GB1601784A - Photoelectrophoretic migration imaging process and dispersion - Google Patents

Description

(54) PHOTOELECTROPHORETIC MIGRATION IMAGING PROCESS AND DISPERSION (71) We, EASTMAN KODAK COMPANY, a Company organized under the laws of the State of New Jersey, United States of America of 343 State Street, Rochester, New York 14650, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to photoelectrophoretic migration imaging processes and photoelectrophoretic migration imaging dispersions for use in such processes.

Known photoelectrophoretic migration imaging processes typically employ a layer of electrical charge-bearing photoconductive particles, i.e., electrically photosensitive particles, positioned between two spaced electrodes, one of which is transparent. To achieve image formation in these processes, the charge-bearing photosensitive particles positioned between the two spaced electrodes are subjected to the influence of an electric field and exposed to activating radiation.

As a result, the charge-bearing electrically photosensitive particles are caused to migrate electrophoretically to the surface of one or other of the spaced electrodes, and an image pattern is obtained on the surface of these electrodes. Typically, a negative image is formed on one electrode, and a positive image is formed on the opposite electrode.Image discrimination occurs in the various photoelectrophoretic migration imaging processes as a result of a net change in charge polarity of either the exposed or the unexposed electrically photosensitive particles so that the image formed on one electrode surface is composed ideally of electrically photosensitive particles of one charge polarity, either negative or positive polarity, and the image formed on the opposite polarity electrode surface is composed ideally of electrically photosensitive particles having the opposite charge polarity.

Regardless of the particular photoelectrophoretic migration imaging process employed, it is apparent that an essential component of any such process is the electrically photosensitive particles. Of course, to obtain an easy-to-read, visible image it is important that these electrically photosensitive particles be coloured as well as electrically photosensitive. Accordingly, as is apparent from the technical literature regarding photoelectrophoretic migration imaging processes, work has been carried on in the past and is continuing to find particles which possess both useful levels of electrical photosensitivity and which exhibit good colorant properties.

Yellow electrically photosensitive particles are useful in photoelectrophoretic migration imaging processes. Such particles are particularly useful in photoelectrophoretic polychrome migration imaging processes based on a subtractive multicolour system. However, yellow electrically photosensitive particles disclosed in the prior art have been considered unsatisfactory for one reason or another. For an example the widely used Indofast yellow particles result in the formation of polychrome images in which the red and green colour purity is less than desired.

We have now discovered that trans-epindolidione having the structure

is highly useful in an electrophotosensitive particle in photoelectrophoretic migration imaging processes. Trans-epindolidione exhibits 1) an excellent yellow hue which is particularly useful in photoelectrophoretic polychrome migration imaging processes based on subtractive multicolour system, 2) minimal undesirable particle interaction in polychrome imaging compositions 3) high speed, 4) light fast images and 5) images of high density.

These properties of trans-epindolidione are especially surprising and unexpected in view of the fact that in our search for an acceptable yellow electrically photosensitive particle, we tested many structurally similar materials such as cis-epindolindione and diquinolonopyridone which provide very poor or no image and/or images of the wrong colour when used in electrophoretic migration imaging systems.

When used in a photoelectrophoretic migration imaging process, the chargebearing electrically photosensitive particles may be positioned between two spaced electrodes; preferably these particles are contained in an electrically insulating carrier such as an electrically insulating liquid or an electrically insulating, liquefiable matrix material, e.g., a thixotropic or a heat- and/or solvent-softenable material, which is positioned between the spaced electrodes. While so positioned between the spaced electrodes, the photosensitive particles used in the invention are subjected to an electric field and exposed to a pattern of activating radiation.

As a consequence, the charge-bearing, electrically photosensitive particles undergo a radiation-induced variation in their charge polarity and migrate to one or the other of the electrode surfaces to form on at least one of these electrodes an image pattern representing a positive-sense or negative-sense image of the original radiation exposure pattern.

According to the present invention, there is provided a photoelectrophoretic migration imaging process which comprises subjecting electrically photosensitive particles positioned between two electrodes to an applied electric field and exposing the particles to an image pattern of radiation thereby obtaining image formation on at least one of the electrodes, wherein at least a portion of the particles comprise trans-epindolidione.

