US3592643A - Photoconductive cadmium sulfide composition and process of preparing - Google Patents

Abstract

A PHOTOCONDUCTIVE POLYMER COMPOSITION AND PROCESS OF PREPARING IS DESCRIBED IN WHICH THE COMPOSITION IS COMPRISED OF A SUBSTANTIALLY NON-PHOTOCONDUCTIVE POLYMER HAVING DISPERSED THEREIN PHOTOCONDUCTIVE PRECIPITATED CADMIUM SULFIDE PARTICLES NO GREATER THAN ABOUT 1/2 MICRON, THE SURFACES OF SAID CADMIUM SULFIDE PARTICLES BEING PRE-COATED WITH A DISPERSING AGENT.

Description

United States Patent 3,592,643 PHOTOCONDUCTIVE CADMIUM SULFIDE COM- POSITION AND PROCESS OF PREPARING John J. Bartfai, Schenectady, N.Y., assiguor to General Electric Company No Drawing. Filed Aug. 15, 1968, Ser. No. 752,751 Int. 61. Gtl3g 5/00 U.S. C]. 96-15 12 Claims ABSTRACT OF THE DISCLOSURE This invention is concerned with an improved photoconductive composition of matter process of preparing the composition and articles prepared therefrom useful in the recording, storing and reproducing of photographic images, technical data, and the like.

Briefly stated, the instant photoconductive composition is comprised of a substantially non-photoconductive polymer having dispersed therein photoconductive cadmium slfide particles no greater than about /2 micron, the surface of said cadmium sulfide particles being pro-coated with a dispersing agent. The cadmium sulfide particles used are precipitated in solution by the reaction of a water-soluble cadmium salt, preferably a cadmium halide with a water-soluble sulfide, preferably a monovalent metal halide or sulfide.

The invention also embraces the use of such composition in making composite articles, for instance, tapes, sheets, slides, disks, etc., suitable for recording, storing and reproducing photographic images and technical data, employing the above composition of matter as the photoconductive layer in which such images and data are recorded, stored and reproduced.

The general principles of recording and reproducing photographic images, technical data, etc. are well known in the prior art. One method employs a recording medium comprised of a photoconductive polymer film layer supported by a heat-resistant base, which if not conductive, may have interposed therebetween a layer of conductive material. The photoconductive polymer film layer may be formed from a heat-deformable composition comprised of a substantially non-photoconductive theromplastic polymer having dispersed therein an insoluble photoconductive material in particle form. When an electrostatic charge is imparted to the photoconductive thermoplastic layer by means of a high voltage corona discharge, and it is then exposed to a light image having light and dark portions to be produced, those portions of the charged film exposed to light become photoconductive and substantially lose their charge. Those portions of the film not exposed to light retain an electrostatic charge in areas corresponding to the dark portions of the image. This latent electrostatic image on the film can be developed by a number of methods and, correspondingly, can be retrieved by a number of techniques. By one method charged, dry, fusible pigment particles are deposited on the exposed film surface and are attracted to the electrostatic charge retained by the unilluminated portions of the exposed film to make the image visible. The image may be fixed by heating the particles to fuse them onto the film surface.

By another method, the image is developed by heating 3,592,643 Patented July 13, 1971 the exposed thermoplastic photoconductive layer, particularly the surface thereof with, for instance, direct application of heat or by heat generated by radio frequency energy acting on the conducting layer, whereby the heat causes only the top thermoplastic charged layer to fuse or melt, and become liquid. When this happens, the charges are attracted to the conducting layer, positioned under the thermoplastic layer, thus deforming the surface of the thermoplastic upper layer into various depressions, hills, ridges, etc. Thereafter, the heated surface is cooled or allowed to cool immediately to set or solidify these hills, ridges, and other deformations in the thermoplastic layer. The recording medium thus treated can now be read or projected visually by passing a beam of light through it in cooperation with a special optical system for conversion into an image or can be optically converted into the desired information or data in the form of electrical signals. The image can be viewed directly, projected on a screen, transmitted electronically for viewing on a television screen elsewhere, or can be simply stored on film. Such a method is more fully described and claimed in Gaynor US. Pat. No. 3,291,601, dated Dec. 13, 1961 and assigned to the assignee of this invention.

There are certain requirements in the utilization of insoluble photoconductors in polymer films used in recording media. Particle size should be of the order of the wavelength of light or smaller to keep light scattering at a minimum for low noise and to insure adequate resolving power. Substantially uniform dispersion of the particles in the non-photoconductive polymer matrix is necessary to preclude agglomeration, the eflect of which would be the same as that of larger particles. In addition, the suspension of the photoconductor in the polymer solution should be stable for a period of time sufficient to permit the casting of films.

