JP7495776B2 - Adhesive sheet with release liner - Google Patents

Description

The present invention relates to an adhesive sheet having a foam substrate and an adhesive sheet with a release liner that includes the adhesive sheet.

Generally, adhesives (also called pressure-sensitive adhesives; the same applies below) are in a soft solid (viscoelastic) state at temperatures around room temperature, and have the property of easily adhering to an adherend when pressure is applied. Taking advantage of these properties, adhesives are widely used in a variety of fields for purposes such as joining and fixing, for example in the form of a substrate-attached adhesive sheet in which an adhesive layer is provided on at least one side of the substrate.

Adhesive sheets with a substrate using a foam having a cellular structure as the substrate (foam substrate-attached adhesive sheets) can be advantageous in terms of shock absorption and conformability to irregularities, compared to adhesive sheets with a substrate made of a plastic film having no cellular structure. In addition, such foam substrate-attached adhesive sheets can be advantageous in terms of waterproofing and sealing properties, compared to adhesive sheets with a nonwoven fabric substrate. For this reason, foam substrate-attached adhesive sheets can be preferably used for joining and fixing components in portable electronic devices. Patent Document 1 is an example of a technical document related to foam substrate-attached adhesive sheets.

JP 2015-155528 A

In recent years, from the viewpoints of increasing the size of the information display unit of portable electronic devices, improving design, and increasing design freedom, there has been a demand to bond the display unit or the display protection member to the housing within a narrower area. However, when the width of the adhesive sheet is narrowed, the bonding reliability of the parts by the adhesive sheet (e.g., pressing adhesive force) tends to decrease. Therefore, the object of the present invention is to provide an adhesive sheet that is suitable for narrowing the width and can exhibit good bonding reliability.

According to the present specification, a pressure-sensitive adhesive sheet with a release liner is provided, which comprises a pressure-sensitive adhesive sheet having a first adhesive surface and a second adhesive surface, and a release liner having a second release surface that contacts the second adhesive surface. The pressure-sensitive adhesive sheet includes a foam substrate, a first adhesive layer disposed on the first surface side of the foam substrate, and a second adhesive layer disposed on the second surface side of the foam substrate. In the pressure-sensitive adhesive sheet, the first adhesive surface is constituted by the first adhesive layer, and the second adhesive surface is constituted by the second adhesive layer. In the pressure-sensitive adhesive sheet, the arithmetic mean roughness Ra _A1 of the first adhesive surface is 800 nm or less, and the arithmetic mean roughness Ra _A2 of the second adhesive surface is 400 nm or less. The pressure-sensitive adhesive sheet constituting such a pressure-sensitive adhesive sheet with a release liner has a highly smooth adhesive surface, so that the adhesive surface can be efficiently brought into intimate contact with the surface of the adherend. This can improve the bonding reliability (e.g., pressing adhesive force).

In a preferred embodiment of the PSA sheet with release liner, the second release surface has an arithmetic mean roughness Ra _R2 of 400 nm or less. By placing the second adhesive surface of the PSA sheet in contact with the second release surface having such smoothness, the smoothness of the second adhesive surface can be suitably maintained or improved.

The release liner-attached PSA sheet disclosed herein may have a configuration in which the release liner has a first release surface on the opposite side to the second release surface, and the first release surface is in contact with the first adhesive surface. The arithmetic mean roughness Ra _R1 of the first release surface is preferably 800 nm or less. By placing the first adhesive surface of the PSA sheet in contact with the first release surface having such smoothness, the smoothness of the first adhesive surface can be suitably maintained or improved.

In a preferred embodiment of the technology disclosed herein, the 15% compressive strength of the adhesive sheet constituting the adhesive sheet with release liner is 2 N/cm2 or ^more . With such an adhesive sheet, the pressure force applied when attached to an adherend can be efficiently utilized to closely press the adhesive surface against the adherend surface, and the effect of the high smoothness of the adhesive surface tends to be better exhibited.

The adhesive layer in the technology disclosed herein preferably has a storage modulus at 23°C of 0.02 MPa or more and less than 0.20 MPa. Such an adhesive layer has appropriate flexibility, so that the highly smooth adhesive surface can be effectively brought into intimate contact with the surface of the adherend. In addition, the adhesive layer has appropriate cohesiveness, so that the adhesive strength can be easily increased.

The adhesive sheet constituting the adhesive sheet with release liner disclosed herein can form a reliable joint structure even in a narrow width, and is therefore suitable for use in joining parts of portable electronic devices. The adhesive sheet can be preferably used, for example, as a fixing member for joining the display unit or display unit protection member to the housing of a portable electronic device.

FIG. 1 is a schematic cross-sectional view illustrating a configuration of a pressure-sensitive adhesive sheet according to an embodiment. FIG. 2 is an explanatory diagram showing an evaluation sample used for measuring a pressure adhesive force. 3 is a cross-sectional view taken along line III-III in FIG. 2.

Preferred embodiments of the present invention will be described below. Matters necessary for carrying out the present invention other than those specifically mentioned in this specification can be understood by those skilled in the art based on the teachings on carrying out the invention described in this specification and the common general technical knowledge at the time of filing. The present invention can be carried out based on the contents disclosed in this specification and the common general technical knowledge in the relevant field.
In the drawings below, components and parts having the same function may be denoted by the same reference numerals, and duplicate descriptions may be omitted or simplified.

In this specification, the term "adhesive" refers to a material that exhibits a soft solid (viscoelastic) state in a temperature range near room temperature and has the property of adhering to an adherend by pressure, as described above. The adhesive here may generally be a material that has a property of satisfying a complex tensile modulus E ^* (1 Hz) < 10 7 ^dyne /cm ² (typically a material that has the above property at 25°C), as defined in, for example, "CA Dahlquist, "Adhesion: Fundamental and Practice", McLaren & Sons, (1966) P. 143". In addition, in this specification, the "base polymer" of the adhesive refers to the main component (i.e., a component that occupies 50% by weight or more of the rubber-like polymer) of the rubber-like polymer (a polymer that exhibits rubber elasticity in a temperature range near room temperature) contained in the adhesive. In this specification, the term "main component" refers to a component that occupies 50% by weight or more, unless otherwise specified.

<Adhesive sheet with release liner>
The pressure-sensitive adhesive sheet with release liner disclosed herein includes a pressure-sensitive adhesive sheet having a first adhesive surface and a second adhesive surface, and a release liner having a second release surface that contacts the second adhesive surface. The pressure-sensitive adhesive sheet is configured as a double-sided pressure-sensitive adhesive sheet (double-sided pressure-sensitive adhesive sheet) having a first pressure-sensitive adhesive layer and a second pressure-sensitive adhesive layer on the first and second surfaces of a foam substrate, respectively. The first pressure-sensitive adhesive surface of the pressure-sensitive adhesive sheet is configured by the first pressure-sensitive adhesive layer. That is, the surface of the first pressure-sensitive adhesive layer is the first pressure-sensitive adhesive surface. Similarly, the second pressure-sensitive adhesive surface of the pressure-sensitive adhesive sheet is configured by the second pressure-sensitive adhesive layer. The concept of pressure-sensitive adhesive sheet here includes a long shape such as a tape shape and other various external shapes.

The release-liner adhesive sheet disclosed herein may have, for example, a cross-sectional structure as shown in FIG. 1. The adhesive sheet 10 constituting this release-liner adhesive sheet 100 has a substrate 15 made of a foam sheet, and a first adhesive layer 11 and a second adhesive layer 12 provided on the first surface 15A and the second surface 15B (both of which are non-releasable) of the substrate 15, respectively. The release liner 20 constituting the release-liner adhesive sheet 100 has a first surface and a second surface as release surfaces, i.e., a first release surface 20A and a second release surface 20B, respectively. The release-liner adhesive sheet 100 shown in FIG. 1 is formed by superposing the release liner 20 and the adhesive sheet 10 so that the second release surface 20B abuts on the surface (second adhesive surface 10B) of the second adhesive layer 12, and then spirally winding the sheet in such a manner that the first adhesive layer 11 is on the inner circumferential side, so that the surface (first adhesive surface 10A) of the first adhesive layer 11 abuts on the first release surface 20A of the release liner 20. That is, the first adhesive surface 10A and the second adhesive surface 10B of the adhesive sheet 10 are protected by the first release surface 20A and the second release surface 20B of one release liner 20, respectively.

In the example shown in FIG. 1, the release-liner-attached adhesive sheet 100 is wound with the first adhesive layer 11 on the inner periphery side, but this is not limited to the orientation, and the second adhesive layer 12 may be wound with the inner periphery side. The release-liner-attached adhesive sheet disclosed herein may have a form in which the first adhesive surface 10A and the second adhesive surface 10B of the adhesive sheet 10 are protected by two independent release liners. That is, the first release surface abutting the first adhesive surface and the second release surface abutting the second adhesive surface may be on two different release liners. The form of the release-liner-attached adhesive sheet is not limited to the roll form shown in FIG. 1, and may be in the form of a sheet. For example, a plurality of adhesive sheets having a first adhesive layer and a second adhesive layer on both sides of a substrate and a plurality of release liners having a first release surface and a second release surface on both sides may be alternately stacked.

<Adhesive sheet>
The adhesive sheet disclosed herein has an arithmetic mean roughness Ra _A1 of the first adhesive surface of 800 nm or less, and an arithmetic mean roughness Ra _A2 of the second adhesive surface of 400 nm or less. Since both the first adhesive surface and the second adhesive surface of such an adhesive sheet have high smoothness, a highly reliable bonding structure can be formed. The orientation of the adhesive sheet (i.e., which of the adherends the first adhesive surface and the second adhesive surface are attached to) can be set according to the material and surface condition of the adherend, the mode and purpose of bonding, etc.

Here, in this specification, unless otherwise specified, the arithmetic mean roughness Ra refers to the arithmetic mean roughness obtained using a non-contact surface roughness measuring device. As a non-contact surface roughness measuring device, an optical interference type surface roughness measuring device is used. As a specific measuring device, a Wyko NT-9100 manufactured by Veeco or an equivalent product can be used. The specific measurement operation and measurement conditions can be set according to the measurement conditions described in the examples below, or so as to obtain results equivalent to or corresponding to those obtained when the measurement conditions are followed.

In the technology disclosed herein, the arithmetic mean roughness Ra _A2 of the second adhesive surface may be preferably 300 nm or less, more preferably 200 nm or less (e.g., 160 nm or less). As Ra _A2 decreases, it becomes easier to increase the adhesion between the second adhesive surface and the adherend surface, and a more preferable bonding state tends to be realized. In addition, the arithmetic mean roughness Ra _A1 of the first adhesive surface may be preferably 600 nm or less, more preferably 400 nm or less, even more preferably 300 nm or less, for example, 250 nm or less. As Ra _A1 decreases, it becomes easier to increase the adhesion between the first adhesive surface and the adherend surface, and a higher performance bonding tends to be realized. The technology disclosed herein may be preferably implemented in an embodiment in which R _A1 and R _A2 are both 400 nm or less (more preferably 300 nm or less, for example 250 nm or less). The lower limits of R _A1 and R _A2 are not particularly limited. Alternatively, from a practical viewpoint, the technology disclosed herein may also be preferably implemented in an embodiment in which one or both of R _A1 and R _A2 are 20 nm or more (e.g., 50 nm or more). The arithmetic mean roughnesses Ra _A1 and Ra _A2 of the first adhesive surface and the second adhesive surface can be controlled by the method and conditions for producing the adhesive layer constituting each adhesive surface, the type of adhesive constituting the adhesive layer, the surface smoothness of the release liner in contact with the adhesive surface, etc.

