EP0194776A2

EP0194776A2 - Multicolor toner images in electrography

Info

Publication number: EP0194776A2
Application number: EP86301263A
Authority: EP
Inventors: Gregory L. C/O Minnesota Mining And Zwadlo; Kevin M. C/O Minnesota Mining And Kidnie
Original assignee: Minnesota Mining and Manufacturing Co
Current assignee: 3M Co
Priority date: 1985-03-07
Filing date: 1986-02-21
Publication date: 1986-09-17
Anticipated expiration: 2006-02-21
Also published as: ZA86851B; KR940004213B1; DE3681942D1; KR860007563A; AU5298886A; EP0194776B1; CA1251827A; AU581957B2; JPS61205954A; DK105986A; EP0194776A3; DK105986D0

Abstract

There is disclosed here a method of transferring an assembly of overlaid color toner images from a primary electrostatic imaging surface (e.g., a photoconductor) to a receptor sheet by embedding and encapsulating the toner particles in a transparent binder. The transfer is accomplished in one step with no loss in image quality, either in color or in resolution, and presents a glossy final result. The final encapsulated image is also protected from abrasion and contamination.

Description

BACKGROUND TO THE INVENTION

Field of Invention

The invention relates to successively developed images and most preferably colored images composed by overlaying two or more separate and/or differently colored toned images, the total composite being subsequently transferred from the primary image forming surface to a receptor surface. In the most general form of the invention the toners may differ in readable properties other than color. The invention particularly concerns methods improving the efficiency of the transfer step and the quality of the resulting images.

Background of the Art

Multicolor toner images produced by successive toner transfer from a photoconductor to a single receptor are well known in the art both for powder toners with constituents intended to improve resolution on transfer and for use with magnetic brush development (U.S. 3,833,293). US 3,612,677 discloses a machine designed to provide good registration when using successive color image transfer, and US 3,804,619 discloses special powder toners to overcome difficulties toners have in 3 color successive transfer.
The production of multi-colored images by overlaying toned images on a photoconductor surface is also known. Thus US 3,337,340 discloses liquid developers designed to minimize the "bleeding away of charge on the photoconductor surface" which occurs when recharging of an already toned surface is attempted. U.S. 4,155,862 and U.S. 4,157,219 disclose liquid toner formulations and apparatus for producing multicolor composite toned images on a photoconductor surface. U.S. 4,275,136 emphasizes the difficulties in ensuring that overlaid toner layers on a photoconductor adhere to one another. The addition of zinc or aluminum hydroxides coated on the colorant particles is used to solve the problem. No transfer of composite images is disclosed in these references.
Many methods are used to aid the efficient transfer of toner from a photoconductor surface after toner development to a receptor sheet. U.S. 3,157,546 discloses overcoating a developed toner image while it is still on the photoconductor. A liquid layer having a concentration of about 5% of a film-forming material in a solvent is used at between 10 and 50 microns wet thickness. After drying, transfer is carried out to a receptor surface which has a mildly adhesive surface. Defensive Publication T879,009 discloses a liquid toner image first developed on a photoconductor and then transferred to a receptor sheet whose surface is coated with a polymer layer easily softenable by residual solvent in the developed image which thus adheres the image to the receptor surface. U.S. 4,066,802 discloses the transfer of a multitoned image from a photoconductor, first to an adhesive carrier sheet, and then to a receptor. The second stage involves the application of heat and pressure with a "polymeric or plasticizing sheet" between the image on the carrier sheet and the receptor surface. U.S. 4,064,285 also uses an intermediate carrier sheet which has a double coating on it comprising a silicone release layer underneath and a top layer which transfers to the final receptor with the multicolor image and fixes it under the influence of heat and pressure. U.S. 4,337,303 discloses methods of transferring a thick (high optical density) toned image from a photoconductor to a receptor. High resolution levels of the transferred images are claimed ( 200 1/mm). It is required to dry the liquid toned image and encapsulate the image in a layer coated on the receptor. Curing of the encapsulating layer is required with some formulations. The materials of this layer are chosen to have explicit physical properties which provide not only complete transfer of the thick toner image but also ensure encapsulation of it.
U.S. 4,477,548 teaches the use of a protective coating over toner images. The coating is placed on the final image and is not involved in any image transfer step. The coating may be a multifunctional acrylate, for example.
Transfer of certain types of composite multitoned images is disclosed in the art. U.S. 3,140,175 deposits microbeads containing a dye and a photoconductor on one electrode, exposes them through a colored original and then applies field between a first and second electrode causing separation of charged and uncharged beads and transfer of the colored image to a receptor surface at the second electrode. U.S. 3,376,133 discloses laying down different colored toners sequentially on a photoconductor which is charged only once. The toners have the same charge as that on the photoconductor and replace the charge conducted away in image areas. However, it is disclosed that subsequent toners will not deposit over earlier ones. The final image of several toners is transferred to a receptor and fixed. U.S. 3,862,848 discloses normal sequential color separation toned images transferred to an intermediate receptor (which can be a roller) by "contact and directional electrostatic field" to give a composite multitoned image. This composite image is then transferred to a final receptor sheet by contact and a directional electrostatic field.

