Adhesive sheets and electronic devices

Abstract

The present invention provides an adhesive sheet having a water-dispersible acrylic adhesive layer that can maintain good appearance quality even when kept in a specific low-temperature range that is below 0°C but not extremely low. [Solution] An adhesive sheet having an adhesive layer is provided. The adhesive layer comprises a water-dispersible acrylic polymer and a water-soluble crosslinking agent. The water-dispersible acrylic polymer is a polymer synthesized by emulsion polymerization using a reactive emulsifier having a radically polymerizable functional group. The gel fraction of the adhesive layer is less than 90%.

Description

[Technical Field]

[0001] This invention relates to adhesive sheets and electronic devices. [Background technology]

[0002] Generally, adhesives (also called pressure-sensitive adhesives; the same applies hereinafter) exhibit a soft solid (viscoelastic) state at temperatures around room temperature and have the property of easily adhering to a substrate under pressure. Taking advantage of this property, adhesives are widely used in various applications, such as joining, fixing, and protecting components within electronic devices, including portable electronic devices such as smartphones. Furthermore, adhesive sheets having an adhesive layer containing pigments such as carbon black are used for purposes such as preventing light leakage and reflection from light sources such as backlight modules of liquid crystal display devices in electronic devices and self-emissive elements such as organic EL (electroluminescence), as well as for purposes such as concealing the substrate, adjusting the appearance of the substrate through the adhesive sheet, and improving design. Patent document 1 is cited as a document relating to this type of technology.

[0003] Furthermore, in recent years, from the perspective of environmental considerations and reducing reliance on petroleum resources, there has been a demand to reduce the amount of organic solvents used in the manufacture of adhesives. For example, the amount of organic solvents used can be reduced by employing solvent-free adhesives such as active energy ray curing adhesives, hot melt adhesives, and emulsion adhesives. Among these, emulsion-type acrylic adhesives are preferred because the desired properties can be suitably obtained by highly controlling the polymerization reaction of acrylic monomers. Patent documents 2 to 4 disclose adhesives containing water-dispersible acrylic polymers obtained by emulsion polymerization. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2020-66655 [Patent Document 2] Japanese Patent Publication No. 2007-154078 [Patent Document 3] Japanese Patent Publication No. 2011-90193 [Patent Document 4] Japanese Patent Publication No. 2007-177003 [Overview of the project] [Problems that the invention aims to solve]

[0005] It is also preferable to reduce the amount of organic solvent used in adhesives that can be used in the above-mentioned electronic devices. However, as a result of the inventors' investigations, it was found that when an adhesive is formed using a water-dispersible acrylic polymer instead of an acrylic polymer synthesized by solution polymerization as described in Patent Document 1, the appearance quality deteriorates when stored in a specific low-temperature range that is below 0°C but not extremely low. Specifically, even if an adhesive containing a water-dispersible acrylic polymer has a good appearance under normal temperature conditions after production, if it is kept in the above-mentioned specific low-temperature range for a certain period of time, pinholes that were not previously observed will appear. When such pinholes occur, the design of the adhesive sheet may be impaired, and if used in the display of a portable electronic device, for example, it may cause display defects in the display. It should be noted that the phenomenon of pinholes occurring when an adhesive formed using a water-dispersible acrylic polymer is kept in the above-mentioned specific low-temperature range is a new problem discovered by the inventors and is different from any of the problems in Patent Documents 1 to 4.

[0006] The present invention was created in view of the above circumstances, and aims to provide an adhesive sheet having an adhesive that contains a water-dispersible acrylic polymer and can maintain good appearance quality even when held in a specific low temperature range that is below 0°C but not extremely low. Another related object is to provide an electronic device including the above adhesive sheet. [Means for solving the problem]

[0007] According to the present specification, an adhesive sheet having an adhesive layer is provided. The adhesive layer includes a water-dispersible acrylic polymer and a water-soluble crosslinking agent. The water-dispersible acrylic polymer is a polymer synthesized by emulsion polymerization using a reactive emulsifier having a radically polymerizable functional group. Further, the adhesive layer has a gel fraction of less than 90%. According to the adhesive of the above configuration, the use of the water-dispersible acrylic polymer reduces the amount of organic solvent, and good appearance quality can be maintained even when it is maintained in a specific low temperature range that is lower than 0°C but not extremely low. For example, the generation of pinholes (hereinafter, also referred to as "low-temperature pinholes") due to being maintained in the above specific low temperature range can be suppressed.

[0008] In some embodiments, the content of the water-soluble crosslinking agent in the adhesive layer is more than 0.05 parts by weight with respect to 100 parts by weight of the water-dispersible acrylic polymer. According to the adhesive sheet in which the content of the water-soluble crosslinking agent in the adhesive layer satisfies the above requirements, the generation of low-temperature pinholes tends to be more effectively suppressed.

[0009] In some embodiments, the water-soluble crosslinking agent includes at least one selected from the group consisting of a carbodiimide-based crosslinking agent, an epoxy-based crosslinking agent, and an oxazoline-based crosslinking agent. By using the above types of water-soluble crosslinking agents as the water-soluble crosslinking agent, the generation of low-temperature pinholes tends to be more effectively suppressed.

[0010] In some embodiments, a silane-based monomer is copolymerized in the water-dispersible acrylic polymer. The technology disclosed herein can be preferably implemented using a water-dispersible acrylic polymer copolymerized with a silane-based monomer.

[0011] In some embodiments, a ketone group-containing monomer is copolymerized in the water-dispersible acrylic polymer. The technology disclosed herein can be preferably implemented using a water-dispersible acrylic polymer copolymerized with a ketone group-containing monomer.

[0012] In some embodiments, the monomer component constituting the above-mentioned water-dispersible acrylic polymer has a carboxyl group-containing monomer content of less than 4% by weight. By using a water-dispersible acrylic polymer in which the amount of carboxyl group-containing monomer used is thus limited, the occurrence of low-temperature pinholes can be better suppressed.

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

[0014] In some embodiments, the adhesive sheet is a substrate-less double-sided adhesive sheet comprising the adhesive layer described above. The effects of the technology disclosed herein can be preferably exhibited in a substrate-less double-sided adhesive sheet. Furthermore, because a substrate-less double-sided adhesive sheet does not have a substrate, it can be made thinner, which is advantageous in terms of making products to which the double-sided adhesive sheet is applied thinner, smaller, and saving space. In addition, a substrate-less double-sided adhesive sheet allows the effects of the adhesive layer, such as adhesive strength, to be expressed to the maximum extent.

[0015] In some other embodiments, the adhesive sheet is a double-sided adhesive sheet with a substrate having the adhesive layer on both sides of the supporting substrate. The double-sided adhesive sheet with a substrate is advantageous in terms of processability and handling.

[0016] In some embodiments, the adhesive sheet has a 180-degree peel strength of 3.0 N / 20 mm or more against a stainless steel plate. Adhesive sheets that satisfy this characteristic have good adhesive strength while exhibiting pigment-added effects such as light shielding and design properties, and are therefore preferably used in applications where a certain level of adhesive strength is required.

[0017] The adhesive sheets disclosed herein can be preferably used to join components of electronic devices, including, for example, home appliances, office automation equipment, and portable electronic devices such as smartphones. For example, electronic devices have parts that are visible to the user and may require excellent appearance quality. The adhesive sheets disclosed herein suppress deterioration of appearance quality due to the occurrence of low-temperature pinholes even when kept in the temperature environment of the specific low-temperature range described above, for example, during storage or transportation. Therefore, they can be applied to visible parts of electronic devices to achieve a surface with good appearance quality.

[0018] As described above, this specification provides an electronic device using any of the adhesive sheets disclosed herein, in other words, an electronic device including such adhesive sheet. [Brief explanation of the drawing]

[0019] [Figure 1] This is a schematic cross-sectional view showing one example of the structure of an adhesive sheet. [Figure 2] This is a schematic cross-sectional view showing another example of the configuration of the adhesive sheet. [Figure 3] This is a schematic cross-sectional view showing another example of the configuration of the adhesive sheet. [Figure 4] This is a schematic cross-sectional view showing another example of the configuration of the adhesive sheet. [Figure 5] This is a schematic exploded perspective view showing an example of the configuration of a display device. [Modes for carrying out the invention]

[0020] Preferred embodiments of the present invention are described below. Matters other than those specifically mentioned herein but necessary for carrying out the present invention can be understood by those skilled in the art based on the teachings on carrying out the invention described herein and the common technical knowledge at the time of filing. The present invention can be carried out based on the contents disclosed herein and the common technical knowledge in the art. Furthermore, in the following drawings, members and parts that perform the same function may be denoted by the same reference numerals, and redundant explanations may be omitted or simplified. Also, the embodiments shown in the drawings are schematic for the purpose of clearly illustrating the present invention and do not necessarily accurately represent the size or scale of the adhesive sheets of the present invention that are actually provided as products.

[0021] In this specification, "adhesive" refers to a material that, as described above, exhibits a soft solid (viscoelastic) state at temperatures around room temperature and has the property of easily adhering to a substrate under pressure. The adhesive referred to here is generally defined as having a complex tensile modulus E, as defined in "CA Dahlquist, “Adhesion: Fundamentals and Practice”, McLaren & Sons, (1966) p. 143". * (1Hz) < 10 7 dyne / cm 2 It may be a material having properties that satisfy the above conditions (typically, a material having the above properties at 25°C).

[0022] In this specification, "water-dispersible adhesive composition" refers to a composition in which at least a portion of the adhesive-forming components are dispersed in water. Water-dispersible includes suspended and emulsified states. The concept of the above-mentioned water-dispersible adhesive composition includes what is called an emulsion-type adhesive composition. In this specification, an adhesive (layer) formed from a water-dispersible adhesive composition may be referred to as a water-dispersible adhesive (layer).

[0023] In this specification, "acrylic polymer" means a polymer containing more than 50% by weight of monomer units derived from acrylic monomers as monomer units constituting the polymer. The above-mentioned acrylic monomer means a monomer having at least one (meth)acryloyl group in one molecule.

[0024] In this specification, "(meth)acryloyl" comprehensively refers to acryloyl and methacryloyl. Similarly, "(meth)acrylate" comprehensively refers to acrylate and methacrylate, and "(meth)acrylic" comprehensively refers to acrylic and methacrylic.

[0025] <Example of adhesive sheet configuration> The adhesive sheet disclosed herein may be an adhesive sheet with a substrate having the adhesive layer on one or both sides of a non-peelable substrate (support substrate), or it may be a substrate-less adhesive sheet (i.e., an adhesive sheet without a non-peelable substrate) in which the adhesive layer is held by a release film. The concept of an adhesive sheet as used herein may include adhesive tapes, adhesive labels, adhesive films, etc. The adhesive sheet disclosed herein may be in roll form or sheet form. Alternatively, it may be an adhesive sheet processed into various shapes.

[0026] Figures 1 and 2 show examples of the configuration of a double-sided adhesive type substrate-less adhesive sheet (substrate-less double-sided adhesive sheet). The adhesive sheet 1 shown in Figure 1 has a configuration in which both sides 21A and 21B of the substrate-less adhesive layer 21 are protected by release films 31 and 32, with at least the adhesive layer side being the release surface. The adhesive sheet 2 shown in Figure 2 has a configuration in which one surface (adhesive surface) 21A of the substrate-less adhesive layer 21 is protected by a release film 31 with both sides being release surfaces. When this is wound, the other surface (adhesive surface) 21B of the adhesive layer 21 comes into contact with the back surface of the release film 31, so that the other surface 21B is also protected by the release film 31.

[0027] Figure 3 schematically shows the structure of an adhesive sheet according to another configuration example. This adhesive sheet 3 is configured as a single-sided adhesive sheet with a base material, comprising a sheet-like support base material 10 having a first surface 10A and a second surface 10B, and an adhesive layer 21 provided on the first surface 10A side. The adhesive layer 21 is fixedly provided on the first surface 10A side of the support base material 10, that is, without any intention to separate the adhesive layer 21 from the support base material 10. Before use, as shown in Figure 3, the adhesive sheet 3 may be in a form in which the surface (adhesive surface) 21A of the adhesive layer 21 is protected by a release film 31, at least on the side facing the adhesive layer 21, which is the release surface.

[0028] Furthermore, the structure of an adhesive sheet relating to another configuration example is schematically shown in Figure 4. This adhesive sheet 4 is configured as a double-sided adhesive sheet with a base material, comprising a sheet-like support base material 10 having a first surface 10A and a second surface 10B, a first adhesive layer 21 fixedly provided on the first surface 10A side, and a second adhesive layer 22 fixedly provided on the second surface 10B side. Before use, the adhesive sheet 4 may be in a form in which the surface (first adhesive surface) 21A of the first adhesive layer 21 and the surface (second adhesive surface) 22A of the second adhesive layer 22 are protected by release films 31 and 32, as shown in Figure 4.

[0029] The technology disclosed herein can preferably be implemented in a substrate-less form from the viewpoint of reducing the thickness of the adhesive sheet. A substrate-less adhesive sheet is advantageous in that it is easy to make thin layers and can maximize adhesive properties such as adhesive strength. Alternatively, in embodiments in which the adhesive sheet disclosed herein is in a substrate-attached form, the presence of a substrate makes it easier to obtain excellent processability and handling properties.

