US 3317409 A
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Description (OCR text may contain errors)
y 2, 1967 A. F. KA SPAUL ET AL 3,317,409
ELECTROLYTI G EDECTROPHOTOGRAPHY Filed April 16, 1963 FIG.2
ALFR ED FKASPAUL-INVENTORS JOHN W. CHRISTENSEN- FM ASW TH ElR ATTORNEY United States Patent O 3,317,409 ELECTROLYTIC ELECTROPHOTOGRAPHY Alfred F. Kaspaul, Fairview, Pa., and John W. Christensen, New Canaan, Conn., assignors to Minnesota Mining and Manufacturing Company Filed Apr. 16, 1963, Ser. No. 273,471 6 Claims. (Cl. 204-18) This invention relates to the production of images on photoconductive surfaces.
More particularly, this invention relates to a method for producing images on photoconductive surfaces which have been precoated with a light transmissive layer of metal, to the images so produced, and also to methods for producing such metal precoated photoconductive surfaces.
The art has heretofore generally appreciated that photoconductive surfaces can be used to record visible images by'selectively depositing image-forming materials on a photoconductive surface bearing a latent image by utilizing the difference in electrical conductivity between illuminated and non-illuminated areas. The general procedure is to employ electrophotographic techniques and deposit material selectively upon photoconductive surface, either concurrently with or shortly after the exposure of such surface to a light image.
It is an object of this invention to provide a process for producing reproductions using photoconductive layers. Another object of the invention is to provide a process for making positive prints. Another object is to provide a photoconductive copy sheet which has a metallic imageforming-layer. Other objects will be apparent to those skilled in the art from the accompanying disclosure.
In accordance with the objects of the invention, it has now been discovered that images can be formed on photoconductive surfaces by selectively removing metal from a photoconductive surface precoated with a light-transmissive film of metal, with the result that a visible image is produced. Thus, for example, one can directly produce positive images on a zinc oxide photoconductive surface by selectively removing metal from a thin precoated layer of metal upon such photoconductive surface.
In practicing the invention, strongly photoconductive material which has been deposited in the form of a layer upon a conductive substrate is coated with a uniform lighttransmissive film of metal over the entire surface, to form a multi-layered light-sensitive sheet. The sheet is then exposed to a light image of the matter to be recorded, for a time suflicient to effect a change in the conductivity of the photoconductor. Thereafter, the latent image existing as areas of differential conductivity in the photoconductor layer is developed by electrolytically removing metal from the upper metallic layer.
Suitable semi-conductive photoconductive materials include highly conductive cadmium sulfide, cadmium selenide, antimony sulfide (Sb S zinc oxide, zinc sulfide, indium oxide, and the like. The photoconductors should have electrical conductivity on exposure to light of at least about mho per cm. and preferably between about 10* and 10* mho per cm. A preferred photoconductive material for purposes of this invention because of its inherent physical properties, such as its decay time or light memory, is highly conductive zinc oxide.
Suitable photoconductive layers are conveniently produced by bonding to the surface or the like of a conductive sheet, such as a metal sheet, which may be backed with paper, if desired. The photoconductor is mixed with a suitable organic bonding agent and applied in admixture to the surface of the sheet, and dried. Suitable bonding agents include a copolymer of styrene and butadiene known as Pliolite, polystyrene, chlorinated rubber, silicone resin, rubber hydrochloride, polyvinylidene chloride and the like. The binder should be a good insulator and as nonconductive as the photoconductor in the dark, having conductivity no greater than 10- mho per cm. Preferably, the binders should be soluble in conventional organic solvents so that they can be applied to the surface in solution. The bonding agents should also have adequate bonding strength between the photoconductor and the support, and preferably should be hydrophobic or Water-insoluble. Preferably, the photoconductor layer is sufficiently conductive that a low voltage can be used during development. Usually the thickness of the layer is between about 0.1 and about 5 mils, and preferably between about 0.5 and about 2 mils.
Using vacuum vapor deposition techniques and preferably pressures not greater than about 10- mm. Hg, the selected metal is vaporized and allowed to deposit upon the surface of the photoconductor. Enough metal is deposited to produce a thin layer which is about 10 to 90 percent and preferably from about 40 to percent optically transmissive. Thinner metal films can be used when the metal is dark colored. In general, transmissive metal films suitable for the purposes of the invention have thicknesses of from about to Angstrom units, depending on the metal used.
