US3446722A - Electrophoretic refining of photosensitive particles - Google Patents

Description

y 27, 1969 s. STEIN ETAL 3,446,722

BLECTFOPHORETIC REFINING OF PHOTOSENST'IIVE PARTICLES Filed Oct. 27, 1966 Sheet of 2 FIG.

INVENTORS IRA S. STEIN VSEVOLOD TULAGIN VSEVOLOIZQS. IHAJLOV May 27, 1969 s. STEIN ETAL 3,446,722 ELECTROPHORETIC REFININQ OF PHOTOSENSITIVE PARTICLES Filed Oct. 27, 1966 Sheet Z of 2 INVENTORS IRA s. STEIN VSEVOLOD TULAGIN FIG. 2

vs v LCD 5. IHAJL'OV 2 ahrromvzrs United States Patent Oflice 3,446,722 Patented May 27, 1969 U.S. Cl. 204-180 9 Claims ABSTRACT OF THE DISCLOSURE Electrically photosensitive particle dispersions are refined by exposing particular colored particles to the appropriate colored. light. Then particles which are suitably photosensitive will migrate to the surface of a blocking electrode wherethey are collected; and particles which are too low in sensitivity will remain on the surface of the transparent injecting electrode and are discarded.

This invention relates in general to imaging methods.

More specifically, the invention concerns the preparation of improved imaging materials and imaging methods utilizing the materials.

. Recently, there has been developed an electrophoretic imaging system capable of producing color images which utilizes electrically photosensitive particles. This process is described in detail and claimed in applications Ser. No. 384,737, now Patent No. 3,384,565; 384,681 now abancloned and 384,680 now abandoned, all filed July 23, 1964. In such imaging systems, variously colored light absorbing particles are suspended in a non-conductive liquid carrier. The suspension is placed between electrodes, subjected to a potential dilference and exposed to an image. 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 this system is the suspended particles which must be electrioally 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 are used, producing a single colored image equivalent to conventional black-and-White photography. Ina polychromatic system, the images are produced in natural, color because mixtures of particles of two or more different colors which are each sensitive to light of a specific wave-length or narrow range of wave-lengths are used. Particles used in this system must have both intense pure colors and be highly photosensitive.

In a subtractive polychromatic system, the particles are selected so that those of different colors respond to different wave-lengths in the visible spectrum corresponding to their absorption bands. Also, it is important that the paricles do not have substantial overlap in their spectral response curves, thus allowing for color separation and subtractive multicolor image formation. In a typical subtractive multicolor system, the particle dispersion should include cyan colored particles sensitive mainly to red light, magenta particles sensitive mainly to green light and yellow particles sensitive mainly to blue light. When mixed together in a carrier liquid, these particles produce a black appearing liquid. When one or more of the particles are caused to migrate from one electrode toward the other electrode, they leave behind particles which produce a color equivalent to the color of the impinging light. Thus, for example, red light exposure causes the cyan colored particles to migrate, leaving behind the magenta and yellow particles which combine to produce red in the final image. In the same manner, blue and green colors are reproduced by removal of yellow and magenta, respectively. When white light impinges upon the mix, all particles migrate, leaving behind the color of the white or transparent substrate. No exposure leaves behind all particles which combine to produce a black image. This is an ideal technique of subtractive color imaging in that the particles perform the dual functions final image colorant and photosensitive medium.

Excellent images may be produced utilizing a wide variety of particles consisting either of a single pigment or a composite of several ingredients. However, it has been found that many pigments or composite particles as purchased commercially and milled to appropriate sizes or as synthesized, contain impurities which degrade images produced therefrom. The primary problem has been found to be undesirable spectral response in some of the particles in an imaging mix. Where some of the particles of a given color respond to a broader range of wavelengths than would be desired, these particles will migrate from the base electrode in response to the improper light thereby decreasing density of that color in the final image. On the other hand, where the particles fail to respond to light of the proper wave-length, these particles will remain on the base electrode While most of the particles of that color migrate away. Thus, in the final image, an undesired color cast will remain in inappropriate areas. For example, if a portion of the cyan colored particles are not fully responsive to red light, they will not migrate when struck by red light and will remain on the base electrode. This area then, which should appear red in the final image, will have a bluish cast due to a portion of the cyan particles remaining with the magenta and yellow particles.

