In electrophotographic and electrostatic printing processes (collectively electrographic processes), an electrostatic image is formed on the surface of a photoreceptive element or dielectric element, respectively. The photoreceptive element or dielectric element may be an intermediate transfer drum or belt or the substrate for the final toned image itself, as described by Schmidt, S. P. and Larson, J. R. in Handbook of Imaging Materials Diamond, A. S., Ed: Marcel Dekker: New York; Chapter 6, pp 227–252, and U.S. Pat. Nos. 4,728,983, 4,321,404, and 4,268,598.
In electrostatic printing, a latent image is typically formed by (1) placing a charge image onto a dielectric element (typically the receiving substrate) in selected areas of the element with an electrostatic writing stylus or its equivalent to form a charge image, (2) applying toner to the charge image, and (3) fixing the toned image. An example of this type of process is described in U.S. Pat. No. 5,262,259.
In electrophotographic printing, also referred to as xerography, electrophotographic technology is used to produce images on a final image receptor, such as paper, film, or the like. Electrophotographic technology is incorporated into a wide range of equipment including photocopiers, laser printers, facsimile machines, and the like.
Electrophotography typically involves the use of a reusable, light sensitive, temporary image receptor, known as a photoreceptor, in the process of producing an electrophotographic image on a final, permanent image receptor. A representative electrophotographic process, discharged area development, involves a series of steps to produce an image on a receptor, including charging, exposure, development, transfer, fusing, cleaning, and erasure.
In the charging step, a photoreceptor is substantially uniformly covered with charge of a desired polarity to achieve a first potential, either negative or positive, typically with a corona or charging roller. In the exposure step, an optical system, typically a laser scanner or diode array, forms a latent image by selectively discharging the charged surface of the photoreceptor to achieve a second potential in an imagewise manner corresponding to the desired image to be formed on the final image receptor. In the development step, toner particles of the appropriate polarity are generally brought into contact with the latent image on the photoreceptor, typically using a developer electrically-biased to a potential of the same polarity as the toner polarity and intermediate in potential between the first and second potentials. The toner particles migrate to the photoreceptor and selectively adhere to the latent image via electrostatic forces, forming a toned image on the photoreceptor.
In the transfer step, the toned image is transferred from the photoreceptor to the desired final image receptor; an intermediate transfer element is sometimes used to effect transfer of the toned image from the photoreceptor with subsequent transfer of the toned image to a final image receptor. The image may be transferred by physical pressure and contact of the toner, with selective adhesion to a target intermediate or final image receptor as compared to the surface from which it is transferred. Alternatively, the toner may be transferred in a liquid system optionally using an electrostatic assist as discussed in more detail below. In the fusing step, the toned image on the final image receptor is heated to soften or melt the toner particles, thereby fusing the toned image to the final receptor. An alternative fusing method involves fixing the toner to the final receptor under pressure with or without heat. In the cleaning step, residual toner remaining on the photoreceptor is removed.
Finally, in the erasing step, the photoreceptor charge is reduced to a substantially uniformly low value by exposure to light of a particular wavelength band, thereby removing remnants of the original latent image and preparing the photoreceptor for the next imaging cycle.
Two types of toner are in widespread, commercial use: liquid toner and dry toner. The term “dry” does not mean that the dry toner is totally free of any liquid constituents, but connotes that the toner particles do not contain any significant amount of solvent, e.g., typically less than 10 weight percent solvent (generally, dry toner is as dry as is reasonably practical in terms of solvent content), and are capable of carrying a triboelectric charge. This distinguishes dry toner particles from liquid toner particles.
A typical liquid toner composition generally includes toner particles suspended or dispersed in a liquid carrier. The liquid carrier is typically nonconductive dispersant, to avoid discharging the latent electrostatic image. Liquid toner particles are generally solvated to some degree in the liquid carrier (or carrier liquid), typically in more than 50 weight percent of a low polarity, low dielectric constant, substantially nonaqueous carrier solvent. Liquid toner particles are generally chemically charged using polar groups that dissociate in the carrier solvent, but do not carry a triboelectric charge while solvated and/or dispersed in the liquid carrier. Liquid toner particles are also typically smaller than dry toner particles. Because of their small particle size, ranging from sub-micron to about 5 microns, liquid toners are capable of producing very high-resolution toned images.
