US3782932A - Electrophoretic imaging process using transparent particles - Google Patents

Abstract

AN IMAGE IS FORMED PHOTOELECTROPHORETICALLY BY EXPOSING AN IMAGING SUSPENSION TO A LIGHT IMAGE IN THE PRESENCE OF AN ELECTRIC FIELD. THE RESULTING IMAGE FORMED IS CONTACTED WITH A SECONDARY SUSPENSION CONTAINING AN ABRASIVE INGREDIENT IN TH PRESENCE OF SAID ELECTRIC FIELD AND EXPOSURE.

Description

Jan. 1, 1974 v. TU LAGIN 3,782,932

ELECTROPHORETIC IMAGING PROCESS USING TRANSPARENT PARTICLES Original Filed 001;. 5, 1968 United States Patent O1 hoe 3,782,932 Patented Jan. 1, 1974 Int. Cl. G03g 13/22 US. Cl. 965-13 4 Claims ABSTRACT OF THE DISCLOSURE An image is formed photoelectrophoretically by exposing an imaging suspension to a light image in the presence of an electric field. The resulting image formed is contacted with a secondary suspension containing an abrasive ingredient in the presence of said electric field and exposure.

CROSS-REFERENCE TO RELATED APPLICATIONS This application is a continuation of application Ser. No. 764,717, filed Oct. 3, 1968, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to an imaging system and, more specifically, to an electrophoretic imaging system.

In photoelectrophoretic imaging colored photosensitive particles are suspended in an insulating carrier liquid. This suspension is then placed between a pair of electrodes, subjected to a potential difference and exposed to a light image. Ordinarily in carrying out the process the imaging suspension is placed on a transparent electrically conductive plate in the form of a thin film and exposure is made through the transparent plate while a second generally cylindrically shaped electrode is rolled across the imaging suspension. The particles are believed to hear an initial charge when suspended in the liquid, which causes them to be attracted to the transparent base electrode and, upon exposure, to change polarities by exchanging charge with the base electrode such that the exposed particles then migrate away from the base electrode to the roller electrode thereby forming images on both of the electrodes by particle subtraction, each image being complementary one to the other. The process may be used to produce both polychromatic and monochromatic images. In the latter instance a single color photoresponsive particle may be used in the suspension or a number of differently colored photoresponsive particles may be used all of which respond to the same Wavelength of light upon exposure. An extensive and detailed description of the photoelectrophoretic imaging technique described above may be found in US. Pat. Nos. 3,384,565, 3,384,566, and 3,383,993.

Although it has generally been found that good quality images can be produced according to the above described procedures, due to the nature of the imaging suspension degrees of difficulty have been encountered in producing high contrast images demonstrating the proper color separation. Various methods have been described whereby the overall color quality of both polychromatic and monochromatic imaging systems are enhanced however none have yet to provide a technique whereby all of the above mentioned difiiculties have been eliminated.

an imaging system which will overcome the above noted disadvantages.

It is a further object of this invention to provide a high quality electrophoretic imaging system.

Yet, still a further object of this invention is to provide a high contrast polychromatic and monochromatic imaging system.

Another object of this invention is to provide a novel process capable of producing high quality color images.

The foregoing objects and others are accomplished in accordance with the present invention generally speaking by providing an imaging suspension of photoelectrophoretic imaging particles in an insulating carrier liquid. The suspension comprising the colored light absorbing particles is placed in an electrode system containing at least one transparent electrode and exposed selectively to actinic radiation while a voltage is applied by way of the electrodes across the imaging suspension. As a result of particle migration within the system an image is formed on the surface of the transparent electrode and unwanted particles migrate and adhere to the surface of the remaining electrode in the system. This electrode supporting the unwanted particles is either removed from the configuration and replaced by still another similar electrode or the surface is cleaned of the unwanted pigmented particles before continuing with the process. The surface of the substituted or cleaned roller, whichever the case may be, is coated with a thin layer of a suspension of substantially colorless or transparent particles in an insulating carrier liquid. The newly coated electrode is then contacted with the imaging suspension and the exposure step repeated, again applying a voltage across the imaging suspension. The latter image enhancement step may be repeated a number of times depending, for example, upon the degree of color purity desired.

