Double-sided adhesive tape

Abstract

The purpose of the present invention is to provide a double-sided adhesive tape capable of achieving both a high holding force relative to a load, and excellent flexibility. The present invention is a double-sided adhesive tape having a foam base material and an adhesive layer on both surfaces of the foam base material, wherein the residual compressive stress thereof is less than or equal to 21.0 kPa, and the inclined holding time until the double-sided adhesive tape peels off and the weight falls is 500 hours or more when a test piece is obtained by cutting out the double-sided adhesive tape to a size of 25mm × 25mm, bonding one surface thereof to a glass plate (width 50mm × length 75mm × thickness 3mm), and thereafter, bonding an aluminum plate (width 40mm × length 40mm × thickness 2mm) to the other surface thereof, and leaving the same for 24 hours in a 23°C, 50%RH environment, and then attaching a 1-kg weight to the center of the aluminum plate so that the load is applied to the double-sided adhesive tape and the aluminum plate, and holding the test piece at an inclination angle of 45° at 60°C and 90% RH.

Description

double-sided adhesive tape

[0001] The present invention relates to a double-sided pressure-sensitive adhesive tape.

[0002] Adhesive tapes are widely used to fix electronic components. Specifically, adhesive tapes are used to fix the surface cover panels of display devices such as televisions and monitors to their housings. Such adhesive tapes are used in the shape of, for example, a picture frame and are arranged around the periphery of the display screen.

[0003] In recent years, the pursuit of design and functionality has led to increasingly narrower framed display devices such as televisions and monitors, and expectations for bezel-less display devices are also rising. In the conventional manufacturing of display devices, cover panels were sometimes fixed to the housing by fitting or screwing, but fitting or screwing is difficult for display devices with increasingly narrow framed displays, so there is an increasing demand for fixing with adhesive tape, and adhesive tape is also becoming thinner and narrower.

[0004] As an example of an adhesive tape that can be used in such a display device, Patent Documents 1 and 2 describe an impact-absorbing tape in which an acrylic adhesive layer is integrally laminated on at least one surface of a base layer, and the base layer is a cross-linked polyolefin resin foam sheet having a specific degree of cross-linking and an aspect ratio of bubbles.

[0005] JP 2009-242541 A JP 2009-258274 A

[0006] In recent years, display devices such as televisions and monitors have become larger, and the weight of components to be fixed, such as cover panels and housings, has also increased. As a result, adhesive tapes, particularly those that have become thinner and narrower, are now subjected to much heavier loads than before. Furthermore, in applications such as wall-mounted televisions, display devices are increasingly being installed tilted forward (e.g., at about 45°) from the vertical direction, and adhesive tapes are required to have excellent holding power against tilt loads (i.e., not easily peeling off even when installed tilted forward).

[0007] Furthermore, in recent years, the proportion of liquid crystal panels used in display devices that use the IPS (In Plane Switching) method has been increasing. In display devices that use the IPS method, stress caused by unevenness in the housing is transmitted to the liquid crystal panel via the adhesive tape, which tends to cause uneven display on the screen. Therefore, adhesive tapes used in display devices that use the IPS method are required to have flexibility that can alleviate the stress transmitted from the housing.

[0008] While increasing the strength is necessary to improve the holding power against load, increasing flexibility is also necessary to reduce display unevenness. However, because these are mutually exclusive physical properties, it has been difficult to achieve both strength and flexibility in the substrate.

[0009] An object of the present invention is to provide a double-sided pressure-sensitive adhesive tape that can achieve both high holding power against a load and excellent flexibility.

[0010] Disclosure 1 relates to a double-sided pressure-sensitive adhesive tape having a foam substrate and pressure-sensitive adhesive layers on both sides of the foam substrate, the double-sided pressure-sensitive adhesive tape having a residual compressive stress of 21.0 kPa or less, and which is prepared by cutting the double-sided pressure-sensitive adhesive tape to a size of 25 mm x 25 mm, bonding one side to a glass plate (50 mm wide x 75 mm long x 3 mm thick), bonding the other side to an aluminum plate (40 mm wide x 40 mm long x 2 mm thick), and allowing the tape to stand for 24 hours in an environment of 23°C and 50% RH. When a 1 kg weight is attached to the center of the aluminum plate so as to apply a load to the double-sided pressure-sensitive adhesive tape and the aluminum plate, and the aluminum plate is held at a 45° angle at 60°C and 90% RH, the inclined holding time until the pressure-sensitive adhesive tape peels and the weight drops is 500 hours or more. Disclosure 2 relates to a double-sided pressure-sensitive adhesive tape according to Disclosure 1, having a shear tensile strength at 23°C of 0.35 MPa or more.

[0014] Disclosure 3 is the double-sided pressure-sensitive adhesive tape of Disclosure 1 or 2, wherein the pressure-sensitive adhesive layer contains a (meth)acrylic copolymer, the (meth)acrylic copolymer having a weight-average molecular weight of 1,000,000 or more, the pressure-sensitive adhesive layer having a gel fraction of 25% by mass or more, and the pressure-sensitive adhesive layer having a shear storage modulus (G') at 180°C of 15 kPa or more. Disclosure 4 is the double-sided pressure-sensitive adhesive tape of Disclosure 1, 2, or 3, wherein the foam substrate has a first foam layer and a second foam layer laminated on at least one surface of the first foam layer and having a lower expansion ratio than the first foam layer. Disclosure 5 is the double-sided pressure-sensitive adhesive tape of Disclosure 4, wherein the first foam layer contains a resin (R1) having a loss tangent (tan δ) at 23°C of 0.9 or more. Disclosure 6 is the double-sided pressure-sensitive adhesive tape of Disclosure 5, wherein the first foam layer contains the resin (R1) in a proportion of 10% by mass or more. Disclosure 7 is the double-sided pressure-sensitive adhesive tape of Disclosure 4, 5, or 6, wherein the expansion ratio of the first foam layer is 8 or more. Disclosure 8 is the double-sided pressure-sensitive adhesive tape of Disclosure 4, 5, 6, or 7, wherein the thickness of the first foam layer is 550 μm or more. Disclosure 9 is the double-sided pressure-sensitive adhesive tape of Disclosure 4, 5, 6, 7, or 8, wherein the aspect ratio of the cells of the first foam layer (average major axis of the cells / average minor axis of the cells) is 2.3 or more.

[0023] Disclosure 10 is the double-sided pressure-sensitive adhesive tape of Disclosure 4, 5, 6, 7, 8, or 9, wherein the expansion ratio of the second foam layer is 2 or less. Disclosure 11 is the double-sided pressure-sensitive adhesive tape of Disclosure 4, 5, 6, 7, 8, 9, or 10, wherein the thickness of the second foam layer is 15 μm or more. Disclosure 12 is the double-sided pressure-sensitive adhesive tape of Disclosure 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or 11, further comprising a resin layer composed of a polyester-based resin. The present invention is described in detail below.

[0011] The inventors focused on the residual compressive stress and the tilted holding time until peeling in a double-sided adhesive tape having a substrate and an adhesive layer, and discovered that by adjusting these within an appropriate range, it is possible to achieve excellent resistance to load and reduce display unevenness that occurs in display devices, thereby completing the present invention.

[0012] The double-sided pressure-sensitive adhesive tape of the present invention comprises a foam substrate and pressure-sensitive adhesive layers on both sides of the foam substrate. The use of a foam substrate can enhance the flexibility of the double-sided pressure-sensitive adhesive tape.

[0013] The double-sided pressure-sensitive adhesive tape of the present invention preferably has a shear tensile strength of 0.35 MPa or more at 23°C. A shear tensile strength of 0.35 MPa or more allows the double-sided pressure-sensitive adhesive tape of the present invention to exhibit higher holding power against shear load. A more preferred lower limit of the shear tensile strength is 0.39 MPa, and an even more preferred lower limit is 0.45 MPa. Furthermore, the double-sided pressure-sensitive adhesive tape of the present invention preferably has an upper limit of the shear tensile strength at 23°C of 1.0 MPa. A shear tensile strength of 1.0 MPa or less allows the double-sided pressure-sensitive adhesive tape of the present invention to have appropriate flexibility and reduce display unevenness. A more preferred upper limit of the shear tensile strength is 0.8 MPa, and an even more preferred upper limit is 0.7 MPa. The shear tensile strength of the double-sided pressure-sensitive adhesive tape can be measured by the shear tensile test described below. FIG. 1 is a schematic diagram showing a shear tensile test of a double-sided pressure-sensitive adhesive tape. First, the double-sided pressure-sensitive adhesive tape 1 of the present invention was cut into a size of 25 mm x 25 mm, and two SUS plates 4 each measuring 125 mm x 50 mm and 2 mm thick were laminated as shown in Figure 1. This laminate was pressed with a 5 kg weight for 10 seconds to bond the two SUS plates 4 together, and then left to stand at 23°C and 50% RH for 24 hours to prepare a test sample in which the two SUS plates 4 were bonded together via the double-sided pressure-sensitive adhesive tape 1. After fixing one of the SUS plates 4 in place, the upper part of the other SUS plate 4 was pulled at a speed of 12.7 mm / min in a direction perpendicular to the lamination direction of the SUS plates (the direction indicated by the arrow in Figure 1) under conditions of 23°C and 50% RH. This test can be performed using, for example, a precision universal testing machine such as the AUTOGLAPH AGS-X manufactured by Shimadzu Corporation. The stress (MPa) at which the double-sided pressure-sensitive adhesive tape 1 broke was measured, and this value was taken as the shear tensile strength. The breakage of the double-sided pressure-sensitive adhesive tape 1 means that the foam substrate is broken between layers.

[0014] Examples of methods for adjusting the shear tensile strength within the above-mentioned range include a method of changing the thickness of the foam substrate (increasing the thickness increases the shear tensile strength), a method of changing the expansion ratio of the foam substrate (reducing the expansion ratio increases the shear tensile strength), a method of adjusting the type and content of a resin (R1) described below contained in the foam substrate (reducing the content increases the shear tensile strength), and a method of selecting the cell structure of the foam substrate (using a foam substrate with a closed cell structure increases the shear tensile strength).

