This application is based on Japanese Patent applications No.2000-350210 filed in Japan on Nov. 16, 2000, No.2000-350223 filed in Japan on Nov. 16, 2000, No.2000-370479 filed in Japan on Dec. 5, 2000, No.  2000-370703 filed in Japan on Dec. 5, 2000 and No.2000-372483 filed in Japan on Dec. 7, 2000, the entire contents of which are hereby incorporated by reference. 
1.  Field of the Invention
The present invention relates to an image display medium, and particularly relates to a reversible image display medium, in which image displaying and image erasing operations can be repeated. 
2.  Description of the Background Art
At present, image display is performed, e.g., in the following manners.  A person uses a pencil, a pen, paints or the like, and manually writes or draws characters, pictures or the like on an image display medium such as paper sheet.  Also, a computer, a word processor or the like is used to display text, graphics or the like on a display such as a CRT display, or output them on a medium such as a paper sheet via a printer for display. 
A copying machine or the like may be used for producing duplication, on a medium of paper or the like, of the texts, pictures, graphics or the like, which are produced on the medium of paper or the like by a person or by a printer.  A facsimile machine may be used for sending such contents (texts, pictures, graphics and others) prepared in the above manner for producing duplication on another medium of paper or the like. 
The above image display, which is performed to display the texts, pictures or the like on the image display medium of paper or the like by a pencil, pen or the like, or by an image forming apparatus such as a printer, a copying machine or a facsimile machine operating in a electrophotographic method, an ink-jet method, a heat transfer method or the like, can achieve clear image display in a high resolution, and thus can achieve easy-on-the-eyes display. 
However, it is impossible to repeat display and erasure of the images on the image display medium of paper or the like.  In the case where the paper is used for writing characters or the like by a pencil, the characters can be erased by an eraser to a certain extent.  However, it is difficult to erase completely the characters or the like written in an ordinary density, although it may be possible when written in a light density.  The medium of paper or the like can not be reused except for the case of using the rear surface of the medium, which is not yet used for the image display. 
Accordingly, the medium of paper or the like bearing images will be abandoned or burnt when it is not longer required.  This results in consumption of a large mount of resources.  The printer, copying machine or the like also consume consumable products or materials such as toner or ink.  For obtaining the new display medium of paper or the like as well as toner, ink or the like, energies and resources are required for producing them.  This is contrary to the current demand for reduction in environmental loads. 
In contrast to the above, the image display by a display such as a CRT display can repeat the image display and the image erasure.  However, the resolution, clarity and precision of images are restricted, as compared with the images displayed by the printer or the like on the paper medium or the like.  Thus the image display by a display is improper especially when used for displaying the text documents mainly composed of letters because of low resolution.  If it is used for displaying sentences which continue in less than the frame-size volume, it will do.  However, if the sentences continue in twice or more times the frame-size volume, they may be difficult to read and to understand.  Due to the relatively low resolution and the light emission from the display, operations for a long time are likely to be hard to eyes. 
Electrophoretic display (EPD) and Twist ball-type display (TBD) have been proposed as an image display method allowing repetition of the image display and image erasure.  Further displaying method was recently proposed, which is disclosed in xe2x80x9cJapan Hardcopy ""99, the book of the thesis, pp.  249-252xe2x80x9d. 
In the electrophoretic display method, two substrates including at least one transparent substrate are opposed together with a spacer therebetween to form a closed space therebetween, and the space is filled with a display liquid formed of a dispersion medium and electrophoretic particles, which are dispersed in the dispersion medium and are different in color from the medium.  The image display is performed by an application of an electrostatic field and in a color of the particles or a color of the dispersion medium. 
The display liquid is usually formed of isoparaffin-contained dispersion medium, particles of titanium dioxide or the like, dyes applying contrast in color to the particles, and an additive such as a surface active-agent, or a charge applying agent. 
In the electrophoretic display, the display is performed by utilizing contrast between particles of a high refractive index (e.g., titanium dioxide particles) and colored insulating liquid, and therefore the particles can not hide the colored liquid to a high extent, resulting in a low contrast. 
Furthermore, there is a limitation on the kind of dye which is dissolved in a high concentration in a nonpolar solvent of high resistance which allows the electrophoresis of particles.  A dye showing a white color is not found.  Nor known is a black dye having a high extinction coefficient.  Therefore the background portion becomes colored so that it is difficult to achieve a good contrast by a white background.  When white particles for formation of images are placed into a colored liquid, the colored liquid may be moved between the substrate and the layer of white particles which are moved to the image observation side substrate, or the colored liquid may come into between the white particles, thereby lowering the contrast.  The electrophoretic particles can scarcely uniformly adhere to the image observation side substrate, and thus the resolution is low. 
