US3169061A

US3169061A - Electrostatic printing

Info

Publication number: US3169061A
Application number: US106897A
Authority: US
Inventors: Hudson Kenneth Clifford
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1961-05-01
Filing date: 1961-05-01
Publication date: 1965-02-09
Anticipated expiration: 1982-02-09

Description

Feb. 9, 1965 K. c. HuDsQN ELEcTRosTATIc PRINTING Filed May 1, 1951 www INVENTR. MVA/57H @f/afan mi;

United States Patent O smal' Deiaware Filed May ll, 1961, Ser. No. 166,897 S Claims. (Ci. 96--1) This invention relates to improved methods of electrostatic recording and more specifically to improved methods of and means for preparing and utilizing surface modulations on a thermoplastic photoconductive layer.

In some methods of reproducing images, such as by Schlieren optics, or by the use of an eidophor image, surface modulations on a transparent slide or lm can be projected onto a viewing screen as a visible image. One method for preparing surface modulated tape is described in Thermoplastic Recording by W. E. Glenn, Journal of Applied Physics, volume 30, Number l2', December 1959. Briefly this method includes writing with an electron beam onto a high-melting base film coated with a transparent conductive coating and overcoated with a thin film of lowmelting insulating thermoplastic. Writing with the electron beam lays down a charge pattern on the thermoplastic in accordance with the information to be stored. The iilm is then heated to the softening point of the thermoplastic. Electrostatic forces between the charges on the film and a ground plane depress the surface where the charges occur. The film is then cooled below the softening point of the thermoplastic to freeze the surface modulations. One ofthe disadvantages of this method lies in the fact that vSchlieren optic projectors are relatively complex mechanisms which must be carefully manufactured to close tolerances. Y

Accordingly, it is a general object of this invention to provide improved methods and apparatus for recording and reproducing visible images.

A still further object is to provide improved methods of image reproduction wherein surface modulations are projected as light images on a viewing screen. Yet another object is to provide improved methods and apparatus for recording and reproducing images' on a photoconductive layer obviating the need for Schlieren optic projection. i

These and other objects and advantages are accomplished in accordance with this invention by producing surface modulations on a layer, and edge lighting the layer. The layer'may comprise a transparent thermoplastic photoconductive-material. It is well known that the resistivity of a photoconductive layer will decrease with increasing temperature. The photoconductive material preferably has a resistivity in darkness at its softening or` melting temperature of at least 109 ohm-centimeters. Inflight it has a resistivity at least two orders of magnitude less than the dark resistivity.

By way of example, an electrostatic charge image is electrophotographically produced on the surface of the layer, for example, by producing an overall electrostatic charge on the surface and then exposing the surface to incident radiation. Because of the difference in 'the resistivity of the layer in darkness and in light the incident radiation substantially reduces or removes completely the charge in irradiated areas thereby forming the electrostatic image. The layer may then be heated to at least its softening point whereupon surface modulations corresponding to the charge image form on the surface.

Once the surface modulations are so formed the layer is allowedto cool thereby freezing themodulations. The surface modulated layer then passes to a projection station where the surface modulation image is converted into a visible light image. This is accomplished by providing means for illuminating, preferably with collimated light,

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an edge of the layer whereupon the surface modulation image appears as a pattern of light and shadow. After projection the surfacer modulation image can be erased by heat whereupon the layer is ready for reuse.

This invention also includes apparatus for producing the aforementioned visible image. ln general, the apparatus includes means for producing a substantially uniform electrostatic charge on the surfacev of the thermoplastic photoconductive layer, means for producing a light image incident upon the surface of the layer, means for heating the layer at least to its softening point, and an optical projection station is provided for converting the surface modulations into a projected light image by illuminating an edge of the photoconductive layer.

Other objects and advantages appear from the following detailed description and the accompanying drawings wherein:

FIG. 1 is a schematic diagram in elevation of apparatus for recording and projecting surface modulation images on a thermoplastic photoconductive layer in accordance with this invention. v

FIG. 2 is an enlarged fragmentary schematic View taken along the line 2 2 of FIG. 1 and illustrating the formation of the surface modulation image on the layer of FIG. 1.

