EP0656569A1

EP0656569A1 - Liquid electrostatic developers with reduced dispersant emissions

Info

Publication number: EP0656569A1
Application number: EP94117009A
Authority: EP
Inventors: Alan Sander Dubin; Sanford Morton Marcus; James Rodney Larson; Steven Paul Schmidt
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1993-11-08
Filing date: 1994-10-27
Publication date: 1995-06-07
Also published as: JPH07181750A

Abstract

An electrostatic liquid developer consisting essentially of a nonpolar liquid having a Kauri-butanol value of less than 30, an average boiling point of 150°C to 400°C, a boiling point range of less than 12°C, and a viscosity of less than 20 cps at 30°C; thermoplastic resin particles having an average by area particle size of less than 10 µm; and a nonpolar liquid soluble ionic or zwitterionic charge director compound.

Description

TECHNICAL FIELD

This invention relates to electrostatic liquid developers comprising resin particles, a charge director compound and a nonpolar liquid having a narrow boiling point range.

BACKGROUND OF THE INVENTION

It is known that a latent electrostatic image can be developed with toner particles dispersed in an insulating nonpolar liquid. Such dispersed materials are known as liquid toners or liquid developers. Useful liquid developers comprise a thermoplastic resin and nonpolar liquid. Generally a suitable colorant is present such as a dye or pigment. The colored toner particles are dispersed in the nonpolar liquid which generally has a high-volume resistivity in excess of 10⁹ ohm centimeters, a low dielectric constant below 3.0, and a high vapor pressure. The toner particles are less than 30 µm average size as determined using the Malvern Particle Sizer described below. After the latent electrostatic image has been formed and developed by the liquid toners, the image may subsequently be transferred to a carrier sheet.
The liquid developers of the prior art comprise resin particles dispersed in mixed hydrocarbon liquids, which have wide molecular weight ranges and hence wide boiling point ranges. These developers have the disadvantage of having high vapor emissions and high viscosities resulting in environmental contamination and low toner particle mobility which retards image quality.
It has been found that the above disadvantages can be overcome and improved developers prepared by using a nonpolar liquid having a narrow boiling point range.

SUMMARY OF THE INVENTION

In accordance with this invention there is provided an electrostatic liquid developer consisting essentially of:

(A) a nonpolar liquid having a Kauri-butanol value of less than 30, an average boiling point between 150°C and 400°C, preferably between 225°C and 325°C, a boiling point range of less than 12°C, preferably less than 10°C, and a viscosity of less than 20 cps at 30°C;
(B) thermoplastic resin particles having an average by area particle size of less than 10 µm, and
(C) an ionic or zwitterionic charge director compound soluble in the nonpolar liquid (A).

DETAILED DESCRIPTION OF THE INVENTION

Throughout the specification the below-listed terms have the following meanings:
In the claims appended hereto "consisting essentially of" means the composition of the electrostatic liquid developer does not exclude unspecified components which do not prevent the advantages of the developer from being realized. For example, in addition to the primary components, there can be present additional components, such as a colorant, fine particle size oxides, adjuvant, e.g., polyhydroxy compound, polybutylene succinimide, aromatic hydrocarbon, etc.
Conductivity is the conductivity of the developer measured in pmhos/cm at 5 hertz and 5 volts.

