TW202500700A - Adhesive Sheet - Google Patents

Abstract

The present invention provides an adhesive sheet, which comprises: a layer (adhesive layer) containing heat-expandable microspheres, which has excellent peelability after heating and excellent transparency. The adhesive sheet of the embodiment of the present invention comprises: an adhesive layer comprising an adhesive and heat-expandable microspheres, the adhesive comprising a base polymer, the heat-expandable microspheres comprising a shell formed by a resin and a volatile substance contained in the shell, and the absolute value of the difference between the refractive index of the base polymer and the refractive index of the resin is 0 to 0.018.

Description

Adhesive Sheet

The present invention relates to an adhesive sheet.

In the process of manufacturing electronic parts, as an adhesive sheet used for the purpose of temporarily fixing the workpiece, there is known an easily peelable adhesive sheet that exhibits adhesiveness when temporarily fixing and exhibits peelability when fixing is not required. As one of such adhesive sheets, an adhesive sheet composed of thermally expandable microspheres in an adhesive layer has been studied (for example, Patent Document 1). The adhesive sheet exhibits the desired adhesive force at a relatively low temperature represented by room temperature. On the other hand, the thermally expandable microspheres expand due to heating, and produce unevenness on the surface of the adhesive layer, thereby reducing the adhesive force. For such an adhesive sheet, the adhered body can also be peeled off by the action of gravity alone.

The adhesive layer containing the heat-expandable microspheres as described above is sometimes required to have high transparency. For example, when aligning the pattern surface of the semiconductor through the adhesive sheet, there is a problem that it is difficult to use an adhesive sheet with poor transparency. In particular, in recent years, there has been a tendency for the ambient temperature during processing to become higher, and the opportunity to use heat-expandable microspheres with excellent heat resistance has increased, but the adhesive layer containing heat-expandable microspheres with excellent heat resistance has a tendency to reduce transparency. [Prior technical literature] [Patent literature]

Patent document 1: Japanese Patent Publication No. 2001-131507

[Problem to be solved by the invention] The present invention is completed to solve the above-mentioned previous problem, and its purpose is to provide an adhesive sheet, which comprises: a layer (adhesive layer) containing heat-expandable microspheres and having excellent peelability after heating and excellent transparency. [Technical means for solving the problem]

[1] The adhesive sheet of the embodiment of the present invention comprises: an adhesive layer including an adhesive and heat-expandable microspheres, the adhesive comprising a base polymer, the heat-expandable microspheres comprising a shell formed by a resin and a volatile substance contained in the shell, and the absolute value of the difference between the refractive index of the base polymer and the refractive index of the resin is 0 to 0.018. [2] The adhesive sheet as described in [1] above, wherein the resin forming the shell may also contain constituent units derived from carboxyl-containing monomers. [3] The adhesive sheet as described in [2] above, wherein the content ratio of constituent units derived from carboxyl-containing monomers in the resin forming the shell may be 21 mol% to 36 mol% relative to 100 mol of the resin. [4] The adhesive sheet as described in [1] above, wherein the resin forming the shell may also contain constituent units derived from methacrylic acid. [5] The adhesive sheet as described in any one of [1] to [4] above, wherein the base polymer in the adhesive may also contain constituent units derived from hydroxyl-containing monomers and/or constituent units derived from carboxyl-containing monomers. [Effect of the invention]

According to the present invention, it is possible to provide an adhesive sheet having a layer (adhesive layer) containing heat-expandable microspheres and having excellent releasability after heating and excellent transparency.

A. Overview of Adhesive Sheet Fig. 1(a) is a schematic cross-sectional view of an adhesive sheet according to one embodiment of the present invention. The adhesive sheet 100 has an adhesive layer 10. The adhesive sheet of the present invention may be composed of only the adhesive layer 10, or may have any appropriate layer in addition to the adhesive layer.

FIG. 1( b ) is a schematic cross-sectional view of an adhesive sheet of another embodiment of the present invention. The adhesive sheet 200 also includes a substrate 20, and an adhesive layer 10 is disposed on at least one side of the substrate 20. FIG. 1( c ) is a schematic cross-sectional view of an adhesive sheet of another embodiment of the present invention. The adhesive sheet 300 includes an adhesive layer 10 and another adhesive layer 30 disposed on at least one side of the adhesive layer 10. The substrate 20 may be disposed between the adhesive layer 10 and the other adhesive layer 30 as shown in the example shown in the figure. Although not shown, the substrate may be omitted and the adhesive sheet may be composed of the adhesive layer and the other adhesive layers. Although not shown, the adhesive sheet may further include an elastic layer (described later in item E) that can impart elasticity to the adhesive sheet, a peeling pad that can be releasably disposed on the adhesive layer, etc. as layers other than the adhesive layer. In addition, the adhesive layer may be disposed on both sides of the substrate.

The adhesive layer comprises an adhesive containing a base polymer and thermally expandable microspheres.

The heat-expandable microspheres are composed of a shell formed by a resin and a volatile substance (typically an organic solvent) contained in the shell. The heat-expandable microspheres can expand at a specified temperature by the volatility of the volatile substance. In the adhesive layer containing such heat-expandable microspheres, the heat-expandable microspheres expand due to heating, thereby generating unevenness on the adhesive surface (i.e., the surface of the adhesive layer), and the adhesive force is reduced or eliminated. When the adhesive tape of the present invention is used as a temporary fixing sheet for a workpiece during processing of electronic parts (e.g., ceramic capacitors), the workpiece exhibits the adhesiveness required for temporary fixing when the workpiece is subjected to a prescribed processing. When the adhesive tape is peeled off from the workpiece after processing, the adhesive force is reduced or disappears due to heating, thereby exhibiting good peeling properties.

The absolute value of the difference between the refractive index of the base polymer in the adhesive and the refractive index of the resin constituting the shell is 0 to 0.018. If it is within this range, the diffuse reflection of visible light in the adhesive layer can be reduced, and an adhesive layer with excellent transparency can be formed. The absolute value of the difference between the refractive index of the base polymer in the adhesive and the refractive index of the resin constituting the shell is preferably 0 to 0.015, more preferably 0 to 0.006, and further preferably 0 to 0.005. If it is within this range, the above effect becomes significant.

In this specification, the refractive index of the base polymer and the refractive index of the resin are calculated based on the monomer composition constituting the base polymer or the resin, and are obtained as the sum of the calculated values (the molar fraction of each monomer composition constituting the polymer × the refractive index value of the monomer). The refractive index of the monomer is the refractive index value of the sodium D line at 20°C.

The adhesion (normal adhesion) at 23°C when the adhesive layer of the adhesive sheet is attached to polyethylene terephthalate is preferably 0.5 N/20 mm or more, more preferably 1 N/20 mm to 20 N/20 mm, further preferably 2 N/20 mm to 20 N/20 mm, further preferably 4 N/20 mm to 20 N/mm. If it is within this range, for example, an adhesive sheet useful as a temporary fixing sheet used in the manufacture of electronic parts can be obtained. In this specification, the adhesion refers to the adhesion in a state where the adhesion is not reduced due to the expansion of the heat-expandable microspheres, and refers to the adhesion in a state where it has not undergone a thermal history of more than 40°C. The adhesive force refers to the adhesive force measured according to the method of JIS Z 0237:2009 (lamination conditions: 2 kg roller reciprocating once, peeling speed (stretching speed): 300 mm/min, peeling angle 180°).