[0030] <Adhesive layer> The inventors have found that pinholes (low-temperature pinholes) formed by storing an adhesive made using a water-dispersible acrylic polymer at low temperatures for a certain period of time are not particularly observed when the storage temperature is set between room temperature and around 0°C, but become noticeable when the storage temperature falls below 0°C. In addition, adhesives using water-dispersible acrylic polymers generally contain emulsifiers used in the emulsion polymerization of the acrylic polymer, and therefore tend to absorb moisture more easily than adhesives using acrylic polymers synthesized by solution polymerization. Based on these findings, the inventors hypothesized that the cause of low-temperature pinholes in adhesives made using water-dispersible acrylic polymers is that when the adhesive is cooled to a temperature below 0°C, the water contained in the adhesive freezes into ice crystals, and these ice crystals gradually grow and push aside the surrounding adhesive, or that the strain accumulated in the surrounding adhesive due to the growth of the ice crystals is relieved after thawing, resulting in a partial absence of adhesive within the adhesive layer, which is then observed as a pinhole. The inventors have found through experiments that low-temperature pinholes are less likely to occur when the storage temperature of the adhesive is extremely low (for example, when stored at a temperature range where ice crystals are unlikely to grow, such as around -80°C) or when the time the adhesive is kept at a low temperature is relatively short (a short period for ice crystal growth). These tendencies are consistent with the above consideration that the growth (coarsening) of ice crystals in the adhesive layer is a factor in the occurrence of low-temperature pinholes.

[0031] According to the technology disclosed herein, in an adhesive layer containing a water-dispersible acrylic polymer with a gel fraction of less than 90%, the water-dispersible acrylic polymer is a polymer synthesized by emulsion polymerization using a reactive emulsifier having radically polymerizable functional groups, and the adhesive layer contains a water-soluble crosslinking agent, thereby suppressing the occurrence of low-temperature pinholes. The reason for this effect is thought to be that the use of a reactive emulsifier reduces the amount of free emulsifier, suppressing the occurrence of locally highly hygroscopic areas (which can become ice crystal formation points) due to the uneven distribution of emulsifiers, and the use of a water-soluble crosslinking agent more uniformly crosslinks the adhesive layer containing the water-dispersible acrylic polymer, suppressing the occurrence of locally highly hygroscopic areas due to the density of crosslinking, thereby effectively suppressing the occurrence of low-temperature pinholes. It should be noted that the technology disclosed herein is not limited to the above considerations.

[0032] (Water-dispersible acrylic polymer) The adhesive layer disclosed herein comprises a water-dispersible acrylic polymer. In this specification, "water-dispersible acrylic polymer" means an acrylic polymer synthesized in a form dispersed in water, and more specifically, an acrylic polymer synthesized by emulsion polymerization. By using a water-dispersible polymer as an adhesive-forming component, it is possible to reduce the amount of organic solvent used and, consequently, eliminate organic solvents. Furthermore, water-dispersible acrylic polymers can be obtained by polymerization using acrylic monomers while highly controlling the reaction, and have the advantage of being easy to obtain desired properties due to the high degree of freedom in molecular design.

[0033] The above water-dispersible acrylic polymer is typically included as a base polymer in the adhesive layer. Here, the "base polymer" of the adhesive layer refers to the main component of the rubbery polymer contained in the adhesive layer, and is not construed in any limited sense other than this. The above rubbery polymer refers to a polymer that exhibits rubber elasticity in the temperature range near room temperature. Also, in this specification, the "main component" refers to a component contained in an amount exceeding 50% by weight unless otherwise specified. Hereinafter, the water-dispersible acrylic polymer may be simply referred to as an acrylic polymer.

[0034] As the acrylic polymer, for example, a polymer of a monomer raw material (monomer component) containing an alkyl (meth) acrylate as a main monomer and further containing a comonomer copolymerizable with the main monomer is preferred. Here, the main monomer refers to a component that occupies more than 50% by weight of the monomer composition in the above monomer raw material.

[0035] As the alkyl (meth) acrylate, for example, a compound represented by the following formula (1) can be preferably used. CH2=C(R 1 )COOR 2 (1) Here, R 1 in the above formula (1) is a hydrogen atom or a methyl group. Also, R 2 is a linear alkyl group having 1 to 20 carbon atoms (hereinafter, such a carbon atom number range may be represented as "C 1-20 "). From the viewpoint of the storage elastic modulus of the adhesive, etc., an alkyl (meth) acrylate in which R 2 is a linear alkyl group of C 1-14 is preferred, and an alkyl (meth) acrylate in which R 2 is a linear alkyl group of C 1-10 is more preferred.

[0036] R 2 is C 1-20Examples of alkyl(meth)acrylates, which are chain-like 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, isooctyl(meth)acrylate. Examples include alkyl(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, eicosyl(meth)acrylate, etc. These alkyl(meth)acrylates can be used individually or in combination of two or more. Preferred examples of alkyl(meth)acrylates include n-butyl acrylate (BA) and 2-ethylhexyl acrylate (2EHA).

[0037] The technology disclosed herein is the same as the above R 2 C 1-20 Alkyl (meth)acrylate (C) is a chain-like alkyl group. 1-20 This can preferably be carried out using an acrylic polymer in which chain-like alkyl (meth)acrylates constitute approximately 50% by weight or more of the monomer components. In some preferred embodiments, the above C 1-20 The proportion of linear 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. 1-20The upper limit of the proportion of linear 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 (for example, 98.5% by weight or less) in embodiments using submonomers.

[0038] In some embodiments, the monomer component is R of formula (1) above. 2 C 4-10 Alkyl (meth)acrylate (C) is a chain-like alkyl group. 4-10 It is preferable that it contains a linear alkyl (meth)acrylate. 4-10 As for linear alkyl (meth)acrylates, C 4-8 It is more preferable to use linear 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), which is a chain-like alkyl group. 4-10 It is even more preferable to use a linear alkyl acrylate, C 4-8 It is particularly preferable to use a linear alkyl acrylate. 4-10 The linear alkyl (meth)acrylate can be used individually or in combination of two or more types. 4-10 The proportion of linear 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. 4-10 The upper limit of the proportion of linear alkyl (meth)acrylate is 100% by weight, and from the viewpoint of obtaining good cohesive force, it is appropriate to set it to approximately 95% by weight or less in some embodiments, and it may also be approximately 90% by weight or less.

[0039] In some embodiments, the above monomer component is C 6-10 It is preferable that it contains a linear alkyl (meth)acrylate. 6-10 As for linear alkyl (meth)acrylates, C 7-9It is more preferable to use a chain-like alkyl (meth)acrylate, and even more preferable to use a C8 chain-like alkyl (meth)acrylate. 6-10 As the linear alkyl (meth)acrylate, linear alkyl acrylate is preferably used. 6-10 The linear alkyl (meth)acrylate can be used alone or in combination of two or more. In some preferred embodiments, the C content of the alkyl (meth)acrylate contained in the monomer component is 6-10 The proportion of the linear 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. The above C in the alkyl (meth)acrylate 6-10 The upper limit of the proportion of linear alkyl (meth)acrylate is 100% by weight, and from the viewpoint of obtaining good cohesive force, it is appropriate to set it to approximately 95% by weight or less in some embodiments, and it may also be approximately 90% by weight or less.

[0040] C above 6-10 As the linear alkyl (meth)acrylate, 2EHA is preferably used. In the embodiment using 2EHA, the proportion of 2EHA is not particularly limited, as described above. 4-10 In the linear alkyl (meth)acrylate, it is appropriate to have 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, 95-100% by weight).

[0041] The above alkyl (meth)acrylate is the above C 4-10 If it contains a linear alkyl (meth)acrylate, other alkyl (meth)acrylates (R of formula (1) above) 2 is less than C4 or C 10 It may also contain an alkyl (meth)acrylate, which is a superchain alkyl group. This allows for a good balance between adhesiveness and cohesiveness. Other alkyl (meth)acrylates include R of formula (1) above. 2 C 1-3Alkyl (meth)acrylates, which are chain-like 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 above other alkyl (meth)acrylates can be used individually or in combination of two or more. The proportion of other alkyl (meth)acrylates in the alkyl (meth)acrylate contained in the above monomer component is preferably about 30% by weight or less, for example, it may be about 20% by weight or less, or about 15% by weight or less. Furthermore, from the viewpoint of obtaining the effect of other alkyl (meth)acrylates, the proportion of the above other alkyl (meth)acrylates is preferably about 1% by weight or more in the total amount of the above alkyl (meth)acrylate, for example, it may be about 5% by weight or more, or about 10% by weight or more.

[0042] The secondary monomers copolymerizable with the main monomer, alkyl(meth)acrylate, can be useful for introducing crosslinking sites into acrylic polymers or for enhancing the cohesive strength of acrylic polymers. Examples of secondary monomers include functional group-containing monomer components such as those listed below, which can be used individually or in combination of two or more. Carboxylate-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 containing epoxy groups: for example, glycidyl (meth)acrylate, methylglycidyl (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, N-(meth)acryloylmorpholine. Silane monomers: Monomers containing alkoxysilyl groups, such as 3-(meth)acryloxypropyltrimethoxysilane, 3-(meth)acryloxypropyltriethoxysilane, 3-(meth)acryloxypropylmethyldimethoxysilane, and 3-(meth)acryloxypropylmethyldiethoxysilane.

[0043] The above functional group-containing monomers can be used individually or in combination of two or more. Among the above functional group-containing monomers, carboxyl group-containing monomers, carbonyl group-containing monomers (typically keto group-containing monomers), hydroxyl group-containing monomers, cyano group-containing monomers, and silane-based monomers are preferred because they can suitably achieve the introduction of crosslinking points and improvement of cohesive force as described above.

[0044] When a functional group-containing monomer is copolymerized with an acrylic polymer, the proportion of the functional group-containing monomer to the total monomer components constituting the acrylic polymer is not particularly limited. Generally, from the viewpoint of achieving a good balance between cohesiveness and tackiness, it is preferable to have a proportion of functional group-containing monomer of about 0.1% by weight or more, for example, it may be about 0.5% by weight or more, or about 1% by weight or more. Furthermore, considering the tackiness due to alkyl (meth)acrylate, in some embodiments, it is preferable to have a proportion of the above functional group-containing monomer of about 40% by weight or less, for example, it may be 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.

[0045] In some preferred embodiments, the monomer component constituting the acrylic polymer includes a carboxyl group-containing monomer. The inclusion of a carboxyl group-containing monomer in the monomer component makes it easier to obtain an adhesive sheet exhibiting good adhesive properties (such as cohesive force). It can also be advantageous in improving the adhesion between the adhesive layer and the adherend. The carboxyl group-containing monomer can be used alone or in combination of two or more. Acrylic acid and methacrylic acid are preferably used as carboxyl group-containing monomers.

[0046] 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 force, etc., in some embodiments, the proportion of the carboxyl group-containing monomer is preferably about 0.1% by weight or more, more preferably about 0.5% by weight or more, even more preferably about 1% by weight or more, and particularly preferably about 1.5% by weight or more. Also, from the viewpoint of adhesive properties such as adhesion strength, in some embodiments, the proportion of the carboxyl group-containing monomer is appropriate to be about 15% by weight or less, may be about 12% by weight or less, and may be, for example, 10% by weight or less. In some preferred embodiments, the proportion of the carboxyl group-containing monomer in the total monomer components is preferably 6% by weight or less or less, more preferably 5% by weight or less or less, even more preferably 4% by weight or less or less (for example, 3.5% by weight or less), may be 3% by weight or less, and may be 2.5% by weight or less. Reducing the proportion of carboxyl group-containing monomers in this way can be advantageous in terms of suppressing the hygroscopicity of adhesives using water-dispersible acrylic polymers and better inhibiting the occurrence of low-temperature pinholes.

[0047] In embodiments in which the above monomer component includes a carboxyl group-containing monomer as a functional group-containing monomer, the proportion of the carboxyl group-containing monomer to the total functional group-containing monomers (the total functional group-containing monomers including the carboxyl group-containing monomer) used as a copolymer component of the acrylic polymer is preferably 30% by weight or more, more preferably 50% by weight or more, more preferably 70% by weight or more, even more preferably 80% by weight or more, and particularly preferably 90% by weight or more. For example, it may be 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 to the total functional group-containing monomer is 100% by weight, and may be, for example, 95% by weight or less.

[0048] In some embodiments, the carboxyl group-containing monomer preferably contains acrylic acid (AA). The use of acrylic acid makes it easier to obtain good adhesive properties (adhesion, cohesiveness, etc.) and excellent polymer dispersion stability. In some embodiments, the content of acrylic acid in the monomer component is preferably about 0.1% by weight or more, more preferably about 0.5% by weight or more, and even more preferably about 1% by weight or more. Also, from the viewpoint of adhesive properties such as adhesion, in some embodiments, the proportion of acrylic acid is appropriate to be 10% by weight or less, and may be, for example, 7% by weight or less. In some preferred embodiments, the content of acrylic acid in the monomer component is preferably 5% by weight or less or less than 5% by weight, more preferably 4% by weight or less or less than 4% by weight (e.g., 3.5% by weight or less), more preferably 3% by weight or less, may be 2.5% by weight or less, may be 2% by weight or less (e.g., less than 2% by weight), may be 1.75% by weight or less, and may be 1.5% by weight or less. Reducing the amount of acrylic acid used in this way can be advantageous in terms of suppressing the hygroscopicity and unevenness of said hygroscopicity in adhesives using water-dispersible acrylic polymers, and better suppressing the occurrence of low-temperature pinholes.

[0049] In embodiments where acrylic acid is used as the carboxyl group-containing monomer, the proportion of acrylic acid to the total carboxyl group monomer is appropriately 30% by weight or more, preferably 50% by weight or more (for example, more than 50% by weight), and may be 60% by weight or more. Furthermore, there is no particular upper limit to the proportion of acrylic acid to the total carboxyl group monomer, and in some embodiments, it may be 90% by weight or less, 80% by weight or less, or 70% by weight or less.

[0050] In some preferred embodiments, acrylic acid (AA) and methacrylic acid (MAA) are used in combination as carboxyl group-containing monomers. By using AA and MAA in combination, structures derived from the carboxyl group-containing monomers can be more uniformly present in the adhesive using a water-dispersible acrylic polymer. This can be advantageous in terms of suppressing the hygroscopicity and unevenness of said hygroscopicity of the adhesive using a water-dispersible acrylic polymer, and better suppressing the occurrence of low-temperature pinholes. Furthermore, an adhesive containing an acrylic polymer with such monomer composition (i.e., copolymer composition) may provide a higher-performance adhesive sheet (e.g., one with better rebound 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 higher, even more preferably 0.5 or higher, and particularly preferably 1.0 or higher (e.g., greater than 1.0 or 1.2 or higher). Furthermore, the above weight ratio (AA / MAA) is more preferably approximately 5 or less, for example, it may be 4 or less, 3 or less, or 2 or less. When the AA / MAA ratio is within the above range, it is easier to obtain the effect of more uniformly distributing the structure derived from the carboxyl group-containing monomer within the adhesive layer. In addition, it tends to improve rebound resistance, and the adhesive properties tend to be more stable over time after the adhesive sheet is manufactured.

