Liquid electrographic developers

Description

[0001] The present invention relates to a liquid electrographic developer composition comprising an electrically insulating carrier liquid, a toner dispersed in the carrier, a quaternary ammonium polymer to control the electrical charge on the pigment, and a polar copolymer to impart charge to the toner dispersion.

[0002] In the art of electrography, charge images are formed on a-substrate and thereafter rendered visible by contact with an electrographic developer composition. These compositions are formulated in dry or liquid form, each presenting its own unique requirements and advantages.

[0003] A conventional liquid electrographic developer is a dispersion of toner particles, usually of a pigment and a binder resin, in an electrically insulating carrier liquid such as a volatile hydrocarbon fraction. The binder resin serves to bind the pigment to the final copy sheet, usually upon the application of heat. The developer can also contain a stabilizer additive to maintain a stable dispersion of the toner particles. Liquid developers also often contain a charge-control agent to maintain a constant charge level.

[0004] Liquid electrographic developers have been proposed which contain polymers uniformly dispersed in the carrier liquid. For example U.S. patent 3 788 995 describes a liquid developer comprising, dispersed in the carrier liquid, at least one polymer containing units of at least two monomers, one of which is a polar monomer selected from inter alia, sulfoalkyl acrylates and methacrylates, and their metal and amine salts, and another of which is a solubilizing monomer such as an alkyl acrylate having 8 to 22 carbon atoms in the alkyl portion. Units of an insolubilizing monomer such as styrene or an alkyl acrylate having up to 4 carbon atoms in the alkyl portion, may also be present. The stabilizing monomer is used in sufficient quantity to give the copolymers a specified minimum dispersibility. It is preferred to disperse uniformly in the carrier liquid a second copolymer having up to 20% by weight of units from a polar monomer, units from an insolubilizing monomer, and possibly, units from a solubilizing monomer.

[0005] A particularly useful liquid developer is disclosed in US Patent 4 273 849. This developer comprises an electrically insulating carrier liquid having stably dispersed therein (1) toner particles of a halogenated polymer, and having dissolved therein (2) a copolymer of a quaternary ammonium salt monomer and a solubilizing monomer, this copolymer being free of alkali metal and alkaline earth metal cations, and (3) an addition copolymer of a polar monomer and a solubilizing monomer, the amount of solubilizing monomer is each copolymer being sufficient to make the copolymer soluble in the carrier liquid. The toner particles (1) are negatively charged and the copolymer (2) is believed to act both as a dispersing agent for.them and as a charge-control agent. The copolymer (3) is added to stabilize the dispersion.

[0006] The aforementioned liquid developers of US '849 have certain drawbacks. As the developers are used through a number of copy sequences, their charge per unit of mass of dispersed toner increases, indicating that the quaternary ammonium charge-control copolymer deposits on an electrostatic image at a lower rate than the toner. This uneven depletion rate, and consequential increase in charge per unit mass in the developer, presents difficulty in developer replenishment and causes nonuniform image density from copy to copy. In particular, one cannot simply replenish from one concentrate, use the developer, and replenish again from the same concentrate with any assurance that the balance of components in the developer after each replenishment will approximate the balance of the components in a fresh developer.

[0007] It is the object of the present invention to provide an electrographic liquid developer composition comprising negatively or positively charged dispersed toner particles and a dissolved polymeric charge-control agent in which the charge of the developer per unit mass of toner is stabilized so as to increase less rapidly during use.

[0008] The present invention provides a liquid electrographic developer composition comprising an electrically insulating liquid carrier which contains dispersed toner particles and in which at least two addition copolymers are uniformly dispersed, the dispersed copolymers comprising a first carrier-soluble copolymer as a charge-control agent which contains units of a quaternary ammonium salt monomer and units of a solubilizing monomer, and a second carrier-soluble copolymer as a charging agent which contains units of a polar monomer and units of a solubilizing monomer characterized in that the first copolymer additionally contains units of an insolubilizing monomer having and acidic function of formula -COOH, -S0₃H or P0₃HR, where R is hydrogen or alkyl.

[0009] When R is alkyl in the-P0₃HR group of the first copolymer, it is preferably 1-10 carbon alkyl such as methyl, propyl, amyl or hexyl. Most preferably, alkyl is 1-4 carbon alkyl.

[0010] The first carrier-soluble copolymer is believed to function as a charge-control agent and the second as a charging agent.

