JP7668136B2 - Adhesive sheet, repeatedly bent laminated member, and repeatedly bent device - Google Patents

Description

The present invention relates to an adhesive sheet for a device that is repeatedly bent, as well as a repeatedly bent laminate member and a repeatedly bent device.

In recent years, bendable displays have been proposed as displays for electronic devices, which are a type of device. In addition to displays that are curved only once, repeatedly bent displays that are intended to be repeatedly bent (folded) have also been proposed.

In the above-mentioned repeatedly bendable display, it is conceivable that one bendable member (flexible member) constituting the flexible display and another flexible member are bonded together by the adhesive layer of an adhesive sheet. However, when a conventional adhesive sheet is used in a repeatedly bendable display, a problem occurs in that peeling occurs at the interface between the adhesive layer and the adherend in the bent portion.

Patent Document 1 discloses an adhesive that aims to prevent peeling during repeated bending, but the effect is not necessarily sufficient.

JP 2016-108555 A

The present invention has been made in consideration of the above-mentioned circumstances, and aims to provide an adhesive sheet that can prevent peeling from occurring at the interface between the adhesive layer and the adherend even when applied to a repeatedly flexed device and flexed for a long period of time, as well as a repeatedly flexed laminate member and a repeatedly flexed device that can prevent peeling from occurring at the interface between the adhesive layer and the adherend even when flexed for a long period of time.

To achieve the above object, first, the present invention provides an adhesive sheet having an adhesive layer for bonding one flexible member and another flexible member that constitute a device that is repeatedly bent, the adhesive layer being formed by laminating a first adhesive layer and a second adhesive layer, the adhesive strength of the second adhesive layer side at 23°C being relatively greater than the adhesive strength of the first adhesive layer side, the storage modulus G'(-20) of the adhesive of the adhesive layer as a whole at -20°C being 0.1 MPa or less, and the adhesive sheet being bent so that the first adhesive layer is on the outside and the second adhesive layer is on the inside (Invention 1).

In the above invention (Invention 1), by having the above-mentioned configuration and physical properties, when a laminate formed by bonding one flexible member to another flexible member with the above-mentioned adhesive layer is left in a bent state for a long period of time with the first adhesive layer on the outside and the second adhesive layer on the inside, peeling at the interface between the adhesive layer and the adherend at the bent portion is suppressed, and excellent flex resistance is obtained.

In the above invention (Invention 1), it is preferable that the storage modulus G'(-20) at -20°C of the adhesive constituting the first adhesive layer is 0.040 MPa or more and 0.1 MPa or less (Invention 2).

In the above inventions (Inventions 1 and 2), it is preferable that the storage modulus G'(-20) at -20°C of the adhesive constituting the second adhesive layer is 0.100 MPa or more and 0.500 MPa or less (Invention 3).

In the above inventions (Inventions 1 to 3), it is preferable that the adhesive strength of the second adhesive layer to the triacetyl cellulose film at 23°C is 1 N/25 mm or more and 30 N/25 mm or less (Invention 4).

In the above inventions (Inventions 1 to 4), it is preferable that the gel fraction of the adhesive in the entire adhesive layer is 40% or more and 99% or less (Invention 5).

In the above inventions (Inventions 1 to 5), it is preferable that the adhesive constituting the first adhesive layer and the adhesive constituting the second adhesive layer are acrylic adhesives (Invention 6).

In the above inventions (Inventions 1 to 6), it is preferable that the adhesive sheet has two release sheets, and the adhesive layer is sandwiched between the release sheets so as to be in contact with the release surfaces of the two release sheets (Invention 7).

Secondly, the present invention provides a repeatedly bent laminated member comprising one flexible member and another flexible member that constitute a device that is repeatedly bent, and an adhesive layer that bonds the one flexible member and the other flexible member to each other, characterized in that the adhesive layer is made of the adhesive layer of the adhesive sheet (Inventions 1 to 7) (Invention 8).

Secondly, the present invention provides a repeatedly bending device (Invention 9) characterized by comprising the above-mentioned bent laminated member (Invention 8).

The adhesive sheet according to the present invention can prevent peeling from occurring at the interface between the adhesive layer and the adherend, even when applied to a repeatedly bent device and bent for a long period of time. Furthermore, the repeatedly bent laminate member and repeatedly bent device according to the present invention can prevent peeling from occurring at the interface between the adhesive layer and the adherend, even when bent for a long period of time.

1 is a cross-sectional view of a pressure-sensitive adhesive sheet according to one embodiment of the present invention. FIG. 2 is a cross-sectional view of a repeatedly bent laminated member according to one embodiment of the present invention. 1 is a cross-sectional view of a repeated bending device according to an embodiment of the present invention. FIG. 1 is an explanatory diagram (side view) illustrating a static bending test.

Hereinafter, an embodiment of the present invention will be described.
[Adhesive sheet]
The pressure-sensitive adhesive sheet according to one embodiment of the present invention has a pressure-sensitive adhesive layer for bonding one flexible member and another flexible member that constitute a repeatedly flexing device, and is preferably formed by laminating a release sheet on one or both sides of the pressure-sensitive adhesive layer. The repeatedly flexing device and the flexible member will be described later.

The adhesive layer is formed by laminating a first adhesive layer and a second adhesive layer. When the adhesive sheet according to this embodiment is applied to a repeatedly bent device, it is bent so that the first adhesive layer is on the outside and the second adhesive layer is on the inside. The adhesive strength of the second adhesive layer side of the adhesive sheet according to this embodiment at 23°C is preferably relatively greater than the adhesive strength of the first adhesive layer side. In addition, in the adhesive sheet according to this embodiment, it is preferable that the storage modulus G'(-20) of the adhesive of the adhesive layer as a whole at -20°C is 0.1 MPa or less. The method for measuring the storage modulus in this specification is as shown in the test example described later.

In the adhesive sheet according to the present embodiment, the adhesive layer as a whole has a low storage modulus G' (-20) at -20°C, so that the strain generated by repeated bending, i.e., stress (which is particularly large at low temperatures), can be suppressed to a small value, and the adhesive sheet exhibits a property of being easily bent and easily following the bending of the adherend. On the other hand, if the storage modulus is merely lowered, the adhesion to the adherend tends to decrease. In particular, if the adhesion to the adherend located on the inside of the bend decreases, peeling is likely to occur at the interface between the adhesive layer and the adherend. Therefore, in the adhesive sheet according to the present embodiment, the adhesive layer has a structure in which a first adhesive layer located on the outside of the bend and a second adhesive layer located on the inside of the bend are laminated, and the adhesive strength of the second adhesive layer side is relatively greater than that of the first adhesive layer side. This makes it possible to lower the storage modulus G' (-20) of the entire adhesive layer, while increasing the adhesive strength of the adhesive layer (second adhesive layer) on the side in contact with the adherend on the side where peeling is likely to occur due to bending. As a result, when a laminate formed by bonding one flexible member to another flexible member with the adhesive layer of this embodiment is left in a bent state for a long period of time, peeling is suppressed at the interface between the adhesive layer and the adherend at the bent portion, and excellent bending resistance is obtained. In particular, excellent bending resistance is exhibited even when the laminate is left in a bent state for a long period of time at low temperatures (e.g., -20°C) or under durable conditions (e.g., 80°C, dry).

In order to make the adhesive strength of the second adhesive layer side at 23°C relatively greater than that of the first adhesive layer side, for example, the storage modulus G'(-20) of the adhesive constituting the second adhesive layer can be made higher than the storage modulus G'(-20) of the adhesive constituting the first adhesive layer, within the range in which the storage modulus G'(-20) of the entire adhesive layer can be maintained at the above value. In order to make the storage modulus G'(-20) of the adhesive constituting the second adhesive layer higher than the storage modulus G'(-20) of the adhesive constituting the first adhesive layer, for example, the glass transition temperature (Tg) of the main polymer (particularly (meth)acrylic acid ester polymer) of the adhesive constituting the second adhesive layer can be made higher than the glass transition temperature (Tg) of the main polymer (particularly (meth)acrylic acid ester polymer) of the adhesive constituting the first adhesive layer. However, this is not limited to this.

From the viewpoint of the above-mentioned flex resistance, the storage modulus G'(-20) of the adhesive as the entire adhesive layer at -20°C is preferably 0.1 MPa or less, more preferably 0.090 MPa or less, particularly preferably 0.085 MPa or less, and even more preferably 0.080 MPa or less. From the viewpoint of cohesive strength, the storage modulus G'(-20) of the adhesive as the entire adhesive layer is preferably 0.040 MPa or more, more preferably 0.050 MPa or more, particularly preferably 0.060 MPa or more, and even more preferably 0.070 MPa or more.

