JP2001322353A - Thermally reversible multicolor recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an improved thermally reversible multicolor recording medium, with which information develops colors and is visually recognized with heat energy even with total three color phases or two single hues and a mixed hue obtained through the polymerization between the two single hues and, in addition, the developed color information is decolored. SOLUTION: In this thermally reversible multicolor recording medium, respectively at least two thermally reversible color image forming recording layers (2) and (3) are provided on both the sides of a heat insulating transparent support (1) having a total light transmittance of 85% or higher and a heat conductive white layer having a lightness index of 88 or more and a heat conductivity of 0.7 W/m deg.C or higher is provided on either one side of the thermally reversible recording layer, the preferable heat insulating properties of the support (1) is 0.5 W/m deg.C or less is the heat conductivity and the white layer (4) is preferably provided on the at least denser developed color layer side between the two thermally reversible recording layers (2) and (3).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermoreversible multicolor recording medium capable of displaying or erasing at least two color images individually or simultaneously by writing thermal energy (for example, thermal head). The recording medium has been improved so that it can be displayed and erased more quickly with a clearer color image. It is also effective when used in combination with various cards that can be rewritten or reused.

[0002]

2. Description of the Related Art As one of the rewritable marking techniques, a visible image is formed and displayed by giving heat (energy) along with light, magnetism, etc., and is erased by giving thermal energy again thereto. What is known as a thermoreversible recording medium that enables this repetition is known.

A thermoreversible recording medium is a physical change type in which an image is displayed by physically changing according to the composition of the recording layer when heat energy is applied, and a chemical change is performed by a chemical change. There are two types of chemical change types
The present invention relates to the latter type, which can be displayed as a black image or a colored image like a normal printed matter. A large number of the latter techniques of the chemical change type have been disclosed by patent applications as in the former technique. For example, JP
There are 246091, JP-A-7-17132 and JP-A-7-137449. JP-A-3-246
No. 091 discloses a thermal paper in which heat-sensitive paint layers of the same composition are laminated on both sides of a base paper. Japanese Unexamined Patent Publication (Kokai) No. 7-17132 discloses that the same and / or different locations on one side or both sides of a support (paper, metal foil, nonwoven fabric, synthetic paper, synthetic resin, etc.) of any transparency (opaque to transparent) are described. Discloses a reversible thermosensitive recording material provided with a reversible thermosensitive recording layer of the same color or a different color. And JP-A-7-
JP-A-137449 discloses a thermoreversible thermosensitive recording medium in which heat-sensitive layers of different colors are provided on both sides of a transparent or translucent resin base material, and a transparent overcoat layer is further provided on the heat-sensitive layer. Have been.

[0004]

The above publications have slightly different contents, but all of them have a small amount of information recorded on only one conventional surface (problem). Use at least two of the same or different colors
The solution is achieved by providing two (reversible) thermosensitive recording layers to form a (thermoreversible) thermosensitive recording medium. The present invention is the same as providing a thermoreversible recording layer of different colors on both sides of the substrate as in the publication, but each of the two colors is developed as a unique color, and the two colors thus developed are further developed. A thermoreversible multicolor recording medium that expresses information with another color, that is, a total of three colors as a mixed hue by polymerizing it, and that can easily and clearly see the color development only from one side, This has been found through intensive studies. It can be solved by the following means.

[0005]

That is, the present invention provides a first aspect of the present invention.
Wherein at least two thermoreversible recording layers (2, 3) for forming a color image are provided on both sides of a heat-insulating transparent support (1) having a total light transmittance of 85% or more, and A brightness index of 88 or more on one side of the thermoreversible recording layer,
A thermoreversible multicolor recording medium comprising a heat conductive white layer (4) having a heat conductivity of 0.7 W / m ° C. or higher. And dependent on claim 1, claims 2 to 4 are also provided as preferred inventions. Hereinafter, the present invention will be described in detail with the following embodiments.

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is a reversible recording medium which chemically performs color development and decoloration. First, at least two thermoreversible recording layers (2, 3) for forming a color image (hereinafter referred to as "reversible recording layers"). HR layer) will be described. As for the color development by the reversible recording medium, at least two are mainly selected from red, blue, green and yellow color development sources, and the components constituting them can be exemplified as follows. Of course, black is also used as a color source, but in this case it is used as information recording in two single colors, black and other colors, and it can not be practically used in three hues including mixed hues. become.

