This invention is generally directed to liquid developer compositions and, in particular, to liquid developers containing high molecular weight ionic or zwitterionic ammonium block copolymers. More specifically, in embodiments the present invention relates to liquid developers with charge directors derived from the alkylation or protonation of poly-2-ethylhexylmethacrylate-co-N',N'dimethylamino-2-ethylmethacrylate (EHMA-DM AEMA) A-B diblock copolymers which form inverse micelies with the ammonium ionic or polar end of the block copolymer directed or faced inward and the nonpolar EHM A tail pointing in a direction outward toward the hydrophobic hydrocarbon vehicle selected for the liquid developer, and wherein the number average molecular weight, determined, for example, from by dividing the number of moles of monoinitiator into the number of grams of acrylic monomer being initiated by the charged molar quantity of monoinitiator, of the charge director is from about 70,000 to about 200,000, preferably from about 80,000 to about 150,000, and more preferably about 85,000 to 100,000.
With the aforementioned molecular weights, there are enabled liquid developers with a number of advantages such as high particle charge with low conductivities. The low conductivities result primarily from the larger micelies which originate from the high molecular weight charge director. The large micelie reduces the conductivity, it is believed, in, for example, the following manner: 1) the electrophoretic mobility is reduced as the size of the micelie increases due to viscous drag; and 2) as the size of the micelie increases, the number of micelies decreases at the same total mass loading of the charge director, resulting in a decrease in the micelie charge density. For example, the effect of charge director molecular weight on the electrophoretic mobility, size, and charge density of micelies formed from the AB diblock ammonium charge directors is illustrated in the following Table.
______________________________________ Conductivity Micelle Charge of 0.1% Charged Density of (by weight) Micelle 0.1% (by Charge Charge Electro- weight) Director Director in phoretic Charge Molecular NORPAR 15 Mobility Director Weight (M.sub.n) (ps/cm) (E-6 cm.sup.2 /Vs) (.mu.C/cm.sup.3) ______________________________________ Very Low 43 11 3.5 (2K) Low (4K) 43 5.4 5.1 Medium 6 2.5 1.9 (25K) Medium 2 2.2 1.0 (50K) High (93K) 0.6 1.5 0.5 ______________________________________
Furthermore, it has been determined that these high molecular weight charge directors result in low conductivity liquid toner dispersions with high particle charge. For example it has been found that a developer charged with a 93,519 molecular weight AB diblock EHMA-DMAEMA.HBr enables particles with a mobility greater than 4 E-10 m.sup.2 /Vs measured, for example, by the ESA method disclosed herein, and a conductivity of a 1 percent developer solids liquid toner dispersion measured with a Scientifica AC conductivity meter disclosed herein of about less then 4 ps/centimeter. The corresponding liquid toner dispersion charged with a 4,000 molecular weight AB diblock EHMA-DMAEMA-HBr enables particles with a mobility of less than 3.5 E-10 m.sup.2 /Vs and a conductivity greater than 8 ps/centimeters. The developers of the present invention can be selected for a number of known imaging and printing systems, such as xerographic processes, wherein latent images are rendered visible with the liquid developer illustrated herein. The image quality, solid area coverage and resolution for developed images usually require sufficient toner particle electrophoretic mobility. The mobility for effective image development is primarily dependent on the imaging system used. The electrophoretic mobility is primarily directly proportional to the charge on the toner particles and inversely proportional to the viscosity of the liquid developer fluid. A 10 to 30 percent change in fluid viscosity caused, for instance, by a 5.degree. C. to 15.degree. C. decrease in temperature could result in a decrease in image quality, poor image development and background development, for example, because of a 5 percent to 23 percent decrease in electrophoretic mobility. Insufficient particle charge can also result in poor transfer of the toner to paper or other final substrates. Poor or unacceptable transfer can result in, for example, poor solid area coverage if insufficient toner is transferred to the final substrate, and can also lead to image defects such as smears and hollowed fine features. To overcome or minimize such problems, the liquid toners of the present invention were arrived at after substantial research efforts, and which toners result in, for example, sufficient particle charge for transfer and maintain the mobility within the desired range of the particular imaging system employed. Advantages associated with the present invention include a high developer particle charge, a low conductivity; and further increasing the desired negative charge on the developer particles, and in embodiments providing a charge director that is superior to similar charge directors like tetraalkyl quaternary ammonium block copolymers, lecithin, and metal salts of petroleum fractions. The superior charge can result in improved image development and superior image transfer. The low conductivity of the dispersions obtained in the present invention improve the developability of the liquid toner dispersion as the high concentration of mobile ions in high conductivity liquid dispersions compete with the toner particles for the latent electrostatic image in the xerographic process. The high concentration reduction of mobile ions can also disrupt other steps in the xerographic printing process such as the electrostatic transfer of the image from the image bearing member to a substrate. In some desirable applications of the xerographic printing process, a subsequent electrostatic image is applied to the image bearing member over a previously developed image. In this process, often referred to as an image-on-image process, a high concentration of mobile ions in the first image would distort the electrostatic latent image being developed in the subsequent development.