In order to set the storage modulus G'(-20) of the adhesive as a whole to the above value, the storage modulus G'(-20) of the adhesive constituting the first adhesive layer is preferably 0.1 MPa or less, more preferably 0.090 MPa or less, particularly preferably 0.085 MPa or less, and even more preferably 0.080 MPa or less. From the viewpoint of cohesive strength, the storage modulus G'(-20) of the adhesive constituting the first adhesive layer is preferably 0.040 MPa or more, more preferably 0.050 MPa or more, particularly preferably 0.060 MPa or more, and even more preferably 0.070 MPa or more.

On the other hand, in order to make the adhesive strength of the second adhesive layer side at 23°C relatively greater than that of the first adhesive layer side, the storage modulus G'(-20) of the adhesive constituting the second adhesive layer is preferably 0.100 MPa or more, more preferably 0.105 MPa or more, particularly preferably 0.110 MPa or more, and even more preferably 0.115 MPa or more. In order to set the storage modulus G'(-20) of the adhesive of the entire adhesive layer to the above value, the storage modulus G'(-20) of the adhesive constituting the second adhesive layer is preferably 0.500 MPa or less, more preferably 0.300 MPa or less, particularly preferably 0.200 MPa or less, and even more preferably 0.140 MPa or less.

From the viewpoint of the above-mentioned flex resistance, the storage modulus G'(25) of the adhesive as a whole adhesive layer at 25°C is preferably 0.080 MPa or less, more preferably 0.060 MPa or less, particularly preferably 0.040 MPa or less, and even more preferably 0.030 MPa or less. From the viewpoint of cohesive strength, the storage modulus G'(25) of the adhesive as a whole adhesive layer is preferably 0.010 MPa or more, more preferably 0.014 MPa or more, particularly preferably 0.018 MPa or more, and even more preferably 0.020 MPa or more.

In order to set the storage modulus G'(25) of the adhesive as a whole to the above value, the storage modulus G'(25) of the adhesive constituting the first adhesive layer is preferably 0.080 MPa or less, more preferably 0.060 MPa or less, particularly preferably 0.040 MPa or less, and even more preferably 0.025 MPa or less. From the viewpoint of cohesive strength, the storage modulus G'(25) of the adhesive constituting the first adhesive layer is preferably 0.010 MPa or more, more preferably 0.012 MPa or more, particularly preferably 0.016 MPa or more, and even more preferably 0.018 MPa or more.

From the viewpoint of exerting adhesive strength on the second adhesive layer side, the storage modulus G'(25) of the adhesive constituting the second adhesive layer is preferably 0.200 MPa or less, more preferably 0.100 MPa or less, particularly preferably 0.080 MPa or less, and even more preferably 0.050 MPa or less. From the viewpoint of cohesive strength, the storage modulus G'(25) of the adhesive constituting the second adhesive layer is preferably 0.012 MPa or more, more preferably 0.020 MPa or more, particularly preferably 0.031 MPa or more, and even more preferably 0.036 MPa or more.

The storage modulus G'(85) of the adhesive as a whole at 85°C is preferably 0.060 MPa or less, more preferably 0.040 MPa or less, particularly preferably 0.020 MPa or less, and even more preferably 0.018 MPa or less, from the viewpoint of preventing the storage modulus G' at -20°C from becoming too high. Furthermore, the storage modulus G'(85) of the adhesive as a whole at the adhesive layer is preferably 0.005 MPa or more, more preferably 0.008 MPa or more, particularly preferably 0.010 MPa or more, and even more preferably 0.012 MPa or more. This results in excellent flex resistance at high temperatures.

In order to set the storage modulus G'(85) of the adhesive as a whole to the above value, the storage modulus G'(85) of the adhesive constituting the first adhesive layer is preferably 0.060 MPa or less, more preferably 0.030 MPa or less, particularly preferably 0.018 MPa or less, and even more preferably 0.012 MPa or less. From the viewpoint of cohesive strength, the storage modulus G'(85) of the adhesive constituting the first adhesive layer is preferably 0.002 MPa or more, more preferably 0.004 MPa or more, particularly preferably 0.006 MPa or more, and even more preferably 0.008 MPa or more.

From the viewpoint of preventing the storage modulus G' of the second adhesive layer from becoming too high at low temperatures, the storage modulus G'(85) of the adhesive constituting the second adhesive layer is preferably 0.100 MPa or less, more preferably 0.080 MPa or less, particularly preferably 0.040 MPa or less, and even more preferably 0.030 MPa or less. From the viewpoint of bending resistance at high temperatures, the storage modulus G'(85) of the adhesive constituting the second adhesive layer is preferably 0.006 MPa or more, more preferably 0.012 MPa or more, particularly preferably 0.019 MPa or more, and even more preferably 0.021 MPa or more.

The gel fraction of the adhesive in the entire adhesive layer is preferably 40% or more, more preferably 50% or more, particularly preferably 60% or more, and even more preferably 70% or more. The gel fraction of the adhesive in the entire adhesive layer is preferably 99% or less, more preferably 95% or less, particularly preferably 90% or less, even more preferably 85% or less, and most preferably 80% or less. This makes it easier for the adhesive in the entire adhesive layer to satisfy the above-mentioned physical properties. The method for measuring the gel fraction in this specification is as shown in the test examples described later.

FIG. 1 shows a specific configuration of one example of the pressure-sensitive adhesive sheet according to this embodiment.
As shown in Fig. 1, the adhesive sheet 1 is composed of two release sheets 12a and 12b, and an adhesive layer 11 sandwiched between the two release sheets 12a and 12b so as to be in contact with the release surfaces of the two release sheets 12a and 12b. In this specification, the release surface of the release sheet refers to the surface of the release sheet that has releasability, and includes both a surface that has been subjected to a release treatment and a surface that exhibits releasability even without being subjected to a release treatment.

In this embodiment, the adhesive layer 11 is a laminate of one first adhesive layer 111 and one second adhesive layer 112. However, the present invention is not limited to this.

1. Component 1-1. Adhesive Layer When the adhesive sheet 1 according to this embodiment is applied to a repeatedly bent device, it is bent so that the first adhesive layer 111 is on the outside and the second adhesive layer 112 is on the inside. The adhesive strength of the second adhesive layer 112 side of the adhesive sheet 1 according to this embodiment at 23° C. is relatively greater than the adhesive strength of the first adhesive layer 111 side.

The adhesive constituting the first adhesive layer 111 and the adhesive constituting the second adhesive layer 112 are not particularly limited as long as they satisfy the above-mentioned physical properties, and may be, for example, any of acrylic adhesives, polyester adhesives, polyurethane adhesives, rubber adhesives, silicone adhesives, etc. The adhesive may be any of emulsion type, solvent type, or solventless type, and may be any of cross-linked type or non-cross-linked type. Among them, acrylic adhesives are preferred because they easily satisfy the above-mentioned physical properties and have excellent adhesive properties and optical properties, and solvent-type acrylic adhesives are particularly preferred. The acrylic adhesive is preferably a cross-linked type, and more preferably a thermally cross-linked type.

The adhesive constituting the first adhesive layer 111 and the adhesive constituting the second adhesive layer 112 may be the same type or different types, but both are preferably acrylic adhesives, particularly solvent-based acrylic adhesives, and even more preferably thermally crosslinked acrylic adhesives. On the other hand, it is preferable that the adhesive constituting the first adhesive layer 111 and the adhesive constituting the second adhesive layer 112 have different monomer compositions of the main polymer ((meth)acrylic acid ester polymer) in the acrylic adhesive, thereby expressing the above-mentioned difference in physical properties.

The following describes a case where the adhesive constituting the first adhesive layer 111 and the adhesive constituting the second adhesive layer 112 are both acrylic adhesives, but the present invention is not limited to this.

The adhesive constituting the first adhesive layer 111 and the adhesive constituting the second adhesive layer 112 are preferably adhesives obtained from an adhesive composition containing a (meth)acrylic acid ester polymer (A), and are particularly preferably adhesives obtained by crosslinking an adhesive composition containing a (meth)acrylic acid ester polymer (A) and a crosslinking agent (B) (hereinafter sometimes referred to as "adhesive composition P"). Such adhesives are likely to satisfy the above-mentioned physical properties and are likely to provide good adhesive strength. In this specification, (meth)acrylic acid means both acrylic acid and methacrylic acid. The same applies to other similar terms. In addition, the concept of "copolymer" is also included in "polymer".

(1) Component of Pressure-Sensitive Adhesive Composition P (1-1) (Meth)acrylic Acid Ester Polymer (A)
The (meth)acrylic acid ester polymer (A) preferably contains, as monomer units constituting the polymer, an alkyl (meth)acrylic acid ester and a monomer having a reactive functional group in the molecule (reactive functional group-containing monomer).

The (meth)acrylic acid ester polymer (A) can exhibit favorable adhesion by containing an alkyl (meth)acrylic acid ester as a monomer unit constituting the polymer. As the alkyl (meth)acrylic acid ester, an alkyl (meth)acrylic acid ester having 1 to 20 carbon atoms is preferable. The alkyl group may be linear or branched, or may have a cyclic structure.

