JP7576478B2 - Adhesive for repeatedly flexed devices, adhesive sheet, repeatedly flexed laminate member, and repeatedly flexed device - Google Patents

Description

The present invention relates to an adhesive and 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 possible to bond one bendable member (flexible member) constituting the repeatedly bendable display to another flexible member with an adhesive layer. However, when a conventional adhesive is used in a repeatedly bendable display, problems arise in that lifting or peeling occurs at the interface between the adhesive layer and the adherend.

Patent Document 1 discloses an adhesive that aims to prevent the adhesive layer from lifting or peeling even when repeatedly bent.

JP 2016-108555 A

However, even when using the above-mentioned adhesives, it was not possible to sufficiently prevent lifting or peeling caused by repeated bending.

The present invention has been made in consideration of the above-mentioned circumstances, and aims to provide an adhesive and adhesive sheet for a repeatedly flexed device that suppresses lifting and peeling when applied to a repeatedly flexed device, as well as a repeatedly flexed laminate member and a repeatedly flexed device that suppresses lifting and peeling.

In order to achieve the above object, first, the present invention provides an adhesive for repeatedly bending devices, for bonding one flexible member and another flexible member that constitute a device that is repeatedly bent, characterized in that the storage modulus G'(-30) of the adhesive at -30°C is 0.20 MPa or less, the creep compliance value measured when a stress of 4500 Pa is applied to the adhesive is defined as minimum creep compliance J(t) _min (MPa ^-1 ), the maximum creep compliance value measured during the application of a stress of 4500 Pa from the measurement of the minimum creep compliance J(t) _min until 1200 seconds has elapsed is defined as maximum creep compliance J(t) _max (MPa ^-1 ), and the creep compliance variation value ΔlogJ(t) calculated from the following formula (I) is 2.60 or less (Invention 1).
ΔlogJ(t)=logJ(t) _max -logJ(t) _min ...(I)

In the above invention (Invention 1), since the storage modulus G'(-30) at -30°C is small as described above, it is possible to reduce the stress generated by repeated bending in a low temperature environment (e.g., -30°C). On the other hand, since the creep compliance fluctuation value is small as described above, it is possible to suppress excessive distortion of the adhesive regardless of the temperature environment when repeatedly bending or bending for a long period of time. As a result, when applied to a repeatedly bending device, lifting or peeling is unlikely to occur at the interface between the adhesive layer and the adherend in low to high temperature environments, and excellent bending resistance is obtained.

In the above invention (Invention 1), it is preferable that the creep compliance value measured 1200 seconds after continuously applying a stress of 4500 Pa to the pressure-sensitive adhesive is defined as creep compliance J(t) (s = 1200) (MPa ^-1 ), and the creep compliance value measured 100 seconds after the stress applied to the pressure-sensitive adhesive is then reduced to 0 Pa is defined as creep compliance J(t) (s = 1300) (MPa ^-1 ), and the creep recovery rate calculated from the following formula (II) is 90% or more (Invention 2).
Creep recovery rate (%) = (1 - J (t) (s = 1300) / J (t) (s = 1200)) x 100 ... (II)

In the above inventions (Inventions 1 and 2), it is preferable that the storage modulus G'(85) at 85°C is 0.005 MPa or more (Invention 3).

In the above inventions (Inventions 1 to 3), it is preferable that the storage modulus change rate (-30/25), which is the value obtained by dividing the storage modulus G' (-30) at -30°C by the storage modulus G' (25) at 25°C, is 5.0 or less (Invention 4).

In the above inventions (Inventions 1 to 4), it is preferable that the storage modulus change rate (-30/85), which is the value obtained by dividing the storage modulus G' (-30) at -30°C by the storage modulus G' (85) at 85°C, is 10.0 or less (Invention 5).

In the above inventions (Inventions 1 to 5), it is preferable that the gel fraction is 40% or more and 95% or less (Invention 6).

In the above inventions (Inventions 1 to 6), it is preferable that the adhesive contains a main adhesive agent and a low molecular weight component, the weight average molecular weight of the low molecular weight component is 400 or more and 100,000 or less, and the glass transition temperature (Tg) of the low molecular weight component is -40°C or less (Invention 7).

In the above inventions (Inventions 1 to 7), it is preferable that the adhesive is an acrylic adhesive (Invention 8).

Secondly, 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, characterized in that the adhesive layer is made of the adhesive for repeatedly bent devices (Inventions 1 to 8) (Invention 9).

In the above invention (Invention 9), 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 10).

Thirdly, 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 for repeatedly bent devices (Inventions 1 to 8) (Invention 11).

Fourthly, the present invention provides a repeatedly flexed device (Invention 12) characterized by comprising the repeatedly flexed laminated member (Invention 11).

When the adhesive and adhesive sheet for a repeatedly flexed device according to the present invention are applied to a repeatedly flexed device, lifting or peeling is unlikely to occur at the interface between the adhesive layer and the adherend. In addition, the repeatedly flexed laminate member and repeatedly flexed device according to the present invention are unlikely to lift or peeling at the interface between the adhesive layer and the adherend.

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 dynamic bending test.

Hereinafter, an embodiment of the present invention will be described.
[Adhesive for repeatedly bending devices]
The pressure-sensitive adhesive for a repeated bending device according to the present embodiment (hereinafter, sometimes simply referred to as "pressure-sensitive adhesive") is an adhesive for bonding one flexible member and another flexible member constituting a repeated bending device. The repeated bending device and the flexible member will be described later.

The pressure-sensitive adhesive according to this embodiment preferably has a storage modulus G'(-30) at -30°C of 0.20 MPa or less, the creep compliance value measured when a stress of 4500 Pa is applied to the pressure-sensitive adhesive is defined as the minimum creep compliance J(t) _min (MPa ^-1 ), the maximum creep compliance _value measured during the continued application of a stress of 4500 Pa until 1200 seconds after the measurement of the minimum creep compliance J(t) min is defined as the maximum creep compliance J(t) _max (MPa ^-1 ), and the creep compliance variation value ΔlogJ(t) calculated from the following formula (I) is preferably 2.60 or less.
ΔlogJ(t)=logJ(t) _max -logJ(t) _min ...(I)
The details of the method for measuring the storage modulus G' and the method for measuring the creep compliance J(t) are as shown in the test examples described later. Also, "when a stress of 4500 Pa is applied to the adhesive" refers to the time when the stress is applied to the adhesive and the stress reaches 4500 Pa.

