JPH08127177A - Reversible thermal recording medium - Google Patents

Abstract

PURPOSE: To obtain a reversible thermal recording medium in which hue change of a color developing state or discoloring failure do not occur by forming a recording layer forming binder by shape memory resin. CONSTITUTION: The recording layer forming binder of a reversible thermal recording medium comprising a recording layer 2 containing colorant, developer and binder as main ingredients in such a manner that a color developing state 3 is formed by heating and melting, and a discoloring state is formed by heating at a temperature lower than the color developing temperature is formed by shape memory resin. Thus, the medium in which the resin is used as the binder is heated to instantaneously color develop, and the state 3 is always stable even at the ambient temperature. The state 3 can be discolored by heating at the color developing temperature or lower, and the discolored state is stable even at the ambient temperature.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reversible thermochromic composition utilizing a color-forming reaction between an electron-donating color-forming compound and an electron-accepting color-developing compound. The present invention also relates to a reversible thermosensitive recording medium using this composition.

[0002]

2. Description of the Related Art Conventionally, an electron-donating color forming compound (hereinafter referred to as
A thermosensitive recording medium utilizing a color-developing reaction between a color former (also referred to as a color former) and an electron-accepting color-developing compound (hereinafter also referred to as a color former) is widely known, and the output of a computer, a facsimile, an automatic ticket vending machine. Widely applied to printers of scientific measuring instruments, printers for CRT medical measurement, etc. But,
The color development of all conventional products is irreversible, and the color development and the color erasing cannot be alternately repeated. On the other hand, according to the patent publication, there have been proposed some thermal recording media that utilize color reaction between a color former and a color developer to reversibly perform color development and decolorization. For example, Japanese Unexamined Patent Publication (Kokai) No. 60-193,691 discloses a color developer using a combination of gallic acid and phloroglucinol. The color-developing material obtained by thermally coloring this material is decolorized with water or steam. However, in the case of this heat-sensitive recording medium, there is a problem in that it is difficult to make it water resistant, and there is a problem in recording storability, and furthermore, there is a problem that the color erasing device for erasing the color developing body becomes large. JP 61-23
Japanese Patent No. 7684 discloses a rewritable optical recording medium using a compound such as phenolphthalene, thymolphthalene or bisphenol as a developer. This product forms a color-developing material by heating and slowly cooling it, while it can be decolored by heating the color-developing material once to a temperature higher than the color-developing temperature and then rapidly cooling it. However, in the case of this recording medium,
The coloring and decoloring steps are complicated, and the decolored body obtained by decoloring the colored body still has some coloring, and a colored image with good contrast cannot be obtained. JP-A-62-140881, JP-A-62-138568 and JP-A-62-138556 disclose a homogenous solution of a color former, a developer and a carboxylic acid ester. This product shows a completely colored state at a low temperature and a completely decolored state at a high temperature, and can maintain the colored or decolored state at an intermediate temperature between them, and by printing with a thermal head on this medium, the colored background White characters (decolored body) can be recorded on (colored body). Therefore, in the case of this recording medium, since the image to be recorded is a negative image, its use is limited, and it is necessary to hold the image within a specific temperature range for storing the recorded image.