SUMMARY OF THE INVENTION

Images are formed by successive charging and toning of at least two electrostatic images on a temporary image sheet. Preferably each toning is effected with a toner absorbing radiation in a different portion of the electromagnetic spectrum than toner used in any other toning step, forming a composite image comprising at least two toners on said temporary image sheet, contacting said composite image with a transparent polymeric resin binder, pressing said composite image in contact with said binder against a receptor sheet with sufficient pressure to encapsulate substantially all of said toner in said binder, releasing said pressure, and removing said receptor sheet along with the binder having the composite image encapsulated therein from said temporary image sheet. The same toner may be used in these sequences to provide a composite of information on a single sheet, or the toners may differ in their mechanically readable properties by other than color differences. For example, the toners may absorb differing wavelengths of radiation outside the visible spectrum. Magnetic properties, luminescence and conductivity differences may also provide the basis for mechanically differentiable properties that can be read.
The image of at least two toners on the temporary image sheet may be contacted with the binder in a number of ways. For example, the binder may already exist as a surface layer on the receptor sheet and the toner image is brought into contact with that surface layer. The binder may also be applied as a separate layer on the toner image (e.g., by coating from a liquid composition). A film of the binder may also be laid over the toner image or between the toner image and the receptor sheet.
A composite multicolored image is produced by overlaying on a primary imaging surface a succession of liquid toned images of differing colors produced by separate charging, exposing and toning procedures. The primary imaging surface may be a photoconductor addressed with an optical image or a charge retaining surface addressed with electrical styli. The entire composite toned image is transferred to a receptor sheet by techniques which result in the toner particles being embedded in and completely encapsulated by a transparent binder yet retaining the high color quality and resolution stemming from the liquid toners used.
The term "encapsulate" in the present invention means the complete immersion of substantially all of a toner particle in the binder so that no part of it projects through the binder surface. There should be substantially no voids or air spaces between the binder and the toner particle surface so that effectively there is optical contact between the two. It is also desired that substantially all of the toner particles (> 95%) should be completely immersed in the binder. The term "substantially all" means that less than 5% of the total volume of toner per square centimeter projects through the free surface of the binder after transfer to the receptor. Preferably less than 2% of the total volume of the toner projects through the free surface of the binder.
The overlay of several toner images (commonly 3 or 4) results in a thick composite of toners in certain areas and little toner in others so that the encapsulating procedure must be able to accommodate thick toner layers. The encapsulating materials are chosen with physical properties explicity suitable to this purpose. Two main embodiments are disclosed:

A) The composite toner layer on the primary imaging surface is coated with an encapsulating layer of a film-forming transparent binder. After drying, this binder layer is contacted with a receptor sheet to which it transfers when pressure and optionally heat is used. The receptor surface is optionally coated with a thin layer of pressure sensitive adhesive. The primary imaging surface optionally, but need not be, advantageously coated with a silicone release layer to ensure complete release of the toner, but choice of the binder material can also ensure the required complete release.
B) The encapsulating material is present as a binder layer already on the receptor _I surface. This surface is brought into contact with the composite toner image under heat and pressure, and encapsulated transfer occurs. Again the primary imaging surface may advantageously be coated with a silicone release layer.

