JPH08127184A - Rewritable heat-sensitive recording medium - Google Patents

Abstract

PURPOSE: To provide a heat-sensitive recording medium which has thermal head resistance, mechanical resistance and favorable printing sensitivity. CONSTITUTION: In a rewritable heat-sensitive recording medium 1, a recording layer 3 which consists mainly of resin and an organic low molecular substance scattered within the resin and changes reversibly transparency by depending on a temperature and an overcoat layer 4 are provided on a substrate 2, and an overcoat layer is constituted of photosetting epoxy resin of cationic polymerization and its film thickness is set to 0.5-2.0μm. Then since the photosetting epoxy resin of the cationic polymerization is applied to an overcoat layer and the overcoat layer is free from an effect by oxygen inhibition at the time of photosetting, even if the film thickness is set up at 2.0μm or thinner, defective hardening is not generated. Therefore, thermal head resistance or mechanical durability are possessed sufficiently, heat from the thermal head is apt to be transferred and an improvement in printing sensitivity is contrived also.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rewritable thermosensitive recording medium having a recording layer whose transparency reversibly changes depending on temperature and an overcoat layer for protecting the recording layer, and more particularly, The present invention relates to an improvement of a thermal recording medium which has thermal head resistance and mechanical resistance and has good printing sensitivity.

[0002]

2. Description of the Related Art Thermosensitive recording media of this type include polyvinyl chloride and the like, as described in JP-A-55-154198, JP-A-5-85045 and US Pat. No. 4,268,413. Thermosetting resin or a thermosetting resin obtained by curing a hydroxyl-modified vinyl chloride-vinyl acetate copolymer with an isocyanate compound, and a recording layer containing an organic low-molecular substance such as a higher fatty acid dispersed in the resin as a main component There is known a structure in which is provided on a support.

The heat-sensitive recording medium is constructed by utilizing the property that the light absorption characteristic (light scattering property) of the recording layer reversibly changes depending on temperature.
That is, the recording layer is specific temperature T ₀ 2 two states transition temperature higher temperature than the T ₁ of the room temperature near, T ₂ (where T ₁ <
T ₂ ), and when the recording layer is heated to T ₂ or higher and held and then cooled to T ₀ or lower, it becomes cloudy and its light scattering property increases to reach the maximum light blocking state, while it is in this cloudy state. After heating the recording layer to T ₁ or more and less than T ₂ and holding it, T
_When it is cooled below ₀ , its light scattering property decreases and it becomes transparent. Moreover, the maximum light-shielding state and the transparent state of the recording layer can be stably maintained under a temperature condition of T ₀ or less, and these states can be reversibly changed. When set to, the recording medium can be used as a rewritable recording medium that can be differentiated from the other state.

Since the recording layer can be made transparent in the temperature range from T ₁ to T ₂ , this temperature range is referred to as a transparentization temperature range.

By the way, since a heating means such as a thermal head is applied as a printing means for the heat-sensitive recording medium, every time when the printing operation is repeated, a mark by the printing means (formed when the thermal head or the like is contacted is formed on the surface of the recording layer. Microscopic unevenness) is likely to remain, and the image quality deteriorates as a recorded image is repeatedly formed, and the surface of the recording layer is partially shaved to form dust, which can be applied to printing means such as a thermal head. There was a defect that it was easy to adhere.

Therefore, in JP-A-63-221087, an overcoat layer made of a heat-resistant material such as a fluorine-based resin or a silicone-based resin is provided on the recording layer to reduce the friction on the surface, thereby improving the printability. There has been proposed a method of improving and preventing deterioration of image quality due to repeated rewriting and adhesion of dust to printing means such as a thermal head. However, even if this method is applied, the adhesive force between the recording layer and the overcoat layer decreases when the printing operation is repeated, and the overcoat layer peels off with time to deteriorate the image quality. JP-A-133781 proposes a means for improving this problem by providing an intermediate layer made of a thermoplastic resin or a thermosetting resin between the recording layer and the overcoat layer.