When the adhesive layer of the adhesive sheet is adhered to polyethylene terephthalate, the adhesive force is preferably reduced to 0.3 N/20 mm or less (preferably 0.2 N/20 mm or less, more preferably 0.1 N/20 mm) by heating. The heating temperature is preferably 70°C to 300°C, more preferably 100°C to 280°C.

The adhesive sheet may further include any other appropriate layers. In one embodiment, the adhesive sheet also includes a primer layer disposed between the adhesive layer and the substrate. If the primer layer is provided, an adhesive sheet having excellent followability to an adherend can be obtained. Furthermore, if the adhesive layer including the heat-expandable microspheres is heated, the heat-expandable microspheres expand and deform in the thickness direction, but the primer layer suppresses the deformation in the direction of the substrate, thereby improving the releasability. Furthermore, another adhesive layer may be provided on the side of the substrate opposite to the adhesive layer, that is, the adhesive sheet may sequentially include an adhesive layer, a primer layer configured as needed, a substrate, and another adhesive layer.

The thickness of the adhesive sheet is preferably 3 μm to 300 μm, more preferably 5 μm to 150 μm, and further preferably 10 μm to 100 μm.

The total light transmittance of the adhesive sheet is preferably 70% or more, more preferably 80% or more, further preferably 85% or more, further preferably 90% or more, and particularly preferably 95% or more. The upper limit of the total light transmittance of the adhesive sheet is, for example, 98%. The haze of the adhesive sheet is preferably 70% or less, more preferably 60% or less, further preferably 50% or less, particularly preferably 40% or less, and most preferably 30% or less. The lower limit of the haze of the adhesive sheet is, for example, 10%.

B. Adhesive layer The adhesive layer includes heat-expandable microspheres. In practice, the adhesive layer also includes an adhesive.

The thickness of the adhesive layer is preferably 1 μm to 50 μm, more preferably 3 μm to 30 μm, and further preferably 5 μm to 20 μm. If it is within this range, an adhesive sheet can be obtained that is difficult to be embedded in the resin of the semiconductor chip even by pressurization during the sealing step. In one embodiment, the thickness of the adhesive layer is 2 μm to 15 μm. If it is within this range, the water emanation from the adhesive layer can be appropriately adjusted to make the sealing resin precursor have good fluidity.

The elastic modulus of the adhesive layer at 25°C based on the nanoindentation method is preferably less than 100 MPa, more preferably 0.1 MPa to 50 MPa, and further preferably 0.1 MPa to 10 MPa. If it is within this range, an adhesive sheet with appropriate adhesion can be obtained. The elastic modulus based on the nanoindentation method refers to the elastic modulus obtained from the load-indentation depth curve obtained by continuously measuring the load and the indentation depth of the indentation head when the indentation head is pressed into the sample during loading and unloading. In this specification, the elastic modulus based on the nanoindentation method refers to the elastic modulus measured in the above manner with the measurement conditions set as load: 1 mN, loading/unloading speed: 0.1 mN/s, and holding time: 1 s.

The total light transmittance of the adhesive layer is preferably 70% or more, more preferably 80% or more, more preferably 85% or more, further preferably 90% or more, and particularly preferably 95% or more. The upper limit of the total light transmittance of the adhesive layer is, for example, 98%. The haze of the adhesive layer is preferably 70% or less, more preferably 60% or less, further preferably 50% or less, particularly preferably 40% or less, and most preferably 30% or less. The lower limit of the haze of the adhesive layer is, for example, 10%.

B-1. Heat-expandable microspheres As the heat-expandable microspheres, any appropriate heat-expandable microspheres can be used as long as they can expand by heating to the extent that the adhesive layer surface is uneven. As the heat-expandable microspheres, microspheres composed of a shell and a volatile substance (typically an organic solvent) contained in the shell can be used.

The foaming starting temperature of the above-mentioned heat-expandable microspheres is above 80°C, preferably 90°C to 260°C, more preferably 100°C to 220°C, particularly preferably 120°C to 200°C, and most preferably 120°C to 180°C. In one embodiment, the foaming starting temperature of the above-mentioned heat-expandable microspheres is above 120°C. Even if the present invention uses heat-expandable microspheres with a high foaming temperature, it is also advantageous in terms of excellent transparency as described above. In this specification, the temperature at which the heat-expandable microspheres begin to expand is equivalent to the foaming starting temperature. In more detail, the foaming starting temperature can be, for example, the temperature at which the thickness of the adhesive layer begins to change due to the expansion of the heat-expandable microspheres. In addition, the foaming start temperature may be, for example, the lowest temperature at which the adhesive force of the adhesive layer reaches 10% or less relative to the normal adhesive force of the adhesive layer (adhesion to PET film at 23°C). The adhesive force at this time refers to the adhesive force measured according to the method based on JIS Z 0237:2000 (lamination conditions: 2 kg roller reciprocating once, peeling speed: 300 mm/min, peeling angle 180°). In addition, the adhesive force at the foaming start temperature (and the measured sample after heating) is measured after the measured sample is restored to room temperature (23°C).

The shell is made of resin. If resin is used, heat-expandable microspheres that are easily softened and expanded by heating can be obtained. In addition, the shell formed by the resin has a density close to that of the adhesive, so it is also advantageous in that it is easy to disperse with high uniformity in the adhesive layer. In one embodiment, an acrylic resin composed of one or more (meth) alkyl esters (e.g., (meth) alkyl esters of C1-20) is used as monomer components.

As the resin forming the shell, for example, a resin having a constituent unit derived from a monomer capable of free radical polymerization can be used. Examples of the monomer include: nitrile monomers such as acrylonitrile (refractive index: 1.391), methacrylonitrile (refractive index: 1.400), α-chloroacrylonitrile, α-ethoxyacrylonitrile, and fumaric acid nitrile; carboxyl group-containing monomers such as acrylic acid (refractive index: 1.421), methacrylic acid (refractive index: 1.431), itaconic acid, maleic acid, fumaric acid, and citric acid; vinylidene chloride (refractive index: 1.426); vinyl acetate; methyl acrylate, methyl methacrylate (refractive index: 1.414), ethyl acrylate (refractive index: 1.406), methyl (Meth)acrylates such as ethyl acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, tert-butyl (meth)acrylate, isobutyl (meth)acrylate, cyclohexyl (meth)acrylate, benzyl (meth)acrylate, β-carboxyethyl acrylate, 2-ethylhexyl acrylate (refractive index: 1.436), 2-hydroxyethyl acrylate (refractive index: 1.453); styrene monomers such as styrene, α-methylstyrene, chlorostyrene; amide monomers such as acrylamide, substituted acrylamide, methacrylamide, substituted methacrylamide, etc. The polymer composed of these monomers may be a homopolymer or a copolymer. Preferably, it is a copolymer.

The refractive index of the resin forming the shell can be adjusted by the composition of the monomer components constituting the resin.

The refractive index of the resin constituting the shell is preferably 1.30 to 1.50, more preferably 1.35 to 1.45, further preferably 1.38 to 1.42, further preferably 1.39 to 1.42, and particularly preferably 1.40 to 1.42. If the refractive index of the resin constituting the shell is within this range and the refractive index of the base polymer of the adhesive is adjusted at the same time, the diffuse reflection of visible light in the adhesive layer can be reduced, forming an adhesive layer with excellent transparency.

