JP3556239B2 - Reversible thermosensitive recording medium and recording method - Google Patents

Description

[0001]
[Industrial applications]
The present invention relates to a reversible thermosensitive recording medium using a polymer liquid crystal for a recording layer and a recording method.
[0002]
[Prior art]
In recent years, there has been a demand for a visual display on the main body of a magnetic card every time information such as a balance is used. For example, in a method using a leuco dye, a dye precursor and a developer are dispersed in a binder and applied to the surface of a substrate, and the dye and the developer react by heating with a thermal head or the like. , Develops color.
[0003]
In addition to the heat-sensitive recording medium using a dye as described above, a heat-sensitive recording medium of a low melting point metal thin film destruction type as disclosed in JP-A-59-199284 has recently been put to practical use.
[0004]
In the recording medium as described above, the once printed portion cannot be printed again, and a printing method that can be rewritten as many times as necessary is required due to the space and recycling of the card.
[0005]
Conventionally, as a rewritable thermosensitive display medium, a method using an organic low-molecular substance, a method using a leuco dye, a method using a polymer liquid crystal, and the like have been proposed. JP-A-63-39376 discloses a heat-sensitive recording medium in which an organic low-molecular substance is dispersed in a resin matrix and the transparency reversibly changes depending on the temperature. This method has disadvantages in that heat must be applied for a long time when the recorded image is erased, and the contrast between the recorded image and the background is poor. JP-A-2-188293 proposes a reversible recording system using a leuco dye which is originally a reversible reaction. In this method, similarly, heat must be applied for a long time during erasing.
[0006]
On the other hand, a medium capable of reversible recording by orienting or moving magnetic powder by an external magnetic field has been proposed. JP-A-53-44425, JP-A-60-27003, and JP-A-3-273521 disclose reversible recording systems using magnetic powder. In this method, recording and erasing can be performed in a short time, but there is a disadvantage that the contrast of an image is poor.
[0007]
Japanese Patent Application Laid-Open No. 2-16082 discloses a recording medium using a side-chain type polymer nematic liquid crystal, capable of recording by rapid cooling by applying heat above the isotropic phase transition temperature (Tc), and erasing recorded information by raising the temperature and gradually cooling. ing. In the case of a polymer nematic liquid crystal, there is a disadvantage that the contrast of an image is poor because recording is performed due to a change in opacity-transparency. In Japanese Patent Application Laid-Open No. 4-174415, reversible recording is realized by using a polymer liquid crystal of a copolymer of a nematic monomer and a cholesteric monomer.
[0008]
However, a reversible thermosensitive recording medium using a polymer cholesteric liquid crystal must be formed on a visible light-absorbing substrate due to the characteristics of the cholesteric liquid crystal. Is poor and visibility is poor.
[0009]
Therefore, improvement of this point is desired.
[0010]
[Problems to be solved by the invention]
An object of the present invention is to provide a reversible thermosensitive recording medium and a recording method which use a polymer cholesteric liquid crystal for a recording layer, have high sensitivity, good contrast, and excellent image stability.
[0011]
[Means for Solving the Problems]
Such an object is achieved by the following (1) to (4) of the present invention.
(1) A reversible thermosensitive recording medium having a recording layer containing a polymer liquid crystal on a support,
The support is a visible light absorbing support colored black ,
The polymer liquid crystal is a polymer cholesteric liquid crystal represented by the following formula 3, and having a glass transition temperature of 40 to 100 ° C and an isotropic phase transition temperature of 160 ° C or more ,
By applying heat to the recording layer, which is a reversible thermosensitive recording layer colored by cholesteric orientation, a transparent printed image was formed, and the black color of the visible light absorbing support appeared in the printed image. Reversible thermosensitive recording medium.
[0012]
Embedded image
[0013]
[In the formula 3, R ₁ represents a cholesterol group represented by the formula 4, and R ₂ represents a biphenyl group, a cyanophenyl group or a methoxyphenyl group.