[0007]

By applying this means, the rewriting resistance and the deterioration of the image quality are certainly reduced (that is, the thermal head resistance and the mechanical resistance are improved), but on the other hand, the recording layer Since there is an intermediate layer and an overcoat layer on top, the heat from the heating means such as the thermal head is hard to be transferred to the recording layer, which leads to a decrease in printing sensitivity and a low printing density in a thermal printer with low printing energy. There was a problem that became.

Therefore, the present inventors have found a radical polymerization type photo-curable urethane acrylate resin as a material which is unlikely to cause a decrease in the adhesive force between the recording layer and the overcoat layer without interposing the intermediate layer. Tried. However, this photocurable urethane acrylate resin has a drawback that its curing is insufficient unless the dry film thickness is set to 2 μm or more due to the influence of oxygen inhibition. Has poor thermal head resistance and mechanical resistance, and cracks or scratches are likely to occur on the surface of the overcoat layer. Also, when the film thickness is 2 μm or more, heat from the above heating means is difficult to transfer and printing sensitivity decreases. Turned out to have a problem.

On the other hand, a nitrocellulose resin was found as a heat resistant resin whose film thickness can be set thin, and an attempt was made to apply the same, but the above problems could not be solved because of its poor mechanical resistance.

The present invention has been made in view of these problems, and its object is to provide a thermal recording medium having thermal head resistance and mechanical resistance and having good printing sensitivity. Especially.

[0011]

That is, the invention according to claim 1 is a recording layer containing a resin and an organic low-molecular substance dispersed in the resin as a main component, the transparency of which changes reversibly depending on temperature. And on the premise of a rewritable heat-sensitive recording medium provided on a support with an overcoat layer provided on this recording layer, the overcoat layer is composed of a cationic polymerization type photocurable epoxy resin, and, The film thickness is set to 0.5 μm to 2.0 μm.

In the heat-sensitive recording medium according to the present invention, the cationic polymerization type photo-curable epoxy resin is used as the constituent material of the overcoat layer, so that oxygen during photo-curing is applied. Since there is no influence of inhibition, curing failure does not occur even if the film thickness is set to 2.0 μm or less. Therefore, it has sufficient thermal head resistance and mechanical resistance as described above, and since the heat from the heating means such as the thermal head is easily transferred to the recording layer, the printing sensitivity can be improved.

In such technical means, conventional supports can be applied as they are as the support for supporting the recording layer and the like, and examples thereof include plastic films, sheets and papers. Etc. may be colored with a dye or a pigment. Specific examples of the plastic film, sheet and the like include a material group having excellent heat resistance and mechanical strength such as polyethylene terephthalate, polyethylene naphthalate and polycarbonate. Further, the above plastic film, sheet, paper, etc. may be provided with a thin film of gold, silver, copper, tin, nickel, chromium or the like to impart light reflectivity. As a film forming means of the metal material which becomes the light reflecting layer,
Any method such as a vacuum vapor deposition method, an ion plating method, a sputtering method and a CVD method can be applied. In this case, in order to improve the adhesive strength between the light reflecting layer and the recording layer, it is preferable to provide an adhesive layer between them. As this adhesive layer,
For example, the polyparabanic acid resin described in JP-A-5-85044 may be mentioned. An anchor layer may be provided on the support for the purpose of improving the adhesive strength between the support and the light reflection layer. Conventionally known materials can be used for this anchor layer, and examples thereof include polyester resins, acrylic polyol resins, and thermosetting resins obtained by reacting these resins with isocyanate.

Next, as a resin material constituting a part of the recording layer, the refractive index thereof is similar to that of the organic low molecular weight substances described below and they are not compatible with each other, excellent in mechanical strength and excellent in film forming ability. And a thermoplastic resin or a thermosetting resin, and specifically, polyester, polyvinyl chloride, vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinyl acetate-vinyl alcohol copolymer, vinyl chloride-vinyl acetate. -Maleic acid copolymer, polyvinyl chloride-based resin such as vinyl chloride-acrylate copolymer or polyvinylidene chloride, vinylidene chloride-vinyl chloride copolymer, polyvinylidene chloride-based resin such as vinylidene chloride-acrylonitrile copolymer, Polyamide resin, silicon resin, polyacrylate or polymethacrylate resin and the like, May be applied are such resins more mixed and, also may be applied to this or the like resin alone or more in thermosetting resins which have been reacted with isocyanates.