[0015] The double-sided pressure-sensitive adhesive tape of the present invention has a residual compressive stress of 21.0 kPa or less. Because the residual compressive stress is 21.0 kPa or less, the double-sided pressure-sensitive adhesive tape of the present invention has excellent flexibility (stress relaxation property) and can reduce display unevenness occurring in display devices. Display unevenness in display devices occurs when stress is transmitted to the liquid crystal panel due to steps or irregularities (e.g., about 0.4 mm) in the housing, resulting in differences in light transmission behavior. Normally, such stress gradually relaxes over time, but in some cases, a certain amount of stress remains without relaxation. Therefore, in order to reduce display unevenness as a display device product, it is preferable to reduce the stress that remains even after a sufficient amount of time (about 24 hours) has passed since the assembly of the display device (bonding of the liquid crystal panel). Figures 2 and 3 show schematic cross-sectional views of a portion where a liquid crystal panel is fixed to a housing via double-sided pressure-sensitive adhesive tape. Figure 2 shows the initial state immediately after bonding. Figure 3 shows the saturated state after sufficient time has passed since bonding. In Figure 2, stress due to steps and unevenness in the housing 3 is transmitted to the liquid crystal panel 2 via the double-sided adhesive tape 1, causing tilt. On the other hand, in Figure 3, the stress is alleviated by the double-sided adhesive tape 1 over time, and the tilt of the liquid crystal panel 2 that occurred immediately after bonding also becomes smaller. The residual compressive stress in this state affects the display unevenness on the screen. The preferred upper limit of the residual compressive stress is 18.5 kPa, and more preferably 17.0 kPa. Furthermore, the preferred lower limit of the residual compressive stress of the double-sided adhesive tape of the present invention is 1.0 kPa. By having the residual compressive stress of 1.0 kPa or more, the double-sided adhesive tape of the present invention has appropriate strength and can exert a higher holding force against a load. The more preferred lower limit of the residual compressive stress is 1.5 kPa, and even more preferably 2.5 kPa.

[0016] The residual compressive stress is measured by the following compression test. First, five double-sided adhesive tapes cut into a size of 20 mm wide x 20 mm long are stacked and bonded together to prepare a test specimen. The prepared test specimen is compressed using a precision universal testing machine (e.g., Shimadzu Corporation's "AUTOGLAPH AGS-X") under conditions of 23°C, 50% RH, and a compression rate of 12.7 mm / min until the compressive displacement per adhesive tape becomes 0.4 mm (2.0 mm total for the five tapes) (corresponding to the state immediately after bonding (Figure 2)). This state is maintained, and the stress is measured after 6 hours (corresponding to the saturated state (Figure 3)).

[0017] Examples of methods for adjusting the residual compressive stress include a method for adjusting the expansion ratio of the foam substrate (increasing the expansion ratio to reduce the residual compressive stress), a method for adjusting the aspect ratio of the foam substrate (increasing the aspect ratio to reduce the residual compressive stress), a method for adjusting the type or content ratio of a resin (R1) described later contained in the foam substrate (reducing the residual compressive stress by selecting one with a large loss tangent or increasing the content), and a method for adjusting the thickness of the foam substrate (reducing the residual compressive stress by increasing the thickness).

[0018] The double-sided pressure-sensitive adhesive tape of the present invention is prepared by cutting the double-sided pressure-sensitive adhesive tape into a size of 25 mm x 25 mm, laminating one side to a glass plate (50 mm wide x 75 mm long x 3 mm thick), laminating an aluminum plate (40 mm wide x 40 mm long x 2 mm thick) to the other side, and allowing the tape to stand for 24 hours in an environment of 23°C and 50% RH. A 1 kg weight was attached to the center of the aluminum plate so that a load was applied to the double-sided pressure-sensitive adhesive tape and the aluminum plate, and the plate was held at a 45° angle at 60°C and 90% RH. The lower limit of the tilted holding time until the pressure-sensitive adhesive tape peeled and the weight dropped was 500 hours. By ensuring this tilted holding time of 500 hours or more, the double-sided pressure-sensitive adhesive tape of the present invention can exhibit high holding power against a load.

[0019] The tilt retention time is measured by the following 45° tilt retention test. Figure 4 shows a schematic diagram of the 45° tilt retention test for double-sided pressure-sensitive adhesive tape. First, double-sided pressure-sensitive adhesive tape 1 is cut into a size of 25 mm x 25 mm, and one side is attached to glass plate 7 (50 mm wide x 75 mm long x 3 mm thick). A 2 kg rubber roller is reciprocated from the other side at a speed of 300 mm / min. Next, aluminum plate 8 (40 mm wide x 40 mm long x 2 mm thick) is attached to the other side. A 5 kg weight is applied from the aluminum plate 8 side for 10 seconds to bond the tape. The test piece is then left to stand for 24 hours in an environment of 23°C and 50% RH to prepare a test specimen. A 1 kg weight 9 is attached to the center of aluminum plate 8 so that a load is applied to double-sided pressure-sensitive adhesive tape 1 and aluminum plate 8. The test piece is held at a 45° tilt at 60°C and 90% RH, and the time until the adhesive tape peels and the weight 9 falls is measured.

[0020] Examples of methods for adjusting the slope retention time include a method for adjusting the storage modulus of the pressure-sensitive adhesive layer at high temperatures (e.g., about 180°C) described below (increasing the storage modulus to increase the slope retention time), a method for adjusting the molecular weight or molecular weight distribution of the acrylic copolymer contained in the pressure-sensitive adhesive layer (increasing the molecular weight or narrowing the molecular weight distribution to increase the slope retention time), a method for changing the tackifier resin contained in the pressure-sensitive adhesive layer described below (selecting a tackifier resin with a high softening point to increase the slope retention time), a method for adjusting the content of the tackifier resin contained in the pressure-sensitive adhesive layer described below (reducing the content to increase the slope retention time), a method for adjusting the gel fraction of the pressure-sensitive adhesive layer (increasing the gel fraction to increase the slope retention time), and a method for adjusting the shear tensile strength of the foam substrate (adjusting the shear tensile strength to increase the slope retention time).

[0021] In order for the double-sided pressure-sensitive adhesive tape of the present invention to simultaneously satisfy the above-mentioned shear tensile strength, residual compressive stress, and slope holding time conditions, it is conceivable to adjust the foam substrate and the pressure-sensitive adhesive layer, for example, as follows. That is, it is conceivable to adopt a closed-cell structure for the foam substrate, incorporate a resin (R1) with a large loss tangent, increase the aspect ratio of the cells, and increase the thickness of the foam substrate. It is also preferable to increase the slope holding time for the pressure-sensitive adhesive layer, particularly to increase the storage modulus at high temperatures. In addition, the above-mentioned conditions can be simultaneously satisfied, for example, by adjusting the expansion ratio of the foam substrate and the content of the resin (R1) with a large loss tangent.

[0022] The foam substrate may have a single-layer structure or a multi-layer structure. The foam substrate preferably includes a first foam layer and a second foam layer having a lower expansion ratio than the first foam layer. The second foam layer is preferably disposed adjacent to the first foam layer, and more preferably integrated with the first foam layer. The foam substrate does not need to have a uniform density internally; the expansion ratio of at least one surface portion is preferably lower than that of the center portion. By using a foam substrate having the above-described structure, the double-sided pressure-sensitive adhesive tape of the present invention can be peeled and removed without leaving any residue (e.g., a portion of the first foam layer that has been torn off) on the adherend during rework, thereby exhibiting excellent reworkability. Furthermore, even if a hard resin layer is laminated on the first foam layer, the flexibility of both adhesive surfaces is not impaired. Furthermore, by using a foam substrate having the above-described structure, the shear tensile strength of the foam substrate can be increased, thereby further improving its holding power against load.

[0023] The foam substrate may have the second foam layer on only one side of the first foam layer, or may have the second foam layer on both sides of the first foam layer. In particular, since excellent reworkability can be exhibited on both adhesive surfaces, it is preferable to have the second foam layer on both sides of the first foam layer. In this case, the resin composition, physical properties, thickness, etc. of the second foam layers on both sides may be the same or different. Furthermore, pressure-sensitive adhesive tapes are usually provided in a rolled state and are unwound from the roll for use. When hard resin layers are laminated on both sides of the first foam layer, wrinkles and creases may occur during winding. By including the second foam layer, it is possible to suppress wrinkles and creases during winding while also exhibiting reworkability.

[0024] The foam substrate may have other layers in addition to the first foam layer and the second foam layer, but from the viewpoint of preventing the manufacturing process from becoming complicated, it is preferable that no other layers are present between the first foam layer and the second foam layer.

[0025] The first foam layer may have an open-cell structure or a closed-cell structure, but preferably has a closed-cell structure. The closed-cell structure of the first foam layer increases the strength of the first foam layer, suppressing deformation and interlayer fracture, and further improving the holding power of the resulting double-sided pressure-sensitive adhesive tape against load. The cell structure can be confirmed, for example, by observation at a magnification of 150 to 200 times using an optical microscope (e.g., Keyence Corporation's "VHX-6000").

[0026] The first foam layer is not particularly limited, and examples thereof include a polyurethane foam layer, a polyolefin foam layer, a rubber-based foam layer, an acrylic foam layer, etc. Among these, a polyurethane foam layer or a polyolefin foam layer is preferred, and a polyolefin foam layer is more preferred, because they can exhibit excellent stress relaxation properties and strength.

[0027] Examples of the polyolefin foam layer include foam layers made of resins such as polyethylene-based resins, polypropylene-based resins, and polybutadiene-based resins. Among these, foam layers made of polyethylene-based resins are preferred because they are easy to obtain flexible polyolefin foam layers. Examples of foam layers made of polyethylene-based resins include foam layers made of low-density polyethylene, high-density polyethylene, linear low-density polyethylene, ethylene-α-olefin copolymers, and mixtures thereof.

[0028] The first foam layer preferably contains a resin (R1) having a loss tangent (tan δ) of 0.9 or more at 23° C. When the first foam layer contains the resin (R1), the stress relaxation properties of the first foam layer are further improved, and the flexibility of the obtained double-sided pressure-sensitive adhesive tape is further improved.

[0029] The loss tangent (tan δ) of the resin (R1) at 23°C may be 0.9 or more, with a preferred lower limit of 1.0 and a preferred upper limit of 3.0. When the loss tangent (tan δ) of the resin (R1) at 23°C is within the above range, the resulting double-sided pressure-sensitive adhesive tape has higher holding power against load and better flexibility. The loss tangent (tan δ) of the resin (R1) at 23°C is more preferably 1.1 at its lower limit, more preferably 2.5 at its upper limit, even more preferably 1.2 at its lower limit, and even more preferably 2.0 at its upper limit. The loss tangent (tan δ) of the resin (R1) at 23°C is measured by dynamic viscoelasticity measurement at a frequency of 1.0 Hz and a heating rate of 5°C / min. Examples of measuring instruments include a Rheometrics Dynamic Analyze RDA-700 manufactured by Rheometrics.