For purposes of this invention, light transmission through a metal layer on a photoconductive surface can be measured using a tungsten filament light source, by comparing the perpendicular transmission through an uncoated inch thick transparent glass plate to the same plate coated by vacuum vapor techniques with the particular metal being coated, the metal deposition upon such glass plate being carried out under the same conditions as, and preferably simultaneously with, the deposition of such metal upon the particular photoconductive surface being coated with this metal.
One suitable method for coating a photoconductive surface by vacuum vapor deposition is as follows: Sheets of photoconductive paper, which are to be coated with a layer of, say cobalt, are circumferentially mounted on a rotatable stage in the bell jar of a vacuum apparatus. Between successive pieces of paper are placed the glass comparative optical transmission plates described above. Beneath an area near the edge of the stage where plates and sheets are circumferentially mounted is placed a heated source of cobalt vapor. Between the stage and the cobalt vapor source, a shutter arrangement is mounted which is suitable for controlling the exposure time of the materials to be coated by cobalt vapor. Also beneath the stage at a distance from the cobalt vapor is mounted a tungsten filament light source so arranged that when the stage is rotating, paper and glass being vapor-coated pass vertically over the filament. A photocell is positioned vertically above the stage over the tungsten filament. Cobalt deposits both upon the glass plates and upon the photoconductive sheets during coating. When the optical transmission of the tungsten light source through the glass is reduced to the desired level (say, 50 percent), compared to the orginal transmission through the glass as measured by the photocell, the vapor deposition is discontinued.
In general, any conductive electrolyzable metal can be used for purposes of this invention, such as copper, vanadium, zinc, bismuth, cadmium, titanium, silver, nickel, and the like. Preferably, the metal is dark in color for contrast; however, if readout of the image is to be made by other than optical viewing, such as by electronic readout or the like, even very thin, non-optically contrasting metal films are useful. Preferred metals for the purpose of the invention are cobalt, titanium and vanadium.
In the case of metals which have heats of vaporization lower than about 50 kilocalories per mole, an intermediate step is desirable and sometimes necessary before such metals are vapor-coated upon a photoconductive substrate. This intermediate step comprises depositing upon the sur face to be metal-coated by vacuum vapor deposition an intermediate nucleating layer having from about 10 to 10 atoms per square centimeter of a metal or metal alloy having a heat of vaporization exceeding 0 kilocalories per mole. Such a layer is discontinuous and substantially optically transparent. The technique used is similar to that disclosed in United States Patent No. 2,754,230.
Such a preliminary deposition appears to create nucleation sites, thereby enhancing subsequent deposition and adherence of metals having heat of vaporization lower than 50 kilocalories per mole so as to produce metal films which are from about 40 to 70 percent optically transmissive.
It is theorized that, and there is evidence to substantiate that, metal films vapor-coated in accordance with the teachings of this invention are not continuous but are rather in the form of little islets having probable separation from one another of, say, about Angstrom units. The presence of these metal islets in such amounts that the optical transmissivity of the film is in the range of about 40 to 70 percent produces coatings which have little or no lateral electrical conductivity. Below about 40 percent they may form an electrically continuous surface giving rise to approximately bulk values of conductivity.
In general, evaporated films display very low conductivity until a certain critical optical transmission level is reached, after which the conductivity increases very rapidly as a function of decrease of percent optical transmission until values approaching conventional bulk conductivity values of the particular metal being vapor-coated are obtained. Naturally, the bulk conductivity value for any given metal film has to be determined empirically in each case since it varies with the type of evaporated metal involved, residual gas pressure in the vacuum apparatus during vapor deposition, rate of metal vaporization, the surface condition of the particular substrate involved (e.g., the presence of nucleation sites), and other factors. For example, vanadium films on glass plates having an optical transmissivity of the order of 20 percent or less have resistance of 100 ohms/square or less, while films having 50 percent transmission have over 1000 ohms/ square resistance. At 70 percent optical transmissivity, the resistance is about 5000 ohms/ square.
It appears that for purposes of the present invention greatest optical contrast in a developed image is achieved by using evaporated metal films in which the maximum amount of metal vapor is deposited which is just insufiicient to produce a bulk value for conductivity. Since bulk conductivity values are dependent upon so many variables, as mentioned above, it is usually simpler and more expedient simply to deposit sufiicient metal by vacuum vapor deposition upon a photoconductive surface until there is produced a metal film having optical transmission characteristics of from about 40 to 70 percent.