In practicing this invention, a thermoplastic photoconductive insulating layer is employed which preferably has a high degree of light transmissivity and which has a narrow temperature range over which transition occurs from the solid to a softened state and vice versa. By way of illustration such `a layer may be prepared from the Vfollowing materials.

Example I The 7 parts of the leuco base of malachite green are dissolved in the polystyrene solution. The remaining materials are made intoa second solution comprising chlorinated paraffin dissolved in the methyl ethyl ketone. The, two solutions are then mixed together and coated on a suitable substrate such as, for example, metal foil or metallized transparent film. v

' A Vpreferred substrate comprises highmelting point film such as, for example, one sold .under the trademark Mylar or Cronar. A conductive surface can be readily produced on such a film by vacuum deposition of a metal such as', for example, copper or aluminum. A photoconductiver layer can be readily produced on the metallized film by well known methods, such yas roll coating, W coating or dip coating. Once the coating on the film is dried, a highly flexible photoconductive film is provided.l

Since, for the vpurposes of this invention the substrate need not be transparent, metal foil, such as polished aluminum or other opaque material may be employed in lieu of metallized `film.

Heat may be applied to a photoconductive layer `on a metallized film or other substrate t0v accelerate the dry-4 maximum photoconductive response to visible light of about 6360 A. and will have another response peak at tial of about $5,600 volts.

about 4200 A. It will also have a relatively sharp thermoplastic transition point at about 50 centrigade.

Surface modulations can be produced on the layer of Example I by employing electrostatic printing techniques. One Vspecific method includes the following steps:

(1) Electrostatic charging-With the substrate of Example I grounded or on a grounded plate, a substantially uniform electrostatic charge is applied to the layer by passing thereover a corona generating device which includes at least one line wire to which is applied a poten- Some photoconductive layers are more efficiently charged with negative polarity, others with positive polarity. However, with respect to the photoconductive layers specifically described herein either polarity of charge may be employed with about equal efficiency. This step is carried out in darkness or in safe light to which the layer is insensitive.

(2) Exposing-With a photographic transparency resting on the layer, it is exposed for about one second to about 200 foot candles of light from a tungsten lamp to produce an electrostatic image consisting of charged areas on the layer which correspond to the dark areas of the transparency. Exposure time can suitably range from about 0.1 to about 10 seconds depending on light intensity during exposure. Projection exposure techniques can be employed with equal facility.

(3) Developizzg.-A visible surface modulated image is produced on the layer by heating it and its substrate for about 3 to 15 seconds at a temperature of about 140 centigrade which will raise the temperature of the layer to at least 50 centigrade. Time and temperature are not critical in this step so long as temperature and/ or time are not sufficient to discharge the latter before surface modulations are produced. Surface modulations can be pro-j v duced by placing the layer and its substrate on -a hot plate (140 C.) and observing the layer while it is illuminated with low-angle safe-light (yellow). As soon as ripples are seen to form on the surface of the layer it is removed from the hot plate and allowed to cool whereby a surfacemodulation image freezes in the layer. If desired, the layer and substrate may be removed from the hot plate and contacted to a metal plate which will function as a heat sink to accelerate freezing of the surface modulated image. The freezing step will require only about one-half the time required to develop the surface modulation image. Once a surface modulation image is produced as described in this step there is no longer any need to maintain the photoconductive layer in darkness or under safe-light illumination. The foregoing surface modulated layer can be prepared for re-use by simply heating the layer at a temperature above its softening or melting point until the surface modulations disappear.

An alternative method for producing surface modulations includes the charging step and combines the exposing step and developing steps described heretofore'. The exposure light source in this method is one chosen to produce sufficient light of a wavelength to which the photoconductive layer is sensitive while at the same time producing sufficient infra-red to heat the layer to atleast its softening temperature. Some light sources foi-'this purpose may require more time for properly heating the layer than for producing the latent electrostatic image thus resulting in overexposure. In such a case, exposure time can be lengthened by using appropriate filters to cut out a portion of the light to which the layer is sensitive. In this way exposure time and heating time can be appropriately balanced to provide for producing the latent electrostatic image and the surface modulation image in a preferably accomplished by illuminating one edge of the photoconductive layer. High contrast in the pattern of light and shadow can be ensured by collimating the light, for example, by imaging the light onto the edge of the layer through a lens and a narrow slot in a baffle. Light penetrating the layer from one edge which strikes a surface modulation at a sufficient angle will emerge from the layer. Light within the layer which does not strike a surface modulation at a sufficient angle will be reflected back into the layer. In this manner the surface modulation image is converted into a pattern of light and shadow which can be viewed in situ or imaged onto a projection screen.

