JP3398770B2 - Thermal display - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat-sensitive display, and more particularly to a rewritable heat-display capable of repeatedly writing and erasing information by heating, which is particularly advantageous for prepaid cards and credit cards.

[0002]

2. Description of the Related Art In recent years, a rewritable material has been required as a heat-sensitive recording material in order to save paper resources. One of the most influential materials is, for example, one in which a thermosensitive layer in which an organic low molecular weight substance such as higher alcohol or higher fatty acid is dispersed in a resin such as polyester is provided on a support.
It is known in the publications such as 4-119377 and JP-A-55-154198. Recording (image formation) and erasing with this type of reversible thermosensitive material utilize the change in transparency depending on the temperature of the thermosensitive layer. As with conventional irreversible thermosensitive recording materials, image formation with a thermal head, hot stamp, etc. In addition, it has the characteristic that it can be erased.

Display bodies using such a reversible thermosensitive material are roughly classified into (1) colored layer type (a reversible thermosensitive material layer and a colored layer, and an air layer provided between both layers). And (2) a reflective layer type (provided with a reversible thermosensitive material layer and a reflective layer). In the above (1), a black or dark color material is used as the coloring layer, and an air layer is formed between the coloring layer and the reversible thermosensitive material layer, so that an image of white and black or dark color with high contrast is obtained. Is trying to get. On the other hand, in the above (2), by providing a metal layer such as aluminum as a reflective layer, it is possible to obtain a higher contrast in a white and silver image than in the colored layer type.

However, when comparing the display bodies of (1) and (2), the colored layer type does not have the reflection of light as seen in the reflective layer type, and the familiar white and black images can be produced. However, it also has some drawbacks as follows. Compared to the reflective layer type display, the contrast is lower (white is thin), the mechanical strength is weak because it has an air layer, especially for large area display because it is vulnerable to shear stress that sandwiches the air layer. In order to obtain an unsuitable and practical contrast, it is necessary to increase the thickness of the reversible thermosensitive material layer to the maximum, but on the other hand, the thermal head is used for erasing to simplify the recording device. To make it possible, it is necessary to use a thin reversible heat-sensitive material layer. Therefore, the colored layer type is not suitable for erasing with a thermal head, and can be used only for erasing with a hot plate or the like. On the other hand, there has been an idea of providing a thin film layer of a high-refractive index substance by paying attention to the difference (about 0.5) in the refractive index between the reversible thermosensitive recording material and air (Japanese Patent Laid-Open No. HEI-2). -175280
Issue). However, with this method, the effect of improving the contrast is small, and it is still insufficient.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a thermosensitive display having a contrast improved by using a reversible thermosensitive material whose transparency reversibly changes by heating.

[0006]

According to the present invention, a layer of a reversible thermosensitive material whose transparency reversibly changes with temperature, one or more layers made of a transparent material, and a colored layer are provided in this order. In the thermosensitive display, the transparent material layer has a refractive index different from that of the adjacent layer across at least one interface between the reversible thermosensitive material and the colored layer. At least a part of the first interface between any of the layers is inclined with respect to the surface of the reversible thermosensitive material, and the reversible thermosensitive material is present at the second interface on the colored layer side of the first interface. It is characterized in that the angle of light incident from the material layer side is changed by the inclined portion of the first interface so that the light is incident on the angle range of total reflection at the second interface. In the thermosensitive display of the present invention, the first interface has at least a pair of inclined surfaces having different orientations, and the extension of each inclined surface of the pair is on the layer side (that is, on the surface side) of the reversible thermosensitive material. ) It is advantageous that they are designed to meet each other.

The present inventors have studied from various angles in order to solve the above-mentioned problems, and as a result, the reason why the low refractive index layer represented by the air layer contributes to the improvement of contrast is that the high refractive index is high. It was found that the total reflection that occurs when light enters the low refractive index layer from the layer is the main cause of improving the contrast, and the wider the angle range in which the total reflection occurs, the higher the contrast. Then, instead of widening this total reflection angle range, one of the interfaces between the layer of the reversible thermosensitive material and the colored layer is tilted with respect to the surface of the layer of the reversible thermosensitive material, By changing the angle of the light coming from the side of the layer, it was found that it is possible to make the light easier to enter into the angle range of total reflection at another interface. Furthermore, by constructing this inclined surface not by one but by a pair of inclined surfaces prepared so as to face each other and intersect on the side of the reversible thermosensitive material, the totally reflected light can be efficiently converted into the original reversible thermosensitive material. I found it possible to return to the layer. The present invention is based on these.

