DE68922214T2 - Electrostatic process for producing multicolor images from a single toner bath. - Google Patents

Description

This invention relates generally to electrostatic imaging systems and, more particularly, to a method for providing a multi-color image from a single toner medium using microencapsulated toner.

Conventional electrostatic multi-color imaging systems use a separate toner bath to develop each desired color. This use of separate toner baths is relatively expensive from the standpoint of equipment complexity, cost, maintenance, and extended processing time. It also requires multiple mechanical registrations to produce the multi-color image - a requirement fraught with the potential for errors.

As an alternative to the use of toners and electrostatic imaging, a recent development in the industry uses an imaging paper sheet that is completely coated on one side with microencapsulated color precursors. A portion of the microcapsules on the sheet are selectively cured by exposure to light. The microcapsules containing the desired color precursors in the image areas have liquid cores which remain uncured. These uncured microcapsules are then broken to release liquid color precursor. The color precursor thus released is contacted with a color developer to provide the color image, generally by transferring it to a developer sheet by pressure contact of the imaging sheet with the developer sheet. Alternately, the capsules containing color precursors are applied directly to a layer of developer material, which itself had previously been applied to a paper carrier.

By way of illustration, one such transfer imaging system containing microencapsulated dye precursors is disclosed in U.S. Patent No. 4,554,235 to Mead Corporation. In a variation of this type of system, U.S. Patent No. 4,501,809 to Mitsubishi Paper Company discloses a recording sheet containing two different types of light and pressure sensitive microcapsules - one set containing dye precursors and the other set containing dye developers. Upon rupture of uncured microcapsules on the recording sheet after selective exposure of the recording sheet to light in imagewise registration with an image to be copied, a dye image is formed on the recording sheet.

The color recording systems illustrated by the above-cited patents share a common disadvantage. Both systems use an imaging or developer sheet that contains microcapsules over a full surface of the sheet. Since in many color imaging applications the desired color image rarely fills the entire sheet and in fact often fills less than half of the full sheet, there is a substantial amount of waste attributable to the unused microcapsules and associated color precursors or developers contained in the unimaged areas of the sheet. In addition, there is a substantial waste of time and energy attributable to the need to photocure the "unused" waste microcapsules using, for example, a scanning laser.

In view of the above, a new system for multi-colour imaging using microcapsules would be highly desired by the colour imaging community, which does not require a such substantial waste of microcapsules and associated coloring materials, plus wasted time and energy due to the need to photocure waste microcapsules.

In one aspect, the present invention relates to a color imaging method comprising the steps of:

(a) forming a latent image on a photoreceptor substrate in any of a variety of known ways, for example by depositing a charge on a photoconductor and imagewise discharging or imagewise depositing a charge on a dielectric material,

(b) electrostatically depositing a toner mixture onto a charged or uncharged surface of the substrate to produce a toner image which is a positive or reversal image compared to the latent image, the toner mixture comprising at least two different toners, each toner comprising a different color precursor contained in photosensitive toner particles,

(c) selectively photohardening or photosoftening at least a portion of the toner particles by imagewise exposure to appropriate radiation wavelengths to provide harder toner particles and softer, breakable toner particles,

(d) transferring the harder toner particles and the fragile toner particles to a copy surface,

(e) breaking at least a portion of the toner particles on the copy surface to release (a) color precursor from the breakable toner particles, and

(f) contacting the color precursor(s) on the copy surface with a developer to form a color image on the copy surface.

In a further aspect, step (d) of the above method is effected prior to performing step (c). It is also within the scope of the invention to perform step (e) prior to step (d), as well as there being other orders of steps.

In yet another aspect, the present invention relates to a developer-bearing microcapsule toner particle comprising a shell and a core, the shell being made of a polymer and the core comprising a colorless chromogenic material as a color precursor and additionally containing a radiation sensitive material, the chromogenic material being capable of becoming colored upon contact with a developer, the shell having a charge characteristic which renders the toner particle electrostatically depositable, the shell having a developer on the outer surface thereof, the shell of the microcapsule being breakable to release the chromogenic material, whereby the chromogenic material is brought into contact with the developer and reacted to produce a color image.

In yet another aspect, the present invention relates to a mixture of the above developer-bearing microcapsule toner particles, the mixture comprising at least two, but usually three or four types of toner particles, each of the types containing a different color precursor (preferably selected from the group consisting of cyan, yellow, magenta and optionally additionally black), each of the particle types additionally containing a photosensitive composition which is photosensitive at wavelengths different from the wavelengths of photosensitivity of the photosensitive composition contained in each of the other types of particles in the mixture.

In yet another aspect, the present invention includes a process for producing the above developer-bearing microcapsule toner particles, which comprises coating a microcapsule toner particle with a color developer by contacting a microcapsule toner particle with an acidic or complexing developer.

In yet another aspect, the present invention relates to a color imaging method comprising the steps of:

(a) forming a latent image on a photoconductive or dielectric substrate in any of a variety of known ways, for example by depositing a charge on a photoconductor and imagewise discharging or imagewise depositing a charge on a dielectric material,

(b) electrostatically depositing a toner mixture onto a charged or uncharged surface of the substrate to form a toner image which is a positive or reversal image compared to the latent image, the toner mixture comprising at least two different toners, each toner comprising a color precursor contained in photosensitive toner particles, the toner particles containing a developer on the surface thereof,

(c) selectively photohardening or photosoftening at least a portion of the toner particles by imagewise exposure to appropriate radiation wavelengths to provide harder toner particles and softer, breakable toner particles,

(d) transferring the harder toner particles and the fragile toner particles to a copy surface and

(e) fracturing at least a portion of the toner particles on the copy surface to release color precursor(s) from the frangible toner particles, whereby the developer is brought into contact with the released color precursor(s) and reacted to form a color image on the copy surface.