The invention also provides a photoelectrophoretic migration imaging dispersion comprising a carrier and electrically photosensitive particles wherein at least a portion of the particles comprise trans-epindolidione.

In photoelectrophoretic migration imaging, trans-epindolidione results in excellent high density yellow images. In trimixes this material does not result in muddy coloured images. This indicates the absence of substantial particle-particle interaction between trans-epindolidione and the other electrophotosensitive components of the trimix.

Methods of making trans-epindolidione are described, for example in U.S.

Patent 3,334,102 and Jaffe et al., Journal of Organic Chemistry 33(11), 4004(1968), wherein epindolidiones are disclosed as pigments in coating compositions and as colorants for materials such as plastics, rubber paper and linoleum.

Trans-epindolidione exhibits certain other properties which are advantageous for its use in photoelectrophoretic migration imaging processes. For example, it is insoluble or only slightly soluble in conventional organic solvents which is advantageous in those embodiments of such processes wherein the electrically photosensitive particles are dispersed in an electrically insulating carrier such as a conventional aliphatic hydrocarbon liquid to form an electrophoretic migration imaging suspension.

In general, electrically photosensitive particles for use in electrophoretic migration imaging processes have an average particle size within the range of from .01 micron to 20 microns, preferably from .01 to 5 microns. Typically, these particles are composed of one or more colorants such as those described in the present invention. However, these electrically photosensitive particles may also contain various non-photosensitive materials such as electrically insulating polymers, charge control agents, various organic and inorganic fillers, as well as various additional dyes or pigments to change or enhance various colorant and physical properties of the electrically photosensitive particle.In addition, such electrically photosensitive particles may contain other photosensitive materials such as various sensitizing dyes and/or chemical sensitizers to alter or enhance their response characteristics to activating radiation.

The electrically photosensitive particles may be dispersed simply as a dry powder between two spaced electrodes and then subjected to a typical electrophoretic migration imaging operation such as that described in U.S. Patent No. 2,758,939. In general, however, the electrically photosensitive particles are dispersed in an electrically insulating carrier, such as an electrically insulating liquid, or an electrically insulating, liquefiable matrix material e.g., a heat- and/or solvent-softenable polymer or a thixotropic polymer. Typically, when a dispersion of electrically photosensitive particles in an electrically insulating carrier is used between the spaced electrodes of an electrophoretic migration imaging system, it is conventional to employ from 0.05 part to 2.0 parts of electrically photosensitive particles for each 10 parts by weight of electrically insulating carrier.

As indicated above, when the electrically photosensitive particles used in the present invention are dispersed in an electrically insulating carrier, such carrier may assume a variety of physical forms and may be selected from a variety of different materials. For example, the carrier may be a matrix of an electrically insulating, normally solid polymeric material capable of being softened or liquefied upon application of heat, solvent, and/or pressure so that the electrically photosensitive particles dispersed therein can migrate through the matrix.In another more typical embodiment of the invention, the carrier may comprise an electrically insulating liquid such as decane; paraffin; Sohio Odourless Solvent 3440 (a Kerosene fraction marketed by the Standard Oil Company, Ohio); various isoparaffinic hydrocarbon liquids such as those sold under the trademark Isopar G by Exxon Corporation and having a boiling point in the range of from l450C to 186 C; various halogenated hydrocarbons such as carbon tetrachloride and trichloromonofluoromethane; various alkylated aromatic hydrocarbon liquids such as the alkylated benzenes, for example, xylenes, and other alkylated aromatic hydrocarbons such as are described in U.S. Patent No. 2,899,335.An example of one such useful alkylated aromatic hydrocarbon liquid which is commercially available is Solvesso 100 made by Exxon Corporation ("Solvesso" is a Registered Trade Mark). Solvesso 100 has a boiling point in the range of from 157"C to 1770C and is composed of 9 percent xylene, 16 percent of other monoalkyl benzenes, 34 percent dialkyl benzenes, 37 percent trialkyl benzenes, and 4 percent aliphatics.

Typically, whether solid or liquid at normal room temperatures, i.e., about 22"C, the electrically insulating carrier used in the present invention is a material having a resistivity greater than 109 ohm-cms, preferably greater than 1012 ohm-cm.

