GB1561545A

GB1561545A - Method of making flexible plates and plates made thereby

Info

Publication number: GB1561545A
Application number: GB46227/76A
Authority: GB
Original assignee: Coulter Systems Corp
Current assignee: Coulter Systems Corp
Priority date: 1975-11-17
Filing date: 1976-11-05
Publication date: 1980-02-20
Also published as: NL7612686A; LU76149A1; AT368644B; IT1066512B; FR2331820B1; DK497876A; IL50833A; CH615626A5; FR2331820A1; IL50833A0; MX143806A; JPS5278504A; DE2652252C2; DD127741A5; SE430546B; SE7612135L; ATA824676A; DE2652252A1

Description

(54) IMPROVED METHOD OF MAKING FLEXIBLE LITHOGRAPHIC PLATES AND PLATES MADE THEREBY (71) We, COULTER SYSTEMS CORPORATION formerly, Coulter Information Systems, Inc. a corporation organized and existing under the laws of the State of Illinois, at 35 Wiggins Avenue, Bedford, Massachusetts, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention is concerned generally with the production of a flexible, directimaging lithographic printing plate which is highly effective and yet more economical than many known lithographic plates.The particular aim of the invention is to provide a direct imaging plate which is to be used in offset printing and which can be discarded after one use, if desired because it is so economical to make.

According to the present invention there is provided a method of making a flexible lithographic printing plate from an electrophotographic member having a flexible organic polymer substrate, an ohmic layer upon the substrate and a sputtered inorganic, oriented crystalline electrically anisotropic, photoconductive coating upon the ohmic layer, which method comprises charging the surface of the electrophotographic member in darkness, immediately thereafter exposing the surface to a projected radiant image to form a latent charge image on said surface, toning the surface with a hydrophobic toner to develop the latent image and treating the surface with a material that renders only non-imaged areas hydrophilic while maintaining the integrity of the photoconductive coating.

The invention also comprises a flexible lithographic printing plate made by the method set out in the preceding paragraph.

Preferred embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a fragmentary sectional view on an exaggerated scale taken through a flexible lithographic printing plate that is made in accordance with the invention Figure 2 is a similar view showing a variation of the invention; and Figure 3 is a block diagram showing the steps of manufacture of the method of the invention illustrating variations thereof.

The electrophotographic member forms the base for the printing plate of the invention and comprises a flexible substrate that is a synthetic resin polymer such as commercially available polyester film carrying an inorganic ohmic layer and an inorganic photoconductive coating on top of the ohmic layer. A typical structure has the substrate formed of transparent polyester sheeting about 0.005 inch thick.

One example is polyethylene glycol terephthalate. The sheeting is sputtered on one surface with a layer of indium-tin oxide to a thickness of about 0.3 ,um. The proportions of indium oxide to tin oxide are about nine to one, respectively. This ohmic layer is an aid to charging only, is transparent to a relatively high degree to visible light, and does not impair the flexibility of the substrate. The coating of photoconductive material is applied on top of the ohmic layer by r.f. sputtering in which the targets used are of the chemical substrate being sputtered, thus avoiding reactive type sputtering. This achieves excellent stoichiometry.The anode which is used in a preferred process is a rotating drum over which a long length of the ohmic layer bearing substrate is transported continuously the anode is not maintained at ground potential so that there is in effect a negative bias voltage between the anode and ground giving rise to a second dark space or Langmuir sheath at the anode through which the sputtering of the photoconductive material must take place.

The coating which is laid down is a deposit of uniformly-sized highly oriented crystals of a wholly inorganic photoconductor forming an extremely dense, abrasive-resistant, highly lightsensitive, high grain, electrically anisotropic coating that is typically about 3500 Angstroms thick. The combined transparency of the entire member is between 70 and 85% for visible light.

The preferred photoconductive coating is cadmium sulfide (CdS) because it is substantially panchromatic. Other materials which have been coated successfully by the same method to achieve electrophotographic members that could form the base for the printing plate of the invention are zinc sulfide (ZnS) and arsenic trisulfide (As2S3). These materials have spectral responses differing from that of cadmium sulfide and are imaged with light which takes best advantage of their spectral properties. Mixtures of these materials have been coated to achieve particular spectral characteristics.