NONPOLAR LIQUID

The nonpolar liquid (A) is a hydrocarbon having a straight carbon chain of from 9 to 30, preferably 12 to 18, carbon atoms and isomers thereof. Some useful nonpolar liquids are n-nonane, n-decane, n-undecane, n-dodecane, n-tridecane, n-tetradecane, n-pentadecane, n-hexadecane, n-heptadecane, n-octadecane, and branched hydrocarbons such as 2-methyl-nonane, 2-methyl-undecane, 3-methyl-undecane, 4-methyl-undecane, 5-methyl-undecane, 2,3-dimethyl-dodecane, 2,4-dimethyl-dodecane, 2,5-dimethyl-dodecane, 3,5-dimethyl-dodecane, 2,2-dimethyldodecane, 3,3-dimethyl-dodecane, 4,4-dimethyl-dodecane, 4,5-dimethyl-dodecane, 4-methyltetradecane, and 8-ethyltridecane, and 4,6,8-trimethyldodecane.
Some preferred liquids are n-dodecane, n-tridecane, n-tetradecane, n-pentadecane, n-hexadecane, n-heptadecane, and n-octadecane, as obtained for example from Eastman Kodak, Rochester, New York or from Humphrey Chemical Company, New Haven, Connecticut. Some preferred mixtures include a mixture of n-tetradecane, n-pentadecane, and n-pentadecane, and a mixture of n-pentadecane, n-hexadecane and n-heptadecane.
Nonpolar liquids with suitable narrow boiling ranges can be obtained by fractional distillation of mixed hydrocarbon liquids with wide molecular ranges and, hence wide boiling ranges. Examples of commercial hydrocarbon mixtures that can be so purified are Isopars® and Norpars® (Exxon), and Sol B series (Shell). Distillation would be conducted by methods known to those skilled in the art.
Stringent manufacturing specifications keep impurities, such as sulfur, acids, carboxyl, and chlorides, to a few parts per million. The nonpolar liquids are substantially odorless, possessing only a very mild paraffinic odor and have excellent odor stability.
Useful nonpolar liquids have average boiling points between 150°C and 400°C, preferably between 225°C and 325°C and a boiling point range of less than 12°C, preferably less than 10°C. All of the nonpolar liquids have an electrical volume resistivity in excess of 10⁹ ohm centimeters and a dielectric constant below 3.0. The vapor pressures at 25°C are less than 5 Torr. While these are the preferred nonpolar liquids, the essential characteristics of all suitable nonpolar liquids in addition to boiling point range and viscosity are the electrical volume resistivity and the dielectric constant. In addition, a feature of the nonpolar liquids is a Kauri-butanol value less than 30, preferably in the vicinity of 27 or 28, determined by ASTM D 1133.
The ratio of thermoplastic resin to nonpolar liquid is such that the combination of ingredients becomes fluid at the working temperature. The nonpolar liquid is present in an amount of 85 to 99.9% by weight, preferably 97 to 99.5% by weight, based on the total weight of liquid developer. The total weight of solids in the liquid developer is 0.1 to 15%, preferably 0.5 to 3.0% by weight. The total weight of solids in the liquid developer is solely based on the resin, including any components dispersed therein, and any pigment component present.

THERMOPLASTIC RESIN PARTICLES

Useful thermoplastic resins or polymers (B) include: ethylene vinyl acetate (EVA) copolymers (Elvax® resins, E. I. du Pont de Nemours and Company, Wilmington, DE), copolymers of ethylene and an α,β-ethylenically unsaturated acid selected from the group consisting of acrylic acid and methacrylic acid, copolymers of ethylene (80 to 99.9%)/acrylic or methacrylic acid (20 to 0%)/alkyl (C₁ to C₅) ester of methacrylic or acrylic acid (0 to 20%), polyethylene, polystyrene, isotactic polypropylene (crystalline), ethylene ethyl acrylate series sold under the trademark Bakelite® DPD 6169, DPDA 6182 Natural and DTDA 9169 Natural by Union Carbide Corp., Stamford, CN; ethylene vinyl acetate resins, e.g., DQDA 6479 Natural and DQDA 6832 Natural 7 also sold by Union Carbide Corp.; Surlyn® ionomer resin by E. I. du Pont de Nemours and Company, Wilmington, DE, etc., or blends thereof, polyesters, polyvinyl toluene, polyamides, styrene/butadiene copolymers and epoxy resins. The synthesis of copolymers of ethylene and an α,β-ethylenically unsaturated acid of either acrylic acid or methacrylic acid is described in UPS. Patent 3,264,272, the disclosure of which is incorporated herein by reference. For the purposes of preparing the preferred copolymers, the reaction of the acid containing copolymer with the ionizable metal compound, as described in the above patent, is omitted. The ethylene constituent is present in about 80 to 99.9% by weight of the copolymer and the acid component in about 20 to 0.1% by weight of the copolymer. A preferred copolymer is ethylene (89% by weight)/methacrylic acid (11% by weight). The acid numbers of the copolymers range from 1 to 120, preferably 54 to 90. Acid No. is milligrams potassium hydroxide required to neutralize 1 gram of polymer. The melt index (g/10 min) of 10 to 500 is determined by ASTM D 1238, Procedure A. Preferred copolymers of this type have an acid number of 66 and 54 and a melt index of 100 and 500 determined at 190°C, respectively.
Other resins include acrylic resins, such as a copolymer of acrylic or methacrylic acid (optional but preferred) and at least one alkyl ester of acrylic or methacrylic acid wherein alkyl is 1-20 carbon atoms, e.g., methyl acrylate (50-90%)/methacrylic acid (0-20%)/ethylhexyl methacrylate (10-50%); and other acrylic resins including Elvacite® acrylic resins, E. I. du Pont de Nemours and Company, Wilmington, DE or blends of resins, polystyrene; polyethylene; and modified resins disclosed in U.S. Patent 4,798,778, the disclosure of which is incorporated herein.
In addition, the resins have the following preferred characteristics:

1. Able to disperse the colorant, e.g., pigment, etc.;
2. Substantially insoluble in the dispersant liquid at temperatures' below 40°C, so that the resin will not dissolve or solvate in storage;
3. Able to solvate at temperatures above 50°C, whereby the resins forming the toner particles will become swollen, or gelatinous, or softened;
4. Able to be ground to form particles between 0.1 µm and 5 µm, in diameter (preferred size), e.g., determined by Horiba CAPA-500 centrifugal particle analyzer; and between 1 µm and 15 µm in diameter, e.g., determined by Malvern 3600E described below;
5. Able to form a particle (average by area) of less than 10 µm, e.g., determined by Horiba CAPA-500 centrifugal automatic particle analyzer, manufactured by Horiba Instruments, Inc., Irvine, CA: solvent viscosity of 1.24 cps, solvent density of 0.76 g/cc, sample density of 1.32 using a centrifugal rotation of 1,000 rpm, a particle size range of 0.01 to less than 10 µm, and a particle size cut of 1.0 µm, and about 30 µm average particle size, e.g., determined by Malvern 3600E Particle Sizer; and
6. Able to fuse at temperatures in excess of 70°C.

The Malvern 3600E Particle Sizer manufactured by Malvern, Southborough, MA uses laser diffraction light scattering of stirred samples to determine average particle sizes. Since the Horiba and Malvern instruments use different techniques to measure average particle size the readings differ. The following correlation of the average size of toner particles in micrometers (µm) for the two instruments is:

Value Determined By Malvern 3600E Particle Sizer Expected Range For Horiba CAPA-500

30 9.9 + 3.4

20 6.4 + 1.9

15 4.6 + 1.3

10 2.8 + 0.8

5 1.0 + 0.5

3 0.2 + 0.6
This correlation is obtained by statistical analysis of average particle sizes for 67 liquid electrostatic developer samples (not of this invention) obtained on both instruments. The expected range of Horiba values was determined using a linear regression at a confidence level of 95%. In the claims appended to this specification the particle size values are as measured using the Horiba instrument.