Although not particularly limited, the adhesive sheet disclosed herein suitably has a 15% compressive strength of 2 N/cm ² or more. According to an adhesive sheet exhibiting such a 15% compressive strength, the pressure (pressing force) applied to the adhesive sheet is prevented from being absorbed by the deformation (compression) of the adhesive sheet itself, and the highly smooth adhesive surface can be suitably brought into close contact with the surface of the adherend. This allows the effect of the high smoothness of the adhesive surface to be better exerted. This is particularly significant in the application mode in which the adhesive sheet is compressed between the adherends for a predetermined time, such as when preparing an evaluation sample for measuring the pressing adhesive force in the examples described later. In a preferred embodiment, the 15% compressive strength of the adhesive sheet can be 5 N/cm ² or more (more preferably 10 N/cm ² or more, even more preferably 20 N/cm ² or more, for example 30 N/cm ² or more, or 40 N/cm ² or more). The 15% compressive strength of the pressure-sensitive adhesive sheet is usually 150 N/ ^cm2 or less, preferably 120 N/ ^cm2 or less, and more preferably 100 N/ ^cm2 or less (e.g., 70 N/ ^cm2 or less). When the 15% compressive strength of the pressure-sensitive adhesive sheet is low, the shock absorption of the pressure-sensitive adhesive sheet tends to improve. From this viewpoint, in one embodiment of the technology disclosed herein, the 15% compressive strength of the pressure-sensitive adhesive sheet can be, for example, less than 30 N/ ^cm2 , may be less than 20 N/ ^cm2 , or may even be less than 10 N/ ^cm2 .

Here, the 15% compressive strength of the adhesive sheet refers to the load (load at a compression rate of 15%) when a measurement sample, which is a 40 mm square cut of the adhesive sheet and stacked to a thickness of about 2 mm, is sandwiched between a pair of flat plates and compressed by a thickness equivalent to 15% of the original thickness. In other words, the 15% compressive strength refers to the load when the measurement sample is compressed to a thickness equivalent to 85% of the original thickness, with the original thickness of the measurement sample being 100%.
The compressive strength of the pressure-sensitive adhesive sheet at any compression ratio is measured in accordance with JIS K 6767. As a specific measurement procedure, the measurement sample is set at the center of the pair of flat plates, and the gap between the flat plates is narrowed to continuously compress the sample to any compression ratio, and the flat plates are stopped at that point, and the load is measured after 10 seconds have passed. The compressive strength of the pressure-sensitive adhesive sheet can be controlled, for example, by the configuration of the pressure-sensitive adhesive sheet or the type of foam base material (constituting material, degree of crosslinking, density, size and shape of air bubbles, thickness, etc.).

(Foam Substrate)
The foam substrate constituting the pressure-sensitive adhesive sheet disclosed herein is a substrate having a portion with bubbles (cell structure), and is typically a substrate containing at least one layer of laminar foam (foam layer). The foam substrate may be a substrate composed of one or more foam layers. The foam substrate may be, for example, a substrate substantially composed of only one or more foam layers. Although not particularly limited, a suitable example of the foam substrate in the technology disclosed herein is a foam substrate composed of a single layer (one layer) of foam layer.

The thickness Hs of the foam substrate is not particularly limited and can be appropriately set depending on the strength, flexibility, and intended use of the adhesive sheet. From the viewpoint of thinning the joint, the thickness Hs of the foam substrate is usually 700 μm or less, preferably 400 μm or less, and more preferably 300 μm or less. From the viewpoint of processability when processing the adhesive sheet into a narrow width, a foam substrate having a thickness Hs of 200 μm or less (typically 180 μm or less, for example 160 μm or less) can be preferably used. Also, from the viewpoint of impact resistance of the adhesive sheet, the thickness Hs of the foam substrate is 50 μm or more, preferably 60 μm or more, and more preferably 80 μm or more. The technology disclosed herein can be preferably implemented in an embodiment in which the thickness Hs of the foam substrate is 100 μm or more (typically more than 100 μm, preferably 120 μm or more, for example 130 μm or more).

The density D (referring to apparent density, hereinafter the same unless otherwise specified) of the foam substrate is not particularly limited, and may be, for example, 0.1 g/cm ³ or more and 0.9 g/cm ³ or less. From the viewpoint of impact absorption, the density D of the foam substrate is suitably 0.8 g/cm ³ or less, and preferably 0.7 g/cm ³ or less (e.g., 0.6 g/cm ³ or less). From the viewpoint of durability against impact, the density D of the foam substrate is preferably 0.12 g/cm ³ or more, more preferably 0.15 g/cm ³ or more, and even more preferably 0.2 g/cm ³ or more (e.g., 0.3 g/cm ³ or more). In one embodiment, for example, a foam substrate having a density D in the range of 0.3 g/cm ³ or more and 0.8 g/cm ³ or less can be preferably adopted. The density D (apparent density) of the foam substrate can be measured in accordance with JIS K 6767.

The average bubble diameter of the foam substrate is not particularly limited, and may be, for example, 300 μm or less, preferably 200 μm or less, more preferably 150 μm or less. In order to achieve high waterproof and dustproof performance even in a narrow width, in one embodiment, a foam substrate having an average bubble diameter of 120 μm or less (more preferably 100 μm or less, for example 80 μm or less, and even 70 μm or less) may be preferably used. The lower limit of the average bubble diameter is not particularly limited, but from the viewpoint of impact resistance, it is usually appropriate to have a diameter of 10 μm or more, and preferably 20 μm or more (for example 30 μm or more). The average bubble diameter here refers to the average bubble diameter in terms of a sphere obtained by observing the cross section of the foam substrate with an electron microscope.

The compressive strength of the foam substrate is not particularly limited. For example, a foam substrate having a 25% compressive strength C ₂₅ of 20 kPa or more (preferably 40 kPa or more, more preferably 100 kPa or more, for example 150 kPa or more) can be used. The C ₂₅ of the foam substrate can be, for example, 1300 kPa or less (typically 1000 kPa or less). Here, the 25% compressive strength C ₂₅ of the foam substrate is measured in the same manner as the 15% compressive strength of the pressure-sensitive adhesive sheet, except that the foam substrate is cut into a 30 mm square shape, stacked to a thickness of about 2 mm, and the compressibility of the measurement sample is 25%. The compressive strength of the foam substrate can be controlled, for example, by the degree of crosslinking or density of the material constituting the foam substrate, the size or shape of the bubbles, etc.

In a preferred embodiment of the technology disclosed herein, a foam substrate having a _C25 of 800 kPa or less (preferably 600 kPa or less, for example 500 kPa or less) may be preferably used. As the _C25 of the foam substrate decreases, the impact absorption of the pressure-sensitive adhesive sheet having the foam substrate tends to increase. In addition, a foam substrate having a _C25 of 50 kPa or more (more preferably 300 kPa or more, for example 400 kPa or more) may be preferably used. As the _C25 of the foam substrate increases, the durability of the foam substrate against impacts such as dropping tends to improve. A pressure-sensitive adhesive sheet having such a foam substrate tends to be better prevented from tearing due to impacts such as dropping, even if it is narrow. Better results can be achieved with a PSA sheet having a foam substrate in which the relationship between _C25 [kPa] and apparent density D [g/ ^cm3 ] satisfies the following formula: ₁₅₀ ≦C25×D≦400 (e.g., 200≦ _C25 ×D≦350, preferably 240≦ _C25 ×D≦300).

The tensile elongation of the foam substrate is not particularly limited. For example, a foam substrate having a tensile elongation in the machine direction (MD) of 200% to 800% (more preferably 400% to 600%) can be suitably used. A foam substrate having a tensile elongation in the transverse direction (TD) of 50% to 800% (more preferably 200% to 500%) is also preferred. The elongation of the foam substrate is measured in accordance with JIS K 6767. The elongation of the foam substrate can be controlled, for example, by the degree of crosslinking, apparent density (expansion ratio), etc.

The tensile strength of the foam substrate is not particularly limited. For example, a foam substrate having a tensile strength in the machine direction (MD) of 5 MPa to 35 MPa (preferably 10 MPa to 30 MPa) can be suitably used. A foam substrate having a tensile strength in the transverse direction (TD) of 1 MPa to 25 MPa (more preferably 5 MPa to 20 MPa) is preferred. The tensile strength of the foam substrate is measured in accordance with JIS K 6767. The tensile strength of the foam substrate can be controlled, for example, by the degree of crosslinking, apparent density (expansion ratio), etc.

The material of the foam substrate is not particularly limited. Usually, a foam substrate including a foam layer formed from a foam of a plastic material (plastic foam) is preferred. The plastic material (meaning including rubber materials) for forming the plastic foam is not particularly limited and can be appropriately selected from among known plastic materials. The plastic material can be used alone or in an appropriate combination of two or more types.

Specific examples of plastic foams include polyolefin resin foams such as polyethylene foams and polypropylene foams; polyester resin foams such as polyethylene terephthalate foams, polyethylene naphthalate foams, and polybutylene terephthalate foams; polyvinyl chloride resin foams such as polyvinyl chloride foams; vinyl acetate resin foams; polyphenylene sulfide resin foams; amide resin foams such as aliphatic polyamide (nylon) resin foams and fully aromatic polyamide (aramid) resin foams; polyimide resin foams; polyether ether ketone (PEEK) foams; styrene resin foams such as polystyrene foams; urethane resin foams such as polyurethane resin foams; and the like. In addition, rubber resin foams such as polychloroprene rubber foams may be used as plastic foams.

A preferred example of the foam is a polyolefin resin foam (hereinafter also referred to as "polyolefin foam"). As the plastic material (i.e., polyolefin resin) constituting the polyolefin foam, various known or commonly used polyolefin resins can be used without any particular limitation. Examples include polyethylene such as low density polyethylene (LDPE), linear low density polyethylene (LLDPE), high density polyethylene (HDPE), polypropylene, ethylene-propylene copolymer, ethylene-vinyl acetate copolymer, etc. Examples of LLDPE include Ziegler-Natta catalyst-based linear low density polyethylene, metallocene catalyst-based linear low density polyethylene, etc. Such polyolefin resins can be used alone or in appropriate combination of two or more types.

Suitable examples of the foam substrate in the technology disclosed herein include polyolefin foam substrates such as polyethylene foam substrates substantially composed of polyethylene resin foam and polypropylene foam substrates substantially composed of polypropylene resin foam, from the viewpoint of impact resistance, waterproofness, dust resistance, etc. Here, polyethylene resin refers to a resin having ethylene as the main monomer (i.e., the main component of the monomers), and may include HDPE, LDPE, LLDPE, etc., as well as ethylene-propylene copolymers and ethylene-vinyl acetate copolymers in which the copolymerization ratio of ethylene exceeds 50% by weight. Similarly, polypropylene resin refers to a resin having propylene as the main monomer. As the foam substrate in the technology disclosed herein, a polyethylene foam substrate may be preferably used.

The method for producing the plastic foam (typically a polyolefin-based foam) is not particularly limited, and various known methods can be appropriately adopted. For example, the plastic foam can be produced by a method including a molding step, a crosslinking step, and a foaming step of the plastic material or the plastic foam. In addition, a stretching step can be included as necessary.
Examples of methods for crosslinking the plastic foam include a chemical crosslinking method using an organic peroxide or an ionizing radiation crosslinking method using ionizing radiation, and these methods can be used in combination. Examples of the ionizing radiation include electron beams, α-rays, β-rays, and γ-rays. The dose of the ionizing radiation is not particularly limited, and can be set to an appropriate dose in consideration of the target physical properties (e.g., degree of crosslinking) of the foam base material.

The foam base material may contain various additives such as fillers (inorganic fillers, organic fillers, etc.), antioxidants, antioxidants, UV absorbers, antistatic agents, lubricants, plasticizers, flame retardants, surfactants, etc., as necessary.

The foam substrate in the technology disclosed herein may be colored in order to impart desired design properties and optical properties (e.g., light blocking properties, light reflectivity, etc.) to the adhesive sheet comprising the foam substrate. For this coloring, known organic or inorganic colorants can be used alone or in appropriate combination of two or more.

