Pressure-sensitive adhesive sheet and electronic device

Abstract

To provide a double-sided adhesive pressure-sensitive adhesive sheet that exhibits good shear characteristics in a configuration including a pressure-sensitive adhesive layer containing a water-dispersible acrylic polymer.SOLUTION: An adhesively double-faced PSA sheet having a PSA layer is provided. The PSA layer comprises a water-dispersed acrylic polymer, a tackifier resin and a water-soluble crosslinking agent. The pressure-sensitive adhesive layer has a thickness of 50 μm or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an adhesive sheet and an electronic device. [Background technology]

[0002] Generally, adhesives (also referred to as pressure-sensitive adhesives; the same applies hereinafter) are in a soft solid (viscoelastic) state at temperatures around room temperature and have the property of easily adhering to an adherend when pressure is applied. Taking advantage of these properties, adhesives are widely used in a variety of applications, for example, for the purpose of joining, fixing, and protecting components within electronic devices, including mobile electronic devices such as smartphones. Furthermore, adhesive sheets having an adhesive layer containing a pigment such as carbon black are used for the purposes of preventing light leakage and anti-reflection from light sources such as backlight modules of liquid crystal display devices in electronic devices, and self-luminous elements such as organic electroluminescence (EL), as well as for the purposes of concealing the adherend, adjusting the appearance of the adherend through the adhesive sheet, and for design purposes.

[0003] In recent years, there has been a demand for a reduction in the amount of organic solvents used in the production of adhesives, from the perspective of environmental considerations and reducing the use of petroleum resources. For example, the amount of organic solvent used can be reduced by adopting solventless adhesives such as active energy ray-curable adhesives, hot-melt adhesives, and emulsion adhesives. Among these, emulsion-type acrylic adhesives are preferred because they allow the desired properties to be suitably obtained by highly controlling the polymerization reaction of the acrylic monomer. Patent Documents 1 to 4 are examples of technical documents disclosing adhesives containing water-dispersible acrylic polymers obtained by emulsion polymerization. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2018-168387 [Patent Document 2] Japanese Patent Application Laid-Open No. 2013-249322 [Patent Document 3] Japanese Patent Application Publication No. 2023-054669 [Patent Document 4] Japanese Patent Application Laid-Open No. 2007-154078 Summary of the Invention [Problem to be solved by the invention]

[0005] It is also preferable to reduce the amount of organic solvent used in the PSA that can be used in the above-mentioned electronic devices, etc. However, double-sided PSA sheets having a PSA layer formed using a water-dispersible acrylic polymer instead of an acrylic polymer synthesized by solution polymerization tend to lack the ability (shear properties) to properly maintain the bonded state when a shear stress is applied to the bonded portion of the PSA sheet.

[0006] The present invention has been made in view of the above circumstances, and aims to provide a double-sided pressure-sensitive adhesive sheet that exhibits good shear properties when configured with a pressure-sensitive adhesive layer containing a water-dispersible acrylic polymer. Another related aim is to provide an electronic device that includes the pressure-sensitive adhesive sheet. [Means for solving the problem]

[0007] According to the present specification, there is provided a double-sided pressure-sensitive adhesive sheet having a pressure-sensitive adhesive layer. The pressure-sensitive adhesive layer contains a water-dispersible acrylic polymer, a tackifier resin, and a water-soluble crosslinking agent. The thickness of the pressure-sensitive adhesive layer is 50 μm or less. With this configuration, the use of the water-dispersible acrylic polymer reduces the amount of organic solvent, while realizing a pressure-sensitive adhesive sheet with good shear properties.

[0008] In some embodiments, the PSA layer preferably contains, as the tackifier resin, a tackifier resin selected from the group consisting of rosin-based tackifier resins and terpene-based tackifier resins, having a softening point of 100° C. or higher and 160° C. or lower. A tackifier resin that satisfies the above type and softening point requirements is more likely to produce a PSA sheet that exhibits better shear properties.

[0009] In some embodiments, the water-soluble crosslinking agent may preferably be a polyfunctional crosslinking agent having two or more functional groups reactive with a carboxyl group or a carbonyl group, and the technology disclosed herein can be suitably carried out using such a water-soluble crosslinking agent.

[0010] In some embodiments, the monomer components constituting the water-dispersible acrylic polymer have a carboxyl group-containing monomer content of 0.5% by weight or more and 12% by weight or less. A water-dispersible acrylic polymer having a carboxyl group-containing monomer content within this range is likely to produce a PSA sheet exhibiting better shear properties.

[0011] In some embodiments, a silane-based monomer is copolymerized into the water-dispersible acrylic polymer. The technology disclosed herein can be suitably implemented using a water-dispersible acrylic polymer copolymerized with a silane-based monomer.

[0012] In some embodiments, the water-dispersible acrylic polymer is copolymerized with a keto group-containing monomer. The technology disclosed herein can be suitably implemented using a water-dispersible acrylic polymer copolymerized with a keto group-containing monomer.

[0013] In some embodiments, the water-dispersible acrylic polymer may be a polymer synthesized by emulsion polymerization using a chain transfer agent. The technology disclosed herein can be preferably implemented using a water-dispersible acrylic polymer obtained using a reactive emulsifier having a radical polymerizable functional group and a chain transfer agent.

[0014] In some embodiments, the PSA sheet is a substrate-less double-sided PSA sheet comprising the PSA layer. The effects of the technology disclosed herein can be preferably achieved in a substrate-less double-sided PSA sheet. Furthermore, since a substrate-less double-sided PSA sheet does not have a substrate, it can be made thinner, which is advantageous in terms of making products to which the double-sided PSA sheet is applied thinner, more compact, and more space-saving. Furthermore, a substrate-less double-sided PSA sheet can maximize the effects of the PSA layer, such as adhesive strength. For example, when comparing double-sided PSA sheets with similar total thicknesses, a substrate-less double-sided PSA sheet can achieve better properties (e.g., adhesive strength, shear adhesive strength, etc.).

[0015] In some other embodiments, the PSA sheet is a substrate-attached double-sided PSA sheet having the PSA layer on both sides of a supporting substrate. Double-sided PSA sheets having a substrate are advantageous in terms of processability and handleability.

[0016] The PSA sheet disclosed herein is a double-sided adhesive sheet (double-sided PSA sheet) that exhibits excellent performance (shear properties) in maintaining a bonded state when a shear stress is applied to a bonded portion formed by the PSA sheet. Therefore, the PSA sheet can be preferably used, for example, to bond components of electronic devices, including home appliances, office automation equipment, and mobile electronic devices such as smartphones. For example, in electronic devices (particularly mobile electronic devices), deformation (bending, twisting, etc.) of the electronic device due to an external force can cause shear stress to be applied to a bonded portion formed by the double-sided PSA sheet. The PSA sheet disclosed herein can maintain a bonded state of the bonded portion even in such cases, and can effectively prevent peeling or misalignment of the bonded portion.

[0017] As described above, the present specification provides an electronic device using any of the pressure-sensitive adhesive sheets disclosed herein, in other words, an electronic device including the pressure-sensitive adhesive sheet. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a cross-sectional view schematically showing one example of the configuration of a pressure-sensitive adhesive sheet. [Figure 2] FIG. 10 is a cross-sectional view schematically showing another example of the configuration of the pressure-sensitive adhesive sheet. [Figure 3] FIG. 10 is a cross-sectional view schematically showing another example of the configuration of the pressure-sensitive adhesive sheet. [Figure 4] FIG. 1 is an exploded perspective view schematically illustrating a configuration example of a display device. DETAILED DESCRIPTION OF THE INVENTION

[0019] Preferred embodiments of the present invention are described below. Matters necessary for carrying out the present invention other than those specifically mentioned in this specification can be understood by those skilled in the art based on the teachings for carrying out the invention described in this specification and the common general technical knowledge at the time of filing. The present invention can be carried out based on the contents disclosed in this specification and the common general technical knowledge in the relevant field. Furthermore, in the following drawings, components and parts that perform the same function may be denoted by the same reference numerals, and redundant explanations may be omitted or simplified. Furthermore, the embodiments shown in the drawings are schematic to clearly explain the present invention and do not necessarily accurately represent the size or scale of the pressure-sensitive adhesive sheet of the present invention that is actually provided as a product.

[0020] As used herein, the term "adhesive" refers to a material that exhibits a soft solid (viscoelastic) state in a temperature range around room temperature and has the property of easily adhering to an adherend by pressure, as described above. The adhesive referred to here is generally a material having a complex tensile modulus E * (1Hz)<10 7 dyne / cm 2 The material may be a material having the properties satisfying the above (typically, a material having the above properties at 25°C).

[0021] In this specification, the term "water-dispersed PSA composition" refers to a composition in which at least a portion of the PSA-forming components are dispersed in water. The water-dispersed type also includes suspended and emulsified states. The concept of the water-dispersed PSA composition includes what is called an emulsion-type PSA composition. In this specification, a PSA (layer) formed from a water-dispersed PSA composition may also be referred to as a water-dispersed PSA (layer).

[0022] As used herein, the term "acrylic polymer" refers to a polymer containing more than 50% by weight of monomer units derived from an acrylic monomer as the monomer units constituting the polymer. The acrylic monomer refers to a monomer having at least one (meth)acryloyl group in one molecule.

[0023] As used herein, "(meth)acryloyl" refers collectively to acryloyl and methacryloyl. Similarly, "(meth)acrylate" refers collectively to acrylate and methacrylate, and "(meth)acrylic" refers collectively to acrylic and methacrylic.

[0024] <Adhesive sheet configuration example> The PSA sheet disclosed herein is a double-sided adhesive PSA sheet (double-sided PSA sheet), and may be a substrate-less PSA sheet (i.e., a double-sided PSA sheet without a non-releasable substrate) in which the PSA layer is held by a release film, or may be a substrate-attached double-sided PSA sheet in which the PSA layer is provided on both sides of a non-releasable substrate (support substrate). The concept of PSA sheet here may include those referred to as PSA tapes, PSA labels, PSA films, etc. The PSA sheet disclosed herein may be in the form of a roll or sheets. Alternatively, the PSA sheet may be processed into various shapes.

[0025] Examples of the configuration of a substrate-less double-sided pressure-sensitive adhesive sheet are shown in Figures 1 and 2. Pressure-sensitive adhesive sheet 1 shown in Figure 1 has a configuration in which both sides 21A and 21B of substrate-less pressure-sensitive adhesive layer 21 are protected by release films 31 and 32, respectively, with at least the pressure-sensitive adhesive layer side being the release surface. Pressure-sensitive adhesive sheet 2 shown in Figure 2 has a configuration in which one surface (adhesive surface) 21A of substrate-less pressure-sensitive adhesive layer 21 is protected by release film 31, with both surfaces being release surfaces; when this is rolled up, the other surface (adhesive surface) 21B of pressure-sensitive adhesive layer 21 abuts against the back surface of release film 31, so that the other surface 21B is also protected by release film 31.

[0026] The structure of a pressure-sensitive adhesive sheet according to another configuration example is shown schematically in Fig. 3. This pressure-sensitive adhesive sheet 4 is configured as a substrate-attached double-sided pressure-sensitive adhesive sheet comprising a sheet-like support substrate 10 having a first side 10A and a second side 10B, a first pressure-sensitive adhesive layer 21 fixedly provided on the first side 10A side, and a second pressure-sensitive adhesive layer 22 fixedly provided on the second side 10B side. Before use, the pressure-sensitive adhesive sheet 4 may be in a form in which the surface (first adhesive surface) 21A of the first pressure-sensitive adhesive layer 21 and the surface (second adhesive surface) 22A of the second pressure-sensitive adhesive layer 22 are protected by release films 31 and 32, as shown in Fig. 3.

[0027] The technology disclosed herein can be preferably implemented in a substrate-less form from the viewpoint of reducing the thickness of the PSA sheet. Substrate-less PSA sheets are advantageous in that they can be easily thinned and can maximize adhesive properties such as adhesive strength. Alternatively, in an embodiment in which the PSA sheet disclosed herein is in a substrate-attached form, the presence of a substrate makes it easier to obtain excellent processability, handleability, and the like.

[0028] <Adhesive layer> (Water-dispersible acrylic polymer) The PSA layer disclosed herein contains a water-dispersible acrylic polymer. In this specification, "water-dispersible acrylic polymer" refers to an acrylic polymer synthesized in a form dispersed in water, and more specifically, to an acrylic polymer synthesized by emulsion polymerization. By using a water-dispersible polymer as a PSA-forming component, it is possible to reduce the amount of organic solvent used, and ultimately to achieve organic solvent-free production. Furthermore, water-dispersible acrylic polymers can be obtained by polymerizing acrylic monomers while highly controlling the reaction, and have the advantage of being easily able to obtain desired properties due to the high degree of freedom in molecular design.

[0029] The water-dispersible acrylic polymer is typically contained in the pressure-sensitive adhesive layer as a base polymer. Here, "base polymer" refers to the main component of the rubber-like polymer contained in the pressure-sensitive adhesive layer, and is not intended to be limited in any other sense. The rubber-like polymer refers to a polymer that exhibits rubber elasticity in a temperature range around room temperature. In this specification, "main component" refers to a component contained in an amount of more than 50% by weight, unless otherwise specified. Hereinafter, the water-dispersible acrylic polymer may be simply referred to as an acrylic polymer.

[0030] The acrylic polymer is preferably a polymer of a monomer raw material (monomer component) that contains, for example, an alkyl(meth)acrylate as a main monomer and may further contain a secondary monomer copolymerizable with the main monomer, where the main monomer is a component that accounts for more than 50% by weight of the monomer composition in the monomer raw material.

[0031] As the alkyl(meth)acrylate, for example, a compound represented by the following formula (1) can be suitably used. CH2=C(R 1 )COOR 2 (1) Here, R in the above formula (1) 1 is a hydrogen atom or a methyl group. 2 is a chain alkyl group having 1 to 20 carbon atoms (hereinafter, this range of carbon atoms is referred to as "C 1-20From the viewpoint of the storage modulus of the adhesive, R 2 C 1-14 alkyl(meth)acrylates, which are chain alkyl groups of the formula R 2 C 1-10 Alkyl (meth)acrylate, which is a chain alkyl group, is more preferred.

[0032] R 2 C 1-20 Examples of alkyl(meth)acrylates, which are chain alkyl groups, include methyl(meth)acrylate, ethyl(meth)acrylate, propyl(meth)acrylate, isopropyl(meth)acrylate, n-butyl(meth)acrylate, isobutyl(meth)acrylate, s-butyl(meth)acrylate, pentyl(meth)acrylate, isopentyl(meth)acrylate, hexyl(meth)acrylate, heptyl(meth)acrylate, 2-ethylhexyl(meth)acrylate, octyl(meth)acrylate, and isooctyl(meth)acrylate. Examples of alkyl (meth)acrylates include butyl (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, nonadecyl (meth)acrylate, and eicosyl (meth)acrylate. These alkyl (meth)acrylates can be used alone or in combination of two or more. Suitable examples of alkyl (meth)acrylates include n-butyl acrylate (BA) and 2-ethylhexyl acrylate (2EHA).

[0033] The technology disclosed herein is 2 C 1-20 Alkyl (meth)acrylate (C 1-20In some preferred embodiments, the above-mentioned C can be preferably carried out using an acrylic polymer in which the chain alkyl (meth)acrylate accounts for approximately 50% by weight or more of the monomer components. 1-20 The proportion of the chain alkyl (meth)acrylate in the monomer component may be approximately 75% by weight or more, approximately 90% by weight or more, or approximately 95% by weight or more. 1-20 In an embodiment in which a secondary monomer is used, the upper limit of the proportion of the chain alkyl (meth)acrylate may be, for example, 99.9% by weight or less, 99.5% by weight or less, or 99% by weight or less (e.g., 98.5% by weight or less).

[0034] In some embodiments, the monomer component is R 2 C 4-10 Alkyl (meth)acrylate (C 4-10 It is preferable that the compound contains a chain alkyl (meth)acrylate. 4-10 As the chain alkyl (meth)acrylate, C 4-8 It is more preferable to use a chain alkyl (meth)acrylate, and from the viewpoint of adhesive properties, R 1 is a hydrogen atom and R 2 C 4-10 Alkyl acrylate (C 4-10 It is more preferable to use a chain alkyl acrylate), and C 4-8 It is particularly preferable to use a chain alkyl acrylate. 4-10 The chain alkyl (meth)acrylate may be used alone or in combination of two or more. 4-10 The proportion of the chain alkyl (meth)acrylate is preferably about 50% by weight or more, more preferably 70% by weight or more, and even more preferably 80% by weight or more. 4-10The upper limit of the proportion of chain alkyl (meth)acrylate is 100% by weight, and in some embodiments, from the viewpoint of obtaining good cohesive strength, it is appropriate to set it to approximately 95% by weight or less, and it may also be set to approximately 90% by weight or less.