In one embodiment, the resin forming the shell includes a constituent unit derived from a carboxyl-containing monomer. If a resin including a constituent unit derived from a carboxyl-containing monomer is used, a heat-expandable microsphere having a shell with high affinity to the adhesive in the adhesive layer can be formed. The heat-expandable microsphere having a shell with high affinity to the adhesive is not easy to agglomerate and can be appropriately dispersed in the adhesive layer. The diffuse reflection of light of the adhesive layer including such heat-expandable microspheres is suppressed, thereby having excellent transparency. In addition, if a carboxyl-containing monomer is used, the refractive index of the resin forming the shell can be easily adjusted. In addition, an adhesive layer having preferably viscoelastic properties even at high temperatures, that is, an adhesive layer having excellent heat resistance (high foaming temperature) can be formed.

In the resin forming the shell, the content of the constituent unit derived from the carboxyl group-containing monomer is preferably 10 mol% to 50 mol%, more preferably 15 mol% to 45 mol%, and further preferably 21 mol% to 36 mol% relative to 100 mol of the resin. Within this range, the above effect becomes significant. In addition, a heat-expandable microsphere having a shell with a well-adjusted refractive index can be obtained. When the content of the constituent unit derived from the carboxyl group-containing monomer is too high, it may be difficult to expand by heating.

In one embodiment, the resin forming the shell includes a constituent unit derived from methacrylic acid. If a resin including a constituent unit derived from methacrylic acid is used, a thermally expandable microsphere having a shell with high affinity for the adhesive in the adhesive layer can be formed. In addition, methacrylic acid has excellent reactivity with other monomers constituting the resin, so if methacrylic acid is used, the refractive index of the resin becomes easy to adjust.

In the resin forming the shell, the content of the constituent units derived from methacrylic acid is preferably 10 mol% to 50 mol%, more preferably 15 mol% to 45 mol%, and further preferably 18 mol% to 40 mol%, based on 100 mol of the resin. Within this range, the above-mentioned effect becomes remarkable.

The sp value of the resin forming the shell is preferably 10.0 (cal/cm ³ ) ^1/2 to 15.0 (cal/cm ³ ) ^1/2 , more preferably 12.0 (cal/cm ³ ) ^1/2 to 14.0 (cal/cm ³ ) ^1/2 , and even more preferably 12.5 (cal/cm ³ ) ^1/2 to 14.0 (cal/cm ³ ) ^1/2 . Within this range, the thermally expandable microspheres can be dispersed preferably in the adhesive layer. As a result, an adhesive layer with excellent transparency can be formed. The sp value is determined by the Fedors method (Hideki Yamamoto, "The Fundamentals of sp Value, Applications and Calculation Methods", Information Organization Co., Ltd., published on April 3, 2006, pages 66-67). Specifically, the sp value is calculated from the evaporation energy Δe (cal) of each atom or atomic group forming the polymer at 25°C and the molar volume ΔV (cm3) of each atom or atomic group forming the polymer at 25°C by the following formula. In the case where the polymer is a copolymer, its sp value is calculated as follows: Calculate the sp value of the homopolymer of each constituent unit constituting the copolymer, and add the values obtained by multiplying the sp values by the molar fraction of each constituent unit. sp value = (ΣΔe/ΣΔv) ^1/2

The thickness of the shell is preferably 15 μm or less, more preferably 7 μm or less, further preferably 5 μm or less, and particularly preferably 4 μm or less. Furthermore, the lower limit of the thickness of the shell is preferably 1 μm or more, more preferably 2 μm or more. If it is within this range, it is possible to produce thermally expandable microspheres that are not easily destroyed by unexpected external forces. Furthermore, by setting the upper and lower limits of the thickness of the shell to the above range, the deviation of the temperature during foaming can be reduced.

The volatile substance contained in the shell is typically an organic solvent. Examples of the organic solvent include: hydrocarbons consisting only of hydrogen atoms and carbon atoms such as propane, cyclopropane, butane, cyclobutane, isobutane, pentane, cyclopentane, neopentane, isopentane, hexane, cyclohexane, 2-methylpentane, 2,2-dimethylbutane, heptane, cycloheptane, octane, cyclooctane, methylheptanes, trimethylpentanes, etc.; and hydrofluoroethers such as C ₃ F ₇ OCH ₃ , C ₄ F ₉ OCH ₃ , C ₄ F ₉ OC ₂ H ₅ , etc. These organic solvents may be used alone or in combination of two or more.

The content ratio of the above-mentioned organic solvent relative to the weight of the heat-expandable microspheres before heating is preferably 5% by weight to 35% by weight, and more preferably 10% by weight to 30% by weight. If it is within this range, an adhesive tape in which the heat-expandable microspheres are dispersed in the adhesive layer with high uniformity can be obtained. When the content ratio is less than 5% by weight, due to reasons such as low density, the heat-expandable microspheres are easily concentrated and distributed on the surface of the adhesive layer during the manufacture of the adhesive layer, and excessive unevenness may be generated on the surface of the adhesive layer after heating. When the content exceeds 35% by weight, the density is high, so it settles in the adhesive layer. Even if heated, sufficient concavity and convexity cannot be formed on the surface of the adhesive layer, and the desired releasability may not be obtained. In addition, adhesive residue may be generated.

The particle size of the heat-expandable microspheres before heating is preferably 0.5 μm to 80 μm, more preferably 5 μm to 45 μm, further preferably 10 μm to 20 μm, and particularly preferably 10 μm to 15 μm. Therefore, the particle size of the heat-expandable microspheres before heating is preferably 6 μm to 45 μm, more preferably 15 μm to 35 μm in terms of average particle size. The particle size and average particle size are values obtained by the particle size distribution measurement method in the laser scattering method.

The heat-expandable microspheres preferably have a moderate strength that does not break until the volume expansion rate becomes preferably 5 times or more, more preferably 7 times or more, and further preferably 10 times or more. When such heat-expandable microspheres are used, the adhesive force can be effectively reduced by heat treatment.

The content of the heat-expandable microspheres in the adhesive layer can be appropriately set according to the desired adhesive force reduction, etc. The content of the heat-expandable microspheres is, for example, 1 to 150 parts by weight, preferably 10 to 130 parts by weight, and further preferably 25 to 100 parts by weight, relative to 100 parts by weight of the base polymer forming the adhesive layer.

The arithmetic surface roughness Ra of the adhesive layer before the thermally expandable microspheres are expanded (i.e., before being heated) is preferably 500 nm or less, more preferably 400 nm or less, and further preferably 300 nm or less. Within this range, an adhesive sheet having excellent adhesion to an adherend can be obtained.

The above-mentioned adhesive layer is preferably an adhesive composed of a base polymer having a dynamic storage modulus at 80°C in the range of 5 kPa to 1 MPa (more preferably 10 kPa to 0.8 MPa). If it is such an adhesive layer, an adhesive sheet having moderate adhesiveness before heating and whose adhesiveness is easily reduced by heating can be formed. Furthermore, the dynamic storage modulus can be measured using a dynamic viscoelasticity measuring device (e.g., the product name "ARES" manufactured by Rheometrics) under the measuring conditions of a frequency of 1 Hz and a heating rate of 10°C/minute.