[0051] Furthermore, it is preferable that the acrylic polymer is copolymerized with a silane monomer. As the silane monomer, an alkoxysilyl group-containing monomer is preferably used. An alkoxysilyl group-containing monomer is typically an ethylenically unsaturated monomer having at least one (preferably two or more, for example, two or three) alkoxysilyl groups in one molecule, and specific examples are as described above. A silane monomer can be used alone or in combination of two or more. By copolymerizing the silane monomer, a crosslinked structure can be introduced into the adhesive containing the acrylic polymer by a condensation reaction of silanol groups (silanol condensation). This can be advantageous from the viewpoint of suppressing the growth of ice crystals in the adhesive and inhibiting the occurrence of low-temperature pinholes. Note that silane monomers are also called silane coupling agents.

[0052] When a silane monomer is copolymerized with an acrylic polymer, the proportion of the silane monomer in the monomer component is appropriately 0.001% by weight or more, preferably 0.005% by weight or more, and more preferably 0.01% by weight or more. In some embodiments, the proportion of the silane monomer in the monomer component is appropriately 0.1% by weight or less, preferably 0.05% by weight or less, and may be, for example, 0.03% by weight or less. By using an appropriate amount of silane monomer, it is easy to obtain the effect of improving appearance quality retention by suppressing the occurrence of low-temperature pinholes.

[0053] In some embodiments, it is preferable that the acrylic polymer is copolymerized with a keto group-containing monomer. Specific examples of keto group-containing monomers are as described above. Keto group-containing monomers can be used individually or in combination of two or more. Keto group-containing monomers tend to be relatively hydrophobic among monomers that can introduce reaction sites with crosslinking agents into water-dispersible acrylic polymers. Therefore, by crosslinking a water-dispersible acrylic polymer into which keto groups have been introduced with a water-soluble crosslinking agent that reacts with the keto groups, a well-balanced crosslinked structure can be formed within and between emulsion particles. This can be advantageous from the viewpoint of suppressing the growth of ice crystals in the adhesive and inhibiting the occurrence of low-temperature pinholes.

[0054] When a keto group-containing monomer is copolymerized with an acrylic polymer, the proportion of the keto group-containing monomer in the monomer component is appropriately set to approximately 0.01% by weight or more, preferably 0.05% by weight or more, and may also be 0.10% by weight or more, or 0.12% by weight or more. Furthermore, in some embodiments, the proportion of the keto group-containing monomer in the monomer component is appropriately set to about 10% by weight or less, preferably 5.0% by weight or less, and may also be 3.0% by weight or less, 1.0% by weight or less, or 0.50% by weight or less, 0.30% by weight or less, or 0.20% by weight or less. By using an appropriate amount of keto group-containing monomer, it is easy to obtain the effect of improving appearance quality retention by suppressing the occurrence of low-temperature pinholes.

[0055] Furthermore, other copolymerization components besides the aforementioned sub-monomers can be used for purposes such as increasing the cohesive strength of acrylic polymers. Such copolymerization components include, for example, vinyl ester monomers such as vinyl acetate and vinyl propionate; aromatic vinyl compounds such as styrene, substituted styrene (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), and arylalkyl ( Examples of copolymerizable monomers include aromatic ring-containing (meth)acrylates such as meth)acrylate (e.g., benzyl (meth)acrylate); olefin 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; vinyl ether monomers such as methyl vinyl ether and ethyl vinyl ether; and others. Other examples of copolymerizable monomers not listed above include monomers having multiple functional groups in a single molecule (polyfunctional monomers).

[0056] The amount of copolymer components other than the above-mentioned sub-monomers is not particularly limited and can be appropriately selected according to the purpose and application. For example, it is preferably 10% by weight or less of the monomer composition of the acrylic polymer, but it may also be 3% by weight or less, or less than 1% by weight (for example, 0% by weight or more and less than 1% by weight).

[0057] While not particularly limited, from the viewpoint of adhesion to the adherend, it is appropriate that the acrylic polymer be designed such that its glass transition temperature (Tg) 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 force, the Tg of the acrylic polymer may be, for example, approximately -75°C or higher, and may be approximately -70°C or higher. The Tg of the acrylic polymer can be adjusted by the type and ratio of monomers used in the synthesis of the polymer.

[0058] The Tg of an acrylic polymer is determined by Fox's formula, based on the composition of the monomer components used in the synthesis of the polymer. Fox's formula is a relationship 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, as shown below. 1 / Tg = Σ(Wi / Tgi) In Fox's equation above, Tg represents the glass transition temperature of the copolymer (unit: K), Wi represents the weight fraction of monomer i in the copolymer (weight-based copolymerization ratio), and Tgi represents the glass transition temperature of the monomer i homopolymer (unit: K).

[0059] The glass transition temperature of the homopolymer used in calculating Tg shall be the value specified in publicly available documents. For example, for the monomers listed below, the following values ​​shall be used as the glass transition temperature of the homopolymer of the monomer. 2-Ethylhexyl acrylate -70℃ n-butyl acrylate -55℃ Methyl methacrylate 105℃ Methyl acrylate 8℃ Vinyl acetate 32℃ Acrylic acid 106℃ Methacrylic acid 228℃

[0060] For the glass transition temperatures of monomer homopolymers other than those exemplified above, the values ​​listed in "Polymer Handbook" (3rd edition, John Wiley & Sons, Inc., 1989) shall be used. If multiple values ​​are listed in this document, the highest value shall be adopted. If the values ​​are not listed in the Polymer Handbook, the values ​​obtained by the measurement method described in Japanese Patent Publication No. 2007-51271 shall be used.

[0061] The acrylic polymers disclosed herein are synthesized by emulsion polymerization. The method of emulsion polymerization is not particularly limited, except that a reactive emulsifier having a radically polymerizable functional group is used, and various monomer supply methods, polymerization conditions, materials used, etc., similar to those of conventionally known general emulsion polymerization can be appropriately employed. For example, as a monomer supply method, a batch supply method in which all monomer raw materials are supplied at once, a continuous supply (dropping) method, a divided supply (dropping) method, etc., can be appropriately employed. The monomer raw materials may also be added dropwise in the form of an aqueous emulsion. The polymerization temperature can be, for example, 20°C or higher (usually 40°C or higher), and it is appropriate to set it to 100°C or lower (usually 80°C or lower).

[0062] The initiator used for 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 consisting of a combination of peroxide and reducing agent can be used. Polymerization initiators can be used individually or in combination of two or more. The amount of polymerization initiator used can be the usual amount and is not particularly limited. For example, it can be selected from a range of about 0.005 parts by weight or more (preferably 0.01 parts by weight or more) and 1 part by weight or less (preferably 0.8 parts by weight or less) per 100 parts by weight of monomer component.

[0063] The emulsion polymerization of acrylic polymers disclosed herein is carried out in the presence of a reactive emulsifier having a radically polymerizable functional group. By emulsion polymerization of monomer raw materials in the presence of a 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, i.e., emulsifier that is easily mobile within the adhesive layer, is reduced. This is thought to suppress the formation of locally highly hygroscopic areas (points where ice crystals form) due to the uneven distribution of emulsifier within the adhesive layer (e.g., uneven distribution at grain boundaries of emulsion particles), thereby reducing the occurrence of low-temperature pinholes. However, the technology disclosed herein is not limited to the above considerations.

[0064] As reactive emulsifiers, for example, those having a structure in which a radically polymerizable functional group is introduced into an anionic surfactant or a nonionic surfactant can be used. Reactive emulsifiers can be used individually or in combination of two or more. The type of radically polymerizable functional group that the reactive emulsifier possesses is not particularly limited and may include, for example, alkenyl groups, acryloyl groups, methacryloyl groups, vinyl groups, vinyl ether groups (vinyloxy groups), allyl ether groups (allyloxy groups), etc. Specific examples of alkenyl groups include propenyl groups and isopropenyl groups (CH2=C(CH3)-). The concept of propenyl groups as used here includes 1-propenyl groups (CH3-CH=CH-) and 2-propenyl groups (CH2=CH-CH2-; sometimes called allyl groups).

[0065] Examples of anionic reactive surfactants include polyoxyethylene (allyloxymethyl) alkyl ether sulfate (e.g., ammonium salt), polyoxyethylene nonylpropenylphenyl ether sulfate (e.g., ammonium salt), alkylallyl sulfosuccinate (e.g., sodium salt), methacryloxypolyoxypropylene sulfate (e.g., sodium salt), and polyoxyalkylene alkenyl ether sulfate (e.g., ammonium salt in which the terminal end of the alkenyl group is an isopropenyl group). When an anionic reactive surfactant forms a salt, the salt may be a metal salt such as a sodium salt, or a nonmetal salt such as an ammonium salt or an amine salt. Examples of nonionic reactive surfactants include polyoxyethylene nonylpropenylphenyl ether.

[0066] Commercially available reactive surfactants include the following product names from Daiichi Kogyo Seiyaku Co., Ltd.: "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", Examples include "Aqualon AR-10", "Aqualon AR-20", "Aqualon AR-1025", "Aqualon AR-2020", ADEKA's product names "Adekaria Soap SE-10N" and "Adekaria Soap SR-1025", Kao's product names "Latemul PD-104", "Latemul PD-420", "Latemul PD-430" and "Latemul PD-450", Sanyo Chemical Industries' product names "Eleminol JS-20" and "Eleminol RS-3000", and Nippon Emulsifier Co., Ltd.'s product name "Antox MS-60".

[0067] In this specification, the term "containing a reactive emulsifier" is used to mean that the reactive emulsifier is included in a state after its radically polymerizable functional group has reacted. In the technologies disclosed herein, the reactive emulsifier is typically included in a water-dispersible adhesive composition or adhesive layer in a form in which at least a portion is incorporated into an acrylic polymer as described above.

[0068] The amount of reactive emulsifier used in emulsion polymerization is not particularly limited. Considering polymerization stability and dispersion stability of the polymerized product, in some embodiments, the amount of reactive emulsifier used is usually appropriate to be 0.1 parts by weight or more per 100 parts by weight of monomer raw material, preferably 0.5 parts by weight or more, and may also be 1.0 part by weight or more, or 1.5 parts by weight or more. Alternatively, the amount of reactive emulsifier used can be, for example, 10 parts by weight or less per 100 parts by weight of monomer raw material. From the viewpoint of limiting the amount of free emulsifier, in some embodiments, the amount of reactive emulsifier used is usually 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. By appropriately setting the amount of reactive emulsifier used within the above range, it is possible to suppress the occurrence of low-temperature pinholes while obtaining the effect of emulsifier content.

[0069] During the polymerization described above, a chain transfer agent (which may also be known as a molecular weight modifier or degree of polymerization modifier) ​​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. Such chain transfer agents can be used individually or in combination of two or more. When using a chain transfer agent, the amount used can be approximately 0.001 parts by weight or more per 100 parts by weight of the monomer component, for example, approximately 0.005 parts by weight or more, or approximately 0.01 parts by weight or more, or for example, approximately 5 parts by weight or less, for example, approximately 1 part by weight or less, or approximately 0.1 parts by weight or less. By setting the amount of chain transfer agent used within an appropriate range, the desired polymerization rate can be obtained.

[0070] The above emulsion polymerization method allows for the preparation of a polymerization solution (acrylic polymer emulsion) in which an acrylic polymer is dispersed in water. Typically, from the viewpoint of dispersion stability, a pH adjusting agent such as aqueous ammonia can be added to the polymerization solution to adjust the pH to an appropriate range (for example, a range of pH 6 to 9).

[0071] (Crosslinking agent) The adhesive layer of the adhesive sheet disclosed herein contains a water-soluble crosslinking agent. The use of a water-soluble crosslinking agent allows for more uniform crosslinking of the adhesive layer containing a water-dispersible acrylic polymer, suppressing the occurrence of locally highly hygroscopic areas (areas that can become sources of low-temperature pinholes) due to uneven crosslinking density, and improving the maintenance of appearance during low-temperature storage. As the water-soluble crosslinking agent, one that is provided as a water-soluble crosslinking agent by a manufacturer, etc., or one that maintains a uniform appearance (no phase separation or turbidity is observed) even after the flow subsides when the crosslinking agent and ion-exchanged water are stirred and mixed in a 1:1 weight ratio at room temperature (e.g., 25°C). Furthermore, unless otherwise specified, the term "crosslinking agent" in this specification refers to a crosslinking agent added after the synthesis of the acrylic polymer (also called an external crosslinking agent). The adhesive layer may contain the above crosslinking agent in the form after the crosslinking reaction, in the form before the crosslinking reaction, in a partially crosslinked form, or in intermediate or complex forms therebetween. Typically, the above crosslinking agent is contained in the adhesive layer exclusively in the form after the crosslinking reaction.

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

[0073] Known carbodiimide crosslinking agents include low-molecular-weight or high-molecular-weight compounds having two or more carbodiimide groups. Examples of commercially available water-soluble carbodiimide crosslinking agents include the CarbodiLite V series (aqueous solution type) manufactured by Nisshinbo Chemical Co., Ltd., such as "CarbodiLite V-02," "CarbodiLite V-02-L2," and "CarbodiLite V-04."