After the light-transmissive layer has been deposited on the photoconductive layer, the multi-layer sheet is ready for use to produce records of light images. The sheets are conveniently stored in the absence of bright light so as to achieve the dark conductivity state. i
To make a reproduction of an image using the photoconductive sheet construction of the invention, the sheet is exposed to a radiation pattern or light image, for example, by means of daylight or an incandescent lamp. The radiation may be actinic light, ultraviolet light, or other radiation such as X-rays which effects a change in the conductivity of the photoconductor layer, herein referred to generally and inclusively as light. The resulting differentially conductive image is then developed to visible (readable) form.
To form a visible image the surface of the light-transmissive metal film is contacted with a liquid or gel electrolyte, usually aqueous in character, and electrolyzed. The electrolyte itself need have no special characteristics, but can be any soluble salt or salt-like material which ionizes in solution. Suitable electrolytes include CaSO Na SO NaCl, CoCl KClO LiCl, NH Cl, Mg(NO NaNO MgCl Mg(ClO) KCl, or HCl. Preferred electrolytes are those in which electrochemical oxidation of the metal results in formation of a soluble salt of the metal, or an insoluble salt which is colorless or nearly transparent.
If the photoconductive surface upon'which the evaporated metal layer rests is one having a long decay time, that is, one which is capable of storing a latent image for a sufficiently long period of time after removal of the light source bearing the image to be recorded, then the electrolyte solution can be contacted with the surface of the evaporated metal film after latent image formation before the image in the photoconductive material has faded. However, if the photoconductive material has a very short decay time, then the electrolyte solution neeeds to be in contact with the evaporated film over the photoconductive surface during the time when the image-bearing light is projected upon the photoconductive surface bearing the evaporated metal layer. In either situation, a current is made to flow through the electrolyte solution. Usually an inert electrode in contact with the electrolyte solution is made the cathode and the electrically conductive substrate beneath the photoconductive material is made the anode.
Those areas of the photoconductive material which are exposed to light are more conductive than those which are not, with the general result that electron flow through regions which have been exposed to light is greater than elsewhere on the photoconductive surface. The difference in conductivity is at least ten-fold, and generally as much as -fold or more. The passage of electrons through the electrolyte solution, through evaporated metal film, and through the photoconductive surface results in the selective removal of the evaporated metal layer upon the photoconductive surface, with the effect that metal is oxidized away from the original evaporated metal film in those regions where the intensity of incident light is greatest. As a consequence, there is developed out of the original continuous evaporated metal film a visible positive image (i.e., one wherein the light-struck areas are light in hue compared to those areas which were not lightstruck).
The rate of metal removal is dependent upon the current density and duration. These values can vary within wide ranges. Generally, D.C. voltages less than 100 volts are used, and less than 50 volts is preferred. When one considers that the path of current flow is not only through the relatively low resistance path of the metal but also through the higher resistance of the binder wet by electrolyte the range of useful current densities from .1 to 5 milliamperes (or even higher), i.e., 1:50, is not surprismg. The range of high to low resistance paths available before exposure is represented by optical transmissivity and is 40 to 70% or 1:1.75. The thickness of the zinc oxide layer is 0.1 to 5 mils or 1 to 50 which represents the total length through which the current must flow and whose resistivity is proportional to the thickness. The thickness of the metal film to be removed also varies.
Cobalt, titanium and vanadium metal films give the best quality images and are preferred. Titanium images are of relatively low optical contrast, but highly stable against fingerprints and high --humidity. Vanadiumimages display high resolution, superior stability and excellent contrast.
Good optical contrast is generally obtained with cadmium films, with somewhat limited stability against fingerprints and high humidity. However, cadmium images can be protected by spraying with an acrylic resin and then display indefinite resistance to high humidity, fingerprints and the like. Occasionally, poor contrast is obtained with cadmium films, apparently owing to the fact that the light yellow cadmium oxide formed during the oxidation process is sometimes not completely removed from the metal surface. Other metals are also useful and can provide films of various colors, depending on the nature of the metal.
The invention is further described by reference to the accompanying drawing. FIGURE 1 is a diagrammatical illustration of the copy sheet of the invention, with the top layerthereof cut back to show an intermediate layer. FIGURE 2 is a diagrammatic representation of the copy sheet of the invention, with a latent image present therein. FIGURE 3 is a diagrammatic representation of the copy sheet after development, with an image appearing thereon. In the drawing, the relative thickness of the layers of the copy sheet has been greatly exaggerated, for clarity.
