JP2000037954A - Reversible heat sensitive recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a reversible heat sensitive recording medium which is capable of making a full color display rapidly and has a wide variable range of a display color and further can easily control temperature. SOLUTION: This reversible heat sensitive recording medium is structured of a sheetlike base layer 2, an intermediate layer 3, a reversible heat sensitive recording layer 4 and a protecting layer 5. The recording layer 4 is composed mainly of a low molecular cholesteric liquid crystalline compound which shows a cholesteric phase at a higher temperature than a room temperature to reflect a light of a visible wavelength range in response to a temperature and becomes a solid in such a state that it reflects a light by quenching the compound from the temperature and a nematic liquid crystalline compound.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reversible thermosensitive recording medium, and more particularly, to a two-dimensional recording medium on which information can be written and erased under a specific heating condition.

[0002]

2. Description of the Related Art In recent years, interest in resource saving and recycling has been increasing, and it is desired that a recording medium such as paper can be used repeatedly. In an approach to such technical development, a rewritable thermosensitive recording material that can be recorded and erased by a heating means such as a thermal head has attracted attention. This type of reversible thermosensitive recording material is used for the purpose of recycling a recording medium,
It can also be used for the purpose of visualizing internal information recorded on a C card, a magnetic card, an optical card, etc., and various applications are possible.

Hitherto, as reversible thermosensitive recording materials, leuco dyes / color-reducing agents, organic low molecular weight / polymer resin matrices, and polymer cholesteric liquid crystals have been known. The leuco dye / developing agent develops color by opening a lactone ring contained in the leuco dye molecule, and decolorizes by closing the ring. The lactone ring opens by rapid cooling after the temperature rise, and closes by slow cooling. Such a leuco dye / developing colorant is applied on a sheet material, and information is written using a thermal head, and erased by passing between heat rollers.

As the organic low molecular weight / polymer resin matrix, those using BA (behenic acid) as the organic low molecular weight compound and PVCA (vinyl chloride-vinyl acetate copolymer) as the high molecular weight compound are known. . This material can be switched between transparent and scattering depending on the heating temperature, and that state is maintained after cooling. This material is applied on a sheet material, and information is written using a thermal head.

[0005] A polymer cholesteric liquid crystal has been known in which a vinyl compound having a cholesteric liquid crystal compound in a side chain is polymerized. After the material is heated to a temperature higher than the crystallization temperature, the display color can be changed and fixed by rapidly cooling from a certain temperature.

Low molecular cholesteric liquid crystals are disclosed in Adv. , Mat
er. , 1997, 9 (14), 1102-1104, some materials are known. After heating above the melting point, rapid cooling from a certain temperature changes and fixes the display color. Can be.

[0007]

With the above-mentioned leuco dye / color reducing agent, the color that can be displayed is determined by the leuco dye, and a full-color display of an arbitrary image cannot be made. In the case of an organic low molecular weight / polymer resin matrix, display is performed by transmission / scattering, so that full-color display cannot be performed. In the polymer cholesteric liquid crystal, in principle, the display color can be changed according to the heating temperature, but the color change requires a time on the order of minutes, which has been a major obstacle to practical use. The low-molecular-weight cholesteric liquid crystal can change the display color according to the heating temperature, but has a problem that the temperature range in which the color is changed is narrow, color unevenness occurs, and the reproducibility is poor.

Accordingly, an object of the present invention is to provide a novel reversible thermosensitive thermosensitive color display which can perform color display at high speed, has a wide variable range of display colors, facilitates temperature control, and has little color unevenness and good reproducibility. It is to provide a recording medium.

[0009]

In order to achieve the above object, the present inventors have conducted various studies. As a result, by using a low-molecular-weight cholesteric liquid crystal compound and a nematic liquid crystal composition together, a variable range of display colors can be obtained. And the temperature control becomes easy, and the present invention has been achieved. That is, the reversible thermosensitive recording medium according to the present invention comprises a sheet-like base layer and a recording layer, and the recording layer reflects a cholesteric phase at a temperature higher than room temperature and reflects light in a visible wavelength range according to the temperature. A low-molecular cholesteric liquid crystal compound which solidifies in a reflection state by being rapidly cooled from the temperature, and a nematic liquid crystal compound.

