JPH0881655A - Adhesive film using polystyrene film as substrate and method for producing the same - Google Patents

Abstract

PURPOSE: To provide an adhesive film maintaining a stable adhesive force with time, excellent in punching processability and dimensional stability, and especially suitable as a film for protecting the surface of a green sheet for ceramic IC packages, and further to provide the method for producing the same. CONSTITUTION: A preliminarily formed acrylic adhesive layer is laminated to one surface of a polystyrene film substrate by a transfer method, etc., to produce the adhesive film. The sheet suitable for protecting the surface of a green sheet is obtained by controlling the gel fraction of the adhesive layer to 90-95%. If desired, a release sheet layer is formed on the adhesive layer.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pressure-sensitive adhesive film which is excellent in punching workability and dimensional stability and is used for surface protection in fields requiring high processing precision such as electronic materials, and a method for producing the same. . Adhesive films and sheets used as surface protection films in fields requiring high processing accuracy such as electronic materials, especially ceramic IC chips, can be punched with high precision and have excellent dimensional stability against changes in humidity. Sex is required. Stretched hard vinyl chloride, cellophane,
Diacetate, triacetate, and the like, and polyethylene terephthalate, stretched polypropylene, and the like, which have excellent humidity resistance, are used as the base material of the adhesive film.

Japanese Patent Laid-Open Nos. 3-131677 and 4-266982 disclose adhesive tapes based on polystyrene. This product disperses a silicone release agent in an organic solvent such as hexane and heptane, and immediately after applying it to one side of a polystyrene film, it is dried immediately to prevent the polystyrene film from being attacked by the solvent. To prevent this, an acrylic adhesive or a non-acrylic adhesive is applied to the other surface.

Further, Japanese Unexamined Patent Publication No. 4-261485 discloses an adhesive tape using a biaxially stretched film of a styrene polymer having a syndiotactic structure as a base material. In this adhesive tape, the adhesive is also used. The layer is formed by applying an adhesive solution to a base film using an organic solvent.

[0004]

Stretched hard vinyl, cellophane, diacetate, triacetate and the like have excellent punching properties, but have poor dimensional stability due to moisture absorption. Moisture resistance of polyethylene terephthalate, stretched polypropylene, etc. is good, but the punching precision is poor, and the chips of the film tend to remain without being completely punched. As described above, the conventionally used ones cannot satisfy both punching workability and dimensional stability.

Further, if the thickness of the base material is made thin in order to improve the punching workability, it becomes easy for air to be entrapped between the adherend and the adherend when it is adhered to the adherend, and the function as a surface protective film is achieved. There is a problem of disappearing. In particular, in the manufacturing process of a laminated (multi-layer type) ceramic IC package, an adhesive tape that does not cause the destruction of the adhesive layer during the punching process has been required for surface protection of the green sheet that constitutes each layer of the IC package. . The green sheet is a mixture of alumina as a main raw material, an organic binder and an organic solvent, and constitutes a ceramic IC package through a stacking and sintering process.

In the laminated ceramic IC package, a two-dimensional circuit is formed in each layer in order to increase the density of the circuit, and an electrical connection is arranged between the layers to form a three-dimensional circuit. In order to electrically connect each layer, punching is performed in the state of the green sheet, a through hole is opened, and a conductive substance is poured therein. During this punching process, the circuit formed on the surface of the green sheet may be contaminated by punched dust.

For this reason, various adhesive tapes have been conventionally used for protecting the surface of the green sheet before firing, but with the miniaturization of through holes due to the high density of the circuit, these adhesive tapes are There is a problem in that the adhesive layer adheres to the inner wall of the hole due to the destruction of the adhesive layer during the through-hole punching process, and the adhered adhesive agent impedes the filling of the paste, so that the filling cannot be completed completely. On the other hand, if the cohesive force of the pressure-sensitive adhesive is greatly increased in order to eliminate the adhesion of the pressure-sensitive adhesive to the inner wall, the pressure-sensitive adhesive force is reduced and the sticking to the green sheet becomes insufficient. for that reason,
There is a need for a surface-protective adhesive tape that can be punched through the adhesive layer without destroying the adhesive layer together with the base material of the protective tape in the micro-punching process, and that also has sufficient adhesive strength when sticking to the green sheet. Was there.