It has also been found that some particles of a given color are more negative and others more positive than the average. It is preferred in an imaging mix that all the particles have a single polarity so that they will initially adhere to the base electrode and migrate away therefrom upon exposure to light. If the polarity of some of the particles of a given color is incorrect, they may either adhere too strongly to the base electrode and not migrate when struck by appropriate light or may migrate away from the base electrode even when not contacted by appropriate light. These characteristics will either produce an incorrect color balance or lower color contrast.

Thus, there is a continuing need for improved imaging particles for use in electrophoretic imaging systems and for methods of eliminating particles having incorrect spectral or electrical characteristics from electrophoretic imaging mixes.

It is, therefore, an object of this invention to provide an electrophoretic imaging system overcoming the abovenoted disadvantages.

It is another object of this invention to provide a method for eliminating particles having undesirable spetcral or electrical characteristics from an electrophoretic imaging mix.

It is another object of this invention to provide an electrophoretic imaging system capable of producing images of improved color balance and color saturation.

It is still another object of this invention to provide a process for separating colored particles which are suitable for use in electrophoretic imaging from a particle suspension.

The above objects, and others, are accomplished by providing a method for electrophoretically separating suit able particles from unsuitable particles in a particulate suspension. In accordance with one embodiment of this invention, the particulate suspension to be purified is placed between a pair of electrodes one of which is conductive and the other of which is substantially insulating. One of the electrodes is at least partially transparent. Light of the wave-length to which the desired particles only.

should respond is projected upon the suspension through the transparent electrode while an electric field is maintained between the electrodes through the suspension. Particles which respond to the specific wave-length of light used will migrate to the substantially insulating electrode. Undesired particles will be left behind on the conductive electrode. These undesired particles are those which have low sensitivity to the wave-length used or electrical characteristics which prevent their migration. This failure to migrate may be due to many characteristics such as chemical impurities which are very close in their physical and chemical properties to the desired pigment so as to make their separation by conventional chemical or physical means difficult. Also, where the particles are composites made up of several components those particles which do not have the desired distribution of the different components will tend to remain on the conductive electrode. After exposure, the electrodes may be separated and the migrated particles removed from the insulating electrode for later use in electrophoretic imaging processes.

In a similar manner, photosensitive particles which have excessively broad photosensitive response can be eliminated. Typically, a suspension of the particles to be purified is placed between a conductive and an insulating electrode. An electric field is established between the electrodes across the suspension and light of course to which the particles should not respond is allowed to fall on the suspension through one of the electrodes. For example, cyan particles should not respond to blue light so that if blue light is allowed to fall on cyan particles in this system only those particles with excessive spectral range will migrate. After exposure, the electrodes are separated. The particles which remain on the conductive electrodes are then suitable for use in electrophoretic imaging systems.

The invention will be further understood upon reference to the drawings in which:

FIGURE 1 show a schematic representation of an electrophoretic system which may be used to purify pigments and which may be also used to form images; and

FIGURE 2 shows a schematic representation of a continuous electrophoretic particle purification system.

