US3142562A

US3142562A - System and method for making records

Info

Publication number: US3142562A
Application number: US857159A
Authority: US
Inventors: Frederick L Blake
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 1959-12-03
Filing date: 1959-12-03
Publication date: 1964-07-28
Anticipated expiration: 1981-07-28

Description

y 1954 F. L.BLAKE 3,142,562

SYSTEM AND METHOD FOR MAKING RECORDS Filed Dec. 3. 1959 2 Sheets-Sheet 1 /0 FIG 4 mmvron Heder/b/r L. B/a/re y 28, 1964 F. L. BLAKE 3,142,562

SYSTEM AND METHOD FOR MAKING RECORDS Filed Dec. 3, 1959 2 Sheets-Sheet 2 VOLTAGE SOURCE 25 VOLTAGE 22 SOURCE F69 MM 1 VOLTAGE SOURCE IN VEN TOR. Frederick L. B/a/re AID/5 United States Patent 3,142,562 SYSTEM AND METHOD FOR MAKING RECORDS Frederick L. Blake, Scottsdale, Ariz., assignor to Motorola, Inc, Chicago, 111., a corporation of Illinois Filed Dec. 3, 1959, Ser. No. 857,159 8 Claims. (*Cl. 961) This invention relates generally to electrical recording sytsems. In particular, it relates to a combination electrical-photochemical-catalytic system in which local areas of a recording medium are acted on by light energy and catalyst material which is moved to the recording medium by an electrical field, to provide records in the form of printing, designs and images, for example.

The catalytic system and materials of the invention can be applied in various arts and in various physical forms. One application, for example, is telegraphic and radio message transmitting systems of the type wherein messages are printed on a recording medium in the form of a sheet by applying electrical energy to the sheet through a scanning stylus or other marking device. In one Well known type of recording system for such applications, the record on the sheet is formed by an electrolytic chemical reaction which produces a colored product, and the energy which causes the electrolytic reaction is supplied by the current which is applied to the sheet by the stylus.

Photography is another application for electrical recording systems. In known electrophotography systems, current which is area-modulated in accordance with the light and dark variations of the subject passes through a recording medium, and this modulated current causes an electrolytic chemical reaction which produces dark and light elemental areas on the medium in the form of an image. The recording medium is usually only one layer in a multilayer structure, which also includes suitable electrodes and a photoconductive layer that provides the current modulation mentioned above.

The sensitivity of electrolytic recording systems is limited by the fact that the electrolytic reaction requires a specific amount of current in accordance with Faradays law, and no practical way of avoiding this limitation on sensitivity has been developed. With respect to message printing applications, this means that the printing speeds which can be obtained are limited, and for electrophotography applications it means that the film is slow and requires long exposures. Of course, the current can be increased by raising the applied voltage in order to speed up the electrolytic reaction, but unduly high voltages may cause sparking and even breakdown of the recording sheet. More current can be used if the electrical resistance of the recording sheet is minimized, and this can be accomplished by wetting the sheet with a liquid electrolyte. However, an unduly wet sheet is difficult to handle, and special storage facilities or special Wet processing is required to make the sheet ready for use. Although dry electrosensitive recording sheets are available for facsimile and printing applications at the present time, these are not sufiiciently sensitive for many applications, and they usually cause undesirable sparking.

It is one object of the present invention to provide an electrical recording system in which the action of catalyst material affords a gain in sensitivity as compared to wholly electrolytic recording systems.

Another object of the invention is to provide an electrical recording medium which is substantially drier than known electrolytic recording mediums and which is more sensitive than such mediums.

Another object of the invention is the provision of sensitive materials for an electrical recording medium which are stable in normal ambient conditions, and which are rendered highly unstable only at the time the record is formed so that the medium can be stored and handled in a practical manner.

A further object of the invention is to provide materials and compositions for an electrocatalytic recording system which are sufficiently noncritical with respect to the amounts, purity and methods involved in the formulation thereof as to permit economical manufacture of the recording medium on a commercially practical basis.

A feature of the invention is the provision of a recording medium with a stable coloring ingredient which undergoes a rapid photochemical change to a contrasting color when contacted by catalyst material and also ex posed to radiation such as light. Since it is the combination of light and catalyst which produces the color change rather than electricity, the sensitivity of the recording medium is not limited by Faradays law.

Another feature of the invention is the provision of a recording medium as described in the preceding paragraph wherein a small amount of the catalyst material causes a larger amount of the coloring ingredient to change in color rapidly in the presence of radiation such as light, thereby providing a gain in sensitivity.

A further feature of the invention is the provision of an electrical recording system as described above in which the catalyst material is brought into efiective contact with the coloring ingredient of the recording medium by electrodepositing photosensitive ions of the catalyst onto the recordin medium, and converting the deposited ions to the active catalyst by exposure to light. Since it is the combination of electrical energy and light energy which places the catalyst in eiiective contact with the coloring ingredient, either the light or the electricity can be modulated so as to control the coloring process such that images, designs, printing or other records are formed on the recording medium.