In the present invention, the recording information is obtained by using a mixture of a low-molecular cholesteric liquid crystal compound and a nematic liquid crystal compound as the reversible thermosensitive recording material used for the recording layer while using the sheet-like base layer. By applying a suitable heating temperature, a desired color can be displayed at a high speed, and a full-color display can also be performed. The display information is erased by reheating. Further, the display colors are various, color unevenness is small, the temperature range in which colors are changed is wide, and temperature control is easy.

As the nematic liquid crystal compound, a general nematic liquid crystal compound is preferable, and a liquid crystal cyanobiphenyl compound, a liquid crystal phenylcyclohexyl compound, a liquid crystal ester compound, a liquid crystal tolan compound,
Liquid crystal pyrimidine compounds, liquid crystal bicyclohexyl compounds, liquid crystal phenyldioxane compounds, mixtures thereof, and the like can be used. A chiral material having a substituent having an asymmetric carbon may be added to these liquid crystal compounds. As the low-molecular cholesteric liquid crystalline compound, for example, dicholesteryl 10,12-docosadiindionate can be used.

The liquid crystal composition used in the present invention is preferably colorless and hardly crystallized. Further, since it is solid at room temperature, the melting point (or clearing point) is 50 to 50%.
Those having a temperature of about 150 ° C. are preferred.

[0013] The recording layer may be provided on the front surface and the back surface of the base layer. Information can be displayed on both sides of the medium.

Further, in the present invention, an intermediate layer having a smooth surface in contact with the recording layer may be provided between the base layer and the recording layer. In order to direct the helical axis in a direction perpendicular to the base layer when the liquid crystal exhibits a cholesteric phase, the surface of the base layer is preferably substantially flat. By interposing the intermediate layer, a material having a rough surface such as plain paper can be used as the base layer. In addition, the direction of the helical axis of the liquid crystal is well aligned in the direction perpendicular to the base layer, and a display with high reflectance can be performed. Further, if the function of absorbing visible light is imparted to the intermediate layer, a display with black background can be performed. If the intermediate layer is provided with a function of reflecting light of a specific wavelength, display with a specific color as a background can be performed. Even if the function of totally reflecting incident light is given to the intermediate layer, if the recording medium is tilted so that the reflected light of light incident from a specific light source does not directly enter the eyes, color display with a black background Can be confirmed.

Further, in the present invention, the recording layer may be formed of a composite film of a liquid crystalline compound and a polymer resin. With such a composite film, the mechanical strength of the recording layer is increased, and the medium is more resistant to bending and friction. Further, the recording layer may include a spacer. The thickness of the recording layer can be made uniform, and the thickness of the recording layer can be kept constant when display information is erased with a heat roller.

Further, in the present invention, a protective layer may be provided on the recording layer. With this protective layer, the recording layer is mechanically or chemically protected from the outside.

Further, in the recording medium according to the present invention, a part of the recording layer may be an irreversible thermosensitive recording area, and specific information may be written in this area in advance. For example, by writing in advance the company name and decorative design,
It can be used as a specific formatted recording medium.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of a reversible thermosensitive recording medium according to the present invention will be described with reference to the accompanying drawings. In each of the embodiments described below, specific substance names are described. However, this is only one example, and the present invention is not limited to these substances. It is possible to use the following materials.

(First Embodiment, see FIG. 1) In FIG. 1, a reversible thermosensitive recording medium 1 comprises a base layer 2, an intermediate layer 3, a recording layer 4, and a protective layer 5 from below. The base layer 2 is made of paper, a flexible sheet material such as polycarbonate or PET (polyethylene terephthalate), and a white PET film is used in the first embodiment. By using a flexible sheet material, paper-like handling, such as bending or binding a plurality of sheets, becomes possible.

The intermediate layer 3 contains a component having a function of absorbing visible light, and the surface in contact with the recording layer 4 is formed smoothly. Specifically, an isopropyl alcohol solution obtained by dispersing carbon black in a silicone resin (YR3370 manufactured by Toshiba Silicone Co., Ltd.) and further mixing a catalyst (CR15 manufactured by Toshiba Silicone Co., Ltd.) is applied onto the base layer 2.
After drying and heat curing, a film having a thickness of 5 μm was formed.