In yet another aspect, the present invention relates to a color imaging method comprising the steps of:

(a) forming a latent image on a photoconductive or dielectric substrate in any of a variety of known ways, for example by depositing a charge on a photoconductor and imagewise discharging or imagewise depositing a charge on a dielectric material,

(b) electrostatically depositing a toner mixture onto a charged or uncharged surface of the substrate to produce a toner image which is a positive or reverse image compared to the latent image, the mixed toner composition comprising at least two different toners, each of the toners comprising a color precursor contained in photosensitive toner particles, the toner particles containing a developer on the surface thereof,

(c) selectively photohardening or photosoftening at least a portion of the toner particles by imagewise exposure to appropriate radiation wavelengths to provide harder toner particles and softer, breakable toner particles,

(d) breaking at least a portion of the toner particles on the substrate to release color precursors from the breakable toner particles, wherein the developer is brought into contact with the released color precursor and reacted to produce a color image on the substrate, and

(e) Transferring the colour image to a copy surface.

In yet another aspect, the present invention relates to a color imaging method comprising the steps of:

(a) forming a latent image on a photoconductive or dielectric substrate in any of a variety of known ways, for example by depositing a charge on a photoconductor and imagewise discharging or imagewise depositing a charge on a dielectric material,

(b) electrostatically depositing a toner mixture on a charged or uncharged surface of the substrate to form a toner image which is a positive or reversal image compared to the latent image, the toner mixture comprising at least two different toners, each toner comprising a color precursor contained in photosensitive toner particles, the toner particles containing a developer on the surface thereof,

(c) transferring the colour image to a copy surface,

(d) selectively photohardening or photosoftening at least a portion of the toner particles by imagewise exposure to suitable wavelengths of radiation to provide harder toner particles and softer, frangible toner particles, and

(e) fracturing at least a portion of the toner particles on the copy surface to release color precursors from the frangible toner particles, whereby the developer is brought into contact with the released color precursor(s) and reacted to form a color image on the copy surface.

In accordance with the present invention, it has now been surprisingly discovered that multi-color images can be produced using a single toner medium. The toner medium is a mixture of 2, 3, 4 or more types of color-forming toner particles which are photosensitive. The relative simplicity and economy of this technique suggests that it will provide significant benefits to the community of those working in the field of color imaging systems. Key advantages of this invention include the ability to (a) use a single toner bath for multi-color imaging, (b) selectively limit the use of toner on the imaging or developer sheet to areas on the sheet where an image is desired, and (c) avoid the need for multiple mechanical registrations for multi-color imaging.

The toner composition useful in the present invention is a toner mixture. This mixture contains at least two different types of toner particles to provide at least two (preferably at least three or four) different color precursors. As used herein, the term "toner particles" is intended to refer to any of a variety of particle shapes that can be used to contain or carry and isolate color precursors. Typical examples of particle shapes are microcapsules, microsponges, softenable solid particles, and emulsion micelles. A "toner mixture" or mixture of toners refers to a mixture of different color-producing toner particles or toners that enable multi-color imaging using a single toner mixture.

When total color imaging capability is desired, typically three or four (cyan, yellow, magenta, and optionally black) color precursors are used, with each toner particle preferably containing one color precursor. Other color precursors (e.g. red, green, or blue) may be used if desired. Either a liquid or a dry toner mixture may be used.

The process of the present invention provides the above-described advantageous result by using a multi-step process of color imaging. In the first two steps, a latent image and then a non-color toner image are formed in a typically electrostatic manner on a support, typically a drum, fabric or sheet. In the subsequent steps, the desired color is developed by exploiting the photosensitivity differences of the toner particles containing the individual color precursors. These photosensitivity differences are suitably created by using a different photoinitiator for each individual color precursor used in the toner mixture.

In a typical electrostatic process, the latent image is formed by known means. First, a bulk positive or negative charge is typically applied to a surface of a photoreceptor substrate, suitably a photoconductive drum, web or sheet, by means of a corona. Portions of the surface of the photoreceptor are then selectively discharged. This selective discharge is suitably effected using light (desirably using a laser light source). The surface of the selectively discharged photoconductor contains a latent image on either the charged portions of the surface (in positive development) or on the uncharged portions of the surface (in reverse development). (An alternating method of forming a latent image typically uses an ion generating cartridge or charging head ("writer") to deposit charges on a dielectric substrate to provide charged and uncharged portions of the substrate, as is known in the art.) Once the latent image has been formed on the photoreceptor, a toner mixture having a charge characteristic that is either opposite (for positive development) or equal to (for reverse development) the charge on the selectively discharged photoreceptor is then applied to the surface of the photoreceptor. Typically, the toner mixture is applied to the photoconductive surface from a liquid toner bath or, in the case of dry toner, by means of a magnetic brush. A variety of development methods are useful and known to practitioners in the art. The photosensitive toner image on the photoreceptor is then selectively hardened (i.e., photopolymerized) (or in some embodiments, softened, i.e., photodepolymerized) by exposure to radiation of a specified wavelength. This photopolymerization or photodepolymerization is performed to cause only toner particles containing desired color precursors to be frangible for the release of color precursors. For example, if a yellow image is desired, the toner image will be exposed to wavelengths of light that will harden the toner particles containing cyan, magenta, and black color precursors. Likewise, if a green image is desired, the toner image will be exposed to wavelengths of light that will selectively harden the toner particles containing magenta and black color precursors. All known colors can be produced by exposing toner particles to appropriate wavelengths of light and then completing the imaging process as well. In addition, the deliberate production of partially hardened Toner particles result in intensity variations of the color produced.

The toner image, which is composed of both hardened (or harder) and frangible (or softer) toner particles, is then transferred to a copy sheet by known methods. For example, this transfer is conveniently carried out by passing the substrate to be printed, such as a copy paper sheet or a transparent film, between the photoreceptor and a transfer corona, whereby the toner particles are transferred from the photoreceptor to the copy sheet.

The frangible toner particles of those that make up the color image are, once on the copy sheet, broken up, typically by irradiation, heat, pressure, or combinations of these methods (preferably by pressure), to release the desired color precursors. These desired color precursors are then developed by contacting them with a developer.

