Liquid developer compositions with copolymers

Abstract: A positively charged liquid developer comprised of a nonpolar liquid, thermoplastic resin particles, pigment, a charge director, and a charge control agent comprised of a PEO:PPO, preferably in a solid form. The developer is suitable for use in electrographic imaging processes.

Description: This invention is generally directed to liquid developer compositions suitable for use in electrographic image on image printing processes, particularly color image printing processes. Developers can discharge the electrostatic charge by exposing it to a modulated beam of radiant energy. Other methods are also known for forming latent electrostatic images such as, for example, providing a carrier with a dielectric surface and transferring a preformed electrostatic charge to the surface. After the latent image has been formed, the image is developed by colored toner particles dispersed in a nonpolar liquid. The image may then be transferred to a receiver sheet. Also known are ionographic imaging systems. Insufficient particle charge can result in poor image quality and also can result in poor transfer of the liquid developer or solids thereof to paper or other final substrates. Poor transfer can, for example, result in poor solid area coverage if insufficient toner is transferred to the final substrate and can also cause image defects such as smears and hollowed fine features. Conversely, overcharging the toner particles can result in low reflective optical density images or poor color richness or chroma since only a few very highly charged particles can discharge all the charge on the dielectric receptor causing too little toner to be deposited. To overcome or minimize such problems, the liquid toners, or developers of the present invention were arrived at after substantial research, and which developers result in, for example, sufficient particle charge to enable effective transfer but not so much charge as to yield images with lower optical densities and lower residual voltages because of excess toner charge. An advantage associated with the present invention includes controlling the increase of the desired positive charge on the developer particles. JP-A-07-325434 discloses a liquid developer for electrophotography comprising a carrier, a charge director contained in the carrier and toner particles dispersed in the carrier, said toner particles comprising a thermoplastic resin, a colorant and a charge control agent. The charge director may comprise a block copolymer of polyethylene glycol and polypropylene glycol. US-A-5304450 discloses a process for the preparation of polymer particles comprising the steps of dissolving a polymer or copolymer in a solvent containing block or graft steric stabilizers, and precipitating polymer particles by the addition of a component in which the polymer is insoluble. The polymer particles may be used for the preparation of liquid toners. The developers of the present invention in embodiments provide images with excellent, for from 1.3 to 1.4 reflective optical density (ROD) and/or especially lower residual voltages (V out ), for example less than 45, and for example from 25 to 45. Higher reflective optical densities provide images with deeper, richer desirable color or more extended chroma. Lower residual image voltages enable the printing of subsequently applied layers to a higher reflective optical density and decrease or eliminate image defects, such as smearing and shifts in L*a*b* color space (hue shifts), when one colored layer is overlaid on a second layer of different color. Series-Capacitance Data was utilized as a means of measuring the total charge in the liquid developer formulation, and which measurements indicate that placing too much charge on the toner or developer particles can cause lower RODs to occur, which is a manifestation of inferior image quality because less chroma occurs. Moreover, there can be added to the liquid developers of the present invention in embodiments thereof charge directors of the formulae as illustrated in US-A-5563015, especially a mixture of Alohas, an abbreviated name for aluminum-di-tertiary butyl salicylate and EMPHOS PS-900™, or Alohas alone. It is an object of the present invention to provide positively charged liquid developers wherein developed image defects, such as smearing, loss of resolution and loss of density, and color shifts in prints having magenta images overlaid with black or other suitable colored images are eliminated or minimized. It is another object of the present invention to provide positively charged liquid developers with higher reflective optical density (ROD) and/or lower residual (V out ) for developed images. It is a further object of the present invention to provide liquid toners