The adhesive according to this embodiment has a small storage modulus G' (-30) at -30°C as described above, which can reduce the stress generated by repeated bending in a low-temperature environment (e.g., -30°C). On the other hand, the small creep compliance fluctuation value as described above can prevent the adhesive from being overly distorted regardless of the temperature environment when repeatedly bent or when bent for a long time. In an adhesive that is overly distorted, the internal structure of the adhesive changes with deformation due to bending, and the adhesive does not return to its original state even when the bending is released. However, the adhesive according to this embodiment can suppress such over-strain. Although the above two physical values are generally contradictory, the adhesive according to this embodiment has both the above two physical values, and the stress against deformation is small even at a very low temperature of -30°C, and the distortion due to deformation is small. As a result, when applied to a repeatedly bent device, lifting or peeling is unlikely to occur at the interface between the adhesive layer and the adherend in low to high temperature environments, and excellent bending resistance is obtained.

From the viewpoint of the above-mentioned bending resistance, the storage modulus G'(-30) of the adhesive according to this embodiment at -30°C is preferably 0.20 MPa or less, more preferably 0.15 MPa or less, particularly preferably 0.11 MPa or less, and even more preferably 0.10 MPa or less. From the viewpoint of cohesive strength, the storage modulus G'(-30) of the adhesive constituting the above-mentioned adhesive layer at -30°C is preferably 0.01 MPa or more, more preferably 0.02 MPa or more, particularly preferably 0.04 MPa or more, and even more preferably 0.08 MPa or more.

From the viewpoint of the bending resistance, the creep compliance fluctuation value ΔlogJ(t) is preferably 2.60 or less, more preferably 2.58 or less, particularly preferably 2.56 or less, and even more preferably 2.54 or less. The lower limit of the creep compliance fluctuation value is not particularly limited, but is usually preferably 1.50 or more, particularly preferably 2.00 or more, and even more preferably 2.20 or more.

The minimum creep compliance J(t) _min of the pressure-sensitive adhesive according to this embodiment has a lower limit of preferably 1 MPa ^-1 or more, more preferably 5 MPa ^-1 or more, particularly preferably 10 MPa ^-1 or more, and even more preferably 15 MPa ^-1 or more. When the minimum creep compliance J(t) _min has a lower limit as described above, the creep compliance fluctuation value ΔlogJ(t) easily satisfies the above value. The upper limit of the minimum creep compliance J(t) _min is not particularly limited, but is usually preferably 1000 MPa ^-1 or less, particularly preferably 100 MPa ^-1 or less, and even more preferably 20 MPa ^-1 or less.

In addition, the maximum creep compliance J(t) _max of the pressure-sensitive adhesive according to this embodiment is preferably 10000 MPa ^-1 or less as an upper limit, more preferably 8000 MPa ^-1 or less, particularly preferably 6000 MPa ^-1 or less, and even more preferably 5500 MPa ^-1 or less. By setting the upper limit of the maximum creep compliance J(t) _max as above, the creep compliance fluctuation value ΔlogJ(t) easily satisfies the above value. The lower limit of the maximum creep compliance J(t) _max is not particularly limited, but is usually preferably 1100 MPa ^-1 or more, particularly preferably 2000 MPa ^-1 or more, and even more preferably 4000 MPa ^-1 or more.

In the pressure-sensitive adhesive of this embodiment, the creep compliance value measured 1200 seconds after continuously applying a stress of 4500 Pa to the pressure-sensitive adhesive is defined as creep compliance J(t) (s = 1200) (MPa ^-1 ), and the creep compliance value measured 100 seconds after the stress applied to the pressure-sensitive adhesive is then reduced to 0 Pa is defined as creep compliance J(t) (s = 1300) (MPa ^-1 ), and the creep recovery rate calculated from the following formula (II) is preferably 90% or more, more preferably 92% or more, particularly preferably 94% or more, and even more preferably 96% or more.
Creep recovery rate (%) = (1 - J (t) (s = 1300) / J (t) (s = 1200)) x 100 ... (II)

The adhesive according to this embodiment exhibits the above creep recovery rate, so that even if a repeatedly bent device to which the adhesive is applied is left in a bent state for a long period of time, it has excellent recovery from the bent state and is less likely to leave flexion marks on the adhesive layer.

There is no particular upper limit to the creep recovery rate, but it is usually preferable that it be 100% or less, more preferably 99% or less, and even more preferably 98% or less.

The creep compliance J(t) (s=1200) of the pressure-sensitive adhesive according to this embodiment is preferably 1100 MPa ^-1 or more as a lower limit, more preferably 2000 MPa ^-1 or more, particularly preferably 4000 MPa ^-1 or more, and even more preferably 4600 MPa ^-1 or more. This results in a pressure-sensitive adhesive with more appropriate stress relaxation properties. From the viewpoint of the cohesive strength of the pressure-sensitive adhesive, the upper limit of creep compliance J(t) (s=1200) is usually preferably 10000 MPa ^-1 or less, particularly preferably 8000 MPa ^-1 or less, and even more preferably 6000 MPa ^-1 or less.

In addition, the upper limit of the creep compliance J(t) (s=1300) of the pressure-sensitive adhesive according to this embodiment is preferably 800 MPa ^-1 or less, more preferably 400 MPa ^-1 or less, particularly preferably 200 MPa ^-1 or less, and even more preferably 180 MPa ^-1 or less. This results in a pressure-sensitive adhesive with superior restorability. The lower limit of the creep compliance J(t) (s=1300) is not particularly limited, but is usually preferably 1 MPa ^-1 or more, particularly preferably 10 MPa ^-1 or more, and even more preferably 100 MPa ^-1 or more.