Cadmium sulfide is a well-known stable photoconductor which is sensitive to light and to wavelengths extending into the infrared region of the spectrum. However, presently available photoconductive cadmium sulfide powders have an average particle size of 2 microns. Polymer films deposited from non-photoconductive polymers which contain cadmium sulfide powders of this size, although photosensitive, have extremely low resolution. Attempts to improve the resolution by reducing particle size by mechanical disintegration result in almost complete loss of photoconductivity. In addition, for most applications the photoconductivity of the cadmium sulfide powder must be enhanced by doping it with a compound such as copper chloride which requires one or more firing operations. Such an operation causes the particles to agglomerate and further lower resolving power.

The present invention provides a process for producing photoconductive polymer compositions which use finely divided photoconductive cadmium sulfide that is produced without a firing operation and without mechanical disintegration. In addition, substantially uniform dispersions of this finely divided cadmium sulfide in polymer solutions are produced. The cadmium sulfide of the present invention is formed by reacting a water-soluble cadmium salt with a water-soluble sulfide salt. Specifically an aqueous solution of a cadmium salt, preferably a cadmium halide, is admixed with an aqueous solution of a sulfide salt, preferably a monovalent metal sulfide or ammonium sulfide, to precipitate the cadmium sulfide. The recovered wet precipitate is then coated with a dispersing agent which enables it to be dipesrsed uniformly in the non-photoconductive polymer solution.

Briefly stated, the process of the preferred embodiment of the present invention produces a photoconductive polymer composition by admixing an aqueous solution of a sulfide, preferably a member selected from the group consisting of a monovalent metal sulfide and ammonium sulfide, with an aqueous solution of a cadmium salt, preferably a cadmium halide, to precipitate cadmium sulfide, recovering said wet cadmium sulfide precipitate and admixing it with a solution of a dispersing agent to coat said precipitate with said agent, recovering the coated cadmium sulfide particles, dispersing said coated particles in a solution of a substantially non-photoconductive polymer to form a substantially uniform dispersed medium, and depositing a film from said dispersed medium.

In carrying out the instant process, an aqueous solution of a cadmium salt, preferably a cadmium halide, is admixed with an aqueous solution of a sulfide salt, preferably a monovalent metal sulfide or ammonium sulfide, to precipitate cadmium sulfide. Preferably, at least substantially equimolar proportions of the reactants are used. Representative of the monovalent metal sulfides useful in the present invention are sodium sulfide, and potassium sulfide. Of the cadmium halides, i.e., cadmium chloride, cadmium iodide, cadmium bromide and cadmium fiuoride, cadmium chloride is preferred because of the ease with which the chloride salts are Washed from the cadmium sulfide precipitate. The concentration of the aqueous solutions of the reactants are not critical but with increasing concentration finer particles are precipitated.

An added advantage of the present invention is that the cadmium sulfide is easily photosensitized by the addition of a dopant such as a copper salt, as for example cupric chloride, to the cadmium salt solution to yield copper doped cadmium sulfide by co-precipitation. Other metallic salts, e.g. silver, can also be used to further sensitize and change the spectral response of the cadmium sulfide.

The reaction to precipitate cadmium sulfide is preferably carried out in a blender, such as a Waring Blendor, to produce maximum agitation during precipitation thus producing extremely fine particles of cadmium sulfide. The precipitate may be recovered in any conventional manner and should be cleaned to remove the soluble salts generated by the reaction. This may be accomplished by Washing it with distilled water and centrifuging. To insure that the precipitate is limited to a particle size of about 0.5 micron or less, the precipitate may be filtered to remove any larger sized particles which may be present.

The wet precipitate is then dispersed in a solution of a dispersing agent. If such solution is an organic solution, the precipitate should be pre-washed with a compatible organic solvent to remove any water present. Besides having good dispersing properties the dispersing agent is preferably a solid at room temperature, transparent to the wavelengths used for photoconductivity and substantially insoluble in the solvent used for forming solutions of the non-photoconductive polymer. Representative of the dispersing agents useful in the present invention are hydrogenated rosin esters especially the glycerol ester of hydrogenated rosin sold under the trademark of Staybelite Ester 10. Other useful dispersing agents include a hard resinous polyol having a molecular Weight of about 1600 and a hydroxyl content of about sold under the trademark of R1100 and water white woodrosin.