Another feature of the invention is to provision of a sensitive coloring ingredient for a recording medium which is a water insoluble, crystalline pigment having a stable state of characteristic color, and which changes rapidly to a contrasting color when contacted by catalyst metal and exposed to light rich in ultraviolet. The coloring ingredient is comparatively insensitive to light and other radiation in the absence of the catalyst metal, so the recording medium can be stored, shipped and used without special precautions for protecting it from such radiatron.

A still further feature of the invention is the provision of a metal catalyst for the recording system described above which has a catalytically inactive ionic form that is convertible to the catalytically active free metal by exposure to light, thereby permitting the formation of images on the recording medium by optical modulation.

Certain embodiments of the invention are illustrated in the accompanying drawings in which:

FIG. 1 is a view of a recording sheet in the form of a homogeneous film in accordance with one embodiment of the invention;

FIG. 2 shows a recording sheet comprising a coating on a conductive support which forms another embodiment of the invention;

FIG. 3 illustrates schematically the manner in which a recording sheet in accordance with the invention can be inscribed employing a printer unit including a scanning electrode and a source of ultraviolet light;

FIG. 4 is a view of a portion of the printer unit of FIG. 3 showing in particular a multi-element electrode in contact with the recording sheet;

FIG. 5 shows a multi-layer sandwich for electro photography applications in accordance with another embodiment of the invention, and parts of the layers are broken away to better illustrate the construction;

sneaeea FIG. 6 is a schematic view showing the sandwich of FIG. in a camera and connected to a voltage source;

FIG. 7 is a cross-sectional view of the multi-layer sandwich of FIG. 5;

FIG. 8 is a cross-sectional view of a sandwich similar to that of FIGS. 5 and 7 and having in addition a photoconductive layer; and

FIG. 9 is a cross-sectional View of a modified construction for the sandwich of FIGS. 5 and 7.

The basic chemical components of the recording system of the invention include catalyst material, and a sensitive coloring material which undergoes a photochemical change of color when contacted by the catalyst and exposed to light. The sensitive coloring material is contained in a sheet for facsimile and printing applications, and is contained in a layer of a multi-layer sandwich for photographic applications. The catalyst comes into contact with the sensitive material in an active form only at the time when the mark or image is produced. Until that time, the catalyst must not act on the sensitive material. This is accomplished by separating the catalyst from the recording medium and applying the catalyst to the sensitive material at the proper time. The catalyst is a metal, and the metal is converted to photosensitive metal ions which are moved by an electric field to the recording medium. The catalyst does not affect the color of the sensitive material until it is reduced to the active free metal form. This reduction or activation of the catalyst is accomplished by exposing the photosensitive ions which have been deposited on the recording medium to light. Since the change of the sensitive material to a contrasting color is also a photochemical change, the activation of the catalyst and the color change of the sensitive material can be accomplished simultaneously by a single exposure to light.

In a particular message printing application, a scanning electrode is made of the catalyst metal, and as the electrode moves across the recording sheet it is electrically pulsed in such a way that photosensitive metal ions are deposited on the recording sheet at selected points. By building up a pattern of these discrete deposits of photosensitive metal ions, characters made up of patterns of dots are formed on the sheet when it is exposed to light.

In photographic applications, photosensitive catalyst metal ions are moved by an electric field from one layer of a sandwich structure to the sensitive coloring material which is in a different layer of the sandwich. The current which deposits the metal ions may be area modulated so that in places where a comparatively large amount of metal is deposited the image will be dark and in places where less metal, or none at all, is deposited, the image will be light.

Alternatively, the current may be unmodulated so that the photosensitive metal ions are deposited uniformly, and the conversion of the metal ions to the free metal form may be modulated by exposure to the light from the subject that is being photographed. In this case, the current is continued after the exposure to remove the residual metal ions from the image forming layer, and then the image is developed by exposure to strong unmodulated light which is rich in ultraviolet.

One of the most important aspects of this catalytic system is that each atom of the catalyst metal which is brought into efiective contact with the sensitive material as described above will act on a larger amount of the sensitive material. The sensitive material can be photochemically changed to a different valence state having a contrasting color. This photochemical reaction is an extremely slow reaction in the absence of catalyst material, so the color change of the sheet occurs only in those places where the catalyst metal has been deposited and activated.

It should be noted that the reaction of the sensitive material to form a colored product is not electrolytic and therefore is not subject to Faradays law. Electrolytic current is used to deposit the metal ions on the sensitive material, but once the metal ions are deposited and reduced to free metal, each metal atom catalyzes the photochemical reaction of a larger amount of the sensitive material. Therefore, for a given amount of current, a mark of greater color density can be achieved than is possible with a wholly electrolytic system. Thus, the catalytic system of the invention provides amplification, or a gain in sensitivity.

In order to incorporate the sensitive material in a conductive sheet or layer, certain other components besides the catalyst and the sensitive material are employed. In one embodiment, the sensitive material is dispersed in a plastic binder, and the plastic is selected so that it can be cast either as a separate film or as a coating on a conductive support. The plastic is impregnated with a highly ionizable salt dissolved in a slow-drying solvent which couples the salt to the plastic binder. The salt erves as an electrolyte which renders the sheet highly conductive. It has also been found desirable to include a stabilizing agent in the recording medium which serves to make the marks or image more permanent.