The storage modulus G'(85) at 85°C of the adhesive according to this embodiment is preferably 0.005 MPa or more, more preferably 0.010 MPa or more, particularly preferably 0.020 MPa or more, and even more preferably 0.022 MPa or more. This allows the adhesive layer to maintain its cohesive force without becoming too soft even at high temperatures. The storage modulus G'(85) at 85°C of the adhesive according to this embodiment is preferably 0.10 MPa or less, more preferably 0.08 MPa or less, particularly preferably 0.06 MPa or less, and even more preferably 0.04 MPa or less. This makes it easier to obtain an adhesive with a low storage modulus G' at low temperatures.

The storage modulus G'(25) at 25°C of the adhesive according to this embodiment is preferably 0.15 MPa or less, more preferably 0.10 MPa or less, particularly preferably 0.07 MPa or less, and even more preferably 0.04 MPa or less. This makes it easier to exhibit suitable adhesiveness. On the other hand, the storage modulus G'(25) at 25°C is preferably 0.010 MPa or more, more preferably 0.015 MPa or more, particularly preferably 0.020 MPa or more, and even more preferably 0.026 MPa or more. This provides excellent repeated bending properties in the standard environmental temperature range, as well as good processability in punching and other processes.

The adhesive according to this embodiment preferably has a storage modulus change rate (-30/25), which is the value obtained by dividing the storage modulus G' (-30) at -30°C by the storage modulus G' (25) at 25°C, of 5.0 or less, more preferably 4.0 or less, particularly preferably 3.5 or less, and even more preferably 3.3 or less. This makes it easier to prevent the modulus from becoming high at low temperatures while maintaining appropriate adhesive strength at room temperature.

The storage modulus change rate (-30/25) is preferably 1.50 or more, more preferably 2.00 or more, particularly preferably 2.50 or more, and even more preferably 3.00 or more. This makes it easier to obtain an adhesive with excellent cohesive strength.

The pressure-sensitive adhesive according to this embodiment has a storage modulus change rate (-30/85), which is the value obtained by dividing the storage modulus G' (-30) at -30°C by the storage modulus G' (85) at 85°C, of preferably 10.0 or less, more preferably 7.0 or less, particularly preferably 5.0 or less, and even more preferably 4.5 or less. This makes it easier to achieve both a low modulus at low temperatures and a high modulus at high temperatures.

The storage modulus change rate (-30/85) is preferably 1.20 or more, more preferably 2.00 or more, particularly preferably 2.50 or more, and even more preferably 3.00 or more. This makes it easier to obtain an adhesive with excellent cohesive strength.

The gel fraction of the adhesive according to this embodiment is preferably 40% or more, more preferably 50% or more, particularly preferably 60% or more, and even more preferably 70% or more. This allows the adhesive to exhibit a suitable cohesive force capable of withstanding repeated bending. As a result, the aforementioned bending resistance is more excellent. On the other hand, the gel fraction of the adhesive according to this embodiment is preferably 95% or less, more preferably 90% or less, particularly preferably 85% or less, and even more preferably 80% or less. This is presumably to suppress the destruction of the crosslinked structure in the adhesive due to repeated bending, and suppress the adhesive layer itself from becoming cloudy. The method for measuring the gel fraction in this specification is as shown in the test example described later.

The adhesive according to the present embodiment is not particularly limited as long as it satisfies the above physical properties, but it is preferable that it contains a low molecular weight component together with the adhesive base. The low molecular weight component preferably has a weight average molecular weight of 400 or more and 100,000 or less, and a glass transition temperature (Tg) of -40°C or less. An adhesive containing a low molecular weight component having such physical properties together with the adhesive base is likely to satisfy the above-mentioned physical properties, particularly the storage modulus G' (-30) at -30°C. The weight average molecular weight in this specification is a value measured in terms of standard polystyrene by gel permeation chromatography (GPC). The glass transition temperature (Tg) in this specification is a value measured by a differential scanning calorimeter at a heating and cooling rate of 20°C/min.

From the viewpoint of increasing the storage modulus G'(85) of the resulting adhesive or decreasing the creep compliance fluctuation value, the weight average molecular weight (Mw) of the low molecular weight component is preferably 400 or more, more preferably 1000 or more, particularly preferably 2500 or more, and even more preferably 3500 or more. From the viewpoint of decreasing the haze value of the resulting adhesive, the weight average molecular weight of the low molecular weight component is preferably 100,000 or less, more preferably 50,000 or less, particularly preferably 20,000 or less, and even more preferably 14,000 or less.

The glass transition temperature (Tg) of the low molecular weight component is preferably −40° C. or lower, more preferably −45° C. or lower, particularly preferably −50° C. or lower, and even more preferably −52° C. or lower. The glass transition temperature (Tg) of the low molecular weight component is preferably −80° C. or higher, more preferably −70° C. or higher, particularly preferably −65° C. or higher, and even more preferably −60° C. or higher.
Specific examples of the low molecular weight component will be described later.

The type of adhesive according to this embodiment is not particularly limited as long as the above-mentioned physical properties are satisfied, and may be, for example, any of acrylic adhesives, polyester adhesives, polyurethane adhesives, rubber adhesives, silicone adhesives, etc. Furthermore, 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.

Specifically, the adhesive according to this embodiment is preferably an adhesive obtained from an adhesive composition (hereinafter sometimes referred to as "adhesive composition P") containing a (meth)acrylic acid ester polymer (A) as an adhesive base, a low molecular weight component (B), and optionally a crosslinking agent (C). Such an adhesive is likely to satisfy the above-mentioned physical properties and is 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 term "polymer" also includes the concept of "copolymer."

(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. Among these, from the viewpoint of lowering the glass transition temperature (Tg) of the (meth)acrylic acid ester polymer (A) and lowering the elasticity of the resulting adhesive, (meth)acrylic acid esters having an alkyl group with 4 to 12 carbon atoms are preferred, and (meth)acrylic acid esters having an alkyl group with 5 to 10 carbon atoms are particularly preferred. Specifically, n-butyl (meth)acrylate and 2-ethylhexyl (meth)acrylate are preferred, and 2-ethylhexyl acrylate is particularly preferred. These may be used alone or in combination of two or more.

