JP2645535B2 - Thermosensitive laminated sheet and information recording body using the same - 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 laminated sheet capable of peelably adhering information recording surfaces of a recording material to each other and an information recording body using the same. The present invention relates to a heat-sensitive laminated sheet used for transmitting more information at a postage, and an information recording medium using the same.

[0002]

2. Description of the Related Art In recent years, mailing of personal privacy documents such as bills and payment notices of the Social Insurance Agency, banks, securities companies, etc. has increased sharply. Such notices relate to the privacy of the individual and the contents of the notice differ depending on the destination, so that the sending operation requires careful attention.

[0003] Therefore, recently, efforts have been made to save labor in shipping work by managing data for each destination by a computer. That is, the data input to the computer is printed out continuously for each recipient at the time of shipping. At this time, the destination and the message are printed on one side of the continuous recording paper at the same time, separated for each recipient, and sent out in a sealed envelope with a window so that only the recipient can be seen from the outside. By doing so, it is possible to prevent inconsistency between the destination and the message, and it is possible to omit operations such as address book.

[0004] However, since the postage of a sealed letter is higher than that of a postcard, various postcards capable of concealing information inside have been proposed and put to practical use. As an example of these proposals, there is a postcard in which a heat-sensitive adhesive layer capable of recording information on the surface is formed on one side of a paper support (Japanese Utility Model Laid-Open No. 1-148371). This sealed postcard is folded into two and becomes a postcard size. After recording information on the surface of the heat-sensitive adhesive, it is folded in two and the heat-sensitive adhesive is thermocompressed with an adhesive strength capable of peeling off each other. is there.

[0005] However, the above-mentioned postcard has a drawback that the recorded contents are transferred to the other surface because information is recorded on the surface of the heat-sensitive adhesive layer, that is, the peeled surface.
Since it is difficult to adjust the adhesive strength of the heat-sensitive adhesive layer, there are drawbacks such as peeling curl, which makes it difficult to read information. Therefore, in order to make the peeling of the heat-sensitive adhesive layers good, a method of adjusting the temperature and pressure of the thermocompression bonding is considered, but it is extremely difficult to adjust the peelability by this method,
There is a disadvantage that spontaneous peeling occurs due to poor adhesion, and a recording material such as direct mail in which the entire surface of the information recording surface is covered with ink cannot be heat-sensitively bonded, and an integrated product cannot be obtained.

In addition, we have already recorded information on two postcard-sized recording materials, and then heat-bonded the information recording surfaces of the respective recording materials to each other in a releasable manner. Although a sheet and an information recording body using the same have been proposed (Japanese Patent Application No. 2-245381), in the case of an information recording body using this heat-sensitive adhesive sheet, two sheets of information-recorded heat-sensitive adhesive sheets are used. When sandwiched between the information recording surfaces of a postcard-sized recording material and thermally bonded, the hot-melt synthetic resin coating layer protrudes from the cut portion of the recording body, and covers the peeling interface between the support and the synthetic resin coating layer with resin. Therefore, it is unsatisfactory in that it is easy to cause poor opening and that the adhesion between the surface of the recording material covered with the high-concentration ink and the synthetic resin coating layer is poor.

[0007]

SUMMARY OF THE INVENTION Accordingly, the present inventors have
As a result of intensive studies to solve the above-mentioned drawbacks, as a result, heat on one side of the support without spontaneous peeling, and easily peelable, does not cause melt-out by heat of 100 ° C or less. On both sides of the formed product provided with the plastic resin layer,
It has been found that excellent results can be obtained by using a heat-sensitive laminated sheet having an excellent adhesive property and a heat-sensitive adhesive layer which can be heat-sensitively bonded to a recording material with heat of 100 ° C. or less by using the present invention. Reached.

Accordingly, a first object of the present invention is to provide a heat-sensitive laminated sheet which can have more information on a postcard without destroying recorded information. A second object of the present invention is to dispose of a seal postcard, to have excellent openability of the postcard, to be able to peel off without generating peeling curl, and to transmit more information.
An object of the present invention is to provide an information recording medium that functions as a postcard.