[0011] In a preferred embodiment of the invention, the charge of the developer per unit mass is further stabilized against change when the toner contains a neutral or basic pigment.

[0012] The liquid carrier employed in a developer of the invention has a low electric constant and a high electrical resistance such that it will not disturb or destroy the electrostatic charge pattern being developed.

[0013] Carrier liquids should have a dielectric constant of less than about 3, have a volume resistivity greater than 10¹⁰ ohm-cm and be chemically stable. Suitable carrier liquids include halogenated hydrocarbon solvents such as trichlorotrifluoroethane; hydrocarbon solvents are useful, such as isoparaffinic hydrocarbons and cyclohydrocarbons such as cyclohexane. Preferably, the liquid carrier is a combination of two different electrically insulating liquids.

[0014] Toner is dispersed in the carrier and serves as the marking particle constituent which is attracted to an electrostatic charge image on an element being developed. The toner comprises a colorant and a binder polymer, if necessary, for adhesion of the colorant to the image-bearing element. Colorants can be selected from one or more of a variety of dyes and pigments. Carbon black is a preferred colorant, but virtually any of the compounds in the "Colour Index", Second Edition, 1956, Volumes I and II, may, in principle, be used.

[0015] Preferred toners comprise neutral or basic pigments such as neutral or basic carbon black pigments. The neutrality or basicity of a pigment is determined as the pH of the pigment by the ASTM D 1512-60(76) method, details of which are reported in the "Analysis of Carbon Black" by B. Schubert et al, Encyclopedia of Industrial ChemicalAnalysis, Volume B (reprint pages 51-52, pH), John Wiley and Sons, Inc. Neutral or basic pigments according to this method exhibit a pH of 7 and higher.

[0016] Binder polymers which can be employed with the toner particles, if desired, include a variety of materials such as halogenated polyolefins, addition polymers such as acrylic polymers and condensation polymers. Preferably, such polymers are polyesterionomers such as disclosed in US Patent 4,252,921.

[0017] The developer of the present invention includes, as a charge-control agent, a carrier-soluble addition copolymer. "Carrier-soluble" refers to its ability to dissolve in the electrically insulating carrier, in the absence of the toner constituents, at the concentration level at which it is employed in the developer. The monomer constituents of the polymer include at least the following: a quaternary ammonium salt monomer, a solubilizing monomer, and a monomer having an acidic function selected from the group consisting of -COOH, -S0₃H and -P0₃HR.

[0018] Examples of suitable quaternary ammonium salt monomers include:

methacryloyloxyethyltrimethylammonium p-toluene sulfonate;

dimethyloctadecylvinylbenzylammonium chloride;

2-vinyl-N-methylpyridinium methyl sulfate;

N-methyl-N-methacryloyloxyethylpiperidinium bromide;

N-methacryloyloxyethylpyridinium nitrate.

[0019] Representative solubilizing monomers include the following: alkylstyrenes and alkoxystyrenes having 3 to 10 carbon atoms in the alkyl group; alkyl acrylates and methacrylates having 8 to 22 carbon atoms in the alkyl group; vinyl alkyl ethers having 8 to 22 carbon atoms in the alkyl group; and vinyl esters of alkanoic acids having 6 to 22 carbon atoms in the alkyl group.

[0020] Specific solubilizing monomers which can be so used include: t-butylstyrene, 4-pentylstyrene, 4-hexylstyrene, 4-octylstyrene, p-amyloxystyrene, lauryl acrylate, lauryl methacrylate, hexadecyl methacrylate, octadecyl methacrylate, eicosyl acrylate, docosyl methacrylate, vinyl caprate, vinyllaurate, vinyl palmitate, vinyl stearate, vinyl eicosate and vinyl docosate.

[0021] Suitable insolubilizing monomers having an acidic function include addition polymerizable polymers such as olefinic and acrylic monomers having the requisite acidic function. Representative acidic monomers include:

Monomers containing -COOH:

acrylic acid

methacrylic acid

vinyl benzoic acid

styryl acetic acid

Monomers containing -S0₃H:

sulfoethyl methacrylamide

sulfoethyl methacrylate

styrene sulfonic acid

Monomers containing -P0₃HR:

vinylbenzylethylphosphate

vinylbenzylphosphonic acid

vinylphosphonic acid

vinylethylphosphonate

[0022] In addition to the quaternary ammonium, solubilizing and acidic monomers, the charge control polymer set forth above can include additional other monomers to adjust, for example, the degree of solubility of the polymer in the carrier, as well as to provide other desirable properties.