[0074] Epoxy crosslinking agents are compounds having two or more epoxy groups in one molecule, typically having three to five epoxy groups in one molecule. Specific examples of epoxy crosslinking agents include (poly)ethylene glycol diglycidyl ether, glycerol polyglycidyl ether, and polyglycerol polyglycidyl ether. Commercially available epoxy crosslinking agents include "Epiclon CR-5L" from DIC Corporation and "Denacol EX-313," "Denacol EX-512," "Denacol EX-810," "Denacol EX-821," "Denacol EX-830," and "Denacol EX-850" from Nagase ChemteX Corporation. Among these known epoxy crosslinking agents, those that are water-soluble can be used.

[0075] Known oxazoline crosslinking agents include compounds having one or more oxazoline groups in one molecule. The oxazoline group may be a 2-oxazoline group, a 3-oxazoline group, or a 4-oxazoline group. Examples of commercially available water-soluble oxazoline crosslinking agents include the product names "Epocross WS-500" and "Epocross WS-700" manufactured by Nippon Shokubai Co., Ltd.

[0076] Known hydrazide-based crosslinking agents include hydrazino group-containing compounds having two or more hydrazino groups (H2N-NH-) as crosslinkable functional groups. Examples of water-soluble hydrazide-based crosslinking agents include polycarboxylic acid polyhydrazides such as oxalic acid dihydrazide, malonic acid dihydrazide, glutaric acid dihydrazide, succinic acid dihydrazide, and adipic acid dihydrazide, as well as hydantoins such as 1,3-bis(hydrazinocarbonoethyl)-5-isopropylhydantoin.

[0077] Examples of known aziridine crosslinking agents include trimethylolpropantris[3-(1-aziridinyl)propionate] and trimethylolpropantris[3-(1-(2-methyl)aziridinylpropionate)]. A commercially available aziridine crosslinking agent is, for example, "Chemitite PZ-33" manufactured by Nippon Shokubai Co., Ltd. Among these known aziridine crosslinking agents, those that are water-soluble can be used.

[0078] Examples of known isocyanate crosslinking agents include bifunctional or polyfunctional isocyanate compounds. Blocked isocyanate crosslinking agents, in which the isocyanate group is blocked, may also be used. Commercially available products include the "Barnock DNW" series from DIC Corporation, the "Aquanate" series from Tosoh Corporation, the "Takenate WD" series from Mitsui Chemicals, and the "Elastron BN" series from Daiichi Kogyo Seiyaku Co., Ltd. Among these known isocyanate crosslinking agents, those that are water-soluble can be used.

[0079] In some preferred embodiments, the water-soluble crosslinking agent used 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 polyfunctional 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, and carbodiimide-based crosslinking agents, epoxy-based crosslinking agents, and oxazoline-based crosslinking agents are even more preferred. These water-soluble crosslinking agents tend to favorably balance the effect of suppressing low-temperature pinholes with other adhesive properties (e.g., adhesive strength).

[0080] The content of the water-soluble crosslinking agent is not particularly limited. For example, the content of the water-soluble crosslinking agent can be in the range of approximately 0.001 parts by weight to approximately 20 parts by weight per 100 parts by weight of the acrylic polymer. From the viewpoint of suppressing the occurrence of low-temperature pinholes, in some embodiments, the content of the water-soluble crosslinking agent per 100 parts by weight of the acrylic polymer is appropriate to be more than approximately 0.01 parts by weight, and from the viewpoint of making it easier to obtain a better low-temperature pinhole suppression effect by facilitating the progress of crosslinking, it is advantageous to be more than approximately 0.05 parts by weight, preferably more than approximately 0.1 parts by weight or more, and more preferably more than approximately 0.3 parts by weight or more. By using a moderately large amount of water-soluble crosslinking agent, the crosslinking density of the adhesive can be increased, suppressing the coarsening of ice crystals during low-temperature storage, and the effect of suppressing low-temperature pinholes can be better exhibited. In some embodiments, the content of the water-soluble crosslinking agent may be approximately 0.5 parts by weight or more, or approximately 0.8 parts by weight or more, 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. Also, 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 preferably achieve both suppression of low-temperature pinholes and adhesiveness. 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 per 100 parts by weight of the acrylic polymer may be approximately 1 part by weight or less (e.g., less than 1 part by weight), or less than 0.1 parts by weight.

[0081] In some embodiments, the 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 good crosslinked structure within and between the acrylic polymer particles, enabling more uniform crosslinking of the adhesive layer. This suppresses the occurrence of areas that can become sources of low-temperature pinholes due to uneven crosslinking density, thereby better maintaining appearance quality. In such embodiments, the ratio of the amount of crosslinking agent C to the amount of silane monomer S (weight ratio (C / 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 larger the weight ratio (C / S), the more favorably the effect of the crosslinked structure based on the crosslinking agent can be exhibited. Furthermore, in some embodiments, the above 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 above weight ratio (C / S), the more favorably the effect of the crosslinked structure based on the silane monomer can be exhibited.

[0082] (Adhesive-forming resin) In some embodiments, the adhesive layer can contain a tackifying resin. This can enhance the adhesive strength of the adhesive sheet. 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 individually or in combination of two or more.

[0083] Examples of the rosin-based tackifying resins mentioned above include rosins such as gum rosin, wood rosin, and tall oil rosin, as well as stabilized rosins (for example, stabilized rosins obtained by disproportionation or hydrogenation treatment of the above rosins), polymerized rosins (for example, polymers of the above rosins, typically dimers), and modified rosins (for example, unsaturated acid-modified rosins modified with unsaturated acids such as maleic acid, fumaric acid, and (meth)acrylic acid). Examples of the rosin derivative tackifying resins mentioned above include esterified rosin resins (for example, rosin esters such as stabilized rosin esters and polymerized rosin esters), phenol-modified rosin resins (phenol-modified rosin), and their esterified products (phenol-modified rosin esters). Examples of the above-mentioned petroleum-based tackifying resins include aliphatic petroleum resins, aromatic petroleum resins, copolymer petroleum resins, alicyclic petroleum resins, and their hydrides. Examples of the terpene-based tackifying resins mentioned above include α-pinene resin, β-pinene resin, aromatically modified terpene resin, and terpene phenol resin. Examples of the ketone-based tackifying resins mentioned above include ketone-based resins formed by the 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. Preferred examples of rosin-based tackifying resins include stabilized rosin esters and polymerized rosin esters. Preferred examples of terpene-based tackifying resins include terpene phenol-based resins.

[0085] The softening point of the tackifying resin used is not particularly limited. From the viewpoint of improving cohesive force, the softening point of the tackifying resin may be, for example, 80°C or higher, or 90°C or higher. In some embodiments, the softening point of the tackifying resin is preferably 100°C or higher, more preferably 120°C or higher, and even more preferably 130°C or higher. In some preferred embodiments, the tackifying resin may include a high-softening-point tackifying resin having a softening point of 140°C or higher. The softening point of the above high-softening-point tackifying resin may be 145°C or higher, for example, 150°C or higher, or 155°C or higher. By using the above high-softening-point tackifying resin, both tackiness and cohesiveness can be suitably achieved. There is no particular upper limit to the softening point of the tackifying resin, but from the viewpoint of compatibility and low-temperature characteristics, it is usually appropriate to 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 tackifying resin referred to herein is defined as the value measured based on the softening point test method (ring-ball method) specified in JIS K5902 and JIS K2207.

[0087] Some preferred embodiments include the tackifying resin comprising one or more terpene-based tackifying resins (typically terpene phenol resins). The techniques disclosed herein can preferably be implemented, for example, in an embodiment where, with a total amount of tackifying resin of 100% by weight, approximately 25% by weight or more (more preferably approximately 30% by weight or more) is a terpene phenol resin. Approximately 50% by weight or more of the total amount of tackifying resin may be terpene phenol resins, and approximately 80% by weight or more (for example, approximately 90% by weight or more) may be terpene phenol resins. Substantially all of the tackifying resin (for example, approximately 95-100% by weight, and even more precisely, approximately 99-100% by weight) may be terpene phenol resins.

[0088] In some other embodiments, the tackifying resin includes a rosin-based tackifying resin. For example, if the total amount of the tackifying resin is 100% by weight, then approximately 25% by weight or more (more preferably approximately 30% by weight or more) of it may be a rosin-based tackifying resin, or approximately 50% by weight or more of the total amount of the tackifying resin may be a rosin-based tackifying resin, or approximately 80% by weight or more (for example, approximately 90% by weight or more) may be a rosin-based tackifying resin. Substantially all of the tackifying resin (for example, approximately 95-100% by weight, and even more precisely, approximately 99-100% by weight) may be a rosin-based tackifying resin.

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

[0090] Such tackifying resin emulsions may be prepared using surfactants (emulsifiers) as needed. One or more surfactants can be appropriately selected from those used in the preparation of acrylic polymer emulsions. Generally, the use of anionic or nonionic surfactants is preferred. The amount of surfactant used is not particularly limited as long as it is sufficient to prepare the tackifying resin into an emulsion. For example, it can be about 0.2 parts by weight or more (preferably 0.5 parts by weight or more) per 100 parts by weight (based on solid content) of tackifying resin, or about 10 parts by weight or less (preferably 5 parts by weight or less).

[0091] When the adhesive layer contains a tackifying resin, the content of the tackifying resin is not particularly limited. In some embodiments, from the viewpoint of suitably exhibiting its effects, the content of the tackifying resin (based on solid content) is usually appropriate to be approximately 1 part by weight or more per 100 parts by weight of acrylic polymer, but may also be approximately 5 parts by weight or more, or approximately 10 parts by weight or more. In some preferred embodiments, the content of the tackifying resin per 100 parts by weight of acrylic polymer is approximately 15 parts by weight or more, more preferably approximately 20 parts by weight or more, even more preferably approximately 25 parts by weight or more, particularly preferably approximately 30 parts by weight or more, and may also be approximately 35 parts by weight or more. Furthermore, from the viewpoint of improving the retention of appearance quality during low-temperature storage (prevention of low-temperature pinholes), in some embodiments, the content of the tackifying resin per 100 parts by weight of acrylic polymer may be, for example, approximately 80 parts by weight or less, preferably 60 parts by weight or less, preferably approximately 50 parts by weight or less, more preferably 45 parts by weight or less, even more preferably 40 parts by weight or less, and may also be 30 parts by weight or less, 20 parts by weight or less, or 15 parts by weight or less. By limiting the amount of tackifying resin added, the amount of additives such as emulsifiers that may inevitably be included in conjunction with the tackifying resin is also reduced, and a decrease in appearance quality retention caused by such additives can be prevented or suppressed. In addition, by keeping the amount of tackifying resin used within an appropriate range, the effect of improving adhesive strength can be effectively obtained. In some other embodiments, the content of the tackifying resin per 100 parts by weight of acrylic polymer may be approximately 10 parts by weight or less (for example, less than 10 parts by weight), less than 5 parts by weight, less than 3 parts by weight, or less than 1 part by weight. The technology disclosed herein can be implemented in a manner in which the adhesive layer substantially does not contain a tackifying resin.

[0092] (Pigment) The adhesive layer disclosed herein may contain pigments as needed. By including an appropriate amount of pigment, the adhesive layer can be given properties and functions such as light shielding, concealment of the adherend, adjustment of the appearance of the adherend through the adhesive sheet, and design properties. As pigments, 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 dioxide, silica, lithium fluoride, calcium fluoride, barium sulfate, alumina, zirconia, iron oxide-based, iron hydroxide-based, chromium oxide-based, spinel-type calcined-based, chromic acid-based, chromium vermilion-based, Prussian blue-based, aluminum powder-based, bronze powder-based, silver powder-based, and calcium phosphate can be used, as well as organic pigments such as phthalocyanine-based, azo-based, condensed azo-based, azo lake-based, anthraquinone-based, perylene / perinone-based, indigo-based, thioindigo-based, isoindolinone-based, azomethine-based, dioxazine-based, quinacridone-based, aniline black-based, and triphenylmethane-based pigments can be used. Black pigments such as aniline black, perylene black, titanium black, and cyanine black may be used as pigments. Among these, pigments containing inorganic particles (typically inorganic pigments) are preferred. Note that the term "inorganic pigment" encompasses all pigments that are mainly composed of inorganic materials and is not limited to those composed solely of inorganic materials. Pigments can be used individually or in combination of two or more.

[0093] From the viewpoint of light shielding and light transmission reduction, 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, and chromium complexes are preferably used. The black pigments can be used individually or in combination of two or more.

[0094] In some preferred embodiments, carbon black is used as the pigment. By using carbon black, light transmittance can be efficiently reduced, and a highly light-shielding adhesive can be preferably obtained. As carbon black, any type generally referred to as carbon black (furnace black, channel black, acetylene black, thermal black, lamp black, pine soot, etc.) can be used without particular limitation. From the viewpoint of maintaining appearance quality, it is preferable to use surface-modified carbon black having functional groups such as carboxyl groups, amino groups, sulfonic acid groups, and silicon-containing groups (e.g., alkoxysilyl groups, alkylsilyl groups) as the carbon black. Such surface-modified carbon black is also called self-dispersing carbon black, and the addition of dispersants can be eliminated or reduced. The above carbon black can be used alone or in combination of two or more types. Self-dispersing carbon black can be manufactured, for example, based on the methods described in JP 2017-171732 and JP 2018-30968 and the common technical knowledge of those skilled in the art, or commercially available products can be used.

[0095] Furthermore, in some embodiments, the pigments include metal oxides such as titanium dioxide (rutile-type titanium dioxide, anatase-type titanium dioxide, etc.), 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, etc.), barium carbonate, and zinc carbonate; and hydroxyl compounds such as aluminum hydroxide, calcium hydroxide, magnesium hydroxide, and zinc hydroxide. Inorganic materials such as silicic acid compounds (aluminum silicate, magnesium silicate, calcium silicate, etc.), 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, hydrated halloysite, etc., as well as organic materials such as acrylic resins, polystyrene resins, polyurethane resins, amide resins, polycarbonate resins, silicone resins, urea-formaldehyde resins, and melamine resins can be used. These can be used as non-black pigments, such as white. The above non-black pigments can be used individually 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 of the amount of black pigment A to the amount of non-black pigment B (weight ratio (A / 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 3000 nm or less, and may be approximately 1000 nm or less. From the viewpoint of improving light shielding properties, the volume-average particle size of the pigment is suitable to be approximately 500 nm or less, preferably approximately 300 nm or less, more preferably approximately 250 nm or less, and even more preferably 200 nm or less (for example, approximately 150 nm or less). Also, 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, or 80 nm or more.

