Radiation-curable adhesive sheet

Abstract

To provide a radiation-curable adhesive sheet excellent in storage stability, capable of preventing a progress of curing in storage, and capable of maintaining excellent step absorbency and stress relaxation for a long period of time.SOLUTION: A radiation-curable adhesive sheet 1 includes an adhesive layer 10 configured to cure by radiation exposure. The adhesive layer 10 includes a photoinitiator 11a and a cross-linking agent 11b. The curing by the radiation exposure is caused by a reaction by the photoinitiator 11a and the cross-linking agent 11b. In the radiation curable adhesive sheet 1, pKa of the photoinitiator 11a is 5 to 12.7.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a radiation-curable pressure-sensitive adhesive sheet, and more particularly to a radiation-curable pressure-sensitive adhesive sheet that can be used to bond a transparent optical member to another optical member. [Background technology]

[0002] Image display devices such as liquid crystal display devices or organic EL display devices are composed of optical component laminates in which transparent cover members such as polarizing films, retardation films, and cover glasses, as well as various other transparent optical components, are laminated. An adhesive sheet made of a transparent pressure-sensitive adhesive layer is used to bond these optical components. Specifically, a pressure-sensitive adhesive sheet is placed between two optical components to be bonded, and the two optical components are bonded by pressing them together to form an optical component laminate. Furthermore, pressure-sensitive adhesive sheets having a pressure-sensitive adhesive layer provided on one side of a substrate film are commonly used as surface protection films in the manufacturing process of optical products to prevent scratches and dirt from adhering to the optical components.

[0003] For example, in image display devices equipped with input devices such as touch panels, a transparent, conductive printed layer made of patterned ITO (indium tin oxide) or other materials is formed on the surface of optical components. Furthermore, silver or copper wiring is formed around the periphery. A black frame-shaped concealing area is typically printed around the periphery of the transparent cover member. Adhesive sheets used to bond optical components with such printed layers and wiring are required to have high fluidity in the adhesive layer and exhibit step absorption properties that prevent air bubbles from remaining in the step. Furthermore, when optical components with printed step differences are bonded together via an adhesive sheet, stress distortions applied to the adhesive around the printed step differences can easily cause display unevenness around the periphery of the image display panel, so excellent stress relaxation properties are also required.

[0004] On the other hand, plastic films used as optical components contain gases such as carbon dioxide, and gas may be generated under high temperature conditions during the manufacturing process. In this case, if the pressure-sensitive adhesive layer is soft, the generation of gas cannot be suppressed, causing the pressure-sensitive adhesive layer to float and generate bubbles. Therefore, in order to suppress the generation of gas from the plastic film, the adhesive sheet is also required to have a high elastic modulus and hardness of the pressure-sensitive adhesive layer, thereby improving adhesion reliability.

[0005] As a pressure-sensitive adhesive sheet that combines the above-mentioned step absorbency, stress relaxation properties, and adhesive reliability, a pressure-sensitive adhesive sheet having a pressure-sensitive adhesive layer that is cured by irradiation with radiation (sometimes referred to in this specification as a "radiation-curable pressure-sensitive adhesive sheet") has been widely used (see, for example, Patent Document 1). Before curing, a radiation-curable pressure-sensitive adhesive sheet has high fluidity and is in a state that is excellent in step absorbency and stress relaxation properties, allowing it to conform sufficiently to steps, and can relieve stress distortion caused by steps and suppress display unevenness, and has the advantage that curing is completed by subsequent irradiation with radiation, thereby improving adhesive reliability. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. WO2016 / 170875 Summary of the Invention [Problem to be solved by the invention]

[0007] Radiation-curable adhesive sheets are sometimes shipped in the form of adhesive sheets containing a photopolymerization initiator and a crosslinking agent and stored for a relatively long period of time, but there have been problems in that curing gradually progresses during storage due to environmental factors such as heat and light, resulting in a decrease in level difference absorption and stress relaxation properties, and the occurrence of air bubbles and uneven display.

[0008] The present invention was conceived in light of the above circumstances, and aims to provide a radiation-curable pressure-sensitive adhesive sheet that can inhibit the progression of curing during storage, can maintain excellent step absorbability and stress relaxation properties for a long period of time, and has excellent storage stability. [Means for solving the problem]

[0009] As a result of intensive research to achieve the above object, the present inventors have found that by incorporating a photopolymerization initiator having a pKa within a specific range as the photopolymerization initiator contained in a radiation-curable pressure-sensitive adhesive sheet, the progress of curing during storage can be suppressed, excellent level difference absorbency and stress relaxation properties can be maintained for a long period of time, and excellent storage stability can be achieved. The present invention was completed based on these findings.

[0010] That is, a first aspect of the present invention provides a radiation-curable pressure-sensitive adhesive sheet. The radiation-curable pressure-sensitive adhesive sheet of the first aspect of the present invention has a pressure-sensitive adhesive layer that is cured by irradiation with radiation, the pressure-sensitive adhesive layer containing a photopolymerization initiator and a crosslinking agent, and the curing by irradiation with radiation is achieved by a reaction between the photopolymerization initiator and the crosslinking agent. In the radiation-curable pressure-sensitive adhesive sheet of the first aspect of the present invention, the pressure-sensitive adhesive layer has high fluidity and excellent level difference absorption and stress relaxation properties before curing. Therefore, the pressure-sensitive adhesive layer can sufficiently conform to levels such as those formed on the surface of an optical component using a patterned transparent conductive printed layer such as ITO, silver or copper wiring, and a black concealing portion formed in a frame shape around the periphery of a transparent cover member, filling the levels without leaving any air bubbles or gaps, thereby alleviating stress distortion caused by the level differences and suppressing display unevenness. Subsequent irradiation with radiation promotes curing through a reaction between the photopolymerization initiator contained in the pressure-sensitive adhesive layer and the crosslinking agent, thereby improving adhesion reliability.

[0011] In the radiation-curable pressure-sensitive adhesive sheet according to the first aspect of the present invention, the photopolymerization initiator has a pKa of 5 to 12.7. When stored for a relatively long period of time, radiation-curable pressure-sensitive adhesive sheets have the problem that, due to environmental factors such as heat and light, curing gradually progresses due to reactions of the photopolymerization initiator and crosslinking agent contained in the pressure-sensitive adhesive layer, resulting in a decrease in level difference absorbency and stress relaxation properties. The photopolymerization initiator having a pKa of 5 to 12.7 is advantageous in that it can inhibit the progress of curing of the pressure-sensitive adhesive layer during storage of the radiation-curable pressure-sensitive adhesive sheet according to the first aspect of the present invention, can maintain excellent level difference absorbency and stress relaxation properties for long periods of time, and has excellent storage stability.

[0012] In the radiation-curable pressure-sensitive adhesive sheet according to the first aspect of the present invention, the pKa of the photopolymerization initiator is preferably 5.1 or more, more preferably 5.2 or more, and may be 5.3 or more, or 5.4 or more, from the viewpoints of being able to suppress the progress of curing of the pressure-sensitive adhesive layer during storage, maintaining excellent level difference absorbency and stress relaxation properties for a long period of time, and achieving excellent storage stability. Furthermore, the pKa of the photopolymerization initiator is preferably 12.5 or less, more preferably 12 or less, and may be 11.5 or less, 11 or less, 10.5 or less, 10 or less, 9.5 or less, 9 or less, 8.5 or less, or 8.0 or less, from the viewpoints of being able to suppress the progress of curing of the pressure-sensitive adhesive layer during storage, maintaining excellent level difference absorbency and stress relaxation properties for a long period of time, and achieving excellent storage stability.

[0013] In the radiation-curable pressure-sensitive adhesive sheet according to the first aspect of the present invention, the pKa of the photopolymerization initiator is preferably a calculated pKa calculated from the chemical structural formula. This configuration is preferred because it allows simple and accurate calculation regardless of the physical properties of the photopolymerization initiator, and there is no measurement error due to measurement conditions, making it easy to compare photopolymerization initiators.

[0014] In the radiation-curable pressure-sensitive adhesive sheet according to the first aspect of the present invention, the photopolymerization initiator is preferably a photopolymerization initiator having an amino group and an aromatic ring in the molecule. The photopolymerization initiator is preferably a photopolymerization initiator having two or more amino groups and an aromatic ring in the molecule. The photopolymerization initiator is preferably an aminoacetophenone-based photopolymerization initiator. Photopolymerization initiators having the above-described chemical structure are preferred because their pKa is easily controlled to a range of 5 to 12.7 due to the amino group and aromatic ring in the chemical structure. Photopolymerization initiators having the above-described chemical structure but not having a pKa in the range of 5 to 12.7 are not considered photopolymerization initiators according to the first aspect of the present invention. [Effects of the Invention]

[0015] The radiation-curable pressure-sensitive adhesive sheet of the present invention can inhibit the progression of curing during storage, can maintain excellent level difference absorbability and stress relaxation properties for a long period of time, and has excellent storage stability. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a cross-sectional view showing one embodiment of the radiation-curable pressure-sensitive adhesive sheet of the present invention. [Figure 2] FIG. 2 is a schematic diagram showing steps for carrying out one embodiment of the method for producing a radiation-curable pressure-sensitive adhesive sheet of the present invention, in which (a) is a pressure-sensitive adhesive layer forming step, (b) is a pressure-sensitive adhesive layer curing step, (c) is a solution applying step, (d) is a solution penetrating step, and (e) is a drying step. [Figure 3] FIG. 3 is a cross-sectional view showing one embodiment of the radiation-curable pressure-sensitive adhesive sheet of the present invention produced by the method of FIG. [Figure 4] FIG. 4 is a cross-sectional view of an optical member laminate showing an example of the simplest embodiment using the radiation-curable pressure-sensitive adhesive sheet of the present invention. [Figure 5] FIG. 5 is a schematic diagram showing steps for carrying out one embodiment of a method for producing an optical component laminate using the radiation-curable pressure-sensitive adhesive sheet of the present invention shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0017] A first aspect of the present invention provides a radiation-curable pressure-sensitive adhesive sheet having a pressure-sensitive adhesive layer that is cured by irradiation with radiation. The pressure-sensitive adhesive layer contains a photopolymerization initiator and a crosslinking agent, and the curing by irradiation with radiation occurs through a reaction between the photopolymerization initiator and the crosslinking agent. The photopolymerization initiator has a pKa of 5 to 12.7. Hereinafter, in this specification, the radiation-curable pressure-sensitive adhesive sheet of the first aspect of the present invention may be referred to as the "radiation-curable pressure-sensitive adhesive sheet of the present invention" or simply as the "pressure-sensitive adhesive sheet of the present invention." Furthermore, the pressure-sensitive adhesive layer of the radiation-curable pressure-sensitive adhesive sheet of the present invention may be referred to as the "pressure-sensitive adhesive layer of the present invention," and the photopolymerization initiator and crosslinking agent contained in the pressure-sensitive adhesive layer of the present invention may be referred to as the "photopolymerization initiator of the present invention" and the "crosslinking agent of the present invention," respectively.

[0018] The form of the radiation-curable pressure-sensitive adhesive sheet of the present invention is not particularly limited, as long as the adhesive surface is an adhesive surface (adhesive layer surface) provided by the pressure-sensitive adhesive layer of the present invention. For example, it may be a single-sided pressure-sensitive adhesive sheet having only one adhesive surface, or a double-sided pressure-sensitive adhesive sheet having adhesive surfaces on both sides. Furthermore, when the radiation-curable pressure-sensitive adhesive sheet of the present invention is a double-sided pressure-sensitive adhesive sheet, it may have a form in which both adhesive surfaces are provided by the pressure-sensitive adhesive layer of the present invention, or a form in which one adhesive surface is provided by the pressure-sensitive adhesive layer of the present invention and the other adhesive surface is provided by a pressure-sensitive adhesive layer other than the pressure-sensitive adhesive layer of the present invention (another pressure-sensitive adhesive layer). From the viewpoint of bonding adherends together, a double-sided pressure-sensitive adhesive sheet is preferred.

[0019] The radiation-curable pressure-sensitive adhesive sheet of the present invention may be a so-called "substrate-less" pressure-sensitive adhesive sheet that does not have a substrate (substrate layer), or may be a pressure-sensitive adhesive sheet of a type that has a substrate. In this specification, a "substrate-less" pressure-sensitive adhesive sheet may be referred to as a "substrate-less pressure-sensitive adhesive sheet," and a pressure-sensitive adhesive sheet of a type that has a substrate may be referred to as a "substrate-attached pressure-sensitive adhesive sheet." Examples of the substrate-less pressure-sensitive adhesive sheet include a double-sided pressure-sensitive adhesive sheet consisting only of the pressure-sensitive adhesive layer of the present invention, and a double-sided pressure-sensitive adhesive sheet consisting of the pressure-sensitive adhesive layer of the present invention and another pressure-sensitive adhesive layer (a pressure-sensitive adhesive layer other than the pressure-sensitive adhesive layer of the present invention). Examples of the substrate-attached pressure-sensitive adhesive sheet include a single-sided pressure-sensitive adhesive sheet having the pressure-sensitive adhesive layer of the present invention on one side of the substrate, a double-sided pressure-sensitive adhesive sheet having the pressure-sensitive adhesive layer of the present invention on both sides of the substrate, and a double-sided pressure-sensitive adhesive sheet having the pressure-sensitive adhesive layer of the present invention on one side of the substrate and another pressure-sensitive adhesive layer on the other side. The "substrate (substrate layer)" refers to a support, and is the part that is attached to the adherend together with the pressure-sensitive adhesive layer when the radiation-curable pressure-sensitive adhesive sheet of the present invention is used (attached) to an adherend. The separator (release liner) that is peeled off when the pressure-sensitive adhesive sheet is used (applied) is not included in the above substrate.

[0020] Hereinafter, embodiments of the present invention will be described with reference to the drawings, but the present invention is not limited to these and is merely an example. FIG. 1 is a cross-sectional view showing one embodiment of the radiation-curable pressure-sensitive adhesive sheet of the present invention.

[0021] The radiation-curable adhesive sheet 1 according to one embodiment of the present invention shown in FIG. 1 is composed of an adhesive layer 10, a release sheet S1 bonded to one main surface 10A of the adhesive layer 10, and a release sheet S2 bonded to the other main surface 10B of the adhesive layer 10.

[0022] In FIG. 1, the adhesive layer 10 is made of an adhesive base material and has two opposing main surfaces (first main surface 10A and second main surface 10B), and may be a single layer or a laminated structure of two or more layers.

[0023] The thickness of the adhesive layer 10 is not particularly limited, but is usually 5 μm to 500 μm, preferably 5 μm to 400 μm, and more preferably 5 μm to 350 μm. If the thickness of the adhesive layer 10 is within this range, it conforms to the steps without leaving any air bubbles, relieves stress distortion caused by the steps, and suppresses display unevenness, which is preferable in terms of excellent step absorbability and stress relaxation properties.