The (meth)acrylic acid ester polymer (A) preferably contains 60% 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, more preferably 80% by mass or more, particularly preferably 90% by mass or more, even more preferably 95% by mass or more, and most preferably 98% by mass or more. This allows the (meth)acrylic acid ester polymer (A) to be given suitable adhesion, and makes it easy to reduce the elastic modulus of the resulting adhesive. In addition, the (meth)acrylic acid ester polymer (A) preferably contains 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. This allows other monomer components to be introduced into the (meth)acrylic acid ester polymer (A) in the desired amount.

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 (C) described below, or with the crosslinking agent (C) and the low molecular weight component (B) via the reactive functional group derived from the reactive functional group-containing monomer, thereby forming a crosslinked structure (three-dimensional network structure), and an adhesive having the desired cohesive strength is obtained. The adhesive is likely to satisfy the creep compliance fluctuation value and creep recovery rate 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 and/or carboxyl group-containing monomers are preferred, and it is particularly preferred to use a combination of hydroxyl group-containing monomers and carboxyl group-containing monomers. Hydroxyl group-containing monomers make it easy to adjust the crosslink density and easily satisfy the creep compliance fluctuation value and creep recovery rate described above. In addition, carboxyl group-containing monomers can improve the adhesive strength of the resulting pressure-sensitive adhesive and increase adhesion to the adherend.

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 above-mentioned physical properties, (meth)acrylic acid hydroxyalkyl esters having a hydroxyalkyl group with 1 to 4 carbon atoms are preferred. Specifically, for example, 2-hydroxyethyl (meth)acrylate and 4-hydroxybutyl (meth)acrylate 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 reactive functional group-containing monomers as monomer units constituting the polymer in a total amount of 0.1% by mass or more, particularly preferably 0.5% by mass or more, and even more preferably 1.0% by mass or more. The (meth)acrylic acid ester polymer (A) preferably contains reactive functional group-containing monomers as monomer units constituting the polymer in a total amount of 10% by mass or less, more preferably 7% by mass or less, particularly preferably 5% by mass or less, and even more preferably 2% by mass or less. Components derived from reactive functional group-containing monomers tend to increase the elastic modulus by inducing water solidification at low temperatures, but if the upper limit of the content of reactive functional group-containing monomers is as above, this tendency can be suppressed and the storage modulus G' at low temperatures can be reduced. In addition, when the (meth)acrylic acid ester polymer (A) contains reactive functional group-containing monomers as monomer units in the above amount, the storage modulus G' described above can be set in a suitable range. Furthermore, when the (meth)acrylic acid ester polymer (A) contains the reactive functional group-containing monomer as a monomer unit in the above amount, the degree of crosslinking can be made relatively strong, the creep compliance fluctuation value can be reduced, and the creep recovery rate can be increased.

The (meth)acrylic acid ester polymer (A) preferably contains hydroxyl group-containing monomer as a monomer unit constituting the polymer in an amount of 0.01% by mass or more as a lower limit, more preferably 0.1% by mass or more, particularly 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 hydroxyl group-containing monomer as a monomer unit constituting the polymer in an amount of 10% by mass or less as an upper limit, more preferably 7% by mass or less, particularly preferably 4.8% by mass or less, and even more preferably 1.8% by mass or less. When the (meth)acrylic acid ester polymer (A) contains hydroxyl group-containing monomer as a monomer unit in the above amount, it becomes easier to satisfy the above-mentioned storage modulus G', creep compliance fluctuation value, and creep recovery rate.

The (meth)acrylic acid ester polymer (A) may not contain a carboxyl group-containing monomer as a monomer unit constituting the polymer, but if it contains a carboxyl group-containing monomer, it is preferable that it contains 0.005% by mass or more, more preferably 0.01% by mass or more, particularly preferably 0.1% by mass or more, and even more preferably 0.2% by mass or more. In addition, if the (meth)acrylic acid ester polymer (A) contains a carboxyl group-containing monomer as a monomer unit constituting the polymer, it is preferable that it contains 7% by mass or less as an upper limit, more preferably 4% by mass or less, particularly preferably 1.5% by mass or less, and even more preferably 0.8% by mass or less. If the (meth)acrylic acid ester polymer (A) contains the carboxyl group-containing monomer as a monomer unit in the above amount, the resulting pressure-sensitive adhesive will have better flex resistance when repeatedly applied to a flex device.

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 form 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 400,000 or more, more preferably 700,000 or more, particularly preferably 900,000 or more, and even more preferably 1,100,000 or more. This can prevent a decrease in the storage modulus G' at high temperatures and contribute to a decrease in the creep compliance fluctuation value. On the other hand, the weight average molecular weight of the (meth)acrylic acid ester polymer (A) is preferably 3,000,000 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. This can prevent the storage modulus G' at low temperatures from becoming too high. 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) Low molecular weight component (B)
The low molecular weight component (B) may be any component having the weight average molecular weight and glass transition temperature (Tg) described above. For example, it is preferable to use an acrylic oligomer, a silicone oligomer, an olefin oligomer, or the like. When the pressure-sensitive adhesive according to the present embodiment is an acrylic pressure-sensitive adhesive and uses a (meth)acrylic acid ester polymer (A) as a main adhesive agent, the (meth)acrylic acid ester polymer (A) and From the viewpoint of compatibility, it is preferable to use an acrylic oligomer, which makes it easier to satisfy the above-mentioned physical properties.

In addition, it is preferable that the low molecular weight component (B) has a reactive functional group. By having the low molecular weight component (B) have a reactive functional group, it reacts with the functional group of the (meth)acrylic acid ester polymer (A) or the crosslinking agent (C) described later to form part of a crosslinked structure, and a predetermined cohesive force is obtained. As a result, the creep compliance fluctuation value and creep recovery rate described above are more likely to be satisfied. In addition, by forming part of the crosslinked structure and being incorporated into the adhesive, the low molecular weight component (B) can suppress bleeding out from the adhesive and prevent a decrease in adhesive strength.

Examples of the reactive functional group possessed by the low molecular weight component (B) include a hydroxyl group, a carboxyl group, an epoxy group, and a thiol group. Among these, a hydroxyl group, which has excellent reactivity with the crosslinking agent (C) described below, particularly an isocyanate-based crosslinking agent, or an epoxy group, which has excellent reactivity with the carboxyl group possessed by the (meth)acrylic acid ester polymer (A), is preferred.