The sensitive material employed in the recording medium is preferably a solid, crystalline pigment, which is water insoluble, and which is changed by a photochemical reaction to a lower or higher valence state which has a contrasting color. The preferred sensitive material is titanium dioxide (TiO which is an excellent white pigment. This pigment will darken very gradually upon exposure to ultraviolet light for a long time, but the darkening is so slow as to be negligible when the exposure is for a limited time. When incorporated in a recording sheet, the titanium dioxide pigment gives the sheet a white color which provides an excellent background for inscription, and the white sheet will not darken appreciably in daylight. Also, in the absence of a catalyst, the sheet is comparatively insensitive to other radiation such as bombardment by electrons or gamma radiation. It has been found that when titanium dioxide crystals are contacted with traces of free metal in the presence of light, the crystals darken very rapidly. Although the mechanism of the color change is not fully understood as yet, it is thought that the dark color is produced by photoreduction of TiO at the surface of the crystal to Ti O thus producing F centers in the crystal which give it a dark color. It is believed that the metal is adsorbed on the crystal and catalyzes the photoreduction so that it takes place very quickly.

The selection of the catalyst metal to be employed for facsimile and photography applications is limited by the fact that the metal has to be applied to the recording medium in a controlled manner as described above. For facsimile and printing applications, it is possible to make the recording electrode of the catalyst metal and deposit ions of the metal on the sheet as previously mentioned. For photography, the catalyst may be provided in the form of a layer of metal separated from the sensitive layer by a barrier layer, and metal ions are moved from the metal layer through the barrier layer to the sensitive layer by applying voltage across the layers. However, the catalyst ions deposited on the recording medium must be reduced to the free metal or Zero valance state in order to be effective. For this reason, silver, gold or lead is employed as the catalyst metal because ions of these metals deposited on the recording medium are reduced to free metal by light. Thus, there are really two photochemical reactions which take place in marking the recording medium. First, the metal ions which are deposited on the recording medium are photochemically reduced to free metal, and this free metal then catalyzes the photochemical color change of the sensitive material.

Metal oxides other than titanium dioxide have been found to change color rapidly in the presence of ultraviolet light and metal such as silver. Examples are zinc oxide, aluminum oxide, lead oxide, tin oxide, vanadium oxide, and molybdenum oxide. It has also been found that cuprous thiocyanate (CuSCN) is darkened when contacted by silver and exposed to strong light in accordance with the invention. A class of materials which may be used as the sensitive material for the recording sheet of the invention is compounds of metals of groups 2, 3, 4, and 6 of the Periodic Table after Mendeleev, which have a stable state of characteristic color and which change to a lower or higher valence state of contrasting color upon exposure to strong light for a long time. Two or more compounds of the above class may be used in a mixture if desired.

The best results have been obtained using titanium dioxide as the sensitive material in the recording shee and silver as the metal catalyst. Also, it has been found that it is preferable to expose the recording medium to light which is rich in ultraviolet for purposes of developing the marks or image on the recording medium. An ultraviolet lamp is suitable for this purpose, but even bright daylight which is rich in ultraviolet will develop the marks or images satisfactorily.

For the hinder or carrier material, it is desirable to use a substance which can be rendered conductive. Suitable materials are high molecular weight organic substances (mol. wt. in the range of about 10,000 to 100,000) which are water soluble and contain a large percentage of polar groups. Natural products within this classification are starches, dextrins, algins, gelatins and cellulose derivatives such as methyl cellulose, hydroxyethylcellulose and carboxymethylcellulose. Synthetic substances of this class are polyfunctional plastics such as polyvinyl alcohol, polyacrylic acid and polyvinyl pyrrolidones. The preferred binder material is polyvinyl alcohol since this material is only slightly hygroscopic, and forms tough, smooth films which provide a good surface for recording.

The preferred electrolyte materials are ammonium salts and substituted ammonium salts. Examples of suitable ammonium salts are ammonium nitrate, ammonium citrate, ammonium levulinate, ammonium silicofluoride and ammonium bifluoride. Examples of suitable substituted ammonium salts are triethylenetetramine acetate or nitrate, tniethanolamine acetate or nitrate, piperidine nitrate, and dodecylamine acetate. Other electrolyte salts can he employed, but the salts named above have been found to be compatible with the other ingredients and they make the recording medium highly conductive. In selecting the electrolyte salt, attention must be given to the chemical properties of the catalyst metal. For example, where the catalyst metal is silver, silver ions will be transported to the recording sheet, and the electrolyte salt must not interfere with the silver ions. For example, ammonium chloride is not used because the chloride ion will precipitate the silver ions before they have a chance to act on the sensitive material. Of the electrolyte salts named above, 8.1114 m-onium nitrate has given the most satisfactory results because of its high solubility and ionization, and also because of its compatibility with polyvinyl alcohol.

