WO2024195837A1

WO2024195837A1 - Optical adhesive sheet

Info

Publication number: WO2024195837A1
Application number: PCT/JP2024/011103
Authority: WO
Inventors: 蒼一朗古賀; 智史岩田; 直樹関岡; 普史形見
Original assignee: 日東電工株式会社
Priority date: 2023-03-23
Filing date: 2024-03-21
Publication date: 2024-09-26

Abstract

An optical adhesive sheet (10) includes a base polymer and an oligomer. The oligomer is a polymer of a monomer component containing a (meth)acrylic acid ester monomer, and the monomer component further includes an acidic group-containing monomer. The optical adhesive sheet (10) has a shear storage modulus at -30°C of 350 kPa or less, and an adhesive strength F1 (N/20 mm) to glass in a first peel test satisfies 8.0≤F1.

Description

Optical Adhesive Sheet

The present invention relates to an optical adhesive sheet.

A display panel has a laminated structure that includes, for example, a pixel panel, a polarizing film, a touch panel, a cover film, etc. In such a display panel, for example, a transparent adhesive sheet (optical adhesive sheet) is used to bond each layer in the laminated structure.

Furthermore, in recent years, the development of repeatedly foldable display panels for smartphones and tablet terminals has progressed. Furthermore, the development of rollable display panels has also progressed. In flexible devices such as such foldable display panels and rollable display panels, each layer in the laminate structure is made so that it can be repeatedly folded. In flexible devices, optical adhesive sheets are used to bond each layer together (see, for example, Patent Document 1 below). The optical adhesive sheet for flexible devices described in Patent Document 1 has excellent flexibility so that it can withstand repeated folding, and also has excellent adhesion to the adherend.

JP 2020-122140 A

On the other hand, optical adhesive sheets for flexible devices are required to have excellent flexibility while also having excellent adhesive properties. Specifically, at the bent points of a foldable display panel, a relatively large shear force is applied in the direction along the adherend, making the optical adhesive sheet prone to peeling. Also, when a rollable display panel is in a rolled state, the optical adhesive sheet is prone to peeling due to the continuous shear force in the direction along the adherend. In other words, optical adhesive sheets for such applications are desired to have improved adhesive properties to the adherend.

The present invention provides an optical adhesive sheet that is suitable for flexible device applications and has excellent flexibility and adhesive properties.

The present invention [1] is an optical adhesive sheet comprising a base polymer and an oligomer, wherein the oligomer is a polymer of a monomer component containing a (meth)acrylic acid ester monomer, and the monomer component further contains an acidic group-containing monomer, and the optical adhesive sheet has a shear storage modulus at -30°C of 350 kPa or less, and an adhesive strength F1 (N/20 mm) to glass in the first peel test described below satisfies the following formula (1).
8.0≦F1 (1)
(First peel test)
A polyethylene terephthalate film is attached to one side of the optical adhesive sheet, and a glass plate is attached to the other side, and a measurement sample is prepared by heating and pressurizing. The measurement sample is left at room temperature for 30 minutes, and then a test piece (the polyethylene terephthalate film with the optical adhesive sheet) is peeled off from the glass plate in the measurement sample. The measurement conditions are as follows: the peel angle of the test piece from the glass plate is 180°, the tensile speed is 300 mm/min, and the peel length is 50 mm in an environment of 25°C and 55% relative humidity.

The present invention [2] includes the optical adhesive sheet described in the above [1], in which the adhesive strength F2 (N / 20 mm) to glass in the second peel test described below satisfies the following formula (2).
6.0≦F2 (2)
(Second Peel Test)
The measurement conditions are the same as those of the first peel test, except that the pulling speed is 60 mm/min.

The present invention [3] includes the optical adhesive sheet described in [1] or [2] above, in which the (meth)acrylic acid ester monomer further includes an alicyclic alkyl group-containing (meth)acrylic acid ester monomer.

The present invention [4] includes an optical adhesive sheet according to any one of [1] to [3] above, in which the acidic group in the acidic group-containing monomer is a carboxy group and/or a phenolic hydroxy group.

The present invention [5] includes the optical adhesive sheet according to any one of [1] to [4] above, in which the glass transition temperature of the homopolymer of the acidic group-containing monomer is 40°C or higher.

The present invention [6] includes an optical adhesive sheet according to any one of [1] to [5] above, in which the content of the acidic group-containing monomer in the monomer component is 1 mass% or more and 20 mass% or less.

The present invention [7] includes an optical adhesive sheet according to any one of [1] to [6] above, in which the amount of the oligomer is 0.3 parts by mass or more and less than 1.5 parts by mass per 100 parts by mass of the base polymer.

The present invention [8] includes an optical adhesive sheet according to any one of [1] to [7] above, which has a haze of 1% or less.

As described above, the optical adhesive sheet of the present invention has a shear storage modulus at -30°C of 350 kPa or less. Such an optical adhesive sheet has excellent flexibility and can relieve stress that occurs in the optical adhesive sheet and the adherend when it is deformed (stress relaxation). Stress relaxation in the optical adhesive sheet can ensure the conformability of the optical adhesive sheet to the adherend, and stress relaxation in the adherend can suppress damage such as cracking of the adherend.

In addition, in the optical adhesive sheet of the present invention, as described above, the oligomer is a polymer of a monomer component containing a (meth)acrylic acid ester monomer, and the monomer component further contains an acidic group-containing monomer, and the adhesive strength F1 (N/20 mm) to glass in the first peel test satisfies 8.0≦F1. In other words, the adhesive strength is excellent. The excellent adhesive strength of the optical adhesive sheet makes it possible to suppress peeling of the optical adhesive sheet from an adherend that is repeatedly deformed.

Optical adhesive sheets like the ones described above are suitable for flexible device applications.

1 is a schematic cross-sectional view of one embodiment of an optical adhesive sheet of the present invention. Fig. 2 shows an example of a method for using the optical adhesive sheet of the present invention. Fig. 2A shows a step of attaching the optical adhesive sheet to a first adherend, Fig. 2B shows a step of joining the first adherend and the second adherend via the optical adhesive sheet, and Fig. 2C shows an aging step.

One embodiment of the optical adhesive sheet of the present invention will be described with reference to Figure 1.

The optical adhesive sheet 10 has a sheet shape with a predetermined thickness and extends in a direction (plane direction) perpendicular to the thickness direction. The optical adhesive sheet 10 has a first adhesive surface 11 and a second adhesive surface 12 opposite the first adhesive surface 11.

FIG. 1 exemplarily shows a state in which release liners L1 and L2 are attached to a first adhesive surface 11 and a second adhesive surface 12 of an optical adhesive sheet 10. The first release liner L1 is disposed on the first adhesive surface 11. The second release liner L2 is disposed on the second adhesive surface 12.

The optical adhesive sheet 10 is an optically transparent adhesive sheet that is placed at the light passing portion of a flexible device. An example of a flexible device is a flexible display panel. An example of a flexible display panel is a foldable display panel and a rollable display panel. A flexible display panel has a layered structure including, for example, a pixel panel, a polarizing film, a touch panel, and a cover film. The optical adhesive sheet 10 is used, for example, for bonding between each layer in the layered structure of a flexible display panel. The release liners L1 and L2 are each peeled off at a predetermined timing when the optical adhesive sheet 10 is used.

The optical adhesive sheet 10 is formed from an adhesive composition. The adhesive composition includes a base polymer and an oligomer. That is, the optical adhesive sheet 10 includes a base polymer and an oligomer. The oligomer is a polymer of a monomer component containing a (meth)acrylic acid ester monomer, and the monomer component further includes an acidic group-containing monomer. The optical adhesive sheet 10 also has a shear storage modulus at -30°C of 350 kPa or less. Furthermore, the optical adhesive sheet 10 has an adhesive strength F1 (N/20 mm) to glass in a first peel test described below that satisfies the following formula (1).

8.0≦F1 (1)

As described above, the optical adhesive sheet 10 has a shear storage modulus of 350 kPa or less at -30°C. Such an optical adhesive sheet has excellent flexibility and can relieve stress that occurs in the optical adhesive sheet and the adherend when it is deformed (stress relaxation). Stress relaxation in the optical adhesive sheet can ensure the conformability of the optical adhesive sheet to the adherend, and stress relaxation in the adherend can suppress damage such as cracking of the adherend.