Particularly in cases where heat and pressure transfer is used, no further fixing of the transferred image is required. Transferred images are of high gloss and show good color purity, high resolution, and high maximum density capability. The encapsulation process also provides some protection against abrasion and chemical contamination of the image.
It is one aspect of the invention to provide a method of complete transfer of a multitoned image from an electrographic image surface to a receptor surface.
It is a further aspect of the invention to provide a method to carry out such a transfer with no loss in sharpness or color rendition.
It is yet another aspect of the invention to provide a method for transferring and heat fixing a multitoned image to a receptor surface to give a high gloss result.
The invention finds utility in a wide range of applications where multicolored toner images are assembled by overlaying on an electrographic surface. Examples are color proofing for the printing industry, colored map making and colored overhead transparencies.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides a method for the efficient and complete transfer of a multitoned image from an electrographic imaging surface to a receptor surface.
The term multitoned image means an image formed by successive overlaying of two or more toners which are differentially readable by mechanical means, using for example, light absorption, UV or IR absorption, magnetic properties, conductivity, luminescence, etc. For a preferred embodiment, the toners are distinguishable from one another by color differences. The term color is inclusive of radiation within 200 nm of the visible portion of the spectrum which can be mechanically distinguished. This includes the near infrared and near ultraviolet. The preferred embodiment uses three or four toners for the color reproduction of natural color scenes, but the transfer of two or more color content images are contemplated in the practice of the present invention.
The invention relates to a method of transferring multitoned images from an electrographic surface to a receptor surface by encapsulating the image in a surface receptor layer of a film-forming binder which is substantially transparent to visible light or to other radiation (near UV, near IR) which may be used to read the final image.
The electrographic surface may be a photoconductor or a dielectric surface suitable for receiving and retaining charge (e.g., from an electrostatic stylus). Photoconductors may be chosen from inorganic types such as selenium and its alloys, zinc oxide and lead oxide dispersions, cadmium sulfide to antimony sulfide or from organic materials such as phthalocyanine pigments, polyvinyl carbazoles, and particularly bis-benzocarbazolyl phenylmethane as disclosed in U.S. 4,361,637. Particularly in the case of photoconductors, these surfaces may be colored or opaque. Even organic photoconductors may have a substantial color. Such colored materials are unsuitable as the final image carrying surface particularly when natural colored images are required. Transfer of the images to a suitable receptor surface such as paper, clear plastic, light diffusing plastic, glass, etc. is therefore important to the final quality of the image.
Apart from its film-forming and transparent properties, the encapsulating binder should have the following properties:

a) releasable from the electrographic surface under heat/pressure
b) adhesive to the receptor surface under heat/pressure
c) should completely embed and encapsulate the toner particles of the image under heat/pressure without disturbing the image
d) should encapsulate a wide range of toners including those which are not capable of fusible fixing to a receptor surface.