First, for each color, generally, a precursor of an electron-donating dye (hereinafter, referred to as a color-forming agent) as a color-forming source, and an electron-accepting compound (hereinafter, a color-developing agent) having a color-deteriorating effect on the color-forming agent with temperature. ) Are used as main components, and this is mixed and dispersed with a binder resin to form each layer.
In the present invention, the types of the color former, the developer and the binder resin are not particularly limited, and those known in general literature and patent application publications are used in combination. The following is an example for reference.

For the red color developing agent, for example, for red, fluoran lactone compounds such as 2-chloro-6-diethylaminofluoran lactone and 3-methyl-6-diethylaminofluoran lactone. In the blue color former, 3- (4-diethylamino-2-methylphenyl) -3- (1-ethyl-2) is used.
Phthalide compounds such as -methylindol-3-yl) -4-azaphthalide. Green color formers include 7- (N, N-dibenzylamino) -3- (N, N-diethylamino) fluoranlactone and 7- (N-octylamino) -3-.
Fluoran lactone compounds such as (N, N-diethylamino) fluoran lactone. Fluoran lactone compounds such as 3-methoxy-6-methoxy fluoran lactone for the yellow color developing agent. In addition, as a black color developing agent, a fluoran lactone compound such as 7- (2-chlorophenylamino) -3- (diethylamino) fluoran lactone is exemplified. Each of the above color formers is usually colorless or lightly colored, and is different from ink dyes and pigments.

The color developer generally comprises a phosphoric acid group, a carboxylic acid group, an aromatic group and the like and a long-chain alkyl group having a carbon number of 10 or more. Specific examples include N-behenyloylaminophenol, 4- (N-behenoylamino) phenoxyacetic acid, 2-octadecylpentanoic acid, and eicosylphosphonic acid.

The binder resin is excellent in transparency, hydrophilicity, compatibility with a color former and a developer, adhesion with the transparent supports (1) and (4), heat resistance and weather resistance. It is better to choose from those that are. Specifically, for example, a modified polymer containing a hydroxyl group (polyvinyl alcohol-based polymer generally referred to) obtained by appropriately saponifying a vinyl acetate group in a copolymer of polyvinyl acetate or ethylene and vinyl acetate, and acrylamide A copolymer of maleic anhydride and ethylene; a modified form of cellulose (each group bond of ethyl, hydroxy, carboxy, etc.), starch and the like.

[0011] At least two colors are selected from each of the three components, and each is laminated on both sides of the heat-insulating transparent support. The total light transmittance (diffuse transmittance + parallel light transmittance) (hereinafter referred to as Tt) is 85% or more, preferably 90% or more.
% Of the transparent support (1) (hereinafter referred to as TI sheet) having heat insulating properties. Here, the reason why the TI sheet requires transparency of Tt ≦ 85% is as follows.

In other words, it is a matter of course that the information of each single hue can be more clearly recognized, but the main object of the present invention is to make the image information overlap two single colors and the two colors with the TI sheet overlapping. Is expressed in a total of three hues that can be visually recognized by mixed hues, and for this purpose, the mixed hues must be more clearly visible. Therefore, in the TI sheet of less than 85%, each color lacks a sharp difference, and it becomes difficult to clearly see what kind of information it is, especially in a detailed expression. Here, specifically, the mixed hue is visually recognized in each of hues of red + blue in violet, green + blue in turquoise, yellow + blue in green, and yellow + red in orange. As described above, the necessary transparency may be determined by specifying Tt. However, even in the range of Tt, it is assumed that the haze (cloudiness) is generally caused by poor surface condition of the TI sheet or a certain haze. (It is easy to develop when such an additive is mixed)), the chromaticity is affected, and as a result, the above-mentioned clear visibility may be adversely affected. Therefore, when strictly selecting the sheet, it is desirable to determine the sheet in consideration of the haze. The haze ratio (diffuse transmittance ÷ Tt × 100) is preferably 3% or less when numerically exemplified.