[0024] The total light transmittance of the entire pressure-sensitive adhesive layer 10 is not particularly limited, but is preferably 80% or more, and more preferably 90% or more, as measured in accordance with JIS K7361. The higher the total light transmittance of the pressure-sensitive adhesive layer 10, the better. Furthermore, the haze value is preferably 1.5% or less, more preferably 1% or less, and even more preferably 0.8% or less.

[0025] The pressure-sensitive adhesive layer 10 contains a photopolymerization initiator 11a and a crosslinking agent 11b. The photopolymerization initiator 11a and the crosslinking agent 11b are preferably dissolved in the pressure-sensitive adhesive layer 10. Here, "dissolved" means, for example, that the photopolymerization initiator 11a and the crosslinking agent 11b are dissolved to an extent that the transparency of the pressure-sensitive adhesive layer 10 can be maintained, i.e., to an extent that cloudiness due to light scattering does not occur. Specifically, the photopolymerization initiator 11a and the crosslinking agent 11b are preferably contained in the pressure-sensitive adhesive layer 10 so that the haze value of the photopolymerization initiator 11a and the crosslinking agent 11b is 1.5% or less, preferably 1% or less, and more preferably 0.8% or less. The present invention also encompasses an embodiment in which only one of the photopolymerization initiator 11a and the crosslinking agent 11b is dissolved in the pressure-sensitive adhesive layer 10.

[0026] The pKa of the photopolymerization initiator 11a contained in the pressure-sensitive adhesive layer 10 is in the range of 5 to 12.7. When the pKa of the photopolymerization initiator 11a is in the range of 5 to 12.7, the progress of curing during storage can be suppressed, excellent step absorbability and stress relaxation properties can be maintained for a long period of time, and excellent storage stability can be achieved.

[0027] The storage stability of the pressure-sensitive adhesive layer 10 can be evaluated by the rate of change (%) of residual stress. For example, the residual stress (N / cm) after storing the pressure-sensitive adhesive layer 10 at 50° C. for 4 weeks can be evaluated by the rate of change (%) of residual stress. 2The rate of change in residual stress [(residual stress after storage at 50°C for 4 weeks - initial residual stress) / (initial residual stress) x 100] is preferably 70% or less, more preferably 60% or less, and may be 55% or less, 50% or less, 45% or less, 40% or less, 35% or less, or 30% or less. A configuration in which the rate of change in residual stress is 70% or less is preferred in that excellent level difference absorbability and stress relaxation properties can be maintained during use of the radiation-curable pressure-sensitive adhesive sheet of the present invention. Each component will be described in detail below.

[0028] <Adhesive layer> The pressure-sensitive adhesive layer of the present invention is cured by irradiation with radiation, and the photopolymerization initiator of the present invention and the crosslinking agent of the present invention undergo polymerization and crosslinking reactions, resulting in curing. Before curing, the pressure-sensitive adhesive layer of the present invention has high fluidity and excellent level difference absorption and stress relaxation properties. Therefore, the pressure-sensitive adhesive layer of the present invention can adequately conform to levels, such as those formed on the surface of an optical component using a patterned transparent conductive printed layer such as ITO, silver or copper wiring, and a black concealing portion formed in a frame shape around the periphery of a transparent cover member, filling the gaps without leaving any air bubbles, thereby alleviating stress distortion caused by the level difference and suppressing display unevenness. Subsequent irradiation with radiation promotes curing through a reaction between the photopolymerization initiator of the present invention contained in the pressure-sensitive adhesive layer of the present invention and the crosslinking agent of the present invention, thereby improving adhesion reliability.

[0029] The adhesive base material contained in the adhesive composition for forming the adhesive layer of the present invention is not particularly limited as long as it has adhesive properties suitable for optical applications. For example, an appropriate material can be selected from acrylic adhesives, rubber adhesives, silicone adhesives, polyester adhesives, urethane adhesives, epoxy adhesives, and polyether adhesives. From the viewpoints of transparency, processability, and durability, it is preferable to use an acrylic adhesive. The adhesive base material can be any of the above adhesives, either alone or in combination of two or more. The acrylic polymer used as the base polymer of the acrylic adhesive is not particularly limited, but is preferably a homopolymer or copolymer of a monomer primarily composed of a (meth)acrylic acid alkyl ester. Herein, the term "(meth)acrylic" is used to mean either or both of "acrylic" and "methacrylic," and the same applies to other cases. In the present invention, the term "acrylic polymer" is used to include not only the above-mentioned (meth)acrylic acid alkyl ester but also other monomers copolymerizable therewith.

[0030] When the PSA base material contains an acrylic polymer that is an acrylic PSA, the acrylic polymer preferably contains, as the main monomer unit in the largest amount by weight, a monomer unit derived from an acrylic acid alkyl ester having a linear or branched alkyl group and / or a methacrylic acid alkyl ester having a linear or branched alkyl group.

[0031] Examples of the (meth)acrylic acid alkyl ester having a linear or branched alkyl group to form the monomer unit of the acrylic polymer, i.e., the (meth)acrylic acid alkyl ester having a linear or branched alkyl group contained in the monomer component to form the acrylic polymer, include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, s-butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, pentyl (meth)acrylate, isopentyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, Examples of the alkyl (meth)acrylate include alkyl (meth)esters having a linear or branched alkyl group having 1 to 20 carbon atoms, such as octyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, isooctyl (meth)acrylate, nonyl (meth)acrylate, isononyl (meth)acrylate, decyl (meth)acrylate, isodecyl (meth)acrylate, undecyl (meth)acrylate, dodecyl (meth)acrylate, tridecyl (meth)acrylate, tetradecyl (meth)acrylate, pentadecyl (meth)acrylate, hexadecyl (meth)acrylate, heptadecyl (meth)acrylate, octadecyl (meth)acrylate, isostearyl (meth)acrylate, nonadecyl (meth)acrylate, and eicosyl (meth)acrylate. As the alkyl (meth)acrylate for the acrylic polymer, one type of alkyl (meth)acrylate may be used, or two or more types of alkyl (meth)acrylates may be used. In this embodiment, the (meth)acrylic acid alkyl ester for the acrylic polymer is preferably at least one selected from the group consisting of n-butyl acrylate, 2-ethylhexyl acrylate, and isostearyl acrylate.

[0032] The proportion of monomer units derived from a (meth)acrylic acid alkyl ester having a linear or branched alkyl group in the acrylic polymer is preferably 50% by weight or more, more preferably 60% by weight or more, more preferably 70% by weight or more, more preferably 80% by weight or more, and more preferably 90% by weight or more. That is, the proportion of a (meth)acrylic acid alkyl ester in the monomer component composition of the raw material for forming the acrylic polymer is preferably 50% by weight or more, more preferably 60% by weight or more, more preferably 70% by weight or more, more preferably 80% by weight or more, and more preferably 90% by weight or more.

[0033] The acrylic polymer contained in the PSA base material may contain a monomer unit derived from an alicyclic monomer. Examples of alicyclic monomers that form the monomer units of the acrylic polymer, i.e., alicyclic monomers contained in the monomer components for forming the acrylic polymer, include (meth)acrylic acid cycloalkyl esters, (meth)acrylic acid esters having a bicyclic hydrocarbon ring, and (meth)acrylic acid esters having a tricyclic or higher hydrocarbon ring. Examples of (meth)acrylic acid cycloalkyl esters include cyclopentyl (meth)acrylate, cyclohexyl (meth)acrylate, cycloheptyl (meth)acrylate, and cyclooctyl (meth)acrylate. Examples of (meth)acrylic acid esters having a bicyclic hydrocarbon ring include bornyl (meth)acrylate and isobornyl (meth)acrylate. Examples of (meth)acrylic acid esters having three or more hydrocarbon rings include dicyclopentanyl (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 monomer for the acrylic polymer, one type of alicyclic monomer may be used, or two or more types of alicyclic monomers may be used. In this embodiment, the alicyclic monomer for the acrylic polymer is preferably at least one selected from the group consisting of cyclohexyl acrylate, cyclohexyl methacrylate, isobornyl acrylate, and isobornyl methacrylate.

[0034] The proportion of the monomer unit derived from the alicyclic monomer in the acrylic polymer is preferably 5 to 60% by weight, more preferably 10 to 50% by weight, and more preferably 12 to 40% by weight, from the viewpoint of realizing appropriate flexibility in the adhesive base material formed containing the acrylic polymer.

[0035] The acrylic polymer contained in the PSA base material may contain a monomer unit derived from a hydroxyl group-containing monomer. The hydroxyl group-containing monomer is a monomer having at least one hydroxyl group in the monomer unit. When the acrylic polymer in the PSA base material contains a hydroxyl group-containing monomer unit, the PSA base material is more likely to have adhesive properties and appropriate cohesive strength. In addition, the hydroxyl group can also serve as a reaction site with the crosslinking agent described below.

[0036] Hydroxyl-containing monomers for forming the monomer units of the acrylic polymer, i.e., hydroxyl-containing monomers contained in the monomer components for forming the acrylic polymer, include, for example, hydroxyl-containing (meth)acrylic esters, vinyl alcohol, and allyl alcohol. Examples of hydroxyl-containing (meth)acrylic esters include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, hydroxyoctyl (meth)acrylate, hydroxydecyl (meth)acrylate, hydroxylauryl (meth)acrylate, and (4-hydroxymethylcyclohexyl)methyl (meth)acrylate. As the hydroxyl-containing monomer for the acrylic polymer, one type of hydroxyl-containing monomer may be used, or two or more types of hydroxyl-containing monomers may be used. In this embodiment, the hydroxyl group-containing monomer for the acrylic polymer is preferably at least one selected from the group consisting of 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl acrylate, 2-hydroxypropyl methacrylate, 4-hydroxybutyl acrylate, and 4-hydroxybutyl methacrylate.

[0037] The proportion of monomer units derived from hydroxyl group-containing monomers in the acrylic polymer is preferably 1 wt% or more, more preferably 2 wt% or more, more preferably 3 wt% or more, more preferably 7 wt% or more, more preferably 10 wt% or more, more preferably 15 wt% or more. The proportion of monomer units derived from hydroxyl group-containing monomers in the acrylic polymer is preferably 35 wt% or less, more preferably 30 wt% or less. These configurations regarding the proportion of hydroxyl group-containing monomers are suitable for achieving adhesiveness and appropriate cohesive strength in a pressure-sensitive adhesive base material formed containing the acrylic polymer.

[0038] The acrylic polymer contained in the PSA base material may contain a monomer unit derived from a nitrogen atom-containing monomer. The nitrogen atom-containing monomer is a monomer that has at least one nitrogen atom in the monomer unit. When the acrylic polymer in the PSA base material contains a nitrogen atom-containing monomer unit, the PSA base material is more likely to have hardness and good adhesive reliability.

[0039] The nitrogen atom-containing monomer for forming the monomer unit of the acrylic polymer, i.e., the nitrogen atom-containing monomer contained in the monomer component for forming the acrylic polymer, can be, for example, N-vinyl cyclic amides and (meth)acrylamides. Examples of the N-vinyl cyclic amide as the nitrogen atom-containing monomer include N-vinyl-2-pyrrolidone, N-vinyl-2-piperidone, N-vinyl-3-morpholinone, N-vinyl-2-caprolactam, N-vinyl-1,3-oxazin-2-one, and N-vinyl-3,5-morpholinedione. Examples of (meth)acrylamides as nitrogen atom-containing monomers include (meth)acrylamide, N-ethyl(meth)acrylamide, N-isopropyl(meth)acrylamide, Nn-butyl(meth)acrylamide, N-octyl(meth)acrylamide, N,N-dimethyl(meth)acrylamide, N,N-diethyl(meth)acrylamide, N,N-dipropyl(meth)acrylamide, and N,N-diisopropyl(meth)acrylamide. As the nitrogen atom-containing monomer for the acrylic polymer, one type of nitrogen atom-containing monomer may be used, or two or more types of nitrogen atom-containing monomers may be used. In this embodiment, N-vinyl-2-pyrrolidone is preferably used as the nitrogen atom-containing monomer for the acrylic polymer.

[0040] The proportion of the nitrogen-atom-containing monomer-derived monomer unit in the acrylic polymer is preferably 1% by weight or more, more preferably 3% by weight or more, and more preferably 5% by weight or more, from the viewpoint of realizing appropriate hardness, adhesiveness, and transparency in the pressure-sensitive adhesive base material formed containing the acrylic polymer. Also, the proportion of the nitrogen-atom-containing monomer-derived monomer unit in the acrylic polymer is preferably 30% by weight or less, more preferably 25% by weight or less, from the viewpoint of realizing sufficient transparency in the pressure-sensitive adhesive base material formed containing the acrylic polymer and realizing good adhesion reliability by preventing the material from becoming too hard.

[0041] The acrylic polymer contained in the PSA base material may contain a monomer unit derived from a carboxyl group-containing monomer. The carboxyl group-containing monomer is a monomer having at least one carboxyl group within the monomer unit. When the acrylic polymer in the PSA base material contains a carboxyl group-containing monomer unit, the PSA base material may achieve good adhesive reliability. Furthermore, the carboxyl group can also serve as a reaction site with the crosslinking agent described below.

[0042] Examples of the carboxyl group-containing monomer for forming the monomer unit of the acrylic polymer, i.e., the carboxyl group-containing monomer contained in the monomer component for forming the acrylic polymer, include (meth)acrylic acid, itaconic acid, maleic acid, fumaric acid, crotonic acid, and isocrotonic acid. As the carboxyl group-containing monomer for the acrylic polymer, one type of carboxyl group-containing monomer may be used, or two or more types of carboxyl group-containing monomers may be used. In this embodiment, acrylic acid is preferably used as the carboxyl group-containing monomer for the acrylic polymer.

[0043] The proportion of the monomer units derived from carboxyl group-containing monomers in the acrylic polymer is preferably 0.1 wt% or more, more preferably 0.5 wt% or more, from the viewpoint of ensuring good adhesive reliability in a pressure-sensitive adhesive base material formed containing the acrylic polymer by utilizing the contribution of the interaction between polar groups and carboxyl groups when polar groups are present on the adherend surface. Also, the proportion of the monomer units derived from carboxyl group-containing monomers in the acrylic polymer is preferably 20 wt% or less, more preferably 15 wt% or less, from the viewpoint of preventing the pressure-sensitive adhesive base material formed containing the acrylic polymer from becoming too hard and realizing good adhesive reliability.