[0014]
Embedded image
[0015]
a and d each represent an integer of 2 to 4, b and e each represent 0 or 1, and c and f each represent 1; x is 0.1 to 0.7, and n is 2.5 to 10. ]
(2) The reversible thermosensitive recording medium according to (1), wherein the polymer liquid crystal has selective reflection in a visible light region.
(3) The reversible thermosensitive recording medium according to the above (1) or (2), wherein the thickness of the recording layer is 10 μm or less.
(4) A transparent print image is formed on the recording layer by applying heat using the reversible thermosensitive recording medium according to any of the above (1) to (3), and the visible light absorbing property is applied to the print image. A recording method in which the black color of the support is made to appear, the background is a cholesteric color, and the printed image is a black image.
[0016]
In addition, T. Ueno, T .; Nakamura, C.I. Tani, Japan Display '86, 290 (1986) discloses a thermo-optic method for applying a polymer cholesteric liquid crystal, which is a copolymer of a polysiloxane having a cholesterol group and a group having a nematic liquid crystal property, to a liquid crystal display. It discloses properties and discloses that the copolymer layer is a reversible recording layer. Although the specific structure of the copolymer is not shown here, it is common to that of the present invention in that it has polysiloxane as a main chain and has a cholesteric side chain and a nematic side chain via a spacer. However, this has a glass transition temperature (Tg) of 25 ° C., which is lower than that of the present invention. The isotropic phase transition temperature (Tc) is 150 ° C., which is also lower than that of the present invention. When the copolymer layer is used as the recording layer in the above document, the film thickness is 20 μm, and the recording layer is thicker than the present invention. Therefore, it is considered that the sensitivity is lower than that of the present invention. Further, in this case, the visibility of the recording spot is poor, and it is necessary to increase the contrast ratio using a circular polarizer. However, this is not necessary in the present invention, and the contrast is sufficient even by direct observation, and the visibility is excellent.
[0017]
Therefore, the effects of high sensitivity and high contrast in the present invention can be obtained only by using the polymer cholesteric liquid crystal of Chemical Formula 3 having a Tg of 40 to 100 ° C., and its structure is included in Chemical Formula 3. However, they cannot be obtained by using those described in the above-mentioned documents whose Tg is different from that of the present invention.
[0018]
In fact, as described in the Examples below, using a polymer cholesteric liquid crystal of the copolymer having the same Tg, the same Tc as that shown in the above-mentioned document, and having a similar Tc, and contained in the chemical formula 3, the same as that of the present invention. When recording was carried out under the conditions, it is clear from the fact that recording characteristics are inferior and that image stability cannot be obtained with the same Tg as in the above document.
[0019]
[Specific configuration]
Hereinafter, a specific configuration of the present invention will be described in detail.
[0020]
The reversible thermosensitive recording medium of the present invention has a recording layer formed of a polymer cholesteric liquid crystal having a glass transition temperature of at least 40 ° C. on a support.
[0021]
Reversible recording can be performed on this recording layer. The above polymer cholesteric liquid crystal exhibits a cholesteric alignment liquid crystal phase at a temperature equal to or higher than the glass transition temperature (Tg) and lower than the isotropic phase transition temperature (Tc), and thus selectively reflected light is obtained. Then, when the temperature becomes equal to or higher than the isotropic phase transition temperature (Tc), it indicates an isotropic phase and becomes transparent. By utilizing such a phenomenon, reversible recording is realized. More specifically, it is considered that when a reversible thermosensitive recording layer colored by cholesteric orientation is provided and appropriate heat is applied by a thermal head or the like, the liquid crystal group is randomly or nematically oriented. Further, when the reversible thermosensitive recording layer is cooled down to room temperature, it becomes cloudy in the case of random orientation and becomes transparent in the case of nematic orientation. In the reversible thermosensitive recording layer of 10 μm or less according to the present invention, the transparent layer is fixed to take a nematic orientation and the base black appears, or it becomes cloudy due to random orientation, but the recording layer is thin and cannot be concealed and becomes transparent. It is considered that the black color of the groundwork appears due to the visibility. The cooling temperature for fixing this layer may be generally equal to or lower than the glass transition temperature (Tg). In addition, the color of the base is usually black as described above because the color of the visible light absorbing support appears. Accordingly, a cholesteric color with a vivid background and a high-contrast image with a black image can be obtained.
