water dampening system has been removed. The ink used is a Driographic black ink (3M Driographic Ink Code 33-A). 1,000 impressions are made without any image breakdown, and the background remains clean throughout the entire run.

EXAMPLE II

The procedure of Example I is repeated except that the electophotographic base is omitted and a plain sheet of paper is substituted in order to test whether exposure of the ink repellent sheet to the corona discharge produces sufficient adhesion in the image areas to retain ink thereon. In this case, in the absence of the electrophotographic sheet, the ink retention is completely unsatisfactory and in fact the surface is repellent to the ink.

EXAMPLE III

The procedure of Example I is repeated except the ink repellent master is made by using a plain paper base and coating it with the same organopolysiloxane. Then instead of having the image superimposed on the ink repellent layer, it is imposed on a separate electrostatic master charged by a corona discharge as in Example I. The image is superimposed on this separate master and light exposed to dissipate the charge in the exposed or non-image areas. Toner powder is applied to this separate master as in Example I and the image reproduced thereon. Then prior to fusing the toner image from the separate master is superimposed on the organopolysiloxane surface of the coated paper and applied by hand pressure so that the toner image is transferred from the separate master to the surface of the ink repellent siloxane layer. The coated paper having the toner imprint thereon is then fused in accordance with the procedure of Example I. The resultant plate is used in running off 2,000 copies of positive prints to give good, clear images with the background remaining clear throughout the entire run. This procedure may be repeated using various materials such as aluminum sheet, plastic sheet, etc., in place of the paper. In each case a plate is made which gives good, clear images when used for printing as in Example I.

EXAMPLE IV

The procedure of Example III is repeated using in place of the plain paper base the same electrostaticallly chargeable paper as used in Example I and an electrostatic charge is applied to this as in Example I prior to the transfer of the toner image as in Example III. The resultant plate gives equivalent printing results as compared with Example III.

EXAMPLE V

The procedure of Example I is repeated using in place of the toner of that example a toner having the composition: 50 percent of a pentaerythrityl abietate resin; 35 percent of a polyamide resin similar to that used in Example I: 5 percent stearic acid; 5 percent carbon black and 5 percent Nigrosine resin dye. Excellent printing results are obtained.

EXAMPLE VI

The procedure of Example I is repeated using in place of the toner of that example a toner having the composition: 25 percent of the polyamide used in Example I; 21 percent of a maleic anhydride resin condensation product; 24 percent stearamide; 16 percent of butyl laurate; and 14 percent of Nigrosine resin dye. Again excellent printing results are obtained.

EXAMPLE VII

The procedure of Example I is repeated a number of times using mixtures of zinc oxide and the polysiloxane as the coating material. Mixtures containing 50 percent, 40 percent and 25 percent by weight of zinc oxide are used individually to give excellent printing results in each case.

While certain features of this invention have been described in detail with respect to various embodiments thereof, it will, of course, be apparent that other modifications can be made within the spirit and scope of this invention and it is not intended to limit the invention to the exact details shown above except insofar as they are defined in the following claims.

Claims

What is claimed is:

1. A planographic printing plate capable of being imaged, having oleophobic non-image and oleophilic image areas, and is suitable for accepting oil-base ink only in the image areas and printing therefrom, while rejecting the oil-base ink in the non-image areas, comprising:

an electrophotographic base comprising a flexible support over which is applied a photoconductive layer;

an oleophobic layer superimposed on said electrophotographic base and adhered thereto which is highly repellent to said oil-base ink when dry, said oleophobic layer being further characterized as comprising a cured elastomeric organopolysiloxane having a ratio of organic groups to silicon atoms in the range of about 1.95:1 to about 2.1:1 and having an adhesive release value, when dry, of less than about 100 grams per inch; an oleophilic layer of fused toner in the image areas of said printing plate, superimposed on and adhered to said oleophobic layer, whereby said printing plate selectively accepts said oil-base ink in the image areas and repels said oil-base ink from the non-image areas.

2. The plate of claim 1 in which said electrophotographic base is zinc oxide-coated.

3. The printing plate of claim 1 in which said oleophobic layer comprises a cured elastomeric polymer of dimethyl polysiloxane.

4. The printing plate of claim 3 having a release value of less than 30 grams per inch.

5. The process of preparing a lighographic plate comprising the steps of:

superimposing on an electrophotographic base a coating of an oleophobic material comprising a cured elastomeric organopolysiloxane having a ratio of organic groups to silicone atoms in the range of about 1.95:1 to about 2.1:1 and having an adhesive release value, when dry, of less than about 100 grams per inch;

drying and curing said oleophobic coating by heating at a temperature in the range of 30.degree.C -200.degree.C;

applying a blanket electrostatic charge to the surface of said electrophotographic base;

exposing said charged surface to a pattern of light and shadow;

superimposing on said oleophobic coating an image corresponding to said pattern of light and shadow formed with a toner material comprising a pigment and an organic binder; and

heating said superimposed image at a temperature in the range of 89.degree.C - 10.degree.C thereby fusing and permanently affixing the toner composition to said oleophobic surface.

6. The process of claim 5 in which said electrophotographic base is zinc oxide dispersed in a resin binder.

7. The process of claim 5 in which said toner material is applied by superimposing the image on said electrostatically charged electrophotographic base coated with said oleophobic coating, exposing said image to actinic light for at least one-half second thereby dissipating the electrostatic charge on said coated base in the areas under the non-image areas and retaining the electrostatic charge on the said coated base under the image areas; then applying a toner suspended in a liquid medium to said coated base whereby the toner particles are retained and adhered in electrostatically charged areas of said base and not being retained in those areas no longer electrostatically charged; and heating said toner-bearing areas at a temperature in the range of 100.degree.-130.degree. C. whereby said toner is fused and permanently affixed to said ink repellent surface.