For example, when the adhesive sheet disclosed herein is used for light blocking purposes, the visible light transmittance of the foam substrate is not particularly limited, but is preferably 0% to 15%, and more preferably 0% to 10%, similar to the visible light transmittance of the adhesive sheet described below. Furthermore, when the adhesive sheet disclosed herein is used for light reflection purposes, the visible light reflectance of the foam substrate is preferably 20% to 100%, and more preferably 25% to 100%, similar to the visible light reflectance of the adhesive sheet.

The visible light transmittance of the foam substrate can be determined by using a spectrophotometer (e.g., a spectrophotometer model "U-4100" manufactured by Hitachi High-Technologies Corporation) to measure the intensity of light at a wavelength of 550 nm irradiated from one side of the foam substrate and transmitted to the other side. The visible light reflectance of the foam substrate can be determined by using the above spectrophotometer to measure the intensity of light at a wavelength of 550 nm irradiated from one side of the foam substrate and reflected. The visible light transmittance and visible light reflectance of the pressure-sensitive adhesive sheet can also be determined by a similar method.

When the pressure-sensitive adhesive sheet disclosed herein is used for light-shielding purposes, the foam substrate is preferably colored black. The black color is preferably 35 or less (e.g., 0 to 35) in L* (lightness) as defined by the L*a*b* color system, more preferably 30 or less (e.g., 0 to 30). Note that a* and b* as defined by the L*a*b* color system can each be appropriately selected according to the value of L*. There are no particular limitations on a* and b*, but both are preferably in the range of -10 to 10 (more preferably -5 to 5, and even more preferably -2.5 to 2.5). For example, it is preferable that both a* and b* are 0 or approximately 0.

In this specification, the L*, a*, and b* defined in the L*a*b* color system are determined by measurement using a colorimeter (for example, a colorimeter manufactured by Minolta, product name "CR-200"). The L*a*b* color system is a color space recommended by the International Commission on Illumination (CIE) in 1976, and refers to the color space known as the CIE1976 (L*a*b*) color system. The L*a*b* color system is also defined in JIS Z 8729 in the Japanese Industrial Standards.

Examples of black colorants that can be used when coloring the foam substrate black include carbon black (furnace black, channel black, acetylene black, thermal black, lamp black, etc.), graphite, copper oxide, manganese dioxide, aniline black, perylene black, titanium black, cyanine black, activated carbon, ferrite (non-magnetic ferrite, magnetic ferrite, etc.), magnetite, chromium oxide, iron oxide, molybdenum disulfide, chromium complexes, composite oxide-based black pigments, anthraquinone-based organic black pigments, etc. From the standpoint of cost and availability, carbon black is an example of a black colorant that is preferred. The amount of black colorant used is not particularly limited, and can be adjusted appropriately to impart the desired optical properties.

When the pressure-sensitive adhesive sheet disclosed herein is used for light reflection applications, the foam substrate is preferably colored white. The white color is preferably an L* (brightness) of 87 or more (e.g., 87 to 100) as defined by the L*a*b* color system, and more preferably 90 or more (e.g., 90 to 100). The a* and b* defined by the L*a*b* color system can each be appropriately selected according to the value of L*. For example, it is preferable that a* and b* are both in the range of -10 to 10 (more preferably -5 to 5, and even more preferably -2.5 to 2.5). For example, it is preferable that a* and b* are both 0 or approximately 0.

Examples of white colorants used to color the foam substrate white include titanium oxide (rutile titanium dioxide, anatase titanium dioxide, and other titanium dioxides), zinc oxide, aluminum oxide, silicon oxide, zirconium oxide, magnesium oxide, calcium oxide, tin oxide, barium oxide, cesium oxide, yttrium oxide, magnesium carbonate, calcium carbonate (light calcium carbonate, heavy calcium carbonate, and other), barium carbonate, zinc carbonate, aluminum hydroxide, calcium hydroxide, magnesium hydroxide, zinc hydroxide, aluminum silicate, and magnesium silicate. Examples of white colorants include inorganic white colorants such as calcium silicate, barium sulfate, calcium sulfate, barium stearate, zinc oxide, zinc sulfide, talc, silica, alumina, clay, kaolin, titanium phosphate, mica, gypsum, white carbon, diatomaceous earth, bentonite, lithopone, zeolite, sericite, and hydrated halloysite, and organic white colorants such as acrylic resin particles, polystyrene resin particles, polyurethane resin particles, amide resin particles, polycarbonate resin particles, silicone resin particles, urea-formaldehyde resin particles, and melamine resin particles. The amount of white colorant used is not particularly limited, and can be adjusted appropriately to impart the desired optical properties.

The surface of the foam substrate may be subjected to an appropriate surface treatment as necessary. This surface treatment may be, for example, a chemical or physical treatment to enhance adhesion to an adjacent material (e.g., an adhesive layer). Examples of such surface treatments include corona discharge treatment, chromate treatment, ozone exposure, flame exposure, ultraviolet irradiation treatment, plasma treatment, application of a primer, etc.

(Adhesive)
The adhesive sheet disclosed herein has an adhesive layer on at least one surface of a foam substrate. The type of adhesive constituting the adhesive layer is not particularly limited. The adhesive may be an adhesive formed from an adhesive composition containing one or more types of base polymers (adhesive polymers) selected from, for example, acrylic, polyester, urethane, polyether, rubber, silicone, polyamide, fluorine, etc. (main component of the polymer component, i.e., a component that occupies 50% by weight or more). The technology disclosed herein may be preferably implemented, for example, in the form of an adhesive sheet having an acrylic adhesive.

Here, "acrylic adhesive" refers to an adhesive that uses an acrylic polymer as the base polymer. "Acrylic polymer" refers to a polymer that has a monomer (hereinafter sometimes referred to as "acrylic monomer") having at least one (meth)acryloyl group in one molecule as the main constituent monomer component (the main component of the monomer, i.e., a component that accounts for 50% by weight or more of the total amount of monomers that constitute the acrylic polymer). Furthermore, in this specification, "(meth)acryloyl group" refers to an acryloyl group and a methacryloyl group in a comprehensive sense. Similarly, "(meth)acrylate" refers to an acrylate and a methacrylate in a comprehensive sense.

The acrylic polymer is typically a polymer containing an alkyl(meth)acrylate as a main monomer component. As the alkyl(meth)acrylate, for example, a compound represented by the following formula (1) can be suitably used.
_CH2 =C( ^R1 ) ^COOR2 (1)
Here, R ¹ in the above formula (1) is a hydrogen atom or a methyl group. R ² is an alkyl group having 1 to 20 carbon atoms. An alkyl (meth)acrylate in which R ² is an alkyl group having 2 to 14 carbon atoms (hereinafter, this range of carbon atoms may be referred to as C _2-14 ) is preferred, since it is easy to obtain a pressure-sensitive adhesive having excellent adhesive properties. Specific examples of _C1-20 alkyl groups include methyl, ethyl, propyl, isopropyl, n-butyl, isobutyl, s-butyl, t-butyl, n-pentyl, isoamyl, neopentyl, n-hexyl, n-heptyl, n-octyl, isooctyl, 2-ethylhexyl, n-nonyl, isononyl, n-decyl, isodecyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, isostearyl, nonadecyl, and eicosyl groups.

In a preferred embodiment, of the total amount of monomers used in the synthesis of the acrylic polymer, approximately 50% by weight or more (typically 50 to 99.9% by weight), more preferably 70% by weight or more (typically 70 to 99.9% by weight), for example approximately 85% by weight or more (typically ⁸⁵ to 99.9% by weight) is occupied by one or more selected from C _2-14 alkyl (meth)acrylates (more preferably C _4-10 alkyl (meth)acrylates. Particularly preferably, n-butyl acrylate and/or 2-ethylhexyl acrylate). According to an acrylic polymer obtained from such a monomer composition, a pressure-sensitive adhesive exhibiting good adhesive properties is easily formed, which is preferable. In some embodiments, from the viewpoint of cohesion and the like, the proportion of C _2-5 alkyl (meth)acrylates in the total amount of the monomers can be approximately 50% by weight or more. The proportion of the _C2-5 alkyl (meth)acrylate may be, for example, about 60% by weight or more, about 75% by weight or more, or about 90% by weight or more. A suitable example of the _C2-5 alkyl (meth)acrylate is n-butyl acrylate.

Although there is no particular limitation, the acrylic polymer is preferably one in which an acrylic monomer having a hydroxyl group (-OH) (hydroxyl group-containing acrylic monomer) is copolymerized. An acrylic polymer having such a copolymer composition is preferable because it is easy to obtain an adhesive with an excellent balance between adhesive strength and cohesive strength and excellent removability.

The hydroxyl group-containing acrylic monomers can be used alone or in combination of two or more. Specific examples of hydroxyl group-containing acrylic monomers include hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 2-hydroxyhexyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, 8-hydroxyoctyl (meth)acrylate, 10-hydroxydecyl (meth)acrylate, and 12-hydroxylauryl (meth)acrylate. Further examples include (4-hydroxymethylcyclohexyl)methyl acrylate, polypropylene glycol mono(meth)acrylate, N-hydroxyethyl (meth)acrylamide, and N-hydroxypropyl (meth)acrylamide. Among these, hydroxyalkyl (meth)acrylates are preferred, and hydroxyalkyl (meth)acrylates in which the alkyl group in the hydroxyalkyl group is linear and has 2 to 4 carbon atoms are particularly preferred.

The hydroxyl-containing acrylic monomer is preferably used in the range of approximately 0.001 to 10% by weight of the total amount of monomers used in the synthesis of the acrylic polymer. This makes it possible to realize an adhesive sheet with a higher level of balance between the adhesive strength and the cohesive strength. Even better results can be achieved by using approximately 0.01 to 5% by weight (e.g., 0.05 to 2% by weight) of the hydroxyl-containing acrylic monomer. Alternatively, the acrylic polymer in the technology disclosed herein may not be one in which the hydroxyl-containing acrylic monomer is copolymerized.

The acrylic polymer in the technology disclosed herein may be copolymerized with monomers other than those mentioned above (other monomers) within a range that does not significantly impair the effects of the present invention. Such monomers can be used for the purpose of, for example, adjusting the glass transition temperature of the acrylic polymer, adjusting the adhesive performance (for example, peelability), etc. For example, examples of monomers that can improve the cohesive strength and heat resistance of the adhesive include sulfonic acid group-containing monomers, phosphoric acid group-containing monomers, cyano group-containing monomers, vinyl esters, aromatic vinyl compounds, etc. In addition, examples of monomers that can introduce functional groups that can become crosslinking base points into the acrylic polymer or contribute to improving the adhesive strength include carboxy group-containing monomers, acid anhydride group-containing monomers, amide group-containing monomers, amino group-containing monomers, imide group-containing monomers, epoxy group-containing monomers, (meth)acryloylmorpholine, vinyl ethers, etc. For example, an acrylic polymer in which a carboxy group-containing monomer is copolymerized as the other monomer is preferable.

Examples of sulfonic acid group-containing monomers include styrenesulfonic acid, allylsulfonic acid, 2-(meth)acrylamide-2-methylpropanesulfonic acid, (meth)acrylamidepropanesulfonic acid, sulfopropyl(meth)acrylate, (meth)acryloyloxynaphthalenesulfonic acid, and sodium vinylsulfonate.
An example of the phosphate group-containing monomer is 2-hydroxyethyl acryloyl phosphate.
Examples of the cyano group-containing monomer include acrylonitrile and methacrylonitrile.
Examples of vinyl esters include vinyl acetate, vinyl propionate, and vinyl laurate.
Examples of aromatic vinyl compounds include styrene, chlorostyrene, chloromethylstyrene, α-methylstyrene, and other substituted styrenes.