[0009]

SUMMARY OF THE INVENTION The above objects of the present invention are attained by a heat-sensitive adhesive for heat-sensitively bonding information recording surfaces of one or more recording materials (E) having information recording surfaces (Ea, Eb). In the laminated sheet, the thermosensitive laminated sheet has a thermoplastic resin layer (B) having a Vicat softening temperature of 45 ° C. or more on one surface of the support (A) so as to be easily peelable but not spontaneously peeled. A heat-sensitive laminated sheet having a four-layer structure in which a heat-sensitive adhesive layer (C, C ′) having a glass transition temperature of 30 to 70 ° C. or a Vicat softening temperature of 40 to 65 ° C. is formed on both surfaces of the formed product. (D), wherein 180 is an interface between the support (A) and the thermoplastic resin layer (B).
The peel strength is 10 to 150 g / 50 mm at a peeling speed of 500 mm / min in an environment of 20 ° C. and 65% RH, and inserted between the information recording surfaces (Ea, Eb) of the recording material (E). One of the interfaces between the heat-sensitive adhesive layers (C, C ′) and the information recording surfaces (Ea, Eb) when the heat-sensitive bonding is performed.
80 ° peel strength, 20 ° C., under an environment of 65% RH, was achieved by the heat-sensitive laminated sheet and the information recording medium using the same, characterized in Rukoto Do and 300 g / 50 mm or more at a peeling speed of 500 mm / min.

Hereinafter, the heat-sensitive laminated sheet of the present invention and an information recording medium using the same will be described in detail with reference to the drawings, but the present invention is not limited thereto. Figure 1
It is a sectional lineblock diagram of a heat sensitive laminated sheet of the present invention. FIG. 2 is a cross-sectional view of the information recording medium of the present invention when the heat-sensitive laminated sheet of FIG. 1 is used.

In the drawing, reference numeral A denotes a support, B denotes a thermoplastic resin layer, C and C ′ denote a heat-sensitive adhesive layer, D denotes a heat-sensitive laminated sheet of the present invention, and E denotes a recording body of the present invention together with D. The recording materials Ea and Eb represent the information recording surface of the recording material. The support in the present invention is appropriately selected from materials such as a transparent or opaque synthetic resin film, a composite of paper and plastic such as impregnated paper, or a light absorbing sheet colored with dye and / or pigment. Can be used.

The thermoplastic resin layer (B) provided on one side of the support (A) has a 180 ° peel strength at a peel speed of 500 mm / min under an environment of 20 ° C. and 65% RH, which has a 180 ° peel strength. At the interface with A), it is 10 to 150 g / 50 mm, and the other surface of the support (A) and the thermoplastic resin layer
(B) Thermal adhesive layer (C, C ') provided on top
The interface between the support (A) and the thermoplastic resin layer (B) is actual.
It does not peel off qualitatively, and is in contact with the heat-sensitive adhesive layer (C, C ′) layer.
Peel strength at the interface between the broadcast recording surface E _a and E _b are said strip
It is necessary to be 300 g / 50 mm or more under the same conditions as in the above. In addition, the measurement of the above-mentioned peel strength is based on JISZ-02.
Perform according to 37.

As the thermoplastic resin forming the thermoplastic resin layer, at least one resin selected from a thermoplastic elastomer, an ionomer resin, and a low molecular weight polyolefin resin having a Vicat softening temperature of 45 ° C. or more is used. Is preferred. The above-mentioned Vicat softening temperature is
It is measured according to ASTM D-1525-70.

The synthetic resin used for the heat-sensitive adhesive layers (C, C ′) provided on the surfaces of A and B has a glass transition temperature of 30 to 70 ° C. or a Vicat softening temperature of 40 to 6 ° C.
It is preferable to use at least one resin selected from ethylene / vinyl acetate copolymer resin, polyester resin, ethylene / acrylic acid copolymer resin, ethylene / ethyl acrylate copolymer resin at 5 ° C.

The above adhesive layer may contain an inorganic pigment in a range of 20% by weight or less, but preferably contains 3 to 15% by weight. By adding an inorganic pigment as described above, when manufacturing a recording body with the heat-sensitive laminated sheet of the present invention sandwiched between recording materials, it is only possible to prevent the molten resin from protruding from between the recording materials. In addition, it is possible to improve the winding preservability during the production of the laminated sheet of the present invention.