[0013] Another one necessary for the TI sheet is described below.
One condition is heat insulation. This heat insulation property is
The two hues emitted from each of the HR layers provided on both sides of the light source are indispensable to each other so that they are not affected by each other at the time of input operation of information heat energy and are not simultaneously colored, but are independently and independently colored. belongs to. Due to the adiabatic properties, each hue develops color rapidly and the color density becomes deep, and as a result, the two-color mixed hue becomes deeper and sharper. If this heat insulating property is numerically exemplified, as described in claim 3, the thermal conductivity is 0.5 W / m ° C. or less, preferably 0.4 W / m ° C. or less.
/ M ° C or lower, and more preferably 0.3 W / m ° C or lower. The TI sheet also needs to have a function of supporting the entire thermoreversible multicolor recording medium, and its thickness is generally set to be larger than the thickness of a heat conductive white layer described later. However, the above-mentioned transparency is also necessary, and it is better to determine the thickness so as to satisfy the whole as well as the heat insulating property.

A TI that effectively expresses the above two requirements
An example of the sheet is as follows. Biaxially stretched films of crystalline polymers such as polyethylene terephthalate (PET), polyethylene naphthalate, polyether ketone,
Polycarbonate, polymethyl methacrylate, linear polyolefin, cyclic olefin-based polymer (eg, norbornene-only polymer or copolymer with ethylene, tetracyclododecene-only polymer or copolymer with ethylene, etc.), polyether sulfone, polyarylate Films such as homo-amorphous polymers are included. Among them, those having a higher Tt are amorphous polymer films. still,
In the case of a cyclic olefin-based polymer, Tt is high at 91% or more, but haze is generally slightly worse than other polymers. A small amount of a primary plasticizer, for example, dioctyl adipate, diethyl phthalate, or the like, mixed with the polymer and molded into a film may improve the haze. The TI sheet may be, for example, a sheet provided with an air phase using two of the above-described polymer sheets, or a closed cell in a solution of the above-described polymer or another transparent resin. Or a fine transparent glass bead mixed therein, cast into a sheet, and dried to form the sheet. The combined use of the air phase, the closed cells, and the glass beads has a large effect of increasing the heat insulating property without impairing the transparency.

Next, a TI sheet (hereinafter referred to as an HR sheet) in which the HR layers of the two colors are provided on both surfaces is further provided with the H layer.
Any one surface of the R layer may have a lightness index of 88 or more, preferably 90 or even 92 or more, and a thermal conductivity of 0.7 W or more.
/ M ° C or higher, preferably 0.8 W / m ° C or higher, 0.9 W
The provision of the thermally conductive white layer (4) having a temperature of / m ° C or higher will be described.

The HR sheet itself is provided with a support from both sides.
When thermal energy is applied by a mulled head or the like, each color is sharply colored to obtain an information image. However, since the HR sheet itself has high transparency, it is difficult to intuitively and quickly visually recognize the HR sheet as it is. Therefore, in order to solve this problem and visually recognize with a higher sharpness difference, first, a heat conductive white layer (4) having a lightness index of 88 or more is used and provided. This brightness index (hereinafter referred to as L value) is, in other words, whiteness, and the higher the white color, the easier the above-mentioned problem can be solved. In the present invention, the whiteness is specified by a numerical value, and at least 88 is required to be solved with sufficient satisfaction.

Further, the heat conductive white layer may include the TI
Unlike sheets, it is required to have better thermal conductivity. This is because heat energy for coloring is radiated from the heat conductive white layer side when the HR layer provided on the TI sheet is on the heat conductive white layer side, so that the heat conduction is performed. The higher the sex, the better. That is, the higher the thermal conductivity, the faster and sharper the color is formed even with low thermal energy. The minimum thermal conductivity required to achieve this smoothly and quickly is at least 0.7 W / m ° C in terms of thermal conductivity, and the higher the higher, the greater the effect.