It has been determined that by treating the image formed according to the above described process with a secondary dispersion or suspension comprising an insulating solvent having suspended therein a number of substantially colorless or transparent particles that the overall color quality and image contrast of the final image produced may be substantially enhanced. It is hypothesized that as the result of the apparent. scrubbing or abrasive action of the suspended particles there is produced a disturbance within the imaging zone during the re-exposure phase of the process. This abrasive action, it is speculated, contributes to the breaking up of particle aggregates and the apparent layer structure of the suspension in a manner which substantially enhances the color separation and contrast of the final image.

The system of the present invention employs intensely colored pigment particles which serve both as the colorant and as the photosensitive material which apparently undergoes a net change in charge polarity upon exposure to activating radiation by interaction with one of the electrodes. No additional photosensitive elements or materials are necessary thus providing a very simple and inexpensive imaging technique. As a result of the mixture of two or more difi'erently colored pigment particles, each of which is sensitive only to light of a specific wavelength, polychromatic images are produced. It has been found that the particles respond in the regions of the spectrum of their principal light absorption with the cyan, magenta and yellow particles responding to red, green and blue light respectively. Thus the system is most suited to subtractive color synthesis. Although the process of the present invention is especially suited for enhancing the color separation and image contrast of a polychromatic imaging system it is likewise suitable for the enhancement of the images resulting from a monochromatic system. In the latter instance a single color photoresponsive particle may be used in the imaging suspension or a number of differently colored photo-responsive particles may be used all of which respond to the same wavelength of light exposure.

3 DESCRIPTION OF INVENTION The invention is further illustrated in the accompanying drawing in which there is seen a transparent electrode generally designated 1 which, in this instance is made of a layer of optically transparent glass 2 overcoated with a thin layer of tin oxide 3. Tin oxide coated glass of this nature is commercially available under the trade name NESA glass. This electrode shall hereafter be referred to as the injecting electrode. Coated on the surface of the injecting electrode is a thin layer 4 of the imaging suspension of the present invention comprising finely divided photosensitive pigment particles dispersed in an insulating carrier liquid. The term photoresponsive for purposes of this application refers to the properties of a particle which once attracted to the injecting electrode will migrate away from this electrode under the influence of an applied electric field when exposed to activating radiation. A further detailed explanation of the apparent mechanism of this operation is disclosed in US. Pat. Nos. 3,384,565, 3,384,566, and 3,383,993 herein incorporated by reference.

Above the liquid imaging suspension is passed a blocking electrode generally designated 5 which is represented herein as a roller electrode having a conductive central core 11 which is covered by a layer of blocking electrode material 12. The core 11 is connected to a power source 6 which in turn is connected by a switch 7 to the injecting electrode 1. By closing switch 7 an electric field may then be established across the imaging suspension between electrodes 5 and 1. The pigment suspension is exposed by way of the projector mechanism consisting of a light source 8, transparency 9, and a lens system 10. For purposes of this illustration a color transparency is used. Switch 7 is closed and the blocking electrode 5 rolled across the upper surface of the injecting electrode 1. The light exposure causes the exposed particles originally attracted to the injecting electrode to migrate through the liquid carrier and adhere to the surface of the blocking electrode material leaving behind a particulate image on the surface of the injecting electrode which is a duplicate of the transparency 9. Upon completion of the exposure step the pigment particles on the surface of the blocking electrode roller are removed by slowly rotating brush 14 against the surface of the roller. As stated above in place of cleaning the roller electrode a clean electrode may be substituted therefor. In either case the cleaned or new roller is coated with a thin layer of a suspension comprising an insulating carrier liquid having suspended therein the substantially colorless or transparent particles of the present invention. The roller electrode is then passed back across the surface of the injecting electrode while again exposing the imaging suspension in the same manner as discussed above. When the cycle is complete the blocking electrode roller may again be cleaned such as by brush 13, or replaced as the case may be, and the entire cycle repeated. The cleanup brushes are merely exemplary of the various means which might be used to remove the migrated particles from the blocking electrode surface. Any other suitable cleaning means may be used such as wiping, scraping, air blasting or using a spray solvent. The cycle including the image enhancement step may be repeated as many times as desired. When the desired image is obtained it may be fixed such as by placing a lamination over its surface or by virtue of the inclusion in the carrier liquid of a dissolved binder material, such as parafiin wax, which will come out of solution as the carrier liquid evaporates. Since the carrier liquid may somewhat evaporate during the more extended process a spray nozzle 15 may be provided to add additional carrier liquid between cycles to the imaging suspension. The additional carrier liquid will maintain the suspension in optimum conditions for particle migration during the ensuing steps.