Referring now to FIGURE 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, commercially available under the name NESA glass. This electrode will hereinafter be referred to as the injecting electrode. Coated on the surface of injecting electrode 1 is 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 actinic electromagnetic radiation, For a detailed theoretical explanation of the apparent mechanism of operation of electrophoretic imaging, see the above mentioned applications Ser. Nos. 384,737 now Pat. No. 3,384,565; 384,681 now abandoned; and 384,680 now abandoned, the disclosures of which are incorporated herein by reference. Where this system is to be used to separate desirable from undesirable photosensitive particles, the suspension will comprise particles of a single color dispersed in a nonconductive carrier liquid. Where the system is to be used for polychromatic imaging, the suspension will comprise a mixture of two or more different colored particles in the carrier liquid. Adjacent to the electrode 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 between electrodes 1 and S. A projector made up of a light source 8 an element 9 which may be a transparency or filter and a lens 10 is provided to expose the dispersion 4 to light of a desired color or to a light image of the original to be reproduced. Where the system is used to separate desired from undesired particles, element 9 will be a suitable filter which permits only light of the desired color to reach the suspension 4. Where the system is being used to produce a polychromatic copy, element 9 will comprise a natural color transparency, such as a Kodachrome transparency. In this exemplary instance, electrode 5 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 I electrode material 12, which may be Baryta paper. The

particle suspension as 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 light exposure. This light exposure causes appropriate particles originally attracted to electrode 1 to migrate through the liquid and adhere to the surface of the blocking electrode, leaving behind particles on the injecting electrode surface which do not respond to the impinging light. Where the system is used for separating desirable from undesirable particles, the desirable particles may be deposited either on the injecting electrode 1 or on the blocking" electrode 5, depending upon whether it is preferred that the particles respond to the impinging light or not, as'discussed above. Where the system is used for image formation, ordinarily a subtractive polychromatic image conforming to the original remains on injecting electrode 1 while unneeded particles migrate to blocking electrode 5.

Where the system is used for separating desired from undesired particles, if the light which struck the suspension is of a color which the given particles should absorb, then the particles on blocking electrode 5 are removed and saved for later use in polychromatic electrophoretic imaging, If the light which impinged upon the suspension was of a color which the particles should not absorb, then the desired particles remain on the injecting electrode 1. The particles are removed therefrom and saved for later use in electrophoretic imaging. In each case, the undesired particles are removed from the other electrode and discarded.

Where the system was used for image formation, the image may be transferred to a receiving surface and fixed by any suitable method, e.g., the methods described incopending application Ser. No. 459,860 filed May 28, 1965.

The system shown in FIG. 1 is capable of producing good separation of particles and of producing excellent images. However, since this is a batch type process and produces only small amounts of purified pigments, it may be desired to make the separation process continuous.