The methods of this invention may be embodied in an apparatus such as that illustrated in FIG. 1. As shown in the figure an endless belt 11 is carried on four rollers 13, 15, 17 and 19. One or more of these rollers, such as roller 13, may be driven by any suitable means, such as motor 1d, to transport the belt 11 in the direction lof the arrow 21. The endless belt 11 preferably comprises a suitable substrate such as, for example, aluminum foil, copper coated, or aluminized transparent film 23 on which a photoconductive thermoplastic layer 25 such as that described in Example I has been laid down.

A substantially uniform electrostatic charge is produced on the photoconductive layer 25 as it passes under a corona generating source 28. This source 28 may comprise an array of fine parallel wires 27 supported in a shield 29. The wires 27 are connected to a high voltage source 31 and 25000 or more volts are applied to the wires 27 to generate thereby corona for charging the photoconductive layer 25. Charging will be enhanced if, during generation of corona, the film 23 is at ground potential. When a metal film 23 is employed as a substrate, it can be Vcarried over a grounded conductive roller, such as roller 15, during charging. With metallized plastic film as a substrate, a strip along one edge of the film 23 can be bare of the photoconductive layer 25. In this instance a grounded connection can be maintained in conductive contact with the bared strip of the transparent film 23.

The photoconductive layer 25 next passes to an exposure station where it is exposed to a light image from a cathode ray tube 32, for example that of a flying spot Vscanner connected to a suitable receiver 33. In the alternative, the required exposure could be provided for with :an ordinary light image projector. The electrostatic charge in all areas of the photoconductive layer struck K by light is substantially reduced or dissipated. In this way, a latent electrostatic image is formed on the photoconductive layer 25.

In the next step, the photoconductive layer is passed through a heating zone. In a preferred embodiment, this zone includes dielectric heating elements 37 connected to a suitable radio frequency source 39. In the alternative, heating could be accomplished by other means such as a hot filament, radiant heat or hot air. When the photoconductive layer is heated at least up to its softening point, the surface thereof becomes modulated as depicted in much exaggerated form in FIG. 2. In areas where charges remain after exposure to the light image, the charges cause depressions to form in the surface of the layer. Such depressions are illustrated at 41 and 43. FIG. 2 also shows in det-ail how a strip 23 of a metallized surface Yon a film 23 can be left bare so that a ground connection can be contacted thereto.

In FIG. 1, when the endless belt 11 passes out of the heating zone formed bythe dielectric heating elements 37, it quickly becomes cooled to ambient temperature and the surface modulations on the photoconductive layer 25 are frozen in place. Cooling of the belt 11 can be accelerated if the roller 13 is formed of heatconductive material such as,l for example, copper so that it can function as a heat sink for the belt( 11.

The frozen surface modulations on the endless beltk 11 are passed to a viewer or projector 45. As shown more clearly in FIG. 2, this projector will include, for example, a point light source 47, a lens 49 and a slotted baille 51 for collimating the light from the source 47 and imaging it onto one edge ofthe surface-modulated photoconductive layer 25. Light penetrating the layer 25 Willbe reflected by the surface modulations thereon and those light rays which strike a surface modulation at a sufficient angle will emerge from the layer. This condition is represented in FIG. 2 by the arrows 53; Thus, wherever light rays emerge from the layer 25, illuminated areas will be produced thereon While all other areas will remain dark. The pattern of light and shadow so produced can be viewed in situ or, if desired, can be projected onto a viewing screen 57 by means of a projection lens 55. Another alternative comprises substituting a conventional television pick-up tube for the viewing screen 57. Such an alternative will provide means for recording the pattern of light and shadow and for display thereof at one or more remote locations with conventional close-circuit television apparatus.