Other configurations may be used similar to that disclosed in the illustration and the configuration represented should be understood as merely being illustrative of the present invention. For example, the roller type blocking electrode could use a replaceable web material or continuous belt which could be cleaned as discussed. Thus, a number of ways are available to execute the imaging steps herein described such as cleaning the unwanted particles from the electrode surface, replacing the blocking electrode after each pass or just replacing the blocking electrode material.

When used in the course of the present invention the term injecting electrode should be understood to mean that it is an electrode which is capable of exchanging charge with the photosensitive particles of the imaging suspension when the suspension is exposed to light thus allowing for a net change in charge polarity of the par ticles. By the term blocking electrode is meant one which is capable of storing the electric charge on its surface, thus it may be thought of as retarding the injection of electric charges into the above mentioned photosensitive particles when the particles come into contact with the surface of the electrode. It is preferred that the injecting electrode be composed of an optically transparent material such as glass coated with a conductive material such as tin oxide, copper iodide, gold or the like; however, other suitable materials including many semiconductive materials which are ordinarily not thought of as conductive but which are still capable of accepting injected charge carriers of the proper polarity under the influence of the applied field, may be used within the course of the present invention. The use of more conductive materials however allows for cleaner charge separation and prevents possible charge buildup on the electrode, this characteristic tending to diminish the interior electrode field. The blocking electrode, on the other hand, is selected so as to prevent or greatly retard the charge exchange of the photosensitive pigment particles when the particles reach the surface of this electrode. The blocking electrode base generally will consist of a material which is fairly high in electrical conductivity. Typical materials are conductive rubber, and various metal foils such as steel, aluminum, copper and brass. Preferably the core of the blocking electrode will have a high electrical conductivity in order to establish the required polarity differential. However, if a low conductivity material is used a separate electrical connection may be made to the back of the blocking layer of the electrode. Although a blocking electrode material may not necessarily be used in these systems, the use of such a material is preferred because of the markedly improved results which it is capable of producing. It is preferred that the blocking layer when used be either an insulator or a semiconductor which will prevent the passage of sufficient charge carriers under the influence of the applied field so as to prevent the discharge of those particles bound to its surface thereby preventing particle oscillation within the system. Although the blocking electrode does allow for passage of some charge carriers it would still be considered to come within the class of preferred materials if it does not allow for the passage of sufficient carrier to recharge the majority of pigment particles to the opposite polarity. Exemplary of the preferred blocking layer materials used are baryta paper which consists of paper coated with a suspension of barium sulfate in a gelatin solution, Tedlar, a polyvinyl fluoride, and polyurethane. Other suitable materials having a resistivity of about 10 ohm cm. or greater may be used as the blocking electrode material. Typical materials in this resistivity range include cellulose acetate coated papers, polystyrene, polytetrafluoroethylene and polyethylene terephthalate. The baryta paper, Tedlar and the other materials used as the blocking layer may be wetted on their back surfaces with tap water or coated with an electrically conductive material. The blocking electrode layer, when utilized, may be a separate replaceable layer which is either taped to the blocking electrode core or held by mechanical fasteners or other devices which are capable of simply holding the layer on the electrode. In the alternative, the layer may be an integral part of the electrode itself being either adhesively bonded, laminated, spray coated or otherwise applied to the surface of the electrode.