[0098] The volume-average particle diameter of the pigment is the volume-average particle diameter based on the laser diffraction-scattering method. Specifically, for a dispersion containing the pigment, it refers to the particle size at 50% of the cumulative value of the particle size distribution measured using a particle size distribution analyzer based on the laser scattering-diffraction method (50% volume-average particle diameter). As a measuring device, for example, the "Microtrac MT3000II" product manufactured by Microtrac-Bell or an equivalent product can be used.

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

[0100] [Measurement of pigment particle size in adhesive layer] The adhesive sample is rapidly frozen in a liquid nitrogen atmosphere, and using an ultramicrotome (Leica model "UC7"), the sample is cut to a thickness of approximately 100 nm in a freezing atmosphere at -30°C to obtain ultrathin sections. The obtained ultrathin sections are observed using a transmission electron microscope (TEM; Hitachi High-Technologies Corporation, accelerating voltage 100 kV). One field of view (6 μm × 6 μm square) of the TEM image, magnified approximately 3,000 times, is subjected to image processing (binarization) to identify particles, and the area fraction of each particle is calculated for all identified particles. Then, the equivalent diameter of a circle is calculated from the area of ​​each particle. The equivalent diameter of a circle refers to the diameter of a circle (true circle) that has the same area as the area of ​​the particle being measured. This operation is performed in four different fields of view in the TEM image (N=4), and the particles classified by equivalent diameter are histogrammed on a number basis to obtain the particle size distribution (number basis). The number of particles used as the basis for calculating the particle size distribution is determined by counting the number of particles present within the field of view described above. From the obtained particle size distribution, the average particle size (TEM average particle size) [nm] based on TEM observation is calculated. When identifying particles, particles that fall at the edges of the image are excluded from the analysis. Image analysis software such as imageJ can be used.

[0101] The form in which the pigment is added is not particularly limited; for example, the pigment is preferably incorporated into the adhesive composition in the form of a dispersion containing the pigment. A preferred dispersion is one in which at least a portion of the pigment is dispersed in water or an aqueous liquid mainly composed of water (typically an aqueous dispersion). By incorporating a dispersion containing at least a portion of the pigment into the adhesive composition to form an adhesive layer, a high-quality appearance with minimal color unevenness is easily obtained. The pigment dispersion is preferably used in the form of a dispersion that is liquid at room temperature (e.g., 23°C).

[0102] The pigment content in the adhesive layer can be set considering the dispersibility of the pigment in the adhesive layer, the thickness of the adhesive layer, the optical properties to be achieved (light transmission such as light shielding), the design, and other required adhesive properties. In some embodiments, the pigment content (on a solid content basis) in the adhesive layer (e.g., black pigment, more specifically carbon black) is appropriately about 0.01 parts by weight or more per 100 parts by weight of acrylic polymer. From the viewpoint of effectively obtaining the effects of pigment addition (e.g., light shielding and design), it is preferably about 0.1 parts by weight or more, more preferably about 1 part by weight or more, and may be about 3 parts by weight or more, or about 4 parts by weight or more, or about 4.5 parts by weight or more. In some embodiments, the pigment content in the adhesive layer may be about 5 parts by weight or more (e.g., more than 5 parts by weight) per 100 parts by weight of acrylic polymer, or it may be 7 parts by weight or more, or 9 parts by weight or more. Adhesives containing a large amount of pigment in this way are suitable for applications where high light shielding is required. Furthermore, in some embodiments, the pigment content may be approximately 30 parts by weight or less per 100 parts by weight of acrylic polymer, preferably approximately 15 parts by weight or less, more preferably 10 parts by weight or less (for example, less than 10 parts by weight), from the viewpoint of better suppressing the occurrence of low-temperature pinholes, it may be approximately 8 parts by weight or less, approximately 6 parts by weight or less, or approximately 5 parts by weight or less (for example, less than 5 parts by weight). In some embodiments, the pigment content may 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 parts by weight or less per 100 parts by weight of acrylic polymer. The technology disclosed herein can also be suitably implemented in embodiments in which the adhesive layer does not contain pigment.

[0103] (Dispersant) The 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 self-dispersing carbon black described above is used as the pigment, a configuration without a dispersant can be preferably adopted. The dispersant is not particularly limited, but one or more types can be appropriately selected from known or conventional dispersants depending on the purpose. For example, when a pigment is used, one or more suitable types that can disperse the pigment well in the adhesive layer can be used. For example, one or more types that show good pigment dispersion performance can be appropriately selected and used from the following anionic, cationic, nonionic, and amphoteric surfactants and polymer compounds (which may be resins).

[0104] Examples of anionic surfactants used as dispersants include alkyl sulfates such as lauryl sulfate and octadecyl sulfate; fatty acid salts; polysulfonates; polycarboxylates; alkylbenzene sulfonates such as nonylbenzenesulfonate and dodecylbenzenesulfonate; naphthalene sulfonates such as dodecylnaphthalenesulfonate; naphthalene sulfonic acid formalin condensates; alkyldiphenyl ether disulfonates such as dodecyldiphenyl ether disulfonate; polyoxyethylene alkyl ether sulfates such as polyoxyethylene octadecyl ether sulfate and polyoxyethylene lauryl ether sulfate; polyoxyethylene alkylphenyl ether sulfates such as polyoxyethylene laurylphenyl ether sulfate; polyoxyethylene styrene 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 an anionic surfactant forms a salt, the salt may be a metal salt (preferably a monovalent metal salt) such as a sodium salt, potassium salt, calcium salt, or magnesium salt, an ammonium salt, or an amine salt. These anionic surfactants can be used individually or in combination of two or more.

[0105] Cationic surfactants used as dispersants include alkylamine salts and quaternary ammonium salts. Specific examples include stearylamine acetate, trimethyl coconut ammonium chloride, trimethyl beef tallow ammonium chloride, dimethyl dioleyl ammonium chloride, methyl oleyl diethanol chloride, tetramethyl ammonium chloride, laurylpyridinium chloride, laurylpyridinium bromide, laurylpyridinium disulfate, cetylpyridinium bromide, 4-alkyl mercaptopyridine, poly(vinylpyridine)-dodecyl bromide, and dodecylbenzyltriethylammonium chloride. These cationic surfactants can be used individually 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 triisostearate, polyoxyethylene sorbitan monooleate, and polyoxyethylene sorbitan trioleate; polyoxyethylene glyceryl ether fatty acid esters; and polyoxyethylene-polyoxypropylene block copolymers. These nonionic surfactants can be used individually or in combination of two or more.

[0107] Examples of amphoteric surfactants used as dispersants include alkylbetaines such as alkyldimethylaminoacetic acid betaine; aminocarboxylate salts; alkylimidazolines; and the like. These amphoteric surfactants can be used individually or in combination of two or more.

[0108] Furthermore, resins (which may be polymer 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 acid; styrene copolymers such as (meth)acrylic acid-styrene copolymers and styrene-maleic acid copolymers; polyvinyl alcohol; polyvinylpyrrolidone; polysiloxanes; polyalkylene oxide derivatives such as ethylene oxide-propylene oxide adduct compounds; phosphate ester resins; long-chain polyaminoamide phosphates; and modified versions thereof. These resins can be used individually or in combination of two or more.

[0109] The form in which the dispersant is added is not particularly limited; it may be included in the pigment dispersion before being incorporated into the adhesive composition, or it may be supplied at the same time as the addition of the pigment to the adhesive composition, or before or after the addition of the pigment.

[0110] The amount of dispersant in the adhesive layer can be set to an appropriate range depending on the purpose, for example, taking into account the dispersibility of the pigment when a pigment is used. In some embodiments, the amount of dispersant per 100 parts by weight of pigment in the adhesive layer can be approximately 0.01 parts by weight or more (for example, approximately 0.1 parts by weight or more), and it is appropriate to be approximately 1 part by weight or more, for example, it may be approximately 2 parts by weight or more, for example, it may be approximately 3 parts by weight or more, or it may be approximately 4 parts by weight or more. Also, in some embodiments, the amount of dispersant per 100 parts by weight of pigment in the adhesive layer can be approximately 100 parts by weight or less, it is appropriate to be approximately 60 parts by weight or less, it may be approximately 30 parts by weight or less, or it may be approximately 10 parts by weight or less. By limiting the amount of dispersant used, it is possible to prevent or suppress a decrease in appearance quality retention caused by the content of dispersant. From this viewpoint, in some embodiments, the amount of the above-mentioned dispersant per 100 parts by weight of pigment in the adhesive layer may be less than 5 parts by weight, less than 3 parts by weight, or less than 1 part by weight. The technology disclosed herein can preferably be carried out in a manner in which the adhesive layer is substantially free of a dispersant.

[0111] (Thickening agent) In some embodiments, the adhesive layer contains a thickening agent. By including a thickening agent, the coatability of the adhesive composition can be improved, making it easier to form an adhesive layer of good quality. In this specification, a thickening agent refers to a component that, when added to an aqueous dispersion in which at least a portion of the adhesive-forming material is dispersed in water, has the effect of increasing the viscosity (thickening effect) compared to when the component is not added. The thickening agent is not particularly limited and examples include polyacrylic acids, carboxylic acid copolymers, urethane compounds, polyvinyl alcohols, celluloses (e.g., hydroxycellulose), polyethers (e.g., polyethylene glycol), etc. These can be used individually or in combination of two or more. For example, the use of polyacrylic acids and carboxylic acid copolymers is preferred. As the carboxylic acid copolymers, copolymers containing carboxyl group-containing acrylic monomers as monomer units are used.

[0112] In embodiments where the adhesive layer contains a thickener, the content of the thickener in the adhesive layer is appropriately set to obtain the desired viscosity characteristics. In some embodiments, the content of the thickener is suitable to be 0.01 parts by weight or more per 100 parts by weight of the acrylic polymer, and from the viewpoint of effectively obtaining the viscosity adjustment effect, 0.1 parts by weight or more is preferred, 0.5 parts by weight or more is more preferred, and 0.8 parts by weight or more is even more preferred. Also, in some embodiments, the content of the thickener is, for example, approximately 20 parts by weight or less per 100 parts by weight of the acrylic polymer, suitable to be 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 additives) The adhesive layer may optionally contain various additives, as long as the effects of the present invention are not significantly impaired. Examples of such optional additives include pH adjusters, leveling agents, crosslinking aids, release modifiers, plasticizers, softeners, fillers, rust inhibitors, preservatives, fungicides, antistatic agents, anti-aging agents, UV absorbers, antioxidants, and light stabilizers. Such additives can be conventionally used by ordinary methods and do not particularly characterize the present invention, so a detailed explanation is omitted.

[0114] (Adhesive composition) The adhesive layer typically consists of an adhesive formed from a water-dispersible adhesive composition. A water-dispersible adhesive composition typically contains water or a water-based mixed solvent or dispersion medium (aqueous solvent or aqueous dispersion medium) as a dispersion medium. Typically, the proportion of water in the volatile components of a water-dispersible adhesive composition is approximately 90% by weight or more, preferably 95-100% by weight. From the viewpoint of environmental considerations and organic solvent elimination, in some embodiments, it is preferable that the water-dispersible adhesive composition is substantially free of organic solvents. Here, "substantially free of organic solvents" means that the amount of organic solvent in the adhesive composition is less than 1% by weight (e.g., less than 0.1% by weight). A water-dispersible adhesive composition can be prepared, for example, by mixing other components (such as a pigment dispersion) with an aqueous dispersion of an acrylic polymer. As the aqueous dispersion of the acrylic polymer, for example, a polymerization reaction solution obtained by emulsion polymerization, or a polymerization reaction solution that has been treated as necessary, such as pH adjustment (e.g., neutralization), adjustment of non-volatile content, or adjustment of viscosity, can be used.

[0115] (Formation of the adhesive layer) The adhesive layer disclosed herein can be formed by conventionally known methods. For example, a method can be employed in which an adhesive layer is formed by applying an adhesive composition to a release surface and drying it. In the case of an adhesive sheet having a support substrate, for example, a method can be employed in which an adhesive layer is formed by directly applying (typically coating) the adhesive composition to the support substrate and drying it (direct method). Alternatively, a method can be employed in which an adhesive layer is formed on a release surface by applying an adhesive composition to the release surface and drying it, and then the adhesive layer is transferred to the support substrate (transfer method). As the release surface, for example, the surface of a release film described later can be preferably used. The adhesive layer disclosed herein is typically formed continuously.

[0116] The adhesive composition can be applied using conventionally known coaters such as gravure roll coaters, die coaters, reverse roll coaters, kiss roll coaters, dip roll coaters, bar coaters, knife coaters, and spray coaters. Alternatively, the adhesive composition may be applied by impregnation or curtain coating methods. From the viewpoint of promoting the crosslinking reaction and improving manufacturing efficiency, it is preferable to dry the adhesive composition under heating. The drying temperature can be, for example, around 40 to 150°C, and is usually preferably around 60 to 130°C. After drying the adhesive composition, aging may be performed for the purpose of adjusting the migration of components within the adhesive layer, promoting the crosslinking reaction, and alleviating any strain that may exist within the adhesive layer.