Referring to FIGURE 1, numeral designates an electrically conductive carrier, having a photoconductive layer ll bonded thereto. 14 designates an intermediate nucleating layer of metallic atoms. 16 represents a vapordeposited light-transmissive metallic layer.
In FIGURE 2, the multi-layer copy sheet construction of the invention, having electrically conducting base layer 10, photoconductive layer 12, intermediate nucleating layer 14 and light-transmissive metallic layer 16, has been exposed to illumination over a portion of its surface, in the form of an image as shown, using actinic radiation sufiicient to cause at least 10-fold increase in the conductivity of the photoconductor in the area illuminated, the latent image being invisible to the eye and existing as an area of increased conductivity in the photoconductive layer. In this instance, the illumination was made using a transparent mask in which the image of the numeral 3 was opaque, and the remainder transparent, the result being that in the area designated 18, conductivity of the photoconductive sheet remains the same as that of the;
dark adapted layer, while the portion designated 19 has increased electrical conductivity.
FIGURE 3 shows the copy sheet of FIGURE 2 after development by electrolysis, in which the image on photoconductive layer 12 consists of that part of light-transmissive layer 16 which has not been removed by electrolytic oxidation of the metal, leaving a contrasting image 20 corresponding to the latent image 18. In this instance, electrolytic oxidation was carried out using an electrolyte which effectively removed all of the metallic layer 16, but which converted the nucleating layer 14 to an insoluble oxide of the metal employed for nucleation, which remains upon the surface of the photoconductor 12 as a layer 22 which is substantially invisible to the eye.
The invention is further illustrated by reference to the following specific examples, in which all parts are by weight unless otherwise specified.
Example I A light sensitive copy sheet is prepared as follows: A suspension of about 46 parts by weight of French Process Zinc Oxide microcrystals in a solution of 27 parts of toluene, 40 parts of acetone, and 11 parts of Pliolite S-7 (a resinous copolymer of 30 weight percent of butadiene and 70 weight percent styrene) serving as a binder, the mixture having been ground in a ball mill until smooth, is wet-coated on a sheet of 1 mil. gauge aluminum foil which is backed with paper. After drying, the firmly bonded smooth white coating of zinc oxide is found to be about 0.8 mil. in thickness. The dried photoconductive layer of zinc oxide is placed in a chamber of a vacuum coating apparatus, maintaining the chamber under pressure of about 10 torr, and cleaned by glow discharge in argon. The sheet is then prenucleated by vapor depositing an invisible coating of nichrome on the surface by electrically heating a tungsten filament over which is suspended a U-shaped piece of nichrome wire until the nichrome is at a temperature at which it vaporizes sufficient nichrome being used to deposit 10 -10 atoms/ sq. cm. or about 0.01 mg. for every sq. cm. of surface to be covered. Then, utilizing a test glass coated at the same time, and a photocell which reads the optical transmission of the test glass with respect to a beam of light which is passed through the glass and into the photocell, the surface of the photoconductive layer which was prenucleated with nichrome is coated with vanadium by evaporating vanadium (also conveniently from a vanadium wire hung over a tungsten heater) until a layer of about 50 percent optical transmissivity has been deposited. Surfaces thus prepared appear to be a medium gray color by visual inspection.
The copy sheet thus prepared is used as follows: An image is formed upon the dark-adapted (24 hours) copy sheet, using a photographic negative bearing indicia to be copied. The copy sheet with negative held in contact therewith is exposed for about 5 seconds to 700-foot candles illumination. The exposed sheet is immediately developed using an electrolyte containing enough oxalic acid or potassium oxalate in aqueous solution to have sp. resistance=300 ohm-cm. The electrolyte is held in a sponge which is made the cathode; the aluminum base layer of the photoconductive copy sheet is made the anode. A 60-volt direct current potential is applied between the metallic conductive base layer of the sheet and the sponge containing the electrolytic solution.
Development is carried out for 1 second at an averagecurrent density suflicient that approximately 60 millicoulombs of electrical charge per square inch are used, A positive image, which can be viewed by reflection, is produced upon the surface of the copy sheet corresponding to the indicia projected thereon.
When development is carried out for 2 and 3 seconds, respectively, approximately 90 to millicoulombs are used. The images are of somewhat greater contrast; however, oxidation carried out beyond the point at which all of the metal has been removed or oxidized in the illuminated areas is obviously without additional effect.