In the thermally conductive white layer, first, an L value of 8
The expression of whiteness of 8 or more is generally achieved by, for example, adding a whitening agent such as titanium oxide, calcium carbonate, magnesium sulfate, zirconium oxide to a polymer exemplified in the TI sheet.
It is carried out by mixing alumina, antimony trioxide, zinc oxide, kaolin, mica, sodium zirconium sulfate, zirconium acetyl acetate and the like into the resin. These whiteners have higher thermal conductivity than the resin, and together with whitening,
It is also very convenient because of its higher thermal conductivity. Therefore, depending on the amount of the whitening agent used, L
The value can be controlled by changing the thermal conductivity as well as the value. Generally, the amount used is achieved at about 60 to 95% by weight based on the resin. In addition, in addition to the whitening agent, if a heat conductive additive such as alumina nitride, boron nitride, beryllium nitride or the like is added, the heat conductive white layer having a higher heat conductivity can be obtained. can do.

Next, the means for forming the respective HR layers on the TI sheet surface and the means for providing the heat conductive white layer (4) on one side of the obtained HR sheet will be described.

In the formation of the HR layer, first, any of the appropriate color formers, developers and binders for forming the HR layer.
The ratio of the resin is determined, and the ratio is roughly as follows. The color former is 15 to 40% by weight, the color developer is 85 to 60% by weight, and the binder resin is 1 to 10% by weight based on the total amount of the color former and the color developer. Here, in order to improve the forming characteristics of the color-forming layer and the color-forming / decoloring characteristics, for example, a dispersant, a surfactant, a lubricant, an antioxidant, an ultraviolet absorber, a light stabilizer,
A trace amount of a color stabilizer, a decolorization accelerator, a sensitizer, etc. may be added.

When the ratio is determined, the ratio is first applied to one side of the TI sheet. The coating is generally performed in the following procedure. First, a desired amount of resin binder is added to a solvent (diluent).
Dissolve in Next, a predetermined amount of a color former and a developer are separately added to the dissolution solution or a mixture of both in advance is added with stirring and uniformly dispersed. Finally, the solution viscosity is adjusted so as to be easy to apply and to obtain a desired layer thickness. Co
After the teng, drying is performed once, and in some cases, two or three times, to obtain an HR layer by obtaining a desired layer thickness. H on one side
After the formation of the R layer, HR layers of other colors are formed in the same manner. The coating means is appropriately selected from spin coating, roll coating, spray coating, screen printing, and the like. Here, when the HR layer is too thin, the color density is lowered. On the other hand, when the HR layer is too thick, especially when the two-color mixed color is visually recognized,
The color density that should be visually recognized as a mixed color is visually recognized differently. Therefore, a reasonable layer thickness is 5
A range of about 20 μm is a good standard.

Next, the heat conductive white layer (4) specified above is formed on one side of the obtained HR sheet.
This forming means mixes the whitening agent with the polymer exemplified in the heat-insulating transparent support (1), for example, and forms a film. The obtained film is laminated on the HR layer.
Further, a film obtained by applying a white paint obtained by mixing the white agent to a solution of the resin exemplified as the binder resin on the polymer film exemplified as the heat insulating transparent support (1) is similarly laminated. I do. Further, the white paint is directly applied to the HR layer and dried to form the HR layer. Of these, a method in which the white paint is directly applied to the HR layer and dried is preferred. Here, it is not specified on which side of the HR sheet the thermal conductive white layer is formed, but it is preferable to provide the thermal conductive white layer on the side of the layer having a deeper color. For example, the blue side for red and blue,
Blue and green are blue, yellow and red are red, blue and yellow are blue, and red and green are red. This is because a lighter color system is provided on the viewing side (viewing side) so that a clearer information image can be visually recognized.

As described above, the intended thermoreversible multicolor recording medium can be obtained, which may be used as it is, or may be subjected to the following surface protection treatment. In other words, protection is provided on the side on which the developed color image is viewed, that is, on the surface on the opposite side where the heat conductive white layer is provided. (A protection from deterioration in color development / decoloration performance due to the environment, damage during use / work process, etc.). This protective layer is preferably higher in transparency and higher in thermal conductivity than the TI sheet, and it is better to determine the components to be used and the layer thickness based on these values. For example, as the component, for example, an acrylic-epoxy system that binds a silicone component,
A UV-curable precursor such as an acrylic / urethane-based or acrylic / silicone-based precursor is coated so as to have a film thickness of about 0.1 to 10 μm, and then cured by UV. Also high hardness
A protective film may be formed by sputtering an inorganic compound, such as BN or ALN, which is water-resistant and thermally conductive, such as BN or ALN, to form an ultrathin film of about 500 to 1000 °.