CHARGE DIRECTOR COMPOUNDS

Suitable hydrocarbon liquid soluble ionic or zwitterionic charge director compounds, which are generally used in an amount of 0.25 to 1,500 mg/g, preferably 2.5 to 400 mg/g developer solids, include: lecithin, Basic Calcium Petronate®, Basic Barium Petronate®, Neutral Barium Petronate, oil-soluble petroleum sulfonate, manufactured by Sonneborn Division of Witco Corp., New York, NY; alkyl succinimide (manufactured by Chevron Chemical Company of California), etc.; sodium dioctylsulfo succinate (manufactured by American Cyanamid Co.), ionic charge directors such as zirconium octoate, copper oleate, iron naphthenate, etc.; nonionic charge directors, e.g., polyethylene glycol sorbitan stearate, nigrosine, triphenyl methane type dyes and Emphos® D70-30C and Emphos® F-27-85, sold by Witco Corp., New York, NY, sodium salts of phosphated mono- and diglycerides with unsaturated and saturated acid substituents, respectively. Other useful negative charge directors include AB diblock copolymers disclosed in Assignee's U.S. Patent 5,035,972, issued July 30, 1991. Other useful positive charge directors include salts of acid containing AB diblock copolymers disclosed in Assignee's U.S. Patent 5,130,221, issued July 14, 1992.

ADDITIONAL COMPONENTS

As indicated above, additional components that can be present in the electrostatic liquid developer are colorants, such as pigments or dyes and combinations thereof, which are preferably present to render the latent image visible, though this need not be done in some applications. The colorant, e.g., a pigment, may be present in the amount of up to about 60 percent by weight based on the total weight of developer solids, preferably 0.01 to 30% by weight based on the total weight of developer solids. The amount of colorant may vary depending on the use of the developer. Suitable pigments which may be used to advantage are well known to those skilled in the art.
Other ingredients may be added to the electrostatic liquid developer, such as fine particle size oxides, e.g., silica, alumina, titania, etc.; preferably in the order of 0.5 µm or less can be dispersed into the liquefied resin. These oxides can be used alone or in combination with the colorant. Metal particles can also be added.
Another additional component of the electrostatic liquid developer is an adjuvant which can be selected from the group consisting of polyhydroxy compound which contains at least 2 hydroxy groups, polybutylene succinimide, metallic soaps, and aromatic hydrocarbon having a Kauri-butanol value of greater than 30. Such adjuvants are well known to those skilled in the art. The adjuvants are generally used in an amount of 1 to 1000 mg/g, preferably 1 to 200 mg/g developer solids.
The particles in the electrostatic liquid developer have an average by area particle size of 10 µm or less (Horiba instrument). The average particle size determined by the Malvern 3600E Particle Sizer can vary depending on the use of the liquid developer. The resin particles of the developer may or may not be formed having a plurality of fibers integrally extending therefrom although the formation of fibers extending from the toner particles is preferred. The term "fibers" as used herein means toner particles formed with fibers, tendrils, tentacles, threadlets, fibrils, ligaments, hairs, bristles, or the like.

