US3804508A

US3804508A - Photoelectrophoretic apparatus for heat fixing an image

Info

Publication number: US3804508A
Application number: US00294423A
Authority: US
Inventors: V Mihajlov; L Carreira
Original assignee: Individual
Current assignee: Individual
Priority date: 1965-05-28
Filing date: 1972-10-02
Publication date: 1974-04-16
Anticipated expiration: 1991-04-16

Abstract

Apparatus for photoelectrophoretic imaging utilizing thermoadhesive layers of image transfer and fixing.

Description

United States Patent [191 Mihajlov et al.

PHOTOELECTROPHORETIC APPARATUS FOR HEAT FIXING AN IMAGE Inventors: Vsevolod S. Mihajlov, 175

Farmbrook; Leonard M. Carreira, 17 Burning Tree Ln., both of Penfield, NY. 14526 Filed: Oct. 2, 1972 Appl. No.: 294,423

Related US. Application Data Division of Ser. No. 12,364, Feb. 18, 1970, Pat. No. 3,705,797, which is a continuation of Ser. No. 808,921, March 20, 1969, abandoned, which is a continuation-in-part of Ser. Nos. 459,860, May 28, 1965, abandoned, and Ser. No. 677,706, Oct. 24, 1967, abandoned, and Ser. No. 677,707, Oct. 24,

I967, abandoned.

US. Cl 355/3, 355/4, 96/1.2,

[ Apr. 16, 1974 [51] lint. Cl. G03g 15/00 [58] Field of Search 355/3 P, 4, 17; 96/1 R, 96/1.3, 1.2

[56] References Cited UNiTED STATES PATENTS 3,474,019 lO/l969 Krieger et a1 355/3 P UX 3,384,565 5/1968 Tulagin et al.... 96/l.3 X

3,275,436 /1966 Mayer 96/1 X 2,955,035 10/1960 Walkup et al. 96/l.4

Primary Examiner-Robert P. Greiner [5 7] ABSTRACT Apparatus for photoelectrophoretic imaging utilizing thermo-adhesive layers of image transfer and fixing.

6 Claims, 3 Drawing Figures PATENTEDAPR 16 1974 SHEU 2 U? 2 PHOTOELECTROPHORETTC APPARATUS FOR HEAT FIXING AN IMAGE CROSS-REFERENCE TO RELATED APPLICATIONS BACKGROUND OF THE INVENTION This invention relates in general to imaging systems and more specifically, to an improved electrophoretic imaging system.

There has been recently develoed an electrophoretic imaging system capable of producing color images which utilizes electrically photosensitive particles. This process is described in detail and claimed in copending applications Ser. Nos. 384,737 now U.S. Pat. No. 3,384,565, issued May 21, 1968; 384,681 now U.S. Pat. No. 3,384,566, issued May 21, 1968 and 384,680 now U.S. Pat. No. 3,383,993, issued May 21, 1968, all filed July 23, 1964. In such an imaging system, variously colored lighteab sorbing particles are suspended in a nonconductive liquid carrier. The suspension is placed between electrodes, subjected to a potential difference and exposed to an image. As these steps are completed selective particle migration takes place in image configuration, providing a visible image at one or both of the electrodes. An essential component of the system is the suspended particles which must be electrically photosensitive and which apparently undergo a net change in charge polarity upon exposure to activating electromagnetic radiation, through interaction with one of the electrodes. In a monochromatic system, particles of a single color may be used, producing a single colored image equivalent to conventional black-and-v white photography. In a polychromatic system, the images are produced in natural color because mixtures of particles of two or more different colors which are each sensitive only to light of a specific wave-length or narrow range of wave-lengths are used. Particles usedin this system must have both intense and pure colors and be highly photosensitive.

After the exposure and particle migration steps are completed, the electrodes are separated and the carrier liquid is allowed to evaporate. This leaves images on one or both of the electrodes made up of selectively deposited particles. The carrier liquid may contain a small proportion of a wax or other binder which would serve to bind the particles together in the images. However, if more than a very small amount of binder material is used, undesirable interference with the imaging process takes place. Thus, the images are at this time in a fragile and easily damaged condition. It has been suggested that a transparent sheet be laminated over the images, or a transparent binder resin be sprayed over the images to form a protective coating. While, when carefully done, these techniques will protect the image, the image is often damaged during the application of the protective material. These protective techniques are not suitable for a mechanized system. Also, when it is desired to transfer the image from the electrode material to a receiving sheet, the dangers of smudging or otherwise damaging an unfixed image is very great. Thus, there is a continuing need for a better system for fixing the particulate image formed on the electrode surface and/or for permitting transfer of said image to a receiving sheet.