The above-mentioned heat-expandable microspheres can be manufactured by any appropriate method. In one embodiment, the above-mentioned heat-expandable microspheres are obtained by suspension polymerization. Suspension polymerization is usually carried out as follows: a monomer (shell forming material) and an organic solvent are dispersed in an aqueous dispersion medium containing a dispersant, and the monomer is polymerized in the presence of an organic solvent. In addition, a dispersion stabilizer for stabilizing the dispersion can also be used. Examples of the dispersion stabilizer in the aqueous dispersion medium include inorganic particles such as silicon dioxide, magnesium hydroxide, calcium phosphate, and aluminum hydroxide. Furthermore, as the dispersion stabilizing aid, for example, a condensation product of diethanolamine and aliphatic dicarboxylic acid, polyvinyl pyrrolidone, methyl cellulose, polyethylene oxide, polyvinyl alcohol, various emulsifiers, etc. can be used.

The properties of the heat-expandable microspheres such as particle size and organic solvent content can be controlled by the above-mentioned suspension polymerization conditions, the types of mixed components, and the amount of addition. For example, by reducing the amount of dispersant added and slowing down the stirring speed during polymerization, heat-expandable microspheres with large particle sizes can be obtained. In addition, if the amount of monomers added is increased and the stirring speed during polymerization is slowed down, heat-expandable microspheres with thick shells can be obtained.

B-2. Adhesive As the adhesive constituting the above-mentioned adhesive layer, any appropriate adhesive can be used as long as the effect of the present invention can be obtained. As the above-mentioned adhesive, for example, acrylic adhesive, silicone adhesive, vinyl alkyl ether adhesive, polyester adhesive, polyamide adhesive, urethane adhesive, fluorine adhesive, styrene-diene block copolymer adhesive, active energy ray curing adhesive, etc. can be cited. Among them, acrylic adhesive, rubber adhesive or silicone adhesive is preferred, and acrylic adhesive is more preferred.

The refractive index of the base polymer is preferably 1.30 to 1.50, more preferably 1.35 to 1.46, further preferably 1.38 to 1.43, and particularly preferably 1.39 to 1.42. Within this range, diffuse reflection of visible light in the adhesive layer can be reduced, and an adhesive layer with excellent transparency can be formed.

In one embodiment, the base polymer (preferably an acrylic polymer) in the adhesive contains constituent units derived from hydroxyl-containing monomers and/or constituent units derived from carboxyl-containing monomers. If such a base polymer is used, an adhesive layer with a better adjusted refractive index can be easily obtained. In addition, if such a base polymer is used, the above-mentioned thermal expansion microspheres can be better dispersed in the adhesive layer. As a result, an adhesive layer with excellent transparency can be formed.

In the above base polymer, the content ratio of the constituent unit derived from the hydroxyl group-containing monomer is preferably 1 mol% to 20 mol%, more preferably 2 mol% to 10 mol%, relative to 100 mol of the base polymer. Within this range, the above effect becomes significant. In addition, a thermally expandable microsphere having a shell with a well-adjusted refractive index can be obtained.

In the above-mentioned base polymer, the content ratio of the constituent unit derived from the carboxyl group-containing monomer is preferably 1 mol% to 20 mol%, and more preferably 2 mol% to 10 mol%, relative to 100 mol of the base polymer. Within this range, the above-mentioned effect becomes significant. In addition, a thermally expandable microsphere having a shell with a well-adjusted refractive index can be obtained.

The hydroxyl value of the above-mentioned base polymer having a hydroxyl group is preferably 0 to 50, and more preferably 20 to 30. Furthermore, the acid value of the above-mentioned base polymer having a carboxyl group is preferably 10 to 100, and more preferably 20 to 50. If a base polymer having a hydroxyl value and an acid value within such a range is used, the above-mentioned thermally expandable microspheres can be better dispersed in the adhesive layer. As a result, an adhesive layer having excellent transparency can be formed. Furthermore, the hydroxyl value and acid value of the polymer in the adhesive layer can be measured by extracting the solvent-soluble components in the adhesive layer. Specifically, the solvent-soluble components can be extracted by the following method. (i) The adhesive layer is placed in a solvent to prepare a solution sample in which the solvent-soluble components in the adhesive layer are dissolved in the above solvent. As the solvent, any one solvent selected from chloroform (CHCl ₃ ), dichloromethane (CH ₂ Cl ₂ ), tetrahydrofuran (THF), acetone, dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), methanol, ethanol, toluene and water or a mixed solvent containing two or more selected therefrom in any ratio can be used, taking into account polarity, etc. Typically, about 30 mL of the solvent is added to about 0.2 g of the adhesive layer, and the mixture is stirred at a temperature ranging from room temperature to about the boiling point of the solvent used for about 30 minutes to 12 hours. If necessary, for example, when the extraction efficiency of the component to be analyzed is low, a solvent in an amount substantially equal to that of the aliquoted solution may be added to the sample after the aliquoted solution is aliquoted, stirred, and the solution may be aliquoted. This operation may be repeated once or more to prepare a solution sample. (ii) The solvent may be removed from the solution sample by evaporation or the like to extract a solvent-soluble polymer. In addition, the solvent-soluble polymer may contain solvent-soluble components such as low molecular weight components of the unreacted crosslinking agent that are not the target of the measurement. In this case, a solvent-soluble polymer consisting only of the target of the measurement may be prepared by immersing the solution sample in a solvent in which only the polymer component is insoluble (reprecipitation method), performing molecular weight separation by gel filtration chromatography using the solution sample (preparative liquid chromatography method), or the like.

The sp value of the base polymer is preferably 10.0 (cal/cm ³ ) ^1/2 to 15.0 (cal/cm ³ ) ^1/2 , more preferably 10.0 (cal/cm ³ ) ^1/2 to 14.0 (cal/cm ³ ) ^1/2 , and even more preferably 10.0 (cal/cm ³ ) ^1/2 to 12.0 (cal/cm ³ ) ^1/2 . Within this range, the thermally expandable microspheres can be preferably dispersed in the adhesive layer. As a result, an adhesive layer with excellent transparency can be formed.

(Acrylic adhesive) As the above-mentioned acrylic adhesive, for example, there can be cited: an acrylic adhesive using an acrylic polymer (homopolymer or copolymer) as a base polymer using one or more (meth) alkyl esters of acrylic acid as monomer components. Specific examples of (meth) alkyl esters of acrylic acid include: methyl (meth) acrylate, ethyl (meth) acrylate, propyl (meth) acrylate, isopropyl (meth) acrylate, butyl (meth) acrylate, isobutyl (meth) acrylate, sec-butyl (meth) acrylate, tert-butyl (meth) acrylate, pentyl (meth) acrylate, hexyl (meth) acrylate, heptyl (meth) acrylate, octyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, isooctyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, 1-oct ... 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 and the like C1-20 alkyl (meth)acrylates. Among them, alkyl (meth)acrylates having a linear or branched alkyl group with 4 to 18 carbon atoms can be preferably used.

In one embodiment, the acrylic polymer comprises a constituent unit derived from a monomer having a homopolymer glass transition temperature (Tg) of 80° C. or higher (preferably 90° C. or higher, and further preferably 100° C. or higher). When such a polymer is used, an adhesive layer having a moderate elastic modulus can be formed. Examples of the monomer include cyclohexyl methacrylate (Tg: 83°C), dicyclopentyl acrylate (Tg: 120°C), dicyclopentyl methacrylate (Tg: 175°C), isobutyl acrylate (Tg: 94°C), isobutyl methacrylate (Tg: 150°C), tert-butyl methacrylate (Tg: 118°C), methyl methacrylate (Tg: 105°C), trihydroxymethylpropane triacrylate (Tg: >250°C), styrene (Tg: 80°C), acrylonitrile (Tg: 97°C), and N-acryloylmorpholine (Tg: 145°C). Among them, methyl methacrylate is preferred. The content ratio of the constituent units derived from the monomer having a glass transition temperature (Tg) of 80° C. or higher in the homopolymer is preferably 1 to 20 parts by weight, more preferably 1 to 10 parts by weight, relative to 100 parts by weight of the base polymer (acrylic polymer).

