DE3750273T2 - Electrostatic printing plate based on silver. - Google Patents

Description

The invention relates to electrostatic printing and, more particularly, to an improved electrostatic printing stencil adapted for use with conventional silver halide photographic techniques in the manufacture of the printing stencil.

Electrostatic printing is well known in the art and has been proposed as an alternative to other printing techniques. In an electrostatic printing process, one first prepares a "stencil" capable of selectively retaining electrostatic charges to form the desired image. The stencil is exposed to a corona discharge which forms a latent electrostatic image and is contacted with dry or liquid toner of the opposite electrostatic charge to develop the image. The toned image is then transferred to a substrate, typically paper, to which the toner is fused to fix the image, and the stencil is returned to the next printing cycle.

In US Patent 4,069,759 it was proposed that an improved electrostatic printing stencil could be made by dispersing a common photographic silver halide salt in an insulating polymer (e.g. gelatin) and coating the dispersion on a conductive substrate. The coating is imagewise exposed and developed to effect reduction of the exposed silver halide to metallic silver. The unexposed silver halide is then dissolved and removed from the coating to fix the image. While the stencil proposed in US Patent 4,069,759 has many advantages and the use of common photographic, aqueous silver halide chemistry when gelatin is selected as the insulating polymer, gelatin has been found to be too sensitive to humidity to be of practical use in a typical workplace. Gelatin rapidly absorbs moisture from the air and ceases to function as an insulating medium at moderate to high humidity, but does provide a conductive path that grounds surface charges deposited on the stencil during the electrostatic printing process.

US-A-1981102 relates to binders for silver halide materials with improved mechanical properties. The disclosed binders include polyacrylic acid, other polymeric carboxylic acids, copolymers of carboxylic acids with styrene and the salts of these polymers.

Thus, there is a need for an improved electrostatic printing stencil that offers the advantages of being based on common photographic silver halide chemistry and providing improved insulation properties under conditions of relative humidity commonly encountered during printing.

Brief description of the invention

The invention provides a photosensitive composition adapted for use in the manufacture of an electrostatic printing stencil, the composition consisting essentially of a photographic silver halide salt dispersed in an insulating polymeric binder which is swellable in aqueous photographic developer solutions having a pH higher than about 8½ and which retains significant insulating properties under relative humidity conditions normally encountered during the printing process. The composition has an insulating value such that it carries an apparent macroscopic electric field of at least five (5) volts/micrometer (5 V/µm) as measured by an electrostatic surface voltage probe 2 seconds after full charging of its surface which has been allowed to equilibrate for 1 hour at 50% relative humidity. Common photographic gelatin, virtually the only medium commonly used for wet processing, retains about one (1) volt/micrometer or less after equilibration under these test conditions. Since the binder is swellable under pH conditions greater than about 8½, common aqueous silver halide developing solutions can be used to process the stencil for use in electrostatic printing.

Copolymers of acrylic and methacrylic acid having acid numbers in the range of 70 to 160 are a preferred binder that can be selected in the practice of the invention. The silver halide/binder composition is typically coated on a conductive substrate that can be applied to a flexible support for use as an electrostatic stencil. After the stencil is exposed to actinic light, the stencil is developed to obtain a silver image using conventional aqueous silver halide developing and fixing chemistry.

In a second embodiment, a diffusion transfer film is prepared by coating the polymeric binder containing development nuclei on a conductive support and overcoating the binder with a conventional silver halide photographic emulsion. The photosensitive element is exposed and then developed using conventional transfer techniques to provide an imaged electrostatic stencil.

In a third embodiment, the invention relates to a film comprising a conductive substrate having a A coating consisting essentially of a silver grain image dispersed in a synthetic insulating polymeric binder which is swellable in aqueous solutions having a pH greater than 8½, the binder being a copolymer of an unsaturated carboxylic acid monomer and an aromatic monomer and having ionizing carboxylic acid groups, the composition having an insulating value such that it carries an apparent macroscopic electric field of at least about five volts/micrometer (5 V/µm) measured 2 seconds after full charge of its surface which has been left to equilibrate for 1 hour at 50% relative humidity at 20°C.

As used herein, the term "electrostatic stencil" refers to the film element used for electrostatic printing, whether the film contains silver particles in the form of the desired image and thus is ready for the printing process, or contains silver halide particles that still need to be exposed and/or developed.

Short description of the drawings

Fig. 1 is a schematic partial view of an electrostatic stencil sheet in which a photographic silver halide salt is dispersed in the insulating binder to form the photosensitive layer 1.

Fig. 2 shows the template of Fig. 1 in which a latent image has been formed and developed.

Fig. 3 shows the template of Fig. 2 after the image has been fixed.

Fig. 4 shows the template of Fig. 3 after charging.

Fig. 5 illustrates the template of Fig. 4 in which toner particles have been attracted to the loaded surface.

Fig. 6 is a schematic partial view of a second embodiment in which the photosensitive layer 8 is a diffusion transfer film.

Fig. 7 shows the embodiment of Fig. 6 in which the diffusion transfer film has been imaged and development has begun.

Fig. 8 shows the embodiment of Fig. 7 after the development has been completed.

Fig. 9 shows the embodiment of Fig. 8 after the photosensitive layer 8 has been removed, at a time when it is ready to be used as an electrostatic stencil.

Detailed description of the invention

The use of conventional aqueous silver halide photographic chemistry ideally meets the requirements of producing electrostatic stencils, particularly when high resolution is required for high quality halftone or continuous tone applications. Sharp image resolution can be achieved by the fine grain size of the silver that can be obtained by using aqueous photographic chemistry well known in the art.

Isolation binders which may be selected for the practice of the invention are "swellable" in aqueous solutions having a pH greater than about 8½, typically in the range of 9 to 14, which are common in conventional aqueous developing solutions used in the Silver halide photography. By "swellable" is meant that the binder readily absorbs water and actually swells in this pH range similar to gelatin. When using preferred polymers described below, swelling is achieved by ionizing acidic groups (usually carboxylic acid groups chemically bonded to the isolation binder) by basic solutions at a pH of about 8.5 or more. This property allows the aqueous developer solution to be brought into close (reducing) contact with the silver halide. When using negative-working silver halide emulsions, the exposed silver halide is reduced to metallic silver by developer solutions and complexing reagents dissolve the unexposed silver halide salt. When using positive-working silver halide emulsions (e.g. those formed by such well-known techniques as solarization or chemical fogging), the unexposed silver halide is reduced to metallic silver and the exposed silver halide is removed.

In the embodiment described in more detail below, in which negative-working silver halide is dispersed in the isolating binders provided by the invention, the developer swells the binder at a pH of about 8.5 and reduces exposed silver halide to metallic silver, and complexing reagents, usually in a fixing solution, remove unexposed silver halide. In the diffusion transfer embodiment, in which negative-working photosensitive silver halide is present in an emulsion layer (usually gelatin) separated from the isolating binder containing a fine dispersion of development nuclei, the developer solution having a pH above about 8.5 swells both the emulsion layer and the isolating binder provided by the invention, thereby reducing the exposed silver halide to metallic silver in the emulsion layer and the unexposed silver halide is dissolved with complexing agents ("silver solvents"). The complexed, unexposed silver halide then diffuses into the swollen binder layer, where the silver ions are selectively reduced to silver metal on the development nuclei.