[0035] In some embodiments, the monomer component is C 6-10 It is preferable that the compound contains a chain alkyl (meth)acrylate. 6-10 As the chain alkyl (meth)acrylate, C 7-9 It is more preferable to use a chain alkyl (meth)acrylate, and it is even more preferable to use a C8 chain alkyl (meth)acrylate. 6-10 As the chain alkyl (meth)acrylate, a chain alkyl acrylate is preferably used. 6-10 The chain alkyl (meth)acrylate may be used alone or in combination of two or more. In some preferred embodiments, the C occupies the alkyl (meth)acrylate contained in the monomer component. 6-10 The proportion of the chain alkyl (meth)acrylate is approximately 50% by weight or more (for example, more than 50% by weight), and may be 60% by weight or more, 70% by weight or more, or 80% by weight or more. 6-10 The upper limit of the proportion of chain alkyl (meth)acrylate is 100% by weight, and in some embodiments, from the viewpoint of obtaining good cohesive strength, it is appropriate to set it to approximately 95% by weight or less, and it may also be set to approximately 90% by weight or less.

[0036] Above C 6-10 As the chain alkyl (meth)acrylate, 2EHA is preferably used. In the embodiment in which 2EHA is used, the proportion of 2EHA is not particularly limited, and may be any proportion other than the above C 4-10 In the chain alkyl (meth)acrylate, the content is suitably more than 50% by weight, preferably 70% by weight or more, more preferably 90% by weight or more, and even more preferably 95% by weight or more (for example, 95 to 100% by weight).

[0037] The alkyl (meth)acrylate is 4-10 When a chain alkyl (meth)acrylate is contained, other alkyl (meth)acrylate (R 2 is less than C4 or C 10 The alkyl (meth)acrylate may contain a chain alkyl group of longer than 100 MPa (alkyl (meth) acrylate). This allows a good balance between adhesion and cohesion. The other alkyl (meth) acrylates include those represented by the formula (1) R 2 C 1-3 Alkyl (meth)acrylates, which are chain alkyl groups, can be preferably used. Specific examples include methyl acrylate (MA), methyl methacrylate (MMA), and ethyl acrylate (EA). Among these, MA is more preferred. The other alkyl (meth)acrylates can be used alone or in combination of two or more. The proportion of the other alkyl (meth)acrylates in the alkyl (meth)acrylates contained in the monomer component is preferably about 30% by weight or less, and may be, for example, about 20% by weight or less, or about 15% by weight or less. Furthermore, from the viewpoint of obtaining the effects of the other alkyl (meth)acrylates, the proportion of the other alkyl (meth)acrylates is preferably about 1% by weight or more of the total alkyl (meth)acrylates, and may be, for example, about 5% by weight or more, or about 10% by weight or more.

[0038] The secondary monomer copolymerizable with the alkyl (meth)acrylate main monomer can be useful for introducing crosslinking points into the acrylic polymer and for increasing the cohesive strength of the acrylic polymer. As the secondary monomer, for example, the following functional group-containing monomer components can be used alone or in combination of two or more: Carboxy group-containing monomers: for example, ethylenically unsaturated monocarboxylic acids such as acrylic acid, methacrylic acid, carboxyethyl (meth)acrylate, carboxypentyl (meth)acrylate, and crotonic acid; ethylenically unsaturated dicarboxylic acids such as maleic acid, itaconic acid, and citraconic acid, and their anhydrides (maleic anhydride, itaconic anhydride, etc.). Hydroxyl group-containing monomers: for example, hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, and 2-hydroxybutyl (meth)acrylate; unsaturated alcohols such as vinyl alcohol and allyl alcohol. Amide group-containing monomers: for example, (meth)acrylamide, N,N-dimethyl(meth)acrylamide, N-butyl(meth)acrylamide, N-methylol(meth)acrylamide, N-methylolpropane(meth)acrylamide, N-methoxymethyl(meth)acrylamide, N-butoxymethyl(meth)acrylamide. Amino group-containing monomers: for example, aminoethyl (meth)acrylate, N,N-dimethylaminoethyl (meth)acrylate, t-butylaminoethyl (meth)acrylate. Monomers having an epoxy group: for example, glycidyl (meth)acrylate, methyl glycidyl (meth)acrylate, allyl glycidyl ether. Cyano group-containing monomers: for example, acrylonitrile, methacrylonitrile. Keto group-containing monomers: for example, diacetone (meth)acrylamide, diacetone (meth)acrylate, acetoacetoxyethyl (meth)acrylate, acetoacetoxybutyl (meth)acrylate, vinyl methyl ketone, vinyl ethyl ketone, allyl acetoacetate, vinyl acetoacetate. Monomers having a nitrogen atom-containing ring: for example, N-vinyl-2-pyrrolidone, N-methylvinylpyrrolidone, N-vinylpyridine, N-vinylpiperidone, N-vinylpyrimidine, N-vinylpiperazine, N-vinylpyrazine, N-vinylpyrrole, N-vinylimidazole, N-vinyloxazole, N-vinylmorpholine, N-vinylcaprolactam, and N-(meth)acryloylmorpholine. Silane-based monomers: for example, alkoxysilyl group-containing monomers such as 3-(meth)acryloxypropyltrimethoxysilane, 3-(meth)acryloxypropyltriethoxysilane, 3-(meth)acryloxypropylmethyldimethoxysilane, and 3-(meth)acryloxypropylmethyldiethoxysilane.

[0039] The functional group-containing monomers can be used alone or in combination of two or more. Among the functional group-containing monomers, carboxy group-containing monomers, carbonyl group-containing monomers (typically keto group-containing monomers), hydroxy group-containing monomers, cyano group-containing monomers, and silane-based monomers are preferred because they can suitably introduce crosslinking points and improve cohesive strength as described above.

[0040] When a functional group-containing monomer is copolymerized with an acrylic polymer, the proportion of the functional group-containing monomer in the total monomer components constituting the acrylic polymer is not particularly limited. Generally, in order to achieve a good balance between cohesive strength and adhesiveness, the proportion of the functional group-containing monomer is preferably about 0.1% by weight or more, for example, about 0.5% by weight or more, or about 1% by weight or more. Furthermore, in consideration of the adhesive effect of alkyl (meth)acrylate, in some embodiments, the proportion of the functional group-containing monomer is preferably about 40% by weight or less, for example, about 30% by weight or less, about 20% by weight or less, 10% by weight or less, 5% by weight or less, or 3% by weight or less.

[0041] In some preferred embodiments, the monomer component constituting the acrylic polymer contains a carboxyl group-containing monomer. When the monomer component contains a carboxyl group-containing monomer, it becomes easier to obtain a pressure-sensitive adhesive sheet that exhibits good adhesive properties (cohesion, etc.). This can also be advantageous in improving the adhesion between the pressure-sensitive adhesive layer and the adherend. The carboxyl group-containing monomer can be used alone or in combination of two or more. As the carboxyl group-containing monomer, acrylic acid and methacrylic acid are preferably used.

[0042] When a carboxyl group-containing monomer is copolymerized with an acrylic polymer, the proportion of the carboxyl group-containing monomer in the total monomer components is not particularly limited. From the viewpoint of cohesive strength, etc., in some embodiments, the proportion of the carboxyl group-containing monomer is, for example, preferably about 0.1 wt% or more, more preferably about 0.5 wt% or more, even more preferably about 1 wt% or more, and particularly preferably about 1.5 wt% or more. Furthermore, from the viewpoint of adhesive properties such as adhesive strength, in some embodiments, the proportion of the carboxyl group-containing monomer is suitably about 15 wt% or less, and may be about 12 wt% or less, for example, 10 wt% or less. In some preferred embodiments, the proportion of the carboxyl group-containing monomer in the total monomer components is preferably 6 wt% or less or less than 6 wt%, more preferably 5 wt% or less or less than 5 wt%, even more preferably 4 wt% or less or less than 4 wt% (e.g., 3.5 wt% or less), and may be 3 wt% or less, or may be 2.5 wt% or less.

[0043] In an embodiment in which the monomer component contains a carboxyl group-containing monomer as a functional group-containing monomer, the proportion of the carboxyl group-containing monomer in the total functional group-containing monomers (total functional group-containing monomers including the carboxyl group-containing monomer) used as a copolymerization component of the acrylic polymer is, from the viewpoint of effectively exhibiting the effect of copolymerizing the carboxyl group-containing monomer, suitably 30% by weight or more, preferably 50% by weight or more, more preferably 70% by weight or more, even more preferably 80% by weight or more, particularly preferably 90% by weight or more, for example, 95% by weight or more, 97% by weight or more, 98% by weight or more, or 99% by weight or more (for example, 99.9% by weight or more). The upper limit of the proportion of the carboxyl group-containing monomer in the total functional group-containing monomer is 100% by weight, and may be, for example, 95% by weight or less.

[0044] In some embodiments, the carboxyl group-containing monomer preferably includes acrylic acid (AA). The use of acrylic acid facilitates the attainment of favorable adhesive properties (e.g., adhesive strength, cohesive strength), and also facilitates the attainment of excellent polymer dispersion stability. In some embodiments, the content of acrylic acid in the monomer components is, for example, preferably about 0.1 wt % or more, more preferably about 0.5 wt % or more, and even more preferably about 1 wt % or more. Furthermore, from the viewpoint of adhesive properties such as adhesive strength, in some embodiments, the proportion of acrylic acid is suitably 10 wt % or less, and may be, for example, 7 wt % or less. In some preferred embodiments, the content of acrylic acid in the monomer components is preferably 5 wt % or less or less than 5 wt %, more preferably 4 wt % or less or less than 4 wt % (e.g., 3.5 wt % or less), more preferably 3 wt % or less, and may be 2.5 wt % or less, 2 wt % or less (e.g., less than 2 wt %), 1.75 wt % or less, or 1.5 wt % or less.

[0045] In an embodiment using acrylic acid as the carboxyl group-containing monomer, the proportion of acrylic acid in all carboxyl group-containing monomers is suitably 30% by weight or more, preferably 50% by weight or more (e.g., more than 50% by weight), and may be 60% by weight or more. There is no particular upper limit to the proportion of acrylic acid in all carboxyl group-containing monomers, and in some embodiments, it may be 90% by weight or less, 80% by weight or less, or 70% by weight or less.

[0046] In some preferred embodiments, acrylic acid (AA) and methacrylic acid (MAA) are used in combination as the carboxyl group-containing monomer. The combined use of AA and MAA allows for a more uniform distribution of the structure derived from the carboxyl group-containing monomer in a PSA using a water-dispersible acrylic polymer. This can be advantageous from the viewpoint of more uniform crosslinking of a PSA using a water-dispersible acrylic polymer and suppressing a decrease in shear properties due to uneven crosslink density. Furthermore, a PSA containing an acrylic polymer with such a monomer composition (i.e., copolymer composition) can provide a PSA sheet with higher performance (e.g., better repulsion resistance). The weight ratio of AA to MAA (AA / MAA) can be, for example, in the range of approximately 0.1 to 10, more preferably approximately 0.3 or more, even more preferably 0.5 or more, and particularly preferably 1.0 or more (e.g., greater than 1.0 or 1.2 or more). Furthermore, the weight ratio (AA / MAA) is more preferably approximately 5 or less, and may be, for example, 4 or less, 3 or less, or 2 or less. By ensuring that the AA / MAA ratio is within the above range, it is easy to obtain the effect of more uniformly distributing the structure derived from the carboxyl group-containing monomer in the pressure-sensitive adhesive layer, and it is also easy to obtain the effect of improving repulsion resistance, and after the pressure-sensitive adhesive sheet is produced, it is easy to obtain excellent stability of the adhesive properties over time.

[0047] In some embodiments, it is preferable that a silane monomer is copolymerized with the acrylic polymer. An alkoxysilyl group-containing monomer is preferably used as the silane monomer. The alkoxysilyl group-containing monomer is typically an ethylenically unsaturated monomer having at least one (preferably two or more, e.g., two or three) alkoxysilyl group in one molecule, specific examples of which are as described above. The silane monomers can be used singly or in combination of two or more. Copolymerizing the silane monomer allows a crosslinked structure to be introduced into the adhesive containing the acrylic polymer through a condensation reaction of silanol groups (silanol condensation). Generally, in adhesives containing a water-dispersible acrylic polymer, crosslinks due to a water-soluble crosslinking agent tend to be formed between emulsion particles, while crosslinks due to the silanol condensation are also likely to be formed inside the emulsion particles. Therefore, in a PSA containing a water-dispersible acrylic polymer and a water-soluble crosslinking agent, by using a water-dispersible acrylic polymer copolymerized with a silane monomer as the water-dispersible acrylic polymer, the PSA can be crosslinked more uniformly, and deterioration of shear properties due to uneven crosslink density can be suppressed. The silane monomer is also called a silane coupling agent.

[0048] When a silane monomer is copolymerized with an acrylic polymer, the proportion of the silane monomer in the monomer components is suitably about 0.001 wt% or more, preferably 0.005 wt% or more, and more preferably 0.01 wt% or more. In some embodiments, the proportion of the silane monomer in the monomer components is suitably about 0.1 wt% or less, preferably 0.05 wt% or less, and may be, for example, 0.03 wt% or less. By using an appropriate amount of silane monomer, a pressure-sensitive adhesive with good shear properties tends to be obtained.

[0049] In some embodiments, it is preferable that a keto group-containing monomer is copolymerized with the acrylic polymer. Specific examples of the keto group-containing monomer are as described above. The keto group-containing monomer can be used alone or in combination of two or more. Keto group-containing monomers tend to be relatively hydrophobic among monomers that can introduce reactive sites with crosslinkers into water-dispersible acrylic polymers. Therefore, by crosslinking a water-dispersible acrylic polymer into which a keto group has been introduced with a water-soluble crosslinking agent that reacts with the keto group, a balanced crosslinked structure can be formed within and between emulsion particles, and a decrease in shear properties due to uneven crosslink density can be suppressed.

[0050] When a keto group-containing monomer is copolymerized with an acrylic polymer, the proportion of the keto group-containing monomer in the monomer components is suitably about 0.01 wt% or more, preferably 0.05 wt% or more, or may be 0.10 wt% or more, or may be 0.12 wt% or more. In some embodiments, the proportion of the keto group-containing monomer in the monomer components is suitably about 10 wt% or less, preferably 5.0 wt% or less, or may be 3.0 wt% or less, 1.0 wt% or less, or may be 0.50 wt% or less, 0.30 wt% or less, or 0.20 wt% or less. Using an appropriate amount of the keto group-containing monomer tends to make it easier to obtain a pressure-sensitive adhesive with good shear properties.

[0051] Furthermore, for the purpose of increasing the cohesive strength of the acrylic polymer, copolymerization components other than the above-mentioned minor monomers can be used. Examples of such copolymerization components include vinyl ester monomers such as vinyl acetate and vinyl propionate; aromatic vinyl compounds such as styrene, substituted styrenes (α-methylstyrene, etc.) and vinyltoluene; cycloalkyl(meth)acrylates such as cyclohexyl(meth)acrylate, cyclopentyl(meth)acrylate and isobornyl(meth)acrylate; aryl(meth)acrylates (e.g., phenyl(meth)acrylate), aryloxyalkyl(meth)acrylates (e.g., phenoxyethyl(meth)acrylate), arylalkyl(meth)acrylates, and the like. Examples of the copolymerizable monomer include aromatic ring-containing (meth)acrylates such as benzyl (meth)acrylate; olefin-based monomers such as ethylene, propylene, isoprene, butadiene, and isobutylene; chlorine-containing monomers such as vinyl chloride and vinylidene chloride; isocyanate group-containing monomers such as 2-(meth)acryloyloxyethyl isocyanate; alkoxy group-containing monomers such as methoxyethyl (meth)acrylate and ethoxyethyl (meth)acrylate; and vinyl ether-based monomers such as methyl vinyl ether and ethyl vinyl ether. Examples of other copolymerizable monomers other than those mentioned above include monomers having multiple functional groups in one molecule (polyfunctional monomers).

[0052] The amount of copolymerization components other than the above-mentioned secondary monomers is not particularly limited and may be appropriately selected depending on the purpose and application. For example, it is preferably 10% by weight or less of the monomer composition of the acrylic polymer, may be 3% by weight or less, or may be less than 1% by weight (for example, 0% by weight or more and less than 1% by weight).