Hereinafter, the present invention will be described in more detail with reference to the accompanying drawings, but prior to this, the description will be started from the contrast improvement (especially the improvement due to the difference in refractive index) which has been considered so far. To

Means for improving the contrast of a display using a reversible thermosensitive material whose transparency reversibly changes upon heating are, as described in the prior art, two types (shown in FIG. 1A). Colored layer type, FIG. 1 (b)
The reflective layer type shown in 1) is roughly divided into two types, but these have in common that the contrast is improved by reflecting the light scattered by the reversible thermosensitive material layer 1. However, while the reflective layer type simply reflects both scattered light and unscattered light equally,
The colored layer type is different in that only scattered light can be selectively reflected. In the figure, 2 is an air layer,
Reference numeral 6 represents a colored layer, and 7 represents a reflective layer.

In the colored layer type, reflection occurs at the interface between the reversible thermosensitive material layer and the low refractive index layer. Figure 2 (a)
In the transparent reversible thermosensitive material layer 1a, the incident light causes only a small amount of partial reflection at the interface as shown in Fig. 1, but as shown in Fig. 1 (b), the reversible thermosensitive material in the clouded state is shown. In the layer 1b, incident light is scattered inside the reversible thermosensitive material 1b, and light incident on the interface in a direction oblique to a certain angle is totally reflected. This total reflection emphasizes the whiteness of the cloudy state. In FIG. 2, 2'and 2 "are air layers (refractive index n = 1), and the paths of light are shown by the solid line and the path of transmitted light or totally reflected light, and by the broken line. This is the path of partially reflected light.

As shown in FIG. 3, the above-mentioned total reflection occurs when light enters from the high refractive index substance 3 side at the interface between the high refractive index substance 3 and the low refractive index substance 4. And the critical angle θc at this time is the refractive index (n ₁ ,
The ratio of n ₂ ) is determined by the following equation. θc = 1 / sin (n ₂ / n ₁ ) Here, for example, the refractive index of the reversible thermosensitive material (high refractive index substance) is set to 1.5, and an air layer is applied to the low refractive index substance to make the refractive index 1 Since the critical angle θc is about 42 °, total reflection occurs in a wide range of 42 ° to 90 ° (FIG. 4). When a low-refractive index material other than air is used, the minimum refractive index is about 1.3, so the critical angle is 60 °, and total reflection occurs within the range of 60 ° to 90 °, which is not as great as the air layer. However, the contrast is improved.

As another factor for improving the contrast, the change in reflectance depending on the incident angle in the partial reflection angle region also contributes to the improvement in contrast (FIG. 4). The contribution of this contrast improvement is greater when the light enters the low refractive index material from the high refractive index material than when the light is incident. This is due to the difference between having and not having the angular region of total internal reflection. This difference in partial reflection due to the incident angle is almost unknown even if the difference in refractive index is simply raised, and the reflectance increases at all angles, so it is not as effective as the contrast improvement by total reflection.

In the present invention, the efficiency of the reflection method utilizing the difference in refractive index as described above is further improved.
Tilting the interface between the low-refractive index layer and the high-refractive index layer with respect to the surface of the reversible thermosensitive material layer changes the angle of the confusion light coming from the reversible thermosensitive material layer, and total reflection at another interface. It is possible to improve the contrast by making it easier for light to enter the angle range.

The structure of the present invention and the light path at that time are shown in FIG. 5A and 5B are both shown in FIG.
The contrast can be improved as compared with the conventional method shown in. However, in FIG. 5A, the light that is totally reflected at the bottom surface (second interface S ₂ ) once hits the lateral wall or hits the first interface S ₁ again. At this time, total reflection may occur again depending on the refractive index of the material on the first interface, and as it is, it may not be possible to return to the layer of the reversible thermosensitive material. In the figure, 5 is a scattering source. Therefore, as described above, it is possible to effectively return the light to the layer of the reversible thermosensitive material by attaching the opposite inclined surfaces by removing the wall (FIG. 5 (b)). That is, by using total reflection using a so-called triangular prism, it is possible to selectively reflect only the light that is incident due to confusion.