[0022]
On the other hand, the erasure of the printed image is realized by processing the printed portion at a temperature between the glass transition temperature (Tg) and the isotropic phase transition temperature (Tc) of the polymer liquid crystal. This is because the phase appearing by printing becomes the liquid crystal phase again by this heat treatment. This heat treatment may be performed using an oven, a heat roller, or the like.
[0023]
As described above, the polymer cholesteric liquid crystal of Chemical Formula 3 has a glass transition temperature (Tg) of 40 to 100 ° C. By using a material having a Tg of 40 ° C. or higher, high sensitivity and high contrast can be obtained. Further, the stability of the recorded image and the background at normal temperature is increased, and a high contrast can be maintained for a long time, so that the image stability is extremely excellent. On the other hand, if the Tg is lower than 40 ° C., such an effect cannot be obtained, and in particular, the above-mentioned image stability cannot be obtained. Moreover, isotropic phase transition temperature (Tc) is at 160 ° C. or higher, preferably Ru 180 ° C. to 200 DEG ° C. der. When Tc is lowered, the above-mentioned image stability is particularly likely to be deteriorated.
[0024]
The thickness of the recording layer may be 10 μm or less, preferably 1 to 5 μm. On the other hand, if the recording layer is too thick, it becomes difficult to obtain sufficient sensitivity.
The recording layer is preferably composed of only the polymer cholesteric liquid crystal of Chemical Formula 3, but in some cases, a resin such as polyvinyl chloride, polyvinyl acetate, epoxy resin, phenoxy resin, acrylic resin, polyurethane, or polyester is recorded. 80 wt% or less of the entire layer may be contained.
[0025]
The chemical formula 3 will be described in detail. R ₁ represents a cholesterol group of formula 4, and R ₂ represents a biphenyl group, a cyanophenyl group or a methoxyphenyl group. The substitution position of the phenyl group in the biphenyl group represented by R ₂ is not particularly limited, but preferably includes a 4-biphenyl group and the like. Further, biphenyl groups having different phenyl group substitution positions may be mixed. When R ₂ is a cyanophenyl group, the substitution position of the cyano group is not particularly limited, but preferably includes a 4-cyanophenyl group and the like, and a cyanophenyl group having a different substitution position of the cyano group. May be mixed. Further, when R ₂ is a methoxyphenyl group, the position of substitution of the methoxy group is not particularly limited, but preferably includes a 4-methoxyphenyl group and the like. May be mixed.
[0026]
In Chemical Formula 3, a and d each represent an integer of 2 to 4, and particularly preferably 3. Usually, a and d are preferably the same, but may be different in some cases. B and e each represent 0 or 1, and c and f each represent 1. x is 0.1 to 0.7. n represents an average degree of polymerization, and is preferably 2.5 to 10, and more preferably 3 to 7.
[0027]
Specific examples of the polymer cholesteric liquid crystal of Chemical formula 3 are shown below.
[0028]
Also shows in the Chemical Formula 3 in embodiment _{R 1, R 2, a,} d, b, e, c, f, by a combination of x and n.
[0029]
[0030]
The polymer cholesteric liquid crystal of Chemical formula 3 is a known compound. H. Kreuzer, et al. , Mol. Electron. , 3, 1 , 9 (1987); H. Kreuzer, et al. , Mol. Cryst. Liq. Cryst. , 199 , 345 (1991).
[0031]
The polymer cholesteric liquid crystal having a Tg of 40 ° C. or higher in Chemical Formula 3 may be used alone or in combination of two or more. Further, the polymer cholesteric liquid crystal of Chemical Formula 3 having a Tg of less than 40 ° C. can be used in combination.