When the reactive functional group is a hydroxyl group, the amount of hydroxyl groups in the low molecular weight component (B) is preferably 1 to 1000 mgKOH/g, more preferably 5 to 100 mgKOH/g, particularly preferably 10 to 70 mgKOH/g, and even more preferably 15 to 30 mgKOH/g. When the reactive functional group is an epoxy group, the amount of epoxy groups in the low molecular weight component (B) is preferably 0.01 to 100 meq/g, more preferably 0.1 to 10 meq/g, particularly preferably 0.7 to 7 meq/g, and even more preferably 1.2 to 3 meq/g.

The acrylic oligomer is a polymer of one or more acrylic monomers and, if desired, other monomers. The acrylic oligomer preferably contains, as monomer units constituting the oligomer, an alkyl (meth)acrylate ester and a monomer containing a reactive functional group.

The content of the low molecular weight component (B) in the adhesive composition P is preferably 0.1 parts by mass or more, more preferably 1 part by mass or more, particularly preferably 5 parts by mass or more, and even more preferably 10 parts by mass or more, relative to 100 parts by mass of the (meth)acrylic acid ester polymer (A). This contributes to lowering the storage modulus G' of the resulting adhesive, particularly the storage modulus G' (-30) at -30°C. The content is also preferably 80 parts by mass or less, more preferably 50 parts by mass or less, particularly preferably 30 parts by mass or less, and even more preferably 20 parts by mass or less. This makes it possible to suppress an increase in creep compliance fluctuation value and a decrease in adhesive strength due to bleed-out.

(1-3) Crosslinking agent (C)
The crosslinking agent (C) crosslinks the (meth)acrylic acid ester polymer (A), or the (meth)acrylic acid ester polymer (A) and the low molecular weight component (B), to form a three-dimensional network structure, triggered by heating the adhesive composition P containing the crosslinking agent (C), etc. This improves the cohesive strength of the resulting adhesive, making it easier to satisfy the creep compliance fluctuation value and creep recovery rate described above.

The crosslinking agent (C) may be any that reacts with the reactive functional groups of the (meth)acrylic acid ester polymer (A), or the (meth)acrylic acid ester polymer (A) and the low molecular weight component (B), 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. The crosslinking agent (C) may be used alone or in combination of two or more.

Among the above, isocyanate-based crosslinking agents are particularly excellent in reactivity with hydroxyl groups, and epoxy-based crosslinking agents are particularly excellent in reactivity with carboxyl groups. Therefore, when the (meth)acrylic acid ester polymer (A) and/or the low molecular weight component (B) have hydroxyl groups, it is preferable to use an isocyanate-based crosslinking agent, and when the (meth)acrylic acid ester polymer (A) and/or the low molecular weight component (B) have carboxyl groups, it is preferable to use an epoxy-based crosslinking agent. Also, when the (meth)acrylic acid ester polymer (A) has hydroxyl groups and carboxyl groups, it is preferable to use an isocyanate-based crosslinking agent and an epoxy-based crosslinking agent in combination.

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.

Examples of epoxy crosslinking agents include 1,3-bis(N,N-diglycidylaminomethyl)cyclohexane, N,N,N',N'-tetraglycidyl-m-xylylenediamine, ethylene glycol diglycidyl ether, 1,6-hexanediol diglycidyl ether, trimethylolpropane diglycidyl ether, diglycidylaniline, and diglycidylamine. Among these, N,N,N',N'-tetraglycidyl-m-xylylenediamine is preferred from the viewpoint of reactivity with carboxy groups.

The content of the crosslinking agent (C) in the adhesive composition P is preferably 0.001 parts by mass or more, more 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 preferably 30 parts by mass or less, more preferably 10 parts by mass or less, particularly preferably 5 parts by mass or less, and even more preferably 1 part by mass or less. When the content of the crosslinking agent (C) is within the above range, the creep compliance fluctuation value and creep recovery rate described above are more likely to be satisfied.

When an isocyanate-based crosslinking agent and an epoxy-based crosslinking agent are used in combination, the content of the isocyanate-based crosslinking agent in the adhesive composition P is preferably 0.0009 parts by mass or more, more preferably 0.009 parts by mass or more, particularly preferably 0.09 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 preferably 20 parts by mass or less, more preferably 9 parts by mass or less, particularly preferably 4 parts by mass or less, and even more preferably 0.8 parts by mass or less.

When an isocyanate-based crosslinking agent and an epoxy-based crosslinking agent are used in combination, the content of the epoxy-based crosslinking agent in the adhesive composition P is preferably 0.0001 parts by mass or more, more preferably 0.001 parts by mass or more, particularly preferably 0.01 parts by mass or more, and even more preferably 0.1 parts by mass or more, relative to 100 parts by mass of the (meth)acrylic acid ester polymer (A). The content is preferably 10 parts by mass or less, more preferably 1 part by mass or less, particularly preferably 0.8 parts by mass or less, and even more preferably 0.4 parts by mass or less.

(1-4) 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 constituting 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 low molecular weight component (B) and, if desired, a crosslinking agent (C), and further adding additives, if 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 resulting polymer and adjust the molecular weight distribution. Therefore, the adhesive is less likely to become biased due to long-term bending, and the adhesive has excellent recovery from a bent state.

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 low molecular weight component (B), and optionally the crosslinking agent (C), additives and 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) Manufacture of adhesive The adhesive according to the present embodiment is preferably obtained from the adhesive composition P, and more preferably obtained by crosslinking the adhesive composition P. Crosslinking of the adhesive composition P can usually be carried out 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), or the (meth)acrylic acid ester polymer (A) and the low molecular weight component (B) are sufficiently crosslinked via the crosslinking agent (C) to form a crosslinked structure, and an adhesive is obtained.

In the adhesive according to this embodiment, it is preferable to lower the storage modulus G', particularly at low temperatures, by lowering the glass transition temperature (Tg) of the (meth)acrylic acid ester polymer (A) and adding a low molecular weight component (B), while at the same time lowering the creep compliance fluctuation value by carrying out thorough crosslinking. This makes it easier to achieve both the storage modulus G' at -30°C (-30) and the creep compliance fluctuation value described above.