Adhesive tapes based on polystyrene films described in JP-A-3-131677 and JP-A-4-266982 have a release agent dissolved in an organic solvent, and this is applied to one side of the polystyrene film. Use by directly applying. As the organic solvent, one which has little influence on polystyrene is selected, but it is necessary to dry the solvent instantaneously, and for this purpose, it is necessary to raise the drying temperature considerably.

Since the adhesive is applied to the other surface of the polystyrene film, even if the adhesive solution using such an organic solvent is applied, the same problem as described above occurs. If it is thin, it is easily affected by solvent and heat, and strict process control is required.

When the solvent solution of the adhesive is directly applied to the polystyrene film substrate, the curing agent such as isocyanate contained in the adhesive in solution and the plasticizer contained in the polystyrene film use the solvent as a medium. It reacts and therefore the adhesive strength decreases over time. Further, in the method of directly coating and drying the film base material, it is necessary to cure the adhesive for about one week after the drying in order to crosslink the pressure-sensitive adhesive, and there is a problem that the productivity is extremely low. The present invention solves the above-mentioned conventional problems, maintains stable adhesive strength over time, is excellent in punching processability and dimensional stability, and is particularly suitable as an adhesive film suitable for protecting a green sheet surface and good in productivity. It is an object to provide a manufacturing method thereof.

[0011]

The pressure-sensitive adhesive film of the present invention is characterized in that an acrylic pressure-sensitive adhesive layer formed in advance is laminated on one surface of a film base material having good punchability, and its constitution is as shown in FIG. -1.

Polyethylene terephthalate, diacetate, triacetate and the like can be used as the film substrate having a good punching property, but it is preferable to use a polystyrene film, particularly a stretched polystyrene film having excellent processability. As the polystyrene film, a styrene homopolymer, a copolymer of styrene and an acrylic ester butadiene, or a film made of a mixture of these and a plasticizer or a plastic polymer can be used, and the tensile strength and the tear strength can be used. 2-1 to improve
It is preferably stretched 0 times.

The thinner the film is, the more excellent the fine workability is. However, since the strength is decreased, a break occurs when re-peeling. Therefore, a film having a thickness of 10 to 50 μm is easy to handle as a surface protective film and punched. 15 in terms of sex,
Those having a thickness of ˜30 μm are particularly preferable. The surface of the film base on which the pressure-sensitive adhesive layer is laminated is preferably subjected to a surface treatment such as corona treatment in order to improve wettability. After corona treatment, 45 dyn / cm or more is preferable, 43
If it is less than dyn / cm, adhesive residue on the adherend is likely to occur during re-peeling.

As the pressure-sensitive adhesive, a copolymer containing an acrylic acid alkyl ester as a main component to which a required amount of a crosslinking agent or the like is added is used. As the cross-linking agent, an ordinary one such as isocyanate is used, and the gel fraction of the pressure-sensitive adhesive after forming the pressure-sensitive adhesive layer can be adjusted by increasing or decreasing the addition amount of the cross-linking agent.

In the present invention, the gel fraction of the pressure-sensitive adhesive is calculated as follows. About 2 g of the pressure-sensitive adhesive is sampled from the pressure-sensitive adhesive layer laminated on the film substrate as a measurement sample, and immersed in 50 ml of tetrahydrofuran (THF) for 24 hours under constant temperature and humidity (23 ° C., 65%). After the immersion, the THF solution containing the soluble component of the adhesive was distilled off, and the insoluble component was removed at 120 ° C.
Oven for 2 hours. The gel fraction is calculated by the following formula [Equation 1], where A is the weight of the pressure-sensitive adhesive sample used as the measurement sample and B is the weight of the pressure-sensitive adhesive insoluble matter after drying.

[0016]

[Equation 1]

When the adhesive film of the present invention is used as an adhesive tape for protecting the surface of a green sheet of a ceramic IC package, the gel fraction of the adhesive is 90 to 95%.
It is preferable to adjust When the gel fraction is 90% or less,
Due to cohesive failure of the adhesive, glue breaks during punching of the green sheet and the adhesive remains in the through holes. 95
When it is more than%, the adhesive strength (tack) is lost and it becomes difficult to attach it to the green sheet.

