Replenishable liquid electrographic developers containing wax and method of preparing same

Abstract

 A replenishable liquid electrographic developer and method for manufacture of the developer are provided in which a stable dispersion comprising wax particles, a dispersing liquid and a dispersing agent for the wax is combined with a liquid electrographic toner formulation. Toner images formed from the resulting developer exhibit improved abrasion resistance.

Description

[0001] The present invention relates to replenishable liquid electrographic developers comprising dispersed wax particles and a dispersing agent for the wax exhibiting defined solubility characteristics, and to a method of preparing such liquid electrographic developers.

[0002] Electrographic imaging and development processes have been extensively described in both the patent and other literature. These processes have in common the steps of forming a latent electrostatic image on an insulating electrographic element, such as a photoconductive insulating layer coated on a conductive support. The electrostatic latent image is then rendered visible by a development step in which the charge image-bearing surface of the electrographic element is brought into contact with a suitable developer composition.

[0003] Liquid developer compositions of the type described in U.S. Patent 4,202,785 are highly useful for developing latent electrostatic images. These developers comprise a stable dispersion of dispersed components and dissolved components in an electrically insulating carrier liquid. The dispersed components include charged particles known as toner particles comprising a colorant or pigment, such as carbon black, and usually a thermoplastic resin binder. The dissolved components include organic liquids from previous steps in the preparation of the liquid developer, as well as optional components such as charge control agents.

[0004] Liquid developers such as those disclosed in the aforementioned U.S. 4,202,785, are particularly useful in microimaging where a high degree of image resolution is essential. The preference for liquid developers as opposed to dry developers for micro-imaging stems in part from the smaller toner particle size obtainable in liquid dispersions, as well as the ease with which such smaller particles can be handled and transported in liquid dispersions.

[0005] A problem with liquid developers and especially when they are used to develop images on plastic film supports, is that the images are not sufficiently resistant to abrasion or scratching.

[0006] When wax is dispersed in a liquid developer to improve the abrasion resistance of developed images, problems are encountered in forming stable dispersions and in the replenishment of the liquid developer during use.

[0007] The present invention is directed to a solution of these problems and provides a liquid developer containing a wax dispersion that provides good abrasion resistance for the developed images and which has improved stability and improved replenishment capability.

[0008] The liquid electrographic developer of the invention comprises an electrically insulating organic carrier liquid containing dispersed constituents and dissolved constituents, said dissolved constituents comprising an electrically insulating organic dispersing liquid which forms a solution with said carrier liquid, said dispersed constituents comprising:

a) optionally, at least one colorant,

b) at least one thermoplastic resin,

c) wax particles, and

d) a dispersing agent for said wax particles which is insoluble in said solution of carrier liquid and dispersing liquid, but soluble in said dispersing liquid alone.

[0009] The present invention also provides a method for preparing the above-defined liquid electrographic developer which comprises the steps of

i) dispersing said wax particles in a solution comprising said dispersing agent dissolved in said organic dispersing liquid,

ii) admixing said wax dispersion with said carrier liquid to form said solution of dispersing liquid and carrier liquid whereby said dispersing agent becomes a dispersed, insoluble constituent, and

iii) before, during or after steps (i) or (ii), admixing said colorant and thermoplastic resin with said organic carrier liquid.

[0010] While the prior art has recognized that wax particles can be added to developer compositions for other purposes, it apparently has not recognized that certain classes of wax particles will improve abrasion resistance of toner images. For example U.S. Patent No. 4,081,391 teaches the addition of waxes, including polyethylene wax, to developer compositions comprising a carrier liquid, pigment or dye particles, and a thermoplastic graft copolymer to produce images displaying superior transferability as a result of the wax component. The method by which the wax is added to the liquid developer entails dissolving the wax in hot solvent and subsequently cooling the solution to crystalize the wax (so-called quenching). Wax particles can also be precipitated from solution by introducing a non-solvent for the wax.

[0011] It has been found, however, that quenching or precipitation techniques form non-aqueous wax dispersions which are not sufficiently stable. Also the wax particle sizes are too large for a high degree of image resolution. It has been found also that the use of dispersing agents dissolved in the dispersion liquid advantageously provides wax particles of submicron size. In this regard, British Patent 1,312,844 teaches that in liquid developers a dispersing agent which is soluble in the carrier liquid for the developer can be employed to disperse pigments and other additives in the liquid. British Patent 1,312,844 also discloses wax as a developer additive.