The recording layer 4 is a liquid crystal layer containing a low-molecular-weight cholesteric liquid crystal compound and a nematic liquid-crystal compound. Specifically, a low-molecular-weight cholesteric liquid crystal represented by the following chemical structural formula (A) is added to 100 parts by weight of toluene. 9 parts by weight of a neutral compound and 1 part by weight of a nematic liquid crystalline ester compound represented by the following chemical structural formula (B) are mixed and dissolved, and this solution is applied on the intermediate layer 3 by a blade, and dried by heating to a thickness of 10 μm. The reversible thermosensitive recording layer 4 was obtained. The thickness of the recording layer 4 is preferably in the range of 3 to 50 μm, more preferably in the range of 6 to 30 μm.

[0022]

Embedded image

[0023]

Embedded image

The protective layer 5 is made of a resin material. Specifically, a polyester film having a thickness of 3 μm is superimposed on the recording layer 4 and bonded by heating at 100 ° C. (Aron Alpha).

In the recording medium 1 having the above configuration, 65
When heated to １２５125 ° C., the liquid crystal compound exhibits a cholesteric phase in which the helical axis is oriented perpendicular to the intermediate layer 3 and reflects light having a specific wavelength according to the temperature. About 65 ° C
Shows red at about 100 ° C, blue at about 120 ° C,
By quenching from those temperatures, it solidifies in its reflective state.

Further, if the mixture is heated to about 130 ° C. or more and then rapidly cooled, it becomes transparent. That is, when the recording medium 1 is heated to 130 ° C. or higher by a heat roller or the like and then rapidly cooled, the entire surface of the recording layer 4 becomes transparent. At this time, a black image is displayed for an observer who is viewed from the direction of arrow A because visible light is absorbed by the intermediate layer 3.

When the recording medium 1 is partially heated and quenched by using a conventionally known thermal head, the heated portion shows a reflection color corresponding to the heating temperature.
In FIG. 1, reference numeral 4a indicates a portion in a transparent state, and reference numeral 4b indicates a portion left as a cholesteric phase. Therefore, when writing is performed at 100 ° C. by the thermal head, when viewed from the direction of arrow A, a green display can be observed on a black background. 65 ° C, 100 ° C, 120 ° C
When writing is performed selectively, full-color display is possible. In the first embodiment, in a portion where low reflectance is desired to be displayed, the reflectance can be reduced as a result by mixing black display portions.

In the recording medium 1, the display color of the liquid crystal is observed with black background. In the middle layer 3,
When a colorant that reflects visible light in a specific wavelength range is added, the display color of the liquid crystal is observed with a certain color as the background. If a display with white background is to be performed, each small area of blue, green, and red is displayed in a mosaic shape in a portion having no display information, and a white background can be obtained by performing macroscopic white display. It is possible.

In the first embodiment, the melting point of the base layer 2 is 200 ° C. or higher, and the melting point of the intermediate layer 3 is 200 ° C.
C. or higher, the crystallization temperature of the protective layer 5 is 200 ° C., and the melting point of the recording layer 4 is 130 ° C. Therefore, even if the recording layer 4 is liquefied by being heated to 130 ° C. or more at the time of writing and erasing, if the base layer 2, the intermediate layer 3 and the protective layer 5 are maintained at a temperature lower than their melting points, their mechanical The target strength is not reduced, and the thickness of the recording layer 4 can be maintained against the pressure applied from the thermal head. If a spherical spacer is mixed in the recording layer 4, the thickness can be more reliably maintained.

In the first embodiment, the function of absorbing the entire wavelength region of visible light is provided by adding carbon black to the intermediate layer 3. For example, the intermediate layer 3 reflects blue light. If the liquid crystal compound is written so as to reflect yellow light, blue and white display can be performed. Of course, colorful display of a plurality of colors is also possible by the method. In this method, the amount of white reflection is
This is the sum of the amount of reflection of the blue color of the intermediate layer 3 and the amount of yellow reflection of the recording layer 4, which is greater than the amount of reflection when white display is performed by arranging the three primary colors of blue, green, and red in a mosaic manner, thereby enabling brighter display. . If the reflection color of the intermediate layer 3 is set to a plurality of colors, a more colorful display can be performed.

