Process Of Stabilizing A Migration Image Comprising Selenium Particles

Abstract

An image comprising migration material residing on a metallic conductive substrate and formed in accordance with the migration imaging process is stabilized and fixed onto the substrate by heating the substrate and the migration material to produce a chemical reaction therebetween resulting in a permanent stable image having high density and resolution.

Parent Case Text

This application is a continuation-in-part of our copending application, Ser. No. 590,959 filed Oct. 31, 1966 and now abandoned.

Description

BACKGROUND OF THE INVENTION

This invention relates in general to imaging, and more specifically, to an improved migration imaging system.

There has been recently developed a migration imaging system capable of producing high quality images of high density, continuous tone and high resolution. This system is described and claimed in copending applications, Ser. Nos. 837,591 and 837,780 both filed June 30, 1969. In a typical embodiment of this imaging system, a migration imaging structure consisting of a conducting substrate with a layer of softenable or soluble material containing migration material is coated onto the conductive substrate. An electrostatic latent image is formed on the surface of the layer. The softenable layer is then developed by dipping the plate into a solvent which attacks only the soluble layer. A portion of the migration material migrates through the softenable layer as it is softened or dissolved, leaving an image on the conductive substrate. Through the use of various materials, either positive-to-positive or positive-to-negative images may be made depending on the materials used and the charging polarities. Those particles in the softenable layer which do not migrate to the conductive substrate are washed away by the solvent with the softenable layer.

Three basic migration imaging structures exist: A layered configuration, which comprises a conductive substrate, a layer of softenable material and an overcoating of migration material (usually particulate) embedded in the upper surface of the softenable layer; a binder structure, in which the migration material is dispersed throughout the soluble layer which overcoats a conducting substrate; and finally an overcoated structure, in which a conductive substrate is overcoated with a layer of softenable material followed by an overcoating of migration material and a second overcoating of softenable material which sandwiches the migration material. The migration imaging process consists of the combination of steps which include charging, exposing and development with a solvent liquid or vapor or a combination of vapor followed by liquid. If vapor development is used alone, the softenable layer may be stripped away leaving the migration image on the substrate. The characteristics of these images are dependent on such process steps as charging potential, light exposure and development as well as the particular combination of process steps. High density, continuous tone and high resolution are some of the photographic characteristics possible. The image is characterized as a fixed or unfixed powder image which can be used in a number of applications such as microfilm, hard copy, optical masks and stripout applications using adhesive materials. Alternative embodiments of these concepts are further described in the above cited copending applications.

Another recently developed imaging system, utilizes non-photoconductive particles contained in a non-photoconductive soluble layer on a conductive substrate. In this system, an electrostatic latent image is formed such as by corona charging through a mask or stencil. When the imaged sheet is exposed to a solvent for the softenable layer only, the particles migrate to the substrate in image configuration. The unwanted particles are washed away with the soluble layer. This system is also described and claimed in copending applications referred to above.

To prevent abrasion of the image formed by the migration imaging method or loss of density, it is necessary to fix the image during development or by additional steps after development. In fixing during development, the developing liquid softens the conducting substrate or a thin film on the substrate so that the image particles can become embedded in the substrate or thin film. In fixing after development, the developing liquid evaporates leaving a coating of dissolved plastic over the image. Thus, by using additional process steps after development, the image can be fixed by either overcoating the image particles or by embedding them in the conducting substrate or in a thin film on the substrate. As techniques require additions to the solvent developer or a special coating step, it can be seen that there is a definite need for a simple and efficient image stabilizing step for migration images which avoids softening or overcoating the substrate, and yet produces images having high resolution and excellent density.

It is, therefore, an object of this invention to provide a method of stabilizing migration images which overcome the above noted disadvantages.

It is another object of this invention to provide a simple and effective method of stabilizing migration images.

It is yet another object of this invention to provide a method of stabilizing selenium containing images.

It is a further object of this invention to provide an improved migration imaging process.