The electrophotographic member is in nowise modified from its originally manufactured form for the purpose of making the printing plate of the invention.

The member is thus invested with all of its inherent benefits and advantages. It is almost indestructible and can be stored under any conditions of light, moisture and temperature without any effect. It is flexible, enabling ready mounting and manipulating. Its surface is abrasionresistant so that a printing plate manufactured therefrom has an unusually long useful life.

In making a plate by the method of the invention, the electrophotographic member is charged, exposed to a light image and toned. The toned image may be fixed by thermal fusing, if needed. Thereafter the surface is treated by suitable reagents which render the non-imaged areas hydrophilic and the toner hydrophobic. Most toners in use today are inherently hyaropnobic before and after fixing. The plate is mounted in the press and the press is set into operation. In an alternate form of the invention, the toner need not be fixed, but is removed by washing after the non-imaged areas have been treated and the plate then run in the press without the presence of toner. In another form, the plate is run in the press without washing and the toner is removed by the first few impressions made.

In Figure 1 there is illustrated a printing plate 10 constructed in accordance with method of the invention, the same being shown in the process of use. The basic electrophotographic film itself is constituted by the elements 12, 14 and 16, these being, respectively the polyester substrate, the ohmic layer and the photoconductive coating. The electrophotographic film, generally designated F is charged in darkness by suitable corona producing means, imaged by radiation to produce a latent electrostatic image and toned. The toner will adhere to the portions of the surface where the charge has not been dissipated by the projected light image and thereby produce a visible image represented by the toned areas 18 in Figure 1.

In this condition, the film F is in the form of a transparency with a prqiectible image.

The image may be fixed or removable, depending upon the process which is followed thereafter.

In one successful method, the toner was fixed by thermally fusing it to the surface of the photoconductive coating 16. It should be appreciated that most toners are made out of carbon and/or organic resin particles and are hydrophobic both before and after fusing. The hard crystalline surface of the photoconductive coatings of the films like F is also hydrophobic. Thus, if oil or greasebased ink were applied to the surface it would wet the entire surface, imaged and non-imaged portions alike.

Thereafter, in order to render the nonimaged portions hydrophilic with out changing the hydrophobic property of the toned areas, the film surface is coated with a material that reacts with or adheres firmly to the non-imaged areas only. Such a material is shown at 20 in Figure 1 extending between the toned areas 18. A compound may be formed that is hydrophilic. The plate 10 now is complete. Once installed in an offset press it can be used to print many copies of the image. The ink adheres firmly only to the imaged areas as shown at 22. If any of the material 20 had mechanically adhered to the areas 18, the first few impressions would remove it.

Another method of makmg a plate is by removing the toned image after treatment of the film F. In Figure 2 the same reference characters are used to designate equivalent elements of Figure 1. The plate 10' differs from the plate 10 only in the respect that the toner which is used to provide the visible image is of a type which can be easily washed away or "pulled" off the surface of coating 16. Since the surface of the photoconductive coating 16 is naturally hydrophobic, removal of the protective coating represented by the toner exposes the original surface and this surface readily accepts ink and repels water.

It is emphasized that the dimensions in Figures 1 and 2 are exaggerated to aid in the explanation.

In Figure 2, the toned image represented by the toner such as 18 in Figure 1 has been removed in one of two ways. The toner can be washed off by the use of a rinsing or swabbing with a hydrocarbon solvent such as an isoparafinnic hydrocarbon sold under the Trademark "Isopar" by Exxon Company U.S.A. or it can be removed by being pulled off in a press. In this latter case the plate with soft masking toner applied and not fixed, but treated, is installed in the press. The first few impressions will pull the toned image off, the bare hydrophobic photoconductor surface. Ink fills the resulting spaces, as shown in Figure 2.