For the purpose of modifying cohesive force, heat resistance, crosslinking property, etc., the acrylic polymer may contain units corresponding to other monomers copolymerizable with the alkyl (meth)acrylate as required. Examples of such monomers include: carboxyl group-containing monomers such as acrylic acid, methacrylic acid, carboxyethyl acrylate, carboxypentyl acrylate, itaconic acid, maleic acid, fumaric acid, and crotonic acid; acid anhydride monomers such as maleic anhydride and itaconic anhydride; hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, hydroxybutyl (meth)acrylate, hydroxyhexyl (meth)acrylate, hydroxyoctyl (meth)acrylate, and octyl (meth)acrylate. Monomers containing hydroxyl groups, such as hydroxydecyl acrylate, hydroxylauryl (meth)acrylate, and (4-hydroxymethylcyclohexyl)methyl methacrylate; monomers containing sulfonic acid groups, such as styrene sulfonic acid, allyl sulfonic acid, 2-(meth)acrylamide-2-methylpropanesulfonic acid, (meth)acrylamide propanesulfonic acid, (meth)acrylate sulfopropyl ester, and (meth)acryloyloxynaphthalenesulfonic acid; (meth)acrylamide, N,N-dimethyl (meth)acrylamide (N-substituted) amide monomers such as amine, N-butyl (meth)acrylamide, N-hydroxymethyl (meth)acrylamide, and N-hydroxymethylpropane (meth)acrylamide; (meth)acrylic acid aminoalkyl ester monomers such as (meth)acrylic acid aminoethyl ester, (meth)acrylic acid N,N-dimethylaminoethyl ester, and (meth)acrylic acid tert-butylaminoethyl ester; (meth)acrylic acid methoxyethyl ester, (meth)acrylic acid ethoxyethyl ester, (Meth)acrylic acid alkoxyalkyl ester monomers such as ethyl ester; maleimide monomers such as N-cyclohexylmaleimide, N-isopropylmaleimide, N-laurylmaleimide, and N-phenylmaleimide; N-methyliconimide, N-ethyliconimide, N-butyliconimide, N-octyliconimide, N-2-ethylhexyliconimide, N-cyclohexyliconimide, N-laurylmaleimide, and N-phenylmaleimide; Iconimide monomers such as 1,2-dioxadiazole; succinimide monomers such as N-(meth)acryloyloxymethylenesuccinimide, N-(meth)acryloyl-6-oxyhexamethylenesuccinimide, and N-(meth)acryloyl-8-oxyoctamethylenesuccinimide; vinyl acetate, vinyl propionate, N-vinylpyrrolidone, methylvinylpyrrolidone, vinylpyridine, vinylpiperidone, vinylpyrimidine, etc. Vinyl monomers such as d-pyridine, vinyl piperazine, vinyl pyrazine, vinyl pyrrole, vinyl imidazole, vinyl oxazole, vinyl morpholine, N-vinyl carboxylic acid amides, styrene, α-methylstyrene, N-vinyl caprolactam; cyanoacrylate monomers such as acrylonitrile and methacrylonitrile; acrylic monomers containing epoxy groups such as glycidyl (meth)acrylate; glycol acrylate monomers such as polyethylene glycol (meth)acrylate, polypropylene glycol (meth)acrylate, methoxyethylene glycol (meth)acrylate, methoxypolypropylene glycol (meth)acrylate; acrylate monomers having heterocyclic rings, halogen atoms, silicon atoms, etc. such as tetrahydrofurfuryl (meth)acrylate, fluoro(meth)acrylate, polysilicone (meth)acrylate; hexanediol di(meth)acrylate, (poly)ethylene glycol di(meth)acrylate, (poly)propylene glycol di(meth)acrylate, etc. The present invention can be used as a monomer of the present invention, for example, a polyfunctional monomer such as tri(meth)acrylate, neopentyl glycol di(meth)acrylate, pentaerythritol di(meth)acrylate, trihydroxymethylpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, dipentaerythritol hexa(meth)acrylate, epoxy acrylate, polyester acrylate, urethane acrylate, or the like; an olefin monomer such as isoprene, butadiene, or isobutylene; or a vinyl ether monomer such as vinyl ether, or the like. Such monomers can be used alone or in combination of two or more.

(Additives) The above-mentioned adhesive may contain any appropriate additives as needed. Examples of such additives include crosslinking agents, adhesive imparting agents, plasticizers, pigments, dyes, fillers, anti-aging agents, conductive materials, antistatic agents, ultraviolet absorbers, light stabilizers, stripping regulators, softeners, surfactants, flame retardants, antioxidants, etc.

As the above-mentioned adhesive-imparting agent, any appropriate adhesive-imparting agent can be used. As the adhesive-imparting agent, for example, an adhesive-imparting resin is used. Specific examples of the adhesive-imparting resin include: rosin-based adhesive-imparting resins (for example, unmodified rosin, modified rosin, rosin phenol-based resins, rosin ester-based resins, etc.), terpene-based adhesive-imparting resins (for example, terpene-based resins, terpene phenol-based resins, styrene-modified terpene-based resins, aromatic-modified terpene-based resins, hydrogenated terpene-based resins), hydrocarbon-based adhesive-imparting resins (for example, aliphatic hydrocarbon resins, aliphatic cyclic hydrocarbon resins, aromatic hydrocarbon resins ( For example, styrene-based resins, xylene-based resins, etc.), aliphatic-aromatic petroleum resins, aliphatic-aliphatic cyclopentane petroleum resins, hydrogenated hydrocarbon resins, coumarone-based resins, coumarone-indene-based resins, etc.), phenol-based adhesive-imparting resins (for example, alkylphenol-based resins, xylene formaldehyde-based resins, cresol-formaldehyde resins, phenol-formaldehyde varnishes, etc.), ketone-based adhesive-imparting resins, polyamide-based adhesive-imparting resins, epoxy-based adhesive-imparting resins, elastic system adhesive-imparting resins, etc.

The amount of the adhesion agent added is preferably 5 to 100 parts by weight, more preferably 10 to 50 parts by weight, relative to 100 parts by weight of the base polymer.

As the above-mentioned crosslinking agent, for example, in addition to isocyanate crosslinking agents, epoxy crosslinking agents, melamine crosslinking agents, and peroxide crosslinking agents, urea crosslinking agents, metal alkoxide crosslinking agents, metal chelate crosslinking agents, metal salt crosslinking agents, carbodiimide crosslinking agents, oxazoline crosslinking agents, aziridine crosslinking agents, amine crosslinking agents, etc. Among them, preferably an isocyanate crosslinking agent or an epoxy crosslinking agent.