[0044] The acrylic polymer contained in the pressure-sensitive adhesive base material may have a crosslinked structure derived from a crosslinking agent. The crosslinked structure of the pressure-sensitive adhesive base material increases the viscosity and tends to improve shape stability. Examples of crosslinking agents include copolymerizable crosslinking agents such as polyfunctional (meth)acrylates and thermosetting crosslinking agents. The acrylic polymer may have a crosslinked structure derived solely from a polyfunctional (meth)acrylate, a crosslinked structure derived solely from a thermosetting crosslinking agent, or a crosslinked structure derived from both a polyfunctional (meth)acrylate and a thermosetting crosslinking agent. The crosslinking agent described above has already formed a crosslinked structure in the acrylic polymer forming the pressure-sensitive adhesive layer of the present invention as a result of the progress of a crosslinking reaction. On the other hand, the crosslinking agent of the present invention contained in the pressure-sensitive adhesive layer of the present invention is an unreacted crosslinking agent before the formation of a crosslinked structure. Therefore, the crosslinking agent that forms a crosslinked structure in the acrylic polymer forming the pressure-sensitive adhesive layer of the present invention does not correspond to the crosslinking agent of the present invention contained in the pressure-sensitive adhesive layer of the present invention. In this specification, the crosslinking agent that forms a crosslinked structure in the acrylic polymer may be referred to as a "first crosslinking agent," and the crosslinking agent of the present invention may be referred to as a "second crosslinking agent."

[0045] Examples of the polyfunctional (meth)acrylate used as the first crosslinking agent include 1,6-hexanediol di(meth)acrylate, butanediol di(meth)acrylate, (poly)ethylene glycol di(meth)acrylate, (poly)propylene glycol di(meth)acrylate, neopentyl glycol di(meth)acrylate, pentaerythritol di(meth)acrylate, pentaerythritol tri(meth)acrylate, dipentaerythritol hexa(meth)acrylate, trimethylolpropane tri(meth)acrylate, tetramethylolmethane tri(meth)acrylate, allyl (meth)acrylate, and vinyl (meth)acrylate. As the polyfunctional (meth)acrylate for the acrylic polymer, one type of polyfunctional (meth)acrylate may be used, or two or more types of polyfunctional (meth)acrylates may be used. In this embodiment, as the polyfunctional (meth)acrylate for the acrylic polymer, at least one selected from the group consisting of 1,6-hexanediol diacrylate, dipentaerythritol hexaacrylate, and trimethylolpropane triacrylate is preferably used.

[0046] The proportion of the polyfunctional (meth)acrylate-derived monomer unit as the first crosslinking agent in the acrylic polymer is preferably 0.001% by weight or more, more preferably 0.005% by weight or more, and may be 0.01% by weight or more, 0.015% by weight or more, 0.02% by weight or more, 0.025% by weight or more, 0.03% by weight or more, 0.035% by weight or more, 0.04% by weight or more, 0.045% by weight or more, 0.05% by weight or more, 0.06% by weight or more, 0.07% by weight or more, 0.08% by weight or more, 0.09% by weight or more, or 0.1% by weight or more. The proportion of the polyfunctional (meth)acrylate-derived monomer unit in the acrylic polymer is preferably 1% by weight or less, more preferably 0.9% by weight or less, and may be 0.8% by weight or less, 0.7% by weight or less, 0.6% by weight or less, or 0.5% by weight or less. These configurations regarding the proportion of the polyfunctional (meth)acrylate are suitable for realizing appropriate hardness, adhesiveness, and shape stability in a pressure-sensitive adhesive base material formed containing the acrylic polymer.

[0047] Examples of thermosetting crosslinking agents used as the first crosslinking agent include isocyanate-based crosslinking agents, epoxy-based crosslinking agents, metal chelate-based crosslinking agents, melamine-based crosslinking agents, peroxide-based crosslinking agents, urea-based crosslinking agents, metal alkoxide-based crosslinking agents, metal salt-based crosslinking agents, carbodiimide-based crosslinking agents, oxazoline-based crosslinking agents, aziridine-based crosslinking agents, and amine-based crosslinking agents. The adhesive base material may contain one type of thermosetting crosslinking agent or two or more types of thermosetting crosslinking agents. Preferably, at least one selected from the group consisting of isocyanate-based crosslinking agents and epoxy-based crosslinking agents is used.

[0048] Examples of isocyanate-based crosslinking agents include lower aliphatic polyisocyanates, alicyclic polyisocyanates, and aromatic polyisocyanates. Examples of lower aliphatic polyisocyanates include 1,2-ethylene diisocyanate, 1,4-butylene diisocyanate, and 1,6-hexamethylene diisocyanate. Examples of alicyclic polyisocyanates include cyclopentylene diisocyanate, cyclohexylene diisocyanate, isophorone diisocyanate, hydrogenated tolylene diisocyanate, and hydrogenated xylene diisocyanate. Examples of aromatic polyisocyanates include 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate, and xylylene diisocyanate. Further, examples of the isocyanate crosslinking agent include commercially available products such as trimethylolpropane / tolylene diisocyanate adduct (trade name "Coronate L", manufactured by Nippon Polyurethane Industry Co., Ltd.), trimethylolpropane / hexamethylene diisocyanate adduct (trade name "Coronate HL", manufactured by Nippon Polyurethane Industry Co., Ltd.), and trimethylolpropane / xylylene diisocyanate adduct (trade name "Takenate D-110N", manufactured by Mitsui Chemicals, Inc.).

[0049] Examples of epoxy crosslinking agents (polyfunctional epoxy compounds) include N,N,N',N'-tetraglycidyl-m-xylylenediamine, diglycidylaniline, 1,3-bis(N,N-diglycidylaminomethyl)cyclohexane, 1,6-hexanediol diglycidyl ether, neopentyl glycol diglycidyl ether, ethylene glycol diglycidyl ether, propylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, sorbitol polyglycidyl ether, glycerol polyglycidyl ether, pentaerythritol polyglycidyl ether, polyglycerol polyglycidyl ether, sorbitan polyglycidyl ether, trimethylolpropane polyglycidyl ether, adipic acid diglycidyl ester, o-phthalic acid diglycidyl ester, triglycidyl tris(2-hydroxyethyl)isocyanurate, resorcinol diglycidyl ether, and bisphenol-S-diglycidyl ether. Further, examples of the epoxy crosslinking agent include epoxy resins having two or more epoxy groups. In addition, examples of the epoxy crosslinking agent include commercially available products such as "Tetrad C" (manufactured by Mitsubishi Gas Chemical Company, Inc.).

[0050] When the PSA base material contains a thermosetting crosslinking agent as the first crosslinking agent for crosslinking between acrylic polymers, the content of the thermosetting crosslinking agent in the PSA base material is preferably 0.001 parts by weight or more, more preferably 0.01 parts by weight or more, per 100 parts by weight of the acrylic polymer in the PSA base material, from the viewpoint of improving the shape stability of the PSA base material and achieving sufficient adhesive reliability to the adherend. Furthermore, the content of the thermosetting crosslinking agent in the PSA base material is preferably 10 parts by weight or less, more preferably 5 parts by weight or less, per 100 parts by weight of the acrylic polymer in the PSA base material, from the viewpoint of imparting appropriate flexibility to the PSA base material and achieving good adhesive strength.

[0051] When the pressure-sensitive adhesive base material contains the above-mentioned acrylic polymer as a pressure-sensitive adhesive, the content of the acrylic polymer in the pressure-sensitive adhesive base material is, for example, 85 to 100 wt %.

[0052] The pressure-sensitive adhesive base material may contain a polymerization initiator in addition to the monomer for forming the acrylic polymer and the first crosslinking agent. Examples of the polymerization initiator include a photopolymerization initiator and a thermal polymerization initiator. The pressure-sensitive adhesive base material may contain one type of polymerization initiator or two or more types of polymerization initiators. The polymerization initiator functions as a catalyst for promoting the polymerization and crosslinking reaction of the monomer components and crosslinking agent to form an acrylic polymer. Therefore, once the acrylic polymer is formed, it is deactivated and decomposed, and it is believed that it does not remain in the pressure-sensitive adhesive layer of the present invention, or if it remains, it is only in trace amounts. On the other hand, the photopolymerization initiator of the present invention is contained in the pressure-sensitive adhesive layer of the present invention together with the acrylic polymer, and promotes the crosslinking reaction of the crosslinking agent of the present invention. Therefore, the photopolymerization initiator for forming the acrylic polymer does not fall under the photopolymerization initiator of the present invention, and those with a pKa not falling within the range of 5 to 12.7 may also be used without limitation. In this specification, the photopolymerization initiator for forming the acrylic polymer may be referred to as the "first photopolymerization initiator," and the photopolymerization initiator of the present invention may be referred to as the "second photopolymerization initiator."

[0053] Examples of the first photopolymerization initiator include benzoin ether-based photopolymerization initiators, acetophenone-based photopolymerization initiators, aminoacetophenone-based photopolymerization initiators, α-ketol-based photopolymerization initiators, aromatic sulfonyl chloride-based photopolymerization initiators, photoactive oxime-based photopolymerization initiators, benzoin-based photopolymerization initiators, benzyl-based photopolymerization initiators, benzophenone-based photopolymerization initiators, ketal-based photopolymerization initiators, and thioxanthone-based photopolymerization initiators. Examples of the benzoin ether-based photopolymerization initiator include benzoin methyl ether, benzoin ethyl ether, benzoin propyl ether, benzoin isopropyl ether, benzoin isobutyl ether, and 2,2-dimethoxy-1,2-diphenylethan-1-one. 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. Examples of aminoacetophenone-based photopolymerization initiators include 2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropan-1-one, 2-methyl-1-phenyl-2-morpholinopropan-1-one, 2-methyl-1-[4-(hexyl)phenyl]-2-morpholinopropan-1-one, 2-ethyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1, 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1, and 2-(dimethylamino)-2-[(4-methylphenyl)methyl]-1-[4-(4-morpholinyl)phenyl]-1-butanone. Examples of α-ketol photopolymerization initiators include 2-methyl-2-hydroxypropiophenone, 1-hydroxycyclohexylphenyl ketone, 2-hydroxy-1-(4-(4-(2-hydroxy-2-methylpropionyl)benzyl)phenyl)-2-methylpropan-1-one, and 1-[4-(2-hydroxyethyl)phenyl]-2-methylpropan-1-one. Examples of aromatic sulfonyl chloride photopolymerization initiators include 2-naphthalenesulfonyl chloride.Examples of photoactive oxime-based photopolymerization initiators include 1-phenyl-1,1-propanedione-2-(o-ethoxycarbonyl)-oxime. Examples of benzoin-based photopolymerization initiators include benzoin. Examples of benzyl-based photopolymerization initiators include benzil. Examples of benzophenone-based photopolymerization initiators include benzophenone, benzoylbenzoic acid, 3,3'-dimethyl-4-methoxybenzophenone, and polyvinylbenzophenone. Examples of ketal-based photopolymerization initiators include benzil dimethyl ketal. Examples of thioxanthone-based photopolymerization initiators include thioxanthone, 2-chlorothioxanthone, 2-methylthioxanthone, 2,4-dimethylthioxanthone, isopropylthioxanthone, 2,4-diisopropylthioxanthone, and dodecylthioxanthone.

[0054] The amount of the first photopolymerization initiator used is not particularly limited, but is preferably 0.001 to 1 part by weight, and more preferably 0.01 to 0.50 parts by weight, relative to 100 parts by weight of all monomer units of the acrylic polymer (total amount of monomer components constituting the acrylic polymer).

[0055] Examples of the thermal polymerization initiator include azo-based polymerization initiators, peroxide-based polymerization initiators, redox-based polymerization initiators, etc. Examples of the azo-based polymerization initiator include 2,2'-azobisisobutyronitrile (AIBN), 2,2'-azobis-2-methylbutyronitrile (AMBN), 2,2'-azobis(2-methylpropionic acid) dimethyl, 4,4'-azobis-4-cyanovaleric acid, 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), 2,2'-azobis(2,4-dimethylvaleronitrile), 1,1'-azobis(cyclohexane-1-carbonitrile), 2,2'-azobis(2,4,4-trimethylpentane), etc. Examples of peroxide polymerization initiators include benzoyl peroxide, t-butyl hydroperoxide, di-t-butyl peroxide, t-butyl peroxybenzoate, dicumyl peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, and 1,1-bis(t-butylperoxy)cyclododecane.

[0056] The amount of the thermal polymerization initiator used is not particularly limited, but is preferably 0.05 to 0.5 parts by weight, and more preferably 0.1 to 0.3 parts by weight, relative to 100 parts by weight of all monomer units of the acrylic polymer (total amount of monomer components constituting the acrylic polymer).

[0057] In addition to the PSA base material, the PSA composition may further contain, as necessary, additives such as an ultraviolet absorber, an antirust agent, an antistatic agent, a crosslinking accelerator, a silane coupling agent, a tackifying resin, an antioxidant, a filler, a colorant such as a pigment or a dye, an antioxidant, a chain transfer agent, a plasticizer, a softener, a surfactant, etc. Examples of tackifying resins include rosin derivatives, polyterpene resins, petroleum resins, and oil-soluble phenols.

[0058] The crosslinking agent (second crosslinking agent) of the present invention contained in the pressure-sensitive adhesive layer of the present invention can be any of the crosslinking agents exemplified above as the first crosslinking agent, and is preferably a polyfunctional (meth)acrylate, which is a crosslinking agent polymerizable by radiation (radiation curable). As the second crosslinking agent, one type of crosslinking agent or two or more types of crosslinking agents may be used. Furthermore, the first crosslinking agent and the second crosslinking agent may be the same crosslinking agent or a combination of different crosslinking agents. In this embodiment, the crosslinking agent (second crosslinking agent) of the present invention is preferably trimethylolpropane tri(meth)acrylate, dipentaerythritol hexa(meth)acrylate, or 1,6-hexanediol di(meth)acrylate.

[0059] The content of the crosslinking agent of the present invention (second crosslinking agent) contained in the pressure-sensitive adhesive layer of the present invention can be appropriately set as long as it can be cured by irradiation with radiation and provide excellent adhesive reliability. It is not particularly limited, but it is, for example, an upper limit of 20% by weight or less (for example, 0.001 to 20% by weight, 0.001 to 18% by weight, 0.001 to 16% by weight, 0.001 to 14% by weight, 0.001 to 12% by weight) relative to the pressure-sensitive adhesive layer of the present invention (100% by weight). The range can be selected from a range with a lower limit of 0.001% or more (e.g., 0.001 to 20% by weight, 0.005 to 20% by weight, 0.01 to 20% by weight, 0.05 to 20% by weight, 0.1 to 20% by weight, 0.5 to 20% by weight, 1 to 20% by weight, 1.1 to 20% by weight, 1.2 to 20% by weight, 1.3 to 20% by weight, etc.).