The color development and the decoloring operation of the thermoreversible multicolor recording medium can be performed, for example, as follows. A DC voltage of, for example, about 20 V is applied to the thermal head that has read the image. Heat energy (for example, 90 to 120 ° C)
Immediately after the occurrence of the color, the color is developed according to the image. This is done for each side of the medium. After printing, place at room temperature. The heated part is quenched, so immediately both sides (HR
Layer), each of which produces a color image. When viewed from the opposite side of the heat conductive white layer, a two or three color image of each of two single colors or a mixture thereof is clearly displayed. When erasing them, the entire medium is heated (at a heating roll or the like) to at least the temperature at which erasing is started. This time, when it is cooled in a state of cooling, it is erased and returns to the original state. Generally, the coloring and decoloring are systematized as hardware.

Incidentally, the present invention basically comprises providing one HR layer on each side of the TI sheet as described above, for example, so that the HR layer does not overlap one side of the sheet. The two HR layers can also be a thermoreversible multicolor recording medium composed of a total of four layers provided with corresponding two different HR layers on the other side.

[0026]

The present invention will be described in more detail with reference to the following examples together with comparative examples.

The color development, decoloring operation and color development referred to in this example
The degree is measured and measured as follows:^*a^*b^*
This is shown in a color system. ◎ Coloring operation: The thermoreversible multicolor recording medium obtained in each example was
Using it, a solid image of 2 cm square from one side, the other side
So that solid images of the same shape can be colored at independent positions
And the 2cm square solid images are emitted at the same position.
It is laid out so that it can be colored, and a sharp strain is used as a color source.
Of "Shoin WD-1200U" (Wapro)
-Apply heat to both sides from the mulled head. ◎ Decoloring operation: A heated roller at 100 ° C.
And the whole is heated to that temperature, kept at room temperature and slowly cooled. ◎ Measurement of degree of color development… Manufactured based on JIS Z8729
Color difference meter “CR-32” manufactured by Minolta, Inc.
1 ", the side opposite to the thermally conductive white layer 4 (visually
Side), each chromaticity L at three color development positions^*, A^*, B^*To
Measure. Where L^*Larger (brightness index / L value) is whiter
Increases the tinge and lightens the color. On the contrary, it becomes smaller
This increases the darkness and the color intensity. Ma
A^*, B ^*Is chromaticity indicating hue and saturation, and L^*a^*b^*
A in the light of the color system chromaticity diagram^*Is the red direction, -a^*Is green
Direction, and b^*Is yellow, -b^*Indicates the blue direction
Will be. For the measurement, a measurement sample was placed on a white plate (L^*
= 92.92, a^*= 0.14, b^*= 1.63)
And carried out on the basis of this.

The lightness index L value indicating the whiteness of the heat conductive white layer 4 is a value obtained by measurement with the color difference meter “CR-321”.

Further, the thermal conductivity (W / m ° C.) (hereinafter referred to as Tc) is a value measured by a laser method thermal constant measuring apparatus manufactured by Rigaku Corporation.

The total light transmittance (Tt%) is a value measured by a turbidity meter “NDH2000” manufactured by Nippon Denshoku Industries Co., Ltd.

(Example 1) First, red and blue H
An R layer composition was prepared. ● Composition solution for red: 40 parts by weight of 2-chloro-6-diethylaminofluorolactone powder as a red color developing agent, and 90 parts by weight of a 2.5% by weight aqueous solution of polyvinyl alcohol were added thereto and thoroughly mixed and dispersed ( A liquid). On the other hand, N-
100 parts by weight of behenyloylaminophenol powder and 2.5% by weight aqueous solution of polyvinyl alcohol (PVA) 4
Then, 00 parts by weight were added and sufficiently mixed and dispersed (Solution B). Then, 65 parts by weight of the liquid A and 250 parts by weight of the liquid B were collected, and 100 parts by weight of a 10% by weight aqueous solution of PVA and 200 parts by weight of water were added thereto and mixed well to obtain a composition for red. ● Composition liquid for blue: In place of the above-mentioned red coloring agent, as a blue coloring agent
Except for using 3- (4-diethylamino-2-methylphenyl) -3- (1-ethyl-2-methylindol-3-yl) -4-azaphthalide, each was prepared under the same conditions as described above and used for blue. A liquid was obtained.