In addition, as described above, in the optical adhesive sheet 10, the oligomer is a polymer of a monomer component containing a (meth)acrylic acid ester monomer, the monomer component contains an acidic group-containing monomer, and the adhesive strength F1 (N/20 mm) to glass in the first peel test satisfies 8.0≦F1. In other words, the adhesive strength is excellent. The excellent adhesive strength of the optical adhesive sheet makes it possible to suppress peeling of the optical adhesive sheet from an adherend that is repeatedly deformed.

The optical adhesive sheet 10 described above is suitable for flexible device applications. In other words, the optical adhesive sheet 10 is suitable for achieving good repeated deformation of the flexible device in which the optical adhesive sheet 10 is used.

The shear storage modulus of the optical adhesive sheet 10 at -30°C is 350 kPa or less, preferably 330 kPa or less, more preferably 310 kPa or less, and even more preferably 300 kPa or less, from the viewpoint of stress relaxation during deformation (bending, curving, etc.) of the optical adhesive sheet 10, and is, for example, 150 kPa or more, preferably 200 kPa or more, and more preferably 250 kPa or more, from the viewpoint of ensuring the cohesive force of the optical adhesive sheet 10 in the low temperature range. The shear storage modulus is determined by dynamic viscoelasticity measurement, and is specifically described in the examples described later. Methods for adjusting the shear storage modulus of the optical adhesive sheet 10 include, for example, selecting the type and adjusting the amount of the base polymer, oligomer, and crosslinking agent, and adjusting the molecular weight of the base polymer and oligomer.

From the viewpoint of improving the adhesiveness of the optical adhesive sheet 10, the glass transition temperature (Tg) of the oligomer is, for example, 50°C or higher, preferably 60°C or higher, more preferably 65°C or higher, even more preferably 100°C or higher, particularly preferably 120°C or higher, and, for example, 160°C or lower, preferably 150°C or lower, more preferably 140°C or lower, even more preferably 135°C or lower. The Tg of the oligomer is preferably higher than the Tg of the base polymer. Methods for adjusting the Tg of the oligomer include adjusting the monomer composition of the oligomer and adjusting the molecular weight.

For the Tg of an oligomer, the glass transition temperature (theoretical value) calculated based on the Fox formula below is used. The Fox formula is a relational expression between the glass transition temperature Tg of a polymer (oligomer) and the glass transition temperature Tgi of a homopolymer of a monomer constituting this polymer (oligomer). In the Fox formula below, Tg represents the glass transition temperature (°C) of a polymer (oligomer), Wi represents the weight fraction of a monomer mi constituting the polymer (oligomer), and Tgi represents the glass transition temperature (°C) of a homopolymer formed from the monomer mi. Literature values can be used for the glass transition temperature of homopolymers. For example, the glass transition temperatures of various homopolymers are listed in "Polymer Handbook" (4th edition, John Wiley & Sons, Inc., 1999). On the other hand, the glass transition temperature of a homopolymer of a monomer can also be calculated by the method specifically described in JP 2007-51271 A.

Fox formula: 1/(273+Tg)=Σ[Wi/(273+Tgi)]

In the first peel test described below, the adhesive strength F1 (adhesive strength at 300 mm/min) of the optical adhesive sheet 10 to glass is 8.0 N/20 mm or more, preferably 8.5 N/20 mm or more, more preferably 8.8 N/20 mm or more, even more preferably 9.0 N/20 mm or more, particularly preferably 9.2 N/20 mm or more, and for example, 15 N/20 mm or less, preferably 13 N/20 mm or less, more preferably 11 N/20 mm or less, from the viewpoint of suppressing peeling of the optical adhesive sheet 10 from the adherend. Examples of methods for adjusting the adhesive strength F1 of the optical adhesive sheet 10 to glass include, for example, selecting the type of base polymer in the optical adhesive sheet 10, adjusting the molecular weight, and adjusting the blending amount. Note that the selection of the type of base polymer includes adjusting the composition of the monomer that forms the base polymer. In addition, for example, the selection of the type of components other than the base polymer in the optical adhesive sheet 10 and adjusting the blending amount of these components can also be mentioned. Components other than the base polymer include a crosslinking agent, a silane coupling agent, and an oligomer. The method for adjusting the adhesive strength F1 of the optical adhesive sheet 10 to glass is also the same as for the adhesive strength of the optical adhesive sheet 10 to glass in other peel tests described below.

(First peel test)
A measurement sample is prepared by laminating one side of the optical adhesive sheet 10 to a polyethylene terephthalate film, laminating the other side to a glass plate, and performing a heating and pressure treatment.
After leaving the measurement sample at room temperature for 30 minutes, the test piece (the polyethylene terephthalate film with the optical adhesive sheet) is peeled off from the glass plate of the measurement sample. The measurement conditions are as follows: temperature 25°C, relative humidity 55%, peel angle of the test piece from the glass plate is 180°, tensile speed is 300 mm/min, and peel length is 50 mm.

Details of the first peel test and the method for measuring the adhesive strength F1 (initial adhesive strength) of the optical adhesive sheet 10 to glass in the first peel test are specifically described in the Examples below (the same applies to the other peel tests described below and the adhesive strength of the optical adhesive sheet 10 to glass in the other peel tests).

In the second peel test described below, the adhesive strength F2 (adhesive strength at 60 mm/min) of the optical adhesive sheet 10 to glass is, from the viewpoint of suppressing peeling of the optical adhesive sheet 10 from the adherend, for example, 5.0 N/20 mm or more, preferably 6.0 N/20 mm or more, more preferably 6.5 N/20 mm or more, even more preferably 6.8 N/20 mm or more, particularly preferably 7.0 N/20 mm or more, and for example, 12 N/20 mm or less, preferably 10 N/20 mm or less, more preferably 9.0 N/20 mm or less, even more preferably 8.0 N/20 mm or less.

(Second Peel Test)
The measurement conditions are the same as those of the first peel test, except that the pulling speed is 60 mm/min.

The ratio (F2/F1) of the adhesive strength F2 to the adhesive strength F1 is, from the viewpoint of ensuring a stable adhesive strength in the optical adhesive sheet 10, for example, 0.5 or more, preferably 0.6 or more, more preferably 0.65 or more, even more preferably 0.7 or more, and for example, 1.2 or less, preferably 1.0 or less, more preferably 0.9 or less, even more preferably 0.8 or less.

The difference ΔH (= δH ₂ - δH ₁ ) between the hydrogen bond term δH ₁ of the HSP of the base polymer and the hydrogen bond term δH ₂ of the HSP of the oligomer is, for example, 0.15 or more, preferably 0.4 or more, more preferably 0.6 or more, and even more preferably 0.8 or more, from the viewpoint of appropriately lowering the compatibility between the base polymer and the oligomer and sufficiently unevenly distributing the oligomer on and near the adhesive surfaces 11, 12, and from the viewpoint of preventing the compatibility between the base polymer and the oligomer from becoming too low, it is, for example, 1.3 or less, preferably 1.2 or less, and more preferably 1.1 or less. By ensuring the compatibility between the base polymer and the oligomer, the haze can be reduced in the optical adhesive sheet 10. Examples of methods for adjusting the δH ₁ of the base polymer include adjusting the monomer composition of the base polymer. Examples of methods for adjusting the δH ₂ of the oligomer include adjusting the monomer composition of the oligomer.

HSP stands for Hansen solubility parameter, and δH is the hydrogen bond term in the Hansen solubility parameter that represents the energy derived from the hydrogen bonding force between molecules.

The δH of a polymer can be calculated from the molar fraction of the monomers that form the polymer and the hydrogen bonding term of the monomers. The δH of an oligomer can be calculated in a similar manner. The hydrogen bonding term of a monomer can be calculated, for example, using computer software HSPiP (Hansen Solubility Parameters in Practice). The method for calculating δH is specifically described in the Examples below.

The haze of the optical adhesive sheet 10 is, for example, 1% or less, preferably 0.8% or less, more preferably 0.5% or less, and for example, 0.01% or more. The haze of the optical adhesive sheet 10 can be measured using a haze meter in accordance with JIS K7136 (2000).

The total light transmittance of the optical adhesive sheet 10 is, for example, 60% or more, preferably 80% or more, more preferably 85% or more, and for example, 100% or less. The total light transmittance of the optical adhesive sheet 10 can be measured in accordance with JIS K 7375 (2008).