Examples of the carboxy group-containing monomer include acrylic acid, methacrylic acid, carboxyethyl (meth)acrylate, carboxypentyl (meth)acrylate, itaconic acid, maleic acid, fumaric acid, crotonic acid, and isocrotonic acid.
Examples of the acid anhydride group-containing monomer include maleic anhydride, itaconic anhydride, and acid anhydrides of the above-mentioned carboxy group-containing monomers.
Examples of amide group-containing monomers include acrylamide, methacrylamide, diethylacrylamide, N-vinylpyrrolidone, N,N-dimethylacrylamide, N,N-dimethylmethacrylamide, N,N-diethylacrylamide, N,N-diethylmethacrylamide, N,N'-methylenebisacrylamide, N,N-dimethylaminopropylacrylamide, N,N-dimethylaminopropylmethacrylamide, and diacetoneacrylamide.
Examples of amino group-containing monomers include aminoethyl (meth)acrylate, N,N-dimethylaminoethyl (meth)acrylate, and N,N-dimethylaminopropyl (meth)acrylate.
Examples of imide group-containing monomers include cyclohexylmaleimide, isopropylmaleimide, N-cyclohexylmaleimide, and itaconimide.
Examples of epoxy group-containing monomers include glycidyl (meth)acrylate, methyl glycidyl (meth)acrylate, and allyl glycidyl ether.
Examples of vinyl ethers include methyl vinyl ether, ethyl vinyl ether, and isobutyl vinyl ether.

Such "other monomers" may be used alone or in combination of two or more. Usually, the total amount of the other monomers is preferably about 40% by weight or less (typically 0.001 to 40% by weight) of the total amount of monomers used in the synthesis of the acrylic polymer, and more preferably about 30% by weight or less (typically 0.01 to 30% by weight, for example 0.1 to 10% by weight). When a carboxyl group-containing monomer is used as the other monomer, its content can be, for example, 0.1 to 10% by weight of the total amount of monomers, and is usually 0.2 to 8% by weight, for example 0.5 to 5% by weight. In some embodiments, from the viewpoint of cohesion and the like, the content of the carboxyl group-containing monomer can be 1.5% by weight or more, may be 2.5% by weight or more, or may be 3.5% by weight or more. Furthermore, when vinyl esters (e.g., vinyl acetate) are used as the other monomers, their content can be, for example, 0.1 to 20% by weight of the total amount of the monomers, and is usually 0.5 to 10% by weight.

The copolymer composition of the acrylic polymer is suitably designed so that the glass transition temperature (Tg) of the acrylic polymer is −15° C. or lower (typically −70° C. to −15° C.), preferably −25° C. or lower (e.g. −60° C. to −25° C.), and more preferably −40° C. or lower (e.g. −60° C. to −40° C.). Setting the Tg of the acrylic polymer to the above-mentioned upper limit or lower is preferable from the viewpoint of impact resistance of the pressure-sensitive adhesive sheet. The Tg of the acrylic polymer can be adjusted by appropriately changing the monomer composition (i.e., the type and amount ratio of the monomers used in the synthesis of the polymer). In some embodiments, from the viewpoint of increasing the cohesiveness of the pressure-sensitive adhesive layer, the Tg of the acrylic polymer may be −58° C. or higher, −55° C. or higher, or −53° C. or higher.

In this specification, the Tg of a polymer refers to the Tg calculated based on the copolymerization composition of the polymer by the Fox formula, which is a relational expression 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 the above Fox formula, Tg represents the glass transition temperature (unit: K) of the copolymer, Wi represents the weight fraction of monomer i in the copolymer (copolymerization ratio on a weight basis), and Tgi represents the glass transition temperature (unit: K) of a homopolymer of monomer i.

The glass transition temperature of the homopolymer used in calculating Tg is a value described in a publicly known document. For example, for the monomers listed below, the following values are used as the glass transition temperatures of the homopolymers of the monomers.
2-Ethylhexyl acrylate -70℃
n-Butyl acrylate -55℃
Ethyl acrylate -22℃
Methyl acrylate 8℃
Methyl methacrylate 105℃
Cyclohexyl methacrylate 66℃
2-Hydroxyethyl acrylate -15℃
4-Hydroxybutyl acrylate -40℃
Vinyl acetate 32℃
Styrene 100℃
Acrylic acid 106℃
Methacrylic acid 228℃

For the glass transition temperatures of homopolymers of monomers other than those listed above, the values given in "Polymer Handbook" (3rd Edition, John Wiley & Sons, Inc., 1989) shall be used. For monomers for which multiple values are given in this document, the highest value shall be used.

For monomers for which the glass transition temperature of the homopolymer is not described in the above literature, the value obtained by the following measurement method shall be used (see JP 2007-51271 A). Specifically, 100 parts by weight of monomer, 0.2 parts by weight of azobisisobutyronitrile, and 200 parts by weight of ethyl acetate as a polymerization solvent are charged into a reactor equipped with a thermometer, a stirrer, a nitrogen inlet tube, and a reflux condenser, and stirred for 1 hour while circulating nitrogen gas. After removing oxygen from the polymerization system in this way, the temperature is raised to 63°C and the reaction is carried out for 10 hours. The mixture is then cooled to room temperature to obtain a homopolymer solution with a solid concentration of 33% by weight. This homopolymer solution is cast onto a release liner and dried to prepare a test sample (sheet-shaped homopolymer) with a thickness of about 2 mm. The test sample is punched out into a disk with a diameter of 7.9 mm, sandwiched between parallel plates, and the viscoelasticity is measured in shear mode using a viscoelasticity tester (TA Instruments, ARES) while applying a shear strain of 1 Hz frequency in the temperature range of -70 to 150°C and at a heating rate of 5°C/min. The peak top temperature of tan δ (loss tangent) is taken as the glass transition temperature.

Although not particularly limited, the acrylic polymer preferably has a ratio of 50% by weight or more (more preferably 70% by weight or more, for example 85% by weight or more) of monomers having a homopolymer glass transition temperature of -45°C or less in the total amount of monomers used in the synthesis of the acrylic polymer. Acrylic polymers having such a copolymer composition tend to have improved impact resistance. The upper limit of the ratio is not particularly limited, and may be 100% by weight of the total amount of monomers. From the viewpoint of cohesiveness of the adhesive, it is usually appropriate to set the ratio of monomers having a homopolymer glass transition temperature of -45°C or less to 99% by weight or less in the total amount of monomers, and preferably to 97% by weight or less.

In the technology disclosed herein, the method for obtaining an acrylic polymer is not particularly limited, and various polymerization methods known as methods for synthesizing acrylic polymers, such as solution polymerization, emulsion polymerization, bulk polymerization, and suspension polymerization, can be appropriately adopted. For example, solution polymerization can be preferably used. As a monomer supply method when performing solution polymerization, a lump-sum charging 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 adopted. The polymerization temperature can be appropriately selected depending on the type of monomer and solvent used, the type of polymerization initiator, etc., and can be, for example, about 20°C to 170°C (typically 40°C to 140°C).

The initiator used in the polymerization can be appropriately selected from known or conventional polymerization initiators depending on the type of polymerization method. For example, azo-based polymerization initiators such as 2,2'-azobisisobutyronitrile, 2,2'-azobis(2-methylpropionamidine) disulfate, and 2,2'-azobis(2-amidinopropane) dihydrochloride can be preferably used. Other examples of polymerization initiators include persulfates such as potassium persulfate and ammonium persulfate; peroxide-based initiators such as benzoyl peroxide, t-butyl hydroperoxide, and hydrogen peroxide; substituted ethane-based initiators such as phenyl-substituted ethane; aromatic carbonyl compounds; and the like. Still other examples of polymerization initiators include redox-based initiators formed by combining a peroxide with a reducing agent. Examples of such redox-based initiators include a combination of a peroxide such as hydrogen peroxide and ascorbic acid, a combination of a peroxide such as hydrogen peroxide and an iron (II) salt, and a combination of a persulfate and sodium hydrogen sulfite. The polymerization initiator can be used alone or in combination of two or more. The amount of the polymerization initiator used may be a normal amount, and can be selected, for example, from the range of about 0.005 to 1 part by weight (typically 0.01 to 1 part by weight) per 100 parts by weight of the total monomer components.

The solvent (polymerization solvent) used in the solution polymerization can be appropriately selected from known or conventional organic solvents. For example, any one of the following solvents or a mixture of two or more solvents can be used: aromatic compounds (typically aromatic hydrocarbons) such as toluene and xylene; acetate esters such as ethyl acetate and butyl acetate; aliphatic or alicyclic hydrocarbons such as hexane, cyclohexane, and methylcyclohexane; halogenated alkanes such as 1,2-dichloroethane; lower alcohols (e.g., monohydric alcohols having 1 to 4 carbon atoms) such as isopropyl alcohol, 1-butanol, sec-butanol, and tert-butanol; ethers such as tert-butyl methyl ether; ketones such as methyl ethyl ketone and acetylacetone. For example, a polymerization solvent (which may be a mixture of solvents) having a boiling point in the range of 40°C to 150°C (preferably 60°C to 150°C, typically 70°C to 130°C) can be used.

According to the solution polymerization, a polymerization reaction liquid in the form of an acrylic polymer dissolved in an organic solvent is obtained. The adhesive layer in the technology disclosed herein may be formed from an adhesive composition containing the above-mentioned polymerization reaction liquid or an acrylic polymer solution obtained by subjecting the reaction liquid to an appropriate post-treatment. The above-mentioned acrylic polymer solution may be prepared by preparing the above-mentioned polymerization reaction liquid to an appropriate concentration as necessary. Alternatively, an acrylic polymer solution prepared by synthesizing an acrylic polymer by a polymerization method other than solution polymerization (e.g., emulsion polymerization, photopolymerization, bulk polymerization, etc.) and dissolving the acrylic polymer in an organic solvent may be used.

The weight average molecular weight (Mw) of the acrylic polymer is not particularly limited, and may be, for example, in the range of 10×10 ⁴ to 500×10 ^4. Since it is easy to balance the adhesive properties, the Mw of the acrylic polymer is preferably in the range of 10×10 ⁴ to 150×10 ⁴ , more preferably in the range of 15×10 ⁴ to 100×10 ⁴ , and even more preferably in the range of 20×10 ⁴ to 75×10 ⁴ (for example, 30×10 ⁴ to 60×10 ⁴ ). The Mw of the acrylic polymer is determined as a value converted into standard polystyrene by performing GPC (gel permeation chromatography) on the solvent-soluble portion of the acrylic polymer (for example, tetrahydrofuran-soluble portion).

The adhesive composition (e.g., an acrylic adhesive composition) in the technology disclosed herein may contain a tackifier resin. There are no particular limitations on the tackifier resin, and various tackifier resins such as rosin-based, terpene-based, phenol-based, hydrocarbon-based, epoxy-based, polyamide-based, elastomer-based, and ketone-based resins can be used alone or in combination of two or more.

Specific examples of rosin-based tackifying resins include unmodified rosins (raw rosins) such as gum rosin, wood rosin, and tall oil rosin; modified rosins (hydrogenated rosin, disproportionated rosin, polymerized rosin, other chemically modified rosins, etc.) obtained by modifying these unmodified rosins through hydrogenation, disproportionation, polymerization, etc.; and various other rosin derivatives. Examples of the above rosin derivatives include rosin esters such as unmodified rosin esterified with alcohols (i.e., rosin esters), modified rosin (hydrogenated rosin, disproportionated rosin, polymerized rosin, etc.) esterified with alcohols (i.e., modified rosin esters); unsaturated fatty acid modified rosins obtained by modifying unmodified rosin or modified rosin (hydrogenated rosin, disproportionated rosin, polymerized rosin, etc.) with unsaturated fatty acids; unsaturated fatty acid modified rosins obtained by modifying rosin esters with unsaturated fatty acids. rosin esters; rosin alcohols obtained by reducing the carboxyl groups in unmodified rosin, modified rosin (hydrogenated rosin, disproportionated rosin, polymerized rosin, etc.), unsaturated fatty acid modified rosins, or unsaturated fatty acid modified rosin esters; metal salts of rosins (particularly rosin esters) such as unmodified rosin, modified rosin, and various rosin derivatives; rosin phenolic resins obtained by adding phenol to rosins (unmodified rosin, modified rosin, various rosin derivatives, etc.) using an acid catalyst and then thermally polymerizing the resulting mixture; etc.