Examples of (meth)acrylic acid alkyl esters having an alkyl group with 1 to 20 carbon atoms include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, n-butyl (meth)acrylate, n-pentyl (meth)acrylate, n-hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, isooctyl (meth)acrylate, n-decyl (meth)acrylate, n-dodecyl (meth)acrylate, myristyl (meth)acrylate, palmityl (meth)acrylate, and stearyl (meth)acrylate. These may be used alone or in combination of two or more.

In the adhesive constituting the first adhesive layer 111, from the viewpoint of setting the storage modulus G' (-20) of the adhesive as a whole to the above value, it is preferable to use a monomer having a considerably low glass transition temperature (Tg) among the above monomers. Specifically, it is preferable to use a monomer having a glass transition temperature (Tg) of -55°C or less, more preferably a monomer having a glass transition temperature (Tg) of -60°C or less, particularly preferably a monomer having a glass transition temperature (Tg) of -65°C or less, and even more preferably a monomer having a glass transition temperature (Tg) of -70°C or less. Examples of monomers having such a low glass transition temperature (Tg) include 2-ethylhexyl acrylate (Tg: -70°C), ethyl carbitol acrylate (Tg: -67°C), etc., and among these, 2-ethylhexyl acrylate is preferable. The lower limit of the above glass transition temperature (Tg) is not particularly limited, but is about -100°C.

In the adhesive constituting the second adhesive layer 112, from the viewpoint of setting the storage modulus G' (-20) of the adhesive as a whole to the above value and making the adhesive strength of the second adhesive layer side at 23°C relatively greater than that of the first adhesive layer side, it is preferable to use, among the above monomers, a monomer having a fairly low glass transition temperature (Tg) (hereinafter sometimes referred to as "extremely low Tg monomer") in combination with a monomer having a relatively low glass transition temperature (Tg) and capable of exhibiting high adhesiveness (hereinafter sometimes referred to as "low Tg monomer"). The extremely low Tg monomer is as described above as a monomer having a fairly low glass transition temperature (Tg). As the low Tg monomer, it is preferable to use a monomer having a glass transition temperature (Tg) of -10 to -65°C, more preferably a monomer having a glass transition temperature (Tg) of -20 to -60°C, particularly preferably a monomer having a glass transition temperature (Tg) of -25 to -55°C, and even more preferably a monomer having a glass transition temperature (Tg) of -30 to -55°C. Examples of low Tg monomers include n-butyl acrylate (Tg: -54°C), methoxytriethylene glycol acrylate (Tg: -57°C), lauryl acrylate (Tg: -27°C), etc., of which n-butyl acrylate is preferred.

The compounding ratio (by mass) of the very low Tg monomer to the low Tg monomer is preferably 90:10 to 10:90, more preferably 80:20 to 20:80, particularly preferably 70:30 to 30:70, and even more preferably 60:40 to 40:60. This allows for an optimal balance between the storage modulus G' and adhesive strength of the resulting adhesive layer.

The (meth)acrylic acid ester polymer (A) preferably contains 60% by mass or more, more preferably 80% by mass or more, particularly preferably 90% by mass or more, and even more preferably 96% by mass or more of (meth)acrylic acid alkyl esters having an alkyl group with 1 to 20 carbon atoms as monomer units constituting the polymer. If the (meth)acrylic acid alkyl ester is contained in the amount above, it is easy to adjust the storage modulus G' to a low value while imparting a predetermined adhesiveness to the (meth)acrylic acid ester polymer (A). In addition, it is preferable to contain 99.9% by mass or less of (meth)acrylic acid alkyl esters having an alkyl group with 1 to 20 carbon atoms, particularly preferably 99.5% by mass or less, and even more preferably 99.0% by mass or less. By containing the (meth)acrylic acid alkyl ester in the amount above or less, it is possible to introduce a desired amount of other monomer components into the (meth)acrylic acid ester polymer (A).

The (meth)acrylic acid ester polymer (A) contains a reactive functional group-containing monomer as a monomer unit constituting the polymer, and reacts with the crosslinking agent (B) described below via the reactive functional group derived from the reactive functional group-containing monomer, forming a crosslinked structure (three-dimensional network structure) and obtaining an adhesive with the desired cohesive strength. The adhesive is likely to satisfy the physical properties (and gel fraction) related to the storage modulus G' described above.

Preferable examples of reactive functional group-containing monomers contained in the (meth)acrylic acid ester polymer (A) as monomer units constituting the polymer include monomers having a hydroxyl group in the molecule (hydroxyl group-containing monomers), monomers having a carboxyl group in the molecule (carboxyl group-containing monomers), and monomers having an amino group in the molecule (amino group-containing monomers). These reactive functional group-containing monomers may be used alone or in combination of two or more.

Among the reactive functional group-containing monomers, hydroxyl group-containing monomers or carboxyl group-containing monomers are preferred, and hydroxyl group-containing monomers are particularly preferred. This makes it easier to satisfy the physical properties related to the storage modulus G' described above.

Examples of hydroxyl group-containing monomers include (meth)acrylic acid hydroxyalkyl esters such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 3-hydroxybutyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate. Among the above, from the viewpoint of ease of satisfying the physical property related to the storage modulus G' described above, (meth)acrylic acid hydroxyalkyl esters having a hydroxyalkyl group having 1 to 4 carbon atoms are preferred. Specifically, for example, 2-hydroxyethyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, and the like are preferred, and in particular, 2-hydroxyethyl acrylate or 4-hydroxybutyl acrylate are preferred. These may be used alone or in combination of two or more.

Examples of carboxyl group-containing monomers include ethylenically unsaturated carboxylic acids such as acrylic acid, methacrylic acid, crotonic acid, maleic acid, itaconic acid, and citraconic acid. Among these, acrylic acid is preferred from the viewpoint of the adhesive strength of the resulting (meth)acrylic acid ester polymer (A). These may be used alone or in combination of two or more.

The (meth)acrylic acid ester polymer (A) preferably contains 0.1% by mass or more of reactive functional group-containing monomer as a monomer unit constituting the polymer, more preferably 0.4% by mass or more, and even more preferably 0.8% by mass or more. The (meth)acrylic acid ester polymer (A) preferably contains 10% by mass or less of reactive functional group-containing monomer as a monomer unit constituting the polymer, more preferably 6% by mass or less, even more preferably 3% by mass or less, even more preferably 2% by mass or less, and most preferably 1.4% by mass or less.

When the (meth)acrylic acid ester polymer (A) contains the reactive functional group-containing monomer as a monomer unit in the above amount, the crosslinking reaction with the crosslinking agent (B) gives the resulting adhesive an appropriate cohesive strength, making it easier to satisfy the physical properties related to the storage modulus G' (and gel fraction) described above, particularly the storage modulus G' at 80°C (80).

It is also preferable that the (meth)acrylic acid ester polymer (A) does not contain a carboxyl group-containing monomer as a monomer unit constituting the polymer. Since a carboxyl group is an acid component, by not containing a carboxyl group-containing monomer, even if the object to which the pressure-sensitive adhesive is applied contains an object that may be damaged by acid, such as a transparent conductive film such as tin-doped indium oxide (ITO), a metal film, or a metal mesh, the damage caused by the acid (corrosion, change in resistance, etc.) can be suppressed.

Here, "free of carboxyl group-containing monomers" means that the polymer does not substantially contain carboxyl group-containing monomers, and in addition to not containing any carboxyl group-containing monomers at all, it is acceptable to contain carboxyl group-containing monomers to the extent that the carboxyl group does not cause corrosion of the transparent conductive film, metal wiring, etc. Specifically, it is acceptable to contain carboxyl group-containing monomers as monomer units in the (meth)acrylic acid ester polymer (A) in an amount of 0.1% by mass or less, preferably 0.01% by mass or less, and more preferably 0.001% by mass or less.

If desired, the (meth)acrylic acid ester polymer (A) may contain other monomers as monomer units constituting the polymer. As the other monomer, a monomer that does not contain a reactive functional group is preferable so as not to inhibit the above-mentioned action of the reactive functional group-containing monomer. Examples of such monomers include non-reactive nitrogen atom-containing monomers such as N-acryloylmorpholine and N-vinyl-2-pyrrolidone, (meth)acrylic acid alkoxyalkyl esters such as methoxyethyl (meth)acrylate and ethoxyethyl (meth)acrylate, vinyl acetate, and styrene. These may be used alone or in combination of two or more.

The polymerization mode of the (meth)acrylic acid ester polymer (A) may be a random copolymer or a block copolymer.