A latent electrostatic image can be developed with toner particles comprised of resin, pigment, and charge adjuvant dispersed in an insulating nonpolar liquid. The aforementioned dispersed materials are known as liquid toners or liquid developers. A latent electrostatic image may be generated by providing a photoconductive layer with a uniform electrostatic charge and subsequently discharging 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, it is developed by colored toner particles dispersed in a nonpolar liquid. The image may then be transferred to a receiver sheet.
Useful liquid developers can comprise a thermoplastic resin, colorant like pigment or dye, and a dispersant nonpolar liquid. The colored toner particles are dispersed in a nonpolar liquid which generally has a high volume resistivity in excess of 10.sup.9 ohm-centimeters, a low dielectric constant, for example below 3.0, and a high vapor pressure. Generally, the toner particles are less than 10 microns (.mu.m) average by area size as determined by the Horiba Capa 500 or 700 particle sizers.
Since the formation of images depends, for example, on the difference of the charge between the toner particles in the liquid developer and the latent electrostatic image to be developed, it has been found desirable to add a charge director compound and charge adjuvants which increase the magnitude of the charge, such as polyhydroxy compounds, amino alcohols, polybutylene succinimide compounds, aromatic hydrocarbons, metallic soaps, and the like to the liquid developer comprising the thermoplastic resin, the nonpolar liquid and the colorant.
U.S. Pat. No. 5,019,477, the disclosure of which is totally incorporated herein by reference, discloses a liquid electrostatic developer comprising a nonpolar liquid, thermoplastic resin particles, and a charge director. The ionic or zwitterionic charge directors may include both negative charge directors, such as lecithin, oil-soluble petroleum sulfonate and alkyl succinimide, and positive charge directors such as cobalt and iron naphthanates. The thermoplastic resin particles can comprise a mixture of (1) a polyethylene homopolymer or a copolymer of (i) polyethylene and (ii) acrylic acid, methacrylic acid or alkyl esters thereof, wherein (ii) comprises 0.1 to 20 weight percent of the copolymer; and (2) a random copolymer of (iii) selected from the group consisting of vinyl toluene and styrene and (iv) selected from the group consisting of butadiene and acrylate.
U.S. Pat. No. 5,030,535 discloses a liquid developer composition comprising a liquid vehicle, a charge control additive and toner particles. The toner particles may contain pigment particles and a resin selected from the group consisting of polyolefins, halogenated polyolefins and mixtures thereof. The liquid developers are prepared by first dissolving the polymer resin in a liquid vehicle by heating at temperatures of from about 80.degree. C. to about 120.degree. C., adding pigment to the hot polymer solution and attriting the mixture, and then cooling the mixture so that the polymer becomes insoluble in the liquid vehicle, thus forming an insoluble resin layer around the pigment particles, may be selected from known thermoplastics, including fluoropolymers.