The organic low molecular weight substance dispersed in the resin material contains at least one atom of oxygen, sulfur, nitrogen or halogen and has 10 to 4 carbon atoms.
An organic compound having a molecular weight of 0 and a molecular weight of 100 to 700 and a melting point of 50 to 300 ° C. is exemplified, and specific examples thereof include higher alcohols such as alkanol, carcandiol, halogenalkanol, and halogenalkanediol; Higher aliphatic amines; alkanes, alkenes, alkynes and halogen-substituted compounds thereof; cyclic compounds such as cycloalkanes, cycloalkenes, cycloalkynes; saturated carboxylic acids, unsaturated carboxylic acids, dicarboxylic acids or their esters, amides, Ammonium salt; saturated or unsaturated halogen fatty acid or ester thereof,
Amides, ammonium salts; acrylic carboxylic acids or their esters, amides, ammonium salts; halogenallyl carboxylic acids or their esters, amides, ammonium salts; thioalcohols or their carboxylic acid esters; thiocarboxylic acids or these There are esters, amides, ammonium salts and the like, and these can be used alone or in admixture of two or more.

Then, an organic solution in which these resin materials and organic low-molecular substances are dissolved, such as methyl ethyl ketone, methyl isobutyl ketone, chloroform, ethanol, benzene, toluene, tetrahydrofuran, carbon tetrachloride, or a solution in which the above resin materials are dissolved. A dispersion liquid in which an organic low-molecular substance is finely dispersed is coated on the support or a support provided with a light reflection layer and an adhesive layer and dried to reversibly change its transparency due to temperature dependence. The recording layer is formed to change. Incidentally, a dye, an ultraviolet absorber, an infrared absorber or the like may be added to the recording layer for the purpose of improving the optical properties of the recording layer.
A plasticizer may be added for the purpose of improving the plasticity of the recording layer.

Next, the cationic polymerization type photocurable epoxy resin constituting the above overcoat layer will be explained. This resin has an epoxy group and the compounds exemplified below or a polymerization catalyst such as dioxane sulfonium salt is present. It has the property of being cationically polymerized and cured by UV irradiation under the conditions.

[0018]

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[0019]

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[0020]

Embedded image Then, the resin material having an epoxy group, the polymerization catalyst is added to a solution prepared by dissolving in ethanol, methanol, isopropyl alcohol or other solvent, the recording layer by microgravure coating or gravure coating method or wire bar coating method. It is coated on the surface, irradiated with ultraviolet rays from a metal halide or mercury lamp, and then dried to form an overcoat layer. The solvent is preferably one that does not attack the recording layer.

Here, it is necessary to set the film thickness of the overcoat layer to 2.0 μm or less as described above from the viewpoint of preventing the deterioration of the printing sensitivity. If the thickness is less than 0.5 μm, the mechanical strength will be insufficient. Therefore, the thickness of the overcoat layer is 0.5
It is necessary to set the thickness to μm to 2.0 μm. Further, since the above-mentioned photocurable epoxy resin of the cationic polymerization type has a high transparency, the visibility of the recording layer may be deteriorated by the presence of the overcoat layer even if the recording layer shows a cloudy state or a transparent state due to temperature dependence. Absent.

In order to prevent sticking by printing means such as a thermal head and prevent deterioration during printing, a lubricant such as silicone oil, paraffin wax, polyethylene wax, polytetrafluoro wax, or carbonic acid is used in the overcoat layer. A lubricant having a particle size of 0.1 μm to 0.2 μm such as calcium, metal stearate, stearic acid amide, polytetrafluoroethylene, or silica may be blended. The mixing ratio is 1% by weight to 10% by weight based on the epoxy resin constituting the overcoat layer. The inventions according to claims 2 and 3 are made for such technical reasons.

That is, the invention according to claim 2 is premised on the rewritable heat-sensitive recording medium according to claim 1, and is selected from silicone oil, paraffin wax, polyethylene wax, and polytetrafluoro wax. 1% by weight to 10% by weight of lubricant in the overcoat layer
Characterized by being blended in a weight percentage,
The invention according to claim 3 has a particle size of 0.1 μm selected from calcium carbonate, stearic acid metal salt, stearic acid amide, polytetrafluoroethylene, and silica.
0.2 μm of lubricant is 1% by weight in the overcoat layer.
It is characterized in that it is blended at a ratio of 10 to 10% by weight.

