Radiation imagery chemistry: process – composition – or product th – Electric or magnetic imagery – e.g. – xerography,... – Post imaging process – finishing – or perfecting composition...
Reexamination Certificate
2001-05-11
2003-07-08
Chapman, Mark A. (Department: 1753)
Radiation imagery chemistry: process, composition, or product th
Electric or magnetic imagery, e.g., xerography,...
Post imaging process, finishing, or perfecting composition...
C430S111300, C430S111310
Reexamination Certificate
active
06589703
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to electrography and more particularly it relates to electrographic methods and apparatus using developer compositions comprised of hard magnetic carrier particles for development of electrostatic charge images.
BACKGROUND OF THE INVENTION
In electrography, an electrostatic charge image is formed on a dielectric surface, typically the surface of the photoconductive recording element. Development of this image is typically achieved by moving it into proximity with a developer composition comprising a mixture of pigmented resinous material, known as toner, and magnetically attractable particles, known as carrier. For a two-component developer comprising a mixture of toner particles and carrier particles, the carrier particles serve as sites against which the non-magnetic toner particles can impinge and thereby acquire a triboelectric charge opposite to that of the electrostatic image, when using a charged area development configuration. When the electrostatic image is in proximity with the developer mixture, the toner particles are stripped from the carrier particles to which they had formerly adhered (via triboelectric forces) by the relatively strong electrostatic forces associated with the charge image. In this manner, the toner particles are deposited on the electrostatic image to render it visible. Both contact toning and non-contact toning are known in the art. The present invention can be used with either contact toning or with non-contact toning, and, hereinafter, the term “contact”, or its equivalent forms, is used for convenience to describe the developer being in proximity to and in developing relationship with the electrostatic image so that development of the image with toner occurs. Therefore, it should be understood that the scope of the present invention is not limited to contact toning. Discharged area development is also known to the art, and the invention is equally applicable to both charged area development and to discharged area development.
It is generally known to apply developer compositions of the above type to electrostatic images by means of a magnetic applicator comprising a cylindrical sleeve of non-magnetic material having a magnetic core positioned within. The core usually comprises a plurality of parallel magnetic strips which are arranged around the core surface to present alternating north and south oriented magnetic fields. These fields project radially, through the sleeve, and serve to attract the developer composition to the sleeve outer surface to form what is commonly referred to in the art as a “brush” or “nap”. Either or both of the cylindrical sleeve and the magnetic core are rotated with respect to each other to cause the developer to advance from a supply sump to a position in proximity to the electrostatic image to be developed. After development, the toner depleted carrier particles are returned to the sump for toner replenishment.
Conventionally, carrier particles made of soft magnetic materials have been employed to carry and deliver the toner particles to the electrostatic image. U.S. Pat. Nos. 4,546,060, 4,473,029 and 5,376,492, the teachings of which are incorporated herein by reference in their entirety, teach the use of hard magnetic materials as carrier particles and also apparatus for the development of electrostatic images utilizing such hard magnetic carrier particles. These patents require that the carrier particles comprise a hard magnetic material exhibiting a coercivity of at least 300 Oersteds when magnetically saturated and an induced magnetic moment of at least 20 EMU/gm when in an applied magnetic field of 1000 Oersteds. The terms “hard” and “soft” when referring to magnetic materials have the generally accepted meaning as indicated on page 18 of Introduction To Magnetic Materials by B. D. Cullity published by Addison-Wesley Publishing Company, 1972.
The hard magnetic carrier materials represent a great advance over the use of soft magnetic carrier materials in that the speed of development is remarkably increased with good image development. Speeds as high as four times the maximum speed utilized in the use of soft magnetic carrier particles have been demonstrated.
In the methods taught by the foregoing patents, a developer comprising carrier particles of a hard magnetic material is moved in the direction of the electrostatic image to be developed by high speed rotation of the multi-pole magnetic core within the sleeve, with the developer being disposed on the outer surface of the sleeve. Rapid pole transitions on the sleeve are mechanically resisted by the carrier because of its high coercivity. Chains of carrier particles comprising the nap of the carrier (with toner particles disposed on the surface of the carrier particles), rapidly “flip” on the sleeve in order to align themselves with the magnetic field reversals imposed by the rotating magnetic core, and as a result, move with the toner on the sleeve through the development zone in actual contact with, or in proximity and developing relationship to, the electrostatic image on a photoconductor. As mentioned previously, this interaction of the developer with the charge image is referred to as “contact” or “contacting” hereinafter for purposes of convenience. Further, the sleeve may also be rotated to increase the velocity of the developer. See also, U.S. Pat. No. 4,531,832, the teachings of which are also incorporated herein in their entirety, for further discussion concerning such a process.
The rapid pole transitions, for example as many as 467 per second at the sleeve surface when the magnetic core is rotated at a speed of 2000 rpm, create a highly energetic and vigorous movement of developer as it moves through the development zone. This vigorous action constantly recirculates the developer to the sleeve surface and then back to the outside of the nap to provide toner for development. This flipping action also results in a continuous feed of fresh toner particles to the image. As described in the above-described patents, this method provides high density, high quality images at relatively high development speeds.
Various materials have been used to treat the surfaces of the toner component of the developer composition to enhance toner performance in an electrographic process. In recent years, polymers with lower melting points, particularly polyester based resins, have been used as a toner resin since they complement the high speed printing equipment and systems recently developed by the printer/copier industry. Also, various other toner addenda, such as low molecular weight polyethylene and polypropylene waxes, have been used to modify the toner resin and improve performance. Incorporating the foregoing materials into the toner can adversely impact the powder flow properties of the developer composition and eventually image quality, and, therefore, the use of silica and/or other metal oxides as a surface treatment for toner to promote flowability has become increasingly important. Also, silica and other metal oxides have been used to reduce adhesion of the toner particles to the dielectric surface bearing the toned electrostatic image, which reduced adhesion can result in better transfer of the toned image to a receiver for the same, such as a paper sheet.
Such uses of surface treated toner are mentioned, for example, in U.S. Pat. No. 5,286,917, which discloses the use of silica in connection with a one-component developer to increase fluidity of the toner. Silica and other surface treatment agents are said to be used for the same or similar reasons in U.S. Pat. Nos. 5,729,805; 4,982,689; and 4,377,332.
A problem associated with the development systems disclosed in the foregoing patents concerns their use of a stationary, i.e., non-rotating, magnetic core and a developer which includes a soft magnetic carrier and surface-treated toner. For example, these patents disclose that the desired low force of adhesion, low coefficient of friction property, associated with the surface treatment also requires a roughe
Fields Robert D.
Jadwin Thomas A.
Stelter Eric C.
Chapman Mark A.
Heidelberger Druckmaschinen AG
Wood John L.
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