The foregoing objects and others are accomplished in accordance with this invention by forming a migration image on a substrate followed by heating or chemically reacting the image forming material with the conductive substrate so as to produce a reaction between the substrate and imaging material, resulting in a permanent, stable image having high density and resolution. The advantages of this improved method will become apparent upon consideration of the following disclosure of the invention; especially when taken in conjunction with the accompanying drawings wherein:

FIG. 1A is a schematic sectional view of a layered structure for carrying out the invention.

FIG. 1B is a schematic sectional view of binder structure used in carrying out the invention.

FIG. 1C is a schematic sectional view of an overcoated structure for carrying out the invention.

FIG. 2A is a schematic sectional view of the structure of FIG. 1A during the charging step.

FIG. 2B is a schematic sectional view of the structure of FIG. 1A during the exposure step.

FIG. 2C is a schematic sectional view of the structure of FIG. 1A during the development step.

FIG. 2D is a schematic sectional view of the structure of FIG. 1A following development.

FIG. 2E is a schematic sectional view of the structure of FIG. 1A during the stabilizing step.

FIG. 1A shows a migration imaging plate comprising a conductive substrate 11 having thereon a softenable layer 12 overlaying the conductive substrate, and a layer 13 comprising migration material usually in particulate form.

The substrate 11 upon which the softenable plastic and particulate migration material are formed may be any suitable conductive substrate which will react chemically with the migration material. Typical substrates are copper, chromium, brass, cadmium, silver and gold. The substrate may be in any form such as a metallic sheet, web, foil, cylinder or the like. If desired, the conductive metal may be coated over an insulator such as paper, glass or plastic.

The softenable plastic layer 12 may be any suitable material which is softened in a liquid or vapor solvent; and in addition, is substantially electrically inert during the imaging and developing cycle. Typical materials are Staybelite Ester 10, a partially hydrogenated rosin ester, Foral Ester, a hydrogenated rosin triester and Neolyne 23, an alkyd resin, all from Hercules Powder Co.; SR 82, SR 84, silicone resins, both obtained from General Electric Corporation; Sucrose Benzoate, Eastman Chemical; Velsicol X-37, Hydrogenated Velsicol X-37, Velsicol Chemical Corp., Hydrogenated Piccopale 100, a highly branched polyolefin, Piccotex 100, polystyrene-vinyl toluene, Piccolastic A-75, 100 and 125, all polystyrenes, Piccodine 2215, a polystyrene-olefin copolymer, all from Pennsylvania Industrial Chemical Co.; Araldite 6060 and 6071, epoxy resins of Ciba; R5061A, a phenyl-methyl silicone resin from Dow Corning; Epon 1001, a bisphenol A-epichlorohydrin epoxy resin, from Shell Chemical Corp., and PS-2, PS-3, both polystyrenes and ET-693, a phenol-formaldehyde resin, from Dow Chemical. Other materials useful as the softenable layer are described in copending application, Ser. No. 837,780 filed June 30, l969, which is incorporated herein by reference.

The above group of materials is not intended to be limiting, but merely illustrative of materials suitable for the softenable plastic layer. The softenable plastic layer may be of any suitable thickness. In general, the thicker the layer the greater the potential needed for charging. A thickness from about 1 to 4 microns has been found satisfactory, but layers outside this range will also work.

The material 13, which constitutes the migration material, may be any suitable migratable material which reacts with the metal selected for the substrate. Typical migration materials are photoconductors such as particulate vitreous selenium, and alloys of selenium such as tellurium and selenium, cadmium sulfide, cadmium sulfoselenide and arsenic triselenide. Other migration material, photoconductive or non-photoconductive, is described in the above mentioned copending application, Ser. No. 837,780. Of course, the migration material is selected on the basis of its reactivity with the metal employed at the substrate. The size of the migration particles range from about 0.01 to 1.5 microns in diameter and may be prepared by vacuum evaporation techniques such as those disclosed in copending application, Ser. No. 423,167, filed on Jan. 4, 1965, and now abandoned. Another convenient method of forming the particulate migration layer is by simply dusting or cascading the material on glass carrier beads over the soluble layer softened by solvent vapor. This method is disclosed in copending application, Ser. No. 483,675, filed on Aug. 30, 1965. The thickness of the migration layer is from about 0.2 to 14 microns with the thicker layers being in the binder form.