[0030] Examples of the resin (R1) include polyethylene-based resins. Among these, α-olefin copolymers such as ethylene-α-olefin copolymers and linear low-density polyethylene obtained by copolymerizing ethylene with a small amount of an α-olefin as needed are preferred because they further improve flexibility. Examples of the α-olefins include propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-heptene, and 1-octene. Among these, α-olefins having 4 to 10 carbon atoms are preferred.

[0031] The preferred lower limit of the content of the resin (R1) in the first foam layer is 10% by mass. When the content of the resin (R1) is 10% by mass or more, the stress relaxation property of the first foam layer is further improved, and the flexibility of the resulting double-sided pressure-sensitive adhesive tape is further improved. The preferred lower limit of the content of the resin (R1) is 15% by mass, and even more preferably 20% by mass. The preferred upper limit of the content of the resin (R1) is 50% by mass. When the content of the resin (R1) is 50% by mass or less, the strength of the first foam layer is increased, deformation and interlayer fracture can be suppressed, and the resulting double-sided pressure-sensitive adhesive tape will have a further improved holding power against load. The preferred upper limit of the content of the resin (R1) is 40% by mass, and even more preferably 30% by mass.

[0032] Examples of the polyurethane foam layer include a polyurethane foam layer made of a urethane resin composition derived from polyisocyanate and polyol, and such a polyurethane foam layer can be produced by heat-curing the urethane resin composition.

[0033] The preferred lower limit of the expansion ratio of the first foam layer is 8 times. When the expansion ratio of the first foam layer is 8 times or more, the first foam layer can have appropriate flexibility, and the flexibility and stress relaxation properties of both adhesive surfaces of the double-sided pressure-sensitive adhesive tape are further improved. The more preferred lower limit of the expansion ratio of the first foam layer is 10 times, and even more preferred lower limit is 15 times. The preferred upper limit of the expansion ratio of the first foam layer is 40 times. When the expansion ratio of the first foam layer is 40 times or less, the strength of the first foam layer is sufficiently increased, and the holding power of the double-sided pressure-sensitive adhesive tape against a load is further improved. The more preferred upper limit of the expansion ratio of the first foam layer is 35 times, and even more preferred upper limit is 30 times. The expansion ratio of the foam layer can be calculated as the reciprocal of the density of the foam layer. The density of the foam layer can be measured using an electronic hydrometer (e.g., "ED120T" manufactured by Mirage) in accordance with, for example, JIS K 6401 (when polyurethane is used) or JIS K 6767 (when polyethylene urethane is used).

[0034] The preferred lower limit of the thickness of the first foam layer is 550 μm. When the thickness of the first foam layer is 550 μm or more, the first foam layer can have appropriate flexibility, and the flexibility and stress relaxation properties of both adhesive surfaces of the double-sided pressure-sensitive adhesive tape are further improved. The more preferred lower limit of the thickness of the first foam layer is 600 μm, and even more preferred is 650 μm. Furthermore, the preferred upper limit of the thickness of the first foam layer is 2500 μm. When the thickness of the first foam layer is 2500 μm or less, deformation when a load is applied can be suppressed, and the holding power of the double-sided pressure-sensitive adhesive tape against the load is further improved. The more preferred upper limit of the thickness of the first foam layer is 2000 μm, and even more preferred is 1500 μm. Note that, in this specification, thickness can be measured using a dial thickness meter (e.g., the "ABS Digimatic Indicator" manufactured by Mitutoyo Corporation).

[0035] The average major axis of the cells in the first foam layer preferably has a lower limit of 500 μm and an upper limit of 1500 μm. When the average major axis of the cells in the first foam layer is 500 μm or more, the flexibility of the foam substrate is increased, and the resulting double-sided pressure-sensitive adhesive tape can further reduce display unevenness on a display device. When the average major axis of the cells in the first foam layer is 1500 μm or less, the strength of the foam substrate can be ensured. The average major axis of the cells in the first foam layer is more preferably 600 μm, more preferably 1200 μm, and even more preferably 700 μm.

[0036] The average minor axis diameter of the cells in the first foam layer is preferably 150 μm at the lower limit and 400 μm at the upper limit. When the average minor axis diameter of the cells in the first foam layer is 150 μm or more, the flexibility of the foam substrate is increased, and the resulting double-sided pressure-sensitive adhesive tape can further reduce display unevenness on a display device. When the average minor axis diameter of the cells in the first foam layer is 400 μm or less, the strength of the foam substrate can be ensured. The average minor axis diameter of the cells in the first foam layer is more preferably 170 μm at the lower limit and 300 μm at the upper limit, and even more preferably 200 μm at the lower limit and 250 μm at the upper limit.

[0037] The aspect ratio of the cells of the first foam layer preferably has a lower limit of 2.3. When the aspect ratio of the cells of the first foam layer is 2.3 or more, the cells of the first foam layer are flattened, thereby increasing the flexibility of the foam substrate, and the resulting double-sided pressure-sensitive adhesive tape has improved stress relaxation properties, thereby further reducing display unevenness in a display device. The aspect ratio of the cells of the first foam layer preferably has a lower limit of 2.7, and more preferably 2.9. The aspect ratio of the cells of the first foam layer preferably has an upper limit of 15. When the aspect ratio of the cells of the first foam layer is 15 or less, the strength of the foam substrate can be ensured. The aspect ratio of the cells of the first foam layer more preferably has an upper limit of 12, and even more preferably has an upper limit of 10. In this specification, the term "aspect ratio of bubbles" refers to the value obtained by dividing the average minor diameter of the bubbles by the average major diameter of the bubbles (average major diameter of bubbles / average minor diameter of bubbles).

[0038] The average major axis of the cells of the first foam layer, the average minor axis of the cells of the first foam layer, and the aspect ratio of the cells of the first foam layer can be determined, for example, as follows. Specifically, the first foam layer is sliced ​​along a plane parallel to the MD and thickness directions using a razor (manufactured by Feather Corporation) to obtain an MD-cut sample. The obtained MD-cut sample is photographed using a digital microscope (e.g., Keyence Corporation's "VHX-6000") under conditions such as a magnification of 200x and a measurement screen size of 1.8 mm x 1.3 mm. From the obtained photographed images, the cell with the longest major axis and the cell with the second longest major axis are selected, and the major and minor axes of these cells are measured to calculate the aspect ratio. This procedure is performed for three photographed images, and the average of the aspect ratios of a total of six cells is taken as the aspect ratio of the cells of the first foam layer.

[0039] The second foam layer is not particularly limited, and may be a layer having the same bubble structure, layer structure, resin composition, physical properties, etc. as the first foam layer, or may be a layer having a different bubble structure, layer structure, resin composition, physical properties, etc. from the first foam layer.

[0040] The expansion ratio of the second foam layer may be smaller than that of the first foam layer. The upper limit of the expansion ratio of the second foam layer is preferably 2. When the expansion ratio of the second foam layer is 2 or less, the strength of the second foam layer is sufficiently increased, and the holding power of the double-sided pressure-sensitive adhesive tape against a load is further improved. The upper limit of the expansion ratio of the second foam layer is more preferably 1.8, and even more preferably 1.6.

[0041] The preferred lower limit of the thickness of the second foam layer is 15 μm. If the thickness of the second foam layer is 15 μm or more, the holding power of the obtained double-sided pressure-sensitive adhesive tape against load is further improved. The more preferred lower limit of the thickness of the second foam layer is 17 μm, and even more preferred lower limit is 20 μm. Furthermore, the preferred upper limit of the thickness of the second foam layer is 100 μm. If the thickness of the second foam layer is 100 μm or less, the flexibility and stress relaxation properties of both adhesive surfaces of the obtained double-sided pressure-sensitive adhesive tape are further improved. The more preferred upper limit of the thickness of the second foam layer is 60 μm, and even more preferred upper limit is 40 μm.

[0042] The preferred lower limit of the expansion ratio of the entire foam substrate is 4 times, and the preferred upper limit is 30 times. When the expansion ratio of the entire foam substrate is within the above range, the holding power against a load of the obtained double-sided pressure-sensitive adhesive tape is further improved, and the flexibility is also further improved. The more preferred lower limit of the expansion ratio of the entire foam substrate is 7 times, and the more preferred upper limit is 20 times, and the even more preferred lower limit is 8 times, and the even more preferred upper limit is 15 times. When the foam substrate has a first foam layer and a second foam layer, the expansion ratio of the entire foam substrate may be calculated from the following formula (1) using the densities of the first foam layer and the second foam layer measured in accordance with JIS K 7222. Expansion ratio (times) = 1 / (ρ １ ×r １ +ρ ２ ×r ２ ) (1) (ρ １ : density of the first foam layer (kg / m ３ ), r １ ρ: ratio of the thickness of the first foam layer to the thickness of the entire foam substrate２ : density of the second foam layer (kg / m ３ ), r ２ : ratio of the thickness of the second foam layer to the thickness of the entire foam substrate)

[0043] The preferred lower limit of the total thickness of the foam substrate is 580 μm, and the preferred upper limit is 2600 μm. When the total thickness of the foam substrate is within the above range, the resulting double-sided pressure-sensitive adhesive tape has improved holding power against a load and improved flexibility. The more preferred lower limit of the total thickness of the foam substrate is 630 μm, the more preferred upper limit is 2100 μm, the even more preferred lower limit is 650 μm, and the even more preferred upper limit is 1600 μm.

[0044] The ratio of the thickness of the first foam layer to the thickness of the second foam layer in the foam substrate (thickness of the first foam layer / thickness of the second foam layer) preferably has a lower limit of 1.0 and an upper limit of 400. When the thickness ratio is within the above range, the holding power of the obtained double-sided pressure-sensitive adhesive tape against a load is further improved, and the flexibility of both adhesive surfaces is also further improved. The lower limit of the thickness ratio is more preferably 3.0, and the upper limit is more preferably 150, and even more preferably 5.0, and even more preferably 40.

