JP7316061B2 - foam sheet and adhesive tape - Google Patents

Description

The present invention relates to foam sheets and adhesive tapes.

Conventionally, foam sheets, for example, are excellent in shock-absorbing, heat-insulating, waterproof and moisture-proof properties. It is used for various purposes such as cushioning material for buffering and adhesive sheet base material. As a specific foamed sheet, Patent Literature 1 discloses a crosslinked polyolefin resin foamed sheet obtained by foaming and crosslinking an expandable polyolefin resin sheet containing a thermal decomposition type foaming agent (see Patent Literature 1). ).

WO2005/007731

By the way, in recent years, small electronic devices such as smartphones and wearable terminals are required to have high waterproof performance. For this reason, a gasket material is placed in the gap between the display panel and the housing and around openings such as the SIM card slot from the viewpoint of improving waterproofness. A gasket material is required to be flexible enough to conform to minute steps on the surface of an adherend, and to be adhesive enough to be temporarily fixed to an arrangement location.
As a material that satisfies such physical properties, an adhesive tape that includes a foam sheet and an adhesive layer provided on at least one surface of the foam sheet is known. Among them, the single-sided adhesive tape, which has an adhesive on one side of the foam sheet, is easier to peel off from the adherend than the double-sided adhesive tape. It can be easily peeled off when disassembling and repairing, etc., and is excellent in operability. However, although the single-sided pressure-sensitive adhesive tape has the advantage of being excellent in operability, it has been confirmed that the adhesiveness to the adherend is poor and the waterproofness is lowered.
Then, this invention makes it a subject to provide the foam sheet|seat which is excellent in waterproofness, and an adhesive tape provided with this foamed sheet|seat.

As a result of extensive studies by the present inventors, a foam sheet having an average cell diameter of 40 to 200 μm and a 50% compression strength B after 3 hours from the start of compression of 25 to 800 kPa, and an adhesive comprising the foam sheet It has been found that the tape solves the above problems. That is, the present invention is as follows.

[1] A foamed sheet having an average cell diameter of 40 to 200 μm and a 50% compressive strength B of 25 to 800 kPa after 3 hours from the start of compression.
[2] The foam sheet according to [1] above, wherein the cell flatness represented by the following formula (1) is 50 to 1000%.
100×average bubble diameter in MD direction×average bubble diameter in TD direction/(average bubble diameter in ZD direction) ² (1)
[3] The foam sheet according to [1] or [2] above, which has a 50% compressive strength A of 50 to 800 kPa.
[4] The foam sheet according to any one of [1] to [3] above, which has a glossiness of 20 to 80%.
[5] The foam sheet according to any one of [1] to [4] above, which has a contact angle of 80 to 120°.
[6] The foam sheet according to any one of [1] to [5] above, which has a gel fraction of 30 to 70% by mass.
[7] The foam sheet according to any one of [1] to [6] above, which has a closed cell rate of 70% or more.
[8] The foam sheet according to any one of [1] to [7] above, which has an expansion ratio of 2 to 20 cc/g.
[9] The foam sheet according to any one of [1] to [8] above, containing a polyolefin resin.
[10] The foamed sheet according to any one of [1] to [9] above, which contains 0.7 to 3 parts by mass of a lubricant with respect to 100 parts by mass of the resin contained in the foamed sheet.
[11] The foam sheet according to any one of [1] to [10] above, which has a thickness of 0.03 to 1.5 mm.
[12] An adhesive tape comprising the foamed sheet according to any one of [1] to [11] above and an adhesive layer provided on at least one surface of the foamed sheet.

ADVANTAGE OF THE INVENTION According to this invention, the foam sheet|seat which shows favorable waterproofness, and an adhesive tape provided with this foamed sheet|seat can be provided.

[Foam sheet]
The foamed sheet of the present invention has an average cell diameter of 40 to 200 μm and a 50% compressive strength B of 25 to 800 kPa after 3 hours from the start of compression. The foamed sheet of the present invention will be described in detail below.

<50% Compressive Strength B>
The foamed sheet of the present invention has a 50% compressive strength B (hereinafter simply referred to as 50% compressive strength B) after 3 hours from the start of compression is 25 to 800 kPa. In the foamed sheet of the present invention, the 50% compressive strength B is thus kept within a certain range. This means that the foamed sheet of the present invention has a constant repulsive force even after a certain period of time has passed since the start of compression, thereby maintaining good adhesion of the foamed sheet to the adherend. Sagging and waterproofness are improved. If the 50% compressive strength B is less than 25 kPa, the repulsive force of the foam cannot be maintained for a long period of time, resulting in poor waterproofness. When the 50% compressive strength B exceeds 800 kPa, the flexibility of the foam sheet is deteriorated. Water resistance deteriorates.
The 50% compressive strength B of the foamed sheet is preferably 25-500 kPa, more preferably 30-200 kPa.
The 50% compressive strength B can be adjusted within a desired range by adjusting the average cell diameter, cell flatness, expansion ratio, etc. of the foamed sheet, which will be described later.
The 50% compressive strength B is the value of the 50% compressive strength when the foam sheet is compressed to 50% thickness and held in that state for 3 hours. Compressive strength is measured according to JIS K6767.

<50% Compressive Strength A>
The 50% compressive strength A of the foamed sheet of the present invention is preferably 50-800 kPa, more preferably 60-600 kPa, still more preferably 70-500 kPa. When the 50% compressive strength A of the foam sheet is at least these lower limits, the repulsive force of the foam sheet increases the adhesion to the adherend and improves the waterproofness. When the 50% compressive strength A of the foam sheet is at most these upper limits, the flexibility of the foam sheet is high, the followability of the foam sheet to the surface shape of the adherend is improved, and the waterproof property is improved. In addition, if it is equal to or less than the upper limit, the ratio of compressive strength B to compressive strength A (compressive strength B/compressive strength A) increases, suppressing changes in physical properties of the foam sheet over time in flexibility and repulsive force. Improves stability.

From the viewpoint of suppressing changes in physical properties such as flexibility and repulsive force of the foam sheet over time and maintaining waterproofness for a long time, the ratio of compressive strength B to compressive strength A (compressive strength B/compressive strength A) is 0. 0.05 or greater is preferred, 0.2 or greater is more preferred, and usually 1 or less.