[0117] (Thickness of the adhesive layer) The thickness of the adhesive layer is not particularly limited. Depending on the application and intended use, a configuration having an adhesive layer with an appropriate thickness of, for example, 300 μm or less may be adopted. From the viewpoint of avoiding the adhesive sheet becoming excessively thick, in some embodiments, the thickness of the adhesive layer is preferably about 100 μm or less, more preferably about 70 μm or less, and even more preferably about 50 μm or less. In some preferred embodiments, the thickness of the adhesive layer can be about 35 μm or less, for example, about 25 μm or less, about 20 μm or less, about 15 μm or less, or about 10 μm or less (for example, less than 10 μm). An adhesive layer with limited thickness can well meet the demands for thinning and weight reduction. The adhesive layer disclosed herein has a gel fraction limited to less than 90%, making it easier to obtain good adhesive properties (adhesion strength, etc.) even in a configuration with limited thickness. On the other hand, in thin adhesive layers, pinholes and the like tend to be more noticeable, but the adhesive sheet disclosed herein is less prone to pinholes and the like even when stored in a low-temperature environment, and can therefore be preferably applied to configurations that have a thin adhesive layer as described above. Furthermore, in some embodiments, it is advantageous for the thickness of the adhesive layer to be approximately 1 μm or more, appropriate to be 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 also be approximately 15 μm or more. By making the adhesive layer thicker than the predetermined thickness, it is easier to obtain adhesive strength suitable for fixing members, etc. In some embodiments, the thickness of the adhesive layer may be 20 μm or more, 30 μm or more, or 40 μm or more. In a double-sided adhesive sheet with a substrate having a first adhesive layer and a second adhesive layer on each side of the substrate, the first adhesive layer and the second adhesive layer may be the same thickness or may have different thicknesses.

[0118] (Gel fraction of the adhesive layer) In the adhesive sheet disclosed herein, the gel fraction of the adhesive layer is less than 90%. According to the technology disclosed herein, in a configuration having an adhesive layer with such a limited gel fraction, the occurrence of low-temperature pinholes can be suppressed and the appearance retention of the adhesive sheet during low-temperature storage can be improved. By keeping the gel fraction of the adhesive layer from being too high, it becomes easier to obtain adhesive strength to the adherend (for example, adhesive strength suitable for fixing a component). In some embodiments, it is advantageous for the gel fraction of the adhesive layer to be less than 85%, preferably 80% or less (e.g., less than 80%), more preferably 77% or less, and may also be 75% or less, 73% or less, 70% or less, 65% or less, 60% or less, or 55% or less. Furthermore, from the viewpoint of easily obtaining appropriate cohesive force, the gel fraction of the adhesive is appropriate to be 10% or more, and preferably 20% or more. From the viewpoint of increasing the crosslinking density of the adhesive and suppressing the coarsening of ice crystals during low-temperature storage, in some embodiments, the gel fraction of the adhesive layer is appropriately 25% or more, advantageously 35% or more, preferably 40% or more, may be 45% or more, may be 50% or more, may be 60% or more, may be 65% or more, and may be 70% or more. The gel fraction of the adhesive layer can be measured by the method described in the examples below.

[0119] The light transmittance of the adhesive layer may vary depending on the intended use of the adhesive sheet and whether or not the adhesive layer contains pigment. In some embodiments, the light transmittance of the adhesive layer at a wavelength of 550 nm (550 nm light transmittance) is approximately 80% or less. For example, by including a pigment in the adhesive layer, an adhesive layer with reduced light transmittance can be formed. The 550 nm light transmittance of the 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, approximately 30% or less, approximately 20% or less, or 10% or less. In some preferred embodiments, the 550 nm light transmittance of the adhesive layer 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. An adhesive sheet having an adhesive layer exhibiting the above-mentioned 550nm light transmittance can exhibit excellent light-shielding and substrate concealment properties. In some embodiments, the lower limit of the 550nm light transmittance of the adhesive layer may be 0.1% or more, or 0.3% or more, from the viewpoint of productivity and other properties such as adhesive strength. In some embodiments, the 550nm light transmittance of the adhesive layer may be 1% or more, 3% or more, or 5% or more. Furthermore, for adhesive sheets for applications where the light transmittance of the adhesive layer is not required or transparency is required, the 550nm light transmittance of the adhesive layer is appropriately over 70%, preferably over 80% (e.g., over 82%), more preferably over 85%, and may be over 90%. In adhesive sheets for such applications, there is no particular upper limit to the 550nm light transmittance of the adhesive layer. For example, in some embodiments, the 550nm light transmittance of the adhesive layer may be 98% or less, or 95% or less. The 550nm light transmittance of the adhesive layer can be measured in the same manner as the 550nm light transmittance of the adhesive sheet described in the examples below.

[0120] <Supporting base material> In embodiments where the adhesive sheet disclosed herein is in the form of a single-sided adhesive type or a double-sided adhesive type adhesive sheet with a substrate, the substrate supporting the adhesive layer can be a resin film, paper, cloth, rubber sheet, foam sheet, metal foil, or a composite thereof. Examples of paper include Japanese paper, kraft paper, glassine paper, fine paper, synthetic paper, and topcoat paper. Examples of cloth include woven or nonwoven fabrics made from various fibrous materials individually or in blends. Examples of the above-mentioned fibrous materials include cotton, rayon, 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.

[0121] The term "nonwoven fabric" as used herein primarily refers to nonwoven fabrics used for adhesive sheets, particularly in the field of adhesive tapes and other adhesive sheets. Typically, this refers to nonwoven fabrics produced using general-purpose paper machines (sometimes referred to as "paper"). Furthermore, the term "resin film" as used herein typically refers to a non-porous resin sheet, distinct from (i.e., excluding) nonwoven fabrics and woven fabrics. Such resin films may also be non-foamed. Here, a non-foamed resin film refers to a resin film that has not undergone any intentional treatment to become foamed. Specifically, a non-foamed resin film may have a foaming ratio of less than 1.1 times (e.g., less than 1.05 times, typically less than 1.01 times). The above-mentioned resin film may be an unoriented film, a uniaxially oriented film, or a biaxially oriented film.

[0122] As the support substrate constituting the adhesive sheet with a substrate, a base film containing a resin film can preferably be used. The base film is typically an independently shape-retaining (independent) component. The support substrate in the art disclosed herein may be substantially composed of such a base film. Alternatively, the support substrate may include auxiliary layers in addition to the base film. Examples of such auxiliary layers include a colored layer, a reflective layer, an undercoat layer, an antistatic layer, etc., provided on the surface of the base film.

[0123] The above-mentioned resin film is a film whose main component is a resin material (for example, a component that is present in the resin film in an amount exceeding 50% by weight). Examples of resin films include polyolefin resin films such as polyethylene (PE), polypropylene (PP), and ethylene-propylene copolymer; polyester resin films such as polyethylene terephthalate (PET), polybutylene terephthalate (PBT), and polyethylene naphthalate (PEN); vinyl chloride resin films; vinyl acetate resin films; polyimide resin films; polyamide resin films; fluororesin films; cellophane; and the like. The resin film may also be a rubber film such as natural rubber film or butyl rubber film. Among these, polyester films are preferred from the viewpoint of handling and processability, and PET films are particularly preferred among them.

[0124] The resin film can contain a coloring agent. This allows for adjustment of the light transmittance (light-blocking properties, etc.) of the resin film. Adjusting the light transmittance (e.g., vertical light transmittance) of the resin film can also be useful in adjusting the light transmittance of the substrate containing the resin film, and further, the light transmittance of the adhesive sheet containing the substrate. Conventional pigments and dyes can be used as the coloring agent. The color of the coloring agent is not particularly limited.

[0125] In some embodiments, black colorants can be preferably used because the light-shielding properties (e.g., vertical light transmittance) can be efficiently adjusted with a small amount of colorant. Specific examples of black colorants include those exemplified as black pigments that can be contained in the adhesive layer. Although not particularly limited, in some embodiments, pigments with a volume-average particle diameter of 10 nm to 500 nm, more preferably 10 nm to 120 nm (e.g., particulate black colorants such as carbon black) can be used. In some other embodiments, the resin film may contain a white colorant such as titanium dioxide. The amount of colorant used in the resin film is not particularly limited and can be adjusted as appropriate to impart the desired optical properties. The amount of colorant used is usually appropriate to be about 0.1 to 30% by weight of the resin film, for example, 0.1 to 25% by weight (typically 0.1 to 20% by weight).

[0126] The above resin film may contain various additives as needed, such as fillers (inorganic fillers, organic fillers, etc.), dispersants (surfactants, etc.), anti-aging agents, antioxidants, UV absorbers, antistatic agents, lubricants, and plasticizers. The proportion of each additive is usually less than 30% by weight (for example, less than 20% by weight, typically less than 10% by weight).

[0127] The above-mentioned resin film may have a single-layer structure, or it may have a multilayer structure of two, three, or more layers. From the viewpoint of shape stability, a single-layer structure is preferred for the resin film. In the case of a multilayer structure, it is preferable that at least one layer (preferably all layers) is a layer having a continuous structure of the above-mentioned resin (e.g., polyester resin). The method for manufacturing the resin film is not particularly limited and may be any conventionally known method as appropriate. For example, conventionally known general film molding methods such as extrusion molding, inflation molding, T-die casting, and calender roll molding can be used as appropriate.

[0128] The support substrate may be colored by a colored layer disposed on the surface of a base film (preferably a resin film). In a substrate having a base film and a colored layer as described above, the base film may or may not contain a coloring agent. The colored layer may be disposed on either one surface of the base film, or on both surfaces. In a configuration where colored layers are disposed on both surfaces of the base film, the composition of these colored layers may be the same or different.

[0129] The colored layer may be a single-layer structure consisting of one layer, or it may be a multilayer structure including two, three or more sub-colored layers. A multilayer colored layer 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 colored layers intended to provide light-shielding properties, a multilayer structure is particularly beneficial from the viewpoint of preventing the occurrence of pinholes and improving the reliability of light leakage prevention.

[0130] The overall thickness of the colored layer is usually appropriate to be 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 containing two or more sub-colored layers, the thickness of each sub-colored layer is usually preferably about 1 μm to 2 μm.

[0131] The thickness of the support substrate is not particularly limited. From the viewpoint of avoiding the adhesive sheet becoming excessively thick, the thickness of the support substrate can be, for example, approximately 200 μm or less (for example, approximately 100 μm or less). Depending on the purpose and manner of use of the 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 (for example, approximately 5 μm or less). There is no particular lower limit to the thickness of the support substrate. From the viewpoint of the handling and processability of the adhesive sheet, the thickness of the support substrate is usually appropriate to be approximately 2 μm or more, preferably approximately 5 μm or more, for example, approximately 10 μm or more.

[0132] The surface of the support substrate may be subjected to conventionally known surface treatments such as corona discharge treatment, plasma treatment, ultraviolet irradiation treatment, acid treatment, alkali treatment, or application of a primer. Such surface treatments may be intended to improve the adhesion between the support substrate and the adhesive layer, in other words, the anchoring ability of the adhesive layer to the support substrate.

[0133] <Release film> In the technologies disclosed herein, a release film (also called a release liner) can be used when forming an adhesive layer, manufacturing an adhesive sheet, storing the adhesive sheet before use, distributing it, and processing it into shape. The release film is not particularly limited, and for example, a release film having a release treatment layer on the surface of a substrate (release film substrate) such as a resin film or paper, or a release film made of a low-adhesion material such as a fluoropolymer (polytetrafluoroethylene, etc.) or a polyolefin resin (polyethylene, polypropylene, etc.) can be used. The release treatment layer may 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.

[0134] <Characteristics of adhesive sheets, etc.> The total thickness of the adhesive sheet disclosed herein (including an adhesive layer, and further including a support substrate in configurations having a support substrate, but not including a release film) is not particularly limited. The total thickness of the adhesive sheet can be, for example, approximately 500 μm or less, or approximately 300 μm or less. From the viewpoint of thinning and weight reduction, in some embodiments, the thickness of the adhesive sheet is usually suitable to be 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). Furthermore, the thickness of the adhesive sheet can be approximately 1 μm or more, for example, approximately 3 μm or more is appropriate, preferably approximately 5 μm or more, more preferably approximately 10 μm or more, even more preferably approximately 12 μm or more, and may also be approximately 15 μm or more. In some embodiments, the thickness of the adhesive sheet may be 20 μm or more, 30 μm or more, or 40 μm or more. Adhesive sheets having a thickness of a predetermined value or more have an adhesive layer with sufficient thickness, making it easier to obtain properties based on the adhesive layer (e.g., adhesive properties such as adhesive strength), and also tend to have excellent handling properties. In the case of a substrate-less adhesive sheet, the thickness of the adhesive layer becomes the total thickness of the adhesive sheet.

[0135] The light transmittance of the adhesive sheet disclosed herein may vary depending on the intended use of the adhesive sheet. In some embodiments, the 550nm light transmittance of the 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 550nm light transmittance of the 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. The adhesive sheet exhibiting the above 550nm light transmittance can exhibit excellent light shielding properties. In addition, in some embodiments, the lower limit of the 550nm light transmittance of the adhesive sheet may be 0.1% or more, or 0.3% or more, from the viewpoint of productivity and other properties such as adhesive strength. In some embodiments, the 550nm light transmittance of the adhesive layer may be 1% or more, 3% or more, or 5% or more. Furthermore, for adhesive sheets intended for applications where light transmittance is not limited or transparency is required, the 550nm light transmittance of the adhesive sheet is appropriately greater than approximately 70%, preferably greater than approximately 80% (e.g., greater than approximately 82%), more preferably greater than approximately 85%, and may be greater than approximately 90%. In such adhesive sheets, there is no particular upper limit on the 550nm light transmittance. For example, in some embodiments, the 550nm light transmittance of the adhesive sheet may be 98% or less, or 95% or less. The 550nm light transmittance of the adhesive sheet can be measured by the method described in the examples below.

[0136] The adhesive strength of the adhesive sheets disclosed herein may vary depending on the intended use and application location, and is not limited to a specific range. In some embodiments, the adhesive sheet may have a 180-degree peel strength (adhesion to SUS) to a stainless steel plate of, for example, approximately 1.0 N / 20 mm or more, approximately 2.0 N / 20 mm or more, preferably approximately 3.0 N / 20 mm or more, more preferably 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, approximately 8.0 N / 20 mm or more, approximately 10 N / 20 mm or more, or approximately 12 N / 20 mm or more. The upper limit of the above adhesion to SUS is not particularly limited and may be, for example, around 30 N / 20 mm or less. The above-mentioned adhesion strength to SUS is the 180-degree peel strength to a stainless steel plate measured according to JIS Z 0237, and can be measured specifically by the method described in the examples below.