The gel fraction of the optical adhesive sheet 10 is, for example, 60% by mass or more, preferably 70% by mass or more, more preferably 75% by mass or more, from the viewpoint of ensuring the cohesive force of the optical adhesive sheet 10 in the high temperature region, and is, for example, 90% by mass or less, preferably 85% by mass or less, more preferably 83% by mass or less, and even more preferably 82% by mass or less, from the viewpoint of ensuring the flexibility of the optical adhesive sheet 10. Examples of methods for adjusting the gel fraction of the optical adhesive sheet 10 include selecting the type of base polymer in the optical adhesive sheet 10, adjusting the molecular weight, and adjusting the blending amount. Examples of methods for adjusting the gel fraction include selecting the type of crosslinking agent and adjusting the blending amount. In addition, the method for measuring the gel fraction is specifically described in the examples described later.

<Base polymer>
The base polymer is an adhesive component of the optical adhesive sheet 10. Examples of the base polymer include acrylic polymer, silicone polymer, polyester polymer, polyurethane polymer, polyamide polymer, polyvinyl ether polymer, vinyl acetate/vinyl chloride copolymer, modified polyolefin polymer, epoxy polymer, fluoropolymer, and rubber polymer. Preferably, acrylic polymer is used from the viewpoint of ensuring good transparency and adhesiveness. The base polymer may be used alone or in combination of two or more kinds.

An acrylic polymer is a polymer of a monomer component (first monomer component) that contains 50% or more by mass of (meth)acrylic acid ester monomer. Note that "(meth)acrylic" means acrylic and/or methacrylic.

(Meth)acrylic acid ester monomers include, for example, (meth)acrylic acid ester monomers having an alkyl group with 1 to 20 carbon atoms (alkyl group-containing (meth)acrylic acid ester monomers) and (meth)acrylic acid ester monomers having a hydroxy group (hydroxy group-containing (meth)acrylic acid ester monomers). (Meth)acrylic acid ester monomers having an alkyl group with 1 to 20 carbon atoms include, for example, (meth)acrylic acid ester monomers having a chain alkyl group (chain alkyl group-containing (meth)acrylic acid ester monomers) and (meth)acrylic acid ester monomers having an alicyclic alkyl group (alicyclic alkyl group-containing (meth)acrylic acid ester monomers).

Examples of (meth)acrylic acid ester monomers containing a chain alkyl group include methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, s-butyl (meth)acrylate, t-butyl (meth)acrylate, pentyl (meth)acrylate, isopentyl (meth)acrylate, neopentyl (meth)acrylate, n-hexyl (meth)acrylate, heptyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, n-octyl (meth)acrylate, isooctyl (meth)acrylate, nonyl (meth)acrylate, isononyl (meth)acrylate, decyl (meth)acrylate, isodecyl (meth)acrylate, undecyl (meth)acrylate, and lauryl (meth)acrylate.

Examples of the alicyclic alkyl group-containing (meth)acrylic acid ester monomer include the alicyclic alkyl group-containing (meth)acrylic acid ester monomer in the second monomer component described below.

As the alkyl group-containing (meth)acrylic acid ester monomer, from the viewpoint of balancing the flexibility and adhesive strength required for an optical adhesive sheet for flexible device applications in the optical adhesive sheet 10, preferably, at least one selected from a first alkyl group-containing (meth)acrylic acid ester monomer having an alkyl group of 8 to 12 carbon atoms and at least one selected from a second alkyl group-containing (meth)acrylic acid ester monomer having an alkyl group of 1 to 4 carbon atoms are used. The first alkyl group-containing (meth)acrylic acid ester monomer is preferably n-octyl acrylate (NOAA). The second alkyl group-containing (meth)acrylic acid ester monomer is preferably n-butyl acrylate (BA). In other words, a combination of NOAA and BA is more preferably used.

The content of the alkyl group-containing (meth)acrylic acid ester monomer in the first monomer component is, for example, 80% by mass or more, preferably 85% by mass or more, more preferably 88% by mass or more, and for example, less than 100% by mass, preferably 99% by mass or less, from the viewpoint of balancing flexibility and adhesive strength in the optical adhesive sheet 10. When the first alkyl group-containing (meth)acrylic acid ester monomer and the second alkyl group-containing (meth)acrylic acid ester monomer are used in combination, the content of the first alkyl group-containing (meth)acrylic acid ester monomer in the monomer component is, for example, 60% by mass or more, preferably 65% by mass or more, more preferably 68% by mass or more, and for example, 85% by mass or less, preferably 80% by mass or less, more preferably 75% by mass or less. The content of the second alkyl group-containing (meth)acrylic acid ester monomer in the monomer component is, for example, 10% by mass or more, preferably 15% by mass or more, more preferably 18% by mass or more, and, for example, 35% by mass or less, preferably 30% by mass or less, more preferably 25% by mass or less.

Hydroxy group-containing (meth)acrylic acid ester monomers include, for example, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, 8-hydroxyoctyl (meth)acrylate, 10-hydroxydecyl (meth)acrylate, 12-hydroxylauryl (meth)acrylate, and (4-hydroxymethylcyclohexyl)methyl (meth)acrylate. 4-hydroxybutyl acrylate (4HBA) is preferred.

The content of the hydroxyl group-containing (meth)acrylic acid ester monomer in the first monomer component is, from the viewpoint of introducing a crosslinked structure into the acrylic polymer and ensuring the cohesive force in the optical adhesive sheet 10, for example, 1 mass% or more, preferably 3 mass% or more, more preferably 5 mass% or more, and from the viewpoint of adjusting the polarity of the acrylic polymer (related to the compatibility between the various additive components in the optical adhesive sheet 10 and the acrylic polymer), for example, 15 mass% or less, preferably 12 mass% or less, more preferably 10 mass% or less.

The first monomer component may also contain a copolymerizable monomer that is copolymerizable with the (meth)acrylic acid ester monomer. Examples of the copolymerizable monomer include a monomer having a polar group. Examples of the polar group-containing monomer include a hydroxy group-containing monomer (excluding hydroxy group-containing (meth)acrylic acid ester monomer), a monomer having a nitrogen atom-containing ring, and an acidic group-containing monomer. Preferably, a monomer having a nitrogen atom-containing ring is used. In the first monomer component, the polar group-containing monomer can modify the acrylic polymer, such as by introducing a crosslinking point into the acrylic polymer and ensuring the cohesive force of the acrylic polymer. The copolymerizable monomer may be used alone or in combination of two or more kinds.

　Examples of monomers having a nitrogen atom-containing ring include N-vinyl-2-pyrrolidone, N-methylvinylpyrrolidone, N-vinylpyridine, N-vinylpiperidone, N-vinylpyrimidine, N-vinylpiperazine, N-vinylpyrazine, N-vinylpyrrole, N-vinylimidazole, N-vinyloxazole, 4-acryloylmorpholine, N-vinyl-2-caprolactam, N-vinyl-1,3-oxazin-2-one, N-vinyl-3,5-morpholinedione, N-vinylpyrazole, N-vinylisoxazole, N-vinylthiazole, and N-vinylisothiazole. N-vinyl-2-pyrrolidone (NVP) is preferred.

When a monomer having a nitrogen atom-containing ring is used, the content of the monomer having a nitrogen atom-containing ring in the first monomer component is, from the viewpoint of ensuring the cohesive strength of the optical adhesive sheet 10 and ensuring the adhesion of the optical adhesive sheet 10 to the adherend, for example, 0.5% by mass or more, preferably 1% by mass or more, more preferably 1.5% by mass or more, and from the viewpoint of adjusting the glass transition temperature of the acrylic polymer and adjusting the polarity of the acrylic polymer (related to the compatibility of various additive components in the optical adhesive sheet 10 with the acrylic polymer), for example, 10% by mass or less, preferably 5% by mass or less, more preferably 3% by mass or less.

Examples of the acidic group-containing monomer include the acidic group-containing monomer in the second monomer component described below.

The first monomer component may contain other copolymerizable monomers in addition to those mentioned above. Examples of the other copolymerizable monomers include epoxy group-containing monomers, cyano group-containing monomers, alkoxy group-containing monomers, and aromatic vinyl compounds. The other copolymerizable monomers may be used alone or in combination of two or more kinds.