[0045] The ratio of the expansion ratio of the first foam layer to the expansion ratio of the second foam layer in the foam base material (expansion ratio of the first foam layer / expansion ratio of the second foam layer) preferably has a lower limit of 1.3 and an upper limit of 100. When the ratio of the expansion ratio of the first foam layer to the expansion ratio of the second foam layer is within the above range, the holding power of the obtained double-sided pressure-sensitive adhesive tape against a load is further improved, and the flexibility of both adhesive surfaces is further improved. The ratio of the expansion ratio of the first foam layer to the expansion ratio of the second foam layer is more preferably 3.0 lower limit, more preferably 80 upper limit, even more preferably 8.0 lower limit, and even more preferably 60 upper limit.

[0046] The method for producing the foam base material is not particularly limited, and may be a method in which the first foam layer and the second foam layer are separately produced and then pressure-bonded or laminated together via a pressure-sensitive adhesive layer or the like, but a method in which a foamable composition for forming the first foam layer and a foamable composition for forming the second foam layer are subjected to multi-layer extrusion is preferred. The multi-layer extrusion method allows the first foam layer and the second foam layer to be laminated without an adhesive layer or the like, and is therefore preferred from the viewpoint of preventing the production process from becoming complicated.

[0047] For example, the multilayer extrusion method may involve first separately extruding the foamable composition forming the first foam layer and the foamable composition forming the second foam layer, and then merging the separately extruded compositions in a molten state in a die to form layers, thereby obtaining a laminate sheet having a laminate of multiple layers composed of multiple compositions. Examples of the foamable composition forming the first foam layer and the foamable composition forming the second foam layer include compositions containing a polyethylene-based resin or the like and a thermally decomposable foaming agent, as described above. The expansion ratio of the resulting foam layer can be adjusted by varying the type and amount of the thermally decomposable foaming agent. Next, at least one surface of the resulting laminate sheet is irradiated with ionizing radiation to crosslink the polyethylene-based resin or the like. The expansion ratio of the resulting foam layer can be adjusted by varying the degree of crosslinking of the polyethylene-based resin or the like. Furthermore, the crosslinked laminate sheet can be foamed by heating or the like to obtain a foam substrate having the first foam layer and the second foam layer. The crosslinked laminate sheet may be stretched during and / or after foaming by heating or the like.

[0048] The pressure-sensitive adhesive layers are laminated on both sides of the foam substrate. The pressure-sensitive adhesive layers do not need to be laminated directly adjacent to the foam substrate, and other layers may be interposed therebetween. The pressure-sensitive adhesive layers on both sides may have the same composition, physical properties, thickness, etc., or may be different.

[0049] Examples of the pressure-sensitive adhesive layer include an acrylic pressure-sensitive adhesive layer, a rubber-based pressure-sensitive adhesive layer, a urethane pressure-sensitive adhesive layer, and a silicone-based pressure-sensitive adhesive layer. Among these, the pressure-sensitive adhesive layer is preferably an acrylic pressure-sensitive adhesive layer because it is relatively stable against light, heat, moisture, etc. and can adhere to various adherends (it has low adherend selectivity). That is, the pressure-sensitive adhesive layer preferably contains a (meth)acrylic copolymer. In this specification, "(meth)acrylic" means acrylic or methacrylic.

[0050] The (meth)acrylic copolymer preferably has at least one structural unit selected from the group consisting of structural units derived from n-butyl(meth)acrylate and structural units derived from 2-ethylhexyl(meth)acrylate, from the viewpoints of improving initial tack and thereby improving ease of application at low temperatures, and of increasing the glass transition temperature (Tg) of the (meth)acrylic copolymer and further improving the bulk cohesive strength of the pressure-sensitive adhesive layer. It is more preferable that the (meth)acrylic copolymer have both structural units derived from n-butyl(meth)acrylate and structural units derived from 2-ethylhexyl(meth)acrylate. In this specification, "(meth)acrylate" means acrylate or methacrylate.

[0051] The content of the structural units derived from n-butyl (meth)acrylate in the (meth)acrylic copolymer is preferably 4% by mass at the lower limit and 80% by mass at the upper limit. By setting the content of the structural units derived from n-butyl acrylate within this range, the resulting double-sided pressure-sensitive adhesive tape can achieve both high adhesive strength and tackiness. The content of the structural units derived from n-butyl (meth)acrylate is more preferably 10% by mass at the lower limit and 70% by mass at the upper limit, and even more preferably 20% by mass at the lower limit and 60% by mass at the upper limit.

[0052] The content of the structural units derived from 2-ethylhexyl (meth)acrylate in the (meth)acrylic copolymer is preferably 10% by mass at the lower limit and 100% by mass at the upper limit. By setting the content of the structural units derived from 2-ethylhexyl (meth)acrylate within this range, the bulk cohesive strength of the pressure-sensitive adhesive layer is further improved, and the resulting double-sided pressure-sensitive adhesive tape can exhibit higher adhesive strength. The content of the structural units derived from 2-ethylhexyl (meth)acrylate is more preferably 30% by mass at the lower limit and 80% by mass at the upper limit, and even more preferably 50% by mass at the lower limit and 60% by mass at the upper limit.

[0053] The (meth)acrylic copolymer may optionally contain structural units derived from other copolymerizable polymerizable monomers other than n-butyl (meth)acrylate and 2-ethylhexyl (meth)acrylate. Examples of the other copolymerizable polymerizable monomers include (meth)acrylic acid alkyl esters having an alkyl group containing 1 to 3 carbon atoms, (meth)acrylic acid alkyl esters having an alkyl group containing 13 to 18 carbon atoms, and functional monomers. Examples of the (meth)acrylic acid alkyl esters having an alkyl group containing 1 to 3 carbon atoms include methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, and isopropyl (meth)acrylate. Examples of the (meth)acrylic acid alkyl esters having an alkyl group containing 13 to 18 carbon atoms include tridecyl (meth)acrylate and stearyl (meth)acrylate. Examples of the functional monomer include hydroxyalkyl (meth)acrylate, glycerin di(meth)acrylate, glycidyl (meth)acrylate, 2-(meth)acryloyloxyethyl isocyanate, (meth)acrylic acid, itaconic acid, maleic anhydride, crotonic acid, maleic acid, and fumaric acid. Among these, from the viewpoint of increasing the bulk cohesive strength and elastic modulus at high temperatures of the pressure-sensitive adhesive layer, hydroxyl group-containing monomers such as hydroxyalkyl (meth)acrylate and glycerin di(meth)acrylate are preferred. Specific examples of the hydroxyalkyl (meth)acrylate include 2-hydroxyethyl (meth)acrylate. These copolymerizable other polymerizable monomers may be used alone, or two or more types may be used in combination.

[0054] The weight-average molecular weight (Mw) of the (meth)acrylic copolymer preferably has a lower limit of 1,000,000. When the weight-average molecular weight of the (meth)acrylic copolymer is 1,000,000 or more, the storage modulus and bulk cohesive strength of the pressure-sensitive adhesive layer at 180°C are further improved, and peeling of the pressure-sensitive adhesive layer when a load is applied can be further suppressed. The weight-average molecular weight of the (meth)acrylic copolymer more preferably has a lower limit of 1,100,000, and even more preferably has a lower limit of 1,300,000. The weight-average molecular weight of the (meth)acrylic copolymer preferably has an upper limit of 2,000,000. When the weight-average molecular weight of the (meth)acrylic copolymer is 2,000,000 or less, the pressure-sensitive adhesive layer has poor interface wettability, and therefore interfacial peeling when a load is applied can be further suppressed. The weight-average molecular weight of the (meth)acrylic copolymer more preferably has an upper limit of 1,900,000, even more preferably has an upper limit of 1,800,000, and even more preferably has an upper limit of 1,750,000.

[0055] The ratio of the weight average molecular weight (Mw) to the number average molecular weight (Mn) of the (meth)acrylic copolymer (molecular weight distribution: Mw / Mn) preferably has a lower limit of 1.05 and an upper limit of 5.0. When the molecular weight distribution of the (meth)acrylic copolymer is 5.0 or less, the proportion of low molecular weight components is reduced, the shear storage modulus and bulk cohesive strength of the pressure-sensitive adhesive layer at 180°C are further improved, and peeling of the pressure-sensitive adhesive layer when a load is applied can be further suppressed. A more preferred upper limit of the molecular weight distribution of the (meth)acrylic copolymer is 4.5.

[0056] In this specification, the weight average molecular weight (Mw) and number average molecular weight (Mn) are the weight average molecular weight and number average molecular weight in terms of standard polystyrene measured by GPC (Gel Permeation Chromatography). Specifically, the obtained acrylic copolymer solution is diluted 50 times with tetrahydrofuran (THF) and the resulting diluted solution is filtered through a filter (material: polytetrafluoroethylene, pore diameter: 0.2 μm). The obtained filtrate is supplied to a gel permeation chromatograph (Waters, 2690 Separations Model), and GPC measurement is performed under conditions of a sample flow rate of 1 mL / min and a column temperature of 40 ° C. to measure the polystyrene-equivalent molecular weight of the (meth)acrylic copolymer, and this molecular weight is the weight average molecular weight (Mw) and number average molecular weight (Mn) of the (meth)acrylic copolymer. The column used may be, for example, GPC KF-806L (manufactured by Showa Denko K.K.), and the detector may be, for example, a differential refractometer, etc. Furthermore, the weight average molecular weight (Mw) and number average molecular weight (Mn) of the obtained (meth)acrylic copolymer may be used to calculate the molecular weight distribution (Mw / Mn) of the (meth)acrylic copolymer.

[0057] The method for adjusting the molecular weight distribution (Mw / Mn) of the (meth)acrylic copolymer is not particularly limited, and examples thereof include a method using living radical polymerization, a method of performing constant temperature polymerization, and a method of performing polymerization while adjusting the amount of monomer input in boiling point polymerization.

[0058] The (meth)acrylic copolymer can be obtained by radically reacting a monomer mixture containing n-butyl(meth)acrylate, 2-ethylhexyl(meth)acrylate, and other copolymerizable monomers in the presence of a polymerization initiator. As a method for radically reacting the monomer mixture, i.e., a polymerization method, a conventionally known method can be used, such as solution polymerization (boiling point polymerization or constant temperature polymerization), emulsion polymerization, suspension polymerization, and bulk polymerization.

[0059] From the viewpoint of exerting high adhesive strength, the pressure-sensitive adhesive layer preferably further contains a tackifying resin.