For example, even when heat higher than the temperature required for heat-sensitive bonding is applied, if the inorganic pigment is added, the melt viscosity of the adhesive layer is high, so that the protrusion is prevented. Furthermore, since blocking can be prevented by the addition of the inorganic pigment, it is possible to improve the long-term winding storage stability.

The recording material (E) used in the information recording medium of the present invention can be appropriately selected from known materials such as paper used for ordinary postcards and computer printing paper. The information recording medium of the present invention sandwiches the heat-sensitive laminated sheet (D) of the present invention between information recording surfaces of a recording material (E),
It is heat-sealed at a temperature of 100 ° C. or less and integrated into a postcard size.

Further, the information recording medium of the present invention can be formed, for example, by sticking together the information recording surfaces of two postcard-sized recording materials (E) as shown in FIG. And, as shown in FIG. 4, the recording material (E), which is simultaneously printed on the printing paper of the computer on three sides at a time, is folded in S-shape or Z-fold.
Opposing information recording surfaces can also be integrated.
Therefore, in the latter case, since four information recording surfaces can be provided inside the postcard, the postcard having a larger amount of information can be obtained.

Although the case where the heat-sensitive laminated sheet (D) of the present invention is used for a postcard has been described in detail above, it is obvious that the heat-sensitive laminated sheet (D) of the present invention can be used for applications other than a postcard. It is. As an example of such an application, for example, if a document to be put in a sealed letter is folded in two, and the heat-sensitive laminated sheet (D) of the present invention is inserted between the information recording surfaces and heat-sensitively bonded, the sealed letter is printed without leaving any trace. Even when opened, a trace will remain on the document itself, and it is not possible to see through the contents of the document from the outside from the outside, so it will be more effective in preventing privacy than conventional mailing by sealed letter be able to.

[0020]

As described in detail above, the heat-sensitive laminated sheet of the present invention has excellent heat-sensitive adhesiveness to a recording material, can be peeled off without causing peeling curl, and transfers recorded information to another surface. Since the recording surfaces of the information are not peeled off, the recording surfaces of the recording materials on which the information is recorded are preferably adhered to each other in a releasable manner. In addition, the recording medium of the present invention can send more than twice the amount of information at a postage fee of a postcard, especially by setting the size to a postcard size, and as an information transmission means excellent in privacy protection. Useful.

[0021]

EXAMPLES The present invention will be described in more detail with reference to the following Examples, but it should not be construed that the invention is limited thereto.

Embodiment 1 FIG. On one surface of a 16 μm thick polyethylene terephthalate film support (A), a thermoplastic resin layer (B) made of an ionomer resin having a Vicat softening temperature of 55 ° C. and not protruding at a temperature of 100 ° C. or less. ) Is formed so as to have a thickness of 10 μm, and a polyester resin having a glass transition temperature of 35 ° C. and an inorganic pigment which can be heat-sensitively bonded to a recording material at 90 ° C. on both surfaces of the obtained laminate. Each of the mixed liquids in which the solid content ratio of the inorganic pigment / inorganic pigment was adjusted to 90/10
It was coated and dried to a thickness of μm to obtain a heat-sensitive laminated sheet.

Next, as shown in FIG. 3, the information recording surfaces (Ea, Eb) of the recording material having a basis weight of 90 g / m ^{2 on} which the information from the computer is printed are contacted with each other so that the information recording surfaces (Ea, Eb) are in contact with each other.
Then, fold back to the opposite side at fold line O, insert the previously prepared heat-sensitive laminated sheet between the opposing surfaces, pass between heat rolls having a surface temperature of 100 to 110 ° C., and heat-seal the sheet sufficiently. Cool.

The postcard prepared as described above is a support (A)
Although there was no spontaneous peeling at the interface between the resin and the thermoplastic resin layer (B), the film could be easily peeled off at the interface without causing peeling curl. Therefore, the recorded information was not destroyed at all and could be read well. Next, the 180-degree peel strength of this recording medium was set to 20 ° C. and 6 ° C.
When measured at a peeling speed of 500 mm / min in an environment of 5% RH, the peeling strength at the interface between the support (A) and the thermoplastic resin layer (B) was 40 g / 50 mm. The peel strength at the interface with the adhesive layer (C, C ′) was 300 g / 50 mm or more.