[0012] However, the use of a wax dispersion containing a dissolved dispersing agent for the wax, presents a problem in replenishing the developer. That is, as the developer is used to produce developed images, the developer becomes depleted of toner particles and must be replenished. Ideally, it is desirable during replenishment to add a concentrated solution of the original developer formulation to the spent developer containing carrier liquid and dissolved components. In the case of the wax dispersion, however, the replenishment problem is increased by the retention of dissolved dispersing agent in the carrier liquid, making it more difficult to simply add a concentrate of original developer without upsetting the delicate equilibrium of developer components.

[0013] In accordance with the present invention, the replenishment problem is alleviated by causing constituents of the added wax dispersion to deplete imagewise from the liquid developer during use together with the other developer constituents. Hence, the developer, when spent, contains little or no disproportionate concentration of components from the wax dispersion, thereby facilitating repeated replenishment without significant loss in image resolution and scratch resistance. How this is achieved is discussed below.

[0014] In the liquid developers and method of forming the liquid developers of the present invention, two liquids (dispersing liquid and carrier liquid), the dispersing agent and the wax are selected from known materials according to the following criteria:

1) the dispersing agent is soluble in the dispersing liquid alone,

2) the dispersing liquid is soluble in and forms a solution with the carrier liquid,

3) the dispersing agent is insoluble in the solution of dispersing liquid and carrier liquid, and

4) the wax is insoluble in both the dispersing liquid and the solution of the two liquids.

[0015] As is well known to those skilled in the art, the solubility of materials in liquids is dependent on a variety of factors such as temperature, pressure, purity, and the chemical nature of the materials themselves. Accordingly, solubility is not absolute but relative, and for purposes of this invention, is to be determined at room temperature. For the purpose of the present invention, therefore, one employs a dispersing agent that is more soluble in the dispersing liquid alone than in the dispersing liquid-carrier liquid solution. Preferably, a substantial difference in solubility exists so that upon mixing a solution of dispersing agent in dispersing liquid with the carrier liquid -- as will occur in the preparation of the liquid developers -- a substantial portion of the dispersing agent will precipitate out and become a dispersed constituent in the carrier liquid-dispersing liquid solution.

[0016] As can be seen from the solubility characteristics of the defined dispersing agent for the wax, it exists in the present liquid developer as a dispersed constituent so that during electrographic development, it electrophoretically deposits and forms part of the toner image along with the other dispersed constituents.

[0017] The liquid developers of the present invention exhibit several advantageous properties including ease of replenishment owing to the solubility characteristics of the wax dispersion constituents, superior abrasion-resistance, and excellent image resolution.

[0018] As noted above, the developer constituents, particularly the electrically insulating liquids, the wax and the dispersing agent for the wax are selected from known materials in accordance with the criteria set forth. In the following paragraphs, exemplary materials are set forth to guide those skilled in the art in selecting materials for use in practicing the invention.

[0019] Electrically insulating organic liquids from which the carrier liquid is selected include halogenated hydrocarbon liquids, for example, fluorinated lower alkanes, such as trichloromonofluoromethane, trichlorotrifluoroethane, etc., having a boiling range typically from about 2° to about 55°C. Other hydrocarbon liquids are useful, such as isoparaffinic hydrocarbons having a boiling range of from about 145° to about 185°C, such as Isopar G^* (Exxon Corp.) or cyclohydrocarbons such as cyclohexane. Additional carrier liquids which are useful in certain situations include polysiloxanes, odorless mineral spirits, octane, and the like.

[0020] The dispersing liquid employed in the developer of the present invention is also electrically insulating and therefore can be selected from the same materials listed above for the carrier liquid as well as other known liquids such as Solvesso 100⁰ (an alkylated aromatic hydrocarbon mixture having a major aromatic component and a boiling range of from 150°-185°C sold by Exxon Corporation.) However, in each case the dispersing liquid is different from the carrier liquid so as to satisfy the solubility criteria 1-4 set forth above for both liquids, the dispersing agent, and wax. For a given dispersing agent, therefore, one can screen potential liquid pairs to determine which liquids together form a solution. With respect to the liquids which satisfy the solution criterion, a further screening is carried out to determine the solubility of the dispersing agent in each of the liquids and in the liquid-liquid solution. In liquid-liquid pairs that satisfy the criteria of the present invention, one of the liquids will readily dissolve the dispersing agent, while the liquid-liquid solution will precipitate out a substantial portion, if not substantially all, the dispersing agent dissolved in the dissolving liquid alone. In such instances, the liquid in which the dispersing agent is soluble is the "dispersing liquid" and the second liquid is the "carrier liquid".