In a preferred aspect, the toner particle of the present invention is "color self-developing." As used herein, the term "color self-developing" is intended to mean a microcapsule toner particle which will produce color upon rupture of the toner particle without the need for any external additives. To produce a color image, the microcapsule toner particle is ruptured by any of various means, such as pressure on a surface of a substrate such as a piece of paper, thereby releasing core material from the toner particle and bringing the core material and the developer contained on the outer surface of the shell of the particle into contact. Upon contact of this developer with the chromogenic material contained in the core of the toner particle, such as an electron donating leuco dye, in the region of the ruptured particle color. A desired color image is formed by selectively breaking a plurality of such toner particles in an appropriate combination to produce the desired color.

The color precursors useful in the present invention are preferably oil-soluble color formers which will produce a color by reacting with a developer material in the presence of a carrier oil. Essentially any of the color precursors conventionally used in carbonless paper can be used in the present invention. Generally, these materials are colorless electron-donating types of compounds. Representative examples of such color formers include substantially colorless compounds having a lactone, a lactam, a sulfone, a spiropyran, an ester or an amido structure in their partial skeleton. Triarylmethane compounds, bisphenylmethane compounds, xanthene compounds, thiazine compounds, spiropyran compounds and the like are specific. If desired, mixtures of the respective color precursors can be used.

Some representative leuco dye precursors which give yellow, cyan and magenta images are shown below. Precursors for yellow color: Precursor to cyan color: Precursor to magenta:

The color precursors used in the present invention must be non-absorbent to the exposure to radiation used to cure the photosensitive encapsulant, since the color precursors must either be present in the encapsulant or the exposure radiation must penetrate through the color precursor to expose the encapsulant. Therefore, colorless, electron-donating types of compounds are preferred for use in the present invention. Of course, it is difficult to obtain a completely colorless color precursor, and a small degree of coloration can be tolerated in the color precursors as long as it does not interfere with the exposure.

Developer materials useful in the present invention include those conventionally used in carbonless paper technology and are well known. Illustrative specific examples are clay minerals such as acid clay, activated clay, attapulgite, etc., organic acids such as tannic acid, gallic acid, propyl gallate, etc., acidic polymers such as phenol-formaldehyde resins, phenol-acetylene condensation resins, condensates between an organic carboxylic acid having at least one hydroxy group and formaldehyde, etc., metal salts of aromatic carboxylic acids such as zinc salicylate, tin salicylate, zinc 2-hydroxynaphthoate, zinc 3,5-di-tert-butyl salicylate, oil-soluble metal salts of phenol-formaldehyde novolac resins (e.g. see US Patents 3,672,935, 3,732,120 and 3,737,410), such as zinc-modified oil-soluble phenol-formaldehyde resins and mixtures thereof.

The location of the developer is not very critical and can vary over a wide range as long as the developer is separated from the color precursor until the color precursor is released. For example, the developer can be kept on a separate developer sheet or otherwise outside the toner particle. On the other hand, the developer can be contained within the toner particle in separate mini-microcapsules to maintain separation from the color precursor. In yet another way, the developer material can be coated on individual toner particles, creating self-developing particles.

The toner particles, which comprise what is referred to herein as a "toner mixture" or "mixture of toners", in a preferred embodiment typically have a shell and a core. The core preferably contains the dye precursor and the photosensitive composition. The shell is generally positively or negatively charged and can be made from various materials known in the art. Typical shell materials include, for example, melamine resins, urethanes or urea-formaldehyde. The average size of the particles is generally between about 0.1 and about 100 µm (microns), preferably between 0.5 and 20 µm. For liquid toners, an average toner particle size is suitably between about 0.1 and about 10 µm, whereas a particularly suitable particle size for dry toners is between about 1 and about 20 µm.

In a preferred embodiment, a developer is adsorbed, coated or otherwise bound to the outer surface of the shell of the toner particle.

To prepare the developer-bearing toner particles of the present invention, an aqueous dispersion of tiny microcapsule toner particles is typically combined with an acidic developer or a complexing developer. In a preferred method of preparing the developer-bearing toner particles, an aqueous solution of citric acid is added to an aqueous dispersion of microcapsule toner particles. The concentration of the toner particles in the aqueous dispersion is usually between about 5 and about 50 wt.%, preferably between about 5 and about 25 wt.%, and most preferably between about 10 and about 20 wt.%. The citric acid is suitably added to provide a citric acid concentration of between about 1 and about 10 wt.%, preferably between about 3 and about 7 wt.%, most preferably about 5 wt.%, based on the total weight of the aqueous dispersion plus citric acid. The toner particle dispersion is then spray dried in a suitable manner. The resulting product is a dry, free-flowing powder of toner particles wherein each toner particle is coated with citric acid which acts as an acidic developer.

If desired, the dry toner particles prepared as described above and containing the developer on the individual microcapsules can be dispersed in a non-polar organic solvent such as ISOPAR G or ISOPAR H, products of Exxon Corporation, preferably in conjunction with other toner additives such as dispersants and/or charge directing agents as known in the art, to provide a liquid reprographic toner composition. When such a liquid composition is used, it is preferred that the dispersed particles be in a non-polar organic medium having a low dielectric constant of 3.5 or less and a high electrical resistivity of 10⁹ Ω cm or more. Suitable organic media include n-paraffin hydrocarbons, cycloaliphatic Hydrocarbons, aromatic hydrocarbons, halogenated aliphatic hydrocarbons and preferably isoparaffin hydrocarbons, such as the ISOPAR compounds mentioned above.

In an alternative embodiment, a dry powder of microcapsule toner particles is coated with a developer as follows. First, dry toner particles are suspended in an organic medium such as tetrahydrofuran (also referred to as "THF") to provide a particle concentration of up to about 50% by weight based on the total weight of the particles plus tetrahydrofuran. A developer such as citric acid is then added to the resulting suspension in an amount of between about 0.1 and about 3%, preferably about 1%, based on the suspension plus citric acid. The tetrahydrofuran is then removed by evaporation or under vacuum to provide toner particles with a developer on the surface of each particle.