[0032]
Further, the polymer cholesteric liquid crystal of Chemical Formula 3 has selective reflection in the visible light region, and exhibits a cholesteric color serving as the background of the recording layer as described above. Usually, the above wavelength region is preferably from 400 to 700 nm, but may be from 400 to 750 nm in some cases, and may include the infrared region.
[0033]
The support used in the present invention is made of a material that absorbs visible light, and may be paper or a plastic film provided with a light absorbing layer, or a plastic film such as black polyethylene terephthalate (PET). The light absorbing layer may be a layer containing carbon black or the like, and the black plastic film may be a layer into which carbon black is kneaded.
[0034]
The thickness of the support is 50 to 500 μm, preferably 100 to 300 μm.
[0035]
In the present invention, it is preferable to provide a protective layer on the recording layer. This can protect the thermal head from damage due to heat and pressure, thereby improving durability. The material of the protective layer is not particularly limited as long as it does not impair the reversibility of the heat-sensitive recording layer, and thermoplastic, thermosetting, and ultraviolet curable resins can be used. Single or copolymers of vinyl, polybutyral, polystyrene, polyurethane, polyvinyl alcohol, cellulose and the like can be used.
[0036]
The thickness of the protective layer may be 0.5 to 30 μm, preferably 1 to 10 μm.
[0037]
In addition, the plastic thermosensitive recording medium of the present invention may be provided with a magnetic recording layer, an irreversible thermosensitive recording layer, a fixed information recording layer, and the like, if necessary.
[0038]
In the reversible thermosensitive recording medium of the present invention, a coating solution of a polymer cholesteric liquid crystal is applied on a support and dried to form a recording layer. The coating liquid is prepared by including a polymer cholesteric liquid crystal in an organic solvent [for example, toluene, tetrahydrofuran (THF), isophorone or a mixed solvent thereof]. The content of the polymer cholesteric liquid crystal is about 10 to 80 wt%. The coating method is not particularly limited, but may be a bar coating method, a gravure coating method, a roll coating method, an air knife coating method, or the like.
[0039]
In most cases, the recording layer exhibits a cholesteric color after being applied and dried. However, depending on the polymer cholesteric liquid crystal, the recording layer develops a color by a shift force using a heat roller or the like set at a temperature of Tg or more and less than Tc. You may do so.
[0040]
When a protective layer is provided if necessary, the recording layer may be provided, and then the protective layer may be provided by application using a coating solution containing a predetermined resin in accordance with the recording layer.
[0041]
【Example】
Hereinafter, the present invention will be described specifically with reference to Examples.
[0042]
Example 1
The polymer liquid crystal Nos. Using 1 (Tg = 56 ° C., Tc = 220 ° C.), 1 part by weight of this was dissolved in 2 parts by weight of toluene to prepare a coating liquid for a recording layer. This recording layer coating solution was applied to a visible light absorbing substrate (188 μm thick black polyethylene terephthalate film) with a Mayer bar so as to have a thickness of 3 μm, dried, and dried to give a red cholesteric color. A recording layer was produced. The maximum reflection wavelength of this reversible thermosensitive recording layer was 680 nm. The prepared reversible thermosensitive recording medium was printed using a thermal head manufactured by TDK Corporation, and a clear black printed image was obtained. The printing conditions were an applied voltage of 13 V, a pulse width of 0.8 msec, a resistance value of 392 Ω, and a scanning speed of 83 mm / sec. Next, when processed in an oven at 150 ° C. for 1 minute, the printed image was erased and returned to the original cholesteric color. When the same printing and erasing were repeated again, reversible printing and erasing were possible in the same manner.