[0021] In connection with the above screening of liquids, the dispersing agent of choice will be relatively insoluble in the carrier liquid alone as well as in the carrier liquid-dispersing liquid solution. In many instances, therefore, the screening process can be modified by simply determining the solubility of the chosen dispersing agent in each of two different liquids alone.

[0022] Examples of useful liquid pairs include any combination from the following lists of dispersing liquids and carrier liquids:

[0023] Waxes employed in the present invention generally include low molecular weight waxes having a softening point of about 60°C to about 130°C. Preferably the waxes are polyolefin waxes such as polyethylene waxes which are available commercially from Eastman Chemical Products, Inc. under the Epolene^* E series polyethylene waxes; Shamrock Chemicals Company (for example, wax types S-394 and S-395); and American Hoechst (for example, VP Ceridust° wax). Other suitable waxes are disclosed in U.S. Patent No. 4,081,391. Other waxes which are useful include carnauba wax, ethylene-propylene copolymer waxes, ester waxes, and amide waxes.

[0024] The dispersing agent which is soluble in and employed to disperse the wax in the dispersing liquid, but which is insoluble in the dispersing liquid-carrier liquid solution, is generally a resinous material such as an ethylene copolymer material that is compatible with the wax. The dispersing agent serves several purposes in the present invention. In its dissolved mode it facilitates obtaining proper wax particle size, while in its dispersed mode it electrophoretically migrates to the charge image and forms part of the toner image. Dispersing agents which are particularly useful with preferred polyethylene waxes include ethylene copolymer materials such as ethylene/vinyl acetate copolymers of varying vinyl acetate content available commercially from DuPont as the Elvax° resin series. Among the Elva_X⁸ resins, the vinyl acetate content appears to affect the solubility of the material in the organic liquids selected for use above. For example, vinyl acetate levels below about 18 percent tend to make the ethylene/vinyl acetate copolymers insoluble in Solvesso 100°. Other useful dispersing agents for the wax include ethylene-acrylic acid copolymers, ethylene-vinyl chloride copolymers, propylene-vinyl acetate copolymers, ethylene-isobutene copolymers, ethylene-maleic anhydride copolymers, and ester modifications of those copolymers having acid functions.

[0025] The ratio, by weight, of the wax to dispersing agent in the present liquid developers can be varied to provide optimum results in terms of colloidal stability and wax particle size. Ratios of wax to dispersing agent from about 1:2 to 4:1 have been successfully employed with a ratio of about 2:1 being preferred.

[0026] Liquid developers according to the present invention also comprise, as a dispersed constituent, a thermoplastic resin binder for the toner image components. Useful resins include polyesters, polycarbonates, polyolefins, halogenated polyolefins, acrylic resins, and resins such as disclosed in U.S. Patent No. 4,052,325.

[0027] Preferred liquid developers include as the binder constituent the thermoplastic polyesteriono- mers disclosed in U.S. Patent No. 4,202,785. In such preferred developers, one of the dispersed components is a polyesterionomer binder resin composed of a polyester terpolymer in which three types of diacid recurring units are employed, one of which comprises an ionic diacid recurring unit.

[0028] The binder concentration can vary widely with a useful concentration range, based on the weight of the total dispersed constituents, being about 10 percent to about 90 percent. A preferred concentration range is from about 40 to about 60 percent.

[0029] The present developers also comprise colorants as optional dispersed components. Useful results may be obtained from virtually any of a wide variety of known dyes or pigment materials. Particularly good results are obtained by using various kinds of carbon black pigments.

[0030] A representative list of colorants may be found, for example, in Research Disclosure, Vol. 109, May, 1973 (published by Industrial Opportunities Ltd.; Homewell, Havant; Hampshire, P09 1EF, United Kingdom), in an article entitled "Electrophotographic Elements, Materials and Processes".