Although the insulating binders are swellable in the developing solution, the insulating properties do not degrade in a drastic manner as does gelatin under typical humidity conditions encountered in the workplace. As a consequence, the binders will retain an applied charge during electrostatic printing and it is not necessary to provide special humidity controls or drying of the stencil before each printing cycle, as would be necessary when using a gelatin binder.

The binders are generally characterized by being capable of supporting an apparent macroscopic electric field of at least 5 volts per micrometer, and preferably at least 30 volts per micrometer, as measured by an electrostatic surface voltage probe two (2) seconds after the surface is fully charged, after the probe has been equilibrated, and thus absorbs moisture at 50% relative humidity and at 20°C. Equilibration for testing will normally occur within about 60 minutes. Gelatin, in contrast, is significantly inferior, exhibiting an apparent macroscopic electric field on the order of about one (1) volt per micrometer or less in this test procedure.

It has been found that synthetic polymers having an acid number of about 70 to 160 are particularly useful in practice of the invention. A preferred class of polymers contains 10 to 25 weight percent acrylic or methacrylic acid to provide swellability. The polymer will typically also contain styrene or other aromatic monomers which are incompatible with water, thus making the polymer less hydrophilic to atmospheric moisture. Generally, the polymer will also contain monomers such as suitable acrylic and methacrylic acid esters which impart clarity, flexibility, strength, processability, etc. to the film. Other comonomers such as alkenes of 2 to 12 carbon atoms, haloolefins, vinyl acetate, vinyl ethers of 3 to 12 carbon atoms, methacrylamide, and the like may also be useful.

Preferred polymers are copolymers containing styrene and acrylic acid or methacrylic acid monomers and preferably also an acrylic acid or methacrylic acid ester monomer. Especially preferred are polymers containing 25 to 35 wt.% styrene, 10 to 25 wt.% acrylic acid or methacrylic acid, with the balance comprising acrylic acid or methacrylic acid esters. The molecular weight of the preferred copolymers will typically be in the range of 25,000 to 150,000. These polymers are compatible with silver halide dispersions, form reasonably curable films that have clarity and are readily available from commercial sources, or they can be prepared using conventional techniques such as free radical polymerization in suspension or emulsion. Equivalent polymers useful in the practice of the invention will be readily apparent to those skilled in the art. These polymers include acrylic acid and methacrylic acid polymers and copolymers, and include commercially available polymers such as Carboset® 525 and Carboset® 526, manufactured by B.F. Goodrich Company, and Joncryl® 67, manufactured by Johnson & Johnson.

A preferred class of polymers are terpolymers and tetrapolymers of (1) a styrene-type monomer, (2) a acrylate-type monomer and (3) an unsaturated carboxyl-containing monomer. The first component imparts hardness and moisture resistance to the polymer, the second imparts flexibility and plasticity to the polymer backbone, and the third alkali swellability. The styrene-type monomer will typically be styrene, an α-substituted styrene having a 1 to 6 carbon alkyl group, and one in which the benzene ring has functionally substituted groups such as nitro, alkoxy, acyl, carboxy, sulfo or halogen, with simple compounds such as styrene, α-methylstyrene, p-methylstyrene and p-tert-butylstyrene being preferred. The acrylate-type component includes alkyl and hydroxyalkyl acrylates and methacrylates wherein the alkyl group has from 1 to 12, preferably from 1 to 6, carbon atoms, such as methyl methacrylate, ethyl methacrylate, ethyl acrylate, hydroxypropyl methacrylate, hydroxyethyl methacrylate and hydroxyethyl acrylate and mixtures thereof. The unsaturated carboxyl-containing monomer will typically be a monomer having from 3 to 15 carbon atoms, preferably from 3 to 6 carbon atoms, and includes cinnamic acid, crotonic acid, sorbic acid, itaconic acid, maleic acid, fumaric acid, or more preferably acrylic acid or methacrylic acid, their corresponding half esters or their corresponding anhydrides.

When selecting this class of polymers in the practice of the invention, the ratio of the three monomer components is selected such that the conductive film element has the following properties: the silver halide, when incorporated into the conductive film element, is processable by conventional aqueous photographic techniques; the electrostatic stencil made therefrom retains deposited charges in the non-silver areas under ambient relative humidity conditions; and the electrostatic stencil is flexible and durable, but not sticky. Typical proportions used to obtain these results are shown in Table 1: Table 1 Binder Component Wide Range (wt.%) Preferred Range Styrene Type Acrylate Type Carboxylic Acid Type

Polymers within this class also generally offer the advantage of being insensitive to Isopar®, the carrier commonly used in liquid toner systems.

Insulating polymeric binders described above are prepared by conventional free radical polymerization techniques as illustrated in the examples. These polymers are soluble in basic solutions and can be coated from aqueous solutions of triethylamine, ammonia or potassium hydroxide and the like. These polymers are compatible with silver halide dispersions and will form reasonably durable films exhibiting clarity. It may be desirable to modify the binder (crosslinking, hardening, plasticizing, acid adjustment, etc.) prior to aqueous photographic processing to thereby control swelling or improve durability. Various modifiers may be added for these purposes. Typical modifiers include aldehydes, multifunctional aziridines and epoxides. The diglycidyl ether of 1,4-butanediol is a preferred modifier for this class of polymers in the practice of the invention.

Equivalent polymers that achieve the balance of the properties described above will be apparent to those skilled in the art and can be selected in the practice of the invention.

The photosensitive silver halide selected for dispersion in the binder can be any of the well-known salts used in photographic applications. Representative useful salts include silver chloride, silver bromide, silver iodide, silver chlorobromide, silver iodobromide and silver chloroiodobromide, either alone or in mixtures. Precipitation of the halide is carried out in gelatin in the usual manner. The amount of gelatin present should be limited, or subsequently reduced by rinsing, to avoid negative results of the invention. Generally, amounts of gelatin as large as 3 to 15 grams per mole of silver can be tolerated in the electrostatic printing stencils without adverse effects.

The grain size distribution and sensitization of the silver halide can be controlled to adapt the silver halides to the selected class of photographic process, generally including continuous toning, X-ray toning, lithographic, photomicrography, direct positive toning, and the like. Typically, the silver halide dispersions are sensitized with conventional compounds such as sulfur, gold, rhodium, selenium, and the like, or with organic sensitizing dyes such as cyanine, 1,1'-diethyl-4,4'-cyanine iodide, methine and polymethine cyanine dyes, cryptocyanines, merocyanines, and the like. Other additives commonly used in silver halide photographic compositions may also be present if desired.

To prepare the dispersion of the silver halide in the insulating polymeric binder, the binder is usually first dissolved in an aqueous solution containing amines such as ammonia or triethylamine. If desired, an alcohol such as methanol, ethanol or isopropanol may be added to facilitate solubilization of the polymer. Ketones such as methyl ethyl ketone may be used as a co-solvent. An aqueous Solution of the silver halide salt is added to the dissolved binder in the desired amounts. The appropriate proportions of silver halide to binder will depend on application details, but will generally be such that the surface of the stencil immediately above the developed silver is discharged significantly faster than silver-free areas. Weight ratios of silver to polymeric binder in the range of 0.5:1 to 3:1 will typically give useful results. A preferred range is 1.7:1 to 2.3:1.