JP-A-2-188294 and JP-A-2-188294
No. 188293 discloses salts of gallic acid and higher aliphatic amines that reversibly develop and reduce color, and salts of bis (hydroxyphenyl) acetic acid or butyric acid and higher aliphatic amines as color developers. The one used is shown. This product can be thermally colored in a specific temperature range and decolorized by heating at a higher temperature than that, but since its color-developing action and color-reducing action occur competitively, these actions must be thermally controlled. It is difficult to obtain good image contrast. As described above, the conventional reversible thermosensitive recording medium utilizing the reaction between the color former and the color developer has various problems and is still unsatisfactory. The present inventors previously used an organic phosphoric acid, a carboxyl compound or a phenol compound having a long-chain aliphatic group as a color developer, and by combining this with a leuco compound as a color former, the coloring and decoloring of the compound can be performed. It can be easily performed only by heating, and its coloring and erasing states can be maintained at room temperature, and the erasing temperature is lower than the erasing temperature. Therefore, a reversible thermochromic composition that can be repeated many times and a reversible thermosensitive recording medium containing the same in a recording layer have been proposed (Japanese Patent Application No. 3-35).
5078). The reversible thermosensitive recording medium proposed by the present inventors is a reversible thermosensitive recording medium having many advantages that are incomparable to conventional reversible thermosensitive recording media, but the temperature of the colored image is lower than the coloring temperature. When the color is erased with, it is not completely erased and there is a density difference (erased residue) with the background part, and it can be seen that it looks like a stain. Furthermore, the density of the erased state is made as close as possible to the density of the background part. Has also been found to have a problem that the erasing work takes time.

[0004]

DISCLOSURE OF THE INVENTION The present invention is a reversible thermosensitive recording medium utilizing the reaction between a color former and a developer, and is a reversible ink that does not cause a hue change in the color-developed state or defective decolorization. It is an object to provide a heat-sensitive recording medium.

[0005]

Means for Solving the Problems The present inventors have completed the present invention as a result of intensive studies to solve the above problems. That is, according to the present invention, a recording layer containing a color developer, a color developer and a binder as main components is used,
In the reversible thermosensitive recording medium, a color-forming state is formed by melting and a decoloring state is formed by heating to a temperature lower than the color-forming temperature, wherein the recording layer forming binder is a shape memory resin. A thermal recording medium is provided. The reversible thermosensitive recording medium in which the shape memory resin is used as the binder for forming the recording layer of the present invention instantly develops color upon heating, and the state of color development is stable even at room temperature. The color-developed state can be decolored by heating at a temperature below the color-developing temperature, and the color-erased state is stable even at room temperature. With respect to the reversible thermosensitive recording medium of the present invention, the principle of coloring and erasing, that is, image formation and image erasing will be described with reference to the graph of FIG. The vertical axis of the graph represents the color density and the horizontal axis represents the temperature. The solid line 1 represents the image forming process by heating.
A broken line 3 shows an image erasing process by heating.
A is the density in the completely erased state, B is the density in the saturated coloring state when heated to a temperature of T ₁ or higher, C is the density at the temperature of T ₀ or lower in the saturated coloring state, and D is T _0. ~ T
_It shows the concentration when erased by heating at a temperature between ₁ .

The reversible thermosensitive recording medium of the present invention is in a colorless state (A) at a temperature of T ₀ or lower. To record
It may be heated to a temperature of T ₁ or higher with a thermal head or the like,
Color development (B) is performed to form a recorded image. Even if the recorded image is returned to the temperature of T ₀ or less according to the solid line 2, the state (C) is maintained as it is, and the recording memory property is not lost. In addition,
The color density at the time of forming a recorded image increases with the temperature increase at a temperature of T ₁ or higher and reaches the saturation density at point B. Next, in order to erase the recorded image, the formed recorded image may be heated to a temperature between T _{0 and} T _{1, which} is lower than the coloring temperature, and the state becomes colorless (D). This state is maintained as it is even if the temperature is returned to T ₀ or lower (A). That is, the process of forming the recorded image reaches C by the path of the solid line ABC and the recording is held. During the process of erasing the recorded image, the erased state is maintained up to A by the path of the broken line CDA. The behavioral characteristics of formation and erasure of the recorded image are reversible and can be repeated many times. FIG. 2 is an explanatory view showing an example of image formation and image deletion by the reversible thermosensitive recording medium of the present invention, in which 1 is a support and 2 is a recording layer. The image forming steps (A) → (B) are achieved by recording and printing at a temperature of T ₁ or higher in FIG. 1 by a heat source for image formation, for example, the thermal head 4. Next, the image erasing step (B) →
(A) is achieved by heating the image erasing heat source, for example, a heating roller 5 to a temperature between T _{0 and} T ₁ . In FIG. 2, 3 indicates a color image.