In the surface-protective pressure-sensitive adhesive tape for a green sheet, the thickness of the pressure-sensitive adhesive layer is such that the SP adhesive strength is 80 ± 30 g / 2.
It is preferable to appropriately adjust it so as to be 5 mm. If the SP adhesive strength is 50 g / 25 mm or less, the tape peels off when the adherend is conveyed, and if it is 110 g / 25 mm or more, adhesive residue or breakage of the base material occurs during re-peeling, resulting in a significant reduction in product yield.

The back surface (the surface opposite to the surface on which the pressure-sensitive adhesive layer is provided) of the base material of the pressure-sensitive adhesive film of the present invention can be subjected to a solvent-resistant treatment with an acrylic back-treatment agent or the like. For example,
When used as an adhesive tape for protecting the surface of a green sheet, an acrylic back treatment agent is used in an amount of 1 to 10 g / m ² (d
ry) and coat with a coater such as a gravure roll coater.
The coating amount of the back surface treating agent is preferably 3 to 8 g / m ² .

This back surface treatment layer prevents the base material from being eroded by the solvent in the base material when the green sheets are stacked in the green sheet manufacturing process. Examples of the acrylic back treating agent include copolymers of methyl methacrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, isooctyl acrylate and the like with styrene, vinyl acetate, N-vinylpyrrolidone, 2-hydroxyethyl acrylate and the like. To be As the solvent of the acrylic resin solution, IPA, ethyl acetate or the like is preferable in consideration of the influence on the base film. Further, PE wax, silicon amide or the like may be added to improve the removability from the back surface of the adherend.

The pressure-sensitive adhesive film of the present invention is characterized in that a pre-formed acrylic pressure-sensitive adhesive layer is laminated on one side of a film substrate having good punchability, and the pressure-sensitive adhesive layer formed in advance in a layer form is transferred. And the like are laminated on the base material. The specific manufacturing method will be described below using a polystyrene film as a base material as an example.

An acrylic pressure-sensitive adhesive layer is transferred to one side of the polystyrene film. To transfer, apply acrylic adhesive to the release paper whose surface has been subjected to a release treatment to a specified thickness, dry it, and attach it to a polystyrene film, and then remove the release paper. Done by. . By transferring the pressure-sensitive adhesive layer in this way, since the solvent of the pressure-sensitive adhesive is dried on the release paper, the solvent does not come into contact with the polystyrene film, and the polystyrene film does not receive heat for drying. .

Further, since the adhesive solution is dried and solidified on the release paper, when transferred to the polystyrene film in this state, the solvent as a reaction medium between the curing agent of the adhesive and the plasticizer of the polystyrene film is removed. Since they do not exist, they do not react and stable adhesive strength is obtained.

When the release sheet layer is formed on the pressure-sensitive adhesive layer surface, for example, the release-treated surface of the release paper having the surface subjected to the release treatment is attached. In a usual adhesive tape, a release agent dissolved or dispersed in a solvent is directly applied to the surface of the base material opposite to the adhesive layer and dried, but in the present invention, the release agent is applied to a polystyrene film. Instead of drying, the release-treated surface of another release paper is attached to the adhesive layer, so the solvent of the release agent does not come into contact with the polystyrene film, and the effect of heat to dry the release agent There is no.

As the release paper, it is possible to use, for example, one having a release layer formed by coating and drying a release agent such as silicone on the film surface of polyethylene terephthalate, polypropylene, cellophane, triacetate or the like. Since the release paper is a part of the adhesive film product and is peeled off at the time of use, if the thickness is too thin, it is difficult to peel off because the stiffness is weak, and if it is too thick, it is bulky and therefore has a thickness of 15 to 50 μm. It is preferable because it is easy to handle.

The pressure-sensitive adhesive film having the release sheet layer on the acrylic pressure-sensitive adhesive layer is prepared by applying and drying the acrylic pressure-sensitive adhesive to a predetermined thickness on a release paper whose surface is release-treated, and then drying the pressure-sensitive adhesive. It can also be directly produced by laminating the layer with a polystyrene-based film or the like serving as a film substrate.

The pressure-sensitive adhesive film of the present invention having a peelable sheet layer on the acrylic pressure-sensitive adhesive layer obtained by these methods has a structure in which the pressure-sensitive adhesive layer is not exposed, so that it is stacked as a sheet or wound as a roll. You can also do it. When using this pressure-sensitive adhesive film, after peeling the peelable sheet, the pressure-sensitive adhesive film is attached to the surface of the green sheet or the like.