Then, as a TI sheet 1, a 15 cm square 2
Axial stretched PET film (thickness 100 μm, haze ratio 1.
26%), and both sides are degreased and washed. First, the above-mentioned composition liquid for red is coated on one side with a bar coater and dried to form a 3 μm-thick HR layer 2 for red. On the other side, the above-mentioned blue composition liquid was applied in the same manner to provide a blue HR layer 3 having a thickness of 3 μm.

A white titanium dioxide powder (particle size: 0.25 μm) was used as a heat conductive white layer (hereinafter referred to as a white layer).
m) 10% by weight of PVA so that 85 parts by weight of PVA resin is contained at 16.5% by weight based on titanium dioxide powder.
An aqueous solution A was added and mixed to prepare a white paint liquid, which was directly applied to the blue HR layer side with a bar coater and dried.
The thickness of the laminated white layer 4 was 10 μm.

Then, the sheet-like red and blue 2 obtained above are obtained.
Regarding the color thermoreversible recording medium, first, the red H
Heat (source) by "Shoin WD-1200U" in R layer 2
5 and then the same heat source 6 from the white layer 4 side performed blue coloring at two locations. Then, from the side to be viewed (the HR layer 2 for red), the coloring positions of red, blue, and a mixture of both colors (total three places)
The chromaticity L ^* , a ^* , and b ^{* of each} were measured. The results are shown in Table 1 together with the Tt and Tc of the formed TI sheet 1 and the L value and Tc of the white layer 4. For reference, the configuration, measurement position, and the like of the two-color thermoreversible recording medium are illustrated in FIG. In this figure, (1-A) is a sectional view of the medium, and (1-A)
(B) is a plan view seen from the viewing side. In addition, it was also confirmed that the color was erased immediately after the color development was performed. When the coloring and erasing operations were repeated 150 times, the degree of coloring of each color was also measured, but it was also confirmed that the value of L ^* a ^* b ^* did not change.

(Table 1)

(Example 2) First, green and blue H
An R layer composition was prepared. ● Green composition liquid ・ ・ Prepared under the same conditions as in Example 1 except that 7- (N, N-dibenzylamino) -3- (N, N-diethylamino) fluorolactone is used as a green color former. A composition solution was obtained. ● Blue composition liquid: Prepared under the same conditions as in Example 1 above.

Using the same PET film as in the first embodiment, a green composition solution was similarly coated on one side with a green HR.
The blue HR layer and the green HR layer having a film thickness of 3 μm were provided by coating the layer and the other side with a blue composition liquid and drying.

On the blue HR layer side, the same white paint solution as in Example 1 was applied in the same manner, dried and a white layer having the same thickness was laminated. A reversible recording medium was obtained. In this case, the coloring operation was performed from both sides in the same manner, and the chromaticities L ^* , a ^* , and b ^* at three locations were measured. The results are summarized in Table 1 as in Example 1. still,
A decoloring operation was performed after color development, and it was also confirmed that the color was immediately deleted as in Example 1. Also at the time of repeated coloring operations and color erasing operation and a 150 times, but was also measured color of each color L ^* a ^*
It was also confirmed that there was no change in the value of b ^* .

(Example 3) First, yellow and red H
An R layer composition was prepared. A yellow liquid composition was prepared under the same conditions as in Example 1 except that 3-methoxy-6-methoxyfluoranlactone was used as a yellow color developing agent. ● Composition solution for red: Obtained and prepared under the same conditions as in Example 1.

On the other hand, 10% by weight of dioctyl adipate as a plasticizer was added and mixed to a polymer (Tg 145 ° C.) obtained by ring-opening polymerization of norbornene and hydrogenation, and the mixture was molded into a film. Thickness 100μm, size 15c
An m-square cyclic olefin-based polymer film was used as a TI sheet, and the yellow composition solution and the red composition solution were applied and dried on both surfaces in the same manner as in Example 1 to form a corresponding yellow HR layer. An HR sheet was formed by laminating the HR layer for red.