In the first embodiment, the temperature change range of the display color is 65 to 125 ° C., and the reflection wavelength is 600 to 440.
in the nm range. When printing was performed using a hot stamp, the variable range of the reflection color was wide and good recording characteristics were exhibited.

(Thermal Printer, see FIGS. 5 and 6) FIG. 5 shows a thermal printer for writing information on the recording medium 1. The printer includes transport rollers 11, 1 in a housing 10 along a traveling direction B of the recording medium 1.
2, heat rollers 13 and 14, a cooler 15, a thermal head 16, and a platen 17 are provided.

The recording medium 1 enters the printer from the entrance 10a, and the heat rollers 13, 12
14, where it is heated to 130 ° C. or higher, and further quenched by the cooler 15. Thus, the recording on the recording medium 1 is erased. Next, the recording medium 1 is conveyed between the platen 17 and the thermal head 16, where necessary information is written. The recording medium 1 heated by the thermal head 16 to be in the display state is naturally cooled rapidly after the heating from the thermal head 16 is stopped, the writing is fixed, and the recording medium 1 is discharged from the outlet 10b.

After the heating element provided on the thermal head 16 has passed, the recording medium is rapidly cooled by natural cooling. Therefore, cooling means for the recording medium is not necessary, but the operation can be performed more reliably. In order to perform this, a cooler 15 may be provided as in the above-described embodiment, or another cooler may be provided downstream of the thermal head 16.

As shown in FIG. 6, the thermal head 16 includes three heating elements 16r, 16g and 16b on the recording medium 1.
Are arranged in parallel with each other in the direction of arrow C perpendicular to the traveling direction B. The heating element 16r is for writing red, the heating element 16g is for writing green, and the heating element 16b is for writing blue. Many pixel components are arranged along the traveling direction B.

The thermal head 16 is configured to reciprocate in a direction C perpendicular to the feed direction B of the recording medium 1 in synchronization with the advance of the recording medium 1. Each heating element 16
b, 16g, and 16r are turned on and off based on the image information for each color while moving in the C direction, and by repeating heating and non-heating, the image is printed on the recording medium 1 by the number of lines equal to the number of pixels. After writing, one full-color image is finally reproduced on the recording medium 1. The writing by the heating element is performed in descending order of temperature, that is, the heating element for blue 16b,
The heating is preferably performed in the order of the green heating element 16g and the red heating element 16r. Although it is possible to write three colors with one heating element, it is preferable to write the three colors separately because the temperature control becomes complicated.

(Laser printer, see FIG. 7) Information can be written on a recording medium using the laser printer shown in FIG. In this case, in order to convert the energy of the laser beam into heat, it is preferable to disperse an infrared absorbent in the intermediate layer 3 and / or the protective layer 5. The intermediate layer 3 and / or the protective layer 5 may be formed from an infrared absorbing material.

This laser printer has semiconductor lasers 31b, 31g, 31 for writing blue, green and red.
r is modulated by the drive circuit 33, and the laser beams emitted from each are modulated by the collimator lenses 32b, 32g,
The light enters the polygon mirror 34 via 32r. The polygon mirror 34 is driven to rotate in the direction of arrow c, and the laser beam is deflected based on this rotation, scans the recording medium 1 in a straight line, and is conveyed in the direction of arrow B. Is written. Although not shown, the laser printer is also provided with an optical element such as an fθ lens.

The colors to be written are the semiconductor lasers 31b, 31
g, 31r by controlling the radiant energy. Therefore, it is also possible to write by controlling the energy of the laser beam for each color by using one semiconductor laser. However, it is easier to control the energy if writing is performed for each color using three semiconductor lasers.

(Second Embodiment) This second embodiment is similar to FIG.
Has the same structure as the recording medium 1 shown in FIG. 2, and the same material as that of the first embodiment is used for the base layer 2 and the intermediate layer 3. The recording layer 4 is composed of 100 parts by weight of toluene, 8.5 parts by weight of the low-molecular-weight cholesteric liquid crystal compound represented by the chemical structural formula (A), and the following chemical structural formula (C).
1.5 parts by weight of a nematic liquid crystalline cyanobiphenyl compound shown by the formula (1) was mixed and dissolved, and this solution was applied on the intermediate layer 3 by a blade and dried by heating to obtain a reversible thermosensitive recording layer 4 having a thickness of 10 μm. The thickness of the recording layer 4 is preferably in the range of 3 to 50 μm, more preferably 6 μm.
３０30 μm. The protective layer 5 is the same as in the first embodiment.