Any suitable insulating carrier liquid may be used in the course of the present invention either as the carrier for the electrophoretic imaging particles or the substantially transparent particle used in the image enhancement step. Typical insulating carrier liquids include decane, dodecane, and tetradecane, molten paraflin wax, molten beeswax and other molten thermoplastic materials, mineral oil, Sohio Odorless Solvent, a kerosene fraction available from the Standard Oil Company of Ohio and Isopar G, a long chain saturated aliphatic hydrocarbon commercially available from the Humble Oil Company of New Jersey and mixtures thereof.

Any suitable ingredient may be used as the abrasive additive to the cleanup or image enhancement suspension of the present invention with the additive generally being a solid, substantially colorless or transparent organic or inorganic material. Typical particles utilized to provide the effect desired include inorganic materials such as barium sulfate, zinc oxide, silica dioxide, sodium chloride, barium titanate, and titanium dioxide, and organic materials such as pyrene, triphenols, phenanthrene, anthracene, tetracyanopyrene, tetrabromopyrene, tetranitropyrene, polyethylene, polypropylene, polyvinylchloride, polyethylene terephthalate, polyvinylfiuoride, polystyrene, polybutylmethacrylate, polytetrafiuoroethylene, 2,6-bis(2- hydroxy-3-tert-butyl-5-methyl-benzyl)-4 methylphenol, polyurethane, copolymers and mixtures thereof. Although not considered critical the particle size of the added ingredient generally will be at least about 0.10 microns or larger. The concentration of the particles present in the cleaning suspension will vary depending upon the specific components utilized and the particles chosen. Generally, the concentration will range from about 0.5 to about 25 percent by weight of the solvent suspension. Inasmuch as the added particle or ingredient is transparent or substantially colorless if the particles were to become a part of the resulting image, the presence of the particle would not interfere with the color of the image produced.

A wide range of voltages may be applied between the electrodes in the system. For good image resolution, high image density and low background it is preferred that the potential applied be such as to create an electric field having a maximum value of at least about 300 volts across the imaging suspension. The applied potential necessary to obtain this field of strength will, of course, vary depending upon the interelectrode gap and upon the thickness and type of blocking material used on the blocking electrode surface. For the very highest image qualities the maximum field may be as large as 5000 volts. The upper limit of field strength is limited only by the electrical conduction of the suspension and the dielectric properties of the blocking material.

In the polychromatic system, the particles are selected so that those of different colors respond to different wavelengths in the visible spectrum corresponding to their principal absorption and further so that their spectral response curves do not have substantial overlap, thus allowing for color separation and subtractive multicolor image formation. Several different particles are employed namely a cyan colored particle sensitive mainly to red light, a magenta colored particle sensitive primarily to green light and a yellow colored particle primarily sensitive to blue light. While this is the simplest combination, additional particles having different absorption maxima may be added to improve color synthesis. When mixed together in the carrier liquid, these particles produce a substantially black liquid and when one or more of the particles are caused to migrate from the injecting electrode to the blocking electrode they leave behind particles which produce a color equivalent to the color of the impinging light source. Thus, for example, red light exposure causes the cyan colored pigment to migrate'thereby leaving behind the magenta and yellow pigments which combine to produce red in the final image. In the same manner blue and green colored light is reproduced by removal of yellow and magenta pigment respectively and, of course, when white light impinges upon the mix all pigments migrate leaving behind the color of the white or transparent substrate. A dark exposure leaves behind all pigments which combine to produce a black image. It should be recognized that this is an ideal technique of subtractive color imaging in that the particles are not only each c0mposed of but one component but in. addition they perform a dual function in that they act both as the final image colorant and the photosensitive medium of the system. Accordingly, the system presents virtually the ultimate in eliminating the complexity of prior art methods of subtractive the color imaging.