The first monomer component preferably includes a first alkyl group-containing (meth)acrylic acid ester monomer having an alkyl group with 8 to 12 carbon atoms, a second alkyl group-containing (meth)acrylic acid ester monomer having an alkyl group with 1 to 4 carbon atoms, a hydroxy group-containing (meth)acrylic acid ester monomer, and a monomer having a nitrogen atom-containing ring. More preferably, it includes NOAA, BA, 4HBA, and NVP.

The base polymer preferably has a crosslinked structure. Examples of methods for introducing a crosslinked structure into a base polymer include a first method and a second method. In the first method, a base polymer having a functional group capable of reacting with a crosslinking agent and a crosslinking agent are blended into an adhesive composition, and the base polymer and the crosslinking agent are reacted in an optical adhesive sheet. In the second method, a first monomer component forming the base polymer contains a multifunctional compound as a crosslinking agent, and the first monomer component is polymerized to form a base polymer in which a branched structure (crosslinked structure) is introduced into the polymer chain. These methods may be used in combination.

The crosslinking agent used in the first method above may be, for example, a compound that reacts with functional groups (such as hydroxyl groups and carboxyl groups) contained in the base polymer. Examples of the crosslinking agent include isocyanate crosslinking agents, peroxide crosslinking agents, epoxy crosslinking agents, oxazoline crosslinking agents, aziridine crosslinking agents, and carbodiimide crosslinking agents. Isocyanate crosslinking agents are preferred because they are highly reactive with the hydroxyl groups and carboxyl groups in the base polymer and allow easy introduction of a crosslinked structure. The crosslinking agents may be used alone or in combination of two or more types.

In the first method, the amount of crosslinking agent per 100 parts by mass of base polymer is, from the viewpoint of ensuring the cohesive strength of the optical adhesive sheet 10, for example, 0.01 parts by mass or more, preferably 0.05 parts by mass or more, more preferably 0.1 parts by mass or more, and from the viewpoint of ensuring good tackiness in the optical adhesive sheet 10, for example, 5 parts by mass or less, preferably 1 part by mass or less, more preferably 0.2 parts by mass or less.

In the second method, the first monomer component (including a polyfunctional compound and a monofunctional monomer for introducing a crosslinked structure) may be polymerized in one step or in multiple steps. In the multiple step polymerization method, first, a monofunctional monomer for forming a base polymer is polymerized (preliminary polymerization), thereby preparing a prepolymer composition containing a partial polymer (a mixture of a polymer with a low degree of polymerization and an unreacted monofunctional monomer). Next, a polyfunctional compound is added to the prepolymer composition, and then the mixture containing the partial polymer and the polyfunctional compound is polymerized (main polymerization). In the main polymerization, a previously prepared oligomer (described below) can be blended with the mixture containing the partial polymer and the polyfunctional compound. A silane coupling agent (described below) can also be blended with the mixture.

Examples of polyfunctional compounds include polyfunctional monomers and oligomers that contain two or more ethylenically unsaturated double bonds in one molecule. Examples of polyfunctional monomers include polyfunctional (meth)acrylates.

Examples of polyfunctional (meth)acrylates include difunctional (meth)acrylates, trifunctional (meth)acrylates, and tetrafunctional or higher polyfunctional (meth)acrylates.

Examples of bifunctional (meth)acrylates include ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, 1,9-nonanediol di(meth)acrylate, glycerin di(meth)acrylate, ethoxylated bisphenol A diacrylate (BPAEODE), and neopentyl glycol di(meth)acrylate.

Examples of trifunctional (meth)acrylates include trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, and tris(acryloyloxyethyl)isocyanurate.

Examples of tetrafunctional or higher polyfunctional (meth)acrylates include ditrimethylolpropane tetra(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol monohydroxypenta(meth)acrylate, alkyl-modified dipentaerythritol penta(meth)acrylate, and dipentaerythritol hexa(meth)acrylate.

Examples of polyfunctional oligomers include urethane (meth)acrylate oligomers, polyester (meth)acrylate oligomers, polyether (meth)acrylate oligomers, polyol (meth)acrylate oligomers, epoxy (meth)acrylate oligomers, polyethylene glycol di(meth)acrylate, and polypropylene glycol di(meth)acrylate.

The polyfunctional compound may be used alone or in combination of two or more kinds. As the polyfunctional compound, preferably, a polyfunctional monomer is used. More preferably, a polyfunctional (meth)acrylate having tetrafunctional or more is used. Even more preferably, dipentaerythritol hexaacrylate is used.

In the second method, the amount of the polyfunctional compound per 100 parts by mass of the monofunctional monomer of the first monomer component is, from the viewpoint of ensuring the cohesive strength of the optical adhesive sheet 10, for example, 0.02 parts by mass or more, preferably 0.05 parts by mass or more, more preferably 0.07 parts by mass or more, and from the viewpoint of ensuring good tackiness in the optical adhesive sheet 10, for example, 3 parts by mass or less, preferably 1 part by mass or less, more preferably 0.5 parts by mass or less.

The base polymer can be formed by polymerizing the first monomer component. Examples of the polymerization method include solution polymerization, emulsion polymerization, and solvent-free photopolymerization (e.g., ultraviolet polymerization). Examples of the solvent for solution polymerization include ethyl acetate and toluene. A chain transfer agent may be used in the polymerization. Examples of the polymerization initiator include a thermal polymerization initiator and a photopolymerization initiator. For example, in the second method, the polymerization initiator is added first during the preliminary polymerization and then added second during the main polymerization. The polymerization initiator may be used alone or in combination of two or more kinds. The amount of the polymerization initiator to be mixed with respect to 100 parts by mass of the first monomer component is, for example, 0.03 parts by mass or more, preferably 0.05 parts by mass or more, more preferably 0.07 parts by mass or more, and, for example, 1 part by mass or less, preferably 0.5 parts by mass or less, more preferably 0.3 parts by mass or less.

Examples of thermal polymerization initiators include azo polymerization initiators and peroxide polymerization initiators. Examples of azo polymerization initiators include 2,2'-azobisisobutyronitrile, 2,2'-azobis-2-methylbutyronitrile, 2,2'-azobis(2-methylpropionate)dimethyl, 4,4'-azobis-4-cyanovaleric acid, azobisisovaleronitrile, and 2,2'-azobis(2-amidinopropane)dihydrochloride. Examples of peroxide polymerization initiators include dibenzoyl peroxide, t-butyl permaleate, and lauroyl peroxide.

Examples of photopolymerization initiators include radical photopolymerization initiators, cationic photopolymerization initiators, and anionic photopolymerization initiators. Examples of radical photopolymerization initiators include acylphosphine oxide photopolymerization initiators, acetophenone photopolymerization initiators, and benzoin ether photopolymerization initiators. Examples of acylphosphine oxide photopolymerization initiators include bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide, bis(2,4,6-trimethylbenzoyl)-2,4-di-n-butoxyphenylphosphine oxide, 2,4,6-trimethylbenzoyldiphenylphosphine oxide, and bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide. Examples of acetophenone-based photopolymerization initiators include 2,2-diethoxyacetophenone, 2,2-dimethoxy-2-phenylacetophenone, 1-hydroxycyclohexyl phenyl ketone, 4-phenoxydichloroacetophenone, and 4-(t-butyl)dichloroacetophenone.

The weight average molecular weight of the base polymer is, for example, 100,000 or more, preferably 300,000 or more, and more preferably 500,000 or more, from the viewpoint of ensuring the cohesive force of the optical adhesive sheet 10. The weight average molecular weight of the base polymer is measured by gel permeation chromatography (GPC) and calculated in terms of polystyrene.

The Tg of the base polymer is, for example, 0°C or lower, preferably -10°C or lower, more preferably -20°C or lower, and for example, -80°C or higher. The Tg of the base polymer can be determined by the above Fox formula (theoretical glass transition temperature).

The content of the base polymer in the optical adhesive sheet 10 is, from the viewpoint of adequately expressing basic properties such as adhesiveness, for example, 80% by mass or more, preferably 90% by mass or more, more preferably 95% by mass or more, and from the viewpoint of ensuring the content of other components in the optical adhesive sheet 10, for example, 99.9% by mass or less, preferably 99.5% by mass or less, more preferably 99.0% by mass or less.