In FIG. 1, when the surface modulated photoconductive layer 25 has served its purpose in the projector 45, it may again be heated to atemperature above its softening point to erase the surface modulations therefrom. To this end, additional dielectric heating elementsY 37 connected to the source 39 are provided. Complete erasure of the surface modulations from the photoconductive layer Vmay re- Y quire a higher temperature or a longer heating cycle than that produced by heating element 37. y

Once the surface modulations have been erased, the endless belt is ready to be recycled to produce another projected image. v

In operating the device of FIG. l, movement of the endless belt 11 is desirably controlled by `a start-stop cycling means 61. Such control'is desirable in the case Where images projected on to thescreen 57 are viewed for dilerent periods of time than required for exposure of the electrostatic images. In addition, such control can provide for regulation of exposure from cathode ray tube 32, and for regulation of heating with the RF heating elements 37 and 37. The drive motor 14 thus provides frame by frame movement of the endless belt 11. During such movement, the endless belt 11 passes under the charging wires 27 and the photoconductive layer 25 becomes electrostatically charged. Since this layer 25 is a good insulator in darkness, it will retain its charge untilstruck by light from the cathode ray tube 32.

Under control of thel cycling means 61 the endless belt 11 is stopped during exposure to .a light image from the cathode ray-,tube 32. During this exposure, an image being viewed on the screen 57 may be .retained for a time in excess of that required for complete exposure of the frame to an image from a cathode ray tube 32. To prevent over-exposure, the cycling means 61 is coupled to a switch 62 in the control circuit of the cathode ray tube 62. Again, due to the high dark resistivity of the photoconductive layer 25, ythe electrostatic image produced thereon by exposure Will be retained for a considerable length of time. f

The exposed frame of the photoconductive layer next passes between the RF heating elements 37 and comes to rest therebetween. The elements 37 are prefer-ably of a size toheat one complete frame at a time. Excessive heating at this time would dissipate the charge image on the photoconductive layer 25. To prevent excessiveA heating, switchingmeans 63 is provided in the RF heating circuit controlled by the cycling means 61. RF energy may be applied to the heating elements 37 for only a fraction of a second and then cut off whereby the photoconductive layer 25 is quickly ,heated and cooled to produce and freeze, in-

In order to erase the surface modulated image from lthe photoconductive layer 25, the image frame is next cycled Ato pass fromthe Schlieren projector 45 into the space between the RF heating elements 37. At this station, heat will generally be applied to the photoconductive layer 25 for a time in excess of that employed to produce the surface modulated rimage in order that complete erasure thereof may be ensured. Control for thisr purpose is provided by switching means 63' in they-RF heating circuit controlled by the cycling means 61. Uponi erasure of the surface modulated image from the frameof the photoconductive layer 25 by means of the heating elements 37" that frame is` ready for recycling to produce. another image. y

In lieu of the combination of resinous materials, polystyrene and chlorinated paraffin, set forth in Example I, many other resinous materials or combinations thereof maybe employed in the thermoplastic photoconductive layer described herein. Suitable resinous materials include the following:

'(6) Hydrocarbon resins s uch as Piccotex 120, Pennsylvania Industrial Chemical Co., Clairton, Pa.

v (7) Acrylates and acrylic copolymers, such as Acryloid A-, l0l, Rohm and Haas Co., Philadelphia, Pa. j (8) Epoxy'resins, such asEpon 1002, Shell Chemical Co., Houston, Texas. (9) Thermoplastic hydrocarbon terpene resins, Vsuch as Piccolyte S-l35, Pennsylvania Industrial Chemical Co.

Various combinations of resinous materials can be employed to provide enhanced flexibility in the thermoplastic layers. AFor example, mixtures of polyvinyl chloride With `chlorinated parains or hydrocarbon terpene resins will provide highly flexible layers.

To provide a substantially transparent photoconductivek layer a dye-intermediate is selected which is soluble in the selected resin. vThe leuco base of malachte green set forth in Example l is only one of a large class of suitable dye intermediates. f

It has the formula:

` op, i /ona ort,` H \CH3 In general, the suitable dye intermediates have the basic formula:

wherein R1 and R2 are selected from the class consisting of monoalkylamino, di-alkylamino, monoarylamino, and alkylarylamino; X is selected fromthe class consisting of 7 Y A wherein R3 is selected from the class consisting of H, (8) Bis-(4,4etl1ylbenzylaminophenyl) phenyl methane: OH, CH3, OCI-I3, R1 and CII Y N- N wherein R4 and R5 are selected from the class consisting C/ lV of H, H, CH3 and oCH3; and Y is H except when ,H2 H QH# X-l-Y is double bonded oxygen. l