When the electrically photosensitive particles used in the present invention are incorporated in a carrier, such as one of the above-described electrically insulating liquids, various other addenda may also be incorporated in the resultant imaging suspension. For example, various charge control agents may be incorporated in the suspension to improve the uniformity of charge polarity of the dispersed electrically photosensitive particles. Such charge control agents are well known in the field of liquid electrographic developers where they are employed for purposes substantially similar to that described herein. Thus, extensive discussion of these charge control agents herein is deemed unnecessary. The charge control agents may be polymers which are incorporated by admixture thereof into the liquid carrier of the suspension.In addition to, and possibly related to, the aforementioned enhancement of uniform charge polarity, it has been found that the charge control agents often provide more stable suspensions, i.e., suspensions which exhibit substantially less settling out of the dispersed photosensitive particles.

In addition to the foregoing charge control agents, various polymeric binders such as natural, semi-synthetic or synthetic resins, may be dispersed or dissolved in the electrically insulating carrier to serve as a fixing material for the final photosensitive particle image formed on one of the spaced electrodes used in electrophoretic migration imaging systems. Here again, the use of such fixing material is conventional and well known in the closely related art of liquid electrographic developers so that extended discussion thereof is unnecessary herein.

The process of the present invention will be described in more detail with reference to the accompanying drawing, which illustrates a typical apparatus with which the migration imaging process of the invention may be performed.

The drawing shows a transparent electrode I supported by two rubber drive rollers 10 capable of imparting a translating motion to electrode I in the direction of the arrows. Electrode 1 may be composed of a layer of optically transparent material, such as glass or an electrically insulating, transparent polymeric support such as polyethylene terephthalate, covered with a thin, optically transparent, conductive layer such as tin oxide, indium oxide or nickel. Optionally. depending upon the particular type of electrophoretic migration imaging process desired, the surface of electrode 1 may bear a "dark charge exchange" material, such as a solid solution of an electrically insulating polymer and 2,4,7 - trinitro - 9 - fluorenone as described in U.S. Patent No. 3.976,485.

Spaced opposite electrode 1 and in pressure contact therewith is a second electrode 5, an idler roller which serves as a counter electrode to electrode 1 for producing the electric field used in the electrophoretic migration imaging process.

Typically, electrode 5 has on the surface thereof a thin, electrically insulating layer 6. Electrode 5 is connected to one side of a power source 15 by switch 7. The opposite side of the power source 15 is connected to electrode 1 so that as an exposure takes place, switch 7 is closed and an electric field is applied between the electrodes to electrically photosensitive particles 4 dispersed in an electrically insulating carrier such as described hereinabove.

The electrically photosensitive particles 4 may be positioned between the electrodes 1 and 5 by applying them to either or both of the surfaces of electrodes 1 and 5 prior to the imaging process or by injecting the particles between the electrodes during the electrophoretic migration imaging process.

As shown in the drawing, exposure of the electrically photosensitive particles 4 takes place by use of an exposure system consisting of light source 8, an original image 11 to be reproduced, such as a photographic transparency, a lens system 12, and any necessary or desirable radiation filters 13, such as colour filters, whereby the electrically photosensitive particles are irradiated with a pattern of activating radiation corresponding to the original image 11. Although the photoelectrophoretic migration imaging system represented in the drawing shows electrode 1 to be transparent to activating radiation from light source 8, it is possible to irradiate the electrically photosensitive particles 4 in the nip 21 between electrodes 1 and 5 without either of the electrodes being transparent.In such a system, although not shown in the drawing, the exposure source 8 and lens system 12 is arranged so that image material 4 is exposed in the nip 21 between electrodes I and 5.

As shown in the drawing, electrode 5 is a roller electrode having a conductive core 14 connected to the power source 15. The core is in turn covered with an insulating layer 6, for example, baryta paper. The insulating layer 6 serves to prevent or at least substantially reduce the capability of the electrically photosensitive particles 4 to undergo a radiation induced charge alteration upon interaction with the electrode 5. Hence, the term "blocking electrode" may be used, as is conventional in the art of electrophoretic migration imaging, in respect of the electrode 5.

Although electrode 5 is shown as a roller electrode and electrode 1 is shown as essentially a translatable, flat plate electrode, either or both of these electrodes may assume a variety of different shapes such as a web electrode, rotating drum electrode and plate electrode, which are well known in the field of electrophoretic migration imaging. In general, during a typical electrophoretic migration imaging process wherein the electrically photosensitive particles 4 are dispersed in an electrically insulating, liquid carrier, electrodes 1 and 5 are spaced such that they are in pressure contact or very close to one another, e.g. less than 50 microns apart.