<Average bubble diameter>
The foamed sheet of the present invention has an average cell diameter of 40 to 200 μm. If the average cell diameter exceeds 200 µm, it becomes difficult to adjust the compressive strength B to the desired range, and the surface of the foamed sheet tends to become rough, resulting in deterioration of the waterproofness of the foamed sheet. If the average cell diameter is less than 40 μm, the production of the foamed sheet becomes difficult, and the effects obtained by reducing the average cell diameter are reduced. The foamed sheet preferably has an average cell diameter of 50 to 170 μm, more preferably 50 to 130 μm. By adjusting the average cell diameter to a relatively small value in this way, the number of cell walls per unit length is increased, which is thought to improve waterproofness. The average cell diameter of the foamed sheet can be adjusted, for example, by the gel fraction described below.
The average cell diameter of the foam sheet is the average value of the average cell diameter in the MD direction and the average cell diameter in the TD direction.

<Bubble Flatness>
The foamed sheet of the present invention preferably has a cell flattening ratio of 50 to 1000%. When the cell oblateness is in such a range, it becomes easier to adjust the above-described compressive strength B to a desired range, and the waterproofness of the foam sheet is improved. The bubble flattening ratio is represented by the following formula (1), and is a value that reflects the shape of the bubbles.
100×average bubble diameter in MD direction×average bubble diameter in TD direction/(average bubble diameter in ZD direction) ² (1)
For example, when the flatness of the bubble is close to 100%, the bubble has a nearly spherical shape. It is considered that the plastic deformation of the foam sheet is suppressed when the cell flatness is within the above range, and the compressive strength B is easily adjusted to the above desired range.
The cell flattening ratio of the foamed sheet is preferably 100 to 500%, more preferably 100 to 400%, from the viewpoint of facilitating adjustment of the compressive strength B to a desired range.
The cell flattening ratio of the foamed sheet can be adjusted within a desired range by adjusting various manufacturing conditions such as the draw ratio when manufacturing the foamed sheet.

The average bubble diameter in the MD direction, the average bubble diameter in the TD direction, and the average bubble diameter in the ZD direction are obtained by the following methods.
A foam sheet cut into 50 mm squares is prepared as a foam sheet sample for measurement. After immersing it in liquid nitrogen for 1 minute, it is cut in the thickness direction along the MD and TD with a razor blade. Take a 200-fold enlarged photograph of this cross section using a digital microscope (manufactured by Keyence Corporation "VHX-900"), and take a 2 mm long cut surface in each of the MD direction, TD direction, and ZD direction. The bubble diameter is measured for the bubbles of , and the operation is repeated 5 times. Then, the average value of all the bubbles was taken as the average bubble diameter in the MD direction, TD direction and ZD direction.
The MD direction means machine direction and is a direction that coincides with the extrusion direction and the like, while the TD direction means transverse direction and is a direction orthogonal to the MD direction. Moreover, the ZD direction is the thickness direction of the foam sheet and is a direction perpendicular to both the MD direction and the TD direction.

<Gel fraction>
The gel fraction of the foamed sheet of the present invention is preferably 30 to 70% by mass, more preferably 33 to 65% by mass, still more preferably 38 to 60% by mass. When the gel fraction is at least these lower limits, sufficient cross-linking is formed, and by foaming this, a foamed sheet with a small average cell diameter can be obtained. Moreover, when the gel fraction is at most these upper limits, it becomes easier to secure the flexibility of the foam sheet.
The gel fraction can be measured according to the method described in Examples below.

<Expansion ratio>
The expansion ratio of the foamed sheet of the present invention is preferably 2 to 20 cc/g, more preferably 3 to 15 cc/g, still more preferably 4 to 10 cc/g. When the foaming ratio is within such a range, appropriate flexibility is ensured, the surface shape of the adherend can be easily followed, and waterproofness can be easily improved. The expansion ratio is represented by the reciprocal of the apparent density.

<Closed cell rate>
The foam sheet of the present invention preferably has closed cells. Having closed cells means that the ratio of closed cells to all cells (referred to as "closed cell ratio") is 70% or more. If the foam sheet used in the present invention has closed cells, it becomes easier to ensure waterproofness. The closed cell ratio of the foamed sheet of the present invention is preferably 70% or more, more preferably 80% or more, still more preferably 90% or more, and 100% or less.
The closed cell content can be determined according to ASTM D2856 (1998). Commercially available measuring instruments include a dry automatic density meter Accupic 1330 and the like.

More specifically, the closed cell content is measured in the following manner. A test piece having a square shape with a side of 5 cm and a constant thickness is cut out from the foam sheet. The thickness of the test piece is measured, the apparent volume _V1 of the test piece is calculated, and the weight _W1 of the test piece is measured. Next, the apparent volume _V2 occupied by the bubbles is calculated based on the following formula. The density of the resin forming the test piece is 1 g/cm ³ .
Apparent volume occupied by bubbles V ₂ = V ₁ - W ₁
Subsequently, the test piece is submerged in distilled water at 23° C. to a depth of 100 mm from the water surface, and a pressure of 15 kPa is applied to the test piece for 3 minutes. After that, remove the test piece from the water, remove the moisture adhering to the surface of the test piece, measure the weight W ₂ of the test piece, and calculate the open cell rate F ₁ and closed cell rate F ₂ based on the following formula do.
Open-cell rate F ₁ (%) = {(W ₂ -W ₁ )/V ₂ } x 100
Closed cell rate F ₂ (%) = 100 - F ₁

<Glossiness>
The glossiness of the foamed sheet of the present invention is preferably 20 to 80%. When the glossiness is within such a range, the surface is relatively smooth, and minute gaps at the interface between the foamed sheet and the adherend are reduced, and waterproofness is likely to be enhanced. The glossiness of the foamed sheet is more preferably 25-80%, more preferably 30-80%. By adjusting the compressive strength B to the range described above while maintaining such a glossiness, it is possible to obtain a foamed sheet having better waterproofness. Glossiness can be adjusted by adjusting the average cell diameter, cell flatness, etc. of the foamed sheet, for example.
The glossiness of the foam sheet is measured according to JIS Z 8741 with a gloss meter TC-108DP/A manufactured by Tokyo Denshoku Co., Ltd. using a TD5 cm×MD10 cm square foam sheet. Considering the surface roughness of the specimen, the measurement method adopts Method 2 (incident light angle of 75°).

<Contact angle>
The contact angle of the foamed sheet of the present invention is preferably 80-120°. When the contact angle is in such a range, the water repellency of the foam sheet is enhanced, and it becomes easy to prevent water from entering due to capillary action through minute gaps at the interface between the foam sheet and the adherend. As a result, the foam sheet is waterproof. become more sexual. The contact angle of the foam sheet is more preferably 85-120°, more preferably 95-120°. The contact angle can be adjusted by the composition of the foam sheet, for example, by adding a lubricant to the foam sheet as described later.
The contact angle is a value determined by a droplet method, and can be measured in detail by the method described in Examples.