Advantageous properties for the binder include a glossy finish after transfer, and capability to receive an embossed surface finish, both of which are aided by thermoplastic properties.
In the preferred.embodiment, liquid toned multitoned images are used because of their high resolution and good tone gradation. Liquid toners can have very small particle sizes (< 1 micrometer) and the embedding and encapsulating of such small particles without disturbing the image puts high demands on the binder.
Two main modes for the method of transfer are disclosed here. In Mode A, the multitoned electrographic element surface is coated with a film-forming binder from a suitable solvent. After evaporating the solvent the layer of encapsulated toner is transferred to the receptor surface by heat and pressure. In the other Mode B, the film-forming binder is first coated and dried on the receptor surface. Transfer is again accomplished by heat and pressure between the toned electrographic surface and the coated receptor surface.
In both modes the surface of the electrographic material optionally may be coated with a release layer such as a film-forming silicone, before the imaging process begins.
Selection of suitable binders for Mode A. use the following principles. The filler/particulate interaction of the materials of this invention allow complete encapsulation of even thick deposits of very fine particulate. The dispersion or solution of the filler material is able to completely wet the toner particles for this encapsulation without interacting with the photoconductor surface. The use of a thick film of binder (3-100 micrometers) allows for complete transfer to even very rough surfaces, such as plain paper. These binder materials have advantages in the process since they are very similar in properties to toner dispersions and therefore highly compatible with the toners.
Examples of binder/solvent systems suitable to this mode of the transfer process are acrylic resin dispersions in cycloaliphatic solvents, e.g., cyclohexane, polyvinyl alcohol dispersions in methanol/water, and poly(vinyl acetal) resins (e.g., polyvinyl butyral, Butvar™; dissolved-in lower alcohols.
Criteria for binders suitable to use in Mode B are presented in U.S. 4,337,303. Transfer by adhesion depends on the surface characteristics of the receptor coatings, and specific receptor-toner interactions: it usually requires a "tacky" material. On the other hand, encapsulation depends on bulk mechanical properties of the material comprising the receptor coating, and according to the present invention this material should be a viscoelastic liquid under the conditions of transfer. As defined by Ferry [Viscoelastic Properties of Polymers, 2nd Ed., New York, Wiley (1970) p. 18], such materials may yet possess sufficiently high viscosities so as to appear to be solids cs semi-solids, but may be recognized rheologically by a hign value of the loss tangent, i.e., tan is much greater than 1 (Ferry, pp. 49 ff), preferably greater than 10 and Newtonian complex dynamic melt viscosity less than about 1,7 x 10³ poise.
Rheological evaluation of receptor materials wherein toner transfer occurred by adhesion or by encapsulation is carried out on a Rhoemetrics, Inc. mechanical spectrometer. This instrument is calibrated to yield rheological functions in agreement with the standard published ones as described in Meissner, Pure and Applied Chemistry, 42, pp. 575-7 (1975), for the IUPAC standard low density polyethylene sample A.
Examples of suitable binders are epoxy end-capped polyethers (e.g. Shell Chemical Co. Epan" 1001 and 1007) and copolymers of medium molecular weight polymethyl methacrylate with triethylene glycoldiacrylate.
In both modes of practice of the invention, the dry thickness of the receptor layer should be in the range 3 micrometers to 100 micrometers and preferably in the range 10 micrometers-to 50 micrometers. If the layer is too thin, it cannot effectively encapsulate the thick composite layers of toners, and loss of toner in the image results. If the layer is too thick, the toners in the process of encapsulation during transfer have more ability to move their position and can cause edge movement in the image with possible color fringing. With the correct choice of layer thickness and material the transferred image can retain resolving power levels up to 200 1/mm.
Liquid toners are well known in the art. To varying degrees, all liquid toners can be used. As is known in the art, the charge pattern for each previous toner image should be discharged prior to laying down a charge pattern for the next toner image. Because the toner images tend to be very thin, this is usually easily accomplished even through the toner itself. It can be relatively conductive as the conductivity of the toner will enable easier discharge through the image.
Drying of the applied liquid toner image provides significant advantages to the process. The actual process step of drying may, however, cover a range of degrees of removal of liquid carrier from the applied toner image. As toner compositions vary significantly in their components, there is no single operative characterization that can be made to describe the optimum drying conditions or the optimum degree of drying. Some general remarks can be made on the subject however.
It is generally better to remove more liquid components from the liquid toner during the drying process than to effect only incidental drying. That is, whatever the percentage of liquid in the toner as applied to the substrate, the greater the percentage of liquid removed, the better the effects upon the imaging process. For example, different deposited toner images may comprise from 90-10% liquid carrier when applied. Different percentages of this liquid should be removed in order to optimize drying. In some instances removal of at least 75% of the carrier liquid may be sufficient. In other toners, removal of more than 95% of the liquid must be effected. Generally then, at least 75% of the carrier liquid should be removed before application of pressure and/or heat. Preferably at least 85%, more preferably at least 95%, and most preferably approximately 100% (greater than 99%) of all original carrier liquid should be removed during the drying process. A range of 75 to 100% of the liquid is generally removed prior to application of pressure, usually 85-100%, more preferably 95-100%.
A few physical procedures can be performed to assist in determining optimum drying conditions. For example, one test which is used is to first dry the applied toner, then apply a clear liquid (consisting of the liquid used as the carrier in the toner) and then quickly apply shear force to the dried image, e.g., resulting from flow of the liquid over the dried image at a speed of 5 cm/sec. If the image of a 1 mm dot is smeared or distorted to increase its dimension in the direct of shear by more than 2%, then it is less than optimally dried. The test must be run with a minimum dwell time of the clear liquid on the dried image, as for example about 5 seconds or less.
Some liquid toners change their reflective characteristics during drying. For example, when applied and during drying, the liquid toner image remains highly reflective. Once optimum drying has been achieved, the image has a matte appearance. Reflectivity is reduced by at least 25% and some times by at least 40% in this optical change during drying. This evaluative technique tends to be dependent upon the individual characteristics of the toner and is not universal to all toners.
The temperature of transfer according to the process of the present invention is defined as a temperature below l8µ°C. It is preferred that the transfer process occurs at temperatures up to 130°C. (above which temperature typical support materials, e.g., polyester films, tend to soften and deform); it is most preferred that the range 20°-70°C. be used, both to conserve energy and to limit the extremes of temperature to which the receptor or photoreceptor, on which the image is originally developed, is subjected. Amorphous selenium, a photoconductor of choice for many applications, crystallizes when heated above 65°C., thereby forfeiting its photoconductive properties. Other useful photoconductors, such as amorphous chalcogenides, or dispersions of inorganic pigments, such as lead oxide, are also damaged when subjected to high pressures, as is necessary in some toner transfer techniques of the prior art. For example, transfer of toner to a thermoplastic receptor by the adhesive mechanism requires typically the application of pressure of 50 to 150 kg/cm²; similar forces are required for the pressure fusing of dry toner deposits. On the other hand, in carrying out the process of the present invention, the toner is encapsulated on application of, typically, 0.3 to 5 kg/cm² although a pressure range of 0.1 to 50 kg/cm² may be used. Generally a range of 0.1 to 20 kg/cm² is preferred.
The invention will now be illustrated by the following examples.