However, where the electrically photosensitive particles 4 are dispersed simply in an air gap between electrodes 1 and 5 or in a carrier such as a layer of heatsoftenable or other liquefiable material coated as a separate layer on electrode 1 and/or 5, these electrodes may be spaced more than 50 microns apart during the imaging process.

The strength of the electric field imposed between electrodes 1 and 5 during the photoelectrophoretic migration imaging process of the present invention may vary considerably; however, it has generally been found that optimum image density and resolution are obtained by increasing the field strength to as high a level as possible without causing electrical breakdown of the carrier medium in the electrode gap. For example, when electrically insulating liquids such as isoparaffinic hydrocarbons are used as the carrier in the imaging apparatus of the drawing, the applied voltage across electrodes 1 and 5 may be within the range of from 100 volts to 4 kilovolts or higher.

As explained hereinabove, image formation occurs in photoelectrophoretic migration imaging processes as the result of the combined action of activating radiation and electric field on the electrically photosensitive particles. For best results, field application and exposure to activating radiation occur concurrently.

However, as would be expected, by appropriate selection of various process parameters such as field strength, activating radiation intensity, incorporation of suitable light sensitive addenda in or together with the electrically photosensitive particles e.g. by incorporation of a persistent photoconductive material, it is possible to alter the timing of the exposure and field application events so that one may use sequential exposure and field application events rather than concurrent field application and exposure events.

When disposed between the imaging electrodes 1 and 5 of the drawing the electrically photosensitive particles 4 exhibit an electrostatic charge polarity, either as a result of triboelectric interaction of the particles or as a result of the particles interacting with the carrier in which they are dispersed, for example, an electrically insulating liquid.

Image discrimination occurs as a result of the combined application of electric field and activating radiation on the electrically photosensitive particles. That is, in a typical imaging operation, upon application of an electric field between electrodes I and 5, the charge-bearing, electrically photosensitive particles 4 are attracted in the dark to either electrodes 1 or 5, depending upon which of these electrodes has a polarity opposite to that of the original charge polarity acquired by the electrically photosensitive particles. Upon exposing the particles 4 to activating electromagnetic radiation, it is theorized that there occurs neutralization or reversal of the charge polarity associated with either the exposed or unexposed particles.In typical photoelectrophoretic migration imaging systems wherein electrode 1 bears a conductive surface, the exposed, electrically photosensitive particles 4, upon coming into electrical contact with the conductive surface, undergo an alteration (usually a reversal) of their original charge polarity as a result of the combined application of electric field and activating radiation. Alternatively, in the case where the surface of electrode 1 bears a dark charge exchange material as described in U.S. Patent No. 3,976,485, the charge polarity of the unexposed particles is reversed, while maintaining the original charge polarity of the exposed electrically photosensitive particles, as these particles come into electrical contact with the dark charge exchange surface of electrode 1.In any case, upon the application of an electric field and activating radiation to the electrically photosensitive particles 4 disposed between electrodes 1 and 5 of the apparatus shown in the drawing, image discrimination may be obtained so that an image pattern is formed by the electrically photosensitive particles which corresponds to the original pattern of activating radiation. Typically, using the apparatus shown in the drawing, a visible image is obtained on the surface of electrode 1 and a complementary image pattern is obtained on the surface of electrode 5.

Subsequent to the application of the electric field and the exposure ot activating radiation, the images which are formed on the surfaces of electrodes I and 5 may be temporarily or permanently fixed to these electrodes or may be transferred to a final image receiving material. Fixing of the final particle image can be effected by various techniques, for example, by applying a resinous coating over the surface of the image bearing substrate. For example, if the electrically photosensitive particles 4 are dispersed in a liquid carrier between electrodes 1 and 5. the images formed on the surfaces of electrodes 1 and 5 may be fixed by incorporating a polymeric binder material in the carrier liquid.Many such binders (which are well known for use in liquid electrophotographic liquid developers) are known to acquire a charge polarity upon being admixed in a carrier liquid and therefore will, themselves, electrophoretically migrate to the surface of one or the other of the electrodes. Alternatively, a coating of a resinous binder (which has been admixed in the carrier liquid), may be formed on the surfaces of electrodes 1 and 5 upon evaporation of the liquid carrier.