The weight average molecular weight of the (meth)acrylic acid ester polymer (A) is preferably 600,000 or more, more preferably 800,000 or more, particularly preferably 1,000,000 or more, and even more preferably 1,200,000 or more. The weight average molecular weight of the (meth)acrylic acid ester polymer (A) is preferably 2,500,000 or less, more preferably 2,000,000 or less, particularly preferably 1,700,000 or less, and even more preferably 1,400,000 or less. When the weight average molecular weight of the (meth)acrylic acid ester polymer (A) is within the above range, it is easy to satisfy the physical properties (and gel fraction) related to the storage modulus G' described above, particularly the storage modulus G'(25) at 25°C and the storage modulus G'(85) at 85°C. The weight average molecular weight in this specification is a value measured by gel permeation chromatography (GPC) in terms of standard polystyrene.

In the adhesive composition P, the (meth)acrylic acid ester polymer (A) may be used alone or in combination of two or more.

(1-2) Crosslinking agent (B)
The crosslinking agent (B) crosslinks the (meth)acrylic acid ester polymer (A) and forms a three-dimensional network structure when the pressure-sensitive adhesive composition P containing the crosslinking agent (B) is heated or the like, which is a trigger. This improves the cohesive strength of the resulting pressure-sensitive adhesive, and makes it easier to satisfy the above-mentioned physical properties related to the storage modulus G' and the gel fraction.

The crosslinking agent (B) may be any that reacts with the reactive group of the (meth)acrylic acid ester polymer (A), and examples thereof include isocyanate-based crosslinking agents, epoxy-based crosslinking agents, amine-based crosslinking agents, melamine-based crosslinking agents, aziridine-based crosslinking agents, hydrazine-based crosslinking agents, aldehyde-based crosslinking agents, oxazoline-based crosslinking agents, metal alkoxide-based crosslinking agents, metal chelate-based crosslinking agents, metal salt-based crosslinking agents, and ammonium salt-based crosslinking agents. Among the above, it is preferable to use an isocyanate-based crosslinking agent that has excellent reactivity with the reactive functional group-containing monomer. The crosslinking agent (B) may be used alone or in combination of two or more.

The isocyanate-based crosslinking agent contains at least a polyisocyanate compound. Examples of polyisocyanate compounds include aromatic polyisocyanates such as tolylene diisocyanate, diphenylmethane diisocyanate, and xylylene diisocyanate, aliphatic polyisocyanates such as hexamethylene diisocyanate, alicyclic polyisocyanates such as isophorone diisocyanate and hydrogenated diphenylmethane diisocyanate, and their biurets, isocyanurates, and adducts which are reactants with low-molecular active hydrogen-containing compounds such as ethylene glycol, propylene glycol, neopentyl glycol, trimethylolpropane, and castor oil. Among these, trimethylolpropane-modified aromatic polyisocyanates, particularly trimethylolpropane-modified tolylene diisocyanate or trimethylolpropane-modified xylylene diisocyanate, are preferred from the viewpoint of reactivity with hydroxyl groups and carboxyl groups.

The content of the crosslinking agent (B) in the adhesive composition P is preferably 0.01 parts by mass or more, more preferably 0.06 parts by mass or more, and even more preferably 0.12 parts by mass or more, based on 100 parts by mass of the (meth)acrylic acid ester polymer (A). The content is preferably 5 parts by mass or less, more preferably 2 parts by mass or less, more preferably 1 part by mass or less, and even more preferably 0.4 parts by mass or less. When the content of the crosslinking agent (B) is within the above range, the physical properties related to the storage modulus G' and the gel fraction described above are easily satisfied. The content of the crosslinking agent in the first adhesive layer 111 is preferably higher than the content of the crosslinking agent in the second adhesive layer 112.

(1-3) Various Additives Various additives that are commonly used in acrylic adhesives, such as a silane coupling agent, an ultraviolet absorber, an antistatic agent, a tackifier, an antioxidant, a light stabilizer, a softener, a filler, a refractive index adjuster, etc., may be added to the adhesive composition P as desired. Note that polymerization solvents and dilution solvents described below are not included in the additives that constitute the adhesive composition P.

It is preferable that the adhesive composition P contains the above-mentioned silane coupling agent. This improves the adhesion of the resulting adhesive layer to the flexible member that is the adherend, resulting in more preferable adhesive strength.

As the silane coupling agent, an organosilicon compound having at least one alkoxysilyl group in the molecule, which has good compatibility with the (meth)acrylic acid ester polymer (A) and has optical transparency, is preferred.

Examples of such silane coupling agents include polymerizable unsaturated group-containing silicon compounds such as vinyltrimethoxysilane, vinyltriethoxysilane, and methacryloxypropyltrimethoxysilane; silicon compounds having an epoxy structure such as 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropylmethyldimethoxysilane, and 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane; mercaptopropyltrimethoxysilane, 3-mercaptopropyltriethoxysilane, and 3-mercaptopropyldimethoxymethylsilane; Examples of such compounds include capryl group-containing silicon compounds, amino group-containing silicon compounds such as 3-aminopropyltrimethoxysilane, N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, and N-(2-aminoethyl)-3-aminopropylmethyldimethoxysilane, 3-chloropropyltrimethoxysilane, and 3-isocyanatepropyltriethoxysilane, as well as condensates of at least one of these compounds with alkyl group-containing silicon compounds such as methyltriethoxysilane, ethyltriethoxysilane, methyltrimethoxysilane, and ethyltrimethoxysilane. These compounds may be used alone or in combination of two or more.

The content of the silane coupling agent in the adhesive composition P is preferably 0.01 parts by mass or more, particularly preferably 0.05 parts by mass or more, and even more preferably 0.1 parts by mass or more, per 100 parts by mass of the (meth)acrylic acid ester polymer (A). The content is also preferably 1 part by mass or less, particularly preferably 0.5 parts by mass or less, and even more preferably 0.3 parts by mass or less. When the content of the silane coupling agent is within the above range, the adhesive layer obtained has improved adhesion to the flexible member that is the adherend, and has greater adhesive strength.

(2) Production of adhesive composition P The adhesive composition P can be produced by producing a (meth)acrylic acid ester polymer (A), mixing the obtained (meth)acrylic acid ester polymer (A) with a crosslinking agent (B), and adding additives as desired.

The (meth)acrylic acid ester polymer (A) can be produced by polymerizing a mixture of monomers constituting the polymer by a normal radical polymerization method. The polymerization of the (meth)acrylic acid ester polymer (A) is preferably carried out by a solution polymerization method using a polymerization initiator as desired. By polymerizing the (meth)acrylic acid ester polymer (A) by a solution polymerization method, it becomes easy to increase the molecular weight of the obtained polymer and adjust the molecular weight distribution, and it becomes possible to reduce the production of low molecular weight substances. Therefore, even if the gel fraction is relatively small and the degree of crosslinking is relaxed, the adhesive is unlikely to become biased due to repeated bending, and an adhesive with excellent bending resistance is easily obtained. However, the present invention is not limited to this, and the polymerization may be carried out without a solvent.

Polymerization solvents used in the solution polymerization method include, for example, ethyl acetate, n-butyl acetate, isobutyl acetate, toluene, acetone, hexane, methyl ethyl ketone, etc., and two or more types may be used in combination.

Examples of polymerization initiators include azo compounds and organic peroxides, and two or more of them may be used in combination. Examples of azo compounds include 2,2'-azobisisobutyronitrile, 2,2'-azobis(2-methylbutyronitrile), 1,1'-azobis(cyclohexane 1-carbonitrile), 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobis(2,4-dimethyl-4-methoxyvaleronitrile), dimethyl 2,2'-azobis(2-methylpropionate), 4,4'-azobis(4-cyanovaleric acid), 2,2'-azobis(2-hydroxymethylpropionitrile), and 2,2'-azobis[2-(2-imidazolin-2-yl)propane].

Examples of organic peroxides include benzoyl peroxide, t-butyl perbenzoate, cumene hydroperoxide, diisopropyl peroxydicarbonate, di-n-propyl peroxydicarbonate, di(2-ethoxyethyl)peroxydicarbonate, t-butyl peroxyneodecanoate, t-butyl peroxypivalate, (3,5,5-trimethylhexanoyl)peroxide, dipropionyl peroxide, and diacetyl peroxide.

In addition, in the above polymerization process, the weight average molecular weight of the resulting polymer can be adjusted by adding a chain transfer agent such as 2-mercaptoethanol.

Once the (meth)acrylic acid ester polymer (A) is obtained, the crosslinking agent (B) and, if desired, additives and a dilution solvent are added to the solution of the (meth)acrylic acid ester polymer (A) and thoroughly mixed to obtain a solvent-diluted adhesive composition P (coating solution).

When any of the above components is in a solid state, or when precipitation occurs when the component is mixed with other components in an undiluted state, the component may be dissolved or diluted in a dilution solvent before mixing with the other components.