These alternative methods are illustrated in the block diagram of Figure 3. All methods start with the electrophotographic member of film F. The film F is imaged at 30 and toned at 32. One variation of the method follows the arrow 34 and provides for a fixing step at 36 which may comprise thermal fusing. Toners with liquid carriers are available that dry with a practically fixed condition. This is included by the reference to "fixing" at 36. Thereafter the treatment for rendering the non-imaged areas hydrophilic takes place at 38. As mentioned, nothing need be done to almost all toners since they are inherently hydrophobic. Then the resulting plate is mounted in the press and run as shown at 40. The first few impressions will clean any treatment material from the toned image. A second method follows arrows 42 and 44.In this case, where a soft masking toner is used, there is no fixing step like 36. After toning the plate is treated at 38' in the identical step as 38, the image washed at 46 to remove the toner and such of the treatment material as may have adhered, and the plate mounted in the press and run as at 40'. The third variation of method follows arrows 42 and 48, the plate being mounted in the press and run at 40" without washing the image.

As for the toners, for the method exemplified in Figure 1 a commercial "fusible" toner is used. This is a type of toner, either in powder form or dispersed in a hydrocarbon, which is fused by a heat lamp after deposit. Such toners are available through toner speciality companies or through supply houses furnishing toners for many office duplicating machines in use today and the makers themselves. Typical sources are Philip A. Hunt Chemical Corp., Dennison Copier Corp., Surface Processes Corp. and Imaging Systems Corp. and Imaging Systems Corp., all of the USA. The range of properties of these toners is wide.

The same companies can furnish the socalled "soft-masking" toners that do not adhere tenaciously. Even fusible toners can be removed if not fixed. Such toners can be washed off surfaces with suitable solvents, such as Isopar, referred to above. Figure 2 shows a plate made with such a toner.

The treatment materials which can be used for rendering the non-imaged surfaces of the plate hydrophilic are of several different types including, 1. silicate polymers such as sodium silicate, potassium silicate and mixtures in various concentrations; 2. sodium aluminum hydroxide complexes; 3. polyvinyl alcohol treated to render it insoluble in water; 4. commercial etch mixtures containing ferrocyanide, such as used conventionally to treat zinc oxide plates; 5. aqueous chromic acid solution containing chromate, and/or dichromate ions and hydrogen ions In some instances where the material used to treat the surface adheres to any degree to the toned images, the toned images are best applied with the soft masking toner or any other type of toner which is easily removed after treatment of the surface and will carry the adhered material with it.Plates with fixed images are preferred if multiple uses are planned.

Stored plates should be readily identified by their images.

One treatment for the making of a plate 10 is the use of sodium silicate, also known as water glass. This material adheres to the photoconductive coating surface and forms a glass-like film that is smooth, hard and continuous. The running of a plate made with this coating material will yield many thousands of copies without showing any visible signs of wear or deterioration of the image. The reason for preferential adherence to the photoconductive coating surface, cadmium sulfide, for example, is not fully known. Sodium silicate is an inorganic polymer comprising long chains of silicate ions connected by sodium ions. It seems to act as a surfactant and forms a continuous film which is not crystalline dry.

It is believed that the silicate polymer does not adhere to the toner because the toner is for the most part organic and not compatible with the aqueous solution of the silicate and/or metasilicate. Thus, oil-based inks will adhere to the toner, but the silicate will not.

Polyvinyl alcohol is soluble in water and forms a soluble film upon evaporation of its water. Such a film is desirable on the surface of the non-imaged areas but is of no value if soluble. If treated with a mixture of boric acid [H3BO3] and sodium tetraborate [Na2B407] polyvinyl alcohol can be made water insoluble. Like the water-glass, the water insoluble polyvinyl alcohol shows preferential adhesion to the non-imaged areas of the surface of the film F and does not adhere as well to the toned images.

Adhesion is not as good as that of the silicate and/or metasilicate material, but many hundreds of impressions can be made before deterioration of the image.

The sodium aluminum hydroxide complex is useful likewise for short runs of a plate. It can be made by intermixing aluminum sulfate [Al2(SO3)3] with sodium hydroxide [NaOH]. There will be a precipitate of aluminum hydroxide [Al2(OH)3] resulting, but if additional sodium hydroxide is added, the precipitate disappears leaving a complex chemical which is probably some form of sodium aluminum hydroxide such as NaAI (OH)4.

Another important conversion solution which can be applied to the surface of the toned electrophotographic film comprises an aqueous chromic acid solution containing chromate ions in sufficient concentration so as to form one or more chromium containing compounds by reaction with at least the portion of the fully crystalline inorganic photoconductive substance located at the free surface of the photoconductive layer or with an additional portion thereof extending into said layer.