As indicated, trans-epindolidione has an especially useful yellow hue. Hence, trans-epindolidione is particularly suited for use in polychrome imaging processes which employ a mixture of two or more differently coloured electrically photosensitive particles, e.g., a mixture of cyan particles which are principally sensitive to red light, magenta particles which are principally sensitive to green light, and yellow particles consisting at least partially of trans-epindolidione which are principally sensitive to blue light.When such a mixture of multicoloured electrically photosensitive particles is formed, for example, in an electrically insulating carrier liquid, the liquid mixture exhibits a black colouration. Preferably the specific cyan, magenta, and yellow particles selected for use in such a polychrome imaging process are chosen so that their spectral response curves do not appreciably overlap whereby colour separation and subtractive multicolour image reproduction can be achieved.

The following examples illustrate the invention. The stated parts and percentages are by weight unless otherwise stated.

Examples Image Evaluation Apparatus An image evaluation apparatus was used in each of the succeeding examples to carry out the photoelectrophoretic migration imaging process described herein.

This apparatus was a device of the type illustrated in the drawing. In this apparatus.

a translating transparent polyethylene terephthalate support coated with a 0.1 mil thick conductive cermet (Cr. SiO) layer served as electrode 1 and was in pressure contact with a 10 centimetre diameter aluminium roller 14 covered with Kodak type III coated paper 6 which served as electrode 5. Electrode 1 was supported by two 2.8 cm diameter rubber drive rollers 10 positioned beneath electrode 1 such that a 2.5 cm opening, symmetric with the axis of the aluminium roller 14, existed to allow exposure of electrically photosensitive particles 4 to activating radiation. The original transparency 11 to be reproduced consisted of adjacent strips of clear (WO)*, red (W29)*, green (W61)* and blue (W47B)* filters. The original was taped to the back side of electrode 1.The exposing activating radiation was supplied from a light source 8 consisting of a Kodak 'Ektagraphic' AV434 projector with 1 kilowatt Xenon arc lamp, (Kodak and 'Ektagraphic' are Registered Trade Marks).

The light source was modified with a Kodak No. 5 flexible M-carbon eleven step 0.3 neutral density step tablet. The voltage between the electrodes (1 and 5) was about 2 kv. Electrode 1 was negative polarity in the case where electrically photosensitive particles 4 carried a positive electrostatic charge, and electrode 1 was positive in the case where electrically photosensitive electrostatically charged particles were negatively charged. The translational speed of electrode 1 was variable about 25 cm per second. Residence time in exposure zone for each dispersion tested was about 10 milliseconds.The log of light intensity in the action zone was as follows: Log 1 Filters erg/cm2/sec WO*Clear 5.34 W29* Red 4.18 W61*Green 4.17 W47B* Blue 4.15 *Refers to Wratten Filter Numbers. ("Wratten" is a Registered Trade Mark).

In the following Examples, image formation occurs on the surfaces of electrode 1 and electrode 5 after simultaneous application of light exposure and electric field to the electrically photosensitive particles 4. In this image evaluation apparatus, the particles to be evaluated for use were admixed with a liquid carrier as described below to form a liquid carrier as described below to form a liquid imaging dispersion which was placed in the nip 21 between the electrodes I and 5.

If the particles being evaluated for use possessed a useful level of electrical photosensitivity, a negative-appearing image reproduction of original Il on electrode 5 and a complementary image on electrode 1 were obtained.

Imaging Dispersion Preparation Imaging dispersions were prepared to evaluate trans-epindolidione as well as other structurally similar materials. A stock solution of the dispersion components shown below was prepared. The stock solution was prepared simply by combining the listed components.

Dispersion Components Isopar G ("Isopar" is a Registered Trade Mark) 2.2 g Solvesso 1.3 g Piccotex 100* 1.4 g pvt** 0.1 g *Styrene-vinyl toluene copolymer.

**Poly(vinyltoluene - co - laurylmethacrylate - co - lithium methacrylate co - methacrylic acid) 56/40/3.6/0.4.

A 5 g aliquot of the stock solution was combined in a closed container with 0.045 g of trans-epindolidione and 12 g of Hamber 440 stainless steel balls. The preparation was then milled three hours on a paint shaker.