[0023] Quaternary ammonium salt polymers useful in the liquid developers of this invention typically have an inherent viscosity in the range from 0.1 to 0.8. The determination is made at a concentration of 0.25 gram of polymer in 100 ml of chloroform at a temperature of 25°C. The resultant polymers contain recurring units derived from one or more solubilizing monomers, from one or more quaternary ammonium salt monomers and from one or more insolubilizing acidic monomers. A typical quaternary ammonium salt copolymer used in the liquid developers of the invention contains from 85 to 97 mole percent of solubilizing monomer units, from 10 to 2 mole percent acidic insolubilizing monomer units, and from 5 to 1 mole percent of quaternary ammonium salt monomer units.

[0024] The solubility of the quaternary ammonium salt copolymer in the carrier liquid can be controlled by adjusting the amount of solubilizing and insolubilizing (acidic) monomers which are used. If the solubilizing monomer has a relatively long alkyl group, rendering a polymer containing it very soluble in hydrocarbon carrier liquids, the insolubilizing monomer may have a relatively short alkyl group. On the other hand, a relatively short alkyl group on the solubilizing monomer requires a somewhat longer alkyl group on the insolubilizing monomer.

[0025] The relative amount of quaternary ammonium salt monomer can be varied to provide polymers having different charge properties. when incoporated into a liquid developer.

[0026] The developer of the present invention also contains, as a charging agent, a carrier-soluble polar addition copolymer of the type disclosed in US Patents 4,273,849 and 3,849,165. As above, "carrier-soluble" refers to the ability of the charging copolymer to dissolve in the liquid carrier, in the absence of the toner constituents, at the concentration level which is employed in the developer.

[0027] The soluble polar copolymer is an addition copolymer of a polar monomer and at least one solubilizing monomer. The amount of polar monomer is at least 1.5x 10-⁴ moles/gm of copolymer. It is preferred for the solubilizing monomer to be present to the extent of at least 35 weight % and for the polar monomer to be present in an amount not in excess of 20 weight %, based on the weight of this copolymer.

[0028] Monomers for addition copolymerizing with the polar monomer to obtain a soluble polar copolymer are the solubilizing monomers previously described and, if desired, insolubilizing monomers such as styrene, vinyl toluene, ethyl acrylate, methyl acrylate, butyl acrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, vinyl acetate, vinyl propionate and vinyl butyrate, as described by Stahly et al, US Patent 3,849,165. The relative amounts of the comonomers are selected to produce a polar addition copolymer of the desired solubility, as described above with respect to the quaternary ammonium salt copolymers.

[0029] Suitable polar monomers include sulfoalkyl acrylates and methacrylates, metal salts of sulfoalkyl acrylates and methacrylates, and amine salts of sulfoalkyl acrylates and methacrylates. Preferably, the sulfoalkyl groups contain from 1 to about 4 carbon atoms. Also suitable are metal salts and amine salts of acrylic and methacrylic acids, and of vinylbenzoic acid, vinylphenylacetic acid, 9(10)-acrylamidostearic acid, monoalkylphthalic acid, and styrenesulfonic acid, the metals of said salts being from Groups I, Ila, Ilb and VIII of the periodic table, and mixtures thereof.

[0030] Preferred polar monomers are acrylic monomers. In this sense, the term "acrylic" broadly includes methacrylic, and salts and esters of such accrylic, acids.

[0031] Specific examples of such acrylic polar monomers include sulfoethyl methacrylate; sulfoethyl acrylate; sulfopropyl methacrylate; sulfobutyl methacrylate; sulfoethyl methacrylate, sodium salt; sulfoethyl methacrylate, partial sodium salt; sulfopropyl methacrylate, sodium salt; sulfobutyl methacrylate, potassium salt; sulfoethyl methacrylate, lithium salt; sulfoethyl methacrylate, copper salt; sulfopropyl methacrylate, dimethylammonium salt; sulfoethyl methacrylate diethylammonium salt; sulfomethyl acrylate, dimethylammonium salt; sodium methacrylate; sodium acrylate; lithium methacrylate; potassium acrylate; barium methacrylate; zinc methacrylate; cobalt methacrylate; ferrous acrylate; magnesium methacrylate; zinc acrylate; and mixtures thereof.