SUMMARY OF THE INVENTION It is, therefore, an object of this invention to provide a method of fixing a particulate electrophoretic image which overcomes the above-noted disadvantages.

It is another object of this invention to provide a method for of an electrophoretic image from damage.

It is another object of this invention to provide a method of transferring an electrophoretic image to a receiving sheet.

It is still another object of this invention to provide an electrophoretic imagingmethod capable of producing images which may be handled without damage thereto.

It is still another object of this invention to provide a continuous method of forming electrophoretic images and protecting them against abrasion damage.

The foregoing objects and others are accomplished in accordance with this invention by providing a thermoadhesive layer, which when brought into contact with an electrophoretic particulate image in a softened state, will permit the particles to be embedded in the layer and be permanently held by the layer when it is permitted to reharden. This thermo-adhesive layer may be coated on the electrode upon which an image is to be formed. After the image is formed, the thermoadhesive layer is softened and the image is pressed against the layer, thereby embedding the particles therein. When the thermo-adhesiveis permitted to reharden, the particles are permanently set therein and are thereby protected against damage from contact with other objects. Alternatively, the thermo-adhesive layer may be coated on a receiving sheet. After an electrophoretic image is formed on an electrode, the receiving sheet is heated to soften the thermo-adhesive and brought into contact with the image. The adhesive picks up the particulate image and when rehardened provides a protective image matrix.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages of this improved electrophoretic imaging process and apparatus will become apparent upon consideration of the following detaileddisclosure of the invention; especially when taken in conjunction with the accompanying drawings wherein:

FIG. 11 shows a side view of a simple exemplary system for carrying out the process of this invention wherein a thermo-adhesive image fixing layer is coated on the. imaging electrode;

FIG. 2 shows a simple exemplary receiving sheet capable of accepting transfer of an electrophoretic particulate image from an imaging electrode; and,

FIG. 3 shows a continuous system for forming, transferring and fixing electrophoretic particulate images.

Referring now to FIG. 1, there is seen a transparent electrode generally designated 1 which, in this exemplary instance, is made up of a layer of optically transparent glass 2 overcoated with a thin optically transparent layer 3 of tin oxide, commerically available under 3 the name NESA glass. This electrode will hereafter be referred to as the injecting electrode. On the surface of injecting electrode 1 is a layer of a thermo-adhesive material 13, coated on, for example, a conductive cellophane film; layer 13 is capable of becoming tacky when heated by infrared lamp schematically shown at 14.

Over the solidified thermo-adhesive layer 13 is coated a thin layer 4 of finely divided photosensitive particles dispersed in an insulating liquid carrier. The term photosensitive, for the purposes of this application, refers to the properties of a particle which, once attracted to the injecting electrode, will migrate away from it under the influence of an applied electric field when it is exposed to activating electromagnetic radiation. For a detailed theoretical explanation of the apparent mechanism of operation of the imaging process, see the above-mentioned copending application. Ser. Nos. 384,737 now U.S. Pat. No. 3,384,565; 384,681 now U.S. Pat. No. 3,384,566; and 384,680 now U.S. Pat. No. 3,383,993, the disclosures of which are incorporated herein by reference. Adjacent to the liquid suspension 4 is a second electrode 5, hereinafter called the blocking electrode, which is connected to one side of the potential source 6 through a switch 7. The opposite side of potential source 6 is connected to the injecting electrode 1 so that when switch 7 is closed, an electric field is applied across the liquid suspension 4 and thermo-adhesive layer 13 between