The cadmium sulfide particles coated with a dispersing agent are then recovered and are dispersed in a solution of the substantially non-photoconductive polymer.

The amount of cadmium sulfide incorporated in the non-photoconductive polymer film may var widely depending on its particle size, its photosensitivity and the particular requirements of the recording technique used. For most applications the cadmium sulfide particles may be present in a dispersed condition in the non-photoconductive polymer in an amount ranging from about /2 to about by weight of the polymer. Specifically, a polymer film having the cadmium sulfide dispersed therein an amount in excess of 5.5% by weight of the polymer is relatively opaque and would require a reflective type of optical readout system.

The specific non-photoconductive polymer used depends on the particular requirements of the information recording technique employed. For satisfactory results, the nonphotoconductive polymer should be a good insulator and have a high electrical resistivity. It should be stable at the elevated temperatures to which it may be exposed during the recording process. In addition, it should be a solid at room temperatures, free from cold flow, and preferably water white (i.e. water clear).

Representative of the non-photoconductive polymers useful in the present invention is polystyrene. The term polystyrene as used herein includes unsubstituted and substituted polystyrenes, that is, polystyrenes that are prepared from styrene monomers substituted, for instance, in the ortho, para, meta, symmetrical, and asymmetrical positions by any substituent which does not significantly affect its non-photoconductivity. Substantially, non-photoconductive copolymers of a styrene are also useful in the present invention, especially where the styrene residue preferably but not essentially predominates, for instance, copolymers of butadiene-1,3-and styrene; ethyl acrylate and styrene, etc., where the monomers other than styrene comprise from about 0.1 to 25 percent, by Weight, of the total weight of the latter and the styrene. Other useful non-photoconductive polymers are polyvinyl acetate resins and silicone resins.

Where the recording medium requires that its photoconductive polymer layer be heat-deformable, the medium is usually a structure composed of the photoconductive film supported on a base which, if not conductive, has interposed therebetween a conducting layer. For such type of medium, the thermoplastic polymer should have certain specific characteristics. In addition to being a good insulator and having high electrical resistivity; it is usually desirable that its specific resistivity be greater than about 10 ohm-centimeters when in the liquid state at the time it is heated to effect formation of any depressions or ridges or other deformations in the surface thereon. The thermoplastic polymer must also be stable at elevated temperatures at which it will be deformed by the heat necessary to develop the charge on the thermoplastic layer. Of major importance, the thermoplastic polymer of the recording medium must be capable of having a fairly sharp melting point in order that the developing of the proper image on the thermoplastic layer proceed with a minimum of control difiiculties. For a broad spectrum of use, the photoconductive thermoplastic layer of the medium should be solid at temperatures of at least 65 C., but should be capable of being converted to the liquid or fused state at temperatures of at least about C. depending on the support backing layer. If the supporting base layer is resistant to temperatures well above 200 C. or higher, it is then possible to use thermoplastic compositions having softening or liquid points well above the minimum 85 C. recited above. Since the recording medium may have backings which do not have the thermal resistance that some of the inflexible backings may have, it is essential that the liquid temperature of the thermoplastic layer be within a range of temperature somewhat lower than the temperature at which the backing itself may :be fused or lose its backing strength. If the backing is, for instance, an optically clear glass or some optically clear heat-resistant polymer such as an aromatic polyester or aromatic polyamide having a fusion temperature above C. or higher (as, for instance, those described in Journal of Polymer Science, volume 40, pages 289418, November 1959), it is apparent that higher melting thermoplastic compositions can be employed within the scope of the invention.

Since the recording medium may comprise at least two layers and more often three layers, it is also essential that the thermoplastic layer have good adhesion to the backing material, whether it is the supporting backing or the conducting layer. Since one embodiment of the recording medium comprises a three-layer structure composed of the backing material, the upper surface photoconductive thermoplastic layer and an intermediate conducting layer (for instance, a thin film of metal, or of a metal oxide, or metal salt), it is additionally important that the thermoplastic layer have good adhesion to the conducting layer or the conducting surface.