[0032] The liquid electrographic developers of this invention preferably contain from 0.01 percent to 5 percent by weight of soluble polar copolymer, from 0.01 percent to 5 percent by weight of quaternary ammonium salt copolymer, and from 0.01 percent to 5 percent by weight of toner. Preferred developers contain from 0.02 percent to 0.5 percent by weight of the soluble acrylic polar copolymer, from 0.02 percent to 0.5 percent by weight quaternary ammonium salt copolymer, and from 0.02 percent to 0.5 percent by weight toner.

[0033] In the following examples, developers of the present invention are evaluated for replenishability, R. Replenishability R is defined for the purpose of the present invention as the ratio of initial charge/mass to final charge/mass after a 1/2 liter reservoir of the developer has been employed to form a electrographic images and replenished. The extent of imaging and replenishment was predetermined so as to use and replenish three times the number of marking particles in the original developer. (For example, if the developer contains 0.2 gram toner, 0.6 gram is developed and replenished.) The replenisher employed as a 5x concentrate of the original developer. The charge before and after in coulombs was measured with a Keithly Instruments Capacitance Bridge (available from Teltex Instruments, Inc., Pennsylvania). The charge measured is divided by the toner weight in kilograms to give charge per mass in coulombs per kilogram toner.

[0034] The following developer components were employed in an Isopar G -Solvesso 100 liquid carrier system with a pigment concentration of 0.7 gram per liter developer. (Solvesso 100 is a trademark for an alkylated aromatic liquid and Isopar G is a trademark for an isoparaffinic hydrocarbon liquid.) The solvesso 100 concentration was approximately 21 grams per liter of developer.

[0035] Each of the developers also contained 0.5 part plasticizer, 0.25 part wax and 0.125 part dispersing agent for the wax.

Examples 1-4

[0036] In this example, replenishability R of control liquid developers containing quaternary ammonium charge-control polymers as described by Merrill, US Patent 4,273,849, column 5, was compared with R for liquid developers within the scope of the invention.

[0037] The developer of the invention corresponded to Developer A above. The charge-control polymer employed in the control developer was either (a) an addition terpolymer of a solubilizing monomer, an insolubilizing monomer which contained no acidic groups and a quaternary ammonium monomer or (b) an addition copolymer of a monomer and a quaternary ammonium polymer. In all other respects, the control developers and developers of the invention were identical.

[0038] The results in terms of R values are reported in Table 1.

[0039] These results indicate that, when the insolubilizing monomer is acidic, developer replenishability, R, is improved relative to charge-control polymers containing either nonacidic insolubilizing monomer or no insolubilizing monomer.

Examples 5-7:

[0040] These examples illustrate that replenishability, R, is further enhanced by the use of non-acidic colorants.

[0041] The procedure of Examples 1-4 was repeated employing developer A with either Raven 1255@ carbon black, which was neutralized by washing, or neutral Regal 300@ carbon black. The concentration of addition polar copolymer and quaternary ammonium charge-control polymer was also varied as indicated.

[0042] Replenishability values, R, are reported in Table 2.

[0043] The R values for Examples 5-7 are to be compared with the R value for the developer in Example 2, an otherwise identical developer. The results indicate that basic or neutral pigments improve the replenishability in the developers of the present invention compared with acidic pigments.

[0044] "Electrography" and "electrographic" as used herein are broad terms which include image-forming processes involving the development of an electrostatic charge pattern formed on a surface with or without light exposure, and thus include electrophotography and other processes.

Claims

1. A liquid electrographic developer composition comprising an electrically insulating liquid carrier which contains dispersed toner particles and in which at least two addition copolymers are uniformly dispersed, the dispersed copolymers comprising a first carrier-soluble copolymer as a charge-control agent which contains units of a quaternary ammonium salt monomer and units of a solubilizing monomer, and a second carrier-soluble copolymer as a charging agent which contains units of a polar monomer and units of a solubilizing monomer characterized in that the first copolymer additionally contains units of an insolubilizing monomer having an acidic function of formula -COOH, -S0₃H or -P0₃HR, where R is hydrogen or alkyl.