[0041]

Embedded image

In the second embodiment, the temperature change range of the display color is 65 to 125 ° C., and the reflection wavelength is 670 to 46.
It changed in the range of 0 nm. When printing was performed using a hot stamp, the variable range of the reflection color was wide and good recording characteristics were exhibited. It goes without saying that images can be written and erased by the thermal printer and the laser printer.

Referring to FIG. 2, a reversible thermosensitive recording medium 1 includes a base layer 2 and a recording layer 4.
1, light absorbing layer 7 provided on the back surface of protective layer 5 and base layer 2
It is constituted by. The recording layer 41 is formed of a composite film of a low-molecular cholesteric liquid crystal compound and a nematic liquid crystal compound and a polymer resin, is separated into a liquid crystal composition 41a and a resin film 41b, and is further formed of a resin, an inorganic oxide, or the like. The spherical spacer 6 is mixed.

In the third embodiment, since a polymer composite film is used as the recording layer 41, the mechanical strength of the recording layer 41 is high, and damage due to external forces such as bending and friction can be minimized. . Further, even when the low-molecular-weight cholesteric liquid crystal compound is heated to a temperature at which the compound becomes an isotropic phase, the deterioration is small.

Next, specific examples of the materials and the production method will be described below. On a transparent PES (polyethersulfone) film, a silica spacer 6 having an average particle size of 15 μm was dispersed in ethanol and spray applied. Next, 5 wt% of a nematic liquid crystalline bicyclohexyl compound represented by the following chemical structural formula (D) is mixed with the low molecular weight cholesteric liquid crystalline compound represented by the chemical structural formula (A), and a photopolymerization initiator DAROCUR 1173 ( Ciba-Geigy)
Acrylate R having aromatic ring added with 3 wt% of
712 (manufactured by Nippon Kayaku Co., Ltd.) at a weight ratio of 17: 3 to prepare a liquid crystal composition. This liquid crystal composition is applied on the base layer 2 and a protective layer 5 having a thickness of 2
A μm transparent polyethersulfone film was overlaid.

[0046]

Embedded image

Next, while pressing the protective layer 5, 0.02
The composite film (recording layer 41) having a thickness of 15 μm was formed by irradiating ultraviolet rays of mW / cm ² for 1 hour. Also,
On the back surface of the base layer 2, a black paint is applied to form a light absorbing layer 7.
Was provided.

In the third embodiment, the temperature change range of the display color is 60 to 110 ° C., and the reflection wavelength is 660 to 450.
in the nm range. When printing was performed using a hot stamp, the variable range of the reflection color was wide and good recording characteristics were exhibited. It goes without saying that images can be written and erased by the thermal printer and the laser printer.

(Fourth Embodiment, See FIG. 3) A reversible thermosensitive recording medium 1 according to a fourth embodiment shown in FIG.
Of the intermediate layer 3, the recording layer 4 and the protective layer 5, respectively.
Is provided. With this recording medium 1, information can be displayed on both sides.

Specifically, a transparent PES having a thickness of 200 μm
Aluminum was formed on both surfaces of the (polyethersulfone) film to a thickness of about 600 angstroms to form an intermediate layer 3 that reflects light. Next, the average particle size is 8 μm.
And 9 parts by weight of the low-molecular-weight cholesteric liquid crystal compound represented by the chemical structural formula (A) and 1 part by weight of a nematic liquid crystalline phenyldioxane compound represented by the following chemical structural formula (E): 100 parts by weight of toluene were mixed and dissolved, and this solution was applied on the intermediate layer 3 by a blade, and dried by heating to obtain a reversible thermosensitive recording layer 4 having a thickness of 8 μm. Further, a polyester film having a thickness of 5 μm was superimposed on the recording layer 4 and was bonded by heating at 100 ° C.