The polymeric binder containing the silver halide is usually applied to a conductive substrate as a solution or dispersion in a carrier solvent, usually an aqueous solution containing basic amines or sodium or potassium hydroxide as described above. The coating method can be any conventional method, including spraying, brushing, roller coating or dip coating, flowing over the surface, pick-up by dipping, spin coating, air brush coater coating, wire bar coating, or any other suitable method. The film thickness can be adjusted accordingly and after drying is usually about 0.02 to about 0.3 mil (0.5-7.5 µm), preferably about 0.04 to about 0.20 mil (1.0 to 5.0 µm). Depending on the application, the conductive support may be: a metal plate such as aluminum, copper, zinc, silver or the like, a conductive polymeric film, a support such as paper, glass, synthetic resin and the like coated with a metal, metal oxide or metal halide by vapor deposition or chemical deposition, a support coated with a conductive polymer, or a support coated with a polymeric binder containing a metal, metal oxide, metal halide, conductive polymer, carbon or other conductive fillers.

In addition to the components described above, various common photographic additives such as developing reagents, superadditives, antifoggants, coating aids such as saponin, alkylarylsulfonic acids or sulfoalkyl succinic acids, plasticizers such as glycerin or 1,5-pentanediol, antistatic agents, antistaining agents, antihalation dyes, undercoating layers, underlying or supporting layers and the like can be added to the stencils as appropriate. Positive images can be obtained by reversal development of the silver halide using either light fogging or a chemical fogging agent or using silver halide emulsions which give direct positive images using the prefogging technique. Direct positive emulsions have been described by Leersmaker, US Patent 2,184,013, Illingsworth, US Patent 3,501,307, and elsewhere.

Turning now to the drawings, Figure 1 illustrates an electrostatic printing stencil in which the photosensitive layer 1 contains sensitized silver halide dispersed in the insulating polymeric binder according to the invention. Layer 1 generally has a thickness of 0.5 to 7.5 micrometers, however the thickness can be less or greater for specifically intended applications. A thin layer 2 of an adhesion activator such as gelatin, which is optional, assists in the adhesion of the photosensitive layer to the conductive substrate 3, which in turn is mounted on the supporting substrate 4.

The stencil is exposed imagewise using any method commonly used with silver halide photographic materials, such as by imaging with actinic light, a cathode ray tube or laser. For negative working emulsions, the latent image 5 is then formed by reducing the exposed silver halide particles to metallic silver using a conventional aqueous developer solution as illustrated in Fig. 2. A conventional aqueous fixing solution such as sodium thiosulfate is then used to remove the unexposed silver halide particles as shown in Fig. 3. The developed stencil is then ready for use in the electrostatic printing process.

Figure 4 illustrates the stencil of Figure 3 after it has been charged by a corona discharge which has deposited positive charges 6 on the stencil surface. The region of the film containing silver 5 provides a current path for overlying charges to travel to ground, thereby forming a latent image of the charges remaining on the stencil surface. Alternatively, charging can be achieved using a negative corona discharge, shielded corotrons, scorotrons, radioactive source, contact electrodes such as electrically biased semiconductor rubber rollers and the like.

The latent image is then developed with liquid or dry toner 7 of opposite polarity as illustrated in Figure 5. Cascade, magnetic brush, powder mist, liquid, magnetic dry and wet development techniques are suitable. Representative dry toners that may be used include Kodak Ektaprint K toner, Hitachi HI toner HMT-414, Canon NP-350F toner and Toshiba T-SOP toner. Examples of suitable liquid toners are Savin 24 toner, Canon LBP toner and James River Graphics T1818 toner. The latent image thus developed ("toned") is transferred to the usual substrate, typically paper, where it is fixed in the usual manner.

Figs. 6 to 9 illustrate a second embodiment wherein conventional diffusion transfer techniques as described in US Patents 2,352,104 and 2,983,606 are used to transfer an imaged electrostatic stencil with dispersed silver in the previously In this embodiment, the insulating synthetic binder 2, having a thickness of about 0.25 to 3 micrometers, contains dispersed development nuclei and a photosensitive layer 8 containing silver halide salts dispersed in a hydrophilic colloid overlying the binder, wherein the ratio of silver to binder 9 is 1:1 to 5:1. A conductive layer 3 and substrate 4 as described hereinbefore are used. Suitable development nuclei are well known in the art and are typically: (1) a metal such as silver, gold and rhodium, (2) sulfides, selenides, tellurides, polysulfides or polyselenides of metals including silver, zinc, chromium, gallium, iron, cadmium, cobalt, nickel, manganese, lead, antimony, bismuth, arsenic, copper, rhodium, palladium, platinum, lanthanum and titanium, (3) readily reducible silver salts which form silver nuclei during development such as silver nitrate or silver citrate, (4) inorganic salts which react with the incoming diffusing silver salts to form nuclei and (5) organic compounds which (a) contain a labile sulfur atom and which can therefore form sulfide nuclei during development including mercaptans, xanthates, thioacetamide, dithiooxamide and dithiobiurate or (b) are reducing agents such as hydrazine derivatives or silanes, and which when evaporated onto silicic acids or barium sulfate, silver nuclei can be formed. Likewise, the hydrophilic colloid can be any of the substances commonly used in diffusion transfer processes such as gelatin, phthalate-substituted gelatin, cellulose derivatives such as carboxymethylcellulose and hydroxymethylcellulose, and other high molecular weight hydrophilic colloidal substances such as dextrin, soluble starch, polyvinyl alcohol or polystyrene sulfonic acid.

Referring to Fig. 7, a photosensitive layer 8 is exposed in a conventional manner to form a latent image with the sensitized silver halide. In negative working emulsions, the photosensitive layer is then treated with a developing reagent which reduces the exposed silver halide in the area 10 to metallic silver, and with an aqueous solvent composition which converts silver halide in the unexposed areas to form a soluble silver halide complex which diffuses into the binder of layer 9 where it contacts the development nuclei and is reduced to insoluble silver particles 11 to form a silver image. Layer 8 is then removed as illustrated in Fig. 9 to give an electrostatic stencil ready for printing in the usual manner. Developing baths for the diffusion transfer process are well known in the art and are described, for example, in U.S. Pat. No. 5,444,444. This is described in, for example, Photographic Silver Halide Diffusion Processes by Andre Rott and Edith Weyde (Focal Press, 1972) and Modern Photographic Processing, Volume 2 by Grant Haist (Wiley, 1979).

Many additional embodiments will be apparent to those skilled in the art. For example, a positive-working silver halide emulsion can be used in conjunction with the diffusion transfer coating 8 as illustrated in Figures 6-9, and the exposed silver halide can be complexed in aqueous solutions to diffuse into the insulating binder layer 9 where it is reduced by the development nuclei to form the desired silver image. Likewise, a separate photosensitive film can be used in place of the coating 8 and can be brought into operative association with the insulating binder 9 before or after imaging as in photomechanical transfer. The photosensitive silver halide emulsion layer or coating 8 and the insulating polymeric binder layer 9 may also contain compounds commonly used in diffusion transfer systems, provided that the specific component does not adversely affect the insulating properties of the binder or the conductive properties of the silver-containing region 11 of the electrostatic printing stencil are influenced. Thus, if desired, suitable antifogging agents such as tetraazaindenes and mercaptotetrazoles, coating aids such as saponin and polyalkylene oxides, hardening agents such as formaldehyde and chrome alum and plasticizers can be used. The substrate 4 can also be transparent if the stencil is to be used as a phototool or for graphic art applications.