2. A developer according to claim 1 wherein said acidic insolubilizing monomer is acrylic acid, methacrylic acid, vinyl benzoic acid, styrene acetic acid, sulfoethyl methacrylamide, sulfoethyl methacrylate, styrene sulfonic acid, vinylbenzylethylphosphate, vinylbenzylphosphonic acid, vinylphosphonic acid or vinylethylphosphonate.

3. A developer according to claim 1 or 2 wherein the solubilizing monomers in the dispersed copolymers are independently selected from t-butylstyrene, 4-pentylstyrene, 4-hexylstyrene, 4-octylstyrene, p-amyloxystyrene, lauryl acrylate, lauryl methacrylate, hexadecyl methacrylate, octadecyl methacrylate, eicosyl acrylate, docosyl methacrylate, vinyl caprate, vinyl laurate, vinyl palmitate, vinyl stearate, vinyl eicosate and vinyl docosate.

4. A developer according to any of the preceding claims wherein the quaternary ammonium salt monomer in the first carrier-soluble copolymer is selected from methacryloyloxyethyltrimethyl- ammmonium p-toluene sulfonate, dimethyloctadecylvinylbenzylammonium chloride, 2-vinyl-N-methylpyridinium methyl sulfate, N-methyl-N-methacryloyloxyethylpiperidinium bromide, and N-methacryloyloxyethylpyridinium nitrate.

5. A developer according to any of the preceding claims wherein the polar monomer in the second carrier-soluble copolymer is selected from sulfoethyl methacrylate, sulfoethyl acrylate; sulfopropyl methacrylate; sulfobutyl methacrylate; sulfoethyl methacrylate, sodium salt; sulfoethyl methacrylate, partial sodium salt; sulfopropyl methacrylate, sodium salt; sulfobutyl methacrylate, potassium salt; sulfoethyl methacrylate, lithium salt; sulfoethyl methacrylate, copper salt; sulfopropyl methacrylate, dimethylammonium salt; sulfoethyl methacrylate diethylammonium salt; sulfomethyl acrylate, dimethylammonium salt; sodium methacrylate; sodium acrylate; lithium methacrylate; potassium acrylate; barium methacrylate; zinc methacrylate; cobalt methacrylate; ferrous acrylate; magnesium methacrylate; and zinc acrylate.

6. A developer according to any of the preceding claims wherein the concentrations of the toner particles, the first carrier-soluble copolymer, and the second carrier-soluble copolymer are each from 0.01 to 5 percent, by weight of the total developer.

7. A developer according to claim 6 wherein the said concentrations are all in the range 0.02 to 0.5 percent by weight of the total developer.

8. A developer according to any of the preceding claims wherein the toner particles comprise a neutral pH or basic pH colorant.

9. A developer according to claim 8 wherein the toner particles further comprise a polyesterionomer as a binder for the colorant.

10. A developer according to any of the preceding claims wherein the carrier comprises a second electrically insulating liquid.

11. A developer according to claim 10 wherein the electrically insulating liquid carrier is a mixture of an isoparaffinic hydrocarbon liquid and an alkylated aromatic liquid.

Ansprüche

1. Flüssige elektrographische Entwicklerzusammensetzung mit einem elektrisch isolierenden flüssigen Träger, der dispergierte Tonerteilchen enthält und in dem mindestens zwei Additionscopolymere gleichförmig dispergiert sind, wobei die dispergierten Copolymeren aus einem ersten im Träger löslichen Copolymer als Ladungssteurmittel, das Einheiten eines quaternären Ammoniumsalzmonomeren und' Einheiten eines löslich machenden Monomeren enthält sowie eine zweiten im Träger löslichen Copolymer als Ladungsmittel, das Einheiten eines polaren Monomeren und Einheiten eines löslich machenden Monomeren enthält, bestehen, dadurch gekennzeichnet, daß das erste Copolymer zusätzlich Einheiten eines unlöslich machenden Monomeren mit einer sauren Funktion der Formeln -COOH, -S0₃H oder -P0₃HR, worin R für Wasserstoff oder Alkyl steht, enthält.

2. Entwickler nach Anspruch 1, dadurch gekennzeichnet, daß das saure, unlöslich machende Monomer aus Acrylsäure, Methacrylsäure, Vinylbenzosäure, Styrolessigsäure, Sulfoethylmethacrylamid, Sulfoethylmethacrylat, Styrolsulfonsäure, Vinylbenzylethylphosphat, Vinylbenzylphosphonsäure, Vinylphosphonsäure oder Vinylethylphosphonat besteht.