Specific examples of the isocyanate crosslinking agent include: low-order aliphatic polyisocyanates such as butyl diisocyanate and hexamethylene diisocyanate; alicyclic isocyanates such as cyclopentyl diisocyanate, cyclohexyl diisocyanate, and isophorone diisocyanate; aromatic isocyanates such as 2,4-toluene diisocyanate, 4,4'-diphenylmethane diisocyanate, and xylylene diisocyanate; trihydroxymethylpropane/toluene diisocyanate trimer adduct (manufactured by Nippon Polyurethane Industries, Ltd., trade name "CORONATE L"), trihydroxymethylpropane/hexamethylene diisocyanate trimer adduct (manufactured by Nippon Polyurethane Industries, Ltd., trade name "CORONATE The content of the isocyanate crosslinking agent can be set to any appropriate amount according to the desired adhesion, elasticity of the adhesive layer, etc., and is typically 0.1 to 20 parts by weight, and more preferably 0.5 to 10 parts by weight, relative to 100 parts by weight of the base polymer.

Examples of the epoxy crosslinking agent include N,N,N',N'-tetraglycidyl-m-xylylenediamine, diglycidylaniline, 1,3-bis(N,N-glycidylaminomethyl)cyclohexane (manufactured by Mitsubishi Gas Chemical Co., Ltd., trade name "TETRAD C"), 1,6-hexanediol diglycidyl ether (manufactured by Kyoeisha Chemical Co., Ltd., trade name "Epolight 1600"), neopentyl glycol diglycidyl ether (manufactured by Kyoeisha Chemical Co., Ltd., trade name "Epolight 1500 NP"), ethylene glycol diglycidyl ether (manufactured by Kyoeisha Chemical Co., Ltd., trade name "Epolight 40E"), propylene glycol diglycidyl ether (manufactured by Kyoeisha Chemical Co., Ltd., trade name "Epolight 1500 NP"), 70P"), polyethylene glycol diglycidyl ether (manufactured by NOF Corporation, trade name "EPIOL E-400"), polypropylene glycol diglycidyl ether (manufactured by NOF Corporation, trade name "EPIOL P-200"), sorbitol polyglycidyl ether (manufactured by Nagase ChemteX Corporation, trade name "Denacol EX-611"), glycerol polyglycidyl ether (manufactured by Nagase ChemteX Corporation, trade name "Denacol EX-314"), pentaerythritol polyglycidyl ether, polyglycerol polyglycidyl ether (manufactured by Nagase ChemteX Corporation, trade name "Denacol EX-512"), sorbitan polyglycidyl ether, trihydroxymethylpropane polyglycidyl ether, diglycidyl adipate, diglycidyl phthalate, triglycidyl-tri(2-hydroxyethyl)isocyanurate, resorcinol diglycidyl ether, bisphenol-S-diglycidyl ether, epoxy resins having two or more epoxy groups in the molecule, etc. The content of the epoxy crosslinking agent can be set to any appropriate amount according to the desired adhesion, elasticity of the adhesive layer, etc., and is typically 0.01 to 10 parts by weight, and more preferably 0.03 to 5 parts by weight, relative to 100 parts by weight of the base polymer.

As the above-mentioned plasticizer, any appropriate plasticizer can be used. Specific examples of plasticizers include: trimellitic acid ester plasticizers, pyromellitic acid ester plasticizers, polyester plasticizers, adipic acid plasticizers, etc. Among them, trimellitic acid ester plasticizers (for example, tri(n-octyl) trimellitic acid ester, tri(2-ethylhexyl) trimellitic acid ester, etc.) or pyromellitic acid ester plasticizers (for example, tetra(n-octyl) pyromellitic acid ester, tetra(2-ethylhexyl) pyromellitic acid ester, etc.) are preferred. Plasticizers can be used alone or in combination of two or more. The content of the plasticizer is preferably 1 to 20 parts by weight, and more preferably 1 to 5 parts by weight, relative to 100 parts by weight of the base polymer.

C. Substrate Examples of the substrate include resin sheets, nonwoven fabrics, paper, metal foils, woven fabrics, rubber sheets, foam sheets, and laminates thereof (particularly laminates containing resin sheets). Examples of the resin constituting the resin sheet include polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polybutylene terephthalate (PBT), polyethylene (PE), polypropylene (PP), ethylene-propylene copolymer, ethylene-vinyl acetate copolymer (EVA), polyamide (nylon), wholly aromatic polyamide (aromatic polyamide), polyimide (PI), polyvinyl chloride (PVC), polyphenylene sulfide (PPS), fluorine-based resin, polyetheretherketone (PEEK), and the like. Examples of nonwoven fabrics include: nonwoven fabrics made of heat-resistant natural fibers such as Manila hemp; synthetic resin nonwoven fabrics such as polypropylene resin nonwoven fabrics, polyethylene resin nonwoven fabrics, and ester resin nonwoven fabrics. Examples of metal foils include: copper foils, stainless steel foils, and aluminum foils. Examples of paper include: Japanese paper, kraft paper, and the like. The substrate may be a single layer or multiple layers. When the substrate is multiple layers, the composition of each layer may be the same or different.

The thickness of the substrate can be set to any appropriate thickness according to the desired strength or flexibility, the purpose of use, etc. The thickness of the substrate is preferably 1000 μm or less, more preferably 1 μm to 1000 μm, further preferably 1 μm to 500 μm, particularly preferably 3 μm to 300 μm, and most preferably 5 μm to 250 μm.

The substrate may be subjected to surface treatment. Examples of the surface treatment include corona treatment, chromic acid treatment, ozone exposure, flame exposure, high-voltage electric shock exposure, ionizing radiation treatment, and coating treatment using a primer.

As the above-mentioned organic coating material, for example, the materials described in "Plastic Hard Coating Material II (CMC Publishing, (2004))" can be cited. It is preferred to use a urethane polymer, and it is more preferred to use polyacrylic urethane, polyester urethane or their precursors. The reason is that the coating and coating on the substrate are simple, and there are many industrial options and they can be obtained cheaply. The urethane polymer is, for example, a polymer of a reaction mixture containing an isocyanate monomer and a monomer containing an alcoholic hydroxyl group (for example, a hydroxyl-containing acrylic compound or a hydroxyl-containing ester compound). In the organic coating material, as an arbitrary additive, a chain extender such as polyamine, an anti-aging agent, an oxidation stabilizer, etc. can be included. The thickness of the organic coating layer is not particularly limited, and is, for example, appropriately about 0.1 μm to 10 μm, preferably about 0.1 μm to 5 μm, and more preferably about 0.5 μm to 5 μm.

D. Other adhesive layer As the above-mentioned other adhesive layer, any appropriate adhesive layer can be formed. As adhesives forming other adhesive layers, for example, there can be cited: rubber adhesives, acrylic adhesives, vinyl alkyl ether adhesives, silicone adhesives, polyester adhesives, polyamide adhesives, urethane adhesives, fluorine adhesives, styrene-diene block copolymer adhesives, etc. The adhesive can be formulated with known or commonly used additives such as plasticizers, fillers, surfactants, anti-aging agents, and adhesion-imparting agents.

The thickness of the other adhesive layer is preferably 300 μm or less, more preferably 1 μm to 300 μm, and further preferably 5 μm to 100 μm.

E. Elastic layer The adhesive sheet of the present invention may also have an elastic layer. The elastic layer may be disposed on one side of the adhesive layer. When the adhesive sheet has a substrate, the elastic layer may be disposed between the adhesive layer and the substrate. By having an elastic layer, the followability to the adherend is improved. Furthermore, when the adhesive sheet having an elastic layer is heated during peeling, the deformation (expansion) of the adhesive layer in the surface direction is restrained, and the deformation in the thickness direction takes precedence. As a result, the peeling property is improved.