[0053] From the viewpoint of adhesion to an adherend, it is suitable, although not particularly limited, that the acrylic polymer be designed so that the glass transition temperature (Tg) of the polymer is −25° C. or lower. In some embodiments, the Tg of the acrylic polymer is preferably −40° C. or lower, more preferably −50° C. or lower, and may be −55° C. or lower. In some embodiments, from the viewpoint of cohesive strength, the Tg of the acrylic polymer is, for example, about −75° C. or higher, and may be about −70° C. or higher. The Tg of the acrylic polymer can be adjusted by the types and amount ratios of monomers used in synthesizing the polymer.

[0054] The Tg of an acrylic polymer can be determined based on the composition of the monomer components used in the synthesis of the polymer using the Fox equation, which is a relational expression, as shown below, between the Tg of a copolymer and the glass transition temperature Tgi of a homopolymer obtained by homopolymerizing each of the monomers constituting the copolymer. 1 / Tg=Σ(Wi / Tgi) In the above Fox formula, Tg represents the glass transition temperature (unit: K) of the copolymer, Wi represents the weight fraction of monomer i in the copolymer (copolymerization ratio by weight), and Tgi represents the glass transition temperature (unit: K) of the homopolymer of monomer i.

[0055] The glass transition temperature of the homopolymer used to calculate Tg is determined based on the value described in the publicly available literature. For example, for the following monomers, the following values ​​are used as the glass transition temperatures of the homopolymers of the monomers: 2-Ethylhexyl acrylate -70℃ n-Butyl acrylate -55℃ Methyl methacrylate 105℃ Methyl acrylate 8℃ Vinyl acetate 32℃ Acrylic acid 106℃ Methacrylic acid 228℃

[0056] For the glass transition temperatures of homopolymers of monomers other than those exemplified above, the values ​​described in "Polymer Handbook" (3rd Edition, John Wiley & Sons, Inc., 1989) shall be used. If multiple values ​​are described in this document, the highest value shall be adopted. If the value is not described in the Polymer Handbook, the value obtained by the measurement method described in JP 2007-51271 A shall be used.

[0057] The acrylic polymer disclosed herein is synthesized by emulsion polymerization. The emulsion polymerization method is not particularly limited, and can be carried out by appropriately adopting various monomer supply methods, polymerization conditions, materials used, and the like similar to those of conventionally known general emulsion polymerizations. For example, the monomer supply method can be a batch feed method in which all monomer raw materials are supplied at once, a continuous feed (dropping) method, a divided feed (dropping) method, or the like. The monomer raw materials may be added dropwise in the form of an aqueous emulsion. The polymerization temperature can be, for example, about 20°C or higher (usually 40°C or higher), and is suitably about 100°C or lower (usually 80°C or lower).

[0058] The initiator used in the polymerization can be appropriately selected from conventionally known polymerization initiators depending on the type of polymerization method. For example, azo initiators, persulfate initiators, peroxide initiators, substituted ethane initiators, carbonyl initiators, redox initiators formed by combining peroxides with reducing agents, etc. can be used. The polymerization initiators can be used alone or in combination of two or more. The amount of the polymerization initiator used is not particularly limited, as long as it is a normal amount. For example, it can be selected from the range of about 0.005 parts by weight or more (preferably 0.01 parts by weight or more) and about 1 part by weight or less (preferably 0.8 parts by weight or less) per 100 parts by weight of the monomer components.

[0059] Emulsion polymerization of the monomer raw material is usually carried out in the presence of a surfactant (emulsifier). Known anionic surfactants, nonionic surfactants, cationic surfactants, etc. can be used as the emulsifier. Usually, anionic or nonionic surfactants are preferred. A surfactant (emulsifier) ​​having a reactive functional group (typically a radically polymerizable functional group) may also be used. Hereinafter, a surfactant having a reactive functional group will be referred to as a reactive surfactant or reactive emulsifier, while a general surfactant without a reactive functional group will be referred to as a non-reactive surfactant or non-reactive emulsifier. One surfactant can be used alone, or two or more surfactants can be used in combination. A reactive emulsifier and a non-reactive emulsifier can also be used in combination.

[0060] Examples of non-reactive anionic surfactants include alkyl sulfates such as lauryl sulfate and octadecyl sulfate; fatty acid salts; alkylbenzenesulfonates such as nonylbenzenesulfonate and dodecylbenzenesulfonate; naphthalenesulfonates such as dodecylnaphthalenesulfonate; alkyldiphenyletherdisulfonates such as dodecyldiphenyletherdisulfonates; polyoxyethylene alkylether sulfates such as polyoxyethyleneoctadecylethersulfonate and polyoxyethylenelaurylethersulfonate; polyoxyethylene alkylphenylether sulfates such as polyoxyethylenelaurylphenylethersulfonate; polyoxyethylene styrenated phenylether sulfate; sulfosuccinates such as laurylsulfosuccinate and polyoxyethylenelaurylsulfosuccinate; polyoxyethylene alkylether phosphates; polyoxyethylene alkylether acetates; etc. When anionic surfactants form salts, these salts can be, for example, metal salts (preferably monovalent metal salts) such as sodium salts, potassium salts, calcium salts, magnesium salts, etc., ammonium salts, amine salts, etc.

[0061] Examples of non-reactive nonionic surfactants include polyoxyethylene alkyl ethers such as polyoxyethylene lauryl ether and polyoxyethylene stearyl ether; polyoxyethylene alkyl phenyl ethers such as polyoxyethylene octylphenyl ether and polyoxyethylene nonylphenyl ether; sorbitan fatty acid esters such as sorbitan monolaurate, sorbitan monostearate, and polyoxyethylene sorbitan monolaurate; polyoxyethylene glyceryl ether fatty acid esters; polyoxyethylene-polyoxypropylene block copolymers; and the like.

[0062] Examples of reactive emulsifiers that can be used include those having a structure in which a radically polymerizable functional group is introduced into an anionic surfactant or a nonionic surfactant. These reactive emulsifiers can be used alone or in combination of two or more. The type of radically polymerizable functional group possessed by the reactive emulsifier is not particularly limited, and may be, for example, an alkenyl group, an acryloyl group, a methacryloyl group, a vinyl group, a vinyl ether group (vinyloxy group), an allyl ether group (allyloxy group), or the like. Specific examples of alkenyl groups include a propenyl group and an isopropenyl group (CH═C(CH)—). The concept of a propenyl group as used herein includes a 1-propenyl group (CH—CH═CH—) and a 2-propenyl group (CH═CH—CH—; sometimes referred to as an allyl group).

[0063] Examples of anionic reactive surfactants include polyoxyethylene (allyloxymethyl) alkyl ether sulfates (e.g., ammonium salts), polyoxyethylene nonylpropenyl phenyl ether sulfates (e.g., ammonium salts), alkyl allyl sulfosuccinates (e.g., sodium salts), methacryloxypolyoxypropylene sulfates (e.g., sodium salts), polyoxyalkylene alkenyl ether sulfates (e.g., ammonium salts in which the alkenyl group ends at an isopropenyl group), etc. When anionic reactive surfactants form salts, the salts may be, for example, metal salts such as sodium salts, or non-metal salts such as ammonium salts or amine salts. Examples of nonionic reactive surfactants include polyoxyethylene nonylpropenyl phenyl ether.

[0064] Commercially available reactive surfactants include those manufactured by Daiichi Kogyo Seiyaku Co., Ltd. under the trade names "Aqualon HS-05," "Aqualon HS-10," "Aqualon HS-1025," "Aqualon HS-20," "Aqualon KH-10," "Aqualon KH-1025," "Aqualon KH-05," "Aqualon BC-0515," "Aqualon BC-10," "Aqualon BC-1025," "Aqualon BC-20," "Aqualon BC-2020," "Aqualon RN-20," "Aqualon RN-30," "Aqualon RN-50," and Examples of such products include Aqualon AR-10, Aqualon AR-20, Aqualon AR-1025, and Aqualon AR-2020, ADEKA Corporation products under the trade names Adeka Rear Soap SE-10N and Adeka Rear Soap SR-1025, Kao Corporation products under the trade names Latemul PD-104, Latemul PD-420, Latemul PD-430, and Latemul PD-450, Sanyo Chemical Industries, Ltd. products under the trade names Eleminol JS-20 and Eleminol RS-3000, and Nippon Nyukazai Co., Ltd. product under the trade name Antox MS-60.

[0065] In embodiments in which a surfactant is used, the surfactant preferably contains a reactive emulsifier. In other words, at least a portion of the surfactant used is preferably a reactive emulsifier. By emulsion polymerizing the monomer raw material in the presence of the reactive emulsifier, the reactive emulsifier can react and be incorporated into the acrylic polymer. By incorporating the reactive emulsifier into the acrylic polymer, the amount of free emulsifier is reduced. This can be advantageous from the standpoint of improving the shear properties and water resistance of the PSA sheet.

[0066] In this specification, "containing a reactive emulsifier" is used to mean containing the reactive emulsifier in a state after its radically polymerizable functional group has reacted. By using a reactive emulsifier as at least a part of the surfactant, the reactive emulsifier is typically contained in the water-dispersible PSA composition or PSA layer in a form in which at least a part of the reactive emulsifier is incorporated into the acrylic polymer as described above.

[0067] The amount of surfactant (emulsifier) ​​used in emulsion polymerization is not particularly limited. In consideration of polymerization stability and dispersion stability of the polymerization reaction product, in some embodiments, the amount of surfactant used is typically 0.1 parts by weight or more per 100 parts by weight of the monomer raw material, preferably 0.5 parts by weight or more, and may be 1.0 parts by weight or more, or may be 1.5 parts by weight or more. Furthermore, the amount of surfactant used may be, for example, 10 parts by weight or less per 100 parts by weight of the monomer raw material. From the viewpoint of adhesive properties, it is desirable to limit the amount of surfactant used (especially non-reactive surfactants). From this viewpoint, the amount of surfactant used is typically preferably 5 parts by weight or less, and may be 4 parts by weight or less, 3 parts by weight or less, or 2.5 parts by weight or less.

[0068] In embodiments in which a reactive emulsifier is used in emulsion polymerization, the amount of the reactive emulsifier is not particularly limited. In consideration of polymerization stability and dispersion stability of the polymerization reaction product, in some embodiments, the amount of reactive emulsifier used is typically 0.1 parts by weight or more per 100 parts by weight of the starting monomer material, preferably 0.5 parts by weight or more, and may be 1.0 parts by weight or more, or even 1.5 parts by weight or more. Furthermore, the amount of reactive emulsifier used can be, for example, 10 parts by weight or less per 100 parts by weight of the starting monomer material. From the viewpoint of limiting the amount of free emulsifier, in some embodiments, the amount of reactive emulsifier used is typically preferably 5 parts by weight or less, more preferably 4 parts by weight or less, even more preferably 3 parts by weight or less, and particularly preferably 2.5 parts by weight or less.

[0069] During the polymerization, a chain transfer agent (which may also be understood as a molecular weight regulator or polymerization degree regulator) may be used as needed. Examples of chain transfer agents include mercaptans such as dodecyl mercaptan (dodecanethiol), lauryl mercaptan, glycidyl mercaptan, 2-mercaptoethanol, mercaptoacetic acid, 2-ethylhexyl thioglycolate, and 2,3-dimercapto-1-propanol, as well as α-methylstyrene dimer. These chain transfer agents may be used alone or in combination of two or more. When a chain transfer agent is used, the amount used may be approximately 0.001 parts by weight or more, for example, approximately 0.005 parts by weight or more, or approximately 0.01 parts by weight or more, or approximately 5 parts by weight or less, for example, approximately 1 part by weight or less, or approximately 0.1 part by weight or less, per 100 parts by weight of the monomer components. By adjusting the amount of the chain transfer agent used within an appropriate range, a desired polymerization rate can be obtained.

[0070] According to the emulsion polymerization, it is possible to prepare a polymerization solution (acrylic polymer emulsion) in the form of an emulsion in which an acrylic polymer is dispersed in water. Usually, from the viewpoint of dispersion stability, etc., a pH adjuster such as aqueous ammonia is added to the polymerization solution to adjust the pH to an appropriate range (for example, a range of about pH 6 to 9).

[0071] (Crosslinking agent) The pressure-sensitive adhesive layer of the pressure-sensitive adhesive sheet disclosed herein contains a water-soluble crosslinking agent. Use of a water-soluble crosslinking agent allows for more uniform crosslinking of the pressure-sensitive adhesive layer containing a water-dispersible acrylic polymer than when using a water-insoluble crosslinking agent such as an oil-soluble crosslinking agent or a water-dispersible crosslinking agent. Increasing the uniformity of the crosslinking in the pressure-sensitive adhesive layer can be advantageous from the viewpoint of improving shear properties by preventing the occurrence of areas where shear stress is concentrated due to variations in the density of crosslinks in the pressure-sensitive adhesive when shear stress is applied to the bonded portion of the pressure-sensitive adhesive sheet. The water-soluble crosslinking agent may be one provided by a manufacturer or one that maintains a uniform appearance (no phase separation or turbidity) even after the flow subsides when the crosslinking agent and ion-exchanged water are stirred and mixed at room temperature (e.g., 25°C) in a 1:1 weight ratio. In addition, unless otherwise specified, the term "crosslinking agent" used herein refers to a crosslinking agent (also referred to as an external crosslinking agent) that is added after the synthesis of the acrylic polymer (post-addition). The pressure-sensitive adhesive layer may contain the crosslinking agent in a form after crosslinking reaction, a form before crosslinking reaction, a partially crosslinked form, an intermediate or composite form thereof, etc. The crosslinking agent is typically contained in the pressure-sensitive adhesive layer exclusively in a form after crosslinking reaction.

[0072] The type of water-soluble crosslinking agent is not particularly limited, and can be appropriately selected from various known crosslinking agents such as carbodiimide-based crosslinking agents, epoxy-based crosslinking agents, oxazoline-based crosslinking agents, hydrazide-based crosslinking agents, aziridine-based crosslinking agents, isocyanate-based crosslinking agents, melamine-based crosslinking agents, peroxide-based crosslinking agents, urea-based crosslinking agents, metal alkoxide-based crosslinking agents, metal chelate-based crosslinking agents, metal salt-based crosslinking agents, amine-based crosslinking agents, etc. The water-soluble crosslinking agents can be used alone or in combination of two or more.

[0073] Known carbodiimide crosslinking agents include low molecular weight compounds or high molecular weight compounds having two or more carbodiimide groups. Commercially available water-soluble carbodiimide crosslinking agents include the Carbodilite V series (aqueous solution type) manufactured by Nisshinbo Chemical Inc., trade names of which include "Carbodilite V-02," "Carbodilite V-02-L2," and "Carbodilite V-04."

[0074] An epoxy-based crosslinking agent is a compound having two or more epoxy groups in one molecule, typically a compound having 3 to 5 epoxy groups in one molecule. Specific examples of epoxy-based crosslinking agents include (poly)ethylene glycol diglycidyl ether, glycerol polyglycidyl ether, polyglycerol polyglycidyl ether, etc. Commercially available epoxy-based crosslinking agents include "Epicron CR-5L" manufactured by DIC Corporation, and "Denacol EX-313," "Denacol EX-512," "Denacol EX-810," "Denacol EX-821," "Denacol EX-830," and "Denacol EX-850" manufactured by Nagase ChemteX Corporation. Of these known epoxy-based crosslinking agents, those that fall under the category of water-soluble crosslinking agents can be used.

[0075] Known oxazoline crosslinking agents include compounds having one or more oxazoline groups in one molecule. The oxazoline group may be any of a 2-oxazoline group, a 3-oxazoline group, and a 4-oxazoline group. Commercially available water-soluble oxazoline crosslinking agents include, for example, products manufactured by Nippon Shokubai Co., Ltd., such as "Epocross WS-500" and "Epocross WS-700."

[0076] Known hydrazide crosslinking agents include hydrazino group-containing compounds having two or more hydrazino groups (HN-NH-) as crosslinkable functional groups. Water-soluble hydrazide crosslinking agents include polycarboxylic acid polyhydrazides such as oxalic acid dihydrazide, malonic acid dihydrazide, glutaric acid dihydrazide, succinic acid dihydrazide, and adipic acid dihydrazide, and hydantoins such as 1,3-bis(hydrazinocarbonoethyl)-5-isopropylhydantoin.

[0077] Examples of known aziridine crosslinking agents include trimethylolpropane tris[3-(1-aziridinyl)propionate] and trimethylolpropane tris[3-(1-(2-methyl)aziridinylpropionate)]. Commercially available aziridine crosslinking agents include, for example, "ChemiTite PZ-33" manufactured by Nippon Shokubai Co., Ltd. Among these known aziridine crosslinking agents, those that fall under the category of water-soluble crosslinking agents can be used.