Other suitable dye intermediates which conform to the above basic formula include the following: (2) The leuco base of crystal violet, tris(4,4',4".di

mcthylaminophenyl) methane:

(9) Bis-(4,4'-dmethylaminophenyl) 2,4dihydroxy C H3 CH3 I phenyl methane C H3 C H3 C H3 C H3 H2 H2 H2 H2 C-C C-C f \0 CH3 CH3 -35 \C-C/ III \C-C/ Gn 1li \CH3 (11) Tris-(4,4',4"-phenylaminophenyl) methane:

(4) BiS-(4,4dimethylaminophenyl) 4" hydroxyphenyl methane:

(12) Bis-(4,4ethylphenylmino phenyl) phenyl meth- H ane: v

, C H3 /C H3 Qi-Ge C H3 II C H3 5 5 (6) 4,4bis(dimethylamino `benzophenone (Michlers ketone) Y t CH3 60 Iii O /CHB l C CH3 (7) Bis-(4,4dmethy1arninophenyl) 4" tolyl meth- 65 ane: (13) Bisf(4,4'methy1aminophenyl) 4 tolyl methane:

$H3 I CH3 CH3 CH3 H 111 G-@4 Ca III CH3 75 y H `t9 l@ (14) Bis-(4,4 dimethylaminophenyl) I- 2",4" dirne- (21) Bis-(4,4'methylarninopheny1) 2",4" dimethoxythoxy-phenyl methane: Y phenyl methane: t

(CHg l, OCB-3 00H3 y 00H3 CH3 CH3 H H l /Nn i N\ CH3-N -C- -N-CHa CH3 H CH3 10 y I Bis-(4,4' dimethylaminophenyl) 2,,41 Bis (4,4. methy1aminopheny1) 2"4/1 meth methane: ane:

CH3 f 15 CH3 CH3 /CI-n y H f @E l @l L Cla H CH3 v s Il 3 (16) Bis-(4,4phenylaminophenyl) 4 ethylaninophenyl methane; (23) 4,4b1s(ethyl-benzylarnino) benzophenone:

02H6 v 02H5 o 02H5 l N \N-#l-N/ Y CH2 CIHz methane: 5 v 02H5 o 02H5 CHfNQ-lQ'NfCHa (25) Bis-(4,4'-ethy1-benzy1amin0pheny1 2",4" dihyv H droxyphenyl methane: t (18) Bis-(4,4methylaminopheny1)-4" Vmethoxyphenyl 45 OH methane:

* OCH,

(26) Tris-(4,4,4"-ethylphenylaminophenyl) methane:

(20) Bis-(4,4methylaminophenyl) 2,4 dihydroXyphenyl methane:

f Photoconductive compositions are conveniently pre- ,OE w pared, forV example, by t, dissolving a quantity of the resinous material, in, a suit-a'ble'solvent such as, for eX- l l |V n ample, methyl ethyl ketone, toluene o1' mixtures thereof CHB-N-OvCll-@e-NTCHS and, when the resinous material is completely dissolved,

adding Ito the solution a quantity of theV dye intermediate.

The proportion of dye intermediate to resinous material may vary over a Wide range. The choice of resinous material as -Well as the dye intermediate can change the optimum ratio for a given use. ln many instances, it is desirable that a photoconductive layer or coating be as transparent as possible. For such purposes 0.8 part by Weight or less of dye intermediate for each part by Weight of resinous material can be employed. For some purposes, the color of a photoconductive lm or coating may not be of major concern. For such purposes, up to 1.4 parts by Weight or more of dye intermediate for each part by Weight of resinous material may be ernployed. The solubility of a particular dye intermediate in a particular resin should also be taken into consideration. In some instances, if a solution is prepared containing too much dye intermediate the excess thereof will, upon drying, crystallize out of solution which generally is undesirable.