<Oligomer>
The oligomer is a polymer of a monomer component (second monomer component) containing a (meth)acrylic acid ester monomer. The oligomer may be used alone or in combination of two or more kinds.

When two or more oligomers are used in combination, at least one of the oligomers needs to satisfy the specified parameters (e.g., glass transition temperature, difference ΔH between the hydrogen bond term δH1 of the HSP of the base polymer and the hydrogen bond term δH2 of the HSP of the oligomer (= δH2 - δH1)). In other words, when two or more oligomers are used in combination, oligomers that do not satisfy the specified parameters (e.g., glass transition temperature, difference ΔH between the hydrogen bond term δH1 of the HSP of the base polymer and the hydrogen bond term δH2 of the HSP of the oligomer (= δH2 - δH1)) may also be included within a range that does not impair the effects of the present invention. Preferably, all oligomers used in combination satisfy the specified parameters.

The (meth)acrylic acid ester monomer in the second monomer component may, for example, be an alkyl group-containing (meth)acrylic acid ester monomer. The alkyl group-containing (meth)acrylic acid ester monomer may, for example, be a chain alkyl group-containing (meth)acrylic acid ester monomer, and an alicyclic alkyl group-containing (meth)acrylic acid ester monomer. The (meth)acrylic acid ester monomer in the second monomer component preferably includes an alicyclic alkyl group-containing (meth)acrylic acid ester monomer.

The (meth)acrylic acid ester monomer containing a chain alkyl group in the second monomer component may be, for example, the chain alkyl group-containing (meth)acrylic acid ester monomer described above in the first monomer component. Preferably, a chain alkyl group-containing (meth)acrylic acid ester monomer having an alkyl group with 1 to 6 carbon atoms is used. More preferably, a chain alkyl group-containing methacrylic acid ester monomer having an alkyl group with 1 to 6 carbon atoms is used. Even more preferably, methyl methacrylate (MMA) is used. MMA has a high glass transition temperature of the homopolymer and is relatively compatible with the base polymer.

When the second monomer component contains a (meth)acrylic acid ester monomer containing a chain alkyl group, the proportion of the (meth)acrylic acid ester monomer containing a chain alkyl group in the second monomer component is, from the viewpoint of ensuring a high Tg of the oligomer and adjusting the compatibility of the oligomer with the base polymer, for example, 20 mass% or more, preferably 30 mass% or more, more preferably 40 mass% or more, even more preferably 45 mass% or more, particularly preferably 47 mass% or more, and for example, 60 mass% or less, preferably 55 mass% or less, more preferably 50 mass% or less, even more preferably less than 50 mass%.

Examples of the (meth)acrylic acid ester monomer containing an alicyclic alkyl group in the second monomer component include (meth)acrylic acid cycloalkyl ester monomers, (meth)acrylic acid ester monomers having a bicyclic aliphatic hydrocarbon ring, and (meth)acrylic acid ester monomers having a tricyclic or higher aliphatic hydrocarbon ring. Preferred are (meth)acrylic acid cycloalkyl ester monomers, (meth)acrylic acid ester monomers having a bicyclic aliphatic hydrocarbon ring, and (meth)acrylic acid ester monomers having a tricyclic or higher aliphatic hydrocarbon ring, and (meth)acrylic acid ester monomers having a tricyclic or higher aliphatic hydrocarbon ring, which contain a condensed ring (condensed ring-containing (meth)acrylic acid ester monomer).

Examples of (meth)acrylic acid cycloalkyl ester monomers include cyclopentyl (meth)acrylate, cyclohexyl (meth)acrylate, methylcyclohexyl (meth)acrylate, tert-butylcyclohexyl (meth)acrylate, cycloheptyl (meth)acrylate, cyclooctyl (meth)acrylate, and cyclododecyl (meth)acrylate. Examples of (meth)acrylic acid esters having a bicyclic aliphatic hydrocarbon ring include isobornyl (meth)acrylate. Examples of (meth)acrylic acid esters having a tricyclic or higher aliphatic hydrocarbon ring include dicyclopentanyl (meth)acrylate, dicyclopentenyl (meth)acrylate, dicyclopentanyloxyethyl (meth)acrylate, tricyclopentanyl (meth)acrylate, 1-adamantyl (meth)acrylate, 2-methyl-2-adamantyl (meth)acrylate, and 2-ethyl-2-adamantyl (meth)acrylate.

As the alicyclic alkyl group-containing (meth)acrylic acid ester monomer in the second monomer component, preferably, a (meth)acrylic acid cycloalkyl ester monomer and a condensed ring-containing (meth)acrylic acid ester monomer are included. More preferably, cyclohexyl methacrylate (CHMA) and dicyclopentanyl methacrylate (DCPMA) are included.

The proportion of the alicyclic alkyl group-containing (meth)acrylic acid ester monomer in the second monomer component is, from the viewpoint of compatibility with the base polymer and high Tg, for example, 30% by mass or more, preferably 40% by mass or more, more preferably 45% by mass or more, and from the viewpoint of the polymerizability of the second monomer component, for example, less than 100% by mass, preferably 95% by mass or less, more preferably 60% by mass or less, even more preferably 50% by mass or less, and particularly preferably less than 50% by mass.

When the second monomer component contains a chain alkyl group-containing (meth)acrylic acid ester monomer, the mass ratio of the alicyclic alkyl group-containing (meth)acrylic acid ester monomer to the chain alkyl group-containing (meth)acrylic acid ester monomer in the second monomer component is, from the viewpoint of increasing the Tg of the oligomer and adjusting the compatibility of the oligomer with the base polymer, for example, 0.6 or more, preferably 0.8 or more, more preferably 0.9 or more, and for example, 5.0 or less, preferably 3.0 or less, more preferably 1.5 or less.

The proportion of the alkyl group-containing (meth)acrylic acid ester monomer in the second monomer component is, from the viewpoint of increasing the Tg of the oligomer, for example, 80% by mass or more, preferably 85% by mass or more, more preferably 88% by mass or more, even more preferably 90% by mass or more, and particularly preferably 95% by mass or more, and from the viewpoint of the polymerizability of the second monomer component, for example, less than 100% by mass, preferably 99.5% by mass or less, more preferably 99% by mass or less, even more preferably 98% by mass or less, and particularly preferably 97% by mass or less.

The second monomer component contains an acidic group-containing monomer as a polar group-containing monomer. Examples of the acidic group-containing monomer include a carboxyl group-containing monomer, a phenolic hydroxyl group-containing monomer, a sulfonic acid group-containing monomer, and a phosphoric acid group-containing monomer. Preferred are a carboxyl group-containing monomer and a phenolic hydroxyl group-containing monomer. In other words, in the acidic group-containing monomer, the acidic group is preferably a carboxyl group or a phenolic hydroxyl group.

Examples of the carboxyl group-containing monomer include acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, crotonic acid, and isocrotonic acid. Examples of the carboxyl group-containing monomer include carboxyl group-containing (meth)acrylic acid ester monomers. Examples of the carboxyl group-containing (meth)acrylic acid ester monomers include carboxyethyl (meth)acrylate and carboxypentyl (meth)acrylate. Examples of the carboxyl group-containing monomer include acrylic acid (AA) and methacrylic acid (MAA).

An example of a phenolic hydroxy group-containing monomer is a phenolic hydroxy group-containing (meth)acrylic acid ester monomer. An example of a phenolic hydroxy group-containing (meth)acrylic acid ester monomer is hydroxyphenyl (meth)acrylate (HQMA: hydroquinone mono(meth)acrylate).

The proportion of the acidic group-containing monomer in the second monomer component is, from the viewpoint of ensuring the cohesive strength of the optical adhesive sheet 10 and the adhesive strength of the optical adhesive sheet 10 to the adherend, for example, 0.1% by mass or more, preferably 1% by mass or more, more preferably 1.5% by mass or more, even more preferably 2% by mass or more, and particularly preferably 2.5% by mass or more, and from the viewpoint of adjusting the glass transition temperature of the oligomer and avoiding the risk of corrosion of the adherend by acid, for example, 20% by mass or less, preferably 15% by mass or less, more preferably 12% by mass or less, even more preferably 10% by mass or less, and particularly preferably 8% by mass or less.

