Radiation imagery chemistry: process – composition – or product th – Electric or magnetic imagery – e.g. – xerography,... – Post imaging process – finishing – or perfecting composition...
Reexamination Certificate
1999-07-14
2002-11-26
Rodee, Christopher (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Electric or magnetic imagery, e.g., xerography,...
Post imaging process, finishing, or perfecting composition...
C430S137130, C428S405000, C427S127000
Reexamination Certificate
active
06485877
ABSTRACT:
BACKGROUND OF THE INVENTION:
The present invention relates to magnetic particles and a magnetic carrier for an electrophotographic developer comprising the magnetic particles, and more particularly, to magnetic particles for use as an electrophotographic magnetic carrier in an electrophotographic developer, which have an excellent durability and a stable charging property, an electrophotographic magnetic carrier for an electrophotographic developer, and an electrophotographic developer using the electrophotographic magnetic carrier.
In electrophotographic developing methods, a photosensitive member composed of a photoconductive material such as selenium, OPC (organic semiconductor), a-Si or the like has been used to form an electrostatic latent image thereon by various means. Then, by using a magnetic brush method or the like, a toner having a polarity reverse to that of the latent image is attached thereon to form the latent image by the electrostatic force.
As is well known in the art, in the above developing methods, there have been used support particles called a magnetic carrier. The magnetic carrier acts for imparting an appropriate positive or negative electrical quantity to the toner by frictional electrification, and transferring the toner into a developing zone near the surface of the photosensitive member by a developing sleeve in which magnets are accommodated, using the magnetic force thereof.
In recent years, the electrophotographic developing method has been widely applied to copying machines or printers. In these apparatuses, it has been demanded to meet various requirements including not only reproduction of thin lines, small characters, photographs, color originals or the like, but also a high image quality, a high image grade, a high copying or printing speed, a continuous image formation or the like. The requirements for these properties have been estimated to become increased more and more in future.
In order to satisfy not only the applicability to various objectives but also the high image quality and the high image grade, the reduction in a particle size of the toner particles and the magnetic carrier particles, has been studied. In particular, it has been strongly demanded to provide magnetic carrier particles having an average particle size as small as 10 to 50 &mgr;m.
On the other hand, in order to satisfy the high copying or printing speed and the continuous image formation, it has been strongly demanded to enhance the durability of these particles as developer. In the case of the magnetic carrier, there has been proposed such a method which comprises iron particles obtained by a mechanical pulverization method, an electrolytic method, a reduction method, a heat-decomposition method, a sintering method or the like; granulating and then heat-sintering various ferrite fine particles or magnetite fine particles to form granulated sintered particles; dispersing magnetic particles or magnetic particle and non-magnetic particles in a binder resin to form composite particles (hereinafter referred to merely as “magnetic core particles”); and then coating the surfaces of the obtained magnetic core particles with various resins. The above magnetic carrier has been already put into practice.
There is no end of a demand for the enhancement in properties of the electrophotographic developers. In order to continuously obtain a clear image, it is desired that the charge amount of the magnetic carrier is kept unchanged and stable even after the magnetic carrier is used for a long period of time. Specifically, when the magnetic carrier is used for a long period of time, there arises such a problem that the coating resin layer is peeled off from the surfaces of the magnetic core particles, so that the charging property of the magnetic carrier is deteriorated, whereby the magnetic carrier cannot impart an appropriate charge to the toner. Therefore, it has been demanded that the coating resin layer can be prevented from being peeled off from the surfaces of the magnetic core particles in order to enhance the durability of the magnetic carrier, thereby allowing the magnetic carrier to show a more stable charging property.
Hitherto, in order to enhance the durability of the magnetic carrier, there have been proposed a magnetic carrier obtained by coating the surfaces of magnetic core particles with a silicone resin (Japanese Patent Publication (KOKOKU) No. 2-3181(1990), Japanese Patent Application Laid-Open (KOKAI) Nos. 62-66269(1987) and 3-242657(1991), etc.); a magnetic carrier obtained by coating the surfaces of magnetic core particles with a silicone resin containing a silane-based coupling agent (Japanese Patent Application Laid-Open (KOKAI) No. 5-107819(1993), etc.); or the like.
At the present time, it has been strongly required to provide an electrophotographic magnetic carrier having an excellent durability and a stable charging property. However, such a magnetic carrier has not been obtained yet.
That is, in the production of the above-mentioned conventional magnetic carriers, when the coating resin layer is formed on the surfaces of the magnetic core particles, fatty acid metal salts, especially organic tin compounds, have been used together with the silicone resin in order to cure the resin. The more the amount of the organic tin compound used is increased, the more the resin can be cured more readily. As a result, it becomes possible to form a uniform and satisfactory coating resin layer without causing the aggregation between the magnetic carrier particles. However, when the amount of the organic tin compound used is as large as not less than 0.4% by weight based on the weight of the resin solid content, the obtained coating resin layer becomes brittle, so that upon a long-term use of the magnetic carrier, the coating resin layer tends to be peeled off from the surfaces of the magnetic core particles, resulting in change in charge amount, i.e., unstable charge amount of the magnetic carrier.
On the other hand, when the amount of the organic tin compound used is reduced, it is difficult to cure the coating resin and, therefore, to form a uniform and satisfactory coating resin layer on the surfaces of the magnetic core particles. In addition, there arises such a problem that upon forming the coating resin layer or upon subsequent heat-treatments, the magnetic carrier particles are aggregated together, thereby deteriorating the yield. This phenomenon becomes more remarkable in the case where it is intended to sufficiently and uniformly coat the magnetic core particles having a small particle size especially not more than 50 &mgr;m, with a silicone resin.
As a result of the present inventor's earnest studies, it has been found that by coating each surface of the magnetic core particles with a silicone resin composition comprising at least one metal alkoxide, at least one silane-based coupling agent and a silicone resin, the obtained magnetic particles are useful as an electrophotographic magnetic carrier for an electrophotographic developer. The present invention has been attained on the basis of the finding.
SUMMARY OF THE INVENTION
It is an object of the present invention to produce magnetic particles having an excellent durability and a stable charging property with a high yield without using any organic tin compound.
It is another object of the present invention to provide an electrophotographic magnetic carrier for an electrophotographic developer which has an excellent durability and, therefore, a stable charging property.
It is a further object of the present invention to provide an electrophotographic developer having an excellent durability.
To accomplish the aims, in a first aspect of the present invention, there are provided magnetic particles having an average particle size of 10 to 200 &mgr;m. which comprise magnetic core particles, and a coating layer formed on each of said magnetic core particles, comprising at least one metal alkoxide represented by the general formula (I):
(RO)
n
M (I)
wherein R is a C
1
to C
16
alkyl group;
Nixon & Vanderhye P.C.
Rodee Christopher
Toda Kogyo Corporation
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