[0028]

In the pressure-sensitive adhesive film of the present invention, since the acrylic pressure-sensitive adhesive layer formed in advance is laminated with the polystyrene-based film, the solvent of the pressure-sensitive adhesive does not come into contact with the polystyrene-based film and the heat for drying is received. Therefore, the physical properties of the film do not change. In addition, since the pressure-sensitive adhesive layer formed in advance is laminated with the polystyrene film, the plasticizer in the polystyrene film does not react with the curing agent (isocyanate) in the pressure-sensitive adhesive and does not deteriorate.

Since the releasable sheet layer is formed on the acrylic pressure-sensitive adhesive layer, the polystyrene-based film does not come into contact with the solvent of the release agent, and there is no influence of heat for drying the release agent. . When a polystyrene film is used as a base material, it can be punched with high precision.
In addition, it is excellent in dimensional stability and can be manufactured without changing physical properties. By adjusting the gel fraction of the acrylic pressure-sensitive adhesive layer to a specific range, it does not cause the pressure-sensitive adhesive layer to break during fine punching, and has excellent properties as a surface-protective pressure-sensitive adhesive tape for green sheets. A film can be obtained.

[0030]

EXAMPLES Next, the adhesive film of the present invention and the method for producing the same will be described in more detail with reference to Examples.

Example 1 2 to 3 parts by weight of an aromatic isocyanate was added as a cross-linking agent to an acrylic pressure-sensitive adhesive copolymerized with 5 parts by weight of acrylic acid as a pressure-sensitive adhesive and 95 parts by weight of butyl acrylate as main components, and solid content was added. It was diluted with toluene so that the concentration became 10%. The pressure-sensitive adhesive was applied to a release paper made of a polyester film (thickness 25 μm) so as to have a thickness of 3 to 5 μm after drying, and then dried to form a pressure-sensitive adhesive layer.

A polystyrene film (made by Asahi Kasei Corp., trade name "OPS film" thickness 25
After the pressure-sensitive adhesive layer on the surface of the release paper was attached to one surface of (μm), the release paper was peeled off to transfer the pressure-sensitive adhesive layer. A silicone paper of polyethylene terephthalate film treated with silicone (manufactured by Toyo Metallizing Co., Ltd., product side: Serape BK, thickness 25 μm) is attached to the pressure-sensitive adhesive layer, and the polystyrene film of the present invention is used as a substrate. Obtained adhesive film and put it in a room at 23 ℃
It was left for a day and cured.

Comparative Example 1 The same coating solution as that used in Example 1 was directly applied to a polystyrene film.

Comparative Example 2 An adhesive film was obtained in the same manner as in Example 1 except that a biaxially stretched polyethylene terephthalate film (manufactured by Dia foil Co., trade name: S-100, thickness 25 μm) was used as a base film. I was cured under the same conditions.

Comparative Example 3 An adhesive film was obtained in the same manner as in Example 1 except that a biaxially oriented polypropylene film (manufactured by Toyobo Co., Ltd., trade name: Bylene film OT, thickness 25 μm) was used as the base film. I was cured under the same conditions.

Comparative Example 4 Cellophane as a base film (manufactured by Nimura Chemical Co., Ltd., trade name: Taiko cellophane, thickness 25 μm)
An adhesive film was obtained and cured under the same conditions as in Example 1 except that m) was used.

Performance Evaluation With respect to the films alone used in the above-mentioned Example 1 and Comparative Examples 1 to 3, edge tear resistance was measured as a substitute property of punching property, and water absorption was measured. (1) Measurement of edge tear resistance According to JIS C2318. (2) Measurement of water absorption rate According to ASTM D570. Table 1 shows the above results.

[0038]

[Table 1]

As is clear from Table 1, the pressure-sensitive adhesive film obtained in Example 1 has the lowest edge tear resistance, and therefore has excellent punching workability, low water absorption, and excellent dimensional stability. The thing of Comparative Example 1 cannot be used as an adhesive film because the polystyrene film is attacked by the solvent of the adhesive. Further, in Comparative Examples 2 and 3, the punching workability was poor,
It can be seen that Comparative Example 4 is inferior in dimensional stability.