Various common methods can be selected for toning the electrostatic printing stencil. If the toner particles are electrically conductive and substantially neutral, or oppositely charged to the latent image, they will adhere to the charged latent image. If the toner is charged with the same polarity as the latent charged image, the toner will adhere to the uncharged area. A developing electrode can be used to improve the quality of the toned image, i.e., to facilitate uniform toning of the solid image areas that have latent electrostatic charge, and to prevent background toning in image areas that do not contain charge. Transfer of the toned image to the desired substrate, typically paper, can be facilitated by applying a corona discharge of opposite polarity to the opposite side of the substrate. Alternatively, toner transfer can be achieved with a conductive roller having an electric bias, adhesive film and paper, and the like. The thus transferred toner image can be fixed by a method commonly known in the art. Usually, heat fixing, solvent fixing, pressure fixing, and the like are used. If necessary, the surface of the stencil can be cleaned using a cleaning device such as a brush, cloth, blade, vacuum knife, and the like to remove the remaining toner image.

Electrostatic printing stencils offer several advantages over those described in the prior art. Since conventional aqueous development and fixing techniques remove byproducts that are soluble in the solution for such purposes, the stencil does not contain byproducts that could interfere with the insulating properties of the binder or the conductive path of the developed silver image, a situation that can occur when using the dry silver halide development techniques described in U.S. Patent 4,069,759. Also, the insulating properties of the binder selected in accordance with the invention are less sensitive to moisture that can affect the electrostatic printing process, and thus the stencil can be used repeatedly or after storage without necessarily having to heat the stencil to remove moisture or to perform special moisture controls.

High resolution can be obtained using the electrostatic printing stencils provided by the invention, achieving results comparable to those obtained in high quality lithographic, flexographic and copier printing. While halftone imaging will normally be selected for these applications, it is possible to tailor a stencil for continuous toner applications since the density of the developed silver will vary with the light intensity used to image the film, as in conventional photography.

The following examples further illustrate various embodiments of the invention and are not to be construed as limiting the invention. Other embodiments will be apparent to those skilled in the art. In the examples, all parts and percentages are by weight and all Temperatures are given in degrees Celsius unless otherwise stated.

Unless otherwise stated, the silver halide emulsions were negative working and sensitized with gold and sulfur containing compounds in the usual manner. The silver chloride was doped with 0.13 millimoles of RhCl3 per mole of silver.

Production of polymers

The general procedure for preparing the polymers is illustrated by preparing Polymer A [styrene/methyl methacrylate/ethyl acrylate/methacrylic acid in a weight ratio of 30:10:40:20] as shown below.

To a 5-liter flask equipped with a high-speed stirrer, reflux condenser, addition funnel, and thermometer were added 788 grams of deionized water, 5 grams of Duponol WAQE (sodium lauryl sulfate), 35.2 grams of styrene, 11.7 grams of methyl methacrylate, 46.9 grams of ethyl acrylate, 23.4 grams of methacrylic acid, and 0.5 grams of octyl mercaptan. The flask was purged with nitrogen and heated to 60°C for 15 minutes. 0.02 grams of ferrous ammonium sulfate, 0.28 grams of ammonium persulfate, and 0.28 grams of sodium bisulfite were added to the flask while the mixture was emulsified and maintained at 69-74°C. A mixture of 316.5 grams of styrene, 105.5 grams of methyl methacrylate, 422 grams of ethyl acrylate, 211 grams of methacrylic acid and 5.10 grams of octyl mercaptan was added to the flask over a period of 140 minutes while a solution containing 2.06 grams of ammonium persulfate, 0.52 grams of sodium bisulfite and 19.4 grams of Duponol WAQE in 1000 grams of deionized water was also added during the 140 minutes. The polymerization was continued for an additional hour and the emulsion was allowed to cool slowly to room temperature. A 5% calcium acetate solution was added, The polymer was filtered to remove excess water, washed repeatedly with deionized water and filtered until the filtrate became clear, and then dried under vacuum. Polymers BI were prepared in a similar manner. The polymer compositions and acid numbers are given below in Table 2. The acid numbers are defined as the milligrams of potassium hydroxide neutralized per gram of polymer, determined by potentiometric titration. Table 2 Monomers Polymers a S = Styrene MMA = Methyl methacrylate EA = Ethyl acrylate AA = Acrylic acid MAA = Methacrylic acid AN = Acid number

Examples 1-11 show the charge retention of the different polymers when treated with different silver halides and can be used at different silver halide-polymer ratios.

Examples 1-6

A solution was prepared from the following ingredients:

Polymer 0.5 grams

Triethylamine 0.3 grams

Water 3.2 grams

a Example 1 = Polymer A

Example 2 = Polymer B

Example 3 = Polymer C

Example 4 = Polymer D

Example 5 = Polymer E

Example 6 = Polymer F

To this solution was added, with stirring, 12.5 grams of a 15.4% solution of a silver chloride emulsion (AgCl granules doped with 0.13 millimoles of RhCl3 per mole of AgCl and having an average edge length of 0.13 to 0.17 µm) containing 3.3 grams of gelatin per mole of silver chloride. A copper-plated polyester base was coated with the dispersion using a doctor blade. The dried films were 2.4 µm thick and contained 90 mg of silver chloride per dcm2, with a silver ion to polymer ratio of 2.8 to 1. The unexposed films were then pan treated according to the following procedure: 1 minute in a commercial lithographic developer (CUFD, EI Du Pont de Nemours and Company), at 32.2°C; 30 seconds in 30% sodium thiosulfate fixer and 15 seconds in 2% acetic acid stop bath at 25ºC; then washed with cold water and dried at 125ºC for 10 minutes. The processed films were mounted on a flat plate, the copper layer was grounded and incubated at 24ºC and at a given relative humidity for one hour. equilibrated. They were then charged by corona discharge (using a double-wire corotron) at 8.2 kV. Charging was stopped (at time = zero) and the charge was allowed to decay. Electrostatic voltages were determined using an electrostatic surface probe. The results are given in volts per um in Table 3 below. Table 3 Example Time(s) rel. humidity = 23% rel. humidity = 50%

Examples 7-10

A solution was prepared from the following ingredients:

Polymer 32.7 grams

Triethylamine 11.5 grams

Water 131 grams

a Example 7 = Polymer G

Example 8 = Polymer H

Example 9 = Polymer I

Example 10 = RESYN 28-1300 (National Starch Co.), carboxylated poly(vinyl acetate) with acid number 67.