electrodes

1 and 5. An image projector made up of a light source 8, a transparency 9, and a lense 10 is provided to expose the dispersion 4 to a light image of the original transparency 9 to be reproduced. Electrode is made in the form of a roller having a conductive central core 11 connected to the potential source 6. The core is covered with a layer of a blocking electrode material 12, which may be, for example, Baryta paper. The pigment suspension is exposed to'the image to be reproduced while a potential is applied across the blocking and injecting electrodes by closing switch 7. Roller 5 is caused to roll across the top surface of injecting electrode 1 with switch 7 closed during the period of image exposure. This light exposure causes exposed pigment particles originally attracted to electrode 1 to migrate through the liquid and adhere to the surface of the blocking electrode, leaving behind a pigment image on the surface of the thermo-adhesive layer 13 which is a duplicate of the original transparency 9. Particles adhering to the surface of a blocking electrode 5 may be cleaned therefrom and the exposure steps repeated, if desired. The additional steps of exposing and cleaning the blocking electrode have been found to increase color purity and color balance. After exposure, the relatively volatile carrier liquid evaporates off, leaving behind the pigment image. At this time, the pigment image is very susceptible to damage by contact with any object since it consists of, in effect, loosely held particles. The image maybe fixed or set by heating the thermo-adhesive layer vl3, as by infrared lamp 14. Heating the thermo -adhesive layer causes it to become tacky as further explained below and causes the image particles to be embedded therein. The thermo-adhesive layer is allowed to harden and may then be stripped from the injecting electrode and handled without fear of smudging or other damage from surface contact.

In an alternative process, the pigment suspension 4 may be coated directly on the NESA glass surface 13,

omitting the intermediate thermo-adhesive layer 13. After the imaging steps are carried out as discussed above, a particulate image remains on the NESA glass surface. The particulate image may then be transferred to a receiving sheet such as is shown in FIG. 2. This receiving sheet comprises a base layer 15 which may be paper, cellophane, or other suitable materials. On this base layer is coated a layer of a thermo-adhesive material 16, the same as layer 13 discussed above. This layer may be made tacky by heating by contact, or by infra red radiation, and be brought into contact with the particulate image on the electrode 1. Or, the solidifed thermo-adhesive layer may be brought into contact with the particulate image and then heated. In either case, the particles become embedded in the thermo-adhesive layer and are fixed therein when the layer is allowed to reharden. The receiving sheet is then removed from the injecting electrode and may be handled in any conventional manner.

FIG. 3 shows an exemplary system for continuously forming a photoelectrophoretic image, transferring the image to a receiving sheet and fixing the image thereon. In the embodiment'of this figure, the transparent electrode 1 and the blocking electrodes are the same as in the above-discussed embodiment of FIG. 1. Here, however, a tractor 18 is coupled to the blocking electrode 5 to automatically transfer and fix the positive image formed on the NESA glass surface 3. The tractor 18 comprises a frame 19.which supports the blocking electrode 5 and image transfer means for movement across the imaging surface. The transfer means consists of a continuous web 20 of transfer material, e.g., paper, on which is coated a thermo-adhesive layer. The web is mounted on supply roller 21 and is adapted to pass in contact with heating guide roller 22 and cooling guide roller 23 on its way to take-up roller 24. Heating guide roller 22 is provided with internal heating means capable of heating the thermo-adhesive layer above its softening temperature as it passes said roller. This heating means, schematically shown at 25 may comprise any conventional means, e.g., a pipe admitting steam to the interior of the roller, electrical resistance heating means connected to a power supply or Peltier junction heating means connected to a power supply. Cooling guide roller 23 is supplied with coolant sufficient to cool the thermo-adhesive below its softening point as it passes said roller. The cooling means, schematically shown at 26, may comprise any conventional means, e.g., cooling water piped to the roller, or Peltier junction means connected to a power supply. In operation, a tri-mix is applied to the transparent electrode 1 as at 4. The tri-mix is exposed to an image and the tractor and blocking electrode are moved from left to right across the imaging surface. As the blocking electrode passes the imaging surface, unwanted particles migrate to the blocking electrode surface leaving a positive particulate image on the NESA glass surface 3. As the tractor reaches the NESA surface, web 20 contacts the NESA surface 3 without relative movement with respect thereto. The thermo-adhesive surface on web 20 is softened by heat applied at roller 22. The particulate image becomes embedded in the softened surface of the thermo-adhesive layer. Then as the thermoadhesive passes cooling roller 23, it is rehardened and wound up on take-up roll 24. When the tractor and blocking electrode reac the end of their travel, brush 27 cleans unwanted pigment from the surface of blocking electrode 5. The tractor is then raised slightly and returned to the starting position without again contacting the transparent electrode surface. The dashed line 28 schematically indicates the path taken by axle 20 of the roller electrode during the imaging and return movements. As can be seen, the device is capable of continuously forming, transferring, fixing and storing photoelectrophoretic images.