Further, settling and condensation of particles are liable to occur due to a very large difference in specific gravity between the particles and the dispersion medium in the display liquid.  This is liable to lower the display contrast.  Further, it is difficult to display the images with high stability for a long time, and remaining of last images is liable to occur.  Further, the degree of charging of the particles in the liquid significantly changes with time, which also impairs the stability of the image display. 
In the twist ball-type display method, images can be displayed in specified colors using an image display medium containing numerous microcapsules enclosing not only an insulating liquid but also fine spheric particle so processed that a half of their surface and the other surface portion show colors or an optical density which differs from each other.  Images are displayed in predetermined colors by rotating the fine spheric particles in the microcapsules due to an electric field strength or magnetic strength. 
However, according to the twist ball-type display, images are displayed using fine spherical particles in the insulating liquid within the microcapsules.  This makes it difficult to attain good contrast.  Further, the resolution is low since spaces are formed between the microcapsules.  In the manufacture of microcapsules, difficulty is entailed in reducing the size of microcapsules to increase the resolution. 
The xe2x80x9cJapan Hardcopy ""99, the book of the thesis, pp.  249-252xe2x80x9d discloses an image displaying method wherein a closed space is formed by placing two substrates as opposed to each other and as spaced from each other, i.e. the two substrates being a laminate of electrodes and a charge transporting layer, the space being used to enclose the electrically conductive toner and insulating particles which are different in color from the toner, an electrostatic field being applied to inject charges into the electrically conductive toner so that the toner is moved by a Coulomb force applied thereto to display images. 
However, the foregoing image displaying method utilizing the charge injection phenomenon poses problems.  When the electrically conductive toner carrying the injected charges is moved, insulating particles (e.g. white particles mixed together to form the color of background) interfere with the movement of the toner particles, making their movement so difficult that some of them may stop their movement.  This results in failure to obtain satisfactory image density and good contrast and in reduction of image display rate.  To overcome these problems, a high voltage drive is necessitated.  The resolution is determined by the electrodes and is so limited.  Furthermore, it is essential to use electrodes, charge-injection layer and electrically conductive toner, which results in limited manufacture. 
Accordingly, an object of the invention is to provide a reversible image display medium, which allows repeating of image display and image erasure, and thereby can reduce consumption of image display mediums of paper or the like relating to the conventional image display and consumable materials such as developers and ink so that a current demand for reduction in environmental loads can be satisfied. 
Another object of the invention is to provide a reversible image display medium, which allows image display in good contrast and high quality. 
Still another object of the invention is to provide a reversible image display medium, which allows image display in high resolution and high quality, and more specifically, in high resolution as compared with the electrophoretic display and the twist ball-type display, and also in higher resolution when image display is performed based on an electrostatic latent image without employing opposite electrodes. 
Yet another object of the invention is to provide a reversible image display medium, which allows stable image display of high quality for a long time. 
Further another object of the invention is to provide a reversible image display medium, which can suppress remaining of last image(s), and therefore can exhibit good reversibility so that an image of good quality can be displayed. 
Further another object of the invention is to provide a reversible image display medium, which can reduce a drive voltage required for image display. 
A further object of the invention is to provide a reversible image display medium, which allows quick image display. 
The present invention relates to a reversible image display medium, which basically has the following structure. 
The reversible image display medium comprises:
two substrates opposed to each other with a predetermined gap therebetween;
one or more developer accommodating cells formed between the two substrates, and each having a periphery surrounded by a partition wall; and
a dry developer contained in each of the cell(s), the dry developer containing at least two kinds of frictionally chargeable dry developing particles having different chargeable polarities and different optical reflection densities. 
The invention provides reversible image display mediums having such basic structure and the following features. 
(1) First Reversible Image Display Medium
This reversible image display medium is such that the dry developer has an average charge quantity of 1.0 xcexcC/g to 10 xcexcC/g as determined by an electric field separation measuring method. 
(2) Second Reversible Image Display Medium
This reversible image display medium is such that at least one kind of the developing particles among the two kinds of the frictionally chargeable dry developing particles having different chargeable polarities and different optical reflection densities are magnetic developing particles having a coercive force of 50 oersteds to 250 oersteds. 
The term xe2x80x9ccoercive forcexe2x80x9d refers to a strength of residual magnetic field of the magnetic developing particles. 
(3) Third Reversible Image Display Medium
This reversible image display medium is such that at least one kind of the developing particles among the two kinds of the frictionally chargeable dry developing particles having different chargeable polarities and different optical reflection densities are magnetic developing particles having an optical reflection density of 1.0 or more. 