[0078] Examples of known isocyanate crosslinking agents include bifunctional or higher polyfunctional isocyanate compounds. So-called blocked isocyanate-type isocyanate crosslinking agents, in which the isocyanate group is blocked, may also be used. Commercially available products include the "Burnoc DNW" series manufactured by DIC Corporation, the "Aquanate" series manufactured by Tosoh Corporation, the "Takenate WD" series manufactured by Mitsui Chemicals, Inc., and the "Elastron BN" series manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd. Among these known isocyanate crosslinking agents, those that fall under the category of water-soluble crosslinking agents may be used.

[0079] In some preferred embodiments, the water-soluble crosslinking agent is selected from the group consisting of carbodiimide-based crosslinking agents, epoxy-based crosslinking agents, oxazoline-based crosslinking agents, hydrazide-based crosslinking agents, and aziridine-based crosslinking agents. These are multifunctional crosslinking agents having two or more functional groups that react with carboxyl groups or carbonyl groups. For example, in embodiments including an acrylic polymer having carboxyl groups, the crosslinking agent can form a crosslinked structure with good reactivity with the carboxyl groups of the acrylic polymer. Among the water-soluble crosslinking agents, carbodiimide-based crosslinking agents, epoxy-based crosslinking agents, oxazoline-based crosslinking agents, and hydrazide-based crosslinking agents are more preferred, with carbodiimide-based crosslinking agents, epoxy-based crosslinking agents, and oxazoline-based crosslinking agents being even more preferred. These water-soluble crosslinking agents tend to favorably achieve both shear properties and other adhesive properties (e.g., adhesive strength).

[0080] The content of the water-soluble crosslinking agent is not particularly limited. The content of the water-soluble crosslinking agent can be, for example, within a range of approximately 0.001 to approximately 20 parts by weight per 100 parts by weight of the acrylic polymer. From the viewpoint of improving cohesive strength, in some embodiments, the content of the water-soluble crosslinking agent per 100 parts by weight of the acrylic polymer is suitably greater than approximately 0.01 parts by weight. From the viewpoint of easily obtaining better effects, it is advantageous to have the content be greater than approximately 0.05 parts by weight, preferably greater than approximately 0.1 parts by weight or more, and more preferably greater than approximately 0.3 parts by weight. By appropriately increasing the amount of the water-soluble crosslinking agent used, good shear holding strength tends to be easily obtained. In some embodiments, the content of the water-soluble crosslinking agent may be greater than approximately 0.5 parts by weight or greater than approximately 0.5 parts by weight, or may be greater than approximately 0.8 parts by weight, per 100 parts by weight of the acrylic polymer. In some embodiments, the content of the water-soluble crosslinking agent may be approximately 1 part by weight or more, approximately 2 parts by weight or more, approximately 3 parts by weight or more, or approximately 4 parts by weight or more. Furthermore, in some embodiments, the content of the water-soluble crosslinking agent may be approximately 15 parts by weight or less, approximately 10 parts by weight or less, approximately 8 parts by weight or less, or approximately 6 parts by weight or less, per 100 parts by weight of the acrylic polymer. By appropriately limiting the amount of crosslinking agent used, it is possible to achieve a favorable balance between shear properties and other properties (e.g., adhesive strength to SUS, as described below). In some preferred embodiments, the content of the crosslinking agent per 100 parts by weight of the acrylic polymer may be approximately 5 parts by weight or less, approximately 4.5 parts by weight or less, approximately 3.5 parts by weight or less, approximately 2.5 parts by weight or less, or approximately 1.5 parts by weight or less. In some other embodiments, the content of the crosslinking agent relative to 100 parts by weight of the acrylic polymer may be approximately 1 part by weight or less (for example, less than 1 part by weight), or may be less than 0.1 parts by weight.

[0081] In some embodiments, the pressure-sensitive adhesive layer may contain an acrylic polymer copolymerized with a silane monomer (silane coupling agent) and the above-mentioned crosslinking agent (external crosslinking agent). This allows for the formation of a favorable crosslinking structure within and between acrylic polymer particles, enabling the pressure-sensitive adhesive layer to be more uniformly crosslinked. This suppresses stress concentration due to the density of crosslinks, making it easier to obtain favorable shear properties. In such embodiments, the ratio (weight ratio (C / S)) of the amount of crosslinking agent C to the amount of silane monomer S is not particularly limited. For example, the weight ratio (C / S) may be 1 or more, 10 or more, 30 or more, 50 or more, 80 or more, 100 or more, 120 or more, 150 or more, 180 or more, 200 or more, or 220 or more. The greater the weight ratio (C / S), the more effectively the crosslinked structure based on the external crosslinking agent can be exerted. In some embodiments, the weight ratio (C / S) may be 1000 or less, 500 or less, 300 or less, 200 or less, 170 or less, 150 or less, 130 or less, 100 or less, or 70 or less. The smaller the weight ratio (C / S), the more favorably the effect of the crosslinked structure based on the silane-based monomer can be exerted.

[0082] (tackifying resin) The PSA layer disclosed herein contains a tackifying resin, which can increase the adhesive strength of the PSA sheet and improve its shear properties. Examples of tackifying resins include rosin-based tackifying resins (including rosin derivative tackifying resins), petroleum-based tackifying resins, terpene-based tackifying resins, and ketone-based tackifying resins. These can be used alone or in combination of two or more.

[0083] Examples of the rosin-based tackifying resin include rosins such as gum rosin, wood rosin, and tall oil rosin, as well as stabilized rosins (e.g., stabilized rosins obtained by disproportionating or hydrogenating the above-mentioned rosins), polymerized rosins (e.g., polymers, typically dimers, of the above-mentioned rosins), and modified rosins (e.g., unsaturated acid-modified rosins modified with unsaturated acids such as maleic acid, fumaric acid, and (meth)acrylic acid). Examples of the rosin derivative tackifying resin include esterified products of the rosin-based resins (e.g., rosin esters such as stabilized rosin ester and polymerized rosin ester), phenol-modified products of the rosin-based resins (phenol-modified rosin), and esterified products thereof (phenol-modified rosin ester). Examples of the petroleum-based tackifying resin include aliphatic petroleum resins, aromatic petroleum resins, copolymer petroleum resins, alicyclic petroleum resins, and hydrogenated versions of these. Examples of the terpene-based tackifying resin include α-pinene resin, β-pinene resin, aromatic modified terpene-based resin, and terpene phenol-based resin. Examples of the ketone-based tackifying resin include ketone-based resins obtained by condensation of ketones (e.g., aliphatic ketones such as methyl ethyl ketone, methyl isobutyl ketone, and acetophenone; alicyclic ketones such as cyclohexanone and methylcyclohexanone) with formaldehyde.

[0084] Examples of tackifying resins that can be preferably used include rosin-based tackifying resins and terpene-based tackifying resins. Suitable examples of rosin-based tackifying resins include stabilized rosin esters and polymerized rosin esters. Suitable examples of terpene-based tackifying resins include terpene phenolic resins. Rosin-based tackifying resins and terpene-based tackifying resins may be used in combination.

[0085] The softening point of the tackifier resin used is not particularly limited. From the viewpoint of improving cohesive strength, etc., the softening point of the tackifier resin may be, for example, 80°C or higher, or may be 90°C or higher. In some embodiments, the softening point of the tackifier resin is preferably 100°C or higher, and from the viewpoint of improving shear properties, it is more preferably 120°C or higher, and even more preferably 130°C or higher. In some preferred embodiments, the tackifier resin may contain a high-softening-point tackifier resin having a softening point of 140°C or higher. The softening point of the high-softening-point tackifier resin may be 145°C or higher, for example, 150°C or higher, or even 155°C or higher. The use of the high-softening-point tackifier resin can favorably achieve both adhesiveness and cohesiveness. There is no particular upper limit for the softening point of the tackifier resin, but from the viewpoints of compatibility, low-temperature properties, etc., it is usually appropriate that the softening point be 200°C or lower, preferably 180°C or lower, more preferably 170°C or lower, and even more preferably 160°C or lower.

[0086] The softening point of the tackifier resin referred to here is defined as a value measured based on the softening point test method (ring and ball method) specified in JIS K5902 and JIS K2207.

[0087] Some preferred embodiments include those in which the tackifier resin comprises one or more terpene-based tackifier resins (typically terpene phenol-based resins). The technology disclosed herein can be preferably implemented, for example, in an embodiment in which, assuming the total amount of tackifier resins to be 100% by weight, approximately 25% by weight or more (more preferably approximately 30% by weight or more) of the tackifier resins is terpene phenol-based resin. Approximately 50% by weight or more of the total amount of tackifier resins may be terpene phenol-based resins, or approximately 80% by weight or more (e.g., approximately 90% by weight or more) may be terpene phenol-based resins. Substantially all of the tackifier resins (e.g., approximately 95 to 100% by weight, or even approximately 99 to 100% by weight) may be terpene phenol-based resins.

[0088] In some other embodiments, the tackifier resin comprises a rosin-based tackifier resin. For example, assuming the total amount of tackifier resins to be 100% by weight, approximately 25% by weight or more (more preferably approximately 30% by weight or more) of the total amount of tackifier resins may be rosin-based tackifier resins, approximately 50% by weight or more of the total amount of tackifier resins may be rosin-based tackifier resins, or approximately 80% by weight or more (e.g., approximately 90% by weight or more) of the total amount of tackifier resins may be rosin-based tackifier resins. Substantially all of the tackifier resins (e.g., approximately 95 to 100% by weight, or even approximately 99 to 100% by weight) may be rosin-based tackifier resins.

[0089] In some embodiments, it is preferable to use a water-dispersed tackifier resin (also referred to as a tackifier resin emulsion) as the tackifier resin. In such embodiments, the PSA composition used to form the PSA layer contains the tackifier resin in the form of an emulsion in which the tackifier resin is dispersed in water. For example, by mixing an aqueous emulsion of an acrylic polymer with an emulsion of the tackifier resin, a PSA composition containing these components in a desired ratio can be easily prepared. It is preferable to use a tackifier resin emulsion that is at least substantially free of aromatic hydrocarbon solvents (more preferably, substantially free of aromatic hydrocarbon solvents and other organic solvents).

[0090] Such a tackifier resin emulsion may be prepared using a surfactant (emulsifier) ​​as needed. Surfactants that can be used in preparing the tackifier resin emulsion can be selected from one or more surfactants similar to those that can be used in preparing acrylic polymer emulsions. Anionic surfactants or nonionic surfactants are usually preferred. The amount of surfactant used is not particularly limited as long as it is an amount that allows the tackifier resin to be prepared in the form of an emulsion. For example, the amount can be about 0.2 parts by weight or more (preferably 0.5 parts by weight or more) and about 10 parts by weight or less (preferably 5 parts by weight or less) per 100 parts by weight (solids basis) of the tackifier resin.

[0091] The content of the tackifier resin in the pressure-sensitive adhesive layer is not particularly limited. In some embodiments, the content of the tackifier resin (based on solid content) is typically about 1 part by weight or more per 100 parts by weight of the acrylic polymer, from the viewpoint of optimally exhibiting its effects, and may be about 5 parts by weight or more, or may be about 10 parts by weight or more. In some preferred embodiments, the content of the tackifier resin per 100 parts by weight of the acrylic polymer is about 15 parts by weight or more, more preferably about 20 parts by weight or more, even more preferably about 25 parts by weight or more, particularly preferably about 30 parts by weight or more, or may be about 35 parts by weight or more. In some embodiments, the content of the tackifier resin per 100 parts by weight of the acrylic polymer may be, for example, about 80 parts by weight or less, suitably 60 parts by weight or less, preferably about 50 parts by weight or less, more preferably 45 parts by weight or less, even more preferably 40 parts by weight or less, or may be 30 parts by weight or less, 20 parts by weight or less, or 15 parts by weight or less. By setting the amount of the tackifier resin within an appropriate range, better shear properties tend to be obtained. In some other embodiments, the content of the tackifier resin relative to 100 parts by weight of the acrylic polymer may be approximately 10 parts by weight or less (for example, less than 10 parts by weight), or may be less than 5 parts by weight.

[0092] (pigment) The pressure-sensitive adhesive layer disclosed herein can contain a pigment as needed. By incorporating an appropriate amount of pigment, the pressure-sensitive adhesive layer can be endowed with properties and functions such as light-blocking properties, concealment of the adherend, adjustment of the appearance of the adherend through the pressure-sensitive adhesive sheet, and design properties. Examples of pigments that can be used include inorganic pigments such as carbon black, graphite, activated carbon, ferrite (non-magnetic ferrite, magnetic ferrite, etc.), magnetite, molybdenum disulfide, chromium complex, copper oxide, manganese dioxide, zinc carbonate, zinc oxide, zinc sulfide, talc, kaolin, calcium carbonate, titanium oxide, silica, lithium fluoride, calcium fluoride, barium sulfate, alumina, zirconia, iron oxide, iron hydroxide, chromium oxide, fired spinel, chromate, chrome vermilion, Prussian blue, aluminum powder, bronze powder, silver powder, and calcium phosphate; and organic pigments such as phthalocyanine, azo, condensed azo, azo lake, anthraquinone, perylene-perinone, indigo, thioindigo, isoindolinone, azomethine, dioxazine, quinacridone, aniline black, and triphenylmethane. As the pigment, black pigments such as aniline black, perylene black, titanium black, cyanine black, etc. may be used. Among them, pigments containing inorganic particles (typically inorganic pigments) are preferably used. Note that inorganic pigments include all pigments that are mainly composed of inorganic materials, and are not limited to those composed only of inorganic materials. The pigments may be used alone or in combination of two or more.

[0093] From the viewpoint of light blocking properties and reduced light transmittance, in some embodiments, black pigments such as carbon black, graphite, copper oxide, manganese dioxide, aniline black, perylene black, titanium black, cyanine black, activated carbon, ferrite (non-magnetic ferrite, magnetic ferrite, etc.), magnetite, chromium oxide, iron oxide, molybdenum disulfide, chromium complexes, etc. are preferably used. The black pigments may be used alone or in combination of two or more.

[0094] In some preferred embodiments, carbon black is used as a pigment. The use of carbon black can efficiently reduce light transmittance, resulting in a highly light-blocking adhesive. Carbon blacks commonly referred to as carbon blacks (furnace black, channel black, acetylene black, thermal black, lamp black, pine soot, etc.) can be used without particular limitation. From the viewpoint of maintaining the quality of appearance, it is preferable to use surface-modified carbon blacks having functional groups such as carboxyl groups, amino groups, sulfonic acid groups, and silicon-containing groups (e.g., alkoxysilyl groups, alkylsilyl groups). Such surface-modified carbon blacks are also called self-dispersing carbon blacks, and they eliminate the need for or reduce the amount of dispersant added. The above carbon blacks can be used alone or in combination of two or more. Self-dispersing carbon blacks can be produced based on the methods described in, for example, JP 2017-171732 A and JP 2018-30968 A and the common general technical knowledge of those skilled in the art. Commercially available products can also be used.

[0095] In some embodiments, the pigment may be selected from the group consisting of metal oxides such as titanium oxide (rutile titanium dioxide, anatase titanium dioxide, and the like), zinc oxide, cerium oxide, aluminum oxide, silicon oxide, zirconium oxide, magnesium oxide, calcium oxide, tin oxide, barium oxide, cesium oxide, and yttrium oxide; carbonate compounds such as magnesium carbonate, calcium carbonate (light calcium carbonate, heavy calcium carbonate, and the like), barium carbonate, and zinc carbonate; and hydroxides such as aluminum hydroxide, calcium hydroxide, magnesium hydroxide, and zinc hydroxide. Examples of suitable pigments include inorganic materials such as silicates; silicate compounds such as aluminum silicate, magnesium silicate, and calcium silicate; barium sulfate, calcium sulfate, barium stearate, zinc oxide, zinc sulfide, talc, clay, kaolin, titanium phosphate, mica, gypsum, white carbon, diatomaceous earth, bentonite, lithopone, zeolite, sericite, and hydrated halloysite; and organic materials such as acrylic resins, polystyrene resins, polyurethane resins, amide resins, polycarbonate resins, silicone resins, urea-formalin resins, and melamine resins. These pigments can be used as white or other non-black pigments. The non-black pigments can be used alone or in combination of two or more.

[0096] In some embodiments, one or more black pigments may be used in combination with one or more non-black pigments. In such embodiments, the ratio (weight ratio (A / B)) of the amount of black pigment A to the amount of non-black pigment B is not particularly limited. For example, the weight ratio (A / B) may be 0.001 or more, 0.01 or more, or 0.1 or more, and the weight ratio (A / B) may be 100 or less, 10 or less, or 1 or less.