As a further requirement, it is important in those cases where the recording medium will be in the form of a tape and thus will be rolled upon itself and stored, that the thermoplastic layer be substantially free of cold flow that will cause any change in the configuration of the recorded information on the thermoplastic surface, such as any depressions or hills or other deformations on the surface of the thermoplastic layer. Since the storage might take place under conditions where the temperature might rise to from 40 C. to 50 (3., this cold flow must be non-existent or very low even at these temperatures. Any significant cold flow will magnify the storage problems to a point where the tape may not be capable of storage in reel condition as is the usual movie film. As a still further requirement, it is essential that the thermoplastic layer have good resistance towards oxygen attack so that it maintains high electrical resistivity during processing of the tape. Again, in those cases where a tape is involved, and because the tape may be rolled up on itself, it is also essential that the thermoplastic layer should be non-tacky and should not stick to any other surface with which it might come in contact in the rolled-up state.

The backing material for the recording medium may be either a flexible composition or may be a rigid inflexible material. Examples of rigid materials which can be employed (keeping in mind that optical clarity, heat resistance, and radiation resistance are usually the required properties) are, for instance, glass (in the form of plates, slides, disks, etc.): unsaturated polyester resins (formed from the reaction of a polyhydric alcohol, such as ethylene glycol, and an alpha-unsaturated alphabeta-dicarboxylic acid or anhydride, for instance, maleic acid, maleic anhydride, etc.). One can also employ metals such as aluminum, nickel, chromium, etc., where the metal serves both as a conducting layer and as a reflective surface which can be read-out optically by reflection.

Examples of flexible materials which can advantageously be employed as the backing material are, for instance, polyethylene terephthalate, such polyethylene terephthalate being sold under the name of Cronar.

In many instances, there is interposed between the thermoplastic layer and the backing, a conducting layer which can be used as a means for heating the thermo plastic layer. Among such conducting layers (which should be thin enough to be optically clear if interposed between the base and the thermoplastic layer) may be mentioned the various metals, for instance, iron, chromium, tin, nickel, etc., metallic oxides, such as stannic oxide, cuprous oxide, etc., salts, for instance, cuprous iodide etc. In using the conducting layer, it is generally preferable that the layer of metal or metal compound applied to the base layer be no thicker than is required to obtain a transparent film thereon. For this reason, it has been found that the metal film is advantageously of the order of about to 100 angstroms (A) or 0.001 to 0.01 micron thick, and that it should have a resistivity of between 1,000 and 10,000 ohms per square centimeter for optimum ratio frequency heating if that is the method used for developing the deformable pattern.

The thickness of the photoconductive polymer layer can vary widely but advantageously is approximately 4 to microns thick. The base layer thickness can also vary widely as long as it has the proper electrical and radiation resistance flexibility, strength, heat resistance, etc.; this base layer can be generally from about 50 to 400 microns or more in thickness.

The conducting layer is advantageously applied to the backing by the well-known method of volatilizing the metal or metal compound in a vacuum at elevated temperatures and passing the backing in proximity to the vapors of the metal or metal compounds so as to deposit an even, thin, adherent film of the metal or metal compound on the backing and preferably while the entire assembly is still under vacuum.

The particular solvents employed to deposit the photoconductive polymer film of the present invention may be varied widely and will depend on the type of polymers and resinous compositions employed in the mixture of ingredients. Included among such solvents are aromatic hydrocarbon solvents, e.g., toluene, xylene, benzene, etc. Solids weight concentrations of from 10 to 30 percent of the composition in the solvent are advantageously used.

Because the thermoplastic layer is capable of being heated to the liquid state (at which time it develops the surface deformations by action of the induced electric field on a charged portion of the liquid and the pattern of ripples thus produced frozen into a permanent record by promptly cooling the liquid thermoplastic layer to the solid state), it is possible to employ such recording material many times over by merely subjecting the surface layer to the action of heat at a temperature high enough to cause fusion of the upper layer to a smooth surface, thus erasing the information stored in the aforesaid thermoplastic layer. All parts and percentages used herein are by weight unless otherwise noted.