TONER PREPARATION

The electrostatic liquid developer can be prepared by a variety of processes as described in US Patent 4,707,429. For example, into a suitable mixing or blending vessel, e.g., attritor, heated ball mill, heated vibratory mill such as a Sweco Mill manufactured by Sweco Co., Los Angeles, CA, equipped with particulate media, for dispersing and grinding, Ross double planetary mixer manufactured by Charles Ross and Son, Hauppauge, NY, etc., or a two-roll heated mill (no particulate media necessary) are placed the thermoplastic resin and nonpolar liquid described above. Generally the resin, nonpolar liquid and optional colorant are placed in the vessel prior to starting the dispersing step. Optionally the colorant can be added after homogenizing the resin and the nonpolar liquid. Polar additive, similar to that described in Mitchell, U.S. Patent 4,631,244, can also be present in the vessel, e.g., up to 100% based on the weight of polar additive and nonpolar liquid.
The dispersing step is generally accomplished at elevated temperature, i.e., the temperature of ingredients in the vessel being sufficient to plasticize and liquefy the resin but being below that at which the nonpolar liquid or polar additive, if present, degrades and the resin and/or colorant decomposes. A preferred temperature range is 80 to 120°C. Other temperatures outside this range may be suitable, however, depending on the particular ingredients used.
The presence of the irregularly moving particulate media in the vessel is preferred to prepare the dispersion of toner particles. Other stirring means can be used as well, however, to prepare dispersed toner particles of proper size, configuration and morphology. Useful particulate media are particulate materials, e.g., spherical, cylindrical, etc., selected from the group consisting of stainless steel, carbon steel, alumina, ceramic, zirconia, silica, and sillimanite. Carbon steel particulate media are particularly useful when colorants other than black are used. A typical diameter range for the particulate media is in the range of 0.04 to 0.5 inch (1.0 to approx. 13 mm).
After dispersing the ingredients in the vessel, with or without a polar additive present, until the desired dispersion is achieved, typically 1 hour with the mixture being fluid, the dispersion is cooled, e.g., in the range of 0°C to 50°C. Cooling may be accomplished, for example, in the same vessel, such as the attritor, while simultaneously grinding with particulate media to prevent the formation of a gel or solid mass; without stirring to form a gel or solid mass, followed by shredding the gel or solid mass and grinding, e.g., by means of particulate media; or with stirring to form a viscous mixture and grinding by means of particulate media. Additional liquid may be added at any step during the preparation of the liquid electrostatic toners to facilitate grinding or to dilute the toner to the appropriate % solids needed for toning. Additional liquid means nonpolar liquid, polar liquid or combinations thereof.
Cooling is accomplished by means known to those skilled in the art and is not limited to cooling by circulating cold water or a cooling material through an external cooling jacket adjacent the dispersing apparatus or permitting the dispersion to cool to ambient temperature. The resin precipitates out of the dispersant during the cooling. Toner particles of average particle size (by area) of less than 10 µm, as determined by a Horiba centrifugal particle size analyzer or other comparable apparatus, are formed by grinding for a relatively short period of time.
After cooling and separating the dispersion of toner particles from the particulate media, if present, by means known to those skilled in the art, it is possible to reduce the concentration of the toner particles in the dispersion, impart an electrostatic charge of predetermined polarity to the toner particles, or a combination of these variations. The concentration of the toner particles in the dispersion is reduced by the addition of additional nonpolar liquid as described previously above. The dilution is normally conducted to reduce the concentration of toner particles to between 0.1 to 15 percent by weight, preferably 0.3 to 3.0, and more preferably 0.5 to 2 weight percent with respect to the nonpolar liquid. One or more ionic or zwitterionic charge director compounds (C), of the type set out above, can be added to impart a charge. The addition may occur at any time during the process; preferably at the end of the process, e.g., after the particulate media, if used, are removed and the concentration of toner particles is accomplished. If a diluting nonpolar liquid is also added, the charge director compound can be added prior to, concurrently with, or subsequent thereto. If an additional adjuvant compound of a type described above has not been previously added in the preparation of the developer, it can be added prior to or subsequent to the developer being charged. Preferably the adjuvant compound is added after the dispersing step.
Other process embodiments for preparing the electrostatic liquid developer are well known to those of ordinary skill in the art.

INDUSTRIAL APPLICABILITY

The liquid electrostatic developers of this invention demonstrate improved image quality, resolution, solid area coverage (density), and toning of fine details, evenness of toning, and reduced squash independent of charge director or pigment present and further exhibit reduced dispersant emissions. The developers of the invention are useful in copying, e.g., making office copies of black and white as well as various colors; or color proofing, e.g., a reproduction of an image using the standard colors: yellow, cyan, magenta together with black as desired; highlight color copying, e.g., copying of two colors, usually black and a highlight color for letterheads, underlining, etc. In copying and proofing the toner particles are applied to a latent electrostatic image and can be transferred, if desired. Other uses envisioned for the liquid electrostatic developers include: digital color proofing, lithographic printing plates, and resists.

EXAMPLES

In the following controls and examples, parts and percentages are by weight unless otherwise noted. Melt indices are determined by ASTM D 1238, Procedure A; and the average particle sizes by area were determined by a Malvern 3600 Particle Sizer, or the Horiba CAPA 500 centrifugal particle analyzer. Mobilities were determined by an electrokinetic sonic analysis instrument (Matec, Inc., Hopkinton, MA).
Image quality of the developers of the invention was determined on a Savin 870 copier with Xerox 4024 paper in standard operating mode.