The color developer used in the present invention basically has a structure showing a color developing ability capable of developing color in the molecule and an alkyl chain having 5 or more carbon atoms for controlling cohesive force between molecules. A compound having a structural part, which is an organic phosphoric acid or aliphatic carboxylic acid or phenol having an aliphatic group having 12 or more carbon atoms, or a metal salt of mercaptoacetic acid having an aliphatic group having 10 to 18 carbon atoms, or carbon Number 5-8
It is an alkyl ester of caffeic acid having an alkyl group of. The aliphatic group includes a linear or branched alkyl group and alkenyl group, and may have a substituent such as halogen, an alkoxy group and an ester group. Among the developers used in the composition of the present invention shown above, particularly preferable are long-chain alkylphosphonic acid, long-chain α-hydroxy fatty acid, long-chain alkylthiomalic acid, long-chain alkylmalonic acid, etc. The invention is not limited to these. The color former of the present invention is a colorless or light-colored dye precursor exhibiting an electron donating property, and is not particularly limited, and conventionally known ones such as triphenylmethanephthalide compound, fluoran compound, phenothiazine compound, leuco aura. Mining compounds, indolinophthalides, etc. are used.
Specific examples of the color developing agent and the color developing agent used in the present invention are described in Japanese Patent Application No. 3-355078 and Japanese Patent Application No. 4-191643.
And Japanese Patent Application No. 4-207604.

The recording layer of the reversible thermosensitive recording medium of the present invention contains a color former and a developer as essential components. The color of the composition is obtained by cooling a color former composition formed by heating, melting and mixing a developer and a color developer to room temperature. Since this color-forming material composition has an erasing temperature region on the lower temperature side than the melting temperature, rapid cooling is preferable for cooling from the melt-colored state to room temperature while maintaining the color development. Slow cooling often causes some color loss when passing through the color removal temperature range, resulting in a decrease in density. It is considered that in the color former composition, the molecules of the color former and the developer interact with each other to open the lactone ring of the color former to develop a color. The composition in a state of being rapidly cooled from a molten state,
In addition to the chromophore molecule, it contains a developer molecule and a chromophore molecule that are not directly involved in chromophore formation. In the composition of the present invention, the color former composition at room temperature is in a solidified state due to the cohesive force between these molecules. The formation of such an aggregate structure is mainly due to the cohesive force acting between the developer molecule forming the color former and the excess developer molecule not forming the color former. It is presumed that the formation of such an aggregate structure is related to the decoloring phenomenon of the color former composition. The color-forming material composition can be decolored by heating its color-developed state to a specific temperature range. In this decoloring process, the aggregate structure of the color developing state changes, and eventually the developer molecules are separated and crystallized from the color former composition to form crystals of the developer alone, resulting in a stable decolored state. Confirmed by X-ray. As described above, in the color former composition formed on the reversible thermosensitive recording medium of the present invention, it is clear that the alkyl chain portion of the color developer plays a large role in the formation of the aggregated colored state and the decoloring process thereof. This is a characteristic of the color former composition formed on the reversible thermosensitive recording medium of the present invention.

In the recording layer of the reversible thermosensitive recording medium of the present invention, a binder component is also an essential component. In the present invention, a shape memory resin is used as the binder for forming the recording layer. The shape memory resin is a resin at room temperature, and changes into rubber characteristics at a temperature slightly higher than room temperature. Specifically, for example, polynorbornene shape memory resin (manufactured by Zeon Corporation), styrene-butadiene copolymer shape memory resin (manufactured by Asahi Kasei Corporation), polyurethane shape memory resin (manufactured by Mitsubishi Heavy Industries, Ltd.), trans-1,4.
-Polyisoprene shape memory resin (manufactured by Kuraray Co., Ltd.) and the like.