[0051]

Embedded image

In the fourth embodiment, the temperature change range of the display color is 58 to 95 ° C., and the reflection wavelength is 580 to 430 n.
m. When printed with a hot stamp, the variable range of the reflection color was wide and a good recording medium was shown. It goes without saying that images can be written and erased by the thermal printer and the laser printer.

(Comparative Example) This comparative example has the same structure as the recording medium 1 shown in FIG. 1, and the base layer 2 and the intermediate layer 3 are made of the same material as in the first embodiment. . The recording layer 4, 10 parts by weight of the liquid crystal compound represented by the chemical structural formula (A) was mixed and dissolved in 100 parts by weight of toluene, and this solution was applied on the intermediate layer 3 with a blade.
It was dried by heating to form a reversible thermosensitive recording layer 4 having a thickness of 10 μm. The protective layer 5 is the same as in the first embodiment.

In this comparative example, the temperature change range of the display color is 75 to 110 ° C., and the reflection wavelength is 550 to 410 nm.
Range. After printing with hot stamp,
Since the variable range of the reflected color and the temperature change width are narrow, it is difficult to control the temperature and color unevenness occurs.

(Cooling Temperature Dependency of Reflection Color) FIG. 4 shows the results of measuring the cooling temperature dependence of the reflection color using the recording media of the first to fourth embodiments and the comparative example. For the measurement, each recording medium was placed on a hot stage, the temperature was raised to 130 ° C. to melt the liquid crystal compound, and then the temperature was lowered by 5 ° C. and held for 2 minutes, and light was irradiated from an angle of 20 °. Then, the reflection spectrum was measured by a spectrophotometer (instant multi-photometry system: MCPD-2000, manufactured by Otsuka Electronics Co., Ltd.) installed at a position of 90 degrees (directly above the recording medium).

In the graph of FIG. 4 showing the measurement results, the vertical axis represents the maximum peak wavelength in the reflection spectrum, and the horizontal axis represents the temperature at that time. As is clear from FIG. 4, the recording media of Embodiments 1 to 4 have a wider variable range of reflected color and a wider range of temperature change than those of the comparative example.

(Other Embodiments) The reversible thermosensitive recording medium according to the present invention is not limited to the above embodiment, but can be variously modified within the scope of the invention.
In particular, various low-molecular-weight cholesteric liquid crystal compounds and nematic liquid crystal compounds constituting the recording layer can be used in addition to those represented by the chemical structural formulas (A) and (B) to (E).

[Brief description of the drawings]

FIG. 1 is a sectional view showing a recording medium according to first and second embodiments of the present invention.

FIG. 2 is a sectional view showing a recording medium according to a third embodiment of the present invention.

FIG. 3 is a sectional view showing a recording medium according to a fourth embodiment of the present invention.

FIG. 4 is a graph showing the cooling temperature dependence of the reflection color of a recording medium.

FIG. 5 is a schematic configuration diagram showing a thermal printer.

FIG. 6 is a plan view showing a thermal head of the thermal printer.

FIG. 7 is a schematic perspective view showing a laser printer.

[Description of Signs] 1 ... recording medium 2 ... base layer 3 ... intermediate layer 4 ... recording layer 5 ... protective layer 41 ... recording layer (composite film)

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2H026 AA09 AA21 BB48 DD48 FF11 FF13 2H111 AA32 BA04 BA37

Claims (5)

[Claims] 1. A sheet-like base layer, which exhibits a cholesteric phase at a temperature higher than room temperature, reflects light in a visible wavelength range according to the temperature, and rapidly cools from that temperature to solidify in a reflection state. A reversible thermosensitive recording medium, comprising: a recording layer containing a molecular cholesteric liquid crystal compound and a nematic liquid crystal compound. 2. The reversible thermosensitive recording medium according to claim 1, wherein the recording layer is provided on a front surface and a back surface of the base layer. 3. The reversible thermosensitive recording medium according to claim 1, wherein the recording layer comprises a composite film of the liquid crystalline compound and a polymer resin. 4. The recording medium according to claim 1, wherein an intermediate layer having a smooth surface in contact with the recording layer is provided between the base layer and the recording layer. Reversible thermosensitive recording medium. 5. The reversible thermosensitive recording medium according to claim 1, wherein a protective layer is provided on the recording layer.