By forming such a prism into a shape arranged along the surface of the reversible thermosensitive material layer as shown in FIG. 6A, the layers of the low refractive index substance and the high refractive index substance are simply superposed. It is possible to reflect light much more efficiently than when combined. Further, since the inclined surface does not have to be a flat surface in particular, for example, a shape in which convex portions having a semi-circular shape as shown in FIG. Materials of these shapes are marketed under the names of prism sheet and lenticular lens sheet, respectively, so that a simple experiment for improving the contrast can be performed based on these materials. As another shape, a corrugated cross section (FIG. 6C) is also effective. As a condition of these shapes, about 50% or more of the entire area of the inclined surface is inclined at an angle of 20 ° to 80 ° with respect to the surface of the layer of the reversible thermosensitive material, and more preferably 30 ° to 70 °. Is desirable. In the present invention, the surface of the reversible heat-sensitive material is a surface of a layer made of a material which is minimum necessary for reversibly changing the transparency by heat, excluding a support, a protective layer and the like. Refers to the light source and the side of the observer when looking at.

The "reversible heat-sensitive material" in the present invention means
It is a material in which the confusion of light changes reversibly with temperature.
In particular, a reversible heat-sensitive material capable of stably maintaining a transparent state and a cloudy state at room temperature is preferable. As an example of such a material, "a material composed of a resin base material and an organic low molecular weight substance dispersed in the resin base material and reversibly changing the cloudiness and the transparent state by heating" described in the related art. (JP-A-4-
239690), "a material consisting of a binary composition showing a phase diagram of the lower critical eutectic temperature type, which reversibly changes between a transparent state and a cloudy state by controlling a cooling time after heating" ( JP-A-61-258853) and the document "Polymer liquid crystal having thermotropic liquid crystallinity". These reversible heat-sensitive materials are provided with a protective layer, a support and the like depending on the purpose of use. When a transparent material is used as the support, it can be used as one of the layers of the transparent material of the present invention. It is also effective to tilt the back surface of the layer of reversible thermosensitive material with respect to the surface.

The transparent material in the present invention may have the minimum transparency regardless of the presence or absence of color. Specifically, gases (air and other non-corrosive gases), liquids such as water and alcohol, and solid resins such as fluororesins such as polytetrafluoroethylene and polyvinylidene fluoride, polyester resins and vinyl chloride. Resin,
In addition to polyolefin resin, acrylic resin, epoxy resin, urethane resin, ice and lithium fluoride as inorganic substances,
Of PbO, Sb ₂ S ₃ , Fe ₂ O ₃ , CdS, ZnS, diamond, etc., highly transparent ones can be appropriately used. Among these materials, the materials are used in combination so as to sandwich the interface and have different refractive indexes.

A protective layer may be provided on the layer of the reversible heat-sensitive material to protect the layer. Protective layer (thickness 0.1 to 5 μm) laminated on the layer of reversible thermosensitive material
Examples of the material include silicone rubber, silicone resin (Japanese Patent Laid-Open No. 63-221087), polysiloxane graft polymer (described in Japanese Patent Application No. 62-152550), ultraviolet curable resin or electron beam curable resin (special Japanese Patent Application No. 63-310600) and the like.
In any case, a solvent is used at the time of coating, but it is desirable that the solvent does not dissolve the layer of the reversible thermosensitive material.
As the solvent in which the resin of the reversible thermosensitive material layer and the organic low molecular weight substance are difficult to dissolve, n-hexane, methyl alcohol,
Examples thereof include ethyl alcohol and isopropyl alcohol, and alcohol-based solvents are particularly preferable in terms of cost.

[0019]

EXAMPLES The present invention will be described in more detail by way of examples. All parts and percentages here are by weight.

Example 1 (Preparation of reversible thermosensitive film) Behenic acid 7 parts Eicosane diacid 3 parts Diisodecyl phthalate 2 parts Vinyl chloride-vinyl acetate copolymer resin (manufactured by UCC) on a polyester film having a thickness of about 50 μm. VYHH) 40 parts THF 150 parts Toluene 15 parts A liquid having a composition was applied with a wire bar and dried by heating to provide a reversible thermosensitive material layer having a thickness of about 15 μm. A 75% butyl acetate solution of urethane acrylate-based UV-curable resin (Unidick C7-157, manufactured by Dainippon Ink and Chemicals, Inc.) 10 parts Toluene 10 parts was applied with a wire bar, and after heating and drying, 80
A protective layer is formed by curing with a W / cm UV lamp, and becomes a cloudy state at a second specific temperature of about 100 ° C. or higher,
A reversible thermosensitive material (film) having a characteristic that it becomes transparent at a temperature of about 70 ° C. to 100 ° C.
It was created.