The thermo-adhesive layers 13 and 116 as described above may comprise any suitable materials. The only requirements are that they be solid at room temperatures and be capable of softening and becoming tacky at reasonably elevated temperatures. When the layer is to be coated directly on the injecting electrode surface, there is the further requirement that they have proper conductive characteristics. Where the thermo-adhesive layer is coated on the blocking electrode surface the layer should have a resistivity between and 10 ohm centimeters. The thermo-adhesive layer preferably comprises a binder material and a thermo-solvent for the binder. The thermo-solvent comprises a material that is solid at room temperature and melts slightly above room temperature, thereby causing the binder-solvent layer to be tacky and permit particles in contact therewith to be embedded therein. Where the final image is to be viewed by projection, thethermoadhesive layer should be transparent.

Any suitable mixture of binder resin and thermosolvent may be used in the process of this invention. Optimum results have been obtained with mixtures of Vinylite VYNS, a vinyl chloride-vinyl copolymer, available from Union Carbide Corporation, and Santolite Ml-IP, an aryl sulfonamide-formaldehyde copolymer available from Monsanto; polyvinylpyrrolidonevinylacetate copolymer and Santolite MI-IP, and Vinylite VHYHO and Aroclor 4465, a blend of chlorinated biphenyls and chlorinated triphenyls. Best results have been obtained with, and therefore, the preferred formulation is, a mixture of 1 part by weight EXON-470,

a vinylchloride-vinylacetate copolymer available from Firestone, about 1 part by weight Santicizer l-I-l, a sulfonimide resin available from Monsanto, dissolved in about 10 parts acetone. This formulation is preferably coated to a thickness of about 0.5 mil and dried. Any other suitable mixture of binder resinand thermosolvent may be used. Typical binder resin materials include polyethylenes, polystyrenes, copolymers of vinylchloride and vinylacetate, copolymers of vinylpyrrolidone and vinylacetate, polyvinyl methacrylates, polyvinyl propylene, polyvinylchloride, cellulose acetate, chlorinated rubber, and mixtures and copolymers thereof. Typical thermo-solvents having melting points slightly above room temperature include (with melting temperatures in parentheses) triphenyl phosphate (48C.); dicyclohexyl phthalate, (63C.); diphenyl phthalate (69C.); Aroclor 5442 (46-52C.) a chlorinated polyphenyl available from Monsanto; Santicizer 3 (58C.), N-ethyl-p-toluenesulfonamide, available from Monsanto; Santolite MHP (62C.) a sulfonamideformaldehyde resin available from Monsanto; Santicizer l-I-i (82C.) N-cyclohexylep-toluenesulfonamide, available from Monsanto; acenaphthene (94C.) acetanilide (113C); o-acetoacetotoluidide (105C); 0- acetotoluidide 101C); o-chloroaceto-acetanilide 103C.); .2-chloro-4-nitroaniline (106C); pdibromobenzene (87C.); p,p-methylenedianiline (93C.); alpha-naphthol (95C.); beta-naphthol 6 (C.); Z-naphthylamine (ll0C.), m-nitroaniline (112C); 4-nitrobiphenyl (97C.); sorbitol hexoacetate (98C.); 2,4-toluenediamine (97C.) and mixtures thereof.

Any suitable ratio of binder to thermo-solvent may be used. Ratios of from 0.5 to 4 parts binder resin for each part of thermo-solvent may be used. For optimum results, a ratio of binder resin to thermo-solvent of about 1:1 is preferred. This ratio may vary depending upon the particular binder resin and thermo-solvent selected. The thickness of the thermo-adhesive layer should preferably be between 0.1 and 4 mils. The optimum balance between effective transfer and economy of materials has been found to occur with a thermoadhesive thickness of about 0.5 mil.

DESCRIPTION OF PREFERRED EMBODIMENTS The following examples further specifically define the present invention with respect to the use of thermoadhesive layers to fix and/or transfer electrophoretic images. The parts and percentages are by weight unless otherwise indicated. The examples below are intended to illustrate various preferred embodiments of the electrophoretic image fixing and transferring process of this invention.