Typically, the core of the toner particles contains a photocurable, photosensitive composition(s). The viscosity of the core of the toner particles is substantially increased upon exposure to appropriate radiation wavelengths by mechanisms such as cross-linking or polymerization. When the toner particles are broken up, the photosensitive composition that polymerized upon exposure to radiation will flow very little, if at all, whereas the unexposed or dimly exposed photosensitive composition can flow relatively freely. As a direct result, the chromogenic material (i.e., color precursor) reacts with the developer in inverse proportion to the level of exposure to the appropriate radiation wavelength to form the desired color in the desired image area.

In an alternative embodiment, the photosensitive composition may be a highly viscous composition which undergoes a substantial decrease in viscosity upon exposure to actinic radiation of the appropriate wavelength. In this case, the chromogenic material disposed in or on the exposed toner particle is made available to the developer upon breakage of the particle.

The photosensitive composition includes a material that undergoes a change in viscosity upon exposure to light, either alone or in combination with a photoinitiator. The photosensitive composition may be photocurable, such as a monomer, dimer or oligomer that is polymerized to a higher molecular weight compound, or it may be a polymer that is further polymerized, e.g., by crosslinking. Alternatively, it may be a composition that is depolymerized or otherwise made less viscous upon exposure to light. Suitable radiation-curable materials include materials that are curable by free radical-initiated, chain-extended addition polymerization or ionic polymerization.

Representative photosensitive compositions are ethylenically unsaturated organic compounds. These compounds contain at least one ethylenic group per molecule. Typically they are liquid at room temperature and can also act as a carrier oil for the chromogenic material in the toner core. A preferred group of radiation-curable materials are ethylenically unsaturated compounds having two or more ethylenic groups per molecule. Representative examples of these compounds include ethylenically unsaturated acid esters of polyhydric alcohols such as trimethylolpropane triacrylate or trimethacrylate, acrylate prepolymers derived from the partial reaction of pentaerythritol with Acrylic or methacrylic acid or acrylic or methacrylic acid esters, isocyanate-modified acrylate, methacrylate and itaconic acid esters of polyhydric alcohols, etc.

Some typical examples of photosoftenable materials that can be used in other embodiments are photolyzable compounds such as certain diazonium compounds, poly(3-oximino-2-butanone methacrylate) that undergoes main chain scission upon UV irradiation, poly(4'-alkylacylophenones), and certain resins containing a quinonediazide moiety.

In accordance with the invention, photoinitiators are optionally used to selectively photoharden or photosoften the toner particles as desired. The photoinitiator is typically responsive to a specific wavelength and/or amount of actinic radiation or actinic radiation. These, alone or in conjunction with a sensitizer, are compounds which absorb the exposure radiation and generate free radicals with or without the aid of a co-initiator. When a system based on ionic polymerization is used, the photoinitiator can be of the anion or cation generating type, depending on the type of polymerization. Suitable photoinitiators include alkoxyphenyl ketones, Michler's ketone, acylated oximine ketones, polycyclic quinones, benzophenones, substituted benzophenones, xanthones, thioxanthones, halogenated compounds such as polynuclear aromatic chlorosulfonyl and chloromethyl compounds, heterocyclic chlorosulfonyl and chloromethyl compounds, chlorosulfonyl and chloromethyl benzophenones and fluorenones, haloalkanes, halophenylacetophenones, photoreducible dye/reducing agent redox couples, halogenated paraffins (e.g. brominated or chlorinated paraffin) and benzoin alkyl ethers.

The amount of photoinitiator used in the photosensitive composition to effect polymerization (ie photocuring) or depolymerization (i.e., photosoftening) of the photosensitive composition in the toner particles, if used, will depend on the particular photosensitive composition selected, the particular photoinitiator selected, and the desired rate of photocuring or photosoftening. The photoinitiator is preferably used in an amount between about 0.1 and about 30 (preferably between about 1 and about 10) weight percent, based on the total weight of the toner particles.

Other additives may be used in the toner particles, such as carrier oils, e.g., odorless petroleum or alkylated biphenyls. Curing or crosslinking agents may also be used. These are free radical generators, such as thermal initiators, which, when reacting with the photosensitive composition, polymerize or crosslink it. After the composition has been selectively exposed to chemically active radiation and the particles have been broken up in the presence of a developer material, the chromogenic material and the developer react to produce color in the form of an image, the curing or crosslinking agent then reacts with the released photosensitive composition and hardens it, thereby preventing image diffusion or image quality loss. In the case of certain curing or crosslinking agents, it may be desirable to heat the image to accelerate curing. A curing or cross-linking agent is preferably selected which is relatively ineffective at room temperature (for good storage stability) and which can be rapidly activated by heating to temperatures above room temperature.

A particularly useful class of thermal initiators that are reactive with ethylenically unsaturated compounds are organic peroxides. Suitable peroxides include diacyl peroxides, ketone peroxides, peroxydicarbonates, alkyl peroxides, Allyl hydroperoxides and sulfonyl peroxides. Bisazides, perborates and diazo compounds are also useful as thermal initiators.

The process of the present invention is expected to have commercial application in the production of full color prints, transparencies and slides, as well as full color computer generated images and full color xerographic copies.

The following examples are intended to illustrate the present invention without limiting its scope.