[0060] Examples of the tackifying resin include rosin resins, rosin ester resins, hydrogenated rosin resins, terpene resins, terpene phenol resins, coumarone-indene resins, alicyclic saturated hydrocarbon resins, C5 petroleum resins, C9 petroleum resins, and C5-C9 copolymer petroleum resins. Among these, from the viewpoint of compatibility with the base polymer, rosin resins or terpene resins are preferred, and rosin resins or terpene resins having hydroxyl groups are more preferred. These tackifying resins may be used alone or in combination of two or more.

[0061] Examples of the rosin resin having a hydroxyl group include Pencel D-135 and Superester A-75 (both manufactured by Arakawa Chemical Industries, Ltd.), etc. Examples of the terpene resin having a hydroxyl group include YS Polystar G150 and YS Polystar T160 (both manufactured by Yasuhara Chemical Co., Ltd.), etc.

[0062] The softening point of the tackifier resin preferably has a lower limit of 70°C and an upper limit of 170°C. If the softening point of the tackifier resin is 70°C or higher, the pressure-sensitive adhesive layer has improved holding power against a load. If the softening point of the tackifier resin is 170°C or lower, the pressure-sensitive adhesive layer has poor interface wettability, which can further suppress interfacial peeling when a load is applied. A more preferred lower limit of the softening point of the tackifier resin is 120°C. In this specification, the term "softening point of the tackifier resin" refers to the softening temperature measured according to JIS K2207 (ring and ball method).

[0063] The tackifier resin preferably has a lower limit of 25 mgKOH / g for its hydroxyl value. When the tackifier resin has a hydroxyl value of 25 mgKOH / g or more, it has poor interaction with the interface of the pressure-sensitive adhesive layer, and therefore interfacial peeling when a load is applied can be further suppressed. A more preferred lower limit for the hydroxyl value of the tackifier resin is 30 mgKOH / g, and an even more preferred lower limit is 35 mgKOH / g. The upper limit for the hydroxyl value of the tackifier resin is not particularly limited. In this specification, the hydroxyl value of the tackifier resin can be measured according to JIS K1557 (phthalic anhydride method).

[0064] The content of the tackifier resin relative to 100 parts by mass of the (meth)acrylic copolymer is preferably 5 parts by mass at the lower limit and 60 parts by mass at the upper limit. By ensuring that the content of the tackifier resin is within this range, the adhesive strength of the resulting double-sided pressure-sensitive adhesive tape becomes more sufficient. The lower limit of the tackifier resin content is more preferably 10 parts by mass, more preferably 50 parts by mass at the upper limit, even more preferably 15 parts by mass, and even more preferably 40 parts by mass at the upper limit.

[0065] The pressure-sensitive adhesive layer may be added with a crosslinking agent to form a crosslinked structure between the main chains of the resins (e.g., the (meth)acrylic copolymer, the tackifying resin, etc.) constituting the pressure-sensitive adhesive layer. In this case, the crosslinking agent may be added to both sides of the pressure-sensitive adhesive layer, or to only one side of the pressure-sensitive adhesive layer. By adjusting the type and amount of the crosslinking agent, it becomes easier to adjust the shear storage modulus and gel fraction at 180°C of the pressure-sensitive adhesive layer. Examples of the crosslinking agent include isocyanate-based crosslinking agents, aziridine-based crosslinking agents, epoxy-based crosslinking agents, and metal chelate-type crosslinking agents. Of these, isocyanate-based crosslinking agents are preferred from the viewpoint of adhesion to the substrate.

[0066] The pressure-sensitive adhesive layer may contain a silane coupling agent to further improve adhesive strength. Examples of the silane coupling agent include epoxy silanes, acrylic silanes, methacrylic silanes, amino silanes, and isocyanate silanes.

[0067] The pressure-sensitive adhesive layer may contain a colorant to impart light-shielding properties. Examples of the colorant include carbon black, aniline black, titanium oxide, etc. Among these, carbon black is preferred because it is relatively inexpensive and chemically stable.

[0068] The pressure-sensitive adhesive layer may contain conventionally known fine particles and additives, such as inorganic fine particles, conductive fine particles, antioxidants, foaming agents, organic fillers, and inorganic fillers, as needed.

[0069] The pressure-sensitive adhesive layer preferably has a gel fraction of 25% by mass or less. A gel fraction of 25% by mass or more of the pressure-sensitive adhesive layer increases the storage modulus and bulk strength of the pressure-sensitive adhesive layer at 180°C, thereby preventing peeling of the pressure-sensitive adhesive layer when a load is applied. A more preferred lower limit of the gel fraction of the pressure-sensitive adhesive layer is 30% by mass, and an even more preferred lower limit is 40% by mass. The preferred upper limit of the gel fraction of the pressure-sensitive adhesive layer is 80% by mass. A gel fraction of 80% by mass or less of the pressure-sensitive adhesive layer further prevents interfacial peeling when a load is applied due to poor interfacial wettability of the pressure-sensitive adhesive layer. A more preferred upper limit of the gel fraction of the pressure-sensitive adhesive layer is 75% by mass, and an even more preferred upper limit is 70% by mass. The gel fraction of the pressure-sensitive adhesive layer can be measured, for example, by the following method. Specifically, a test piece is prepared by cutting a double-sided pressure-sensitive adhesive tape into a 50 mm x 100 mm flat rectangular shape. The test piece is immersed in ethyl acetate at 23° C. for 24 hours, then removed from the ethyl acetate and dried at 110° C. for 1 hour. The mass W of the test piece after drying ２ The gel fraction of the pressure-sensitive adhesive layer can be obtained by measuring the mass of the pressure-sensitive adhesive layer and calculating the gel fraction using the following formula (2). Note that no release film for protecting the outermost pressure-sensitive adhesive layer is laminated on the test piece. Gel fraction (mass%) = 100 × (W ２ -W ０ ) / (W １ -W ０ ) (2) (W ０ : Mass of the substrate, W １ : mass of test piece before immersion, W ２ : Mass of test piece after immersion and drying)

[0070] The pressure-sensitive adhesive layer preferably has a lower limit of a shear storage modulus (G') at 180°C of 15 kPa. If the pressure-sensitive adhesive layer has a shear storage modulus of 15 kPa or more at 180°C, the bulk cohesive strength of the pressure-sensitive adhesive layer is further improved, and peeling of the pressure-sensitive adhesive layer when a shear load or an oblique load is applied can be further suppressed. The pressure-sensitive adhesive layer's shear storage modulus at 180°C is more preferably 18 kPa, and even more preferably 21 kPa. The pressure-sensitive adhesive layer's shear storage modulus at 180°C is preferably 50 kPa, and even more preferably 50 kPa. If the pressure-sensitive adhesive layer has a shear storage modulus of 50 kPa or less at 180°C, the pressure-sensitive adhesive layer can have poor interfacial wettability, thereby further suppressing interfacial peeling when a shear load or an oblique load is applied. The pressure-sensitive adhesive layer's shear storage modulus at 180°C is more preferably 40 kPa, and even more preferably 32 kPa. The shear storage modulus of the pressure-sensitive adhesive layer at 180°C can be measured using a viscoelasticity measuring device (for example, "Rheometrics Dynamic Analyze RDA-700" manufactured by Rheometrics Corporation) under the conditions of shear mode, measurement temperature of -40 to 200°C, temperature rise rate of 3°C / min, frequency of 10 Hz, and strain of 0.10%.

[0071] The gel fraction and shear storage modulus at 180°C of the pressure-sensitive adhesive layer can be adjusted within the above ranges, for example, by adjusting the composition, weight average molecular weight, molecular weight distribution (weight average molecular weight / number average molecular weight) etc. of the (meth)acrylic copolymer contained in the pressure-sensitive adhesive layer, or by adjusting the type and amount of the crosslinking agent and tackifying resin contained in the pressure-sensitive adhesive layer.

[0072] The thickness of the pressure-sensitive adhesive layer preferably has a lower limit of 20 μm and an upper limit of 100 μm. When the thickness of the pressure-sensitive adhesive layer is 20 μm or more, the adhesive strength of the pressure-sensitive adhesive layer is sufficient. When the thickness of the pressure-sensitive adhesive layer is 100 μm or less, the stress relaxation properties of the foam substrate can sufficiently contribute to the stress relaxation properties of the double-sided pressure-sensitive adhesive tape as a whole. The thickness of the pressure-sensitive adhesive layer is more preferably 25 μm lower limit, more preferably 80 μm upper limit, even more preferably 30 μm lower limit, even more preferably 70 μm upper limit, still more preferably 35 μm lower limit, even more preferably 65 μm upper limit.

[0073] The double-sided pressure-sensitive adhesive tape of the present invention may have layers other than the foam substrate and the pressure-sensitive adhesive layer, if necessary.

[0074] The double-sided pressure-sensitive adhesive tape of the present invention preferably further comprises a resin layer. By including a resin layer in the double-sided pressure-sensitive adhesive tape of the present invention, it is possible to prevent the foam substrate from stretching and breaking during handling. Furthermore, since this can impart excellent reworkability to the resulting double-sided pressure-sensitive adhesive tape, it is preferable that the resin layer has a higher shear strength than the foam substrate. The double-sided pressure-sensitive adhesive tape of the present invention may comprise only one resin layer, or may comprise multiple resin layers. From the viewpoint of preventing wrinkles and creases during winding, it is preferable that the double-sided pressure-sensitive adhesive tape comprise only one resin layer.

[0075] Examples of resins constituting the resin layer include polyester resins such as polyethylene terephthalate, acrylic resins, polyethylene resins, polypropylene resins, polyvinyl chloride, epoxy resins, silicone resins, phenolic resins, polyimides, polyesters, polycarbonates, etc. Among these, acrylic resins, polyethylene resins, polypropylene resins, and polyester resins are preferred because of their excellent flexibility. Among polyester resins, polyethylene terephthalate is preferred.

[0076] The resin constituting the resin layer may be a thermoplastic resin. The thermoplastic resin is not particularly limited, and examples thereof include styrene (co)polymers, olefin (co)polymers, vinyl chloride (co)polymers, polyether ester triblock (co)polymers, polyester (co)polymers, urethane (co)polymers, amide (co)polymers, and acrylic (co)polymers. Among these, the thermoplastic resin is preferably an acrylic (co)polymer, a styrene (co)polymer, or an olefin (co)polymer, from the viewpoint of being able to exhibit strength, elongation, flexibility, and self-adhesiveness as an elastic body, and being able to exhibit excellent reworkability while further improving the adhesion between the resin layer and the substrate. Furthermore, an acrylic (co)polymer or a styrene (co)polymer is more preferred, and a styrene (co)polymer is even more preferred.