In FIG. 1B, the binder form of the structure is shown in which the migration particles 13 are dispersed throughout soluble layer 12.

The structure of FIG. 1C shows the overcoated structure in which the migration particles 13 are sandwiched between two layers of soluble matrix material 12 which overlays conductive substrate 11. Both the binder and overcoated structure shown in FIGS. 1B and 1C, respectively contain essentially the same basic materials as illustrated for the layered structure shown in FIG. 1A.

In FIG. 2A the layered structure of FIG. 1A is uniformly charged over its entire surface by a corona discharge device 14, such as that shown in U.S. Pat. No. 2,777,957 to Walkup. The potential required for migration imaging has been shown to depend on a number of factors. For example, the form of the imaging structure such as the three illustrated in FIGS. 1A, 1B and 1C, the thickness and material used in the soluble layer, the type of migration material used, the developing solvent, the combination of process steps, the polarity of the potential and the light exposure, etc. If the potential is too high, the migration particles are usually deposited on the conducting substrate randomly without regard to light exposure. If, on the other hand, the potential is too low, none of the particles are deposited. In general, the potential may range from a few volts to 400 volts with a soluble layer of about 2 microns in thickness depending upon the material used. Generally, it may be said that the potential increases with the thickness of the soluble matrix layer for a given matrix material. For a few combinations of material, images can be obtained with potentials for only one polarity. For some combinations of migration materials and soluble layers, the maximum potential is higher for positive than for negative polarity. For example, this was observed with selenium vacuum evaporated on several different matrix materials.

Other methods of forming an electrostatic image on the surface of the photoconductive layer are also included within the scope of this invention. Such methods include corona charging through a stencil as shown in copending application, Ser. No. 483,675, filed on Aug. 30, 1965. In addition, the migration imaging structure may be charged through a liquid by an electrode using a low viscous liquid such as a silicone oil.

In FIG. 2B the imaging or exposure step takes place with exposing light 15 selectively impinging upon the charged surface containing, for example, photoconductive particles 13. The exposure for migration images depends upon the photoconductor, potential and its polarity, the combination of the process steps in the form of the imaging structure and the material of the soluble layer and solvent used in development. As in xerographic imaging, any amount of light suitable to activate photoconductor material 13, is usually sufficient to form an image. For example, the minimum exposure for maximum density with 4,000 angstrom light is approximately 1.5 .times. 10.sup.11 photons/cm..sup.2 with a structure consisting of selenium vacuum evaporated on Staybelite, 2 microns thick. This same exposure discharges a 50 micron conventional xerographic selenium plate from 600 volts to 500 volts.

In FIG. 2C the development step for the migration imaging structure is illustrated, wherein the structure is developed by immersing in a solvent for soluble layer 12. The solvent liquid 16 may be applied to the structure by spraying, pouring or dipping the structure into the liquid. The development time is not particularly critical inasmuch as the solvent is selected so as to dissolve only the softenable or soluble layer and be relatively neutral with regard to the photoconductive particles and conducting substrate. The development time is divided essentially into two parts; the time for imagewise migration of the particles to the conducting substrate and the time for flushing away the unmigrated particles. The development time ranges from less than 1 second with a layered structure 3 microns thick, such as that illustrated in FIG. 1A, to about 45 seconds using a binder structure such as that illustrated in FIG. 1B having a binder structure about 12 microns thick. The flushing time, and hence the developing time, can be reduced by increasing the relative motion between the solvent and imaging structure.