FIG. 2 shows a schematic representation of a continuous system for separating desired from undesired electrically photosensitive particles. Here, the injecting electrode 20 is in the form of a transparent cylinder which may be glass having a thin transparent coating of tin oxide on the exterior surface thereof. Two blocking electrode rollers 21 and 22 are arranged so as to rotate in virtual contact with injecting electrode 20*. Blocking electrode rollers 21 and 22 each consists of a conductive core 23 and 24, respectively, and a surface layer of blocking electrode material, such as Baryta paper, 25 and 26, respectively. Any conventional means may be provided for rotating injecting electrode cylinder 20 and blocking electrode rollers 21 and 22 in unison. For example, synchronous motors or gearing may be provided to rotate cylinder 20 and rollers 25 and 26 at speeds such that their surfaces pass with no difference in surface speed. Pro jection means 27 and 28 are provided to project light of desired spectral characteristics onto the suspension at the points where blocking electrode roller 21 contact injecting electrode cylinder and at the point where blocking electrode roller 22 contacts injecting electrode cylinder 20. These projectors 27 and 28 will include suitable filters to vary the spectral characteristics of the light emitted therefrom. Variable slit means 29 and 30 may be provided to limit the amount of light falling on the suspension. The Width of this slit may be varied so as to vary the quantity of light impinging on the suspension. A potential is applied between the blocking electrode rollers 21 and 22 and the injecting electrode cylinder 20 by means of potential source 31 connected to the conductive core of blocking electrodes 21 and 22 and to the conductive surface of injecting electrode cylinder 20 by means of wiper 32 and switch 33. In operation, a uniform layer of the particles to be purified dispersed in an insulating carrier liquid is coated onto the surface of injecting electrode cylinder 20 from reservoir 34. To illustrate the operation of this system consider the case where reservoir 34 contains crude cyan colored particles to be refined suspended in an insulating carrier liquid. When the cyan particles reach the nip between roller 21 and cylinder 20 they are exposed to light of a color to which they should not respond. Where the particles are cyan colored, this light may be blue and green. Particles which have excessively broad spectral response will respond to the blue and green light and migrate to the surface of blocking electrode roller 21. The particles which do not have this excessive response remain on the surface of injecting electrode cylinder 12. If the insulating carrier liquid has evaporated excessively, the surface of injecting electrode cylinder 20 may be remoistened by means of spray means 35 which applied additional insulating carrier liquid. When the particle suspension reaches the nip between blocking electrode roller 22 and injecting electrode cylinder 20, the suspension is exposed to light to which the particles should respond. In the case of cyan particles the light would be red in color. Particles which have suitable photosensitivity will migrate to the surface of blocking electrode roller 22. Particles which are either insensitive or too low in sensitivity will remain on the surface of injecting electrode cylinder 20. As blocking electrode roller 22 continues to rotate, the desired particles reach doctor blade 36 which removes them from the surface of roller 22 dropping them into container 37. The particles which reach container 37 are only those which have high photosensitivity and the desired spectral response characteristics. As is further pointed out in the example below these particles have in general superior imaging characteristics when compared to the original particles as purchased or synthesized. Meanwhile, the undesired particles which have excessive spectral response are removed from the surface of blocking electrode roller 21 by doctor blade 38 and dropped into container 39. Similarly, those particles which were relatively insensitive and remained on injecting electrode cylinder 20 reach doctor blade 40 and are removed from the injecting electrode cylinder to container 41. Particles reaching containers 39 and 41 may be reprocessed or discarded. Using this system, any desired quantity of crude particles may be separated into suitable and unsuitable portions. After particles of one color have been refined, particles of other colors may be similarly refined merely by changing the filters used in projection means 27 and 28 so that light of the appropriate color is produced. In each instance, light of colors to which the specific particles should not respond is projected onto the suspension at the nip between roller 21 and cylinder 20 while light of a color to which the specific particle should respond is projected onto the suspension at the nip between roller 22 and cylinder 20. Of course, each of rollers 21 and 22 and cylinder 20 could be in the form of a belt entrained around rollers, if desired.

Any suitable insulating liquid may be used as the carrier for the pigment particles in this system. Typical carrier liquids include decane, dodecane, N-tetradecane,

parafiin, beeswax or other thermoplastic materials, Sohio Odorless Solvent 3440 (a kerosene fraction) and Isopar-G (a long chain saturated aliphatic hydrocarbon). Voltages ranging from about 300 to about 5,000 volts of either positive or negative polarity may be used to produce good separation and excellent images in systems of the sort shown in the figures.