[0097] The particle size of the pigment is not particularly limited. In some embodiments, the volume average particle size of the pigment is approximately 3,000 nm or less, and may be approximately 1,000 nm or less. From the viewpoint of improving light-blocking properties, the volume average particle size of the pigment is suitably approximately 500 nm or less, preferably approximately 300 nm or less, more preferably approximately 250 nm or less, even more preferably approximately 200 nm or less, and may be approximately 150 nm or less (e.g., 120 nm or less). In some embodiments, the volume average particle size of the pigment is usually approximately 10 nm or more, and may be approximately 30 nm or more, 50 nm or more, 80 nm or more, or 100 nm or more (e.g., more than 100 nm). By using a pigment having an appropriate particle size within the above range, a pressure-sensitive adhesive layer with good appearance quality can be preferably obtained.

[0098] The volume average particle size of the pigment is the volume average particle size determined by the laser diffraction / scattering method, and specifically refers to the particle size at 50% of the cumulative value in the particle size distribution measured for a pigment-containing dispersion using a particle size distribution measuring device based on the laser diffraction / scattering method (50% volume average particle size). The measuring device can be, for example, a Microtrac MT3000II manufactured by Microtrac-Bell or an equivalent.

[0099] Although not particularly limited, in some embodiments in which the pressure-sensitive adhesive layer contains a pigment, the average particle diameter of the pigment particles dispersed in the pressure-sensitive adhesive layer may be approximately 150 nm or less. The average particle diameter of the pigment particles dispersed in the pressure-sensitive adhesive layer here refers to the average particle diameter determined from the number-based particle size distribution obtained by TEM observation, specifically, measured using ultrathin frozen sections of the pressure-sensitive adhesive. A small average particle diameter of pigment particles in the pressure-sensitive adhesive layer tends to limit the amount of large-diameter particles, and the maximum diameter of particles present in the pressure-sensitive adhesive layer also tends to be small. Furthermore, limiting the amount of large-diameter particles with a relatively small specific surface area means that a certain amount of pigment particles in the pressure-sensitive adhesive layer have a predetermined or greater light absorption area. This makes it easier to achieve excellent pigment addition effects. For example, when the pigment is a black pigment such as carbon black, the pressure-sensitive adhesive layer can exhibit excellent light transmittance reduction and, therefore, light-blocking properties. In some preferred embodiments, the average particle size is less than 130 nm, and may be approximately 120 nm or less, approximately 110 nm or less, approximately 100 nm or less, or even 90 nm or less. The lower limit of the average particle size is not particularly limited, and is suitably approximately 10 nm or more. From the viewpoint of limiting the amount of small particles with low light absorption that cause diffraction and scattering, the average particle size is preferably approximately 50 nm or more, more preferably approximately 70 nm or more, and even more preferably approximately 80 nm or more, and may be, for example, 90 nm or more. The average particle size determined from the number-based particle size distribution by TEM observation is specifically measured by the following method.

[0100] [Measurement of pigment particle size in adhesive layer] The pressure-sensitive adhesive sample was rapidly frozen under a liquid nitrogen atmosphere, and then cut into approximately 100 nm thick ultrathin sections using an ultramicrotome (Leica model "UC7") in a frozen atmosphere at -30°C. The obtained ultrathin sections were then observed using a transmission electron microscope (TEM; Hitachi High-Technologies Corporation, accelerating voltage 100 kV). Image processing (binarization) was performed on one field of view (6 μm × 6 μm square) of the TEM image magnified approximately 3,000 times to identify particles, and the area fraction of each particle was calculated for all identified particles. The circle-equivalent diameter was then calculated from the area of ​​each particle. The circle-equivalent diameter refers to the diameter of a circle (perfect circle) having the same area as the area of ​​a single particle being measured. This procedure was performed on four different fields of view in the TEM image (N=4), and the particles classified by circle-equivalent diameter were plotted on a histogram based on the number to obtain a particle size distribution (based on the number). The number of particles that serves as the basis for calculating the particle size distribution is determined by counting the number of particles present within the above-mentioned field of view. From the obtained particle size distribution, the average particle size based on TEM observation (TEM average particle size) [nm] is determined. Note that when identifying particles, particles on the edge of the image are omitted during analysis. ImageJ, for example, can be used as image analysis software.

[0101] The form of pigment addition is not particularly limited. For example, the pigment is preferably blended into the PSA composition in the form of a dispersion containing the pigment. Such a dispersion is preferably a dispersion (typically an aqueous dispersion) in which at least a portion of the pigment is dispersed in water or an aqueous liquid primarily composed of water. Blending a dispersion in which at least a portion of the pigment is dispersed into a PSA composition to form a PSA layer makes it easier to obtain a high-quality appearance with minimal color unevenness. The pigment dispersion is preferably used in the form of a dispersion that is liquid at room temperature (e.g., 23°C).

[0102] The content of the pigment in the pressure-sensitive adhesive layer can be determined taking into consideration the dispersibility of the pigment in the pressure-sensitive adhesive layer, the thickness of the pressure-sensitive adhesive layer, the desired optical properties (light transmittance, such as light-blocking properties), and the desired adhesive properties, such as design. In some embodiments, the content (solids basis) of the pigment (e.g., black pigment, more specifically, carbon black) in the pressure-sensitive adhesive layer is suitably approximately 0.01 parts by weight or more per 100 parts by weight of the acrylic polymer. From the viewpoint of effectively obtaining the effects of adding the pigment (e.g., light-blocking properties, design properties, etc.), the content is preferably approximately 0.1 parts by weight or more, more preferably approximately 1 part by weight or more, and may be approximately 3 parts by weight or more, approximately 4 parts by weight or more, or approximately 4.5 parts by weight or more. In some embodiments, the content of the pigment in the pressure-sensitive adhesive layer may be approximately 5 parts by weight or more (e.g., more than 5 parts by weight), 7 parts by weight or more, or even 9 parts by weight or more per 100 parts by weight of the acrylic polymer. Such a high-pigment-containing pressure-sensitive adhesive is suitable, for example, for applications requiring high light-blocking properties. In some embodiments, the content of the pigment can be approximately 30 parts by weight or less per 100 parts by weight of the acrylic polymer. From the viewpoint of adhesive properties such as adhesive strength, the content is preferably approximately 15 parts by weight or less, more preferably 10 parts by weight or less (e.g., less than 10 parts by weight). From the viewpoint of easily obtaining higher shear properties, the content may be approximately 8 parts by weight or less, approximately 6 parts by weight or less, or approximately 5 parts by weight or less (e.g., less than 5 parts by weight). In some embodiments, the content of the pigment can be approximately 3 parts by weight or less, approximately 2 parts by weight or less, approximately 1 part by weight or less, approximately 0.5 parts by weight or less, or approximately 0.1 part by weight or less per 100 parts by weight of the acrylic polymer. The technology disclosed herein can also be suitably implemented in an embodiment in which the adhesive layer does not contain a pigment.

[0103] (dispersant) The pressure-sensitive adhesive layer disclosed herein may or may not contain a dispersant. For example, when no pigment is used or when a self-dispersing pigment such as the above-mentioned self-dispersing carbon black is used as the pigment, a configuration without a dispersant can be preferably adopted. The dispersant is not particularly limited, and one or more known or commonly used dispersants can be appropriately selected and used depending on the purpose. For example, when a pigment is used, one or more suitable dispersants that can disperse the pigment well in the pressure-sensitive adhesive layer can be used. For example, one or more suitable surfactants that can disperse the pigment well in the pressure-sensitive adhesive layer can be appropriately selected and used from the following anionic, cationic, nonionic, and amphoteric surfactants and polymeric compounds (which may be resins) that exhibit good pigment dispersion performance.

[0104] Examples of anionic surfactants used as dispersants include alkyl sulfates such as lauryl sulfate and octadecyl sulfate; fatty acid salts; polysulfonates; polycarboxylates; alkylbenzenesulfonates such as nonylbenzenesulfonate and dodecylbenzenesulfonate; naphthalenesulfonates such as dodecylnaphthalenesulfonate; naphthalenesulfonic acid formalin condensates; alkyl diphenyl ether disulfonates such as dodecyl diphenyl ether disulfonate; polyoxyethylene alkyl ether sulfates such as polyoxyethylene octadecyl ether sulfate and polyoxyethylene lauryl ether sulfate; polyoxyethylene alkyl phenyl ether sulfates such as polyoxyethylene lauryl phenyl ether sulfate; polyoxyethylene styrenated phenyl ether sulfate; sulfosuccinates such as lauryl sulfosuccinate and polyoxyethylene lauryl sulfosuccinate; polyoxyethylene alkyl ether phosphates; polyoxyethylene alkyl phosphate sulfonates; polyoxyethylene alkyl ether acetates; and the like. When the anionic surfactant forms a salt, the salt may be, for example, a metal salt (preferably a salt of a monovalent metal) such as sodium salt, potassium salt, calcium salt, or magnesium salt, an ammonium salt, an amine salt, etc. These anionic surfactants may be used alone or in combination of two or more.

[0105] Examples of cationic surfactants used as dispersants include alkylamine salts and quaternary ammonium salts. Specific examples include stearylamine acetate, trimethyl palm ammonium chloride, trimethyl tallow ammonium chloride, dimethyl dioleyl ammonium chloride, methyl oleyl diethanol chloride, tetramethyl ammonium chloride, lauryl pyridinium chloride, lauryl pyridinium bromide, lauryl pyridinium disulfate, cetyl pyridinium bromide, 4-alkyl mercaptopyridine, poly(vinylpyridine)-dodecyl bromide, and dodecyl benzyl triethyl ammonium chloride. These cationic surfactants can be used alone or in combination of two or more.

[0106] Examples of nonionic surfactants used as dispersants include polyoxyethylene alkyl ethers such as polyoxyethylene lauryl ether, polyoxyethylene cetyl ether, polyoxyethylene stearyl ether, and polyoxyethylene oleyl ether; polyoxyethylene alkylphenyl ethers such as polyoxyethylene octylphenyl ether and polyoxyethylene nonylphenyl ether; sorbitan fatty acid esters such as sorbitan monolaurate, sorbitan monopalmitate, sorbitan monostearate, and sorbitan monooleate; polyoxyethylene sorbitan fatty acid esters such as polyoxyethylene sorbitan monolaurate, polyoxyethylene sorbitan monopalmitate, polyoxyethylene sorbitan monostearate, polyoxyethylene sorbitan tristearate, polyoxyethylene sorbitan triisostearate, polyoxyethylene sorbitan monooleate, and polyoxyethylene sorbitan trioleate; polyoxyethylene glyceryl ether fatty acid esters; polyoxyethylene-polyoxypropylene block copolymers; etc. These nonionic surfactants can be used alone or in combination of two or more.

[0107] Examples of amphoteric surfactants used as dispersants include alkyl betaines such as alkyl dimethyl amino acetic acid betaine, amino carboxylates, alkyl imidazolines, etc. The above amphoteric surfactants can be used alone or in combination of two or more.

[0108] Resins (which may be polymeric compounds) used as dispersants include polyurethane resins; polyester resins; polyamide resins such as unsaturated polyamides; (meth)acrylic resins such as poly(meth)acrylic acid esters and (meth)acrylic acid-(meth)acrylate copolymers; polycarboxylic acid (salts) such as amine salts, ammonium salts, and alkylamine salts of polyacrylic acid and polycarboxylic acids; styrene copolymers such as (meth)acrylic acid-styrene copolymers and styrene-maleic acid copolymers; polyvinyl alcohol; polyvinylpyrrolidone; polysiloxane; polyalkylene oxide derivatives such as ethylene oxide-propylene oxide adducts; phosphate ester resins; long-chain polyaminoamide phosphates; and modified products thereof. These resins may be used alone or in combination of two or more.

[0109] The form of addition of the dispersant is not particularly limited, and the dispersant may be contained in the pigment dispersion before being blended into the pressure-sensitive adhesive composition, or may be supplied at the same time as the pigment is added to the pressure-sensitive adhesive composition, or before or after the pigment is added.

[0110] The content of the dispersant in the PSA layer can be set within an appropriate range depending on the purpose. For example, in embodiments using a pigment, the dispersibility of the pigment can be taken into consideration. In some embodiments, the amount of dispersant per 100 parts by weight of pigment in the PSA layer can be approximately 0.01 parts by weight or more (e.g., approximately 0.1 parts by weight or more), suitably approximately 1 part by weight or more, for example, approximately 2 parts by weight or more, for example, approximately 3 parts by weight or more, or even approximately 4 parts by weight or more. In some embodiments, the amount of dispersant per 100 parts by weight of pigment in the PSA layer can be approximately 100 parts by weight or less, suitably approximately 60 parts by weight or less, or may be approximately 30 parts by weight or less, or may be approximately 10 parts by weight or less. By limiting the amount of dispersant used, it is possible to prevent or suppress deterioration in appearance quality retention due to the presence of the dispersant. From this perspective, in some embodiments, the content of the dispersant per 100 parts by weight of pigment in the PSA layer can be less than 5 parts by weight, or less than 3 parts by weight, or even less than 1 part by weight. The technology disclosed herein can be preferably practiced in an embodiment in which the PSA layer does not substantially contain a dispersant.

[0111] (thickener) In some embodiments, the pressure-sensitive adhesive layer contains a thickener. The inclusion of a thickener can improve the coatability of the pressure-sensitive adhesive composition, facilitating the formation of a pressure-sensitive adhesive layer with good quality. As used herein, the term "thickener" refers to a component that, when added to an aqueous dispersion in which at least a portion of a pressure-sensitive adhesive-forming material is dispersed in water, increases the viscosity (thickening effect) compared to when the component is not added. The thickener is not particularly limited, and examples thereof include polyacrylic acids, carboxylic acid copolymers, urethane compounds, polyvinyl alcohols, celluloses (e.g., hydroxycellulose), polyethers (e.g., polyethylene glycol), and the like. These can be used alone or in combination of two or more. For example, polyacrylic acids and carboxylic acid copolymers are preferably used. As the carboxylic acid copolymers, copolymers containing a carboxyl group-containing acrylic monomer as a monomer unit are used.

[0112] In the embodiment in which the adhesive layer contains a thickener, the content of the thickener in the adhesive layer is appropriately set so as to obtain the desired viscosity characteristics. In some embodiments, the content of the thickener is suitably 0.01 parts by weight or more relative to 100 parts by weight of the acrylic polymer, and from the viewpoint of effectively obtaining the viscosity adjusting effect, it is preferably 0.1 parts by weight or more, more preferably 0.5 parts by weight or more, and even more preferably 0.8 parts by weight or more. In some embodiments, the content of the thickener is, for example, approximately 20 parts by weight or less relative to 100 parts by weight of the acrylic polymer, suitably approximately 10 parts by weight or less, preferably approximately 5 parts by weight or less, more preferably approximately 3 parts by weight or less, even more preferably approximately 2 parts by weight or less, and may be approximately 1 part by weight or less.

[0113] (Other added ingredients) The pressure-sensitive adhesive layer may optionally contain various additives to the extent that the effects of the present invention are not significantly impaired. Examples of such optional additives include pH adjusters, leveling agents, crosslinking aids, release adjusters, plasticizers, softeners, fillers, rust inhibitors, preservatives, mildew inhibitors, antistatic agents, antiaging agents, UV absorbers, antioxidants, light stabilizers, etc. As for such various additives, conventionally known ones can be used in the usual way, and since they do not particularly characterize the present invention, detailed explanations will be omitted.

[0114] (Adhesive composition) The PSA layer typically comprises a PSA formed from a water-dispersed PSA composition. Water-dispersed PSA compositions typically contain water or a water-based mixed solvent or dispersion medium (aqueous solvent or aqueous dispersion medium) as a dispersion medium. The proportion of water in the volatile components contained in a water-dispersed PSA composition is typically, for example, approximately 90% by weight or more, and preferably approximately 95 to 100% by weight. From the perspective of environmental considerations and eliminating organic solvents, in some embodiments, the water-dispersed PSA composition is preferably substantially free of organic solvents. Here, "a PSA composition substantially free of organic solvents" means that the amount of organic solvent in the PSA composition is less than 1% by weight (e.g., less than 0.1% by weight). Water-dispersed PSA compositions can be prepared, for example, by mixing other components (e.g., pigment dispersions) with an aqueous dispersion of an acrylic polymer. Examples of aqueous dispersions of acrylic polymers that can be used include polymerization reaction solutions obtained by emulsion polymerization, and those obtained by subjecting the polymerization reaction solutions to treatments such as pH adjustment (e.g., neutralization), nonvolatile content adjustment, and viscosity adjustment, as necessary.