The reversible thermosensitive recording medium of the present invention using these shape memory resins as the binder for the recording layer instantly develops a color upon heating, and when cooled to a temperature below room temperature, a colored image is stably retained. When the color-developed image is heated to a temperature lower than the color-developing temperature, the image is immediately decolorized and the density becomes almost the same as the background. As described above, when the shape memory resin is used as the binder of the recording layer, the color memory, the color erasing property, and the retention of the color image are good because the shape memory resin softens at the color developing temperature and the color erasing temperature and becomes rubbery. It is presumed that the color developing agent and the color developing agent are easily melted during the color development, and the color developing agent is easily aggregated during the color erasing to facilitate the separation from the color developing agent. Further, the shape memory resin for forming the recording layer of the reversible thermosensitive recording medium of the present invention is desired to be soluble in water or an organic solvent. In the case of the type dispersed in water or an organic solvent, the color density and the storability of the color image are adversely affected by the influence of the surfactant of the dispersant. Examples of the organic solvent that dissolves the shape memory resin that can be used in the present invention include alcohols,
Glycol alkyl ethers, toluene, xylene,
Methyl ethyl ketone, ethyl acetate, dichloroethane, chlorobenzene, tetrahydrofuran, dioxane, tetrahydropyran, dimethylformamide, N-methylpyrrolidone and the like can be mentioned.

In the reversible thermosensitive recording medium of the present invention, the proportion of each component in the composition constituting the recording layer is such that the color former 1
0.5 to 20 parts by weight, preferably 1 to 10 parts by weight, and 0.5 to 20 parts by weight of binder with respect to parts by weight,
It is preferably 1 to 10 parts by weight. The higher the developer ratio, the lower the decolorization start temperature, and the lower the ratio, the lower the thermal sensitivity. Further, when the ratio of the binder is high, the thermal sensitivity is low, and when the ratio is low, the binding property tends to be low. The reversible thermosensitive recording medium of the present invention comprises a support and a recording layer containing the composition, and a protective layer is usually formed on the recording layer to improve durability. You don't have to. The support is paper, synthetic paper, plastic film, a composite of these, glass, or the like, as long as it can hold the recording layer. The recording layer may be in any form as long as the above composition is present, and the developer may be mixed and melted to form a film, which may be cooled to form the recording layer. However, it is usually preferable to sufficiently disperse the color developer and the color developer in the binder resin to form the recording layer, and a reversible thermosensitive recording medium having a long life can be obtained by this method. Further, in this case, the recording layer can be formed according to a conventionally known method. That is, the color-developing agent and the color-developing agent may be uniformly dispersed or dissolved in water or an organic solvent together with the binder resin, and this may be applied and dried on the support. In the reversible thermosensitive recording medium of the present invention, various additives, such as a dispersant, a surfactant and a polymer, which are commonly used in thermosensitive recording paper, are used for the purpose of improving coating properties or recording properties, if necessary. Cationic conductive agent, filler, color image stabilizer, antioxidant, light stabilizer,
A lubricant or the like can be added to the recording layer.

The protective layer prevents the surface from being deformed or discolored by heat and pressure when heat is applied, and also has chemical resistance,
It can also have a role of improving water resistance, abrasion resistance, head matching property, and the like. Therefore, it is preferable that the material for forming the protective layer has excellent heat resistance and high strength, such as silicone rubber and silicone resin (JP-A-63-221).
No. 087), polysiloxane graft polymer (JP-A-62-152550, JP-A-63-317385).
No.), a thermosetting resin, an ultraviolet curable resin, an electron beam curable resin, etc. are used, but those formed by an aqueous emulsion of a water-soluble polymer or a hydrophobic polymer compound may be used. By forming such a protective layer, heat resistance is improved as described above, and resistance to contact with organic solvents, plasticizers, oils, sweat, water, etc. is also increased, and image formation and erasing can be performed without problems even in a bad environment. It is possible to obtain a repeatable recording medium. Further, by containing a light stabilizer in the protective layer, a recording medium having significantly improved light resistance of the image and the background can be obtained, and addition of a polymeric cationic conductive agent enables antistatic property, and further the protective layer By adding an organic or inorganic filler and a lubricant to the above, sticking caused by contact with a thermal head or the like can be prevented, and a thermal recording medium having high reliability and head matching can be obtained. The protective layer may be formed by uniformly dispersing or dissolving the components of the protective layer in water or an organic solvent, and uniformly coating and drying the composition on the recording layer. The thickness of the protective layer is about 0.5 to 10 μm. Is good.