To this solution was added with stirring 74.2 grams of the silver chloride as in Examples 1-6, but containing 33.3 grams of gelatin per mole of silver chloride. A copper-plated polyester base was coated with the dispersion as in the previous examples. The dried film was 4 µm thick and had a silver weight of 80 mg per dcm². The silver ion to binder ratio was 1.15 to 1. Films were developed in a commercial X-ray film developer (MXD, EI Du Pont de Nemours and Company) and fixer (thiosulfate) at room temperature. They were treated with 2% acetic acid, rinsed with water and dried at 125°C for 10 minutes. After equilibration at 24°C and 37% relative humidity, the processed films were charged by corona discharge as in the previous examples. The results are given in volts per um in Table 4 below. Table 4 Example time(s)

Example 11

Example 9 was repeated except that a silver iodobromide emulsion (AgBr0.985 I0.015 with an average grain volume of 0.0185 µm³) containing 13.3 grams of gelatin per mole of silver halide replaced the silver chloride. The dried film had a coating thickness of 4 µm and contained 80 mg of silver halide per dcm². The silver ion to binder ratio was 1.15 to 1. The film was processed and charged as in Examples 7-10. At 24°C and 37% relative humidity, the electrostatic charges present per µm in the polymer regions were 80, 56, 47 and 40 volts per µm for 2, 30, 60 and 120 seconds, respectively.

Examples 12-17 demonstrate the use of different conductive substrates with two different insulating polymers.

Example 12

Polymer J (methacrylamide/methyl methacrylic acid/ethyl acrylate/methacrylic acid in a ratio of 4.2:42.8:43:10) was prepared as follows: a mixture of 4.2 grams of methacrylamide, 42.8 grams of methyl methacrylate, 43 grams of ethyl acrylate, 10 grams of methacrylic acid, and 0.1 gram of VAZO 64 initiator (azobisisobutyronitrile) in 666 grams of t-butanol was heated to reflux under a nitrogen atmosphere for two hours. An additional 0.1 gram of VAZO was added, reflux was continued for an additional two hours, two additional additions of 0.1 gram of VAZO were made, and reflux was continued for a total reaction time of 8 hours. The polymer was precipitated in cold water, rinsed with water and dried to a white powder.

A solution was prepared from the following ingredients:

Polymer J 5.0 grams

Triethylamine 0.5 grams

Water 35.0 grams

To 5 grams of the polymer solution was added, with stirring, 9.9 grams of an ortho-sensitized silver iodobromide emulsion as in Example 11, in which the gelatin content was 13 grams of gelatin per mole of silver halide and the silver halide content was 11.7%. Aluminum was coated with the dispersion under red safelight conditions using a wire wound rod and a 6.0 µm coating was obtained after drying.

The coating was handled and processed under red safelight. Images were made by contact exposure to halftone and resolution targets in a vacuum frame using a tungsten lamp at 56 inches (lamp output = 10 foot candle power, 12 feet from the lamp bulb). This example was exposed for one second, processed in the pan for 1 minute under a nitrogen atmosphere in the developer below:

0.01% potassium bromide

0.05% sodium sulphite

1.00% Hydroxylamine hydrochloride

0.01% Dimezone-S

1.00% Hydroquinone

5.40% potassium carbonate

5.40% potassium bicarbonate

Deionized water

It was then fixed for 2 minutes, stopped in 2% acetic acid for 2 minutes, rinsed in distilled water for 2 minutes (all at 26ºC), blown dry and heated for 1 minute at 125ºC.

The image consists of black silver imaging, with the coating exposed and a white background unexposed. Resolution was at least 101 line pairs per millimeter. Charge uptake and decay in the dark were determined using a Monroe Model 276A static charge analyzer. The exposed areas showed an initial uptake of 8 volts, the same as bare aluminum, and did not decay for 60 seconds; the unexposed areas initially picked up 153 volts, which decayed to 100 volts for 10 seconds, 92 volts for 20 seconds, 75 volts for 60 seconds. This charge difference between the exposed and unexposed areas is useful for electrostatic toning.

The electrostatic stencil was charged to a maximum recording charge with a positive corona while the aluminum support was electrically grounded. After a few seconds of decay, the grounding was broken and the plate was immersed in a dispersion of negatively charged black toner particles in Isopar®, a non-polar hydrocarbon liquid with a Kauri-Butanol value of about 27 - Exxon Corp. Toner was attracted to the white non-silver portions of the image, making the overall stencil appear black. It was then gently rinsed in an Isopar® tray, water removed, rewetted with Isopar®, covered with paper and passed under the positive corona to facilitate toner transfer to the paper. The image was transferred normally (toner was transferred where the stencil was silver-free) and had 6 line pairs/millimeter resolution when the stencil remained completely wet with Isopar®.

Example 13

The procedure of Example 12 was repeated with the following modifications: Copper-clad Kapton® (polyimide film, E.I. Du Pont de Nemours and Company) was coated with the emulsion to achieve a thickness of 5.7 µm and the processed film was heated for 5 minutes at 125°C. The fully coated electrostatic stencil thus prepared was mounted on a Savin 770 copy drum, charged and toned; the image was transferred to paper as in Example 12 to obtain 100-150 copies of black toner image with a resolution of 20 line pairs per millimeter.

Example 14

Example 12 was repeated except that 9.9 grams of polymer solution was used, giving a silver ion to polymer ratio of 0.58 to 1, and copper plated Kapton® was coated with the dispersion to a coating thickness of 5.7 microns. Subsequent treatment was the same as in Example 13. It was found that the stencil with the higher percentage polymer (Example 14) was better charged and toned; however, the image coating had a greater tendency to delaminate.

Example 15

Example 12 was repeated except that the conductive substrate used was aluminized Mylar® (polyester film, E.I. Du Pont de Nemours and Company). This resulted in an intact image with no significant anchoring or quality problems.

Example 16

Polymer K was prepared in the same manner as Polymer J, except that 4.2 grams of methacrylamide, 21.8 grams of methyl methacrylate, 64 grams of ethyl acrylate, and 10 grams of methacrylic acid were used. The films were prepared, imaged, processed, charged, and toned as in Example 12. The initial charge pickup was 55 volts; after 10 depressions it was 16 volts.

Example 17

In this example, the same coating and processing as in Example 13 was used except that the image was heated at 125°C for 10 minutes. A coating thickness of 1.8 µm was achieved. Image areas that air dried prior to heating (A) were slightly hazy; areas that were wet when placed in the oven (B) became transparent after heating. The black silver image had a resolution of 228 line pairs per millimeter. The charge uptake and charge decay of the image were determined using a Monroe Model 276A static charge analyzer at various relative humidities as shown in Table 5 below. Data are in volts per µm. Table 5 Relative humidity 0 seconds 10 seconds 20 seconds 30 seconds

The copper layer of the electrostatic stencil of Example 17 was electrically grounded and the image was positively charged with a corona under ambient conditions. After a few seconds, the grounded image was immersed in a toner bath consisting of negatively charged toner particles in Isopar®, dewatered, lightly rinsed with Isopar®, and the wet image was transferred to paper using a negative corona behind the paper. The toner image was positive with respect to the original image, negative with respect to the stencil, and the resolution was 16 line pairs per millimeter. The electrostatic stencil was recharged and toned and the toner image was allowed to dry. The toner was removed from the stencil using clear tape, giving a clean positive image with respect to the original, with a resolution of 50 line pairs per millimeter.