[0024]

According to the rewritable heat-sensitive recording medium according to the invention described in claims 1 to 3, since a cationic polymerization type photocurable epoxy resin is applied as a constituent material of the overcoat layer, Since there is no influence of oxygen inhibition in (2), even if the film thickness is set to 2.0 μm or less, curing failure does not occur. Therefore, it has sufficient thermal head resistance and mechanical resistance, and the heat from the heating means such as the thermal head is easily transferred to the recording layer, so that the printing sensitivity can be improved.

Further, since the above-mentioned photocurable epoxy resin of the cationic polymerization type has a high transparency, even if the recording layer shows a cloudy state or a transparent state due to the temperature dependence, the visibility of the recording layer becomes visible due to the presence of the overcoat layer. It does not decrease. Therefore, it is possible to improve the contrast between the cloudy portion and the transparent portion in the recording layer.

[0026]

EXAMPLES Examples of the present invention will be described in detail below.

In addition, all the parts of the compounding components described in Examples and Comparative Examples are parts by weight.

[Embodiment 1] A thermal recording medium 1 according to this embodiment, as shown in FIG. 1, is a support 2 made of transparent polyethylene terephthalate having a thickness of 100 μm, and a recording provided on the support 2. The layer 3 and the overcoat layer 4 directly formed on the recording layer 3 constitute the main part.

The thermosensitive recording medium is manufactured through the following steps.

That is, the following coating solution for recording layer was uniformly applied on a support 2 made of transparent polyethylene terephthalate having a thickness of 100 μm by a wire bar and dried at 100 ° C. for 3 minutes to give a film thickness of 20 μm. The recording layer 3 was formed.

(Coating liquid for recording layer) Behenic acid 2 parts Sebacic acid 1 part Hydroxyl vinyl chloride-vinyl acetate-vinyl alcohol copolymer [Sekisui Chemical Co., Ltd. trade name Eslec A] 12 parts Tetrahydrofuran 80 parts Toluene 20 Part Isocyanate [Duranate 24A, trade name, manufactured by Asahi Kasei Co., Ltd.] 1 part Next, a coating liquid for an overcoat layer having the following composition was applied onto the recording layer 3 using a wire bar, and a metal halide lamp (120W × 4) was used. UV irradiation for 4 seconds under a lamp), followed by drying at 80 ° C. for 10 minutes to form an overcoat layer 4 having a film thickness of 1.5 μm to obtain the thermosensitive recording medium 1 shown in FIG.

(Coating liquid for overcoat layer) Cationic polymerization type photocurable epoxy resin containing polymerization catalyst [Asahi Denka Co., Ltd. trade name ADEKA OPTOMER 550] 10 parts Ethanol 10 parts [Example 2] Overcoat It is substantially the same as the thermosensitive recording medium according to Example 1 except that the following composition was applied as the coating liquid for the coat layer.

(Coating liquid for overcoat layer) Cationic polymerization type photocurable epoxy resin containing a polymerization catalyst [Asahi Denka Co., Ltd. trade name ADEKA OPTOMER 550] 10 parts ethanol 10 parts silicone oil 0.5 parts [Comparative Example 1] The following composition was applied as a coating liquid for the overcoat layer, which was dried after being irradiated with ultraviolet rays to give a dry film thickness of 2.5.
The thermal recording medium is substantially the same as that of Example 1 except that an overcoat layer having a thickness of μm is formed.

(Coating liquid for overcoat layer) Radical polymerization type photo-curable urethane acrylate resin [trade name U-6HA manufactured by Shin-Nakamura Chemical Co., Ltd.] 10 parts polymerization initiator [α-hydroxyisobutyl manufactured by Merck Co., Ltd.] Phenon brand name Darocur 1173] 0.5 part Ethanol 10 parts [Comparative Example 2] The following composition was applied as a coating liquid for the overcoat layer and dried at 80 ° C for 5 minutes to give a dry film thickness of 0.5.
The thermal recording medium is substantially the same as that of Example 1 except that an overcoat layer having a thickness of μm is formed.