EXAMPLE 1 Preparation of individual toners, followed by preparation of a toner mixture and multi-color imaging using the toner mixture (A) (1) Aqueous preparation of toner particles forming blue color

Blue color-forming toner particles photosensitive to near ultraviolet radiation were prepared in water in the following manner. A solution was prepared by dissolving 5.0 g of ethylene-maleic anhydride copolymer (molar ratio 1:1, MW 80,000) and 1.0 g of sodium hydroxide in 45 g of water with stirring and heating at 90°C for 2 hours. Then 100 g of water was added and the solution was cooled to 55°C. The pH was adjusted from 4.3 to 4.0 with 10% sulfuric acid and the temperature was maintained at 55°C until the solution was used. The toner core solution was prepared by first mixing 60.14 g of trimethylolpropane triacrylate (TMPTA) and 16.55 g of methyl methacrylate (MMA). To this was added 4.52 g of COPIKEM IX (a product of Hilton-Davis), a precursor for blue dye, which was Heating to 75ºC and stirring. After the dye precursor was dissolved, this solution was allowed to cool to room temperature. Then 5.20 g of Michler's ketone, a UV sensitive photoinitiator, was added with stirring, which was continued until the photoinitiator dissolved. 37.53 g of CYMEL 385 (a modified melamine-formaldehyde resin, a product of Cyanamid) was heated to about 50ºC.

The ethylene-maleic anhydride copolymer solution was placed in a jacketed mixer heated to 55ºC using circulating water. The mixer power setting was controlled to 40 V using a variable transformer. The core solution was then added and the mixer power setting was increased to 90 V for 45 seconds to finely disperse the core liquid into small droplets. The mixer power was reduced to 40 V and the CYMEL 385 (a modified melamine-formaldehyde resin, a product of American Cyanamid) was added to the mixer. Stirring and heating were then continued at 55ºC for 2 hours.

The blue color-forming toner particles were later isolated as a dry powder by spray drying.

(A) (2) Aqueous production of toner particles forming magenta color

Magenta color-forming toner particles which were photosensitive to blue light were prepared in water in the following manner. A solution was prepared by dissolving 5.0 g of ethylene-maleic anhydride copolymer and 1.0 g of sodium hydroxide in 45.0 g of water by stirring and heating at 85°C for 2 hours. To this was added 100 g of water and the temperature was adjusted to 55°C. The pH was adjusted from 4.27 to 4.00 with 10% sulfuric acid and the temperature was kept at 55ºC until the solution was used. The toner core solution was prepared by first mixing 60.11 g of trimethylolpropane triacrylate and 16.56 g of methyl methacrylate. To this was added 4.52 g of COPIKEM XX (a product of Hilton-Davis), a magenta dye precursor, which was dissolved by heating to 75ºC and stirring. After the dye precursor had dissolved, the mixture was cooled to room temperature and 2.64 g of camphorquinone and 2.37 g of triethanolamine were added. Stirring was continued until the photoinitiator and hydrogen donor dissolved.

The solution prepared from the ethylene-maleic anhydride copolymer was placed in a jacketed mixer which was heated and maintained at 55ºC by circulating water. The mixer power setting was controlled to 40 V by a variable transformer. The core solution was then added and the mixer power setting was increased to 90 V for 45 seconds to disperse the core liquid into small droplets. The mixer power was reduced to 40 V and 37.4 g of CYMEL 385 (a modified melamine-formaldehyde resin, a product of American Cyanamid) which had been preheated to about 50ºC was added to the mixer. Stirring and heating at 55ºC were then continued for 2 hours.

The toner particles forming the magenta color were later isolated as a dry powder by spray drying.

(B) Preparation of the toner mixture and electrostatic photoselective formation of a multi-color image

A liquid mixed toner was prepared by combining 2.0 g of the dry blue color-forming toner powder prepared as in section (A) (1) , 2.0 g of the dry magenta color forming toner powder prepared as in section (A) (2), and 196 g of a low dielectric constant liquid hydrocarbon, ISOPAR G (a product of Exxon Chemical Company). This mixture was first stirred in a beaker and then transferred to a shaker jar and shaken.

A charged latent image was created on an electrostatic paper (a product of Versatec Inc.) by means of a steel piece (1-1/2 inches wide, 3 inches long) connected to a DC power supply set at 750 volts. The electrostatic paper was placed on a flat aluminum base plate and the steel piece, connected to the positive lead or terminal of a power supply, was held in contact with the paper surface for 60 seconds with the power on. The paper was then immersed in the liquid mixed toner. When the paper was removed, an uncolored toner image was visible which exactly corresponded in area and location to the point of contact with the charged steel piece. The toner image on the sheet was allowed to dry at room temperature.

Color image-wise exposure of the non-color toner image was carried out in the following manner (see Table I below). The area where the toner had been deposited was covered with a contact mask (Mask A) which was divided into four areas, with areas 1 and 4 being opaque and areas 2 and 3 being transparent. The mask was covered with a glass bandpass filter (Filter A) (Model No. 51800, a product of Oriel Corporation) which only transmitted light with wavelengths between 225 and 400 nm (UV). The toner image area was then irradiated through Filter A and Mask A with a mercury lamp. Thus, areas 2 and 3 were exposed to light from 225 to 400 nm and areas 1 and 4 were not. Filter A and Mask A were was then removed and the toner image area was then covered by a mask (mask B). Mask B had four areas corresponding to areas 1 through 4 of mask A, except that in mask B areas 1 and 3 were opaque and areas 2 and 4 were transparent. This mask was then covered with a glass long pass filter (filter B) (model no. 51482, a product of Oriel Corporation) which only transmitted light with wavelengths greater than 420 nm. The toner image area was then irradiated through filter B and mask B with the same mercury lamp as before. Therefore, areas 2 and 4 were exposed to light with wavelengths greater than 420 nm and areas 1 and 3 were not. Filter B and mask B were then removed.

In the areas exposed to UV light (225 to 400 nm through filter A), the blue color-forming toner particles were cured because they contained a photoinitiator sensitive to UV light. In the areas exposed to light with wavelengths greater than 420 nm, the magenta color-forming toner particles were cured because they contained a photoinitiator sensitive to blue light.