In the reversible thermosensitive recording medium of the present invention, an undercoat layer may be provided between the support and the recording layer, if necessary. The undercoat layer is installed for the purpose of improving the heat insulating property, improving the adhesion between the support and the recording layer, and improving the resistance of the support to the solvent when the recording layer is formed. You just have to decide whether or not. One of the important roles of the undercoat layer is to improve heat insulation, but this is to make the applied heat energy useful for forming a heat record and erasing heat without waste. Can be done sharply. To form an undercoat layer for the purpose of heat insulation, fine hollow particles made of an organic or inorganic material may be coated on a support,
Specifically, a fine hollow body having a particle size of about 10 to 50 μm formed of glass, ceramics, or plastic may be well dispersed in a solvent together with a binder resin, and uniformly coated and dried on the support. It should be noted that the applied heat energy can be saved by using a heat insulating support instead of providing the heat insulating layer, and a plastic film, synthetic paper or the like is used as the heat insulating support. The formation of a recorded image is not particularly limited by a hot pen, a thermal head, laser heating or the like depending on the purpose of use. Similarly, the erasing of the recorded image is not particularly limited as long as the erasing temperature conditions such as the heating roller, the sheet heating element, the constant temperature bath, the warm air, and the thermal head are given. It is also possible to form the recorded image by another thermal head set at the recording temperature while erasing the recorded image by the thermal head set at the erasing temperature.

[0014]

EXAMPLES The present invention will now be described in more detail with reference to examples, but the present invention is not limited to these examples. All parts below are based on weight. Example 1 The following composition was crushed and dispersed by a ball mill to a particle size of 1 to 3 μm to prepare a coating liquid for forming a recording layer. 2- (o-chloroanilino) -6-dibutylaminofluorane 3.5 parts Octadecylphosphonic acid 12 parts Styrene-butadiene copolymer shape memory resin (manufactured by Asahi Kasei Corporation) 10 parts Tetrahydrofuran 90 parts A white PET film (Lumirror E-20, manufactured by Toray Industries, Inc.) having a thickness of 100 μm was coated with a wire bar and dried to a coating thickness of about 6.0 μm, and a reversible thermosensitive recording sheet was obtained. The recording sheet thus prepared was brought into contact with a hot plate set at 130 ° C. for 20 seconds to obtain a color image, and the color density was measured with a densitometer RD-914 manufactured by Macbeth. Next, apply the colored recording sheet to a hot plate set at 70 ° C for 30
The image was erased by contacting for 2 seconds, and the erasing density was measured with RD-914. For comparison, the density of the background part was also measured by the same method. Further, the density of the image portion after the color image was placed in a constant temperature bath at 40 ° C. and stored for 48 hours was measured in the same manner as above, and the image residual rate was calculated by the following formula. The results are shown in Table 1.

Formula A = (C / B) × 100 A: Image residual ratio (%) B: Image density before storage C: Image density after storage Example 2 Exactly the same as Example 1 using the following composition A reversible thermosensitive recording medium was prepared by the method of 1. and evaluated in the same manner as in Example 1. The results are shown in Table 1. 2-anilino-3-methyl-6-dibutylaminofluorane 3 parts Eicosylthiomalic acid 10 parts Styrene-butadiene copolymer shape memory resin (manufactured by Asahi Kasei Corporation) 10 parts Tetrahydrofuran 90 parts Example 3 The following composition was prepared. A reversible thermosensitive recording medium was prepared in exactly the same manner as in Example 1, and evaluated in the same manner as in Example 1. The results are shown in Table 1.