These chromium containing compounds are characterized by being water receptive yet are substantially water insoluble in order that such compounds are retained on said surface at least for the duration of the subsequent lithographic printing run and maintain the non-printing areas of the printing plate surface water receptive during same.

Water receptive yet substantially water insoluble chromium containing compounds can be formed with for instance cadmium, in the case where the fully crystalline inorganic photoconductive substance coating is formed of cadmium sulfide, by the application thereto of chromic acid in aqueous solution to cause liberation of cadmium ions from said photoconductive substance. The liberated cadmium ions in turn combine with chromate ions existing in the aqueous chromic acid solution to form substantially water insoluble cadmium chromates. For this reaction it is necessary to employ the chromic acid in such concentration that the chromate ions provided by same are present in a quantity exceeding the concentration required for the formation of chromates with the available cadmium.

The chromic acid in aqueous solution for application to the photoconductive layer can be prepared by for instance dissolving chromium trioxide in water or by dissolving in water, a chromate capable of liberating chromate ions in an acidic solution, such as for instance sodium bichromate and the like, where the acidity of aqueous solution can be provided by the addition thereto of an acid such as for instance sulphuric acid and the like in such quantity that the hydrogen ion concentration in the solution is in excess of the concentration required for the liberation of cadmium ions.

Whilst in the above described method of rendering the fully crystalline inorganic photoconductive substance water receptive in the unprotected areas of the photoconductive layer free of toner deposits in accordance with this invention, some portion of the chromium containing compounds formed during the reaction may be present as free precipitate in the bulk of the chromic acid solution. It has been found that surprisingly a substantial portion of the chromium containing compounds upon formation are retained at the surface of the photoconductive layer in the non-printing areas of the printing plate surface; and, depending; on the extent of reaction, such chromium containing compounds can also extend into such photoconductive layer.It has been found that such retained chromium containing compounds are characterized by a surprisingly strong degree of adhesion which enables the employment of the printing plate in accordance with this invention for extended lithographic printing run lengths, such as in excess of 100,000 copies, as the water receptivity of the non-printing areas of such printing plate surface is maintained during such run lengths by the continued adherence thereto of the water receptive chromium containing compounds.

Whilst not wishing to be bound by any theory, it is believed that the surprisingly strong adhesion exhibited by the aforementioned chromium containing compounds is attained substantially in view of the physical nature of the fully crystalline inorganic photoconductive substance such as for instance cadmium sulfide comprising the photoconductive layer and the method of forming or depositing such photoconductive substance in closely packed and highly oriented crystalline form as has been described in the foregoing, which allows substitution within the crystal lattice to occur in a particularly regular and uniform manner.

The above disclosed process of conversion in accordance with this invention that is of treating the photoconductive layer with the aqueous chromic acid solution can be carried out conveniently by immersion followed by rinsing in clear water to remove unreacted chromic acid and non-adhering chromium containing compounds as well as other byproducts which may have formed during the reaction depending on the type of chromate and acid employed in preparing the aqueous chromic acid solution. The reaction time necessary for the production of a functionally adequate water receptive surface depends on the concentration of chromate ions and hydrogen ions in the aqueous chromic acid solution as has been disclosed in the foregoing, as well as on the temperature of such solution.It has been found that for convenience of operation it is advantageous to maintain the aqueous chromic acid solution at around room temperature or in the range from about 60 degrees F to about 80 degrees F and to adjust the chromate ion and hydrogen ion concentration therein for a reaction time in the range from a few seconds to a few minutes, preferably about 30 seconds.

EXAMPLE 1 The electrophotographic member was a film made as described above using a polyester substrate, an indium-tin oxide layer of about 0.3 ,um and a photoconductive coating of cadmium sulfide of about 0.3 ,am thick. Such a film rinsed in a solution of the sodium aluminum hydroxide complex as described above provided a coating that rejected oil-based inks. A piece of this film about 19x48 cms, was placed in an A. B. Dick small offset printer and run without an image. The machine passed 5600 impressions without signs of inking on its surface when the plate was removed. There was no sign of wear or that the plate could not continue indefinitely without taking up ink on the surface.