EXAMPLE 1 The preparation of photoconductive cadmium sulfide and subsequent coating of the particles was carried as follows. Into a Waring Blendor containing 200 ml. of H 0, 22.8 g. CdCl and 0.33 g. of CuCl were added. With agitation, there was added all of a solution comprised of 35.2 g. of a 24% aqueous solution of (NH S and 150 ml. of H 0. Agitation was continued for 5 min. The CdS slurry was removed from the blender and placed in a large centrifuge operated at 2400 r.p.m. The liquid was decanted off and the CdS was redispersed in the blender in 500 ml. H O. The water was changed by centrifugation and decantation for at least 10 times until the pH was neutral and a test for chloride ion showed only a trace. The wet CdS was redispersed in 500 ml. of acetone using the microblender, and then centrifuged. This was repeated at least once. Acetone washing was followed by three washings with 500 ml. of methyl ethyl ketone in the microblender. The methyl ethyl ketone slurry was passed successively through fine filters (Gelman Velspar 5.0a and 0.9,41.) to remove any large particles. A cake of precipitate formed in the filter allowing generally only extremely small particles to pass, most of which were determined under a microscope to be less than 0.2 micron. Following filtration, an aliquot of slurry was removed and evaporated to dryness to determine the weight of CdS in the methyl ethyl ketone slurry. For each gram of CdS in the slurry, 2 g. of Staybelite Ester 10 were @Hl'. ployed for coating the CdS. The CdS slurry and appropriate weight of Staybelite were placed in the microblender and agitated. The contents were transferred to a beaker and with magnetic stirring on a hot plate. After cooling, the residue was crushed to break up adhering ester film and form a coarse powder.

The following formulation and procedure was used to form photoconductive films.

20 g. Polystyrene sold under the trademark Polystyrene PS2 by the Dow Chemical Co. and identified as hav ing a 20,000 average molecular weight 12 g. Polyalphamethylstyrene sold under the trademark 276-V9 by the Dow Chemical Co. (used as a plasticizer) 40 ,ul. Surfactant (to control evaporation) sold under the trademark DC-ll Silicone Additive by the Dow Chemical Co.

0.99 g. of the above prepared photoconductor (composed of /3 CdS doped with 0.0025 mole copper and Staybelite Ester 10) g. 1,1,2-trichloroethane.

The mixture of trichloroethane solvent, polystyrene and polyalphamethylstyrene plasticizer was agitated for min. in a speed controlled microblender operated at 30 v. Then, 0.99 g. of the coated CdS particles were added and agitated for 30 min. at 80 v. The contents were removed from the blender and DC11 silicone was added. After shaking for 15 min. to disperse, the dispersion was filtered through a 5.0 filter (Gelman Velspar) to remove any foreign material.

Films were cast using a circular doctor knife with a 0.004 in. gap on 2 x 2 in. glass plate coated with a conductive tin oxide layer. The cast films were air-dried for 5 min., placed in an 80 C. oven for 15 min., and finally dried at 145 C, for 15 min. During the drynig cycles, the films were covered with a 6-inch glass cylinder with a 3-inch opening at the top. The height of the cylinder was 2 inches and the opening at the top was covered with a 500-mesh screen. The screen allowed the solvent to escape but prevented dust from falling on the films. The dried films were removed from the oven and allowed to cool under the dust cover. Electrodes were then wiped clean and film thickness measured. Film thickness was approximately 25 microns and each cast film contained 1% of the photoconductor. These films were also examined under a microscope which showed that most of the dispersed particles were smaller than 0.2 micron and only a relatively few particles were larger than 0.9 micron indicating some incomplete dispersion.

The resolving power of the. resulting recording medium, i.e. the above deposited photoconductive film on the glass base having interposed therebetween a layer of tin oxide was determined using a resolution target which was a negative containing line patterns at spatial frequencies of 12.5, 25, 50 and 100 line pairs/mm. To record the lines, the film was first charged negatively by corona discharge of 30 v. per micron of film thickness. The target was then placed in contact with the charged film surface and illuminated with a Honeywell Model 65C flashlamp at a distance of about 3 feet. Exposure time was 4 see. in conjunction with neutral density filters of 5% transmission. The film was then recharged positively by corona discharge of 20 volts per micron of film thickness. Heat was applied by passing 850 watts through the tin oxide coating for 0.2 second to develop the image pattern. The developed pattern, i.e. fined formations formed only in the film surface which had been exposed to light, was determined under a microscope to have good correspondence with the spatial frequencies of the negative at 12.5, 25 and 50 line pairs/mm, The film failed to resolve the 100 line pairs/mm. pattern, undoubtedly due to light scattering from relatively large particles or aggregates of photoconductor.

Photoconductive film was also formed with a commercially available copper doped cadmium sulfide (containing about the same amount of copper) having a particle size of about 2 microns, the smallest particle size exhibiting photoconductive properties which is commercially available. The films were prepared as previously set forth except that they were not coated with a dispersing agent.

The revolving power of these films could not be determined in the same manner as above since it was found to be below the lowest spatial frequency of the target, i.e. below 12.5 line pairs/mm. By another test, i.e. using a 100 and a 200 mesh per linear inch screen as a target, the resolving power was found to be 8 line pairs/mm.