[Adhesive sheet]
The adhesive sheet of this embodiment has an adhesive layer for bonding one flexible member and another flexible member that constitute a repeatedly bending device, and the adhesive layer is made of the adhesive described above.

FIG. 1 shows a specific configuration of one example of the pressure-sensitive adhesive sheet according to this embodiment.
As shown in Fig. 1, an adhesive sheet 1 according to one embodiment 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. Note that the release surface of the release sheet in this specification refers to a 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.

(1) Constituent Element (1-1) Adhesive Layer The adhesive layer 11 is composed of the adhesive according to the above-mentioned embodiment, and is preferably composed of an adhesive obtained by crosslinking the adhesive composition P.

The thickness of the adhesive layer 11 in the adhesive sheet 1 according to this 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 floating or peeling is less likely to occur at the interface between the adhesive layer and the adherend. In addition, the thickness of the adhesive layer 11 is preferably 300 μm or less as an upper limit, more preferably 150 μm or less, particularly preferably 90 μm or less, and from the viewpoint of obtaining a thinner repeated bending device, it is further preferably 40 μm or less. When the upper limit of the thickness of the adhesive layer 11 is as above, the adhesive layer and the adherend are easily maintained in contact with each other, so that the bending resistance is more excellent, and the stress applied to the adhesive layer is relatively small, so that the adhesive layer is easily restored from the bent state. The adhesive layer 11 may be formed as a single layer or as a laminate of multiple layers.

(1-2) Release Sheet 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 in 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. It is preferable that one of the release sheets 12a and 12b 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.

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 (2-1) Haze Value The haze value of the pressure-sensitive adhesive layer 11 in the pressure-sensitive adhesive sheet 1 according to this embodiment is preferably 30% or less, more preferably 10% or less, particularly preferably 1% or less, and even more preferably 0.8% or less. This makes it possible to suppress changes in the appearance of the pressure-sensitive adhesive layer 11 due to bending. In addition, the lower limit of the haze value is not particularly limited, but is preferably 0% or more, more preferably 0.01% or more, and particularly preferably 0.03% or more. The haze value in this specification is a value measured in accordance with JIS K7136:2000.

(2-2) Adhesive strength The adhesive strength of the adhesive sheet 1 according to the present embodiment to soda lime glass is preferably 0.1 N/25 mm or more as a lower limit, more preferably 1.0 N/25 mm or more, particularly preferably 1.3 N/25 mm or more, and even more preferably 1.4 N/25 mm or more. When the lower limit of the adhesive strength of the adhesive sheet 1 is as described above, lifting or peeling due to bending at the interface between the adhesive layer and the adherend is less likely to occur. On the other hand, the upper limit of the adhesive strength is not particularly limited, but is usually preferably 60 N/25 mm or less, more preferably 40 N/25 mm or less, and in terms of reworkability that allows the adhesive sheet to be reattached when the adhesive sheet is misattached, it is particularly preferably 20 N/25 mm or less, and even more preferably 10 N/25 mm or less. In addition, the adhesive strength in this specification basically refers to the adhesive strength measured by the 180-degree peeling method in accordance with JIS Z0237:2009, and the specific test method is as shown in the test examples described later.

(3) Manufacturing of adhesive sheet As an example of manufacturing the adhesive sheet 1, the case where the adhesive composition P is used will be described. A coating solution of the adhesive composition P is applied to the release surface of one of the release sheets 12a (or 12b), and the adhesive composition P is thermally crosslinked by heat treatment to form a coating layer, and then the release surface of the other release sheet 12b (or 12a) is superimposed on the coating layer. If a curing period is required, a curing period is provided, and if no curing period is required, the coating layer becomes the adhesive layer 11 as it is. This results in the adhesive sheet 1. The conditions for the heat treatment and curing are as described above.

In another example of the production of the adhesive sheet 1, a coating solution of the adhesive composition P is applied to the release surface of one of the release sheets 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 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 the coating layer and the release sheet 12b with the coating layer are bonded together so that both coating layers are in contact with each other. If a curing period is required, a curing period is provided, and if no curing period is required, the laminated coating layer becomes the adhesive layer 11 as it is. This results in the above-mentioned adhesive sheet 1. According to this production example, stable production is possible even when the adhesive layer 11 is relatively thick.

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 composed of the adhesive described above, or is the adhesive layer 11 of the adhesive sheet 1 described above.

The repeatedly bent laminated member 2 is either a repeatedly bent device itself or a member constituting part of a repeatedly bent device. The repeatedly bent device is preferably a 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, 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, 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.

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 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 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 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 that when the device is repeatedly bent or when it is left in a bent state for a long period of time, lifting or peeling is unlikely to occur at the interface between the adhesive layer 11 and the adherend (the first bendable member 21 and the second bendable member 22). In addition, when the adhesive constituting the adhesive layer 11 satisfies the creep recovery rate described above, the device also has excellent recovery from a bent state.

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 constructed 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 cover film 31, polarizing film 33, touch sensor film 35, organic EL element 37, and TFT substrate 39 correspond to the flexible member.

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.

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) 98.5 parts by mass of 2-ethylhexyl acrylate, 1.0 part by mass of 4-hydroxybutyl acrylate, and 0.5 parts by mass of acrylic acid were copolymerized by a solution polymerization method to prepare a (meth)acrylic acid ester polymer (A). The molecular weight of this (meth)acrylic acid ester polymer (A) was measured by the method described below, and the weight average molecular weight (Mw) was 1,200,000.

2. Preparation of adhesive composition A mixture of 100 parts by mass (solid content equivalent; the same applies below) of the (meth)acrylic acid ester polymer (A) obtained in the above step 1 and a hydroxyl group-containing acrylic oligomer (B1; product name "ARUFON (registered trademark)" manufactured by Toagosei Co., Ltd.) as a low molecular weight component (B) was prepared. A coating solution of a pressure-sensitive adhesive composition was obtained by mixing 15 parts by mass of 1,2-dimethyl-2,4-dimethylphenyl ...