[0115] (Formation of adhesive layer) The pressure-sensitive adhesive layer disclosed herein can be formed by a conventionally known method. For example, a method can be employed in which a pressure-sensitive adhesive composition is applied to a surface (release surface) having releasability and then dried to form a pressure-sensitive adhesive layer. For pressure-sensitive adhesive sheets having a supporting substrate, a method (direct method) can be employed in which a pressure-sensitive adhesive composition is directly applied (typically coated) to the supporting substrate and then dried to form a pressure-sensitive adhesive layer. Alternatively, a method (transfer method) can be employed in which a pressure-sensitive adhesive composition is applied to a surface (release surface) having releasability and then dried to form a pressure-sensitive adhesive layer on the surface, and then the pressure-sensitive adhesive layer is transferred to a supporting substrate. For example, the surface of a release film described below can be preferably used as the release surface. The pressure-sensitive adhesive layer disclosed herein is typically formed continuously.

[0116] The pressure-sensitive adhesive composition can be applied using a conventionally known coater such as a gravure roll coater, a die coater, a reverse roll coater, a kiss roll coater, a dip roll coater, a bar coater, a knife coater, a spray coater, etc. Alternatively, the pressure-sensitive adhesive composition may be applied by impregnation, a curtain coating method, or the like. From the viewpoints of promoting the crosslinking reaction, improving production efficiency, etc., the pressure-sensitive adhesive composition is preferably dried under heating. The drying temperature can be, for example, about 40 to 150° C., and is usually preferably about 60 to 130° C. After drying the pressure-sensitive adhesive composition, aging may be further carried out for the purposes of adjusting component migration within the pressure-sensitive adhesive layer, promoting the crosslinking reaction, and alleviating distortion that may exist within the pressure-sensitive adhesive layer, etc.

[0117] (Adhesive layer thickness) In the pressure-sensitive adhesive sheet disclosed herein, the thickness of the pressure-sensitive adhesive layer is 50 μm or less. By limiting the thickness of the pressure-sensitive adhesive layer to 50 μm or less, a pressure-sensitive adhesive sheet that exhibits good shear properties while using a water-dispersible acrylic polymer can be suitably realized. In some preferred embodiments, the thickness of the pressure-sensitive adhesive layer can be approximately 45 μm or less, and may be, for example, approximately 40 μm or less, approximately 37 μm or less, or approximately 35 μm or less. From the viewpoint of reducing the thickness and weight of the pressure-sensitive adhesive sheet, in some embodiments, the thickness of the pressure-sensitive adhesive layer may be approximately 25 μm or less, approximately 20 μm or less, approximately 15 μm or less, or approximately 10 μm or less (e.g., less than 10 μm). Furthermore, from the viewpoint of adhesive strength and the like, in some embodiments, it is advantageous for the thickness of the pressure-sensitive adhesive layer to be approximately 1 μm or more, and appropriately approximately 3 μm or more, preferably approximately 5 μm or more, more preferably approximately 10 μm or more, even more preferably approximately 12 μm or more, and may be approximately 15 μm or more. By making the pressure-sensitive adhesive layer have a predetermined thickness or more, adhesive strength suitable for fixing members, etc. is easily obtained. In some embodiments, the thickness of the pressure-sensitive adhesive layer may be 20 μm or more, 30 μm or more, or 32 μm or more. Note that in a substrate-attached double-sided pressure-sensitive adhesive sheet having a first pressure-sensitive adhesive layer and a second pressure-sensitive adhesive layer on each side of the substrate, the first pressure-sensitive adhesive layer and the second pressure-sensitive adhesive layer may have the same thickness or different thicknesses.

[0118] (Light transmittance of adhesive layer) In the pressure-sensitive adhesive sheet disclosed herein, the light transmittance of the pressure-sensitive adhesive layer may vary depending on the intended use of the pressure-sensitive adhesive sheet and whether or not the pressure-sensitive adhesive layer contains a pigment. In some embodiments, the pressure-sensitive adhesive layer has a light transmittance at a wavelength of 550 nm (550 nm light transmittance) of approximately 80% or less. For example, by incorporating a pigment into the pressure-sensitive adhesive layer, a pressure-sensitive adhesive layer with reduced light transmittance can be formed. The 550 nm light transmittance of the pressure-sensitive adhesive layer may be approximately 70% or less, approximately 60% or less, approximately 50% or less (e.g., less than 50%), approximately 40% or less, 30% or less, 20% or less, or 10% or less. In some preferred embodiments, the 550 nm light transmittance of the pressure-sensitive adhesive layer is less than 10%, or may be 5% or less, 3% or less, 1.5% or less, 1.0% or less, or 0.1% or less. A pressure-sensitive adhesive sheet having a pressure-sensitive adhesive layer exhibiting the above-mentioned 550 nm light transmittance can exhibit excellent light-blocking properties and adherend concealment properties. Furthermore, in some embodiments, the lower limit of the 550 nm light transmittance of the pressure-sensitive adhesive layer may be 0.1% or more, or even 0.3% or more, from the viewpoints of productivity, adhesive strength, and other properties. In some embodiments, the 550 nm light transmittance of the pressure-sensitive adhesive layer may be 1% or more, 3% or more, or even 5% or more. Furthermore, in pressure-sensitive adhesive sheets intended for applications where the light transmittance of the pressure-sensitive adhesive layer does not need to be limited or transparency is required, the 550 nm light transmittance of the pressure-sensitive adhesive layer is suitably, for example, greater than approximately 70%, preferably greater than approximately 80% (e.g., greater than approximately 82%), more preferably greater than approximately 85%, and may even be greater than approximately 90%. In pressure-sensitive adhesive sheets intended for such applications, the upper limit of the 550 nm light transmittance of the pressure-sensitive adhesive layer is not particularly limited. For example, in some embodiments, the 550 nm light transmittance of the pressure-sensitive adhesive layer may be 98% or less, or may be 95% or less. The 550 nm light transmittance of the pressure-sensitive adhesive layer can be measured in the same manner as the 550 nm light transmittance of the pressure-sensitive adhesive sheet described in the Examples below.

[0119] (Gel fraction of adhesive layer) In the pressure-sensitive adhesive sheet disclosed herein, the gel fraction of the pressure-sensitive adhesive layer is not particularly limited. The gel fraction of the pressure-sensitive adhesive layer may be, for example, 98% or less, 95% or less, or 90% or less. From the viewpoint of easily obtaining adhesive strength to an adherend (e.g., adhesive strength suitable for fixing a member), in some embodiments, the gel fraction of the pressure-sensitive adhesive layer is suitably less than 90%, advantageously less than 85%, preferably 80% or less (e.g., less than 80%), more preferably 77% or less, may be 75% or less, may be 73% or less, may be 70% or less, may be 65% or less, may be 60% or less, may be 55% or less, or may be 50% or less. Furthermore, the gel fraction of the pressure-sensitive adhesive may be 0%, or it is appropriate that it is 10% or more from the viewpoint of improving shear properties (e.g., shear holding strength), it is advantageous that it is 20% or more, it is preferably 25% or more, it is more preferably 30% or more, it may be 35% or more, it may be 40% or more, it may be 45% or more, it may be 50% or more, it may be 60% or more, it may be 65% or more, or it may be 70% or more.

[0120] The gel fraction of the pressure-sensitive adhesive layer can be determined by wrapping a sample of weight W1 taken from the pressure-sensitive adhesive layer in a porous tetrafluoroethylene resin sheet, immersing it in ethyl acetate at room temperature for one week, drying it, measuring the weight W2 of the ethyl acetate-insoluble portion, and substituting W1 and W2 into the following formula: gel fraction [%] = W2 / W1 x 100. As the porous tetrafluoroethylene resin sheet, Nitto Denko Corporation's "Nitoflon (registered trademark) NTF1122" (average pore size 0.2 μm, porosity 75%, thickness 85 μm) or an equivalent product can be used.

[0121] <Supporting base material> In embodiments in which the PSA sheet disclosed herein is in the form of a substrate-attached double-sided PSA sheet, the substrate supporting the PSA layer can be a resin film, paper, cloth, rubber sheet, foam sheet, metal foil, a composite thereof, or the like. Examples of paper include Japanese paper, kraft paper, glassine paper, fine paper, synthetic paper, and top-coated paper. Examples of cloth include woven fabrics and nonwoven fabrics made from various fibrous materials, either alone or in combination. Examples of the fibrous materials include cotton, staple fiber, Manila hemp, pulp, rayon, acetate fiber, polyester fiber, polyvinyl alcohol fiber, polyamide fiber, and polyolefin fiber. Examples of rubber sheets include natural rubber sheets and butyl rubber sheets. Examples of foam sheets include foamed polyurethane sheets and foamed polychloroprene rubber sheets. Examples of metal foils include aluminum foil and copper foil.

[0122] The term "nonwoven fabric" as used herein refers to a nonwoven fabric for adhesive sheets that is primarily used in the field of adhesive tapes and other adhesive sheets, and typically refers to a nonwoven fabric (sometimes referred to as "paper") that is produced using a general papermaking machine. The term "resin film" as used herein typically refers to a non-porous resin sheet that is distinguished from, for example, nonwoven fabrics and woven fabrics (i.e., does not include nonwoven fabrics or woven fabrics). Such resin films may be non-foamed. A non-foamed resin film here refers to a resin film that has not been intentionally treated to form a foam. Specifically, a non-foamed resin film may be a resin film with an expansion ratio of less than 1.1 (e.g., less than 1.05, typically less than 1.01). The resin film may be a non-stretched film, a uniaxially stretched film, or a biaxially stretched film.

[0123] In some embodiments, a plastic substrate can be preferably used as the support substrate constituting the substrate-attached double-sided PSA sheet. When the support substrate is a plastic substrate, better shear properties (e.g., one or both of shear adhesive strength and shear holding strength) are more likely to be obtained than, for example, when the support substrate is a nonwoven fabric. Here, the term "plastic substrate" refers to a substrate that includes a resin film as a base film. The base film is typically a member that can independently maintain its shape (independent). The support substrate in the technology disclosed herein can be a plastic substrate essentially composed of such a base film. Alternatively, the support substrate can be a plastic substrate that includes an auxiliary layer in addition to the base film. Examples of the auxiliary layer include a colored layer, a reflective layer, an undercoat layer, an antistatic layer, etc., provided on the surface of the base film.

[0124] The resin film is a film whose main component is a resin material (e.g., a component contained in the resin film in an amount exceeding 50% by weight). Examples of resin films include polyolefin-based resin films such as polyethylene (PE), polypropylene (PP), and ethylene-propylene copolymer; polyester-based resin films such as polyethylene terephthalate (PET), polybutylene terephthalate (PBT), and polyethylene naphthalate (PEN); vinyl chloride-based resin films; vinyl acetate-based resin films; polyimide-based resin films; polyamide-based resin films; fluororesin films; cellophane; and the like. The resin film may also be a rubber-based film such as a natural rubber film or a butyl rubber film. Among these, polyester films are preferred from the viewpoints of handleability and processability, and PET films are particularly preferred.

[0125] A colorant can be contained in the resin film. This allows the light transmittance (light blocking property, etc.) of the resin film to be adjusted. Adjusting the light transmittance (for example, perpendicular light transmittance) of the resin film can also be useful for adjusting the light transmittance of a substrate containing the resin film, and further, the light transmittance of a pressure-sensitive adhesive sheet containing the substrate. Conventionally known pigments and dyes can be used as the colorant. There are no particular restrictions on the color of the colorant.

[0126] In some embodiments, a black colorant may be preferably used because a small amount of the colorant can efficiently adjust the light-blocking properties (e.g., vertical light transmittance). Specific examples of black colorants include those exemplified as black pigments that can be contained in the pressure-sensitive adhesive layer. While not particularly limited, in some embodiments, a pigment (e.g., a particulate black colorant such as carbon black) with a volume-average particle diameter of 10 nm to 500 nm, more preferably 10 nm to 120 nm, can be used. In other embodiments, the resin film may contain a white colorant such as titanium oxide. The amount of colorant used in the resin film is not particularly limited and can be adjusted appropriately to impart the desired optical properties. The amount of colorant used is typically approximately 0.1 to 30 wt % of the weight of the resin film, and can be, for example, 0.1 to 25 wt % (typically 0.1 to 20 wt %).

[0127] The resin film may contain various additives, such as fillers (inorganic fillers, organic fillers, etc.), dispersants (surfactants, etc.), antioxidants, antioxidants, UV absorbers, antistatic agents, lubricants, plasticizers, etc. The blending ratio of the various additives is usually about less than 30% by weight (for example, less than 20% by weight, typically less than 10% by weight).

[0128] The resin film may have a single layer structure, or a multilayer structure of two, three or more layers. From the viewpoint of shape stability, the resin film preferably has a single layer structure. In the case of a multilayer structure, at least one layer (preferably all layers) preferably has a continuous structure of the resin (e.g., polyester-based resin). The method for producing the resin film is not particularly limited and may be any conventionally known method, as appropriate. For example, conventionally known general film forming methods such as extrusion molding, inflation molding, T-die casting molding, and calendar roll molding may be appropriately used.

[0129] The supporting substrate may be colored by a colored layer disposed on the surface of a base film (preferably a resin film). In such a substrate having a configuration including a base film and a colored layer, the base film may or may not contain a colorant. The colored layer may be disposed on either one surface of the base film, or may be disposed on both surfaces. In a configuration in which colored layers are disposed on both surfaces of the base film, the configurations of the colored layers may be the same or different.

[0130] The colored layer may have a single layer structure consisting of a single layer, or a multilayer structure including two, three, or more sub-colored layers. A colored layer having a multilayer structure including two or more sub-colored layers can be formed, for example, by repeatedly applying (e.g., printing) a colored layer-forming composition. The color and amount of colorant contained in each sub-colored layer may be the same or different. For a colored layer intended to impart light-blocking properties, a multilayer structure is particularly useful from the viewpoint of preventing pinholes from occurring and increasing the reliability of preventing light leakage.

[0131] The overall thickness of the colored layer is usually about 1 μm to 10 μm, preferably about 1 μm to 7 μm, and can be, for example, about 1 μm to 5 μm. In a colored layer including two or more sub-colored layers, the thickness of each sub-colored layer is usually preferably about 1 μm to 2 μm.

[0132] The thickness of the support substrate is not particularly limited. From the viewpoint of preventing the pressure-sensitive adhesive sheet from becoming excessively thick, the thickness of the support substrate can be, for example, approximately 200 μm or less (e.g., approximately 100 μm or less). Depending on the purpose and manner of use of the pressure-sensitive adhesive sheet, the thickness of the support substrate may be approximately 70 μm or less, approximately 30 μm or less, or approximately 10 μm or less (e.g., approximately 5 μm or less). There is no particular lower limit for the thickness of the support substrate. From the viewpoint of the handleability and processability of the pressure-sensitive adhesive sheet, the thickness of the support substrate is usually approximately 2 μm or more, preferably approximately 3 μm or more, and may be, for example, approximately 5 μm or more, or approximately 10 μm or more.

[0133] The surface of the support substrate may be subjected to a conventionally known surface treatment such as corona discharge treatment, plasma treatment, ultraviolet irradiation treatment, acid treatment, alkali treatment, application of a primer, etc. Such a surface treatment may be a treatment for improving the adhesion between the support substrate and the pressure-sensitive adhesive layer, in other words, the anchoring ability of the pressure-sensitive adhesive layer to the support substrate.

[0134] <Release film> In the technology disclosed herein, a release film (also referred to as a release liner) can be used during the formation of a pressure-sensitive adhesive layer, the production of a pressure-sensitive adhesive sheet, and the storage, distribution, and shaping of a pressure-sensitive adhesive sheet before use. The release film is not particularly limited, and examples that can be used include release films having a release treatment layer on the surface of a substrate (release film substrate) such as a resin film or paper, and release films made of low-adhesion materials such as fluorine-based polymers (polytetrafluoroethylene, etc.) and polyolefin-based resins (polyethylene, polypropylene, etc.). The release treatment layer can be formed by surface-treating the substrate with a release treatment agent such as a silicone-based, long-chain alkyl-based, fluorine-based, or molybdenum sulfide.

[0135] <Characteristics of adhesive sheets> The thickness of the adhesive sheet disclosed herein (thickness including the adhesive layer and, in configurations having a supporting substrate, further including the supporting substrate but excluding the release film; also referred to as total thickness) is not particularly limited. The total thickness of the adhesive sheet can be, for example, approximately 500 μm or less, and may be approximately 300 μm or less. From the viewpoint of thinness and weight reduction, in some embodiments, the thickness of the adhesive sheet is usually approximately 200 μm or less, preferably approximately 100 μm or less, more preferably approximately 70 μm or less, and even more preferably approximately 50 μm or less. In some preferred embodiments, the thickness of the adhesive sheet may be approximately 35 μm or less, approximately 25 μm or less, approximately 20 μm or less, approximately 15 μm or less, or approximately 10 μm or less (e.g., less than 10 μm). The thickness of the pressure-sensitive adhesive sheet can be approximately 1 μm or more, for example, approximately 3 μm or more, preferably approximately 5 μm or more, more preferably approximately 10 μm or more, even more preferably approximately 12 μm or more, and may be approximately 15 μm or more. In some embodiments, the thickness of the pressure-sensitive adhesive sheet may be 20 μm or more, 30 μm or more, or even 40 μm or more. A pressure-sensitive adhesive sheet having a thickness of a predetermined value or more can have a pressure-sensitive adhesive layer with sufficient thickness, making it easier to obtain properties based on the pressure-sensitive adhesive layer (e.g., adhesive properties such as adhesive strength) and also tends to be easy to handle. In a substrate-less pressure-sensitive adhesive sheet, the thickness of the pressure-sensitive adhesive layer is the total thickness of the pressure-sensitive adhesive sheet.