A typical toner particle for a liquid toner composition generally comprises a visual enhancement additive (for example, a colored pigment particle) and a polymeric binder. The polymeric binder fulfills functions both during and after the electrophotographic process. With respect to processability, the character of the binder impacts charging and charge stability, flow, and fusing characteristics of the toner particles. These characteristics are important to achieve good performance during development, transfer, and fusing. After an image is formed on the final receptor, the nature of the binder (e.g. glass transition temperature, melt viscosity, molecular weight) and the fusing conditions (e.g. temperature, pressure and fuser configuration) impact durability (e.g. blocking and erasure resistance), adhesion to the receptor, gloss, and the like.
In addition to the polymeric binder and the visual enhancement additive, liquid toner compositions can optionally include other additives. For example, charge directors can be added to impart an electrostatic charge on the toner particles. Dispersing agents can be added to provide colloidal stability, aid fixing of the image, and provide charged or charging sites for the particle surface. Dispersing agents are commonly added to liquid toner compositions because toner particle concentrations are high (inter-particle distances are small) and electrical double-layer effects alone will not adequately stabilize the dispersion with respect to aggregation or agglomeration. Release agents can also be used to help prevent the toner from sticking to fuser rolls when those are used. Other additives include antioxidants, ultraviolet stabilizers, fungicides, bactericides, flow control agents, and the like.
U.S. Pat. No. 4,547,449 to Alexandrovich, et al. discloses liquid electrographic developers comprising an electrically insulating liquid carrier, toner, a charge-control agent and a charging agent. The charge-control agent is a carrier-soluble, addition copolymer of a quaternary ammonium salt monomer, a monomer having —COOH, —SO3H or —PO3HR acidic function wherein R is hydrogen or alkyl, and a solubilizing monomer. The charging agent is a carrier-soluble, addition polar copolymer. The disclosed developers are stated to exhibit improved replenishability as evidenced by reduced buildup of charge in the developers during the course of use and repeated replenishment. Specifically, this patent noted that the prior art exhibited drawbacks relating to the stability of their charge as they are used through a number of copy sequences. In particular, the charge of the developer per unit of mass of dispersed toner of the prior art increases, indicating that the quaternary ammonium charge-control copolymer deposits on an electrostatic image at a lower rate than the toner. This uneven depletion rate and consequential increase in charge per unit mass in the developer presents difficulty in developer replenishment and causes nonuniform image density from copy to copy. The invention as described therein is asserted to stabilize the charge of the developer per unit mass of toner is so that, after a period of use, the buildup of charge per unit of mass is significantly reduced. Such stability is stated to be achieved when the quaternary ammonium salt charge-control polymer in the developer composition contains an insolubilizing monomer having an acidic function selected from the group consisting of —COOH, —SO3H or —PO3HR acidic function wherein R is hydrogen or alkyl.
Charge directors, including certain quaternary ammonium salts, are disclosed in Beyer, U.S. Pat. No. 3,417,019 and Tsuneda, U.S. Pat. No. 3,977,983 for liquid developers.
U.S. Pat. No. 5,627,002 to Pan, et al. discloses a positively charged liquid developer comprised of a nonpolar liquid, thermoplastic resin particles, pigment, a charge control agent, and a charge director comprised of a cyclodextrin or a cyclodextrin derivative containing one or more organic basic amino groups. This patent states that the hollow interiors provide these cyclic molecules with the ability to complex and contain, or trap a number of molecules or ions, such as positively charged ions like benzene ring containing hydrophobic cations, which are known to insert themselves into the cyclodextrin cavities.