Example 1

(Mode A.)

A photoreceptor comprising 40 parts of bis-(N-ethyl-1,2-benzocarbazol-5-yl) phenylmethane (hereinafter BBCPM) as disclosed in U.S. 4,361,637, and 60 parts of poly 4,4-isopropylidene diphenylene carbonate coated as about a 10 micrometer thick layer as a charge generating layer on a support was topcoated with a 1-1/2% solution in heptane of Syl-off 23 as a release layer and dried to give a release layer coating weight of 0.04 g/_m ².
The photoreceptor layer was positively charged, exposed to a suitable imaging light, and developed, sequentially with Panacopy PAKU-SSTK yellow, cyan, and magneta liquid toners^*, designated here Y-1, C-l, and M-1 respectively, to give a full color image on the photoreceptor. 100g of acryloid NAD 10 from Rohm & Haas (a thermoplastic acrylic resin dispersion in naphtha) was mixed with 175g of cyclohexane. This dispersion was coated onto the toned photoreceptor surface at a thickness of about 33 micrometers and dried. No image distortion was observed. The photoreceptor was contacted to a sheet of conventional printing paper through a hot roller at 110°C and a pressure of 3 kg/cm² and at a speed of 5.0 cm/sec and separated. Complete transfer of the toned image in registration was obtained without distortion. The photoreceptor was reusable and no additional fixing of the image was required. Thinner coatings of the encapsulating material resulted in intermittent transfer and thicker amounts did not provide any advantage.

^*Toners: Y-1 azo pigment CI 21105 in polymethacrylate binder
C-1 Phthalocyanine pigment CI 74160 in a polyester binder
M-1 Pigment CI 4516:1 in a modified rosin binder (i.e., a natural resin binder which is esterified or hydrogenated to thicken it)

Example 2

(Mode A.)

An encapsulated image was made on the photoreceptor as in Example 1. A conventional printing paper was coated with a thin layer of polyacrylate pressure adhesive and contacted to the image area. Upon separation, all the toner image transferred to the paper. The thick dispersion film eliminated any tackiness in the untoned areas.