A similar plate about 16x25.4 cm. was imaged by exposure to a test pattern and toned with a toner of the fusible variety and fused. The plate was installed in the same machine and was run for about 1000 impressions before wear of the image was detected.

The treatment solution was made for both plates using the following ingredients, mixed as explained above: Aluminum sulfate-160 grams in 500 cc of water.

Sodium hydroxide-- 50 grams in 800 cc of water.

No critical temperatures were necessary.

EXAMPLE 2 An aqueous solution of polyvinyl alcohol having a concentration of between 10/ and 24, by weight was coated on a plate, dried and then immersed in a solution at room temperature of boric acid and sodium tetraborate in the following proportions: Polyvinyl alcohol solution 1000 cc.

Boric acid 30 grams.

Sodium tetraborate 100 grams.

The same plate was used as described in Example 1, imaged in the same manner. The polyvinyl alcohol is applied and dried.

Thereafter the plate is dipped into the boric acid/sodium tetraborate solution once or twice, permitting drying between, and thereafter placed on the same A.B. Dick off-set press. A run of about 100 impressions was made with little deterioration of the image before the machine was stopped.

Some signs of wear were visible on the image of the plate.

The size of the plate was about 16 cm by 25.4 cm.

EXAMPLE 3 The same type of electrophotographic member as in Examples 1 and 2, having the dimensions 25.4 cm. by 33 cm. is imaged using a fusible toner thereafter fused permanently to the surface. An aqueous solution of sodium silicate is prepared having a concentration of 2% by volume of a stock solution which is 40"92" Baume.

The stock solution is 1.4 grams/cc. The plate was held flat on a steel surface by means of a vacuum apparatus and a cotton swab wetted with the sodium silicate solution was wiped onto the surface carrying the images. Air or heat lamp drying is effected in a few minutes. Placed on the A.B. Dick offset printing press, the plate of this example gave excellent results, being removed after running several thousands of copies with excellent impressions and showing no signs of wear. One test with an identical plate was permitted to produce 40,000 impressions without showing any signs of wear.

EXAMPLE 4 The plate and solution were the same as in Example 3 except that the plate was imaged with soft masking toner. The concentration of the sodium silicate was 1.5% by volume. The imaged plate was prepared by submerging it in the solution and then permitting it to air dry. Thereafter the toner was wiped off using a large cotton swab dipped in Isopar G, a hydrocarbon solvent of Exxon Company.

The plate was placed in the A.B. Dick offset printing press and run to produce many thousands of copies of excellent quality without showing signs of wear.

Identical plates were tried without an Isopar wash and, after the first few impressions, gave the same results as Example 4.

Many other examples of application of sodium silicates were tried. Toners used were fusible and soft masking types. Where the latter were used they were either wiped off before running or placed directly in the offset printing press. lhe silicate solutions tried were concentrations of as little as .1% and as much as 5% with no extraordinary criticality noted. The most desirable range appeared to be a concentration between 1% and 2%. Applications were by wiping the material onto the surface or by dipping. A single application was all that was needed, although the thickness of the resulting coating was controlled to a large extent by the number of applications, with drying between each coat. From one to four separate applications were tried successfully.

In addition to commercial toners of the type described, marking pen ink was used as a toner and treated as soft masking toner, that is, wiped off with a solvent before running the plate in the press or permitted to be worn off by running directly.

Scanning electron micrographs and estimates of molecular dimensions indicate that a typical silicate coating is about 700 Angstroms in thickness. This is achieved by three applications of a .5% silicate solution or a single application of a 2% solution.

EXAMPLE 5 The photo conductive member was prepared as described in the foregoing using the polyester substrate, indium-tin oxide ohmic layer of about 300 Angstroms and a photoconductive coating consisting of cadmium sulfide of about 3000 Angstroms thickness.

An electrostatic latent image was produced on the surface of the photoconductive coating by applying a uniform negative electrostatic charge to the surface by means of a corona discharge device and then exposing at 50ft. candle seconds to radiation comprising a light pattern representing the subject matter to be reproduced.

The electrostatic latent image was developed by immersion in a liquid dispersion of electroscopic marking particles to tone the image areas by attraction thereto of such electroscopic marking particles.