[0031] The colorant concentration, when colorant is present, varies widely with a useful concentration range, by weight of the total dispersed constituents, being about 10 to about 90 percent. A preferred concentration range is from about 35 to about 45 percent.

[0032] Optionally, the developers of the present invention include various charge control agents to enhance uniform charge polarity on the developer toner particles.

[0033] Various charge control agents have been described heretofore in the liquid developer art. Examples of such charge control agents may be found U.S. Patent No. 3,788,995 which describes various polymeric charge control agents, such as terpolymers. Other useful charge control agents include phosphonate materials described in U.S. Patent No. 4,170,563 and quaternary ammonium polymers described in U.S. Patent No. 4,229,513.

[0034] Various non-polymeric charge control agents may also be employed such as, for example, the various metal salts described, for example, in U.S. Patent No. 3,417,019. Other charge control agents known in the liquid developer art may also be employed.

[0035] Examples of preferred polymeric charge control agents for use in the developer of present invention include poly(styrene-co-lauryl methacrylate-co-sulfoethyl methacrylate), poly(vinyltoluene-co-lauryl methacrylate-co-lithium methacrylate-co-methacrylic acid), poly(styrene-co-lauryl methacrylate-co-lithium sulfoethyl methacrylate), poly(vinyltoluene-co-lauryl methacrylate-co-lithium methacrylate), poly(stryene-co-lauryl methacrylate-co-lithium methacrylate), poly(t-butylstyrene-co-lauryl methacrylate-co-lithium methacrylate), poly(t-butylstyrene-co-lithium methacrylate) or poly(vinyl toluene-co-lauryl methacrylate-co- methacryloyloxyethyltrimethylammonium p-toluene sulfonate).

[0036] The amount of charge control agent used will vary depending upon the particular charge control agent and its compatibility with the other components of the developer. It is usually desirable to employ an amount of charge control agent within the range of from about 0.01 to about 10.0 weight percent based on the total weight of a working strength liquid developer composition. The charge control agent may be added in the liquid developer simply by dissolving or dispersing the charge control agent in the carrier liquid at the time concentrates of the components are combined with the carrier liquid to form a working strength developer.

[0037] In the present method, a dispersion of wax is prepared in a solution of dispersing liquid and dispersing agent. Prior art techniques can be modified to prepare the present dispersions. One technique for forming such dispersion is disclosed, for example, in U.S. Patent No. 4,081,391 wherein the hot wax solution is quenched to insolubilize the wax constituent. The addition of soluble dispersing agent to the quenching medium improves the colloidal stability of the wax dispersion and prevents reagglomeration of wax particles. Preferably, however, the wax dispersion is prepared by solvent ball-milling a mixture of wax, dispersing agent and dispersing liquid following the general ball-milling technique disclosed in Patent No. 4,052,325. According to this technique, a quantity of wax and dispersing agent is added to a dispersing liquid such as Solvesso 100^* to form an initial dispersion of wax in a solution of Solvesso 100° and dispersing agent. This mixture is milled with stainless steel milling beads about 0.3 cm in diameter for a suitable time to produce wax particles in the desired particle size range. In this regard, two days of milling has produced wax particles of 5p or less; one week of milling has produced wax particles of 2p or less, and 2-3 weeks has produced submicron particles. In this regard, milling the dispersion to obtain particles in the submicron range is preferred (e.g., particles in the range from about .05 to 1.0 micron range). The concentration of wax (i.e., total solids excluding milling media) in the mixture varies from about 5 to about 15 percent. An advantage of ball-milling is that the amount of wax necessary to achieve comparable scratch resistance in developed toner images is considerably less than the amount of wax in liquid developers prepared from quenched or precipitated wax dispersions.