<Asker C hardness>
The Asker C hardness of the foamed sheet of the present invention is preferably 25-70, more preferably 30-60. When the Asker C hardness of the foamed sheet is in such a range, the unevenness of the surface is easily smoothed when the foamed sheet is attached to the adherend by pressure, and as a result, the waterproofness is improved. The Asker C hardness can be measured using an Asker C hardness tester.

<Dimensions of foam sheet>
The thickness of the foam sheet is preferably 0.03-2.0 mm. When the thickness is 2.0 mm or less, the foam sheet can be made thinner and can be suitably used for miniaturized electronic devices. Further, when the thickness is 0.03 mm or more, it becomes easy to ensure waterproofness and flexibility of the foam sheet. From such a point of view, the thickness is more preferably 0.05 to 1.5 mm, even more preferably 0.1 to 1.0 mm.

<Polyolefin resin>
The foamed sheet of the present invention contains a resin, preferably a polyolefin resin. By using a polyolefin-based resin, it becomes easy to adjust the average cell diameter within the above range while ensuring appropriate flexibility of the foam sheet.
Examples of polyolefin resins include polyethylene resins, polypropylene resins, ethylene-vinyl acetate copolymers, etc. Among these, polyethylene resins are preferred.

[Polyethylene resin]
Examples of polyethylene resins include polyethylene resins polymerized with polymerization catalysts such as Ziegler-Natta compounds, metallocene compounds, and chromium oxide compounds. Preferably, polyethylene resins polymerized with a metallocene compound polymerization catalyst are used.

Moreover, as polyethylene resin, linear low-density polyethylene is preferable. By using linear low-density polyethylene, it becomes possible to impart flexibility to the foam sheet and to make the foam sheet thinner. This linear low-density polyethylene is more preferably obtained using a polymerization catalyst such as a metallocene compound. In addition, linear low-density polyethylene is obtained by copolymerizing ethylene (for example, 75% by mass or more, preferably 90% by mass or more with respect to the total amount of monomers) and a small amount of α-olefin if necessary. Linear low-density polyethylene is more preferred.
Specific examples of α-olefins include propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-heptene, and 1-octene. Of these, α-olefins having 4 to 10 carbon atoms are preferred.
The density of the polyethylene resin, for example, the linear low-density polyethylene described above, is preferably 0.870 to 0.910 g/cm ³ , more preferably 0.875 to 0.907 g/cm ³ , and more preferably 0.880 to 0.905 g/cm 3 . /cm ³ is more preferred. A plurality of polyethylene resins can be used as the polyethylene resin, and a polyethylene resin having a density other than the density range described above may be added.

(metallocene compound)
Examples of metallocene compounds include compounds such as bis(cyclopentadienyl) metal complexes having a structure in which a transition metal is sandwiched between π-electron unsaturated compounds. More specifically, tetravalent transition metals such as titanium, zirconium, nickel, palladium, hafnium, and platinum have one or more cyclopentadienyl rings or analogues thereof as ligands. can be mentioned.
Such a metallocene compound has uniform properties of active sites and each active site has the same degree of activity. Polymers synthesized using metallocene compounds have high uniformity in terms of molecular weight, molecular weight distribution, composition, and composition distribution. proceed to As a result, even if the foam sheet is made thin, the thickness can be easily made uniform because the sheet can be stretched uniformly.

Examples of ligands include cyclopentadienyl rings and indenyl rings. These cyclic compounds may be substituted with hydrocarbon groups, substituted hydrocarbon groups or hydrocarbon-substituted metalloid groups. Hydrocarbon groups include, for example, methyl group, ethyl group, various propyl groups, various butyl groups, various amyl groups, various hexyl groups, 2-ethylhexyl groups, various heptyl groups, various octyl groups, various nonyl groups, and various decyl groups. , various cetyl groups, and phenyl groups. The term "various" means various isomers including n-, sec-, tert- and iso-.
Moreover, you may use what polymerized the cyclic compound as an oligomer as a ligand.
Furthermore, in addition to the π electron system unsaturated compounds, monovalent anion ligands such as chlorine and bromine or divalent anion chelate ligands, hydrocarbons, alkoxides, arylamides, aryloxides, amides, arylamides, phosphides, and aryls Phosphide or the like may also be used.

Examples of metallocene compounds containing tetravalent transition metals and ligands include cyclopentadienyltitanium tris(dimethylamide), methylcyclopentadienyltitanium tris(dimethylamide), bis(cyclopentadienyl)titanium dichloride, dimethyl and silyltetramethylcyclopentadienyl-t-butylamide zirconium dichloride.
A metallocene compound, in combination with a specific co-catalyst (co-catalyst), exhibits its action as a catalyst in the polymerization of various olefins. Specific co-catalysts include methylaluminoxane (MAO), boron-based compounds, and the like. The ratio of the cocatalyst used to the metallocene compound is preferably 100,000 to 1,000,000 mol times, more preferably 50 to 5,000 mol times.
When the linear low-density polyethylene described above is used as the polyolefin resin contained in the foam sheet, the linear low-density polyethylene may be used alone, or may be used in combination with other polyolefin resins. For example, it may be used in combination with other polyolefin resins described below. When other polyolefin resin is contained, the ratio of other polyolefin resin to linear low-density polyethylene (100 parts by mass) is preferably 40 parts by mass or less, more preferably 30 parts by mass or less.

[Polypropylene resin]
Examples of polypropylene resins include polypropylene and propylene-α-olefin copolymers containing 50% by mass or more of propylene. These may be used individually by 1 type, and may use 2 or more types together.
Specific α-olefins constituting the propylene-α-olefin copolymer include ethylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-heptene, 1- Octene and the like can be mentioned, and among these, α-olefins having 6 to 12 carbon atoms are preferred.

[Ethylene-vinyl acetate copolymer]
The ethylene-vinyl acetate copolymer used as the polyolefin resin is, for example, ethylene-acetic acid containing preferably 6 to 40% by mass, more preferably 12 to 35% by mass, still more preferably 20 to 33% by mass of vinyl acetate. A vinyl copolymer is mentioned.
In addition to ethylene and vinyl acetate, the ethylene-vinyl acetate copolymer used in the present invention may contain vinyl alcohol produced by partially hydrolyzing vinyl acetate.
Examples of such ethylene-vinyl acetate copolymer include "Ultrasen" manufactured by Tosoh Corporation, "Evaflex" manufactured by DuPont Mitsui Polychemicals, Ltd., "UBE Polyethylene" manufactured by Ube Industries, Ltd., Asahi Kasei Chemicals Co., Ltd. The company's "Suntech" etc. are mentioned.