As stated above while the process of the present invention is especially suitable for enhancing the color separation and image contrast of a polychromatic imaging system it is likewise suitable for the enhancement of the images produced from a monochromatic system. In the latter instance a single color photoresponsive particle may be used in the imaging suspension or a number of differently colored photoresponsive particles may be used all of which respond to the same wavelength of light exposure.

It is desirable to use electrophoretic pigment particles which are relatively small in size because smaller particles produce better and more stable pigment dispersions in the liquid carrier and in addition are capable of producing images of higher resolution than is the case with particles of larger sizes. Even where the pigments are not commercially available in small particle sizes the particle size may be reduced by conventional techniques such as ball milling or the like. When the particles are suspended in the liquid carrier they may take on a net electrostatic charge and as a result may be attracted towards one of the electrodes in the system depending on the polarity of the charge with respect to that of the electrodes. It is not necessary that the particles take on only one polarity of charge but instead particles of both polarities may be present. In such a case some of the particles in the suspension will initially move towards the injecting electrode while others will move toward the blocking electrode; however, this particle migration takes place uniformly over the entire area and the effect of imagewise, exposure-induced migration is superimposed upon it. Thus, the apparent bipolarity of the suspension in no way affects the imaging capability of the system except for the fact that it subtracts some of the particles uniformly from the imaging suspension before imagewise modulation of the particles takes place. Thus a portion of the suspended particles are removed from the system as potential image formers. This effect is overcome by merely forming an initial suspension of sufiiciently' high particle concentration so that the system is still capable of producing the desired intense images. It has been found that with some of the suspensions described above either polarity of potential may be applied to the electrodes during imaging.

Typical photosensitive pigment particles which may be used to form the pigment mix of the present invention are disclosed in US. Pat. 3,384,488 the disclosure of which is hereby incorporated by reference. The percentage of pigment in the insulating liquid carrier is generally not considered to be critical; however, for reference purposes it is noted from about 2 to about 10 percent pigment by weight has been found to be highly suited for the above disclosed process.

As stated above, ones the particle image has been formed it may be fixed to the respective electrode such as by spraying a binder onto the surface, by laminating an overlay over the imaged surface, or by including a binder in the liquid suspension. Generally, it will be preferable to transfer the image from the electrode and fix it on a secondary surface so that the electrode may be reused. Such a transfer step may be carried out by adhesive pickoff techniques, such as with adhesive tape or by electrostatic field transfer. In the latter instance a roller electrode is passed over the particle image on the injecting electrode with the polarity of the potential applied being reversed from that originally applied during the imaging phase of the process. This transfer electrode may be covered with a paper sleeve which may be conveniently removed from the electrode core following image transfer.

Although various electrode spacings may be employed, spacings of less than about 1 mil and extending down to where the electrodes are substantially in virtual contact resulting from the electrodes being pressed together are preferred. The latter condition constitutes a particularly preferred form of the invention.

PREFERRED EMBODIMENTS the roller. A Tedlar sleeve is wrapped around the roller electrode to provide the blocking surface. The roller is approximately 2 /2 inches in diameter and it is moved across the plates surface at about 1.4 cm. per second. The NESA plate employed is roughly 3 inches square and is exposed with a light intensity of about 1800 foot candles. Exposure is made with a 3200 K. lamp through a Kodachrome positive transparency which is placed between the white light source and the NESA glass substrate.

Example I An imaging suspension comprising equal amounts of .Watchung Red B, a barium salt of 1-(4'-methyl-5'-chloroazobenzene-2'-sulfonic acid)-2hydroxy-3-naphth0ic acid,