The glass transition temperature of the homopolymer of the acidic group-containing monomer is, for example, 40°C or higher, preferably 60°C or higher, more preferably 80°C or higher, and even more preferably 100°C or higher, from the viewpoint of increasing the Tg of the oligomer. The upper limit of the glass transition temperature of the homopolymer of the acidic group-containing monomer is not particularly limited, and is appropriately adjusted within a range that ensures the flexibility of the adhesive composition according to the content of the oligomer in the adhesive composition (optical adhesive sheet 10) described later. The upper limit of the glass transition temperature of the homopolymer of the acidic group-containing monomer is, for example, 300°C. Specifically, the glass transition temperature of the homopolymer of AA is 106°C, and the glass transition temperature of the homopolymer of MAA is 228°C.

The pKa (acid dissociation constant) of an acidic group-containing monomer is an index that quantitatively represents the strength of the acidity of the acidic group-containing monomer, and the smaller the pKa, the stronger the acidity. The pKa (acid dissociation constant) of an acidic group-containing monomer is, for example, 12 or less, preferably 10 or less, more preferably 8 or less, even more preferably 7 or less, particularly preferably 6 or less, and most preferably 5 or less. Specifically, the pKa of AA is 4.66, and the pKa of MAA is 4.25.

The second monomer component may contain a polar group-containing monomer other than the above-mentioned acid group-containing monomer. Preferably, it does not contain a polar group-containing monomer other than the acid group-containing monomer. Examples of polar group-containing monomers include hydroxy group-containing monomers and monomers having a nitrogen atom-containing ring.

The proportion of the polar group-containing monomer in the second monomer component is, for example, 10% by mass or less, preferably 5% by mass or less, more preferably 1% by mass or less, and even more preferably 0% by mass, from the viewpoint of ensuring the adhesive strength of the optical adhesive sheet 10 to the adherend and adjusting the compatibility of the oligomer with the base polymer.

The second monomer component preferably includes an alkyl group-containing (meth)acrylic acid ester monomer and an acidic group-containing monomer. More preferably, it is composed of an alkyl group-containing (meth)acrylic acid ester monomer and an acidic group-containing monomer. The alkyl group-containing (meth)acrylic acid ester monomer is preferably an alicyclic alkyl group-containing (meth)acrylic acid ester monomer used alone, or a combination of a chain alkyl group-containing (meth)acrylic acid ester monomer and an alicyclic alkyl group-containing (meth)acrylic acid ester monomer. Preferably, a combination of a chain alkyl group-containing (meth)acrylic acid ester monomer and an alicyclic alkyl group-containing (meth)acrylic acid ester monomer is used.

Specific examples of the second monomer component include a combination of CHMA and AA, a combination of DCPMA, MMA and MAA, and a combination of DCPMA, MMA and HQMA.

As described above, the oligomer is obtained by polymerizing a monomer component (second monomer component) containing a (meth)acrylic acid ester monomer. Examples of the polymerization method include solution polymerization, emulsion polymerization, and solvent-free photopolymerization (e.g., ultraviolet polymerization). For example, ethyl acetate and toluene are used as the solvent for solution polymerization. In the polymerization, a chain transfer agent may be used to adjust the molecular weight. Examples of the polymerization initiator include the above-mentioned thermal polymerization initiator and photopolymerization initiator. The polymerization initiator may be used alone or in combination of two or more kinds. The amount of the polymerization initiator used is, for example, 0.05 parts by mass or more, preferably 0.1 parts by mass or more, and for example, 1 part by mass or less, preferably 0.5 parts by mass or less, relative to 100 parts by mass of the second monomer component.

The oligomer has a weight average molecular weight Mw of 1000 or more and 30000 or less. From the viewpoint of high adhesion on the surface (adhesive surfaces 11, 12) of the optical adhesive sheet 10, the weight average molecular weight Mw of the oligomer is, for example, 4300 or more, preferably 4500 or more, more preferably 4700 or more, and even more preferably 4900 or more, and from the viewpoint of uneven distribution of the oligomer on the surface of the optical adhesive sheet 10 and its vicinity (mobility to the surface), it is, for example, 10000 or less, preferably 8000 or less, more preferably 6000 or less, and even more preferably 5800 or less. The weight average molecular weight Mw of the oligomer is measured by gel permeation chromatography (GPC) and calculated in terms of polystyrene.

The content of the oligomer in the adhesive composition (optical adhesive sheet 10) is, from the viewpoint of sufficiently increasing the adhesive strength of the optical adhesive sheet 10, for example, 0.1 parts by mass or more, preferably 0.3 parts by mass or more, more preferably 0.4 parts by mass or more, and even more preferably 0.5 parts by mass or more, relative to 100 parts by mass of the base polymer. From the viewpoint of ensuring the transparency of the optical adhesive sheet 10, it is, for example, less than 3 parts by mass, preferably 2 parts by mass or less, more preferably 1.5 parts by mass or less, even more preferably less than 1.5 parts by mass, particularly preferably 1.0 parts by mass or less, and most preferably 0.8 parts by mass or less. If the content of the oligomer in the optical adhesive sheet 10 is too large, the shear storage modulus at -30°C in the optical adhesive sheet 10 may increase and the flexibility may decrease.

The adhesive composition preferably contains a silane coupling agent. Examples of silane coupling agents include epoxy silane coupling agents. The content of the silane coupling agent in the adhesive composition is, for example, 0.1 parts by mass or more, preferably 0.2 parts by mass or more, more preferably 0.3 parts by mass or more, and for example, 5 parts by mass or less, preferably 3 parts by mass or less, more preferably 1 part by mass or less, relative to 100 parts by mass of the base polymer.

The adhesive composition may contain other components as required. Examples of the other components include solvents, tackifiers, plasticizers, softeners, antioxidants, fillers, colorants, UV absorbers, antioxidants, surfactants, and antistatic agents. Examples of the solvent include polymerization solvents used as required during polymerization of the acrylic polymer, and solvents added to the polymerization reaction solution after polymerization. Specifically, ethyl acetate and toluene are used as the solvent.

The optical adhesive sheet 10 can be manufactured, for example, by applying the above-mentioned adhesive composition onto a first release liner L1 to form a coating film, and then irradiating the coating film with ultraviolet light or drying the coating film. The optical adhesive sheet 10 may also be manufactured by applying the above-mentioned adhesive composition onto a first release liner L1 to form a coating film, laminating a second release liner L2 onto the coating film, and then irradiating the coating film between the release liners with ultraviolet light or drying the coating film.

The first release liner L1 may be, for example, a release liner having a release treatment layer on the surface of the liner substrate, or a release liner formed from a low-adhesion material. Examples of the liner substrate include resin film and paper. Examples of the resin of the resin film include polyester resin and polycarbonate resin. Examples of the polyester resin include polyethylene terephthalate (PET) and polybutylene terephthalate. The release treatment layer can be formed by surface treating the liner substrate with a release treatment agent. Examples of the release treatment agent include silicone release treatment agents, long-chain alkyl release treatment agents, and fluorine release treatment agents. Examples of the low-adhesion material include polyolefin resins and fluorine-based polymers. Examples of the polyolefin resin include polyethylene, polypropylene, and cycloolefin polymer (COP). Examples of the fluorine-based polymer include polytetrafluoroethylene.

　Examples of methods for applying the adhesive composition include roll coating, kiss roll coating, gravure coating, reverse coating, roll brushing, spray coating, dip roll coating, bar coating, knife coating, air knife coating, curtain coating, lip coating, and die coating. The drying temperature of the coating film is, for example, 50°C to 200°C. The drying time is, for example, 5 seconds to 20 minutes.

Examples of the second release liner L2 include a release liner having a release treatment layer on the surface of the liner substrate, and a release liner made from a low-adhesion material. Specifically, the second release liner L2 is the same as that described above as the first release liner L1.

In this manner, an optical adhesive sheet 10 can be produced in which the adhesive surfaces 11, 12 are covered and protected by the release liners L1, L2.

The method of using the optical adhesive sheet 10 will be described with reference to Figures 2A to 2C.

As shown in FIG. 2A, the optical adhesive sheet 10 is attached to one surface in the thickness direction of the first member 21 (adherend). The first member 21 is, for example, a part of the laminated structure of a flexible display panel. Specifically, the first member 21 can be a pixel panel, a polarizing film, a touch panel, or a cover film (the same applies to the second member 22 described below).