Example 3

(Mode A.)

Cyan (C-2) and Magenta (M-2) toners were developed and encapsulated as in Example 1. Primed polyester was contacted through a hot roller to the image and complete transfer of the image was obtained with good transparency.

Toner C-2

A pigment dispersion was prepared by first mixing

4 g Cyan colorant CI 74160 (Dupont),
2 g Cobalt oleate (Mooney Chemicals), and
300 mL isodecane, Isopar G™ (Exxon Co.) and mixing separately a binder composition of
5 g ethylene/vinyl acetate copolymer AC405 (Allied Chemicals),
4 g polyterpene resin, Z7115 (Arizona Chemical Co.), and
100 mL cyclohexane

Toner M-2

Pigment dispersion prepared by mixing in a Silverson disperser

4.5g Magenta colorant CI 15865 (Dupont)
3 g Calcium neodecanoate, Ca Ten Cem" (Mooney Chemicals)
300 mL isodecane, Isopar G" (Exxon Co) mix separately
3g ethylene vinyl acetate copolymer, AC 405 (Allied Chemicals)
3g polyterpene resin, Z7115 (Arizona Chemical Co.)
100 mL cyclohexane (warmed)

Example 4

(Mode A.)

An image was made as in Example 1. The same acryloid dispersion was coated onto the photoconductor while Krome Kote printing paper (clay treated coated stock for color printing) was contacted while wet to the surface using 0.338 kg/cm² pressure without heat. After drying, the paper was separated from the photoconductor. All the toner was firmly encapsulated in the binder transferred to the paper resulting in a full color, high quality image.

Example 5

(Mode A.)

A BBCPM/polycarbonate photoreceptor as in Example 1, but without a release layer was multicolor imaged as in Example 1. A dispersion of polyvinyl alcohol in methanol/water was used in place of the acryloid dispersion. Transfer was accomplished as in example 2.
Upon separation, complete transfer of the image was obtained without having to add any release properties to the photoreceptor with special coatings.

Example 6

(Mode B.)

1.5g of Irgacure 651 (a cationic photocuring agent by Ciba-Geigy) was dissolved in 50g butyl acetate. 50g of a 60% solution of an epoxy oligomer (the reaction product of DER 332, itaconic acid and acrylic acid in a molar ratio of 4:3:2 in butyl acetate) was added. This solution was coated onto Matchprint® (3M) positive base paper using a #28 wire wound rod to give a dried coating thickness of about 18 microns. The coating was allowed to dry overnight and was relatively tack-free at room temperature. An organic photoconductor consisting of

40 parts bis-(N-ethyl-1,2-benzocarbazol-5-yl)phenyl methane
50 parts binder, Makrolon 5705 (high molecular weight polycarbonate resin)
9.5 parts polyester, Vitel 222
0.5 parts red sensitizing dye (Electron accepting cyanine dye)

²

Claims

1. A process for electrographic multitoned image transfer comprising the steps of

a) producing on the surface of an electrographic sheet a liquid toned image comprising at least two different toners absorbing radiation in different portions of the electromagnetic spectrum,

b) contacting said surface with a receptor layer comprising a thermoplastic film-forming binder having a dry thickness in the range of 3 to 100 micrometers.

c) applying between said electrographic sheet and said receptor layer a pressure of between 0.1 kg/cm² and 50 kg/c_m ² at a temperature in the range 20°C to 130°C,

d) releasing the pressure, and

e) separating said receptor layer from said one surface of said electrographic sheet, said receptor layer carrying said liquid toned image embedded in it and encapsulated by it.

2. A process as recited in claim 1 wherein said pressure is between 0.3 kg/cm² and 5 kg/cm², and said liquid toned image is dried prior to the application of pressure between said electrographic sheet and said receptor layer.

3. A process as recited in claim 1 wherein said receptor layer is contacted to said one surface of said electrographic sheet in step (b) by coating onto it a solution of said thermoplastic film-forming binder in a suitable solvent, drying off said solvent, and then contacting said receptor layer with a support surface.