[0077] When the resin constituting the resin layer is a thermoplastic resin, the upper limit of the tensile modulus of the resin layer is preferably 200 MPa. Using a flexible resin with a tensile modulus of 200 MPa or less for the resin layer ensures the flexibility of the resulting double-sided pressure-sensitive adhesive tape as a whole, making it easier to wind the double-sided pressure-sensitive adhesive tape into a roll and significantly improving handleability. The tensile modulus of the resin layer can be measured using a method in accordance with JIS K 7161. Specifically, for example, a dumbbell-shaped test specimen is prepared by punching out the resin layer using a punching blade (e.g., "Tensile No. 1 Dumbbell" manufactured by Kobunshi Keiki Co., Ltd.). The tensile modulus of the obtained test specimen is measured at a tension speed of 100 mm / min using, for example, an "Autograph AGS-X" manufactured by Shimadzu Corporation. The tensile modulus is calculated from the slope of the tensile strength between 1 and 3% strain.

[0078] The thickness of the resin layer is preferably 10 μm at the lower limit and 100 μm at the upper limit. When the thickness of the resin layer is 10 μm or more, the resin layer is less likely to break even when pulled. When the thickness of the resin sheet is 100 μm or less, a decrease in conformity to the adherend can be suppressed. The thickness of the resin layer is more preferably 15 μm at the lower limit, 80 μm at the upper limit, 20 μm at the lower limit, 60 μm at the upper limit, 25 μm at the lower limit, and 50 μm at the upper limit.

[0079] The resin layer may be colored. By coloring the resin layer, light-blocking properties can be imparted to the obtained double-sided pressure-sensitive adhesive tape. Examples of methods for coloring the resin layer include a method of kneading particles of carbon black, titanium oxide, or the like, or fine bubbles into the resin constituting the resin layer, and a method of applying ink to the surface of the resin layer.

[0080] The double-sided pressure-sensitive adhesive tape of the present invention can be produced, for example, by the following method. First, a solvent is added to an acrylic copolymer, a tackifying resin, etc. to prepare a solution of pressure-sensitive adhesive A. This solution of pressure-sensitive adhesive A is applied to the release-treated surface of a release film, and the solvent in the solution is dried to remove, thereby forming a pressure-sensitive adhesive layer. This pressure-sensitive adhesive layer is then pressed onto the surface of the foam substrate using a rubber roller or the like to bond it. In a similar manner, a pressure-sensitive adhesive layer is bonded to the other surface of the foam substrate, thereby obtaining a double-sided pressure-sensitive adhesive tape having pressure-sensitive adhesive layers on both sides of the foam substrate, and in which the surface of the pressure-sensitive adhesive layer is covered with a release film.

[0081] The preferred lower limit of the overall thickness of the double-sided pressure-sensitive adhesive tape of the present invention is 600 μm, and the preferred upper limit is 3000 μm. When the overall thickness of the double-sided pressure-sensitive adhesive tape of the present invention is 600 μm or more, the resulting double-sided pressure-sensitive adhesive tape has sufficient adhesive strength and also sufficient stress relaxation properties. When the overall thickness of the double-sided pressure-sensitive adhesive tape of the present invention is 3000 μm or less, the resulting double-sided pressure-sensitive adhesive tape can achieve sufficient adhesion and fixation, and the flexibility of both adhesive surfaces can also be more sufficient. A more preferred lower limit of the overall thickness of the double-sided pressure-sensitive adhesive tape of the present invention is 700 μm, a more preferred upper limit is 1700 μm, an even more preferred lower limit is 850 μm, and an even more preferred upper limit is 1500 μm. Note that a certain thickness or more may be required to fill gaps between components, and it has been even more difficult to solve the problem of the present invention within such design constraints, i.e., to achieve both high load retention and excellent flexibility.

[0082] The double-sided pressure-sensitive adhesive tape of the present invention is used, for example, to secure components in electronic devices. Examples of such electronic devices include televisions, monitors, portable electronic devices, and in-vehicle electronic devices. The double-sided pressure-sensitive adhesive tape of the present invention is particularly suitable for securing components in display devices such as televisions and monitors, particularly relatively large display devices. Specifically, it is used, for example, to secure the surface cover panel of such display devices to the housing. The double-sided pressure-sensitive adhesive tape of the present invention exhibits high holding strength against shear loads and tilting loads. Furthermore, the double-sided pressure-sensitive adhesive tape of the present invention has excellent flexibility and can suppress unevenness in the screen display caused by steps or irregularities in the housing of the display device. Therefore, it is suitable for use even when securing components in relatively large display devices using a narrow double-sided pressure-sensitive adhesive tape. The shape of the double-sided pressure-sensitive adhesive tape of the present invention for these applications is not particularly limited, and examples include rectangular, frame-shaped, circular, oval, and doughnut-shaped. The double-sided pressure-sensitive adhesive tape of the present invention may also be used for vehicle interiors and the interior and exterior of home appliances (e.g., TVs, monitors, air conditioners, refrigerators, etc.).

[0083] According to the present invention, it is possible to provide a double-sided pressure-sensitive adhesive tape that can achieve both high holding power against a load and excellent flexibility.

[0084] FIG. 1 is a schematic diagram showing a shear tensile test of a double-sided adhesive tape; FIG. 2 is a schematic cross-sectional view showing an initial state when a liquid crystal panel is fixed to a housing; FIG. 3 is a schematic cross-sectional view showing a saturated state when a liquid crystal panel is fixed to a housing; FIG. 4 is a schematic diagram showing a 45° inclined holding test of a double-sided adhesive tape; FIG. 5 is a schematic cross-sectional view of a test sample used for evaluating screen display unevenness; and FIG. 6 is a schematic diagram showing an actual device holding test.

[0085] The following examples will explain the present invention in more detail, but the present invention is not limited to these examples.

[0086] (Preparation of Acrylic Copolymer A) 159 parts by mass of ethyl acetate as a solvent and 75 parts by mass of n-butyl (meth)acrylate (BA), 35 parts by mass of 2-ethylhexyl (meth)acrylate (2EHA), 0.1 parts by mass of 2-hydroxyethyl acrylate (HEA), and 5 parts by mass of acrylic acid (AAc) were added to a reactor equipped with a thermometer, a stirrer, and a cooling tube. After nitrogen substitution, the reactor was placed in a water bath set at 60 ° C. and heated to initiate reflux. 30 minutes after the start of reflux, 0.050 parts by mass of azobisisobutyronitrile as a polymerization initiator was added to the reactor and allowed to react for 6 hours. Thereafter, ethyl acetate was added to the reactor and the mixture was cooled while diluting, thereby obtaining a solution of acrylic copolymer A. The obtained acrylic copolymer A solution was diluted 50 times with tetrahydrofuran (THF), and the resulting diluted solution was filtered through a filter (material: polytetrafluoroethylene, pore diameter: 0.2 μm). The obtained filtrate was supplied to a gel permeation chromatograph (Waters, "2690 Separations Model"), and GPC measurement was performed under conditions of a sample flow rate of 1 mL / min and a column temperature of 40°C. The polystyrene-equivalent molecular weight of acrylic copolymer A was measured, and the weight average molecular weight (Mw) was calculated. The weight average molecular weight (Mw) was 1,400,000. A GPC KF-806L (Showa Denko KK) was used as the column, and a differential refractometer was used as the detector.

[0087] (Preparation of Acrylic Copolymer B) A solution of acrylic copolymer B was obtained in the same manner as for acrylic copolymer A, except that the temperature of the water bath was set to 50° C. The weight average molecular weight of the obtained acrylic copolymer B was 1,600,000.

[0088] (Preparation of Acrylic Copolymer C) A solution of acrylic copolymer C was obtained in the same manner as in acrylic copolymer A, except that the blending amount of acrylic acid was changed to 3 parts by mass. The weight average molecular weight of the obtained acrylic copolymer C was 1,400,000.

[0089] (Preparation of Acrylic Copolymer D) A solution of acrylic copolymer D was obtained in the same manner as for acrylic copolymer A, except that the solvent was changed to a mixed solvent of 100 parts by mass of ethyl acetate and 50 parts by mass of toluene and the polymerization initiator was changed to 0.14 parts by mass of azobisisobutyronitrile. The weight average molecular weight of the obtained acrylic copolymer D was 800,000.

[0090] (Example 1) (1) Preparation of foam substrate As a foamable composition for forming the first foam layer, low-density polyethylene ("UBF polyethylene F420" manufactured by Ube Maruzen Polyethylene Co., Ltd., density 0.920 g / cm ３A composition was used that consisted of: 75 parts by mass of an α-olefin copolymer (manufactured by Mitsui Chemicals, Inc., loss tangent at 23°C: 0.1) as the resin (R1); 25 parts by mass of an α-olefin copolymer (manufactured by Mitsui Chemicals, Inc., loss tangent at 23°C: 1.3) as the resin (R1); 8.0 parts by mass of azodicarbonamide as a thermal decomposition type blowing agent; 1 part by mass of zinc oxide as a decomposition temperature regulator; and 0.5 parts by mass of 2,6-di-t-butyl-p-cresol as an antioxidant. The foamable composition that formed the second foam layer consisted of 100 parts by mass of polyethylene (manufactured by Ube Maruzen Polyethylene Co., Ltd., "UBF polyethylene F420"); 1 part by mass of zinc oxide as a decomposition temperature regulator; and 0.5 parts by mass of 2,6-di-t-butyl-p-cresol as an antioxidant. The foamable composition that formed the first foam layer and the foamable composition that formed the second foam layer were fed into an extruder for multilayer extrusion molding and melt-kneaded at 130°C. After melt-kneading, a long sheet-like foam base fabric with a thickness of approximately 0.06 mm was extruded, in which a layer of the foamable composition forming the first foam layer was laminated on both sides with a layer of the foamable composition forming the second foam layer. Next, both sides of the long sheet-like foam base fabric were crosslinked by irradiating them with an electron beam at an acceleration voltage of 500 kV at 4.0 Mrad. The crosslinked foam base fabric was continuously fed into a foaming oven maintained at 250 ° C. using hot air and an infrared heater, heated and foamed, and stretched at an MD stretch ratio of 3.5 times. This resulted in a foam base fabric in which a second foam layer was laminated on both sides of the first foam layer. The thickness and density of the first and second foam layers were measured in accordance with JIS K 7222, and the expansion ratio was calculated using the obtained density. Furthermore, the expansion ratio of the entire foam base fabric was calculated using the expansion ratio of the first foam layer and the second foam layer using the following formula (1): The measurement results are shown in Table 1. Expansion ratio (times) = 1 / (ρ １ ×r １ +ρ ２ ×r ２ ) (1) (ρ １ : density of the first foam layer (kg / m ３ ), r １ ρ: ratio of the thickness of the first foam layer to the thickness of the entire foam substrate ２ : density of the second foam layer (kg / m ３ ), r ２: ratio of the thickness of the second foam layer to the thickness of the entire foam substrate)

[0091] (2) Preparation of Pressure-Sensitive Adhesive Solution To the solution of acrylic copolymer A obtained in "(Preparation of Acrylic Copolymer)" above, 15 parts by mass of rosin ester resin Pencel D-135 (manufactured by Arakawa Chemical Industries, Ltd.) and 15 parts by mass of terpene phenol resin YS-Polystar G150 (manufactured by Yasuhara Chemical Co., Ltd.) as tackifier resins, and 30 parts by mass of ethyl acetate as a solvent were added per 100 parts by mass of the solid content of the acrylic copolymer, and the mixture was thoroughly stirred to obtain a pressure-sensitive adhesive solution.