Examples of the dilution solvent include aliphatic hydrocarbons such as hexane, heptane, and cyclohexane; aromatic hydrocarbons such as toluene and xylene; halogenated hydrocarbons such as methylene chloride and ethylene chloride; alcohols such as methanol, ethanol, propanol, butanol, and 1-methoxy-2-propanol; ketones such as acetone, methyl ethyl ketone, 2-pentanone, isophorone, and cyclohexanone; esters such as ethyl acetate and butyl acetate; and cellosolve-based solvents such as ethyl cellosolve.

The concentration and viscosity of the coating solution thus prepared are not particularly limited as long as they are within the range that allows coating, and can be appropriately selected depending on the situation. For example, the adhesive composition P is diluted so that the concentration is 10 to 60 mass %. Note that the addition of a dilution solvent or the like is not a necessary condition for obtaining the coating solution, and if the adhesive composition P has a viscosity that allows coating, it is not necessary to add a dilution solvent. In this case, the adhesive composition P becomes a coating solution in which the polymerization solvent for the (meth)acrylic acid ester polymer (A) itself serves as the dilution solvent.

(3) Manufacturing of adhesive In this embodiment, the adhesive constituting the first adhesive layer 111 and the adhesive constituting the second adhesive layer 112 are preferably each formed by crosslinking the adhesive composition P. The crosslinking of the adhesive composition P can usually be performed by a heat treatment. This heat treatment can also serve as a drying treatment for volatilizing a diluting solvent and the like from a coating film of the adhesive composition P applied to a desired object.

The heating temperature for the heat treatment is preferably 50 to 150°C, and more preferably 70 to 120°C. The heating time is preferably 10 seconds to 10 minutes, and more preferably 50 seconds to 2 minutes.

After the heat treatment, a curing period of about 1 to 2 weeks may be provided at room temperature (e.g., 23°C, 50% RH) if necessary. If this curing period is required, the adhesive will be formed after the curing period has elapsed. If no curing period is required, the adhesive will be formed after the heat treatment has been completed.

By the above heat treatment (and curing), the (meth)acrylic acid ester polymer (A) is sufficiently crosslinked via the crosslinking agent (B) to form a crosslinked structure, and an adhesive is obtained.

Here, an example of a design guideline for making the adhesive constituting the first adhesive layer 111 and the adhesive constituting the second adhesive layer 112 will be described. The adhesive constituting the first adhesive layer 111 is preferably a (meth)acrylic acid ester polymer (A) having a fairly low glass transition temperature (Tg) crosslinked at a high crosslink density by a crosslinking agent (B). On the other hand, the adhesive constituting the second adhesive layer 112 is preferably a (meth)acrylic acid ester polymer (A) having a not so low glass transition temperature (Tg) crosslinked at a slightly lower crosslink density by a crosslinking agent (B). As a result, in the adhesive constituting the first adhesive layer 111, the glass transition temperature (Tg) of the (meth)acrylic acid ester polymer (A) is quite low, so that the storage modulus G' at low temperatures is reduced. In addition, the high crosslink density suppresses the reduction in the storage modulus G' at high temperatures. On the other hand, in the adhesive constituting the second adhesive layer 112, the crosslink density is somewhat low, so that the adhesive strength is improved.

(4) Thickness of the Adhesive Layer The thickness of the adhesive layer 11 in the adhesive sheet 1 according to the present embodiment (measured according to JIS K7130) is preferably 1 μm or more as a lower limit, more preferably 5 μm or more, particularly preferably 10 μm or more, and even more preferably 15 μm or more. When the lower limit of the thickness of the adhesive layer 11 is as above, the desired adhesive strength is easily exerted, and the bending resistance is more excellent.

The thickness of the adhesive layer 11 is preferably 300 μm or less as an upper limit, more preferably 150 μm or less, and particularly preferably 90 μm or less, and from the viewpoint of obtaining a thinner repeatedly flexed device, is preferably 40 μm or less. When the upper limit of the thickness of the adhesive layer 11 is as above, the stress generated in the adhesive layer due to repeated flexing is prevented from becoming too large, and the flex resistance can be improved.

The thickness of the first adhesive layer 111 preferably has a lower limit of 0.5 μm or more, more preferably 3 μm or more, particularly preferably 5 μm or more, and even more preferably 8 μm or more. When the lower limit of the thickness of the first adhesive layer 111 is as above, the storage modulus G'(-20) of the adhesive of the adhesive layer as a whole is more likely to satisfy the above value.

The thickness of the first adhesive layer 111 is preferably 150 μm or less as an upper limit, more preferably 75 μm or less, particularly preferably 45 μm or less, and from the viewpoint of obtaining a thinner repeatedly bent device, more preferably 20 μm or less. When the upper limit of the thickness of the first adhesive layer 111 is as described above, the stress generated in the adhesive layer due to repeated bending can be prevented from becoming too large, and bending resistance can be improved. The first adhesive layer 111 may be formed as a single layer, or may be formed by laminating multiple layers.

The thickness of the second adhesive layer 112 preferably has a lower limit of 0.5 μm or more, more preferably 3 μm or more, particularly preferably 5 μm or more, and even more preferably 8 μm or more. When the lower limit of the thickness of the second adhesive layer 112 is as described above, it becomes easier to achieve the desired adhesive strength on the second adhesive layer 112 side.

The upper limit of the thickness of the second adhesive layer 112 is preferably 150 μm or less, more preferably 75 μm or less, and particularly preferably 45 μm or less, and from the viewpoint of obtaining a thinner repeatedly bent device, is preferably 20 μm or less. When the upper limit of the thickness of the second adhesive layer 112 is as described above, the stress generated in the adhesive layer due to repeated bending can be prevented from becoming too large, and bending resistance can be improved. The second adhesive layer 112 may be formed as a single layer, or may be formed by laminating multiple layers.

The ratio of the thickness of the first adhesive layer 111 to the thickness of the second adhesive layer 112 is preferably 90:10 to 10:90, more preferably 80:20 to 20:80, more preferably 70:30 to 30:70, and even more preferably 60:40 to 40:60. This makes it easier to satisfy the physical properties of the storage modulus G' and adhesive strength described above.

The release sheets 12a and 12b protect the adhesive layer 11 until the adhesive sheet 1 is used, and are peeled off when the adhesive sheet 1 (adhesive layer 11) is used. In the adhesive sheet 1 according to this embodiment, one or both of the release sheets 12a and 12b are not necessarily required.

Examples of the release sheets 12a and 12b that can be used include polyethylene films, polypropylene films, polybutene films, polybutadiene films, polymethylpentene films, polyvinyl chloride films, vinyl chloride copolymer films, polyethylene terephthalate films, polyethylene naphthalate films, polybutylene terephthalate films, polyurethane films, ethylene vinyl acetate films, ionomer resin films, ethylene-(meth)acrylic acid copolymer films, ethylene-(meth)acrylic acid ester copolymer films, polystyrene films, polycarbonate films, polyimide films, and fluororesin films. Crosslinked films of these films can also be used. Furthermore, laminated films of these films can also be used.

It is preferable that the release surfaces of the release sheets 12a and 12b (particularly the surfaces that come into contact with the adhesive layer 11) are subjected to a release treatment. Examples of release agents used in the release treatment include alkyd-based, silicone-based, fluorine-based, unsaturated polyester-based, polyolefin-based, and wax-based release agents.

Of the release sheets 12a and 12b, it is preferable that one of the release sheets is a heavy release type release sheet with a large release force, and the other is a light release type release sheet with a small release force. In particular, it is preferable that the release sheet 12a in contact with the first adhesive layer 111, which has a relatively small adhesive force, is a light release type release sheet, and the release sheet 12b in contact with the second adhesive layer 112, which has a relatively large adhesive force, is a heavy release type release sheet. This makes it possible to increase the difference in release force between the release sheets 12a and 12b, resulting in excellent handling properties. Specifically, when the release sheet 12a that is to be released first is peeled off from the first adhesive layer 111, it is possible to effectively prevent the second adhesive layer 112 and the release sheet 12b from peeling off from each other, which is called a tearful separation.

There are no particular limitations on the thickness of the release sheets 12a and 12b, but it is usually about 20 to 150 μm.

2. Physical Properties (1) Haze Value The haze value of the adhesive layer 11 at 25°C is preferably 10% or less, more preferably 5% or less, particularly preferably 3% or less, and even more preferably 1% or less. The above haze value of the adhesive layer 11 makes it excellent in light transmittance and suitable for use in a repeatedly bent display. The lower limit of the haze value is not particularly limited, and is preferably 0% or more, more preferably 0.1% or more. The haze value in this specification is a value measured in accordance with JIS K7136:2000.