4. A process as recited in claim 1 wherein said receptor layer is permanently adhered to a support surface.

5. A process as recited in claim 1 wherein said one surface of said electrographic sheet comprises a thin release layer of a film-forming silicone.

6. A-process as recited in claim 2 wherein said thermoplastic film-forming binder is chosen from the group consisting of acrylic resin dispersions, polyvinyl alcohol, and poly(vinyl acetal) resins.

7. A process as recited in claim 6 wherein said solvent is chosen from the group consisting of cycloaliphatic solvents, C_l-C₆ alcohols, and aqueous alcohol.

8. A process as recited in claim 4 wherein the material of said thermoplastic film-forming binder has a Newtonian complex dynamic melt viscosity of less than about 1.7 x 10³ poise and a loss tangent greater than 10 at the temperature of transfer.

9. A process as recited in claim 3 wherein said support surface comprises one of two major surfaces of a support sheet comprising paper, clear plastic, light diffusing plastic, glass, or opaque plastic.

10. A process as recited in claim 3 wherein the material of said thermoplastic film-forming binder is chosen from the group consisting of photo-cured epoxy oligomers.

11. A process for electrographic multitoned image transfer comprising the steps of

a) producing on the surface of an electrographic sheet a liquid toned image comprising at least two different toners which are differentially readable by mechanical means,

c) applying between said electrographic sheet and said receptor layer a pressure of between 0.1 kg/c_m ² and 50 kg/c_m ² at a temperature in the range 20°C to 130°C,

d) releasing the pressure, and

12. A process as recited in claim 11 wherein said pressure is between 0.3 kg/cm² and 5 kg/c_m ².

13. A process as recited in claim 11 wherein said receptor layer is contacted to said one surface of said electrographic sheet in step (b) by coating onto it a solution of said thermoplastic film-forming binder in a suitable solvent and drying off said solvent, and then contacting said receptor layer with a support surface.

14. A process as recited in claim 11 wherein said receptor layer is permanently adhered to a support sheet.

15. A process as recited in claim 11 wherein said one surface of said electrographic sheet comprises a thin release layer of a film-forming silicone.

16. A process for electrographic multitoned image transfer comprising the steps of

a) producing on the surface of an electrographic sheet a first liquid toned image,

b) producing on said surface with a toned image a second liquid toned image on said first liquid toned image,

c) contacting said surface with a receptor layer comprising a thermoplastic film-forming binder having a dry thickness in the range of 3 to 100 micrometers.

d) applying between said electrographic sheet and said receptor layer a pressure of between 0.1 kg/cm² and 50 kg/cm² at a temperature in the range 20°C to 130°C,

e) releasing the pressure, and

f) separating said receptor layer from said one surface of said electrographic sheet, said receptor layer carrying said liquid toned image embedded in it and encapsulated by it.

17. A process as recited in claim 16 wherein said pressure is between 0.3 kg/cm² and 5 kg/cm², and said first and second liquid toned images are dried prior to the application of pressure between said electrographic sheet and said receptor layer.

l8. A process as recited in claim 16 wherein said receptor layer is contacted to said one surface of said electrographic sheet in step (b) by coating onto it a solution of said thermoplastic film-forming binder in a suitable solvent and drying off said solvent.

19. A process as recited in claim 16 wherein said receptor layer is permanently adhered to a support surface.

20. An apparatus for the formation of electrographic multitoned images comprising

a) means for establishing a charge on an electrophotographic surface,

b) means for imagewise removing charge on said electrophotographic surface,

c) means for producing on said electrophotographic surface a liquid toned image comprising at least two different toners absorbing radiation in different portions of the electromagnetic spectrum,

d) means for contacting said electrophotographic surface with a receptor layer comprising a thermoplastic film-forming binder having a dry thickness in the range of 3 to 100 micrometers.

e) means for applying between said electrographic sheet and said receptor layer a pressure of between 0.1 kg/cm² and 50 kg/cm² at a temperature in the range of 20°C to 130°C,

f) means for separating said receptor layer from said electrographic surface so that said receptor layer carries said liquid toned image embedded in it and encapsulated by it.