Further illustrations of compositions which can be used to form transparent photoconductive layers exhibiting thermoplastic properties which are useful in the same manner as described in connection with Example I include the following solutions:

xample Il 2.5 parts by Weight bis-(4-4 dimethyl aminophenyl) phenyl methane and 5.0 parts by weight of a styrene-butadiene copolymer such as, for example, Pliolite S-SB by the Goodyear Tire and Rubber Co., Akron, Ohio,

dissolved in 412 parts by Weight of methyl ethyl ketone.

A layer made from this solution has a softening temperature of about 55 to 57 C.

Example III 2.5 parts by Weight of bis-(4,4-dimethylaminophenyl) phenyl methane and 5.0 parts by Weight of styrene-butadiene copolymer (Pliolite S-SD) dissolved in 42.0 parts by Weight of methyl ethyl ketone.

A layer made from this solution has a softening temperature of about 56 to 58 C.

Example l V 1.5 parts by Weight of bis-(4,4dirnethylarninophenyl)` phenyl methane and 5.0 parts by Weight of styrene-butadiene copolymer (Pliolite S-5 dissolved in 42.0 parts by Weight of methyl ethyl ketone.

A layer made from this solution has a softening point of about 54 to 56 C.

Example V 1.0 part by weight of tris-(4,4',4"-dimethyl-aminophenyl) methane and 5.0 parts by weight of styrene-butadiene copolymer (Pliolite S-5 dissolved in 42.0 parts by Weight of methyl ethyl ketone.

A layer made from this solution has a softening point of about 85 to 87 C.

Example VI 1.0 part by Weight of tris-(4,4,4"-dimethyl-arninophenyl) methane and 12 5.0 'partsby weight of a hydrocarbon resin such as, for example, Piccotex P-l20, Pennsylvania Industrial Chemical Corp., Clairton, Pa.,

and 1.6 parts by Weight of a `polyvinyl chloride copolymer such as, for example, Geon 400X-1l0, B. F. Goodrich Chemical Co., Akron, Ohio,

dissolved in 50.6 parts by weight of methyl ethyl ketone.

A layer made from this solution has a softening point of about 50 to 52 C.

Example VII A layer made from this solution has a softening point of about 50 C.

Example VIII 1.() part by weight of tris-(4,4,4"-dimethyl-arninophenyl) methane and 5.0 parts by weight of a high styrene copolymer (Marbon M-llOO TMV) and 1.6 parts by weight of a polyvinyl chloride copolymer (Geon 400X-) dissolved in 50.0 parts by weight of methyl ethyl ketone.

A layer made from this solution has a softening point of about 48 to 50 C.

Example 1X 1.0 part by Weight of bis-(4,4dimethylaminophenyl) phenyl methane and 5.0 parts by Weight of a hydrocarbon resin (Piccotex P-lOO) and 1.6 parts by Weight of a vinyl chloride copolymer (Vinylite VMCH) dissolved in 50.0 parts by weight of methyl ethyl ketone.

A layer made from this solution has a softening point of about 40 C.

Example X 1.0 part by Weight of tris-(4,4',4"-dimethyl-aminophenyl) methane and 5.0 parts by weight of a polystyrene resin such as, for

example, StyronPSJZ, The Dow Chemical Co., Midland, Michigan,

and 1.6 parts by weight of a vinyl chloride copolymer (Vinylite VMCH) dissolved in 50.0 parts by Weight of methyl ethyl ketone.

I3 A layer made from this solution has a softening point of about 52 C.

Example Xl 1.0 part by weight of tris-(4,4',4dimethyl aminophenyl) methane and 5.0 parts by weight of polystyrene resin (Styron PS-2)k and 1.6 parts by weight of polyvinyl chloride copolymer (Geon LOOX-100) dissolved in 50.0 parts of weight of methyl ethyl ketone.

A layer made from this solution has a softening point of about 47 C.

Example X Il 1.0 part by Weight of tris-(4,4,4dimethyl aminophenyl) methane and 5.()k parts by weight of a hydrocarbon resin (PiccoteX P-izo) and 1.6 parts by weight of a vinyl chloride copolymer (Vinylite VMCH) dissolved in 50.0 parts by weight of methyl ethyl ketone.