EXAMPLE 2 In this example the cadmium sulfide precipitate was prepared in substantially the same manner as disclosed in Example 1 except that 24 grams of sodium sulfide were used instead of ammonium.

The formulation and procedure used to form the photoconductive films was the same as that disclosed in Example 1. The resulting films had a resolving power as good 8 as the films prepared with the cadmium sulfide precipitate in Example 1.

EXAMPLE 3 The formulation and procedure used in this example to prepare the photoconductive film was the same as that set forth in Example 2 utilizing the precipitated cadmium sulfide except that in this instance the cadmium sulfide was not sensitized with a doping agent and 5.5% of the coated cadmium sulfide was used.

For comparison purposes, films were prepared in the same manner with commercially available copper doped cadmium sulfide which had been ball milled from a particle size of 2 microns to a particle size of 0.5 micron.

The photo response of both films was determined by a photo decay method. Each film was corona charged to 36 V./;/. in the dark and then exposed in a monochromator with an incident light intensity of 12 microwatts/cm. at 450 millimicrons. The light was projected on the charged films through a semi-transparent probe which measured the surface potential. After a 5 second exposure, the films containing the precipitated cadmium sulfide lost volts whereas the film containing the mechanically reduced cadmium sulfide did not display any measurable decay indicating the substantial absence of photoconductivity.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A process for producing a photoconductive cadmium sulfide polymer composition consisting essentially of precipitating cadmium sulfide from solution by admixing an aqueous solution of a member selected frrom the group consisting of a monovalent metal sulfide and ammonium sulfide with an aqueous solution of a cadmium halide, recovering the wet precipitated cadmium sulfide comprised substantially of particles no greater than about Me microns, coating the wet precipitated cadmium sulfide with a polymeric dispersing agent, and dispersing said coated cadmium sulfide particles in a solution of a substantially n0n-ph0t0c0nductive polymer to form a substantially uniform dispersed photoconductor medium, said polymeric dispersing agent having the characteristics of being a solid at room temperature, transparent to the wavelengths used for photoconductivity and substantially insoluble in the solvent used for forming said solution of non-photoconductive polymer, and said non-photoconductive polymer being electrically insulating when exposed to radiation to which cadmium sulfide is photo sensitive.

2. A process according to claim 1 wherein a sensitizing agent is coprecipitated with said cadmium sulfide.

3. A process according to claim 1 wherein a film is deposited from said dispersed medium.

4. A process according to claim 1 wherein said substantially non-photoconductive polymer is selected from the group consisting of unsubstituted and substituted polystyrenes, copolymers of styrene wherein the monomers other than styrene comprise from about 0.1 to 25 percent by weight of the copolymer, polyvinyl acetate resins and silicon resins.

5. A process according to claim 1 wherein said dispersing agent is the glycerol ester of hydrogenated rosin.

6. A process according to claim 1 wherein said monovalent metal sulfide is sodium sulfide.

7. A process according to claim 1 wherein said cadmium halide is cadmium chloride.

8. A photoconductive polymer composition consisting essentially of a substantially non-photoconductive polymer having dispersed therein precipitated cadmium sulfide particles no greater than about /2 micron, the surfaces of said carmium sulfide particles being pre-coated with a polymeric dispersing agent, said dispersing agent having the characteristics of being a solid at room tempera ture, and transparent to the wavelengths used for photoconductivity, and said nonphotoconductive polymer being electrically insulating when exposed to radiation to which said cadmium sulfide is photosensitive.

9. A composition according to claim 8 wherein said substantially non-photoconductive polymer is selected from the group consisting of unsubstituted and substituted polystyrene, copolymers of styrene wherein the monomers other than styrene comprise from about 0.1 to 25% by Weight of the copolymer, polyvinyl acetate resins and silicone resins,

10. A composition to claim 8 wherein said dispersing agent is the glycerol ester of hydrogenated rosin.

11. A recording medium comprising a base supporting member, a thermoplastic photoconductive layer comprised of the composition of claim 8 and in intermediate conducting layer selected from the class consisting of metals, metal oxide, and metal salts in contact with both the base member and the thermoplastic photoconductive layer.

12. A composition according to claim 8 wherein said cadmium sulfide is sensitized by a photosensitizing agent co-precipitated therewith.

References Cited UNITED STATES PATENTS 1/1958 Flasch. 4/1968 Corrsin et a1. 96l.5

GEORGE F. LESMES, Primary Examiner M. B. WITTENBERG, Assistant Examiner US. Cl. X.R.