The solvent developer liquid 16 may comprise any suitable solvent for the soluble layer 12. Typical solvents are Freon TMC (duPont); trichloroethylene, chloroform, ethyl ether, xylene, dioxane, benzene, toluene, cyclohexane, 1,1,1-trichloroethane, pentane, n-heptane, Odorless Solvent 3440 (Sohio); Freon 113 (duPont), m-xylene, carbon tetrachloride thiophene, diphenyl ether, p-cymene, cis-2,2-dichloroethylene, nitromethane, ethanol, ethyl acetate, methyl ethyl ketone, ethylene dichloride, methylene chloride, 1,1-dichloroethylene, trans 1,2-dichloroethylene and super naptholite, (Buffalo Solvents and Chemicals). Other developer liquids are described in copending application, Ser. No. 837,780.

After developing in the solvent liquid as shown in FIG. 2C, the photoconductor 13 is formed in image configuration on substrate 11 as shown in FIG. 2D. At this point in order to stabilize image 13, and at the same time increase the density of the image, a stabilizing step which comprises reacting image 13 with substrate 11 is carried out as shown in FIG. 2E. The stabilizing step involves heating the image bearing substrate 11 with any suitable heating means such as a conducting coil 19 in order to react the substrate 11 with the photoconductor material and cause a chemical reaction between said photoconductive material and the substrate. Other heating means such as hot air, gas burners, etc., may of course, be used. During the heating step, the photoconductor material in the image area agglomerates usually producing an initial reduction in density due to fading, but after the reaction with the substrate, the photoconductive material appears to wet and spread over the substrate resulting in a stable image having high density and illustrated by 13'.

As already mentioned above, the substrate may take any form or configuration as long as the reactive metal is at the exposed surface to receive the migration material after development.

For the purpose of illustrating the invention, a conventional migration imaging member containing a copper substrate, and having a 2-micron layer of Staybelite Ester 10, a 50 percent hydrogenated glycerol rosin ester of the Hercules Powder Company, overlaying the copper substrate, with a 0.2-micron layer of vapor deposited selenium deposited in the upper surface of the Staybelite is treated as follows:

The plate is first charged by a corona charging device to a positive potential of about 100 volts (FIG. 2A). The plate is then exposed to an optical image of about 10 foot-candle-seconds in the illuminated areas using a tungsten lamp (FIG. 2B). Development of the plate is carried out by immersion in Freon 113, a halogenated hydrocarbon available from the E. I. duPont de Nemours Co., Inc. for about 2 seconds and then removed and dried in air (FIGS. 2C and 2D). The particulate image is then stabilized by heating the substrate to a temperature of about 100.degree. C. for about 1 minute to react the copper substrate with the selenium particulate image. This reaction yields a black crystalline material having a melting point of about 1,000.degree. C.

During the heating, the selenium in the image areas agglomerates producing an initial reduction in density-fading, but after the reaction, the selenium appears to wet and spread over the copper substrate. The fading can be prevented by first converting the surface of selenium in the image areas to a crystalline form, followed by heating to produce the chemical reaction at the copper-selenium surface. This crystallization can be produced by exposure of the image to any known element or compound which will crystallize the surface of the selenium or selenium alloy in the image areas. These agents include vapor treatments with mercury, iodine, chlorine, bromine, fluorine, amines such as hexylamine, etc. For Example, exposure to mercury vapors for about 5 minutes is usually sufficient to convert the surface to a crystalline form.

The reaction temperature is that temperature at which the selenium will react with the given substrate. This temperature is only critical with respect to the substrate in that it should not exceed a temperature which will warp or buckle the substrate.

Generally, temperatures in the range of about 90.degree. to 350.degree. C. are sufficient to react the selenium or selenium containing alloy with the substrate.

