Radiation imagery chemistry: process – composition – or product th – Electric or magnetic imagery – e.g. – xerography,... – Post imaging process – finishing – or perfecting composition...
Reexamination Certificate
1998-06-12
2003-09-30
Dote, Janis L. (Department: 1753)
Radiation imagery chemistry: process, composition, or product th
Electric or magnetic imagery, e.g., xerography,...
Post imaging process, finishing, or perfecting composition...
C430S111320, C430S137130
Reexamination Certificate
active
06627370
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to electrostatography. More particularly, it relates to the carrier particles that are used in two-component developers.
BACKGROUND OF THE INVENTION
In electrostatography, an electrostatic charge image is formed on a dielectric surface, typically the surface of a photoconductive recording element or photoconductor. Development of this image is commonly achieved by contacting it with a dry, two-component developer comprising a mixture of pigmented resinous electrically insulative particles known as toner, and magnetically attractable particles, known as carrier.
The carrier particles serve as sites against which the non-magnetic toner particles can impinge and thereby acquire a triboelectric charge. The toner particles are held on the surface of the relatively larger-sized carrier particles by the electric force generated by the friction of both particles as they impinge upon and contact one another during mixing interactions.
During contact between the electrostatic image and the developer mixture, the toner particles are stripped away from the carrier particles to which they had formerly adhered (via triboelectric forces) by the relatively strong attractive force of the electric field formed by the charge image which overcomes the bonding forces between the toner particles and the carrier particles. In this manner, the toner particles are attracted by the electrostatic forces associated with the charge image and deposited on the electrostatic image to render it visible.
Conventionally, carrier particles made of soft magnetic materials have been employed to carry and deliver the toner particles to the electrostatic image. This “soft” carrier is typically unoxidized or partially oxidized iron or steel powder. Carriers of this type suffer from various problems such as charge instability, relative humidity performance and resistance change and other problems. For example, as the developer is used, the carrier surface changes due to either the toner particles adhering to the surface of the soft carrier, referred to in the art as “carrier scumming”, or due to the breaking or brittle fracture of the iron oxide off the surface of the carrier particles.
It is known in the art to incorporate charge control agents onto the surface of soft magnetic carrier particles. Reference is made, for example, to U.S. Pat. Nos. 5,215,848; 5,171,653; 5,230,980; 4,868,082; 5,340,677; and 5,346,771. A typical reason for incorporating charge control agents in this type of carrier is to lower the initially high charge observed with the developer system.
After aging of the developer by the repeated cycling of the developer, there is essentially no difference between the disteryl ammonium methyl sulfate treated and untreated carriers described in the '653 patent. More recently, hard magnetic materials have been used to carry and deliver the toner particles to the electrostatic image. Many of the problems encountered with the “soft” type of particle are solved with the “hard” ferrites as carriers. However, because the magnetic attraction between the permanent magnetic core and the permanently magnetic hard ferrite carrier is so high, the developer station has to be significantly modified. In the case of soft ferrites and iron oxide powder carriers, the developer station shell is rotated around a fixed magnetic core. When developers based on hard ferrite carrier particles are used, the magnetic core of the development roller, which contains between 4 to 30 magnetic material arranged sequentially in north-south pole alignment, is rotated. This causes the chains of the magnetic carrier particles, which form the development brush, to flip end-to-end at very high rates. In other words, it is well recognized that the hard magnetic material carriers are not analogous to the soft carrier materials.
U.S. Pat. No. 4,546,060 to Miskinis et al, and U.S. Pat. No. 4,473,029 to Fritz et al, teach the use of hard magnetic materials as carrier particles and an apparatus for the development of electrostatic images utilizing such hard magnetic carrier particles, respectively. These patents require that the carrier particles comprise a hard magnetic material, meaning a magnetic material exhibiting a coercivity of at least 300 Oersteds when magnetically saturated and an induced magnetic moment of at least 20 EMU/g 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 biggest impact of the use of hard ferrite carrier particles is that extremely high mechanical agitation takes place as the core of the development shell is rotated. Unlike the case with soft carriers which have no flipping of the carrier chains, the number of developer chain flips can range between 5,000 to 25,000 flips per minute when using hard ferrite carriers. Due to this high mechanical agitation, the aging of the developer takes place at a very rapid rate.
The developer aging is characterized by the loss of charge in the developer which causes dusting of the toner from the development shell due to high centrifugal force as well as increase in image density and image fog. It is to a solution to this problem that the present invention is directed.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided a carrier, for an electrcostatographic developer, comprising particles of a hard magnetic ferrite material as the core, the core having a coating of a polymer resin, said resin in turn having a coating of an organic conductive material.
The developer compositions of the invention have several advantages. They exhibit no need for preconditioning; a longer developer life in that the charge to mass ratio does not change significantly over an extended period, e.g. 100,000 copies in an electrophotographic copying machine. In addition, the developer is less prone to “dusting”. This implies that the electrostatic forces that hold the toner to the carrier surface is sufficient to substantially prevent the formation of toner dust in the copying machine. Still further, the charging rate, where the term “charging rate” is used to describe how rapidly a toner added to developer equilibrates to its highest charge, is increased. As a result of these properties, there is a consistently high image quality achieved for long periods when using the developers of the invention.
DETAILED DESCRIPTION OF THE INVENTION
The compositions of the invention have three essential components. First is the hard ferrite magnetic carrier particles. These particles are coated with a polymeric resin to control the triboelectric charging. The final component is a organic conductive material coating on the polymeric resin. Except for the organic conductive material coating, the hard magnetic carrier particles coated with resin are known in the art. It has been found that incorporation of small levels of organic conductive materials on the surface of the polymeric resin coating the particles improves the charge stability of electrcostatographic developers and other properties. Incorporation of organic conductive material on the carrier surfaces can be achieved by solution coating methods. As described in more detail below, a solution of a organic conductive material is prepared in a solvent such as methanol, water or isopropyl alcohol etc. The solution is added to the carrier which has been previously coated with desired amounts of a polymer resin to ensure correct tribocharging for toner; the solvent is then removed leaving the coating of the organic conductive material on the resin. Other methods for forming the coating may also be useful.
Representative hard carrier particles include magnetic material gamma ferric oxides and the hard ferrites compounds of barium and/or strontium such as, BaFe
12
O
19
and SrFe
12
O
19
disclosed in U.S. Pat. No. 4,042,518. Another hard ferrite ma
Anderson James Hunter
DiPrima Donna Anne
Tyagi Dinesh
Dote Janis L.
Kessler Laurence P.
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