(Embodiment 2) In this embodiment, ceramic I
The pressure-sensitive adhesive film of the present invention used as a pressure-sensitive adhesive tape for protecting the surface of the green sheet of the C package and the method for producing the same will be described.

As a base film, a biaxially stretched polystyrene film (Asahi Kasei Kogyo Co., Ltd., trade name "OPS-R", thickness 16 μm) is provided on one side with an acrylic resin back treatment agent (Sakata Inx Co., Ltd. trade name "XGS-215". )) Was applied by a gravure roll coater so that the weight after drying was 5 g / m ² . On the opposite side of the base film, 45 dyn /
Corona treatment was applied so as to be cm.

As an adhesive, 95 parts by weight of butyl acrylate, 4.5 parts by weight of acrylic acid, and 0.5 parts by weight of 2-hydroxyethyl methacrylate were used, and aromatic isocyanate (manufactured by Nippon Polyurethane Company, trade name "Coronate L") was used. 1) by weight was added to dilute with toluene so that the solid content concentration was 10%. As the release sheet layer, a polyethylene terephthalate film (manufactured by Toyo Metallizing Co., Ltd., trade name “Serapeal BK type”, thickness 25 μm) which has been subjected to release treatment by a conventional method is used, and the thickness of the adhesive after drying on the film. Was 4 μm, and dried sufficiently with a dryer at 100 ° C.

Next, the corona-treated surface of the substrate film and the pressure-sensitive adhesive layer surface of the release sheet having the pressure-sensitive adhesive layer formed thereon were bonded together by a laminator to obtain a pressure-sensitive adhesive tape. The structure of the obtained adhesive tape is shown in FIG.

(Embodiment 3) In this embodiment, ceramic I
The pressure-sensitive adhesive film of the present invention used as a pressure-sensitive adhesive tape for protecting the surface of the green sheet of the C package and the method for producing the same will be described.

As a base film, a biaxially stretched polystyrene film (Asahi Kasei Kogyo Co., Ltd., trade name "OPS-R", thickness 16 μm) is provided on one side with an acrylic resin back treatment agent (Sakata Inx Co., Ltd. trade name "XGS-215". )) Was applied by a gravure roll coater so that the weight after drying was 5 g / m ² . On the opposite side of the base film, 45 dyn /
Corona treatment was applied so as to be cm.

As an adhesive, 95 parts by weight of butyl acrylate, 4.5 parts by weight of acrylic acid, and 0.5 parts by weight of 2-hydroxyethyl methacrylate were used for aromatic isocyanate (manufactured by Nippon Polyurethane Co., trade name "Coronate L"). 1.2 parts by weight was added and diluted with toluene so that the solid content concentration was 10%.

As a release sheet layer, a polyethylene terephthalate film (manufactured by Toyo Metalizing Co., Ltd., trade name "Serapeal BK type", thickness 25 μm) subjected to a release treatment by a conventional method is used, and this adhesive is dried on the film. The coating was applied so that the thickness afterwards would be 4 μm, and was sufficiently dried by a dryer at 100 ° C. Next, the corona-treated surface of the substrate film and the pressure-sensitive adhesive layer surface of the release sheet having the pressure-sensitive adhesive layer formed thereon were bonded together by a laminator to obtain a pressure-sensitive adhesive tape. The structure of the obtained adhesive tape is shown in FIG.

Comparative Example 5 A pressure-sensitive adhesive tape was obtained in the same manner as in Example 2 except that the amount of aromatic isocyanate added in the pressure-sensitive adhesive was changed to 0.6 part by weight.

Comparative Example 6 A pressure-sensitive adhesive tape was obtained in the same manner as in Example 2 except that the amount of aromatic isocyanate added in the pressure-sensitive adhesive was changed to 1.4 parts by weight.

Performance Evaluation The gel fraction of the pressure-sensitive adhesive layer was measured for these three types of pressure-sensitive adhesive tapes, and the release sheet layer was adhered to the surface of the green sheet after peeling, and the following evaluation items were evaluated.