3. Entwickler nach Anspruch 1 oder 2, dadurch gekennzeichnet, daß die löslich machenden Monomeren in den dispergierten Copolymeren unabhängig voneinander ausgewählt sind aus t.-Butylstyrol, 4-Pentylstyrol, 4-Hexylstyrol, 4-Octylstyrol, p-Amyloxystyrol, Laurylacrylat, Laurylmethacrylet, Hexadecylmethacrylat, Octadecylmethacrylat, Eicosylacrylat, Docosylmethacrylat, Vinylcaprat, Vinyllaurat, Vinylpalmitat, Vinylstearat, Vinyleicosat und Vinyldocosat.

4. Entwickler nach einem der vorstehenden Ansprüche, dadurch gekennzeichnet, daß das quaternäre Ammoniumsalzmonomer des ersten, im Träger löslichen Copolymeren ausgewählt ist aus Methacryloyloxyethyltrimethylammonium - p - toluolsulfonat, Dimethyloctadecylvinylbenzylammoniumchlorid, 2 - Vinyl - N - methylpyridiniummethylsulfat, N - Methyl - N - methacryloyloxyethylpiperidiniumbromid und N - Methacryloyloxyethylpyridiniumnitrat.

5. Entwickler nach einem der vorstehenden Ansprüche, dadurch gekennzeichnet, daß das polare Monomer des zweiten, im Träger löslichen Copolymeren ausgewählt ist aus Sulfoethylmethacrylat, Sulfoethylacrylat; Sulfopropylmethacrylat; Sulfobutylmethacrylat; Sulfoethylmethacrylat, Natriumsalz; Sulfoethylmethacrylat, Teilnatriumsalz; Sulfopropylmethacrylat, Natriumsalz; Sulfobutylmethacrylat, Kaliumsalz; Sulfoethyl - methacrylat, Lithiumsalz; Sulfoethylmethacrylat, Kupfersalz; Sulfopropylmethacrylat, Dimethylammoniumsalz; Sulfoethylmethacrylat, Diethylammoniumsalz; Sulfomethylacrylat, Dimethylammoniumsalz; Natriummethacrylat; Natriumacrylat; Lithiummethacrylat; Kaliumacrylat; Bariummethacrylat; Zinkmethacrylat; Cobaltmethacrylat; Ferroacrylat; Magnesiummethacrylat und Zinkacrylat.

6. Entwickler nach einem der vorstehenden Ansprüche, dadurch gekennzeichnet, daß die Konzentrationen der Tonerteilchen, des ersten im Träger löslichen Copolymeren und des zweiten im Träger löslichen Copolymeren jeweils bei 0,01 bis 5 Prozent, bezogen auf das Gewicht des gesamten Entwicklers, liegen.

7. Entwickler nach Anspruch 6, dadurch gekennzeichnet, daß die angegebenen Konzentrationen jeweils im Bereich von 0,02 bis 0,5 Prozent, bezogen auf das Gewicht des gesamten Entwicklers, liegen.

8. Entwickler nach einem der vorstehenden Ansprüche, dadurch gekennzeichnet, daß die Tonerteilchen eine Farbkomponente von neutralem oder basischem pH-Wert enthalten.

9. Entwickler nach Anspruch 8, dadurch gekennzeichnet, daß die Tonerteilchen des weiteren ein Polyesterionomer als Bindemittel für die Farbkomponente enthalten.

10. Entwickler nach einem der vorstehenden Ansprüche, dadurch gekennzeichnet, daß der Träger eine zweite elektrisch isolierende Flüssigkeit umfaßt.

11. Entwickler nach Anspruch 10, dadurch gekennzeichnet, daß der elektrisch isolierende flüssige Träger aus einer Mischung aus einem flüssigen isoparaffinischen Kohlenwasserstoff und einer alkylierten aromatischen Flüssigkeit besteht.

Revendications

1. Composition de révélateur électrographique liquide comprenant un porteur liquide isolant de l'électricité qui contient des particules de révélateur dispersées et dans lequel sont dispersés de façon uniforme au moins deux copolymères d'addition, les copolymères dispersés comprenant un premier copolymère soluble dans le porteur comme agent de contrôle de charge composé de motifs d'un monomère sel d'ammonium quaternaire et de motifs d'un monomère solubilisant, et un deuxième copolymère soluble dans le porteur comme agent de charge composé de motifs de monomère polaire et de motifs de monomère solubilisant, caractérisé en ce que le premier copolymère contient de plus des motifs d'un monomère insolubilisant ayant une fonction acide de formule -COOH, -S0₃H ou -P0₃HR, où R est un atome d'hydrogène ou un groupe alkyle.