(2) Adhesive strength The adhesive strength of the adhesive sheet 1 according to the present embodiment on the second adhesive layer 112 side at 23° C. to soda lime glass is preferably 0.5 N/25 mm or more as a lower limit, more preferably 1.0 N/25 mm or more, particularly preferably 2.0 N/25 mm or more, and even more preferably 3.0 N/25 mm or more. When the lower limit of the adhesive strength of the adhesive sheet 1 to soda lime glass is as described above, the bending resistance is more excellent. On the other hand, the upper limit of the adhesive strength is not particularly limited, but reworkability may be required. From this viewpoint, the adhesive strength is preferably 20 N/25 mm or less, more preferably 15 N/25 mm or less, and particularly preferably 10 N/25 mm or less. The adhesive strength in this specification basically refers to the adhesive strength measured by a 180-degree peeling method in accordance with JIS Z0237:2009, and the specific test method is as shown in the test example described later.

The adhesive strength of the first adhesive layer 111 side of the adhesive sheet 1 according to this embodiment to soda lime glass at 23°C is preferably 0.1 N/25 mm or more, more preferably 0.4 N/25 mm or more, particularly preferably 0.8 N/25 mm or more, and even more preferably 1.2 N/25 mm or more, from the viewpoint of bending resistance. On the other hand, the upper limit is not particularly restricted, but from the viewpoint of compatibility with other physical properties, it is preferably 18 N/25 mm or less, more preferably 10 N/25 mm or less, particularly preferably 5 N/25 mm or less, and most preferably 2.5 N/25 mm or less.

In addition, the adhesive strength of the second adhesive layer 112 side of the adhesive sheet 1 according to this embodiment to soda-lime glass at 23°C is greater than the adhesive strength of the first adhesive layer 111 side to soda-lime glass at 23°C.

The adhesive strength of the adhesive sheet 1 according to this embodiment to the triacetyl cellulose film on the second adhesive layer 112 side at 23°C is preferably 1 N/25 mm or more as a lower limit, more preferably 2 N/25 mm or more, particularly preferably 3 N/25 mm or more, and even more preferably 5 N/25 mm or more. When the lower limit of the adhesive strength of the adhesive sheet 1 to the triacetyl cellulose film is as above, the bending resistance is more excellent. On the other hand, the upper limit of the adhesive strength is not particularly limited, but reworkability may be required. From this viewpoint, the adhesive strength is preferably 30 N/25 mm or less, more preferably 25 N/25 mm or less, and particularly preferably 20 N/25 mm or less.

The adhesive strength of the adhesive sheet 1 according to this embodiment on the side of the first adhesive layer 111 to the triacetyl cellulose film at 23°C is preferably 0.5 N/25 mm or more, more preferably 1.0 N/25 mm or more, particularly preferably 1.5 N/25 mm or more, and even more preferably 2.0 N/25 mm or more, from the viewpoint of bending resistance. On the other hand, the upper limit is not particularly restricted, but from the viewpoint of compatibility with other physical properties, it is preferably 27 N/25 mm or less, more preferably 16 N/25 mm or less, particularly preferably 11 N/25 mm or less, and most preferably 6 N/25 mm or less.

In addition, the adhesive strength of the adhesive sheet 1 according to this embodiment to the triacetyl cellulose film on the second adhesive layer 112 side at 23°C is greater than the adhesive strength of the adhesive sheet 1 according to this embodiment to the triacetyl cellulose film on the first adhesive layer 111 side at 23°C.

Whether soda lime glass or triacetyl cellulose film is used as the adherend, the adhesive strength of the second adhesive layer 112 side is preferably 1.1 times or more, more preferably 1.5 times or more, and particularly preferably 1.8 times or more, relative to the adhesive strength of the first adhesive layer 111 side. There is no particular upper limit, but taking into consideration compatibility with other physical property values, it is preferably 10 times or less, more preferably 5 times or less, and particularly preferably 3 times or less.

3. Manufacturing of adhesive sheet As a manufacturing example of the adhesive sheet 1, the case where the above-mentioned adhesive composition P is used will be described. As a manufacturing example of the adhesive sheet 1, a coating solution of the adhesive composition P for forming the first adhesive layer 111 is applied to the release surface of one release sheet 12a, and a heat treatment is performed to thermally crosslink the adhesive composition P, forming a coating layer, to obtain a release sheet 12a with a coating layer. Also, a coating solution of the adhesive composition P for forming the second adhesive layer 112 is applied to the release surface of the other release sheet 12b, and a heat treatment is performed to thermally crosslink the adhesive composition P, forming a coating layer, to obtain a release sheet 12b with a coating layer. Then, the release sheet 12a with a coating layer and the release sheet 12b with a coating layer are bonded together so that both coating layers are in contact with each other. Here, a plurality of release sheets with coating layers may be prepared, and the coating layers may be bonded together in a desired number and in a desired stacking order. When a curing period is required, a curing period is provided, whereas when no curing period is required, the laminated coating layers are left as they are to become the first adhesive layer 111 and the second adhesive layer 112. This provides the adhesive sheet 1 having the adhesive layer 11, which is a laminate of the first adhesive layer 111 and the second adhesive layer 112. The conditions for the heat treatment and curing are as described above.

In addition, the coating layer for forming the first adhesive layer 111 and the coating layer for forming the second adhesive layer 112 may each be sandwiched between two release sheets, and when the coating layer for forming the first adhesive layer 111 and the coating layer for forming the second adhesive layer 112 are bonded together, one of the release sheets may be peeled off.

Methods for applying the coating solution of the adhesive composition P include, for example, bar coating, knife coating, roll coating, blade coating, die coating, and gravure coating.

[Repeatedly bent laminated member]
As shown in Figure 2, the repeatedly bent laminated member 2 of this embodiment is composed of a first flexible member 21 (one flexible member), a second flexible member 22 (another flexible member), and an adhesive layer 11 located between them and bonding the first flexible member 21 and the second flexible member 22 to each other.

The adhesive layer 11 in the repeatedly bent laminated member 2 is the adhesive layer 11 of the adhesive sheet 1 described above. The adhesive layer 11 is located between the first flexible member 21 and the second flexible member 22 so that when the repeatedly bent laminated member 2 is bent, the first adhesive layer 111 is on the outside of the bend and the second adhesive layer 112 is on the inside of the bend.

The repeatedly bent laminate member 2 is a repeatedly bent device itself, or a member constituting part of a repeatedly bent device. The repeatedly bent device is preferably a display (repeatedly bent display) that can be repeatedly bent (including folded), but is not limited to this. Examples of such repeatedly bent devices include organic electroluminescence (organic EL) displays, electrophoretic displays (electronic paper), liquid crystal displays that use a plastic substrate (film) as a substrate, foldable displays, etc., and may also be touch panels.

The first flexible member 21 and the second flexible member 22 are members that can be repeatedly bent (including folded), and examples thereof include cover films, gas barrier films, hard coat films, polarizing films (polarizing plates), polarizers, retardation films (retardation plates), viewing angle compensation films, brightness enhancement films, contrast enhancement films, diffusion films, semi-transmissive reflective films, electrode films, transparent conductive films, metal mesh films, flexible glass, film sensors (touch sensor films), liquid crystal polymer films, light-emitting polymer films, film-like liquid crystal modules, organic EL modules (organic EL films, organic EL elements), electronic paper modules (film-like electronic paper), TFT (Thin Film Transistor) substrates, etc.

At least one of the first flexible member 21 and the second flexible member 22 may be a triacetyl cellulose film or a laminate having a triacetyl cellulose film on the adhesive layer 11 side. Triacetyl cellulose films generally have low adhesion to adhesive layers, but the adhesive layer 11 in this embodiment provides excellent flex resistance even when the adherend is a triacetyl cellulose film.

The Young's modulus of the first flexible member 21 and the second flexible member 22 is preferably 0.1 to 10 GPa, more preferably 0.5 to 7 GPa, and even more preferably 1.0 to 5 GPa. Having the Young's modulus of the first flexible member 21 and the second flexible member 22 in this range makes it easy to repeatedly bend each flexible member.

The thickness of the first flexible member 21 and the second flexible member 22 is preferably 10 to 3000 μm, more preferably 25 to 1000 μm, and even more preferably 50 to 500 μm. Having the thickness of the first flexible member 21 and the second flexible member 22 within this range makes it easy to repeatedly bend each flexible member.

To manufacture the repeatedly bent laminated member 2, as an example, one release sheet 12a of the adhesive sheet 1 is peeled off, and the exposed adhesive layer 11 (first adhesive layer 111 or second adhesive layer 112) of the adhesive sheet 1 is attached to one side of the first flexible member 21.

Then, the other release sheet 12b is peeled off from the adhesive layer 11 of the adhesive sheet 1, and the exposed adhesive layer 11 (the second adhesive layer 112 or the first adhesive layer 111) of the adhesive sheet 1 is bonded to the second flexible member 22 to obtain the repeatedly bent laminate member 2. As another example, the bonding order of the first flexible member 21 and the second flexible member 22 may be reversed.

[Repeated bending device]
The repeatedly bent device according to the present embodiment includes the repeatedly bent laminate member 2, and may be configured to include only the repeatedly bent laminate member 2, or may include one or more repeatedly bent laminate members 2 and other flexible members. When one repeatedly bent laminate member 2 and another repeatedly bent laminate member 2 are laminated, or when the repeatedly bent laminate member 2 and another flexible member are laminated, it is preferable to laminate them via the adhesive layer 11 of the adhesive sheet 1 described above.