that enable excellent image characteristics, and which toners enhance the positive charge of the resin, such as ELVAX®, based colored toners. The present invention provides a positively charged liquid developer containing a nonpolar liquid, a resin, a colorant, a charge director and a charge control agent, wherein said charge control agent comprises a poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) triblock copolymer. The present invention further provides an imaging method which comprises forming an electrostatic latent image followed by the development thereof, wherein said development is conducted with the liquid developer as defined above. Preferred embodiments of the present invention are set forth in the claims. In embodiments, the present invention is directed to liquid developers comprised of a nonpolar liquid, a pigment, a resin, preferably a thermoplastic resin, a PEO:PPO charge control agent, and a charge director, such as the aluminum salts of alkylated salicylic acid, like, for example, hydroxy bis[3,5-tertiary butyl salicylic] aluminate, or a mixture of the aluminum salts of alkylated salicylic acid, like, for example, hydroxy bis[3,5-tertiary butyl salicylic] aluminate and EMPHOS PS-900™,reference US-A-5563015. In a preferred aspect, the present invention relates to a positively charged liquid developer comprised of a nonpolar liquid, thermoplastic resin particles, the triblock copolymer charge control agent, an optional charge adjuvant, an optional pigment, and a charge director comprised of a mixture of I. a nonpolar liquid soluble organic phosphate mono and diester mixture derived from phosphoric acid and isotridecyl alcohol, and II. a nonpolar liquid soluble organic aluminum complex, or mixtures thereof of the formulae wherein R 1 is selected from the group consisting of hydrogen and alkyl groups, and n represents a number, such as from 1 to 6. Of importance with respect to the present invention is the selection of the PEO:PPO charge control agent, which agent is mixed with the toner resin and pigment, and thereafter a charge director is added thereto. PEO:PPO examples are poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) triblock copolymers of the formula (CH 2 -CH 2 -O) x -(CH 2 -CHCH 3 -O) y -(CH 2 -CH 2 -O) x wherein x and y represent the average number of ethylene oxide and propylene oxide repeat units in each of their respective blocks or segments. In the triblock copolymer formula representative of the Pluronic series available from BASF, the preferred PEO:PPO:PEO triblock copolymer charge control agent is Pluronic F-108 (Table 1) in which x and y are about 132 and 52, respectively, when the average triblock copolymer molecular weight (M w ) is about 14,600 and the two ethylene oxide blocks are of about equal length and comprise about 80 weight percent of the total triblock copolymer molecular weight. Specifically, the BASF F108 is believed to possess an M w of 14,600 with 30 weight percent of PEO and a melting point of 56°C. Examples of PEO-PPO-PEO triblock copolymer compositions available from BASF are illustrated in Table 1, wherein L designates liquid and F designates prill or spherical pellets (solid), and x and y are the average degrees of polymerization or DPs for the PEO and PPO blocks, respectively. Pluronic PEO-PPO-PEO Triblock Copolymer Compositions BASF PEO-PPO-PEO Triblock Copolymer Approximate Total M w of Triblock Copolymer Approximate M w of Propylene Oxide Block Approximate Wt. % of Both Ethylene Oxide Blocks Approximate DP ave for PEO (x) & PPO (y) Blocks x y L-35 1900 900 50 11.3 15.5 F-77 6600 2100 70 51.1 36.2 F-87 7700 2400 70 60.2 41.3 F-127 12600 3600 70 102.1 62.0 F-38 4700 900 80 43.1 15.5 F-68 8400 1800 80 74.9 31.0 F-88 11400 2400 80 102.1 41.3 F-98 13000 2700 80 116.9 46.5 F-108 14600 3000 80 131.7 51.7 Triblock copolymers can be of three physical forms including solids, pastes, and liquids and tend to be solids at molecular weights of 4,700 and higher when the ethylene oxide content is about 80 weight percent and the propylene oxide content is about 20 weight percent (F-38 in Table 1). Generally, as the propylene oxide content increases and/or the triblock copolymer molecular weight decreases, the triblock copolymer tends to become paste like and eventually a liquid at very low molecular weights. The preferred triblock PEO-PPO-PEO copolymer charge control agents for the invention liquid developers are solids with low PPO contents (less than or equal to 50 weight percent and more preferably less than or equal to 30 weight percent). Less preferred triblock PEO-PPO-PEO copolymer charge control agents are pastes which are mixtures of liquids and solids. The solid triblock copolymers are less likely to be washed out of the toner particle (solid phase) into the