[0136] The light transmittance of the pressure-sensitive adhesive sheet disclosed herein may vary depending on the intended use of the pressure-sensitive adhesive sheet. In some embodiments, the 550 nm light transmittance of the pressure-sensitive adhesive sheet is approximately 80% or less, may be approximately 70% or less, may be approximately 60% or less, may be approximately 50% or less (e.g., less than 50%), may be approximately 40% or less, may be 30% or less, may be 20% or less, or may be 10% or less. In some preferred embodiments, the 550 nm light transmittance of the pressure-sensitive adhesive sheet is less than 10%, may be 5% or less, may be 3% or less, may be 1.5% or less, may be 1.0% or less, or may be 0.1% or less. Pressure-sensitive adhesive sheets exhibiting the above 550 nm light transmittance can exhibit excellent light-blocking properties. Furthermore, in some embodiments, the lower limit of the 550 nm light transmittance of the pressure-sensitive adhesive sheet may be 0.1% or more, or may be 0.3% or more, from the viewpoints of productivity and other properties such as adhesive strength. In some embodiments, the 550 nm light transmittance of the pressure-sensitive adhesive layer may be 1% or more, 3% or more, or 5% or more. Furthermore, in pressure-sensitive adhesive sheets for applications where light transmittance restrictions are not required or transparency is required, the 550 nm light transmittance of the pressure-sensitive adhesive sheet is suitably, for example, greater than approximately 70%, preferably greater than approximately 80% (e.g., greater than approximately 82%), more preferably greater than approximately 85%, and may even be greater than approximately 90%. In such pressure-sensitive adhesive sheets, the upper limit of the 550 nm light transmittance is not particularly limited. For example, in some embodiments, the 550 nm light transmittance of the pressure-sensitive adhesive sheet may be 98% or less, or even 95% or less. The 550 nm light transmittance of the pressure-sensitive adhesive sheet can be measured by the method described in the Examples below.

[0137] In some embodiments of the pressure-sensitive adhesive sheet disclosed herein, the pressure-sensitive adhesive sheet preferably has a shear holding strength sufficient to prevent the measurement sample from falling off the adherend in a shear holding strength test (application area: width 10 mm, length 20 mm, measurement temperature: 80°C, load: 1000 g, measurement time: 1 hour) performed by the method described in the Examples below. A pressure-sensitive adhesive sheet having such shear holding strength is more likely to maintain an appropriate bonded state by the pressure-sensitive adhesive sheet against stress in the shear direction. From the viewpoint of improving shear properties, in some embodiments, the length (displacement distance) by which the upper end of the measurement sample displaces from the initial application position after the measurement time has elapsed is preferably less than 0.6 mm / h, more preferably 0.5 mm / h or less (e.g., 0.4 mm / h or less), even more preferably 0.3 mm / h or less, and may even be less than 0.3 mm / h (e.g., 0.2 mm / h or less). The lower limit of the displacement distance is 0 mm / h. In some embodiments, taking into consideration the balance with other properties such as adhesive strength, the displacement distance may be greater than 0 mm / h, for example, 0.1 mm / h or greater.

[0138] In some embodiments of the pressure-sensitive adhesive sheet disclosed herein, the pressure-sensitive adhesive sheet preferably has a shear adhesive strength of 2.0 MPa or more, as measured by the method described in the Examples below. A pressure-sensitive adhesive sheet having such a shear adhesive strength is more likely to maintain an appropriate bonded state against stress in the shear direction. For example, a pressure-sensitive adhesive sheet that combines the above-mentioned preferred shear holding strength with a shear adhesive strength of 2.0 MPa or more is more preferred. From the viewpoint of improving shear properties, in some embodiments, the shear adhesive strength of the pressure-sensitive adhesive sheet is preferably 2.2 MPa or more, more preferably 2.5 MPa or more, and may be 2.8 MPa or more, 3.0 MPa or more, 3.5 MPa or more, or even 4.0 MPa or more. There is no particular upper limit to the shear adhesive strength. In some embodiments, taking into consideration the balance with other properties (e.g., impact absorption, etc.), the shear adhesive strength of the pressure-sensitive adhesive sheet may be, for example, 10 MPa or less, 8 MPa or less, or 6 MPa or less.

[0139] The adhesive strength of the PSA sheet disclosed herein may vary depending on the intended use and the application location, and is therefore not limited to a specific range. From the viewpoint of obtaining good adhesion to the adherend, in some embodiments, the PSA sheet suitably has a 180-degree peel strength (adhesion strength to SUS) against a stainless steel plate of, for example, approximately 1.0 N / 20 mm or more, preferably approximately 2.0 N / 20 mm or more, more preferably approximately 3.0 N / 20 mm or more, and may be approximately 4.0 N / 20 mm or more, approximately 5.0 N / 20 mm or more, approximately 6.0 N / 20 mm or more, approximately 7.0 N / 20 mm or more, or approximately 8.0 N / 20 mm or more. In some preferred embodiments, from the viewpoint of the joining reliability of the members, the PSA sheet suitably has an adhesive strength to SUS of approximately 10 N / 20 mm or more, may be approximately 12 N / 20 mm or more, or may be approximately 15 N / 20 mm or more. The upper limit of the adhesive strength to SUS is not particularly limited and may be, for example, about 30 N / 20 mm or less. The adhesive strength to SUS is the 180-degree peel strength against a stainless steel plate measured in accordance with JIS Z 0237, and specifically, can be measured by the method described in the examples below.

[0140] <Application> The pressure-sensitive adhesive sheet disclosed herein has the advantage of being excellent in maintaining a bonded state when a shear stress is applied to the bonded portion of the pressure-sensitive adhesive sheet (shear properties), and can be preferably used, for example, for bonding components of electronic devices, including home appliances, office automation equipment, and portable electronic devices such as smartphones, or for imparting optical properties (e.g., light-blocking properties) to the above-mentioned electronic devices. Application to portable electronic devices is particularly preferred. For example, when a portable electronic device is placed in a pocket (e.g., a back pocket) of clothing, external forces may be applied due to the wearer's movements or posture (e.g., sitting), causing deformation (flexural deformation, etc.), which may result in shear stress being applied to the bonded portion of the pressure-sensitive adhesive sheet. The pressure-sensitive adhesive sheet disclosed herein can also be preferably used in an embodiment in which the above-mentioned shear stress may be applied.

[0141] Non-limiting examples of the portable electronic device include mobile phones, smartphones, tablet PCs, notebook PCs, various wearable devices (e.g., wristwear devices worn on the wrist like a wristwatch, modular devices worn on a part of the body with a clip or strap, eyewear devices including eyeglasses (monocular and binocular, including head-mounted devices), clothing devices attached to shirts, socks, hats, etc. as accessories, earwear devices attached to the ears like earphones, etc.), digital cameras, digital video cameras, audio equipment (portable music players, IC recorders, etc.), calculators (calculators, etc.), portable game devices, electronic dictionaries, electronic organizers, e-books, in-car information devices, portable radios, portable televisions, portable printers, portable scanners, portable modems, etc. In this specification, the term "portable" does not simply mean that a device is portable, but rather means that the device has a level of portability that allows it to be carried relatively easily by an individual (average adult). Examples of the electronic device include personal computers (desktop, notebook, tablet, etc.), televisions, etc. These may be ones that incorporate a display device such as a liquid crystal display or an organic EL display.

[0142] In some embodiments, the adhesive sheet can be used for the purpose of fixing a pressure-sensitive sensor to other members in an electronic device equipped with a pressure-sensitive sensor, such as the above-mentioned portable electronic device, etc. In some embodiments, the adhesive sheet can be used for fixing a pressure-sensitive sensor to other members in an electronic device (typically a portable electronic device) equipped with a function that enables an absolute position to be specified on a plate (typically a touch panel) corresponding to the screen using a device for indicating a position on the screen (typically a pen-type or mouse-type device) and a device for detecting the position.

[0143] The pressure-sensitive adhesive sheet disclosed herein is also suitable for use in applications where it is disposed on the back surface of a display screen (display unit) such as a touch panel display in an electronic device such as a portable electronic device. By disposing the pressure-sensitive adhesive sheet according to some embodiments on the back surface of the display screen (display unit), it is possible to prevent a decrease in visibility of the display screen regardless of the manner in which the electronic device is used. For example, by disposing the pressure-sensitive adhesive sheet on the back surface of a display screen (display unit) such as a touch panel display in the electronic device, it is possible to prevent light reflection through the display screen. The above-mentioned reflection may be caused by a metal member disposed on the back side of the display screen. However, by using the pressure-sensitive adhesive sheet according to some embodiments (for example, a pressure-sensitive adhesive layer or a pressure-sensitive adhesive sheet having the pressure-sensitive adhesive layer in an embodiment with limited light transmittance) for example, to bond the metal member to the display unit, it is possible to simultaneously achieve bonding of the members and imparting light-blocking properties.

[0144] Materials (adherend materials) to which the pressure-sensitive adhesive sheets disclosed herein can be attached include, but are not limited to, metal materials such as copper, silver, gold, iron, tin, palladium, aluminum, nickel, titanium, chromium, zinc, etc., or alloys containing two or more of these, various resin materials (typically plastic materials) such as polyimide resins, acrylic resins, polyethernitrile resins, polyethersulfone resins, polyester resins (polyethylene terephthalate resins, polyethylene naphthalate resins, etc.), polyvinyl chloride resins, polyphenylene sulfide resins, polyetheretherketone resins, polyamide resins (so-called aramid resins, etc.), polyarylate resins, polycarbonate resins, and liquid crystal polymers, and inorganic materials such as alumina, zirconia, soda glass, quartz glass, and carbon. Among these, metal materials such as copper, aluminum, and stainless steel, and resin materials (typically plastic materials) such as polyimide resins, aramid resins, and polyphenylene sulfide resins are widely used. The above materials may be materials for components constituting products such as electronic devices. The adhesive sheet disclosed herein may be attached to a component made of the above materials when used. The above materials may also be materials constituting a fixing target (e.g., a back surface member such as an electromagnetic wave shield or a reinforcing plate) for the pressure-sensitive sensor, display unit, or other such device. The fixing target refers to an object to which the adhesive sheet is attached, i.e., an adherend. The back surface member refers to a member disposed on the opposite side of the front surface (viewing side) of the pressure-sensitive sensor or display unit in, for example, a portable electronic device, and may be, for example, a member constituting the support unit 540 disposed on the back surface of the display device 500 shown in FIG. 4 described below. The fixing target may have either a single-layer structure or a multi-layer structure, and the surface to which the adhesive sheet is attached (the attachment surface) may be subjected to various surface treatments. Although not particularly limited, an example of the object to be fixed is a back surface member having a thickness of 1 μm or more (typically 5 μm or more, for example 60 μm or more, or even 120 μm or more) and 1500 μm or less (for example 800 μm or less).

[0145] In some embodiments, the component or material to which the PSA sheet is attached may be optically transparent (a light-transmitting adherend). Since the adhesive surface of a PSA sheet attached to a light-transmitting adherend can be viewed through the light-transmitting adherend, a high appearance quality is desirable. The 500 nm light transmittance of the light-transmitting adherend may be, for example, greater than 50%, such as 70% or more. In some preferred embodiments, the 500 nm light transmittance of the adherend is 80% or more, more preferably 90% or more, and may be 95% or more (e.g., 95 to 100%). Such a material may be a resin film (e.g., a polyester resin film such as a PET film) disposed on the back surface of an image display unit of various devices, such as a portable electronic device. The PSA sheet disclosed herein may be preferably used in an embodiment in which it is attached to an adherend (e.g., a component) having a 500 nm light transmittance of a predetermined value or higher, as described above. The 500 nm light transmittance may be measured in the same manner as for the 500 nm light transmittance of a PSA sheet.

[0146] In some embodiments, the pressure-sensitive adhesive sheet is used in a manner in which it is attached to a metal member. Examples of materials for the metal member include the metal materials exemplified above as the adherend material. Such metal members are, for example, members or articles having a surface (adhesive sheet attachment surface) formed from a metal material such as aluminum or stainless steel, and preferred examples include metal members such as stainless steel members and aluminum members. By attaching the pressure-sensitive adhesive sheet to an area of ​​the metal member surface that needs to be concealed, the area of ​​the metal member can be concealed. The pressure-sensitive adhesive sheet may cover the entire surface of the metal member, or may cover only a portion of the surface (e.g., a portion of the area that needs to be concealed). The metal member may, for example, be a member constituting the support unit 540 of the display device 500 shown in FIG. 4, which will be described later. The metal member is preferably one of the adherends of the pressure-sensitive adhesive sheet.

[0147] The pressure-sensitive adhesive sheet according to some embodiments can be preferably used in electronic devices (typically portable electronic devices) that require specific optical properties. For example, the pressure-sensitive adhesive sheet can be preferably used in electronic devices that include various light sources such as LEDs (light emitting diodes) or light-emitting elements such as self-emitting organic EL devices. For example, the pressure-sensitive adhesive sheet can be preferably used in electronic devices (typically portable electronic devices) that include organic EL display devices or liquid crystal display devices that require specific optical properties.

[0148] FIG. 4 is an exploded perspective view schematically showing an example of the configuration of a display device. As shown in FIG. 4, display device 500 included in portable electronic device 400 includes display unit 520 configured from a cover member, an organic EL unit, etc., and support unit 540. Display device 500 is configured to further include adhesive sheet 530. In this configuration example, adhesive sheet 530 fixes the members that configure display unit 520 and support unit 540. Note that support unit 540 is configured to include a substrate (a metal plate such as a stainless steel plate or an aluminum plate) and the like. The adhesive sheet disclosed herein is preferably used as a component of the above-mentioned display device.

[0149] The matters disclosed in this specification include the following: [1] A double-sided adhesive sheet having an adhesive layer, the pressure-sensitive adhesive layer contains a water-dispersible acrylic polymer, a tackifying resin, and a water-soluble crosslinking agent, The pressure-sensitive adhesive sheet, wherein the pressure-sensitive adhesive layer has a thickness of 50 μm or less. [2] The PSA sheet according to [1] above, wherein the tackifier resin comprises a tackifier resin having a softening point of 100°C or higher and 160°C or lower selected from the group consisting of rosin-based tackifier resins and terpene-based tackifier resins. [3] The pressure-sensitive adhesive sheet according to [1] or [2] above, wherein the water-soluble crosslinking agent is a polyfunctional crosslinking agent having two or more functional groups that react with a carboxy group or a carbonyl group. [4] The pressure-sensitive adhesive sheet according to any one of [1] to [3] above, wherein the content of a carboxy group-containing monomer in the monomer components constituting the water-dispersible acrylic polymer is 0.5% by weight or more and 12% by weight or less. [5] The pressure-sensitive adhesive sheet according to any one of [1] to [4] above, wherein a silane monomer is copolymerized in the water-dispersible acrylic polymer. [6] The pressure-sensitive adhesive sheet according to any one of the above [1] to [5], wherein the water-dispersible acrylic polymer is copolymerized with a keto group-containing monomer. [7] The pressure-sensitive adhesive sheet according to any one of the above [1] to [6], wherein the water-dispersible acrylic polymer is a polymer synthesized by emulsion polymerization using a chain transfer agent. [8] The pressure-sensitive adhesive sheet according to any one of [1] to [7] above, which is a substrate-less double-sided pressure-sensitive adhesive sheet comprising the pressure-sensitive adhesive layer. [9] The pressure-sensitive adhesive sheet according to any one of [1] to [7] above, which is a substrate-attached double-sided pressure-sensitive adhesive sheet having the pressure-sensitive adhesive layer on both sides of a supporting substrate.

[10] An electronic device comprising the pressure-sensitive adhesive sheet according to any one of [1] to [9] above. [Example]

[0150] Several examples of the present invention will be described below, but it is not intended that the present invention be limited to those shown in these examples. In the following description, "parts" and "%" are by weight unless otherwise specified.