Examples 1--3 Three separate portions of crude trans-epindolidione pigment were extracted three different ways as follows: Extractions A - Trans-epindolidione was extracted with hot dimethylsulphoxide then extracted with hot xylene.

B - Trans-epindolidione was extracted with dimethylsulphoxide at room temperature for 12 hours/then extracted with ether at room temperature.

C - Trans-epindolidione was extracted with boiling ethanol for 2 hours.

The pigments of each extraction were orange-yellow. Three different dispersions were formed according to the above procedures and the sensitometric characteristics of each were determined using a densitometer containing a Kodak No. 5 flexible M-Carbon, 11 step wedge (0.3 ND). Each extraction exhibit good sensitometry, and each exhibited electrophotosensitivity by forming complementary images on each electrode. The yellow colour density was good considering both Dmax and Dmin.

In all pigment samples resulting from Extractions A, B and C the sample colour was orange-yellow and the charge on the dispersed particles was positive.

Example 4 Two grams of crude trans-epindolidione was combined with 50 ml of dimethylformamide (DMF).

The mixture was stirred at reflux for 2 hours, then cooled and filtered. The fine yellow material was slurried with water and filtered. The pigment was then stirred for 30 minutes in 150 ml boiling water, filtered, and dried in vacuo at 75 .

An imaging dispersion was formed and the sensitometric characteristics thereof were determined as described above. The sensitivity of the dispersion was 0.5 ergs/cm2. An image having excellent Dmin and Dmax was obtained.

Example 5 In the course of our search for acceptable yellow electrically photosensitive particles, many particles were tested before discovering the usefulness of transepindolidione in electrophoretic migration imaging processes. Many of the unsuitable particles tested were structurally related to trans-epindolidione and/or possessed a yellow colour. A number of such unsuitable particles are presented in Table I to emphasize the unexpected as well as the unpredictable nature of the present invention. Imaging dispersions of each of the listed particles were prepared and tested according to the procedures already described in relation to Example 1.

The data of the table shows the colour and/or quality of the images formed with each particle tested. Particles 1--9 and 13-15 are yellow and form poor or no images in electrophoretic imaging processes. Particles 10 are brown-red and form a poor image. Particles 11 and 12 exemplify the type disclosed in the aforementioned U.S. Patent No. 3,474,020. This data shows that while these particles form fair to good images, such images are not yellow. It is also worth noting that particles 11 and 12 would be unsuitable for use in subtractive multicolour imaging systems because of their colour. The image quality resulting from the use of each of Table I particles is determined upon the basis of a visual observation of the image which took into account Admin, DmaX, and image colour.

TABLE 1 Number Particle Image Quality

No Image No Image Poor No Image Poor Fair (Pale Yellow) No Image TABLE I (cont.) Number Particle Image Quality

No Image Poor (Pale Yellow) Poor (Pale Brown-Red) Fair (Red-Violet Image)

Good (Blue-Violet Image) Poor (Yellow Image) Poor Poor Example 6 To compare a prior art three-colour imaging dispersion (trimix) with a threecolour imaging dispersion (trimix) according to this invention. two imaging dispersions were prepared.

Each dispersion was prepared and tested as in Example 1. Each contained 2"-, by weight cyan blue GTNE and 2qd by weight Sandarin Brilliant Red. One contained 2% by weight Indofast Yellow (CI No. 70600). The other contained 20 by weight trans-epindolidione. The Indofast Yellow trimix produced an image with magenta-like reds and blue-like greens. The trans-epindolidione trimix gives a much truer red and a more saturated green.

The Indofast Yellow trimix image had a much higher D min than the transepindolidione trimix.

WHAT WE CLAIM IS: 1. A photoelectrophoretic migration imaging process which comprises subjecting electrically photosensitive particles positioned between two electrodes to an applied electric field and exposing the particles to an image pattern of radiation, thereby obtaining image formation on at least one of the electrodes.

wherein at least a portion of the particles comprise trans-epindolidione.

2. A process as claimed in Claim 1, wherein the particles are contained in an electrically insulating carrier.

3. A process as claimed in Claim 2, wherein the carrier is a liquid.

4. A process as claimed in Claim 2 or Claim 3, wherein the carrier also contains electrically photosensitive cyan and magenta particles so that polychrome image formation is obtained on at least one of the electrodes.