In the repeatedly bent device according to this embodiment, the adhesive layer is made of the adhesive described above, so peeling is unlikely to occur at the interface between the adhesive layer and the adherend at the bent portion, even when the device is repeatedly bent or left in a bent state for a long period of time.

Figure 3 shows an example of a repetitive bending device in this embodiment. Note that the repetitive bending device according to the present invention is not limited to this repetitive bending device.

As shown in FIG. 3, the repeatedly bent device 3 according to this embodiment is configured by laminating, from the top, a cover film 31, a first adhesive layer 32, a polarizing film 33, a second adhesive layer 34, a touch sensor film 35, a third adhesive layer 36, an organic EL element 37, a fourth adhesive layer 38, and a TFT substrate 39. The above-mentioned cover film 31, polarizing film 33, touch sensor film 35, organic EL element 37, and TFT substrate 39 correspond to a flexible member. The repeatedly bent device 3 according to this embodiment may be bent with the cover film 31 side on the inside, or may be bent with the cover film 31 side on the outside.

At least one of the first adhesive layer 32, the second adhesive layer 34, the third adhesive layer 36, and the fourth adhesive layer 38 is the adhesive layer 11 of the adhesive sheet 1 described above. It is preferable that two or more of the first adhesive layer 32, the second adhesive layer 34, the third adhesive layer 36, and the fourth adhesive layer 38 are the adhesive layer 11 of the adhesive sheet 1 described above, and it is most preferable that all of the adhesive layers 32, 34, 36, and 38 are the adhesive layer 11 of the adhesive sheet 1. When the adhesive layer 11 is used, it is arranged so that the first adhesive layer 111 is located on the outside of the bending direction of the repeated bending device 3, and the second adhesive layer 112 is located on the inside.

Here, for example, when a triacetyl cellulose film is laminated on one or both sides of the polarizing film 33, the first adhesive layer 32 and/or the second adhesive layer 34 that contacts the triacetyl cellulose film are preferably the adhesive layer 11 of the adhesive sheet 1 described above.

The above-described embodiments are described to facilitate understanding of the present invention, and are not described to limit the present invention. Therefore, each element disclosed in the above embodiments is intended to include all design modifications and equivalents that fall within the technical scope of the present invention.

For example, either or both of the release sheets 12a and 12b in the adhesive sheet 1 may be omitted, and a desired flexible member may be laminated in place of the release sheets 12a and/or 12b.

The present invention will be explained in more detail below with reference to examples, but the scope of the present invention is not limited to these examples.

Example 1
1. Preparation of (meth)acrylic acid ester polymer (A) (1) Preparation of (meth)acrylic acid ester polymer (A1) for first pressure-sensitive adhesive layer 99 parts by mass of 2-ethylhexyl acrylate and 1 part by mass of 4-hydroxybutyl acrylate were copolymerized by solution polymerization to prepare (meth)acrylic acid ester polymer (A1). The molecular weight of this (meth)acrylic acid ester polymer (A1) was measured by the method described below, and the weight average molecular weight (Mw) was 1,200,000.

(2) Preparation of (meth)acrylic acid ester polymer (A2) for second pressure-sensitive adhesive layer 49 parts by mass of n-butyl acrylate, 50 parts by mass of 2-ethylhexyl acrylate, and 1 part by mass of 4-hydroxybutyl acrylate were copolymerized by solution polymerization to prepare (meth)acrylic acid ester polymer (A2). The molecular weight of this (meth)acrylic acid ester polymer (A2) was measured by the method described below, and the weight average molecular weight (Mw) was 1,200,000.

2. Preparation of adhesive composition (1) Preparation of adhesive composition for first adhesive layer 100 parts by mass (solid content equivalent; the same applies below) of the (meth)acrylic acid ester polymer (A1) obtained in the above step 1 (1), 0.25 parts by mass of trimethylolpropane-modified xylylene diisocyanate (XDI; manufactured by Soken Chemical Industries, Ltd., product name "TD-75") as a crosslinking agent (B), and 0.20 parts by mass of 3-glycidoxypropyltrimethoxysilane as a silane coupling agent were mixed, thoroughly stirred, and diluted with methyl ethyl ketone to obtain a coating solution of an adhesive composition for a first adhesive layer.

(2) Preparation of adhesive composition for second adhesive layer 100 parts by mass of the (meth)acrylic acid ester polymer (A2) obtained in the above step 1(2), 0.14 parts by mass of trimethylolpropane-modified xylylene diisocyanate (XDI; manufactured by Soken Chemical & Engineering Co., Ltd., product name "TD-75") as the crosslinking agent (B), and 0.20 parts by mass of 3-glycidoxypropyltrimethoxysilane as a silane coupling agent were mixed, thoroughly stirred, and diluted with methyl ethyl ketone to obtain a coating solution of an adhesive composition for a second adhesive layer.

3. Manufacturing of adhesive sheet (1) Formation of coating layer for first adhesive layer The coating solution of the adhesive composition for the first adhesive layer obtained in the above step 2(1) was applied by a knife coater to the release-treated surface of a light-release type release sheet (manufactured by Lintec Corporation, product name "SP-PET381031"), one side of which was a polyethylene terephthalate film that had been release-treated with a silicone-based release agent. The coating layer was then heat-treated at 90°C for 1 minute to form a coating layer.

(2) Formation of Coating Layer for Second Pressure-Sensitive Adhesive Layer The coating solution of the pressure-sensitive adhesive composition for the second pressure-sensitive adhesive layer obtained in step 2(2) above was applied by a knife coater to the release-treated surface of a heavy-release release sheet (manufactured by Lintec Corporation, product name "SP-PET752150"), one side of which had been release-treated with a silicone-based release agent. The coating layer was then heat-treated at 90°C for 1 minute to form a coating layer.

(3) Manufacturing of adhesive sheet The exposed surface of the coating layer obtained in the above step 3(1) and the exposed surface of the coating layer obtained in the above step 3(2) were laminated so as to be in contact with each other, and cured for 7 days under conditions of 23°C and 50% RH to obtain an adhesive sheet. That is, an adhesive sheet having a total adhesive thickness of 25 μm and consisting of a light release type release sheet/first adhesive layer (thickness: 12 μm)/second adhesive layer (thickness: 13 μm)/heavy release type release sheet was obtained. The thickness of the adhesive layer was measured using a constant pressure thickness gauge (manufactured by Techrock Co., Ltd., product name "PG-02") in accordance with JIS K7130.

Here, the formulations (solid content equivalent) of the pressure-sensitive adhesive composition when the (meth)acrylic acid ester polymer (A) is taken as 100 parts by mass (solid content equivalent) are shown in Table 1. The details of the abbreviations and the like shown in Table 1 are as follows.
[(Meth)acrylic acid ester polymer (A)]
2EHA: 2-ethylhexyl acrylate 4HBA: 4-hydroxybutyl acrylate BA: n-butyl acrylate

Comparative Example 1
An adhesive sheet having a single adhesive layer (thickness: 25 μm) was produced in the same manner as in Example 1, using only the coating solution of the adhesive composition for the second adhesive layer of Example 1.

Comparative Example 2
100 parts by mass of the (meth)acrylic acid ester polymer (A1) obtained in step 1 (1) of Example 1, 0.35 parts by mass of trimethylolpropane-modified xylylene diisocyanate (XDI; manufactured by Soken Chemical & Engineering Co., Ltd., product name "TD-75") as the crosslinking agent (B), and 0.20 parts by mass of 3-glycidoxypropyltrimethoxysilane as the silane coupling agent were mixed, thoroughly stirred, and diluted with methyl ethyl ketone to obtain a coating solution of a pressure-sensitive adhesive composition.

Using the coating solution of the above adhesive composition, an adhesive sheet having a single adhesive layer (thickness: 25 μm) was produced in the same manner as in Example 1.

Comparative Example 3
An adhesive sheet having a single adhesive layer (thickness: 25 μm) was produced in the same manner as in Example 1, using only the coating solution of the adhesive composition for the first adhesive layer of Example 1.

The weight average molecular weight (Mw) is a polystyrene-equivalent weight average molecular weight measured by gel permeation chromatography (GPC) under the following conditions (GPC measurement).
<Measurement conditions>
GPC measuring device: Tosoh Corporation, HLC-8020
GPC columns (passed in the following order): TSK guard column HXL-H manufactured by Tosoh Corporation
TSK gel GMHXL (x2)
TSK gel G2000HXL
Measurement solvent: tetrahydrofuran Measurement temperature: 40°C

[Test Example 1] (Measurement of gel fraction)
The pressure-sensitive adhesive sheets produced in the Examples and Comparative Examples were cut to a size of 80 mm x 80 mm, the pressure-sensitive adhesive layer was wrapped in a polyester mesh (product name: Tetoron mesh #200), and the mass was measured using a precision balance. The mass of the mesh alone was subtracted to calculate the mass of the pressure-sensitive adhesive alone. This mass was designated M1.