Examples 18-24

These examples illustrate the properties of films formed by dispersing a silver salt in gelatin binder versus those formed by dispersing the same silver salt in the improved insulating media of the present invention. In all cases, the silver salt used consisted of AgCl grains doped with 0.13 millimoles of RhCl3 per mole of AgCl and having an average edge length of 0.13 to 0.17 µm. Charge retention was measured after development of the unexposed films.

(i) Films containing gelatin binders

A silver chloride dispersion was prepared by adding 3610 grams of silver chloride nuggets (granules doped with 0.13 millimoles of RhCl3 per mole of AgCl and having an average edge length of 0.13 to 0.17 µm) containing 13.3 grams of gelatin per mole of AgCl to 3045 grams of water, adjusting the pH to 6.7 with 130 grams of 0.1 N sodium hydroxide, heating and stirring for one hour at 45°C, and adding 214 grams of a solution prepared by mixing 165.2 grams of 0.1 N sodium hydroxide, 32.1 grams of tetraazaindene stabilizer*, and 16.7 grams of water.

* Tetraazaindene = 4-hydroxy-2-2methyl-[1,2,4]triazole[2,3]pyrimidine

Gelatin was swollen in water at 20ºC and then dissolved in additional water at 50ºC to give a 15 wt% solution. 295 grams of the gelatin solution was then added to 705 grams of the AgCl solution to give a net 17.63 wt% emulsion. Formaldehyde hardener was added at a concentration of 5 grams of formaldehyde per 1000 grams of emulsion. An indium tin oxide coated polyester substrate (surface resistivity of A film of 500 ohms per area, 5 mil thick polyester base was coated with the emulsion using a laboratory coater. The films were processed in the tray using standard reagents in the following order: developer, stop, fix, stop, rinse, dry. The types of gelatin used and the coating thickness obtained after processing are summarized in Table 6.

(ii) Films with improved polymeric binders

A solution was prepared using the following ingredients:

Polymer 2.00 grams

Water 10.44 grams

Isopropanol 3.20 grams

Potassium hydroxide 0.30 grams

Potassium bicarbonate 0.06 grams

Acid Violet 520 dye 0.10 grams

To this solution was added, with stirring, 54 grams of AgCl nuggets containing 10 grams of gelatin per mole of AgCl. A gel-backed indium tin oxide coated polyester substrate (surface resistivity of 500 ohms per area, 5 mil thick polyester base) was coated with the dispersion using a wire-wound rod. The films were processed according to the procedure described for the gelatin films. The polymers used and the coating weights obtained are summarized in Table 6. Table 6 Example Binder Coating thickness Deionized gelatin Rousselot ILLS Non-deionized gelatin Polymer a after processing

(iii) Determination of cargo retention

Samples of the above films were mounted on an aluminum plate and an electrical connection was made from the conductive indium tin oxide substrate to ground using conductive copper tape. The films were equilibrated in a glove box at a given relative humidity as measured with an Omega hand held hygrometer (model RH-201) for one hour and then charged by corona discharge with a double wire corotron, applying 6 kV to the corotron. Voltages were determined using an electrostatic surface voltage probe. The results are summarized in volts per µm in Table 7. Table 7 Example Time(s) T = 24ºC; relative humidity = 11%

Films containing gelatin binders were heated to determine the effect on electrostatic properties. Films of Examples 18-21 were dried at 100°C for 10 minutes and then conditioned at 48% relative humidity for 1 or 10 minutes after which electrostatic data were obtained. These data are summarized in volts per µm below in Table 8. Table 8 Example time min

(iv) Results of toning

Films of Examples 18-24 were toned with liquid electrostatic toner containing carbon black pigment in a modified Savin 870 copier under identical conditions, temperature being 19°C and relative humidity 48%. The time from corona charging to toning was 15 seconds. The double wire corotron was applied with a bias of 6 kV and the developing electrode was maintained at ground potential. Transfer of the toner from the film surface to the offset enamel paper was accomplished by application of a bias transfer roller. After the toner was transferred to the paper, it was thermally fused at 100°C in an oven. Optical reflection density measurements were made using a Macbeth RD918 densitometer and are given in Table 9 below. Table 9 Example ambient conditions heated a Heated for 10 minutes at 100ºC, then conditioned for 1 minute at ambient conditions prior to toning.

Examples 26 and 27 illustrate the use of a commercially available resin as an insulating polymeric binder.

Example 25

In 35 grams of water were dissolved 2.5 grams of Carboset® 526 (copolymer of ethyl acrylate/methyl methacrylate/acrylic acid in a ratio of 17:71:12, B.F. Goodrich Co.) and 0.59 grams of triethylamine. Equal amounts of the polymer solution and silver halide emulsion of Example 12 were mixed and were coated at 60 mg per dcm² on copper-plated Kapton®. Exposure and development according to the procedure of Example 12 gave a black silver image with a clear background and good resolution. Charge build-up and charge decay studies as a function of relative humidity were performed on coatings of pure Carboset® 526 at 36.90 mg per dcm² on copper under the same conditions as in Example 17. At relative humidity of 4 to 72%, Carboset® 526 retained charge at least as well or better than Polymer J of Example 12.

Example 26

Example 25 was repeated using Carboset® 525 (copolymer of ethyl acrylate/methyl methacrylate/acrylic acid in a ratio of 56:37:7, B.F. Goodrich Co.). An image was produced, but it was weaker than that of Example 25.

Example 27

A film was prepared as in Example 1 except that the AgCl emulsion contained 13.3 grams of gelatin per mole of AgCl and the final coating weight was 120 mg per dcm2. The film was exposed and processed in MXD (EI Du Pont de Nemours and Company) rapid access X-ray film developer to obtain a variety of developed silver amounts. Development was determined by a Panalyzer 4000 (Panametrics, Division of Esterline Corp.). Surface resistivity in the silver image areas was measured with a Fluka 77 multimeter (John Fluke Mfg. Co., Inc.) between two samples separated by 1 cm. Voltage absorption in the silver image areas was measured with a Monroe 276A static test meter. The results are given in Table 10 below. Table 10% developed silver resistance voltage absorption capacity (Ohm) (Volt)

Example 28

Indium tin oxide coated Mylar® (polyester film) was coated with a 1.8 mg per dcm2 backed polyvinylidene chloride resin at 200 feet per minute using a spray air brush coater and heat treated at 170ºC at 20 fpm for a dwell time of 8 minutes. This was overcoated with a gelatin layer of 0.8-1.0 mg per dcm2 at 200 fpm using a spray air brush coater and heat relaxed at 145ºC at 45 fpm for a dwell time of 3.5 minutes.

A solution of Polymer E was prepared by adding 450 grams of isopropyl alcohol (95%), 450 grams of methyl ethyl ketone and 132 grams of potassium hydroxide pellets to 2314 grams of water with stirring. To this solution was added 600 grams of Polymer E with rapid stirring, stirring was continued until it (Polymer E) was largely dissolved (15 minutes). To this was added 54 grams of potassium bicarbonate. A silver chloride dispersion was prepared by adding 3610 grams of silver halide nuggets (grains doped with 0.13 millimoles of RhCl3 per mole of AgCl and having an average edge length of 0.13 to 0.17 µm) containing 10 grams of gelatin per mole of AgCl to 2300 grams of water and adjusting the pH to 6.7 by adding 130 grams of 0.1 N sodium hydroxide and 15 grams of 0.1 N sulfuric acid. This was heated at 45°C for one hour and 214 grams of a solution prepared by mixing 386 grams of 0.1 N sodium hydroxide, 75 grams of tetraazaindene stabilizer and 39 grams of water was added. This was diluted to 25% silver chloride with 614 grams of water.