[Content of each resin]
When a polyethylene resin is used as the polyolefin resin, the amount of the polyethylene resin relative to the total amount of the resin is preferably 30% by mass or more, more preferably 50% by mass or more, still more preferably 60% by mass or more, and even more preferably 70% by mass or more. It may consist essentially of polyethylene resin. Waterproofness improves as content of polyethylene resin is 30 mass % or more.

Furthermore, when the foamed sheet contains a polyolefin resin as the resin, the resin contained in the foamed sheet may be the polyolefin resin alone, or may contain a resin other than the polyolefin resin. When the foam sheet contains a resin other than a polyolefin resin, the ratio of the polyolefin resin to the total amount of resin is preferably 60% by mass or more, more preferably 70% by mass or more, and even more preferably 80% by mass or more.
Examples of resins other than the polyolefin resin used for the foamed sheet include various elastomers such as styrene thermoplastic elastomers, ethylene propylene thermoplastic elastomers such as EPDM, and rubber components.

<Lubricant>
The foamed sheet of the present invention preferably contains a lubricant. By containing the lubricant, it becomes easier to adjust the contact angle of the foamed sheet within the desired range described above, and it becomes easier to obtain a highly waterproof foamed sheet. The content of the lubricant in the foam sheet is preferably 0.7 to 3 parts by mass, more preferably 0.8 to 2 parts by mass, per 100 parts by mass of the resin contained in the foam sheet.
Examples of lubricants include higher fatty acids such as stearic acid and palmitic acid, higher alcohols such as palmityl alcohol and stearyl alcohol, calcium stearate, zinc stearate, barium stearate, aluminum stearate, magnesium stearate, and sodium palmitate. metal salts of higher fatty acids, higher fatty acid esters such as butyl stearate and glyceryl monostearate, and higher fatty acid amides such as oleic acid amide, stearic acid amide and erucic acid amide. Here, the term “higher grade” means 9 or more carbon atoms, preferably 9 or more and 30 or less carbon atoms.
Among these, the lubricant is preferably at least one selected from higher fatty acids and metal salts of higher fatty acids, from the viewpoint of easily adjusting the contact angle to a desired range and enhancing the effect of improving waterproofness. Among them, a metal salt of a higher fatty acid makes it easier to control the average cell diameter within a desired range, so the lubricant preferably contains at least a metal salt of a higher fatty acid. It is more preferable to use together.
Among the above higher fatty acids, stearic acid is particularly preferred, and among higher fatty acid metal salts, zinc stearate and calcium stearate are particularly preferred.

<Pyrolytic foaming agent>
The foamed sheet of the present invention is preferably formed by foaming a foamable composition containing the above resin and a thermally decomposable foaming agent.
Organic foaming agents and inorganic foaming agents can be used as thermal decomposition type foaming agents. Examples of organic foaming agents include azodicarbonamide, azodicarboxylic acid metal salts (barium azodicarboxylate, etc.), azo compounds such as azobisisobutyronitrile, nitroso compounds such as N,N'-dinitrosopentamethylenetetramine, hydra Hydrazine derivatives such as zodicarbonamide, 4,4'-oxybis(benzenesulfonylhydrazide) and toluenesulfonylhydrazide, semicarbazide compounds such as toluenesulfonylsemicarbazide, and the like can be mentioned.
Examples of inorganic foaming agents include ammonium acid, sodium carbonate, ammonium hydrogencarbonate, sodium hydrogencarbonate, ammonium nitrite, sodium borohydride, anhydrous monosoda citric acid, and the like.
Among these, azo compounds are preferred, and azodicarbonamide is more preferred, from the viewpoints of obtaining fine air bubbles, economy and safety. These may be used individually by 1 type, and may use 2 or more types together.
The content of the thermal decomposition type foaming agent in the foamable composition is preferably 1 to 20 parts by mass, more preferably 2 to 18 parts by mass, and even more preferably 4 to 15 parts by mass with respect to 100 parts by mass of the resin.

The foamable composition preferably contains the above-described lubricant in addition to the resin and the thermally decomposable foaming agent. It may contain additives generally used for foam sheets such as fillers.

[Method for producing foamed sheet]
The method for producing the foamed sheet of the present invention is not particularly limited. It can be produced by stretching in at least one of the MD directions. More specifically, the manufacturing method includes the following steps (1) to (4).
Step (1): A step of mixing a resin, a thermally decomposable foaming agent, an optional lubricant, and other additives to form a sheet-like foamable composition Step (2): Sheet step (3): heating the crosslinked foamable composition to foam the thermally decomposable foaming agent to form a foamed sheet; Step (4): a step of stretching the foam sheet in either or both of the MD direction and the TD direction

In step (1), the method of forming the sheet-like foaming composition is not particularly limited, but for example, a resin, a thermally decomposable foaming agent, a lubricant blended as necessary, and other additives are used. The foamable composition may be molded by supplying it to an extruder, melt-kneading it, and extruding the foamable composition from the extruder into a sheet.
As a method for cross-linking the foamable composition in the step (2), a method of irradiating the sheet-like foamable composition with ionizing radiation such as an electron beam, α-ray, β-ray, and γ-ray is used. The irradiation dose of the ionizing radiation may be adjusted so that the degree of cross-linking of the resulting foamed sheet falls within the above desired range, and is preferably 5 to 15 Mrad, more preferably 6 to 13 Mrad. The accelerating voltage of the ionizing radiation is preferably 200-1000 kV, more preferably 300-800 kV.
In step (3), the heating temperature for heating the foamable composition to foam the thermally decomposable foaming agent may be at least the foaming temperature of the thermally decomposable foaming agent. It is preferably 220 to 280°C.

The stretching of the foam sheet in the step (4) may be performed in both the MD and TD directions, or may be performed in only one direction, but is preferably performed in both directions. The foamed sheet may be stretched after foaming the sheet-like foaming composition to obtain the foamed sheet, or may be performed while foaming the sheet-like foaming composition. In the case where a foamed sheet is obtained by foaming a sheet-like foamable composition and then the foamed sheet is stretched, the foamed sheet is continuously stretched while maintaining the molten state at the time of foaming without cooling the foamed sheet. The foam sheet may be stretched, and after the foam sheet is cooled, the foam sheet may be heated again to be in a molten or softened state and then stretched. By stretching the foam sheet, it becomes easier to make it thinner.
In step (4), the draw ratio of the foamed sheet in one or both of the MD and TD directions is preferably 1.1 to 5.0 times, more preferably 1.2 to 4.0 times, 1.5 to 3.3 times is more preferable. Moreover, when the draw ratio is set to the above lower limit or more, the flexibility of the foamed sheet tends to be improved. On the other hand, when it is below the upper limit, it is possible to prevent the foam sheet from being broken during stretching, and the foaming gas from being released from the foam sheet during foaming to prevent the expansion ratio from significantly decreasing, and the flexibility of the foam sheet to be improved. It becomes easier to make the quality uniform.
The foam sheet may be heated to, for example, 100 to 280.degree. C., preferably 150 to 260.degree. C. during stretching.
The foam sheet obtained as described above may be cut into a desired shape by a known method such as punching.