3. Production of adhesive sheet The obtained coating solution of the adhesive composition was applied by a knife coater to the release-treated surface of a heavy release type release sheet (manufactured by Lintec Corporation, product name "SP-PET752150"), one side of which was a polyethylene terephthalate film 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.

Then, the coating layer on the heavy release type release sheet obtained above and a light release type release sheet (manufactured by Lintec Corporation, product name "SP-PET381130"), one side of which was a polyethylene terephthalate film treated with a silicone-based release agent, were attached so that the release-treated surface of the light release type release sheet was in contact with the coating layer, and the sheet was cured for 7 days under conditions of 23°C and 50% RH to produce an adhesive sheet having an adhesive layer with a thickness of 25 μm, i.e., an adhesive sheet having a configuration of heavy release type release sheet/adhesive layer (thickness: 25 μm)/light release type release sheet. The thickness of the adhesive layer was measured in accordance with JIS K7130 using a constant pressure thickness gauge (manufactured by Techlock Corporation, product name "PG-02").

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 AAc: acrylic acid
[Low molecular weight component (B)]
B1: Hydroxyl-containing acrylic oligomer (manufactured by Toagosei Co., Ltd., product name "ARUFON (registered trademark) UH-2000", hydroxyl content: 20 mg KOH/g, Mw: 11000, Tg: -55°C)
B2: Epoxy group-containing acrylic oligomer (manufactured by Toagosei Co., Ltd., product name "ARUFON UG-4010", epoxy group content: 1.4 meq/g, Mw: 2900, Tg: -57°C)
B3: Non-functional acrylic oligomer (manufactured by Toagosei Co., Ltd., product name "ARUFON UP-1000", Mw: 3000, Tg: -77°C)
[Isocyanate-based crosslinking agent (C1)]
XDI: Trimethylolpropane modified xylylene diisocyanate (manufactured by Soken Chemical Industries, Ltd., product name "TD-75")
TDI: Trimethylolpropane modified tolylene diisocyanate (manufactured by Toyochem Co., Ltd., product name "BHS8515")
[Epoxy-based crosslinking agent (C2)]
N,N,N',N'-Tetraglycidyl-m-xylylenediamine
[Silane coupling agents]
SC1: 3-glycidoxypropyltrimethoxysilane SC2: 1,6-bis(trimethoxysilyl)hexane

[Examples 2 to 4, Comparative Examples 1 to 4]
Except for changing the types and ratios of the monomers constituting the (meth)acrylic acid ester polymer (A), the weight average molecular weight (Mw) of the (meth)acrylic acid ester polymer (A), the type and amount of the low molecular weight component (B), the type of the isocyanate-based crosslinking agent (C1), the amount of the epoxy-based crosslinking agent (C2), and the type of the silane coupling agent as shown in Table 1, pressure-sensitive adhesive sheets were produced in the same manner as in 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 800 μm. A cylindrical body having a diameter of 8 mm (height of 800 μm) was punched out from the resulting laminate of pressure-sensitive adhesive layers 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 "MCR302") in accordance with JIS K7244-6, and the storage modulus G' (-30) at -30°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
Heating rate: 5° C./min
Distortion: 1%
Normal force: 1.0N
Measurement temperature: -30℃~140℃

Based on the obtained results, the storage modulus change rate (-30/25), which is the value obtained by dividing the storage modulus G'(-30) by the storage modulus G'(25), and the storage modulus change rate (-30/85), which is the value obtained by dividing the storage modulus G'(-30) by the storage modulus G'(85), were calculated. The results are shown in Table 2.

Test Example 3 (Creep Compliance Measurement)
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 800 μm. A cylindrical body having a diameter of 8 mm (height of 800 μm) was punched out from the resulting laminate of pressure-sensitive adhesive layers to form a sample.

A viscoelasticity measuring device (manufactured by Anton Paar, product name "MCR302") was used to measure the creep compliance J(t) (MPa ^-1 ) of the above sample under the following conditions while continuously applying a stress of 4500 Pa. From the measurement results, the value when a stress of 4500 Pa was applied was defined as the minimum creep compliance J(t) _min (MPa ^-1 ), and the maximum creep compliance J(t) _max (MPa ^-1 ) measured up to 1200 seconds after the measurement of the minimum creep compliance J(t) _min was derived.
Measurement temperature: 25°C
Measurement points: 1000 points (logarithmic plot)

From the obtained minimum creep compliance J(t) _min (MPa ^-1 ) and maximum creep compliance J(t) _max (MPa ^-1 ), the creep compliance variation value ΔlogJ(t) was calculated according to the following formula (I).
ΔlogJ(t)=logJ(t) _max -logJ(t) _min ...(I)

In addition, a viscoelasticity measuring device (manufactured by Anton Paar, product name "MCR302") was used to continuously apply a stress of 4500 Pa to the above sample under the following conditions, and then the applied stress was reduced to 0 Pa and this state was maintained for a while. During this time, the creep compliance J(t) (MPa ^-1 ) was measured. The creep compliance value measured 1200 seconds after the start of application of a stress of 4500 Pa was taken as creep compliance J(t) (s = 1200) (MPa ^-1 ), and the creep compliance value measured 100 seconds after the applied stress was reduced to 0 Pa was taken as creep compliance J(t) (s = 1300) (MPa ^-1 ).
Measurement temperature: 25°C
Measurement points when stress is applied: 1000 points (logarithmic plot)
Measurement points when stress is removed: 1000 points (logarithmic plot)

The creep recovery rate (%) was calculated from the obtained creep compliance J(t) (s = 1200) (MPa ^-1 ) and creep compliance J(t) (s = 1300) (MPa ^-1 ) according to the following formula (II). The results are shown in Table 2.
Creep recovery rate (%) = (1 - J (t) (s = 1300) / J (t) (s = 1200)) x 100 ... (II)

[Test Example 4] (Haze value measurement)
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 5] (Measurement of adhesive strength)
The light release type release sheet was peeled off from the adhesive sheet 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 laminate of heavy release type release sheet/adhesive layer/PET film. The obtained laminate was cut into a width of 25 mm and a length of 110 mm.