Examples of terpene-based tackifying resins include terpene resins such as α-pinene polymers, β-pinene polymers, and dipentene polymers; modified terpene resins obtained by modifying these terpene resins (phenol-modified, aromatic-modified, hydrogen-modified, hydrocarbon-modified, etc.); and the like. Examples of the modified terpene resins include terpene phenol resins, styrene-modified terpene resins, aromatic-modified terpene resins, and hydrogenated terpene resins.

Examples of phenolic tackifying resins include condensates of various phenols such as phenol, m-cresol, 3,5-xylenol, p-alkylphenol, and resorcin with formaldehyde (e.g., alkylphenol resins, xylene formaldehyde resins, etc.); resols obtained by addition reaction of the above phenols with formaldehyde using an alkaline catalyst; novolacs obtained by condensation reaction of the above phenols with formaldehyde using an acid catalyst; etc.

Examples of hydrocarbon-based tackifying resins include various hydrocarbon resins such as aliphatic hydrocarbon resins, aromatic hydrocarbon resins, aliphatic cyclic hydrocarbon resins, aliphatic/aromatic petroleum resins (styrene-olefin copolymers, etc.), aliphatic/alicyclic petroleum resins, hydrogenated hydrocarbon resins, coumarone resins, and coumarone-indene resins. Examples of aliphatic hydrocarbon resins include polymers of one or more aliphatic hydrocarbons selected from olefins and dienes having about 4 to 5 carbon atoms. Examples of the olefins include 1-butene, isobutylene, and 1-pentene. Examples of the dienes include butadiene, 1,3-pentadiene, and isoprene. Examples of aromatic hydrocarbon resins include polymers of vinyl group-containing aromatic hydrocarbons having about 8 to 10 carbon atoms (styrene, vinyltoluene, α-methylstyrene, indene, methylindene, etc.). Examples of aliphatic cyclic hydrocarbon resins include alicyclic hydrocarbon resins obtained by cyclodimerizing and then polymerizing so-called "C4 petroleum fractions" or "C5 petroleum fractions"; polymers of cyclic diene compounds (cyclopentadiene, dicyclopentadiene, ethylidene norbornene, dipentene, etc.) or hydrogenated products thereof; aromatic hydrocarbon resins or alicyclic hydrocarbon resins obtained by hydrogenating the aromatic rings of aliphatic/aromatic petroleum resins; etc.

In the technology disclosed herein, the tackifier resin may preferably have a softening point (softening temperature) of about 100°C or higher (preferably about 120°C or higher, more preferably about 135°C or higher). A pressure-sensitive adhesive sheet with better repulsion resistance can be realized by using a pressure-sensitive adhesive containing a tackifier resin with a softening point equal to or higher than the above-mentioned lower limit. Among the tackifier resins exemplified above, terpene-based tackifier resins (e.g., terpene phenol resins), rosin-based tackifier resins (e.g., esterified products of polymerized rosin), and the like having such softening points can be preferably used. The tackifier resin may preferably be used in an embodiment containing, for example, a terpene phenol resin with a softening point of 135°C or higher. In addition, a pressure-sensitive adhesive containing a tackifier resin with a softening point of 140°C or higher can achieve particularly excellent repulsion resistance. For example, a terpene phenol resin with a softening point of 140°C or higher may preferably be used. The upper limit of the softening point of the tackifier resin is not particularly limited, and may be, for example, about 200°C or lower (typically about 180°C or lower). The softening point of the tackifier resin can be measured based on the softening point test method (ring and ball method) specified in JIS K2207.

In some embodiments, a tackifier resin having at least one polar group in the molecule may be preferably used. Suitable examples of such polar groups include hydroxyl groups and carboxyl groups. Of these, tackifier resins having a hydroxyl group are preferred. For example, at least one tackifier resin having at least one polar group (preferably a hydroxyl group) in the molecule and selected from terpene phenol resins, alkylphenol resins, rosin-modified phenol resins, and xylene formaldehyde resins may be preferably used.

The amount of tackifier resin used is not particularly limited and can be set appropriately depending on the desired adhesive performance (adhesive strength, etc.). For example, it is preferable to use about 10 to 100 parts by weight (more preferably 15 to 80 parts by weight, and even more preferably 20 to 60 parts by weight) of tackifier resin per 100 parts by weight of acrylic polymer on a solids basis.

Examples of suitable compositions for the acrylic adhesive disclosed herein include a composition containing 20 to 60 parts by weight of a tackifier resin with a softening point of 120°C or higher per 100 parts by weight of acrylic polymer, and a composition containing 10 to 50 parts by weight of a tackifier resin with a softening point of 135°C or higher per 100 parts by weight of acrylic polymer. Acrylic adhesives with such compositions tend to provide a good balance between repulsion resistance and flexibility.

A crosslinking agent may be used in the adhesive composition as necessary. The type of crosslinking agent is not particularly limited, and may be appropriately selected from known or conventional crosslinking agents (e.g., isocyanate-based crosslinking agents, epoxy-based crosslinking agents, oxazoline-based crosslinking agents, aziridine-based crosslinking agents, melamine-based crosslinking agents, peroxide-based crosslinking agents, urea-based crosslinking agents, metal alkoxide-based crosslinking agents, metal chelate-based crosslinking agents, metal salt-based crosslinking agents, carbodiimide-based crosslinking agents, amine-based crosslinking agents, etc.). The crosslinking agent may be used alone or in combination of two or more. The amount of the crosslinking agent used is not particularly limited, and may be selected, for example, from a range of about 10 parts by weight or less (e.g., about 0.005 to 10 parts by weight, preferably about 0.01 to 5 parts by weight) per 100 parts by weight of the acrylic polymer. An example of a crosslinking agent that can be preferably used is an isocyanate-based crosslinking agent. The amount of the isocyanate-based crosslinking agent used per 100 parts by weight of the acrylic polymer may be, for example, about 1 part by weight or more, or about 1.5 parts by weight or more. In some embodiments, the amount of the isocyanate-based crosslinking agent used per 100 parts by weight of the acrylic polymer may be, for example, about 2.5 parts by weight or more, or about 3.5 parts by weight or more.

The adhesive composition may contain a crosslinking aid as necessary. The crosslinking aid may also be understood as a curing aid. The crosslinking aid is not particularly limited, but for example, a compound containing two or more hydroxyl groups in the molecule may be used. Hereinafter, a compound containing two or more hydroxyl groups in the molecule may also be referred to as a hydroxy compound. The hydroxy compound may be used alone or in combination of two or more. The hydroxy compound may be used in combination with various crosslinking agents, and may be preferably used in combination with an isocyanate-based crosslinking agent, for example.

As the hydroxy compound, a compound having two or more hydroxyl groups in the molecule can be used without any particular limitation, and examples of the compound that can be used include aliphatic polyols, alicyclic polyols, aromatic polyols, polyether polyols, polyester polyols, polycarbonate polyols, polycaprolactone polyols, acrylic polyols, and polyurethane polyols. As the hydroxy compound, a compound containing two or more hydroxyl groups in the molecule and one or more nitrogen atoms in the molecule can be used. Hereinafter, such a hydroxy compound may be referred to as a "nitrogen atom-containing hydroxy compound." Examples of the nitrogen atom-containing hydroxy compound include N,N,N',N'-tetrakis(2-hydroxyethyl)ethylenediamine, N,N,N',N'-tetrakis(2-hydroxypropyl)ethylenediamine, N,N,N',N'-tetrakis(2-hydroxyethyl)trimethylenediamine, N,N,N',N'-tetrakis(2-hydroxypropyl)trimethylenediamine; polyoxyethylene condensates of ethylenediamine, polyoxypropylene condensates of ethylenediamine, polyoxyethylene-polyoxyethylene ... Examples include polyoxyalkylene condensates of alkylene diamines such as dipropylene condensates; dialcoholamines such as diethanolamine, dipropanolamine, diisopropanolamine, N-methyldiethanolamine, N-methyldiisopropanolamine, N-ethyldiethanolamine, N-ethyldiisopropanolamine, N-butyldiethanolamine, and N-butyldiisopropanolamine; and trialcoholamines such as triethanolamine, tripropanolamine, and triisopropanolamine.

The amount of the hydroxy compound used relative to 100 parts by weight of the acrylic polymer can be, for example, 0.01 parts by weight or more, 0.02 parts by weight or more, or 0.03 parts by weight or more. The amount used can be, for example, 10 parts by weight or less, 5 parts by weight or less, 1 part by weight or less, 0.5 parts by weight or less, or 0.1 parts by weight or less. When an isocyanate-based crosslinking agent and a hydroxy compound are used in combination, the amount of the hydroxy compound used can be, for example, 1/500 or more of the amount of the isocyanate-based crosslinking agent used, 1/200 or more, or 1/100 or more. In addition, the amount of the hydroxy compound used is usually suitably 1/3 or less of the amount of the isocyanate-based crosslinking agent used, and may be 1/10 or less, 1/25 or less, or 1/50 or less.

The adhesive composition may contain various additives that are common in the field of adhesive compositions, such as leveling agents, plasticizers, softeners, fillers, colorants (pigments, dyes, etc.), antistatic agents, antiaging agents, UV absorbers, antioxidants, light stabilizers, etc., as necessary. As for such various additives, those that are conventionally known can be used in the usual manner, and they do not particularly characterize the present invention, detailed explanations are omitted.

(Adhesive Layer)
As a method for forming a pressure-sensitive adhesive layer on a foam substrate, various conventionally known methods can be applied. For example, a method of directly applying a pressure-sensitive adhesive composition to a foam substrate (direct method), a method of applying a pressure-sensitive adhesive composition to a suitable release surface to form a pressure-sensitive adhesive layer on the release surface, and a method of transferring the pressure-sensitive adhesive layer by laminating it to a foam substrate (transfer method), etc. may be used in combination with these methods. Also, different methods may be adopted for the first pressure-sensitive adhesive layer and the second pressure-sensitive adhesive layer. The pressure-sensitive adhesive composition can be applied using a known or conventional coater such as a gravure roll coater, a reverse roll coater, a kiss roll coater, a dip roll coater, a bar coater, a knife coater, or a spray coater. When a pressure-sensitive adhesive composition containing a solvent is used, it is preferable to dry the pressure-sensitive adhesive composition under heating from the viewpoint of promoting a crosslinking reaction and improving production efficiency.

The 23°C storage modulus of the adhesive layer disclosed herein is not particularly limited, and may be, for example, 0.01 MPa to 0.60 MPa or less. In one embodiment, the 23°C storage modulus of the adhesive layer may be 0.40 MPa or less, and is preferably 0.20 MPa or less. When the 23°C storage modulus of the adhesive layer is low, the flexibility of the adhesive layer increases, and therefore the adhesive surface tends to be easily brought into close contact with the surface of the adherend. This is particularly meaningful in an adhesive sheet having a high smoothness of the adhesive surface before application to the adherend. High flexibility of the adhesive layer is also preferable from the viewpoint that the smoothness of the adhesive layer can be easily maintained or improved by abutting the adhesive layer of the adhesive sheet before use (i.e., before application to the adherend) against a highly smooth release surface. The adhesive sheet disclosed herein can also be implemented in an embodiment in which the 23°C storage modulus of the adhesive layer is 0.15 MPa or less or 0.10 MPa or less. In one aspect of the technology disclosed herein, the 23°C storage modulus of the adhesive layer can be, for example, 0.02 MPa or more, preferably 0.05 MPa or more, and more preferably 0.07 MPa or more. Such an adhesive layer has moderate cohesiveness and is therefore easy to increase adhesive strength. It can also be advantageous in terms of improving processability when processing the adhesive sheet into a narrow width and suppressing the adhesive from spilling out. The 23°C storage modulus of the adhesive layer can be adjusted by the composition of the adhesive layer, the manufacturing method, etc.