[0038] Several techniques can be employed to prepare a working strength developer comprising the aforementioned wax dispersion. For example, as disclosed in U.S. Patent No. 4,202,785, one or more developer concentrates are prepared for each of the other developer components (a concentrate is a concentrated solution or dispersion of one or more developer components in a suitable electrically insulating liquid vehicle not necessarily the developer carrier liquid). The concentrates and the above-prepared wax dispersion are then admixed in a preselected sequence, the admixture slurried with the carrier liquid to dilute the components, and the slurry homogenized to form the working strength developer. The liquid employed in forming each concentrate is preferably the same organic liquid employed to form the wax dispersion so that upon combining the concentrates and wax dispersion, only one liquid component in addition to the carrier liquid is introduced into the working strength developer. Alternatively, the wax dispersion is combined with an already formed working strength developer including carrier liquid having dispersed and dissolved components, followed by homogenization as necessary. In general, useful working strength developers of the present invention contain from about 0.05 to about 15% by weight of dispersed components and from about 99.95 to about 85% by weight of liquid and dissolved components. Best results are generally obtained wherein the dispersed components are present in the range of from about 0.1 to about 3% by weight and the liquid and dissolved components are present in the range of from about 99.9% to about 97% by weight of the resultant developer compositions.

[0039] Liquid developers in which the present wax dispersions are usefully employed are generally those comprising an electrically insulating organic carrier liquid having dispersed components and dissolved components. The dispersing liquid employed in the preparation of the wax dispersion may also be present in such developers before the addition of the wax dispersion. Alternatively, the dispersing liquid is first introduced upon addition of the wax dispersion. In all cases, of course, the criteria set forth above for the dispersing liquid, carrier liquid, wax and dispersing agent must be satisfied.

[0040] The present liquid developers are employed to develop electrostatic (charge) images carried by various types of elements. Such elements are either photoconductive themselves or are adapted to receive charge images, as disclosed in U.S. Patent No. 3,519,819. Preferably, the developers are employed with photoconductive film elements containing arylmethane photoconductor compositions such as disclosed in copending U.S. Patent Number 4,301,226.

[0041] The following examples are included for a further understanding of the invention.

Example 1 Preparation of Liquid Developer

[0042] Shamrock Chemicals Corporation S-394⁰ polyethylene wax, Solvesso 100° alkylated aromatic dispersing liquid and Elvax 210° ethylene/vinylacetate dispersing agent were combined in a ratio by weight of 2:1 wax:Elvax° and a total wax plus dispersing agent concentration, based on the weight of the total mixture, of ten percent (10%). The mixture was ball-milled for 2 weeks and 2.63 grams of the mixture added to the following ball-milled Solvesso 100° concentrate:

A) 11.67 gms of a pigment-Solvesso 100° concentrate containing 6% pigment and 4.8% stabilizer polymer, based on the weight of the concentrate.

B) 8.4 gms of a thermoplastic resin-Solvesso 100° concentrate containing 10% thermoplastic resin, based on the weight of the concentrate.

C) 1.23 gms of a charge control agent-Solvesso 100° concentrate containing 10% charge control agent, based on the weight of the concentrate.

[0043] To the resulting admixture of wax dispersion and concentrates was added sufficient Isopar G^* isoparaffinic hydrocarbon carrier liquid and the resulting mixture homogenized to form one liter of a working strength liquid developer containing the following components and concentrations:

Example 2

[0044] The developer of Example 1 containing Shamrock S-395° polyethylene wax in place of S-394⁰, and similar developers varying only in wax and dispersing agent concentration were employed to form toned images on photoconductive recording film element with toner fusion temperatures set at 110°C. The film element comprised a film support, an electrically conducting layer overlying the support and a photoconductive layer overlying the conductive layer. Toners containing 0.2 to 0.3 parts wax by weight per 1.0 part of pigment exhibited highest scratch resistance. Image resolution was about 406 lines/mm, the same resolution as a control developer formulation having no wax, indicating that the wax was appropriately dispersed in a submicron range of particle size.

Example 3 Replenishability

[0045] The developer of Example 1 was employed to develop electrophotographic images on photoconductive film elements. During such use, the concentration of dispersed components was monitored. Replenishment of the developer took place continually when the concentration of dispersed components fell below a preselected level. The replenishing liquid consisted of a 5x (five times) concentrate of the original developer. After approximately four developer turnovers (i.e., the dispersed constituents had been depleted and replenished four times), the image resolution and abrasion resistance of toner images were essentially the same as exhibited by images produced from the original developer.