(4) Fourth Reversible Image Display Medium
This reversible image display medium is such that when at least the two kinds of the frictionally chargeable dry developing particles having different chargeable polarities and different optical reflection densities are represented by a and b, the reversible image display medium satisfies conditions represented by formula 1 and formula 2 given below concerning the cell:
PDxe2x89xa7[1/xcfx81a+(1xe2x88x92Tc)/(Tcxc3x97xcfx81b)]xc3x97Ma/(Dxc3x97f)xe2x80x83xe2x80x83Formula 1 
PDxe2x89xa7{1/xcfx81b+Tc/[(1xe2x88x92Tc)xc3x97xcfx81a]}xc3x97Mb/(Dxc3x97f)xe2x80x83xe2x80x83Formula 2 
wherein:
D means a cell gap (height or thickness of a space defining the cell between the pair of substrates),
xcfx81a is a true specific gravity of the particle a, and xcfx81b is a true specific gravity of the particle b;
Ma is an amount of the particles a adhering to the substrate per unit area of the substrate, more specifically, a required weight of the particles a adhering to the substrate per unit area of the substrate on image observation side when a solid image display having a contemplated image density is performed by using the particles a;
Mb is an amount of the particles b adhering to the substrate per unit area of the substrate, more specifically, a required weight of the particles b adhering to the substrate per unit area of the substrate on the image observation side when a solid image display having a contemplated image density is performed by using the particles b;
Tc is a mixing ratio by weight of the particles a to all particles in the cell;
PD is a ratio of the volume of all particles (volume of all particles in the cell) to a cell volume (volume of the cell space);
f is (the cell volume)/(display areaxc3x97D) (the display area is an effective region area Sdsp for image display in a portion forming the cell in the substrate on the image observation side). 
(5) Fifth Reversible Image Display Medium
This reversible image display medium is such that at least one kind of the developing particles among the two kinds of the frictionally chargeable dry developing particles having different chargeable polarities and different optical reflection densities are non-magnetic developing particles, and an amount of the non-magnetic developing particles of 3 xcexcm or less in particle size in volume particle size distribution of the non-magnetic developing particles is 0.5 wt % or less. 
(6) Sixth Reversible Image Display Medium
This reversible image display medium is such that at least one kind of the developing particles among the two kinds of the frictionally chargeable dry developing particles having different chargeable polarities and different optical reflection densities are magnetic developing particles, and an amount of the magnetic developing particles of 4 xcexcm or less in particle size in volume particle size distribution of the magnetic developing particles is 1 wt % or less. 
(7) Seventh Reversible Image Display Medium
This reversible image display medium is such that a third component (e.g., fluidization agent or a component predominantly containing a fluidization agent) is added to at least one kind (e.g. non-magnetic developing particles) among the two kinds of the frictionally chargeable dry developing particles having different chargeable polarities and different optical reflection.  In each of the third component-added developing particles, an proportion of the third component to the developing particles of 5 xcexcm or less in particle size in volume particle size distribution of the developing particles is different from an proportion of the third component to the developing particles of more than 5 xcexcm in particle size in the volume particle size distribution such that an adhesion of the particles can be reduced to a larger extent than when employing the same proportion of the third component to the developing particles of 5 xcexcm or less in particle size as the proportion of the third component to the developing particles of more than 5 xcexcm in particle size. 
(8) Eighth Reversible Image Display Medium
This reversible image display medium is such that third components (e.g., fluidization agent or a component predominantly containing a fluidization agent) are added to at least one kind (e.g. non-magnetic developing particles) among the two kinds of the frictionally chargeable dry developing particles having different chargeable polarities and different optical reflection densities.  In each of the third component-added developing particles, the third component to be added to the developing particles of 5 xcexcm or less in particle size in volume particle size distribution of the developing particles is different from the third component to be added to the developing particles of more than 5 xcexcm in particle size such that an adhesion of the particles can be reduced to a larger extent than when using the same third component to be added to the developing particles of 5 xcexcm or less in particle size as the third component to be added to the developing particles of more than 5 xcexcm in particle size. 
(9) Ninth Reversible Image Display Medium
This reversible image display medium is such that when the two kinds of the frictionally chargeable dry developing particles having different chargeable polarities and different optical reflection densities are represented by a and b (volume average particle size of the particles axe2x89xa7volume average particle size of the particles b), and a ratio of volume average particle size in the developing particles a and b (=volume average particle size of the developing particles a/volume average particle size of developing particles b) is in a range of 1 to 10. 
(10) Tenth Reversible Image Display Medium
This reversible image display medium is such that at least one kind of the developing particles among the two kinds of the frictionally chargeable dry developing particles having different chargeable polarities and different optical reflection densities are magnetic developing particles, and a ratio of volume average particle size in the two kinds of dry developing particles (=volume average particle size of the magnetic developing particles/volume average particle size of the other developing particles) is in a range of 0.5 to 10. 
If no problem arises, a combination of at least two features of the foregoing reversible image display mediums can be employed.