The dispersion was prepared as follows: Polystyrene resin 100 grms.

was melted and Pigment Red Cl 53 20 grms.

was added. The mixture was stirred in a heated blender until homogenized. The mixture was cooled and then crushed to form pigmented resin particles within the range 20200u. The following mixture was then prepared: Pigmented resin 20 grms.

Alkyd resin 40 grms.

Isoparaffinic hydrocarbon 140 grms.

The mixture was ball milled for 48 hours.

This formed the toner concentrate. The liquid dispersion was prepared by dispersing the above concentrate in isoparaffinic hydrocarbon dispersant in the proportion 5-20 grms. of concentrate per 1 liter of dispersant. The toner or electroscopic marking particles in dispersed form were within the size range l-5u.

The polystyrene resin had a melting point of 150"C, molecular weight about 5000, acid value below 1.

The alkyd resin was a safflower oil extended glycerol based long oil alkyd, oil length 64%, solids content 69-71% in aliphatic hydrocarbon solvent, acid value 6-10.

After toning the surface of the photoconductive coating was rinsed in clear isoparaffinic hydrocarbon dispersant, dried and heated to a temperature of about l500-1600C to fuse the toner deposits.

An aqueous chromic acid solution was prepared as follows: Sulfuric acid (concentrated) 114 grms.

Water 40 grms.

Potassium bichromate 5 grms.

were mixed and then diluted with an equal volume of water.

The imaged photoconductive member was immersed for 30 seconds in the above aqueous chromic acid solution held at the temperature of about 70 degrees F and then rinsed in water.

The above treatment rendered water receptive the surface of the photoconductive layer free of toner deposits to form the non-printing areas of the printing plate surface whereas the printing areas thereon were formed by the fused ink receptive toner deposits.

The thus prepared lithographic printing plate was placed on the plate cylinder of an offset duplicator. Using black jobbing offset ink and standard fountain solution several thousand copies of excellent quality and high resolution were produced.

EXAMPLE 6 The potassium bichromate of Example 5 was replaced by an equal weight of ammonium bichromate.

EXAMPLES 7 and 8 The 114 grms. of concentrated sulfuric acid of Examples 5 and 6 was replaced by 70 grms. of 40 M,, aqueous solution of hydrofluoric acid. The immersion time of 30 seconds of Examples 5 and 6 was increased to 50 seconds.

EXAMPLE 9 Example 5 was repeated with the exception that the aqueous chromic acid solution comprised the following mixture: Sulfuric acid (concentrated) 36 grms.

Water 1100 grms.

Sodium bichromate 36 grms.

The solution was held at 70 degrees F and the immersion time was 1.5 minutes.

EXAMPLE 10 The mixture of Example 9 was diluted with and equal volume of water to form the aqueous chromic acid solution. The solution was held at 70 degrees F and immersion time was 3 minutes.

EXAMPLE 11 Example 5 was repeated with the exception that the aqueous chromic acid solution comprised the following: Chromic acid anhydride 60 grms.

Water 100 grms.

In this solution the chromate ions and the hydrogen ions were both provided by chromic acid formed by dissolving the chromic acid anhydride in water.

The solution was held at 75 degrees F and the immersion time was 1.75 minutes.

EXAMPLE 12 In Example 11 the temperature of the aqueous chromic acid solution was raised to 90 degrees F and the immersion time was reduced to 20 seconds.

EXAMPLE 13 In Example 11 the temperature of the aqueous chromic acid solution was raised to 105 degrees F and the immersion time was reduced to 6 seconds.

EXAMPLE 14 In Example 12 the aqueous chromic acid solution was diluted with an equal volume of water. The immersion time was increased to 50 seconds.

EXAMPLES 15-21 In Examples 5 to 11, the liquid dispersion of electroscopic marking particles was prepared as follows: Pigment Blue Cl 15 100 grms.

Alkyd resin 400 grms.

Isoparaffinic hydrocarbon 300 grms.

were balled milled for 48 hours to form the toner concentrate. The liquid dispersion was prepared by dispersing the above concentrate in isoparaffinic hydrocarbon dispersant in the porportion 15-25 grms. of concentrate per 1 liter of dispersant. The electroscopic marking particles in the dispersed form were within the size range 0.5--2u.