However, the present manufacturing method is not limited to the above, and a foamed sheet may be obtained by a method other than the above. For example, instead of irradiating ionizing radiation, cross-linking may be performed by adding an organic peroxide to the foamable composition in advance and heating the foamable composition to decompose the organic peroxide. good.

[Adhesive tape]
The pressure-sensitive adhesive tape of the present invention comprises the foam sheet described above and a pressure-sensitive adhesive layer provided on at least one surface of the foam sheet. The pressure-sensitive adhesive tape of the present invention is preferably a single-sided pressure-sensitive adhesive tape comprising a foam sheet and a pressure-sensitive adhesive layer on one side of the foam sheet. Compared to double-sided adhesive tape, single-sided adhesive tape is easier to separate from the adherend. It can be easily peeled off from the surface and has excellent operability. In general, single-sided pressure-sensitive adhesive tapes can be easily peeled off and have excellent operability, but they do not have sufficient adhesion to adherends and are inferior in waterproofness. However, as described above, since the foam sheet of the present invention has excellent waterproof properties, the single-sided pressure-sensitive adhesive tape comprising the foam sheet of the present invention can be easily peeled off from the adherend and also has excellent waterproof properties.

<Adhesive>
The pressure-sensitive adhesive layer is preferably formed from the pressure-sensitive adhesive described below. The adhesive is not particularly limited, and for example, an acrylic adhesive, a urethane adhesive, a rubber adhesive, or the like can be used. From the viewpoint of reworkability and waterproofness, an acrylic adhesive is preferable.
In the present invention, when the pressure-sensitive adhesive tape has pressure-sensitive adhesive layers on both sides of the foam sheet, the pressure-sensitive adhesives used for the two pressure-sensitive adhesive layers may be the same or different.

(Acrylic adhesive)
Hereinafter, one embodiment of the acrylic pressure-sensitive adhesive used in the pressure-sensitive adhesive layer will be described in more detail. The acrylic pressure-sensitive adhesive is a pressure-sensitive adhesive containing an acrylic polymer obtained by polymerizing a polymerizable monomer containing a (meth)acrylic acid alkyl ester-based monomer (A).
In this specification, the term "(meth)acrylic acid alkyl ester" refers to a concept including both acrylic acid alkyl ester and methacrylic acid alkyl ester, and the same applies to other similar terms. . In addition, the term "polymerizable monomer" includes not only compounds having no repeating units, but also compounds that copolymerize with (meth)acrylic acid alkyl ester-based monomers (A), other monomers described later themselves as repeating units. It refers to a concept that can also include those with

<<(Meth)acrylic acid alkyl ester-based monomer (A)>>
(Meth)acrylic acid alkyl ester-based monomer (A) is an ester of (meth)acrylic acid and an aliphatic alcohol, and the number of carbon atoms in the alkyl group of the aliphatic alcohol is preferably 2 to 14, more preferably Alkyl esters derived from fatty alcohols that are 4-10 are preferred.

Specific (meth)acrylic acid alkyl ester-based monomers (A) include, for example, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl ( meth)acrylate, t-butyl (meth)acrylate, pentyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, n-octyl (meth)acrylate, isooctyl (meth)acrylate Acrylate, nonyl (meth)acrylate, decyl (meth)acrylate, undecyl (meth)acrylate, dodecyl (meth)acrylate, tridecyl (meth)acrylate, tetradecyl (meth)acrylate and the like.
Among these, n-butyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, 2-ethylhexyl (meth)acrylate and n-octyl (meth)acrylate are preferred, n-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate or combinations thereof are more preferred.
The (meth)acrylic acid alkyl ester-based monomers may be used alone, or two or more of them may be used in combination.

The (meth)acrylic acid alkyl ester-based monomer (A)-derived structural unit constitutes the main component in the adhesive, and the content thereof is generally 30% by mass or more, preferably 30% by mass or more, based on the total amount of the adhesive. is 40% by weight or more, more preferably 45% by weight or more. Thus, by increasing the content of the (meth)acrylic acid alkyl ester-based monomer (A), it is possible to impart desired adhesive strength to the adhesive.
The content of the structural unit derived from the (meth)acrylic acid alkyl ester-based monomer (A) in the pressure-sensitive adhesive is substantially the same as the content of the (meth)acrylic acid alkyl ester-based monomer (A) in the pressure-sensitive adhesive composition described later. are the same, they can be expressed interchangeably. The same applies to components other than the component (A) described below.

<<Polar group-containing vinyl monomer (B)>>
The polymerizable monomer preferably contains a polar group-containing vinyl monomer (B) in addition to the (meth)acrylic acid alkyl ester monomer (A). The polar group-containing vinyl monomer (B) has a polar group and a vinyl group.

Examples of the polar group-containing vinyl monomer (B) include (meth)acrylic acid, carboxylic acids containing a vinyl group such as itaconic acid, and anhydrides thereof, vinyl monomers having a hydroxyl group, nitrogen-containing vinyl monomers, and the like. .
Vinyl monomers having a hydroxyl group include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, caprolactone-modified (meth)acrylate, polyoxyethylene (meth)acrylate, and polyoxypropylene (meth)acrylate.
Nitrogen-containing vinyl monomers include (meth)acrylonitrile, N-vinylpyrrolidone, N-vinylcaprolactam, N-vinyllauryllactam, (meth)acryloylmorpholine, (meth)acrylamide, dimethyl(meth)acrylamide, N-methylol ( meth)acrylamide, N-butoxymethyl(meth)acrylamide, dimethylaminomethyl(meth)acrylate and the like.
Among the polar group-containing vinyl monomers (B), carboxylic acids containing a vinyl group such as (meth)acrylic acid and itaconic acid, and anhydrides thereof are preferred, (meth)acrylic acid is more preferred, and acrylic acid is further preferred. preferable. These polar group-containing vinyl monomers (B) may be used alone or in combination of two or more.

When the polar group-containing vinyl monomer (B) is used, the content of the structural unit derived from the polar group-containing vinyl monomer (B) in the adhesive is 100 of the structural unit derived from the (meth)acrylic acid alkyl ester monomer (A). It is preferably 1 to 15 parts by mass, more preferably 2 to 12 parts by mass, and still more preferably 3 to 10 parts by mass. By setting the content of the polar group-containing vinyl monomer (B) within such a range, it becomes easier to adjust the Tg, cohesive strength, adhesive strength, etc. of the pressure-sensitive adhesive layer to appropriate ranges.