The solvent couples the electrolyte salt to the plastic. The solvent should be very slow drying, it must be compatible with the binder material that is employed, and it must be reasonably insensitive to variations in humidity and temperature. The selected solvents are slightly hygroscopic and because of this they serve to plasticize the binder material. In cases where the electrolyte salt itself is sufficiently hygroscopic to plasticize the binder (such as triethanolamine acetate), it may be possible to omit the solvent. Suitable solvents are polyhydroxy alcohols having a molecular weight below about 200, such as glycerine, ethylene glycol, diethylene glycol, triethylene glycol and propylene glycol. Mannitol and so-rbitol are also possible solvents, but these are not as satisfactory as the others. The preferred solvent for use with the polyvinyl alcohol binder is glycerine.

It has been found that by including a stabilizing agent in the recording medium, the color density and contrast of the marks or image upon ageing is significantly improved. This stabilizing agent is an organic complexing compound which forms stable complexes with the changed form of the sensitive material. The preferred stabilizing agent is ammonium succinate. This material improves the permanence of the mark or image formed on the recording medium, improves resolution, and decreases bleeding of the mark or image upon ageing. Examples of other suitable stabilizing agents are ammonium oxalate, cupferron, an ion exchange resin known in the trade as AM9 (American Cyanamid), O-hydroxyquinoline and the ammonium salt of ethylenediaminetetracetic acid. A distinctive characteristic of these stabilizing materials is that they are all complexing agents for transition metal ions.

For facsimile applications, the recording sheet can be made in the form of a thin homogeneous film 10 as illustrated in FIG. 1. This film does not necessarily contain any paper or fibrous material. However, fibrous material can be included in the composition, and this tends to give the recording sheet additional body and more of the appearance of paper. Still another way to make the recording sheet is to apply a coating 11 of the recording medium on a conductive support 12. The support 12 can be paper which has been impregnated with graphite or metal particles to render it conductive, or it can be a laminate of paper and metal foil such as is sometimes used for packaging purposes.

A typical composition and method for making recording sheets in the form of a film as in FIG. 1 is as follows:

The above ingredients are combined and agitated in a blender in the following manner. The water, glycerine, ammonium nitrate and ammonium succinate are mixed and agitated until the salts are dissolved. Then the polyvinyl alcohol and antifoaming agent are added and agitation is continued until all of the constituents are blended. The titanium dioxide is then blended in slowly with agitation until a smooth homogeneous mix is obtained. The composition is milled in a ball mill for about one hour in order to reduce foam and further disperse the insoluble titanium dioxide particles. The mixture is then poured onto a smooth surface such as a glass plate and is doctored with a blade or rod having about a .020 inch clearance between the glass surface and the edge of the doctoring tool. This will produce a film which dries to a thickness of about .003 to .005 inch. Drying is best ac complished at room temperature for about 24 hours. If oven drying is employed, the temperature should not go above about F. because above this temperature the film may be distorted. When the film is dry, it can be peeled off the glass, and it is tough and pliable. It has a white color, and is readily markable with a silver electrode employing pulses having an amplitude of about volts, a pulse width of about 50 to 500 microseconds and a pulse interval of about 1600 microseconds.

Variations in the proportions of the ingredients given above are permissible as shown by the ranges stated in Table I set forth above. The ranges in the table are based on 300 milliliters (304 grams) of the 10% aqueous solution of polyvinyl alcohol. For the glycerine, the range is primarily determined by the effect of humidity. Below the minimum amount of 6 grams, the film becomes too dry if the relative humidity is below about 30 percent. Above the maximum amount of about 25 grams the film becomes too wet if the relative humidity exceeds about 30 percent.

For the ammonium nitrate, using less than the minimum amount of grams the film is too nonconductive, and above the maximum amount of about 15 grams the ammonium nitrate tends to crystallize on the film surface. The variation in the proportion of water employed is not critical since its chief influence is to control the viscosity of the composition so that it can be cast as a film conveniently. The stabilizing agent has little efiect below the minimum amount of 2 grams, and it tends to crystallize on the film surface if used in amounts above the maximum of about 6 grams. The sensitivity of the film decreases unduly if less titanium dioxide is used than the stated mini mum of 45 grams, and above the maximum amount of 65 grams of titanium dioxide the film becomes slightly abrasive such that marks may be formed on the film mere- 1y by contacting it with the silver electrode Without applying voltage to it. Of course, this is not satisfactory. Also, when the film is too abrasive the contact resistance of the electrode traveling across the film is too high and this reduces the efficiency of the system. When the preferred proportions of titanium dioxide and binder are employed, the titanium dioxide particles are protected by the binder and the recording surface is smooth. The antifoarning material is used to reduce the tendency of the composition to foam as a result of the blending, and the amount used is not critical.

The preferred proportions and ranges of the ingredients in the film of FIG. 1 when it is ready for use are given in the following table on the basis of percentage of the total composition by weight.

Table II Ingredients Preferred Range, Percentage percent Sensitive Pigment 50 40-60 Binder 25 20-30 Solvent 11 -15 Electrolyte Salt. 3 5-10 Stabilizing Agenh-.- 4 2-5 Antifoaming Agent 2 As He quired.