[0137] <Application> The adhesive sheet disclosed herein can maintain good appearance quality even when kept in a low-temperature environment (e.g., a low-temperature environment of about -20°C to -3°C) for a certain period of time (e.g., more than one day, more than three days, or even more than one week), and is therefore suitable for various applications, such as those in which the adhesive sheet attached to the surface of an object is visible. For example, electronic devices have parts that are visible to the user and may require excellent appearance quality, and the adhesive sheet disclosed herein can be applied to visible parts of electronic devices to achieve a surface with good appearance quality. The adhesive sheet disclosed herein is suitable for applications such as joining components of electronic devices including home appliances, office automation equipment, and portable electronic devices such as smartphones, and for applications that impart optical properties (e.g., light shielding) to the above-mentioned electronic devices.

[0138] Non-exclusive examples of the above-mentioned portable electronic devices include mobile phones, smartphones, tablet computers, notebook computers, various wearable devices (e.g., wristwear-type devices worn on the wrist like watches, modular devices attached to a part of the body with clips or straps, eyewear-type devices including glasses (monocular and binocular, including head-mounted types), clothing-type devices attached to shirts, socks, hats, etc. as accessories, earwear-type 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 consoles, electronic dictionaries, electronic organizers, e-books, in-car information systems, portable radios, portable televisions, portable printers, portable scanners, portable modems, etc. In this specification, "portable" means not merely being able to carry something, but having a level of portability that allows an individual (a typical adult) to carry it relatively easily. Examples of the above-mentioned electronic devices include personal computers (desktop, notebook, tablet, etc.) and televisions. These may incorporate display devices such as liquid crystal or organic EL.

[0139] In some embodiments, the adhesive sheet may be used to fix a pressure sensor to other components within an electronic device equipped with a pressure sensor, such as the aforementioned portable electronic device. In some embodiments, the adhesive sheet may be used to fix a pressure sensor to other components within an electronic device (typically a portable electronic device) that has a function to specify an absolute position on a screen-corresponding panel (typically a touch panel) using a device for indicating a position on a screen (typically a pen-type or mouse-type device) and a device for detecting a position.

[0140] Furthermore, the adhesive sheets disclosed herein are also suitable for use on the back surface of display screens (display units) such as touch panel displays in electronic devices such as portable electronic devices. By placing an adhesive sheet according to one of the embodiments on the back surface of the display screen (display unit), a decrease in the visibility of the display screen can be prevented regardless of how the electronic device is used. For example, it can be placed on the back surface of a display screen (display unit) such as a touch panel display in the electronic device to prevent reflection of light through the display screen. The above-mentioned reflection can occur due to a metal member placed on the back side of the display screen, but by using an adhesive sheet according to one of the embodiments (for example, an adhesive sheet having an adhesive layer or an adhesive sheet having an adhesive layer with limited light transmittance), for example, to bond the metal member and the display unit, bonding of the members and providing light shielding can be achieved simultaneously.

[0141] The materials to which the adhesive sheets disclosed herein are attached (adhered materials) are not particularly limited, but include, for example, metallic 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 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, liquid crystal polymers, etc. (typically plastic materials); and inorganic materials such as alumina, zirconia, soda glass, quartz glass, and carbon. Among these, metallic materials such as copper, aluminum, and stainless steel, and resin materials such as polyimide resins, aramid resins, and polyphenylene sulfide resins (typically plastic materials) are widely used. The above materials may be materials for components that make up products such as electronic devices. The adhesive sheet disclosed herein may be used by being attached to a component made of the above materials. The above materials may also be materials that make up the object to which the pressure sensor or display unit is fixed (for example, a back surface component such as an electromagnetic wave shield or a reinforcing plate). The object to which the adhesive sheet is fixed refers to the object to which the adhesive sheet is attached, i.e., the adherend. The back surface component refers to a component that is located on the opposite side of the front surface (viewing side) of the pressure sensor or display unit in, for example, in a portable electronic device, and may be a component that makes up the support part 540 located on the back surface of the display device 500 shown in Figure 5 below. The object to which the adhesive sheet is fixed may be in the form of a single-layer structure or a multi-layer structure, and the surface to which the adhesive sheet is attached (the adhesive surface) may be subjected to various surface treatments. While not particularly limited, an example of an object to be fixed is a back surface member with a thickness of 1 μm or more (typically 5 μm or more, e.g., 60 μm or more, and even 120 μm or more) and 1500 μm or less (e.g., 800 μm or less).

[0142] In some embodiments, the member or material to which the adhesive sheet is attached may be light-transmitting (light-transmitting substrate). Since the adhesive surface of the adhesive sheet attached to the light-transmitting substrate can be seen through the light-transmitting substrate, it is desirable that it has good appearance quality. The 500nm light transmittance of the above light-transmitting substrate may be greater than, for example, 50%, and 70% or more. In some preferred embodiments, the 500nm light transmittance of the above substrate may be 80% or more, more preferably 90% or more, and 95% or more (for example, 95-100%). Such a material may be a resin film (for example, a polyester resin film such as PET film) that is placed on the back surface of the image display part of various devices such as portable electronic devices. The adhesive sheet disclosed herein may preferably be used in a manner in which it is attached to a substrate (e.g., a member) with a 500nm light transmittance of 500nm or more than a predetermined value. The 500nm light transmittance may be measured in the same manner as the 500nm light transmittance of the adhesive sheet.

[0143] Furthermore, in some embodiments, the adhesive sheet is used in a manner in which it is attached to a metal member. Examples of the material of the metal member include the metal materials exemplified above as adherend materials. Such a metal member is, for example, a member or article having a surface (adhesive sheet attachment surface) formed from a metal material such as aluminum or stainless steel, and preferred examples include metal members made of stainless steel or aluminum. By attaching the adhesive sheet to an area on the surface of the metal member where concealment is required, that area of ​​the metal member can be concealed. The adhesive sheet may cover the entire surface of the metal member, or it may cover a part of the surface (for example, a part of the surface where concealment is required). The metal member may be, for example, a member constituting the support part 540 of the display device 500 shown in Figure 5, which will be described later. Preferably, the metal member is one of the adherends of the adhesive sheet.

[0144] Adhesive sheets according to several embodiments can be preferably used in electronic devices (typically portable electronic devices) that require predetermined optical properties. For example, adhesive sheets 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 ELs. For example, they can be preferably used in electronic devices (typically portable electronic devices) that are equipped with organic EL display devices or liquid crystal display devices that require predetermined optical properties.

[0145] Figure 3 is a schematic exploded perspective view showing an example of the configuration of a display device. As shown in Figure 5, the display device 500 provided by the portable electronic device 400 comprises a display unit 520 composed of a cover member and an organic EL unit, etc., and a support unit 540. The display device 500 further includes an adhesive sheet 530. In this example configuration, the adhesive sheet 530 fixes the components constituting the display unit 520 and the support unit 540. The support unit 540 is composed of a substrate (a metal plate such as a stainless steel plate or an aluminum plate), etc. The adhesive sheet disclosed herein is preferably used as a component of the display device as described above.

[0146] The matters disclosed herein include the following: [1] An adhesive sheet having an adhesive layer, The above adhesive layer comprises a water-dispersible acrylic polymer and a water-soluble crosslinking agent. The above-mentioned water-dispersible acrylic polymer is a polymer synthesized by emulsion polymerization using a reactive emulsifier having a radically polymerizable functional group. An adhesive sheet in which the gel fraction of the adhesive layer described above is less than 90%. [2] The adhesive sheet according to [1] above, wherein the adhesive layer contains more than 0.05 parts by weight of the water-soluble crosslinking agent per 100 parts by weight of the water-dispersible acrylic polymer. [3] The adhesive sheet according to [1] or [2] above, wherein the water-soluble crosslinking agent comprises at least one selected from the group consisting of carbodiimide-based crosslinking agents, epoxy-based crosslinking agents, and oxazoline-based crosslinking agents. [4] The adhesive sheet according to any one of [1] to [3] above, wherein a silane monomer is copolymerized in the water-dispersible acrylic polymer. [5] The adhesive sheet according to any one of [1] to [4] above, wherein a keto group-containing monomer is copolymerized in the water-dispersible acrylic polymer. [6] The adhesive sheet according to any one of [1] to [5] above, wherein the monomer component constituting the water-dispersible acrylic polymer contains less than 4% by weight of carboxyl group-containing monomers. [7] The adhesive sheet according to any one of [1] to [6] above, wherein the water-dispersible acrylic polymer is a polymer synthesized by emulsion polymerization using a chain transfer agent. [8] The adhesive sheet according to any one of [1] to [7] above, which is a substrate-less double-sided adhesive sheet consisting of the adhesive layer described above. [9] The adhesive sheet described above is an adhesive sheet with a substrate, comprising the adhesive layer and a support substrate, as described in any of [1] to [7] above.

[10] An adhesive sheet according to any of [1] to [9] above, wherein the 180-degree peel strength to stainless steel plate is 3.0 N / 20 mm or more.

[11] Electronic device containing any of the adhesive sheets described in [1] to

[10] above. [Examples]

[0147] The following describes some embodiments of the present invention, but the present invention is not intended to be limited to those shown in these embodiments. In the following description, "parts" and "%" refer to weight unless otherwise specified.

[0148] <Example 1> (Preparation of acrylic polymers) In a reaction vessel equipped with a thermometer, stirrer, nitrogen inlet tube, and reflux condenser, 73 parts of deionized water and 0.07 parts of emulsifier E1 (product name "Aqualon KH-1025", manufactured by Daiichi Kogyo Seiyaku Co., Ltd., an anionic reactive surfactant) were added and stirred at 60°C for 1 hour under a nitrogen atmosphere. After adding 0.1 parts of a polymerization initiator (product name "VA-057", manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), a monomer emulsion was 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 (product name "KBM-503", manufactured by Shin-Etsu Chemical Co., Ltd.) and 1.88 parts of the above emulsifier E1 (product name "Aqualon KH-1025") with 30 parts of deionized water, and this monomer emulsion was polymerized dropwise at 60°C for 4 hours. Furthermore, after holding the mixture at 60°C for 3 hours, it was cooled to room temperature and the pH was adjusted to 6.0 using 10% aqueous ammonia as a pH adjusting agent to prepare an acrylic polymer emulsion containing a water-dispersible acrylic polymer.

[0149] (Preparation of adhesive composition) To the obtained acrylic polymer emulsion, 1 part of a thickener (product name "Aron B-500", manufactured by Toagosei Co., Ltd., carboxylic acid copolymer) and 1 part of a crosslinking agent C1 (product name "Carbodilite V-04", manufactured by Nisshinbo Chemical Co., Ltd., water-soluble carbodiimide crosslinking agent) were added to 100 parts of the above water-dispersible acrylic polymer, based on solid content, and mixed. Deionized water and 10% aqueous ammonia were added to the resulting mixture to adjust the pH to 9.0 and the solid content concentration to 40%, and the mixture was stirred at 2000 rpm for 5 minutes using an Awatori Rentaro (manufactured by Shinky Co., Ltd.), followed by vacuum degassing at 2200 rpm for 5 minutes to obtain a water-dispersible adhesive composition.

[0150] (Making adhesive sheets) The above adhesive composition was applied to the release surface of a silicone-release-treated polyethylene terephthalate (PET) film (product name "Diafoil MRF38", manufactured by Mitsubishi Chemical Corporation, thickness 38 μm), and dried at 100°C for 3 minutes to form an adhesive layer with a thickness of 15 μm. The release surface of a release-treated polyester film (product name "Diafoil MRF25", manufactured by Mitsubishi Chemical Corporation, thickness 25 μm) was laminated onto this adhesive layer. In this way, a substrate-less double-sided adhesive sheet was obtained in which both sides were protected by the above two release films.

[0151] <Examples 3-14, 20-21 and Comparative Examples 1-6> The type of emulsifier, the type and amount of water-soluble crosslinking agent, and the thickness of the adhesive layer were changed as shown in Tables 1 to 3. The adhesive composition for each example was prepared in the same manner as in Example 1, and the substrate-less adhesive sheet for each example was obtained in the same manner as in Example 1, except that the adhesive composition was used.

[0152] <Example 2> In a reaction vessel equipped with a thermometer, stirrer, nitrogen inlet tube, and reflux condenser, 73 parts of deionized water and 0.07 parts of emulsifier E2 (product name "Aqualon HS-10", manufactured by Daiichi Kogyo Seiyaku Co., Ltd., an anionic reactive surfactant) were added and stirred at 60°C for 1 hour under a nitrogen atmosphere. After adding 0.1 parts of a polymerization initiator (product name "VA-057", manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), a monomer emulsion was 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), 0.15 parts of diacetone acrylamide (DAAM), 0.035 parts of t-dodecanethiol (chain transfer agent), and 1.88 parts of the above reactive emulsifier E2 (product name "Aqualon HS-10") with 30 parts of deionized water. This monomer emulsion was polymerized dropwise at 60°C for 4 hours. Furthermore, after holding at 60°C for 3 hours, the mixture was cooled to room temperature and the pH was adjusted to 6.0 using 10% aqueous ammonia as a pH adjuster to prepare an acrylic polymer emulsion containing a water-dispersible acrylic polymer. To the obtained acrylic polymer emulsion, 1 part of a thickener (product name "Aron B-500", manufactured by Toagosei Co., Ltd., carboxylic acid copolymer) and 1 part of a crosslinking agent C4 (adipic acid dihydrazide) were added to 100 parts of the above water-dispersible acrylic polymer, based on solid content, and mixed. Deionized water and 10% aqueous ammonia were added to the obtained mixture to adjust the pH to 9.0 and the solid content concentration to 40%, and the mixture was stirred at 2000 rpm for 5 minutes using an Awatori Rentaro (manufactured by Shinky Co., Ltd.), followed by vacuum degassing at 2200 rpm for 5 minutes to obtain a water-dispersible adhesive composition. A substrate-less adhesive sheet according to this example was obtained in the same manner as in Example 1, except that this adhesive composition was used.