[0060] The photopolymerization initiator of the present invention (second photopolymerization initiator) contained in the pressure-sensitive adhesive layer of the present invention has a pKa of 5 to 12.7. When stored for a relatively long period of time, radiation-curable pressure-sensitive adhesive sheets have the problem that, due to environmental factors such as heat and light, curing gradually progresses through the reaction of the photopolymerization initiator and crosslinking agent contained in the pressure-sensitive adhesive layer, resulting in a decrease in level difference absorbency and stress relaxation properties. The photopolymerization initiator of the present invention having a pKa of 5 to 12.7 is advantageous in that it can inhibit the progress of curing of the pressure-sensitive adhesive layer of the present invention during storage of the radiation-curable pressure-sensitive adhesive sheet of the present invention, maintain excellent level difference absorbency and stress relaxation properties for a long period of time, and provide excellent storage stability.

[0061] The pKa (acid dissociation constant) of a photopolymerization initiator is an index for quantitatively expressing the acidity of the photopolymerization initiator; the smaller the pKa, the stronger the acidity. The reason and mechanism for the improved storage stability of the pressure-sensitive adhesive layer of the present invention due to the pKa of the photopolymerization initiator (second photopolymerization initiator) being in the range of 5 to 12.7 is unclear, but it is thought that when the acid value of the pressure-sensitive adhesive is high, the second crosslinker (particularly a polyfunctional (meth)acrylate) undergoes a Michael addition reaction, gradually progressing the crosslinking reaction. It is presumed that the acid value in the pressure-sensitive adhesive layer can be adjusted by adding the photopolymerization initiator (second photopolymerization initiator) of the present invention having a pKa of 5 to 12.7, thereby suppressing the crosslinking reaction over time. Note that this mechanism is speculative and should not be construed as limiting the scope of the present invention.

[0062] In the radiation-curable pressure-sensitive adhesive sheet of the present invention, the pKa of the photopolymerization initiator of the present invention (second photopolymerization initiator) is preferably 5.1 or more, more preferably 5.2 or more, and may be 5.3 or more, or 5.4 or more, from the viewpoints of being able to suppress the progress of curing of the pressure-sensitive adhesive layer during storage, being able to maintain excellent level difference absorbency and stress relaxation properties for a long period of time, and having excellent storage stability. Furthermore, the pKa of the photopolymerization initiator is preferably 12.5 or less, more preferably 12 or less, and may be 11.5 or less, 11 or less, 10.5 or less, 10 or less, 9.5 or less, 9 or less, 8.5 or less, or 8.0 or less, from the viewpoints of being able to suppress the progress of curing of the pressure-sensitive adhesive layer during storage, being able to maintain excellent level difference absorbency and stress relaxation properties for a long period of time, and having excellent storage stability.

[0063] The pKa of a photopolymerization initiator can be measured by known methods, such as neutralization titration, absorptiometry, or capillary electrophoresis. However, these methods may not be applicable depending on the physical properties of the photopolymerization initiator. Measurement errors may also occur due to measurement conditions such as temperature and concentration. To solve these problems, it is preferable to use a calculated pKa calculated from the chemical structural formula of the photopolymerization initiator. The calculated pKa can be calculated easily and accurately regardless of physical properties as long as the chemical structural formula is known, and there is no measurement error due to measurement conditions, making it easy to compare photopolymerization initiators. Therefore, it is preferable to use a calculated pKa calculated from the chemical structural formula as the pKa of the photopolymerization initiator of the present invention.

[0064] The calculated pKa is not particularly limited, but for example, a chemical structural formula can be drawn using ChemDraw 19.0, and the chemical structural formula can be calculated using a calculation module. For example, the calculation module can be Molecular Networks' cheminformatics platform MOSES. MOSES is a calculation module developed, maintained, and owned by Molecular Networks GmbH (Erlangen, Germany).

[0065] The photopolymerization initiator (second photopolymerization initiator) of the present invention contained in the pressure-sensitive adhesive layer of the present invention can be any of the first photopolymerization initiators exemplified above, with a pKa of 5 to 12.7. Specifically, a photopolymerization initiator with a calculated pKa of 5 to 12.7 calculated using a calculation module can be used by drawing the chemical structural formula of the photopolymerization initiator using ChemDraw 19.0. Furthermore, when the pKa of the first photopolymerization initiator is 5 to 12.7, the first and second photopolymerization initiators may be the same or different photopolymerization initiators.

[0066] Specifically, the photopolymerization initiator (second photopolymerization initiator) of the present invention is preferably a photopolymerization initiator having an amino group and an aromatic ring in the molecule, and more preferably a photopolymerization initiator having two or more amino groups and an aromatic ring in the molecule. Furthermore, the photopolymerization initiator of the present invention is preferably an aminoacetophenone-based photopolymerization initiator. It is believed that the pKa of a photopolymerization initiator having the above-described chemical structure is easily controlled to a range of 5 to 12.7 due to the amino group and aromatic ring in the chemical structure. It should be noted that even if a photopolymerization initiator has the above-described chemical structure, if its pKa is not within the range of 5 to 12.7, it does not fall under the category of the photopolymerization initiator of the present invention.

[0067] More specifically, the photopolymerization initiator (second photopolymerization initiator) of the present invention is preferably a photopolymerization initiator having a chemical structure of the following formula (1). [ka]

[0068] In the above formula (1), Ar is -SR 5 Or -N(R 6 )(R 7 ), and R 5 represents a hydrogen atom or an alkyl group.

[0069] R 1 and R 2 R each independently represents an alkyl group having 1 to 8 carbon atoms. 1 and R 2 may be bonded to each other to form an alkylene group having 2 to 9 carbon atoms. R 1 and R 2 The alkyl group represented by may be straight-chain, branched or cyclic, and is preferably straight-chain or branched. R 1 and R 2 The alkyl group represented by may be unsubstituted or may have a substituent, such as an aryl group, a heterocyclic group, a nitro group, a cyano group, a halogen atom, -OR a , -SR a, -COR a , -COOR a , -OCOR a , -NR a R b , -NHCOR a , -CONR a R b , -NHCONR a R b , -NHCOOR a , -SO2R a , -SO2OR a , -NHSO2R a Examples include: R a and R b Each of R independently represents a hydrogen atom, an alkyl group, an aryl group, or a heterocyclic group. The substituent is preferably an aryl group. In particular, R 1 and R 2 Preferably, one of the groups is an unsubstituted alkyl group and the other is an alkyl group substituted with an aryl group. Examples of the halogen atom include a fluorine atom, a chlorine atom, a bromine atom, and an iodine atom. R a and R b The number of carbon atoms in the alkyl group represented by is preferably 1 to 20. The alkyl group may be linear, branched, or cyclic, but is preferably linear or branched. Aryl groups and R as substituents a and R b The number of carbon atoms in the aryl group represented by is preferably 6 to 20, more preferably 6 to 15, and even more preferably 6 to 10. The aryl group may be a monocyclic ring or a condensed ring. In addition, some or all of the hydrogen atoms in the aryl group may be substituted with an alkyl group having 1 to 8 carbon atoms. R a and R b The heterocyclic group represented by is preferably a 5-membered ring or a 6-membered ring. The heterocyclic group may be a monocyclic ring or a condensed ring. The number of carbon atoms constituting the heterocyclic group is preferably 3 to 30, more preferably 3 to 18, and still more preferably 3 to 12. The number of heteroatoms constituting the heterocyclic group is preferably 1 to 3. The heteroatom constituting the heterocyclic group is preferably a nitrogen atom, an oxygen atom, or a sulfur atom.

[0070] R 3 and R 4 R each independently represents a hydrogen atom, an alkyl group having 1 to 12 carbon atoms, an alkoxy-substituted alkyl group having 2 to 4 carbon atoms, or an alkenyl group having 3 to 5 carbon atoms. 3 and R 4 may be bonded to each other to form an alkylene group having 3 to 7 carbon atoms, and the alkylene group may have -O- or -N(R 8 )- may be included. 8 represents an alkyl group having 1 to 4 carbon atoms.

[0071] R 6 and R 7 R each independently represents a hydrogen atom, an alkyl group having 1 to 12 carbon atoms, an alkoxy-substituted alkyl group having 2 to 4 carbon atoms, or an alkenyl group having 3 to 5 carbon atoms. 6 and R 7 may be bonded to each other to form an alkylene group having 3 to 7 carbon atoms, and the alkylene group may have -O- or -N(R 9 )-, where R 9 represents an alkyl group having 1 to 4 carbon atoms.

[0072] Examples of the photopolymerization initiator (second photopolymerization initiator) of the present invention include 2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropan-1-one (calculated pKa: 5.56), 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1 (calculated pKa: 7.68), 2-methyl-1-phenyl-2-morpholinopropan-1-one (calculated pKa: 5.56 ... Examples include 2-ethyl-1-[4-(hexyl)phenyl]-2-morpholinopropan-1-one (calculated pKa: 5.56), 2-ethyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1 (calculated pKa: 7.96), and 2-(dimethylamino)-2-[(4-methylphenyl)methyl]-1-[4-(4-morpholinyl)phenyl]-1-butanone (calculated pKa: 7.70).

[0073] Commercially available products of the photopolymerization initiator (second photopolymerization initiator) of the present invention include Omnirad907, Omnirad 369E, and Omnirad 379 (trade names: all manufactured by IGM Resins BV).

[0074] The content of the photopolymerization initiator of the present invention (second photopolymerization initiator) contained in the pressure-sensitive adhesive layer of the present invention can be appropriately set as long as it can be cured by irradiation with radiation and provide excellent adhesive reliability, and is not particularly limited. For example, the content of the photopolymerization initiator of the present invention (second photopolymerization initiator) can be an upper limit of 2% by weight or less (e.g., 0.0001 to 2% by weight, 0.0001 to 1.8% by weight, 0.0001 to 1.6% by weight, 0.0001 to 1.4% by weight, 0.0001 to 1.2% by weight, 0.0001 to 1.0% by weight, 0.0001 to 0.8% by weight, 0.0001 to 0.6% by weight, 0.0001 to 1 ... The range can be selected from a range with a lower limit of 0.0001% or more (for example, 0.0001 to 2% by weight, 0.0002 to 2% by weight, 0.0003 to 2% by weight, 0.0004 to 2% by weight, 0.0005 to 2% by weight, 0.0006 to 2% by weight, 0.0007 to 2% by weight, 0.0008 to 2% by weight, 0.0009 to 2% by weight, 0.001 to 2% by weight, 0.0012 to 2% by weight, 0.0014 to 2% by weight, 0.0016 to 2% by weight, 0.0018 to 2% by weight, 0.002 to 2% by weight, etc.).

[0075] The pressure-sensitive adhesive layer of the present invention preferably further comprises a hindered amine light stabilizer (HALS) in addition to the photopolymerization initiator (second photopolymerization initiator) of the present invention and the crosslinking agent (second crosslinking agent) of the present invention. The pressure-sensitive adhesive layer of the present invention preferably comprises a hindered amine light stabilizer, since this further improves the storage stability of the pressure-sensitive adhesive layer and allows excellent level difference absorbability and stress relaxation properties to be maintained for a long period of time. Examples of the hindered amine light stabilizer include low-molecular-weight ones, high-molecular-weight ones, N-alkyl-type ones, and NH-type ones. The hindered amine light stabilizer may be used alone or in combination of two or more types.

[0076] Examples of the low-molecular-weight hindered amine light stabilizer include a mixture of 70% by weight of a reaction product (molecular weight 737) of decanedioic acid bis(2,2,6,6-tetramethyl-1(octyloxy)-4-piperidinyl) ester, 1,1-dimethylethyl hydroperoxide, and octane with 30% by weight of polypropylene; bis(1,2,2,6,6-pentamethyl-4-piperidyl)[[3,5-bis(1,1-dimethylethyl)-4-hydroxyphenyl]methyl]butyl malonate (molecular weight 685); a mixture of bis(1,2,2,6,6-pentamethyl-4-piperidyl)sebacate and methyl-1,2,2,6,6-pentamethyl-4-piperidylsebacate (molecular weight 509); bis(2,2,6,6-tetramethyl-4-piperidyl)sebacate and methyl-1,2,2,6,6-pentamethyl-4-piperidylsebacate (molecular weight 509); tetrakis(2,2,6,6-tetramethyl-4-piperidyl) sebacate (molecular weight 481); tetrakis(2,2,6,6-tetramethyl-4-piperidyl)-1,2,3,4-butanetetracarboxylate (molecular weight 791); tetrakis(1,2,2,6,6-pentamethyl-4-piperidyl)-1,2,3,4-butanetetracarboxylate (molecular weight 847); a mixture of 2,2,6,6-tetramethyl-4-piperidyl-1,2,3,4-butanetetracarboxylate and tridecyl-1,2,3,4-butanetetracarboxylate (molecular weight 900); a mixture of 1,2,2,6,6-pentamethyl-4-piperidyl-1,2,3,4-butanetetracarboxylate and tridecyl-1,2,3,4-butanetetracarboxylate (molecular weight 900), and the like can be mentioned.

[0077] Examples of the high molecular weight hindered amine light stabilizer include poly[{6-(1,1,3,3-tetramethylbutyl)amino-1,3,5-triazine-2,4-diyl}{(2,2,6,6-tetramethyl-4-piperidyl)imino}hexamethylene{(2,2,6,6-tetramethyl-4-piperidyl)imino}] (molecular weight 2,000 to 3,100); polymer of dimethyl succinate and 4-hydroxy-2,2,6,6-tetramethyl-1-piperidineethanol (molecular weight 3,100 to 4,000); N,N',N",N"'-tetrakis-(4,6-bis-(butyl-(N-methyl- Examples include a mixture of (2,2,6,6-tetramethylpiperidin-4-yl)amino)-triazin-2-yl)-4,7-diazadecane-1,10-diamine (molecular weight 2,286) and the polymer of the above-mentioned dimethyl succinate and 4-hydroxy-2,2,6,6-tetramethyl-1-piperidineethanol; and a polycondensate of dibutylamine, 1,3,5-triazine, N,N'-bis(2,2,6,6-tetramethyl-4-piperidyl-1,6-hexamethylenediamine, and N-(2,2,6,6-tetramethyl-4-piperidyl)butylamine (molecular weight 2,600 to 3,400).

[0078] When the pressure-sensitive adhesive layer of the present invention contains a hindered amine light stabilizer, it is preferable to use a hindered amine light stabilizer with a pKa of 5 to 12.7. A hindered amine light stabilizer with a pKa of 5 to 12.7 is suitable in that it can inhibit the progress of curing of the pressure-sensitive adhesive layer of the present invention during storage of the radiation-curable pressure-sensitive adhesive sheet of the present invention, can maintain excellent level difference absorbability and stress relaxation properties for a long period of time, and provides excellent storage stability.