EXAMPLE I

An imaging plate such as that illustrated in FIG. 1A is prepared by roll-coating a 2 -micron layer of Staybelite Ester 10 (Hercules Powder Company) on a 3 -mil Mylar polyester film (E. I. duPont de Nemours & Co., Inc.) having a thin coating of copper about 0.1 micron thick. A thin layer of vitreous selenium approximately 1 micron in thickness, is then deposited onto the Staybelite by inert gas deposition using the process set forth in copending patent application, Ser. No. 423,167, filed on Jan. 4, 1965. The plate is then electrostatically charged under dark room conditions to a positive potential of about 60 volts by means of a corona discharge device described by Carlson in U.S. Pat. No. 2,588,699. The charged plate is then exposed to an optical image with an energy in the illuminated areas of about 10 foot-candle-seconds by means of a tungsten chamber and a weak blue filter. The plate is then developed by immersing it in a bath of cyclohexane for about 2 seconds. The plate is removed from the developer bath and dried. An excellent image corresponding to the projected image is observed on the plate. This image comprises a thin layer of selenium particles in image configuration on a copper substrate.

EXAMPLE II

The plate of Example I is then placed in a sealed glass chamber and exposed to vapors of mercury for about 5 minutes. At the end of this time, the plate is removed from the glass chamber and heated by hot air to a temperature of about 100.degree. C. for several minutes, at which time the selenium and the substrate react, with the selenium appearing to wet and spread over the copper substrate. The resultant image shows high density, excellent contrast and is resistant to abrasion and thoroughly stable at relatively high temperatures.

EXAMPLE III

An imaging plate using the copper coated Mylar substrate as in Example I is roll-coated with a 2 -micron layer of Piccotex 100, (Pennsylvania Industrial Chemical Company). This plate is coated with a selenium layer and developed in cyclohexane as in Example I. The plate is then exposed and developed as in Example II and shows a stable image following mercury vapor treatment and heat stabilization as set forth in Example II.

EXAMPLES IV - IX

The procedures set forth in Examples I and II are carried out with a series of plates which are prepared, imaged, developed and stabilized under varying conditions with the results and process parameters set forth in the table below for all of the samples prepared and tested in the examples. ##SPC1##

Although specific components and proportions have been stated in the above description of the preferred embodiment of this invention, other suitable materials and procedures such as those listed above, may be used with similar results. In addition, other materials may be added which synergize, enhance or otherwise modify the images.

Other modifications and ramifications would appear to those skilled in the art upon reading the disclosure. These are intended to be included within the scope of this invention.

Claims

What I claim is:

1. A method for stabilizing a migration image comprising

a. providing a migration imaging member having a conductive substrate comprising a metallic layer and an electrically insulating solvent soluble overlayer containing migration material, said migration material comprising selenium particles

b. forming an electrostatic latent image on said member;

c. developing said image by applying a solvent for said electrically insulating layer to said member wherein a portion of the selenium particles deposit in image configuration on said conductive substrate and wherein another portion of selenium particles and said electrically insulating layer are removed from said substrate; and

d. heating said substrate and said selenium particles residing thereon to a temperature not exceeding the temperature which will warp or buckle said substrate whereby a chemical reaction occurs between said metallic layer and said selenium particles.

2. The method as defined in claim 1 wherein said electrostatic latent image is formed by uniformly charging said migration imaging member and selectively illuminating said charged member with a pattern of activating radiation.

3. The method of claim 1 wherein the conductive substrate comprises copper.

4. The method of claim 1 wherein the conductive substrate comprises brass.

5. The method of claim 1 wherein the conductive substrate comprises cadmium.

6. The method of claim 1 wherein the conductive substrate comprises silver.

7. The method of claim 1 wherein the conductive substrate comprises gold.

8. The method of claim 1 wherein the migration material comprises vitreous selenium.

9. The method of claim 1 wherein the migration material comprises a vitreous alloy containing at least 50 percent selenium by weight.

10. The method of claim 8 wherein the developed image of claim 1 is treated with a crystallizing agent prior to step (d).

11. The method of claim 10 wherein the crystallizing agent comprises vapors of mercury.

12. The method of claim 1 wherein the conductive substrate comprises chromium.