A layer made from this solution has a softening point of about 63 to 65 C.

kVarious modifying agents may be added to the foregoing compositions to vary the physical properties or appearance thereof provided they do not interfere with the electrical properties. Flexibility can be enhanced, for example, by including in a composition, such as that of Example I, a small amount of a plasticizer, such as, for example tricresyl phosphate, butyl phthalyl-butyl-glycolate, tris-(2,3-dibromo-propyl) phosphate, or di-(Z- ethylhexyl) phthalate. Such a composition can be coated on a flexible substrate or can be formed into self-supporting flexible tilms. A self-supporting film may be produced by flow-coating a mirror-finish metal plate with the composition -to form a photo-conductive coating on the plate. The coating is then physically stripped from the plate and thus provides a self-supporting photo-conductive film. Additional solvents can also be added, such as, for example, toluene to produce the desired coating thickness of the dry finished thermoplastic photoconductive layer.

When a composition is prepared wherein a dye intermediate is dissolved in a non-halogenated resin, enhanced photo-conductive response can often be obtained or at least ensured by including in the composition at least a trace amount of a compatible non-volatile halogenated compound such as, for example, tris-(2,3-dibromopropyl) phosphate or any compatible chlorinatedV hydrocarbon.

Many of the compositions contemplated herein, when coated on a substrate or formed into a film, may have a tendency to form color which may be undesirable under some circumstances. Color formation in a lm or coating can be substantially retarded by including in the compositions a small amount of stabilizer for the dye intermediate thereof. A specific example of a suitable stabilizer is one have the formula (CsHit) 2-51'1-(S-V-CHa-COOC8H17) 2 (Thermolite 20, Metal and Thermit Corp., Rahway, Nl). Other materials such as pyrocatechol, 2 hydroxy-4- methoxy benzophenone, andr2,2dihydroxy 4 methoxy benzophenone may also be used. Some compositions including such a stabilizer will remain substantially colorless for a considerable time unless subjected to intense 'n ultra-violet radiation.

What is claimed is:

l. A method of image reproduction employing a thermoplastic transparent photoconductive layer having a resistivity in darkness at its softening temperature of at least 109 ohm-centimeters and 'a resistivity when irradiated of at least two orders of magnitude less than said resistivity in darkness; said method comprising the steps of: producing a blanket electrostatic charge on said layer, directing an optical image incident upon said charged layer thereby producing on'said layer a latent electrostatic image, forming on said layer a surface modulation image in response -to deformation ofa surface of said layer by the electrostatic forces`of said electrostatic image illuminating an edge of said layer with light penetrating said edge to produce from said surface modulation image a pattern of light and shadow, removing said surface modulation image from said layer, and then repeating said steps on saidy layer.

2. A method of image reproduction employing a thermoplastic transparent photoconductive layer having a resistivity in darkness at its softening temperature of at least 109 ohm-centimeters and a resistivity when irradiated of at least two orders of magnitude less than said resistivity in darkness; ysaid method comprising the steps of: producing a blanket electrostatic charge on said layer, directing an optical image incident upon said charged layer thereby producing on said layer a latent electrostatic image, heating said layer to at least said softening ternperature to produce thereon surface modulations in conformity with said electrostatic image in response to deformation of a surface of said layer by the electrostatic forces of said electrostatic image, cooling said layer to freeze said surface modulations, illuminating an edge of said layer with light penetrating said edge to produce from said `surface modulations a pattern of light and shadow, heating said layer at least to the softening point thereof to remove said surface modulations, and then repeating said steps on said layer.

3. Image reproduction apparatus employing a transparent thermoplastic photoconductive layer; said apparatus comprising means for applying a substantially uniform electrostatic charge to a surface of said layer, means for exposing said layer to a lightv image to produce thereon a latent electrostatic image, means for heating said layer to at least its softening temperature to produce surface modulations thereon in conformity with said electrostatic image and a source of illuminationv positioned and arranged with respect to the layer so that theV light rays therefrom are transmitted at an edge of said layer with light penetrating said edge to produce thereon a pattern of light and shadow from said surface modulations.

4. The apparatus of claim 3 including additional heating means for erasing said surface modulations from said layer.

5. The apparatus of claim 3 including means for cooling said layer to freeze said surface modulations.

` 6. Image reproduction apparatus employing a substantially transparentk endless layer of thermoplastic photoconductive material; said apparatus comprising, means for transporting said endless layer along a predetermined path, means adjacent said path for applying a substantially uniform electrostatic charge to a surface of said layer, means for exposing said layer to a light image to produce thereon a latent electrostatic image, means for heating said layer to produce surface modulations thereon in conformity with said latent electrostatic image, and a source of illumination positioned and arranged with respect to the layer so that the light rays therefrom are transmitted at an edge of said layer with light penetrating said edge to produce a patternof light and shadow from said surface modulations.