To 630 grams of the 25% AgCl solution, 247 grams of the polymer solution (15%) were slowly added with stirring. Before coating, 6.7 grams of EPI-REZ 5022 (diglycidyl ether of 1,4-butanediol, Celanese Corp.) were added and coated onto the above-treated indium tin oxide Mylar® film with 15 mg per dcm² polymer coating weight using a laboratory coater at 60 fpm. This product was dried for 30 seconds at 10ºC, 60 seconds at 30ºC and 60 seconds at 50ºC. The total dry coating weight was 103 mg per dcm².

After exposure and development as in Examples 18-24, the developed, exposed silver image had a surface resistance of 50-100 ohms and a voltage draw of 1 volt measured 2 seconds after charging. The unexposed non-silver portion of the image had a voltage draw of 242 volts measured 2 seconds after charging, 206 volts after 15 seconds and 190 volts after 30 seconds at 19% relative humidity. Toning in a modified Savin 870 office copier as described in Examples 18-24 gave 5-98% spots and 150 lines per millimeter resolution. The image transferred to paper had Dmax of 2.4 and Dmin of 0.03.

Example 29

In this example, the invention is illustrated by a diffusion transfer film. The following solution:

Water 3116 grams

Ammonium hydroxide (29%) 84 grams

Isopropyl alcohol (95%) 400 grams

400 grams of ground Polymer A were added with vigorous stirring. The solution was left without stirring until the polymer was dissolved (overnight). To 1720 grams of the polymer solution, 600 grams of a 2% zinc sulfate solution were added over 1 minute with rapid stirring; then 210 grams of a 1.062% sodium sulfide solution were added over 5 seconds with stirring; then 520 grams of a 2.59% solution of Acid Violet 520 (antihalation dye) were added over 30 seconds. This product was diluted to 4% by adding 1250 grams of water. Before coating, 31 grams of EPI-REZ 5022 (diglycidyl ether of 1,4-butanediol, Celanese Corp.) was added. The solution was coated onto 5 mil thick Mylar® (polyester) previously dusted with indium tin oxide using an air brush coater under the following conditions: 200 fpm, 4 inch air brush coater pressure. This product was dried at 85ºC. This film was then heat relaxed in a separate step at 145ºC and 45 fpm, giving a residence time of 3.5 minutes at 145ºC. This film was overcoated with a blue sensitized, high contrast camera sensitive emulsion of AgCl0.8Br0.195I0.005 (average grain volume of 0.01 µm3) dispersed 2:1 in gelatin using a bar coater at 80 fpm. The final binder layer coating weight was 9.3 mg per dcm2; the emulsion layer was 73.6 mg per dcm2. The silver ion to polymer ratio was 3.0 to 1. The film was exposed and developed with very slight agitation for 1 minute in Agfa CP297B (Agfa-Gaevert) diffusion transfer developer at 28ºC, stirred for 1 minute in 10% acetic acid stop solution at 28ºC, removing much of the top gelatin layer, rinsed in water at 15ºC and dried at room temperature.

The unexposed areas gave developed silver in the polymeric binder layer with a surface resistance of 20-35 ohms and a voltage response of 0 volts. The exposed areas were free of silver in the polymeric binder layer and after charging the electric field response at 38% relative humidity was 150 volts for 2 seconds; 104 volts for 15 seconds; 91 volts for 30 seconds. Toning in a modified Savin 870 copier as described in Examples 18-24 gave 4-98% halftone spots/150 lines per inch halftone. Dmax was 2.5 and Dmin was 0.01.

Examples 30-31

These examples compare the properties of diffusion transfer films containing either gelatin or a styrene-acrylic tetrapolymer as a binder in the receptor layer.

(i) Diffusion transfer film with gelatin binder in the receptor layer (Example 30)

60 grams of Rousselot III gelatin was added to 1360 ml of deionized water and stirred at room temperature with vigorous stirring for 20 minutes. The suspension was heated to 52°C for 30 minutes and then cooled to 35°C. 106 ml of a 0.15 M zinc sulfate solution and 6 ml of a 0.15 M ferrous sulfate solution were added over a period of 1 minute. 336 ml of a 0.05 M sodium sulfide solution were added through an orifice such that the addition time was approximately 2 minutes. The following aqueous solutions were then added:

15% solution of Polystep B-27

(Stepan Chemical Co.) 60 ml

1.33 M formaldehyde 40 ml

0.264 M Chromium potassium sulfate 40 ml

The solution was applied directly to the conductive side of the indium tin oxide coated Mylar® at a coating weight of 0.7 to 1.0 grams per m² of gelatin.

An ortho-sensitized, high contrast camera-sensitive emulsion of AgCl0.7Br0.3I (average grain volume of about 0.025 um3) was coated on the gelatin layer at a silver coating weight of 3.1 grams per m2. The emulsion contained no hardener.

The multilayer film was exposed imagewise to a tungsten light and developed in commercially available Agf a PMT developer (type CP297B) for 60 seconds at about 20ºC with gentle agitation. The emulsion layer was then removed with pressurized water at 38ºC. The sample was washed in water at 38ºC for 2 minutes and dried at room temperature.

(ii) Diffusion transfer film with improved polymeric binders (Example 31)

To a solution of 4.0 grams of Polymer E and 2.5 grams of triethylamine in 80 grams of water was added 6 ml of a 4% aqueous zinc sulfate solution over 1 minute, then 19.2 ml of a 0.23% sodium sulfide solution over 5 seconds. After stirring for 5 minutes, the precipitate was filtered off and the solution containing the zinc sulfide seeds was applied to the conductive side (surface resistivity = 500 ohms per area) of indium tin oxide coated Mylar® to give 7 mg per dcm2 of clean, colorless polymeric receptor layer containing 1% zinc sulfide seeds. This was heated at 125°C for 10 minutes to improve adhesion to the conductive substrate.

A blue-sensitized, high contrast, camera-sensitive emulsion of AgCl0.8Br0.195I0.005 (average grain volume of 0.01 um³) dispersed 2:1 in gelatin was coated without hardener over the polymeric receptor layer at a coating weight of 69 mg per dcm².

The multilayer coating was exposed to light image by image and developed in commercial Kodak PMT-D developer (Eastman Kodak Co., Chicago, Ill.) modified with 12.5% potassium hydroxide and 5% potassium carbonate for 60 seconds at 28ºC with gentle agitation. The developed image was stirred in 10% acetic acid stop solution at 28ºC for 30 seconds, during which most of the top gelatin layer was removed. The black positive diffusion transfer image in the receptor layer remained on the conductive support and was rinsed with water at 40ºC to remove gelatin and silver residues, dried, heated at 125ºC for 5 minutes to drive off volatile contaminants. The image had Dmax of 3.0-3.5 and low Dmin.