The toner image area was then brought into contact with a developer sheet (20#, white, NCR Paper (TM) from Appleton Papers, Inc.) and pressure was then applied to break up the toner particles that had not been cured. This resulted in an image that had purple (subtractive combination of blue and magenta), white, magenta and blue areas. The purple color was created in area 1, which was not irradiated during either exposure. Therefore, neither type of toner particle was cured. The white area was created in area 2, which was irradiated by both exposures, curing both types of toner particles. The magenta color was created in area 3, which was irradiated only during the first exposure, so that only the blue color was formed. toner particles were hardened. The blue color was created in area 4, which was only irradiated during the second exposure, so that only the toner particles forming the magenta color were hardened.

The results regarding the color produced for each of the different areas of the image are summarized in Table I below. TABLE I Range Mask A First exposure Filter A Mask B Second exposure Filter B Color former cured Color produced opaque transparent none both Blue Magenta Purple White

It can be seen that the resulting colors included purple, magenta and blue as well as a portion of the image with the white coloring of the paper.

EXAMPLE 2 Proposed example of multi-color imaging using a toner mixture containing microencapsulated precursors for three different colors

In an analogous manner to that described above, multi-color images are produced using a single toner bath composed of a mixture of three different encapsulated toners, each containing either a cyan, magenta or yellow dye precursor. All three toners are prepared from a toner mixture deposited side by side during electrostatic imaging. Each color-forming toner in the mixture contains a specific photoinitiator (or photoinitiator-sensitizer system) sensitive to a given wavelength different from the photoinitiator(s) (generally two or more) contained in the other color toner particles in the mixture. Three or more different lasers, each with a wavelength corresponding to that which initiates a reaction with one of the photoinitiators, are then used to selectively harden toner particles in the toner image. The toned and exposed image is then developed as described in the previous examples to provide a multicolor image. Thus, for example, in a given area, laser irradiation producing light curing of cyan-producing toner particles would result in only the color red in the final image by releasing yellow and magenta, whereas yellow would be produced by an area in which both the cyan-producing toner particles and the magenta-producing toner particles were cured by the appropriate laser exposures. A lamp and filter could also be used in place of the lasers if desired. Imaging could also be done by transmitted or reflected light. Likewise, a black-producing toner could be included in the toner mixture and used in the same way if desired.

EXAMPLE 3 Aqueous production of blue color-forming toner particles

Blue color-forming toner particles which were photosensitive to near ultraviolet rays were prepared in water in the following manner. A solution was prepared by dissolving 5.0 g of ethylene-maleic anhydride copolymer (molar ratio 1:1; MW 80,000) and 1.0 g of sodium hydroxide in 45.0 g of water with stirring and heating at 90°C for 2 hours. Then 100 g of water was added and the solution was cooled to 55°C. The pH was adjusted from 4.3 to 4.00 with 10% sulfuric acid and the temperature was maintained at 55°C until the solution was used. The toner core solution was prepared by first mixing 60.14 g of trimethylolpropane triacrylate (TMPTA) and 16.55 g of methyl methacrylate (MMA). To this was added 4.52 g of COPIKEM IX (a product of Hilton-Davis), a dye precursor for blue, which was dissolved by heating to 75°C and stirring. After the dye precursor was dissolved, this solution was allowed to cool to room temperature. Then 5.20 g of Michler's ketone, a UV sensitive photoinitiator, was added with stirring, which was continued until the photoinitiator dissolved. 37.53 g of CYMEL 385 (a modified melamine-formaldehyde resin, a product of American Cyanamid) was heated to about 50ºC.

The ethylene-maleic anhydride copolymer solution was placed in a jacketed mixer heated to 55ºC using circulating water. The mixer power setting was controlled to 40 V using a variable transformer. The core solution was then added and the mixer power setting was increased to 90 V for 45 seconds to finely disperse the core liquid into small droplets. The mixer power was reduced to 40 V and the CYMEL 385 was added to the mixer. Stirring and heating were then continued at 55ºC for 2 hours.

The blue color-forming toner particles were later isolated as a dry powder by spray drying.

EXAMPLE 4 Spray drying of an aqueous suspension of toner particles treated with citric acid and demonstration of color self-development using these particles

20 ml of an aqueous suspension of magenta color-forming toner particles containing about 20% solids was diluted to 50% with water. To this was added 1.0 g of anhydrous citric acid. The mixture was stirred at room temperature for 15 minutes to dissolve the citric acid. The solution was then spray dried and the solids, pale pink particles, were collected. Rubbing and breaking the particles against a sheet of uncoated paper caused the development of a deep magenta color.

EXAMPLE 5 Production of color self-developing toner particles in a non-aqueous medium and color development therewith

Another way to produce color self-developing toner particles was to dissolve the citric acid in THF (tetrahydrofuran) and add this to the particles along with a dispersant. The system was then diluted with ISOPAR G, an isoparaffinic liquid, and the THF was removed by rotary evaporation. A batch of these particles was prepared by treating 0.1 g of particles with 5 drops of a 1% solution of citric acid in THF (1.8 mg citric acid). 0.05 g of ZELEC UN (an acid phosphate ester, a product of EI du Pont de Nemours & Co.) was also added as a dispersant. The mixture was then diluted with 10 g of ISOPAR and the THF was removed. These toner particles were coated on uncoated paper. After the ISOPAR had evaporated, a weighing paper was placed on the top of this sheet and pen pressure was applied. This resulted in magenta color developing lines on the uncoated paper.

EXAMPLE 6 Production of a color self-winding toner mixture and electrostatic, photoselective production of a multi-color image

Magenta-forming, color self-developing toner particles and yellow-forming, color self-developing toner particles were isolated as powders by spray drying as in Examples 1 or 2. A mixture of spray dried, color self-developing toner particles made from one part magenta-forming particles and one part yellow-forming particles was prepared. 3.0 g of a 5% ISOPAR H solution of SOLSPERSE 21000 (a dispersant manufactured by ICI) was added to 1.17 g of the toner particle mixture. 60 g of ISOPAR H was added and the entire mixture sonicated to form a dispersion of toner particles. 0.2 g of ZELEC UN in 5 ml of ISOPAR H isoparaffinic liquid was added, followed by further sonication. The resulting toner particle dispersion was used as a toner mixture to electrostatically form images.