2-anilino-3-methyl-6- [N-ethyl-N- (P-toluidino)] fluorane 3 parts docosylphosphonic acid 10 parts polyurethane shape memory resin (manufactured by Mitsubishi Heavy Industries, Ltd.) 10 parts 3, 4-Dihydro-2H-pyran 30 parts Tetrahydrofuran 60 parts Example 4 A reversible thermosensitive recording medium was prepared in the same manner as in Example 1 using the following composition, and evaluated in the same manner as in Example 1. The results are shown in Table 1. 2- (o-chloroanilino) -6-diethylaminofluorane 3.5 parts α-hydroxyoctadecylphosphonic acid 12 parts trans-1,4-polyisoprene shape memory resin (manufactured by Kuraray Co., Ltd.) 10 parts toluene 40 parts tetrahydrofuran 50 parts comparison Example 1 A reversible thermosensitive recording medium was prepared in the same manner as in Example 1 except that the following resin was used instead of the shape memory resin in Example 1, and evaluated in the same manner as in Example 1. The results are shown in Table 1.
Shown in

Vinyl Chloride / Vinyl Acetate Copolymer (VMCH manufactured by Union Carbide) Comparative Example 2 Reversible in the same manner as in Example 2 except that the following resin was used instead of the shape memory resin in Example 2. A thermal recording medium was prepared and evaluated in the same manner as in Example 2. The results are shown in Table 1.
Shown in Acrylic resin (manufactured by Mitsubishi Rayon Co., Ltd., BR-107) Comparative Example 3 A reversible thermosensitive recording medium was prepared in the same manner as in Example 3 except that the following resin was used instead of the shape memory resin in Example 3. Evaluation was made in the same manner as in Example 3. The results are shown in Table 1.
Shown in Polyester resin (manufactured by Toyobo Co., Ltd., Byron 200) Comparative Example 4 A reversible thermosensitive recording medium was prepared in the same manner as in Example 4 except that the following resin was used instead of the shape memory resin in Example 4, and Example 4 was used. It evaluated by the same method as. The results are shown in Table 1.
Shown in Vinyl chloride / vinyl acetate copolymer (manufactured by Denki Kagaku Kogyo,
Denka vinyl # 1000A)

[0018]

[Table 1]

[0019]

As described above, in the reversible thermosensitive recording medium of the present invention, the use of the shape memory resin as the binder for forming the recording layer improves the decoloring property and brings the decoloring density close to the background density. You can

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between color density and temperature of a reversible thermosensitive recording medium of the present invention, and is an explanatory view of the principle of color development and color erasing. The solid line (A → B → C) indicates the image forming process, and the broken line (C
→ D → A) shows an image erasing process.

FIG. 2 is an explanatory diagram of an image forming process and an image erasing process.

[Explanation of symbols]

 1 Support 2 Reversible Thermosensitive Recording Layer 3 Colored Image 4 Thermal Head 5 Heating Roller

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Keiji Taniguchi 1-3-6 Nakamagome, Ota-ku, Tokyo Within Ricoh Co., Ltd. (72) Kyoji Tsutsui 1-3-6 Nakamagome, Ota-ku, Tokyo Shares Inside Ricoh Company (72) Inventor Fumio Kawamura 1-3-6 Nakamagome, Ota-ku, Tokyo Inside Ricoh Company, Ltd.

Claims (2)

[Claims] 1. A reversible thermosensitive recording medium having a recording layer containing an electron-donating color-forming compound, an electron-accepting color-developing compound and a binder as main components, wherein the recording layer-forming binder is a shape memory resin. Characteristic reversible thermosensitive recording medium. 2. A reversible thermosensitive recording medium, wherein the shape memory resin according to claim 1 is soluble in an organic solvent.