Claims

1.- A liquid electrographic developer comprising an electrically insulating organic carrier liquid containing dispersed constituents and dissolved constituents, said dissolved constituents comprising an electrically insulating organic dispersing liquid which forms a solution with said carrier liquid, said dispersed constituents comprising at least one thermoplastic resin and wax particles, and characterized in that said developer also contains a dispersing agent for said wax particles which is insoluble in said solution of carrier liquid and dispersing liquid, but soluble in said dispersing liquid alone.

2. The developer of Claim 1 wherein said wax particles comprise a polyolefin wax, carnauba wax, ester wax or an amide wax.

3. The developer of Claim 1 wherein said wax particles comprise a polyethylene wax.

4. The developer of Claim 2 or 3 wherein said dispersing liquid comprises an alkylated aromatic liquid and said carrier liquid comprises an isoparaffinic hydrocarbon liquid.

5. A liquid electrographic developer comprising an electrically insulating organic carrier liquid containing dispersed constituents and dissolved constituents, said dissolved constituents comprising

i) an electrically insulating organic dispersing liquid which forms a solution with said carrier liquid, and

ii) a charge control agent,

said dispersed constituents comprising

a) at least one colorant;

b) at least one thermoplastic resin,

c) polyethylene wax particles, and

d) an ethylene copolymer dispersing agent for said wax particles which is insoluble in said solution of carrier liquid and dispersing liquid, but soluble in said dispersing liquid alone.

6. The developer of Claim 5 wherein said dispersing liquid comprises an alkylated aromatic liquid and said carrier liquid comprises an isoparaffinic hydrocarbon liquid.

7. The developer of Claim 5 or 6 wherein said dispersing agent comprises an ethylene/vinylacetate copolymer.

8. The developer of Claim 6 wherein said dispersing agent comprises an ethylene/vinylacetate copolymer having a vinyl acetate content of at least 18 percent.

9. The developer of Claim 5 or 8 wherein the concentration of polyethylene wax particles is from about 0.01 to about 10 grams per liter of developer.

10. The developer of Claim 9 wherein the particle size of said polyethylene wax particles is from about 0.05 to about 1 micron.

11. The developer of Claim 7 wherein said thermoplastic resin is a polyesterionomer resin comprising ionic diacid recurring units.

12. The developer of Claim 5 wherein said charge control agent comprises a polymeric charge control agent.

13. The developer of Claim 12 wherein said charge control agent comprises poly(styrene-co-lauryl methacrylate-co-sulfoethyl methacrylate), poly(vinyl-toluene-co-lauryl methacrylate-co-lithium methacrylate-co-methacrylic acid), poly(styrene-co-lauryl methacrylate-co-lithium sulfoethyl methacrylate), poly(vinyltoluene-co-lauryl meth- - acrylate-co-lithium methacrylate), poly(stryene-co-lauryl methacrylate-co-lithium methacrylate), poly-(t-butylstyrene-co-lauryl methacrylate-co-lithium methacrylate), poly(t-butylstyrene-co-lithium methacrylate) or poly(vinyl toluene-co-lauryl methacrylate-co-methacryloyloxyethyl trimethylammonium p-toluene sulfonate).

14. The developer of Claim 13 wherein the concentration of said charge control agent is in the range from about 0.01 to about 10.0 weight percent based on the weight of said developer.

15. The developer of Claim 10 wherein the ratio of polyethylene wax to dispersing agent is from about 1:2 to about 4:1.

16. A method for preparing a liquid electrographic developer comprising an electrically insulating organic carrier liquid containing dispersed constituents and dissolved constituents, said dissolved constituents comprising an electrically insulating organic dispersing liquid which forms a solution with said carrier liquid, said dispersed constituents comprising at least one thermoplastic resin and wax particles; the method being characterized by

i) dispersing said wax particles in a solution comprising a dispersing agent dissolved in said organic dispersing liquid, said dispersing agent being relatively insoluble in said solution of carrier liquid and dispersing liquid, but soluble in said dispersing liquid alone,

ii) admixing said wax dispersion with said carrier liquid to form said solution of dispersing liquid and carrier liquid

whereby said dispersing agent becomes a dispersed, insoluble constituent, and

iii) before, during or after steps (i) or (ii), admixing said thermoplastic resin with said organic carrier liquid.

17. The method of Claim 16 wherein said wax particles are dispersed in said solution of dispersing agent in dispersing liquid by solvent ball-milling.