Developer for electrostatic image 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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C430S109400, C430S110400, C430S111410

Reexamination Certificate

active

06534229

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a developer for electrostatic image development which is employed in electrophotographic methods, electrostatic recording methods, and electrostatic printing methods.
2. Description of Related Art
Various electrophotographic methods have been disclosed in, for example, U.S. Pat. No. 2,297,691, Japanese Examined Patent Application, Second Publication No. Sho 42-23910, and Japanese Examined Patent Application, Second Publication No. Sho 43-24748; commonly, an electrostatic latent image is formed on an electrostatic latent image bearing medium such as a photoconductive photosensitive medium or the like by means of charge or light exposure, and then this electrostatic latent image is developed employing a toner composition containing a colorant in a binder resin, and the resulting toner image is transferred to a support medium such as transfer paper or the like and fixed, and a visible image is thus formed.
Various development methods have been known and incorporated as part of the electrophotographic method, and are roughly classified into a two-component development method employing microparticles (20-500 &mgr;m) such as iron powder, ferrite powder, nickel powder, glass powder and the like, and a single-component development method employing a developer made of a toner only.
Typical examples of the two-component development method include a cascade method described in U.S. Pat. No. 2,618,552 and a magnetic brush development method described in U.S. Pat. No. 2,874,063. In these methods, a carrier takes partial charge of functions such as stirring, transfer and charging of the developer and, therefore, the function of the carrier and that of the toner are clearly separated. Accordingly, the two-component development method has been widely employed at present because it can control charging of the toner and form the developer layer comparatively easily and also facilitates more rapid processing.
In concert with the recent evolution of an information-oriented society, needs such as an improvement in the quality of the printed image as well as more rapid processing, an increase in density and the long-term storage stability of recording have been realized in the fields of electrophotography, electrostatic recording and electrostatic printing. Thus, great hopes are entertained of an improvement in the properties of a toner capable of recording an electrostatic latent image on a non-printed medium.
Apparatuses employing such an electrostatic image development method include copying machines, printers and the like. The processing speed varies depending on the manufacturer and type, but many apparatuses have the processing speed corresponding to 30 sheets per minute in the case of an office printer or about 60-100 sheets per minute for an office copying machine, when converted to printing speed using A-4 size paper. As a result of increases in speed for developing apparatus, conventional developing apparatus has recently been superseded by a high-speed apparatus which has a processing speed corresponding to 100 sheets per minute when converted to printing speed using A-4 size paper in a longitudinal direction and 140 sheets per minute when converted to printing speed using A-4 size paper in a lateral direction, and which has a fixing speed of 30 m per minute.
A toner for a two-component developer, which is suited for use in high-speed printing, should ideally meet certain important requirements such as stable charge behavior during mass-circulation printing with a resulting stable image with less fogging as well as solving the problem of scattered toner so as to not contaminate the inside of the machine during long-term use. These objects are largely dependent on the charging properties of the toner.
With respect to contamination of the inside of the apparatus, scattering of the toner is a serious problem in apparatus for high-speed printing. That is, scattering of the toner is a significant problem because it causes not only contamination of the printed image but also contamination of the inside of the apparatus, thereby impairing maintainability and further causing various apparatus problems.
Although a polyester resin has recently been employed with view to bringing about high durability, polyester resin was not found to be particularly suited for the preparation of a positive-charge toner used for high-speed printing because it exhibits strong negative-charge properties as compared with conventionally used styrene-acrylic resin. Therefore, a toner has never been obtained which possesses all the qualities required of a toner for high-speed printing, such inhibiting the scattering of the toner.
BRIEF SUMMARY OF THE INVENTION
An object of the present invention is to provide a developer for electrostatic image development, which exhibits stable charge behavior during printing even when using a polyester resin and does not cause contamination of the inside of the apparatus because of less scattering of the toner, and also facilitates easy maintenance.
Another object of the present invention is to provide a developer for electrostatic image development, which can attain the objects described above during printing at high speeds exceeding 20 or 30 m per minute.
As a result of diligent research with the object of solving the problems described above, the present inventors have found that the proportion (% by volume) of a toner having a comparatively large particle diameter in a range of from 12 &mgr;m to 16 &mgr;m, inclusive, and its specific charge amount exert a large influence on the amount of the toner scattered, thus obtaining the present invention.
That is, the present invention provides a developer for electrostatic image development comprising colored resin particles comprising at least a binder resin and a colorant, and a magnetic carrier, wherein a proportion of said colored resin particles having a particle diameter in a range of from 12 &mgr;m to 16 &mgr;m, inclusive, in said developer and a specific charge amount of said colored resin particles meet the following expression (1):
5.18+0.12×A<B  (1)
(where A represents a proportion (% by volume) of colored resin particles having a particle diameter in a range of from 12 &mgr;m to 16 &mgr;m, inclusive, and B represents a specific charge amount Q/M (&mgr;C/g) of colored resin particles having a particle diameter in a range of from 12 &mgr;m to 16 &mgr;m, inclusive).
According to the present invention, in the developer for electrostatic image development comprising colored resin particles comprising at least a binder resin and a colorant, and a magnetic carrier, said colored resin particles meet the expression (1) and, therefore, contamination of the inside of the apparatus hardly occurs during long-term use because of less scattering of the toner, thus making it possible to obtain an image with less contamination (fogging) of the printed image due to scattering of the toner.
Particularly, the developer for electrostatic image development containing the colored microparticles (toner) which meet the expression (1) can be effectively employed as a developer for developing apparatus for printing at high speeds exceeding 20 or 30 m per minute.
DETAILED DESCRIPTION OF THE INVENTION
In general, the toner is made of colored resin particles comprising a binder resin, a colorant, a releasing agent and a charge control agent, and scattering of the toner is a phenomenon where, when the specific charge amount of the toner is reduced, a Coulomb force between the toner and carrier cannot overcome a centrifugal force due to rotation of a magnetic brush, causing the toner to scatter from the carrier. However, since the processing speed during the developing and the charge amount of the toner are inextricably linked in this phenomenon, only a quantitative argument has hitherto been made.
The target particle diameter of the toner to be provided as the product is about 10 &mgr;m, and the toner is generally obtained in the form of a powder hav

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