The alkyd resin was a safflower oil modified urethane alkyd, oil length 67pub, solids content 60 ,4 in aliphatic hydrocarbon solvent, acid value below 2.

After toning the surface of the photoconductive layer was rinsed in clear isoparaffinic hydrocarbon dispersant and air dried.

The toned photoconductive member was immersed in the aqueous chromic acid solution and then rinsed in water.

The thus prepared lithographic printing plate was placed on the plate cylinder of an offset duplicator. The toner deposits were ink receptive and formed the printing image areas.

EXAMPLES 2229 In each of Examples 15 to 21, after immersing the toned photoconductive member in the aqueous chromic acid solution and rinsing in water, the member was air dried following which the toner image deposits formed by the electroscopic marking particles were removed from the photoconductive layer by wiping with a pad soaked in isoparaffinic hydrocarbon whereby the underlying portions of the surface of the photoconductive layer became exposed. The surface was air dried.

The thus prepared lithographic printing plates were placed on the plate cylinder of an offset duplicator. In each, the exposed surface of the photoconductive layer was ink receptive and formed the printing image areas.

It is pointed out that the basic concept is a plate which is based upon a tough, durable abrasion resistant structure. The polyester substrate and its coatings include nothing which can be worn out or rendered unusable. No plate constructed in accordance with the invention has yet to show signs of wear on the conductive surface of the substrate, no matter how many impressions have been made with it.

The silicate coating likewise is long-lasting and when evenly applied enable-runs of 100,000 imprints to be readily achieved.

The polyester substrate 12 of the film F was about .005 inch so that the resulting plate is of the order of .006 inch in the tests made. The plates were thus quite thin and could be wrinkled if carelessly handled.

Without sacrifice of any of the physical or electrostatic properties, a slightly thicker polyester substrate could be used to decrease the tendency to wrinkle and enable the plate to more readily conform to the cylinder surface of a press. In contrast with conventional plates, the imaged plate can be washed off before fixing and/or treatment, corrected at that time, imaged and additional images added.

In the claims which follow, in referring to the treatment the word "coating" is used to mean an actual physical covering as well as the products of a chemical reaction.

WHAT WE CLAIM IS: 1. A method of making a flexible lithographic printing plate from an electrophotographic member having a flexible organic polymer substrate, an ohmic layer upon the substrate and a sputtered inorganic, oriented crystalline electrically anisotropic, photoconductive coating upon the ohmic layer, which method comprises charging the surface of the electrophotographic member in darkness, immediately thereafter exposing the surface to a projected radiant image to form a latent charge image on said surface, toning the surface with a hydrophobic toner to develop the latent image and treating the surface with a material that renders only the non-imaged areas hydrophilic while maintaining the integrity of the photoconductive coating.

2. A method according to claim 1 in which the toner is fixed after toning and before said treating step.

3. A method according to claim 1 in which the toned image is removed after said treating step, together with any treatment material adhered thereto.

4. A method according to claim 1 in which the toner is soluble in a solvent which has no effect upon the material used to treat the surface and is removed from the imaged areas together with any of said material adhered thereto after treatment.

5. A method according to any one of claims 1 to 4 in which the material with which the said surface is treated comprises a silicate polymer.

6. A method according to any one of claims I to 4 in which the material with which the said surface is treated comprises a sodium aluminium hydroxide complex.

7. A method according to any one of claims 1 to 4 in which the material with which the said surface is treated comprises a potassium ferrocyanide reacted etchant.

8. A method according to any one of claims 1 to 4 in which the material with which the said surface is treated comprises a polyvinyl alcohol rendered insoluble in water.

9. A method according to any one of claims 1 to 4 in which the material with which said surface is treated comprises a silicate polymer of the group consisting of sodium silicate, potassium silicate and mixtures thereof.

10. A method according to claim 9 in which the silicate is an aqueous solution from 0.1 to 10% by volume of a 40 to 420 Baume stock solution.

11. A method according to claim 9 or 10 in which said material comprises a solution of sodium silicate and the treatment comprises applying a coating to said surface and drying the same.