5. A process as claimed in any one of the preceding claims, wherein the particles have an average size of from 0.01 to 20 microns.

6. A process as claimed in any one of Claims 1 to 4, wherein the particles have an average size of from 0.01 to 5 microns.

7. A process as claimed in Claim 1 substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.

8. A process as claimed in Claim I substantially as hereinbefore described in any one of Examples 1 to 4.

9. A process as claimed in Claim 1 substantially as hereinbefore described in Example 6.

10 An image whenever recorded by a process as claimed in any one of the preceding claims.

11. A photoelectrophoretic migration imaging dispersion comprising a carrier and electrically photosensitive particles wherein at least a portion of the particles comprise trans-epindolidione.

12. A dispersion as claimed in Claim 11, wherein the carrier is a liquid.

13. A dispersion as claimed in Claim 11 or Claim 12, wherein the electrically photosensitive particles also comprise electrically photosensitive cyan and magenta particles.

14. A dispersion as claimed in any one of Claims 11 to 13, wherein the particles have an average size of from 0.01 to 20 microns.

15. A dispersion as claimed in any one of Claims Il to 13, wherein the particles have an average size of from 0.01 to 5 microns.

16. A dispersion as claimed in Claim 11 substantially as hereinbefore described in any one of Examples 1 to 4.

17. A dispersion as claimed in Claim 11 substantially as hereinbefore described in Example 6.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (17)

**WARNING** start of CLMS field may overlap end of DESC **. Example 6 To compare a prior art three-colour imaging dispersion (trimix) with a threecolour imaging dispersion (trimix) according to this invention. two imaging dispersions were prepared. Each dispersion was prepared and tested as in Example 1. Each contained 2"-, by weight cyan blue GTNE and 2qd by weight Sandarin Brilliant Red. One contained 2% by weight Indofast Yellow (CI No. 70600). The other contained 20 by weight trans-epindolidione. The Indofast Yellow trimix produced an image with magenta-like reds and blue-like greens. The trans-epindolidione trimix gives a much truer red and a more saturated green. The Indofast Yellow trimix image had a much higher D min than the transepindolidione trimix. WHAT WE CLAIM IS:

1. A photoelectrophoretic migration imaging process which comprises subjecting electrically photosensitive particles positioned between two electrodes to an applied electric field and exposing the particles to an image pattern of radiation, thereby obtaining image formation on at least one of the electrodes.

wherein at least a portion of the particles comprise trans-epindolidione.

2. A process as claimed in Claim 1, wherein the particles are contained in an electrically insulating carrier.

3. A process as claimed in Claim 2, wherein the carrier is a liquid.

4. A process as claimed in Claim 2 or Claim 3, wherein the carrier also contains electrically photosensitive cyan and magenta particles so that polychrome image formation is obtained on at least one of the electrodes.

5. A process as claimed in any one of the preceding claims, wherein the particles have an average size of from 0.01 to 20 microns.

6. A process as claimed in any one of Claims 1 to 4, wherein the particles have an average size of from 0.01 to 5 microns.

7. A process as claimed in Claim 1 substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.

8. A process as claimed in Claim I substantially as hereinbefore described in any one of Examples 1 to 4.

9. A process as claimed in Claim 1 substantially as hereinbefore described in Example 6.

10 An image whenever recorded by a process as claimed in any one of the preceding claims.

11. A photoelectrophoretic migration imaging dispersion comprising a carrier and electrically photosensitive particles wherein at least a portion of the particles comprise trans-epindolidione.

12. A dispersion as claimed in Claim 11, wherein the carrier is a liquid.

13. A dispersion as claimed in Claim 11 or Claim 12, wherein the electrically photosensitive particles also comprise electrically photosensitive cyan and magenta particles.

14. A dispersion as claimed in any one of Claims 11 to 13, wherein the particles have an average size of from 0.01 to 20 microns.

15. A dispersion as claimed in any one of Claims Il to 13, wherein the particles have an average size of from 0.01 to 5 microns.

16. A dispersion as claimed in Claim 11 substantially as hereinbefore described in any one of Examples 1 to 4.

17. A dispersion as claimed in Claim 11 substantially as hereinbefore described in Example 6.