[0079] The pKa (acid dissociation constant) of a hindered amine light stabilizer is an index for quantitatively expressing the acidity of the hindered amine light stabilizer, with a smaller pKa indicating a stronger acidity. It is believed that when the acid value of the adhesive is high, the second crosslinker (particularly a polyfunctional (meth)acrylate) undergoes a Michael addition reaction, gradually progressing the crosslinking reaction. It is presumed that when the pKa of the hindered amine light stabilizer of the present invention is in the range of 5 to 12.7, the acid value in the adhesive layer is adjusted, suppressing the crosslinking reaction over time. Note that this mechanism is merely presumed and should not be construed as limiting the scope of the present invention.

[0080] In the radiation-curable pressure-sensitive adhesive sheet of the present invention, the pKa of the hindered amine light stabilizer is preferably 5.1 or more, more preferably 5.2 or more, and may be 5.3 or more, or 5.4 or more, from the viewpoints of being able to suppress the progress of curing of the pressure-sensitive adhesive layer during storage, maintaining excellent level difference absorbency and stress relaxation properties for a long period of time, and achieving excellent storage stability. Furthermore, the pKa of the hindered amine light stabilizer is preferably 12.5 or less, more preferably 12 or less, and may be 11.5 or less, 11 or less, 10.5 or less, 10 or less, or 9.5 or less, from the viewpoints of being able to suppress the progress of curing of the pressure-sensitive adhesive layer during storage, maintaining excellent level difference absorbency and stress relaxation properties for a long period of time, and achieving excellent storage stability.

[0081] The pKa of the hindered amine light stabilizer is preferably measured using a calculated pKa, as in the case of the photopolymerization initiator of the present invention. Specifically, it is preferable to use a chemical structural formula of the hindered amine light stabilizer drawn using ChemDraw 19.0, and calculate the calculated pKa of the chemical structural formula using a calculation module to obtain a calculated pKa of 5 to 12.7.

[0082] When the pressure-sensitive adhesive layer of the present invention contains a hindered amine-based light stabilizer, the content thereof can be appropriately set as long as it can be cured by irradiation with radiation to provide excellent adhesive reliability, and is not particularly limited. However, from the viewpoints of being able to suppress the progress of curing of the pressure-sensitive adhesive layer during storage, being able to maintain excellent level difference absorbability and stress relaxation properties for a long period of time, and being excellent in storage stability, it is preferable that the content be, for example, an upper limit of 8% by weight or less (for example, 0.001 to 8% by weight, 0.001 to 7.5% by weight, 0.001 to 7.0 ... The range can be selected from a range with a lower limit of 0.001% or more (for example, 0.001 to 6.5% by weight, 0.001 to 6.0% by weight, 0.001 to 5.5% by weight, 0.001 to 5.0% by weight, 0.001 to 4.5% by weight, 0.001 to 4.0% by weight, etc.) or a range with a lower limit of 0.001% or more (for example, 0.001 to 8% by weight, 0.002 to 8% by weight, 0.003 to 8% by weight, 0.004 to 8% by weight, 0.005 to 8% by weight, 0.006 to 8% by weight, 0.007 to 8% by weight, 0.008 to 8% by weight, 0.009 to 8% by weight, 0.01 to 8% by weight, etc.).

[0083] The photopolymerization initiator of the present invention has a pKa in the range of 5 to 12.7, and preferably the pressure-sensitive adhesive layer of the present invention also contains a hindered amine light stabilizer, thereby suppressing the progress of curing during storage, allowing excellent level difference absorbability and stress relaxation properties to be maintained for a long period of time, and providing excellent storage stability. The storage stability of the pressure-sensitive adhesive layer of the present invention can be evaluated by the rate of change (%) of residual stress. For example, the residual stress (N / cm) after storing the pressure-sensitive adhesive layer at 50°C for 4 weeks can be evaluated by the rate of change (%) of residual stress. 2 The rate of change in residual stress [(residual stress after storage at 50°C for 4 weeks - initial residual stress) / (initial residual stress) x 100] is preferably 70% or less, more preferably 60% or less, and may be 55% or less, 50% or less, 45% or less, 40% or less, 35% or less, or 30% or less. A configuration in which the rate of change in residual stress is 70% or less is preferred in that excellent level difference absorbability and stress relaxation properties can be maintained during use of the radiation-curable pressure-sensitive adhesive sheet of the present invention.

[0084] The method for forming the pressure-sensitive adhesive layer of the present invention is not particularly limited, but one embodiment (sometimes referred to herein as "first embodiment") may involve, for example, applying (coating) a pressure-sensitive adhesive composition containing the pressure-sensitive adhesive base material, the photopolymerization initiator of the present invention (second photopolymerization initiator), and the crosslinking agent of the present invention (second crosslinking agent), and optionally a hindered amine light stabilizer (HALS) and other additives, onto a support, and drying and curing the resulting pressure-sensitive adhesive composition layer; or applying (coating) the pressure-sensitive adhesive composition onto a support, and irradiating the resulting pressure-sensitive adhesive composition layer with active energy rays to cure it. If necessary, the pressure-sensitive adhesive composition may be further dried by heating. In order to prevent the crosslinking reaction of the photopolymerization initiator of the present invention and the crosslinking agent of the present invention from progressing due to irradiation with active energy rays, it is preferable to dry and cure the pressure-sensitive adhesive composition layer.

[0085] The support is not particularly limited, but a plastic film is preferred. Examples of materials for the plastic film include polyester resins such as polyethylene terephthalate (PET), acrylic resins such as polymethyl methacrylate (PMMA), polycarbonate, triacetyl cellulose (TAC), polysulfone, polyarylate, polyimide, polyvinyl chloride, polyvinyl acetate, polyethylene, polypropylene, ethylene-propylene copolymers, and cyclic olefin polymers such as "Arton" (a cyclic olefin polymer manufactured by JSR Corporation) and "Zeonor" (a cyclic olefin polymer manufactured by Zeon Corporation). These plastic materials may be used alone or in combination of two or more. The support may be a release sheet. The release sheet is not particularly limited, but examples thereof include plastic films whose surfaces are treated with a release agent such as a silicone-based, long-chain alkyl-based, fluorine-based, or molybdenum sulfide-based release agent.

[0086] The pressure-sensitive adhesive composition can be applied (coated) using a known coating method, and examples of such coaters include a gravure roll coater, a reverse roll coater, a kiss roll coater, a dip roll coater, a bar coater, a knife coater, a spray coater, a comma coater, and a direct coater.

[0087] The drying and curing temperature is preferably 40 to 200° C., more preferably 50 to 180° C., and even more preferably 60 to 170° C. The drying and curing time can be appropriately selected from among various times, and is, for example, 5 seconds to 20 minutes, preferably 5 seconds to 10 minutes, and more preferably 10 seconds to 5 minutes.

[0088] Examples of the active energy rays include ionizing radiation such as α-rays, β-rays, γ-rays, neutron beams, and electron beams, and ultraviolet rays, with ultraviolet rays being particularly preferred. The irradiation energy, irradiation time, irradiation method, and the like of the active energy rays are not particularly limited, and may be appropriately set so as to achieve the desired viscosity and viscoelasticity depending on the thickness of the pressure-sensitive adhesive layer, etc.

[0089] When the pressure-sensitive adhesive layer formed above is photocured by irradiation with the above-mentioned active energy rays and / or ultraviolet light described below, it is preferable that the main surface not facing the support is further laminated with another support (including a release sheet) to block oxygen, which inhibits photocuring.

[0090] As another embodiment of the method for forming a pressure-sensitive adhesive layer of the present invention (sometimes referred to herein as "second embodiment"), for example, the pressure-sensitive adhesive layer is preferably formed by a method including the following steps. (a) forming a pressure-sensitive adhesive layer made of a pressure-sensitive adhesive base material on a support (pressure-sensitive adhesive layer forming step); (b) curing the pressure-sensitive adhesive layer (pressure-sensitive adhesive layer curing step); (c) preparing a solution of the photopolymerization initiator of the present invention and the crosslinking agent of the present invention, and applying the solution to one surface of the cured pressure-sensitive adhesive layer (solution application step); (d) allowing the photopolymerization initiator of the present invention and the crosslinking agent of the present invention contained in the solution to penetrate into the pressure-sensitive adhesive layer from the one surface in the thickness direction (solution penetration step); (e) The pressure-sensitive adhesive layer is dried (drying step).

[0091] In the method of the first embodiment, when the pressure-sensitive adhesive composition contains a first photopolymerization initiator that forms a crosslinked structure in the pressure-sensitive adhesive base material and a photopolymerization initiator (second photopolymerization initiator) of the present invention, the light absorption wavelength bands of the two photopolymerization initiators must be sufficiently separated so that only the curing reaction caused by the first photopolymerization initiator proceeds and the curing reaction caused by the second photopolymerization initiator does not proceed. The need to distinguish between these wavelength ranges significantly narrows the design flexibility of the pressure-sensitive adhesive. Furthermore, even when a thermal polymerization initiator is used as the polymerization initiator for forming the crosslinked structure in the pressure-sensitive adhesive base material, curing due to the reaction of the second photopolymerization initiator may proceed during thermal curing, requiring strict control of the curing reaction. Another problem is that thick pressure-sensitive adhesive layers require a long time for thermal curing, reducing production efficiency.

[0092] On the other hand, in the method of the second embodiment, the curing reaction by the first photopolymerization initiator is completed before the second photopolymerization initiator is added. Therefore, it is not necessary to combine the first photopolymerization initiator with the second photopolymerization initiator in a way that prevents the curing reaction by the second photopolymerization initiator from proceeding, or to set strict curing conditions. This significantly increases the design flexibility of the adhesive. In other words, because the curing reactions of the first and second photopolymerization initiators are separated, the degree of freedom in selecting the combination of the first and second photopolymerization initiators is extremely high, and it is even possible for the light absorption wavelength bands of the two photopolymerization initiators to overlap or be similar. Furthermore, it is possible to combine the same photopolymerization initiator as the first and second photopolymerization initiators, which was previously impossible.

[0093] This allows for a high degree of freedom in the combination of the first and second photopolymerization initiators, and even allows for the use of the same photopolymerization initiator. This is advantageous in that it avoids the use of polymerization initiators that may cause coloration or deterioration of the pressure-sensitive adhesive layer. Furthermore, because the curing reactions of the first and second photopolymerization initiators are separate and do not interfere with each other, strict condition setting for controlling each curing reaction is not required. For example, with the first photopolymerization initiator, it is not necessary to set conditions to prevent the second photopolymerization initiator from proceeding. Once the curing conditions that impart excellent step absorption and stress relaxation properties to the pressure-sensitive adhesive layer are determined, the adhesive reliability to be imparted to the pressure-sensitive adhesive layer can be easily controlled by changing the coating conditions and curing conditions of the second photopolymerization initiator solution. Furthermore, even when a thermal polymerization initiator and a second photopolymerization initiator are combined, the two curing reactions are separate, so there is no need to control the curing caused by the second photopolymerization initiator reaction during thermal curing.

[0094] 2(a) to 2(e) are diagrams schematically showing steps for carrying out one embodiment of the manufacturing method according to the second embodiment of the present invention. First, as shown in Fig. 1(a), an adhesive layer 10a made of a transparent adhesive base material is formed on a support S1 (adhesive layer forming step). As the support, the support exemplified in the first embodiment can be used.

[0095] In the second embodiment, the method for forming the pressure-sensitive adhesive layer is not particularly limited, but can be carried out by the same method as in the first embodiment.

[0096] Next, the pressure-sensitive adhesive layer 10a is cured (pressure-sensitive adhesive layer curing step). In FIG. 1(b), 10 denotes the pressure-sensitive adhesive layer obtained by curing the pressure-sensitive adhesive layer 10a. The method for curing the pressure-sensitive adhesive layer 10a is not particularly limited, but examples thereof include heating the pressure-sensitive adhesive layer 10a and curing the pressure-sensitive adhesive layer 10a by irradiating it with active energy rays. If necessary, the pressure-sensitive adhesive layer 10a may be further heated and dried. Examples of active energy rays include ionizing radiation such as α rays, β rays, γ rays, neutron rays, and electron beams, as well as ultraviolet rays, with ultraviolet rays being particularly preferred.

[0097] The conditions for curing the pressure-sensitive adhesive layer 10a can be appropriately selected depending on the embodiment so that the pressure-sensitive adhesive layer 10 has desired properties. For example, in the radiation-curable pressure-sensitive adhesive sheet of the present invention, the heating temperature, time, or amount of active energy ray irradiation can be appropriately set so that the pressure-sensitive adhesive layer 10 exhibits high fluidity and excellent step absorbency and stress relaxation properties.

[0098] FIG. 1(b) shows an embodiment in which the pressure-sensitive adhesive layer 10a is irradiated with ultraviolet light U to cure the pressure-sensitive adhesive layer 10a. While ultraviolet light may be irradiated directly onto the pressure-sensitive adhesive layer 10a, it is preferable to irradiate the layer through a support to block oxygen, which inhibits curing by UV irradiation. FIG. 1(b) shows an embodiment in which ultraviolet light is irradiated onto the pressure-sensitive adhesive layer 10a through a support S2. When irradiating the pressure-sensitive adhesive layer 10a with ultraviolet light through a support, another support S2 (including a release sheet) is attached to the main surface of the pressure-sensitive adhesive layer 10a opposite to the main surface facing the support S1, and ultraviolet light is irradiated through the support. The illuminance and duration of ultraviolet light irradiation are appropriately determined depending on the composition of the pressure-sensitive adhesive base material, the thickness of the pressure-sensitive adhesive layer, and the like. For ultraviolet light irradiation, a high-pressure mercury lamp, a low-pressure mercury lamp, a metal halide lamp, or the like can be used.

[0099] Next, as shown in FIG. 1(c), after peeling and removing the support S2, a solution 12 of a photopolymerization initiator 11a (second photopolymerization initiator) and a crosslinking agent 11b (second crosslinking agent) is applied to one side of the pressure-sensitive adhesive layer 10 (solution application step). The solution 12 of the photopolymerization initiator 11a and the crosslinking agent 11b is not particularly limited as long as it is liquid and can be applied to and penetrate the pressure-sensitive adhesive layer. For example, when the photopolymerization initiator 11a and / or the crosslinking agent 11b are liquid, they may be applied as a solution as is, or a solution in which the photopolymerization initiator 11a and the crosslinking agent 11b are mixed. Alternatively, the photopolymerization initiator 11a and the crosslinking agent 11b may be dissolved in a solvent. FIG. 1(c) illustrates an embodiment in which a solution 12 in which the photopolymerization initiator 11a and the crosslinking agent 11b are dissolved in a solvent 13 is applied to one side of the pressure-sensitive adhesive layer 10.