The same white paint liquid as in Example 1 (90 parts by weight of white titanium dioxide powder and 14.5% by weight of PVA resin) was applied to the blue HR layer side and dried. Then, a white layer having a thickness of 8 μm was formed to obtain a sheet-like two-color thermoreversible recording medium of green and blue. In this case, the coloring operation was performed from both sides in the same manner, and the chromaticities L ^* , a ^* , and b ^* at three locations were measured. The results are summarized in Table 1 in the same manner as in Example 1. In addition, it was also confirmed that the color was erased immediately after the color erasing operation was performed as in Example 1. When the coloring and erasing operations were repeated 150 times, the degree of coloring of each color was also measured, but it was also confirmed that the value of L ^* a ^* b ^* did not change.

(Comparative Example 1) As a TI sheet, Tt = 8
First, the formation of the red and blue HR layers and the formation of the white layer on the blue HR layer side were performed under the same conditions as in Example 1 except that a PET film having 0.03% and Tc of 0.43 W / m ° C. was used. Then, a two-color thermoreversible recording medium for comparison was obtained. Similarly, the chromaticities L ^* , a ^* , and b ^* at three locations were measured, and the results are summarized in Table 1. The difference from Example 1 is evident, which is the result of the influence of two factors that the TI sheet did not have the required transparency and did not have sufficient heat insulation. It can be said.

Comparative Example 2 White titanium dioxide powder (particle size: 0.25 μm) 80 for the white layer in Example 1
By weight, the PVA resin is 60 parts by weight based on the titanium dioxide powder.
A 10% by weight aqueous solution of PVA was added and mixed so as to be contained in an amount of 10% by weight to prepare a comparative coating liquid, and the formation of the red and blue HR layers and the blue H
The coating was applied to the R layer side and dried. However, the coating was performed three times. The thickness of the laminated layer is 50 μ
m and L values were 84.31 and Tc was 0.58 W / m ° C. About this, it developed similarly and visually recognized first. Although red was not much different from that of Example 1, blue was slightly recognized visually, and the mixed color of both was also substantially seen in red hue. And, similarly, each chromaticity L ^* , a at three places
^{* And} b ^* were measured and the results are summarized in Table 1.

[0044]

As described above, the present invention has the following advantages.

First, image information can be represented by three hues of any two hues of red, blue, green, and yellow, and a mixture of hues by superposition of these two hues, and by viewing it from one surface. It became easy to see immediately.

Further, the color image composed of each of the three hues is displayed deeper and more vividly. Further, the developed color image is completely easily erased, and the repetitive life thereof can be used longer.

[Brief description of the drawings]

FIG. 1 is a side sectional view of a thermoreversible recording medium of Example 1 (1-
(A) and the plane (1-B).

[Explanation of symbols]

 1 TI sheet (PET film) 2 Red HR layer 3 Blue HR layer 4 White layer

Claims (4)

[Claims] 1. At least two thermoreversible recording layers (2, 3) for forming a color image are provided on both sides of a heat-insulating transparent support (1) having a total light transmittance of 85% or more, and A brightness index of 88 or more on one side of the thermoreversible recording layer,
A thermoreversible multicolor recording medium comprising a thermally conductive white layer (4) having a thermal conductivity of 0.7 W / m ° C or higher. 2. The thermoreversible recording medium according to claim 1, wherein at least two thermoreversible recording layers forming a color image are formed of at least two color developing layers selected from the group consisting of red, blue, yellow and green. Color recording medium. 3. The heat insulating transparent support (1) has a heat insulating property as follows:
3. The thermoreversible multicolor recording medium according to claim 1, which has a thermal conductivity of 0.5 W / m.degree. C. or less. 4. The heat-conductive white layer (4) according to claim 1, wherein the heat-conductive white layer (4) is provided on a side of the at least two thermoreversible recording layers (2, 3) which is closer to a deeper color forming layer. 2. The thermoreversible multicolor recording medium according to claim 1.