The elastic layer includes a base polymer, and as the base polymer, the polymer exemplified as the base polymer constituting the adhesive layer can be used. In one embodiment, the elastic layer can include natural rubber, synthetic rubber, synthetic resin, etc. Examples of the synthetic rubber and synthetic resin include: nitrile-based, diene-based, and acrylic-based synthetic rubbers; thermoplastic elastomers such as polyolefin-based and polyester-based; ethylene-vinyl acetate copolymers; urethane; polybutadiene; soft polyvinyl chloride, etc. The base polymer constituting the elastic layer can be the same as or different from the base polymer forming the adhesive layer. The elastic layer can be a foamed film formed from the base polymer. The foam film can be obtained by any appropriate method. Furthermore, the elastic layer and the adhesive layer can be distinguished by the difference in base polymers and/or the presence or absence of thermally expandable microspheres (the elastic layer does not contain thermally expandable microspheres). More specifically, in the case where the elastic layer and the adhesive layer are formed of different base polymers and the interface between the elastic layer and the adhesive layer can be identified by cross-sectional observation, the boundary between the elastic layer and the adhesive layer is defined by the interface. In addition, when the interface between the elastic layer and the adhesive layer cannot be identified by cross-sectional observation, the region where the thermally expandable microspheres are observed by cross-sectional observation is the adhesive layer.

The elastic layer may contain any appropriate additives as required. Examples of additives include crosslinking agents, vulcanizers, adhesive imparting resins, plasticizers, softeners, fillers, and anti-aging agents. When a hard resin such as polyvinyl chloride is used as the base polymer, it is preferred to use a plasticizer and/or softener in combination to form an elastic layer with desired elasticity.

The thickness of the elastic layer is preferably 3 μm to 200 μm, more preferably 5 μm to 100 μm. If it is within this range, the above functions of the elastic layer can be fully exerted.

The tensile modulus of the elastic layer at 25°C is preferably 0.2 MPa to 500 MPa, more preferably 0.3 MPa to 500 MPa, and further preferably 0.5 MPa to 500 MPa. Within this range, the elastic layer can fully exert the above-mentioned function. The tensile modulus can be measured according to JIS K 7161:2008.

F. Method for producing adhesive sheet The adhesive sheet of the present invention can be produced by any appropriate method. For example, the adhesive sheet of the present invention can be produced by directly coating a composition containing an adhesive and heat-expandable microspheres on a substrate (in the case of obtaining an adhesive sheet without a substrate, any appropriate substrate), or by transferring a coating layer formed by coating a composition containing an adhesive and heat-expandable microspheres on any appropriate substrate to a substrate. The composition containing an adhesive and heat-expandable microspheres can contain any appropriate solvent. Furthermore, after forming an adhesive coating layer using a composition containing an adhesive, heat-expandable microspheres are sprinkled on the adhesive coating layer, and then the heat-expandable microspheres are embedded in the coating layer using a laminating press or the like, thereby forming an adhesive layer containing heat-expandable microspheres.

When the adhesive layer has the elastic layer, the elastic layer can be formed by, for example, coating a composition for forming the elastic layer on the substrate or the adhesive layer.

As a coating method for each of the above-mentioned compositions, any appropriate coating method can be adopted. For example, each layer can be formed by drying after coating. As a coating method, for example, coating methods using a multi-layer coater, a die coater, a gravure coater, a coater, etc. can be cited. As a drying method, for example, natural drying, heat drying, etc. can be cited. The heating temperature during heat drying can be set to any appropriate temperature according to the characteristics of the substance to be dried. Example

The present invention is specifically described below by way of examples, but the present invention is not limited to the examples. The evaluation method in the examples is as follows. Furthermore, in the following evaluation, an adhesive sheet from which a peeling pad is peeled off is used. In addition, in the examples, "parts" and "%" are based on weight unless otherwise specified.

<Evaluation method> (1) Transparency evaluation A4-sized adhesive sheets were prepared and placed on the same newspapers. The transparency was evaluated based on the visibility of the text on the newspapers through the adhesive sheets. If all the text in the A4 size could be read, it was rated as excellent (○); if less than 10% of the text could not be read, it was rated as good (Δ); if more than 10% could not be read, it was rated as unacceptable (×). (2) Foaming start temperature An adhesive tape (20 mm × 50 mm) was placed on a hot plate (digital hot plate PC-400D manufactured by AS ONE) with the adhesive layer on the hot plate side. A glass plate was placed on the adhesive tape to produce a hot plate/adhesive tape/glass plate sandwich structure. The temperature of the hot plate is varied within the range of 50℃ to 300℃, and the lowest temperature of the hot plate at which the expansion rate of the adhesive layer (the thickness of the adhesive tape after heating at the specified temperature for 1 minute/the thickness of the unheated adhesive tape at room temperature) is above 105% is taken as the foaming starting temperature.

[Production Example 1] Preparation of Thermally Expandable Microspheres A 150 g of sodium chloride, 70 g of colloidal silica containing 20% by weight of silica active ingredient (manufactured by Nissan Chemical Co., Ltd., trade name "SNOWTEX"), 1 g of polyvinyl pyrrolidone, and 0.5 g of a condensate of diethanolamine and adipic acid were added to 600 g of distilled water, and the pH of the obtained mixture was adjusted to 2.8-3.2 to obtain an aqueous solution. 36 g of acrylonitrile, 36 g of methyl methacrylate, and 28 g of methacrylic acid were added to the above aqueous solution as an oil-based additive to form a shell material. Furthermore, 1 g of ethylene glycol dimethacrylate was added as a crosslinking agent to obtain a reaction solution. The reaction solution was added to a pressure-resistant reaction vessel equipped with a homomixer (manufactured by Tokushiki Kagaku Kogyo Co., Ltd., trade name "TK Homomixer"), and 70 g of isobutane (boiling point: -11.7°C) and 5 g of initiator (diisopropoxydicarbonate) as an organic solvent intended to be enclosed in the shell were added to the pressure-resistant reaction vessel. The homomixer was rotated under the specified initial stirring conditions (stirring speed: 6000 rpm, stirring time: 2 minutes), and after stirring the above mixture, it was heated to 60°C while stirring at 80 rpm and reacted for 24 hours. The solid component obtained by filtering the reaction solution after the reaction was placed at room temperature under a nitrogen flow for 1 week to obtain thermally expandable microspheres. Furthermore, the obtained thermal expansion microspheres were measured using the trade name "SALD-2000J" manufactured by Shimadzu Corporation, and the average particle size was 14 μm. In addition, by X-ray CT (Xradia 520versa manufactured by ZEISS (measurement conditions: tube voltage 60 KV tube current 83 μA, pixel size 0.20 μm/pixel)), it was found that the isobutane solvent in the thermal expansion microspheres contained 12% by weight relative to the weight of the thermal expansion microspheres. In addition, the above-mentioned X-ray CT was used for measurement, and the thickness of the shell of the thermal expansion microspheres was 2 μm.

[Production Examples 2 to 7] Production of Heat-Expandable Microspheres B to G Heat-expandable microspheres were obtained in the same manner as in Production Example 1 except that the monomer components were set as shown in Table 1.