Further, as a result of conducting an opening test of 1,000 sheets of the produced postcards, the heat-sensitive adhesiveness to the recording material was good, the adhesive did not protrude due to the heat at the time of heat-sensitive bonding, and the openability was good. Met. Furthermore, as a result of winding up the produced heat-sensitive laminated sheet and storing it in an environment of 40 ° C. for one month, it was confirmed that there was no occurrence of blocking and the long-term storage property was excellent. The total weight of the postcard was within the standards of the Postal Law, and it was confirmed that the postcard could be used as a postcard.

Embodiment 2 FIG. A heat-sensitive laminated sheet was obtained in exactly the same manner as in Example 1, except that an ethylene / vinyl acetate copolymer resin having a Vicat softening temperature of 58 ° C. was used as a heat-sensitive adhesive instead of the polyester resin having a glass transition temperature of 35 ° C. Was. The produced postcard was the same as in Example 1, and had sufficient performance as a postcard.

Comparative Example 1 A heat-sensitive laminated sheet was produced in exactly the same manner as in Example 1 except that a thermoplastic elastomer having a Vicat softening temperature of 43 ° C. was used as the thermoplastic resin, and was heat-sensitively bonded to a recording material to obtain a postcard. Regarding the obtained postcard, the 180 ° peel strength between the support (A) and the thermoplastic resin layer (B) was set at 20 ° C.
When measured at a peeling speed of 500 mm / min in an environment of 65% RH, the peeling force is 500 g / 50 mm or more, and the peeling force at the time of opening the postcard is high. Was difficult to formally peel off . Ma
In addition , even in a sample in which regular peeling occurred between the support and the thermoplastic resin layer, the paper layer of the recording material was broken during the opening of the postcard, and the openability of the postcard was poor. Furthermore , even the postcards that were peeled off properly had severe curls, and could not be read as it was in a tubular shape.

Comparative Example 2 Except for using an ethylene / ethyl acrylate copolymer resin having a Vicat softening temperature of 50 ° C. as a thermoplastic resin, a heat-sensitive laminated sheet was prepared in exactly the same manner as in Example 1, and the postcard was heat-sensitively bonded to a recording material to form a postcard. Obtained. Regarding the obtained postcard, the 180-degree peel strength between the support (A) and the thermoplastic resin layer (B) was increased to 20 ° C. and 65% RH.
When measured at a peel strength of 500 mm / min under the environment described above, the peel strength is 200 g / 50 mm or more. Since the peeling force at the time of opening the postcard is high, peeling curl occurs, and it is difficult to read the recorded information as it is. there were.

Comparative Example 3 A heat-sensitive laminated sheet was produced in exactly the same manner as in Example 1, except that a polyester resin having a glass transition temperature of 28 ° C. was used as the heat-sensitive adhesive layer resin, and a postcard was obtained by heat-sensitive bonding with a recording material. In this case, upon heat-sensitive bonding with the recording material, the heat-sensitive adhesive resin melts and protrudes from the cut portion of the recording medium, and the molten resin covers the peeling interface, so that the postcard cannot be opened. Further, when the heat-sensitive laminated sheet was wound up and stored in an environment of 40 ° C. for one month, it could not be used because blocking occurred.

Comparative Example 4 A heat-sensitive laminated sheet was prepared in exactly the same manner as in Example 1, except that an ethylene / vinyl acetate copolymer resin having a Vicat softening temperature of 38 ° C. was used as the heat-sensitive adhesive layer resin, and the postcard was heat-sensitively bonded to a recording material. I got
This case is also the same as that of Comparative Example 3, in which the heat-sensitive adhesive resin melts during the heat-sensitive bonding with the recording material, protrudes from the cut portion of the recording medium, and the molten resin covers the peeling interface. Opening was not possible. Further, when the heat-sensitive laminated sheet was wound up and stored in an environment of 40 ° C. for one month, it could not be used because blocking occurred.

Comparative Example 5 A heat-sensitive laminated sheet was prepared in exactly the same manner as in Example 1 except that a polyester resin having a glass transition temperature of 75 ° C. was used as the heat-sensitive adhesive layer resin, and a postcard was obtained by heat-sensitive bonding with a recording material. In this case, the heat-sensitive adhesiveness to a recording material whose information recording surface is covered with high-concentration ink is poor, and peeling occurs simultaneously between the recording material and the heat-sensitive adhesive layer and between the support and the thermoplastic resin layer. Therefore, it was not easy to open normally.