Subsequently, a reversible heat-sensitive material (film) in which a device having the structure shown in FIG.
The letter 1 was visually confirmed. When each lenticular lens sheet or prism sheet (Diaart, manufactured by Mitsubishi Rayon Co., Ltd.) 8 is set so that the uneven surface faces the reversible thermosensitive film, the whiteness is improved compared to when neither is set. It could be confirmed. Further, when a reversible thermosensitive sheet in which blocks of 8 mm square were printed instead of letters was set in the apparatus and the reflection density was measured using Macbeth RD914, the results are as shown in Table 1, and good results were obtained visually. Was obtained.

[0022]

[Table 1]

Example 2 The same reversible thermosensitive sheet as in Example 1 was visually observed while the apparatus of FIG. 7 was immersed in water. As a result, the whiteness of the reversible thermosensitive sheet was improved as compared with the case where neither the lenticular lens sheet nor the prism sheet was set.

[0024]

According to the present invention, it is possible to provide a thermosensitive display having high contrast.

[Brief description of drawings]

FIG. 1A is a schematic view of a colored layer type display body, and FIG. 1B is a schematic view of a reflective layer type display body.

2 (a) and 2 (b) are diagrams for explaining a light path when the reversible thermosensitive material layer is in a transparent state and a cloudy state.

FIG. 3 is a diagram for explaining light reflection at an interface between a high refractive index substance and a low refractive index substance.

FIG. 4 is a graph showing a relationship between a critical angle and reflection.

FIG. 5 is an explanatory diagram for explaining a relationship between an inclination of an interface and a light path.

6 (a), (b) and (c) are diagrams of an example of a lens or the like which is effective when the first interface has periodicity.

FIG. 7 is a diagram showing an example of a thermal display according to the present invention.

[Explanation of symbols]

1, 1a, 1b Reversible thermosensitive material layer 2, 2 ', 2 "Air layer 3 High refractive index substance 4, 4a, 4b Low refractive index substance 5 Scattering source 6 Coloring layer 7 Reflective layer S ₁ First interface S ₂ Second interface

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-5-201146 (JP, A) JP-A-5-254246 (JP, A) JP-A-3-7377 (JP, A) JP-A-6- 48032 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) B41M 5/36

Claims (6)

(57) [Claims] 1. A layer of a reversible thermosensitive material whose transparency reversibly changes with temperature, one or more layers of a transparent material, and a coloring layer are provided in this order, and the layer of the transparent material is at least an interlayer. A thermosensitive display having a refractive index different from that of the adjacent layer across one of the interfaces of the reversible thermosensitive material and the colored layer. At least a part of the first interface is inclined with respect to the surface of the layer of the reversible thermosensitive material, and the layer of the reversible thermosensitive material is provided at the second interface which is closer to the colored layer than the first interface. A thermosensitive display, characterized in that the angle of light incident from the side of is changed by the inclined portion of the first interface so that the light is incident on the angle range of total reflection at the second interface. 2. The thermosensitive display according to claim 1, wherein the first interface has at least one pair of different inclined surfaces of inorganic material, and the extension of each inclined surface of the pair intersects on the side of the layer of the reversible thermosensitive material. . 3. In two kinds of materials that are adjacent to each other across the first interface, the transparent material on the surface side or the reversible thermosensitive material has a lower refractive index than the transparent material on the other side, and the second material In two kinds of transparent materials that are adjacent to each other across the interface of
The thermosensitive display according to claim 1 or 2, wherein the transparent material on the surface side has a higher refractive index than the transparent material or the colored layer on the other side. 4. The first interface according to claim 1, wherein at least 50% or more of the total area of the first interface has an inclination of 20 ° to 80 ° with respect to the surface of the layer of the reversible thermosensitive material. The thermosensitive display according to item 1. 5. The heat-sensitive display body according to claim 1, wherein the first interface has a shape having a periodicity. 6. A reversible heat-sensitive material layer comprising a resin base material and an organic low molecular weight substance dispersed in the resin base material as main components, and the state of cloudiness and transparency is reversibly changed by heating. The thermosensitive display according to any one of claims 1 to 5.