U.S. Pat. No. 5,411,834 to Fuller discloses 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. The acid derivative contains pendant fluoroalkyl or pendant fluoroaryl groups, and the charge adjuvant is associated with or combined with said resin and said optional pigment. In certain embodiments, it is stated that “it is important that the thermoplastic resin, copolymers with pendant fluorinated groups as illustrated herein, and the optional second charge adjuvant be sufficiently compatible that they do not form separate particles, and that the charge adjuvant be insoluble in the hydrocarbon to the extent that no more than 0.1 weight percent be soluble in the nonpolar liquid.” See column 8, lines 44–50.
U.S. Pat. No. 6,018,636 to Caruthers discloses an imaging system wherein changes in toner developability of toners in a liquid toner system are determined and compensated for by sensing the toner concentration and liquid toner volume in a tank, based on changes in the toner concentration and toner mass in the tank. Based on measurements made of the toner and/or a test printed image, adjustments can be made, such as creating a new voltage differential or adding toner and/or liquid carrier material to the tank.
U.S. Pat. No. 5,722,017 to Caruthers discloses a liquid developing material replenishment system wherein an apparatus for developing an electrostatic latent image with a liquid developing material includes a liquid developing reservoir for providing a supply of operative liquid developing material to the developing apparatus, and a liquid developing material supply is coupled to the liquid developing material reservoir for providing a supply of liquid developing concentrate to the liquid developing material reservoir for replenishing the supply of operative liquid developing material in the liquid developing reservoir. A developed image having a large proportion of printed image area or having substantially a single color will cause a greater depletion of marking particles and/or charge director in the liquid developing material supply tank as compared to a developed image with a small amount of printed image area or of a single color. This patent explains that                while the rate of the replenishment of the liquid developing material may be controlled by simply monitoring the level of liquid developer in the supply reservoir 116, in advanced systems the rate of replenishment of the liquid carrier, the marking particles, and/or the charge director components of the liquid developing material is controlled in a more sophisticated manner to maintain a predetermined concentration of the marking particles and the charge director in the operative solution stored in the supply reservoir 116. One exemplary replenishment systems of this nature include systems which measure the conductivity of the operative liquid developing material and add selective amounts of charge director compound to the reservoir as a function of the measured a conductivity, as disclosed in detail in U.S. Pat. No. 4,860,924, incorporated by reference herein. Another system of this nature is disclosed in commonly assigned U.S. patent application Ser. No. 08/551,381, also incorporated by reference herein, which describes control of the amount of carrier liquid, charge director and/or marking particles in a liquid developing material reservoir in response to the amount of each component depleted therefrom as a function of the number of pixels making up each developed image.See column 14, line 48 to column 15, line 3.        
U.S. Pat. No. 4,860,924 to Simms, et. al. discloses a copier wherein charge director is supplied to a liquid developer in response to a conductivity measurement thereof. Toner concentrate deficient in charge director is supplied to the liquid developer in response to an optical transmissivity measurement thereof. Conductivity is measured by a pair of spaced electrodes immersed in the developer and through which a variable alternating current is passed. A variable capacitor neutralizes the inherent capacitance of the electrodes. A phase sensitive detector is provided with a reference voltage having the same phase shift as that caused by capacitive effects. The conductivity measurement is corrected in response to a developer temperature measurement.
U.S. Pat. No. 4,935,328 to El-Sayed discloses an electrostatic liquid developer stated to have improved negative charging characteristics consisting essentially of (A) nonpolar liquid having a Kauri-butanol value of less than 30, present in a major amount, (B) thermoplastic resin particles having an average by area particle size of less than 10 μm, (C) charge director compound, and (D) at least one soluble solid or liquid organic monofunctional amine compound of the formula: Rn NH3-n wherein R is alkyl, cycloalkyl or alkylene, or substituted alkyl, the alkyl, cycloalkyl, alkylene or substituted alkyl group being of 1 to 50 carbon atoms, and n is an integer of 1 to 3. The electrostatic liquid developer is useful in copying, making proofs including digital color proofs, lithographic printing plates, and resists.