[0151] Example 1 (Preparation of Acrylic Polymer) A reaction vessel equipped with a thermometer, a stirrer, a nitrogen inlet tube, and a reflux condenser was charged with 73 parts of ion-exchanged water and 0.07 parts of emulsifier E1 (trade name "Aqualon KH-1025", manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd., an anionic reactive surfactant) as a reactive emulsifier, and the mixture was stirred at 60°C for 1 hour under a nitrogen atmosphere. To this was added 0.1 part of a polymerization initiator (trade name "VA-057", manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and then a monomer emulsion prepared by emulsifying 85 parts of 2-ethylhexyl acrylate (2EHA), 13 parts of methyl acrylate (MA), 1.25 parts of acrylic acid (AA), 0.75 parts of methacrylic acid (MAA), 0.035 parts of t-dodecanethiol (chain transfer agent), 0.02 parts of 3-methacryloxypropyltrimethoxysilane (trade name "KBM-503", manufactured by Shin-Etsu Chemical Co., Ltd.), and 1.88 parts of the reactive emulsifier E1 (trade name "Aqualon KH-1025") in 30 parts of ion-exchanged water was added dropwise at 60°C over 4 hours to polymerize the mixture. After further maintaining the temperature at 60°C for 3 hours, the mixture was cooled to room temperature and adjusted to pH 6.0 using 10% aqueous ammonia as a pH adjuster, thereby preparing an acrylic polymer emulsion containing a water-dispersible acrylic polymer.

[0152] (Preparation of Pressure-Sensitive Adhesive Composition) To the resulting acrylic polymer emulsion, 1 part of a thickener (trade name "Aron B-500", manufactured by Toagosei Co., Ltd., a carboxylic acid copolymer), 10 parts of a tackifier resin T1 (trade name "Tamanol E-200NT", manufactured by Arakawa Chemical Industries, Ltd., a terpene phenol resin with a softening point of 145°C), and 1 part of a crosslinker C1 (trade name "Carbodilite V-04", manufactured by Nisshinbo Chemical Inc., a water-soluble carbodiimide crosslinker) were added and mixed per 100 parts of the water-dispersible acrylic polymer (based on solids). Ion-exchanged water and 10% aqueous ammonia were added to the resulting mixture to adjust the pH to 9.0 and the solids concentration to 40%. The mixture was stirred at 2000 rpm for 5 minutes in a mixer (manufactured by Thinky Corporation), followed by vacuum degassing at 2200 rpm for 5 minutes to obtain a water-dispersible pressure-sensitive adhesive composition.

[0153] (Preparation of adhesive sheet) The above-mentioned pressure-sensitive adhesive composition was applied to the release-treated surface of a silicone release-treated polyethylene terephthalate (PET) film (trade name "Diafoil MRF38", manufactured by Mitsubishi Chemical Corporation, thickness 38 μm) and dried at 100°C for 3 minutes to form a 15 μm thick pressure-sensitive adhesive layer. The release-treated surface of a release-treated polyester film (trade name "Diafoil MRF25", manufactured by Mitsubishi Chemical Corporation, thickness 25 μm) was then bonded to this pressure-sensitive adhesive layer. In this way, a substrate-less double-sided pressure-sensitive adhesive sheet was obtained, both sides of which were protected by the above-mentioned two release films.

[0154] <Examples 2 to 17, 21 to 27 and Comparative Examples 1 to 11> The type of emulsifier, the type and amount of crosslinking agent, and the thickness of the adhesive layer were changed as shown in Tables 1 to 4. A pressure-sensitive adhesive composition according to each example was prepared in the same manner as in Example 1, and a substrate-less pressure-sensitive adhesive sheet according to each example was obtained in the same manner as in Example 1, except for using the pressure-sensitive adhesive composition.

[0155] Example 18 A reaction vessel equipped with a thermometer, a stirrer, a nitrogen inlet tube, and a reflux condenser was charged with 73 parts of ion-exchanged water and 0.07 parts of emulsifier E2 (trade name "Aqualon HS-10", manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd., an anionic reactive surfactant) as a reactive emulsifier, and the mixture was stirred at 60°C for 1 hour under a nitrogen atmosphere. To this, 0.1 parts of a polymerization initiator (trade name "VA-057", Fujifilm Wako Pure Chemical Industries, Ltd.) was added, and then a monomer emulsion containing 85 parts of 2-ethylhexyl acrylate (2EHA), 13 parts of methyl acrylate (MA), 1.25 parts of acrylic acid (AA), 0.75 parts of methacrylic acid (MAA), 0.15 parts of diacetone acrylamide (DAAM), 0.035 parts of t-dodecanethiol (chain transfer agent), and 1.88 parts of the reactive emulsifier E2 (trade name "Aqualon HS-10") emulsified in 30 parts of ion-exchanged water was added dropwise over 4 hours at 60 ° C. to polymerize. After maintaining the temperature at 60 ° C. for 3 hours, the mixture was cooled to room temperature and adjusted to pH = 6.0 using 10% aqueous ammonia as a pH adjuster, thereby preparing an acrylic polymer emulsion containing a water-dispersible acrylic polymer. To the resulting acrylic polymer emulsion, 1 part of a thickener (trade name "Aron B-500" manufactured by Toagosei Co., Ltd., carboxylic acid copolymer), 35 parts of a tackifier resin T1 (trade name "Tamanol E-200NT" manufactured by Arakawa Chemical Industries, Ltd., softening point 145°C, terpene phenol resin), and 1 part of a crosslinker C4 (adipic acid dihydrazide) were added and mixed per 100 parts of the water-dispersible acrylic polymer (solids basis). Ion-exchanged water and 10% aqueous ammonia were added to the resulting mixture to adjust the pH to 9.0 and the solids concentration to 40%. The mixture was stirred at 2000 rpm for 5 minutes in a mixer (Thinky Corporation), followed by vacuum degassing at 2200 rpm for 5 minutes to obtain a water-dispersible pressure-sensitive adhesive composition. A substrateless pressure-sensitive adhesive sheet according to this example was obtained in the same manner as in Example 1, except for using this pressure-sensitive adhesive composition.

[0156] <Examples 19 to 20> The amount of crosslinker C4 used was changed as shown in Table 2. Otherwise, the pressure-sensitive adhesive composition of each example was prepared in the same manner as in Example 18, and the substrate-less pressure-sensitive adhesive sheet of each example was obtained in the same manner as in Example 1, except for using the pressure-sensitive adhesive composition.

[0157] Example 28 The pressure-sensitive adhesive composition of Example 2 was applied to the release-treated surface of a silicone release-treated polyethylene terephthalate (PET) film (trade name "Diafoil MRF38", manufactured by Mitsubishi Chemical Corporation, thickness 38 μm) and dried at 100°C for 3 minutes to form a 15 μm thick pressure-sensitive adhesive layer. Two sheets of this pressure-sensitive adhesive layer-attached release film were prepared and attached to the first and second surfaces of a 12 μm thick polyethylene terephthalate (PET) film, to obtain a substrate-attached double-sided pressure-sensitive adhesive sheet having a structure of pressure-sensitive adhesive layer (thickness 15 μm) / PET film (thickness 12 μm) / pressure-sensitive adhesive layer (thickness 15 μm), with both sides protected by the two release films.

[0158] <Examples 29 to 31, Comparative Examples 12 to 13> Except for changing the thickness of each pressure-sensitive adhesive layer as shown in Tables 3 and 4, the same procedure as in Example 28 was carried out to obtain a substrate-attached double-sided pressure-sensitive adhesive sheet according to each example.

[0159] <Examples 32 to 34> In preparing the water-dispersed PSA compositions, pigments were further added in the types and amounts (based on solids content) shown in Table 2. The pigment used was "SA Black A4048" manufactured by Mikuni Shikiso Co., Ltd. The water-dispersed PSA compositions of each example were obtained in the same manner as in Example 1, and substrate-less PSA sheets of each example were obtained in the same manner as in Example 1, except for using the PSA compositions.

[0160] The materials used in the above examples and comparative examples are listed below. Emulsifier E1: Anionic reactive surfactant (trade name "Aqualon KH-1025", manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.) Emulsifier E2: Anionic reactive surfactant (trade name "Aqualon HS-10", manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) Emulsifier E3: Anionic reactive surfactant (trade name "Hitenol NF-08", manufactured by Daiichi Kogyo Seiyaku Co., Ltd., non-reactive) Tackifying resin T1: terpene phenol resin (trade name "Tamanol E-200NT", manufactured by Arakawa Chemical Industries, Ltd., softening point 145°C) Tackifying resin T2: rosin ester resin (product name "Super Ester E-865NT", manufactured by Arakawa Chemical Industries, Ltd., softening point 160°C) Tackifying resin T3: rosin ester resin (product name "Super Ester NS-100H", manufactured by Arakawa Chemical Industries, Ltd., softening point 100°C) Crosslinker C1: Water-soluble carbodiimide crosslinker (trade name "Carbodilite V-04", manufactured by Nisshinbo Chemical Inc.) Crosslinker C2: Water-soluble epoxy crosslinker (trade name "Denacol EX-313", manufactured by Nagase ChemteX Corporation) Crosslinker C3: Water-soluble oxazoline crosslinker (trade name "Epocross WS-500", manufactured by Nippon Shokubai Co., Ltd.) Crosslinker C4: Water-soluble hydrazide crosslinker (adipic acid dihydrazide) Crosslinker C5: Epoxy-based crosslinker (trade name "Tetrad C", manufactured by Mitsubishi Gas Chemical Company, Inc., hydrophobic) Crosslinking agent C6: Isocyanate-based crosslinking agent (trade name "Coronate HX", manufactured by Tosoh Corporation, hydrophobic) Crosslinker C7: Water-dispersible carbodiimide crosslinker (trade name "Carbodilite E-05", manufactured by Nisshinbo Chemical Inc., water-dispersible) Pigment: Carbon black aqueous dispersion (product name "SA Black A4048", manufactured by Mikuni Color Co., Ltd., uses polymeric dispersant, volume average particle diameter 110 nm)

[0161] <Evaluation method> (Adhesion to SUS) A measurement sample was prepared by attaching a 50 μm thick PET film to one adhesive side of a pressure-sensitive adhesive sheet (double-sided pressure-sensitive adhesive sheet) under a measurement environment of 23°C and 50% RH, and then cutting the sheet to a size of 20 mm wide and 100 mm long. The adhesive side of the prepared measurement sample was then pressed against the surface of a stainless steel plate (SUS304BA plate) using a 2 kg roller, moving back and forth once, under a 23°C and 50% RH environment. After leaving the sheet under the same environment for 30 minutes, the peel strength (adhesion to SUS) [N / 20 mm] was measured using a universal tension and compression tester in accordance with JIS Z 0237:2000 at a tension speed of 300 mm / min and a peel angle of 180°. The measurement results are shown in Tables 1 and 2. The universal tension and compression tester may be, for example, a Minebea "Tension and Compression Tester, TG-1kN" or an equivalent. Note that the PET film backing is not required for single-sided pressure-sensitive adhesive sheets.

[0162] (shear adhesive strength) A measurement sample was prepared by cutting a pressure-sensitive adhesive sheet (double-sided pressure-sensitive adhesive sheet) into a size of 10 mm x 10 mm. In an environment of 23°C and 50% RH, each adhesive side of the measurement sample was placed on the surface of two stainless steel plates (SUS304BA plates) and pressed together by rolling a 2 kg roller back and forth once. After leaving the sample in the same environment for two days, the shear adhesive strength [MPa] was measured using a tensile tester at a tensile speed of 10 mm / min and a peel angle of 0°. A universal tension and compression tester (product name "TG-1kN", manufactured by Minebea Co., Ltd.) can be used as the tensile tester. In the case of a single-sided pressure-sensitive adhesive sheet (single-sided pressure-sensitive adhesive sheet), the non-adhesive side of the sheet can be fixed to the stainless steel plate with an adhesive or the like, and the rest of the measurement can be performed in the same manner as above.

[0163] (shear holding strength) A pressure-sensitive adhesive sheet (double-sided pressure-sensitive adhesive sheet) was cut to a size of 10 mm x 20 mm to prepare a measurement sample. In an environment of 23°C and 50% RH, each adhesive side of the measurement sample was placed on the surface of two stainless steel plates (SUS304BA plates) as adherends, and a 2 kg roller was rolled back and forth once to press the samples together. The adherends to which the measurement samples had been attached were then suspended in an environment at 80°C with the length of the measurement sample aligned vertically. A 1000 g load was applied to the free end of the adherend, and the adherend was left in this state at 80°C for one hour. After one hour, the length (displacement length, hereinafter also referred to as displacement distance) [mm / h] of the measurement sample from its initial attachment position was measured. Measurements were performed using three samples for each pressure-sensitive adhesive sheet (i.e., N = 3), and the arithmetic average was calculated. The "Shear Retention Test" column in Tables 1 to 4 shows the slippage distance or whether or not the sample fell (if any of the three samples fell within one hour, it is marked as "Falled"). If the slippage distance is less than 0.6 mm / h, the sample is evaluated as "Pass," and if the slippage distance is 0.6 mm / h or more or the sample falls within one hour, the sample is evaluated as "Fail."

[0164] (550nm light transmittance) The light transmittance [%] of the PSA sheet at a wavelength of 550 nm was measured using a commercially available transmittance measuring device in accordance with JIS K 7136:2000. The measurement results are shown in Tables 1 to 4. The transmittance measuring device used was a spectrophotometer manufactured by Hitachi (device name "U4150 spectrophotometer") or an equivalent device.

[0165] [Table 1]

[0166] [Table 2]

[0167] [Table 3]

[0168] [Table 4]

[0169] As shown in Tables 1 to 4, the PSA sheets of Examples 1 to 34, which contained a PSA layer containing a water-dispersible acrylic polymer, a tackifying resin, and a water-soluble crosslinking agent and had a thickness limited to 50 μm or less, all exhibited good shear adhesive strength of 2.0 MPa or more, and all had acceptable shear holding strengths. On the other hand, in the PSA layers using a water-dispersible acrylic polymer, Comparative Examples 1 to 2, 6 to 9, which did not use a water-soluble crosslinking agent, and Comparative Examples 10 to 13, which had PSA layers that were too thick, all failed the shear holding strength test. Furthermore, Comparative Examples 3 to 5, and 9, which did not use a tackifying resin, were unable to achieve sufficient shear adhesive strength.

[0170] Although specific examples of the present invention have been described above in detail, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and alterations of the specific examples exemplified above. [Explanation of symbols]

[0171] 1,2,4 Adhesive sheet 10A front page 10B Second side (back) 21 Adhesive layer (first adhesive layer) 21A Adhesive surface (first adhesive surface) 21B Adhesive surface (second adhesive surface) 22 Adhesive layer (second adhesive layer) 22A Adhesive surface (second adhesive surface) 31,32 Release film

Claims

1. A double-sided adhesive sheet having an adhesive layer, the pressure-sensitive adhesive layer contains a water-dispersible acrylic polymer, a tackifying resin, and a water-soluble crosslinking agent, A pressure-sensitive adhesive sheet, wherein the pressure-sensitive adhesive layer has a thickness of 50 μm or less.

2. The pressure-sensitive adhesive sheet according to claim 1 , wherein the tackifier resin comprises a tackifier resin having a softening point of 100° C. or higher and 160° C. or lower selected from the group consisting of rosin-based tackifier resins and terpene-based tackifier resins.

3. The pressure-sensitive adhesive sheet according to claim 1 or 2, wherein the water-soluble crosslinking agent is a polyfunctional crosslinking agent having two or more functional groups that react with a carboxy group or a carbonyl group.

4. The pressure-sensitive adhesive sheet according to claim 1 or 2, wherein the content of a carboxy group-containing monomer in the monomer components constituting the water-dispersible acrylic polymer is 0.5% by weight or more and 12% by weight or less.

5. The pressure-sensitive adhesive sheet according to claim 1 or 2, wherein a silane-based monomer is copolymerized with the water-dispersible acrylic polymer.

6. The pressure-sensitive adhesive sheet according to claim 1 or 2, wherein the water-dispersible acrylic polymer is copolymerized with a keto group-containing monomer.

7. The pressure-sensitive adhesive sheet according to claim 1 or 2, wherein the water-dispersible acrylic polymer is a polymer synthesized by emulsion polymerization using a chain transfer agent.

8. The pressure-sensitive adhesive sheet according to claim 1 or 2, which is a substrate-less double-sided pressure-sensitive adhesive sheet comprising the pressure-sensitive adhesive layer.

9. The pressure-sensitive adhesive sheet according to claim 1 or 2, which is a substrate-attached double-sided pressure-sensitive adhesive sheet having the pressure-sensitive adhesive layer on both sides of a supporting substrate.

10. An electronic device comprising the pressure-sensitive adhesive sheet according to claim 1 or 2.

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