developer carrier fluid (liquid phase) and will be better retained within the toner or solids particles and/or on the surface of the toner particles wherein the charge control agent can easily perform its normal function of modulating toner charging. Higher PPO content (> 50 weight percent) triblock copolymers are more hydrophobic and thus are more likely to be hydrocarbon carrier fluid miscible which increases the probability of some charge control agent wash out from the surface of the solid particles. Maintaining the charge control agent in the particles, or on the particle surface enables maximum developer charge modulation and minimum charge exchange between undesirably located charge control agent in the carrier fluid and desirably located charge director in the carrier fluid. Charge exchange between components in the carrier fluid can cause undesirable high supernatant conductivities and low optical density in prints obtained from liquid developers participating in the charge exchange. In principle, higher molecular weight PEO-PPO-PEO triblock copolymers may also be selected. For example, when the PEO content is maintained at 80 weight percent and the x and y values are 1,056 and 416, respectively, a triblock copolymer molecular weight of about 117,000 results. The triblock polymer charge control agents can be selected as mixtures, for example from 1 to 99 weight percent of one triblock, and from 99 to 1 of a second triblock. In embodiments, the M w of the polypropylene oxide block and the polyethylene oxide block are from 2,000 to 50,000 at any weight percent composition for each block wherein the resulting PEO-PPO-PEO triblock copolymer is a solid or paste. In embodiments of the present invention, the PEO:PPO is selected in various effective amounts, such as for example from 0.05 to 10, and preferably from 3 to 7 weight percent based on the total weight percent of the solids of resin, pigment, and PEO:PPO. For example, when 5 weight percent of PEO:PPO is selected, 55 weight percent of resin, and 40 weight percent of pigment is selected. Examples of nonpolar liquid carriers or components selected for the developers of the present invention include a liquid with an effective viscosity of, for example, from 0.5 to 500 mPa·s (0.5 to 500 centipoise), and preferably from 1 to 20 mPa·s (1 to 20 centipoise), and a resistivity equal to or greater than 5 x 10 9 ohm/cm, such as 5 x 10 13 . Preferably, the liquid selected is a branched chain aliphatic hydrocarbon. A nonpolar liquid of the ISOPAR® series (manufactured by the Exxon Corporation) may also be used for the developers of the present invention. These hydrocarbon liquids are considered narrow portions of isoparaffinic hydrocarbon fractions with extremely high levels of purity. The liquids selected generally have an electrical volume resistivity in excess of 10 9 ohm-centimeters and a dielectric constant below 3.0 in embodiments of the present invention. Moreover, the vapor pressure at 25°C is preferably less than 10 Torr. The amount of the liquid employed in the developer of the present invention is, for example, from 85 to 99.9 percent, and preferably from 90 to 99 percent by weight of the total developer dispersion, however, other effective amounts may be selected. The total solids, which include resin, pigment and the PEO:PPO Y charge control additive content of the developer in embodiments is, for example, 0.1 to 15 percent by weight, preferably 0.3 to 10 percent, and more preferably, 0.5 to 10 percent by weight. Typical suitable thermoplastic toner resins can be selected for the liquid developers of the present invention in effective amounts, for example, in the range of 99.9 percent to 40 percent, and preferably 80 percent to 50 percent of developer solids comprised of thermoplastic resin, pigment, charge control agent, and in embodiments other components that may comprise the toner. Generally, developer solids include the thermoplastic resin, pigment and charge control agent. Examples of resins include polyesters, especially the SPAR polyesters, commercially available, and see for example US-A-3590000; reactive extruded polyesters, with a gel amount of from 10 to 40 percent, and other gel amounts, or substantially no gel, reference US-A-5376494; ethylene vinyl acetate (EVA) copolymers (ELVAX® resins, E.I. DuPont de Nemours and Company, Wilmington, Delaware); copolymers of ethylene and an alpha, beta-ethylenically unsaturated acid selected from the group consisting of acrylic acid and methacrylic acid; copolymers of ethylene (80 to 99.9 percent), acrylic or methacrylic acid (20 to 0.1 percent)/alkyl (C1 to C5) ester of methacrylic or acrylic acid (0.1 to 20 percent). The liquid developers of the present invention preferably contains a colorant dispersed in the resin