Example 1

An unpigmented toner was prepared by adding 294 grams of a copolymer of ethylene (91%) and methacrylic acid (10%), melt index at 190°C of 500, Acid No. of 60, 6 grams of aluminum stearate, and 1 kilograms of Isopar® L (Exxon Chemical Co., Houston, Texas) to a Union Process 1S Attritor, Union Process Company, Akron, Ohio, charged with 0.1875 inch (4.76 mm) diameter carbon steel balls. The mixture was milled at 100°C for 1 hour then cooled to ambient temperature. An additional 500 g of Isopar® L were added and the mixture was milled for 3 hours. The particle size was 6.6 microns measured with an Malvern 3600 E Particle Sizer. Isopar® L was removed and replaced by pure n-tetradecane (b.p. 253°C; Eastman Kodak, Rochester, NY) (Sample 1A) and by Norpar® 15 (b.p. 204-316°C; Exxon) (Sample 1B-Control) with 3 repeated centrifugations, decantations, and dilutions to 1.5% solids. Developer samples 1A and 1B were charged with Basic Barium Petronate® (Witco, New York, NY) to a level which resulted in a conductivity of 20 pmhos/cm. The electrophoretic mobilities of the toners were measured and are reported in Table 1.

Example 2

A toner was prepared by adding 255 grams of Elvacite® 2014 (DuPont Co., Wilmington, DE), 45 grams of cyan pigment NB D 7010 (BASF Corp, Parsippany, NJ)), and 1 kilogram of Isopar® L to a 1S Attritor as described in Example 1. The mixture was milled at 100°C for 1 hour then cooled to ambient temperature. An additional 500 g of Isopar® L were added and the mixture was milled for 3 hours. The particle size was 7.2 microns measured with an Malvern 3600 E Particle Sizer. Isopar® L was removed and replaced by pure n-tetradecane (b.p. 253°C; Eastman Kodak, Rochester, NY) (Sample 2A) and by a mixture (90/10 by mole) of n-hexadecane and n-dodecane (b.p. 216-287°C; both Eastman Kodak, Rochester, NY) (Sample 2B-Control) with 3 repeated centrifugations, decantations, and dilutions to 1.5% solids. Developer Samples 2A and 2B were charged with Emphos® D70-30C Witco, New York, NY) to a level which resulted in a conductivity of 20 pmhos/cm. The electrophoretic mobilities of the toners were measured and are reported in Table 1.

TABLE 1

Sample	Vapor Pressure(Torr@ 25°C)^a	ESA Mobility (10x¹⁰ m²/V Sec)
1A	0.010	-5.3
1B(Control)	0.08	-3.8
2A	0.010	2.65
2B(Control)	0.012	1.42

a Vapor pressures for pure hydrocarbons were obtained from "TRC Thermodynamic Table - Hydrocarbons, Thermodynamics Center, The Texas A&M University System, College Station, TX". Vapor pressures for Norpar 15 and Isopar V were obtained from Exxon product literature.

Example 3

A toner was prepared by adding 240 grams of the copolymer of Example 1, 3 grams of aluminum stearate, 57 grams of carbon black Sterling NS (Cabot Corp, Boston, Massachusetts) and 1 kilogram of Isopar® L to a 1S Attritor as described in Example 1. The mixture was milled at 100°C for 1 hour then cooled to ambient temperature. An additional 500 g of Isopar® L were added and the mixture was milled for 3 hours. The particle size was 8.0 microns measured with a Malvern 3600 E Particle Sizer. Isopar® L was removed to bring the toner concentration to 55% solids. For Sample 3A the toner was diluted to 1.5% solids in pure n-hexadecane (b.p. 287°C; Eastman Kodak). For Sample 3B (Control) the toner was diluted to 1.5% solids in Isopar® V (b.p. 255-295; Exxon). To each was added Basic Barium Petronate® (Witco Corp., New York, NY) in the amount of 50 milligrams/gram of toner solids. Properties of these toners are reported in Table 2. Image quality was determined using a Savin 870 copier with Xerox 4024 paper in standard operating mode and the results are presented in Table 3. It is clear that the images obtained with the toner dispersed in n-hexadecane gave superior images with higher image density, lower background, and higher resolution.