All of the following Examples l-IV are carried out in an apparatus of the general type illustrated in FIG. 1. In different examples, however, the thermo-adhesive layer may be placed on the NESA glass substrate, or on the blocking electrode surface, or on a separate receiving sheet such as shown in FIG. 2. The NESA glass surface is connected in serieswith a switch, a potential source, and a conductive center of a roller having a coating of Baryta paper on its surface. The roller is approximately 2 inches in diameter and is moved across the plate surface at about 1.45 centimeters per second. The plate employed is roughly 3 inches square and is exposed with a light intensity'of 8,000 foot candles as measured on the uncoated NESA glass surface. Unless otherwise indicated, 7 percent by weight of the indicated pigments in each example are suspended in Sohio Odorless Solvent 3440, a kerosene fraction available from Standard Oil of Ohio, and the magnitude of the applied potential is 2,500 volts. Exposure is made with a 3,200l(. lamp through a multicolor Kodachrome transparency. I

EXAMPLE I Equal parts by weight of Vinylite VYNS, a vinylchloride vinylacetate copolymer available from Union Carbide Corporation and Santolite MI-IP, an aryl sulfonimide formaldehyde copolymer available from Monsanto are mixed in anacetone solvent and coated to a thickness of about 10 microns ona cefiophanefilm 70600, available from I-larmon Colors Company, is dispersed in about 100 parts Sohio Odorless Solvent 3440, and coated onto the thermo-adhesive coated cellophane layer on the NESA glass surface. A potential is imposed across the suspension, and image is formed as discussed above. After the image is formed, residual carrier is allowed to evaporate and the coated cellophane surface is heated until the thermo-adhesive softens with an infrared lamp. A roller having a fluorocarbon coated surface is rolled across the thermo-adhesive layer to press the particulate image into the thermoadhesive surface. The heating is then stopped and the thermo-adhesive surface is allowed to reharden. An excellent image results, with a tough surface resistant to abrasion damage.

EXAMPLE ll Equal portions of a vinyl pyrrolidone-vinylacetate copolymer is mixed with Santolite MHP in an acetone solvent condition. The solution is coated onto a Baryta paper blocking electrode surface to a thickness of about 10 microns and allowed to harden thereon. A trimix is prepared, comprising a cyan pigment, Cyan Blue GTNF, the beta form of copper phthalocyanine, C. I. No. 74160, available from Collway Colors Company; a magenta pigment, Quindo Magenta RV-6803, a quinacridone pigment available from Harmon Color Co.; and a yellow pigment Algol Yellow GC, l,2,5,6-di(C,C'- diphenyl)-thiazole-anthraquinone, C. I. No. 67300, available from General Dyestuffs Co., in about 100 parts Sohio Odorless Solvent 3440. This tri-mix is coated onto the NESA glass substrate and an image is produced as in Example 'I. The blocking electrode is cleaned of residual unwanted pigments, then the surface is heated until it becomes tacky..Then the roller is rolled across the NESA surface picking up and embedding the image particles therefrom. The thermoadhesive is allowed to re-harden. The Baryta paper carrying the pigment containing adhesive layer is removed from the roller electrode and examined. An image of excellent quality with a tough, abrasion resistant surface is seen.

EXAMPLE III Equal proportions of Vinylite VYNS and Aroclor 4465, a blend of chlorinated bi-phenyls and chlorinated moved from the platen and pressed down on the particulate image on the NESA sheet. Upon removal of the sheet and re-hardening of the thermo-adhesive, an excellent image is seen with'a hard abrasion resistant surface.

EXAMPLE IV Equal proportions of EXON 470, and Santicizer l-H are dissolved in acetone and the solution is coated onto a long paper web and allowed to dry. The web is wound 6 on a roller and placed in a device such as shown in FIG.

3. A tri-mix prepared as in Example I is coated ono the NESA glass electrode. An'image is produced as in Example I. Unwanted pigment particles migrate to the blocking electrode and are removed therewith, leaving a particulate image remaining on the NESA surface. As the tractor passes over the NESA plate, the particulate image is pressed into the heated thermo-adhesive surface. As the paper web is wound past the cooled roller, the thermo-adhesive hardens, resulting in a tough image surface which can be wound onto the takeup roller. The image operations may then be repeated without delay.