[Table 1] Thermal expansion microspheres A Thermal expansion microspheres B Thermal expansion microspheres C Thermal expansion microspheres D Monomer ingredients Weight Morelby Weight Morelby Weight Morelby Weight Morelby Methyl Methacrylate (MMA) 36 0.27 34 0.23 Methacrylic acid (MAA) 28 0.24 twenty four 0.19 20 0.21 Acrylonitrile (AN) 36 0.50 34 0.42 45 0.51 Acrylic acid (AA) 18 0.17 Methacrylonitrile (MAN) 60 0.79 55 0.49 Vinylidene chloride Calculate refractive index 1.407 1.409 1.406 1.395 SP value 12.9 13.9 12.7 13.6 Thermal expansion microspheres E Thermal expansion microspheres F Thermal expansion microspheres G Monomer ingredients Weight Morelby Weight Morelby Weight Morelby Methyl Methacrylate (MMA) Methacrylic acid (MAA) 40 0.36 Acrylonitrile (AN) 36 0.50 60 0.65 Acrylic acid (AA) Methacrylonitrile (MAN) 64 0.50 55 0.64 40 0.35 Vinylidene chloride Calculate refractive index 1.408 1.411 1.394 SP value 13.5 12.6 13.8

[Production Example 8] Preparation of acrylic copolymer I 2-ethylhexyl acrylate (EHA), ethyl acrylate (EA), methyl methacrylate (MMA), and 2-hydroxyethyl acrylate (HEA) were added to a toluene solvent in a weight ratio of 30:70:5:4, and 0.5 parts by weight of tert-butyl peroxy-2-ethylhexanoate (manufactured by NOF Corporation, trade name: PERBUTYL O (registered trademark)) was added as a polymerization initiator. The mixture was stirred at 80°C to 90°C for 12 hours to obtain a toluene solution of acrylic copolymer I having a solid content concentration of 40%. The refractive index (calculated refractive index) of acrylic copolymer I was 1.413, and the SP value was 10.1.

[Production Example 9] Preparation of acrylic copolymer II 2-Ethylhexyl acrylate (EHA), ethyl acrylate (EA), methyl methacrylate (MMA), and acrylic acid (AA) were added to a toluene solvent in a ratio of 30:70:5:5.5 (weight ratio), and 0.5 parts by weight of tert-butyl peroxy-2-ethylhexanoate (manufactured by NOF Corporation, trade name: PERBUTYL O (registered trademark)) was added as a polymerization initiator, and stirred at 80°C to 90°C for 12 hours to obtain a toluene solution of acrylic copolymer II having a solid content concentration of 40%. In addition, the refractive index (calculated refractive index) of acrylic copolymer II was 1.413, and the SP value was 10.1.

[Example 1] 100 parts by weight of acrylic copolymer I as a base polymer, 2 parts by weight of an isocyanate crosslinking agent (manufactured by Tosoh Corporation, trade name "CORONATE L"), 35 parts by weight of thermally expandable microspheres A, and 210 parts by weight of toluene were mixed to prepare a composition for forming an adhesive layer. 100 parts by weight of acrylic copolymer I, 2 parts by weight of an isocyanate crosslinking agent (manufactured by Tosoh Corporation, trade name "CORONATE L"), and 210 parts by weight of toluene were mixed to prepare a composition for forming an elastic layer. The composition for forming an elastic layer was applied to a PET film (thickness: 38 μm) as a substrate and dried to form an elastic layer with a thickness of 20 μm. Furthermore, the adhesive layer-forming composition is applied on the elastic layer and dried to obtain an adhesive sheet (adhesive layer (thickness: 30 μm)/elastic layer (20 microns)/substrate). Furthermore, another adhesive layer is formed on the side of the substrate opposite to the elastic layer. The obtained adhesive tape is provided for the above evaluation. The results are shown in Table 2.

[Examples 2 to 7, Comparative Examples 1 to 2] Except that the types of base polymer and heat-expandable microspheres in the adhesive layer were set to the composition shown in Table 2, an adhesive tape was obtained in the same manner as in Example 1. The obtained adhesive tape was subjected to the above evaluation. The results are shown in Table 2.

[Table 2] Embodiment 1 Embodiment 2 Embodiment 3 Embodiment 4 Embodiment 5 Adhesive layer Base polymer Acrylic copolymer I Acrylic copolymer I Acrylic copolymer I Acrylic copolymer I Acrylic copolymer I Base polymer formulation amount (parts by weight) 100 100 100 100 100 Crosslinking agent CORONATE L CORONATE L CORONATE L CORONATE L CORONATE L Amount of crosslinking agent (parts by weight) 2 2 2 2 2 Thermal expansion microspheres Microsphere Preparation Example A Microsphere Preparation Example B Microsphere Preparation Example C Microsphere Preparation Example D Microsphere Preparation Example E Thermal expansion microspheres dosage (parts by weight) 35 35 35 35 35 Thermal expansion microsphere foaming starting temperature (℃) 90 180 140 110 80 The absolute value of the difference between the refractive index of the base polymer and the refractive index of the resin 0.006 0.004 0.007 0.018 0.005 Thickness(μm) 30 30 30 30 30 Transparency Rating ○ ○ ○ ○ ○ Embodiment 6 Embodiment 7 Comparison Example 1 Comparison Example 2 Adhesive layer Base polymer Acrylic copolymer I Acrylic copolymer II Acrylic copolymer I Acrylic copolymer II Base polymer formulation amount (parts by weight) 100 100 100 100 Crosslinking agent CORONATE L CORONATE L CORONATE L CORONATE L Amount of crosslinking agent (parts by weight) 2 2 2 2 Thermal expansion microspheres Microsphere Preparation Example F Microsphere Preparation Example D Microsphere Preparation Example G Microsphere Preparation Example G Thermal expansion microspheres dosage (parts by weight) 35 35 35 35 Thermal expansion microspheres foaming starting temperature (℃) 120 110 130 130 The absolute value of the difference between the refractive index of the base polymer and the refractive index of the resin 0.002 0.018 0.019 0.019 Thickness(μm) 30 30 30 30 Transparency Rating ○ ○ × ×

10: Adhesive layer 20: Base material 30: Other adhesive layers 100, 200, 300: Adhesive sheet

FIG. 1( a ) to ( c ) are schematic cross-sectional views of an adhesive sheet according to an embodiment of the present invention.

10: Adhesive layer

20: Base material

30: Other adhesive layers

100,200,300: Adhesive sheet

Claims (5)

An adhesive sheet, comprising: an adhesive layer including an adhesive and heat-expandable microspheres, the adhesive including a base polymer, the heat-expandable microspheres consisting of a shell formed by a resin and a volatile substance contained in the shell, the absolute value of the difference between the refractive index of the base polymer and the refractive index of the resin is 0 to 0.018. The adhesive sheet of claim 1, wherein the resin forming the shell comprises constituent units derived from carboxyl-containing monomers. The adhesive sheet of claim 2, wherein the resin forming the shell has a content ratio of constituent units derived from carboxyl group-containing monomers of 21 mol% to 36 mol% relative to 100 mol of the resin. The adhesive sheet of claim 1, wherein the resin forming the shell comprises constituent units derived from methacrylic acid. The adhesive sheet of claim 1, wherein the base polymer in the adhesive comprises constituent units derived from hydroxyl-containing monomers and/or constituent units derived from carboxyl-containing monomers.