The 0°C storage modulus of the adhesive layer disclosed herein is not particularly limited and may be, for example, about 0.05 MPa to 10 MPa. In one embodiment, the 0°C storage modulus of the adhesive layer may be 1.5 MPa or less, preferably 1.2 MPa or less, and may be 1.0 MPa or less. With an adhesive layer having such a 0°C storage modulus, the adhesion between the adhesive surface and the surface of the adherend can be improved more quickly. Therefore, the adhesion between the adhesive surface and the surface of the adherend can be effectively improved even with a shorter pressure-bonding time. With an adhesive sheet having a high smoothness of the adhesive surface before attachment to the adherend, the improvement of the adhesion can be particularly efficiently promoted. The adhesive sheet disclosed herein can also be implemented in an embodiment in which the 0°C storage modulus of the adhesive layer is 0.80 MPa or less or 0.60 MPa or less. In one embodiment of the technology disclosed herein, the 0°C storage modulus of the adhesive layer may be, for example, 0.065 MPa or more, preferably 0.08 MPa or more. Such an adhesive layer is preferable from the viewpoints of improving processability when processing the adhesive sheet into a narrow width and suppressing the adhesive from overflowing. The adhesive sheet disclosed herein can also be implemented in an embodiment in which the 0°C storage modulus of the adhesive layer is 0.10 MPa or more, 0.20 MPa or more, or 0.30 MPa or more. In some embodiments, the 0°C storage modulus of the adhesive layer may be, for example, 0.55 MPa or more, 0.75 MPa or more, or 0.85 MPa or more. The adhesive sheet disclosed herein can quickly and well adhere to the surface of the adherend even at such a 0°C storage modulus due to the high smoothness of the first adhesive surface and the second adhesive surface. The 0°C storage modulus of the adhesive layer can be adjusted by the composition of the adhesive layer, the manufacturing method, etc.

The 23°C storage modulus and 0°C storage modulus of the adhesive layer can be the storage modulus at 23°C and 0°C obtained by dynamic viscoelasticity measurement of the adhesive constituting the adhesive layer. A specific measuring device that can be used is ARES manufactured by TA Instruments or an equivalent product. The specific measurement operation and measurement conditions can be set according to the measurement conditions described in the examples below, or to obtain results equivalent to or corresponding to those obtained when the measurement conditions are followed.

The thickness of the adhesive layer is not particularly limited and can be set according to the intended use and target performance of the adhesive sheet. For example, the thickness of the adhesive layer can be about 5 μm to 150 μm. From the viewpoint of achieving a high level of balance between thinning of the adhesive sheet and adhesive performance, the thickness of the adhesive layer is appropriately about 10 μm to 100 μm, preferably about 15 μm to 90 μm, and more preferably about 20 μm to 80 μm. From the viewpoint of thinning of the adhesive sheet, the thickness of the adhesive layer may be 50 μm or less, and may further be 35 μm or less.

The total thickness Ht of the adhesive sheet disclosed herein (thickness from the first adhesive surface to the second adhesive surface) is not particularly limited. The total thickness Ht of the adhesive sheet is usually 70 μm or more, preferably 80 μm or more, and more preferably 90 μm or more (e.g., 150 μm or more). By making the total thickness Ht of the adhesive sheet equal to or greater than the above-mentioned lower limit, the adhesive sheet can exhibit excellent impact resistance, waterproofness, dust resistance, and the like. In addition, the total thickness Ht of the adhesive sheet is usually 800 μm or less, preferably 500 μm or less, and more preferably 400 μm or less (e.g., 350 μm or less). By making the total thickness Ht of the adhesive sheet equal to or less than the above-mentioned upper limit, it can be advantageous in terms of thinning, miniaturization, weight reduction, resource saving, and the like of the product.

In the adhesive sheet disclosed herein, the proportion of the foam substrate thickness Hs to the total thickness Ht of the adhesive sheet is not particularly limited. From the viewpoint of effectively achieving both impact resistance and adhesive performance, it is usually appropriate to set Hs/Ht to approximately 20% or more and approximately 80% or less, and preferably to approximately 30% or more and approximately 70% or less (e.g., approximately 40% or more and approximately 60% or less).

The adhesive sheet disclosed herein may further include layers other than the foam substrate and the adhesive layer (such as an intermediate layer and an undercoat layer; hereinafter, also referred to as "other layers"), provided that the effects of the present invention are not significantly impaired. For example, the other layers may be provided between the foam substrate and the surface (adhesive surface) of the adhesive layer.

<Release Liner>
The release liner constituting the release-liner-attached pressure-sensitive adhesive sheet disclosed herein (hereinafter, sometimes referred to as "final liner") may be, for example, a release liner having a release treatment layer on the surface of a liner substrate such as a plastic film or paper (which may be resin-impregnated paper or resin-laminated paper); a release liner made of a low-adhesion material such as a fluorine-based polymer (polytetrafluoroethylene, etc.) or a polyolefin-based resin (polyethylene, polypropylene, etc.); etc. The release treatment layer may be formed by surface-treating the liner substrate with a release treatment agent. Examples of the release treatment agent include silicone-based release treatment agents, long-chain alkyl-based release treatment agents, fluorine-based release treatment agents, molybdenum (IV) sulfide, etc. In one embodiment, a release liner having a release treatment layer made of a silicone-based release treatment agent may be preferably used.

As the above-mentioned final liner, a release liner having a release treatment layer on the surface of a plastic film as a liner substrate can be preferably used, since it is easy to realize a highly smooth release surface. Examples of materials for the plastic film include polyester-based resins such as polyethylene terephthalate and polyethylene naphthalate, cellulose resins such as triacetyl cellulose, acetate-based resins, polysulfone-based resins, polyethersulfone-based resins, polycarbonate-based resins, polyamide-based resins, polyimide-based resins, polyolefin-based resins, cyclic polyolefin resins (norbornene-based resins, etc.), (meth)acrylic-based resins, polyvinyl chloride-based resins, polyvinylidene chloride-based resins, polystyrene-based resins, polyvinyl alcohol-based resins, polyarylate-based resins, polyphenylene sulfide-based resins, and mixtures thereof. From the viewpoint of dimensional stability and strength, a plastic film containing a polyester-based resin film (typically a polyethylene terephthalate film) can be preferably used.

The release-liner adhesive sheet disclosed herein typically has a first release surface abutting the first adhesive surface and a second release surface abutting the second adhesive surface on the same or different final liners. From the viewpoint of simplifying the configuration of the release-liner adhesive sheet and saving resources, a configuration having the first release surface and the second release surface on one final liner can be preferably adopted. A typical example of such a configuration is a configuration in which the first and second surfaces of one final liner are used as the first and second release surfaces.

The arithmetic mean roughness Ra _R1 of the first release surface and the arithmetic mean roughness Ra _R2 of the second release surface of the final liner are not particularly limited. In one embodiment, the arithmetic mean roughness Ra _R1 of the first release surface is suitably 1000 nm or less, preferably 700 nm or less, more preferably 500 nm or less, and even more preferably 300 nm or less (e.g., 250 nm or less). A small arithmetic mean roughness Ra _R1 of the first release surface is advantageous in reducing the arithmetic mean roughness Ra _A1 of the first pressure-sensitive adhesive layer in contact with the first release surface or suppressing an increase. In another embodiment, the arithmetic mean roughness Ra _R2 of the second release surface is preferably 300 nm or less, more preferably 200 nm or less (e.g., 150 nm or less). A small arithmetic mean roughness Ra _R2 of the second release surface is advantageous in reducing the arithmetic mean roughness Ra _A2 of the second pressure-sensitive adhesive layer in contact with the second release surface or suppressing an increase. The technology disclosed herein can be preferably implemented in an embodiment in which Ra _R1 and Ra _R2 are both 400 nm or less (more preferably 300 nm or less, for example 250 nm or less). The lower limits of Ra _R1 and Ra _R2 are not particularly limited. Alternatively, from a practical standpoint, the technology disclosed herein can be preferably implemented in an embodiment in which one or both of Ra _R1 and Ra _R2 are 20 nm or more (for example 50 nm or more). Ra _R1 and Ra _R2 can be controlled by the configuration of the release liner. For example, in a release liner having a release treatment layer on the surface of a liner substrate, they can be controlled by the constituent material and manufacturing conditions of the liner substrate, the constituent material and manufacturing conditions of the release treatment layer, etc.

The thickness of the final liner is not particularly limited, and may be, for example, about 10 μm to about 500 μm. In consideration of the balance between flexibility and handleability (e.g., ease of separation from the adhesive sheet), the thickness of the final liner is usually appropriate to be 15 μm or more (preferably 25 μm or more, more preferably 30 μm or more, for example 45 μm or more), and is appropriate to be 250 μm or less (preferably 150 μm or less, more preferably 100 μm or less).

The first adhesive layer of the adhesive sheet constituting the release-liner adhesive sheet disclosed herein may be an adhesive layer formed on the first release surface of a release liner (i.e., a final liner) included in the release-liner adhesive sheet and transferred to a foam substrate, or an adhesive layer formed on the release surface of a member not included in the release-liner adhesive sheet may be transferred to a foam substrate. The member may be a release liner (hereinafter also referred to as a "process liner") that is the same or different in composition as the final liner and is used only in the manufacturing process of the release-liner adhesive sheet. Similarly, the second adhesive layer of the adhesive sheet constituting the release-liner adhesive sheet disclosed herein may be an adhesive layer formed on the second release surface of a final liner and transferred to a foam substrate, or an adhesive layer formed on the release surface of a member not included in the release-liner adhesive sheet (e.g., a process liner) that is transferred to a foam substrate.

The process liner is not particularly limited, and can be selected from the above-mentioned examples of the final liner. The arithmetic mean roughness Ra _P of the surface (release surface) of the process liner is not particularly limited, as long as the above-mentioned preferred arithmetic mean roughnesses Ra _A1 and Ra _A2 can be realized on the first adhesive surface and the second adhesive surface of the release liner-attached PSA sheet. For example, the arithmetic mean roughness Ra _P of the release surface of the process liner may be larger than one or both of the arithmetic mean roughness Ra _R1 of the first release surface of the final liner and the arithmetic mean roughness Ra _R2 of the second release surface. In one embodiment, the process liner can be preferably a release liner having a release treatment layer on the surface of a liner substrate (e.g., a liner substrate laminated with polyethylene on paper) made of resin-impregnated paper or resin-laminated paper. In another embodiment, a release liner having the same structure as the final liner can also be used as the process liner.

The adhesive sheet disclosed herein may have desired optical properties (transmittance, reflectance, etc.). For example, an adhesive sheet used for light blocking purposes preferably has a visible light transmittance of 0% or more and 15% or less (more preferably 0% or more and 10% or less). Also, an adhesive sheet used for light reflecting purposes preferably has a visible light reflectance of 20% or more and 100% or less (more preferably 25% or more and 100% or less). The optical properties of the adhesive sheet can be adjusted, for example, by coloring the foam substrate as described above.

The adhesive sheet disclosed herein is preferably halogen-free from the viewpoint of preventing metal corrosion, etc. The fact that the adhesive sheet is halogen-free can be an advantageous feature, for example, when the adhesive sheet can be used to fix electric/electronic components. In addition, it is also preferable from the viewpoint of reducing the environmental load, since it can suppress the generation of halogen-containing gas during combustion. A halogen-free adhesive sheet can be obtained by adopting measures, either alone or in appropriate combination, such as intentionally not using halogen compounds as raw materials for the foam base material or adhesive, using a foam base material that is not intentionally mixed with halogen compounds, and not using additives derived from halogen compounds when additives are used.

The object (adherend) to which the adhesive sheet disclosed herein is attached is not particularly limited. The adhesive sheet disclosed herein can be used in a form in which it is attached to an adherend made of, for example, metal materials such as stainless steel (SUS) and aluminum; inorganic materials such as glass and ceramics; resin materials such as polycarbonate (PC), polymethyl methacrylate (PMMA), polypropylene, polyethylene terephthalate (PET), acrylonitrile butadiene styrene copolymer resin (ABS), high impact polystyrene (HIPS), PC-ABS blend resin, and PC-HIPS blend resin; rubber materials such as natural rubber and butyl rubber; and composite materials thereof.