2. Révélateur selon la revendication 1 dans lequel ledit monomère insolubilisant à caractère acide est l'acide acrylique, l'acide méthacrylique, l'acide vinylbenzoïque, l'acide styrène acétique, le sulfoéthyl méthacrylamide, le méthacrylate de sulfoéthyle, l'acide styrène sulfonique, le vinylbenzyléthylphosphate, l'acide vinylbenzylphosphonique, l'acide vinylphosphonique ou le vinyléthylphosphonate.

3. Révélateur selon la revendication 1 ou 2 où les monomères solubilisants dans les copolymères dispersés sont choisis parmi le t-butylstyrène, le 4-pentylstyrène, le 4-hexylstyrène, le 4-octylstyrène, le p-amyloxystyrène, l'acrylate de lauryle, le méthacrylate de lauryle, le méthacrylate d'hexadécyle, le méthacrylate d'octadécyle, l'acrylate d'eicosyle, le méthacrylate de docosyle, le caprate de vinyle, le laurate de vinyle, le palmitate de vinyle, le stéarate de vinyle, l'eicosenate de vinyle et le docosanoate de vinyle.

4. Révélateur selon l'une quelconque des revendications précédentes dans lequel le monomère sel d'ammonium quaternaire du premier copolymère soluble dans le porteur est choisi parmi le p-toluène sulfonate de métacryloyloxyéthyltriméthylammonium, le chlorure de diméthyloctadécylvinylbenzyl- ammonium, le méthylsulfate de 2 - vinyl - N - méthylpyridinium, le bromure de N - méthyl - N - méthacryloyloxyéthyl piperidinium et le nitrate de N - méthacryloyloxyéthylpyridinium.

5. Révélateur selon l'une quelconque des revendications précédentes dans lequel le monomère polaire du deuxième copolymère soluble est choisi parmi le méthacrylate de sulfoéthyle, l'acrylate de sulfoéthyle, le méthacrylate de sulfopropyle, le méthacrylate de sulfobutyle, le sel de sodium du méthacrylate de sulfoéthyle, le sel partiel de sodium du méthacrylate de sulfoéthyle, le sel de sodium du méthacrylate de sulfopropyle, le sel de potassium du méthacrylate de sulfobutyle, le sel de lithium du méthacrylate de sulfoéthyle, le sel de cuivre du méthacrylate de sulfoéthyle, le sel de diméthylammonium du méthacrylate de sulfopropyle, le sel de diéthylammonium du méthacrylate de sulfoéthyle, le sel de diméthylammonium d'acrylate de sulfométhyle, le méthacrylate de sodium, l'acrylate de sodium, le méthacrylate de lithium, l'acrylate de potassium, le méthacrylate de baryum, le méthacrylate de zinc, le méthacrylate de cobalt, l'acrylate ferreux, le méthacrylate de magnésium, et l'acrylate de zinc.

6. Révélateur selon l'une quelconque des revendications précédentes dans lequel les concentrations en particules de révélateur, en premier copolymère soluble dans le porteur et en deuxième copolymère soluble dans le porteur sont chacune comprises entre 0,01 et 5 pour cent en poids par rapport au poids total du révélateur.

7. Révélateur selon la revendication 6 dans lequel lesdites concentrations sont toutes comprises entre 0,02 et 0,5 pour cent en poids par rapport au poids total du révélateur.

8. Révélateur selon l'une quelconque des revendications précédentes dans lequel les particules de révélateur comprennent un colorant de pH neutre ou basique.

9. Révélateur selon la revendication 8 dans lequel les particules de révélateur comprennent de plus un polyesterionomère comme liant du colorant.

10. Révélateur selon l'une quelconque des revendications précédentes dans lequel le porteur comprend un deuxième liquide isolant de l'électricité.

11. Revélateur selon la revendication 10 dans lequel le liquide porteur isolant de l'électricité est un mélange de liquide hydrocarboné isoparaffinique et de liquide aromatique alkylé.