[0100] The photopolymerization initiator 11a and crosslinking agent 11b in the solution 12 penetrate into the pressure-sensitive adhesive layer 10 in the thickness direction (solution penetration step). This state is shown in FIG. 1(d). When the solution 12 is a solution in which the photopolymerization initiator 11a and the crosslinking agent 11b are dissolved in the solvent 13, the solvent 13 penetrates into the surface of the pressure-sensitive adhesive layer 10 and causes it to swell, and the photopolymerization initiator 11a and the crosslinking agent 11b penetrate into the pressure-sensitive adhesive layer 10 while still dissolved in the solvent. The photopolymerization initiator 11a and the crosslinking agent 11b become "dissolved" within the pressure-sensitive adhesive layer 10.

[0101] Furthermore, as the photopolymerization initiator 11a and the crosslinking agent 11b penetrate the pressure-sensitive adhesive layer 10a, a concentration gradient may be formed in the thickness direction. Therefore, the concentrations of the photopolymerization initiator 11a and the crosslinking agent 11b on the side where the solution 12 is applied may be higher than those on the opposite side. This state is shown in FIG. 1(d).

[0102] Thereafter, the pressure-sensitive adhesive layer 10 is dried to obtain the pressure-sensitive adhesive sheet 1a shown in FIG. 1(e) (drying step). If the solution 12 is a solution in which a photopolymerization initiator 11a and a crosslinking agent 11b are dissolved in a solvent 13, the permeated solvent 13 is evaporated in the drying step. By drying the pressure-sensitive adhesive layer 10, the pressure-sensitive adhesive layer 10 returns to a state close to that before application. Therefore, changes in the physical properties of the pressure-sensitive adhesive layer 10 due to the photopolymerization initiator 11a and the crosslinking agent 11b can be minimized. When the pressure-sensitive adhesive layer 10 is dried, the permeation of the photopolymerization initiator 11a and the crosslinking agent 11b into the pressure-sensitive adhesive layer 10 stops, and the concentration gradients of the photopolymerization initiator 11a and the crosslinking agent 11b are fixed.

[0103] Preferably, the pressure-sensitive adhesive layer 10a contains a first photopolymerization initiator and a first crosslinking agent, and the pressure-sensitive adhesive layer curing step involves curing by a reaction between the first photopolymerization initiator and the first crosslinking agent. When the pressure-sensitive adhesive layer 10a contains a first photopolymerization initiator and a first crosslinking agent, both or either one of them may remain after the pressure-sensitive adhesive layer curing step. In this case, the solution 12 may contain only either the second photopolymerization initiator or the second crosslinking agent. However, it is preferable that the solution 12 contain both the second photopolymerization initiator and the second crosslinking agent.

[0104] The solvent is not particularly limited as long as it can dissolve the second photopolymerization initiator and the second crosslinking agent and can swell the adhesive layer 10. However, aqueous solvents have poor wettability to the adhesive layer, and the photopolymerization initiator and the crosslinking agent do not easily penetrate into the adhesive layer, so non-aqueous solvents are preferred. The non-aqueous solvent is not particularly limited, but examples thereof include esters such as methyl acetate, ethyl acetate, isopropyl acetate, and butyl acetate; aromatic hydrocarbons such as toluene, xylene, and ethylbenzene; ketones such as acetone, methyl ethyl ketone, and methyl isobutyl ketone; alicyclic ketones such as cyclopentanone and cyclohexanone; aliphatic hydrocarbons such as hexane, heptane, and octane; alicyclic hydrocarbons such as cyclohexane; halogenated hydrocarbons such as chloroform, dichloromethane, and 1,2-dichloroethane; ethers such as diethyl ether, dimethoxyethane, tetrahydrofuran, and dioxane; amides such as N,N-dimethylformamide and N,N-dimethylacetamide; nitriles such as acetonitrile, propionitrile, and benzonitrile; and alcohols such as methanol, ethanol, propanol, isopropanol, n-butanol, isobutanol, sec-butanol, and tert-butanol. Esters, aromatic hydrocarbons, ketones, and alcohols are preferred. The solvents can be used alone or in combination of two or more.

[0105] The total concentration of the second photopolymerization initiator and the second crosslinking agent in the solution can be appropriately set depending on the desired step absorbency, stress relaxation properties, and adhesion reliability to be imparted to the pressure-sensitive adhesive layer 10, and can be appropriately selected, for example, from 95% by weight or less (e.g., 0.1 to 95% by weight, 0.1 to 90% by weight, 0.1 to 85% by weight, 0.1 to 80% by weight, 0.1 to 70% by weight, 0.1 to 60% by weight, 0.1 to 50% by weight, 0.1 to 40% by weight, etc.), or from 0.1% by weight or more (e.g., 0.1 to 95% by weight, 0.2 to 95% by weight, 0.3 to 95% by weight, 0.4 to 95% by weight, 0.5 to 95% by weight, 1 to 95% by weight, 1.5 to 95% by weight, 3 to 95% by weight, 5 to 95% by weight, etc.). If the concentrations of the second photopolymerization initiator and the second crosslinking agent in the solution are within this range, the second photopolymerization initiator and the second crosslinking agent can be dissolved while the adhesive layer 10 is sufficiently swollen, thereby imparting excellent step absorbability, stress relaxation properties, and adhesive reliability to the adhesive layer 10.

[0106] Specifically, the concentration of the second crosslinking agent in the solution can be set appropriately depending on the desired curing properties to be imparted to the pressure-sensitive adhesive layer 10, and can be selected appropriately from the range of, for example, 95% by weight or less (e.g., 1 to 95% by weight, 1 to 90% by weight, 1 to 85% by weight, 1 to 80% by weight, 1 to 75% by weight, 1 to 70% by weight, 1 to 65% by weight, 1 to 60% by weight, etc.), or for example, 1% by weight or more (e.g., 1 to 95% by weight, 3 to 95% by weight, 4 to 95% by weight, 5 to 95% by weight, 6 to 95% by weight, 7 to 95% by weight, 8 to 95% by weight, 9 to 95% by weight, 10 to 95% by weight).

[0107] The concentration of the second photopolymerization initiator in the solution can be appropriately set depending on the desired curing properties to be imparted to the pressure-sensitive adhesive layer 10, and can be, for example, an upper limit of 20% by weight or less (e.g., 0.001 to 20% by weight, 0.001 to 19% by weight, 0.001 to 18% by weight, 0.001 to 17% by weight, 0.001 to 16% by weight, 0.001 to 15% by weight, 0.001 to 14% by weight, 0.001 to 13% by weight, The range can be selected from a range with a lower limit of 0.001% or more (e.g., 0.001 to 12% by weight, etc.) or a range with a lower limit of 0.001% or more (e.g., 0.001 to 20% by weight, 0.002 to 20% by weight, 0.003 to 20% by weight, 0.004 to 20% by weight, 0.005 to 20% by weight, 0.006 to 20% by weight, 0.007 to 20% by weight, 0.008 to 20% by weight, 0.009 to 20% by weight, 0.01 to 20% by weight, etc.).

[0108] When the pressure-sensitive adhesive layer of the present invention contains a hindered amine light stabilizer (HALS), the hindered amine light stabilizer may be further dissolved in solution 12. By including a hindered amine light stabilizer in solution 12, the pressure-sensitive adhesive layer of the present invention containing the hindered amine light stabilizer in addition to the second photopolymerization initiator and the second crosslinking agent can be produced by a single coating process, thereby improving production efficiency. The concentration of the hindered amine light stabilizer in the solution can be appropriately set depending on the desired storage stability to be imparted to the pressure-sensitive adhesive layer 10, and may be, for example, an upper limit of 20 wt % or less (e.g., 0.001 to 20 wt %, 0.001 to 19 wt %, 0.001 to 18 wt %, 0.001 to 17 wt %, 0.001 to 16 wt %, 0.001 to 15 wt %, 0.001 to 14 wt %, 0.001 to 13 wt %). , 0.001 to 12% by weight, etc.) or a range with a lower limit of 0.001% or more (for example, 0.001 to 20% by weight, 0.002 to 20% by weight, 0.003 to 20% by weight, 0.004 to 20% by weight, 0.005 to 20% by weight, 0.006 to 20% by weight, 0.007 to 20% by weight, 0.008 to 20% by weight, 0.009 to 20% by weight, 0.01 to 20% by weight, etc.).

[0109] If the concentrations of the second photopolymerization initiator, second crosslinking agent, and hindered amine light stabilizer are higher than the above ranges, bleeding may occur or distribution variations may occur in terms of coating uniformity. Also, if the concentrations are lower than the above ranges, more solvent than necessary may be required, which may result in a decrease in adhesive properties due to residual solvent or excessive swelling of the adhesive, resulting in appearance defects (surface irregularities).

[0110] It is also preferable that solution 12 further contains an ultraviolet absorber. By including an ultraviolet absorber in solution 12 in addition to the second photopolymerization initiator and the second crosslinking agent, a pressure-sensitive adhesive sheet containing the ultraviolet absorber can be produced by a single coating process, thereby improving production efficiency. Furthermore, when the pressure-sensitive adhesive sheet of the present invention containing an ultraviolet absorber is irradiated with ultraviolet light, the ultraviolet absorber absorbs the ultraviolet light and generates heat, thereby accelerating the curing reaction and improving adhesion reliability, which is even more preferable. The concentration of the ultraviolet absorber in the solution can be set appropriately depending on the desired ultraviolet absorption properties to be imparted to the pressure-sensitive adhesive layer 10a, and can be selected from a range, for example, with an upper limit of 50% by weight or less (e.g., 1 to 50% by weight, 1 to 45% by weight, 1 to 40% by weight, 1 to 35% by weight, 1 to 30% by weight, 1 to 25% by weight, 1 to 20% by weight, 1 to 15% by weight, etc.) or a lower limit of 1% or more (e.g., 1 to 50% by weight, 2 to 50% by weight, 3 to 50% by weight, 4 to 50% by weight, 5 to 50% by weight).

[0111] The solution 12 can be applied (coated) to the adhesive layer 10 using a known coating method, such as a gravure roll coater, reverse roll coater, kiss roll coater, dip roll coater, bar coater, knife coater, spray coater, comma coater, or direct coater.

[0112] The amount of the solution 12 applied to the pressure-sensitive adhesive layer 10 can be appropriately set depending on the desired level difference absorbency, stress relaxation properties, and adhesive reliability to be imparted to the pressure-sensitive adhesive layer 10, and may be, for example, 1 to 1000 μg / cm 2 , preferably 1 to 500 μg / cm 2 , more preferably 1 to 300 μg / cm2 , and more preferably 1 to 100 μg / cm 2 When the amount of solution 12 applied is within this range, the second photopolymerization initiator and the second crosslinking agent can be dissolved, and sufficient level difference absorbability, stress relaxation properties, and adhesive reliability can be imparted to the pressure-sensitive adhesive layer 10.

[0113] After applying the solution 12 to the pressure-sensitive adhesive layer 10, the solution may be left to stand, if necessary, to allow the second photopolymerization initiator and second crosslinking agent to penetrate. The standing time is not particularly limited and can be appropriately selected, for example, within 15 minutes, and can be selected, for example, from the range of 1 second to 10 minutes, preferably 5 seconds to 5 minutes. The standing temperature can be room temperature (approximately 10 to 30°C). When left to stand under the above conditions, the solution can be sufficiently penetrated into the pressure-sensitive adhesive layer 10.

[0114] The heating temperature in the drying step is preferably 40 to 200°C, more preferably 50 to 180°C, and even more preferably 60 to 170°C. The drying time can be appropriately set, for example, 5 seconds to 20 minutes, preferably 5 seconds to 10 minutes, and more preferably 10 seconds to 5 minutes. By drying under the above conditions, the pressure-sensitive adhesive layer 10 can be restored to a state close to that before application. If necessary, a standing time may be provided to further homogenize the second photopolymerization initiator, the second crosslinking agent, and the pressure-sensitive adhesive layer. The standing time is not particularly limited and can be appropriately selected, for example, within 30 days, and can be appropriately selected, for example, between 1 hour and 15 days, and preferably between 24 hours and 10 days. By standing, the pressure-sensitive adhesive layer 10 stabilizes the second photopolymerization initiator, the second crosslinking agent, and the pressure-sensitive adhesive layer, and variation in characteristic evaluation can be suppressed.

[0115] FIG. 3 is a cross-sectional view showing one embodiment of the radiation-curable pressure-sensitive adhesive sheet of the present invention produced by the production method of the second embodiment described above.

[0116] The radiation-curable adhesive sheet 1b according to one embodiment of the present invention shown in Figure 3 is composed of an adhesive layer 10, a release sheet S1 bonded to one main surface 10A of the adhesive layer 10, and a release sheet S2 bonded to the other main surface 10B of the adhesive layer 10.

[0117] In Fig. 3, the adhesive layer 10 is made of an adhesive base material and has two opposing main surfaces (first main surface 10A and second main surface 10B), and may be a single layer or a laminate structure of two or more layers. The adhesive layer 10 can be formed by the above-mentioned adhesive layer forming step and adhesive layer curing step, and corresponds to the adhesive layer 10 in Fig. 2 above. Therefore, the adhesive layer 10 is preferably a cured adhesive layer.

[0118] The term "single layer" means that the adhesive layer is not a laminated structure. For example, forming an adhesive layer made of a transparent adhesive base material and then forming another adhesive layer made of the same transparent adhesive base material on top of that constitutes a laminated structure, not a single layer.

[0119] The thickness of the adhesive layer 10 is not particularly limited, but is usually 5 μm to 500 μm, preferably 5 μm to 400 μm, and more preferably 5 μm to 350 μm. If the thickness of the adhesive layer 10 is within this range, it conforms to the steps without leaving any air bubbles, relieves stress distortion caused by the steps, and suppresses display unevenness, which is preferable in terms of excellent step absorbability and stress relaxation properties.

[0120] The total light transmittance of the entire pressure-sensitive adhesive layer 10 is not particularly limited, but is preferably 80% or more, and more preferably 90% or more, as measured in accordance with JIS K7361. The higher the total light transmittance of the pressure-sensitive adhesive layer 21, the better. Furthermore, the haze value is preferably 1.5% or less, more preferably 1% or less, and even more preferably 0.8% or less.

[0121] The pressure-sensitive adhesive layer 10 contains a photopolymerization initiator 11a and a crosslinking agent 11b. The photopolymerization initiator 11a and the crosslinking agent 11b are preferably dissolved in the pressure-sensitive adhesive layer 10. Here, "dissolved" means, for example, that the photopolymerization initiator 11a and the crosslinking agent 11b are dissolved to an extent that the transparency of the pressure-sensitive adhesive layer 10 can be maintained, i.e., to an extent that cloudiness due to light scattering does not occur. Specifically, the photopolymerization initiator 11a and the crosslinking agent 11b are preferably contained in the pressure-sensitive adhesive layer 10 so that the haze value of the photopolymerization initiator 11a and the crosslinking agent 11b is 1.5% or less, preferably 1% or less, and more preferably 0.8% or less. The present invention also encompasses an embodiment in which only one of the photopolymerization initiator 11a and the crosslinking agent 11b is dissolved in the pressure-sensitive adhesive layer 10.