Next, the adhesive wrapped in the polyester mesh was immersed in ethyl acetate at room temperature (23°C) for 24 hours. The adhesive was then removed and air-dried for 24 hours in an environment with a temperature of 23°C and a relative humidity of 50%, and then dried in an oven at 80°C for 12 hours. After drying, the mass was weighed using a precision balance, and the mass of the adhesive alone was calculated by subtracting the mass of the mesh alone. This mass was designated M2. The gel fraction (%) was expressed as (M2/M1) x 100. The results are shown in Table 2.

[Test Example 2] (Measurement of storage modulus G')
The pressure-sensitive adhesive layers of the pressure-sensitive adhesive sheets prepared in the Examples and Comparative Examples were laminated together to form a laminate having a thickness of 0.2 mm. The resulting laminate of pressure-sensitive adhesive layers was cut into a 1 cm square to form a sample.

The storage modulus G' of the above sample was measured under the following conditions by a torsional shear method using a viscoelasticity measuring device (manufactured by Anton Paar, product name "MCR301") in accordance with JIS K7244-1, and the storage modulus G' (-20) at -20°C, the storage modulus G' (25) at 25°C, and the storage modulus G' (85) (MPa) at 85°C were obtained. The results are shown in Table 2.
Measurement frequency: 1Hz
Nomar Force: 1N
Measurement temperature: -20℃~100℃

[Test Example 3] (Measurement of haze value)
The adhesive layer of the adhesive sheet produced in the examples and comparative examples was attached to glass to prepare a measurement sample. After background measurement was performed on the glass, the haze value (%) of the measurement sample was measured using a haze meter (manufactured by Nippon Denshoku Industries Co., Ltd., product name "NDH5000") in accordance with JIS K7136:2000. The results are shown in Table 2.

[Test Example 4] (Measurement of adhesive strength)
One release sheet was peeled off from the adhesive sheets obtained in the Examples and Comparative Examples, and the exposed adhesive layer was attached to an easy-adhesion layer of a polyethylene terephthalate (PET) film (manufactured by Toyobo Co., Ltd., product name "PET A4300", thickness: 100 μm) having an easy-adhesion layer to obtain a release sheet/adhesive layer/PET film laminate. The obtained laminate was cut into a width of 25 mm and a length of 110 mm.

On the other hand, the following two types of adherends were prepared.
(1) Soda-lime glass plate (manufactured by Nippon Sheet Glass Co., Ltd., product name "Soda-lime glass", thickness: 1.1 mm)
(2) Triacetyl cellulose (TAC) film (Konica Minolta, product name "KC8UAW", thickness: 80 μm)

The release sheet was peeled off from the laminate in an environment of 23°C and 50% RH, and the exposed adhesive layer was attached to each of the above-mentioned adherends, and pressure was applied for 20 minutes at 0.5 MPa and 50°C in an autoclave manufactured by Kurihara Seisakusho. After that, the laminate was left for 24 hours under conditions of 23°C and 50% RH, and the adhesive strength (N/25 mm) was measured when the laminate of the PET film and the adhesive layer was peeled off from the adherend using a tensile tester (Tensilon manufactured by Orientec Co., Ltd.) at a peel speed of 300 mm/min and a peel angle of 180 degrees. The measurements were performed under conditions other than those described here in accordance with JIS Z0237:2009. For the adhesive sheet manufactured in Example 1, both the adhesive strength of the first adhesive layer side and the adhesive strength of the second adhesive layer side were measured. The results are shown in Table 2.

[Test Example 5] (Evaluation of bending resistance)
In an environment of 23°C and 50% RH, one release sheet was peeled off from the adhesive sheet prepared in the examples and comparative examples, and the exposed adhesive layer (the first adhesive layer in Example 1) was attached to one side of a polyethylene terephthalate (PET) film (manufactured by Toyobo Co., Ltd., product name "PET50TA063", thickness: 100 μm). Next, the other release sheet was peeled off, and the exposed adhesive layer (the second adhesive layer in Example 1) was attached to a triacetyl cellulose (TAC) film (manufactured by Konica Minolta Co., Ltd., product name "KC8UAW", thickness: 80 μm). Then, the sheet was pressurized at 0.5 MPa and 50°C for 20 minutes in an autoclave manufactured by Kurihara Seisakusho Co., Ltd., and then left for 24 hours under conditions of 23°C and 50% RH. The thus obtained laminate consisting of PET film/adhesive layer/TAC film was cut into a piece having a width of 50 mm and a length of 200 mm, which was used as a sample.

The obtained sample was held for 480 hours in an environment of -20°C and 50% RH and an environment of 80°C and dry, as shown in Figure 4, between two holding plates (distance between each plate: 4 mm) made of standing glass plates, with the PET film on the outside and the TAC film on the inside. After this static bending test, it was visually confirmed whether or not there was peeling at the interface between the pressure-sensitive adhesive layer and the adherend at the bent portion of the test piece. Then, the bending resistance was evaluated based on the following criteria. The results are shown in Table 2.
◯: No peeling △: Partial peeling (air bubbles)
×: Peeling occurred

As can be seen from Table 2, when the adhesive layer of the adhesive sheet of the embodiment was attached to two flexible members and left in a bent state for a long period of time, no peeling occurred at the interface between the adhesive layer and the flexible members, and the adhesive layer had excellent bending resistance. This effect was fully demonstrated in an environment of -20°C and 50% RH, and in an environment of 80°C and dry.

The present invention is suitable for bonding one flexible member (particularly a triacetyl cellulose film or a laminate containing a triacetyl cellulose film) to another flexible member that constitutes a repeatedly bending device.

REFERENCE SIGNS LIST 1... Adhesive sheet 11... Adhesive layer 111... First adhesive layer 112... Second adhesive layer 12a, 12b... Release sheet 2... Repeatedly bent laminated member 21... First flexible member 22... Second flexible member 3... Repeatedly bent device 31... Cover film 32... First adhesive layer 33... Polarizing film 34... Second adhesive layer 35... Touch sensor film 36... Third adhesive layer 37... Organic EL element 38... Fourth adhesive layer 39... TFT substrate S... Test piece P... Holding plate

Claims (8)

A pressure-sensitive adhesive sheet having a pressure-sensitive adhesive layer for bonding one flexible member and another flexible member that constitute a device that is repeatedly bent,
the pressure-sensitive adhesive layer is formed by laminating a first pressure-sensitive adhesive layer and a second pressure-sensitive adhesive layer,
an adhesive strength of the second adhesive layer side at 23° C. is relatively greater than an adhesive strength of the first adhesive layer side;
the pressure-sensitive adhesive layer as a whole has a storage modulus G'(-20) at -20°C of 0.1 MPa or less;
the storage modulus G'(-20) at -20°C of the pressure-sensitive adhesive constituting the second pressure-sensitive adhesive layer is 0.100 MPa or more and 0.500 MPa or less;
An adhesive sheet, characterized in that it is bent so that the first adhesive layer is on the outside and the second adhesive layer is on the inside. The adhesive sheet according to claim 1, characterized in that the storage modulus G'(-20) at -20°C of the adhesive constituting the first adhesive layer is 0.040 MPa or more and 0.1 MPa or less. 3. The pressure-sensitive adhesive sheet according to claim 1 , wherein the adhesive strength of the second pressure-sensitive adhesive layer side to a triacetyl cellulose film at 23° C. is 1 N/25 mm or more and 30 N/25 mm or less. 4. The pressure-sensitive adhesive sheet according to claim 1 , wherein the pressure-sensitive adhesive layer as a whole has a gel fraction of 40% or more and 99% or less. The pressure-sensitive adhesive sheet according to any one of claims 1 to 4 , wherein the pressure-sensitive adhesive constituting the first pressure-sensitive adhesive layer and the pressure-sensitive adhesive constituting the second pressure-sensitive adhesive layer are acrylic pressure-sensitive adhesives. The pressure-sensitive adhesive sheet has two release sheets,
6. The pressure-sensitive adhesive sheet according to claim 1 , wherein the pressure-sensitive adhesive layer is sandwiched between the two release sheets so as to be in contact with the release surfaces of the release sheets. A flexible member and another flexible member that constitute a device that is repeatedly bent;
A repeatedly bent laminated member including a pressure-sensitive adhesive layer that bonds the one bent member and the other bent member to each other,
A repeatedly bent laminated member, characterized in that the pressure-sensitive adhesive layer is made of the pressure-sensitive adhesive layer of the pressure-sensitive adhesive sheet according to any one of claims 1 to 6 . A repeatedly flexed device comprising the repeatedly flexed laminate member according to claim 7 .

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