Comparative Example 6 Except for using an ethylene / ethyl acrylate copolymer resin having a Vicat softening temperature of 66 ° C. as the heat-sensitive adhesive layer resin, a heat-sensitive laminated sheet was prepared in exactly the same manner as in Example 1 and heat-sensitively bonded to a recording material. I got a postcard. This case is also the same as in the case of Comparative Example 5, in which the heat-sensitive adhesiveness to a recording material whose information recording surface is covered with high-concentration ink is inferior, and the recording material and the heat-sensitive adhesive layer and the support and the thermoplastic resin Normal peeling was not possible because peeling occurred simultaneously in both layers.

Comparative Example 7 A thermosensitive laminated sheet was produced in exactly the same manner as in Example 1 except that an acrylic resin having a glass transition temperature of 55 ° C. was used as the heat-sensitive adhesive layer resin, and a postcard was obtained by heat-sensitive bonding with a recording material. This case is also the same as in the case of Comparative Example 5, in which the heat-sensitive adhesiveness to a recording material whose information recording surface is covered with high-concentration ink is inferior, and the recording material and the heat-sensitive adhesive layer and the support and the thermoplastic resin Normal peeling was not possible because peeling occurred simultaneously in both layers.

Comparative Example 8 A heat-sensitive laminated sheet was prepared in exactly the same manner as in Example 1 except that an ionomer resin having a Vicat softening temperature of 58 ° C. was used as the heat-sensitive adhesive layer resin, and a postcard was obtained by heat-sensitive bonding with a recording material. This case is also the same as in the case of Comparative Example 5, in which the heat-sensitive adhesiveness to a recording material whose information recording surface is covered with high-concentration ink is inferior, and the recording material and the heat-sensitive adhesive layer and the support and the thermoplastic resin Normal peeling was not possible because peeling occurred simultaneously in both layers.

Comparative Example 9 A heat-sensitive laminated sheet was prepared in exactly the same manner as in Example 1, except that a mixed solution in which the solid content ratio of the polyester resin and the inorganic pigment was adjusted to 79/21 was used as the heat-sensitive adhesive, and the heat-sensitive adhesive was heat-sensitively bonded to the recording material. I got a postcard. In this case, since the amount of the inorganic pigment added was large, the heat-sensitive adhesiveness to the recording material was poor, the printed surface of the recording material peeled off in the opening test was slightly white, and the information was poorly interpreted.

Table 1 shows a list of conditions for producing postcards obtained in Example 1, Example 2, and Comparative Examples 1 to 9, and Table 2 shows the evaluation results thereof.

[Table 1]

[0037]

[Table 2] The heat-sensitive adhesiveness to the recording material in the table, the legibility of information, the degree of peeling curl, the suitability for winding the sheet, the postcard opening rate, and the peeling force were evaluated as follows.

When the heat-sensitive adhesive postcard on the recording material was opened, the degree of mixing of white spots (bubbles) between the surface of the recording material on which information was recorded and the heat-sensitive adhesive layer was visually evaluated. ：: no white spots Δ: slightly more white spots ×: very many white spots

Readability of Information The readability of the recorded information was visually determined with the postcard opened as it was. ：: Good readability Δ: Slightly difficult to read ×: Poor readability (Readable defects are caused by transfer to the other side of recorded information, lack of recorded information due to paper layer destruction, curl, and poor adhesion.)

Peeling curl degree The curl degree generated when the postcard was opened was visually evaluated. ：: No curl △: Slightly curl ×: Preservability of very strong curl (cylindrical) sheet After storing the rolled heat-sensitive laminated sheet at 40 ° C., It was evaluated visually. ：: no blocking △: slight blocking: very strong blocking

The transparency of the concealable postcard was measured. If it is 3% or less, there is no practical problem. Postcard Opening Rate An openability test was performed on 1,000 postcards, and the opening rate was calculated by the following formula. Opening rate (%) = [number of postcards that can be peeled off properly / total number of postcards (1000)] × 100 Peeling strength postcards (n = 3) are sampled at 50 mm × 140 mm, and the temperature is 20 ° C. according to JIS Z-0237. , 65% R
The peeling resistance was measured at a peeling speed of 500 mm / min in an environment of H, and the average value of the three sheets was indicated as a peeling force.