U.S. Pat. No. 5,026,621 discloses a toner for electrophotography which comprises as main components a coloring component and a binder resin which is a block copolymer comprising a functional segment (A) consisting of at least one of a fluoroalkylacryl ester block unit or a fluoroalkyl methacryl ester block unit, and a compatible segment (B) consisting of a fluorine-free vinyl or olefin monomer block unit. The functional segment of block copolymer is oriented to the surface of the block polymer, and the compatible segment thereof is oriented to be compatible with other resins and a coloring agent contained in the toner whereby the toner is provided with both liquid repelling and solvent soluble properties.
Moreover, in U.S. Pat. No. 4,707,429 there are illustrated, for example, liquid developers with an aluminum stearate charge additive. Liquid developers with charge directors are also illustrated in U.S. Pat. No. 5,045,425. Further, stain elimination in consecutive colored liquid toners is illustrated in U.S. Pat. No. 5,069,995. Additionally, of interest are U.S. Pat. Nos. 4,760,009 and 5,034,299.
The disclosures of each of the U.S. patents mentioned herein are totally incorporated herein by reference.
In copending patent application U.S. Ser. No. 986,316, the disclosure of which is totally incorporated herein by reference, there is illustrated a process for forming images which comprises (a) generating an electrostatic latent image; (b) contacting the latent image with a developer comprising a colorant and a substantial amount of a vehicle with a melting point of at least about 25.degree. C., which developer has a melting point of at least about 25.degree. C., the contact occurring while the developer is maintained at a temperature at or above its melting point, the developer having a viscosity of no more than about 500 centipoise and a resistivity of no less than about 10.sup.8 ohm-cm at the temperature maintained while the developer is in contact with the latent image; and (c) cooling the developed image to a temperature below its melting point subsequent to development.
In copending patent application U.S. Ser. No. 249,827, filed concurrently herewith, the disclosure of which is totally incorporated herein by reference, there is illustrated a liquid developer with AB copolymer charge directors.
In U.S. Pat. No. 5,306,591, there is disclosed a liquid developer comprised of thermoplastic resin particles, a charge director, and a charge adjuvant comprised of an imine bisquinone; and U.S. Pat. No. 5,308,731 discloses a liquid developer comprised of a liquid, thermoplastic resin particles, a nonpolar liquid soluble charge director, and a charge adjuvant comprised of a metal hydroxycarboxylic acid.
In copending patent application U.S. Ser. No. 185,341, the disclosure of which is totally incorporated herein by reference, there is illustrated a liquid developer comprised of a liquid, thermoplastic resin particles, a nonpolar liquid soluble charge director comprised of a zwitterionic quaternary ammonium block copolymer wherein both cationic and anionic sites contained therein are covalently bonded within the same polar repeat unit in the quaternary ammonium block copolymer.
Further, illustrated in copending patent applications U.S. Ser. No. 200,988 is a positively charged liquid developer comprised of thermoplastic resin particles, optional pigment, a charge director, and a charge adjuvant comprised of a polymer of an alkene and unsaturated acid derivative; and wherein the acid derivative contains pendant ammonium groups, and wherein the charge adjuvant is associated with or combined with said resin and said optional pigment; in U.S. Ser. No. 204,012 is a negatively charged liquid developer comprised of thermoplastic resin particles, optional pigment, a charge director, and an insoluble charge adjuvant comprised of a copolymer of an alkene and an unsaturated acid derivative, and wherein the acid derivative contains pendant fluoroalkyl or pendant fluoroaryl groups, and wherein the charge adjuvant is associated with or combined with said resin and said optional pigment; and in U.S. Ser. No. 204,016 is a liquid developer comprised of thermoplastic resin particles, optional pigment, and a charge director comprised of a mixture of an organic anioinic complex phosphate ester and organic aluminum complex, or mixtures thereof of the formulas ##STR1## wherein R.sub.1 is selected from the group consisting of hydrogen and alkyl, and n represents a number, the disclosures of which are totally incorporated herein by reference.