[0043]
Example 2
The polymer liquid crystal Nos. 2 (Tg = 50 ° C., Tc = 216 ° C.), 1 part by weight thereof was dissolved in 2 parts by weight of toluene to prepare a recording layer coating liquid. This recording layer coating solution was applied to a visible light absorbing substrate (188 μm thick black polyethylene terephthalate film) with a Mayer bar to a thickness of 3 μm, dried, and reversible heat sensitive to a green cholesteric color. A recording layer was produced. The maximum reflection wavelength of this reversible thermosensitive recording layer was 520 nm. The prepared reversible thermosensitive recording medium was printed using a thermal head manufactured by TDK Corporation, and a clear black printed image was obtained. The printing conditions were the same as in Example 1.
[0044]
Next, when processed in an oven at 150 ° C. for 1 minute, the printed image was erased and returned to the original cholesteric color. When the same printing and erasing were repeated again, reversible printing and erasing were possible in the same manner.
[0045]
Example 3
The polymer liquid crystal Nos. 4 (Tg = 47 ° C., Tc = 196 ° C.), 1 part by weight thereof was dissolved in 2 parts by weight of toluene, and a reversible thermosensitive recording exhibiting a blue-green cholesteric color in the same manner as in Examples 1 and 2. Layers were made. The maximum reflection wavelength of this reversible thermosensitive recording layer was 436 nm. On the other hand, when printing was performed in the same manner as in Examples 1 and 2, a clear black printed image was obtained. Similarly, the printed image could be erased, and printing and erasing could be repeated.
[0046]
Comparative Example 1
In the chemical formula 3, R ₁ = cholesterol group, R ₂ = methoxyphenyl group, a = 3, b = e = 0, c = f = 1, d = 4, x = 0.35, n = 5 (Tg = 25 (Tc = 141 ° C.), 1 part by weight of the polymer liquid crystal was dissolved in 2 parts by weight of toluene, and a reversible thermosensitive recording exhibiting a blue-green cholesteric color was performed in the same manner as in Examples 1 to 3. Layers were made. The maximum reflection wavelength of this reversible thermosensitive recording layer was 460 nm. On the other hand, when printing was attempted in the same manner as in Examples 1 to 3, the sharpness of the printed image was inferior to those of Examples 1 to 3, and the image after printing gradually disappeared.
[0047]
Example 4
On the recording layer of each of the reversible thermosensitive recording media of Examples 1 to 3, a protective layer having a thickness of 2 μm (dry film thickness) was further formed by a bar coating method using a 20 wt% aqueous solution of an aqueous urethane resin as a coating liquid.
[0048]
In the recording medium provided with the protective layer, the same good reversible recording as in Examples 1 to 3 was possible, and the durability was further improved.
[0049]
【The invention's effect】
According to the present invention, a reversible thermosensitive recording medium having high sensitivity and high contrast can be realized. Further, the image stability after recording is excellent.

Claims (4)

In a reversible thermosensitive recording medium having a recording layer containing a polymer liquid crystal on a support,
The support is a visible light absorbing support colored black ,
The polymer liquid crystal is a polymer cholesteric liquid crystal represented by the following formula 1 and having a glass transition temperature of 40 to 100 ° C and an isotropic phase transition temperature of 160 ° C or more ,
By applying heat to the recording layer, which is a reversible thermosensitive recording layer colored by cholesteric orientation, a transparent printed image was formed, and the black color of the visible light absorbing support appeared in the printed image. Reversible thermosensitive recording medium.
[In the formula 1, R ₁ represents a cholesterol group represented by the formula ₂ , and R ₂ represents a biphenyl group, a cyanophenyl group or a methoxyphenyl group.
a and d each represent an integer of 2 to 4, b and e each represent 0 or 1, and c and f each represent 1; x is 0.1 to 0.7, and n is 2.5 to 10. ] The reversible thermosensitive recording medium according to claim 1, wherein the polymer liquid crystal has a selective reflection in a visible light region. 3. The reversible thermosensitive recording medium according to claim 1, wherein the thickness of the recording layer is 10 μm or less. A transparent print image is formed on the recording layer by applying heat using the reversible thermosensitive recording medium according to any one of claims 1 to 3, and the black color of the visible light-absorbing support appears on the print image. Recording method for obtaining an image in which the background is cholesteric color and the printed image is black.