The adhesive sheet included in the adhesive sheet with release liner disclosed herein can exhibit good bonding reliability even in a narrow width, and therefore can be suitable as an adhesive sheet used for purposes such as bonding and fixing in portable devices for which there is a strong demand for narrower widths. In addition, since the adhesive sheet disclosed herein contains a foam substrate, it can be excellent in shock absorption, waterproofing, dust resistance, and the like. By utilizing such characteristics, it can be preferably applied to electronic device applications, such as for fixing the display part of a portable electronic device, fixing a display part protective member of a portable electronic device, fixing a key module member of a mobile phone, fixing a decorative panel of a television, fixing a battery pack of a personal computer, and waterproofing the lens of a digital video camera. Particularly preferred applications include applications for portable electronic devices. In particular, it can be preferably used in portable electronic devices having a liquid crystal display device. For example, in such portable electronic devices, it is suitable for applications such as bonding a display part (which may be the display part of a liquid crystal display device) or a display part protective member to a housing.

The display protection member is typically a member having an area that exhibits light transmission in the thickness direction (hereinafter also referred to as a "light-transmitting member"), and may also be referred to as a lens. Here, in this specification, the term "lens" is a concept that includes both those that exhibit light refraction and those that do not exhibit light refraction. In other words, the term "lens" in this specification also includes light-transmitting members that do not have a refraction effect, such as a protective panel that simply protects the display of a portable electronic device. The protective panel can also be understood as a display protection member or display cover member that has light transmission. When the material of the protective panel is glass, the protective panel can also be referred to as a "cover glass". However, the material of the protective panel or the lens is not limited to glass, and may be any material that exhibits light transmission.

In this specification, the term "portable electronic device" refers to electronic devices that are generally portable and used, and is not limited to any other devices. Here, "portable" does not mean that the device is merely portable, but rather that the device has a level of portability that allows an individual (average adult) to carry it relatively easily. Examples of "portable electronic devices" include mobile phones, smartphones, tablet PCs, and notebook PCs. Such portable electronic devices may be so-called wearable terminals (for example, wristband types such as wristwatches, head-mounted types such as glasses, etc.). The portable electronic devices may have one or more functions, such as a telephone, a clock, a camera, glasses, a personal computer or other information terminal, health management tools such as a blood pressure monitor, a pulse monitor, and a pedometer, a music player, a video player, audio recording, and video recording.

The adhesive sheet disclosed herein can be used in the form of a joining member processed into various shapes for joining or fixing components constituting a portable electronic device (for example, joining a display unit or a display unit protection member to a housing, preferably joining a display unit protection member (typically a protection panel) having optical transparency to a housing). A preferred form of such a joining member is one having a narrow portion less than 2.0 mm in width, and an average width W [mm] of the narrow portion less than 1.0 mm (more preferably 0.7 mm or less, even more preferably 0.5 mm or less, for example 0.3 mm or less). According to the adhesive sheet disclosed herein, even when used as a joining member having a shape including such a narrow portion (for example, a frame shape), good performance (pressure adhesive force, shock absorption, etc.) can be exhibited. The average width W [mm] of the narrow portions of the adhesive sheet is obtained by dividing the total area of the narrow portions included in the adhesive sheet by the total length. When the width of the narrow portions is constant, the width of the narrow portions and the above average width are the same.

The narrow portion is typically linear. The term "linear" here refers to a concept that includes straight lines, curved lines, folded lines (e.g., L-shaped), rings such as frame-shaped and circular rings, and composite or intermediate shapes of these. The ring-shaped shape is not limited to those that are formed by curves, but includes rings that are partially or entirely formed in straight lines, such as a shape that follows the periphery of a rectangle (frame-shaped) or a shape that follows the periphery of a sector. The length of the narrow portion is not particularly limited. For example, in a form in which the length of the narrow portion is 10 mm or more (typically 20 mm or more, e.g., 30 mm or more), the effect of applying the technology disclosed herein can be suitably exerted. The width of the narrow portion may be constant or may vary partially.

The adhesive sheet disclosed herein can be used, for example, as a fixing member for joining a display unit or a display protection member of a portable electronic device to a housing, taking advantage of the characteristic that the adhesive sheet easily exhibits good bonding performance even when made narrow. Therefore, according to this specification, there is provided a fixing member for fixing a display unit or a display protection member of a portable electronic device to a housing, which includes any of the adhesive sheets disclosed herein.

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

<Experimental Example 1>
<Preparation of Pressure-Sensitive Adhesive Composition A>
Into a reaction vessel equipped with a stirrer, a reflux condenser, a thermometer, a dropping device, and a nitrogen inlet tube, 2.9 parts of acrylic acid, 5 parts of vinyl acetate, 92 parts of n-butyl acrylate, 0.1 part of 2-hydroxyethyl acrylate, and 30 parts of ethyl acetate and 120 parts of toluene as polymerization solvents were charged, and the mixture was stirred for 2 hours while introducing nitrogen gas.
After removing oxygen from the polymerization system in this manner, 0.2 parts of 2,2'-azobisisobutyronitrile (AIBN) was added, and the temperature was raised to 60°C to carry out a polymerization reaction for 6 hours to obtain a polymer solution containing a polymer. The solid content of this polymer solution was 40.0%, and the Mw of the polymer was 50 x ¹⁰⁴ .
To the above polymer solution, 10 parts of Arakawa Chemical Industries Co., Ltd.'s trade name "Pensel D-125" (rosin-based tackifier resin, solid content 100%), 10 parts of Arakawa Chemical Industries Co., Ltd.'s trade name "Super Ester A-100" (rosin-based tackifier resin, solid content 100%), 5 parts of Eastman Chemical Co.'s trade name "Foralin 8020F" (rosin-based tackifier resin, solid content 100%), and 15 parts of Arakawa Chemical Industries Co., Ltd.'s trade name "Tamanol 803L" (terpene phenol resin, solid content 100%) were added relative to 100 parts of the polymer in the polymer solution, and thoroughly stirred until dissolved. Furthermore, aromatic polyisocyanate (trade name "Coronate L", Tosoh Corporation, solid content 75%) was added as a crosslinking agent in a ratio of 2.0 parts relative to 100 parts of the polymer in the above polymer solution, and thoroughly stirred to obtain a solvent-based pressure-sensitive adhesive composition A.

(Storage modulus measurement)
The adhesive composition A was applied to the first release surface of the release liner L1 described later, and dried at 100°C for 2 minutes to form an adhesive layer having a thickness of 50 μm on the first release surface. The adhesive layers having a thickness of 50 μm were stacked together to prepare a laminated adhesive sample having a thickness of about 2 mm. The laminated adhesive sample was punched into a disk shape having a diameter of 7.9 mm, and the sample was sandwiched and fixed between parallel plates. Dynamic viscoelasticity measurement was performed under the following conditions using a viscoelasticity tester (manufactured by TA Instruments, ARES), and the 23°C storage modulus and 0°C storage modulus were obtained. As a result, the 23°C storage modulus of the adhesive layer (hereinafter also referred to as "adhesive layer A") formed from the adhesive composition A was 0.09 MPa, and the 0°C storage modulus was 0.50 MPa.
[Storage modulus measurement conditions]
・Measurement mode: Shear mode ・Temperature range: -70℃ to 150℃
Heating rate: 5°C/min
Measurement frequency: 1Hz

<Preparation of Release Liners L1 and L2>
A release treatment layer using a silicone-based release treatment agent was formed on the first and second surfaces of a 38 μm thick polyethylene terephthalate (PET) film to obtain a release liner L1 having a first and second release surface. A liner substrate having a thickness of 125 μm, in which polyethylene was laminated on both sides of a wood-free paper, was prepared, and a release treatment layer using a silicone-based release treatment agent was formed on the first and second surfaces of the liner substrate to obtain a release liner L2 having a first and second release surface. Here, the second release surface of the release liners L1 and L2 is formed so that the peel strength against the adhesive layer formed from the above-mentioned adhesive composition A is higher (so that peeling is heavier) than that of the first release surface of the release liners L1 and L2.

The arithmetic mean roughness of each release surface of release liners L1 and L2 is shown in Table 1. These values were measured in the same manner as the surface smoothness measurement of the adhesive surface described below, except that the non-measurement surface of each release liner was placed in close contact with a glass slide and the four sides were fixed with masking tape (Nitto Denko Corporation, No. 720A).

<Preparation of Pressure-Sensitive Adhesive Sheet with Release Liner>
Using the pressure-sensitive adhesive composition A and release liners L1 and L2 prepared above, pressure-sensitive adhesive sheets with release liners were prepared according to the following procedure.

(Example 1)
The adhesive composition A was applied to the first release surface (light release surface) of the release liner L2, and dried at 100° C. for 2 minutes to form an adhesive layer (first adhesive layer) having a thickness of 50 μm. The adhesive composition A was applied to the second release surface (heavy release surface) of the release liner L1, and dried at 100° C. for 2 minutes to form an adhesive layer (second adhesive layer) having a thickness of 50 μm. The second adhesive layer formed on the second release surface of the release liner L1 was bonded to the second surface of a 150 μm-thick foam substrate (polyethylene foam sheet with corona discharge treatment on both sides, density 0.56 g/cm ³ , 25% compressive strength (C ₂₅ ) 800 kPa, average bubble diameter 55 μm; hereinafter referred to as "foam substrate B1"). This release liner L1 was left on the second adhesive layer as it was and used as the final liner. The first pressure-sensitive adhesive layer formed on the first release surface of the release liner L2 was then bonded to the first surface of the foam substrate B1, and the release liner L2 was then removed to expose the first pressure-sensitive adhesive layer. This laminate (first pressure-sensitive adhesive layer/foam substrate B1/second pressure-sensitive adhesive layer/release liner L1) was then wound around a core material with the first pressure-sensitive adhesive layer facing inward, thereby obtaining a pressure-sensitive adhesive sheet with a release liner according to this example, in which the first and second adhesive surfaces were protected by the first and second release surfaces of the release liner L1, respectively.

(Example 2)
Two release liners L2 were prepared. The adhesive composition A was applied to the first release surface (light release surface) of the first release liner L2, and dried at 100° C. for 2 minutes to form an adhesive layer (first adhesive layer) having a thickness of 50 μm. The adhesive composition A was applied to the second release surface (heavy release surface) of the second release liner L2, and dried at 100° C. for 2 minutes to form an adhesive layer (second adhesive layer) having a thickness of 50 μm. The second adhesive layer formed on the second release surface of the second release liner L2 was bonded to the second surface of the foam substrate B1. The second release liner L2 was left on the second adhesive layer as it was and used as the final liner. The first adhesive layer formed on the first release surface of the first release liner L2 was bonded to the first surface of the foam substrate B, and then the first release liner L2 was removed to expose the first adhesive layer. This laminate (first adhesive layer/foam substrate B1/second adhesive layer/release liner L2) was wound around a core material with the first adhesive layer facing inward, thereby obtaining an adhesive sheet with release liner according to this example, in which the first adhesive surface and the second adhesive surface are protected by the first release surface and the second release surface of the release liner L2, respectively.

The release-lined adhesive sheets of Examples 1 and 2 were cured in an oven at 50°C for one day, and then the following measurements and evaluations were carried out.