[0092] (3) Preparation of Double-Sided Adhesive Tape The obtained adhesive solution was applied to a 50 μm thick polyethylene terephthalate (PET) resin sheet ("Lumirror X30" manufactured by Toray Industries, Inc.) and dried at 110 ° C for 5 minutes to produce an adhesive sheet having a 23 μm thick adhesive layer. One side of a foam substrate was bonded to this adhesive layer to produce a laminate in which the resin sheet and the foam substrate were laminated via the adhesive layer. Next, a 50 μm thick release film was prepared, and the obtained adhesive solution was applied to the release-treated surface of this release film and dried at 110 ° C for 5 minutes to produce an adhesive sheet (a) having a 40 μm thick adhesive layer, which was then bonded to the surface of the resin sheet of the laminate. Furthermore, an adhesive sheet (b) having the same configuration as the adhesive sheet (a) was bonded to the surface of the foam substrate of the laminate in the same manner. The laminate was then aged by heating at 40 ° C for 48 hours. This resulted in a double-sided adhesive tape having the resin sheet as the resin layer, a first adhesive layer on the side of the foam substrate having the resin layer and a second adhesive layer on the other side, with the adhesive layers on both sides covered with release films.

[0093] Examples 2 to 4 Double-sided pressure-sensitive adhesive tapes were prepared in the same manner as in Example 1, except that in the above-mentioned "(1) Preparation of foam substrate", the content (parts by mass) of the resin (R1) forming the first foam layer was changed to the content (parts by mass) shown in Table 1.

[0094] (Examples 5, 7, 8, 11, and 13) Double-sided pressure-sensitive adhesive tapes were produced in the same manner as in Example 1, except that in the above-mentioned "(1) Preparation of foam substrate," the amount of foaming agent, line speed, tension, etc. were adjusted when producing the foam substrate, thereby changing the expansion ratio, thickness, and aspect ratio of the first foam layer as shown in Table 1.

[0095] (Example 6, Comparative Examples 1 and 2) Double-sided pressure-sensitive adhesive tapes were produced in the same manner as in Example 1, except that in the above-mentioned "(2) Preparation of Pressure-sensitive Adhesive Solution", the pressure-sensitive adhesive layer was formed using the acrylic copolymers shown in Tables 1 and 2 instead of acrylic copolymer A.

[0096] Example 9 In the above-mentioned "(1) Preparation of foam substrate", the resin (R1) used in the foamable composition forming the first foam layer was changed to 25 parts by mass of styrene-ethylene-butylene-styrene block copolymer (SEBS) (manufactured by Asahi Kasei Corporation, "S.O.E (registered trademark) S1609", loss tangent at 23°C 0.9). Otherwise, a double-sided pressure-sensitive adhesive tape was prepared in the same manner as in Example 1.

[0097] Example 10 In the above-mentioned "(1) Preparation of foam substrate", for the foamable composition forming the first foam layer, 25 parts by mass of Hybrar (registered trademark) 7311F (manufactured by Kuraray Co., Ltd. (loss tangent at 23°C: 0.1)) was used in place of 25 parts by mass of the α-olefin copolymer that was the resin (R1). A double-sided pressure-sensitive adhesive tape was prepared in the same manner as in Example 1 except for the above.

[0098] (Example 12) A double-sided pressure-sensitive adhesive tape was produced in the same manner as in Example 1, except that in the above-mentioned "(1) Preparation of foam substrate", 2.0 parts by mass of azodicarbonamide was further added as a thermal decomposition type foaming agent to the foamable composition forming the second foam layer.

[0099] (Example 14) A double-sided pressure-sensitive adhesive tape was produced in the same manner as in Example 1, except that in the above-mentioned "(1) Preparation of foam substrate", only the foamable composition that forms the first foam layer was used, and the line speed and tension were appropriately adjusted to produce a foam substrate having only the first foam layer with the thickness shown in Table 1.

[0100] (Example 15) In the above-mentioned "(3) Preparation of double-sided adhesive tape," a double-sided adhesive tape without a resin layer was prepared. That is, a 50 μm-thick polyethylene terephthalate (PET) film was prepared as a release film, and the obtained adhesive solution was applied to the release-treated surface of this release film and dried at 110°C for 5 minutes to prepare an adhesive sheet (a) with a 40 μm-thick adhesive layer. This was then bonded to the surface of the foam substrate prepared in Example 1. Furthermore, in a similar manner, an adhesive layer (b) having the same configuration as the adhesive sheet (a) was also bonded to the other surface of the foam substrate. Thereafter, the tape was aged by heating at 40°C for 48 hours. A double-sided adhesive tape without a resin layer was obtained in the same manner as in Example 1, except as described above.

[0101] (Example 16) In the above-mentioned "(1) Preparation of foam substrate", the foamable composition forming the first foam layer was the same as in Example 1, except that the same low-density polyethylene and resin (R1) as in Example 1, and ethylene-vinyl acetate copolymer (EVA) (manufactured by Tosoh Corporation, "Ultrathene 636", vinyl acetate content 19%, density 0.941 g / cm ３ , MFR 2.5 g / 10 min (190°C), and loss tangent at 23°C 0.1) were used in the amounts (parts by mass) shown in Table 1. Except for this, double-sided pressure-sensitive adhesive tapes were prepared in the same manner as in Example 1.

[0102] (Comparative Example 3) A double-sided pressure-sensitive adhesive tape was produced in the same manner as in Example 1, except that in the above-mentioned "(1) Preparation of foam substrate", the resin (R1) was not contained in the foamable composition forming the first foam layer.

[0103] Comparative Example 4 A double-sided pressure-sensitive adhesive tape was produced in the same manner as in Example 1, except that in the above-mentioned "(1) Preparation of foam substrate", the blending amounts (parts by mass) of the resin and resin (R1) in the foamable composition forming the first foam layer were changed as shown in Table 2, and the line speed and tension were appropriately adjusted to change the aspect ratio of the bubbles in the first foam layer as shown in Table 2.

[0104] (Comparative Example 5) The foamable composition forming the first foam layer was composed of 100 parts by mass of low-density polyethylene (manufactured by Ube Maruzen Polyethylene Co., Ltd., "UBE Polyethylene F420"), 4.5 parts by mass of azodicarbonamide as a thermal decomposition type foaming agent, 1 part by mass of zinc oxide as a decomposition temperature regulator, and 0.5 parts by mass of 2,6-di-t-butyl-p-cresol as an antioxidant. The composition was fed into an extruder and melt-kneaded at 130 ° C. to extrude a long sheet-like foam raw web having a thickness of approximately 0.2 mm. Next, both sides of the long sheet-like foam raw web were crosslinked by irradiating them with an electron beam at an acceleration voltage of 500 kV for 4.0 Mrad. The crosslinked foam raw web was continuously fed into a foaming furnace maintained at 250 ° C. using hot air and an infrared heater, where it was heated and foamed, and stretched at an MD stretch ratio of 2.5 times to obtain a polyethylene foam. A 40 μm thick sheet made of a styrene-acrylic copolymer (manufactured by Fujikura Chemical Industries, Ltd., "SCT-126") was heat-sealed to the obtained polyethylene foam. A double-sided pressure-sensitive adhesive tape was produced in the same manner as in Example 1, except for the above.

[0105] (Comparative Example 6) 90 parts by mass of polypropylene glycol (PPG) (weight average molecular weight 1000) and 10 parts by mass of 1,5-pentanediol were used as the polyol, and 0.7 parts by mass of an amine catalyst (manufactured by Sankyo Air Products Co., Ltd., "DABCO LV33") and 1 part by mass of a foam stabilizer (manufactured by Toray Dow Corning Co., Ltd., "SZ5740M") were added to 100 parts by mass of the polyol and stirred. Polyisocyanate (manufactured by Tosoh Corporation, "Dinuclear Monomeric MDI") was added thereto after adjusting the isocyanate index to 85. Thereafter, 0.2 g / cm ３ The mixture was mixed with nitrogen gas and stirred until the temperature reached 100°C, yielding a solution containing fine bubbles. The solution was applied to a predetermined thickness on a 50 μm-thick release film (Nippa Corporation, "V-2") using an applicator, and the foam raw materials were reacted to obtain a polyurethane foam (700 μm thick). A 40 μm-thick sheet made of acrylic resin (Kuraray Co., Ltd., "LA250") was heat-sealed to the obtained polyurethane foam. A double-sided pressure-sensitive adhesive tape was produced in the same manner as in Example 1, except for the above.

[0106] The meanings of the abbreviations shown in Tables 1 and 2 are as follows: SEBS: styrene-ethylene-butylene-styrene block copolymer PE: polyethylene EVA: ethylene-vinyl acetate copolymer St-Ac: styrene-acrylic copolymer

[0107] For the obtained double-sided pressure-sensitive adhesive tapes, the gel fraction of the pressure-sensitive adhesive layer, the shear storage modulus of the pressure-sensitive adhesive layer at 180°C, the shear tensile strength of the pressure-sensitive adhesive tape at 23°C, the residual compressive stress of the pressure-sensitive adhesive tape, and the inclined retention time of the pressure-sensitive adhesive tape were measured using the methods described above. The results are shown in Tables 1 and 2. The test time for the 45° inclined retention test conducted to measure the inclined retention time of the pressure-sensitive adhesive tapes shown in Tables 1 and 2 was a maximum of 500 hours, and if the weight did not fall after 500 hours, the inclined retention time was expressed as "500 or more."