Any suitable electrically photosensitive particles may be improved by the separation process of this invention and will produce polychromatic images of higher quality than images produced from the crude particles. Typical particle materials include: Algol Yellow GC, 1,2,5,6-di- (c,c'-diphenyl)-thiazole-anthraquinone; Graphthol Rhodamine, the molybdenum lake of 3,6, bis(diethylamino)- 9,2-carbethoxy phenyl xanthenonium chloride; Bonodur Red B, 1-(4'-ethyl-5'-chloroazobenzene-2-sulfonic acid)- 2-hydroxy-3-naphthoic acid calcium lake; Indanthrene Brilliant Orange RK, 4,10-dibromo-6,l2-anthanthrone; Calcium Lithol Red, a calcium lake of l-(2-azonaphthalene-1'-sulfonic acid)-2-naphthol; Indofast Violet Lake, dichloro-9,18-isoviolanthrone; Cyan Blue GTNF, the beta form of copper phthalocyanine; Indofast Yellow Toner, fiauanthrone; Cyan Green 15-3100, a chlorinated copper phthalocyanine; Methyl Violet, a phosphotungstomolybdic lake of 4 (N,N,N' trimethylanilino) methylene- N",N"-dimethylanilimium chloride; Diane Blue, 3,3' methoxy 4,4 diphenyl bis(1" azo 2" hydroxy- 3"-naphthanilide; Monolite Fast Blue GS, a mixture of alpha and beta metal-free phthalocyanine; Duol Carmine, a calcium lake of 1-(4'-methylazobenzene- 2' sulfonic acid)-2hydroxy-3-naphthoic acid; Naphthol Red B, 1-(2- methoxy 5 nitrophenylazo) 2 hydroxy 3" nitro- 3-naphthanilide; Quindo Magenta RV-6803, a substituted quinacridone; Vulcan Fast Red BBE, 3,3'-dimethoxy-4,4'- biphenyl bis l-phenyl-3"-methyl-4"-azo-2"-pyrazolin- 5 "-one); Watchung Red B, 1- (4-methyl-5'-chloroazobenzene-2'-sulfonic acid)-2-hydroxy-3-naphthoic acid; and mixtures thereof. Other typical electrically photosenstive pigments include the following, described in the noted copending applications; 8,13-dioxodinaphtho-(1,2-2,3')- furan-6-carbox-4"-methoxyanilide (Ser. No. 421,377, filed Dec. 28, 1964); 1-cyano-2,3-(3'-nitro)-phthaloyl-7,8-benzopyrrocoline (Ser. No. 445,235, filed Apr. 2, 1965, now Patent 3,402,177); N 2" pyridyl 8,13 dioxodinaphtho-(1,2-2,3')-furan-6-carboxamide (Ser. No. 421,281, filed Dec. 28, 1964); various quinacridones as disclosed in application Ser. No. 468,935, filed July 1, 1965, now abandoned; anthraquinones as disclosed in application Ser. No. 467,344, filed June 28, 1966; azo pigments as disclosed in application Ser. No. 473,607, filed July 21, 1965; dioxazine pigments as disclosed in application Ser. No. 519,104, filed Jan. 6, 1966, now abandoned; and phthalocyanines as disclosed in application Ser. No. 560,- 603, filed June 27, 1966.

The following examples further specifically define the present invention with respect to the method of separating suitable from unsuitable photosensitive particles and of producing images using said particles. Parts and percentages are by weight unless otherwise indicated. Examples below are intended to illustrate various preferred embodiment of the processes of the present invention.

All of the following examples are carried out in the apparatus of the general type illustrated in FIGURE 1 with the particle mix coated on a NESA glass substrate through which exposure is made. The NESA glass surface is connected in series of a switch, a potential source, and the 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 4 centimeters per second. The plate employed is roughly 3 inches square and is exposed with a light intensity of about 36 foot candles as measured on the uncoated NESA glass surface. All particles which have a relatively large particle size as received commercially or as made are ground in a ball mill for about 48 hours to reduce their size in order to provide a more stable dispersion which improves the resolution of the final images. In each of the examples relating to the separation of suitable from unsuitable particles, about 7 parts of the specific particles to be refined are suspended in about 100 parts Sohio Odorless Solvent 3440. In each case, the suspension is exposed to light of the desired color by projection utilizing appropriate color filters. In the portions of the examples in which polychromatic images are produced, about 8 parts by weight of a mixture of the three different colored particles is dispersed in about 100 parts Sohio Odorless Solvent 3440. In the imaging examples, the suspension is exposed to a natural color image by means of a conventional Kodachrome transparency.

EXAMPLE I A sample of a cyan pigment, Monolite Fast Blue GS, a mixture of the alpha and beta forms of metal-free phthalocyanine, is divided into two portions. The first portion is dispersed in Sohio Odorless Solvent 3440 and is coated onto the NESA glass substrate of a device similar to that shown in FIGURE 1. A red filter, Wratten 29, is interposed between the light source and the NESA substrate. A potential of about 3,000 volts is maintained across the suspension during exposure as the roller electrode is passed across the suspension. Since cyan pigments should be sensitive to red light, the most photosensitive particles migrate to the roller electrode, leaving the less sensitive particles on the NESA substrate. The NESA substrate is cleaned, and the particles adhering to the roller electrode are removed and resuspended in Sohio Odorless Solvent 3440. The exposure step is repeated, however in this instance a blue filter, Wratten 47b, is in place between the light source and the NESA substrate. Since the cyan particles should not respond to blue light, after exposure only those particles having undesirably broad spectral response have migrated to the blocking electrode. The purified particles remain on the NESA substrate.