A latent image was created on a piece of dielectric paper (4008-F electrographic paper, a product of Versatec, Inc.) using a corona charging technique. A sheet of MYLAR with the letter "O" cut into it was placed on the dielectric paper. A corona connected to a +9000 V power supply was passed over the cut area several times, resulting in the formation of a charged area on the paper in the shape of the cut "O". The dielectric paper was seconds in the above toner mixture. The paper bearing a toned, colorless image was then dried to remove isoparaffinic liquid ISOPAR H by briefly placing it in a heating cabinet at a temperature of 80ºC.

A portion of the toner image was covered by an opaque mask and the paper was exposed to the output of a 100 W BLAKRAY lamp (manufactured by Ultra-Violet Products) for 2 minutes at a distance of 5 inches. The light was filtered using a long-pass filter that only allowed wavelengths longer than 420 nm to pass (Oriel Corporation, Model #51482). The yellow-producing, self-developing toner particles containing camphorquinone (a photoinitiator sensitive to 480 ± 20 nm light) were cured in the irradiated areas. The magenta-producing self-developing toner particles were not hardened in the irradiated or masked areas of the toner image because these magenta-producing particles contained Michler's ketone as a photoinitiator to make these particles sensitive to 350 ± 40 nm light.

The selectively hardened toner image on the paper was passed through a squeegee roller after the toner image was covered with a piece of weighing paper to prevent toner particles from sticking to the rollers during print development. The final electrostatically produced image was red where it was shielded from light (the subtractive combination of magenta and yellow) and magenta where the yellow-producing, self-color-developing toner particles had been hardened by irradiation.

Claims (35)