[0122] In Figure 3, dotted line X-X' is a line that divides the pressure-sensitive adhesive layer 10 into two equal parts in the thickness direction. If the thickness of the pressure-sensitive adhesive layer 10 is not uniform, dotted line X-X' is a line that divides the thickness at each point into two equal parts.

[0123] The photopolymerization initiator 11a and the crosslinking agent 11b are formed by the penetration of the photopolymerization initiator 11a and the crosslinking agent 11b into the pressure-sensitive adhesive layer 10 through the solution application step, the solution penetration step, and the drying step, and a concentration gradient of the photopolymerization initiator 11a and the crosslinking agent 11b can be generated in the thickness direction of the pressure-sensitive adhesive layer 10 as shown in FIG. 3. Therefore, when the single-layer pressure-sensitive adhesive layer 10 is divided into two equal parts in the thickness direction, the concentrations of the photopolymerization initiator 11a and the crosslinking agent 11b in the region corresponding to the first main surface 10A, one of the two main surfaces, will be different from the concentrations of the photopolymerization initiator 11a and the crosslinking agent 11b in the region corresponding to the second main surface 10B. The case where the photopolymerization initiator 11a and the crosslinking agent 11b are not present in the region with the lower concentration (i.e., the concentration is 0) is also within the scope of the present invention.

[0124] The concentrations of the photopolymerization initiator 11a and the cross-linking agent 11b in the region to which the first principal surface belongs and the concentrations of the photopolymerization initiator 11a and the cross-linking agent 11b in the region to which the second principal surface belongs are taken to mean the average concentrations in each region if there is a concentration gradient within each region.

[0125] FIG. 3 shows an embodiment in which the first main surface 10A faces the support S1, and the concentrations of the photopolymerization initiator 11a and the crosslinking agent 11b in the region to which the second main surface 10B belongs are higher than the concentrations of the photopolymerization initiator 11a and the crosslinking agent 11b in the region to which the first main surface 10A belongs, and can be obtained by applying a solution 12 to the second main surface 10B and allowing the photopolymerization initiator 11a and the crosslinking agent 11b to penetrate in a dissolved state into the pressure-sensitive adhesive layer 10 from the first main surface 10B to a depth in the thickness direction.

[0126] The radiation-curable pressure-sensitive adhesive sheet of the present invention can be used to bond a transparent optical member to another optical member in an image display device such as a liquid crystal image display device or an organic electroluminescence (EL) image display device. Examples of the optical member include a polarizing film, a retardation film, a transparent cover member such as a cover glass, and various other transparent optical members. The optical member of the present invention also includes a glass substrate on which a transparent conductive layer such as a patterned ITO film is formed.

[0127] FIG. 4 is a cross-sectional view of an optical component laminate showing an example of the simplest embodiment using the radiation-curable pressure-sensitive adhesive sheet of the present invention. Referring to FIG. 4, the optical component laminate 2 comprises an optically transparent first optical component S3 and a second optical component S4 bonded to the first optical component S3 via an optically transparent pressure-sensitive adhesive layer 10. The optical component laminate 2 is obtained by peeling the supports S1 and S2 from the pressure-sensitive adhesive sheet 1b shown in FIG. 3 and bonding the first and second optical components. The transparent first optical component S3 and second optical component S4 can be composed of a polarizing film, a retardation film, or other optical film used in optical display devices, or a transparent cover member such as a cover glass on the viewing side of an optical display device. The first optical component S3 is bonded to the first main surface 10A of the pressure-sensitive adhesive layer 10, and the second optical component S4 is bonded to the second main surface 10B of the pressure-sensitive adhesive layer 10.

[0128] 5 is a diagram schematically illustrating steps for carrying out one embodiment of a method for producing an optical component laminate using a radiation-curable pressure-sensitive adhesive sheet of the present invention. In this embodiment, as shown in FIG. 5(a), a substrate 20 (hereinafter sometimes simply referred to as "substrate 20") consisting of an optical component laminate 2a and an optical component is used. In this embodiment, the optical member laminate 2a is obtained by laminating an optical member S4 to the main surface 10B of the pressure-sensitive adhesive sheet 1a produced by the production method of the second embodiment of the present invention, and peeling off the support S1.

[0129] 5(a), in this embodiment, the pressure-sensitive adhesive layer 10 is cured in the pressure-sensitive adhesive layer curing step, and the photopolymerization initiator 11a and the crosslinking agent 11b are dispersed in a dissolved state. The present invention also encompasses an embodiment in which only one of the photopolymerization initiator 11a and the crosslinking agent 11b is dissolved in the pressure-sensitive adhesive layer 10.

[0130] In this embodiment, the optical member laminate 2a includes an optical member S4, but the optical member S4 may be a support S2 (release sheet). In this embodiment, a concentration gradient of the photopolymerization initiator 11a and the crosslinking agent 11b exists in the thickness direction from the main surface 10B of the pressure-sensitive adhesive layer 10 that contacts the optical member S4.

[0131] 5(a), in this embodiment, the main surface 20A to be bonded to the optical component laminate 2a has a printed layer 30. Examples of the printed layer 30 include a transparent, conductive printed layer made of patterned ITO (indium tin oxide), and a black concealing portion formed in a frame shape around the periphery of a transparent cover member. The present invention also encompasses the use of a substrate 20 without a printed layer 30.

[0132] Next, the pressure-sensitive adhesive layer 10 of the optical component laminate 2a is bonded to the main surface 20A of the substrate 20. Bonding can be performed by a known method, such as under heat and pressure in an autoclave. The pressure-sensitive adhesive layer 10 of the optical component laminate 2a is cured in the pressure-sensitive adhesive layer curing step. However, it is in a state prior to the curing reaction between the photopolymerization initiator 11a (second photopolymerization initiator) and the crosslinking agent 11b (second crosslinking agent). This provides high fluidity and excellent step absorption and stress relaxation properties. Therefore, the pressure-sensitive adhesive layer 10 is bonded to the main surface 20A of the substrate 20 so as to completely fill the step between the printing layer 30. Furthermore, the pressure-sensitive adhesive layer 10 adequately relieves stress distortion caused by the step 30, thereby suppressing display unevenness when the optical component laminate is used in an image display device.

[0133] Next, the pressure-sensitive adhesive layer 10 is cured by a reaction between the photopolymerization initiator 11a and the crosslinking agent 11b. For example, the pressure-sensitive adhesive layer 10 may be cured by irradiating it with active energy rays. If necessary, the pressure-sensitive adhesive layer 10 may be further dried by heating. Examples of active energy rays include ionizing radiation such as α rays, β rays, γ rays, neutron rays, and electron beams, and ultraviolet rays, with ultraviolet rays being particularly preferred.

[0134] The conditions for curing the pressure-sensitive adhesive layer 10 may be appropriately set, for example, the heating temperature and time, or the amount of active energy ray irradiation, so that the pressure-sensitive adhesive layer 10 exhibits a high elastic modulus and excellent adhesive reliability.

[0135] FIG. 5(c) shows an embodiment in which the pressure-sensitive adhesive layer 10 is irradiated with ultraviolet light U to cure the pressure-sensitive adhesive layer 10. Irradiation with ultraviolet light U decomposes the photopolymerization initiator 11a to generate radicals or ions, which initiate the polymerization and cross-linking reaction of the cross-linking agent 11b. The ultraviolet light may be irradiated through the optical member S4. In addition, when the optical member S4 is a support member S2 (release sheet), the ultraviolet light may be irradiated through the support member S2, and the optical member S4 may be laminated after the support member S2 is peeled off. Furthermore, the ultraviolet light may be irradiated from the substrate 20 (optical member) side. FIG. 5(c) shows an embodiment in which the pressure-sensitive adhesive layer 10 is irradiated with ultraviolet light U through the optical member S4.

[0136] By curing the pressure-sensitive adhesive layer 10, an optical member laminate 2b is obtained as shown in Fig. 5(d). In Fig. 5(d), 10c is the pressure-sensitive adhesive layer 10 after curing.

[0137] By curing the adhesive layer 10, the crosslinking agent 11b crosslinks and polymerizes to form a crosslinked structure 11c, thereby forming an adhesive layer 10c. The adhesive layer 10c has an improved elastic modulus and improved adhesive reliability to the substrate 20. Therefore, the adhesive layer 10c suppresses the generation of gases such as carbon dioxide due to heating of the substrate 20 (plastic film), preventing the generation of bubbles.

[0138] 5(d), the pressure-sensitive adhesive layer 10c has a higher crosslink density on the main surface 10B side where it contacts the optical member S4 than on the opposite main surface 10A. This configuration is preferable in that it can improve flexibility when the optical member laminate 2b is used as a flexible image display device in which the main surface 10B side is bent outward. That is, when a flexible display is bent, a tensile stress is generally applied to the outside and a compressive stress is applied to the inside, with the stress on the outside being stronger than the stress on the inside. Therefore, by positioning the main surface 10B of the pressure-sensitive adhesive layer 10c on the outside when bending the flexible display, durability against bending can be improved.

[0139] In this embodiment, the optical member laminate 2b has, for example, the following configuration. An optical member laminate (2b) including a substrate (20) made of an optical member and a pressure-sensitive adhesive layer (10c), The pressure-sensitive adhesive layer 10c is laminated on a main surface of the substrate 20 made of the optical member, the pressure-sensitive adhesive layer 10c is a single layer made of a transparent pressure-sensitive adhesive base material cured by a reaction between the second polymerization initiator 11a and the second cross-linking agent 11b, and having two opposing main surfaces; When the single layer of adhesive layer is divided into two equal parts in the thickness direction, The density of the cross-linked structures 11c in the region to which the first principal surface 10A, one of the two principal surfaces, belongs is different from the density of the cross-linked structures 11c in the region to which the second principal surface 10B, the other principal surface, belongs.

[0140] In this embodiment, the pressure-sensitive adhesive layer 10c is laminated so as to fill in the step between the main surface 20A of the substrate 20 and the printing layer 30. [Example]

[0141] The present invention will be described in more detail below based on examples, but the present invention is not limited to these examples.

[0142] (Preparation of adhesive sheet A) A monomer mixture consisting of 57 parts by weight of butyl acrylate (BA), 23 parts by weight of 4-hydroxybutyl acrylate (4HBA), 8 parts by weight of 2-hydroxyethyl acrylate (HEA), and 12 parts by weight of cyclohexyl acrylate (CHA) was blended with 0.035 parts by weight of a photopolymerization initiator (trade name "Omnirad184", manufactured by IGM Resins BV) and 0.035 parts by weight of a photopolymerization initiator (trade name "Omnirad651", manufactured by IGM Resins BV). The mixture was then irradiated with ultraviolet light until the viscosity reached approximately 20 Pa s (measurement conditions: BH viscometer No. 5 rotor, 10 rpm, measurement temperature 30°C), yielding a prepolymer composition in which some of the monomer components had polymerized.

[0143] Next, 0.03 parts by weight of dipentaerythritol hexaacrylate (DPHA) was added to and mixed with the prepolymer composition to obtain an acrylic pressure-sensitive adhesive composition. The acrylic pressure-sensitive adhesive composition was applied to the release-treated surface of a release film (trade name "MRF#38", manufactured by Mitsubishi Plastics, Inc.) so that the thickness of the pressure-sensitive adhesive layer after formation was 250 μm to form a pressure-sensitive adhesive composition layer, and then a release film (trade name "MRN#38", manufactured by Mitsubishi Plastics, Inc.) was attached to the surface of the pressure-sensitive adhesive composition layer. Thereafter, an illuminance of 5 mW / cm was applied. 2 , Light amount: 1500mJ / cm 2The pressure-sensitive adhesive composition layer was photocured under the conditions of UV irradiation, to form a pressure-sensitive adhesive sheet A.

[0144] Example 1 The release film on one main surface (referred to as "side 1") of PSA sheet A was peeled off, and an ethyl acetate solution containing 80 wt% trimethylolpropane triacrylate (TMPTA) and 0.5 wt% photopolymerization initiator (Omnirad 369E, manufactured by IGM Resins BV) diluted to a concentration of 80 wt% was applied to the exposed side using a wire wound rod type #12 bar coater manufactured by RD Specialties (target wet coating thickness: 27 μm). After application, PSA sheet A was heated and dried in an oven at 110°C for 2 minutes to volatilize and remove the solvent, yielding PSA sheet A containing a PSA layer containing dissolved TMPTA. The chemical structure of the photopolymerization initiator (Omnirad 369E, 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1) was drawn using ChemDraw 19.0, and the calculated pKa was calculated using the calculation module of the chemoinformatics platform MOSES (Molecular Networks GmbH, Erlangen, Germany). It is 7.68.

[0145] Example 2 An adhesive sheet A including an adhesive layer in which TMPTA was dissolved was obtained in the same manner as in Example 1, except that the concentration of the photopolymerization initiator (Omnirad 369E, manufactured by IGM Resins BV) in the ethyl acetate solution was set to 1.0 wt %.

[0146] Example 3 An adhesive sheet A including an adhesive layer in which TMPTA was dissolved was obtained in the same manner as in Example 1, except that the concentration of the photopolymerization initiator (Omnirad 369E, manufactured by IGM Resins BV) in the ethyl acetate solution was set to 2.0 wt %.

[0147] Example 4 An adhesive sheet A containing an adhesive layer having TMPTA dissolved therein was obtained in the same manner as in Example 1, except that a photopolymerization initiator (Omnirad 907, manufactured by IGM Resins BV) was used and the concentration of the photopolymerization initiator in the ethyl acetate solution was set to 0.025 wt %. The chemical structure of the photopolymerization initiator (Omnirad 907, 2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropan-1-one) was drawn using ChemDraw 19.0, and the calculated pKa was calculated to be 5.56 using the chemoinformatics platform MOSES (Molecular Networks GmbH, Erlangen, Germany).

[0148] Example 5 An adhesive sheet A including an adhesive layer in which TMPTA was dissolved was obtained in the same manner as in Example 1, except that a photopolymerization initiator (Omnirad 907, manufactured by IGM Resins BV) was used and the concentration of the photopolymerization initiator in the ethyl acetate solution was 1.0 wt %.

[0149] Example 6 An adhesive sheet A including an adhesive layer in which TMPTA was dissolved was obtained in the same manner as in Example 1, except that a photopolymerization initiator (Omnirad 907, manufactured by IGM Resins BV) was used and the concentration of the photopolymerization initiator in the ethyl acetate solution was 2.0 wt %.