Imaging tests are then made with both the stock pigment and the purified pigment, Two dispersions are prepared by dispersing about 7 parts of each pigment in about 100 parts Sohio Odorless Solvent 3440. Each suspension is coated onto a NESA substrate and imaged as described above using a conventional black-and-white transparency between the projection lamp and the NESA substrate. In each case, an image is produced on the NESA substrate which is a positive copy of the original in cyan colored areas against the transparent background. The purified pigment exhibits higher photosensitivity and good density. The unpurified pigment produces an image having some undesirable deposition of pigment particles in background areas.

EXAMPLE II A sample of a magenta pigment, Watchung Red B, l- (4' methyl 5' chloroazobenzene 2 sulfonic acid)- 2-hydroxy-3-naphthoic acid, C.I. No. 15865, is divided into two portions. The first portion is purified as described in Example I. Here the first exposure is by means of green light through a Wratten 61 filter. Since magenta pigments should be sensitive to green light, the more sensitive particles migrate to the blocking electrode. These particles are removed from the blocking electrode and resuspended. This suspension is then exposed to red light by means of a Wratten 29 filter. Since magenta pigments should not be sensitive to red light, the purified pigments remain on the injecting electrode with only the undesirably sensitive pigments migrating to the blocking electrode.

Two suspensions are then prepared, the first consisting of about 7 parts of the stock pigment dispersed in Sohio Odorless Solvent 3440 and the second consisting of about 7 parts of the purified pigment dispersed in Sohio Odorless Solvent 3440. Each of these suspensions is imaged as described above, with a conventional blackand-white transparency placed between the light source and the NESA electrode. In each case in image conforming to the original is formed on the NESA substrate, appearing magenta-colored against the transparent background. The image produced by the purified pigments is of higher overall quality with less undesirable pigment deposition in background areas.

EXAMPLE I11 An example of a yellow pigment, N-2"-pyridyl-8,13-

dioxodinaphtho-(1,2-2',3')-furan 6 carboxamide, prepared by the process described in copending application Ser. No. 421,281, filed Dec. 28, 1964, is divided into two portions. The first portion is purified by the method generally described in Example I. A suspension prepared from this pigment is first exposed to blue light by means of a Wratten 47b filter. Since a yellow pigment should be sensitive to blue light, the preferred particles migrate to the blocking electrode leaving insensitive particles behind on the injecting electrode. The particles are removed from the blocking electrode, resuspended and re-exposed, this time to red light by means of a Wratten 29 filter. Since a yellow pigment should not be sensitive to red light, the preferred pigments remain on the NESA electrode with those pigments having excessive spectral response migrating to the blocking electrode.

Two suspensions are then prepared for imaging tests, the tfirst consisting of about 7 parts of the stock pigment dispersed in about parts Sohio Odorless Slovent 3440 and the second consisting of about 7 parts of the purified pigment suspended in about 100 parts Sohio Odorless Solvent 3440. Images are prepared from each dispersion as described above, using a conventional black-and-White transparency between the light source and the NESA electrode. The image produced from the purified pigment is generally superior to that produced with the stock pigment, showing higher photosensitivity and better background.

EXAMPLE IV Two trimixes are prepared as follows:

(a) About 4 parts of stock Monolite Fast Blue GS, about 4 parts of stock Watching Red B and about 4 parts of N-2"-pyridyl-8,13-dioxodinaphthol-( 1,2-2,3 -furan-6- carboxamide as prepared in the laboratory, are dispersed in about 100 parts Sohio Odorless Solvent 3440.