Next, as shown in FIG. 2B, one side in the thickness direction of the first member 21 is bonded to the other side in the thickness direction of the second member 22 via the optical adhesive sheet 10 on the first member 21. The second member 22 is, for example, part of a laminated structure of a flexible display panel.

Next, as shown in FIG. 2C, the optical adhesive sheet 10 between the first member 21 and the second member 22 is aged. Aging increases the bonding strength between the optical adhesive sheet 10 and the members 21 and 22. The aging temperature is, for example, 20°C to 160°C. The aging time is, for example, 1 minute to 21 days. When aging is performed using autoclave treatment (heat and pressure treatment), the temperature is, for example, 30°C to 80°C, the pressure is, for example, 0.1 to 0.8 MPa, and the treatment time is, for example, 15 minutes or more.

The present invention will be specifically described below with reference to examples. However, the present invention is not limited to the examples. Furthermore, the specific numerical values of the compounding amounts (contents), physical properties, parameters, etc. described below can be replaced with the upper limits (numerical values defined as "equal to or less than") or lower limits (numerical values defined as "equal to or more than") of the corresponding compounding amounts (contents), physical properties, parameters, etc. described in the above-mentioned "Form for carrying out the invention."

Example 1
<Preparation of Prepolymer Composition>
In a reaction vessel equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen gas inlet tube, 70 parts by mass of n-octyl acrylate, 20 parts by mass of n-butyl acrylate, 8 parts by mass of 4-hydroxybutyl acrylate (4HBA), and 2 parts by mass of N-vinyl-2-pyrrolidone were added to a monomer mixture, and 0.05 parts by mass of a photopolymerization initiator (product name "Omnirad 184", 1-hydroxycyclohexyl phenyl ketone, manufactured by IGM Resins) and 0.05 parts by mass of a second photopolymerization initiator (product name "Omnirad 819", bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide, manufactured by IGM Resins) were then added. The mixture was then irradiated with ultraviolet light under a nitrogen atmosphere to polymerize a portion of the monomer components in the mixture to obtain a prepolymer composition. A black light was used for the ultraviolet irradiation. The ultraviolet irradiation was continued until the viscosity of the prepolymer composition reached 10 to 20 Pa·s. This viscosity was measured using a Brookfield viscometer (product name "TVB-10M", manufactured by Toki Sangyo Co., Ltd.) with rotor No. 22, rotor rotation speed of 6 rpm, and temperature of 30° C. The obtained prepolymer composition was a partial polymer containing acrylic polymer P1 and unreacted monomer components (residual monomers). The weight average molecular weight of acrylic polymer P1 in the prepolymer composition was about 4.3 million.

<Preparation of Oligomer>
First, in a reaction vessel equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen gas inlet tube, a mixture containing 95 parts by mass of cyclohexyl methacrylate (CHMA), 5 parts by mass of acrylic acid (AA), 3 parts by mass of α-thioglycerol as a chain transfer agent, 0.3 parts by mass of azobisisobutyronitrile as a thermal polymerization initiator, and ethyl acetate as a solvent (solid content concentration 26% by mass) was reacted at 72°C to 74°C for 6 hours under a nitrogen atmosphere (polymerization reaction). Next, the reaction solution was heated at 90°C for 12 hours to volatilize and remove ethyl acetate, the chain transfer agent, and unreacted monomers. This resulted in the solid oligomer used in Example 1. Table 1 shows the glass transition temperatures of the oligomers used in Example 1.

<Preparation of Pressure-Sensitive Adhesive Composition>
To the prepolymer composition, 0.5 parts by mass of the oligomer, 0.11 parts by mass of dipentaerythritol hexaacrylate, 0.02 parts by mass of a photopolymerization initiator (trade name "Omnirad 819", bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide, manufactured by IGM Resins), and 0.5 parts by mass of a silane coupling agent (trade name "KBM-403", 3-glycidoxypropyltrimethoxysilane, manufactured by Shin-Etsu Chemical Co., Ltd.) were added and mixed per 100 parts by mass of the monomer component in the prepolymer composition (monomer component forming the base polymer), to prepare a pressure-sensitive adhesive composition. The relative number of parts of the oligomer per 100 parts by mass of the base polymer in the pressure-sensitive adhesive layer described below is shown in Table 1 as "blended amount (parts by mass)".

<Formation of Pressure-Sensitive Adhesive Layer>
Next, a pressure-sensitive adhesive composition was applied to the release-treated surface of a first release liner (product name "Diafoil MRE # 75", polyethylene terephthalate film, thickness 75 μm, manufactured by Mitsubishi Chemical Corporation) one side of which had been treated for silicone release, to form a coating film. Next, the release-treated surface of a second release liner (product name "Diafoil MRE # 75", polyethylene terephthalate film, thickness 75 μm, manufactured by Mitsubishi Chemical Corporation) one side of which had been treated for silicone release was bonded to the coating film on the first release liner. Next, the coating film between the release liners was irradiated with ultraviolet light, and the coating film was photocured to form an adhesive layer (thickness 50 μm). In the ultraviolet light irradiation, a black light was used as the irradiation light source, the irradiation intensity was about 2.5 mW / cm ² , and the irradiation time was 16 minutes. In this manner, an optical adhesive sheet (thickness 50 μm) of Example 1 with a release liner was prepared.

Examples 2 to 4 and Comparative Examples 1 to 5
As shown in Table 1, except for changing the type and composition ratio of the monomers used in the preparation of the oligomer, the oligomers used in each Example and Comparative Example were prepared in the same manner as the oligomer used in Example 1. DCPMA represents dicyclopentanyl methacrylate, MMA represents methyl methacrylate, MAA represents methacrylic acid, and HQMA represents hydroxyphenyl (meth)acrylate. The Tg of the oligomers used in each Example and Comparative Example is as shown in Table 1.

In addition, in preparing the adhesive composition, the optical adhesive sheets with release liner of Comparative Examples 1 to 5 were prepared in the same manner as the optical adhesive sheet with release liner of Example 1, except that the type and amount of oligomer added was changed as shown in Table 1. Note that in Comparative Example 1, no oligomer was added.

<Evaluation>
[Oligomer Tg]
The Tg of the oligomers used in each of the Examples and Comparative Examples was calculated based on the Fox formula. The values are shown in Table 1.

[Hydrogen bond term of HSP]
The hydrogen bond term of the Hansen solubility parameter (HSP) was calculated for each of the base polymer and the oligomers used in each of the examples and comparative examples, and the difference (ΔH) between the base polymer and each of the oligomers was calculated.

First, the hydrogen bond term of HSP was calculated for each monomer forming the oligomer by computer software HSPiP (Hansen Solubility Parameters in Practice). Next, the hydrogen bond term (δH2) of the oligomer was calculated from the molar fraction of the monomer in the oligomer and the hydrogen bond term of the monomer. For example, the hydrogen bond term of the oligomer used in Example 1 was calculated to be 5.25 MPa ^1/2 from the molar fraction of CHMA (molecular weight 154.2) of 0.8905 and the hydrogen bond term of 4.4 MPa ^1/2 , and the molar fraction of AA (molecular weight 72.1) of 0.1095 and the hydrogen bond term of 12.2 MPa ^1/2 .

Similarly, the hydrogen bond term (δH1) of the HSP of the base polymer was determined. The value was 5.07 MPa ^1/2 . The difference ΔH (= δH2 - δH1) between the δH2 of the ligomer in the optical adhesive sheet and the δH1 of the base polymer was calculated and shown in Table 1.

[Shear storage modulus]
The dynamic viscoelasticity was measured for the optical pressure sensitive adhesive sheets of each Example and Comparative Example.

A measurement sample was prepared for each optical adhesive sheet. Specifically, first, multiple pieces of optical adhesive sheet cut from the optical adhesive sheet were laminated together to prepare a sample sheet with a thickness of approximately 1.0 mm. Next, this sheet was punched out to obtain cylindrical pellets (diameter 7.9 mm) that served as the measurement sample.

Then, for each measurement sample prepared, a dynamic viscoelasticity measurement was performed using a dynamic viscoelasticity measuring device (name: Advanced Rheometric Expansion System (ARES), manufactured by Rheometric Scientific) after fixing the sample to a parallel plate jig with a diameter of 7.9 mm. In this measurement, the measurement mode was shear mode, the measurement temperature range was -65°C to 200°C, the heating rate was 5°C/min, and the frequency was 1 Hz. The shear storage modulus at -30°C was read from the measurement results. The results are shown in Table 1.