12. A method according to claim 11 in which at least one further said coating is applied after the first dries.

13. A method according to any one of claims 1 to 4 in which the material with which said surface is treated comprises an aqueous chromic acid solution containing chromate ions and hydrogen ions in sufficient concentration to form at least one water receptive yet substantially water insoluble chromium containing compound by reaction with at least the non-imaged part of said photoconductive coating.

14. A method according to any one of claims 1 to 4 in which material with which said surface is treated comprises an aqueous chromic acid solution the chromate ions are of which being provided by chromium compounds selected from the group comprising potassium bichromate, sodium bichromate, ammonium bichromate and chromic acid anhydride and the hydrogen ions thereof being provided by acids selected from the group comprising sulphuric acid, hydrofluoric acid and chromic acid formed by dissolving chromic acid anhydride in water.

15. A method according to claim 13 or 14 in which the treated surface is rinsed with water after treatment.

16. A lithographic printing plate comprising, in combination, an electrophotographic member including a flexible, organic polymer substrate. ohmic layer upon the substrate and a sputtered, inorganic, oriented crystalline, electrically anisotropic photoconductive coating upon the ohmic layer, with an image formed on said latter surface by image and non-image areas, said image areas being hydrophobic, and said non-image areas having thereon a coating of hydrophilic material irremovably adherent thereto.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. and when evenly applied enable-runs of 100,000 imprints to be readily achieved. The polyester substrate 12 of the film F was about .005 inch so that the resulting plate is of the order of .006 inch in the tests made. The plates were thus quite thin and could be wrinkled if carelessly handled. Without sacrifice of any of the physical or electrostatic properties, a slightly thicker polyester substrate could be used to decrease the tendency to wrinkle and enable the plate to more readily conform to the cylinder surface of a press. In contrast with conventional plates, the imaged plate can be washed off before fixing and/or treatment, corrected at that time, imaged and additional images added. In the claims which follow, in referring to the treatment the word "coating" is used to mean an actual physical covering as well as the products of a chemical reaction. WHAT WE CLAIM IS:

1. A method of making a flexible lithographic printing plate from an electrophotographic member having a flexible organic polymer substrate, an ohmic layer upon the substrate and a sputtered inorganic, oriented crystalline electrically anisotropic, photoconductive coating upon the ohmic layer, which method comprises charging the surface of the electrophotographic member in darkness, immediately thereafter exposing the surface to a projected radiant image to form a latent charge image on said surface, toning the surface with a hydrophobic toner to develop the latent image and treating the surface with a material that renders only the non-imaged areas hydrophilic while maintaining the integrity of the photoconductive coating.

17. A printing plate according to claim 16,

in which the image is formed by areas of hydrophobic toner adherent to said surface.

18. A printing plate according to claim 16, in which the toner image is formed by areas of hydrophobic toner permanently adherent to said surface.

19. A printing plate'according to claim 17 in which the toner is removable.

20. A printing plate according to any one of claims 16 to 19 in which said hydrophilic coating comprises a silicate polymer.

21. A printing plate according to any one of claims 16 to 19 in which said hydrophilic coating comprises a silicate polymer of the group consisting of sodium silicate, potassium silicate and mixtures thereof.

22. A printing plate according to any one of claims 16 to 19 in which said hydrophilic coating comprises sodium silicate.

23. A printing plate according to any one of claims 16 to 19 in which said hydrophilic coating comprises a sodium aluminium hydroxide complex.

24. A printing plate according to any one of claims 16 to 19 in which said hydrophilic coating comprises a ferrocyanide-reacted substance.

25. A printing plate according to any one of claims 16 to 19 in which said hydrophilic coating comprises insoluble polyvinyl alcohol.

26. A printing plate according to any one of claims 16 to 19 in which said hydrophilic coating comprises at least one water receptive yet substantially water insoluble chromium compound formed by reaction of an aqueous chromic acid solution with at least part of said fully crystalline inorganic photoconductive substance contained in said photoconductive layer.

27. A printing plate according to any one of claims 16 to 26 in which the substrate is a polyester.

28. A method in accordance with any one of claims 1--15 in which the substrate is a polyester.

29. A printing plate made by the method claimed in any one of claims 1--15 or 28 and substantially as hereinbefore described with reference to the accompanying drawing.

30. A method of making a flexible lithographic printing plate substantially as hereinbefore described with reference to the accompanying drawing.