≪Other monomers≫
The polymerizable monomer may contain monomers other than (A) and (B) described above. Other monomers include styrene-based monomers, polyfunctional monomers, carboxylic acid vinyl esters such as vinyl acetate, and the like. Styrenic monomers include, for example, styrene, α-methylstyrene, o-methylstyrene, and p-methylstyrene.
Examples of polyfunctional monomers include monomers having two or more vinyl groups, preferably polyfunctional (meth)acrylates having two or more (meth)acryloyl groups. The use of polyfunctional monomers allows the formation of networks in acrylic polymers.
Specific polyfunctional monomers include hexanediol di(meth)acrylate, polyethylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, ethoxylated bisphenol A di(meth)acrylate, tris(2-hydroxyethyl). isocyanurate triacrylate, ethoxylated trimethylolpropane triacrylate, proxied trimethylolpropane triacrylate, proxied glyceryl triacrylate, neopentyl glycol adipate diacrylate, and the like.
When using other monomers, the content of structural units derived from other monomers in the adhesive is 0.5 parts per 100 parts by mass of structural units derived from (meth)acrylic acid alkyl ester monomer (A). to 15 parts by mass, more preferably 1 to 7 parts by mass, and even more preferably 1 to 5 parts by mass.
The weight average molecular weight of the acrylic polymer measured by the GPC method is not particularly limited, but is, for example, 200,000 to 1,000,000, preferably 400,000 to 900,000.

<Tackifying resin>
The acrylic pressure-sensitive adhesive may contain a tackifying resin from the viewpoint of improving adhesive strength. As the tackifying resin, a tackifying resin with low polymerization inhibition such as hydrogenated terpene resin, hydrogenated rosin, disproportionated rosin resin, rosin ester polymer, and petroleum resin is preferred. Among these, rosin-based resins are preferred, and rosin ester-based polymers are particularly preferred, since tackifying resins containing a large number of double bonds inhibit the polymerization reaction.

The softening point of the tackifier resin may be about 95°C or higher from the viewpoint of improving the cohesive strength and adhesive strength of the adhesive, but preferably includes 120°C or higher, such as 95°C or higher and 120°C. A temperature of less than 120° C. or more and a temperature of 120° C. or more and 150° C. or less may be used together. The softening point may be measured by the ring and ball method defined in JISK2207.
The content of the tackifying resin in the acrylic pressure-sensitive adhesive is preferably 2 to 40 parts by mass, more preferably 4 to 35 parts by mass, and still more preferably 5 to 25 parts by mass with respect to 100 parts by mass of the acrylic polymer. be.

<Crosslinking agent>
When the resin constituting the acrylic pressure-sensitive adhesive has a hydroxyl group or a carboxyl group, a cross-linking agent may be used to form a cross-linked structure between main chains from the viewpoint of improving adhesiveness.
Examples of cross-linking agents include isocyanate-based cross-linking agents, epoxy-based cross-linking agents, aziridine-based cross-linking agents, and metal chelate-type cross-linking agents. Among these, isocyanate-based cross-linking agents and epoxy-based cross-linking agents are preferred.

Examples of isocyanate-based cross-linking agents include polyisocyanate compounds. Specific examples of polyisocyanate compounds include aromatic polyisocyanates such as tolylene diisocyanate, diphenylmethane diisocyanate and xylylene diisocyanate; aliphatic polyisocyanates such as hexamethylene diisocyanate; and alicyclic polyisocyanates such as isophorone diisocyanate and hydrogenated diphenylmethane diisocyanate. etc. Also included are biuret and isocyanurate forms thereof, as well as adduct forms which are reactants with low-molecular-weight active hydrogen-containing compounds such as ethylene glycol, propylene glycol, neopentyl glycol, trimethylolpropane and castor oil.
These may be used individually by 1 type, and may be used in combination of 2 or more type.

Examples of epoxy-based cross-linking agents include bisphenol A type epoxy compound, bisphenol F type epoxy compound, 1,3-bis(N,N-diglycidylaminomethyl)toluene, N,N,N',N'-tetraglycidyl -4,4-diaminodiphenylmethane, N,N,N',N'-tetraglycidyl m-xylenediamine, 1,3-bis(N,N-diglycidylaminomethyl)cyclohexane, 1,6-diglycidyl n-hexane etc.

The peel strength of the pressure-sensitive adhesive layer used in the present invention may be adjusted by the amount of the cross-linking agent.
The amount of the cross-linking agent added to the adhesive is preferably 0.5 parts by mass or more, more preferably 1 part by mass or more, and preferably 7 parts by mass or less, relative to 100 parts by mass of the acrylic polymer. More preferably, it is 5 parts by mass or less.

<Other ingredients>
The acrylic pressure-sensitive adhesive used in the present invention contains various additives conventionally used in pressure-sensitive adhesives, such as plasticizers, softeners, pigments, dyes, photopolymerization initiators, and flame retardants, in addition to the components described above. You may

(rubber adhesive)
The rubber pressure-sensitive adhesive contains a rubber component, and it is preferable to use a styrene-isoprene block copolymer as the rubber component. The styrene-isoprene block copolymer preferably has a diblock ratio of 25 to 70% by weight, more preferably 30 to 65% by weight, and still more preferably 45 to 60% by weight. Here, diblock means a diblock composed of styrene and isoprene. The styrene-isoprene block copolymer exhibits sufficient adhesive strength when the diblock ratio is 25% or more, and tends to increase the shear strength when the diblock ratio is 70% by mass or less. The styrene-isoprene block copolymers include those having three or more blocks such as triblocks composed of styrene, isoprene, and styrene blocks in addition to diblocks.

Although the amount of styrene in the styrene-isoprene block copolymer is not particularly limited, it is preferably 14 to 24% by mass, more preferably 15 to 18% by mass. When the styrene content is 14% by mass or more, the adhesive becomes highly cohesive and tends to increase the shear strength. Further, when the content is 24% by mass or less, the cohesive force becomes an appropriate level, and the adhesive force is easily expressed.
The molecular weight of the styrene-isoprene block copolymer is not particularly limited, but the weight average molecular weight is preferably 100,000 to 400,000, more preferably 150,000 to 250,000. The term "mass average molecular weight" as used herein refers to a molecular weight measured as polystyrene equivalent molecular weight by GPC (gel permeation chromatography).