(1) Adhesive Residue in Through Hole A through hole punched with a punch having a diameter of 100 μm was visually inspected with a microscope. (Number of repetitions r = 20) (2) Adhesiveness After the adhesive tape was actually attached to the green sheet and left for 2 weeks under constant temperature and humidity (23 ° C, 65%), the end and hole parts were attached as a sticking condition. Visual inspection was performed for peeling and peeling. (Repeat number r = 20)

(3) Removability The surface state of the green sheet when the adhesive tape was actually attached to the green sheet and left for 2 weeks under constant temperature and humidity (23 ° C., 65%) and then the tape was peeled off by hand. Was visually inspected. (Repeat number r = 20) The above results are shown in Table 2.

[0053]

[Table 2]

As is clear from Table 2, the pressure-sensitive adhesive sheet in which the gel fraction of the pressure-sensitive adhesive layer is adjusted to 90 to 95% is excellent in adhesive residue in through holes, sticking property and removability. However, the pressure-sensitive adhesive sheet of Comparative Example 5 having a low gel fraction has a good sticking property, but the sticking agent adheres to the inner wall of the through hole due to the cohesive failure of the pressure-sensitive adhesive during the punching process, and the removability is also poor.

On the other hand, the pressure-sensitive adhesive sheet of Comparative Example 6 having a high gel fraction had good removability, but had poor sticking property to the green sheet, and the film and the pressure-sensitive adhesive layer were peeled off due to anchor destruction during punching. The punching residue of the adhesive layer occurs on the inner wall of the through hole.

[0056]

In the pressure-sensitive adhesive film of the present invention, since the acrylic pressure-sensitive adhesive layer formed in advance is laminated with the polystyrene-based film, when the polystyrene-based film is provided with the pressure-sensitive adhesive layer or the release layer, the polystyrene-based film is Since it is not affected by heat, the physical properties do not change. Further, since the plasticizer in the polystyrene film does not react with the curing agent (isocyanate) in the pressure-sensitive adhesive, the pressure-sensitive adhesive has excellent stability over time and does not deteriorate. or,
In the method of directly applying the adhesive to the base film, the curing period of the adhesive is required to be one week, whereas according to the transfer method of the present invention, the curing period of the adhesive is only one day, which is short. The sex is very good. Furthermore, when a polystyrene film is used as a base material, it can be punched with high precision and has excellent dimensional stability, so that it can be used in fields requiring high processing precision such as in the electronic equipment field. It can be suitably used as a surface protective film. Then, by adjusting the gel fraction of the acrylic pressure-sensitive adhesive layer to a specific range, it is possible to prevent the adhesion of glue in the through holes due to the destruction of the pressure-sensitive adhesive layer at the time of fine punching, and to prevent conduction defects after sintering. It is possible to obtain a pressure-sensitive adhesive film having excellent properties as a pressure-sensitive adhesive tape for protecting the surface of a green sheet of a ceramic IC package which can be finely punched.

[Brief description of drawings]

FIG. 1 is a diagram illustrating a basic configuration of an adhesive film of the present invention.

FIG. 2 is a diagram illustrating a constitution of an adhesive film of the present invention used as an adhesive tape for protecting a surface of a green sheet.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Makoto Miura 3535 Kurohama, Hasuda City, Saitama Sekisui Chemical Co., Ltd.

Claims (6)

[Claims] 1. A pressure-sensitive adhesive film obtained by laminating a pre-formed acrylic pressure-sensitive adhesive layer on one surface of a polystyrene-based film. 2. The pressure-sensitive adhesive film according to claim 1, further comprising a peelable sheet layer formed on the acrylic pressure-sensitive adhesive layer. 3. The gel fraction of the acrylic pressure-sensitive adhesive layer is 90.
It is -95%, The adhesive film of Claim 1 or 2 characterized by the above-mentioned. 4. A method for producing an adhesive film, which comprises laminating an acrylic adhesive layer formed in advance on one surface of a polystyrene film. 5. An acrylic pressure-sensitive adhesive layer formed on a release paper surface is transferred to one side of a polystyrene film, and a release-treated surface of a release sheet having a surface subjected to a release treatment is attached to the acrylic pressure-sensitive adhesive layer surface. The method for producing an adhesive film according to claim 4, wherein the adhesive film is combined. 6. The method for producing an adhesive film according to claim 4, wherein an acrylic adhesive layer is formed on one surface of the peelable sheet, and the adhesive layer and the film substrate are bonded together.