As previously mentioned, it is possible to include fibrous material in the composition. For this purpose, a finely divided wood cellulose fiber product such as that sold under the trademark Solka Floc can be used. The purpose of including this material in the film is to give the film more dimensional rigidity, prevent the film from sweating in conditions of high relative humidity, and give the film the feel and texture of paper. The following composition which includes such fibrous material is given by way of example:

The recording sheet can also be made in the form of a laminate as illustrated in FIG. 2. The sheet 10 includes a plastic film 11 on a conductive support 12 which may be paper impregnated with graphite or metal, and alternatively may be a paper-metal laminate. One satisfactory way to impregnate paper with graphite is to spray it with or dip it in a solution of colloidal graphite. Such a solution is commercially available under the trademark Aquadag. The proportions of the ingredients for the film when it is applied as a coating on a conductive paper backing are somewhat different than given above, particularly with respect to the glycerine content, because the paper absorbs a rather large percentage of the glycerine and more must be used. The following composition is given by way of example:

The ingredients are mixed together in a blender and then milled for about one-half hour by the same procedure as described above in connection with the description of the film of PEG. 1. The composition is then ap lied to the graphite impregnated paper.

in providing the embodiment in which the plastic film is coated on a metal foil-paper laminate, the film has the same composition as when it is made without any backing. It has been found that the best adhesion is obtained when a very thin coating of polyvinyl alcohol is baked onto the metal foil, and then the sensitive composition is applied to the baked coating. The sensitive composition may be cast or sprayed onto the baked coating.

FIGS. 3 and 4 illustrate a portion of a printer unit with which the recording sheets of FIGS. 1 and 2 may be employed. The utility of the recording sheet is not restricted to any specific apparatus, and the printer mechanism of FIGS. 3 and 4- is shown only for the purpose of giving an example of suitable apparatus. As illustrated in FIG. 3, the recording sheet 10 is a long strip which can be supplied as a roll. The sheet 10 is moved past a scanning mechanism 26 by

rollers

27 and 28 which are driven by a motor through suitable gears (not shown). The motor also drives one of the

pulleys

29 and 30 of the scanning mechanism 26. A continuous belt 31 is driven by the pulleys 29 and 3t and an electrode unit 32 is carried by the belt 31 and moves across the width of the sheet 11 once for each revolution of the belt. Additional electrode units may be spaced along the belt so that as one unit leaves the right side of the sheet, another is in position at the left side ready to start the next line.

The electrode unit 32 is provided with several printing fingers 33 as illustrated in FIG. 4. These fingers are pieces of wire made of the catalyst metal such as silver as described above. As viewed in FIG. 4, the right ends of the fingers 33 contact the recording sheet 10, and the left ends of the fingers contact the conductors 34 which extend along the face of an insulating panel 36. The printing pulses are supplied through the conductors 34 to the fingers 33, and the fingers apply the pulses to the recording surface of the sheet 19. Electrical contact may be made to the back of the sheet it by a metal plate 37 which extends across the width of the sheet. Electrical contact may also be made to the same side of the sheet as is in contact with the fingers 33 by making the roller 28 of conductive material.

A positive pulse applied to any one or" the metal fingers 33 causes metal ions to be deposited on a very small area of the sheet It). By applying the pulses to the printing fingers according to a code, these metal deposits are formed in a pattern which corresponds to the characters of a message as illustrated in FIG. 1. The characters are :made visible by exposing the sheet to ultraviolet light, and for this purpose a light source 38 may be provided in the printer unit as illustrated in FIG. 3. However, the record may be developed by exposing the sheet to ul- 9 traviolet light after removing it from the printer unit, if desired.

The recording system of the invention can also be applied to photography. Images have ben recorded employing a multi-layer sandwich structure 13 as illustrated in FIGS. and 7. In this structure, there is a transparent laminated electrode 14 consisting of a transparent glass or plastic body 15 with a very thin coating 16 of material such as tin oxide which is conductive and also transparent. One electrical connection 21 is made to the conductive coating 16. Transparent laminates of this type are available commercially under the trademarks NESA and EC. A layer 17 containing the catalyst metal is in contact with the electrode 14. Contacting the layer 17 there is a transparent barrier layer 13, and an image forming layer 19 which contains a dispersion of the sensitive material contacts the barrier layer. Electrical contact is made to the image forming layer 19 by the electrode 20.

Electrical connections

21 and 22 are made to the electrode layers 16 and 2t), and voltage is applied across the electrodes from the voltage source 23. Current flows transversely through the structure and forms ions of the catalyst metal in layer 17. Electrode layer 16 is made positive and layer 19 is made negative so that these metal ions move through the barrier layer 18 to the image forming layer 19 under the influence of the applied electrical field.

In order to record an image in the layer 19, the deposition of catalyst metal in the image forming layer can be modulated optically as previously mentioned. This process will be described with reference to FIGS. 5 to 7. The sandwich 13 is placed in a camera enclosure 40 provided with a lens 41 and a shutter 42 as illustrated schematically in FIG. 6. The transparent electrode 14 faces the shutter 42 so that when the shutter is opened, light from the subject to be photographed will be directed onto the sandwich 13 as represented by the arrows 43 and 44 in FIG. 6. With the shutter closed, direct current voltage is applied to the sandwich from the source 23 by closing the switch 24 for a time sufficient to ionize and move all or nearly all of the metal catalyst from layer 17 to layer 19. The barrier layer 18 is extremely thin and is pervious to the metal ions so that they quickly pass through the barrier and into the image forming layer 19. The metal ions have less mobility in the image forming layer so that nearly all of the ionic catalyst is concentrated near the upper surface of layer 19 adjacent the barrier a short time after the voltage is applied to the sandwich.

