Reversible image display medium

A reversible image display medium comprising:

• two substrates opposed to each other with a gap therebetween;
• one or more developer accommodating cells formed between the two substrates, each having a periphery surrounded by a partition wall;
• and
• a dry developer contained in each of the cell(s), wherein
• the dry developer contains at least two kinds of frictionally chargeable dry developing particles having different chargeable polarities and different optical reflection densities, and
• wherein
• at least one kind, among the two kinds, of the developing particles are magnetic developing particles having a coercive force of 50 oersteds to 250 oersteds.

A reversible image display medium comprising:

• two substrates opposed to each other with a gap therebetween;
• one or more developer accommodating cells formed between the two substrates, each having a periphery surrounded by a partition wall;
• and
• a dry developer contained in each of the cell(s), wherein
• the dry developer contains at least two kinds of frictionally chargeable dry developing particles having different chargeable polarities and different optical reflection densities, and
• wherein
• at least one kind, among the two kinds, of the developing particles are magnetic developing particles having an optical reflection density of 1.0 or more.

A reversible image display medium comprising:

• two substrates opposed to each other with a gap therebetween;
• one or more developer accommodating cells formed between the two substrates, each having a periphery surrounded by a partition wall;
• and
• a dry developer contained in each of the cell(s), wherein
• the dry developer contains at least two kinds of frictionally chargeable dry developing particles a and b having different chargeable polarities and different optical reflection densities, and
• wherein
• conditions represented by following formula 1 and formula 2 are satisfied concerning the cell:
• PD≦[1/ρa+(1−Tc)/(Tc×ρb)]×Ma/(D×f) Formula 1
• PD≧{1/ρb+Tc/[(1−Tc)×ρa]}×Mb/(D×f) Formula 2
• wherein:
• D is a cell gap (height or thickness of a space defining the cell between the two substrates);
• ρa is a true specific gravity of the particle a, and ρb 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, and 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, and 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 space defining the cell);
• and
• f is (the cell volume)/(display area×D) (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).

A reversible image display medium comprising:

• two substrates opposed to each other with a gap therebetween;
• one or more developer accommodating cells formed between the two substrates, each having a periphery surrounded by a partition wall;
• and
• a dry developer contained in each of the cell(s), wherein
• the dry developer contains at least two kinds of frictionally chargeable dry developing particles having different chargeable polarities and different optical reflection densities, and
• wherein
• at least one kind of the dry developing particles are non-magnetic developing particles, and an amount of the non-magnetic developing particles having a particle size of 3 μm or less in volume particle size distribution of the non-magnetic developing particles is 0.5 wt % or less.

A reversible image display medium comprising:

• two substrates opposed to each other with a gap therebetween;
• one or more developer accommodating cells formed between the two substrates, each having a periphery surrounded by a partition wall;
• and
• a dry developer contained in each of the cell(s), wherein
• the dry developer contains at least two kinds of frictionally chargeable dry developing particles having different chargeable polarities and different optical reflection densities, and
• wherein
• at least one kind of the dry developing particles are magnetic developing particles, and an amount of the magnetic developing particles having a particle size of 4 μm or less in volume particle size distribution of the magnetic developing particles is 1 wt. % or less.

A reversible image display medium comprising:

• two substrates opposed to each other with a gap therebetween;
• one or more developer accommodating cells formed between the two substrates, each having a periphery surrounded by a partition wall;
• and
• a dry developer contained in each of the cell(s), wherein
• the dry developer contains at least two kinds of frictionally chargeable dry developing particles having different chargeable polarities and different optical reflection densities, and
• wherein
• a third component is added to at least one kind of the dry developing particles, and in each of the third component-added developing particles, an addition proportion of the third component to the developing particles having a particle size of 5 μm or less in volume particle size distribution of the developing particles is different from an addition proportion of the third component to the developing particles having a particle size of more than 5 μm in the volume particle size distribution such that an adhesion of the particles of 5 μm or less in particle size can be reduced to a larger extent than when employing the same addition proportion of the third component to the developing particles of 5 μm or less in particle size as the proportion of the third component to the developing particles of more than 5 μm in particle size.

A reversible image display medium comprising:

• two substrates opposed to each other with a gap therebetween;
• one or more developer accommodating cells formed between the two substrates, each having a periphery surrounded by a partition wall;
• and
• a dry developer contained in each of the cell(s), wherein
• the dry developer contains at least two kinds of frictionally chargeable dry developing particles having different chargeable polarities and different optical reflection densities, and
• wherein
• third components are added to at least one kind of the dry developing particles, and in each of the third component-added developing particles, the third component to be added to the developing particles having a particle size of 5 μm or less in volume particle size distribution of the developing particles is different from the third component to be added to the developing particles having a particle size of more than 5 μm in the volume particle size distribution such that an adhesion of the particles of 5 μm or less in particle size can be reduced to a larger extent than when adding the same third component to the developing particles of 5 μm or less in particle size as the third component to be added to the developing particles of more than 5 μm in particle size.

A reversible image display medium comprising:

• two substrates opposed to each other with a gap therebetween;
• one or more developer accommodating cells formed between the two substrates, each having a periphery surrounded by a partition wall;
• and
• a dry developer contained in each of the cell(s), wherein
• the dry developer contains at least two kinds of frictionally chargeable dry developing particles having different chargeable polarities and different optical reflection densities, and
• wherein
• at least one kind, among the two kinds, of the developing particles are magnetic developing particles, and a ratio of volume average particle size in the two kinds of the 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.