Radiation imagery chemistry: process – composition – or product th – Electric or magnetic imagery – e.g. – xerography,... – Post imaging process – finishing – or perfecting composition...
Reexamination Certificate
2002-01-15
2003-06-24
Dote, Janis L. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Electric or magnetic imagery, e.g., xerography,...
Post imaging process, finishing, or perfecting composition...
C430S111330
Reexamination Certificate
active
06582870
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a carrier for an electrophotographic developer and an electrophotographic developer containing the same which get rid of the problem of carrier scattering and provide high image quality.
BACKGROUND OF THE INVENTION
Size reduction of carrier particles has been demanded for coping with the advanced technology of obtaining high image quality and the increasing trend to full color printing. With regard to full color printing, in particular, a toner concentration should be increased to carry out development over a wide area. Carrier size reduction results in an increased specific surface area and improved toner holding properties, which will make it possible to prevent toner scattering and thereby to produce high quality images free from fog for an extended period of time.
As the carrier size reduces, the magnetic brush formed becomes softer, making it possible to meet the image quality requirements, such as reproducibility of fine transverse lines (perpendicular to the paper transport direction) with no scratches and halftone uniformity.
Carrier size reduction is of necessity accompanied by the problem of carrier scattering. Carrier particles scattered on a photoreceptor cause an image defect called white spots and give scratches to the photoreceptor which also result in a image defect called white streaks. Hence, it has been difficult to obtain a high quality toner image without involving the problem of carrier scattering. A number of proposals have been made to date to accomplish these conflicting subjects.
For example, Japanese Patent 2,769,854 specifies a carrier having an average particle size of 20 to 60 &mgr;m in terms of percents passing a 250-mesh, a 350-mesh and a 400-mesh screen and a magnetization in a magnetic field of 3 kOe. As proposed, scattering of a carrier could be controlled by reducing the proportion of fine particles. However, to specify the magnetic characteristics of a carrier in a magnetic field of 3 kOe does not cope with the practical electrophotographic development system because the actual magnetic field for forming a magnetic brush on a sleeve is 1 kOe at the highest. From this viewpoint, the achievements in preventing carrier scattering while obtaining high image quality are unsatisfactory.
Japanese Patent 2,832,013 discloses a resin-coated carrier having a weighted average particle size of 30 to 65 &mgr;m, in which the carrier core has a specific average surface pore size. It seems that the patent specifies the surface porosity to represent the surface properties of carrier particles in terms of voids (gaps) among carrier particles and surface pores of the carrier particles. However, the voids among carrier particles are so small that a toner is not smoothly transferred to a photoreceptor and that the magnetic brush tends to be hard, failing to obtain satisfactorily high image quality such as reproducibility of fine lines. In addition, not grasping the particle size distribution and magnetic characteristics of the carrier which are important factors concerning carrier scattering, the patent fails to prevent carrier scattering satisfactorily.
Japanese Patent 2,854,317 specifies a carrier having a weighted average particle size of 20 to 60 &mgr;m in terms of percents passing a 250-mesh, a 350-mesh, a 400-mesh and a 500-mesh screen and magnetic characteristics in a magnetic field of 3 kOe, i.e., a magnetization, a residual magnetization, and a coercive force. Although the particle size distribution is specified, such a high magnetic field as 3 kOe is not actually obtained on a sleeve. Therefore, the countermeasure against carrier scattering cannot be seen as sufficient, still less consistent with image quality.
Japanese Patent 3,029,180 proposes a carrier having a median diameter (D
50
) of 15 to 45 &mgr;m which is specified in terms of a particle size distribution (such as proportions of 22 &mgr;m or smaller particles and 16 &mgr;m or smaller particles), a specific surface area measured by an air permeation method, and an arithmetic surface area calculated from an average particle size and a specific gravity. To increase a specific surface area is effective to improve toner holding properties but insufficient against carrier scattering. Seeing that smooth toner movement through carrier particles is of importance for developability, the technique disclosed is insufficient for image quality.
JP-A-10-198077 discloses a carrier having a 50% diameter D
50
(volume basis) of 30 to 80 &mgr;m which has specific ratios of 10% diameter D
10
, 50% diameter D
50
and 90% diameter D
90
, contains not more than 3% of 20 &mgr;m or smaller particles, and has a magnetization of 52 to 65 emu/g in a magnetic field of 1 kOe. This is an attempt to prevent carrier scattering by specifying the particle size distribution and the magnetization in a magnetic field of 1 kOe. However, when a toner concentration varies, and the resistance of the developer reduces, carrier scattering occurs easily. Because of the low magnetization, the margin against carrier scattering cannot be seen as sufficiently broad.
JP-A-2001-27828 proposes a developer and an image forming apparatus which exhibit satisfactory toner transport and provide a high quality image. The carrier used therein has a weighted average particle size of 35 to 55 &mgr;m, a 22 &mgr;m or smaller particles content of 0 to 15%, and a 88 &mgr;m or greater particles content of 0 to 5% and is coated with a specific resin layer for fluidity improvement. The carrier preferably has a magnetization of 70 to 120 emu/g in a magnetic field of 1 kOe. While improvement of developer's fluidity is important, an excessive increase of magnetic characteristics leads to disadvantages arising from a hard magnetic brush, and the effects of soft development using small particles are impaired. In other words, although an increased magnetization is highly effective in preventing carrier scattering, a countermeasure against reduction of image quality due to a hardened magnetic brush is insufficient.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a carrier for an electrophotographic developer and an electrophotographic developer containing the carrier which get rid of the problem of carrier scattering and provide high image quality.
As a result of extensive investigations, the present inventors have found that the above object is accomplished by a carrier the core of which is spherical magnetic particles having specific powder characteristics and magnetic characteristics.
The present invention provides a carrier for an electrophotographic developer comprising spherical magnetic core particles which have a volume average particle size of 25 to 45 &mgr;m, an average void size of 10 to 20 &mgr;m, a volume based particle size distribution having less than 1% of 22 &mgr;m or smaller particles, a magnetization of 67 to 88 emu/g in a magnetic field of 1 kOe, and a difference of 10 emu/g or smaller in magnetization in a magnetic field of 1 kOe between scattered particles and remaining particles.
The present invention also provides an electrophotographic developer comprising the carrier and a toner having an average particle size of 4 to 11 &mgr;m.
The present invention produces the following advantageous effects.
(1) The carrier, having a small-size core to form a soft magnetic brush, secures stable development to provide high quality images for an extended period of time.
(2) Having a given void size among particles, the carrier charges a toner with a sharp rise when a developer is replenished with a toner. As a result, toner scattering hardly occurs, and fog-free toner images are obtained. Further, the toner can be transported smoothly from a sleeve to a photoreceptor to form an image with improve halftone uniformity, fine transverse line reproducibility and the like.
(3) The increased magnetic characteristics of the carrier broadens the margin against carrier scattering. Further, the increased magnetic characteristics and a given void size maintain the magnetic brush s
Kayamoto Kanao
Sato Yuji
Shinmura Issei
Dote Janis L.
Powdertech Co., Ltd.
Young & Thompson
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