Then the shutter is actuated to admit enough light from the subject to form a latent image. There is no longer any reflective metal layer in the path of the light because the metal of layer 17 has migrated to layer 19 and is now in ionic form. The light passes through the

transparent layers

15, 16 and 18 and is directed onto the image forming layer 19. The light reduces the photosensitive metal catalyst ions to free metal according to the intensity distribution of the light over the area of the image forming layer 19. Thus, the density of the free catalyst metal in layer 19 after the exposure varies over the area of the layer providing a latent image.

The current supplied from the source 23 may either be continued or stopped during the exposure. After the exposure, current is passed through the sandwich for a time sufficient to remove the residual metal ions from the layer 19. These residual metal ions move through the layer 19 and are deposited on the negative electrode 20. The catalyst which was reduced to free metal by the exposure is absorbed on the pigment crystals in the image forming layer and does not re-ionize to any undesirable extent. When the residual metal ions have been removed from layer 19, the current flow is stopped. The image is developed by exposing the sandwich to strong light which is rich in ultraviolet, and this causes the sensitive pigment to change to a contrasting color in the places where it is in contact with the metal catalyst. In elemental areas of the layer 19 where the density of the catalyst metal is highest, the contrast is greatest, and in elemental areas where there is no catalyst metal, the layer 19 retains its original color. When the sensitive coloring ingredient is titanium dioxide and the catalyst is silver, the original color of layer 19 is white and the contrasting color is black or dark brown. In between these two extremes there are variations in the gray scale in accordance with the area distribution of the catalyst metal deposited in layer 19. The structure comprising layers 18-20 can be peeled off of the electrode 14 either before or after developing the image, if desired.

A satisfactory construction for the sandwich 13 of FIGS. 5 and 7 will be discussed by way of example. The transparent electrode 14 consisting of

layers

15 and 16 is available commercially as mentioned above. A very thin film of silver is vapor deposited over the conductive layer 16 of the transparent electrode to form the catalyst layer 17. Gold and lead may also be used for the catalyst layer 17 as described above. A satisfactory thickness for the layer 17 is about 50 angstrom units. A thin coating of transparent organic film-forming material such as polyvinyl alcohol is applied over the catalyst layer 17 to form the barrier layer 13. A satisfactory thickness for layer 18 is 1 micron. The polyvinyl alcohol adheres uniformly and intimately to the layer 17 and thus provides good electrical contact between the layers. The layer 18 is dried, and then the recording medium is cast over layer 18 to form the image forming layer 19. The composition of the recording medium is in accordance with the previous description, and the preferred composition is set forth in Tables I and II above. A suitable thickness for the layer 19 is about 1 mil. The electrode 29 serves the same function as the conductive backing 12 of the recording sheet of FIG. 2, and it may be made in any of the ways discussed in connection with FIG. 2. The sandwich will operate satisfactorily if a coating of graphite is painted over the layer 19 to form the electrode layer 20. If the electrode 20 has a metal layer, it can be placed on the image forming layer before it dries to insure uniform and intimate adhesion between these layers. The

conductors

21 and 22 are connected to the

conductive layers

16 and 20. It should be noted that the thickness of the layers is exaggerated in the drawings for purposes of illustration, and

the layers are not drawn to scale.

It has been found that it is also possible to record images with the catalytic recording system of the invention by area modulating the current flow through the sandwich. A sandwich 13a adapted for such operation is shown in FIG. 8. A layer 25 of photoconductive material is provided between

layers

16 and 17 of the sandwich 13a as shown in FIG. 8. In all other respects, the sandwich 13a is the same as sandwich 13. With this arrangement, no mechanical shutter is needed because the time of the exposure is controlled electrically. A voltage pulse is applied across the

electrodes

16 and 20, and light from the subject area-modulates the conductivity of the photoconductive layer. The current which fiows transversely through the structure is area-modulated in accordance with the intensity distribution of the light. Thus, the amount of catalyst metal which ionizes and moves to the image forming layer 19 varies over the area of the sandwich to provide a latent image. This image is developed by exposing the layer 19 to strong light as previously explained.

The sandwich 13b of FIG. 9 is a modified construction in which no separate photoconductive layer is provided, and in which the catalyst is incorporated in a carrier medium of plastic material. A satisfactory method for making the sandwich 13b illustrated in FIG. 9 is as follows. Silver nitrate in the amount of 10 milligrams is dissolved in milliliters of a 7 percent aqueous solution of polyvinyl alcohol. A transparent film of this material is cast on the NE-SA or EC glass electrode 14 and is dried to provide the catalyst layer 17. A thin transparent film of polyvinyl alcohol is then applied over the catalyst layer 17 and is dried to form the barrier layer 18. Thirty grams of titanium dioxide is dispersed in toluene and this mixture is ball milled for about 4 hours. Forty-five grams of a plastic material known in the trade as Pliolite is then added to the titanium dioxide and toluene mixture, and this substance is milled further until the titanium dioxide pigment is properly dispersed. The dispersion is then diluted with toluene until the consistency is proper for casting purposes, and the resulting material is cast as a layer 19 over the polyvinyl alcohol layer 18. After the layer 19 has dried, a graphite electrode 26 is painted on over the layer 19. The plastic layers 17, 18 and 19 provide uniform and intimate surface contact with the electrode layers 16 and 20 so that there is good electrical contact between all layers of the sandwich.