The rubber-based pressure-sensitive adhesive preferably contains a tackifying resin in addition to the rubber component described above. Various tackifying resins can be used as the tackifying resin used in the rubber-based adhesive, but petroleum-based resins, terpene resins, and coumarone resins are preferably used. The tackifying resin may be used alone or in combination of two or more. However, at least one selected from petroleum-based resins and terpene resins and coumarone resins may be used in combination. preferable. Such a combination of tackifying resins makes it easier to adjust the peel strength within the above range.
Examples of petroleum resins include aliphatic petroleum resins (C5 petroleum resins), alicyclic petroleum resins, and aromatic petroleum resins. family-based petroleum resins are preferred. Further, it is preferable to use a petroleum-based resin having a softening point of about 90 to 120°C.
As the terpene resin, those having a softening point of about 80 to 120° C. can be used, but those having a softening point of less than 100° C. are preferable from the viewpoint of ensuring adhesive strength. As the coumarone resin, one having a softening point of preferably 110 to 130° C., more preferably 115 to 125° C. is used in order to ensure cohesion.

The tackifying resin is preferably 60 to 250 parts by mass, more preferably 100 to 200 parts by mass, even more preferably 110 to 180 parts by mass, based on 100 parts by mass of the rubber component. By setting the blending amount of the tackifying resin within the above range, it becomes possible to improve the cohesive force and impart an appropriate peel adhesive force.
When a petroleum-based resin and at least one selected from a terpene resin and a cumarone resin are used in combination, the petroleum-based resin is preferably 50 to 200 parts by mass, preferably 60 to 150 parts by mass, with respect to 100 parts by mass of the rubber component. Parts by mass are preferable, and 60 to 110 parts by mass are more preferable. On the other hand, the terpene resin is preferably 10 to 70 parts by mass, more preferably 20 to 60 parts by mass, and even more preferably 30 to 50 parts by mass with respect to 100 parts by mass of the rubber component. Furthermore, the coumarone resin is preferably 10 to 60 parts by mass, more preferably 15 to 50 parts by mass, and even more preferably 20 to 40 parts by mass with respect to 100 parts by mass of the rubber component.
The rubber-based pressure-sensitive adhesive may contain softeners, antioxidants, fillers, etc., if necessary.

(urethane adhesive)
The urethane-based adhesive is not particularly limited, and examples thereof include urethane resins obtained by reacting at least a polyol with a polyisocyanate compound. Examples of the polyols include polyether polyols, polyester polyols, polycarbonate polyols, polycaprolactone polyols, and the like. Examples of the polyisocyanate compound include diphenylmethane diisocyanate, tolylene diisocyanate, hexamethylene diisocyanate, and the like. These urethane pressure-sensitive adhesives may be used alone or in combination of two or more.
Moreover, as the urethane-based adhesive, a urethane resin obtained by reacting a polyurethane polyol and a polyfunctional isocyanate-based curing agent may be used. Examples of the polyurethane polyol include those obtained by reacting the above-described polyol with a polyisocyanate compound, or those obtained by reacting a polyol, a polyisocyanate compound, and a chain extender such as a diamine. As the polyfunctional isocyanate-based curing agent, any compound having two or more isocyanate groups may be used, and the isocyanate compounds described above can be used.
In addition to the urethane resin, the urethane-based adhesive may contain the fine particles described above, and the urethane-based adhesive may contain a tackifying resin, a softening agent, an antioxidant, a filler, etc., as necessary. may contain.

[Method for producing pressure-sensitive adhesive layer]
When an acrylic adhesive is used as the adhesive constituting the adhesive layer, the adhesive composition containing the polymerizable monomer and the polymerization initiator is heated and refluxed, and then the polymer is crosslinked. can be obtained by Moreover, it is also possible to obtain by irradiating the adhesive composition containing the polymerizable monomer described above with light to polymerize the polymerizable monomer.
The pressure-sensitive adhesive composition may contain at least one of the above-described tackifying resin, fine particles, and other components, if necessary.

The method of forming the adhesive layer on at least one surface of the foamed sheet is not particularly limited. a method of applying an adhesive using a brush, a method of transferring an adhesive layer formed on a release sheet to a foam sheet, and the like.

<Application of adhesive tape>
Although the use of the adhesive tape is not particularly limited, it is preferably used, for example, as a sealing material inside electronic equipment because of its excellent waterproofness. Moreover, it can be suitably used inside various portable electronic devices in which the space for arranging the foam sheet is small. Examples of electronic equipment include liquid crystal displays, organic EL displays, mobile phones, cameras, game machines, electronic notebooks, and personal computers.

The present invention will be described in more detail with reference to examples, but the present invention is not limited by these examples.

[Measuring method]
The measurement method and evaluation method of each physical property are as follows.

<50% Compressive Strength B>
The foam sheet was compressed to a thickness of 50% and held in that state for 3 hours to measure the 50% compressive strength. Compressive strength was measured according to JIS K6767.

<50% Compressive Strength A>
Measured according to JIS K6767.

<Expansion ratio>
The apparent density of the foamed sheet was measured according to JIS K7222, and the reciprocal thereof was taken as the foaming ratio.

<Average bubble diameter, bubble flatness>
The average cell diameter of the foamed sheet was measured by the method described in the specification.

<Gel fraction>
A test piece of about 100 mg is taken from the foamed sheet, and the weight A (mg) of the test piece is precisely weighed. Next, this test piece was immersed in 30 cm ³ of xylene at 120° C. and allowed to stand for 24 hours, filtered through a 200-mesh wire mesh to collect the undissolved matter on the wire mesh, vacuum dried, and weighed the insoluble matter. Accurately weigh B (mg). From the obtained values, the gel fraction (% by mass) was calculated according to the following formula.
Gel fraction (% by mass) = (B/A) x 100

<Closed cell rate>
The closed cell rate of the foam sheet was measured by the method described in the specification.

<Asker C hardness>
The Asker C hardness of the foamed sheet was measured with an Asker C hardness tester (Asker CL-150L, manufactured by Kobunshi Keiki Co., Ltd.).

<Glossiness>
The glossiness of the foam sheet was measured according to JIS Z 8741 with a TC-108DP/A gloss meter manufactured by Tokyo Denshoku Co., Ltd. using a TD5 cm×MD10 cm square foam sheet. Considering the surface roughness of the specimen, method 2 (incident light angle of 75°) was adopted as the measurement method.

<Contact angle>
Water was collected in a syringe and a 3.0 μL drop was made at the tip of the needle. The prepared droplets were brought into contact with the foam sheet to form droplets on the foam sheet. An image of the droplet was taken 1 second after the droplet was formed on the foam sheet. By analyzing the image of this droplet, the contact angle was calculated by the θ/2 method. The average value of 10 measurements was taken as the contact angle. In addition, the foam sheet was kept in the measurement environment (23° C., relative humidity 50%) for 24 hours before the measurement.