particles. Colorants, such as pigments or dyes and mixtures thereof, are preferably present to render the latent image visible. The colorant, preferably a pigment, may be present in the toner in an effective amount of, for example, from 0.1 to 60 percent, and preferably from 10 to 50, and in embodiments 40 percent by weight based on the total weight of solids contained in the developer. The amount of colorant used may vary depending on the use of the developer. Examples of pigments which may be selected include carbon blacks available from, for example, Cabot Corporation, FANAL PINK™, PV FAST BLUE™, pigments as illustrated in US-A-5223368, and other known pigments. Examples of charge directors present in various effective amounts of, for example, from 0.001 to 5, and preferably from 0.005 to 1 weight percent or parts, include aluminum di-tertiary-butyl salicylate; hydroxy bis[3,5-tertiary butyl salicylic] aluminate; hydroxy bis[3,5-tertiary butyl salicylic] aluminate mono-, di-, tri- or tetrahydrates; hydroxy bis[salicylic] aluminate; hydroxy bis[monoalkyl salicylic] aluminate; hydroxy bis[dialkyl salicylic] aluminate; hydroxy bis[trialkyl salicylic] aluminate; hydroxy bis[tetraalkyl salicylic] aluminate; hydroxy bis[hydroxy naphthoic acid] aluminate; hydroxy bis[monoalkylated hydroxy naphthoic acid] aluminate; bis[dialkylated hydroxy naphthoic acid] aluminate wherein alkyl preferably contains 1 to 6 carbon atoms; bis[trialkylated hydroxy naphthoic acid] aluminate wherein alkyl preferably contains 1 to 6 carbon atoms; bis[tetraalkylated hydroxy naphthoic acid] aluminate wherein alkyl preferably contains 1 to 6 carbon atoms; and the like in admixture with EMPHOS PS-900™. To further increase the toner particle charge and, accordingly, increase the mobility and transfer latitude of the toner particles, charge adjuvants can be added to the toner particles. For example, adjuvants, such as metallic soaps like aluminum or magnesium stearate or octoate, fine particle size oxides, such as oxides of silica, alumina, titania, and the like, paratoluene sulfonic acid, and polyphosphoric acid, may be added. These types of adjuvants can assist in enabling improved toner charging characteristics, namely, an increase in particle charge that results in improved electrophoretic mobility for improved image development and transfer to allow superior image quality with improved solid area coverage and resolution in embodiments. The adjuvants can be added to the toner particles in an amount of from 0.1 percent to 15 percent of the total developer solids, and preferably from 3 percent to 7 percent of the total weight percent of solids contained in the developer. The liquid electrostatic developer of the present invention can be prepared by a variety of processes such as, for example, mixing in a nonpolar liquid the thermoplastic resin, charge control agent, and colorant in a manner that the resulting mixture contains, for example, 30 to 60 percent by weight of solids; heating the mixture to a temperature of from 40°C to 110°C until a uniform dispersion is formed; adding an additional amount of nonpolar liquid sufficient to decrease the total solids concentration of the developer to 10 to 30 percent by weight; cooling the dispersion to 10°C to 30°C; adding the aluminum charge director compound to the dispersion; and diluting the dispersion. Disclosed is a positively charged liquid developer comprised of a nonpolar liquid, a resin, a pigment, a charge director and a charge control agent comprised of a poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) triblock copolymer; a developer wherein the poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) triblock copolymer is a solid, and is of the formula (CH 2 -CH 2 -O) x -(CH 2 -CHCH 3 -O) y -(CH 2 -CH 2 -O) x wherein x and y represent the number of ethylene oxide and propylene oxide repeat segments, respectively; a developer wherein x is from 43 to 1,056 and y is from 16 to 416, and the triblock copolymer possesses a (M w ) molecular weight range of from 4,700 to 11,7000 when the triblock copolymer has a composition of about 80 percent polyethylene oxide (PEO) and about 20 percent polypropylene oxide (PPO); a liquid developer wherein the liquid has a viscosity of from 0.5 to 20 mPa·s (0.5 to 20 centipoise) and resistivity equal to or greater than about 5 x 10 9 , and the resin has a volume average particle diameter of from 0.1 to 30 µm (microns); a developer wherein the resin is a copolymer of ethylene and vinyl acetate; a developer wherein the pigment is present in an amount of from 0.1 to 60 percent by weight based on the total weight of the developer solids; a developer wherein the pigment is carbon black, cyan, magenta, yellow or mixtures thereof; a developer wherein the charge control agent is present in an