TABLE 2

Sample	Viscosity (cp)^a	Charge/Mass (µC/gram)^b Bulk - Particle	Mobility (µcm/vsec)^c
			Zeta III	Indigo
3A	3.01	63 - 20	0.030	0.46
3B(Control)	6.66	47 - 17	0.009	0.10

a Haake CV3 viscometer (Haake Inc., Saddlebrook, NJ) using rotating shear with a coaxial tool.
b Indigo Charge Meter (Indigo Ltd., Rehovot, Israel). Bulk measurement was made with 1.5% toner. Particle data was obtained from the difference in charge of the bulk and the supernatant. All measurements were performed at a field of 1000 v/mm.
c Zetasizer III (Malvern Ins., Malvern, England) at a field of 15 v/mm. Indigo Mobility Apparatus (Indigo Ltd., Rehovot, Israel) at 1700 v/mm)

TABLE 3

Sample	Conductivity (pmho/cm)	Density Image/Background	Resolution (lp/mm)
3A	8	1.2/0.0	7
3B(Control)	4	0.8/0.2	6

The electrostatic liquid developer when used to develop an electrostatic image results in improved image quality, reduced squash, improved solid area coverage independent of the pigment and charge director compound present.

Claims

An electrostatic liquid developer consisting essentially of:
(A) a nonpolar liquid having a Kauri-butanol value of less than 30, an average boiling point of 150°C to 400°C, a boiling point range of less than 12°C, and a viscosity of less than 20 cps,;

(B) thermoplastic resin particles having an average by area particle size of less than 10 µm; and

(C) an ionic or zwitterionic charge director compound soluble in the nonpolar liquid (A).
The developer of claim 1 wherein the nonpolar liquid has a boiling point range of less than 10°C.
The developer of Claim 1 wherein the nonpolar liquid is n-tetradecane.
The developer of claim 1 wherein the nonpolar liquid is n-pentadecane.
The developer of claim 1 wherein the nonpolar liquid is n-hexadecane.
The developer of claim 1 wherein the nonpolar liquid is n-heptadecane.
The developer of claim 1 wherein the nonpolar liquid is a mixture of n-tetradecane, n-pentadecane, and n-hexadecane.
The developer of claim 1 wherein the nonpolar liquid is a mixture of n-pentadecane, n-hexadecane, and n-heptadecane.
The developer of claim 1 further comprising up to about 60% by weight of a colorant based on the total weight of developer solids.
The developer of claim 9 wherein the colorant is a pigment.
The developer of claim 9 wherein the colorant is a dye.
The developer of claim 1 wherein a fine particle size oxide is present.
The developer of claim 1 wherein component (A) is present in the amount of 85 to 99.9% by weight, by weight based on the total weight of the liquid developer, the total weight of the developer solids is 0.1 to 15% and component (C) is present in the amount of 0.25 to 1500mg/g of developer solids.
The developer of claim 1 further comprising an adjuvant selected from the group consisting of polyhydroxy compounds, polybutylene succinimide, metallic soaps, and an aromatic hydrocarbon.
The developer of claim 1 wherein the thermoplastic resin component (B) is a copolymer of at least one alkyl ester of acrylic or methacrylic acid wherein alkyl is 1 to 20 carbon atoms and acrylic or methacrylic acid.
The developer of claim 1 wherein the thermoplastic resin component is a copolymer of methyl methacrylate (50-90%)/methacrylic acid (0-20%)/ethylhexyl acrylate (10-50%).
The developer of claim 16 wherein the thermoplastic resin component is a copolymer of methyl methacrylate (67%)/methacrylic acid (3%)/ethylhexyl acrylate (30%).
The developer of claim 1 wherein the thermoplastic resin component is a copolymer of ethylene (89%)/methacrylic acid (11%) having a melt index at 190°C of 100.
The developer of claim 1 wherein the particles have an average particle size by area of less than 15 µm.