<Arithmetic mean roughness of adhesive surface>
In an environment of 23°C and 50% RH, the adhesive sheet constituting the adhesive sheet with release liner was separated from the release liner. The second adhesive surface of this adhesive sheet was attached to a slide glass S1112 No. 2 (manufactured by Matsunami Glass Industry Co., Ltd.), and the arithmetic mean roughness Ra of the first adhesive surface (surface to be measured) was measured under the following conditions. Similarly, the first adhesive surface of the adhesive sheet separated from the release liner was attached to the slide glass, and the arithmetic mean roughness Ra of the second adhesive surface (surface to be measured) was measured. The measurement was performed within 10 minutes after the adhesive surface to be measured was separated from the release liner. The results are shown in Table 1.
[Arithmetic mean roughness measurement conditions]
Apparatus: Optical interference type surface roughness measuring device (Veeco, Wyko NT-9100)
Measurement area/time: 622 μm x 467 μm
(Objective lens: 10x, FOV (internal lens): 1.0x)
Measurement mode: VSI (Vertical Scan Interferometry)
Back scan: 5 μm
Measurement distance: 10 μm
Threshold: 0.1%
・Scan speed: 1x (Single scan)

<15% Compressive Stress of Adhesive Sheet>
Using the method described above, the 15% compressive stress was measured for each of the pressure-sensitive adhesive sheets constituting the pressure-sensitive adhesive sheets with release liner according to Examples 1 and 2. The results are shown in Table 1.

<Pressure Adhesive Strength>
The adhesive sheet according to each example was cut into a window frame shape (picture frame shape) with a short side of 59 mm, a long side of 113 mm, and a width of 0.5 mm as shown in Figures 2 and 3 to obtain a window frame-shaped adhesive sheet 30. Using this window frame-shaped adhesive sheet 30, a rectangular glass plate 42 (Gorilla glass manufactured by Corning was used) with a short side of 59 mm, a long side of 113 mm, and a thickness of 1 mm was bonded to a window frame-shaped polycarbonate plate (PC plate) 44 with a thickness of 2 mm and a rectangular shape with a width of 70 mm and a length of 130 mm and an opening 44A with a short side of 54 mm and a long side of 95 mm in the center, to prepare an evaluation sample 40 shown in Figures 2 and 3. The above bonding was performed by sandwiching the window frame-shaped adhesive sheet 30 between the glass plate 42 and the PC plate 44 and pressing them together with a load of 50 N for 10 seconds. Note that Figure 3 is a cross-sectional view taken along the III-III line in Figure 2. Figure 2 corresponds to the view seen from the arrow II in Figure 3.

The measurement of the pressing adhesive force was performed under the conditions of 23°C and 50% RH for the evaluation sample after the above-mentioned lamination and after keeping it under the conditions of 23°C and 50% RH for 1 hour. Specifically, the evaluation sample was set in a universal tensile compression tester (apparatus name "tensile compression tester, TG-1kN" manufactured by Minebea Co., Ltd.) with the PC board 44 side facing up. Then, the tip 46A (having a flat surface with a diameter of 10 mm) of the round bar 46 was abutted against the glass plate 42 with a position shifted by 15 mm from the center C of the opening 44A to one long side side as the pressing center P, and the round bar 46 was lowered at a speed of 10 mm/min to press the glass plate 42 in a direction away from the PC plate 44. Then, the maximum stress observed until the glass plate 42 and the PC plate 44 were separated was measured as the pressing adhesive force. The results are shown in Table 1.

As shown in Table 1, the adhesive sheet of Example 1, which has a highly smooth first adhesive surface and a second adhesive surface, achieved an increase of approximately 20% in the pressing adhesive force measured by the above method compared to the adhesive sheet of Example 2, which has a less smooth adhesive surface.
The pressure adhesive force was measured in the same manner as above, except that the pressure center P was set at the center C of the opening 44A, and the increase rate of the pressure adhesive force in Example 1 compared to Example 2 was less than half of that in Example 2. This result shows that the effect of increasing the smoothness of the adhesive surface is more pronounced in an embodiment in which the pressure is applied to a position shifted from the center C to the long side.
The first adhesive surface in both Examples 1 and 2 is the surface of an adhesive layer transferred from the first release surface (Ra _R1 = 1054 nm) of the release liner L2 serving as a process liner. Therefore, the difference in the arithmetic mean roughness Ra _A1 of the first adhesive surface between Examples 1 and 2 is believed to be due to the difference in the arithmetic mean roughness Ra _R1 of the first release surface of the final liner in contact with these adhesive surfaces.

(Examples 3 and 4)
A 200 μm thick foam substrate (polyethylene foam sheet corona discharge-treated on both sides, density 0.22 g/cm ³ , 25% compressive strength (C ₂₅ ) 70 kPa; hereinafter referred to as "foam substrate B2") was used instead of the foam substrate B1 used in Examples 1 and 2. In all other respects, the same procedures as in Examples 1 and 2 were repeated to obtain PSA sheets with release liners according to Examples 3 and 4.

The release-lined adhesive sheets of Examples 3 and 4 were cured in an oven at 50°C for one day, and then the same measurements and evaluations were carried out as for the release-lined adhesive sheets of Examples 1 and 2. The results are shown in Table 2.

As shown in Table 2, it was confirmed that the adhesive sheets of Examples 3 and 4 using foam substrate B2 also had the effect of improving the pressing adhesive strength by increasing the smoothness of the adhesive surface, similar to the adhesive sheets of Examples 1 and 2 using foam substrate B1. Specifically, the adhesive sheet of Example 3, which has a first adhesive surface and a second adhesive surface with high smoothness, showed an increase in pressing adhesive strength of approximately 15% compared to the adhesive sheet of Example 4, which has adhesive surfaces with lower smoothness.

<Experimental Example 2>
<Preparation of Pressure-Sensitive Adhesive Composition B>
A mixture of 70 parts of n-butyl acrylate, 30 parts of 2-ethylhexyl acrylate, 3 parts of acrylic acid, and 0.07 parts of 4-hydroxybutyl acrylate was polymerized in toluene with 0.08 parts of AIBN as a polymerization initiator to obtain a polymer solution containing a polymer with a Mw of 40× ¹⁰⁴ .
To the above polymer solution, 30 parts of a rosin-based tackifier resin (manufactured by Arakawa Chemical Industries, Ltd., product name "PENSEL D-125") and 2 parts of an aromatic polyisocyanate (manufactured by Tosoh Corporation, product name "CORONATE L", solid content 75%) as a crosslinking agent were added per 100 parts of the polymer in the polymer solution, and the mixture was thoroughly stirred to obtain a solvent-based pressure-sensitive adhesive composition B.

<Preparation of Pressure-Sensitive Adhesive Composition C>
A reaction vessel equipped with a stirrer, a thermometer, a nitrogen gas inlet tube, a reflux condenser, and a dropping funnel was charged with 95 parts of n-butyl acrylate, 5 parts of acrylic acid, and toluene as a polymerization solvent. The mixture was refluxed with nitrogen at room temperature for 1 hour, and then heated to 60° C., 0.2 parts of AIBN was added as a polymerization initiator, and a polymerization reaction was carried out at about 63° C. for 7 hours to obtain a polymer solution containing a polymer with a Mw of about 50× ¹⁰⁴ .
To the above polymer solution, 20 parts of a terpene phenol resin (manufactured by Arakawa Chemical Industries, Ltd., product name "Tamanol 803L") and 30 parts of a xylene formaldehyde resin having a hydroxyl group (manufactured by Mitsubishi Gas Chemical Company, Inc., product name "Nikanol H-80") were added as a tackifier resin relative to 100 parts of the polymer in the polymer solution, 4 parts of an aromatic polyisocyanate (manufactured by Tosoh Corporation, product name "Coronate L", solid content 75%) was added as a crosslinking agent, and 0.05 parts of a nitrogen atom-containing hydroxy compound (manufactured by Asahi Denka Corporation, product name "EDP-300", a propylene oxide adduct of ethylenediamine) was further added as a hydroxy compound, and the mixture was thoroughly stirred to obtain a solvent-based pressure-sensitive adhesive composition C.

In the same manner as in the measurement of the storage modulus of the adhesive layer A described above, the adhesive compositions B and C were used to form the adhesive layers B and C, and the storage modulus at 23°C and the storage modulus at 0°C were determined. The results obtained are shown in Table 3 together with the results of the measurement of the storage modulus of the adhesive layer A.

(Examples 5 to 8)
PSA sheets with release liner according to Examples 5 to 8 were obtained in the same manner as in Examples 1 to 4, except that PSA composition B was used instead of PSA composition A.

(Examples 9 to 12)
PSA sheets with release liner according to Examples 9 to 12 were obtained in the same manner as in Examples 1 to 4, except that PSA composition C was used instead of PSA composition A.

The release-lined adhesive sheets of Examples 5 to 12 were cured in an oven at 50°C for one day, and then the same measurements and evaluations were carried out as for the release-lined adhesive sheets of Examples 1 to 4. The results are shown in Tables 4 and 5.

As can be seen from the comparison between Example 5 and Example 6, and between Example 7 and Example 8, it was confirmed that the use of adhesive composition B also had the effect of improving the pressing adhesive strength for each of the substrates B1 and B2 by increasing the smoothness of the adhesive surface (Table 4). In addition, as can be seen from the comparison between Example 9 and Example 10, and between Example 11 and Example 12, it was confirmed that the use of adhesive composition C also had the effect of improving the pressing adhesive strength for each of the substrates B1 and B2 by increasing the smoothness of the adhesive surface (Table 5).

Specific examples of the present invention have been described in detail above, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and variations of the specific examples given above.

REFERENCE SIGNS LIST 10 Adhesive sheet 10A First adhesive surface 10B Second adhesive surface 11 First adhesive layer 12 Second adhesive layer 15 Foam substrate 15A First surface 15B Second surface 20 Release liner 20A First release surface 20B Second release surface 30 Window frame-shaped adhesive sheet 40 Evaluation sample 42 Glass plate 44 Polycarbonate plate (PC plate)
44A Opening 46 Round bar 46A Tip 100 Adhesive sheet with release liner

Claims (6)

an adhesive sheet having a first adhesive surface and a second adhesive surface;
A pressure-sensitive adhesive sheet with a release liner, comprising a release liner having release treatment layers on both sides of a plastic film, the release liner having a second release surface that contacts the second adhesive surface and a first release surface opposite the second release surface that contacts the first adhesive surface,
The pressure-sensitive adhesive sheet is
A foam substrate;
a first pressure-sensitive adhesive layer disposed on a first surface side of the foam substrate and constituting the first adhesive surface;
a second pressure-sensitive adhesive layer disposed on a second surface side of the foam substrate to constitute the second adhesive surface, wherein the first adhesive surface has an arithmetic mean roughness Ra _A1 of 50 nm or more and 250 nm or less , and the second adhesive surface has an arithmetic mean roughness Ra _A2 of 50 nm or more and 160 nm or less ,
The 25% compressive strength _C25 of the foam substrate is 400 kPa or more,
the release liner has an arithmetic mean roughness Ra _R1 of the first release surface of 50 nm or more and 250 nm or less, and an arithmetic mean roughness Ra _R2 of the second release surface of 50 nm or more and 150 nm or less;
a pressure-sensitive adhesive constituting the first pressure-sensitive adhesive layer and the second pressure-sensitive adhesive layer is an acrylic pressure-sensitive adhesive having an acrylic polymer as a base polymer,
A pressure-sensitive adhesive sheet with a release liner, wherein the first pressure-sensitive adhesive layer and the second pressure-sensitive adhesive layer have a storage modulus at 23° C. of 0.02 MPa or more and 0.15 MPa or less . 2. The pressure-sensitive adhesive sheet with a release liner according to claim 1 , wherein the pressure-sensitive adhesive sheet has a 15% compressive strength of 2 N/ ^cm2 or more. 3. The pressure-sensitive adhesive sheet with a release liner according to claim 1, wherein the pressure-sensitive adhesive layer has a storage modulus at 0°C of 0.05 MPa or more and less than 1.5 MPa. The pressure-sensitive adhesive sheet with a release liner according to any one of claims 1 to 3 , wherein 50% by weight or more of the total amount of monomers used in the synthesis of the acrylic polymer is a _C2-5 alkyl (meth)acrylate. The pressure-sensitive adhesive sheet with a release liner according to any one of claims 1 to 4 , wherein the acrylic polymer has a glass transition temperature of -60°C or higher and -40°C or lower. The pressure-sensitive adhesive sheet with a release liner according to claim 1 , which is used for bonding components of a portable electronic device.

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