7. The apparatus of claim 6 including additional heating. means for erasing said surface modulations from said` k (References on following page) References Cited in the le of this patent Y UNITED STATES PATENTS Hatherell et al Feb. I20, 1934 Wenschow Mar. 28, 1939 Gaspar Nov. 18, 1941 Beeber et al. Aug. 24, 1948 Fuchs Dec. 30, 1952 Middleton Dec. 22, 1953 Isborn June 19, 1956 Grieg et al Feb. 21, 1956 Miles Dec. 4, 1956 Giaimo June 7, 1960 Norton May 23, 1961 Dreyfoos et al Sept. 18, 1962 Boldebuck Nov. 13, 1962 FOREIGN PATENTS f 592,152 Belgium June 22, 1961) 598,591 Belgium Dec. 28, 1960 OTHER REFERENCES Hutter et al.: -Electrostatic Imaging and Recording, Journal of the S.M.P.T.E., vol. 69, January 1960, pages 32-35.

Electronic Industries, February 1960, pages 7649.

Bovey: Effects of Ionizing Radiation on Natural and Synthetic High Polymers, Interscience (1958), pages 65-69.

Claims

1. A METHOD OF IMAGE REPRODUCTION EMPLOYING A THERMOPLASTIC TRANSPARENT PHOTOCONDUCTIVE LAYER HAVING A RESISTIVITY IN DARKNESS AT ITS SOFTENING TEMPERATURE OF AT LEAST 10**9 OHM-CENTIMETERS AND A RESISTIVITY WHEN IRRADIATED AT LEAT TWO ORDERS OF MAGNITUDE LESS THAN SAID RESISTIVITY IN DARKNESS; SAID METHOD COMPRISING THE STEPS OF: PRODUCING A BLANKET ELECTROSTATIC CHARGE ON SAID LAYER, DIRECTING AN OPTICAL IMAGE INCIDENT UPON SAID CHARGED LAYER THEREBY PRODUCING ON SAID LAYER A LATENT ELECTROSTATIC IMAGE, FORMING ON SAAID LAYER A SURFACE MODULATION IMAGE IN RESPONSE TO DEFORMATION OF A SURFACE OF SAID LAYER BY THE ELECTROSTATIC FORCES OF SAID ELECSTROSTATIC IMAGE ILLUMINATING AN EDGE OF SAID LAYER WITH LIGHT PENETRATING SAID EDGE TO PRODUCE FROM SAID SURFACE MODULATION IMAGE A PATTERN OF LIGHT AND SHADOW, REMOVING SAID SURFACE MODULATION IMAGE FROM SAID LAYER, AND THEN REPEATING SAID STEPS ON SAID LAYER.

3. IMAGE REPREDUCTION APPARATUS EMPLOYING A TRANSPARENT THERMOPLASTIC PHOTOCONDUCTIVE LAYER; SAID APPARATUS COMPRISING MEANS FOR APPLYING A SUBSTANTIALLY UNIFORM ELECTROSTATIC CHARGE TO A SURFACE OF SAID LAYER, MEANS FOR EXPOSING SAID LAYER TO A LIGHT IMAGE TO PRODUCE THEREON A LATENT ELECTROSTATIC IMAGE, MEANS FOR HEATING SAID LAYER TO AT LEAST ITS SOFTENING TEMPERATURE TO PRODUCE SURFACE MODULATION THEREON IN CONFORMITY WITH SAID ELECTROSTATIC IMAGE AND A SOURCE OF ILLUMINATION POSITIONED AN ARRANGED WITH RESPECT TO THE LAYER SO THAT THE LIGHT RAYS THEREFROM ARE TRANSMITTED AT AN EDGE OF SAID LAYER WITH LIGHT PENETRATING SAID EDGE TO PRODUCE THEREON A PATTERN OF LIGHT AND SHADOW FROM SAID SURFACE MODULATIONS.