[Adhesive strength]
For each of the optical pressure-sensitive adhesive sheets of the Examples and Comparative Examples, the adhesive strength to an adherend was measured by the following peel test.

(First peel test)
A measurement sample was prepared for each optical adhesive sheet. Specifically, the first release liner was first peeled off from the optical adhesive sheet, and the exposed surface of the optical adhesive sheet was attached to a polyethylene terephthalate film (product name "Lumirror S10", thickness 25 μm, manufactured by Toray Industries, Inc.) whose surface had been plasma-treated to obtain a laminate. In the plasma treatment, a plasma irradiation device (product name "AP-TO5", manufactured by Sekisui Kogyo Co., Ltd.) was used, and the voltage was set to 160 V, the frequency was set to 10 kHz, and the treatment speed was set to 5000 mm / min. Next, a test piece (width 20 mm x length 100 mm) was cut out from the laminate (PET film / optical adhesive sheet / second release liner). Next, the second release liner was peeled off from the optical adhesive sheet in this test piece, and the exposed surface of the optical adhesive sheet was attached to a glass plate (alkali glass, manufactured by Matsunami Glass Co., Ltd.). Next, the glass plate with the optical adhesive sheet (test piece) was subjected to a heating and pressurizing treatment for 15 minutes under conditions of a temperature of 50° C. and a pressure of 0.5 MPa. This caused the test piece to be pressure-bonded to the glass plate. In this manner, a measurement sample was prepared.

Next, the measurement sample was left to stand at room temperature for 30 minutes, after which the test piece (polyethylene terephthalate film with optical adhesive sheet) was peeled off from the glass plate of the measurement sample, and the force required for peeling (peel strength) was measured. The measurement conditions were a temperature of 25°C, a relative humidity of 55%, a peel angle of the test piece from the glass plate of 180°, a tensile speed of 300 mm/min, and a peel length of 50 mm. A tensile tester (product name "Autograph AG-50NX plus", manufactured by Shimadzu Corporation) was used for the measurement. The average measured peel strength is shown in Table 1 as adhesive strength F1 (N/20 mm).

(Second Peel Test)
The measurement conditions were the same as those of the first peel test, except that the pulling speed was 60 mm/min.
The average value of the measured peel strength is shown in Table 1 as adhesive strength F2 (N/20 mm).

[Gel fraction]
For each of the optical pressure-sensitive adhesive sheets of the examples and comparative examples, the gel fraction was measured as follows.

First, about 500 mg of an adhesive sample was taken from the optical adhesive sheet between the release liners. Next, the mass (W1) of the adhesive sample was measured. Next, the adhesive sample was immersed in about 40 g of ethyl acetate in a container for 7 days. Next, all components that were insoluble in ethyl acetate (insoluble parts) were collected. Next, the insoluble parts were dried at 130°C for 2 hours (removal of ethyl acetate). Next, the mass (W2) of the insoluble parts was measured. Then, the gel fraction (mass%) of the adhesive sheet after photocuring was calculated based on the following formula. The values are shown in Table 1.

Gel fraction (mass%) = (W2/W1) x 100

[Haze]
For each of the optical pressure-sensitive adhesive sheets of the examples and comparative examples, the haze was measured as follows.

First, a sample for measurement was prepared. Specifically, the first release liner was peeled off from the optical adhesive sheet, and then the sheet was attached to an alkaline glass (thickness 1.0 mm, total light transmittance 92%, haze 0.4%, manufactured by Matsunami Glass Co., Ltd.). Next, the first release liner was peeled off from the optical adhesive sheet on the glass. This produced a sample for measurement. Next, the haze of the optical adhesive sheet in the sample was measured using a haze meter (product name "HM-150", manufactured by Murakami Color Research Laboratory). The measurement was in accordance with JIS K7136 (2000). In this measurement, the sample was placed in the device so that light was shining on the alkaline glass side of the sample. The measured haze is shown in Table 1.

<Considerations>
The optical adhesive sheets of Examples 1 to 4 have a shear storage modulus of 350 kPa or less at -30°C, are excellent in flexibility, and can relieve stress generated in the optical adhesive sheet and the adherend when used in a flexible device. On the other hand, the optical adhesive sheet of Comparative Example 3 has a shear storage modulus of more than 350 kPa at -30°C, is poor in flexibility, and is not suitable for flexible device applications.

In addition, in the optical adhesive sheets of Examples 1 to 4, the oligomer is a polymer of a monomer component containing a (meth)acrylic acid ester monomer, the monomer component contains an acidic group-containing monomer, and the adhesive strength F1 (N/20 mm) to glass in the first peel test satisfies 8.0≦F1. In other words, it has excellent adhesiveness, and when used in a flexible device, peeling from the adherend can be suppressed. On the other hand, the optical adhesive sheet of Comparative Example 1 does not contain an oligomer, the optical adhesive sheet of Comparative Example 2 has an oligomer that is a polymer of a monomer component containing a (meth)acrylic acid ester monomer, and the monomer component does not contain an acidic group-containing monomer, and the optical adhesive sheet of Comparative Example 4 has an oligomer that is a polymer of a monomer component containing a (meth)acrylic acid ester monomer, and the monomer component contains an acidic group-containing monomer, but the adhesive strength F1 (N/20 mm) to glass in the first peel test is 8.0>F1 for the optical adhesive sheets of Comparative Examples 1, 2, and 4. In other words, it has poor initial adhesion and is not suitable for flexible device applications.

The above invention is provided as an exemplary embodiment of the present invention, but this is merely an example and should not be interpreted as limiting. Modifications of the present invention that are obvious to those skilled in the art are intended to be included in the scope of the claims below.

The optical adhesive sheet of the present invention is suitable for use in light passing areas in flexible devices (e.g., flexible display panels such as foldable display panels and rollable display panels).

10 Optical adhesive sheet 11 First adhesive surface 12 Second adhesive surface L1, L2 Release liner 21 First member 22 Second member

Claims

An optical adhesive sheet comprising a base polymer and an oligomer,
The oligomer is a polymer of a monomer component containing a (meth)acrylic acid ester monomer,
The monomer component further includes an acidic group-containing monomer,
The optical adhesive sheet has a shear storage modulus of 350 kPa or less at −30 ° C.,
An optically adhesive sheet having an adhesive strength F1 (N/20 mm) to glass in the first peel test described below that satisfies the following formula (1).
8.0≦F1 (1)
(First peel test)
A polyethylene terephthalate film is attached to one side of the optical adhesive sheet, and a glass plate is attached to the other side, and a measurement sample is prepared by heating and pressurizing. The measurement sample is left at room temperature for 30 minutes, and then a test piece (the polyethylene terephthalate film with the optical adhesive sheet) is peeled off from the glass plate in the measurement sample. The measurement conditions are a temperature of 25°C, a relative humidity of 55%, a peel angle of the test piece from the glass plate of 180°, a tensile speed of 300 mm/min, and a peel length of 50 mm.
The optical adhesive sheet according to claim 1, wherein the adhesive strength F2 (N/20 mm) to glass in the second peel test described below satisfies the following formula (2).
6.0≦F2 (2)
(Second Peel Test)
The measurement conditions are the same as those of the first peel test, except that the pulling speed is 60 mm/min.
The optical adhesive sheet according to claim 1, wherein the (meth)acrylic acid ester monomer further includes an alicyclic alkyl group-containing (meth)acrylic acid ester monomer.
The optical adhesive sheet according to claim 1, wherein in the acidic group-containing monomer, the acidic group is a carboxy group and/or a phenolic hydroxy group.
The optical adhesive sheet according to claim 1, wherein the homopolymer of the acidic group-containing monomer has a glass transition temperature of 40°C or higher.
The optical adhesive sheet according to any one of claims 1 to 5, wherein the content of the acidic group-containing monomer in the monomer component is 1 mass% or more and 20 mass% or less.
The optical adhesive sheet according to any one of claims 1 to 5, wherein the amount of the oligomer blended is 0.3 parts by mass or more and less than 1.5 parts by mass per 100 parts by mass of the base polymer.
The optical adhesive sheet according to any one of claims 1 to 5, having a haze of 1% or less.