amount of from 0.05 to 10 weight percent based on the weight of the developer solids of resin, pigment and charge control agent; a developer wherein the liquid for the developer is an aliphatic hydrocarbon; a developer wherein the aliphatic hydrocarbon is a mixture of branched hydrocarbons of from 8 to 16 carbon atoms, or a mixture of normal hydrocarbons of from 8 to 16 carbon atoms; a developer wherein the aliphatic hydrocarbon is a mixture of branched hydrocarbons of from 8 to 16 carbon atoms; a developer wherein the resin is an alkylene polymer, a styrene polymer, an acrylate polymer, a polyester, or mixtures thereof; an imaging method which comprises forming an electrostatic latent image followed by the development thereof with the liquid developer illustrated herein; an ionographic imaging method which comprises charging a receptor followed by the development thereof with the developer illustrated herein; a developer wherein the charge director is a nonpolar liquid soluble organic aluminum complex; a developer wherein the charge director is comprised of a mixture of I. a nonpolar liquid soluble organic phosphate mono and diester mixture derived from phosphoric acid and isotridecyl alcohol, and II. a nonpolar liquid soluble organic aluminum complex, or mixtures thereof of the formulas wherein R 1 is selected from the group consisting of hydrogen and alkyl groups, and n represents a number; a developer containing a charge adjuvant; a positively charged liquid developer comprised of a nonpolar liquid, thermoplastic resin particles, a pigment, a charge director, and a charge control agent comprised of poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) copolymer with an Mw of from greater than 4,700 to 117,000 when the PPO content is less than or equal to 50 percent; a developer wherein the copolymer is of the formula (CH 2 -CH 2 -O) x -(CH 2 -CHCH 3 -O) y -(CH 2 -CH 2 -O) x wherein x and y represent the average number of consecutive structural repeat units in a block of polyethylene oxide (PEO) or polypropylene oxide (PPO); a developer wherein the weight percent of PEO-PPO-PEO polyethylene oxide/polypropylene oxide/polyethylene oxide is 40/20/40 and the Mw thereof is 12,000; wherein the weight percent of PEO-PPO-PEO is 40/20/40 and the Mw thereof is 6,000; wherein the weight percent of PEO-PPO-PEO is 30/20/50 and the M w thereof is 12,000; wherein the weight percent of PEO-PPO-PEO is 60/20/20 and the M w thereof is 12,000; wherein the weight percent of PEO-PPO-PEO is 30/20/50 and the M w thereof is 12,000; wherein the weight percent of PEO-PO-PEO is 30/40/30 and the M w thereof is 6,000; wherein the weight percent of PEO-PPO-PEO is 50/20/30 and the M w thereof is 12,000; or wherein the weight percent of PEO-PPO-PEO is 10/40/50 and the M w thereof is 6,000; a developer wherein the poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) triblock copolymer charge control agent is a solid, and x and y are greater than 11.3 and 15.5 for PEO and PPO, respectively, when the triblock copolymer M w is greater than 1,900 and the PPO composition is less than or equal to 50 weight percent; a positively charged liquid developer wherein the charge control is comprised of mixtures of said poly(ethylene oxide-b-propylene oxide-b-ethylene oxide). The toner particle size can range from 0.1 to 3.0 µm and the preferred particle size range is from 0.5 to 1.5 µm. Particle size, when measured, was measured by a Horiba CAPA-500 centrifugal automatic particle analyzer manufactured by Horiba Instruments, Inc., Irvine, CA. The total developer charge (Q in microcoulombs) was measured using the series-capacitor technique. The charge in all samples was measured at 400 volts for 0.05 second. The electrical properties of liquid developers can be reviewed using a series-capacitor method, which is a well-established method for determining the dielectric relaxation time in partially conductive materials as, for example, might be found in "leaky" capacitors, reference US-A-5459077. Two series capacitors can be used. One is comprised of a dielectric layer (MYLAR®) which corresponds to the photoreceptor, the other is comprised of a layer of liquid (ink). Although a constant bias voltage is maintained across the two capacitors, the voltage across the ink layer decays as the charged particles within it move. Measurement of the external currents allows the observation of the decay of voltage across the ink layer. Depending on the composition of the ink layer, this reflects the motion of charged species, in real time, as in the various, actual liquid immersion development processes of this invention. Application of a co-developed theoretical analysis, together with a knowledge of the dielectric thicknesses of the MYLAR® and ink layers, the applied bias voltage and the