Images can be recorded in layer 19 of the sandwich 13b of FIG. 9 in the manner described in connection with FIG. 6. That is, sflver ions are uniformly moved from layer 17 through layer 18 to the image forming layer 19 by applying voltage across the

electrodes

16 and 20 with the sandwich in the dark. Then the exposure is made to reduce some of the silver ions to free silver, thus forming a latent image in the layer 19. The residual silver ions are removed from layer 19 by continuingthe current after the exposure, and the image is developed by directing strong ultraviolet light onto the layer 19.

Tests conducted with the sandwich of the invention show that the titanium dioxide pigment in the layer 19 exhibits photoconductivity. That is, the transverse conductivity of the layer varies with the intensity of light directed on it. The photoconductivity of the layer 19 has little effect when images are formed by optical modulation as discussed above. However, images may be recorded with the sandwich of FIG. 9 by using the photoconductivity of the layer 19 to area-modulate the flow of catalyst ions from the layer 17 through the barrier layer 18 to the image forming layer 19. In this case, the image is recorded in the manner described in connection with FIG. 7 except that the titanium dioxide in the layer 19 serves to area-modulate the current and no separate photo-conductive layer is provided.

It is apparent that the catalytic recording system of the invention has various applications, and in each of the applications it has certain advantages over known systems. In facsimile and message printing applications, one of the most important advantages of the catalytic system is that it is more sensitive than known systems. Because of this it is possible to make the recording sheet substantially dry and yet mark it without sparking at high printing speeds and relatively low voltages. For example, messages can be printed in the apparatus of FIGS. 3 and 4 without sparking at a speed of 20 inches per second and marking voltages of 25 to 200 volts. The printing has satisfactory contrast, resolution and permanence. No wet processing is required either before or after marking, and to protect the sheet during storage, simple packaging such as a plastic wrapper can be used. In photopraphy applications, one of the main advantages of the catalytic system of the invention over photographic film of the silver halide type is that the recording medium is not sensitive to light or other radiation either before or after the image is formed. This is particularly advantageous in conditions where the ambient radiation level is high. Another advantage is that no wet chemical processing is needed to develop the image, as is the case in silver halide photography. The catalytic recording medium of the invention is more sensitive than entirely electrolytic recording mediums previously proposed for electrophotography, and it can be used in a much drier condition than such electrolytic systems.

I claim:

1. A method of making visible records, including the steps of electrolytically applying ions of a metal selected from the group consisting of silver, gold and lead to portions of a recording sheet comprising a compound selected from the group consisting of cuprous thiocyanate, and oxides of titanium, zinc, aluminum, lead, tin, vanadium and molybdenum dispersed in an organic binder containing a polyhydroxy alcohol and a non-photosensitive nitrate salt which renders said recording sheet electrically conductive, said ions being applied to said sheet by applying direct current electric potential from an external potential source to produce a potential difference between a conductive electrode which is in contact with said sheet and a solid element containing the selected metal which is distinct from but in contact with said sheet to produce current through said sheet which transfers ions of the metal from said solid element to said sheet by electrolysis, and exposing said sheet and the metal ions thereon to light rich in ultraviolet.

2. A method of making visible records, including the steps of electrolytically applying silver ions to a recording sheet which comprises titanium dioxide material dispersed in an organic binder containing a polyhydroxy alcohol and a non-photosensitive nitrate salt which renders said recording sheet electrically conductive, said silver ions being applied to said sheet by applying direct current electric potential from an external potential source to produce a potential difierence between a conductive electrode which is in contact with said sheet and a solid element of silver material which is distinct from but in contact with said sheet to produce current through said sheet which transfers silver ions from said solid element to said sheet by electrolysis, and exposing said recording sheet and the silver ions thereon to light rich in ultraviolet.

3. A method of making visible records, which comprises moving a recording stylus comprised of silver material over and in contact with a recording sheet comprising titanium dioxide pigment dispersed in a watersoluble organic binder containing a polyhydroxy alcohol and a non-photosensitive nitrate salt which renders said recording sheet electrically conductive, applying direct current electric potential from an external potential source to produce a potential difference between said recording stylus while said stylus is in contact with said sheet and a conductive electrode while such electrode is in contact with said sheet to produce current through said sheet which transfers silver ions from said recording stylus to said sheet by electrolysis, and exposing said sheet and said silver ions thereon to light rich in ultraviolet.