The results of the above Examples and Comparative Examples show that the thermoplastic resin layer (B) uses a synthetic resin having a Vicat softening temperature in a specific range of 45 ° C. or higher, and the heat-sensitive adhesive layers (C, C ′). ) Has a glass transition temperature of 30 to 70 ° C.,
Or, by using a synthetic resin having a specific range of Vicat softening temperature of 40 to 65 ° C, discardability of the seal postcard,
This demonstrates that it is possible to obtain a postcard that is excellent in the openability of a postcard and the heat-sensitive adhesiveness to a recording material, can be peeled off without causing peeling curl, and can read information normally and easily.

[Brief description of the drawings]

FIG. 1 is a sectional configuration view of a heat-sensitive laminated sheet of the present invention.

FIG. 2 is a cross-sectional configuration diagram of an information recording medium of the present invention when the heat-sensitive laminated sheet of FIG. 1 of the present invention is used.

FIG. 3 is a diagram showing an example of a recording material capable of producing a recording medium of the present invention by performing S-shaped folding.

FIG. 4 is a diagram showing an example of a recording material capable of producing a recording medium of the present invention by folding in a Z shape.

5 is a cross-sectional configuration diagram of an information recording body using the recording material of FIG.

[Explanation of symbols]

Reference Signs List A support B thermoplastic resin layer C heat-sensitive adhesive layer C 'heat-sensitive adhesive layer D heat-sensitive laminated sheet E recording material Ea information recording surface Eb information recording surface

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification number Agency reference number FI Technical indication location C09J 7/02 JJA C09J 7/02 JJA JKF JKF JKP JKP JLE JLE D21H 27/00 D21H 5/00 B

Claims (6)

(57) [Claims] 1. A heat-sensitive laminated sheet for heat-sensitively bonding information recording surfaces of one or more recording materials (E) each having an information recording surface (Ea, Eb), wherein the heat-sensitive laminated sheet comprises a support ( On one side of A), the Vicat softening temperature is 45 ° C. so that it can be easily peeled but does not spontaneously peel.
A heat-sensitive adhesive layer (C, C ′) having a glass transition temperature of 30 to 70 ° C. or a Vicat softening temperature of 40 to 65 ° C. is formed on both surfaces of the formed product provided with the thermoplastic resin layer (B). A heat-sensitive laminated sheet (D) having a four-layer structure, wherein the 180-degree peel strength at the interface between the support (A) and the thermoplastic resin layer (B) is 20 ° C. and 65% RH. 10 to 150 g / 50 m at a peeling speed of 500 mm / min.
m and the information recording surface (E) of the recording material (E).
a and Eb), the 180-degree peel strength at the interface between each of the heat-sensitive adhesive layers (C, C ′) and the information recording surface (Ea, Eb) is 20 ° C. atmosphere of 65% RH, the heat-sensitive laminated sheet, wherein Rukoto Do and 300 g / 50 mm or more at a peeling speed of 500 mm / min. 2. The support (A) of the heat-sensitive laminated sheet is selected from a transparent or opaque synthetic resin film, a composite of paper and plastic, and a light-absorbing sheet colored with a dye and / or a pigment. The heat-sensitive laminated sheet according to claim 1, wherein 3. The resin according to claim 1, wherein the resin of the thermoplastic resin layer (B) is at least one resin selected from a thermoplastic elastomer, an ionomer resin, and a low molecular weight polyolefin resin. 2. The heat-sensitive laminated sheet according to 1. 4. The resin for the heat-sensitive adhesive layer (C, C ′) is selected from ethylene / vinyl acetate copolymer resin, polyester resin, ethylene / acrylic acid copolymer resin, and ethylene / ethyl acrylate copolymer resin. The heat-sensitive laminated sheet according to any one of claims 1 to 3, which is at least one type of resin. 5. An information recording surface (Ea, Ea) of one or more recording materials (E) having at least two or more information recording surfaces.
An information recording medium, characterized in that the heat-sensitive laminated sheet (D) according to claim 1 is sandwiched between b) and integrated at a temperature of 100 ° C or less. 6. The information recording medium according to claim 5, wherein the information recording medium can be mailed as a postcard.