(b) A second suspension is prepared as in part (a) above, except that in this instance each of the pigments has been purified as described in Examples I-III.

Each of these trimixes is coated onto a NESA substrate of a device of the sort shown in FIGURE 1. Each suspension is exposed to a natural color image by means of a Kodachrome transparency placed between the light source and the NESA electrode. After exposure, each suspension is found to have produced a full color image on the NESA electrode conforming to the original. The purified pigments are found to be more sensitive than unpurified pigments. The image produced by the purified pigments is found to have superior color balance and little deposition of unwanted pigments in background areas.

Although specific components and proportions have been described in the above examples, other suitable materials, as listed above, may be used with similar results. In addition, other materials may be added to the particles, or to the particle suspensions to synergize, enhance, or otherwise modify their properties. Typically, the particles or suspensions may be dyesensitized or electrically sensitized, if desired.

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

What is claimed is:

1. A method of refining photoelectrophoretic imaging particles which comprises the steps of:

(a) placing between a pair of electrodes, at least one of which is at least partially transparent, a suspension comprising particles of a single color dispersed in an insulating carrier liquid,

(b) subjecting said suspension to an applied electric field while exposing said suspension through said transparent electrode to light of limited spectral characteristics whereby particles responsive to said light migrate to one electrode leaving non-responsive particles on said other electrode, and

(c) separating said electrodes and removing particles from one electrode for later use in photoelectrophoretic imaging processes.

2. The method of claim 1 wherein said particles removed from said one electrode are resuspended and steps (a), (b), and (c) are repeated using light of diiferent spectral characteristics whereby said particles are further refined.

3. The method of claim 1 wherein said particles are cyan in color and are subjected to red light whereby insensitive particles remain on one electrode while sensitive particles migrate to the other electrode from which they are removed for later use in electrophoretic imaging.

4. The method of claim 1 wherein said particles are cyan in color and are subjected to blue and green light whereby overly sensitive particles migrate to one electrode, leaving particles substantially insensitive to blue and green light on said other electrode, from which other electrode they are removed for later use in electrophoretic imaging.

5. An apparatus for refining electrophoretic imaging particles which comprises:

(a) an at least partially transparent first electrode having a conductive surface;

(b) a second electrode having a substantially insulating surface;

() means to bring said second electrode into virtual contact with said first electrode;

(d) a third electrode having a substantially insulating surface;

(e) means to bring said third electrode into virtual contact with said first electrode;

(f) means to introduce a suspension comprising particles in a carrier liquid between said first and second electrodes;

(g) means to impose an electric field across said suspension between said first and second electrodes;

(h) projection means to direct light of a first color onto said suspension between said first and second electrodes through said transparent first electrode whereby the portion of said particles responsive to said light migrate to said second electrode, leaving the remaining particles on said first electrode;

(i) means to introduce that portion remaining on said first electrode between said first and third electrodes;

(j) means to impose an electric field across said suspension between said first and third electrodes; and

(k) projection means to direct light of a second color onto said suspension between said first and third electrodes through said transparent first electrode whereby the portion of said particles responsive to said light migrates to said third electrode leaving the remaining particles on said first electrode.

6. An apparatus according to claim 5 further including means to resuspend said particles in said carrier liquid before said particles are introduced between said first and and third electrodes.

.7. An apparatus according to claim 5 further including means to clean each electrode of adhering particles after exposure.

8. An apparatus according to claim 5 wherein each electrode has an endless surface.

9. An apparatus according to claim 5 further including drive means to continuously move each electrode surface whereby the surfaces of said first and second electrodes and of said rfirst and third electrodes come into virtual contact without relative surface motion at contact.

References Cited UNITED STATES PATENTS 1,435,886 11/1922 Acton et al 204- 2,485,335 10/1949 Tyson 204-180 HOWARD S. WILLIAMS, Primary Examiner.

A. C. PRESCOTT, Assistant Examiner.

US. Cl. X.R.

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