4. A method of making visible records, which comprises providing a recording medium in the form of a plastic sheet comprising a binder of water-soluble plastic of the polyvinyl type, a polyhydroxy alcohol, a nonphotosensitive nitrate salt, and a compound selected from the group consisting of cuprous thiocyanate and oxides of titanium, zinc, aluminum, lead, tin, vanadium and molybdenum, electrolytically applying to said sheet ions of a metal selected from the group consisting of silver, gold and lead by applying direct current electric potential from an external potential source to produce a potential difference between a conductive electrode in contact with one side of said sheet and a solid element containing the selected metal in contact with the other side of said sheet to produce current through said sheet which transfers ions of the selected metal from said solid element to said sheet by electrolysis, and exposing said sheet and said metal ions thereon to light rich in ultraviolet.

5. A method of making visible records which comprises, providing a recording medium in the form of a layer of an organic binder containing titanium dioxide and a nonphotosensitive nitrate salt and a polyhydroxy alcohol distributed through an area thereof, electrolytically applying silver ions to said recording medium from a solid layer containing silver which is distinct from but in contact with said recording medium by applying direct current electric potential from an external potential source to produce a potential difference between said silver-containing layer and an electrode in contact with said recording medium and thereby transfer silver ions from said solid layer to said recording medium throughout said area thereof, exposing said recording medium to a pattern of light from a subject for forming a latent image in said recording medium by reducing silver ions in said area to silver metal in accordance with the intensity distribution of the light, subsequently removing residual silver ions from the recording medium by passing electric current between electrodes on opposite sides of said recording medium, and exposing said recording medium to light rich in ultraviolet which has a substantially uniform intensity distribution over the area of said recording medium, with such light being effective to produce a visible color change in said recording medium corresponding to said latent image.

6. A recording sheet having a recording medium in the form of a layer comprising a polyvinyl plastic binder material, a polyhydroxy alcohol material having a molecular weight less than about two-hundred and a non-photosensitive nitrate salt distributed in said binder material for rendering the same electrically conductive, and a compound selected from the group consisting of cuprous thiocyanate and oxides of titanium, zinc, aluminum, lead, tin, vanadium and molybdenum distributed in said binder in an amount sufiicient to produce a color change in elemental areas of said recording medium when recording signals are applied thereto with a stylus electrode made of a metal selected from the group consisting of silver, lead and gold and the recording sheet is exposed to light rich in ultraviolet.

7. A recording sheet having a recording medium in the form of a layer comprising a binder of polyvinyl plastic, a polyhydroxy alcohol material having a molecular weight less than about two-hundred and a non-photosensitive nitrate salt distributed in said plastic binder for rendering the same electrically conductive, and titanium dioxide dispersed in said binder in an amount sufficient to produce a color change in elemental areas of said recording medium when silver ions are applied to said medium and said recording sheet is exposed to light rich in ultraviolet.

8. A recording sheet having a recording medium in the form of a layer, comprising a binder of polyvinyl alcohol, a mixture of glycerine and ammonium nitrate distributed in said binder for increasing the electrical conductivity thereof, titanium dioxide material dispersed in said binder in an amount sufficient to produce a color change in said recording sheet when silver is applied thereto and said sheet is exposed to light rich in ultraviolet, and said binder further containing ammonium succinate therein in an amount sufiicient to stabilize such a color change in said sheet.

References Cited in the file of this patent UNITED STATES PATENTS 2,229,091 Kline Ian. 21, 1941 2,283,558 Kline May 19, 1942 2,294,146 Wise Aug. 25, 1942 2,310,946 Finch Feb. 16, 1943 2,319,765 Talmey May 18, 1943 2,339,267 Hogan et al. Jan. 18, 1944 2,476,800 Blackburn July 19, 1949 2,528,005 Kline Oct. 31, 1950 2,776,251 Schwartz Jan. 1, 1957 2,901,348 Dessauer et al Aug. 25, 1959 2,983,654 Dalton May 9, 1961 3,052,541 Levinos Sept. 4, 1962 3,072,541 Shely et al. Jan. 8, 1963 3,088,883 Robillard May 7, 1963 FOREIGN PATENTS 188,030 Great Britain Oct. 23, 1922 464,112 Great Britain Apr. 12, 1937 537,593 Great Britain June 27, 1941 113,072 Australia May 22, 1941

Claims

1. A METHOD OF MAKING VISIBLE RECORDS, INCLUDING THE STEPS OF ELECTROLYTICALLY APPLYING IONS OF A METAL SELECTED FROM THE GROUP CONSISTING OF SILVER, GOLD AND LEAD TO PORTIONS OF A RECORDING SHEET COMPRISING A COMPOUND SELECTED FROM THE GROUP CONSISTING OF CUPROUS THIOCYANATE, AND OXIDES OF TITANIUM, ZINC, ALUMINUM, LEAD, TIN, VANADIUM AND MOLYBDENUM DISPERSED IN AN ORGANIC BINDER CONTAINING A POLYHYDROXY ALCOHOL AND A NON-PHOTOSENSITIVE NITRATE SALT WHICH RENDERS SAID RECORDING SHEET ELECTRICALLY CONDUCTIVE, SAID IONS BEING APPLIED TO SAID SHEET BY APPLYING DIRECT CURRENT ELECTRIC POTENTIAL FROM AN EXTERNAL POTENTIAL SOURCE TO PRODUCE A POTENTIAL DIFFERENCE BETWEEN