observed current, enables the measurement of the total collected charge (Q). EXAMPLES Controls 1A and 1B = 40 Percent of Rhodamine Y Magenta; No CCA: One hundred sixty-two (162.0) grams of ELVAX 200W® (a copolymer of ethylene and vinyl acetate with a melt index at 190°C of 2,500, available from E.I. DuPont de Nemours & Company, Wilmington, Del.), 108.0 grams of the magenta pigment (Sun Rhodamine Y 18:3 obtained from Sun Chemicals) and 405 grams of ISOPAR-M® (Exxon Corporation) were added to a Union Process O1 attritor (Union Process Company, Akron, Ohio) charged with 4.76 millimeters (0.1857 inch) diameter carbon steel balls. The mixture was milled in the attritor which was heated with running steam through the attritor jacket at 56°C to 86°C for 2 hours. 675 Grams of ISOPAR-G® were added to the attritor at the conclusion of 2 hours, and cooled to 23°C by running water through the attritor jacket, and ground in the attritor for an additional 2 hours. Additional ISOPAR-G®, about 300 grams, was added and the mixture was separated from the steel balls. To 293.54 grams of the mixture (14.308 percent solids) were added 2,499.46 grams of ISOPAR-G® (Exxon Corporation), and 7.0 grams of 1:1 Alohas/PS-900 (Witco) charge director (3 weight percent in ISOPAR-M®) to provide a charge director level of 5 milligrams of charge director per gram of toner solids (Control 1A). After print testing the Control 1A developer, an additional 7.0 grams of 1:1 Alohas/PS-900 (Witco) charge director (3 weight percent in ISOPAR-M®) were added to this developer to give a charge director level of 10 milligrams of charge director per gram of toner solids (Control 1B). The Control 1B developer was then print tested in the same way as was the Control 1A developer. The charge of the resulting liquid toner or developer after print testing, in the Xerox Corporation 8936 throughout unless otherwise indicated, was measured by the series capacitance method and was found to be 0.30 for the Control 1A developer and 0.26 for the Control 1B developer. Alohas is an abbreviated name for hydroxy bis(3,5-di-tertiary butyl salicylic) aluminate monohydrate, reference for example US-A-5366840 and US-A-5324613. Examples 1A and 1B = 40 Percent of Rhodamine Y Magenta; 5 Percent of PEO:PPO (Pluronic F-108): One hundred forty-eight point five (148.5) grams of ELVAX 200W® (a copolymer of ethylene and vinyl acetate with a melt index at 190°C of 2,500, available from E.I. DuPont de Nemours & Company, Wilmington, Del.), 108.0 grams of the magenta pigment (Sun Rhodamine Y 18:3), 13.5 grams of the charge additive PEO:PPO (Pluronic F-108), and 405 grams of ISOPAR-M® (Exxon Corporation) were added to a Union Process O1 attritor (Union Process Company, Akron, Ohio) charged with 4.76 millimeters (0.1857 inch) diameter carbon steel balls. The mixture was milled in the attritor which was heated with running steam through the attritor jacket at 56°C to 86°C. for 2 hours. 675 Grams of ISOPAR-G® were added to an attritor at the conclusion of the 2 hours, and cooled to 23°C by running water through the attritor jacket, and ground in the attritor for an additional 2 hours. Additional ISOPAR-G®, about 300 grams, was added and the mixture was separated from the steel balls. To 289.94 grams of the mixture (14.486 percent solids) were added 2503.06 grams of ISOPAR-G® (Exxon Corporation), and 7.0 grams of 1:1 Alohas/PS-900 (Witco) charge director (3 weight percent in ISOPAR-M®) to provide a charge director level of 5 milligrams of charge director per gram of toner solids (Example 1A). After print testing the Example 1A developer, an additional 7.0 grams of 1:1 Alohas/PS-900 (Witco) charge director (3 weight percent in ISOPAR-M®) were added to this developer to give a charge director level of 10 milligrams of charge director per gram of toner solids (Example 1 B). The Example 1B developer was then print tested on the 8936 of Control 1A. The charge of the resulting liquid toner or developer after print testing was measured by the series capacitance method, and was found to be 0.40 for the Example 1A developer and 0.46 for the Example 1B developer. The Xerox ColorgrafX System 8936 is a 36 inch wide multiple pass ionographic printer. The printer parameters were adjusted to obtain a contrast of 50 and a speed of 2.0 ips by inputting values on the control panel. After single pass prints were made with the above parameter settings using the standard test printing mode (sail patterns), the residual development voltage was measured using an Electrostatic Volt Meter (Trek Model No. 565). This value is shown as residual voltage [(V out )]. This parameter is valuable because it is a measurement used to predict the amount of undesired color shifting (also referred to as staining) of the developed toner layer upon subsequent