[0108] <Evaluation> The double-sided pressure-sensitive adhesive tapes obtained in the Examples and Comparative Examples were evaluated as follows. The results are shown in Tables 1 and 2.

[0109] (1) Evaluation of Screen Display Unevenness (1-1) Preparation of Test Sample Figure 5 shows a schematic cross-sectional view of a test sample used to evaluate screen display unevenness. Four sheets of step-forming tape 5 (manufactured by Sekisui Chemical Co., Ltd., "3810RS", width 20 mm x length 140 mm x thickness 0.1 mm) were stacked and bonded to a SUS plate 4 (width 40 mm x length 140 mm) that had been washed with ethanol and then wiped dry. The double-sided adhesive tape 1 obtained in the above "(3) Preparation of Double-Sided Adhesive Tape" was cut into a flat rectangular shape 20 mm wide x 140 mm long, and was bonded to the step-forming tape 5 and the SUS plate 4 so as to create a slope. Furthermore, a polycarbonate (PC) plate 6 (manufactured by Takiron C.I. Co., Ltd., "PC1600") measuring 40 mm in width, 140 mm in length, and 3 mm in thickness was attached to the double-sided adhesive tape 1 from above, and a 2 kg rubber roller was used to roll it back and forth once at a speed of 300 mm / min, thereby pressing each layer together and producing a test sample having a step formed by the step-forming tape 5.

[0110] (1-2) Evaluation of Screen Display Unevenness The obtained test sample was left to stand for 24 hours under conditions of 23°C and 50% RH, and the surface of the PC board 6 was closely observed using a digital microscope (Keyence Corporation, "VR-3000", 12x magnification), and evaluation was performed by the following method. That is, with the length direction of the test sample as the x-axis (unit: μm, the position of the lower end of the test sample is x=0, and the direction from the lower end to the upper end of the test sample is positive), the length was divided into sections every 5000 μm starting from x=0, and the thickness of the entire test sample was measured at positions every 5000 μm (x=0, 5000, 10000, ...), and the amount of change in thickness for each section was calculated. For each section, the absolute value of the difference in thickness change from the adjacent section was calculated, and the larger of the absolute values ​​of the differences from the sections on both sides was used as the absolute value of the difference in thickness change for that section (for example, if the thickness change from 0 to 5000 was 50, the thickness change from 5000 to 10000 was 70, and the thickness change from 10000 to 15000 was 90, then the thickness change from 5000 to 10000 was 40). The largest of the thickness changes for all sections was used as the slope (μm), and the obtained slope was used to evaluate screen display unevenness according to the following criteria: ◎: The slope was 11 μm or less. ○: The slope was greater than 11 μm and less than 13 μm. ×: The slope was greater than 13 μm.

[0111] (2) Evaluation of Actual Machine Retention Properties The obtained double-sided adhesive tape 1 was cut into a 3 mm wide piece, and the first adhesive layer was attached parallel to two short sides and one long side of an aluminum plate 8 (260 mm wide x 500 mm long x 3 mm thick). Then, a second adhesive layer was attached to a glass plate 7 (200 mm wide x 400 mm long x 3 mm thick) so that the edges were aligned with the double-sided adhesive tape 1. A 2 kg rubber roller was used to press the glass plate 7 side back and forth at a speed of 300 mm / min. The roller was then left to stand for 24 hours at 23°C and 50% RH to obtain a measurement sample. An actual machine retention test was performed on the obtained measurement sample. A schematic diagram of the actual machine retention test is shown in FIG. 6. A laminate consisting of a glass plate 7, double-sided adhesive tape 1, and aluminum plate 8 was hung at a 45° angle from a metal rod 12 by a string 11 in an environment of 60°C and 90% RH, and then a 3.5 kg weight 10 was hung from the center of the glass plate 7 so that a load was applied to the double-sided adhesive tape 1 and the glass plate 7, and the time until the weight 10 fell (holding time) was measured. Evaluation was performed according to the following criteria: ⊚: The weight did not fall even after 250 hours of holding. △: The holding time was 50 hours or more but less than 250 hours. ×: The holding time was less than 50 hours.

[0112] (3) Evaluation of Reworkability The obtained double-sided pressure-sensitive adhesive tape was cut into pieces measuring 5 mm x 125 mm. A 2 kg rubber roller was rolled back and forth over each of the pressure-sensitive adhesive layers on both sides at a speed of 300 mm / min to attach the pieces to a glass plate. The pieces were then cured for 24 hours in an environment of 23°C and 50% RH to produce a glass plate / double-sided pressure-sensitive adhesive tape / glass plate laminate. The obtained laminate was split by slicing the center of the first foam layer with a razor (manufactured by Feather Corporation). This resulted in a sample of glass plate / first pressure-sensitive adhesive layer / resin layer / adhesive layer / second foam layer / first foam layer (approximately half the thickness) (referred to as the "first pressure-sensitive adhesive layer sample") and a sample of first foam layer (approximately half the thickness) / second foam layer / second pressure-sensitive adhesive layer / glass plate (referred to as the "second pressure-sensitive adhesive layer sample"). For each of the two samples obtained, the adhesive tape was peeled off by pulling it at a high speed (approximately 1000 mm / min) in a direction at an angle of 30° from the glass plate, and the reworkability of the adhesive tape was evaluated according to the following criteria. ⊚: The double-sided adhesive tape was peeled off without breaking. ◯: The double-sided adhesive tape was peeled off, but the foam substrate broke once during peeling. ×: The foam substrate broke two or more times during peeling of the double-sided adhesive tape. Note that even if the evaluation is "×", the double-sided adhesive tape of the present invention can still be used without any problems depending on the application.

[0113] (4) Wrinkle Evaluation The obtained double-sided adhesive tape was cut into a size of 50 mm wide x 125 mm long, and the first adhesive layer was pressed against a SUS plate (SUS304 plate washed with ethanol and then wiped dry) using a 2 kg rubber roller reciprocated once at a speed of 300 mm / min to obtain a test sample. For the obtained test sample, the second adhesive layer of the double-sided adhesive tape was turned upward, and the release film was peeled off. A separator (manufactured by San-A Corporation, "WHCZ11CM", width 50 mm x length 125 mm x thickness 1.1 mm) was gently placed on the exposed second adhesive layer. After leaving the sample at 40°C for 48 hours, the separator was peeled off, and the depth of wrinkles that had developed in the second adhesive layer was measured using a digital microscope (manufactured by Keyence Corporation, "VR-3000", 12x magnification) and evaluated according to the following criteria: ⊚: No wrinkles were present in the second adhesive layer. Good: Wrinkles were present in the second pressure-sensitive adhesive layer, but the depth of all wrinkles was 60 μm or less. Poor: Wrinkles were present in the second pressure-sensitive adhesive layer, and wrinkles deeper than 60 μm were present. Even if the evaluation is "Poor", the double-sided pressure-sensitive adhesive tape of the present invention can be used without problems depending on the application.

[0114]

[0115]

[0116] According to the present invention, it is possible to provide a double-sided pressure-sensitive adhesive tape that can achieve both high holding power against a load and excellent flexibility.

[0117] REFERENCE SIGNS LIST 1 double-sided adhesive tape 2 liquid crystal panel 3 housing 4 SUS plate 5 tape for forming a step 6 polycarbonate (PC) plate 7 glass plate 8 aluminum plate 9 1 kg weight 10 3.5 kg weight 11 kite string 12 metal rod

Claims

1. A double-sided adhesive tape having a foam substrate and adhesive layers on both sides of the foam substrate, The residual compressive stress is 21.0 kPa or less, and, A test specimen obtained by cutting the double-sided adhesive tape to a size of 25 mm x 25 mm, attaching one side to a glass plate (width 50 mm x length 75 mm x thickness 3 mm), and attaching an aluminum plate (width 40 mm x length 40 mm x thickness 2 mm) to the other side, and leaving it standing for 24 hours in an environment of 23°C and 50% RH, is subjected to the following test: a 1 kg weight is attached to the center of the aluminum plate so that a load is applied to the double-sided adhesive tape and the aluminum plate, and the specimen is held at a 45° incline at 60°C and 90% RH. The inclination holding time until the adhesive tape peels off and the weight falls is 500 hours or more. A double-sided adhesive tape characterized by the following features.

2. The double-sided adhesive tape according to claim 1, wherein the shear tensile strength at 23°C is 0.35 MPa or more.

3. The adhesive layer contains a (meth)acrylic copolymer, The weight-average molecular weight of the (meth)acrylic copolymer is 1 million or more. The adhesive layer has a gel fraction of 25% by mass or more. The adhesive layer has a shear storage modulus (G') of 15 kPa or more at 180°C. The double-sided adhesive tape according to claim 1 or 2.

4. The double-sided adhesive tape according to claim 1 or 2, wherein the foam substrate comprises a first foam layer and a second foam layer having a lower foaming ratio than the first foam layer, laminated on at least one surface of the first foam layer.

5. The double-sided adhesive tape according to claim 4, wherein the first foam layer contains a resin (R1) having a loss tangent (tanδ) of 0.9 or more at 23°C.

6. The double-sided adhesive tape according to claim 5, wherein the content of the resin (R1) in the first foam layer is 10% by mass or more.

7. The double-sided adhesive tape according to claim 4, wherein the foaming ratio of the first foam layer is 8 times or more.

8. The double-sided adhesive tape according to claim 4, wherein the thickness of the first foam layer is 550 μm or more.

9. The double-sided adhesive tape according to claim 4, wherein the aspect ratio of the bubbles in the first foam layer (average major diameter of bubbles / average minor diameter of bubbles) is 2.3 or more.

10. The double-sided adhesive tape according to claim 4, wherein the foaming ratio of the second foam layer is 2 times or less.

11. The double-sided adhesive tape according to claim 4, wherein the thickness of the second foam layer is 15 μm or more.

12. Furthermore, the double-sided adhesive tape according to claim 1 or 2, having a resin layer composed of a polyester resin.