In an environment of 23°C and 50% RH, the heavy release type release sheet was peeled off from the laminate, and the exposed adhesive layer was attached to a soda lime glass plate (manufactured by Nippon Sheet Glass Co., Ltd., product name "Soda Lime Glass", thickness: 1.1 mm), and pressurized for 20 minutes at 0.5 MPa and 50°C in an autoclave manufactured by Kurihara Seisakusho Co., Ltd. After leaving it under conditions of 23°C and 50% RH for 24 hours, 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 (manufactured by Orientec Co., Ltd., Tensilon) 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. The results are shown in Table 2.

[Test Example 6] (Evaluation of bending resistance)
Under an environment of 23°C and 50% RH, the light release type release sheet was peeled off from the adhesive sheet prepared in the examples and comparative examples, and the exposed adhesive layer was attached to one side of a polyethylene terephthalate (PET) film (manufactured by Toyobo Co., Ltd., product name "PET50TA063", thickness: 100 μm). Next, the heavy release type release sheet was peeled off, and the exposed adhesive layer was attached to a polyimide film (thickness: 50 μm). Then, after pressing for 20 minutes at 0.5 MPa and 50°C in an autoclave manufactured by Kurihara Seisakusho Co., Ltd., it was left for 24 hours under conditions of 23°C and 50% RH. The laminate consisting of the PET film/adhesive layer/polyimide film obtained in this way was cut into a width of 50 mm and a length of 200 mm, which was used as a sample.

As shown in Figure 4, both ends of the obtained sample were fixed to two holding plates of a bending tester with a constant temperature and humidity chamber (manufactured by Yuasa System Co., Ltd., product name "CL09-typeD01-FSC90"). At this time, the polyimide film side of the sample was fixed on the inside of the bend. The sample was then bent 200,000 times in temperature environments of -30°C, 25°C, and 80°C, with a bending diameter (diameter) of 3 mm, a stroke of 80 mm, and a bending speed of 60 rpm.

After the dynamic bending test, the sample was visually inspected for peeling at the interface between the adhesive layer and the adherend at the bent portion. The bending resistance was evaluated according to the following criteria. The results are shown in Table 2.
⊚: No line marks were present at the bent portion.
◯: Line marks are present in the bent portion, but when viewed from directly in front, the line marks are barely visible.
△: Line marks were present at the bent portion, and the line marks were visible even when viewed from the front. Or, minute air bubbles were generated at the bent portion.
×: Lifting or peeling occurred at the bent portion.

As can be seen from Table 2, the adhesive layer of the adhesive sheet of the embodiment had excellent flex resistance, with little lifting or peeling occurring at the interface between the adhesive layer and the adherend when flexed for long periods of time, regardless of whether the adhesive layer was in a low, normal, or high temperature environment.

The present invention is suitable for bonding one flexible member to another flexible member that constitutes a repeatedly bending device.

REFERENCE SIGNS LIST 1... Adhesive sheet 11... 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 (12)

A pressure-sensitive adhesive for repeatedly bending devices for bonding one flexible member and another flexible member that constitute a device that is repeatedly bent, comprising:
The pressure-sensitive adhesive has a storage modulus G'(-30) of 0.20 MPa or less at -30 ° C.;
The adhesive for use in a repeatedly flexed device is characterized in that the creep compliance value measured when a stress of 4500 Pa is applied to the adhesive is defined as minimum creep compliance J(t) _min (MPa ^-1 ), the maximum creep compliance value measured during the application of a stress of 4500 Pa until 1200 seconds after the measurement of the minimum creep compliance _J (t) min is defined as maximum creep compliance J(t) _max (MPa ^-1 ), and the creep compliance fluctuation value Δlog J(t) calculated from the following formula (I) is 2.60 or less.
ΔlogJ(t)=logJ(t) _max -logJ(t) _min ...(I) The adhesive for repeated bending devices according to claim 1, characterized in that the creep compliance value measured 1200 seconds after continuously applying a stress of 4500 Pa to the adhesive is defined as creep compliance J(t) (s = 1200) (MPa ^-1 ), and the creep compliance value measured 100 seconds after the stress applied to the adhesive is then reduced to 0 Pa is defined as creep compliance J(t) (s = 1300) (MPa ^-1 ), and the creep recovery rate calculated from the following formula (II) is 90% or more.
Creep recovery rate (%) = (1 - J (t) (s = 1300) / J (t) (s = 1200)) x 100 ... (II) The pressure-sensitive adhesive for repeated bending devices according to claim 1 or 2, characterized in that the storage modulus G'(85) at 85°C is 0.005 MPa or more. The pressure-sensitive adhesive for repeated bending devices according to any one of claims 1 to 3, characterized in that the storage modulus change rate (-30/25), which is the value obtained by dividing the storage modulus G'(-30) at -30°C by the storage modulus G'(25) at 25°C, is 5.0 or less. The pressure-sensitive adhesive for repeated bending devices according to any one of claims 1 to 4, characterized in that the storage modulus change rate (-30/85), which is the value obtained by dividing the storage modulus G' (-30) at -30°C by the storage modulus G' (85) at 85°C, is 10.0 or less. The pressure-sensitive adhesive for repeated bending devices according to any one of claims 1 to 5, characterized in that the gel fraction is 40% or more and 95% or less. The pressure-sensitive adhesive contains a main adhesive agent and a low molecular weight component,
The weight average molecular weight of the low molecular weight component is 400 or more and 100,000 or less,
The pressure-sensitive adhesive for repeated bending devices according to any one of claims 1 to 6, characterized in that the glass transition temperature (Tg) of the low molecular weight component is -40°C or lower. The adhesive for repeated bending devices according to any one of claims 1 to 7, characterized in that the adhesive is an acrylic adhesive. 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,
9. An adhesive sheet, comprising the adhesive layer for use in a repeatedly flexed device according to claim 1. The pressure-sensitive adhesive sheet has two release sheets,
10. The pressure-sensitive adhesive sheet according to claim 9, 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 flexed laminated member, characterized in that the pressure-sensitive adhesive layer is made of the pressure-sensitive adhesive for a repeatedly flexed device according to any one of claims 1 to 8. A repeatedly bent device comprising the repeatedly bent laminated member according to claim 11.

Images