development passes. The reflective optical density (ROD), a color intensity measurement of chroma, was measured with a MacBeth 918 color densitometer using the substrate paper background as a reference. The paper used to test print these images was Rexham 6262. A series of measurements were accomplished with the following results: For Control 1A, which contained 40 weight percent of Rhodamine Y magenta pigment and zero weight percent of CCA, and wherein the milligrams of charge director per gram of toner solids was 5/1; 1:1 by weight of Alohas/PS-900, the total charge of the developer in microcoulombs was 0.30, the reflective optical density was 1.36, and the residual voltage was 65V. For Control 1B, which contained 40 weight percent of Rhodamine Y magenta pigment and zero weight percent of CCA, and wherein the milligrams of charge director per gram of toner solids was 10/1; 1:1 by weight of Alohas/PS-900, the total charge of the developer in microcoulombs was 0.26, the reflective optical density was 1.34, and the residual voltage was 55V. For Example 1A, which contained 40 weight percent of Rhodamine Y magenta pigment and 5 weight percent of PEO:PPO (Pluronic F-108) CCA, and wherein the milligrams of charge director per gram of toner solids was 5/1; 1:1 by weight of Alohas/PS-900, the total charge of the developer in microcoulombs was 0.40, the reflective optical density was 1.35, and the residual voltage was 42V. For Example 1B, which contained 40 weight percent of Rhodamine Y magenta pigment and 5 weight percent of PEO:PPO (Pluronic F-108) CCA, and wherein the milligrams of charge director per gram of toner solids was 10/1; 1:1 by weight of Alohas/PS-900, the total charge of the developer in microcoulombs was 0.46, the reflective optical density was 1.32, and the residual voltage was 35V. For improved image quality in multilayered images, it is preferred that RODs increase or stay the same, which permits more intense color or chroma, and V outs decrease, which minimize color staining or hue shifts of a magenta image after overcoating said magenta image with a yellow toner. The thickness of a developed layer, for example yellow, is dependent upon the charging level (proportional to applied voltage) on the dielectric receptor. Since a constant voltage is generally applied to the dielectric receptor in development of all layers in a multilayered image, large residual voltages, as might occur after development of the magenta layer, add to the applied voltage resulting in a thicker yellow layer. A thicker yellow layer overlaid on the thinner magenta layer can cause the latter to color shift towards orange. Review of the measurements and data presented herein indicates that increasing the charge director level in the no CCA magenta control developers, Controls 1A and 1B, failed to increase the developer charging levels (total Q), and reflective optical densities (ROD) of the developed magenta images remained essentially constant. When 5 percent PEO:PPO (Pluronic F-108) CCA was incorporated into what was otherwise the same magenta developer formulations as were used in Controls 1A and 1B, Examples 1A and 1B magenta developers were produced with charging levels of 0.40 and 0.46 versus 0.30 and 0.26 for the corresponding Control developers 1A and 1B when using the same charge director (CD) and levels thereof. Although the RODs of the developed magenta layers decreased only slightly in Examples 1A and 1B versus Controls 1A and 1B, the residual voltages (V out ) on the developed magenta toner layers decreased significantly to 42 and 35 volts, down from 65 and 55 volts in the corresponding no CCA developers in Controls 1A and 1B. By increasing the magenta developer charging level in Example 1 B to 0.46 from 0.26 in Control 1 B, it is believed that the conductivity of the developer also increased slightly causing the developed magenta layer residual voltage in Example 1 B to decrease, while having little effect on reflective optical density (ROD of 1.32) versus Control 1B (ROD of 1.34). Side by side inspection of Example 1B and Control 1B (magenta images overcoated with yellow images) images indicated a visually observable color shift of the Control 1B image toward orange versus the Example 1B image when both sets of prints were made using identical machine printing parameters. The incorporation of the PEO:PPO (Pluronic F-108) charge control agents into the developer formulation modulates the initial developer charging level to a new developer charging level having essentially the same reflective optical density, but a lower residual voltage as was found for the PEO:PPO (Pluronic F-108) developers. Thus, this charge control agent simultaneously tunes the charging level (total Q), reflective optical density (ROD) and residual voltage (V out ).