The following table shows the results obtained when various metals are used as light-transmissive coatings on a zinc oxide photoconductive sheet. In each case, a transparent negative is employed, through which the sheet is exposed to light of the intensity and for the duration set forth. The electrolyte which is employed is dissolved in water to give a solution which at 25 C. has specific resistance of about 300-ohm centimeters. The metalcoated photoconductive copy sheets are dark-adapted for periods ranging from 24 to 48 hours prior to exposure. The time of development of these exposed copy sheets ranges from one second to several minutes, depending upon the voltage and amperage employed. Generally speaking, about 50-60 millicoulombs per square centimeter give good contrast. It is found that the use of higher voltage at shorter development times promotes improved contrast. In each case, a positive picture is obtained from the negative transparency.
While a sponge containing the aqueous electrolyte is conveniently made the cathode, the exposed copy sheets can also be developed by immersing them in an electrolyte solution contained in a shallow pan, with a stainless steel cathode. The aluminum layer of the copy sheet is made the anode. In the latter case, development can be observed as it proceeds, if desired.
TABLE I Opt. Development Metal 1 Trans, Exposure, Time, Remarks on Visual Observation percent Sec/F6 Sec.
Volt Milliarnps Electrolyte 30 20/150 Good contrast, gray tones. 50 /150 Low contrast. 28 /150 Good contrast, dark gray tones. 30 15/150 Good contrast. 30 20/150 Good contrast, dark gray. 55 30/150 Fair contrast, light gray tones. 54 30/150 Do. 50 5/700 1 Excellent contrast. 48 3/700 1 D 50 120/1 Low contrast. 50 120/1 180 D0.
1 A light transmissive coating of the metal named was vapor-coated on the surface of a zinc oxide-pliolite photoconductive layer supported on an aluminum foil sheet backed with paper.
2 Exposed and developed using a commercially available microfilm readerprinter; 8% inch wide sheet used; 11 inches passed under a sponge wet with the electrolyte in 4.5 seconds for development.
3 Wiped by hand with a sponge wet with electrolyte and connected to a battery; metal backing of the copy What is claimed is:
1. A process for making a reproduction which comprises exposing to a light image a copy sheet comprising a substrate consisting of an electrically conductive base sheet, an intermediate photoconductive layer and a surface layer overlaying said photoconductive layer consisting essentially of a light-transmissive metallic film having about 40 to 70 percent optical transmissivity, to create a latent image in the photoconductive layer of the said copy sheet, wetting the surface of the copy sheet with an aqueous electrolyte solution, and creating a direct current electrical potential between the electrolyte as the cathode and the conductive base layer of the copy sheet as the anode to convert the said light-transmissive metallic layer electrolytically to a visible record of the said light image.
2. A method according to claim 1, wherein the lighttransmissive metal film has about 50 percent optical transmission.
3. A copy sheet suitable for recording light images by electrophotographic techniques, comprising an electrically conductive substrate, a layer of photoconductive material bonded to said substrate, comprising a particulate photoconductor and an insulating resinous binder, and a lighttransmissive metal layer having about 40 to 70 percent optical transmissivity adhered to the surface of said photoconductive layer.
4. A copy sheet according to claim 3, wherein the said light-transmissive metal is vanadium.
5. A copy sheet suitable for recording light images by electrophotographic techniques, comprising an electrically conductive substrate, an intermediate photoconductive layer comprising a particulate photoconductor and an insulating resinous binder bonded to said conductive sub strate, a nucleating layer consisting of about 10 to 10 metal atoms per square centimeter distributed uniformly and adherently over the surface of said photoconductor, and a light-transmissive metal layer having about 40 to percent optical transmissivity distributed uniformly over and upon said nucleating metal and tightly adherent to said photoconductive layer.
6. A copy sheet according to cliam 5, wherein the nucleating metal atoms are nickel, and the light-transmissive film is vanadium.
References Cited by the Examiner UNITED STATES PATENTS 2,912,592 11/1959 Mayer 96-1 X 3,010,883 11/1961 Johnson et al. 204-18 3,085,051 4/1963 Hamm et al. 961 X 3,127,331 3/1964 Neher 96-1 X 3,127,333 3/1964 Bonrud 96-1 X 3,199,086 8/1965 Kallman et al. 96-1 NORMAN G. TORCHIN, Primary Examiner. D. PRICE, R. MARTIN, Assistant Examiners.