(Coating liquid for overcoat layer) Nitrocellulose resin [trade name GB22 manufactured by Toyo Ink Mfg. Co., Ltd.] 10 parts Methyl isobutyl ketone 10 parts "Evaluation test" Next, the thermal recording media according to Examples 1 and 2 And the thermal recording media according to Comparative Examples 1 and 2 were evaluated as follows. The evaluation items are resistance to 10-time repeated printing by a thermal head, saturated printing energy indicating printing sensitivity, and OD values of a printed portion OD and an unprinted portion in a thermal recording medium.

The evaluation of the resistance against 10-time repeated printing by the thermal head is visually judged by the presence or absence of scratches, and the OD value is the reflection OD value when the thermal recording medium is placed on the aluminum plate. The print area OD represents the print density at saturated print energy. This measurement was performed with a Macbeth optical densitometer 918.

[0037]

[Table 1] From Table 1, the resistance against repeated printing 10 times by the thermal head required for the thermal recording medium is such that the thickness of the overcoat layer is set to 1.5 μm or more.
2 and Comparative Example 1 are satisfied, but the thickness of the overcoat layer is set as thin as 0.5 μm.
About scratches occurred. An experiment was also conducted in which the film thickness of the overcoat layer was set to 1.5 μm using the coating liquid for the overcoat layer applied in Comparative Example 2, which was the same as in the example. It has been confirmed that there are scratches.

Regarding the saturated printing energy representing the printing sensitivity, the saturated printing energy of the thermal recording medium according to Comparative Example 1 was 0.35 mJ, and the saturated printing energy of the thermal recording media according to Examples 1 and 2 was 0.30 mJ. It was confirmed that the printing sensitivity was lower than that of the conventional one.

It should be noted that the transparency of the unprinted portion of each thermal recording medium did not differ as a whole, and good transparency was obtained. Also,
There was no difference in the density of the printed part, and good contrast was obtained as a whole.

From the above evaluation test, the thermal recording media according to Examples 1 and 2 have better printing sensitivity than the thermal recording media according to Comparative Example 1, and the thermal recording media according to Comparative Example 2 have better printing sensitivity. The resistance to the thermal head was good. That is, in each of the examples, it was confirmed that resistance to the thermal head, good printing sensitivity and transparency were simultaneously provided.

[0041]

According to the rewritable heat-sensitive recording medium according to the present invention, it is possible to improve the print sensitivity while having sufficient thermal head resistance and mechanical resistance.
Further, it is possible to improve the contrast between the cloudy part and the transparent part in the recording layer.

Therefore, it is possible to provide a thermosensitive recording medium having a good rewriting resistance and capable of obtaining a recorded image having a good print density and a high contrast even when a thermal printer having a low print energy is used. Have

[Brief description of drawings]

FIG. 1 is a structural cross-sectional view of a thermosensitive recording medium according to an example.

[Explanation of symbols]

 1 Thermal recording medium 2 Support 3 Recording layer 4 Overcoat layer

Claims (3)

[Claims] 1. A recording layer comprising a resin and an organic low-molecular substance dispersed in the resin as a main component, the transparency of which reversibly changes depending on temperature, and an overcoat layer provided on the recording layer. In a rewritable heat-sensitive recording medium having a support and a support, the overcoat layer is composed of a cationic polymerization type photocurable epoxy resin, and the film thickness is 0.5 μm.
A rewritable thermosensitive recording medium, characterized in that the rewritable thermal recording medium is set to ˜2.0 μm. 2. A lubricant selected from silicone oil, paraffin wax, polyethylene wax, and polytetrafluoro wax is blended in the overcoat layer in a proportion of 1% by weight to 10% by weight. The rewritable thermosensitive recording medium according to claim 1. 3. A particle diameter of 0.1 μm to 0.2 μm selected from calcium carbonate, stearic acid metal salt, stearic acid amide, polytetrafluoroethylene, and silica.
2. The rewritable heat-sensitive recording medium according to claim 1, wherein said lubricant is blended in said overcoat layer in a proportion of 1% by weight to 10% by weight.