1. A color imaging process comprising the steps of: (a) forming a latent image on a photoconductive or dielectric substrate, (b) electrostatically depositing a toner mixture onto a charged or uncharged surface of the substrate to produce a toner image which is a positive or reversal image compared to the latent image, the toner mixture comprising at least two different toners, each toner comprising a color precursor contained in photosensitive toner particles, (c) selectively photohardening or photosoftening at least a portion of the toner particles by imagewise exposure to appropriate radiation wavelengths to provide harder toner particles and softer, breakable toner particles, (d) transferring the harder toner particles and the fragile toner particles to a copy surface, (e) breaking at least a portion of the toner particles on the copy surface to release (a) color precursor from the fragmentable toner particles and (f) contacting the color precursor(s) on the copy surface with a developer, thereby causing the released color precursor(s) and the developer to react to produce a color image on the copy surface. 2. A method according to claim 1, characterized in that the toner mixture comprises at least three types of toner particles, each of the types containing a different color precursor and each of the types additionally containing a radiation-sensitive composition. 3. A method according to claim 2, characterized in that each of the types of toner particles contains a different color precursor selected from the group consisting of cyan, yellow, magenta and optionally additionally black. 4. A method according to claim 2, characterized in that the radiation-sensitive material is a photocurable or photosoftenable material and contains a photoinitiator for each of the types of toner particles, the photoinitiator being sensitive to a given wavelength of light which is different from the photosensitivity of any other photoinitiator contained in any of the other types of toner particles. 5. Process according to claim 4, characterized in that the radiation-sensitive composition is light-curable and consists essentially of a photoinitiator and a polymerizable or crosslinkable material. 6. Process according to claim 4, characterized in that the radiation-sensitive material is light-softenable and consists essentially of a depolymerizable material. 7. Process according to claim 6, characterized in that the radiation-sensitive material additionally contains a photoinitiator. 8. A color imaging process comprising the steps of: (a) forming a latent image on a photoconductive or dielectric substrate, (b) electrostatically depositing a toner mixture onto a charged or uncharged surface of the substrate to produce a toner image which is a positive or reversal image compared to the latent image, the toner mixture comprising at least two different toners and each toner comprising a color precursor contained in photosensitive toner particles, (c) selectively photohardening or photosoftening at least a portion of the toner particles by imagewise exposure to appropriate radiation wavelengths to provide harder toner particles and softer, breakable toner particles, (d) breaking at least a portion of the toner particles on the substrate to release (a) color precursor from the breakable toner particles and (e) transferring the released ink precursor to a copy surface and (f) contacting the color precursor(s) on the copy surface with a developer, thereby causing the released color precursor(s) and the developer to react to produce a color image on the copy surface. 9. The method of claim 8, characterized in that the toner mixture comprises at least three types of toner particles, each of the types containing a different color precursor and each of the types additionally containing a radiation-sensitive composition. 10. The method of claim 9, characterized in that each of the types of toner particles contains a different color precursor selected from the group consisting of cyan, yellow, magenta and optionally additionally black. 11. A method according to claim 9, characterized in that the radiation-sensitive material is a photocurable or photosoftenable material and contains a photoinitiator for each of the types of toner particles, the photoinitiator being sensitive to a given wavelength of light which is different from the photosensitivity of any other photoinitiator contained in each of the types of toner particles. 12. A method according to claim 11, characterized in that the radiation-sensitive composition is light-curable and consists essentially of a photoinitiator and a polymerizable or cross-linkable material. 13. Process according to claim 11, characterized in that the radiation-sensitive material is light-softenable and consists essentially of a depolymerizable material. 14. Method according to claim 13, characterized in that the radiation-sensitive material additionally contains a photoinitiator. 15. A color imaging method comprising the steps of: (a) forming a latent image on a photoconductive or dielectric substrate, (b) electrostatically depositing a toner mixture onto a charged or uncharged surface of the substrate to form a toner image that is a positive or reversal image compared to the latent image, the toner mixture comprising at least two different toners, each toner comprising a color precursor contained in the photosensitive toner particle, (c) transferring the coloured image to a copy surface, (d) selectively photohardening or photosoftening at least a portion of the toner particles by imagewise exposure to appropriate radiation wavelengths to provide harder toner particles and softer, breakable toner particles, (e) breaking at least a portion of the toner particles of the copy surface to release (a) color precursor from the breakable toner particles and (f) contacting the color precursor(s) on the copy surface with a developer, thereby causing the released color precursor(s) and the developer to react to produce a color image on the copy surface. 16. Method according to claim 15, characterized in that the Toner mixture comprises at least three types of toner particles, each of the types containing a different color precursor and each of the types additionally containing a radiation-sensitive composition. 17. The method of claim 16, characterized in that each of the types of toner particles contains a different color precursor selected from the group consisting of cyan, yellow, magenta and optionally additionally black. 18. A method according to claim 16, characterized in that the radiation-sensitive material is a photocurable or photosoftenable material and includes a photoinitiator for each type of toner particles, the photoinitiator being sensitive to a given wavelength of light which is different from the photosensitivity of any other photoinitiator included in any of the other types of toner particles. 19. Process according to claim 18, characterized in that the radiation-sensitive composition is light-curable and consists essentially of a photoinitiator and a polymerizable or crosslinkable material. 20. Process according to claim 18, characterized in that the radiation-sensitive material is light-softenable and consists essentially of a depolymerizable material. 21. Method according to claim 20, characterized in that the radiation-sensitive material additionally contains a photoinitiator. 22. The method of claim 1, characterized in that the toner mixture consists of particle shapes selected from the group consisting of microcapsules, microsponges, softenable solid particles, emulsion micelles and combinations thereof. 23. A developer-bearing, color self-developing, electrostatically depositable microcapsule toner particle comprising a shell and a core, the shell being made of a polymer and the core comprising a colorless chromogenic material as a color precursor and additionally containing a radiation sensitive material, the chromogenic material being capable of becoming colored upon contact with a developer, the shell having a charge characteristic to render the toner particle electrostatically depositable, the shell having a developer on the outer surface, the shell of the toner particle being frangible to release the chromogenic material, the released material being brought into contact with the developer and reacted to produce a colored image. 24. Microcapsule toner particles according to claim 23, characterized in that the developer is an acidic developer or a complexing developer. 25. Microcapsule toner particles according to claim 24, characterized in that the developer is an acidic developer selected from the group consisting of citric acid, oxalic acid, maleic acid, gluconic acid, acrylic acid, methacrylic acid and malonic acid. 26. Microcapsule toner particles according to claim 24, characterized in that the developer is a complexing developer selected from the group consisting of zinc, cobalt and nickel salts of organic acids. 27. A mixture of microcapsule toner particles according to claim 23, characterized in that the mixture comprises at least two types of toner particles, each of the types containing a different color precursor, each of the types of toner particles additionally containing a photosensitive composition which is photosensitive at wavelengths which are different from the wavelengths of photosensitivity of the photosensitive composition present in all other types of particles are contained in the mixture. 28. A mixture according to claim 27, characterized by at least three types of toner particles, each of the types containing a different color precursor selected from the group consisting of cyan, yellow, magenta and black. 29. A method for producing the developer-bearing microcapsule toner particles according to claim 23, comprising coating a microcapsule toner particle with a color developer by contacting the microcapsule toner particle with an acidic or complexing developer. 30. A process according to claim 29, characterized in that the contacting is carried out in an aqueous medium, followed by spray drying to provide dry, developer-bearing microcapsule toner particles. 31. A method according to claim 29, characterized in that the contacting is carried out in an organic medium, followed by removal of the organic medium to provide dry, developer-bearing microcapsule toner particles. 32. The method of claim 30 or 31, additionally comprising dispersing the dry developer-bearing microcapsule toner particles in a non-polar organic solvent. 33. A color imaging process comprising the steps of: (a) forming a latent image on a photoconductive or dielectric substrate, (b) electrostatically depositing a toner mixture onto a charged or uncharged surface of the substrate to produce a toner image which is a positive or reversal image compared to the latent image, the toner mixture comprising at least two different toners, each toner comprising a color precursor contained in photosensitive toner particles which Toner particles carry a developer on the surface, (c) selectively photohardening or photosoftening at least a portion of the toner particles by imagewise exposure to appropriate radiation wavelengths to provide harder toner particles and softer, breakable toner particles, (d) transferring the harder toner particles and the fragile toner particles to a copy surface and (e) fracturing at least a portion of the toner particles on the copy surface to release color precursor(s) from the frangible toner particles, whereby the developer is brought into contact with the color precursor(s) and reacted to form a color image on the copy surface. 34. A color imaging process comprising the steps of: (a) forming a latent image on a photoconductive or dielectric substrate, (b) electrostatically depositing a toner mixture onto a charged or uncharged surface to produce a toner image which is a positive or reversal image compared to the latent image, the toner mixture comprising at least two different toners, each toner comprising a color precursor contained in photosensitive toner particles, the toner particles carrying a developer on their surface, (c) selectively photohardening or photosoftening at least a portion of the toner particles by imagewise exposure to appropriate radiation wavelengths to provide harder toner particles and softer, breakable toner particles, (d) breaking at least a portion of the toner particles on the substrate to release color precursor(s) from the breakable toner particles, whereby the developer is brought into contact with the color precursor(s) and reacted to form a color image on the substrate, (e) Transferring the colour image to a copy surface. 35. A color imaging process comprising the steps of: (a) forming a latent image on a photoconductive or dielectric substrate, (b) electrostatically depositing a toner mixture onto a charged or uncharged surface of the substrate to produce a toner image which is a positive or reversal image compared to the latent image, the toner mixture comprising at least two different toners, each of the toners comprising a color precursor contained in photosensitive toner particles, the toner particles containing a developer on their surface, (c) transferring the toner image to a copy surface, (d) selectively photohardening or photosoftening at least a portion of the toner particles by imagewise exposure to appropriate radiation wavelengths to provide harder toner particles and softer, breakable toner particles, and (e) fracturing at least a portion of the toner particles on the copy surface to release color precursor(s) from the frangible toner particles, whereby the developer is brought into contact with the color precursor(s) and reacted to form a color image on the copy surface.