The receptor areas corresponding to the unexposed image had 8.8 mg per dcm2 of finely divided black silver metal dispersed in 6.5 mg per dcm2 of polymer matrix. The silver to polymer ratio of 1.34 to 1 is above the threshold of about 1.2, and the surface resistivity in silver-containing areas was very low, 5 to 14 ohms. The areas corresponding to the exposed image were fairly clean, nearly colorless, and had a surface resistivity of greater than 107 ohms. The stencil was toned using a modified Savin 870 copying machine as in Examples 18-24. With a 50 volt developing electrode potential, the background of the toner image transferred to paper (corresponding to the silver areas of the stencil) was completely toner-free, with halftone patches of 2-95%/150 lines per inch halftone.

(iii) Electrostatic data

Data were obtained for the diffusion transfer films of Examples 30-31 at various relative humidities according to the procedure described in Examples 18-24. The temperature was 22°C in all cases. The results in volts per µm are summarized in Table 11. Table 11 Example time(s) relative humidity = 11%

The diffusion transfer film with gelatin binder - Example 30 - was heated at 100°C for 10 minutes, then conditioned at 48% relative humidity for 1 or 10 minutes. The electrostatic data in volts per µm obtained immediately after conditioning are given below in Table 12. Table 12 Conditioning time Time(s) 1 min 10 min

(iv) Toning results

Films of Examples 30-31 were toned at 21°C and 43% relative humidity as in Examples 18-24. Optical reflection densities as measured in Examples 18-24 are given in Table 13 below. Table 13 Optical Density Example Ambient Conditions Heated a Heated at 100ºC for 10 minutes, followed by conditioning for 1 minute at ambient conditions.

Example 32

The Polymer E solution containing ZnS developing nuclei as described in Example 31 was coated onto gelatin-backed polyester film at 28 mg per dcm2 to give a clear, colorless coating. A piece of Kodak PMT negative paper was imagewise developed. The exposed PMT paper and receptor polymer/nucleus coating were fed into the nip of a laminator with the paper emulsion side opposite the nucleus coating and the sheets were separated. Kodak PMT-D developer was introduced into the nips between the sheets, the sheets were wet laminated at 1 meter per minute under low nip pressure, the laminate was left at room temperature for 30 seconds and then the sheets were separated to produce a black positive image with Dmax of 0.7 and Dmin of 0.02 in the receptor coating and a strong negative image on the PMT paper. This illustrates the well-known photomechanical transfer process and can be used to produce a silver image in Polymer E.

Claims (25)

1. A light-sensitive composition consisting essentially of a photographic silver halide salt dispersed in a synthetic insulating polymeric binder capable of swelling in aqueous solutions having a pH greater than about 8½, the composition having an insulating value such that it carries an apparent macroscopic electric field of at least about five volts/micrometer (5 V/µm) measured 2 seconds after full charging of its surface which has been left to equilibrate for 1 hour at a relative humidity of 50%. 2. The composition of claim 1, wherein the binder is swellable in aqueous solutions having a pH in the range of about 9 to 14 and has an insulation value of at least about 30 V/µm. 3. The composition of claim 2, wherein the binder has an acid number of about 70 to 160. 4. A composition according to claim 3, wherein the binder is a copolymer of an aromatic monomer and acrylic or methacrylic acid. 5. The composition of claim 4, wherein the binder is a copolymer containing 10 to 50 wt.% of a styrene monomer, 5 to 50 wt.% of a carboxylic acid monomer, and 0 to 85 wt.% of an acrylate monomer. 6. The composition of claim 5, wherein the binder is a copolymer containing 25 to 35 weight percent of a styrene monomer, 10 to 25 weight percent of a carboxylic acid monomer, and 40 to 65 weight percent of an acrylate monomer. 7. A composition according to claim 4 having a weight ratio of silver ion to binder in the range of about 0.5 to 3 parts silver to 1 part binder. 8. An electrostatic printing stencil comprising a conductive substrate bearing a coating of a photosensitive composition according to claims 1 to 7. 9. A diffusion transfer film comprising developing nuclei dispersed in a binder, wherein the binder is a synthetic insulating polymeric binder capable of swelling in aqueous solutions having a pH greater than about 8½, and the film has an insulating value such that it carries an apparent macroscopic electric field of at least about five volts/micrometer (5 V/µm) measured 2 seconds after full charging of its surface which has been left at 50% relative humidity for 1 hour to equilibrate. 10. The film of claim 9, wherein the binder is swellable in aqueous solutions having a pH in the range of about 9 to 14 and has an insulation value of at least about 30 V/µm. 11. The film of claim 10, wherein the binder has an acid number of about 70 to 160. 12. A film according to claim 11, wherein the binder is a copolymer of an aromatic monomer and acrylic or methacrylic acid. 13. The film of claim 12, wherein the binder is a copolymer containing 10 to 50 wt.% of a styrene monomer, 5 to 50 wt.% of a carboxylic acid monomer, and 0 to 85 wt.% of an acrylate monomer. 14. The film of claim 13, wherein the binder is a copolymer containing 25 to 35 wt.% of a styrene monomer, 10 to 25 wt.% of a carboxylic acid monomer, and 40 to 65 wt.% of an acrylate monomer. 15. The film of claim 12 having a weight ratio of silver ion to binder in the range of about 0.5 to 3 parts silver to 1 part binder. 16. A film comprising a conductive substrate bearing a coating consisting essentially of a silver grain image dispersed in a synthetic insulating polymeric binder swellable in aqueous solutions having a pH greater than about 8½, the binder being a copolymer of an unsaturated carboxylic acid monomer and an aromatic monomer and having ionizing carboxylic acid groups, the composition having an insulating value such that it carries an apparent macroscopic electric field of at least about five volts/micrometer (5 V/µm) measured 2 seconds after full charging of its surface which has been left to equilibrate for 1 hour at 50% relative humidity. 17. The film of claim 16, wherein the binder is swellable in aqueous solutions having a pH in the range of about 9 to 14 and has an insulation value of at least about 30 V/µm. 18. The film of claim 16, wherein the binder has an acid number of about 70 to 160. 19. A film according to claim 16, wherein the binder is a copolymer of an aromatic monomer and acrylic or methacrylic acid. 20. The film of claim 16 having a silver to binder weight ratio in the range of about 0.5 to 3 parts silver to 1 part binder. 21. The film of claim 16, wherein the binder is a copolymer containing 10 to 50 wt.% of a styrene monomer, 5 to 50 wt.% of a carboxylic acid monomer, and 0 to 85 wt.% of an acrylate monomer. 22. The film of claim 21, wherein the binder is a copolymer containing 25 to 35 wt.% of a styrene monomer, 10 to 25 wt.% of a carboxylic acid monomer, and 40 to 65 wt.% of an acrylate monomer. 23. The film of claim 21, wherein the binder is swellable in aqueous solutions having a pH in the range of about 9 to 14 and has an insulation value of at least about 30 V/µm. 24. The film of claim 23, wherein the binder has an acid number of about 70 to 160. 25. The film of claim 24, wherein the binder is a copolymer containing 25 to 35 wt.% of a styrene monomer, 10 to 25 wt.% of a carboxylic acid monomer, and 40 to 65 wt.% of an acrylate monomer.