Electrographic toner, transfer process and development...

Radiation imagery chemistry: process – composition – or product th – Electric or magnetic imagery – e.g. – xerography,... – Post imaging process – finishing – or perfecting composition...

Reexamination Certificate

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Details

C430S114000, C430S116000

Reexamination Certificate

active

06190817

ABSTRACT:

FIELD OF THE INVENTION
The present invention concerns an electrographic toner, a transfer process, and a development system for developing electrostatic images.
RELATED TECHNOLOGY
Conventional development methods use either dry or liquid toners. Both cases are associated with certain inconveniences.
In the case of dry toners, it is difficult to achieve reliable triboelectric deposition of toner particles, since each individual toner particle must be brought into contact with the deposition surface in a reproducible manner. Triboelectric deposition surfaces are subject to wear due to the fact that toner material is rubbed off and minute, strongly attached dust particles are deposited. Uncharged toners result in undesirable dust buildup. Toner charged with the wrong polarity affects the sharpness of edges and the uniformity of the background. The choice of materials for toners and equipment surfaces is limited due to the triboelectric requirements for all components. In addition, toner particles with 5 &mgr;m diameter and smaller represent a health hazard by inhalation as fine dust.
Liquid developing processes are normally based on electrophoresis. The liquid toner used in this process is a non-conductive dielectric carrier liquid with suspended toner particles. The toner particles are charged via deposition of ions from the liquid, and when the liquid is brought into the proximity of the charging surface, the charged toner particles are attracted by the electric field of the latent image on the charging surface. The toner particles are transferred to the charging surface together with the carrier liquid, and the carrier liquid is then released to the atmosphere.
In order not to destroy the latent image on the charging surface, organic carrier media with a very high volume resistivity are used. A commonly used carrier medium is available from Exxon under the name Isopar. The known liquid carrier media, however, represent a problem from the point of view of occupational safety and the environment. Furthermore, the choice of materials for toner and equipment surfaces is limited due to the requirements for all components because of the electrostatic charges and compatibility with the carrier liquid. In addition, the toner is not handled economically, since the toner material is only 3% of the total developer volume.
SUMMARY OF THE INVENTION
An object of the invention is to provide a toner that can be used with liquid carrier media that are safer for the operators and the environment, as well as to provide a developing process and a developing system for this toner.
The present invention therefore provides an electrographic toner with a plurality of polymer particles wherein each polymer particle has functional groups on its surface, which functional groups can dissociate into two components in a carrier liquid, with one component of each group firmly attached to the surface, so that the individual polymer particles carry an electrostatic charge in the carrier liquid.
The toner according to the present invention can be used in combination with carrier liquids that are non-toxic, slightly volatile, incombustible, and do not attack the equipment surfaces. If the polymer particles are suspended in the carrier liquid, part of the functional groups on the surface of the polymer particles is dissociated, and each polymer particle carries an electric charge that is positive or negative depending on the type of the functional groups. Suitable carrier liquids may include aqueous solutions or other media capable of stabilizing the dissociation of the charge carriers. Surfactants or protective colloids, for example, can be used for stabilizing the suspensions in both aqueous and non-aqueous systems.
Preferably all polymer particles have basically the same number of such functional groups on their surfaces, which dissociate into two components in a carrier liquid, so that each polymer particle in the carrier liquid carries a precisely defined electric charge, determined by its chemical composition. This allows the amount of toner material to be increased in the overall volume of the developer, since, contrary to conventional liquid developing methods, there is no danger of the background purity being affected by individual particles with insufficient charge. The higher concentration of the toner in the developer saves carrier liquid and, since less carrier liquid is transferred onto the printing substrate together with the toner during printing, the remaining carrier liquid is more easily removed from the printing substrate, resulting in gentler treatment of the printing substrate.
Uniform chargeability of the polymer particles is achieved in a simple manner by having basically spherical polymer particles of basically the same size. The average diameter of the polymer particles is preferably less than 7 &mgr;m, better less than 2 &mgr;m, and the size variation is preferably approximately 10% or less. The dissociatable functional groups can either be uniformly distributed over the volume of the polymer particles or located only on the surface of the polymer particles.
Polymer particles with these properties can be prepared, for example, using heterophase polymerization, such as suspension or emulsion methods. The functional groups on the particle surface that carry the electric charges of one polarity in the carrier liquid can be, for example, phosphonate, —SO
3
—, —CO
2
—, or —NR
3
+, where R is an organic radical. The carriers of the corresponding countercharges, dissociated in the carrier liquid, are preferably alkali metal or alkali earth ions, halides, small charged molecules, or polyions. Particles with such functional groups are known per se, however, they have not typically been used in toners, but only, for example, as demineralizing agents in ion exchange columns.
In a single-component developer, all polymer particles have a coloring agent. In the case of a two-component developer, a first kind of polymer particles containing a coloring agent and a second kind of polymer particles that does not necessarily contain a coloring agent are provided. If they are suspended in the carrier liquid and the countercharges are dissociated in the carrier liquid or removed therefrom, the two kinds of polymer particles have opposite polarities, with the second kind forming the carrier particles for the first kind. The number of charges per carrier particle, as well as the size and mass of the carrier particles, can be different from those of the toner particle. Often the carrier particles have a considerably larger diameter than the toner particles. The carrier particles can be electrically conducting and/or magnetized, which increases the flexibility of the system compared to a single-component system.
An electrographic developing system to be used with the above-described toner contains a movable component, which can be, for example, a continuous belt running around one or more rollers, or a rotating cylinder, and it is partially immersed in a bath with an electrode opposite the portion of the movable part that is immersed in the bath. According to the present invention, a membrane or a diaphragm is located between the surface of the component and the electrode, which membrane can be traversed by atoms or small molecules, but not by macroscopic particles such as the toner particles. The counterions dissolved in the carrier liquid are attracted toward the electrode through the membrane by a suitable voltage, and the toner particles are attracted to the developer surface of the movable component. The developer surface of the movable component has a potential selected so that the toner particles adhere to it electrostatically.
Since the counterions are permanently dissociated in the carrier liquid, the toner particles and their counterions can be physically separated using electrolysis according to the process of the present invention prior to being transferred to the developer surface. This results in an even layer of uniformly charged toner particles on the developer surface, providing outstand

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