[0153] <Examples 15-17> The type and amount of water-soluble crosslinking agent were changed as shown in Table 2. The adhesive composition for each example was prepared in the same manner as in Example 2, and the substrate-less adhesive sheet for each example was obtained in the same manner as in Example 1, except that the adhesive composition was used.

[0154] <Comparative Example 7> In a reaction vessel equipped with a condenser, nitrogen inlet tube, thermometer, and stirrer, an emulsion was prepared by emulsifying 56 parts by weight of 2EHA, 40 parts of n-butyl methylate, 2 parts of 2-hydroxyethyl methacrylate, 2 parts of acrylic acid, and 2 parts of emulsifier E4 (product name "ADEKA Soap SE-10N", manufactured by ADEKA, an ether sulfate-type nonionic anionic surfactant) with 180 parts of water. This emulsion was then charged and the mixture was purged with nitrogen for 1 hour under stirring. Thereafter, the internal bath temperature was controlled to 25°C during polymerization. 0.1 parts of hydrogen peroxide (30%) was added, followed by the addition of 1 ml of an aqueous ascorbic acid solution consisting of 0.05 parts ascorbic acid and 10 parts water to initiate polymerization. Five hours after the start of polymerization, the remaining aqueous ascorbic acid solution was added dropwise over 2 hours, and the reaction was allowed to mature for another 2 hours to complete. Subsequently, the mixture was neutralized with 10% aqueous ammonia to prepare an acrylic polymer emulsion containing a water-dispersible acrylic polymer. To the obtained acrylic polymer emulsion, 1 part of a thickener (product name "Aron B-500", manufactured by Toagosei Co., Ltd., carboxylic acid copolymer) and 1.5 parts of a crosslinking agent C1 (product name "Carbodilite V-04", manufactured by Nisshinbo Chemical Co., Ltd., water-soluble carbodiimide crosslinking agent) were added to 100 parts of the above water-dispersible acrylic polymer, based on solid content, and mixed. Deionized water and 10% aqueous ammonia were added to the obtained mixture to adjust the pH to 9.0 and the solid content concentration to 25%, and the mixture was stirred at 2000 rpm for 5 minutes using an Awatori Rentaro (manufactured by Shinky Co., Ltd.), followed by vacuum degassing at 2200 rpm for 5 minutes to obtain a water-dispersible adhesive composition. A substrate-less adhesive sheet according to this example was obtained in the same manner as in Example 1, except that this adhesive composition was used.

[0155] <Examples 18-19> In preparing the water-dispersible adhesive composition, the types and amounts (based on solid content) of pigments shown in Table 2 were further added. The pigment used was "SA Black A4048," a product of Mikuni Pigment Co., Ltd. Otherwise, the water-dispersible adhesive composition for each example was obtained in the same manner as in Example 1, and the substrate-less adhesive sheet for each example was obtained in the same manner as in Example 1, except that the adhesive composition was used.

[0156] <Examples 22-27> In preparing the water-dispersible adhesive composition, tackifying resins of the types and amounts (based on solid content) shown in Table 2 were further added. A water-dispersible adhesive composition for each example was obtained in the same manner as in Example 1, and a substrate-less adhesive sheet for each example was obtained in the same manner as in Example 1, except that the adhesive composition was used.

[0157] <Example 28> A water-dispersible adhesive composition was prepared by the same method as described in Example 1. The adhesive composition was applied to the release surface of a silicone-release-treated polyethylene terephthalate (PET) film (product name "Diafoil MRF38", manufactured by Mitsubishi Chemical Corporation, 38 μm thick), and dried at 100°C for 3 minutes to form an adhesive layer 6 μm thick. Two of these adhesive-layered release films were prepared and laminated to the first and second surfaces of a PET film (product name "Lumirror 5A", manufactured by Toray Industries, Inc.) with a thickness of approximately 4 μm, resulting in a double-sided adhesive sheet with a substrate having a structure of adhesive layer (6 μm thick) / PET film (approximately 4 μm thick) / adhesive layer (6 μm thick), with both sides protected by the two release films, and a total thickness of approximately 16 μm.

[0158] <Comparative Example 8> A double-sided adhesive sheet with a substrate according to this example was obtained in the same manner as in Example 28, except that a water-dispersible adhesive composition prepared by the same method as described in Comparative Example 1 was used.

[0159] The materials used in the above examples and comparative examples are shown below. Emulsifier E1: Anionic reactive surfactant (product name "Aqualon KH-1025", manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) Emulsifier E2: Anionic reactive surfactant (product name "Aqualon HS-10", manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) Emulsifier E3: Nonionic reactive surfactant (product name "Aqualon RN-20", manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) Emulsifier E4: Nonionic anionic reactive surfactant (product name "ADEKA Soap SE-10N", manufactured by ADEKA Corporation) Emulsifier E5: Anionic surfactant (product name "Latemul E-118B", manufactured by Kao Corporation, non-reactive) Emulsifier E6: Nonionic surfactant (product name "Emulgen A-60", manufactured by Kao Corporation, non-reactive) Crosslinking agent C1: Water-soluble carbodiimide crosslinking agent (product name "Carbodilite V-04", manufactured by Nisshinbo Chemical Co., Ltd.) Crosslinking agent C2: Water-soluble epoxy crosslinking agent (product name "Denacol EX-313", manufactured by Nagase ChemteX Corporation) Crosslinking agent C3: Water-soluble oxazoline-based crosslinking agent (product name "Epocross WS-500", manufactured by Nippon Shokubai Co., Ltd.) Crosslinking agent C4: Water-soluble hydrazide crosslinking agent (adipic acid dihydrazide) Crosslinking agent C5: Epoxy-based crosslinking agent (product name "Tetrad C", manufactured by Mitsubishi Gas Chemical Company, hydrophobic) Crosslinking agent C6: Isocyanate-based crosslinking agent (product name "Coronate HX", manufactured by Tosoh Corporation, hydrophobic) Crosslinking agent C7: Water-dispersible carbodiimide-based crosslinking agent (product name "Carbodilite E-05", manufactured by Nisshinbo Chemical Co., Ltd.) Pigment: Carbon black aqueous dispersion (product name "SA Black A4048", manufactured by Mikuni Pigment Co., Ltd., uses polymer-based dispersant, volume average particle size 110 nm) Tackifying resin T1: Terpene phenol resin (product name "Tamanol E-200NT", manufactured by Arakawa Chemical Industries, Ltd., softening point 145℃) Tackifying resin T2: Rosin ester resin (product name "Super Ester E-865NT", manufactured by Arakawa Chemical Industries, Ltd., softening point 160℃)

[0160] <Evaluation Method> (Adhesion to SUS) Under a measurement environment of 23°C and 50%RH, a 50μm thick PET film was attached to one adhesive side of an adhesive sheet (double-sided adhesive sheet) as a backing, and the sheet was cut to a size of 20mm wide and 100mm long to prepare a measurement sample. The prepared measurement sample was then pressed against the surface of a stainless steel plate (SUS304BA plate) by applying pressure with a 2kg roller for one back-and-forth motion under the same environment of 23°C and 50%RH. After leaving this in the same environment for 30 minutes, the peel strength (adhesion to SUS) [N / 20mm] was measured using a universal tensile and compression tester in accordance with JIS Z 0237:2000, under conditions of a tensile speed of 300mm / min and a peel angle of 180 degrees. The measurement results are shown in Tables 1 and 2. For the universal tensile and compression tester, for example, the "Tensile and Compression Tester, TG-1kN" manufactured by Minebea Co., Ltd. or an equivalent product can be used. Note that for single-sided adhesive sheets, the PET film backing is not necessary.

[0161] (Gel fraction) The gel fraction of the adhesive layer was determined by wrapping a sample of weight W1 taken from the adhesive layer in a porous sheet made of tetrafluoroethylene resin, 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 × 100. The porous sheet made of tetrafluoroethylene resin used was "Nitoflon (registered trademark) NTF1122" (average pore size 0.2 μm, porosity 75%, thickness 85 μm) or an equivalent product manufactured by Nitto Denko Corporation. "Nitoflon (registered trademark) NTF1122" was used in this measurement.

[0162] (550nm light transmittance) The light transmittance [%] of the adhesive sheet at a wavelength of 550 nm was measured using a commercially available transmittance meter in accordance with JIS K 7136:2000. The measurement results are shown in Tables 1 and 2. A spectrophotometer manufactured by Hitachi, Ltd. (device name "U4150 type spectrophotometer") or an equivalent device was used as the transmittance meter.

[0163] (Appearance quality evaluation) An evaluation sample was obtained by cutting a release film-backed adhesive sheet to a size of 100 mm x 100 mm. The release film (in the case of a double-sided adhesive sheet with two release films, the release film on the easily peelable side) was peeled off from this evaluation sample. The evaluation sample was held flat at the midpoint between a point light source and a projection screen, which were positioned at a distance of approximately 100 cm (approximately 50 cm from the point light source), and positioned so that the angle of the exposed adhesive layer surface of the evaluation sample with respect to the light rays from the point light source was approximately 90 degrees. The evaluation sample was positioned with the adhesive layer surface from which the release film had been peeled facing the point light source. In a darkroom at 23°C and 50% RH, the point light source was turned on, and the image projected onto the screen through the evaluation sample was visually observed to identify any visible pinholes (size 0.02 mm) present in the evaluation sample. 2The presence or absence of the above was evaluated. As a point light source, for example, a "Xenon Lamp C2577" manufactured by Hamamatsu Photonics can be used. For each example, 10 evaluation samples were prepared and the above evaluation was performed, and the number of samples in which a visible pinhole was confirmed (NG number) was counted. If the number of samples in which a visible pinhole was confirmed was 5 or more (out of 10 samples), it was judged to be a failure. The above-mentioned appearance quality evaluation (pre-low temperature treatment evaluation) was performed on the adhesive sheets of all examples and comparative examples, and no samples had visible pinholes (i.e., number of NGs / 10 samples = 0 / 10 samples). After confirming this, the evaluation samples were stored at -5°C for one week, then taken out and left to stand for 24 hours in an environment of 23°C and 50% RH, and the above-mentioned appearance quality evaluation (post-low temperature treatment evaluation) was performed. The results are shown in Tables 1 to 3. In the column for the water solubility of the crosslinking agent, "Y" indicates a water-soluble crosslinking agent, and "N" indicates a non-water-soluble crosslinking agent.

[0164] [Table 1]

[0165] [Table 2]

[0166] [Table 3]

[0167] As shown in Tables 1 to 3, in Examples 1 to 28, which included a water-dispersible acrylic polymer and a water-soluble crosslinking agent, and in which a reactive emulsifier was used during the polymerization of the water-dispersible acrylic polymer, the adhesive layer with a gel fraction of less than 90% showed no pinholes or a small number of pinhole-causing samples after low-temperature treatment at -5°C for one week, and exhibited good appearance quality. On the other hand, in Comparative Examples 1-2 and 8, which used a non-reactive emulsifier during the polymerization of water-dispersible acrylic polymers, Comparative Example 5, which did not use a crosslinking agent, and Comparative Examples 3, 4, and 6, which did not use a water-soluble crosslinking agent (only oil-soluble or water-dispersible crosslinking agents were used), pinholes occurred in the adhesive layer with a gel fraction of less than 90% after low-temperature treatment, resulting in poor appearance quality. Comparative Example 7, with a gel fraction of 90% or more, had low adhesion to SUS.

[0168] From the above results, it can be seen that an adhesive containing a water-dispersible acrylic polymer synthesized by emulsion polymerization using a reactive emulsifier having a radically polymerizable functional group and a water-soluble crosslinking agent can be realized in a configuration with a gel fraction of less than 90% and can maintain good appearance quality even when stored in a specific low temperature range that is below 0°C but not extremely low, as well as an adhesive sheet having said adhesive.

[0169] Although specific examples of the present invention have been described in detail above, these are merely illustrative and do not limit the scope of the claims. The technologies described in the claims include various modifications and changes to the specific examples illustrated above. [Explanation of Symbols]

[0170] 1, 2, 3, 4 Adhesive sheets 10 Supporting base material 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. An adhesive sheet having an adhesive layer, The adhesive layer comprises a water-dispersible acrylic polymer and a water-soluble crosslinking agent. The aforementioned water-dispersible acrylic polymer is a polymer synthesized by emulsion polymerization using a reactive emulsifier having a radically polymerizable functional group. An adhesive sheet in which the gel fraction of the adhesive layer is less than 90%.

2. The adhesive sheet according to claim 1, wherein the adhesive layer contains more than 0.05 parts by weight of the water-soluble crosslinking agent per 100 parts by weight of the water-dispersible acrylic polymer.

3. The adhesive sheet according to claim 1 or 2, wherein the water-soluble crosslinking agent comprises at least one selected from the group consisting of carbodiimide-based crosslinking agents, epoxy-based crosslinking agents, and oxazoline-based crosslinking agents.

4. The adhesive sheet according to claim 1 or 2, wherein a silane monomer is copolymerized in the water-dispersible acrylic polymer.

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

6. The adhesive sheet according to claim 1 or 2, wherein the monomer component constituting the water-dispersible acrylic polymer contains less than 4% by weight of carboxyl group-containing monomers.

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

9. The adhesive sheet according to claim 1 or 2, wherein the adhesive sheet is an adhesive sheet with a substrate that includes the adhesive layer and a support substrate.

10. The adhesive sheet according to claim 1 or 2, wherein the 180-degree peel strength to a stainless steel plate is 3.0 N / 20 mm or more.

11. An electronic device comprising the adhesive sheet according to claim 1 or 2.

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