C.I. No. 15,865, monolite Fast Blue G.S., the alpha form of metal-free phthalocyanine, 0.1. No. 74,100, and a yellow pigment Algol Yellow G.C., l,2,5,6-di(C,C'-diphenyl) thiazole anthraquinone, C.I. No. 67,300, in mineral oil "is prepared with the total pigment constituting about 8 percent by weight of the suspension. The pigments are magenta, cyan, and yellow, respectively. The resulting mixture is coated on a NESA glass substrate and exposed as discussed above such that a positive polychrome image is projected onto the tri-mix as the roller moves across the surface of the NESA glass. The roller is held at a negative potential of about 2500 volts with respect to the NESA glass substrate. A suspenson comprising a 10% solution of zinc oxide in mineral oil is coated on a second roller similar to that used above and the second roller subsequently passed over the NESA substrate 6 times under similar conditions. The roller surface is cleaned after each pass and the zinc oxide suspension reapplied. After completion of the 6 passes it is found that an excellent quality full color positive image with all colors well separated is left behind on the NESA substrate. Potential application and exposure are both continued during the entire process.

Example H The process of Example I is repeated with the exception that barium sulfate is substituted for the zinc oxide. An image demonstrating good color separation is produced.

8 Example III A process of Example I is repeated with the exception that titanium dioxide is substituted for the zinc oxide. Similar results are obtained.

Example IV An imaging suspension comprising a metal-free phthalocyanine pigment, Monolite Fast Blue G.S., is prepared, 7 parts by weight of the photosensitive particles being dispersed in a mineral oil carrier. The resulting suspension is coated on a NESA glass substrate and the remainder of the process completed according to the procedure of Example I. The resulting image produced demonstrates the capabilities of the present invention in a monochrome imaging process.

Although the present examples were specific in terms of conditions and materials used, any of the above mentioned materials may be substituted where applicable with similar results being realized. In addition to the step used in the process of the present invention, other steps or modificationsmay be used, if desirable. For example, the pigment used as the additive in the suspension of the image enhancing step may be photoconductive and thus this step in the process will also serve the purpose of an additional imaging step. In addition, other materials may be incorporated in the imaging suspension and other facets of the invention which will enhance, synergize or otherwise desirably affect the properties therein described. For example, various sensitizers may be utilized in conjunction with the imaging suspension.

Anyone skilled in the art will have other modifications occur to them based on the teachings of the present invention. These modifications are intended to be encompassed within the scope of this invention.

What is claimed is:

1. A photoelectrophoretic imaging process comprising:

(a) subjecting a layer of an imaging suspension to an applied electric field between two electrodes, said suspension comprising a plurality of finely divided particles in a substantially insulating carrier liquid each of said particles comprising an electrically photosensitive pigment,

(b) substantially simultaneously exposing said suspen sion to an image with a source of electromagnetic radiation whereby an image is formed on an electrode and a complimentary image is formed on the other of said electrodes,

(c) replacing said other electrodes with an electrode having coated on its surface a suspension comprising transparent particles in a substantially insulating carrier liquid, and

(d) re-exposing said imaging suspension while substantially simultaneously reapplying an electric field to the resulting configuration to improve said image.

2. The process as disclosed in claim 1 wherein the re-exposing of the imaging suspension in the presence of an electric field is repeated at least once.

3. A photoelectrophoretic imaging process comprising:

(a) subjecting a layer of an imaging suspension to an applied electric field between two electrodes, said suspension comprising a plurality of at least two differently colored finely divided particles in a substantially insulating carrier liquid each of said particles comprising an electrically photosensitive pigment the principal light absorption band of which substantially coincides with its principal photosensitive response and said pigment being the primary colorant for said particle,

(b) substantially simultaneously exposing said suspension to an image with a source of electromagnetic radiation whereby an image is formed on one electrode and a complimentary image is formed on the other of said electrodes,

(c) replacing said other electrode with an electrode having coated on its surface a suspension comprising transparest particles dispersed in a substantially insulating carrier liquid, and

(d) re-exposing said imaging suspension while substantially simultaneously reapplying an electric field t0 the resulting configuration to improve said image.

4. The process as disclosed in claim 3 wherein steps (0) and (d) are repeated at least once.

References Cited UNITED STATES PATENTS ROLAND E. MARTIN, JR., Primary Examiner 0 US. Cl. X.R.

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