<Waterproof>
For the waterproof evaluation, the test sample prepared by the following method is submerged in water at a depth of 10 cm under the conditions of a temperature of 23°C and a relative humidity of 50%. It was done by The presence or absence of inundation was visually confirmed.
In addition, this waterproofness evaluation was performed after curing for 6 hours under conditions of a temperature of 23° C. and a relative humidity of 50% after preparation of the test sample.

(Preparation of test samples)
(1) Production of adhesive (A) A reactor equipped with a thermometer, a stirrer and a cooling tube was charged with 70 parts by mass of butyl acrylate, 25 parts by mass of 2-ethylhexyl acrylate and 5 parts by mass of acrylic acid, and 80 parts by mass of ethyl acetate. Parts by mass were added, and the mixture was replaced with nitrogen. The reactor was then heated to initiate reflux. Subsequently, 0.1 part by mass of azobisisobutyronitrile was added as a polymerization initiator into the reactor, and the mixture was refluxed at 70° C. for 5 hours to obtain an acrylic copolymer solution.
The weight-average molecular weight of the obtained acrylic copolymer was measured by GPC method using "2690 Separations Model" manufactured by Water Co. as a column, and it was 500,000.
1 part by mass of an isocyanate cross-linking agent and 10 parts by mass of a rosin ester polymer were added to a solution of the obtained acrylic copolymer (100 parts by mass) and stirred to obtain an adhesive (A).
(2) Preparation of single-sided adhesive tape A release paper with a thickness of 150 μm is prepared, the adhesive (A) that forms an adhesive layer is applied to the release treatment surface of the release paper, and dried at 100 ° C. for 5 minutes. A pressure-sensitive adhesive layer having a thickness of 20 μm was formed on the release paper. The pressure-sensitive adhesive layer was attached to one surface of the foamed sheets obtained in Examples and Comparative Examples, and the release paper was peeled off to obtain a single-sided pressure-sensitive adhesive tape having a pressure-sensitive adhesive layer with a thickness of 20 μm.
(3) Preparation of Test Sample A single-sided adhesive tape was punched out into a frame shape with an outer circumference of 75 mm long, 50 mm wide, and 2.0 mm wide. Next, the resulting frame-shaped adhesive tape was sandwiched between two acrylic plates having an arithmetic mean roughness Ra of 1.0 μm and a maximum height roughness Rz of 5.0 μm. Then, bolts were tightened until the clearance between the two acrylic plates was halved to prepare a test sample.

Each component used for manufacturing the foamed sheet in each example and comparative example is as follows.
[resin]
- Resin A: linear low-density polyethylene resin (LLDPE) (manufactured by Exxon Chemical Co., trade name "EXACT3027", density: 0.900)
[Pyrolytic foaming agent]
・Azodicarbonamide average particle size 15 μm
[Lubricant]
・Stearic acid ・Zinc stearate ・Calcium stearate [Antioxidant]
・Phenolic antioxidant: 2,6-di-t-butyl-p-cresol

[Example 1]
(Manufacture of foam sheet)
Each component constituting the foamable composition shown in Table 1 was supplied to an extruder in the parts by mass shown in Table 1, melt-kneaded at 130°C to obtain a foamable composition, and then the foamable composition was obtained. The composition was extruded into a sheet. Next, both sides of the sheet-like foamable composition were irradiated with 5.5 Mrad of electron beams at an acceleration voltage of 500 kV to crosslink the foamable composition, and then the crosslinked foamable composition was heated to 250°C with hot air and an infrared heater. A foamed sheet was obtained by continuously feeding into a foaming furnace held at a temperature of 1.5 mm and heating and foaming. The resulting foamed sheet was then continuously discharged from the foaming furnace. Then, while maintaining the temperature of both sides of the foam sheet at 200 to 250° C., the foam sheet is stretched in the TD direction at a draw ratio of 1.1 times, and the foam sheet is fed into the foaming furnace. The foamed sheet was stretched in the MD direction at a draw ratio of 1.1 times by winding up the foamed sheet at a higher winding speed than the speed (supply speed). Thereby, a foamed sheet (thickness: 0.5 mm) of Example 1 was obtained. The resulting foamed sheet was evaluated according to the evaluation method described above, and the results are shown in Table 1.

[Examples 2 to 4 and Comparative Examples 1 to 6]
A foamed sheet was obtained in the same manner as in Example 1, except that the composition of the foamable composition, the manufacturing conditions, etc. were changed as shown in Table 1. The resulting foamed sheet was evaluated according to the evaluation method described above, and the results are shown in Table 1.
It should be noted that the foam sheet of Comparative Example 4 had too strong a repulsive force, and the measurement of the 50% compressive strength B and the evaluation of the waterproof time could not be performed.

The pressure-sensitive adhesive tape comprising the foamed sheet of the present invention, in which the average cell diameter and the 50% compressive strength B three hours after the start of compression were within a certain range, exhibited excellent waterproofness. On the other hand, the pressure-sensitive adhesive tape provided with a foam sheet having a compressive strength B outside the predetermined range was inferior in waterproofness.

Claims (11)

The average cell diameter, which is the average value of the average cell diameter in the MD direction and the average cell diameter in the TD direction, is 40 to 200 μm, and the 50% compression strength B after 3 hours from the start of compression is 25 to 800 kPa. A foam sheet containing at least one polyolefin resin selected from the group consisting of resins, polypropylene resins, and ethylene-vinyl acetate copolymers. 2. The foam sheet according to claim 1, wherein the cell flatness represented by the following formula (1) is 50 to 1000%.
100×average bubble diameter in MD direction×average bubble diameter in TD direction/(average bubble diameter in ZD direction) ² (1) 3. The foam sheet according to claim 1, wherein the 50% compressive strength A is 50-800 kPa. The foam sheet according to any one of claims 1 to 3, having a glossiness of 20 to 80%. The foam sheet according to any one of claims 1 to 4, which has a contact angle of 80 to 120°. The foam sheet according to any one of claims 1 to 5, having a gel fraction of 30 to 70% by mass. The foam sheet according to any one of claims 1 to 6, which has a closed cell rate of 70% or more. The foam sheet according to any one of claims 1 to 7, which has an expansion ratio of 2 to 20 cc/g. The foamed sheet according to any one of claims 1 to 8 , containing 0.7 to 3 parts by mass of a lubricant with respect to 100 parts by mass of the resin contained in the foamed sheet. The foam sheet according to any one of claims 1 to 9 , having a thickness of 0.03 to 1.5 mm. An adhesive tape comprising the foam sheet according to any one of claims 1 to 10 and an adhesive layer provided on at least one surface of the foam sheet.