Although specific compoents and proportions have been stated in the above description of preferred embodiments of the thermo-adhesive layer, other suitable materials, as listed above, may be used with similar results. In addition, other materials may be added to the mixture to synergize, enhance, or otherwise modify its properties. For example, whiteners may be added to the thermo-adhesive to brighten the image, especially where an inexpensive grade of paper is used for transfer. Where the thermo-adhesive layer is used on the injecting electrode, additives may be included to increase the conductivity of the layer as desired.

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

What is claimed is:

1. A photoelectrophoretic imaging apparatus comprising:

a. a smooth optically transparent first electrode adapted to support one side of a layer of imaging suspension;

b. a second electrode adapted to contact the side of said suspension opposite said first electrode;

c. a layer tackifiable by the addition of heat provided on the surface of one of said electrodes;

d. means to apply an electrical potential between said electrodes and means to expose said suspension to an image with electromagnetic radiation through said first electrode whereby an image may be formed on said layer tackifiable by heat; and

e. means to heat said layer to a tacky state whereby said particles are embedded in said layer.

2. The apparatus of claim 1 wherein said tackifiable layer coated on the surface of one of said electrodes having a thickness of from about 0.1 mil to about 4 mils.

3. The apparatus of claim 2 wherein said tackifiable layer is coated on said second electrode.

4. The apparatus of claim 3 wherein said tackifiable layer coated on said second electrode having a resistivity from about 10 to about 10" ohm centimeters.

5. The apparatus of claim 1 wherein said tackifiable layer coated on the surface of one of said electrodes is a solid layer comprising a resinous binder and a thermo-solvent for said resinous binder.

6. The apparatus of claim 2 wherein said tackifiable Patent No. 80 508 Dated April 16, 1974 Inventofl's) Vsevolod S. Mihaj 10v, et a1 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

6 Column 1, line 20, delete "develoed and insert ---developed--.

Column 2, line l4 delete "of" and insert --protecting.

Column 4, line 66, delete "reac" and insert -reach-.

Column 5, lines '41 & 42, delete "sulfonimide" and insert -sulfonaI nide-o Column 6 lines 52 & 53, delete "sulfonimide" and insert --sulfonamid.e--.

Column 6, line 60, delete "BBe" and insert --BBE-. I

Column 8, line 13, delete "compoents". and insert --component-s-.

Signed and "sealed this 22nd day of October 1974.

(SEAL) Attest:

MCCOY M. GIBSON JR. c. MARSHALL DANN Attesting Officer Commissioner of Patents FORM Po-mso (10-69) 3 uscoMM-Dc suave-ps9 U.5 GOVERNMENT PRINTING OFF CE: 1969 0-366-334,

Patent No.

Column 4,

v UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION 3,804, 508 Dated April 16 1974- VseVOl d S. Mihaj lov, et al It is certified that error appears in the aboveidentified patent jan d that said Letters Patent are hereby corrected as shewnbelowz Columnl, line 20, delete develoed' and insert --developed--.

Column 2, linel4 delete f'of" and insert -protecting-.

line 66, delete "re ac" and insert -reach- Column 5, lines 41 & 42 delete "sulfonimid e" and insert -sulfonamide.

Celu n m 6 lines 52 & 53, delete "sulfonimide" and insert -sulfonamide--.

Column 6, line 60, delete "BBe" and insert BBE---.

"Column 8, line l3, delete "compoents"v and insert --component s-.

S ig ned; and sea1ed this 22nd day of October 1974.

Attestv C. MARSHALL DANN Att est m Off icer Commissioner of Patents USCOMM-DC 60376-P69 u.s. GOVERNMENT mnmns orncz 190 o-ass-au.

Claims

2. The apparatus of claim 1 wherein said tackifiable layer coated on the surface of one of said electrodes having a thickness of from about 0.1 mil to about 4 mils.
3. The apparatus of claim 2 wherein said tackifiable layer is coated on said second electrode.
4. The apparatus of claim 3 wherein said tackifiable layer coated on said second electrode having a resistivity from about 108 to about 1015 ohm centimeters.
5. The apparatus of claim 1 wherein said tackifiable layer coated on the surface of one of said electrodes is a solid layer comprising a resinous binder and a thermo-solvent for said resinous binder.
6. The apparatus of claIm 2 wherein said tackifiable layer is coated on said first electrode.