Claims: A positively charged liquid developer containing a nonpolar liquid, a resin, a colorant, a charge director and a charge control agent, wherein said charge control agent comprises a poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) triblock copolymer. The developer of claim 1, wherein said poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) triblock copolymer is a solid, and is of the formula (CH 2 -CH 2 -O) x -(CH 2 -CHCH 3 -O) y -(CH 2 -CH 2 -O) x wherein x and y represent the number of ethylene oxide and propylene oxide repeat segments, respectively. The developer of claim 2, wherein x is from 43 to 1,056 and y is from 16 to 416, and said triblock copolymer possesses a (Mw) molecular weight range of from 4,700 to 11,7000 when the triblock copolymer has a composition of about 80 percent polyethylene oxide (PEO) and about 20 percent polypropylene oxide (PPO). The liquid developer of claim 2, wherein said liquid has a viscosity of from 0.5 to 20 mPa.s (0.5 to 20 centipoise) and a resistivity equal to or greater than 5 x 10 9 ohm-cm, and said resin has a volume average particle diameter of from 0.1 to 30 µm (microns). The developer of any of claims 1 to 4, wherein the pigment is present in an amount of from 0.1 to 60 percent by weight based on the total weight of the developer solids and the charge control agent is present in an amount of from 0.05 to 10 weight percent based on the weight of the developer solids of resin, pigment and charge control agent. The developer of any of claims 1 to 5, wherein the liquid for said developer is an aliphatic hydrocarbon, preferably a mixture of branched hydrocarbons of from 8 to 16 carbon atoms, or a mixture of normal hydrocarbons of from 8 to 16 carbon atoms. The developer of any of claims 1 to 6, wherein the resin is selected from the group consisting of an alkylene polymer, a styrene polymer, an acrylate polymer, a polyester, a copolymer of ethylene and vinyl acetate and mixtures thereof. The developer of any of claims 1 to 7, wherein said charge director is a nonpolar liquid-soluble organic aluminum complex. The developer of any of claims 1 to 7, wherein said charge director comprises a mixture of I. a nonpolar liquid-soluble organic phosphate mono and diester mixture derived from phosphoric acid and isotridecyl alcohol, and II. a nonpolar liquid-soluble organic aluminum complex, or mixtures thereof of the formulae wherein R 1 is selected from the group consisting of hydrogen and alkyl groups, and n represents a number. An imaging method which comprises forming an electrostatic latent image followed by the development thereof, wherein said development is conducted with the liquid developer of any of claims 1 to 9.

Inventors: BONSIGNORE FRANK J, CHAMBERLAIN SCOTT D, GIBSON GEORGE A, KNAPP CHRISTOPHER M, PAN DAVID H, SPIEWAK JOHN W, BONSIGNORE, FRANK J., CHAMBERLAIN, SCOTT D., GIBSON, GEORGE A., KNAPP, CHRISTOPHER M., PAN, DAVID H., SPIEWAK, JOHN W.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1997
Main classification: G03G9
IPC codes: G03G9