[0150] Example 7 An adhesive sheet A containing an adhesive layer in which TMPTA was dissolved was obtained in the same manner as in Example 1, except that a photopolymerization initiator (Omnirad 907, manufactured by IGM Resins BV) was used and the concentration of the photopolymerization initiator in the ethyl acetate solution was 4.0 wt %.

[0151] Example 8 An adhesive sheet A containing an adhesive layer in which TMPTA was dissolved was obtained in the same manner as in Example 1, except that a photopolymerization initiator (Omnirad 907, manufactured by IGM Resins BV) was used and the concentration of the photopolymerization initiator in the ethyl acetate solution was 8.0 wt %.

[0152] Example 9 An adhesive sheet A containing an adhesive layer in which TMPTA was dissolved was obtained in the same manner as in Example 1, except that toluene was used as the solvent and the concentration of the photopolymerization initiator (Omnirad 369E, manufactured by IGM Resins BV) in the toluene solution was 2.0 wt %.

[0153] Example 10 An ethyl acetate solution of a photopolymerization initiator (Omnirad 907, manufactured by IGM Resins BV) diluted to a concentration of 1.0 wt % was applied using a wire wound rod type #3 bar coater manufactured by RD Specialties (target wet coating thickness: 7 μm) in the same manner as in Example 1, except that an adhesive sheet A containing an adhesive layer having TMPTA dissolved therein was obtained.

[0154] Example 11 An adhesive sheet A containing an adhesive layer having TMPTA dissolved therein was obtained in the same manner as in Example 1, except that an ethyl acetate solution of a photopolymerization initiator (Omnirad 907, manufactured by IGM Resins BV) diluted to a concentration of 1.0 wt % was applied using a wire wound rod type #20 bar coater manufactured by RD Specialties (target wet coating thickness: 46 μm).

[0155] (Comparative Example 1) An adhesive sheet A including an adhesive layer having TMPTA dissolved therein was obtained in the same manner as in Example 1, except that a photopolymerization initiator (Omnirad 184, manufactured by IGM Resins BV) was used and the concentration of the photopolymerization initiator in the ethyl acetate solution was 1.0 wt %. The chemical structure of the photopolymerization initiator (Omnirad 184, 1-hydroxycyclohexyl-phenyl ketone) was drawn using ChemDraw 19.0, and the calculated pKa was 12.8, calculated using the chemoinformatics platform MOSES (Molecular Networks GmbH, Erlangen, Germany).

[0156] (Comparative Example 2) An adhesive sheet A including an adhesive layer in which TMPTA was dissolved was obtained in the same manner as in Example 1, except that a photopolymerization initiator (Omnirad 184, manufactured by IGM Resins BV) was used and the concentration of the photopolymerization initiator in the ethyl acetate solution was 2.0 wt %.

[0157] (Comparative Example 3) An adhesive sheet A containing an adhesive layer having TMPTA dissolved therein was obtained in the same manner as in Example 1, except that a photopolymerization initiator (Omnirad 184, manufactured by IGM Resins BV) was used and the concentration of the photopolymerization initiator in the ethyl acetate solution was 4.0 wt %.

[0158] Comparative Example 4 An adhesive sheet A containing an adhesive layer having TMPTA dissolved therein was obtained in the same manner as in Example 1, except that a photopolymerization initiator (Omnirad 127D, manufactured by IGM Resins BV) was used and the concentration of the photopolymerization initiator in the ethyl acetate solution was 1.0 wt %. The chemical structure of the photopolymerization initiator (Omnirad 127D, 2-hydroxy-1-(4-(4-(2-hydroxy-2-methylpropionyl)benzyl)phenyl)-2-methylpropan-1-one) was drawn using ChemDraw 19.0, and the calculated pKa was 12.84, calculated using the chemoinformatics platform MOSES (Molecular Networks GmbH, Erlangen, Germany).

[0159] (Comparative Example 5) An adhesive sheet A including an adhesive layer in which TMPTA was dissolved was obtained in the same manner as in Example 1, except that a photopolymerization initiator (Omnirad 127D, manufactured by IGM Resins BV) was used and the concentration of the photopolymerization initiator in the ethyl acetate solution was 2.0 wt %.

[0160] (Comparative Example 6) An adhesive sheet A containing an adhesive layer having TMPTA dissolved therein was obtained in the same manner as in Example 1, except that a photopolymerization initiator (Omnirad 127D, manufactured by IGM Resins BV) was used and the concentration of the photopolymerization initiator in the ethyl acetate solution was 4.0 wt %.

[0161] <Evaluation of residual stress change rate> The release films were peeled off from the pressure-sensitive adhesive sheets obtained in Examples 1 to 11 and Comparative Examples 1 to 6 to prepare test pieces measuring 40 mm in width and 20 mm in length. These test pieces were then wound lengthwise to prepare cylindrical pieces (the length of the cylinder was 40 mm). This was done so that there were no internal voids. The cylinder was clamped at both ends of 10 mm in the lengthwise direction with the chucks of an autograph (AG-X plus 200N, manufactured by Shimadzu Corporation) (the distance between the chucks was 20 mm). The autograph was used to pull the piece until the distance between the chucks reached 80 mm, and the distance between the chucks was maintained at 80 mm for 300 seconds. The residual stress [N / cm ] was calculated using the formula [stress after 300 seconds of holding / length × adhesive thickness]. 2 ] was requested. The residual stress at the initial stage of the pressure-sensitive adhesive sheet production and the residual stress after storing the pressure-sensitive adhesive sheet in a thermostatic chamber at 50°C for 4 weeks were determined, and the rate of change (%) in the residual stress was calculated using the following formula. Residual stress change rate (%) = (residual stress after 4 weeks storage at 50°C - initial residual stress) / (initial residual stress) x 100 The storage stability of the pressure-sensitive adhesive sheet was evaluated according to the following criteria, and the results are shown in Table 1. ◎ Residual stress change rate is 30% or less ○: Residual stress change rate is over 30% and 50% or less △: Residual stress change rate is over 50% and 70% or less × Residual stress change rate is over 70% and 100% or less ××...Residual stress change rate exceeds 100%

[0162] [Table 1]

[0163] Example 12 A pressure-sensitive adhesive sheet A including a pressure-sensitive adhesive layer having dissolved TMPTA was obtained in the same manner as in Example 1, except that the concentration of the photopolymerization initiator (Omnirad 369E, manufactured by IGM Resins BV) in the ethyl acetate solution was 2.0 wt % and a hindered amine light stabilizer (Tinuvin 770DF, manufactured by BASF) was dissolved in the ethyl acetate solution at 0.2 wt %. Measurement of the rate of change in residual stress showed that the storage stability was improved compared to Example 3, which did not contain a hindered amine light stabilizer.

[0164] Variations of the present invention are listed below. [Appendix 1] A pressure-sensitive adhesive sheet having a pressure-sensitive adhesive layer that is cured by irradiation with radiation, the pressure-sensitive adhesive layer contains a photopolymerization initiator and a crosslinking agent, The curing by irradiation with radiation is caused by a reaction between the photopolymerization initiator and the crosslinking agent, A radiation-curable pressure-sensitive adhesive sheet, wherein the photopolymerization initiator has a pKa of 5 to 12.7. [Appendix 2] A radiation-curable pressure-sensitive adhesive sheet according to Appendix 1, wherein the pKa of the photopolymerization initiator is a calculated pKa calculated from the chemical structural formula of the photopolymerization initiator. [Appendix 3] The radiation-curable pressure-sensitive adhesive sheet according to appendix 1 or 2, wherein the photopolymerization initiator has an amino group and an aromatic ring in the molecule. [Appendix 4] The radiation-curable pressure-sensitive adhesive sheet according to Appendix 3, wherein the photopolymerization initiator has two or more amino groups and an aromatic ring in the molecule. [Appendix 5] The radiation-curable pressure-sensitive adhesive sheet according to Appendix 3 or 4, wherein the photopolymerization initiator is an aminoacetophenone-based photopolymerization initiator. [Appendix 6] A pressure-sensitive adhesive sheet having a pressure-sensitive adhesive layer that is cured by irradiation with radiation, the pressure-sensitive adhesive layer is cured by a reaction between a first photopolymerization initiator and a first crosslinking agent, the pressure-sensitive adhesive layer contains a second photopolymerization initiator and a second crosslinking agent, the curing by irradiation with radiation is caused by a reaction between the second photopolymerization initiator and the second crosslinking agent, 6. The radiation-curable pressure-sensitive adhesive sheet according to any one of claims 1 to 5, wherein the second photopolymerization initiator has a pKa of 5 to 12.7. [Appendix 7] The radiation-curable pressure-sensitive adhesive sheet according to Appendix 6, wherein the second photopolymerization initiator and the second crosslinking agent are dissolved in the pressure-sensitive adhesive layer. [Appendix 8] The radiation-curable pressure-sensitive adhesive sheet according to appendix 6 or 7, wherein the second photopolymerization initiator is the same as the first photopolymerization initiator. [Appendix 9] The pressure-sensitive adhesive layer is a single layer having two opposing main surfaces, When the single layer of adhesive layer is divided into two equal parts in the thickness direction, 9. The radiation-curable pressure-sensitive adhesive sheet according to any one of Appendices 1 to 8, wherein the concentrations of the photopolymerization initiator (second photopolymerization initiator) and the crosslinking agent (second crosslinking agent) in a region to which a first main surface, one of the two main surfaces, belongs are different from the concentrations of the polymerization initiator (second photopolymerization initiator) and the crosslinking agent (second crosslinking agent) in a region to which a second main surface, the other of the two main surfaces, belongs. [Appendix 10] A radiation-curable adhesive sheet according to Appendix 9, wherein the single-layer adhesive layer has a concentration gradient of the photopolymerization initiator (second photopolymerization initiator) and the crosslinking agent (second crosslinking agent) in the thickness direction. [Appendix 11] The radiation-curable pressure-sensitive adhesive sheet according to any one of Appendices 1 to 10, wherein the pressure-sensitive adhesive layer has a thickness of 5 to 500 μm. [Appendix 12] A method for producing a radiation-curable pressure-sensitive adhesive sheet according to any one of Appendices 1 to 11, comprising: forming a pressure-sensitive adhesive layer made of a pressure-sensitive adhesive base material on a support; Curing the pressure-sensitive adhesive layer, preparing a solution of a photoinitiator (second photoinitiator) and a crosslinker (second crosslinker); The solution is applied to one surface of the cured pressure-sensitive adhesive layer, the photopolymerization initiator (second photopolymerization initiator) and the crosslinking agent (second crosslinking agent) contained in the solution are allowed to penetrate into the pressure-sensitive adhesive layer from the one surface in a thickness direction; Drying the adhesive layer 1. A method for producing a radiation-curable pressure-sensitive adhesive sheet, comprising the steps of: [Appendix 13] The solution containing the photopolymerization initiator (second photopolymerization initiator) and the crosslinking agent (second crosslinking agent) is a solution in which the photopolymerization initiator and the crosslinking agent are dissolved in a solvent, Attachment 13. The method for producing a pressure-sensitive adhesive sheet according to claim 12, comprising the step of evaporating the solvent of the solution by drying the pressure-sensitive adhesive layer. [Appendix 14] The method for producing a radiation-curable pressure-sensitive adhesive sheet according to Appendices 12 or 13, wherein the support is a release sheet. [Appendix 15] The method for producing a radiation-curable pressure-sensitive adhesive sheet according to any one of Appendices 12 to 14, further comprising the step of attaching a release sheet to the surface of the pressure-sensitive adhesive layer opposite the support. [Appendix 16] A substrate made of an optical member; An optical member laminate comprising: a pressure-sensitive adhesive layer is laminated on a main surface of the substrate made of the optical member, 12. An optical member laminate, wherein the pressure-sensitive adhesive layer is a cured product of the pressure-sensitive adhesive layer of the radiation-curable pressure-sensitive adhesive sheet according to any one of claims 1 to 11. [Appendix 17] The main surface of the substrate made of the optical member has a printing layer, 17. The optical member laminate according to claim 16, wherein the pressure-sensitive adhesive layer is laminated so as to fill in any step between the main surface of the substrate made of the optical member and the printing layer. [Appendix 18] The optical member laminate according to appendix 16 or 17, wherein the cured product is a product obtained by a reaction between the polymerization initiator and a crosslinking agent. [Industrial Applicability]

[0165] The radiation-curable pressure-sensitive adhesive sheet of the present invention is useful as a pressure-sensitive adhesive sheet having a transparent pressure-sensitive adhesive layer that can be used to bond a transparent optical member to another optical member. [Explanation of symbols]

[0166] 1. Radiation-curable adhesive sheet 1a Radiation-curable adhesive sheet 1b Radiation-curable adhesive sheet 2 Optical component laminate 2a Optical component laminate 2b Optical component laminate 10a Adhesive layer (before curing) 10 Adhesive layer (after curing) 10c Adhesive layer (after curing) S1, S2 Support (release sheet) U: Ultraviolet light 11a Photopolymerization initiator 11b Crosslinker 11c Crosslinked structure 12 Photopolymerization initiator and crosslinker solution 13 Solvents S3, S4 Optical components 20 Substrate (optical component) 30 printing layer

Claims

1. A pressure-sensitive adhesive sheet having a pressure-sensitive adhesive layer that is cured by irradiation with radiation, the pressure-sensitive adhesive layer is photocured by a reaction between a first photopolymerization initiator and a first crosslinking agent, and contains a second photopolymerization initiator and a second crosslinking agent, and is a single layer having two opposing main surfaces; when the single-layer pressure-sensitive adhesive layer is divided into two equal parts in a thickness direction, the concentrations of the second photopolymerization initiator and the second crosslinking agent in a region to which a first main surface, one of the two main surfaces, belongs are different from the concentrations of the second photopolymerization initiator and the second crosslinking agent in a region to which the other second main surface belongs; the curing by irradiation with radiation is caused by a reaction between the second photopolymerization initiator and the second crosslinking agent, A radiation-curable pressure-sensitive adhesive sheet, wherein the pKa of the second photopolymerization initiator is 5 to 12.

7.

2. The radiation-curable pressure-sensitive adhesive sheet according to claim 1 , wherein the pKa of the second photopolymerization initiator is a calculated pKa calculated from the chemical structural formula of the second photopolymerization initiator.

3. 3. The radiation-curable pressure-sensitive adhesive sheet according to claim 1, wherein the second photopolymerization initiator is a photopolymerization initiator having an amino group and an aromatic ring in the molecule.

4. The radiation-curable pressure-sensitive adhesive sheet according to claim 3 , wherein the second photopolymerization initiator is a photopolymerization initiator having two or more amino groups and an aromatic ring in the molecule.

5. The radiation-curable pressure-sensitive adhesive sheet according to claim 3 or 4, wherein the second photopolymerization initiator is an aminoacetophenone-based photopolymerization initiator.

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